Photoelectrochemical (PEC) water splitting for the production of hydrogen as a renewable energy carrier remains one of the holy grails of chemistry and major challenges in materials science. Central to this challenge is the stability-efficiency trade-off: efficient photocatalysts are unstable in aqueous solution while stable photocatalysts lack the efficiency for commercial viability. Here we combine a suite of surface sensitive techniques with density functional theory to investigate the role of dilute N in enhancing the stability of III-V photocathodes, with a focus on GaP_1minus;xN_x (x < 0.02) as a prototype system to study general PEC processes. The investigation provides site-specific, mechanistic understanding of the competing reactions of photocorrosion and hydrogen evolution at the photcathode-electrolyte interface. GaN(-H) sites at the GaPN surface provide enhanced stability relative to GaP and introduce catalytic sites for hydrogen evolution that are not available on either GaP or GaN, while oxidized and defect sites are susceptible to anodic oxidation. New directions in material and interface design to optimize the photocathode for sunlight-driven water splitting with sustained high efficiency are identified. Surface treatments, such as incorporation of a thin, uniform nitride surface layer, should be particularly advantageous in this pursuit.

Photoelectrochemical (PEC) water splitting can be used to store solar energy in the form of hydrogen. The performance of a PEC water splitting device is best evaluated using the solar-to-hydrogen (STH) efficiency. Knowledge of practical operation limits can provide researchers with a means to assess and guide research directions in the field. Previous studies have calculated maximum efficiencies based on solar absorption limits with the framework used to study photovoltaic (PV) devices. While these numbers provide a starting point for understanding the limits of PEC water splitting devices, these studies neglect additional losses that are specific to PEC systems and thus can overestimate practical solar conversion efficiencies for this application. This work presents results of STH efficiency calculations for single and dual absorber systems over a wide range of band gaps that take into account the effects of various system losses including absorption limits, material defect losses, shunt losses, and reaction overpotentials. Comparing maximum STH values for devices with precious vs. non-precious metal catalysts or minimal vs. significant photovoltage losses illustrates the need for researchers to focus on these issues. Additionally, improvement in performance with the addition of a small bias is shown by calculating an applied bias photon conversion efficiency for each device configuration. These results provide insight into the intricacies of PEC device functioning as well as define obtainable efficiency values representative of the current state of materials research in the field.

We are developing an artificial photosynthetic system that will utilize sunlight and water as the inputs and produce hydrogen and oxygen as the outputs. We are taking a modular, parallel development approach in which 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.

Converting solar energy directly into hydrogen is the goal of a photoelectrochemical water-splitting cell (PEC). Such a solar cell would then replace the conventional route via a photovoltaic solar cell (or other renewable energy source) coupled with a water electrolysis system. Even though the benchmark PEC cells reached hydrogen production efficiency 12%, the current challenge is to fabricate them cheaply enough to compete with other hydrogen production solutions. In this respect, metal oxide semiconductors such as hematite Fe2O3 (or Cu2O) show good light absorption and favourable band position for oxygen (or hydrogen) evolution, but have intrinsic drawbacks in poor carrier mobility and large overpotentials.
We review the understanding of the energy band alignment for the semiconductor-electrolyte interface. We describe a simple kinetic model of charge transfer from semiconductor surface states to the electrolyte or recombination from surface states with conduction band electrons. The numerical solution of the kinetic model under steady-state or transient conditions enables extraction of the rate constants for charge transfer and recombination processes. We examined optical losses of the PEC cell stack and also the effect of nanostructured PEC electrodes on the light absorption and charge transfer.

Heteroepitaxial growth of III-V semiconductor nanowires (NWs) on Si holds promise for their integration with dissimilar and highly-mismatched photoactive substrates. Such a strategy enables an optimal bandgap combination of 1.7 / 1.1 eV, especially important for efficient photoelectrolysis of water to generate renewable hydrogen and oxygen. In this context, the energy-conversion properties of nanowire-arrays are first evaluated by non-aqueous photoelectrochemistry with one-electron, reversible, redox species. Selected-area growth of n-GaAs NW-arrays on GaAs and Si substrates were carried out in a metal-organic chemical vapor deposition (MOCVD) reactor. Near-unity optical absorption and minimal reflection of GaAs nanowire-arrays were observed at both normal and off-normal incidence, very useful for solar-tracking. Near-unity carrier-collection efficiencies were realized by the radial, rectifying semiconductor/liquid junction. These nanowire photoanodes exhibited overall inherent photoelectrode energy-conversion efficiencies of ~8.1% under 100 mW×cm^-2 of simulated Air Mass 1.5 illumination, with open-circuit photovoltages of 590±15 mV and short-circuit current densities of 24.6±2.0 mA×cm^-2. The current-voltage characteristics of radial p⁺-n GaAs homo-junctions will also be discussed. Subsequently, this NW-on-Si method can be applied to ternary or quaternary systems that have a 1.7 eV direct band gap, e.g. GaAs_0.78P_0.22.

Photocatalytic reactions on TiO2 have over the last 30 years attracted tremendous scientific and technological interest. A main research direction using TiO2 based materials is still a use for direct splitting of water into H2 and O2 to generate the potential fuel of the future, hydrogen. In order to achieve a maximum turn-over rate (by creating a high surface area), usually nanoparticles are used either suspended in the reaction environment or compacted to a photoelectrode. Over the past decades various 1D and highly defined TiO2 morphologies were explored for their photocatalytic performance and were found in many cases superior to nanoparticles. This includes nanotubes or wires grown by hydrothermal or template methods, or by anodic oxidation. Several of these advanced morphologies can directly be grown on conductive substrates such as wires, rods or self-organized anodic structures and therefore can be directly used as photo-anodes. The presentation will focus mainly on the aspects of recently synthesized advanced nanostructures (namely, highly ordered nanotube arrays or mesosponge structures) in view of their photoelectrochemical water splitting potential. Fabrication of the nanostructures will be briefly discussed, but emphasis will be on novel modification approaches and use of the structures to significantly enhance their H2 production yield. Recent results will be shown on electronic doping, catalytic doping, junction formation and band-gap engineering.

The slow kinetics of the water oxidation half-reaction limit the efficiency of current solar water splitting technologies for hydrogen fuel generation. We study electrocatalysts for the oxygen evolution reaction (OER) in a thin film geometry, enabling simple and direct comparison of the activity of different catalyst materials. We report the solution synthesis, structural/compositional characterization, and OER electrocatalytic properties of ~2 nm-thick films of NiO_x, CoO_x, Ni_yCo_1-yO_x, Ni_0.9Fe_0.1O_x, IrO_x, MnO_x, and FeO_x. The thin-film geometry enables the use of quartz-crystal microgravimetry, voltammetry, and steady-state Tafel measurements to study the intrinsic electrocatalytic activity and electrochemical properties of the oxides. 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 layered Ni_0.9Fe_0.1OOH oxyhydroxide species with nearly every Ni atom electrochemically active. In contrast to previous reports that showed synergy between Co and Ni oxides for OER catalysis, Ni_yCo_1-yO_x showed decreasing activity relative to the pure NiO_x films with increasing Co content. This finding is explained by the suppressed in situ formation of the layered active oxyhydroxide with increasing Co.
The high OER activity and simple synthesis make these Ni-based catalyst thin films potentially useful for incorporating with semiconductor photoelectrodes for direct solar-driven water splitting. Towards this goal, we use in situ spectroelectrochemistry to quantify the optical properties of the catalysts at electrode potentials where water oxidation is taking place. We propose a simple model of the composite semiconductor-catalyst system that accounts for parasitic optical absorption in the catalyst layer and calculate a figure-of-merit for the different catalysts as a function of thickness to inform composite semiconductor-catalyst device design.
(Trotochaud, L.; Ranney, J.K.; Williams, K.N.; Boettcher, S.W. J. Am. Chem. Soc.2012, 134, 17253.)

Solar thermal fuels make use of molecules that undergo reversible photo-isomerization to store solar energy and convert it into thermal energy. Because solar thermal fuels produce no emissions and can store and convert energy within one material, they are an attractive option for a renewable alternative energy source. The trans- to cis-azobenzene photo-isomerization has drawn attention as a candidate material for solar thermal fuels. However, both isomers are photoactive in similar regions of the solar spectrum, and the metastable cis-isomer exhibits a higher absorption coefficient, leading to a photo-stationary state (dynamic equilibrium of the two directions of photoisomerization) with a significant amount of the lower energy trans isomer and a resulting energy storage capacity of only twenty percent of the maximum value. We evaluate possible solutions to this problem, modifying absorption properties and isomerization of the two isomers by: (i) designing close-packed semi-crystalline azobenzene/template nanostructures, (ii) functionalizing azobenzene, and (iii) using photo-sensitizers. Using time dependent density functional theory (TDDFT) and Casida&’s equation, we calculate the absorption properties of trans- and cis-photoisomers and examine how much gain we can get in storage efficiency of solar thermal fuels.

Photoelectrochemical cells based on III-V semiconductor photocathodes have demonstrated efficient conversion of solar energy to hydrogen. However, photocorrosion of the electrode in electrolyte solution remains an issue, in part because the complex chemistry active at the electrode-electrolyte interface remains poorly understood. We use first-principles molecular dynamics simulations to study the structure, stability, and chemical activity of GaP/InP(001) semiconductor electrodes in contact with water. We find that surface oxygen and hydroxyl can fundamentally change the electronic and chemical properties of water at the interface, inducing spontaneous dissociative adsorption of water. In addition, the formation of a strong hydrogen-bond network at the interface leads to fast surface hydrogen transport, which can act a self-healing mechanism to passivate carrier traps. Specific implications for understanding reaction kinetics and photocorrosion in III-V cathodes will be discussed. Performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52- 07NA27344.

In this work, we have carried out a systematic study of mechanisms of Au nanoparticle/GaP-catalyzed photoreduction of CO2 and water vapor over a wide range of wavelengths (254nm, 365nm, 532nm, 633nm). When the photon energy matches the plasmon resonance of the Au nanoparticles (free carrier absorption), we observe a significant enhancement in the photocatalytic activity due to the intense local electromagnetic fields created by the surface plasmons of the Au nanoparticles. These intense electromagnetic fields enhance absorption in the GaP, thereby enhancing the photocatalytic activity in the visible range. We use 13CO2 isotopes in order to verify the origin of carbonaceous species (i.e., CH4) produced by the catalytic process. We model the plasmon excitation at the Au nanoparticle-GaP interface using finite difference time domain (FDTD) simulations, which provides a rigorous analysis of the electric fields and charge at the Au nanoparticle-GaP interface.

The long term goal of our research is the direct conversion of carbon dioxide and water with visible light to a liquid fuel in a nanoscale assembly. Focusing on robust inorganic molecular light absorbers and metal oxide nanocatalysts, geometries are explored that afford the coupling of the components across a proton transmitting nanoscale silica layer under separation of the water oxidation catalysis from all other photosynthetic processes. Using recently developed heterobinuclear charge-transfer units anchored on silica as visible light chromophores, Co₃O₄ nanoparticles as multi-electron catalysts for water oxidation,^[1,2] and core-shell geometry for separating the O₂ evolution catalysis from light absorber and reduction chemistry, we are developing an assembly for closing the photosynthetic cycle on the nanoscale.
Starting out with spherical Co₃O₄(4 nm)/SiO₂(2 nm) core/shell particles, we have developed the materials chemistry for embedding of molecular wires of type oligo(paraphenylenevinylene) (OPPV) into the silica shell for controlled hole transport from a visible light sensitizer on the outside to the Co₃O₄ catalyst core on the inside. Transient optical absorption spectroscopy revealed efficient hole injection into the embedded OPPV (3 aryl units) molecules followed by fast (microsecond or less) transfer to the Co oxide particle.^[3] The efficient hole transport contrasts with hole injection times into metal oxide catalysts on the order of milliseconds previously reported in the literature. The result opens up an approach for using nanoscale silica layers with embedded organic wire molecules for separating the water oxidation catalyst from light absorber and reductive chemistry by a reactant/product impermeable barrier. While the spherical core/shell particles were adequate for demonstrating hole transport across embedded wire molecules, Co oxide/silica core/shell nanotubes afford the proper geometry for realizing water oxidation catalysis (inside Co₃O₄ tube) in a separate space from light absorber (outside of SiO₂ shell). We have developed synthetic methods for preparing Co₃O₄ nanotubes in a range of sizes and a solvothermal method for preparing dense silica shells with embedded OPPV with precise thickness and high uniformity. The core/shell nanotube design has the proper geometry for closing of the photosynthetic cycle under separation of oxygen from reduced products.
Building on our previous work on Ir oxide cluster catalyst,^[4] monitoring of visible light sensitized water oxidation at Co₃O₄ nanoparticles by transient ATR-FT-IR spectroscopy revealed reaction intermediates in aqueous solution, providing direct insight into elementary steps of the multi-electron chemistry on the catalyst surface. In parallel, we have developed binuclear ZrOCo(II) units covalently anchored on the silica surface for visible light induced reduction of CO₂ to CO and formate. These heterobinuclear charge-transfer chromophores, of which we developed over a dozen todate offer hellip;.

REFERENCES
[1] F. Jiao and H. Frei. Nanostructured Cobalt Oxide Clusters in Mesoporous Silica as Efficient Oxygen-Evolving Catalysts. Angew. Chem. Int. Ed. 48, 1841-1844 (2009).
[2] F. Jiao and H. Frei. Nanostructured Cobalt and Manganese Oxide Clusters as Efficient Water Oxidation Catalysts. Energy Environ. Sci. 3, 1018-1027 (2010).
[3] H.S. Soo, A. Agiral, A. Bachmeier, and H. Frei. Visible Light-Induced Hole Injection into Rectifying Molecular Wires Anchored on Co3O4 and SiO2 Nanoparticles. J. Am. Chem. Soc. 134, 17104 (2012).
[4] N. Sivasankar, W.W. Weare, and H. Frei. Direct Observation of a Hydroperoxide Surface Intermediate upon Visible Light Sensitized Water Oxidation at Ir Oxide Nanocluster Catalyst by Rapid-Scan FT-IR Spectroscopy. J. Am. Chem. Soc. 133, 12976-12979 (2011).

In the development of artificial photosynthesis, catalytic fixation of CO₂ remains a significant challenge to be overcome. A series of heterobimetallic complexes featuring directly bonded cobalt and zirconium atoms has shown significant promise in CO₂ cleavage. The resulting carbon monoxide ligand has shown abnormally high stability; completion of the catalytic cycle requires removal of this CO. Ultrafast time-resolved infrared spectroscopy has been applied to explore the photochemical behavior of this system. The behavior of short-lived metal-to-metal charge transfer states was shown to be critical to future directions in ligand choice. In initial systems, rapid thermalization of the charge transfer state shields otherwise labile ligands.

The efficient reduction of CO2 to value added products presents an ongoing series of challenges to many fields of chemical science. Combining a homogeneous transition-metal complex electrocatalyst with a specific p-type semiconductor photocathode in an electrochemical cell is a particularly appealing approach to CO2 reduction since it allows for direct solar energy harvesting. Toward this goal, we have synthesized a class of manganese carbonyl complexes bearing bidentate phosphine ligands and have evaluated their ability to function as electrocatalysts for the reduction of carbon dioxide to CO. Carbon monoxide can be converted into a variety of liquid fuels when combined with hydrogen by using Fischer-Tropsch chemistry. This talk will discuss the electrochemical and photophysical properties of these manganese bisphosphine carbonyl complexes as well as their chemical and energy efficiency as catalysts for two-electron CO2 reduction at glassy carbon electrodes. The potential of using these catalysts for photoelectrochemical CO2 reduction will also be addressed.

Due to limited amount of energy resources and their depletion as well as environmental concern, the researchers have been seeking for clean and renewable energy sources. Carbon dioxide (CO2) is one of the major green house gases and produced by the consumption of fossils fuels. So the managing CO2 emission is one of the major technological as well as political challenges. The photcatlytic reduction of CO2 to value-added chemicals such as methanol and CO using solar energy is an attractive option for the capturing of green house gas and at the same time to solve problem of shortage of sustainable energy. We here investigated the zirconate-based photocatalysts for the reduction of CO2. The photocatalysts have been prepared using wet-chemical methods, which have been characterized by many techniques, such as XRD, XPS, Raman, SEM, BET, UV-vis absorption spectroscopy. Different products (such as methanol, ethanol, methane, and CO) have been obtained via photoduction of CO2. The heterojunction fabrication of zirconate with PbS and/or Au nanoparticles can change the yield of different photoreduction products. Detailed mechanism was thoroughly studied in this work. We envision this would afford a viable approach for the photoreduction of CO2 and a better understanding of the related mechanism, which would facilitate the development of novel photocatalysts for the photoreduction of CO2.

