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

Due to its ∼2.4 eV band gap, BiVO₄ is a very promising photoanode material for harvesting the blue portion of the solar light for photoelectrochemical (PEC) water splitting applications. In WO₃/BiVO₄ heterojunction films, the electrons photoexcited in BiVO₄ are injected into WO₃, overcoming the lower charge carriers' diffusion properties limiting the PEC performance of BiVO₄ 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 BiVO₄ excitation, a favorable electron transfer from photoexcited BiVO₄ to WO₃ 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 BiVO₄. PEC measurements reveal the implication of these wavelength-dependent ultrafast interactions on the performances of the WO₃/BiVO₄ heterojunction.