Dunwei Wang, Boston College
Song Jin, University of Wisconsin-Madison
Juan Bisquert, Universitat Jaume I
Joel W. Ager III, Lawrence Berkeley National Laboratory
D2: Metal Oxides for Solar Fuels - TiO2
Tuesday PM, April 22, 2014
Westin, 2nd Floor, Metropolitan II
2:30 AM - *D2.01
Semiconductor Nanowires for Artificial Photosynthesis
Peidong Yang 1
1UC, Berkeley Berkeley USAShow Abstract
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.
3:00 AM - D2.02
Observation and Alternation of Surface States on Metal Oxide Photoelectrodes
Chun Du 1 Ming Zhang 1 Ji-Wook Jang 1 Yang Liu 1 Gang-Yu Liu 1 Dunwei Wang 1
1Boston College Chestnut Hill USAShow Abstract
3:15 AM - *D2.03
TiO2 Nanotube Array Based Photoelectrochemical Water Splitting
Peng Wang 1 Zhonghai Zhang 1
1KAUST Thuwal Saudi ArabiaShow Abstract
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.
3:45 AM - D2.04
Nitrogen and Transition Metal Codoped Titania Nanotube Arrays for Visible Light Sensitive Photoelectrochemical Water Oxidation
Tomiko M Suzuki 1 Gaku Kitahara 1 Takeo Arai 1 Yoriko Matsuoka 1 Takeshi Morikawa 1
1Toyota Central Ramp;D Labs, Inc. Nagakute, Aichi JapanShow Abstract
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 . 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 . 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 . 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 . 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.
 P. Roy, et. al., Angew. Chem. Int. Ed., 50 (2011) 2904.  R. Asahi, T. Morikawa, et al., Science, 293 (2001) 269.  Y. Gai, J. Li, et al., Phys. Rev. Lett., 102 (2009) 036402., W. Zhu, et al., Phys. Rev. Lett., 103 (2009) 226401.  S. Sato, T. Arai, et al., J. Am. Chem. Soc., 133, (2011) 15240.
4:30 AM - *D2.05
Rapid Flame Processing of Metal Oxides Photoanodes for Enhanced Solar Water-Splitting
In Sun Cho 1 Lili Cai 1 Manca Logar 1 Pratap M Rao 1 Chi Hwan Lee 1 Robert Sinclair 2 Fritz B. Prinz 1 Xiaolin Zheng 1
1Stanford University Stanford USA2Stanford University Stanford USAShow Abstract
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.
5:00 AM - D2.06
Atomic Level In-Situ Characterization of Metal/TiO2 Photocatalysts Under Light Irradiation in Water Vapor
Liuxian Zhang 1 Peter A. Crozier 1
1Arizona State University Tempe USAShow Abstract
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). 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.  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.
. Miller, B.K.; Crozier, P.A. Microscopy and Microanalysis., 2013 DOI: 10.1017/S1431927612014122.
. Zhang, L.; Miller, B.K.; Crozier, P.A. Nanoletter 2013, DOI: 10.1021/nl304333h
5:15 AM - D2.07
Cold Gas Sprayed Semiconductor-Based Electrodes for the Photo-Induced Water Oxidation
Iris Herrmann-Geppert 1 2 Thomas Emmler 2 Henning Gutzmann 1 Peter Bogdanoff 3 Thomas Dittrich 3 Thomas Klassen 1 2
1Helmut-Schmidt-University Hamburg Germany2Helmholtz-Zentrum Geesthacht Geesthacht Germany3Helmholtz-Zentrum Berlin Berlin GermanyShow Abstract
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.
5:30 AM - D2.08
Highly-Efficient Capillary Photoelectrochemical Water Splitting Using Cellulose Nanofiber-Templated TiO2 Photoanodes
Zhaodong Li 1 Chunhua Yao 2 Yanhao Yu 1 Zhiyong Cai 2 Xudong Wang 1
1University of Wisconsin-Madison Madison USA2USDA Forest Service Madison USAShow Abstract
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).
D1: Overview of Solar Hydrogen Production by PEC
Tuesday AM, April 22, 2014
Westin, 2nd Floor, Metropolitan II
9:00 AM - *D1.01
III-V Surface Treatments and Catalysis for Photoelectrochemical Water Splitting
John A Turner 1
1National Renewable Energy Lab Golden USAShow Abstract
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.
9:30 AM - *D1.02
Critical Metrics and Fundamental Materials Challenges for Renewable Hydrogen Production Technologies
Eric Miller 1 Sara Dillich 1 Erika Sutherland 1 Katie Radolph 2 David Peterson 2 Chris Ainscough 2 Sarah Studer 1
1US Department of Energy Washington USA2US Department of Energy Golden USAShow Abstract
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.
10:00 AM - *D1.03
Materials for Efficient Photoelectrochemical Water Splitting: The U.S. Department of Energy PEC Working Group
Heli Wang 1 Eric L Miller 2
1NREL Golden USA2US Department of Energy Washington USAShow Abstract
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.
