Design of a graphene oxide-SnO2 nanocomposite with superior catalytic efficiency for the synthesis of β-enaminones and β-enaminoesters

Article abstract of DOI:10.1039/c5ra03363b

A graphene oxide (GO)-SnO₂-based nanocomposite was synthesized by decorating the graphene oxide surface with SnO₂ nanoparticles via a solvothermal process. The nanocomposite was characterized using Fourier transform infrared spectra (FTIR), FT-Raman spectroscopy, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Field-emission Scanning electron microscopy (FE-SEM), Energy dispersive X-ray spectroscopy (EDS), Transmission electron microscopy (TEM) and N₂ adsorption/desorption study. The FE-SEM and TEM images demonstrate the uniform distribution of the SnO₂ nanoparticles on the GO surface and high-resolution transmission electron microscopy (HRTEM) confirms an average particle size of 8-12 nm. The GO-SnO₂ nanocomposite has been found to be an extremely efficient catalyst for the synthesis of β-enaminones and β-enaminoesters in methanol solvent and also, in solventless conditions. The GO-SnO₂ nanocomposites exhibited synergistically more superior catalytic efficiency compared to pure graphene oxide and SnO₂ nanoparticles. The reaction conditions were optimized by changing different parameters such as catalyst, solvent, catalyst loading, and temperature. It has been found that the catalyst gave higher activity under solventless conditions than methanol. The GO-SnO₂ composite was recycled for up to four cycles with minimal loss in activity.

Full text of DOI:10.1039/c5ra03363b

RSC Advances
View Article Online
View Journal | View Issue
PAPER
Design of a graphene oxide-SnO2 nanocomposite
with superior catalytic efficiency for the synthesis
of b-enaminones and b-enaminoesters†
Cite this: RSC Adv., 2015, 5, 39193
Aniket Kumar,a Lipeeka Rout,a Rajendra S. Dhaka,b Saroj L. Samala
a
*
and Priyabrat Dash
A graphene oxide (GO)–SnO2-based nanocomposite was synthesized by decorating the graphene oxide
surface with SnO2 nanoparticles via a solvothermal process. The nanocomposite was characterized using
Fourier transform infrared spectra (FTIR), FT-Raman spectroscopy, X-ray diffraction (XRD), X-ray
photoelectron spectroscopy (XPS), Field-emission Scanning electron microscopy (FE-SEM), Energy
dispersive X-ray spectroscopy (EDS), Transmission electron microscopy (TEM) and N2 adsorption/
desorption study. The FE-SEM and TEM images demonstrate the uniform distribution of the SnO2
nanoparticles on the GO surface and high-resolution transmission electron microscopy (HRTEM)
confirms an average particle size of 8–12 nm. The GO–SnO2 nanocomposite has been found to be an
extremely efficient catalyst for the synthesis of b-enaminones and b-enaminoesters in methanol solvent
and also, in solventless conditions. The GO–SnO2 nanocomposites exhibited synergistically more
superior catalytic efficiency compared to pure graphene oxide and SnO2 nanoparticles. The reaction
conditions were optimized by changing different parameters such as catalyst, solvent, catalyst loading,
and temperature. It has been found that the catalyst gave higher activity under solventless conditions
than methanol. The GO–SnO2 composite was recycled for up to four cycles with minimal loss in activity.
Received 23rd February 2015
Accepted 21st April 2015
DOI: 10.1039/c5ra03363b
www.rsc.org/advances
carbonyl compounds with amines catalyzed by chlorides,17,18
metal oxides,19 aurates,20 SiO2 and it's composite,16,21–23 ionic
liquids,24 and triates.25–27 Owing to its high surface to volume
1. Introduction
Fine chemicals are considered to be chemicals that are manu-
factured to high and well-dened standards of purity. These
chemicals are a highly multifaceted class of intermediates for
the synthesis of heterocyclic and biologically active
compounds.1,2 b-enaminones have emerged as one of the nest
bio-active intermediates in the eld of organic chemistry. In
particular, the enaminones and enaminoester moieties have
attracted much interest, because they are fundamental and
versatile starting materials for the synthesis of heterocycles,3
alkaloids,4,5 g-amino alcohol,6 quinolines,7 azocompounds8 and
a,b-amino acids.9,10 In addition, they serve as a basic synthon for
various pharmaceutical drug molecules that control physio-
logical actions, such as anti-bacterial,11,12 anticonvulsant,13 anti-
tussive,14 anti-inammatory,15 anti-tumor drugs.16 The scientic
community thus seeks for economic synthesis of such ne
chemicals. The conventional synthesis protocols used for b-
enaminones and b-enaminoesters involved condensation of
ratio and lack of residual energy, metallic nanoparticles have
also been a topic of interest for b-enaminone synthesis.28–30 The
basic drawbacks which the previous reaction strategies undergo
include requirements for particular reaction environment,25
hygroscopic triate precursors,25,27 use of homogeneous cata-
lyst,31–33 formation of side products, requirement of extended
reaction time,34,35 non-reusability of the catalyst,33 use of
expensive reagents,36,37 and use of toxic reagents and
solvents.38,39 Therefore, improved synthesis routes in terms of
minimal side-products, product purity and catalyst recovery are
highly desirable in ne chemical synthesis.40 Keeping all this in
mind, the development of reusable heterogeneous supported
catalyst under non-toxic and solventless condition has become
more desirable.41,42 In order to meet these requirements, sup-
ported metal oxide nanocomposites are found to be preferable
materials as solid-state catalysts owing to their nanometric size
that contributes to the dramatic increase in their surface area.
Graphene oxide, a two-dimensional sheet of sp2 hybridized
carbon, has received increasing attention as it possesses similar
properties to that of graphene, as well as the availability of polar
functional groups such as hydroxyl, carboxyl, etc. on its planar
surface. Because of its high surface area, thermal stability,
mechanical and electrical properties, it can be used as a
aDepartment of Chemistry, National Institute of Technology, Rourkela, Odisha, India,
769008
bDepartment of Physics, Indian Institute of Technology, Delhi, New Delhi, India,
110016
† Electronic supplementary information (ESI) available: See DOI:
10.1039/c5ra03363b
This journal is © The Royal Society of Chemistry 2015
RSC Adv., 2015, 5, 39193–39204 | 39193

