| Summary: | In the present work, three kinds of nanosized SnO<sub>2</sub> samples were successfully synthesized via a hydrothermal method with subsequent calcination at temperatures of 500 °C, 600 °C, and 700 °C. The morphology and structure of the as-prepared samples were characterized using X-ray diffraction, transmission electron microscopy, selected area electron diffraction, Brunauer–Emmett–Teller analysis, and X-ray photoelectron spectroscopy. The results clearly indicated that the SnO<sub>2</sub> sample calcined at 600 °C had a higher amount of chemisorbed oxygen than the SnO<sub>2</sub> samples calcined at 500 °C and 700 °C. Gas sensing investigations revealed that the cataluminescence (CTL) sensors based on the three SnO<sub>2</sub> samples all exhibited high selectivity toward H<sub>2</sub>S, but the sensor based on SnO<sub>2</sub>−600 °C exhibited the highest response under the same conditions. At an operating temperature of 210 °C, the SnO<sub>2</sub>−600 °C sensor showed a good linear response to H<sub>2</sub>S in the concentration range of 20–420 ppm, with a detection limit of 8 ppm. The response and recovery times were 3.5 s/1.5 s for H<sub>2</sub>S gas within the linear range. The study on the sensing mechanism indicated that H<sub>2</sub>S was oxidized into excited states of SO<sub>2</sub> by chemisorbed oxygen on the SnO<sub>2</sub> surface, which was mainly responsible for CTL emission. The chemisorbed oxygen played an important role in the oxidation of H<sub>2</sub>S, and, as such, the reason for the SnO<sub>2</sub>−600 °C sensor showing the highest response could be ascribed to the highest amount of chemisorbed oxygen on its surface. The proposed SnO<sub>2</sub>-based gas sensor has great potential for the rapid monitoring of H<sub>2</sub>S.
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