Nanocrystalline Complex Oxides Zn_xCo_3–xO₄: Cobalt and Zinc Ions Impact on Large Growth of Conductivity

Abstract

Cobalt oxide based nanostructures are perspective materials for gas sensors, photocatalysts, and other devices for ecology applications due to the high concentration of chemisorbed oxygen and catalytic activity in oxidation reactions. Finely dispersed nanocrystalline oxides Zn x Co3–x O4 (0 ≤ x ≤ 1) with a high level of conductivity have been synthesized by the chemical precipitation of oxalates with subsequent thermal treatment. Comprehensive studies of the morphology, electrophysical properties, nature and concentration of defects in the obtained materials were carried out. It is shown that the introduction of zinc atoms to the Co3O4 structure leads to a sharp increase in conductivity by more than 5 orders of magnitude. In addition, the zinc-induced interplay of Co2+ spin centers between tetrahedral and octahedral sites was revealed using the EPR method. The correlation between the conductivity and the concentration of Co2+ ions in tetrahedral and octahedral environments was established for the first time. The obtained results open up the possibility of fine-tuning the electronic properties of nanostructured Zn x Co3–x O4 by variation of zinc concentration in the samples.

Zn Modification of Co3O4 Nanofibers: Effect on Structure and Conductivity

“…The substitution of a Co 3+ ion in an octahedral environment with a Co 2+ ion should be accompanied by the formation of a hole and formation of the acceptor level in the band gap of oxide (or enhancement of its concentration), respectively. There was a similar effect in the Zn x Co 3– x O 4 nanocrystals …”

“…The activation energy at temperatures below 300 K is about 0.23 ± 0.01 eV, and at temperatures above 300 K the activation energy rises to 0.37 ± 0.05 eV. The change in the activation energy may be associated with the transition to the hopping mechanism of conduction at low temperatures, which is sometimes observed in nanostructured metal oxides. , However, the change in the temperature dependences of the conductivity is very similar to what was observed in nanocrystalline Zn x Co 3– x O 4 . As in the case of nanocrystalline samples, the appearance of a region with an activation energy of 0.4 eV may be associated with the introduction of zinc atoms into an octahedral environment in the spinel structure.…”

“…In this temperature range, the temperature dependence of the conductivity of cobalt oxide has an activation form with an activation energy of 0.67 eV, which is characteristic of cobalt oxide. 10 When zinc is added, two sections with different activation energies are distinguished on the temperature dependence of the conductivity.…”

“…For nanocrystalline cobalt oxide composites with zinc, zinc can be embedded in both tetrahedral positions and octahedral positions of the spinel lattice. 10 In the case of nanofibers of Co , Sigma-Aldrich, 99,9%) in a volume ratio of 1:1 were used as solvents in the preparation of the polymer solution; polyvinylpyrrolidone (PVP) (Sigma−Aldrich, Mw = 1300000) was used as a polymer.…”

“…It is known that the doping of wide-band p-type semiconductors, which also includes Co 3 O 4 , is extremely inefficient . One of the possibilities for a significant increase in the conductivity of cobalt oxide is the introduction of zinc. − Thus, the articles ,− report an increase in the conductivity of cobalt oxide nanocrystalline Zn x Co 3– x O 4 from two to 6 orders of magnitude with the introduction of zinc atoms.…”