Monolayer Hf2N2I2 with a Direct Band Gap and Broad Acid-Base Adaptability for Photocatalytic Water Splitting

Photocatalytic materials are often hindered by inefficient charge transport, weak light absorption, and large reaction barriers. Herein, the water-splitting performance of monolayer Hf2N2I2 is systematically investigated using first-principles calculations. The material exhibits a direct band gap of 2.56 eV and suitable band-edge positions for overall water splitting. pH-dependent band alignment further shows thermodynamic feasibility over a wide pH range of 0-12, indicating good adaptability from acidic to mildly alkaline conditions. Monolayer Hf2N2I2 also shows strong light absorption, with a maximum absorption coefficient of 8.35 × 105 cm-1, and excellent carrier transport, including an electron mobility of 1064.85 cm2 V-1 s-1 along the x direction and a hole mobility of 295.55 cm2 V-1 s-1 along the y direction. Ab initio molecular dynamics simulations at 300 and 500 K confirm its thermal stability. The Hf site is identified as the most favorable HER active center, with ΔGH* = 0.92 eV, while the *O → *OOH step is the potential-limiting step for OER. Moreover, the calculated solar-to-hydrogen efficiency reaches 16.04%, exceeding the 10% practical benchmark. These results suggest that monolayer Hf2N2I2 possesses favorable intrinsic properties for further exploration in photocatalytic water splitting.