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Fe-doped and C-doped anatase TiO2 films were made by spray pyrolysis. For Fe:TiO2, a small sub-bandgap photoresponse is observed which is attributed to the presence of additional states located just above the valence band. Although no visible-light photoresponse is observed for carbon-doped TiO2 due to the low carbon content, the photocurrent at hν > Eg is significantly larger than for undoped TiO2. At the same time, the donor density of oxidized C-doped TiO2 is > 1.9× 1019 cm−3, compared to 3.2× 1017 cm−3 for undoped TiO2. Assuming that only light absorbed in the depletion layer contributes to the photocurrent, the photoresponse of C-doped anatase (at 330 nm) is 16 times larger than that predicted for undoped TiO2 under similar conditions. The strong enhancement of the absorption is most likely caused by a change in the electronic structure of the material due to the presence of carbon and/or related defects. Photoluminescence measurements suggest that the defects present in oxidized carbon-doped anatase resemble those present in undoped, reduced TiO2.

Thin, smooth anatase TiO2 films with a thickness of about 100 nm have been made by reactive electron beam evaporation of reduced TiO2. The influence of the oxygen partial pressure is investigated in the range from 2×10−5 to 2×10−3 mbar, and the influence of the substrate temperature is studied in the range from 150 to 300 °C. The structure and properties of the films are investigated with x-ray diffraction, transmission electron microscopy, Raman spectroscopy, impedance spectroscopy, and capacitance–voltage profiling. Depending on the type of substrate, two different morphologies are obtained. On titanium-coated fused silica, grains with a preferred (004) orientation and sizes ranging from 40 to 60 nm in directions parallel to the surface, and from 20 to 125 nm perpendicular to the surface are found. On fused silica, a monolayer of randomly oriented single crystal disks with lateral sizes between 0.2 and 2 μm is found. These films may provide a convenient alternative to single crystals in fundamental stud...

An "ink" (cyanamide) infiltrated anodic aluminum oxide (AAO) stamp is found capable of printing carbon nitride films featuring regular microstructures of the stamp onto the substrates via in situ "chemical vapor deposition". A photocurrent density of 30.2 μA cm(-2 --) at 1.23 VRHE is achieved for a film on a conductive substrate, which is so far the highest value for pure carbon nitride based photoelectrochemical devices.

Recent progress in the development of bismuth vanadate (BiVO4) photoanodes has firmly established it as a promising material for solar water splitting applications. Performance limitations due to intrinsically poor catalytic activity and slow electron transport have been successfully addressed through the application of water oxidation co-catalysts and novel doping strategies. The next bottleneck to tackle is the modest optical absorption in BiVO4, particularly close to its absorption edge of 2.4 eV. Here, we explore the modification of the BiVO4 surface with Ag@SiO2 core-shell plasmonic nanoparticles. A photocurrent enhancement by a factor of ~2.5 is found under 1 sun illumination (AM1.5). We show that this enhancement consists of two contributions: optical absorption and catalysis. The optical absorption enhancement is induced by the excitation of localized surface plasmon resonances in the Ag nanoparticles, and agrees well with our full-field electromagnetic simulations. Far-field effects (scattering) are found to be dominant, with a smaller contribution from near-field plasmonic enhancement. In addition, a significant catalytic enhancement is observed, which is tentatively attributed to the electrocatalytic activity of the Ag@SiO2 nanoparticles.

We demonstrate a photoelectrochemical/photovoltaic (PEC/PV) device based on a combined configuration with a bismuth vanadate (BiVO 4 ) photoanode in the front and a thin film solar cell as power source in the rear. An optical model of this PEC/PV configuration is built by our in-house developed Advance Semiconductor Analysis (ASA) software. Micromorph (a-Si:H/nc-Si:H) cells are investigated in the optical model in reference to the photoanode filtered solar spectrum. The optimum micromorph cell with 400 nm a-Si:H i-layer is confirmed to have the best performance when combined with BiVO 4 photoanode. A solar-to-hydrogen efficiency of 5.2%, the highest conversion efficiency of metal oxide materials ever reported is achieved.

