Preparation Of Tin Oxide Colloidal Quantum Dot Films And Their Sensing Properties Toward Hydrogen Sulfide

Metal oxide semiconductor based gas sensors have been widely utilized due to their high sensitivity, low detection limit, simple use and long lifetime. However there are still some issues such as high operation temperature and poor selectivity. Current research is mainly focused on the usage in special activities of low-dimensional materials to decrease the operation temperature, develop high performance and low power gas sensor.Colloidal quantum dots?CQDs? are “quasi-zero dimensional” inorganic semiconductor nanocrystals with quantum confinement effect. Their characteristics of extremely large surface-to-volume ratio, tunable physicochemical properties and excellent film-forming properties at room temperature make CQDs promising sensing materials for gas sensor. Here we employed the solvothermal method to synthesize SnO₂ CQDs, which were spin-coated into films with inorganic ligand exchange treatment at room temperature to fabricate SnO₂ gas sensor, the sensors were preferentially sensitive to H2 S at a low operation temperature?70oC?.First, we investigated the synthesis conditions of SnO₂ CQDs. The XRD, HRTEM and UV-vis characterization results showed that the obtained products dispersed in toluene were tetragonal SnO₂ nanocrystals around 2-3 nm in diameter with strong quantum confinement effect. The SnO₂ CQDs based gas sensors were fabricated by spin coating with AgNO3 ligand exchange treatment at room temperature. The optimal sensor showed a response of 29 to 50 ppm H2 S when operated at 70 oC, with the response and recovery times being 37 s and 127 s, respectively. Meanwhile, a complete depletion flat band model of SnO₂ CQDs gas sensor was built to discuss the sensing mechanism. Based on the experimental results and theoretical calculation, a quantitative relationship between SnO₂ Fermi level variation??EF? and H2 S gas concentration?CH2S? of ?EF = 0.01 InCH2 S + 0.05 was obtained.In order to optimize the sensor performance, we studied the influences of inorganic salt ligand?such as NH4 Cl, NaNO₂ and CuCl2? on sensor sensing properties. The sensors treated by CuCl2 ligand with a heating treatment at 200 oC exhibited the highest response of 1755 toward 50 ppm H2 S at 70 oC, with the response and recovery times of 48 s and 35 s. XPS result suggested the enhanced sensing mechanism might be related to the incorporation of copper ions. CuO was obtained by the hydrolysis and decomposition reactions of copper ions, leading p-n heterojunctions between p-type CuO and n-type SnO₂. Upon exposure of H2 S, a significant variation in sensor resistance would be observed due to the conversion of p-n heterojunction, leading to the enhancement of sensor response. Our research results may provide experimental and theoretical references for the study of various oxide quantum dots based gas sensors.