Work function-tunable graphene/WO3 heterojunctions for high-performance photoelectrochemical cell: UV-treatment effect and defective graphene

Designing hybrid photoelectrodes with graphene capable of efficient charge-carrier transfer and excellent chemical stability is an effective strategy for developing high-performance photoelectrochemical (PEC) cells. However, it remains unclear how the PEC properties are enhanced and how to control the junction properties in graphene/metal oxide heterojunction-based photoelectrodes. Here, we develop a deterministic junction to enhance PEC performance in graphene/tungsten trioxide (WO3)-based photoelectrodes by tuning the work function of graphene. It reveals that the band structure of graphene/WO3 heterojunctions can be modified by ultraviolet (UV) treatment on WO3 thin films. This modification is supported by the observation that a single-layer graphene/UV-treated WO3 photoelectrode exhibits significantly enhanced PEC activities toward the oxygen evolution reaction (OER) (evidenced by features like a threefold increase in photocurrent, an onset potential shift, and improved stability), whereas the single-layer graphene/untreated WO3 electrode demonstrates improved properties for the hydrogen evolution reaction (HER). Additionally, defects in the graphene layer formed during PEC water splitting contribute to high catalytic activities of the graphene/WO3 photoelectrode due to a decrease in the Gibbs free energy for OER and HER. These results emphasize that band structure engineering via work function tuning in graphene/WO3 heterostructures can provide multiple benefits for high PEC performance and long-term stability.