Facile synthesis of silver-deposited silanized magnetite nanoparticles and their application for catalytic reduction of nitrophenols

Article abstract of DOI:10.1016/j.apcata.2011.11.006

We have demonstrated a facile fabrication of silver-deposited silanized magnetite (Fe₃O₄/SiO₂@Ag) beads, along with their catalytic performance in the reduction of nitrophenols. Initially, 283 ± 40 nm sized spherical magnetite (Fe₃O₄) particles composed of ～13 nm superparamagnetic nanoparticles were synthesized, and then they were silanized following the modified Sto?ber method. Silica-coated magnetic (Fe₃O₄/SiO₂) nanoparticles are then resistant to oxidation and coagulation. In order to deposit silver onto them, Fe₃O₄/SiO₂ nanoparticles were dispersed in a reaction mixture consisting of ethanolic AgNO₃ and butylamine. With this simple and surfactant-free fabrication method, we can avoid any contamination that might make the Fe ₃O₄/SiO₂@Ag particles unsuitable for catalytic applications. The as-prepared Fe₃O₄/SiO₂@Ag particles were accordingly used as solid phase catalysts for the reduction of 4-nitrophenol (4-NP) in the presence of sodium borohydride. The reduction of other nitrophenols such as 2-nitrophenol (2-NP) and 3-nitrophenol (3-NP) were also tested using the Fe₃O₄/SiO₂@Ag nanoparticles as catalysts, and their rate of reduction has been found to follow the sequence, 4-NP>2-NP>3-NP. The Fe₃O₄/SiO ₂@Ag particles could be separated from the product using an external magnet and be recycled a number of times after the quantitative reduction of nitrophenols.

Full text of DOI:10.1016/j.apcata.2011.11.006

Applied Catalysis A: General 413–414 (2012) 170–175
Contents lists available at SciVerse ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Facile synthesis of silver-deposited silanized magnetite nanoparticles and their
application for catalytic reduction of nitrophenols
a,∗
a
b
b,∗∗
Kuan Soo Shin , Young Kwan Cho , Jeong-Yong Choi , Kwan Kim
a
Department of Chemistry, Soongsil University, Seoul 156-743, Republic of Korea
Department of Chemistry, Seoul National University, Seoul 151-742, Republic of Korea
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 18 August 2011
3 4 2
We have demonstrated a facile fabrication of silver-deposited silanized magnetite (Fe O /SiO @Ag)
beads, along with their catalytic performance in the reduction of nitrophenols. Initially, 283 ± 40 nm
sized spherical magnetite (Fe3O4) particles composed of ∼13 nm superparamagnetic nanoparticles
were synthesized, and then they were silanized following the modified Stöber method. Silica-coated
magnetic (Fe3O4/SiO2) nanoparticles are then resistant to oxidation and coagulation. In order to
deposit silver onto them, Fe3O4/SiO2 nanoparticles were dispersed in a reaction mixture consisting
of ethanolic AgNO3 and butylamine. With this simple and surfactant-free fabrication method, we
can avoid any contamination that might make the Fe3O4/SiO2@Ag particles unsuitable for catalytic
applications. The as-prepared Fe3O4/SiO2@Ag particles were accordingly used as solid phase catalysts
for the reduction of 4-nitrophenol (4-NP) in the presence of sodium borohydride. The reduction of
other nitrophenols such as 2-nitrophenol (2-NP) and 3-nitrophenol (3-NP) were also tested using
the Fe3O4/SiO2@Ag nanoparticles as catalysts, and their rate of reduction has been found to follow
the sequence, 4-NP>2-NP>3-NP. The Fe3O4/SiO2@Ag particles could be separated from the product
using an external magnet and be recycled a number of times after the quantitative reduction of
nitrophenols.
Received in revised form 18 October 2011
Accepted 3 November 2011
Available online 12 November 2011
Keywords:
Fe3O4 particle
Silanization
Silver deposition
Catalytic reduction
Nitrophenol
©
2011 Elsevier B.V. All rights reserved.
1
. Introduction
nanoparticles, various procedures have been employed to obtain
silica–metal composites. This is because colloidal silica, which is
thermostable and resistant to coagulation, avoids the aggrega-
tion of the metal particles. Guo et al. [29] reported a general
route to construct multifunctional Fe O /metal hybrid nanos-
tructures using 3-aminopropyltrimethoxysilane (APTMS) as a
linker.
