Nanocomposites Based on Polyelectrolytes-Multiwalled Carbon Nanotubes Coated with a Silica Shell

This work reports a simple and reproducible method for silica-coated multiwalled carbon nanotubes (MWCNTs) nanostructures by the sol-gel method using tetraethoxysilane and 3-aminopropyltriethoxysilane as organosilicon precursors of silica. The synthetic method is based on a noncovalent functionalization of the MWCNTs by the adsorption of the monolayers of different polyelectrolytes, such as poly(allylamine hydrochloride) and poly(sodium 4-styrene sulfonate), which are positively and negatively charged, respectively. The role of these polyelectrolytes is the dispersion and interaction with both the nanotubes and the silicon precursors, influencing directly the silica shell growth. Two different morphology types of the silica-coated multiwalled carbon nanotubes composites were obtained on multiwalled carbon nanotubes. In both case the samples displayed uniformity in the silica layer. Finally, the investigation of the photoluminescence spectra of the samples suggested the presence of some incorrect oxygen bond, or vacancy of oxygen or even presence of nitrogen in the network, which is a typical feature of materials prepared by the sol-gel process at room temperature.


Introduction
Since the discovery of fullerene by Kroto, and carbon nanotubes (CNTs) by Iijima, the synthesis, characterization and theoretical studies of carbon nanostructures and related materials have been very active research fields [1][2][3][4][5] .The CNTs consist of graphitic sheets, which are rolled up into a cylindrical shape, and are characterized by chemical inertness due to the strong covalent sp 2 bonds of the carbon atoms on the nanotube surface, a unique one-dimensional structure and nanometer size.These features impart unusual properties to the nanotubes, including exceptionally high tensile strength [6][7][8][9] , high flexibility, low mass density, high thermal conductivity and electronic properties ranging from metallic to semiconducting, depending on the arrangement of hexagonal rings along the tubular surface [10][11][12][13] .A tremendous amount of work has been done on different aspects of CNT technology [6][7][8] .However, manipulation and processing of CNTs has been limited by the formation of big bundles held strongly together via van der Waals interactions, and their hydrophobicity and poor solubility in organic solvents, preventing their application in various areas [13][14][15] .The application of CNTs in various composites, the fabrication of films and macrofibers from CNTs, and the production of electronic, optical and optoelectronic components and devices often requires surface modification through functionalization of CNTs to overcome such difficulties of the chemical processability 4,[13][14][15] .Different strategies have been suggested to overcome these problems of processability by surface modification, and can be divided into two main categories involving either noncovalent functionalization or covalent functionalization 16,17 .Noncovalent CNT modification concerns the physical adsorption of aromatic molecules and/or wrapping of polymers to the surface of the CNTs, allowing them to be dispersed into water efficiently 17 .Noncovalent functionalization methods are mainly based on supramolecular complexation using various adsorption forces, such as van der Waals forces, π-π interactions and charge-transfer interactions 18 .Covalent functionalization occurs by using a highly reactive reagent [18][19][20] .The main strategies used for covalent functionalization have included halogenation 20 , cycloaddition 21,22 , radical addition 23 , ozonolysis 24 and oxidation processes 4,8 .For several years, the oxidation process by mixed acids has been the standard method to break down and functionalize carbon nanomaterials 3,4,25 .One typical example of covalent functionalization is the carboxylation of CNTs, which usually requires a tedious reaction process under strongly acidic conditions.The carboxylic acid group (COOH) is considered the prototype chemical group for such purpose because it can either serve as an anchor group for further functionalization, or be created by the covalent attachment of further groups 10,11,17 .However, covalent functionalization causes fragmentation and defects in the hexagonal lattice of graphite, which affect the intrinsic structural properties of the nanotubes, including mechanical, thermal and electrical properties, thus the performances of CNT hybrid materials do not reach the goal level 26 .Another important strategy toward obtaining composites involving heterostructures of CNTs is the noncovalent functionalization of CNTs.The main advantage of noncovalent functionalization compared with covalent functionalization is that it does not destroy the conjugated system of the CNT walls, and therefore it does not affect the final structural properties of the material 13 .Functionalization of the CNTs was used successfully to assembly silica-coated gold nanoparticles and gold nanorods 27,28 as well as CdTe QDs 13,29 and CdSe/ZnS 30 onto the surface of CNTs.The interest in the preparation of nanocomposites of polymers, nanoparticle or/and silica and carbon nanotubes (CNTs) has increased tremendously in recent years due to the synergistic effects resulting from the combination of these two classes of materials 31 .Combinations of nanomaterials that exhibit different properties, highly desirable for simultaneous and efficient technological applications, can be performed by assembling "building blocks" of several materials into nanocomposites, thereby providing bi-, tri-or multifunctionality 32 .Recently, silica-coated CNT composites are of interest because they are useful for the advancement of nanoscale sensors and electric devices as well as optical 29 , mechanical reinforcement 9 and templates for fabrication of silica nanotubes 33 .The silica-coated multiwalled carbon nanotubes (MWCNTs) show the ability to prevent charge transfer and improve the nanomechanical properties of polymeric composites because the bending strength of CNTs is improved by the silica shell 29,30 .Several methods have been explored to fabricate silica-coated CNT nanocomposites, such as synthesis of silica on noncovalently (using π-π interaction) or covalently (by forming a σ bond) functionalized 16 .However, despite using the techniques of functionalization of CNTs, many studies did not obtain composites with coated carbon nanotubes individually, with a homogeneous layer of silica, and some methods are comparatively complicated, or even are not suitable for large-scale production 16,34 .
In this paper, we report on SiO 2 deposition on MWCNTs by the combination of noncovalent functionalization and sol-gel method to generate nanocomposites consisting of CNTs coated with a SiO 2 shell.Our approach is based on the noncovalent functionalization of CNTs, which has led to a high level of dispersion of the resulting products.The use of different polyelectrolytes, positively or negatively charged, allows us to investigate the influence of reagent order on the morphology of the resulting CNT-SiO 2 hybrid nanocomposites.

