Synthesis, characterization and catalytic application of Au NPs-reduced graphene oxide composites material: An eco-friendly approach

Article abstract of DOI:10.1016/j.catcom.2013.06.021

In this paper, we have reported the synthesis of Au NPs-reduced graphene oxide (rGO) composites under microwave irradiation using ascorbic acid as eco-friendly reducing agent. The formation of Au NPs-rGO composites was confirmed by UV/Visible spectroscopy

Full text of DOI:10.1016/j.catcom.2013.06.021

Catalysis Communications 40 (2013) 139–144
Contents lists available at SciVerse ScienceDirect
Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Short Communication
Synthesis, characterization and catalytic application of Au NPs-reduced
graphene oxide composites material: an eco-friendly approach
Ponchami Sharma a, Gitashree Darabdhara a, Tallapareddy Muralikrishna Reddy a, Ashwini Borah a,
a
a
a,
Pranjal Bezboruah b, Pranjal Gogoi b, , Najrul Hussain , Pinaki Sengupta , Manash R. Das
⁎
⁎
a
Materials Science Division, CSIR-North East Institute of Science and Technology, Jorhat-785006, Assam, India
Medicinal Chemistry Division, CSIR-North East Institute of Science and Technology, Jorhat-785006, Assam, India
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 25 April 2013
Received in revised form 16 June 2013
Accepted 18 June 2013
Available online 22 June 2013
In this paper, we have reported the synthesis of Au NPs-reduced graphene oxide (rGO) composites under
microwave irradiation using ascorbic acid as eco-friendly reducing agent. The formation of Au NPs-rGO com-
posites was confirmed by UV/Visible spectroscopy, DRIFT spectroscopy, TGA, XRD analysis, SAXS and HRTEM.
HRTEM analysis results show that the particle size of the Au NPs formed on rGO nanosheets were in the range
of 2-20 nm which is also supported by SAXS analysis results. The synthesized Au NPs-rGO composites were
successfully utilized for the aerobic oxidation of benzyl alcohols to their corresponding aldehydes.
© 2013 Elsevier B.V. All rights reserved.
Keywords:
Graphene oxide
Gold nanoparticle
Ascorbic acid
Aerobic oxidation
1. Introduction
A number of methods have been reported for synthesis of metal
NPs-graphene composites. Among them solution chemistry approach
Graphene, the single-layered two-dimensional allotrope of car-
bon, has created a new renaissance in the field of nanoscience and
nanotechnology [1,2]. The closely packed honeycomb of sp2 hybrid-
ized carbon network enriches graphene with extraordinary electrical,
thermal and mechanical properties [3–9]. Graphene-based materials
modified with metal nanoparticles (NPs) have in particular got inter-
est for the development of advanced materials for a number of appli-
cations. However, the individual graphene sheets have a tendency to
restack to form layered graphite structure due to strong van der
Waals interaction. Insertion of the metal NPs into the graphene sheets
is proved to be very effective in preventing such restoring. In view of
this recent research, efforts have focused on the synthesis of metal
NPs-graphene oxide (GO)/graphene composite materials [10]. Metal-
lic NPs are important in different area such as catalysis, microelec-
tronics, light emitting diodes, photovoltaic cells, surface-enhanced
Raman scattering (SERS) or localized surface plasmon resonance
(LSPR) and also in medical or biological applications [11]. Moreover,
metal NPs show changes in their electronic, optical and catalytic proper-
ties, depending on the method of synthesis [12]. Thus, insertion of metal
NPs onto graphene-based matrix represents an important study for the
exploration of their properties and applications.
[13–17], electrochemical deposition of metal NPs onto graphene
sheets [18], ultrasonication [19], microwave irradiation [20], etc., are
most widely used. Typically, most of the method involves the simul-
taneous reduction of the metal salt and GO nanosheets [13–17].
NaBH4 and hydrazine hydrate are most widely used reducing agent
for the synthesis of the metal NPs-GO/reduced graphene oxide
(rGO) composite materials. However, toxicity is the major disadvan-
tage of these two reagents. Nowadays, searching for an efficient, less
toxic, eco-friendly reducing agent is of great significance for the prep-
aration metal NPs-GO/rGO composite materials. Ascorbic acid is an
environment friendly reducing agent, which is widely used in redox
reactions. The excess acid can be easily removed from the reaction
mixture by decomposition using H2O2.
