Study On NH₃ And NO₂ Gas Sensors Based On Metal Oxide Nanomaterials

Ammonia（NH₃）and Nitric dioxide（NO₂）are two kinds of nitrogen-containing inorganic gaseous compounds emitted in the process of energy utilization.They have the characteristics of high chemical activity and high harmfulness,and they will pose a serious threat to human health and ecological environment when they enter the atmospheric environment,thus it is urgent to detect them accurately and quickly.Gas sensors based on metal oxide semiconductor have significant advantages in detection range,sensitivity,volume and cost,which are suitable for the detection of low concentration NH₃ and NO₂.However,the test environment is complex and changeable,and at the same time to meet the needs of accurate and rapid detection,the performance of gas sensors needs to be further improved.In this paper,based on the physical and chemical properties of NH₃ and NO₂ gases,the gas sensing mechanism of gas sensors and the key factors affecting their performances,WO₃ and In₂O₃ were selected as basic materials,respectively.Through morphology control,chemical modification of sensing materials and sensor device optimization,the accurate and rapid detection toward low concentration NH₃ and NO₂ was realized.The primary research work is summarized as follows:（1）Study on the enhancement of NH₃ and NO₂ gas sensing performances based on the morphology control of gas sensing materialsWO₃ has a wide band gap,high baseline resistance,and many acidic sites on the surface,which is suitable for the detection of reducing gas NH₃,while In₂O₃ has a high conductivity and low baseline resistance,which has obvious advantages in the detection of oxidizing gas NO₂.The effect of morphology on the NH₃ and NO₂gas sensing performances was firstly investigated.By controlling the hydrothermal synthesis conditions,WO₃ and In₂O₃ with nanoparticle,nanorod,nanosheet,and microsphere structures were prepared.The gas sensing test results are summarized as follows:the WO₃ nanosheets exhibited the optimum sensing performances toward NH₃,which achieved high response of 16.95 to 100 ppm NH₃ under the working temperature of300℃,and it also showed reliable repeatability and good selectivity.For NO₂,the In₂O₃flower-like microspheres exhibited the optimum gas sensing performances,which reached high response value of 695 to 5 ppm NO₂ under 100℃,and it also showed reliable repeatability and good selectivity.Their better gas sensing performaces were mainly attributed to the large specific surface area and chemisorbed oxygen species,which was benefical for the gas adsorption and diffusion of NH₃ and NO₂,and also promoted the chemical recations,thus finally improving the gas sensing performances.（2）Study on the enhancement of NH₃ and NO₂ gas sensing performances based on the chemical modification of gas sensing materialsThe noble metal Ru has excellent selective catalytic oxidation ability for NH₃,and the difference of work function between Ru and WO₃ can promote the migration of free electrons,increase the charge density at the interface,and then increase the proportion of surface chemisorbed oxygen species.Therefore,Ru loading on WO₃ surface can further improve the response and selectivity to NH₃.In this paper,Ru-modified WO₃nanosheets were successfully synthesized through a facile hydrothermal method followed by the impregnation process.3 wt%Ru-WO₃ nanosheets exhibited a high response of 40.3 to 100 ppm NH₃ under the optimal working temperature of 250℃,which was 2.4 times higher than that of pure WO₃nanosheets.Moreover,the sensor showed a good response even at the low concentration of 1 ppm,and also exhibited high selectivity and reliable repeatability toward NH₃.Herein,the excellent and enhanced NH₃ gas sensing performances of Ru-modified WO₃ nanosheets were mainly owing to the synergistic effects of electronic and chemical sensitization of Ru modification.When the low-valent alkali metal is doped into In₂O₃ as an acceptor impurity,in order to maintain the electrical neutrality,it is going to compensate the holes in the form of oxygen vacancies,thereby increasing the proportion of surface chemisorbed oxygen species.At the same time,it can improve the alkalinity of the material and facilitate the adsorption of NO₂ gas molecules.Therefore,it is expected to improve the response value and selectivity of In₂O₃ to NO₂.In this paper,Rb-doped In₂O₃ flower-like microspheres were synthesized via a facile one-step solvothermal method along with the subsequent thermal treatment.The sensors based on 1 mol%Rb-doped In₂O₃exhibited ultrahigh response of 1502 to 5 ppm NO₂ under the rather low working temperature of 75℃,which was 2.2 times higher than that of pure In₂O₃.Even at the low concentration of 0.1 ppm NO₂,a high response value of 10.2 was achieved and the theoretical detection limit was down to 3.5 ppb.Moreover,1 mol%Rb-doped In₂O₃exhibited high selectivity and reliable repeatability toward NO₂.The excellent and enhanced NO₂ gas sensing properties were mainly owing to the increased surface chemisorbed oxygen species induced by Rb doping.（3）Study on the enhancement of NH₃ and NO₂ gas sensing performances based on the optimization of sensor devicesThe MEMS fabrication process reduces the size of sensing film to the micrometer level,which can effectively shorten the gas diffusion path,thereby greatly reducing the response/recovery time.And it can also improve the receptor function,transducer function and utility of sensing materials to target gases,thus reducing the detection limit.At the same time,the power consumption is reduced to 6-10%of the traditional form.In this paper,based on the obtained NH₃ and NO₂ gas sensing materials,the gas sensing film was deposited on the micro-hotplate chip by the dispensing method,and the thickness was controlled by the number of dispensing times,and then packaged into MEMS gas sensors,respectively.On the premise of high response value and high selectivity to NH₃ and NO₂,the MEMS sensors improved the detection ability to low concentration gases,which exhibited a good response to NH₃ as low as 0.25 ppm and NO₂as low as 25 ppb.At the same time,the MEMS gas sensor shortens the response/recovery time to NH₃ and NO₂,and the response time to NH₃ and NO₂is reduced to 8 s and 110 s,respectively,thus achieving rapid detection of NH₃ and NO₂.In addition,the MEMS gas sensors showed excellent long-term stability,the sensor performances did not show significant attenuation after 90 days of testing,revealing that the MEMS gas sensors obtained in this work have a good practical application prospect.