Co sub(3)O sub(4) nanostructures: the effect of synthesis conditions on particles size, magnetism and transport properties

Surfactant-free Co sub(3)O sub(4) 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 degree C and 800 degree C. X-ray diffraction (XRD) and helium gas pycnometry indicated the formation of single phase Co sub(3)O sub(4) 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 Co sub(3)O sub(4) nanoparticles synthesized at 300 degree C and 800 degree C, respectively. The corresponding decrease in the specific surface area from 39 m super(2) g super(-1) to similar to 2 m super(2) g super(-1) was confirmed by gas adsorption analysis using BET techniques. Magnetic susceptibility measurements revealed a dominant antiferromagnetic (AFM) ordering and the Neel 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 mu sub(B) to 6.16 mu sub(B)) substantially larger than the value of 3.9 mu sub(B) expected for Co super(2+) ions in the normal spinel structure of Co sub(3)O sub(4) were extracted from the inverse susceptibility data. This finding was rationalized by taking into account the disordered distribution of Co super(2+) and Co super(3+) ions in the Co sub(3)O sub(4) inverse spinel structures ([(Co super(2+)) sub(1-x)(Co super(3+)) sub(x)] super(tet)[(Co super(2+)) sub(x) (Co super(3+)) sub(2-x)] super(oct)O sub(4)) where the inversion degree (x) depends on the synthesis temperature. Transport measurements using hot pressed pellets of Co sub(3)O sub(4) nanoparticles indicated p-type semiconducting behavior and drastic reductions in the thermal conductivity with decreasing average particle size.