A hybrid photovoltaic/photoelectrochemical (PV/PEC) water-splitting device with a benchmark solar-to-hydrogen conversion efficiency of 5.2 % under simulated air mass (AM) 1.5 illumination is reported. This cell consists of a gradient-doped tungsten–bismuth vanadate (W:BiVO_4) photoanode and a thin-film silicon solar cell. The improvement with respect to an earlier cell that also used gradient-doped W:BiVO4 has been achieved by simultaneously introducing a textured substrate to enhance light trapping in the BiVO4 photoanode and further optimization of the W gradient doping profile in the photoanode. Various PV cells have been studied in combination with this BiVO_4 photoanode, such as an amorphous silicon (a-Si:H) single junction, an a-Si:H/a-Si:H double junction, and an a-Si:H/nanocrystalline silicon (nc-Si:H) micromorph junction. The highest conversion efficiency, which is also the record efficiency for metal oxide based water-splitting devices, is reached for a tandem system consisting of the optimized W:BiVO_4 photoanode and the micromorph (a-Si:H/nc-Si:H) cell. This record efficiency is attributed to the increased performance of the BiVO_4 photoanode, which is the limiting factor in this hybrid PEC/PV device, as well as better spectral matching between BiVO_4 and the nc-Si:H cell.

The design of a fluidized bed atomic layer deposition (ALD) reactor is described in detail. The reactor consists of three parts that have all been placed in one protective cabinet: precursor dosing, reactor, and residual gas treatment section. In the precursor dosing section, the chemicals needed for the ALD reaction are injected into the carrier gas using different methods for different precursors. The reactor section is designed in such a way that a homogeneous fluidized bed can be obtained with a constant, actively controlled, reactor pressure. Furthermore, no filters are required inside the reactor chamber, minimizing the risk of pressure increase due to fouling. The residual gas treatment section consists of a decomposition furnace to remove residual precursor and a particle filter and is installed to protect the pump. In order to demonstrate the performance of the reactor, SiO2 particles have been coated with TiO2 using tetrakis-dimethylamino titanium (TDMAT) and H2O as precursors. Experiments with varying pulse times show that saturated growth can be obtained with TDMAT pulse times larger than 600 s. Analysis of the powder with High-Angle Annular Dark-Field Scanning Transmission Electron Microscopy (HAADF-STEM) and energy dispersive X-ray spectroscopy confirmed that after 50 cycles, all SiO2 particles were coated with a 1.6 nm homogenous shell of TiO2.

The development of new energy materials that can be utilized to make renewable and clean fuels from abundant and easily accessible resources is among the most challenging and demanding tasks in science today. Solar-powered catalytic water-splitting processes can be exploited as a source of electrons and protons to make clean renewable fuels, such as hydrogen, and in the sequestration of CO2 and its conversion into low-carbon energy carriers. Recently, there have been tremendous efforts to build up a stand-alone solar-to-fuel conversion device, the "artificial leaf", using light and water as raw materials. An overview of the recent progress in electrochemical and photo-electrocatalytic water splitting devices is presented, using both molecular water oxidation complexes (WOCs) and nano-structured assemblies to develop an artificial photosynthetic system.

Bismuth vanadate (BiVO4) is a promising semiconductor material for photoelectrochemical water splitting showing good visible light absorption and a high photochemical stability. To improve the performance of BiVO4, it is of key importance to understand its photophysics upon light absorption. Here we study the carrier dynamics of BiVO4 prepared by the spray pyrolysis method using broadband transient absorption spectroscopy (TAS), in thin films as well as in a photoelectrochemical (PEC) cell under water-splitting conditions. The use of a dual-laser setup consisting of electronically synchronized Ti:sapphire amplifiers enable us to measure the femtosecond to microsecond time scales in a single experiment. On the basis of this data, we propose a model of carrier dynamics that includes relaxation and trapping rates for electrons and holes. Hole trapping occurs in multiple phases, with the majority of the photogenerated holes being trapped with a time constant of 5 ps and a small fraction of this hole trapping ...