10:30 AM - D1.04
Stabilizing Si and GaAs Photoanodes for Water Oxidation with Thick TiO2
Shu Hu 1 2 Matthew Shaner 1 2 Joseph Beardslee 1 Bruce Brunschwig 2 3 Nathan S Lewis 1 2 3
1California institute of technology Pasadena USA2Joint Center for Artificial Photosynthesis Pasadena USA3California Institute of Technology Pasadena USAShow Abstract
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.
10:45 AM - D1.05
Artificial Photosynthesis from a Silicon Based Monolithic PV/PEC Device
Wilson Smith 1 Ibadillah A Digdaya 1 Lihao Han 2 Fatwa F Abdi 3 1 Bernard Dam 1 Miro Zeman 2 Arno HM Smets 2
1Delft University of Technology Delft Netherlands2Delft University of Technology Delft Netherlands3Helmholtz-Zentrum Berlin Berlin GermanyShow Abstract
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.
11:30 AM - *D1.06
Sunlight-Driven Hydrogen Formation by Membrane-Supported Photoelectrochemical Water Splitting
Nathan S. Lewis 1
1California Institute of Technology Pasadena USAShow Abstract
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.
12:00 PM - D1.07
Photoelectrochemical Water Splitting Using Adapted Thin Film Silicon Tandem Junction Solar Cells
Felix Urbain 1 Karen Wilken 1 Oleksandr Astakhov 1 Vladimir Smirnov 1 Jan Philipp Becker 1 Friedhelm Finger 1 Uwe Rau 1 Jamp;#252;rgen Ziegler 2 Bernhard Kaiser 2 Wolfram Jaegermann 2
1Forschungszentrum Juelich GmbH Juelich Germany2Technical University of Darmstadt Darmstadt GermanyShow Abstract
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.
12:15 PM - D1.08
High-Performance Silicon Photoanodes Passivated with Thin Ni Films for Water Splitting
Michael James Kenney 1 Ming Gong 1 Yanguang Li 1 Justin Wu 1 Ju Feng 1 Mario Lanza 1 Hongjie Dai 1
1Stanford Stanford USAShow Abstract
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.
12:30 PM - D1.09
Amorphous Si Thin Film Based Photocathode for Efficient Solar Hydrogen Production
Yongjing Lin 1 2 Corsin Battaglia 2 Mathieu Boccard 3 Zhibin Yu 2 Mark Hettick 2 1 Christophe Ballif 3 Joel Ager 1 Ali Javey 2 1
1Lawrence Berkeley National Lab Berkeley USA2University of California Berkeley Berkeley USA3Ecole Polytechnique federale de Lausanne Lausanne SwitzerlandShow Abstract
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.
Dunwei Wang, Boston College
Song Jin, University of Wisconsin-Madison
Juan Bisquert, Universitat Jaume I
Joel W. Ager III, Lawrence Berkeley National Laboratory
D4: Metal Oxides for Solar Water Splitting - Hematite
Wednesday PM, April 23, 2014
Westin, 2nd Floor, Metropolitan II
2:30 AM - D4.01
Enhanced Photocatalytic Water Splitting by Plasmonic TiO2-Fe2O3 Co-Catalyst Under Visible Light Irradiation
Wei Hsuan Hung 1 Tzu-Ming Chien 1
1Feng Chia University Seatwen TaiwanShow Abstract
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).
2:45 AM - D4.02
TiO2/Hematite Core-Shell Hierarchical Nanosturctured Array for Photoelectrochemical Water Splitting
Jih-Sheng Yang 1 Jih-Jen Wu 1
1National Cheng Kung University Tainan TaiwanShow Abstract
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.
3:00 AM - D4.03
Mixed Metal Oxides on Hematite: Understanding the Nature of Heterojunctions and Surface Treatments Toward Enhanced Sunlight-Driven Water Splitting
Matthew T. Mayer 1 Ludmilla Steier 1 S. David Tilley 1 Michael Graetzel 1
1Ecole Polytechnique Federale de Lausanne (EPFL) Lausanne SwitzerlandShow Abstract
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 (NiFeOx, CoFeOx) 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.
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
3:15 AM - D4.04
Assessing the Optimal Temperature and Light Intensity for Water Splitting on Hematite Photoanodes
Xiaofei Ye 1 Jing Yang 1 2 Madhur Boloor 1 Nicholas A. Melosh 1 William C. Chueh 1
1Stanford University Stanford USA2Peking University Beijing ChinaShow Abstract
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.
4:15 AM - D4.06
Flat Band Potential Engineering in Photoanodes for Solar Water Splitting
Beniamino Iandolo 1 Haixiang Zhang 2 Bjoern Wickman 1 Igor Zoric 1 Gavin Conibeer 2 Anders Hellman 1
1Chalmers University of Technology Gamp;#246;teborg Sweden2School of Photovoltaic and Renewable Energy Engineering, University of New South Wales Sydney AustraliaShow Abstract
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.
