Preparation And Photocatalytic Ability Of Titania With Ordered Spherical/Macroporous Micro-nano Structure

Recently,semiconductors have been applied widely in photodegradation of the environmental pollutants. The photocatalysis process utilizing semiconductor photocatalyst shows many advantages, such as direct utilization of the solar rays, mild reaction conditions and production only CO₂ and H2O. Therefore they have been studied extensively in the photocatalytic field. Titania is one of the most promising photocatalysts. It can completely destruct the undesirable contaminants (pollutants) in both liquid and gaseous phase by using solar or artificial light illumination. However it has some disadvantages:(1) The efficiency of photocatalytic reaction is limited by the absorption capability for visible light of titania and by the high recombination rate of photoinduced electron hole pairs formed in the photocatalytic process. The low light quantum yield limits its practical application. (2) The band gap of anatase titania is 3.2ev. Therefore it can only absorpt UV light with the wavelength below 387nm, which accounts for only 4% of solar rays. (3) Most photocatalyst is powder. In the using process, the situations of inactivation and agglomeration often happen. Furthermore it is difficulty to recycling use. In order to realize the practical application, we must solve the immobilization problem of photocatalyst. But after the photocatalyst is immobilized, special surface area is reduced. The adsorption of dye and utilization efficiency of solar is decreased. Therefore the photocatalytic activity is low. In order to realize the practicality of photocatalysis technology, it is crucial to improve the photocatalytic effect and solve the immobilization problem. We must discover a new photocatalyst with high photocatalytic activity, and it is easy to disperse and recycle.In this paper, titania with ordered spherical/macroporous micro-nano structure were prepared by colloidal crystal templating process and sol-gel technology on the basis of reviewing many literatures, which consists of ordered hollow Ag/TiO₂ spheres, ordered hollow TiO₂ spheres, TiO₂ inverse opal and TiO₂/Ag inverse opal. We also investigated their photocatalytic ability.The preparation of template is the basis of this template-directed method. Monodisperse polystyrene spheres with diameter of 216nm were prepared by emulsifier-free polymerization, and then assembled into template by one-substrate vertical deposition at room temperature or 4℃. At room template, hexagonal array appears in most part of the template. At a lower temperature such as at 4℃, relatively large areas of square array were observed in the formed colloidal crystal. It is because that thermal motion of particles is inhibited at low temperature and dislocation is happened. We also compared two deposition methods, one-substrate vertical deposition and two-substrate vertical deposition. The positions of PBG with different incident angle of light were calculated using Bragg's law. The transmission spectrum shows that the experimental result is in good agreement with the calculation. The position of the PBG shifted to shorter wavelengths as the incident angle of light increased. It proved that the structure of PS template is high- quality.The colloidal crystal template was coated with Ag by an electroless plating method. The silver shell was coated for different time, and the morphologies are compared. Subsequently the resultant PS/Ag spheres and PS spheres were coated with TiO₂ by sol-gel technology. Finally we successfully prepared ordered hollow Ag/TiO₂ spheres and ordered hollow TiO₂ spheres. From the scanning electron microscope, we find that the morphology of PS/Ag is crucial to that of the resultant ordered hollow Ag/TiO₂ spheres. Sometimes square array was observed in the samples of PS/Ag spheres and ordered hollow Ag/TiO₂ spheres. XRD and XPS patterns show that titania is anatase and silver is existed as simple substance. The successful preparation of ordered hollow structure supplies a new way for other ordered hollow materials.We prepared high-quality titania inverse opal by combining colloidal crystal template with an two-substrate infiltration technique. Furthermore, we also prepared TiO₂/Ag inverse opal by depositing silver nanoparticles. Compared with the dropwise infiltrated sample, the optimized sol-gel infiltration method can obtain high-quality titania inverse opals with close packing open-pore structures. Titania precursors with different concentration were prepared, the resultant samples from these precursors were compared. The experiment results prove that low concentration titania precursor is more suitable for the structure because of the high permeability. The transmission spectra of the resultant titania inverse opal and the dropwise infiltrated sample were measured. Compared with the dropwise infiltrated sample, the titania inverse opal exhibits more obvious characteristics of photonic crystal, which is attributed to the uniformity of the sample morphology as well as the elimination of large area structural distortion.We evaluated the photocatalytic ability of ordered hollow Ag/TiO₂ spheres. For comparison, we also evaluated the photocatalytic activity of flat TiO₂ film, disordered porous Ag/TiO₂ film and ordered hollow TiO₂ spheres. The results show that the as-prepared ordered hollow Ag/TiO₂ sphere exhibits more efficient photocatalytic activity. The differences between ordered hollow Ag/TiO₂ nanostructure and other reference samples are the existence of ordered hollow nanostructure and the addition of Ag nanoparticles. The porous nanostructure can provide large surface area, which increases the absorption of dye and accelerates the reaction rate. When the hollow spheres are arranged in ordered hexagonal structure, and the size of the spheres is comparable to the wavelength of UV light, the interaction of light with the sample is enhanced. Furthermore, ordered hollow structure is composed by nanocrystal titania of 20nm. Small size can increase surface area, therefore improve photocatalytic ability. The appropriate amount of Ag nanoparticle deposition can reduce the recombination of electron and hole, increase the hydroxyl radical. Therefore it has a direct effect on the photocatalytic activity. We also evaluate the photocatalytic activity of the resultant TiO₂ inverse opal and TiO₂/Ag inverse opal obtained by two-substrate infiltration technique. Compared with the dropwise infiltrated sample, the optimized titania inverse opal exhibits higher photocatalytic activity. It indicates that high-quality inverse opal structure is very important to improve the photocatalytic efficiency. Because of its inherent structural and physical properties, such ordered nanostructure can offer large surface area and porosity, increasing the absorption of dye and accelerating the diffusion rate. Furthermore, slow-photon is another factor leading to the improved photocatalytic efficiency. The reproducibility of the photocatalyst indicates the photocatalytic activity of the sample has not decreased in the several reactions. The optimized titania inverse opal as a photocatalyst has the advantages of enhanced photocatalytic activity and good reproducibility. The photocatalytic activity of TiO₂/Ag inverse opal is a little better than TiO₂ inverse opal. We believe that the coverage of silver nanoparticles decrease the irradiation on titania surface, which leads to the loss of light energy. As a new photocatalysts, titania with ordered spherical/macroporous micro-nano structure exhibits higher photocatalytic activity compared with titania film and disordered porous Ag/TiO₂ film. Furthermore it can solve the immobilization problem. The photocatalyst can be dispersed and recycled conveniently. If it can be further investigated and be utilized in practical application, it will play an important role in environment pollution.