Preparation And Gas Sensing Properties Of Tungsten Oxide Nanowires/Porous Silicon Composite

Recently, nitrogen dioxide (NO2) in the atmosphere has been a great threat on theenvironment and human life. Tungsten oxide (WO3) has been considered as one of themost interesting materials in the field of NO2sensors owing to its low cost andhigh-sensitivity. Unfortunately, gas sensors based on WO3materials usually work at ahigh temperature above200oC owing to high activation energy of reaction with gasmolecules. The high operating temperature is unfavorable for power saving anddevice integration. Porous silicon fabricated simply and at low cost byelectrochemical etching of silicon in HF solution, shows a variety of interestingproperties, including room temperature gas sensing properties and easy integration. Ithas become an ideal candidate material for potential low-power gas sensors.Therefore, a new gas sensor based on low-dimensional nanomaterials/porous siliconcomposites may be beneficial in reducing operating temperature or enhancing gasresponse for their synergetic enhancement or heterojunction effects. To the best of ourknowledge, studies in this area are not a lot.In this paper, we demonstrated the synthesis of WO3nanowires/porous siliconcomposites through the thermal evaporation without catalysts. The factors influencedthe morphology of the composites were body vacuum, source temperature, substratetemperature and argon-oxygen ratio. The morphology, crystal structures and gassensing properties of the WO3nanowires/porous silicon composites were analyzedand investigated by field emission scanning electron microscope (FESEM), X-raydiffraction (XRD), transmission electron microscope (TEM), Raman spectroscope andgas sensing characteristics measurement. The diameters and lengths of WO3nanowires, which were monoclinic phase, were40-60nm and20-30μm, respectively.The aspect ratio (length/diameter) of nanowires can be range of500-750. The sensormade of WO3nanowires/porous silicon composite exhibited a high response (~3.32),fast response/recovery (~175/44s) and excellent selectivity toward2ppm NO2at anoptimum operating temperature of100oC.However, the density of WO3nanowires in pores of porous silicon is lower thanthat on surface. Thus, WO3nanowires/porous silicon composites were also preparedby thermal annealing of sputter-deposited tungsten film in this paper. The WO3 nanowires/porous silicon composites were investigated by field emission scanningelectron microscope (FESEM) and X-ray diffraction (XRD). The morphology of theWO3nanowires/porous silicon composites was greatly affected by the annealingtemperature and thickness of the tungsten film. Dense and uniformly distributed WO3nanowires, which were monoclinic phase and had some oxygen vacancies, werecompletely contained inside the silicon pores. The diameters and lengths of WO3nanowires were typically from20to30nm and1to2μm, respectively. In addition,the gas sensing properties were also investigated. It was found the sensor based onWO3nanowires/porous silicon composites had a high response (~4.76) and selectivitytoward2ppm NO2at an optimum operating temperature of150oC. The lowestconcentration of NO2detected was250ppb.Modulations of the potential barriers at both networked nanowireshomojunctions and heterojunctions between porous silicon and tungsten oxide, aswell as modulations of the depletion width along the nanowires, are likely to beresponsible for the good gas sensing properties at a low operating temperature.