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International Journal of Advanced Chemical Science and Applications (IJACSA)
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ISSN (Print):2347-7601, ISSN (Online): 2347-761X, Volume -3, Issue -2, 2015
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Multiwalled carbon nanotube hybrids derived from functionalization
of amphiphilic poly(amidomine) dendrimers for improved
dispersibility and electrical conductivity
1G. Vimala,
2E. Murugan
1Department of Chemistry Pachaiyappa’s College for Women, Kanchipuram – 631 501. Tamil Nadu, India. 2Department of Physical Chemistry, University of Madras Guindy, Chennai – 600 025, Tamil Nadu, India
Email: [email protected],
[Received :15th Feb.2015; Accepted: 15th April 2015]
Abstract - Multiwalled carbon nanotubes (MWCNTs)-
amphiphilic poly(amidoamine) (PAMAM) dendrimers
(APAMAM) based hybrids were developed through
individual functionalization of PAMAM with generation 4
(G4) and generation 5 APAMAM (G5) on MWCNTs. The
two hybrids were used as matrices and deposited with
silver (Ag) nanoparticles (NPs) to develop another two
types of new MWCNT based hybrids viz., MWCNT-
APAMAM (G4)-AgNPs, and MWCNT-APAMAM (G5)-
AgNPs. The degree of covalent functionalization of
APAMAM (G4)/(G5) in MWCNTs and deposition of AgNPs
in APAMAM (G4)/(G5) were examined by Fourier
transform infrared spectroscopy, Raman spectroscopy,
thermogravimetric analysis, zeta potential, scanning and
high-resolution transmission electron microscopy, and
energy-dispersive spectroscopy. Further, based on the
results of x-ray diffraction and high-resolution
transmission electron microscopy, the size of AgNPs
present in MWCNTs-APAMAM (G4)-AgNPs was
determined to be in the range of 6 nm, and 2 nm for
MWCNTs-APAMAM and (G5)-AgNPs respectively. The
MWCNTs-APAMAM (G5) showed better dispersibility in
aqueous and various polar and nonpolar organic solvents,
and especially the dispersed homogeneous solution formed
from water and DMSO was stable for 9 months. The
MWCNTs-APAMAM (G4)/(G5)-AgNPs showed an
electrical conductivity 13-19 times higher than pristine
MWCNTs.
[Key words: MWCNTs, amphiphilic poly(amidoamine),
AgNPs Raman spectroscopy, thermogravimetric analysis,
zeta potential ]
I. INTRODUCTION
The research on carbon nanotubes (CNTs) is an
attracting topic nowadays due to their unique structures,
excellent mechanical, electrical properties and
considerable potential applications [1-3]. In practice, the
insolubility and weak dispersibility of CNTs in common
solvents have limited their applications especially in the
fields of development of composite materials and
bottom-up hybrid nanomaterials or devices [4,5]. Few
researchers have focused their attention on tube
functionalization and modification in order to improve
the solubility and surface functionality. Especially,
functionalization also introduces the field of CNTs-
based nanochemistry. Much progress has been made in
this area, based on the fundamental work performed by
Smalley [3] and Haddon [4]. To date, two
methodologies, namely noncovalent and covalent, have
been developed to functionalize CNTs with a variety of
organic, inorganic metallic, biochemical and polymeric
structures [6]. Generally, the attachment of small or
large molecules to the CNTs by covalent methods is
more stable and effective. With regard to the covalent
methods, carboxylic acid groups formed at the ends and
defect sites of CNTs are commonly used for precursor
functionalization in the formation of amine, ester and
organometallic structures [7]. Among the various
covalent CNT functionalization, binding polymers to the
CNTs is a very attractive area, because the individual
properties of the two materials can be combined to give
one hybrid material.
Particularly, functionalization of nanotubes with
symmetrical polymer like dendrimers represents a
promising strategy to introduce sufficient amount of
functional groups onto the CNT surfaces for succeeding
in processing with a limited number of sp2 carbon
atoms attacked [8]. Dendrimers are globular, highly
branched macromolecules with a three-dimensional
dendritic architecture. A Poly(amidoamine) (PAMAM)
dendrimer can introduce a dense outer amine shell
through a cascade-type generation [9-11]. Because of
their low melting viscosity, high solubility and
abundance of functional groups dendrimers have
potential applications in a wide range of fields from drug
delivery to material coatings [12-14]. Amphiphilic
dendrimers act as unimolecular micelles that attract non-
polar compounds to their hydrophobic regions and
counter ions to their hydrophilic charged regions [15,
16]. Ford et al [17] reported hydrophobically modified
poly(propyleneimine) (PPI) dendrimer containing
quaternary ammonium and tertiary amine functionality
and the same used for catalysis. Shim et al [18] reported
that poly(styrene) and poly(4-vinylpyridine) brushes can
be functionalized with MWCNTs through solution
International Journal of Advanced Chemical Science and Applications (IJACSA)
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ISSN (Print):2347-7601, ISSN (Online): 2347-761X, Volume -3, Issue -2, 2015
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polymerization. Further, they also observed that the said
polymer fabrication derived from MWCNTs was
dispersed only in hydrophobic solvent such as toluene. It
may be mentioned here that the importance of
solubilization of CNTs in aqueous/organic phase has
gained much attention due to its numerous applications,
specifically in biomedical applications which include
biosensors, antimicrobial, anticancer activity,
nanoprobes, and nanotweezers, etc. Similarly, several
conducting CNT polymer composites, viz., CNT poly
(3-octylthiophene), CNT polyaniline (CNT-PANI), and
MWCNT sulfonated polyaniline (MWCNT-SPAN)
were also reported [19-21]. Recently, it was realized that
the integration of CNTs with metal nanoparticles (NPs)
generates a new family of novel materials with more
advanced properties and applications than the pristine
precursors. As a result, hybrid materials of CNTs with
metal NPs have attracted greater attention due to their
promising applications as electronics, catalysts,
biosensors, imaging and therapeutics [22-24]. Recent
advances have revealed that dendrimers can be
covalently functionalized on the surface of CNTs for
subsequent metal or metal oxide nanoparticle synthesis
and assembly for further electrical and biological
applications. This implies that by combining the
dendrimers surface functionality and unique molecular
recognition ability with the electronic properties of
CNTs, it may be possible to generate various complex
composite nanodevices for a wide range of electrical and
biomedical applications [25-32]. Although few studies
on amphiphilic hybrid nanospheres and macromolecular
nanowires were reported, particularly those based on
covalent linkage of polymer building blocks, have been
rarely studied [33-35]. More particularly, studies
pertinent to amphiphilic dendrimers functionalized CNT
composite with metal nanoparticles are accountable. In
our previous study, we developed new nanohybrids viz.,
MWCNTs functionalized with amphiphilic
poly(propyleneimine) dendrimer carrying silver
nanoparticles (AgNPs) and demonstrated that they have
better dispersibility and antimicrobial activity [36].
