High-density assembly of gold nanoparticles with zwitterionic carbon nanotubes and their electrocatalytic activity in oxygen reduction reaction

Article abstract of DOI:10.1039/c2cc34785g

Gold nanoparticles (AuNPs) were assembled with high density onto multi-walled carbon nanotubes, which were functionalized with zwitterionic poly(imidazoliumsulfonate). The AuNP/zwitterionic CNT hybrids exhibited decent electrocatalytic activity in oxygen reduction reaction as the AuNP-based catalysts.

Full text of DOI:10.1039/c2cc34785g

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ARTICLE TYPE
High-Density Assembly of Gold Nanoparticles with Zwitterionic Carbon
Nanotubes and Their Electrocatalytic Activity in Oxygen Reduction
Reaction
Youn Soo Kim, Areum Cha, Ju Yeon Shin, Hyun Ji Jeon, Jun Ho Shim, Chongmok Lee,* and Sang-gi
Lee*
5
Received (in XXX, XXX) Xth XXXXXXXXX 200X, Accepted Xth XXXXXXXXX 200X
DOI: 10.1039/b000000x
Gold nanoparticles (AuNPs) were assembled with high
density onto multi-walled carbon nanotubes, which were
were coated with charged ligands, thereby allowing the charge-
induced assembly of MNPs and CNTs with high density without
1
0
functionalized with zwitterionic poly(imidazoliumsulfonate). 50 significant alteration of size and morphology. On the other hand,
the presence of any anion-exchange between the imidazolium salt
and the ligands that are coated onto the MNPs facilitate the
production of ligand-free MNPs, leading to their aggregation. As
a proof-of-concept, we investigated the assembly of pre-
The AuNP/zwitterionic CNT hybrids exhibited decent
electrocatalytic activity in oxygen reduction reaction as the
AuNP-based catalysts.
Owing to their unique properties, carbon nanotubes (CNTs) and 55 synthesized sodium citrate-coated gold nanoparticles (AuNPs)
1
2
2
3
3
4
4
5
0
5
0
5
0
5
metal nanoparticles (MNPs) are arguably the two most common
building blocks for current nanosciences. The assembly of these
two characteristic nanomaterials could lead to a successful
integration of their properties that have many potential
onto
functionalized
poly(vinylbutylimidazolium
zwitterionic
poly(vinylimidazoliumpropylsulfonate)-
CNTs
(PolyZWION-f-CNTs)
and
chloride)-functionalized
CNTs
(PolyVBIM(Cl)-f-CNTs). In zwittterionic PolyZWION-f-CNTs,
1
6
0
the imidazolium cation and sulfonate anion are covalently bond
in a molecule, which prevents intermolecular anion exchange,
and allows the charged AuNPs to assemble onto the CNT
surfaces through the electrostatic interactions between
zwitterionic imidazolium sulfonate and citrate ion. With the
PolyVBIM(Cl)-f-CNTs, the interionic reactions between
imidazolium chloride and the sodium citrates could afford NaCl
and imidazolium citrate salt, and which allows many of the
AuNPs to be naked, thus enabling easy aggregation (Figure 1).
applications ranging from catalysts to highly efficient fuel cells.
Several routes have been developed to assemble the MNPs with
CNTs. Examples include direct chemical deposition with and
without the aid of reducing agents, electrochemical deposition,
and the assembly of pre-synthesized MNPs onto CNTs. Among
these methods, the assembly of pre-synthesized MNPs on the
CNTs has received a great deal of attention because the MNPs
are synthesized in advance and their size and shape are controlled
by well-known procedures. To assemble MNPs with CNTs
effectively, one or more of the hydrophobic, van der Waals,
electrostatic, and - stacking intermolecular interactions
between the functionalities of CNT and MNP surfaces have been
65
1,2
utilized. However, the necessary modification of the MNP and
CNT surfaces for these interactions often adversely affects the
electrical property of the MNPs and CNTs. Herein we report
high-density assembly of gold nanoparticles onto multi-walled
CNTs, functionalized with zwitterionic poly(vinylimidazolium
propylsulfonate), that showed excellent electrocatalytic activity in
oxygen reduction reaction, one of the key factors in the
performance of fuel cells.
