Sodium-Poor, Hydroxyl-Rich, Defective Na<sub>2</sub>Ti<sub>3</sub>O<sub>7</sub> Prepared by γ-Irradiation and Its Enhanced Proton Conductivity
Tosapol Maluangnont, Tanagorn Sangtawesin, Phieraya Pulphol, Orawan Khamman, Pakpoom Reunchan, Kazuma Gotoh, Naratip Vittayakorn
Abstract
High Resolution Image Download MS PowerPoint Slide The use of γ-irradiation to tailor the physicochemical properties of materials is not widely applied to layered alkali metal oxides. Herein, we show that γ-irradiation (up to 400 kGy) of Na 2 Ti 3 O 7 leads to a sodium-poor, hydroxyl-rich analogue where the layered structure, plate-like morphology, and textural properties are preserved. The deintercalation of sodium ions modifies the Ti–O bond lengths and expands the unit cell; the latter is supported by density functional theory (DFT) calculations. 23 Na solid-state NMR suggests the transport of the symmetric, 7-fold Na2 sites to an intermediate environment, which is closer to the asymmetric, 9-fold Na1 sites. An 8 wt % mass loss (1.4 mol water/mol titanate) is observed, indicating an increased concentration of protons/hydroxyls. These hydroxyl groups (i.e., lattice protons) possess higher thermal stability than solely surface-adsorbed ones in the nonirradiated sample. At 200–400 kGy, the proton conduction (50 °C and ∼70% RH) of ∼10 –6 S·cm –1 is 1 order of magnitude larger than that in the nonirradiated sample; the relaxation time decreases from 30 to 2–6 μs with γ-irradiation. The γ-dose dependence of dielectric loss is also present and analyzed using the Jonscher universal power law, indicating the low-frequency dispersion behavior characteristics of high charge densities.