The oxidation of benzyl alcohol is an important reaction for the
production of benzaldehyde, which is widely used in perfumes, phar-
maceuticals, dyestuffs, agrochemicals and as plastic additives [21].
Benzaldehyde can be obtained by many processes as liquid phase
chlorination and oxidation of toluene, partial oxidation of benzyl
alcohol, alkali hydrolysis of benzal chloride and the carbonylation of
benzene. However, most of the process provides benzaldehyde with
trace contamination from chlorine, which is not suitable in perfumes
and pharmaceuticals. On the other hand, in the catalytic vapor-phase
oxidation of benzyl alcohol to benzaldehyde [22], a very significant
carbon loss in the form of carbon oxides is a major problem. A few
processes have been reported for the liquid phase oxidation of benzyl
alcohol to benzaldehyde over Pd [23], heteropoly acid [24] and
⁎
Corresponding authors. Fax: +91 376 2370011.
E-mail addresses: gogoipranj@yahoo.co.uk (P. Gogoi), mnshrdas@yahoo.com
(M.R. Das).
1566-7367/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.catcom.2013.06.021

140
P. Sharma et al. / Catalysis Communications 40 (2013) 139–144
Ni-Al-hydrotalcite [25], using molecular oxygen or aqueous H2O2 as
terminal oxidant. Recently, Au NPs is considered as the catalyst of
choice for the oxidation of benzyl alcohol [26–32]. Although these
methods often show excellent activity, many of these are associated
with several shortcomings such as low yield, use of organic solvent,
transition-metal, etc., there is a need to develop a convenient and
more active catalyst for the benign oxidation of benzyl alcohol. In this
paper, we report the catalytic application of Au NPs-rGO for the oxida-
tion of benzyl alcohol to benzaldehyde under organic-solvent-free con-
dition, where O2 is used as an oxidant (Scheme 1). The Au NPs-rGO
composites were synthesized using eco-friendly ascorbic acid as a
reducing agent under microwave irradiation which avoids the use of
toxic reducing agents such as NaBH4, hydrazine hydrate, etc.
3. Results and discussion
3.1. Characterization of GO nanosheets
The complete characterization of the synthesized GO nanosheets
from graphite powder was reported in the previously published
paper [14].
3.2. Characterization of AuNP-rGO composites and rGO nanosheets
3.2.1. UV/Visible spectroscopy
The formation of Au NPs on the rGO nanosheets by microwave
irradiation using ascorbic acid as reducing agent was primarily mon-
itored by UV/Visible spectroscopy as shown in Fig. 1. GO nanosheets
shows two characteristic peaks at 241 nm and 308 nm due to
π → π* and n → π* transitions, respectively. The n → π* peak disap-
pears upon reduction of GO to rGO. Similar observation was also
obtained in the UV/Visible spectra of AuNPs-rGO composites. The ap-
pearance of a characteristic surface plasmon resonance (SPR) band at
545 nm indicates the formation of Au NPs on the rGO nanosheets.
2. Experimental
2.1. Preparation and characterization of GO nanosheets
GO nanosheet was synthesized from powder graphite adopting
the Hummer and Offeman [33] method. A detailed synthesis method
of GO nanosheets is discussed in previously published paper [14].
3.2.2. Thermogravimetric analysis
The simultaneous conversion of GO to rGO nanosheets by ascorbic
acid with the formation of Au NPs is reflected in the thermogravimetric
(TGA) curves. It is observed from the Fig. 2 that the thermal stability of
GO nanosheet is much lower than the raw graphite. The main weight
loss takes place around 200 °C (37.72%) due to pyrolysis of the labile
oxygen-containing functional groups [13]. The weight loss in this region
decreases up to 17.17% upon formation of Au NPs-rGO composites due
to the reduction of these oxygen containing functional groups. More-
over, the weight loss associated with high temperature pyrolysis of Au
NPs-rGO composites above 600 °C is similar to that for graphite and
rGO, which results from pyrolysis of the carbon skeleton of rGO
nanosheets [13].
