Unraveling the role of Ni and Co deposition on BiVO4 thin films: Surface chemistry insights into enhanced photoelectrochemical water splitting

Abstract

Bismuth vanadate (BiVO4, BVO) is one of the most promising photoanode materials for photoelectrochemical (PEC) water splitting due to its suitable bandgap, good visible light absorption, and favorable band alignment for oxygen evolution reaction (OER). Cocatalysts are commonly applied to BVO to accelerate surface reaction kinetics, enhance charge extraction, and suppress carrier recombination. In this work, we investigate the effect of Ni- and Co-based cocatalyst deposition on the PEC performance of hydrothermally synthesized BVO thin films. Thin layers of Ni, Co, and their combinations (~3 nm) were deposited using magnetron sputtering, and their influence on OER was evaluated in 0.1 M potassium phosphate buffer (pH = 8). Linear sweep voltammetry revealed significant enhancement in photocurrent density upon cocatalyst deposition. The sample modified with sequential Ni then Co deposition exhibited the highest photocurrent density of 2.43 mA/cm2 at 1.23 V vs RHE, approximately threefold higher than the unmodified BiVO4 film. Furthermore, a cathodic shift of ~0.15 V in onset potential was observed for all cocatalyst-modified samples. Mott-Schottky analysis indicated an order-of-magnitude increase in carrier density and more positive flat band potentials for cocatalyst-modified films. XPS analysis revealed that Co remained on the surface as Co(OH)2, while Ni predominantly migrated into the bulk as metallic Ni when deposited before Co, forming a conductive bridge between the BVO matrix and the surface-active catalytic sites. This spatial configuration facilitates charge separation and enhances OER kinetics. These findings highlight the critical role of cocatalyst composition,‚deposition order, and film architecture in developing efficient and durable BVO -based photoanodes for solar-driven OER.