Defect Engineering Of P-type Metal Oxides For Gas Sensing Application

p-Type metal oxide semiconductors(MOSs)have rich structure diversity,excellent surface properties and photocatalytic properties for volatile organic compounds(VOCs).However,the inherent low sensitivity of p-type MOS gas sensors limit their applications.The study of the defects closely related to charge exchange between gas and MOS interface is of great significance for the rational design of high performance MOS gas sensors.Herein,the main research results and innovations of present thesis are listed as follow:1.The VOCs response sensitivity of p-CuCrO2 gas sensor was enhanced by vacuum annealing.CuCrO2(CCO)nanoparticles with different particle sizes have been used for p-type MOSs sensitive materials.The results show that the single ionized oxygen vacancy(Vo· defect)plays an important role in enhancing the charge exchange between VOCs and CCO nanoparticles.To further prove this viewpoint,more Vo·defects were introduced by vacuum annealing.VOCs sensing test show that sensitivities positively correlate with the concentration of oxygen vacancies.Defects characterization and literatures show that the possible sensitization mechanism are:1)suppressing the hole concentration of the sensor channel;2)the unpaired electrons in Vo· providing active sites for chemisorption of oxygen and VOCs molecules.In addition,Vo· defect-rich CCO sensor exhibit good reproducibility and stability at medium operating temperature(<325?).It was found that the o· defects induced by defect engineering(vacuum annealing)can reasonably improve the gas sensing performance of p-type ternary oxide sensor.However,the vacuum heat-treatment will increase the defect concentration of the whole sensor,and the presence of oxygen vacancies in the inner defect will increase the baseline resistance.Thus the inner oxygen vacancy will limiting the further improvement of the gas sensing performance of the p-type sensor.2.Enhancement of sensitivity of p-CuAlO2 sensor by Ar&H2 plasma treatment method,which is used to tail the surface defect of p-CuAlO2 sensor.Compared with other defect engineering methods,plasma treatment can accurately produce surface Vo defects without affecting the bulk performance,which is essential for fully exploring the potential of charge exchange between gas and MOS gas sensors.Upon plasma treatment time of 30 minutes,the responses to acetone,ehanol and other VOCs were significantly enhanced.The defect characterizations indicate that the Vo defects on the sensor surface increase significantly after Ar&H2 plasma treatment.The gas sensitivities positively correlate with the concentration of Vo defects,which is consistent with the latest progress in Vo as active sites for VOCs adsorption and redox reactions.Because of Ar&H2 plasma treatment only produce oxygen vacancies on the ultimate surface of p-CuAIO2 sensor,oxygen vacancies are continuously compensated at high temperature and air atmosphere,resulting in poor stability of p-CuAlO2 sensor.3.In order to improve the long-time stability of p-type sensor,Sc3+ doped p-type NiO has been used to enhance the sensing performance of VOCs.When the optimum doping rate is 7.4 at.%,the response to 100 ppm acetone increases from 8.2(pure NiO)to 109.4,and the detection limit reaches 10 ppb.The defect characterizations indicate that VO defects have a significant effect on the VOCs response of p-type NiO sensor,rather than Ni3+.The heterogeneous dopant Sc can change the surface coordination properties,form a large number of stable Vo sites on the surface.However,their high working temperature made it difficult for practical applications,especially in the fields of pattern recognition and wearable devices.4. By adjusting the resistance of p-CuScO2 sensor,the effect of adsorbed water on ammonia response of p-CuScO2 sensor was studied.Room temperature MOS gas sensors provide broad prospects for environmental monitoring in the era of the Internet of Things(IoT).CuScO2,a delafossite with unique oxygen gap intercalation ability,was used to modulate its resistance by air annealing for nearly 8 orders of magnitude without changing its morphology and structure.The gas-sensing results show that CuScO2 exhibits n-type response to ammonia molecules in room temperature and humid air,while it exhibits normal p-type response to ammonia and other volatile organic compounds in dry air.The presence of water plays an important role in the response polarity of CuScO2 sensors.The unique sensing mechanism may be used to design high sensitivity room temperature sensors working in humid environment.The unique n-/p-type conversion contains abundant nolecular correlation features,and has important potential value in the field of pattern recognition.