A facile synthesis methodology for preparation of Ag-Ni-reduced graphene oxide: a magnetically separable versatile nanocatalyst for multiple organic reactions and density functional study of its electronic structures

Article abstract of DOI:10.1039/C8RA08235A

Here, we report a simple ‘in situ' co-precipitation reduction synthesis method for the preparation of nanocatalysts composed of Ag, Ni nanoparticles, and reduced graphene oxide (RGO). First-principles calculations based on Density Functional Theory (DFT) were performed to obtain the electronic structures and properties of Ag-Ni-graphene superlattice and to understand the interfacial interactions which exist at the interface between Ag, Ni, and graphene. The catalytic performance of the synthesized catalysts (Ag_xNi_(1?x))_yRGO_(100?y) were evaluated for four reactions (i) reduction of 4-nitrophenol (4-NP) in the presence of excess NaBH₄ in aqueous medium, (ii) A3 coupling reaction for the synthesis of propargylamines, (iii) epoxidation of styrene, and (iv) ‘Click reaction' for the synthesis of 1,2,3-triazole derivatives. For all of these reactions the catalyst composed of Ag, Ni, and RGO, exhibited significantly higher catalytic activity than that of pure Ag, Ni, and RGO. Moreover, an easy magnetic recovery of this catalyst from the reaction mixture after completion of the catalytic reactions and the good reusability of the recovered catalyst is also reported here. To the best of our knowledge, this is the first time the demonstration of the versatile catalytic activity of (Ag_xNi_(1?x))_yRGO_(100?y) towards multiple reactions, and the DFT study of its electronic structure have been reported.

Full text of DOI:10.1039/C8RA08235A

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A facile synthesis methodology for preparation of
Ag–Ni-reduced graphene oxide: a magnetically
separable versatile nanocatalyst for multiple
organic reactions and density functional study of its
electronic structures†
Cite this: RSC Adv., 2018, 8, 37774
Madhurya Chandel, Priyanka Makkar, Barun Kumar Ghosh, Debabrata Moitra
*
and Narendra Nath Ghosh
Here, we report a simple ‘in situ’ co-precipitation reduction synthesis method for the preparation of
nanocatalysts composed of Ag, Ni nanoparticles, and reduced graphene oxide (RGO). First-principles
calculations based on Density Functional Theory (DFT) were performed to obtain the electronic
structures and properties of Ag–Ni-graphene superlattice and to understand the interfacial interactions
which exist at the interface between Ag, Ni, and graphene. The catalytic performance of the synthesized
catalysts (AgxNi(1ꢀx))yRGO(100ꢀy) were evaluated for four reactions (i) reduction of 4-nitrophenol (4-NP) in
the presence of excess NaBH4 in aqueous medium, (ii) A3 coupling reaction for the synthesis of
propargylamines, (iii) epoxidation of styrene, and (iv) ‘Click reaction’ for the synthesis of 1,2,3-triazole
derivatives. For all of these reactions the catalyst composed of Ag, Ni, and RGO, exhibited significantly
higher catalytic activity than that of pure Ag, Ni, and RGO. Moreover, an easy magnetic recovery of this
catalyst from the reaction mixture after completion of the catalytic reactions and the good reusability of
the recovered catalyst is also reported here. To the best of our knowledge, this is the first time the
demonstration of the versatile catalytic activity of (AgxNi(1ꢀx))yRGO(100ꢀy) towards multiple reactions, and
the DFT study of its electronic structure have been reported.
Received 5th October 2018
Accepted 4th November 2018
DOI: 10.1039/c8ra08235a
rsc.li/rsc-advances
catalytic activity towards multiple organic reactions as well as
easy magnetic recovery from reaction mixtures, is limited.10,12
1. Introduction
To develop an efficient, but versatile and magnetically
separable catalyst, we have synthesized the catalysts which are
composed of varying amounts of Ag, Ni nanoparticles, and
RGO. We have investigated the catalytic activity of the synthe-
sized catalysts towards four important reactions: (i) reduction of
4-nitrophenol (4-NP) in the presence of NaBH4, (ii) A3 coupling
reaction for the synthesis of propargylamines, (iii) epoxidation
of styrene, and (iv) ‘Click reaction’ for the synthesis of 1,2,3-
triazole derivatives in aqueous medium. We have chosen to
develop a versatile catalyst for these reactions because of the
following reasons:
(i) Reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-
AP): nitrophenols and their derivatives are widely used, or
produced in many industries, such as dye-based industries,
textiles, leather, pesticides, pharmaceuticals, iron and steel
manufacturing, foundries, renery, etc.8,11,13 Effluents of these
industries contain a signicant amount of 4-NP and other
nitrophenols. These effluents, when coming in contact with
natural water bodies, cause water pollution, because these
nitrophenols are harmful to the kidney, liver, central nervous
systems, etc.11,14,15 Therefore, to address this environmental
For the last couple of decades, the applications of graphene and
graphene-based materials have been explored extensively in
myriad elds including electronics, sensors, solar cells, struc-
tural materials, catalysis, microwave absorbing materials,
supercapacitors, etc.1–5 Graphene has proved its capability to
become a material of paramount importance by exhibiting
several excellent properties (such as, electrical conductivity,
mechanical strength, chemical stability, lightweight, etc.).1,2,6,7
Several researchers have reported the application of graphene
or Reduced Graphene Oxide (RGO) as the support to construct
heterogeneous catalysts, where catalytically active sites are
immobilized on the surface of graphene or RGO sheets.6–11
However, to the best of our knowledge, to date, the development
of graphene-based catalysts, which can exhibit versatile high
Nano-materials Lab, Department of Chemistry, Birla Institute of Technology and
Science, Pilani K. K. Birla Goa Campus, Goa-403726, India. E-mail: naren70@
yahoo.com; Fax: +91 832 2557033; Tel: +91 832 2580318
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c8ra08235a
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pollution issue removal or destruction of 4-NP (or similar high surface area support and developing magnetically sepa-
nitrophenols) from the polluted aquatic system is very crucial. rable catalyst are attractive strategies.10,14,18,34,37,38 Therefore, to
On the other hand, reduction of 4-NP produces 4-AP, which is develop a catalyst which will exhibit catalytic activity towards all
an important chemical for many pharmaceutical industries for of the above mentioned organic reactions along with easy
the production of antipyretic and analgesic drugs, etc.10,11,14,16 magnetic separation, we have designed the catalyst where Ag
Moreover, several researchers have investigated the reduction of and Ni nanoparticles are intimately coexisting, and these
4-NP to 4-AP as a model reaction to evaluate the catalytic activity nanoparticles are immobilized on the surface of nanometer
of variety of nanostructured catalysts because this reaction is thin sheets of RGO. The reasons for choosing Ni and Ag nano-
easy to perform, convincing, and trustworthy for this particles as catalytically active components of this catalyst is
purpose.8,9,11,13,14,16
that Ni and Ag nanoparticles can participate in these reactions
(ii) A3 coupling reaction: this reaction can be used to by stabilizing intermediates through the formation of coordi-
synthesize propargylamines through a one-pot reaction using nation complexes and thus help the reactions to proceed
the combination of three starting materials (e.g. aldehyde, forward.22,23,39–41 Moreover, as Ni nanoparticles are magnetic in
amine, and alkyne). Propargylamines are an important class of nature, the presence of Ni introduces a magnetic character in
compound for the preparation of several biologically active the catalyst and makes the catalyst magnetically separable. RGO
nitrogen compounds, and also crucial intermediates for many provides a high surface area, which helps the reactant mole-
pharmaceuticals and natural products.17,18 Traditional methods cules to get adsorbed on the surface of the catalyst and to come
for propargylamines synthesis (such as the nucleophilic addi- in contact with catalytically active sites (here Ag and Ni). It is
tion of Grignard reagents or lithium acetylide to imines, etc.) also expected that the high conducting nature of RGO will help
involve multistep synthesis or purication.19–21 Use of moisture in the electron transfer process of these catalysis reactions.1,3,6
sensitive reagents in a regulated reaction condition is one of the
Herein, we are reporting an ‘in situ’ co-precipitation reduc-
limitations associated with these traditional methods which tion technique for the preparation of a versatile catalyst, ((Agx-
limits the large-scale reaction.18,21 To avoid these problems, the Ni(1ꢀx))yRGO(100ꢀy)), for (i) reduction reaction of 4-nitrophenol
coupling of aldehydes, amines, and alkynes via A3 coupling (4-NP) in presence of NaBH4, (ii) A3 coupling reaction for the
reaction over the heterogeneous catalysts has emerged as synthesis of propargylamines, (iii) epoxidation reaction of
a promising approach to synthesize propargylamines.17,18,22,23
styrene, and (iv) ‘Click reaction’ for the synthesis of 1,2,3-tri-
(iii) Epoxidation of styrene: epoxidation is an important azole derivatives in aqueous medium and magnetic recovery
reaction for the chemical industries because of the wide range and reusability of the catalyst. We have also performed the rst-
applications of epoxy resins, including synthesis of many principles calculations based on Density Functional Theory
commodities, useful intermediates for the synthesis of many (DFT) to understand how the interfacial interactions between
pharmaceutical products, and ne chemicals.24,25 Conventional Ag, Ni, and RGO inuence their electronic structures, due to
methodologies for the epoxidation of styrene (such as the non- which the synergistic effect originates and the synthesized
catalytic process using chlorine, co-epoxidation process, catalyst exhibits superior catalytic activity. To the best of our
organic peroxide and peracid-based catalytic process, etc.) suffer knowledge, this is the rst time the catalyst, containing Ag, Ni
from some disadvantages, such as the production of the large nanoparticles and RGO, has been reported which can exhibit
amount of undesirable by-products, harsh reaction conditions, catalytic property for these four reactions.
