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Structural, magnetic, and textural properties of iron
oxide-reduced graphene oxide hybrids and their
use for the electrochemical detection of chromium
Anand Prakash, Sudeshna Chandra, D. Bahadur
*
Department of Metallurgical Engineering and Materials Science, Indian Institute of Technology Bombay, Mumbai-400 076, India
A R T I C L E I N F O
A B S T R A C T
Article history:
Superparamagnetic Fe3O4 nanoparticles were anchored on reduced graphene oxide (RGO)
Received 5 March 2012
nanosheets by co-precipitation of iron salts in the presence of different amounts of graph-
Accepted 2 May 2012
ene oxide (GO). A pH dependent zeta potential and good aqueous dispersions were
Available online 16 May 2012
observed for the three hybrids of Fe3O4 and RGO. The structure, morphology and microstructure of the hybrids were examined by X-ray diffraction, transmission electron microscopy (TEM), Fourier transform infrared spectroscopy, Raman and X-ray photoelectron
spectroscopy. TEM images reveal lattice fringes (d311 = 0.26 nm) of Fe3O4 nanoparticles with
clear stacked layers of RGO nanosheets. The textural properties including the pore size distribution and loading of Fe3O4 nanoparticles to form Fe3O4–RGO hybrids have been controlled by changing the concentration of GO. An observed maximum (10 nm) in pore
size distribution for the sample with 0.25 mg ml1 of GO is different from that prepared
using 1.0 mg ml1 GO. The superparamagnetic behavior is also lost in the latter and it
exhibits a ferrimagnetic nature. The electrochemical behavior of the hybrids towards chromium ion was assessed and a novel electrode system using cyclic voltammetry for the
preparation of an electrochemical sensor platform is proposed. The textural properties
seem to influence the electrochemical and magnetic behavior of the hybrids.
2012 Elsevier Ltd. All rights reserved.
1.
Introduction
Graphene, a two-dimensional sheet of sp2-hybridized carbon
atoms arranged in a honeycomb lattice, exhibits remarkable
surface, electronic and mechanical properties [1,2]. Owing to
these fascinating properties, graphene has a range of applications in nano-electronics, catalysis and waste water treatments [3–5]. On the other hand, superparamagnetic Fe3O4
nanoparticles (NPs) have been brought into sharp focus due
to their potential biomedical applications such as hyperthermia treatment of cancer [6], contrast agent for magnetic resonance imaging [7], magnetic separation [8], targeted drug
delivery [9] and waste water treatments [10]. The NPs are required to be chemically stable, uniform in size with high spe-
cific surface area and well dispersed in liquid media for all
applications. But due to anisotropic dipolar attraction, pristine NPs of Fe3O4 tend to aggregate into large clusters thereby
losing the specific properties associated with single-domain
magnetic nanostructures [11]. This can be effectively overcome by loading the magnetic nanoparticles (MNPs) in carbon-based matrix [12]. Carbon nanotubes (CNTs) loaded
with MNPs using different chemical approaches and their
applications in electrochemical sensing [13], solid phase
extraction [14], catalytic properties [15], etc. have been well
explored. Due to similarities between CNTs and graphene, it
is anticipated that graphene sheets if loaded with Fe3O4 NPs
might give an extra feature of magnetic moiety with enhanced properties of the hybrids.
* Corresponding author: Fax: +91 22 2572 3480.
E-mail address: [email protected] (D. Bahadur).
0008-6223/$ - see front matter 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.carbon.2012.05.002
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Recently, Yang et al. [5] have reported the preparation of
graphene oxide (GO)–Fe3O4 hybrids containing 13.4 wt.%
Fe3O4 by a chemical precipitation method which showed a very
high loading efficiency of a cancer drug. Yu et al. [16] decorated
reduced graphene oxide (RGO) nanosheets with Fe3O4 NPs by
high temperature decomposition of the precursor iron(III) acetylacetonate and proposed their use as a magnetic resonance
contrast agent. He et al. [17] reported the attachment of surface-modified Fe3O4 NPs to GO by covalent bonding. Chen
et al. [18] have developed an electromechanical actuator based
on graphene and graphene/Fe3O4 hybrid paper by separately
mixing aqueous suspension of Fe3O4 NPs and GO and reducing
it with hydrazine hydrate (80%) at 90 C. Wang et al. [19] prepared Fe3O4 graphene composites by a gas/liquid interface
reaction at 180 C with enhanced cycling performance for lithium-ion batteries. Zhang et al. [20] have developed 3D hierarchical porous Fe3O4/graphene composites with high lithium
storage capacity and for controlled drug delivery. Hu et al.