Recently, we have shown that Ag can be deposited onto the sil-
ica beads, without using a linker like APTMS, by soaking them in
Magnetic nanoparticles are a class of nanoparticle which can
be manipulated using magnetic fields [1–3]. Such particles com-
monly consist of magnetic elements such as iron, nickel or cobalt
and their chemical compounds [4–7]. These particles have been the
focus of much research recently because they possess attractive
properties which could see potential use in catalysis, biomedicine,
magnetic resonance imaging, data storage, and environmental
remediation [8–15]. Nonmagnetic metal nanoparticles have also
attracted a great deal of interest today [16–20]. Among others,
gold and silver nanoparticles are receiving great attention due to
their unique optical properties associated with surface plasmon
resonance [21–25]. Metallic nanoshells composed of a magnetic
core and a concentric noble metal shell find many applications
in trace analysis and in heterogeneous catalysis [26–28]. Unfortu-
nately, these colloids are generally unstable, owing to aggregation
of the metal nanoparticles. In order to improve the stability of metal
3
4
ethanolic solutions of AgNO and butylamine [30,31]. The extent of
3
silvering could be adjusted by varying the relative concentrations
of butylamine and AgNO . The Ag-deposited silica (SiO @Ag) beads
3
2
were then used as efficient surface-enhanced Raman scattering
(SERS) substrates that could be used as core materials of SERS-based
biosensors [31]. We have also demonstrated the facile synthesis
of Ag-deposited Fe O3 (Fe O @Ag) particles and their application
2
2
3
as solid phase catalysts for the reduction of 4-nitrophenol (4-
NP) to 4-aminophenol (4-AP) in the presence of NaBH4 [8]. The
Fe O (hematite) particles used earlier were commercial products
2
3
with very irregular shape and size distribution. Accordingly, it was
difficult to enjoy fully the properties of superparamagnetism, as
would be expected from nanometer sized magnetite (Fe O ) parti-
∗
Corresponding author. Tel.: +82 2 8200436; fax: +82 2 8244383.
Corresponding author. Tel.: +82 2 8806651; fax: +82 2 8891568.
E-mail addresses: kshin@ssu.ac.kr (K.S. Shin), kwankim@snu.ac.kr (K. Kim).
∗
∗
3
4
cles. Bare Fe O4 nanoparticles are, however, likely to form a large
3
0
926-860X/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.apcata.2011.11.006

K.S. Shin et al. / Applied Catalysis A: General 413–414 (2012) 170–175
171
aggregation and be easily oxidized or dissolved in an acid condi-
2. Experimental
tions during the treatment procedure. Therefore, a protective layer
on top of the Fe O4 nanoparticles is required. In that sense, silica
2.1. Materials
3
has been reported to be not only one of the ideal protective lay-
ers for Fe O nanoparticles due to its high chemical and thermal
stability but also a suitable supporting matrix to immobilize Ag
nanoparticles [32].
Ferric chloride six hydrate (FeCl ·6H O), tetraethyl orthosilicate
3
4
3
2
(TEOS, 98%), silver nitrate (AgNO , 99.8%), n-butylamine (C H N,
3 4 11
99%), 2-NP (99%), 3-NP (99%), 4-NP (99%) and sodium borohydride
Aminophenol is an important intermediate in the preparation
of several analgesic and antipyretic drugs such as acetaminophen,
acetanilide, phenacetin, and so forth [33]. It is a strong reducing
agent and is used as a photographic developer. It is also used as a
corrosion inhibitor in paints and anticorrosion-lubricating agent in
fuels. In the dye industry, aminophenol is used as a wood stain
and as a dyeing agent for fur and feathers [34]. Due to the sig-
nificance of aminophenols, there is a demand for direct catalytic
reduction of nitrophenols [35]. Indeed, several research groups
have investigated the catalytic reduction of nitrophenols with
NaBH4 to aminophenols, using a number of noble metal nanopar-
ticles [36–39]. In these studies, catalytic metal nanoparticles were
mostly deposited onto dendrimer/polymers or resin beads before
conducting the catalytic reduction of nitrophenol with sodium
borohydride. Recently, Mandal et al. [38,39] reported the prepara-
tion of spongy type gold or dendritic Ag nanostructures for such
catalytic reactions. Although these nanostructures may possess
high catalytic activity, it is still difficult to avoid their contamina-
tion caused by the formation of undesired byproduct in the reaction
medium.
(NaBH , 99%) were purchased from Aldrich and used as received.