cost-effectiveness, separation problem associated with homo-
geneous catalysts, etc.10,26,27 Therefore, till date, several
researchers are engaged in developing novel catalysts for
2. Experimental
2.1. Materials
chlorine-free epoxidation reaction.27–30
(iv) Click reaction: synthesis of triazoles by ‘Click reaction’
has gained immense attention due to the use of triazoles as Acetone, hydrazine hydrated (N2H4$H2O), polyvinylpyrrolidone
biologically active chemicals, agrochemicals, pharmaceuticals, (PVP), sodium hydroxide (NaOH), sodium nitrate, sulphuric
drug molecules with signicant anti-HIV activity, antimicrobial acid (H2SO4), hydrochloric acid (HCl), sodium nitrate (NaNO3),
activity against Gram-positive bacteria, etc.31–34 Several 4-nitrophenol (4-NP), and nickel(II)nitrate hexahydrate
researchers have developed various types of heterogeneous (Ni(NO3)2$6H2O) were purchased from Fischer Scientic. Silver
catalysts for the synthesis of triazole derivatives.23,32,34,35 nitrate (AgNO3), potassium permanganate (KMnO4), sodium
However, in some cases the limitations associated with these azide (NaN3), 30% H2O2 solution, ethylene glycol, acetonitrile,
procedures are the requirement of elevated reaction tempera- methanol, dichloromethane, toluene, chloroform, diethyl-
ture, the formation of undesired products, low yields, use of amine, and aniline were purchased from Merck, India. Sodium
homogeneous catalysts which are difficult to separate from borohydride (NaBH4), aldehydes, piperidine, phenylacetylene,
products, etc.31,35,36
styrene, styrene oxide, cyclohexene oxide, epichlorohydrin, tert-
Though several nanostructured catalysts exhibit high cata- butyl hydroperoxide (TBHP in 5–6 M decane), and graphite
lytic activity due to their high surface area, their tendency to powder (mean particle size of <20 mm) were purchased from
form agglomerates and difficulty in separation by easy ltration Sigma-Aldrich. All the chemicals were used without further
process are major limitations for large-scale applications. To purication. Distilled water was used throughout the
overcome these limitations immobilization of nanocatalysts on experiment.
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reaction, the initial concentration of NaBH4 was very high, and
it remained almost constant throughout the reaction. It is
a well-established fact that the metal nanoparticle catalyzed
reduction reaction of 4-NP in the presence of excess NaBH4
proceeds via pseudo rst-order kinetics.3,11,14,33 As the absor-
bance of 4-NP is proportional to its concentration, the ratio of
the absorbance of 4-NP At (measured at time t) to Ao (at t ¼ 0) is
equal to Ct/Co (where Co is the initial concentration and Ct is the
concentration at time t of 4-NP). The apparent rate constant
(kapp) was determined using the following equations:3,14
2.2. Synthesis of ((AgxNi(1ꢀx))yRGO(100ꢀy)
)
We have synthesized the nanocatalysts composed of the
different amount of Ag nanoparticle, Ni nanoparticle, and RGO,
in two steps. In the rst step, graphene oxide (GO) was prepared
by using the modied Hummers method (method of GO
synthesis is provided in ESI†).42 In the next step, a mixture of
ethylene glycol (EG) and PVP (molar ratio 2 : 1) was prepared,
and the calculated amount of GO sheets were dispersed in this
mixture. In this dispersion AgNO3 and Ni(NO3)$6H2O were
added according to the compositions of the nanocatalyst (i.e.
(AgxNi(1ꢀx))yRGO(100ꢀy), where x ¼ 0–1, and y ¼ 0–100). Aer the
addition of metal nitrate salts, the mixture was stirred until the
metal salts were dissolved. In this mixture, NaOH pellets were
added with stirring till the pH of the mixture became ꢁ10, and
then N2H4 was added dropwise keeping the metal ion: N2H4
molar ratio 1 : 40. Aer complete addition of N2H4, this reaction
mixture was reuxed at 85 ꢂC for 15 min and then cooled to
room temperature. The precipitate thus formed was magneti-
cally separated from the reaction mixture by using a magnet
externally. The collected precipitate was washed with distilled
water until the pH of the washing reached ꢁ7. Finally, the
precipitate was washed with acetone and dried at 60 ꢂC for 10 h.
As a representative, the detail of the synthesis procedure for
(Ag0.27Ni0.73)37RGO63 preparation has been described in ESI.†
To perform the control experiments, we have also prepared pure
Ag, and pure Ni nanoparticles using the aforesaid method,
without adding RGO. Pure RGO was also synthesized by
reducing GO with N2H4. Several compositions of catalysts,
containing only Ag and Ni nanoparticles (i.e. AgxNi(1ꢀx)), were
also prepared using the same method. The code of the catalysts
and the corresponding compositions are listed in Table S1
(ESI†).
dCt/dt ¼ ꢀkappCt
(1)
(2)
ln(Ct/Co) ¼ ln(At/Ao) ¼ ꢀkapp
t
The value of kapp was calculated from the slope of the ln(At/
Ao) vs. time plot.
2.3.2 Synthesis of propargylamines by A3 reaction. The
catalytic activity of the synthesized catalyst (AgxNi(1ꢀx))y-
RGO(100ꢀy) was tested for the synthesis of propargylamine via an
A3 coupling reaction. In a typical reaction, in a 25 ml round
bottomed ask 2 mmol aldehyde, 2.4 mmol amine, 3 mmol
phenylacetylene, and 50 mg catalyst were mixed with 10 ml
acetonitrile under the nitrogen atmosphere. Then, the reaction
mixture was stirred for 12 h at 100 ꢂC. The progress of the
reaction was monitored by Thin Layer Chromatography (TLC).
Aer completion of the reaction, the reaction mixture was
allowed to cool down, and from the reaction mixture catalyst
was recovered magnetically. The solvent was removed using
a rotavapor and the collected crude product was puried using
a gel column chromatography with petroleum ether and ethyl
acetate (10 : 2) as eluent. The catalysis reaction was performed
at different reaction conditions and it was observed that the
highest yield was obtained when the reaction temperature was
ꢂ
100 C, reaction time was 12 h and acetonitrile as the solvent.
2.3. Catalytic activity tests
Aer optimizing the reaction conditions, the A3 coupling
reaction was also performed by using different types of alde-
hydes and amines to demonstrate the effectiveness of the
synthesized catalyst.
2.3.1 Reduction of 4-nitrophenol to 4-aminophenol. To
investigate the catalytic activity of the synthesized catalysts the
reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP) was
performed in the presence of excess NaBH4 in an aqueous
medium. The reaction was carried out in the presence of cata-
lysts (AgxNi(1ꢀx))yRGO(100ꢀy) with different compositions. In
a typical run, 4.5 ml of 9 mM aqueous solution of 4-NP was
mixed with 1.5 ml H2O and 1 ml of 0.2 M NaBH4 solution. To
this solution, 1 ml of an aqueous suspension of the catalyst
(0.1 g lꢀ1) was added. 4 ml of this reaction mixture was trans-
ferred immediately in a quartz cuvette and the absorbance
spectra of the solution were recorded using a UV-Vis spectro-
photometer (V-570, Jasco, Japan) at an interval of 1 min. Before
the addition of NaBH4, the aqueous solution of 4-NP exhibited
the maximum absorption (lmax) at 317 nm. When the aqueous
solution of NaBH4 was added to the solution of 4-NP, the color
of the solution became dark yellow due to the formation of
phenolate ion and lmax peak was red-shied to 400 nm.3,14 Aer
addition of the catalyst, the color of the reaction mixture started
to fade with the progress of time. The progress of the reduction
of 4-NP was determined by monitoring the gradual decrease of
the intensity of the lmax peak (at 400 nm) with time. In this
2.3.3 Epoxidation of styrene. The catalytic activity of the
synthesized (AgxNi(1ꢀx))yRGO(100ꢀy) catalyst was tested towards
the epoxidation of styrene using TBHP as an oxidizing agent. In
a typical experiment, 5 mmol styrene, 4 ml acetonitrile, and
25 mg catalyst were mixed in a round bottomed ask, and the
mixture was sonicated for 5 min. In this mixture, 12.5 mmol of
TBHP was added and then the reaction mixture was reuxed
ꢂ
under the nitrogen atmosphere at 100 C. The progress of the
reaction with time was monitored by drawing 0.1 ml aliquot
from the reaction mixture and the catalyst was separated from
the reaction mixture using a magnet externally. Then the reac-
tion mixture was analyzed using a gas chromatograph (GC). The
conversion and product selectivity were calculated using the
following eqn (3) and (4) respectively:10,15
moles of reactant converted
Conversion ð%Þ ¼
ꢃ 100 (3)
moles of reactant feed
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calculate the ground state structures, binding energy, Density of
States (DOS), and Projected Density of states (PDOS) using
a plane wave set and pseudopotentials. DFT was used with
Generalized Gradient Approximation (GGA)44 and parameter-
ized by Perdew, Burke, and Ernzerhof (PBE).45 Kohn–Sham
orbitals were expanded in a plane-wave basis set up to the
kinetic energy cutoff 25 Ry (340 eV). The convergence criterion
for the self-consistence calculation was 10ꢀ7 Ry per Bohr
moles of product formed
Product selectivity ð%Þ ¼
ꢃ 100
moles of reactant converted
(4)
The reaction was conducted under different reaction condi-
tions. It was observed that the highest amount of styrene
conversion was achieved when the reaction was performed at
ꢂ
(0.0257 eV Aꢀ1). For calculating the nal electronic properties of
100 C for 10 h with 1 : 2 styrene : TBHP ratio. Therefore, this
˚
reaction condition was used when the reaction was performed
with the catalysts having different compositions.
