Green fabrication of reduced graphene oxide decorated with Ag nanoparticles (rGO/Ag NPs) nanocomposite: A reusable catalyst for the degradation of environmental pollutants in aqueous medium

Article abstract of DOI:10.1016/j.molliq.2020.114302

In this current work, a bio-inspired green approach has been followed to synthesize Ag nanoparticles (NPs), which were immobilized over reduced graphene oxide (rGO) as a suitable support. In the stepwise preparation of the bionanocomposite (rGO/Ag NPs) by

Full text of DOI:10.1016/j.molliq.2020.114302

Journal of Molecular Liquids 319 (2020) 114302
Contents lists available at ScienceDirect
Journal of Molecular Liquids
journal homepage: www.elsevier.com/locate/molliq
Green fabrication of reduced graphene oxide decorated with Ag
nanoparticles (rGO/Ag NPs) nanocomposite: A reusable catalyst for the
degradation of environmental pollutants in aqueous medium
b
c,
Saba Hemmati a, Majid M. Heravi a, , Bikash Karmakar , Hojat Veisi
⁎
⁎
a
Department of Chemistry, School of Science, Alzahra University, PO Box 1993891176, Vanak, Tehran, Iran
b
Department of Chemistry, Gobardanga Hindu College, India
Department of Chemistry, Payame Noor University, Tehran, Iran
c
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 6 August 2020
Received in revised form 5 September 2020
Accepted 9 September 2020
Available online 11 September 2020
In this current work, a bio-inspired green approach has been followed to synthesize Ag nanoparticles (NPs),
which were immobilized over reduced graphene oxide (rGO) as a suitable support. In the stepwise prepa-
ration of the bionanocomposite (rGO/Ag NPs) by using Menthapulegium flower extract, initially graphene
oxide (GO) was reduced to rGO followed by in situ reduction of Ag+ ions to Ag NPs. In these reductions, dif-
ferent biomolecules such as, polyphenols, flavonoids, alkaloids, terpenoids and mild acids were employed
as the natural reductant. The oxygenated functions in the aforementioned biomolecules, efficiently, assisted
the capping and stabilizing the AgNPs. The as-synthesized nanocomposite was fully characterized using dif-
ferent techniques such as, UV–Vis spectroscopy, scanning electron microscopy (SEM), energy-dispersive X-
ray spectroscopy (EDX), wavelength dispersive X-ray spectroscopy (WDX) elemental mapping, transmis-
sion electron microscopy (TEM), X-ray diffraction (XRD) and inductively coupled plasma (ICP). Diameter
of the biosynthesized Ag NPs were found to be in the range of 20–25 nm. The constituent elements were
found to be homogeneously dispersed as found from elemental mapping study. This novel prepared
nano-composite showed excellent water dispersibility due to the hydrophilicity of biomolecules attached
to them, which is essential criterion in heterogeneous catalysis. This composite was used as an effective cat-
alyst in the reductive degradation of water contaminated by organic dyes such as methyl Orange (MO) and
rhodamin B (RhB) using NaBH4 as reductive agent at room temperature. The progress of reduction was
monitored by UV–Vis spectroscopy. Both the reactions followed pseudo-unimolecular kinetics and the cor-
responding rate constants were found being 0.098 s−1 and 0.096 s−1 respectively. The material was signif-
icantly robust as justified by the leaching as well as hot-filtration tests and its reusability for several times
without any appreciable loss in its activity.
Keywords:
Silver
Graphene oxide
Nanocatalyst
Reduction
Waste-water treatment
© 2020 Published by Elsevier B.V.
1. Introduction
delivery targeting cancer cell and other deadly ailments [4], dis-
posal of heavy and toxic metals, degradation of environmental con-
In the last few decades nanotechnology has been considered as
one of the most emerging field in science. This technology has ap-
peared extremely promising, due to its extensive applications in
different arena like physics, chemistry, biology, engineering and
their interdisciplinary regions [1–3]. Nanosciences have been
exploited in different fields like applicative pharmacology, drug
taminants, water purifications [5], electronics [6], imaging contrast
agents [7] sensor and catalysis [8–11]. Scientist are particularly in-
terested in nanocatalysis, based on the unique criteria of the mate-
rials, like NPs have extremely small dimension, high surface to
volume ratio, exceptional shape dependant properties, uniformity
in particle size distribution, truly heterogeneous nature and reus-
ability with almost constant efficiency [12–15]. They satisfy almost
all the criteria as for an ideal catalyst. Green chemistry offers a co-
hesive set of twelve principles and has engrossed widespread in-
terest in view of sustainability [16]. One of the most important
future perspectives of green chemistry is designing ecological or bio-
inspired catalysts in pursuing important organic transformations. The
⁎
Corresponding authors.
E-mail addresses: mmheravi@alzahra.ac.ir (M.M. Heravi), hojatveisi@yahoo.com
(H. Veisi).
https://doi.org/10.1016/j.molliq.2020.114302
0167-7322/© 2020 Published by Elsevier B.V.

