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Volume 59, Issue 9, May 2011, Pages 3313-3320

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Volume 59, Issue 9, May 2011, Pages 3313-3320 lu L eo and Su sea fo ine Abstract 1. Introduction to facilitate the strong mechanical strength of the CNTs has been the key issue in improving the mechanical proper- parts, ships, aircrafts and satellites. Therefore, the robust formation of CNTAAl co...

Volume 59, Issue 9, May 2011, Pages 3313-3320
lu L eo and Su sea fo ine Abstract 1. Introduction to facilitate the strong mechanical strength of the CNTs has been the key issue in improving the mechanical proper- parts, ships, aircrafts and satellites. Therefore, the robust formation of CNTAAl composites is desired. There have been several research studies aimed at improving the mechanical strength of Al by incorporating CNTs [7–11]. CNTs are not easily mixed with Al due to the large difference between the surface tensions of the ⇑ Corresponding author at: BK21 Physics Division, Department of Energy Science and Center for Nanotubes and Nanostructured Compos- ites, Sungkyunkwan Advanced Institute of Nanotechnology, Sungkyunk- wan University, Suwon 440-746, South Korea. Tel.: +82 31 299 6507. E-mail address: leeyoung@skku.edu (Y.H. Lee). Available online at www.sciencedirect.com Acta Materialia 59 (2011) 3313–33 Polymer composites based on carbon nanotubes (CNTs) have been commonly utilized due to the extraordinary ther- mal, electrical and mechanical properties of CNTs [1–3]. The composite performance strongly depends on the dis- persion, length, crystallinity and degree of alignment of the CNTs, as well as the binding characteristics between the CNT surface and the host polymer. The percolation limit of the CNT content in improving electrical conductiv- ity has ranged widely, from 0.0021 to 9.5 wt.%, mainly due to the various dispersion conditions of CNTs [4]. Control of the anchoring of host polymers on the surface of CNTs ties of composites. Poor control of the anchoring process often leads to degraded mechanical properties [5]. Mechanically strong metal composites have attracted interest recently due to green energy requirements [6]. Although aluminum is known as a rust-free light material and is used in car parts and buildings, its use is still limited mainly due to its poor mechanical strength as compared to its iron counterpart. A CNT-based Al composite with enhanced mechanical strength could be utilized to improve the fuel efficiency of vehicles by reducing the vehicle weight. Its applications may be further extended to electronic The wetting of a metal on carbon nanotubes is fundamentally difficult due to the unusually large difference between their surface ten- sions and is a bottleneck for making metal–carbon nanotube (CNT) composites. Here, we report a simple method to enhance the wetta- bility of metal particles on the CNT surface by applying aluminum, which is the material with the largest surface tension. This method involves two steps: (i) Al nanoparticles are decorated on multiwalled carbon nanotubes by electroplating and (ii) Al powder is further spread on Al-electroplated CNTs, followed by high-temperature annealing to accommodate complete wetting of the aluminum. The large surface tension difference is overcome by forming strongAlAC covalent bonds initiated by defects of the CNTs. The decrease in theD-band intensity, the G-band shift in the Raman spectroscopy and the formation of AlAC covalent bonds, as confirmed by X-ray photoelectron spectroscopy, were in agreement with our structural model of CNTAvacancyAOAAl determined by density functional calculations. Crown Copyright � 2011 Published by Elsevier Ltd. on behalf of Acta Materialia Inc. All rights reserved. Keywords: Carbon nanotubes; Aluminum; Electroplating; Interface; Nanocomposite Improving the wettability of a Kang Pyo So a,b, Il Ha Lee a,b, Dinh Seong Chu Lim a,b, Kay Hy aBK21 Physics Division, Department of Energy Science Sungkyunkwan Advanced Institute of Nanotechnology, bR&D