Synthesis And Characterization Of Highly Active Nano TiO2, And Its Visible Light Photocatalytic Properties

As worldwide environment pollution and energy resource crisis are graving gradually today, effective utilization of solar energy to solve pollution problems has became of great interest in many countries. The synthesis and applications of photocatalyst with high quantum-efficiency and high activity, in which solar energy can be utilized efficiently, have became hot research topics in the field of material, chemistry and environment science.In this dissertation, the basic principle about semiconductor photocatalysis was reviewed briefly at first. The methods, research progress and main problems about improving the total quantum-efficiency and utilization efficiency of solar energy for semiconductor photocatalysis have been summarized, the advantages/disadvantages and the developing trends about the preparation route of nanosized TiO₂ prepared by wet chemistry has been evaluated. Then, a series of nanosized TiO₂ photocayalysts with high visible-light photocatalytic activity (VLPA) was successfully prepared by high-temperature hydrolysis solvothermal method (HHSM) using titanium alkoxides as starting material. Subsequently, the effect of synthesis temperature, solvent component and heat-treatment condition on physical properties and photocatalytic activity of nanosized TiO₂ has been studied. The degradation mechanism and dynamics of the as-prepared nanosized TiO₂ for organic targets including methyl orange (MO) and 4-cholophenol (4-CP) solution was analyzed. Furthermore, the origin of photocatalytic activity under visible light irradiation and stability of as-prepared nanosized TiO₂ was discussed. In this thesis, a series of important conclusions and valuable achievements with practical application have been obtained, which have given experiment data for the degradation of organic pollutants by using sunlight energy.A kind of the HHSM method to prepare nanosized TiO₂ with high activity and thermal stability was studied. In this method, under reaction condition of high-pressure steams in which the water dissolved in organic solvent from gas phase, the TNB undergoes high temperature hydrolysis-condensation followed by solution -recrystallization process, finally to form the nanosized anatase TiO₂. In the condition of temperature range 165-270℃ and various solvents with different polarity including acetone, ethanol, pyridine and their mixture of acetone-ethanol-pyridine, single anatase TiO₂ with high thermal stability, highly VLPA under λ≥450 nm, small crystallize size of about 9.3-11.7 nm, high BET surface area of about 117-150m²/g and low aggregation was successfully synthesized. It has been confirmed that the synthesis temperature, solvent components and ratio of H₂O:TNB were key preparation parameters which mainly influence the phase type, the crystalline size, the BET surface area and VLPA. In all solvents system, 240℃/6h was found to be the bestsynthesis condition for better photocatalytic performance. For the C-240 sample prepared by acetone-ethanol-pyridine solvent, under the condition of irradiation with visible light (k > 450 nm) for 60 min on the 20mg/L MO solution, the degradation rate was found to be above 90% which is as high as about 9 times of that of commercial Degussa P-25. The crystalline phase of single anatase type can maintain until calcined at 800 °C. The BET surface areas for C-240 samples before and after calcinations at 600 °C was about 129 m2/g and 69 m2/g, respectively, which are about 3 times and 1.5 times higher than that of commercial P-25. Above result demonstrated that the as-prepared C-240 samples have excellent BET surface areas and thermal stability.By the means of ultrasonic desorption, DTA-TGA, UV-Vis, FT-IR and GC-MS measurements, it has been revealed that, besides H2O and residue solvents adsorbed on the surface of nanosized TiC*2 synthesized by the HHSM, the surface adsorption compounds may contain aromatic compounds formed under high-pressure steam condition. According to the weight-loss and peak site from the DTA-TGA curves, heat-treatment temperature were selected for as-prepared samples. For example, the series of A-240/T samples, which T are 180°C, 250°C and 365°C, respectively, were selected to get rid of surface adsorption compounds step by step. All samples of A-240/180, A-240/250, A-240 and A-240/365 have better VLPA than that P-25, and their VLPA order on both MO and 4-CP solutions decrease in the sequence of A-240/180, A-240/250, A-240 and A-240/365. After visible light irradiation for 3h, the A-240/180 has demonstrated a mineralization rate of about 70% on 20mg/L 4-CP solution, which is an order higher than that of P-25. Nevertheless, the photocatalytic activity under UV-Vis irradiation showed a different trend which A-240/365 had the best photocatalytic activity. The A-240/365 have obviously better activity than that of P-25 under UV-Vis irradiation. The results indicated that surface adsorption status had direct relation with and important effect on the photocatalytic activity.The results from XRD, TEM. BET and diffuse reflective spectra (DRS) measurements have revealed that, for samples after heat-treatment at different temperatures, the crystalline type remain unchanged and the crystalline size and BET surface area changed little while their diffuse reflective absorption spectra (DRAS) showed great difference, hi visible-light range of 400-700 nm, visible-light absorption decreased in the sequence