Alignment of Single-Walled Carbon Nanotubes with Ferroelectric

Jul 1, 2012 - The scanning electron microscope (SEM) is utilized to investigate the alignment of the SWCNTs in the smectic A phase. Fourier Transform ...
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Alignment of Single-Walled Carbon Nanotubes with Ferroelectric Liquid Crystal Yongxia Zhao,†,‡ Yanmeng Xiao,†,‡ Shuliang Yang,†,‡ Jingwei Xu,†,* Wei Yang,†,* Meiye Li,† Dongmei Wang,† and Yunchun Zhou† †

The State Key Laboratory of Electroanalytic Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun, 130022, People's Republic of China ‡ Graduate School of the Chinese Academy of Sciences, Beijing, 100039, People's Republic of China

ABSTRACT: Single-walled carbon nanotubes (SWCNTs) are aligned by using a ferroelectric liquid crystal (FLC) named [4(3)-(S)-methyl-2-(S)-chloropentanoyloxy)]-4′-nonyloxy-biphenyl. The scanning electron microscope (SEM) is utilized to investigate the alignment of the SWCNTs in the smectic A phase. Fourier Transform Infrared spectroscopy (FTIR) and Raman spectroscopy are utilized to investigate the interaction between these two species. The SEM images show that SWCNTs can be well aligned along the smectic layers. A new peak appears at 1702 cm−1 in the FTIR spectra in the SWCNT hybrids, which indicate that charge transfer occurring from the hexagonal rings of the SWCNTs to the CO groups of the FLC molecules that contact directly with the SWCNTs. To our knowledge, this is the first time we observe the charge transfer effect by FTIR. Similarly, a new peak at 763 cm−1 is also found in the Raman spectrum of the SWCNT hybrid that results from the charge transfer effect between the SWCNTs and the C−Cl groups of the FLC. The π−π stacking and charge transfer effect together make the SWCNTs align unidirectionally along the smectic layers instead of in the direction of the molecular long axis.

1. INTRODUCTION The carbon nanotube (CNT), as a new member of the carbon allotrope family, was first discovered by Iijima in 1991.1 The combination of extraordinary mechanical, thermal, and electronic performance exhibited by CNTs make them ideal for a broad range of applications, such as conductive and highstrength composites, energy-storage and energy-conversion devices, field emitters, transistors, sensors, gas storage media, and molecular wires.2,3 Control of nanotube orientation becomes a very important issue in most attempts to apply CNTs in devices or new materials. Some alignment methods, such as magnetic4 and electric5 field alignment, shear flow6 or other mechanical techniques (i.e., molecular combing7), as well as aligned growth,8−12 have been proposed. The applications of these methods, however, are limited either by dependence on “external force” or restriction to a narrow range of size scales. Without aligning them in the desired direction, their realistic application as one-dimensional conductors or semiconductors has been restricted.3,13 Liquid crystals (LCs) have anisotropic physical properties owing to molecular self-assembly, and the variation of © 2012 American Chemical Society

molecular ordering and orientation occur quickly with temperature. These properties make LCs templates to provide a straightforward, even more efficient method for controlling the order and orientation of CNTs. So far, most of the CNT/ LC hybrids have been implemented by dispersing the nanotubes into the nematic LC phase, which exhibits solely orientational order, while their molecular centers of mass are isotropically distributed.14 Patrick et al.14 reported a variety of methods for postsynthesis organization of single- and multiwalled carbon nanotubes (SWCNT and MWCNT) using thermotropic nematic LC media under the external alignment force (magnetic or electric field, substrate grooves, etc.). Lagerwall et al.15,16 utilized the lyotropic nematic LC phases with rod-shaped and disk-shaped micelles to induce the alignment of SWCNTs. These reports demonstrate that the orientational order of the nematic LC host can be transferred to the CNT guest. Received: December 7, 2011 Revised: May 21, 2012 Published: July 1, 2012 16694

