Defect engineering in ZnIn2X4 (X=S, Se, Te) semiconductors for improved photocatalysis

ZnIn2S4 has emerged as a material of interest for semiconductor-based chalcogenide photocatalysts due to its visible light absorption, chemical and thermal stability, and low cost. However, the photocatalytic activity of ZnIn2S4 is affected by the limited range of visible light absorption and ultrafast recombination of solar light-induced holes and electrons. While previous studies have considered the consequences of metal doping, metal deposition, and vacancy engineering on the photocatalytic activity of ZnIn2S4, a comprehensive understanding of native point defects and how they affect electronic and photocatalytic properties remains elusive. Here, we present a density functional theory (DFT) investigation of defect energetics in ZnIn2X4 (X=S, Se, Te) compounds in both bulk and ultrathin phases. Using both semi-local and hybrid DFT functionals, properties of interest such as the electronic band gap and band edges, optical absorption spectra, and carrier mobilities are first computed for defect-free structures. Although ultrathin ZnIn2S4 shows lower absorption compared to other chalcogenides, it exhibits sufficient overpotential for oxidation and reduction reactions for photocatalytic water splitting. Formation energies of all possible vacancies, self-interstitials, and anti-site substitutional defects are then computed for all structures, as a function of chemical growth conditions, charge state, and Fermi level (EF), which leads to the identification of the lowest energy acceptor and donor type defects and their corresponding shallow or deep level nature. DFT results show that these metal sulfide photocatalysts are prone to ZnIn and InZn anti-site substitutions, which pin the equilibrium EF close to the conduction band edge, indicative of n-type conductivity. While ZnIn does not create deep defect levels in ZnIn2X4, most of the stable native defects do create deep levels, which could adversely affect solar absorption. Finally, we report the influence of defects on the photocatalytic hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) on ultrathin ZnIn2X4. Our results suggest that a metal interstitial defect could substantially boost HER and OER on the surface of ultrathin ZnIn2X4. Overall, this systematic first principles investigation can help drive the experimental design and defect engineering of ZnIn2X4 compounds for a variety of photocatalytic applications.