of A-240/180, A-240/250 and A-240/365. Above sequence has a good agreement with the change trend of VLPA. This result revealed that the main cause of the different VLPA resulted from the different visible light absorption, which resulted from the different surface adsorption compounds under heat-treatment process. Using visible light obtained with different cut-off wavelength in the 400-800 nm as irradiation light source, for samples with different heat-treatment process, the VLPA results on MO solution also revealed that the VLPA order have a similar trend as that of light absorption from the DRAS. Furthermore, present sample has good photocatalytic activity under visible light irradiationup to A. > 650 nm. Therefore, the VLPA of as-prepared samples can be well adjusted and optimized by appropriately heat-treatment process.Three series of A-240, E-240 and C-240 nanosized TiO2 sample, which were synthesized in acetone, ethanol and acetone-ethanol-pyridine solvents, respectively, are similar in single anatase phase, crystalline sizes and BET surface area. But their photocatalytic activity under both visible-light and UV-Vis light irradiation decreased in the sequence of A-240, E-240 and C-240. Moreover, they had different DTA-TGA curves and differential temperature peaks. These results indicated that they should have different surface adsorption compounds and adsorption status. Similar to A-240/T sample, above samples were treated with different heat-treatment process. The results from photocatalytic activity of samples with different heat-treatments showed that activity decreased in sequence of C-240/250 > C-240/180 > C-240 > > C-240/365 and E-240/250 > E-240/180 > E-240 > > E-240/365, which was different from that of the heat-treated A-240/T samples. Furthermore, after heat treatment for above three kinds of samples prepared from different solvents, the visible light adsorption and photocatalytic activity of the samples A-240/180, C-240/250 and E-240/250 with the highest activity in each series under visible light irradiation showed a similar trend by A-240/180 > C-240/250 > > E-240/250. This verified the relationship between the visible light adsorption and photocatalytic activity.In order to investigate the mineralization activity of samples under visible and UV-Vis light irradiation, respectively, the total organic carbon (TOC) method was used to investigate the photodegradation mineralization efficiency and dynamics of both A-240/180 and C-240/180 samples on MO and 4-CP solution. Under visible-light and UV-Vis light irradiation, the mineralization dynamic formulas of A-240/180 sample on 4-CP solution was [(TOCo-TOC)/TOCo]Vls = 0.69[l-exp(-0.025/)] and [(TOCo-TOC)/rOCo]uv = l^exp(-0.026/), respectively. After irradiation for 3 hours, the mineralization on 20 mg/L and 60 mg/L 4-CP solution can reach about 70% and 100% respectively, and the mineralization under visible-light irradiation is about two times as that reported presently. Under same irradiation time of 3 hours, the mineralization rates of both C-240/180 and A-240/180 samples on 20mg/L MO solution can reach 60-70%, and the corresponding dynamic formulas are [(TOCo-TOCyrOCo]c.240/i80 = 0.63[l-exp(-0.0150] and [(TOCo-TOC)/rOCo]A.240/i80 = 0.72[l^exp(-0.033/)], respectively. Therefore, nanosized TiO2 synthesized by the HHSM method have excellent mineralization rate under both visible-light and UV-Vis light irradiation, and therefore is one kind of promising photocatalyst with high VLPA performance.Photocatalytic degradation process of on MO under visible light irradiation with the aid of nanosized TiO2 samples synthesized under different solvents was investigated by UV-Vis spectrum and TOC measurement. Results showed that, under the attack of free hydroxyl radicals, at first the azo bonds of MO molecule were broken up to form aromaticring intermediate. Then, oxidization process was taken place to form the chain compounds. Finally, the MO was mineralized as the form of CO2 and H2O. Due to different photocatalytic activity, intermediate and photodegradation process may be different for various samples. Taking sample E-240/180 with the lowest photocatalysis for example, azo bonds were broken up to form p-aminobenzenesulfonate (p-ABS) and A^-dimethyl -/7-benzenediamine. The latter was further oxidized to N,7V-dimethyl-4-nitrobenzenamine (NMPA), and subsequently NMPA was broken down and mineralized slowly. For samples C-240/180 and A-240/180 with relatively high photocatalytic activity, however, azo bonds were directly broken up to NMPA and/?-ABS. For sample A-240/180 with the highest photocatalytic activity, intermediate compounds could be photodegraded directly and mineralized instantly, as comparable example of sample C-240/180 with a slightly low photocatalytic property, NMPA need further oxidization process to form p-nitroaniline before its following photodegradation and mineralization process.For photocatalyst, stability is one of the most important parameter for practical application. The results of as-prepared sample C-240 and heat-treated samples C-240/180 and C-240/365 revealed that the photocatalytic activity maintained nearly consistent after exposed in air for 150 days, it only decreases about 10% after 340 days exposured. Photocatalytic activity under visible-light irradiation maintained unchanged after recycled use for 6 times. These results all showed that present photocatalysts have excellent climate-resistance and recycle stability, and possess important value in practical application.