Dye-sensitized solar cells (DSCs) have long been envisaged as a cost-effective alternative to conventional inorganic solid state photovoltaic technologies, but further optimization of the cells performance and large scale manufacturing processes is still underway. The advent of printed electronics has opened a new window of opportunities for the development of innovative lightweight, flexible, biodegradable paper-like substrates for DSCs [1,2]. Recent studies also suggest the use of one-dimensional morphologies such as nanotubes, nanowires and fibers in an attempt of improving electron transport in the semiconductor, providing a larger surface area for dye adsorption and enhancement of the light harvesting efficiency. Among these alternatives, photoanodes made of cellulose fibers embedded with conductive and photochromic nanoparticles, and with a much lower tortuosity of the pores compared to mesoporous nanoparticulated films, apparently offers a decisive advantage. In the present work we are exploiting the potential of incorporating carbon nanostructures and titania nanoparticles into cross-linked microfibrillated cellulose fibers, and use this new material architecture as a semiconductor for DSCs. It is well know that graphite, and in particular, expanded graphite (EG) has good electronic conductivity and high surface area [3], which may provide efficient electron transport paths within the TiO₂/cellulose composite semiconductor. To this extent, EG/P25 TiO₂ composite nanoparticles were prepared by ultrasound-assisted method and used to fill the interfiber space among cellulose fibers layers of commercial paper samples, thus promoting electric contact. Simultaneously, nanosized TiO₂ particles were deposited and grafted on cellulose fibers surface by using a sol-gel method at low temperature (<100 °C) and titanium isopropoxide as the TiO₂ percursor. Finally, the paper-like semiconductors were sintered at moderate temperatures (<200 °C) to complete the formation of TiO₂ and then sensitized with N719 ethanol dye solution. Complete DSCs were characterized by means of electrochemical impedance spectroscopy to elucidate how composition and topography of the composite semiconductor impact on its global performance. Under one-third Sun - typical lower-light, real-world light conditions, enhanced photocurrent densities were obtained with DSCs made of graphite/TiO₂ decorated commercial bleached Eucalyptus globulus kraft paper grafted on FTO conductive glass.
Acknowledgements: the Portuguese National Science Foundation (FCT) under the contract PTDC/EQU-EQU/101397/2008 and Programa Ciecirc;ncia 2007. LEPAE, CEFT and DEMM at FEUP are acknowledged for the much appreciated facilities and financial support.
[1] B. Wang, L.L. Kerr, Solar Energy Materials & Solar Cells, 2011, 95, 2531.
[2] K. Fan, T. Peng, J. Chen, X. Zhang, R. Li, Journal of Materials Chemistry, 2012, 22, 16121.
[3] Y.-S. Wei, Q.-Q. Jin, T.-Z. Ren, Solid-State Electronics, 2011, 63,76.

Forty years after the first reported photoelectrochemcial (PEC) water splitting experiment, the promise of hydrogen production from photoelectrochemical water splitting remains just that, a promise. Thousands of papers later and no material system has been identified that could fulfill the promise of hydrogen production from the direct splitting of water using sunlight as the only energy input.
Recent technonomic analysis studies indicate that for a commercially viable PEC-based water splitting system, the solar-to-hydrogen conversion efficiencies need to approach 20%. The highest efficiency to-date for water splitting using visible light is the GaAs/GaInP2 PV/PEC tandem cell with a published efficiency of 12.4%. This is not surprising since III-V based solar cells have the highest reported photovoltaic efficiency. Unfortunately, this material system has not shown the necessary long-term stability. Stabilizing the system using surface treatments or solution additives may be possible but another approach is to identify III-V materials with inherently greater stability.
Our previous studies of dilute III-V nitrides showed that they have greater stability over the pure phosphides, but suffer from poor electronic properties. The pure nitride materials used in white-light LEDs have shown better electronic properties and these alloys appear to be a fruitful area of research.
We have grown InGaN alloys with bandgaps close to the range necessary for efficient water splitting and characterized their properties for PEC water splitting. This report will summarize our efforts on these materials and their application to tandem cells for photoelectrochemical water splitting.

Efficient dye-sensitized photocathodes offer new opportunities for converting sunlight into storable energy cheaply and sustainably. We are developing dye-sensitized NiO cathodes for use in tandem dye-sensitized solar cells and for the photo-reduction of carbon dioxide or water to high energy products (solar fuels). Despite the infancy and complexity of this research area, we have brought about a number of exciting developments which have improved our understanding of the system and allowed us to substantially improve the photoconversion efficiency. Addressing the main limitations to p-type dye-sensitized solar cells, by improving the quality of the NiO electrodes, substituting the triiodide/iodide electrolyte for more suitable alternatives and engineering new dyes specifically for the p-type system, has enabled us to substantially increase the efficiency of the device and IPCE&’s exceeding 64% are now possible. We are now developing this idea further using the lessons we have learnt from solar cells, to address the issue of solar fuel production. Here, the kinetic balance is even more critical and so we are simultaneously developing new methods to monitor the charge-transfer rates under conditions which are as close as possible to working devices. Highlights from recent work involving new porphyrin, pthalocyanine and bodipy-based photocatalysts and new NiO morphologies, alongside results from fundamental studies on the charge-transfer mechanism using transient absorption spectroscopy (including TR-IR) will be presented. It is anticipated that, once we have fully optimised the kinetic balance in the cell, the versatility of the system will allow us to develop the electrode for photoreduction of water or carbon dioxide to produce solar fuels.
[1] Li, L. et al. Adv. Mater., 2010, 15, 1759-1762. [2] E. A. Gibson et al., Angew. Chem. Int. Ed. 2009, 48, 4402 -4405. [3] Windle, C. D. et al. Chem. Commun., 2012, 48, 8189-8191

Next-generation solar cells based on new concepts and/or novel materials are currently attracting wide interests. In this lecture, several examples of next generation photovoltaics based on the electrochemical systems are presented.
Among new types of solar cells, dye-sensitized solar cells (DSSCs) have received much attention as the low-cost solar cells. However, the energy conversion efficiency should be improved for the practical use. In order to improve the energy conversion efficiency, the extension of absorption range of the sensitizers to near-infrared regions is an important issue. In our study, panchromatic photoelectric conversion up to around 1000 nm has been accomplished by the use of new sensitizer DX. The panchromatic DSSC with DX is useful for a series-connected tandem solar cell. We prepared the mechanical stack tandem solar cell showing a high overall power conversion efficiency (eta;) of about 12%.
Hybrid solar cells composed of conjugated polymers and TiO₂ have a possibility of achieving high performance solar cells. With this in mind, polymer-sensitized solar cells (PSSCs) were constructed by the use of novel soluble polythiophene derivatives with hydrophilic anchoring units, which allow the polymer to penetrate into the TiO₂ nanostructure and to bond to the TiO₂ surface. The PSSCs are composed of the photoanode, the Pt-coated counter electrode, and typical liquid electrolyte with iodide. The PSSCs sensitized with two types of the polymers yielded higher IPCE values in the visible region because of dual-sensitization.
We have developed organic photovoltaics based on the surface complexes formed of TiO₂ with dicyanomethylene compounds (TCNX). The surface complexes exhibit broad absorption bands in the visible to near-infrared region due to interfacial charge-transfer transitions from the surface bound TCNX to the conduction band of TiO₂. In the solar cell, charge separation occurs directly by the charge-transfer transitions. It was found that the spectral sensitivity of the solar cell can be controlled by adjusting the π-conjugation length of TCNX. Ionization potential measurements revealed that the effects arise from the increase of the HOMO energy of the surface bound TCNX with extension of the π-conjugation system and the resultant red-shift of the charge transfer absorption band. In order to increase the energy conversion efficiency, effects of coadsorbents on TiO₂ and cations in the electrolyte were investigated.
Since the mechanisms of DSSC include electrochemical reaction, it can be hybridized with an electrochemical storage battery. We have reported a three-electrode solar rechargeable battery, namely “energy-storable dye-sensitized solar cell (ES-DSSC)”, composed of the photoanode, the counter electrode and the charge-storage electrode. The ES-DSSC not only generates output power, but also stores the electricity by itself. The partial shadowing effect on the output voltage were studied on series-connected cells. The output voltage of the two DSSCs connected in series significantly decreased not only when both cells were shadowed, but also when either one of the cells was shadowed. These results indicate that the ES-DSSCs can stabilize output power under various photoirradiation conditions.
Acknowledgements: I would like to thank Prof. T. Kubo, Prof. S. Uchida, Prof. J. Fujisawa, Dr. J. Nakazaki, Dr. T. Kinoshita, Dr. Y. Saito, Dr. M. Sasaki, Dr. M. Nagata, Mr K. Akitsu for their collaborations. This work was supported by the Funding Program for World-Leading Innovative R&D on Science and Technology (FIRST Program) from the Japanese Government.

The most pressing problems of our planet are rapid decline of natural energy resources, increasing population, and energy demand. It has now been realized that one of the main source of energy i.e. nuclear is catastrophe energy sources, thus the realization of new technologies to power our planet is paramount. Recognizing solar energy as a major natural resource abundantly available (2200 thermal kilowatt hours (kWh) per square meter) that can be fully exploited for the benefit of the mankind. Consequently the development of nanomaterial based technologies to convert solar energy into electricity is paramount, and among them Dye sensitized solar cells (DSSCs) are front runner. The counter electrode in DSSCs is one of the vital components, as it reduces the oxidized redox shuttle, generated after the dye regeneration. Since transparent connot;ductive oxide (TCO) substrates exhibit insufficient electron transfer kinetics for redox shuttle reduction, Platinum (Pt) is coated onto the TCO substrate (platinized cathode) to catalyze the cathodic reduction. Further to achieve high open circuit potential, transparency and to eliminate the possibilities of electrode corrosion cobalt based electrolytes are being employed. The sluggish kinetics of Pt reducing cobalt complexes at the counter electrode was one bottleneck limiting efficiency in cobalt based DSSCs and recently we have demonstrated that DSSCs employing counter electrodes based on poly(alkylthiophenes) outperform DSSCs fabricated with platinized counter electrodes. Such a carbon based electrocatalyst is attractive not only for its improved activity but also as it is much cheaper than platinum. The improved performance of DSSCs fabricated with these counter electrodes is due to lower charge transfer resistance owing to their ultrahigh surface area morphology and also because they avoid the formation of a passivation layer at electrolytes-electrode interface. There is ample room for future developments of counter electrodes based on semiconducting. Device fabricated using nanoporous poly(3,4-propylenedioxythiophene) [PProDOT] based cathode not only resulted in cost reduction but also resulted 20% higher light to electricity conversion.
These polymers can also be an effective candidate and easy alternative to rival TCO coatings. Recently we have observed impressive preliminary results based on these polymers to act as flexible cathode in DSSCs fabrication, this will having the merit of being flexible, less expensive and more environmentally friendly in processing and in manufacturing.

Zero-, one-, and two-dimensional nanostructured carbon allotropes, i.e., fullerenes, single-walled carbon nanotubes (SWNT) and graphenes in combination with electron-donating conjugated molecules are promising building blocks for artificial photosynthesis and solar energy conversion. In this talk I will focus on the various aspects of covalently and noncovalently-linked composites of porphyrins with fullerenes, SWNT and graphenes. In particular, the linkage structures between the porphyrin and nanocarbons exerted a substantial impact on their interaction between the components in the excited states. I will also highlight our recent developments of the hybrid materials of fullerenes and SWNT, where SWNT is utilized as scaffolds or wires of self-assembled fullerenes for photoelectrochemical devices. Finally, the effects of charge-separated state of donor—acceptor linked molecules on membrane activity of biological living cells will be presented. 1) Acc. Chem. Res. 2009, 42, 1809; 2) J. Am. Chem. Soc. 2009, 131, 3198; 3) J. Phys. Chem. C (Feature Article) 2009, 113, 9029; 4) J. Phys. Chem. Lett. (Perspective) 2010, 1, 1020; 5) Adv. Mater. 2010, 22, 1767; 6) J. Am. Chem. Soc. 2011, 133, 7684; 7) Angew. Chem. Int. Ed. 2011, 50, 4615; 8) J. Am. Chem. Soc. 2011, 133, 10736; 9) J. Am. Chem. Soc. 2012, 134, 6092; 10) Chem. Commun. (Feature Article) 2012, 48, 4032.

The exploring of ecologically benign carbon-free energy sources is one of the greatest challenges expecting to address the escalating global energy demand. The energy harvested from the sunlight offers desirable approach toward fulfilling needs in sustainable clean energy needs [1]. The solar energy can be converted into chemical energy stored within chemical bond of reduced compounds such as hydrogen and hydrides.
As majority of biological energy harvesting and transformation processes occurs at nanoscale, it is very attractive to construct complex artificial hybrids and devices including soft biological materials and inorganic nanoscale materials which share similar dimensions. For example, a naturally occurring light-harvesting protein phycocyanin (Pc) was recently exploited to enhance oxygen evolution on hematite thin-film photo electrode [2]. Such assembly shows long term stability and thus constitutes a promising hybrid photoanode for photoelectrochemical applications.
Here, we report on the application of light-driven proton pump bacteriorhodopsin (bRh) as a visible light harvester on Pt@TiO2 photocatalyst for solar hydrogen production.
Here we report on application of a light-driven proton pump bacteriorhodopsin (bRh) as a building block providing it's biological functionality (light trapping and proton pumping) to construct a nano-bio device. In particular, bR was interfaced with 1.5 nm Pt nanoparticles supported on TiO2 photocatalyst. In addition to dye-synthesizing role, bR molecules can provide protons, which are consequently reduced on platinum nanoparticles. Photoelectrochemical measurements in the presence of a redox electrolyte shows a 50% increase in photocurrent density when TiO2 electrodes were modified with bRh. Such increase in photocurrent combined with transient absorption studies demonstrates an achievable charge injection from the membrane protein molecules to TiO2 semiconductor nanoparticles. The light-driven H2 evolution was detected both under monochromatic 560 nm and white light illumination using different electron donors. The turnover rate of the hybrid photocatalyst was found to increase by 24 times in the presence of white light compared to monochromatic illumination. Such an increase can be attributed to the source of additional electrons resulting from UV excitation of TiO2 nanoparticle. Hence, the nano-bio hybrid triad containing a visible light harvesting protein bRh and a semiconductor photocatalyst doped with Pt nanoparticles represents a promising system for solar hydrogen generation.
M. G. Walter, E. L. Warren, J. R. McKone, S. W. Boettcher, Q. Mi, E. A. Santori, N. S. Lewis, Chem. Rev. 110, 6446-6473 (2010)
D. K. Bora, E. A. Rozhkova, K. Schrantz, P. Wyss, A. Braun, T. Graule, E. C. Constable, Adv. Funct. Mater. 22, 490-502 (2012).

Hematite provides most of the needs of a good photocatalyst material. It has an appropriate bandgap, is a low cost material and is photoelectrochemically stable. Yet, the efficiencies of converting the solar to chemical energy are still relatively low due to the high electron-hole recombination rate. One of the possibilities to enhance the efficiency is using nano-bio interfaces, like dye [1] or protein coated [2] semiconductors. The light harvesting protein C-phycocyanin (PC) which can be found in the photosystem of cyanobacteria and red algae could fulfill this task [2], due to its favorable photochemical properties such as high molar absorption coefficient and wide UV-Visible absorption.
The main goal of this work was to functionalize the hematite film surface and to determine its efficiency and stability under illumination (AM 1.5) in harsh and protein friendly environment, as well as to quantify the generated H2. XRD, SEM, voltammetry, and GC chromatography were used to characterize the films.
Enzymatic cross-linking and co-polymerization of tyrosine with tyrosinase was used for the first time to immobilize PC on the surface of hematite. It resulted in the in situ formation of melanin which stabilized the protein structure on the semiconductor surface and enabled 2 fold increase of the photocurrent at pH 7. Laccase assay was used as an indirect method to prove the native conformation of the protein on the hematite surface after immobilization and electrochemical treatment. The long term stability of the protein coating was proven during a 24h current density measurement. The surface structures of the PC-melanin coated hematite films show an interesting fractal pattern which is most probably in favor of the high current density increase under illumination with visible light. In 30 min operation time ~1200 ppm H2 was generated with 1 cm2 active surface in PBS.
Operating the water splitting device in a protein friendly environment can extend its lifetime compared with those used in strongly alkaline electrolyte.
Acknowledgement
Funding for this research was provided by the Swiss State Secretariat for Education and Research project Sciex 10.013 (NISHP - Nanobio-Interfaces for Photocatalytic Solar Hydrogen) and VELUX Foundation (BioPEC - Biomimetic Photoelectrochemical Cells for Solar Hydrogen Generation).
[1] O'Regan, B. and Grätzel, M., Nature, 1991. 353(6346): p. 737-740.
[2] Bora, D.K., Rozhkova, E. A., Schrantz, K., Wyss, P.P., Braun, A., Graule, T., Constable, C.C., Advanced Functional Materials, 2012. 22(3): p. 490-502.

We have recently demonstrated that plant pigments in the betalain family are promising sensitizers of TiO₂ that result in high photon-to-electron quantum yields and extended light-harvesting. Found in plants of the order Caryophylalles, purple betacyanins and yellow betaxanthins are bioavailable pigments which serve as photoprotectants and antioxidants. Recently, some members of this family of pigments have been shown to undergo single-step two-electron oxidation. When adsorbed on nanoparticulate TiO₂, these pigments display remarkably broadened absorption spectra that may result from self-assembly. Dye-sensitized solar cells consisting of betalain-sensitized TiO₂ show high values of incident photon-to-current conversion efficiency, IPCE(lambda;), suggestive of carrier multiplication, that track the spectrum of the TiO₂-adsorbed dye. We postulate that high IPCE values are the result of single-step, two-electron interfacial electron transfer. In this work we consider two betacyanins: betanin from red beet root (beta vulgaris) and amaranthin from red Hopi dye (amaranthus cruentus), and the betaxanthin, indicaxanthin (also from beet root) as sensitizers of TiO₂. We investigate the basis for broadened absorption spectra of these dyes on nanoparticulate TiO₂ by measuring the diffuse reflectance spectrum as a function of nanoparticle morphology, i.e. with differing proportions of exposed crystal facets, and in the presence of co-adsorbents which prevent dye aggregation. We demonstrate fluorescence resonance energy transfer (FRET) from yellow indicaxanthin to purple betanin and show that FRET can be used to extend light-harvesting of betalain-sensitized TiO₂ films. We also present spectroelectrochemical measurements that investigate the nature of the interfacial photoredox chemistry of betanin on a nanocrystalline TiO₂ electrode. Optical spectra and IPCE(lambda;) are reported for DSSCs containing dye cocktails as a means to extend light-harvesting.

