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ISRN Condensed Matter Physics
Volume 2012 (2012), Article ID 140842, 7 pages
Fullerene Nanotubes Fabricated with Light Irradiation as a Critical Influence Factor
1Key Laboratory of Rubber-Plastics, Ministry of Education, Qingdao University of Science and Technology, 53 Zhengzhou Road, Qingdao, Shandong 266042, China
2School of Polymer Science and Engineering, Qingdao University of Science and Technology, 53 Zhengzhou Road, Qingdao, Shandong 266042, China
Received 3 August 2012; Accepted 21 August 2012
Academic Editors: V. Kochereshko and S. Krukowski
Copyright © 2012 Yongtao Qu et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
Crystalline fullerene C60 nanotubes were prepared simply via liquid-liquid interfacial precipitation using the mixture of C60-saturated pyridine and isopropyl alcohol. C60-saturated pyridine solution was exposed to visible light to promote the growth of fullerene C60 nanotubes. The average diameters of the fullerene particles in C60-pyridine colloid solution after irradiation were characterized by dynamic light scattering. After light irradiation, an outer separated layer of pyridine surrounds the fullerene particles because of the charge transfer complexes formation. The mean ratios of inner diameter to outer diameter of fullerene C60 nanotubes fabricated at different irradiation time and wavelength were given in this paper for the first time. On the basis of the relationship between the average diameters of the fullerene particles in C60-pyridine colloid solution and the mean ratio of inner diameter to outer diameter of FNTs fabricated after irradiation, outer separated layer of pyridine surrounding the fullerene particles was supposed to play an important role in the formation process of fullerene C60 nanotubes.
Since the discovery of carbon nanotube (CNT), one-dimensional (1D) nanometer-scale materials have been extensively studied owing to their unique structures and physical properties, which lead them to a range of potential applications in the field of nanometer-scale devices. C60 is a well-known fullerene prototype, and its zero-dimensional structure has been generally accepted . It has been expected that the 1D tube of C60, prepared via the self-assembly of zero dimensional C60, will possess the novel optoelectronic and magnetic properties. Fullerene C60 nanofibers (FNFs) have been attracting much attention in recent years [2–5] and found a number of potential applications, such as field-effect transistors , solar cells , fuel cell catalyst carriers , and electrodes [9, 10].
Recently, various synthesis methods have been developed for the preparation of fullerene C60 nanotubes (FNTs), such as solution evaporation , template technique , surfactant-assisted method , and liquid-liquid interfacial precipitation (LLIP) method [12, 13]. Compared to other synthesis methods, the LLIP is simple and financially viable. FNTs can be achieved at room temperature and without the need for catalysts or surfactants. However, the formation mechanism of the FNTs during the simple and easy to operate LLIP is not clear up till now. So far, several possible mechanisms have been proposed, such as “2+2” cycloaddition , core dissolution mechanism [14, 15], concentration depletion mechanism , and C60-pyridine colloid as a precursor .
In this study, we have found that the visible light irradiation before the mixing of C60-saturated pyridine and isopropyl alcohol (IPA) plays an important role in the formation of FNTs. During the irradiation, there is a color change process of C60-saturated pyridine solution. The average diameters of the fullerene particles in C60-saturated pyridine solution would change greatly after the addition of IPA. The mean ratios of inner diameter to outer diameter of FNTs depending on the irradiation time were collected in this paper for the first time. On the basis of the relationship between the average diameters of the fullerene particles in C60-pyridine colloid solution and the mean ratios of inner diameter to outer diameter of FNTs fabricated after irradiation, a possible growth mechanism is proposed to describe the formation process of FNTs.
2. Results and Discussion
Figure 1 shows FT-IR spectra of FNTs (a) and pristine C60 powder (b), respectively. Both spectra showed characteristic sharp absorption peaks of C60 (526, 576, 1182, and 1428 cm−1), confirming that the nanotubes are composed of C60 molecules . However, as shown in spectra (a), some modifications of the base line were observed even after drying in vacuum, which may be related to the presence of the solvent pyridine or IPA molecules.