Changes in the optical absorption and electrical conductivity of dense and mesoporous anatase TiO2 films were measured in situ as a function of electrode potential during electrochemical lithium intercalation. A special two-electrode geometry was used for the conductivity measurements, in which the contacts were separated by a small gap bridged by the TiO2. When electrons are injected, an accumulation layer is formed and the conductivity increases several orders of magnitude. A monotonic increase of the optical absorption with wavelength confirms the presence of (partially) delocalized electrons. Insertion of lithium ions results in the formation of the Li0.5TiO2 phase and a decrease of the overall conductance. The specific conductivity of the Li0.5TiO2 phase is (9.1±0.2) S/cm, significantly lower than that of Li-doped anatase TiO2. This is corroborated by the absorption spectrum of Li0.5TiO2, which shows two pronounced peaks around 440 and 725 nm and no characteristic free-electron features. At potential...

We study the initial interaction of adsorbed H2O with P-rich and Ga-rich GaP(100) surfaces. Atomically well defined surfaces are prepared by metal-organic vapour phase epitaxy and transferred contamination-free to ultra-high vacuum, where water is adsorbed at room temperature. Finally, the surfaces are annealed in vapour phase ambient. During all steps, the impact on the surface properties is monitored with in-situ reflection anisotropy spectroscopy (RAS). Photoelectron spectroscopy and low-energy electron diffraction are applied for further in-system studies. After exposure up to saturation of the RA spectra, the Ga-rich (2 x 4) surface reconstruction exhibits a sub-monolayer coverage in form of a mixture of molecularly and dissociatively adsorbed water. For the p(2 x 2)/c(4 x 2) P-rich surface reconstruction, a new c(2 x 2) superstructure forms upon adsorption and the uptake of adsorbate is significantly reduced when compared to the Ga-rich surface. Our findings show that microscopic surface reconstructions of GaP(100) greatly impact the mechanism of initial interface formation with water, which could benefit the design of e.g. photoelectrochemical water splitting devices., Comment: 18 pages, 10 figures, accepted in New Journal of Physics

Metal oxides are generally very stable in aqueous solutions and cheap, but their photochemical activity is usually limited by poor charge carrier separation. Here we show that this problem can be solved by introducing a gradient dopant concentration in the metal oxide film, thereby creating a distributed n(+)-n homojunction. This concept is demonstrated with a low-cost, spray-deposited and non-porous tungsten-doped bismuth vanadate photoanode in which carrier-separation efficiencies of up to 80% are achieved. By combining this state-of-the-art photoanode with an earth-abundant cobalt phosphate water-oxidation catalyst and a double- or single-junction amorphous Si solar cell in a tandem configuration, stable short-circuit water-splitting photocurrents of ~4 and 3 mA cm(-2), respectively, are achieved under 1 sun illumination. The 4 mA cm(-2) photocurrent corresponds to a solar-to-hydrogen efficiency of 4.9%, which is the highest efficiency yet reported for a stand-alone water-splitting device based on a metal oxide photoanode.

One of the main challenges in developing highly efficient nanostructured photoelectrodes is to achieve good control over the desired morphology and good electrical conductivity. We present an efficient plasma-processing technique to form porous structures in tungsten substrates. After an optimized two-step annealling procedure, the mesoporous tungsten transforms into photoactive monoclinic WO3. The excellent control over the feature size and good contact between the crystallites obtained with the plasma technique offers an exciting new synthesis route for nanostructured materials for use in processes such as solar water splitting.

