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

The visible light induced photoelectrochemical properties of the pressed powder electrodes n-BiVO 4, p-Co 3 O 4, and n-BiVO 4 /p-Co 3 O 4 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 BiVO 4 /Co 3 O 4 . At a bias of 0.1 V vs Ag/AgCl the n-type photocurrent of BiVO 4 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 BiVO 4 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 BiVO 4 /Co 3 O 4 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 BiVO 4 −Co 3 O 4 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-BiVO 4 by p-Co 3 O 4 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.