In recent years, visible-light-driven hydrogen evolution from water using a photocatalyst has attracted significant attention as a potential means of supplying hydrogen from renewable resources. For efficient solar energy conversion, a photocatalyst that permits light absorption at wavelengths longer than 600 nm (band gap smaller than 2 eV) is highly desirable. Assuming overall water splitting with a quantum yield of unity, the theoretical potential of photocatalysts with an absorption edge of 600 and 650 nm can achieve, respectively, 16.2 and 20.6% solar energy conversion. Thus, the development of â?o600-nm-class photocatalystsâ? is an important mission for solar energy conversion. Oxynitrides of LaTiO₂N, Ta₃N₅, and BaTaO₂N are active photocatalysts that have band gaps of 2.1, 2.0, 1.8 eV, respectively. Our group has attempted to improve the photocatalytic activities by several ways including a new precursor route, surface modification by cocatalysts that act as efficient gas evolution sites, forming a solid solution with a wide gap metal oxide, and so on. For example, single-crystalline LaTiO₂N particles with mesoporous and microporous architecture can be prepared by heating well-grown La₂Ti₂O₇ particles synthesized by a flux method. The thus-prepared LaTiO₂N modified with a cobalt oxide cocatalyst photocatalytically generates oxygen from water with an apparent quantum yield of nearly 30% at 440 nm. In this presentation, recent development of oxynitride-type photocatalysts, which have an absorption band longer than 600 nm, will be given.

Close inspection of the pre-edge in oxygen NEXAFS spectra of titanium oxynitride photocatalysts with 20 nm particle size reveals an additional eg resonance in the valence band that went unnoticed in previous TiO2 anion doping studies. The spectral weight of this Ti(3d)-O(2p) hybridized state with respect to and located between the readily established t2g and eg resonances scales with the photocatalytic decomposition power, suggesting that this resonance bears co-responsibility for the photocatalytic performance of titanium oxynitrides at visible light wavelengths. Anodic oxidation of hematite (α-Fe2O3) nanoparticulate films at 600 mV in KOH electrolyte forms a species at the surface which causes a new transition in the upper Hubbard band between the regions of the hybridized Fe(3d)-O(2p) and Fe(4sp)-O(2p) states, as evidenced by oxygen NEXAFS spectra. This transition, not known for pristine α-Fe2O3, is at about the same x-ray energy at which pristine Si doped Si:Fe2O3 has such transition. These states coincide with the onset of an oxidative dark current wave as observed in the cyclic voltamogram. Electrochemical oxidation at 200 mV does not form such an extra NEXAFS feature. A 100 nm thick pulsed laser deposited blue, non-stoichiometric WO3-δ film grows on TiO2 (110) in [220] direction. Oxidative treatment at 400°C turns the film color from blue to yellow and improves the film quality considerably, as shown by improvement of the Kiessig oscillations in the x-ray reflectometry curves. Detailed analysis of resonant valence band photoemission spectra of the as-prepared non-stoichiometric film and oxidized yellow film suggests that a transition near the Fermi energy originates from the non-stoichiometry, i.e. oxygen deficiency and insofar poses electronic defect states which partially can be eliminated by heat treatment in oxygen. We will also present very recent unpublished in-situ electrochemical studies on WO3 and Fe2O3 with ambient pressure XPS and liquid electrolyte NEXAFS spectroscopy. 1)A. Braun, K.K. Akurati, G. Fortunato, F.A. Reifler, A. Ritter, A.S. Harvey, A. Vital, T. Graule, J. Phys. Chem. C 2010, 114, 516â?"519. 2)D.K. Bora, A.Braun , S.Erat, R.Löhnert, A.K. Ariffin, R.Manzke, K.Sivula, J. Töpfer, T.Graule, M.Grätzel, E.Constable, J. Phys. Chem. C 2011, 115, 5619â?"5625. 3)A. Braun , S. Erat, X. Zhang, Q. Chen, F. Aksoy, R. Löhnert, Z. Liu, T. Graule, S.S. Mao, J. Phys. Chem. C 2011, 115, 16411â?"16417.