Metal oxides have emerged as attractive candidates for photoelectrochemical water splitting, mainly due to their good stability in aqueous solutions, easy synthesis, and low cost. One of the most promising metal oxide photoanodes is bismuth vanadate (BiVO4). This metal oxide evolves oxygen under illumination with visible light, and is stable in aqueous solutions with pH between 3 and 11. However, in the past, BiVO4 suffered from low charge injection efficiency. This problem has been solved by functionalizing its surface with water oxidation catalysts (e.g. CoPi, RhO2, FeOOH, NiFeOx, MnOx)1-5, resulting in significant performance improvements. However, the true nature of semiconductor-catalyst interactions and the improvement mechanisms are still unclear, and requires further attention. In this study, we used intensity modulated photocurrent spectroscopy (IMPS) to examine the surface charge carrier dynamics of spray-deposited bismuth vanadate (BiVO4) photoanodes and the effect of depositing different co-catalysts on its surface. An LED was used to illuminate the sample with a modulated intensity, and the real and imaginary parts of the opto-electrical impedance were recorded. To interpret the resulting spectra, we used a model developed by Peter et al. that allows one to distinguish the rate constants for surface recombination and charge injection into the electrolyte. We found that the photocurrent of BiVO4 is mainly limited by surface recombination. This is evident by the significant reduction of the surface recombination, when CoPi and NixMnyOz were deposited on the BiVO4 photoanodes. Surprisingly, we did not observe any enhancement of charge transfer rate constant into the electrolyte. This result suggests that BiVO4 is already a good oxidation catalyst by itself, which is consistent with the relatively positive potential of the valence band edge, resulting in a high thermodynamic driving force. This observation contradicts the common assumption that these catalysts are functioning as electrocatalysts, and shows that their role on our BiVO4 is mainly that of a surface passivation layer. Based on this we predict that the modification of BiVO4 surface with RuOx, a well-known (non-passivating) oxygen evolution catalyst, does not improve the performance; our experiments show that this is indeed the case. These results allow us to develop a modified carrier dynamics model, which will be discussed here. We further examined the nature of the surface states with a combination of cyclic voltammetry and spectroelectrochemistry. We also compared BiVO4 prepared by various deposition techniques—both chemical and physical routes—to demonstrate their influence on the nature of the surface of BiVO4. The semiconductor/co-catalyst/electrolyte interface was also investigated by depositing CoPi onto those differently prepared BiVO4 photoanodes. Finally, the diverse effects of porous (ion-permeable) and dense (non-ion-permeable) co-catalyst on the photoelectrochemical performance of such semiconductor/co-catalyst systems will be discussed. [1] Park, Y.; McDonald, K. J.; Choi, K.-S. Chem. Soc. Rev. 2013, 42 (6), 2321. [2] Kim, T. W.; Choi, K.-S. Science (80-. ). 2014, 343 (6174), 990. [3] Ma, Y.; Le Formal, F.; Kafizas, A.; Pendlebury, S. R.; Durrant, J. R. J. Mater. Chem. A 2015, 3 (41), 20649. [4] Zhong, D. K.; Choi, S.; Gamelin, D. R. J. Am. Chem. Soc. 2011, 133 (45), 18370. [5] Trotochaud, L.; Young, S. L.; Ranney, J. K.; Boettcher, S. W. J. Am. Chem. Soc. 2014, 136 (18), 6744.

