Revealing the high-temperature stability and phase transformation in isolated TiO2 nanotubes using in situ heating TEM

Abstract

High crystallinity is key to the performance of most electronic materials, especially semiconductors. When derived from amorphous phase, heat treatment is commonly used for achieving the desired structural order and crystal phase. Understanding thermally driven phase transformations is essential for assessing material viability. Titanium dioxide (TiO₂) nanotube arrays, fabricated via anodic oxidation, are a promising low dimensional wide band-gap semiconductor potentially useful for a myriad of applications. Nonetheless, previous studies primarily focused on the array film geometry, obscuring the nanoscale processes in single nanotubes. Herein, we report the efficacy of in situ heating transmission electron microscopy and electron energy loss spectroscopy to dynamically elucidate the processes of crystallization and phase transformation in a single amorphous TiO₂ nanotube. The study reveals that crystallization initiates at 300 °C to form anatase and rutile phases, with brookite emerging at 550 °C. Remarkably, the nanotubes retain this unique three-phase structure even at 950 °C. Supported by ex situ X-ray diffraction and Raman spectroscopy results, this systematic and real-time exploration demonstrates that a complete transition to rutile, the most stable phase, could be prevented in isolated nanotubes, unlocking new applications.