Synthesis of glucose-mediated Ag-γ-Fe2O3 multifunctional nanocomposites in aqueous medium - a kinetic analysis of their catalytic activity for 4-nitrophenol reduction

Article abstract of DOI:10.1039/c5gc00941c

This paper reports the synthesis of γ-Fe₂O₃ supported Ag nanoparticles (NPs) in aqueous medium by following a green approach. The presence of Fe₂O₃ in the gamma phase and silver in the nanocomposite has been confirmed by Raman spectroscopy, EDAX and XPS analyses. The presence of Ag in the nanocomposite is also indicated by UV spectroscopy. In the process of in situ generation of glucose mediated Ag NPs on the γ-Fe₂O₃ matrix, the size of γ-Fe₂O₃ nanoclusters reduced from 11.6 ± 1.6 to 9 ± 1 nm as was estimated from HRTEM analysis. Glucose served as an effective stabilizer for both Ag and γ-Fe₂O₃ in the nanocomposite. At lower concentrations of Ag (0.15-1.2 μM) the reduction of 4-nitrophenol (4-Nip) follows pseudo-first-order kinetics and the second order rate constant for this process was found to be 5.28 × 10³ dm³ mol^-1 s^-1. Whereas, at higher concentrations (3.2-28.9 μM), it follows zero-order kinetics and occurred with a rate constant of 1 × 10^-2 mol dm^-3 s^-1. The amount of silver in the nanocomposite is found to influence the kinetics of the catalytic reduction in a complex scheme following the Langmuir-Hinshelwood mechanism. The recyclability of the as-synthesized nanocomposite up to 7 cycles and the catalytic effect even at a very low silver concentration (0.15 μM) associated with high surface area and superparamagnetism suggest it to be a cost effective and environmentally friendly potential catalytic system.

Full text of DOI:10.1039/c5gc00941c

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Synthesis of glucose-mediated Ag - γ-Fe O multifunctional
2
3
nanocomposites in aqueous medium - a kinetic analysis of their
catalytic activity for 4-nitrophenol reduction
Received 00th January 20xx,
Accepted 00th January 20xx
a,b
* a,b
a,c
DOI: 10.1039/x0xx00000x
Mandeep Kaloti, Anil Kumar
and Naveen K. Navani
www.rsc.org/
This paper reports the synthesis of γ-Fe
approach. The presence of Fe in gamma phase and silver in the nanocomposite has been confirmed by Raman
spectroscopy, EDAX and XPS analyses. The presence of Ag in the nanocomposite is also indicated by UV spectroscopy. In
the process of in situ generation of glucose mediated Ag NPs on γ-Fe matrix reduced the size of γ-Fe nanoclusters
from 11.6 ± 1.6 to 9 ± 1 nm as was estimated from HRTEM analysis. Glucose served as an effective stabilizer for both Ag
and γ-Fe in the nanocomposite. At lower concentrations of Ag (0.15 - 1.2 µM) the reduction of the 4-nitrophenol (4-
Nip) follows pseudo-first-order kinetics and the second order rate constant for this process was found to be 5.28 × 10 dm
2 3
O supported Ag nanoparticles (NPs) in aqueous medium by following the green
2 3
O
2
O
3
2 3
O
2
O
3
3
3
-
1
-1
mol s . Whereas, at higher concentrations (3.2 - 28.9 µM), it follows zero-order kinetics and occurred with a rate
-2
-3 -1
constant of 1 × 10 mol dm s . The amount of silver in the nanocomposite is found to influence the kinetics of the
catalytic reduction in a complex scheme following the Langmuir-Hinshelwood mechanism. The recyclability of the as
synthesized nanocomposite up to 7 cycles and the catalytic effect even at very low silver (0.15 µM) associated with high
surface area and superparamagnetism suggests it to be the cost effective and environmental friendly potential catalytic
system.
2
6
porous silicon , silica,
27,28
28 29
silver and graphene oxide have
Introduction
been extensively used for coating of MNPs.
Metal nanoparticles (NPs) are finding increasing applications in
energy conversion, petrochemical industries and oil refineries
as they catalyze various conversions like C-C cross coupling,
hydrogenation-dehydrogenation, electron transfer reaction
During the last few decades, magnetic nanoparticles (MNPs)
have attracted wide attention due to its unique properties
such as superparamagnetism, low Curie temperature, high
1
coercivity, high magnetic susceptibility and high surface area.
3
0
and hydrocarbon oxidation. The repetitive use of the
catalyst, however, requires the metal NPs to be loaded onto a
supporting material having nano/microstructures so as to
produce an effective heterogeneous catalyst. Recently, the use
These features render them the great potential for various
3
2
applications like magnetic storage devices, ferrofluids,
4
magnetic separation/biomolecular purification, MRI contrast
5
6
agents, targeted drug delivery, hyperthermia, immunoassay,
7
8
biosensing, tissue regeneration, cell sorting, molecular
9
10
3 4
of iron oxide (Fe O ) NPs as supporting material has drawn a
1
1
12
great deal of attention because of their biocompatibility,
reusability and economic viability. Moreover, these particles
exhibit high magnetization, which makes them suitable for
biomedical as well as catalytic applications allowing their easy
interaction in live cells and catalysis. For most of the
biomedical applications, it is desirable to make them water
soluble containing biocompatible components.
For achieving these features, MNPs have generally been
coated by a variety of organics and inorganics. Organics
separation using
a
magnet. In this context, the
1
3
generally included the natural (glucose/dextran, chitosan,
14 functionalization of the MNPs with Ag NPs to form binary
hybrids has been widely studied in order to investigate their
1
5
16
alginate and starch ) and synthetic (poly(vinyl alcohol)
3
applications in negative magnetoresistance, in catalysis for
1
1
PVA) , poly(ethyleneglycol) (PEG) , poly(vinylpyrrolidone)
7
18
(
3
2
1
PVP) , poly(acrylic acid) (PAA) , Poly(N-isopropylacrylamide)
9
20
performing epoxidation of styrene
and 4-nitrophenol
(
3
3
reduction and for biological applications like antimicrobial
37
activity and drug release. Compared to Pt, Pd, Ru and
2
PolyNIPAAM) ,
1
22
(PLGA) ,
(
poly(lactic-co-glycolic
acid)
3
4
35
33
36
2
3
24
poly(methylacrylic acid) (PMAA) and polyethyleneimine )
33,38-39
39-43
, silver
10
25 Au
is relatively inexpensive and possesses
polymers. Inorganic materials such as quantum dots, gold,
good physical and chemical properties including the catalytic
as well as antimicrobial activity. In these studies the catalytic
reduction of 4-nitrophenol (4-Nip) to 4-aminophenol (4-Ap)
has been considered as a standard benchmark reaction for
a.
Centre of Nanotechnology, Indian Institute of Technology Roorkee, Roorkee-
47667, India.
2
b.
c.
Department of Chemistry, Indian Institute of Technology Roorkee, Roorkee-
47667, India. *E-mail: anilkfcy@iitr.ac.in
easy assessment of catalytic activity of metal NPs in aqueous
33-43
2
medium.
Previous studies on such systems have employed
8,41-44
magnetite NPs as a matrix/support to the Ag NPs.
