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Nano Res
1
Synthesis of hexagonal- and triangular Fe3O4
nanosheets via seed-mediated solvothermal growth
Chunhui Li (), Ruixue Wei, Yanmin Xu, Ailing Sun and Liuhe Wei ()
Nano Res., Just Accepted Manuscript • DOI: 10.1007/s12274-014-0421-3
http://www.thenanoresearch.com on January 22, 2014
© Tsinghua University Press 2014
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Nano Research DOI 10.1007/s12274‐014‐0421‐3
1
Synthesis of He xagonal- and Triangular Fe3O4
Nanosheets via Seed-Mediated Solvothermal
Growth
Chunhui Li*, Ruixue Wei, Yanmin Xu, Ailing Sun and
Liuhe Wei*
College of Chemistry and Molecular Engineering,
Zhengzhou Key Laboratory of Elastic Sealing Materials,
Zhengzhou University, Zhengzhou 450001, P R China.
Anisoprotic Fe3O4 nanosheets enclosed by {111} facets have
been prepared via a two-step microemulsion solvothermal
method. The presence and number of twin face are of crucial
important in the formation of hexagonal- and triangular
nanosheets. The material showed excellent catalytic activity in
the synthesis of quinoxaline in many solvents.
2
Synthesis of Hexagonal- and Triangular Fe3O4 Nanosheets via Seed-Mediated Solvothermal Growth
Chunhui Li (), Ruixue Wei, Yanmin Xu, Ailing Sun, Liuhe Wei () College of Chemistry and Molecular Engineering, ZhengZhou Key Laboratory of Elastic Sealing Materials, Zhengzhou University,
Zhengzhou, 450001 (P.R. China).
Received: day month year / Revised: day month year / Accepted: day month year (automatically inserted by the publisher)
©The Author(s) 2010. This article is published with open access at Springerlink.com
ABSTRACT Hexagonal‐ and triangular monodisperse Fe3O4 nanosheets have been synthesized via a two‐step
microemulsion solvothermal approach in which uniform Fe3O4 nanoparticle are prepared firstly and then
these hydrophobic nanocrystals are dispersed in identical microemulsion environment as seeds for further
re‐growth through secondary solvothermal process. The growth of anisotropic morphologies has been
explained by the presence and orientation of twin plane in the face‐centered cubic Fe3O4 which direct the
shape of the growing particles. Herein, reentrant grooves resulted from twin planes are favorable sites for the
addition of adatoms, leading to anisotropic growth. Triangular nanosheets are believed to contain one twin
face which directs the growth of the primary particles in two dimensions. Hexagonal nanosheets are believed
to contain two parallel planes that allow the growth edges to regenerate one another. The growth mechanism
is evidenced by the analysis of HRTEM results and the as‐prepared Fe3O4 nanoparticle has been proven an
effective catalyst in the synthesis of quinoxaline.
KEYWORDS
Fe3O4 nanocrystal, solvothermal synthesis, anisotropic growth, twin plane.
1. Introduction
Magnetic nanoparticles have been of great
interest because of its fascinating features and
wide range of extensive applications in
high‐density data storage, ferrofluids, medicine,
and as catalyst [1‐4]. The size and shape of
magnetic particles determine their physical and
chemical features, which may function as a
foundation for the development of new fields
[5‐7]. As a conventional magnetic material,
magnetite (Fe3O4) has yielded a great deal of
literature featuring numerous techniques and
nanoparticle morphologies. The exploration of
the novel routes for preparing Fe3O4 nanocrystals
Nano Res (automatically inserted by the publisher) DOI (automatically inserted by the publisher) Research Article
————————————
Address correspondence to Chunhui Li, E-mail: [email protected], [email protected]
3
with desirable size and morphology has been of
scientific and technological interest [8‐10].