Solar powered electrolysis of water is a promising way to harvest and store solar energy in the form of large quantities of hydrogen. The photovoltage of semiconducting materials is used in photoelectrochemical cells or photovoltaic devices to provide the required thermodynamic potential for water splitting (1.23V) and the over-potentials for the oxygen evolution reaction (OER) and the hydrogen evolution reaction (HER) under illumination. In order to keep the electrochemical overvoltages as low as possible, highly active electrocatalysts are deposited onto the semiconducting absorbers. Since the catalytic layer could potentially shadow the semiconducting light absorber, crucially important is balancing catalytic activity with optical transparency. The development of low cost, but highly efficient, transparent and stable electro-catalysts is currently a challenging issue for solar water splitting devices. Among the electro-catalytic materials being pursued, manganese oxide is an especially promising material due to its low cost, high activity and stability in alkaline electrolytes. In this contribution, we have investigated electrodeposited and annealed α-Mn2O3 layers as catalysts for the OER in alkaline electrolytes (pH14) and evaluated their suitability for use in photoelectrochemical devices. The electrochemically deposited α-Mn2O3 layers are highly porous and we could vary the layer thickness over a wide range (nm to µm). In cyclo-voltammetry experiments, all samples show an anodic oxidation process before the OER starts. This can be explained by a partial oxidation of Mn3+ to Mn4+ as verified by in-line XPS studies. From current-voltage measurements and the specific capacity of these electrodes, we were able to determine the specific current densities involved in the OER (related to the effective accessible electrochemical surface area, ECSA) which dependence on the layer thickness. We found that the specific OER activity increases from ca. 0.01mA/cm2 (ECSA) for very thin layers to ca. 0.2 mA/cm2 (ECSA) for µm thick layers (at an overvoltage of 350mV). In order to investigate this effect in more detail we also prepared well-defined Mn2O3 layers with different thicknesses in the nm range by atomic layer deposition. First results are presented and compared to the electrochemically deposited materials. With respect to “benchmark” catalysts (e.g. Fe/Ni) the specific activity of the electrodeposited α-Mn2O3 is only moderate but high current densities can still be achieved at low over-potentials. This is attributed to the high electrochemically active surface areas of the electrodes (10mA/cm2 (electrode surface) at 340mV over-voltage). Long-term tests revealed high stability of the electrodes operated in pH14 electrolyte at 10mA/cm2. Together with the light transmission spectra of the α-Mn2O3-electrodes, in total, these data allows us to assess the applicability of Mn2O3 as a catalyst to combine with various light absorbing materials. As an example, the influence of Mn2O3 deposition on the photoelectrochemical performance of semiconducting BiVO4 and Fe2WO6 electrodes will be discussed in terms of charge carrier dynamics at the solid/electrolyte interface.

Part I: Basic Principles: Introduction.- Principles of Photoelectrochemical Cells.- Photoelectrochemical Measurements.- Part II: Materials Properties and Synthesis: Nanostructured alpha-Fe2O3 Photoanodes.- Mixed Metal Oxide Photoelectrodes and Photocatalysts.- Combinatorial Identification and Optimization of New Oxide Semiconductors.- Part III: Devices and Device Characterization.- Multijunction Approaches to Photo-electrochemical Water Splitting.- Part IV: Future Perspectives.- Economic and Business Perspectives.- Emerging Trends in Water Photoelectrolysis.

Bismuth vanadate (BiVO4) thin films are deposited by a low-cost and scalable spray pyrolysis method. Its performance under AM1.5 illumination is mainly limited by slow water oxidation kinetics. We confirm that cobalt phosphate (Co-Pi) is an efficient water oxidation catalyst for BiVO4. The optimum thickness of BiVO4 is 300 nm, resulting in an AM1.5 photocurrent of 1.9 mA/cm2 at 1.23 V vs. RHE when catalyzed with Co-Pi. Once the water oxidation limitation is removed, the performance is limited by low charge separation efficiency. This causes more than 60% of the electron-hole pairs to recombine before reaching the respective interfaces. The slow electron transport is shown to be the main cause of this low efficiency, and future efforts should therefore be focused on addressing this key limitation.

In this chapter, the basic principles of photoelectrochemical water splitting are reviewed. After a brief introduction of the photoelectrochemical cell and the electrochemical reactions involved, the electronic structure and properties of semiconductors are discussed. The emphasis is on metal oxide semiconductors, and special attention is given to the presence of ionic point defects in these materials. This is followed by a closer look at the semiconductor/electrolyte interface. The energy conversion efficiency and different definitions of the quantum efficiency are treated next. The chapter concludes with a brief outline of the material’s requirements and challenges facing the development of highly efficient photoelectrodes.

