Efficient degradation of azo dyes using Ag and Au nanoparticles stabilized on graphene oxide functionalized with PAMAM dendrimers

Article abstract of DOI:10.1039/c3nj01050c

Herein, we report the stabilization of silver and gold nanoparticles (Ag/Au NPs) on graphene oxide (GO) functionalized with PAMAM dendrimers. The grafting of the PAMAM dendrimers on GO has been investigated using TGA and Raman spectral studies and the stabilization of the Ag/Au NPs on the dendritic structures has been confirmed using XRD, UV-Vis and FT-IR spectra, SEM and TEM studies. The catalytic activity of the prepared nanocatalysts towards the degradation of organic azo dyes, namely methyl orange and congo red, has been tested. The prepared nanocatalysts were found to exhibit excellent catalytic activity towards the complete degradation of both methyl orange and congo red within only a few seconds.

Full text of DOI:10.1039/c3nj01050c

NJC
View Article Online
View Journal | View Issue
PAPER
Efficient degradation of azo dyes using Ag and Au
nanoparticles stabilized on graphene oxide
functionalized with PAMAM dendrimers†
Cite this: New J. Chem., 2014,
3
8, 1551
a
b
a
Rajendiran Rajesh, S. Senthil Kumar and Rengarajan Venkatesan*
Herein, we report the stabilization of silver and gold nanoparticles (Ag/Au NPs) on graphene oxide (GO)
functionalized with PAMAM dendrimers. The grafting of the PAMAM dendrimers on GO has been
investigated using TGA and Raman spectral studies and the stabilization of the Ag/Au NPs on the
dendritic structures has been confirmed using XRD, UV-Vis and FT-IR spectra, SEM and TEM studies.
The catalytic activity of the prepared nanocatalysts towards the degradation of organic azo dyes, namely
methyl orange and congo red, has been tested. The prepared nanocatalysts were found to exhibit
excellent catalytic activity towards the complete degradation of both methyl orange and congo red
within only a few seconds.
Received (in Montpellier, France)
th September 2013,
Accepted 21st December 2013
4
DOI: 10.1039/c3nj01050c
www.rsc.org/njc
light-active photocatalysts, whereas their practical applications are
still limited since the quantum yield is still not satisfactory.
1
. Introduction
11,12
The textile manufacturing process involves the preparation and Thus, the development of a simple method/material for the
utility of various dyes, and partly introduces these dyestuffs into efficient degradation of azo dyes has gained enormous significance.
aquatic systems and thus leading to environmental pollution
and health hazards. Azo dyes have been extensively used in nanoparticles (NPs), as they exhibit enhanced catalytic activity
textile, leather and paper industries and constitute ca. 50% of due to their large surface to volume ratios.
Extensive research has been carried out to explore metal
1
–5
1
3–15
In particular,
all dyes produced and thus, the efficient degradation of azo dyes noble metal NPs have gained more significance because of their
6
has become a challenging task for environmental chemists. Many closely packed structure and valence bands in which electrons
1
6
different methodologies are being adopted to treat dye effluents, are free to move. Towards the interest and progress in
7
8
which include adsorption tactics, biological degradation,
heterogeneous catalysis, immobilization of these metal NPs
9
10
17
18
Fenton-like reactions, and photocatalysis. The adsorption onto solid supports, that include silica, indium tin oxide,
1
9
20
methodology resulted in the transfer of pollutants from waste alumina and insoluble copolymers, has received significant
water to solid wastes and the traditionally employed biological attention. However, immobilization of metal NPs into these solid
degradation delivered a poor efficiency. In the case of Fenton- surfaces tend to show less catalytic activity as a consequence of less
like reactions, the subsequent treatment of a ferrous slurry and interaction between the catalyst and the substrate. The interaction
2 2
also the requirement of H O led to complexity. The thorough of the substrate with catalyst becomes less predominant as the
degradation of azo dyes is said to be achieved using photo- catalytic sites are buried inside the solid matrix. This could be
catalysis, where the most employed photocatalytic material, overcome by the introduction of a flexible open-arch structure on
21
TiO , is active only in the ultraviolet range because of its wide the solid support which can stabilize the metal NPs.
2
band gap. Immense efforts have been focused towards the
Dendrimers are considered to be one of the most appropriate
modification of TiO
2
with an objective of developing visible encapsulating agents for the stabilization of metal NPs due to
their large size and the presence of unique three dimensional
a
architectures of the dendrons that prevent leaching of the NPs
Department of Chemistry, Pondicherry University, Puducherry-605014, India.
E-mail: venkatesanr.che@pondiuni.edu.in; Fax: +91-413-2655987;
Tel: +91-413-2654415
22
during the course of the reaction. Amongst the dendrimers,
(poly)amidoamine (PAMAM) dendrimers are preferable for
catalytic applications because of their regular structure and
chemical versatility, possessing intrinsic amino and amido
functionalities. Conventional methods available for the preparation
of these branched dendrimers are tedious and highly expensive.
