| Volatile organic compounds(VOCs)are important precursors for the formation of ozone,which are causing slow but irreversible damage to human health,agricultural production,and ecosystems,and more.VOCs,as the primary factor leading to excessive air quality in urban areas of China,has been officially included in the "14th Five-Year Plan" governance focus.The situation of prevention and control is extremely severe.At present,the VOCs detection method based on spectral analysis has high sensitivity and can distinguish different types of VOCs,but the equipment is sophisticated and expensive,and the maintenance cost is high,making it difficult to achieve large-area gridded online monitoring.The side-heating gas sensor based on semiconductor oxide has fast response and is easy to integrate,but it has high energy consumption and poor selectivity,and is easily interfered by conventional flammable/toxic gases in life.For semiconductor gas sensors based on nanomaterials,the academic circle usually improves their sensitivity to one VOC gas through technical means such as doping/recombination,and cannot achieve air quality assessment through efficient detection of multiple types of VOCs.In response to the above problems,this paper carried out the design and construction of low-power MEMS micro-heaters and the fusion preparation of semiconductor micro/nano composite sensitive films,systematically evaluated its sensitivity to a variety of VOCs,and achieved a series of innovative achievements.Details as follows:(1)Based on the MEMS process,a low-power micro-heater with a cantilever beam structure was designed and fabricated,and a two-dimensional ultra-thin ordered porous film was prepared in situ on its surface by a template-assisted method,realizing the fusion manufacturing of sensor devices.Through the optimization of the process parameters,the high-efficiency unity of the structural integrity of the MEMS microheater and the uniformity of the film are realized.Compared with other synthesis technologies,this technology is simpler and more convenient,and can be perfectly integrated with MEMS micro heaters while maintaining the specific shape of the material.This study provides a new method and idea to design and prepare gas sensors,which will promote the further development of subsequent gas sensor research and applications.(2)Based on the MEMS micro-heater,a Ni-doped SnO2 ordered porous sensitive film was constructed in situ,and the ultra-sensitive sensing of various common VOCs gases such as alcohols,benzenes,aldehydes and ketones was realized.The cantilever beam structure makes the heat distribution in the sensitive area of the sensor highly uniform and has extremely high heating efficiency(10℃/1 mW).The detection limit of the sensor for VOCs can be as low as 5 ppb at an operating temperature of 250℃,and the response/recovery time is less than 10 s.Furthermore,based on in situ Raman spectroscopy and XPS surface analysis,the mechanism of Ni doping-induced enhancement of gas sensitivity was deeply explored.The developed Ni-SnO2 sensor has a high comprehensive sensitivity to VOCs gas,which can provide a material basis and technical approach for the development of air quality assessment sensors.(3)Based on the in situ preparation method of sacrificial templates,ZnO/WO3 ordered nanoporous sensing membranes with high sensitivity,fast response and reproducibility were fabricated on rough curved commercial ceramic tubes.Experimental results demonstrate that the nanocomposite film has sensitive response,fast response/recovery,and reproducible detection performance.The 5%ZnO/WO3 composite sensor can detect the concentration of 0.1 ppm with a response time of only seconds and high signal reproducibility(RSD=3.49%).The combination between ZnO and WO3 creates a large heterojunction,which is beneficial to increase the crosssensitivity of the gas sensor to VOCs while suppressing the response to other kinds of interfering gases.The strategy of preparing nanocomposite sensing films in situ is not only beneficial to optimize the performance of gas sensors by adjusting the nanostructure and chemical composition,but also provides a technical route for the development of cross-sensitive gas sensors. |
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