Preparation and electrochemical properties of MnO2 nanosheets attached to Au nanoparticles on carbon nanotubes

Article abstract of DOI:10.1039/c0dt01073a

A wet chemical route for the preparation of MnO₂ nanosheet/Au nanoparticle/MWNT hybrid materials is developed. The Au nanoparticles are prepared by reducing AuCl₄^- with citrate and attached to thiol-modified MWNTs. Owing t

Full text of DOI:10.1039/c0dt01073a

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PAPER
Preparation and electrochemical properties of MnO2 nanosheets attached to
Au nanoparticles on carbon nanotubes
Li Wei, Changqing Li, Haibin Chu and Yan Li*
Received 20th August 2010, Accepted 10th November 2010
DOI: 10.1039/c0dt01073a
A wet chemical route for the preparation of MnO2 nanosheet/Au nanoparticle/MWNT hybrid
materials is developed. The Au nanoparticles are prepared by reducing AuCl4- with citrate and attached
to thiol-modified MWNTs. Owing to the reducing property and the binding ability to Mn-containing
species of capping agents surrounded the Au nanoparticles, the MnO2 nanosheets are formed on the
surface of Au nanoparticles. The ternary nanocomposites of MnO2/Au/MWNT have been
characterized by transmission electron microscopy, X-ray diffraction, X-ray photoelectron
spectroscopy, and FT-IR spectroscopy. The affiliation of MnO2 nanosheets into the hybrids remarkably
enhances the electrocatalytic performance of Au nanoparticle/MWNT towards the oxygen reduction
reaction. The specific capacitance of the ternary hybrids is also increased dramatically comparing with
that of Au/MWNT.
transfer number increases along with the decrease of the size
Introduction
of Au particles, the contribution of the two-electron reduction
Carbon nanotubes (CNTs) are hollow nano-sized tubes of con-
pathway of O2 to hydrogen peroxide is still unavoidable at low
centric graphitic carbon capped by fullerene-like hemispheres.1
CNTs have intrinsic properties, such as low density, high surface
area, high stability, high electrical conductivity, and stability in
acid/basic media. Owning to their excellent properties CNTs
have been intensively studied in the last decade.2–6 Inorganic
nanomaterials are of particularly interest due to their unique
electronic, optical, magnetic and catalytic properties. Importantly,
these features differ from those of the bulk materials and can
be further tuned by controlling their composition, size and
morphology.7 CNTs can be used as support materials for the
dispersion and stabilization of inorganic nanomaterials.8–13 Such
hybrids of CNTs and inorganic materials may integrate the
unique properties of both components8 and have found various
applications in catalysis,2,9,10 energy conversion,11–16 sensor,17,18
and analysis.19 Using CNTs instead of carbon black to support
the nanosized inorganic catalysts in electrocatalytic processes is
advantageous owing to the higher conductivity and stability of
CNTs.12
potential even when the Au nanoparticles are smaller than 3 nm.21
Manganese oxides have an excellent catalytic activity towards the
chemical disproportionation of hydrogen peroxide into water and
molecular oxygen. Therefore, a combined use of manganese oxide
nanostructures together with Au nanoparticles might provide
efficient electrocatalysts towards ORR. EI-deab et al. studied the
activity of binary catalysts of Au nanoparticles and manganese
oxide (MnOx) nanoparticles electrodeposited onto glassy carbon
electrode (GCE) towards ORR in alkaline medium.22,23 They
showed that the combined use of MnOx and Au nanoparticles
resulted in the occurrence of the ORR at a potential close to
that obtained at Pt electrodes. However, the addition of MnOx
decreases the conductivity of the catalysts, which is unfavourable
for the electrocatalysis.
Our strategy for this study is combining CNTs, Au nanopar-
ticles, and nanostructured MnO2 in a controlled way to obtain
superior functions of the materials. Here, MnO2 nanosheets were
attached to Au nanoparticles supported on carbon nanotubes
through a wet chemical route. The formation mechanism of the
nanostructured composites was discussed. The electrocatalytic
properties of the as-obtained hybrid materials towards ORR was
studied on bare GCE using cyclic voltammetry (CV) and liner
scanning voltammetry (LSV). The results show that the integration
of CNTs and Au with MnO2 is promising to obtain improved
electrocatalytic performance.
