Dielectric and electrical properties of sol-gel γ-Fe2O3 nanoparticles: polaron hopping and a thermal crossover of the transport regime

Abstract

γ-Fe2O3 nanoparticles prepared by the sol-gel method (supercritical drying in ethanol) were characterized by XRD, TEM/HRTEM and Raman spectroscopy, which indicate a single-phase cubic spinel structure (maghemite-like) made of quasi-spherical, essentially single-crystalline nanoparticles (crystallite size of about 9–12 nm from XRD; mean TEM diameter of about 7 nm). Their electrical and dielectric properties were studied by impedance spectroscopy between 300 and 600 K. Below ≈ 420 K the ac conductivity is dispersive and follows Jonscher’s power law, with a frequency exponent s that varies non-monotonically with temperature (minimum near 340 K); above ≈ 440 K it is frequency independent. The thermally activated conductivity, the low activation energies at low temperature (100– 300 meV from the ac data; 315 meV from the impedance fits) and the behavior of s(T) are consistent with charge transport by hopping of small polarons between Fe 2+ and Fe 3+ ions, with a probable crossover from correlated-barrier-hopping/overlapping-large-polaron to non-overlapping small-polaron tunneling; Mott’s variable-range hopping is ruled out. The Fe 2+ carriers are attributed to the reducing synthesis medium and to the oxygen-deficient surface of the ~7 nm particles, and the hopping barrier to the reorganization energy of the Fe 2+ /Fe 3+ electron transfer. The activation energy increases to 0.73 eV above 440 K and the conductivity rises about sevenfold between 420 and 440 K, whereas an Arrhenius extrapolation predicts only ≈ 2.5; we tentatively interpret this step as the thermally activated release of additional Fe 2+ carriers (loss of surface hydroxyl groups and oxygen) combined with the activation of the inter-particle contacts of the aerogel network, which carry the long-range current.

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