Alternatively, researchers have started grafting these dendrimers on
b
Environmental & Analytical Chemistry Division, School of Advanced Sciences,
VIT University, Vellore-632014, India
†
Electronic supplementary information (ESI) available: TGA data, FT-IR spectra,
1
H-NMR, SEM, TEM, surface analyser, detailed UV-Vis kinetics of the catalytic
degradation of MO and CR, effect of catalytic dosage and reusability. See DOI:
1
0.1039/c3nj01050c
This journal is ©The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2014
New J. Chem., 2014, 38, 1551--1558 | 1551

View Article Online
Paper
NJC
Scheme 1 Schematic illustration of the synthesis of Ag/Au NPs stabilized on GO grafted with PAMAM dendrimers. (A) Grafting of the PAMAM dendrimers
on the GO surface, (B) stabilization of the Ag and Au NPs on the GO-PAMAM dendrimer support.
solid supports such as silica, CNTs, insoluble polystyrene chloride, ethylenediamine (EDA), methacrylate (MA) were
copolymers, chitosan surfaces etc., using Michael addition purchased from Merck, India. Graphite powder of particle size
reactions which is extremely cost effective and also these solid 150 mm was obtained from Sigma Aldrich. All chemicals were
supported dendrimers serve as excellent platforms for a variety used without further purification. Double distilled water was
2
3–26
of heterogeneous catalytic reactions.
used throughout the experiments.
In the present investigation, we have grafted varying branches of
PAMAM dendrimers [up to three generations (G1,2,3PAMAM)] on
2.2 Synthesis of Ag/Au NPs/GO-G1,2,3PAMAM
the surface of graphene oxide (GO) followed by the stabilization of Grafting of the G1,2,3PAMAM dendrimers on GO and the
silver and gold NPs (Ag/Au NPs) on the GO supported PAMAM stabilization of the Ag/Au NPs on the GO supported G1,2,3PAMAM
dendritic structure by a simple chemical reduction method was carried out using the procedure reported earlier with minor
2
7
(
Scheme 1). Our focus here is to collectively utilize the large modifications. GO was prepared based on a modified Hummers
2
8
surface area and high electron density of GO, flexible structure and Offeman’s method. GO (0.5 g) was dispersed in 30 mL of
of the PAMAM dendrimers and the catalytic activity of the SOCl2 and stirred for 24 h at 70 1C and the solution was
Ag/Au NPs, towards the degradation of organic azo dyes. subsequently filtered and washed with anhydrous tetrahydrofuran
Ag/Au NPs stabilized on all three generations of PAMAM (THF), and dried under vacuum for 24 h at room temperature, to
dendrimers (Ag/AuNPs/GO-G1,2,3PAMAM) were found to effectively obtain acylated GO (GO-COCl) (0.4953 g).
degrade the dyes methyl orange and congo red, in the presence of
GO-COCl was treated with 20 mL of ethylenediamine in an
ultrasonic bath (40 kHz) at 60 1C for 5 h and the mixture was
stirred for another 24 h. The solid obtained was separated using
a 0.22 mm Millipore polycarbonate membrane filter under
vacuum and then washed with anhydrous methanol. The
resulting solid, after repeated washing and filtration was dried
NaBH in a very short time.
4
2. Experimental section
2
.1 Chemicals
HAuCl
Á3H
congo red, potassium permanganate, hydrogen peroxide, thionyl 20 mL of methylacrylate and 50 mL of methanol were mixed in
4
2 2
O, silver nitrate, sodium borohydride, methyl orange, overnight under vacuum and this yielded 0.5020 g of GO-NH .
1
552 | New J. Chem., 2014, 38, 1551--1558
This journal is ©The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2014

View Article Online
NJC
Paper
a 250 mL three-necked round-bottomed flask to which 0.1 g of the same equipment. The surface area was calculated by a chem.
GO-NH in 20 mL methanol was added dropwise within 20 min BET pulsar TPR/TPD surface analyser.
2
with continuous stirring. The solution was treated in an ultrasonic
bath (40 kHz) at 50 1C for 7 h and the mixture was stirred for
another 24 h. The filtered solid product was dispersed in methanol,
filtered and then washed thrice with methanol in order to ensure
that no addition free reagents were present along with the product.
Then the product was dried overnight to yield GO grafted with
dendrimer generation 0.5, (GO-G0.5PAMAM, 0.1011 g).