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NMP in the hybrid solution was evaporated by using Ar gas and then drying in a vacuum oven before the experiments. 2.2. Instrument. The characterization of the purification and cutting effect of the SWCNTs was carried out by TEM (Philips-FEI Tecnai F20, 200 kV) and high resolution SEM (Philips-FEI XL30-SFEG). FTIR spectra were obtained in the absorbance mode using a Bruker IFS 66v/s spectrophotometer that equipped with a MCT detector. The hybrid materials sandwiched between two pieces of CaF2 plates were heated to isotropic phase on a hot-stage and then cooled to SmA phase to align the SWCNTs. As reference, the same was done to pure FLC. The inner surface of each plate was coated with a thin layer of polyimide unidirectionally rubbed in order to induce a homogeneous orientation of the FLC molecules. All of the samples were kept for 10 min at the desired temperature before spectral collection. The spectrum of the shortened SWCNTs was taken at room temperature with its sample ground with KBr and pressed into pellets for the detection. All of the spectra have a resolution of 4 cm−1 and an accumulation of 100 scans. To preserve the alignment of the SWCNTs in the SmA phase, the hybrid materials sandwiched between the CaF2 plates were plunged into liquid nitrogen and quenched for several hours for the SEM and Raman experiments.18 As reference, the pure FLC was quenched in the same way. To observe the fracture surface of the samples in the SEM experiments, one of the CaF2 plates was split after the two CaF2 plates was separated. All samples for SEM measurements were coated with Au and carried out with 45° tilt angle. Three different laser wavelengths were used to investigate the Raman spectra. The visible Raman spectra with the excitation wavelengths of 514.5 and 633 nm were recorded on a Horiba Jobin Yvon LabRAM HR Raman spectrometer. Meanwhile, the near-infrared Raman spectra were acquired on a Nicolet FT-Raman 960 spectrometer with an excitation wavelength of 1064 nm. For the FLC and SWCNT hybrid samples, one of the CaF2 plate was used for the Raman spectroscopy after the quenched CaF2 cell was separated. About the SWCNT sample, the SWCNTs were directly put on the CaF2 plate for the detection. All of the Raman spectra have a resolution of 4 cm−1. In addition, the textures of the SmA phases for the pure FLC and the hybrid materials were captured using a polarized optical microscope (Shanghai Changfang Optical Instrument Co., Ltd., China) equipped with a digital color video camera.

Smectic LC that has a characteristic layered structure has received little attention as a template for CNTs relative to nematic LC.17 Theoretically, the two-dimensionally ordered Smectic LC may result in a different alignment of CNTs. In other words, it may align the CNTs in the direction of the extended layer instead of along the long axis of the molecular director like that in the nematic LC. In this work, we align SWCNTs by using a ferroelectric liquid crystal (FLC) named [4-(3)-(S)-methyl-2-(S)-chloropentanoyloxy)]-4′-nonyloxy-biphenyl (3M2CPNOB, Figure 1) that exhibits a smectic A

Figure 1. Chemical Structure of 3M2CPNOB.

(SmA) phase between 61.1 and 48.8 °C in the cooling process. The scanning electron microscope (SEM) together with a simple freezing method is used to observe the alignment of the SWCNTs in the SmA phase. Fourier Transform Infrared spectroscopy (FTIR) and Raman spectroscopy are utilized to investigate the interaction between the FLC molecules and the SWCNTs.

2. EXPERIMENTAL SECTION 2.1. Materials. 2.1.1. The 3M2CPNOB. The synthesized procedure and characterization of the properties of the FLC followed our previous work.18 2.1.2. Purification and Cutting of SWCNTs. The SWCNTs (Diameter: 1−2 nm, Length: 1−3 μm, Purity: >90%) that produced by catalytic chemical vapor decomposition method using CH4 as the carbon source and Co as the catalyst were purchased from XF NANO, INC. To suspend the SWCNTs easily, it was prerequisite to purify and cut the SWCNTs before dispersing them in the FLC. A four-step procedure was adopted to do this. First, 500 mg of as-produced SWCNTs was heattreated at 300 °C for 1 h in a flow of dry air in a furnace to remove the amorphous carbon.17,19 Then, the heat-treated sample was refluxed in 50 mL 2.6 N HNO3 solution at 140 °C for 48 h to remove metal catalysts.20,21 Third, a liquid nitrogen quenching method was implemented to eliminate vapor-grown carbon nanofibers and MWCNTs by freezing the SWCNTs in the liquid nitrogen for about 10 h.22,23 Fourth, sonication 100 mg of as above treated SWCNTs in the mixture of 16 g (NH4)2S2O8 and 200 mL 96% H2SO4 for 5 h to obtain lengthcontrolled SWCNTs.21 The resultant suspension was diluted with 3 L of deionized water, and the cut SWCNTs were collected on a 220 nm pore filter membrane (type: polytetrafluoroethylene (PTFE); Haining Yanguanzhenghao Filter Factory, China). The shortened SWCNTs were dispersed in 1-methyl-2-pyrrolidinone (NMP, 24 mg/L) by ultrasonication for 10 h. The solution was centrifuged at 10,000 rpm for 1 h to separate the cut and isolated SWCNTs from SWCNT bundles. The supernatant solution was collected, and its concentration was determined to be 0.02 mg/mL by measuring the mass of the deposited SWCNTs after evaporation of the NMP. 2.1.3. Preparation of SWCNT/FLC Hybrids. SWCNT/FLC hybrid suspensions in NMP with various mass ratios up to 1 wt % of SWCNTs were obtained by dissolving the FLC into the SWCNT suspension, and followed by ultrasonication. The

