Optimal Doping for Enhanced SnO 2 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 SnO 2 crystal and grain size, its sensing performance to ethanol vapor (0.1–50 ppm) exhibited an optimum with respect to SiO 2 content. The thermal stability and morphology of SiO 2 ‐doped SnO 2 nanoparticles were evaluated by sintering at 200–900 °C for 4–24 h in air. At low SiO 2 content, sintering of SnO 2 was prevented only partially resulting in small sinter necks (bottlenecks) between SnO 2 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 SnO 2 SiO 2 heterojunctions rather than the decreasing SnO 2 crystallite and grain sizes with increasing SiO 2 doping. At high SiO 2 contents, SnO 2 sintering was inhibited as its grains were separated effectively by dielectric SiO 2 ; this resulted in isolated SnO 2 nanocrystals with drastically reduced sensitivity, thereby effectively being insulators.