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Impact of sonication pretreatment on carbon nanotubes: A transmission electron microscopy study
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Impact of sonication pretreatment on carbon nanotubes: A transmission electron microscopy study Marta D. Rossell a,b,*, Christian Kuebel c,d, Gabriele Ilari a, Felix Rechberger b, Florian J. Heiligtag b, Markus Niederberger b, Dorota Koziej b, Rolf Erni a a
Electron Microscopy Center, Empa, Swiss Federal Laboratories for Materials Science and Technology, U¨berlandstrasse 129, 8600 Du¨bendorf, Switzerland b Laboratory for Multifunctional Materials, Department of Materials, ETH Zu¨rich, Wolfgang-Pauli-Strasse 10, 8093 Zu¨rich, Switzerland c Institute of Nanotechnology, Karlsruhe Institute of Technology, Campus North, Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany d Karlsruhe Nano Micro Facility, Karlsruhe Institute of Technology, Campus North, Hermann-von-Helmholtz-Platz 1, 76344 EggensteinLeopoldshafen, Germany
A R T I C L E I N F O
A B S T R A C T
Article history:
Sonication treatments are commonly used for debundling and dispersing carbon nano-
Received 22 March 2013
tubes (CNTs) in liquid media prior to chemical functionalization. However, this step may
Accepted 8 May 2013
lead to the stripping of the outer graphitic layers and the scission of the CNTs, and can
Available online 21 May 2013
therefore have a deleterious effect on the achievable properties of the functionalized CNTs. Thus, knowledge on the structural integrity of the modified CNTs is required to understand its influence on the device performance of hybrid nanocarbon-based composites. Here we report on the impact of a sonication pretreatment on the structure of multiwalled CNTs, and on the role of the induced modifications on the subsequent attachment of ferrimagnetic Fe3O4 nanoparticles. Decoration of the CNTs with Fe3O4 nanoparticles is achieved by a microwave-assisted synthesis route involving the reaction of iron acetylacetonate with 2-pyrrolidinone. Employing a combination of atomic resolution transmission electron microscopy, electron energy-loss spectroscopy, energy-filtered transmission electron microscopy and electron tomography, we provide evidence that significant degradation of the CNT structure takes place during the dispersion process. Moreover we find that the sp2 system is more heavily disrupted at the interface between the CNTs and the surface-deposited nanoparticles suggesting that nucleation of Fe3O4 preferentially occurs at the nanotube defect sites. Ó 2013 Elsevier Ltd. All rights reserved.
1.
Introduction
The outstanding structural, mechanical and electronic properties of carbon nanotubes (CNTs) make them very
attractive for various applications in modern nanodevices. But in order to take full advantage of their properties and to extend the scope of their application spectrum, CNTs often need to be modified and integrated with other materi-
* Corresponding author at: Electron Microscopy Center, Empa, Swiss Federal Laboratories for Materials Science and Technology, ¨ berlandstrasse 129, 8600 Du¨bendorf, Switzerland. U E-mail address: [email protected] (M.D. Rossell). 0008-6223/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.carbon.2013.05.024
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als. A crucial step towards modification of CNTs is the functionalization of their relatively inert sidewalls by attachment of reactive surface species. In particular, covalent modification strategies are frequently used to functionalize CNTs and are associated with a transformation of sp2into sp3-hybridized carbon atoms [1]. As such, both chemical modification by strong acids and ultrasonication pretreatments alter the tube walls by introducing a considerable amount of defects and dangling bonds [2]. These treatments are commonly used together because sonication, apart from successfully debundling and dispersing CNTs, has proven to be highly effective in increasing the formation of defect sites for the attachment of functional groups. Thus, various functional groups (such as hydroxyl, carbonyl, carboxyl, amine, fluorine, etc.) can covalently attach to these locations providing active nucleation sites for high-loading of nanoparticles [3]. However, care has to be taken because prolonged sonication even in inert media causes fractionation of the multiwalled CNTS (MWCNTs) and stripping of the outer graphitic layers [4]. With increasing sonication times MWCNTs get shorter and thinner, and ultimately transform into amorphous carbon. Obviously, such detrimental structures disrupt the p electron system of the CNTs resulting in severe degradation of their charge-carrier mobility as well as their mechanical properties, thereby hampering the potential of the covalent hybrid structures for practical applications. Hence, the study of the interfaces between CNTs and metal or metal-oxide particles is of particular interest, not only in regard of their intrinsic structure and properties, but also to understand the resulting device performance. For this, transmission electron microscopy (TEM) is ideally suited as it provides the spatial resolution to study the structure of materials at atomic level. Besides, apart from the actual image information, spectral information can also be obtained to study the electronic structure of the CNTs. However, only little is known about the structure of functionalized nanotubes at the atomic scale [5,6], and most of the available reports explore the growth of nanotubes from catalytically active metal particles where the nucleation and growth of the tube occur through such an interface [6–8]. In this work we study the impact of a sonication treatment for the preparation of a stable dispersion of MWCNTs in 2-pyrrolidinone on the nanotube sidewall integrity. Subsequently, the attachment of iron-oxide nanoparticles is achieved by a simple and rapid microwave-assisted synthesis route. The magnetic functionalization of CNTs with Fe3O4 nanoparticles provides hence a mean to control the position and orientation of the nanotubes by using a magnetic field. We combine aberration-corrected high-resolution transmission electron microscopy with electron energy loss spectroscopy and electron tomography to learn more on the atomic and electronic structure of these technologically important metal-oxide/carbon interfaces. Our results show evidence that the sp2 nanotube structure is heavily disrupted, especially at the interfaces with the Fe3O4 nanoparticles.
2.
Experimental
2.1.
Materials
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MWCNTs grown by catalytic chemical vapor deposition having an average outer diameter of 15 nm and carbon purity exceeding 95% were purchased from Nanocyl S.A. (Belgium). Fe(III) acetylacetonate (P99.9%) and 2-pyrrolidinone (99%) were supplied by Sigma–Aldrich. All the chemicals were used without further purification.
2.2.
Synthesis of the Fe3O4 nanoparticles on MWCNTs
Crystalline Fe3O4 nanoparticles on MWCNTs were synthesized using a microwave-assisted non-aqueous sol–gel method in a CEM Discover reactor operating at a frequency of 2.45 GHz [9,10]. The synthesis described below involving the reaction of iron acetylacetonate with 2-pyrrolidinone is based on a previously published route [11] which we extended to microwave heating. Similar to N-methyl-2-pyrrolidinone [12] also 2-pyrrolidinone is a suitable solvent for the preparation of stable carbon nanotube dispersions. In the first step of the procedure 5 mL of 2-pyrrolidinone with 5.6 mg of MWCNTs was sonicated in a cup-horn type Branson Sonifier 250 sonicator operating at 20 kHz during 10 min until a black dispersion was obtained. The resulting dispersion was kept in nitrogen flow for 10 min and transferred into a glovebox under argon atmosphere (O2 and H2O <0.1 ppm). In the glovebox, 60 mg of Fe(III) acetylacetonate was dissolved in 0.75 mL of 2pyrrolidinone and mixed with the pre-sonicated 2-pyrrolidinone dispersion of MWCNTs. The reaction mixture was transferred into a 10 mL glass tube and sealed with a Teflon cap. During microwave heating the reaction was stirred with a stir bar. The heat treatment was performed at 200 °C for 5 min. The hybrid material was extracted by centrifugation and washed three times with ethanol. Finally, the CNTs decorated with Fe3O4 nanoparticles were dispersed in ethanol and a drop of the suspension was air-dried onto a lacey carboncoated copper grid for TEM characterization. Magnetic separability of the sample was tested in ethanol solution by placing a magnet near the glass bottle, as shown in Fig. 1a and b. The black powder was strongly attracted towards the magnet in a few seconds, demonstrating high magnetic sensitivity.