Recently, imidazolium-based ionic liquids (ILs) have been
3
utilized as reaction media and stabilizers for MNPs. In addition,
the incorporation of the IL moieties onto the CNT surfaces could
provide a novel class of functional, and addressable CNTs that
may serve to create tailored, amendable interfaces between CNTs
4
and their surroundings. During our ongoing study on the
5
hybridization of ILs with CNTs, we envisioned that the
70
Figure 1.Schematic presentation of the charge-induced assembly and
ligand exchange-driven aggregation of charge AuNPs onto the
PolyZWION-f-MWCNTs and PolyVBIM(Cl)-f-MWCNTs.
zwitterionic imidazolium salt-functionalized CNTs could have
electrostatic interactions with the pre-synthesized MNPs, which
This journal is © The Royal Society of Chemistry [year]
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To demonstrate our hypothesis, we first investigated the 45 rather than their size altered, and the solution color became clear
solution-state interactions between sodium citrate-coated AuNPs
and the CNT-free homopolymeric PolyZWION and
PolyVBIM(Cl) (synthetic procedure, see SI). No solution-color
change occurred when the zwitterionic PolyZWION (ca. 8.8 mg)
after 5 h (Figure S2b). In contrast, upon addition of
PolyVBIM(Cl)-f-MWCNTs, the solution color was immediately
changed to dark blue as observed with homopolymeric
PolyVBIM(Cl), and the solution color also became clear after 5 h
5
0
5
0
was added to an aqueous solution of AuNPs (20 mL, 0.04 M) 50 (Figure S2c). The transmission electron microscopy (TEM)
with an average diameter of 10 nm, prepared by NaBH reduction
images of the precipitates indicated that the functionality of the
modified CNTs is important factor for the assembly of AuNPs
(Figure 3).
4
6
of HAuCl in the presence of trisodium citrate. This suggested
4
stabilization of AuNPs in the presence of zwitterionic
7
1
1
2
PolyZWION
(b in Figure 2). Interestingly, addition of
homopolymeric PolyVBIM(Cl) changed the solution color
immediately to blue color (c in Figure 2). The UV-vis absorbance
spectra of the mixture of AuNPs and PolyZWION indicated that
the absorbance band of the aqueous AuNP solution at 510 nm (a
in Figure 2) was almost maintained with diminished peak
intensity (c in Figure 2). In contrast, from a mixture of AuNPs
and PolyVBIM(Cl), the absorbance peak at 510 nm was shifted to
a longer wavelength of 580 nm (c in Figure 2). The observed
solution color change and UV-vis results clearly indicated that
the size of the AuNPs was almost retained in the presence of
zwitterionic PolyZWION, whereas the interactions of AuNPs
with PolyVBIM(Cl) caused aggregation of the AuNPs in
8
solution.
0
0
0
0
0
.8
.6
.4
.2
.0
a
b
c
Figure 3. TEM (inset: STEM) images of (a) AuNP@COOH-f-MWCNTs,
(b) AuNP@PolyVBIM(Cl)-f-MWCNTs, and (c) AuNP@PolyZWION-f-
MWCNTs. (d) SEM image of AuNP@PolyZWION-f-MWCNTs.
5
5
As shown in Figure 3a, the AuNPs were not sufficiently
assembled with the COOH-functionalized MWCNTs, and AuNPs
of different size were deposited onto the CNT surfaces to form
the AuNP@COOH-f-MWCNTs. With the PolyVBIM(Cl)-f-
MWCNTs, the AuNPs were not deposited onto the CNT surfaces,
but rather were aggregated in the proximity of the CNTs (Figure
(
a) (b) (c)
6
6
7
7
8
0
5
0
5
0
4
00 450 500 550 600 650 700
Wavelength / nm
Figure 2. UV-vis absorbance spectra and the corresponding photos (inset)
of the aqueous solution of (a) AuNPs, (b) a mixture of AuNPs and
PolyZWION, and (c) a mixture of AuNPs and PolyVBIM(Cl).
