Highly efficient and simultaneous catalytic reduction of multiple dyes using recyclable RGO/Co dendritic nanocomposites as catalyst for wastewater treatment

Article abstract of DOI:10.1039/c6ra23621a

Development of a low cost, highly efficient and easily retrievable catalyst with improved reusability is a major challenge in the area of advanced catalysts. In this study, we report a simple one-step approach for the fabrication of a reduced graphene oxide (RGO)/Co dendritic nanocomposite. The structure and morphology of the as synthesized material are thoroughly examined by XRD, Raman, FTIR, TEM, and SEM. The magnetic properties of the RGO/Co dendritic nanocomposite reveal that it exhibits ferromagnetic behavior at room temperature with high saturation magnetization. The catalytic activity of the RGO/Co dendritic nanocomposite was investigated for the reduction of different dyes namely, 4-nitrophenol, methylene blue, methyl orange and rhodamine B individually, and their mixture in the presence of a sufficient amount of NaBH₄. RGO/Co dendritic nanocomposite exhibits excellent catalytic activity as compared to the bare Co dendritic structure. The catalyst could be easily separated by an external magnet and recycled magnetically with no major loss of catalytic activity upto five cycles. The high catalytic efficiency, low cost and easy recycle technique make RGO/Co dendritic nanocomposite a proficient catalyst for degradation of organic dyes.

Full text of DOI:10.1039/c6ra23621a

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Highly efficient and simultaneous catalytic reduction of multiple
dyes using recyclable RGO/Co dendritic nanocomposites as
catalyst for the wastewater treatment
Prasanta Kumar Sahooa, Dinbandhu Thakura, D. Bahadura and Bharati Panigrahyb*
Development of a low cost, high efficient and easy retrievable catalyst with improved reusability is a major challenge in the
area of advanced catalysts. In this study, we report a simple one-step approach for the fabrication of reduced graphene
oxide (RGO)/ Co dendritic nanocomposite. The structure and morphology of the as synthesized material are thoroughly
examined by XRD, Raman, FTIR, TEM, SEM. Magnetic property of RGO/Co dendritic reveal that it exhibits ferromagnetic
behavior at room temperature with high saturation magnetization. Catalytic activity of RGO/Co dendritic nanocomposite
was investigated for the reduction of different dyes namely, 4-nitrophenol, methylene blue, methyl orange and rhodamine
B individually, and their mixture in presence of sufficient amount of NaBH4. RGO/Co dendritic nanocomposite exhibits
excellent catalytic activity as compared to the bare Co dendritic structure. The catalyst could be easily separated by an
external magnet and recycled magnetically with no major loss of catalytic activity upto five cycles. The high catalytic
efficiency, low cost and easy recycle technique make RGO/Co dendritic nanocomposite a proficient catalyst for
degradation of organic dyes.
as assisting ligands, and with different semiconductors and
their nanocomposites.10-13 But utilization of light radiation in
1. Introduction
Use of dyes in industries can cause several problems when it is
discharged into rivers or enter the environment.1-3 Dyes are
generally toxic, affecting the photosynthetic activity of aquatic
organisms. Therefore, to get rid of dye pollutants from waste
water are a serious issue and have attracted extensive
attention in current years.4,5 In the past decades, different
types of waste water purification methods, such as chemical,
physico-chemical and biological methods has been applied for
removal of dye containing wastes.6-8 However, these
techniques are usually not striking due to the time-consumed
during the processes and are also uneconomical.9 Several
scientists concentrate on the photo-oxidation of dyes by
irradiating UV/visible light source with oxidizing agents that act
the degradation process limits its application on large scale
basis. On the other hand, noble or precious metal
nanoparticles (e.g., Pd, Pt, Au, Ag, and Ru) based catalysts have
attracted significant attention over the last few years due to
their ability to degrade a dye in presence of NaBH4.7,14-22
However, noble metal nanoparticles are unstable and
susceptible to irreversible aggregation in the solution due to
their high surface energy resulting from the high surface-to-
volume ratio, which leads to a significant reduction of catalytic
activity.23
In order to tackle the above mentioned drawback and avoid
the aggregation of noble metal nanoparticles, researchers
have adopted an effective strategy by incorporating solid
supports (such as silica, polymers and carbon allotropes) which
improve the agglomeration and stability of the noble metal
nanoparticles and enables the customization of impactful
complex reagents.24 Carbon allotropes, for example carbon
nanotubes (CNTs), graphene derivatives such as graphene
oxide or reduced graphene oxide have been regarded as
efficient co-catalyst due to their exceptional capability of being
able to load a variety of nanoparticles.25 Out of the above
mentioned solid supports, two dimensional (2D) reduced
graphene oxide (RGO) has specifically attracted a lot of
attention to the research community due to their fascinating
characteristics such as high electrical conductivity, large
specific surface area, flexible structures and mechanical
a.Department of Metallurgical Engineering and Materials Science, Indian Institute
of Technology Bombay, Powai, Mumbai-400076, India
b.Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore,
560012, India
† Electronic Supplementary Information (ESI) available: The tables contain rate
constants, reduction conditions, time and yields for the reduction reactions of
different dyes catalyzed by various catalysts. The table contain room temperature
(RT) magnetic data of the Co DND and the RGO/Co DND nanocomposite. FTIR and
XPS spectra of GO and RGO/Co DND nanocomposite. Reaction paths for the
catalytic reduction of 4-NP in presence of RGO/Co DND nanocomposite and NaBH4.
N2 adsorption-desorption isotherms of Co DND and RGO/Co DND nanocomposite.
UV-Vis absorption spectra and kinetic plot of the reduction of MB in presence of Co
DND and RGO physical mixture. XRD, XPS, SEM and TEM study of RGO/Co DND
nanocomposite before reduction of MB and after 5th cycle reduction of MB in
presence of NaBH4. UV-Vis spectra of MB before reduction and after 10 min of
reduction in presence of RGO/Co DND nanocomposite and NaBH4.
