Computational study of surface orientation effect of wurtzite GaN on CH4 and CO sensing mechanism

Abstract

The adsorption behaviors of gas molecules CH4 and CO on polar (0001), (000–1) facets and nonpolar (10-10), (11–20) facets of wurtzite GaN were explored in details by first-principles calculations. Among these important low-index facets, CH4 on the (0001) facet exhibits the lowest adsorption energy of −1.57 eV, signifying the highest density of adsorbed CH4 molecules under the same methane concentration in air. Meanwhile, the C–H bond is weakened revealed by the bond length elongation from 1.0956 Å to 1.1020 Å during adsorption, consistent with charge redistribution results. The adsorption energies for CO on the (0001) and (000–1) facets are calculated to be −2.17 eV and −3.89 eV, much lower than those on nonpolar facets. Obviously, the corresponding C–O bond is activated during adsorption, proved by the bond length elongated from 1.1531 Å to 1.2003 Å and 1.1715 Å, respectively, in line with charge redistribution results. Particularly, the co-adsorption simulation verifies the surface redox reaction between CO and O2 on + c-plane GaN. Therefore, we predict superior performance of (0001) GaN in detection for high concentration CH4 and (000–1) GaN in selective detection for low concentration CO.