Multiwalled Carbon Nanotube

Liu et al.26 have reported the use of a concentrated H2SO4/HNO3 mixture to cut the highly tangled long ropes of CNTs into short, open-ended pipes and ...
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J. Phys. Chem. B 2007, 111, 5337-5343

5337

Fabrication of “Tadpole”-like Magnetite/Multiwalled Carbon Nanotube Heterojunctions and Their Self-Assembly under External Magnetic Field Baoping Jia and Lian Gao* State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai, Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, China ReceiVed: January 26, 2007; In Final Form: March 14, 2007

Novel “tadpole”-like Fe3O4/multiwalled carbon nanotube (MWCNT) heterojunctions were successfully synthesized by position-selectively attaching Fe3O4 sphere on the tips of MWCNTs through a straightforward and effective polyol-medium solvothermal method. Transmission and scanning electron microscopy (TEM and SEM) and X-ray diffraction (XRD) investigations show these Fe3O4 spheres are constructed with tiny nanocrystallites (∼5 nm in average diameter), which were preferentially aggregated in an oriented pattern on the open ends of the MWCNT template. Magnetic investigation indicates this novel Fe3O4/MWCNT hybrid presents superparamagnetic behavior. The size and corresponding magnetic performance of these magnetite/ MWCNT hybrids can be adjustable to some extent for specific applications through altering the reaction parameters. Furthermore, these tadpolelike nanocomposites can orient and self-assemble into one-dimensional structure under external magnetic field, displaying great potential in precise manipulation and organization of carbon nanotube-based structures into integrated functional system.

1. Introduction Carbon nanotubes (CNTs), as the focus of intense investigation since their discovery, have become one of the most promising candidates for building blocks in molecular or nanoscale devices due to their unique tubular structures and fascinating properties.1-4 A range of novel CNT-based functional nanocomposites and nanodevices have been fabricated through various methods and have shown wide potential applications in electronic, optical, catalytic, and photonic devices.5-11 The next challenge for scientists is to precisely control and assemble these functional parts into an integrated system through “bottom-up” routes. But how to place the nanotubes at desired locations has been one of the long-standing unsolved problems.12 Recently, the modifications of CNTs with magnetic materials via different strategies provide a feasible option for manipulating CNTs with the assistance of an external magnetic field. Korneva et al.13 have filled self-prepared CNTs (average outer diameter of 300 nm) with Fe3O4 nanoparticles and easily oriented them in a magnetic field. Correa-Duarte et al.14 successfully aligned magnetic-particle-attached carbon nanotubes under a low magnetic field. Other reports also show that magnetic nanoparticles could be encapsulated in or attached on the surface of CNTs. Although there have been a number of reports about successful synthesis of magnetic CNTs, the location and amount of magnetic materials loaded on CNTs are still difficult to control. Indiscriminate filling or coating with magnetic materials is unfavorable for the precise control of CNTs and would hinder further organization of these nanodevices into an integrated system. In some cases, clean CNTs are required and undue modification would have some negative influence on the performance of nanodevices. Therefore, the functionalization of CNTs in particular positions (like the tip) is more favorable for the development of CNT-based devices. * To whom correspondence should be addressed: tel 0086-2152412718; fax 0086-21-52413122; e-mail [email protected].

