Nanowires, with their unique capability to bridge the nanoscopic and macroscopic worlds, have already been demonstrated as important materials for different energy conversion. One emerging and exciting direction is their application for solar to fuel conversion. The generation of fuels by the direct conversion of solar energy in a fully integrated system is an attractive goal, but no such system has been demonstrated that shows the required efficiency, is sufficiently durable, or can be manufactured at reasonable cost. One of the most critical issues in solar water splitting is the development of suitable photoelectrodes with high efficiency and long-term durability in an aqueous environment. Semiconductor nanowires represent an important class of nanostructure building block for direct solar-to-fuel application because of their high surface area, tunable bandgap and efficient charge transport and collection. Nanowires can be readily designed and synthesized to deterministically incorporate heterojunctions with improved light absorption, charge separation and vectorial transport. Meanwhile, it is also possible to selectively decorate different oxidation or reduction catalysts onto specific segments of the nanowires to mimic the compartmentalized reactions in natural photosynthesis.
Recently, We have developed a fully integrated system of nanoscale photoelectrodes assembled from inorganic nanowires for direct solar water splitting. Similar to the photosynthetic system in a chloroplast, the artificial photosynthetic system comprises two semiconductor light absorbers with large surface area, an interfacial layer for charge transport, and spatially separated cocatalysts to facilitate the water reduction and oxidation. Under simulated sunlight, a 0.12% solar-to-fuel conversion efficiency is achieved, which is comparable to that of natural photosynthesis. The result demonstrates the possibility of integrating material components into a functional system that mimics the nanoscopic integration in chloroplasts. It also provides a conceptual blueprint of modular design that allows incorporation of newly discovered components for improved performance.

TBD

In this presentation, we show that by varying the voltages during two-step anodization the morphology of the hierarchical top-layer/bottom-tube arrays TiO2 (TiO2 NTs) can be finely tuned between nanoring/nanotube, nanopore/nanotube, and nanohole-nanocave/nanotube morphologies, which allows us to optimize the photoelectrochemical (PEC) water splitting performance on the hierarchical TiO2 NTs. The optimized photocurrent density and photoconversion efficiency of the hierarchical TiO2 NTs were 1.59 mA cmminus;2 at 1.23 V vs. RHE and 0.84% respectively, which are the highest values ever reported on pristine TiO2 materials under illumination of AM 1.5G. The top porous layer of the hierarchical TiO2 NTs was found to have characteristics of photonic crystal, which was utilized to combine with plasmonic Au nanocrystals to produce visible-light active composite material. The selection of the Au nanocrystals is so that their surface plasmonic resonance (SPR) wavelength matches the photonic band gap of the photonic crystal and thus the SPR of the Au receives remarkable assistance from the photonic crystal substrate. Under visible light illumination (>420nm), the designed material produced a photocurrent density of ~150 mu;A cm-2, which is the highest value ever reported in any plasmonic Au/TiO2 system under visible light irradiation. Additionally, palladium nanocrystals were deposited onto the TiO2 NTs (Pd/TiO2 NTs) and, because of formation Schottky junctions between TiO2 and Pd, the Pd/TiO2 NTs showed significantly higher water contaminant decompsotiion activities than the TiO2 NTs.

The anodization of a titanium metal sheet to form aligned titanium dioxide nanotube (TNT) arrays are of considerable research interest in field such as photocatalysts, solar cells, and sensors [1]. Due to its wide band gap (3.2 eV), only small fraction of the solar light can be absorbed, so that it is an important issue to develop new TNT arrays with enhanced photocatalytic activities under visible light irradiation. In the research field of TiO2 particles, impurity doping such as nitrogen is one of typical approaches to extend spectral response of TiO2 to visible light region [2]. Moreover, codoping of nitrogen and metal ion possesses potential to induce formation of new states which are close to the valence band and conduction band edges, respectively [3]. The codoping approach is an efficient way to absorb wider spectrum of solar irradiation by TiO2 for photoelectrochemical water splitting for solar hydrogen generation and CO2 reduction [4]. In this work, we report on the fabrication of titania nanotube arrays codoped with nitrogen and transition metals such as Fe, V, Cr, and Co (N,M-TNT) for the visible light-driven photoelectrochemical water oxidation.
Vertically aligned N,M-TNT were successfully prepared for the first time via an anodization process using low concentration transition metal (0.05-0.13 at%)-Ti alloys and a subsequent nitridation process.
Photoelectrochemical measurements were performed in a 3-electrode cell containing 0.1 M KOH with a N,M-TNT photoanode, a platinum cathode, and an Ag/AgCl reference electrode. The codoping of nitrogen and transition metal substantially improved the photocurrent of the TNT photoanode under visible light irradiation. The rate of increase in the photocurrent was dependent on transition metal species and it was found that codoping of iron showed the highest enhancement. N, Fe(0.13at%) codoped TNT photoanode yielded a visible-light-induced water oxidation with a photocurrent density of 0.76 mA/cm2 at 0.6 V (vs. Ag/AgCl) under visible light irradiation, which was 13 times and 5 times higher than that of Fe(0.13 at%)-TNT and N-TNT, respectively. Incident photon to current conversion efficiency (IPCE) in the visible light region of 400-700 nm was enhanced by the codoping of N and 0.13 at%-Fe, and it was measured to be 2.6% at 400 nm (at 0.6 V vs. Ag/AgCl). Oxygen detection was also conducted by a fluorescence measurement system.
The photoelectrochemical water oxidation activity of codoped TNT could be further improved by optimizing the amount of doping, kind of dopant, and nanotube structure. This scalable method for codoping to TNT can also be extended to other metal oxide nanotubes.
References
[1] P. Roy, et. al., Angew. Chem. Int. Ed., 50 (2011) 2904. [2] R. Asahi, T. Morikawa, et al., Science, 293 (2001) 269. [3] Y. Gai, J. Li, et al., Phys. Rev. Lett., 102 (2009) 036402., W. Zhu, et al., Phys. Rev. Lett., 103 (2009) 226401. [4] S. Sato, T. Arai, et al., J. Am. Chem. Soc., 133, (2011) 15240.

Photoelectrochemical (PEC) water-splitting is the simplest and cleanest route that directly converts sun light to hydrogen and potentially it will enable a low-cost production of hydrogen. One of the biggest challenges for the realization of the PEC water-splitting is to develop an efficient photoanode having a good light absorption, fast charge transport and transfer properties simultaneously. Typically metal oxides are considered to be good candidates because of their excellent photochemical stability and low-cost. However, their poor material quality such as large amount of defects, low surface area, low charge carrier&’s mobility/conductivity, which largely originated from the preparation method, limits the charge transport and transfer properties.
In this talk, i will present two novel flame processing techniques, i.e., flame reduction and doping, for metal-oxide photoanodes that greatly improve the charge transport and transfer properties, hence enhancing the PEC water-splitting performance. First, we developed a rapid flame reduction method to generate controllable amount of oxygen vacancies in TiO2 nanowires (NWs) that leads to nearly three times improvement in the PEC water-splitting performance. The flame reduction method has unique advantages of a high temperature (>1000 oC), ultra-fast heating rate, tunable reduction environment, and open-atmosphere operation, so it enables rapid formation of oxygen vacancies (<1min) near the surface region without damaging the nanowire morphology and crystallinity, and even applicable to various metal oxides. Second, we designed an ex-situ novel doping method which combines versatile solution phase chemistry and rapid flame annealing process (i.e., Sol-Flame) to dope TiO2 NWs with cobalt (Co). The sol-flame doping method not only preserves the morphology and crystallinity of the TiO2 NWs, but also allows fine control over the Co dopant profile by varying the concentration of Co precursor solution. In addition, the sol-flame doping is a general method to dope metal dopants into the metal oxides NWs regardless of their synthesis method. Finally, we extended the sol-flame doping method to codope TiO2 NWs with tungsten and carbon (W, C) by sequentially annealing W-precursor coated TiO2 nanowires in flame and CO gas. This is the first experimental demonstration that codoped TiO2:(W, C) nanowires outperform monodoped TiO2:W and TiO2:C and double the saturation photocurrent of undoped TiO2 for PEC water-splitting. Given the good controllability and versatility of the flame processing methods, it can be applied to other metal oxide photoanodes such as Fe2O3, WO3 and BiVO4 to further improve their PEC water-splitting performance.

TiO2 is a semiconducting oxide used as a UV-light photocatalyst with potential applications to degradation of organics and solar fuel generation. The photocatalytic activity can be significantly enhanced via the deposition of metal particles onto the oxide surface. Photogenerated electrons are transferred to the metal while the holes remain in the TiO2 valence band thus suppressing electron-hole pair recombination. It is now recognized that atomic level in situ observations of catalytic nanomaterials are critical for understanding structure-reactivity relations because the active form of the material may exist only under reaction conditions. We have undertaken a series of in situ TEM experiments to develop a fundamental understanding of metal particle/TiO2 structure changes in reaction conditions. Such an analysis is performed under in situ conditions in the presence of light and reactants in an environmental transmission electron microscope (ETEM).[1] Here we employ a modified ETEM with a broadband light source to study the behavior of metal particles on TiO2 semiconductor surfaces under photoreaction conditions. Insights from these experiments can help in the design of photocatalysts with better performance and stability. Preliminary experiments showed that the surfaces of anatase nano particles becomes disordered in water vapor under light exposure in the electron microscopes. [2] In this study we investigate the changes that occur in a variety of supported metal systems including Pt/TiO2, one of the most efficient metal/TiO2. Pt coupled anatase nanoparticles were prepared by photodeposition. Light induced surface and interface changes will be presented. Catalytical properties before and after structure change are tested by measuring H2 production under Xenon lamp using gas chromatography. Structure-reactivity relationships will also be discussed for the Pt system and a number of transition metals.
References:
[1]. Miller, B.K.; Crozier, P.A. Microscopy and Microanalysis., 2013 DOI: 10.1017/S1431927612014122.
[2]. Zhang, L.; Miller, B.K.; Crozier, P.A. Nanoletter 2013, DOI: 10.1021/nl304333h

One of the most challenging tasks in photo assisted water splitting for hydrogen generation is the development of low cost, but highly efficient photoelectrodes. Identifying suitable catalysts and processes opens up the way to build photoelectrochemical cells for large-scale hydrogen production.
In this contribution the potential of cold gas spraying for the production of photoelectrodes employing semiconductors for the water oxidation reaction (OER) is presented. Conventional methods of coating usually employ wet chemical methods with subsequent calcination steps to obtain strong binding between the catalyst particles and the substrate. In the cold gas spraying process particles are accelerated to high velocities by a pressurized gas. The nitrogen used as process gas is preheated and then expanded in a De Laval type nozzle. By impact on the substrate the particles deform and break up and thus can build an efficient interface to the back contact (analyzed by cross-section SEM).
Cold gas spraying is a method for the direct coating of surfaces and does not require additives that have to be removed afterwards e.g. by a calcination step but allows the direct formation of a working electrode ensemble.
For the coating process only particles in the µm-range can be utilized. First investigation were performed with P25 TiO2 which was agglomerated to particles with a size of approximately 20 µm. The films yielded seven times higher photocurrents than comparable doctor blade references. This approach was extended to WO3 which shows high activity in the photo-induced water oxidation. Due to the impact on the substrate during the cold gas spraying the particles break up which form a porous film. Furthermore the substrate is deformed so that a caldera-type substrate structure is formed which enables an embedment of the TiO2 particles in the substrate.
Interestingly, in the physical-chemical analysis (Raman, XRD, UPS, XPS) indications were found that the catalyst surface is changed due to the cold gas spray process. Spray parameters and the film thickness on the substrate were varied in order to investigate the influence of the operation properties on the photoelectrocatalytic properties of the TiO2 and WO3 coatings. These findings are compared to films obtained from the established wet-chemical deposition methods.

High porosity three dimensional (3D) nanofiber networks for PEC photoanode development, offer extremely large surface area, excellent charge transport properties, as well as long optical paths for efficient light absorption. 3D cellulose nanofiber networks have been attracting increasing attention in nanomanufacturing owing to their great abundance, low-cost, degradability and bio-compatibility. Here, we used 3D cellulose nanofiber as templates for fabricating PEC photoanode via atomic layer deposition of TiO2. After annealing the cellulose-TiO2 core-shell nanostructure, anatase TiO2 nanotube 3D network was achieved, which offers tremendous surface area for PEC water splitting. Annealing the core-shell structure in vacuum can preserve the carbon from cellulose in TiO2 and make the TiO2 network into “black” so that realizes photoactivity in visible light region. Furthermore, based on the excellent hydrophilic property of cellulose, A novel capillary PEC setup is created as well. Such low-cost and large-area technique for creating “out-of-water” PEC electrode materials might have a potential value for solar energy application for their interaction that between reaction sites and light is not limited by the volume, surface and depth of electrolyte (water).

The GaAs/GaInP2 PV/PEC tandem cell has shown to be a high-efficiency water splitting system, but this material system has not shown the necessary long-term stability and interfacial catalysis and energetics are still an issue. Stabilizing the system using surface treatments or solution additives has improved the stability but band-edge energetics and surface catalysis are still important challenges.
This report will discuss our recent results in modifying the band-edge energetics and attaching homogenous catalysts for hydrogen evolution.

The US Department of Energy&’s (DOE) Fuel Cell Technologies Office has made significant progress in fuel cell technology advancement and cost reduction. Encouragingly, rollouts of fuel-cell vehicles by major automotive manufacturers are scheduled over the next several years. With these rollouts, enabling technologies for the widespread production of affordable renewable hydrogen become increasingly important. Near-term utilization of current reforming and electrolytic processes is necessary for early hydrogen markets, but transitioning to industrial-scale renewable hydrogen production remains essential to the longer term. Central to the long term vision is a portfolio of renewable hydrogen conversion processes, including, for example, the direct photoelectrochemical and thermochemical routes, as well as photo-assisted electrochemical routes. DOE utilizes technoeconomic analyses to assess the long-term viability of these emerging hydrogen production pathways and to help identify key materials- and system-level cost drivers. Sensitivity analysis from the technoeconomic studies will be discussed in connection with the metrics and fundamental materials properties that have direct impact on hydrogen cost. It is clear that innovations in macro-, meso- and nano-scale materials are all needed for pushing forward the state-of-the-art. These innovations, along with specific research and development pathways for advancing materials systems for the renewable hydrogen conversion technologies are discussed.

Development of durable photoelectrochemical (PEC) water splitting devices with high solar-to-hydrogen (STH) conversion efficiency has been a significant materials challenge for decades. Critical requirements on semiconductor materials&’ band gap, band edge, optoelectronic efficiency, and stability must be satisfied simultaneously. While earth-abundant metal oxide semiconductors can be stable, STH efficiencies have been limited by issues related to the wide band gap, band-edge mismatch and the poor opotoelectronic quality in these materials. Tandem cell configurations have been developed to address the band-edges mismatch, but more focus is needed on overcoming the efficiency limitations due to absorption, charge mobility, recombination, interfacial kinetics, etc.
Crystalline III-V materials offer an alternative pathway to efficient STH conversion. Over a range of compositions, these materials have suitable band gaps and optoelectronic quality. In addition, the band edge mismatch has been successfully addressed using monolithic PEC/PV tandem cell designs. Stability, however, remains a key issue. NREL, working with other members of the U.S Department of Energy PEC Working Group (including the Lawrence Livermore National Laboratory, and the University of Nevada at Las Vegas) have been investigating the corrosion of III-V materials and interfaces with a goal to develop surface modification methods for mitigating corrosion. Approaches have included coatings, ion bombardment, surface nitridation as well as electrolyte treatments.
Significant materials challenges remain, and effective usage of resources is needed. The PEC Working Group facilitates progress by bringing together diverse PEC researchers with common interests and goals, promoting collaborative activities, resource sharing, and joint publications.

An artificial photosynthetic system that produces fuels from sun light requires water oxidation components. For efficient solar hydrogen production, a promising strategy is to stabilize a wide variety of non-oxide semiconductors, like Si and GaAs, against photocorrosion. Particularly, protecting 1.7 eV band-gap semiconductors can promise efficient photoanodes for water oxidation. Besides, stabilizing Si photoanodes will open up options of 1.7 eV band-gap photocathodes for hydrogen evolution. Here, we will show that thick layers of TiO₂ coated on Si and GaAs by atomic layer deposition stabilize both in 1M base.
With one-electron redox couples, bare TiO₂ coated Si or GaAs does not conduct holes, but only conduct electrons. When the TiO₂ surface is deposited and intermixed with a metallic layer, it efficiently conducts holes from semiconductor to liquid. Consequently, Si and GaAs water-oxidation anodes can be stabilized in strong, corrosive base, and Si photoanodes have demonstrated to continuously and stably oxidize water for over 100 hours at photocurrent densities of >30 mA×cm^-2 with ~100% internal quantum efficiency. Such facile hole conduction through thick, insulating TiO₂ was enabled by, and depended upon, the presence of metallic films or islands that were deposited on the TiO₂ surface, and was independent of thickness for TiO₂ overlayers ranging from 4.26-142.5 nm in thickness. Hole conduction appears to rely on the presence of mid-band-gap defect states that are induced in the TiO₂ overlayers by invasive metal contacts.

Hydrogenated amorphous silicon carbide (a-SiC:H) has shown promising activities as a photocathode for photoelectrochemical (PEC) water splitting. This material has many promising advantages for large-scale utilization since it is compromised entirely of earth abundant materials and can be fabricated in industrial processing techniques. Therefore, it is of paramount importance to identify and overcome the performance limitations for this material in order to address the global environmental and energy demands.
One limitation for a-SiC:H photocathodes is the non-ideal alignment of the conduction and valence band edge positions. This requires a bias voltage to be applied to drive water splitting, which can be overcome by integrating a PV cell under the photocathode. The challenges for this PV/PEC integration require matching the Vop and Jsc of the PV cell with the Vonset and Jplateua of the photocathode, while at the same time managing the spectral utilization of the sun. To improve the PV matching with the PEC films, we have fabricated several unique single and tandem junction PV cells with both amorphous silicon and nano-crystalline silicon, showing enhanced current matching and performance.
In addition, we have utilized several surface passivation techniques to reduce corrosion during the PEC testing. Using both ALD and RF sputtering depositions, we deposited thin transparent conducting layers on the surface of the a-SiC:H photocathode, which showed improved onset potentials, saturated photocurrent densities and enhanced stability.
Finally, we have investigated various hydrogen evolution catalysts deposited on the passivated a-SiC:H photocathodes, showing significantly enhanced water splitting capabilities at reduced bias potentials. Electronic band diagrams have been developed to explain the activity (or non-activity) of different catalysts.
Overall, we have been able to identify and address significant hurdles in the development a-SiC:H photocathodes for solar water splitting, and herein report our recent advances with regards to PV integration, surface passivation, and hydrogen evolution catalysis.

We are developing an artificial photosynthetic system that will only utilize sunlight and water as the inputs and will produce hydrogen and oxygen as the outputs. We are taking a modular, parallel development approach in which the three distinct primary components-the photoanode, the photocathode, and the product-separating but ion-conducting membrane-are fabricated and optimized separately before assembly into a complete water-splitting system. The design principles incorporate two separate, photosensitive semiconductor/liquid junctions that will collectively generate the 1.7-1.9 V at open circuit necessary to support both the oxidation of H2O (or OH-) and the reduction of H+ (or H2O). The photoanode and photocathode will consist of rod-like semiconductor components, with attached heterogeneous multi-electron transfer catalysts, which are needed to drive the oxidation or reduction reactions at low overpotentials. The high aspect-ratio semiconductor rod electrode architecture allows for the use of low cost, earth abundant materials without sacrificing energy conversion efficiency due to the orthogonalization of light absorption and charge-carrier collection. Additionally, the high surface-area design of the rod-based semiconductor array electrode inherently lowers the flux of charge carriers over the rod array surface relative to the projected geometric surface of the photoelectrode, thus lowering the photocurrent density at the solid/liquid junction and thereby relaxing the demands on the activity (and cost) of any electrocatalysts. A flexible composite polymer film will allow for electron and ion conduction between the photoanode and photocathode while simultaneously preventing mixing of the gaseous products. Separate polymeric materials will be used to make electrical contact between the anode and cathode, and also to provide structural support. Interspersed patches of an ion conducting polymer will maintain charge balance between the two half-cells. The modularity of the system design approach allows each piece to be independently modified, tested, and improved, as future advances in semiconductor, polymeric, and catalytic materials are made. Hence, this work will demonstrate a feasible and functional prototype and blueprint for an artificial photosynthetic system, composed of only inexpensive, earth-abundant materials, that is simultaneously efficient, durable, manufacturably scalable, and readily upgradeable.

For the application as photocathodes in integrated photoelectrochemical water splitting devices the thin film silicon technology stands out as an attractive choice, because it combines low-cost production, earth-abundance and versatility. Since the electrochemical potential to electrolyze water generally lies above 1.23 V, great importance is given to the latter characteristic, as thin film silicon solar cells can be adjusted to provide an extended range of achievable voltages, without impairing device efficiency. Nevertheless, as integrated water splitting devices additionally require chemical-resistant electrodes, stability issues of the silicon solar cells in contact with aqueous solutions need to be addressed.
We report on the optimization and usage of thin film silicon tandem junction solar cells. Tandem junction solar cells consist of two sub-cells connected in series. In this work, we investigate two types of tandem solar cells: (i) two amorphous (a-Si:H/a-Si:H) sub-cells with an open circuit voltage VOC of 1.87 V and a solar conversion efficiency of 10.0% (ii) and amorphous connected to microcrystalline (a-Si:H/µc-Si:H) sub-cells with a VOC of 1.42 V and an efficiency of 10.8%.
a-Si:H and µc-Si:H layers were deposited by plasma enhanced chemical vapor deposition, using a mixture of SiH4, H2, CH4, B(CH3)3 and PH3 gases. The optical band gap E04 was evaluated using photothermal deflection spectroscopy measurements and the crystallinity ICRS of µc-Si:H was determined by means of Raman spectroscopy. Solar cells were investigated by current-voltage measurements under AM 1.5 illumination. The photoelectrochemical performance of the electrodes was evaluated in an aqueous 0.1M H2SO4 solution under Xe halogen lamp irradiation (100 mW/cm2).
By carrying out cyclic voltammetry measurements, we demonstrate the performance of the developed silicon based photocathodes, with respect to photocurrent densities and onset potentials for water reduction. a-Si:H/µc-Si:H photocathodes with a Pt back contact, for instance, exhibit a photocurrent onset potential of 1.3 V vs. the reversible hydrogen electrode (RHE) and a high photocurrent of 9.0 mA/cm2 at 0 V vs. RHE. However, the poor stability of the photocathodes, evaluated using chronoamperometric measurements, suggests that the application of protective layers on the silicon surface will be essential. During operation at 0 V vs. RHE, photocathodes without back contacts, i.e. direct contact of the silicon surface to the acidic electrolyte, generate stable photocurrents only for two hours. In this regard, various back contact interface designs are investigated, including silicon-silicon (µc-SiC:H, µc-SiOx:H), silicon-metal (Pt, Ag, Al, Ni, Mo) and silicon-TCO-metal interfaces. The corrosion behavior of both single layers and complete photovoltaic devices are studied in a broad pH range and in different electrolyte concentrations. Thereby, the electrochemical stability of the respective interfaces is evaluated.