Department of Biotechnology, Indian Institute of Technology Roorkee,
Roorkee-247667, India.
2
Electronic Supplementary Information (ESI) available: [optical spectra, FTIR,
FESEM images and their elemental mapping, kinetic control experiments,
kinetic trace for the reduction of 4-Nip at an intermediate concentration of
However, the use of magnetite poses several challenges as it is
unstable in air and tends to undergo phase transformation to
2
.4 µM Ag exhibiting transition between first- and zero-order kinetics and
yield Fe
2
O
3
with a reduction in magnetization and loss of
1,45
depicting the role of glucose as reducing agent as compared to NaBH
DOI: 10.1039/b000000x
4
.See
dispersibility.
The development of an efficient, workable
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ARTICLE
Journal Name
catalyst requires having the support material to be fairly stable on an atomic force microscope (AFM) supplied by Molecular
in natural environment with relatively high magnetization Tools and Devices for Nanotechnology equipped with NOVA
(without any change in phase) for easy separation, coated with software using semi-contact mode. Electron micrographs and
inexpensive metal catalyst at very low concentration. In the selected area electron diffraction (SAED) patterns were
literature we have came across a report employing commercial recorded on a transmission electron microscope (FEI-TECNAI
γ-Fe
Ag NPs (52 wt %).
The present MS for the first time has reported the synthesis of Zetasizer ZS90 equipped with 632 nm He–Ne laser. Infrared
glucose-mediated Ag- γ-Fe nanocomposites with a specific (IR) spectra were recorded in the mid IR range (4000 - 400
objective of using γ-Fe
2 3
O NPs loaded with a relatively much higher amount of G2 20 S-TWIN) at an accelerating voltage of 200 kV. The zeta
4
5
potential of nanocomposites was recorded on a Malvern
2
O
3
−
1
2 3
O as a support material owing to its cm ) in KBr medium on a Thermo Nicolet Nexus Fourier
relatively higher magnetization and biocompatibility in transform infrared (FTIR) spectrometer. Raman measurements
aqueous medium. The in situ generation of Ag NPs at its lower were made on an inVia Renishaw spectrophotometer
+
concentration (0.6 – 3.2 µM) by the reduction of Ag using equipped with Ar ion 514 nm laser having a resolution of ± 1
-1
glucose as a stabilizer as well as reducing agent in the presence cm and a 2.5 µm confocal resolution. Surface area of the
of γ-Fe results in the production of fairly smaller γ-Fe samples was analyzed using five point Brunauer–Emmett–
2
O
3
2 3
O
nanoclusters. These nanocomposites have been characterized Teller (BET) method on a NOVA 2200e High Speed Automated
in terms of their size, optical, magnetic and catalytic Surface Area Analyzer. The X-ray photoelectron spectroscopy
properties.
was performed on a PHI 5000 Versa Probe II supplied by
945
The present work makes use of non-toxic precursors and ULVAC-PHI Inc., Japan equipped with monochromatic Al K
α
aqueous medium to design a catalytic system consisting of radiation source. Magnetic measurements of the samples
biocompatible magnetic nanoparticles support loaded with were carried out on Vibrating Sample Magnetometer (VSM)
non-toxic catalyst (Ag) at its low concentrations. This catalyst using PAR 155 and MCFMS-3 cryogenic free miniVSM system
has been successfully utilized for the conversion of hazardous with pulse cryocooler at 300 K.
material like 4-Nip to useful environmental benign product, 4-
Ap and demonstrates several advantages over the existing Methodology
catalytic systems in terms of being efficient, recyclable and Synthesis of γ-Fe O NPs and silver coated γ-Fe O (Ag – γ-Fe O )
2
3
2
3
2
3
magnetically separable, requiring very small amount of the
catalyst besides being environmentally benign. All these
features suggest it to be environmental friendly green material
for performing sustainable catalytic processes. The preliminary
biological experiments designed with this nanocomposite have
demonstrated its efficient antimicrobial activity.
nanocomposites
The γ-Fe NPs (SB) were prepared by using an earlier
46
2 3
O
reported method with a slight modification and is described
briefly. Ferrous sulphate (0.75 g) and ferric sulphate (0.46 g)
0
were dissolved in 100 mL DIW at 40 C and the pH of the
solution was maintained at 3 using ammonium hydroxide
(
25%) and stirred for 30 min in order to precipitate the iron
hydroxides. The pH of this solution was further increased by
adding ammonium hydroxide solution under stirring to 10.5.
After about 30 min it resulted in the formation of blackish
brown NPs. These particles could be separated magnetically
and washed with DIW several times till the pH is around 9.5.
The particles, thus produced were found to be negatively
charged using zeta probe of about -27mV.
Experimental section
Reagents
Ferric sulphate hydrate (Thomas Baker), ferrous sulphate
heptahydrate, silver nitrate and glucose (BDH), 4-nitrophenol
(
99 %) (SISCO research lab Pvt. Ltd), ammonium hydroxide
25%) and sodium borohydride (Merck). All chemicals used
2 3
To the above synthesized γ-Fe O NPs (SB), silver nitrate of
(
varied concentrations (0.15 - 28.9 µM) were added slowly. This
reaction mixture was stirred for about 15 min followed by
sonication for another 10 min. Thereafter, glucose solution,
having about 5 times higher concentration to that of silver
were of analytical grade. All water used in the experiments
was of Millipore grade deionized water (DIW).
Equipment
nitrate, was added drop-wise and stirred further for 30 min at
0
0 C. This excess amount of the glucose was added to ensure
The electronic spectra and the kinetic studies were recorded
on a Shimadzu UV-VIS 1601S spectrophotometer equipped
with temperature controller. The X-ray diffraction patterns of
powder samples were recorded on a Bruker AXS D8 Advance
X-ray diffractometer using Cu Kα radiation (1.5418 Å) as the X-
ray source. The morphology and elemental analysis of the
nanocomposites were studied on a field emission scanning
electron microscope (FESEM, FEI-QUANTA 200-FEG) equipped
with an energy dispersion X-ray spectrometer using an
accelerating voltage of 20 kV. The surface morphology of the
nanostructures was analyzed recording their 2D and 3D images
5
the complete reduction of Ag NPs. The Ag coated particles
were then washed with DIW in order to remove any
free/unbound Ag NPs. The resulting Ag
–
2 3
γ-Fe O
nanocomposites were subjected to further characterization. In
a control experiment, Ag NPs were also generated in the
+
absence of matrix using the same concentration of Ag and
†
glucose (Fig. S1, ESI ). These NPs have absorption maximum at
around 396 nm.
