Chemical top-down synthesis of amphiphilic superparamagnetic Fe3O4 nanobelts from exfoliated FeOCl layers

Article abstract of DOI:10.1039/c4dt02176b

A novel chemical top-down method has been employed to prepare tiny Fe₃O₄ nanobelts. Uniform Fe₃O₄ nanobelts were obtained by solvothermal reduction of exfoliated FeOCl layers with the assistance of high polar solvents and a PEO-PPO-PEO stabilizer. We also studied the superparamagnetic properties of Fe₃O₄ nanobelts, which exhibit a magnetic saturation value as high as ～53 emu g^-1 at room temperature and a magnetic anisotropy constant up to 6.6 × 10⁴ erg cm^-3. Furthermore the as-obtained product, which is amphiphilic and exhibits excellent dispersibility and redispersibility in most of the common solvents, could be readily used in various realms of biomedicine and biotechnology.

Full text of DOI:10.1039/c4dt02176b

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Chemical “top-down” synthesis of amphiphilic
superparamagnetic Fe3O4 nanobelts from
exfoliated FeOCl layers†
Cite this: Dalton Trans., 2014, 43,
16173
Received 18th July 2014,
Accepted 9th September 2014
Xiao Wei, Juan Su, Xin-Hao Li* and Jie-Sheng Chen
DOI: 10.1039/c4dt02176b
www.rsc.org/dalton
A novel chemical “top-down” method has been employed to
Besides one-dimensional nanostructures, the surface modi-
prepare tiny Fe3O4 nanobelts. Uniform Fe3O4 nanobelts were fication is another aspect that should be considered for their
obtained by solvothermal reduction of exfoliated FeOCl layers with practical applications, especially in the realms of drug delivery
the assistance of high polar solvents and a PEO–PPO–PEO stabil- and magnetic resonance imaging (MRI). Consider the success
izer. We also studied the superparamagnetic properties of Fe3O4 in fabricating magnetic nanoparticles in oil phase, much
nanobelts, which exhibit a magnetic saturation value as high efforts have been devoted to replacing the hydrophobic ligands
as ∼53 emu g−1 at room temperature and a magnetic anisotropy of as-obtained magnetic nanoparticles with polar ones to
constant up to 6.6 × 104 erg cm−3. Furthermore the as-obtained make them hydrophilic for biological use. As an alternative,
product, which is amphiphilic and exhibits excellent dispersibility amphiphilic magnetic nanoparticles that are dispersible in
and redispersibility in most of the common solvents, could be both polar and nonpolar solvents should be ideal candidates
readily used in various realms of biomedicine and biotechnology.
for these applications. Although amphiphilic magnetic
spheres with a size larger than 100 nm have been prepared in
our previous work,20 the large size of these materials limited
their applications in in situ hyperthermia. This is because only
tiny nanoparticles smaller than 50 nm can penetrate through
the cell membrane. Developing an efficient method for fabri-
cating fine nanostructures with an amphiphilic surface is thus
highly alluring but remains a great challenge.
A variety of techniques have been exploited to prepare mag-
netic nanostructured particles in the past.21–24 Especially the
colloidal chemical synthesis14,25–28 of nanoparticles has been
intensively pursued because both the size and the shape of the
nanoparticles may be finely controlled through this synthetic
technique. Generally, for synthesis of nanoparticles, the
solution colloidal chemical approaches can be designated
“bottom-up” methods. “Top-down” methods are mostly con-
ventional physical techniques, through which the preparation
of uniform nanoparticles with controlled sizes and shapes
from bulk phase is hard to achieve. However, the advantage
of “top-down” methods for synthesizing anisotropic 1D or 2D
nanostructures is obvious, whilst “bottom-up” methods
usually tend to boost the formation of nanoparticles with low
structural anisotropy.
Introduction
One-dimensional (1D) nanostructures, including nanowires,
nanorods and nanobelts, have drawn much attention because
they show great promise in the manufacture of nanoelectronic
devices, sensors and field emission devices, as well as in
spintronic and medical applications.1–10 Among the one-
dimensional nanostructures, magnetic nanorods, nanobelts
and nanowires with an elongated structure exhibit magnetic
anisotropies. Most of the applications of magnetic materials in
technology and industry nowadays are probably determined by
the magnetic anisotropy, that is, the dependence of magnetic
properties on a preferred direction in response to thermal or
magnetic stimuli. Therefore, the one-dimensional magnetic
nanostructures are potentially applicable as hard magnetic
materials and ferrofluids in magnetic recording, magnetic
hyperthermia and related realms.11–19 However, the
approaches for the synthesis of uniform one-dimensional
magnetic nanostructures, especially via solution-phase
approaches, are really limited at the moment.