In previous studies, solvothermal strategy has
been widely utilized to synthesize many kinds of
nanoparticles with uniform size and shape,
including monodisperse Fe3O4 nanocrystals size
of about 20 nm and microspheres in the range of
200‐800 nm [11‐13]. In most cases, the
morphologies of these kinetically favorable
particles are spherical and the exposed surfaces
are not specific crystal plane which may be
crucial to their features such as catalytic property
[14, 15]. Meanwhile, controlling the shape of
nanoparticles is also an equally important aspect
of desirable catalyst synthesis [15, 16]. However,
the challenge to synthetically and systematically
control both the morphology and exposed plane
of anisotropic Fe3O4 nanostructures with a simple
method still remains up to date [17‐19]. The
development of the mechanistic explanation is
expected to permit more elaborate control of
nanocrystal shape such that high yields of
shape‐specific particles can be obtained.
Compared with other transition metal oxides
extensively, Fe3O4 NP has attracted more
attention due to its property and application in
organic synthesis for it is inexpensive, relatively
non‐toxic and environmental friendly. Additional,
magnetic Fe3O4 NPs can be recovered easily after
the reaction by simple magnetic separation [20].
In this paper, hexagonal‐ and triangular
monodisperse Fe3O4 nanosheets enclosed by {111}
planes were successfully synthesized through a
two‐step microemulsion solvothermal process. In
the first step, near‐spherical monodisperse 7‐8
nm Fe3O4 nanoparticles formed through a
kinetics process. In the secondary step, the
formation of anisotropic Fe3O4 nanosheets is a
thermodynamic process and all the exposed
surfaces of the triangular and hexagonal
nanosheets are {111} planes, which have the
lowest surface energy for face‐centered cubic (fcc)
Fe3O4. The mechanism relied on the presence of
twin planes has been proposed to explain the
appearance of anisotropic fcc Fe3O4 nanoparticles
which can be supported by detailed analysis of
high resolution transmission electron microscopy
(HRTEM) results. The catalytic performance of
the as‐obtained materials was investigated in the
synthesis of quinoxaline.
2. Experimental
2.1. Synthesis of Fe3O4 NP “seeds”. In a typical
synthesis, 1 g NaOH, 10 mL H2O, 6 mL ethanol,
and 10 mL oleic acid (OA) were mixed together to
form an even solution. After stirring for half an
hour, an aqueous solution of 2 mmol FeSO4∙7H2O
(560 mg in 14 mL de‐ionized water) was then
added. After further stirring for 15 min, the
solution was transferred into an autoclave and
kept at 180 oC for 10 h. The system was then
allowed to cool to room temperature. The Fe3O4
black product obtained at the bottom of the
autoclave which can be well‐dispersed in hexane
was deposited by adding ethanol for next step.
2.2. Synthesis of triangular Fe3O4 nanosheets. 1
g NaOH, 10 mL H2O, 6 mL ethanol, and 10 mL
oleic acid (OA) were mixed together followed by
stirring for half an hour. The solution was mixed
with Fe3O4 NP “seeds” to form a black system
and then 0.6 mmol FeSO4 in 14 mL H2O was
added. After further stirring for 30 min, the
solution was transferred into an autoclave and
kept at 180 oC for 10 h.
2.3. Synthesis of hexagonal Fe3O4 nanosheets.
The synthetic procedure of hexagonal Fe3O4
nanosheets was similar to above expect that the
agitation time before solvothermal process was
shorten from 30 min. to 1 min.
2.4 Characterizations. The crystal structure and
phase purity were characterized by using a X’Pert
PRO X‐ray diffractometer (XRD) with Cu Kα
radiation (λ = 1.5406 Å) in the 2θ range 10o to 70o.
The Fourier transform infrared (FT‐IR) spectra of
4
the sample were collected on a PerkinElmer
Spectrum GX from 4000 to 400 cm‐1.
