The results show that ascorbic acid as reductant and urea as precipitator supplied a relatively steady environment during the synthesis process and led to the formations of Fe3O4 tetrahe
Trang 1Synthesis, characterization and growing mechanism of monodisperse
Fe 3 O 4 microspheres
Yingdi Lva, Hui Wanga, , Xiaofang Wanga, Jinbo Baib
a
Key Laboratory of Synthetic and Natural Functional Molecule Chemistry (Ministry of Education), Department of Chemistry, Northwest University, Xi’an 710069, PR China
b Laboratory MSS/MAT, CNRS UMR 8579, Ecole Centrale Paris, 92295 Chatenay Malabry, France
a r t i c l e i n f o
Article history:
Received 14 January 2009
Received in revised form
8 March 2009
Accepted 31 March 2009
Communicated by S Uda
Available online 8 April 2009
PACS:
61.10.Eq
81.16.Dn
81.07-b
81.07.Lk
81.10.Dn
Keywords:
A1 Nanostructures
A2 Growth from solutions
A2 Hydrothermal crystal growth
B1 Nanomaterials
B2 Iron oxide
a b s t r a c t
Monodisperse Fe3O4microspheres assembled by a number of nanosize tetrahedron subunits have been selectively synthesized through the hydrothermal process The synthesized Fe3O4microspheres have good dispersibility The subunits made up of microspheres were uniform in size and like-tetrahedron in shape The average diameter of each Fe3O4microsphere is about 50–55mm The length of each edge of tetrahedron is about 100 nm A series of experiments had been carried out to investigate the effect of reductant, precipitator and reaction time on the formation of Fe3O4microsphere and tetrahedron subunits The results show that ascorbic acid as reductant and urea as precipitator supplied a relatively steady environment during the synthesis process and led to the formations of Fe3O4 tetrahedron subunit and monodisperse Fe3O4microspheres As the reaction time increased from 3 to 24 h, the Fe3O4 microspheres tended towards dispersion and becoming large in size from 10–20 to 50–55mm, and the subunits formed Fe3O4 microspheres that varied from spheroid to tetrahedron and from a small nanoparticle (20–30 nm) to a large one (90–110 nm) A reasonable explanation for the formations of the
Fe3O4 microsphere and the tetrahedron subunit was proposed through Ostwald ripening and the attachment growth mechanism, respectively
&2009 Elsevier B.V All rights reserved
1 Introduction
It has been accepted that the special structure and
character-istics of microspheres assembled by nanoparticles endow them
with potential application such as catalysis, medicine, electronics
and contrast agents in MIR[1] Thus, the synthesis of magnetic
particles with specific size and well-defined morphologies has
become a hot topic in the related research field Among magnetic
particles, Fe3O4have been extensively investigated
Fe3O4as one of the most important transition magnetic metal
oxides has received increasing attention due to its extensive
applications It has been considered as an ideal material for
magnetic data storage[2], a candidate for biological application
such as a tag for sensing and imaging [3], and a drug-delivery
carrier for antitumor therapy [4] The synthesis of Fe3O4
nanocrystals with different sizes and shapes has attracted
considerable interest in recent years So a number of Fe3O4
nanocrystals, such as nanoparticles, nanowires, nanorods,
nano-belts, nanopyramids, and nanotubes via different synthesis methods have been reported by different research groups[5–11] Although the fabrication of monodisperse magnetic particles have been pioneered by Matijevic’s group in the early 1980s[12], no more attention has been paid to the synthesis of monodisperse-sphere microparticles assembled by nanoparticles In this paper, we report a new synthesis method of Fe3O4 microspheres assembled
by a specific nanoparticles via hydrothermal method at a low temperature, in which FeCl36H2O, ascorbic acid, urea and poly-ethylene glycol 6000(PEG-6000) were used as precursor, reductant, precipitator and surfactant, respectively The effects of the reductant and the precipitator on the formation of products were discussed In addition, the function of the surfactants in the process of self-assembled Fe3O4microspheres was also investigated
2 Experimental section 2.1 Materials
Hexahydrated ferric chloride (FeCl36H2O), sodium hydroxide (NaOH) and urea were bought from Rgent Company in Tianjin
Contents lists available atScienceDirect
journal homepage:www.elsevier.com/locate/jcrysgro
Journal of Crystal Growth
0022-0248/$ - see front matter & 2009 Elsevier B.V All rights reserved.
doi:10.1016/j.jcrysgro.2009.03.046
Corresponding author Tel.: +86 29 8836 3115; fax: +86 29 8830 3798.
