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Structural, magnetic, and textural properties of ironoxide-reduced graphene oxide hybrids and theiruse 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

Article history:

Received 5 March 2012

Accepted 2 May 2012

Available online 16 May 2012

0008-6223/$ - see front matter � 2012 Elsevihttp://dx.doi.org/10.1016/j.carbon.2012.05.002

* Corresponding author: Fax: +91 22 2572 348E-mail address: dhirenb@iitb.ac.in (D. Bah

A B S T R A C T

Superparamagnetic Fe3O4 nanoparticles were anchored on reduced graphene oxide (RGO)

nanosheets by co-precipitation of iron salts in the presence of different amounts of graph-

ene oxide (GO). A pH dependent zeta potential and good aqueous dispersions were

observed for the three hybrids of Fe3O4 and RGO. The structure, morphology and micro-

structure of the hybrids were examined by X-ray diffraction, transmission electron micros-

copy (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 dis-

tribution and loading of Fe3O4 nanoparticles to form Fe3O4–RGO hybrids have been con-

trolled by changing the concentration of GO. An observed maximum (�10 nm) in pore

size distribution for the sample with 0.25 mg ml�1 of GO is different from that prepared

using 1.0 mg ml�1 GO. The superparamagnetic behavior is also lost in the latter and it

exhibits a ferrimagnetic nature. The electrochemical behavior of the hybrids towards chro-

mium 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 applica-

tions in nano-electronics, catalysis and waste water treat-

ments [3–5]. On the other hand, superparamagnetic Fe3O4

nanoparticles (NPs) have been brought into sharp focus due

to their potential biomedical applications such as hyperther-

mia treatment of cancer [6], contrast agent for magnetic res-

onance imaging [7], magnetic separation [8], targeted drug

delivery [9] and waste water treatments [10]. The NPs are re-

quired to be chemically stable, uniform in size with high spe-

er Ltd. All rights reserved

0.adur).

cific surface area and well dispersed in liquid media for all

applications. But due to anisotropic dipolar attraction, pris-

tine 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 over-

come by loading the magnetic nanoparticles (MNPs) in car-

bon-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 en-

hanced properties of the hybrids.

.

4210 C A R B O N 5 0 ( 2 0 1 2 ) 4 2 0 9 – 4 2 1 9

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) ace-

tylacetonate and proposed their use as a magnetic resonance

contrast agent. He et al. [17] reported the attachment of sur-

face-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] pre-

pared Fe3O4 graphene composites by a gas/liquid interface

reaction at 180 �C with enhanced cycling performance for lith-

ium-ion batteries. Zhang et al. [20] have developed 3D hierar-

chical 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 compos-

ites 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 dif-

ficult 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 com-

plex 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 elec-

trochemical properties of the hybrids. To resolve such issues,

in the present work we provide an easy one-step co-precipita-

tion route for synthesizing Fe3O4–RGO hybrids at relatively low-

er 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 (FeCl3Æ6H2O-

23648-9) and Ferrous chloride tetrahydrate (FeCl2Æ4H2O-

22029-9) 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 meth-

od [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 hy-

drate was added and the mixture was again stirred for 4–5 h

at 90 �C to ensure complete reduction of GO. The above reac-

tion mixture was cooled and washed several times with mil-

liQ 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 exper-

imental 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 dif-

fractometer PW3040/60 with CuKa radiation). The NPs distri-

bution, morphologies and selected area electron diffraction

(SAED) patterns of the Fe3O4–RGO hybrids were characterized

by transmission electron microscopy (TEM) using a JEOL JEM-

2100 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 hy-

brids 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 measure-

ments, 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 hy-

brids 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 electrochemi-

cal 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 elec-

trode (GCE, diameter u = 2 mm), Pt-wire and Ag/AgCl elec-

trodes were used as working, counter and reference

C A R B O N 5 0 ( 2 0 1 2 ) 4 2 0 9 – 4 2 1 9 4211

electrodes, respectively with CHI1140A electrochemical work-

station (CHI110, Austin, TX). All electrochemical measure-

ments 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 (001) diffraction peak corresponding

to graphite oxide (see Supplementary information, Fig. S1)

indicates complete exfoliation of RGO nanosheets in the hy-

brids. Moreover, the full width at half maximum (FWHM) of

(311) 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 (311)

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 ob-

served 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 de-

creases (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 prepara-

tion 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 (311) peak.

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 (311) plane of the cubic

Fe3O4, which is consistent with the d-spacing of (311) XRD

peak, while the interlayer distance of stacked RGO nano-

sheets was found to be 4.3 A, corresponding to the spacing

of the (002) planes of graphite. The observed basal spacing

is higher than that of well-ordered graphite (3.35 A). The high-

er basal spacing may be due to the presence of residual oxy-

gen-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 ex-

pected, 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 cm�1 (C–O stretching vibration of epoxide)

and 1732 cm�1 (C=O stretching of carbonyl and carboxyl

groups at edges of the GO networks) [27]. The band at

1620 cm�1 is attributed to the skeletal vibration of graphitic

domains [28]. All the absorption bands related to oxygen-con-

taining functional groups of GO disappear in the spectrum of

RGO nanosheets thereby, confirming the reduction of above-

mentioned functional groups by hydrazine hydrate. The band

at �1532 cm�1 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 cm�1

(G-bands), which are characteristic of sp2 bonded, honey-

comb-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 cm�1, 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 struc-

tural changes occur in carbon framework of GO which is re-

flected 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 exfoli-

ated GO and are in agreement with previous results reported

for RGO nanosheets obtained from exfoliated GO [31]. The sig-

nificant 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 cm�1 which indicates the presence of

Fe3O4 NPs in the hybrids [33].