S. Hemmati, M.M. Heravi, B. Karmakar et al.
Journal of Molecular Liquids 319 (2020) 114302
concept of greener nanoscience is a practical application of advanced
nanotechnology [17–20]. Following this coalescence between nano-
technology and green chemistry, the biogenic synthesis of NPs has
evolved as an outstanding approach in the design of eco-friendly
catalysts.
isolated from the reaction mixture by centrifugation and reused sev-
eral times without significant decrease in catalytic activity. Robust-
ness of the material was further investigated using hot filtration
and leaching test.
Extract of different parts of plants like fruits, barks, flowers, roots
leaves contain several phytochemicals like polyphenols, flavonoids,
alkaloids, terpenoids and mild acids which have been found to be
very effective in the biometric conversion of metal salts into corre-
sponding NPs [21–26]. In the recent past, this green metric protocol
for the synthesis of noble metal NPs has been quite popular [27–30].
Among the different noble metals Ag NPs are relatively less expensive
as compared to Au, Pd and Pt. They also find extensive applications in di-
verse fields like biological transmission electron microscopy, colorimetric
DNA sensor, animal husbandry, agriculture, household, packaging, optics,
electronics and catalysis [31,32]. Thereby, ample studies are going on for
the biogenic synthesis of Ag NPs [33–38]. Nevertheless, due to ultrafine
size, large specific surface area and strong electrostatic attraction, the
NPs are frequently found to form nano-clusters by self-aggregation
which considerably reduces their catalytic efficiency. This can be re-
stricted by homogeneous immobilization of the NPs over a suitable sup-
port [39–41].
Among various suitable materials as support, graphene and its deriv-
atives have gained utmost importance due to their high thermal and
mechanical stability, exceptional electrical conductivity with incredibly
large surface area and adsorption ability. These unique materials are
endowed with atomic width and a densely packed two-dimensional
honeycomb like structures [42].
In continuation to our current venture on the design and synthesis of
sustainable catalytic nanomaterials [39–50], we have synthesized Ag
NPs following biogenic technique and incorporated them over the sur-
face of bioreduced graphene oxide (rGO/Ag NPs). For the green reduc-
tion of GO and Ag+ ions we exploited the Menthapulegium flower
extract, a rich source in polyphenols, terpenoids, alkaloids and polysac-
charide moieties. The plant has high ornamental and medicinal values
being used as anti-hypertensive, antitussive and expectorant in tradi-
tional medicine [51,52].
The increasing social development has upgraded the human
lifestyle but at the cost of environmental pollution. The lack in
consciousness and inadequate control measures has caused serious
damage to the ecological parameters. Contamination of harsh
chemicals, heavy metals, drugs and pharmaceuticals, hormones
and dyes into wastewater effluent and thereafter into natural wa-
ters has been a major issue. A bulk amount of synthetic dyes is re-
leased into water by textile, dye, paint, print and paper industries
as a result of incomplete quenching of pigments and successive
washing of colored materials [53]. Even at a minute concentration
the dyes are highly diffusible into water which obstructs the pene-
tration of sunlight. This, in turn, decreases the dissolution of oxy-
gen into water, causes death of photosynthetic organisms which
ultimately leads to disruption of aquatic ecosystem. Hence, the
wastewater treatment by disposal of these organic dyes following
suitable techniques is of great importance, in view of sustainable
management [54–56]. In this regard, catalytic reduction of dye-
stuffs involving nanomaterials nowadays, is considered as one of