Similarly, in our another study, we reported that an
amphiphilic multiwalled carbon nanotube polymer
hybrid with improved conductivity and dispersibility
produced by functionalization with poly (vinyl benzyl)
triethyl ammonium chloride [37]. Further, we developed
novel MWCNTs hybrid catalysts for effective catalysis
of 4-nitrophenol reduction [38]. In this study, we have
developed MWCNTs nanohybrids via effective
functionalization of amphiphilic PAMAM dendrimer
(APAMAM) with generation number 4 (G4) and
generation 5 (G5) and thus produced 2 types of
corresponding hybrids with integration of all 3 smart
nanomaterials viz., MWCNTs-APAMAM (G4) and
MWCNTs-APAMAM (G5). These two hybrids in turn
were used as a individual matrices and deposited with
AgNPs, and thus yielded the another two types of
corresponding hybrids viz., MWCNTs-APAMAM (G4)-
AgNPs, and MWCNTs-APAMAM (G5)-AgNPs. The
resulting MWCNTs hybrids were thoroughly
characterized with spectral, thermal, microscopic,
electrical conductivity, and dispersibility studies so as to
identify the superior amphiphilic MWCNTs-APAMAM
hybrid material with high functionalized yield, good
dispersibility, and improved electrical conductivity
which can be used for fabrication of electronic materials,
sensors, and materials for biomedical applications.
II. MATERIALS AND METHODS
A. Materials
MWCNTs with purity greater than 95% were purchased
from Sigma-Aldrich. Hydrochloric acid (HCl, Merck),
potassium permanganate (KMnO4, Merck), methylene
chloride (CH2Cl2, Merck),
tetrabutylammoniumbromide (TBAB, Alfa aesar), acetic
acid (CH3COOH, 99.8%, Merck), Thionyl chloride
(Merck), N,N’-dicyclohexylcarbodiimide (DCC,
Aldrich), PAMAM (G4 & G5) dendrimers (Symo-Chem,
Netherland), tributylamine (Merck), silver acetate
(Merck), toluene (SRL), tetrahydrofuran (THF, Merck),
chloroform (CHCl3, Merck), dimethylsulphoxide
(DMSO, Merck) were of analar grade of 99% purity and
were used as such for the reactions.
B. Characterization
To ascertain the functionalization of amphiphilic
dendrimers on MWCNTs, the pristine MWCNTs and 4
types of MWCNTs hybrids viz., MWCNTs-APAMAM
(G4), MWCNTs-APAMAM (G5), MWCNTs-APAMAM
(G4)- AgNPs, and MWCNTs-APAMAM (G5)-AgNPs
were characterized with spectroscopy, thermal and
microscopic techniques. Fourier transform infrared
spectra (FTIR) were recorded on a Bruker Tensor-27
FTIR spectrophotometer with OPUS software in the
range 4000 to 400 cm-1. The pellet for analysis was
made by taking equal amount of each MWCNTs hybrids
and KBr (1:1 ratio). Similarly, all the MWCNTs hybrids
were used for the thermogravimetric analysis and
Raman studies. The thermogravimetric analysis (TGA)
was carried out on SDT Q600 V20.5 Build 15
instrument at a heating rate of 10oC/min from 50 to 800
o C under nitrogen atmosphere. Raman spectra were
recorded on a Witec Confocal Raman instrument (CRM
200) with Argon ion laser (514.5 nm). X-ray diffraction
(XRD) patterns were recorded on a X’perts Highscore
plus/Pan Analytical, Philips X-ray diffractometer,
equipped with Cu Kα photon source (40 KeV, 20 mA,
λ= 1.5418 Ao) and scanned at the rate of 10o min-1 over
the range of 10o - 80o (2θ). Scanning electron
microscopy (SEM) and energy dispersive spectroscopy
(EDS) measurements were carried out on a HITACHI S-
3000H scanning electron microscope instrument
interfaced with EDS DX – 4 energy diffraction
spectrometer and both the analyses were performed
using the MWCNTs hybrid through accelerating voltage
of 2 kV. That is, the samples for analyses were prepared
by taking equal amount and were spread on the surface
of double-sided adhesive tape, one side being already
adhered to the surface of a circular copper disc pivoted
by a rod and the spread samples were sputtered with
International Journal of Advanced Chemical Science and Applications (IJACSA)
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gold prior to SEM observation. After the SEM
observation, using the respective MWCNTs hybrid the
elemental analysis was also performed with EDS and the
percentage of elements was observed (semi-
quantitative). High resolution transmission electron
microscopy (HRTEM) analysis was performed on JEOL
3010 transmission electron microscope operating at 200
kV. The MWCNTs hybrid samples to be analyzed was
initially sonicated with acetone for few minutes and then
one drop of each sample (suspension) was placed on a
glow discharged carbon-coated grid and the sample used
for HRTEM observation after evaporating the solvent.
