Heterojunction electrodes were fabricated by layer-by-layer deposition of WO 3 and BiVO 4 on a conducting glass, and investigated for photoelectrochemical water oxidation under simulated solar light. The electrode with the optimal composition of four layers of WO 3 covered by a single layer of BiVO 4 showed enhanced photoactivity by 74% relative to bare WO 3 and 730% relative to bare BiVO 4 .According to the flat band potential and optical band gap measurements, both semiconductors can absorb visible light and have band edge positions that allow the transfer of photoelectrons from BiVO 4 to WO 3 . The electrochemical impedance spectroscopy revealed poor charge transfer characteristics of BiVO 4 , which accounts for the low photoactivity of bare BiVO 4 . The measurements of the incident photon-to-current conversion efficiency spectra showed that the heterojunction electrode utilized effectively light up to 540 nm covering absorption by both WO 3 and BiVO 4 layers. Thus, in heterojunction electrodes, the photogenerated electrons in BiVO 4 are transferred to WO 3 layers with good charge transport characteristics and contribute to the high photoactivity. They combine merits of the two semiconductors, i.e. excellent charge transport characteristics of WO 3 and good light absorption capability of BiVO 4 for enhanced photoactivity.

In this paper, in order to increase the power conversion efficiency we demonstrated the selective growth of "nanoforest" composed of high density, long branched "treelike" multigeneration hierarchical ZnO nanowire photoanodes. The overall light-conversion efficiency of the branched ZnO nanowire DSSCs was almost 5 times higher than the efficiency of DSSCs constructed by upstanding ZnO nanowires. The efficiency increase is due to greatly enhanced surface area for higher dye loading and light harvesting, and also due to reduced charge recombination by providing direct conduction pathways along the crystalline ZnO "nanotree" multi generation branches. We performed a parametric study to determine optimum hierarchical ZnO nanowire photoanodes through the combination of both length-wise growth and branched growth processes. The novel selective hierarchical growth approach represents a low cost, all solution processed hydrothermal method that yields complex hierarchical ZnO nanowire photoanodes by utilizing a simple engineering of seed particles and capping polymer.

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Hexagonal WO 3 (hex-WO 3 ) nanowires with high aspect ratio and crystallinity have been prepared for the first time by a microwave-assisted hydrothermal method. By using X-ray diffraction, scanning electron microscopy, transmission electron microscopy and high resolution transmission electron microscopy, the phase and morphology of the products were identified, which were controlled by reaction temperature, holding time and added salts. Uniform hex-WO 3 nanowires with a diameter of 5-10 nm and lengths of up to several micrometres were synthesized by a microwave-assisted hydrothermal process at 150 C for 3 h in a solution containing (NH 4 ) 2 SO 4 as a capping reagent and Na 2 WO 4 as a starting material. The aspect ratio and specific surface area of hex-WO 3 nanowires were 625 and 139 m 2 g À1 , respectively, which represented one of the highest values reported for WO 3 . The electrocatalytic activity for hydrogen evolution reaction of hex-WO 3 nanowires was also investigated by cyclic voltammetry and linear sweep voltammetry. The results demonstrated that hex-WO 3 nanowires were a promising electrocatalyst for the hydrogen evolution reaction (HER) from water. Experimental Synthesis and characterizationTo synthesize 1D WO 3 , 2 g analytical grade sodium tungstate dihydrate (Na 2 WO 4 $2H 2 O) were dissolved in 45 ml distilled water under stirring at room temperature and 5 ml 3 M HCl was

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