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.