E-mail address: huiwang@nwu.edu.cn (H Wang).
Trang 2Ascorbic acid was bought from Northwest Geological Institute of
Nonferrous Metals in Xi’an Polyethylene glycol 6000 (PEG-6000)
was bought from Kermel Company in Tianjin All the reagents are
AR
2.2 Synthesis methods
In a typical procedure, 0.003 mol FeCl36H2O, 0.003 mol
ascorbic acid, 0.015 g PEG-6000 and 0.6 g urea were dissolved in
a deionized water of 25 ml to get an orange solution of 0.12 mol/L
Fe3+ions The precursor solution was sealed into a 50 ml
Teflon-lined autoclave, followed by the hydrothermal treatment at 120 1C
for 3 or 8 or 24 h and then cooled to room temperature naturally
Three kinds of black solid products could be obtained at three
various times, respectively These hydrothermal products would
be collected after washing with deionized water several times and
subsequently dried in vacuum at 60 1C for 6 h The
above-described experiment conditions are denoted as the standard
condition The other conditions (under the standard condition)
were invariable if changed the species of the precipitator or the
amounts of the reductant
2.3 Characterizations
X-ray diffraction (XRD) patterns were recorded on a Rigaku
(Japan) D/Max r-A X-ray diffractometer with Cu Ka radiation
(50 kV, 300 mA) at room temperature in air Scanning electron
microscopy (SEM) images were taken with Quanta 400 ESEM-FEG
instrument operated at 25 kV, transmission electron microscope
(TEM) images were taken with JEOL JEM-3010 instrument
operated at 300 kV They were used to characterize morphology,
particle sizes, and compositions of the products The specific
surface areas of the samples were measured with a surface
analyzer (JW-K) by the absorption of N2at liquid N2temperature
(BET method)
3 Result and discussion
Ascorbic acid had been reported as a relatively weak reductant
in many studies [13,14] Urea also had been reported as an
effective precipitator in the previous research due to its alkaline
reaction by slow hydrolysis at 70 1C [15,16] This characteristic
made the pH of the reaction system change in a narrow range So
we selected ascorbic acid as reductant and urea as precipitator In
addition, if ascorbic acid and urea are simultaneously utilized, a
very soft reaction that favors the growth of crystal will take place
However, the changes in the number of the reductant and the
species of precipitator would influence the synthesis of product
3.1 Effect of reductant and precipitator
Fig 1shows the SEM and TEM images of the product prepared
at 120 1C for 24 h under the standard condition.Fig 2shows the
XRD patterns of the various products synthesized by the
above-mentioned methods in Section 2.2 It is apparent fromFig 1a that
the Fe3O4product synthesized in this work presents in the form of
a large amount of microspheres, and all the microspheres are
monodisperse with a relatively uniform size XRD pattern shown
inFig 2(a)-2 indicates the product to be a cubic-structured Fe3O4
according to the standard pattern (JCPDS Card no 85-1436,
a=8.393 A) The average diameter of the microspheres shown in
Fig 1b is between 50 and 55mm and each of them is very round
The existing small holes and chippings were due to outside force
effect originating from SEM test on the surface of microspheres
Fig 1c shows an image of individual microsphere with a clear surface and the surface is smooth Fig 1d reveals the specific surface condition of the microspheres shown in Fig 1c The surface of microspheres was made of a number of small subunits, each of which is like a tetrahedron crystal in shape and uniform in size and packed densely.Fig 1e clearly exhibits that each of the subunits is a nanometer size of regular tetrahedron crystal with four symmetrical horns, and TEM imagine in Fig 1f further supports this result The TEM imagine also shows that the length