The study presents modeling of optical and photo-current responses of hybrid complexes assembled from semiconductor quantum dots (QDs), nanowires (NWs), metal nanoparticles (NPs), and photosynthetic molecules. QDs and NWs can be arranged into light-harvesting complexes [1,2]. In these complexes, nanocrystals are coupled via energy transfer (FRET). Consequently, this coupling creates a flow of excitons from QDs to NWs. Excitons harvested in NWs can be ionized and used to create photo-voltage. Using kinetic equations for excitons, we model exciton transport in QD-NW and NP-NW complexes and explain the origin of a blue shift of exciton emission observed in the experiment [3]. Another system of our interest is a complex composed of natural photosynthetic reaction centers, semiconductor QDs, and metal NPs [4,5]. We show that, by using superior optical properties of nanoparticles and involving energy transfer, one can strongly enhance an efficiency of light harvesting in natural photosynthetic systems [6-8]. Potential applications of hybrid exciton-plasmon structures are in artificial photosynthetic systems, photovoltaic devices, and sensors. [1] J. Lee, A. O. Govorov, and N. A. Kotov, Nano Letters 5, 2063 (2005). [2] P. Hernandez-Martinez and A. O. Govorov, Phys. Rev. B 78, 035314 (2008). [3] J. Lee, P. Hernandez, J. Lee, A. O. Govorov, and N. A. Kotov, Nature Materials 6, 291 (2007). [4] A. O. Govorov and I. Carmeli, Nano Lett. 7, 620 (2007). [5] A. O. Govorov, Adv. Mater., 20, 4330 (2008). [6] S. Mackowski, S. Wörmke, A.J. Maier, T.H.P. Brotosudarmo, H. Harutyunyan, A. Hartschuh, A.O. Govorov, H. Scheer, C. Bräuchle, Nano Lett. 8, 558 (2008). [7] I. Nabiev, A. Rakovich, A. Sukhanova, E. Lukashev, V. Zagidullin, V. Pachenko, Y. Rakovich, J. F. Donegan, A.B. Rubin, and A.O. Govorov, Angew. Chemie, 49, 7217 (2010). [8] I.Carmeli, L. Lieberman, L. Kraversky, Z. Fan, A. O. Govorov, G. Markovich, and S. Richter, Nano Letters, 10, 2069 (2010).

We have performed a ‘structure-property relationship&’ study of energy transport in porphyrin-based metal-organic frameworks (MOFs) for light-harvesting applications. Two MOF materials, DA-MOF and F-MOF, constructed from especially designed Zn(II) porphyrin struts, namely [5,15-Bis[4-(pyridyl)ethynyl]-10,20-diphenylporphinato]zinc(II) or DA-ZnP and [5,15-dipyridyl-10,20-bis(pentafluorophenyl)porphinato]zinc(II) or F-ZnP, have been considered. The photoinduced singlet exciton migration in the two MOFs was investigated using fluorescence quenching experiments utilizing ferrocene-based quenchers coordinated to Zn-ions of the struts. Within its lifetime, the exciton is found to migrate very efficiently in DA-MOF, as far as ~45 struts (net displacement ~60 nm), whereas only poorly in F-MOF, up to ~3 struts (net displacement ~3 nm). Theoretical investigation, within the framework of Forster rate equation, indicates high energy-transport anisotropy in both materials. Efficient energy migration in DA-MOF is primarily attributed to an enhanced overlap integral between the normalized absorption and emission spectra (OI=4.07eV-1) as well as higher exciton couplings (J<=0.004 eV) owing to their more conjugated DA-ZnP struts, compared to F-ZnP struts in F-MOF (OI=0.46 eV-1, J<=0.002 eV). The distance dependent energy transfer rates between DA-ZnP struts were used to determine chemically accessible spacers that will yield nearly unidirectional energy transport in novel MOFs based on DA-ZnP struts.

Applicability of the Ga(Sbx)N1minus;x alloys for practical realization of photoelectrochemical water splitting is investigated using first-principles density functional theory incorporating the local density approximation and generalized gradient approximation plus the Hubbard U parameter formalism. Prior results with calculations revealed that a relatively small concentration of Sb impurities is sufficient to achieve a significant narrowing of the band gap, enabling absorption of visible light.1 Theoretical results predict that Ga(Sbx)N1minus;x alloys with 2 eV band gaps straddle the potential window at moderate to low pH values, thus indicating that dilute Ga(Sbx)N1minus;x alloys could be potential candidates for splitting water under visible light irradiation. Theoretical computations with Sb composition beyond 7% change the electronic band gap from direct to indirect.
Experimental synthesis is carried out using metal organic chemical vapor deposition using trimethyl gallium (TMGa) and Trimethyl Antimony (TMSb) and ammonia. Crystalline GaSbxN1-x films were obtained at x values ranging from 0-5%. The synthesis was carried out on different planar substrates and GaN nanowires. Optical measurements confirm that severe band gap reduction occurs with incorporation of antimony in to GaN as predicted by the theoretical calculations. X-Ray Diffraction (XRD) results confirm the lattice expansion at small concentrations of antimony. This presentation will highlight our results with both synthesis and photoelectrochemical characterization of GaSbxN1-x alloys.
Acknowledgements: Financial support from US Department of Energy (DE-FG02-07ER46375) and NSF (DMS1125909).
1. R.M. Sheetz, E. Richter, A.N. Andriotis, C. Pendyala, M.K. Sunkara and M. Menon, “Visible light absorption and large band gap bowing in dilute alloys of gallium nitride with antimony”, Phys. Rev. B 84, 075304 (2011)

Bio-inspired photosynthetic systems serve to guide the design of constructs for solar energy conversion based upon the oxidation of water and subsequent use of reducing equivalents to synthesize energy-rich compounds such as hydrogen or fuels based on reduced carbon. In order to establish the principles by which artificial photosynthesis and natural photosynthesis can be engineered to operate at higher efficiency, we are designing and assembling a tandem, two-junction photochemical cell based upon Grätzel-type photoelectrodes sensitized by pigments inspired by those used in water-oxidizing photosystem II (PSII) and in bacterial photosynthesis. The photoanode, inspired by PSII, will contain a mimic of the water oxidizing side of PSII reaction center. Upon photoexcitation, electrons are injected into semiconductors such as SnO2. The photoelectrode model of bacterial reaction centers will be sensitized by low potential naphthalocyanines/phthalocyanines, which absorb light in the near IR region of the spectrum. Upon photoexcitation, these dyes are designed to inject electrons into semiconductors having sufficiently negative conduction bands to effectively drive the reduction of protons to hydrogen.
(2) portions of this abstract and lecture were presented at the American Chemical Society Meeting in Philadelphia, PA, August 2012, and at other national and international conferences.

The generation of storable hydrogen fuel from light in the manner of photosynthesis is a promising solution to the clean energy problem. To date, some of the highest performance from the photocathode half-cell of a water-splitting photoelectrochemical (PEC) cell has been demonstrated with crystalline p-InP, a direct bandgap material that has demonstrated solar-to-hydrogen conversion efficiencies of over 14% [1]. Unfortunately, the high cost of epitaxial growth substrates and processes inhibits the application of the crystalline p-InP photocathode to a large-scale industrial deployment, requiring cost-oriented innovation despite its high performance. By analogy with solar cells, a thin-film approach would address these challenges by utilizing the benefits of the InP material while decreasing the use of expensive materials and processes. Here we demonstrate such an approach, using non-epitaxial growth methods along with a nanotexturing and atomic layer deposition protection to create a viable thin-film, polycrystalline InP photocathode. First, a recently explored MOCVD process [2] was used to grow 2-3mu;m of thin-film poly-InP on a Mo substrate. Then, an unpatterned RIE process we developed for crystalline InP [3] was employed to form tightly spaced nanopillars across the film, gaining benefits in light trapping, charge collection, and other aspects of a high surface area structure. A conformal TiO2 protection and passivation layer was then deposited through atomic layer deposition, and a Pt co-catalyst was deposited through sputtering. Current-voltage measurements performed in aqueous 1M HClO4 show photocurrent at the reversible potential of 12 mA/cm2 and an onset potential of 380mV vs. the RHE, compared to 33 mA/cm2 photocurrent and 650 mV onset potential for crystalline controls. Stability of nanostructured devices is also demonstrated in acidic environments to beyond 24 hours. We believe this is the first demonstration of a thin-film poly-InP photoelectrochemical device for H2 generation.
This material is based upon work performed by the Joint Center for Artificial Photosynthesis, a DOE Energy Innovation Hub, supported through the Office of Science of the U.S. Department of Energy under Award Number DE-SC0004993.
Thin film deposition was performed at the Molecular Foundry, which is supported by the Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231.
[1] A. Heller, Science (1984)
[2] Zheng, M., Yu, Z., Joon Seok, T., Chen, Y.-Z., Kapadia, R., Takei, K., Aloni, S., et al. Journal of Applied Physics, (2012)
[3] Lee, M. H., Takei, K., Zhang, J., Kapadia, R., Zheng, M., Chen, Y.-Z., Nah, J., et al. Angew. Chem. Int. Ed. (2012)

Electrochemical coupling layer-by-layer (ECC-LbL) assembly is introduced as a novel fabrication methodology for preparing layered thin films. This method allows us to covalently immobilize functional units, such as porphyrin, fullerene, and fluorene, into thin films of desired thicknesses and designable sequences for both homo- and hetero-assemblies while ensuring efficient layer-to-layer electronic interactions. Films were prepared using a conventional electrochemical set-up by a simple and inexpensive process where various layering sequences can be obtained and photovoltaic functions of a prototype p/n heterojunction device were demonstrated. This method allows us to covalently immobilize functional units into thin films of requisite thickness (number of layers) and sequence for both homo- and hetero-assemblies while ensuring efficient layer-to-layer electronic communications using a conventional electrochemical set-up in a simple and inexpensive process. Because the reaction site for coupling is independent from the functional unit, this method is applicable in the presence of many kinds of organic functional groups. Therefore, the ECC-LbL should be a powerful method for constructing robust and well designed organic layered structures, with potential for various types of organic photo-current devices.

Photoelectrochemistry allows us to store the solar energy as chemical energy by splitting water into hydrogen by using suitable semiconductors. Since the early 1970s there is a long quest for efficient photoelectrodes. Practically 1.8 to 2.0V is required to split water taking into account the overpotentials and other efficiency loss processes. Single band gap photoabsorbers do not provide enough photovoltage to drive this energetic reaction. Multiple band gap systems or tandem cells can suitably match and effectively utilize the solar spectrum, finally leading to higher efficiencies.
Tandem cells made out of silicon have the distinct advantage that they consist of a widely available material, they are based on a proven technology and they are able to provide the necessary voltage needed for the water splitting reaction. Here we investigate photovoltaic tandem cells made up of amorphous and microcrystalline silicon (a-Si/µ-Si) for hydrogen production. We have shown previously, that it is possible to achieve efficiencies well above 6%, with these types of cells. But the photovoltages were not sufficient to drive the hydrogen evolution reaction on their own. Here we present our recent investigations on the modification of these devices, in order to understand which modifications are necessary to achieve higher working voltages, by variation of the catalyst material and the electrolyte, by the optimization of catalyst deposition techniques and by using additionally electronically better adapted buffer layers between the absorber and the electrolyte. The surfaces are characterized with respect to their photoelectrochemical (PEC) characteristics by cyclic voltammetry and with respect to their chemical composition and their electronic structure by X-ray photoelectron spectroscopy.

The development of photochemical systems capable of splitting water into hydrogen and oxygen has attracted significant interest motivated by the need to secure the future supply of clean and sustainable energy [1]. Due to the complex chemistry involved in four-electron oxidation of water to dioxygen [2], the major challenge in photoelectrochemical water splitting is the development of cheap, efficient and stable photoanodes. Recently, we have been developing photoanodes based on a novel class of visible-light photoactive inorganic/organic hybrid materials - TiO₂ modified at the surface with polyheptazine (also known as “graphitic carbon nitride”). As we have shown, the optical absorption edge of the TiO₂-polyheptazine hybrid is red-shifted into the visible (2.3 eV; ~540 nm) as compared to the bandgaps of both of the single components - TiO₂ (3.2 eV; ~390 nm) and polyheptazine (bandgap of 2.9 eV; ~428 nm), which is due to the formation of an interfacial charge-transfer complex between polyheptazine (donor) and TiO₂ (acceptor) [3]. In other words, the direct optical charge transfer leads to generation of electrons with a relatively negative potential in the conduction band of TiO₂, while the holes photogenerated in the polyheptazine layer can drive photooxidation of water, as evidenced by visible light-driven evolution of dioxygen on hybrid electrodes modified with iridium or cobalt oxide nanoparticles acting as oxygen evolution co-catalysts [3-6]. Importantly, polyheptazine is highly stable, and at the same time it offers a possibility for further functionalization with transition metal-based catalytic sites enabling chemical transformations along multi-electron pathways.
References
[1] N.S. Lewis, D.G. Nocera, Proc. Natl. Acad. Sci. U.S.A.2006
, 103, 15729.
[2] H. Dau, C. Limberg, T. Reier, M. Risch, S. Roggan, P. Strasser, ChemCatChem2010, 2, 724.
[3] M. Bledowski, L. Wang, A. Ramakrishnan, A.; O.V. Khavryuchenko, V.D. Khavryuchenko, P.C. Ricci, J. Strunk, T. Cremer, C. Kolbeck, R. Beranek, Phys. Chem. Chem. Phys.2011, 13, 21511
[4] L. Wang, M. Bledowski, A. Ramakrishnan, D. König, A. Ludwig, R. Beranek, J. Electrochem. Soc.2012, 159 (7), H616.
[5] M. Bledowski, L. Wang, A. Ramakrishnan, A.; O.V. Khavryuchenko, A. Bétard, R. Beranek, ChemPhysChem2012, 13, 3018.
[6] M. Bledowski, L. Wang, A. Ramakrishnan, R. Beranek, J. Mater. Res.2012, DOI:10.1557/jmr.2012.297.

The efficient harvesting of low-energy solar irradiation for solar fuels and photovoltaic applications demands the development of efficient photon upconversion systems, preferably built from the earth abundant constituents. We have utilized Sn(IV) porphyrins as templates for the synthesis of self-upconverting donor/acceptor systems. Their ultimate photophysical properties can be tuned in a desired manner by changing the nature of the triplet excitation energy acceptor, as demonstrated via the axial incorporation of either anthracene or perylene moieties. In case of the latter, the selective low-energy excitation of the porphyrin Q-band, along with the favorable energetics, is expected to generate the upconverted perylene-based delayed fluorescence driven by the bimolecular triplet-triplet annihilation and is a subject of the ongoing research.

Nature sustains itself by converting solar energy, in series of reactions between light harvesting components, electron transfer pathways, and redox-active centers. As an artificial system mimicking such solar energy conversion, porous chalcogenide aerogels (chalcogels) encompass the above components into a common architecture. We present the ability to tune the redox properties of chalcogel frameworks containing biological Fe4S4 clusters. We have investigated the effects of [SnnS2n+2]4- linking blocks ([SnS4]4-, [Sn2S6]4-, [Sn4S10]4-) on the electrochemical and electrocatalytic properties of the chalcogels, as well as on the photophysical properties of incorporated light-harvesting dyes, tris(2,2&’-bipyridyl)ruthenium(II) (Ru(bpy)32+ ). The various thiostannate linking blocks do not alter significantly the chalcogel surface area (90-310 m2/g) or the local environment around the Fe4S4 clusters as indicated by 57Fe Mössbauer spectroscopy. However, the varying charge density of the linking blocks greatly affects the reduction potential of the Fe4S4 cluster and the electronic interaction between the clusters. We find that when the Fe4S4 clusters are bridged with the adamantane [Sn4S10]4- linking blocks, the electrochemical reduction of CS2 and the photochemical production of hydrogen are enhanced. In addition, we show that the further improvement is made by the addition of the third transition metals into the chalcogels of Fe4S4 clusters linked with the adamantane [Sn4S10]4- blocks in the ability to reduce protons both electrocatalycally and photochemically. The capability to tune the properties of biomimetic chalcogels presents a novel avenue to control the function of multifunctional chalcogels for a wide range of electrochemical or photochemical processes relevant to solar fuels.

Polymeric carbon nitride bulk films were prepared on silicon templates, prestructured by XeF2 dry etching [1, 2], by polycondensation of dicyandiamide precursors at 550° C [3]. After chemical split-off of micrometer thick overlayers by chemical etching in HF, ultrathin polymeric carbon nitride nanosheets were obtained, partially covering the surface area. Analysis by scanning electron microscopy confirms that the structures are embedded into the corrugated surface topography of the silicon supports. Alternatively, lamella-like thin-films could be prepared by mechanical smoothening of the precursor material at the transition to the liquid phase at about 220° C and subsequent thermal polycondensation. Upon light-induced hydrogen evolution in aqueous acidic electrolytes, a significant positive shift of the photocurrent onset by 0.2 - 0.3 V was observed for p-type silicon substrates. For n-type silicon, hydrogen evolution in the dark occurs at current densities approximately two times higher than for clean silicon reference samples. These results confirm the (photo)electrocatalytic activity of the carbon nitride deposits and allow, for the first time, to assess this activity quantitatively. Compositional analysis by photoelectron spectroscopy and energy dissipative X-ray measurements will be presented and compared to the corresponding analysis of bulk films. Properties at the interfacial region will be discussed in terms of Fermi-level equilibration between the inorganic substrate and the organic surface layer. Based on recent photoluminescence and absorption measurements, it will be shown that short carrier lifetimes of the non-conjugated system necessitate ultrathin surface coverage by polymeric carbon nitride in order to achieve optimized activity of the metal-free catalyst. Further enhancements by employment of co-catalytic materials for both the hydrogen and the oxygen evolution reaction will be finally outlined.
[1] M. Lublow, S. Kubala, J.-F. Veyan, and Y. J. Chabal, J. Appl. Phys. 111, 084302 (2012).
[2] J.-F. Veyan, D. Aureau, Y. Gogte, P. Campbell, X.-M. Yan, and Y. J. Chabal, J. Appl. Phys. 108, 114913 (2010).
[3] F. Yang, M. Lublow, S. Orthmann, C. Merschjann, T. Tyborski, M. Rusu, S. Kubala, A. Thomas, R.Arrigo, M. Hävecker, Th. Schedel-Niedrig, ChemSusChem 5, 1227 (2012).