[21] and Cheng et al. [22] synthesized graphene–Fe3O4 composites at high temperatures with improved reversible capacity
and cyclic stability for lithium-ion batteries. Since the above
reaction was carried out at a very high temperature, it was difficult to control pore size distribution for desired applications.
Despite significant efforts, multifunctional hybrid materials
that take an advantage of the superior properties of graphene
and Fe3O4 have been largely unexplored. Also, there is a need
to address the existing challenges and problems in the field of
graphene–Fe3O4 hybrids, some of which are (1) relatively complex synthesis method (2) control on size, surface properties
and coverage density of MNPs on graphene and (3) effect of
GO concentration on their microstructural, magnetic and electrochemical properties of the hybrids. To resolve such issues,
in the present work we provide an easy one-step co-precipitation route for synthesizing Fe3O4–RGO hybrids at relatively lower temperature (90 C). During the reaction, reduction of GO
takes place simultaneously with nucleation and growth of
Fe3O4 NPs on RGO nanosheets. We have also investigated the
pore size distribution, superparamagnetic behavior and loading
amount of MNPs on RGO by controlling the amount of GO. The
electrochemical detection of metal ions was carried out using
the synthesized hybrids as electrode material.
2.
Experimental
2.1.
Materials
Graphite powder with a particle size of 45 lm (product No.
496596, 99.99 %), Ferric chloride hexahydrate (FeCl3Æ6H2O23648-9) and Ferrous chloride tetrahydrate (FeCl2Æ4H2O22029-9) were obtained from Sigma–Aldrich. Hydrazine
hydrate (N2H4, 80%), H2SO4 (98%), HCl (35%), H2O2 (30%), KMnO4
and other chemical reagents were purchased from Thomas
Baker, India and used without further purification. A dialysis
tubing cellulose membrane used in the purification of GO,
was purchased from Sigma–Aldrich (D 9402-100 FT).
2.2.
Synthesis of Fe3O4–RGO hybrids
Aqueous suspension of GO was prepared by Hummer’s method [23–25] from natural graphite involving graphite oxidation,
followed by ultrasonication (see Supplementary information).
In a typical procedure, graphite oxide (125 mg) was dispersed
in 500 mL of milliQ water (0.25 mg/mL) and ultrasonicated for
4–5 h. It was then centrifuged at 5000 rpm for 20 min and the
supernatant was collected for further synthesis of Fe3O4–RGO
hybrids. The aqueous suspension of GO was purged with N2
gas and was vigorously stirred for 30 min. FeCl3Æ6H2O
(1.838 g, 0.0216 mol) and FeCl2Æ4H2O (0.703 g, 0.0108 mol) were
dispersed separately in 20 mL milliQ water and were added to
the suspension of GO. Stirring of the mixture was continued
at 80 C for 30 min and then, 10 mL of ammonia solution
(NH4OH) was quickly injected into the reaction mixture and
was stirred for another 30 min. To this, 10 mL of hydrazine hydrate was added and the mixture was again stirred for 4–5 h
at 90 C to ensure complete reduction of GO. The above reaction mixture was cooled and washed several times with milliQ and the unreacted precursors were removed using a
magnet. We refer the above synthesized Fe3O4–RGO hybrid
as Fe3O4–RGO-1 in the text (Fig. 14). By adopting same experimental procedure and fixing all other parameters including
weight of reagents, we prepared Fe3O4–RGO-2 (0.5 mg/mL–
500 mL of aqueous suspension of GO) and Fe3O4–RGO-3
(1.0 mg/mL–500 mL of aqueous suspension of GO).
2.3.
Instruments and measurements
The crystallographic structures of the Fe3O4–RGO hybrids
were analyzed by X-ray diffraction (XRD, Philips powder diffractometer PW3040/60 with CuKa radiation). The NPs distribution, morphologies and selected area electron diffraction
(SAED) patterns of the Fe3O4–RGO hybrids were characterized
by transmission electron microscopy (TEM) using a JEOL JEM2100 facility. Raman measurements were performed on Lab
RAM HR 800 micro-Raman spectroscopy using the 514.5 nm
line of an Argon (Ar+) laser. The magnetic properties of the hybrids were measured with vibrating sample magnetometer
(Lake Shore, VSM-7410). XPS measurements were performed
using an ESCA Probe (MULTILAB from Thermo VG Scientific)
with a monochromatic Al Ka radiation (Energy = 1486.6 eV).
