Revisiting the dissolution-recrystallization mechanism of rutile growth from protonated titanate nanotubes

Rutile, as the most stable natural phase of TiO₂, is an important industrial chemical. The hydrothermal phase transition from protonated titanate towards rutile is a promising method for the low-temperature synthesis of rutile not only owing to the modest reaction conditions but also the possibility to achieve rutile with a high surface-area. Previous studies have proposed a dissolution-recrystallization growth mechanism to explain such phase transitions, based on indirect evidence. However, the lack of direct proof leaves the detailed mechanism to still be clarified and makes the precise control of rutile synthesis an impossible task. Herein, the solubility of protonated titanate in acid was thoroughly studied with time-tracking experiments, using combined spectroscopies, electron microscopy and other techniques, under conditions that are relevant for the phase transition towards rutile. We achieved the complete dissolution of 0.1 M (based on Ti) PTNs in 3 M HCl within 12 h. Hydrothermal treatment of the formed solution yielded rutile with a rod-like morphology with an average diameter of about 9 nm, similar to the morphology of the direct hydrothermal product of protonated titanate nanotubes. These observations indicated that dissolution of titanate is one key step in rutile growth. This was further proven in systems with a higher feeding amount of protonated titanate nanotubes and in systems with commercial P25 TiO₂ as the Ti source. Our findings verified that the growth followed a dissolution-recrystallization mechanism by revealing the previously overlooked solubility of titanate.