Direct photo-electrolysis by generated electron-hole pairs in semiconductor materials using solar power has been attracted as a method for producing hydrogen (H₂) gas from water. The material utilized in this technology has to satisfy both a proper band-gap value and suitable conduction and valence band positions. A proper band-gap value is essential to use solar spectrum. Also, conduction band-edge must be more negative than hydrogen-evolving potential and valence band-edge must be positive than oxygen-evolving potential for water splitting. Although oxide-semiconductor materials have been studied previously, some problems regarding large band-gap and instability are still remained. In this study, we introduced 3C-SiC, which satisfies the band-edge levels and has a proper band-gap of 2.2 eV.
In this experiment, n-type 3C-SiC (N doping density: 3.6 × 10¹⁸ cm^-3) was used as a photoanode and the properties for water splitting were analyzed. Also, the generation of H₂ gas from a counter electrode was confirmed by using a gas chromatograph. First of all, cyclic voltammetry (CV) measurement was performed for confirming photoreaction. An electrochemical cell for the measurements consists of platinum (Pt) or copper (Cu) as a counter electrode and Ag/AgCl as a reference electrode. 0.01 M HCl is used as electrolyte. When the 3C-SiC sample was illuminated, we obtained photo-current densities of 0.2 mA/cm² at an external bias of 1 V (vs. Ag/AgCl) and it increased to 19.7 mA/cm² at 2 V. A photo-current of 0.007 mA/cm² was also obtained even without external bias (0 V). In order to examine visible light response of 3C-SiC, an ultraviolet (UV) filter cutting whole wavelength below 400 nm was inserted between the light source and the 3C-SiC sample. In this case with the UV filter, similar photo-current levels were obtained, 0.006 mA/cm² at 0 V, 0.15 mA/cm² at 1 V and 12 mA/cm² at 2 V, indicating that the 3C-SiC anode has high visible light response. Next, the gases produced on the counter electrode were analyzed by gas chromatograph. Little H₂ gas was produced at external bias below 1.5 V. However, when the total current flow amount was 0.1 C (at 1.5 V), the hydrogen composition in the cell increased to 811 ppm. It indicates that sufficient photo anodic reaction occurs to produce H₂ gas using 3C-SiC. However, it is small amount because the 3C-SiC semiconductor has stacking faults, which shorten the carrier lifetime. Lastly, decomposition of 3C-SiC by anodic oxidation remains problems that must be solved.
In summary, photo-current was observed when light was illuminated on n-type 3C-SiC. We demonstrated the efficiency of sample for visible light response and H₂ gas generation.

Direct conversion of solar energy into storable fuels such as hydrogen has significant potential in providing clean and sustainable energy source. Photoelectrochemical (PEC) water splitting is a promising approach to converting solar energy into hydrogen. In this report, photoelectrodes consisted of vertically aligned tantalum nitride (Ta3N5) nanowire arrays have been fabricated for PEC water splitting. Ta3N5 is a suitable photoelectrode material for PEC water splitting because of its proper band positions for water reduction and oxidation, broad visible light absorption spectrum, and stability in an aqueous environment. Vertically aligned Ta3N5 nanowires were grown in situ on a Ta metal substrate by trough-mask anodization and high-temperature nitridation. The single crystalline 1D nanostructures with low contact barriers make them ideal building blocks of PEC cells towards highly efficient solar energy conversion. Under irradiation of AM 1.5G simulated sunlight, the Ta3N5 nanowire photoelectrode yielded a photocurrent density of 3.8 mA/cm2 at 1.23 V vs RHE, which was ~3.2 times higher than that of a planar Ta3N5 photoelectrode. A maximum incident photon-to-current conversion efficiency of 41.3% was achieved at 440 nm under 1.23 V vs RHE. Furthermore, high and stable photocurrent was demonstrated by modifying the Ta3N5 nanorods with cobalt phosphate co-catalyst.

For the past 40 years, there has been considerable increase in the development of hydrogen from water using sunlight beginning with the work of Fujishima and Honda in 1972. The initial water-splitting research focused on bulk semiconductors, such as titanium dioxide and CdS; however, these bulk materials have low solar hydrogen evolution rates compared with high surface area materials. Previous research completed on titanium dioxide with various surfactants showed greater solar hydrogen production with larger surface area and pore volume. The present study employed mesoporous titanium dioxide in solar simulated hydrogen evolution with methanol as sacrificial reagent. The benefit of this higher surface semiconductor was indicated to lead to higher solar hydrogen evolution rates. This study focused on tuning the level of anatase and rutile phases in the mesoporous material, which affects the amounts of hydrogen produced from water-splitting. The two phases in these materials were controlled by the aging time length using evaporation induced self-assembly (EISA) method. The synthesis, characterization, and an application for photocatalytic water-splitting will be presented.

A solar thermal fuel absorbs sunlight and stores the energy chemically via an induced structural change, which can later be reversed to release the energy as heat. Azobenzene molecules have a cis-trans photoisomerization with these properties, and hydrogen-bonding and packing via attachment to rigid template structures have shown promise in increasing the energy stored and the length of time it can be stored [A Kolpak et al, Nano Lett. 11, 3156-3162 (2011)]. Other important factors in determining the efficiency of a solar thermal fuel are the absorption cross-section and the quantum yield for photoisomerization, which must also be optimized for a successful material. We employ time-dependent density-functional theory (TDDFT) and the GW/Bethe-Salpeter formalism to calculate the optical absorption and dynamics in the excited-state to address these two factors. We use excited-state forces to map out potential-energy surfaces and follow the structural change after absorption for azobenzene-derived materials, to correlate the efficiency of photoisomerization with the functionalization and template.

Bismuth vanadate, BiVO₄ (BVO), is an attractive photoanode material for water oxidation due to its ~2.4 eV band gap and stability in basic and neutral conditions [1]. Thin BVO films made by spin coating from solution followed by calcination have been reported to AM 1.5G photocurrent densities of up to 2.9 mA cm 2 with an onset potential against reversible O2/H2O (OER) of 0.7 V [2]. While these results are quite promising, we note that the deposition techniques by this work and others employed provide only limited control over the structure and composition of the BVO. Here we employed both chemical vapour deposition (CVD) and physical vapour deposition methods to synthesis BVO thin films with an emphasis on controlling stoichiometry. Vapour transport was performed using Bi metal and V2O5 powders as precursors in a 2-zone furnace under air flow with the film composition controlled by the temperature and oxygen pressure in the second zone. PVD deposition was performed by co-sputtering using V and Bi2O3 targets with the Bi/V ratio controlled by the target bias and the post annealing temperature. Both methods yielded continuous crystalline films up to 500 nm thick. The BVO was single phase (monoclinic) as confirmed by X-ray diffraction and Raman spectroscopy. Stoichiometry was measured by SEM/EDAX and x-ray photoelectron spectroscopy. We have investigated a wide range of Bi:V ratios, ~60:40 to ~40:60, with non-BVO phases forming outside this range. Photoelectrochemical (PEC) measurements (AM1.5 and pH 7 in buffer with a sulphite sacrificial acceptor salt) showed photoanodic response in all cases with minority hole transport to the surface, although some amount of photocathodic behaviour was also observed. In many cases, back illumination through the transparent back contact yielded larger currents. For both deposition methods, the best PEC response was found for V-rich films; Bi-rich films had very low current densities. This finding is somewhat in contrast to the prediction that Bi rich BVO should be n-type and V-rich p-type. These findings will be discussed in terms of the bulk and surface recombination properties, as measured by quantum efficiency and surface photovoltage, and in terms of the surface Fermi level as evaluated by ultraviolet photoelectron spectroscopy.
This material is based upon work performed by the Joint Center for Artificial Photosynthesis, a DOE Energy Innovation Hub, supported through the Office of Science of the U.S. Department of Energy under Award Number DE-SC0004993.
[1] Y. Park , K. J. McDonald and K.-S. Choi, Chem. Soc. Rev. DOI: 10.1039/C2CS35260E (2013)
[2] R. Saito, Y. Miseki, and K. Sayama, Chem. Commun. 32 3833 (2012).

Titanium dioxide is a wide-band gap semiconductor that finds a broad range of applications in environmental and energy sectors. The anatase form of TiO₂ is of particular interest because of its use in 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. A unique feature of these films is that they exhibit ~100% {001} facets at the external surface.¹ The TiO₂ films were grown hydrothermally on gold, quartz, and Pyrex substrates in the presence of fluoride, which acts as a crystallographic controlling agent. Platinum nanoparticles (3-5 nm) were deposited on the surface of the {001} textured films by the photoreduction of chloroplatinic acid under UV irradiation using methanol as an electron donor. Grazing angle XRD measurements and SEM images demonstrate the preferential growth of {001} facets and confirm the presence of platinum nanoparticles at the surface of the film. The surface area of the films was estimated by atomic force microscopy, which yields a lower limit for the total active area. Compared to TiO₂ materials such as P25 with surface areas on the order of 50 m² g^-1, our films exhibit an area close to the projected area of the measured surface. The np-Pt⁰-laced anatase films were tested for their ability to generate a gaseous mixture of hydrogen and oxygen from deionized and degassed water under irradiation with 254-nm light. Hydrogen production was studied as a function of irradiation time and hydrogen evolution rates were established. When normalized to surface area, the hydrogen evolution rates of these films were on the order of 1-3 mmol hr^-1 m^-2, ~10 times more photoactive than P25 powder under similar conditions.² Despite their apparent low surface area, H₂(g) evolution rates of np-Pt⁰ {001} textured anatase films exceed those of np-Pt⁰ mesoporous anatase films or suspended anatase powders. 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. Ichimura, A.S.; Mack, B. M.; Usmani, S. M.; Mars, D. G. Chem. Mater. 2012, 24, 2324-2329.
2. Chen, X.; Shen, S.; Guo, L.; Mao, S. S. Chem. Rev. 2010, 110, 6503-6570.

Solar energy is the only renewable energy source capable of meeting the rapidly expanding global demand for energy (15 TW demand; 120,000 TW theoretical maximum capacity). One of the most promising conversion strategies is hydrogen fuel production, wherein the solar energy is stored as stable chemical bonds. Solar-driven water oxidation (artificial photosynthesis), which splits water into oxygen and hydrogen, produces dihydrogen from an abundant natural resource. However, the realization of an efficient water splitting system depends on the catalysts used to facilitate the oxygen and hydrogen evolution reactions at the anode and cathode, respectively. For many catalysts, including the widely used heterogeneous metal oxides (e.g. α-Fe2O3, WO3, cobalt phosphate), the chemical environment of the active catalyst species is poorly understood, due in large part to the difficulty of making the appropriate spectroscopic measurements on the microscopic materials in situ. To investigate the mechanism of water oxidation on a wide variety of catalysts, we have designed a spectroelectrochemical cell for in situ electron paramagnetic resonance (EPR) spectroscopy. The quartz cell permits simultaneous photochemical and electrochemical excitation of both homogeneous and heterogeneous catalyst samples. We present preliminary EPR measurements of trapped intermediates, generated photo- and/or electrochemically under working conditions, for several transition metal water oxidation catalysts. Our results have demonstrated the applicability of EPR spectroscopy to the study of transition metal solar fuels catalysts on real electrodes capable of splitting water.

There has been a growing interest in the light induced water splitting and hydrogen production. A number of factors should be considered while choosing the correct materials in order to increase or maximize the photoelctrochemical performance. Surface nanostructuring is one of such factors which enable to maximize the performance by preventing the surface oxidation of active materials. In the current study, a number of manganese loaded TaON and doped TaON semiconductor electrodes were fabricated. Conventional and simple wet impregnation was applied to prepare the composites by using a solution containing the metal oxide precursor. Amount of loaded metal, decomposition temperature and precursor salts were varied to have a comprehensive idea about the parameters suitable for optimum performance. These composites were heated to different temperatures and different manganese oxide species were formed on the surface. NEXAFS experiments performed at the synchrotron facility confirmed that the majority of manganese species were in Mn4+ state. PXRD patterns show that there was some non uniform line broadening which may be caused by the tantalum displacement by manganese in the crystal lattice. STEM images, SEM and EDX mapping indicated the uniform coverage of manganese species onto the surface of semiconductor material. Five weight percentage of manganese was the highest loading possible for the uniform distribution and increasing the manganese content further caused agglomeration which can be seen clearly from SEM. Thermal decomposition behavior was of the composite materials was investigated by TG-MS technique. The electrodes were fabricated using an ITO substrate and electrophoretic deposition technique. The active electrodes were prepared by post calcinations treatment of the deposited materials.

We have recently developed an easy, low-cost method to attach transition metal and/or lanthanide ions to the surface of TiO2 nanorods (NR). We utilize the gram-scale preparation of TiO2 NR reported by Hyeon, therefore, the metallated NR can be prepared in large quantities. TiO2 NR can be metallated with quantitative precision, and we demonstrate titration of the TiO2 NR surface with metal ions. Metal ions can be added in well-defined combinations. The metallated TiO2 NR were characterized with TEM, powder XRD, UV-visible, XAS, XPS, and EDS spectroscopy. The data indicate the presence of single site metal ions on the surface of the TiO2 NR, and we believe this is the first example of a new class of materials. The materials are unique in that it becomes possible to merge nanocrystal science with coordination chemistry to formulate new catalysts with the major benefit of using the electronic structure and surface properties of the well-defined host nanocrystal to affect catalysis at the single site centers. Some preliminary data for the visible light photocatalytic decomposition of organic dyes will be presented. Our results show that Co(II)-TiO2 NR exhibit turnover frequency ~21X greater than TiO2 NR. Ultimately, these materials may be excellent precursors for synthesis of hybrid nanocrystals and have utility for solar fuels catalysis.

One of the key challenges in photoelectrochemical cell (PEC) is the slow transfer of electrons across energetic barriers at interfaces, leading to a large overpotential at room temperature. Increasing the operating temperature is a promising approach to accelerate electron transfers across the electrochemical interface. We have designed an elevated-temperature solid-state PEC based on a heterostructure between a semiconductor light absorber and a wide band-gap mixed ionic and electronic conductor (MIEC). The heterojunction between the light absorber and the MIEC, on one hand, can efficiently separate charge carriers. On the other hand, the MIEC/gas interface suppresses the reverse current at elevated temperatures. In this talk, we will present both theoretical and experiment results on the semiconductor-MIEC heterojunction for water-splitting. The temperature-dependent current-voltage characteristics of the semiconductor/MIEC heterojunction, with and without illumination, shows promising characteristics for elevated-temperature water splitting. The experiment result is interpreted using a temperature-dependent theoretical model.

From the beginning of PEC research many effort was devoted to understand the physical chemistry of underlying phenomenon associated with the photoelectrochemcial process. The semiconductor - electrolyte interface plays an important role in the functionality of photosensitive semiconductor. The functionality in turn represented in the form of obtained photocurrent from an electrode which finally determined its efficiency. The origin of photo current lies in the formation of electron hole pair by light having energy higher than the band gap of the semiconductor. In this case, holes formed have distinct role in the water oxidation process. Impedance spectroscopy technique has already been employed to study the formation of electron hole pair in PEC process [The Journal of Physical Chemistry Letters2012 3 (17), 2517-2522]. But the use of soft X - ray absorption spectroscopy for studying the role of holes is rare. The objective of our study was to probe the formation of hole at SC interface and to understand the nature of holes with NEXAFS spectroscopy by looking into the VB of semiconductor. For the same, we have employed atmospheric pressure chemical vapor deposited si - hematite film on Silicon nitride substrate. For the in - situ study, we have used a specially designed liquid flow cell able to operate in UHV condition [base pressure: 3 X 10 -9 torr]. The X - ray spectra have been taken during chronoamperometric condition (applying bias) and exposure of AM 1.5 solar simulated white lights. On careful analysis of X - ray spectra at O K edge, we have found that on increasing the bias and light on condition at around 300 mV, two new spectral features evolved before the pre edge. We identify these as an O 2p hole transition into the charge transfer band and an Fe 3d type hole into the upper Hubbard band [Ref:Journal of Physical Chemistry C, 2012, 116 (32), pp 16870-16875]

Photoelectrochemical water splitting could provide a sustainable means of producing hydrogen fuel.¹ To make this process efficient and economical, a hydrogen evolution reaction (HER) catalyst composed of inexpensive and abundant materials is required for the photocathode.
Our previous studies have demonstrated that nanostructured crystalline molybdenum disulfide (MoS₂) has high catalytic activity for the HER.^{2, 3} More recent studies have shown that amorphous molybdenum sulfide catalysts also exhibit high HER activity, requiring only ~200 mV overpotential to achieve a current density of 10 mA/cm²_geometric.^4-6 Furthermore, these catalysts can be synthesized via simple wet chemical procedures and easily deposited onto any desired substrate. These advantages could make amorphous molybdenum sulfide catalysts easy to incorporate into a functional water splitting device.
However, many questions remain about the properties of amorphous molybdenum sulfide and the origins of its catalytic activity. Ex-situ characterization reveals that as deposited, the catalyst is predominantly composed of amorphous MoS₃, but after electrochemical testing, the material changes to more closely resemble MoS₂. We have employed in-situ ambient pressure x-ray photoelectron spectroscopy (XPS) to better understand these changes to the catalyst under reaction conditions and attempt to identify the active sites. Using this technique, we measure changes in this material&’s composition and chemical state during catalysis. Next, we show that this amorphous molybdenum sulfide catalyst can be combined with silicon to make a high-performing photocathode for solar water splitting. Based on our enhanced understanding of this catalyst, we propose strategies for further improving its performance in a functional water splitting device.
1. M. G. Walter et al., Chem. Rev. 110, 6446 (2010)
2. T. F. Jaramillo et al., Science 317, 100 (2007)
3. Z. Chen et al., Nano Letters 11, 4168 (2011)
4. J. D. Benck et al., ACS Catal., 1916 (2012)
5. D. Merki et al., Chem. Sci. 2, 1262 (2011)
6. H. Vrubel et al., E&ES 5, 6136 (2012)

The electrochemical (EC) properties of single-crystalline p-type 3C-SiC films on p-Si and n-Si substrates were investigated as photoelectrodes in H2SO4 aqueous solutions in the dark and under various illumination conditions. In a first step the SiC films were etched by HF solution and aqua-regia/HF solution, respectively. Before and after the EC characterization the samples were analyzed with regard to their composition and their electronic structure by X-ray photoelectron spectroscopy (XPS).
The EC measurements indicate that the p-type 3C-SiC films on p-Si substrates can generate a cathodic photocurrent, which corresponds to hydrogen production, and are additionally able to generate an anodic photocurrent, which corresponds to oxygen evolution. From the XPS-data it is deduced that the anodic reaction at least partially leads to an oxidation of the SiC surface to silicon oxides, carbon oxides and carbonates, whereas no change in surface composition is observed after the cathodic reaction.
From the combination of quantum-efficiency measurements with band-energy diagrams the observed photoelectrochemical (PEC) behavior can be well explained.
EC measurements for the system p-3C-SiC on n-Si show a photocurrent only in the anodic regime, corresponding to the oxygen evolution reaction. This behavior will be compared to the p-3C-SiC on p-Si. It can also be understood by the construction of the related band-energy diagram.
Furthermore, the effect of surface modification on the PEC behavior of SiC by Pt-catalyst deposition and thin tin oxide layers will be discussed.
[1] Q.-B. Ma, B. Kaiser, J. Ziegler, D. Fertig, and W. Jaegermann, J. Phys. D: Appl. Phys. 45, 325101 (2012).