Nowadays, the substitution of Pt in fuel cell and other elec-
trochemical devices is attracting great attention. Au is believed
to be one of the most promising candidate materials for non-
Pt-based electrocatalysts in oxygen reduction reaction (ORR).20
Yet, in acidic media a 2-electron reduction of oxygen takes
place on the Au surface, while the peroxide intermediate is
further reduced only at high over-potentials. Although the electron
In addition, MnO2 is considered to be an important candidate
material for supercapacitors. The electrochemical performance of
MnO2 depends on their crystallographic forms, specific surface
area, morphology and electrical conductivity.24,25 However, the low
Beijing National Laboratory for Molecular Sciences, State Key Laboratory
of Rare Earth Materials Chemistry and Applications, Key Laboratory for the
Physics and Chemistry of Nanodevices, College of Chemistry and Molecular
Engineering, Peking University, Beijing, 100871, China
2332 | Dalton Trans., 2011, 40, 2332–2337
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The Royal Society of Chemistry 2011
©

electrical conductivity of MnO2 is unfavourable for fast electron
transfer during the electrochemical processes. The combination
of MnO2 with CNTs may compensate the poor conductivity.26
Therefore, we also investigated the capacitance properties of the
hybrids.
Electrochemical measurements
Electrochemical measurements were performed on an electro-
chemical workstation (CHI660C) by cyclic and linear scanning
voltammetry technique. Ag/AgCl/KCl(saturated) electrode and
Pt wire were used as reference and counter electrodes, respectively.
For the liner scanning voltammetry measurements, rotating disk
electrode (Pine Research Instrument) with glassy carbon (GC)
substrate was used as working electrode. The catalyst-modified
electrode was prepared by transferring 10 mL suspension of the
samples onto the glassy carbon electrode with a diameter of
3.0 mm (5.0 mm for RDE). After the electrode was dried, the
hybrid material was covered with a thin layer of Nafion to ensure
the adherence to the electrode surface. ORR was carried out in
O2-saturated 0.1 M KOH aqueous solutions at room temperature.
The capacitance performance of the samples was investigated
with similar procedure in 0.5 M Na2SO4 aqueous solution while
a saturated calomel electrode (SCE) was used as the reference
electrode instead.
Experimental
Thiolation of MWNTs
Purified MWNTs with diameters of 10–20 nm were purchased
from Shenzhen Nanotech Port Co. Ltd, China. The as-received
MWNTs were oxidized by refluxing at 130 ◦C in concentrated
nitric acid for 12 h, followed by washing with deionized water
for several times and dried at 70 ◦C for 10 h. The surface
thiolation of MWNTs were performed by the dicyclohexycarbodi-
imide (DCC)-assisted condensation reaction between carboxyl
groups on the surface of oxidized MWNTs and amino groups
of NH2(CH2)2SH. Typically, 0.1 g oxidized MWNTs were first
dispersed in 100 ml N,N-dimethylformamide (DMF) by sonica-
tion, then 0.6 g DCC and 0.2 g NH2(CH2)2SH were ultrasonically
dissolved in the suspension. The condensation reaction was carried
out by stirring the reaction solution at 50 ◦C for 6 h. After
that, the thiol-modified MWNTs (s-MWNTs) were collected by
centrifugation and washed with ethanol, then dried in vacuum at
room temperature for 6 h.
Results and discussion
1. Self-assembly of Au nanoparticles on MWNTs
TEM image of Au particles anchored on MWNTs by sonication
method was shown in Fig. 1a. In the citrate-reduction procedure,
the Au nanoparticles were produced by the reduction of AuCl4
-
and stabilized by dicarboxy acetone molecules.27 When Au sol
was mixed with s-MWNTs suspension, Au nanoparticles linked
to the CNTs by the strong coordination interaction between Au
and S. Under the sonication condition, the presence of transverse
shear force can help to obtain a suspension with good dispersion
and thus increase the combination chance of Au nanoparticles
with the outside walls of s-MWNTs.
Preparation of Au nanoparticle/MWNT hybrid materials
The Au colloid was prepared via reduction of HAuCl4 with sodium
citrate as reported by Turkevich.27 For a typical procedure, 25 mL
0.01 wt% HAuCl4 solution was stirred and heated to boiling, then
1 mL 1 wt% sodium citrate solution was added quickly. After being
boiled for 10 min, the solution was cooled to room temperature.
Subsequently, the Au colloid was mixed with 5 mg s-MWNTs
dispersed in 25 mL deionized water under sonication.
Synthesis of MnO2 nanosheets on the surface of Au nanoparticles
attached to MWNTs
10 mL 0.01 M KMnO4 solution was added to the Au/MWNT
composite suspension. The mixture was kept at 70 ◦C for 2 h
under mechanical stirring. Then the suspension was centrifuged
at 12 000 rpm for 20 min, washed with deionized water and ethanol
and dried in vacuum at room temperature.
