Crystallization of anatase TiO2 in niobium potassium phosphate glasses

Abstract

In this work, the glass forming ability was studied in potassium phosphate glasses with increasing amounts of TiO2 in order to obtain a glass-ceramic with photocatalytic properties. The first studied series has been the binary system (100-x) KPO3-xTiO2. Homogeneous and transparent glasses could be obtained with x varying from 10 to 30 mole%. Since the photocatalytic anatase phase could not be precipitated in this system, the ternary system KPO3-TiO2-Nb2O5 was investigated in order to incorporate higher TiO2 contents without spontaneous crystallization under cooling. Thermal properties of all glass samples were investigated by DSC and allowed identifying an increase of glass transition temperatures with increasing TiO2. For all compositions, exothermic events related with crystallization were also observed and suitable heat-Treatments resulted in specific crystalline phases identified by X-ray diffraction. Selective precipitation of the anatase titanium oxide was successfully obtained from the glass compostion 35KPO3-25Nb2O5-40TiO2 (KN25T40) after heat treatment at 720°C for 2 h, suggesting the possibility of obtaining a glass-ceramic for photocatalytic applications. Structural investigations by Raman were also performed on glasses and glass-ceramics and allowed to point out the glass intermediary behavior of TiO2 in the phosphate vitreous network where TiO4 and TiO6 octahedra are inserted inside the phosphate network with TiO6 clusters identified at higher TiO2 contents. Raman analysis also identified anatase TiO2 in the KN25T40 glass-ceramic.