| At present, the air pollution bring out many disasters to the people. NO2 is a common air pollutants released from industrial complexes and automobiles, and can cause damage to the health of the people. Besides, the detection of NO2 concentration in exhaled gases has become standard medical practice for monitoring chronic obstructive pulmonary disease. Therefore, there is an urgent need for the development of highly sensitive, lower power consuming and portable NO2 detectors with the low detection limit and fast recovery rate. Metal oxide semiconductors have been considered as a promising gas-sensing materials due to their high stability, low production cost and the simplicity of their use. NiO, as a typical p-type semiconductor, has attracted great attention owing to its good stability and excellent electrical properties. Especially, because of the high adsorption energy of NO2 on a NiO surface and the related drastic change of electronic structure, NiO shows a great potential for NO2 gas sensing. However, high selectivity, fast recovery rate and high sensitivity are difficult to achieve simultaneously at room temperature in the case of bare NiO-based gas sensors. Therefore, in this work, in order to further improve the room-temperature sensing performances of NiO-based sensors, the modulation of defect and interface structure have been carried out, and the relationship between the sensing performances and defect structure (grain boundaries, Ni vacancies) or the heterointerface have been established.The single-crystalline (SC) NiO hexagonal nanosheet and the nanoparticle self-assembled polycrystalline (PC) NiO nanosheet were prepared by the solvethermal methods and a comparative study on their room temperature gas-sensing performances was further investigated. Compared to the PC NiO, the SC NiO exhibited enhanced sensitivity to NO2 at room temperature. And the response of SC NiO to 30 ppm NO2 is 1.13, which is two times than that of the PC NiO. On the basis of the grain boundary scattering theory, the surface-adsorbed NO2 molecules introduce the surface states that can remarkably enhance the free carrier scattering at the extra grain boundary of PC NiO, leading to the high scattering potential. After being exposed to NO2, the carrier mobility of PC NiO decreases. However, due to the absence of grain boundary in the SC NiO nanosheets, the chemisorption of NO2 gives less influence on the carrier mobility so that the SC NiO performs the largely enhanced sensitivity.Then, based on the mesoporous single-crystalline NiO nanosheets, the NiO with different amounts of Ni vacancies were prepared by using different annealing temperature (400-600?). The gas-sensing studies revealed that the room-temperature sensing properties of NiO increased with the concentration of Ni vacancies. The room-temperature response of NiO reaches maximam at the annealing temperature of 500?, and the response decrease a little with further increasing the annealing temperature. On the basis of in-situ DRIFTS, the Ni vacancies as the main active sites participate in the gas-solid reaction and then determine the room-temperature sensing properties. Due to the equivalent defect ionization rate at room temperature, the variation of conductivity aroused by the adsorption of NO2 molecules on the doubly negatively charged Ni vacancies increases with increasing the amount of the Ni vacancies, leading to enhanced sensitivity of NiO.We constructed the self-assembled heterojunction nanocomposites through annealing the mixtures with different molar ratios of two kinds of precursors. When the molar ratio of SnO2 to NiO is 1:1, the room-temperature NO2 response reach maximam. And the response of NiSn11 to 30 ppm NO2 is 6.41, which is five times larger than the bare NiO (1.17). The existence of interface bonds and the formation of heteroj unction was also confirmed by the XPS and the HRTEM results. The gas sensing studies revealed that the response values incresed with increasing the amount of interface bonds. Moreover, it was found that the change in the contact potential aroused by NO2 adsorption plays an important role in modulating the mobility of holes in the heteroj unction nanocomposites.Afterwards, owing to the high electron mobility of rGO, rGO-NiO were prepared by the solvethermal method to improve the recovery rate of NiO-based sensors. And a careful comparison study between the rGO-NiO nanocomposite and pristine NiO for room temperature NO2 sensing had been done. It was found that the nanocomposites exhibited enhanced sensitivity in the high concentration range, and the recovery rate was also improved. According to the fitting results of the recovery curves of NiO and rGO-NiO nanocomposite, the recovery time of rGO-NiO (1330 s) is shorter than that of NiO (3300 s). The enhanced recovery rate was attributed to the high electron mobility of rGO and the effective electron transfer between the rGO and the NiO.At last, in order to satisfy the requirement for practical applications, ternary nanocomposites were successfully prepared by incorporating the n-type semiconductor into the NiO-rGO nanocomposites. The preparation of ternary nanocomposites was designed to improve the response of the sensors and ensure the fast recovery rate. Compared with the bare NiO and the NiO-rGO nanocomposites, the as-prepared ternary nanocomposites presented a largely enhanced response and a fast recovery rate towards NO2 at room temperature, and the response of ternary nanocomposites to 30 ppm NO2 is 21.9, and the recovery time obtained by the fitting results is 1251 s, implying that the ternary nanocomposites could be used for room-temperature gas sensing application.In our work, the room-temperature NO2 sensing properties of NiO-based nanocomposites were largely enhanced due to the effective manupulation of the defect and heterointerface structure. Moreover, the relationship between the microstructure and the sensing performances have been established, and the room-temperature sensing mechanism was also detailed, which could help for the further enhancement of the sensing properties of p-type metal-oxide semiconductors and exploring more room-temperature sensing materials with better sensing performances. |
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