Co3O4 nanostructures: the effect of synthesis conditions on particles size, magnetism and transport properties

Abstract

Surfactant-free Co₃O₄ nanostructures with various particle size ranges were synthesized via the solution combustion method using cobalt nitrate solution as a cobalt precursor and urea as a combustion fuel. Control over average particles size range was achieved by tuning the reaction ignition temperature between 300 °C and 800 °C. X-ray diffraction (XRD) and helium gas pycnometry indicated the formation of single phase Co₃O₄ nanoparticles with a spinel structure. Transmission electron microscopy (TEM) studies revealed an increase of the size range from 5–8 nm to 200–400 nm for Co₃O₄ nanoparticles synthesized at 300 °C and 800 °C, respectively. The corresponding decrease in the specific surface area from 39 m² g⁻¹ to ∼2 m² g⁻¹ was confirmed by gas adsorption analysis using BET techniques. Magnetic susceptibility measurements revealed a dominant antiferromagnetic (AFM) ordering and the Néel temperature decreases with a decreasing average particle size range from 31 K (200–400 nm) to 25 K (5–18 nm). Interestingly, effective magnetic moments (ranging from 4.12 μ_B to 6.16 μ_B) substantially larger than the value of 3.9 μ_B expected for Co²⁺ ions in the normal spinel structure of Co₃O₄ were extracted from the inverse susceptibility data. This finding was rationalized by taking into account the disordered distribution of Co²⁺ and Co³⁺ ions in the Co₃O₄ inverse spinel structures ([(Co²⁺)_1−x(Co³⁺)_x]^tet[(Co²⁺)_x(Co³⁺)_2−x]^octO₄) where the inversion degree (x) depends on the synthesis temperature. Transport measurements using hot pressed pellets of Co₃O₄ nanoparticles indicated p-type semiconducting behavior and drastic reductions in the thermal conductivity with decreasing average particle size.