Preparation And Performances Of Photosensitive/Gas-sensitive Semiconductor Nanomaterials

Semiconductor nanomaterial has great application prospects in various functional devices such as light detectors and gas sensors owing to its distinctive and outstanding optical, electric and photoelectric characteristics. Looking for simple processes to prepare semiconductor nanomaterials with excellent performances is the key to opening up its applications and putting it into practical uses. This dissertation focused on metal oxide semiconductor and conducting polymer nanomaterials. Semiconductors and semiconductor composites with optimal component and structure were prepared using electrospinning technique or solution synthesis. The properties of photosensitive, gas-sensitive and the PN junction formed between the interface of N-type metal oxide semiconductor and P-type conducting polymer in the composite were investigated and discussed.Zinc oxide (ZnO) is one of the most important metal oxide semiconducting materials. To improve the photosensitive properties and make ZnO nanofibers able to detect organic gases with high sensitivities at room temperature. Au nanoparticles were introduced to the ZnO nanofibers to obtain Au/ZnO nanofibers using electrospinning technique. Photosensing tests show that Au nanoparticle decoration enhances the photoresponsive properties and the Au/ZnO responds not only to UV light but also to visible light. Gas sensing tests demonstrate that the sensitivity of Au/ZnO nanofiber sensor is greater than that of ZnO nanofiber sensor. The sensitivity to 5 ppm ethanol reaches 0.31 using 0.20 mol.% Au/ZnO nanofiber sensor. The improvement of performance is attributed to the enhanced photocatalytic reactions of organic gases on the Au/ZnO nanofibers surface. In addition, the effect of UV wavelength on the sensitivities to different organic gases were also examined and analyzed.Polyaniline (PANI) is one kind of organic conducting polymers. It is known that the morphology of nanostructured PANI plays a very important role in their properties. PANI nanotube is a promising candidate to improve its functional applications. Here, a new method, which combing the electrospinning technique and the chemical polymerization, for synthesis of PANI nanotubes was presented. Mn2O3 nanofibers prepared by electrospinning were used as reactive template to initiate the polymerization of aniline in acid solution. During the polymerization, the Mn2O3 nanofibers were simultaneously removed because of its oxidant role. As-prepared PANI nanotubes show an average diameter of 80 nm and inner diameter of 38 nm. It is found that the PANI nanotube sensor can detect as low as ppb level of ammonia in air at room temperature. The sensitivities to 25 ppb and 50 ppb ammonia are 0.11 and 0.33, respectively, and the response time is about 50 s. The PANI nanotube sensor shows good reversibility and the current can recover through flushing the test chamber with air blow for about 100 s.To further improve the sensitivity to ammonia, PN junction formed between P-type semiconductor and N-type semiconductor is adopted to improve the performance of PANI-based gas sensors. In the work, one kind polyaniline/titanium dioxide (PANI/TiO2) composite nanofibers, where P-type PANI and N-type TiO2 nanoparticles are distributed along the entire area of nanofibers, was fabricated and the sensing performances to ammonia were investigated. To prepare this kind of composite nanostructure, Mn3O4/TiO2 nanofibers were first prepared by electrospinning, and then the Mn3O4 in the Mn3O4TiO2 nanofibers acted as an oxidant to oxidatively polymerize aniline. As a result, Mn3O4 was completely consumed while PANI/TiO2 composite nanofibers were formed. Gas sensing tests show that the PANI/TiO2 composite nanofiber sensors possess better sensing performance than that of pure PANI. The improved contact interface between PANI and TiO2, along with the combined effects of ammonia on the conductivities of PANI and the PN junction pathway are the main reasons for the enhanced sensitivity. It is also revealed that the sensitivity of the sensor made from the composite nanofibers strongly depends on the constituent content. The highest sensitivity to 25 ppb ammonia is 0.43, which is obtained at an optimal TiO2 content of 40.74 wt.% in the PANI/TiO2 composite nanofibers.ZnO is a typical N-type semiconductor, and PANI is a P-type semiconductor. Novel photoelectric devices can be fabricated based on the PN junction formed at the interface of the two semiconductors. In the work, N-type ZnO nanorods array were synthesized via a hydrothermal method, and P-type PANI thin film was prepared by combining the rapid mixing reaction and the dilute in situ deposition method. Then the current-voltage characteristics were examined by sandwich-type configurations of FTO-ZnO-FTO, FTO-PANI-FTO and FTO-ZnO/PANI-FTO. The current-voltage characteristics of the FTO-ZnO/PANI-FTO configuration demonstrates a nonlinear and asymmetric curve, where the cut-in voltage is about 0.5 V and the reverse breakdown voltage is around-27 V, which verify the formation of PN junction between the ZnO nanorods and the PANI thin film. Effects of UV illumination and ammonia exposure on the current-voltage characteristics were also investigated. In addition, a unique photoresponsive configuration (PANI/ZnO), where the current decreases upon UV irradiation, was constructed.