Oxidation-enabled SnS conversion to two-dimensional porous SnO₂ flakes towards NO₂ gas sensing

Tin dioxide (SnO₂)-based electronic materials and gas sensors have attracted extensive attention from academia and industry. Herein we report the preparation of two-dimensional (2D) porous SnO₂ flakes by thermal oxidation of 2D SnS flakes that serve as a self-sacrificial template. An oxidation-enabled, temperature-dependent matter conversion from SnS through three-phase SnS-SnS₂-SnO₂ (400 °C) and two-phase SnS₂-SnO₂ (600 °C) to pure-phase SnO₂ (≥800 °C) is disclosed by means of combined XRD, TG-DSC and XPS studies. Meanwhile, the associated chemical reactions and the mass and heat changes during this solid-state conversion process are clarified. The as-prepared 2D SnO₂ flakes exhibit structural porosity with tunable pore sizes and crystallite sizes/crystallinity, resulting in superior potential for NO₂ sensing. At the optimized operating temperature of 200 °C, the prototype gas sensors made of porous SnO₂ flakes show competitive sensing parameters in a broad NO₂ concentration range of 50 ppb-10 ppm in terms of high response, faster response/recovery speeds, and good selectivity and stability. A sensing mechanism involving the adsorption and desorption of NO₂/O₂ molecules and the possible surface reactions is further rationalized for the SnO₂ NO₂ gas sensors.