The properties of thin film InVO4 photoanodes for water splitting have been studied. Compact films of InVO4 were prepared by spray pyrolysis and are found to be stable between pH 3 and 11. Although the indirect bandgap is 3.2 eV, a modest amount of visible light absorption is observed. The origin of this absorption is attributed to the presence of deep donor states at ?0.7 eV below the conduction band. These donor states presumably correspond to oxygen vacancies, which form as a result of small but unavoidable deviations of In:V from the ideal 1:1 stoichiometry during the wet-chemical synthesis process. Shallow donors are absent in this material, in contrast to what is normally observed for metal oxides. The deep donor model explains the much stronger visible light absorption of powders compared to thin films. The defect chemical reactions that lead to the formation of the deep donors are shown, and are supported by photoluminescence data

Novel thin fi lm composite photocathodes based on device-grade Cu(In,Ga) Se 2 chalcopyrite thin fi lm absorbers and transparent conductive oxide Ptimplemented TiO 2 layers on top are presented for an effi cient and stable solar-driven hydrogen evolution. Thin fi lms of phase-pure anatase TiO 2 are implemented with varying Pt-concentrations in order to optimize simultaneously i) conductivity of the fi lms, ii) electrocatalytic activity, and iii) light-guidance toward the chalcopyrite. Thereby, high incident-photon-tocurrent-effi ciencies of more than 80% can be achieved over the full visible light range. In acidic electrolyte (pH 0.3), the most effi cient Pt-implemented TiO 2 ‐Cu(In,Ga)Se 2 composite electrodes reveal i) photocurrent densities up to 38 mA cm −2 in the saturation region (−0.4 V RHE, reversible hydrogen electrode), ii) 15 mA cm −2 at the thermodynamic potential for H 2 -evolution (0 V RHE), and iii) an anodic onset potential shift for the hydrogen evolution (+0.23 V RHE). It is shown that the gradual increase of the Ptconcentration within the TiO 2 layers passes through an effi ciency- and stability-maximum of the device (5 vol% of Pt precursor solution). At this maximum, optimized light-incoupling into the device-grade chalcopyrite light-absorber as well as electron conductance properties within the surface layer are achieved while no degradation are observed over more than 24 h of operation.

To enhance the visible light absorption of anatase TiO2 photo-electrodes, the material was doped with carbon by two different methods: i) by spray pyrolysis under a CO2 atmosphere, and ii) by a post-deposition thermal treatment in a hexane-containing environment. For the hexane-treated samples, most of the carbon is located at the surface, from which it can be removed by re-oxidation at elevated temperatures. In addition, both methods seem to result in the presence of small amounts of carbon in the bulk of the material, as deduced from a small red-shift of the absorption edge of TiO2.

The photoluminescence (PL) of poly(3-octyl-thiophene) (P3OT) thin films applied on TiO2 substrates is compared to the PL of P3OT films applied on quartz. Quenching of excitons occurs at the P3OT/TiO2 interface and not at the P3OT/quartz interface. Yet, in the former case the PL intensity is stronger than in the latter. In particular, P3OT films less than 5 nm thick lumines much more when applied on TiO2, which is in striking contrast to what one expects. For films thicker than 10 nm, the increase of the PL as function of the film thickness is the same for TiO2 and for quartz, which indicates that the PL enhancement originates at the interface. The dissociation of excitons at the P3OT/TiO2 interface yields positive polarons in P3OT, which is not the case at the P3OT/quartz interface. We postulate that interaction between positive polarons and excitons explain the observed enhancement of the PL at the P3OT/TiO2 interface.

