Insights into the enhancement mechanism of FeOOH/VO-BiVO4 photoanodes for water oxidation with intensity-modulated photocurrent spectroscopy

To overcome the rapid charge recombination and sluggish water oxidation kinetics, FeOOH/V O -BiVO 4 photoanodes were rationally prepared. The highest photocurrent density of 3.5 mA cm −2 was generated by the FeOOH/V O -BiVO 4 photoanodes at 1.23 V RHE , which was 3.5-fold higher than that of the pristine BiVO 4 . The dominate enhancement mechanism can be ascribed to significantly increased charge transfer efficiency by the FeOOH overlayers. Intensity-modulated photocurrent spectroscopy revealed that such increased charge transfer efficiency was resulted from the strong suppression of surface recombination by the FeOOH overlayers. Rate constant of surface recombination was strongly reduced to approximately 10 −2  s −1 at 0.93–1.23 V RHE . The strongly suppressed surface recombination was attributed to the storage capacity of the FeOOH for the photogenerated holes. Meanwhile, oxygen vacancies just had a minor impact on the kinetics of charge transfer and surface recombination but slightly increased the charge separation efficiency in the bulk. This work clarifies the roles of oxygen vacancies and the FeOOH overlayers in the increased photocurrent density, facilitating the rational design of high-performance BiVO 4 -based photoanodes.