The photoelectrochemical (PEC) conversion of water to hydrogen fuel at an illuminated semiconductor surface is a promising solution to the intermittency problem faced by solar energy. However, semiconductor stability is a serious issue for this approach due to the harsh conditions in which PEC hydrogen production is carried out. The field is currently limited to the use of wide bandgap oxides for water splitting anodes and they suffer from poor performance under visible light illumination. Silicon is a promising photoanode material but its sensitivity to anodic corrosion has hindered its use in OER applications. To address the stability problem, an ultrathin film of Ni (~ 2 nm) was used to form a metal-insulator-semiconductor (MIS) Schottky diode with n-Si while also serving as an anticorrosion layer and an active OER catalyst. The electrode performed well and was able to achieve 55 mA/cm2 and 500 mV of photovoltage under ~ 2 suns of illumination. In addition to the high performance, the electrode was very robust and able to pass 10 mA/cm2 of photocurrent for at least 80 hours with no sign of performance decay in a mixed electrolyte of potassium borate and lithium borate. The addition of lithium ions to the electrolyte was found to greatly enhance the stability of the nickel film.

Solar hydrogen production by photoelectrochemical water splitting holds great promise for efficient solar energy harvesting and storage. To achieve spontaneous water splitting, developing efficient photoelectrodes with both high photovoltage and high photocurrent is highly desirable. However, current studied photocathodes such as p-Si, p-Cu2O and p-GaP have photovoltage lower than half of 1.23 V, the minimum voltage required for water splitting. Here we present a photocathode using amorphous Si thin film with TiO2 encapsulation layer for efficient solar hydrogen production. With platinum as catalyst, a photocurrent onset potential of 0.93 V vs reversible hydrogen electrode potential and saturation photocurrent of 11.6 mA/cm2 are measured. Importantly, this a-Si photocathodes exhibit an impressive photocurrent of ~6.1 mA/cm2 at a large positive bias of 0.8 V vs RHE, which is the highest for all reported photocathodes at such positive potential. Ni-Mo alloy is demonstrated as an alternative low-cost catalyst with onset potential and saturation current similar to those obtained with platinum. This low-cost photocathode with high photo-voltage and current is a highly promising candidate for future tandem water splitting cells.

In this study, we introduced a plasmonic TiO2-Fe2O3 co-catalyst photoelectrode for improving water splitting process. The absorption of incident photons and the separation of photo-generated electron-hole pairs can been enhanced due to the wide range energy absorption and strong built-in electric field from the combination of these two metal oxide semiconductors with silver plasmonic nanoparticles(NPs). Plasmonic TiO2-Fe2O3 co-catalyst photoelectrode are fabricated through the precipitation and solution processing method. 28 times higher photocurrent has been observed with the optimization ratio of plasmonic TiO2-Fe2O3 co-catalyst with respect to the pure TiO2 under the visible light irradiation. The enhancement mechanism in the plasmonic co-catalyst system is investigated by different combinations of electrode structure. The crystallinity and absorption band edge of TiO2-Fe2O3 co-catalyst have been respectively characterized by the X-ray diffractometer (XRD) and ultraviolet - visible absorption spectroscopy (UV-VIS).

In this work, TiO2/hematite (α-Fe2O3) core-shell hierarchical nanostructured arrays were constructed on fluorine-doped tinoxide (FTO) substrates for use in photoelectrochemical (PEC) water splitting. The rutile TiO2 nanorod (NR) array was grown on the FTO substrate by hydrothermal method and the hematite thin layer were subsequently formed on the surface of NR array using chemical vapor deposition (CVD). Compared to the thin-film hematite photoanode, a five-fold enhancement of the photocurrent density is measured in the PEC water splitting cell with the TiO2 NR/hematite core-shell array photoanode. An optimized photocurrent density of 1.1 mA/cm2 at 1.23 V versus reversible hydrogen electrode (RHE) under AM 1.5 at 100 mW cm-2 is obtained. The one-dimensional rutile TiO2 NR arrays provide a large surface area for the deposition of hematite layer with the well-controlled thickness. The photoholes can therefore efficiency transport to the surface of the photoanode for water oxidation. The dynamics of charge transfer were investigated using photocurrent transient and electrochemical impedance spectroscopy (EIS) measurements. The results will be reported in detail in the presentation.

As a result of recent intense efforts toward using hematite (Fe₂O₃) as photoelectrode material for sunlight-driven water splitting, researchers have advanced the understanding of its various shortcomings (including short hole diffusion length, poor light absorption coefficient, large overpotential for water oxidation, surface-state mediated electron-hole recombination, Fermi level pinning). Despite these intrinsic inadequacies, many (or all) of these challenges could potentially be addressed through the use of heterostructure designs, the interfacing of one or more materials on the hematite surface. An overlayer on hematite can produce a number of effects that may manifest as a desirable cathodic shift in the photocurrent onset potential (i.e. a reduction in the observed oxygen evolution overpotential). For instance, a known oxygen evolution catalyst may improve the kinetics of charge transfer, but may alternatively act to decrease surface recombination¹ and/or reduce Fermi level pinning.² A p-type overlayer can create a built-in junction that generates enhanced photovoltage.³
Distinguishing between these effects is an important challenge toward learning how to design advanced photoelectrodes. To this end, we performed a series of studies employing mixed metal oxide overlayers, seeking to produce performance-enhanced hematite photoelectrodes and to develop methods of understanding the nature of the improvements. Hematite was interfaced with a variety of mixed metal oxides with a focus on amorphous (NiFeO_x, CoFeO_x) and spinel ferrite (CuFe₂O₄, CaFe₂O₄, NiFe₂O₄) compounds, materials chosen from candidates exhibiting good oxygen-evolution catalytic activity, p-type conductivity, or both. We will present our findings on which treatments gave promising improvements along with studies toward a mechanistic understanding of each different heterojunction behavior.
Relevant references:
1) Badia-Bou, L.; Mas-Marza, E.; Rodenas, P.; Barea, E. M.; Fabregat-Santiago, F.; Gimenez, S.; Peris, E.; Bisquert, J. J. Phys. Chem. C 2013, 117, 3826
2) Du, C.; Yang, X.; Mayer, M. T.; Hoyt, H.; Xie, J.; McMahon, G.; Bischoping, G.; Wang, D. Angew. Chem. Int. Ed. Engl. 2013, doi: 10.1002/anie.201306263
3) Lin, Y.; Xu, Y.; Mayer, M. T.; Simpson, Z. I.; McMahon, G.; Zhou, S.; Wang, D. J. Am. Chem. Soc. 2012, 134, 5508

Photoelectrochemical cells (PECs) have attracted enormous attention for solar hydrogen generation. An overwhelming majority of the work has focused on room-temperature PECs, as it is believed that the efficiency of photovoltaic devices decreases with increasing temperature. While that the photovoltage generally decreases with temperature due to the rising intrinsic carrier concentration, electrocatalytic activity and minority/majority carrier transport properties in many materials actually improve with temperature. Hematite, in particular, is a promising material for elevated-temperature PECs (achieved through moderate concentration) because both the minority and majority carrier mobilities increase exponentially with temperature as a result of improved electron hopping dynamics. Therefore, room temperature is not likely the optimal temperature to operate hematite-based PECs.
In this work, we used various Ti-doped hematite thin films grown by pulsed-laser deposition as a model system to study the effect of temperature and light intensity on the photoelectrochemical properties. To eliminate possible microstructural effects across multiple samples, all of the photoanodes were dense and smooth (roughness < 1nm). These hematite-based photoanodes were characterized in a temperature-controlled photoelectrochemical cell (5 - 80 ^oC) using a concentrated solar simulator (up to 10 suns). The photovoltage decreased as expected with increasing temperature. However, at the same time, we observed a significant enhancement in the photocurrent under the synergetic effect of temperature and light intensity. Tuning both temperature and light intensity opens up the opportunity to further enhance PEC efficiency, and to understand the semiconductor/liquid interface.

Sunlight assisted water splitting is an appealing route towards sustainable, large-scale production of hydrogen. In a photoelectrochemical cell, gaseous hydrogen and oxygen are evolved at the cathode and anode respectively, of which at least one is a photoactive semiconductor. The oxygen evolution reaction (OER) usually needs a high overpotential, which reduces the solar-to-chemical energy conversion significantly. Efforts to alleviate this issue have so far focused mostly on modifying the semiconductor/electrolyte interface through addition of co-catalysts. An interesting, yet less explored strategy involves improving the intrinsic properties of the semiconductor. By pursuing this avenue, we show how engineering the flat band potential in the semiconductor can lead to a decrease in overpotential comparable to the best results obtained using OER co-catalysts. To this end, we utilize as a model system photoanodes based on flat, thin films of hematite (Fe ₂O₃), a semiconductor known to suffer from high values of overpotential for the OER. We demonstrate cathodic (i.e. towards more negative potential) shifts of the OER onset potential of up to 200 mV by tuning fabrication parameters (oxidation time) of the photoanodes. Electrochemical impedance spectroscopy (EIS) measurements reveal that variation of the oxidation time has very little effect on OER dynamics. This in turn indicates that oxidation time affects the transport of photo-generated charges through the semiconductor. Mott-Schottky analysis establishes a strong correlation between the observed shift of onset potential for the OER and a shift of flat band potential in hematite. We further explore the changes in the transport properties of the fabricated photoanodes by using a variety of techniques, including optical spectroscopy, conductivity measurements, XPS, XRD and TEM.

Hematite (α-Fe2O3) has been investigated for decades for photoelectrochemical (PEC) water splitting because it has a band gap of 2.1 eV, is composed of abundant elements, is environmentally benign, is stable throughout a wide range of pH, and shows potential for up to 15% solar-to-hydrogen efficiency. While hematite has many desirable properties, several challenges remain, including sluggish oxygen evolution reaction (OER) kinetics and severe mismatch between hematite&’s absorption depth and minority carrier collection length. This mismatch precludes its use in planar films, and nanostructured architectures are required to reach large photocurrents.
We report on the fabrication of hematite thin films and hematite-coated scaffolds and their use in PEC water splitting, focusing specifically on the roles of film thickness, dopants, interfaces, and nanostructured architecture. Hematite thin films and coatings were fabricated with tight control over the thickness by successive ionic layer adsorption and reaction (SILAR), wherein a substrate is alternately dipped between iron-containing and oxidizing baths analogously to atomic layer deposition. SILAR growth rates were ~0.5 nm per cycle, and the deposited iron hydroxide was then annealed to form hematite. Annealing at 450 °C was sufficient for the hydroxide-to-hematite phase transformation, but photocurrents were in the microamp range. Annealing at 775 °C resulted in much larger photocurrents because of diffusion of Sn from the F:SnO2 (FTO) substrate into the hematite. Sn increases the conductivity of the film and may enhance OER kinetics.
Photocurrent initially increased with film thickness due to enhanced light absorption, but saturated at ~0.7 mA/cm2 (all currents reported at 1.23 V vs RHE) after about 120 cycles due to limitations in charge collection. Photocurrent increased to 1.1 mA/cm2 upon addition of an ultrathin (<10 nm) TiO2 interlayer between the FTO and hematite, which likely improves the interface and reduces shunting. Additionally, depth-profiled XPS showed that annealing causes diffusion of both Ti and Sn through the hematite. The surface is particularly Ti-rich, indicating the importance of Ti in passivating surface traps or catalyzing OER.
To increase light absorption while maintaining hematite thickness similar to the collection length, we have coated mesostructured Sb:SnO2 (ATO) scaffolds with hematite. Inverse-opal type scaffolds were constructed by partially filling the pore space between a close-packed monolayer of micron-diameter spheres with ATO, and then removing the spheres. This scaffold can increase surface area by up to 85% compared to a flat film. Photocurrent increased by ~15% to 1.25 mA/cm2, which was limited by ATO conductivity and imperfect scaffold formation. Appropriate use of nanostructured architectures, such as multilayer opals or nanowire arrays, and interfacial treatments will increase efficiency of PEC water splitting with hematite.

Due to a short charge collection length, nanostructuring is the key approach to achieve both efficient light absorption and charge collection in hematite based photoanodes for photocatalytic water oxidation. The overall water oxidation efficiency of ultrathin hematite electrodes, however, has been poor. The poor performance is known to be due to electron/hole recombination in the bulk as well as at surface trap states. We utilize atomic layer deposition (ALD) to make conformal thin film hematite electrodes with controllable dimensions and composition. These models systems allowed us to show that either doping or modification of the contacting substrate with an oxide underlayer lead to substantially improved performance; however the cause of the improvement is distinct. A combination of photoelectrochemical, spectroscopy and microscopy measurements was employed to gain insight in the material properties which lead to the improvement. For example, Raman line-shape analysis combined with absorption and photoelectrochemical measurements demonstrated a strong correlation between the degree of film crystallinity and the size of crystallites with the water splitting efficiency. In addition, the surface properties were altered by doping which improved the water oxidation to surface-state recombination branching ratio. Recent results on the identification of the surface structure and states will also be presented.

Hematite (α-Fe₂O₃) is an attractive material for the photoelectrochemical (PEC) oxidation of water due to its broad absorption of light in the visible (bandgap ~2 eV), chemical stability, and great elemental abundance. However, current α-Fe₂O₃-based photoanodes are impractical for solar fuel production because of the short hole collection length, relatively rapid rate of charge carrier recombination, and slow water oxidation kinetics. However, there remains significant room for improvement: it is thought that an optimal α-Fe₂O₃ photoanode could obtain a 4x improvement in current density and significantly reduced onset potential as compared to the current state-of-the-art. Recently, efforts have been made to improve the performance of α-Fe₂O₃ photoanodes through doping, catalytic enhancement, and nanostructuring. Here, we report the enhanced PEC performance of α-Fe₂O₃ photoanodes by utilizing atomic layer deposition (ALD) to produce epitaxial α-Fe₂O₃ films which conformally coat highly crystalline, transparent, and conductive support structures. The control of α-Fe₂O₃ epitaxy opens routes to control and optimize crystallographic orientation, thereby improving both charge collection efficiency and catalytic interfacial processes. In addition, the sharp and well-defined structure between photoanode and conductive support afforded by the epitaxy is capable of reducing the density of interfacial trap states, therefore lowering instances of recombination as electrons pass into the current collector. Critically, we show that it is possible to employ epitaxial ALD of α-Fe₂O₃ on high aspect ratio nanowire architectures to allow for increased optical absorption while maintaining a short charge transfer pathway within the photoanode itself.

Hematite has emerged as a promising anode material for photoelectrochemical (PEC) water splitting due to its visible light suitable band gap energy and excellent stability under caustic condition. Considerable effort has been devoted to investigate the kinetics of the interfacial charge transfer from hematite surface for water oxidation by different characterization techniques. These works pointed out the critical role of surface states on both hole accumulation and recombination processes. However, the detailed mechanisms are still unclear. In our work, nanostructured hematite films were made by low cost dip coating procedure and its photoelectrochemical property was affected dramatically after oxygen plasma post treatment. XPS and valence band PES measurements of the hematite samples revealed a variation of structural defects on hematite surface as a function of plasma treating period, which matches both variations of photocurrent density and of surface states investigated by impedance spectroscopy. These findings demonstrate strong correlation among surface state, crystal defects and performance.

Water oxidation is a thermodynamically and kinetically demanding reaction and has been identified as a key step in future solar photocatalytic light harvesting technologies. Developing earth-abundant, chemically stable and low cost electro-catalysts to facilitate water oxidation has been an on-going challenge.
Since the water oxidation reaction releases protons, it is anticipated that its activation energy will be influenced by pH. Towards this goal, we have studied the pH dependence of two promising catalysts. Thin films of manganese oxide (MnOx) and iron oxide (Fe2O3) were prepared by a facile technique, photochemical metal-organic deposition (PMOD). They were tested for electrocatalytic activity in water oxidation over a range of neutral (pH=6) to alkaline (pH=13) solutions both with and without buffering. Both electro-catalysts are stable throughout the whole pH range of the experiments. In the unbuffered solution and at pH>10.5, the onset potential of the oxygen evolution reaction for MnOx is 0.1 V lower than that of Fe2O3. However, at pH<10.5 their onset potential is almost the same and invariable with respect to change in pH. This is in contrast with the buffered solution where the onset potential changes with a Nernstian slope almost over the entire pH range. This observation suggests a change in the mechanism of water oxidation around pH=10.5, which we plan to identify and investigate by future spectro-electrochemical studies. Our results are relevant for better understanding the influence of protons on water oxidation at the metal oxide catalyst surfaces.

Vertically aligned SnSe nanosheets are successfully synthesized on different substrates (silicon, quartz, and fluorine-doped tin oxide glass) via a non-catalytic vapor phase synthesis method for the first time. Such substrate independent feature could benefit the fabrication and application of various nanodevices due to the considerably enhanced surface area. The SnSe nanosheets have the thickness of ~ 20minus;30 nm and the lateral dimension of several mu;m. The analyses using X-ray diffraction and high-resolution transmission electron microscopy demonstrate that nanosheets are single crystalline with an orthorhombic crystal structure of the Pnma 62 space group. Two-dimensional nanosheets are formed due to the anisotropic atomic bonding nature of the SnSe crystal, which is apparently different from the oriented attachment growth or the exposed plane suppressing growth. They also reveal faceted edge planes, which is elucidated in detail based upon the difference in the surface energy of each atomic plane. SnSe nanosheets show a direct band gap of ~1.1 eV, ideally meeting the requirements as a high-performance light absorbing material for solar cell applications.

Mo-doped BiVO4 films were prepared by a simple electrodeposition and annealing procedure and studied as oxygen evolving photoanodes for application in a water splitting photoelectrochemical cell. Undoped BiVO4 was known as a promising photoanode that has a direct bandgap of 2.4 eV and the appropriate valence band position for O2 evolution. Its conduction band edge position and flat band potential are fairly negative compared with most other narrow bandgap (i.e., Eg < 2.6 eV) oxide-based photoanode materials, located just short of the thermodynamic level for H2. As a result, complete water splitting with BiVO4 requires only a small amount of external bias. However, the poor electron-hole separation yield by the low mobility of electron was known to be one of the main limiting factors for BiVO4-based photoanodes. To improve the electron transport properties, Mo-doped BiVO4 electrodes were prepared by an electrochemical route. The electrochemical route provided an effective way of doping BiVO4, and the optimally doped sample, BiV0.97Mo0.03O4, increased the electron-hole separation yield from 0.18 to 0.58 at 0.6 V vs. RHE, which is a record high separation yield achieved for BiVO4-based photoanodes. As a result, BiV0.97Mo0.03O4 generated impressive photocurrents, for example, 2 mA/cm2 at a potential as low as 0.4 V vs. RHE for sulfite oxidation, which has fast oxidation kinetics and, therefore, the loss of holes by surface recombination is negligible. In addition, photocurrent of Mo-doped BiVO4 generated by front-side illumination becomes comparable to the photocurrent from back-side illumination, while photocurrent of undoped BiVO4 generated by front-side illumination is typically significantly lower than the photocurrent from back-side illumination. This feature will make this Mo-doped BiVO4 suitable for the construction of a tandem structure where front-side illumination is necessary. For photooxidation of water, BiV0.97Mo0.03O4 was paired with FeOOH as an oxygen evolution catalyst (OEC) to improve the poor catalytic ability of BiV0.97Mo0.03O4 for water oxidation. The resulting BiV0.97Mo0.03O4/FeOOH photoanodes generated a significantly improved photocurrent for water oxidation compared to previous reported results, but the photocurrent of BiV0.97Mo0.03O4/FeOOH for water oxidation could not reach the photocurrent of BiV0.97Mo0.03O4 for sulfite oxidation. In order to examine the cause, the effects of Mo-doping on the interaction between BiVO4 and FeOOH and the effects of FeOOH on the electron-hole separation yield of BiV0.97Mo0.03O4 were investigated in detail, which provided new insights into semiconductor-OEC interactions.

Fulvalene diruthenium tetracarbonyl, FvRu2(CO)4 which was originally reported in 1997 [1] is getting renewed attention due to its potential for photo-isomeric storage of solar energy. When this molecule absorbs a photon in the near UV (~3.5 eV), its conformation changes from the stable ground state to a meta-stable isomer about 1 eV higher in energy. The stored energy is recovered as the molecule is nudged back to its ground state conformation with an external stimulus like heat. One of the hurdles for commercialization is that ruthenium (Ru) is a scarce element. Other shortcomings include non-optimal absorption spectrum to harvest broad range of solar wavelengths and storage of only a small fraction (~30%) of the absorbed photon energy. The goal of this research is to find earth abundant alternatives which can address these issues.
Elements chosen to replace Ru are manganese (Mn), vanadium (V) and chromium (Cr). Valence electrons of these transition metals are known to give rise to magnetic interactions which can increase the stored energy. Another modification explored is the substitution of carbonyls with halogens such as chloride (Cl) or bromide (Br). Halogens can help to modify storage properties of the molecule further. Proposed molecules are investigated by quantum mechanical calculations using the DFT method with B3LYP functional and Pople type basis sets augmented with polarization functions. Atomic models consist of fulvalene incorporated with one of Mn, V or Cr with halogen ligands, for example fulvalene dichromium dichloride (FvCr2Cl2).
Test case for the calculations is the original molecule FvRu2(CO)4. Calculated properties are stored energy of 1.4 eV, forward energy barrier of 3.0 eV, and reverse energy barrier of 1.6 eV. These values agree reasonably well with the experimental results [1]. In the case of the proposed molecules, calculated values of the stored energy are 1.0 eV for FvMn2Br2, 1.8 eV for FvCr2Cl2, and 2.0 eV for FvV2Br2. Spin densities of the atoms indicate magnetic coupling between the two transition metal atoms in the molecule, as expected.
Overall, results suggest that Ru can be replaced with earth abundant elements some of which enhance energy storage capability of the original fulvalene molecule. Calculations pertaining to other storage properties are in progress and results will be reported during the presentation.
[1] R. Boese et. al, "Photochemistry of fulvalene tetracarbonyl diruthenium and its derivatives; efficient light energy storage devices," J. Am. Chem. Soc., 1997, 119 (29), p. 6757

Titanium dioxide (TiO2) has been extensively investigated as a photoanode for photoelectrochemical (PEC) water splitting, owing to its high photocatalytic activity, proper band-edge positions, superior photo-chemical stability, low-cost and non-toxicity. However, the poor charge transport and transfer properties of TiO2 strongly limit its PEC performance. Here, we present novel flame processing methods, i.e., ‘flame-reduction&’ and ‘sol-flame doping&’, to improve the charge transport and transfer properties of TiO2 nanowires (NWs) by introducing controllable amount of oxygen vacancies and metal ion dopants into the TiO2 lattice, and study their impacts on PEC performance. The flame processing methods have unique advantages of high temperature (>1000 oC), ultra-fast heating rate, and open-atmosphere operation, which enables rapid formation of oxygen vacancies or diffusion of dopants (less than one minute) without damaging the nanowire morphology and crystallinity, while allowing fine control over the concentration of the oxygen vacancies and metal ion dopants. Both flame reduction and doping greatly improve the charge transport and transfer properties of TiO2 NWs, which leads to nearly three times improvement in the PEC water-splitting performance. We believe that the good controllability and versatility of our flame processing methods will impact on various metal oxide photoanodes to further improve their PEC water splitting performance.

As a new direction for artificial photosynthesis, polynuclear photocatalysts containing an oxo-bridged binuclear chromophore coupled with a multi-electron transfer catalyst anchored in mesoporous silica scaffold have been developed in our laboratory. This system can not only control precisely the redox potential by selecting appropriate metals but achieve robustness by covalently anchoring all-inorganic units on silica surface. In this study, we have developed a new photochemical method for coupling of an iridium oxide nanocluster next to a binuclear ZrOCo unit anchored on SBA-15 mesopore surface. Particularly, the photochemical deposition allowed minimize the interference of IrO_x with photon absorption by binuclear MMCT chromophores, and the blocking of IrO_x with Zr sites for CO₂ reduction. As a result, for the first time we closed the photosynthetic cycle (i.e., the photoreduction of CO₂ to CO under oxidation of water to O₂) at a precisely defined nanoscale polynuclear unit.

