Mussel-inspired green synthesis of silver nanoparticles on graphene oxide nanosheets for enhanced catalytic applications

Article abstract of DOI:10.1039/c3cc00115f

We report a facile green approach to the synthesis of silver nanoparticles (Ag NPs) on the surface of graphene oxide nanosheets functionalized with mussel-inspired dopamine (GO-Dopa) without additional reductants or stabilizers at room temperature. The resulting hybrid Ag/GO-Dopa exhibits good dispersity and excellent catalytic activity in the reduction of nitroarenes.

Full text of DOI:10.1039/c3cc00115f

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Mussel-inspired green synthesis of silver nanoparticles
on graphene oxide nanosheets for enhanced catalytic
applications†
Eun Kyung Jeon,a Eunyong Seo,a Eunhee Lee,a Wonoh Lee,b Moon-Kwang Umb
and Byeong-Su Kim*a
Cite this: DOI: 10.1039/c3cc00115f
Received 6th January 2013,
Accepted 12th February 2013
DOI: 10.1039/c3cc00115f
www.rsc.org/chemcomm
We report a facile green approach to the synthesis of silver nano- have been widely exploited in various fields ranging from versatile
particles (Ag NPs) on the surface of graphene oxide nanosheets surface modifications, biomineralization, and even to Li-ion
functionalized with mussel-inspired dopamine (GO-Dopa) without battery. In particular, its redox-active catechol group can undergo
additional reductants or stabilizers at room temperature. The resulting spontaneous oxidation to quinone under mild solution conditions,
hybrid Ag/GO-Dopa exhibits good dispersity and excellent catalytic thus serving as a reducing agent to reduce the metal precursor
activity in the reduction of nitroarenes.
to NPs, as suggested in previous reports. For example, Lee and
co-workers demonstrated that Cu and Ag can form spontaneously
Graphene, a two-dimensional aromatic carbon monolayer, has on the dopamine and polydopamine-coated surfaces.15
recently received significant attention in various fields of science
In this communication, we report a facile synthetic approach
and engineering on account of its unique electrical, optical, thermal to integrating the mussel-inspired chemical motif of dopamine
and mechanical properties.1–5 Among various types of graphene and (Dopa) to the surface of GO to afford GO-Dopa with the aim of
related structures, solution processable graphene oxide (GO) typi- inducing the spontaneous formation of NPs on the surface of
cally prepared by chemical exfoliation possesses unique advantages GO at room temperature without the use of any additional
for many applications owing to its facile synthetic nature in a reducing agents or stabilizers. As a representative example, we
controlled, scalable, and reproducible manner.6–9 The abundant demonstrate the synthesis of Ag NPs on the surface of graphene
surface functional groups provide GO with excellent aqueous nanosheets. Furthermore, we exploit the excellent catalytic
dispersity and also offer opportunities for further chemical mod- activity of the resulting hybrid Ag/GO-Dopa toward the reduction
ifications. Taking full advantage of the surface functional groups, of a series of model nitroarenes.
as well as its large specific surface area and stability, GO nanosheets
To functionalize the dopamine molecules on the surface
are emerging as promising supports for graphene-based hybrid of GO, we initially prepared GO according to the modified
nanomaterials for a variety of applications, including sensors, Hummers method.16,17 The catechol group in dopamine was
optical and electronic devices, energy conversion and storage, selectively protected with an acetonide group (Dopa*) to prevent
and catalysts. Toward this end, recent studies have endeavored to undesirable side reactions such as the self-polymerization of
anchor metal, metal oxide, semiconducting, and magnetic nano- dopamine that might hinder the attachment of catechol on the
particles (NPs) on the surface of GO either by hosting preformed GO.18,19 After the preparation of Dopa*, the free amine group of
NPs on graphene or by growing NPs in situ with the metal precursor Dopa* reacted with the carboxylic acid and/or epoxide groups on
in the presence of graphene nanosheets. However, many of the surface of GO through an N-ethyl-N0-(3-dimethyl aminopropyl)-
these approaches still require additional reducing agents carbodiimide methiodide (EDC)-mediated reaction, followed by
and/or stabilizers with proper thermal treatment.10–14
Inspired by the adhesive properties of 3,4-dihydroxy-L- afford the GO-Dopa with free catechol groups (Fig. 1).