Preparation of MWCNTs/Polyelectrolyte Suspension
In this study, we have produced composites based on the silica coating of MWCNTs.The synthetic process required initial nanotube functionalization for the dispersion in water of the MWCNTs, which were prepared by noncovalent functionalization using the polymer-wrapping technique by the adsorption of the monolayers of polyelectrolytes.The first composite prepared was based on functionalized MWCNTs with a cationic polyelectrolwyte.The previous reported route was adapted 29 , changing the reagent concentrations.In the second one, we employed the coating by using an anionic polyelectrolyte, PSS, for the subsequent growing of the silica shell.
The dispersion of MWCNTs by functionalization using cationic polyelectrolyte was performed using 0.045 g PAH as a polymer-wrapping agent for 0.030 g of the nanotubes in 50 mL of water under sonication for 4 h.For purification, the excess PAH was removed by several centrifugation/ redispersion cycles, and then the supernatant was filtered in a microporous membrane and redispersed in distilled water by brief sonication.PAH provides stable dispersions of CNTs with amine functionalities.The material was labeled as PAH-MWCNTs.
The functionalization using anionic polyelectrolyte was performed using 0.030 g of MWCNTs in 50 mL of 1.0 mol L -1 NaCl solution, where it was sonicated (40 kHz) for 3 h.Then, 0.100 g of PSS was added, and the resulting solution was stirred for 6 h.The dispersion was centrifuged for 1 h (3600 rpm), and the supernatant was discarded.The MWCNTs were redispersed in distilled water, filtered in microporous membrane and diluted in 50 mL of distilled water, to afford the material labeled PSS-MWCNTs 13 .

Coating of SiO 2 on MWCNTs
Silica coating using PAH-MWCNTs as template was carried out according to a modified version of the method described by Giersig et al. 27 .In a typical procedure, 400 mL of ethanol containing 84.5 µL TEOS was added to 100 mL of an aqueous solution of PAH-MWCNTs containing citric acid (1.85 × 10 -3 mol -1 pH 3) under sonication for 1 h.Finally, APS (7.2 µL) was added and the mixture, and homogenized by magnetic stirring for 30 min.After that, a solution of NH 4 OH (4.2%) was added dropwise until pH 8 was reached.Subsequently, the solution was aged for 10 h at room temperature, obtaining the silica coating.The products were separated by centrifugation (3600 rpm), and the precipitates were vacuum-filtered, washed in ethanol and redispersed in 50 mL of absolute ethanol.
In a typical synthesis of silica coating using PSS-MWCNTs as template, 30 mL of water containing PSS-MWCNTs was sonicated for 1 h.The above mixture was added to 80 mL anhydrous ethanol, and further sonicated for 1.5 h to form a stable dispersion.Immediately, 1 mL of NH 4 OH and 0.2 mL of APS were added to the as-prepared MWCNTs dispersion.After that, a TEOS solution (0.8 mL TEOS in 40 mL ethanol) was added dropwise under vigorous stirring.The reaction mixture was stirred for another 12 h.Finally, the CNTs solution was centrifuged and washed with ethanol and once more redispersed in water.The resultant hybrid materials were designed as PAH-MWCNT-SiO 2 and PSS-MWCNT-SiO 2 .The processes described above lead to the formation of a uniform and thick layer of silica on every individual MWCNT.

