Study On Modification And Photocatalytic Performance Of TiO₂-based Nanosheets

Photocatalysis is one of the most promising processes in controlling of organic pollutants in recent years, it is a clean technology in water system treatment and the final decomposition products of organic compounds can be only CO2and water. TiO2-based photocatalysts have many advantages such as stable photocatalytic activity, high physical and chemical stability, and non-toxicity, so they are widely used for pollution control both in water and air system. However, the traditional TiO2photocatalysts have some shortcomings, i.e., the narrow range of light absorbing, and the low efficient use of photogenerated electron-hole pairs. The further application of TiO2-based photocatalysts must face and solve these problems.Nanostructures of TiO2have been actively exploited to overcome these drawbacks. Among many kinds of TiO2-based nanomaterials, TiO2-based nanosheets (TNS) have attracted many attentions because of high photoactivities. Hydrothermal preparation of TiO2-based nanosheets under strongly alkaline condition is widely employed by the researchers for its high productive rate and convenience. The high photocatalytic performance is mainly due to three superiorities. The first, because of the sheet-like structure, the photoexcited electrons and holes can easily transport to the surface of TNS to participate in the photoreactions; The second, the high surface area (8times of that of P25), so that TNS have more opportunities to contact with the pollution molecules in water system; The third, TNS have a negative charged surface, and strong adsorption capacity for many organic, i.e., cationic dyes used in industries. However, TNS still have the similar shortcomings with regular TiO2-based materials. The narrow range of light absorbing and the low efficient use of photogenerated electron-hole pairs are limiting the application of TNS. The modification of TNS may provide breakthroughs to solve these problems and obtain photocatalysts with higher photocatavities. In this research, we prepared different kinds of modified TNS:1) TNS with higher BET surface area (1.23times of that of P25) was abtained by adding CTAB in the process of hydrothermal reaction, providing nanosheets with larger size, higher photocurrent, and lower FL intensity. The adsorption test suggests that the CT1-TNS have higher adsorption of crystal violet (CV) than that of TNS. According to Langmuir model, the highest adsorption amount by monolayer (?max) of CT1-TNS (60.9mg/g) is1.24times of that of TNS (49.1mg/g). The photoactivities of CT1-TNS in degrading CV and Rhodamine B (RhB) are1.61and1.62times of that of TNS, respectively.2) Carbon deposited TiO2-based nanosheets (C-TNS) photocatalysts were prepared by a two-step hydrothermal treatment of Degussa P25using glucose as the carbon precursor. The carbon deposition has significantly enhanced the visible light absorbing ability and dye molecules adsorption ability of TNS. The XPS result suggests that, the carbon deposition results in the decrease of hydroxyl oxygen, and the deposited carbon mainly exists in the form of carbonaceous species. When the starting glucose solution concentration is2g/L, the obtained C-TNS possesses the highest visible light photoactivity in degrading RhB, which is12times and3times of that of P25and the pure TNS, respectively. The enhancement of visible light photoactivity is due to the strong visible light absorbing ability of carbon species and the dye absorption enhancement by carbon deposition. It is suggested that carbon deposition is a promising way to enhance the visible light photoactivity of TiO2-based nanosheets.3) CeO2@TNS photocatalysts were prepared by a one-pot hydrothermal method using cerium nitrate as the cerium precursor. Cerium ions co-exist in two forms of Ce3+and Ce4+. The result shows that the binding energy of Ti and O elements changed, and the content of Ti3+increased obviously. Appropriate amount of CeO2can significantly inhibit the recombination of electron-hole pairs, which is proved by the decline in the intensity of the fluorescence spectra. CeO2@TNS with cerium ratio of0.5%(molar ratio) possesses the highest photocataytic activity when degrading Rhodamine B (RhB), which is5times and1.6times of P25and TNS, respectively. It is suggested that cerium ions are efficient electron trappers to improve the separation of electrons and holes.4) Ho2O3@TNS and Gd2O3@TNS were prepared by hydrothermal method. Ho2O3/Gd2O3@TNS photocatalysts have well developed nanosheet structure and relativiely low degree of crystallinity. The deposited HO2O3and Gd2O3have changed the surface structure of TNS, and Ti-O-Ho/Gd bonds were formed. The compositions of HO2O3and Gd2O3have inhibited transformation of P25to nanosheets during the hydrothermal reaction, and cause the decrease in the BET surface and the adsorption ability. Ho2O3@TNS with cerium ratio of1.0%(molar ratio) possesses the highest UV-Vis photocataytic activity when degrading Rhodamine B (RhB), which is3.94 times and1.67times of P25and TNS, and its visible photoactiviy is1.90times and12.38times of P25and TNS. Gd2O3@TNS with cerium ratio of0.5%(molar ratio) possesses the highest UV-Vis photocataytic activity when degrading RhB, which is1.60times and5.13times of P25and TNS, and its visible photoactiviy is1.98times and12.90times of P25and TNS.