phenylalanine (dopamine), which is found abundantly in marine FT-IR spectroscopy clearly indicates the successful surface
the subsequent deprotection using trifluoroacetic acid (TFA) to
mussel foot proteins, dopamine and its polymer, polydopamine, functionalization of GO to GO-Dopa (Fig. 2a). For example, GO
shows a broad peak at 3300 cmꢀ1 arising from the abundant
a Interdisciplinary School of Green Energy, KIER-UNIST Advanced Center for Energy,
Ulsan National Institute of Science and Technology (UNIST), Ulsan 689-798,
Korea. E-mail: bskim19@unist.ac.kr
b Composites Research Center, Korea Institute of Materials Science, 797 Changwon-
daero, Changwon, Gyungnam 642-831, Korea. E-mail: wonohlee@kims.re.kr
† Electronic supplementary information (ESI) available: Details of experimental
procedures and additional data. See DOI: 10.1039/c3cc00115f
O–H stretching vibrations. Moreover, the peaks due to CQO
stretching, and conjugated CQC are visible at 1730 and
1630 cmꢀ1, respectively.20 After the reaction of GO with Dopa*,
a peak appeared at 2900 cmꢀ1, which corresponds to C–H
stretching of the alkyl group of dopamine, while the intensity
of the peak at 1730 cmꢀ1 decreased and shifted because of the
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Fig. 1 (a) Synthetic route to acetonide-protected dopamine (Dopa*). (i) N-carbethoxy-
phthalimide, TEA, MeOH. (ii) p-TSA, benzene. (iii) Hydrazine hydrate, DCM, MeOH.
(b) Schematic representation of the preparation of GO-Dopa and Ag/GO-Dopa.
Fig. 3 Representative AFM and TEM images of Ag/GO-Dopa: (a) GO, (b) GO-
Dopa, (c–e) Ag/GO-Dopa, with an inset image of the Ag NPs. (f) Size distribution
histogram of the Ag NPs. Scale bar in AFM images is 2.5 mm.
graphene sheets possibly due to the nucleation sites such as
carboxylic acid groups coordinating with Ag+ ions, the Ag NPs
generally featured a wide size distribution, low crystallinity, and
poor colloidal stability; this clearly indicates the importance of the
controlled growth mediated by the dopamine motif (Fig. S1 in ESI†).
According to transmission electron microscopy (TEM)
images, we confirmed that Ag NPs with an average particle size
of 7.71 ꢁ 1.34 nm are well dispersed on the GO-Dopa sheets
Fig. 2 (a) FT-IR and (b) UV/vis spectra of GO (black), GO-Dopa* (red), GO-Dopa
(blue) and Ag/GO-Dopa (green).
formation of the amide bond (i.e.–CONH–) on the surface of GO. (Fig. 3d and e). We also found no free Ag NPs formed outside
Because of their structural similarities, the spectra of GO-Dopa the graphene sheet. The Ag NPs are crystalline with lattice
and GO-Dopa* are very similar, however, more pronounced O–H spacings of 1.20 and 2.36 Å, which correspond to the (311) and
stretching vibration is observed upon the regeneration of (111) peaks, respectively. The edges and basal planes of the
the catechol groups using TFA. Further characterization by graphene sheets are all decorated with Ag NPs, although there
UV/vis spectroscopy was performed to analyze the surface func- is relatively higher density at the edges; this observation can be
tionalization of GO (Fig. 2b). Interestingly, the absorbance attributed to the higher distribution of surface functional
maxima (lmax) of functionalized GO blue-shifted from that of groups such as carboxylic acid groups at the edges that
GO (lmax = 230 nm). The conjugation of aromatic dopamine are functionalized with dopamine. In addition, XRD analysis
contributed to the shift of the lmax of GO-Dopa* and GO-Dopa, supports the successful formation of Ag NPs with a broad peak
confirming the successful surface functionalization of GO.
corresponding to the GO nanosheet at around 201 (Fig. S2 in
Atomic force microscopy (AFM) images support the function- ESI†). ICP/MS measurement confirmed that approximately
alization of the surfaces of the GO sheets. The GO-Dopa sheets 2.4% of the Ag/GO-Dopa is composed of Ag NPs. Furthermore,
are thicker than the GO sheets owing to the surface functiona- we also demonstrated that the distribution and sizes of the Ag
lization of dopamine. It is also interesting to note that the NPs could be controlled by changing the reaction time, solution
GO-Dopa sheets were stacked due to the presence of the surface pH, or concentration of the Ag precursor (Fig. S3 in ESI†).
anchoring catechol groups, increasing the adhesion of each
After careful characterization of the hybrid Ag/GO-Dopa,
graphene sheet. Once the stable GO-Dopa suspension was it was employed for the catalytic reduction of a series of nitroarenes
prepared, Ag NPs were easily formed in situ by simple mixing in the presence of NaBH4 because of the well-known catalytic activity
with aqueous AgNO3 solution at room temperature for 3 h. of Ag NPs in this reaction. First, we studied the reduction of
Individual Ag NPs with a height of 3.30 nm were deposited on 4-nitrophenol (4-NP) to 4-aminophenol (4-AP) to evaluate the cataly-
the functionalized GO sheets (Fig. 3c).
