The Impact of Different Metal Dopants on the Structural, Dielectric, and Electrical Characteristics of Bentonite: Electrical Measurements Supported by Tight-Binding Calculations

Abstract

Composite dielectric materials with different dielectric characteristics are crucial for energy storage devices. In this study, a composite material of 5 wt.% metal (M)/bentonite (Bento) (M = Cd, Cu, Fe, or Zn) was synthesized using a wet impregnation technique to examine the effect of metal incorporation in bentonite. X-ray diffraction, scanning electron microscopy, energy-dispersive x-ray analysis, and impedance analysis were carried out to investigate the structural, morphological, electrical, and dielectric properties of the samples. A low-frequency dielectric response provided evidence of dielectric relaxation, which was associated with local charge polarizations occurring at the grain (or particle) site. A reduction in the dielectric constant was observed in metal-doped composites, attributed to the variation in grain size and the resulting change in the dipole number per unit volume generated by doping. Interestingly, the AC conductivity dispersion decreased for dopants with a larger ionic radius, in line with qualitative expectations. Tight-binding calculations demonstrated that the changes in binding energy, hardness, and the positioning of the highest occupied molecular orbital and lowest unoccupied molecular orbital of Bento were responsible for the differences in dielectric properties resulting from the incorporation of different metals into Bento. Thus, our results demonstrate the successful metal doping-induced tunability of the dielectric and structural properties of Bento, with significant implications for its use as supercapacitor (optimal dopant is Fe, as it induces larger permittivity), and hence its polarizability. The lowest permittivity was achieved for Cd doping, suggesting its potential use for insulator applications.