Formation of Nickel Oxide Nanoribbons in the Cavity of Carbon

or filling operation is carried out before the dissolution of the template. .... such a dark image is indicated by the arrow (a) in Figure 1. Apar...
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5682

J. Phys. Chem. B 2001, 105, 5682-5688

Formation of Nickel Oxide Nanoribbons in the Cavity of Carbon Nanotubes Keitaro Matsui, Bhabendra K. Pradhan, Takashi Kyotani,* and Akira Tomita Institute of Multidisciplinary Research for AdVanced Materials, Tohoku UniVersity, 2-1-1 Katahira, Aoba-Ku, Sendai 980-8577, Japan ReceiVed: February 7, 2001; In Final Form: April 9, 2001

An attempt was made to encapsulate nickel into the cavity of the carbon nanotubes, which were prepared by the pyrolytic carbon deposition on the nanochannels of anodic aluminum oxide film. For nickel loading, the MOCVD (metal organic chemical vapor deposition) method using nickelocene was applied to the carboncoated nanochannels and then the nickel/carbon nanotube composites were liberated from the anodic oxide film by alkaline treatment. The resultant carbon nanotubes contained numerous numbers of NiO nanoribbons with about 4 nm thickness and 20 nm width. The length of the longest nanoribbon was about 800 nm. The crystal structure of the NiO nanoribbons was determined by using the electron diffraction technique. Furthermore, the mechanism of the nanoribbon formation was discussed in relation to the formation processes of the nickel/carbon nanotube composites.

Introduction A template technique is a promising method to synthesize nanosized materials with ordered structures. Especially using uniform and straight nanochannels of an anodic aluminum oxide film as a template, many researchers have fabricated a variety of one-dimensional nanomaterials, e.g., nanotubes, nanowires, or nanorods.1-7 We demonstrated that carbon nanotubes can also be prepared by the template method in the following manner; low : pyrolytic carbon deposition on the inner walls of the nanochannels in an anodic oxide film and then liberation of the carbon from the film by dissolving the template.8,9 Since then many other groups have utilized this template technique for the production of carbon nanotubes.10-16 The most striking feature of the carbon nanotubes thus prepared is uniformity of each tube size such as length, diameter, and thickness. Furthermore, with this template technique, encapsulation of other material into the cavity of carbon nanotubes can be easily achieved, if the encapsulation or filling operation is carried out before the dissolution of the template. Such other material never clings onto the outer surface of the carbon nanotubes. This is because there is no other space for the material to be loaded except on the inner surface of the carbon-coated anodic oxide film. With this filling technique, we and then Martin et al. have prepared Pt-, Pt/Ru-, Fe-, and Ag-filled carbon nanotubes in which the metal or metal oxide was present as nanorods, nanoparticles, or nanocrystals.11,17-20 Very recently, we have attempted to insert nickel into the tubes by metal organic chemical vapor deposition (MOCVD) of nickelocene. Wire-like nickel was formed in the cavity of carbon nanotubes as a result. Some of our results are briefly reported in a short communication.21 The most surprising finding in this report was that the thickness of wire-like nickel was as thin as 4 nm and it was less than the inner diameter (20 nm) of the tubes. Much attention has been paid to the fabrication of metal nanowires, because of their fundamental and practical impor* Correspondingauthor.Tel.&Fax81-22-217-5626.E-mail: kyotani@tagen. tohoku.ac.jp.

tance. Recently, such nanowires have often been prepared by the template technique using anodic aluminum oxide films,6,7 polycarbonate track etched membranes,22,23 or nanochannels glass.24 In all of the cases, the obtained diameter of nanowires always corresponds to the inner diameter of the nanochannels in a template. No one has ever found what we reported in the communication, i.e., the formation of nanowires whose diameter is much smaller than the inner diameter of the template channels. Since then we have further investigated the details of both the morphology and the structure of the wire-like nickel. In the present study, we report the results of such detailed analysis and clarify why the diameter of the wire-like nickel looked smaller than the inner diameter of the channels. Furthermore, we determine the crystal structure of such nickel materials and discuss their formation process. Experimental Section We prepared anodic aluminum oxide films with a channel diameter of 30 nm. The details of the preparation method were described elsewhere.9 In addition to this type of anodic oxide films, we used commercially available films (Whatman Int. Limited, Anodisc 13) with a channel diameter in the range 200400 nm, which are much larger than that of the above tailormade films. These two types of anodic films were employed to examine the effect of channel size on the formation of wirelike nickel. The films were subjected to the pyrolytic decomposition of propylene (1.2% in N2) at 800 °C for 3 h, which resulted in the uniform carbon coating on the inner walls of the template nanochannels. The nickel loading into the nanochannels of the carbon-coated films was carried out by the same method as in the previous communication.21 Briefly, the carbon-coated films were subjected to nickelocene (Ni(C5H5)2) MOCVD under H2 flow at 275 °C for 1 h. After this MOCVD, the nickeldeposited film was treated with 10 M NaOH solution at 150 °C in an autoclave for 6 h to dissolve the aluminum oxide template. The Ni/carbon nanotube composites were then obtained as an insoluble fraction. The microscopic features of the composites were observed with a transmission electron microscope (TEM; JEOL, JEM-2010) equipped with an energydispersive X-ray spectrometer (Noran, EDS Voyager) under an