of each edge of tetrahedron is about 100 nm The selected area electron diffraction pattern indicates the nanosize tetrahedron to
be a single-crystalline structure
To investigate the formation factors of Fe3O4microspheres, a series of experiments had been carried out in this work The results show that ascorbic acid plays an important roll in the formation of Fe3O4 microspheres It is found that Fe3O4 could
be formed in the presence of ascorbic acid, and in contrast, not formed in the absence of ascorbic acid under the standard condition The reason for it may be explained that an appropriate amount of ascorbic acid (0.003 mol), forming a gentle reductant, could slowly reduce Fe3+ to Fe2+ by the following reaction 2Fe3++C6H8O6=2Fe2++C6H6O6+2H2+, and presenting a gradual reduction effect at a moderate temperature of 120 1C; and subsequently, the reduced Fe3+ (Fe2+) and the unreduced Fe3+
contributed to the formation of Fe3O4 The further experiment also found that the amount of ascorbic acid was an important impact factor in our system As the content of ascorbic acid was increased
to 0.005 mol with other condition as constant, FeCO3 could be formed in the reaction system due to existing diffraction peak assigned to FeCO3 (JCPDS Card no 29-696, a=8.393 A) in the products, as described inFig 2(b) This may be due to the fact that the increase of ascorbic acid decreased the pH value in the system (about pH 6–5), and this relatively low pH value favored not only the reduction of all Fe3+ to Fe2+ but also the reaction of urea hydrolysis (NH2)2CO+3H2O=2NH3H2O+CO2, led to producing much more CO2to form FeCO3 While the pH values were 8–9, the content of ascorbic acid was 0.003 mol under the standard condition It is concluded that acidic solution was not favorable to the formation of Fe3O4 In addition, comparing the XRD patterns
of FeCO3sample with FeCO3standard inFig 2(b), another crystal phase assigned to FeO can be observed in the sample (JCPDS Card
no 6-615, a=8.393 A) A reasonable explanation for it is that the excessive ascorbic acid could reduce almost all Fe3+to Fe2+, and formed Fe(OH)2due to urea hydrolysis and further generated FeO under hydrothermal condition This change process could be also described as follows: Fe2++2NH3H2O=Fe(OH)2+2NH4 and Fe(OH)2=FeO+H2O As the quantity of ascorbic acid decreased to 0.001 mol in the reaction system, the XRD pattern inFig 2(a)-4 indicated the formation of Fe2O3 The reason may be that, on one hand, the amount of ascorbic acid added in the reaction system was relatively lower; on the other hand, a part of ascorbic acid in the system reacting with O2dissolved in water made the amount
of ascorbic acid to be too low to reduce Fe3+to Fe2+, and finally led
to the formation of Fe2O3 Urea as precipitator was indispensable to this reaction system, without urea Fe3O4 would not produce As the precipitator changed into sodium hydroxide (NaOH) the morphologies of precipitated Fe3O4occurred change A number of microspheres of
Fe3O4were formed in the (NaOH) system, but the surface of each microsphere shown inFig 3a is not tight and rather rough.Fig 3b clearly shows sheet structures on the surface and there are many interspaces among sheets The sheets have obvious edges It indicates that different pH value affected the formation rate of
Fe3O4and further changed the microsphere of Fe3O4, namely, the nucleation rate of Fe3O4 could be controlled by pH [17] One reason for it may be that, when urea was used as precipitator, pH
Trang 3Fig 1 The SEM and TEM imagines of Fe3O4 products synthesized at 120 1C for 24 h under the standard condition: a solution of 25 ml with 0.003 mol FeCl3 6H2O, 0.003 mol ascorbic acid, 0.015 g PEG-6000 and 0.6 g urea.
