Local structure of stoichiometric and oxygen-deficient A 2Ti6O13 (A = Li, Na, and K) studied by X-ray absorption spectroscopy and first-principles calculations

Kanchanawarin, Jarin; Limphirat, Wanwisa; Promchana, Pratya; Sooknoi, Tawan; Maluangnont, Tosapol; Simalaotao, Kodchakorn; Boonchun, Adisak; Reunchan, Pakpoom; Limpijumnong, Sukit; T-Thienprasert, Jiraroj

doi:10.1063/1.5052032

Local structure of stoichiometric and oxygen-deficient A 2Ti6O13 (A = Li, Na, and K) studied by X-ray absorption spectroscopy and first-principles calculations

Author(s)

Kanchanawarin, Jarin

Limphirat, Wanwisa

Promchana, Pratya

Sooknoi, Tawan

Maluangnont, Tosapol

Simalaotao, Kodchakorn

Boonchun, Adisak

Reunchan, Pakpoom

Limpijumnong, Sukit

T-Thienprasert, Jiraroj

Date Issued

October 21, 2018

Type

Article

DOI

10.1063/1.5052032

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.

Citation

Journal of Applied Physics, 124(15), 2018