FTIR spectroscopy as a tool for nano-material characterization by materialresearch


									                                                               Infrared Physics & Technology 53 (2010) 434–438

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                                                       Infrared Physics & Technology
                                               journal homepage: www.elsevier.com/locate/infrared

FTIR spectroscopy as a tool for nano-material characterization
Charles Baudot a,⇑, Cher Ming Tan b, Jeng Chien Kong a
    STMicroelectronics Asia Pacific Pte Ltd., 5A Serangoon North Avenue 5, Singapore 554574, Singapore
    School of EEE, Nanyang Technological University, Block S2, Nanyang Avenue, Singapore 639798, Singapore

a r t i c l e          i n f o                           a b s t r a c t

Article history:                                         Covalently grafting functional molecules to carbon nanotubes (CNTs) is an important step to leverage the
Received 10 June 2010                                    excellent properties of that nano-fiber in order to exploit its potential in improving the mechanical and
Available online 22 September 2010                       thermal properties of a composite material. While Fourier Transform Infra Red (FTIR) spectroscopy can
                                                         display the various chemical bonding in a material, we found that the existing database in FTIR library
Keywords:                                                does not cover all the bonding information present in functionalized CNTs because the bond between
FTIR characterization                                    the grafted molecule and the CNT is new in the FTIR study. In order to extend the applicability of FTIR
Carbon nanotube
                                                         to nano-material, we present a theoretical method to derive FTIR spectroscopy and compare it with
Covalent functionalization
Epoxy fillers
                                                         our experimental results. In particular, we illustrate a method for the identification of functional mole-
                                                         cules grafted on CNTs, and we are able to confirm that the functional molecules are indeed covalently
                                                         grafted on the CNTs without any alterations to its functional groups.
                                                                                                                           Ó 2010 Elsevier B.V. All rights reserved.

1. Introduction                                                                            load transfer and heat transfer between the epoxy matrix and
                                                                                           CNT fillers, covalent bonding between the polymer and the nano-
   Infrared spectroscopy is a crucial tool to characterize the struc-                      tubes must be present. Consequently, we intend to apply FTIR to
ture of matter at the molecular scale. Based on the singular reso-                         examine the presence of such covalent bonds.
nant vibrational modes of different branches or body parts of a                                Blake et al. [8] demonstrated that there is indeed a covalent
molecule, one can reveal its constituting elements and its bonding                         bond between functional molecules and CNTs by grafting silicon
arrangement. The reverse process is also exploited because a spe-                          and tin based molecules on the nanotubes and by looking for
cific molecule possesses a unique infrared spectrograph equivalent                          C–Si and C–Sn vibration modes in the FTIR spectrum. However,
to a finger print which can unveil its identity. FTIR spectroscopy                          such an affirmation cannot be done if purely carbon based func-
analysis is a method based on the principle of infrared spectros-                          tional molecules are grafted as it is the case in our present study.
copy, and it has extended its area of application to the study of                          C–Si and C–Sn vibrations occur in a very specific narrow range,
nano-scaled objects during the last decade.                                                almost independently of the remaining molecular structure,
   A common approach to exploit the unique properties of nano-                             because of the relatively big difference in atomic masses of the
particles and to improve its workability is to functionalize the sur-                      two atoms. However, carbon to carbon bonds exist in a very wide
face of the latter [1–6]. However, the application of FTIR in the                          range of wavenumbers due to the huge amount of existing organic
characterization of functionalized nanoparticles could be difficult                         molecules and their various configurations. Coleman et al. [9] used
because it may not reveal the chemical bonds between the nano-                             another alternative to demonstrate the presence of covalent bond-
particles and the functional molecules based on a database of                              ing. They grafted thiolic functional molecules on the CNTs and
known vibrational frequencies. This is because the chemical bond                           dispersed gold nanoparticles in a solution containing the function-
between the functionalized molecules and the nano-material is                              alized CNTs (f-CNTs). Then, using an atomic force microscope to
usually novel, and thus, a strategy must be developed in order to                          scan a sample of the dispersed f-CNTs on a surface, they showed
provide evidence about the resonant vibrational information be-                            the CNTs decorated with nanoparticles. However, this method is
tween the grafted molecules and the nano-sized particles.                                  only usable to demonstrate that covalent bonds indeed occur on
   In this work, we study the covalent grafting of epoxide mole-                           CNTs. It is a destructive method that alters the characteristics of
cules on CNTs [7]. This strategy is for the exploitation of CNTs as                        the functional molecules and it is only compatible with the grafting
fillers in epoxy composite materials for its mechanical and heat                            of functional groups that can bond to nanoparticles which do not
dispersion enhancements. In order to ensure a better mechanical                            necessarily suit the targeted application.
                                                                                               The method that we present here is a combined experimental
    ⇑ Corresponding author. Tel.: +65 6427 7368; fax: +65 6427 7379.                       and theoretical FTIR analysis of f-CNTs to unveil the occurrence
      E-mail address: charles.baudot@st.com (C. Baudot).                                   of a chemical covalent bond between functional molecules and

