Synthesis Of TiO₂ Nano-crystal Sol In Aqueous Solution At Room Temperature And Study On The Preparation Of Its Photocatalytic Film

TiO₂ has been widely studied and applied in environment protection due to its excellent photocatalytic activity and physicochemical characteristics.Especially in the time when the pollution problem becomes more and more conspicuous,the study on TiO₂ catalyst has more practical significance.Film is an important application pattern of TiO₂ catalyst,which is more effectively to utilize light and easier to recycle compared with TiO₂ powder,and can be facilely deposited on a variety of substrates such as glass slide,porcelain bricks,metal panels,etc.Resently,in order to extend the application of TiO₂ film on heat-intolerant organic substrates and realize the large-area coating in the preparation of TiO₂ film,the research on its low-temperature preparation method has been attracting more and more attention and become a prosperous research area in the field of material,chemistry and environment science.In this paper,the low-temperature preparation of TiO₂ film and the approaches to improve its photocatalysis have been studied.By means of sol-gel method,TiO₂ sol containing nanometer crystal grains(called TiO₂ nano-crystal sol)was synthesized via improving formula and control process,and thus crystalline TiO₂ film could be prepared from the TiO₂ nano-crystal sol without any high-temperature treatment. Based on this,the preparation methods of TiO₂ film with higher photocatalysis were explored from three aspects such as constructing porous structure,coupled semiconductor and dye sensitization.The main research contents and results are as follows:(1)Synthesized TiO₂ nano-crystal sol in aqueous solution at room temperature, realized low-temperature preparation of crystalline TiO₂ film.The reaction formula with water as primary solvent was designed(where more ratio of Ti(OC₄H₉)₄:H₂O:C₂H₅OH:HCl=1:185:10:0.1)and suited control process was improved to synthesize TiO₂ nano-crystal sol through sol-gel method.¹³CNMR and XAFS were adopted to analyse the hydrolyzation behavior of Ti(OC₄H₉)₄ and the crystal structure of TiO₂ colloidal particle.The results showed that Ti(OC₄H₉)₄ hydrolyzes completely and no organic radicals were linked to Ti atoms in the final sol. Hydrate titanium ions polymerized slowly and bonded orientedly,and thus anatase TiO₂ crystal grains grew in the aqueous solution,and stable TiO₂ nano-crystal sol was formed subsequently(Mean particle diameter is 28.7 nm).The resultant sol was coated on the substrate,and crystalline TiO₂ film with well photocatalytic activity was obtained at low temperature.(2)Fabricated porous surface morphology of crystalline TiO₂ film at low temperature,increased photocatalytic activity of the film.Styrene-acrylate spheres and 4,4'-isopropylidenediphenol(BPA)were used as templates to control the surface morphology of TiO₂ film and added into TiO₂ nano-crystal sol respectively.The film was prepared from resultant solution,and then the template was preferentially extracted by organic solvent.So the porous anatase TiO₂ film was obtained at low temperature(dried at room temperature or 60℃).The influence of the formula and control process on the morphology of the film was investigated and the preparation mechanism of porous TiO₂ film was discussed.The adsorption properity as well as photocatalytic performance were also studied.Results are as follows:When rigid styrene-acrylate spheres was used as a template,suitable control of heterocoagulation between styrene-acrylate spheres and TiO₂ colloidal particle was favorable to the dispersion of styrene-acrylate spheres in the film as well as the formation of porous structure with certain wall thickness.The mass ratio of TiO₂ sol and styrene-acrylate emulsion affected the morphology of the pores;the aging time was a dominant factor for the formation of the dense pores on the surface; styrene-acrylate spheres could be eliminated effectively by the ultrasonic oscillation. The appropriate proportion was:15 g TiO₂ sol+0.2 g styrene-acrylate emulsion+10 g H₂O;the solutions are aged for 15 d,and the film was soaked into toluene and vibrated by ultrasonic for 10 min.When flexible BPA was used as a template,the resultant solution was a two phase mixture consisting of BPA and TiO₂ colloidal particles.The control of phase separation of these two substances in film could be achieved via adjusting the dosage of the components in the solution,and thus TiO₂ film with homogeneous and continuous porous structure was obtained.With the decrease of BPA concentration, the effect region of BPA was changed from the inside region of the film to the surface region of the film.Low concentration of TiO₂ colloidal particle and high content of ethanol was propitious to form porous structure.The appropriate proportion was:4 g TiO₂ sol+0.07 g BPA+10 g C₂H₅OH.The porosity enhanced the adsorption properity of the film,and more TiO₂ particles could contact with the pollutant molecule.So the photocatalytic activity was increased via the adsorption followed by photocatalytic oxidation.(3)Designed laminated coupled semiconductor at low temperature,increased photocatalytic activity of the film.Poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonic acid)(PEDT/PSS,an organic semiconductor)and SnO₂ were used to increase the photocatalytic activity of TiO₂ film respectively.The dual-layer coupled semiconductor film was prepared at low temperature(dried at 60 or 100℃),which used corresponding semiconductor film as an underlayer and TiO₂ film as a surface layer.The spectroscopy study on the film indicated that these dual-layer structure formed heterojunction,and there was an overlapping band in the coupled semiconductor film.The underlayer semiconductor induced a better charge separation due to electron transfer from TiO₂ layer to underlayer semiconductor,and thus the recombination of photon-generated electron-hole pair was suppressed.Hence,the photocatalytic activity of coupled semiconductor film was increased significantly compared with TiO₂ film,and the highest could increase about 3 times.(4)Established TiO₂-CuTsPc sensitizing system at low temperature,extended the light response region of the film.Copper(â…¡)phthalocyanine tetrasulfonic acid(CuPcTs)was added into TiO₂ sol, and phthalocyanine sensitized TiO₂ sol(TiO₂-CuTsPc sol)was synthesized via the adsorption of CuTsPc on the TiO₂ colloidal particles due to electrostatic attraction. TiO₂-CuTsPc film was then prepared by coating the resultant sol on the substrate at room temperature.The influence of water bath time and the CuTsPc concentration on the CuPcTs loading in the film were investigated,and the relationship between CuPcTs loading and photocatalysis was studied,Furthermore,the loading mechanism of CuTsPc was also discussed.The results showed that the sensitization of CuTsPc could extend the light response region of the film to 600-700 nm,and thus increase the visible-light photocatalytic activity of the film.UV-Vis absorption spectra indicated that prolonging the time in water bath or raising the CuPcTs concentration in TiO₂ sol were helpful for increasing the amount of CuPcTs supported in TiO₂ film.The visible-light photocatalytic activity of the film was increased with the amount of CuPcTs,but too much CuPcTs would decrease the photocatalytic activity.When 0.036 g CuPcTs was added into 20 g TiO₂ sol and the mixture was heated in the water bath at 80℃for 8 h,the photocatalytic activity could reach the highest.CuPcTs could be loaded in the film uniformly and firmly via adsorption on the TiO₂ colloidal particles,and this ensured the stability for repeated use of TiO₂-CuTsPc film.The photocatalytic activity of the film became stable after being used 2～3 times.