2.3.
Instrumentation and computational details
Electron diffraction (ED), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), electron energy-loss spectroscopy (EELS), and energy-filtered transmission electron microscopy (EFTEM) were performed using a JEOL 2200FS TEM/STEM microscope operated at 200 keV and equipped with an in-column Omega-type energy filter. For the EELS data acquisition, the convergence and collection semiangles were set to 10.8 and 10 mrad, respectively. For these values, the energy resolution measured as the full width at half maximum of the zero-loss peak is 1 eV and the dispersion was set to 0.16 eV/channel.
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8, 30 and 30 eV energy slits and 2, 50 and 15 s acquisition times, respectively. For electron tomography a JEOL single axis tomography holder was used and zero-loss filtered TEM images were manually recorded at 2° tilt intervals between 30° and +30°, and at 1° tilt intervals elsewhere over a range from 73° to 73°. The images were aligned using the StackReg plugin for the image processing software ImageJ. The 3D volume reconstruction was computed using 50 cycles of the simultaneous iterative reconstruction technique (SIRT) implemented in TomoJ, which is also an ImageJ plugin. Finally, the 3D voxel projection and the orthoslices were generated with the Amira visualization program. The HRTEM data in Fig. 3 was obtained at 300 keV on a FEI Titan 80-300 microscope equipped with a third-order spherical aberration corrector on the imaging lens system. Through-focal series of crystalline Fe3O4 nanoparticles on MWCNTs were recorded employing a focus step of 0.45 nm, exposure times of 1 s, and a 23.25 pm/pixel sampling. The MacTempas (TotalResolution) software was used to reconstruct the exit-plane waves (EPWs) according to the experimental parameters and to determine the cation displacements in the EPW phase images. Atomic positions were extracted by using the peak finding routine and fitting them as two dimensional Gaussian peaks. Displacements were calculated as a difference vector between each cation and the best-fit lattice calculated using a least-squares routine. Visualization of the local atomic displacements was carried out using a custom Mathematica code. Although 200 and 300 keV electrons used in the present TEM study carry sufficient energy to cause knock-on damage of carbon nanomaterials, in all experiments performed the electron dose was kept small enough to minimize the impact of radiation damage. The success of this approach is confirmed in the imaging study of the pristine MWCNTs, which does not reveal local damage of the graphitic structure.
3.
Fig. 1 – (a) Photograph of the synthesized MWCNT/Fe3O4 composite dispersed in ethanol, and (b) its response to a magnet. (c) TEM image of the composite with insert showing a histogram of the particle size distribution. (d) Corresponding ED pattern. The Fe3O4 and the MWCNT reflections are indicated in white and black, respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this book.)
Zero-loss filtered images were recorded with a 15 eV energy slit. EFTEM was applied to analyze the elemental distribution of Fe, O and C. The collection angle of the EFTEM images was 25 mrad. The conventional three-window technique was used to obtain element specific images. Fe, O and C elemental maps were obtained from the Fe-M2,3, O-K and C-K edges with
Results and discussion
Sonication- and microwave-assisted functionalization of MWCNTs with magnetite nanoparticles leads to discontinuous coverage of the nanotube surface. This is in agreement with reported density functional theory studies revealing that the interactions between single-walled carbon nanotubes (SWCNTs) and Fe3O4 are very weak [13]. The TEM image of the synthesized MWCNT/Fe3O4 composite in Fig. 1c shows that the nanoparticles are quasi-spherical and relatively monodisperse with the majority of the particles in the 4– 10 nm size range. The crystallinity of the nanoparticles was assessed by electron diffraction; the ED pattern in Fig. 1d shows well-defined diffraction rings corresponding to the cubic spinel structure of Fe3O4 (Joint Committee on Powder Diffraction Standards, JCPDS, card 19-0629). Likewise, EELS analysis confirmed that the nanoparticles are Fe3O4. Fig. 2a and b depict the oxygen K-edge and iron L-edge EELS spectra of the magnetite nanoparticles, respectively. Both spectra display distinct features whose position and shape are characteristic for Fe3O4 [14]. A comparative energy-loss near-edge structure (ELNES) study of the carbon K-edge was performed on bare CNTs
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Fig. 2 – (a) Oxygen K-edge, (b) iron L-edge, and (c) carbon K-edge spectra of the MWCNT/Fe3O4 composite. For comparison, the carbon K-edge of the MWCNT/Fe3O4 composite (solid black line) is plotted against the carbon K-edge of a bare-untreated MWCNT sample (gray filled spectrum) and a bare-sonicated MWCNT sample (red line). HRTEM images of a (d) bare-untreated MWCNT, (e) bare-sonicated MWCNT, and (f) sonicated MWCNT decorated with a Fe3O4 NP. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
(before and after the sonication treatment) and decorated CNTs. As ELNES probes the unoccupied local projected density of states it can be used to characterize the nature and geometry of the carbon bonds and the overall quality of the CNTs. Fig. 2c shows three EELS spectra obtained from a sonicated CNT sample loaded with Fe3O4 nanoparticles (solid black line), bare-sonicated (solid red line) and bare-untreated (gray filled spectrum) CNTs. During acquisition of the experimental spectra, the probe was scanned in a frame containing several tenths or hundreds of nanotubes in random orientations. Thus, the data obtained is an average over many individual nanostructures and the spectral features are expected to be independent of any polarization dependence [15]. The background in all spectra was subtracted by fitting a decaying power-law function to an energy window just in front of the edge onset. For comparison, the background subtracted spectra were normalized with respect to the peak intensity at 292 eV, which also resulted in the same continuum intensity between 310 and 340 eV for all spectra. All three spectra exhibit the spectral fingerprint of sp2-hybridized carbon with high intensity peaks at 285.4 and 292 eV that correspond to excitation of a 1 s core electron to the unoccupied p* and r* orbitals, respectively. Here, at an energy resolution of 1 eV, we cannot resolve the r* fine structure, in particular the sharp onset due to an excitonic core hole-valence state
interaction, but the broader r* peak at 292 eV due to more delocalized r* states. The spectra obtained from the bare-untreated and baresonicated CNT samples are indeed very similar. The most striking difference between these spectra is found in the relative intensity of the p* and r* peaks. Thus, the p* peak intensity is reduced relative to the r* peak for the sonicated CNTs. A similar behavior was observed in SWCNTs by X-ray absorption spectroscopy (XAS) [16] and was associated with the incorporation and release of C–O and C–H bonds. This is consistent with a destruction of the C–C p and r bonds during the sonication treatment and the attachment of functional groups on the CNT sidewalls. Such sonication-induced structural damage of the CNT graphitic planes is readily apparent in the HRTEM image of Fig. 2e when compared with an untreated CNT sample (Fig. 2d). Yet, another interesting feature of the bare-sonicated CNT spectrum is the presence of a small peak at 288.6 eV, between the p* and r* peaks (highlighted with a red arrow in Fig. 2c). This feature observed on previous XAS studies of graphite oxide [17] and doped graphene [18] supports the presence of oxygenated groups on the sidewalls of the sonicated CNTs. A larger decrease of the p*/r* intensity ratio occurs after loading the CNTs with Fe3O4 nanoparticles suggesting that microwave-assisted CNT functionalization further promotes