View Article Online
RSC Advances
Paper
signicant supporting material and has already shown many structural and catalytic properties of the GO–SnO2 nano-
promising applications.43 Moreover, the design of GO-based composite were thoroughly investigated by various techniques.
nanocomposites, generated by decorating GO surface with The GO–SnO2 nanocomposite demonstrates remarkable activity
metal oxide nanoparticles, have further extended its applica- with high yield of products for a wide variety of amine and
tions. These GO–metal oxide nanocomposites have been found dicarbonyl precursors. All the reactions proceeded in a shorter
as promising materials for fuel cells,44 sensors,45 solar cells,46 period of time compared to traditional catalysts. Moreover, the
lithium batteries,47 and organic synthesis.48 In context to catal- catalyst can be recycled and reused up to four cycles with
ysis, the presence of various functional groups, larger surface minimal loss in activity in solventless system.
area, and high thermal strength makes GO as a promising
material for catalysis. These oxygenated functional groups can
serve as nucleation sites for metal and metal oxide ions to form
GO-based nanocomposites. The main advantages of nano-
2. Experimental
2.1 Materials
Graphite powder, CDCl3 and SnCl4$5H2O were purchased from
Sigma-Aldrich. H2O2 (30%), isopropanol, ethanol, NaNO3,
H2SO4 (98%), HCl, KMnO4, and silica gel (100–200 mesh) were
purchased from Hi-Media. All chemicals were used as received
without further purication. 18 MU cm Milli-Q water was used
throughout the synthesis.
composite materials, such as high thermal stability, high
strength and chemical resistance make them great choice for
designing the catalytic system. Though GO based systems has
been studied for various catalytic reactions such as coupling
reaction,49,50 reduction reaction51 and oxidation reaction,52,53 to
the best of our knowledge, the use GO-based system for ne
chemical syntheses has not been demonstrated so far.
2.2 Preparation of GO
Among various metal oxide nanoparticles, tin oxide (SnO2) is
a wide band gap n-type semiconductor and is found to be
suitable material for different applications such as sensors,54
hydrogenation catalyst,55 barrier layers for solar cells,56 opto-
electronic devices,57 liquid crystal displays58 and as anode
material in lithium-based batteries.59 Moreover, recently, they
have found potential application in catalysis.60–62 Tin oxide has
been widely used as a catalyst in chemical reactions, including
oxidation and synthesis of vinyl ketone, because of its excellent
catalytic properties in the adsorption and dissociation of
various molecules.63 Also, as tin(IV) oxide exhibits redox catalytic
properties, these may be modied substantially by the incor-
poration of other molecules to further upgrade its catalytic
efficiency.63,64 Additionally, SnO2 nanostructures could have
additional advantages in enhancing the catalytic activities
because such structures can potentially not only have high
surface area but also helps in more efficient transport for the
reactant molecules to get to the active sites. Therefore, combi-
nation of GO with SnO2 can lead to a potential catalytic system
for various reactions.
As a part of our own work towards the development of
improved catalytic systems, we have considered the use of gra-
phene oxide (GO) as a support for various metal oxide nano-
particles and use them as a catalyst in organic synthesis. In this
direction, we were interested in investigating GO–SnO2 based
system as a novel catalyst for ne chemical synthesis. Generally,
GO is densely surrounded by polar oxygen-containing func-
tional groups, such as hydroxyl, epoxy, and carboxyl groups, and
it was envisaged that these groups would be able to coordinate
to the SnO2 species and prevent them from aggregating and
leaching. Moreover, due to its pronounced cations exchange
capacity and intercalation ability, GO would be an ideal catalyst
support material, and for the intercalation of catalytically active
GO was prepared from natural graphite powder through
modied Hummer's method.66–68 In a typical synthesis, 1 g of
graphite was added to 25 ml to conc. H2SO4 (98% w/w), followed
by stirring at room temperature for 24 h. Aer that 100 mg of
NaNO3 was added to the mixture and stirred for 30 min.
Subsequently, the mixture was kept below 5 ꢀC using an ice
bath, and 3 g of KMnO4 was slowly added to the mixture and the
mixture was stirred for 2 h under ice-water bath. About 250 ml
distilled water and 20 ml H2O2 (30%) was added to dilute the
solution at room temperature. The suspending solution was
allowed to precipitate for 12 h and later on, the upper super-
natant was collected and centrifuged. Successively, the GO
powders were washed with 10% HCl and distilled water three
times. The obtained GO was dispersed in distilled water to get a
stable brown solution.
2.3 Preparation of GO–SnO2 nanocomposites
Graphene oxide–SnO2 nanocomposites were prepared via a
solvothermal method. In a typical synthesis, a required amount
of 0.02 g of SnCl4$5H2O and 50 mg of GO were added in 50 ml of
isopropanol, followed by sonication for 1 h to obtain a
completely dispersed solution. Aerward, 2 ml of deionized
water was added dropwise, followed by stirring for another
30 min. Aer that, the aqueous dispersion was transferred into
a 10ꢀ0 ml Teon-lined stainless-steel autoclave and heated at
180 C for 16 h. The products were then collected by centrifu-
gation and washed several times with water and ethanol in
order to remove the chloride ion. The resulting precipitates
were dispersed in water for characterizations and further use.
2.4 Characterization
SnO2 species.65 Herein, we report for the rst time the design of The catalyst was analyzed by X-ray diffraction study using PHI-
GO–SnO2 nanocomposite as an efficient catalyst for the LIPS PW 1830 X-ray diffractometer with Cu Ka source. The
synthesis of b-enaminones and enaminoesters. The nano- compositional information of the products was performed
composite used was prepared by a simple solvothermal method using EDX (JEOL JSM-6480 LV). Raman spectra were recorded
that is inexpensive and requires a lesser amount of time. The using a BRUKER RFS 27 spectrometer with 1064 nm wavelength
39194 | RSC Adv., 2015, 5, 39193–39204
This journal is © The Royal Society of Chemistry 2015

