Exploring the Different Roles of Particle Size in Photoelectrochemical and Photocatalytic Water Oxidation on BiVO₄

Water oxidation on visible-light-active bismuth vanadate (BiVO₄) has commonly been demonstrated to be viable in powder suspension (PS) and particulate photoelectrochemical (PEC) systems. Here, we demonstrate that particle size reduction, which is known to be efficacious in promoting charge carrier extraction and boosting surface active sites, has an opposite effect on BiVO₄'s photoactivity in the two systems. With three BiVO₄ samples of distinctive particle sizes, smaller BiVO₄ particle size is shown to be beneficial for enhancing PEC photocurrent generation, but deleterious for photocatalytic O₂ evolution on suspended BiVO₄. Such contrary effect of particle size in the PEC and PS systems is revealed to be due to the different governing factors of the systems: charge transport in the former and charge separation in the latter. Smaller particle size was found to enrich the interparticle and the particle/FTO substrate contacts which improve charge transport and charge collection efficiency in BiVO₄ particulate electrode. On the contrary, larger particle size is necessary for improved photocatalytic O₂ evolution because of increased crystallinity and greater band bending, which are essential for charge separation.