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.

Fig. 4 – Raman spectra of hybrids Fe3O4–RGO-1, Fe3O4–RGO-

2, and Fe3O4–RGO-3.

4212 C A R B O N 5 0 ( 2 0 1 2 ) 4 2 0 9 – 4 2 1 9

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

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

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.

C A R B O N 5 0 ( 2 0 1 2 ) 4 2 0 9 – 4 2 1 9 4213

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 con-

tent and loading of these NPs on RGO nanosheets provide cru-

cial 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 informa-

tion 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 g�1) at an ap-

plied 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 g�1). 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 coerciv-

ity 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 ob-

tained (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) mea-

surement 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 val-

ues 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 isoelec-

tric point (IEP) have been determined. It has been observed

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.

4214 C A R B O N 5 0 ( 2 0 1 2 ) 4 2 0 9 – 4 2 1 9

that IEP of all the hybrids is nearly independent of GO concen-

tration 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 posi-

tive 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 car-

ried out, as shown in Fig. 8a–c. There is a need to control the

random and wide distribution of pores to utilize the proper-

ties of Fe3O4–RGO hybrids. In the present case, the narrow dis-

tribution of pores in Fe3O4–RGO hybrids has been achieved by

varying the concentration of GO at relatively lower tempera-

ture. 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 dis-

tribution 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 concentra-

tion 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 param-

eters 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 diam-

eter decrease continuously. However, the BET specific surface

area of the hybrids followed a different behavior. The concen-

tration of GO in Fe3O4–RGO-3 is higher than Fe3O4–RGO-2, but

the observed specific surface area of Fe3O4–RGO-3 is margin-

ally lower than Fe3O4–RGO-2. The sample Fe3O4–RGO-3 exhib-

its 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 vol-

ume 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 voltammo-

grams (CVs) of bare GCE, Fe3O4–RGO-1, Fe3O4–RGO-2 and

Fe3O4–RGO-3 modified glassy carbon electrodes (GCEs) in

0.1 M PBS in presence of 0.1 M FeðCNÞ3�6 =FeðCNÞ4�6 and the po-

tential was scanned between �0.5 and 1.0 V. The observed re-

dox peaks and their peak-to-peak potential separation

(DEp = Epa�Epc) are related to the electron transfer (ET) coeffi-

cient, and a low DE value indicates a fast ET for a single-

electron 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

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.

C A R B O N 5 0 ( 2 0 1 2 ) 4 2 0 9 – 4 2 1 9 4215

presence of two well-resolved anodic and cathodic peaks

which were not seen in Fe3O4–RGO-1 and Fe3O4–RGO-3 modi-

fied 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

Table 1 – Textural analysis of Fe3O4–RGO hybrids with different

Hybrids BET (m2/g) Singpore

Fe3O4–RGO-1 (0.25 mg/mL) 137.12Fe3O4–RGO-2 (0.5 mg/mL) 207.99Fe3O4–RGO-3 (1.0 mg/mL) 204.89

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� 105Þ � 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 determina-

tion of the diffusion coefficient is important to determine the

rate at which the electroactive species moves onto the elec-

trode surface. The values of D was calculated using the above

GO concentration.

le point adsorption totalvolume of pores (cm3/g)

BJH desorption average porediameter (4 V/A) (A)

0.33 82.50.25 46.10.22 43.1

1.0 0.8 0.6 0.4 0.2 0.0 -0.2 -0.4 -0.6

-8

-6

-4

-2

0

2

4

6

8

> >

Cur

rent

/10-5

A

Potential/V vs. Ag/AgCl

Fe3O4-RGO-2 with 30 nM of Cr+3

Fig. 11 – Cyclic voltammograms of the Fe3O4–RGO-2 modified

electrode depicting stability towards chromium ion.

Fig. 12 – CV of Fe3O4–RGO-2 modified electrode in 0.1 M

FeðCNÞ3�6 =FeðCNÞ4�6 with 30 nM Cr3+ at different scan rates.

Linear relation of the anodic and cathodic peak currents vs.

the square root of the scan rate is shown in the inset.

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).

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

presence of 0.1 M FeðCNÞ3�6 =FeðCNÞ4�6 .

Fig. 10 – Electrochemical response of the modified electrodes

towards chromium.

4216 C A R B O N 5 0 ( 2 0 1 2 ) 4 2 0 9 – 4 2 1 9

equation and was found to be 2.38 · 10�10, 19.38 · 10�10 and

10.16 · 10�10 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 observa-

tion that it is more selective towards chromium.

Henceforth, all the electrochemical studies were per-

formed 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

0.1 M FeðCNÞ3�6 =FeðCNÞ4�6 in presence of 30 nM Cr3+ ions at var-

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 line-

arity 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

Fig. 14 – Schematic representation of RGO loaded with Fe3O4 NPs.

C A R B O N 5 0 ( 2 0 1 2 ) 4 2 0 9 – 4 2 1 9 4217

diffusion controlled mass transfer reactions. The plot of Ipagainst 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) con-

centrations 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 sur-

face area plays an important role in electrochemical perfor-

mance 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 hy-

brid 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 disor-

der, 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 coeffi-

cient 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 depen-

dent and can flip on both sides of IEP as compared to GO. More

4218 C A R B O N 5 0 ( 2 0 1 2 ) 4 2 0 9 – 4 2 1 9

importantly, the IEP of Fe3O4–RGO hybrids are nearly indepen-

dent 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 tri-

valent cations in biological systems and waste water.

Acknowledgments

Financial supports from Department of Science and Technol-

ogy (DST) and Department of Information Technology (DIT),

Government of India are gratefully acknowledged. The

authors are thankful to the Centre for Research in Nanotech-

nology & 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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