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[Display omitted] Metal-organic polymers (MOPs) can enhance the photoelectrochemical (PEC) water oxidation performance of BiVO 4 photoanodes, but their PEC mechanisms have yet to be comprehended. In this work, we constructed an active and stable composite photoelectrode by overlaying a uniform MOP on the BiVO 4 surface using Fe2+ as the metal ions and 2,5-dihydroxyterephthalic acid (DHTA) as ligand. Such modification on the BiVO 4 surface yielded a core-shell structure that could effectively enhance the PEC water oxidation activity of the BiVO 4 photoanode. Our intensity-modulated photocurrent spectroscopy analysis revealed that the MOP overlayer could concurrently reduce the surface charge recombination rate constant (k sr) and enhance the charge transfer rate constant (k tr), thus accelerating water oxidation activity. These phenomena can be ascribed to the passivation of the surface that inhibits the recombination of the charge carrier and the MOP catalytic layer that improves the hole transfer. Our rate law analysis also demonstrated that the MOP coverage shifted the reaction order of the BiVO 4 photoanode from the third-order to the first-order, resulting in a more favorable rate-determining step where only one hole accumulation is required to overcome water oxidation. This work provides new insights into the reaction mechanism of MOP-modified semiconductor photoanodes. [ABSTRACT FROM AUTHOR]

[Display omitted] • Enhanced photoelectrochemical water oxidation on Bi 2 WO 6 photoanodes by heterojunction and facet-engineering. • The heterojunction of WO 3 /Bi 2 WO 6 improved charge separation and transfer. • Crystalline {2 0 0} and {1 3 1} orientations on WO 3 /Bi 2 WO 6 and Bi 2 WO 6 films, respectively. • A 1st-order and a 2nd-order charge reaction kinetics were discovered on {2 0 0} and {1 3 1} facets, respectively. Charge separation and transfer kinetics play a significant role in photocatalytic water oxidation, which is sensitive to the crystal facets. Herein, the bismuth tungstate (Bi 2 WO 6) films were used as a model photoanode to investigate how the photogenerated charges react on {1 3 1} and {2 0 0} facets under constantly applied bias in a photoelectrochemical (PEC) cell. On the WO 3 electron transport layer, the Bi 2 WO 6 films have a {2 0 0} orientation while possessing a {1 3 1} orientation on the fluorine-doped tin oxide substrate. It is discovered that {2 0 0} facets on the WO 3 /Bi 2 WO 6 heterojunction display a low overpotential of PEC water oxidation, showing a 1st-order reaction behavior and faster charge transfer rate constant. Differently, water oxidation on the {1 3 1} facet on Bi 2 WO 6 displays a 2nd-order reaction behavior. Further, density functional theory calculations were applied to demonstrate how the rate-determining step and the major formed intermediate correlate with the charge accumulation and reaction orders. The findings of this work shed fundamental light on how to rationally design and precisely modulate surface charge density and reaction kinetics for efficient (photo)electrocatalysts. [ABSTRACT FROM AUTHOR]

[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]