Increasing water pollution due to continuous release of waste from the plastic, textile and dye industries are dangerous and toxic for living beings. Getting rid of these chemicals from industries is a challenging task and advanced oxidation processes like photocatalysis helps us to achieve this with ease. Dye degradation by photocatalysis is traditionally carried out by chemical decomposition processes. However, photocatalysis by semiconductor nanoparticles have a brighter future ahead as they provide tunable high absorption, stability and surfaces area besides being inexpensive and nontoxic. In photocatalytic reactions semiconductors are generally used as catalysts in presence of ultraviolet light and sunlight. Presently not many materials exist with an appropriate band gap position necessary for practical applications. Most photocatalytic materials possess transition-metal cations with a d0 electronic configuration or metal cations with d10 electronic configuration. These ions have empty d or sp orbitals which form the bottom of the conduction bands. The top of the valence bands of metaloxide photocatalysts with d0 or d10 metal cations consist of O2p orbitals, which are located at about +3 eV. This band gap is too wide to absorb any visible light. Hence in this report we focus our attention on ZnO-GaN heterostructures with lower effective band gap in visible region having potential photocatalytic applications.
This work reports on a solid solution of GaN and ZnO with a wurtzite structure. This material contains the typical metal cations with d10 configuration and is expected to show photocatalysis under visible light irradiation. To obtain a self-supported mode with high structural stability optimization of growth parameters have been carried out to a single phase wurtzite structure of ZnO-GaN composite. GaN-ZnO nanospheres (diameter~250 nm) have been grown with potential applications in environmental science. The products were characterized by XRD, SEM, DRS, FTIR and Raman spectroscopy. The effective band gap is lower than 3.34 eV because of the presence of Zn3d and N2p electrons in the upper valence band which provides p-d repulsion for the valence band maximum, resulting in narrowing of the band gap. Raman spectra reveals the A1 (LO)_GaN to be redshifted to 729 cm^-1 for the ZnO-GaN solid solution as compared to GaN. A series of weak anomalous modes appears in the low energy spectrum (138 cm^-1 to 253 cm^-1). DRS studies reveal the approximate band gap values for the composite and elemental mapping shows a uniform distribution of cations. The results will be presented in detail.
Reference: J. Am. Chem. Soc. 2005, 127, 8286-8287.

Hierarchical flower-like TiO2 / RuO2 nanorod heterostructure was synthesized. Amorphous TiO2 layer was firstly spin-coated onto RuO2 nanorods on Si wafer prepared by RF sputtering. The amorphous layer was transformed to flower-like titanate under acetic acid solvothermal reaction. The titanate flower with the diameter of ~2mu;m was composed of needle-like petals. After heating at 700 oC for 1 h, the titanate further transfer to anatase without morphology change. Furthermore, the TiO2 flower / RuO2 nanorods heterostructure was used as working electrode for photoelectrochemical water splitting in 1 M KOH aqueous solution. It showed enhanced photocurrent (3.63 mA/cm2 at 1.23 V vs RHE) compared with that of TiO2 nanoparticles / RuO2 nanorods (0.91 mA/cm2 at 1.23 V vs RHE) under the irradiation of a Xe lamp. The enhanced performance can be ascribed to the large surface area of flower-like TiO2, and the good conductivity of RuO2 nanorod. Furthermore, the TiO2 / RuO2 heterostructure can reduce the recombination of excited electrons and holes significantly. These suggest the hierarchical heterostructure that TiO2 flower / RuO2 nanorods can be employed as a potential electrode material for photoelectrochemical water splitting.

The production of stable, all-solid dye sensitized solar cells requires the deposition of a solid hole conductor. One of the main problems is inefficient pore filling of the nano-structured metal oxide electrode. Copper thiocyanate (CuSCN) is considered one of the most promising hole conductor materials, due to its reasonable hole conductivity (ge;5×10minus;4 S/cm), chemical stability and transparency in the visible range. Electrodeposition is a simple, efficient method for coating complex shapes. Jin at el. [Electrochimica Acta 53, 6048-6054 (2008)] suggested the use of electrochemical deposition in aqueous mild basic electrolytes, preventing acid damage to metal oxide electrodes. However, CuSCN coverage on ZnO nanowires was not entirely homogeneous. In this work we present complete superfilling of CuSCN onto ZnO nanowires electrode, using pulsed electrodeposition in aqueous mild basic electrolytes. The method uses recurrent potential pulses combined with periods of zero potential. The zero-potential interval discharges the electrolyte charged layer and regenerates the ion concentration near the cathode surface, resulting in denser nucleation and more uniform coating. The pulse sequence was found to determine the deposition rate and the p-doping level by controlling S/Cu ratio in the deposited CuSCN films. Current-voltage characteristics showed excellent diode properties, indicating close interfacial ZnO/CuSCN contact. Pulsed electrodeposition offers the advantages of low cost, room temperature deposition in a one-pot process, combined with fast deposition rates, making it highly accessible for lab-scale deposition as well as scale up for industrial processes.

For maximized efficiency of photoanodes and photocathodes for water splitting, oxygen/hydrogen evolution catalysts must be effectively interfaced with semiconductor light absorbers. In addition, many photoelectrodes are unstable during operation at high or low pH (convenient regimes for operating a water-splitting device), but the effects of various catalysts and the nature of the catalyst/semiconductor interface on this degradation process have not been clearly demonstrated. Here, the photoelectrochemical performance and degradation behavior of BiVO4 photoanodes are correlated with the chemistry, structure, and morphology of thin catalyst layers deposited by various methods (atomic layer deposition, sputtering, and spin coating). BiVO4 single crystals were synthesized hydrothermally, and the structure and morphology of oxide catalyst surface layers of different thickness and crystallinity were investigated with high resolution transmission electron microscopy (TEM). These data were correlated to the photoelectrochemical oxygen evolution performance and durability at various pH values by assembling the crystals into thin film photoelectrodes. The results indicate that the thickness of the catalyst layer as well as the dispersion of the catalyst material exhibit controlling effects on the photocurrent and lifetime. Overall, this study shows that understanding the atomic-level details of the semiconductor/catalyst interface and their effect on performance could lead to rational design of better photoelectrodes.

Semiconductor materials are generally very ideal for harvesting and converting solar energy into electro-chemical energy. Scheelite type bismuth vanadate (BiVO4) with a bandgap (Eg) ~ 2.4 eV has been found to be potentially suitable visible light photocatalyst. However, its practical application as photocatalyst is significantly limited due to rapid recombination of generated electron-hole pairs, slow hole transfer kinetics, and poor electrical conductivities. The photocatalytic performance can be enhanced by making noble metal- BiVO4 heterostructures. Incorporation of metal particles such as gold (Au) and silver (Ag) onto the surfaces of BiVO4 particles can effectively enhance its photocatalytic activity by charge transfer between semiconductor and metal nanoparticles and thereby reducing the electron hole recombination. Moreover, the surface plasmon resonance (SPR) of the loaded metal nanoparticles provides additional visible-light absorption, which can also contribute to the overall photocatalytic activities. The conventional routes for fabricating such heterostructures e.g. physical mixing, electro-chemical deposition, molecular interaction, sputtering, etc have some limitations in terms of stability and performance. In contrast, the click chemistry approach offers significant advantages such as control over the formation with minimal aggregation, versatility to functionalize variety of surfaces and fabricating the variety of mono/tailored multilayer nanostructures of different nanoparticles on array of substrates. Herein, we have reported the click chemistry approach for fabrication of Ag/Au-BiVO4 heterostructures. The photo electrochemical measurements of these heterostructures revealed better performance in terms of activity and stability in both acidic and basic medium electrolytes due to strong covalent linkages provided by triazole ring between the surface moieties.

Solar fuels are a key technology that could enable a sustainable future. The details of how absorbers, protective layers and thin catalyst films will be integrated into a single photon-to-fuel device demands much work. Here the stability of Si solar cells underneath Ir, Ti and TiO₂ stacks for the hydrogen evolution reaction in basic solutions was investigated. We show that a thin layer of Ti protects Si from basic conditions while maintaining excellent electrical conductivity to an ultrathin layer of Ir. Silicon is etched in basic conditions, and the protection of silicon with appropriate oxide/metal layers allows for silicon devices to be integrated into a basic solar fuels system, where the best OER catalysts are stable¹. We hypothesize that the Ti layer partially oxidizes, expanding and creating a thin dense oxide layer that is stable in basic conditions. Ongoing work is probing the use of TiO₂ as an optically transparent protective layer in place of Ti².
The activity of ultrathin films of Ir on Si was also investigated. We observe ultrathin layers of Ir (2-3 nm) on Si show higher HER activities than thick films (50 nm) on Ti. The activity of Ir toward HER in ultrathin films is of relevance for making “optically transparent” catalysts as well as general research into Pt group catalyst modification³. We hypothesize this is due to electron donation from Si to the more electronegative Ir, resulting in higher HER activity. We are in the process of probing this further by looking at the thickness dependence of the activity of Ir on Si.
(1) Trotochaud, L.; Ranney, J. K.; Williams, K. N.; Boettcher, S. W. J. Am. Chem. Soc. 2012, 134, 17253-61.
(2) Seger, B.; Pedersen, T.; Laursen, A. B.; Vesborg, P. C. K.; Hansen, O.; Chorkendorff, I. J. Am. Chem. Soc. 2013, 135, 1057-64.
(3) Strmcnik, D.; Uchimura, M.; Wang, C.; Subbaraman, R.; Danilovic, N.; Vliet, D. Van Der; Paulikas, A. P.; Stamenkovic, V. R.; Markovic, N. M. 2013, 5.

Highly ordered, vertically aligned, one side-opened, and regularly porous anatase TiO2 nanotube arrays have been facilely grown by anodizing Ti foil in mixed viscous solvents of ethylene glycol and glycerol. By changing the volume ratio of two solvents, we have controlled the structural properties of TiO2 nanotube arrays such as tube diameters, wall thicknesses, and tube lengths. Our prepared TiO2 nanotube arrays have been found to have enhanced (004) planes, which are reactive in catalysis reactions. We have demonstrated that TiO2 nanotube arrays grown in 2:1 (v/v) ethylene glycol and glycerol have the lowest band-gap energy and the largest mean crystallite diameter. TiO2 nanotube arrays grown on Ti foil have been directly employed for photocatalytic materials and the working electrode of photovoltaic dye-sensitized solar cells. Among our prepared samples, TiO2 nanotube arrays grown in 2:1 (v/v) ethylene glycol and glycerol have shown the best photocatalytic activity for the degradation of methylene blue and the highest photovoltaic conversion efficiency of 4.08%.

Ta₃N₅ is a visible light absorber with a favorable band gap of 2.1 eV and conduction and valence bands that straddle the redox potentials for water reduction/oxidation, thermodynamically enabling the semiconductor to achieve unassisted water splitting under solar illumination.¹ Ta₃N₅, however, is unstable due to the thermodynamically favored nitrogen anion oxidation by photogenerated holes to form a Ta₂O₅ hole-blocking layer at the surface of the nitride.² Thus, the Ta₃N₅ photoanode degrades unless water oxidation is more kinetically favored, which can be facilitated by an oxygen evolution reaction (OER) catalyst that increases reaction kinetics and acts as a hole-scavenger within the semiconductor.³
Several highly active OER catalysts were chosen as candidates for the protection of Ta₃N₅ and were deposited by various methods such as spray pyrolysis, dip-coating, electron-beam evaporation and gas-phase reactive deposition. However, the presence of a good OER catalyst on a photoanode does not necessarily correlate with good performance of the catalyst/photoanode combination, since interfacial phenomena between the semiconductor and catalyst could have detrimental effects on the overall performance.⁴ Furthermore, since catalysts are being deposited onto an absorber, it is important that a large portion of the incident light is transmitted through the catalyst layer. Thus, it was found that catalyst OER activity trends did not necessarily translate to high oxygen evolution once the catalysts were deposited on Ta₃N₅.
Interestingly, it was found that Ta₃N₅ photoanodes can be stabilized by the addition of IrO₂ colloidal catalyst nanoparticles. In acidic electrolytes, IrO₂ is the only known acid-stable OER catalyst.⁵ Furthermore, the photocurrent derived from the IrO₂/Ta₃N₅ was the highest among the systems studied in this investigation. This result is particularly promising as the most efficient photocathodes are generally more stable in acid than in base, allowing for the same electrolyte to be used for a tandem device.
References
(1) Chun et al, J. Phys. Chem. B 2003, 107 , 1798-1803.
(2) Ishikawa et al, J. Phys. Chem. B 2004, 108, 11049-11053.
(3) Cong et al, Chem. Mater. 2012, 24, 579-586.
(4) Seabold et al, Chem. Mater. 2012, 23, 1105-1112.
(5) McCrory et al, J. Am. Chem. Soc. 2013, DOI: 10.1021/ja407115p.

Fe(NO₃)₃^.9H₂O and Bi(NO₃)₃^.5H₂O were used as starting materials to synthesize Bi₂Fe₄O₉ nanoparticles by a sol-gel process in the presence of PEG400. The as-prepared nanoparticles were annealed at different temperatures ranging from 400 to 800°C and a pure orthorhombic phase could be obtained at 700-800°C. Scanning electron microscope images revealed that the morphology of the nanoparticles changed with different PEG concentrations. Moreover, the sample containing 24g/L PEG400 showed good dispersivity and uniformity and had an average particle size centered around 200 nm. The results of UV-vis diffuse rflectance spectroscopy indicated that the band gap value of the Bi₂Fe₄O₉ nanoparticles was determined to be 2.1 eV, which allowed the efficient absorption of visible light. The Bi₂Fe₄O₉ nanoparticles exhibited excellent photocatalytic degradation of methyl orange under visible light irradiation.

Electrolysis of water is the decomposition of water into oxygen and hydrogen gas due to electric current being passed through the water. Production of hydrogen from water requires large amounts of energy so it is uneconomic compared to the production from coal or natural gas. For this reason, H2 gas produced using renewable energy sources rather than fossil fuels, such as H2 generated from water utilizing solar radiation, has thus attracted much interest as a clean energy carrier. Ever since Fujishima and Honda reported photoelectrochemical (PEC) water splitting using a titanium oxide electrode in 1972,[1] many researchers has intensively studied water splitting using semiconductor photoelectrodes or photocatalysts. Since photocatalytic or PEC water splitting resembles photosynthesis in green plants it is regarded as a form of artificial photosynthesis. Most attention photocatalyst material is Fe2O3 and titanium oxide. Also, efficiency, reliability, and cost of photocatalyst are important, and efficiency is biggest issue. Titanium oxide has reliability with low cost, but has low efficiency of light absorption that does not absorb visible light by 3.2 eV band gap. Recently reported, surface plasmon resonance to boost visible light absorption of titanium oxide photocatalyst, and enhanced hydrogen production of photochemical cell[2]. The resonance of plasmon and meta-structure same effect in electromagnetic. Meta-structure has light absorption rate higher than the surface plasmon[3].
In our study, it is fabrication of high-efficiency artificial photosynthesis electrochemical cell by focusing high efficiency plasmon generation in meta-structure, using the meta-structure of titanium oxide photocatalyst. Our propose photoelectrode of combined photocatalyst with meta-structure has Au layer thickness of 50 ~ 100 nm on FTO, and titanium oxide layer deposited thickness of 10 ~ 200nm, and Au dot is formed on titanium oxide layer. Briefly, titanium oxide photocatalyst is located between Au dot and the Au layer, and hydrogen production efficiency is improved by the plasmon resonance energy of meta-structure transfer in the titanium oxide photocatalyst. We report improvement of photocatalytic efficiency and dependent of light absorption by the titanium oxide thickness and the Au dot size.
[1] A. Fujishima, K. Honda, Nature 238, 37 (1972).
[2] Syed Mubeen, Joun Lee, Martin Moskovits, Nature Nano 8, 247 (2013)
[3] Jing Wang, Yiting Che,n Min Qiu, J. A. Physics 109, 074510 (2011)

Electrochemical reduction of carbon dioxide to high-value fuels and chemicals is a particularly attractive solution to partner with renewable, carbon-neutral energy sources by mitigating intermittency and energy storage problems inherent in solar and wind power. Sn, an appealing catalyst for this reaction, has been shown to primarily produce formic acid from CO2. Our work reports the production of methane from CO2 on Sn electrodes. This work was accomplished by utilizing a highly sensitive electrochemical cell for identification and quantification of reduced products. Both polycrystalline Sn and nanostructured Sn electrodes were studied over a range of potentials. Formic acid, carbon monoxide and hydrogen were confirmed as the major products formed on Sn. However, while no other products were observed on polycrystalline Sn, methane was detected at potentials as early as -1.15V vs RHE on the nanostructured Sn electrodes. This is the first time methane has been reported as a product formed on Sn surfaces for CO2 reduction. Furthermore, characterization of the Sn catalysts via electrochemical impedance spectroscopy (EIS) suggests that the production of methane does not result from an increase of the surface area of the electrodes. These results suggest that the introduction of undercoordinated sites on Sn metals may lead to a stronger binding energy with CO, a key intermediate for CO2 reduction, and allow for the formation of more reduced products.

Properties of electrode-electrolyte interface are one of very important aspects in photoelectrochemical solar-to-fuel energy conversion technology, yet, its understanding is less developed compared to bulk properties of electrolyte and electrode due to the challenging aspects in theoretical modeling and experimental characterization. However, technological advancement in the past decade made it possible to access atomistic information at the electrode-electrolyte interfaces.
Very recently, it was suggested that proton transport at the electrolyte-electrode interface could influence on overall hydrogen production efficiency via interplay between electrode surface and co-catalysts [1]. In fact, such a phenomenon has been experimentally demonstrated by Esposito et al on platinized Si/SiOx MIS device [2].
In the talk, implication of this finding to solar-to-hydrogen conversion efficiency, to corrosion resistance, and to a new design paradigm, which will take full advantage of these findings, will be discussed. This work was performed under the auspices of the US Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.
[1] B. Wood, E. Schwegler, W.-Ih Choi, and T. Ogitsu, JACS 135, 15774 (2013)
[2] D. Esposito, I. Levin, T. Moffat, A. A. Talin, Nat. Mat. 12, 562 (2013)

The demand for cost and energy efficient processes drives the need for inexpensive and earth-abundant catalysts. One important aspect in increasing the use of catalysts is replacing expensive elements, such as platinum, with relatively inexpensive elements. Transition metal dichalcogenides have shown high catalytic activity towards the hydrogen evolution reaction (HER) and could serve as replacement materials for platinum catalysts. Phase-pure transition metal pyrite electrodes prepared via thermal sulfidation show promising catalytic performance. The role of morphology on the performance of these transition metal pyrite catalysts will be discussed.

Discovering improved electrocatalysts for the oxygen evolution reaction (OER) is of great importance for efficient solar fuels generation, regenerative fuel cells, and recharging metal air batteries. The slow kinetics of the 4-electron OER requires large overpotentials to drive water oxidation at appreciable current densities. Among the numerous compositions investigated, mixed metal oxides in the (Ni-Fe)Ox and (Ni-Co)Ox composition spaces are among the most active and most studied OER catalysts. Although this technologically important reaction has been studied for more than 50 years, many of the mechanistic details remain under investigation. Lacking a robust fundamental understanding of the basic science and mechanistic details of multi-electron heterogeneous electrocatalysis, an efficient high-throughput synthesis and property screening methodology is well-suited to discovering the requisite new catalytic materials. We have established high throughput methods to systematically investigate the performance of pseudo-quaternary material libraries as OER electrocatalysts. We report a new Ce-rich family of active catalysts composed of earth abundant elements, which was discovered using high-throughput methods to produce 5456 discrete compositions in the (Ni-Fe-Co-Ce)Ox composition space. The activity and stability of this new OER catalyst was verified by re-synthesis and extensive electrochemical testing of samples in a standard format in 1.0 M NaOH, as well as by operation in a photovoltaic-powered electrolyzer for more than 100 hours. The most interesting variations in activity lie in a pseudoternary cross-sectional plane containing 665 compositions. Our detailed investigation of this psuedoternary cross-section has revealed systematic trends in Tafel slopes and electrochemical signals with composition, which provide a connection between the previously known Ni-Fe and newly discovered Ni-Co-Ce catalysts. Characterization of selected compositions by XRD, XPS, SEM, TEM, EDS, XRF mapping, and EXAFS both as-synthesized and after electrochemical testing, reveal important differences in nanostructure and stability along with the observed differences in electrochemical performance under OER conditions.

Quaternary mixed metal oxide combinatorial libraries targeting members of the Bi-containing Mullite phase AB2O5 (A = Bi, Sm, Gd, La; B = Mn,Cr, Ni were prepared by ink jet printing and screened for PEC performance using an in-house fabricated scanning droplet cell. Structural and UV-visible optical absorption data will also be presented.

There is a growing interest in sustainably producing chemical fuels
through the electrochemical splitting of water to O2 and H2, using solar
energy as an electron source. Earth abundant, active catalysts are
needed to drive these electrochemical reactions if the process is to be
feasible on an industrial scale. Additionally, the photoelectrochemical
cell must be stable in solution, which is often operated at extremes
in pH. Atomic layer deposition of manganese oxide thin films holds
promise to address both of these issues. Atomic layer deposition allows
for conformal and pinhole free coverage of highly structured surfaces
with high control of film thickness, thereby providing a tunneling
protection layer for the light absorber from solution. Additionally,
manganese oxide has been shown to be highly catalytically active
for both the water oxidation reaction and the oxygen reduction
reaction.

Water and sunlight playing in harmony in presence of a semiconductor, such as iron oxide (α-Fe₂O₃), can provide a clean, unlimited, sustainable and renewable energy free from carbon produced by photoelectrochemical (PEC) cells. Among many metal oxides pointed as candidates for this application (such as TiO₂, WO₃ and ZnO) the fundamental characteristics of α-Fe₂O₃, such as abundance, excellent chemical stability in an aqueous environment and favorable optical band gap, emerged it as a promising material. Although α-Fe₂O₃ meets many requirements, the efficiency reported until now is far from theoretical prediction. In the last years, our research group has developed and applied hematite vertical nanorod to split water into molecular hydrogen and oxygen. This work describesthe influence of temperature of thermal treatment and use of WO₃ modifying the hematite films surface. The formation of nanorods distributed along of conductor glass substrate was observed by top-view of scanning electron microscopy images. Hematite films modified with WO₃ were analyzed by X-ray diffraction and their crystallographic arrangement was identified using JCPDS catalog. Besides, using the diffraction data it was found that the rod growth occurs preferentially oriented at the highly conductive (001) basal plane of hematite, which is perpendicular to the substrate. The good photoeletrochemical performance of hematite photoanode modified with WO₃ was attributed to the structural, morphological and catalytic properties working in great harmony.
Acknowledgements
We gratefully acknowledge financial support from the Brazilian agencies of FAPESP (Grants 2011/19924-2, 2012/19926-8 and 2013/05471-1), CAPES, CNPq (Grants no. 473669/2012-9), Instituto Nacional em Eletrocirc;nica Orgacirc;nica (INEO), NanoBioMed Brazil Network (CAPES), CEM- UFABC and CDMF.