GO-G0.5PAMAM suspended in 20 mL methanol was added
dropwise into 40 mL of 1 : 1 methanol–ethylenediamine
solution. The solution was placed in an ultrasonic bath (40 kHz)
at 50 1C for 5 h and the mixture was stirred for another 24 h at the
same temperature. The solid was then filtered and washed with
methanol thrice to yield GO grafted with dendrimer generation
3
. Results and discussion
3.1 Covalent grafting of G1,2,3PAMAM dendrimers on GO
The functionalization of graphene oxide with PAMAM dendrimers
was demonstrated in our previous report, by investigating the
27
weight loss of the contents using thermogravimetric analysis
between 200 1C and 800 1C (ESI,† Fig. S1). Further the covalent
links between PAMAM dendrimers and GO surface were verified
by FT-IR spectroscopy (ESI,† Fig. S2). The formation of a GO single
layer and the covalent immobilization of the G1,2,3PAMAM
dendrimers on GO has been sequentially monitored by Raman
spectroscopy. A laser excitation of 532 nm was employed and
completely dried samples were used for measurements. The
Raman spectra of pure graphite, GO-G1PAMAM, GO-G2PAMAM
and GO-G3PAMAM are shown as Fig. 1 (a, b, c and d respectively).
1
.0 (GO-G1PAMAM). The steps were repeated using methylacrylate
and ethylenediamine until the desired number of generations
up to generation 3) was achieved. The dendrimer-modified
GO (GO-G2,3PAMAM) were then washed thrice with methanol
25 mL), and water (25 mL) respectively.
0 mg of each GO grafted with the dendrimers of all three
(
À1
The second order 2D peak at 2732 cm for pure graphite in
(
Fig. 1a is found to become increasingly diminished after each
introduction step of more dendritic sites. In Fig. 1c and d, the
second order 2D peaks have completely disappeared and this
indicated the efficient grafting of second and third generation
dendrimers on GO. Furthermore, the Raman spectra of all the
samples exhibited two predominant peaks, at 1357 cm and
5
generations (GO-G1,2,3PAMAM), were dispersed in de-ionized
water upon sonication in a 100 mL two neck round bottom
flask. To each of these GO-G1,2,3PAMAM solutions, 10 mL of
29
0.01 M AgNO
3
solution has been added dropwise and the
À1
mixtures were stirred for 1 h. This was followed by the subsequent
À1
30–33
1568 cm , corresponding to the D and G band, respectively.
addition of 10 mL of 0.01 M NaBH and stirring was continued for
3
2
4
The D and G band arise from the activation of sp and sp carbons
in the graphite sheets, respectively during the first order scattering
process and the intensity ratio of the D to G bands expresses the
sp /sp carbon ratio, a measure of disorder. In our PAMAM-
grafted GO samples the D/G ratios were found to be 0.730, 0.987,
another 6 h under a nitrogen atmosphere. The obtained solid
products were washed thrice with water and further dried under
vacuum to yield Ag NPs/GO-G1,2,3PAMAM respectively.
Au NPs/GO-G1,2,3PAMAM were synthesized using a similar
procedure. To each of the GO-1,2,3PAMAM solutions, 10 mL of
2
3
34
1.018, 1.03 for pure graphite, GO-G1PAMAM, GO-G2PAMAM,
0
.01 M HAuCl
4
Á3H
2
O was added dropwise and stirred for 12 h,
GO-G3PAMAM respectively, reflecting the functionalization of
the PAMAM dendrimers on the GO surfaces.
followed by the subsequent addition of 10 mL of 0.5 M NaBH
4
and the stirring was continued for another 24 h. The filtered
solids were washed thrice with water and the products (Au NPs/
GO-G1,2,3PAMAM) were dried overnight under vacuum. All the
obtained nanocatalysts were insoluble in water or any other
organic solvents and can be dispersed in water and other
solvents upon sonication under high frequency.
3.2 Characterization of Ag/Au NPs stabilized on PAMAM
dendrimer-grafted GO
The stabilization of the Ag and Au NPs on GO functionalized
with PAMAM dendritic structures has been investigated by
2.3 Measurements
The sequence of dendrimer functionalization was monitored
using a Raman spectrometer (Witec Confocal Raman instrument
(CRM200) with Ar ion laser 514.5 nm). A Nicolet 6700 FT-IR
spectrometer was used to record the IR spectra using the KBr pellet
method. The crystalline nature of the Ag/Au NPs/GO-G1,2,3PAMAM
nanocatalysts were investigated using an X-pertPRO Pananalytical
a
X-ray diffractometer with Cu K radiation. The surface morphology
of the Ag/Au NPs/GO-G1,2,3PAMAM nanocatalysts were analysed by
SEM (HITACHI S-3400N Scanning Electron Microscope). A Tecnai
F-30 field emission Transmission Electron Microscope (TEM) was
employed to capture the TEM images of the Ag/Au NPs. The UV-Vis
absorption spectra of the prepared nanocatalysts were recorded
at room temperature with an Ocean optics (HR4000) spectro-
Fig. 1 Raman spectra of (a) pure graphite, (b) GO-G1PAMAM, (c) GO-
photometer and also the catalytic activity was examined using G2PAMAM, (d) GO-G3PAMAM.
