Engineering of Oxide Materials for Improved Photoelectrochemical Water Oxidation

Global climate warming and environment pollution have spurred scientists to develop new high-efficient and environmental-friendly energy technologies. Hydrogen is an ideal fuel for fuel cell applications. Hydrogen has to be produced from renewable and carbon-free resources using nature energies such as sunlight if one thinks of clean energy and environmental issues. In this regard, a photoelectrochemical (PEC) cell consisting of semiconductor photoelectrodes that can harvest light and use this energy directly for splitting water is a more promising way for hydrogen generation. Abundant and inexpensive oxide semiconductor such as ZnO has been recognized as a promising photoelectrode, but the photoconversion efficiency is substantially limited by its large band gap and rapid charge recombination. Recently, doping with 4d transition metal, such as Mo, has been carried out to remarkably enhance the PEC performance of many photoanodes including TiO₂, BiVO₄, and Fe₂O₃. Nevertheless, 1-D Mo-doped ZnO nanostructures have not been reported for PEC water splitting. We report the first demonstration of cobalt phosphate (Co-Pi) assisted Zn_1-xMo_xO nanorods (NRs) as visible-light-sensitive photofunctional electrodes to fundamentally improve the performance of ZnO NRs for PEC water splitting. The maximum photoconversion efficiency could be successfully achieved as high as 1.05%, with the significant photocurrent density of 1.4 mA cm^-2. More importantly, in addition to achieve the maximum incident photon to current conversion efficiency (IPCE) value of 86%, it could be noted that the IPCE of Zn_1-xMo_xO photoanodes at the monochromatic wavelength of 450 nm is up to 12%. Our PEC performances are comparable to those of many oxide-based photoanodes in recent reports. The improvement in photoactivity of PEC water splitting may be attributed to the enhanced visible-light absorption, increased charge-carrier densities, and improved interfacial charge-transfer kinetics due to the synergistic effects of Mo incorporation and Co-Pi modification, thus contributing to photocatalysis. The new design of constructing highly photoactive Co-Pi assisted Zn_1-xMo_xO photoanodes enriches the doping community and sheds light on developing high efficiency photoelectrodes for solar-hydrogen field.

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