Colloids and Surfaces A: Physicochem. Eng. Aspects 372 (2010) 146–150
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Colloids and Surfaces A: Physicochemical and
Engineering Aspects
journal homepage: www.elsevier.com/locate/colsurfa
Effect of ligand structure on the catalytic activity of Au nanocrystals
Kang Yeol Lee a,1 , Young Wook Lee b , Joon-Hwa Lee a , Sang Woo Han b,∗
a
b
Department of Chemistry and Research Institute of Natural Science, Gyeongsang National University, Jinju 660-701, Republic of Korea
Department of Chemistry and KI for the NanoCentury, KAIST, Daejeon 305-701, Republic of Korea
a r t i c l e
i n f o
Article history:
Received 27 July 2010
Received in revised form 2 September 2010
Accepted 12 October 2010
Available online 20 October 2010
Keywords:
Au nanoparticles
4,4 -Bipyridine
4-Nitrophenol reduction
Electron transfer
Ligand structure
a b s t r a c t
The surface coating-dependent catalytic activity of icosahedral Au nanoparticles (IAuNPs) for the reduction of 4-nitrophenol by NaBH4 in aqueous solution was investigated. Cetyltrimethylammonium bromide
(CTAB)-stabilized IAuNPs with average size of 50 nm were prepared by three-step seeding protocol. The
CTAB was then exchanged with several amine derivatives such as pyrrole, pyridine, 2,2 -bipyridine,
and 4,4 -bipyridine. The catalytic reaction rates of nanoparticles are in the order of IAuNPs@4,4 bpy > IAuNPs@2,2 -bpy > IAuNPs@pyridine > IAuNPs@pyrrole > IAuNPs@CTAB, revealing that the catalytic
activity of the nanoparticles highly depends on the type and structure of the ligand molecules.
© 2010 Elsevier B.V. All rights reserved.
1. Introduction
In the past decades, tremendous research interest has been
focused on the noble metal nanoparticles because of their unique
optical and electrical properties [1] and their promising applications in catalysis [2], optics [3], sensors [4], and solar cells [5].
Particularly, Au nanoparticles (AuNPs) have potentially high catalytic activity although bulk Au is typically an ineffective catalyst
[6]. In fact, stabilized AuNPs were frequently employed as active
units in catalysis [7].
For preparing stable AuNPs, surface-stabilizers such as alkylthiolate, poly(vinyl pyrrolidone), and cetyltrimethylammonium
bromide (CTAB) have been widely used. However, application of
AuNPs to various fields such as catalysis, optics, and sensors can
be potentially restricted owing to these stabilizers. For instance,
Stowell and Korgel [2] and El-Sayed and coworker [8] demonstrated that ligand stabilization could decrease the catalytic activity
of nanoparticles in some cases. On the basis of this fact, choice of an
appropriate capping ligand which can enhance both stability and
activity of the particles is crucial in the application of nanoparticles
to catalysis.
Here we report on the surface coating-dependent catalytic
activity of polyhedral metal nanoparticles, i.e., icosahedral AuNPs
(IAuNPs). Due to their well-defined polyhedral morphology, there
are a number of catalytic active sites such as corners and edges
on the surfaces of the IAuNPs [9,10]. Although IAuNPs have many
potentially active sites, they have weak catalytic activity due to
the presence of many stabilizers on their surfaces. In fact, CTAB
has been used as a stabilizer in the preparation of IAuNPs, which
could decrease the catalytic activity of the particles owing to the
inhibition of close contact between reactants and surface of catalysts. In this work, the surface of IAuNPs has been modified
with various small organic amine additives such as pyrrole, pyridine, 2,2 -bipyridine, and 4,4 -bipyridine through ligand exchange
reaction in order to improve the catalytic activity of IAuNPs,
and ligand-dependent catalytic activity was investigated. Amine
derivatives were chosen as ligand molecules because they can be
bound strongly to the AuNPs surfaces through the Au–N bond
and their adsorption structures have been well studied [11–14].
To investigate how the nature of the stabilizer has an influence
on the catalytic activity of IAuNPs, the reduction of 4-nitrophenol
(4-NP) by NaBH4 has been chosen as a model reaction because
this reaction is rapid and can be easily characterized [15–17].
