The Studying Of Photoelectric Gas Sensing Properties On Nano-material ZnO

The gas sensor based on semiconductor materials has been extensively used to detect and monitor a huge variety of gases and vapors. However, owing to the high activation energy of reaction with gas molecules, appropriate high temperature was needed for the gas sensor to get good sensitivity. The high temperature operation of gas sensor with many defects restricts its application. The first defect is its high-energy comsume, which restricts this kind of gas sensor cann't intergrate in portable device. The seconed defet is its high-danger, cann't applicate in explosive environment. Therefore, the light irradiation is introduced, to get gas sensors to work at room temperature. The photoelectric gas sensor has been fabricated with the ZnO nano-materials, owing to its distinguished photoelectric performance and surfaced properties. The various kinds of nanomaterials based on ZnO have been prepared, and the relation between gas sensing performance and the structure of materials have been explored in order to understand: how to enhance the gas sensitivity, how to obtain the selectivity of photoelectric gas sensor. This study would supply the theory to fabricate the new generation of gas sensor operating at room temperature. In this paper, the doping and surface modifaction etc. were applied to mediate the properties of materials, in order to fabracte the photoelectric gas sensor with high sensitivity and high selectivity.The main results are illuminated as follows:1. The key factors on influencing the gas sensivity have been analysed about photoelectric gas sensor. The ZnO nanorods of different sizes (40, 100, 300 nm) and nanoparticles (6 nm in diameter) were prepared, and their sensing properties to formaldehyde at room temperature with and without the UV light irradiation (0.155 mW/cm2) were measured. The transient photovoltage and photoluminescence technology were used to investigate the dependence of photo-generated charge efficiency on the size of ZnO. The results demonstrated that the gas response of the samples without UV light irradiation is dominated by the surface-to-volume ratio of the materials; while under the illumination of UV light, it was controlled by both the surface-to-volume ratio and the photo-generated charge efficiency. Furthermore, the dependent of material morphology on gas sensitivity have been studied. The one-dimensional materials can support enough space for the separation of electron-hole pairs, and increase delocalization of charge carriers, because the charge is free to move throughout the length of the crystal. What is more, the one-dimensional materials own high surface-to-volume ratio. The NR-40 own high sensitivity to formaldehyde, and the detect limit lower than 0.8 ppm. It is the appropriate nanostructure for fabricating the UV light-assisted gas sensor.2. The doping of transition metal is used to improve the gas molecules adsorption and then increase the activation of gas sensor. The cobalt ion and copper ion have been doped into the ZnO. The decrease of photoconductivity in ambient air or in water vapor atmosphere has been observed when the cobalt-doped zinc oxide nanobelts are irradiated with 630 nm light. This kind of negative photoconductivity is attributed to the photodesorption of water molecules from nanobelts'surface. This result supplies a potential method to detect water vapor with a low concentration in environment. Copper-doped ZnO (1mol %) nanocrystals were synthesized using sol-gel method. The high sensing to ethanol and acetone in air have been achieved when the copper-doped ZnO was irradiated with UV light. The detection limit of gas sensor to ethanol and acetone is as low as 40 ppm and 10 ppm, respectively.3. Azo-ZnO hybrid films were prepared by functionalizing the ZnO macropore films with azo pigment (1,1'- (biphenyl-4,4'-diylbis (diazene-2,1-diyl)) dinaphthalen-2-ol). The oxygen sensing characteristics of hybrid films and pure ZnO film were measured under the irradiation of UV light. Because the functionalization promotes the photo-generation-charge, the results show that the sensitivity of hybrid film is about 520 times higher than that of pure ZnO film. Our results demonstrate that the functionalization with azo pigment is a promising approach for enhancing the oxygen sensitivity of ZnO under the irradiation of UV light.4. The photoelectric gas sensor with selectivity were fabricated based on the RuN3 because of its selectivity to C=O group. The RuN3/ZnO was prepared, under the irradiation of visible, the electrons were been filled into ZnO from the photo-generation exctions in RuN3 and induced the photocurrent. Because of the vibration coupling between the exciton and formaldehyde molecules, the energy of exctions transfers into formaldehyde molecules when it was adsorbed ZnO/RuN3, which decreases the photocurrent intensity. Therefore, the photocurrent intensity of ZnO/RuN3 is negative response to formaldehyde or acetone (decreased as increasing the formaldehyde concentron (<40 ppm)), however, the photocurrent intensity is positive response (increased as increasing the concentration) to other gas: water, ethanol, ether, ethyl acetate. It is demonstrated that the selectivity to formaldehyde can be obtained in ZnO/RuN3 when illuminated with visible light.