High‐resolution transmission electron
microscopy (HRTEM) was performed using a FEI
Tecnai G2 F20 S‐Twin microscope working at an
acceleration voltage of 200 kV. XPS analyses were
performed with a THERMO ESCALAB 250Xi.
3. Results and Discussion
3.1. Design of reaction strategy. As mentioned
above, the experimental part of this work consists
of two steps. In the first step, Fe3O4 NP “seeds”
range of 7‐8 nm has been prepared according to
traditional microemulsion solvothermal reaction
in which oleic acid (OA) was used as surfactant.
Meanwhile, OA molecules adsorbed onto the
surface of the as‐prepared nanoparticles and
these nanoparticles became hydrophobic. Since
the amount of OA is relatively less than that of
water plus alcohol, the homogeneous solution
should be a positive microemulsion [21, 22].
When Fe3O4 nanoparticles “seeds” were
transferred into the above solution, the Fe3O4 NP
was embedded into the “oil core” owing to their
hydrophobic characteristics. That is to say, Fe3O4
nanoparticles “seeds” can be dispersed stably in
microemulsion followed by secondary
solvothermal process. Furthermore, the size of
“oil core” is comparable to the size of Fe3O4 NP
“seeds” which encouraged us to deduce that each
“oil core” contain only one Fe3O4 NP as seeds for
further growth (Figure 1).
3.2. Structure and formation mechanism of
Fe3O4 nanosheets. The XRD patterns of Fe3O4 NP
“seeds”, triangular and hexagonal nanoparticles
are very similar and presented in Fig. 2. All peaks
of the nanostructures can be well matched with
the peaks of pure fcc Fe3O4 (JCPDS Card No.
76‐1849) with lattice constants a = 8.4 nm.
The FT‐IR spectrum of the 7‐8 nm Fe3O4 NP
“seeds” (Fig. S1 in the Electronic Supplementary
Material (ESM)) and all the peaks give direct
support to the existence of a C17H31COO‐ unit
coating outside the Fe3O4 NP “seeds” [23, 24].
Figure 1 Formation mechanism of anistropic Fe3O4
nanosheets.
Figure 2 XRD patterns of the Fe3O4 nanoparticles.
The XPS spectra of C 1s, Fe 2p core level give
further proofs for the chemical structure of the
OA coated Fe3O4 nanoparticles in Figure 3. Two C
1s peaks located at 288.0 and 284.8 eV for Fe3O4
nanoparticles. Peak at 284.8 eV can be ascribed to
the carbon atoms in the aliphatic chain (C‐C), and
peak at 288.0 eV belonged to the carboxylate
(‐COO‐) moiety, which was consistent with the
data obtained from carboxylateds in the literature
previously reported. C 1s peak corresponded to
carboxylic carbon (‐COOH), which located at 290
eV, did not appear in the spectrum, indicating the
absence of free acid on the Fe3O4 nanoparticles.
The bonding energy at 710.6 eV is the
characteristic peak from Fe 2P3/2 core level
electrons. The Fe 2p1/2 peaks at 724.4 eV can be
attributed to the carboxylate‐Fe bond [25]. The
XPS results accord with the FTIR data, indicating
the formation of chemical bonds between the
5
Fe3O4 nanoparticles and the oxygen atoms of the
carboxylic acid.
Figure 3 XPS spectra of (a) C 1s peaks, (b) Fe 2p peaks.
As shown in Figure 4, Fe3O4 NP “seeds” have
quasi‐spherical shape with diameter of 7‐8 nm.
Figure 4 TEM images of Fe3O4 NP ”seeds”.
Figure 5(a) shows the TEM image of regular
triangular Fe3O4 nanosheets size of about 16 nm,
which is obviously bigger than 7‐8 nm Fe3O4 NP
“seeds” after re‐growth under solvothermal
condition. Further details concerning the
structure of the Fe3O4 triangular nanosheets are
obtained by the high‐resolution TEM (HRTEM).