Department, Chonju Machinery Re Received 16 November 2010; received in revised Available onl 1359-6454/$36.00 Crown Copyright � 2011 Published by Elsevier Ltd. on beh doi:10.1016/j.actamat.2011.01.061 minum on carbon nanotubes oc Duong a,b, Tae Hyung Kim a,b, k An b, Young Hee Lee a,b,⇑ Center for Nanotubes and Nanostructured Composites, ngkyunkwan University, Suwon 440-746, South Korea rch Center, Chonju 561-844, South Korea rm 27 January 2011; accepted 31 January 2011 2 March 2011 www.elsevier.com/locate/actamat 20 alf of Acta Materialia Inc. All rights reserved. ria two materials as the surface tension of Al is 955 mN m�1, which is almost 20 times larger than that of CNTs (45.3 mN m�1) [12,13]. An additional technical barrier to formulate CNTAAl composites with high mechanical strength is the high oxidation capability of Al, which causes Al particles to be easily oxidized and lose their metal characteristics. Because of these difficulties, the wet- tability of Al on the CNT surface has been extremely dif- ficult to realize. Therefore, the issue remains of how to overcome the large difference in surface tension and improve the wetta- bility of Al on the CNT surface, which would be a simple way to generate CNTAAl composites with high mechanical strength. In this study, we adopted the strategy of forming strong AlAC covalent bonding between Al and the CNT surface. For this, we applied an electroplating method to coat Al nanoparticles on vertically aligned multiwalled CNTs, which eventually enhances the wettability of Al on the CNTs. Strong covalent bonds were formed between Al and the CNT surface during the electrodeposition. This covalent bond improves aluminum wettability on the CNT surface. Defect-associated nucleation of Al on the CNT surface was observed by Raman spectroscopy and X-ray photoelectron spectroscopy, and was also supported by density functional theory (DFT) calculations. 2. Materials and methods 2.1. Synthesis of vertically aligned carbon nanotubes A thin (less than 100 nm) layer of Ni was deposited onto a TiN/Si (100) substrate using an RF magnetron sputter. The chamber was evacuated to a pressure of 1 � 10 �6 Torr and the substrate temperature during the deposi- tion was maintained at 350 �C by a graphite heater. The Ni-coated TiN/Si substrate was then moved into a thermal chemical vapor deposition (CVD) chamber. The CNTs were grown on Ni-coated TiN/Si substrates using thermal CVD at 650 �C with a gas mixture with C2H2 (20%) and Ar (80%). The CVD chamber pressure was held at 5.5 Torr. The CVD chamber was heated by halogen lamps at a heating rate of 54 �C min�1 until the temperature reached the growth temperature of 650 �C. The growth temperature was fixed during the growth time of 20 min. The experiment procedure has been described in detail else- where [14]. 2.2. Al electroplating The electrolyte was prepared by dissolving 1.17 M anhy- drous aluminum chloride into a mixture of two solutions of tetrahydrofuran (THF):benzene = 8:2. To increase the solution conductivity and solubility of aluminum chloride, 0.23 M lithium aluminum hydride was added to the electro- lyte [15]. The electroplating cell consisted of three elec- 3314 K.P. So et al. / Acta Mate trodes: vertically aligned carbon nanotube as a working electrode, Pt mesh as a counter electrode and Ag/AgCl as a reference electrode. N2 gas was flowed through the cell for more than an hour to remove moisture and oxygen. The prepared electrolyte was injected into the cell by a med- ical syringe and the CNT electrode was inserted into the cell and stabilized for 20 min. The counter electrode was placed parallel to the working electrode with the reference elec- trode in the middle. CV measurements were taken with a scan rate of 50 mV s�1. Electroplating was applied for 5 min at a given applied voltage. The electrolyte was rinsed with ethanol after electroplating and moisture was removed by acetone followed by drying in a vacuum oven overnight. 