3. RESULTS AND DISCUSSION 3.1. Characterization of the Purified SWCNTs. The TEM images for the as-produced and purified SWCNTs are shown in Figure 2. In Figure 2a, it is clearly shown that metal nanoparticles coexist with SWCNTs. In Figure 2b, nearly no

Figure 2. TEM images of (a) as-produced SWCNTs and (b) purified SWCNTs. 16695

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Figure 3. The FTIR spectra of (a) the shortened SWCNTs. (b) The shortened SWCNTs, 3M2CPNOB, and different mass ratios from 0.1 to 1 wt % of the SWCNT hybrids. The arrows indicate the positions of the important peaks.

molecules and the hexagonal rings of the SWCNTs. Charge transfer with SWCNTs acting as electron-donor meanwhile the carbonyl group of the FLC molecules acting as electronacceptor makes the CO stretching vibration of carbonyl groups that touch directly with the SWCNTs moves to lower frequency. The frequency of the carbonyl groups that do not touch directly with the SWCNTs keeps unchanged. The charge transfer effect in above-mentioned references either only calculated theoretically or observed by Raman spectroscopy. To our knowledge, it is the first time to observe the charge transfer effect by FTIR. In addition, the intensity of the new peak changes regularly as the variation of mass ratios of the SWCNT hybrids. To accurately quantify the regularity of the variation, we illustrate the relative intensity of the carbonyl groups at 1702 and 1769 cm−1 (I1702/I1769) in different mass ratios of SWCNT hybrids in Figure 4. The relative intensity increases as the mass ratio

impurities can be observed, which indicates that the impurities have been effectively removed in the process of the purification. 3.2. The FTIR. The enlarged FTIR spectrum of the shortened SWCNTs is shown in Figure 3a. Except the inherent CC stretching mode at 1578 cm−1, some other functional groups have been inducted into the SWCNTs in the purification and/or cutting processes. The peak at 1738 cm−1 that corresponding to the carbonyl group demonstrates that some carbon atoms in the defect regions of the SWCNT wall have been oxidized. However, the intensity of all of the peaks is very weak so that in Figure 3b, the normal spectrum looks like a baseline and no peaks could be seen at all. For the spectra of the SWCNT hybrids, the most obvious difference to the pure FLC is that a new peak at 1702 cm−1 appears in all mass ratios of the SWCNT hybrids. Moreover, this new peak does not exist in the spectrum of the shortened SWCNTs at all and should result from the interaction between the SWCNTs and the FLC molecules. Aromatic compounds are known to interact with graphite, and consequently with the graphitic sidewalls of CNTs via the so-called π−π stacking.24,25 But this kind of interaction is only physical adsorption, which will not induce the appearance of a new peak. The charge transfer effect between CNTs and the molecules contacted with them may result in the formation of this peak. Barros et al.26 reported that SWCNTs behaved as donors, with charge transfer occurring from hexagonal rings of the nanotubes to the carboxyl groups that were oxidized by the nitric acid in the purification process. Rao et al.27 investigated the effects of exposing SWCNTs to some dopants and demonstrated that the SWCNTs would either act as electronacceptor when the dopants were the typical electron-donor (potassium, rubidium) or act as electron-donor when the dopants were typical electron-acceptor (iodine, bromine). Zhao et al.25 studied the interaction between CNTs and aromatic organic molecules such as benzene (C6H6), cyclohexane (C 6 H 12 ), and 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ: C8N2O2Cl2) by using first principles calculations. Their results illuminated that except the π−π stacking there was charge transfer effect between the electronegative DDQ and SWCNTs. Similar work has also been reported by others.24,28,29 These investigations demonstrate the possibility of the charge transfer between the electronegative FLC