View Article Online
Paper
RSC Advances
incident laser light. Field emission scanning electron micros-
copy (FESEM) of the sample was recorded by Nova NanoSEM/
FEI. Transmission electron micrographs (TEM) of the sample
was recorded using PHILIPS CM 200 equipment using carbon
coated copper grids. Nitrogen adsorption/desorption isotherm
was obtained at 77 K on a Quantachrome Autosorb 3-B appa-
ratus. The specic surface area and pore size distribution were
acquired by emulating BET equation and BJH method, respec-
tively. 1H NMR and 13C NMR spectra were recorded on a Bruker
spectrometer at 400 MHz using TMS as an internal standard.
FTIR spectra of the product were recorded using a Perkin-Elmer
FTIR spectrophotometer using NaCl support. A commercial
electron energy analyzer (PHOIBOS 150 from Specs GmbH,
Germany) and a non-monochromatic Mg Ka X-ray source (hv ¼
1253.6 eV) have been used to perform XPS measurements with
the base pressure of <1 ꢁ 10ꢂ9 mbar. Catalytic reactions were
monitored by thin layer chromatography on 0.2 mm silica gel F-
254 plates. All the reaction products are known compounds and
have been identied by comparing their physical and spectral
characteristics with the literature reported values.
Fig. 2 FTIR spectra of (a) GO and (b) GO–SnO2 nanocomposite.
deposition of SnO2 nanoparticles onto the GO surface. Addi-
tionally, it also suggests that SnO2 nanoparticles possibly acted
as a spacer to separate the GO sheets resulting in a decrease of
the restacking problems in the composite. By Scherrer equa-
tion, the average crystallite size of SnO2 nanoparticles was
found to be around 10 nm from (110) and (101) diffraction peak
of GO–SnO2, which is in a good agreement with the HRTEM
ndings as discussed in the later part of the discussion.
Further, the nature of chemical groups and the deoxyge-
nating degree of oxygen functional groups in GO and GO–SnO2
samples were characterized using FT-IR spectroscopy (Fig. 2a
and b). As depicted in Fig. 2a. FT-IR spectrum of GO revealed the
presence of oxygenated functional groups such as structural
–OH near 3412 cmꢂ1, C–O–C at 1051 cmꢂ1, and C]O in
carboxylic acid moieties at 1726 cmꢂ1. Other characteristic
vibrations were the O–H deformation peak at 1405 cmꢂ1 and the
C–O stretching peak at 1043 cmꢂ1. These results suggest the
abundance of hydroxyl groups on the surface of GO. The peak
around 1182 cmꢂ1 is attributed to C–H bending vibration of the
aromatic ring present in GO–SnO2 nanocomposite.69,70
However, in GO–SnO2 composite, the O–C–O vibration and
structural O–H peak disappeared and the intensity of the C–O–C
and C]O peaks slightly decreased, suggesting minimal
reduction of oxygen-containing functional groups of GO in
GO–SnO2 nanocomposite. Furthermore, the presence of Sn–O–
Sn symmetric stretching (668 cmꢂ1) and Sn–O asymmetric
stretching (539 cmꢂ1) in GO–SnO2, as opposed to that of GO,
demonstrate the successful incorporation of SnO2 nano-
particles in GO nanosheets.
3. Results and discussion
3.1 Structure and morphology characterization
In this work, graphene oxide was synthesized using hummer's
methods as discussed in the experimental section. Later on, a
solvothermal method was employed for the fabrication of
GO–SnO2 composite using SnCl4$5H2O. The presence of various
chemical groups on both GO and GO–SnO2 was characterized by
XRD and FTIR spectroscopy. Fig. 1 shows the typical XRD
pattern of as prepared GO and GO–SnO2 nanocomposite. The X-
ray diffraction peak of GO show a sharp peak at 2q ¼ 10.5ꢀ
which relates to the (002) reection of stacked GO sheets. The
diffraction peaks (100), (110), (101), (200), (211), (310) and (301)
as seen in the XRD prole in Fig. 1b can be indexed to the rutile
phase of SnO2 (JCPDS no. 41-1445). It is interesting to note that
no distinct peaks related to graphene oxide were observed in the
powder X-ray diffraction pattern, which could be due to low
crystallinity of GO in the GO–SnO2 composite and sufficient
The surface composition and chemical states of the indi-
vidual species in GO–SnO2 nanocomposite were evaluated by
XPS analysis. Fig. 3a shows the XPS survey scan of GO–SnO2 in
the wide energy range, revealing the presence of carbon, oxygen
and tin species without the detection of any other hetero
elements. Two symmetrical peaks at 488 eV and 496.5 eV in the
Sn 3d spectrum (Fig. 3b) can be attributed to Sn 3d5/2 and Sn
3d3/2 binding energy, respectively, which are, in well agreement
with the energy splitting reported for bare SnO2.71 These values
Fig. 1 XRD pattern of GO and GO–SnO2 nanocomposite.
This journal is © The Royal Society of Chemistry 2015
RSC Adv., 2015, 5, 39193–39204 | 39195

View Article Online
RSC Advances
Paper
Fig. 3 (a) XPS survey spectra of GO–SnO2 nanocomposite, (b) Sn 3d core level XPS spectra of GO–SnO2 nanocomposite, (c) C 1s XPS spectra of
GO and (d) C 1s XPS spectra GO–SnO2 nanocomposite.
correspond to the 3d binding energy of Sn(IV) ions. The spin– GO. No apparent aggregation of SnO2 nanoparticles on the GO
orbit splitting of Sn 3d is observed about 8.5 eV while the area sheet were observed. This type of coverage of SnO2 nanoparticle
ratio of the two peaks (3d5/2 and 3d3/2) is 1.5. The interaction of on GO surface decreases the restacking of graphene oxide
SnO2 with GO in GO–SnO2 can be conrmed by the presence of sheets, thereby increase the stability of individual graphene
an O 1s peak at 533 eV in the XPS spectra (Fig. 3a), which oxide sheets.73 Additionally, the contour of individual SnO2
corresponds to the oxygen species in the SnO2. In addition, an nanoparticles can be seen in Fig. 4d. Fig. 4e shows the high-
increase in the peak-to-peak separation from 8.2 ꢃ 1 eV in bare resolution TEM image of GO–SnO2 composite where the
SnO2 nanoparticles72 to 8.5 eV in the GO–SnO2 composite lattice fringe of each SnO2 nanoparticle can be clearly observed.
suggest the binding of SnO2 with GO nanosheets. For GO, the From the size distribution chart, the average size of the SnO2
main peak positioned at around 284.5 eV, is assigned to non- nanoparticles was found to be around 8–12 nm (Fig. 4f). This
oxygenated ring carbon molecules, while alternate peaks at lattice-resolved HRTEM image shows interplanar spacing of
286.7, 287.8 and 289.1 eV are assigned to the oxygen-containing 0.33 nm and 0.26 nm, which corresponds to (110) and (101)
groups (C–OH), (C]O), and (O]C–OH), respectively (Fig. 3c). planes of rutile phases of SnO2, respectively.74 The corre-
The C 1s XPS range of the GO–SnO2 demonstrates that there is sponding selected area electron diffraction (SAED) pattern
no signicant change in intensity of the oxygenated function- shown in Fig. 4g have sharp diffraction spots indicating single
alities in comparison to that of GO as shown in Fig. 3d, crystalline nature of SnO2 nanoparticles in the nanocomposite.
demonstrating that no effective change in functional group The EDS spectrum of the GO–SnO2 (Fig. 4h) exhibits the pres-
present on GO happened during the nanocomposite synthesis. ence of C, O, and Sn elements suggesting the formation of
The detailed morphology and internal structure of GO–SnO2 GO–SnO2 nanocomposite via solvothermal process demon-
composite were further studied by FE-SEM and TEM as shown strated in the work.
in Fig. 4a–e. The FE-SEM image of GO (Fig. 4a) shows well-
To obtain further understanding of the structural changes
established layered structure with sub-micrometre size pores. occurs during the chemical processing from GO to GO–SnO2,
It is observed that the layered structure of GO was maintained in Raman spectra were obtained and are shown in Fig. 5. The
the composite aer decoration with SnO2 nanoparticles Raman spectrum of pure graphene oxide displays a D-band at
(Fig. 4b). Fig. 4c displays the TEM image of GO–SnO2 composite 1310 cmꢂ1, typically assigned to defects and disarranged
under a low magnication, where numerous SnO2 nano- structures of GO lattice.75 Another peak at 1603 cmꢂ1 was also
particles can be seen attached onto the GO surface. It was found seen which is assigned to the G-band obtained due to the
that small sized uniform SnO2 nanoparticles are anchored on vibration of sp2-bonded carbon atoms in the hexagonal lattice.
39196 | RSC Adv., 2015, 5, 39193–39204
This journal is © The Royal Society of Chemistry 2015