Characterization
Transmission electron microscopy (TEM, JEOL-200 CX) and
high-resolution TEM (HRTEM, Tecnai F30) were employed to
examine the morphology and the size of the hybrid materials.
X-ray diffraction (XRD) measurements were performed on a
Rigaku Dmax-2000 X-ray diffractometer using Cu-Ka radiation
˚
(l = 1.5406 A) with an accelerating voltage of 40 kV. The IR
spectra of the samples were measured on a Magna-IR 750 Fourier
transform infrared (FTIR) spectrometer at a resolution of 4 cm-1.
X-ray photoelectron spectroscopy (XPS) measurements were
performed on a AXIS Ultra X-ray photoelectron spectroscope
with Al-Ka radiation (hn = 1486.71 eV).
Fig. 1 (a) TEM image of Au particles anchored on MWNTs with sonica-
tion method; (b, c) TEM and HRTEM images of MnO2/Au/MWNT
hybrid materials; (d) XRD patterns for MnO2/Au/MWNT (A) and
Au/MWNT (B).
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2. Structure of MnO2 nanosheet/Au nanoparticle/MWNT
hybrids
Au nanoparticles probably results from the lack of protection due
to the oxidation of the stabilizer molecules on the surface of Au
nanoparticles after heating in KMnO4 solution at 70 ◦C.
Fig. 1b and 1c show the TEM and HRTEM images of the
MnO2/Au/MWNT hybrid materials prepared via heating Au
nanoparticle/MWNT in KMnO4 solution at 70 ◦C for 2 h. It
can be seen from Fig. 1b that nanosheets with less than 100 nm
in length were attached onto the surface of Au nanoparticles
anchored on MWNTs. The HRTEM characterization showed
that the nanosheets were about ~10 nm in width and 100 nm in
length. Additionally, the lattice fringes with interplanar distances
of approximately 0.23 nm correspond to the {211} planes of a-
MnO2.
Fig. 1d shows XRD patterns for Au/MWNT and MnO2/
Au/MWNT hybrids. Both samples show characteristic peak of
MWNTs at 2q = 25.78 (002), the other peaks in Fig. 1d(B) can
be indexed to a cubic phase Au (JCPDS no. 04-0784). Fig. 1d(A)
shows a broad diffraction peak at 2q = 12.0 and a shoulder peak at
2q = 36.7, which correspond to {110} and {400} planes of a-MnO2
(JCPDS 44-0141), respectively.
4. Formation mechanism of MnO2 nanosheets
Typical XPS spectra of MnO2/Au/MWNT hybrid materials is
shown in Fig. 3a. The oxidation state of manganese can be
evaluated from Mn 2p spectrum. From Fig. 3a we can see that Mn
2p3/2 and Mn 2p1/2 peaks are centered at 642.6 eV and 654.4 eV,
respectively, with a spin-energy separation of 11.8 eV, which is
in accordance with the reported data of Mn 2p3/2 and Mn 2p1/2
in MnO2.24,28 In addition, no Mn 2p3/2 signal of Mn2O3 (641.2
0.2 eV) and KMnO4 (647 eV) were observed in the XPS spectra,
indicating that the oxidation state of Mn is +4.
3. Effect of reaction time on the morphology of the hybrid
materials
The reaction time not only determines the size and the density
of MnO2 nanosheets, but also affects the aggregation and size
variation of Au nanoparticles. As the reaction time was prolonged
from 2 to 8 h, the density of MnO2 nanosheets was remarkably
increased (Fig. 2). Meanwhile, the average length of the nanosheets
increased from 35 to 50 nm. Once the reaction time was extended
to 16 h, the size of Au nanoparticles increased, accompanied by
the length shortening of MnO2 nanosheets. The size increase of
Fig. 3 (a) XPS spectrum of Mn 2p for MnO2/Au/MWNT hybrid
material; (b) FTIR spectra of s-MWNTs (A), Au/MWNT (B) and
MnO2/Au/MWNT (C).
The change of surface group of Au NPs can be verified
from FTIR spectra (Fig. 3b). For s-MWNTs, the bands located
at 1735 cm-1 and 1580 cm-1 are ascribed to the u(C
mode
O)
of amide groups and the stretching mode of C C in CNTs,
respectively. For Au/MWNT, the band around 1706 cm-1 is
attributed to the u(C
of stabilizer molecules on Au surface.