10.1021/jp010496m CCC: $20.00 © 2001 American Chemical Society Published on Web 05/24/2001

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Figure 2. High-magnification TEM images of the nickel inserts taken from different parts (a-c) in Figure 1. Wire-like nickels with a thickness of 4 and 6 nm ((a) and (b), respectively) and a thin plate-like nickel whose width is almost the same as the inner diameter of the carbon nanotube (c) are observed.

Figure 1. Low-magnification TEM image of nickel/carbon nanotube composites prepared with the anodic oxide film with a channel diameter of 30 nm. The high-magnification images of the substances as indicated by the arrows (a-c) are shown in Figure 2.

accelerating voltage of 200 kV. The crystal structure of the nickel compounds formed in the tubes was examined by a highresolution transmission electron microscopic (HRTEM) and electron diffraction for selected area (SAD) techniques. Results and Discussion Appearance of Nickel Inserts in Nanotubes. Figure 1 shows a low-magnification TEM image of the Ni/carbon nanotube composites prepared from the anodic oxide film with a channel diameter of 30 nm. This image exhibits the presence of many carbon nanotubes with an outer diameter of 30 nm. In the cavity of these nanotubes, many inserts were observed. It should be noted that there is no deposit outside of the nanotubes. As for the appearance of the inserts, most of them look like wires and some are particles with a size of less than 20 nm. Among the wire-like substances, the width of the thinnest ones is about 4 nm. They are most noticeable because their dark line images present a striking contrast to other parts. A typical example of such a dark image is indicated by the arrow (a) in Figure 1. Apart from these wire-like inserts, there are many plate-like substances in the tubes and the width of some of them is almost equal to the inner diameter of the tubes. One of the examples for the plate-like substances is indicated by the arrow (c) in the TEM image. Figure 2 shows high-magnification TEM images of these inserts, which were taken from the three parts (a-c) in Figure 1. Parts a and b of Figure 2 show wire-like materials with 4 and 6 nm in width, respectively. In the case of Figure 2c, a plate-like material is observed and both the sides are almost in contact with the inner wall. The TEM observation of many different sights of the composites revealed that about a half of the nanotubes contained several types of inserts such as wire, plate, and particle and the other half were empty. Among the filled nanotubes, about 80% of the inserts were wire- or plate-like, and the rest were particles with a size more than 10 nm. The length of the wire- or platelike materials varied from 10 to several hundreds nanometers,

Figure 3. TEM images of the nickel/carbon nanotube composite observed on the TEM sample stage at three different tilt angles.

and the longest one reached about 800 nm. Many of the short wires were present at angles to the tube axis. Energy-dispersive X-ray spectra confirmed the presence of nickel in the wire-like substances. From the SAD analyses of such wire- and plate-like nickels and particles, the first two were identified as NiO and most of the last as Ni metal. The details of the SAD analysis of wire-like NiO will be described later. Although we reported in the short communication21 that the chemical form of wire-like materials was Ni metal, the detailed reevaluation of those results has revealed the conclusions already reported to be incorrect. Morphology of Wire-Like Nickel Oxide. To investigate the morphology of wire-like NiO, we observed a single wire with TEM at a different tilt angle from -20 to +20°. Figure 3 shows three TEM images of Ni/carbon nanotube composites observed at the three different angles (-20, 0, and +20°). The first image at a stage angle of -20° exhibits a wire-like NiO of about 4 nm in width. In this image, the wire looks as if it is floating in the tube cavity. With increasing tilt angle, the width apparently increases, and at 20°, the width of the substance on the right side becomes close to the inner diameter of the carbon nanotube. If the shape of this material were a wire, its width should not have increased with the tilt angle. Thus, the results obtained from Figure 3 suggest that the shape of the wire-like NiO is not a wire, but a long and narrow plate or a nanoribbon with a thickness of 4 nm.