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Fig. 3 – (a) Reconstructed phase image of a MWCNT/Fe3O4 interphase. Inset: magnified view of a unit cell from the particle with overlaid model of the Fe3O4 structure along [1 structure along [1 0 1] showing the (light and dark blue) Fe and (yellow) O columns. (b) Reconstructed phase image with peak positions determined by the Gaussian peak fitting function. (c) Corresponding displacement map illustrating no clear displacement pattern. The vector magnitudes are given by the color scale. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
the formation of structural defects and C–O bonds. Besides, the decrease of the p* peak intensity is accompanied by a broadening of the r* peak and by the smearing of the higher energy features above 293 eV characteristic of multi-shell nanotubes (Fig. 2c, black spectrum). The slight decrease of
the p* peak could indicate that a small fraction of the carbon atoms form sp3-type bonds with Fe atoms. Yet, electron diffraction does not show the presence of a distinguishable carbide phase. From the phase contrast image in Fig. 2f it is difficult to obtain atomic-level information on how the Fe3O4 nanoparticle attach to the CNT. This is due to the image delocalization affecting phase contrast micrographs in conventional highresolution electron microscopes, but also to the low electron scattering power of the light carbon atoms. In order to derive direct structural information of such soft–hard matter interfaces, focal series of atomic-resolution micrographs were recorded using an aberration-corrected transmission electron microscope (see Section 2.3) and were processed to retrieve the complex electron wavefunction at the exit plane of the specimen [19,20]. The phase images of the restored exit-plane waves (EPWs) were then used to extract structural information of the metal-oxide/carbon interfaces. These images are directly interpretable and display enhanced bright-atom contrast in the image plane, allowing detection of light-atom columns close to heavy-atom columns. As an example, Fig. 3a shows an EPW phase image of a 5.5 nm magnetite nanoparticle sitting on the surface of a sonicated MWCNT. The particle exhibits the Fd-3 m space group and is oriented with its [101] zone axis perpendicular to the image plane. A magnified view of a unit cell from the particle with an overlaid model is shown in the inset. The cation columns are represented by dark blue (Fe3+) and light blue (Fe2+) spheres, and the oxygen planes of the Fe3O4 parcolumns by yellow spheres. The ð111Þ ticle are parallel to the highly defective sidewalls of the CNT. Incomplete carbon shells indicate partial stripping of the outer graphitic layers. Yet, the graphitic-like structure is more heavily disrupted and bent under the nanoparticle suggesting that nucleation of Fe3O4 on the nanotube surface preferentially occurs at defect sites. Additionally, Fig. 3a also reveals that the nanoparticle exhibits exposed crystalline {1 1 1} surface facets but appears rounded at the interface with the CNT. Although the particle-vacuum interface raises the impression that a structural modification or a reconstruction occurs at the surface of the Fe3O4 particle, one has to consider that Fig. 3a is the result of a focal series reconstruction, which always also represents a time average. As surface atoms are predicted to be most mobile [21], a conclusive picture about the actual surface configuration of the Fe3O4 particle is not possible based on the phase image in Fig. 3a. Unit-cell scale maps of the cation displacements were extracted from reconstructed phase images using Gaussian fitting of atomic columns positions followed by calculation of the relative displacements of these columns and a best-fit lattice obtained using a least-squares routine [22,23]. Fig. 3b shows a magnification of the nanoparticle with peak positions determined by the Gaussian peak fitting function. The obtained displacement map obtained this way (Fig. 3c) exhibits no distinctive displacement pattern. This result, which could be observed in all analyzed interfaces, is not unexpected since the graphitic sheets between the grown metal-oxide nanoparticle and the underlying nanotube are much damaged preventing lattice-mismatch-induced strain. This damage is also readily visible in the carbon elemental map of Fig. 4 as grooves on the CNT surfaces where the
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Fig. 4 – Zero-loss filtered TEM image of the MWCNTs decorated with Fe3O4 nanoparticles and corresponding EFTEM elemental maps of C (green), Fe (red), and O (blue) obtained from the C-K, Fe-M2,3, and O-K edges. The white arrow indicates the presence of grooves on a MWCNT. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
nanoparticles are attached (see, e.g., marked location with a white arrow in Fig. 4). As a matter of fact, the damage of the graphitic layers is so severe that holes occasionally form at the sidewalls of the sonicated CNTs. When this occurs, during microwave synthesis, the iron precursor can penetrate through the open holes into the CNT channel and small Fe3O4 nanoparticles crystallize inside the tubes. One of these particles is highlighted with a red arrow in the zero-loss filtered TEM image of Fig. 5a. Besides, three larger Fe3O4 nanoparticles located on the outer surface of the MWCNT are also present. However, only from simply two-dimensional projected images like the one in Fig. 5a it is very difficult, if not impossible, to precisely determine the location of these small particles with respect to the nanotube structure. In order to obtain the 3D positions of the particles, electron tomography is required [24,25]. This technique was already successfully used to reconstruct the 3D structure of functionalized CNTs [26–29] and other carbonaceous nanocomposites [30]. Fig. 5b shows the volume visualization of the same CNT as the one presented in Fig. 5a. The nanotube exhibits an average outer diameter of 18 nm and a wall thickness of about 6 nm, thus leaving an empty channel of 6 nm. Several slices through the 3D volume reconstruction obtained across the center of the nanotube channel and at the positions marked in Fig. 5b are presented in Fig. 5c and d, respectively. A movie of the full series of orthoslices along the 3D tomography
reconstruction is shown in Movie S1 of the Supplementary data. Our results reveal that a 2.5 nm particle is indeed located inside the nanotube (see Fig. 5c and slice 2 in Fig. 5d) while a larger 6.5 nm particle is found at the outer surface but in close proximity. Moreover, it can be seen that the nanotube sidewall between both particles is not straight but has undergone significant deformation. A groove is clearly visible underneath the large particle significantly reducing the diameter of the tube channel at this position (as shown in slices 2 and 3 in Fig. 5d). We believe that during the microwave synthesis of Fe3O4 the iron precursor penetrated through this damaged area and the small particle crystallized inside the nanotube. Subsequently, the defective graphitic layers provided active nucleation sites for the growth of the large particle found outside the nanotube.
4.
Conclusions
This work shows that sonication treatments can substantially alter the sp2 structure of CNTs by introducing defect sites at their sidewalls, but also by partially removing graphitic layers which in some cases even leads to open holes. We use the differences in the p*/r* intensity ratio seen in the ELNES of the carbon K-edge of three different CNT samples to obtain information about the structure, defects and doping of CNTs. A
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[31], biomedical imaging [32] or even tissue engineering purposes [33].
Acknowledgments Financial support by EMPA and ETH Zu¨rich is gratefully acknowledged. This work was supported by funding from the Swiss COST office under the SBF project number C10.0089. G.I. and R.E. acknowledge support by the Swiss National Science Foundation under project number 200021_134571. Part of this work was performed at the Karlsruhe Nano Micro Facility (KNMF, www.kit.edu/knmf) of the Forschungszentrum Karlsruhe which is acknowledged for provision of access to the FEI Titan 80-300 microscope at their laboratories.
Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.carbon. 2013.05.024.
