Towards Understanding the True Nature of Hydrogen Treatment in Metal Oxide Photoelectrodes

Abstract

Hydrogen treatment (H-treatment) on photocatalysts (by simple post annealing under H2 atmosphere) has received much attention, since Chen et al. reported the dramatically improved water splitting and dye degradation performance on hydrogenated black TiO2 in 2011 (1). Recently, the Li group reported that this H-treatment can also be applied to improve the photoelectrochemical (PEC) water splitting performance of various metal oxides such as TiO2, WO3, Fe2O3, and BiVO4 (2-3). Despite these reported improvements and proposed explanations, it is still not clear to what extent the lattice and electronic structure of the oxides are modified upon H-treatment to obtain such a performance enhancement. To elucidate the effect of H-treatment in oxides, here we compared the photocurrent and photoconductivity of H-treated BiVO4 and tungsten (W)-doped BiVO4. Doping increases the charge carrier density and conductivity, but at the same time it may produce defect states that can act as recombination sites. As a result, one would expect the saturated photocurrent increase, but the onset potential to shift to the positive direction (i.e., decreased photovoltage) due to decreased carrier lifetime. This is exactly what we observed for W-doped BiVO4, which is also consistent with our previous work (4). However, we observed the opposite for the H-treated BiVO4; the onset potential shifted to the negative direction, and the saturated photocurrent increased. This suggests that while H-treatment may increase the charge carrier density (and conductivity), it also increases the carrier lifetime, which is atypical for doping. We further verified this by performing time-resolved microwave conductivity (TRMC) and direct conductivity measurement. Finally, we succeeded to detect hydrogen in the BiVO4 lattice by using nuclear reaction analysis (NRA). The ability to do this will no doubt advance the understanding behind the true enhancement mechanism of H-treatment and provide additional pathways for highly efficient photoelectrodes.