BiVO(4) films were prepared by a simple electrodeposition and annealing procedure and studied as oxygen evolving photoanodes for application in a water splitting photoelectrochemical cell. The resulting BiVO(4) electrodes maintained considerable photocurrent for photo-oxidation of sulfite, but generated significantly reduced photocurrent for photo-oxidation of water to oxygen, also decaying over time, suggesting that the photoelectrochemical performance of BiVO(4) for water oxidation is mainly limited by its poor catalytic ablity to oxidize water. In order to improve the water oxidation kinetics of the BiVO(4) electrode, a layer of FeOOH was placed on the BiVO(4) surface as an oxygen evolution catalyst using a new photodeposition route. The resulting BiVO(4)/FeOOH photoanode exhibitied significantly improved photocurrent and stability for photo-oxidation of water, which is one of the best among all oxide-based phoatoanode systems reported to date. In particular, the BiVO(4)/FeOOH photoanode showed an outstanding performance in the low bias region (i.e., E < 0.8 V vs RHE), which is critical in determining the overall operating current density when assembling a complete p-n photoelectrochemical diode cell. The photocurrent-to-O(2) conversion efficiency of the BiVO(4)/FeOOH photoanode is ca. 96%, confirming that the photogenerated holes in the BiVO(4)/FeOOH photoanode are indeed excusively used for O(2) evolution.

In this article, we present recent advances that we have achieved toward improving the properties of anodically formed semiconducting TiO 2 nanotubes as well as nanowire arrays as electrodes for oxidative photoelectrochemistry. The morphology, crystallinity, composition, and illumination geometry of nanotube or nanowire arrays are critical factors in their performance as photoelectrodes. We discuss the key aspects relating to each factor and the advances achieved in improving each. With respect to the more fully investigated nanotube arrays, the ability to control the morphological parameters such as pore size, tube length, and wall thickness of the nanotube architecture has enabled high performance in applications such as water photoelectrolysis, photocatalysis, dye-sensitized solar cells, and heterojunction TiO 2 -polymer hybrid solar cells. We begin by reviewing the photoelectrochemical performance of state-of-the-art nanotube arrays fabricated on planar substrates. We then present more recent results related to the growth of TiO 2 nanotube arrays on nonplanar substrates designed in such a way that reflected light normally lost to free space is instead directed to a different point on the device, in turn improving overall photoconversion efficiency. Insofar as the crystallinity of the nanotubes is concerned, the use of a high-temperature oxygen or air-ambient anneal to crystallize the nanotube arrays is disadvantageous, since it results in a thick barrier layer where recombination losses occur and also because it precludes compatibility with polymeric substrates. In this regard, we discovered a twostep fabrication process for synthesis of crystallized nanotube arrays at low-temperatures. The photoelectrochemical applications of TiO 2 are limited by its large electronic band gap. We briefly review the cationic and anionic doping approaches popularly used to modify the TiO 2 band gap. We consider the use of ternary oxide systems containing titania as both a structural support and corrosion-inhibitor, in particular fabrication and performance of n-type Ti-Fe-O nanotubes and p-type copper-rich Cu-Ti-O nanotubes, with a note on our recent synthesis of iron oxide nanotube arrays by anodic oxidation of iron. Fabrication and photoelectrochemical properties of CdS-TiO 2 and CdTe-TiO 2 nanotube array heterojunction photoelectrodes are discussed. The article concludes by examining low temperature synthesis, and resulting properties, of single crystal vertically oriented TiO 2 nanowire arrays on transparent conductive glass substrates; preliminary investigation of these nanowire array photoelectrodes for water photolysis reveals them to have low series resistance and provide excellent separation of photogenerated charges.

A bare WO 3 electrode and a WO 3 electrode coupled with a layer of Co-Pi oxygen evolution catalyst (OEC) were prepared to investigate the effect of Co-Pi OEC on the selectivity of photo-oxidation reactions and photostabilities of WO 3 photoanodes. WO 3 photoanodes have been reported to produce peroxo species as well as O 2 during photooxidation reactions, and the accumulation of peroxo species on the surface is known to cause a gradual loss of photoactivity of WO 3 . The photocurrent to O 2 conversion efficiencies of the WO 3 and WO 3 /Co-Pi OEC electrodes were obtained by simultaneously measuring the photocurrent and O 2 gas generated during illumination at 0.8 V vs Ag/AgCl. The result shows that the presence of OEC increases the photocurrent to O 2 conversion efficiency from approximately 61% to approximately 100%. The complete suppression of peroxo formation provided the WO 3 /Co-Pi OEC photoelectrode with long-term photostability. The photocurrent-potential characteristics show that the presence of OEC effectively reduces the electron-hole recombination near the flat band potential region and shifts the onset potential of photocurrent by 0.17 V to the negative direction. However, when the applied potential became more positive than approximately 0.35 V vs Ag/AgCl, the WO 3 /Co-Pi OEC electrode produced less initial photocurrent than the bare WO 3 electrode. Mott-Schottky plots reveal the presence of interface states at the WO 3 /OEC junction that induce more electron-hole recombination when the Fermi level moves below these states. Regardless of the adverse effect on recombination present at 0.8 V vs Ag/AgCl, the WO 3 /Co-Pi OEC achieved a more efficient and sustainable solar to O 2 conversion owing to the ability of Co-Pi OEC to significantly increase the photocurrent to O 2 conversion efficiency and prevent the photocurrent decay of the WO 3 electrode.