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Size and shape controlled synthesis of Ni/Al Layered Double
Hydroxides magnetic nanoparticles
1
Luan Lingyu, Liu Shangying, Zhang Jie, Zhu Wenxia, Sun Dejun
Key Laboratory for Colloid and Interface Chemistry of the Ministry of Education, Shandong
University, Jinan, Shandong, People’s Republic of China (250100)
E-mail:lyluan@mail.sdu.edu.cn
Abstract
We investigated the influence of the peptization process on the size and shape of layered double hydroxide
magnetic nanoparticles. In this paper, we prepared 1:1 Ni/Al layered double hydroxide magnetic
nanoparticles in aqueous solution. By adjusting the peptization temperature and time, we can efficiently
control the particle size, shape and size polydispersity, which are important for the formation of oriented
films, the liquid-crystalline phase behavior and the stability of Pickering emulsions. The particle growth
followed the mechanisms of Ostwald ripening and imperfect oriented attachment. The suspensions peptized
at lower temperatures were not well dispersed; while the particles in suspensions peptized at higher
temperatures were apt to form larger aggregates. Peptization time also affected the polydispersity.
Peptization at 130℃ for 24 h proved to be the optimal synthesis conditions and magnetic measurements
showed that this material is paramagnetic.
Keywords: Layered Double Hydroxides; Peptization; Magnetic nanoparticles; Ostwald ripening; Imperfect
oriented attachment.
1. Introduction
Layered double hydroxides (LDHs) are a class of inorganic compounds with the general formula
[M
2+
1-x
M
3+
x
(OH)
2
]
x+
A
n-
x/n
·mH
2
O, where M
2+
and M
3+
are divalent and trivalent metal cations, such as
Mg
2+
,Co
2+
, Ni
2+
, Al
3+
, Fe
3+
, Cr
3+
, etc. Structurally, LDHs can be characterized as containing
hydrotalcite-like layers, where some divalent metal cations have been partially substituted by trivalent
metal cations (isomorphic substitution) to form positively charged sheets. The charge is compensated by
exchangeable anions in the interlayers. [1, 2] Such structural characteristics and compositional variability
allow LDHs to have potential applications in wide areas. Our group has done much work concerning Mg/Al
LDH, such as their stability [3], liquid-crystalline phase behavior [4, 5], rheological properties [6] and the
properties of liquid paraffin-in-water emulsions stabilized solely by Mg/Al LDH particles [7, 8].
Recently, magnetic nanoparticles have been extensively investigated because of their applications in
electronics (magnetic fluids, magnetic memories, etc.), biotechnology (cell separation, automated DNA
extraction, etc.) [9] and medicine (magnetic resonance imaging, drug delivery carriers, etc.) [10]. All these
applications require different material properties, such as particle shape; a narrow particle size distribution;
thermal, chemical and colloidal stability; magnetic characteristics and low-toxicity [11].
Considering the special requirements for the applications of magnetic nanoparticles and the structural
characteristics and variability of cations of LDHs, we attempted to synthesize stable homogeneous
suspensions of nano-sized layered double hydroxide particles containing magnetic metal cations.
Non-steady co-precipitation [12-14], co-precipitation of mixed salt solutions at varied or constant pH,
followed by peptizing at a certain temperature, has been successfully used to synthesize LDHs. Our group
has reported that co-precipitating Mg
2+
/Al
3+
(2:1) in NH
3
·H
2
O solution at pH 9.5 and then peptizing the
obtained precipitate at 80℃ for 24 h results in well-dispersed colloidal LDH suspensions. [4] However, due
to the existence of large aggregates which settle in aqueous solution, the LDHs synthesized in this way are
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Supported by Specialized Research Fund for the Doctoral Prigram of Higher Education.(No. 20060422021).