Herein, we show the proof-of-principle application of the
“top-down” method for preparing uniform nanobelts. We
applied a novel chemical “top-down” method combined with a
solvothermal process to transfer exfoliated FeOCl layers to
nearly uniform superparamagnetic Fe3O4 nanobelts. The
Fe3O4 nanobelts exhibited a magnetic saturation value as high
School of Chemistry and Chemical Technology, Shanghai Jiao Tong University,
Shanghai 200240, P. R. China. E-mail: xinhaoli@sjtu.edu.cn;
Fax: +86 021 5474 1297
†Electronic supplementary information (ESI) available: SEM image of FeClO,
TEM image of Fe3O4 aggregates obtained without an exfoliating process and
magnetic hysteresis loops of Fe3O4 nanobelts. See DOI: 10.1039/c4dt02176b
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as ∼53 emu g−1 at room temperature and a magnetic It is obvious that the belt structure of the final Fe3O4 nanobelts
anisotropy constant of up to 6.6 × 104 erg cm−3. Moreover, the was mainly replicated from the layered precursor, exfoliated
excellent dispersibility of the tiny Fe3O4 nanobelts in both FeClO nanosheets. P123 stabilized the as-formed nanosheets
polar and non-polar solvents was also investigated in this or nanobelts during the whole synthetic process.
work.
Results and discussion
Experimental
Fig. 1 shows the transmission electron microscopy (TEM)
images of Fe3O4 nanobelts. It is seen that the width (∼17 nm,
Fig. 1A) and the thickness (1–4 nm, Fig. 1C) of the nanobelts
are nearly uniform while the length of the nanobelts is
FeOCl was selected as a possible precursor for the synthesis of
nanobelts due to its layered structure and unique composition
containing Fe and O species simultaneously. Inorganic layered
polydisperse, ranging from 30 to 200 nm. The selected-area
materials, such as layered double hydroxides (LDH) and
electron diffraction (SAED) pattern as shown in Fig. 1 further
confirms the formation of crystalline Fe3O4 nanobelts.
FeOCl, have been widely used in catalysis, adsorption, nano-
composite formation, and drug delivery.29–32 Recently, there
All these TEM results suggested the success of our chemical
were several reports on the preparation of stable suspensions
“top-down” method for transferring bulk FeOCl solid into tiny
nanobelts. Further extending our principle to the synthesis
of metal oxide nanobelts or nanosheets is still in progress by
of LDHs,29,33,34 and nanoparticles have also been obtained
using the layered materials as templates or scaffolds.35,36
However, further transferring these layered materials into even
solvothermal reduction of exfoliated small sheets of specific
smaller nanostructures via chemical redox reactions has never
LDHs.
been attempted till now.
It is worth noting that the process of exfoliating the layered
In our experiment, the layered millimeter or micrometer
FeOCl is of key importance for the synthesis of nearly uniform
sized bulk FeOCl (Fig. S1†) was firstly prepared by following
Fe3O4 nanobelts. The intense sonication of FeOCl solid before
the procedures described elsewhere.37 Typically, a mixture of
the solvothermal reaction is also highly required to get a stable
12.986 g of FeCl3 and 9.582 g of Fe2O3 was sealed in a glass
FeOCl colloidal solution and finally Fe3O4 nanobelts; other-
tube under vacuum and heated at 385 °C for a week. The as-
wise, we probably could get large micrometer sized aggregates
rather than Fe3O4 nanobelts (Fig. S3†). Moreover, the tempera-
obtained FeOCl product was washed thoroughly with acetone
and finally dried under vacuum. Further exfoliation of FeOCl
was conducted via sonication for 300 cycles of 5 s on and 5 s
off pulses with an output power of 400 W in ethylenediamine (en)
and block-copolymer poly(ethylene glycol)–block-poly(pro-
pylene glycol)–block-poly(ethylene glycol) (PEO–PPO–PEO or
P123). The as-formed stable colloidal suspension was then
sealed in a 50 mL PTFE-lined stainless steel autoclave and
heated at 140 °C for 12 h under static conditions followed by
slow cooling to room temperature. The resulting product was
thoroughly washed with absolute ethanol and a mixture of
ethanol and deionized water to remove all residual reagents,
and separated through centrifugation. The whole formation
mechanism of Fe3O4 nanobelts is summarized in Scheme 1. In
the reaction process, Fe2+ cations were formed via reduction of
a fraction of Fe3+ ions by the ethylenediamine, then smaller
Fe3O4 nanobelts were formed. Therefore, our synthetic
approach can be ascribed to the chemical “top-down” method.
Scheme 1 Synthetic pathways for the synthesis of Fe3O4 nanobelts
(blue) via a two-step method: (1) exfoliating FeOCl solid (red) into FeOCl
layers with the presence of en and P123 followed by (2) solvothermal
reduction of exfoliated FeOCl layers into Fe3O4 nanobelts (blue).
Fig. 1 Face-view (A) and side-view (C) TEM images, simulated models
(B) and SAED pattern (D) of typical Fe3O4 nanobelts.
