Photocatalytic and Photoelectrochemical Water Oxidation over Metal-Doped Monoclinic BiVO4 Photoanodes

Abstract

The visible-light-induced water oxidation ability of metal-ion-doped BiVO4 was investigated and of 12 metal ion dopants tested, only W and Mo dramatically enhanced the water photo-oxidation activity of bare BiVO4; Mo had the highest improvement by a factor of about six. Thus, BiVO4 and W- or Mo-doped (2 atom?%) BiVO4 photoanodes about 1 mu m thick were fabricated onto transparent conducting substrate by a metalorganic decomposition/spin-coating method. Under simulated one sun (air mass 1.5G, 100 mW?cm-2) and at 1.23 V versus a reversible hydrogen electrode, the highest photocurrent density (JPH) of about 2.38 mA?cm-2 was achieved for Mo doping followed by W doping (JPH approximate to 1.98 mA?cm-2), whereas undoped BiVO4 gave a JPH value of about 0.42 mA?cm-2. The photoelectrochemical water oxidation activity of W- and Mo-doped BiVO4 photoanodes corresponded to the incident photon to current conversion efficiency of about 35 and 40?% respectively. Electrochemical impedance spectroscopy and MottSchottky analysis indicated a positive flat band shift of about 30 mV, a carrier concentration 1.62 times higher, and a charge-transfer resistance reduced by 34-fold for W- or Mo-doped BiVO4 relative to undoped BiVO4. Electronic structure calculations revealed that both W and Mo were shallow donors and Mo doping generated superior conductivity to W doping. The photo-oxidation activity of water on BiVO4 photoanodes (undoped<W doped<Mo doped) was in accordance with the results from electrochemical impedance spectroscopy, MottSchottky analysis, and theoretical electronic structural calculations. Thus, Mo or W doping enhanced the photocatalytic and photoelectrochemical water oxidation activity of monoclinic BiVO4 by drastically reducing its charge-transfer resistance and thereby minimizing photoexcited electronhole pair recombination.