Water splitting into hydrogen and oxygen using semiconductor-based heterogeneous photocatalysis plays a key role in a promising path to clean and sustainable energy production. The GaN/ZnO alloy has been shown to be an efficient visible-light photocatalyst for water splitting [1], although the microscopic structural motifs at its aqueous interface are not well understood.
We use density functional theory based molecular dynamics (DFTMD), to investigate the microscopic structural and electronic properties of aqueous interfaces of GaN, ZnO, and representative GaN/ZnO alloy structures. The nonpolar Wurtzite facets, which are found to be catalytically more active compared to other surfaces [2], are used as model surfaces. The properties of the aqueous GaN interface are in agreement with our earlier work [3]. We find that water adsorption on these surfaces is substantially dissociative. The surface anions (N or O) act as bases accepting protons from dissociated water molecules while the corresponding hydroxide ions bond with surface cations (Ga or Zn). Surface N-sites show stronger basic character and are protonated more readily than surface O-sites. All surface Ga atoms are bonded to hydroxide ions while about 50% of surface Zn atoms are bonded to hydroxide ions and intact water molecules are adsorbed on the remaining 50% of surface Zn atoms. Hydroxide ions adsorbed on cation sites are expected to act as catalytic centers for water oxidation [3]. These centers exhibit varying inner sphere water environments depending on local alloy composition and surface corrugation.
We calculate the redox level alignment at the aqueous GaN and ZnO interfaces taking into account the various microscopic structural motifs sampled in DFTMD simulations and the quasiparticle corrections obtained from many-body perturbation theory within the GW approximation. Our approach, following techniques used for semiconductor heterointerfaces, directly yields the alignment of the water 1b1 level to the semiconductor valence band edge, from which the flat-band redox level alignment follows. The energy of conduction and valence band edges of hydrated GaN and ZnO surfaces is lower compared to bare GaN and ZnO surfaces. The redox level alignment is also found to be sensitive to the microscopic structural details such as the degree of dissociation of interfacial water.
This research was carried out at Brookhaven National Laboratory under Contract No. DE-AC02-98CH10886 with the U.S. Department of Energy.
[1] K. Maeda, K. Teramura, D. L. Lu, T. Takata, N. Saito, Y. Inoue, K. Domen, Nature 2006, 440, 295
[2] D. Wang, A. Pierre, M. G. Kibria, K. Cui, X. Han, K. H. Bevan, H. Guo, S. Paradis, A.-R. Hakima, Z. Mi, Nano Letters 2011, 11, 2353
[3] X. Shen, Y. A. Small, J. Wang, P. B.Allen, M. V. Fernandez-Serra, M. S. Hybertsen, J. T. Muckerman, Journal of Physical Chemistry C 2010, 114, 13695

Solar water splitting on semiconductor photocatalysts has received much attention as a means of production of renewable and storable hydrogen on a large scale. A photocatalyst can generate both hydrogen and oxygen on the surface when the band gap straddles the reduction and oxidation potentials of water. Two different photocatalysts can also be connected in series so that hydrogen and oxygen are generated on the different photocatalysts. This reaction scheme is often called Z-scheme water splitting. In addition, photocatalytic materials are applicable to photoelectrochemical water splitting when being immobilized on a conductive substrate by appropriate methods. Similar to the photocatalytic water splitting, the top of the valence band must be more positive than the oxygen evolution potential to allow a photoanode to generate oxygen. On the other hand, a photocathode works for hydrogen evolution when the conduction band edge is more negative than the hydrogen evolution potential. An external voltage can be applied between a photoelectrode and a counter electrode to compensate for the potential deficiency to drive redox reactions on a counter electrode. Alternatively, a photoanode and a photocathode can be connected in series as in Z-scheme water splitting. In the tandem configuration, the maximum photocurrent and the working potential of the photoelectrodes are theoretically determined by the intersection of the steady current-potential curves of the respective photoelectrodes. Therefore, it is very important to develop photoelectrodes that generate high photocurrent with a small external voltage.
The author has proven that some (oxy)nitrides and (oxy)chalcogenides work as promising photocatalysts and photoelectrodes for water splitting under visible light irradiation. (Ga_1-xZn_x)(N_1-xO_x) and TaON, the absorption edge wavelengths of which are approximately 500 nm, can split water into hydrogen and oxygen in the stoichiometric ratio. Recently, a photocatalyst with a 600-nm absorption edge can also be active for overall water splitting with proper modifications. Additionally, a novel method was developed for fabrication of electrodes of photocatalyst powders based on particle transfer. Photoanodes of IrO₂-modified LaTiO₂N fabricated by particle transfer generates significantly higher photocurrent than those prepared by the conventional electrophoretic deposition. The enhanced performance should result from a smaller barrier at the interface among the conductor and the semiconductor particles. Direct formation of nanorods on a metal foil is also very effective in reducing resistances of carrier transfer between a conductive substrate and a photoelectrode material, as in the case of vertically aligned Ta₃N₅ nanorod arrays. Some p-type (oxy)chalcogenides exhibit excellent photoelectrochemical performance in hydrogen evolution under sunlight. Recent progress on non-oxide semiconductors for water splitting will be presented in the talk.

Conversion of CO2 into useful carbon sources such as CO, HCOOH, HCHO, CH3OH and CH4 is a potential means of solving energy and environmental problems. CO2 can be adsorbed on solid bases, indicating that solid base catalysts determine, alter, and control the structure of CO2 on their surface. Layered double hydroxides (LDHs) have attracted an increasing interest because of their potential applications as CO2 adsorbents, ion exchangers, base catalysts, and precursors of well-mixed oxides for various catalytic applications. Ebitani et al. have reported that Mg-Al hydrotalcite showed the activity for the aldol addition of carbonyl compounds in aqueous media because of the high water tolerance of the base sites on the surface. If CO2 is first adsorbed on the photocatalyst surface and then activated under photoirradiation, LDHs would function as photocatalysts in water. In this study, we carried out the photocatalytic conversion of CO2 in water in the presence of various LDHs.
LDHs were synthesized by the following coprecipitation method; aqueous solution containing chloride salts of metal components were slowly added to an aqueous sodium carbonate solution at room temperature. The pH was kept constant at 10 by adding an aqueous sodium hydroxide solution. The resulting suspension was transferred into a stainless steel autoclave with inner Teflon vessel and aged under hydrothermal condition. Photocatalytic conversion of CO2 was carried out using a flow system reactor. 1.0 g of catalyst powder was suspended in 1.0 L of ultra-pure water. This suspension was irradiated from within the reactor with a 400 W high pressure Hg lamp under CO2 gas flow at a flow rate of 15 mL min-1.
Considerable amount of CO was obtained over these LDHs in the photocatalytic conversion of CO2. Photocatalytic hydrogen evolution from water competitively took place in all cases. Ni-Al LDH exhibited high selectivity to CO because the amount of H2 evolved was relatively low. Ni-Al LDH (Ni/Al = 4) which was aged for 20 h under hydrothermal condition at 383 K after coprecipitation showed the highest activity for the evolution of CO. GC/MS analysis for the photocatalytic conversion of 13C-labeled CO2 clarified that 13C-CO (m/z = 29) was evolved in priority to 12C-CO (m/z = 28). Accordingly, CO2 introduced in the gas phase was adsorbed on the surface of LDHs, and then reduced into CO in the photocatalytic conversion of CO2 in water.

Nitrogen-doped oxide nanosheets were prepared by exfoliating N-doped layered perovskite oxide, CsAE2Ta3O10-xNy (AE:Ca, Sr, Ba) via proton exchange and two-step intercalation of ethylamine (EA) and tetrabutylammonium (TBA) ions. Inter-calation of EA is essential for preparing nanosheets since TBA+ ions (the exfoliation reagent) are not intercalated into the protonated form without EA intercalation. Monolayer nanosheets could be prepared by the above processes, although some bilayer or trilayer nanosheets were also produced. The AE2Ta3O10-xNy nanosheets exhibited photo-catalytic activity for H2 evolution from water under visible light irradiation. In contrast, CsAE2Ta3O10-xNy exhibited very low photocatalytic activity for H2 evolution under the visible light irradiation, even when methanol was added to water as a sacrificial agent. The improved photocatalytic activity originates from the characteristics of nanosheets such as their molecular thickness and large surface area. The Sr1.5Ba0.5Ta3O10-xNy nanosheet showed the highest photocatalytic activity among the AE2Ta3O10-xNy nanosheets prepared, and then Rh-loaded Sr1.5Ba0.5Ta3O10-xNy nanosheet showed the photocatalytic activity for oxygen and hydrogen production from water under visible light irradiation.

Surface/interface structure of metal/semiconductor materials plays an important role in determining efficiency of photoelectrochemical and photocatalytic solar energy harvesting and storage.
This talk will introduce the latest research activities in our group1-11), focusing on our challenges on the scientific and technological possibilities of nano-photocatalytic materials for solar chemical conversion. Efforts to explore suitable materials and to optimize their energy band configurations for specific applications, and the design and fabrication of advanced photocatalytic materials in the framework of nanotechnology will be introduced, taking examples of recent research progress on Ag3PO4 and other newly developed materials system. Specifically, design and control of surface/interface nano-structures of metal/semiconductors from both experimental and theoretical approaches for efficient solar energy harvesting will be introduced and discussed in detail.
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Solar driven hydrogen production by semiconductor photocatalytic water splitting is a promising way to provide clean and renewable energy towards the low-carbon future. For the commercial use, we still lack high efficient photocatalyst and good understanding of how, why and when the photoexcited carriers reacted on the catalytst surface. Here we report a mesoporously structured single-crystalline anatase TiO2 nanobelt, which possesses an amazing H2 production rate of 65.4 mmolg-1h-1 and the high quantum efficiency (QE) of 94% under solar light irradiation. The novel material was obtained by applying hydrogen plasma etching process on single-crystalline anatase TiO2 nanobelts surface, which generated abundant hexagonally shaped nanopores (5-20nm) with fresh {101} and {001} facets on the surface. The newly formatted facets provided reaction sites for water oxidation and H2 evolution respectively. Such structure efficiently separated the photo-excited electron-hole pairs in space and suppressed the back reactions. The structure optimized samples obtained 25% improvement of QE in solar driven water splitting reactions. Our approach provides a facile way in crystalline facet engineering by morphological control to attain highly efficient photocatalyst for environmental protection and solar energy conversion use.

Charge Transfer Complexes (CTC&’s), electronic states formed at the interface between donor and acceptor materials, play a crucial role as intermediate electronic stepping stones in the photovoltaic processes occurring in so-called organic solar cells based on polymer:fullerene donor:acceptor bulk heterojunctions. While CTC&’s are intensively studied in polymer:fullerene systems [1-3], this contribution aims to discuss in more detail the ‘universal&’ effect of CTC&’s in light induced charge transfer mechanisms for a broad variety of donor:acceptor material systems e.g. polymer:polymer, small molecule:fullerene and metal oxide:polymer (i.e. solid state Grätzel-cells) and how these findings can be related to other material systems for artificial photosynthesis.
A powerful technique used to investigate the universality of CTC&’s in various materials systems is Fourier Transform Photocurrent Spectroscopy (FTPS). FTPS is an ultrasensitive technique able to measure the External Quantum Efficiency over 9 order of magnitude [4] capable of detect the weak photocurrent produced from the direct excitation of the CTC ground state.
(1)Benson-Smith et al. Formation of a Ground-State Charge-Transfer Complex in Polyfluorene//[6, 6]-Phenyl-C61 Butyric Acid Methyl Ester (PCBM) Blend Films and Its Role in the Function of Polymer/PCBM Solar Cells. Adv. Funct. Mater. (2007) vol. 17 (3) pp. 451
(2)Vandewal et al. On the origin of the open-circuit voltage of polymer-fullerene solar cells. Nature Materials (2009) vol. 8 (11) pp. 904-909
(3)Deibel et al. Role of the charge transfer state in organic donor-acceptor solar cells. Advanced Materials (2010)
(4)Vandewal et al. Fourier-Transform Photocurrent Spectroscopy for a fast and highly sensitive spectral characterization of organic and hybrid solar cells. Thin Solid Films (2008) vol. 516 (20) pp. 7135-7138

Artificial photosynthetic systems made of all-inorganic components are attractive from a durability standpoint, but the use of solid inorganic materials faces the challenge of facile and precise tuning of the electronic properties of the photoactive components. We have developed an inorganic molecular approach by assembling oxo-bridged heterobinuclear light absorbing units covalently anchored on a silica nanopore surface. A dozen different units utilizing mostly first row transition metal centers, e.g. TiOCo(II) have been synthesized and characterized in the past few years. The units are anchored on the nanochannel surfaces of mesoporous silica such as SBA-15, or on the convex surface of silica nanoparticles. They possess a visible light absorbing metal-to-metal charge transfer transition (MMCT, e.g. Ti(IV)OCo(II) to Ti(III)OCo(III)) suitable for driving a multi-electron catalyst for water oxidation coupled to the donor center, or for activating a reduction reaction on the acceptor metal. With a wide range of metal centers to select from, the redox potential of donor and acceptor can be matched with the potential of the catalyst. We have recently demonstrated the closing of the photosynthetic cycle of CO2 reduction by H2O upon photoexcitation of the MMCT transition of a ZrOCo(II) unit coupled to an IrOx nanocluster for water oxidation. CO and O2 were directly observed. The hierarchical structure of the assembly was achieved by MMCT-driven photodeposition. Time resolved optical absorption studies of the excited state electron transfer of such heterobinuclear unit revealed an unusually slow back electron transfer of a few microseconds. We attribute the long lifetime of the excited MMCT state to fast intersystem crossing upon absorption of a photon.
For closing of the photosynthetic cycle on the nanoscale under separation of the sites for water oxidation from chromophore and the CO2 reduction chemistry, a Co3O4 core - SiO2 shell nanotube construct is being developed. The 2-3 nm thin silica layer serves as a proton conducting, gas impermeable membrane. Proton transport and separation properties have been quantitatively established by electrochemical measurements. Tightly controlled electron transfer from light absorber across the nanoscale silica separation layer to the Conot;3O4 water oxidation catalyst is achieved by hole transfer through rectifying p-oligo(phenylenevinylene) wire molecules embedded in the silica shell. Hole transport times were measured by transient optical absorption spectroscopy to be microseconds or faster. Water oxidation induced at the Co3O4 surface in liquid water was monitored by time-resolved FT-IR spectroscopy in the attenuated total reflection configuration. Two surface intermediates were detected, an Co(IV)=O and a superoxo species, which provide the first direct observation of the stepwise oxidation of water on the metal oxide catalyst surface under reaction conditions.

High-efficiency photoelectrochemical water-splitting devices require integrating electrocatalysts (ECs) onto light-absorbing semiconductors (SCs), but the energetics and charge-transfer processes at SC|EC interfaces are poorly understood. In order to study ECs on photoanodes, we fabricate model EC-coated single-crystal TiO₂ electrodes and directly probe SC|EC interfaces in situ using a new dual-electrode photoelectrochemistry technique to independently monitor and control the potential/current at both the SC and the EC.¹ We discover that redox-active ion-permeable ECs such as Ni(OH)₂/NiOOH yield “adaptive” SC|EC junctions where the effective Schottky barrier height changes in situ with the oxidation level of the EC. In contrast, dense, ion-impermeable IrO_x ECs yield constant-barrier-height “buried” junctions. Conversion of dense, thermally deposited NiO_x on TiO₂ into ion-permeable Ni(OH)₂/NiOOH correlated with increased apparent photovoltage and fill-factor. A new theory of adaptive EC|SC junctions is proposed and applied via numerical simulation to understand this behavior.² The theory can also be used to understand catalyst-modified hydrogen-evolving photocathodes as well as catalyst-modified visible-light-absorbing oxides such as BiVO₄, and experiments are underway to directly test the predictions. These results provide new insight into the dynamic behavior of SC|EC interfaces that help guide the design of efficient SC|EC devices. They also illustrate a new class of adaptive semiconductor junctions.
(1) Lin, F.; Boettcher, S. W. Adaptive semiconductor-electrocatalyst junctions in water splitting photoanodes. Nat. Mater. 2013, Accepted.
(2) Mills, T. J.; Boettcher, S. W. Theory and simulations of electrocatalyst-coated semiconductor electrodes for solar water splitting. Submitted. 2013.

The slow kinetics of the water oxidation half-reaction limit the efficiency of current solar water splitting technologies for hydrogen fuel generation.¹ We have studied a variety of electrocatalysts for the oxygen evolution reaction (OER) in a thin film geometry, enabling simple and direct comparison of the activity of different catalyst materials.² Ni_0.9Fe_0.1O_x was found to be the most active water oxidation catalyst in basic media, passing 10 mA cm^-2 at an overpotential of 336 mV with a Tafel slope of 30 mV dec^-1 and intrinsic OER activity roughly an order of magnitude higher than IrO_x control films and similar to or better than the best known OER catalysts in basic media. The high activity is attributed to the in situ formation of a layered Ni_yFe_1-yOOH oxyhydroxide species with nearly every Ni atom electrochemically active. These thin-film catalysts are ideal for integration with light-absorbing photoanodes, and we have measured their optical properties under operating conditions and shown using a simple model that optical absorption by the catalyst drives optimal film thicknesses to the ultra-thin sub-nanometer range.³
Fe plays a critical, but yet not understood role, in enhancing the activity of the NiOOH catalyst. We report electrochemical, electrical, and surface spectroscopic measurements of Ni and mixed Ni-Fe hydroxides to investigate the changes in electronic properties, OER activity, and structure of films as a result of Fe inclusion.⁴ Through-film conductivity measurements show little change with Fe addition, indicating that an increase in film conductivity is not the dominant mechanism for enhanced activity. The addition of Fe shifts Ni redox peaks to higher potentials and significantly increases OER activity. Measurements of activity as a function of film thickness indicate that Fe exerts an electron withdrawing effect on Ni centers, similar to that observed for noble metal electrodes. Cyclic voltammetry of Ni(OH)₂/NiOOH kept rigorously Fe-free shows unique Ni redox peaks and no significant OER current until >400 mV overpotential, suggesting that most previous reports of highly-active Ni(OH)₂-based OER catalysts are affected by Fe impurities. We show that under rigorously Fe-free conditions, the β-NiOOH structure is less active for OER, and that the increase in activity previously reported with aging in KOH electrolyte is rather due to incorporation of Fe impurities. These results have significant implications for the design and study of Ni(OH)₂-based OER electrocatalysts and batteries.
¹Trotochaud, L; Boettcher, S.W. Scripta Mat.2013, In Press. http://dx.doi.org/10.1016/j.scriptamat.2013.07.019
²Trotochaud, L.; Ranney, J.K.; Williams, K.N.; Boettcher, S.W. J. Am. Chem. Soc.2012, 134, 17253.
³Trotochaud, L.; Mills, T.J.; Boettcher, S.W. J. Phys. Chem. Lett.2013, 4, 931.
⁴Trotochaud, L; Young, S.L.; Boettcher, S.W. In preparation.

The high photocatalytic activity of anatase is markedly enhanced by the presence of a rutile interface. Typically, such heterojunctions are formed by sintering admixtures and result in biphasic particles or agglomerates. Given the importance of the heterojunction there have been surprisingly few studies on its spatial manipulation in relation to the anode and incident light. Sputter coating, dip-coating and doctor blading, (relatively expensive or time consuming methods) have been used to demonstrate the potential value in orientating the heterojunction perpendicularly to the incident light. These layerwise techniques maximize the area of the heterojunction relative to the volume of the two phases and spatially orientate the two phases, thereby optimizing charge carrier flow direction and path length but the interface area is limited to the abrupt change in phase composition when coating solutions or targets are changed.
Here, we have shown that phytic acid can be used to simultaneously template anatase nanoparticle formation and dope the Tio2 with phosphate to yield a photocatalyst with high specific surface area, enhanced light adsorption properties into the visible spectrum and lowered l.E.P. By varying the amount of phytic acid, the ratio of rutile:anatase in the resulting reaction mixture was easily tuned. Electrophoretic deposition in organic solvents, was used to alter zeta potential and create layered coatings that varied in phase gradient with coating thickness from biphasic mixtures. This single step technique offers a rapid and practical route to creating complex biphasic microstructures without recourse to expensive and time consuming equipment and techniques that could quite easily be scaled up for industrial purposes. We achieved heterogeneous deposition initially of a rutile rich layer and subsequently a concentration gradient of an increasing proportions of anatase and went on to quantify photoelectrical efficacy. These hierarchically structured films were used to construct DSSCs; these demonstrated 185% higher short current density than cells made using the industry standard material, P25.
To the best of our knowledge, this is the first time that an in situ method using a mixed phase nanocrystalline Tio2 starting material has been used to construct oriented heterojunctions on an electrode.

Since the reported visible light driven H2 production of the self-doped TiO2 and black TiO2,[1-3] the enhanced visible light response photocatalyst has attracted more and more attention. However, severe preparation conditions strongly limit it further practical application. Herein, we developed a simple and facile chemical reduction route to synthesize the colored TiO2 for enhanced solar-driven H2 production. Stable TiO2@TiO2-x core-shell nanocrystals can be prepared via reductant at mild temperature in large scale like 100 grams. The TiO2-x shell thickness can be tuned by reaction time and temperature. The color of TiO2 sample also gradually changes from white to blue and black. The H2 production was investigated under full solar spectrum, it can produce about 0.6 mmol/hour H2 per 0.1 g TiO2@TiO2-x nanocrystals, which is 7 fold better than that of P25 TiO2 nanocrystals. There is a broad absorption band in the visible light region (400-900 nm) for TiO2@TiO2-x, this absorption band results the H2 evolution rate of ~20 micro-mol/hour per 0.1g TiO2@TiO2-x. XPS investigation implies that the valence band raise with the reduction degree. The Ti3+ signal can be detected through electron paramagnetic resonance (EPR) technique. And then the Ti3+ signal disappears due to too high concentration of oxygen vacancy with the increase of reduction degree. Theoretical calculation shows there is a new vacancy band just below the conduction band. That results in that TiO2@TiO2-x nanocrystals possess a narrow band gap have visible light response for photocatalytic activity.
Acknowledgement. The financial support from the National Natural Science Foundation of China (No. 61176016), Science and Technology Department of Jilin Province (No. 20121801) and Returnee startup fund of Jilin is gratefully acknowledged. Z.S. thanks the support of the “Hundred Talent Program” of CAS, and Innovation and Entrepreneurship Program of Jilin.
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[2] X. B. Chen, L. Liu, P. Y. Yu, S. S. Mao, Science 2011, 331, 746.
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Control over the crystal habit of titanium dioxide has been of recent interest due to the relationship between surface energy and photocatalytic activity.¹ Anatase TiO₂ with primarily {001} facets exposed at the surface is of particular interest because it is highest in energy and therefore may be more effective in certain photochemical reactions. Potential applications of {001} faceted anatase include dye-sensitized solar cells, photo-oxidation chemistry, and splitting water to produce hydrogen and oxygen. In this work, we test a new platform for photocatalytic hydrogen evolution consisting of nanoparticle Pt⁰-laced anatase films that are oriented with the c-axis perpendicular to the substrate. TiO₂ was grown hydrothermally on gold and fused silica substrates in the presence of fluoride, which acts as a crystallographic controlling agent to prepare films that exhibit ~100% {001} facets at the external surface.² A disperse layer of platinum nanoparticles (3-7 nm in diameter) was deposited at the {001} surface by functionalization with 1,8-octanedithiol prior to the photoreduction of Pt(II) in methanol/water (50/50 vol %). Grazing angle X-ray diffraction and scanning electron microscopy demonstrate the preferred c-axis oriented film growth and dominant {001} facets at the film surface. High resolution SEM confirms the disperse nature of the deposited platinum nanoparticles. Compared to TiO₂ materials such as anatase powders and Degussa P25 with surface areas on the order of 50 m²/g, our films exhibit an area close to the projected area of the substrate. Hydrogen evolution rates as high as 6.9 mmol hr^-1 m^-2 were obtained for TiO₂(Pt) on fused silica films in pure water using 254-nm light, corresponding to an apparent quantum yield of 8.2 %. When methanol was used as an electron donor, hydrogen evolution rates of 5.1 mmol hr^-1 m^-2 (7.4 % APY) were achieved using 365-nm light. The relative size and surface coverage of the platinum nanoparticles and the electronic properties of the substrate were shown to have significant effects on hydrogen evolution rates. The remarkable ability of our films to generate H₂(g) may be due to effective electron-hole pair separation at the {001} surface, which is facilitated by highly dispersed np-Pt⁰ acting as electron traps.
1. Chen, X.; Shen, S.; Guo, L.; Mao, S. S. Chem. Rev. 2010, 110, 6503-6570.
2. Ichimura, A.S.; Mack, B. M.; Usmani, S. M.; Mars, D. G. Chem. Mater. 2012, 24, 2324-2329.

