Fabrication And Characteristic Of ZnO Nanowires Array For Gas Sensing

Zinc oxide (ZnO) is an important n-type semiconductor material with a direct band gap, which has a wide application in optoelectronic devices, e.g., fabricating field effect transistors (FETs), gas sensors, solar cells. Among the various ZnO nanostructures, ZnO nanowires own higher electrical conductivity and larger surface area, which have a promising future in fabricating ultra-high sensitive gas sensors. However, there are still many problems in gas sensors based on ZnO nanowires, e.g., low sensitivity, high working temperature and long response-recovery time. The listed drawbacks, to some extent, result in the slow progress in ZnO materials. Therefore, two methods are used for improving the gas-sensing properties of ZnO nanowires, one is adjusting the band structure and carrier concentration by means of metallic material with good catalytic property and the other is increasing the surface area of ZnO for accelerating the reaction processes of adsorption-desorption of gas molecules. And thus, it is important to study the sensing mechanism of metallic material with good catalytic property in preparation ZnO nanowires and prepare these composites with the larger surface area. The dissertation is aiming at a comprehensive study on controllable preparation of ZnO nanowires array and its gas-sensitive performance. Combined with density functional theory (DFT) and hydrothermal method, two kinds of sensitizing technologies, doping and decorating of catalytic Pd are obtained to investigate how to adjust the morphology and energy band structure of ZnO nanowires. The different sensitizing methods are compared and the features and advantages are concluded. The intrinsic mechanism in enhancing the nanowires’sensitivity by noble metal Pd as sensitive material, is revealed. The theory of improving the gas sensitivity of ZnO nanowires could be established from theoretically illustrating to experimentally validating. In the end, a method of preparing ultra-long nanowires array was proposed and gas-sensitivity measurement was carried out. Main conclusions of dissertation could be obtained as folio wings:(1) The first-principle was chosen to investigate the absorbed mechanism of gas molecule on the surface of ZnO nanowires decorated with Pd atom. The result indicates that a strong interaction is emerged between Pd atom and the O atom of hydroxy in ethanol molecule, and the corresponding density of states (DOS) exhibits obvious change near the Femi level, when ethanol molecule absorbed on Pd decorated ZnO nanowires (Pd-ZnO-NWs) surface. For ZnO nanowires with an oxygen vacancy (ZnO-NWs-D), its band gap width decreased from0.83eV to0.75eV, indicating that higher active locations are generated in oxygen vacancy, and the system shows more metallic features. According to further investigations on ethanol, acetone, methanol molecule adsorbed on Pd-ZnO-NWs-D surface, it is found that the adsorption energy of ethanol is the biggest while the band gap width is smallest, which makes the electrons transfer from valence band to conduction band more easily to participate in electron exchange, and it is beneficial for enhancing the sensitivity of sensors.(2) Hydrothermal method was used to prepare the Pd-doped ZnO nanowires array on interdigital electrodes, different doping ratio are compared, specifically,0%,0.25%,0.5%,1%and2%. It is proved that with the rise of solution concentrations of PdCl2, the diameter of nanowires was changing from big to small then to big. The reason is that when the concentration of PdCl2is low, Pd2+plays an active role in the crystal growth of ZnO nanowires along [0001] crystal orientation, while when the concentration reaches a certain value, with the consumption of Pd2+, the growth get restricted by Cl-along the same direction. At last, the results of gas-sensitive test indicate that Pd-doped (0.25at.%) device shows the utmost sensitivity in ethanol. Compared to pure ZnO nanowires (450℃and3.91), Pd-doped device shows better operating temperature (325℃) and response (8.17).(3) By surface decorating, ZnO nanowires array decorated with Pd nanoparticles are prepared (the loading quantity is0.5,1.3and3.1at.%). And samples are used to test sensitivity of ethanol, acetone, methanol, methane, carbon monoxide exposed to200ppm. The testing result shows that Pb decorated ZnO nanowires array has the best selectivity for ethanol when carbon monoxide and methane are interfering gases, and when the amount of Pd is1.3at.%, the sensitivity and response-recovery time to ethanol of this device are4.23,9s and9s, respectively, and its detecting limitation is as low as1ppm.(4) In order to increase surface area of ZnO nanowires, an ultra-long ZnO nanowires array (11μm,6h) is prepared using microfluidic technology. It is proved that the inhomogeneous velocity field shows great impact on the ZnO growth in theories and experiments. As a whole, It is observed that the diameter of nanowires in middle areas is bigger than that of two sides, and nanowires in the entrance section is bigger than the exit area. It is confirmed that dissipation speed of PEI on heating has great relationship with the diameter of ZnO nanowires. At last, the prepared ultra-long ZnO nanowires array was used to take measurement for volatile organic compounds (VOC) gases. It is shown that, at475℃, for200ppm of acetone, methanol and ethanol gases, the sensitivity of nanowires array could be8.26,3.58,4.09respectively, corresponding response-recovery time could be9s and5s,9s and10s,7s and11s.Both theoretical mechanism and experimental studies in the dissertation could be used to illustrate the key role of catalyst Pd played in improving the gas sensitivity of ZnO nanowires. And it even provides meaningful understanding for in-depth study of the gas sensitivity based on ZnO nanomaterials by other metallic catalyst. The obtained microfluidic technology could provide new strategy and method for constructing novel gas sensors.