Electrophoretic measurement was carried out with a zeta
potential analyzer (Zetaplus 3+). The electrical
conductivity of MWCNT was measured using a four-
probe resistivity/Hall measurement system (HL5500PC,
Bio-Rad). Sheet samples of 40-60 μm thickness were
prepared by pumping 5 mg of MWCNTs between two
iron plates at a pressure of 150 KN/cm2. The bulk
electrical conductivity of each MWCNTs hybrid was
measured at room temperature using a programmable
curve tracer (Sony Tektronix 370A). Specimens were
polished on both sides into a thickness of 1 mm, and a
very small amount of silver paste (of thickness about
0.05 mm) was applied on the sample surface to reduce
the contact resistance between the samples and
electrodes. To minimize any potential problems
associated with silver paste, the samples were heated at
40oC to remove solvent quickly. Then, the edges of the
samples were ground again to remove silver paste
attached on them. The dispersibility of all the
amphiphilic hybrids were examined with water and
different organic solvents.
C. Synthesis
1) Functionalization of MWCNTs
Pristine MWCNTs (200 mg) and 15 mL CH2Cl2 were
taken in a 100 mL round-bottomed flask and the mixture
was dispersed in ultrasonicator (Cole Parmer) for 10
min. Then, 0.25 g of TBAB in 5 mL H2O, 5 mL acetic
acid and 0.065 g KMnO4 in 5 mL H2O were mixed
together and the resulting solution was added to the flask
[37]. Then the resulting mixture was stirred vigorously
at 25oC for 48 hrs. Then it was diluted with 1000 mL of
deionized water and the resulting product was filtered
under vacuum by 0.2 m Teflon membrane. Then
washing and centrifugation of resulting functionalized
materials were performed continuously until the pH of
the filtrate showed 7 (at least 10 cycles were required).
The resulting filtrate was dried in vacuum and thus
obtained 0.198 g of functionalized MWCNTs
(MWCNTs-COOH) (2).
2) Synthesis of MWCNTs-APAMAM (G4), MWCNTs-
APAMAM (G5), MWCNTs- APAMAM (G4)-AgNPs
and MWCNTs-APAMAM (G5)-AgNPs hybrids 0.08 g
of MWCNTs-COOH (2) was dispersed in 5 ml of DMF
by ultrasonication for 10 min and the dispersed solution
was refluxed with 10 ml of solution ; washed with
acetone, filtered, and dried in vacuum for 24 hrs yielding
the 0.092 g of acid chloride functionalized MWCNTs
product (MWCNTs-COCl) (3). This product in turn was
transferred to two different 100 ml RB flasks and
dispersed separately in 5 ml of DMF through sonication
for 3 min.
Then 100 mg of PAMAM (G4) and PAMAM (G5) was
added dropwise to the respective containers and 0.96 g
of DCC was also added to each container. The
respective mixture solution was refluxed for 48 hrs in
dry nitrogen atmosphere. The solutions were washed
with acetone, filtered, and dried in vacuum at 60oC and
thus yielded black powders viz., 0.188 g of MWCNTs-
PAMAM (G4) (4) and 0.275 g of MWCNTs-PAMAM
G5(5). These two hybrids were converted into
amphiphilic form through quaternization reaction. For
quaternization reaction, 0.1 g of MWCNTs-PAMAM
(G4) and MWCNTs-PAMAM (G5) hybrids were taken
individually in two different 100 ml RB flasks and
stripped in 5 ml of DMF, 10 ml of tributylamine and 10
ml of methyl iodide followed by deaeration under N2
atmosphere and again the reaction mixtures were gently
refluxed for 72 hrs under nitrogen atmosphere at 80oC.
The resulting quaternized product was centrifuged and
dried under vacuum to obtain MWCNTs-APAMAM
(G4) (6) with 0.138g and MWCNTs-APAMAM (G5)
hybrid (7) with 0.245 g (Scheme 1).
Hundred milligrams of MWCNTs-APAMAM (G4) (6)
and MWCNTs-APAMAM (G5) (7) was taken
individually in two different 50 ml RB flask and
dispersed with addition of 10 ml deionized water in each
container. To this 10 ml of silver acetate aqueous
solution (0.01 mol L-1) was added dropwise in each
container. The resulting solution was magnetically
stirred for 24 hrs, washed, filtered and dried and thus
obtained a respective blackish product in the form of
powder, viz. MWCNTs-APAMAM (G4)-AgNPs (8) and
MWCNTs-APAMAM (G5)-AgNPs (9) hybrids
(Scheme 1). In order to remove the loosely adsorbed
AgNPs, the MWCNTs-APAMAM (G4)/(G5)-AgNPs
hybrids were thoroughly washed individually with
deionized water and then centrifuged [39].