This journal is ©The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2014
New J. Chem., 2014, 38, 1551--1558 | 1553

View Article Online
Paper
NJC
Fig. 2 The XRD patterns of (a) pure graphite, (b) GO-G3PAMAM, (c–e) Ag
NPs/GO-G1,2,3PAMAM respectively, (f–h) Au NPs/GO-G1,2,3PAMAM,
respectively.
Fig. 3 UV-Visible spectrum of (a) graphene oxide, (b) GO-G3PAMAM, (c)
Ag NPs/GO-G3PAMAM, (d) Au NPs/GO-G3PAMAM.
the PAMAM dendritic structures through these functional
groups.
The SEM images of pure graphite (Fig. S3a, ESI†) shows a
flake-like structure, illustrating its multi-layered morphology.
In the case of the PAMAM-grafted GO shown in Fig. S3b and c
comparison of their crystallinity. The X-ray diffraction patterns
of pure graphite and PAMAM-grafted GO are shown in Fig. 2a
and b. The diffraction pattern observed at 26.451 for pure
graphite in Fig. 2a was found to disappear in Fig. 2b and this
illustrates that the PAMAM-grafted graphene could be
composed of mostly single or few layers. The XRD patterns in
Fig. 2(c–e) at (2y) 38.21, 44.31, 64.41 and 77.41 can be assigned to
the (111), (200), (220) and (311) crystalline planes of Ag,
respectively, and thus the Ag NPs were found to exist in the
(ESI†), the flake-like structure has collapsed into diffused snow
like features and this could be due to the disruption of graphite
layers upon dendrimer substitution. The SEM images of Ag
NPs/GO-G1,2,3PAMAM presented in Fig. 4(a–c) are indicative of
the formation of Ag NPs with a colloid-like morphology. The
stabilized Ag NPs were mostly spherical in shape. Fig. 4(d–f)
depicts the SEM images of Au NPs/GO-G1,2,3PAMAM and the
Au NPs were also found to be spherical in shape. Unlike the Ag
NP colloids, the Au NPs supported on all three generations of
dendrimers were well separated and dispersed in the dendritic
structure.
The surface morphology and size of the synthesized Ag/
AuNPs were also examined using HR-TEM images. Fig. 5(a–c)
and Fig. 5(d–f) shows the HR-TEM images of Ag and Au NPs/
GO-G3PAMAM, respectively. Fig. 5(a and b) provides evidence
that the Ag NPs are well dispersed across the dendritic PAMAM
structures and the size of the Ag NPs has been estimated to be
B20 nm from Fig. 5c. The crystalline nature of the Ag NPs
3
5
face centered cubic structure. Similarly, the peaks observed in
Fig. 2(f–h) at (2y) 38.21, 44.51, 64.61 and 77.51 can be assigned
to the (111), (200), (220) and (311) crystalline planes of
Au, respectively, which also exists in the face centered cubic
3
6
structure. The broadening of the Bragg’s peaks indicates the
formation of NPs and for both Ag and Au NPs the peak
corresponding to the (111) plane was more intense than the
other planes, suggesting that (111) is the predominant orienta-
tion. The mean size of the Ag and Au NPs were calculated using
the Scherrer formula D = Kl/b
of the particle, K is the shape dependent Scherrer’s constant, l
is the wavelength of radiation, b is the full peak width and y is
Using the full width half maxima
FWHM) of the (111) peak, the size of both the Ag and Au NPs
s
cos y, where D is the average size
s
3
7–41
the diffraction angle.
(
were calculated to be 20 nm, which is in good agreement with
the particle size obtained from the TEM analysis.
The UV-Visible spectra of graphene oxide, GO-G3PAMAM, Ag
NPs/GO-G3PAMAM and Au NPs/GO-G3PAMAM are shown in
Fig. 3 (a, b, c and d, respectively). The absorbance at 240 nm in
Fig. 3a is attributed to the p–p* transition of graphene oxide
and the shifting of the same to 300 nm in Fig. 3b could be due
to the n–p* transition of PAMAM dendrimer. The peaks at
417 nm and 529 nm (Fig. 3c and d) are due to the strong surface
plasmon resonance characteristics of the Ag and Au NPs,
3
5,38,39
respectively.
The stabilization of Ag/Au NPs can also be
evidenced from the FT-IR spectral investigations (Fig. S2, ESI†).
The disappearance of peaks corresponding to NH
functionalities in the range between of 3000 to 3050 cm in
2
and –OH
À1
Fig. 4 SEM images of (a–c) Ag NPs/GO-G1,2,3PAMAM and (d–f) Au NPs/
Fig. S2c and d (ESI†) shows that the metal NPs are stabilized on GO-G1,2,3PAMAM, respectively.
1
554 | New J. Chem., 2014, 38, 1551--1558
This journal is ©The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2014

View Article Online
NJC
Paper
temperature and the reaction was monitored with the help of
an UV-Visible spectrometer. MO and CR were found to
exhibit absorption maxima of 465 and 495 nm, respectively,
and hence the catalytic degradation of these dyes was
monitored by observing the change in absorbance of these
peaks and then the pseudo-first order rate constants have
been calculated.