4
Ammonia solution (28–30 wt%) was obtained from Samchun Pure
Chemical Company. Other chemicals, unless specified, were of
reagent grade, and highly purified water, of resistivity greater than
18.0 Mꢀ cm, was used throughout the experiments.
2.2. Synthesis of Fe O particles
3
4
Spherically shaped Fe O4 particles were synthesized follow-
3
ing the protocols of Deng et al. [40]. Initially, FeCl ·6H O (1.35 g,
3
2
5 mmol) was dissolved in ethylene glycol (40 mL) to form a clear
solution, followed by the addition of sodium acetate (3.6 g) and
polyethylene glycol (1.0 g). The mixture was stirred vigorously for
◦
30 min and then refluxed at 200 C for 24 h. After cooling to room
temperature, the black products were washed several times with
◦
ethanol and then dried at 60 C for 6 h. The formation of Fe O was
3
4
confirmed by X-ray diffraction (XRD). According to Transmission
electron microscopy (TEM) images, the size of Fe O4 particles was
3
approximated to be 283 ± 40 nm in diameter.
In this work, we have firstly synthesized spherical Fe O4 (mag-
2.3. Preparation of Fe O /SiO @Ag beads
3
3
4
2
netite) particles with a mean diameter of 283 ± 40 nm, but are
actually composed of ∼13 nm superparamagnetic particles, and
then the as-prepared Fe O particles are coated with SiO2 onto
Silver can be deposited directly onto Fe O4 (magnetite) par-
ticles. However, a more homogeneous deposition of Ag and the
3
3
4
which Ag nanoaggregates are finally assembled to endow them
with both the superparamagnetism and catalytic activity. With
this simple and surfactant-free fabrication method, we can avoid
increased dispersion stability are accomplished once Fe O4 is
3
coated with SiO2 beforehand [41]. Accordingly, Fe O4 particles
3
(10 mg) were dispersed in 30 mL ethanol into which water (8 mL),
NH3 (30%, 0.5 mL) and TEOS (0.5 mL) were added with sonication.
After 1 h, the black precipitate was collected with a perma-
nent magnet, and rinsed with ethanol three times. Subsequently,
2 mg of silica-coated Fe O4 (Fe O /SiO ) particle was placed in a
any contamination that might make the Ag-deposited SiO -coated
2
Fe O particles (hereafter, denoted by Fe O /SiO @Ag) unsuitable
3
4
3
4
2
for catalytic applications. The Fe O /SiO @Ag nanoparticles are sel-
3
4
2
dom used as catalysts for the reduction of nitrophenols by NaBH4.
We demonstrate the usefulness of those Fe O /SiO @Ag nanopar-
3
3
4
2
polypropylene container into which 10 mL of a silvering solution
3
4
2
◦
ticles especially in the catalytic reduction of 4-NP to 4-AP by
sodium borohydride in aqueous solution. Other nitrophenols like
was added, and then incubated for 40 min at 50 C with vigorous
shaking. The 4 mM of AgNO3 and 4 mM of butylamine in abso-
lute ethanol were used as a silvering mixture. The polypropylene
container was used to avoid nonspecific silvering of the reaction
vessel. The Fe O /SiO @Ag particles were separated from the reac-
2
-nitrophenol (2-NP) and 3-nitrophenol (3-NP) were also tested
with NaBH4 using Fe O /SiO @Ag nanoparticles as catalyst. The
3
4
2
hybrid magnetic nanoparticles prepared in this work can be sepa-
rated from the product using a neodium magnet and can be recycled
a number of times after the quantitative reduction of nitrophenols.
3
4
2
tion mixture by using a neodium magnet, rinsed with ethanol, and
redispersed in purified water (2 mg/10 mL).
Fig. 1 depicted the performance of Fe O /SiO @Ag nanoparticles
3
4
2
as catalysts in the reduction of nitrophenols to aminophenols by
NaBH4.
2.4. Catalytic reduction of nitrophenols
−
7
In a typical reaction, 2.5 mL of 2.0 × 10 M nitrophenols (2-
NP, 3-NP, and 4-NP) were mixed with 0.02 mg of Fe O /SiO @Ag
3
4
2
nanoparticles at room temperature in a glass vial. A fresh prepared
aqueous solution of 0.1 mL NaBH4 (1.0 M) was then added with
constant stirring. In order to avoid magnetic condensation, the stir-
ring was performed with a mechanical stirrer. Then, UV–visible
(UV–vis) absorption spectra were recorded with time to monitor
the change in the reaction mixture.