the structures, and empirical dispersion-corrected density
functional theory (DFT-D2) approach, which was proposed by
Grimme46,47 was employed. To account for the intermolecular
interactions and van der Waals (vdW) interactions, the Grimme-
D2 correction was used.12
2.3.4 Synthesis of 1,4-disubstituted 1,2,3-triazoles by ‘Click
reaction’. For the synthesis of 1,4-disubstituted 1,2,3-triazoles
by ‘Click reaction’, the synthesized materials (AgxNi(1ꢀx))y-
RGO(100ꢀy) were used as catalyst. Here, 2-phenyl-2-(4-phenyl-1H-
1,2,3-triazol-1-yl) ethanol was synthesized in aqueous medium
as a model reaction. In a typical synthesis, 1 mmol styrene
oxide, 1.5 mmol sodium azide, 1 mmol phenylacetylene, and
25 mg of (AgxNi(1ꢀx))yRGO(100ꢀy) were mixed with 3 ml water in
a round bottomed ask. This reaction mixture was reuxed at
100 ꢂC for 12 h with constant stirring. Then the reaction mixture
was allowed to cool down to room temperature, and the catalyst
was separated magnetically. The product was separated from
the reaction mixture by ltration, followed by recrystallization.
To determine the optimized catalyst composition, the reactions
were performed using the catalysts having different amounts of
In the present study ve systems were investigated (i)
superlattice of pure Ag with face-centered cubic structure (space
ꢀ
group Fm3m), (ii) pure Ni with face-centered cubic structure
ꢀ
(space group Fm3m), (iii) graphene superlattice, (iv) Ag–Ni
interface, and (v) Ag–Ni-graphene superlattice. Ultraso pseu-
dopotential for these systems were constructed by using 17, 16,
and 4 electrons for Ag (4p64d105s1), Ni (3p63d84s2), and C
(2s22p2), respectively. From the Quantum ESPRESSO website,
the pseudopotentials of Ag, Ni, and C were chosen.48 To select
the k-points mesh, the Monkhorst–Pack approach was used49
and the details of k points for each system are provided in the
computational details section in the ESI.† The Brillouin zone
integration of these systems was performed with Methfessel–
Paxton smearing technique50 for Ag (111) slab, and Ni (111) slab,
Marzari–Vanderbilt smearing technique51 for graphene, Ag–Ni
interface, and Ag–Ni-graphene superlattice. Here, the smearing
parameter was 0.005 Ry. The Binding energy of Ag–Ni-graphene
superlattice was calculated from the difference between the
total energy of the Ag–Ni-graphene superlattice and the sum of
each system alone (Ag–Ni interface and graphene). The sizes of
the unit cells of the simulated systems are listed in Table S2
(ESI†). The details of the sample input les for the geometric
optimization of Ag, Ni, graphene, Ag–Ni, and Ag–Ni-graphene
superlattice are provided in the computational details section
in ESI.†
Ag, Ni, and RGO. As (Ag0.27Ni0.73)37RGO63 was found to be the
most effective catalyst, the Click reaction was also carried out
using different types of epoxy with this as catalyst.
2.3.5 Magnetic recovery of the catalyst and recycling. Aer
each reaction cycle, the catalyst was separated from the reaction
mixture by applying an external permanent magnet (N35 grade
NdFeB magnet having energy product BHmax ¼ 33–36 MGO) and
then the catalyst washed with distilled water and ethanol. It was
observed that no unreacted molecule was remained adsorbed in
ꢂ
the catalyst. Aer washing, the catalyst was dried at 100 C for
8 h in an oven, and then used for the next reaction cycle.
2.4. Characterization
The synthesized pure Ni, pure Ag, AgxNi(1ꢀx), and (AgxNi(1ꢀx))y-
RGO(100ꢀy) catalysts were characterized by X-ray diffraction
(XRD), thermogravimetric analysis (TGA), Fourier Transform
Infrared spectra (FT-IR), Raman, Field Emission Scanning
Electron Microscope (FESEM), energy dispersive X-ray analysis
(EDX), and Vibrating Sample Magnetometer (VSM). The prod-
ucts which were obtained from catalysis reactions were char-
acterized by Liquid Chromatography-Mass Spectrometer (LC-
MS), Proton Nuclear Magnetic Resonance (1H NMR), Gas
Chromatography (GC), and Differential Scanning Calorimetric
(DSC) (details of the characterization are discussed in ESI†).
3. Results and discussion
3.1. Structure and morphology of the synthesized
(AgxNi(1ꢀx))yRGO(100ꢀy) catalyst
The room temperature wide-angle XRD was used to identify the
crystalline phases which were present in the synthesized cata-
lysts. As a representative, XRD patterns of GO, pure Ag, pure Ni,
Ag0.50Ni0.50, Ni30RGO70, and (Ag0.27Ni0.73)37RGO63 are shown in
Fig. 1. The XRD patterns of other compositions of the catalysts
are shown in Fig. S1–S2 (ESI†). In case of pure Ag nanoparticles,
diffraction peaks at 2q ¼ 38.2ꢂ, 44.3ꢂ, 64.6ꢂ, and 77.3ꢂ, which
correspond to the (111), (200), (220), and (222) planes of FCC
phase of Ag (ICDD no. 04–0783), were observed. XRD pattern of
2.5. First-principles calculations
We have performed the rst-principles quantum mechanical pure Ni sample showed the peaks at 2q ¼ 44.3ꢂ, 51.7ꢂ and 76.4ꢂ
calculations based on DFT to obtain the electronic structures of corresponding to (111), (200), and (220) planes of FCC phase of
Ni, Ag, Ag–Ni interface, and Ag–Ni-graphene superlattice. Here, Ni (ICDD no. 04–0850). In the XRD patterns of the AgxNi(1ꢀx)
we have used Quantum ESPRESSO computational package43 to catalysts, the characteristic diffraction peaks of the only Ag and
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TGA and DTA of the synthesized catalysts were performed to
understand the effect of temperature on the catalysts and also
to determine the amount of RGO present in the catalyst (Fig. 2).
In the TGA-DTA thermogram of pure GO following points were
observed (i) ꢁ11% weight loss occurred in the temperature
ꢂ
range of 30–110 C, which might be due to the loss of surface
adsorbed water, (ii) in the temperature range of 110–290 ꢂC,
ꢁ31% weight loss in TGA and a sharp exothermic peak at 240 ꢂC
in DTA indicated the removal of oxygen-containing functional
groups (e.g. epoxy, carbonyl, carboxylic group) from GO in this
temperature range, (iii) the complete decomposition of GO
occurred in the temperature range of 300–625 ꢂC. The
exothermic nature of this decomposition was indicated by the
presence of an exothermic peak at 460 ꢂC in the DTA. The TGA
thermogram of pure RGO showed (i) ꢁ5% weight loss in 30–
110 ꢂC temperature range due to the removal of adsorbed water,
and (ii) a continuous weight loss till the complete decomposi-
tion of RGO in the temperature range of 110–625 ꢂC. DTA of
RGO also showed an exothermic peak at 500 ꢂC. From TGA-DTA
thermograms of the synthesized catalysts two major points were
observed (i) similar to GO and RGO, the loss of surface adsorbed
water (ꢁ5 wt%) occurred in the temperature range of 30–110 ꢂC,
(ii) in the temperature range of 110–550 ꢂC, the continuous
weight loss due to the decomposition of RGO from the catalyst.
This major weight loss step is similar to pure RGO. Here, the
important points were noted (i) for the (AgxNi(1ꢀx))yRGO(100ꢀy)
samples, no distinguishable weight loss step in the temperature
range of 110–290 ꢂC was observed, which was noticed in case of
pure GO. This fact indicated that (AgxNi(1ꢀx))yRGO(100ꢀy)
samples contain only a few numbers of oxygen-containing
functional groups, and GO was converted to RGO via the
partial reduction of these groups during the synthesis of the
catalysts. (ii) the decomposition of RGO occurred in the cata-
lysts at a relatively lower temperature (ꢁ550 ꢂC) than that RGO
(ꢁ625 ꢂC). This could be due to the presence of Ag and Ni
particles in the catalyst, which facilitated the decomposition of
carbon, (iii) in case of the catalysts ((AgxNi(1ꢀx))yRGO(100ꢀy)),
aer complete decomposition of RGO, the remaining amount of
the undecomposed catalyst matched well with the expected
Fig. 1 Room temperature wide angle powder XRD pattern of (a) GO,
(b) pure Ni, (c) Ni30RGO70, (d) pure Ag, (e) Ag0.50Ni0.50, and (f)
(Ag0.27Ni0.73)37RGO63.
Ni were present (Fig. S1, ESI†). In case of pure GO sample, an
intense peak at 2q ¼ 9.76ꢂ and a small peak at 2q ¼ 42.14ꢂ were
observed which represented the (001) and (101) planes of GO,
respectively.3,4,10 In the XRD pattern of (AgxNi(1ꢀx))yRGO(100ꢀy)
samples, all the peaks which are characteristic diffraction peaks
of Ag and Ni were present indicating the presence of pure Ni
and Ag nanoparticles in these samples (Fig. S2, ESI†). The
crystallite size of Ag and Ni particle was calculated by Scherrer's
equation using the diffraction peak corresponding to (111)
plane of Ag and (111) plane of Ni. The crystallite sizes of Ni and
Ag nanoparticles were ꢁ11 nm and ꢁ15 nm, respectively. It was
also observed that in the XRD patterns of (AgxNi(1ꢀx))yRGO(100ꢀy)
samples no diffraction peaks of GO were present. Therefore, it
was concluded that during the synthesis of these nanocatalysts
by the ‘in situ’ co-precipitation reduction technique GO was
converted to RGO. TGA, FT-IR, and Raman spectroscopy of
these samples also indicated this conversion of GO to RGO.
