Adsorption and Sensing Performance toward Methanol Vapor on SnS/SnS₂In-Plane Heterostructures

Group-IV monochalcogenides MX (M = Ge, Sn; X = S, Se) show great potential in gas-sensing applications due to their unique two-dimensional (2D) geometry with a puckered layer and high oxidation resistance. In terms of gas-sensing response modulation, creating 2D heterojunctions has been demonstrated to be highly effective. In this work, a unique in-plane heterostructure of SnS/SnS2 was prepared by thermally induced phase transition from n-type SnS2 to p-type SnS for a promising methanol sensor. Differing from common vertically stacked van der Waals heterojunctions, plentiful grain boundaries and lattice defects are generated in the SnS/SnS2 lateral heterostructure, inducing further structural symmetry breaking, due to the obvious lattice mismatch between orthogonal SnS and hexagonal SnS2. The special local irregularity with a grain boundary relaxation effect existed around the highly active sites of the generated vacancy defects and the lattice Sn atoms at SnS/SnS2 grain boundaries, which can trigger strong adsorption of methanol molecules on the lateral heterostructured sensing surface. Meanwhile, the SnS/SnS2 lateral heterojunctions with strong in-plane covalent bonding show significant modulation on the electron transport characteristic. Resultantly, the sensor based on the as-prepared SnS/SnS2 in-plane heterostructure is revealed to display a 7.1 times enhancement in sensing response toward 25 ppm methanol at room temperature compared with a pristine SnS sensor and shows the capability to detect rarefied methanol vapor as low as 100 ppb.