tic activity of hybrid Ag/GO-Dopa. The successful conversion of the
As suggested in previous reports, the catechol groups of reaction was monitored via UV/vis spectroscopy; specifically, the
dopamine oxidize to quinone, which offers the electrons to absorption peak of 4-NP at 400 nm decreases concomitantly with
reduce Ag ions to form Ag NPs on the surface of GO-Dopa. the increase in a new absorption peak at 300 nm, which indicates the
Moreover, the GO-Dopa nanosheet also plays a role as a formation of the reduced product, 4-AP. From the UV/vis spectra, the
stabilizer for anchored Ag NPs, preventing the reaggregation pseudo-first-order reaction kinetics were applied to determine
of the resulting Ag NPs. We observed that the resulting the reaction rate constant. From the linear relation of ln(Ct/C0) with
Ag/GO-Dopa exhibits fairly good aqueous dispersity and stability t, we determined that the rate constant, k, for this reaction is
for several months. As a control experiment, pristine GO was also 0.364 minꢀ1 and the turnover frequency (TOF) is 14.6 minꢀ1, which
subjected to the identical reaction conditions in order to form Ag is significantly higher than those reported earlier (typically
NPs on the graphene sheet. Although Ag NPs did form on the 10ꢀ1 to 10ꢀ3) under similar reaction conditions (Fig. 4b).21,22
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a facile and general means of producing metal/graphene nano-
structures with different functionalities for various applications,
which is the subject of our ongoing endeavour.
We synthesized hybrid Ag/GO-Dopa catalysts using an Ag
precursor and the GO sheets functionalized with a mussel-
inspired biological motif, i.e. dopamine. The dopamine-
functionalized GO sheets acted as both the reducing agents
and stabilizers for the reduction of Ag ions at room tempera-
ture. The GO-Dopa sheets were covered with Ag nanoparticles
that possess good dispersity and stability in aqueous solution.
Hybrid Ag/GO-Dopa exhibits superior catalytic activity toward
the reduction of a series of nitroarenes because of the interplay
between the high surface area of the graphene nanosheets and
the catalytic activity of the Ag nanoparticles. We anticipate that
the approach presented in this study will provide a facile means
of preparing nanoparticles on graphene nanosheets for
advanced electronic, energy, and sensor applications.
Fig. 4 (a) Time-dependent UV/vis absorption spectra for the reduction of 4-NP
over Ag/GO-Dopa catalyst in aqueous media at 298 K. (b) Plot of ln(Ct/C0) versus
time for the reduction of 4-NP. Reaction conditions: 1.0 mol% catalyst and
800 equiv. NaBH4. (c) Plot of ln(Ct/C0) versus time for the reduction of 4-NP with
three different catalysts. (d) Plot of ln(Ct/C0) versus time for the reduction of 4-NP
at three different temperatures.
This research is supported by WCU Program through the
Korea Science and Engineering Foundation funded by the
MEST (R31-2008-000-20012-0), the NRF (2012R1A1A2040782),
and the Principal Research Program in the KIMS.
Notes and references
To elucidate the catalytic activity of Ag/GO-Dopa, we performed
control experiments using GO and GO-Dopa. The GO and
GO-Dopa showed a negligible catalytic activity with a rate
constant of 0.004 and 0.003 minꢀ1, respectively (Fig. 4c).
Although the catalytic activity of GO is noticeable, it is still
significantly less reactive than the hybrid Ag/GO-Dopa. We also
found that the catalytic activity of Ag/GO-Dopa is affected by the
temperature of the reaction (Fig. 4d). The rate constants
obtained at various temperatures were plotted to provide the
activation energy of the reaction; the Arrhenius plot revealed
the activation energy of approximately 40.0 kJ molꢀ1. This value
is considerably smaller than that of the Au/GO system for the
same reaction (85.9 kJ molꢀ1).23
We further investigated the catalytic activity of the Ag/GO-Dopa
composite towards the reduction of other nitroarenes. We found
that hybrid Ag/GO-Dopa retains excellent catalytic activity in the
reduction of a series of nitroarenes regardless of the types and
positions of the substituents (Fig. S4 in ESI†). We propose that
the high catalytic activity of Ag/GO-Dopa originates from three
factors: (1) the high surface area and enhanced adsorption of
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Finally, in order to demonstrate the versatility of this bio-
inspired approach, we synthesized NPs of other metals such as
Cu and Au on the surface of graphene nanosheets based on
the protocol described for Ag/GO-Dopa (Fig. S5 in ESI†). As
characterized by TEM and XRD, highly water-dispersible
Cu/GO-Dopa and Au/GO-Dopa were successfully prepared at room
temperature without using external reducing agents. This result
further highlights the potential of our bio-inspired approach to be
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