BiVO4 is one of the most promising photoanode materials for water-splitting systems. Nitrogen incorporation into a BiVO4 surface overcomes the known bottleneck in its charge-transfer kinetics into the electrolyte. We explored the role of nitrogen in the surface charge recombination and charge-transfer kinetics by employing transient photocurrent spectroscopy at the time scale of surface recombination and water oxidation kinetics, transient absorption spectroscopy, and X-ray photoelectron spectroscopy. We attributed the activity enhancement mechanism to the accelerated V5+/V4+ redox process, in which incorporated nitrogen suppresses a limiting surface recombination channel by increasing the oxygen vacancies.

BiVO4 is one of the most promising photoanode candidates to achieve high-efficiency water splitting. However, overwhelming charge recombination at the interface limits its water oxidation activity. In this study, we show that the water oxidation activity of the BiVO4 photoanode is significantly boosted by the TiO2 overlayer prepared by atomic layer deposition. With a TiO2 overlayer of an optimized thickness, the photocurrent at 1.23 V RHE increased from 0.64 to 1.1 mA·cm–2 under front illumination corresponding to 72% enhancement. We attribute this substantial improvement to enhanced charge separation and suppression of surface recombination due to surface-state passivation. We provide direct evidence via transient photocurrent measurements that the TiO2 overlayer significantly decreases the photogenerated electron-trapping process at the BiVO4 surface. Electron-trapping passivation leads to enhanced electron photoconductivity, which results in higher photocurrent enhancement under front illumination rather than back illumination. This feature can be particularly useful for wireless tandem devices for water splitting as the higher band gap photoanodes are typically utilized with front illumination in such configurations. Even though the electron-trapping process is eliminated completely at higher TiO2 overlayer thicknesses, the charge-transfer resistance at the surface also increases significantly, resulting in a diminished photocurrent. We demonstrate that the ultrathin TiO2 overlayer can be used to fine tune the surface properties of BiVO4 and may be used for similar purposes for other photoelectrode systems and other photoelectrocatalytic reactions.

Postsynthetic treatments of BiVO 4 photoanodes have recently shed light on better understanding and improving the photoanodes for water splitting. We demonstrate that a mild heat treatment of BiVO 4 under O 2 flow at 200 °C improves its water oxidation activity. Charge separation and charge injection efficiencies increase along with the decreased charge transfer resistances across the BiVO 4 /electrolyte interface. The depletion region width and the band bending have shown to increase after annealing, while charge carrier density remains unchanged. Transient photocurrent measurements further confirm the reduced charge carrier recombination as a result of enlarged band bending. The surface states decrease and the fraction of vanadium ions increases in the surface region after the heat treatment. Since the surface of the BiVO 4 photoanodes is generally vanadium deficient, it is suggested that such a treatment can improve the BiVO 4 photoanodes performance prepared by other methods as well.

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Photogenerated charge carrier dynamics of BiVO4 have been investigated by ultrafast transient absorption spectroscopy (TAS), and a numerical modeling has been applied to reveal the origins of the dynamical behavior. The numerical model, based on rate equations, presents the possibility of both photogenerated hole and electron absorption dynamics below 500 nm, as opposed to the generally suggested photogenerated hole absorption mechanism. The investigations done in the ultrafast time regime show that the positive transient absorption peak at 470 nm exhibits inverse behavior as compared to the broad-band feature represented at 550 nm under anodic bias, in the presence of a hole scavenger and at increasing excitation pump power. A combination of TAS findings under various conditions with the numerical modeling reveals that both electron and hole absorption are possible in the spectral region above 500 nm whereas electron absorption at the excited state is the dominant process at shorter wavelengths. Moreover, the major changes in transient absorption response take place in the ultrafast time scale, and overall recombination dynamics is a reflection of the ultrafast recombination mechanism.