Up to now, only limited research related to the positionrestricted modification of CNTs with inorganic materials has been reported. Liu et al.15 have reported the synthesis of only tip-decorated CNT-ZnO heterojunction arrays by water-assisted chemical vapor deposition (CVD) of carbon on a zinc foil, which acts as both the substrate for the CNT growth and a zinc source. Ravindran et al.16,17 conjugated amine-modified CdSe@ZnS nanoparcles onto the ends of acid-treated multiwalled carbon nanotubes (MWCNT) through an ethylenecarbodiimide coupling procedure. Other reports18,19 about foreign materials (such as Co3O4 and noble metal) beaded onto MWCNTs also show the possibility of controlling the position of foreign materials along the carbon nanotubes. It would be of great significance if the benefits of position-restricted modification and magnetic performance can be integrated on CNT-based composites; the resulting new type of magnetic CNTs would bear many advantages superior to the reported magnetic CNT-based hybrids. For example, if we could attach considerable magnetic material exclusively on the tips of CNTs, then precise manipulation and further installation of CNT devices on the aimed substrate or joint would be feasible with the assistance of a magnetic field. However, this kind of position-selectivedecorated magnetic CNTs and their potential applications have been much less reported. The main purpose of our present work is to realize end-only functionalization of CNTs with magnetic materials. Our consideration mainly contains the following aspects. First, two basic modes for connecting foreign materials with CNTs, noncovalent and covalent linkage, have been intensively investigated. The former is based on physical attraction, while the latter is focused on the formation of a covalent bond between functional groups (on the surface of CNT) and foreign materials, which is preferable for position-restricted modification of CNTs. According to our understanding, the location and amount of covalent bonding is strongly dependent on the distribution and density of functional groups on the surface of CNTs, which

10.1021/jp070675p CCC: $37.00 © 2007 American Chemical Society Published on Web 04/17/2007

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Figure 1. TEM images of MWCNTs (a) before and (b) after acid treatment for 20 h.

hints that if we could confine the distribution and density of the functional groups, we could control the place being decorated. Second, the self-aggregating capability of magnetic materials also has a great effect on the final results, which means that if the magnetic materials tend to aggregate within a small area, then the decoration could be restricted to limited positions. In this paper, we demonstrate a straightforward and effective solvothermal approach for position-selective attaching of Fe3O4 spheres onto the tips of MWCNTs. As a result, novel “tadpole”like magnetite/MWCNT nanostructures were fabricated. Further investigation indicates that this novel heterojunctionlike structure can be oriented and self-assembled under an external magnetic field, displaying great potential in precise manipulation and organization of CNT-based devices into an integrated system. 2. Experimental Section Materials. All chemicals were analytical-grade and were used as received. Pristine MWCNTs synthesized by the CVD method were kindly provided by Chengdu Institute of Organic Chemistry, Chinese Academy of Sciences. Cutting and Dispersion of MWCNTs. The pristine MWCNTs were first dispersed into a mixture of concentrated nitric acid and sulfuric acid (volume ratio ) 1:3) with constant stirring for 1 h. Then, the black solution was treated with sonication for 20 h. Afterward, the solution was diluted with distilled water and rinsed for several times until the pH value reached neutral. The resulting MWCNTs were separated from the solution by filtration and dried in vacuum at 60 °C for 2 h for further use. Attaching Fe3O4 Spheres on MWCNTs. The generation of Fe3O4 was carried out by a polyol-medium solvothermal method according to ref 21 with some modification. Typically, FeCl3‚ 6H2O (0.54 g, 2 mmol) was dissolved into 70 mL of ethylene glycol to form a stable orange solution. Then pretreated MWCNTs (100 mg) were dispersed in the solution by sonication for 3 h. After that, NaAc (3.6 g) and poly(ethylene glycol) (PEG1000) (1.0 g) were added with constant stirring for 30 min until they were completely dissolved. The as-formed viscous slurry was transformed into a Teflon-lined stainless steel autoclave of 80 mL capacity and maintained at 200 °C for 8 h. After the sample was cooled to ambient temperature, black precipitates were collected after being rinsed with pure ethanol and water repeatedly and dried in vacuum at 60 °C for 10 h.