Sunlight is a nearly inexhaustible energy source, but its large-scale use requires an efficient storage solution. One promising storage concept is the direct conversion of light into chemical energy, for example by producing hydrogen through solar-powered water electrolysis. In order to provide the required minimum photovoltage of 1.23 V, photoelectrochemical cells can be made from suitable semiconducting materials that form a semiconductor/liquid junction. Another option is the application of photovoltaic devices (buried junctions) whose front- and back-contacts are functionalized with electrocatalysts for the hydrogen and oxygen evolution reactions (HER, OER). In both cases the development of low cost, but highly efficient electro-catalysts is a challenging task. Especially for the HER, alternative materials are in demand in order to avoid the current need for expensive noble metal catalysts in acidic electrolytes. In this contribution we investigated the structural and electrochemical properties of MoS2 nano-layers as an earth abundant catalyst for electrochemical hydrogen evolution. The MoS2 nano layers were prepared by impregnating multi walled carbon nanotubes (MWCNT) by a wet chemical process. The electrochemical activity towards the HER was investigated by cyclic voltammetry and electrochemical mass spectroscopy analysis (EMS). The absence of any H2S signal in EMS shows that no or only marginal corrosion occurs under hydrogen evolution conditions in acidic electrolytes. From DFT calculations it has been predicted that the edges of the hexagonal layered MoS2 are the active centers for the HER. In order to verify this prediction, we studied samples with different nanotube diameters and for comparison also commercial MoS2 powders with different particle sizes. As a descriptor for the ratio between edges and basal planes we used the current waves which are due to oxidation processes at the edges and planes and correlate it with the activity for the HER. The same was done with the E2g and A1g Raman signals which also reflect the amount of edges and basal planes in the MoS2. In the light of this analysis the influence of the crystalline structure on the catalytic stability and activity will be discussed. Finally, in order to demonstrate the concept of solar water splitting we deposited MWCNT-MoS2 as a catalyst for the HER onto a a-Si/µc-Si triple-junction solar cell. The “superstrate geometry” of these solar cells enables illumination from the transparent glass substrate side so that no shadowing or scattering effects caused by the catalyst or by gas bubbles influence the performance. RuO2 or Co2O3OER catalysts were used for these ‘artificial leafs’ and the efficiency under AM 1.5 illumination was determined. The figure shows transmission electron spectroscopy analysis which reveals the formation of thin MoS2 layers wrapping multi walled carbon nano tubes.

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The adsorption and release of nitrogen oxide (NO) molecules at the surface of Fe-doped TiO2 nanoparticles, made by a template-free sol–gel process, has been studied. Fe3+ ions are found to be highly effective NO adsorption sites in Fe/TiO2. Up to [similar]89 μmol g−1 of the adsorbed NO can be released by exposure to trace amounts of H2O, which exceeds the water-induced release capacity of other NO storage materials by at least a factor of 2. The surprising efficiency of water as a trigger molecule for NO release is due to its large dipole moment, which causes a much stronger coordination to the Fe3+ species than is the case for NO. In addition, a new IR band at 1840 cm−1 has been found for Fe-doped TiO2 nanoparticles exposed to NO gas. This band is assigned to the stretch vibration of an NO species coordinatively bonded to an Fe3+ site. In contrast to Fe2+–NO vibrations in e.g.zeolites, Fe3+–NO vibrations are seldom observed, and this is the first time that convincing evidence is reported for the presence of Fe3+–NO in Fe/TiO2. The implications of these findings for the application of Fe-doped TiO2 as a novel NO storage and release material are briefly discussed

The properties of thin film InVO4 photoanodes for water splitting have been studied. Compact films of InVO4 were prepared by spray pyrolysis and are found to be stable between pH 3 and 11. Although the indirect bandgap is 3.2 eV, a modest amount of visible light absorption is observed. The origin of this absorption is attributed to the presence of deep donor states at ?0.7 eV below the conduction band. These donor states presumably correspond to oxygen vacancies, which form as a result of small but unavoidable deviations of In:V from the ideal 1:1 stoichiometry during the wet-chemical synthesis process. Shallow donors are absent in this material, in contrast to what is normally observed for metal oxides. The deep donor model explains the much stronger visible light absorption of powders compared to thin films. The defect chemical reactions that lead to the formation of the deep donors are shown, and are supported by photoluminescence data

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The direct conversion of solar energy into hydrogen represents an attractive but challenging alternative for photo-voltaic solar cells. Several metal oxide semiconductors are able to split water into hydrogen and oxygen upon illumination, but the efficiencies are still (too) low. The operating principles of photo-electrochemical devices for water splitting, their main bottlenecks, and the various device concepts will be reviewed. Materials properties play a key role, and the advantages and pitfalls of the use of interfacial layers and dopants will be discussed. Special attention will be given to recent progress made in the synthesis of nanostructured metal oxides with high aspect ratios, such as nanowire arrays, which offers new opportunities to develop efficient photo-active materials for solar water splitting.