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Scheme 1: Synthesis of MWCNTs-APAMAM (G4),
MWCNTs-APAMAM (G5), MWCNTs-APAMAM
(G4)-AgNPs and MWCNTs-APAMAM (G5)-AgNPs
hybrids
III.RESULTS AND DISCUSSION
1) FTIR
The FTIR spectra of MWCNTs-COOH, MWCNTs-
COCl, MWCNTs-PAMAM (G4), MWCNTs-APAMAM
(G4), MWCNTs-PAMAM (G5), MWCNTs-APAMAM
(G5), MWCNTs-APAMAM (G4)-AgNPs, and
MWCNTs-APAMAM (G5)-AgNPs are shown in Fig. 1
a-i respectively. The FTIR spectrum for MWCNTs-
COOH is shown in Fig. 1a, and it shows new broad and
intense peaks at 3400, 1730, and 1262 cm-1 and these
peaks were due to O-H, C=O, and C-O groups and thus
indicate the surface functionalization of COOH onto the
MWCNTs. The FTIR spectrum for MWCNTs-COCl is
shown in Fig. 1b and the characteristic peaks viz.,
C=O(str) and C-Cl(str) were noticed at 1725 cm-1 and
642 cm-1, respectively. In the spectrum of MWCNTs-
PAMAM (G4) and MWCNTs-PAMAM (G5), the
condensation of surface amino group of PAMAM
(G4)/(G5) dendrimers with MWCNTs-COCl showed the
characteristic peaks of N-H(str) and C=O(str) at 3430
and 1642 cm-1, respectively and are shown in Fig. 1c
and 1e, respectively. The formation of amphiphilic
character on the MWCNTs-APAMAM (G4) and
MWCNTs-APAMAM (G5) was evident through the
appearance of intense peak for C-N+ (str) at 1154 cm-1,
the decreased peak intensity of N-H(str) at 3430 cm-1,
and increased of peak intensity for C-H2(str) at 2922
and 2851 cm-1 (Fig. 1 d & f). After deposition of
AgNPs on MWCNTs-APAMAM (G4) and MWCNTs-
APAMAM (G5), the characteristic peaks were shifted
from 3430 to 3445 cm-1 and from 1154 to 1160 cm-1 is
certainly an indication for the formation/deposition of
AgNPs on respective MWCNTs amphiphilic hybrids
(Figure 1 g & h). Similar observations were also
reported by Li et al [40] in which they stated that the
characteristic amine group band was shifted from 3368
cm-1 to 3420 cm-1 and thus established the formation of
AgNPs in the preparation of stable silver colloids. A
analog study was also performed by Cao et al [41] and
they observed that the N-H band observed at 1655 cm-1
for d-MWCNTs was shifted to 1565 cm-1 in d-
MWCNTs/ Ag thus confirming the binding of Ag onto
the –NH2 groups
Fig. 1. FTIR spectra of (a) MWCNTs-COOH, (b)
MWCNTs-COCl, (c) MWCNTs-PAMAM (G4), (d)
MWCNTs-APAMAM (G4), (e) MWCNTs-PAMAM
(G5), (f) MWCNTs-APAMAM (G5), (g) MWCNTs-
APAMAM (G4)-AgNPs, and (h) MWCNTs-APAMAM
(G5)-AgNPs.
2) TGA Analysis
The MWCNTs-APAMAM (G4) and MWCNTs-
APAMAM (G5) hybrids were characterized with TGA
to quantify the amount of APAMAM (G4) and
APAMAM (G5) functionalized onto MWCNTs. The
weight loss curves for pristine MWCNTs, MWCNTs-
COOH, MWCNTs-PAMAM (G4), MWCNTs-
APAMAM (G4), MWCNTs-PAMAM (G5) and
MWCNTs-APAMAM (G5) hybrids are shown in Fig. 2.
For pristine MWCNTs (Fig. 2a), no weight loss was
noticed up to 800oC. For MWCNTs-COOH (Fig. 2b),
10% weight loss was observed due to the decomposition
of carboxyl groups from 300 to 500oC [42-44]. In the
case of MWCNTs-PAMAM (G4) (Fig. 2c) and
MWCNTs-PAMAM (G5) (Fig. 2e), 32 and 47 % weight
loss was observed due to the decomposition of
dendrimers at 200 to 400oC. Chan Park et al [39]
reported that the functionalization of MWCNTs with
PAMAM dendrimers with –NH2 surface groups having
generation number 2 and 3 were found to be grafted
with 29% and 45% respectively. Hence,
functionalization of dendrimers increased linearly with
the increase of generation number from 4 to 5 [28]. The
amphiphilic hybrid viz., MWCNTs-APAMAM (G4) and
MWCNTs-APAMAM (G5), showed weight loss of
66.5% (Fig. 2d) and 68.5% (Fig. 2f) at 330-400oC and
this is due to the decomposition of amphiphilic
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quaternary ammonium groups. Analogous study
reported that the chloromethyl and quaternary groups
were decomposed in the range of 300-400oC [45].
Fig. 2. TGA curves of (a) Pristine MWCNTs, (b)
MWCNTs-COOH, (c) MWCNTs-PAMAM (G4), (d)
MWCNTs-APAMAM (G4), (e) MWCNTs-PAMAM
(G5) and (f) MWCNTs-APAMAM (G5).