5684 J. Phys. Chem. B, Vol. 105, No. 24, 2001 With this information in mind, we took a closer look at the TEM images in Figure 2. The plate-like materials observed in Figure 2c are very similar to the image of the wire-like NiO at a tilt angle of 20° (Figure 3). From this finding, it can be said that all of both the wire- and plate-like materials formed in the nanotubes are NiO nanoribbons. In the cavity of the nanotubes, these nanoribbons must be present at a variety of angles to the direction of the electron beam under TEM observation. When the angle between the electron beam and nanoribbon plane is close to 0°, in other words, the beam passes parallel to nanoribbon plane, its appearance becomes wire-like. On the other hand, when the angle is near 90°, the appearance becomes thin plate-like. In Figure 1, the wire-like appearance always comes out as a very dark and noticeable image, while the images of the platelike appearance are weak and unclear. It is well-known that for highly crystallized materials the difference in the darkness mostly depends on how much incident electron beam is reduced by Bragg reflection, rather than the difference in the actual thickness of the objects. We can therefore conjecture that many more electrons were diffracted when the electron beam passes parallel to the nanoribbon plane than when the beam passes vertically. If the wire- and plate-like materials are nanoribbons as described above, the number of wire-like appearance should be comparable to that of plate-like appearance. To check this hypothesis, we counted the number of all the nanoribbons found in a wide TEM observation area (2.0 µm × 1.5 µm) where 443 nanoribbons were seen in a few hundred nanotubes. Among them, the number of nanoribbons that appear as a wire with a thickness in the range 4-5 nm was 215, whereas the number of the plate-like shape whose width is more than 18 nm was 83. The rest (145) correspond to the intermediates between wireand plate-like appearance. In this case, the number of wire-like substances was larger than that the plate-like ones. One of the reasons for this discrepancy is as follows: the images of the thin plate-like inserts are so weak and vague that it is very difficult to count all of them correctly, especially on some parts where the density of the nanotubes is very high. Thus, the number of plate-like appearance must have been underestimated. The reason we mistook nanoribbons for nanowires in the previous communication21 is because our attention had been paid only to the noticeable wire-like appearance and we had missed the unclear images of the plate-like one. Crystal Structure of NiO Nanoribbon. As we described in the previous section, the appearance of a nanoribbon depends on the angle between the electron beam and nanoribbon. The top view of a nanoribbon gives a thin plate and the side view shows a wire. To determine the crystal structure of NiO nanoribbons, the SAD measurement of a single nanoribbon from both views is desirable. This measurement requires the SAD analyses at two tilt angles whose difference is 90°. Such large inclination of the sample stage, however, is not possible. We therefore tried to make the SAD analyses for many different nanoribbons that appear as either a wire or a thin plate. Figure 4 shows a typical TEM image of the side view of two nanoribbons and a SAD pattern taken from the area indicated by a circle in the TEM image. The pattern presents a pair of arcs from carbon (002) and two diffused rings from carbon (10) and (11) reflections. In addition to the reflection from the carbon tube, we observed many spots, all of which were identified as the diffraction from fcc NiO. The appearance of the diffraction not as a ring, but as spots, indicates high crystallinity of the NiO nanoribbon. Each of the spots can be

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Figure 4. TEM image of the side view of nanoribbons (a) and the corresponding SAD pattern (b). The SAD pattern was taken from the area indicated by a circle in the TEM image.

assigned the Miller indices of {111}, {200}, {220}, and {222}, as in Figure 4b. From the examination of the spatial arrangement of the spots, it was found that the set of lattice planes giving rise to these spots derives from a single fcc crystal with its [101] direction being oriented toward the direction of the electron beam. In other words, the SAD pattern of Figure 4b corresponds to the diffraction pattern with [101] zone axis. From the position of (111) and (222) spots in the SAD pattern, one of the {111} planes of fcc NiO should run parallel to the nanoribbon axis. This is confirmed by a high-resolution TEM image (Figure 5), which was taken from the nanoribbon in Figure 4a. In the high-resolution image, the lattice fringes in the nanoribbon are clearly observed and the lattice planes are parallel to the nanoribbon axis. The regular spacing of the observed lattice planes was 0.24 nm, which is consistent with the d spacing of (111) of fcc NiO.