2.2. Preparation of Au NPs-rGO composites and rGO nanosheets
The Au NPs-rGO composites material was synthesized by using
environment friendly, nontoxic reducing agent, ascorbic acid under
microwave irradiation. The detailed experimental procedure for syn-
thesis of Au NPs-rGO composites material is described in supporting
information. Synthesis of rGO was also carried out adopting the
same procedure without adding Au-salt to have better understanding
of reduction of GO nanosheets during the formation of Au NPs-rGO
composites.
2.3. Catalytic experiments
3.2.3. X-ray diffraction analysis
The liquid phase oxidation was carried out in an ACE pressure tube
filled with oxygen. Water suspension of Au NPs-rGO composites
(8.1 mg in 2 mL), benzyl alcohol (1 mmol) and K2CO3 (3 mmol)
were placed in a glass ACE pressure tube (10 mL) with a magnetic
stirrer bar. The tube was sealed with a rubber O-ring and threaded
Teflon seal under O2, and the entire mixture was vigorously stirred
at 130 °C (bath temperature) for 4 h. The reaction mixture was
cooled to room temperature and the organic product was extracted
with ethyl acetate (3 × 10 mL). The aqueous phase was separated
and could be used consecutively for another three times for the oxi-
dation of benzyl alcohols (1st recycle 73%, 2nd recycle 74% and 3rd
recycle 70% benzaldehyde was obtained). The upper organic phase
was dried and concentrated under reduced pressure. The product
was purified by column chromatography.
The formation of the Au NPs on rGO nanosheets is further charac-
terized by X-ray diffraction (XRD) analysis as shown in Fig. 3. The
XRD pattern of GO shows a characteristic peak at 2θ value of 11.72.
The intensity of this peak considerably decreases upon formation of
rGO, and two new peaks appear at 2θ value of 23.82 and 42.62. Sim-
ilar observation was also found in the XRD pattern of Au NPs-rGO
composites. Moreover, the prominent peaks in the XRD pattern of at
2θ values of about 38.08°, 44.34°, 64.50°, 77.48° and 81.68° corre-
sponding to d-spacing of the Au NPs are 2.36, 2.04, 1.44, 1.23 and
1.18 Å are assigned to the (111), (200), (220), (311) and (222) crys-
tallographic planes of cubic Au NPs, respectively (JCPDS card No
004-0784). The high intense diffraction peak observed at 38.08°, cor-
responding to the crystalline Au, confirms that the NPs are composed
of pure crystalline Au. The crystallite size of the Au NPs is calculated
O2
Aqueous Suspension of
AuNPs-rGO
CHO
OH
Ace pressure tube
130 oC, 4h
R
R
(external bath temperature)
R= H, OMe, Cl, NO2
Scheme 1. Au NPs-rGO-catalyzed aerobic oxidation of benzyl alcohol.

P. Sharma et al. / Catalysis Communications 40 (2013) 139–144
141
(111)
256 nm
241 nm
AuNPs-rGO
(200)
(220)
545 nm
(311)
(222)
rGO
(002)
308 nm
Au NPs-rGO
GO
(OO1)
rGO
GO
300
400
500
600
700
800
900
1000
Wavelength (nm)
Fig. 1. UV/visible spectra of GO, rGO and Au NPs-rGO composites in water.
by Sherrer equation using PDXL software and found to be ~20 nm,
which is also supported the TEM analysis results.
20
40
60
80
2θ (degree)
3.2.4. Small-angle X-ray scattering analysis
Fig. 3. Powder XRD diffractogram of GO, rGO and Au NPs-rGO composites.
Small-angle X-ray scattering (SAXS) is another technique used to
understand the particle size distribution of the Au NPs on the rGO
nanosheets. The SAXS intensities of the Au NPs on rGO were plotted
as a function of the scattering vector, Q (shown in Fig. 4). The scattering
vector (Q) is defined in terms of scattering angles, θ, and wavelength, λ,
of the radiation (Q = 4π sinθ/λ). The particle size distribution profile of
Au NPs is shown in Fig. 4 inset. The average particle size was found to be
~27 nm.
and 289.90 eV can be assigned to C=C, C−C, C−O, C=O and O−C=
O, respectively. It is observed from the Fig. 5(a) that there is a decrease
of the intensity of the peak of C−O, C = O and O−C=O compared to
GO nanosheets, indicating the removal of oxygen containing functional
groups of the Au NPs-rGO composites after reduction of the GO
nanosheets in presence of Au NPs. The Au 4f spectrum was also detected
and the binding energies of the Au 4f7/2 and Au 4f5/2 electrons of Au° are
found to be at 82.2 and 85.8 eV (peak-to-peak distance 3.6 eV) con-
forms presence of Au° state particles (shown in Fig. 5(b)) [34].
