Synthesis and Performance of Doped Cu-W Oxide Semiconductors for Photoelectrochemical Water Splitting

Abstract

Introduction Hydrogen gas is a promising clean energy fuel owing to its high energy capacity and generation of water as the only by-product when used as a fuel source. Research is currently being conducted to find methods of hydrogen production that are economically, energetically, and environmentally efficient. One potential method of hydrogen production utilizes sunlight to drive both the hydrogen and oxygen evolution reactions from the splitting of water molecules, thus storing solar energy in the chemical bonds of hydrogen gas.1 The water splitting reaction itself is non-spontaneous (ΔG=237 kJ mol-1), requiring the input of energy to drive the forward reaction. The technical challenges for the development of light-harvesting photoanode for PEC systems lie in the key characteristics required for the materials for water splitting to occur. Metal oxides are one of the most widely studied groups of semiconductors for PEC reactions due to their stability in aqueous solution and relatively low band gap energies.4 In this study, the effect of dopants on the electrical properties and water splitting efficiency of a Cu-W oxide substrate are investigated. Experimental Doped Cu-W-O semiconductor films were synthesized using spray deposition on ultrasonically cleaned FTO glass. The solution was spray deposited for 5 seconds and dried for 55 seconds to allow the complete evaporation of solvent. Various number of cycles were tested to optimize the parameters of the spray pyrolysis technique with the doped Cu-W-O solution. The substrate composition was characterized using X-ray diffraction and X–ray photoelectron spectroscopy. The surface morphology of the thin films was examined using scanning electron microscopy. Photoelectrochemical analysis of the doped Cu-W-O thin films was performed in a three electrode setup with a pH 7 electrolyte. The semiconductor thin film, coiled platinum wire, and a saturated calomel electrode were employed as the working, counter, and reference electrodes respectively. Results Investigations indicate successful modification of band gap of Cu-W-O with incorporation of dopant, leading to enhanced light harvesting ability of doped Cu-W oxide photoelectrodes. Such electronic modifications resulted in profound increase in photocurrent density of doped Cu-W oxide films compared to the undoped film. The improved optical absorption and photocurrent density indicate the potential of doped Cu-W oxide semiconductor as an efficient photoanode for PEC water splitting device. Acknowledgements This work was partly performed under auspices of Department of Energy under contract DE-NE0008236.