1350-4495/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved.
                                                    C. Baudot et al. / Infrared Physics & Technology 53 (2010) 434–438                                      435

                                                                                            Once n-butyllithium reagent starts attacking the CNTs, a natural
                                                                                        dispersion of the functionalized nanotubes occurs by electro-
                                                                                        repulsion. After stirring the homogeneous solution for 2 h,
                                                                                        epichlorohydrin is added in controlled amount while stirring is
                                                                                        continuing. During this reaction, while lithium chloride is formed
                                                                                        as a by-product, epoxide branches are grafted on the CNTs by
                                                                                        nucleophilic substitution. Thus, each functionalized site contains
                                                                                        two branches, namely a butyl branch and an epoxide branch as
                                                                                        shown in Fig. 1.
Fig. 1. Sketch showing the grafting of functional molecules on the side wall of a
                                                                                            The butyl branch contributes in producing unevenness on the
carbon nanotube.                                                                        CNT surface that reduces considerably the bundling effect which
                                                                                        is due to both the Van der Waals forces of attraction and the high
CNTs. Pristine CNTs show very weak resonant frequencies in the                          aspect ratio of the nanotubes. Moreover, the epoxide branch con-
infrared spectrum in the 400–4000 cmÀ1 range. However, once                             tributes in improving the solubility of the CNTs in epoxy resins.
molecules are grafted on it, we observe differential infrared                           Furthermore, the epoxy ring contained in the grafted functional
absorption response due to the functional sites. The purpose of this                    molecule participates in the polymerization of the composite when
work is to provide evidence that the observed differential response                     the curing agent is added. Thus a covalent bond is believed to be
is not due to either adsorbed molecules, wrapping molecules or                          created between the CNT and the composite matrix.
any other such kind of residual particles which are not covalently                          Using a Perkin–Elmer System GX 2000 FTIR spectrometer, an
bonded to the CNTs, but it is indeed due to the covalent bonding                        extensive FTIR analysis is performed on samples of f-CNTs. The
of the grafted molecules to the CNTs.                                                   samples are prepared by taking a small amount of f-CNTs in pow-
                                                                                        der form and mix it with potassium bromide to make several thin
                                                                                        and semi-transparent discs. The discs are then analyzed in the
2. Experimental                                                                         spectrometer under ambient conditions using different wavenum-
                                                                                        ber resolutions. First, a broad scan of the sample is performed from
    The chemical procedure for the grafting of the functional mole-                     400 cmÀ1 to 4000 cmÀ1 with a step of 4 cmÀ1 (Fig. 2). Among the
cules to CNTs consists of two steps in a one pot reaction. The first                     various peaks present in the spectrum, several are already clearly
step consists of initiating reactive sites on the CNT walls by using a                  identifiable as belonging to the functional molecules. For example,
lithiated alkyl and the second step, bonding functional molecules                       we observe a peak at 830 cmÀ1 which corresponds to a mono-
containing oxirane rings. The detailed reaction path and methodol-                      substituted epoxide ring vibration. We also observe peaks corre-
ogy is described elsewhere [7]. The CNTs that are used for this                         sponding to the straight chain alkanes or branches of the epoxide
experiment are high purity multiwall nanotubes from Nanothinx                           molecules. The list of known wavenumber ranges obtained from
S.A. The use of organo-metallic agents to initiate reaction sites on                    FTIR databases [13] is listed in Table 1. In order to obtain more de-
CNTs has already been reported [10–12] and n-butyllithium is used                       tailed spectra of the sample, a higher precision FTIR analysis on a
in our work here. This organo-lithium molecule is obtained from                         narrower range of 800–1800 cmÀ1 is performed with a step of
Sigma–Aldrich at a concentration of 2 M in cyclohexane. The sol-                        0.1 cmÀ1 (Fig. 3). In order to visualize the peaks that are masked
vent in which the reaction takes place is high purity anhydrous                         because of the convolution, a first order derivative of the spectrum
cyclohexane from Sigma–Aldrich. The reactions are done at room                          is performed (Fig. 4). The information collected in those spectra re-
temperature under inert atmosphere to prevent the very reactive                         veal peaks which are not identifiable in the database of the pure
butyl alkyl from reacting with the humidity present in air. The                         initial constituents such as the peaks at wavelengths of
glove box used during the experiments consists of an argon atmo-                        1401 cmÀ1 and 1470 cmÀ1, but they are believed to be of crucial
sphere with a humidity level of less than 0.1 ppm and an oxygen                         importance for the characterization of the grafting of functional
level of at most 2 ppm.                                                                 molecules on CNTs.

