Study Of Photoelectric Gas Sensing Propetries Of ZnO Nano-composites

Chemical sensors have played important roles in industrial, medical anddomestic applications in detecting pollutant, toxic and combustible gases. Keyrequirements for chemical sensors contain not only high sensitivity and goodselectivity to a trace targeted gas, but also the abilities of working in a continuousmode at room temperature. In recent years, growing interests have been observed inthe developing of gas sensors that could work at room temperature, which is probablyinspired by the advances in material fabrication and nanotechnology, as well as somenovel concepts of sensing mechanisms, such as gas sensing based on the variation inphotoelectric response. Zinc oxide (ZnO), an intrinsic large band-gap semiconductor,has been recently demonstrated as a suitable candidate for photoelectric gas sensingwith high sensitivity to a broad range of simple gases and organic vapours. However,UV excitation with wavelengths shorter than380nm is required to induce effectivecharge separation for gas sensing process, which greatly hinders their practicalapplication. It is essential to extend the photo-response of the large band-gapsemiconductor into the visible-light region for the design of practical photoelectric gassensors that could efficiently utilize solar spectrum.Until now, various methods have been attempted to shift the photo-response of wideband-gap semiconductor materials into the visible-light region. Cost-effectivephotoelectric materials that could utilize visible-light irradiation efficiently are greatlydesired. Narrow band-gap semiconductors such as CdS, Bi₂S₃, CdSe, PbS, and LnPhave been demonstrated as low-cost sensitizers for large band-gap semiconductorssuch as TiO₂or ZnO. These inorganic sensitizers can be modified in grain size and morphology to produce various nanostructures, which enables the materials with highspecific surface area and flexible photo-response. The heterostructures of the twokinds of semiconductors can utilize visible light and own high surface-to-volume ratio.Thus, they are an attractive option for visible-light-induced photoelectric gas sensors.The heterostructures have been widely used for solar cells and as photocatalystworking under visible-light irradiation.The focus of the thesis is to build composite structures based on ZnO, developedthe best synthetic methods and conditions. The modification of ZnO by combinedwith another semiconductor to extend photoresponding range to the visible region andenhance the absorption of visible light. By constructing a hetero-interface, promotethe separation of charge and inhibit their recombination. We investigated theinfluence of crystal structure, photoelectric stability, optical and electrical propertiesand gas sensing activity of composite material by transient photovoltaic technology(TPV), photocurrent and so on. The relationship between the behavior ofphotogenerated charge (including separation-recombination) and photoelectricactivity is also discussed, which provide a theoretical basis to design and constructvisible-light-irradiated gas sensor. The specific work to accommodate as follows:1. A visible-light-induced gas sensing element based on the heterostructures ofCdS nanoparticles (NPs)/ZnO microcrystals has been successfully developed.Commercial ZnO microcrystals were coated with CdS nanoparticles by asonochemical method to form the heterostructures. Compared with native CdS NPs,the as-obtained CdS NPs/ZnO showed enhanced photocurrent intensity. And theheterostructures-based element exhibited an excellent performance of photoelectricgas sensing to formaldehyde induced by visible-light and the observational detectionlimit is lower than10ppmat room temperature. Moreover, the photocurrent intensityand sensitivity to formaldehyde were controllable by tuning the amount of CdS NPsattached on the surface of ZnO microcrystals. A possible explanation on the sensingmechanism has been proposed. In addition, the gas sensing element possessed goodreproducibility. Our results demonstrate that the heterostructures of CdS NPs/ZnO isan efficient and promising material for visible-light-induced photoelectric gas sensors working at room temperature.2. CdS nanowire/ZnO nanosphere materials (CdS/ZnO) with hierarchicalstructure were synthesized by a three-step solvothermal process. By XRD analysis, allthe diffraction peakscan be indexed as a mixture of hexagonal wurtzite ZnO and thehexagonal wurtzite CdS. FESEM analysis confirmed the growth of ZnO nanosphereson the surface of CdS nanowires (NWs). The high resolution TEM (HRTEM) imagesshow both components with distinguished and coherent interfaces. The transientphotovoltage (TPV) measurements revealed that the interface between CdS and ZnOcan inhibit the recombination of photogenerated excess carriers and prolong thelifetime of excess carriers in CdS/ZnO materials. Moreover, the CdS/ZnO materialsexhibit a dramatic improvement in optoelectronic performance andvisible-light-irradiation gas sensing activity, which gave1order of magnitude largerthan that of CdSNWs in response to formaldehyde. The enhancement of sensingproperties is attributed to the interfacial transport of excess carriers.3. Carbon-doped ZnO microspheres were prepared through a facile hydrothermalprocess. The powder was placed in a furnace and maintained at500C and700C for2h in air. The characteri-zations for the physicochemical properties of preparedsamples under different calcination temperatures were carried out on X-ray diffractionanalysis, scanning electron microscopy, X-ray photoelectron spec-troscopy, UV–visdiffuse reflectance spectra, Raman spectroscopy and photocurrent spectra. By XPSanalysis, the carbon may be incorporating into the interstitial positions of the ZnOlattice and exist in the form of carboxyl carbon. Raman spectroscopy showed that thecalcination condition was the key factor for the carbon doping during the combustionprocess. UV–vis diffuse reflectance spectroscopy showed the absorption edges ofC-ZnO extended to the visible-light region. C-doped ZnO under500C calcinationexhibited the superior UV-activated room-temperature gas sensing activity for thedetection of ethanol. The result of transient photovoltage suggested that the presenceof sp2carbon-type structures could improve the separation extent and restrain therecombination of the photoinduced electron–hole pairs, which increase the number ofphotoinduced oxygen ions on the surface of C-doped ZnO and enhancing of gas sensing activity.