Wavelength-Dependent Ultrafast Charge Carrier Separation in the WO₃/BiVO₄ Coupled System

Abstract

Due to its ~2.4 eV band gap, BiVO<sub>4</sub> is a very promising photoanode material for harvesting the blue portion of the solar light for photoelectrochemical (PEC) water splitting applications. In WO<sub>3</sub>/BiVO<sub>4</sub> heterojunction films, the electrons photoexcited in BiVO<sub>4</sub> are injected into WO<sub>3</sub>, overcoming the lower charge carriers’ diffusion properties limiting the PEC performance of BiVO<sub>4</sub> photoanodes. Here, we investigate by ultrafast transient absorption spectroscopy the charge carrier interactions occurring at the interface between the two oxides in heterojunction systems to directly unveil their wavelength dependence. Under selective BiVO<sub>4</sub> excitation, a favorable electron transfer from photoexcited BiVO<sub>4</sub> to WO<sub>3</sub> occurs immediately after excitation and leads to an increase of the trapped holes’ lifetime in BiVO4. However, a recombination channel opens when both oxides are simultaneously excited, evidenced by a shorter lifetime of trapped holes in BiVO<sub>4</sub>. As a result, PEC measurements reveal the implication of these wavelength-dependent ultrafast interactions on the performances of the WO<sub>3</sub>/BiVO<sub>4</sub> heterojunction.