R E F E R E N C E S
Fig. 5 – (a) Zero-loss filtered TEM image of a MWCNT decorated with three Fe3O4 nanoparticles. The red arrow points at a 2.5 nm particle located inside the nanotube. (b) Voxel projection view from the tomographically reconstructed nanotube using zero-loss filtered TEM images. (c) Orthoslice view across the center of the MWCNT channel. (d) Orthoslices through the 3D tomography reconstruction obtained at different positions as marked in panel b. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
significant decrease of the p*/r* intensity ratio and the presence of a small feature between the p* and r* peaks of the sonicated CNTs assert the formation of functional groups on the CNT sidewalls. Besides, by using aberration-corrected TEM imaging we reveal that the nanotube crystal structure is most altered at the metal-oxide/carbon interfaces. This suggests that defects act as active sites for chemical reaction during the Fe3O4 nucleation. Although sonication- and microwave-assisted synthesis offers a fast and efficient route to produce MWCNT/Fe3O4 composites with highly crystalline nanoparticles, it is important to realize that there may be drawbacks of this approach. For example, the defective structure of the MWCNTs might compromise the mechanical properties of the nanotubes and reduce the strength of the composites preventing their use as reinforcing agents. Likewise, modifications of the sp2 structure might also have a direct impact on the electrical and thermal transport properties of CNTs since defect sites scatter electrons and phonons. However, these magnetic hybrid nanostructures might hold great potential for medical applications, in particular for magnetic guided drug delivery
[1] Chusuei CC, Wayu M. Characterizing functionalized carbon nanotubes for improved fabrication in aqueous solution environments. In: Marulanda JM, editor. Electronic properties of carbon nanotubes. InTech; Rijeka, 2011. p. 57–68. [2] Hwang S-H, Park Y-B, Yoon KH, Bang DS. Smart materials and structures based on carbon nanotube composites. In: Yellampalli S, editor. Carbon nanotubes – synthesis, characterization, applications. InTech; Rijeka, 2011. p. 371–96. [3] Balasubramanian K, Burghard M. Chemically functionalized carbon nanotubes. Small 2005;1:180–92. [4] Lu KL, Lago RM, Chen YK, Green MLH, Harris PJF, Tsang SC. Mechanical damage of carbon nanotubes by ultrasound. Carbon 1996;34:814–6. [5] Rodrı´guez-Manzo JA, Bahnhart F, Terrones M, Terrones H, Grobert N, Ajayan PM, et al. Heterojunctions between metals and carbon nanotubes as ultimate nanocontacts. PNAS 2009;12:4591–5. [6] Pohl D, Scha¨ffel F, Ru¨mmeli MH, Mohn E, Ta¨schner C, Schultz L, et al. Understanding the metal–carbon interface in FePt catalyzed carbon nanotubes. Phys Rev Lett 2011;107:185501. [7] Jensen K, Mickelson W, Han W, Zettl A. Current-controlled nanotube growth and zone refinement. Appl Phys Lett 2005;86:173107. [8] Yoshida H, Takeda S, Uchiyama T, Kohno H, Homma Y. Atomic-scale in situ observation of carbon nanotube growth from solid state iron carbide nanoparticles. Nano Lett 2008;8:2082–6. [9] Bilecka I, Djerdj I, Niederberger M. One-minute synthesis of crystalline binary and ternary metal oxide nanoparticles. Chem Commun 2008;7:886–8. [10] Pinna N, Niederberger M. Surfactant-free nonaqueous synthesis of metal oxide nanostructures. Angew Chem Int Ed 2008;47:5292–304. [11] Li Z, Chen H, Bao H, Gao M. One-pot reaction to synthesize water-soluble magnetite nanocrystals. Chem Mater 2004;16:1391–3.
CARBON
6 1 (2 0 1 3) 4 0 4–41 1
[12] Giordani S, Bergin S, Nicolosi V, Lebedkin S, Blau WJ, Coleman JN. Fabrication of stable dispersions containing up to 70% individual carbon nanotubes in a common organic solvent. Phys Stat Sol (b) 2006;243:3058–62. [13] Yin W-J, Wei S-H, Ban C, Wu Z, Al-Jassim MM, Yan Y. Origin of bonding between the SWCNT and the Fe3O4(001) surface and the enhanced electrical conductivity. J Phys Chem Lett 2011;2:2853–8. [14] Colliex C, Manoubi T, Ortiz C. Electron-energy-lossspectroscopy near-edge fine structure in the iron–oxygen system. Phys Rev B 1991;44:11402–11. [15] Najafi E, Hitchcock AP, Rossouw D, Botton GA. Mapping defects in a carbon nanotube by momentum transfer dependent electron energy loss spectroscopy. Ultramicroscopy 2012;113:158–64. [16] Abbas M, Wu ZY, Zhong J, Ibrahim K, Fiori A, Orlanducci S, et al. X-ray absorption and photoelectron spectroscopy studies on graphite and single-walled carbon nanotubes: oxygen effect. Appl Phys Lett 2005;87:051923. [17] Jeong HK, Noh HJ, Kim JY, Jin MH, Park CY, Lee YH. X-ray absorption spectroscopy of graphite oxide. EPL 2008;82:67004. [18] Schultz BJ, Patridge CJ, Lee V, Jaye C, Lysaght PS, et al. Imaging local electronic corrugations and doped regions in graphene. Nat Commun 2011;2:372. [19] Schiske P. Zur Frage der Bildrekonstruktion durch Fokusreihen. In: Proceedings of the 4th European Region Conference on Electron Microscopy, vol 1. Rome, 1968; p. 145– 146. [20] Kirkland EJ, Siegel BM, Uyeda N, Fujiyoshi Y. Digital reconstruction of bright field phase-contrast images from high-resolution electron-micrographs. Ultramicroscopy 1980;5:479–503. [21] Erni R, Rossell MD, Hartel P, Alem N, Erickson K, Gannett W, et al. Stability and dynamics of small molecules trapped on graphene. Phys Rev B 2010;82:165443. [22] Polking MJ, Han M-G, Yourdkhani A, Petkov V, Kisielowski CF, Volkov VV, et al. Ferroelectric order in individual nanometrescale crystals. Nat Mater 2012;11:700–9.
411
[23] Lubk A, Rossell MD, Seidel J, He Q, Yang SY, Chu YH, et al. Evidence of sharp and diffuse domain walls in BiFeO3 by means of unit-cell-wise strain and polarization maps obtained with high resolution scanning transmission electron microscopy. Phys Rev Lett 2012;109:047601. [24] Midgley PA, Weyland M. 3D electron microscopy in the physical sciences: the development of Z-contrast and EFTEM tomography. Ultramicroscopy 2003;96:413–31. [25] Midgley PA, Dunin-Borkowski RE. Electron tomography and holography in materials science. Nat Mater 2009;8:271–80. [26] Ersen O, Werckmann J, Houlle´ M, Ledoux M-J, Pham-Huu C. 3D electron microscopy study of metal particles inside multiwalled carbon nanotubes. Nano Lett 2007;7:1898–907. [27] Cha JJ, Weyland M, Briere J-F, Daykov IP, Arias TA, Muller DA. Three-dimensional imaging of carbon nanotubes deformed by metal islands. Nano Lett 2007;7:3770–3. [28] Tessonnier J-P, Ersen O, Weinberg G, Pham-Huu C, Su DS, Schlo¨gl R. Selective deposition of metal nanoparticles inside or outside multiwalled carbon nanotubes. ACS Nano 2009;8:2089–91. [29] He Z, Ke X, Bals S, Van Tendeloo G. Direct evidence for the existence of multi-walled carbon nanotubes with hexagonal cross-sections. Carbon 2012;50:2524–9. [30] Gass MH, Koziol KKK, Windle AH, Midgley PA. Fourdimensional spectral tomography of carbonaceous nanocomposites. Nano Lett 2006;6:376–9. [31] Yang F, Hu JH, Yang D, Long J, Luo GP, Jin C, et al. Pilot study of targeting magnetic carbon nanostructures to lymph nodes. Nanomedicine 2009;4:317–30. [32] Choi JH, Nguyen FT, Barone PW, Heller DA, Moll AE, Patel D, et al. Multimodal biomedical imaging with asymmetric single-walled carbon nanotube/iron oxide nanoparticle complexes. Nano Lett 2007;7:861–7. [33] Cunha C, Panseri S, Iannazzo D, Piperno A, Pistone A, Fazio M, et al. Hybrid composites made of multiwalled carbon nanotubes functionalized with Fe3O4 nanoparticles for tissue engineering applications. Nanotechnology 2012;23:465102.
