Optimizing The Sensing Performance Of Nano-sized Metal Oxide Semiconductor By Tunning The Absorbed Oxygen

Metal oxide semiconductor（MOS）materials are attractive in gas sensors,batteries and integrated circuits due to their unique electronic structure and physical and chemical stability.The sensors based on oxide semiconductor materials have been explored for the detection of VOCs due to their advantages of small size,low cost,simple operation,easy preparation,good sensing activity,and real-time monitoring,which play important role in air monitor,food safety,industrial/agricultural production,fire safety,electronic nose,and medical diagnostics.Usually,the response of a sensor mainly depends on the specific chemical reaction between the chemisorbed oxygen and the target gas molecule absorbed on the surface of the semiconductor nanomaterials.The chemisorbed oxygen of semiconductors can directly affect and fundamentally determines the activation energy of a sensing reaction.Thus,the regulation and optimization of the chemisorbed oxygen of oxide semiconductors is the key to improve the gas sensing properties of a metal oxide semiconductor.The sensing properties of the sensor based on single metal oxide semiconductors are often suboptimal,especially in response and selectivity,which are mainly affected by the chemisorbed oxygen of a metal oxide semiconductor.The low sensitivity is mainly due to the insufficient amount of chemisorbed oxygen species on the surface of a metal oxide semiconductor,and the poor selectivity is because the oxidizing ability of the chemisorbed oxygen is too strong,which can not only oxidize the target gas,but also the other reducing interfering gases.In this thesis,we use heterovalent heteroatom doping and solid solution strategies to control the composition of the sensing material and optimize the chemisorbed oxygen of the metal oxide semiconductor,thus increasing their sensing activity.This thesis mainly includes the following three parts:1.We successfully prepared several kinds of alkaline earth metal（Ca,Sr and Ba）doped In₂O₃ porous nanotubes via a low-valent cation doping strategy.The introduction of low-valence alkaline earth metals can create oxygen vacancies and provide more active sites for sensing reaction,increase the amount of surface basic sites,and enrich the chemisorbed oxygen,thereby optimizing the sensing performances.Among the sensing materials,the 5%Ca-In₂O₃ is the best one,which formaldehyde sensing response is about 3.5 and 9 times higher than the response to ethanol and acetone at 100ppm.In addition,the sensing response of 5%Ca-In₂O₃ based sensor（S=116 vs 100ppm）is 10 times higher than that of the pristine In₂O₃ based sensor.Furthermore,the5%Ca-In₂O₃ sensor also gives ultra-short response time（≈1 s）and a low detection limit of 60 ppb at 130°C.2.To further understand the influence of low-valence heteroatom doping on chemisorbed oxygen,and obtain material with excellent sensing performance,Cd Ga₂O₄with low work function was chosen as the matrix material,which is good for formaldehyde sensing in nature.And a series of alkali metal（K,Na）doped Cd Ga₂O₄nanofibers were prepared successfully,which formaldehyde sensing performances were investigated systematically.Compared with those classical semiconductors,including In₂O₃,Zn O and Sn O₂,the Fermi level of Cd Ga₂O₄ is a little high,combined with suitable band gap and carrier type,which is good for formaldehyde sensing.Much more oxygen vacancies would be formed due to the valence difference between Cd and alkali metals when Cd²⁺was heavily replaced by alkali ions.These vacancies will provide more abundant active sites for both chemisorbed oxygen and sensing reactions,tuning the chemisorbed oxygen,thus achieving the enhanced formaldehyde sensing performance.As a result,the 7.5%K doped Cd Ga₂O₄ based sensor shows much greater response to formaldehyde than that of pure Cd Ga₂O₄.The response of 7.5%K-Cd Ga₂O₄based sensor to 10 ppm formaldehyde is as high as 90,which is about 4 times higher than that of pure Cd Ga₂O₄ based sensor at 120°C.Furthermore,the 7.5%K-Cd Ga₂O₄sensor also shows ultra-short response/recovery time（1 s and 62 s）,excellent selectivity,and low detection limit（20 ppb）.It is one of the materials with excellent formaldehyde sensitivity.3.In order to further understand the effect of chemisorbed oxygen,a series of gallium indium(Ga_xIn_2-xO₃)bimetallic oxide solid solution nanofibers with tunable composition were prepared by electrospinning method.The as obtained products exhibit porous characteristics with ultrathin pore wall.After investigated its sensing performance systematically,we found that this material is highly sensitive to hydrogen and does not give cross-response with carbon monoxide and methane.As a result,the Ga_1.2In_0.8O₃ based sensor gives the best sensing performance for hydrogen.The Ga_1.2In_0.8O₃ based sensor shows the highest response（R_a/R_g≈15）to 500 ppm hydrogen gas at 280°C,which is about 6 times as that of pristine In₂O₃.Furthermore,the sensor based on Ga_1.2In_0.8O₃ showed ultra-short response and recovery time（≈1 s and 3 s）,good reproducibility,remarkable stability,low detection limit（2 ppm）and excellent selectivity to hydrogen.The enhanced hydrogen gas sensing performance of this material can be attributed to its novel atomically thin wall,the abundant chemisorbed oxygen with optimal activity and the adsorption and activation of hydrogen.