Since the seminal demonstration of water dissociation using a semiconductor absorbing photons to generate electron-hole pairs (1), hematite as a photoanode has been a material of choice. Itâ?Ts abundance and relatively low cost, its good light absorption capabilities (bandgap of 2.1 eV) and chemical stability are the most noticeable advantages of α -Fe2O3. However, this material exhibits a number of challenges, including its less than ideal electrical vs. optical properties. Indeed, its hole diffusion length is extremely small (2 to 5 nm) (2) as compared to its light penetration depth (αâ?"1 = 118 nm at λ = 550 nm) (3), giving photo-generated charges low probability to reach the semiconductor/liquid interface and therefore participate to water oxidation. The resulting poor external quantum efficiency (EQE) close to the band edge has been identified to be one of the major limitations in context of a tandem device (4). To answer this problem, we present here progresses towards a high efficiency host-guest heterostructure5 that allows the carrier generation close to the surface where a high EQE extremely thin absorber (ETA) is conformaly coated on a tridimensional transparent conductive oxide (3D-TCO) (6). High EQE has been achieved through the improvement of atomic layer deposited (ALD) films by doping the hematite and passivating both the substrate and electrolyte interfaces. (1) Boddy, P. J Electrochem Soc 1968. (2) Kennedy, J.; Frese, K., Jr J Electrochem Soc 1978, 125, 709. (3) Balberg, I.; Pinch, H. Journal of Magnetism and Magnetic Materials 1978, 7, 12â?"15. (4) Brillet, J.; Cornuz, M.; Le Formal, F.; Yum, J.-H.; Graetzel, M.; Sivula, K. J Mater Res 2010, 25, 17â?"24. (5) Sivula, K.; Le Formal, F.; Graetzel, M. Chem Mater 2009, 21, 2862â?"2867. (6) Lin, Y.; Zhou, S.; Sheehan, S. W.; Wang, D. J. Am. Chem. Soc., 2011, 133, pp 2398â?"2401

In this work, we will report on the use of metallic and multimetallic nanoparticles as additives for modifying the surface reactivity of the TiO2 nanoparticles, which have conveniently modified in order to increase the CO2 reduction capability under photo catalytic conditions. For improving these photocatalytic processes, not only must be generated electron-hole pairs, but the electronic transfer must be efficient enough, which requires multi-electronic redox processes, together with the condition that that the oxidation and reduction must occur simultaneously. In order to overcome these problems, the use of the multimetallic particles can be followed for increasing the photocatalyst efficiency. In this contribution, special attention is paid to the silver and silver-copper bimetallic nanoparticles, which reached competitive efficiencies compared to other noble metal bimetallic nanoparticles, such as platinum and platinum-copper systems. The structural and functional characterization as well as the use and reliability of these kind of additive will be presented and discussed considering the viability of efficiency increase and the tune of the production of different sub products for fuel synthesis.

BiVO₄ is considered to be a promising photoanode material for solar water splitting applications. The monoclinic phase has a bandgap of ~2.4 eV, which corresponds to an optical absorption edge at ~520 nm. However, the quantum efficiencies reported so far do not exceed 55% [1-3], and the main performance-limiting factors are still not clear. In this work, photoelectrochemical characterization of spray-deposited, dense films of BiVO₄ results in a quantum efficiency of up to 95% at low light intensities. By comparing front- and back-side illumination, electron transport was found to be the main rate-limiting factor under these conditions [2]. This is further confirmed by a time-resolved microwave conductivity study that reveals a carrier mobility value of ~3 x 10-3 cm²/Vs, which is 2 â?" 3 orders of magnitude lower than is typically observed for metal oxide photoanodes. Intriguingly, Mott-Schottky analysis of BiVO₄ films consistently indicates very high donor densities, which is difficult to reconcile with the high quantum efficiencies and poor electron transport properties. We will present the results of a recent electrochemical impedance spectroscopy study that was carried out to elucidate these contradictory observations. Integrating the quantum efficiencies over the solar spectrum leads to a predicted AM1.5 current density of 3.6 mA/cm². However, the observed photocurrent density under simulated AM1.5 illumination is much lower than the predicted value (~0.4 mA/cm²) due to extensive carrier recombination. Under this condition, poor water oxidation kinetics (hole transfer) become rate-limiting, as evidenced by a significant increase of photocurrent density when hydrogen peroxide is added as a hole scavenger. Based on these insights, we have modified the sample with cobalt-phosphate as a water oxidation catalyst [4] to address the poor hole transfer, and tungsten as a donor-type dopant to enhance electron transport. This has resulted in an AM1.5 photocurrent density of 2 mA/cm² at 1.23 V_RHE. While this is lower than the state-of-the-art photocurrent of 2.8 mA/cm² recently reported by Luo et al., our results have been obtained for a five-times thinner compact film of only 200 nm and a low cost cobalt phosphate as the co-catalyst. In addition, the amount of light scattering in these spray-deposited films is negligible, which is an important advantage since these films would ultimately have to be incorporated in a tandem device structure to achieve overall water splitting. References [1] K. Sayama et al., J. Phys. Chem. B, 110 (2006) 11352 [2] Y. Liang, T. Tsubota, L.P.A. Mooij, and R. van de Krol, J. Phys. Chem. C 115 (2011) 17594 [3] W. Luo et al., Energy Environ. Sci., 4 (2011) 4046 [4] M. Kanan and D. Nocera, Science, 321 (2008) 1072