The degradation of both MO and CR did not proceed in the
4
absence of either NaBH or the Ag/Au NPs catalysts. The
degradation studies were carried out with different concentrations
of NaBH4 and Ag/Au NPs in order to optimize the suitable
conditions for maximum catalytic activity. The maximum
Fig. 5 TEM images of (a–c) Ag NPs/GO-G3PAMAM and (d–f) Au NPs/ catalytic degradation was found to be attained in the presence
GO-G3PAMAM, respectively.
of 50 mL of 0.1 M NaBH
and hence the same concentration of NaBH
4
and 5 mg of Ag/Au NPs/GO-G1,2,3PAMAM
and the nanocatalysts
4
for maintained for further investigation on the catalytic activity of
Ag/Au NPs/GO-G1,2,3PAMAM. Fig. 6a and e shows the catalytic
degradation of MO with time and it can be observed that complete
degradation occurs within 80 s and 150 s in the presence of the
Ag and Au NPs/GO-G3PAMAM nanocatalysts. Fig. 6c and g show
could not be detected by HR-TEM and this could be because
of the Ag NPs being buried inside the PAMAM dendritic
structure. The TEM images of Au NPs/GO-G3PAMAM shown
in Fig. 5(d and e) demonstrates that the Au NPs are well
dispersed across the dendritic structures and Fig. 5f showed
the size of the Au NPs to be B20 nm. Also the Au NPs were
found to be highly crystalline and that can be observed from
the lattice fringe spacing of 0.23 nm (Fig. S4a, ESI†), which is in
agreement with that reported for the spacing between the (111)
plane of fcc Au (0.235 nm) (JCPDS fill No. 04.0784), and the
fringe spacing values are in accordance with that obtained from
the corresponding ln A versus time plots for the same, and the
À3 À1
rate constants were calculated to be 25.9 Â 10
s
and 19.1 Â
À3 À1
10
s
for the degradation of MO in the presence of the Ag and
AuNPs/GO-G3PAMAM nanocatalysts, respectively. Similarly, Fig. 6b
and f show the degradation of CR with time in the presence of the
Ag and Au NPs/GO-G3PAMAM nanocatalysts and the catalytic
degradation occurs within 180 s and 240 s, respectively. The
corresponding ln A vs. time plots are presented as Fig. 6d and h,
and the obtained rate constant values for the reduction are
4
0,41
the XRD pattern of the Au NPs (Fig. 2(f–h)).
area diffraction (SAED) image of the Au NPs is presented in
Fig. S4b, ESI†), which shows bright diffraction rings from
inner to outer, and can be indexed to the (111), (200), (220),
The selected
(
À3 À1
À3 À1
17.8 Â 10
s
and 12 Â 10
s , respectively. The catalytic
efficiency of the Ag and Au NPs stabilized on all three genera-
tions of dendrimers has been evaluated (Fig. S5 to S8, ESI†) and
the data are presented in Table 1. However, the NPs stabilized
on the third generation dendrimers were found to exhibit the
highest catalytic activity towards the degradation of both dyes,
and this could be because of the presence of a higher number
of metal NPs stabilized on the widely expanded third genera-
tion dendritic structures. The degradation constants calculated
for all the prepared nanocatalysts towards the degradation of
MO and CR were found to be comparable with or better than
the reported values (Table 1). The enhanced degradation
observed with our nanocatalysts can be attributed to the
following reasons (i) high surface area of the GO and grafted
PAMAM dendrimer that can adsorb azo dyes, (ii) NaBH4 is
expected to act as the hydride source and the Ag/Au NPs
catalysts are expected to activate the azo nitrogen bond and
also they could bind with the sulphur and oxygen atoms of the
dyes, resulting in the weakening of the azo double bond via
conjugation, (iii) the large number of N, O atoms of the PAMAM
(
311) and (222) planes, respectively, confirming the highly
3
5
crystalline nature of the Au NPs.
The BET surface area of pure graphite, GO, Ag NPs/
GO-G1,2,3PAMAM, and Au NPs/GO-G1,2,3PAMAM were measured
by nitrogen adsorption measurements and the data are depicted in
Table S1, ESI.† The synthesized GO exhibits a significantly
2
À1
increased surface area (230 m g ) than that of pure graphite
2
À1 34
(8.7 m g ). Interestingly, the BET surface area of the Ag NPs
stabilized on GO grafted with G1,2,3PAMAM dendrimers is about
2
NPs stabilized on GO-G1,2,3PAMAM have shown surface areas of
2
level of PAMAM dendritic structures induces the stabilization of a
larger number of Ag and Au NPs, which is the origin for the gradual
increase in the BET adsorption values.
