The role of synthetic parameters in the magnetic behavior
of relative large hcp Ni nanoparticles
A. Kotoulas • M. Gjoka • K. Simeonidis •
I. Tsiaoussis • M. Angelakeris • O. Kalogirou •
C. Dendrinou-Samara
Received: 11 January 2010 / Accepted: 19 April 2010
Springer Science+Business Media B.V. 2010
Abstract The controllable synthesis of relatively
large nickel nanoparticles via thermal decomposition
of nickel acetate tetrahydrate in oleylamine in the
presence of 1-adamantane carboxylic acid (ACA) and
trioctylphosphine oxide (TOPO) is reported. High
crystalline hcp nanoparticles of different sizes have
been prepared at 290 C, whereas at relative lower
temperatures fcc are favored. The particle size was
varying between 50 and 150 nm by properly adjusting
the proportion of the capping ligands. TOPO-to-
ACA ratio was also found to have an influence on the
magnetic properties through the potential formation
of a NiO shell. Pure hcp Ni nanoparticles over 50 nm
in size served as models to illuminate the magnetic
behavior of this metastable hexagonal Ni phase.
Contrary to the net ferromagnetic characteristics of
fcc Ni nanoparticles in the same size range, hexagonal
structured particles exhibit superparamagnetic
behavior at room temperature and a weak ferromagnetic
contribution below 15 K.
Keywords Nickel nanoparticles hcp Nickel
Magnetism Thermal decomposition Surfactants
Introduction
Nickel is a ferromagnetic transition metal that
naturally crystallizes in the face-centered cubic
(fcc) crystal structure. The preparation of nanoparticulate
nickel materials is of great interest due to
their applications in magnetic sensors, electronics,
catalysts (Chen et al. 2007b; Singla et al. 2007) and
recently in biomedicine (Guo et al. 2009). The
synthetic methods of Ni nanoparticles include thermal
decomposition of several precursors, electrochemical
or chemical reduction, and laser pyrolysis
(Hou and Gao 2004). Among them, the most widely
applied is the thermal decomposition of a variety of
precursors such as NiCl2 (Vergara and Mandurga
2002), Ni(acac)2 (acac = acetylacetonate) (Hou and
Gao 2003; Hou et al. 2005; Winnischofer et al. 2008;
Chen et al. 2009) and Ni(COD)2 (COD = cycloocta-
1,5-diene) (Cordente et al. 2003) in the presence of
different kind of surfactants.
Although the cubic structure is the equilibrium
bulk state of Ni, another metastable phase, the
hexagonal close-packed (hcp) structure, is often