2
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Powder samples were obtained by drying the sample in a Optical spectra
vacuum oven at 50 °C for 3 h, which were used for the analysis
The γ- Fe
2
O
3
NPs (SB) used as matrix exhibits a broad
of XRD, BET, FTIR analyses and Raman spectroscopy.
absorption peak at 425 nm and a shoulder at 485 nm. The
+
addition of Ag to the γ-Fe O matrix results in the increase in
2 3
Preparation of samples for various analyses
its absorbance associated with a slight red shift in absorption
+
peaks to 430 and 495 nm respectively. At low [Ag ] (0.6 µM),
For recording the electronic spectra diluted solution of the Ag
–
γ-Fe
2 3
O samples were used. For Raman analysis, the
its in situ reduction by glucose results in a blue shifted
absorption spectrum exhibiting a broad peak at 395 nm and a
shoulder at 499 nm. The drastic change in absorbance upon
-1
instrument was calibrated using silicon reference at 520 cm
prior to use. The powder sample was placed on the glass
substrate and the measurements were carried out using 514
+
the reduction of Ag is understood by the formation of Ag NPs
-
nm argon ion laser in the wavenumber range of 200 - 1200 cm
1
.
on the surface of the matrix, and the resulting nanocomposite
is termed as SPLAg. Moreover, at a higher concentration of
silver (6.4 µM), it exhibited a further blue shifted spectrum
with both peaks shifted to the blue region and this nanosystem
is denoted by SPHAg. The blue shifted spectrum at a higher
concentration of silver might have arisen due to further
All measurements were made at low laser power of 0.5 mW
to avoid any kind of damage to the samples. For AFM and
FESEM analyses, the diluted samples were applied on the glass
substrate, which was dried at 40 °C in vacuum oven. For
FESEM analysis, the prepared sample were further coated with
gold in order to make them conducting. For TEM analysis, a
small diluted sample drop was applied to a 200 mesh carbon
coated copper grid and dried at room temperature. Powdered
samples were used for characterization of FTIR, BET and XPS.
reduction in the size of the Ag – γ- Fe
2 3
O NPs. It is likely that
2 3
the γ- Fe O NPs are also becoming smaller in the presence of
Ag NPs, which was reflected by their increased dispersity. This
aspect was further examined, by plotting the additive spectra
consisting of matrix and Ag NPs, prepared in the absence of
matrix, at its two used concentrations (Fig. S1 curves b and c,
For magnetic measurements,
a known amount of the
powdered samples taken in a polymer capsule were used.
†
ESI ). A comparison of the additive spectra with those obtained
Kinetic conditions for catalytic reduction of 4-Nip
Catalytic activity of silver coated γ-Fe
examined by performing the kinetics of the model reaction,
involving the reduction of 4-Nip at its surface using NaBH as
reducing agent. In these experiments, the concentration of
NaBH (51.1 - 119.3 mM), γ-Fe NPs (28.77 mM) and 4-Nip
62.5 - 193.5 µM) were kept such that these were more than
2 3
O nanocomposites was
+
SPHAg
SB
SPLAg
SPHAg
3
91 nm
4
1
.50
474 nm
79 nm
395 nm
4
4
2
O
3
1.25
1.00
(
ten times higher to those of the concentration of Ag NPs in
order to maintain the pseudo-kinetic conditions. The
thermodynamic parameters were evaluated by performing the
kinetics in the temperature range of 293K to 323K.
429 nm
495 nm
0
0
.75
.50
(d)
c)
b)
424 nm
485 nm
(
(
The catalytic activity of the synthesized Ag
2 3
– γ-Fe O
(
a)
nanocomposites were analyzed spectrophotometrically by
performing the reduction of 4-Nip to 4-Ap. All the kinetic
experiments were performed in 1 cm quartz cuvette. In these
reactions, 0.1mL of 4-Nip solution of varied concentrations (2 -
2
00
300
400
500
600
700
800
Wavelength (nm)
Fig. 1 UV-visible spectra of the SB (a) (supporting matrix) and the matrix coated by
typical low (c) (SPLAg) and high (d) (SPHAg) concentrations of Ag NPs generated in in
situ reaction.
5
.5 mM) was added into 1.98 mL DIW followed by the addition
-3
of freshly prepared 1 mL of 0.2 mol dm NaBH
reaction mixture, 20 µL of the varied amount of Ag on γ-Fe
28.77 mM) was added to keep the total volume at 3.1 mL.
4
. To this
2
O
3
(
†
by the in situ generation of Ag NPs (Fig. S1 d and e, ESI ) clearly
After the addition of the catalyst, the time immediately after
the induction period was considered as to be t = 0 denoted as
shows the latter to be more intense and fairly broad. It
suggests some changes either in their morphology or some
type of interaction between the two resulting in the observed
change(s).
t
0
. The progress of the reaction was then monitored by
following the decrease in absorbance due to consumption of
-Nip at 400 nm.
4
The reusability of the above synthesized nanocatalyst(s) was
also examined by repetitive use of the same catalyst for
performing the above reaction. In these experiments, the
nanocomposite used to catalyze the reaction was separated
magnetically, washed with distilled water and reused seven
times for performing the reaction.
XRD
Fig. 2 shows the XRD pattern of the as synthesized samples:
SB, SPLAg and SPHAg. All the reflections due to SB
2 3
corresponded to γ-Fe O (JCPDS file no. 39-1346) having cubic
structure. The addition of low silver in SPLAg results in the
development of two new peaks corresponding to (110) and
(
200) planes due to Ag NPs. In case of SPHAg, apart to the
Results and discussion
peaks due to γ-Fe
2
O
3
Ag NPs in SPLAg an additional reflection
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ARTICLE
Journal Name
SPHAg
666
(a)
(b)
from (220) planes of Ag matching with its cubic structure
JCPDS file no. 001-1167) was also observed. These data clearly
support the formation of Ag – γ-Fe nanocomposites. The
size of the γ-Fe and Ag NPs in SB, SPLAg and SPHAg were
determined using Debye-Scherrer equation. The size of the γ-
Fe in these samples was found to be 12 ± 1.1, 11.2 ± 0.9,
.9 ± 1, respectively. The size of Ag NPs in SPLAg and SPHAg
was determined to be 6 ± 0.7 and 6.4 ± 0.4 nm, respectively. A
decrease in size of γ-Fe NPs with increasing Ag
A
B
1
1
400
200
718
SB
6
4
2
0
0
0
0
(
(
b)
4
37
5
355
00
2
O
3
1000
800
600
400
2 3
O
437
7
68
18
6
20
6
(
a)
5
00
352
2
O
3
620
SBHAg
SB
8
5
67 571
400 450 500 550 600 650 700
200 400 600 800 1000 1200
-
1
-1
Raman shift (cm )
2
O
3
wavenumber (cm )
concentration is in line with that of the observation made by
the optical absorption data.
Fig. 3 A FTIR spectra of (a) SB and (b) SPHAg; B Raman spectra of (a') SB and (b') SPHAg;
In a control experiment, the Raman spectrum of SB was also
recorded. It displayed all the four peaks as observed in SPHAg,
thus supporting the presence of γ-Fe O in both the samples.
SB
2
3
Moreover, a comparison of the spectra of SB and SPHAg
reveals that the intensity of all the peaks due to vibrational
mode of γ-Fe O in the latter sample are enhanced by about 3
(
a)
2
3
SPLAg
SPHAg
fold even at low laser power (0.5 mW). This mild increase in
the intensity is understood by the SERS effect caused by the
presence of Ag in SPHAg involving the chemical enhancement
(
b)
5
1
mechanism. It may be pointed out that the Fe O4 phase
3
shows the prominent Raman peaks at 305, 513, 534 and 660
-1 50
cm
which are fairly different to those observed in the
(c)
present system, thereby, rules out the presence of Fe O4
3
phase in the present system.
2
0
30
40
50
60
70
80
2
Theta
XPS
Fig. 2 XRD patterns of SB (a), SPLAg (b) and SPHAg (c).
