Phase transition of layered titanate under supercritical N,N-dimethylformamide and supercritical water

Supercritical fluid exfoliation of layered titanate is an efficient way to prepare titania nanosheets. However, a phase transition may occur under high temperature and high pressure conditions. We have studied the phase transition process of H1.07Ti1.73O4·H2O in two solvents (N,N-dimethylformamide (DMF) and water) under supercritical conditions by XRD measurements. Experimental results show that exfoliation is found in supercritical DMF, while in supercritical water exfoliation and the phase transition occur concurrently, transforming it into anatase. An interpretation of the phase transition is proposed: as water is a protic solvent, it can supply H+ under high temperature and high pressure conditions; the phase transition is caused by dissolution and recrystallization, which is confirmed by XRD and SEM results. The suitable solvents shall be selected for the preparation of titania nanosheets by supercritical exfoliation.