Effect of Surfactant Monolayer on Reduction of Fe3O4 Nanoparticles under Vacuum

We study the effects of surfactant monolayer coating on the reduction of Fe 3 O 4 nanoparticles under vacuum thermal annealing. Oleic acid coated and uncoated Fe 3 O 4 nanoparticles were synthesized by a simple coprecipitation technique. In the temperature range of 300−700 °C, the particle size and lattice constant of uncoated Fe 3 O 4 nanoparticles increased from 9 to 18 nm and from 8.357 to 8.446 Å, respectively. On further heating (above 700 °C), Fe 3 O 4 decomposed into γ-Fe 2 O 3 and FeO phases. In the range of 800−1000 °C, the FeO phase was predominant, and its size grew significantly from 30 to 44 nm. Conversion of oleic acid coated Fe 3 O 4 phase to metallic α-Fe commenced at 500 °C and continued up to 800 °C. After vacuum annealing at 800 °C, the magnetic behavior of the sample changed from ferrimagnetic to ferromagnetic. The activation energies for the phase transitions of uncoated and oleic acid coated nanoparticles were estimated to be 30.304 and 17.349 kJ/mol, respectively. Thermogravimetric analysis (TGA) coupled with mass spectrometry revealed that, for coated nanoparticles, effluents such as H 2, CO, and CO 2 from oleic acid facilitate the reduction of Fe 3 O 4 into α-Fe and FeO during vacuum thermal annealing. The interaction between the headgroup of the oleic acid and the oxygen in Fe 3 O 4 is expected to lead to weakened bonding, which could result in a lower activation energy for the reduction of the surfactant-coated nanoparticles. This is a plausible reason for the precipitatation of α-Fe at lower temperature (at 500 °C) in the surfactant-coated system.