In order to further confirm the phase of iron oxide (Fe O or
3
4
γ- Fe O ) present in the as synthesized sample, surface analysis
3
2
FTIR and Raman spectra
of 1.2 µM Ag- γ-Fe O nanocomposite was performed using
2
3
FTIR spectra of pure SB and SPHAg are presented in Fig. S2,
†
ESI and the corresponding spectral data is summarized in
XPS analysis (Fig 4). The survey scan spectrum recorded in the
range of 0-800 eV exhibits the peaks due to different
elements: C, O, Ag and Fe (Fig 4a).The individual narrow scan
of each element was subjected to baseline correction and
multiple peak fitting for C1s and O1s by using Gaussian
†
Table S1, ESI . The IR spectrum of SB exhibits characteristic
-1 48
peaks due to maghemite phase at 630, 571 and 436 cm .
-
1
FTIR spectra of SPHAg exhibits various prominent bands (cm )
at: 437, 567, 630, 808, 973, 1113, 1317, 1400, 1620, 2887,
software
The panel (b) exhibits a doublet due to Fe 2p3/2 and Fe 2p1/2 at
10.9 and 724.6 eV respectively with a characteristic spin-orbit
.
-
1
2
924, 3225 and 3433. Peaks at 630, 567 and 437 cm indicate
the presence of γ-Fe O3 in this sample (Fig. 3A), which are
2
7
4
different to those of the peaks reported for Fe O in this
7
52
3
4
separation of 13.7 eV,
along with a satellite peak at 719.1
region. Other peaks at 1398, 1312, 1116, 1054, 978 and 810
53
eV. The presence of the charge transfer satellite peak
indicates that iron is present in the +III oxidation state,
differentiating it with
-1
cm have been assigned to the asym and sym stretching of
2
SO4 in both the cases, which are contributed by the used
-
4
precursors FeSO4 and Fe (SO ) . In the case of SPHAg, two
8
2
4 3
the Fe O phase which does not show such a peak due to the
3
4
-1
additional peaks are observed at 2857 and 2924 cm , which
54
presence of iron in a mixed oxidation state of +II and +III.
-1
matched with the -CH stretching. A broad peak at 3400 cm
Moreover, the Fe 2p3/2 peak 710.9 eV and the satellite peak
situated around 8 eV higher binding energy to that of Fe 2p3/2
55
can be assigned to -OH stretching arising due to the adsorbed
water on the surface of these particles. Thus, IR spectroscopy
clearly indicated the presence of γ-Fe O in both the samples.
peak are also characteristics of γ-Fe O phase.
2
3
2
3
Panel (c) exhibits doublet due to Ag 3d5/2 and Ag 3d3/2 at
binding energies of 366.6 and 372.6 eV, respectively with a
56
The identification of the iron oxide phase in SPHAg was also
probed by recording their Raman spectrum (Fig. 3 b). Its
Raman spectrum exhibits the presence of four prominent
spin orbit coupling separation of 6 eV. A shift in the Ag 3d
peaks towards lower energy as compare to those of the bulk Ag
57
observed at 368.1 and 374.1 eV indicates the silver in as
-1
peaks at about 352, 500, 666 and 718 cm , all these peaks
matched with those of γ-Fe O3 phase reported in the
2
synthesized nanocomposite to be present in nanodimension.
The high resolution spectrum for C1s region (panel (d)) shows
three peaks at binding energies of 283.4, 284.8 and 287.4 eV.
4
7,49,50
literature.
4
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These peaks have been assigned to C-C, C-O and C=O groups
4
1
respectively. A comparison of these peaks with that of the
pure glucose shows them to be shifted at lower energy
5
4
3
2
1
0000
(a)
2 3
indicating the glucose moiety to be present in the Ag- γ-Fe O
5
8
0000
0000
0000
0000
0
composite.
O1s spectrum shows four bands centered at 528.7, 529.8,
32.4 and 535 eV respectively. The intense band at 528.7 eV
5
and a shoulder at 529.8 eV has been assigned to the lattice
5
9.60
oxygen and anionic species
2 3
in γ-Fe O . The bands at higher
binding energies 532.4 and 535 eV might have been attributed
6
1,62
to the oxygen of glucose and water vapors respectively.
A
0
100 200 300 400 500 600 700 800
slight shift in the binding energy due to glucose functionalities
in C1s and O1s region might have arisen because of its weak
binding with silver and the matrix in the composite.
Binding Energy (eV)
4
4
3
3
2
2
1
1
500
000
500
000
500
000
500
000
480
Fe 2p3/2
710.9
Fe 2p1/2
Ag 3d5/2
4
4
4
4
3
60
40
20
00
80
7
24.6
Ag 3d3/2
XPS analyses of this sample give the atomic % for different
elements as: C 1s (14), O 1s (64.1), Fe 2p (21.81) and Ag 3d
(0.09), from which the % ratio for Fe/Ag is computed to be
3
66.6
372.6
satellite peak
719.1
2
42. Whereas, the calculated atomic % ratio of Fe/Ag, by
taking the concentrations of the precursors used for the
preparation of γ-Fe O and Ag, comes out to 160. It suggested
2
3
360
that fairly high amount of Ag (≥ 66 %) is present on the surface
(
b)
(c)
5
00
340
7
08
712
716
720
724
728
362
364
366
368
370
372
374
376
of the Ag - γ-Fe O nanocomposites.
2 3
Binding energy (eV)
Binding energy (eV)
(
d)
C 1s
(e)
O 1s
AFM
2
83.4 eV
528.7 eV
Fig. 5 shows the AFM images of SB and SPHAg along with their
respective surface roughness and histogram. The AFM image
of SB exhibits aggregated chains of clusters of NPs with the
average surface roughness (roughness distribution) of 13 nm
284.8 eV
529.8 eV
287.4 eV
(
9-19 nm).
532.4 eV
535 eV
The coating of silver causes these chains to be broken and
shows the particles to be smaller, more organized and isolated
as is clearly indicated by their 3D images. The surface
roughness of these particles is reduced to 3.7 nm (1.8 -7 nm).
2
80
282
284
286
288
290
292 524 526 528 530 532 534 536 538
Binding energy (eV)
Binding energy (eV)
Fig. 4 XPS spectra of SPHAg: Survey scan (a); Narrow scans of the elements: Fe 2p (b);
Ag 3d (c); C1s (d) and O 1s (e).
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Journal Name
Fig. 5 2D and 3D AFM images of SB (a, b) and SPHAg (a', b'). Surface roughness of the SB and SPHAg is depicted in c and c' respectively.
