Study On The Gas Sensors Operating At Room Temperature Based On Tungsten Oxide Thin Films/Intermediate-sized Porous Silicon With Ordered Channels

Over the past few decades, with the rapid development of industrialization, airpollution problems have become more and more serious. As the highly toxic gases,the presence of nitrogen dioxide (NO2) and ammonia (NH3) in the atmosphere arevery dangerous to the human beings and environmental protection even in lowconcentrations. In addition, there is an attractive feature associated with thedevelopment of gas sensors operating at room temperature. It has accelerated thedevelopment of new sensing materials and sensor techniques since the past severalyears. So far, the tungsten oxide (WO3) or porous silicon (PS) has been widelyconsidered as a promising gas-sensing material in recent years. Based on the abovebackground, in this paper, the relevant experiment and theory of this novel gas sensorbased on the nanocomposite structure composed of tungsten oxide/PS were studied indetail.The main research work of this paper include:1. The study of the preparationprocess about the intermediate-PS with highly ordered pore channels(intermediate-sized pore:50~200nm), especially, the relationship of the PSmicrostructure with key preparition condition parameters such as etching currentdensity as well as etching time.2. The gas-sensing performances of theintermediate-PS.3. The preparation conditions and sensing characteristics of theWO3/intermediate-PS nanocomposite sensing material were also studied.The intermediate-PS layer was successfully formed on the polished surface basedon the n-type monocrystalline silicon substrate via galvanostatic electrochemicaletching method in a double-tank cell setup. The present research results exhibitetching current density as well as etching time played vital roles in modulating the PSmicrostructure. When the current density was too small, the obtained pore diameterwas also small. The pore diameter, porosity and layer thickness increased quicklywith the increase of etching current density. The straight, smooth and highly orderedintermediated-size pore arrays could be formed at its critical current density withsuitable etching time. The microstructure properties of PS, such as average porosity,layer thickness, mean pore diameter and microstructure was strongly dependent on theapplied etching time. When the etching time was too long, the PS surface was attacked by the electrolyte and caused the dissolution of the top layer. The preparedintermediate-PS showed a large specific surface area with effective diffusion concuits,which was favorable for NO2or NH3gas-sensing application, such as high sensitivity,good reversibility, perfect repeatability, fast reponse/recovery characteristics, etc.Afterward, the tungsten oxide thin films were deposited on the top of optimizedintermediate-PS layer by DC reactive magnetron sputtering method, and facribatedthe nanocomposite gas sensor. It can be observed that the sputtering time was largelyinfluenced the microstructure and the gas-sensing properties of the preparednanocomposite. In comparison with pure PS sensor, the WO3/PS nanocomposite gassensor showed a higher sensitivity and better selectivity. The results also revealed thatthe addition of WO3thin films strongly modified the PS gas-sensing behavior, whichcan be ascribed to the high specific surface area for gas adsorption and stable gasdiffusion concuits.