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journal homepage: www.elsevier.com/locate/carbon
Impact of sonication pretreatment on carbon nanotubes: A transmission electron microscopy study Marta D. Rossell a,b,*, Christian Kuebel c,d, Gabriele Ilari a, Felix Rechberger b, Florian J. Heiligtag b, Markus Niederberger b, Dorota Koziej b, Rolf Erni a a
Electron Microscopy Center, Empa, Swiss Federal Laboratories for Materials Science and Technology, U¨berlandstrasse 129, 8600 Du¨bendorf, Switzerland b Laboratory for Multifunctional Materials, Department of Materials, ETH Zu¨rich, Wolfgang-Pauli-Strasse 10, 8093 Zu¨rich, Switzerland c Institute of Nanotechnology, Karlsruhe Institute of Technology, Campus North, Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany d Karlsruhe Nano Micro Facility, Karlsruhe Institute of Technology, Campus North, Hermann-von-Helmholtz-Platz 1, 76344 EggensteinLeopoldshafen, Germany
A R T I C L E I N F O
A B S T R A C T
Article history:
Sonication treatments are commonly used for debundling and dispersing carbon nano-
Received 22 March 2013
tubes (CNTs) in liquid media prior to chemical functionalization. However, this step may
Accepted 8 May 2013
lead to the stripping of the outer graphitic layers and the scission of the CNTs, and can
Available online 21 May 2013
therefore have a deleterious effect on the achievable properties of the functionalized CNTs. Thus, knowledge on the structural integrity of the modified CNTs is required to understand its influence on the device performance of hybrid nanocarbon-based composites. Here we report on the impact of a sonication pretreatment on the structure of multiwalled CNTs, and on the role of the induced modifications on the subsequent attachment of ferrimagnetic Fe3O4 nanoparticles. Decoration of the CNTs with Fe3O4 nanoparticles is achieved by a microwave-assisted synthesis route involving the reaction of iron acetylacetonate with 2-pyrrolidinone. Employing a combination of atomic resolution transmission electron microscopy, electron energy-loss spectroscopy, energy-filtered transmission electron microscopy and electron tomography, we provide evidence that significant degradation of the CNT structure takes place during the dispersion process. Moreover we find that the sp2 system is more heavily disrupted at the interface between the CNTs and the surface-deposited nanoparticles suggesting that nucleation of Fe3O4 preferentially occurs at the nanotube defect sites. Ó 2013 Elsevier Ltd. All rights reserved.
1.
Introduction
The outstanding structural, mechanical and electronic properties of carbon nanotubes (CNTs) make them very
attractive for various applications in modern nanodevices. But in order to take full advantage of their properties and to extend the scope of their application spectrum, CNTs often need to be modified and integrated with other materi-
* Corresponding author at: Electron Microscopy Center, Empa, Swiss Federal Laboratories for Materials Science and Technology, ¨ berlandstrasse 129, 8600 Du¨bendorf, Switzerland. U E-mail address: [email protected] (M.D. Rossell). 0008-6223/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.carbon.2013.05.024
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als. A crucial step towards modification of CNTs is the functionalization of their relatively inert sidewalls by attachment of reactive surface species. In particular, covalent modification strategies are frequently used to functionalize CNTs and are associated with a transformation of sp2into sp3-hybridized carbon atoms [1]. As such, both chemical modification by strong acids and ultrasonication pretreatments alter the tube walls by introducing a considerable amount of defects and dangling bonds [2]. These treatments are commonly used together because sonication, apart from successfully debundling and dispersing CNTs, has proven to be highly effective in increasing the formation of defect sites for the attachment of functional groups. Thus, various functional groups (such as hydroxyl, carbonyl, carboxyl, amine, fluorine, etc.) can covalently attach to these locations providing active nucleation sites for high-loading of nanoparticles [3]. However, care has to be taken because prolonged sonication even in inert media causes fractionation of the multiwalled CNTS (MWCNTs) and stripping of the outer graphitic layers [4]. With increasing sonication times MWCNTs get shorter and thinner, and ultimately transform into amorphous carbon. Obviously, such detrimental structures disrupt the p electron system of the CNTs resulting in severe degradation of their charge-carrier mobility as well as their mechanical properties, thereby hampering the potential of the covalent hybrid structures for practical applications. Hence, the study of the interfaces between CNTs and metal or metal-oxide particles is of particular interest, not only in regard of their intrinsic structure and properties, but also to understand the resulting device performance. For this, transmission electron microscopy (TEM) is ideally suited as it provides the spatial resolution to study the structure of materials at atomic level. Besides, apart from the actual image information, spectral information can also be obtained to study the electronic structure of the CNTs. However, only little is known about the structure of functionalized nanotubes at the atomic scale [5,6], and most of the available reports explore the growth of nanotubes from catalytically active metal particles where the nucleation and growth of the tube occur through such an interface [6–8]. In this work we study the impact of a sonication treatment for the preparation of a stable dispersion of MWCNTs in 2-pyrrolidinone on the nanotube sidewall integrity. Subsequently, the attachment of iron-oxide nanoparticles is achieved by a simple and rapid microwave-assisted synthesis route. The magnetic functionalization of CNTs with Fe3O4 nanoparticles provides hence a mean to control the position and orientation of the nanotubes by using a magnetic field. We combine aberration-corrected high-resolution transmission electron microscopy with electron energy loss spectroscopy and electron tomography to learn more on the atomic and electronic structure of these technologically important metal-oxide/carbon interfaces. Our results show evidence that the sp2 nanotube structure is heavily disrupted, especially at the interfaces with the Fe3O4 nanoparticles.
2.
Experimental
2.1.
Materials
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MWCNTs grown by catalytic chemical vapor deposition having an average outer diameter of 15 nm and carbon purity exceeding 95% were purchased from Nanocyl S.A. (Belgium). Fe(III) acetylacetonate (P99.9%) and 2-pyrrolidinone (99%) were supplied by Sigma–Aldrich. All the chemicals were used without further purification.
2.2.
Synthesis of the Fe3O4 nanoparticles on MWCNTs
Crystalline Fe3O4 nanoparticles on MWCNTs were synthesized using a microwave-assisted non-aqueous sol–gel method in a CEM Discover reactor operating at a frequency of 2.45 GHz [9,10]. The synthesis described below involving the reaction of iron acetylacetonate with 2-pyrrolidinone is based on a previously published route [11] which we extended to microwave heating. Similar to N-methyl-2-pyrrolidinone [12] also 2-pyrrolidinone is a suitable solvent for the preparation of stable carbon nanotube dispersions. In the first step of the procedure 5 mL of 2-pyrrolidinone with 5.6 mg of MWCNTs was sonicated in a cup-horn type Branson Sonifier 250 sonicator operating at 20 kHz during 10 min until a black dispersion was obtained. The resulting dispersion was kept in nitrogen flow for 10 min and transferred into a glovebox under argon atmosphere (O2 and H2O <0.1 ppm). In the glovebox, 60 mg of Fe(III) acetylacetonate was dissolved in 0.75 mL of 2pyrrolidinone and mixed with the pre-sonicated 2-pyrrolidinone dispersion of MWCNTs. The reaction mixture was transferred into a 10 mL glass tube and sealed with a Teflon cap. During microwave heating the reaction was stirred with a stir bar. The heat treatment was performed at 200 °C for 5 min. The hybrid material was extracted by centrifugation and washed three times with ethanol. Finally, the CNTs decorated with Fe3O4 nanoparticles were dispersed in ethanol and a drop of the suspension was air-dried onto a lacey carboncoated copper grid for TEM characterization. Magnetic separability of the sample was tested in ethanol solution by placing a magnet near the glass bottle, as shown in Fig. 1a and b. The black powder was strongly attracted towards the magnet in a few seconds, demonstrating high magnetic sensitivity.
