MS-1-P-2094 Direct observation of Ti vacancies in Ti0.87O2 nanosheet using low-voltage monochromated TEM
Titania nanosheets [1] are two-dimensional single crystals of a titanium oxide with a thickness of one titanium or two oxygen atoms (Fig. 1a), and they show attractive material properties, such as photocatalytic reactions. The titania nanosheets are synthesized from a layered titanate K0.8Ti1.73Li0.27O4 through a soft chemical procedure (i.e., delamination), and the atomic arrangement of Ti-O layers in the parent crystal are basically preserved in the titania nanosheets. The nanosheets have the composition of Ti0.87O2, including cation vacancies at Li-substituted Ti sites of the parent crystal. In general, atomic vacancies affect the stability of crystal structures and material properties; therefore, it is important to reveal the atomic structure around Ti vacancies and their distribution in the nanosheets.
The observation of atomically thin materials requires not only high spatial resolution but also high sensitivity and low irradiation damage. We found that oxide nanosheets are substantially beam-sensitive, in contrast to a graphene and related materials. For instance we reported the topotactic reduction of a Ti0.87O2 nanosheet to Ti2O3 nanosheet [2].
We performed low-voltage and low-dose TEM observation using Titan3 at 80 kV with an image corrector (CEOS, CETCOR) and a monochromator, whose energy spread is 0.1 eV (FWHM). Attainable information limit under this condition was found to be 90 pm [3]. Figure 1b shows a high-resolution TEM image observed under underfocused condition [4]. The TEM image shows several bright areas as indicated by arrows, and we integrated these portions of the TEM image contrast (Fig. 2a). Based on the experimental results we constructed Ti vacancy structure models, and the atomic positions were optimized using first-principles calculation (the CASTEP code) as shown in Fig. 2b. The multislice simulation result based on the model successfully reproduces the experimental result (see Fig. 2c), and we found that the two oxygen atoms near the Ti vacancy are considered to be desorbed during the TEM observation [4].
[1] T. Sasaki, et al., J. Am. Chem. Soc. 118 (1996) 8329. [2] M. Ohwada, et al., J. Phys. Chem. Lett. 2 (2011) 1820. [3] K. Kimoto, et al., Ultramicrosc. 134 (2013) 86. [4] M. Ohwada, et al., Scientific Reports 3 (2013) 2801.
We thank Drs. Nagai, Ishizuka, Inoke, Lazar, Freitag, Sato and Suenaga for invaluable discussions. This work is supported by Nanotechnology Platform of MEXT and Research Acceleration Program of JSPS.
Fig. 1: A crystal structure model of Ti0.87O2 nanosheet (a), and a high-resolution TEM image of a Ti0.87O2 nanosheet. The TEM image is observed under underfocused condition. White arrows in Fig. 1b indicate several bright areas, suggesting Ti vacancies. |
Fig. 2: (a) Experimental TEM image which is obtained as an average of the portions in Fig. 1b. (b) The atomic arrangement near the Ti vacancy optimized using the first-principles calculation. (c) The multislice simulation of a TEM image based on the optimized structure model. |