Fourier transform infrared spectra (FTIR) were taken on a
JASCO spectrometer (6100 type-A) instrument. The specific
surface area, pore volume and pore size distribution of
the hybrids were measured by ASAP 2020 Micromeritics
instrument. Specific surface areas were determined by
the multipoint Brunauer–Emmet–Teller (BET) method. The
corresponding pore size distribution and total pore volume
were determined by the Brunauer Joyner–Hallenda (BJH)
method applied to the desorption branch. Prior to measurements, the samples were outgassed at 40 C with a heating
rate of 10 C/min for 1 h and then the temperature was raised
up to 50 C and maintained overnight. Zeta potentials of hybrids were measured by zeta potential analyzer, (DelsaNano
C, Beckman coulter Inc.). The content of Fe in hybrids was
measured by inductively coupled plasma–atomic emission
spectrometer (ICP–AES, ARCOS, Germany). The electrochemical measurements (Cyclic Voltammetry) were conducted in a
3-electrode single-cell system in Fe2+/Fe3+ couple electrolyte
with 0.1 M PBS as supporting electrolyte. Glassy carbon electrode (GCE, diameter u = 2 mm), Pt-wire and Ag/AgCl electrodes were used as working, counter and reference
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electrodes, respectively with CHI1140A electrochemical workstation (CHI110, Austin, TX). All electrochemical measurements were carried out at room temperature. To eliminate
the effect of dissolved oxygen, the electrolyte was purged
with nitrogen gas for half an hour.
3.
Results and discussion
Fig. 1 shows the XRD patterns of the magnetic hybrids Fe3O4–
RGO-1, Fe3O4–RGO-2 and Fe3O4–RGO-3 to elucidate the phase
and structural parameters. The observed diffraction peaks
of the hybrids are in good agreement with those reported in
the literature for pure face-centered cubic structured Fe3O4
[20,21]. The absence of (0 0 1) diffraction peak corresponding
to graphite oxide (see Supplementary information, Fig. S1)
indicates complete exfoliation of RGO nanosheets in the hybrids. Moreover, the full width at half maximum (FWHM) of
(3 1 1) diffraction peak of hybrids (inset of Fig. 1) is dependent
on concentration of GO and the average crystallite size of
Fe3O4 NPs in Fe3O4–RGO-3 as estimated from FWHM of (3 1 1)
is larger than Fe3O4–RGO-1 and Fe3O4–RGO-2.
FEG-TEM images of the obtained hybrids (Fe3O4–RGO-1,
Fe3O4–RGO-2 and Fe3O4–RGO-3) are shown in Fig. 2a–f. It is observed that Fe3O4 NPs are anchored on RGO nanosheets in all
three samples. The aggregation of Fe3O4 NPs is maximum for
Fe3O4–RGO-1 (Fig. 2a). With an increase in the concentration
of RGO in Fe3O4–RGO-2 and Fe3O4–RGO-3, aggregation decreases (Fig. 2b) and a good distribution of Fe3O4 NPs is
observed for Fe3O4–RGO-3 (Fig. 2c). The SAED pattern of
Fe3O4–RGO-1 (inset Fig. 2a) shows well-defined rings, which
are characteristic of the cubic structure of nanocrystalline
Fe3O4. As can be seen in TEM micrographs (Fig. 2a–c), after
strong sonication (200 W, 2 h) of hybrids during the preparation of the TEM specimen, Fe3O4 NPs were observed only on
RGO nanosheets. This implies strong interaction between
Fe3O4 NPs and RGO nanosheets.
The TEM micrograph of Fe3O4–RGO-1 (Fig. 2d) on a selected
site demonstrates good crystallinity and clear lattice fringes
Fig. 1 – Powder XRD patterns of as prepared hybrids Fe3O4–
RGO-1, Fe3O4–RGO-2 and Fe3O4–RGO-3 respectively. The top
left inset shows in large view of (3 1 1) peak.
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of the Fe3O4 NPs along with a cross section view of stacked
RGO nanosheets (square across the regions with fringes).