This lecture presents the photocatalytic hydrogen production by utilizing solar energy, mainly discussing three types of reactions including water splitting, biomass reforming and industrial waste reforming. Hydrogen molecule is an energy carrier for solar energy storage, and is ideal for clean energy process, and the hydrogen production utilizing solar energy could eventually realize the so called Hydrogen Economy and as a means for solar energy powering the world. The bottleneck to produce hydrogen through photocatalysis is the developing of high active photocatalysts which have been extensively investigated and explored during last several decades. This talk will focus on the role of cocatalysts in photocatalysis. The crucial roles of dual co-catalysts respectively for both half reactions, oxidation and reduction are highlighted. Three typical catalysts, Pt/TiO₂, Pt-PdS/CdS and CoPi/BiVO₄ are studied for methanol reforming (as a representative reaction of biomass reforming), H₂S reforming and water splitting reaction respectively. Our research shows that the cocatalyst is necessary for hydrogen evolution or/and oxygen evolution, and the coloading of dual co-catalysts are absolutely necessary for improving the photocatalytic activity by reducing the recombination of photogenerated electrons and holes and reducing the activation energy. It is found that the oxidation half reaction is the most difficult part of the photocatalytic hydrogen production, particularly from water splitting reaction, and the cocatalyst to reduce the activation energy for oxygen evolution from water splitting essentially acts as the same role as the electrochemical catalysts to lower the overpotential. By well designing and loading the dual cocatalysts, Pt and PdS on CdS, using Na₂S+Na₂SO₃ (H₂S dissolved in NaOH) as sacrificial reagent, we can achieve a quantum efficiency of the artificial photosynthesis high as 93%. In situ photoelectrochemical measurements, photoluminescence spectroscopy and high resolution transmission electron microscopy characterizations indicate that the exceptionally high quantum efficiency can be attributable to the vital factors including mainly the spatially separated PdS and Pt as the oxidation and reduction active sites respectively; the efficient utilization of the electrons at the shallow trap states of CdS for photocatalytic reactions; and the formation of atomic heterojunctions between the co-catalyst PdS and CdS as these factors could effectively prohibit the recombination of photogenerated electrons and holes and favor the full utilization of the photogenerated electrons.

There has been a growing interest in developing systems that are suitable for the conversion of atmospheric CO2 into hydrocarbons that can used as a fuel using sunlight. This is a process that mimics photosynthesis and has, to date, mostly eluded mankind at a level that could effectively limit our dependence on other fuel sources. In this work we focus on using a ferroelectric material that exhibits a reduction potential of the photoexcited electron around -3V vâ?Ts NHE. This highly reducing electron enables a series of reaction schemes to be utilised in the reduction of CO2 that have not been possible using semiconductors that have a highly oxidising hole. The photocatalytic fixation of CO2 to hydrocarbons was performed using LiNbO3. 0.126 cm2 of powdered LiNbO3 was irradiated with 64.2mW/cm2 light from a mercury lamp with 10ml of water under 30% CO2/air. The LiNbO3was held on a platen above the water to give a gas-solid catalytic reaction. We use GC-MS to determine that formic acid and formaldehyde were present in the water after irradiation for 6 hours. The products where produced at rates of 7.7mmol and 1.1 micro mol /hour. This rate of production enabled the calculation of the efficiency of the catalyst. This was calculated to be ca 2% which compared to 0.17% for TiO2 under the same conditions. The efficiency was calculated from a ratio of light available for reaction and the energy contained in the chemical products. MgO doped LiNbO3 was also tested and found to have an energy conversion efficiency of 0.72%. We explain the differences in the performance for the efficiency between the doped and undoped materials in terms of the majority carriers in the system. The high efficiency of the LiNbO3 system, when compared to TiO2, is explained in terms of the ferroelectric nature of the catalyst. A ferroelectric will effectively separate the reduction and oxidation processes on its surface. This is due to the formation of an internal self developed p-n junction as a result of the spontaneous polarisation due to ion movement in the crystal lattice. This spatial selectivity reduces the chance for back reactions and enables the reaction to move towards completion. A second reason is that the reduction potential of a photoexcited electron is sufficiently high to directly reduce CO2 to CO2+-. This enables a previously unavailable reaction path to occur. The third and final reason is that the high surface charge density of LiNbO3 means that chemisorbed molecules may exhibit anomalous band structures. For example there is evidence that CO2 is not a linear molecule on the surface of a ferroelectric, this bending of the molecule structure would more readily enable the injection of extraction of electrons. In summary we show that LiNbO3 is an attractive material to act as a catalyst for artificial photosynthesis due to inherent materials properties associated with the band locations and ferroelectric properties.

