Unexpected Phase Stability of Anatase TiO2 Nanocrystals at Ultrahigh Temperature via Surface Restructuring

The phase stability of nanocrystals is of great importance to its performance in different applications, and extensive studies have been devoted to understanding the nature of superheating/supercooling, to achieve the desired long-term phase stability in practice. While the fundamental mechanism remains elusive due to the absence of atomic-level insights into dynamic structural evolution of nanocrystals under extreme conditions. Herein, through in situ atomic level spherical aberration-corrected scanning transmission electron microscopy we revealed an abnormal phase stability of the individual single crystalline anatase TiO ₂ at ultra-high temperatures governed by surface effects. The in situ atomic level observations at the exceptionally high temperatures show a highly anomalous phenomenon that the individual anatase TiO ₂ nanorod single crystal could maintain the anatase phase at 1250°C, instead of turning into the rutile phase, which exceeded the reported anatase-to-rutile phase transition point nearly 650°C higher. As the temperature rises above the phase transition point, the surfaces of anatase TiO ₂ nanorods undergo a series of atomic reconstructions, which not only reduce the total energy of the system, but also act as a kinetic “surface locking” effect preventing the rutile nucleation. This work shows that surfaces could have a critical effect on the phase stability of single crystalline nanomaterials, and the proposed “surface locking” mechanism opens up a new way for manipulating the thermal stability of nanocrystals.