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robustness.26 Moreover, graphene sheet prevents the and some other reported precious metal nanoparticle based
agglomeration of the loaded metal nanoparticles as well as catalysts (Table S1 represent the comparison of the reported
enhance the catalytic activity due to the strong synergistic rate constant values of RGO/Co nanocomposite and precious
interaction between the two constituents.27 The existence of metal nanoparticle based catalysts used for different dyes). It
strong covalent interaction of the metal nanoparticles with has been noticed that the catalytic activity of RGO/Co DND
graphene sheets can restrict the poisoning and improve the nanocomposite is superior to that of bare Co DND structure in
chemical stability of the metal nanoparticles. However, case of all four dyes and in some of the dyes; it is higher than
considerable research is required to understand how that of other reported precious metal nanoparticle based
graphene-based composite catalysts loaded with interesting catalysts.
properties can be taken out from the degradation reaction
solution and can be used further for the same application. To
2. ExperimentationSection
solve this issue, designing graphene based composites in the
presence of magnetic nanoparticles as
a magnetically
2.1 Chemicals
separable catalyst could be an interesting approach. In view of
this, RGO based nanocomposites containing noble metal with
magnetic nanoparticles have recently been reported as
catalysts for dye degradation using NaBH4.28-31 Unfortunately,
the cost associated with noble metal based catalysts is very
high, which is a major challenge for developing low cost
catalysts for water treatment and environmental remediation.
One possible solution is replacing the commonly used precious
noble metal catalysts with inexpensive non-precious metal
catalysts such as cobalt (Co) and nickel (Ni). Cobalt
nanoparticles have long been of scientific and technological
attention for its effective ferromagnetic nature which can be
used for industrial applications, such as magnetic fluids,
optoelectronics, and data storage applications.32-35 To the best
of our knowledge, there are a very few reports regarding the
utilization of cobalt nanomaterials for the purpose of dye
degradation mainly because of their difficulty in fabrication
and stabilization.36 Here, our objective is to develop a cost
effective catalyst which can exhibit good catalytic activity
towards the decolorization of multiple dyes and an easy
magnetically retrievable catalyst with improved reusability as
compared to traditional methods such as centrifugation and
filtration.
Natural graphite flakes (particle size, 45 µm) Cobalt (II)
chloride hexahydrate (CoCl2.6H2O) were purchased from
Sigma-Aldrich. 4-NP, MO, MB and RhB of analytical reagent
(AR) grade were purchased from Thomas Baker Chemicals Pvt.
Ltd., India. Rest of the chemicals are analytical grade and
obtained from Merck Specialties Private Limited, India. Milli-Q
water was used for the preparation of aqueous solution in all
our experiments.
2.2 Synthesis of RGO/Cobalt DND nanocomposite
Modified Hummers' method was used for the preparation of
Graphite oxide (GO) from the natural graphite flakes.37
RGO/Co DND nanocomposite was prepared by one-step
chemical reduction method. 50 mg of the obtained GO was
dispersed into 25 ml of absolute ethanol followed by 1 hr of
ultrasonication to get stable GO suspension. 0.476
g
CoCl2.6H2O and 0.183 g cetyltrimethyl ammonium bromide
(CTAB) were dissolved in the above suspension of GO. After
being under magnetic stirring for 30 min at room temperature,
the above mixture was heated to 55 °C, and a solution
composed of 7.5 ml of N2H4.H2O (80 wt %) and 0.6 g NaOH was
added and allowed to react for 30 min.
In this work, we demonstrate a simple low-temperature
method for the large-scale synthesis of bare Co dendritic
(DND) structure and RGO/Co dendritic (DND) nanocomposite.
The structural, microstructural and magnetic properties of
fabricated Co DND and RGO/Co DND nanocomposite were
studied here. The scanning electron microscopy (SEM) and
transmission electron microscopy (TEM) images show that the
as-prepared Co DND and RGO/Co DND nanocomposite exhibit
dendritic morphologies and leaf like flakes are well-arranged
initiating from the centre. Both Co DND and RGO/Co DND
nanocomposite exhibit ferromagnetic behaviour but the
saturation magnetization of RGO/Co dendritic nanocomposite
is lower than that of the bare cobalt DND structure. The
catalytic activity of the Co DND and RGO/Co DND
nanocomposite were also investigated for the reduction of
various organic dyes (4-nitrophenol (4-NP), Methylene Blue
(MB), Methyl Orange (MO) and Rhodamine B (RhB)) and their
Scheme 1 Schematic representation of synthesis of RGO/Co DND nanocomposite by
mixture by NaBH4. The magnetic property of RGO/Co DND one-stepchemicalapproach and the catalysis reaction of multiple dyes by NaBH4 in
presence of RGO/Co DND nanocomposite.
nanocomposite makes it ideal for re-usable catalyst. The
catalytic activity of RGO/Co DND nanocomposite for different
dyes was further compared with the bare Co DND structure
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The as prepared materials were rinsed thoroughly in milli-Q range of 250–800 nm. All the catalytic reactions were
water and ethanol followed by dried at 50 °C for 4 h in a performed at room temperature.
vacuum oven. For comparison study, bare Co DND structure
was synthesized by similar method in the absence of GO. The
3 Results and discussion
formation mechanism of the Co DND structures may follow the
diffusion-limited aggregation (DLA) process through inter-
XRD patterns of Cobalt DND and RGO/Cobalt DND
action between the stochastic diffusive force and the directive
nanocomposite are shown in Fig. 1. The peaks at 2θ values of
force.38 A detailed schematic representation of the synthesis of
41.8, 44.6, 47.5, 62.5 and 76.1 are ascribed to the (100), (002),
RGO/Co DND nanocomposite is presented in the Scheme 1.
(101), (102) and (110) diffraction planes of hexagonal close-
packed (hcp) cobalt lattice, respectively (JCPDS No. 05-0727).