Characterization techniques
Ultraviolet-visible (UV-vis) spectra were registered on a diode array UV-2550 Shimadzu spectrometer.Photoluorescence (PL) spectra were obtained at room temperature using a Shimadzu RF-5301 PC spectrofluorophotometer.The spectrofluorophotometer is equipped with a 150 W xenon lamp.The absorption and fluorescence measurements were typically performed with 10 mm-quartz cuvettes (Shimadzu) using air-saturated solutions at room temperature.The Fourier-transform infrared (FT-IR) spectra were acquired on a PerkinElmer spectrometer (Spectrum GX), in the transmission mode; 32 spectra with 2 cm -1 resolution.Transmission electron microscopy (TEM) was performed on a JEM 2100 FEG-TEM operating at 200 kV.The grid used was 300-mesh Lacey Formvar with ultrathin carbon film.The scanning electron microscopy (SEM) was performed on a FEI Inspect F50.The zeta potential values of the dispersions were determined at a pH of 10.0 using a Delsa Nano C apparatus from Beckman Coulter.The pH values of the MWCNTs dispersions were adjusted to 10 by dropwise addition of 0.1 mol L -1 NaOH solution.

Results and Discussion
A simple, efficient and reproducible method for silica coating of MWCNTs with different SiO 2 coating thicknesses was applied.We have used the sol-gel method starting with TEOS and APS as silicon precursors, and carbon nanotubes coated with polyelectrolytes bearing positive and negative charges.We produced two types of CNT-SiO 2 nanocomposites with different silica shell sizes, PAH-MWCNT-SiO 2 and PSS-MWCNT-SiO 2 .The two processes described above lead to the formation of a uniform layer of silica on individual MWCNTs.The morphological structures of the PAH-MWCNT-SiO 2 and PSS-MWCNT-SiO 2 composites were investigated by TEM and SEM observation.Figure 1 shows the electron microscopy images of the silica-coated MWCNTs.The MWCNTs' thickness after silica coating increased to In the sol-gel process, the acidic or basic catalysts have a significant role in the formation of the final morphology 35,36 .The reactions presented in Figure 2 can be used to explain the morphologies of the PAH-MWCNT-SiO 2 and PSS-MWCNT-SiO 2 .The silica shell in the PAH-MWCNT-SiO 2 is thin because in the acidic catalyst the condensation of the silanol groups occurs more rapidly, which may result in the formation of a smaller silica layer on the surface of the carbon nanotubes, due to lower aggregation, Figure 2a.The acid/base catalysis incorporated more atoms in the structure, increasing the silica layer.The silica shell in the PSS-MWCNT-SiO 2 is formed by basic catalyst, in which the silica sol is based on spherical particles, Figure 2b.The synthesis in this condition produces a thicker layer formed by numerous spherical particles on the surface of carbon nanotubes due to the slower condensation.
The differences in the silica shell can also be related to the acid catalyst, which promotes the protonation of ethoxy groups, while in basic catalysis, the amino (-NH 2 ) basic group reacts preferentially with the silanol, affecting the condensation of these groups.In this case, growth is hindered and the particles formed will be smaller in size 36 .
Figure 3 shows a scheme that represents the two different methods of applied functionalization of MWCNTs with the polyelectrolytes, PAH and PSS, as well as the coated silica layer.Surface modification by noncovalent functionalization using the self-assembly layer-by-layer technique allowed us to obtain PAH-MWCNT-SiO 2 and PSS-MWCNT-SiO 2 via electrostatic interactions between the charged surface of the functionalized carbon nanotube and the charge of prehydrolyzed alkoxide precursor (TEOS and APS).These methods involved the dispersion of MWCNTs in water using two different functionalizations with the cationic and anionic polyelectrolytes, PAH and PSS, as polymer-wrapping agent, furnishing a stable dispersion (Figure 4).The dispersion stability is due to thermodynamic preference between polyelectrolyte and MWCNTs, as compared in the interaction between water and MWCNTs 30 .This process suppressed the MWCNTs'   hydrophobic surface and generated a positive charge when functionalized with the PAH and negative charge with PSS.PAH-MWCNTs with positive charge at pH 3 can be formed via donor-acceptor interactions between MWCNTs and PAH, a weak polyelectrolyte.Indeed, in water at pH =3 the PAH-MWCNTs materials displayed a positive charge (zeta potential (ζ) = +50 mV).PAH has an effective pK a of ca.8-9, being fully protonated in neutral and acidic solutions but partly deprotonated in slightly basic solutions.Multilayers can also be deposited in acidic or neutral aqueous solutions (pH of the PAH solution is 3.1 or 6.5), in which the amino groups of PAH are fully protonated, indicating the prevalence of the positive charge 6 .The high density of sulfonate groups in PSS confers a negative charge to this polyelectrolyte 19 that, as a polyacid, is fully ionized above pH 3.5 37 Indeed, in water at pH = 10 the PSS-MWCNTs materials displayed a negative charge (zeta potential (ζ) = -49 mV).The high zeta potential values indicated that the colloidal dispersion was stable.These values are presented in Table 1.
