Study On The Morphological Structure, Photoelectric Properties And Photocatalytic Performance Of TiO₂ Nanocrystals

The morphological structure and photoelectric properties of TiO2 nanocrystals synthesized by the sol-gel method were characterized by differential thermal analysis-thermogravitery (DTA-TG), X-ray diffraction (XRD), N2 adsorption analysis applying BET method, UV-Visible diffuse reflectance spectroscopy (DRS), surface photovoltage spectroscopy (SPS), photoluminescence spectroscopy (PL) and transient photoconductivity spectroscopy. Terephthalic acid was used as a probe, the active oxidative species ((OH) produced in the supernatant liquid of the irradiated TiO2 nanoparticles suspension containing terephthalic acid were quantitatively investigated by measuring the fluorescence derived by the reaction with terephthalic acid. Rhodamine-B and ethylene were employed as liquid and gas phase reactant respectively to investigate photocatalytic performance of the TiO2 nanoparticles.The results showed that: (1) The crystalline phase and photoelectric properties of photocatalysts were closely connected with the temperature of thermal treatment. With the increase in the temperature of thermal treatment,the crystalline phase of TiO2 transformed from anatase to rutile gradually, the crystalline size increased and the specific surface area decreased. The sample treated at 400℃ (T4) was a mixture of anatase and rutile, anatase phase and rutile phase accounted for 64% and 36％ respectively. Compared to the samples treated at the other temperatures, T4 had the excellent absorbance ability, the strongest photovoltaic response, the longest lifetime of the photoluminescence spectroscopy and transient photoconductivity spectroscopy, which indicated that the efficiency of its photogenerated carriers was greatest. (2) (OH radicals formed at the TiO2/H2O interface was in proportional to the illumination time and obeyed zero-order kinetics, and the formation rate of (OH radicals produced on the TiO2 nanoparticles treated at different temperatures could be expressed as follows: 400℃ > 500℃ > 600℃ > 300℃ > 200℃. (3) The photocatalytic mechanism was different in gas phase and liquid phase. In the liquid-solid system, the photocatalytic performance of TiO2 was mainly dependent on the separation efficiency of its photogenerated carriers and the formation rate of the (OH produced on its surface. High separation efficiency of carriers and great formation rate of (OH facilitated the photo-degradation of RhB, and thererfore T4 showed the excellent photocatalytic performance among the catalysts. In the gas-solid system, the pre-adsorb of organic compounds and oxygen on the catalyst surface was an important factor for the high photooxdation efficency. High surface area and abundant hydroxyl groups of the catalysts facilitated the adsorb of C2H4 and O2, these adsorbates traped the photogenerated carriers to produce active species rapidly,which subsequently attacked C2H4 and mineralized it into CO2 and H2O, therefore, the sample treated at 200℃ showed the highest photoeffectiveness. With the increase in temperature of thermal treatment, surface area of TiO2 particles decrease and the hydroxyl groups desorbed, which was unfavorable for the adsorb of the reactants and led to the decrease of the photocatalytic performance.