Moreover, 4-NP is a typical contaminant of industrial wastewater and employed in a number of industrial processes such
as the manufacture of explosives, drugs, dyes, insecticides, and
pesticides [18].
2. Experimental
∗ Corresponding author. Tel.: +82 42 3502812; fax: +82 42 3502810.
E-mail address: [email protected] (S.W. Han).
1
Current address: Department of Chemistry, Korea University, Seoul 136-701,
Republic of Korea.
0927-7757/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.colsurfa.2010.10.019
HAuCl4 , CTAB, trisodium citrate, pyrrole, pyridine, 2,2 bipyridine, and 4,4 -bipyridine were purchased from Aldrich. Other
chemicals, unless specified, were reagent grade, and Milli-Q water
K.Y. Lee et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 372 (2010) 146–150
Fig. 1. UV–vis spectra of the IAuNPs stabilized with CTAB, pyrrole, pyridine, 2,2 -bpy,
and 4,4 -bpy.
with a resistivity greater than 18.0 M cm was used in the preparation of aqueous solutions.
CTAB-stabilized IAuNPs with average size of ca. 50 nm were
prepared by three-step seeding protocol [10]. In a typical ligand
exchange reaction, 10 ␮L of 5.6 × 10−4 M aqueous solution of amine
derivative (pyrrole, pyridine, 2,2 -bipyridine, and 4,4 -bipyridine)
was added into a microtube containing 200 ␮L of aqueous solution of CTAB-stabilized IAuNPs, and the mixture was stored for
overnight.
The catalytic reduction of 4-NP was studied as follows. In a standard quartz cuvette with a 1 cm path length, 2.745 mL of water,
0.030 mL of the surface-modified Au hydrosol, and 0.150 mL of
1.00 × 10−3 M 4-NP were mixed. Then, 0.0750 mL of 2.00 × 10−1 M
NaBH4 was introduced into this mixture and time-dependent
absorption spectra were recorded every 8 min in the range of
190–1000 nm at 25 ◦ C.
The extinction spectra were recorded with a SINCO S-3100
UV–vis spectrophotometer. Transmission electron microscopy
(TEM) images were obtained with a FEI Technai G2 F30 Super-Twin
transmission electron microscope operating at 300 kV.
3. Results and discussion
IAuNPs stabilized with amine derivatives were prepared by
substitution of CTAB ligand with pyrrole, pyridine, 2,2 -bipyridine
(2,2 -bpy), and 4,4 -bipyridine (4,4 -bpy). The ligand desorption
kinetics depend on the binding strength between the ligand functional group and the inorganic nanocrystal surface. Since the Au–N
bond is considerably more covalent than of Au–N+ bond owing
to charge donation/back-donation mechanism, Au–N bond has a
larger binding energy than Au–N+ [19]. Therefore, CTAB which has
a relatively weak binding strength to Au can be easily stripped
from the nanocrystal surfaces by ligand exchange reaction with
amine additives. The functionalization of the AuNPs with amine
compounds was verified by UV–vis spectroscopy. It is well-known
that the AuNPs exhibit strong plasmon resonance absorption that
is dependent on their size and shape [20,21]. The surface plasmon
band is also influenced by the surrounding media, especially by
the nature of surface capping agent [22]. Fig. 1 shows the UV–vis
absorption spectra of the as-prepared CTAB- and amine derivativesstabilized IAuNPs. Each spectrum exhibits a single strong surface
plasmon peak at ∼530 nm which can be assigned to dipole plasmon resonance [9,23]. It is noticeable that plasmon peaks of all
the amine derivatives-stabilized IAuNPs are narrower than that of
CTAB-stabilized IAuNPs, revealing the effective exchange of CTAB
with amine compounds. Such the variation of bandwidth of sur-
147
face plasmon absorption has been frequently observed for ligand
exchange processes [24].
