The Influence of Crystal Facet Exposure of Metal Oxide Semiconductors for Gas Sensing Performance

Abstract

The surface atomic arrangement and electronic structure of metal oxide semiconductor (MOS) gas-sensing materials directly determine their gas sensing performance. This article systematically reviews recent research advances and underlying mechanisms of exposed crystal facet engineering in optimizing the gas-sensing properties of MOS materials. Firstly, at the microscale level, the sensitization mechanism is elucidated, whereby high-energy crystal facets significantly enhance gas adsorption, surface reactions, and charge transfer efficiency by providing abundant dangling bonds and unsaturated coordination atoms. Secondly, four types of cutting-edge synthesis strategies for achieving facet-oriented exposure are comprehensively categorized and summarized: (i) thermodynamic modulation, including hydrothermal/solvothermal methods utilizing capping agents to reduce surface energy and selective chemical etching based on facet-dependent reactivity; (ii) kinetic control, encompassing nucleation rate manipulation, epitaxial growth techniques, and plasma-assisted synthesis; (iii) soft-template strategies, focusing on the spatial confinement effects induced by bio-templates and microemulsion/micelle methods; (iv) extreme-condition synthesis, analyzing the unique advantages of supercritical fluids and femtosecond laser processing in inducing the formation of specific crystal facets. Finally, the applicability of different synthesis routes is evaluated, and key challenges—such as maintaining the thermodynamic stability of high-energy facets, scalable production techniques, and selective detection in complex gas environments—along with future research directions are outlined. The review serves as a guide for researchers to leverage crystallographic engineering in designing advanced gas sensors. It bridges materials chemistry, surface science, and device engineering, emphasizing how controlling facet exposure can unlock superior performance in real-world applications.