Magnetic iron oxide and iron oxide@gold nanoparticle anchored nitrogen and sulfur-functionalized reduced graphene oxide electrocatalyst for methanol oxidation

Article abstract of DOI:10.1039/c5ra03735b

Fuel cells have been attracting more and more attention in recent decades due to high-energy demands, fossil fuel depletions and environmental pollution throughout world. In this study, we report the synthesis of metallic and bimetallic nanoparticles such

Full text of DOI:10.1039/c5ra03735b

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Magnetic iron oxide and iron oxide@gold nanoparticles
anchored nitrogen and sulfur-functionalized reduced
graphene oxide electrocatalyst for methanol oxidation
Necip Atara,*, Tanju Erena, Mehmet Lütfi Yolab, Hassan Karimi-Maleha,*, Bermali
Demirdögena
aDepartment of Chemical Engineering, Pamukkale University, Denizli, Turkey
bDepartment of Metallurgical and Materials Engineering, Sinop University, Sinop, Turkey
cDepartment of Chemistry, Graduate University of Advanced Technology, Kerman, Iran
ꢀCorresponding authors. E-mail: necipatar@gmail.com (N. Atar); h.karimi.maleh@gmail.com (H. Karimi-
Maleh); Tel:+98-911-254-0112
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ABSTRACT
Fuel cells have been attracting more and more attention in recent decades due to high-energy
demands, fossil fuel depletions and environmental pollution throughout world. In this study,
we report the synthesis of metallic and bimetallic nanoparticles such as spherical iron oxide
nanoparticles [(sp)Fe3O4], rod iron oxide nanoparticles [(rd)Fe3O4] and iron@gold
nanoparticles (Fe3O4@AuNPs) involved L-cysteine functionalized reduced graphene oxide
nanohybrids [(sp)Fe3O4/cis/rGO, (rd)Fe3O4/cis/rGO and Fe3O4@AuNPs/cis/rGO] and their
applications as an electrocatalyst for methanol electro-oxidation. The nanohybrids have been
characterized by transmission electron microscopy (TEM), X-ray photoelectron spectroscopy
(XPS) and X-ray diffraction (XRD). Experimental results have demonstrated that reduced
graphene oxide-supported bimetallic nanoparticles enhanced electrochemical efficiency for
methanol electro-oxidation with regard to diffusion efficiency, oxidation potential and
forward oxidation peak current. Fe3O4@AuNPs/cis/rGO, in comparison to (sp)Fe3O4/cis/rGO
and (rd)Fe3O4/cis/rGO, have showed the most efficiency for methanol electro-oxidation.
Keywords: Methanol oxidation; Fuel cell; Reduced graphene oxide; Nanoparticle;
Characterization
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1. Introduction
Fuel cells are electrochemical cells used to generate electricity from fuels, and recently
have drawn more attention as an alternative method for energy production 1, 2. One example of
these cells is direct methanol fuel cells (DMFCs) which are used for operating portable
electronic devices 3, 4. Methanol is preferred because it has some significant advantages in
supply, transport, and storage 5, 6. More importantly methanol’s energy density is higher than
gaseous hydrogen, and can integrate into existing energy infrastructure 7, 8. Many research
groups have been investigated the electro-oxidation of methanol in direct methanol fuel cells
3, 4, 9, 10. Although Pt and Pt-based alloys (with metals such as Ru, Pd, Au, Ni etc.) are the
most promising anode catalysts in DMFCs, high cost and limited supply of Pt put some
1
restrictions for large-scale applications . Recently, bimetallic nanoparticles (NPs) have been
employed as catalysts in nanotechnology researches 11-15. Bimetallic NPs generally show
better catalytic properties than their monometallic forms 11, 15. Use of NPs has some certain
advantages 16-21. First, bimetallic nanoparticles consist of two different metals which are
combined, and this can create a synergetic effect in specific properties such as size dependent
optic, electronic and catalytic properties. Secondly, use of bimetallic nanoparticles allows
reducing the amount of expensive catalyst, since a less expensive metal can be employed as
the core material 11, 12. Moreover, because surface area is larger and the majority of atoms can
be used at nanoscale, NPs can increase reaction rate and selectivity 1, 11, 22. Studies showed
that the catalytic activity of NPs is determined by some parameters including shape, structure,
composition and architecture 23-27. Over the last decade, additional control and design of
magnetic nanoparticles were achieved by developing core-shell structured nanomaterials like
Fe3O4@AuNPs that we studied in this paper. The core properties of these particles have
providing an increased number of catalytic applications. In the structure of gold-coated
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Fe3O4@AuNPs core-shell nanoparticles, the magnetic Fe3O4 nanoparticles are important
about enhancing the chemical stability by protecting the core from corrosion and displaying
fine compatibility and affinity. The significantly enhanced electrocatalytic performance of
Fe3O4@AuNPs core-shell nanoparticles can be attributed to the large surface area resulting
from the formation of the bimetallic nanoparticle on rGO sheets and also the improved
12
conductivity
.