The particle structures and size distributions can be determined
by TEM measurements. Fig. 2 shows the TEM images of CTAB- and
amine-coated IAuNPs. As shown in the figure, the prepared particles have hexagonal morphology which is a typical projected image
of icosahedral particles [9]. All the particles have well-defined
edges and corners, and they have generally sharper surface features than spheres. Furthermore, size and shape distributions are
very narrow. The measured mean particle sizes were mostly around
50–51 nm; IAuNPs@CTAB (50.4 ± 1.2 nm) (Fig. 2a), IAuNPs@pyrrole
(50.8 ± 1.2 nm) (Fig. 2b), IAuNPs@pyridine (50.4 ± 1.8 nm) (Fig. 2c),
IAuNPs@2,2 -bpy (50.5 ± 1.4 nm) (Fig. 2d), and IAuNPs@4,4 -bpy
(50.9 ± 1.5 nm) (Fig. 2e). These results clearly show that the structural properties of IAuNPs are same among the samples regardless
of the type of stabilizers. On the basis of this fact, catalysis experiments with these particles might exclusively demonstrate the
effect of stabilizers on the catalytic activity of nanoparticles.
To investigate the effect of stabilizer on the catalytic activity of
IAuNPs, the catalytic reduction of 4-NP by NaBH4 was examined.
4-NP is inert to NaBH4 if it is used alone. However, the metal
nanoparticles can effectively catalyze the reduction of 4-NP by
acting as an electron relay system; electron transfer takes place
between 4-NP and NaBH4 through the metal particles. The reaction can be readily monitored by using UV–vis spectroscopy. In
NaBH4 medium, the peak corresponding to 4-NP at 317 nm is
red-shifted to 400 nm due to the formation of 4-nitrophenolate.
The reduction can be visualized by the disappearance of the
400 nm peak with the concomitant appearance of a new peak at
300 nm after the catalytic reaction. This peak has been attributed
to 4-aminophenol (4-AP) [15–17]. Fig. 3 shows successive UV–vis
absorption spectra of the reduction of 4-NP in the presence of
IAuNPs@CTAB, IAuNPs@pyrrole, IAuNPs@pyridine, IAuNPs@2,2 bpy, and IAuNPs@4,4 -bpy. In the absence of IAuNPs, the peak
associated with the 4-nitrophenolate remained unaltered. The
addition and proper mixing of an aliquot of IAuNPs to the reaction
mixture cause the fading and ultimate bleaching of the yellow
color of the 4-NP solution. This was reflected in the UV–vis
spectra (Fig. 3); the intensity of the 400 nm peak decreases
while a new peak at 300 nm corresponding to 4-AP appears.
This shows that the IAuNPs catalyze the reduction reaction.
Because the amount of the nanoparticles added is very small,
the absorption spectra of 4-NP is hardly interfered with that of
the AuNPs. Since the concentration of borohydride ion (BH4 − )
used for the reaction largely exceeded the concentration of 4-NP
and the IAuNPs, pseudo-first-order kinetics with respect to the
concentration of 4-NP could be applied to estimate the rate
constant. Fairly good linear relationships were found between ln
A and reaction time (see insets of Fig. 3). Here, A represents the
absorbance at 400 nm at any reaction time. The pseudo-first-order
rate constants determined from these plots are 8.00 × 10−3 ,
2.16 × 10−2 , 2.42 × 10−2 , 2.54 × 10−2 , and 4.30 × 10−2 min−1 for
the reaction in the presence of IAuNPs@CTAB, IAuNPs@pyrrole,
IAuNPs@pyridine, IAuNPs@2,2 -bpy, and IAuNPs@4,4 -bpy,
respectively (Fig. 4). The reaction rate highly depends on the
type of stabilizer on metal surface; the catalytic activities of
nanoparticles are in the order of IAuNPs@4,4 -bpy > IAuNPs@2,2 bpy > IAuNPs@pyridine > IAuNPs@pyrrole > IAuNPs@CTAB.
