RESUMEN
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 Na2Ti3O7 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. 23Na 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 â¼70% RH) of â¼10-6 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.
RESUMEN
The investigations of temperature-dependent electrical properties in graphitic carbon nitride (g-C3N4) have been largely performed at/below room temperature on devices commonly fabricated by vacuum techniques, leaving the gap to further explore its behaviors at high-temperature. We reported herein the temperature dependence (400 â 35 °C) of alternating current (AC) electrical properties in bulk- and nanosheet-g-C3N4 compacts simply prepared by pelletizing the powder. The bulk sample was synthesized via the direct heating of urea, and the subsequent HNO3-assisted thermal exfoliation yielded the nanosheet counterpart. Their thermal stability was confirmed by variable-temperature X-ray diffraction, demonstrating reversible interlayer expansion/contraction upon heating/cooling with the thermal expansion coefficient of 2.2 × 10-5-3.1 × 10-5 K-1. It is found that bulk- and nanosheet-g-C3N4 were highly insulating (resistivity ρ â¼ 108 Ω cm unchanged with temperature), resembling layered van der Waals materials such as graphite fluoride but unlike electronically insulating oxides. Likewise, the dielectric permittivity ε', loss tangent tan δ, refractive index n, dielectric heating coefficient J, and attenuation coefficient α, were weakly temperature- and frequency-dependent (103-105 Hz). The experimentally determined ε' of bulk-g-C3N4 was reasonably close to the in-plane static dielectric permittivity (8 vs. 5.1) deduced from first-principles calculation, consistent with the anisotropic structure. The nanosheet-g-C3N4 exhibited a higher ε' â¼ 15 while keeping similar tan δ (â¼0.09) compared to the bulk counterpart, demonstrating its potential as a highly insulating, stable dielectrics at elevated temperatures.
RESUMEN
Ball milling of solids under benign conditions leads to surface functionalization without altering the crystal structure and morphology. However, these additional surface functional groups are rarely fixed but instead mobilized across such ball milled solids. This phenomenon, including its effects on electrochemical and electrical properties, has received limited attention. We report herein that dry vibratory ball milling of lepidocrocite-type Cs2Ti6O13 generated hydroxyl groups which subsequently migrated from surfaces to bulk. The increased number of bulk hydroxyl groups is deduced from Raman, IR, and solid state 1H nuclear magnetic resonance spectroscopy, and thermogravimetric analysis. In contrast, the decrease in the relative proportion of surface hydroxyl groups/water and carbon-oxygen species was deduced from X-ray photoelectron spectroscopy. The inaccessible hydroxyl groups in ball milled Cs2Ti6O13 lead to a smaller amount of stored charge and increased charge transfer resistance, according to galvanostatic charge-discharge experiments and electrochemical impedance spectroscopy studies in 1 M Na2SO4. The alternating current electrical properties were also measured, revealing fundamental insights such as the one-dimensional conduction pathway and the relaxation time in microseconds. A model has been proposed for this surface-to-bulk migration of the hydroxyl groups, which competes with surface dangling bonds leading to particle agglomeration.
RESUMEN
Understanding the fundamentals of transport properties in two-dimensional (2D) materials is essential for their applications in devices, sensors, and so on. Herein, we report the impedance spectroscopic study of carbon nitride nanosheets (CNNS) and the composite with anatase (TiO2/CNNS, 20 atom% Ti), including their interaction with atmospheric water. The samples were characterized by X-ray diffraction, N2 adsorption/desorption, solid state 1H nuclear magnetic resonance spectroscopy, thermogravimetric analysis, and transmission electron microscopy. It is found that CNNS is highly insulating (resistivity ρ â¼ 1010 Ω cm) and its impedance barely changes during a 20 min-measurement at room temperature and 70% relative humidity. Meanwhile, incorporating the semiconducting TiO2 nanoparticles (â¼10 nm) reduces ρ by one order of magnitude, and the decreased ρ is proportional to the exposure time to atmospheric water. Sorbed water shows up at low frequency (<102 Hz) with relaxation time in milliseconds, but the response intrinsic to CNNS and TiO2/CNNS is evident at higher frequency (>104 Hz) with relaxation time in microseconds. These two signals apparently correlate to the endothermic peak at ≤110 °C and >250 °C, respectively, in differential scanning calorimetry experiments. Universal power law analysis suggests charge hopping across the 3D conduction pathways, consistent with the capacitance in picofarad typical of grain response. Our work demonstrates that the use of various formalisms (i.e., impedance, permittivity, conductivity, and modulus) combined with a simple universal power law analysis provides insights into water-induced transport of the TiO2/CNNS composite without complicated curve fitting procedure or dedicated humidity control.
RESUMEN
The lepidocrocite-type layered alkali titanate AxMyTi2-yO4 has diverse chemical compositions with variation in charge per formula unit x, the interlayer cation A+, and the intralayer metal M. Despite this multivariable nature, the composition dependence of physical properties is not well explored. We report herein the AC conductivity and the complementary dielectric properties of Cs0.7M0.35Ti1.65O4, K0.8M0.4Ti1.6O4 (M = Zn, Ni), and the mixed-interlayer ion Cs0.6K0.1Zn0.35Ti1.65O4. For Cs0.7Zn0.35Ti1.65O4, the total AC conductivity is â¼7 × 10-8 to 2 × 10-6 S·cm-1 at 200-350 °C, associating with an activation energy Ea â¼ 865 meV. Meanwhile, the conductivity of K0.8Zn0.4Ti1.6O4 is higher by 1 order of magnitude at much lower temperature (25-150 °C) and a smaller Ea â¼ 250 meV. This difference originates from the compositional robustness of the cesium-containing samples, contrasting with the sintering-induced changes in the potassium analogues. For the latter, the loss of the interlayer K+ ion results in (i) generation of carriers due to charge compensation, (ii) reduction of sheet charge density and weakening of electrostatic attraction, and (iii) widening of the interlayer distance, all contributing to a lower Ea in K0.8M0.4Ti1.6O4. The angular frequency dependence of conductivity, dielectric permittivity (up to a colossal value of 109), and dielectric loss follows the universal power law. Our work demonstrates the potential of simple compositional variation for electrical properties tuning, prompting a more in-depth investigation covering a wider range of possible candidates of x, A+, and M in lepidocrocite titanate.