Hierarchical Self-Assembly of SnO₂ Nanoparticles into Porous Microspheres: Exceptionally Selective Ammonia Sensing at...

Herein, porous SnO₂ microspheres in a three-dimensional (3D) hierarchical architecture were successfully synthesized via a facile hydrothermal route utilizing d-(+)-glucose and cetyltrimethylammonium bromide (CTAB), which act as reducing and structure-directing agents, respectively. Controlled adjustment of the CTAB to glucose mole ratio, reaction temperature, reaction time, and the calcination parameters all provided important clues toward optimizing the final morphologies of SnO₂ with exceptional structural stability and reasonable monodispersity. Electron microscopy analysis revealed that microspheres formed were hierarchical self-assemblies of numerous primary SnO₂ nanoparticles of ∼3-8 nm that coalesce together to form nearly monodispersed and ordered spherical structures of sizes in the range of 230-250 nm and are appreciably porous. N₂-sorption measurements further confirmed the high degree of porosity for these structures, with an estimated BET surface area of ∼35 m² g^-1. Taking advantage of these porous structures and large surface area, the ammonia (NH₃) sensing capabilities of the SnO₂ spheres were explored. The gas sensor exhibited a notable response value (S) of ∼20.72 when exposed to 100 ppm of NH₃ gas, all while operating at room temperature (∼27 °C), along with an impressively low detection limit of ∼1 ppm. Based on the comprehensive investigations, the potential mechanism behind the formation of these intricate SnO₂ hierarchical structures along with the factors that make this material exhibit such excellent gas sensing behavior is postulated. Overall, the work provides a facile and possibly a generic route for the synthesis of hierarchical nanostructured materials that holds promise for the development of ultrasensitive gas sensor materials operating at room temperature.