We studied the bimetallic magnetic core-shell Fe3O4@AuNPs with
composing rGO to improve the surface properties and support the catalysis of methanol
oxidation.
Graphene has become interesting for scientist from different fields because of its high
elasticity, thermal conductivity, surface area and mechanical strength, and used as catalyst
support in fuel cells 1. Graphene has a two dimensional structure in which carbon atoms forms
a honeycomb-like structure via sp2 hybridization, and carbon is the only one-atom-thick
24, 28
material
. Graphene shows exceptional properties due to this hexagonal structure and
electron configuration. Graphene oxide (GO) is the oxidation product of graphene and like
graphene it has a two-dimensional (2D) crystal structure. GO shows different chemical and
physical properties than graphene 28, 29. In some cases it necassary to regain graphene’s
desirable properties such as electrical conductivity. For this purpose GO is generally reduced
by thermal annealing or chemical reducing agents forming reduced graphene oxide (r-GO) 30,
31
.
The aim of this study is to synthesize metallic and bimetallic nanoparticles and
functionalized reduced graphene oxide containing nanohybrid [(sp)Fe3O4/cis/rGO,
(rd)Fe3O4/cis/rGO and Fe3O4@AuNPs/cis/rGO] catalysts for methanol electro-oxidation
applications.
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2. Experimental
2.1. Materials
All chemicals that used in the experiments were reagent grade and were used as
received following; Graphite powder (Merck, Germany), sulfuric acid (H2SO4, Merck,
Germany), potassium persulfate (K2S2O8, Merck, Germany), phosphorus pentoxide (P2O5,
Merck, Germany), potassium permanganate (KMnO4, Merck, Germany), hydrogen peroxide
(H2O2, Merck, Germany), ethanol (Merck, Germany), hydrochloric acid (HCl, Sigma–
Aldrich), N-(3-dimethylaminopropyl)-N´-ethylcarbodiimidehydrochloride (EDC, Sigma-
Aldrich, USA), ethanol (Sigma-Aldrich, USA), β-cyclodextrin (Sigma-Aldrich, USA), L-
cysteine (cis, Sigma–Aldrich, USA), HAuCl4 (Sigma-Aldrich, USA), Sodium citrate (Merck,
Germany), isopropyl alcohol (IPA, Sigma-Aldrich, USA), methanol (Merck, Germany),
HPLC grade acetonitrile (MeCN, Sigma-Aldrich, USA), NaBH4 (Merck, Germany),
perchloric acid (HClO4, Sigma-Aldrich, USA), hydrazine hydrate (Merck, Germany) and
other chemicals were reagent grade quality and were used as received. The ultra-pure water
with resistance of 18.3 MΩ cm (Human Power 1+ Scholar purification system) was used in
the experiments of aqueous media.
2.2. Instrumentation
All electrochemical experiments were performed using a BAS-100B electrochemical
analyzer (Bioanalytical System Inc., Lafayette, IL, U.S.) and Gamry Reference 600 work-
station. Argon gas was passed through the solutions during experiments for about 10 min.