2
À1
2
À1
2
À1
90 m g , 330 m g and 365 m g , respectively. Similarly Au
2
À1
2
À1
2
À1
93 m g , 325 m g and 358 m g , respectively. The enhanced
3
.3 Degradation of organic azo dye molecules using Ag/Au
NPs/GO-G1,2,3PAMAM as heterogeneous nanocatalysts
The efficacy of Ag/Au NPs/GO-G1,2,3PAMAM nanocatalyst dendrimers could assist in increasing the number of Ag/Au
towards the degradation of different organic dye molecules NPs, and (iv) the PAMAM dendritic structures containing
using NaBH4 as a reducing agent was investigated. Methyl more hetero atoms are expected to exhibit hydrophilic inter-
orange (MO) and congo red (CR), which are used as commercial actions with the azo dyes and thus assist in bringing the
colorants in the dyeing industry, have been chosen as the dye molecules near the catalytic sites. The plausible mechanism
model substrates for testing the catalytic activity of our nano- for the degradation of MO and CR has been presented in
catalysts. The degradation reaction was carried out at room Scheme 2 and the degradation products were characterized
This journal is ©The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2014
New J. Chem., 2014, 38, 1551--1558 | 1555

View Article Online
Paper
NJC
Fig. 6 UV-Vis kinetics spectra for the degradation of (a) methyl orange and (b) congo red by Ag NPs/GO-G3PAMAM. ln A vs. time plot for the
degradation of (c) methyl orange and (d) congo red by Ag NPs/GO-G3PAMAM. UV-Vis kinetics spectra for the degradation of (e) methyl orange and (f)
congo red by Au NPs/GO-G3PAMAM. ln A vs. time plot for the degradation of (g) methyl orange and (h) congo red by Au NPs/GO-G3PAMAM.
using NMR and FT-IR spectroscopy. Furthermore, all six conditions without any change in its catalytic activity (ESI,†
catalysts could be reused at least 10 more times under similar Fig. S25 and S26).
1
556 | New J. Chem., 2014, 38, 1551--1558
This journal is ©The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2014

View Article Online
NJC
Paper
Table 1 Systematic literature survey of the degradation of methyl orange and congo red azo dyes and corresponding rate constants at room
temperature
Rate constants (k) for the degradation of
k
Entry
Samples
Carrier system
Metals
Methyl orange
Congo red
Ref.
a
À3 À1
À3 À1
À3 À1
À3 À1
1
2
3
4
5
6
7
Ag/GO-G3PAMAMa
Ag/GO-G2PAMAMa
Ag/GO-G1PAMAM
PAMAM dendrimer
PAMAM dendrimer
PAMAM dendrimer
PAMAM dendrimer
PAMAM dendrimer
PAMAM dendrimer
Tannic acid
Ag
Ag
Ag
Au
Au
Au
Ag
Au
Pt
Ti
W
Ti
Cu
Ni
Ti
25.9 Â 10
12.1 Â 10
s
16.8 Â 10
10.9 Â 10
s
This work
This work
This work
This work
This work
This work
42
s
s
À3 À1
À3 À1
8.5 Â 10
s
4.8 Â 10
s
b
À3 À1
À3 À1
Au/GO-G3PAMAMb
19.1 Â 10
s
12.1 Â 10
s
À3 À1
À3 À1
Au/GO-G2PAMAM
9.5 Â 10
7.1 Â 10
s
6.1 Â 10
3.5 Â 10
—
s
b
À3 À1
À3 À1
Au/GO-G1PAMAM
s
s
c
À3
À1
Ag/Au/Pt/tannic acid
583 Â 10 min
À3
À3
À3
À3
À1
À1
4.9 Â 10 min
2.9 Â 10 min
d
À1
À1
À1
8
9
P25TiO
2
/GO
P25
PAH
CNTS
Alumina
Thioacetamide
116 Â 10 min
—
—
—
43
44
45
46
47
48
49
e
TiO
TiO
2
/PW12
/CNTS
46.5 Â 10 min
f
À3
10
11
12
13
14
2
37.3 Â 10 min
g
À3
À1
Alumina/CuO
Thioacetamide/NiS
—
—
—
—
567 Â 10 min
h
À3
À1
72 Â 10 min
i
À3
À1
À1
TiO
P25TiO
2
TiO
SiO
2
(TNA)
12.5 Â 10 min
j
À3
2
/SiO
2
2
Ti
0.003 Â 10 min
a
b
Reactions were catalyzed by Ag NPs stabilized on GO grafted with first, second, third generation PAMAM dendrimers. Reactions were catalyzed
c
d
by Au NPs stabilized on GO grafted with first, second, third generation PAMAM dendrimers. Tannic acid supported Ag/Au/Pt NPs. Graphene
e
f
g
oxide supported P25TiO
2
NPs. Titanium oxide supported tungsten phosphate NPs. Carbon nanotube supported TiO
2
NPs. Alumina supported
h
i
j
k
copper oxide NPs. Nickel sulphate NPs. Titanium oxide nanotube arrays NPs. P25Titanium oxide supported silicone oxide NPs. Apparent
rate constants.
