桥连氧化钨纳米线的可控合成及气敏性质

Controllable Synthesis and Gas Sensing Properties of Bridged Tungsten Oxide Nanowires

氧化钨WO_(3−x)(0≤x<1)具有丰富的氧化态、亚化学计量比晶相以及可逆的光致/电致变色特性,纳米线具有高比表面积和准一维单晶载流子传输通道,WO_(3−x)纳米线结合了上述两者的优异特性,在智能玻璃、能源转换与存储器件和气体传感器等领域有广阔的应用前景。本文从WO_(3−x)的基本性质出发,分析了液相法和气相法(气-液-固、气-固、热氧化)纳米线生长的机制及特点。其中,热氧化法无需催化剂,有望解决纳米线应用的器件化瓶颈,在<500℃下即可实现纳米线尺寸与生长位置的可控生长,实现桥连纳米线器件的高效、原位集成。随后,本文综述了桥连WO_(3−x)纳米线器件在NOx等气体分子检测中的应用进展,梳理了桥连WO_(3−x)纳米线器件在低功耗、高灵敏气体分子检测中的应用,以期为今后高灵敏、低功耗、高集成的氧化物桥连纳米线器件的开发提供参考。

The rapid development of industrialization has resulted in severe environmental problems.A comprehensive assessment of air quality is urgently required all around the world.Among various technologies used in gas molecule detection,including Raman spectroscopy,Fourier transform infrared(FTIR)spectroscopy,mass spectroscopy(MS),electrochemical sensors,and metal oxide semiconductor(MOS)gas sensors,MOS gas sensors possess the advantages of small dimension,low power consumption,high sensitivity,low production cost,and excellent silicon chip compatibility.MOS sensors hold great promise for future Internet of Things(IoT)sensors,which will have a profound impact on indoor and outdoor air quality monitoring.The development of nanotechnology has significantly enhanced the development of MOS gas sensors.Among various nanostructures like nanoparticles,nanosheets and nanowires,the emergence of quasi-one-dimensional(q1D)nanowires/nanorods/nanofibers,with unique q1D geometry(facilitating fast carrier transport)and large surface-to-volume ratio,potentially act as ideal sensing channels for MOS sensors with extremely small dimension,and good stability and sensitivity.These structures have thus been the focus of extensive research.Among the various MOS nanomaterials available,tungsten oxide(WO_(3−x),0≤x<1)nanowires feature the characteristic properties(multiple oxidation states,rich substoichiometric oxides with distinct properties,photo/electrochromism,(photo)catalytic properties,etc.),and unique q1D geometry(single-crystalline pathway for fast carrier transport,large surface-to-volume ratio,etc.).WO_(3−x)nanowires have broad applications in smart windows,energy conversation&storage,and gas sensing devices,and have thus become a focus of attention.In this paper,the fundamental properties of tungsten oxide,synthesis methods and growth mechanism of tungsten oxide nanowires are reviewed.Among various(vapor-liquid-solid(VLS),vapor-solid(VS)and thermal oxidation)growth methods,the thermal oxidation method enables an in situ integration of WO_(3−x)nanowires on predefined electrodes(so-called bridged nanowire devices)via the oxidation of lithographically patterned W film at relatively low growth temperature(~500℃)because of interfacial strain,defects and oxygen on the surface of the W film.The novel bridged nanowire-based sensor devices outperform traditional lateral nanowire devices in terms of larger exposure area,low power consumption via self-heating,and greater convenience in device processing.Recent progress in bridged WO_(3−x)nanowire devices and sensitive NOx molecule detection under low power consumption have also been reviewed.Power consumption of as low as a few milliwatts was achieved,and the detection limit of NO2 was reduced to 0.3 ppb(1 ppb=1×10^(−9),volume fraction).In situ formed bridged WO_(3−x)nanowire devices potentially satisfy the strict requirements of IoT sensors(small dimension,low power consumption,high integration,low cost,high sensitivity,and selectivity),and hold great promises for future IoT sensors.