Title : Solar water splitting by doping-treated BiVO4
Abstract:
Cost-effective solar fuel generation requires suitable photocatalysts for artificial photosynthesis. To develop such appropriate photocatalysts, their atomic structure control is of primary importance since their functionalities (e.g., electronic band frame, electric properties, kinetics, etc.) are governed by their atomic structure. In this regard, BiVO4’s atomic structure has been engineered via P5+ doping and In3+/Mo6+ dual doping. The significantly enhanced photo-responsive characteristics of doping-treated BiVO4 have been studied within experimental and theoretical domains.
Specifically, to reduce the charge-transfer resistance of monoclinic (m-) BiVO4, phosphorous was doped into its vanadium sites for the first time through VO4 and PO4 oxoanion exchange. As a result, the charge-transfer resistance was significantly reduced, and thus solar-to-hydrogen efficiency was enhanced up to 29.3 times, as Figure 1 shows. Notably, this brand-new oxoanion exchange technique can be applied to other various VO4-based semiconductors to improve their electronic, catalytic, and photochemical properties.[1]
Thanks to the phosphorus doping study, it is possible to make m-BiVO4 function as an excellent photocatalyst for water oxidation (2H2O(l) + 4h+→ O2(g) + 4H+) under visible light. However, its conduction band is located at a more positive potential than that of proton reduction (0 VRHE at pH 7, RHE: reversible hydrogen electrode), it is still incapable of evolving H2 (2H+ + 2e−→ H2(g)). This is why overall water splitting (2H2O(l) + 4h+ + 4e− → O2(g) + 2H2(g)) under visible light irradiation over BiVO4-based photocatalysts has never been fully achieved.
To meet this challenge, the band edges of m-BiVO4 was engineered by simultaneously substituting In3+ for Bi3+ and Mo6+ for V5+ in the host lattice of monoclinic BiVO4, which induced partial phase transformation from pure monoclinic BiVO4 to a mixture of monoclinic BiVO4 and tetragonal BiVO4. This In3+/Mo6+ doped BiVO4 has a slightly larger band-gap energy (Eg ~ 2.5 eV) than usual ‘yellow’ monoclinic BiVO4 (Eg ~ 2.4 eV), as supported by the unique color change to green, and higher (more negative) conduction band edge (− 0.1 VRHE at pH 7) than H+/H2 potential (0 VRHE at pH 7). Consequently, as Fig. 2 displays, the In3+/Mo6+ doped BiVO4 is able to split water into H2 and O2 under visible-light irradiation without using any sacrificial reagents (e.g. CH3OH or AgNO3). This outcome is the first example of a pure water-splitting photocatalyst responding to visible light without any noble-metal cocatalyst.
