Preparation And Properties Of ZrSiO₄ @ TiO₂ Photocatalytic Composites

Titanium dioxide is a widely used photocatalyst.The crystal phase of titanium dioxide with high photocatalytic activity is anatase titanium dioxide or the mixed crystals of anatase and rutile titanium dioxides. The photocatalytic activity of pure rutile titanium dioxide is very low, and the anatase to rutile phase transition temperature is also very low. Therefore, the key for anatase titanium dioxide to apply in photocatalytic ceramics is to improve its high temperature stability and the ability to withstand high temperature chemical attack. In this paper, anatase TiO₂ nanoparticles and porous ZrSiO₄ powder were synthesized using hydrothermal method. ZrSiO₄ @ TiO₂ photocatalytic material was prepared by self-assembling these two kinds of powders which were modified by phytic acid. The photocatalytic ceramics were prepared by the photocatalytic materials. XRD, SEM, TEM, DSC, DLS, XPS and BET were used to analyze phase composition, microstructure, partical size and relevant mechanism. The photocatalytic activities of the test materials were characterized by the method to measure degradation ratios of methyl orange solution in the UV. The relevant mechanisms have been discussed.TiO₂ sol was prepared using tetrabutyl orthotitanate (TBOT) as starting material and diethanolamine (DEA) as chelating reagent. The effects of processing temperature and soaking time on the crystallization, particle size distribution and dispersibility of TiO₂ were investigated. Titania-silica composites were formed after adding tetraethoxysilane (TEOS) into TiO₂ sol by hydrothermal processing. The composites were uniform mixed oxides of anatase TiO₂ and amorphous SiO₂ on a nanometer scale. Titania-silica composites with 1:1 molar ratio were maintained as anatase after treatment at 1200℃for 2h because silica inhibited the transition of anatase to rutile and abnormal grain growth of titania nanocrystals. The photocatalytic activity of the composites after treatment at different temperatures was much higher than that of TiO₂ (P25) treated at the same conditions.Porous zircon ultrafine powders were manufactured using zirconium oxychloride(ZrOCl₂), TEOS as starting materials and NaF as mineralizer. The higher content of NaF and rising temperature of hydrothermal treatment led to higher crystallinity of zircon. The increment of crystallinity of zircon decreased with the longer holding time. The morphology of zircon was greatly affected by the pH values. The porous zircon exhibited high specific surface area with a bimodal pore size distribution. Porous zircon ultrafine powders were nestle-like platelets composing of nanofibers. The synthetic mechanism of zircon porous powders was probably not Ostwald ripening but the typeâ…¡aggregation growth of the nanocrystals. The porous zircon exhibited quite good high temperature stability. The crystal phase, morphology and specific surface area did not change obviously after treatment at 750℃for 0.5h. However, the crystal phase was also maintained with significant nanofiber growth and sharp reducement of specific surface area after treatment at 900℃for 0.5h. There was an obvious exothermic reaction at ca. 850℃with some weight loss because of emission of fluorine in the zircon.ZrSiO₄ @TiO₂ composites were self-assembled using phytic acid modified porous zircon and nanotitania powders. The pH value influenced surface charges of the two kinds of particles, which played a key role in the formation of uniform TiO₂ coating on zircon powders. TiO₂ nanoparticles encapsulated ZrSiO₄ ultrafine powders uniformly and penetrated into their slits. The combination of TiO₂ and ZrSiO₄ were strong, therefore, anatase-type titania was stabilized at high temperature. Photocalytic glazed ceramic tiles were obtained by firing tiles with composites coating at moderate high temperature of 900℃for 10min. Both ZrSiO₄ @TiO₂ composites and photocatalytic ceramics showed quite good photocatalytic performance.Cheap silica sol can replace TEOS as silica source to synthesize porous zirconium silicate. Phosphoric acid(H₃PO₄) can replace the expensive phytic acid as surface modifier to self-assemble ZrSiO₄ @TiO₂ photocatalytic composite powders. Both of them can reduce the production costs significantly and are favorable to the promotion of their application.