Preparation And Acetone Sensing Properties Of Electrospun Tin Oxide-Based Nanofibers

In order to deal with the increasingly serious air pollution,a highly efficient sensor with an excellent sensitivity to the VOC air pollutant?acetone?is of great significance.SnO₂-based sensing materials play an important role in the field of gas sensors,and have shown great potential in the fields of environmental pollutant gas detection and flammable/explosive gas alarm.Compared with traditional zero-dimensional and two-dimensional nanomaterials,one-dimensional nanomaterials have great advantages in the identification of signals and electron transport due to their unique structures and characteristics,and have been paid special attention in the field of nanosensors.Electrospinning is a relatively cheap and simple method to prepare one-dimensional nanomaterials.The composition and structure of one-dimensional nanofibers obtained by this method can be easily designed and accurately controlled.The large specific surface area of nanofibers provides more reactive sites on the surface of materials for gas molecules,which is beneficial to improve the gas response.The super length and large length-to-diameter ratio of nanofibers is beneficial to the rapid transmission of electrical signals and obtaining rapid response-recovery characteristics.In this paper,a series of SnO₂-based nanofibers were prepared by electrospinning.To improve the gas sensing characteristics,doping,constructing heterojunctions and noble metal loading were adopted to regulate the compositions and structures of nanofibers.The relationship between structures and gas sensing properties is built by studying the acetone sensing performance of these SnO₂-based nanofibers deeply,which will lay the foundation for expanding the application in the field of environmental monitoring and protection.The details are presented as following three parts:1.Metal ion doping is one of the most widely used methods to improve the gas sensing performance of metal oxide semiconductors.Based on the unique structure and controllable compositions of nanofibers,a series of alkaline earth-doped SnO₂ nanofibers have been prepared by the electrospinning technology combined with high temperature post-treatment with PVP as templates.The mechanism of alkaline-earth metal ion doping was discussed in detail.Firstly,we chose SnO₂ nanofibers as intrinsic materials.Sr²⁺ was selected for interstitial doping to control the grain size of SnO₂ nanofibers by inhibiting the growth of SnO₂ crystals,aiming to further improve the gas sensing performance.When the doping level of Sr²⁺ is 1 at%,the grain size of SnO₂ nanofibers is the smallest.Correspondingly,the acetone sensing performance is the best.The response value?²5?to 100 ppm acetone is 2 times that of pure SnO₂ nanofibers,response and recovery times are about 6 s and 6s.The selectivity and stability are also excellent.In order to further verify the effect of alkaline-earth metal ion doping on the gas sensing performance of SnO₂ nanofibers,we also selected another alkaline-earth metal ion(Ca²⁺)for trace doping.When the doping level of Ca²⁺ is 0.8 at%,the acetone sensing performance of nanofibers is the best.The response value?²7?to 100 ppm acetone is 2.3 times that of pure SnO₂ nanofibers.The response time is about 6 s,and the recovery time is about 3 s.The selectivity and stability are excellent,too.Based on these improved sensing properties,it is found that in addition to the effect of alkaline earth inhibitors on grain growth,the lattice defects and oxygen vacancies caused by the substitution of Ca²⁺ in Sn⁴⁺ sites also plays an important role in the enhancement of gas sensing performance.2.Constructing heterojunctions in gas sensing materials is also an important way to improve the gas sensing properties of semiconductors.Based on the controllable structures and compositions of nanofibers,different nano-heterojunctions have been incorporated into SnO₂ nanofibers through the electrospinning technology,and the effect of different component ratios on the acetone sensing performance was also studied.The mechanism of the gas sensing enhancement caused by heterojunctions was discussed in detail.Firstly,p-Cr₂O₃/n-SnO₂ composite nanofibers with homogeneous distributions were prepared by using the electrospinning technology combined with high temperature post-treatment with PVP as templates,and the acetone sensing properties were investigated in detail.The results show that the introduction of Cr₂O₃/SnO₂ p-n heterojunctions greatly improves the acetone sensing properties of SnO₂ nanofibers.Choosing 100 ppm acetone as the target gas,when the content of Cr₂O₃ in composite fibers is 3 at%,the response value?⁴6,280 oC?of Cr₂O₃/SnO₂ composite nanofibers is the largest.The response time is only 4 s,and the recovery time is about 5 s.The saturated concentration of acetone?20,000 ppm?is much higher than that of pure SnO₂ nanofibers?5,000 ppm?,and the moisture resistance,selectivity and stability are excellent.In order to study the effect of homo-heterojunctions on gas sensing properties,n-CdO/n-SnO₂ composite nanofibers were prepared by electrospinning,and their acetone sensing properties were investigated in detail.The results show that CdO/SnO₂ heterojunction composite nanofibers exhibit significantly improved acetone sensing performance compared to pure SnO₂ nanofibers.The best performance of CdO/SnO₂ composite nanofibers occurs when the CdO content is 5 at%,and the response to 100 ppm acetone?⁵0,280 oC?is 4.2 times that of pure SnO₂ nanofibers?¹2,300 oC?.The response time is about 6 s,the recovery time is only 5 s.The saturated concentration of acetone?20,000 ppm?is much higher than that of pure SnO₂ nanofibers?5,000 ppm?,and the moisture resistance,selectivity and stability are excellent.3.Loading intrinsic materials with noble metals has been proven to be an effective way to improve the gas response and lower the operating temperature.However,the preparation routes of most works belong to multi-step post-treatment methods,namely,preparing intrinsic materials firstly,and then modify the surface with noble metals through dipping,deposition and sputtering,et al.These processes are more cumbersome and cause the low utilization of materials.Since the electrospinning precursors are homogeneously mixed in solution phase,the components of one-dimensional nanomaterials prepared by this one-step direct method are uniformly distributed,which is advantageous for improving the sensing performance and reducing the cost of noble metals.Based on the electrospinning technology,SnO₂-based composite nanofibers loaded with Au were prepared with PVP as templates.The operating temperature to acetone was reduced by the introduction of Au,and the response characteristics were further improved.Firstly,we introduced Au into SnO₂ nanofibers,resulting in a significantly reduced operating temperature and increased response to acetone for Au/SnO₂ composite nanofibers.The response of Au/SnO₂ composite nanofibers to 100 ppm acetone reaches 57 at the optimum working temperature of 220 oC.The response time is 8.5 s,and the recovery time is 7 s.The saturated concentration to acetone reaches 30,000 ppm,and the response signal keeps stable even after 60 days.In addition,we also discussed the role of Au/SnO₂ schottky junctions and noble metal catalysts in the sensing enhancement.In order to improve the acetone selectivity of Au/SnO₂ composite nanofibers,we introduced a third-party non-intrinsic rare earth oxide Eu₂O₃,aiming to utilize the synergistic effect of Eu₂O₃ and Au to improve the selectivity.The response value of as-prepared Eu₂O₃/Au/SnO₂ composite nanofibers to 100 ppm acetone reaches 65 at the optimum temperature of 200 oC.The response time is 11.7 s,and the recovery time is 6.5 s.The lowest detection limit to acetone reaches ppb levels,and the saturated concentration is 30,000 ppm.A wider detecting concentration range for acetone is obtained,and the repeatability is excellent.