The crystal lattice fringes with d-spacing of 0.26 nm (inset
Fig. 2d) can be assigned to the (3 1 1) plane of the cubic
Fe3O4, which is consistent with the d-spacing of (3 1 1) XRD
peak, while the interlayer distance of stacked RGO nanosheets was found to be 4.3 Å, corresponding to the spacing
of the (0 0 2) planes of graphite. The observed basal spacing
is higher than that of well-ordered graphite (3.35 Å). The higher basal spacing may be due to the presence of residual oxygen-containing functional groups, indicating incomplete
reduction of GO in RGO nanosheets [26]. The lattice fringes
of the Fe3O4 NPs were clearly observed in Fe3O4–RGO-1, but
not so well for Fe3O4–RGO-2 and Fe3O4–RGO-3 probably due
to increased content of RGO nanosheets (Fig. 2e and f). As expected, during the synthesis of hybrids, restacking process
among RGO nanosheets is hampered as the Fe3O4 NPs get
anchored randomly on RGO layers which may be responsible
for the porous structure in the hybrids.
The FTIR spectra of the Fe3O4–RGO hybrids along with GO
and RGO are shown in Fig. 3. The observed representative
peaks in GO confirm the presence of the oxygen-containing
functional moieties in carbon frameworks, which include
bands at 1066 cm1 (C–O stretching vibration of epoxide)
and 1732 cm1 (C=O stretching of carbonyl and carboxyl
groups at edges of the GO networks) [27]. The band at
1620 cm1 is attributed to the skeletal vibration of graphitic
domains [28]. All the absorption bands related to oxygen-containing functional groups of GO disappear in the spectrum of
RGO nanosheets thereby, confirming the reduction of abovementioned functional groups by hydrazine hydrate. The band
at 1532 cm1 could be ascribed to formation of –COO after
coating with Fe3O4 NPs [5].
Raman spectra of Fe3O4–RGO hybrids (Fig. 4) reflect
two prominent bands at 1344 (D band) and 1596 cm1
(G-bands), which are characteristic of sp2 bonded, honeycomb-structured carbon allotropes [20]. The D band is either
absent or very weak in perfect graphite and only becomes
active in the presence of defects whereas the prominent G
peak observed at 1575 cm1, corresponds to the first-order
scattering of the E2g mode (in-plane bond-stretching motion
of pairs of C sp2 atoms) [29].
During synthesis of Fe3O4–RGO hybrids, significant structural changes occur in carbon framework of GO which is reflected in terms of shift and intensity ratio (ID/IG) of D and G
band [30]. The intensity ratio of the two bands, ID/IG for these
hybrids has increased as compared to GO (ID/IG 1.01, (see
Supplementary information Table S2) and are in range of
1.4–1.5. This indicates the presence of localized sp3 defects
within the sp2 carbon network upon reduction of the exfoliated GO and are in agreement with previous results reported
for RGO nanosheets obtained from exfoliated GO [31]. The significant increase in ID/IG intensity ratios were observed due to
reduction of exfoliated GO in hybrids. This leads to decrease
in the average size of the sp2 domains and can be explained
if new graphitic domains are created during reduction and
that are smaller in size to the ones present in GO [32]. A small
peak appeared at 680 cm1 which indicates the presence of
Fe3O4 NPs in the hybrids [33].
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Fig. 2 – TEM micrographs of Fe3O4–RGO hybrids with different concentration of GO: (a) 0.25 mg/mL, Fe3O4–RGO-1 (b) 0.5 mg/
mL, Fe3O4–RGO-2 and (c) 1.0 mg/mL, Fe3O4–RGO-3. Corresponding HR-TEM images of hybrids show lattice images of Fe3O4
NPs and stacked layers of RGO (d–f). A typical SAED pattern of Fe3O4–RGO-1 is shown in inset of (a).
Fig. 3 – FTIR spectra of (a) GO, (b) RGO, and Fe3O4–RGO
hybrids with different concentration of GO: (c) Fe3O4–RGO-1,
(d) Fe3O4–RGO-2 and (e) Fe3O4–RGO-3.
Since we have adopted the same synthesis protocol for all
hybrids, XPS was recorded only for Fe3O4–RGO-2 as a
representative and compared with that of RGO nanosheets.
The bands observed in wide scan XPS spectrum of the Fe3O4–
RGO-2 confirm the presence of C1s, O1s, and Fe2p (Fig. 5a).
Deconvolution of the C1s peak (Fig. 5b) of RGO shows that
relative contribution of the components associated with
Fig. 4 – Raman spectra of hybrids Fe3O4–RGO-1, Fe3O4–RGO2, and Fe3O4–RGO-3.
oxygenated functional groups decreased markedly, indicating
the deoxygenation of GO as reported in literature [21]. The C1s
spectra of Fe3O4–RGO-2 (Fig. 5c) shows nonoxygenated carbon
(284.8 eV) and the carbon in C–O (286.2 eV) which confirms
the presence of RGO. The observed O1s peak in RGO at
531.4 eV is shifted to lower binding energy (530.1 eV) due to
attachment of Fe3O4 NPs in Fe3O4–RGO hybrids (Fig. 5d) [31].