View Article Online
Paper
RSC Advances
Fig. 4 FESEM image of (a) GO, (b) GO–SnO2 nanocomposite, (c) TEM image of GO–SnO2 nanocomposite, (d and e) HRTEM image of GO–SnO2
nanocomposite, (f) size distribution curve of GO–SnO2 nanocomposite, (g) SAED pattern of GO–SnO2 nanocomposite and (h) EDX spectra of
GO–SnO2 nanocomposite.
GO in the composite, thus conrming the successful incorpo-
ration of the SnO2 particles onto the GO surface. Furthermore,
these results are consistent with the TEM and XPS results. It is
important to note that similar geometrical connement of
metal oxide nanoparticles decorated on graphene nanosheets
had been reported in various literatures, to suppress the
agglomeration of nanoparticles.77–79
From XRD ndings, it was found that SnO2 possibly act as a
spacer during the composite synthesis resulting in the decrease
of restacking of GO sheets. In reality, this should lead to an
increase in the surface area of the composite.80,81 To gain
Fig. 5 FT-Raman spectra of GO and GO–SnO2 nanocomposite.
insights about this possible impact, N2 adsorption/desorption
isotherm of GO and GO–SnO2 composite were carried out and
are shown in Fig. 6a and b. Both the samples exhibit type IV
isotherm with an associated H3-type hysteresis loop based on
In comparison, the D and G-Raman band of the composite are
positioned at
a
different position like 1290 cmꢂ1 and
the IUPAC classication.78 The average pore diameters in GO
and GO–SnO2 composite were found to be 4.4 nm and 5 nm,
respectively, whereas, BET surface areas were 100 m2 gꢂ1 and
128 m2 gꢂ1, respectively. This higher surface area of GO–SnO2
suggests the successful decoration of SnO2 nanoparticles on GO
surface. Moreover, this observed increase in the surface area
could be one of the factors responsible for the enhanced cata-
lytic activity of the GO–SnO2 composite discussed later in detail.
Additionally, TGA was employed for the estimation of weight
percentage of SnO2 in GO–SnO2 composite (Fig. 7). A weight loss
of ꢄ4 wt% was observed before 100 ꢀC due to the evaporation of
1593 cmꢂ1, respectively. A careful search on the literatures
reveal that when metal nanoparticles are deposited on the
surface of GO, the intensity ratio of D and G-band (ID/IG) usually
increase, suggesting the presence of electronic interaction
between the nanoparticles and GO.76 In our case, the intensity
ratio of composite (ID/IG ¼ 1.19) is found to be larger than that
of the pure graphene oxide (ID/IG ¼ 0.94). Additionally, the peak
position of G-band was shied from 1603 cmꢂ1 in GO to
1593 cmꢂ1 in GO–SnO2 composite. These Raman results
suggest the presence of electronic interaction between SnO2 and
This journal is © The Royal Society of Chemistry 2015
RSC Adv., 2015, 5, 39193–39204 | 39197

View Article Online
RSC Advances
Paper
96% (Table 1, entry 4). In comparison, the reaction using bulk
SnO2 nanoparticle produced a yield 48% (Table 1, entry 3) of the
desired product as compared to graphite (10%) (Table 1, entry 1)
and GO (30%) (Table 1, entry 2). Later on, the impact of various
reaction parameters such as solvent, catalyst loading, temper-
ature, and time was studied (Table 1, entries 6–23). Appropri-
ately,
a blank reaction was performed using 1 mmol
proportionate each of commercially available acetylacetone and
benzylamine. The reaction did not move ahead satisfactory even
aer prolonged reaction time and only traces of product was
obtained (Table 1, entry 5). The inuence of various solvents
like methanol, ethanol, acetonitrile, DCM, THF, DMSO, dioxane
and hexane on the condensation of the model reaction was then
studied (Table 1, entries 6–12). More precisely, it is interesting
to note that the yield of product increased with increase in
polarity of the solvent. Again, polar solvents have very high
affinity towards the hydrophilic pores of GO–SnO2 nano-
composite and thus, the reactions occur with the high catalytic
efficiency.83 Additionally, enhanced reactivity can be attributed
to the strong hydrogen bond interaction in methanol solvent,
which stabilizes the reaction intermediates and enhances the
rate of reaction.84,85 The reaction was also carried out under
solventless conditions to evolve the inuence of the solvent
parameter, and product was obtained in good yield with less
reaction time (Table 1, entry 13). It was observed that the
reaction provided best result when methanol was used as
solvent (96% yield), which was then, employed for further study
along with solventless system (98% yield). The activity of cata-
lyst without solvent remains constant aer 40 min i.e. 98%. In
an effort to demonstrate the catalytic efficiency of GO–SnO2,
various catalyst loadings were then investigated (Table 1, entries
14–17). The yield was found to increase with the increase in
catalyst loading in the range of 5 to 15 wt%. Best result with
respect to yield was obtained when 10 wt% of SnO2 nano-
particles was used (Table 1, entries 15, 17). No signicant
increase in yield happened when catalyst loading was increased
beyond 10 wt% (Table 1, entry 16). Lastly, effect of different
temperatures on the activity for GO–SnO2 composite for model
reaction was studied at varying temperatures ranging from
room temperature (r.t.) to reux condition. It was observed that
with the increase in temperature the yield of desired product
initially increased (Table 1, entries 18–23). However, at a
temperature, i.e., reux condition, the yield was decreased,
reecting 60 ꢀC as the optimum temperature. Hence, the
optimum reaction conditions were catalyst: GO–SnO2
composite, solvent: methanoꢀl and solventless system, catalyst
loading of 10 wt%, temp: 60 C and time: 75 min.
Fig. 6 N2 adsorption/desorption isotherm of SnO2 and GO–SnO2
nanocomposite.
Fig. 7 TGA of GO–SnO2 nanocomposite.
moisture content. In comparison,
a bigger weight loss
happened in the range of 300–500 ꢀC, corresponding to the
decomposition of graphene sheets.82 From the analysis, the
SnO2 content in the composite was found to be ꢄ70 wt%.
3.2 Catalytic activity
At rst, condensation of benzylamine with acetylacetone was
chosen as a model reaction (Scheme 1). In order to nd the best
catalyst, the model reaction was carried out using graphite, GO,
SnO2, and GO–SnO2. Among all, GO–SnO2 composite was found
to be the most efficient catalyst providing an excellent yield of
Scheme 1 A model reaction involving condensation of benzyl amine and acetyl acetone.
39198 | RSC Adv., 2015, 5, 39193–39204
This journal is © The Royal Society of Chemistry 2015