FESEM
analyses of SPLAg and SPHAg shows the presence of Fe, O, C
and Ag in these samples (Fig. 6 b' and c'), in which the atomic
weight ratios of Fe and O corresponds to the structure of γ-
FESEM images of the some representative samples containing
pure matrix (SB) and the matrix containing low and high
concentrations of Ag NPs along with their EDAX analyses are
shown in Fig. 6. The FESEM image of the SB matrix shows it to
consist largely the clusters of spherical NPs (Fig. 6 a). Its EDAX
analysis shows the presence of Fe and O (Fig. 6 a'). The
addition of low amount of silver (SPLAg) causes these particles
to assemble resulting in the formation of nanochains in which
the size of the particles appears to be slightly reduced (Fig. 6
b). Whereas, at higher Ag concentrations (SPHAg) these
nanochains become more compact and the size of the particles
in these chains is further reduced (Fig. 6 c). Whereas, EDAX
Fe
2 3
O and the weight percent of silver was low (0.3%) and high
(
1.04%), respectively. The carbon in these samples is
contributed from the reducing agent glucose used for the
preparation of Ag NPs. The effect of addition of silver
concentration on the morphological changes could be better
visualized by FESEM images recorded at lower magnification
†
Figs. S3 and S4, ESI ). These images clearly show that the large
(
aggregated nanoclusters are broken into smaller units
associated with increased organization and a decrease in the
size of NPs.
Fig. 6 FESEM images and their respective EDAX analysis on the specific location marked by a cross sign in red of SB (a, a'), SPLAg (b, b') and SPHAg (c, c')
TEM
presented in Fig. 7. An analysis of these images indicates that
the TEM micrograph of SB contains largely clusters of NPs.
Whereas, SPHAg seems to have more organized chains of
nanoclusters consisting of spherical NPs. EDAX analyses of the
The morphology and the size/size distribution in these
nanostructures were further analyzed by TEM. TEM
micrographs, elemental and SAED analyses of these images are
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TEM micrographs of SB and SPHAg show the presence of Fe
and O in both the samples (Fig. 7 a'' and 7 b''). However, the
latter sample also shows the presence of C and Ag.
The SAED analyses of SB and SPHAg exhibit the number of
concentric rings containing spots. The indexing of the ring
pattern in SB corresponds to the planes (220), (311), (400),
(
422), (511), (440), (620), (533) and (731) matching with that of
γ-Fe phase (JCPDS file No. 39-1346) (Fig. 7 a'''). The indexing
of the SAED pattern of SPHAg also exhibits the planes (220),
311), (400), (422), (511), (440) and (620) matching with γ-
Fe phase. In addition, it also contains the planes (111) and
2 3
O
(
2 3
O
(200) matching to that of the Ag present in the cubic phase
(JCPDS file No. 001-1167) (Fig. 7 b''').
HR-TEM images of the SB and SPHAg exhibited lattice fringes
with the d-spacing 0.295 nm (Figs. 7 a'''' and 7 b''''), which
matches with the d spacing corresponding to (220) plane of γ-
Fe O phase suggesting it to be present in both the samples. In
2
3
case of SPHAg, the presence of Ag could also be confirmed by
the presence of lattice fringes, which corresponded to the
(111) plane of silver in cubic phase (7 b'''''). From several such
images of: SB, the size of γ-Fe O NPs was found to be 11.6
2
3
±
1.6 nm; whereas, for SPHAg, the size of γ-Fe O and Ag NPs
2 3
were estimated to be 9 ± 1 and 6.6 ± 0.6 nm, respectively.
Fig. 7(B)- (b') TEM images; (b'') EDAX analysis; (b''') SAED pattern and HR-TEM image of
γ-Fe NPs (b'''') and Ag NPs (b''''') in SPHAg.
2 3
O
BET
The observed decrease in the size of nanoparticles was
examined by determining the BET surface area of SB, SPLAg
and SPHAg, which it is found to follow the order: SB (61.721
m²/g) < SPLAg (65.17 m²/g) < SPHAg (75.08 m²/g) after 2 h
degassing. It suggests that the addition of Ag NPs results in an
increase in the surface area indicating the size of these
particles to reduce as is also evidenced by AFM, FESEM and
TEM analyses.
Magnetic measurement
The magnetic behavior of SPLAg and SPHAg along with SB was
analyzed by recording the hysteresis loops (M-H curve) at 300
K and 5 K (Fig. 8). All the samples at 300 K exhibit the
superparamagnetic behavior without any appreciable
Fig. 7(A)- (a') TEM images;(a'') EDAX analysis; (a''') SAED pattern and (a'''') HR-TEM
coercivity and remanance and attain
magnetization at field greater than 0.5 kOe. A comparison of
the values of saturation of magnetization (M in emu/g)
indicates them to increase in the order: SB (59) > SPLAg (63) >
SPHAg (69). Moreover, the value of M for SPHAg is observed
a
saturation
image of SB.
s
s
to be to be about 20 % higher to that of SB, suggesting that the
addition of Ag enhances the value of Ms.
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Journal Name
SPHAg
SPLAg
SB
virtually no change in the absorption due to 4-Nip was noted,
suggesting the absence of involvement of NaBH alone as well
as NaBH on the γ-Fe matrix in the reduction process. This
reaction though appears to be feasible in terms of the
reduction potential of 4-Nip (E for 4-Nip/4-Ap = - 0.76V) and
NaBH (H BO /BH - = - 1.33V) versus NHE contrary to the
60
4
4
0
0
0
4
2 3
O
2
0
4
3
3
4
-3
-2
-1
0
1
2
3
above observation. Obviously, this process in not permitted
kinetically.
Applied field (kOe)
-
-
-
20
40
60
Ag mediated catalytic reduction of 4-Nip
The kinetics of the reaction was examined by changing the
concentration of silver in a wide concentration range (0.15 –
2
8.9 µM) by maintaining the pseudo-kinetic conditions with
Fig. 8 M-H loop of SB, SPLAg and SPHAg at (a) 300 K.
respect to other reactants. Under all the used kinetic
conditions, the 4-Nip upon reduction is converted into 4-Ap.
For a typical case, in which the concentration of the silver has
XRD and Raman spectroscopy of the as synthesized
nanocomposites reveals them to be silver coated γ-Fe O
2
3
been kept at 1.8 µM, NaBH (68.22 mM) and 4-Nip (161.27
4
nanocomposites containing Ag in the cubic phase (Figs. 2 and
). XPS analysis evidently demonstrates the formation of
composite consisting of γ-Fe O and silver phase bound to
µM), the reduction process is presented in Fig. 9 a. It results in
the appearance of two new peaks at 231 and 302 nm and
exhibiting four isosbestic points at 224, 249, 277 and 318 nm
respectively.
3
2
3
glucose. A reduction in the average size of the matrix observed
upon in situ generation of Ag NPs on γ-Fe O matrix is
2
3
In order to analyze the operating mechanism the effect of
concentration of Ag NPs was investigated both at its lower as
well as higher concentrations. At lower concentrations of Ag,
the catalytic reduction followed the first-order kinetics,
whereas, at higher concentration of Ag, the zero-order kinetics
is obeyed. Typical kinetic traces observed at low and high
concentrations of Ag have been recorded in Fig. 9 b and c,
respectively.
understood by the breaking of weak van der Waals non-
covalent interactions within γ-Fe O clusters upon generation
2
3
of Ag. This is also evidenced by the optical absorption
spectrum of Ag coated γ-Fe O nanocomposites at its low and
2
3
high concentration exhibiting increasing blue shift in the
absorption spectra (Fig. 1c and 1d). The surface effects
observed upon increasing addition of Ag causing a reduction in
the size of the Ag – γ-Fe O nanoclusters, suggests them to
2
3
bring a change in the orientation of the surface spin due to
increased supramolecular interactions, resulting in an
enhanced magnetization (Fig. 8).