Impurities peaks are not detected in the spectrum which
confirms the absence of cobalt oxides or hydroxides phases,
signifying that the as-synthesized cobalt heterostructures exist
as a pure hcp phases.39 It has been noticed that relative
intensities of (002) / (100) and (002) / (101) planes in both Co
DND and RGO/Co DND nanocomposite are quite higher than
the standard values, which reveals that dendritic growth takes
place along [001] directions for both Co DND and RGO/Co DND
nanocomposite. In addition, all XRD peaks of RGO/Co DND
nanocomposite are quite broader than the Co DND. This may
be due to the formation of smaller sizes of the Co dendritic
leaves in the RGO/Co DND nanocomposite.
2.3 Characterization
XRD (Philips powder diffractometer PW 3040/60) was used to
study the crystal structure of as-prepared materials by using
CuKα (λ = 1.541 Å) as radiation source. The presence of
functional groups in the as-prepared materials were analysed
by FTIR spectroscopy (JASCO spectrometer, 6100 type-A) in the
range of 400-4000 cm-1. Raman spectroscopy (Lab RAM HR
800) was carried by using Ar+ laser of wavelength 519 nm as an
excitation source. X-ray photoelectron spectroscopy (XPS) was
carried out to investigate the chemical states and surface
composition of the Co DND and RGO/Co DND nanocomposite
by using MULTILAB from Thermo VG Scientific with Al Kα
radiation (1486.6 eV). The specific surface area of the Co DND
and RGO/Co DND nanocomposite were measured by the
multipoint Brunauer−Emmet−Teller (BET) method using
Micromeritics instrument ASAP 2020. SEM (JEOL Model JSM-
7600F) and TEM (JEOL JAM 2100F, accelerating potential 200
kV) were used to investigate the morphological properties of
as-prepared materials. Inductive coupled plasma atomic
emission spectroscopy (ICP-AES, Prodigy-Teledyne Leeman
Labs) was performed for the determination of the chemical
composition of RGO/Co DND nanocomposite. The
magnetization measurements of as-prepared materials were
executed through magnetic properties measurement system
(MPMS-XL, Quantum Design). The catalytic performances of
as-prepared Co DND and RGO/Co DND nanocomposite were
investigated by UV-Vis absorption spectroscopy (Cecil, model
no CE3021).
2.4 Evaluation of catalytic activity
Catalytic activity of the as-prepared Co DND and RGO/Co DND
nanocomposite was determined for the reduction and
degradation of high concentration dyes by using NaBH4 as
reducing agent. Four types of different dyes (4-NP, MB, MO
and RhB) were chosen to examine the degradation efficiency
of our fabricated catalyst. In a typical degradation reaction, 20
ml of aqueous solution of 40 ppm of 4-NP or MB or MO or RhB
dye was taken, separately in a reaction cell. 6 mg of catalyst
was added separately to the different dye solutions followed
by stirring for 10 minutes. 2 ml of freshly prepared NaBH4
solution (1.5 M) was rapidly injected in to the above mixture
dye solutions, separately. In a definite time interval (2
minutes), 1 mL of each reaction solution was diluted with 2 mL
of aqueous solution and placed into a quartz cell from the
reaction system. Then, the UV-Vis spectra of reaction solutions
were collected by using an UV–Vis spectrophotometer in the
Fig.1 XRD patterns of (a) Co DND and (b) RGO/ Co DND nanocomposite.
Fig.2 shows the FTIR spectra of GO and RGO/Co DND
nanocomposite. Presence of many oxygenic functional groups
in the FTIR spectrum of GO (Fig. 2 (a)) indicates that there is a
successful oxidation of graphite.The broad band at 3400 cm-1
comes from the O-H stretching vibration. The 1730 cm-1 band
corresponds to the C=O stretching vibration of the COOH
groups present at the edges of the graphite oxide sheets. The
observed band at 1404, 1222 and 1045 cm-1, can be ascribed
to O-H deformation, C–O (epoxy) stretching vibration peak and
C–O (alkoxy), respectively. The band at 1612 cm−1 exists either
due to the vibration of the adsorbed water molecules or may
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be from the skeletal vibrations of unoxidized graphitic ratio in RGO/Co DND nanocomposite as compared with GO
sheets.40
reveals that there is an effective reduction of GO in the
RGO/Co DND nanocomposite.
Fig.2 FTIR spectra of (a) Co DND and (b) RGO/Co DND nanocomposite.
Fig.3 Raman spectra of (a) GO and (b) RGO/Co DND nanocomposite.
Fig. 2 (b) shows that most of the oxygenic functional groups
are nearly disappear in the RGO/Co DND nanocomposite The chemical states of elements present in the GO and
except less intense C=O stretching vibration of -COOH and O-H RGO/Co DND nanocomposite were investigated by XPS
stretching and bending vibrations of water. This reveals that ananlysis. The XPS spectra of GO and RGO/Co DND
most of the oxygenic functional groups were successfully nanocomposite were shown in Fig. 4 (a). Both XPS spectra
removed from GO in the reduction method transforming into consists up C1s and O1s peaks but RGO/Co DND
RGO. The C=O stretching vibration of GO and RGO/Co DND nanocomposite posses an addition Co 2p peak. This confirmed
nanocomposite are 1730 and 1711 cm-1, respectively. The blue that Co DNDs are formed and deposited on the surface of
shift in the C=O stretching vibration in RGO/Co nanocomposite graphene sheets.
as compared to GO is due to the chemical interaction between
the Co and the residual C=O group present on the RGO sheets
(Fig. S1).27(c) In case of RGO/Co DND nanocomposite, the
absorption spectrum at 1560 cm-1 corresponds to the skeletal
vibration of the graphitic domains.41
Raman spectroscopy is extensively used to observe the
structural changes in carbonaceous materials. Fig. 3 shows the
characteristic Raman spectra of GO and RGO/Co DND
nanocomposite. The well knows D and G bands are present in
the Raman spectra of GO and RGO/Co DND nanocomposite.