Three decades after the demonstration of photoelectrochemical (PEC) water splitting by Fujishima and Honda with TiO2, intensive research is still ongoing to identify a suitable semiconductor to be integrated in an efficient, cost effective, and durable PEC system. Among all candidates, transition metal oxides are still drawing lots of attention as they offer good resistance to corrosion and are inexpensive to produce. The major issue to solve remains their poor optical absorption, which restrains their solar-to-hydrogen efficiency to only a few percent. Numerous attempts have been made to reduce the band gap of transition metal oxides, mainly via incorporation of foreign elements such as nitrogen. Unfortunately, this incorporation method usually leads to an increase in structural defects and poor PEC performances. Thus, it appears that the best research strategy is to focus on metal oxides that already have appropriate optical absorption properties and then fine-tune, if necessary, their material properties such as bulk charge transport and catalytic surface activity. With an optical band gap of 2.2 eV, copper tungstate (CuWO4) should be considered as a potential candidate for PEC hydrogen production. As of today, most publications on CuWO4 report on its fundamental properties but only a handful on its potential as a PEC water splitting material. In the present communication, we report on the synthesis of CuWO4 thin film via low-cost processes for PEC hydrogen production. Our preliminary study indicated that copper tungstate material synthesized using co-sputtering methods at 275°C were amorphous and did not show any photo-response in 0.33M H3PO4 electrolyte under AM1.5G simulated illumination. However, a major improvement in PEC performance was observed after a post-deposition treatment performed at 500°C in pure argon for 8 hours. Indeed, a photocurrent density of approximately 0.4 mA.cm-2 at 1.6 V vs. SCE was measured on annealed CuWO4 samples. Subsequent X-ray diffraction analysis indicated a clear transformation of as-deposited amorphous thin films into a triclinic CuWO4 structure after the annealing step. Electrochemical impedance spectroscopy (EIS) finally revealed that polycrystalline n-type CuWO4 possessed a much lower flat-band potential (-0.35 V vs. SCE) than WO3 (+0.15 V vs. SCE) and ideal surface band-edges that straddle both the hydrogen and oxygen evolution reactions. However, it appeared that the main Achillesâ?T heel of CuWO4 is its bulk charge transfer resistance (approx. 2000 ohm.cm-2, measured by EIS). Current research efforts are focused on resolving this issue by adding dopants and/or conductive carbon nano-tubes into the CuWO4 matrix.

The direct conversion of solar energy to chemical fuels using semiconductor electrodes has been a topic of intense research for several decades. While a plethora of research has been directed at highly efficient n-type semiconductor photoanodes, a much smaller number of p-type semiconducting oxide materials are currently known. Recently, our research efforts have sought to investigate the reduction of the bandgap sizes of early transition-metal oxides via the incorporation of transition metals with d10 electron configurations, that is, specifically Cu+ and Ag+. An extension of these investigations into the photcatalytically-relevant niobates and tantalates has recently been shown to yield a promising class of new semiconductors based on mixed Cu+/Ta5+ and Cu+/Nb5+ metal oxides. While many alkali-metal niobates and tantalates are reported to be highly active photocatalysts, the related copper (I) niobates and copper (I) tantalates have not been well-explored yet owing to the difficulties in their high-purity preparation and structural characterization. This paper presents a detailed investigation of the photoelectrochemistry and electronic structure of this new class of p-type semiconductor materials for photocathode in photoelectrochemical cell (PEC). Photoelectochemical measurements carried out under visible light show a photocathodic current that is characteristic of p-type semiconducting behavior of the copper niobates and copper tantalates working electrode. Electronic strcuture calculations of copper niobate shows that its conduction band consists of Nb (d0) and O (p) orbitals, whereas its valence band is comprised of primarily Cu (d10) orbitals. Similarly, electronic structure calculations of copper tantalate shows that its conduction band consists of Ta (d0) and O (p) orbitals, whereas its valence band is comprise of primarily Cu (d10) orbitals. The niobate and tantalate films were deposited onto fluorine-doped tin oxide (FTO) glass using the doctor blade technique and characterized by X-ray diffraction (XRD), UV-visible spectroscopy and SEM. These new class of p-type semiconductor materials can be used for CO2 reduction to generate chemicals.

Mimicking photosynthesis has been intensively studied primarily for generating carbon footprint-free fuels. Such artificial photosynthesis consists of three key reactions: water oxidation, proton transfer through membranes and wired electron transfer, and electrochemical reduction of carbon dioxide. Various semiconductor photoelectrodes have been studied on their photoelectrochemical water oxidation yet most of them suffer from inactivity for visible light, high charge recombination upon visible light irradiation, and photocorrosion. Very recently, cobalt and nickel-based electrocatalysts have been reported to have high electrocatalytic effect for water oxidation and maintain their catalytic effect even when loaded on semiconductor electrodes. We have found that such effect is highly dependent on the kind of semiconductors. For systematic investigation, we have tested six semiconductor electrodes with different bandgaps and energy levels for their photoelectrochemical water oxidation without or with the electrtocatalysts. In this presentation, the photoelectrochemical characterization of and gaseous oxygen evolution from the semiconductor electrodes will be discussed.

