| At present, air pollution is a serious threat to our production and living space. To alleviate the air pollution, the primary task is to find a way to rapidly and truly detect the toxic and harmful gases. Metal oxide semiconductor (MOS) gas sensor has attracted dramatic attention due to its high sensitivity, low cost, simplicity for production and suitability to portable instrument. However, poor selectivity is a fatal weakness of the MOS gas sensor. To address such problem, various attempts are made for the selectivity enhancement of such gas sensor. Nevertheless, all the present efforts are focused on an engineering level and unable to provide a fundamental and instructive principle to enhance the sensor selectivity. In this way, it is an urgent demand to study the selectivity of the MOS gas sensor in a methodology perspective in terms of the specificities of gas sensing.A workman must sharpen his tools if he is to do his work well. Thus, On the basis of the combinatorial materials science, a high throughput screening platform of gas-sensing materials (HTSP-GM), which can be used for the sensing performance evaluation and sensing mechanism investigation, was firstly designed and fabricated. This platform can be used for the gas sensing investigation in different gas surroundings through a time-dependent (R-t) and temperature-dependent (R-T) way. To identify the validity of this platform, we firstly study the sensing mechanism of La2O3-decorated SnO2nanocrystalline porous film toward formaldehyde according to the R-t test. Then, a new method for rapidly searching the optimal operating temperature (OOT) of MOS gas sensor was built based on the R-T test.Subsequently, in order to know the gas sensing properties of the oxygen vacancy, including the singly ionized oxygen vacancy (SIOV) and doubly ionized oxygen vacancy (DIOV), in different atmospheres (oxygen rich, oxygen free and reducing atmosphere), a temperature-dependent and atmosphere-dependent electron conduction model of SnO2was outlined according to the defect chemistry and the small polaron hopping (SPH) conduction model. In the same time, through the curve fitting between the electron conduction model and the temperature-dependent electrical conductivity (TEC) testing results of SnO2nanocrystalline film. It was found that the experimental results in the ionization stage of SnO2were in a good accordance with the theoretical deductions. Moreover, several characteristic parameters related to the defects nature of oxygen vacancy were obtained according to the curve fitting.Then, for the sake of giving a clear characterization of the metal cation defect in the MOS, an improved electron conduction model of p-and n-doped SnO2was derived according to the defect chemistry and the SPH conduction model. In the same time, TEC measurements by the HTSP-GM were carried out, for the first time, to perform the defect structure studies of the p-type (Li+, Cd2+, Al3+), isovalent (Ti4+) and n-type (Nb5+, W6+) doped SnO2nanocrystalline films in the oxygen free atmosphere. Meanwhile, by combining the improved electron conduction model and the experimental results, we gave a systematic depiction of the metal cation defect from formation, ionization to their influence on electrical conduction.Finally, in terms of the gas sensing mechanism of the oxygen vacancy and metal cation defect, we intensively studied the gas sensing of the defect reaction of the Cd2+doped SnO2and an all-defect gas sensing mechanism was proposed, and following a gas species identification method was established. Moreover, we have applied this method to the gas species identification of WO3gas sensor, which was aimed at exploring its practicability. According to the all-defect gas sensing mechanism, the feature parameters TM of different point defects were extracted from the TEC rate patterns and plotted them as the feature reaction spectrum for the gas species. In the feature reaction spectrum, it contains the TM and d[ln(σT3/2)]/dT that corresponding to the adsorption, desorption and reaction information between the point defect in the WO3and the gases, which could be used for the gas species identification.
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