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

Abstract

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 low-temperature synthesis of rutile, not only due to the modest reaction condition, but also the possibility to achieve the synthesis of rutile with high surface-area. Previous studies have proposed a dissolution-recrystallization growth mechanism to explain such phase transitions, based on indirect evidences. Lack of direct proof leaves the detailed mechanism to be clarified, and makes the precise control of rutile synthesis an impossible task. Herein, the solubility of protonated titanate in acid is thoroughly studied with time-tracking experiments, using combined spectroscopy, electron microscopy and other techniques, under conditions that are relevant to the phase transition towards rutile. We find that complete dissolution of 0.1 M (based on Ti) PTNs is achieved in 3 M HCl within 12 h. Hydrothermal treatment on the formed solution yields rutile with rod-like morphology and average diameters about 9 nm, that are similar to the direct hydrothermal product of protonated titanate nanotubes, in terms of their morphologies. These observations lead to a conclusion that dissolution of titanate is one key step of rutile growth. This conclusion is further proven in systems with higher feeding amount of protonated titanate nanotubes, and in systems starting from commercial P25 TiO2 as Ti source. Our finding verifies a dissolution-recrystallization mechanism, by revealing a previously overlooked solubility of titanate.