O)
Meanwhile, the peak at 1397 cm-1 and 1567 cm-1 are assigned
to the symmetric and asymmetric stretching bands of carboxylate
group in the stabilizer molecules, respectively.29 However, these
characteristic IR adsorption bands of stabilizer molecules on
Au surface disappear in the spectrum of the MnO2/Au/MWNT
composite, as shown in Fig. 3b(C). This is the evidence that the
stabilizer molecules were oxidized by KMnO4 when heating at
◦
70 C, meanwhile, MnO2 nanosheets were formed in situ on the
surfaces of Au nanoparticles by the reduction of KMnO4.
In order to further investigate the formation mechanism
of the MnO2/Au/MWNT ternary hybrids, MnO2/Au and
MnO2/MWNT hybrids were prepared, respectively. As shown
in Fig. 4, MnO2 nanosheets only formed on the surface of Au
Fig. 2 TEM images of MnO2/Au/MWNT hybrid materials prepared by
heating Au/MWNT in 1.67 mM KMnO4 solution at 70 ◦C for 8 h (a) and
16 h (b).
2334 | Dalton Trans., 2011, 40, 2332–2337
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N2-saturated 0.1 M KOH solution (Fig. 5b solid curve) and air-
saturated 0.1 M KOH solution (Fig. 5b dotted curve), we can
find that the oxygen reduction peak appears at ~ -0.39 V for
bare GC electrode, while it appears at more positive potential
for both Au/MWNT (Fig. 5c) and MnO2/Au/MWNT hybrids
(Fig. 5d). Further more, the reduction current of ORR increases
evidently, indicating that Au/MWNT and MnO2/Au/MWNT
hybrids present good catalytic activity towards ORR. Fig. 5e
shows the linear scanning voltammograms of ORR recorded
on RDEs. The result shows that the ORR onset potential on
MnO2/Au/MWNT hybrids is about 0.0 V, whereas it is about
-0.2V on Au/MWNT. In addition, MnO2/Au/MWNT hybrids
display higher limiting current density compared with that of
Au/MWNT. The increase of peak current is attributed to the
catalytic disproportional activity of MnO2. This result is consistent
with the results of previous studies, which revealed that Au
nanoparticles were found to cause a significant positive shift of
the onset potential of the current flow of the ORR, while MnOx
enhanced the catalytic disproportionation of the electrogenerated
hydrogen peroxide.22
A peak at the potential of about -0.45 V is observed for
MnO2/Au/MWNT hybrids in Fig. 5a(C). And the peak did not
disappear when the cyclic voltammogram was carried out in N2-
saturated 0.1 M KOH. So the peak may arise from the reduction
of MnOx. Further research is needed to fully address the reduction
pathway of the ORR.
Fig. 4 TEM images of Au/MnO2 (a) and MnO2/MWNT (b).
Cyclic voltammograms of Au/MWNT and MnO2/Au/MWNT
hybrids were also recorded between -0.2 and 1.0 V vs. SCE in aque-
ous 0.5 M Na2SO4 at a scan rate of 5 mV s-1. MnO2/Au/MWNT
hybrids show better capacitance performance than Au/MWNT as
shown in Fig. 5f. The specific capacitance (SC) of the electroactive
material was calculated using the following equation,32
nanoparticles, while no MnO2 nanosheets are observed on the
surface of MWNTs without Au nanoparticles, but the MWNTs
are different from their raw morphology and there are some
attachments adhered on the surface of MWNTs. Previous studies
-
have revealed that on the surface of MWNTs, MnO4 can be
reduced to MnO2 by oxidizing exterior carbon, forming MnO2
coated MWNTs.13,30 So the attachments on MWNTs may be
q
-
C =
amorphous MnO2 formed by the reduction of MnO4 .
