Visible Light Induced Photoelectrochemical Properties of n-BiVO4and n-BiVO4/p-Co3O4

The visible light induced photoelectrochemical properties of the pressed powder electrodes n-BiVO4, p-Co3O4, and n-BiVO4/p-Co3O4 containing 0.8 wt % cobalt were investigated. At pH 7 flatband potentials of −0.30 and +0.54 V vs NHE were measured for the bismuth vanadate and cobalt oxide, respectively, whereas −0.31 V was obtained for BiVO4/Co3O4. At a bias of 0.1 V vs Ag/AgCl the n-type photocurrent of BiVO4 changes to p-type upon prolonged irradiation, whereas it remains n-type at the much higher bias of 1.0 V vs Ag/AgCl. The change in conductivity type can be rationalized by invoking oxidation of water to a surface peroxide species. From the photocurrent decay of BiVO4 under chopped irradiation the presence of efficient charge recombination is indicated. It can be suppressed by addition of iodide, thiocyanate, or methanol, leading to about twice as large incident-photon-to-current efficiencies (IPCE). Different from that, in the case of the BiVO4/Co3O4 electrode the IPCE values do not change in the presence of iodide or thiocyanate and are 4 times higher. This distinct difference is rationalized by the assumption that the photogenerated charges are efficiently separated at the BiVO4−Co3O4 interface forming a type of n-/p-junction. Whereas electrons migrate to the n-type component, holes move to the p-type material. In summary, modification of n-BiVO4 by p-Co3O4 stabilizes the photocurrent, increases the efficiency of its generation, and leads to a compartmentalization of interfacial reduction and oxidation at the n-type component and p-type component, respectively.