Towards atomic resolution in sodium titanate nanotubes using near-edge X-ray-absorption fine-structure spectromicroscopy combined with multichannel multiple-scattering calculations

Recent advances in near-edge X-ray-absorption fine-structure spectroscopy coupled with transmission X-ray microscopy (NEXAFS–TXM) allow large-area mapping investigations of individual nano-objects with spectral resolution up to E/ΔE = 104 and spatial resolution approaching 10 nm. While the state-of-the-art spatial resolution of X-ray microscopy is limited by nanostructuring process constrains of the objective zone plate, we show here that it is possible to overcome this through close coupling with high-level theoretical modelling. Taking the example of isolated bundles of hydrothermally prepared sodium titanate nanotubes ((Na,H)TiNTs) we are able to unravel the complex nanoscale structure from the NEXAFS–TXM data using multichannel multiple-scattering calculations, to the extent of being able to associate specific spectral features in the O K-edge and Ti L-edge with oxygen atoms in distinct sites within the lattice. These can even be distinguished from the contribution of different hydroxyl groups to the electronic structure of the (Na,H)TiNTs.


Broadening of the O K-edge spectra
Following the prescription used in the MXAN code [1] the O-K edge spectra were broadened with a Lorentzian function of energy-dependent width Γ(E)=Γ c +A s (0.5 + arctan((E−E s )/b)/π). Here Γ c is the natural width of the O 1s hole (0.156 eV) and E s is the step position, which was taken as the plasmon peak of anatase (12.7 eV) [2]. The parameters A s = 1 eV (step height) and b = 2 eV have been adjusted to the experiment. The same parameter values were used for anatase and the (Na,H)TiNTs. Figure S1: Comparison of the Ti 2p XPS core-level spectra recorded on (Na,H)TiNTs (grey line) and on anatase (black line) reference powder used as starting material. The Ti 2p L-edge spectra were recorded with the TXM installed at the undulator beamline U41-XM at BESSY II, Berlin [4][5][6]. The setup of the instrument is analogous to a bright-field light microscope. A focusing spherical grating monochromator (FSGM) at the undulator beamline U41-XM at the electron storage ring BESSY II of the HZB delivers X-rays with photon energy E and the required high spectral resolution of E/ΔE ≥ 4500. A single reflection ellipsoidal shaped mirror condenser [7,4] is used to illuminate an object field of 15-20 μm.

XPS study of the sample
In the best case the HZB TXM provides a high spatial resolution close to 10 nm (half-pitch) [8] and a spectral resolution up to E/∆E ≈ 10 4 . It allows measurements to be taken at room or liquid nitrogen temperature in a vacuum of 10 −7 TORR. The spectra were recorded at room temperature in transmission mode by taking a sequence of images over a range of photon energies covering the investigated absorption edges with E/ΔE ≥ 4500. Note that the exit slit of the monochromator was set to 5 µm, which corresponds to a calculated spectral resolution of E/∆E = 2 × 10 4 . For the present study a zone plate objective with an outermost zone width of 40 nm was used to image the sample onto a cooled back-illuminated soft X-ray CCD camera (Roper Scientific, PI SX1300).
The NEXAFS spectra were normalized since the photon flux varies as a function of photon energy (hν) and time in the object field (x, y). The normalization was performed by dividing the function I(x, y, hν) recorded on the sample by the photon flux curve I o (x + ∆x, y + ∆y, hν) recorded in its sample free proximity at position (x + ∆x, y + ∆y). Both I(x, y, hν) and I o (x, y, hν) were recorded in the same image stack since near each studied nanostructure bare regions permit the measurement of I o .
The elemental compositions of the sample was investigated with a FE-SEM (Carl Zeiss, Supra 35 LV) equipped with an EDS (energy-dispersive X-ray spectrometer) element analysis system.
XPS (X-ray photoelectron spectroscopy) measurements were also performed to determine chemical composition, in a VERSAPROBE PHI 5000 from Physical Electronics, equipped with a monochromatic Al Kα X-ray source with a highly focused beam size that can be set from 10 to 300 µm. The energy resolution was 0.6 eV. For the compensation of the built up charge on the sample surface during the measurements, a dual-beam charge neutralization composed of an electron gun (~1 eV) and argon ion gun (≤10 eV) was used. The relative amount of sodium was evaluated to be 12% in accordance with EDS (11%).