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Fig. 2 XRD pattern (A) of Fe3O4 nanobelts, and FTIR spectra (B) of
Fe3O4 nanobelts and P123. Bars in A: JCPDS No. 88-0315.
ture, at which the precursors were solvothermally treated, is
another key parameter for the preparation of nearly uniform
Fe3O4 nanobelts. Too high a temperature (for example,
>180 °C) usually gives rise to the formation of irregular par-
ticles or aggregates due to the vigorous reaction process, while
too low a temperature (for example, <120 °C) might result in a
longer reaction time or the formation of impurity (α-Fe2O3).
Under optimized conditions (140 °C for 12 h), pure Fe3O4
nanobelts were obtained. The powder X-ray diffraction (XRD)
pattern of Fe3O4 nanobelts (Fig. 2A) indicates that the FeOCl is
completely converted to Fe3O4. The as-obtained Fe3O4 nano-
Fig. 3 (A) Suspensions of Fe3O4 nanobelts various solvents; (B)
amplified image of the transparent dispersion of Fe3O4 nanobelts in
acetonitrile; (C) left: amphiphilic Fe3O4 nanobelts originally dispersed in
belts are pure-phase ferrites with a cubic structure. The pres- water or hexane to which hexane or water was added, right: the thin
film formed at the water–hexane interface; (D) oil (hexane) in water
emulsion stabilized by Fe3O4 nanobelts; (E) amplified images of the cap-
sules in the as-formed emulsion with a size ranged from 20 to 450 µm.
ence of PEO–PPO–PEO in the final product is proved by
infrared (IR) spectroscopy. As shown in Fig. 2B, the typical
stretching vibrations of C–O–C and C–H appear in the IR spec-
trum of the Fe3O4 nanobelts even after thorough washing with
excess amount of ethanol and water. A strong Fe–O peak at
590 cm−1 was also observed. Two new peaks at 1407 and
nanobelts originates from the PEO–PPO–PEO block copolymer.
Gentle shaking or sonication easily moves the amphiphilic
1579 cm−1 suggested the formation of COO– groups after the
Fe3O4 nanobelts to the water–hexane interface (Fig. 3C right),
between the Fe3O4 nanobelt and the organic stabilizer could and a black/brown thin film forms at the interface and
solvothermal reactions. This is to say that a strong interaction
expands upwards along the wall of the glass bottle. As a result,
amphiphilic Fe3O4 nanobelts can be used to stabilize micro-
emulsion (Fig. 3D). Through rigorous shaking, the amphiphi-
lic Fe3O4 nanobelts can stabilize the oil-in-water emulsion or
reverse emulsion to form hollow capsules as shown in Fig. 3E.
be expected via chemical bonds. All these results indicated
that the as-obtained Fe3O4 nanobelts are hybrids of Fe3O4 and
P123.
The presence of P123 on the surface of Fe3O4 nanobelts
could not only keep them from aggregation, but also afford
them an amphiphilic surface. Unlike common nanoparticles, These as-obtained capsules with superparamagnetic properties
and excellent dispersibility in various common solvents could
be good candidates for drug delivery, controlled manipulation
which are either hydrophilic or hydrophobic, the products can
be well dispersed in most of the common solvents. For
instance, the as-synthesized powder of the Fe3O4 nanobelts of liquid droplets and micro-detection/reaction.29
The magnetic properties of the products were investigated
using a superconducting quantum interference device (SQUID)
can be well dispersed in polar and non-polar solvents (Fig. 3A),
and form stable colloids. Under optimal conditions, the col-
loidal solution of the Fe3O4 nanobelts in ethanol remains magnetometer. Fig. 4A shows the temperature dependence of
field-cooled (FC) and zero-field-cooled (ZFC) magnetization
measured with an applied field of 1000 Oe from 300 K to 4 K.
stable for more than four months. More importantly, all these
colloidal dispersions are transparent (Fig. 3B) due to the small
size of the Fe3O4 nanobelts, benefitting for their potential On cooling, the zero-field-cooled magnetization begins to drop
and deviates from the field-cooled magnetization at 65 K, the
applications in MRI or biomedicine.
blocking temperature (TB) of the as-obtained Fe3O4 nanobelts.
The Fe3O4 nanobelts remain in the polar solvent or in the
non-polar solvent (represented by water and hexane, respect- Typical hysteresis loops have also been observed for Fe3O4
nanobelts at 4 K and 300 K (Fig. 4B). It is seen that the Fe3O4
nanobelts are ferromagnetic at 4 K but nearly superpara-
ively, as shown in Fig. 3C left), in which they are originally dis-
persed, without shaking or sonication. This phenomenon
demonstrates the amphiphilic nature of the Fe3O4 nanobelts. magnetic at 300 K, consistent with the small size of the Fe3O4
nanobelts.38,39 At 4 K, the coercive forces (Hc) and the rema-
It is believed that the amphiphilic behaviour of the Fe3O4
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We have successfully prepared amphiphilic superparamagnetic
Fe3O4 nanobelts via a novel chemical “top-down” method.
This synthetic approach, using the exfoliated layered material
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quantity application. The as-obtained product, which exhibits
nearly uniform width, superparamagnetic properties, obvious
magnetic anisotropy and excellent dispersibility and redisper-
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