20
(a)-5
(a)-4 (a)-3
(a)-2
(a)-1
(b)-3
(b)-1 (b)-2
2θ (degree)
2θ (degree)
Fig 2 XRD patterns (a)-1 Fe3O4 after reaction for 3h and (a)-2 after reaction for 24 h with 0.003 mol ascorbic acid, (a)-3 Fe3O4 standard, (a)-4 Fe2O3 after reaction for 24 h
Trang 4value in the (urea) system maintained a relative constant due to
keeping a hydrolytic equilibrium of urea hydrolyzed step-by-step,
so the nucleation rate of Fe3O4 was relatively slower in urea
system than in NaOH system This relative steady environment
during nucleation favored the formation of homogeneous and
tight Fe3O4 In contrast, pH values in NaOH system changed all
through and decreased with the nucleation time of forming Fe3O4,
and further resulted in the formation of Fe3O4microsphere with
the loose and sheet structure The other reason may be that the
crystal seed already existed in NaOH system before Fe3O4
generation due to existing OH and the solid seed would
influence the morphology of final crystal; while there was no
crystal seed before Fe3O4 formation in the system used urea as
precipitator, so the morphology of Fe3O4, formed by gradual hydrolysis of urea, was more uniform in urea system than in NaOH system
3.2 Effect of reaction time
To have further insight into the formation process of the Fe3O4
microspheres, the products formed at different reaction times of 3 and 8 h were characterized by SEM observation and BET method The SEM images are shown in Fig 4 Fig 4a reveals that the microspheres of Fe3O4formed at 120 1C for 3 h are the aggregates
of a lot of spheroidal particles (subunits) and have small opening
2 μμm
50 μm
Fig 3 SEM imagines of Fe3O4 products synthesized at 120 1C for 24 h with a solution of 25 ml with 0.003 mol FeCl3 6H2O, 0.003 mol ascorbic acid, 0.015 g PEG-6000 and 0.4 g NaOH as precipitator.
200nm
d c
50 μμm
a
200nm
b
Fig 4 SEM images of Fe3O4 products synthesized at 120 1C for 3 (a and b) and for 8 h (c and d) with a solution of 25 ml with 0.003 mol FeCl3 6H2O, 0.003 mol ascorbic acid, 0.015 g PEG-6000 and 0.6 g urea as precipitator.
Trang 5on the surface The size of microspheres agglomerates is about
10–20mm (Fig 4a) The spheroidal subunit shown inFig 4b is a
uniform nanospheroid and its average diameter is around
20–30 nm As the reaction time was prolonged to 8 h, it is
obvious fromFig 4c that some monodisperse microspheres with a
size of 15–25mm can be observed and their dispersion became
better than those formed at 120 1C for 3 h The surface of
the microspheres is incompact compared to the surface shown
inFig 4a and the microspheres were also the aggregates of many
of small subunits with a nanosize of 40–60 nm inFig 4d But the
subunit in shape was somewhat close to tetrahedron between
spheroid and tetrahedron, as the image inserted inFig 4d As the
reaction time was further prolonged to 24 h a completely
tetrahedron subunit with a nanosize of 90–110 nm that made up
the monodisperse microspheres was formed, as shown inFig 2e
and f Accordingly, it concludes that as the reaction time
prolonged from 3 to 24 h the aggregated microspheres made up
of subunits tended towards dispersion and becoming large in size
from 10–20 to 15–25 to 50–55mm again, and the subunits tended
to vary from spheroid to tetrahedron and from a small
nanoparticle (20–30 nm) to a large one (90–110 nm) But they
were still the same crystal as Fe3O4due to no difference between
the products of 3 and 24 h supported by the XRD patterns in
Fig 2(a)-1 and (a)-2 which agreement with standard card (JCPDS
Card no 85-1436,a=8.393 A) The attachment growth mechanism
may be introduced in our system Because any nanoparticle has an
active face with high surface energy during crystal growth process, the crystal growth preferentially occurs on this active face[18] So in our reaction system, one spheroidal was attached with subsequent formed Fe3O4 crystal from the four-equipotent directions to decrease its surface area to decrease the surface energy at maximum degree This assumption is further confirmed
by BET results listed inTable 1 As the reaction time was 3, 8 and
24 h, the BET specific surface areas of three Fe3O4 products distinctly tended to decrease, and being 23.9174, 19.8599 and 13.6639 m2g 1, respectively The experimental result indicates that the surface energy was indeed the driven force of this process All this could be explained inFig 5 The spherical subunit generated firstly at the early stage of the fast nucleation in the first
3 h As the reaction time prolonged, a dissolution-recrystallized process happened in this system The formed fresh Fe3O4particle preferentially grew on the surface of the spherical subunit from the four-equipotent direction (step 1) As the reaction time further prolonged, the dissolution-recrystallized process continually occurred on the active face by overlapping layer by layer (step 2) until the subunits of tetrahedron shape were completely formed (step 3) in order to lower-specific surface energy There are many reports concerning the formation mechanism
of inorganic microspheres[19,20] The formation of microspheres after fast nucleation in solution is related to two primary mechanisms: random aggregation and Ostwald ripening In the present work, Ostwald ripening may be involved in the formation
of Fe3O4microspheres Firstly, some Fe3+was reduced by ascorbic acid to Fe2+in our reaction system When the reaction tempera-ture reached 70 1C urea hydrolysis produced OH to increase pH values in the system and subsequently the nanospheroid crystal generated no sooner than the precipitation reaction happened Driven by the minimization of interfacial energy, the spheroidal nanocrystals would act as primary building units to produce larger self-assembled aggregates (Fig 4a) This was a kinetically fast process because our system was aqueous with many surface
Table 1
The results of BET surface area of Fe3O4 synthesized in 3, 8 and 24 h.
