In this Article, we report the effect of postdeposition thermal treatment on reactively cosputtered copper tungstate (CuWO4) thin films and its impact on photoelectrochemical performances. This study indicates that CuWO4 films fabricated at 275 °C were amorphous and did not show significant photoresponse in 0.33 M H3PO4 electrolyte when irradiated with air mass 1.5Global simulated illumination. However, a major improvement in photoelectrochemical performance was observed in identical test conditions after a postdeposition treatment performed at 500 °C in pure argon for 8 h. Indeed, a photocurrent density of approximately 400 μA cm–2 at 1.6 V vs saturated calomel electrode was measured on the annealed CuWO4 samples. Subsequent X-ray diffraction analysis revealed a clear transformation of as-deposited amorphous thin films into a triclinic CuWO4 structure after the annealing step. The polycrystalline CuWO4 films exhibited n-type conductivity, an indirect band gap of 2.25 eV, and a flat-band potential of −0.35 V vs saturated calomel electrode.
We report on the incorporation of molybdenum into tungsten oxide by co-sputtering and its effect on solar-powered photoelectrochemical (PEC) water splitting. Our study shows that Mo incorporation in the bulk of the film (WO 3 :Mo) results in poor PEC performance when compared with pure WO 3 , most likely due to defects that trap photo-generated charge carriers. However, when a WO 3 :Mo/WO 3 bilayer electrode is used, a 20% increase of the photocurrent density at 1.6 V versus saturated calomel reference electrode is observed compared with pure WO 3 . Morphological and microstructural analysis of the WO 3 :Mo/WO 3 bilayer structure reveals that it is formed by coherent growth of the WO 3 :Mo top layer on the WO 3 bottom layer. This effect allows an optimization of the electronic surface structure of the electrode while maintaining good crystallographic properties in the bulk.
SUMMARYFor several decades, the main body of research in photoelectrochemical (PEC) hydrogen production has centered on a small number of semiconductor materials classes, including stable but inefficient metal-oxides, as well as some more efficient materials such as III-V compounds which suffer from high cost and poor stability. While demonstrating some limited success in meeting the rigorous PEC demands in terms of bandgap, optical absorption, band-edge alignment, surface energetics, surface kinetics, stability, manufacturability and cost, none of the 'traditional' PEC semiconductors are adequate for application in water-splitting devices with high performance (greater than 15% solar-to-hydrogen conversion) and long durability (greater than 200 h life). As a result, it is widely held that new semiconductor classes and configurations need to be identified and developed specifically for practical implementation of solar water-splitting. Examples include ternary and quaternary metal-oxide compounds, as well as non-oxide semiconductor materials, such as silicon-carbide and the copper-chalcopyrites. This paper describes recent progress at the University of Hawaii to develop improved semiconductor absorbers and interfaces for solar photoelectrolysis based on polycrystalline tungsten trioxide and polycrystalline copper-gallium-diselenide. Specific advantages and disadvantages of both materials classes in terms of meeting long-term PEC hydrogen production goals are detailed.
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