JEOL 2100 HRTEM (JEOL Ltd., Tokyo, Japan) and ZEISS EVO 50 SEM (GERMANY)
analytic microscopies were used to investigate the morphologies of (sp)Fe3O4/cis/rGO,
(rd)Fe3O4/cis/rGO and Fe3O4@AuNPs/cis/rGO nanocomposites.
XPS analysis were performed on a PHI 5000 Versa Probe (Φ ULVAC-PHI, Inc., Japan/USA)
model with monochromatized Al Kα radiation (1486.6 eV) as an X-ray anode operated at 50
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W. To prepare the samples, one drop of the prepared (sp)Fe3O4/cis/rGO, (rd)Fe3O4/cis/rGO
and Fe3O4@AuNPs/cis/rGO solutions were placed on clear glass and then dried in air.
A Rigaku Miniflex X-ray diffractometer was used for X-ray diffraction measurements of the
(sp)Fe3O4/cis/rGO,
(rd)Fe3O4/cis/rGO
and
Fe3O4@AuNPs/cis/rGO
composites
nanostructures. A scanning speed of 2θ = 20/min with a step size of 0.020 was used to
examine the samples.
2.3. Synthesis of rGO
GO was prepared by the modified Hummers method 12. Typically, 2.5 g of graphite
powder were placed in a flask containing a mixture of 12.5 mL of H2SO4 (98%), 2.5 g of
K2S2O8 and 2.5 g of P2O5. The mixture was kept at 80 °C for 6 h. Then, the mixture was
cooled to room temperature and added with 500 mL of ultra-pure water. The product was
filtered and washed with ultra-pure water and 125 mL of H2SO4 (98%) was added at 0 °C.
Later, 15 g of KMnO4 were added to the stirring suspension which was kept at 20 °C. After
the feeding of KMnO4 was finished, the flask was heated to 50 °C. After 4 h, 500 mL of ultra-
pure water were added to the mixture in an ice bath. The last mixture was stirred for 2 h and
diluted to 1 L with ultra-pure water. After that, the suspension was fed slowly with 20 mL of
H2O2 (30%) and the solution started bubbling. The color of the suspension changed to brilliant
yellow from brownish. The synthesized graphite oxide was filtered and washed with 0.1 M
HCl and ultra-pure water three times. The graphite oxide was collected by an ultracentrifuge.
The as-prepared GO was dispersed into 200 mL water under mild ultrasound yielding a
yellow-brown suspension, then 4 mL hydrazine hydrate (80 wt%) were added and the solution
was heated in an oil bath maintaining at 100 °C under a water-cooled condenser for 24 h.
After the reaction, the prepared rGO product was collected by vacuum filtration.
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2.4. Synthesis of (sp)Fe3O4, (rd)Fe3O4 and Fe3O4@AuNPs
The (sp)Fe3O4 were synthesized in aqueous solution without any surfactants 32. In this
method, ferric and ferrous ions are mixed in highly basic solution at room temperature. FeCl3
(0.0125 M, 97%) and FeCl2 (0.0125 M, 98%), were mixed with 25 ml of ultra-pure water.
Then, 20 ml of NH3 (28%) was added to the mixture and the resulting solutions were stirred
for about 10 min. Here in, black precipitates were immediately formed, which were filtrated
and washed with ultra-pure water for several times to remove impurities. Lastly, the
precipitates were dried in furnace at 80 °C for 6 h. To synthesis the rod shaped Fe
nanoparticles, first of all, 2 mmol of FeCl3·6H2O and 20 mmol of NaOH were mixed with 20
mL of ultra-pure water and 6 mmol of 1-Butyl-3-methylimidazolium Chloride were added
drop by drop during mixing until a homogenous solution were formed. After 20 mins of
o
mixing procedure, the mixture was kept in an autoclave with a heat of 150 C for 6 hours.