When the cationic polyelectrolyte is used for wrapping the MWCNTs, a number of positive charges on the surface of the MWCNTs (see the zeta potential value in Table 1) can attract the silicate anions catalyzed by the hydrolysis of TEOS that will form uniform chains of silica on the surface of the nanotubes after condensation.Then, APS was added to the solution and the pH was adjusted to 8 to form more branched structures of SiO 2 , increasing the thickness of the layer.However, the layer grows until ~5 nm.At acid pH, the composite PAH-MWCNTs-SiO 2 has a positive charge (zeta potential (ζ) = +19 mV) due to protonation of the amine group of the APS precursor.When the polyelectrolyte used to coat the carbon nanotubes has a negative charge, it will repel the silicate anions formed by the hydrolysis of TEOS; for that reason, the APS that has an amino group that can interact with the negatively charged nanotubes was used.Similar results have been reported by others 14 .On the other hand, using PSS-MWCNTs with a negative charge, the APS with the positive charge will interact with the SO 3 -groups from PSS (Figure 3) forming various nucleation centers for the growth of silica.The growth of the silica layer was performed using TEOS, which generates a negatively charged surface (zeta potential (ζ) = -12 mV) due the SiO -groups.In this case, the thickness of the silica layer of the PSS-MWCNT-SiO 2 was 50 nm.
By analyzing the TEM images of the pristine MWCNTs (Figure 5), we can observe the morphology and tubular structure and that the carbon nanotubes have multiple walls, and that they agglomerate due to van der Waals interaction between the nanotubes.The MWCNTs did not contain impurities relative to the presence of catalysts employed during their synthesis; they did not present surface defects.The TEM images in Figure 5c and 5d showed that coating the MWCNTs with PAH and PSS not only dispersed the nanotubes without damaging their structure but also de-agglomerated the MWCNTs bundles and showed an ultrathin polyelectrolyte layer as well as the absence of surface defects on the MWCNTs.
Figure 6(a) and 6(b) displays the FTIR spectra of commercially available PAH, PSS, MWCNTs and the prepared nanocomposites PAH-MWCNTs and PSS-MWCNTs.The pure MWCNTs do not exhibit absorption.The spectra of the MWCNTs coated with PAH and PSS are similar to the pure polyelectrolytes as well.In Figure 6(a), the absorption in the region 1600 cm -1 is assigned to the stretching of the N-H bond.The band at 1069 cm -1 is assigned to the stretching of the C-N bond of the polyelectrolyte (PAH).In Figure 6(b), the bands at 1228 and 1152 cm -1 of the spectrum of PSS correspond to the SO 3 group asymmetric and symmetric vibrations, respectively.PSS-MWCNTs displayed all the typical absorption bands of commercial PSS, indicating successful functionalization of the MWCNTs.
The FTIR spectra of the PAH-MWCNTs-SiO 2 and PSS-MWCNTs-SiO 2 are shown in Figure 6(c).The absorbance band in the region 3400 cm -1 is assigned to the stretching of the O-H bond, and the bands at 1100 and 950 cm -1 are assigned to the asymmetric and symmetric stretching of the Si-O bond of silanol groups, which also confirms the successful coating of silica on MWCNTs.The characteristic bands related to the amino group appear at 1630 cm -1 and 1550 cm -1 and are assigned to N-H bonds.The absorption at 2854 and 2930 cm -1 in Figure 6(a), (b) and (c) should be assigned to the C-H bonds from the CH 2 groups of the polyelectrolytes PAH and PSS.These bands suggest strongly that the MWCNTs were successfully encapsulated using the polyelectrolytes, and also with a silica shell, in the case of nanocomposites, because the absorption bands of these groups are present in the polyelectrolytes and nanotube-coated silica spectra.
An interesting property of organic and inorganic hybrid materials formed by the sol-gel process is the photoluminescence of these materials, which can serve as a probe for the quality of the silica network formed [38][39][40] .Figure 7 shows the PL spectra of the silica and the nanocomposites PAH-MWCNTs-SiO 2 and PSS-MWCNTs-SiO 2 .The composite PAH-MWCNTs-SiO 2 presented an emission band at 426 nm (Figure 7a) while the silica nanoparticles synthesized under the same conditions without the use of functionalized nanotubes showed an emission at 434 nm.The emission spectra of  the PSS-MWCNTs-SiO 2 (Figure 7b) presented an emission band at 392 nm while the nanoparticles synthesized under the same conditions showed emission 395 nm.The origin of the photoluminescence in sol-gel derived silica has three possible mechanisms, defect mechanism, charge-transfer mechanism or the presence of carbon and nitrogen impurities [38][39][40][41] .In addition, the sol-gel reaction used in these preparations can result in the formation of various defect centers, such as incorrect oxygen bond and the vacancy of oxygen that can form emission centers consisting of dioxasilirane =Si(O 2 ) and silylene, =Si: centers 39 .The presence of the nitrogen atoms from the APS and PAH used for the synthesis of MWCNTs-SiO 2 can also have a similar effect as oxygen on the PL properties.In Figure 7, it can be seen that the PL spectra of the silica nanoparticles synthesized by the same method as the MWCNTs-SiO 2 are similar.However, the PL spectra of the PSS-MWCNTs-SiO 2 and PAH-MWCNTs-SiO 2 showed different maximum emission bands because of the local molecular environment, the number of nitrogen groups and pH used in the synthesis [38][39][40][41][42][43] .The PSS-MWCNTs-SiO 2 contains a larger number of nitrogen atoms in the silica layer than the PAH-MWCNTs-SiO 2 , where the nitrogen atoms are on the surface of nanotubes, and in the outside part of the silica layer.These molecular environments are different because of the order of the reagents.Therefore, we can conclude that the silica shell was cross-linked as expected, with some incorrect oxygen bond, or vacancy of oxygen or even presence of nitrogen in the network detected from the analysis of PL spectra of these shells.
The study presented here shows that is possible to prepare nanocomposites based on MWCNTs covered with a thin SiO 2 shell that might be used in different fields of technological applications, such as support for catalysts or even as fillers in polymeric, ceramic or cementitious composites aiming to increase their mechanical properties.