The
reaction with IAuNPs@4,4 -bpy was fastest and its reaction rate
was about 5 times higher than that of IAuNPs@CTAB. A still
doubtful point is whether surface-bound or free ligands can reduce
4-NP or not. To check this possibility, 4-NP reduction reactions
in the presence of amine compounds were carried out under
identical experimental conditions without IAuNPs. The absorbance
at 400 nm of 4-nitrophenolate was not changed by free ligands,
indicating that added amine additives themselves could not reduce
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K.Y. Lee et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 372 (2010) 146–150
Fig. 2. TEM images of the IAuNPs stabilized with (a) CTAB, (b) pyrrole, (c) pyridine, (d) 2,2 -bpy, and (e) 4,4 -bpy. The scale bars represent 50 nm for each image.
the 4-NP. As shown in Fig. 3, the overall reduction process is rather
slower than those observed in the previous studies [16,17]. This
may be ascribed to the relatively larger particle size of IAuNPs
than those of the other AuNPs. In fact, we have chosen IAuNPs
with average size of 50 nm due to their excellent homogeneity
in particle shape and size and also to their invariant structural
properties regardless of the type of stabilizers (Fig. 2).
In the previous studies on the catalytic activity of metal
nanoparticles, it was established that the catalytic activity
was related to binding strength between metal surface and
ligand [25,26], capping ligand stabilization [8,27], and surfaceto-volume ratio [28]. The enhanced catalytic activities of the
amine derivatives-stabilized IAuNPs can be attributed to their
surface characteristics. The diffusion of anionic reactants, i.e., 4-
K.Y. Lee et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 372 (2010) 146–150
149
Fig. 3. Successive UV–vis absorption spectra (8 min interval) of 4-NP by NaBH4 in the presence of (a) IAuNPs@CTAB, (b) IAuNPs@pyrrole, (c) IAuNPs@pyridine, (d) IAuNPs@2,2 bpy, and (e) IAuNPs@4,4 -bpy. The insets show the pseudo first-order plots of −ln A vs time for each sample.
Fig. 4. Plot of rate constant for the reduction of 4-NP vs type of stabilizer.
nitrophenolate and borohydride, to the surface of catalyst particles
can more readily proceed in the case of amine ligand than in the
case of CTAB because the long alkyl chains of CTAB should hinder
the incorporation of ions onto the surface of IAuNPs, whereas amine
stabilizer should induce the close contact between reactant metal
surface. Since the rate-determining step in the electron-transfer
reaction catalyzed by metal nanoparticles is the approach of reactants to the particle surface [29,30], the different ligand chemistry
could differentiate the catalytic activities between nanocatalysts.
On the other hand, the highest catalytic activity of IAuNPs@4,4 bpy among the amine derivative-stabilized IAuNPs can be ascribed
to the structure of adsorbates on the surface of the particles. Due
to the strong affinity of nitrogen to Au surfaces, pyrrole, pyridine,
and 2,2 -bpy take upright orientations on the particle surfaces with
all nitrogen atoms pointing towards the Au surface [11–13]. In the
case of 4,4 -bpy, the molecules adsorb on the Au surface vertically
via only one N atom, with the other N atom pointing outward
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K.Y. Lee et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 372 (2010) 146–150
with respect to the Au surface [14]. As a result, the outermost
surface of the 4,4 -bpy-stabilized particles should provide more
polar environment than other amine compound-stabilized particles, thus facilitating the approaching of ionic reactant species onto
the nanoparticle surfaces, and consequently enforcing the catalytic
activity. All the above experimental results demonstrate unambiguously that the molecular structure of the capping molecule
plays a decisive role for the efficient AuNPs-mediated electron
transfer between charged reactants.
4. Conclusions
We have investigated the effect of stabilizers, i.e., CTAB, pyrrole,
pyridine, 2,2 -bpy, and 4,4 -bpy, on the catalytic activity of the Au
nanoparticles for the reduction reaction of 4-NP with NaBH4 . The
reaction occurs faster in the presence of amine derivative-stabilized
AuNPs than in the presence CTAB-stabilized particles. The reaction
with IAuNPs@4,4 -bpy was fastest and its reaction rate was about 5
times higher than that of IAuNPs@CTAB. These experimental results
show that the structure of the capping ligands plays a crucial role
for the efficient nanoparticle-catalyzed electron transfer reactions.
Acknowledgements
This work was supported by Basic Science Research Program
(KRF-2008-313-C00415, 2008-0062042), Future-based Technology
Development Program (Nano Fields) (2009-0082640), and Pioneer
Research Center Program (2009-0082813) through the National
Research Foundation (NRF) funded by the Korean government
(MEST).
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