DV ×w
On the basis of our experimental result, it is reasonable to
assume that Au nanoparticles play a critical role in the formation
of MnO2 nanosheets. A small quantity of H+ formed during
the formation of Au nanoparticles can promote the formation
of acetone diacetate,31 which located at the outside of protective
agent. The Mn atoms of the MnO6 octahedron, the basic unit
of MnO2, may form bonds with O atoms of the protective
agents on the surface of Au nanoparticles via an intermolecular
hydrogen bond or coordination bond, acting as anchor sites for
the growing of MnO2 nanosheets. Meanwhile, the capping agents
of Au nanoparticles, i e. citrate ions, could act as the reductants
where q is the voltammetric charge, DV is the potential window of
cycling, and w is the loading of the composites. The SC values for
all electrodes are listed in Table 1. It is inferred from Table 1 that
the specific capacitance of MnO2/Au/MWNT is 2–3 times higher
than that of Au/MWNT, meantime the specific capacitance of
MnO2/Au/MWNT increases with the increase of reaction time,
indicating that the MnO2 nanosheets formed on the surface of
Au nanoparticles show good capacity for charge storage. The
nanosheets have a very small thickness of ~10 nm, hence the
specific surface area should be very high. And the nanosheets
are well crystallized. Therefore, theoretically, this kind of MnO2
nanosheets ought to present superior capacitance. In respect that
w is the loading of the composites and only a small amount of
MnO2 present in the composites, if the loading of MnO2 was used
instead, the specific capacitance would be much higher.
-
for MnO4 . Then the formation of MnO2 nanosheets is initiated
at the Au nanoparticles. When the concentration of KMnO4 is
moderate, MnO2 nanosheets can only be formed around the Au
nanoparticles. But if the initiate concentration is too high, the site
selectivity of the growth of nanostructured MnO2 goes off.
5. Electrochemical performances of the MnO2/Au/MWNT
hybrid materials
Table 1 Specific capacitance (SC) of Au/MWNT and MnO2/
Au/MWNT
The electrochemical behaviour of MnO2/Au/MWNT ternary
hybrids towards ORR was studied in O2-saturated 0.1 M KOH
solution at a scan rate of 50 mV s-1. ORR behaviour of bare
GC electrode, Au/MWNT and MnO2/Au/MWNT are shown in
Fig. 5a. Compared with the cyclic voltammograms recorded in
Electrode materials
Specific capacitance /F g-1
Au/MWNT
MnO2/Au/MWNT (2 h)
MnO2/Au/MWNT (8 h)
33.67
99.6
122.1
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Dalton Trans., 2011, 40, 2332–2337 | 2335
©

Fig. 5 (a) CVs for oxygen reduction at bare GC electrode (A), Au/MWNT (B) and MnO2/Au/MWNT (C) in the O2-saturated 0.1 M KOH solution; (b,
c, d) CVs for oxygen reduction at bare GC electrode, Au/MWNT and MnO2/Au/MWNT in the N2-protected (solid curve) or air-saturated 0.1 M KOH
solution (dotted curve); (e) Linear scanning voltammograms obtained at RDE coated with Au/MWNT (A), MnO2/Au/MWNT (B) in O2-saturated
0.1 M KOH solution at potential scan rate of 1 mV s-1; (f) CVs of Au/MWNT (A) and Au/MnO2/MWNT of different reaction time (B, 2 h; C, 8 h) in
0.5 M Na2SO4 aqueous solution.
nanoparticles. The size and density of the nanosheets could
be tuned by increasing the reaction time. Combining the high
Conclusion
conductivity of MWNTs, the catalytic activity of Au nanopar-
ticles and MnO2 nanosheets, the MnO2/Au/MWNT hybrids
show superior electrochemical performance. Attribute to the
assistance of MnO2, the ternary composites show enhanced
electrocatalytic activity toward ORR reaction and might be a
potential substitute for Pt-based electrocatalysts. In addition,
the specific capacitance of MnO2/Au/MWNT is 2–3 times
higher than that of Au/MWNT. These results show that the
introducing of nanostructured MnO2 into the Au/MWNT system
Ternary hybrid materials composed of MnO2 nanosheets, Au
nanoparticles and MWNTs were prepared through a wet chemical
route. The formation mechanism was carefully studied employing
the controlled experiments and by the surface analysis and
spectroscopic data of the products. It was proposed that the
-
reduction of MnO4 and the formation of MnO2 nanosheets
were initiated at the surfaces of Au nanoparticles. The capping
agents on the surfaces of Au nanoparticles play important
roles for the in situ formation of MnO2 nanosheets around Au
2336 | Dalton Trans., 2011, 40, 2332–2337
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©

remarkably improves the electrochemical property. The ternary
nanostructured hybrids of MnO2/Au/MWNT may find potential
applications as electrocatalysts for fuel cells and as materials for
super capacitors.
12 Z. Y. Lin, H. B. Chu, Y. H. Shen, L. Wei, H. C. Liu and Y. Li, Chem.
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Acknowledgements
The authors would like to acknowledge funding from the Ministry
of Science and Technology of China (projects 2011CB933003 and
2007CB936202) and the National Natural Science Foundation of
China (project 50772002)
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