Local structure of stoichiometric and oxygen-deficient A2Ti6O13 (A = Li, Na, and K) studied by X-ray absorption spectroscopy and first-principles calculations

Abstract

Oxygen vacancy defects (VO) in Ti-based oxides play important roles in catalytic processes despite limited knowledge regarding their formation and characterization. Here, we demonstrate the use of X-ray absorption spectroscopy (XAS) measurements to compare the relative proportion of VO defects in as-grown alkali hexatitanate A2Ti6O13 (A = Li, Na, K). Both X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) regions were studied. The similarity of measured XANES spectra of Ti K-edge in all samples indicates the presence of (Ti4+)O6 units in good agreement with reported X-ray diffraction results. The small influence of cations A at the tunnel was observed and can be well reproduced in the simulated spectra. In addition, we present a semi-quantitative approach to intuitively determine the content of VO defects in oxygen-deficient K2Ti6O13-x by in situ time-resolved XAS measurements under reducing conditions (10%H2/Ar, 50-650 °C). The in situ XANES measurements indicate that the oxidation state of bulk Ti remains the same as the as-grown sample, i.e., 4+, at elevated temperatures. By in situ EXAFS measurements, the relative number of VO defects is highest at a reduction temperature of ∼550 °C and slightly decreases after that. To confirm the formation of VO defects, first-principles calculations were independently carried out using a 126-atom K2Ti6O13 supercell with VO at various positions. Based on calculated EXAFS, the removal of the oxygen atom nearest to the tunnel, which is the lowest energy structure, provides a good match to the experimental spectra.

Towards a Greener and Scalable Synthesis of Na2Ti6O13 Nanorods and Their Application as Anodes in Batteries for Grid‐Level Energy Storage

“…The subtle differences in the microstructure of the materials obtained at lower temperature are responsible for the reversible insertion of at least two lithium ions (96 mA h g À1 ). According to our findings and recent firstprinciples calculations, [67] NTO-900 reveals a lower binding energy for twofold coordinated oxygen atoms in the tunnel structure, sitting next to the alkali ion. We suppose that this explains the higher electrochemical performance of the material.…”

“…The relative quantification of NTO-900 and NTO-1100 can be found in the Supporting Information (see Table S2, Supporting Information). These results-together with the changes seen in the Raman spectra (in Figure 5)-lead us to suppose a slight structural modification in the Ti-O sublattice when increasing the synthesis temperature of the NTO sample from 900 to 1100 C. In agreement with this, recent firstprinciples calculations [67] propose a low binding energy for twofold coordinated oxygen atoms in the tunnel structure, sitting next to the alkali ion. Their removal is thus energetically most favorable.…”

Differential Scanning Calorimetric Analysis of Ni Doped Sodium Hexa-titanate

“…Where symbols have their usual meanings and already been explained in our previous papers 14,[22][23][24] . It has been found that a ≠ b ≠ c, and also α = γ = 90 o ≠ β, as the case should be for monoclinic structure, here the value of β is decreasing on increasing the doping percentage of nickel.…”

Temperature‐dependent electrical transport behavior and structural evolution in hollandite‐type titanium‐based oxide

“…Figure 8C shows the variation of half-life time (the time required for E′ to reduce to the middle between its initial and final values) with temperature. 58,59 Moreover, the potassium content in the (3 × 1) tunnel of K 2 Ti 6 O 13 is 2× higher than that in the (2 × 2) tunnel of K 1.36 Ti 8 O 16 . It is known that an increased number of migrating ions and an increased mobility result in an increased ionic conductivity.…”

“…As shown in Figure S10, K 1.36 Ti 8 O 16 contains a (2 × 2) tunnel with an area of 0.3 nm 2 , while K 2 Ti 6 O 13 involves a (3 × 1) tunnel with an area of 0.25 nm 2 . 58,59 Moreover, the potassium content in the (3 × 1) tunnel of K 2 Ti 6 O 13 is 2× higher than that in the (2 × 2) tunnel of K 1.36 Ti 8 O 16 . Hence, when compared to K 1.36 Ti 8 O 16 , it is more difficult for the potassium to transfer in K 2 Ti 6 O 13 .…”