There are emerging technologies of using nanostructured metal oxide semiconductors for solar energy conversion and energy storage. The ability to control the particle size, morphology and composition of nanoparticles is of crucial importance nowadays considering the nanostructured 3d TM compounds in the applications of solar photovoltaic and photoelectrochemical cells, catalysts etc. It is crucial to understand the peculiarities of metal oxides so as to invent strategies to improve the performance of these materials. The L-edge XAS spectra of 3d TM compounds provide direct information of 3d electrons and 3d-orbital. Synchrotron Radiation produces soft X-rays which are optimally suited to study the electronic structure of electrode materials and which can detect electron holes. We show that the L-edge XAS spectra can be simulated well by ligand-field theory, and it provides electronic state information with significantly better resolution. The charge transfer in TM nanoclusters grown in silica nanopores that act as efficient and robust catalysts has been revealed, e.g., the metal-to-ligand charge transfer was evident between a nanoparticle catalyst and surfactant ligands on its surface. The results reveal the electronic structure of the 3d metal compounds in pure form and their variations upon doping. In addition, in-situ XAS and RIXS characterization was demonstrated for real-time study of catalytic reactions. The challenge has been that soft X-rays cannot easily peek into a high pressure catalytic cell or a photoelectrochemical cell (PEC). The unique design of the in-situ cell has overcome the burden. Recently the experiment has been performed for studying, for example, the hole generation in a specifically designed photoelectrochemical cell under operando conditions. The oxygen valence band signature was recorded while tuning the PEC parameters, two different types of holes in the valence band near the Fermi energy are discovered.

Competitive photoinduced oxidation of n-type Si and its chemical etching in alkaline solution is investigated in the search for scalable pattern formation for photoelectrochemical energy conversion. The study is a precursor investigation for the further development of nanoemitter photovoltaic and photoelectrocatalytic solar cells [1-2-3]. Typically, these cells are fabricated by site-selective deposition of materials that form local Schottky junctions [4] in contact with a redox electrolyte or for water splitting an acidic or alkaline solution. The operational energetics of this type of cell are described by pinch off effects [5-6].
Due to the complexity of anodic oxide formation and chemical etching, we restrict the investigation to the initial phase where a transitory photocurrent peak is observed slightly positive from OCP. SPRES data recorded at the U 49/2 beamline at BESSY II show submonolayer silicon surface oxidation and, from the surface core level shift for H-terminated Si, considerable areas with remnant H-termination are deduced, indicating island-type oxide formation. XPS valence band spectra point to the formation of a degenerately doped surface region attributed to H interdiffusion slightly anodic from OCP. Scanning probe microscopy shows the formation of macropores with roughly 300-500 nm diameter and average depth of 5-8 nm. The results are discussed based on chemical and electrochemical dissolution mechanisms.
[1] T. Stempel, M. Aggour, K. Skorupska, A. Munoz, H.-J.Lewerenz, , Electrochem. Comm. 10 (2008) 1184-1186
[2] H.-J. Lewerenz, Electrochimica Acta 56 (2011) 10713-10725
[3] H.-J. Lewerenz, K. Skorupska, A. Munoz, T. Stempel, N. Nüsse, M. Lublow, T. Vo-Dinh, P. Kulesza, Electrochimica Acta 56 (2011) 10726-10736
[4] A. Munoz, H.-J. Lewerenz, Electrochimica Acta 55 (2010) 7772-7779
[5] R.C. Rossi, N.S. Lewis, J. Phys. Chem. B 105 (2001) 12303-12318
[6] M.G. Walter, E.L. Warren, J.R. McKone, S.W. Boettcher, Q. Mi, E. A. Santori, N.S. Lewis, Chemical Reviews 110 (2010) 6446-6473

Photoelectrochemical (PEC) water splitting is an attractive pathway for renewable production of hydrogen (H2), but the efficiency and stability of semiconducting photoelectrodes must be improved. One promising approach to achieve both high efficiency and good electrochemical stability is the metal-oxide-semiconductor (MOS) photoelectrode design.[1] This MOS arrangement generally consists of catalytic metal structures, or collectors, deposited on an oxide-covered semiconductor. Of great importance to this design is the thin oxide layer, which must simultaneously protect the semiconductor from the potentially corrosive electrolyte while mediating minority carrier tunneling between the semiconductor and collectors. Si is a commonly used photovoltaic material that is also attractive for use in MOS photoelectrodes, but Si-based MOS electrodes demonstrated to date have greatly suffered from poor oxide quality and very low solar-to-hydrogen conversion efficiencies.[2]
In this work, we demonstrate drastic improvement in the performance of Si-based MOS photocathodes, achieved by (i.) using high quality SiO2 tunneling layers synthesized by rapid thermal annealing (RTA) and ( ii.) employing a bilayer collector structure. Experimentally, these photocathodes achieve solar-to-hydrogen conversion efficiencies (eta;STH) of ~3%, and modeling efforts show a clear pathway towards eta;STH > 10% using an all-Si device. In order to better understand the interfacial charge transfer mechanisms in these MOS electrodes and further optimize their performance, in-situnot; scanning probe techniques were utilized to evaluate the photoelectrochemical properties of well-defined MOS surfaces with high spatial resolution. Specifically, we have combined scanning electrochemical microscopy (SECM) and laser beam induced current (LBIC) to simultaneously obtain maps of external quantum efficiency (EQE) and catalytic activity with resolution down to 1 mu;m. These measurements reveal the unexpected occurrence of H2(g) evolution off of the SiO2 surface, and suggest the occurrence of a H-spillover assisted hydrogen evolution reaction (HER) mechanism. This finding has important implications for the future development of MOS photoelectrodes, including the possibility of significantly reduced loadings of expensive precious metal catalysts.
References
[1.] H.J. Lewerenz, et al., Electrochem. Acta, 2011, 56, 10726.
[2.] A.G. Munoz and H.J. Lewerenz, ChemPhysChem, 2010, 11, 1603.

Natural light harvesting organisms have evolved to harvest sunlight efficiently. Amongst the various light harvesting complexes, chlorosomes are unique because they consist of assembled bacteria chlorophyll dyes with a minimal protein network. Green sulfur bacteria, which contain chlorosomes, can survive in extremely low light conditions mainly because of efficient light absorption and transfer of energy, facilitated by the assembled dye molecules. Due to these reasons, chlorosomes have been used in dye sensitized solar cells to improve the light absorption and performance.¹
The chlorosome absorption spectrum is fixed and their size is dependent on the organism. In order to tailor the properties, mimics of chlorosomes with artificial dyes are created. Various solution-based techniques have been developed for synthesizing mimics by supramolecular self-assembly.^{2, 3} However, controlling the size of agglomerates and their subsequent deposition on surfaces to fabricate a device is difficult. In this work, a one-step aerosol-based self-assembly technique has been developed for the first time, to assemble and deposit chlorin (Bacteria chlorophyll mimics) agglomerates. The monomers of chlorin dye are dissolved in a volatile solvent. This monomer solution is atomized by electrospray to form monodispersed droplets. The volatile solvent evaporates, thus resulting in an increased concentration of monomers. The increased concentration of monomers promotes self-assembly in the evaporating droplets. The self-assembled agglomerates formed are deposited on a transparent, conducting substrate. The red shift in peak absorbance of the agglomerates is measured using UV-visible spectrophotometer to ascertain the assembly. Another approach that will be discussed is the use of two solvents with differing degrees of volatility. A judicious choice of the solvents in the aerosolized droplets will thus also promote the self-assembly of the monomer. The presentation will discuss the results obtained by varying a range of parameters in the experimental approach to elucidate the self-assembly process in size-controlled aerosolized droplets.
Future work will study the deposition of multiple layers and chromophore mixtures to mimic the excitation energy donor and acceptor functionality found in clorosomes. The energy transfer between the assembled dyes and receptor dyes will be studied by time resolved spectroscopy. These self-assembled structures and a receptor dye layer will then be deposited on columnar TiO2 structures to fabricate a dye sensitized solar cell.
1. Modesto-Lopez, L. B.; et. al., Energ Environ Sci 2010, 3, (2), 216-222.
2. Mass, O.; et. al., J. S. New Journal of Chemistry 2011, 35, (11), 2671-2690.
3. Miyatake, T.; Tamiaki, H. Coordination Chemistry Reviews 2010, 254, (21-22), 2593-2602.

The production of carbon-neutral energy is an imperative emergence of our century that is associated to the projected shortage of fossil fuels, the increase of petrol barrel price and the environmental burden that such massive exploitation of natural resources is imposing to present and future generations.[1-2] Research in the field of artificial photosynthesis for conversion of water to fuel is becoming more and more important as the key issue to the design of efficient catalytic routines that operate with energy and rates commensurate with the solar flux. We report herein the unprecedented deposition of discrete multi-transition metal-oxo polyanionic clusters, i.e. polyoxometalates (POM), on carbon nanotustructure surface (i.e. carbon nanotubes), by the templating electrostatic effect of the charged groups of positively functionalized carbon nanostructures. In particular, immobilized on positively charged carbon nanostructures, polyoxometalate catalysts enable water oxidation at low overpotential and with high efficiency in terms of both rate and productivity.[3] Carbon nanostructures promising in the context of water splitting functional systems due to their electron-acceptor/charge separation character, large surface area, and stability.[4-5] Our vision points to a careful choice/design of the nano-structured support, and to a precise positioning of the catalytic domain on such templates, by tailored synthetic protocols. This is a key point to access single-site catalysis approaching the homogeneous behavior.
[1] N. Armaroli, V. Balzani, ChemSusChem 2011, 4, 21-36.
[2] H. B. Gray, Nature Chem. 2009, 1, 7.
[3] A. Sartorel, M. Carraro et al., J. Am. Chem. Soc, 2008, 130, 5006.
[4] F. M. Toma, A. Sartorel, et al., Nat. Chem., 2010, 2, 826-831
[5] F.M. Toma, A. Sartorel, ChemSusChem, 2011, 4, pp 1447-1451

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. In this talk, I will highlight several recent examples in this lab using semiconductor nanowires and their heterostructures for the purpose of direct solar water splitting.

Plant cellular structures found in leaves represent a unique and diverse source of renewable materials templates. They are easily produced and contain intricate microscale morphologies with interconnected porous cellular and vascular subunits and structural reproducibility within specific types or classes of plants. A major challenge in utilizing living botanical structures as materials templates is that they are filled with water and conventional dehydration strategies often collapse or degrade the intricate botanical structure. This restricts the ability to introduce water reactive precursors into such structures. We have developed a room-temperature chemical method using acidified 2,2-dimethoxypropane to dehydrate water-rich botanical materials (e.g., fern leaves and water-rich jade succulents). This mild dehydration process leaves much of the porous cellular leaf structure intact even with ~90% mass loss. These chemically dehydrated templates have been utilized in the growth of porous and ordered leaf replicate structures consisting of TiO2 and SiO2 via sol-gel precursor impregnation methods. These white oxide products externally and internally (cellular structures) look nearly identical but shrunken intact versions to the original leaf. This presentation will detail the chemical, thermal, and absorption properties of the dried porous templates and porous oxide products and describe recent efforts to use these botanical templates to produce other porous metal oxides (e.g., Co3O4, NiO, and CuO) using both halide and acetate precursor impregnation strategies. The functionalization of porous titania structures with platinum nanoparticles and their use in photocatalytic chemical transformations will also be described. Other porous metal oxides with interconnected pore walls may have use in electrochemical energy storage systems.

Although photosynthesis has occurred for millions of years in every chloroplast within the dimension of several microns, artificial photosynthesis has not been successful to realize similar delicate spatial control in the same length scale. Herein it is demonstrated that a nanotree all-inorganic heterostructure is capable to realize solar-driven water-splitting without any external bias. Using two types of semiconductor nanowires in every nanotree structure to mimic the Z-scheme of photosynthesis, the artificial nanotree strucutre demonstrates the capability to spatially fine-tune the processes of photogeneration, charge separation, and chemical reactions within the length scales of microns. It also would serve as a building block for fully integrated solar-energy conversion membrane with product separation capability and enhanced energy conversion efficiency.

Diamond electrodes have attracted considerable attention for their use in applications requiring stable operation in harsh conditions a large electrochemical potential window or optically transparent electrodes. More recently, carbon electrodes have been proposed as an alternative to metal electrodes in hybrid systems for energy harvesting. For this application, the electrode material is expected to offer a suitable surface to immobilize organic redox groups, proteins or enzymes, and to exhibit an efficient charge transfer to them. In addition, the optical electrode transparency is a desirable requirement for these applications. Metal electrodes, and in particular Au electrodes have been shown to exhibit a fast charge transfer in electrolytic environments, but at the same time they are not able to form stable bonds with organic or bioorganic molecules. Diamond electrodes, in contrast, offer a suitable surface for chemical modification but the lower density of states is expected to considerably affect the efficiency of electron transfer across diamond surfaces. In the case of bio-hybrid systems for energy harvesting, typically composed of photosynthetic proteins immobilized on the electrode&’s surface, such a reduced electron transfer would be detrimental for the generated photocurrent.
In this work, we directly compare the immobilization of photosynthetic reaction centers (RCs) on Au thin films as well as on boron-doped diamond,in view of direct charge transfer under photo-excitation for metallic and semi-conductive electrodes. A voltage-dependent current response was observed under photo-stimulation of RCs confined onto the different surfaces, enhanced by the presence of cytochrome C and coenzyme Q0 in solution. The role of these two species in the charge transfer is similar to the role they play in the natural environment of RCs. Thus, cytochrome C shuttles the low-energy electrons from the electrode to the RCs P-sides while Q0 is responsible for extracting the high energy electron from the Q-side of the RCs and shuttling it in the electrolytic solution. A deeper insight into these processes is provided by studying the photocurrent-signal as a function of the concentration of these two species in solution, in order to investigate the processes taking place at the interface between RCs and electrodes as well as within the species in the electrolytic solution. In addition, our work aims at discussing issues like the experimentally observed dependence of the photocurrent-signal on the voltage applied between references and working electrodes. Finally a complete electrochemical characterization will be presented, comparing both electrode materials in terms of surface coverage, specific interactions between molecules and substrate and electron transfer rate constants.

Energy transfer from the defect state of zinc oxide nanoparticles to the fluorescent dye AlexaFluor 594 (A594) cadaverine has been studied using both steady-state and time- resolved photoluminescence (PL) measurements. The addition of five stoichiometric equivalents of A594 cadaverine completely quenches the visible defect emission from zinc oxide nano crystals. We also find that the entire defect emission of ZnO is reduced without any change in the overall line shape of the emission, demonstrating that the defect emission is from a single electronic state coupled to the phonon modes of the crystal lattice. The energy transfer is modeled using the dynamic quenching model developed by Tachiya (Sadhu, S.; Tachiya, M. J. Phys. Chem. 2009, 113, 19488 - 19492). Remarkably, there is very efficient energy transfer when there is just one adsorbed dye molecule per nanocrystal, regardless of the orientation of the dipole moment of the cadaverine molecule and the distance to the defect state.

