Band engineering in Janus In2NaX monolayers for photocatalytic water splitting

Two-dimensional Janus monolayers offer a promising platform for solar-driven water splitting owing to their intrinsic out-of-plane polarity, which facilitates efficient separation of photogenerated charge carriers. In this work, first-principles calculations based on the FP-LAPW + lo method were employed to investigate the structural, electronic, optical, and photocatalytic properties of the previously unexplored Janus In 2 NaX (X = S, Se, and Te) monolayers. HSE06 calculations predict direct band gaps of 1.58, 1.40, and 0.95 eV for In 2 NaS, In 2 NaSe, and In 2 NaTe, respectively, while spin–orbit coupling induces only minor band-gap reductions without altering the electronic characteristics. GW-BSE calculations reveal excellent UV–visible light absorption, with absorption coefficients reaching approximately 57 × 10 5 cm⁻ 1 and corrected solar-to-hydrogen efficiencies of 18.31–22.41%. Band-edge alignment and HER/OER thermodynamic driving-force analysis demonstrate that In 2 NaS and In 2 NaSe possess sufficient reduction and oxidation potentials for overall water splitting, whereas In 2 NaTe is suitable only for the oxygen evolution reaction because its conduction-band minimum is insufficiently negative for hydrogen evolution. The superior photocatalytic activity of In 2 NaS is further supported by its favorable HER/OER Gibbs free-energy descriptors. Structural optimization, phonon spectra, ab initio molecular dynamics, and elastic constants confirm the energetic, dynamical, thermal, and mechanical stability of the investigated monolayers, identifying In 2 NaS and In 2 NaSe as promising visible-light photocatalysts for sustainable hydrogen production.