Figure 5(b) shows a representative HRTEM
image of a single triangular nanosheet of fcc
Fe3O4, and the top-right inset shows the
corresponding Fast Fourier Transform (FFT)
image. As shown in Fig. 5(b), the lattice spacing
along different direction of 0.3 nm correspond
well to the crystal plane of (220) and the angle
between two lattice directions is 60°.
As is well known that the crystal plane 0)2(2 , )220( ,
)202( , )202( , )022( and )220( of fcc material
belong to the family of crystal planes {220}, six planes are all parallel to [111] direction and have
same lattice spacing. Moreover, the angles
between the adjacent crystal planes should be 60o.
Given all that the electron beam direction can be
indexed to the [111] direction of the fcc Fe3O4
nanoparticles and the exposed face is (111) plane,
which is explicated in the lower‐right inset in Fig.
5(b).
Figure 5 (a) TEM image of triangular Fe3O4 nanosheets, (b)
HRTEM of single triangular nanosheet and FFT (top-right).
TEM image of hexagonal Fe3O4 nanosheets is
shown in Figure 6(a). It is clear to recognize that
the shape of each nanoparticle is a hexagon with
size of 12‐14 nm. HRTEM image of a hexagonal
Fe3O4 nanoparticle is given in Figure 6(b) and
top‐right inset shows the corresponding Fast
Fourier Transform (FFT) image which is similar
to those of regular triangular Fe3O4 nanosheets.
Based on same reasons, the exposed crystal plane
of hexagonal Fe3O4 nanosheets is also (111) plane,
as shown in the lower‐right inset in Fig. 5(b).
Solvothermal and hydrothermal synthesis
routes has been widely and effectively adopted to
produce monodisperse nanostructures many of
which are isotropic quasi‐spherical nanoparticles.
In most cases, the particles morphologies have
been explained by the particular reaction
environment in which they were generated.
Generally, the growth mechanisms often fall into
two general categories. One uses the presence of
organic molecules in the reaction system to
reduce or speed up the addition of adatoms to
specific crystal faces. The second viewpoint
6
suggests that high concentration of surfactant
create micelles, which functioned as templates for
crystal growth and as space‐confining structures
that direct the particle shape physically [26].
Figure 6 (a) TEM image of hexagonal Fe3O4 nanosheets, (b)
HRTEM of hexagonal nanosheet and FFT (top-right).
In this work, both physical‐constriction and
surface‐modification mechanisms are difficult to
explain the observations of triangular and
hexagonal Fe3O4 nanoparticles [5, 11]. So, it may
be necessary to propose a new mechanism for the
growth of anisotropic Fe3O4 nanoparticles. It
seems that a strictly thermodynamic explanation
cannot be convincing, but that the kinetics of
growth, considering the symmetrical
characteristic of Fe3O4 species, plays a major role.
As already known, the surface energies of fcc
follow a general sequence of γ{111} < γ{100} <
γ{110}. On the basis of the surface free energy
minimization principle, octahedral structures
enclosed by eight {111} planes or tetrahedron‐like
structures enclosed by four {111} planes have the
lowest surface energy for fcc crystal structure, if
only obeying thermodynamic factor [27‐30].
But it is not the case and the particle
morphologies reported in literatures and in our
synthesis methods employed closely correspond
to those observed in the synthesis of fcc crystals,
such as Ag, Au and silver halide. Isotropic
particles, wires, and hexagonal and triangular
sheets have been produced for fcc species. The
growth mechanism of anisotropic sheets can be
explained by the formation of twin planes on
[111]‐type faces (Figure 7).
Figure 7 Fe3O4 models for a particle with single and two
parallel twin planes.
The twin planes form easily in fcc crystal
structures including Fe3O4 species, where the
stacking fault energy is lower than most metals,
decreasing the energy required to form a twin
plane. Due to the sixfold symmetry of the fcc
system, these twinned crystals form
hexagonal‐shaped nuclei. At the six surface
where the twin plane terminates, the stacking
fault of the twin plane cause {111} faces to form in
alternating concave and convex orientations,
designated “I‐type” and “II‐type”, respectively
(Figure 8).
