Sodium-Poor, Hydroxyl-Rich, Defective Na₂Ti₃O₇ Prepared by γ-Irradiation and Its Enhanced Proton Conductivity

Abstract

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 solidstate 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.