High Temperature Sintering Preparation Of TiO₂Photocatalytic Ceramics And Their Properties

Titanium dioxide （TiO₂） has been widely investigated as a heterogeneous photocatalystsince1972, due to its excellent functionality, long-term stability, low cost, and nontoxicity.Photocatalytic ceramics are expected to play a significant role in air purification andself-cleaning against growing environmental problems utilizing its photocatalytic activity andphotoinduced hydrophilicity. However, the metastable anatase phase tends to transform intostable rutile phase upon heating. Thermal stability of anatase TiO₂is lower when coated on theglazed ceramic surface compared to the powder alone due to the negative influence ofthe glaze.Therefore, to find a compromise between photoactivity （best at the lower firing temperature）and coating adhesion（best inhigher firing temperature） is the main goalofpractical applicationof photocatalytic ceramics. In this research, highly thermal stable anatase TiO₂powders weresynthesized and applied in ceramics. A novel method has been developed for the first time togenerate TiO₂from the decomposition reaction of titanium phosphate in the glaze.The anatase-to-rutile phase transformation is significantly inhibited by SiO₂orPO3-4modification. The phase transformationstart temperature ofTiO₂increases from550℃toabove950℃when SiO₂was introduced by the sol-gel process, KH₂PO₄was doped byimpregnation technology or mechanical mixing method, and phosphoric acid was introducedby hydrolysis precipitation method.Carbonates and silicates were doped inorder to investigatethe role ofoxides inthe crystalstructure of TiO₂. The results reveal that the common components of ceramic, such as Li2O,Na2O, K2O, MgO, CaO, ZnO, BaO, B2O3, and Al2O3,mainly have inhibitory effects to thegrowth of anatase and the phase transformation of TiO₂.The mixtures of TiO₂and glaze were obtained for the investigation of the phasetransformation of TiO₂in glazes. The results suggest that the start and finish phasetransformation temperatures of TiO₂synthesized by hydrolysis method and Degussa P25TiO₂in glazes are close to the softening temperature and sphere temperature of the glazes,respectively. However, the phase transformation behaviors of TiO₂in fused silica are similar tothat of alone. The eutectic liquid is essential for the phase transformation behavior of anataseTiO₂in glaze. The phase transformation behaviors of SiO₂-modified TiO₂and5wt%KH₂PO₄-modified TiO₂in fused silica and glazes are similar to that of pure TiO₂. TiO₂modified by phosphoric acid has a higher thermal stability in glazes than that ofKH₂PO₄-modified TiO₂.Photocatalytic ceramics were prepared by spin-coating phosphoric acid modified TiO₂ with a P/Ti molar ratio of1:2on the surface of glazed ceramics. Underglaze and firing systemhave little effects on the crystal structure of TiO₂on the surface of glaze layer, but have greateffects on the surface morphology and photoactivity. In addition, the improvement of firingtemperature is favorable for the gloss and abrasion resistance, but unfavorable forphotocatalytic activity and photoinduced hydrophilicity.Three titanium phosphate precursors with different P/Ti molar ratios were respectivelymixed with a low temperature frit （G2） and rapidly firing at different temperatures. TiO₂isfound in all three cases, but undergoes different phase transformation behaviors. The anatasephase is stable up to1000℃when titanium phosphate precursor with a P/Ti molar ratio of1:1is used. The reaction between titanium phosphate and sodium carbonate is related to the P/Timolar ratio of titanium phosphate and the relative amount of sodium carbonate. TiO₂is mainlypresent as the primary decomposition product in the case of P/Ti molar ratio of1:1, but mainlythe secondary decomposition product in the case of P/Ti molar ratio of2:1and4:1. From themicroscopic analysis, phosphate compound has a certainadsorptioncapacity for TiO₂particles,whicheffectivelyprevents the contact ofTiO₂particles and inhibits the phase transformationofTiO₂. The reaction process of titanium phosphate and sodium carbonate under the meltingtemperature of glaze can be regarded as an earlier stage of the sintering process of titaniumphosphate and glaze. On further heating, titanium phosphate compounds dissolve in the liquid,so that precipitation of TiO₂occurs. The inhibiting effect of phosphate on the anatase-to-rutilephase transformation is more significant for the primary TiO₂than that of secondary TiO₂.Photocatalytic ceramics were prepared by spin-coating titanium phosphate precursor witha P/Ti molar ratio of1:1onthe surface ofglazed ceramics and rapidly sintering at1000℃. TiO₂particles are covered by the matrix if directly coated on the low temperature glaze （C2）,whereas TiO₂particles are immobilizing on the surface of glaze with low densification ifdirectly coated on the high temperature glaze （C4）. The problem can be avoided by previouslycoating a thin G2layer on the surface of C4. Anatase TiO₂particles with a size range of about70-130nm are firmly embedded on the coating when rapidly sintered at1000℃for3min. Thedensified coating has a high gloss of77.8%, a good adhesion of rank5B, and a high pencilscratchhardness over6H, which has a veryclose qualityas compared to the surface ofordinaryglazed ceramics. Photocatalytic degradationofMO accomplishes97%after5hUV irradiation, and water contact angle ofthe surface decreases dramatically to9.9°after12h UV irradiation.High temperature sintering preparation of photocatalytic ceramics is achieved by thedecomposition of titanium phosphate in the glaze.