2.5. Instruments
UV–vis absorption spectra were obtained using SCINCO S-4100
and Avantes 3648 spectrometers. The magnetic properties were
measured using a Quantum Design SQUID Magnetometer. TEM
images were obtained on a LIBRA-120 transmission electron micro-
scope at 120 kV. XRD patterns were obtained on a MAC Science
Fig. 1. Schematic representation of the performance of Fe3O4/SiO2@Ag nanoparti-
cles as catalysts in the reduction of nitrophenols to aminophenols by NaBH4.
◦
Co M18XHF-SRA powder diffractometer for a 2ꢁ range of 30–80

1
72
K.S. Shin et al. / Applied Catalysis A: General 413–414 (2012) 170–175
Fig. 2. XRD spectra of (a) Fe3O4, (b) Fe3O4/SiO2, and (c) Fe3O4/SiO2@Ag particles:
TEM images of (d) Fe3O4, (e) Fe3O4/SiO2, and (f) Fe3O4/SiO2@Ag particles.
◦
at an angular resolution of 0.05 using Cu K␣ (1.5406 A˚ ) radia-
tion. Infrared spectra were obtained using a Bruker IFS 113v FT-IR
spectrometer equipped with a Globar light source and a liquid N2-
cooled wide-band mercury cadmium telluride detector.
3
. Results and discussion
3
.1. Characterization of Fe O /SiO @Ag beads
3
4
2
All the XRD peaks of iron oxides synthesized in this work can be
indexed to the magnetite structure of Fe O4 (JCPDS 75-1609), as in
3
Fig. 4. (a) Magnetization curve of the as-prepared Fe3O4 particles measured at
room temperature: Inset shows a magnified view at low applied fields. (b) A similar
magnetization curve measured for Fe3O4/SiO2@Ag particles.
Fig. 2a. The average size of the Fe O4 nanoparticles deduced from
3
Sherrer’s formula is about 13 nm [42]. The TEM image (see Fig. 2d)
shows that Fe O4 particles are spherical with a mean diameter of
3
2
83 ± 40 nm. The discrepancy between the XRD and TEM data can
The existence of polyethylene glycol used during the synthe-
be understood by assuming that the 283-nm sized particles were
actually composed of 13-nm sized Fe O4 nanoparticles [26]. This
suggests that Fe O4 nanoparticles have self-assembled into spher-
ical aggregates. In fact, a close look at the TEM image indicated that
large particles consisted of agglomeration of small particles. Disor-
dered pores existed among the primary nanoparticles but within
spherical aggregates. As explained in Section 2, a more homoge-
neous deposition of Ag and the increased dispersion stability are
sis of Fe O4 particles can be verified from the IR spectrum (shown
3
3
−
1
in Fig. 3a). The peaks around 2930, 2844, and 1444 cm can be
assigned, respectively, to the asymmetric and the symmetric C–H
3
stretching vibration and to the deformation of the –CH – groups. On
2
−
1
the other hand, the peaks at 1386 and 888 cm can be attributed,
respectively, to the deforming and the out-of-plane bending vibra-
tion for both the isolated and bridged surface hydroxyls. In addition,
−
1
the peak centered at ∼595 cm would be indicative of the presence
accomplished once Fe O4 is coated with SiO2 beforehand.
3
of a strong interaction between polyethylene glycols and Fe O par-
3
4
ticles [43]. The successful silanization can also be confirmed from
the IR spectrum (shown in Fig. 3b). The two strong bands appear-
−
1
ing at ∼1230 and ∼1120 cm , as well as a somewhat weaker band
−
1
at ∼801 cm , can be attributed to the vibrational modes involv-
ing the bridging oxygen atoms in Si–O–Si moieties, while the peak
−
1
at ∼948 cm is due to the Si–O stretching vibration of the Si–OH
bonds [44,45]. According to the TEM image (see Fig. 2e), the silica
shell thickness is estimated to be about 50 nm. The deposition of Ag
+
onto silica-coated Fe O (Fe O /SiO ) using a 1:1 molar ratio of Ag
3
4
3
4
2
and butylamine can be confirmed from the corresponding XRD data
shown in Fig. 2c as well as from the TEM image shown in Fig. 2f. The
four XRD peaks of Fe O /SiO @Ag particles appearing at 2ꢁ values
3
4
2
◦
◦
◦
◦
of 38.1 , 44.3 , 64.4 , and 77.3 can be attributed to the reflections
of the (1 1 1), (2 0 0), (2 2 0), and (3 1 1) crystalline planes of cubic Ag,
respectively [46]. According to the Sherrer’s equation, the size of the
Ag nanoparticles is estimated to be 30 nm. Therefore, the average
size of Fe O /SiO @Ag particles is estimated to be about 363 nm.
