Gas sensing behavior of ZnO toward H2 at temperatures below 300°C and its dependence on humidity and Pt-decoration

Abstract The sensing reaction mechanism of ZnO toward H2 was studied, and the effect of humidity and Pt decoration on the sensor performance was also investigated. Sputter-deposited ZnO thin films exhibit a distinct sensing response to H2 at temperatures above 150°C. An inverse V-shape response feature may develop upon the H2 exposure depending on the H2 concentration and the working temperature. The characteristic response feature results from a series of surface processes occurring on the ZnO surface, including ZnO reduction, Zn nanocluster growth, O2 adsorption on Zn nanoclusters and reoxidation of the nanoclusters. The sensor response to H2 is greatly deteriorated in humid ambient. However, the sensor shows a response enhancement toward humid H2 gas mixtures if the sensor surface is prepared to be initially free from H2O adspecies. This suggests that coexistence of hydrogen and water adspecies on the ZnO surface improves the sensor response to H2. When the ZnO thin film is decorated with Pt nanoparticles, the sensor performance is greatly improved both in dry and in humid environments. The improvement is proposed to result from hydrogen spillover from Pt nanoparticles to the ZnO support, which enhances the reaction rate of ZnO reduction by hydrogen adatoms.