Photovoltaic (PV) cells are the most promising man-made devices for direct solar energy utilization. Understanding the charge separation and charge transport in PV materials is crucial for improving the efficiency of the solar cells. Advanced electron paramagnetic resonance (EPR) spectroscopy, especially light-induced multi-frequency EPR has been essential for understanding the mechanisms of the light-induced generation, separation, and recombination of the charge carriers in natural photosynthesis. Here, we use light-induced EPR spectroscopy combined with DFT calculations to study mechanisms of charge separation and charge stabilization in active organic PV materials based on the composites of multiple polymers (P3HT, PCDTBT, and PTB7) and fullerene derivatives (C60-PCBM and C70-PCBM). Time-resolved EPR spectra show strong polarization pattern for all polymer-fullerene blends under study, which is caused by non-Boltzmann population of the electron spin energy levels in the radical pairs. The detection of these polarization patterns allows us to make a comparison with charge separation processes in molecular donor-acceptor systems, as found in natural and artificial photosynthetic centers. In these type of systems the polarization spectra of the charge separated states were observed for the first time and described within the models of spin-correlated radical pairs (SCRP) and sequential electron transfer. The polarization pattern of the SCRP in polymer-fullerene blends allows us to describe the charge separation process like electron jumps or tunneling between neighboring fullerene molecules. The first step of the charge separation process is exciton dissociation and electron transfer to the fullerene molecule neighboring to the polymer. In order to outcompete the recombination process this state cannot live longer than a few picoseconds. Forward electron transfer forms an intermediate radical pair, with the separation distance within 15-20 Å. The third step is electron transfer to the secondary radical pair with a separation of 25-30 Å which is stable for tens-hundreds microseconds. The following electron transfer steps are on the slower time-scale and lead to further charge separation (in the case of organic PV devices - to the current generation) or charge recombination. The analysis presented here helps us to improve our understanding of to the mechanism of charge separation processes in the active organic photovoltaic materials.
This work was supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences, and Biosciences, under Contract DE-AC02-06CH11357.

Currently, the most efficient material for photoelectrochemical (PEC) solar water splitting is GaInP2 at 12.4 % solar-to-hydrogen (STH) conversion. This material is usually grown by metal-organic chemical vapor phase deposition on GaAs substrates for lattice matching. Despite its high efficiency, this material presents challenges for practical PEC applications due to corrosion under operating conditions, thus preventing long-term use. More recently, a nitrogen ion treatment (“N-treatment”) of GaInP2 has been shown to prevent this corrosion while retaining the high efficiency (> 10% STH). The desire to optimize and understand the impact of this treatment has motivated the characterization experiments in this contribution.
We report the results of electronic and chemical structure characterization experiments performed using a ‘tool-chest&’ of experimental techniques on N-treated GaInP2 epitaxial thin films. The electronic structure of the surface is probed by X-ray photoelectron spectroscopy (XPS) before and after removal of surface adsorbates using low-energy (50 eV) Ar+ ion treatments. Also, the amount of N atoms at the surface is larger in samples for which the corrosion treatments proved successful. Additional techniques, probing the surface valence band maximum (UV photoelectron spectroscopy - UPS), conduction band minimum (inverse photoemission spectroscopy - IPES), and surface bandgap (by combining UPS and IPES), have been employed. As a function of Ar+ ion treatment time, we find a significant reduction of the electronic surface bandgap, which will be discussed in view of the presence/removal of surface adsorbates and potential lateral surface inhomogeneities of the GaInP2 thin film surface. While the bulk bandgap is important for the absorption of the solar spectrum, the surface bandgap and the position of the band edges is critical for the water splitting reactions. Thus, this finding might be crucial to overall device performance.
Synchrotron-based X-ray techniques (in particular X-ray emission and absorption spectroscopy) were used to probe the near-surface bulk properties of these materials. Together with the surface-sensitive XPS results, this provides a non-destructive “depth-resolved” picture of the material. We find that significantly more N is implanted into the bulk for successful N-treatments, even exceeding the above-mentioned enhancement at the surface. This N is shown to be chemically bound to Ga and/or In and in the same electronic state regardless of the efficacy of the treatment. Also, the effect of this N on the local electronic structure around the P atoms is investigated. Finally, morphological changes in the structure are examined using a combination of atomic force and scanning electron microscopy, which help us to paint a comprehensive picture of the electronic, chemical, and morphological properties of the surface. This picture will be interpreted with respect to device STH efficiency and corrosion protection.

This presentation describes the fabrication, via immobilisation of photosynthetically active entities within silica materials, of photobiochemical leaf-like nanobio hybrid materials capable of the energy conversion as the principal component of a photobioreactor [1-14] and a biofuel cell [15, 16].
The photosynthetic activity shows that the material was able to produce oxygen for over a month. The photochemical material was also able to reduce CO₂ into carbohydrates. A part of these photosynthates were excreted into the aqueous phase contained within the pores of silica. By a simple extraction method, these products could be recovered. The molecules excreted by the material were mainly polysaccharides composed of rhamnose, galactose, glucose, xylose and mannose units. Considering that the quantity of sugars increased as a function of time, this photosynthetic material holds much promise in the development of new, green chemical processes. For instance, atmospheric CO₂ could be strategically exploited via this kind of artificial leaf-like materials, as a source of carbon to produce valuable compounds or biofuels while the active biomass is continuously reused. These results constitute a significant advance towards the final goal, long-lasting semi-artificial photobioreactors and biofuel cells that can advantageously exploit solar radiation to convert polluting carbon dioxide into useful biofuels, sugars or medical metabolites [1-14] and electricity [15, 16].
[1] J. C. Rooke, A. Léonard, H. Sarmento, J. P. Descy and B. L. Su, J. Mater. Chem., 2008, 18, 2833.
[2] J. C. Rooke, A. Léonard and B. L. Su, J. Mater. Chem., 2008, 18, 1333.
[3] J. C. Rooke, C. Meunier, A. Léonard and B. L. Su, Pure Appl. Chem., 2008, 80, 2345.
[4] C. F. Meunier, P. Van Cutsem, Y. U. Kwon and B. L. Su, J. Mater. Chem., 2009, 19, 4131.
[5] C. F. Meunier, P. Van Cutsem, Y. U. Kwon and B. L. Su, J. Mater. Chem., 2009, 19, 1505.
[6] C. F. Meunier, J. C. Rooke, A. Léonard, P. Van Cutsem and B. L. Su, J. Mater. Chem., 2010, 20, 929.
[7] J. C. Rooke, A. Léonard, C. F. Meunier, H. Sarmento, J. P. Descy and B. L. Su, J. Colloid Interface Sci., 2010, 344, 348.
[8] C.F. Meunier, J.C. Rooke, K. Hadju, P. Van Cutsem, P. Cambier, A. Léonard and B. L. Su, Langmuir, 2010, 26, 6568
[9] C. F. Meunier, Ph. Dandoy and B. L. Su, J. Colloid. Interface Sci., 2010, 342, 211.
[10] A. Léonard, J. C. Rooke, C. F. Meunier, H. Sarmento, J. P. Descy and B. L. Su, Energy Environ. Sci., 2010, 3, 370.
[11] C. F. Meunier, J. C. Rooke, A. Léonard, H. Xie and B. L. Su, Chem. Commun. 2010, 46, 3843
[12] B. L. Su, Q. J. Zhang, D. Bonifazi and J. L. Li, ChemSusChem, 2011, 4, 1327
[13] A. Leonard, Ph. Dandoy, E. Danloy, G. Leroux, J. C. Rooke, Ch. F. Meunier and B. L. Su,
Chem. Soc. Rev., 2011, 40, 860
[14] J. C. Rooke, A. Leonard, Ch. F. Meunier and B. L. Su, ChemSusChem, 2011, 4, 1249
[15] Ch. F. Meunier, X. Y. Yang and B. L. Su, ChemCatChem, 2011, 3, 476
[16] X. Y. Yang, G. Tian, N. Jiang and B. L. Su, Energy Environ. Sci., 2012, 5, 5540

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. Researchers have found that plasma treated or plasma grown hematite films are with improved photoelectrochemical properties. Our works aims to investigate the effect of plasma treatment on nanostructured hematite films prepared by dip coating procedure in our lab, and to uncover the mechanism under it. Photoelectrochemical measurements shows that, in contrast to the samples without post surface treatment, the plasma treated hematite samples displayed significantly changed J-V characteristics with increased dark current density and the onset potentials are andoically shifted by around 200 mV. The decreased photocurrent density and shifted onset can be interpreted by enhanced surface states (recombination centers) raised from defects by plasma or by higher electron density in conduction band due to increased O vacancies. The corresponding changes in the electronic structure are monitored with X-ray photoelectron spectroscopy (XPS). Systematic changes in the oxygen and iron core elevel spectra as well as in the valence band spectra are observed. Particularly the oxygen spectra show characteristic and significant changes depending on the duration of the plasma treatment.

Nickel oxide (NiO) has attracted attention as an electrode material for electrochromic devices, supercapacitors, and Li-ion batteries [1-4]. Because NiO displays p-type conductivity, it has also been used as a photocathode material in tandem photoelectrochemical (PEC) dye-sensitized cells [5]. There has been much effort to improve the conversion efficiency by fabricating films with improved properties and developing more effective sensitizers for p-type NiO. One of the factors limiting the use of NiO for PEC sensitized cell applications is the low hole conductivity in p-NiO. In this presentation, we report a general approach for synthesizing NiO-CdS core-shell nanoparticle films as photocathodes for p-type semiconductor-sensitized PEC cells. The conformal semiconductor layer served several functions: it sensitizes the NiO to visible light, transports the photogenerated holes away from the hole injection site, and suppresses recombination on the NiO surface by passivating NiO surface states. The morphology, crystal structure and composition of the core-shell electrodes were characterized by transmission electron microscopy and X-ray diffraction. Hole transport and recombination kinetics were characterized by using intensity modulated photocurrent and photovoltage spectroscopy. Compared to dye-sensitized NiO cathodes, the CdS-NiO photocathodes exhibited two orders of magnitude faster hole transport and charge-collection efficiencies approaching 100%. These results and others are discussed.
References
[1] Purushothaman, K. K.; Muralidharan, G. J. Sol-Gel Sci. Technol., 2008, 46, 190.
[2] Varghese, B.; Reddy, M. V.; Yanwu, Z.; Lit, C. S.; Hoong, T. C.; Rao, G. V. S.; Chowdari, B. V. R.; Wee, A. T. S.; Lim, C. T.; Sow, C. H. Chem. Mat., 2008, 20, 3360.
[3] Kim, J. H.; Kang, S. H.; Zhu, K.; Kim, J. Y.; Neale, N. R.; Frank, A. J. Chem. Commun., 2011, 47, 5214.
[4] Ellis, B. L.; Lee, K. T.; Nazar, L. F. Chem. Mat., 2010, 22, 691.
[5] Nattestad, A.; Mozer, A. J.; Fischer, M. K. R.; Cheng, Y. B.; Mishra, A.; Bauerle, P.; Bach, U. Nat. Mater., 2010, 9, 31.

A naturally abundant nickel-borate (Ni-Bi) complex is demonstrated to successfully catalyze the photoelectrochemical (PEC) water oxidation of BiVO4 electrodes at 1.23 VRHE with nearly 100% faradaic efficiency for oxygen evolution. Ni-Bi is electrodeposited (ED) and photodeposited (PD) with varying times on BiVO4 electrodes in 0.1 M borate electrolyte with 1 mM Ni2+ at pH 9.2. Surprisingly, optimally deposited Ni-Bi films (ED-10 sec & PD-30 min) display the same layer thickness of ca. 40 nm. Both Ni-Bi films enhance the photocurrent generation of BiVO4 at 1.23 VRHE by a factor of 3 - 4 under AM 1.5-light (100 mW/cm2) along with a nearly 200% increase in the incident and absorbed photon-to-current efficiencies. Impedance analysis further reveals that the charge transfer resistance at BiVO4 is markedly decreased by Ni-Bi deposits. The primary role of Ni-Bi has been suggested as a hole-conductor making photogenerated electrons more mobile and catalyzing a four-hole transfer to water through cyclic changes between the lower and higher Ni oxidation states. Beyond the optimal thickness of Ni-Bi, however, the PEC performance of BiVO4 decreases even to almost zero due to the kinetic bottleneck and charge recombination by thick Ni-Bi films. Under an identical PEC condition (0.1 M, pH 9.2), borate electrolyte (good proton acceptor) is found to be better than nitrate (poor proton acceptor), indicative of a proton-coupled electron transfer pathway in PEC water oxidation.

Photoelectrochemical (PEC) water splitting has attracted significant attention recently due to the potential of converting solar to chemical energy, in the form of hydrogen and oxygen. Much of these efforts have been focussed on metal oxide semiconductors, mainly due to their good stability in aqueous solutions, easy synthesis, and low cost. However, many metal oxides suffer from having poor carrier separation. This is also the case for bismuth vanadate (BiVO₄), one of the most promising metal oxide photoanodes. In the last two years, along with the development of novel water oxidation catalysts, the photoelectrochemical performance of the material has been significantly improved.^1-5 Once the limitation of the slow water oxidation kinetics is removed, the material is limited by poor carrier separation efficiency (< 50%), which is presumably due to the low intrinsic carrier mobility of BiVO₄.¹
In this study, we show a new and original approach for enhancing carrier separation in PEC materials by the application of gradient concentration of tungsten (W)—which acts as a donor type dopant—to create a homojunction in the BiVO₄ photoanode. This dopant concentration difference creates a shift in the Fermi energy level that introduces additional band bending and better carrier separation. As a result, the carrier separation efficiency increases from 50% to 80%, and an AM1.5 photocurrent of 3 mA/cm² is achieved at 1.23 V vs reversible hydrogen electrode (RHE) for the Co-Pi-catalyzed gradient-doped W:BiVO₄ photoanode. When combined with a double-junction amorphous silicon solar cell, this device results in a solar-to-hydrogen (STH) efficiency of ~4.3 %, which represents the highest efficiency achieved for metal oxide photoanodes so far. While this is slightly slower than the recent 4.7% efficiency benchmark reported by Nocera et al.,⁶ it is achieved using a double-junction a-Si solar cell, i.e. we have replaced four layers of the triple-junction water splitting device (p-i-n and TCO layers) with one single oxide layer.
References
[1] F. F. Abdi, R. van de Krol, J. Phys. Chem. C, 2012, 116, 9398.
[2] W. J. Luo, et al., Energy Environ. Sci. 2011, 4, 4046.
[3] S. K. Pilli, et al., Energy Environ. Sci. 2011, 4, 5028.
[4] J. A. Seabold, K. S. Choi, J. Am. Chem. Soc. 2012, 134, 2186.
[5] D. K. Zhong, S. Choi, D. R. Gamelin, J. Am. Chem. Soc. 2011, 133, 18370.
[6] S. Y. Reece, J. A. Hamel, K. Sung, T. D. Jarvi, A. J. Esswein, J. J. H. Pijpers, D. G. Nocera, Science 2011, 334, 645.

Of the various transition metal oxides that show promise in serving as both robust and efficient anodes for the photocatalytic oxidation of water, spinel cobalt oxide (Co₃O₄) has emerged as a particularly potent water oxidation catalyst, whose advantages include high catalytic turnover rates, earth abundance, and a band gap with large extinction coefficient in the visible region of the solar spectrum, suggesting its potential use as both catalyst and light absorber. Yet, the development of an optimized Co₃O₄-based photoanode requires detailed knowledge of the evolution of the charge carrier dynamics following photoexcitation. In this work, the first of its kind on this material, we dissect the temporal evolution of the complex index of refraction of polycrystalline spinel cobalt oxide upon photogeneration of free carriers by measuring the optical pump-induced transient changes in transmittance and reflectance over a broad range of pump and probe wavelengths on timescales ranging from a few picoseconds to hundreds of nanoseconds, and over excitation densities spanning three orders of magnitude. We discuss the observed carrier dynamics in terms of the mechanism underlying electron-hole recombination in the presence of multiple band gaps in this material and their relevance in regards to competition with interfacial electron transfer.