2.3.
Instrumentation and computational details
Electron diffraction (ED), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), electron energy-loss spectroscopy (EELS), and energy-filtered transmission electron microscopy (EFTEM) were performed using a JEOL 2200FS TEM/STEM microscope operated at 200 keV and equipped with an in-column Omega-type energy filter. For the EELS data acquisition, the convergence and collection semiangles were set to 10.8 and 10 mrad, respectively. For these values, the energy resolution measured as the full width at half maximum of the zero-loss peak is 1 eV and the dispersion was set to 0.16 eV/channel.
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8, 30 and 30 eV energy slits and 2, 50 and 15 s acquisition times, respectively. For electron tomography a JEOL single axis tomography holder was used and zero-loss filtered TEM images were manually recorded at 2° tilt intervals between 30° and +30°, and at 1° tilt intervals elsewhere over a range from 73° to 73°. The images were aligned using the StackReg plugin for the image processing software ImageJ. The 3D volume reconstruction was computed using 50 cycles of the simultaneous iterative reconstruction technique (SIRT) implemented in TomoJ, which is also an ImageJ plugin. Finally, the 3D voxel projection and the orthoslices were generated with the Amira visualization program. The HRTEM data in Fig. 3 was obtained at 300 keV on a FEI Titan 80-300 microscope equipped with a third-order spherical aberration corrector on the imaging lens system. Through-focal series of crystalline Fe3O4 nanoparticles on MWCNTs were recorded employing a focus step of 0.45 nm, exposure times of 1 s, and a 23.25 pm/pixel sampling. The MacTempas (TotalResolution) software was used to reconstruct the exit-plane waves (EPWs) according to the experimental parameters and to determine the cation displacements in the EPW phase images. Atomic positions were extracted by using the peak finding routine and fitting them as two dimensional Gaussian peaks. Displacements were calculated as a difference vector between each cation and the best-fit lattice calculated using a least-squares routine. Visualization of the local atomic displacements was carried out using a custom Mathematica code. Although 200 and 300 keV electrons used in the present TEM study carry sufficient energy to cause knock-on damage of carbon nanomaterials, in all experiments performed the electron dose was kept small enough to minimize the impact of radiation damage. The success of this approach is confirmed in the imaging study of the pristine MWCNTs, which does not reveal local damage of the graphitic structure.
3.
Fig. 1 – (a) Photograph of the synthesized MWCNT/Fe3O4 composite dispersed in ethanol, and (b) its response to a magnet. (c) TEM image of the composite with insert showing a histogram of the particle size distribution. (d) Corresponding ED pattern. The Fe3O4 and the MWCNT reflections are indicated in white and black, respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this book.)
Zero-loss filtered images were recorded with a 15 eV energy slit. EFTEM was applied to analyze the elemental distribution of Fe, O and C. The collection angle of the EFTEM images was 25 mrad. The conventional three-window technique was used to obtain element specific images. Fe, O and C elemental maps were obtained from the Fe-M2,3, O-K and C-K edges with
Results and discussion
Sonication- and microwave-assisted functionalization of MWCNTs with magnetite nanoparticles leads to discontinuous coverage of the nanotube surface. This is in agreement with reported density functional theory studies revealing that the interactions between single-walled carbon nanotubes (SWCNTs) and Fe3O4 are very weak [13]. The TEM image of the synthesized MWCNT/Fe3O4 composite in Fig. 1c shows that the nanoparticles are quasi-spherical and relatively monodisperse with the majority of the particles in the 4– 10 nm size range. The crystallinity of the nanoparticles was assessed by electron diffraction; the ED pattern in Fig. 1d shows well-defined diffraction rings corresponding to the cubic spinel structure of Fe3O4 (Joint Committee on Powder Diffraction Standards, JCPDS, card 19-0629). Likewise, EELS analysis confirmed that the nanoparticles are Fe3O4. Fig. 2a and b depict the oxygen K-edge and iron L-edge EELS spectra of the magnetite nanoparticles, respectively. Both spectra display distinct features whose position and shape are characteristic for Fe3O4 [14]. A comparative energy-loss near-edge structure (ELNES) study of the carbon K-edge was performed on bare CNTs
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Fig. 2 – (a) Oxygen K-edge, (b) iron L-edge, and (c) carbon K-edge spectra of the MWCNT/Fe3O4 composite. For comparison, the carbon K-edge of the MWCNT/Fe3O4 composite (solid black line) is plotted against the carbon K-edge of a bare-untreated MWCNT sample (gray filled spectrum) and a bare-sonicated MWCNT sample (red line). HRTEM images of a (d) bare-untreated MWCNT, (e) bare-sonicated MWCNT, and (f) sonicated MWCNT decorated with a Fe3O4 NP. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
(before and after the sonication treatment) and decorated CNTs. As ELNES probes the unoccupied local projected density of states it can be used to characterize the nature and geometry of the carbon bonds and the overall quality of the CNTs. Fig. 2c shows three EELS spectra obtained from a sonicated CNT sample loaded with Fe3O4 nanoparticles (solid black line), bare-sonicated (solid red line) and bare-untreated (gray filled spectrum) CNTs. During acquisition of the experimental spectra, the probe was scanned in a frame containing several tenths or hundreds of nanotubes in random orientations. Thus, the data obtained is an average over many individual nanostructures and the spectral features are expected to be independent of any polarization dependence [15]. The background in all spectra was subtracted by fitting a decaying power-law function to an energy window just in front of the edge onset. For comparison, the background subtracted spectra were normalized with respect to the peak intensity at 292 eV, which also resulted in the same continuum intensity between 310 and 340 eV for all spectra. All three spectra exhibit the spectral fingerprint of sp2-hybridized carbon with high intensity peaks at 285.4 and 292 eV that correspond to excitation of a 1 s core electron to the unoccupied p* and r* orbitals, respectively. Here, at an energy resolution of 1 eV, we cannot resolve the r* fine structure, in particular the sharp onset due to an excitonic core hole-valence state
interaction, but the broader r* peak at 292 eV due to more delocalized r* states. The spectra obtained from the bare-untreated and baresonicated CNT samples are indeed very similar. The most striking difference between these spectra is found in the relative intensity of the p* and r* peaks. Thus, the p* peak intensity is reduced relative to the r* peak for the sonicated CNTs. A similar behavior was observed in SWCNTs by X-ray absorption spectroscopy (XAS) [16] and was associated with the incorporation and release of C–O and C–H bonds. This is consistent with a destruction of the C–C p and r bonds during the sonication treatment and the attachment of functional groups on the CNT sidewalls. Such sonication-induced structural damage of the CNT graphitic planes is readily apparent in the HRTEM image of Fig. 2e when compared with an untreated CNT sample (Fig. 2d). Yet, another interesting feature of the bare-sonicated CNT spectrum is the presence of a small peak at 288.6 eV, between the p* and r* peaks (highlighted with a red arrow in Fig. 2c). This feature observed on previous XAS studies of graphite oxide [17] and doped graphene [18] supports the presence of oxygenated groups on the sidewalls of the sonicated CNTs. A larger decrease of the p*/r* intensity ratio occurs after loading the CNTs with Fe3O4 nanoparticles suggesting that microwave-assisted CNT functionalization further promotes