Understanding the oxygen evolution reaction (OER) activity and stability of the NiFe-based materials is important for achieving low-cost and highly efficient electrocatalysts for practical water splitting. Here, we report the roles of Ni and Fe on the OER activity and stability of metallic NiFe and pure Ni thin films in alkaline electrolyte. Our results support that the Ni(OH)2/NiOOH does not contribute to the OER directly, but it serves as an ideal host for Fe incorporation, which is essential for obtaining high OER activity. Furthermore, the availability of Fe in the electrolyte is found to be important and necessary for both NiFe and pure Ni thin films to maintain enhanced OER performance, while the presence of Ni is detrimental to the OER kinetics. The impacts of Fe and Ni species present in KOH on the OER activity are consistent with the dissolution/re-deposition mechanism we proposed. Stability studies show that the OER activity will degrade under prolonged continuous operation. Satisfactory stability can, however, be achieved with intermittent OER operation in which the electrocatalyst is cycled between a degraded and recovered state. Accordingly, two important ranges, i.e., the recovery range and the degradation range, are proposed. Compared to intermittent OER operation, prolonged continuous OER operation (i.e. in the degradation range) generates a higher NiOOH content in the electrocatalyst, which is likely related to the OER deactivation. If the electrode works in the recovery range for a certain period, i.e., at a sufficiently low reduction potential, where the Ni3+ is reduced to Ni2+, the OER activity can be maintained and even improved, if also Fe is present in the electrolyte.

International audience; The formation of heterostructures has proven to be a viable way to achieve high photoelectrochemical water splitting efficiencies with BiVO4 based photoanodes. Especially, cobalt and nickel based oxides are suitable low cost contact materials. However, the exact role of these contact materials is not yet completely understood because of the difficulty to individually quantify the effects of surface passivation, charge carrier separation and catalysis on the efficiency of a heterostructure. In this study, we used photoelectron spectroscopy in combination with in situ thin film deposition to obtain direct information on the interface structure between polycrystalline BiVO4 and NiO, CoOx and Sn-doped In2O3 (ITO). Strong upwards band bending was observed for the BiVO4/NiO and BiVO4/CoOx interfaces without observing chemical changes in BiVO4, while limited band bending and reduction of Bi and V was observed while forming the BiVO4/ITO interface. Thus, the tunability of the Fermi level position within BiVO4 seems to be limited to a certain range. The feasibility of high upwards band bending through junctions with high work function (WF) compounds demonstrate that nickel oxide and cobalt oxide are able to enhance the charge carrier separation in BiVO4. Similar studies could help to identify whether new photoelectrode materials and their heterostructures would be suitable for photoelectrochemical water splitting.

In devices based on CdS, indium is often used to make Ohmic contacts. Since indium is scarce and expensive, suitable replacement materials need to be found. In this work, we show that sputtered titanium nitride forms an Ohmic contact with n-type CdS. The CdS films, deposited with chemical bath deposition, have a hexagonal crystal structure and are polycrystalline, mostly with a (002) texture. The thickness of the films is ?600 nm, and the donor density is 1.9?×?1016 cm?3. The donor density increases to 1.5?×?1017 cm?3 upon annealing. The contact resistivity of sputtered TiN on CdS is found to be 4.7?±?0.6 ? cm2. This value is sufficiently small to avoid large resistive losses in most CdS device applications. To demonstrate the use of TiN in a CdS device, a Au/CdS/TiN Schottky diode was constructed. The diode has a potential barrier of 0.69 V and an ideality factor of 2.2.