View Article Online
Paper
RSC Advances
Table 1 Study of reaction parameters on model reaction of benzylamine and acetyacetonea
Entry
Solvent
Catalyst
Catalyst loading
Temp (ꢀC)
Yieldb (%)
Effect of catalyst
1.
2.
3.
4.
5.
Methanol
Methanol
Methanol
Methanol
Methanol
Graphite
GO
SnO2
GO–SnO2
None
20 mg
20 mg
10 wt% SnO2
10 wt% GO–SnO2
—
60
60
60
60
60
10
30
48
96
Traces
Effect of solvent
6.
7.
8.
9.
10.
11.
12.
13.
Ethanol
DCM
Acetonitrile
THF
GO–SnO2
GO–SnO2
GO–SnO2
GO–SnO2
GO–SnO2
GO–SnO2
GO–SnO2
GO–SnO2
10 wt%
10 wt%
10 wt%
10 wt%
10 wt%
10 wt%
10 wt%
10 wt%
60
60
60
60
60
60
60
60
84
80
86
80
68
82
20
98
DMSO
Dioxane
Hexane
Solvent less
Effect of catalyst loading
14.
15.
16.
17.
Methanol
GO–SnO2
GO–SnO2
GO–SnO2
GO–SnO2
5 wt% GO–SnO2
10 wt% GO–SnO2
15 wt% GO–SnO2
10 wt% GO–SnO2
60
60
60
60
86
96
88
98
Methanol
Methanol
Solventless
Effect of temperature
18.
19.
20.
21.
22.
23.
Methanol
Methanol
Methanol
Solventless
Solventless
Solventless
GO–SnO2
GO–SnO2
GO–SnO2
GO–SnO2
GO–SnO2
GO–SnO2
10 wt% GO–SnO2
10 wt% GO–SnO2
10 wt% GO–SnO2
10 wt% GO–SnO2
10 wt% GO–SnO2
10 wt% GO–SnO2
r.t
45
Reux
r.t
45
48
78
82
54
80
85
Reux
a
b
Reaction conditions: acetyl acetone (1 mmol), benzaldehyde (1 mmol), solvent (5 ml), solventless system and time (75 min). Isolated yield.
In order to check the generality of the above reaction in methanol solvent and 96% in solventless system (Table 2,
protocol, the optimized conditions were then applied for the entry 5). Similarly excellent yield of the condensation product
enamination of diketones with various amines, providing was obtained in case of aliphatic and acyclic amine such as
excellent yields of the products as shown in Table 2 (entries 1– methylamine, ethylamine, propylamine, butylamine and mor-
33). All the reactions were completed within 20–210 min at 60 ꢀC pholine (Table 2, entries 1–4, 7), when GO–SnO2 nanocomposite
temperature to give the desired products in excellent yields (75– was used as a catalyst. It was found that the reaction with
98%) without forming any side products. The 1H and 13C NMR aliphatic amines proceeds easily in a short period of time as
data of products are provided in supplementary information compared to aromatic amine. From our observations, it could
le. It was exciting to know that the condensation products of b- be justied that the lower nucleophilicity of aromatic amines as
enaminones and b-enaminoesters were obtained in excellent compared to the aliphatic amines could be the reason for higher
yield under solventless conditions in lesser time with respect to yield obtained in case of aliphatic amines. Moreover, the pres-
the systems containing methanol solvent. While a few past ence of electron releasing and withdrawing group is found to be
methodologies for similar reaction conditions showed negative a factor in governing the yield of product. For example, an
results and extremely poor yield,18,86 the system (GO–SnO2) aromatic amine containing an electron donating substituents
developed here was found to be more promising for the such as a methyl group gave good yield (Table 2, entry 10).
b-enaminones and b-enaminoesters synthesis. Acetyl acetone Further, the yield of condensation product obtained for
reacts with aniline to give a good yield of product of around 92% o-nitroaniline and p-nitroaniline (Table 2, entries 8–9)
This journal is © The Royal Society of Chemistry 2015
RSC Adv., 2015, 5, 39193–39204 | 39199

View Article Online
RSC Advances
Paper
Table 2 GO–SnO2 nanocomposite catalyzed synthesis of a library of b-enaminones and b-enaminoestersa
Time (min)/yieldb (%)
Methanol
Entry
R1
R2
Solventless
1
2
3
4
5
6
7
8
CH3
C2H5
C3H7
C4H9
Me
Me
Me
Me
Me
Me
Me
Me
60/94
90/90
60/96
75/98
90/92
75/96
120/82
110/80
100/86
180/72
100/90
45/92
60/88
50/94
75/88
80/92
60/92
140/82
190/70
100/92
220/70
100/94
35/96
50/94
45/92
60/90
75/94
75/96
180/80
210/71
90/92
200/71
90/96
40/98
60/96
30/98
40/98
70/96
40/98
90/86
75/80
90/89
150/74
80/92
30/96
40/92
20/96
30/90
45/94
40/96
100/88
150/76
90/96
180/76
80/96
20/98
30/94
30/94
20/96
25/94
30/94
120/84
180/74
82/96
160/96
75/98
Ph
Ph–CH2
C4H9NO
o-C6H6N2O2
p-C6H6N2O2
C8H11
C7H9
CH3
C2H5
C3H7
9
Me
Me
Me
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
OMe
OMe
OMe
OMe
OMe
OMe
OMe
OMe
OMe
OMe
OMe
OEt
OEt
OEt
OEt
OEt
OEt
OEt
OEt
OEt
OEt
OEt
C4H9
Ph
Ph–CH2
C4H9NO
o-C6H6N2O2
p-C6H6N2O2
C8H11
C7H9
CH3
C2H5
C3H7
C4H9
Ph
Ph–CH2
C4H9NO
o-C6H6N2O2
p-C6H6N2O2
C8H11
C7H9
a
Reaction condition: amine (1 mmol), 1,3-dicarbonyl compound (1 mmol), GO–SnO2 nanocomposite (10 wt%), temperature (60 ꢀC), solvent and
solventless system. b Isolated yield.
precursor, was slightly lower which can be attributed to electron
The proposed mechanism for the condensation reaction is
withdrawing nature of nitro group which supresses the yield of shown in Scheme 2. Typically, the condensation of dicarbonyl
the product. So, the presence of electron withdrawing group is compounds with aliphatic and aromatic amines proceeds
also one of the factors which govern the yield of product.
through an addition–elimination reaction. In GO–SnO2, the GO
The addition of amines to other dicarbonyl compound like surface acts as an acidic site that helps the keto form of the
methyl acetoacetate and ethyl acetoacetate was found to give an dicarbonyl compound (1A) to get converted into more favorable
excellent yield of desired products with same type of aliphatic, and active enol form (1B). The lone pair of nitrogen of the amine
acyclic and aromatic amine under optimized conditions (Table then adds to the enolic carbonic centre to generate a tetrahe-
2, entries 11–33). To conrm the role of the catalyst, a blank dron intermediate aer passing through a four-centered tran-
reaction was carried out under similar reaction conditions with sition state (I, II).87 In the same time, tin oxide has a Lewis acid
model reaction in absence of catalyst, but, no progress was site Sn4+ and Lewis basic sites O2ꢂ 88 The Lewis acid site, Sn(IV)
.
found even aer stirring for a long reaction time. It clearly of SnO2 molecule that anchored onto the GO surface co-
establishes the role of GO–SnO2 catalyst for activation of the ordinate with the other carbonyl oxygen of the diketone,
carbonyl group.
thereby stabilizing the transition state further. Finally, the
39200 | RSC Adv., 2015, 5, 39193–39204
This journal is © The Royal Society of Chemistry 2015

