Rutile to Cubic Phase Transformation Induced By Electron Beam

Abstract

Cubic TiO2 was predicted to be a great visible light absorption material partially due to the smallest bandgap among all the TiO2 polymorphs. The cubic TiO2 can be synthesized using the common TiO2 polymorph i.e. anatase as a precursor, but it mixes with other metastable phases as byproducts including baddeleyite, TiO2 (OI) and TiO2 (OII). The traditional syntheses of cubic TiO2 have to be performed at a high-temperature and high-pressure conditions. Such harsh condition limits the potential applications of the cubic TiO2. In this work, cubic TiO­2 was grown on the surface of a rutile nano-template at room temperature in vacuum. The rutile precursors were made by pulsed laser deposition (PLD). As shown in Figure 1, the growth of cubic TiO2 was induced by the electron beam in an aberration corrected scanning transmission electron microscopy (STEM). Monochromated electron energy loss spectroscopy (EELS) was used to investigate the phase transformation under the electron illumination. The STEM images show that lattice matching relationships are (001)R // (001)C and [100]R // [100]C with only 4.6% lattice mismatch, where R and C refer to rutile and cubic, respectively. Density functional theory (DFT) reveals that rutile-to-cubic TiO2 transformation can be highly favorable if the strain is > 10% at the interface. The driving forces originate from three resources: (1) lattice mismatch at the interface; (2) defective structure of the black rutile; (3) oxygen vacancies created by the electron beam. In addition to knock out oxygen, the electron beam also provides energy for the Ti atomic displacement in the rutile during phase transformation. Figure 1. Phase transformation from rutile to cubic TiO2 by electron beam exposure. Figure 1