The Fe2p XPS spectrum (Fig. 5e) exhibits two peaks at 711.2
and 724.8 eV, corresponding to the Fe2p3/2 and Fe 2p1/2 spin
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Fig. 5 – X-ray photoelectron spectroscopy (XPS) spectra: (a) wide scan, (b) C1s spectra of RGO, (c) C1s spectra of Fe3O4–RGO-2, (d)
O1s spectra of RGO and Fe3O4–RGO-2, and (e) Fe2p spectra of Fe3O4–RGO-2.
orbit peaks of Fe3O4 [21,34]. Further, the M–T data is recorded
for these hybrids, discussed in next section, which is another
indication for formation of the Fe3O4 phase in the RGO matrix.
The room-temperature magnetic properties of the Fe3O4–
RGO hybrids were measured by VSM. The magnetization of
the Fe3O4–RGO hybrids is strongly dependent on Fe3O4 content and loading of these NPs on RGO nanosheets provide crucial information on the magnetic properties. After performing
a series of controlled experiments, it has been observed that
the concentration of GO plays a crucial role in engineering
the loading capacity of the NPs. To get a quantitative information about loading capacity of iron content of NPs in Fe3O4–
RGO hybrids, ICP–AES measurements were carried out and
the loading amount of iron content (wt.%) in RGO nanosheets
was calculated to be 50.8, 45.1 and 38.4% for Fe3O4–RGO-1,
Fe3O4–RGO-2, and Fe3O4–RGO-3, respectively. The decrease
in the loading amount of Fe content is due to an increase in
the concentration of GO. Based on the ICP–AES results, it
may be anticipated that magnetization of Fe3O4–RGO-1
should be higher than Fe3O4–RGO-2 and Fe3O4–RGO-3 hybrids.
The magnetization of Fe3O4–RGO-1 (57.4 emu g1) at an applied field of 2T, is higher than that of Fe3O4–RGO-2 and
Fe3O4–RGO-3 as the amount of RGO increases and that of
Fe3O4 decreases [21,35]. Contrary to the above, inspite of
higher loading wt.% of Fe content in Fe3O4–RGO-2, the
magnetization at an applied field of 2T was found to be
slightly lower than the Fe3O4–RGO-3 (37.9 emu g1). Thus, it
may be assumed that the magnetization is not only attributed
to the loading percentage of Fe content in Fe3O4–RGO hybrids
but also to the crystallite size of Fe3O4 NPs (Supplementary
information, Table S1) and surface properties discussed later.
Further, typical superparamagnetic behavior (zero coercivity and zero remanence) was observed for Fe3O4–RGO-1 and
Fe3O4–RGO-2 whereas in Fe3O4–RGO-3 hybrid having larger
crystallite size, finite value of coercivity (45.5 Oe) was obtained (inset Fig. 6). This indicates that a precise control of
size of the magnetic NPs over GO is also very important to
engineer a complete superparamagnetic Fe3O4–RGO hybrid.
Since the XRD patterns of Fe3O4 (JCPDS card 19-629) and
c-Fe2O3 (JCPDS card 39-1346) are quite similar, it is essential
that the temperature dependent magnetization (M–T) measurement is considered to unambiguously assign the crystal
phase of the hybrids because TC is very sensitive to crystal
phase. The M–T measurements for hybrids at an applied field
of 100 Oe (inset of Fig. 6) were recorded, which reveal that values of magnetization in temperature scan 30–600 C depends
on RGO content but TC was found to be 853 K, which agrees
well with that reported for Fe3O4 whereas the TC of c-Fe2O3
is around 918 K [9]. These results further confirmed that the
phase formed in present investigation is Fe3O4 rather than
c-Fe2O3 and is supported by the XPS spectra for Fe2p (Fig. 5e).
By measuring the zeta potential of Fe3O4–RGO hybrids as a
function of pH, the acidity or basicity of surfaces and isoelectric point (IEP) have been determined. It has been observed
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Fig. 6 – The room temperature magnetization hysteresis
curves of Fe3O4–RGO hybrids with different concentration of
GO: Fe3O4–RGO-1, Fe3O4–RGO-2 and Fe3O4–RGO-3. The
bottom right inset shows the magnetization vs. temperature
along with TC for these hybrids. The top left inset shows in
large view of M–H curve.
that IEP of all the hybrids is nearly independent of GO concentration and is found to be in range of 3.5–3.8 (Fig. 7). This
indicates that at pH < IEP, the Fe3O4–RGO hybrids exhibit positive surface charge and can act as anion exchanger, while at
pH > IEP, the surface charge is negative, which is beneficial
for adsorbing cations.