View Article Online
Paper
RSC Advances
Scheme 2 Possible mechanistic pathway for the formation of b-enaminones.
tetrahedron intermediate undergoes an elimination of the
water molecule to generate the imine moiety. The imine inter-
mediate (IV) aer tautomerization forms the enaminones as the
nal product. It can be concluded that the Sn(IV) of SnO2
nanoparticle help in activation and stabilization of substrate
molecules during enaminones and enaminoester synthesis.
3.3 General procedure for the synthesis of b-enaminones
and b-enaminoesters in solvent media
In a 50 ml round bottom ask, GO–SnO2 nanocomposite
(0.02 g) as a catalyst was taken in methanol (5 ml) and stirred at
room temperature for 10 minutes. Then, a mixture of amine
(1 mmol) and b-diꢀcarbonyl compound (1 mmol) were added to it
and stirred at 60 C under nitrogen atmosphere. The advance-
ment of reaction was monitored by TLC. On completion of the
reaction as indicated by TLC examination, the catalyst was
ltered off. Evaporation of the solvent followed by column
chromatography on silica-gel column gave the pure enami-
nones as the nal products.
Fig. 8 Recyclability chart for GO–SnO2 nanocomposite.
This journal is © The Royal Society of Chemistry 2015
RSC Adv., 2015, 5, 39193–39204 | 39201

View Article Online
RSC Advances
Paper
Table 3 Catalytic performance of different composite materials for b-enaminones and b-enaminoesters
Entry
Catalyst
Solvent
T (ꢀC)
T (min)
Yield (%)
Ref.
1
2
3
4
5
6
7
Ag/Fe3O4@meso-SiO2
B2O3/Al2O3
Cu/AlO(OH)
Graphene oxide–SnO2
Graphene oxide–SnO2
Silver nanoparticles
PPA–SiO2
Methanol
Ethanol
60
60
90
87
94
98
96
90
70
78
29
79
Room temperature
27
60
60
120
105
40
75
120
130
Solventless
Solventless
Methanol
Methanol
Methanol
This work
This work
28
60
Reux temperature
89
aer four cycles, and the yield of the model reaction was 91%
aer 4th run (Fig. 8).
3.4 General procedure for the synthesis of b-enaminones
and b-enaminoesters under solventless condition
In a 50 ml round bottom ask, mixture of amine (1 mmol),
b-dicarbonyl compound (1 mmol), and GO–SnO2 nano-
composite (0.02 g) as catalyst was taken and stirred at 50 ꢀC
under nitrogen atmosphere. The advancement of reaction was
monitored by TLC. On completion of the reaction as indicated
by TLC examination, the mixture was diluted with EtOAc (5 ml)
and ltered. The catalyst was recovered by ltration from
residue. The ltrate was concentrated and product was puried
by column chromatography on silica gel using EtOAc/hexane as
eluent. Recovered catalyst was washed with water followed by
acetone (3 ꢁ 10 ml) and dried at 150 ꢀC for 1 h, before reusing.
The concentrated product was extracted three times with
ethyl acetate (20 ml) and the combined layer were dried over
anhydrous Na2SO4. The solvent was removed in vacuum.
Subsequently, pure b-enaminone as a product was obtained by
3.6 Comparison with other reported systems
Later on, to check the efficiency of our catalyst we have
compared the activity of our catalyst with other reported cata-
lysts. Table 3 shows the comparison of reported catalyst with
our catalyst for the synthesis of b-enaminones and b-enami-
noesters under the same conditions. From Table 3, it can be
seen that our catalyst exhibited higher yields in lesser time
compared to the other reported system such as Cu/AlO(OH),87
Ag/Fe3O4@meso-SiO2, B2O3/Al2O3,90 Ag nanoparticle28 and
PPA–SiO2.91 These results conrmed that our GO–SnO2 catalyst
is more effective and less time consuming for synthesis of
b-enaminones and b-enaminoesters.
doing column chromatography using silica gel and EtOAc/ 4. Conclusions
hexane. The structures of all the products were unambigu-
ously established on the basis of their spectral analysis. All the
In summary, we have successfully synthesized GO–SnO2 nano-
composite catalyst via a solvothermal method. The synthesized
catalyst was then characterized using FTIR, XRD, FESEM, TEM,
BET, XPS, FT-Raman and EDX techniques. The particle size of
nanocomposite was found to be around 8–12 nm. For the rst
time, GO–SnO2 nanocomposite shows remarkably higher cata-
lytic activity for the synthesis of ne chemicals such as
b-enaminones and b-enaminoesters. The nanocomposite cata-
lyst offers several advantages, including high yield, short reac-
tion time, simple work up procedure, ease of separation and
easy recyclability of the catalyst. The catalyst was reused up to
four runs in solventless system without any signicant loss in
activity.
product were identied by comparing with available literatures.
3.5 Reproducibility of catalyst
As discussed earlier, reusability of the heterogeneous catalyst
plays an important role in order demonstrate it as economical.
Because of this, a series of reaction cycles were run in order to
investigate the efficiency of our catalytic system. Recyclability of
the catalyst was studied for the model reaction comprising of
acetylacetone and benzyl amine in methanol and solventless
system. Aer each catalytic cycle, the catalyst was recovered by
centrifugation method, thoroughly washed with polar organic
solvents (ethylacetate) to remove all the organic substances and
nally dried for 10 h before performing the next cycle with an
equal amount of substrate loading. When methanol was used,
the recyclability of the catalyst was found to be undesirable and
the activity was almost lost. This slow decrease of the catalytic
efficiency may be credited to the fact that aer each catalytic run
we continuously lose some amount of catalyst during the cata-
lyst recovery process and hence the effective loading of the
catalyst keeps on decreasing from one catalytic cycle to the
next.89 In methanol medium, the yield of the product decreased
signicantly to 48% aer 4th run (Fig. 8). However, in solvent-
less condition, GO–SnO2 could be recovered and reused several
times. It was observed that there is no signicant loss of activity
Acknowledgements
The authors are thankful to Department of Science & Tech-
nology (DST), Govt. of India, for funding and the Sophisticated
Analytical Instrument Facility, IIT Madras for providing FT-
RAMAN facility. XPS facility at IIT Delhi is partially funded by
FIST grant of DST, India.
References
1 G. J. Reddy, D. Latha, C. Thirupathaiah and K. S. Rao,
Tetrahedron Lett., 2005, 46, 301–302.
39202 | RSC Adv., 2015, 5, 39193–39204
This journal is © The Royal Society of Chemistry 2015