the promising, cost-effective and energy efficient green methods,
[41,57–62]. This strategy successfully transforms the organic pol-
lutants to safer chemicals being tender to natural waters.
2. Experimental
2.1. Preparation of Menthapulegium flower extract
Fresh Menthapulegium flowers were collected and washed thor-
oughly with double-distilled water. 2.0 g of the flower petals were
extracted in 100 mL deionized water by boiling for 20 min. The col-
ored solution was then cooled and filtered through Whatmann-1 fil-
ter paper. It was stored at 4 °C in refrigerator for further use.
2.2. Biogenic synthesis of rGO/Ag NPs nanocomposite using Menthapulegium
flower extract
GO was prepared following a procedure modified by Hummers
et al. [63]. 100 mg of GO was dispersed in the Menthapulegium flower
extract by sonication for 20 min followed by refluxing for 2 h. Com-
plete reduction of GO was confirmed by change in color of the solu-
tion from light brown to black due to excitation of surface Plasmon
resonance. The rGO was isolated by centrifugation followed by
washing with DI water. In the next step, the rGO was dispersed
again in the flower extract by sonication and 10 mL aqueous solution
of AgNO3 (0.05 g/L) was added dropwise. The mixture was then agi-
tated at 100 °C for 30 min. Finally, the rGO/Ag NPs nanocomposite
was retrieved from the reaction mixture in a centrifuge, rinsed thor-
oughly with DI water and subsequently dried in air. ICP-OES tech-
nique was used to quantify the Ag load on the composite, which
was found to be 0.096 mmol/g.
2.3. Reductions of MO and RhB: general procedure
In a 25 mL aqueous solution of MO or RhB (0.0002 M), 0.003 g of
the rGO/Ag NPs nanocomposite was shaken at room temperature for
1 min. Fresh NaBH4 solution (25 mL, 0.2 M) was then added to the
mixture and stirred till the colored solution became colorless. The
progress of reaction was monitored using UV–Vis spectroscopy.
After completion of the reaction, the catalyst was separated from re-
action mixture by centrifugation, washed with H2O/EtOH and recov-
ered being used in next runs.
3. Results and discussion
3.1. Characterization of catalyst by its data analysis
The rGO/Ag NPs nanocomposite was synthesized biogenically in
stepwise fashion. The oxygenated functions including the polyhy-
droxy organic groups present in Menthapulegium flower extract
first competently reduces the GO to rGO under ultrasonic conditions.
The electron rich oxygen functions thereafter transfers their elec-
trons to the surface anchored Ag+ ions to reduce it to Ag NPs in
situ. The tiny NPs also gets capped and stabilized by the biomolecules
of flower extract (Scheme 1). The as synthesized nanocomposite was
characterized with UV–Vis, SEM, EDX, atomic mapping, TEM, powder
XRD and ICP-OES techniques.
Consequently, in this current study we wish to report the cata-
lytic degradation of two harmful organic dyes, Methyl orange (MO)
and Rhodamine B (RhB), using novel composite, bionanocomposite
(rGO/Ag NPs). The reduction was carried out using NaBH4 at aqueous
media and was completely monitored over time dependant UV–Vis
spectroscopy. Both the reactions were completed within 50 s with al-
most quantitative yield. Subsequently, using the UV data, kinetics of
the reactions was also studied. In addition, the catalyst was easily
Fig. 1 represents the overlapping UV spectra of GO and rGO/Ag
NPs. GO exhibits a characteristic absorption maxima at 235 nm at-
tributed to the π → π* transitions of C\\C bonds from aromatic moi-
ety and a shoulder at 280 nm due to the n → π* transition of C_O
function. In its reduction to rGO, the visible change from is observed
from light to dark brown solution, due to reduction of hydrophilic
functional groups like carboxyl, hydroxyl and epoxide. Spectroscop-
ically, this change is displayed from a bathochromic shift from
2

