Preparation And Gas/Humidity Sensing Properties Of Micro/nanostructure TiO₂ On Pure Ti Matrix

With the improvement of human living standards and the rapid development of artificial intelligence technology,gas sensors used in environmental air monitoring,medical diagnosis,military detection and other fields have attracted extensive attention.As the basis of building high-performance gas sensors,sensing materials have always been the focus of research in this field.TiO₂ is a traditional n-type metal oxide semiconductor material,which is widely applied in photocatalysis,solar cells,gas sensors and other fields.However,the problems of long preparation period,poor selectivity and insufficient stability have limited its application as a gas sensor.Surface modification,doping and morphology control are currently adopted to improve the gas sensitivity of TiO₂material.On this basis,the preparation of micro/nano-sized titanium dioxide by in-situ thermal oxidation was proposed,and the gas/humidity sensitivity of TiO₂ materials wasimproved via the surface modification of GO and metal oxides.In this thesis,on the basis of previous literature research and preliminary experimental exploration,research programs were determined.The main research works were carried out as follows:（1）Using pure Ti wafers as matrix,micro/nano-sized titanium dioxide thin films were grown by direct thermal oxidation method.The optimum thermal oxidation temperature was determined to be 1000℃,and the mechanism of thermal oxidation growth of titanium dioxide was analyzed.The titanium dioxide grown bythermal oxidationexhibitedexhibited the ultra-high response of aboutsix orders of magnitude,but it showed little response towardslow humidity level（RH≤30%）.Thus,surface modification with GO was carried out to improve the performance of titanium dioxide grown by thermal oxidation.Subsequently,the composition,crystal structure and morphology of the modified materialswere analyzed,and the effect of GO on the humidity sensitivity of TiO₂ was investigated.The mechanism of humidity sensing reaction was discussed.The results indicated that GO surface modification greatly improved the sensitivity of low humidity level,stability（2 months）,selectivity and the hysteresis characteristics,and accelerated the response recovery time（superior tothe commercial humidity sensor）.GO not only creates abundant carrier transport channels,but also provide a large number of active sites for water molecule adsorption reaction casued by various oxygen-containing functional groups.（2）After depositing metal film by pre-magnetron sputtering,the metal oxide surface-modified TiO₂ film was grown by in-situ thermal oxidationprocess.The composition,crystalstructure and micro-morphology were analyzed.The hydrogen sensing mechanism and response type transition mechanism were discussed.The results showed that the optimum operating temperature and voltage are 300℃and 0.1V,respectively.Under such conditions,the sensor exhibited n-type response,high sensitivity(R_max=24),good stability and reproducibility（1-120 days）and excellent selectivity.In addition,themeasurements showed that operating temperature,operating voltage,hydrogen concentration and surface modifying materials could result inthe inversion of the n/p type response behaviour,which was mainly due to the formation of the inversion layer caused by the higher intergranular barrier.The above research results laid a foundation for the application of titanium dioxide materials by thermal oxidation as gas/humidity sensors,and provided ideas and directions for developing new sensing materials,exploring new preparation techniques and improving sensing performance.