Fig. 2 (a) TGA and (b) DTA curve of GO, RGO, and (Ag0.27Ni0.73 37
)
RGO63
.
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Fig. 3 FESEM images of (a) pure RGO, (b) pure Ni, (c) pure Ag, (d) Ag0.50Ni0.50, (e) (Ag0.27Ni0.73)37RGO63.
amount of Ag and Ni in the catalyst. This is an indication that, The ID/IG ratio of GO, RGO, and the catalyst were 0.98, 1.02, and
the ‘in situ’ co-precipitation reduction method, which we have 1.15, respectively. This increase of ID/IG for RGO and the catalyst
adopted to prepare the catalysts, is capable of producing compared to GO might be due to the decrease in the average
nanocatalysts with the desired composition of Ag, Ni, and RGO. size of sp2 domains upon the reduction of GO during the
The FT-IR spectra of GO, pure RGO, and the catalyst formation of the catalyst.4,10
(Ag0.27Ni0.73)37RGO63 are shown in Fig. S3 (ESI†). In case of GO
FESEM micrographs of the synthesized pure RGO, Ag
and (Ag0.27
the peaks at (i) 1384 cmꢀ1 for the C–O stretching vibration of nanoparticles, Ni nanoparticle, Ag0.50Ni0.50
,
-
–CO2H group, (ii) 1234 cmꢀ1 for the C–O stretching vibration of Ni0.73)37RGO63 nanocomposite are shown in Fig. 3. Nanometer
an epoxy group, (iii) 1053 cmꢀ1 for C–O stretching vibration thin sheets were clearly visible in the micrograph of RGO. The
were observed, which indicated the presence of carboxyl, epoxy, micrographs of pure Ni and Ag showed that in both the cases
and carbonyl groups on the surface of GO sheets.3,52 Moreover, almost spherical nanoparticles, having the particle size of 30–
the peak at 1620 cmꢀ1 represented the contribution of the C]C 50 nm range were attached closely to each other. In case of
skeletal vibration of the graphitic domain of GO.3,52 In the FT-IR Ag0.50Ni0.50 sample, Ag and Ni nanoparticles were randomly
spectra of RGO and the catalyst, the disappearance of the peaks attached to each other. However, the large agglomerations of
at 1730 and 1234 cmꢀ1, and decrease in the intensity of the peak only Ni or only Ag particles were not observed. The micrograph
at 1053 cmꢀ1 indicated the reduction of the functional groups of of (Ag0.27Ni0.73)37RGO63 revealed that closely attached Ag and Ni
GO during the preparation of RGO and the catalysts. It was also particles were immobilized on the surface of RGO sheets. Color
noted that the band at 1620 cmꢀ1 (in GO sample) has been red mapping of Ag0.50Ni0.50 and (Ag0.27Ni0.73 37
) RGO63 also showed
shied to 1545 cmꢀ1 (for RGO) and 1580 cmꢀ1 (for catalyst). The the intimate attachment of Ag and Ni particles (Fig. S5–S7,
red shiing of this band, which corresponds to the C]C skel- ESI†). EDS analysis of the samples also conrmed the compo-
etal vibration, can be interpreted as the partial restoration of p– sition of the composites (Fig. S8 and S9, ESI†).
p conjugation of graphene sheet in the pure RGO as well as the
catalyst.3,52 These results clearly indicated the formation of RGO
from GO during the synthesis of the catalysts.
3.2. Formation mechanism of (AgxNi(1ꢀx))yRGO(100ꢀy)
nanocomposites
Fig. S4 (ESI†) presents the Raman spectra of the pure GO,
In this ‘in situ’ co-precipitation reduction method the formation
of Ni and Ag nanoparticles might occur due to the following
reactions:53–55 When Ni(NO3)2 was dissolved in ethylene glycol,
Ni2+ ion formed the coordination complex with glycol (eqn (5)).
pure RGO, and (Ag0.27Ni0.73)37RGO63 catalyst. In case of GO, two
characteristic peaks at 1337 and 1589 cmꢀ1 were observed
which correspond to the D and G bands, respectively. In the
case of RGO, these bands were shied to 1328 and 1580 cmꢀ1
.
Raman spectra of the catalyst showed the further shiing of
these D and G bands to the lower wave numbers i.e. at 1315 and
1563 cmꢀ1, respectively. The shiing of the Raman shi values
of D and G bands to the lower bands could be attributed to the
formation of RGO in the catalyst due to the reduction of GO.4,10
Ni2+ + 3EG / [Ni(EG)3]2+
(5)
(6)
(7)
[Ni(EG)3]2+ + mN2H4 / [Ni(N2H4)m]2+ + 3EG
[Ni(N2H4)m]2+ + 2OHꢀ / Ni(OH)2 + mN2H4
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2Ni(OH)2 + N2H4 / 2Ni0 + N2 + 4H2O
where EG: ethylene glycol.
Paper
Fig. S10 and S11 (ESI†). The lattice parameters obtained for the
unit cell of Ag and Ni were 4.08 and 3.52 A, respectively. These
values matched well with the reported values. E. Schiavo et al.
(8)
˚
When this complex reacted with N2H4, the ligand (i.e. EG)
was replaced by N2H4 and formed another complex
[Ni(N2H4)m]2+ (eqn (6)). Ni(OH)2 was formed when [Ni(N2H4)m]2+
was reacted with OHꢀ (eqn (7)). The formation of nickel
hydroxide from hydrazine complex occurred because Ni(OH)2 is
more stable than Ni[(N2H4)m]2+, so this transformation is ther-
modynamically feasible.53 Then, the formation of Ni nano-
particles occurred via the reduction of nickel hydroxide by
hydrazine (eqn (8)).
and F. Cinquini et al. have reported the values of lattice
63
˚
˚
parameter of Ag and Ni 4.09 A (ref. 61 and 62) and 3.52 A,
respectively. In the Ag–Ni superlattice, the binding energy
between Ag and Ni was ꢀ5.68 eV, which indicated the existence
of the interaction between Ag and Ni in the interface. In Ag–Ni-
graphene superlattice, the binding energy between Ag–Ni
interface and graphene was ꢀ3.32 eV, which also indicated the
existence of strong interaction between them. The equilibrium
interlayer distance between Ag–Ni interface and graphene was
Ag nanoparticles were produced via the reduction of Ag+ ions
by ethylene glycol and N2H4. Ethylene glycol was converted to
acetaldehyde aer losing one molecule of H2O (eqn (9)). Then,
Ag+ ions were reduced by acetaldehyde and formed Ag nano-
particles (eqn (10)). Simultaneously, N2H4 also reduced Ag+ and
produced Ag nanoparticles (eqn (11)).56–58
˚
3.29 A.
The electronic total charge density for the interface between
Ag and Ni is shown in Fig. 4d, which clearly shows the orbital
overlap between Ag and Ni in the interface. From this plot, it
was observed that due to this interaction electron decient
centres were formed on/near the Ni atoms. The higher value of
electronegativity of Ag (1.93) than that of Ni (1.91) might be
responsible for this. The difference charge density plot of Ag–Ni
(Fig. 4f), where the blue color represents the charge depletion
and the red color represents the charge accumulation, also
indicated the formation of electron decient center on/near Ni
atoms. The charge density distribution plot (Fig. 4e) and the
difference charge density plot (Fig. 4g) of Ag–Ni-graphene
clearly exhibited the interaction between Ag–Ni, C–Ag, and C–
Ni in the interface.
For in-depth understanding of the interactions which exist
in the interfaces between Ag–Ni, and Ag–Ni-graphene, their
Density of States (DOS), Projected Density of States (PDOS), and
band structures were studied (Fig. S12–19, ESI†). Zero band gap
of graphene was clearly shown by the band structure of gra-
phene (Fig. S17, ESI†). This plot also showed that the valence
bands and the conduction bands are either separated by a gap
or overlapped with each other which intersect in two equivalent
points, known as Dirac points in the rst Brillouin zone.64,65 The
band structures of Ag, Ni, and Ag–Ni-graphene also showed
their metallic character (Fig. S12–19, ESI†). In the band struc-
ture of Ag–Ni interface the appearance of new bands near the
Fermi level compared to Ag and Ni indicated the effect of Ag on
the electronic property of Ni. PDOS of Ag–Ni interface (Fig. S15–
S16, ESI†) also indicated the strong hybridization between Ni
3d, Ni 4s, Ag 4d and Ag 5s. The PDOS on the d band of Ni, which
is positioned between ꢀ4 eV to ꢀ5 eV below the Fermi level, was
found to be enhanced, and the width of d-band was broadened,
indicating the hybridization of Ni 3d with Ag 4d states.66 PDOS
of Ag–Ni-graphene (Fig. S18–S19, ESI†) indicated the strong
hybridization between C 2p states of graphene and Ni 3d and Ag
4d at valance and conduction band. Due to this hybridization,
we can predict that the conductivity of the composite will be
increased when graphene will be incorporated in the system.
