Semiconducting BiVO4 and TiO2 thin films for photoelectrochemical water splitting and photodegradation

Abstract

There is increasing concern for the impact of air and water quality on human health. Many air pollutants derive from the burning of fossil fuels; the major water pollutants derive from organics/inorganics produced by industrial processes and domestic activities. Photocatalysis can be applied to the former through alternative fuels, such as H2, which is a renewable and clean fuel that can be generated from photoelectrochemical water splitting. Photocatalysis can be applied to the latter through photodegradation of water pollutants. Semiconducting oxides, particularly as thin films, are used widely in photocatalysis owing to their ease of fabrication, environmental stability, suitable band gap, and appropriate valence or conduction band edge for photocatalytic oxidation or reduction, respectively. BiVO4 and TiO2 are two such photocatalysts that have emerged as promising materials for photocatalytic applications. BiVO4 has a narrow band gap and suitable valence band edge for water oxidation but its photocurrents remain low relative to theoretical limits. Its main drawback is high charge-recombination rates. Thus, BiVO4 thin films decorated with metallic V13O16 have been engineered for the first time. Characterization and DFT simulations indicate that superior photocurrents result from enhanced charge separation between the two phases, which results from ohmic contact and band bending at the interface from V13O16. A Co-Pi co-catalyst coating further increases the photocurrent to 5.0 mA/cm2 at 1.23 V vs. RHE, which is among the highest reported for BiVO4-based photocatalysts. A model explaining charge separation and transfer in terms of the distribution of nanostructural phases is proposed. In contrast, TiO2 has a wide band gap and high charge-recombination rates also limit its photodegradation efficiency. Cr-doped, V-doped, and Cr/V-codoped TiO2 thin films were synthesized and examined for photodegradation of methylene blue. Compared to undoped TiO2, superior photocatalytic performances were observed for 0.01-0.05 mol% Cr, 0.01-0.60 mol% V, and 0.01 mol% Cr/V. The new concept of multivalence charge transfer is proposed to explain the presence of the nonequilibrium valences Ti3+, Cr4+, V3+, and V4+. Further, a model comprehensively summarizing the role of doping on the extrinsic and intrinsic effects on the photocatalytic performance is presented.