View Article Online
Paper
RSC Advances
2 G. Negri, C. Kascheres and A. J. Kascheres, J. Heterocycl. 28 K. D. Bhatte, P. J. Tambade, K. P. Dhake and B. M. Bhanage,
Chem., 2004, 41, 461–491. Catal. Commun., 2010, 11, 1233–1237.
3 K. V. Tarasenko, O. V. Manoylenko, V. P. Kukhar, 29 J. Sun, Z. Dong, P. Li, F. Zhang, S. Wei, Z. Shi and R. Li,
¨
G.-V. Roschenthaler and I. I. Gerus, Tetrahedron Lett., 2010,
Mater. Chem. Phys., 2013, 140, 1–6.
51, 4623–4626.
30 M. Kidwai, S. Bhardwaj, N. K. Mishra, V. Bansal, A. Kumar
and S. Mozumdar, Catal. Commun., 2009, 10, 1514–1517.
31 B. Giuseppe, B. Marcella, L. Manuela, M. Enrico, M. Paolo
and S. Letizia, Synlett, 2004, 10, 239–242.
32 Z. H. Zhang, L. Yin and Y. M. Wang, Adv. Synth. Catal., 2006,
348, 184–190.
33 Z. Zhan-Hui and H. Jin-Yong, J. Braz. Chem. Soc., 2006, 17,
1447–1451.
34 M. E. F. Braibante, H. S. Braibante, L. Missio and
A. Andricopulo, Synthesis, 1994, 898–900.
4 G. Li, K. Watson, R. W. Buckheit and Y. Zhang, Org. Lett.,
2007, 9, 2043–2046.
5 B. A. D. Neto, A. A. M. Lapis, A. B. Bernd and D. Russowsky,
Tetrahedron Lett., 2009, 65, 2484–2496.
6 I. Chaaban, J. V. Greenhill and P. Akhtar, J. Chem. Soc., Perkin
Trans. 1, 1979, 6, 1593–1596.
7 L. J. O. Figueiredo and C. Kascheres, J. Org. Chem., 1997, 62,
1164–1167.
8 T. Mahmud, R. Rehman, A. Gulzar, A. Khalid, J. Anwar,
U. Shaque, Z. Waheed and M. Salman, Arabian J. Chem., 35 Y. Gao, Q. Zhang and J. Xu, Synth. Commun., 2004, 34, 909–
2010, 3, 219–224. 916.
9 C. Cimarelli and G. Palmieri, J. Org. Chem., 1996, 61, 5557– 36 M. A. P. Martins, C. P. Frizzo, D. N. Moreira, F. A. Rosa,
5563.
M. R. B. Marzari, N. Zanatta and H. G. Bonacorso, Catal.
Commun., 2008, 9, 1375–1378.
37 C. P. C. Marin, D. G. Henderson and R. W. Soeder, Synth.
Commun., 1997, 27, 4275–4283.
38 A. R. Katritzky, Y. Fang, A. Donkor and J. Xu, Synthesis, 2000,
2029–2032.
39 E. Haak, Synlett, 2006, 12, 1847–1848.
10 D. Potin, F. Dumas and J. D. Angelo, J. Am. Chem. Soc., 1990,
112, 3483–3486.
11 N. N. Salama, K. R. Scott and N. D. Eddington, Biopharm.
Drug Dispos., 2004, 25, 227–236.
12 I. O. Edaogho, O. A. Phillips, E. E. Udo, S. Samuel and
B. Rethish, Eur. J. Med. Chem., 2009, 44, 967–975.
13 A. Z. El-Hashim, I. O. Edaogho, S. M. Jaffal, M. H. Yousif, 40 P. T. Anastas and R. H. Crabtree, in Handbook of Green
C. I. Ezeamuzie and S. B. Kombian, Life Sci., 2011, 89, 378–
387.
14 A. El-Hashim, S. Youse, I. Edaogho, R. Raghupathy,
Chemistry Green Catalysis, Wiley-VCH, 2009.
41 D. Choudhary, S. Paul, R. Gupta and J. H. Clark, Green Chem.,
2006, 8, 479–482.
M. Yousif and H.-U. Simon, Eur. J. Pharmacol., 2010, 632, 42 H. Sharghi, A. Khoshnood and R. Khalifeh, Iran. J. Sci.
73–78. Technol., 2012, 25–31.
15 D. L. Boger, T. Ishizaki, R. J. Wysocki, S. A. Munk, P. A. Kitos 43 Y. Nishina, J. Miyata, R. Kawaib and K. Gotohb, RSC Adv.,
and O. Suntornwal, J. Am. Chem. Soc., 1989, 111, 6461–6463. 2012, 2, 9380–9382.
16 H. Eshghi, S. M. Seyedi, E. Safaei, M. Vakili, A. Farhadipour 44 G. G. kumar, Z. Awan, K. S. Nahm and J. S. Xavier, Biosens.
and M. Bayat-Mokhtari, J. Mol. Catal. A: Chem., 2012, 363–
364, 430–436.
17 J. Lin and L.-F. Zhang, Monatsh. Chem., 2007, 138, 77–81.
Bioelectron., 2014, 53, 528–534.
45 X. Wang, Q. Wang, Q. Wang, F. Gao, F. Gao, Y. Yang and
H. Guo, ACS Appl. Mater. Interfaces, 2014, 6, 11573–11580.
18 M. M. Khodaei, A. R. Khosropour and M. Kookhazadeh, 46 R. Bajpai, S. Roy, N. Koratkar and D. S. Misra, Carbon, 2013,
Synlett, 2004, 11, 1980–1984. 56, 56–63.
19 Z. H. Zhang, T. S. Li and J. J. Li, Catal. Commun., 2007, 8, 47 L. Dong, M. Li, L. Dong, M. Zhao, J. Feng, Y. Han, J. Deng,
1615–1620.
X. Li, D. Li and X. Sun, Int. J. Hydrogen Energy, 2014, 39,
16116–16122.
48 R. Nie, J. Shi, S. Xia, L. Shen, P. Chen, Z. Hou and F. S. Xiao,
J. Mater. Chem., 2012, 22, 18115–18118.
49 N. Zhang, H. Qiu, Y. Liu, W. Wang, Y. Li, X. Wang and J. Gao,
J. Mater. Chem., 2011, 21, 11080–11083.
50 A. R. Siamaki, A. E. R. S. Khder, V. Abdelsayed, M. S. El-Shall
and B. F. Gupton, J. Catal., 2011, 279, 1–11.
51 K.-C. Hsu and D.-H. Chen, Nanoscale Res. Lett., 2014, 9, 484–
494.
20 A. Arcadi, G. Bianchi, S. D. Giuseppe and F. Marinelli, Green
Chem., 2003, 5, 64–67.
21 B. Das, K. Venkateswarlu, A. Majhi, M. R. Reddy and
K. N. Reddy, J. Mol. Catal. A: Chem., 2006, 246, 276–281.
22 A. R. Gholap, N. S. Chakor, T. Daniel, R. J. Lahoti and
K. V. Srinivasan, J. Mol. Catal. A: Chem., 2006, 245, 37–46.
23 H. Seki and G. I. Georg, J. Am. Chem. Soc., 2010, 10, 15512–
15513.
24 J. Kang, F. Liang, S. G. Sun, Q. Liu and X. H. Bi, Org. Lett.,
2006, 8, 2547–2550.
25 F. Epifano, S. Genovese and M. Curini, Tetrahedron Lett.,
2007, 48, 2717–2720.
52 X. Chen, G. Wu, J. Chen, X. Chen, Z. Xie and X. Wang, J. Am.
Chem. Soc. Rev., 2011, 133, 3693–3695.
53 S. Guo, S. Dong and E. Wang, ACS Nano, 2009, 4, 547–555.
26 M. M. Khodaei, A. R. Khosropour and M. Kookhazadeh, Can. 54 F. Xu, M. Deng, G. Li, S. Chen and L. Wang, Electrochim. Acta,
J. Chem., 2005, 83, 209–212. 2013, 88, 59–65.
27 J. S. Yadav, V. N. Kumar, R. S. Rao, A. D. Priyadarshini, 55 R. Adnan, N. A. Razana, I. A. Rahman and M. A. Farrukh,
P. P. Rao, B. V. S. Reddy and K. Nagaiah, J. Mol. Catal. A:
Chem., 2006, 256, 234–237.
J. Chin. Chem. Soc., 2010, 57, 222–229.
This journal is © The Royal Society of Chemistry 2015
RSC Adv., 2015, 5, 39193–39204 | 39203