4:30 AM - D4.07
Photoelectrochemical Water Splitting with Solution-Deposited Hematite Thin Films and Coated Scaffolds
Ivan Garcia Torregrosa 1 2 Anthony J. Abel 1 Anjli M. Patel 1 Jason B. Baxter 1
1Drexel University Philadelphia USA2Delft University of Technology Delft NetherlandsShow Abstract
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.
4:45 AM - D4.08
Effect of Surface Properties on Photocatalytic Water Oxidation with Hematite Electrodes
Omid Zandi 1 Thomas Hamann 1
1Michigan State University East Lansing USAShow Abstract
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.
5:00 AM - D4.09
Atomic Layer Epitaxy of Hematite on Indium Tin Oxide for Photoelectrochemical Water Splitting
Jonathan D Emery 1 Peijun Gao 2 Christian M Schlepuetz 1 Shannon C Riha 1 Robert PH Chang 2 Alex BF Martinson 1
1Argonne National Laboratory Lemont USA2Northwestern University Evanston USAShow Abstract
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.
5:15 AM - D4.10
Influence of Oxygen Plasma Treatment on the Electronic Structure and Photo-Electrochemical Properties of Iron Oxide Films for Solar Water Splitting Photoanodes
Yelin Hu 1 2 Florent Boudoire 1 3 Iris Hermann-Geppert 4 5 Peter Bogdanoff 6 Giuseppino Fortunato 7 Michael Graetzel 2 Artur Braun 1
1Empa, Swiss Federal Laboratories for Materials Science and Technology Damp;#252;bendorf Switzerland2amp;#201;cole Polytechnique Famp;#233;damp;#233;rale de Lausanne Lausanne Switzerland3University of Basel Basel Switzerland4Helmholtz-Zentrum Geesthacht Geesthacht Germany5Helmut-Schmidt University Hamburg Germany6Helmholtz-Zentrum Berlin famp;#252;r Materialien und Energie Berlin Germany7Empa, Swiss Federal Laboratories for Materials Science and Technology St. Gallen SwitzerlandShow Abstract
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.
5:30 AM - D4.11
pH-Dependence of Electrochemical Water Oxidation by Manganese Oxide and Iron Oxide Thin Films
Shima Haghighat 1 Jahan Dawlaty 1
1University of Southern California Los Angeles USAShow Abstract
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.
D5: Poster Session I
Wednesday PM, April 23, 2014
Marriott Marquis, Yerba Buena Level, Salons 8-9
9:00 AM - D5.02
Vapor Phase Synthesis of Vertically Aligned SnSe Nanosheets
Xing-Hua Ma 1 Ki-Hyun Cho 1 Yun-Mo Sung 1
1Korea University Seoul Republic of KoreaShow Abstract
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.
9:00 AM - D5.03
Electrochemically Prepared Mo-Doped BiVO4 Photoanodes for Efficient Photoelctrochemical Water Splitting
Yiseul Park 1 Donghyeon Kang 2 Kyoung-Shin Choi 2
1Daegu Gyeongbuk Institute of Science and Technology (DGIST) Daegu Republic of Korea2University of Wisconsin-Madison Madison USAShow Abstract
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.
9:00 AM - D5.04
Fulvalene with Earth Abundant Elements
Hal Gokturk 1
1Ecoken San Francisco USAShow Abstract
Fulvalene diruthenium tetracarbonyl, FvRu2(CO)4 which was originally reported in 1997  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 . 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.
 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
9:00 AM - D5.07
Flame Processing of TiO2 Nanowires to Improve Charge Transport and Transfer Properties for Efficient Photoelectrochemical Water Splitting
Lili Cai 1 In Sun Cho 1 Manca Logar 1 2 Chi Hwan Lee 1 Apurva Mehta 3 Pratap M Rao 1 Fritz B Prinz 1 Xiaolin Zheng 1
1Stanford University Stanford USA2Jozef Stefan Institute Ljubljana Slovenia3SLAC National Accelerator Laboratory Menlo Park USAShow Abstract
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.
9:00 AM - D5.08
Photochemical Deposition of Ir Oxide Next to Robust Polynuclear ZrOCo Unit for Closing the Photosynthetic Cycle
Wooyul Kim 1 Beth McClure 1 Guangbi Yuan 1 Heinz Frei 1
1Lawrence Berkeley National Laboratory Berkeley USAShow Abstract
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 IrOx with photon absorption by binuclear MMCT chromophores, and the blocking of IrOx 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.
9:00 AM - D5.10
Wurtzite ZnO-GaN Heterostructured Hollow Nanospheres for Photocatalytic Applications
Sumithra Sivadas Menon 1 B. Kuppulingam 1 K. Baskar 1 Shubra Singh 1
1Anna University Chennai IndiaShow Abstract
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