2.3. Al wetting Al powder (Samjeon Chemicals, Korea) with a size of 75 lm was placed on top of vertically aligned multiwalled CNTs, which were electroplated by Al as described above. This structure was then heat-treated at 700 �C for 1 h in a vacuum furnace (10�5 Torr). 2.4. Measurement The electroplating was performed using a potentiostat (Solartron 1287A). The morphology of the electroplated MWCNTs was characterized by field-emission scanning electron microscopy (6700F, JEOL) and high-resolution transmission electron microscopy (TEM; JEOL 2010F HRTEM, 200 keV). Elemental analysis was obtained by energy-dispersive X-ray spectroscopy (EDX). AlAC covalent bonds were characterized by X-ray diffraction (XRD; Rigaku Rotaflex D/MAX system, Rigaku, Japan, Cu Ka, 1.54 A˚), X-ray photoelectron spectroscopy (XPS; ESCA2000, VG Microtech, UK, Al Ka 1486.6 eV) and Raman spectroscopy (Renishaw, RM1000, 633 nm). 2.5. DFT calculations To interpret the experimental data (8,0) carbon nano- tubes with and without a vacancy were simulated. Al and O atoms were added on the surface of the nanotubes. All models were calculated by the DFT implemented by DMOL3. The numerical atomic basic sets were used with an orbital radius cut-off of 4.8 A˚. The generalized gradient approximation (GGA) for exchange and correlation in the Perdew–Burke–Ernzerh function were employed [16–18]. The Brillouin zone was sampled with a 4 � 4 � 1 irreduc- ible Monkhorst–Pack k-point grid [19]. The convergence threshold for calculation of the self-consistent energy was 10�6 Ha. Our model structures were relaxed until the forces on the atoms were smaller than 0.05 eV A˚�1. 3. Results and discussion 3.1. Schematic diagram of experimental procedure lia 59 (2011) 3313–3320 Fig. 1 shows a schematic of the experimental procedure used to improve the wettability of additional aluminum eria powder after the electroplating of aluminum on multi- walled carbon nanotubes (MWCNTs) to form AlACNT covalent bonds. Vertically aligned MWCNTs synthesized by thermal chemical vapor deposition (TCVD) on a silicon substrate were coated with Al nanoparticles by an electro- plating method. The synthesized MWCNTs had diameters of 40–50 nm and lengths of 3–5 lm, depending on the growth time. Al nanoparticles adhered strongly to the outer MWCNT walls due to the formation of strong AlAC covalent bonds. Additional Al powder was then spread on top of the vertically aligned MWCNT film and further annealed at 700 �C for 1 h to fully accommodate the wet- ting of additional Al. 3.2. Plating phenomena and morphology analysis Fig. 2a shows the current–voltage (CV) data obtained with the aluminum plating solutions. The details of the solution are described in the experimental section. In the THF and benzene solvent alone, a negligible current Fig. 1. Schematic diagram of the Al wetting process on a CNT: (a) vertically aligned CNT, (b) Al electroplating, (c) loading of Al powder on top of the vertically aligned CNT and (d) formation of the Al/CNT composite by heavy Al wetting at 700 �C (10�5 Torr) for 1 h. K.P. So et al. / Acta Mat (�4 lA) and linear behavior of the CV data (inset of Fig. 2b) were observed, confirming that there was insignif- icant chemical reaction of the solvent with a bias up to �3 V. The conductivity of the solvent is poor and therefore no active ionic motion is expected, resulting in simple resis- tor behavior. By adding AlCl3, the current level increased to the order of milliamps (Fig. 2b) but still remained low. AlCl3 was dissolved in the solvent to some degree and Al ions were further adsorbed on the surface upon bias, but no apparent reduction peak was observed, confirmed by the simple capacitive behavior, as shown in Fig. 2b. Actual coating involving the reduction of Al ions took place when catalytic LiAlH4 was added [20]. The current level was increased to 50 mA at �3 V. The reduction of Al ions was clearly observed and began at �1.5 V, as indicated in the inset of Fig. 2a. Figs. 2c and 1d show the current–time transient behav- ior in the early stage of coating. At a low voltage (�1.6 V), though still above the reduction voltage, the cur- rent profile is composed of three steps: