Elucidating the role of surface states of BiVO4 with Mo doping and a CoOOH co-catalyst for photoelectrochemical water splitting

Bismuth vanadate (BiVO4) is a promising material for photoelectrochemical (PEC) water splitting, however, its PEC performance is limited by the high surface and bulk charge recombination rates. Here we present a comprehensive study to elucidate a recombination phenomenon of BiVO4 that arises with Mo doping. The Mo doping produces multiple effects including the formation of MoOx (reduced form of Mo6+) species and oxygen vacancies (VOs) on the surface of the BiVO4 that work in tandem with V4+ species (and MoOx) acting as surface-active intermediates (i-SS) providing improved hole transfer to the electrolyte. In contrast, in the absence of V4+ species, the VOs can act as recombination centers (r-SS). Further, CoOOH co-catalyst coating is used to minimize such recombination centers. Eventually, a photocurrent enhancement of ~37 times (1.1 mA/cm2 at 1.23 V vs. RHE) and a cathodic shift in onset potential of ~500 mV compared to that of pristine BiVO4 (0.03 mA/cm2 at 1.23 V vs. RHE) is obtained. We carried out in-depth PEC analysis using hole scavenger measurements, PEC impedance spectroscopy, and intensity-modulated photocurrent spectroscopy to elucidate the effect of the surface reduction process upon doping, the impact of Vos, MoOx species and CoOOH layer on the enhanced PEC performance.