Increasing the Photocatalytic Activity of BiVO4 by Naked Co(OH)2 Nanoparticle Cocatalysts

Bismuth vanadate (BiVO4 or BVO) is one of the most studied photocatalysts for water oxidation because of its excellent visible light absorption and appropriate band energy positions. However, BVO presents a low charge mobility and a high electron–hole recombination rate. To address these fundamental limitations, this study proposes the coating of previously synthesized phase-pure monoclinic scheelite BVO with different amounts of naked cobalt (further oxidized to cobalt hydroxide) nanoparticles (NPs) via a modified magnetron sputtering deposition. The resulting BVO/Co photocatalysts were investigated for methylene blue (MB) photodegradation, photocatalytic oxygen evolution, and photoelectrochemical (PEC) water oxidation. In the MB photodegradation tests, the BVO/Co sample prepared with a deposition time of 5 min (BVO/Co(5 min)) presented the highest photoactivity (k = 0.06 min−1) compared with the other sputtering investigated times (k = 0.01–0.02 min−1), as well as the pristine BVO sample (k = 0.04 min−1). A similar trend was evidenced for the PEC water oxidation, where a photocurrent density of 23 µA.cm−2 at 1.23 V (vs. RHE) was observed for the BVO/Co(5 min) sample, a value 4.6 times higher compared with pristine BVO. Finally, the BVO/Co(5 min) presented an O2 evolution more than two times higher than that of the pristine BVO. The increased photocatalytic performance was ascribed to increased visible-light absorption, lesser electron–hole recombination, and enhanced charge transfer at the liquid/solid interface. The deposition of Co(OH)2 NPs via magnetron sputtering can be considered an effective strategy to improve the photocatalytic performance of BVO for different target catalytic reactions, including oxygen evolution, water oxidation, and pollutant photodegradation.