The G band typically corresponds to the E2g phonon of C sp2
atoms, whereas the D band initiates from the breathing k-
point phonon of the A1g symmetry as it is related to local
43
defects and disorders in the graphene structure.42,
The
average size of sp2 domains is calculated from the intensity
ratio (ID/IG) of the D band to the G band.44 The ID/IG ratios for
graphite oxide and RGO/Co DND nanocomposite are 0.82 and
1.2 respectively. It has been seen that there is a re-
establishment of a conjugated graphene network (sp2 carbon)
after chemical reduction of graphene oxide. But the size of the
original graphite layer is typically larger than the re-established
graphene network that leads to the increase of the intensity
ratio (ID/IG), consequently.45 Therefore, the increase of ID/IG
Fig.4 (a) XPS spectra of GO and RGO/Co DND nanocomposite, (b) High-resolution Co 2p
core level XPS spectrum of RGO/Co DND nanocomposite, (c) High-resolution
deconvoluted C 1s core level XPS spectra of GO and (d) High-resolution deconvoluted C
1s core level XPS spectra of RGO/Co DND nanocomposite.
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The high-resolution XPS spectra of Co doublet (2p3/2 and Ag
2p1/2) in the RGO/Co DND nanocomposite are shown in the
Fig.4 (b). The doublet Co (2p3/2 and 2p1/2) was centered at
binding energy nearly 778.6 and 793.8 eV in RGO/Co DND
nanocomposite. However, the binding energy of Co 2p for Co
DND in RGO/Co DND nanocomposite are slight higher than the
metallic Co (2p3/2 = 778.1 eV; 2p1/2 = 793.3 eV). The slight
higher binding energy of Co 2p is because of partial oxidation
of Co DND on the surface of RGO sheets.46 In addition, it has
been noticed in Fig.S2 that there is a blue shift in the O 1s peak
of RGO/Co DND nanocomposite. This is due to the covalent
interaction between the Co and the sp2 carbon of the RGO
through the oxygen atom (Co-O-C).47 The deconvoluted C1s
spectra of GO and RGO/Co DND nanocomposite are shown in
the Fig. 4 (c, d). The C 1s deconvoluted XPS spectra of GO
contains oxygenic functional groups (-COOꢀat 288.4 eV, -C=Oꢀat
286.9ꢀeV, and -C-Oꢀat 286.6ꢀeV) at 286-289 eV, which confirms
that graphite is undergoes successful oxidation to GO through
the oxidation process. Whereas there is an effective reduction
in the peak intensities of the oxygenic functional groups in the
C 1s spectra of RGO/Co DND nanocomposite. This confirms
that most of the oxygenic functional groups are successfully
removed from RGO/Co DND nanocomposite and GO is
reduced to RGO in RGO/Co DND nanocomposite through the
reduction process.
SEM images of the Co DND and RGO/Co DND nanocomposite
are shown in the Fig. 5 (a, b).The images show that the as-
prepared Co DND and RGO/Co DND nanocomposite exhibit
dendritic morphologies and most of the leaf like flakes exhibit
radiating arrangement from the centre. A good growth
environment for Co dendrites is confirmed by the uniformity of
crystalline Co structure. The average length of leaves is about
3.8 and 1.7 μm for Co DND and RGO/Co DND nanocomposite,
respectively. In addition, we have noticed that Co DNDs are
distributed throughout the RGO sheets in the RGO/Co DND
nanocomposite. Typical TEM image in Fig. 5 (c) demonstrates
dendritic cobalt microstructures. Existence of a long central
backbone with very sharp secondary branches has been
observed where the secondary branches are grown on both
side of the backbone. Again, the secondary branches are
parallel to each other and arranged with an even spacing. Fig.
5 (d) shows that the dendritic cobalt structures are spreads
over the RGO sheets. It was observed that Co DNDs are
remained attached on the RGO sheets and there were no Co
DNDs outside of the RGO sheets, suggesting that they were
strongly anchored on the surface of RGO sheets. Selected area
electron diffraction (SAED) (inset of Fig. 5 (c, d)) of cobalt
dendritic structure shows hcp structure in both the cases and
matched with the XRD results. Furthermore, the SAED patterns
demonstrate similar pattern in different areas of dendritic
systems. The lattice spacing of 0.22 nm and 0.26 nm are
corresponding to the (100) plane of Co in the Co DND and
RGO/Co DND nanocomposite. This shows 100 lattices were
probable along the [001] direction and the dendrides are
growing parallel to the (001) plane. The higher lattice spacing
of the Co DND in the RGO/Co nanocomposite as compared to
the bare Co DND is due to the smaller leaf size of the dendrite,
Fig.5 SEM images of (a) Co DND and (b) RGO/Co DND nanocomposite; TEM images of
(c) Co DND and (d) RGO/Co DND nanocomposite; Latice fringes of (100) plane in the (e)
Co DND and (f) RGO/Co DND nanocomposite.
which is in good agreement with XRD data. The total amounts
of Co on RGO/Co DND nanocomposite is calculated form ICP-
AES result and it is 60%.
Fig.6 Hysteresis loops of (a) Co DND and (b) RGO/Co DND nanocomposite.
The magnetization (M) versus magnetic field (H) curve of Co
DND and RGO/Co DND nanocomposite, at different applied
magnetic fields is presented in Fig. 6; Table S2 represents the
values for saturation magnetization, ramanent magnetization,
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and coercivity. A magnetic hysteresis curve with well-defined seen by the naked eye, as the decolourizations of all dyes
saturation magnetization is observed for both samples, which occurred within 10 min.
reveals ferromagnetic nature of DNDs at room temperature
(RT).
3.1 Catalytic performances of RGO/ Co DND nanocomposite
Catalytic performances of RGO/Co DND nanocomposite
towards the reduction of various dyes (4-NP, MB, MO and RhB)
was evaluated in presence and absence of reducing agent
(NaBH4). The catalytic reduction of various dyes was also
examined in the presence of bare Co DND and compared with
RGO/Co DND nanocomposite. The catalytic reaction was
process was monitored using UV-Visible spectroscopy.