A critical step for assembling a robust integrated artificial photosystem is the efficient coupling of the sites for carbon dioxide reduction and water oxidation across a physical barrier that separates the products of the two half reactions. The need for a separating membrane is particularly important for CO2 reduction by 6 or more electrons to a liquid fuel if extensive back and cross reactions are to be avoided. A silica wall of a few nanometer depth may possess the desired properties because it transmits protons fast on the time scale of catalytic turnover, yet is impermeable to small molecules. We have developed a solvothermal method to obtain crystalline Co3O4 nanotubes for a range of sizes, from 500 nanometer outer diameter, 200 nm inner diameter and tens of micrometer length, to nanotubes with 10 nm outer and 6 nm inner diameter. Using a visible light sensitization system in close to neutral aqueous solution, the Co3O4 nanotubes were found to exhibit high water oxidation activity. A cylindrical shell of a few nanometer thickness was cast around the Co3O4 nanotube by a modified version of a hydrothermal synthesis method originally introduced by Stoeber. Attaching a visible light chromophore and a reduction catalyst on the outside silica surface opens up an approach for separating the light absorber and fuel forming catalysis from the water oxidation chemistry on the inside of the tube. In order to accomplish efficient directional electron transport between the water oxidation catalyst and the chromophore across the silica wall, the materials chemistry for embedding rectifying molecular wires into the nanometer-thin silica layer has been developed. The wire molecules (oligo para phenylene vinylene) are covalently attached to the Co3O4 surface and extend across the silica layer. Directionality for charge flow is imposed by proper alignment of HOMO and LUMO of the wire molecules with catalyst and chromophore electronic levels. To demonstrate the concept, spherical Co3O4 catalyst core/SiO2 shell particles were used. FT-Raman, infrared and UV-Vis spectra confirmed the presence of the embedded wires. Nanosecond absorption spectroscopy revealed very efficient transfer of charge from the chromophore to the wire molecules at a small overpotential of 150 mV by monitoring the characteristic transient hole absorption at 600 nm. Similar constructs using the nanotube geometry will be presented. The latter are suitable for demonstrating visible light-driven water oxidation catalysis under separation of O2 by the silica wall and as well as evaluation of the charge transport efficiency by transient spectroscopy. This work was supported by the Director, Office of Science, Office of Basic Energy Sciences, Division of Chemical, Geological and Biosciences of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231.

As the seriousness of environmental issue has been grown up, reduction of carbon dioxide (CO2) becomes crucial worldwide. In addition to the attempts for reducing the CO2 consumption, it is important to construct a system for direct CO2 reduction. To reduce the amount of CO2 and to utilize CO2 effectively, so-called artificial photo-synthesis has been intensively investigated. However, several problems persist, such as low efficiency, poor stability, and the need for sacrificial materials or external power input. On the other hand, it was reported that gallium nitride (GaN) can be used as photo-catalyst for water splitting in stead of titanium oxide (TiO2). GaN has an advantage for photo-catalytic reaction because of its wide band gap and low affinity. The research area of nitride semiconductor has been expanded beyond its potential application to highly efficient optical and power devices for energy saving. In the recent study, we have succeeded in realizing direct CO2 conversion with water and UV light illumination using GaN photo-electrode. Here, we report that the CO2 conversion can be improved by replacing the photo-electrode from GaN to AlGaN/GaN film from the viewpoints of both photo current and selectivity of reaction products. The nitride semiconductor films, which were Si-doped n-type gallium nitride (GaN) and intrinsic aluminum gallium nitride (AlGaN), were grown on (0001) sapphire substrate by metalorganic vapor-phase epitaxy (MOVPE). And nickel oxide (NiO) particles were appended to the surface of the nitride semiconductor film as co-catalysts. For counter electrode, a copper (Cu) or indium (In) plate was chosen. In the case of GaN photo-electrode and Cu counter electrode, the generation of formic acid (HCOOH) from CO2 and H2O with 8.8% Faradic efficiency was confirmed. For AlGaN/GaN photo-electrode, both photo current and Faradic efficiency of HCOOH were increased in spite of the decrease of light absorption of long-wavelength side. It is considered that the separation of electron-hole pair exited by light irradiation becomes efficient due to the internal electric field induced in the AlGaN layer. By changing the counter electrode from Cu to In with keeping AlGaN/GaN photo-electrode, the selectivity for HCOOH was enhanced in which its Faradic efficiency of HCOOH reached at ~ 68%.