The EIS measurements (Nyquist plot, Fig. S20, ESI†) of the
HO–(CH2)2–OH / CH3CHO + H2O
2Ag+ + 2CH3CHO / 2Ag0 + 2H+ + CH3COCOCH3 (10)
4Ag+ + N2H4 / 4Ag0 + N2 + 4H+
(9)
(11)
In this ‘in situ’ co-precipitation reduction technique to
synthesize (AgxNi(1ꢀx))yRGO(100ꢀy), hydrazine also reduced GO
(particularly its epoxy groups (eqn (12))1 and converted GO to
RGO. Generally, this reduction process initiated from the edges
of GO akes and then moved towards the basal planes.1 Gao
et al. have proposed the plausible reaction mechanism for the
reduction of the epoxy groups of GO by N2H4. The general
reaction for this reduction reaction can be presented as eqn
(12).1
(12)
In this synthesis route, PVP played the role of a capping
agent, which prevented the agglomeration of Ag and Ni nano-
particles and restricted their growth. Here, the formation of Ag
and Ni nanoparticles and conversion of GO to RGO occurred
simultaneously. The unreduced or partially reduced oxygen-
containing functional groups of RGO helped to immobilize
the Ag and Ni nanoparticles on the surface of nanometer thin
RGO sheets. These metal nanoparticles also acted as a spacer
between the RGO layers and prevented the agglomeration of
RGO sheets.59,60
3.3. First-principles calculations of electronic structure
To understand the interfacial interactions between Ag, Ni, and synthesized catalysts supported this by showing the charge
graphene in the synthesized catalysts, we have performed the transfer resistance (Rct) of (Ag0.27Ni0.73)37RGO63 (0.144U) is
rst-principles calculations based on DFT. The superlattice much lower than that of RGO (2.073U), Ag0.50Ni0.50 (0.275U),
structures of Ag, Ni, graphene, Ag–Ni interface, and Ag–Ni- Ni(0.85U), and Ag (3.11U), indicating the enhanced conductivity
graphene before and aer full relaxation are presented in of (Ag0.27Ni0.73)37RGO63. The DFT calculations showed that the
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Fig. 4 Electronic total charge density plot of (a) graphene, (b) Ni slab, (c) Ag slab, (d) Ag–Ni interface, (e) Ag–Ni-graphene superlattice, difference
charge density plots of (f) Ag–Ni interface, and (g) Ag–Ni-graphene superlattice (where red color represents charge accumulation and blue color
represents charge depletion in difference charge density plot).
strong interfacial interaction and orbital level hybridization the time required to initiate the reaction was ꢁ4 min. This
between Ag, Ni, and graphene in Ag–Ni-graphene superlattice result clearly indicated the benecial effect of RGO. To under-
originate the formation of electron decient Ni centres and stand the effect of the incorporation of Ag in the catalyst
enhancement of electrical conductivity in Ag–Ni-graphene. composition on its catalytic activity, the catalyst was prepared
These factors might play important roles in the intermediate with 5 wt% Ag and 95 wt% Ni (Ag0.05Ni0.95). The catalytic activity
formation, and faster charge transfer process during the catal- of this catalyst was found to be better than that of pure Ni
ysis reactions. Therefore, we can predict that the synthesized particles. In this case, 22 min were required to complete the
catalyst (AgxNi(1ꢀx))yRGO(100ꢀy) will show better catalytic activity reaction with ꢁ4 min initiation time. These results clearly
than that of pure Ag, or pure Ni, or Ag–Ni. This prediction agrees demonstrated that the presence of both Ag and RGO enhanced
well with our experimental results which have been discussed in the catalytic activity of the catalyst. To tune the compositions of
details in Sections 3.4.
the catalyst, catalysts having varying wt% of Ag, Ni, and RGO
were prepared. The change of reaction completion time and the
apparent rate constant (kapp) with changing composition of the
catalyst are listed in Table 1 and UV-visible spectra are shown in
Fig. S21 (ESI†). It has been observed that, the catalyst composed
3.4. Catalytic activity of (AgxNi(1ꢀx))yRGO(100ꢀy)
3.4.1 Reduction of 4-NP in presence of NaBH4. To evaluate
the catalytic activity of the synthesized catalysts, the reduction
reaction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP) in
presence of excess NaBH4 in aqueous medium was investigated
as a model reaction. To understand the effect of incorporation
of RGO and Ag along with Ni in the catalyst composite, rst, the
catalysis reaction was performed using pure Ni nanoparticles as
the catalyst. It has been observed that under this reaction
condition, the reaction was completed within 30 min and the
time required to initiate the reaction was ꢁ8 min. The
Ni40RGO60 catalyst, where Ni nanoparticles are dispersed on the
surface of RGO nanosheets, exhibited a remarkable increase in
catalytic activity. The reaction was completed within 18 min and
of 3 wt% Ag, 57% Ni, and 40% RGO ((Ag0.05Ni0.95)60RGO40)
exhibited the highest catalytic activity with reaction completion
time of 4 min and the reaction started almost instantly soon
aer the addition of the catalyst which is shown in Fig. 5e. In
this case, the kapp was 19.6 ꢃ 10ꢀ3 sꢀ1. This kapp value was found
to be similar and in some cases superior than the values re-
ported in the literature for the different metal nanoparticle
based catalysts (Table S3, ESI†).
The conversion of 4-NP to 4-AP is a six electron transfer
reduction reaction. When this reaction is carried out in the
presence of metal nanoparticle catalyst and excess NaBH4, it
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Table 1 Comparisons of completion time and rate constant of the
synthesized catalysts for the reduction of 4-nitrophenol in presence of
NaBH4
nanoparticles play the role of storing electrons aer ET from
ꢀ
BH4
.
41 The H atoms thus generated attack 4-NP and reduce it.11
Several factors are responsible for the high catalytic perfor-
mance of (Ag0.05Ni0.95)60RGO40 towards this reaction, such as (i)
nanosize of the Ag and Ni particles helps to adsorb BH4ꢀ ions on
their surface, (ii) from the DFT study we have observed that,
presence of Ag atoms creates electron decient sites on/near Ni
atoms. These electron decient Ni atoms become more effective to
Apparent rate
Initiation
time (min) time (min)
Completion constant
Composites
(kapp) sꢀ1
Pure Ni
Ni40RGO60
Pure Ag
8
4
3
4
2
0
5
2
1
0
0
2
1
0
1
30
18
8
22
14
7
15
15
7
4
8
10
10
6
2.46 ꢃ 10ꢀ3
6.66 ꢃ 10ꢀ3
10.32 ꢃ 10ꢀ3
1.50 ꢃ 10ꢀ3
4.75 ꢃ 10ꢀ3
10.03 ꢃ 10ꢀ3
5.76 ꢃ 10ꢀ3
4.83 ꢃ 10ꢀ3
13.87 ꢃ 10ꢀ3
19.60 ꢃ 10ꢀ3
9.10 ꢃ 10ꢀ3
8.39 ꢃ 10ꢀ3
7.86 ꢃ 10ꢀ3
10.83 ꢃ 10ꢀ3
5.10 ꢃ 10ꢀ3
ꢀ
collect electrons from BH4 via electron transfer process and to
generate hydrogen efficiently. Thus the presence of Ag makes the
catalysts more efficient towards the reduction of 4-NP in presence
of NaBH4, (iii) presence of RGO in the catalyst also helps to
enhance its catalytic activity by providing larger surface area. RGO
exhibits high adsorption capacity of 4-NP due to p–p stacking
interaction.3,10 Rout et al. have reported that due to the strong
adsorption of 4-NP onto the surface of RGO, the N]O bond of
Ag0.05Ni0.95
Ag0.15Ni0.85
Ag0.25Ni0.75
Ag0.27Ni0.73
(Ag0.05Ni0.95 90
(Ag0.05Ni0.95 70
(Ag0.05Ni0.95 60
(Ag0.05Ni0.95 50
(Ag0.05Ni0.95 40
)
)
)
)
RGO10
RGO30
RGO40
RGO50
RGO60
)
˚
˚
–NO2 group becomes stretched out (from 1.23 A to 1.28 A) and this
helps to activate the –NO2 group for the reduction reaction and
convert to –NH2 group.9 (iv) DFT study also showed the existence of
hybridization in the orbital level between Ag, Ni, and graphene.
Due to these interfacial interactions, the electrical conductivity of
the catalyst enhances. (EIS measurements also showed the
increase of the conductivity of catalyst due to the presence of RGO
Fig. S20, ESI†). This increase of conductivity, as well as the highly
conjugated p bonding RGO system facilitates the charge transfer
between the catalytically active sites and the reactant molecules
(i.e. 4-NP). These factors improve the efficiency of the catalyst
((Ag0.05Ni0.95)60RGO40).
(Ag0.15Ni0.85 33.5
(Ag0.27Ni0.73 37
(Ag0.37Ni0.63 40.5
)
RGO66.5
) RGO63
)
RGO59.5
9
proceeds via electron transfer mechanism where relaying of
ꢀ
electrons occur from BH4 (donor) to 4-NP (acceptor). In the
aqueous medium generation of hydrogen from BH4ꢀ occurs via
ꢀ
the rst adsorption of BH4 on the surface of catalyst (metal
nanoparticles), followed by electron transfer (ET) to the metal
nanoparticles.11,14,15 According to Ding et al. the metal
Fig. 5 Time dependent UV-Vis spectral changes of the reaction mixture of 4-NP catalyzed by (a) pure Ni, (b) pure Ag, (c) Ag0.05Ni0.95, (d)
Ni40RGO60, (e) (Ag0.05Ni0.95 60
)
RGO40, and (f) pseudo first order kinetic plot of 4-NP reduction reaction with pure Ni, Pure Ag, Ag0.05Ni0.95
,
Ni40RGO60, and (Ag0.05Ni0.95 60
)
RGO40
.