Figure 8 Alternating sides contain I-type and II-type faces.
Similar to the structure reported in literature [27,
30], the I sides are bounded on each side by
slow‐growing II sides in a crystal with a single
twin plane, the I‐sides quickly grow themselves
out of existence, leaving a triangular sheet as
expressed in Figure 9.
Figure 9 Formation of triangular and hexagonal Fe3O4
nanosheets.
Figure 9 also describes the formation of
hexagonal Fe3O4 nanosheets with two parallel
twin planes. The crystallographic directions of
7
the second twin plane can be oriented at a 60o
rotation relative to the first twin plane. This cause
the I‐sides resulted from the second plane to be
matched with the II‐sides of the first twin plane,
and vice versa. That is to say, all six edges of the
particle generated by the intersection of the twin
planes with the surface would have fast‐growing
I‐sides. Each growing face regenerates the two
adjacent faces, preventing them from growing
themselves out and leading sustained growth.
In this work, the detailed structure of side faces
which mentioned as “I‐type” and “II‐type”
cannot be observed because of the sizes of
triangular and hexagonal Fe3O4 nanosheets are
too tiny. The analysis of HRTEM of Fe3O4 triangle
also supports the conclusion. It is not difficult to
discover that three crystal planes (Fig. 10(b)) are
all perpendicular to three sides of the triangular
Fe3O4 NPs, respectively.
Figure 10 Crystal planes of triangular Fe3O4 nanosheet.
Herein, we chose one side as a represent to
explain this phenomenon in detail, which is the
same with the other two sides. As described
above, the main surface of Fe3O4 NPs and the side
surface belong to the family of crystal planes
{110}, so we can name the main surface (111) and
name the side surface )111( . On the basis of
crystallographic rule, the (111) and )111( faces are all
perpendicular to the 0)2(2 plane. So the intersection
line of face (111) and )111( is also perpendicular to the
0)2(2 plane in the light of geometrical principle.
Namely, the 0)2(2 plane is perpendicular to the side
of the Fe3O4 triangle which can be indexed as
]022[ direction (Fig. 10(a)). Likewise, the
crystallographic planes and directions of the
other two sides of the triangular Fe3O4 NP were
labeled in Figure 10 (b).
3.3. Catalytic performance. In order to
investigate the catalytic activity of as‐prepared
Fe3O4 NPs, we report here the synthesis of
quinoxaline from o‐phenylenediamine and
1,2‐diphenylethanedione catalyzed by Fe3O4 NPs
in different solvents (Scheme 1) (detailed
synthetic procedure and characterization were
given in the Electronic Supplementary Material
(ESM)). Controlled experiments results show that
the catalytic performance of three samples (7‐8
nm Fe3O4 NPs, triangular Fe3O4 and hexagonal
nanosheets) are indistinguishable. The can be
comprehended easily for the fact that the sizes
and exposed crystal plane of the three samples
are very similar which are critical to the catalytic
activity of nanomaterials.
Scheme 1 Synthesis of quinoxalines catalyzed by Fe3O4
NPs.
As shown in Table 1, the catalytic activities of
as‐obtained Fe3O4 NPs have little relationship to
the solvent and 1.3 mol‐% of Fe3O4 is enough. In
addition, in the absence of catalyst, the reaction
was slow and gave unsatisfactory yield of
quinoxaline. The above results show that catalytic
performance of as‐prepared Fe3O4 NPs is
comparatively effective for the synthesis of
quinoxaline, no matter which solvent is chosen.
Table 1 Optimization of reaction conditions.