Using photocatalysis for energy applications depends, more than for environmental purposes or selective chemical synthesis, on converting as much of the solar spectrum as possible; the best photocatalyst, titania, is far from this. Many efforts are pursued to use better that spectrum in photocatalysis, by doping titania or using other materials (mainly oxides, nitrides and sulphides) to obtain a lower bandgap, even if this means decreasing the chemical potential of the electron-hole pairs. Here we introduce an alternative scheme, using an idea recently proposed for photovoltaics: the intermediate band (IB) materials [1]. It consists in introducing in the gap of a semiconductor an intermediate level which, acting like a stepstone, allows an electron jumping from the valence band to the conduction band in two steps, each one absorbing one sub-bandgap photon. For this the IB must be partially filled, to allow both sub-bandgap transitions to proceed at comparable rates; must be made of delocalized states to minimize nonradiative recombination; and should not communicate electronically with the outer world. For photovoltaic use the optimum efficiency so achievable, over 1.5 times that given by a normal semiconductor, is obtained with an overall bandgap around 2.0 eV (which would be near-optimal also for water phtosplitting). Note that this scheme differs from the doping principle usually considered in photocatalysis, which just tries to decrease the bandgap; its aim is to keep the full bandgap chemical potential but using also lower energy photons. In the past we have proposed several IB materials based on extensively doping known semiconductors with light transition metals, checking first of all with quantum calculations that the desired IB structure results [2]. Subsequently we have synthesized in powder form two of them: the thiospinel In2S3 and the layered compound SnS2 (having bandgaps of 2.0 and 2.2 eV respectively) where the octahedral cation is substituted at a â?^10% level with vanadium, and we have verified that this substitution introduces in the absorption spectrum the sub-bandgap features predicted by the calculations [3]. With these materials we have verified, using a simple reaction (formic acid oxidation), that the photocatalytic spectral response is indeed extended to longer wavelengths, being able to use even 700 nm photons, without largely degrading the response for above-bandgap photons (i.e. strong recombination is not induced) [3b, 4]. These materials are thus promising for efficient photoevolution of hydrogen from water; work on this is being pursued, the results of which will be presented. [1] A. Luque, A. Martí Phys. Rev. Lett. 78, 1977, 5014. [2] a) P. Palacios et al., Phys. Rev. B 73 (2007) 085206; ibid. Thin Solid Films 515 (2007) 6280; ibid. Phys. Rev. Lett. 101 (2008) 046403. [3] a) R. Lucena et al.: Chem. Maters. 20 (2008) 5125. b) P. Wahnón et al. PCCP, in press (DOI: 10.1039/c1cp22664a). [4] R. Lucena et al., submitted.

High efficient III-V semiconductors have been successfully applied for direct photoelectrochemical (PEC) water splitting systems. Monolithic p-GaInP2/n/p-GaAs PEC-PV tandem cell device demonstrated a 12.4% solar-to-hydrogen (STH) conversion efficiency [1], however the lifetime of the device was a challenge due to the corrosion of the material. This study addresses the corrosion issue of the materials and the stability of the top layer. Therefore, thin films of p-GaInP2 were tested 24h in different solutions (including 3M H2SO4 for comparison) at 1 sun. Different analytical techniques (including SEM, EDX, XPS and ICP) were used to analyze the surfaces and the tested solutions. Samples tested in sulfuric acid experienced extensive corrosion and surface Ga was selectively dissolved, identified both from the corroded surface and tested solutions. The samples tested in ammonium-bearing solutions experienced significant less corrosion, almost identical to that of an as-received one. SEM and EDX investigation also confirmed the significant reduced corrosion with samples tested in ammonium-bearing solutions. In other words, ammonium showed inhibitive effect for p-GaInP2, which is very promising to meet US DOEâ?Ts 2013 goal of 1000h durability at 8% STH efficiency.