Two architectures of TiO2-based hybrid nanostructures were designed to improve the photocatalytic activity of the bare TiO2. The first one was a metal-semiconductor heterojunction and the second one was a TiO2 hybrid with different carbon allotropes. First, the ultra-fine (2 nm) Ag nanocrystallites-decorated TiO2 hollow sphere heterostructures were fabricated by a two-step hydrothermal method. It showed excellent photodegradation performance of the RhB dye. The efficiency could reach up to 100 % in 20 minutes under simulated sunlight irradiation. Second, a comparative photocatalytic study of activated carbon-, grapheme-, carbon nanotubes- and fullerene-TiO2 was conducted under visible light irradiation. An enhanced photodegradation of the RhB dye was achieved by using these hybrid nanostructures over that of only using pure TiO2; and the fullerene-TiO2 showed the best photocatalytic performance. The XRD, FE-SEM, TEM, STEM, HRTEM coupled with EDX, UV-visible absorption and PL spectroscopy, XPS, and electrochemical impedance were carried out to correlate their morphology, microstructures, surface properties and electronic band structures to their photocatalytic activities. In the metal-semiconductor heterostructures, the Ag nanocrystallites were of high crystallinity and distributed evenly on the surface of the TiO2 hollow spheres with compact interfaces. This would reduce the recombination rate of charge carriers and favoured the charge transfer across the interfaces. The increase in number of adsorbed oxygen also facilitated the generation of the hydroxyl radicals; and the reduced surface defects extended the life time of charge carriers upon Ag loading. The Schottky barriers between Ag and illuminated TiO2, upward shift of Fermi level and the increased electron density contributed to the charges separation and transport. In the architecture of carbon materials-TiO2, the superior adsorption property, favorable chemical bond formation (Ti-O-C), narrower band gap, smaller particle size and effective charge carrier separation had significantly promoted the photocatalytic efficiencies. These two types of heterostructures showed excellent photocatalytic performance in environmental remediation by employing more than one function of components. The interactions between TiO2 and Ag/carbon had a great effect on the control of morphology, modification of surface properties and electronic band structures. These key factors involved in photocatalysis will be of great importance to the design of functional and effective photocatalysts in future.

Artificial photosynthesis for a sustainable energy economy is considered as Holy Grail of chemistry whereby water splitting reaction is controlled in the context of Natural photosynthetic process. The main objective of the current investigation is to build a solar fuel generator which is economically viable and operative with and without sunlight. That is to make it functioning during day and night. For achieving the same, we have a developed low cost hydrothermal fabrication route for making thin film electrode of NiO/Ni (OH) 2 functionalized hematite. The photoanode heterostructure to be presented here is very efficient for good hydrogen gas evolution in both dark and light condition. The electrode is prepared by a “two reactor&’&’ hydrothermal modification of a pristine hematite film. The electrode shows a promising photocurrent density of 16mA/cm2 at overpotential of 0.37 V. In addition to this, the electrode shows charge storage capacity once exposed to light and with an application of anodic bias above 400mV in parallel with electrochromic behavior. Also, the water splitting reaction proceeds as a dark reaction after several hours of light exposure. We believe the abrupt increase in current density originates from the oxidized Ni (OH)2 layer which is absent in the case of pn-junction-like devices made by mere deposition of NiO on hematite by thermal annealing. On the other hand, hematite alone does not show such behavior. From gas-chromatographic analysis, the electrode evolves 1.85mmol/L of hydrogen after first 15 minute of operation. Finally, this kind of new PEC electrode offers a low-cost and simple way for the dual purpose applications of water splitting and charge storage.

We have synthesized plasmonically sensitized photocatalytic Au-TiO₂ aerogels that split water at visible wavelengths. These catalytic nanoarchitectures feature highly dispersed and size-stabilized ~5-nm Au nanoparticles incorporated into the aerogel oxide network as plasmonic sensitizers. The guest-host Au-TiO₂ aerogels retain the high surface area and mesoporosity of unmodified TiO₂ aerogels and maintain stable dispersion of the Au particles at high weight fractions (ge;8.5 wt.%). The Au nanoparticles exhibit a broad surface plasmon resonance (SPR) centered at ~550 nm that spans a wide range of the visible spectrum. Incorporating Au nanoparticles into titania aerogels extends photocatalytic activity to visible wavelengths longer than 700 nm,with the wavelength-dependent photoactivity tracking the profile of the SPR. By tuning sol-gel synthesis, aspects of the nanoscale network of the host TiO₂ aerogel can be modified to improve charge-transfer processes in the photocatalytic nanoarchitecture. We will report on the structural and functional characterization of these materials including: optical response, charge-carrier dynamics, and wavelength-dependent photoactivity for generation of O₂ and H₂. We have also made initial investigations of the mechanism of plasmonic sensitization and the fate of reactive carriers in the composite as a function of synthetically varied materials parameters (i.e., Au||TiO₂ interfacial structure and network connectivity) using time-resolved spectroscopy at the picosecond and nanosecond time scale.

Photoelectrochemical (PEC) water splitting is a very attractive strategy for converting solar energy into hydrogen fuel. Designing high-performance photoanodes is essential for efficient solar energy conversion in PEC water splitting. Here, we report an effective approach to improve the PEC performance of ZnO nanorod (NR)-based photoanodes by introducing low-crystalline Ni(OH)2 electrocatalyst nanosheets onto the ZnO surfaces. ZnO NR arrays and Ni(OH)2 nanosheets were grown sequentially by electrochemical deposition, forming a core-shell structure. The ZnO NR cores acted as photon absorber as well as rapid charge transporter; whilst the wrinkled Ni(OH)2 nanosheets largely increased the surface area and facilitated the PEC process by lowering the energy barrier of water oxidation and suppressing electron-hole recombination. As a result, more than one order of magnitude enhancement of PEC efficiency was obtained from the Ni(OH)2/ZnO core-shell NR photoanode compared to bare ZnO NRs. The thickness effect of Ni(OH)2 overcoating was also investigated. It was observed that although the electrocatalytic effect increased monotonically with the amount of Ni(OH)2 coating, too much Ni(OH)2 coverage could reduce the photocatalytic effect by limiting the light absorption. In summary, large-area Ni(OH)2 nanosheet-coated ZnO core-shell nanorod (NR) arrays were successfully synthesized via a well-controlled electrodeposition method and exhibited a drastic enhancement of photocurrent and conversion efficiency compared to bare ZnO NRs-based photoanodes. This research demonstrates that introducing electrocatalysts to conventional PEC photoanode systems and the complex low-crystalline/single-crystalline core-shell nanostuctured configurations could open a new avenue toward designing and fabrication of high-performance PEC photoanodes.

Photoactive semiconductors are critical for the development of photoelectrochemical (PEC) water-splitting systems. In particular, the choice of an efficient n-type semiconductor photoanode material is critical for PEC cell design. TiO2 is the most widely studied photoelectrode. However, the wide band-gap (asymp;3.2) of this compound severely limits its efficiency for PEC fuel generating applications. Alternative materials such as WO3, Fe2O3, as well as III-V compounds, also suffer from limitations for PEC applications. Therefore, new semiconductors need to be discovered and developed.
Here, we use high throughput combinatorial synthesis, measurement and analysis methodologies to rapidly screen the composition-structure-property relationships of MnO-ZnO alloys and allow identification of candidates for a more detailed study in PEC applications. The (MnO)1-x(ZnO)x thin films are synthesized using combinatorial pulsed laser deposition with continuous orthogonal gradients in both chemical composition and substrate temperature. The solubility limit of ZnO into MnO is determined using disappearing phase method and found to decrease with increasing temperature. For example, (MnO)1-x(ZnO)x deposited at 300 °C exhibit different crystalline phases in the range of x<0.2, 0.20.4, corresponding to rocksalt (RS), rocksalt+wurzite, and wurzite (WZ) crystal structures, respectively. Optical measurements indicate the strong reduction of the optical band gap associated with the RS to WZ transition, and are consistent with the first-principles theory prediction of Egap=2.1 eV at a x=0.5 alloy composition. We also find that the optical absorption edges occur at the RS phase boundaries. The electrical conductivity for the Ga-doped (MnO)1-x(ZnO)x samples deposited at 300 °C from a 4% Ga-doped ZnO target exhibits three distinct regions that correspond to the three observed structural phases. In the RS structure (x<0.2), the miscibility gap (0.20.4), the average measured conductivity values are determined to be < 2 S/cm, asymp;10 S/cm and asymp;100 S/cm respectively per atom of Ga. For example the 4% Ga-doped (MnO)0.5(ZnO)0.5 alloy with x=0.5 has a conductivity of 1.7 S/cm. The corresponding electron concentration and mobility determined using Hall effect are 3.1x1019 cm-3 and 0.38 cm2/Vs respectively.
In summary, Ga-doped MnO-ZnO alloys present a promising materials system for water oxidation in a PEC cell. Investigations to determine the performance of these alloys for both regenerative and water-splitting electrochemical applications are ongoing.

Global climate warming and environment pollution have spurred scientists to develop new high-efficient and environmental-friendly energy technologies. Hydrogen is an ideal fuel for fuel cell applications. Hydrogen has to be produced from renewable and carbon-free resources using nature energies such as sunlight if one thinks of clean energy and environmental issues. In this regard, a photoelectrochemical (PEC) cell consisting of semiconductor photoelectrodes that can harvest light and use this energy directly for splitting water is a more promising way for hydrogen generation. In the past, there has been a major research effort studying the synthesis of various manganese compounds aimed at mimicking the oxygen evolving complex of photosystem II. Of these manganese compounds, particular attention has been given to the manganese oxides, which have been shown to be multifunctional for applications of battery, supercapacitors, oxygen electrochemistry, and especially visible light-driven catalysis. Therefore, the low costs of manganese and the envisioned biomimetic character make the development of manganese oxide-based photoelectrodes for solar water splitting especially desirable. However, to date, the PEC activity of manganese oxide is still low due to poor carrier separation and transport. Recently, we report the first demonstration of hierarchically porous Ca-containing MnO2 nanorod bundles as visible-light-sensitive photofunctional nanoelectrodes to fundamentally improve the performance of MnO2 for PEC hydrogen generation. A substantial amount of Ca (up to 7.8 at%) can be in-situ incorporated into the MnO2 lattice via simple electroplating technique because of the exceptionally small feature sizes of quantum rods. The maximum photocurrent could be successfully achieved as high as 0.42 mA cm-2, which is the best value for a MnO2 photoanode. Significantly, Ca-containing MnO2 photoanodes illustrated striking PEC activity in response to visible light with a high incident photon to current conversion efficiency (IPCE) of 7% at a monochromatic wavelength of 450 nm, which is comparable to that from hematite. The improvement in photoactivity of PEC response may be attributed to the enhanced visible-light absorption, increased charge-carrier densities, and large contact area with electrolyte due to the synergistic effects of Ca incorporation and specific mesopore networks, thus contributing to photocatalysis. The new design of constructing highly photoactive Ca-containing MnO2 nanostructures sheds light on developing high efficiency photoelectrodes for solar-hydrogen field.

Metal-organic frameworks (MOFs) are three-dimensional (3D) crystalline materials which consist of a coordinately bonded metal ion and an organic ligand, and an infinite number of design of their crystal structure for applications is possible by the selection of various organic ligands. In addition, the enormously high specific surface area of MOFs which are generated from micropores or mesopores with long-range order opened a new field of solid-state chemistry. Recently, the insertion of functional guest molecules or nanoparticles in the pores and cavities of MOFs has been reported for a number of applications such as hydrogen storage, gas sensing, and chemical conversion catalysis. The guest species within the target size (5-50 Å) can be uniformly embedded throughout the MOF because of the easily controllable physical/chemical conditions of the intra-pores; consequently, MOFs are considered as a new class of host template materials. In addition, the synthesis of metal oxides by using a thermal treatment has been reported elsewhere. The organic ligands are decomposed and removed from the MOFs during calcination; then, the metal ions are oxidised and crystallised by reaction with oxygen atoms in air at high temperature.
Further, we attempted to synthesise a uniformly doped metal oxide nanoparticles by using metal-ion inserted MOFs, i.e. the angstrom-sized pores with long-range order can efficiently prevent agglomeration of dopant metal ions, and the rigid 3D hollow structure can provide a uniform oxidation state of metal ions. In this report, we synthesised multi-metal doped zinc oxide (ZnO) nanoparticle by using MOF-5 as a template. The prepared MOF-5 was transformed to ZnO by calcination, and the inserted metal atoms (iron, cobalt, nickel, and copper) were well-distributed in every wurtzite ZnO nanoparticle with small amount of spinel ZnM2O4 (M: inserted metal). These crystal structures were identified with various measurements, and the atomic arrangement of the wurtzite (-2111) plane and spinel (10-1) plane in Fe-doped ZnO nanoparticle was directly observed by high-resolution scanning transmission electron microscopy (HR-STEM). It was also found that the colour of the synthesised doped ZnO nanoparticles was red-shifted from white to brown, and the induced photon-to-current efficiency (IPCE) result of them showed increased photocatalytic ability within visible light range.

The development of cost effective solar-fuel devices has been an unresolved challenge for the past four decades. Practical systems need to be designed so that light absorbing, catalytic, mass transport and separation components function in an integrated fashion and pure fuel streams can be evolved. Our team is focused on applying systems engineering approaches to optimize integrated solar-hydrogen generators. By combining photovoltaic design principles, light management approaches, optofluidic components for thermal management, and electrolysis cells designed for solar-driven operation we are developing next generation devices that harvest energy from the sun and store it in the form of hydrogen fuel in cost effective manners. This work will highlight technoeconomic considerations that drive the materials selection and device designs, as well as how multiphysics models can inform integration approaches for the fabrication of practical solar-hydrogen systems. The integration solutions explored include: (1) the implementation of microfluidic thermal management schemes to avoid detrimental heating of photovoltaic components and reduce the overpotentials for water splitting reactions, (2) the development of novel solar tracking and concentrator components to reduce materials utilization and (3) the integration of multijunction thin-film silicon photovoltaic components with nanostructured membrane electrode assemblies for the cost effective production of solar-hydrogen.

As a sustainable energy source, photoelectrochemical conversion is a promising approach for massively generating hydrogen gases via solar-driven water splitting. A photoelectrochemical cell (PEC) employs semiconductor absorbers for harvesting sunlight, in which the semiconductor bandgap needs to be small from a viewpoint of maximizing light absorptance. However, this approach requires overpotential for triggering a photoelectrochemical reaction because the amount of photovoltage needed for water splitting is directly proportional to a bandgap. The photocurrent and photovoltage therefore lie in a tradeoff relation which limits a PEC efficiency within a certain value. This feature leads to a maximum efficiency of ~17% for ideal bandgap materials of 2.03 eV (min. energy required for spontaneous reaction) [1,2]. To decrease overpotentials, adopting metal catalysts such as Pt nanoparticles (NPs) results in a decrease of a maximum PEC efficiency because of poor light absorptance (shaded by metal NPs) as well as recombination loss at the interface between photoelectrode and metal NPs.
We present that the photon-to-current efficiency of PEC could be further increased over the theoretical limit via circuit coupling between PEC and thermoelectric (TE) devices. This hybrid system (PEC-TE), consisting of a PEC cell electrically coupled in series with a TE device, was capable of utilizing a full solar spectrum for collecting both photon and phonon energies. Additional use of a TE device achieved a light absorptance identical to a sole PEC while reducing overpotential for water splitting by Seebeck voltage. Henceforth, our PEC-TE system has successfully boosted a water splitting reaction without the additional needs of noble metal catalysts and/or external bias. The PEC-TE basically operates as a sole PEC reversely biased by thermovoltage, in which the output voltage can be readily higher than the bias required for spontaneous reaction. As a result, photon-to-current efficiency of ~22% was achieved, by only harnessing solar energy, at ~18.1 °C temperature gradient across a TE device. This is clearly in contrast to conventional small-bandgap semiconductors which require external electrical bias for cleaving water molecules because the output voltage should be considerably larger than thermodynamic water-splitting potential (~1.23 V).
[1] A. B. Murphy et al. Int. J. hydrogen energy. 31, 1999-2017 (2006).
[2] J. R. Bolton et al. Nature, 316, 495-500 (1985)

Renewable energies such as wind and solar have many attractive qualities, but they currently are technologically limited by intermittency and low energy density storage options for mobile applications. Using renewable energy, H2O, CO2 and the proper catalyst, it is possible to produce fuels and chemicals such as HCOOH, syngas, methane, alcohols and ethylene with a room temperature and pressure electrochemical reaction: the CO2 reduction reaction (CO2RR)1. Coupling CO2RR to syngas with Fischer-Tropsch technology would allow the production of high energy density, liquid fuels as well from CO2, H2O and sunlight. However, CO2RR catalysts typically suffer from poor selectivity, low energy efficiency, instability, and/or low current densities. Understanding the limits and mechanism of existing CO2 electrocatalytic reduction on metals is an ongoing and growing effort, and experimental and theoretical works2 are starting to give some insights into ways of improving beyond our existing metal catalysts. One such theoretical study suggests that there are optimal binding energies for certain adsorbates for the CO2RR as well as scaling relations in adsorbate binding energies that could limit the potential performance of transition metal catalysts. This study explores ways to try and tune binding energies and/or break these scaling relations using new CO2RR catalysts such as chemically modified transition metals and alloys. Modification of a polycrystalline Pt catalyst with a thin Polyaniline (PANI) film has shown increases in CO and formate production by up to a factor of five. The mechanism of this improvement, similar polymer-metal systems, as well as alloys for the CO2RR are investigated.
Acknowledgment
The authors would like to thank the Global Climate and Energy Project (GCEP), the United States Department of Energy (DOE), Chevron and the Stanford Graduate Fellowship (SGF), and the National Science Foundation (NSF) for project and student funding.
References
1. Hori et al. Electrochimica Acta, 39, 1833-1839, 1994.
2. Peterson et al. J. Phys. Chem. Lett. 3, 251minus;258, 2012.

Vertically oriented arrays of silicon microwires (Si MW) have been shown to be a viable option for the photocathode of a photoelectrochemical water splitting device. The array geometry orthogonalizes the direction of light absorption and carrier collection, allowing for light absorption along the axis of the wires and radial carrier collection. Minority carrier collection then takes place at a conformally contacted semiconductor-liquid junction. Despite the promising performance of Si MW, to be competitive with more traditional energy sources, the cost of solar-energy systems must be significantly reduced. Materials and fabrication are two of the major costs of solar energy systems. The Si MW address the question of materials cost because they use less material than planar cells and can be made from gaseous precursors. To address the fabrication cost, herein we demonstrate a low-energy, scalable fabrication route for the manufacture of Si MW arrays.
Previous methods of patterning substrates for the vapor-liquid-solid (VLS) growth of highly ordered, vertically oriented Si MW arrays are energy intensive and expensive; to combat this, we have developed a new templating technique. Previous templating strategies utilized masking, photolithographic exposure, and etching of a thermal SiO₂ overlayer. The new method uses microimprint lithography of a silica sol-gel to directly pattern Si growth substrate wafers. Previous methods required vacuum evaporation of the VLS catalyst, but in the new method, the Cu VLS catalyst was electrodeposited. The resulting arrays were uniform and exhibited electrical performance comparable to Si MW arrays that had been formed using the previous templating process.
The material quality of the Si MW arrays grown from the new method was investigated using regenerative photoelectrochemistry in methyl viologen (MV^2+/+). Averaging across six devices made from arrays grown from the new templating technique, the figures of merit for the devices were: V_oc = 380 ± 10 mV, J_sc = 7 ± 2 mA cm^-2, Phi;_ext,sc = 0.17 ± 0.05, ff = 0.5 ± 0.1 and eta;₈₀₈ = 2.1 ± 0.3%. Thus, the low-energy, scalable fabrication techniques described are capable of producing Si MW arrays with equivalent electrical performance to arrays created via the traditional templating techniques. The new fabrication method is both scalable and robust, allowing for the creation of Si MW arrays of varying pitch and wire diameter, which can potentially lead to the fabrication of more efficient photocathodes for the hydrogen evolution reaction (HER).

Photoelectrochemical (PEC) water splitting could provide a sustainable means of hydrogen fuel production.¹ Recent research in PEC water splitting has focused on developing materials suitable for application in a dual-absorber device configuration due to the high solar-to-hydrogen efficiencies tandem devices could enable.^{2, 3}
Silicon is a promising candidate photocathode material for a tandem PEC device due to its abundance, relatively low cost, excellent charger carrier transport, and near-ideal band structure.^4-6 However, several challenges must be addressed to make silicon photocathodes efficient and economical. The surface of the silicon must be protected to prevent corrosion or oxidation, which can destroy device performance.⁶ The silicon must also be combined with an active catalyst to reduce the kinetic overpotential necessary to drive the hydrogen evolution reaction (HER) at the photocathode surface. To date, the most successful approaches for addressing these challenges have relied on metal oxide protecting layers combined with expensive precious metal catalysts.^4-6
Nanostructured, crystalline molybdenum sulfide (MoS₂) is an inexpensive material that possesses excellent HER activity and stability.^{7, 8} We demonstrate that MoS₂ can confer these benefits to silicon photocathodes, simultaneously addressing the challenges of stability and activity. We fabricate conformal coatings of MoS₂ on silicon using a thermal synthesis. This technique yields photocathodes that remain highly active after more than 100 hours of continuous operation. Although these MoS₂-Si structures show very good performance, additional improvements are necessary to match the performance of precious metal-based devices. To further enhance the performance, we employ nanostructuring to increase the density of catalytically active sites and improve light absorption. These efforts result in molybdenum sulfide-silicon structures with photocurrent onset potentials within ~150 mV of the best reported Pt/Si photocathodes. Based on our findings, we propose strategies for further improving the performance of molybdenum sulfide/silicon photocathodes.
1. M. G. Walter, et al., Chem Rev, 110, 6446 (2010).
2. L. C. Seitz, et al., Submited (2013).
3. M. F. Weber, et al., J Electrochem Soc, 131, 1258 (1984).
4. J. R. McKone, et al., E&ES, 4, 3573 (2011).
5. S. W. Boettcher, et al., JACS, 133, 1216 (2011).
6. B. Seger, et al., JACS, 135, 1057 (2013).
7. Z. Chen, et al., Nano Letters, 11, 4168 (2011).
8. J. Kibsgaard, et al., Nat Mater, 11, 963 (2012).