Formation of amines from aromatic nitro compound through
reduction process by using NaBH4 is a very significant reaction
for the production of various products in the industry. Catalytic
reduction of aromatic nitro compounds has been reported by
many researchers in presence of various forms of materials as
catalysts such as metal, alloys and their composites.48-50 Here,
we have compared the catalytic activity study of Co DND and
RGO/Co DND nanocomposite on the reduction of 4-NP into 4-
amino phenol (4-AP) by NaBH4. The mechanism of catalytic
reduction involves transfer of electrons from donor (BH4−) to
the acceptor (4-NP) on the catalysts surface. The electron
transfer rate depends on the adsorption rate of 4-NP,
interfacial electron transfer and desorption rate of 4-AP on the
catalysts surface. The reaction progress is examined with time
by UV-Vis spectroscopy as both 4-NP and 4-AP absorb in the
UV-Vis region. It has been noticed that the peak intensity at
400 nm corresponding to 4-NP absorption decreases gradually
with time, while a new peak is generated at 300 nm as the
reduction reaction proceeds.51,52 The new peak at 300 nm
corresponds to 4-AP formation.
The slow fading of colour of 4-NP solution indicates that the
reduction reaction progress, which can be observed in a naked
eye. UV-Vis spectra of 4-NP reaction solution were recorded at
Fig.7 UV-Vis absorption spectra of the reduction of (a,b) 4-NP, (c,d) MB, (e,f) MO and
(g, h) RhB by NaBH4 in presence of Co DND and RGO/Co DND nanocomposite.
an interval of 2 minutes in presence of Co DND and RGO/Co
DND nanocomposite as catalysts (Fig. 7 (a,b)). We have
The kinetics of the above mentioned reduction reaction was
noticed that after two days of experiment there was no
expected to follow a pseudo-first-order rate of reaction for
occurrence of reduction reaction in presence of bare RGO or
each dye in the presence of sufficient amount of NaBH4.53, 54
absent of catalysts. But, as the reaction proceeds in presence
So, the rate equation for this reduction reaction can be
of catalysts, there is a gradual decrease of absorption intensity
represented as below:
at 400 nm with increase in absorption intensity at 300 nm. The
absorption at 300 nm corresponds to the generation of 4-AP. It
kt = ln C0 – ln C = ln A0 – ln A
has been observed in Fig. 7 (a, b) that the absorption intensity
at 400 nm decreases more rapidly in case of RGO/Co DND
Here, k is the rate constant. C0 and C are the concentration of
nanocomposite as compared to Co DND. The conversion (%) of
each dye at time t=0 and t = t where as A0 and A are the
4-NP to 4-AP in 10 min is 68 % and 97 % for Co DND and
absorbance of each dye (λmax (4-NP) = 400 nm, λmax (MB) = 663 nm,
RGO/Co DND nanocomposite respectively.
λmax (MO) = 465 nm and λmax (RhB) = 554 nm) at time t=0 and t=0,
The catalytic performance of RGO/Co DND nanocomposite and
respectively. The ratio of Ct to C0 (Ct/C0) was calculated from
Co DND was also examined for the degradation of different
the ratio of the absorbance’s (At/Ao) at 400, 663, 465 and 554
type of dyes (e.g., MB, MO and RhB with the help of excess
nm for 4-NP, MB, MO and RhB, respectively. Fig. 8 shows the
NaBH4. Fig.7 (c-h) shows the catalysis reactions of Co DND and
relation of ln (Ct/C0) versus time (t) in the presence of Co DND
RGO/Co DND nanocomposite for various dyes with time. There
and RGO/Co DND nanocomposite for 4-NP, MB, MO and RhB.
is a gradual decrease in absorbance at λmax (λmax(MB) = 663 nm,
It is obvious from Fig. 8 that ln (Ct/C0) shows a good linear
λmax(MO) = 465 nm and λmax(RhB) = 554 nm ) with time for all three
correlation with the reaction time for Co DND and RGO/Co
dyes. The evolution of the degradation of dye can be clearly
DND nanocomposite confirming pseudo-first-order kinetics.
6 | J. Name., 2012, 00, 1-3
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Fig.9 Mechanism of the catalytic reduction of various dyes in presence of RGO/Co DND
nanocomposite as catalyst.
composite as compared to the bare Co DND (BETCo DND = 9 m2/g
and BETRGO/Co DND = 52 m2/g in Fig. S4) (2) the adsorption of
various dyes on the RGO surface through π−π stacking
interactions which offers more number of dye molecules in the
vicinity of Co DND, leading to an enrichment in the area of
strong contact; (2) the enhancement of local electron density
through electron diffusion process from RGO to Co DND, which
causes increasing the electron-acceptance process by various
Fig.8 The kinetic plots (ln Ct/C0 vs time) for the reduction of various dyes catalyzed by
Co DND and RGO/Co DND nanocomposite.
The rate constant values for Co DND and RGO/Co DND
nanocomposite were calculated from the pseudo-first-order
reaction kinetics (from the slopes of the straight lines of ln
(Ct/C0) vs time plot) and are presented in the Table 1.