Catalyst
[mol-%]
Solvent Time [h] Yield
[%]
1 0 n-Hexane 4 9.2
2 1.3 n-Hexane 4 99.6
3 4 CHCl3 4.5 98.2
4 10 EtOH 2.5 99.3
5 10 n-Hexane 2.5 92.9
6 10 CHCl3 2.5 90.1
7 10 H2O 2.5 81.6
8
4. Conclusions
Anisotropic triangular and hexagonal Fe3O4
nanosheets have been synthesized through
two‐step solvothermal reactions. Hydrophobic
7‐8 nm Fe3O4 NPs formed in the first
solvothermal reaction can be dispersed well in
positive microemulsion as “seeds” to re‐grow to
generate bigger anisotropic NPs during the
secondary solvothermal process. Triangular and
hexagonal Fe3O4 nanosheets exposes {111} planes
and they are not single crystals which is very
common for monodisperse NPs, but twinned
crystals with twin planes. Nuclei contains single
twin face form triangular Fe3O4 nanosheet while
that contains two twin faces form hexagonal
Fe3O4 nanosheet. Detailed research concerning
the relationship between the {220} planes and the
sides of Fe3O4 triangle supports the formation
mechanism. The as‐obtained Fe3O4 NPs showed
excellent catalytic performance in the synthesis of
quinoxaline whichever solvent was chosen.
Acknowledgements
This work was supported by the National
Natural Science Foundation of China (Grant Nos.
20901069, 50873093 and 21271156) and Henan
Province Scientific and Technological Research
Program (092102210054).
Eletronic Supplementary Material:
Supplementary material (IR spectrum of
OA‐coated Fe3O4 nanoparticles; synthesis and
NMR spectrum of quinoxaline) is available in the
online version of this article at
http://dx.doi.org/10.1007/10.1007/s12274‐***‐****‐*
(automatically inserted by the publisher) and is
accessible free of charge..
Open Access: This article is distributed under the
terms of the Creative Commons Attribution
Noncommercial License which permits any
noncommercial use, distribution, and reproduction
in any medium, provided the original author(s)
and source are credited.
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10
Electronic Supplementary Material
Synthesis of Hexagonal- and Triangular Fe3O4 Nanosheets via Seed-Mediated Solvothermal Growth
Chunhui Li (), Ruixue Wei, Yanmin Xu, Ailing Sun, Liuhe Wei () College of Chemistry and Molecular Engineering, ZhengZhou Key Laboratory of Elastic Sealing Materials, Zhengzhou University,
Zhengzhou, 450001 (P.R. China). Supporting information to DOI 10.1007/s12274-****-****-* (automatically inserted by the publisher)
IR spectrum of OA‐coated Fe3O4 nanoparticles
Fig. S‐1 IR spectrum of the Fe3O4 nanoparticle powder.Two strong peaks around 2921cm‐1 and 2850cm‐1 can be
attributed to the asymmetry and symmetry stretching vibration of C‐H bond. Two strong peaks at 1632cm‐1 and 1462cm‐1
were caused by the asymmetry and symmetry stretching vibration of –COO‐ group, which are two characteristic peaks
to identify carboxylic salt in the IR database.
General Procedure for the Synthesis of quinoxaline
1 mmol o‐phenylenediamine, 1 mmol 1,2‐diphenylethanedione, certain amount of Fe3O4 NPs and 5 mL solvent
was stirred at room temperature for a given time. After completion of the reaction (monitored by TLC), a conv
entional permanent magnet was applied to the bottom of the reaction flask to separate Fe3O4 NPs from the sol
ution. Purification was performed by short column chromatography on silica gel eluting with petroleum/ethyl ac
etate (10:1 v/v) to obtain pure quinoxaline as a light yellow solid.
δH 7.300‐7.394 (m, 6H), 7.506‐7.538 (m, 4H), 7.746‐7.798 (m, 2H), 8.156‐8.209 (m, 2H).
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Address correspondence to Chunhui Li, E-mail: [email protected], [email protected]