3.2.5. X-ray photoelectron (XPS) analysis
XPS characterization of the GO nanosheets before and after depo-
sition of the Au NPs was performed to understand the surface chem-
istry of the rGO nanosheets. This characterization also provides the
information of reduction of the oxygen containing functional group
and AuCl4− ions. GO nanosheets shows bands at 285 and 530 eV
due to C1s and O1s, respectively (data not shown). It is described in
our recent publication about the C1s XPS core level spectrum of GO
nanosheets, and it can be deconvoluted into three components with
binding energies at about 283.90, 285.90 and 287.81 eV assigned to
C−C/C−H, C−O and C = O species, respectively [15]. This result
indicates that the GO nanosheets contain large number of carboxyl,
carbonyl, hydroxyl and epoxide functional groups. On the other
hand, the C1s XPS core level spectrum of Au NPs-rGO composites is
depicted in Fig. 5(a), and the peak at 284.47, 285.40, 286.60, 288.13
3.2.6. Elemental analysis
The chemical constituents of GO, rGO and Au NPs-rGO composites
were evaluated by CHN analysis (Table 1). It is observed from the
Table 1 that the percentage of carbon and hydrogen content consider-
ably increases upon formation of rGO which even increases more in
case of Au NPs-rGO composites indicating better reduction of GO in
presence of Au NPs [35].
5
10
0.035
0.030
4
3
2
1
0
0.025
0.020
0.015
0.010
0.005
0.000
10
10
10
10
10
99
96
93
90
Graphite
87
100
80
60
40
0
20
40
60
80
100
120
rGO
20
Particle diameter (nm)
40
30
20
10
GO
0
100
80
60
40
20
AuNP_rGO
0.0071
0.1071
0.2071
0.3071
0.4071
0.5071
100
200
300
400
500
600
700
800
900
1000
Q(1/ang.)
Temperature (oC)
Fig. 4. SAXS curve of intensity against scattering vector (Q). Inset: particle size distribu-
tion profile of Au NPs on rGO nanosheets.
Fig. 2. TGA curve of graphite, rGO, GO and Au NPs-rGO composites.

142
P. Sharma et al. / Catalysis Communications 40 (2013) 139–144
284.47
4f7/2
a
b
4f5/2
286.60
285.40
288.13
289.92
280 282 284 286 288 290 292 294 296
Binding Energy (eV)
78
80
82
84
86
88
90
92
94
Binding Energy (eV)
Fig. 5. XPS spectra of (a) C1s XPS spectra of AuNPs-rGO composites, and (b) Au4f core level XPS spectrum of AuNPs-rGO composites.
Table 1
and fringes of rGO nanosheets are resolved at few regions. The
measured fringe lattice of Au NPs corresponds to the (111) plane is
found to be 0.19 nm (Inset Fig. 6e). Fig. 6f shows that the diffraction
dots are resolved in the selected area of electron diffraction (SAED)
images, implying the crystalline nature of the Au NPs on the rGO
nanosheets.
⁎
CHN analysis of GO, rGO and AuNP-rGO composites.
Materials
Elemental content (wt%)
C/O molar ratio
C
H
O
GO
rGO
44.00
65.46
66.10
2.96
0.93
0.98
53.02
33.54
32.87
1.11
2.60
2.69
Au NP_rGO composites
⁎
As-received basis.
3.3. Catalytic application of Au NPs-rGO composites
The catalytic application of the synthesized Au NPs-rGO composites
towards the oxidation of alcohol has been investigated. A variety of ben-
zyl alcohols are smoothly oxidized to their corresponding aldehydes
with high yields. During the reaction, other functional groups such as
alkyl, methoxy and chloro remain intact (Table 2). The influence of
the reaction conditions (viz. reaction time and temperature) on the pro-
cess performance has also been studied and presented in Table 3. It is in-
teresting to note that benzyl benzoate is the only side product formed.