Table 1The rate of reduction of various dyes under different
catalysts, the correlation coefficients for ln (Ct/C0) - t plots and
the completion time.
dyes; and (3) providing
a surrounding that prevents
aggregation of Co DND, which causes loss of activity as seen
in the case of bare Co DND.57,58 The rate of reduction of dye
(MB) in presence of RGO and Co DND physical mixture
nanocomposite was studied in presence of NaBH4 (Fig. S5). We
have noticed that RGO/Co DND nanocomposite is showing
better catalytic activity as compared to the RGO and Co DND
physical mixture. This is due to the strong synergistic
interaction between the two constituents RGO and Co DND in
the RGO/Co DND nanocomposite, which is not only enhances
the catalytic activity through the existence of strong chemical
interaction between Co DND with RGO but also it improves the
chemical stability of the Co DND against poisoning. The rate
constants of the reduction reactions by using RGO/Co DND
nanocomposite catalyst are also compared with rate constants
of some other reported precious metal nanoparticle based
catalysts for four different dyes (Table S1 represent the
comparison of the reported rate constant values of RGO/Co
nanocomposite and precious metal nanoparticle based
catalysts used for different dyes). It is found that rate
constants of RGO/Co DND nanocomposite in some of the dyes
are higher than that of the noble metal nanoparticle based
catalysts. We have also compared the other catalytic systems
with the RGO/Co DND nanocomposite in terms of reduction
conditions, times and yields for the reduction of MB and 4-NP
(Table S3). Degradation of the mixture of dyes is also studied
by using RGO/Co DND nanocomposite as catalyst. RGO/Co
DND nanocomposite reveals an enhancement in the catalytic
efficiency for the reduction of mixture of dyes. Fig. 10 (a)
shows the degradation of mixture of dyes (4-NP, MB, MO and
RhB) within 15 min in the presence of RGO/Co DND
Catalysts
Co DND
k (min-1)
R2
Time (min)
0.1180 (4-NP)
0.1010 (MB)
0.0910 (MO)
0.1080 (RhB)
0.3310 (4-NP)
0.4627 (MB)
0.2912 (MO)
0.3367 (RhB)
0.99
0.99
0.99
0.99
0.98
0.99
0.97
0.98
10
10
10
10
10
10
10
10
RGO/Co DND
nanocomposite
The catalytic reduction reaction mechanism of various organic
dyes in presence different catalysts was also proposed by the
previous reports.55, 56 The mechanism of the catalytic reduction
of various dyes with RGO/Co DND nanocomposite is shown in
Fig. 9. The catalytic reduction process involves the electron
transfer from the donor to the acceptor through the catalyst
surface. So, the higher surface area and the higher electron
transfer property of the catalyst will greatly enhance the
efficiency of the catalyst. Again, RGO/Co DND nanocomposite
shows higher rate constant values than Co DND in case of all
four dyes. This signifies that the enwrapping bare Co DND with
RGO sheets may enhances the catalytic activity. The details
reaction paths for the catalytic reduction of 4-NP in presence
of RGO/Co DND nanocomposite as catalyst is discussed in the
Fig.S3. Similar reaction paths can be applied for the reduction
of other dyes also.
This type of improvement in the catalytic activity by
compositing with RGO can be attributed to (1) the higher
surface area of the RGO, which enhances the surface are of the
nanocomposite ([RGO/Co DND nanocomposite] = 3 × 10−1
g
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L−1, [4-NP] = 40 ppm, [MB] = 40 ppm, [MO] = 40 ppm and [RhB] conditions or any elaborated setup. The XRD, FTIR, SEM and
= 40 ppm and [NaBH4] = 0.15 M]). The representative sample TEM results confirmed the formation of the dendritic structure
of RGO/Co DND nanocomposite was also examined for and good adhesion of Co DND structure on the reduced
reusability for the degradation of MB dye by NaBH4 in Fig. 10 graphene sheets. Magnetic studies reveal that both the Co
(b). The used catalyst is magnetically separated and was DND and RGO/Co DND nanocomposite display room-
recycled for an additional four times to test the degradation temperature ferromagnetic behaviour. In addition, the
process. After the completion of five cycles, there is a minor RGO/Co DND nanocomposite shows
a superior catalytic
decrease in the degradation percentage value with the activity towards degradation of individual dyes as well as
increasing cycle. This suggests that RGO/Co DND mixture of dyes. The values of rate constant (k) of the
nanocomposite has excellent stability against poisoning by the reduction reactions for RGO/Co DND nanocomposite are
reaction products.
superior to bare Co DND structure in case of all the four dyes.
In order to confirm the stability of the catalysts and the The magnetic property of the RGO/Co dendritic
reaction product formed after catalysis. We have carried out a nanocomposite makes it a promising material for practical
deep characterization (XRD, XPS, SEM and TEM) of the catalyst applications like recyclable catalysts. Moreover, simple
(RGO/Co DND nanocomposite) to see structural and method of preparation, high values of reaction rate constants
morphological change in the catalyst after 5th cycles of and good recyclability suggests that this material will find
degradation of MB in presence of NaBH4 (Fig. S6). It has been promising applications in the field of advanced catalysis
noticed that after 5th cycles, there is no obvious change in the particularly for waste water treatment.
structure (XRD) and morphology (SEM and TEM) of the Co DND
in the RGO/Co DND nanocomposite as compared to the
Acknowledgements
RGO/Co DND nanocomposite before reduction. But, we have
noticed a small amount (1.5 at. %) of Co2B in the XPS study of
Financial support from the Nano-Mission of Department of
the catalyst (RGO/Co DND nanocomposite) after 5th cycles.
Science and Technology (DST), India and IITB-Monash Research
This is due to the interaction of partial oxidized Co DND in
Academy are greatly acknowledged.
RGO/Co DND nanocomposite and NaBH4.59,60 In addition, we
have also investigated the products formed after MB
degradation in presence of RGO/Co DND nanocomposite and
References
NaBH4 through UV-Visible spectroscopy (Fig. S7). MB shows
absorbance maxima at 665 nm in an aqueous solution. The
absorption peak at 665 nm, which is attributed to MB,
vanished after 10 min of reduction of MB and a new peak is
generated at 258 nm.61 The peak around 258 nm is the
characteristic absorption peak of leucomethylene blue (LMB),
which is formed in this reduction process. However, these
results show that RGO/Co DND nanocomposite is a stable and
potential active catalyst for the reduction of different organic
dyes individually as well as mixture in the water treatment
application.
1
Y.J.Horng and D.S. Hu, Environ. Sci. Technol., 1993,27, 1169-
1174.
G.W.Kuo, Water Res., 1992,26, 881-886.
