Study On Zinc Oxide Microwire Ethanol Gas Sensors

Zinc oxide(ZnO) is an important ?-? semiconductor material with a wide direct band gap of 3.37eV and a large binding energy of 60 meV at room temperature, and is widely applicated in optics and electronics fields and so on. As one of of the most concerned semiconductor sensor materials, ZnO has other advantages of good chemical and thermal stability, high electron mobility, good photosensitivity and gas sensitivity. In recent years, lots of gas sensors based on ZnO micro/nanostructures have been explored and fabricated. Among them, one-dimensional (1D) ZnO microwire devices have become a research hotspot in gas sensor field because of the high surface-to-volume ratio and surface activity of the microwire. However, there are still shortcomings such as higher working temperature, longer recovery time in the usual ZnO gas sensing devices, seriously affecting their further application.In this paper, we carried out a series of research work, focusing on the controllable growth of 1D ZnO micro wires and the fabrication, sensing performance and mechanism of ethanol gas sensor based on ZnO microwire. The main research and results are as follows:(1) Synthesizing the hexagonal wurtzite structur ZnO microwires (ZnO HMWs) with smooth surface and uniform diameter by chemical vapor deposition (CVD). The morphologies, crystal structure and optical properties of samples were characterized by scanning electron microscope (SEM), transmission electron microscope(TEM), X-ray diffraction (XRD) and Photoluminescence(PL).(2) Fixing a single ZnO microwire on a slide to fabricate ethanol gas sensor. By the current signals, we obtained the important parameters of sensing response, response-recovery time, repeatability, sensitivity, stability and selectivity. Especially, we explained the working mechanism of the ZnO microwire ethanol sensor at room temperature in detail.(3) By increasing graphite powder ratio in reaction source, the curved side ZnO microwires (ZnO CMWs) were synthesized in CVD system. We thought that the too fast nucleation of ZnO at higher graphite ratio was the reason leading to the nonlinearly radial growth of ZnO microwires. Because the curved surface and larger surface-to-volume ratio are in favor of increasing the carrier density and gas molecule adsorption capacity of the ZnO CMW, the ZnO CMW sensor exhibited higher sensing response and faster response time compared to the ZnO HMW sensor.(4) Using ion-sputtered Au nanoparticles to modify the CVD-fabricated ZnO microwires, the surface-active ZnO microwires (Au/ZnO MWs) were obtained. And the gas sensor based on Au/ZnO MW exhibited fully-enhanced ethanol sensing performance relative to the unmodified ZnO MW device. Especially, the recovery performance and sensitivity have been greatly improved over a wide ethanol concentration range, showing that the sensor has a good application prospect in rapid detection of lower concentration ethanol gas. Our analysis shows that the localized surface plasmon resonance (LSPR) effect of Au nanoparticles, under indoor natural light, enhanced the conductivity of the ZnO MW. And as active media, Au nanoparticles can provide the sites for accelerating desorption of ethanol molecules by reverse spillover effect. Meanwhile, the Au nanoparticles layer would reduce the surface smoothness of the ZnO MW, it is also helpful to improve the performance of Au/ZnO MW ethanol gas sensors.