Doping Effects on BiVO4 Photocatalysts and Photoelectrodes for Photocatalytic Water Splitting

Abstract

Photocatalytic water splitting is one of the key techniques to harvest and utilize solar energy in hydrogen form. Water oxidation is challenging step to be improved for high efficiency in water splitting process. BiVO4 is one of the best water oxidation photocatalyst, and its characteristics are changeable by metal doping. To investigate doping effects on BiVO4 for photocatalytic water splitting, single dopant doping and dual dopants doping are performed at BiVO4 particulate photocatalysts and photoelectrodes. For single-doping effects, various metal ion-doped BiVO4 was investigated and among the 12 metal ion dopants, it was found that doping of only W and Mo dramatically enhanced the water photooxidation activity of a bare BiVO4, with Mo showing the highest improvement by a factor of ca. 6. Thus, for ca. 1 μm thick BiVO4 and 2at% (W or Mo)-doped BiVO4 photoanodes were fabricated onto transparent conducting substrate by metal- organic decomposition/ spin-coating method. Under simulated 1 sun (air mass 1.5G, 100 mw.cm-2) and at 1.23 VRHE, the highest photocurrent density (JPH) ~2.38 mA.cm-2 was achieved for Mo-doping followed by W-doping (JPH ~1.98 mA.cm-2), whereas undoped BiVO4 gave JPH ~0.42 mA.cm-2 The photoelectrochemical water oxidation activity of W- and Mo- doped BiVO4 photoanodes corresponded to the incident to photon current conversion efficiency (IPCE) of ~35% and ~40%, respectively. The electrochemical impedance spectroscopy and the Mott-Schottky analysis indicated a positive flat band shift of ca. 30 mV, more than 2 ~3 times carriers’ concentration, and 3-4 fold reduced charge transfer resistance 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 than W-doping. Photo-oxidation activity of water on BiVO4 photoanodes (undoped<W-doped<Mo-doped) was in full accords with the results of impedance spectroscopy, Mott?Schottky analysis and the 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 the photoexcited electron-hole pair recombination. Dual-doping study achieves the goal by developing ‘greenish’ BiVO4 (GBVOx), Bi1-xInxV1-xMoxO4, via doping-flavored solid state reactions. The new GBVOx photocatalyst has larger band-gap energy than usual ‘yellow’ scheelite-monoclinic BiVO4, as well as higher (more negative) conduction band than H+/H2 potential (0 V vs. RHE). Hence, it is able to extract H2 from pure water via visible light-driven overall water splitting without using any sacrificial reagents (e.g., CH3OH or AgNO3). The density functional theory calculation indicates that In3+/Mo6+ dual doping triggers partial phase transformation from pure m-BiVO4 to a mixture of m-BiVO4 and t-BiVO4, which sequentially leads to unit-cell volume growth, which sequentially leads to unit-cell volume growth, compressive lattice-strain increase, conduction- band edge uplift, and band-gap widening.