After the reaction complete, the mixture was cooled at room temperature. After that the
mixture was filtrated and washed with ultra-pure water and ethanol. Finally the mixture was
o
heated to 380 – 400 C until a dark red (rd)Fe3O4 solution was formed. For preparation
Fe3O4@AuNPs, Fe(NO3)3 (4 mL, 0.01 M) was reduced by 0.1 M ascorbic acid (20 mL) for 20
min at room temperature under a nitrogen atmosphere. The pH of the solution was adjusted to
4.0 and the HAuCl4 (4 mL, 0.01 M) was added at room temperature for 1 h. A dark solid
indicated that the iron oxide core was coated with a gold shell. Then, the product was
separated by a magnet and washed with water. The Fe3O4@AuNPs was dried overnight in a
vacuum oven at 25 °C 14.
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2.5. Preparation of (sp)Fe3O4/cis/rGO, (rd)Fe3O4/cis/rGO and Fe3O4@AuNPs/cis/rGO
nanocomposites
rGO was dissolved in ethanol at 2 mg mL-1. The mixture was sonicated to form a
homogeneous suspension. The prepared rGO suspension was treated with 0.2 M of EDC
solution for 8 h to ensure the surface activation of residual carboxylated groups. EDC
compound provides the most popular and versatile method for labeling or crosslinking to free
carboxylic groups on rGO 33, 34. The EDC molecules are considered zero-length carboxyl-to-
amine crosslinkers. EDC reacts with carboxylic acid groups to form an active intermediate
product that is easily displaced by nucleophilic attack from primary amino groups in the
reaction mixture. The primary amine forms an amide bond with the original carboxyl group,
and an EDC by-product is released as a soluble derivative. Therefore we used EDC for
activation of free carboxylic acid groups of rGO 12, 35 . Then 1.0 mM cysteine was mixed with
the activated rGO suspension at a 1:1 volume ratio and kept stirring for 2 h (cis/rGO). After
that, 1 mg mL-1 of (sp)Fe3O4 solution was mixed with the 0.1 mg mL-1 of cis/rGO solution at
a 1:1 volume ratio. Finally, the mixture was sonicated to generate a homogeneous mixture
[(sp)Fe3O4/cis/rGO]. In a typical experiment of self-assembly, the aqueous dispersion of
(rd)Fe3O4 and Fe3O4@AuNPs (1 mg mL−1) was mixed with the aqueous dispersion of cis/rGO
(0.1 mg mL−1) at a 1:1 volume ratio and sonicated for 15 min to form a homogeneous mixture
[(sp)Fe3O4/cis/rGO, (rd)Fe3O4/cis/rGO and Fe3O4@AuNPs/cis/rGO]. The mixture was then
kept undisturbed under ambient condition for 12 h.
2.6. Procedure for the electrode preparation
Glassy carbon electrodes (GCE) were cleaned by polishing with fine wet emery paper.
They were polished with 0.1 µm and 0.05 µm alumina slurries, respectively on microcloth
pads. The electrodes were sonicated twice in ultra-pure water and then in 50:50 (v/v) IPA and

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MeCN solution. After washing with water, glassy carbon electrode was washed with MeCN
to eliminate any physisorbed materials. Finally, 20 µL of composite suspensions was dropped
onto the GCE and then evaporating the solvent under an infrared heat lamp.
2.7. Electrochemical measurements
Electrocatalytic oxidation of 0.5 mol L−1 methanol on (sp)Fe3O4/cis/rGO,
(rd)Fe3O4/cis/rGO and Fe3O4@AuNPs/cis/rGO was investigated in 0.1 mol L−1 HClO4
solution by cyclic voltammetry (CV) between −0.5 and +1.5 V. The potentials were measured
with respect to the Ag/AgCl electrode as a reference electrode. The counter electrode was a Pt
wire.
The schematic diagram of the preparation of the nanocomposites for methanol oxidation is
shown in Scheme 1.