3
4
2
The XRD peaks of (3 1 1) planes of Fe O , marked with asterisks (*)
3
4
in Fig. 2b and c, are barely seen in the patterns of Fe O /SiO and
Fig. 3. Diffuse reflectance infrared spectra of (a) before and (b) after silanization
3
4
2
onto the Fe3O4 nanoparticles.
Fe3O4/SiO2@Ag particles. This means that once the Fe3O4 particles

K.S. Shin et al. / Applied Catalysis A: General 413–414 (2012) 170–175
173
Fig. 5. UV–vis absorption spectra of 4-nitrophenol (4-NP) taken (a) before and (b)
after immediate addition of NaBH4 and (c) that of 4-aminophenol (4-AP) shown
for comparison. (d) UV–vis absorption spectra of 4-NP taken up to 20 min with the
addition of NaBH4 and Fe3O4/SiO2 particles.
Fig. 6. (a) Successive UV–vis absorption spectra of the reduction of 4-nitrophenol
by NaBH4 in the presence of Fe3O4/SiO2@Ag particles. (b) Plot of ln(At/A0) against
reaction time for Fe3O4/SiO2@Ag nanoparticles in the catalytic reduction of 4-
nitrophenol.
are completely covered by thick layers of SiO , the reflection peaks
2
that corresponding to Fe O4 must almost disappear.
3
Fig. 4a shows the magnetization curve of the as-prepared
Fe O particles measured at room temperature. The curve presents
3
4
observed that, after immediate addition of freshly prepared aque-
a small hysteresis loop, and the magnetic saturation value is
70 emu/g. From a magnified view of the magnetization curve at
low applied fields, the remnant Mr defined as the magnetization at
H = 0 is about 1.3 emu/g, while the coercity Hc defined as the field
magnitude necessary to obtain M = 0 is about 9.5 Oe. It has been
ous solution of NaBH , the peak due to 4-nitrophenol was red
shifted from 317 to 400 nm (see Fig. 5a and b). This peak was
due to the formation of 4-nitrophenolate ions in alkaline condition
4
∼
caused by the addition of NaBH . In the absence of proper catalyst,
4
the thermodynamically favorable reduction of 4-nitrophenol (the
reported that the magnetic Fe O4 particles exhibit superparamag-
standard reduction potentials for 4-nitrophenol/4-aminophenol
3
−
netic behavior when the particle size decreases to below a critical
value, generally around 20 nm. The relatively high Ms value with
relatively low Hc and Mr values in this work can then be attributed
to the small primary particle size of about 13 nm in dimension, as
well as to the oriented attachment of primary particles in large
spherical aggregates. Fig. 4b then shows the magnetization curve
and H BO /BH
are −0.76 and −1.33 V, respectively) was not
3
3
4
observed and the peak due to 4-nitrophenol ions at 400 nm
remains unaltered even a couple of days as reported in the lit-
eratures. The UV–vis absorption spectrum after the addition of
Fe O /SiO beads in the mixture of 4-nitrophenol and NaBH4
3
4
2
also remains unaltered with time, as shown in Fig. 5d, which
means that pure Fe O /SiO beads do not work as catalyst for this
measured for the Fe O /SiO @Ag particles. The magnetic satura-
3
4
2
3
4
2
tion value has decreased nearly to one third of the initial value after
coating with silica and silver, i.e. ∼25 emu/g. A part of the decrease
is due to mass effect of silica and silver, and the remaining part
reaction.
Fig. 6a shows the UV–vis absorption spectra, representing
the reduction of 4-NP to 4-AP by NaBH4 in the presence of
is due to their diamagnetic shielding. Anyhow, the Fe O /SiO @Ag
Fe O /SiO @Ag beads. In the absence of Fe O /SiO @Ag beads,
3 4 2 3 4 2
3
4
2
particles could be magnetically concentrated and readily picked up
using a small magnet.
the aqueous mixture of 4-NP and NaBH4 shows a maximum
absorption at 400 nm, which, as mentioned above, corresponds
to 4-nitrophenolate ion in alkaline conditions, and the peak
remains unaltered with time. However, the addition of small
amount of purified Fe O /SiO @Ag beads (0.02 mg) with stirring
3
.2. Catalytic activity of Fe O /SiO @Ag beads
3 4 2
3
4
2
In order to study the catalytic activity of the Fe O /SiO @Ag, we
causes fading and ultimate bleaching of the yellow color of the
reaction mixture. As shown in Fig. 6a, time-dependent UV–vis
spectra of this catalytic reaction mixture shows the disappearance
of 400 nm peak and the gradual development of the new peak
3
4
2
have chosen the reduction of 2-NP, 3-NP, 4-NP, and 4-NA by NaBH4
as model reactions. UV–vis absorption spectra were recorded with
time to monitor the change in the reaction mixture. It has been

1
74
K.S. Shin et al. / Applied Catalysis A: General 413–414 (2012) 170–175
Fig. 8. The variation of the rate constant of the reduction of 4-nitrophenol by NaBH4
in the presence of Fe3O4/SiO2@Ag nanoparticles up to 20 cycles.
