Facet-Engineered BiVO<sub>4</sub> Photocatalysts for\nWater Oxidation: Lifetime Gain Versus Energetic Loss

A limiting factor\nto the efficiency of water Oxygen Evolution\nReaction\n(OER) in metal oxide nanoparticle photocatalysts is the rapid recombination\nof holes and electrons. Facet-engineering can effectively improve\ncharge separation and, consequently, OER efficiency. However, the\nkinetics behind this improvement remain poorly understood. This study\nutilizes photoinduced absorption spectroscopy to investigate the charge\nyield and kinetics in facet-engineered BiVO<sub>4</sub> (F-BiVO<sub>4</sub>) compared to a non-faceted sample (NF-BiVO<sub>4</sub>) under\noperando conditions. A significant influence of preillumination on\nhole accumulation is observed, linked to the saturation and, thus,\npassivation of deep and inactive hole traps on the BiVO<sub>4</sub> surface. In DI-water, F-BiVO<sub>4</sub> shows a 10-fold increase\nin charge accumulation (∼5 mΔOD) compared to NF-BiVO<sub>4</sub> (∼0.5 mΔOD), indicating improved charge separation\nand stabilization. With the addition of Fe(NO<sub>3</sub>)<sub>3</sub>, an efficient electron acceptor, F-BiVO<sub>4</sub> demonstrates\na 30-fold increase in the accumulation of long-lived holes (∼45\nmΔOD), compared to NF-BiVO<sub>4</sub> (∼1.5 mΔOD)\nand an increased half-time, from 2 to 10 s. Based on a simple kinetic\nmodel, this increase in hole accumulation suggests that facet-engineering\ncauses at least a 50–100 meV increase in band bending in BiVO<sub>4</sub> particles, thereby stabilizing surface holes. This energetic\nstabilization/loss results in a retardation of OER relative to NF-BiVO<sub>4</sub>. This slower catalysis is, however, offset by the observed\nincrease in density and lifetime of photoaccumulated holes. Overall,\nthis work quantifies how surface faceting can impact the kinetics\nof long-lived charge accumulation on metal oxide photocatalysts, highlighting\nthe trade-off between lifetime gain and energetic loss critical to\noptimizing photocatalytic efficiency.