Figure 4. The relative intensities of different mass ratios of the SWCNTs hybrids.

increases and achieves maximum of 94.3% at 0.5 wt % of the SWCNT hybrids then decreases gradually as the mass ratio further increases to 1 wt %. The reason may be that in the lower mass ratios, the SWCNTs dispersed well in the SmA phase and nearly all the SWCNTs could interact with the FLC molecules. With the increase of the mass ratio, the amount of electron donor−acceptor pairs increases also. However, when the mass ratios are greater than 0.5 wt %, the SWCNTs 16696

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aggregate seriously, which results in a decrease of the amount of electron donor−acceptor pairs. So the relative intensity contrarily decreases with the mass ratio increases. 3.3. The Raman Spectroscopy. The line shape and peak position of the G band of the CNT sample in Raman spectrum have been used to estimate the category of the CNTs (metallic or semiconducting).30−32 Only a symmetric and sharp G+ band is observed in the Raman spectra of the SWCNT sample (Figure 5a−c) indicates that the SWCNTs are all achiral ones.30,33 In other words, the SWCNTs are armchair and/or zigzag ones. In addition, the Raman spectrum of the metallic zigzag CNTs only has a broad G− band which also called a Breit−Wigner−Fano31,32 line shape, so it is clearly that the SWCNTs in our experiments are metallic armchair and/or semiconducting zigzag ones. In Figure 5a, there is no evident difference between the Raman spectra of the FLC and SWCNT/FLC hybrid. We do not observe any peaks that derived from the SWCNTs. The reason may be that the signal of the low content SWCNTs is too weak to be detected. In Figure 5b, some difference is found between these two spectra. First, two weak radial breathing mode (RBM) peaks that exist in the spectrum of the SWCNTs also appear in that of the SWCNT/FLC hybrid which indicates the existence of the SWCNTs in the hybrid. Moreover, the intensity of the peak at 322 cm−1 becomes much higher in the spectrum of the SWCNT/FLC hybrid than that in the spectrum of the SWCNTs. Meanwhile, the peak at 630 cm−1 disappears in the spectrum of the SWCNT/FLC hybrid. According to the frequencies of these two peaks, we can attribute them to the deformation and stretching vibrations of the C−Cl group. 34 The changes of these two peaks demonstrate the interaction between the C−Cl group of the FLC molecules and the SWCNTs. In Figure 5c, we can see that the spectrum of the SWCNT/FLC hybrid is the combination of the spectra of SWCNTs and FLC. In the spectrum of the SWCNT hybrid, peaks of the FLC and SWCNTs coexist except that some FLC peaks are covered up by the SWCNT peaks. A new peak at 763 cm−1 appears in the spectrum of the SWCNT hybrid. Because the C−Cl group has vibrational frequency in the range of 600−800 cm−134, and can also arose charge transfer effect with the SWCNTs as electron donors, this new peak should result from the interaction of the SWCNTs and the C−Cl group of the FLC molecules. Due to the limitation of experimental equipments, this new peak is not observed in the FTIR spectra. In addition, the relative intensity of the diameterdependent30,35 RBM peaks changes a lot between the spectra of the SWCNTs and SWCNT/FLC hybrid. The reason may be that the extent of the interaction between the FLC molecules and the SWCNTs that have different diameters is different. In other words, the FLC molecules may interact much more strongly with the SWCNTs that in some specific diameters than others. We do not observe evident peak of the carbonyl group in Figure 5a or 5b. In Figure 5c, since broad M band36 near 1702 cm−1 exists in the Raman spectrum of the SWCNTs, the peak at 1702 cm−1 in the FTIR spectra of the SWCNT hybrid is not observed in the Raman spectrum. 3.4. Alignment of the SWCNTs in the SmA Phase. It is well-known that smectic LC has layered structure and in the SEM image (Figure 6a) the layers align parallel to each other with the calamitic molecules perpendicular to the layers. The image of the shortened SWCNTs is shown in Figure 6b, we can see that all the SWCNTs are cut to the range of 0.5−1 μm. Figure 6c,d show the SEM images of the alignment of