[Here Scheme 1]
3. Results and discussion
3.1. Characterizations of nanocomposites
The
morphologies
of
the
(sp)Fe3O4/cis/rGO,
(rd)Fe3O4/cis/rGO
and
Fe3O4@AuNPs/cis/rGO nanocomposites were investigated by using the JEOL 2100 HRTEM
with an accelerating voltage of 200 keV. A drop of sample solution was deposited on a
polymeric grid dried at room temperature under an argon gas stream. Fig 1A shows the
transparent, wrinkled and planar sheet like morphology of rGO. In Fig 1B, the TEM image of
the (sp)Fe3O4/cis/rGO shows that the sizes of the (sp)Fe3O4 are very similar with a mean
diameter of 18 to 20 nm on a lighter shaded substrate corresponding to rGO sheet. Fig. 1C
shows the size of the (rd)Fe3O4/cis/rGO around 7-10 nm on the lighter shaded rGO sheets. Fig
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1D is the last TEM image that is corresponded to Fe3O4@AuNPs/cis/rGO and shows that the
bimetallic Fe3O4@AuNPs have been seen as dark dots with a mean diameter of 12 to 15 nm
on rGO sheets. Fig. 1E represents the EDX data of Fe3O4@AuNPs/cis/rGO composite to
analyze the materials and the structure of the nanocomposite.
[Here Fig. 1]
SEM characterization is used to investigate the morphologies of the developed
surfaces in present study. In Fig 2A, shows the smooth surface of bare GC electrode. Fig 2B,
2C and 2D show dense layers covered on the GC electrode, indicating that the successful
binding of (sp)Fe3O4/cis/rGO, (rd)Fe3O4/cis/rGO and Fe3O4@AuNPs/cis/rGO composites,
respectively.
[Here Fig. 2]
Fig. 3 shows the XPS spectrum of the nanocomposites, C, N, S, Fe of (sp)Fe3O4 (Fig
3A), Fe of (rd)Fe3O4 (Fig 3B) and Fe3O4@AuNPs (Fig 3C) peaks are prominent, showing that
(sp)Fe3O4, (rd)Fe3O4 and Fe3O4@AuNPs have been functionalized on the L-cysteine
functionalized reduced graphene oxide sheets. The C1s core-level spectra of the
nanocomposites were curve-fitted in Fig. 3. The peaks at 283.4 eV, 284.3 eV and 286.4 eV
are assigned to CH and CN and CONH, respectively 11. The peak located at 397.4 eV in the
N1s narrow region XPS spectrum is corresponded to C–N groups in the covalent attachment of
the carboxyl group of the reduced graphene oxide with the amino group of the cysteine 12. S2p
region was curve-fitted with two components by a doublet (2p1/2 and 2p3/2), owing to the spin-
orbit coupling. The peak at 162.1 eV indicates that the sulfur atom in the nanocomposite was
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grafted with the nanoparticles. The peak at 163.2 eV can be assigned to free mercapto group
in unreacted cysteine. Fig 3A shows the Fe2p region that characterized by doublet 2p1/2 and
2p3/2 signals that appear at 721.2 and 718.8 eV, respectively, corresponding the presence of
Fe3O4 on (sp)Fe3O4/cis/rGO. The peak signals at 720.3 and 718.1 eV in Fig 3B are
corresponded to Fe 2p1/2 and 2p3/2, respectively, indicating the functionalization of (rd)Fe3O4.
Fig 3C shows the peaks at 720.6 and 718.3 eV of Fe2p and peaks at 82.7 and 87.5 eV of Au4f
narrow region spectra of the Fe3O4@AuNPs/cis/rGO composite, respectively 12.