Characterization. The X-ray diffraction pattern (XRD) of the sample was recorded on D/max 2550V X-ray diffractionmeter with Cu KR irradiation at λ ) 1.5406 Å. Transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) observations were performed on a JEM-2100F electron microscope with an accelerating voltage of 200 kV. Scanning electron microscopy (SEM) images of the product were taken on a field emission scanning electron microscope (FESEM, JEOL, JSM-6700F). Magnetic characterization was conducted on a physical property measurement system (PPMS, model 6000). 3. Results and Discussion The pristine MWCNTs in our case were first treated with a mixture of concentrated H2SO4/HNO3. According to previous reports,21-23 the mixed oxidizing acids have been applied as an effective method in opening the ends of CNTs as well as cutting CNTs through destroying their defect sites preferentially. Figure 1 illustrates the typical transmission electron microscopy (TEM) images of MWCNTs before and after acid treatment. It can be clearly seen that the average length of MWCNTs has obviously reduced from several micrometers to hundreds of nanometers after acid treatment for 20 h. As a result, relatively short and straight MWCNTs in the range of ca. 200-600 nm were obtained. It is evident that these shortened MWCNTs are well-separated and kept individual from each other. These nanotubes can be readily dispersed in ethanol and form stable suspensions for months without aggregation. According to our understanding, this transformation can be mainly attributed to reduction of their length and generation of large numbers of hydrophilic groups such as carboxyl on the MWCNTs. Electrostatic repulsion between these functional groups would be favorable for good dispersion of resulted nanotubes. After the second process of decorating the pretreated MWCNTs with magnetite, a powder X-ray diffraction pattern (XRD) of sample (as shown in Figure 2) was recorded. Analysis indicates that the product is a mixture of two phases: cubic Fe3O4 and MWCNTs. All diffraction peaks of cubic Fe3O4 can be readily indexed according to JCPDS file no. 75-0033, and the diffraction peak at 2θ ) 26° can be indexed to (002) reflection of the MWCNTs. No obvious peaks from other phases are observed. The main peaks of Fe3O4 in the XRD pattern are broadened,

“Tadpole”-like Magnetite/MWCNT Heterojunctions

Figure 2. XRD pattern of the tadpolelike magnetite/MWCNT hybrid.

indicating the crystalline portion of the Fe3O4 particle is very small. According to the Scherrer formula, the average size of Fe3O4 particles calculated from (311) is about 5.3 nm. Structure and morphology of the as-produced Fe3O4/MWCNT composite were investigated by TEM and field emission scanning electron microscopy (FESEM). Figure 3 shows typical TEM images of the magnetite-modified MWCNTs. In the low-

J. Phys. Chem. B, Vol. 111, No. 19, 2007 5339 magnification image as shown in Figure 3a, spherical particles with diameters of ca. 100-200 nm and narrow size distribution were well-dispersed on the membrane. Energy-dispersive X-ray (EDX) analysis confirms that only Fe and O element were contained in these beads, and the atomic ratio Fe/O ≈ 3:4, which is coincident with Fe3O4. It is also visible that most of the short MWCNTs were combined with Fe3O4 spheres and only a few of them (less than 5%) were free of Fe3O4 nanoparticles. Close observation (as shown in Figure 3b-e) indicates these Fe3O4 and MWCNTs are organized into interesting “tadpole”-like structures, in which the Fe3O4 sphere, acting as “head”, is peculiarly attached on the tip of MWCNT “tail”. More details can be found in HRTEM images of individual spheres (as shown in Figure 3f). It can be clearly seen that these Fe3O4 spheres are constructed with tiny nanoparticles with an average diameter of 5 nm, which is close to the value calculated by the Scherrer formula. Parallel lattice fringes among almost all crystals and the grain boundaries indicate the oriented-aggregation pattern of these particles, and the prominence of lattice fringe d ≈ 0.48 nm among the observed crystallites agrees well with the (111) lattice planes. Further FESEM images as shown in Figure 4 provided more direct information about threedimensional morphology of the magnetite/MWCNT structure, which is consistent with TEM observation.

Figure 3. (a) Overall TEM image of the magnetite modified MWCNTs. (b-e) High-magnification TEM images of tadpolelike structure. (f) HRTEM image of individual Fe3O4 sphere.

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Figure 4. SEM images of the magnetite/MWCNT structure with different magnification.

Figure 5. Schematic illustration of self-assembly of magnetite beads onto the ends of shortened MWCNTs.