In order to examine the porous structure and surface area
of the hybrids, N2 adsorption/desorption isotherms were carried out, as shown in Fig. 8a–c. There is a need to control the
random and wide distribution of pores to utilize the properties of Fe3O4–RGO hybrids. In the present case, the narrow distribution of pores in Fe3O4–RGO hybrids has been achieved by
varying the concentration of GO at relatively lower temperature. The effect of GO on textural properties of hybrids could
be well understood by analyzing the shape of hysteresis loops
along with the pore size distributions. The increase in the
nitrogen uptake at higher relative pressure (P/Po > 0.3) for
Fig. 7 – pH dependent zeta-potential plots of GO, RGO, Fe3O4–
RGO-1, Fe3O4–RGO-2 and Fe3O4–RGO-3 hybrids at room
temperature.
the hybrids was due to adsorption in mesopores and the
generated N2 isotherms are close to Type IV with an evident
hysteresis loop in the 0.4–0.99 range of relative pressure
(Fig. 8a–c). This indicates that the mesoporous structure of the
Fe3O4–RGO hybrids are according to the IUPAC classification.
The hysteresis loop for Fe3O4–RGO-1 is not saturated at
very high relative pressure (P/Po), and can be ascribed to H-3
type, suggesting aggregates of Fe3O4 NPs on RGO nanosheets
giving rise to slit-shape pores and hence expected to have distribution of pores along with hump as shown in inset of
Fig. 8a [36]. The hysteresis loops for Fe3O4–RGO-2 and Fe3O4–
RGO-3 are also H-3 type with differences in their desorption
branch of isotherms (Fig. 8 b, c) that depend on the concentration of GO. As the concentration of GO increases, the hump in
pore size distribution starts decreasing and finally disappears
(Fig. 8c). This gives a uniform narrow distribution of pores
without hump in Fe3O4–RGO-3. The details of textural parameters of the hybrids are listed in Table 1.
We observe that with an increase in the concentration of
GO in hybrids, the total pore volume and average pore diameter decrease continuously. However, the BET specific surface
area of the hybrids followed a different behavior. The concentration of GO in Fe3O4–RGO-3 is higher than Fe3O4–RGO-2, but
the observed specific surface area of Fe3O4–RGO-3 is marginally lower than Fe3O4–RGO-2. The sample Fe3O4–RGO-3 exhibits larger particle size as seen TEM and X-ray data. Also, it is
reflected in magnetization measurements with nearly similar
value of magnetization as for Fe3O4–RGO-2. While the sample
Fe3O4–RGO-2 is essentially superparamagnetic, the sample
Fe3O4–RGO-3 is more like ferrimagnetic. It appears that the
specific surface area, pore size distribution and total pore volume may be ascribed to the interaction between Fe3O4 NPs
aggregates and RGO nanosheets and folded RGO nanosheets.
Therefore, the porous structure of Fe3O4–RGO-1 may be
due to the Fe3O4 NPs aggregates and their attachment on
RGO nanosheets whereas folded RGO nanosheets and well
distributed Fe3O4 NPs intercalated in RGO are responsible
for porous structure in Fe3O4–RGO-2 and Fe3O4–RGO-3. The
pores generated in Fe3O4–RGO hybrids, are anticipated to be
beneficial for certain applications such as removing heavy
metal cations from water and targeted drug deliveries [10].
With a view to understand the electrochemical properties
of the hybrids and their use in electrochemical sensing, cyclic
voltammetry was performed. Fig. 9 shows cyclic voltammograms (CVs) of bare GCE, Fe3O4–RGO-1, Fe3O4–RGO-2 and
Fe3O4–RGO-3 modified glassy carbon electrodes (GCEs) in
4
0.1 M PBS in presence of 0.1 M FeðCNÞ3
6 =FeðCNÞ6 and the potential was scanned between 0.5 and 1.0 V. The observed redox peaks and their peak-to-peak potential separation
(DEp = EpaEpc) are related to the electron transfer (ET) coefficient, and a low DE value indicates a fast ET for a singleelectron electrochemical reaction. The redox peak current
responses were much larger on the modified electrodes than
the bare electrode, which indicates the electroactive nature
of the hybrid material.