3) Raman Spectral Study
A Raman spectrum can provide qualitative and
quantitative information about the structural change of
electronic properties of MWCNTs due to
functionalization of PAMAM (G4), PAMAM (G5),
APAMAM (G4) and APAMAM (G5). Hence, all
MWCNTs hybrids were analyzed by Raman
spectroscopy along with pristine MWCNTs. The
observed ID/IG values for all MWCNTs hybrids along
with the control are presented in Table 1. The spectra of
MWCNTs-APAMAM (G4), AMWCNTs-PAMAM (G5),
MWCNTs-APAMAM (G4)-AgNPs and MWCNTs-
APAMAM (G5)-AgNPs are shown in Fig. 3 a-d,
respectively. Generally, in Raman spectra the peaks
observed at 1331 and 1559 cm-1 correspond to a
defective carbon band due to disordered sp3 hybridized
carbons in the nanotubes walls D-band and a graphite
carbon band from the sp2-hybridized G-band of
MWCNTs, respectively. The area ratio of D-band to G-
band of MWCNTs is a direct indication for the degree of
modification of MWCNTs. The calculated ID/IG ratio
for MWCNTs-APAMAM (G4) (Fig. 3a) and MWCNTs-
APAMAM (G5) (Fig. 3b) was 1.4 and 1.6, respectively.
This observation confirms the covalent functionalization
of APAMAM (G4) and APAMAM (G5) onto the
MWCNTs. The quantum of covalent functionalization
of dendrimers in MWCNTs is directly related to the
damage of CNTs structure and thereby drastic
disturbance in electronic properties, and this can be
ascertained through increased values of ID/IG in the
Raman spectrum [28]. In fact, the nature of covalent or
non-covalent functionalization on MWCNTs is normally
identified based on the ID/IG value. In our case, the
enhancement of the ID/IG ratio from 0.3 (pristine
MWCNTs) to 1.3 and 1.5 for MWCNTs-PAMAM (G4)
and MWCNTs-PAMAM (G5) hybrid sufficiently
confirmed the higher degree of covalent
functionalization of PAMAM (G4)/(G5) and similarly
further enhancement to 1.4 and 1.6 supported the
covalent functionalization of APAMAM (G4)/(G5).
Similarly, the ID/IG ratio of MWCNTs-APAMAM
(G4)-AgNPs (Fig. 3c) and MWCNTs-APAMAM (G5)-
AgNPs (Fig. 3d) hybrids were 1.42 and 1.67
respectively. However, deposition of AgNPs in their
respective MWCNT hybrids has mildly increased the
values or defects.
TABLE 1. ID/IG RESULTS OF MWCNTs HYBRIDS.
Types of MWCNTs hybrids ID/IGa
Pristine MWCNTs 0.30
MWCNTs-COOH 0.60
MWCNTs-PAMAM (G4) 1.30
MWCNTs-PAMAM (G5) 1.50
MWCNTs-APAMAM (G4) 1.40
MWCNTs-APAMAM (G5) 1.60
MWCNTs-APAMAM (G4)-AgNPs 1.42
MWCNTs-APAMAM (G5)-AgNPs 1.67
Fig. 3. Raman spectra of (a) MWCNTs-APAMAM (G4),
(b) MWCNTs-APAMAM (G5), (c) MWCNTs-
APAMAM (G4)-AgNPs and (d) MWCNTs-APAMAM
(G5)-AgNPs.
4) SEM, EDS, HRTEM and XRD
The surface morphologies of pristine MWCNTs,
MWCNTs-APAMAM (G4), MWCNTs-APAMAM (G5),
MWCNTs-APAMAM (G4)-AgNPs, and MWCNTs-
APAMAM (G5)-AgNPs were studied with SEM, EDS
and HRTEM techniques. The SEM image Fig. 4a
suggested that the pristine MWCNTs entangled together
with a distribution such as fine threads/ropes, whereas
the image of MWCNTs-APAMAM (G4), MWCNTs-
APAMAM (G5) (Fig. 4b, c) showed clear, intense, white
patches distributed homogeneously, and the existence of
MWCNTs was not visible. It confirmed that the
MWCNTs were homogeneously mixed with a complete
coverage of polymers onto the surface. It also strongly
supported the functionalization of APAMAM (G4) and
APAMAM (G5) onto the MWCNTs. The images
obtained from MWCNTs-APAMAM (G4)-AgNPs and
MWCNTs-APAMAM (G5)-AgNPs hybrids are shown
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in Fig. 4 d and e respectively. In Figure 4d and e
distribution of white dots onto the MWCNTs-
APAMAM G4/G5 is seen and these dots are certainly
due to the deposition of AgNPs.
Fig. 4. SEM images of (a) pristine MWCNTs, (b)
MWCNTs-APAMAM (G4), (c) MWCNTs-APAMAM
(G5), (d) MWCNTs-APAMAM (G4)-AgNPs, and (e)
MWCNTs-APAMAM (G5)-AgNPs.
The EDS analysis is one of the most effective surface
characterization techniques for identifying and
quantifying (semi-quantitative) the surface elements.
The percentage of elements in pristine MWCNTs,
MWCNTs-APAMAM (G4), MWCNTs-APAMAM (G5),
MWCNTs-APAMAM (G4)-AgNPs, and MWCNTs-
APAMAM (G5)-AgNPs was determined with EDS and
the observed spectrum along with the percentage of
elements are shown in Fig. 5 a-e respectively. The
results suggested that the percentage of carbon gradually
decreased from pristine MWCNT to MWCNT hybrids
with the induction of sizable percentage of newer
elements in each step of the functionalization. The
carbon (C), nitrogen (N), oxygen (O), iodide (I), and
silver (Ag) peaks appeared in all the spectrum with
varied intensity and the results are given in Table 2.
From the results (weight percentage), it is suggested that
the percentage of carbon noticed in pristine MWCNTs
to MWCNTs-APAMAM (G5) are gradually decreased
from 99% to 80%. Whereas, the percentage of nitrogen
increased from 9.3 % to 11.3%, and the same trend was
also observed for oxygen, iodide, silver and palladium.