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(generated ‘in situ’ from the reaction of aldehyde and amine)
and forms the corresponding propargylamine. The tentative
reaction mechanism is shown in Scheme 1.
3.4.2 A3 coupling reaction. We have synthesized different
propargylamines to demonstrate the catalytic activity of the
synthesized catalysts ((AgxNi(1ꢀx))yRGO(100ꢀy) towards A3
)
In this reaction, the electron decient centres on the Ni,
which are generated due to the presence of Ag in the catalyst,
helps to coordinate –C^C– of phenylacetylene with Ni and
subsequently formation of Ni-acetylide intermediate more effi-
ciently. Here also, RGO helps to adsorb the reactant molecules
on the surface of the catalyst by providing larger surface area
and p–p stacking interaction. The enhancement of the
conductivity of the catalyst due to the presence of RGO facili-
tates the charge transfer between catalytically active sites (i.e.
metal nanoparticles) and reactant molecules. Therefore, the
catalysts which contain Ag, Ni, and RGO show superior catalytic
activity than pure Ag, or pure Ni, or Ni–Ag, or Ni-RGO.
3.4.3 Epoxidation reaction. To achieve a relatively clean
chlorine free reaction condition for the synthesis of styrene
oxide, we have investigated the epoxidation reaction of styrene
in presence of TBHP with the synthesized catalysts. The reaction
can be presented as (14):15,33
coupling reaction. For this purpose, rst the synthesis of N,N-
diethyl-3-phenylprop-2-yn-1-amine was conducted as a model
reaction where phenylacetylene, paraformaldehyde, and dieth-
ylamine were used as starting materials and acetonitrile was
used as the solvent. The reaction can be presented as (13):17,37
(13)
This reaction was performed in presence of the catalysts with
varying compositions. When the reactions were conducted
without any catalyst or with pure RGO no product was formed.
When pure Ni particle was used as a catalyst, ꢁ71% yield was
obtained. Incorporation of Ag nanoparticle in the catalyst
resulted in the enhancement of the yield. For example, the yield
was increased to ꢁ80% when Ag0.50Ni0.50 was the catalyst. To
investigate the effect of RGO in the catalyst, the catalysts were
prepared by immobilizing Ni nanoparticles on the surface of
RGO (e.g. Ni10RGO90, Ni20RGO80, and Ni30RGO70 where the
amount of RGO was 90, 80 and 70 wt%, respectively). Ni30RGO70
showed better catalytic performance (yield ꢁ80%) than that of
pure Ni (yield ꢁ71%). As it has been observed that the presence
of Ag and RGO with Ni enhanced the yield, now the catalysts
were prepared where both Ag and Ni particles were immobilized
on the surface of RGO and found to be benecial in terms of
catalytic activity. The % of yield of the product obtained by the
catalysts having different compositions are listed in Table S4
(ESI†). It was observed that the catalyst (Ag0.27Ni0.73)37RGO63
generated the highest yield of ꢁ95%. The % of yields of the
product which were obtained at different reaction conditions
are listed in Table S5 (ESI†). To demonstrate the applicability of
this catalyst for the A3 reaction, several other propargylamines
were also synthesized by using different amines and aldehydes.
It was observed that reasonably good yield (ꢁ80–87%) was ob-
(14)
The reaction was performed in acetonitrile medium by
varying the reaction conditions (e.g., temperature, reaction
time, styrene : TBHP ratio) and catalyst compositions. When
the reaction was performed without catalyst or with pure RGO
no desirable product was formed. Table S7 (ESI†) summarizes
the effects of catalyst composition on the % of styrene conver-
sion and % of styrene oxide formation. It was observed that the
presence of Ag nanoparticle along with Ni particle in the catalyst
composite was benecial for obtaining better styrene conver-
sion and selectivity of styrene oxide formation (Fig. S22, ESI†).
The catalytic performance of the catalyst was further improved
when Ag and Ni nanoparticles were dispersed on the surface of
RGO. When the reaction was performed in the presence of
(Ag0.27Ni0.73)37RGO63 catalyst the highest amount of styrene
conversion (96%) with 89% styrene oxide formation selectivity
1
were achieved. The catalytic performance of (Ag0.27Ni0.73)37-
tained and listed in Table 2. The LC-MS, H NMR, and FT-IR
RGO63 at different reaction conditions are listed in Table S8
(ESI) (Fig. S23 and S24, ESI†). The catalytic performance of
(Ag0.27Ni0.73)37RGO63 towards the epoxidation reaction of
styrene was found to be comparable and in some cases better
than that of reported catalysts (Table S9, ESI†).
Several researchers have addressed the epoxidation of
styrene on the various catalyst (e.g. Ag, Au, Au–BaTiO3,
etc.).24,25,30,39,40 The plausible reaction mechanism for the
oxidation of styrene to styrene oxide is presented in Scheme 2.
In the rst step of the reaction, a metal alkyl peroxy interme-
diate (Intermediate-I) is formed via interaction between cata-
lytically active metal nanoparticles and TBHP. When styrene
reacts with intermediate I, two type of intermediates (Interme-
diate IIa and Intermediate IIb) are formed which are isomers of
oxametallacyclic species. From Intermediate IIa, epoxide (i.e.
data of the products are provided in ESI (Spectral data
section, Fig. S27–29, ESI†).
The comparison of catalytic activity in terms of % yield of
N,N-diethyl-3-phenylprop-2-yn-1-amine of different reported
catalysts with that of (Ag0.27Ni0.73)37RGO63 is listed in Table S6
(ESI†) and it was observed that the catalytic activity of the
synthesized catalyst is comparable with the reported
catalysts.17,18,23,67
The reaction mechanisms for nanoparticles catalysed A3
coupling reaction, have been proposed by different
researchers.17,68,69 The reaction generally proceeds via the acti-
vation of C– H bond of the alkyne in the rst step by the
formation of a metal acetylide intermediate, which is generated
by the reaction of alkyne and the metal nanoparticles. In the
second step, this metal acetylide reacts with iminium ion
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Table 2 (Ag0.27Ni0.73 37
Paper
)
RGO63 catalyzed A3 coupling reaction of aldehyde, amine, and phenylacetylenea
Entry
Alkyne
Amine
Aldehyde
Product
Yield (%)
95
1
Ph–C^C–H
(HCHO)n
2
3
Ph–C^C–H
Ph–C^C–H
84
88
4
5
Ph–C^C–H
Ph–C^C–H
86
87
6
7
8
Ph–C^C–H
Ph–C^C–H
Ph–C^C–H
80
80
87
(HCHO)n
9
Ph–C^C–H
Ph–C^C–H
83
78
10
11
Ph–C^C–H
80
a
Reaction condition: aldehyde (2 mmol), amine (2.4 mmol), phenylacetylene (3 mmol) catalyst (Ag0.27Ni0.73 37
)
RGO63 50 mg, acetonitrile 10 ml,
reaction temperature 100 ꢂC, reaction time 12 h.
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Scheme 2 Plausible reaction mechanism of formation of styrene
oxide and benzaldehyde from styrene in presence of TBHP catalyzed
by (Ag0.27Ni0.73)37RGO63.
Scheme 1 The tentative reaction mechanism for synthesis of prop-
argylamines via A3 coupling reaction.
Table
3 Synthesis of 2-phenyl-2-(4-phenyl-1H-1,2,3-triazol-1-yl)
ethanol with different synthesized catalysta
styrene oxide) is formed via sharpless mechanism where the
transfer of oxygen occurs to the olenic bond. On the other
hand benzaldehyde forms as a byproduct from Intermediate IIb
via breakage of the C–C bond.24
In the synthesized catalyst (AgxNi(1ꢀx))yRGO(100ꢀy) presence
of electron decient Ni centres helps in the formation of
Intermediates IIa and IIb from Intermediate I via formation of
coordination complexes with metal nanoparticles. Moreover,
the presence of RGO helps to adsorb styrene molecule on the
surface of the catalyst by p–p interaction. Due to these factors,
the catalyst containing Ag, Ni, and RGO showed better catalytic
Catalyst
Yield (%)
Pure Ni
Pure Ag
Ni10RGO90
Ni20RGO80
Ni30RGO70
Ag0.50Ni0.50
(Ag0.15Ni0.85 33.5
(Ag0.27Ni0.73 37RGO63
(Ag0.37Ni0.63 40.5
(Ag0.45Ni0.55 44
55
53
26
66
66
62
89
91
90
92
)
)
)
RGO66.5
RGO59.5
) RGO56
activity than pure Ni, pure Ag, and Ag0.50Ni0.50
.
a
Reaction condition: styrene oxide (1 mmol 0.115 ml), phenylacetylene
3.4.4 Click reaction. We have also performed the synthesis
of 1,4-disubstituted 1,2,3-triazoles by ‘Click reaction’ in the
presence of the synthesized catalysts using styrene oxide, phe-
nylacetylene, and sodium azide as a model substrate. The
reaction can be presented as (15):10,34
(1 mmol 0.110 ml) sodium azide (1.5 mmol 97.5 mg), water (3 ml),
reaction time 12h, reaction temperature ¼ 100 ꢂC, catalyst dose ¼
25 mg.
observed that, when Ni nanoparticle was dispersed on the
surface of RGO, the efficiency of the catalyst was also increased.
For example, when Ni30RGO70 was used as catalyst the yield was
ꢁ66% which is higher than that of pure Ni-catalyzed reaction.
