Self‐Assembled Gallium Sulfide (GaS) Heterostructures Enabling Efficient Water Splitting and Selective Ammonia Sensing

Abstract Herein, a comprehensive validation of the catalytic and sensing capabilities of gallium sulfide (GaS). This study focuses on the self‐assembled heterostructure formed by GaS with its native oxide, revealing novel insights into the crucial role of defects, strain, and surface oxide phases in optimizing the behavior of 2D materials for catalytic and sensing applications. Although the energy barrier for water dissociation on pristine GaS surfaces is prohibitive (+419.3 kJ mol −1 ), surface sulfur vacancies considerably reduce this barrier, transforming defective GaS (GaS x ) into an efficient catalyst for the hydrogen evolution reaction (HER) in alkaline media. Water dissociation is energetically favorable at room temperature on GaS 0.96 surfaces (−147.6 kJ mol −1 ). Correspondingly, the differential free energy for HER on GaS 0.96 in an alkaline medium is found to be −1.56 eV for the hydroxyl adsorption step and +1.28 eV for the desorption step, while all reaction steps are exothermic for its implementation as a catalyst for oxygen evolution reaction (OER). These theoretical models and surface‐science experiments confirm that exposure of GaS surfaces to ambient conditions leads to the inevitable formation of a self‐assembled nanoscale (≈3 nm thick) oxide skin. This native oxide layer stabilizes the surface and, moreover, it also significantly enhances its catalytic and sensing properties by providing additional active sites and improving charge transfer dynamics. The exceptional sensitivity (response of 18% at T = 150 °C) and selectivity for detecting ammonia (NH 3 ) are attributed to both its high affinity for chemisorption and the significant charge‐transfer interactions that enhance the sensor response.