The morphology of the FNTs was analyzed by SEM. As shown in Figure 2(a), the FNTs have the fibril structures with the diameter of about 500 nm, and all the cross sections of FNTs have the rectangle structures. Figure 2(b) shows a TEM image of typical FNTs precipitated in the C60-saturated pyridine and IPA colloidal solution. The FNTs show tubular structure with wide size distribution. To accurately analyze outer diameter size distribution of FNTs, the outer diameter of 99 FNTs have been measured and the diameter size distribution is shown in Figure 3. As shown, most FNTs have the outer diameters ranging from 200 nm to 600 nm, accounting for 89% of all samples. The acquired select area electron diffraction (SAED) pattern of an FNT inset in Figure 2(b) indicates that the FNT is a local single crystal.
To confirm the crystal structure of FNTs, XRD analysis was carried out and the XRD pattern of the FNTs is shown in Figure 4. There are five diffraction peaks, which have been indexed as the (111), (220), (311), (420), and (511) diffraction peaks from an fcc lattice, indicating that the FNTs have a face-centered cubic (fcc) structure with cell dimensions a =. From the SAED pattern, the growth direction of the FNT is , which is in good agreement with the result reported by Minato et al. .
In the process of preparing FNTs, it is found that the freshly prepared C60-pyridine solution is purple-pinkish at the beginning but turns into burgundy over time . Interestingly, the visible light irradiation such as green light centered at 525 nm can speed up the color change process if the C60-pyridine solution is exposed to visible light. However, such color change is not observed in the C60-toluene or C60-m-xylene solution [20, 21]. The color change of C60-pyridine solution depending on the irradiation time was recorded by using the UV-visible spectrophotometer (Figure 5).
As shown in Figure 5 (a–c), after light irradiation, the characteristic valley of C60 located at about 440 nm in the C60 spectrum  progressively disappears with increasing irradiation time. After light irradiation, the absorbance of the freshly prepared purple-pinkish solution (trace a) increases visibly in the region of 400–500 nm (trace b-c), which is consistent with the observed color change. But when the irradiation time increases to 50 min, the absorption in the region of 400–500 nm decreases (trace d) visibly compared with trace c. The inset shows the details of the spectral structure in the 520–550 nm wavelength region, where an intersection point of trace a–d take place at about 536 nm. This intersection point is defined as the isoabsorptive point. The existence of isoabsorptive point means a balanced reaction between two substances in the C60-saturated pyridine solution when irradiated by green light.
This phenomenon is probably related to the formation of the C60-pyridine charge transfer (CT) complexes [23, 24]. Fullerene C60 has the ability to accept multiple electrons, making it become a potential electron accumulator [24, 25]. On the other hand, pyridine has a nitrogen atom with lone pair, which allows the molecule to be an electron donor [19, 26]. It is reasonable that CT complex should exist in the C60-pyridine system under certain condition. The absorption peak in the region of 400–450 nm corresponds to CT complex absorption, and the characteristic valley located at 440 nm and the absorption in the region of about 500–600 nm are corresponds to C60 molecules absorption.
To further understand the color change process, the average diameters of the fullerene particles in C60-saturated pyridine solution were detected by DLS with different irradiation time and the corresponding diameter changes after IPA addition, which are shown in Figure 6.
As shown in Figure 6, the mean diameters of the fullerene particles vary with the irradiation time. Compared to other irradiation time, when irradiated by green light for 30 min, the fullerene particles in C60-saturated pyridine solution show the largest mean diameter about 260 nm. It is speculated that the pristine C60 molecules were surrounded by a large amount of pyridine molecules due to the formation of CT complexes. It is supposed that 30 min is the optimum irradiation time to form CT complexes between pyridine and C60 molecules according to the UV-vis absorption spectra. So some pyridine molecules may even permeate into the loose pristine C60 particles to form CT complexes with C60 molecules. Due to pyridine molecules’ permeation and CT complexes’ formation, the pristine C60 particles in the C60-saturated pyridine solution became looser and showed larger diameters. This hypothesis is supported by the mean diameter decrease after IPA injection.
