Structural properties and electrical conductivity mechanisms of semiconducting quaternary nanocomposites: Effect of two transition metal oxides

Abstract

Several quaternary nanocomposites, doped with two transition metal oxides, of the generalised composition formula 0.4ZnO–0.1P2O5–0.5[xV2O5–(1-x) MoO3] were prepared via the melt quenching process. We investigated the microstructural and conductivity properties of two ternary systems and four quaternary systems to determine the better semiconductor nanocomposite system for wider application purposes. The X-ray diffraction results showed the presence of superposed nanocrystallites within amorphous networks or matrices. Analysis of ultraviolet–visible absorption spectra showed that optical bandgap energy (Eopt) values varied with V2O5 concentration, and there was an inverse relationship between Eopt and average nanocrystallite size. The responsible mechanisms of ac conductivity were examined using the model of Jonscher's universal power-law and Almond–West formalism, and it was found that ac conductivity increased as temperature rose, exhibiting semiconducting features. All the ternary (two examples) and quaternary (four examples) nanocomposite systems established non-linearity in dc conductivity, caused by the small polaron hopping process with dissimilar activation energies at different temperature regions. The power-law exponent (s) of Jonscher's universal power-law revealed that the ac conductivity mechanism could be described by correlated barrier-hopping (CBH) or non-overlapping small polaron tunnelling (NSPT) models for different samples due to major structural alterations. We applied modified CBH and NSPT models (as s > 0.8) to get realistic values of different fitting parameters, and in this process, we also obtained the values of ideal or hypothetical glass transition temperature from the theoretical models. The different activation energies associated with ac conductivity and for the small polaron migration process diminished with an increment in conductivity. Scaled ac conductivity spectra demonstrated that the conductivity relaxation process relied upon the composite structural features and did not depend on temperature. These materials can be used for such applications as gas sensors, even at higher temperatures due to their semiconducting nature and the different valence states of transition metal ions.