The rate constant of this catalytic reaction in the presence of the
−
2
−1
s
Fe O /SiO @Ag beads is 2.5 × 10
, as measured from the plot
3
4
2
of lnA (A = absorbance at 400 nm) versus time (shown in Fig. 6b).
3.3. Reduction of other nitrophenols
Reduction of different nitro compounds was also studied using
NaBH4 in presence of Fe O /SiO @Ag beads. The yellow color of
3
4
2
2
-NP and 3-NP was discharged during the course of reduction
under identical condition. As shown in Fig. 7a, the peak at 390 nm
−
5
corresponding to the absorbance peak of 3-NP (1.0 × 10 M)
decreases gradually on NaBH4 (1.0 M) reduction in presence of
Fe O /SiO @Ag beads (0.02 mg). Again similar situation occurred in
3
4
2
case of 2-NP whose peak in alkaline aqueous solution appeared at
16 nm but disappeared in the course of Ag catalyzed NaBH reduc-
4
4
tion (Fig. 7b). In all cases, the silver plasmon band remained masked
with the absorption band of the nitro compounds until complete
conversion of the corresponding amino derivatives. The rate con-
stants of the catalytic reduction of 2-NP and 3-NP in the presence
−
3
of the Fe O /SiO @Ag beads are determined to be 5.5 × 10 and
3
4
−1
2
−
3
2
.4 × 10
s
, respectively. A comparative study revealed that the
rate of reduction follows the order, 4-NP>2-NP>3-NP (Fig. 7c).
3
.4. Catalytic recyclability of Fe O /SiO @Ag beads
3
4
2
The Fe O /SiO @Ag nanoparticles are very effective for the cat-
3
4
2
alytic reduction of 4-NP. At the end of the reaction, the catalyst
particles remained active and were easily separated from the prod-
uct, 4-AP, using a neodium magnet. After one cycle of reaction,
the particles were washed thoroughly with distilled water and
could be recycled a number of times for the reduction reaction. To
demonstrate magnetic recovery of the Fe O /SiO @Ag nanoparti-
Fig. 7. Successive UV–vis absorption spectra of the reduction of (a) 3-nitrophenol
and (b) 2-nitrophenol by NaBH4 in the presence of Fe3O4/SiO2@Ag particles. (c) Plot
of ln(At/A0) against reaction time for Fe3O4/SiO2@Ag nanoparticles in the catalytic
reduction of 2-nitrophenol, 3-nitrophenol, and 4-nitrophenol.
3
4
2
cles, we increased the volume of reagents and catalysts by 30 times
so that the Fe O /SiO @Ag nanoparticles can be recovered more
3
4
2
at 300 nm which substantiates the formation of the 4-AP (see
Fig. 5c). These results indicate that Fe O /SiO @Ag catalyze the
efficiently using a neodium magnet. The catalytic recyclable usage
of the Fe O /SiO @Ag (0.02 mg) nanoparticles for the reduction
3
4
2
3
4
2
−
5
reduction process. In this experiment, the concentration of the
borohydride ion, used as reductant, largely exceeds (∼100 times)
that of 4-NP. As soon as we added the NaBH , the Fe O /SiO @Ag
of 4-NP (0.06 mL, 1.0 × 10 M) in the presence of NaBH4 (0.2 mL,
1.0 M) and purified water (2.5 mL) was studied. Fig. 8 shows the cat-
alytic efficiency of the Fe O /SiO @Ag nanoparticles after 20 cycles.
4
3
4
2
3
4
2
beads started the catalytic reduction by relaying electrons from the
donor BH4 to the acceptor 4-NP right after the adsorption of both
onto the particle surfaces. As the initial concentration of sodium
borohydride was very high, it remained essentially constant
throughout the reaction. Therefore, for evaluation of the catalytic
rate pseudo-first-order kinetics with respect to 4-NP was used.
