Structural and optical properties of SnO2/SnS nano-heterojunction thin films for gas sensor

This study investigated the structural and optical characteristics of tin dioxide/tin sulfide (SnO₂/SnS) nano-heterojunction thin films fabricated by spray pyrolysis at 400 °C. X-ray diffraction (XRD) confirmed the coexistence of tetragonal SnO₂ and orthorhombic SnS phases, with the crystallite size decreasing from 23.6 nm (SnS) to 16.1 nm for the heterojunction film. Field emission scanning electron microscopy (FESEM) images revealed a morphological transition from compact nano-grains in pure SnS to vertically aligned rod-like structures in the SnO₂/SnS films, with feature sizes ranging from 30 to 270 nm. In terms of optical properties, measurements show an increase in absorbance across the UV–Vis range for the heterojunction film, as well as an increase in the optical energy bandgap from 2.1 eV (SnS) to 2.2 eV (SnO₂/SnS), attributed to the contribution of the wide-bandgap SnO₂ layer. The formation of the p–n heterojunction enhanced charge separation across the junction, representing an advanced strategy for enhancing gas-sensing performance. As a result, the SnO₂/SnS sensor demonstrated a notable improvement in gas response of 2.2 to 20 ppm H₂S at 150 °C, about 62% higher than pure SnS (1.36), with faster response (20 s) and recovery (26 s) times. The gas response further increased sharply with gas concentration, reaching 2.9 at 40 ppm of H₂S for the SnO₂/SnS sample. These results demonstrate the effectiveness of SnO₂/SnS heterostructures in developing the performance of low-cost gas sensors for environmental monitoring applications.