3
b), suggesting that the sodium citrate ligands that were coated
2
5
onto the AuNPs may have reacted with imidazolium chloride to
give the naked AuNPs without citrate capping, which were easily
9
We next investigated the assembly of sodium citrate-coated
AuNPs onto the zwitterionic PolyZWION-f-MWCNTs and
PolyVBIM(Cl)-f-MWCNTs, which were prepared by radical
polymerization of N-allylimidazolium chloride-functionalized
MWCNTs with 3-(3-vinyl-3H-imidazolium-1-yl)propane-1-
aggregated prior to deposition onto the CNT surfaces. In contrast,
the zwitterionic PolyZWION-f-MWCNTs could assemble the
AuNPs onto the CNT surfaces. The TEM and scanning electron
microscopy (SEM) images (Figure 3c and 3d) clearly showed that
the AuNPs were densely decorated onto the CNT surfaces
without significant alteration of particle size to form the
3
3
4
0
5
0
sulfonate
and
1-butyl-3-vinylimidazolium
chloride,
5
c
respectively. Thermogravimetric analysis (TGA) indicated that
about 10 wt% of the polymeric moiety was attached onto the
CNTs (Figure S1). For comparison, the assembly of AuNPs with
carboxylic acid-functionalized MWCNTs (COOH-f-MWCNTs)
was also investigated. The functionalized MWCNTs were added
to an aqueous solution of AuNPs, and the mixture solution was
stirred for 10 h. With COOH-f-MWCNTs, the brown-colored
solution remained after 10 h, indicating that the AuNPs could not
be completely assembled with COOH-f-MWCNTs (Figure S2a).
The addition of the zwitterionic PolyZWION-f-MWCNTs did not
change the solution color, but it became pale after 3 h. This
suggested that the concentration of the AuNPs was decreased,
supported
AuNP@PolyZWION-f-MWCNTs.
The
X-ray
diffraction (XRD) patterns of these AuNPs showed typical face-
centered cubic (fcc) Au diffraction patterns but the peaks for
AuNP@PolyZWION-f-MWCNTs were broader than those of the
AuNP@PolyVBIM(Cl)-f-MWCNTs, indicating that smaller
AuNPs were assembled onto the PolyZWION-f-MWCNTs
1
0a
(
Figure S3). The peaks at 83.1 (4f7/2) and 87.5 eV (4f5/2) in the
X-ray photoelectron spectroscopy (XPS) spectrum clearly
o
10b
confirmed the Au oxidation state of the AuNPs (Figure S4).
The positive zeta potentials of PolyZWION-f-MWCNTs (+42.2
mV) and PolyVBIM(Cl)-f-MWCNTs (+40.6 mV) could support
their electrostatic interactions with the citrate-coated AuNPs
2
|
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having negative zeta potential (-38.1 mV). However, due to the
ligand exchange between the sodium citrate, coated onto the
AuNPs, and the imidazolium chloride in PolyVBIM(Cl)-f-
MWCNTs, the AuNPs could aggregate as depicted in Figure 1.
These AuNP/CNT hybrids have a promising potential in a
variety of applications, particularly in electrocatalytic oxygen
were
covalently
functionalized
with
zwitterionic
poly(vinylimidazolium propylsulfonate), without any significant
change in the particle sizes. In contrast, the use of the
45 conventional
MWCNTs caused severe AuNP aggregation. The AuNPs
assembled onto the zwitterionic MWCNTs exhibited high
electrocatalytic activity for oxygen reduction reaction. This
charge-induced assembly approach of AuNPs with zwitterionic
poly(imidazolium
chloride)-functionalized
5
1
2
reduction reactions. The assessment of the electrocatalytic
activities of the AuNP@COOH-f-MWCNTs,
AuNP@PolyVBIM(Cl)-f-MWCNTs, and AuNP@PolyZWION-f- 50 CNTs may be extrapolated to various MNPs, and studies on this
1
0
MWCNTs for oxygen reduction has revealed that the
AuNP@PolyZWION-f-MWCNTs exhibited higher
electrocatalytic activity than those of the AuNP@COOH-f-
MWCNTs and AuNP@PolyVBIM(Cl)-f-MWCNTs (for details,
see SI).
promising potential are currently underway.
This work was supported by National Research Foundation
(NRF-20120005673) and Seoul R&D program (ST100032)
Notes and references
55 Department of Chemistry and Nano Science, Ewha Womans Unversity,
(
a)
(
b)
0
.0
1
20-750 Seoul, Korea. Fax: +82-2-3277-3419; Tel: +82-23277-4505; E-
0
.05
.00
mail: sanggi@ewha.ac.kr(S.-g. Lee); cmlee@ewha.ac.kr(C. Lee)
† Electronic Supplementary Information (ESI) available: Detail
-0.1
-0.2
-0.3
0
experimental
DOI: 10.1039/b000000x/
Y. S. Kim and A. Cha are equally contributed on this work. Current
address for Dr. J.H. Shim: Department of Chemistry, Daegu University,
Gyeongsan 712-714, Korea.
procedures
and
characterization
data.
