Sodium-Poor,\nHydroxyl-Rich, Defective Na<sub>2</sub>Ti<sub>3</sub>O<sub>7</sub> Prepared by γ‑Irradiation\nand Its Enhanced Proton Conductivity

The use of γ-irradiation\nto tailor the physicochemical properties\nof materials is not widely applied to layered alkali metal oxides.\nHerein, we show that γ-irradiation (up to 400 kGy) of Na<sub>2</sub>Ti<sub>3</sub>O<sub>7</sub> leads to a sodium-poor, hydroxyl-rich\nanalogue where the layered structure, plate-like morphology, and textural\nproperties are preserved. The deintercalation of sodium ions modifies\nthe Ti–O bond lengths and expands the unit cell; the latter\nis supported by density functional theory (DFT) calculations. <sup>23</sup>Na solid-state NMR suggests the transport of the symmetric,\n7-fold Na2 sites to an intermediate environment, which is closer to\nthe asymmetric, 9-fold Na1 sites. An 8 wt % mass loss (1.4 mol water/mol\ntitanate) is observed, indicating an increased concentration of protons/hydroxyls.\nThese hydroxyl groups (i.e., lattice protons) possess higher thermal\nstability than solely surface-adsorbed ones in the nonirradiated sample.\nAt 200–400 kGy, the proton conduction (50 °C and ∼70%\nRH) of ∼10<sup>–6</sup> S·cm<sup>–1</sup> is 1 order of magnitude larger than that in the nonirradiated sample;\nthe relaxation time decreases from 30 to 2–6 μs with\nγ-irradiation. The γ-dose dependence of dielectric loss\nis also present and analyzed using the Jonscher universal power law,\nindicating the low-frequency dispersion behavior characteristics of\nhigh charge densities.