S. Hemmati, M.M. Heravi, B. Karmakar et al.
Journal of Molecular Liquids 319 (2020) 114302
Mentha pulegium flower extract
AgNO3
rGO
GO
+
Et2N
O
NEt2
-
Cl
COOH
SO3Na
NaBH4
NEt2
N
N
RhB
NaBH4
Me2N
MO
rGO/Ag NPs
Et2N
O
SO3Na
H
COOH
HN
NH
Leuco RhB
Me2N
Hydrazine derivative
Scheme 1. Fabrication of rGO/Ag NPs nanocomposite using Menthapulegium flower extract and its catalytic application in the reduction of MO and RhB.
~235 nm peak (GO) to ~274 nm (rGO). Fig. 1 clearly depicts the dif-
ference. [42]. The electronic conjugation within graphene sheet is re-
stored upon green reduction, reflecting increased π-electron density
and structural rearrangements. The appearance of a new peak at
415 nm is corroborated to the Ag NPs being formed from Ag ions.
However, the biomolecules being attached to the material could
not be identified from UV–Vis spectra.
The morphological appearance and microstructures of the pre-
prepared rGO/Ag NPs nanocomposite was investigated by SEM
which is displayed in Fig. 2. The GO image represents a thin transpar-
ent sheet with wrinkled morphology throughout. It looks like a
turned cloth, edges of which are seen to be partly folded (Fig. 2a).
In the SEM image of rGO/Ag NPs nanocomposite the white spherical
balls representing the Ag NPs are seen to be evenly distributed on the
shrink surface of rGO (Fig. 2b). On close observation, a thin layer of
the biomolecules can be seen over the Ag NPs. An EDX analysis of
the nanocomposite documents its exact chemical compositions.
Fig. 1. UV–Vis spectra of GO and the biosynthesized rGO/Ag NPs nanocomposite.
Fig. 3 represents the EDX profile with Ag, C and O as the main
3