Since the percentage of elements was determined by
taking equal amount of each MWCNTs hybrid, it is
logical to compare the quantum of elements (semi-
quantitative) in the analysis.
The generation of amphiphilic character was achieved
through quaternization in MWCNT-PAMAM (G4),
MWCNTs-PAMAM (G5) and it is confirmed from the
appearance of nitrogen and iodide in MWCNT-
APAMAM (G4) and MWCNT- APAMAM (G5), and
deposition of AgNPs in MWCNT- APAMAM (G4)-
AgNPs and MWCNT- APAMAM (G5)-AgNPs. In a
nutshell, the decreasing trend of carbon from MWCNTs
to MWCNTs hybrids and appearance of relevant
elements like nitrogen, iodide, and silver supported the
amount of functionalization and deposition of
APAMAM (G4), APAMAM (G5) and AgNPs onto the
corresponding MWCNTs.
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Fig. 5. EDS spectra of (a) pristine MWCNTs, (b)
MWCNTs-APAMAM (G4), (c) MWCNTs-APAMAM
(G5), (d) MWCNTs-APAMAM (G4)-AgNPs, and (e)
MWCNTs-APAMAM (G5)-AgNPs.
TABLE 2. EDS RESULTS OF MWCNTs HYBRIDS
Types of
MWCNTs
hybrids
EDS (wt %)
C N O I Ag
a 99 - -
b 85 8.5 2.7 3.00 -
c 80 11.3 3.5 4.20 -
d 78 8.5 2.7 3.00 6.00
e 75.1 11.3 3.5 3.0 7.10
The functionalization of APAMAM (G4), and
APAMAM (G5) onto the MWCNTs and deposition of
AgNPs on MWCNTs-APAMAM (G4)/(G5) was also
visualized through the HRTEM images. The image of
the pristine MWCNTs (Fig. 6a) showed a smooth
surface. In contrast, the images of MWCNTs-
APAMAM (G4) (Fig. 6b) and MWCNTs-APAMAM
(G5) (Fig.6c) suggested well distributed/dispersed
MWCNTs with heterogeneous coverage of layers on the
surface, thus proving the functionalization of APAMAM
(G4)/(G5) onto the MWCNTs. In other words, the degree
of debundled CNTs was more in MWCNTs-APAMAM
(G5) due to functionalization of higher generation
APAMAM (G5). The distribution of dense black dots
seen in the Fig. 6d &e is a strong evidence for
deposition of AgNPs onto the surface of MWCNTs-
APAMAM (G4)/(G5). Similarly, from transmission
electron microscopy images the size of the AgNPs
available in MWCNTs-APAMAM (G4)-AgNPs and
MWCNTs-APAMAM (G5)-AgNPs hybrids were
determined as 6 nm 2 nm respectively.
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Fig. 6. HRTEM images of (a) pristine MWCNTs, (b)
MWCNTs-APAMAM (G4), (c) MWCNTs-APAMAM
(G5), (d) MWCNTs-APAMAM (G4)-AgNPs, and (e)
MWCNTs- APAMAM (G5)-AgNPs.
The X-ray diffraction patterns of MWCNTs are
presented in Fig. 7a. The 2θ peaks noted at 26.02o and
42.00o are pertinent to (0 0 2) and (1 1 0) planes of
MWCNTs. Fig. 7b and c are the X-ray diffraction
patterns of the MWCNTs-APAMAM (G4)-AgNPs and
MWCNTs-APAMAM (G5)-AgNPs. According to the
JCPDS database No. 04-0783 the diffraction peaks at 2θ
26.81O, 38.51o , 44.67
o, and 64.8
o are indexed to (0 0 2),
(1 1 1), (2 0 0), and (2 2 0) planes which are the
reflections of AgNPs with face-centered cubic
symmetry. Further, the size of the AgNPs was
determined from the (1 1 1) peak using the Scherrer
equation [46] and found as 6 and 2 nm.
Fig. 7. XRD patterns of (a) pristine MWCNTs, (b)
MWCNTs-APAMAM (G4)-AgNPs, and (c) MWCNTs-
APAMAM (G5)-AgNPs.
5) Zeta potential measurements
The surface modification of MWCNTs was once again
established through zeta potential measurements. That
is, zeta potentials of the MWCNTs, MWCNTs-COOH,
MWCNTs-PAMAM (G4), MWCNTs-PAMAM (G5),
MWCNTs-APAMAM (G4), MWCNTs-APAMAM (G5),
MWCNTs-APAMAM (G4)-AgNPs, and MWCNTs-
APAMAM (G5)-AgNPs at pH 7 were measured
individually with zeta plus instrument in 1 mM sodium
chloride (NaCl) solution and the values are given in
Table 3. The surface potential for MWCNTs is
measured as 20 mV. The surface potential of MWCNTs-
COOH is -9.5 mV and this value became positive in the
case of MWCNTs-PAMAM G4 (28.2 mV) for and
MWCNTs-PAMAM (G5) (36.3 mV). Further, the
hybrids converting into amphiphilic form viz.,
MWCNTs-APAMAM (G4) and MWCNTs-APAMAM
(G5) have lead to increase more positive charge to the
tune of 50 mV and 60.5 mV respectively and this is due
to presence of poly(quaternary) ammonium ions and
also the electrostatic attraction between the APAMAM
(G4)/(G5) and MWCNTs. Furthermore, by the deposition
of AgNPs onto the surface of MWCNTs-APAMAM
(G4)/(G5), the zeta potential has shifted from 50 mV to -
33.6 mV and 60.5 mV to -46.7 mV respectively. The
change of zeta potential at every step strongly supports
the surface modification of MWCNTs by APAMAM
(G4)/(G5) and AgNPs.
