Structural And Defect Control Of Nanooxides And Their Gas Sensing Properties

With the discharge of pollutants in production and living,many toxic volatile gases are produced.In order to detect these harmful gases,gas sensors came into being.The core of gas sensors is sensing materials.ZnO,WO₃ and Co₃O₄ oxide semiconductors are traditional gas sensing materials.Although researchers have investigated the sensing performance of semiconductor gas sensors made by ZnO,WO₃ and Co₃O₄ oxide nanomaterials and achieved great improvement,there are still many deficiencies,which are difficult to meet the practical application requirements of high performance and low power consumption.Thus,there is an urgent need to prepare low-cost sensors to achieve real-time online output to meet the needs of production and life,medical and health,air quality,food safety and other fields.In this paper,the gas sensing properties of ZnO,WO₃ and Co₃O₄ were investigated by means of defect regulation,heterojunction and annealing,and the corresponding gas sensing enhancement mechanism was further explained,obtaining the following conclusions:1.Integration of two oxides to form heterojunction nanostructures reveals drastic effects on chemiresistive gas sensing performance,however transition metal vanadates(TMVs)decoration onto metal oxide backbone material(e.g.ZnO)has not been reported yet.In this contribution,ZnO nanosheets assembled spheres were synthesized by a hydrothermal method and then decorated with different amount of Zn₃(VO₄)₂ via a facile impregnation–calcination process.Material characterization indicates that mesoporous ZnO nanosheets assembled spheres with abundant defects are successfully decorated by different content of Zn₃(VO₄)₂ nanoparticles.Taking acetone(C₃H₆O)as a probe molecule,gas sensing properties of both pristine ZnO(P-ZnO)and Zn₃(VO₄)₂decorated ZnO(V₂-ZnO)were systematically investigated.Compared with P-ZnO,V₂-ZnO exhibits improved acetone response at the optimum operation temperature of350°C with an optimal Zn₃(VO₄)₂ capacity of 1.0%in molar ratio(named as 1.0%V₂-ZnO).The sensor response of 1.0%V₂-ZnO spheres is as high as 303.6 towards 200ppm acetone that is nearly 10 times higher than that of P-ZnO.Simultaneously,1.0%V₂-ZnO spheres show much shorter response/recovery time than that of P-ZnO.The excellent acetone sensing performance of V₂-ZnO spheres is mainly attributed to synergistic effects of heterojunction interface,abundant defects,small thickness,and mesoporous construction induced electronic and chemical sensitization.2.The 2D/2D BWO/WO heterojunction nanosheets of bismuth tungstate Bi₂WO₆(BWO)modified tungsten oxide WO₃(WO)were successfully prepared by a simple hydrothermal-calcination method.The crystal structure and morphology of nanosheets were analyzed by various characterization methods.The characterization results displayed that BWO/WO-3 nanosheets with small size and thin thickness.Furthermore,the sensing properties of three sensor based on BWO,WO and BWO/WO-3 nanosheets were investigated systematically.At the optimal operating temperature(260?),BWO/WO-3 heterojunction sensor reveals excellent DMA response(R_a/R_g=60),which was 7 and 3 times higher than that of BWO and WO,respectively.At the same time,BWO/WO-3 sensor indicates fast response time(<3.6 s)and recovery time(18s)toward 200 ppm DMA.moreover,BWO/WO-3 sensor also possesses the advantages of long-term stability,good selectivity and strong anti-interference ability.After excluding the effects of specific surface area and oxygen vacancy on gas sensing performance,the improvement of sensing performance of BWO/WO-3 nanosheets is mainly due to the appropriate defect sites of N-N heterojunction between BWO and WO,as well as accelerated electron/hole pair recombination.These experimental results demonstrate that nanosheets structured BWO/WO-3 heterojunction has considerable potential for development of DMA gas sensor.3.Polycrystalline Co₃O₄ nanoparticles were synthesized using a hydrothermal method with different calcination times.The microstructure and surface defects of these materials were investigated systematically.Room temperature gas sensing properties of Co₃O₄ nanoparticles were tested towards ammonia(NH₃).The response value of Co₃O₄-2h gas sensor to 200 ppm NH₃ is 102.8 with response and recovery time of 65 s and 208 s,respectively.The Co₃O₄-2h sensor also exhibited high selectivity,good repeatability and long-term stability.After excluding the impact of specific surface area and grain-size effect on sensitivity,the boosted sensing performance of Co₃O₄-2h nanoparticles is mainly attributed to the high Co³⁺concentration and the abundant doubly ionized oxygen vacancies(V_O¨).This work provides a promising strategy to enhance gas sensing properties of P-type oxides by adjusting charge states of oxygen vacancy.