3.4 Influence of catalytic dosage
4. Conclusions
In order to examine the effect of the catalytic dosage on the rate
of degradation, the reaction was carried out with varying
amounts of the Ag/Au NPs/GO-G1,2,3PAMAM nanocatalysts in
Graphene oxide grafted with PAMAM dendrimers served as an
exemplary support for the stabilization of Ag and Au NPs.
All the prepared nanocatalysts, Ag/Au NPs/GO-G1,2,3PAMAM
were found to exhibit excellent catalytic activity towards the
complete degradation of methyl orange and congo red in the
1
mg increments, in the presence of an identical concentration
of NaBH and azo dye. It can be observed from Fig. S9–S24
ESI†), that the rate of degradation increases with the amount
4
(
4
presence of NaBH as a hydride source, in very short reaction
of catalyst added up to 5 mg, and further increases in the
amount of catalyst did not contribute to a significant enhance-
ment in the degradation rate. Hence, 5 mg of the nanocatalysts
has been chosen as the optimum dosage for the efficient
catalytic degradation of MO and CR.
times. The remarkable catalytic activity achieved is attributed to
the combined effect of the high surface area of graphene oxide,
the highly active metal NPs and the hydrophilic dendritic
structure. Furthermore, the catalysts were found to retain their
activity for a high number of cycles owing to their high stability
and reproducibility. These features make the Ag/Au NPs/GO-G1,
2,3PAMAM nanocatalysts good candidates for the degradation
of dye pollution in water.
Acknowledgements
We gratefully acknowledge the University Grants Commission
(UGC), Govt. of India, New Delhi, for funding the research
project (F. No 39-782/2010 (SR)). SSK thank the Tamilnadu State
Council for Science and Technology, Chennai, for the Young
Scientist Visiting Fellowship. The authors also acknowledge the
services rendered by the Central Instrumentation Facility (CIF),
Pondicherry University.
References
1
P. G. Rieger, H. M. Meir, M. Gerle, U. Vogt, T. Groth and
H. J. Knackmuss, J. Biotechnol., 2002, 94, 101.
G. M. Liu, X. Z. Li and J. C. Zhao, Environ. Sci. Technol., 2000,
34, 3982.
Scheme 2 Plausible mechanism for the degradation of methyl orange
and congo red azo dyes by the Ag/Au NPs/GO-G1,2,3PAMAM nanocata-
2
4
lysts using NaBH as the hydride source.
This journal is ©The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2014
New J. Chem., 2014, 38, 1551--1558 | 1557

View Article Online
Paper
NJC
3
4
M. H. Habibi, A. Hassanzadeh and S. Mahdavi, J. Photochem. 28 D. K. Bhui, H. Bar, P. Sarkar, G. P. Sahoo, S. P. De and
Photobiol., A, 2005, 172, 89. A. Misra, J. Mol. Liq., 2009, 145, 33.
C. Hu, X. X. Hu, L. S. Wang, J. H. Qu and A. M. Wang, 29 M. B. Avinash, K. S. Subrahmanyam, Y. Sundarayya and
Environ. Sci. Technol., 2006, 40, 7903. T. Govindaraju, Nanoscale, 2010, 2, 1762.
M. Janus and A. W. Morawski, Appl. Catal., B, 2007, 75, 118. 30 Y. P. Sun, K. Fu, Y. Lin and W. Huang, Acc. Chem. Res., 2002,
5
6
H. Zhang, D. Chen, L. V. Xiaojun, Y. Wang, H. Chang and
J. Li, Environ. Sci. Technol., 2010, 44, 1107.
J. Pengfei, J. Zhang, F. Chen and M. Anpo, Appl. Catal., B,
35, 1096.
31 C. Cioffi, S. Campidelli, C. Sooambar, M. Marcaccio,
G. Marcolongo, M. Meneghetti, D. Paolucci, F. Paolucci,
C. Ehli, G. M. A. Rahman, V. S. Dirk, M. Guldi and M. Prato,
J. Am. Chem. Soc., 2007, 129, 3938.
32 K. N. Kudin, B. Ozbas, H. C. Schniepp, R. K. Prudhomme,
I. A. Aksay and R. Car, Nano Lett., 2008, 8, 36.
33 S. Sarkar, E. Bekyarova, S. Niyogi and C. R. Haddon, J. Am.
Chem. Soc., 2011, 133, 3324.
7
8
9
2009, 85, 148.
F. J. Cervantes, A. G. Espinosa, M. A. Moreno-Reynosa and
J. R. Rangel-Mendez, Environ. Sci. Technol., 2010, 44, 1747.
J. Fernandez, J. Bandara, A. Lopez, P. Buffat and J. Kiwi,
Langmuir, 1999, 15, 185–192.
