Optimal doping for enhanced SnO₂ sensitivity and thermal stability

Tin oxide nanocrystals (5-10 nm) doped with silica (0-15 wt %) were made by flame-spray-pyrolysis direct deposition onto the sensing electrodes and in situ stabilization by rapid flame annealing. Although increased SiO ₂-doping reduced the SnO₂ crystal and grain size, its sensing performance to ethanol vapor (0.1-50 ppm) exhibited an optimum with respect to SiO₂ content. The thermal stability and morphology of SiO₂-doped SnO₂ nanoparticles were evaluated by sintering at 200-900°C for 4-24 h in air. At low SiO₂ content, sintering of SnO₂ was prevented only partially resulting in small sinter necks (bottlenecks) between SnO₂ primary particles (smaller than twice the Debye length). This morphology drastically enhanced the sensitivity toward the analyte by maintaining a thermally stable high surface area and fully depleted connections at the primary particle necks. This enhancement is attributed mostly to the decreasing neck size of the SnO₂-SiO₂ heterojunctions rather than the decreasing SnO₂ crystallite and grain sizes with increasing SiO₂ doping. At high SiO₂ contents, SnO₂ sintering was inhibited as its grains were separated effectively by dielectric SiO₂; this resulted in isolated SnO ₂ nanocrystals with drastically reduced sensitivity, thereby effectively being insulators.