As an earth abundant material, silicon stands out as one of the most promising semiconductors for solar energy driven devices with its outstanding good performance on light absorption, incident photon to electron conversion efficiency, charge transfer property and rich knowledge. Besides of the applications on solar cells and photovoltaic, photoelectrochemical (PEC) water splitting is also worthy to be pursued for silicon based photoelectrodes since it has potential to generate hydrogen as clean energy source by solar energy. However, one of the drawbacks of silicon for PEC water splitting is the stability issue in aqueous solutions. In order to address this problem, the surface protection/passivation of silicon by metal oxide thin layers is investigated.
In this study, we protect the surface of silicon by employing the atomic layer deposition (ALD) to synthesize metal oxide thin films. As photocathode, the electron generated by solar irradiation from silicon would easily flow from its conduction band into the metal oxide protection layer, and then inject into the electrolyte for reduction reactions, if the band structures of silicon and metal oxide is suitable aligned. During this process, this study tries to address following unclear aspects: how does the relative band structures of Si and metal oxide (such as TiO2, Al2O3, ZnO etc.) ALD films, the ALD films quality and thickness, and the chemical potential of the electrolytes affect the overall charge transfer.
On the other hand, when silicon is used as a photoanode, due to the deep valence band positions of metal oxides, it is required to control the protection layer thickness to be thin enough to allow the holes pass through and meanwhile provide fully coverage from the chemical corrosion. Furthermore, the methyl functional group surface modified silicon would be investigated as well to provide more detailed information on the metal oxide/silicon interfaces.
Overall, this work systematically deposits the commonly used metal oxide onto silicon surface by ALD, providing insight of the charge transfer process from silicon to metal oxide and indepth understanding on the metal oxide/silicon interfaces.

Water splitting using photoelectrochemical (PEC) devices based on low-cost hydrogenated amorphous silicon (a-Si:H) thin films has the potential to meet the requirements in high efficiency and long durability for affordable hydrogen production. Incorporation of carbon in the amorphous silicon film should lead to an increase in the corrosion resistance compared to the use of conventional a-Si:H films. The bandgap of amorphous silicon carbide (a-SiC:H) can be readily tuned between 1.8-2.1 eV, suitable for water splitting applications. We report our efforts to develop a-SiC:H thin film photoelectrodes and integrate the a-SiC photoelectrode with Si solar cells into a monolithic, hybrid photovoltaic (PV)/a-SiC device capable of water splitting using sunlight as the only energy source. In this presentation, the basic microstructure and opto-electronic properties of a-SiC thin film are first described, followed then by fabrication of a-SiC photoelectrodes and hybrid PV/a-SiC devices. The PEC properties of both the a-SiC photoelectrode and hybrid device are discussed, particularly the surface modification by metal nanoparticles, which is critical to PEC performance of the hybrid device. We show that with surface modification by ruthenium (Ru) nanoparticles, the photocurrent of the hybrid PV/a-SiC device reaches ~5 mA/cm2, and the good durability of up to ~800 hours in 0.25M H2SO4 electrolyte has been achieved in the metal nanoparticle treated hybrid devices. Finally, we describe a roadmap for achieving the solar-to-hydrogen efficiency of >10%.

n-type tantalum pentoxide (Ta2O5, bandgap; 4.0 eV) has been widely studied for application as a water splitting photocatalyst under ultraviolet (UV) irradiations. Recently, we have developed N-doped Ta2O5 (N-Ta2O5) powder, which absorbs visible light at wavelengths shorter than 520 nm (Eg=2.4eV) [1, 2]. Because the N doping caused a change in the conduction from n-type to p-type which resulted in much negative position of conduction band minimum (ECBM), we have successfully achieved highly selective visible light-induced reduction of CO2 to HOOH for the first time by N-Ta2O5 linked with a ruthenium complex catalyst in acetnitrile (MeCN)/triethanolamine (TEOA) [3, 4]. In this paper, we demonstrate a potential of p-type N-Ta2O5 as a visible-light sensitive photocathode for hydrogen production by water splitting.
Sputtered N-Ta2O5 films were prepared on antimony-doped tin oxide (ATO)-coated glass substrates by sputtering of Ta2O5 in N2/Ar gas mixture. Metallic substance as a H2 evolution co-catalyst was sputtered on the N-Ta2O5 film. These photocathodes were tested in an Ar-purged standard 3-electrode cell, with a Pt wire, Ag/AgCl, and 0.2M K2SO4 as a counter, reference, and electrolyte. A 300 W Xe lamp was used as a light source. As a result, it was found that cathodic photocurrent depended on the kind of cocatalyst and that modification with Pt and Rh largely enhanced photocurrent of the N-Ta2O5 photocathode. We also confirmed that the rate of H2 evolution over Pt/N-Ta2O5 was 17 times higher than that over bare N-Ta2O5 and that Faradaic efficiency (photocurrent efficiency) was calculated to be 78%. These results suggest that surface modification with co-catalyst promotes charge separation which accompanies with photocatalytic hydrogen production.
N-Ta2O5 powders were also prepared by gaseous NH3 treatment of Ta2O5. Photocatalytic reactions for H2 evolution were performed in an aqueous solution with a 100 mM electron donor, pre-purged with Ar gas under visible-light irradiation. As a result, we found that ascorbic acid and ethylenediaminetetraacetic acid operate as electron donors to N-Ta2O5 in a photoexcited state in aqueous solution and that their activity are high compared with TEOA which is well-known as an effective electron donor in organic solvents. The rate for H2 generation over Pt-loaded N-Ta2O5 was 37 times higher than that of N-Ta2O5 powder. The pronounced effect of Pt cocatalyst for N-Ta2O5 is similar to that of the electrode system.
In summary, it was clarified that p-type N-Ta2O5 photocatalyst was effective for H2 evolution in an aqueous solution under visible light irradiation and that loading of Pt or Rh metals as co-catalyst largely enhanced photocatalytic H2 evolution.
References
[1] T. Morikawa, et. al., Appl. Phys. Lett., 96 (2010) 142111. [2] T. M. Suzuki, et al., J. Mater. Chem., 22 (2012) 24584. [3] S. Sato, et. al., Angew. Chem. Int. Ed., 49 (2010) 5101. [4] T. M. Suzuki, et. al., Chem. Commun., 47 (2011) 8673.

From both a scientific and social viewpoint, the conversion of solar energy into fuels has
become one of the important topics that people are trying to address. By taking the advantage of large surface area and effective charge collection, nanowire could be applied as efficient building blocks for a fully integrated system of solar-to-fuel conversion. Here it is shown that precise control of the construction and optimization for nanowire-based structures could be achieved, which leads to attractive energy-conversion properties. The idea of nanowire-based structrue also demonstrates an advance to biomimick the natural photosynthesis by spatially control the whole photosynthetic process at microscopic level. Based on such concept, some further application was described in the field of water splitting and carbon dioxide reduction.

Broadband light absorption enhancement in solar cells using nanoscale structures and novel physical effects has attracted a great deal of attention in recent years. Especially, utilizing plasmonic nanoparticles(NPs) such as Au and Ag NPs is considered as one of the promising methods for significant enhancement of local electromagnetic fields and thus improves the optical properties of the nanostructure devices. In addition, the incident photons can be scattered over a longer propagation path in the active layer by metallic nanostructures. These features can potentially benefit the light absorption and photocurrent generation of polymer solar cells. Thus, the incorporation of metallic NPs in organic solar cells has been conducted for improving photovoltaic performances. However, the plasmonic related scattering and local field enhancement effect are hard to cover the whole wavelength range of active polymer response spectrum by only using single-component metal nanoparticles.
Here, we report a facile method to fabricate polymer solar cells incorporated by Au, Ag nanoparticles and Ag-Au bimetallic nanoparticles. The silica layer between Ag and Au is employed for the enlargement of optical absorption in visible wavelength region. The strategy employing binary metal nanoparticles into will provide improvement in optical properties and performance of polymer solar cells.

Mixed ionic and electronic conducting materials offer tremendous potential in devices such as battery electrodes, chemical sensors, gas separators, electrochromic windows, and, of interest in this study, membranes for photoelectrochemical cells. Our proposed solar-powered water-splitting device design requires a multifunctional membrane that is mixed conducting, visually transparent, stable in water, and has low permeability to O2 and H2. We synthesized a mixed conducting polymer composite material by polymerizing EDOT in an aqueous sPPO matrix. Incorporating this composite material via layer-by-layer (LbL) assembly into water-stable membranes allowed for control of the conductive and optical properties of these membranes. The mechanism of conductivity in these membranes was further studied through EDS and temperature dependent impedance spectroscopy.

Developing robust water splitting catalysts is a key challenge in photocatalytic hydrogen production. In Nature, this process is carried out efficiently during green plant photosynthesis by a protein complex known as photosystem II, and emulating this highly energetic process is a viable gateway to the production of photo-catalysts for water oxidation. Whilst the basic components of the photosystem can be recreated, controlling their spatial arrangement to promote charge separation and water oxidation over recombination and photo-corrosion processes is non-trivial. However, a new bottom up approach involving the integration of nanoscale components, such as protein scaffolds and nanoparticulate catalysts, should allow the design of systems with precisely defined structures that offer greater tunability and flexibility than traditional small molecule systems.
In this work, we present the design of a novel class of artificial, water splitting enzymes and routes to their assembly. The constructs are based on generic architectures comprised of an inorganic nanoparticle co-catalyst within a hollow, photosensitized protein cage. These constructs will be capable of light activated electron transfer between protein-based photoactive cofactors and catalytic sites on the nanoparticle surface, and subsequent water oxidation or proton reduction. The design is modular, and as a result the catalytic activity can be systematically tuned by altering the photoactive cofactor, co-catalyst, or the separation between these key components.

Photoelectrochemical water splitting is one of the candidates for hydrogen gas generation from water. GaN is suitable for the photoelectrochemical electrode due to the band-edge energies. However, the n-type GaN has stability problem that the surface anodic corrosion during the photoelectrochemical reaction. Recently, NiO loading are used to prevent the surface corrosion. High stabilities of n-type GaN surface morphology and of photocurrent were performed using NiO loaded. However, the details of reaction and the role of NiO have not been clarified. We investigated the surface stabilities of the NiO co-catalyst loaded on n-type GaN dependent on the electrolytes in this report.
The working electrode was n-type GaN grown on (0001) sapphire substrates by metal-organic vapor phase epitaxy (MOVPE). The GaN layer was 2.0 µm n-type layer on 2.0 µm undoped layer. The carrier concentration of n-type GaN layer was 2.0×10¹⁷ cm^-3. NiO were deposited on the GaN surface by spin coating and annealing at 280 °C for 1 hour in N₂. The counter and reference electrodes, which were made of Pt, and Ag/AgCl/NaCl, respectively, were used for electrochemical evaluations. The light intensity was controlled as 100 mW/cm² by using 500 W Xe-lamp. The electrolytes were 0.5 mol/L H₂SO₄ (pH 1.0) and 1.0 mol/L NaOH (pH 14.0). The surface morphologies of GaN were observed by Scanning Electron Microscope (SEM).
The flatband potential of NiO loaded sample in H₂SO₄ electrolyte obtained from Mott-Schottky plot shifted 0.06 V to negative direction compared to that without NiO loaded sample. That in NaOH electrolyte shifted 0.18 V to negative direction. The pH dependence of the potential with NiO loaded sample is almost followed the Nernstian relationship of water comparing to that without NiO sample. The results show that the surface of the sample without NiO loaded probably formed Ga-O or Ga-OH by Ga surface oxidation. Thus, the sample without NiO loaded did not show the Nernstian relationship of water which required the OH^- or H⁺ adsorption on the surface along the pH. The ion adsorption on the electrode surface is probably important for the smooth water splitting reaction from the results.
Cyclic voltammetry with light illumination was evaluated in order to clarify the photoelectrochemical properties of the sample with and without NiO loaded. The photocurrent density of the sample without NiO loaded at applied bias between -0.4 V and +0.1 V in NaOH showed hysteresis. However, that for NiO loaded sample did not show the hysteresis. This shows the reaction speed difference, that is, the speed of the sample with NiO loaded is faster than that without NiO loaded. This also support the surface change of with and without NiO loaded.
In summary, NiO co-catalyst changes the surface reaction mechanism probably due to the difference of the interface between the electrolyte and the photoelectrode surface of n-type GaN.

Over the past years, fundamental progress has been made in developing novel material and material architectures for water splitting reactions. However, numerous challenges still remain to exploit the full potential of earth-abundant metal oxide photoanodes. One of the most compelling issues related to metal oxide photoanodes is their light absorption limited to the UV part of the spectrum. Substituting oxygen with sulfur or nitrogen raises the valence band and therefore contributes to the reduction of the metal oxides&’ band gap. Unfortunately, sulfides and nitrides undergo photocorrosion in water. As a result, oxynitrides have recently been proposed as a way to balance between band gap reduction and aqueous stability.[1]
Herein, we report a colloidal wet-chemical approach for preparing oxynitride nanocrystals. By decomposing metal-amino complexes in presence of surfactants rather than using the more common high-temperature nitridation of metal oxides[2], we have achieved a superior control on the oxynitride stoichiometry governing the valence band edge position in the visible range. A careful manipulation of the reaction conditions has enable size and shape control. Highly hyperbranched nanostructrures have been obtained and crystal splitting is hypothesized as the growth mechanism. Finally, we outline the technological potentiality of these colloidal oxynitrides as photoanodes for water splitting by testing their photoactivity and stability at different pH.
1. Wu, Y.; Lazic, P.; Hautier, G.; Persson, K.; Ceder G. Energy Environ. Sci. 2013, 6, 157.
2. (a) Hoang, S.; Guo, S.; Hahn, N. T.; Bard, A. J.; Mullins, C.B. Nano Lett. 2012, 12, 26. (b) Ida, S.; Okamoto, Y.; Matsuka, M.; Hagiwara, H.; Ishihara, T. J. Am. Chem. Soc. 2012, 134, 15773.

Solar photoelectrochemical (PEC) water splitting is a technology for the production of hydrogen, which is an attractive source of renewable and clean energy. Lanthanum titanium oxynitride (LaTiO₂N) is a promising semiconducting visible light absorber with a band gap of about 2.2 eV. This system is particularly desirable for PEC applications since its energy bands are properly aligned to drive reactions of both water oxidation and hydrogen reduction. LaTiO₂N has been mostly studied in the form of powder that is readily available and relatively inexpensive; however, its materials and transport properties, such as an absorption coefficient and charge carriers dynamics cannot be properly determined. Prior LaTiO₂N powder photoelectrodes typically have low chemical stability, often resulting in significant drop in photocurrent after only several minutes of operation. We present methods for the preparation and properties of epitaxial LaTiO₂N thin films grown on insulating (La₂Ti₂O₇) and conductive (La₅Ti₅O₁₇) single crystals by thermal ammonolysis. While optically transparent insulating substrates are suitable for optical and electrical characterizations, conducting substrates open up the possibility of detailed PEC characterization.
The LaTiO₂N films are shown to have a remarkably high absorption coefficient of about 10⁵ cm^-1 above 2.4 eV, a value comparable to the classic direct band gap semiconductor GaAs. The microstructure and composition of LaTiO₂N films were studied by scanning transmission electron microscopy (STEM) in a combination with electron energy loss spectroscopy (EELS). It has been found that typical ammonolysis reaction conditions results in the formation of nitrogen-deficient nonstoichiometric LaTiO₂N films with partially reduced titanium. In addition, it is observed that surface amorphous layer up to 30 nm thick can occur in facile manner if the growth is not properly controlled. Water oxidation photocurrents for unfunctionalized films of about 70 mu;Acm^-2 (at 1 V vs. Ag/AgCl in pH 7 buffer solution) were obtained. An electrochemical signature of the surface amorphous layer was observed. We also demonstrate using spectral ellipsometry that the LaTiO₂N film thickness can be effectively controlled over a range of thicknesses appropriate for PEC applications.

To improve the photoelectrochemical (PEC) performance of BiVO₄, three different modifications (doping, heterojunction, and catalyst deposition) using earth-abundant elements are performed and their effects are compared in a 0.1 M phosphate electrolyte at pH 7 under AM 1.5-light (100 mW/cm²). When a hexavalent element (Cr⁶⁺, W⁶⁺, or Mo⁶⁺) is doped at various levels, the Mo⁶⁺-doping effect is most significant at 10 atomic % with ca. 2 times higher photocurrent generation at the oxygen evolution potential (1.23 V_RHE). Such enhancement is attributed to a decrease in charge transfer resistance (R_ct) by donor doping, resulting in a ca. 2-fold increase in charge separation efficiency (eta;_sep) to ca. 25 %. W⁶⁺ is less effective than Mo⁶⁺ whereas Cr⁶⁺ has a detrimental effect. To further improve the charge separation efficiency of Mo⁶⁺-doped BiVO₄ (Mo-BiVO₄), a ca. 600 nm-thick WO₃ layer is deposited under a similarly thick Mo-BiVO₄ layer. This binary heterojunction (WO₃/Mo-BiVO₄) exhibits eta;_sep of ca. 50 % along with more than 3 times higher photocurrent generation. On the other hand, an oxygen evolving cobalt-phosphate (Co-Pi) catalyst electrodeposited to Mo-BiVO₄ (Mo-BiVO₄/Co-Pi) enhances charge injection efficiency (eta;_inj) from ca. 50 to ca. 70 % at 1.23 V_RHE. These two binaries are coupled into a ternary heterojunction (WO₃/Mo-BiVO₄/Co-Pi) in order to improve the charge transfer efficiencies (eta;_sep and eta;_inj). The PEC performance of this ternary is significantly high with photocurrent density of ca. 2.4 mA/cm² at 1.23 V_RHE (corresponding to the solar-to-hydrogen efficiency of ca. 3 %) due to eta;_sep and eta;_inj of ca. 60 % and 90 %, respectively.

Although it has been successfully demonstrated that carbon quantum dots (QDs) can significantly enhance the photoelectrochemical performance of semiconductors under visible light irradiation, it is highly debatable if such enhancement arises from the “upconversion” luminescence behavior of the carbon QDs. We have proven that the contribution of this “upconversion” luminescence to the improved photoelectrochemical performance is negligible even under direct high intensity NIR laser irradiation. This is achieved through the rational design of a TiO2 electrode synergistically sensitized with carbon and CdS QDs. The matching of band edges between the CdS QDs and the TiO2 nanocrystals would result in the formation of a type II heterojunction, while carbon QDs played the role of an electron-donor sensitizer. The photoinduced charge transfer dynamics were examined by femtosecond transient absorption spectroscopy and electrochemical impedance spectroscopy, and it was found that the addition of carbon QDs contributed significantly to the superior charge separation and transfer properties of the resultant TiO2/CdS/C photoelectrode. Overall, the synergistic effect of carbon and CdS QDs lead to a significant enhancement of the light harvesting efficiency of the electrode.

Global energy consumption increased dramatically over the years, driven by rising standards of living and a growing worldwide population. The increased demand for energy will require significant growth in energy generation capacity, secure energy sources, and a zero carbon emissions motivations. Among the various alternative energy strategies, the production of chemical fuels by solar energy conversion has been considered as one of the major strategies for solving the global energy issues. Various forms of metal oxide-carbon based nanocomposite were synthesized for photocatalytic solar hydrogen production. The carbon is presented in the form of carbon dots and reduced graphene sheets. Carbon dots are novel type of carbon nanomaterial, which have been used to improve the photocatalytic processes due to their favorable upconverted PL (UCPL) feature and electron transfer ability. Reduced graphene sheets assembled into 3D network, are favorable for photocatalytic reaction due to its high surface area, porous structure and superior electrical conductivity. Our findings emphasize on the achievement of tailoring of carbon nanomaterial into 3D network and quantum dot and functionalizing-sensitizing with noble metal nanoparticles to enable visible light (as opposed to UV light) water splitting based on complete solution process. Furthermore, the use of one-dimensional nanostructures features good vectorial electron transport due to decreased grain boundaries. It is noted that other than structural design, it is crucial to attain crystalline phase structures and superior interfacial contact between photocatalyst and co-catalyst to enable optimal synergy for enhanced photocatalytic H2 production performance.

Major barriers to progress in photoelectrochemical (PEC) water splitting include the lack of semiconductors with proper conduction and valence band alignment, significant over potential losses from sluggish water oxidation kinetics, material instability at operating conditions, and the need for quality semiconductor transport properties. We demonstrate, from experiment and theory, that spinel tin nitride (Sn₃N₄) has proper band alignment and we propose it as a solution to the first of these challenges. Tin nitride is an n-type semiconductor with a band gap near 1.7 eV. Because its band edges straddle the redox potentials of water and its composition of Earth-abundant elements make it potentially scalable, tin nitride is worthy of study as a photoanode material.
Sn₃N₄ thin films have been grown by reactive sputtering of tin in a nitrogen atmosphere. The resulting materials were n-type semiconductors with carrier concentrations of ~10¹⁸ cm^-3, conductivities of 0.14 S/cm and mobilities up to 1.2 cm²/Vs. The optical absorption onset was near 1.7 eV, which agrees with theoretical calculations. Work function (4.7 eV) and Hall measurements indicate that Fermi level is 0.1 to 0.2 eV below the conduction band, and that the conduction band minimum is ~0.2 eV above water&’s oxidation potential. GW-corrected DFT-surface calculations that take into account water surface dipole interactions are consistent with experiment.
PEC devices were made by growing Sn₃N₄ on fluorinated tin oxide coated glass. Cobalt oxide catalysts were applied and the devices were tested for stability and photocatalytic activity. Initial results of stability tests indicate that the nitride is stable under slight bias in basic conditions. Early PEC device characterization shows a small but promising photoresponse (~0.1 mA/cm² at 1.23 V vs. RHE) under AM 1.5 illumination in 0.1 M potassium phosphate (pH= 7.25).
There are a number of challenges with this material. First, low defect density, large grain films have not been reported in the literature nor observed in this study. Second, band structure calculations indicate large hole effective masses (~10 m_e) which, in combination with high scattering center densities, could lead to small minority carrier diffusion lengths. Finally, photo-corrosion of the anode is a concern. This issue can be addressed by a conformal coating of a stable catalyst. Short minority carrier diffusion lengths can be increased by growing low defect materials, and arrays of tin nitride nano rods could circumvent the need for large diffusion lengths. Further work will focus on increasing the photocurrent in planar tin nitride devices by increasing the grain size and identifying the proper catalyst.
This work is supported by the U.S. Department of Energy and the Netherlands Organization for Scientific Research (NWO), VENI scheme.

Photovoltaic devices usually exploit mid-range band-gap semiconductors, which absorb in the visible range of the solar spectrum. However, much energy is lost in the IR and near-IR range. Understanding the growth mechanism of small band-gap materials and how to optimize the growth parameters can pave the way to construct highly efficient IR and near IR photovoltaic devices. In this work we combine the advantages of small band-gap, bulk-like PbS deposited by facile, cheap, and direct chemical bath deposition, with the good electronic properties of ZnO, and the large surface area of nanowires, towards low cost photovoltaic devices utilizing IR and near-IR light.
The mechanism of PbS growth by chemical bath deposition on ZnO nanowires was scrutinized using HRTEM. A visible proof is shown for a growth mechanism starting from amorphous Pb(OH)2 layer, that evolves into the 'ion-by ion' growth mechanism. The grain size affects the magnitude of the band-gap and is controlled by the deposition temperatures. Deposition above 40°C results in bulk-like PbS with an optical band-gap of 0.4 eV. Complete PbS coverage of the complex ZnO nanowires architecture was achieved at both high and low temperatures. Photocurrent measurements under white, near-IR (784 nm) and IR (1550 nm) illumination show that despite a 200 meV barrier for electron transfer at the PbS/ZnO interface, extraction of photoelectrons from PbS to the ZnO is feasible. The ability to harvest electrons from a narrow band-gap semiconductor deposited on a large surface-area electrode using wet chemistry can advance the field towards high efficiency, low cost IR and near-IR optoelectronic devices.