Fig. 2. Experimental FTIR spectrum of the functionalized CNTs showing the changes in transmittance of the IR signal over the range of 400–1800 cmÀ1 (inset 2500–
4000 cmÀ1).
436                                                   C. Baudot et al. / Infrared Physics & Technology 53 (2010) 434–438

Table 1                                                                                   3. Theoretical calculations
List of know IR resonant vibrations related to straight chain alkanes, epoxide rings
and water.
                                                                                             In order to identify the significance of each peak, a model of the
  Wavenumber       Deformation type             Fragment                                  f-CNTs must be done and the different vibrational modes of the de-
  range (cmÀ1)
                                                                                          signed macro-molecule must be compared to the experimental re-
  445–825          Water                        O–H out-of-plane bending                  sults. As CNTs are almost chemically inert, a chemical reaction is
                                                                                          more likely to occur on the reactive defect sites of the CNT walls.
  455–455          Straight chain alkanes       C–C skeleton vibration
  485–540          Straight chain alkanes       C–C skeleton vibration
  805–880          Mono-substituted             Ring vibration
                   epoxide ring
  1060–1150        Saturated aliphatic esters   C–O asymmetric stretching
  820–1140         Saturated aliphatic esters   C–O symmetric stretching
  1370–1390        R–CH3                        C–H symmetric deformation
  1440–1480        –CH2–                        C–H scissor vibration
  1475–1500        Mono-substituted             CH2 deformation vibration
                   epoxide ring
  1595–1710        Water                        O–H in-plane bending vibration
  2865–2885        R–CH3                        C–H symmetric stretching
  3000–3700        Water                        O–H stretching vibration                  Fig. 5. (a) Positions 1 and 2 are the locations of the binding sites for the functional
  3230–3550        Water                        Intermolecular hydrogen bond              molecules on the C30 and (b) Cluster model of the functionalization of a C30

                                    Fig. 3. High resolution FTIR spectrum of the functionalized CNTs over the range of 800–1800 cmÀ1.

                                    Fig. 4. First order derivative of the FTIR spectrum shown above over the range of 1300–1550 cmÀ1.
                                                          C. Baudot et al. / Infrared Physics & Technology 53 (2010) 434–438                                                 437

                        Fig. 6. Optimized configurations of the three conformers created by grid scan technique for the functionalized half-fullerene.

Table 2                                                                                       Table 3
Theoretical results of the covalently grafted bond states for different orientations.         Vibrational modes comparison between experiment and theoretical work.

      Configuration      B.E. (eV)a       L(CCNT–Cbutyl) (Å)b     L(CCNT–Cepoxide) (Å)           Wavenumber (cmÀ1)              Fragment deformation
      6a                2.58             1.685                   1.836                          Experimental     Theoretical
      6b                6.61             1.581                   1.571
                                                                                                 489              493          Straight chain alkanes   C–C skeleton vibration
      6c                6.47             1.577                   1.577
                                                                                                 830              843          Epoxide ring             Ring vibration
      Binding energy (absolute value).                                                          1401             1403          CCNT–Cepoxide            C–C stretching
      Bond length, L.                                                                           1470             1475          CCNT–Cbutyl              C–C stretching
                                                                                                1504             1504          Epoxide ring             CH2 bending

