Annealing temperature effects on photoelectrochemical performance of bismuth vanadate thin film photoelectrodes

The effects of annealing treatment between 400 °C and 540 °C on crystallization behavior, grain size, electrochemical (EC) and photoelectrochemical (PEC) oxygen evolution reaction (OER) performances of bismuth vanadate (BiVO₄) thin films are investigated in this work. The results show that higher temperature leads to larger grain size, improved crystallinity, and better crystal orientation for the BiVO₄ thin film electrodes. Under air-mass 1.5 global (AM 1.5) solar light illumination, the BiVO₄ thin film prepared at a higher annealing temperature (500-540 °C) shows better PEC OER performance. Also, the OER photocurrent density increased from 0.25 mA cm^-2 to 1.27 mA cm^-2 and that of the oxidation of sulfite, a hole scavenger, increased from 1.39 to 2.53 mA cm^-2 for the samples prepared from 400 °C to 540 °C. Open-circuit photovoltage decay (OCPVD) measurement indicates that BiVO₄ samples prepared at the higher annealing temperature have less charge recombination and longer electron lifetime. However, the BiVO₄ samples prepared at lower annealing temperature have better EC performance in the absence of light illumination and more electrochemically active surface sites, which are negatively related to electrochemical double-layer capacitance (C_dl). C_dl was 0.0074 mF cm^-2 at 400 °C and it decreased to 0.0006 mF cm^-2 at 540 °C. The OER and sulfide oxidation are carefully compared and these show that the efficiency of charge transport in the bulk (η_bulk) and on the surface (η_surface) of the BiVO₄ thin film electrode are improved with the increase in the annealing temperature. The mechanism behind the light-condition-dependent role of the annealing treatment is also discussed.