S. Hemmati, M.M. Heravi, B. Karmakar et al.
Journal of Molecular Liquids 319 (2020) 114302
WDX data in turn also justifies the successful formation of the
nanocomposite.
To get a deeper insight of the structural morphology of rGO/Ag NPs
nanocomposite, TEM analysis was carried out. As can be seen from
Fig. 5, the black dots spread homogeneously over the transparent rGO
sheet, symbolizes the Ag NPs. Mean diameter of Ag NPs were around
~15–25 nm.
Crystallinity and phase structure of the rGO/Ag NPs nanocompos-
ite was investigated over XRD analysis. Fig. 6 illustrates the single
phase diffractogram of the material which signifies that Ag NPs are
perfectly embedded over rGO and behaves as a single entity. It repre-
sents the four significant sharp diffraction peaks at 2θ = 38.1°, 44.3°,
64.4°, 77.4°, attributed to diffraction over (1 1 1), (2 0 0), (2 2 0) and
(3 1 1) planes of cubic Ag crystalline phases (JCPDS No. 04-0783).
The broad region at 2θ = 10–25° (002) corresponds to the amor-
phous rGO moiety.
3.2. Catalytic reduction of MO and RhB over rGO/Ag NPs nanocomposite
After full characterization of the synthesized nanocomposite, its
catalytic performance was examined. Reduction of two water con-
taminated organic dyes, viz., MO and RhB in presence of NaBH4 as
reducing agent in the presence of rGO/Ag NPs nanocomposite was
investigated. Progress of the reaction was fully monitored using a
UV–Vis spectrophotometer. Dilute aqueous solution of the dye as-
sumed a specific color prior to the addition of borohydride and cat-
alyst. As the reaction started, color of the solution started fading
within seconds, an indication of the degradation. This change in
color is associated with the change in absorption frequency
(λmax) recorded by the UV–Vis spectrophotometer. Fig. 7a displays
the time dependent absorbance plots of the reduction of MO in
presence of 0.003 g of rGO/Ag NPs nanocomposite. As shown in
the plot, the initial absorption maxima at 465 nm gradually dimin-
ishes with time and after a time lapse of 50s, the bell shaped plot
became flattened. This also visibly could be seen from the colorless
reaction mixture. Next, we studied the reaction kinetics for the re-
duction of the two dyes over NaBH4. As the concentration of NaBH4
remained constant throughout the reaction, it can be assumed that
the reaction followed a pseudo-first-order kinetics. Accordingly, it
follows the Lambert-Beer's equation ln(At/A0) = kt, where A0 is the
initial absorbance and At is the absorbance at time t in second. From
the slope of the plot of ln(At/A0) vs t, the rate constant for the cat-
alytic reduction of MO using 0.003 g of catalyst was found to be
0.098 s−1 (Fig. 7b). The similar trend was followed in the reduction
of RhB (λmax 550 nm) affording excellent results in short reaction
time (50s). The results have been displayed in Fig. 8a. In this case
the rate constant was found to be 0.096 s−1 and the corresponding
plot has been presented in Fig. 8b.
Fig. 2. SEM images of (a) GO and (b) the rGO/Ag NPs nanocomposite.
constituent of the material. The presence of C and O could be justified
for the polyphenolic attachments.
The EDX data was further validated with WDX atomic mapping
image. It displays a clear picture of atomic distributions over the
material surface. An X-ray scan of the SEM image segment is used
in the WDX mapping analysis, as shown in Fig. 4. The C, O and Ag
atoms are observed to be uniformly distributed over the surface.
In some regions the Ag atoms seem to form clusters. The EDX and
While concerning about green and sustainable chemistry, re-
usability of the catalyst is an obvious measure. In this regard,
after completion of a fresh batch of probe reactions, the catalyst
was recovered by centrifugation and washed with EtOH:H2O
(1:1). It was then dried and reused in further batches applying
the same conditions. The results revealed that rGO/Ag NPs nano-
composite retained its activity up to 10 successive runs without
significant loss in its catalytic activity in the reduction of MO
and RhB (Fig. 7c and c).
In order to ascertain the robustness of our catalyst, the Ag
leaching was inspected with the 10 time used catalyst. Gratifyingly,
the leaching was insignificant in both reductions (0.01 wt% of the
initial load). Furthermore, the true heterogeneity nature of the
rGO/Ag NPs nanocomposite was assessed through a hot filtration
test in the reduction of MO under already secured optimal condi-
tions. The reaction was stopped after 25 s when about 60% yield
was obtained and the catalyst was isolated from the reaction mix-
ture. Then the catalyst-free hot filtrate was further stirred for
Fig. 3. EDX spectrum of the rGO/Ag NPs nanocomposite.
4

