Acoustic Shock Wave-Induced Rutile to Anatase Phase Transition of TiO₂ Nanoparticles and Exploration of Their Unconventional Thermodynamic Structural Transition Path of Crystallization Behaviors

Titanium dioxide (TiO₂) is one of the most well-known and long-standing polymorphic materials in the transition metal oxide group of materials. The transition from rutile to anatase is one of the long-standing fundamental questions among materials science researchers because seeking the nucleation site at the beginning of the phase transition is highly challenging. Until now, there have been no studies on the unconventional structural phase transition of TiO₂ nanoparticles by acoustic shock waves. In the present study, this work provides the first evidence on the solid-state nanostructure of the rutile-to-anatase phase transition of TiO₂ by acoustic shock waves whereby these phase transition results are evaluated by Raman spectroscopy, thermal calorimetry, X-ray photoelectron spectroscopy, and microscopic techniques. We propose a novel mechanism for the occurrence of the rutile-to-anatase phase transition based on thermophysical properties and shock wave-induced melting concepts. Under shocked conditions, the R–A phase transition occurs because of the anatase phase’s lower interfacial energy (γ^L/A) and surface energy compared to rutile. We strongly believe that the present work can provide in-depth insight into understanding the crystallization concepts of the TiO₂ NPs under extreme conditions, especially with regard to the rutile-to-anatase phase transition.