The result shows that the catalytic activity of the Fe O /SiO @Ag
nanoparticles decreased slightly after 20 cycles of the catalysis
experiments. The reason for slight decrease of the catalytic activ-
3 4 2
−
ity may be that some of Fe O /SiO @Ag nanoparticles are not fully
3
4
2
recovered using an external magnet or the Ag nanoparticles are
detached away from the Fe O /SiO @Ag nanoparticles when they
3
4
2

K.S. Shin et al. / Applied Catalysis A: General 413–414 (2012) 170–175
175
are stirred, as a result, the catalytic efficiency of the Fe O /SiO @Ag
[10] Q.A. Pankhurst, J. Connolly, S.K. Jones, J. Dobson, J. Phys. D: Appl. Phys. 36 (2003)
R167–R181.
3
4
2
nanoparticles decreases.
. Conclusions
In the present study, Fe O /SiO @Ag beads are prepared and
[
[
11] P.N.L. Lens, M.A. Hemminga, Biodegradation 9 (1998) 393–409.
12] J. Giri, A. Ray, S. Dasgupta, D. Datta, D. Bahadur, Bio-Med. Mater. Eng. 13 (2003)
387–399.
4
[
13] T.-J. Yoon, J.S. Kim, B.G. Kim, K.N. Yu, M.-H. Cho, J.-K. Lee, Angew. Chem. Int. Ed.
44 (2005) 1068–1071.
3
4
2
[14] Z. Liao, H. Wang, R. Lv, P. Zhao, X. Sun, S. Wang, W. Su, R. Niu, J. Chang, Langmuir
27 (2011) 3100–3105.
their application for the catalytic reduction of nitrophenols in
the presence of sodium borohydride is examined. In order to
avoid the aggregation of the metal particles, silica was coated on
[
15] J.R. Weertman, in: C.C. Koch (Ed.), Nanostructured Materials: Processing,
Properties and Applications, William Andrews Publishing, Norwich, NY,
2002.
Fe O nanoparticles using the modified Stöber method. Then silver
[16] W.L. Barnes, A. Dereux, T.W. Ebbesen, Nature 424 (2003) 824–830.
[17] C.F. Bohren, D.R. Huffman, Absorption and Scattering of Light by Small Particles,
John Wiley & Sons, New York, 1998.
3
4
was deposited onto Fe O /SiO nanoparticles simply by soaking
3
4
2
them in ethanolic solutions of AgNO3 and butylamine and their
characteristics are examined by TEM, XRD, UV–vis, and magne-
tometer analyses. With this simple and surfactant-free fabrication
of Fe O /SiO @Ag nanoparticles, we can avoid contamination in the
[
18] S.K. Gray, T. Kupka, Phys. Rev. B 68 (2003) 0454151–04541511.
[19] J.M. Oliva, S.K. Gray, Chem. Phys. Lett. 379 (2003) 325–331.
[20] S.A. Maier, P.G. Kik, H.A. Atwater, S. Meltzer, E. Harel, B.E. Kowel, A.A.G.
Requicha, Nat. Mater. 2 (2003) 229–232.
3
4
2
[
[
21] A.J. Haes, R.P. Van Duyne, J. Am. Chem. Soc. 124 (2002) 10596–10604.
22] S. Eustis, M.A. El-Sayed, Chem. Soc. Rev. 35 (2006) 209–217.
final product, which makes them suitable for further catalytic appli-
cations. The catalytic activity of Fe O /SiO @Ag nanoparticles thus
[23] T.R. Jensen, M.D. Malinsky, C.L. Haynes, R.P. Van Duyne, J. Phys. Chem. B 104
2000) 10549–10556.
3
4
2
(
prepared was investigated in the reduction reaction of nitrophenols
2-NP, 3-NP and 4-NP) to aminophenols by NaBH . A comparative
[
24] A.J. Haes, S. Zou, G.C. Schatz, R.P. Van Duyne, J. Phys. Chem. B 108 (2004)
6961–6968.
(
4
study revealed that the rate of reduction follows the order, 4-NP>2-
NP>3-NP. Since the magnetic particles are readily recovered from
the solution phase mixture without centrifugation and/or filtering,
[25] M.D. Malinsky, K.L. Kelly, G.C. Schatz, R.P. Van Duyne, J. Am. Chem. Soc. 123
2001) 1471–1482.
26] K. Kim, J.-Y. Choi, H.B. Lee, K.S. Shin, ACS Appl. Mater. Interfaces 2 (2010)
872–1878.
[27] L. Wang, J. Bai, Y. Li, Y. Huang, Angew. Chem. Int. Ed. 47 (2008) 2439–2442.
