Engineering HfN₂-XC (X = Ge, Sn) van der Waals heterostructures for photocatalytic water splitting with broad pH adaptability

Van der Waals heterostructures provide a viable approach to solar-driven water splitting, which converts sunlight into clean hydrogen fuel to address worldwide energy problems. Herein, based on first-principles calculations, it is predicted that the HfN₂-XC (X = Ge, Sn) heterostructures possess direct band gaps in the range of 1.55–1.88 eV, with type-II band alignments and an interfacial electric field that together promote the separation of photoexcited charge carriers. More importantly, solar‑to‑hydrogen efficiencies of 23% and 28% are achieved for the HfN₂‑GeC and HfN₂‑SnC heterostructures, respectively, which are higher than the standard commercial benchmark of 10%. Moreover, the HfN₂-GeC and HfN₂-SnC heterostructures exhibit favorable band edges for overall water splitting at pH = 0–7 and pH = 0–4, respectively. The heterostructures capture a broader range of visible light than the separate monolayers, indicating their enhanced capability for solar energy conversion. Notably, applying biaxial strain transforms the band alignment from type-II to type-I and decreases the Gibbs free energy change for the HER to approximately 0.10 eV. Our findings provide valuable guidance for the potential application of HfN₂-XC (X = Ge, Sn) heterostructures as photocatalysts for water splitting.