1
1
0 J. Lee, H. S. Shim, M. Lee, J. K. Song and D. Lee, J. Phys.
Chem. Lett., 2011, 2, 2840.
1 A. C. Pradhan, K. M. Parida and B. Nanda, Dalton Trans.,
34 M. Fang, K. Wang, H. Lu, Y. Yang and S. Nutt, J. Mater.
Chem., 2009, 19, 7098.
2
011, 40, 7348–7356.
35 D. Philip, C. Unnib, S. A. Aromala and V. K. Vidhua, Spectro-
chim. Acta, Part A, 2011, 78, 899.
1
1
1
2 J. Zhang, Z. Xiong and X. S. Zhao, J. Mater. Chem., 2011, 21, 3634.
3 P. Kannan and S. A. John, Electrochim. Acta, 2011, 56, 7029. 36 F. Bei, X. Hou, S. L. Y. Chang, G. P. Simon and D. Li, Chem.–
4 M. Lee, P. Amaratunga, J. Kim and D. Lee, J. Phys. Chem. C,
010, 114, 18366.
5 G. Chang, Y. Luo, W. Lu, X. Qin, A. M. Asiri, A. O. Al-Youbi
Eur. J., 2011, 17, 5958.
37 S. S. Shankar, A. Ahmad and M. Sastry, Biotechnol. Prog.,
2003, 19, 1627.
2
1
1
1
1
1
2
and X. Sun, Catal. Sci. Technol., 2012, 2, 800.
6 D. Astruc, F. Lu and J. Ruiz Aranzaes, Angew. Chem., Int. Ed.,
38 J. Huang, Q. Li, D. Sun, Y. Lu, Y. Su, X. Yang, H. Wang,
W. Shao, N. He, J. Hong and C. Chen, Nanotechnology, 2007,
18, 105.
2005, 44, 7852.
7 R. Raja, V. B. Golovko, J. M. Thomas and A. B. Murcia, Chem. 39 H. Wang, X. Qiao, J. Chen and S. Ding, Colloids Surf., A,
Commun., 2005, 2026. 2005, 256, 111.
8 S. Senthil Kumar, G. Baek, S. Lee, C.-H. Jun and D. Lee, 40 S. Basavaraja, S. D. Balaji, A. Lagashetty, A. H. Rajasab and
Electrochem. Commun., 2012, 19, 123. A. Venkataraman, Mater. Res. Bull., 2008, 43, 1164.
9 S. Panigrahi, S. Basu, S. Praharaj, S. Pande, S. Jana, A. Pal 41 G. Xu, X. Qiao, X. Qiu and J. Chen, Colloids Surf., A, 2008,
and S. K. Ghosh, J. Phys. Chem. C, 2007, 111, 4596. 320, 222.
0 B. Sreedhar, P. Surendra Reddy and D. Keerthi Devi, J. Org. 42 N. Gupta, H. P. Singh and R. K. Sharma, J. Mol. Catal. A:
Chem., 2009, 74, 8806.
Chem., 2011, 335, 248.
2
2
1 Y. Jiang and Q. Gao, J. Am. Chem. Soc., 2006, 128, 716.
2 J. Peterson, A. Ebber, V. Allikmaa and M. Lopp, Proc. Est.
Acad. Sci., Chem., 2001, 50, 156.
43 S. M. Torres, L. M. P. Martinez, J. L. Figueiredo, J. L. Faria
and A. M. T. Silva, Appl. Surf. Sci., 2013, 275, 361.
44 P. Niu and J. Hao, Langmuir, 2011, 27, 13590.
45 Y. J. Xu, Y. Zhuang and X. Fu, J. Phys. Chem. C, 2010,
114, 2669.
2
2
3 J. Zohuriaan and M. Mehr, Iran. Polym. J., 2005, 14, 235.
4 B. Pan, D. Cui, F. Gao and R. He, Nanotechnology, 2006,
17, 2483.
46 D. Jana and D. Goutam, RSC Adv., 2012, 2, 9606.
2
2
2
5 C. Zhang, P. Su, M. U. Farooq, Y. Yang, X. Gao and 47 H. Guo, K. Yingchang, D. Wang, K. Lin, R. Shen, J. Chen and
E. Hongjun, React. Funct. Polym., 2010, 70, 129. W. Weng, J. Nanopart. Res., 2013, 15, 1475.
6 G. R. Krishnan and K. Sreekumar, Eur. J. Org. Chem., 2008, 48 T. S. Natarajan, K. Natarajan, H. C. Bajaj and J. R. J. Tayade,
763. Ind. Eng. Chem. Res., 2011, 50, 7753.
7 R. Rajesh and R. Venkatesan, J. Mol. Catal. A: Chem., 2012, 49 H. R. Jafry, M. V. Liga, Q. Li and A. R. Barron, Environ. Sci.
4
359, 88.
Technol., 2011, 45, 1563.
1
558 | New J. Chem., 2014, 38, 1551--1558
This journal is ©The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2014