Then, why did these magnetite spheres preferentially attach on the tips of MWCNTs? To explore this question, it is necessary to discuss the oxidative shortening of MWCNTs as a prerequisite. As a well-known method to introduce acidic groups onto the surface of carbonaceous materials in general, the use of mixtures of oxidizing acids, like a concentrated H2SO4/HNO3 mixture, has been proved effective in cutting and separating carbon nanotubes. According to previous reports,24,25 the breakdown of cavitation bubbles in ultrasonication would generate high temperatures within microscopic domains and lead to localized sonochemistry that attacks carbon nanotubes. This kind of attack would preferentially happen at those defective positions where non-six-membered carbon rings appear and resulted in damage. Meanwhile, carboxylic acid groups (-COOH) were introduced at these positions, whose amount and distribution have great influence on the further derivation of CNTs. Liu et al.26 have reported the use of a concentrated H2SO4/HNO3 mixture to cut the highly tangled long ropes of CNTs into short, open-ended pipes and thus produced many carboxylic groups at the open end. But the exact concentration of carboxylic acid groups was not mentioned. Kim and Sigmund27 have further determined the concentration of COOH groups on MWCNTs (treated with concentrated H2SO4/HNO3) by potentiometric titration. Up to now, there has been no report about the determination of exact concentration of carboxylic acid groups on the tips or sidewall. In most cases, it is suggested that positively charged colloids could be used to detect the locations and concentration of functional groups. According to our understanding, the amount and distribution of COOH can be altered by controlling the period of acid treatment. It is rational to imagine that, at the earlier stage of acid treatment, carboxylic groups would be generated both at the ends and along the sidewall of the carbon nanotube. As the acid treatment proceeds, continuous attack at the defects would enlarge those damages along CNTs and eventually cut the nanotubes, in which not only the amount of COOH group is increased but also the COOH groups that used to be on the sidewall are now on the new open

Figure 6. TEM images of controlling the load of Fe3O4 on MWCNTs through altering the experimental parameters: (a-d) Fe3O4 spheres with an average diameter of ∼80 nm, fabricated with 0.27 mg of starting ferric precursor and 3 h of solvothermal treatment; (e, f) Fe3O4 spheres of ∼40 nm fabricated with PEG-2000 as capping agent.

ends of shortened nanotubes. This kind of transferring would continue as long as the acid treatment goes on, through which most of the COOH groups were redistributed from the sidewall of CNTs onto the tips. And the dominance of COOH groups at the ends of CNTs is of consequence for further position-selective modification of CNTs. In our case, a large number of carboxylic groups on the newly opened ends of shortened MWCNTs could catch and strongly bond with Fe3+ ions in FeCl3 solution through electrostatic attraction. In the following solvothermal process, some of these ferric ions would be in situ reduced into Fe2+ ions and then coprecipitated into Fe3O4 crystallites, through which the position-selective decoration of MWCNTs with magnetite was realized. In our earlier researchs, necklacelike magnetite/MWCNT nanostructure with Fe3O4 beads along MWCNTs were first synthesized as the duration of acid

“Tadpole”-like Magnetite/MWCNT Heterojunctions

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Figure 7. Magnetic properties of magnetite/MWCNT structure. (a) Magnetic hysteresis curves of samples at 300 K; (b) separation from solution under an external magnetic field. (Inset) Well-dispersed solution before magnetic separation.

Figure 8. (a, b) TEM and (c, d) SEM images of the orientation and alignment of tadpolelike magnetite/MWCNT hybrid under an external magnetic field.

treatment was extended.28 And further longer acid treatment would result in a netlike Fe3O4/MWCNT composite instead of unrestrained increase of Fe3O4 load along MWCNTs. All of these facts accord well with our expectation. On the other hand, the self-aggregating nature of these Fe3O4 nanocrystallites is also helpful for the restricted modification of MWCNTs. Generally, the main driving force for oriented aggregation of nanocrystals is attributed to the tendency to decrease the high surface energy through self-organization of adjacent particles in a common crystallographic orientation and joining of these particles at a planar interface. In our case, dipolar interactions between the magnetite nanocrystals also contribute to their

aggregation. At the same time, the nonaqueous solution could slow the aggregating rate of Fe3O4 nanocrystals due to greater viscosity, providing enough time for Fe3O4 nanocrystals to rotate to the low-energy configuration interface.29 The weak capping agent PEG could prevent fast growth or oxidation of Fe3O4 crystallite and be helpful to the oriented aggregation. Figure 5 illustrates the schematic procedure for novel position-selective modification of shortened MWCNTs with Fe3O4 spheres. In practical applications, it is desired that the morphology and performance of nanodevices can be adjusted to meet different needs. Control experiments indicate that the loading capability of magnetic parts in our sample can be altered via

