It is known that the structure and morphology

It is known that AZ 10606120 the structure and morphology of functional materials have a great impact on their physical and chemical properties. It is also true for the sensing performance of metal oxide gas sensors (e.g., In2O3, ZnO, Fe2O3) [35], [36] and [37]. In order to investigate the relationship between morphology and sensing performance, the gas sensing experiments were performed to examine our ZnO microstructures with different morphologies to H2S. At first, the temperature dependent sensing behavior and the optimum operating temperature of the three ZnO sensors to 1000 ppb H2S were investigated by varying the heating voltage to obtain the highest response (Fig. 6). For these three sensors, the optimum temperature is 220 °C and corresponding response is 51.0, 29.9 and 23.3, respectively. Obviously, the response of the three sensors exhibits a trend of “increase-maximum-decay” with the operating temperature. As the temperature increases, a higher response is achieved due to the activation of adsorbed molecular oxygen and lattice oxygen to form active O2−, O− and mobile O2−, respectively [38]. This phenomenon will last until a certain optimum operating temperature is reached, after that exothermic gas adsorption becomes difficult and gas molecules start to be desorbed in a large quantity, resulting in a reduced sensor response. Thus, the optimum operating temperature is a balance between these two contradicting mechanisms. In addition, it should be noted that the sphere-like ZnO microstructure exhibits highest response of 51.0–1000 ppb H2S at 220 °C. The cauliflower-like ZnO microstructure is with a response of 29.9, while the response of sisal-like structure is only 23.3 under same conditions.