Benzoic acid is responsible for the formation of this side product,
which is formed due to the over-oxidation of benzaldehyde. The
3.2.7. High resolution transmission electron microscope analysis
The morphology and structure of the Au NPs-rGO composites
were investigated by high resolution transmission electron micro-
scope (HRTEM). It is observed from Fig. 6a that the rGO nanosheets
are non-uniformly folded, and Au NPs are distributed on that folded
rGO sheets. Fig. 6(b–d) shows that a well-dispersed high density of
Au NPs of the size range from 2 to 20 nm is deposited onto rGO
nanosheets. The HRTEM images of Au NPs embedded on the rGO
nanosheets are shown in Fig. 6e. The crystal lattice of the Au NPs
Fig. 6. TEM images of Au NPs on rGO nanosheets synthesized using 1 × 10−3 mol dm−3 HAuCl4 solution by ascorbic acid reduction process under microwave irradiation: (a–d) TEM
images, (e) HRTEM images along with fringe spacing (0.19 nm) in enlarged form, (f) SAED image of the Au NPs (1 × 10−3 mol dm− HAuCl4 solution).

P. Sharma et al. / Catalysis Communications 40 (2013) 139–144
143
Table 2
4. Conclusion
Au NPs-rGO-catalyzed the oxidation of benzyl alcohols to their corresponding carbonylsa.
d
Au NPs-rGO composites were successfully synthesized using en-
vironment friendly reducing agent, ascorbic acid. The well-dispersed
spherical Au NPs on rGO nanosheets have a size range of ~2-20 nm.
The composites material show promising catalytic activity towards
the halogen- and solvent-free oxidation of substituted benzyl
alcohols by O2 to their corresponding aldehydes with good selectiv-
ity and yields.
Entry
Substrate (1a–e)
Product (2a–e)b
Yield (%)c
TOF (h−1
)
1
2
3
4
5
PhCH2OH (1a)
PhCHO (2a)
75
77
73
67
65
524.04
531.01
510.06
468.14
454.16
4-MeOPhCH2OH (1b)
4-MePhCH2OH (1c)
4-ClPhCH2OH (1d)
3-NO2PhCH2OH (1e)
4-MeOPhCHO (2b)
4-MePhCHO (2c)
4-ClPhCHO (2d)
3-NO2PhCHO (2e)
a
Benzyl alcohol (1 mmol), aqueous suspension of Au-NP-rGO (2 mL), K2CO3
(4 mmol); the oxidation was carried out in an ACE pressure tube filled with oxygen
at 130 °C.
Acknowledgements
b
The products were characterized by comparison of physical and spectral data.
Isolated yield.
TOF: turn over frequency.
c
d
The authors thank the Department of Science and Technology,
New Delhi, for financial support and also the Director, CSIR-North
East Institute of Science and Technology, Jorhat, India, for the inter-
est in this work. P. Sharma and P. Bezboruah are thankful to CSIR,
New Delhi, and UGC, New Delhi, respectively, for senior research
fellowship grant. The authors also acknowledge SAIF, NEHU,
Shillong, for TEM images and Dr. K. R. Patil, Scientist, NCL, Pune for
XPS analysis.
formation of this side product can be considerably lowered by reducing
the reactive temperature and/or the reaction time. It was observed that
with increase in reaction period and temperature, the benzyl alcohol
conversion is increased. However, there is a small decrease in the selec-
tivity for benzaldehyde due to the formation of benzyl benzoate. Results
of the Au NPs-rGO composites catalyzed the oxidation of benzyl
alcohols to their corresponding aldehydes are summarized in Table 2.
The catalytic performance of the Au NPs-rGO is compared with earlier
supported Au catalyst is presented in Table 4. Among all the reported
catalyst, the Au NPs-rGO, using O2 as the oxidizing agent, showed
highest yield of benzaldehyde. TEM and SAXS investigation (shown in
supplementary materials, Figs. S2 and S3) also shows that the morphol-
ogy and size distribution of the Au NPs on the rGO nanosheets remain
unchanged after the performance of the catalytic reaction.
Appendix A. Supplementary data
Supplementary data to this article can be found online at http://
dx.doi.org/10.1016/j.catcom.2013.06.021.
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