Uniformly citrate-assisted deposition of small-sized FeOOH on BiVO4 photoanode for efficient solar water oxidation

•Uniformly citrate-assisted FeOOH overlayer with enriched oxygen vacancy is achieved.•The citrate acts as the bridging agent to anchor the FeOOH on BiVO4 surface.•The citrate-assisted FeOOH overlayer can improve the OER activity of BiVO4 electrode.•The PEC enhancement originates from the holes storage and catalytic effect of FeOOH. Nanoporous BiVO4 is a promising photoanode material for driving the photoelectrochemical (PEC) water splitting to hydrogen energy, whereas its performance is limited by the sluggish surface catalysis reaction. Herein, a facile citrate-assisted method was developed for growing uniform amorphous FeOOH overlayer on nanoporous BiVO4, which significantly improved the water oxidation kinetics and resulted in a remarkable photocurrent density of 3.33 mA cm−2 at 1.23 V (vs. RHE). The activity enhancement was even superior to the well-crystallized α-FeOOH and β-FeOOH modified BiVO4. The citrate serves as the bridging agent that not only optimizes the BiVO4/FeOOH interface facilitating the hole trapping/transport, but also inhibits the particle growth of FeOOH nanolayer and provides more oxygen vacancies to promote the water oxidation. We demonstrate that the uniform FeOOH overlayer acts as the hole storage layer and catalyst, which increases the potential drop in the Helmholtz layer and greatly facilitates the interfacial hole transfer. Moreover, the citrate-assisted modification method was proved to be a general strategy to improve the PEC performance of TiO2 and α-Fe2O3 photoanodes. Our work provides a simple and effective design strategy for constructing uniform semiconductor/electrocatalysts interface that can realize efficient solar-to-hydrogen conversion.