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

Abstract

Surfactant-free Co 3 O 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 C and 800 C. X-ray diffraction (XRD) and helium gas pycnometry indicated the formation of single phase Co 3 O 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 3 O 4 nanoparticles synthesized at 300 C and 800 C, respectively. The corresponding decrease in the specific surface area from 39 m 2 g À1 to $2 m 2 g À1 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 m B to 6.16 m B ) substantially larger than the value of 3.9 m B expected for Co 2+ ions in the normal spinel structure of Co 3 O 4 were extracted from the inverse susceptibility data. This finding was rationalized by taking into account the disordered distribution of Co 2+ and Co 3+ ions in the Co 3 O 4 inverse spinel structures ([(Co 2+ ) 1Àx (Co 3+ ) x ] tet [(Co 2+ ) x (Co 3+ ) 2Àx ] oct O 4 ) where the inversion degree (x) depends on the synthesis temperature. Transport measurements using hot pressed pellets of Co 3 O 4 nanoparticles indicated p-type semiconducting behavior and drastic reductions in the thermal conductivity with decreasing average particle size.

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