Twice Heat-treating To Synthesize TiO₂/Carbon Composites With Visible-light Photocatalytic Activity

In2001, Asahi reported the substitution of N for O in TiO₂crystal lattice can promote thephotocatalytic performance, which caused scientific researchers’ attention on nonmetal doped TiO₂. Apartfrom doping N, doping other nonmetal elements （such as B, F, C, S and I） can also promote the visible-lightphotocatalytic activity of the material. Among these nonmetal elements, carbon attract the researcher’sattention because of its rich source. Generally, there are two mechanisms about carbon: photosensitizationand doping effect. Both of the mechanisms can enhance the visible-light absorption of the catalysts. In thisthesis, we prepared several visible-light active titania/carbon （TiO₂/C） composites with tetrabutyl titanate astitanium precursor and with resorcinol-formaldehyde resin （RF） and polyethylene glycol（PEG）s as carbonprecursors, and studied the effect of heat-treating condition on the material’s structure, morphologies,visible-light absorption ability and photocatalytic performance. The photocatalytic mechanism was alsodiscussed.The detail contents and results are as follows:1. In this part, the TiO₂/carbon （TiO₂/C） composites were synthesized by simply mixing a TiO₂solwith a semi-cured resorcinol–formaldehyde （RF） polymer, followed by drying, heat-treating in an argonatmosphere, and calcinating in air atmosphere at different temperature. As-prepared TiO₂/C compositeswere characterized by means of X-ray diffraction （XRD）, UV–Vis diffuse reflectance spectroscopy（UV–Vis DRS）, thermal analysis （TA）, transmission electron microscopy （TEM）, and X-ray photoelectronspectroscopy （XPS）. The visible-light photocatalytic activity of TiO₂nanoparticles was evaluated on thebasis of the degradation rate of MO and the removal of propylene. With the results of XRD, the particlesize of the TiO₂/C nanocomposities decrease with the increasing of carbon content. By the analysis of XPS,with RF as carbon source, the C element present in different forms in samples at different secondheat-treating temperatures. When the heat-treating temperature is300°C, carbon species （such as C-OR（H）,C=O, C-OOR（H））mainly absorbed on the TiO₂surface; with increasing the heat treatment temperature, Cwas gradually doped into the TiO₂lattice. C was doped in the Ti site, which is disadvantageous to the photocatalytic activity. From the UV-Vis absorption spectra, the as-prepared C modified TiO₂samples hadstrong visible light absorption. The reason for the enhancement visible light absorption and the broadeningof the light response range is the photosensitization of carbon. In addition, we investigated the effect of thefirst heat-treating temperature, the second heat-treating time and temperature on the acticity of thephotocatalyst. The results show that the optimal first heat-treating temperature is600°C and the optimalsecond calcination time and temperature are1hour and400°C, respectively.2. For comparision, we prepared several visible-light active C-TiO₂by the twice heat-treatmentmethod with TiO₂sol as precursor in the absence of extra carbon precursor. The results show that only theC-TiO₂s which have been eventually heat-treated at400°C in air has the obvious visible-lightphotocatalytic activities for the degradation of MO. This can be attributed to the synergistic effect of thesingle electron oxygen vacancies （SETOVs） and modified carbon species facilitating the transfer of thephoto-generated electron.3. TiO₂/C composites were prepared with PEGs （the molecular weights are400and1000, respectively）as carbon source. The results show that the photocatalytic property of carbon modified TiO₂catalysts withPEG as carbon source was effected by the carbon content and doping site. The sample treated at300°C inthe second heat treatment has strong visible light absorption in the wide range form200to800nm, andshows the best activity in the visible light photocatalytic degradtion of MO. By the analysis of XPS, a greatmount of carbon species on the TiO₂surface acts as photo-sensitizer which can enhance the visible lightphotocatalytic activity; but a part of C was doped into Ti site in the TiO₂lattice, which is disadvantageousto the photocatalytic activity （i.e., this doping inactivates the catatlyst）.