step I involves for- mation of the electric double layer on the electrode, corresponding to a rapid decrease in current, step II repre- sents the reduction reaction of ions in the electrode that gives rise to a gradual increase in current and step III involves a diffusion-limited current corresponding to the saturation of nucleation sites for the reduction of ions [21]. These three steps are represented in Fig. 2c. At a low bias of �1.6 V, the formation of an electric double layer of ions took a relatively long time, up to 5 s. The reduction process of Al ions was also slow, resulting in a slight increase in the current. With increasing applied volt- age, the current decreased more rapidly and the reduction of Al ions occurred more quickly. It is worth noting that we did not observe a current decrease corresponding to step III. This phenomenon is different from the observed diffusion-limited model [21– 23]. The current decrease in step III is usually observed when the electrodeposition rate dominates the diffusion rate of ions so that the ion supply to the nucleation sites is limited by the overlap of the diffusion zone. However, vertically aligned carbon nanotubes provide a large surface area. In this case, no diffusion limit is expected when nucle- ation is initiated and the reduction speed or current simply relies on the applied voltage. This behavior was confirmed by the increased current with increasing applied voltage in Fig. 2d. Al reduction took place quickly and thus the cur- rent increased and became saturated at the long time limit. Fig. 3 shows the XRD pattern obtained after electro- plating for 5 min to confirm the formation of Al crystals. As the voltage increased, Al crystal peaks were clearly observed near 38.5� (111) and 44.7� (200) [24]. This clearly confirms the reduction of Al to form Al crystals. The pres- ence of the C(101) peak near 44� even at a high bias voltage indicates that the crystalline inner walls of the MWCNTs were intact [25]. The SEM morphologies of the Al-coated MWCNTs are shown in Fig. 4. The pristine MWCNTs contain carbona- ceous particles, and nanotubes are entangled on the top part (Fig. 4a). The size of the Al particles at �3 V was around 100 nm (Fig. 4b). At �5 V, the carbon nanotubes were completely covered by Al particles (Fig. 4c). A side view of the Al-coated MWCNTs at �3 V is shown in Fig. 4d, demonstrating the uniform deposition of Al parti- cles, even at the bottom positions. This was confirmed by the EDX mapping for carbon and aluminum (Fig. 4e and f). Al particles were uniformly coated over the vertically aligned MWCNTs, independent of depth. 3.3. Morphology analysis after wetting of additional Al powder The morphologies of the pristine MWCNT film are shown in Fig. 5a–c. Al powder placed at the top of the pris- tine MWCNTs remained intact even after the melting of lia 59 (2011) 3313–3320 3315 Al. The side view of the MWCNTs clearly shows no wet- ting of Al on the surface of the MWCNTs. This is expected 3316 K.P. So et al. / Acta Materialia 59 (2011) 3313–3320 from the large difference between the surface tensions of the two materials and the hydrophobicity of the CNTs. Fig. 3. X-ray diffraction analysis after aluminum electroplating at �2 to �5 V vs. Ag/AgCl. Aluminum crystal patterns are clearly shown after electrochemical reduction. Fig. 4. Morphologies after Al electroplating as examined by SEM and EDS electroplating at (b) �3 V and (c) �5 V for 5 min, (d) side view of the sample Fig. 2. Cyclic voltammetry and current–time transient analysis: (a) CV charact a scan range of 0 to �3 V and (b) CV characteristics without LiAlH4. Note transient for 10 s at �2 V to �5 V vs. Ag/AgCl and (d) the high potential cur On the other hand, Al powder placed on top of the Al- coated MWCNTs infiltrated well into the vertically aligned MWCNTs forest. Consequently, due to this wetting pro- cess, the MWCNT forest was unevenly shrunk, resulting in discontinuous cracks in the forest (Fig. 5d). The inset clearly shows a crack formation near the Al wetting region. This phenomenon