Probing the electronic and geometrical structure of nanomaterials by polarization-dependent X-ray absorption in scanning transmission X-ray microscopy

抄録

X-ray absorption near-edge structure (XANES) probes the unoccupied electronic states in a local, i.e. element and orbital-selective way. XANES analysis provides detailed information in the geometrical and electronic structure of active sites, complex matter and nanomaterials, especially when combined with transmission X-ray microscopy. Here we review our recent work on XANES on boron nitride ]1] and titanate nanostructures [2].

The structure of hexagonal boron nitride (hBN) and that of individual boron nitride nanotubes (BNNT) is analyzed using linearly polarized XANES and first-principles calculations. We find that hBN has AA' stacking in agreement with the literature. The spectra of the multi-wall BNNT show a pronounced polarization dependence which is qualitatively well reproduced in the calculations. We show that BNNT have a mixed stacking sequence which explains some of the differences seen between the hBN and BNNT line shapes.

Next we focus on nanostructured titanium dioxide in the TiO2–B phase, which is a promising anode material for lithium ion batteries. The structural and electronic changes between nanoribbons in TiO2–B and the thermodynamic stable anatase phase are studied using nanoscale XANES and first-principles calculations. The oxygen K-edge spectra of the two phases display marked differences which are very well reproduced in the calculations. Strong linear dichroism is observed in single nanoribbons, reflecting preferential O-2p to Ti-3d bond orientation in the low symmetry crystal structures. A simple bond counting model is developed which semiquantitatively accounts for the major dichroic effect. It is shown that the crystal orientation of the nanoparticles can be inferred from the dichroic spectra.

[1] Polarization dependent X-ray absorption near-edge spectra of boron nitride nanotubes, Peter Krüger, Yuya Maekawa, Adam Hitchcock and Carla Bittencourt, Radiat. Phys. Chem. 175, 108129 (2020).

[2] Chemical Bond Modification upon Phase Transformation of TiO2 Nanoribbons Revealed by Nanoscale X-ray Linear Dichroism, P. Krüger, M. Sluban, P. Umek, P. Guttmann and C. Bittencourt, J. Phys. Chem. C 121, 17038-17042 (2017)