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Abstract
Charge kinetics, which includes separation of photogenerated electrons and holes, charge transport to reaction sites, and interfacial charge transfer for chemical reactions, critically determines the photochemical conversion efficiency of a photocatalyst. While various efforts have been invested in the production of photocatalysts with different morphologies or alteration of photocatalysts properties via post-synthesis treatment, fundamental understanding of charge kinetics in relation to the surface and bulk properties of photocatalyst is essential for development of a highly effective photocatalyst. For this purpose, a range of visible-light-active BiVO4 particles with monoclinic scheelite phase were produced using solid-liquid state reaction and studied for their photocatalytic activities, primarily on water oxidation reaction.
The work started with an investigation on the influence of {010}/{110} relative exposure extent on the photooxidation ability of BiVO4 with respect to the charge transfer efficiency of the material. Two dual-faceted BiVO4’s with distinctly different dominant exposed facets, one which was {010}-dominant and the other {110}-dominant, were synthesized. Despite {010} and {110} being proven as the respective active reduction and oxidation sites of BiVO4, the photooxidation activities of {010}-dominant BiVO4 were found to be higher than {110}-dominant BiVO4 for water oxidation and 2,4-dichlorophenoxyacetic acid degradation. The presence of larger reduction functional {010} facets was shown to facilitate electron transfer, which is crucial not only for photoreduction, but also for photooxidation reactions.
The BiVO4 particles with varying {010}/{110} relative exposure extents were further studied for the facet-dependent charge transfer interactions between BiVO4 and reduced graphene oxide (RGO). While improvement of photoelectrochemical (PEC) photocurrent was apparent for all BiVO4 samples in the presence of RGO, the degree of enhancement was revealed to correlate to the exposure extent of {010} facets, attributing to improved charge transfer ability. The experimental results were supported by density functional theory calculations, whereby different electronic properties between graphene/BiVO4{010} and graphene/BiVO4{110} interfaces were predicted to facilitate charge transfer across the former.
Following these findings, the practicability of Argon (Ar) annealing as a post-synthesis treatment to optimize the charge separation and transport aspects of dual-faceted BiVO4 for water oxidation was examined. In addition to reduced band gap, greater local structure distortion, and improved crystallinity, Ar annealing was also shown to effectively instigate formation of oxygen vacancies, which enhance electron transport in BiVO4. The extent of improvements however, was dependent on the annealing temperature.
The last section of the work explored the prospect of dual-faceted BiVO4 microcrystal in comparison to the generally more desirable smaller (or nanosized) particles for water oxidation in powder suspension (PS) and PEC systems. Contrary effects of particle size were displayed in the two systems: larger particles evolved greater amount of O2 in the PS system, whereas particulate electrode made of smaller particles led to higher PEC photocurrent. These findings highlight the different governing factors of the two systems, that is, charge transport for PEC system and charge separation for PS system. Although faceted BiVO4 microcrystal falls short of providing short interparticle distance and good photocatalyst/substrate contact for efficient charge transport predominantly needed in the PEC system, the higher crystallinity and the more significant band-bending-mediated built-in electric field present in the microsized crystal facilitated charge separation and allowed it to outperform the smaller BiVO4 particles for photocatalytic O2 evolution.