Study On The Preparation And Sensing Properties Of Porous Silicon/One-Dimensional Metal-Oxide

It is reported that the major pollutants affecting the city’s air quality are PM10(98.7%), SO2(1.0%) and NOx(0.3%). The NOxare deadly poisons and high-valueenvironmental pollutants in which NO2is a very typical representative of NOx. In thecase of NO2gas sensing materials， low-dimensional nanostructure metal-oxidesemiconductors have attracted much focus owing to their high surface area, orientedelectron conduction and low power consumption.To begin with, a simple overview is given about the definition and classificationof gas sensors, main characteristic parameters and characterizations and the sensingmechanism of nanostructure metal-oxide semiconductors in this paper. Then, asystemic summarizations and analysis are presented about the latest research anddevelopment of four non-traditional gas sensing materials (TeO2, WO3, porous siliconand composites based on porous silicon). The innovative studies on the preparationand sensing properties of porous silicon/TeO2nanowires and porous silicon/WO3nanorods are launched in an orderly way, in which the most important experimentalresults are presented as follows.(1) A novel composite structure of p-porous silicon/p-TeO2nanowires has beensuccessfully synthesized using Te powder as source materials and porous silicon asgrowth substrate by thermal evaporation method. The morphological feature andcrystal structure of the TeO2nanowires grown on porous silicon were characterized byscanning electron microscopy, X-ray diffraction and transmission electron microscopy.As prepared TeO2nanowires with diameters ranging from100to200nm and lengthsup to15μm were covered by a distinctly snow-like over layer along whole length.Gas sensing properties of both composite structure sensor and pristine porous siliconsensor were examined at the working temperature of26–150℃with NO2concentration ranging from0.05to3ppm. Next, we made a comparison of gassensing performances between porous silicon/TeO2nanowires and other reportedTeO2nanowires. Besides, the repeatability and selectivity of the new sensor were alsotested respectively. The results revealed that the composite structure sensor exhibitedhigh response, excellent repeatability and good selectivity to NO2at room temperature.(2) Another novel composite structure of p-porous silicon/p-TeO2nanoneedle has been successfully synthesized using porous silicon covered by a layer floccule asgrowth substrate by thermal evaporation method. The optimum working temperatureis tested and it is room temperature. Gas sensing properties of p-porous silicon/p-TeO2nanoneedle were examined at room temperature with NO2concentration ranging from5to500ppb. The results revealed that the composite structure sensor exhibited lowdetecting limit and it reached5ppb.(3) Hexagonal WO3nanorods are synthesized by a facile hydrothermal process at180℃using sodium tungstate and sodium chloride as starting materials. Themorphology, structure and composition of the prepared nanorods are studied byscanning electron microscopy, X-ray diffraction spectroscopy, and energy dispersivespectroscopy. It is found that the agglomeration of the nanorods is strongly dependenton the pH value of the reaction solution. Uniform and isolated WO3nanorods withdiameter ranging from100–150nm and length up to4μm are obtained at pH=2.5and the nanorods are identified as hexagonal in phase structure. The sensingcharacteristics of WO3nanorods sensor are obtained by measuring the dynamicresponse towards NO2with a concentration in the range of0.5–5ppm and at aworking temperature in the range of25–250℃. The obtained WO3nanorods sensorsare found to exhibit opposite sensing behaviors depending on the working temperature.When exposed to oxidizing NO2gas, the WO3nanorods sensor behaves as n-typesemiconductor as expected when the working temperature was higher than50℃,whereas, it behaves as p-type semiconductor below50℃. The origin of the n–to p-type transition is correlated with the formation of inversion layer at the surface ofWO3nanorods at room temperature.(4) The microstructures and crystalline phases of hexagonal WO3nanorodsannealed at different temperatures were investigated by scanning electron microscopy,transmission electron microscopy, selected area electron diffraction and X-raydiffraction. It was found that a possible agglomeration occurred during annealingprocess. Meanwhile, the XRD patterns indicated that a new crystalline phase ofmonoclinic WO2.83was produced by thermal treatment and the hexagonal structureWO3prevailed together with monoclinic WO2.83at samples annealed by300and400℃/2h, but the characteristic peak of WO2.83disappeared at sample annealed by500℃/2h. Dynamic responses measurements of annealed samples performed at roomtemperature showed that all the annealed WO3nanorods sensors presented p-typebehaviors. We suppose the p-type behavior of WO3nanorods sensor to be that an inversion layer is formed in the space charge layer when the sensor was exposed toNO2at room temperature. Among those annealed nanorods,400℃-annealed samplehas higher response, quicker response and recovery speed and more stablerepeatability than300and500℃annealed samples. This finding is useful for makingnew room temperature NO2sensors based on hexagonal WO3nanorods.(5) A composite structure of p-porous silicon/n-WO3nanorods has beensuccessfully synthesized using three different experimental schemes. Server A, aporous silicon/WO3nanorods sensor was prepared by spin-coating method, in whichthe WO3nanorods were spin-coated directly on the porous silicon surface. We firstinvestigated the response of porous silicon/WO3nanorods sensor by sensing1ppmNO2in the temperature range of25–250℃. Then, gas sensing properties of poroussilicon/WO3nanorods were examined at the the optimum working temperature of200℃. Server B, another porous silicon/WO3nanorods sensor was prepared by thehydrothermal process adopting the preparation process of WO3nanorods. It wasresearched that the morphological transformation and the gas sensing propertiesaffected by Hydrothermal reaction time. Server C, on the basis of Server B, we adjustthe experimental procedure, in which the preparation technology of seed layer and theorder of adding reagents NaCl were changed. The effects of hydrothermal reactiontime, hydrothermal reaction temperature, the addition amount of structure directingagents NaCl and the concentration of seed liquid on morphology and gas sensingproperties of porous silicon/WO3nanorods were studied systematically.