This enhancement of the catalytic activity might be due to the
following reasons (i) dispersion of Ni particles on the RGO
surface prevents the agglomeration of nanoparticles. Therefore,
the reactant molecules get more access to the catalytically active
sites, (ii) the reactant molecules (e.g. styrene oxide, phenyl-
acetylene) are adsorbed more on the surface of the RGO con-
taining catalysts, because of the p–p interaction between RGO
sheets and phenyl group of the reactant molecules, (iii) the high
conductivity of RGO facilitates the electron transfer process
during the reaction.
(15)
First, the reaction was performed without any catalyst or with
pure RGO and no product was obtained. Then the reactions
were carried out in presence of the synthesized catalysts, which
are composed of varying amount of Ni, Ag, and RGO. When pure
Ni and pure Ag nanoparticles were used as a catalyst, the yield
was ꢁ50% and ꢁ48%, respectively. When Ag0.50Ni0.50 was used
as catalyst the product yield was increased to ꢁ62%, which
clearly indicated that the presence of Ag positively inuences
the catalytic activity property of the catalyst. It has also been
As it has been observed that the presence of Ag and RGO
helped in the enhancement of the catalytic activity of Ni, we
have tested the catalytic activity of the catalyst, containing
varying amount of Ag, Ni, and RGO. The change in % of product
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epichlorohydrine) and found the % of the yield of the corre-
sponding products are ꢁ88% and ꢁ67%.
The tentative reaction mechanism for this Click reaction was
proposed by several researchers10,32,33,35 and shown in Scheme 3.
In this reaction, the metal nanoparticles play the role of a cata-
lytically active site and act as a bifunctional catalyst which
combines one pot ring opening of epoxy and 1,3-dipolar cyclo
addition.10,32 Initially, catalytically active metal azide is formed
which helps to activate epoxide and facilitates the delivery of
azide ion during the ring opening of epoxide. Thus, the orga-
noazide intermediate (Intermediate-I) is formed from epoxy and
azide. During the same time, another intermediate, metal-
acetylide complex (Intermediate-II) forms via p complexation
between the metal nanoparticle and phenylacetylene. Then
coordination of organoazide (Intermediate-I) with the metal
centre of metal acetylide (Intermediate II) occurs which
enhances the nucleophilicity of –C^C–. Then 1,3 dipolar cyclo
addition reaction results in the formation of the new C–N bond
between the nucleophilic b-carbon atom of the acetylide and the
terminal electrophilic nitrogen atom of the azide (Intermediate-
III).10,32 Aer protonation, intermediate-III yields the b-hydroxy-
1,2,3-triazole. In this reaction pathway formation of metal azide
via interaction of metal nanoparticles and N3ꢀ, and formation
of the metal-acetylide complex (which forms via p complexation
between the metal nanoparticle and phenylacetylene) play
critical roles. As the DFT study showed that the presence of Ag
causes the formation of electron decient centres near/on Ni
atoms, the formation of metal azide and metal-acetylide occurs
efficiently with these electron decient Ni centres. Moreover,
the presence of RGO in the synthesized catalyst also helps to
absorb the reactant materials (e.g. phenylacetylene, styrene
oxide) on the surface of the catalyst. These factors are
Scheme 3 The tentative reaction mechanism involved for synthesis of
2-phenyl-2-(4-phenyl-1H-1,2,3-triazol-1-yl)ethanol via Click reaction
catalyzed by (Ag0.27Ni0.73)37RGO63 catalyst.
yield with the variation of the composition of the synthesized
catalyst is listed in Table 3. It has been observed that the catalyst
containing 10 wt% Ag, 27% Ni, and 63 wt% RGO (i.e. (Ag0.27
-
Ni0.73)37RGO63) showed the highest catalytic activity with ꢁ91%
yield. The performance of this catalyst for this reaction was
compared with the reported catalysts (Table S10, ESI†) and
found to be comparable and in some cases superior to some of
the reported results. We have also performed the Click reaction
by replacing styrene oxide with cyclohexane oxide (and
Fig. 6 Reusability of (a) (Ag0.05Ni0.95)60RGO40 catalyst for the reduction of 4-NP, and (Ag0.27Ni0.73)37RGO63 catalyst for (b) the synthesis of N,N-
diethyl-3-phenylprop-2-yn-1-amine via A3 coupling reaction, (c) the epoxidation reaction of styrene to styrene oxide, and (d) the synthesis of 2-
phenyl-2-(4-phenyl-1H-1,2,3-triazole-1-yl)ethanol via Click reaction.
37786 | RSC Adv., 2018, 8, 37774–37788
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responsible for the superior catalytic activity of (AgxNi(1ꢀx)
)
the A3 coupling reaction, (Ag0.27Ni0.73)37RGO63 catalyzed reac-
RGO(100ꢀx) than pure Ag, Ni, or Ag0.50Ni0.50
.
tion produced N,N-diethyl-3-phenylprop-2-yn-1-amine with
The details of the spectral analysis of products, which were ꢁ95% yield. This same catalyst showed ꢁ96% styrene conver-
formed in the A3 coupling reaction, epoxidation reaction of sion with ꢁ89% styrene oxide formation selectivity for epoxi-
styrene and Click reaction, are provided in ESI (Spectra data dation of styrene reaction, and ꢁ91% yield for the synthesis of
section, Fig. S25–S29, ESI†).
2-phenyl-2-(4-phenyl-1H-1,2,3-triazol-1-yl)ethanol via Click
reaction. The catalyst also demonstrated its easy magnetic
recovery from the reaction mixture aer completion of the
reactions. The recovered catalysts also exhibited a very good
reusability.
To the best of our knowledge, this is the rst time a catalyst
having composition (AgxNi(1ꢀx))yRGO(100ꢀy) which showed the
versatile catalytic activity towards the aforesaid reactions with
high efficiency and also the investigations on the electronic
structures of this catalyst by DFT calculations have been re-
ported. The easy synthetic methodology high catalytic perfor-
mance with very good reusability and easy magnetic recovery
make the synthesize catalyst an attractive catalyst.
3.5. Reusability
The presence of Ni nanoparticle introduced the magnetic
character in (AgxNi(1ꢀx))yRGO(100ꢀy) nanocatalyst. Using VSM
the saturation magnetization (Ms) and coercivity (Hc) values of
the catalysts were determined. For example, Ms and Hc values of
(Ag0.27Ni0.73)37RGO63, were 9.81 emu gꢀ1 and 146.5 Oe, respec-
tively (Fig. S30, ESI†). The magnetic character of this catalyst
allowed its easy magnetic separation from the reaction mixture.
Aer completion of all the reactions, the catalyst was separated
from the reaction mixture by applying a magnet externally
(Fig. S31, ESI†) and then washed with distilled water for several
times and dried in a vacuum oven at 60 ꢂC for 12 h. The
recovered catalyst was used in the next cycle of the catalysis
reactions. No signicant loss of activity was observed up to 6th
reuse for all four reactions and shown in Fig. 6. XRD patterns
and FESEM micrograph of the reused catalyst also conrmed
that the crystal structure and morphology of the catalyst
remained same aer 6th catalysis cycle (Fig. S32, ESI†). These
results thus clearly demonstrated the structural robustness of
the catalyst.
Conflicts of interest
There are no conicts to declare.
Acknowledgements
Dr N. N. Ghosh gratefully acknowledges DRDO Jodhpur for VSM
analysis. Dr Ghosh is thankful to the Central Sophisticated
Instrumentation Facility (CSIF) of BITS Pilani K. K. Birla Goa
campus for providing the FESEM and LC-MS facility.
4. Conclusion
Notes and references
Here, we have reported a simple ‘in situ’ co-precipitation
reduction technique for the preparation of nanocatalyst con-
taining Ag, Ni, and RGO (AgxNi(1ꢀx))yRGO(100ꢀy). In this catalyst
Ag and Ni particles were immobilized on the surface of the
nanometer thin RGO sheets. Here, RGO provided a highly
conductive support to the catalytically active Ni and Ag nano-
particles. DFT studies were performed to obtain the electronic
structure and electronic properties of Ag–Ni-graphene. This
study helps us to understand the interfacial interactions and
orbital level hybridizations which exist in the interfaces between
Ag, Ni, and RGO. These interfacial interactions create electron
decient centres near/on the Ni atoms and also enhance the
conductivity of the catalyst. The synergistic effect, which arises
from the coexistence of Ag, Ni, and RGO in the catalyst, posi-
tively inuences the catalytic activity of the synthesized
catalysts.
1 X. Gao, J. Jang and S. Nagase, J. Phys. Chem. C, 2009, 114, 832.
2 M. Hu, Z. Yao and X. Wang, Ind. Eng. Chem. Res., 2017, 56,
3477.
3 D. Moitra, B. Ghosh, M. Chandel, R. Jani, M. Patra, S. Vadera
and N. Ghosh, RSC Adv., 2016, 6, 14090.
4 D. Moitra, M. Chandel, B. K. Ghosh, R. K. Jani, M. K. Patra,
S. R. Vadera and N. N. Ghosh, RSC Adv., 2016, 6, 76759.
5 D. Moitra, C. Anand, B. K. Ghosh, M. Chandel and
N. N. Ghosh, ACS Appl. Energy Mater., 2018, 1, 464.
6 X. Fan, G. Zhang and F. Zhang, Chem. Soc. Rev., 2015, 44,
3023.
7 O. C. Compton and S. T. Nguyen, Small, 2010, 6, 711.
8 K. Hareesh, R. Joshi, D. Sunitha, V. Bhoraskar and S. Dhole,
Appl. Surf. Sci., 2016, 389, 1050.
