Chemical modification of TiO2 by H2PO4-/HPO42- anions using the sol-gel route with controlled precipitation and hydrolysis: enhancing thermal stability

Two titanium phosphate materials (T (p) P and T (h) P) have been successfully synthesized by sol-gel route with controlled precipitation and hydrolysis. The T (p) P material was obtained from the reaction between precipitated titania and phosphate buffer solution H2PO (4) (-) /HPO (4) (2-) (pH = 7.3). The T (p) P material was prepared through hydrolysis of titanium in the presence of H2PO (4) (-) /HPO (4) (2) . The probable state of the phosphate anions in titania framework and their effect on the anatase-to-rutile transformation were characterized by ICP-AES, DTA-TG, P-31 NMR, FT-IR, and Raman analysis HRTEM/SEM. FT-IR and P-31 NMR analyses of titanium phosphate T (p) P calcined at low temperature showed that the phosphate species existed not only as Ti-O-P in the bulk TiO2 but also as amorphous titanium phosphates, including bidentate Ti(HPO4)(2) and monodentate Ti(H2PO4)(4). Increased calcination temperature only gave an enrichment of bidentate structure on the titania surface. For the T (p) P material, H2PO (4) (-) /HPO (4) (2-) anions were introduced into the initial solution, before precipitation, what promoted their lattice localization. At high temperatures, all the phosphorus inside the bulk of TiO2 migrated to the surface. The Raman analysis of both samples showed that the bidentate phosphates increased the temperature of the anatase-to-rutile phase transformation to more than 1000 A degrees C with the formation of well crystalline TiP2O7 phase. This phenomenon was more evident for T (p) P sample.