Thus, the most probable location to find grafted molecules on the                              ration 6a remains at its original position, leading to an unstable
CNTs will be a place where defects naturally occur, that is, the
                                                                                              arrangement with a B.E. of about 4 eV less than the two other con-
tip of the CNT. Therefore, the model that we develop consists of                              formations. Correspondingly, the calculated L (CCNT–Cepoxide) in 6a
grafting molecules on both CNT tips and on lateral defect sites
                                                                                              is somewhat larger than 6b and 6c by about 0.26 Å and this also re-
and the different vibrational modes of the resulting structure are                            flects a much weaker covalent bonding in that configuration as
then computed.
                                                                                              compared to those in the 6b and 6c arrangements. Therefore, to
    The first cluster model consists of half a fullerene (C30) to which                        model the covalent functionalization of CNTs, the configuration
are attached a butyl molecule and an epoxide molecule placed on                               6b is chosen in order to determine the IR spectrum of the f-CNTs.
atomic sites where the most active binding regions are deemed                                     We examine the vibrational spectrum of the f-CNTs by means of
to be located. Such locations are found at the junction between                               cluster models with methods based on the density functional the-
pentagonal and hexagonal rings (Fig. 5). In order to avoid any                                ory (DFT) within the generalized gradient approximation (GGA)
geometry reconstruction at the hemi-spherical edge, the dangling                              with the Perdew, Burke and Ernzerhof (PBE) correlation functional
bonds are saturated with hydrogen atoms. After performing a                                   [17]. Our calculations are closed shell and are performed using the
relaxation of the cluster, the Hessian matrix is calculated in order                          Dmol3 module of Materials Studio from Accelrys. The electronic
to evaluate the resonant vibrational frequencies of the molecular                             wavefunctions are expanded in atom-centered basis functions de-
model [14]. The second model consists of a 7-5-5-7 Stone Wales                                fined on a dense numerical grid. The chosen basis set is double
defect site found on the side wall of a CNT [15] on which the same                            numerical plus d-polarization (DND). DND is an all-electron basis
molecules are grafted and the same procedures are performed.                                  set composed of two numerical functions per valence orbital, sup-
    We first investigate the most stable configuration of the models.                           plemented with a polarization d-function on all the non-hydrogen
For example, in the case of the functionalized half-fullerene, we                             atoms. Each basis function is restricted to a cut-off radius. A global
study the optimal arrangement of the molecule displayed in                                    real space cut-off is selected as the maximum value of the cut-offs
Fig. 5b. Grid scan conformer search technique [16] is employed                                specific for every element of the system. Real space cut-offs for
by fixing the half-fullerene and the butyl molecule while rotating                             atomic species are optimized by considering total energies of
the epoxide ring at three different angles with respect to the butyl                          atoms. The chosen cut-off values lead to atomic energies with an
molecule. The same procedure is then repeated for side-wall func-
                                                                                              accuracy of 0.1 eV/atom, allowing calculations without a signifi-
tionalization. Three configurations are created and geometrically                              cant loss of accuracy. The self consistent field (SCF) convergence
optimized for each model. The final configurations of the function-
                                                                                              has been tested and a convergence threshold less than 10À6 Ha
alized half-fullerene are shown in Fig. 6.                                                    has been evaluated to be satisfactory to achieve the structural
    The binding energy (B.E.) and the length (L) of the C–C bond
linking the butyl molecule to the C30 (CCNT–Cbutyl) and the epoxide                               From the simulation of Dmol3, the harmonic wavenumbers ob-
molecule to the C30 (CCNT–Cepoxide) are summarized in Table 2. The
                                                                                              tained are listed in Table 3 together with the corresponding exper-
values of B.E. are calculated based on the following equation:                                imental results. In agreement with the theoretical results, the
B:E: ¼ EðC30 þ butyl þ epoxideÞ À EðC30 Þ À EðbutylÞ À EðepoxideÞ                             covalent bond formation of the epoxide molecule on C30 is dis-
                                                                                              closed by the wavenumber 1403 cmÀ1. Also, the covalent bond for-
                                                                                              mation of butyl molecule on C30 is attributed to the CCNT–Cbutyl
where E(C30 + butyl + epoxide), E(C30), E(butyl) and E(epoxide) are                           stretch at around 1475 cmÀ1. Besides the two crucial wavenum-
the respective total energies of the indicated systems.                                       bers, we also observe that the theoretical results are in agreement
   The conformations 6b and 6c in Table 2 are very close to each                              with the experimental vibrational frequencies of the functional
other in both the B.E. and L. The carbon atom located at position                             molecules such as the ring oscillation and the alkyl stretching.
1 for these two configurations is distorted and a tetrahedral sp3                              However, there are still some peaks that have not been identified
hybridization contributing to the overall structural stability is                             such as the peaks at 873 cmÀ1, 1297 cmÀ1, 1333 cmÀ1 and
formed. On the other hand, the same carbon atom in the configu-                                1358 cmÀ1. We believe that it is because the attachment of the
438                                                     C. Baudot et al. / Infrared Physics & Technology 53 (2010) 434–438

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