Enhanced photoelectrochemical water oxidation on WO3 nanoflake films by coupling with amorphous TiO2

Abstract

High surface states induced photoelectrochemical (PEC) activity and stability issue is the major performance bottleneck for nanostructured WO3 photoanode. Here we report that amorphous TiO2 (α-TiO2) be used as passivator to modify the surface states of WO3 nanoflakes film for improving its PEC water oxidation performance. In comparison with bare WO3 film, the α-TiO2-coupled WO3 (α-TiO2/WO3) film is of higher PEC water oxidation activity and stability. The photocurrent on α-TiO2/WO3 film photoanode is improved by 2 times that reached 1.4 mA/cm2 in 0.1 M Na2SO4 solution at 0.8 V (vs. SCE). After α-TiO2 coupling, the UV–vis absorption edge and intensity of WO3 film do not change obviously, but the formation and oxidation kinetics of H2O2 on WO3 photoanode are modified. The density functional theory calculations indicate the monoclinic WO3(002) has a surface energy of 1.17 J/m2, whereas the α-TiO2/WO3(002) with a much lower surface energy of 0.3 J/m2. Additionally, the length of TiO bond of α-TiO2 is changed from the initial 1.929 Å to a shorter length of 1.886 Å and a longer length of 2.290 Å in α-TiO2/WO3(002) through the O2--W6+ bonding effect at α-TiO2/WO3 interface. Therefore, the enhanced PEC water oxidation performance on α-TiO2/WO3 film can be attributed to that α-TiO2 passivates the surface states of WO3 nanoflakes film, and the formation of H2O2 from the photoexcited-electrons/dissolved-oxygen route is depressed and the PEC oxidation of H2O2 is accelerated.