[Here Fig. 3]
The successful synthesis of the (sp)Fe3O4/cis/rGO, (rd)Fe3O4/cis/rGO and
Fe3O4@AuNPs/cis/rGO composites were also confirmed by XRD patterns in Fig 4. The XRD
pattern of (sp)Fe3O4/cis/rGO composite is shown in Fig. 4A. The intense and narrow peaks at
2θ = 34.16o and 42.23o refers to the (002) and (004) planes of rGO sheets, respectively 36. The
characteristic peaks of (sp)Fe3O4 also have been observed. The peaks at 2θ = 30.08o, 35.16o,
38.24o, 56.83o and 62.57o are corresponded to the (220), (311), (400), (511) and (440) planes
of Fe, respectively. Fig. 4B shows the XRD pattern of (rd)Fe3O4/cis/rGO composite. The
intense and narrow peaks at 2θ = 33.24o and 41.83o refers to the (002) and (004) planes of
rGO sheets, respectively 36. The peaks at 2θ = 32.68o, 37.12o, 46.19o, 54.61o and 61.83o are
corresponded to the (220), (311), (400), (511) and (440) planes of Fe3O4, respectively. The
XRD pattern of Fe3O4@AuNPs/cis/rGO composite is shown in Fig. 4C. The intense and
narrow peaks at 2θ = 22.46o and 36.52o refers to the (002) and (004) planes of rGO sheets,
respectively. The peaks at 2θ = 39.87o and 77.92o are corresponded to the (111) and (311)
planes of Au, respectively. The characteristic peaks of Fe3O4@AuNPs also have been
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observed. The peaks at 2θ = 45.06o, 64.71o and 79.93o are corresponded to the Fe3O4(110)-
Au(200), Fe3O4(200)-Au(220) and Fe3O4(211)-Au(222) planes, respectively.
[Here Fig. 4]
3.2. Electrocatalytic activity for methanol oxidation
The electrocatalytic activities of (sp)Fe3O4/cis/rGO, (rd)Fe3O4/cis/rGO and
Fe3O4@AuNPs/cis/rGO modified GCE were investigated by CV at 100 mV s−1 in a 0.1 mol
L−1 HClO4 solution. Fig 5A shows the voltammograms of L-cysteine functionalized rGO,
(sp)Fe3O4 and (sp)Fe3O4/cis/rGO, respectively. The peak current of 0.5 mol L−1 methanol on
(sp)Fe3O4/cis/rGO modified GCE increases slowly at lower potentials and then quickly
increases at potentials higher than 500 mV. The current density is directly proportional to the
amount of methanol oxidized at the electrode. The observed current density on
(sp)Fe3O4/cis/rGO modified GCE is much higher than (sp)Fe3O4 and cis/rGO modified GC
electrodes. Fig. 5B shows the voltammograms of L-cysteine functionalized rGO, (rd)Fe3O4
and (rd)Fe3O4/cis/rGO, respectively and Fig 5C shows the voltammograms of L-cysteine
functionalized rGO, Fe3O4@AuNPs and Fe3O4@AuNPs/cis/rGO, respectively. Fig. 5B and
5C are similar to Fig. 5A and shows that the current densities on (sp)Fe3O4/cis/rGO (Fig. 5B)
and Fe3O4@AuNPs/cis/rGO (Fig. 5C) modified GC electrodes are much higher than
(rd)Fe3O4 (Fig. 5B) and Fe3O4@AuNPs (Fig. 5C) and cis/rGO (both Fig. 5B and 5C)
modified GC electrodes, which confirms that reduced graphene oxide-supported (sp)Fe3O4
(Fig. 5A), (rd)Fe3O4 (Fig. 5B) and Fe3O4@AuNPs (Fig. 5C) generate more complete
oxidation of methanol to carbon dioxide.
The efficiencies of the (sp)Fe3O4/cis/rGO, (rd)Fe3O4/cis/rGO and Fe3O4@AuNPs/cis/rGO on
methanol oxidation were compared with regard to oxidation potential and current densities in
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Table 1. As shown in Table 1, the oxidation peak potential of methanol for
Fe3O4@AuNPs/cis/rGO was 554 mV, compared with an oxidation potential of 559 mV for
(sp)Fe3O4/cis/rGO and an oxidation potential of 562 mV for (rd)Fe3O4/cis/rGO. This
observation shows that the Fe3O4@AuNPs/cis/rGO catalyst can significantly decrease the
barrier to methanol oxidation and perform better than the other reduced graphene oxide-
supported catalysts in Fig 5D. As given in Fig. 6, for all reduced graphene oxide based
electrocatalysts, the forward oxidation current (I) is proportional to the square root of the scan
rate, suggesting that the oxidation behavior of methanol at all electrodes is controlled by
diffusion processes. The slope for Fe3O4@AuNPs/cis/rGO electrocatalyst is larger than those
for the other electrocatalysts, indicative of a faster diffusion process of methanol on the
surface of Fe3O4@AuNPs/cis/rGO electrocatalyst.