(
[
1
Fe O /SiO @Ag nanoparticles, separated from the product using a
3
4
2
[28] Z. Xu, Y. Hou, S. Sun, J. Am. Chem. Soc. 129 (2007) 8698–8699.
[29] S. Guo, S. Dong, E. Wang, Chem. Eur. J. 15 (2009) 2416–2424.
[30] K. Kim, H.B. Lee, Y.M. Lee, K.S. Shin, Biosens. Bioelectron. 24 (2009)
1864–1869.
neodium magnet, could be recycled a number of times.
Acknowledgement
[
31] K. Kim, H.B. Lee, J.-Y. Choi, K.S. Shin, ACS Appl. Mater. Interfaces 3 (2011)
3
24–330.
This work was supported by National Research Founda-
tion (NRF) of Korea Grant funded by the Korean Government
[
32] L. Li, E.S.G. Choo, X. Tang, J. Ding, J. Xue, Acta Mater. 58 (2010) 3825–3831.
[33] S. Mitchell, Kirk-Othmer Encyclopedia of Chemical Technology, 4th ed., Wiley-
Interscience, NY, 1992.
(
MEST) (Nos. 2011-0001218, 2011-0006737, 2010-0019204, 0409-
0100172, and 2009-0072467).
[
34] M.J. Vaidya, S.M. Kulkarni, R.V. Chaudhari, Org. Process Res. Dev. 7 (2003)
02–208.
2
2
[
[
[
[
[
35] T. Swathi, G. Buvaneswari, Mater. Lett. 62 (2008) 3900–3902.
36] K. Hayakawa, T. Yoshimura, K. Esumi, Langmuir 19 (2003) 5517–5521.
37] J. Ge, T. Huynh, Y. Hu, Y. Yin, Nano Lett. 8 (2008) 931–934.
38] M.H. Rashid, T.K. Mandal, J. Phys. Chem. C 111 (2007) 16750–16760.
39] M.H. Rashid, R.R. Bhattacharjee, A. Kotal, T.K. Mandal, Langmuir 22 (2006)
References
[1] W. Gao, S. Sattayasamitsathit, K.M. Manesh, D. Weihs, J. Wang, J. Am. Chem.
Soc. 132 (2010) 14403–14405.
7
141–7143.
40] H. Deng, X. Li, Q. Peng, X. Wang, J. Chen, Y. Li, Angew. Chem. Int. Ed. 44 (2005)
782–2785.
[
[
2] C. Xu, S. Sun, Polym. Int. 56 (2007) 821–826.
3] G. Leem, S. Sarangi, S. Zhang, I. Rusakova, A. Brazdeikis, D. Litvinov, T.R. Lee,
Cryst. Growth Des. 9 (2009) 32–34.
[
2
[
[
[
41] L. Xia, N.H. Kim, K. Kim, J. Colloid Interface Sci. 306 (2007) 50–55.
42] L.S. Birks, H. Friedman, J. Appl. Phys. 17 (1946) 687–692.
43] W. Zhang, L. Gai, Z. Li, H. Jiang, W. Ma, J. Phys. D: Appl. Phys. 41 (2008)
[
[
[
4] J. Park, J. Joo, S.G. Kwon, Y. Jang, T. Hyeon, Angew. Chem. Int. Ed. 46 (2007)
4
630–4660.
5] J. Park, K. An, Y. Hwang, J.-G. Park, H.-J. Noh, J.-Y. Kim, J.-H. Park, N.-M. Hwang,
T. Hyeon, Nat. Mater. 3 (2004) 891–895.
6] J.R. Davis, Nickel, Cobalt, and there Alloys, ASM International, Materials Park,
225001–225006.
[
[
44] M. Tomozawa, J.-W. Hong, S.-R. Ryu, J. Non-Cryst. Solids 351 (2005) 1054–1060.
45] A. Balamurugan, G. Sockalingum, J. Michel, J. Fauré, V. Banchet, L. Wortham,
S. Bouthors, D. Laurent-Maquin, G. Balossier, Mater. Lett. 60 (2006)
2
000.
[
[
[
7] D.-H. Chen, C.-H. Hsieh, J. Mater. Chem. 12 (2002) 2412–2415.
8] K.S. Shin, J.-Y. Choi, C.S. Park, H.J. Jang, K. Kim, Catal. Lett. 133 (2009) 1–7.
9] X. Wang, H. Ji, X. Zhang, H. Zhang, X. Yang, J. Mater. Sci. 45 (2010) 3981–3989.
3752–3757.
[
46] R. He, X. Qian, J. Yin, Z. Zhu, J. Mater. Chem. 12 (2007) 3783–3786.