Photocarrier Recombination Dynamics in BiVO4 for Visible Light-Driven Water Oxidation

Photocarrier recombination dynamics of BiVO4 powders synthesized at different temperatures were studied by temperature-dependent steady-state and time-resolved photoluminescence (PL). Structural analysis indicates that BiVO4 materials synthesized at low temperatures contain mixed-phase crystals including monoclinic and tetragonal scheelite phase, showing poor photocatalytic performance. Relatively higher synthesis temperatures improve the photocatalyst efficiency by promoting the formation of single-phase monoclinic BiVO4 with larger grains. Excitation-power dependence along with temperature dependence of the PL of BiVO4 suggests that the donor-to-acceptor transitions are the dominant radiative recombination mechanism. Furthermore, hole effective lifetimes observed in PL decays were found in the order of nanoseconds, which is far behind the ideal radiative lifetime of ∼6 μs, calculated theoretically using van Roosbroeck-Shockley relation. This suggests that the photocarrier recombination in BiVO4 occurs predominately nonradiatively via multiphonon emission, plausibly through deep-level defects. In addition, the coexistence of tetragonal and monoclinic phases might indirectly induce additional trap states, leading to an increase of the nonradiative recombination rate and subsequently poor photocatalytic efficiency in samples synthesized at lower temperatures. Thus, the nonradiative recombination which is associated with a short photocarrier lifetime and small holes diffusion length is the most limiting process for BiVO4 performance.