The perfect storm of increasing global population, scarcity of traditional energy resources and associated climate change threaten the future viability of a technologically advanced society. The need for humankind to develop renewable, sustainable energy production methods, for example solar hydrogen, is critical. Developing new materials for photocatalysis and improving the efficiency of known photocatalysts is a fundamental challenge on the path towards sustainable hydrogen production. The biggest advantage of a solar hydrogen economy would be that hydrogen is both a renewable and practically emission free energy source, able to sustain future energy demand, and at the same time an easy way to store and transport energy. One of the major disadvantages that it faces today is that suitable semiconductor materials suffer from a lack of efficiency due to (1) wide band gaps - for the visible region and (2) charge carrier recombination.
One approach to narrow the band gap of metal oxide semiconductors, would be via incorporation of the properties of the “lone pair” of electrons found in heavy post-transition metal oxides (Sn, Pb, Bi) [1]. The heavier group IV elements exhibit two oxidation states, the group oxidation state N and the N-2 oxidation state. The latter - referred to in terms of the lone pair of electrons - gives rise to structural distortions in oxides such as PbO, SnO and BiVO₄ [2]. The formation of the lone pair from metal s electrons (found at the top and bottom of the valence band) and oxygen p states gives rise to a narrower band gap for the valence bands of oxides are typically dominated by the O 2p states alone [3].
We have been investigating SnWO₄, Sn₂TiO₄ and other Sn²⁺ ternary oxides in terms of band structure, dominant charge carrier type and photocatalytic capability as bulk materials and thin films. Using XPS the filled electronic states of different oxide materials can be investigated and our results show that the valence band of SnWO₄ is influenced by the O 2p, Sn 5s and Sn 5p states. Hard x-ray photoelectron spectroscopy (HAXPES) provides further insight into the contribution of the electronic states towards the different bands by taking advantage of varying photoionization cross sections at high incident photon energies. Additionally new synthetic routes, such as microwave heating, are investigated to obtain materials unobtainable with traditional solid-state synthesis and the preliminary results will be discussed.
[1] D.J. Payne et al. Physical Review Letters96 157403 (2006)
[2] D.J. Payne et al. Applied Physics Letters98 212110 (2011).
[3] A. Walsh et al. Chemical Society Reviews40 4455 (2011)

Liquid Light, Inc. is developing technology for the electrochemical and photoelectrochemical conversion of carbon dioxide to commodity chemicals and fuels. Founded in 2009, we are commercializing the electroreduction catalyst technology originally developed by Professor Andrew Bocarsly at Princeton University. These simple homogeneous catalysts are novel in that they are inexpensive, stable, selective for a wide variety of products, and operable at low overpotential. This paper will present our advancements in the field of electrochemical and photoelectrochemical reduction of carbon dioxide, utilizing the technology developed at Princeton and Liquid Light. Specifically, we will highlight the scale-up of production of two chemicals in commercial size, continuous-flow electrochemical reactors. Additionally we will highlight our advancements in carbon dioxide catalysis, showcasing the twenty products produced to-date at our laboratories. While Liquid Light focuses on electrochemical processes for carbon dioxide conversion using clean electricity sources, we sponsor research at Princeton University that focuses on mechanistic understanding of the catalysis and photoelectrochemical system development. This paper will also highlight new photoelectrochemical systems that directly convert light energy into stored chemical energy in the form of such desirable products as methanol, ethanol, and butanol. Importantly, these products are produced directly from carbon dioxide and water.

Artificial photosynthesis has attracted much attention due to potential applications for energy-storing devices using solar energy. Mimicking photosynthesis in plants, photoelectrochemical (PEC) conversion system is suggested to use CO2 and H2O as feedstock chemicals to produce fuels by using solar energy in a sustainable manner. PEC CO2 reduction is a promising way to produce energy-dense carbon-based fuels such as carbon monoxide, formic acid, methanol, etc. However, PEC CO2 conversion system suffers from poor energy conversion efficiency and poor product selectivity, and its success has yet to be realized due to extremely high molecular stability and poor solubility of CO2 in aqueous solutions under ambient temperature and pressure. Herein, we demonstrate a PEC CO2 reduction platform in which an electrode composed of Au nanoparticles as reduction catalysts is powered by monolithic CuInGaS2 thin-film photovoltaic (PV) devices in acetonitrile-water mixture electrolyte. Specifically, Au nanoparticles were prepared by electrochemical synthetic method on solid substrates to increase CO2 reduction activities with selective production for carbon monoxide and formic acid, and CuInGaS2 thin-film PV cell was fabricated by low-cost solution-based preparation method whose high open-circuit voltage is desirable to overcome high overpotential of CO2 reduction reaction. Products were detected by ion chromatography and gas chromatography with TCD and FID, and faradaic efficiencies were evaluated by chronoamperometry. Furthermore, the product selectivity, the energy conversion efficiency, and the stability of the electrode were dramatically improved due to expanded actual surface area and catalytically active sites, compared to the results obtained from a bare Au electrode with the same surface area. This PV-powered solar-fuel device exhibited total faradaic efficiencies of CO2 to C1 chemicals (e.g. carbon monoxide and formic acid) of over 80% as well as 0.7% solar-to-fuel conversion efficiency, which is comparable to examples found in nature.

The development of a cost effective process for the (photo)electrochemical reduction of CO2 could enable a shift to a sustainable energy economy. Utilizing solar energy to enable such a process could generate carbon neutral fuels or chemicals that are conventionally produced from fossil resources. One key to developing such a process is a catalyst capable of performing the conversion at a low overpotential and high selectivity to the desired product, particularly to hydrocarbons and alcohols. Unfortunately, known catalysts do not meet these requirements.
Transition metals have been previously studied as CO2 electro-reduction catalysts,[1] revealing that copper is unique amongst the transition metals because it has an intermediate binding energy to CO and, as a result, produces a mixture of hydrocarbons and alcohols as major products. Despite many detailed studies into copper electrodes,[2] much remains to be learned about the factors that lead to the formation of hydrocarbons and alcohols. This paper will show new insights gained into these electrochemical processes on copper[3] as well as a number of other transition metals: Au, Ag, Zn, Ni, Pt, and Fe. The focus will be on the production of hydrocarbons and/or alcohols on these metals, and theory and experiment agree as to what the bottlenecks are in the process. This paper will provide new design principles to consider that can guide the development of improved CO2 electro-reduction catalysts to synthesize fuels and chemicals in a renewable, sustainable fashion.
References
1. Y. Hori, in Handbook of Fuel Cells: Fundamentals, Technology and Application, ed. A. L. Wolf Vielstich, Hubert A. Gasteiger, VHCWiley, Chichester, 2003, vol. 2, pp. 720-733.
2. M. Gattrell, N. Gupta, A. Co; J. Electroanal. Chem., 2006, 594, 1-19.
3. K. P. Kuhl, E. R. Cave, D. N. Abram, T. F. Jaramillo; Energy Environ. Sci., 2012, 5, 7050-7059.

Solar CO2 conversion has attracted increasing attention not only to reduce anthropogenic carbon emissions but also to convert CO2 to renewable and recyclable chemical fuels. Despite the scientific interest, industrial importance, and even socioeconomic attention, the solar CO2 conversion technology progresses very slowly over the past four decades, particularly because of the limited number of suitable photoelectrodes. Si is one of the most appropriate electrodes because it is an earth abundant element with a narrow band gap of ~ 1.1 eV, high carrier mobility, stability over a wide pH range, non-toxicity, well-established fabrication technique, and commercial availability. Although planar p-Si is promising, charge carrier recombination can occur due mainly to the low diffusion length of the minority carriers in the same absorber thickness. A wire-array geometry, however, possesses long optical paths for efficient photon absorption as well as a higher collection efficiency for the minority carrier. To the best of our knowledge, this is the first report of the fabrication of Sn-coupled p-Si nanowire arrays for solar CO2 conversion. Vertically aligned, free standing p-Si nanowire arrays of varying lengths were grown on p-Si wafers, and coupled with Sn nanoparticles. These heterojunction wire/Sn arrays increased the production of formate dramatically, by more than 10 and 5 times compared to planar p-Si and wire arrays, respectively, with Faradaic efficiencies of ~ 40% in a single cell (88% in an H-type cell).

This presentation will describe new approaches to the synthesis of candidate catalysts for electrochemical oxidation of water to dioxygen. The approaches involve atomic layer deposition of metal-oxide materials on isolated reaction sites within redox-conductive metal-organic frameworks as well as on metallo-porphyrin covered surfaces. The new approaches were developed with the goal of overcoming some potential limitations, both practical and fundamental, presented by catalysts created by conventional methods.

Hydrogen is one of the most promising renewable fuel sources, with a potentially limitless supply available from electrochemical water splitting. A major bottleneck to viable hydrogen generation is finding a non-toxic, earth abundant inexpensive catalyst for efficient hydrogen evolution reaction (HER) in water. Transition metal di-chalcogenides (TMDC), particularly WS2 and MoS2, have been shown to fit these criteria.
Recent work on layered 2D sheets of MoS2 has shown that a lithiation-induced phase transformation from semiconducting to metallic phase can be achieved, which leads to enhanced HER catalysis due to increasing catalytically active sites. (1) However, 2D MoS2 is limited by low catalytic surface area. The use of MoS2/MoOx shell/core nanowires has been shown to overcome these limitations, with the ability to deposit a high surface area, highly conductive architecture on a variety of substrates via CVD. These nanowire arrays have shown great electro-catalytic properties and high stability in acidic environments. (2)
Here, we present the first report on the chemical modification and structural correlation (HRTEM) of CVD synthesized MoS2 (10 nm)/MoOx (80 nm) shell/core nanowires, which leads to a dramatic increase in the electro-catalytic activity. We progressively intercalate the MoS2 nanowires with Lithium, while correlating the catalytic activity with the structural changes using HRTEM. The “as-grown” nanowires are semiconducting with a highly ordered MoS2 shell. After lithium intercalation, this ordered structure is transformed into polycrystalline MoS2 domains with high surface area. These nanowires with high surface area show an on-set potential of ~150 mV vs. RHE, which is 50 mV lower that as-synthesized nanowires, almost double the generated current.
Furthermore, in order to understand origin of high catalytic activity (metallic phase transformation or increase electron density) we doped the as-grown nanowires by exposure to hydrazine (N2H4). We observed a dramatic ~4 times increase in current density and a further decrease in the on-set potential to ~100 mV vs. RHE. These new chemical modification techniques help in understanding the true nature of the electro-catalytic site in MoS2. We also investigated the HER photo-catalytic effect of the MoS2 nanowires using similar chemical modification approaches as above.
References.
1. M. Chhowalla, H. S. Shin, G. Eda, L.-J. Li, K. P. Loh and H. Zhang, Nature Chemistry, 2013, 5, 263-275.
2. Z. Chen, D. Cummins, B. N. Reinecke, E. Clark, M. K. Sunkara and T. F. Jaramillo, Nano Letters, 2011, 11, 4168-4175.

Photoelectrochemical water splitting can be used to store solar energy in the form of hydrogen. However, the efficiency of the overall water splitting reaction is severely limited by the high overpotential costs required for the oxygen evolution half reaction. Furthermore, there exists a need for a nonprecious metal catalyst to drive the oxygen evolution reaction (OER) at low overpotentials to make the process more economical as well as more efficient.
As an alternative to the best known precious metal OER catalysts like RuO2 and IrO2, previous work has focused on manganese oxide and cobalt oxide catalysts.[1-5] We have identified amorphous CoTixOy as a novel, active, nonprecious metal catalyst for OER. Using a simple and scalable sol gel synthesis, thin films (50 nm - 200 nm) of CoTixOy can be deposited on a number of different conductive substrates. This work begins by characterizing the structure and morphology of this material and studying the effect of varying the heat treatment temperature and metals composition on catalytic activity and material stability. Furthermore, we characterize the effect of preparation route and electrochemical testing on the oxidation state of cobalt using ex situ L-edge x-ray absorption spectroscopy (XAS).
We have verified that crystalline CoTiO3 is formed at calcination temperatures 550°C and above, however the lower temperature calcinations which result in an amorphous CoTixOy material produce better catalysts for the OER. A calcination temperature of 150°C for a sample with equal amounts of cobalt and titanium produces a catalyst with a relatively low overpotential (~440 mV) to reach 10 mA/cm2 and loses only 26% of its initial activity over 10,000 oxidation/reduction cycles. All samples with 55% or greater cobalt in the metals composition have high activity for the OER. Furthermore, adjusting the metal composition of this material tunes the redox peak potential that is observed for cobalt on the first oxidative electrochemical sweep. Ex situ XAS reveals that both calcination temperature and metal composition have an effect on the cobalt oxidation state. It is found that the cobalt is in a mixed 2+/3+ state under OER conditions but that achieving this state via adjusted material composition or electrochemical oxidation results in a more active catalyst than via thermal means.
1. Gorlin, Y. & Jaramillo, T.F. J Am Chem Soc 132, 13612-13614 (2010).
2. Brimblecombe, R., Koo, A., Dismukes, G.C., Swiegers, G.F. & Spiccia, L. J Am Chem Soc 132, 2892-2894 (2010).
3. Yeo, B.S. & Bell, A.T. J Am Chem Soc 133, 5587-5593 (2011).
4. Esswein, A.J., Surendranath, Y., Reece, S.Y. & Nocera, D.G. Energy Environ. Sci. 4, 499-504 (2011).
5. Liang, Y. Li, Y. Wang, H. Zhou, J. Wang, J. Regier, T. & Dai, H. Nature Mater. 10, 780-786 (2011).

With the increasing interest in using hydrogen as a sustainable and carbon-free energy carrier, there is a great need to explore and optimize new catalysts suitable for electrochemical water reduction. The scale of global energy demand calls for such hydrogen evolution reaction (HER) catalysts to be fabricated from non-precious materials. In this study, a large group of first-row transition metal dichalcogenides (ME2, M = Fe, Co, Ni; E = S, Se) are introduced as non-precious HER catalysts [1]. Our electrochemical studies reveal their high activity towards the HER in an acidic electrolyte, which is among the most active electrocatalysts based on non-noble materials. These materials effectively enrich the family of hydrogen-producing catalysts, which may find uses in solar-fuel devices.
[1] D. Kong, J. J. Cha, H. Wang, H. R. Lee and Y. Cui, Energy Environmental Science (2013), DOI: 10.1039/C3EE42413H

Efficient photoelectrosynthetic devices for light induced water splitting must deliver a photovoltage in the range of 1.8 V with quantum efficiencies approaching . We have studied thin film silicon tandem cells as possibly economic competitive solutions in their photoelectrochemical performance in contact to electrolyte solutions in comparison to their PV performance to deduce remaining losses related to the involved electrochemistry and strategies to overcome them.
In this work, we investigate two types of tandem solar cells: (i) An amorphous top cell connected to a microcrystalline layer (a-Si:H/µc-Si:H) with VOC up to 1.42 V and an efficiency of 10.8%. (ii) two amorphous (a-Si:H/a-Si:H) sub-cells with an open circuit voltage VOC of about 1.8 V and a solar conversion efficiency of around 10.0%. The solar cells were investigated by current-voltage measurements in 3 and 2 electrode geometry. The photoelectrochemical performance of the electrodes was evaluated in an aqueous solutions of different pH. The exposed n-doped surface of the bottom cell was modified by Pt co-catalysts using sputter deposition or wet chemical reduction of Pt-salts.
The cyclic voltammetry data measured with a potentiostat, show the performance of the silicon based photocathodes, in the achieved photocurrent densities and onset potentials for water reduction. In general, we have found that the photocurrent densities measured for the PV device are also obtained in the PEC cell. However, the onset of photocurrent as well as the steepness of the photocurrent onset corresponding to the fill factor is strongly dependent on the given charge transfer kinetics. For a-Si:H/µc-Si:H photocathodes with Pt co-catalysts the photocurrent onset potential measured in 3 electrode geometry is around 1.3 V vs. the reversible hydrogen electrode (RHE) and a photocurrent of 9.0 mA/cm2 at 0 V vs. RHE is reached. For a-Si:H/a-Si:H photocathodes with Pt co-catalysts we reach photocurrent onset potentials of about 1.7 V vs RHE, the maximum power point is situated at 1.3 V, and the photocurrent is again similar to the PV cell at 6 mA/cm2. If we use a 2 electrode geometry with RuO2 as counter electrode the onset of Uph is at 1.5 V vs RHE and the photocurrent at RHE is 4.5 mA/cm2. These additional losses in comparison to the 3 electrode results are related to still non-identified resistances in the circuit: Removing the remaining losses of 200 mV would allow to approach efficiencies in the range of 10%. However, the poor stability of the photocathodes is still an issue.

Energy from the sun can provide sufficient power for all of our energy needs and a potentially efficient route to storing this energy is to convert sunlight into chemical energy in the form of chemical bonds, which is a form of an artificial photosynthesis process. Considering the abundance of H2O on the planet, water splitting is a natural pathway for artificial photosynthesis. In the last decade, several semiconductors oxides such as Fe2O3, WO3 and SrTiO3 have been considering as promising material for photoanode in a photoelectrochemical cell (PEC). These oxides semiconductors showed high photocurrent and in general, it performance is directly associated to the morphological control at the nanoscale. In this work, we demonstrate an alternative and promising way to produce metal oxide photoanodes with high performance for water oxidation. In this approach, we processed Fe2O3, WO3 and SrTiO3 thin films using a colloidal dispersion of oxides nanoparticles as the precursor and these nanoparticles were synthesized by a non-aqueous method. Using this deposition approach the photoanodes showed a photocurrent density of 1.9 mA.cm-2 for WO3 and 2.4mA.cm-2 for hematite (both performed measured at 1.23 VRHE under AM 1.5 illumination). We intend to address a critical discusses about the impact of the nanostructure control in the semiconductors oxides photoanode performance for water oxidation.

In the context of solar power utilization to meet the dramatically increasing demand of green energy, energy storage in chemical bonds is highly desirable. In particular, sunlight-driven water splitting, H2O(l) + hv --> H2 (g) + ½ O2 (g), resulting in the production of H2(g) is a “Holy Grail” to achieve energy and environmental sustainability. A promising approach for H2 production is the use of a photoelectrochemical (PEC) system consisting of semiconductor light absorbers, a membrane and surface-attached electrocatalysts. To optimize the PEC performance, one must sufficiently understand the interfacial energetics and charge-transfer kinetics at semiconductor/metal/liquid junctions. For the hydrogen evolution reaction (HER), metals in various forms are widely used as catalysts. When a metal catalyst is bound to a semiconductor surface in electrolyte, several critical parameters are still poorly understood, including metal selection, interfacial chemical composition, mixed-barrier-height interfacial energetics, in situ H2 alloying, and light collection. Additionally, the interactions between these various parameters have been barely investigated and remain poorly understood. Existing theoretical models to evaluate the solar-to-hydrogen (STH) energy conversion efficiency have generally assumed HER semiconductor photocatalysis is adequately described by the serial combination of catalyst kinetics (e.g. the Butler-Volmer equation) and photodiode response (e.g. the Shockley diode equation).
We investigated the interfacial effects on energetics and kinetics for solar hydrogen production. We selected silicon and six different metals (Pt, Pd, Au, Ni, Cu, and Pt) as model materials because of the well-controlled electronic properties of Si, the span of metal work functions and catalytic abilities, and the well-documented properties of the respective metal/semiconductor junctions. We characterized the catalytic activities of these metals in the dark and on illuminated p-Si using cyclic voltammetry. The magnitudes and variations in open-circuit voltage for various metals on Si are larger than those predicted for either the Si/metal or the Si/liquid junction. We also observed enhancement in the apparent catalytic rates for several metals deposited on p-Si and tested under illumination. The enhancement cannot be interpreted by a generally adopted diode-electrocatalysis model. These observations indicate that both the HER energetics and kinetics for these metal nanoparticle-catalyzed photocathodes are consequences of synergistic interfacial effects for hydrogen production.

This presentation focuses on the use of soft and hard X-ray photoelectron spectroscopy for experimental understanding and design of interfaces in mesoscopic materials used for energy conversion. In particular the presentation contain investigations on electronic structure and chemical composition of perovskite structures (e.g. CH3NH3PbI3) and molecular materials (e.g. Spiro-OMeTAD hole-conductor) deposited onto mesoporous oxides used in solar cells and Li-ion batteries. The efficiency of the conversion process in these systems is largely dependent on the properties of the interfacial region including atomic and molecular organization as well as orbital composition and energy matching between the materials. Insight into electronic structure of the interface is therefore crucial in order to understand and optimize the function. X-ray based techniques such as photoelectron spectroscopy (PES) are powerful for obtaining such information at an atomic level due to the possibility for element specificity. This contribution reviews some of our recent synchrotron based PES developments for understanding the material interfaces between molecular as well as inorganic materials. Specifically how PES can be used as both a surface and a near surface technique.
Specifically it will be shown how a combination of soft and hard X-ray photoelectron spectroscopy can be used to experimentally understand the molecular orbital structure in molecular materials, the valence electronic structure of CH3NH3PbI3 and similar compounds as well as oxide intermixing in conducting thin films.

GaInP2/GaAs tandem cell is demonstrated to have 12.4 % of solar to hydrogen conversion efficiency without any other energy input except photon. However, its short lifetime is long-standing problem. Recently, NREL group found that nitrogen treated GaInP2 photocathode become corrosion resistive when Pt/Ru cocatalyst nanoparticles are deposited on top. Experiments seem to indicate that all the components are necessary to have extended longevity. In order to better understand the surface of the photoelectrode, we performed the first-principles molecular dynamics (MD) based on density functional theory. Our systematic MD study shows that Pt atoms have a tendency to avoid coordination with surface nitrogen, whereas Ru atoms are free to have bonds with any other elements. This difference leads to the rearrangement of atoms at the catalyst/light-absorber interfaces as well as the shape change of catalyst itself over time. Also some metal atoms such as In and Ga escape the surface and migrate to the catalyst and its ratio varies depending on nitridation and catalyst. We will discuss how our findings are possibly related to the experimentally observed corrosion resistivity.
This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.

Efficient catalysts for oxygen evolution reaction (OER) based on earth abundant elements are necessary for wide spread application of photochemical energy conversion and storage.
Recently, mixed Ni-Fe oxides thin films have been shown to achieve very low OER overpotentials (~0.3 V at 10 mA cm-2) with minimum at approximately 50% of Ni-Fe mixing (1). Subsequent X-ray absorption spectroscopy of these films have revealed mixed NixFe1-xOOHy layered type structures in edge-sharing octahedral arrangement. Our ab-initio calculations identify structures (i.e., y stochiometry) consistent with experiments. Next, we are able to explain observed increase of in Ni(II)/Ni(III) redox transition with increasing Fe content. Finally, detailed calculations of OER activity reveal role of Ni-Fe neighbors in the activity enhancement.
(1) Louie, M. W.; Bell, A. T. J. Am. Chem. Soc. 2013, 135, 12329-12337.

