Study On The Assembly And Gas - Sensing Properties Of Nickel Oxide - Based Multi - Stage Structural Materials

Hydrogen sulfide（H₂S）, as an extremely toxic, corrosive and inflammable gas, is extremely dangerous to the human beings. The acceptable levels of H₂ S in the ambient environment are reported to be within the range < 83 ppb, therefore, early recognition and real time detection within this concentration range of H₂ S is necessary. NiO is one of the most important p-type transition metal oxide semiconductors. NiO nanoparticulate film sensors and thick film sensors with hierarchical structure have been reported to detect H₂S. However, the relatively high working temperature（> 150 ℃） and especially high detection limit（> 1 ppm） of these sensors still need to be further improved in order to satisfy the acceptable detection level of H₂S in indoor air quality monitoring. In order to achieve the detection of ppb-level H₂S at lower temperature,hierarchical NiO nanostructures constructed by simple building unites were designed and in-situ prepared here, the morphology and structure were characterized by the modern instrument analysis methods such as scanning electron microscopy（SEM）,transmission electron microscopy（TEM）, X-ray diffraction（XRD） and X-ray photoelectron spectra. The as-obtained hierarchical NiO arrays film gas sensors were used to measure the gas-sensing performance of H₂S and other gases. The main contents of the thesis are as follows:（1） The precursors of hierarchical NiO in-situ deposited on the ceramic tubes were prepared via hydrothermal method by using nickel sulfate, deionized water and ammonia as reactants without any templates and surfactants. And hierarchical NiO nanowall arrays films were obtained after subsequent calcination at 500 °C in air. The arrays are composed of interconnected smooth nanosheets and the thickness is about 14 nm. The nanosheets are further assembled with abundant sub-nanometer of nanoparticles with pore size in a range of 5.7 to 27.1 nm. The correspondingly wide pore size distribution centered at 5-20 nm, can be regarded as the stacking of the nanoparticles. The NiO nanowall arrays film sensor has good sensitivity and selectivity to H₂S, the response to 10 ppm H₂S is 12.9 at the optimum temperature of 92 ℃, the recovery time is 79 s and the lowest detection limit is 1 ppb.（2） The CdO/NiO nanofilms with wrinkle structure were synthesized by one-pothydrothermal method grown in-situ on ceramic tubes. The wrinkles are composed of porous thin nanosheets with slippy surfaces and the nanofilm is assembled with a large number of tens of nanometer sized particles, and porous nanostructures can also be clearly seen which are resulted from the stacking of the nanoparticles. Obviously the shape and size of the pores are irregular, however, the nanopores are evenly distributed throughout the sheets. The wrinkled structure CdO/NiO nanofilm with the mole ratio of2.01 at% has the best sensitivity and selectivity to H₂S. The response of the CdO/NiO nanofilm sensor to 10 ppm H₂S can reach 18.2, the recovery time is 82 s and the lowest detection limit can be decreased down to 0.5 ppb at 92 ℃.（3） Hierarchically structured, porous CuO/NiO nanowall arrays were grown in-situ on ceramic tubes by a facile hydrothermal reaction. After compositing with CuO, array structure has not changed, which are assembled by smooth nanosheets with a thickness of ⁵4 nm. The pore diameters are in a range of 1.2-6.7 nm. The CuO/NiO nanowall rrays with the mole ratio of 2.84 at% have the higher sensitivity and selectivity to H₂S than pure NiO arrays film sensor. But the optimum working temperature elevates from92 ℃ to 133 ℃, the response is 36.9 when the sensor is exposed to 5 ppm H₂S, the response time reduces from 79 s to 25 s and the lowest detection limit decreases from 1ppb to 0.5 ppb when compared to those of the pure NiO arrays film sensor.（4） The surface statuses of NiO, CdO/NiO and CuO/NiO nanomaterials before and after exposure to the H₂S gas were investigated by XPS. The probable oxidization products of NiO arrays film sensor were analyzed by GC-MS. The H₂S-sensing mechanism could be elucidated by a redox reaction between the H₂S molecules and the chemisorbed oxygen ions and the oxidation product of H₂S is SO₂ in this detection process.