After IPA is added, C60-pyridine colloids will be formed. The mean diameter of C60-pyridine colloids without the irradiation increases significantly to about 1400 nm. If no irradiation, little CT complexes formed. Without the separation of pyridine adsorbed by fullerene molecules around fullerene particles, the fullerene molecules will contact with the polar medium IPA directly, which is energetically unfavorable and they prefer to aggregate . So irradiation plays an important role in the stability of C60-pyridine colloids after IPA injection.
Fullerene particles will not dissolve with the polar solvent IPA because IPA is a bad solvent for fullerenes. However, as shown in Figure 6, the mean diameter of C60-pyridine colloids decreases significantly after IPA injection when the irradiation time is about 20–40 min. This phenomenon may result from the hydrophobic interaction when IPA was injected into C60 pyridine solution [28, 29]. Pyridine was adsorbed by fullerene molecules when CT complexes formed appear to act as a surfactant around fullerene molecules protecting them from IPA [11, 30]. So there will be some repulsive interaction between nonpolar Pyridine and polar IPA. Contrarily, there is no aggregation but size decrease (), as can be seen in Figure 7. The hydrophobic surfactant interactions are thought to act in stabilizing the structures into small-sized sphere-like fullerene particles .
Interestingly, there is little size variation when the irradiation time increases to 60–70 min. Just as shown in UV-vis analysis, excessive light irradiation goes against the formation of CT complexes. So the C60 particles would also be encapsulated by an outer layer of pyridine, and no aggregation would happen when IPA injected. But there is not enough pyridine adsorbed by fullerene particles, and hydrophobic interaction is not strong enough for the size decrease when irradiated for 60–70 min.
The inset in Figure 6 shows the fitted curve to the mean diameters of C60-pyridine colloids after IPA added. The relationship between the irradiation time (x, min) and the mean diameters of the C60-pyridine colloids (y, nm) was fitted by a curve of . The equation suggests that when the irradiation time is 34 min, there are the smallest colloid particles after IPA injected, which means the most stable structure of C60-pyridine colloid.
Figure 8 gives the irradiation time and wavelength dependence of the mean ratio of inner diameter to outer diameter of FNTs.
As shown in Figure 8, when the wavelength of irradiated light centered at 468 nm, the ratio of inner diameter to outer diameter is about 0.67, which is the largest.. This is because the increase absorbance of fullerene C60 dissolved in pyridine solution within the short light wavelength regions, which is beneficial to the formation of CT complexes . Taking no account of wavelength, when irradiated by 30 min, there is the largest ratio of inner diameter to outer diameter of FNTs, which means relatively largest inner diameter and thinnest tube wall.
With the experimental observations mentioned in the above section, a possible growth mechanism describing the formation process of C60 FNTs is proposed. The schematic is shown in Figure 9.
After light irradiation, C60-pyridine CT complexes formed immediately, which play an important role in the nucleation and crystal growth process. Previous works [33, 34] have reported that C60 colloids with narrow size distributions were immediately formed when the solution of fullerenes in a good solvent was injected into a poor solvent. So when IPA was injected into C60-saturated pyridine solution, crystal seeds were formed immediately. Besides the crystal seeds, sphere-like fullerene particles were encapsulated by an outer layer of pyridine because of CT complexes formation, which acted as the C60 suppliers in the crystal growth process. Due to the highly anisotropic nature of these crystal seeds , the growth direction was largely confined to the direction, resulting in one-dimensional structure . During the crystal growth after nucleation, the C60 molecules prefer the corners of the rectangle cross section, as indicated by the red C60 molecules in Figure 9, because the corner sites have a relatively higher free energy . The secondarily preferable sites for C60 molecules are the edges of the cross section, as indicated by the blue molecules. The last ones are the central portion of the cross section, and this crystal growth process will end at the equilibrium point, when the C60 suppliers are too stable to supply C60 molecules to the central portion .