TABLE 3. ZETA POTENTIAL RESULTS OF
MWCNTs HYBRIDS.
Types of MWCNTs hybrids Zeta potential
(mV)
Pristine MWCNTs 20.0
MWCNTs-COOH -9.5
MWCNTs-PAMAM (G4)
28.2
MWCNTs-PAMAM (G5) 36.3
MWCNTs-APAMAM (G4) 50.0
MWCNTs-APAMAM (G5)
60.5
MWCNTs-APAMAM (G4)-AgNPs -33.6
MWCNTs-APAMAM (G5)-AgNPs -46.7
6) Electrical Conductivity Measurements
To confirm the observations established in the zeta
potential study, all MWCNTs hybrids were again
studied for the determination of electrical conductivity
measurements using the four-probe method at room
temperature. The observed values of electrical
conductivity of pristine MWCNTs, MWCNTs-PAMAM
(G4), MWCNTs-PAMAM (G5), MWCNTs-APAMAM
(G4), MWCNTs-APAMAM (G5), MWCNTs-APAMAM
(G4)-AgNPs, and MWCNTs-APAMAM (G5)-AgNPs
are presented in Table 1. Electrical conductivity
measurements could provide information about the
geometric configuration of the MWCNTs that cannot be
extracted by other measurements such as thermal
conductivity or optical spectrum. The recent studies
showed that the MWCNTs were dispersed into polymers
to increase the electrical conductivity of the composites.
Liu et al [47] and Linsunova et al [48] studied the
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electrical conductivity of MWCNTs in aqueous solution
fluids.
From Table 4, it is seen that conductivities of the
pristine MWCNTs was determined as 5.2 S/cm. The
conductivity of the MWCNTs-PAMAM (G4) and
MWCNTs-PAMAM (G5) was 24.78 and 37.64 S/cm,
respectively. The conductivity of the MWCNTs-
APAMAM (G4) and MWCNTs-APAMAM (G5) reaches
a value of 48.98 and 86.12 S/cm respectively, which is 9
and 17 times higher than the pristine MWCNTs.
Because, MWCNTs is an excellent electron acceptor
[49], while APAMAM (G4)/(G5) can be considered as an
good electron donor-acceptor. Therefore, it is inferred
that the enhanced doping effect is associated with
MWCNTs (or) effective charge transfer from
APAMAM (G4)/(G5) to the MWCNTs through induced
chemical bonding.
This kind of interaction, homogeneity with higher
generation dendrimers and compatibility enabled
electron delocalization and enhanced the conductivity of
the amphiphilic hybrid. It was expected that the AgNPs
decoration would have beneficial effect on the electrical
conductivity of MWCNTs because the inherent
electrical conductivity of Ag is 6.3 S/cm is much higher
than that of the pristine MWCNTs. The electrical
conductivity of MWCNTs-APAMAM (G4)/(G5) was
increased from 5.2 to 48.98 S/cm and 86.12 S/cm
respectively, this enhancement was attributed to the
charge transfer between MWCNTs and APAMAM
(G4)/(G5) that were covalently bonded onto the
MWCNTs. The conductivity of MWCNTs increased
significantly to 69.32 and 98.29 S/cm after silver
deposition which is 13 and 19 times higher than the
pristine MWCNTs showing the effectiveness of Ag
favors more electronic transport thus enhanced the
electrical conductivity of the MWCNTs-APAMAM
(G4)/(G5)-AgNPs hybrid.
TABLE 4. ELECTRICAL CONDUCTIVITY
RESULTS OF MWCNTs HYBRIDS.
Types of MWCNTs hybrids
Electrical
Conductivity
(S/cm)
Pristine MWCNTs 5.15
MWCNTs-COOH 5.48
MWCNTs-PAMAM (G4) 24.78
MWCNTs-PAMAM (G5) 37.64
MWCNTs-APAMAM (G4) 48.98
MWCNTs-APAMAM (G5) 86.12
MWCNTs-APAMAM (G4)-AgNPs 69.32
MWCNTs-APAMAM (G5)-AgNPs 98.29
7) Dispersibility of MWCNT-APAMAM (G4) and
MWCNT-APAMAM (G5)
Amphiphilic MWCNT hybrids, viz., MWCNTs-
APAMAM (G4) and MWCNTs-APAMAM (G5) were
prepared and subsequently they were dispersed in water
to study the degree of dispersibility without sonication
under identical experimental conditions at ambient
temperature. To ascertain the degree of dispersibility
and stability, the respective MWCNTs-APAMAM (G4)
and MWCNTs-APAMAM (G5) solutions were
periodically monitored under undisturbed condition upto
8 to 9 months. The dispersibilities of pristine MWCNTs,
MWCNTs-APAMAM (G4) and MWCNTs-APAMAM
(G5) hybrids were studied not only in aqueous phase but
also in various organic solvents, viz., toluene, THF,
chloroform, and dimethyl sulfoxide (DMSO), and the
corresponding photographs are shown in Fig. 8 (i) & (ii)
A-E.
The photograph of pristine MWCNTs dispersed in water
(Fig. 8 (i & ii) A) showed that the MWCNTs settled
down in water due to its higher surface energy, van der
Waals force, and high aspect ratio [50, 51]. In contrast,
the degree of dispersibility of MWCNTs derived from
APAMAM (G4) and APAMAM (G5) in aqueous phase
gradually improved. But the degree of dispersibility of
later was relatively higher than the MWCNTs-
APAMAM (G4) hybrid in irrespective of the solvent.