D.K.Sujoy, B. Jayati, D.R. Akhil and G.K.Arun, Langmuir, 2006,
22, 7265-7272.
O.Legrini, E. Oliveros andM.A. Braun, Chem. Rev.,1993, 93,
671-698.
K.Mohanty, T.J. Naidu, B. C. Meikap and M.N.Biswas, Ind.
Eng. Chem. Res.,2006, 45,5165-5171.
2
3
4
5
6
7
8
9
B. Manu and S. Chaudhari, Bioresour. Technol., 2002, 82
225-231.
,
R. Patel and S. Suresh, J. Hazard. Mater. B,2006, 137, 1729-
1741.
L.G. Devi, S.G. Kumar, K. M. Reddy and C. Munikrishnappa, J.
Hazard. Mater.,2009, 164,459-467.
R.S.Blackburn, Environ.Sci. Technol., 2004,38, 4905-4909.
10 N. Riaz, F.K. Chong, B.K. Dutta, Z.B. Man, M. S. Khan and E.
Nurlaela, Chem. Eng. J., 2012, 185–186, 108-119.
11 A.R. Khataee, M.N. Pons and O. Zahraa, J. Hazard. Mater.,
2009, 168, 451-457.
12 B. Naik, C.H. Manoratne, A. Chandrashekhar, A. Iyer, V. S.
Prasad and N.N. Ghosh, J. Exp. Nanosci.,2013, 8,462-479.
13 G.K. Naik, P. M. Mishra and K. Parida, Chem. Eng. J.,2013,
229, 492-497.
Fig.10 (a) Time dependent UV–Vis spectral changes of mixture of 4-NP,MO, MB and
RhBdyes in presence of NaBH4 catalyzed by RGO/Co DND nanocomposite.(b) The
catalytic performance of MB within five cycles for RGO/Co DND nanocomposite.
14 Y. C. Chang and D. H. Chen, J. Hazard. Mater., 2009, 165
664-669.
,
15 D.Nandi, S. Siwal, M. Choudhary and K. Mallick, Appl. Catal.
A: Gen., 2016, 523, 31-38.
16 N. Pradhan, A. Pal and T. Pal, Langmuir, 2001, 17,1800-1802.
17 S. Saha, A. Pal, S. Kundu, S. Basu and T. Pal, Langmuir,2010,
26, 2885-2893.
18 N. Gupta, H. M. Singh and R.K. Sharma,J. Mol. Catal. A
Chem.,2011, 335,248-252.
4. Conclusions
In summary, a one-step solution route was developed for
synthesis of RGO/Co DND nanocomposite. The synthesis
procedure is very simple and does not require any rigorous
8 | J. Name., 2012, 00, 1-3
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Please do not adjust margins

Please do not adjust margins
RSC Advances
Page 9 of 10
Journal Name
View Article Online
DOI: 10.1039/C6RA23621A
ARTICLE
19 X. Zhang, M. Lin, X. Lin, C. Zhang, H. Wei, H. Zhang and B. 41 T.Szabó, O. Berkesi and I.Dékány, Carbon, 2005, 43, 3186-
Yang, ACS Appl. Mater. Interfaces,2014,
20 M.R. Kim, D. K. Lee and D. J. Jang, Appl. Catal. B,2011, 103
253-260.
6
, 450-458.
3189.
42 F. Tuinstra and J.L. Koenig, J. Chem. Phys., 1970,53, 1126-
1130.
,
21 H. Wang, J. Shen, Y. Li, Z. Wei, G. Cao, Z. Gai, K. Hong, P. 43 D. Graf, F. Molitor, K. Ensslin, C. Stampfer, A. Jungen, C.
Banerjee and S. Zhou, ACS Appl. Mater. Interfaces,2013,
9446-9453.
22 Q. Zhuo, Y. Ma, J. Gao, P. Zhang, Y. Xia, Y. Tian and X. Sun,
Inorg. Chem., 2013,52, 3141-3147.
5
,
Hierold and L. Wirtz,Nano Lett., 2007,7, 238-242.
44 S. Stankovich, D. A. Dikin, R. D. Piner, K. A. Kohlhaas, A.
Kleinhammes, Y. Jia, Y. Wu, S.T. Nguyen and R.S.
Ruoff,Carbon, 2007, 45, 1558-1565.
23 H. Wu, X. Huang, M. Gao, X. Liao and B. Shi, Green 45 C. Xu, X. Wang and J.W. Zhu, J. Phys. Chem. C, 2008, 112,
Chem.,2011, 13, 651-658. 19841-19845.
24 (a) P. Xiao, Y. X. Zhao, T. Wang, Y. Y. Zhan, H. H. Wang, J. L. Li, 46 M. Govindhan, B. Mao and A. Chen, Nanoscale, 2016,
A. Thomas and J. J. Zhu, Chem. Eur. J., 2014, 20, 2872-2878; 1485-1492.
(b) A. Buonerba, A. Noschese and A. Grassi, Chem. Eur. J., 47 P. Wang, Z. G. Liu, X. Chen, F.L. Meng, J.H. Liu and X. J.
2014, 20, 5478-5486; (c) M. J. Li, Z. L. Wu and S. H. Overbury, Huang, J.Mater.Chem.A, 2013, , 9189-9195.
J. Catal., 2011, 278, 133-142; (d) Y. Mei, Y. Lu, F. Polzer, M. 48 T. Y. Tu, J. Zeng, B. Lim and Y. N. Xia, Adv. Mater., 2010, 22
Ballauff and M. Drechsler, Chem. Mater., 2007, 19, 1062- 5188-5192 .
1069;(e) Y. Lu, Y. Mei, M. Drechsler and M. Ballauff, Angew. 49 M. Schrinner, M. Ballauff, Y. Talmon, Y. Kauffmann, J. Thun,
Chem., Int. Ed., 2006, 45, 813-816; ( f ) M. S. Chen and D. W. M. Moller and J. Breu, Science, 2009, 323, 617-620.