View Article Online
RSC Advances
Paper
56 S. Chappel and A. Zaban, Sol. Energy Mater. Sol. Cells, 2002, 74 S. Zhuang, X. Xu, B. Feng, J. Hu, Y. Pang and G. Zhou, ACS
71, 141–152. Appl. Mater. Interfaces, 2014, 6, 613–621.
57 T. W. Kim, D. U. Lee, D. C. Choo, J. H. Kim, H. J. Kim, 75 Q. Guo and X. Qin, ECS Solid State Lett., 2013, M41–M43.
J. H. Jeong, M. Jung, J. H. Bahang, H. L. Park, Y. S. Yoon 76 S. Li, W. Xie, S. Wang, X. Jiang, S. Peng and D. He, J. Mater.
and J. Y. Kim, J. Phys. Chem. Solids, 2002, 63, 881.
Chem. A, 2014, 2, 17139–17145.
58 H. Liu, V. Avrutin, N. Izyumskaya, U. Ozgur and H. Morkoç, 77 S. Yang, W. Yue, J. Zhu, Y. Ren and X. Yang, Adv. Funct.
Superlattices Microstruct., 2010, 48, 458–484. Mater., 2013, 23, 3570–3576.
59 M. S. Niasari, N. Mir and F. Davar, Inorg. Chim. Acta, 2010, 78 X. Liu, J. Cui, J. Sun and X. Zhang, RSC Adv., 2014, 4, 22601–
363, 1719–1726. 22605.
60 Q. Zhao, Z. Zhang, T. Dong and Y. Xie, J. Phys. Chem. B, 2006, 79 Y.-X. Wang, Y.-G. Lim, M.-S. Park, S.-L. Chou, J. H. Kim,
¨
¨
¨
110, 15152–15156.
H.-K. Liu, S.-X. Dou and Y.-J. Kim, J. Mater. Chem. A, 2014,
2, 529–534.
80 H. Song, L. Zhang, C. He, Y. Qu, Y. Tian and T. Lv, J. Mater.
Chem., 2011, 21, 5972–5977.
81 X. S. Zhou, L. J. Wan and Y. G. Guo, Adv. Mater., 2013, 25,
2152.
61 H. Sharghi, S. Ebrahimpourmoghaddam, R. Memarzadeh
and S. Javadpour, J. Iran. Chem. Soc., 2012, 10, 141–149.
62 K. J. Klanbunde, Nanoscale Materials in Chemistry, Wiley,
New York, 2001.
63 N. L. V. Carreno, H. V. Fajardo, A. P. Maciel, A. Valentini,
F. M. Pontes, L. F. D. Probst, E. R. Leite and E. Longo, 82 S. Li, W. Xie, S. Wang, X. Jiang, S. Peng and D. He, J. Mater.
J. Mol. Catal. A: Chem., 2004, 207, 91–96. Chem. A, 2014, 2, 17139–17145.
64 G. C. Bond, L. R. Molloy and M. J. Fuller, J. Chem. Soc., Chem. 83 S. Ray, A. Bhaumik, A. Dutta and C. Mukhopadhyay, Catal.
Commun., 1975, 19, 796–797. Sci. Technol., 2013, 3, 1267–1277.
65 J. Biana, M. Xiaoa, S. J. Wanga, Y. X. Lub and Y. Z. Menga, 84 N. Ahmed and Z. N. Siddiqui, J. Mol. Catal. A: Chem., 2014,
Catal. Commun., 2009, 10, 1529–1533. 394, 232–243.
66 S. J. Park, J. H. An, R. D. Piner, I. Jung, 85 S. Park, J. An, I. Jung, R. D. Piner, S. J. An, X. Li,
D. X. Y. A. Velamakanni, S. B. T. Nguyen and R. S. Ruoff,
Chem. Mater., 2008, 20, 6592–6594.
A. Velamakanni and R. S. Ruoff, Nano Lett., 2009, 9, 1593–
1597.
67 K. N. Kudin, B. Ozbas, H. C. Schniepp, R. K. Prudhomme, 86 M. M. Khodaei, A. R. Khosropour and M. Kookhazadeh,
I. A. Aksay and B. Car, Nano Lett., 2008, 8, 36–41. Russ. J. Org. Chem., 2005, 41, 1445–1448.
68 Y. Xu, K. Sheng, C. Li and G. Shi, ACS Nano, 2010, 4, 4324– 87 S. G. Babu, B. Thomas, A. Nijamudheen, A. Datta and
4330. R. Karvembu, Catal. Sci. Technol., 2012, 2, 1872–1878.
69 S. Min and G. Lu, J. Phys. Chem. C, 2011, 115, 13938–13945. 88 M. Batzill and U. Diebold, Prog. Surf. Sci., 2005, 79, 47–154.
70 L. Wang, Y. Ye, X. Lu, Z. Wen, Z. Li, H. Hou and Y. Song, Sci. 89 Z. N. Siddiqui, N. Ahmed, F. Farooq and K. Khan,
Rep., 2013, 3, 3568.
71 G. Wang, B. Wang, X. Wang, J. Park, S. Dou, H. Ahnb and 90 J. X. Chen, C. F. Zhang, W. X. Gao, H. L. Jin, J. C. Ding and
K. Kim, J. Mater. Chem., 2009, 19, 8378–8384. H. Y. Wu, J. Braz. Chem. Soc., 2010, 21, 1552–1556.
72 F. Li, J. Song, H. Yang, S. Gan, Q. Zhang, D. Han, A. Ivaska 91 M. Nisar, I. Ali, M. R. Shah, M. Qayum, M. Z.-U. Haq,
Tetrahedron Lett., 2013, 54, 3599–3604.
and L. Niu, Nanotechnology, 2009, 20, 455602–455608.
73 H. Seema, K. C. Kemp, V. Chandra and K. S. Kim,
Nanotechnology, 2012, 23, 355705–3557013.
U. Rashid and M. S. Islam, Molecules, 2013, 18, 15182–15192.
39204 | RSC Adv., 2015, 5, 39193–39204
This journal is © The Royal Society of Chemistry 2015