According to this mechanism, it is expected to obtain FNTs with different ratios of inner diameter to outer diameter when the C60 suppliers have different equilibrium points. In this study, it is the most favorable condition for the formation of CT complexes between pyridine and C60 molecules when the C60-saturated pyridine solution was irradiated by light with relatively short wavelength and optimum time. After IPA injection, the most stabilized sphere-like fullerene particles with smallest diameters encapsulated by an outer layer of pyridine were formed immediately. Reaching to the equilibrium point easily, these stable fullerene particles are not likely to supply C60 to the central portion of the tubes because of the outer protect layer of pyridine. As a result, FNTs with relatively thinner walls and clear inner diameters were formed. Some researchers have suggested that the rate of growth of a crystal face is a function of the degree of supersaturation at that face [35, 36]. The protection layer of pyridine outside the fullerene particles extends the duration of the C60 supply stage, which means low supersaturation at the face, indicating that the process of increasing the reproducibility of high-quality FNTs with relatively largest inner diameter and thinnest tube wall is rather slow and steady.
3. Experimental Section
FNTs were prepared at ambient pressure and around room temperature using C60 fullerene powder (99.5% purity, MER Ltd.) based on the experimental procedures described in references [16, 37]. The typical procedure was as follows. After ultrasonication for 10 min in an ice water bath, a purple-pinkish pyridine solution saturated with C60 was prepared. In order to promote the growth of the FNTs, the solution was exposed to visible light such as green light with the center wavelength of 525 nm [20, 32, 38]. After irradiation, the red purple pyridine solution turned into burgundy solution immediately. 2 mL of this brown pyridine solution was put into a transparent 25 mL glass bottle, followed by adding 18 mL of IPA. To obtain suitable diffusion at the interface, the mixture was vigorously agitated in an ultrasonic bath for 1 min before stored at 8°C. Golden-brown cluster fibers were suspended in the solutions after about 12 hours.
Infrared spectroscopy was performed for the specimen dried at room temperature and using an FTIR apparatus (Bruker Vertex 70). Hollow structure of the C60 FNTs was characterized by using transmission scanning electron microscope (TEM, JEOL JEM-2000EX) and scanning electron microscope (SEM, JEOL JSM-6700F). For the purpose of electron microscopic measurement, the specimens were placed on silicon wafer substrates or copper microgrid with carbon film. The XRD measurements were performed at Beijing Synchrotron Radiation Facility (BSRF) at ambient temperature. The X-ray wavelength was 0.06199 nm, and the beam size was 25 μm in diameter. The color change of C60-pyridine solution was recorded by using the UV-visible spectrophotometer (SHIMADZU UV-2400PC). The average diameters of the fullerene particles in C60-pyridine colloid solution were detected immediately with DLS (Zetasizer Nano ZS90).
Irradiated with optimum time, the FNTs with relatively thinner walls and clear inner diameters were prepared via LLIP using the mixture of C60-saturated pyridine and IPA at 8°C. After light irradiation, C60-pyridine CT complexes formed immediately, which play an important role in the nucleation and crystal growth process. A protection layer of pyridine outside the fullerene particles was formed because of CT complex formation. After IPA injected, stable C60-pyridine colloids will be formed, which acted as the C60 suppliers in the crystal growth process. The hydrophobic surfactant interactions might act in stabilizing the structures into small-sized sphere-like fullerene colloid particles. When the irradiation time was 34 min, there were the smallest particles after IPA injection, which means the most stabilization of C60-pyridine colloid and the topmost reproducibility of high quality FNTs with relatively largest inner diameter and thinnest tube wall. On the basis of experimental observations, a possible growth mechanism is proposed to describe the formation process of C60 FNTs.
This work was partially supported by the Program for International S&T Cooperation Projects in the Ministry of Science and Technology of China (2011DFA50430), National Natural Science Foundation of China (50773033 and 50872060), Science Foundation of Shandong Province (Y2007F01 and Q2008F07), and Doctoral Fund of QUST.
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