See
A
B
C
-0.05
0.10
A
B
C
6
6
7
0
5
0
‡
-
-
0.4 -0.2 0.0 0.2 0.4
0.0
0.5
1.0
1.5
1
Reviews: (a) G. G. Wildgoose, C. E. Banks, R. G. Compton, Small,
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X. Peng, J. Chen, J. A. Misewich, S. S. Wong, Chem. Soc. Rev., 2009,
38, 1076.
Potential / V vs SCE
Potential / V vs SCE
(c)
(d) 80
A (n = 3.1)
B (n = 3.5)
C (n = 3.7)
0
.02
Ring
Disk
6
4
0
0
0
.0
-
-
-
-
0.2
2
Selected papers: (a) G. M. Rahman, D. M. Guldi, E. Zambon, L.
Pasquato, N. Tagmatarchis, M. Prato, Small., 2005, 1, 527; (b) G. A.
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H. Pádua, Angew. Chem. Int. Ed., 2011, 50, 8683.
0.4
0.6
0.8
A (n = 2.9)
B (n = 3.5)
C (n = 3.8)
20
0
-
0.4 -0.2 0.0 0.2 0.4 0.6
Potential / V vs SCE
0.05
1/20.10
/ rpm
0.15
-1/2
-
w
75
80
1
2
5
0
Figure 4. (a) RDE and (c) RRDE polarization curves for oxygen
reduction in an O -saturated 0.1 M HClO solution at a scan rate of 10
mV s (rotation speed = 1600 rpm). (b) CVs recorded in 0.1 M HClO
2
4
-
1
3
4
-
1
2
solution deaerated with N purging gas at a scan rate of 50 mV s . (d) K-
L plots for the oxygen reduction obtained from the RDE data presented in
(a). Electrodes used in electrocatalytic oxygen reduction reactions: (A)
AuNP@COOH-f-MWCNTs (green); (B) AuNP@PolyVBIM(Cl)-f-
MWCNTs (blue); and (C) AuNP@PolyZWION-f-MWCNTs (magenta),
and saturated calomel electrode (SCE) as reference electrode.
4
5
T. Fukushima, T. Aida, Chem. Eur. J., 2007, 13, 5048.
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8
9
9
5
0
5
As shown in Figure 4a, the rotating disk electrode (RDE)
polarization curves indicated that the AuNP@PolyZWION-f-
MWCNTs showed at more positive onset potential than those of
the AuNP@COOH-f-MWCNTs and AuNP@PolyVBIM(Cl)-f-
MWCNTs. The cyclic voltammetry (CV) curves showed that the
AuNP@PolyZWION-f-MWCNTs exhibited a higher peak current
density, implying that the AuNPs had a higher surface area
6
7
2
006, 44, 3113; (b) M. Grzelczak, J. Pérez-Juste, P. Mulvaney, L. M.
2
3
3
4
5
0
5
0
Liz-Marzán, Chem. Soc. Rev., 2008, 37, 1783.
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1
0
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(
Figure 4b). The number of electron transfers per O molecule (n)
2
for the AuNP@PolyZWION-f-MWCNTs, that was calculated
from both the oxygen reduction current density at -0.2 V of the
rotating ring-disk electrode (RRDE, Figure 4c) and the slope of
the Koutecky-Levich (K-L) plots (Figure 4d), suggested that the
oxygen reduction reaction proceeded with about four electron
transfers without significant formation of hydrogen peroxide,
which is quite rare in AuNP-catalyzed electrocatalytic oxygen
reduction reaction in acidic media.
In summary, we have successfully assembled pre-synthesized
sodium citrate-coated AuNPs onto zwitterionic MWCNTs, which
11 H. Hinterwirth, W. Lindner, M. Lammerhofer, Anal. Chim. Acta
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13 G. Vázquez-Huerta, G. Ramos-Sánchez, A. Rodrígues-Castellanos, D.
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This journal is © The Royal Society of Chemistry [year]
Journal Name, [year], [vol], 00–00 | 3