Conclusions
In this work, we have successfully synthesized MWCNTs-SiO 2 nanocomposites by simple methods using the selfassembly layer-by-layer technique on the basis of electrostatic interactions between MWCNTs and polyelectrolytes, and the sol-gel process to make a SiO 2 shell on the MWCNTs.The positively charged polyelectrolyte, PAH, and negatively charged, PSS, were used for the dispersion of MWCNTs and interaction with hydrolyzed silicon precursors, APS and TEOS, through electrostatic interactions with the charged surface of the carbon nanotube wrapped with the polyelectrolytes.The condensation of TEOS and APS on the carbon nanotube surface occurs due the preferential nucleation through the electrostatic interactions, and guarantees a homogeneous deposition along the individual carbon nanotubes as shown by the TEM and SEM micrographs.The thickness of the nanocomposites, after silica coating, increased to around 5 nm for the PAH-MWCNT-SiO 2 and 50 nm for the PSS-MWCNT-SiO 2 .The photoluminescence characterization of the resulting composites shows that although the SiO 2 shell well-fitted the MWCNTs, it might contain some incorrect oxygen bonds, or oxygen vacancy or even the presence of nitrogen in the network, which is a typical feature of materials prepared by the sol-gel process at room temperature.

Figure 2 .
Figure 2.. Schematic representation of the sol-gel reaction using (a) acid and (b) basic catalyst.

Figure 3 .
Figure 3. Schematic representation of the two synthetic routes used for the preparation of the PAH-MWCNTs-SiO 2 and PSS-MWCNTs-SiO 2 , including the functionalization with polyelectrolytes PAH and PSS.

Figure 7 .
Figure 7. PL spectra (λ ex = 355 nm) (a) of the silica nanoparticles and PAH-MWCNTs-SiO 2 and (b) silica nanoparticles and PSS-MWCNTs-SiO 2 .The silica nanoparticles were prepared according to the specific procedures used to prepare the nanocomposites.

Table 1 .
Zeta potential value (z) of MWCNTs coated with polyelectrolyte PAH, PSS and silica measured at different pH values.