Harvesting energy directly from sunlight as nature accomplishes through photosynthesis is a very attractive and desirable way to solve the energy challenge. Many efforts have been made to find appropriate materials and systems that can utilize solar energy to produce chemical fuels. One of the most viable options is the construction of a photoelectrochemical cell that can reduce water to H(2) or CO(2) to carbon-based molecules. Bismuth vanadate (BiVO(4)) has recently emerged as a promising material for use as a photoanode that oxidizes water to O(2) in these cells. Significant advancement in the understanding and construction of efficient BiVO(4)-based photoanode systems has been made within a short period of time owing to various newly developed ideas and approaches. In this review, the crystal and electronic structures that are closely related to the photoelectrochemical properties of BiVO(4) are described first, and the photoelectrochemical properties and limitations of BiVO(4) are examined. Subsequently, the latest efforts toward addressing these limitations in order to improve the performances of BiVO(4)-based photoanodes are discussed. These efforts include morphology control, formation of composite structures, composition tuning, and coupling oxygen evolution catalysts. The discussions and insights provided in this review reflect the most recent approaches and directions for general photoelectrode developments and they will be directly applicable for the understanding and improvement of other photoelectrode systems.

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Cobalt-based oxygen evolution catalysts containing phosphates (Co-Pi OEC) were photochemically deposited onto the surface of n-type R-Fe 2 O 3 electrodes to enhance solar O 2 production. R-Fe 2 O 3 films used in this study were prepared by electrodepositing Fe films followed by thermal oxidation at 500 °C. The use of a nonaqueous plating solution made it possible to deposit adherent and uniform Fe films, which is difficult to achieve in an aqueous medium. Photodeposition of Co-Pi OEC was carried out by using photogenerated holes in the valence band of R-Fe 2 O 3 to oxidize Co 2þ ions to Co 3þ ions in a phosphate buffer solution, which resulted in the precipitation of Co-Pi OEC on the R-Fe 2 O 3 surface. Two different deposition conditions, open circuit (OC) and short circuit (SC) conditions, were studied comparatively to understand their effect on the growth and composition of Co-Pi OEC deposits. The results showed that the SC condition where the photoreduction reaction is physically separated from the photo-oxidation reaction significantly increased the yield and nucleation density of Co-Pi OECs, resulting in a better coverage of the R-Fe 2 O 3 surface with Co-Pi OEC nanoparticles. X-ray photoelectron spectroscopy showed that the OC condition resulted in a higher Co 2þ /Co 3þ ratio in the Co-Pi OEC deposits than the SC condition. This difference in composition is due to the simultaneous photoreduction occurring on the R-Fe 2 O 3 surface under OC conditions. Co-Pi OEC improved the photocurrent of R-Fe 2 O 3 electrodes more than Co 2þ ions simply adsorbed on the R-Fe 2 O 3 surface and the Co-Pi OEC deposited under SC conditions resulted in the most pronounced photocurrent enhancement. These results demonstrate the advantages of creating a SC condition for photodeposition of Co-Pi OECs. O 2 detection measurements show that the presence of photodeposited Co-Pi OEC on the R-Fe 2 O 3 surface not only increases the total amount of photocurrent generated by facilitating electron-hole pair separation but also increases the photocurrent to O 2 conversion efficiency by improving O 2 evolution kinetics.

The spectra and dynamics of photogenerated electrons and holes in excited hematite (a-Fe 2 O 3 ) electrodes are investigated by transient absorption (from visible to infrared and from femto-to microseconds), bias-dependent differential absorption and Stark spectroscopy. Comparison of results from these techniques enables the assignment of the spectral signatures of photogenerated electrons and holes. Under the pulse illumination conditions of transient absorption (TA) measurement, the absorbed photon to electron conversion efficiency (APCE) of the films at 1.43 V (vs. reversible hydrogen electrode, RHE) is 0.69%, significantly lower than that at AM 1.5. TA kinetics shows that under these conditions, >98% of the photogenerated electrons and holes have recombined by 6 ms. Although APCE increases with more positive bias (from 0.90 to 1.43 V vs. RHE), the kinetics of holes up to 6 ms show negligible change, suggesting that the catalytic activity of the films is determined by holes with longer lifetimes.

α-Fe 2 O 3 /ZnFe 2 O 4 composite electrodes are prepared via a simple surface treatment performed on nanoparticulate α-Fe 2 O 3 electrodes. The α-Fe 2 O 3 electrodes are uniformly covered with a solution containing Zn 2+ ions which react with Fe 2 O 3 upon heating to form ZnFe 2 O 4 . As Zn 2+ ions do not completely diffuse into the core of Fe 2 O 3 particles under mild heating conditions, the ZnFe 2 O 4 forms only on the surface of Fe 2 O 3 as a shell layer, resulting in the formation of Fe 2 O 3 /ZnFe 2 O 4 composite electrodes. Any unreacted ZnO is removed by dissolution in 1 M NaOH, where ZnFe 2 O 4 and Fe 2 O 3 are stable. X-ray diffraction and energy-dispersive spectroscopy studies show that a crystalline ZnFe 2 O 4 phase forms after the heat treatment and the composite electrode with the best photoelectrochemical performances contains a 1:1 mol ratio of ZnFe 2 O 4 and Fe 2 O 3 . The Fe 2 O 3 /ZnFe 2 O 4 composite electrode shows a significantly enhanced photocurrent response compared to the bare Fe 2 O 3 electrode because ZnFe 2 O 4 has conduction and valence band edges shifted ca. 200 mV from those of Fe 2 O 3 to the negative direction which allows for the efficient separation of electronÀhole pairs at the Fe 2 O 3 /ZnFe 2 O 4 interface. Further improvement in photocurrent is observed when the surface is modified by an Al 3+ treatment that forms thin ZnFe 2Àx Al x O 4 or Fe 2Àx Al x O 3 layers that reduce surface states created by Fe 3+ ions exposed on the surface having imperfect coordination environments. The formation of ZnFe 2 O 4 and Al 3+ -containing layers make the surface less catalytic for O 2 evolution, and therefore, introduction of the Co 2+ ions as oxygen evolution catalysts further improved the over performance of the composite electrodes. KEYWORDS: zinc ferrite (ZnFe 2 O 4 ), iron oxide (Fe 2 O 3 ), water oxidation, photoanode, solar energy conversion dx.