Amorphous NiSn and FeOOH as bifunctional co-catalysts for oxygen reduction and phenol (water) oxidation over BiVO4 under visible light

Abstract

Monoclinic BiVO4 (BiV) has been widely used as a photoanode for water oxidation, but rarely as a photocatalyst for organic oxidation due to slow reaction of O2. In this work, BiV has been modified with poorly crystallized sFe and sNi, where sFe is FeOOH, and sNi is a mixture of Ni(OH)2 and polysulfide. Under light, sFe/BiV and sNi/BiV in aqueous solution were more active than BiV, respectively, not only for phenol oxidation but also for O2 reduction. Importantly, the rate of phenol oxidation obtained for sFe/sNi/BiV was larger than the sum of the rates measured for sFe/BiV and sNi/BiV, by a factor of approximately 1.5. Moreover, on a film electrode, O2 reduction had a current of sFe/sNi/BiV > sNi/BiV > sFe/BiV > BiV, while water (photo)oxidation had a current of sFe/sNi/BiV > sNi/BiV > sFe/BiV > BiV. A possible mechanism is proposed, involving formation of a reduced sulfur species for O2 reduction and an oxidized iron species for phenol oxidation. In sFe/sNi/BiV, there is a mutual promotion between the sNi-mediated electron transfer and the sFe-mediated hole transfer. This results in a further improved efficiency of charge separation for O2 reduction and phenol oxidation.