5342 J. Phys. Chem. B, Vol. 111, No. 19, 2007 changing the experimental parameters such as amount of ferric precursor, duration of solvothermal treatment, and type of capping agent. When we decreased the starting ferric precursor to 1 mmol and shortened the duration of solvothermal treatment to 3 h, the size of Fe3O4 spheres attached on the tip of MWCNT was reduced to ∼80 nm (as shown in Figure 6a-d). It is rational that when the initial Fe3+ concentration decreased dramatically, the quantity of magnetite nanocrystallites would decrease simultaneously. Meanwhile, the shortening of reaction time also can confine the aggregating of Fe3O4 spheres to some extent. Moreover, with the above conditions held constant, replacement of the capping agent with PEG-2000, which would further hinder self-aggregation of Fe3O4 nanoparticles due to a better steric effect, can further reduce the size of Fe3O4 aggregation to ∼40 nm (as shown in Figure 6e,f). All these results indicate that the workability of our novel Fe3O4/MWCNT structure could be altered across a large range via controlling the reaction parameters. Magnetic hysteresis analysis shows the samples have a strong magnetic response to a varying magnetic field (as shown in Figure 7a). The saturation magnetization value of the tadpolelike structures is 58.9 emu/g at 300 K (curve I), which is comparatively lower than that of pure Fe3O4.30 The reduction of saturation magnetization can be mainly attributed to the existence of MWCNT. The M-H curves of all magnetic MWCNTs at 300 K show nonlinear, reversible characteristics with no hysteresis (zero coercivity), exhibiting superparamagnetic behavior. According to the results of TEM and XRD, the average size of our Fe3O4 nanocrystallites is about 5 nm, much smaller than the superparamagnetic critical size of Fe3O4 (Dp ) 25 nm).31 So it is reasonable that our Fe3O4/MWCNT composites show a superparamagnetic response. Interestingly, the saturation magnetization value of these Fe3O4/MWCNT structures decreased to 33.5 and 12.1 emu/g (curves II and III) gradually with the size of Fe3O4 aggregation reduced, which indicates the magnetic properties of this kind of Fe3O4/MWCNT nanodevice can be adjustable to some extent for specific applications. According to our understanding, the relative proportion between Fe3O4 and carbon nanotubes has an important influence on the saturation magnetization of product. It is rational that the proportion of magnetic material would decrease with decreasing size of Fe3O4 aggregates on carbon nanotubes, and the saturation magnetization would also decrease correspondingly. Magnetic separability of the sample is also tested in ethanol by placing a magnet near the glass bottle. The black powder was attracted toward the magnet in a short period (Figure 7b), demonstrating high magnetic sensitivity. Further investigation indicates that these tadpolelike structures can selfassemble into one-dimensional structures under external magnetic field (as shown in Figure 8). This kind of self-assembly can mainly be attributed to the dipolar-dipolar interactions between magnetite spheres under an external magnetic field, which forces them to orient one by one along direction of magnetic field and form chainlike structures. All of these results reveal that the novel tadpolelike Fe3O4/MWCNT structure can be oriented and manipulated under an external magnetic field. It is rational to expect that, if magnetic microelectrode were purposefully set in designed positions, our novel Fe3O4/ MWCNT hybrid could be attracted and precisely self-installed on them as functional nanodevices and further organized into an integrated system, which is the next aim of our work in progress.

