The material properties and photoelectrochemical performance of compact thin-film InVO4 photoanodes prepared by spray pyrolysis are investigated. Nearly phase-pure orthorhombic InVO4 can be obtained by a postdeposition anneal treatment in air between 450−550 °C. Optical absorption spectra indicate that InVO4 has an indirect bandgap of ∼3.2 eV with a pronounced sub-bandgap absorption starting at ∼2.5 eV. A dielectric constant of 50 and a flatband potential of −0.04 V vs RHE are determined, which confirms that this material is able to evolve hydrogen. Few shallow donors are present in this material, which is markedly different from what is usually observed for simple binary oxides. The main photocurrent response occurs in the UV (<400 nm) and the incident photon-to-current conversion efficiency is less than 1%. The impedance data show that the poor photo response is due to a high density of deep donors and a concomitantly small depletion layer. The visible light absorption of InVO4 is attributed to the presence of ionized deep donors in the space charge region of the material, which explains why InVO4 powders show a much stronger visible light absorption than thin films. The defect-chemical origin of the deep donor state is discussed and some general considerations for the use of ternary and more complex metal oxides as photoelectrodes are outlined.

α-Fe2O3thin film photoanodes for solar water splitting were prepared by spray pyrolysis ofFe(AcAc)3. The donor density in theFe2O3films could be tuned between1017–1020 cm-3by doping with silicon. By depositing a 5 nmSnO2interfacial layer between theFe2O3films and the transparent conducting substrates, both the reproducibility and the photocurrent can be enhanced. The effects of Si doping and the presence of theSnO2interfacial layer were systematically studied. The highest photoresponse is obtained forFe2O3doped with 0.2% Si, resulting in a photocurrent of 0.37 mA/cm2at 1.23 VRHEin a 1.0 M KOH solution under 80 mW/cm2AM1.5 illumination.

Fe-doped and C-doped anatase TiO 2 films were made by spray pyrolysis. For Fe:TiO 2 , a small subbandgap 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 TiO 2 due to the low carbon content, the photocurrent at hν > E g is significantly larger than for undoped TiO 2 . At the same time, the donor density of oxidized C-doped TiO 2 is >1.9 × 10 19 cm −3 , compared to 3.2 × 10 17 cm −3 for undoped TiO 2 . 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 TiO 2 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 TiO 2 .

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