Figure 5. The Raman spectra of the 3M2CPNOB (blue line), the SWCNTs (red line), and the SWCNT/FLC hybrid (black line) at different excitation wavelengths of 514.5 nm (a), 633 nm (b), and 1064 nm (c), respectively. Some characteristic peaks have been marked in the spectra.

SWCNTs in 0.1 and 0.5 wt % of SWCNT hybrids, respectively. From these images, we can see that all of the SWCNTs that lie in the smectic layer are aligned unidirectionally. In other words, the SWCNTs unidirectionally aligned perpendicular to the FLC molecules. The well alignment is not only the contribution from π−π stacking, but also from the charge transfer effect between the two species. The sketch map of the interaction between the 16697

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Figure 8. POM images of 3M2CPNOB (a), and SWCNT hybrid (b) in SmA phase.

4. CONCLUSIONS The SEM images provide unambiguous experimental proof that SWCNTs can be well aligned in the smectic LC, like that in the nematic LC. Besides the commonly referred to π−π stacking between the aromatic rings of the LC molecules and the hexagonal rings of the SWCNTs, a new peak appears at 1702 cm−1 in the FTIR spectra in all mass ratios of the SWCNT hybrids relative to the spectrum of the pure FLC, which indicates that the charge transfer occurring from the hexagonal rings of the SWCNTs to the carbonyl groups of the FLC molecules that contact directly with the SWCNTs and makes the CO stretching shifts to lower frequency. To our knowledge, this is the first time we observe the charge transfer effect by FTIR. Similar to the FTIR results, a new peak at 763 cm−1 is also found in the Raman spectrum of the SWCNT hybrid, which results from the charge transfer effect of the SWCNTs and the C−Cl groups of the FLC. The π−π stacking and charge transfer effect together make the SWCNTs align unidirectionally along the smectic layers instead of in the direction of the molecular long axis like that in the nematic LC.

Figure 6. SEM images of (a) pure FLC in SmA phase with the arrow indicates the smectic layers. (b) The shortened SWCNTs. (c,d) The fracture surface of 0.1 and 0.5 wt % of SWCNT hybrids respectively, with the arrow indicates the orientational direction of the SWCNTs.

SWCNTs and the smectic LC molecules is illustrated in Figure 7. The π−π stacking occurs between the core part of the FLC

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AUTHOR INFORMATION

Corresponding Author

Figure 7. The sketch map of the interaction between the SWCNTs and the smectic LC molecules.

*Tel: +86-431-85262643; fax: +86-431-85262649; e-mail: [email protected] (J.X.). Tel: +86-431-85262354; E-mail: [email protected] (W.Y.). Notes

molecules and the hexagonal rings of the SWCNTs, and charge transfer effect occurring from the hexagonal rings of the SWCNTs to the electronegative CO and C−Cl groups of the FLC molecules respectively. SWCNTs aligned in this mode could interact with the FLC molecules more effectively than that aligned parallel to the FLC molecules. Park et al.37 bring up that the core part of the tri-fluorophenyl2 LC molecules anchors helically to the SWCNT wall, meanwhile the tail part repels from the wall to enhance the π−π stacking by maximizing the hexagon−hexagon interactions between the two species. Their standpoint supports our conclusion well. In addition, from Figure 6c,d we can conclude that the layered structure of the SmA phase is not destroyed by the intercalated SWCNTs, which is consistent with the followed POM results. 3.5. The POM. Figure 8a shows a typical fan-shaped texture of the SmA phase of 3M2CPNOB. But to the SWCNT hybrid (Figure 8b), the texture is obviously different from that of the pure FLC. The fan-shaped texture is still keeping whereas the step-like layered structure is clearly seen. This phenomenon indicates that in the aligning process, the smectic layer structure is not destroyed. The addition of SWCNTs only results in some variation of the molecular directors.

The authors declare no competing financial interest.

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REFERENCES

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