Hematite has been identified as a promising material for solar water splitting in a so-called tandem-cell configuration where, given its low bandgap (2.1 eV), it absorbs up to 16% of terrestrial solar illumination. The two main challenges when using metal oxides like hematite are the poor overall charge transport and the very short minority carrier diffusion length. The former can be overcome with proper electronic doping and the latter can be addressed through precise nanostructuring of the material. We have previously developed an attractive way to prepare photoactive hematite films using a simple solution-based colloid technique, but the precise doping of intrinsic iron oxide nanoparticles has remained an ongoing challenge. So far, dopant integration in hematite colloids has been possible only when heating the films to a substantially high temperature of 800 °C, causing drastic changes in morphology and growth of the particles size (not to mention the requirement for temperature resistant substrates). Herein, we discuss the challenges of preparing doped iron oxide nanoparticles and present a method to facilitate dopant integration directly during particle fabrication in order to avoid the high temperature annealing step. We further show these particles employed in highly scalable processes for the preparation of efficient hematite photoanodes for solar water splitting. The synthesis route was chosen to be water-free to avoid iron hydroxide formation further reducing the need for high temperature annealing. Iron oxide nanoparticles are found to be crystalline and properly doped as-synthesized with a fine control over diameter and size distribution. The film preparation consists in printing the solution on a conductive substrate while controlling the necking of the particle, the thickness and the porosity in order to achieve efficient solar energy to hydrogen conversion. 1. Sivula, K.; Le Formal, F. & Grätzel, M. (2011), 'Solar Water Splitting: Progress Using Hematite (alpha-Fe2O3) Photoelectrodes', ChemSusChem 4(4), 432-449. 2. Sivula, K.; Zboril, R.; Le Formal, F.; Robert, R.; Weidenkaff, A.; Tucek, J.; Frydrych, J. & Grätzel, M. (2010), 'Photoelectrochemical Water Splitting with Mesoporous Hematite Prepared by a Solution-Based Colloidal Approach', Journal of the American Chemical Society 132(21), 7436-7444.

Molecular framework materials containing light-harvesting and catalytically competent molecules have been rationally designed. These materials act as an excellent light-harvesting system by combining intra-framework energy migration and interfacial electron transfer quenching. We have also demonstrated that framework materials built from catalytic components are active catalysts in a range of reactions that are relevant to solar energy utilization, including water oxidation, carbon dioxide and proton reduction, and photocatalytic organic transfromations. Our work illustrates the potential of combining photosensitizers, molecular catalysts, and framework structures in developing highly active heterogeneous catalysts for solar energy utilization.

Efficient generation of fuels through direct conversion of solar energy requires the development of suitable photo-electrode materials. Amongst the properties that such materials should possess are: strong visible light absorption, high surface area and low cost. Nano-structured metal oxides with high aspect ratios (such as nanowires) can be produced by a variety of techniques included wet-chemistry and plasma-based methods. It has recently been reported that irradiation of metal surfaces at elevated temperatures by low-energy helium ions (energy below the ion damage threshold) leads to the formation of a fibreform nano-structure, with filament diameter below 20nm [1]. To date, this effect has mainly been studied in the context of nuclear fusion where it is potentially detrimental for the lifetime of the tungsten plasma-facing materials. The helium-induced nano-structure does however present very interesting properties for a photocatalytic surface. The very high level of porosity (up to 90%) indeed leads to significant levels of light absorption across the whole solar spectrum, with measured values of up to 98% of the total solar light [2]. The nano-structure is easily turned into a stoichiometric oxide by oxidation in an oxygen-rich environment. In this contribution, we will describe the formation of nano-structured tungsten and molybdenum trioxide surfaces by low energy helium irradiation and subsequent oxidation. The formation kinetics of the helium-induced nano-structure depends strongly on the surface temperature during plasma exposure and the ion energy (in the range 10-50eV). In addition, the size and porosity of the structure depend on the surface temperature. Thermal oxidation has been used to produce stoichiometric WO3 and MoO3 oxides, the structure of which has been studied by high-resolution secondary electron microscopy and X-Ray diffraction while the optical absorption is measured by a spectrophotometer. Results of the photocatalytic activity of such nano-structured oxides towards water splitting will be presented. References: [1] M.J. Baldwin, et al, Nucl. Fusion, 48 (2008) 035001 [2] S. Kajita et al, Applied Physics Express, 3 (2010) 085204