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Fig. 3 – (a) Reconstructed phase image of a MWCNT/Fe3O4 interphase. Inset: magnified view of a unit cell from the particle with overlaid model of the Fe3O4 structure along [1 structure along [1 0 1] showing the (light and dark blue) Fe and (yellow) O columns. (b) Reconstructed phase image with peak positions determined by the Gaussian peak fitting function. (c) Corresponding displacement map illustrating no clear displacement pattern. The vector magnitudes are given by the color scale. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
the formation of structural defects and C–O bonds. Besides, the decrease of the p* peak intensity is accompanied by a broadening of the r* peak and by the smearing of the higher energy features above 293 eV characteristic of multi-shell nanotubes (Fig. 2c, black spectrum). The slight decrease of
the p* peak could indicate that a small fraction of the carbon atoms form sp3-type bonds with Fe atoms. Yet, electron diffraction does not show the presence of a distinguishable carbide phase. From the phase contrast image in Fig. 2f it is difficult to obtain atomic-level information on how the Fe3O4 nanoparticle attach to the CNT. This is due to the image delocalization affecting phase contrast micrographs in conventional highresolution electron microscopes, but also to the low electron scattering power of the light carbon atoms. In order to derive direct structural information of such soft–hard matter interfaces, focal series of atomic-resolution micrographs were recorded using an aberration-corrected transmission electron microscope (see Section 2.3) and were processed to retrieve the complex electron wavefunction at the exit plane of the specimen [19,20]. The phase images of the restored exit-plane waves (EPWs) were then used to extract structural information of the metal-oxide/carbon interfaces. These images are directly interpretable and display enhanced bright-atom contrast in the image plane, allowing detection of light-atom columns close to heavy-atom columns. As an example, Fig. 3a shows an EPW phase image of a 5.5 nm magnetite nanoparticle sitting on the surface of a sonicated MWCNT. The particle exhibits the Fd-3 m space group and is oriented with its [101] zone axis perpendicular to the image plane. A magnified view of a unit cell from the particle with an overlaid model is shown in the inset. The cation columns are represented by dark blue (Fe3+) and light blue (Fe2+) spheres, and the oxygen planes of the Fe3O4 parcolumns by yellow spheres. The ð111Þ ticle are parallel to the highly defective sidewalls of the CNT. Incomplete carbon shells indicate partial stripping of the outer graphitic layers. Yet, the graphitic-like structure is more heavily disrupted and bent under the nanoparticle suggesting that nucleation of Fe3O4 on the nanotube surface preferentially occurs at defect sites. Additionally, Fig. 3a also reveals that the nanoparticle exhibits exposed crystalline {1 1 1} surface facets but appears rounded at the interface with the CNT. Although the particle-vacuum interface raises the impression that a structural modification or a reconstruction occurs at the surface of the Fe3O4 particle, one has to consider that Fig. 3a is the result of a focal series reconstruction, which always also represents a time average. As surface atoms are predicted to be most mobile [21], a conclusive picture about the actual surface configuration of the Fe3O4 particle is not possible based on the phase image in Fig. 3a. Unit-cell scale maps of the cation displacements were extracted from reconstructed phase images using Gaussian fitting of atomic columns positions followed by calculation of the relative displacements of these columns and a best-fit lattice obtained using a least-squares routine [22,23]. Fig. 3b shows a magnification of the nanoparticle with peak positions determined by the Gaussian peak fitting function. The obtained displacement map obtained this way (Fig. 3c) exhibits no distinctive displacement pattern. This result, which could be observed in all analyzed interfaces, is not unexpected since the graphitic sheets between the grown metal-oxide nanoparticle and the underlying nanotube are much damaged preventing lattice-mismatch-induced strain. This damage is also readily visible in the carbon elemental map of Fig. 4 as grooves on the CNT surfaces where the
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Fig. 4 – Zero-loss filtered TEM image of the MWCNTs decorated with Fe3O4 nanoparticles and corresponding EFTEM elemental maps of C (green), Fe (red), and O (blue) obtained from the C-K, Fe-M2,3, and O-K edges. The white arrow indicates the presence of grooves on a MWCNT. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
nanoparticles are attached (see, e.g., marked location with a white arrow in Fig. 4). As a matter of fact, the damage of the graphitic layers is so severe that holes occasionally form at the sidewalls of the sonicated CNTs. When this occurs, during microwave synthesis, the iron precursor can penetrate through the open holes into the CNT channel and small Fe3O4 nanoparticles crystallize inside the tubes. One of these particles is highlighted with a red arrow in the zero-loss filtered TEM image of Fig. 5a. Besides, three larger Fe3O4 nanoparticles located on the outer surface of the MWCNT are also present. However, only from simply two-dimensional projected images like the one in Fig. 5a it is very difficult, if not impossible, to precisely determine the location of these small particles with respect to the nanotube structure. In order to obtain the 3D positions of the particles, electron tomography is required [24,25]. This technique was already successfully used to reconstruct the 3D structure of functionalized CNTs [26–29] and other carbonaceous nanocomposites [30]. Fig. 5b shows the volume visualization of the same CNT as the one presented in Fig. 5a. The nanotube exhibits an average outer diameter of 18 nm and a wall thickness of about 6 nm, thus leaving an empty channel of 6 nm. Several slices through the 3D volume reconstruction obtained across the center of the nanotube channel and at the positions marked in Fig. 5b are presented in Fig. 5c and d, respectively. A movie of the full series of orthoslices along the 3D tomography
reconstruction is shown in Movie S1 of the Supplementary data. Our results reveal that a 2.5 nm particle is indeed located inside the nanotube (see Fig. 5c and slice 2 in Fig. 5d) while a larger 6.5 nm particle is found at the outer surface but in close proximity. Moreover, it can be seen that the nanotube sidewall between both particles is not straight but has undergone significant deformation. A groove is clearly visible underneath the large particle significantly reducing the diameter of the tube channel at this position (as shown in slices 2 and 3 in Fig. 5d). We believe that during the microwave synthesis of Fe3O4 the iron precursor penetrated through this damaged area and the small particle crystallized inside the nanotube. Subsequently, the defective graphitic layers provided active nucleation sites for the growth of the large particle found outside the nanotube.
4.
Conclusions
This work shows that sonication treatments can substantially alter the sp2 structure of CNTs by introducing defect sites at their sidewalls, but also by partially removing graphitic layers which in some cases even leads to open holes. We use the differences in the p*/r* intensity ratio seen in the ELNES of the carbon K-edge of three different CNT samples to obtain information about the structure, defects and doping of CNTs. A
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[31], biomedical imaging [32] or even tissue engineering purposes [33].
Acknowledgments Financial support by EMPA and ETH Zu¨rich is gratefully acknowledged. This work was supported by funding from the Swiss COST office under the SBF project number C10.0089. G.I. and R.E. acknowledge support by the Swiss National Science Foundation under project number 200021_134571. Part of this work was performed at the Karlsruhe Nano Micro Facility (KNMF, www.kit.edu/knmf) of the Forschungszentrum Karlsruhe which is acknowledged for provision of access to the FEI Titan 80-300 microscope at their laboratories.
Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.carbon. 2013.05.024.
