Chemical Sensors Based On Nanomaterials

With the rapid development of industrialization, our lives are visibly improving in a material sense, yet the environment become worse and even destroyed which threatened severely the people’s living. The pollutant, such as industrial waste, gases, and automobile exhaust, not only pollute the environmen, but also is harm to the health of people. Therefore, the detection and emission control of chemical pollutant is very urgent in many fields. Chemical sensors based on nanomaterials show the better sensing performance than the common sensing materials due to their unique physical and chemical characteristics. They exhibit high sensitivity, good selectivity and stability compared to their bulk courterparts. Therefore, nanomaterials are the good candidates for chemical sensors application to detect harmful substances in many fields.The liquid and gaseous are the major form of chemical pollutants in the industry and our daily life. The gas sensor is the chemical sensor to detect the gas gaseous pollutants and the chemical sensor based on surface enhanced Raman scatting(SERS) is the main part to detect liqiuid pollutants. Therefore, the improvement of sensing performance of gas sensor using nano-technology is the main development direction of gas sensor. Simultaneously, the controllable fabrication of liquid chemical sensor with good sensing performance using nanotechnology is also important. In this thesis, different kind of metal oxide semiconductor sensing materials with various nanostructures were fabricated by template method and hydrothermal method combining with physical vapor deposition process. In addition, large area ordered metal nanomaterials were also fabricated by X-ray interference lithography(XIL) and UV lithography methods. These nanomaterials have very good SERS performance, which can be well applied to detect liqiuid pollutants. Detail works are briefed as followings:(1) Different kinds of multi-lalyer metal oxide semiconductor porous film can be synthesized using Polystyrene(PS) as the soft template combining the magnetron sputtering physical deposition process. The formation of heterojunction structure can be obtained between different materials in the multi-lalyer metal oxide porous film. The sensing properties of material can be improved due to the formation of heterojunction structure. For example, the multi-layer In2O3/Cu O porous thin film was fabricated by this method with p-n heterojunction for detecting different gas(methanol, ethanol, H2 S, NH3 and acetone) detection. These sensing materials exhibit very good sensing performance toward ethanol at 250℃ with very low detection limit. They also show very good selectivity to ethanol. In this work, the relationship between the gas sensing performance and the number of layers was also studied.(2) Different nanomaterials decorated hexagonal Co3O4 nanosheets can be synthesized by hydrothermal method combining with magnetron sputtering physical evaporation process. The Cr-decorated hexagonal Co3O4 nanosheets and Zn O-decorated hexagonal Co3O4 nanosheets were synthesized by this method. The Cr-decorated hexagonal Co3O4 nanosheets shows the enhanced sensing performance to ethanol and also has very short response time(1 s) and recovery time(7 s) due to the good catalytic performance of Cr. Also they still have the high sensitivity even when the ethanol concentration down to 10 ppm. Furthermore, Zn O nanoparticle decorated hexagonal Co3O4 nanosheets exihibit the improved sensing performance and fast response and recovery time to ethanol due to the formation of the p-n junction between Co3O4 and Zn O materials.(3) Large-scale Au nanodisk arrays on Si substrate are successfully fabricated via x-ray interference lithography and followed by electron-beam vapor deposition. The Au nanodisk arrays exhibit a significant, uniform, stable and reproducible surface enhancement on Raman scattering signal, which enables the detection of R6 G as low as 10-8 M with an enhancement factor of 106. The Raman signal enhancement of Au nanodisks is determined by the diameters of nanodisks and the inter-disk distance of nanodisks. The diameters of the nanodisks and the inter-disk distance can be simply optimized to obtain high enhancement in Raman signal by varying exposure time and development time in XIL process. The Au/Ag double-layer bimetal nanodisk arrays are also fabricated which show a significant increase in the Raman signal enhancement than that of the Au nanodisk arrays.(4) Large-area and highly ordered Si nanocone arrays decorated with Ag or Au/Ag nanoparticles have been fabricated via a mask-free lithography with reaction ion etching, followed by metal deposition process. Ultrasensitive surface enhanced Raman scattering signals with an enhancement factor of 1012 were achieved even at the concentration of the Rhodamine 6G as low as 10-15 M. The surface-enhanced Raman spectroscopy(SERS) substrate was also applied on the detection of Sudan I dye and the Raman signals were substantially enhanced as well. The stability of the SERS substrate can be significantly improved by covering Ag nanoparticles with Au thin layer, which maintain a high SERS performance even after one month storage. This nanofabrication process appears to be a feasible approach to prepare uniform and reproducible SERS-active substrates with high sensitivity and stability for practical SERS applications.