Tunable Phase Transformation Behavior of Two-Dimensional TiO2 Revealed by In Situ Transmission Electron Microscopy

The polymorphs and the phase transformation process of two-dimensional (2D) oxide materials, which are the crucial building blocks of high-performance nanodevices, are rarely studied. Here, we directly observed the atomic structural evolution of anatase-to-rutile phase transformation of TiO2 nanosheets using the in situ heating transmission electron microscopy (TEM) technique. The transformation process of the [010] direction and the (013) plane of anatase to [11̅0] and (111̅) of rutile was captured by electron diffraction. The phase transformation was highly affected by the as-prepared nanosheet surface structure by the templating effect. In addition, the dependence of crystallographic orientation with respect to electron beam irradiation was also revealed. Through a different path, the [010] direction and the (004) plane of anatase were transformed to [11̅0] and (111̅) of rutile with nucleation from the edge of the sheet by high-resolution TEM characterization, which had a high electron dose rate. The electron energy loss spectrum (EELS) obtained at different temperatures suggested the reduced valence of Ti, which was possibly due to the formation of oxygen vacancies. This finding could shed light on the unique structure transformation behavior of 2D oxides and the effective control of their polymorph nanostructures by both temperature and electron irradiation, which could facilitate the discovery of new functionalities.