Goodman, Science, 2004, 306, 252-255; (g) E. Lam and J. H. T. 50 B. K. Barman and K. K. Nanda, Dalton Trans., 2015, 44,4215-
Luong, ACS Catal., 2014, , 3393-3410. 4222.
25 (a) S. Nardecchia, D. Carriazo, M. L. Ferrer, M. C. Gutierrez 51 S. Praharaj, S. Nath, S. K. Ghosh, S. Kundu and T. Pal,
8,
1
,
4
and F. del Monte, Chem. Soc. Rev., 2013, 42, 794-830; (b) D.
M. Sun, C. Liu, W. C. Ren andH. M. Cheng, Small, 2013, 9,
1188-1205.
Langmuir, 2004, 20, 9889-9892.
52 J. Zeng, Q. Zhang, J. Y. Chen and Y. N. Xia, Nano Lett., 2010,
10, 30-35.
26 (a) C. C. Huang, C. Li and G. Q. Shi, Energy Environ. Sci., 2012, 53 J. Lee, J. C. Park and H. A. Song, Adv. Mater., 2008, 20, 1523-
, 8848-8868; (b) S. Stankovich, D. A. Dikin, G. H. B. 1528.
Dommett, K. M. Kohlhaas, E. J. Zimney, E. A. Stach, R. D. 54 J. Huang, S. Vongehr, S. Tang, H. Lu and X. Meng, J. Phys.
Piner, S. T. Nguyen and R. S. Ruoff, Nature, 2006, 442, 282- Chem. C, 2010, 114, 15005-15010.
286; (c) X. Z. Zhou, X. Huang, X. Y. Qi, S. X. Wu, C. Xue, F. Y. C. 55 S.K. Ghosh, S. Kundu, M. Mandal and T. Pal, Langmuir, 2002,
Boey, Q. Y. Yan, P. Chen and H. Zhang, J. Phys. Chem. C, 2009, 18, 8756-8760.
113, 10842-10846; (d) Z. Hu, Y. D. Huang, S. F. Sun, W. C. 56 P. Veerakumar, S. M. Chen, R. Madhu, V. Veeramani, C. T.
5
Guan, Y. H. Yao, P. Y. Tang and C. Y. Li, Carbon, 2012, 50, 994-
1004; (e) Z. Hu, J. Li, C. Y. Li, S. J. Zhao, N. Li, Y. F. Wang, F.
Hung and S. B Liu,ACS Appl. Mater. Interfaces, 2015, 7,
24810-24821.
Wei, L. Chen and Y. D. Huang, J. Mater. Chem. B, 2013,
5003-5013.
1,
57 Z, Ji, X, Shen, G. Zhu, H. Zhou and A. Yuan, J. Mater.
Chem.,2012, 22, 3471-3477.
27 (a) C. Z. Zhu and S. J. Dong, Nanoscale, 2013,
(b) G. P. Singh, K. M. Shrestha, A. Nepal, K. J. Klabunde and C.
M. Sorensen, Nanotechnology, 2014, 25, 265701;(c) P. K. 59 B. A. D. Neto, A. S. Lopes, M. Wust, V. E. U. Costa, G. Ebeling
Sahoo, R. Aepuru, H. S. Panda and D. Bahadur, Sci. Rep., and J. Dupont, Tetrahedron Lett., 2005, 46, 6843-6846.
2015, , 17726. (d) E. O. Pentsak, E. G. Gordeev and V. P. 60 P. Krishnan, K. L. Hsueh and S. D. Yim, Appl. Catal., B, 2007,
Ananikov, ACS Catal., 2014, , 3806-3814. 77, 206-214.
28 K.S. Shin, Y.K. Cho, J.Y. Choi and K. Kim,Appl. Catal. A: Gen., 61 P. Veerakumar, S. M. Chen, R. Madhu, V. Veeramani, C.T.
5, 1753-1767; 58 J. Li, C. Y. Liu and Y. Liu, J. Mater. Chem., 2012, 22, 8426-
8430.
5
4
2012, 413– 414, 170-175.
Hung and S. B. Liu, ACS Appl. Mater. Interfaces, 2015,
24810-24821.
7,
29 Y. Wang, H. Li, J. Zhang, X. Yan and Z. Chen,
Phys.Chem.Chem.Phys., 2016, 18, 615-623.
30 P. K. Sahoo, B. Panigrahy and D. Bahadur, RSC Adv., 2014, 4,
48563-48571.
31 S. Li, H. Li, J. Liu, H. Zhang, Y. Yang, Z. Yang, L. Wang and B.
Wang, DaltonTrans., 2015, 44, 9193-9199.
32 J.P. Chen, C.M. Sorensen, K. J. Klabunde and G.C.
Hadjipanayis,Phys. Rev. B, 1995, 51, 11527-11523.
33 G.A. Held, G. Grinstein, H. Doyle, S. Sun and C.B. Murray,
Phys. Rev. B, 2001, 64, 012408.
34 K. Tanabe, Mater. Lett., 2007, 61, 4573-4575.
35 X.B. Yin, N. Fang, X. Zhang, I.B. Martini and B.J. Schwartz,
Appl. Phys. Lett., 2002, 81, 3663-3665.
36 R.A. Soomro, S.T.H. Sherazi, N. Sirajuddin, M. Raza, N.H.K.
Shah, K. R. Hallam and A. Shah, Adv. Mater. Lett., 2014,
191-198.
5,
37 W. S. Hummers and R. E. Offeman, J. Am. Chem. Soc., 1958,
80, 1339-1339.
38 Z. An, J. Zhang and S. Pan, Cryst. Eng. Comm, 2010,12, 500-
506.
39 M.Wu, Z. Pang, X.Liu, G.Li, Y. Ma, Z. Sun, L. Zhang and X.
Chen, J. Alloys Compd., 2012, 513, 245-250.
40 Y.Xu, H.Bai, G. Lu , C. Li and G.Shi, J. Am. Chem. Soc., 2008,
130, 5856-5857.
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