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Figure 5. High-resolution TEM image of a small part of the nanoribbon in Figure 4a.

Figure 6 shows a TEM image of the top view of a nanoribbon and its corresponding SAD pattern. It should be kept in mind that the nanoribbon observed in Figure 6 is different from that in Figure 4. The SAD pattern (Figure 6b) is characterized by the hexagonal arrangement of six strong spots from {220} planes of fcc NiO. Such arrangement is possible only for the diffraction pattern with [111] zone axis. The above SAD analyses of the two different nanoribbons reveal that the side and top views give the [101] and [111] diffraction patterns, respectively. Since the zone axes of [101] and [111] make right angles, a crystal structure of a NiO nanoribbon can be determined. The shape of a whole nanoribbon and its crystal model are schematically illustrated in Figure 7. Since the latter structural model (Figure 7b) was drawn with its nanoribbon axis in accord with the axis of the nanoribbon in Figure 7a, the atomic arrangement of the nanoribbon can be viewed as in Figure 7b. It can be seen from this model that NiO {111} and {202} planes are parallel to the top and side surfaces of the nanoribbon, respectively, and the direction of the ribbon axis is 〈211〉. The proposed crystal model as in Figure 7b was determined from the results of only the two SAD patterns in Figures 4 and 6. We made such SAD analysis also for many other nanoribbons, five ribbons for the side view and seven for the top view. For these analyses, we obtained the same results from the four ribbons for the side view and from the five for the top view. We can thereby conclude that most of the NiO nanoribbons formed in the tubes are highly crystallized and have a crystal structure as proposed in Figure 7. Effect of Alkaline Treatment. It was reported that Ni metal was always the only phase formed in the MOCVD using nickelocene.25,26 The nanoribbons formed in the present work, however, were identified as a highly crystallized fcc NiO. Since nickelocene must be decomposed to Ni metal under the present conditions, the oxide was likely formed when the nickeldeposited carbon/alumina film was exposed to air and/or when the film was treated with the alkaline solution. To clarify this issue, we tried to characterize the nickelcontaining carbon nanotubes before the alkaline treatment. After the MOCVD experiment, the nickel-deposited carbon/alumina film was taken out to the ambient air and broken into fine pieces. The TEM observation showed that some of the nanotubes projected out of broken alumina pieces and also some of the nanotubes were separated from the alumina matrix. A TEM image of such isolated carbon nanotubes containing Ni inserts

Figure 6. TEM image of the top view of nanoribbon (a) and the corresponding SAD pattern (b).

is shown in Figure 8, where three particles with a size of 8-9 nm are observed together with many fine particles with a size of less than 3 nm. The corresponding SAD pattern (Figure 8b) presents several sharp spots, which were identified as the reflection from fcc Ni metal (the spots can be indexed (111), (200), and (311)). Probably, the three large particles in Figure 8a would be responsible for the diffraction spots from Ni metal. Together with these sharp spots, we could observe many weak spots along concentric circles. These spots can be assigned to the reflections from (111), (200), and (220) planes of fcc NiO, and it is likely that these reflections come from the many fine particles in Figure 8a. Thus, we can conclude that upon the air exposure the fine particles were oxidized while the large particles remained unoxidized. Since almost all the particlelike large inserts we observed after the alkaline treatment were Ni metal, these inserts may originate from such large Ni metal particles as observed in Figure 8. In other words, such large

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Figure 8. TEM image of the nickel/carbon nanotube composites before the alkaline treatment (a) and the corresponding SAD pattern (b). Figure 7. Shape (a) and structural model (b) of NiO nanoribbon.

particles, at least some of them, remained unoxidized even during the alkaline treatment. Upon the TEM observation of several hundred different sights of the tubes before the alkaline treatment, we found huge numbers of particles. As long as we observed, there was no highly crystallized ribbon-like nickel. We found only five ribbon-like materials, all of which were ill-crystallized, and their images were not as dark as the nanoribbons observed after the alkaline treatment. The number of the ribbon-like materials at this stage is too few to ascribe them to the origin of a large number of the NiO nanoribbons. Thus, we can safely say that the formation of highly crystallized NiO nanoribbons is likely to occur during the alkaline treatment process. Effect of Channel Size. To examine the effect of the channel size on the formation of nanoribbons, the commercial alumina films with the larger channel size were subjected to the same sequence of the treatments, i.e., carbon deposition, MOCVD, and alkaline treatments. Figure 9 shows one of the TEM images of the resultant Ni/carbon tube composites before the alkaline treatment. Numerous particles with various sizes are observed in the cavity of the tube. From the SAD pattern of this image, the chemical form of these particles was determined to be Ni metal and NiO. There was no other shape of inserts in this case. Once the alkaline treatment was carried out, the appearance in the tube cavity was dramatically changed. The resultant TEM image is shown in Figure 10, where several shapes of inserts are observed such as particle, plate, and even wire. Most of the wire-like materials can be found near or in contact with the tube walls, and their thickness is generally much larger than

