The Preparation Of TiO₂-based Porous Films And The Application In The Photocatalytic Reduction Of CO₂

As we all known, current energy shortages and environmental issues have became theworldwide focus in recent years. Degradation of organic pollutants through photocatalyticoxidation and converting them to a useful material by photocatalytic reduction is a potential wayto solve the environmental pollution problems. Especially, the photocatlytic reduction of CO2into hydrocarbon is a process of converting waste materials to useful one. On the one hand, itcould alleviate the green house effect. On the other hand, it is essential in developing alternativefuels and providing various raw materials for different industries. Many researchers have shownthat CO2could be reduced by water vapor or solvent with photocatalysts. TiO2nanomaterials arethe promising and prospective candidate owing to its advantages, such as strong oxidized ability,stable chemical properties, environmentally friendly, low cost and obtained easily. However, thelower utilization of solar light and the higher combination efficiency of the photo-generatedelectron-holes are still the most important challenges for the chemical scientists. So it is essentialto modify TiO2to improve the photocatalytic efficiency.TiO2thin films obtained by conventional methods often have small surface area, however，the titanate porous network nanotube films synthesised by hydrothermal method have largersurface areas, stronger adsorption capacity and the higher efficiency of light. If using differentmethods of modification，we can obtained the TiO2film with special morphology and structureof large specific surface area. Therefore, in this thesis, depositing the noble metal、coupling withother semiconductors and cocatalysts were used to improve the photocatalytic reduction efficiency of CO2to hydrocarbon fuel. The detailed contents are as follows:(1) Anatase TiO2nanosheet porous films were prepared by calcination of the orthorhombictitanic acid films at400℃. They showed an excellent photocatalytic reduction activity for CO2tomethane, which should be related to their special porous structure and largeBrunauer-Emmet-Teller (BET) surface area. In order to further improve the photocatalyticactivity, Pt nanoparticles were loaded on the TiO2porous films by the photo-reduction method. Itwas found that the loading of Pt expanded the light absorption ability of the porous film, andimproved the transformation efficiency of CO2to methane. The conversion yield of CO2tomethane on Pt/TiO2film from3.71ppm/h cm2reached to20.51ppm/h cm2. Moreover, thetransient photocurrent-time curves showed that the Pt/TiO2nanosheet porous film exhibited ahigher photocurrent, indicating that a higher separation efficiency of the photo-generated chargecarriers was achieved.(2) In order to improve the efficiency of CO2reduction, a facile development of highlyefficient MgO/TiO2network nanotubes films via dip-calcination method is described. It wasfound that they exhibit relatively high CO2photo-reduction efficiency with selective formationof methane compared with pure TiO2network nanotube film. Especially, the0.01-MgO/TNTsexhibited the maximum CH4yield (25.12ppm/h cm2). In particular, MgO plays a critical role inphotoreduction of CO2to CH4. Because it has the strong adsorption ability of CO2and initiatesthe reaction by binding a CO2molecule, forming a magnesium carbonate species on the surface.For further improve the photocatalytic activity, Pt nanoparticles were loaded on0.01-MgO/TNTsfilms by the photo-reduction method. It was found that the loading of Pt notably improved thetransformation efficiency of CO2to methane and the conversion yield reached100.22ppm/h cm2. Therefore，the synergy between Pt and MgO in the nanocomposite played an important role inCO2photocatalytic reduction.(3) TiO2/SrTiO3composite film was in situ grown using the orthorhombic titanic acidnanotubes (TAN) network film as the precursor. It was found that the size and crystallinity ofSrTiO3nanocubes increased with the reaction time increasing, and TAN completely transferredinto SrTiO3when the reaction time is3h. The TiO2/SrTiO3composite network films obtainedwith1h exhibit the best photocatalytic performance for photoreduction of CO2under Hg lampirradiation. All of characterization interpreted that the distribution of SrTiO3nanocubes on thesurface of TiO2nanotubes was uniform and formed close contact between SrTiO3and TiO2,which is beneficial for the fast separation of photo-generated electron-holes, so as to suppress therecombination efficiency. Besides this, the1D nanotube network structure of TiO2/SrTiO3composite, large BET surface areas, and strong adsorption ability were advantageous for thephotocataytic reduction of CO2. And the hierarchical structure promotes to utilizing more lightabsorption. After loading noble metals by photo-reduction method, the reduction efficiency ofCO2to CH4was remarkably improved than as-synthesised TiO2/SrTiO3composite.