S. Hemmati, M.M. Heravi, B. Karmakar et al.
Journal of Molecular Liquids 319 (2020) 114302
Fig. 4. WDX atomic mapping of the rGO/Ag NPs nanocomposite.
another 25 s. Incidentally, the reaction hardly proceeded within
this period (Fig. 7d). This was also an indication for the insignifi-
cant leaching of Ag species into the reaction mixture. In fact, ICP-
OES analysis of the filtrate justified that there was no leaching of
Fig. 5. TEM image of the rGO/Ag NPs nanocomposite.
Fig. 6. XRD pattern of the rGO/Ag NPs nanocomposite.
5

S. Hemmati, M.M. Heravi, B. Karmakar et al.
Journal of Molecular Liquids 319 (2020) 114302
Fig. 7. (a) The absorption profile in the reduction of MO catalyzed over rGO/Ag NPs nanocomposite, (b) Its kinetic plots (lnAt/A0 vs t), (c) reusability of rGO/Ag NPs nanocomposite for the
reduction of MO, and (d) Product yield vs time (i; with catalyst) and the hot filtration run (ii, without catalyst).
Ag species into the reaction solution, which confirms the true het-
erogeneous nature of the composite. The same experiment was
conducted with RhB reduction and similar results were obtained
(Fig. 8d).
Menthapulegium flower extract. Tiny Ag NPs were homogeneously
dispersed and anchored over reduced GO surface exploiting the herb
bio-ingredients. They also help to cap and stabilize the NPs. The protocol
involves mild and clean reaction conditions and devoid of harsh
chemicals and solvents. We demonstrated that rGO/Ag NPs nanocom-
posite could efficiently catalyze the reduction of MO and RhB dyes in
short reaction times, in the presence of the NaBH4 as reductant, being
followed up over UV–Vis spectrophotometer. The excellent recyclabil-
ity, minute leaching and true heterogeneity confirmed the high robust-
ness of our prepared nanocomposite.
3.3. The mechanistic study
A reasonable mechanistic pathway for the reduction of MO and
RhB dyes over rGO/Ag NPs nanocomposite in aqueous medium has
been suggested as depicted in Scheme 2. NaBH4 initially dissociates
to produce borohydride ions which get adsorbed over the surface
of Ag NPs. In the meantime, the dye is also adsorbed over the catalyst
surface via π-π stacking interactions. H− ions are then transferred to
the dye and it is reduced. Eventually, the reduced dye leaves the
nanocomposite surface making the catalytic site free for the next
cycle [64].
In order to display the significant benefit of the mentioned approach
in this study, compared to literature, some of results for MO reduction
are listed in Table 1. The results show that the rGO/Ag NPs nanocompos-
ite is an efficient catalyst considering the rate constant comparing with
the before reported catalysts.
CRediT authorship contribution statement
I have to add that this submission is original and all co-authors are
contributed and aware of the submission and also agree to its publica-
tion in the J Mol Liquid. This work is novel/original and is not under con-
sideration for publication elsewhere.
Seba Hemmati: Investigation, Formal analysis. Majid Heravi:
Supervision, Investigation, Validation. Bikash Karmakar: Writing -
review & editing, Formal analysis. Hojat Veisi: Planning, Writing -
original draft.
Declaration of competing interest
4. Conclusion
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influ-
ence the work reported in this paper.
In summary, we have introduced a water dispersible rGO/Ag
NPs nanocomposite following a biogenic green approach using
6

S. Hemmati, M.M. Heravi, B. Karmakar et al.
Journal of Molecular Liquids 319 (2020) 114302
Fig. 8. (a) The absorption profile in the reduction of RhB catalyzed over rGO/Ag NPs nanocomposite, (b) Its kinetic plots (ln At/A0 vs t), (c) reusability of rGO/Ag NPs nanocomposite for the
reduction of RhB, and (d) Product yield vs time (i; with catalyst) and the hot filtration run (ii, without catalyst).
-
SO3 Na+
-
SO3 Na+
N
N
H
N
N
MO
Hydrazine derivative
H
-
-
BH4
BO2
N
+2e, +2H+
N
+1e, +H+
N
O
N
-
-
BH4
BO2
N
O
N
O
O
O
O
Leuco RhB
RhB
H
Scheme 2. Mechanism for the reduction of MO and RhB by rGO/Ag NPs nanocomposite.
7

S. Hemmati, M.M. Heravi, B. Karmakar et al.
Journal of Molecular Liquids 319 (2020) 114302
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Table 1
Comparison of the catalytic capacities of various catalysts reported in the literature for the
reduction of MO by NaBH4.
Entry
Catalyst
T (K)
k (s−1) × 10−3
Ref.
1
2
3
4
5
6
7
8
9
Fe3O4-g-C3N4-TCT-PAA-Ag
Ag-γ-Fe2O3@CS
TiO2
Fe3O4/Hal-Mel-TEA(IL)-Pd
CuO nanostructure
Co:La:TiO2
SGCN/Fe3O4/PVIs/Pd
SGCN/Fe3O4/PVIs/Pd
rGO/Ag NPs nanocomposite
298
298
303
293
308
303
298
303
298
45
61
52
45
11
77
52
115
98
[31]
[65]
[66]
[61]
[67]
[66]
[68]
[68]
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This work
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Acknowledgments
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SH and MMH are thankful Alzahra University Research Council
and Elites Federation. HV is grateful to Payame Noor University
(PNU) for financial supports and BK thanks Gobardanga Hindu Col-
lege for affiliation.
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