Effect of copper on the hydrogen sulfide gas sensing properties of the tin oxide thin film

Abstract

Detection of hydrogen sulfide gas is important due to the environmental considerations and the health hazards it posses. Sul-fides are known to be toxic for as a low as 100 ppm/air and hence sensors capable of detecting offensive sulfides are needed for the optimization of auto ventilation system of toilet or kitchen, dentistry etc. The present work talks about the different means of improving the sensitivity and selectivity of the tin oxide thin film towards hydrogen sulfide. The tin oxide thin film is modified with an overlayer of copper oxide nano-film deposited by the mono-layer protected copper nanoclusters and the newly developed liquid-liquid interface reaction technique (LLIRT). The other method involves incorporation of copper in the tin oxide matrix using spray pyrolysis. Modification of the thin film tin oxide surface by copper oxide nanofilm results in the enhancement of the sensitivity (s=100) and selectivity towards H/sub 2/S as compared to the pure tin oxide (s=12) thin film. The effect of thickness of the overlayer copper oxide film on the gas sensing properties is correlated. The surface functionalisation with monolayer protected clusters (MPC's) leads to the room temperature detection of H/sub 2/S. The sensing surface is made by first depositing a thin film of tin oxide on to a glass substrate followed by surface functionalisation with monolayer protected copper nanoclusters capped with different capping agents prepared as per the Brust synthesis route. These nanoclusters enhance the sensitivity of the sensor towards hydrogen sulfide. The environment of copper clusters changes after the exposure to H/sub 2/S gas, allowing the access of copper to gas molecules. This further facilitates the electron transfer between cluster to cluster and hence enhances the conductance of the sensor element. The sensitivity of copper clusters capped with different functional groups and with different chain length is established. The sensitivity of the film is calculated as the ratio of change in the conductance to the original conductance. The effect of surface coverage, morphology, oxidation-state and the amount of copper on the sensitivity have been studied. The correlation between copper incorporation and the improvement in the selectivity and sensitivity towards hydrogen sulfide is discussed.