Nonmetal Doped Modification And Wide Spectral Response Of Mesoporous TiO₂Photocatalytic Fiber

In the field of water treatment, TiO2has becoming the hot topic in recent research owing to its advantages of higher photocatalytic activity, stable physicochemical properties, low toxicity and cost effectiveness. However, the wide bandgap of TiO2(3.2eV) and the massive recombination of photogenerated electron-hole pairs have limited its overall photocatalytic efficiency. Moreover, photocatalytic reactions in water primarily take place on the surface of liquid and solid, therefore the physicochemical properties, especially the surface structure of TiO2is another critical factor affecting photocatalytic efficiencies.Previous photocatalytic water treatment researches mostly focused on the preparation of TiO2powders and supported TiO2. TiO2powders are hard for recycling and utilizing and supported TiO2has disadvantages of catalyst dropping and low quantum efficiency. Therefore, it is not only a urgent problem for the need of photocatalytic technology development but also a future direction to develop a novel practical TiO2photocatalyst with both high efficient UV and visible light response and the morphology of easy recyclement and utilization.In this paper, TiO2continuous fibers were prepared by a sol-gel method with titanium tetrachloride (TiCl4) and titanium tetrabutoxide (TBOT) as the titanium source, respectively. Firstly, with TiCl4as titanium source, the reaction process of the sol was analysed, so were the spinnability of sol and the photocatalytic activity of prepared fibers. In order to further improve the visible light photoactivity, the N-doped TiO2continuous fibers were prepared with the diethylamine (Et2NH) as the nitrogen source. For not decreasing the UV photoactivity while improving the visible light photoactivity, UV-visible light responsive wide spectral responsive aminosilane coupling agents modified N-Si codoped TiO2fibers were prepared with TBOT as the titanium source. The fibers were analysed by a lot of characterization technologies such as TG-DTA、XRD、SEM、FT-IR、XPS、UV-vis DRS、N2adsorption-desorption and so on. The factors affecting the sol spinnability and the effect of single doping or codoping on the structure and photocatalytic activity of TiO2were studied systematically. The work of this paper mainly focuses on the development of wide spectral responsive mesoporous TiO2fiber photocatalyst. Such studies can give insights into the preparation of TiO2fiber photocatalyst and its enhanced photoactivity. The content of the experiments can be divided into three parts as follows:In the first section, with TiCl4as titamium source the reaction process and mechanism between TiCl4and CH3OH, the effect of the addition of triethylamine (Et3N) on the reaction process and spinnability and the effect of different amount of Et3N and heat-treated temperature on the structure and photoactivity of TiO2fibers were investigated. A chemical reaction happened and titanium chloroalkoxide and HC1were generated after the dropwise additon of TiCl4into CH3OH, and the condensation was suppressed by the resulted stong acid environment. The added Et3N consumed HC1forming the salt of Et3NHCl, which will promote the hydrolysis and condensation of TiCl4with forming the chain structure of organotitanium polymer, which was the main component of the excellent spinnable sol. The spinnable sol was obtained when the insoluble Et3NHCl was removed by filtration after the resolvent was concentrated and the precipitation was moved to tetrahydrofuran (THF). Results showed that at the molar ratio of Et3N and TiCl3was4, the HC1was removed compeletely and the best spinnability and photocatalytic activity was obtained for the sol and the prepared fibers, respectively. Different results of heat-treated temperatures showed that the most photocatalytic activity of TiO2was obtained at600℃, which was in good crystallinity and without organic residues. Lower temperatures resulted in a low crystallinity and the residues of organics. A temperature heat-treated at700℃resulted in the appearance of rutile, which lowered its photocatalytic activity. A benefit to the preparation of a spinnable sol and high photoactivity TiO2fibers will be provided by the study.In the second section, different N-doped TiO2continuous fibers were prepared with TiCl4as the titanium source and Et2NH as the nitrogen source, and the mechanism of doping was investigated. Results showed that the doping of N did not affect the crystalline phase but suppressed the crystalline growth under the experimental conditions, which was favorable for the improved photocatalytic efficiency. The doping of N maked the pore size distribution of TiO2bigger, while proper pore size would be benefit for the adsorption pre-enrichment and subsequent photocatalytic degradation of organic pollutants in water. XPS results indicated that the surface nitrogen content of TiO2was1.03%when the ratio of N/Ti was1:1. The proper doping of N was favorable for the visible light photoactivity, while the photoactivity was reduced when excessive doping. UV-vis DRS results indicated that with the increase of nitrogen, the red shift was observed and the bandgap decreased from3.2eV to3.14eV. The best photoactivity of TiO2was obtained at the N/Ti ratio of1:1, which was due to the unique mechanism of nitrogen doping. Furthermore, in order to develop practical visible light responsive photocatalyst, we developed a preparation method of TiO2fiber cloth. It was prepared after lapping, gumming, pressing, curing and heat-treatment with N-doped TiO2precursor fibers as raw materials. This invention would play an important role in facilitating the practical application of TiO2photocatalyst. This achievement has been authorized by the national invention patent, and the number is ZL201210036918.2.In the third section, N-Si co-doped TiO2continuous fibers were prepared by a modified sol-gel method with TBOT as the titanium source. Three aminosilane coupling agents, namely3-aminopropyltriethoxysilane (APTES),3-aminopropyltrimethoxysilane (APTMS) and N-(2-aminoethyl)-3-aminopropyltriethoxysilane (AEAPTES), were selected as novel different Si and N dopants. The results indicated that Si and N were incorporated into the lattice of TiO2under proper conditions. In which, proper Si doping enhances surface area, delays the phase transformation from anatase to rutile and improves the UV photocatalytic activity, while proper N doping improves visible light absorption. In the case of APTES as a modifier at a Si/Ti molar ratio of0.15, TiO2fibers with a mixed crystalline phase at an anatase/rutile ratio of77:23and the largest Brunauer-Emmett-Teller (BET) specific surface area were obtained at900℃. It displayed the highest wide spectral responsive photoactivity, and the degradation rate of the azo dye reactive brilliant red X-3B in aqueous solution reached96.6%for90min and96.4%for180min under UV and visible light irradiation, respectively. In addition, the degradation efficiency was still maintained at>90%for5cycles. The results provided a novel and facial way to the preparation and utilization of high photoactivity wide spectral responsive TiO2fibers. The achievement has been published in Journal of alloys and compounds.