Preparation And Gas-sensitive Characterization Of ZnO Nanowire Arrays And Their Derivatives

Zinc oxide(ZnO)is a typical n-type semiconductor and an ideal gas sensing material.Its band gap is(3.37 eV),exciton binding energy 60 meV,and has the advantages of photoelectric response,high electron mobility,good chemical stability and thermal stability,etc.The preparation of ZnO materials and the study of gas-sensitive properties are still research hot spots.In this thesis,the main contents are the preparation of ZnO nanowire arrays and their derivatives and the study of gas-sensitive properties.In order to enhance the gas-sensitive properties of ZnO nanowire arrays,ZnO nanowire arrays(ZnO NRs),nickel-doped ZnO nanowire arrays(Ni-ZnO NRs)and ZnO/TiO2 composite nanowire arrays(ZnO/TiO2 NRs)were grown in situ on FTO-etched electrodes by induced growth process.The morphology,structure and elemental composition of the materials were characterized and analyzed by scanning electron microscopy(SEM),transmission electron microscopy(TEM),X-ray diffraction(XRD),X-ray photoelectron spectroscopy(XPS),and X-ray energy spectroscopy(EDS).Based on it,three gas-sensitive devices were designed and fabricated,and their gas-sensitive properties to toxic and hazardous gases were studied in detail,and their sensitivity mechanisms were analyzed and investigated as follows:(1)Preparation,characterization and gas-sensitive performance of ZnO NRsThe seed layer of ZnO was grown on the FTO etched electrode by drop coating technique.Then the ZnO NRs were induced by hydrothermal method.It is found that the material appeared in the form of nanowire arrays,and the nanowire arrays were uniformly distributed on the surface of the FTO electrode,which exhibited good crystallinity and belonged to the hexagonal structure.The ZnO NRs were then prepared as gas sensors and their gas-sensing performance was further investigate.It revealed that the device had good selectivity and response to 5 ppm of H2S at an operating temperature of 250℃ with a response of 10.4,respectively,2 times higher than that of other toxic and hazardous gases in general.The response-recovery characteristics of the sensor for H2S were then explored and the response-recovery times were 117 and 98 s,respectively.The minimum detection limit of the device for H2S is 53.9 ppb,which was calculated by means of the signal-to-noise ratio.(2)Synthesis,characterization and gas-sensitive performance of Ni-ZnO NRsThe doping of ZnO NRs was carried out by controlling the Ni content in the precursor via a bimetallic source organic precursor solution,and Ni-ZnO NRs with different Ni were obtained.The response of 8 at.%Ni-ZnO NRs to 5ppm H2S was found to be 68.6,being more than 6 times higher than that of other toxic and hazardous gases.It revealed that Ni-ZnO NRs have a unique gas-sensing response and selectivity to H2S.By studying the gas-sensitive performance of different Ni-ZnO NRs to H2S,it exhibits that 8 at.%Ni-ZnO NRs possess the best gas-sensing performance with a response of 68.9 for H2S at the optimum operating temperature of 250℃,which is 6.26 times higher than that of ZnO NRs.The response-recovery time of the sensor for H2S was also further measured,indicating that it had a response time of 75 s and a recovery time of 57 s.The minimum detection limit of the device for H2S gas was calculated by means of the signal-to-noise ratio,which was 3.14 ppb.In the repeatability cycle measurement for H2S,the gas sensor exhibited good stability.Compared with ZnO NRs,the Ni doping effectively enhances the gas sensitivity of the gas sensor.The reason is that Ni doping reduces the activation energy of the material reacting with H2S gas,thus enhancing the performance of the sensor.(3)Fabrication,characterization and gas-sensitive performance of ZnO/TiO2 NRs compositesThe SEM characterization results show that the ZnO/TiO2 NRs sensors have fold-like TiO2 attached to the surface of the ZnO nanowire arrays,and the gas response of the ZnO/TiO2 NRs sensors to different VOCs was investigated.It showed that the sensor has higher response to alcohols and then their gas-sensing property of the ZnO/TiO2 NRs sensor was also investigated for ethanol at different operating temperatures.It is obseved that the response to 100 ppm ethanol at an operating temperature of 300℃ was 281,which was 133 times higher than that of the ZnO NRs sensor.The response recovery time for ethanol was 55 s and the recovery time was 35 s,which was about 0.7 and 0.6 times that of the ZnO NRs and Ni-ZnO NRs sensors,thus suggesting that the gas sensor can adsorb and desorb ethanol gas more rapidly.Finally,the minimum detection limit was calculated by fitting the concentration-response curves for ethanol concentrations of 10,25,50,75,and 100 ppm,and the value was less than 39.7 ppb.The good stability was demonstrated in the subsequent cycling stability test.The electrons in the material can be transferred from TiO2 to ZnO and form a charge region at the interface in that TiO2 has a higher valence band than ZnO.This structure has a certain influence on the improvement of gas-sensitive performance of the material sensor.In addition,the surface structure makes the surface rough with a large number of folds,and then it greatly enhances the specific surface area of the material,which is more favorable for the adsorption of the target gas.It can be concluded that the composite material has great potential for application in the field of low concentration ethanol detection.In summary,two different approaches were tried to enhance the performance of the thin-film ZnO nanowire arrays.The 8%Ni-ZnO NRs have an excellent response to H2S.However,the ZnO/TiO2 NRs possess the best gas-sensing performance with a response for ethanol.The work provides a good reference for the performance enhancement of ZnO-based gas sensors.