Effect of concentration of Ag NPs
Lower concentration range. In the first set of experiments, the
concentration of Ag was varied in the range of 0.15 - 1.2 µM
keeping other reactants at a much higher concentration (more
than ten times) so as to maintain the pseudo-kinetic
conditions. In all the kinetic experiments, an induction period
was noted, the duration of which is observed to reduce with
an increase in the concentration of Ag.
Catalytic reduction of 4-Nip
In control experiments the feasibility of reduction of 4-Nip was
examined by NaBH alone and NaBH on the γ-Fe O matrix in
4
4
2 3
the absence of Ag NPs (Fig. S5, ESI†). Under these condiꢀons
no reduction of 4-Nip was observed up to 180 min and
2
.2
1
.0
0.5
.0
-0.5
1.0
-1.5
2.0
-2.5
First o
Linear
2.0
2
2
1
1
0
0
.5
.0
2.5
2.0
1.5
1.0
0.5
0.0
t0
2
R
1.8
1.6
:
0.998
0
1.4
Induction period
1.2
-
Induction
1.0
0.8
.5 period
0.6
-
240 270 300 33
Time
.0
.5
.0
400
800
1200 1600 2000
Time (s)
(
c)
(
b)
0
100 200 300 400 500 600 700
Time (s)
0
500
1000
1500
Time (s)
2000
2500
4
Fig. 9 (a) Absorption spectra of 4-Nip depicting its catalytic reduction to 4-Ap having [Ag] = 1.8 µM, [NaBH ] = 68.22 mM, [4-Nip] = 161.27 µM. Inset: Digital image exhibiting the
reduction of 4-Nip to 4-Ap. Kinetic traces depicting reduction of 4-Nip recorded at different concentrations of silver (b) 0.3 µM; (c) 3.2 µM, [NaBH
4
] = 68.22 mM, [4-Nip] = 161.27
0
µM at 30 C.
8
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JournC0al Na1Cme
DOI: 10.1039/C5G 094
In this concentration range, the reduction of 4-Nip following first-order behavior. The observed rate constant is, therefore,
pseudo-first-order rate constant was found to vary linearly considered to be the apparent first-order kinetic constant
with the concentration of Ag NPs (Fig. 10 a), from which the (kAp).
second order rate constant was determined from the slope of
3
the graph to be 5.28 × 10 dm mol s .
3
-1 -1
Higher concentration range. Since at higher concentrations of
The used pseudo-conditions, specifically the concentration of Ag (> 2.4 µM) the kinetics starts deviating from the first-order
NaBH4 and 4-Nip were examined by varying their behavior, kinetic experiments at relatively higher
concentrations within a small range. The kinetic plots obtained concentrations of Ag (3.2 - 28.9 µM) were performed by
from these experiments by varying each of the individual keeping the concentrations of other reactants such that the
reactant keeping all other reactants constant are shown in Fig. pseudo-conditions are maintained. The kinetic data of these
1
0 b and c, respectively.
runs exhibit zero-order kinetics (Fig. 10 a').
For NaBH , a change in the concentration from 60 - 119 mM As expected under these conditions, the reduction of 4-Nip
4
did not show any appreciable change in rate constant, but a gets over efficiently within 5 min excluding induction period.
decrease in its concentration to 51.2 mM, however, led to a To further examine the above kinetic behavior, the effect of
slight decrease in rate constant value (Fig 10 b). It suggests the concentration of all other reactants, namely, NaBH (51.2 - 119
4
-
reaction to be independent of the BH concentration.
mM) and 4-Nip (146 - 194 µM) was varied, keeping the
4
For 4-Nip variation (64.5 - 178 µM) a change in the kinetic concentration of silver constant. The kinetic traces of these
behavior is recorded in Fig. 10 c. Interestingly, at lower runs are shown in Fig. 10 b' and c'.
concentrations of 4-Nip (64.5 to 104 µM), the rate constant From these experiments it is evident that a slight variation
increases with an increase in [4-Nip] and was found to be the from the pseudo-conditions results in the marked deviation in
maximum at 113 µM, thereafter, it starts decreasing from 113 the rate constant associated with a mixed kinetic behavior.
to 178 µM. At very low concentrations of 4-Nip the surface is
-
Analysis of thermodynamic parameters
largely occupied by BH , which results in relatively lower
4
values of rate constant. A further increase in the concentration The thermodynamic parameters for the above studied
of 4-Nip the surface gets increasingly occupied by more Nip reaction were obtained by performing the effect of
molecules, causing an increase in the rate constant. The higher temperature on the rate of reduction of 4-Nip for the typical
concentrations of Nip, however, interferes in the reaction with concentrations of Ag at 0.6 µM and 6.4 µM in the lower and
-
BH due to higher binding constant for 4-Nip compared to that higher concentration range, respectively by keeping the
4
-
of BH , as has also been observed in other Ag catalyzed concentrations of NaBH (68.22 mM) and 4-Nip (113 / 161 µM)
4
4
6
reactions, resulting in a decrease in the rate constant. At a constant. These experiments were performed in the
0
fixed concentration of other reactant this is the characteristic temperature range of 20 – 50 C. In these experiments, it was
3
feature of the surface catalyzed reaction following the observed that the induction period decreases with an increase
Langmuir-Hinshelwood model (Fig. 10 c). Since the in temperature. The activation energy and other
-
‡
‡
‡
concentration of BH4 in these experiments is in large excess thermodynamic parameters (ΔH , ΔG and ΔS ) were obtained
68.22 mM) to that of 4-Nip (113 µM), therefore, at low using the thermodynamic formulations derived from the
(
6
4
concentrations of Ag the observed kinetics exhibits the first- absolute reaction rate theory (Fig.11 a and b).