Photoelectrochemical processes for water splitting and CO₂ reduction (artificial photosynthesis) have been under intensified study as a promising path to solar fuel production. The use of co-catalysts in conjunction with photoactive materials has proven to be crucial for high energy conversion efficiency and in some cases also protects light-absorbing materials from photocorrosion. Previously known examples of such systems typically involved the formation of hetero-structured co-catalyst thin films or islands on top of photoactive materials. The feasibility of an alternative configuration, consisting of evenly distributed catalysis sites in close vicinity of light absorbing sites, remains yet to be investigated. Towards the verification of this concept, we selected transition metal tungstates as the model system and examined their catalytic properties for electrochemical and photochemical water oxidation. Initial screening tests showed that, among multiple MWO₄ (M = Mn, Fe, Ni, Co, Cu, Zn, and Cd), cobalt tungstate (CoWO₄) was by far the most active water oxidation catalyst. Further investigation revealed that CoWO₄ catalyst powders synthesized via co-precipitation were amorphous nanoparticles, which could be crystallized by post-synthesis thermal treatment. Diffusive reflectance measurement showed that CoWO₄ had a direct band gap of 2.7 eV and an indirect band gap of 1.4 eV. Crystallographic structure appears to play an important role on the catalytic activity of CoWO₄ for electrochemical water oxidation. In close to neutral electrolyte (0.2 M Na₂WO₄, pH 8.0), amorphous CoWO₄ exhibited much higher catalytic activity than its crystalline counterpart over an overpotential range of 0.25 to 0.45 V. Calculated Tafel slopes are ~ 60 mV/decade for amorphous and ~110 mV/decade for crystalline electrodes, while the exchange current density of crystalline catalysts is two orders of magnitude higher than the amorphous. At ~ 400 mV of overpotential, electrodes using amorphous CoWO₄ as catalyst generated ~1 mA/cm² of steady state current.⁽¹⁾ Our latest study discovered that CoWO₄ was also highly active for [Ru(bpy)₃]²⁺-assisted photochemical water oxidation in solution. In the presence of Na₂S₂O₈ as a sacrificial electron acceptor, the turnover frequency of amorphous CoWO₄ is estimated in the order of 10 s^-1, comparable to the best known water oxidation catalysts such as IrO₂.^(2,3) Additional discussions in this presentation will cover various results from analytical and computational studies that elucidate the different reaction mechanisms on the surface of the amorphous and crystalline catalysts.
(1) H. Jia, J. Stark, L. Q. Zhou, C. Ling, T. Sekito, Z. Markin. RSC Adv. 2012, 2(29), 10874-10881
(2) M. Yagi, E. Tomita, S. Sakita, T. Kuwabara, K. Nagai. J. Phys. Chem. B. 2005, 109, 21489-21491
(3) M. Hara, J. T. Lean, T.E. Mallouk. Chem. Mater. 2001, 13, 4668

Water oxidation (WO) to molecular dioxygen and its inverse reaction oxygen reduction (OR) to water, are the two key reactions involved in water splitting and hydrogen fuel cells respectively. The mastering of both reactions WO and OR catalyzed by Transition Complexes (TMC) is an essential requirement for their potential applications in wide spread energy applications. In both cases the O-O bond formation step is a crucial step that needs to be fully understood as well as all the previous activation pathways. Additionally the oxidation of water to molecular oxygen is also a reaction of interest from a biological perspective since it is the main reaction occurring at the Oxygen Evolving Center of Photosystem II (OEC-PSII), A few Ru and Co complexes have been described recently that are capable of catalyzing the water oxidation and the oxygen reduction reactions, and their performances have been shown to be strongly dependent on, nuclearity, oxidation state and ligand topology.[1] A step forward in the field consists on unraveling the different reaction pathways trough which these reactions proceed. We have tackled this challenging topic by carrying out thorough electrochemical, spectroscopic and kinetic analysis together with O-18 labeling studies and DFT calculations. The combination of all these results gives evidence for mechanisms involving: intramolecular O-O bond formation and breaking, water nucleophilic attack and bimolecular O-O bond formation.[2] From the reduction side we have also develop a few CO2 reduction catalysts as well as proton reduction catalysts, that will also be described. In addition new functionalized materials have been created by heterogenization of catalysts into solid supports that behave as active materials. Overall a whole range of new molecules and materials of interest for artificial photosynthesis will be covered.
REFERENCES
[1] (a) Sala, X.; Rodriguez, M.; Romero, I.; Escriche, L.; Llobet, A. Angew. Chem. Int. Ed. 2009, 48, 2842. (b) Romain, S.; Vigara, L.; Llobet, A. Acc. Chem. Res. 2009, 42, 1944. (c) Fukuzumi, S.; Mandal, S.; Mase, K.; Ohkubo, K.; Park, H.; Benet-Buchholz, J.; Nam, W.; Llobet, A.J. Am. Chem. Soc. 2012, 134, 9906.
[2] (a) Romain, S.; Bozoglian, F.; Sala, X.; Llobet, A., J. Am. Chem. Soc. 2009, 131, .2768. (b) Bozoglian, F.; Romain, S.; Ertem, Cramer, C. J.; Gagliardi, L.; Llobet, A. et al. J. Am. Chem. Soc. 2009, 15176. (c) Sartorel, A.; Miroacute;, P.; Llobet, A.; Bo, C.; Bonchio, M. et al. J. Am. Chem. Soc. 2009, 16051. (d) Sala, X.; Ertem, M. Z.; Cramer, C. J.; Gagliardi, L.; Llobet, A. et al. Angew. Chem. Int. Ed. 2010, 49, 7745. (e) Duan, L.; Bozoglian, F.; Privalov; T.; Llobet, A., Sun, L. et al. Nat. Chem. 2012, 3, 2576. (f) Rigs, S.; Mandal, S. Nam, W.; Llobet, A., Stahl, S. s. Chem. Sci. 2012, in press. (g) Maji, S.; Vigara, L.; Cottone, F.; Bozoglian, F.; Benet-Buchholz, J.; Llobet, A. Angew. Chem. Int. Ed. 2012, 51, 5967-5970.

GaInP2 is a promising photocathode material for solar hydrogen production if its corrosion problem can be resolved. Recently our collaborators succeeded in improving the corrosion resistance of GaInP2 photocathodes by N2+ bombardment. In order to better understand the origin of the protection mechanism, we performed first-principles X-ray emission calculations of Nitrogen K-edge, which were compared to experimentally measured spectra. It turns out that the electronic state of incorporated nitrogen couples with both localized and delocalized states, and that it varies depending on its chemical environment. In particular, when nitrogen forms bonds with anions such as P and N, new peaks rise up at the low-energy side. These are attributed to localized states and would not be visible in spectral feature derived from N-metal bonds. These peaks may explain observed features in the experimental spectra in the low-energy regime once appropriate energy dependent lifetime broadening is applied. We will also discuss how P L2,3 emission edge changes its shape when N-P bonds exist.

Production of clean energy from renewable resources remains one of the greatest challenges for society. Hydrogen is considered a promising energy carrier because of its high energy density. One of the most economical ways to harness hydrogen is from the photocatalytic overall water-splitting reaction (OWS) using solar energy, which does not leave a carbon footprint. During the overall splitting of water, the hydrogen evolution reaction (HER) should take place efficiently, which requires a catalyst. Platinum-group metals (Pt, Rh, Pd, etc.) are excellent HER electrocatalysts. However, it remains challenging to develop highly active HER cocatalysts that have a low cost and utilize abundant materials. Moreover, to achieve overall water splitting, the surfaces of the catalysts should also be insensitive to the back-reaction of the produced H₂ and O₂ that produces H₂O, i.e., the oxygen reduction reaction (ORR). Photocatalytic overall water splitting in a particulate type of system has been achieved only on metal/metal oxide surfaces with a core-shell structure. Tungsten carbide exhibits platinum-like behavior, which makes it an interesting potential substitute for noble metals in catalytic applications. In this study, tungsten carbide nanocrystals (~5 nm) are successfully synthesized through the reaction of tungsten precursors with mesoporous graphitic carbon nitride (mpg-C₃N₄) as the reactive template in flowing inert gas at high temperatures. Systematic experiments that vary the precursor compositions and temperatures used in the synthesis selectively generate different compositions and structures for the final nano-carbide (W₂C or WC) products. Electrochemical measurements demonstrate that the WC phase with a high surface area exhibits both high activity and stability in hydrogen evolution over a wide pH range. The WC sample also shows excellent hydrogen oxidation activity, whereas its activity in oxygen reduction is poor. Hence, makes it a potential cocatalyst candidate for photocatalytic water splitting. These tungsten carbides (WC) are successful cocatalysts for overall water splitting and give H₂ and O₂ in a stoichiometric ratio from H₂O decomposition when supported on a Na-doped SrTiO₃ photocatalyst. We present tungsten carbide (on a small scale) as a promising and durable catalyst substitute for platinum and other scarce noble-metal catalysts in catalytic reaction systems used for renewable energy generation; as a cathode catalyst in water electrolysis, as an anode catalyst in PEFC and as a dual-role cocatalyst in overall water splitting.

Photoelectrochemical (PEC) water splitting relies crucially on the stability of a photoactive material in an aqueous environment in concert with effective and stable catalysts. In order to enable widespread implementation of this technology for global scale solar energy conversion, low cost materials prepared from abundant elements will be required. Cuprous oxide (Cu2O) is one such material that is quite promising due to the abundance of copper, suitable bandgap, and favorable band alignments for reducing water and carbon dioxide. In aqueous media, a protective overlayer is required to prevent corrosion of the Cu2O. We identified that a key source of instability in our composite photocathode structures lies at the interface between the overlayer and the catalyst nanoparticles. Thus, we have studied the properties of the protective overlayers of our structures and investigated different catalysts (as well as different deposition techniques for these catalysts). These studies have led to vastly improved stability of the photocathodes: typically, the chronoamperometric stability measurements are now carried out on the order of tens of hours instead of tens of minutes, as in our previously published work. Moreover, the sustained photocurrents that are obtained correspond to 7% solar-to-hydrogen efficiency in a tandem cell configuration, where the bias is provided by a photovoltaic device (i.e. a dye-sensitized solar cell).

Low-cost alternatives to platinum catalysts for hydrogen generation are in great need for artificial photosynthesis. A promising candidate material is molybdenum disulfide (MoS2), a layered crystal with experimentally verified catalytically active edges [1][2]. The identification of active sites provides an attractive opportunity to enhance the catalytic activity by engineering these materials to maximally expose these active sites. In this study, we successfully synthesized polycrystalline MoS2 and molybdenum diselenide (MoSe2) films with nearly vertically aligned crystal layers that fully expose the edge sites across the entire substrate. These films exhibit the highest packing density of active sites. As a result, the catalytic activity improves greatly and correlates directly with the density of exposed edge sites. Layered chalcogenide films with high-density edge sites offer a new avenue to generate highly efficient hydrogen evolution catalyst.
[1] B. Hinnemann et al., Journal of the American Chemical Society 127, 5308 (2005).
[2] T. F. Jaramillo et al., Science 317, 100 (2007).

Recently, Reece et al. [1] described a triple-junction thin film silicon solar cell equipped with non-precious metal catalysts at the front and back contact of the thin film device to demonstrate water splitting under illumination after immersing the cell into an aqueous (pH7) solution. The disadvantage of this structure however are shadowing effects of the incident light by the catalyst deposited on top of the front layer of the solar cell. To overcome this drawback in our approach we have tested triple-junction silicon solar cells in superstrate configuration as an alternative [2]. Here the solar cell is illuminated from the transparent glass substrate side of the thin film a-Si/µc-Si solar cell so that the front and back contact are accessible to the electrolyte without any shadowing effects. We proofed this concept of solar water splitting by depositing RuO2 and platinum nano particles for the OER and HER. Furthermore we have functionalized these contacts by a novel cathode catalyst layer evolving hydrogen. The catalyst used in our case consisted of carbon supported nano-scaled MoS2 particles embedded in a conductive polymer matrix.
The catalysts have been prepared by impregnating different carbon supports (carbon nanotubes, graphene oxide etc.) by a wet chemical process. The carbon supported molybdenum sulphide particles were characterized by X-ray diffractometry, scanning electron microscopy, Raman and X-ray photoelectron spectroscopy. To obtain inside into the morphology of the catalyst cross section transmission electron micrographs were taken showing a complex intergrowth of the MoS2 particles with the graphene layers of the substrate. The electrochemical activity towards hydrogen evolution was verified by electrochemical mass spectroscopy analysis. The missing of a H2S signal let us conclude that no or marginal corrosion processes occur at this electrodes.
In our contribution the catalytic stability and activity will be discussed as a function of the sample morphology.
First results on the stability of the hydrogen evolving catalyst as part of the light-induced water splitting device will be presented.
References:
[1] Steven Y. Reece, Jonathan A. Hamel, Kimberly Sung, Thomas D. Jarvi, Arthur J. Esswein,
Joep J. H. Pijpers, Daniel G. Nocera, SCIENCE, 2011, 334, 645-648.
[2] Bernd Kaiser, Wolfram Jaegermann, Sebastian Fiechter, Hans Joachim Lewerenz, Bunsen-Magazin, 2011, 13, 104-111.

Thin film hematite photoanodes fabricated by atomic layer deposition (ALD) were further modified with cobalt oxide, also by ALD. The addition of only 1 ALD cycle, <1 monolayer of CoO, resulted in significantly enhanced photoelectrochemical performance towards water oxidation. A cathodic shift of 100-150 mV in the photocurrent onset potential was observed for both flat and nanostructured inverse opal scaffold geometries. The photocurrent of the CoO-coated inverse opal scaffold reached 0.81 and 2.1 mA/cm2 at 1.23 and 1.53 V, respectively, representing one of the best hematite photoanodes with Co-based catalysts. Electrochemical impedance spectroscopy is employed to rationalize the improvement in performance. Furthermore, the optical transparency in the visible region and long-term stability of the ultrathin CoO layer render it a uniquely advantageous catalyst for water oxidation.

Current photocatalysts suffer from low efficiency in converting solar energy directly to chemical fuels. By breaking CoO micro-powders into nanoparticles using femtosecond laser pulses, we transformed non-reactive CoO into an efficient photocatalyst. Distinguished from previous catalysts, CoO nanocrystals decompose pure water at a high rate under visible light without any co-catalysts or sacrificial reagents. At a long wavelength of 532 nm, the nanocrystals demonstrated an energy conversion of 10% and a hydrogen production rate of 50 mmol h-1g-1, both of which are orders of magnitude higher than those of reported catalysts. Impedance measurement reveals that a significant shift in the band-edge positions is responsible for photocatalytic activity of the nanocrystals. It is further proposed that a short carrier diffusion length to particle surfaces, a strong radial electric field associated with band bending, and the p-type nature of CoO all contribute to evolutions of hydrogen and oxygen on the same particle surfaces.

The paper will discuss the need for Solar Fuels and overview the different scientific paths to achieve this goal. Visions and strategies in research in the Swedish Consortium for Artificial Photosynthesis and the European network SOLAR-H2 will be covered. Our research aims for the production of hydrogen from solar energy and water. Water shall be oxidized in a catalytic process using solar energy. The electrons from water shall be used in a second process to reduce protons to hydrogen. We apply a biomimetic approach where we copy key principles from natural enzymes that accomplish partial reactions. Water oxidation using solar energy is carried out by Photosystem II using a catalytic Mn4 complex. In our chemistry we also develop Mn-based catalytic systems and use a photoactive Ru-center to drive oxidative electron transfer. I will describe our recent research on light driven, multi-electron transfer in these Mn systems and a recent water oxidizing catalyst based on a cobalt nano-particle. To accomplish reduction of protons to hydrogen we mimic the di-iron center in hydrogenase enzymes. Some recent results on these biomimetic Fe-Fe complexes will be described.
Magnuson, A., Anderlund, M., Johansson, O., Lindblad, P., Lomoth, R., Polivka, T., Ott, S., Stensjö, K., Styring, S., Sundström, V. and Hammarström L. (2009) Biomimetic and Microbial Approaches to Solar Fuel Generation.
Accounts Chemical Res., 42, 1899-1909
Ott, S., Styring, S., Hammarström, L., and Johansson, O. (2010) Towards Solar Fuels using a biomimetic Approach. Progress in the Swedish Consortium for Artificial Photosynthesis. In “Energy production and storage”; Inorganic Chemical Strategies for a Warming World, edited by Robert Crabtree, Chichester, UK: John Wiley & Sons, Ltd, pp 199-227
Shevchenko, D., Anderlund, M. F., Thapper, A. and Styring, S. (2011) Photo-driven water oxidation with visible light using a cobalt containing catalyst
Energy and Environmental Science, 4, 1284-1287
Risch, M., Shevchenko, D., Anderlund, M.F., Styring, S., Heidkamp, J., Lange, K.M., Thapper, A. and Zaharieva, I. (2012) Atomic structure of cobalt-oxide nanoparticles active in light-driven catalysis of water oxidation
Intl J.Hydr. Research, 37; 8878-8888
S. Kaur-Ghumaan, L. Schwartz, R. Lomoth, M. Stein, S. Ott
Catalytic (2012) Hydrogen Evolution from Mononuclear Ferrous Carbonyl Complexes as Minimal Functional Models of the [FeFe] Hydrogenase Active Site
Angew. Chem. Int. Ed. 49, 8033-8036.

Designing and predicting new functional materials, such as photocatalysts to split water, are challenging tasks. To predict such materials, electronic band gap tuning by doping has been the most popular method recently. However, in general, the introduction of impurities generally creates unwanted defect states in band gap. Hence, poor crystallinity and not so robust photo-current upon doping have been the major obstacles in successfully obtaining efficient photocatalysts. On the other hand, database driven alloy design, rather than doping, can provide a better pathway to predict and design new photocatalysts theoretically. Such efforts pose extra challenges, as it requires crystal structure prediction by computational effort. Here, we&’ll present our recently predicted new alloys which are designed for efficient photocatalysts. We&’ll first start with WO3 and then show the effect of doping and alloying with other elements on its electronic properties. Detail density functional theory calculations have been performed to calculate the electronic properties of these newly designed materials. In this processes we have predicted new multi-cation (2 to 3 cation) oxides, some of them have already been verified experimentally. Finally, we&’ll extend the discussions on the other classes of predicted materials.

Direct conversion of solar energy into storable fuels such as hydrogen has significant potential in providing clean and sustainable energy source. Hydrogen production through photoelectrochemical (PEC) water splitting is one of the attractive ways to convert solar energy into storable chemical energy. Development of efficient, robust and low-cost photocatalysts for water splitting is urgently demanded and is the key for sun light conversion into hydrogen, considered as a green fuel. One promising candidate for this purpose is a metal-free polymeric carbon nitride (g-C3N4) photocatalyst, which has been already proven to produce hydrogen via water splitting under visible light illumination in the presence of an appropriate sacrificial electron donor [1]. Chalcogenides such as p-type chalcopyrite thin films used in thin film heterojunction solar cell devices are of high interest as new photoelectrocatalysts for the hydrogen evolution reaction (HER) under solar light illumination. Here, we have mainly worked on developing and testing of new composite photocathodes. For this, g-C3N4 thin films deposited on chalcopyrite thin films have been successfully achieved using dicyandiamide precursors by a thermal polycondensation process.[2] Morphological and structural properties of g-C3N4 thin films have been investigated using scanning electron microscopy and grazing incidence X-ray diffraction. Additionally, X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy and Raman spectroscopy have been also performed to get information on the chemical surface composition and vibrational properties of our new photocathode composites. Photoelectrochemical investigations clearly prove hydrogen evolution under visible light illumination as detected by our new designed PEC set-up coupled with an on-line mass spectrometry.
[1] X. Wang, K. Maeda, A. Thomas, K. Takanabe, G. Xin, J. M. Carlsson, K. Domen, M. Antonietti, Nature Mat. 8 (2009) 76.
[2] F. Yang, M. Lublow, S. Orthmann, C. Merschjann, T. Tyborski, M. Rusu, S. Kubala, A. Thomas, R. Arrigo, M. Hävecker, Th. Schedel-Niedrig, ChemSusChem 5 (2012) 1227.