To ascertain the electroactivity of the hybrid materials, the
modified electrodes were tested for selective determination of
chromium. Cyclic voltammetry was carried out in presence of
1 nM Cr3+ in the electrolyte at a scan rate of 50 mV/s. As can
be seen from the Fig. 10, CVs of Fe3O4–RGO-2 indicated
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Fig. 8 – Nitrogen adsorption/desorption isotherm and pore size distribution (inset) of the as prepared hybrids (a) Fe3O4–RGO-1,
(b) Fe3O4–RGO-2 and (c) Fe3O4–RGO-3. The filled and open symbols indicate adsorption and desorption branches.
presence of two well-resolved anodic and cathodic peaks
which were not seen in Fe3O4–RGO-1 and Fe3O4–RGO-3 modified electrodes. However, a small redox hump was observed in
the CV of Fe3O4–RGO-3 modified electrodes. From the above
observation, it can be said that the Fe3O4–RGO-2 hybrid with
0.5 mg/mL graphene oxide gives optimum detection limit for
the chromium ions. The anodic peaks appeared at 0.248 and
0.864 V while the cathodic peaks were seen at 0.006 and
0.696 V; DEp were calculated to be 240 and 168 mV. The ratio
of anodic to cathodic peak current (Ipa/Ipc) were calculated
to be 1.04 indicating a reversible reaction on the electrodes.
On the forward anodic scan, the oxidation wave is attributed
to the oxidation of Cr(III) to Cr(IV) species while the peak in
reverse scan is attributed to the reduction of the Cr(IV) to
the parent Cr(III) ions.
The diffusion coefficients of the electrode system were
measured using Randles–Sevcik equation [37] as described
below:
ip ¼ ð2:69 105 Þ n3=2 C D0:5 m0:5
where ip is the peak current density (A/cm2), n is the number of
electrons, C is the concentration (mol/cm3), D is the diffusion
constant (cm2/s) and m is the sweep rate (V/s). The determination of the diffusion coefficient is important to determine the
rate at which the electroactive species moves onto the electrode surface. The values of D was calculated using the above
Table 1 – Textural analysis of Fe3O4–RGO hybrids with different GO concentration.
Hybrids
Fe3O4–RGO-1 (0.25 mg/mL)
Fe3O4–RGO-2 (0.5 mg/mL)
Fe3O4–RGO-3 (1.0 mg/mL)
BET (m2/g)
137.12
207.99
204.89
Single point adsorption total
pore volume of pores (cm3/g)
0.33
0.25
0.22
BJH desorption average pore
diameter (4 V/A) (Å)
82.5
46.1
43.1
4216
CARBON
5 0 ( 2 0 1 2 ) 4 2 0 9 –4 2 1 9
Fig. 9 – Cyclic voltammograms of bare GCE, Fe3O4–RGO-1,
Fe3O4–RGO-2 and Fe3O4–RGO-3 modified GCE in 0.1 M PBS in
4
presence of 0.1 M FeðCNÞ3
6 =FeðCNÞ6 .
Fig. 12 – CV of Fe3O4–RGO-2 modified electrode in 0.1 M
4
3+
at different scan rates.
FeðCNÞ3
6 =FeðCNÞ6 with 30 nM Cr
Linear relation of the anodic and cathodic peak currents vs.
the square root of the scan rate is shown in the inset.
Fig. 10 – Electrochemical response of the modified electrodes
towards chromium.
Fig. 13 – Electrochemical response of Fe3O4–RGO-2 modified
electrode towards successive addition of chromium (III) ions
in the electrolyte. Insets show enlarged plot of cathodic peak
currents vs. potentials at different concentrations of Cr3+
ions (top left) and plot of cathodic peak currents vs. different
concentrations of Cr3+ ions (bottom right).
8
6
Fe3O4-RGO-2 with 30 nM of Cr+3
>
2
0
>
Current/10-5A
4
-2
-4
-6
-8
1.0
0.8
0.6
0.4
0.2
0.0 -0.2 -0.4 -0.6
Potential/V vs. Ag/AgCl
Fig. 11 – Cyclic voltammograms of the Fe3O4–RGO-2 modified
electrode depicting stability towards chromium ion.
equation and was found to be 2.38 · 1010, 19.38 · 1010 and
10.16 · 1010 cm2/s for Fe3O4–RGO-1, Fe3O4–RGO-2 and Fe3O4–
RGO-3, respectively. As can be seen that the Fe3O4–RGO-2 gives
the highest diffusion coefficient which credence the observation that it is more selective towards chromium.