order with respect to 4-Nip and the reaction follows pseudo-
0
0
0
0
0
0
0
0
0
.008
.007
.006
.005
.004
.003
.002
.001
.000
0.0035
Linear Fit
R : 0.9915
First order
Fitting
First Order
(
a)
0.0050
.0045
0.0040
2
(c)
0
0
0
0
0
.0030
.0025
.0020
.0015
0
0
.0035
.0030
0.0025
.0020
0.0015
0
-
0.001
5
0
60 70 80 90 100 110 120
0
.0 0.2 0.4 0.6 0.8 1.0 1.2
60
80
100 120 140 160 180
NaBH4
[ ] (mM)
[Nip] (µM)
[Ag] (µM)
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G
d
re
o
e
n
n
o
C
t a
h
d
e
j
m
u si s
t
t
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DOI: 10.1039/C5G 094
JournC0al Na1Cme
0.016
0.014
0.012
0.010
0.008
0.006
0.004
0.014
0.012
0.010
0.008
0.006
0.004
0.002
0
.014
.012
zero-order
zero order
Fitting
Zero-order
(a')
(c')
0
0.010
0.008
Mixed order
0.006
140 150 160 170 180 190 200
0
5
10 15 20 25 30
Ag]
50 60 70 80 90 100 110 120
NaBH ] (mM)
[
[
(µM
)
4
[Nip] (µM)
0
Fig. 10 Kinetic data exhibiting effect of concentrations of at 30 C: Silver (a) first-order rate constant (k) as a function of [Ag] = (0.15 -1.209 µM), [4-Nip] = 161.27 µM, [NaBH
4
] =
] = 68.22 mM and [Ag] = 0.6
4
] = (51.2 -119.3 mM), [4-Nip] =
6
8.22 mM; NaBH
4
(b) [NaBH
4
] = (51.2 -119.3 mM) having [Ag] = 0.6 µM, [4-Nip] = 161.27 µM and 4-Nitrophenol (c) [4-Nip] = (64.5 -178 µM), [NaBH
4
µM. Rate constant as a function of concentration: Silver (a') [Ag] = (3.2 -28.9 µM), [4-Nip] = 161.27 µM, [NaBH
4
] = 68.22 mM; NaBH (b') [NaBH
4
0
1
4
61.27 µM and [Ag] = 6.4 µM and 4-Nitrophenol (c') [4-Nip] = (146 -194 µM) having [NaBH ] = 68.22 mM and [Ag] = 6.4 µM at 30 C
-
-
-
-
-
-
-
3.8
4.0
4.2
4.4
4.6
4.8
5.0
Linear Fit
2
R : 0.98366
Linear fit
R : 0.97672
-
-
-
-
-
-
-
4.8
5.0
5.2
5.4
5.6
5.8
6.0
(b)
2
0
.003225
0.003300
1/T]
0.003375
0.003450
0.0031
0.0032
0.0033
1/T
0.0034
[
Fig. 11 Plot of ln (k) versus (1/T) for the 4-Nip reduction in the presence of Ag – γ- Fe
µM; (b) high [Ag] = 6.4 µM and [4-Nip] = 161 µM.
2 3 4
O nanocomposite containing [NaBH ] = 68.22 mM: (a) low [Ag] = 0.6 µM and [4-Nip] = 113
Table 1 Calculated thermodynamic parameters for low and high concentration of Ag NPs.
-
1
‡
-1
‡
-1
‡
-1 -1
Catalyst [Ag]
Order of reaction
E
A
(kJ mol )
∆G (kJ mol )
∆H (kJ mol )
∆S (J mol K )
0
6
.6 µM
.4 µM
first-order
zero-order
38.9
25.0
87.8
86.0
36.4
22.4
- 169
- 210
reaction is initiated by the addition of Ag NPs, the duration of
the induction period should be controlled by the 4-Nip
approaching to these sites. Indeed, the induction period
Mechanism
The reduction of 4-Nip is conceived to occur by the donation of reduces with increase in the concentration of Ag (0.15 - 28.9
-
electron from BH4 to the acceptor 4-Nip adsorbed on the µM) from 320 s to 61 s. Increasing sites essentially allows more
surface of the catalyst (Ag – γ-Fe O ) in a bimolecular reaction. molecules of 4-Nip to get adsorbed on their surface to undergo
2
3
In these experiments, the surface of the particles (S) is a reduction. It indirectly determines the concentration of 4-Nip
-
composite of Ag – γ-Fe O (X) containing adsorbed BH and is available for the reaction on its surface.
2
3
4
denoted by S . Since the reaction is initiated only in the Since at low concentrations of Ag the reduction follows first-
X
presence of Ag NPs and γ-Fe O does not contribute to the order kinetics and at high concentrations it follows zero-order
2
3
-
reaction, it is likely that BH initially transfer the electrons to kinetics (Table 1), from the above kinetic data this behavior is
4
Ag NPs, which then reduces 4-Nip to 4-Ap (Scheme 1).
understood by the surface reactions having 4-Nip molecules
The observed change in kinetics with an alteration in adsorbed on the surface of the catalyst. The occurrence of the
concentration of Ag suggests that the sites available on the above reaction could be best explained in terms of Langmuir-
6
4
surface of the catalyst influence the kinetics in a complex Hinshelwood model.
-
scheme. Since BH4 is initially mixed with 4-Nip and the
1
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2 3 2 3
Scheme 1 Mechanism for the reduction of 4-Nip to 4-Ap on silver coated γ-Fe O nanocomposites (Ag – γ-Fe O )
The mechanism for the occurring reaction may be presented At low concentrations of Ag NPs, the rate constant for the
as:
reduction of 4-Nip varies linearly with the concentration of Ag
NPs (Fig. 8 a) associated with a reduction in the induction
period. It evidently suggests the catalytic efficiency in low Ag
NPs concentration range to be quite effective for the reduction
of 4-Nip. It was further verified experimentally by repetitive
use of the catalyst (up to seven times) without any catalytic
loss. The conversion efficiency and the time taken for
complete conversion by the catalyst are presented in Fig. 12.
ꢖꢗꢘꢙ
4˗ꢀꢁꢂ ꢃ ꢄꢅꢆꢇꢈ–ꢈꢉ˗ꢊꢋꢌꢍꢎꢈꢏꢐꢑ
⇌
ꢒ4˗ꢀꢁꢂ ꢓ ꢄ ꢕ ꢚꢛꢛꢜ ꢅ4˗ꢝꢂ ꢓ ꢄ ꢑ
ꢔ
ꢔ
⇌
4˗ꢝꢂ ꢃ ꢄꢔ
The rate of reaction then will be –
ꢞꢒ4-ꢀꢁꢂꢕ
ꢓ
ꢠꢈꢡꢢꢣꢤꢔ
ꢞꢟ
where, ꢤꢔ represents the fraction of bare sites on the matrix The very similar efficiency with a marginal increase in reaction
and ꢒ4-ꢀꢁꢂ ꢓ ꢄꢔꢕ denotes the concentration of the species time after the reuse of the catalyst for 7 times indicates that
consisting of the 4-Nip bound to the surface site. ꢡꢢꢣ the catalyst maintains its catalytic activity in the process of
represents the apparent rate constant of the reaction.
reduction. The above observations clearly demonstrate that
fairly small amount of Ag NPs to be highly effective for the
reduction as compared to the most of the earlier reported
ꢞꢒ4-ꢀꢁꢂꢕ
ꢞꢟ
ꢥ
ꢦ-ꢧꢨꢣꢒ4-ꢀꢁꢂꢕ
ꢓ
ꢠꢈ
1 ꢃ ꢩꢥꢦ-ꢧꢨꢣꢒ4-ꢀꢁꢂꢕꢪ ꢃ ꢅꢥꢦ-ꢢꢣꢒ4-ꢝꢂꢕꢑ
28,41-44
work.
K4-Nip and K4-Ap denote the equilibrium constants between 4-
Nip and the surface and 4-Ap and the surface, respectively.
Thus the rate constant from the above rate law will depend on
the relative values of ꢅꢥꢦ-ꢧꢨꢣꢒ4-ꢀꢁꢂꢕꢑ and ꢅꢥꢦ-ꢢꢣꢒ4-ꢝꢂꢕꢑ. At
low concentration of the catalyst both 4-Nip and 4-Ap are
relatively weakly adsorbed, under these conditions
ꢫꢩꢥꢦ-ꢧꢨꢣꢒ4-ꢀꢁꢂꢕꢪ ꢃ ꢅꢥꢦ-ꢢꢣꢒ4-ꢝꢂꢕꢑꢬ<< 1. Therefore, the
reaction follows the first-order kinetics.