Spectroscopic characterization plays an essential role in the rational design and development of materials for photoelectrochemical (PEC) technologies [1]. In this context, synchrotron based soft x-ray spectroscopies are well suited to probing the local electronic structure and photo-excitation dynamics in PEC systems yielding valuable insights that are complementary to conventional ultra-violet/visible (UV/Vis) techniques. However, the interpretation of core-level spectra is not straight-forward and theoretical methods are indispensable in connecting measured spectra to structural and dynamical models.
We present three case studies of first-principles theoretical methods applied in conjunction with experimental core-level spectroscopy measurements to investigate the electronic structure and dynamical processes in molecular, solid-state and interfacial systems relevant to PEC technologies. In the first, we study two zinc(II)-porphyrin based Donor-π-Acceptor (D-p-A) [2] dyes using a combination of the occupancy-constrained excited electron and core-hole (XCH) [3] density functional theory (DFT) approach and time-dependent density functional theory (TDDFT) simulations. These methods respectively provide a detailed interpretation of measured N K-edge x-ray absorption and ultraviolet / visible spectra [4]. In the second, we investigate the electronic structure of Fe2O3 p-n junctions through simulations of O K-edge x-ray absorption spectra in order to explain the apparent reduction in t2g-eg splitting in the experimentally observed spectral features. Finally, we use a combination of constrained DFT and TDDFT to interpret measured transient core-level shifts in time-resolved femtosecond x-ray photoelectron spectroscopy, investigating the dynamics of the electron injection process from a N3 dye molecule chemisorbed onto a ZnO substrate. These studies illustrate the utility of first-principles methods in guiding the design of better PEC materials.
This work was performed at the Molecular Foundry, supported by the Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231.
References:
[1]Himpsel, F. J., et al, J. Electron . Spectrosc. Relat. Phenom., http://dx.doi.org/10.1016/j.elspec.2012.10.002
[2]Yella, A. et al, Science 2011, 334, 629.
[3] Prendergast, D and Galli, G; Phys. Rev. Lett. 2006, 96, 215502.
[4] Zegkinoglou, I et al, J. Phys. Chem. C, J. Phys. Chem. C, 2013, 117, 13357

Inorganic nanoscale devices have shown promise in the renewable energy context for their robustness and potential scalability. Light harvesting molecules are necessary to achieve artificial photosynthesis, e.g. photo-induced charge transfer reactions which split water. For optimal efficiency in an artificial photosynthetic device, a long excited state lifetime is necessary. In this study, we considered the experimentally synthesized vanadium(V) oxo compound VOL^F (where L^F is N(CH2Ar2)((CH2)2OH) with Ar = (2-hydroxy-5-fluoro-3-methyl)phenyl) which has been observed to remain in the excited state for ~300 ps.
We used linear response time dependent density functional theory (TD-DFT) to compute and interpret static UV-Vis spectra. To assess effects due to finite temperature, we used first principles molecular dynamics simulations to generate multiple starting configurations in different solvent environments. Comparing with experiment, we find good agreement with the observed spectra. Furthermore, we find that the bright state is associated with charge transfer to the vanadium from surrounding ligands. Our results suggest transitions between low lying d-states can give rise to extended excited state lifetimes.

Solar-driven water splitting is a key photochemical reaction in solar fuel production.[1] In nature, water oxidation occurs in Photosystem II, containing a CaMn₄O₄ cluster as the active site.[2] To mimic the function of this protein complex, an amorphous cobalt-phosphate catalyst (Co-Pi) has been recently reported to efficiently promote water splitting.[3]
Density functional theory (DFT) calculations are routinely used to study the electronic and at-omistic structures of these materials as well as the thermodynamics and mechanisms of the electro-chemical oxygen evolution reaction. The accuracy of these theoretical predictions has never been de-termined against high-level quantum chemistry methods. Our coupled cluster (CC) calculations provide a highly accurate computational estimate of the OER thermodynamics for a realistic model reaction pathway and active site of cobalt oxide nanoparticles. We show that hybrid B3LYP and PBE0 functionals can adequately reproduce the CC reference energetics, while plain GGA calculations lead to very important discrepancies. The inclusion of on-site electronic repulsion (DFT+U) substantially im-proves the calculated electronic and structural properties, but there is no value of the U parameter that can lead to the same level of general agreement obtained with hybrid functionals.[4]
Apart from benchmarking the energetics we focus on thermodynamics and kinetics of the Co-Pi model in the reaction conditions. Based on the predicted structures of the catalyst (from the EXAFS data and our recent simulations)[5,6] we established their stability in solution and the termination of the surface active sites. Among the many possible configurations with different oxidation states, charges and ligands we identified the relevant activated intermediates capable to promote the rate-limiting O-O bond formation. We employed ab-initio metadynamics to predict the activation energy and mechanism for the O-O bond formation.
The presented correlations between the reaction mechanism, thermodynamic efficiency and local structure of the active sites can provide key guidelines for the rational design of a superior catalyst.

The low cost and high chemical stability of transition metal oxides makes them attractive candidates for photoelectrochemical water splitting applications. Although unassisted water splitting has been observed for a selected number of wide-bandgap metal oxides, all oxides that absorb in the visible region need a bias potential in order to split water. We recently demonstrated a stand-alone water splitting device composed of a BiVO₄ photoanode biased by a double-junction amorphous silicon cell [1]. The thin-film a-Si/a-Si tandem cell is placed behind the BiVO₄ electrode and absorbs the <2.4 eV photons that are transmitted through the BiVO₄ film. The PV cell generates a bias voltage of ~1 V, resulting in an overall solar-to-hydrogen efficiency of 4.9%. Preliminary stability measurements showed no measurable performance decrease over a period of one hour [1].
While encouraging, the efficiency is still a factor of ~2 below that what is believed needed for practical applications [2]. In the present work, we assess the realizable efficiencies for hybrid PEC/PV tandem devices based on a metal oxide photoanode placed in front of a thin film silicon PV cell. Although any PV cell can be used, we limited ourselves to thin film silicon because it is earth-abundant and low-cost. The I-V characteristics of the photoanode and the PV cell are simulated in order to find the working point of the tandem device. The optimal device design depends on the complex interplay between metal oxide bandgap, the shape of the aborption spectrum, film thickness, charge separation efficiency, catalytic efficiency, energetic positions of the band edges, and overpotentials. The actual performance characteristics of BiVO₄ are used as a starting point, and combined with those of various single- and double-junction a-Si/a-Si, a-Si/µc-Si and nanocrystalline silicon PV cells.
We find that the realizable efficiency of the BiVO₄ /2-jn a-Si device described above is limited to ~6%, leaving little room for further improvement. In contrast, solar-to-H₂ efficiencies as high as 15% can be obtained by combining a nanocrystalline, single-junction silicon solar cell with a 1.8 eV metal oxide absorber. Moreover, our calculations show that the metal oxide doesn&’t have to be ‘perfect&’ in a hybrid photoelectrode: a carrier separation efficiency of 0.7 and a catalytic efficiency of 0.75 is already sufficient to reach an overall STH efficiency of 10%. Previous work by us and others has shown that these values are well within reach [3], which demonstrates that hybrid tandem photoelectrodes based on metal oxide absorbers and thin film silicon PV cells are promising components for efficient solar fuel devices.
[1] F.F. Abdi et al., Nat. Commun. 4:2195 (2013) 1.
[2] J. Keable and B. Holcroft, “Economic and business perspectives”, in: “Photoelectrochemical Hydrogen Production”, R. van de Krol and M. Grätzel (Eds.), Springer (2012).
[3] F.F. Abdi et al., ChemCatChem 5 (2013) 490.

BiVO₄ has emerged as one of the most promising photoanode materials for solar water splitting. The breakthrough is the result of the successful identification of the performance limiting factors in the material: (i) poor catalytic activity and (ii) slow electron transport (low carrier separation efficiency). The first problem is solved by coupling water oxidation co-catalysts on the surface of BiVO₄,^1,2 while the latter is overcome by introducing dopants, such as W and Mo.^3,4 We recently showed that this can be further improved by introducing a gradient in the W dopant concentration in BiVO₄, which has resulted in carrier separation efficiencies as high as 80%.⁵ As a result, water oxidation catalysis and bulk carrier recombination no longer limit the photocurrent of BiVO₄. Instead, modest light absorption—especially for photons with energy close to the bandgap—becomes the main limitation. To illustrate this, the amount of light absorbed in our best W:BiVO₄ samples is only ~5 mA/cm², compared to a theoretical maximum of ~7.5 mA/cm² (assuming that all photons with an energy higher than the bandgap are absorbed and collected). This means that a significant increase of the photocurrent (up to 2.5 mA/cm²) can be achieved when the light absorption is improved.
In this work, we explore the application of core-shell nanoparticles on the surface of 100 nm-thick spray-deposited BiVO₄ films as a route to enhance the optical absorption of the films by near-field plasmonic enhancement. Similar to the work of Thomann et al.,⁶ SiO₂ is used as the shell layer to avoid surface recombination. However, here, Ag is used as a core material because: (i) Ag is less expensive than Au, and (ii) the plasmon frequency of Ag can be tailored between 400 to 550 nm, which is a better match with the absorption spectrum of BiVO₄. Under back-side AM1.5 illumination, we achieve a ~2.5 fold photocurrent improvement when decorating BiVO₄ with Ag/SiO₂ core-shell nanoparticles. We show that this enhancement can be attributed to two factors introduced by the core-shell nanoparticles. The first factor is related to the improved optical absorption in the layer, caused by both the optical near-field enhancement and improved light scattering (also called far-field effects) induced by the excitation of localized surface plasmon resonance in Ag nanoparticles. In addition, a significant non-optical enhancement is also observed, which is tentatively attributed to the reduction of surface recombination.
References
1. F. F. Abdi and R. van de Krol, J Phys. Chem. C 116 (2012) 9398
2. D. K. Zhong, S. Choi and D. Gamelin, J Am. Chem. Soc. 133 (2011) 18370
3. F. F. Abdi, N. Firet and R. van de Krol, ChemCatChem 5 (2013) 490
4. S. K. Pilli et al., Energy Environ. Sci. 4 (2011) 5028
5. F. F. Abdi et al., Nat. Commun. 4:2195 (2013) 1
6. I. Thomann et al., Nano Letters 11 (2011) 3440

In recent years, BiVO4 has been regarded as one of the most promising metal oxide photoanode candidates for water splitting. In JCAP, we have successfully developed sputtering synthesis effort for making high quality, uniform and dense thin film BiVO4 not only for potential large scale application, but also for fundamental electronic characterization such as transient absorption (TA), resonant inelastic X-ray scattering (RIXS) and so on. From our characterization results, we confirm that the electron conductivity is fairly poor in BiVO4 and probe the origin of it.
To overcome this intrinsic problem of BiVO4 and make it more viable for our wireless water splitting device, we use several approaches. 1) We are able to introduce dopants of W and Mo into BiVO4 by co-sputtering. Influence of different doping materials and doping levels will be discussed in the presentation. We are also going to show thermal conductivity measurement, TA and RIXS analysis explore the roles of the dopants in improving electronic properties of BiVO4. 2) We use H2 treatment to make the film more O deficient and thus more n-type conductive. Again, related PEC and electronic characterization will be shown in the presentation. 3) The addition of underlayer/hole blocking layer to facilitate charge separation at the back. 4) The integration of OER catalysts to rapidly extract holes from the surface instead of recombining with electrons. We will compare the traditional Co based catalysts for mild pH operation condition with the Ni-Fe (Ce) based catalyst for extreme basic operation condition. We will also discuss the possibility to stabilize BiVO4 under extreme conditions.

Water splitting using sun light is a promising strategy to harvest abundant solar energy on the Earth. BiVO₄ has appeared as an important semiconducting oxide to split water using sun light, because of its appropriate band gap matching with solar spectrum and stability in aqueous solutions under illumination. Among a variety of preparing techniques, including spray pyrolysis and spin-coating, reactive sputtering is proved to be most reliable as a result of commercially available and scalable deposition in modern industry compared with chemical methods.
Herein we used reactive sputtering method to deposit highly photoactive bismuth vanadate (BiVO₄) photoanode. We investigated the effects of V/Bi ratio, deposition and annealing temperatures, molybdenum doping, cobalt phosphate (Co-Pi) catalyst, and wide-bandgap oxide hole-blocking layer on the crystallography and photoelectrochemical properties of BiVO₄ thin films. The results showed that at vanadium rich side and elevated deposition temperature it was prone to form pure-phase monoclinic BiVO₄, which is more photoactive than tetragonal phase. Although the photocurrent was improved with increased deposition temperature, the situation was not the case after annealing in air for 2 hours at 500°C. Molybdenum dopant can greatly improve the conductivity of electrons. So after doping, the photocurrent could be increased by 3 times, reaching ~1mA/cm². Co-Pi catalyst is proved to be an efficient oxygen evolution catalyst, which can decrease the overpotential of water oxidation. The gate voltage of water oxidation shifted to a much lower value after Co-Pi deposition using electrochemical/photo-assisted electrochemical deposition method. Bulk recombination within photoanode side should also be suppressed to output electrons efficiently. Wider-bandgap tin oxide and tungsten oxide were adopted as hole-blocking layers to reflect holes diffused to the interface. After optimization of experimental conditions including film thickness, we can acquire BiVO₄ photoanode with photocurrent density exceeding 1.8mA/cm² at potential 1.23V vs RHE under AM1.5.
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Mail address: Hahn-Meitner-Platz 1, 14109, Berlin, Germany
Corresponding author&’s email: [email protected]

BiVO₄ is a promising photocatalyst for water splitting using solar energy. Bulk BiVO₄ exists in several crystalline forms; the monoclinic polymorph is believed to have the best photocatalytic properties. Raman spectroscopy is capable of identifying many properties of materials including distinguishing crystalline polymorphs in bulk and thin film solids. Here it was applied to study thin films of BiVO₄ grown by molecular beam epitaxy on (001)-oriented yittria-stabilized cubic zirconia substrate compared to single BiVO₄ crystals. Raman spectra were measured at several temperatures from 10 to 500 K with 9 different excitation laser lines in the range from 325 to 514.5 nm. Temperature evolution of Raman spectra in the range 10-500 K was observed and attributed to the monoclinic phase; soft mode corresponding to ferroelastic transition observed in the crystal. Raman spectra indicate that the BiVO₄ films and crystals have monoclinic structure, as expected for the growth conditions used and consistent with XRD and STEM. Excitation energy dependences of Raman intensities provide the bandgap estimates of 2.4 - 2.5 eV for the crystal and 2.6 - 2.7 eV for the film, the latter being consistent with spectroscopic ellipsometry and EELS data. The larger bandgap for the films is likely due to the strain. Supported in part by the NSF Grant DMR-1006136 and the M. J. Murdock Charitable Trust

Iron based chalcogenides nanocrystrals (NCs) hold considerable promise for both solar energy conversion and electrical storage. Their utilization is hampered by lack of control over crystallinity and nanoscale organization of the homogeneous fabrications, the availability of high quality NCs of these materials will help us understand better both the photophysical processes in them and acquire building blocks to make better mesoscale materials from them. Therefore, two kinds of ~30nm diameter NCs FeSex (X=1, 2) have been synthesized and which display strong photoluminescence indicative of minimal defect density which are previously unavailable. FeSex (X=1, 2) displays photoluminescence (PL) peaks at 447nm and 462nm respectively while quantum yields are as high as 20%. Additionally, the synthetic route for iron based ternary chalcogenides NCs also have also been developed by doping Cr elements in the synthetic process, i.e., FeCr2Se4 and FeCr2S4 are synthesized through using a facile wet chemical method. FeCr2Se4 and FeCr2S4 NCs possessed narrow size distribution with high quality. FeCr2Se4 and FeCr2S4 NCs offer a promising conductivity for integration as electrical application. Importantly, these colloidal NCs can be considered be suitable for biomedical fluorescence marker and as the environmentally benign substitutes for optical, electronic and photovoltaic materials.

Hydrogen is considered as a promising clean energy carrier for the future. Since Fujishima and Honda originally reported that a TiO2 based photoelectrochemical cell could be used to split water into hydrogen and oxygen in 1972,[1] intensive researches have been done to improve the performance of the photoelectrochemical water splitting cell in the past forty years. However, the solar energy conversion efficiency is still low, it is necessary to explore some materials with high conversion efficiency.
A p-n photoelectrochemical cell is a highly desirable approach to realize high conversion efficiency[2]. The photocurrent in a p-n photoelectrochemical cell is determined by the photoelectrode with the lower photocurrent. To date, several p-type semiconductor photocathodes with high solar photocurrent (dozens of mA cm-2) have been developed.[3-4] Some promising visible-light-responsive photoanode materials have also been studied by us and other researchers. Though different methods have been used to improve performance of a photoanode, all of these photoanodes exhibit much lower solar photocurrent (lower than 4 mA cm-2)[5-6] than the photocathodes, which is a bottleneck for water splitting in a p-n photoelectrochemical cell. Therefore, finding an efficient photoanode is a key step in solar water splitting for hydrogen production.
In this study, we prepared the Ta3N5 photoelectrodes by modified thermal oxidation and nitridation method.[7] A high solar photocurrent @1.23 VRHE of 5.5 mA cm-2 was obtianed on the Ta3N5 photoanode by facile thermal or mechanical exfoliation of the surface recombination centers. This strategy can offer guidance to improve photoelectrochemical performance of other materials.
References:
[1] Fujishima, A. ; Honda, K. Nature 1972, 238, 37-38.
[2] Walter, M. G. ; Warren, E. L. ; McKone, J. R. ; Boettcher, S. W. ; Mi, Q. ; Santori, E. A. ; Lewis, N. S. Chem. Rev. 2010, 110, 6446-6473.
[3] Boettcher, S. W.; Warren, E. L. ; Putnam, M. C. ; Santori, E. A.; Tuner-Evans, D.; Kelzenberg, M. D. ; Walter, M. G.; McKone, J. R.; Brunschwig, B. S. ; Atwater, H. A.; Lewis, N. S. J. Am. Chem. Soc. 2011, 133, 1216-1219.
[4] Lee, M. H.; Takei, K.; Zhang, J.; Kapadia, R. ; Zheng, M.; Chen, Y.; Nah, J.; Matthews, T. S. , Chueh, Y.; Ager, J. W. ; Javey, A. Angew. Chem. Int. Ed. 2012, 51, 10760-10764.
[5] Luo, W.; Yang, Z. ; Li, Z. ; Zhang, J. ; Liu, J.; Zhao, Z. ; Wang, Z.; Yan, S.; Yu, T.; Zou, Z. Energy Environ. Sci. 2011, 4, 4046-4051.
[6] Li, Y. ; Takata, T. ; Cha, D.; Takanabe, K. ; Minegishi, T.; Kubota, J.; Domen, K. Adv. Mater. 2013, 25, 125-131.
[7] Li, M. ; Luo, W. ; Cao, D. ; Zhao, X. ; Li, Z. ; Yu, T.; Zou, Z. Angew. Chem. Int. Ed. 2013, 52, 11016-11020

Two-dimensional (2D) layered materials exhibit high anisotropy in materials properties due to the large difference of intra- and inter-layer bonding. This presents opportunities to engineer materials whose properties strongly depend on the orientation of the layers relative to the substrate. Here, using similar growth process reported in our previous study of MoS₂ and MoSe₂ films whose layers were oriented vertically on flat substrates, we demonstrate that the vertical layer orientation can be realized on curved and rough surfaces such as nanowires (NWs) and microfibers. Such structures can increase the surface area while maintaining the perpendicular orientation of the layers, which may be useful in enhancing various catalytic activities. We show vertically-aligned MoSe₂ and WSe₂ nanofilms on Si NWs and carbon fiber paper. We find that MoSe₂ and WSe₂ nanofilms on carbon fiber paper are highly efficient electrocatalysts for hydrogen evolution reaction (HER) compared to flat substrates. Both materials exhibit extremely high stability in acidic solution as the HER catalytic activity shows no degradation after 15,000 continuous potential cycles. The HER activity of MoSe₂ is further improved by Ni doping.

Semiconductor nanowires have been perceived as promising materials for high-efficient and low-cost solar energy conversion. Wafer-scale vertical-aligned p-type and axial p-n junction InP nanowire arrays were grown by metal-organic vapour-phase epitaxy (MOVPE).
The p-InP nanowires were used as cathode for photo-electrochemical hydrogen production from water. Noble-metal-free electro-catalyst MoSx has been deposited onto the nanowires by photochemical deposition. With only ca. 3% surface coverage by nanowires, the maximum photocathode conversion efficiency under AM 1.5G illumination reaches 5.7%, which is higher than most of other nanowire electrodes with precious metals as electrocatalyst.
The axial p-n junction InP nanowire solar cell exhibits an open-circuit voltage (Voc) of 0.73V, a short circuit current (Isc) of 21 mA/cm2 and a fill factor (FF) of 73%, which results in an overall power conversion efficiency of 11.1%[1]. The axial p-n junction solar cell was also tested for hydrogen evolution from water and the result will be compared with those obtained for p-InP nanowire electrode.
1. Y. Cui, J. Wang, S. R. Plissard, A. Cavalli, T. T. T. Vu, R. P. J. van Veldhoven, L. Gao, M. Trainor, M. A. Verheijen, J. E. M. Haverkort, E. P. A. M. Bakkers, Nano Lett. 2013, 13, 4113.

Iron pyrite (FeS₂) is an earth-abundant semiconductor with near-ideal properties as a solar absorber material, due to its high absorption coefficient, abundance and non-toxicity. However, all reported devices suffer from a low open-circuit voltage (V_OC), limiting their efficiency.
We used numerical modeling of Hall effect data and photoemission spectroscopy to demonstrate the existence of a p-type conductive inversion layer at the surface of high-quality n-type single crystals of iron pyrite grown by a flux technique. This hole-rich surface layer results in strong band-bending near the surface and tunneling of majority carriers from the bulk through a narrow potential barrier, thus greatly reducing V_OC. It also explains the universally observed small activation energy and p-type conduction in polycrystalline pyrite thin films. We use low-temperature Hall effect data of single crystals to evaluate surface treatments like annealing in various atmospheres, atomic layer deposition of oxides and sulfides, chemical etching, plasma etching and molecular adsorption. We found treatments that can reduce and potentially eliminate the surface layer by passivating surface states and subsurface defects. This is one crucial task in making pyrite a successful solar absorber material. We further present current-voltage characteristics of pyrite thin film solar cells as a function of temperature and discuss the most promising strategies for surface passivation in context of raising their device efficiency.

Bismuth vanadate (BiVO4) has, over the past decade, been the focus of intensive research as a promising photoanode material in photoelectrochemical (PEC) water splitting devices. To continue to improve overall material performance, stability, and efficiency, a fundamental understanding of the electronic structure of this material is desired. In this work, a comprehensive approach to understanding both valence band (VB) and conduction band (CB) orbital character, as well as photoexcited carrier dynamics, has been undertaken using both experimental and theoretical means. Density functional theory calculations confirm the VB maximum and CB minimum to be comprised primarily of O 2p and V 3d orbitals, respectively. Triplet d-orbital splitting was calculated and ascribed to oxygen ligand anisotropy. To confirm these theoretical findings, arange of optical and x-ray spectroscopies have been applied to study high quality monoclinic BiVO4 (m-BiVO4) thin films (~130 nm thickness) deposited by chemical vapor deposition (CVD) and sputtering. X-ray absorption spectroscopy (XAS) supports the predicted triplet splitting of the CB and the fundamental bandgap was measured to be 2.5 eV via combination of XAS with x-ray emission. Resonant inelastic x-ray scattering (RIXS) provides conclusive evidence that m-BiVO4is an indirect semiconductor. Furthermore, inelastically scattered features (between 0.6 and 1.7 eV) were attributed to splitting of the V d states of the CB. A direct gap of 2.8 eV has also been measured by steady state and transient absorption spectroscopies in the optical range. The excited state carrier lifetimes were measured by transient absorption pump probe spectroscopy over probe wavelengths between 0.9 and 3.5 eV. A significant portion of the excited state species decayed with characteristic lifetimes of 5.2 and 45 ns; however, additional components were observed with lifetimes out to 0.3 ms. Comparison of transient optical spectra with x-ray spectroscopies has allowed the assignment of specific transient features to both free and trapped minority holes, as well as photoexcited majority electrons. Therefore, this work provides a basis for determining carrier lifetimes in BiVO4photoanodes and, thus, for improving material quality and photoelectrochemical conversion efficiencies.