Acoustic Shock\nWave-Induced Rutile to Anatase Phase\nTransition of TiO<sub>2</sub> Nanoparticles and Exploration of Their\nUnconventional Thermodynamic Structural Transition Path of Crystallization\nBehaviors

Titanium dioxide (TiO<sub>2</sub>) is one of the most\nwell-known\nand long-standing polymorphic materials in the transition metal oxide\ngroup of materials. The transition from rutile to anatase is one of\nthe long-standing fundamental questions among materials science researchers\nbecause seeking the nucleation site at the beginning of the phase\ntransition is highly challenging. Until now, there have been no studies\non the unconventional structural phase transition of TiO<sub>2</sub> nanoparticles by acoustic shock waves. In the present study, this\nwork provides the first evidence on the solid-state nanostructure\nof the rutile-to-anatase phase transition of TiO<sub>2</sub> by acoustic\nshock waves whereby these phase transition results are evaluated by\nRaman spectroscopy, thermal calorimetry, X-ray photoelectron spectroscopy,\nand microscopic techniques. We propose a novel mechanism for the occurrence\nof the rutile-to-anatase phase transition based on thermophysical\nproperties and shock wave-induced melting concepts. Under shocked\nconditions, the R–A phase transition occurs because of the\nanatase phase’s lower interfacial energy (γ<sup>L/A</sup>) and surface energy compared to rutile. We strongly believe that\nthe present work can provide in-depth insight into understanding the\ncrystallization concepts of the TiO<sub>2</sub> NPs under extreme\nconditions, especially with regard to the rutile-to-anatase phase\ntransition.