Unconventional Anisotropy in Excitonic Properties and Carrier Mobility in Iodine-Based XTeI (X = Ga, In) Monolayers for Visible-Light Photocatalytic Water Splitting

Two-dimensional materials for efficient visible-light-driven photocatalytic water splitting after the era of TiO₂ and other metal oxides are scarce. Recent years have witnessed an upsurge in the research of nonoxide semiconductors for overall photocatalytic water splitting. Herein, XTeI (X = Ga, In) monolayers are demonstrated to be stable water-splitting photocatalysts. Merely 0.1 (0.03) V of additional voltage is required to drive the oxidation evolution reaction by the GaTeI (InTeI) monolayer. As these monolayers exhibit a higher valence band position than traditional metal oxides, implying a narrower band gap, they are suitable as efficient visible-light-driven photocatalysts with an absorption coefficient of ∼10^–5 cm^–1 in the visible range of the solar spectrum. Anisotropic carrier mobilities favor charge separation and reduce their recombination. Exciton properties have been analyzed in-depth via GW approximation. Exciton spatial extension and exciton binding energy are found to exhibit a highly anisotropic nature. The exciton binding energies of GaTeI and InTeI on different substrates, like SiO₂ and h-BN, range from ∼14 meV to ∼0.2 eV, which are even smaller than those in TMDs. These facilitate easier charge separation, making them favorable photocatalysts.