Research On Gas Sensors Based On NiO Nanomaterials

Recently, the researches related to gas sensors have attracted constantly increasing interests because of the urgent need in the fields of atmospheric and micro environmental air monitoring, industrial production/automobile emission control, coal mine/household/food security, medical diagnosis, military, etc. During various sensors based on different mechanisms, the resistive gas sensors based on metal oxide semiconductors have occupied the significant positions due to their advantages of low-cost, high sensitivity, and simplicity of operation. Meanwhile, they are also the most concerned topics in previous International Meetings on Chemical Sensors（IMCS）. It is well known, excellent sensing material is the foundation for fabricating high-performance gas sensor. The gas sensing performance is closely related to the morphology and component of sensing material. Thus, continually exploring and innovating new functional gas sensing materials are playing important scientific significance for promoting sensing properties of gas sensor. Nickel oxide（Ni O） is an important p-type metal oxide semiconductor with excellent chemical and electric properties. As gas sensing material, its sensitivity is relatively lower compared to n-type metal oxide semiconductors, such as Sn O2, In2O3, Zn O, and WO3. However, Ni O possesses a prominent catalytic activity for volatile organic compounds（VOCs） oxidation. From this point of view, Ni O should be a potential material terrace for fabricating the high-performance VOCs gas sensors. In this thesis, considering the advantages of Ni O, we adopted in-situ doping and surface modification methods to adjust the surface/interface based on designing and synthesizing several Ni O nanostructures with novel morphologies, aiming to realize the efficient recognition of VOCs gases.Combining the assets of hierarchical structure and functional modification of doping, various well-defined Ni O hierarchical nanostructures were firstly constructed via the hydrothermal method. Then, we adopted aliovalent cation in-situ doping method to obtain hierarchical W-doped Ni O flower-like microspheres and Al-doped Ni O nanorod-flowers in order to improve the gas sensing properties of Ni O nanomaterials. The effect of the doping amounts of W and Al in the Ni O on the gas sensing performance was examined to confirm the best doping amounts. The results indicated that the sensor based on 4.0 at% W-doped Ni O sample showed the best gas sensing properties with a ppb-level detection limit（100 ppb）, an ultra-high response and a good selectivity to acetone compared to pure Ni O. The sensor based on 2.15 at% Al-doped Ni O nanorod-flowers showed the greatly enhanced gas sensing performance to ethanol. The improvement of sensing properties can be mainly caused by the decrease of carrier（holes） concentration and the increases of deficient oxygen component and chemisorbed oxygen component.According to the mechanism of interaction between target gases and sensing materials, we chosed α-Fe₂O₃ to fabricate the α-Fe₂O₃/Ni O heterostructures for the enhancement of sensing properties by unique interfacial effect and special performance differs from monocomponent. The two-steps solution method was adopted to synthesize α-Fe₂O₃/Ni O heterostructures with various morphologies. First, well-defined hierarchical Ni O flower-like microspheres and Ni O nanotubes were prepared via the hydrothermal route. Then α-Fe₂O₃ nanorods were assembled on the surfaces of Ni O substrate materials. The testing results showed that, the as-synthesized hierarchical α-Fe₂O₃/Ni O flower-like microspheres exhibited enhanced gas sensing performance to toluene in contrast to pure Ni O. The gas response of α-Fe₂O₃/Ni O flower-like microspheres to 100 ppm toluene was about 18.7, which was 13 times higher than that of pure Ni O at 300 °C. The sensor based on α-Fe₂O₃/Ni O nanotubes possessed better sensing characteristics to toluene than that of α-Fe₂O₃/Ni O flower-like microspheres. The sensor had ppb-level detection limit and excellent selectivity toward toluene, giving a response of 8.8 to 5 ppm target gas, which was about 7.8 times higher than that of pure Ni O nanotubes at 275 °C. The bend of energy bands located in the heterojunction caused by the difference of work functions benefits the transfer of electrons and strengthens space charge region, thus enhancing the gas response of composites to target gas.