This is because, the APAMAM (G5) functionalized
MWCNTs increases the hydrophilic character and hence
promotes the effective dispersibility. To check the
dispersibility and stability of MWCNTs-APAMAM (G5)
hybrid was dispersed in various organic solvents and
compared with the pristine MWCNTs and MWCNTs-
APAMAM (G4). The results in toluene, THF,
chloroform and DMSO (Fig. 8 (ii) B-E) indicated that
the functionalized MWCNTs showed homogeneous
dispersibility due to the presence of alkyl groups present
in the MWCNTs hybrid. Whereas, in water it gives a
clear homogeneous solution (Fig. 8 (ii) F) is due to the
increased polarity (10.2) as well as hydrophilic
attraction due to poly(quaternaryonium ions) present in
the amphiphilic hybrid confirming the effective
dispersibility of MWCNTs. Thus the synergetic action
of (i) attraction of high polar solvents with amphiphilic
MWCNTs hybrid and enriched functionalization of
poly(quaternaryonium ions) led to improved
dispersibility. Hence, the MWCNTs-APAMAM (G5)
hybrid was stable for 9 months in aqueous and other
organic solvents.
Xu et al [52] showed dispersion of poly(N-
isopropylacrylamide) functionalized MWCNTs hybrid
only in aqueous solution. Wang et al [53] reported that
the functionalized MWCNTs with double-hydrophilic
block copolymer viz., poly(ethyleneoxide)-b-poly[2-
(N,N-dimethyl amino)ethylmethacrylate] showed
dispersibility in DMSO and ethanol-water mixtures.
Choi et al [54] reported that the carboxylic acid-
terminated hyperbranched poly(ether-ketone) on
MWCNTs dispersed in polar solvents. We reported that
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28
the MWCNTs functionalized with amphiphilic
poly(propyleneimine) dendrimer showed better
dispersibility in aqueous and organic solvents [36].
Similarly, we reported that the MWCNTs functionalized
with 4 wt% poly (vinylbenzyl) triethylammonium
chloride showed better dispersibility in aqueous and
organic solvents [37]. In the present study, the
MWCNT-APAMAM (G5) hybrid contains both more
hydrophobic and hydrophilic properties which enabled
dispersion of the MWCNTs effectively in
aqueous/organic media with improved conductivity.
Fig. 8. Photograph of dispersibility studies of
(i) MWCNTs-APAMAM (G4) and (ii) MWCNTs-
APAMAM (G5) hybrids in (A) pristine MWCNTs, (B)
toluene, (C) THF, (D) chloroform, (E) DMSO and (F)
water.
IV. CONCLUSIONS
The MWCNTs-amphiphilic poly (amido amine)
dendrimers based hybrids were developed through
individual functionalization of APAMAM (G4),
APAMAM (G5) on MWCNTs. Subsequently, the above
two hybrids were used as a matrices for deposition of
AgNPs to develop two types of new MWCNT-
amphiphilic dendrimers-deposited with AgNPs based
hybrids viz., MWCNT-APAMAM (G4)-AgNPs, and
MWCNT-APAMAM (G5)-AgNPs. The appearance of
C-N+ peak at 1154 cm-1 and shifting of –N-H band
from 3430 to 3420 cm-1 in FTIR indicated the
quarternization and the shifting of N-H and C-N+ peaks
from 3430 to 3445 cm-1, and from 1154 to 1160 cm-1
respectively confirmed deposition of AgNPs on the
respective MWCNTs hybrids. The increased ratio of
ID/IG in the Raman spectrum from MWCNTs-
APAMAM (G4) (1.4) to MWCNTs-APAMAM (G5)
(1.6) strongly supported the covalent functionalization
of APAMAM (G4)/(G5) on MWCNTs. The quantum and
nature of the functionalization of APAMAM (G4)/(G5)
on MWCNTs was established through percentage of
weight loss of dendrimers molecules in TGA. The
decreased weight percentage of carbon from pristine
MWCNTs (99%) to MWCNTs nanohybrids (75.1%)
and appearance of N, I, and Ag peaks in EDS, change of
surface morphology from smooth to heterogeneous with
more black dots observed in SEM and HRTEM
supported more functionalization of APAMAM (G5) and
deposition of AgNPs on MWCNTs-APAMAM hybrids.
Further, based on the results of XRD and HRTEM, the
size of AgNPs present in MWCNTs-APAMAM (G4)-
AgNPs was 6nm and 2 nm for MWCNTs-APAMAM
(G5)-AgNPs. Further, the MWCNTs-APAMAM
(G4)/(G5) hybrid has proved to be 9 and17 times superior
in electrical conductivity than pristine MWCNTs and
further MWCNTs-APAMAM (G4)/(G5)-AgNPs hybrid
has showed 13 to 19 times higher than that of pristine
MWCNT as shown by four-probe conductivity
measurements. The MWCNTs-APAMAM (G5) showed
better dispersibility in aqueous and various polar and
nonpolar organic solvents which is stable for 9 months.
In a nutshell, the MWCNTs-APAMAM (G5) based
hybrid was better in terms of (i) high functionalized
yield, (ii) higher electrical conductivity, and (iii) better
dispersibility in aqueous and organic solvents.
ACKNOWLEDGMENT
The authors gratefully acknowledge the DST-
Nanomission (DST-NSTI), New Delhi, Government of
India for financial assistance and Prof. C.N.R.Rao,
Honorary President of Jawaharlal Nehru Centre for
Advanced Scientific Research, Bangalore, India.
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International Journal of Advanced Chemical Science and Applications (IJACSA)
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ISSN (Print):2347-7601, ISSN (Online): 2347-761X, Volume -3, Issue -2, 2015
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