Solar radiation has great potential as an abundant, clean, and renewable energy source. However, the solar power density at the surface of the earth is relatively diffuse, with average values of approximately 1000W/m2. Thus, in order to compete with other energy sources, improved materials for solar energy harvesting, concentration, and conversion must be realized. In this talk, the advantageous optical and electronic properties of composite nanostructured materials for solar energy harvesting, concentration, and conversion are described. Bio-inspired composites for enhanced light harvesting are introduced, followed by a specific description of the confinement of light via cavity modes in bilayer films of nanoscale thickness. The potential impact of cavity modes in ultrathin films on the design of solar concentrators is described, and the application to a new type of “resonance-shifted” luminescent solar concentrator (RSLSC) is introduced. By spatially varying the thickness of the film so that the bilayer cavity undergoes a resonance shift, near-lossless propagation and collection of emission is observed. The prospects and necessary improvements for further utilization of RSLSCs are discussed. For energy conversion, I discuss our efforts to temporally and spatially resolve exciton generation and charge separation in photovoltaic heterostructures, beginning with the initial absorption of photons to create an electron-hole pair, to the generation and transport of free charge carriers. Specifically, the use of an ultrafast Stark shift in excitonic molecular J-aggregates (a photosynthetic analog material) to monitor charge separation and photovoltaic field formation in nanoscale heterostructures is described.

Intensive research is still ongoing to identify a suitable semiconductor to be integrated in an efficient, cost effective, and reliable photo-electro-chemical (PEC) system. Among all candidates, binary transition metal oxides are still drawing lots of attention as most of them offer good resistance to corrosion and are inexpensive to produce. Unfortunately, no binary oxide has yet fulfilled all aforementioned PEC criteria, despite numerous attempts to improve electrical conductivity (Fe2O3), light absorption (WO3, TiO2) or material stability (Cu2O). Assuming oxide-based PEC systems are the best pathway to low-cost PEC hydrogen production, one will need to search beyond binary oxides to find the ideal photocatlysts. Of course, with the increased number of elements in ternary and quaternary compounds comes the difficulty to identify a unique system capable of satisfying simultaneously all criteria. One possible approach resides in pure combinatorial analysis, where sets of selected elements are automatically blended to form series of compounds. Similar approach has been employed in the past in the field of superconductors to investigate compounds containing multiple cations metal-oxides. While material discoveries in this field used to be quite fortuitous, modern theoretical calculations has since permitted to predict trends to design (rather than discover) new superconducting material classes.
In the present communication, we demonstrate how density functional theory (DFT) can be of great aid in the quest of new oxide-based PEC materials. Starting with tungsten trioxide as host, we show how the addition of impurities can lead to novel tungstate alloys with optimum optoelectronic characteristics. As suggested by DFT, copper was first evaluated to form ternary CuWO4 mineral. Several synthesis processes were used to form this new PEC material (PVD, spray, solid state reaciton), all leading to 2.2 eV band-gap n-type CuWO4 systems capable of generating photocurrent density of 0.5 mA.cm-2 at saturation (1 VSCE, pH10, AM1.5G). Moving further, DFT calculations revealed that the addition of bismuth to form quaternary CuBiW2O8 mineral could suppress the unfilled mid-gap states present in CuWO4 and also would improve the electron mobility. Electrical measurements performed on both CuWO4 and CuBiW2O8 ceramic pellets confirmed this hypothesis, with an increase in electrical conductivity by a factor of 100 after addition of bismuth (from 5.7×10-8 S.cm-1 to 3.8×10-6 S.cm-1). Research is currently focused on CuBiW2O8 thin films synthesis to evaluate the PEC performances of this new material class.

We report a highly efficient WO₃/BiVO₄ core-shell nanowire (NW) photoanode for photoelectrochemical (PEC) water splitting. The saturation photocurrent of the WO₃/BiVO₄ NWs under simulated solar illumination (AM 1.5G, 100 mW/cm²) is about 6 mA/cm², which exceeds that previously reported for planar WO₃/BiVO₄ porous films (3 mA/cm²), and falls in between the AM 1.5G optical limits for WO₃ (4 mA/cm²) and BiVO₄ (7.5 mA/cm²). Briefly, arrays of WO₃ NWs were grown on fluorinated tin oxide substrates by rapid, atmospheric flame vapor deposition, and were then coated with BiVO₄ by a simple drop-casting method. When Co-Pi catalyst was added by photo-assisted electrodeposition, photocurrents of up to 5 mA/cm² were achieved at 1.23 V_RHE, with a low onset bias of only 0.3 V_RHE. The exceptionally high performance of these WO₃/BiVO₄ NWs arises from the combination of the strong light absorption of BiVO₄ and efficient charge transport of WO₃, as well as from the improvement of charge separation by the staggered type-II WO₃/BiVO₄ heterojunction. This significant advance will greatly impact artificial photosynthesis, in which robust and efficient photoanodes are a performance-limiting building block. Moreover, this WO₃/BiVO₄ photoanode is synthesized from cheap materials using low cost and scalable processing.

Sunlight can be harvested and transduced into useful energy using semiconductor-liquid junction solar cells, which generate power through the transfer of electronic energy to molecules dissolved in fluid solution. If two such interfacial electron-transfer events are efficiently coupled, chemical bonds can be formed, an important step to powering our planet with fuels derived from renewable energy. Toward this, free-standing periodic arrays of crystalline silicon microwires partially embedded in a Nafion proton-exchange membrane were used to photogenerate H2, and I3-, from aqueous HI solutions via sunlight-driven artificial photosynthesis. Ordered arrays of Si microwires were grown on planar Si(111) substrates by a chemical-vapor-deposition process, employing Cu growth catalysts. Electrocatalysts were deposited on the Si microwires to catalyze light-driven hydrogen evolution and iodide oxidation at p- and n-type Si, respectively. Open-circuit photovoltages measured for each type of wire array were 400 - 500 mV, with n+p-Si in HCl/H2 and n-Si in acidic I3-/I-. Corrosion of Si was attenuated through methylation of Si atop sites via a two-step chlorination-alkylation surface chemistry procedure. This resulted in stable I- oxidation under continuous simulated sunlight illumination for weeks. Si microwire arrays partially embedded in Nafion proton-exchange membrane were mechanically removed from the Si substrate to yield free-standing devices. Pt electrocatalysts were deposited on the microwire backsides by electron-beam evaporation to complete the functional fuel-forming devices. Solar-to-hydrogen efficiencies and Faradaic yields for product formation were quantified. These systems are sustainable because the HI fuel precursor is inorganic, thus not capable of liberating CO2, and HI can be regenerated in a flow battery / fuel cell as H2 + I2.

Wide-bandgap semiconductors like titanium dioxide (TiO2) and tungsten trioxide (WO3) suffer from the inability to harvest photons with energy lower than their bandgap, therefore limiting their efficiency in solar energy applications. To mitigate this problem, photons of low energy can be upconverted into higher energy light, creating a sub-bandgap sensitization of semiconductors without any chemical modification of the latter. Photoaction spectra and modulated non-coherent light excitation in photoelectrochemistry clearly demonstrate that triplet-fusion based upconversion induces light activation of wide bandgap semiconductors. The combined results represent proof-of-concept that molecular-based upconversion can be integrated into solar cells and solar fuel generating devices. Finally, theoretical and practical aspects of deploying this technology at its highest efficiency will be discussed.

Photoelectrochemical water splitting by semiconductors holds great promise for efficient solar energy harvesting and storage. Constructing an efficient photoelectrochemical cell consisting of photoanode and photocathode for unassisted solar water splitting is particularly interesting and remains highly challenging. The difficulty lies in the mismatch of photocurrent between photoanode and photocathode as well as the lack of sufficient photovoltage for zero-bias water splitting. In this talk, we presented our success of constructing an unassisted solar water splitting cell with hematite-based photoanode and p-InP nanopillar as photocathode. We demonstrate that about 1 mA/cm2 photocurrent is achieved at zero bias under 100 mW/cm2 simulated solar light. The solar to hydrogen conversion efficiency was over 1%, which is among the highest for metal-oxide based n/p photoelectrochemical cell for spontaneous water splitting. The generation of hydrogen and oxygen, stability of this n/p photoelectrochemical cell and design principle to achieve unassisted water splitting will be discussed. We believe our results represent a significant advance toward complete solar water splitting with high efficiency.

Hematite Fe2O3 is a promising photoanode material for photoelectrochemical (PEC) water splitting due to its abundance, low-cost, stability, and substantial sun light harvesting with a bandgap of 2.1 eV. Since the first hematite PEC water splitting demonstration in 1976(1), there have been numerous advancements, particularly through interface engineering, doping, and nanostructuring of hematite (2). One of the fundamental challenges to using hematite to achieve water splitting with high external quantum efficiency is the disparity between the long photon absorption length (hundreds of nm) and the short hole diffusion length (2-4 nm). Thus holes photogenerated far from the semiconductor-liquid junction are lost to recombination. The use of a host-guest approach enables the decoupling of absorption from transport by having a transparent, conductive support (host) with many absorbing thin films (guest), each having a high internal quantum efficiency (3). We present the development of a new host material based upon the atomic layer deposition of niobium doped tin oxide onto porous scaffolds. This new host material is transparent, conductive, and stable over a wide range of pHs. Subsequent hematite deposition leads to composite architectures having significantly enhanced PEC water splitting performance. We show a significantly increased utilization of near band-edge photons, which are key to better harvesting the solar spectrum. Our initial results are near-record setting for a composite approach using hematite.
(1) Hardee, KL.; Bard, AJ. “Application of Chemically Vapor-Deposited Iron-Oxide Films to Photosensitized Electrolysis”. J. Electrochem. Soc. 123.7, 1024-1026 (1976).
(2) Sivula, K.; Le Formal, F.; Grätzel, M. “Solar Water Splitting: Progress Using Hematite Photoelectrodes”. Chem. Sus. Chem. 4, 432-449 (2011).
(3) Hisatomi, T.; Dotan, H.; Stefik, M.; Sivula, K.; Rothschild, A.; Grätzel, M.; Mathews, N. “Enhancement in the Performance of Ultrathin Hematite Photoanodes for Water Splitting by an Oxide Underlayer”. Adv. Mater. 20, 2699-2702 (2012).

Photoelectrochemical (PEC) hydrogen production is of considerable interest as it offers an environmentally “green” and renewable approach to solar fuels production. Efficient PEC water splitting devices require semiconductor photoelectrode materials fulfilling a number of primary requirements such as bandgap, band edge alignment, and corrosion resistance to aqueous electrolyte. We are developing an amorphous silicon carbide (a-SiC) photoelectrode with a band gap around 2.0eV, prepared by plasma enhanced chemical vapour deposition at low temperature (<250C), which is widely used to fabricate silicon-based thin film materials and solar cells in laboratories and industry with low production cost and a variety of substrates (e.g. glass, stainless steel, or plastic). A-SiC photoelectrode has shown good resistance to corrosion in aqueous electrolyte, and so far has tested (survived) for 310 hours. Our simulation has indicated that a-SiC photoelectrode integrated with PV devices could achieve solar-to-hydrogen (STH) conversion efficiency over 15%.
We have developed a hybrid PEC device incorporating an a-Si tandem solar cell and the a-SiC photoelectrode, which produced photocurrent greater than 3.5 mA/cm2 at saturation in three-electrode setup (-1.5 Vs..SEC). This photocurrent was close to that of solid-state version using indium tin oxide (ITO) to replace the electrolyte. However, a short circuit photocurrent density as low as 0.33 mA/cm2 was observed in two-electrode setup using RuO2 thin film counter electrode. This information tells us that surface energetics and/or kinetics at the a-SiC semiconductor-electrolyte interface might limit the photocurrent extraction, and surface modification is needed to improve the charge carrier transportation through the interface. Metal nanoparticles were fabricated by sputtering, and SEM revealed very small (<5nm) nanoparticles formed on the surface of the a-SiC to modify the interface. Hybrid PEC device modified using Ru metal nanoparticles showed promising results, with flatband potential shifted anodically (e.g. 500 mV) resulting in an increase in short circuit photocurrent density in two-electrode setup (greater than 2.0 mA.cm-2). The STH efficiency was increased significantly to around 2.5%. Much cheaper W nanoparticles also had a similar effect on a-SiC photoelectrode. The stability in air of a-SiC modified by metal nanoparticles was good after spending 5 months in air, comparing bare a-SiC itself. Initial analysis on the influence of metal nanoparticles on a-SiC surface suggested the barrier height at the a-SiC/electrolyte interface was reduced by metal nanoparticles due to their low work function as well as the energetic change of a-SiC/electrolyte interface.

The development of photochemical systems capable of splitting water into hydrogen and oxygen has attracted significant interest motivated by the need to secure the future supply of clean and sustainable energy [1]. Due to the complex chemistry involved in four-electron oxidation of water to dioxygen [2], the major challenge in photoelectrochemical water splitting is the development of cheap, efficient and stable photoanodes. Recently, we have been developing photoanodes based on a novel class of visible-light photoactive inorganic/organic hybrid materials - TiO₂ modified at the surface with polyheptazine (also known as “graphitic carbon nitride”). As we have shown, the optical absorption edge of the TiO₂-polyheptazine hybrid is red-shifted into the visible (2.3 eV; ~540 nm) as compared to the bandgaps of both of the single components - TiO₂ (3.2 eV; ~390 nm) and polyheptazine (bandgap of 2.9 eV; ~428 nm), which is due to the formation of an interfacial charge-transfer complex between polyheptazine (donor) and TiO₂ (acceptor) [3]. In other words, the direct optical charge transfer leads to generation of electrons with a relatively negative potential in the conduction band of TiO₂, while the holes photogenerated in the polyheptazine layer can drive photooxidation of water, as evidenced by visible light-driven evolution of dioxygen on hybrid electrodes modified with iridium or cobalt oxide nanoparticles acting as oxygen evolution co-catalysts [3-6]. Importantly, polyheptazine is highly stable, and at the same time it offers a possibility for further functionalization with transition metal-based catalytic sites enabling chemical transformations along multi-electron pathways. Our current attempts at improving the efficiency of kinetic charge separation in such hybrid photoanodes will be discussed.
References
[1] N.S. Lewis, D.G. Nocera, Proc. Natl. Acad. Sci. U.S.A.2006
, 103, 15729.
[2] H. Dau, C. Limberg, T. Reier, M. Risch, S. Roggan, P. Strasser, ChemCatChem2010, 2, 724.
[3] M. Bledowski, L. Wang, A. Ramakrishnan, A.; O.V. Khavryuchenko, V.D. Khavryuchenko, P.C. Ricci, J. Strunk, T. Cremer, C. Kolbeck, R. Beranek, Phys. Chem. Chem. Phys.2011, 13, 21511
[4] L. Wang, M. Bledowski, A. Ramakrishnan, D. König, A. Ludwig, R. Beranek, J. Electrochem. Soc.2012, 159 (7), H616.
[5] M. Bledowski, L. Wang, A. Ramakrishnan, A.; O.V. Khavryuchenko, A. Bétard, R. Beranek, ChemPhysChem2012, 13, 3018.
[6] M. Bledowski, L. Wang, A. Ramakrishnan, R. Beranek, J. Mater. Res.2012, DOI:10.1557/jmr.2012.297.

Solar-powered photoelectrolysis of water is a promising route to produce clean hydrogen fuel from abundant and renewable resources: water and sunlight. Iron oxide ( α-Fe₂O₃) is one of the most promising photoanode candidates because of the unique combination of visible light absorption up to 600 nm, stability in aqueous solutions, abundance and low cost. However, its poor transport properties and short lifetime of minority charge carriers lead to strong bulk recombination that hinders the development of efficient α-Fe₂O₃ photoanodes for water photo-oxidation. Here we show a novel approach to overcome the deleterious tradeoff between light absorption and collection of photogenerated minority charge carriers (holes) by means of resonant light trapping in ultrathin films of high internal quantum efficiency.
The resonant light trapping effect is readily obtained in a simple device configuration comprising quarter-wavelength semiconductor films on back reflector substrates.^sect; This optical stack gives rise to constructive interference inside the film, close to the surface, and destructive interference outside the film, thereby suppressing the reflection and boosting the absorption in the photoanode film. Model calculations predict that up to 71% of the relevant solar spectrum is absorbed in ultrathin (< 50 nm) α-Fe₂O₃ films on ideal back reflector substrates. Most of the photogenerated holes in ultrathin films can reach the surface and oxidize water before recombination takes place. The rest of the light is reflected and easily harvested using photon retrapping schemes such as V-shaped cells. The combination of the resonant light trapping effect together with photon retrapping provides very efficient light harvesting in ultrathin films of high internal quantum efficiency, the enabling key for efficient solar energy conversion.
Water stable photo-oxidation current densities as high as 4 mA cm^-2 were obtained, under 100 mW cm^-2 white light illumination, using a 90⁰ V-shaped cell comprising 26 nm thick Ti-doped α-Fe₂O₃ films on silver-gold alloy (stable at 1M NaOH solution and electrode potential of 1.65 V_RHE) coated substrates.^sect;
Our light trapping strategy is not restricted to α-Fe₂O₃ photoanodes for water splitting. It is applicable to other semiconductors, especially those with high absorption coefficient. Thus, it opens up new opportunities for employing earth-abundant materials that may cost a fraction of conventional semiconductors currently employed in photovoltaic cells. Furthermore, this strategy provides a route for shrinking the photoactive films by more than 90%, in some cases, without compromising their light absorption. This holds great promises for second generation thin film solar cells based on CdTe or CuIn_1-xGa_xSe₂ (CIGS) for which the availability of rare earth elements (Te or In) is a major limitation.
sect; Hen Dotan et al., Resonant light trapping in ultrathin films for water splitting, Nature Materials (in press).