Jia and Gao 4. Conclusion In summary, we developed a simple and effective solvothermal method to synthesize novel tadpolelike Fe3O4/MWCNT structures, in which tiny nanocrystallites with an average diameter of ca. 5 nm were decorated exclusively on the tips of acid-shortened MWCNTs and self-organized into Fe3O4 sphere through an oriented pattern. The preshortening of MWCNTs by oxidizing acid, as prerequisite, would introduce a large number of carboxyl groups at the new open ends of shortened tubes, and the high density of functional groups in these restrictive positions would provide great opportunity for further position-selective modification of MWCNT with Fe3O4. Magnetic investigation indicated that this novel Fe3O4/MWCNT hybrid presents superparamagnetic behavior, which may result from the nanoscale size of particles. Additionally, the morphology and magnetic property of our magnetic MWCNTs can be adjusted to some extent through altering the reaction parameters such as amount of ferric precursor, time of solvothermal treatmen, and type of capping agent, indicating high flexibility of this novel nanostructure in different applications. Furthermore, these tadpolelike composites can also be manipulated and oriented under an external magnetic field and self-assemble into one-dimensional structures due to the dipolar-dipolar interactions between magnetite spheres, displaying great potential in precise manipulation and organization of CNT-based structures into integrated functional devices. Acknowledgment. The project was partly supported by the National Natural Science Foundation of China (50372077, 50572114, and 50602049). References and Notes (1) Treacy, M. M. J.; Ebbesen, T. W.; Gibson, J. M. Nature 1996, 381, 678. (2) Wong, E. W.; Sheehan, P. E.; Lieber, C. M. Science 1997, 277, 1971. (3) Ebbesen, T. W.; Lezec, H. J.; Hiura, H. J.; Bennett, W.; Ghaemi, H. F.; Thio, T. Nature 1996, 382, 54. (4) Dai, H.; Hafner, J. H.; Rinzler, A. G.; Colbert, D. T.; Smalley, R. Nature 1996, 384, 147. (5) Tans, S. J.; Verschueren, R. M.; Dekker, C. Nature 1998, 393, 49. (6) Joachim, C.; Gimzewski, J. K.; Aviram, A. Nature 2000, 408, 541. (7) Collins, P. G.; Arnold, M. S.; Avouris, P. H. Science 2001, 292, 706. (8) Yao, Z.; Postma, H. W. C.; Balents, L.; Dekker, C. Nature 1999, 402, 273. (9) Fuhrer, M. S.; Nygård, J.; Shih, L.; Forero, M.; Yoon, Y. G.; Mazzoni, M. S. C.; et al. Science 2000, 288, 494. (10) Bockrath, M.; Cobden, D. H.; McEuen, P. L.; Chopra, N. G.; Zettl, A.; Thess, A.; et al. Science 1997, 275, 1922. (11) Postma, H. W. C.; Teepen, T.; Yao, Z.; Grifoni, M.; Dekker, C. Science 2001, 293, 76. (12) Dai, L. M.; Patil, A.; Gong, X. Y.; Guo, Z. X.; Liu, L. Q.; Liu, Y.; Zhu, D. B. ChemPhysChem 2003, 4, 1150. (13) Korneva, G.; Ye, H.; Gogotsi, Y.; Halverson, D.; Friedman, G.; Bradley, J. C.; et al. Nano Lett. 2005, 5, 879. (14) Correa-Duarte, M. A.; Grzelczak, M.; Salgueirino-Maceira, V.; Giersig, M.; Liz-Marzan, L. M.; Farle, M.; et al. J. Phys. Chem. B 2005, 109, 19060. (15) Liu, J. W.; Li, X. J.; Favier, F.; Dai, L. M. AdV. Mater. 2006, 18, 1740. (16) Ravindran, S.; Chaudhary, S.; Colburn, B.; Ozkan, M.; Ozkan, C. S. Nano Lett. 2003, 3, 447. (17) Ravindran, S.; Bozhilov, K. N.; Ozkan, C. S. Carbon 2004, 42, 1537. (18) Fu, L.; Liu, Z.; Liu, Y.; Han, B.; Hu, P.; Cao, L.; et al. AdV. Mater. 2005, 17, 217. (19) Quinn, B. M.; Dekker, C. S.; Lemay, G. J. Am. Chem. Soc. 2005, 127, 6146. (20) Ruoff, R. S.; Lorents, D. C.; Chan, B.; Malhotra, R.; Subramoney, S. Science 1993, 259, 346.

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