Sample Reaction time (h) BET surface area/m 2
g 1
Step 1
Overlapping layer by layer
Continuing layer by layer
Fig 5 The schematic sketch of formation process of Fe3O4 tetrahedron crystal.
FeCl3·6H2O
VC Urea +
Form Aggregation
Microspheres
Shape change Subunits
Monodisperse Microspheres Generate
Ostwald Ripening
Microspheres
Fast Nucleation
Subunits
3h
24h
Fig 6 The schematic sketch of formation process of microspheres.
Trang 6hydroxyls and did not allow spheroidal subunit to rotate
adequately to find the low-energy configuration interface and
form perfectly single crystal [21] These aggregates formed
microspheres and continued to grow through Ostwald ripening
As stated early, it is obviously observed fromFigs 4 and 1that the
size of the agglomerates changed from 10–25 to 50–55mm with
increasing time from 3 to 24 h (Figs 4a and b and1c) and the size
of the subunits varied from 20–30 to 90–110 nm (Figs 4b, d and
1f), which was in agreement with the result of Ostwald ripening
In addition, as the time prolonged the microsphere surface
became more and more compact in structure due to the change
of subunit in shape from spheroid to tetrahedron at the cost of
newly formed smaller Fe3O4crystal, and the microspheres became
more and more dispersed (seeFig 2a) because of the continuous
hydrothermal process and the presence of PEG-6000 We found
that polyethylene glycol 6000 (PEG-6000) kept microspheres
monodisperse When PEG-6000 was removed from our system
with the other condition as constant, the microspheres still
existed and most of them aggregated together When we used
0.015 g of poly-vinylpyrrolidone (PVP) instead of PEG-6000, the
laminated structure was formed The results show that PEG-6000
was not only acting as a surfactant but also as a molding agent
Fig 6illustrates the schematic sketch of formation process of the
microsphere
4 Conclusion
We developed a convenient method to synthesize Fe3O4
microspheres assembled by nanosize subunits with a tetrahedron
structure through hydrothermal process Ascorbic acid as
reduc-tant and urea as precipitator played an imporreduc-tant role in the
process of Fe3O4 crystal formation Both of them supplied a
relatively steady environment, which was a crucial factor that
determined the morphologies of subunits and Fe3O4microspheres
formed by the subunits On the other hand, the reaction time was
also an important factor that determined the dispersion of Fe3O4
microspheres and the generation of tetrahedron subunits We also
proposed a reasonable exploration for the growth mechanism of
tetrahedron subunit crystal and the aggregations mechanism of
Fe3O4microsphere in the present work Ostwald ripening and the
attachment growth mechanism were responsible for the
forma-tion of Fe3O4microspheres and the growth of tetrahedron subunit,
respectively The Fe3O4 microsphere made up of tetrahedron
subunit for hydrogen storage as material has a potential applica-tion value[22–24]
Acknowledgments The present work was financially supported by the National Hi-Tech Research and Development Program (863) of China (2007AA05Z116), the National Natural Science Foundation of China (20673082 and 20873099), the Scientific Research Founda-tion for ROCS, SEM (2006331), the Key Project of Science and Technology of Shaanxi Province (2005k07-G2) and the Natural Science Foundation of Shaanxi Education Committee (06JK167)
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