Toluene- and benzene-selective gas sensors based on Pt- and Pd-functionalized ZnO nanowires in self-heating mode

Abstract

Selective toluene and benzene gas sensors, based on Pt- and Pd-functionalized ZnO nanowires (NW) in self-heating mode, are presented. The thickness of the initial sputtered metal layer (5 and 10 nm) and annealing temperature (500–750 °C) were varied to optimize the formation of Pt nanoparticles (NPs), whereas the UV irradiation time was optimized to obtain isolated Pd NPs on the surface of crystalline ZnO NWs. After a series of gas sensing studies and optimization of the gas sensors under the self-heating mode, Pt- and Pd-functionalized ZnO NWs showing most sensitive responses to toluene and benzene gases, respectively, and exhibiting sufficient selectivity for these two amongst other reducing gases were obtained. At room temperature, under an applied voltage of 20 V, the maximum response of Pt-functionalized ZnO NWs (initial thickness, 5 nm; annealing temperature, 600 °C) to 50 ppm toluene is 2.86. Moreover, the maximum response of Pd-functionalized ZnO NWs (UV irradiation time, 5 s) to 50 ppm benzene is 2.20. Further, the sensing mechanisms of these sensors are explained. Chemical sensitization by Pd and Pt, as well as the generation of ZnO/Pd and ZnO/Pt heterointerfaces enhanced the sensing performance of these sensors. The selective sensing of toluene and benzene gases by Pt- and Pd-functionalized ZnO NWs, respectively, is explained in terms of the adsorption phenomena. This study opens a pathway to the fabrication of selective toluene and benzene gas sensors with low power consumption operating in the self-heating mode.