Henceforth, all the electrochemical studies were performed with Fe3O4–RGO-2 modified GCE. CV measurements
was recorded for Fe3O4–RGO-2 in presence of 30 nM Cr3+ for
10 cycles at a scan rate of 50 mV/s (Fig. 11) and it shows no
change in the oxidation/reduction current peak indicating
the stability of the electrode material.
CVs of Fe3O4–RGO-2 modified electrodes were carried out in
4
3+
ions at var0.1 M FeðCNÞ3
6 =FeðCNÞ6 in presence of 30 nM Cr
ious scan rates and are shown in Fig. 12. The anodic and the
cathodic peak current increased with increase in the scan rate.
Both anodic (Ipa) and cathodic (Ipc) peak currents showed linearity with the square root of the scan rate (m) over the entire
range of 10–400 mV/s., which suggest that the redox processes
on Fe3O4–RGO-2 modified GCE electrodes are predominantly
CARBON
5 0 ( 20 1 2 ) 4 2 0 9–42 1 9
4217
Fig. 14 – Schematic representation of RGO loaded with Fe3O4 NPs.
diffusion controlled mass transfer reactions. The plot of Ip
against m1/2 gave straight lines for both anodic and cathodic
peak with r2 of 0.99898 and 0.9999, respectively. The difference
between the anodic and the cathodic peak potential (DEp) was
found to increase with increase in scan rate.
Under the same condition, CVs were again measured using
Fe3O4–RGO-2 modified GCE electrode with varying Cr(III) concentrations ranging from 0.2 to 2 nM (Fig. 13). As can be seen
that the intensity of cathodic peak current increased with an
increase of Cr(III) ion concentration. On further addition of
chromium, the anodic peak potential shifted to a higher
potential and the cathodic peak potential shifted to more
negative side along with rise in peak currents. This behavior
shows the analytical importance of the modified electrode
in the determination of chromium.
Based on all the above results, it is corroborated that surface area plays an important role in electrochemical performance of a system. The interaction of Fe3O4 NPs with RGO
membranes is well-documented [38] and it can be seen that
both the components contribute to the surface area of the hybrid material. Further, the magnetic properties of magnetic
NPs show strong dependence on the average crystallite size,
and magnetization decreases with decreasing crystallite size
due to increasing surface disorder and spin canting effect
[39]. In the present study, the crystallite size of Fe3O4 NPs in
Fe3O4–RGO-3 hybrid is larger than Fe3O4–RGO-2, which results
in reduced surface spin canting effect or surface spin disorder, and possibly cation site distribution in Fe3O4–RGO-3
hybrids. This in turn, is responsible for higher magnetic
moment, finite coercivity (45.5 Oe) (Fig. 6) and lower specific
surface area for Fe3O4–RGO-3 hybrid. The highest surface area
of Fe3O4–RGO-2 hybrid results in optimum electrochemical
detection of chromium. The onset of a new redox peak in
the cyclic voltammograms of Fe3O4–RGO-2 hybrid modified
electrode indicates enhanced electron transfer resulting in
an improved electrochemical response towards chromium
ions. This is further supported by the highest diffusion coefficient of the Fe3O4–RGO-2 hybrids modified electrodes as
calculated by the Randles–Sevcik equation.
4.
Conclusions
A simple method for in situ conversion of iron salts to magnetic
NPs and simultaneous reduction of GO into RGO nanosheets in
aqueous solution has been proposed for the preparation of
Fe3O4–RGO hybrids. The distribution and loading of Fe3O4
NPs on RGO nanosheets, along with distribution of pores is
controlled by altering the GO concentration at relatively lower
temperature. The zeta potentials of the hybrids are pH dependent and can flip on both sides of IEP as compared to GO. More
4218
CARBON
5 0 ( 2 0 1 2 ) 4 2 0 9 –4 2 1 9
importantly, the IEP of Fe3O4–RGO hybrids are nearly independent of GO concentrations. The CVs response suggests that
Fe3O4–RGO-2 is electroactive towards chromium ion and can
be used for its electrochemical sensing. The proposed Fe3O4–
RGO modified electrodes might be useful for a simple and
effective way to develop electrochemical sensors for other trivalent cations in biological systems and waste water.
Acknowledgments
Financial supports from Department of Science and Technology (DST) and Department of Information Technology (DIT),
Government of India are gratefully acknowledged. The
authors are thankful to the Centre for Research in Nanotechnology & Science (CRNTS) for TEM and Raman facilities.
Appendix A. Supplementary data
Supplementary data associated with this article can be found,
in the online version, at http://dx.doi.org/10.1016/j.carbon.
2012.05.002.
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