Solar energy harvesting is an important technological challenge, considering that the energy of sunlight that strikes the earthâ?Ts surface in an hour is sufficient to meet our energy demands for a year. Moreover, an economic and mobile energy storage media that does not significantly affect the current energy infrastructure is necessary to offset the diffuse and intermittent nature of sunlight. These challenges could be resolved by generating transportable solar fuels (like hydrogen or methanol) from abundant sources, e.g. H2O and CO2, utilizing sunlight as the primary energy source. Multiple approaches including photoelectrochemical and photocatalytic methods have been proposed and investigated in the past decades. Irrespective of the approach that is pursued, oxygen evolution from water is the critical reaction, because water is the only cheap, clean and abundant source that is capable of completing the redox cycle for producing either hydrogen (from H2O) or carbonaceous fuels (from CO2) on a terawatt scale. Thus, an effective catalyst for oxygen evolution via water oxidation is the key to accomplish the challenge of efficient solar energy harvesting. Here, we will demonstrate that the morphology and crystal structure have negligible effect on the photocatalytic properties of MnO2 based oxygen evolution catalysts, while the turnover rate is proportional to its surface area (i.e. Mn sites available on the surface). In order to testify the hypothesis, a wide range of manganese oxides with various morphologies and polymorphs are synthesized and their structures are well characterized. The as-synthesized catalysts, such as α-MnO2 nanotubes, α-MnO2 nanowires, and β-MnO2 nanowires, exhibit excellent activities in water oxidation driven by visible light. By calculating the TOFs per surface Mn site, the rates for all the different catalysts are similar (~0.001-0.0005 per second per surface Mn), indicating the negligible morphology and crystal structure effects. Based on this finding, one should expect that the highest activity would be obtained from the manganese oxide with the highest surface area. To further enhance the turnover frequencies (TOFs) that limited by surface area, surface active site with a higher TOF rate compared with Mn4+ is required. Along this direction, we introduce K+ doped MnO2 catalysts into oxygen evolution reaction. By doping MnO2 with K+, we create Mn3+ sites on the surface of mixed manganese oxides. Our preliminary data show that more than one order higher oxygen evolution rates per surface Mn were observed. In order to explore the origin of the enhancement in oxygen evolution activity, detailed structural characterizations have been performed and the results indicate that Mn3+ sites generated by K+ doping may be responsible for the high TOFs.

We confirmed that GaN photocatalyst produced hydrogen continuously for 500 hours without etching of GaN layers after the electric charge of 722 coulombs. The energy conversion efficiency from light to hydrogen (H2+1/2O2=H2O, Î"G=-237.13 kJ/mol) was up to 0.72% in spite of no extra bias applied to this photoelectrolysis system. Hydrogen is environmentally clean energy because it becomes only water after combustion. As a method of hydrogen generation, photoelectrolysis of water is promising. Various oxides have been used as photocatalysts so far. We found that GaN is a desirable material to split water by photoelectrolysis without any bias [1]. Moreover, we can control bandgap by changing group-III content of InGaN, which can absorb not only ultraviolet light but also visible one. Although long-time stability have been studied in a few oxides such as Cu2O and InNiTaO4 [2,3], we have already revealed that deposition of NiO co-catalyst on the GaN layer significantly prevents etching during the photoelectrolysis. In this study, we investigated stability of the GaN photocatalyst with NiO for 500 hours. We used a 3-μm-thick GaN layer on a sapphire substrate grown by metalorganic vapor-phase epitaxy and NiO was deposited on the GaN surface. This GaN working electrode connected to a Pt counterelectrode was dipped into a NaOH solution with the concentration of 1 mol/L and no bias was applied to this system. This GaN electrode was irradiated by light from a 300-W Xe lamp with the energy density of 100 mW/cm2. We performed 10-hour experiments for 50 times. Hydrogen was produced continuously for 500 hours, the total amount of which was 82.2 mL, and its evolution rate was 0.164 mL/h. This total time was longer than that for InNiTaO4 [3], and the evolution rate were superior to those for Cu2O [2], that is, GaN is more adequate for photocatalyst than any other materials. The energy conversion efficiency from light to hydrogen was 0.72% at the maximum. In fact, surface state after the experiment had no clear change compared with that before the experiment by a Nomarski microscope, i.e., the GaN layer still survives. This GaN generated the electric charge of 722 C, which was calculated by the integration of photocurrent for 500 hours. In conclusion, we confirmed that the GaN photocatalyst with NiO shows excellent stability. 1) M. Ono, K. Ohkawa et al., J. Chem. Phys. 126, 054708 (2007). 2) M. Hara, K. Domen et al., Chem. Commun. 357, (1998). 3) Z. Zou, H. Arakawa et al., Chem. Nature 625, (2011).

Hydrogen production using photoelectrochemical water splitting represents an attractive approach to realizing a clean, sustainable energy infrastructure. Unfortunately, finding a semiconductor electrode that offers high solar conversion efficiency while remaining stable under operating conditions has been difficult. In large part, this can be traced to a poor understanding of the complex chemistry active at the electrode-electrolyte interface. In order to understanding the reactive states precursory to photoexcitation, hydrogen evolution, and photocorrosion, we have studied the structure, stability, and chemical activity of III-V semiconductor electrodes in the presence of an electrolyte using a combination of first-principles molecular dynamics simulations and model density-functional calculations. We find that a local bond-topological model is able to capture much of the basic chemistry and structural motifs of the surfaces, allowing efficient study of complex surface morphologies built using simpler models and providing a map by which local structural features might be identified experimentally. Our results point to the particular importance of oxygen-containing surface adsorbates in determining the available reaction pathways for photocorrosion and water dissociation. Explicit modeling of water molecules at the interface reveals the importance of hydrogen bonding in determining the surface structure, which has additional implications for surface reactivity.