[Here Fig. 5]
[Here Fig. 6]
[Table 1]
In the literature, several catalysts were prepared for the fuel cell application. For
example, Pt-Pd electrocatalysts supported on carbon microspheres (CMSs) were used for
methanol oxidation in alkaline media 37. Pt/ CMS and Pd/CMS electrodes showed an activity
of 11.4 mA cm−2 and 2.5 mA cm−2, respectively. Another study is pure Pt and nitrogen
containing carbon nanotubes as platinum catalyst (Pt/N-CNT) showed an activity of 0.167
mA cm−2 and an activity of 13.3 mA cm−2, respectively 38. Recently, the Pt-Pd alloy porous
films comprised of nanodendrites showed enhanced electrocatalytic activities toward the
electrooxidation of methanol in acidic solution 39. So we can say that the anodic current
density of Fe3O4@AuNPs/cis/rGO catalyst is found to be higher than those of the mentioned
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electrodes, which indicate that the developed Fe3O4@AuNPs/cis/rGO catalyst have excellent
catalytic activity on methanol electrooxidation.
The chronoamperometry measurements were performed to test the long-term
stabilities of the as-prepared catalysts at 0.6 V in 0.5M KOH and 1.0 M CH3OH in Fig. 7.
After the methanol oxidation reaction starts, it was found that Fe3O4@AuNPs/cis/rGO
showed the highest oxidation current density at 20 000 s, which is 1.87 and 2.54 times as high
as that observed on (rd)Fe3O4/cis/rGO and (sp)Fe3O4/cis/rGO, respectively. This indicates that
Fe3O4@AuNPs/cis/rGO is more stable for methanol electro-oxidation than (rd)Fe3O4/cis/rGO
and (sp)Fe3O4/cis/rGO, respectively.
[Here Fig. 7]
4. Conclusion
Magnetic spherical iron oxide nanoparticles, rod iron oxide nanoparticles and
iron@gold nanoparticles involved L-cysteine functionalized reduced graphene oxide
nanohybrids were successfully synthesized and characterized. The results show that
(sp)Fe3O4, (rd)Fe3O4 and Fe3O4@AuNPs were highly dispersed on the reduced graphene
oxide nanosheets can more effectively enhance electrocatalytic activity of (sp)Fe3O4,
(rd)Fe3O4 and Fe3O4@AuNPs for the oxidation of methanol into CO2. Notably, the peak
potential of methanol oxidation at Fe3O4@AuNPs/cis/rGO modified GCE is lower than
(sp)Fe3O4/cis/rGO and (rd)Fe3O4/cis/rGO, respectively. Thus, it can be said that
Fe3O4@AuNPs/cis/rGO show better electrocatalytic performance.
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Acknowledgement
This work was supported by The Scientific and Technological Research Council of Turkey
(TUBITAK), the project number of 113Z264. We would like to thank TUBITAK for the
financial support.
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Table 1.Comparison of electrocatalyticactivity of methanol oxidation on bare GCE, rGO,
(sp)Fe3O4/cis/rGO, (rd)Fe3O4/cis/rGO and Fe3O4@AuNPs/cis/rGO (scan rate: 100 mV s−1).
GC Electrode
(sp)Fe3O4/cis/rGO
(rd)Fe3O4/cis/rGO
Fe3O4@AuNPs/cis/rGO
rGO
PeakCurrentDensity (mA cm-2)
E (mV)
559
562
554
567
-
4.91 ± 0.1
6.23 ± 0.3
14.67 ± 0.2
0.64 ± 0.4
-
GCE
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Figure Captions
Scheme 1. The procedure of fabrication of the nanocomposites for methanol oxidation.
Figure 1. TEM image of (A) rGO, (B) (sp)Fe3O4/cis/rGO, (C) (rd)Fe3O4/cis/rGO (D)
Fe3O4@AuNPs/cis/rGO and (E) EDX data of Fe3O4@AuNPs/cis/rGO.