A comparison of the present nanocomposite with those
45,65
somewhat related systems investigated previously
are
presented in Table 2 as regards to their synthesis and
characteristic features. This table clearly reveals that the as
synthesized nanocomposites contains the highest value of
magnetization with the minimum amount of Ag NPs and
exhibits fairly high catalytic activity for the reduction of 4-Nip.
In order to assess the role of glucose as stabilizer and reducing
agent, in a control experiment the in situ generation of Ag was
also carried out by using NaBH4 as reducing agent. By
employing the same concentration of Ag NPs produced by
using NaBH on γ-Fe O matrix as used in the present system,
On the other hand, at high concentrations of catalyst, 4-Nip is
relatively more strongly adsorbed on the surface as compared
to that of the product. Under such conditions
>
ꢩꢥꢦ-ꢧꢨꢣꢒ4-ꢀꢁꢂꢕꢪ ꢭ ꢫꢩꢥꢦ-ꢢꢣꢒ4-ꢝꢂꢕꢪ ꢃ1} and the reaction
would then follow zero-order kinetics.
4
2
3
the rate constant for the catalytic reduction of 4-Nip was
found to be 1.6 times smaller to that of glucose mediated
nanosystems under identical experimental conditions (Fig. S7,
The weak binding of 4-Nip at low concentrations of silver and
relatively stronger binding at its high concentration(s) in the
transition state is also reflected by thermodynamic parameters
†
ESI ). It suggests that besides glucose being biocompatible and
‡
in which the ∆S at lower and higher concentrations are -169
non-toxic serves as a better stabilizer and reducing agent
generating Ag NPs providing more catalytic sites on the surface
of matrix. To the best of our knowledge this is the lowest
concentration of Ag NPs employed for the 4-Nip reduction in
such systems.
-1
-1
and -210 J mol K , respectively. These values suggest the
formation of more ordered structure at higher concentrations
of Ag which is also supported by the AFM, FESEM and TEM
data (Figs. 5-7).
Catalytic efficiency and reusability of the nanocomposites
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 11
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ARTICLE
DOI: 10.1039/C5G 094
JournC0al Na1Cme
3
2
2
2
2
00
80
60
40
20
Fig. 12 (a) Graph showing the variation in the time of completion of full conversion of 4-
Nip to 4-Ap for up to 7 cycles and (b) Digital image showing the easy separation of the
nanocomposite and reusability of the catalyst employing external magnetic field for
large scale reactions.
Zero order
(a)
1
00
8
6
4
2
0
0
0
0
0
200
1
2
3
4
5
6
7
Cycle
2 3
Table 2: Comparison of synthetic protocol and characteristic features of the as synthesized Ag – γ-Fe O nanocomposite with similar reported nanosystems.
-
1
Catalyst
used
Ag wt (%)
employed
52
Method of preparation
and phase
size
Magnetization
(emu/g)
k
app (s )
Order of
reaction
First
Mechanism/
Reference
(Temp)
Model
-
s
-
3
2
Ag-γ-Fe O
3
2
γ-Fe O
3
(commercial)
33 ± 19 nm
Not reported
15 ×10
45
0
Ag deposited in
2
(γ-Fe O
3
)
(35 C)
ethanolic medium using 5-10 nm (Ag)
butylamine as reducing
agent
-
3
Ag
2.5 - 10
Aqueous, High temp.
(350° C) synthesis;
Size of Fe
2
O
3
̴
20 with
2.33-14×10
First
Mechanism
proposed
65
0
supported
iron oxide
microbox
not reported
16.8-28 nm
(Ag)
hysteresis
loop
(20 C)
2 3
mixed Phase (γ-Fe O
*
2 3
and α-Fe O )
containing Ag
-
3
Ag – γ-
0.05-0.43
(0.15-1.2
µM)
Aqueous, Glucose
mediated in situ Ag
9 ± 1 nm (γ-
63
0.6-6.4× 10
First
Zero
Mechanism
analyzed
This work
This work
0
2
Fe O
3
2
Fe O
3
)
(superpara-
magnetic)
69
(30 C)
generation; γ-Fe
2
O
3
6.6 ± 0.6 nm
(Ag)
(Langmuir-
-
3
1
.15 (3.2
µM)
synthesized and
characterized by
Raman, XRD, XPS
9.02 ×10 mol
Hinshelwood
kinetic model)
-
1 -1
(superpara-
magnetic)
L s
0
(30 C)
*
(
composition not reported)
activity. Thus, the as synthesized nanocomposites, being
magnetically separable and recyclable with superparamagnetic
features, having high magnetization
demonstrating efficient catalytic activity indicates it to be
We have successfully synthesized Ag – γ-Fe O multifunctional environmentally benign multifunctional material, promising for
and surface area,
Conclusions
2
3
nanocomposites consisting of both γ-Fe O and Ag in the cubic catalytic and biological applications.
2
3
phase in aqueous medium employing the green approach. The
in situ coating of the Ag on the surface of γ-Fe O matrix
3
2
results in its homogeneous distribution causing a reduction in Acknowledgements
the size of the γ-Fe O3 nanoclusters. The silver coated
2
Financial support of CSIR, New Delhi (Grant no.
1/2758/13/EMR-II) to undertake this work is gratefully
nanocomposite catalyzes the reduction of hazardous 4-Nip to
useful environmental benign product, 4-Ap and exhibits the
concentration dependent kinetics following the Langmuir-
Hinshelwood mechanism. Glucose being environmentally
benign, displaying weak non-covalent interactions with Ag and
γ-Fe O in the composite through –OH and –C=O functional
0
acknowledged. Mandeep Kaloti is grateful to MHRD, New Delhi
for the award of SRF. Thanks are also due to head IIC, IIT
Roorkee for providing the facilities of FESEM, AFM, TEM and
XRD.
2
3
groups, not only serves as an effective stabilizer but also
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Journal Name
Table of Contents Graphic
Synthesis of glucose-mediated Ag - γ-Fe O multifunctional nanocomposites in aqueous medium - a
kinetic analysis of their catalytic activity for 4-nitrophenol reduction
2
3
a,b a,b*
Mandeep Kaloti, Anil Kumar and Naveen K. Navani
a,c
a
Centre of Nanotechnology, Indian Institute of Technology Roorkee, Roorkee-247667,India
Department of Chemistry, Indian Institute of Technology Roorkee, Roorkee-247667, India. E-mail*: anilkfcy@iitr.ac.in
Department of Biotechnology, Indian Institute of Technology Roorkee, Roorkee-247667, India
b
c
Synthesis of glucose-mediated Ag - γ-Fe O nanocomposites in aqueous medium exhibits catalytic activity for 4-nitrophenol reduction to 4-
2
3
aminophenol following Langmuir-Hinshelwood mechanism at lower [Ag] (µM) (0.3, SPLAg; 6.4, SPHAg).
1
1
1
1
0
0
0
0
0
.6
.4
.2
.0
.8
.6
.4
.2
SB
1600
SPHAg
SB
SPHAg
SPLAg
1
1
1
400
200
000
800
600
664.54
501.39
3
91 nm
472 nm
718.81
3
57.02
479 nm
395 nm
4
94.24
352.28
424 nm
485 nm
713.46
6
69.3
400
.0
2
300
600
900
00 300 400 500 600 700
Wavelength ( nm)
-
1
Raman shift cm )
(
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