Surface Modification of Tungsten Oxide-Based Photoanodes for Solar-Powered Hydrogen Production
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1171-S02-01
Surface Modification of Tungsten Oxide-Based Photoanodes for Solar-Powered Hydrogen Production N. Gaillard1, J. Kaneshiro1, E.L. Miller1, L. Weinhardt2,3, M. Bär2,4, C. Heske2, K. -S. Ahn5, Y. Yan5, and M. M. Al-Jassim5 1
Hawaii Natural Energy Institute (HNEI), University of Hawaii at Manoa, Honolulu, HI 96822, USA 2 Department of Chemistry, University of Nevada Las Vegas, NV 89154-4003, U.S.A. 3 Experimentelle Physik II, Universität Würzburg, D-97074 Würzburg, Germany 4 Helmholtz-Zentrum Berlin für Materialien und Energie, Lise-Meitner-Campus, D-14109 Berlin, Germany 5 National Renewable Energy Laboratory (NREL), Golden, CO 80401, U.S.A. ABSTRACT We report on the development of tungsten-oxide-based photoelectrochemical (PEC) water-splitting electrodes using surface modification techniques. The effect of molybdenum incorporation into the WO3 bulk or the surface region of the film is discussed. Our data indicate that Mo incorporation in the entire film (WO3:Mo) results in poor PEC performances, most likely due to defects that trap photo-generated charge carriers. However, compared to a pure WO3 (WO3:Mo)-based PEC electrode, a 20% (100%) increase of the photocurrent density at 1.6 V vs. SCE is observed if the Mo incorporation is limited to the near-surface region of the WO3 film. The resulting WO3:Mo/WO3 bilayer structure is formed by epitaxial growth of the WO3:Mo top layer on the WO3 bottom layer, which allows an optimization of the electronic structure induced by Mo incorporation while maintaining good crystallographic properties. INTRODUCTION More than three decades after the first report of photo-induced water splitting on TiO2based electrodes by Fujishima and Honda1, intensive research is still ongoing to identify a suitable semiconductor to be integrated in an efficient, cost effective, and reliable photo-electrochemical (PEC) system. Many candidate semiconductor materials have been studied for PEC applications, including amorphous silicon-based materials, III-V compounds, and transition metal oxides. One example of the latter is WO3, which offers good corrosion resistance and is inexpensive to produce. However, the relatively high band gap of WO3 (approx. 2.6 eV) results in a lack of sufficient absorption of the solar spectrum necessary for high hydrogen evolution rates. Several experiments have been performed at the University of Hawai’i to reduce the band gap of reactively-sputtered WO3 films by incorporating impurities in the material bulk. For instance, a net 0.5 eV band gap reduction has been achieved using nitrogen as impurity2. However, photoelectrochemical characterizations performed in an 0.33M H3PO4 electrolyte under simulated AM1.5 global light showed a decrease of the saturated photocurrent density from ~2.68 mA.cm-2 (in WO3) to ~0.67 mA.cm-2 (in WO3:N). Additional SEM studies pointed out poor microstructural properties induced by nitrogen incorporation (using N2 gas in addition to
O2 during the reactive-sputtering process) that could affect both, bulk and surface electronic propert
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