US EPA

1743139 

Journal Article 

Effect of Surfactant Monolayer on Reduction of Fe(3)O(4) Nanoparticles under Vacuum 

Ayyappan, S; Gnanaprakash, G; Panneerselvam, G; Antony, MP; Philip, J 

2008 

Yes 

Journal of Physical Chemistry C
ISSN: 1932-7447
EISSN: 1932-7455 

112 

47 

18376-18383 

10.1021/jp8052899 

WOS:000261056500015 

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 degrees 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 angstrom, respectively. On further heating (above 700 degrees C), Fe(3)O(4) decomposed into gamma-Fe(2)O(3) and FeO phases. In the range of 800-1000 degrees 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 alpha-Fe commenced at 500 degrees C and continued up to 800 degrees C. After vacuum annealing at 800 degrees 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 alpha-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 alpha-Fe at lower temperature (at 500 degrees C) in the surfactant-coated system.