Photocatalytic and photoelectrochemical water oxidation over metal-doped monoclinic BiVO₄ photoanodes

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 BiVO ₄; Mo had the highest improvement by a factor of about six. Thus, BiVO₄ and W- or Mo-doped (2 atom %) BiVO₄ photoanodes about 1 μm thick were fabricated onto transparent conducting substrate by a metal-organic 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 (J_PH) of about 2.38 mA cm^-2 was achieved for Mo doping followed by W doping (J _PHâ1.98 mA cm^-2), whereas undoped BiVO₄ gave a J_PH value of about 0.42 mA cm^-2. The photoelectrochemical water oxidation activity of W- and Mo-doped BiVO ₄ photoanodes corresponded to the incident photon to current conversion efficiency of about 35 and 40 % respectively. Electrochemical impedance spectroscopy and Mott-Schottky analysis indicated a positive flat band shift of about 30 mV, a carrier concentration 1.6-2 times higher, and a charge-transfer resistance reduced by 3-4-fold for W- or Mo-doped BiVO ₄ 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, Mott-Schottky 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 electron-hole pair recombination. Performance-enhancing dopants: The photocatalytic and photoelectrochemical water oxidation performance of monoclinic BiVO₄ is dramatically enhanced by doping Mo or W into V sites by improving the charge-transfer characteristics. Mo doping generates superior n-type conductivity than that of W doping (see picture).