R E F E R E N C E S
Fig. 5 – (a) Zero-loss filtered TEM image of a MWCNT decorated with three Fe3O4 nanoparticles. The red arrow points at a 2.5 nm particle located inside the nanotube. (b) Voxel projection view from the tomographically reconstructed nanotube using zero-loss filtered TEM images. (c) Orthoslice view across the center of the MWCNT channel. (d) Orthoslices through the 3D tomography reconstruction obtained at different positions as marked in panel b. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
significant decrease of the p*/r* intensity ratio and the presence of a small feature between the p* and r* peaks of the sonicated CNTs assert the formation of functional groups on the CNT sidewalls. Besides, by using aberration-corrected TEM imaging we reveal that the nanotube crystal structure is most altered at the metal-oxide/carbon interfaces. This suggests that defects act as active sites for chemical reaction during the Fe3O4 nucleation. Although sonication- and microwave-assisted synthesis offers a fast and efficient route to produce MWCNT/Fe3O4 composites with highly crystalline nanoparticles, it is important to realize that there may be drawbacks of this approach. For example, the defective structure of the MWCNTs might compromise the mechanical properties of the nanotubes and reduce the strength of the composites preventing their use as reinforcing agents. Likewise, modifications of the sp2 structure might also have a direct impact on the electrical and thermal transport properties of CNTs since defect sites scatter electrons and phonons. However, these magnetic hybrid nanostructures might hold great potential for medical applications, in particular for magnetic guided drug delivery
[1] Chusuei CC, Wayu M. Characterizing functionalized carbon nanotubes for improved fabrication in aqueous solution environments. In: Marulanda JM, editor. Electronic properties of carbon nanotubes. InTech; Rijeka, 2011. p. 57–68. [2] Hwang S-H, Park Y-B, Yoon KH, Bang DS. Smart materials and structures based on carbon nanotube composites. In: Yellampalli S, editor. Carbon nanotubes – synthesis, characterization, applications. InTech; Rijeka, 2011. p. 371–96. [3] Balasubramanian K, Burghard M. Chemically functionalized carbon nanotubes. Small 2005;1:180–92. [4] Lu KL, Lago RM, Chen YK, Green MLH, Harris PJF, Tsang SC. Mechanical damage of carbon nanotubes by ultrasound. Carbon 1996;34:814–6. [5] Rodrı´guez-Manzo JA, Bahnhart F, Terrones M, Terrones H, Grobert N, Ajayan PM, et al. Heterojunctions between metals and carbon nanotubes as ultimate nanocontacts. PNAS 2009;12:4591–5. [6] Pohl D, Scha¨ffel F, Ru¨mmeli MH, Mohn E, Ta¨schner C, Schultz L, et al. Understanding the metal–carbon interface in FePt catalyzed carbon nanotubes. Phys Rev Lett 2011;107:185501. [7] Jensen K, Mickelson W, Han W, Zettl A. Current-controlled nanotube growth and zone refinement. Appl Phys Lett 2005;86:173107. [8] Yoshida H, Takeda S, Uchiyama T, Kohno H, Homma Y. Atomic-scale in situ observation of carbon nanotube growth from solid state iron carbide nanoparticles. Nano Lett 2008;8:2082–6. [9] Bilecka I, Djerdj I, Niederberger M. One-minute synthesis of crystalline binary and ternary metal oxide nanoparticles. Chem Commun 2008;7:886–8. [10] Pinna N, Niederberger M. Surfactant-free nonaqueous synthesis of metal oxide nanostructures. Angew Chem Int Ed 2008;47:5292–304. [11] Li Z, Chen H, Bao H, Gao M. One-pot reaction to synthesize water-soluble magnetite nanocrystals. Chem Mater 2004;16:1391–3.
CARBON
6 1 (2 0 1 3) 4 0 4–41 1
[12] Giordani S, Bergin S, Nicolosi V, Lebedkin S, Blau WJ, Coleman JN. Fabrication of stable dispersions containing up to 70% individual carbon nanotubes in a common organic solvent. Phys Stat Sol (b) 2006;243:3058–62. [13] Yin W-J, Wei S-H, Ban C, Wu Z, Al-Jassim MM, Yan Y. Origin of bonding between the SWCNT and the Fe3O4(001) surface and the enhanced electrical conductivity. J Phys Chem Lett 2011;2:2853–8. [14] Colliex C, Manoubi T, Ortiz C. Electron-energy-lossspectroscopy near-edge fine structure in the iron–oxygen system. Phys Rev B 1991;44:11402–11. [15] Najafi E, Hitchcock AP, Rossouw D, Botton GA. Mapping defects in a carbon nanotube by momentum transfer dependent electron energy loss spectroscopy. Ultramicroscopy 2012;113:158–64. [16] Abbas M, Wu ZY, Zhong J, Ibrahim K, Fiori A, Orlanducci S, et al. X-ray absorption and photoelectron spectroscopy studies on graphite and single-walled carbon nanotubes: oxygen effect. Appl Phys Lett 2005;87:051923. [17] Jeong HK, Noh HJ, Kim JY, Jin MH, Park CY, Lee YH. X-ray absorption spectroscopy of graphite oxide. EPL 2008;82:67004. [18] Schultz BJ, Patridge CJ, Lee V, Jaye C, Lysaght PS, et al. Imaging local electronic corrugations and doped regions in graphene. Nat Commun 2011;2:372. [19] Schiske P. Zur Frage der Bildrekonstruktion durch Fokusreihen. In: Proceedings of the 4th European Region Conference on Electron Microscopy, vol 1. Rome, 1968; p. 145– 146. [20] Kirkland EJ, Siegel BM, Uyeda N, Fujiyoshi Y. Digital reconstruction of bright field phase-contrast images from high-resolution electron-micrographs. Ultramicroscopy 1980;5:479–503. [21] Erni R, Rossell MD, Hartel P, Alem N, Erickson K, Gannett W, et al. Stability and dynamics of small molecules trapped on graphene. Phys Rev B 2010;82:165443. [22] Polking MJ, Han M-G, Yourdkhani A, Petkov V, Kisielowski CF, Volkov VV, et al. Ferroelectric order in individual nanometrescale crystals. Nat Mater 2012;11:700–9.
411
[23] Lubk A, Rossell MD, Seidel J, He Q, Yang SY, Chu YH, et al. Evidence of sharp and diffuse domain walls in BiFeO3 by means of unit-cell-wise strain and polarization maps obtained with high resolution scanning transmission electron microscopy. Phys Rev Lett 2012;109:047601. [24] Midgley PA, Weyland M. 3D electron microscopy in the physical sciences: the development of Z-contrast and EFTEM tomography. Ultramicroscopy 2003;96:413–31. [25] Midgley PA, Dunin-Borkowski RE. Electron tomography and holography in materials science. Nat Mater 2009;8:271–80. [26] Ersen O, Werckmann J, Houlle´ M, Ledoux M-J, Pham-Huu C. 3D electron microscopy study of metal particles inside multiwalled carbon nanotubes. Nano Lett 2007;7:1898–907. [27] Cha JJ, Weyland M, Briere J-F, Daykov IP, Arias TA, Muller DA. Three-dimensional imaging of carbon nanotubes deformed by metal islands. Nano Lett 2007;7:3770–3. [28] Tessonnier J-P, Ersen O, Weinberg G, Pham-Huu C, Su DS, Schlo¨gl R. Selective deposition of metal nanoparticles inside or outside multiwalled carbon nanotubes. ACS Nano 2009;8:2089–91. [29] He Z, Ke X, Bals S, Van Tendeloo G. Direct evidence for the existence of multi-walled carbon nanotubes with hexagonal cross-sections. Carbon 2012;50:2524–9. [30] Gass MH, Koziol KKK, Windle AH, Midgley PA. Fourdimensional spectral tomography of carbonaceous nanocomposites. Nano Lett 2006;6:376–9. [31] Yang F, Hu JH, Yang D, Long J, Luo GP, Jin C, et al. Pilot study of targeting magnetic carbon nanostructures to lymph nodes. Nanomedicine 2009;4:317–30. [32] Choi JH, Nguyen FT, Barone PW, Heller DA, Moll AE, Patel D, et al. Multimodal biomedical imaging with asymmetric single-walled carbon nanotube/iron oxide nanoparticle complexes. Nano Lett 2007;7:861–7. [33] Cunha C, Panseri S, Iannazzo D, Piperno A, Pistone A, Fazio M, et al. Hybrid composites made of multiwalled carbon nanotubes functionalized with Fe3O4 nanoparticles for tissue engineering applications. Nanotechnology 2012;23:465102.