is ascribed to the volume contraction of eristics of the aluminum plating solution with a scan rate of 50 mV s�1 and the different current level in the y-axis, (c) the low potential current–time rent–time transient at �1.6 V to �2 V vs. Ag/AgCl. melted Al (�22%) during cooling. The side view of the wet area demonstrates that Al powder infiltrated into the sides of the MWCNTs forest (Fig. 5e and f). MWCNTs were embedded in the Al matrix, as shown in the inset of Fig. 5f. Some of the Al powder still remained on top of the MWCNTs due to incomplete melting caused by the preformed aluminum oxide layer. MWCNTs scratched from the sample were further son- icated and dropped onto a TEM grid. Al was uniformly mapping: top views of the (a) pristine MWCNT and Al-MWCNT by electroplated at �3 V and EDX mapping observations of (e) C and (f) Al. coated over all of the MWCNTs, as confirmed by EDX mapping (Fig. 5g–i). MWCNTs still remained intact even after annealing at 700 �C for 1 h. This is in good agreement with the previous results showing that MWCNTs were par- tially degraded, particularly near the defects and edges, but the graphitized CNT wall remained intact during heat treatment even at 800 �C for 1 h [26]. This indirectly sug- gests that our MWCNTs have some defects on their walls which was also confirmed in the Raman spectrum, as will be discussed in next section. Fig. 5. Morphology changes after heavy Al wetting. (a–c) The pristine CNT without Al electroplating: (a) top view, (b) side view and (c) enlarged side view of the white square in (b). (d–f) The corresponding morphology changes with wetting after Al electroplating at �5 V for 5 min. (g) TEM image after heavy Al wetting, and element mapping of (h) C and (i) Al. rms K.P. So et al. / Acta Materialia 59 (2011) 3313–3320 3317 Fig. 6. AlAC covalent bond analysis: (a) Raman spectroscopy analysis in te shift (blue), (c) Al2p XPS peak after Al electroplating at �5 V for 5 min a interpretation of the references to color in this figure legend, the reader is refe of electroplating voltage, (b) D/G intensity ratio (black) and G-band peak nd (d) Al2p peak after further heat treatment at 700 �C for 1 h. (For rred to the web version of this article.) 3.4. AlAC covalent bond analysis by Raman spectroscopy and XPS Information on the presence of defects can be found in the Raman spectra. Fig. 6a shows a D-band near 1330 cm�1 and a G-band near 1590 cm�1 in the pristine sample, which corresponds to contributions from sp3-like defects on the CNT wall or amorphous layers and sp2-like graphitic hexagons, respectively [27]. Interestingly, the intensity of the D-band and the D/G ratio decreased grad- ually with increasing electroplating bias, as shown in Fig. 6b. This implies that the defect density decreased with Al deposition. This is in contrast with a previous report that the electrodeposition of Al with amorphous carbons resulted in an increased D-band intensity [28]. This ambi- guity will be clarified later when the density functional cal- culations are discussed. The G-band peak was upshifted with increasing bias. This is evidence of electron transfer Fig. 7. The optimized structures (top panel) of (a) (8,0) CNTAvacancy, (b) CNTAvacancyAAl, (c) CNTAvacancyAAlAO and (d) CNTAvacan- cyAOAAl. The induced charge (bottom panel) due to interaction of CNTAvacancy with (e) Al, (f) Al@O and (g) OAAl. The induced charge is defined as qinduced = qCNT-adsorbate � qCNT � qadsorbate. The red and blue isosurfaces indicate the regions of charge accumulation and depletion, respectively. The value of the isosurface is ± 0.007 e A˚�3. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) e 0 0 � 3318 K.P. So et al. / Acta Materialia 59 (2011) 3313–3320 Table 1 DFT calculation results of CNT vacancy with Al atom incorporation. Configuration Binding energy (eV) CNTAvacancy CNTAvacancyAAl (CNTAAl) �6.07 (�1.40) CNTAvacancyAAlAO (CNTAAlAO) �10.39 (�6.29) CNTAvacancyAOAAl (CNTAOAAl) �10.50 (�6.20) The binding energy, amount of electron transfer and
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