Figure 2. SEM image of (A) bare GCE, (B) (sp)Fe3O4/cis/rGO/GCE, (C)
(rd)Fe3O4/cis/rGO/GCE and (D) Fe3O4@AuNPs/cis/rGO/GCE.
Figure 3. The narrow region XPS spectra of (A) Fe2p of (sp)Fe3O4/cis/rGO, (B) Fe2p of
(rd)Fe3O4/cis/rGO and (C) Fe2p and Au4f of Fe3O4@AuNPs/cis/rGO and C1s, N1s and S2p of
rGO.
Figure 4. XRD characterizations of (A) (sp)Fe3O4/cis/rGO, (B) (rd)Fe3O4/cis/rGO and (C)
Fe3O4@AuNPs/cis/rGO.
Figure 5. The electrocatalytic activities of (A) cis/rGO, (sp)Fe3O4 and (sp)Fe3O4/cis/rGO, (B)
cis/rGO, (rd)Fe3O4 and (rd)Fe3O4/cis/rGO, (C) cis/rGO, Fe3O4@AuNPs and
Fe3O4@AuNPs/cis/rGO
Fe3O4@AuNPs/cis/rGO.
and
(D)
(sp)Fe3O4/cis/rGO,
(rd)Fe3O4/cis/rGO
and
Figure 6. The relationship of I vs. the square root of scan rate on reduced graphene oxide
supported core–shell nanoparticle modified GCE.
Figure 7. Chronoamperometry results of (sp)Fe3O4/cis/rGO, (rd)Fe3O4/cis/rGO and
Fe3O4@AuNPs/cis/rGO catalysts obtained in 0.5 M KOH and 1.0 M CH3OH at 0.6 V.
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TEM image of (A) rGO, (B) (sp)Fe3O4/cis/rGO, (C) (rd)Fe3O4/cis/rGO (D) Fe3O4@AuNPs/cis/rGO and (E)
EDX data of Fe3O4@AuNPs/cis/rGO.
337x190mm (96 x 96 DPI)

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SEM image of (A) bare GCE, (B) (sp)Fe3O4/cis/rGO/GCE, (C) (rd)Fe3O4/cis/rGO/GCE and (D)
Fe3O4@AuNPs/cis/rGO/GCE.
256x213mm (96 x 96 DPI)

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The narrow region XPS spectra of (A) Fe2p of (sp)Fe3O4/cis/rGO, (B) Fe2p of (rd)Fe3O4/cis/rGO and (C)
Fe2p and Au4f of Fe3O4@AuNPs/cis/rGO and C1s, N1s and S2p of rGO.
277x147mm (96 x 96 DPI)

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XRD characterizations of (A) (sp)Fe3O4/cis/rGO, (B) (rd)Fe3O4/cis/rGO and (C) Fe3O4@AuNPs/cis/rGO.
434x121mm (96 x 96 DPI)

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The electrocatalytic activities of (A) cis/rGO, (sp)Fe3O4 and (sp)Fe3O4/cis/rGO, (B) cis/rGO, (rd)Fe3O4 and
(rd)Fe3O4/cis/rGO, (C) cis/rGO, Fe3O4@AuNPs and Fe3O4@AuNPs/cis/rGO and (D) (sp)Fe3O4/cis/rGO,
(rd)Fe3O4/cis/rGO and Fe3O4@AuNPs/cis/rGO.
552x358mm (96 x 96 DPI)

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The relationship of I vs. the square root of scan rate on reduced graphene oxide supported core–shell
nanoparticle modified GCE.
208x152mm (96 x 96 DPI)

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Chronoamperometry results of (sp)Fe3O4/cis/rGO, (rd)Fe3O4/cis/rGO and Fe3O4@AuNPs/cis/rGO catalysts
obtained in 0.5 M KOH and 1.0 M CH3OH at 0.6 V.
248x168mm (96 x 96 DPI)

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The procedure of fabrication of the nanocomposites for methanol oxidation.
334x212mm (96 x 96 DPI)