9 L. Rout, A. Kumar, R. S. Dhaka, G. N. Reddy, S. Giri and
P. Dash, Appl. Catal., A, 2017, 538, 107.
Here, it has been demonstrated that combination of Ag, Ni,
and RGO in the composition of the catalyst caused signicant 10 D. Moitra, B. K. Ghosh, M. Chandel and N. N. Ghosh, RSC
enhancement of the catalytic activity compared to pure Ag and Adv., 2016, 6, 97941.
Ni nanoparticles towards four important reactions (i) reduction 11 T. Aditya, A. Pal and T. Pal, Chem. Commun., 2015, 51, 9410.
of 4-nitrophenol (4-NP) in the presence of NaBH4, (ii) A3 12 M. Chandel, D. Moitra, P. Makkar, H. Sinha, H. S. Hora and
coupling reaction for the synthesis of propargylamines, (iii)
epoxidation of styrene, and (iv) ‘Click reaction’ for the synthesis 13 H. Hu, J. H. Xin, H. Hu, X. Wang, D. Miao and Y. Liu, J.
of 1,2,3-triazole derivatives in aqueous medium. (Ag0.05 Mater. Chem. A, 2015, 3, 11157.
N. N. Ghosh, RSC Adv., 2018, 8, 27725.
-
Ni0.95)60RGO40 exhibited highest catalytic activity towards the 14 B. Naik, S. Hazra, V. S. Prasad and N. N. Ghosh, Catal.
reduction of 4-nitrophenol with kapp 19.6 1910ꢀ3 sꢀ1. In case of
Commun., 2011, 12, 1104.
This journal is © The Royal Society of Chemistry 2018
RSC Adv., 2018, 8, 37774–37788 | 37787

View Article Online
RSC Advances
Paper
15 B. K. Ghosh, D. Moitra, M. Chandel, H. Lulla and 44 I. Solovyev, P. Dederichs and V. Anisimov, Phys. Rev. B:
N. N. Ghosh, Mater. Res. Bull., 2017, 94, 361. Condens. Matter Mater. Phys., 1994, 50, 16861.
16 Y.-g. Wu, M. Wen, Q.-s. Wu and H. Fang, J. Phys. Chem. C, 45 J. P. Perdew, K. Burke and M. Ernzerhof, Phys. Rev. Lett.,
2014, 118, 6307.
1996, 77, 3865.
17 T. K. Saha and R. Das, ChemistrySelect, 2018, 3, 147.
18 N. Hussain and M. R. Das, New J. Chem., 2017, 41, 12756.
46 S. Grimme, J. Comput. Chem., 2004, 25, 1463.
47 S. Grimme, J. Comput. Chem., 2006, 27, 1787.
´
19 S. Frindy, A. El Kadib, M. Lahcini, A. Primo and H. Garcıa, 48 Quantum ESPRESSO, Files: C.pbe-n-rrkjus_psl.1.0.0.UPF,
Catal. Sci. Technol., 2016, 6, 4306.
Ag.pbe-n-rrkjus_psl.1.0.0.UPF,
and
Ni.pbe-n-
20 A. M. Munshi, M. Shi, S. P. Thomas, M. Saunders,
M. A. Spackman, K. S. Iyer and N. M. Smith, Dalton Trans.,
2017, 46, 5133.
rrkjus_psl.1.0.0.UPF, https://www.quantum-espresso.org/
pseudopotentials, (accessed February 2018).
49 H. J. Monkhorst and J. D. Pack, Phys. Rev. B: Condens. Matter
Mater. Phys., 1976, 13, 5188.
21 X. Zhang and A. Corma, Angew. Chem., 2008, 120, 4430.
22 K. Namitharan and K. Pitchumani, Eur. J. Org. Chem., 2010, 50 M. Methfessel and A. Paxton, Phys. Rev. B: Condens. Matter
3, 411. Mater. Phys., 1989, 40, 3616.
23 N. Salam, A. Sinha, A. S. Roy, P. Mondal, N. R. Jana and 51 N. Marzari, D. Vanderbilt, A. De Vita and M. Payne, Phys. Rev.
S. M. Islam, RSC Adv., 2014, 4, 10001.
Lett., 1999, 82, 3296.
24 D. Nepak and D. Srinivas, Appl. Catal., A, 2016, 523, 61.
25 Q. Wang, C. Li, J. Bai, W. Sun and J. Wang, J. Inorg.
Organomet. Polym. Mater., 2016, 26, 488.
26 V. R. Choudhary, R. Jha and P. Jana, Catal. Commun., 2008,
10, 205.
52 M. Zong, Y. Huang, Y. Zhao, X. Sun, C. Qu, D. Luo and
J. Zheng, RSC Adv., 2013, 3, 23638.
53 Z. G. Wu, M. Munoz and O. Montero, Adv. Powder Technol.,
2010, 21, 165.
54 D. Knetsch and W. Groeneveld, Inorg. Chim. Acta, 1973, 7, 81.
27 D.-H. Zhang, H.-B. Li, G.-D. Li and J.-S. Chen, Dalton Trans., 55 S.-H. Wu and D.-H. Chen, J. Colloid Interface Sci., 2003, 259,
2009, 47, 10527.
282.
28 A. Wang and H. Jing, Dalton Trans., 2014, 43, 1011.
29 B. Sakthivel, D. S. R. Josephine, K. Sethuraman and
A. Dhakshinamoorthy, Catal. Commun., 2018, 108, 41.
30 A. S. Sharma and H. Kaur, Appl. Catal., A, 2017, 546, 136.
56 T. Zhao, R. Sun, S. Yu, Z. Zhang, L. Zhou, H. Huang and
R. Du, Colloids Surf., A, 2010, 366, 197.
¨
57 U. Nickel, A. zu Castell, K. Poppl and S. Schneider, Langmuir,
2000, 16, 9087.
31 A. N. Prasad, B. Thirupathi, G. Raju, R. Srinivas and 58 J. H. Byeon and Y.-W. Kim, Ultrason. Sonochem., 2012, 19,
B. M. Reddy, Catal. Sci. Technol., 2012, 2, 1264. 209.
32 H. Naeimi and Z. Ansarian, Appl. Organomet. Chem., 2017, 59 R. Dhanda and M. Kidwai, J. Mater. Chem. A, 2015, 3, 19563.
31, e3796.
60 L. Liu, J. Xue, X. Shan, G. He, X. Wang and H. Chen, Catal.
Commun., 2016, 75, 13.
33 B. K. Ghosh, D. Moitra, M. Chandel, M. K. Patra, S. R. Vadera
and N. N. Ghosh, Catal. Lett., 2017, 147, 1061.
34 B. K. Ghosh, S. Hazra and N. N. Ghosh, Catal. Commun.,
2016, 80, 44.
˜
´
61 E. Schiavo, A. B. Munoz-Garcıa, V. Barone, A. Vittadini,
M. Casarin, D. Forrer and M. Pavone, Chem. Phys. Lett.,
2018, 693, 28.
35 C. Wang, D. Ikhlef, S. Kahlal, J.-Y. Saillard and D. Astruc, 62 R. Wyckoff, Crystal structures, 1963, vol. 1, p. 85.
Coord. Chem. Rev., 2016, 316, 1.
63 F. Cinquini, F. Delbecq and P. Sautet, Phys. Chem. Chem.
Phys., 2009, 11, 11546.
64 P. Khomyakov, G. Giovannetti, P. Rusu, G. v. Brocks, J. Van
den Brink and P. J. Kelly, Phys. Rev. B: Condens. Matter
Mater. Phys., 2009, 79, 195425.
36 B. A. Kumar, K. H. V. Reddy, B. Madhav, K. Ramesh and
Y. Nageswar, Tetrahedron Lett., 2012, 53, 4595.
37 X. Huo, J. Liu, B. Wang, H. Zhang, Z. Yang, X. She and P. Xi, J.
Mater. Chem. A, 2013, 1, 651.
38 R. N. Baig and R. S. Varma, Chem. Commun., 2013, 49, 752. 65 C. Gong, G. Lee, B. Shan, E. M. Vogel, R. M. Wallace and
39 D.-H. Zhang, G.-D. Li, J.-X. Li and J.-S. Chen, Chem.
Commun., 2008, 29, 3414.
40 X. Wang, Z. Liang, F. Zhang, L. Yang and S. Xu, J. Mater. Sci.,
2013, 48, 5899.
K. Cho, J. Appl. Phys., 2010, 108, 123711.
66 Y.-A. Zhu, D. Chen, X.-G. Zhou, P.-O. Astrand and
W.-K. Yuan, Surf. Sci., 2010, 604, 186.
ꢁ
67 C. Wei, Z. Li and C.-J. Li, Synlett, 2004, 09, 1472.
41 M. Gopiraman, D. Deng, S. Saravanamoorthy, I.-M. Chung 68 G.-P. Yong, D. Tian, H.-W. Tong and S.-M. Liu, J. Mol. Catal.
and I. S. Kim, RSC Adv., 2018, 8, 3014. A: Chem., 2010, 323, 40.
42 W. Hummers and R. Offeman, J. Am. Chem. Soc., 1958, 80, 69 W. Yan, R. Wang, Z. Xu, J. Xu, L. Lin, Z. Shen and Y. Zhou, J.
1339.
Mol. Catal. A: Chem., 2006, 255, 81.
43 P. Giannozzi, S. Baroni, N. Bonini, M. Calandra, R. Car,
C. Cavazzoni, D. Ceresoli, G. L. Chiarotti, M. Cococcioni
and I. Dabo, J. Phys.: Condens. Matter, 2009, 21, 395502.
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