Song Miao, Lu Zexi, Li Dongsheng
Physical and Computational Sciences Directorate, Pacific Northwest National Laboratory, Richland, WA 99352, USA.
Nanoscale. 2020 Nov 26;12(45):23183-23190. doi: 10.1039/d0nr06226j.
Polymorphs widely exist in nature and synthetic systems and are well known to determine material properties. Understanding phase transformation mechanisms among polymorphs enables the design of structures and tuning of phases to tailor material properties. However, current understanding is limited due to the lack of direct observations of the structural evolution at the atomic scale. Here, integrating (semi) in situ transmission electron microscopy and density functional theory, we report atomic structural evolutions of phase transformation from anatase (A) to rutile (R), brookite (B), R-phase, and TiO. Besides the consistent paths with previous reports, we discover several unreported paths, including a [001] direction and (020) plane of anatase to [100]R and (01[combining macron]1)R of rutile, respectively, ([001]A||[100]R, (020)A||(01[combining macron]1)R) and [001]A||[001]B, (020)A||(220)B. Density functional theory analysis elucidates atomic structural evolution during the processes and over 16% of Ti-O bonds break and reform during the processes with energy barriers of ∼0.7-1.0 eV per TiO2 formula unit. Under electron-beam irradiation, anatase particles transform into TiO2-R phase or TiO at high or room temperature, respectively. We also reveal the anisotropic nature of the electron-beam effect, which is seldom discussed: dependence of crystallographic orientation with respect to electron-beam irradiation direction. Understanding the atomic structural evolution sheds light on interpreting and controlling TiO2 polymorphs and intermediate structures for various applications. The revealed electron-beam effects in our work provide guidance for in situ transmission electron microscopy studies.
多晶型物广泛存在于天然和合成体系中，众所周知，它们决定着材料的性能。了解多晶型物之间的相变机制有助于设计结构和调节相以定制材料性能。然而，由于缺乏对原子尺度结构演变的直接观察，目前的认识有限。在此，我们结合（半）原位透射电子显微镜和密度泛函理论，报道了从锐钛矿（A）到金红石（R）、板钛矿（B）、R相和TiO₂的相变的原子结构演变。除了与先前报道一致的路径外，我们还发现了几条未报道的路径，包括锐钛矿的[001]方向和（020）面分别转变为金红石的[100]R和（01̅1）R（[001]A||[100]R，（020）A||（01̅1）R）以及[001]A||[001]B，（020）A||（220）B。密度泛函理论分析阐明了过程中的原子结构演变，在此过程中超过16%的Ti-O键断裂并重新形成，每个TiO₂化学式单元的能垒约为0.7-1.0 eV。在电子束辐照下，锐钛矿颗粒在高温或室温下分别转变为TiO₂-R相或TiO。我们还揭示了电子束效应的各向异性，这一点很少被讨论：晶体取向相对于电子束辐照方向的依赖性。了解原子结构演变有助于解释和控制用于各种应用的TiO₂多晶型物和中间结构。我们工作中揭示的电子束效应为原位透射电子显微镜研究提供了指导。
