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[Display omitted] • Enhanced photoelectrochemical water oxidation on anatase TiO 2 nanotubes photoanodes. • Ultrafast charge separation and parallel charge consumption on TiO 2 films. • Increased doping level and negative shift of flat-band potential on TiO 2 nanotube photoanodes. • A 1st-order and a 2nd-order charge reaction kinetics were discovered on TiO 2 nanotube photoanodes. • A parallel reaction model was established coincided with the reaction order transition. Artificial photosynthesis relies on the photoreactions between the chemical molecules and photogenerated charges. The oxygen evolution reactions are regarded as the most difficult procedure, because they are multiple-step reactions with multiple charges transfer. Notwithstanding the extensive study on photoelectrochemical and photochemical water oxidation, understanding how the surface charge reacts with the chemical molecules is limited, especially the reaction kinetics of the surface charges. We prepared anatase TiO 2 nanotubes array and dense polycrystalline TiO 2 films on Ti substrates, where the photocurrent increased about 4–4.5 times on nanotubes compared with the dense films. Moreover, the surface charge reaction rate and the accumulated charge densities were analyzed by transient surface photovoltage and transient photocurrent techniques. It was found a 2nd-order charge reaction behavior could be found when the charge density was high, differently, a 1st-order charge reaction could be observed when the charge density was low. The nanotubes structures would in situ grow from the Ti substrates, which simultaneously enhanced charge separation and transfer kinetics to improve the PEC performance. [ABSTRACT FROM AUTHOR]

• Uniformly citrate-assisted FeOOH overlayer with enriched oxygen vacancy is achieved. • The citrate acts as the bridging agent to anchor the FeOOH on BiVO 4 surface. • The citrate-assisted FeOOH overlayer can improve the OER activity of BiVO 4 electrode. • The PEC enhancement originates from the holes storage and catalytic effect of FeOOH. Nanoporous BiVO 4 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 BiVO 4 , 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 BiVO 4. The citrate serves as the bridging agent that not only optimizes the BiVO 4 /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 TiO 2 and α-Fe 2 O 3 photoanodes. Our work provides a simple and effective design strategy for constructing uniform semiconductor/electrocatalysts interface that can realize efficient solar-to-hydrogen conversion. [ABSTRACT FROM AUTHOR]