those of the thinnest nanoribbons (4 nm) observed in the carbon nanotubes with 30 nm diameter. By using the tilting technique described in the previous section, it turned out that the wirelike appearance is the outcome of the side view of thin plate substances. The SAD analysis revealed that the large particles are Ni metal and the other thin plates are NiO. It is noteworthy that the SAD patterns from the thin NiO plates appeared as sharp spots, indicating the high crystallinity of these plates. This experiment using the alumina film with the larger channels gives us the following important information. First, the long and narrow NiO plates (nanoribbons) as observed in the nanotubes with the small diameter were not formed but the thin plates with larger width were observed when tube size is as large as more than 200 nm. In other words, the spatial hindrance due to the tube hollow with an inner diameter of 20 nm is required for the formation of such narrow NiO plates (nanoribbons). Second, the highly crystallized NiO plates were not formed during the MOCVD process but during the alkaline treatment. Thus, it is evident that the alkaline treatment plays an important role in the formation of the highly crystallized thin NiO plates including nanoribbons. A crystallization phenomenon was already observed in the case of the ferrocene MOCVD work,18 where upon the alkaline treatment highly crystallized Fe3O4 nanoparticles were formed in the cavity of the nanotubes and some of them were found to be single crystals. Since such alkaline treatment has often been used for the hydrothermal syntheses of Ni-containing inorganic crystals with plate-like shape,27,28 the phenomenon we observed here is not so curious. Formation Process of NiO Nanoribbons. Taking the effect of the alkaline treatment and the role of the tube cavity into

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Figure 9. TEM image of the nickel/carbon nanotube composites prepared from the anodic oxide film with a channel size of 200-400 nm. The image was taken before the alkaline treatment.

Figure 10. TEM image of the nickel/carbon nanotube composites prepared from the anodic oxide film with a channel size of 200-400 nm. The image was taken after the alkaline treatment.

consideration, we can explain the formation of NiO nanoribbons in the following way. Initially, Ni metal particles were deposited in the nanotube hollow by the MOCVD under H2 flow. When these particles were exposed to air, most of the large particles remained unoxidized, but the fine particles were oxidized to NiO. Upon the subsequent alkaline treatment, NiO crystals were formed from these fine NiO particles. It is likely that the fine particles were dissolved into the alkaline solution in the autoclave and these dissolved species deposit and grow as thin plate-like crystal of NiO during the treatment. Such crystallization took place in the cavity of the nanotubes with the inner diameter as small as 20 nm, and consequently, the NiO crystals could grow only toward the direction of the tube axis. Thus, long and narrow NiO plates, that is, NiO nanoribbons, were formed. On the other hand, in the case of the large tubes with a diameter of larger than 200 nm, NiO plates with a size of a few hundred nanometers can be formed without spatial hindrance. We did not observe any NiO nanowires in the present study. The reason for this is as follows. The formation of cylindrical nanowires or nanorods requires the complete filling of the tube cavity with the NiO crystal, but the observed anisotropic crystal growth of NiO thin plates along the tube axis did not satisfy such requirement.

anodic aluminum oxide template. The width of the nanoribbons is almost the same as the inner diameter (20 nm) of the carbon nanotubes, and the thickness is about 4 nm. The length of the longest one reached about 800 nm. The crystal structure of the nanoribbons was determined as follows; NiO {111} and {220} planes are parallel to the top and side surfaces of the nanoribbon, respectively, and the direction of the ribbon axis is 〈211〉. It was found that the alkaline treatment, which was carried out for the template removal, plays a crucial role in the formation of such highly crystallized NiO nanoribbons.

Conclusions Highly crystallized NiO nanoribbons were obtained in the cavity of the carbon nanotubes, which were prepared in the

Acknowledgment. We express our appreciation to the High Voltage Electron Microscope Laboratory of Tohoku University for microscopic analysis. This study was partly supported by the Ministry of Education, Science, Sports, and Culture, Grantin-Aid for Scientific Research on Priority Areas, No. 288 “Carbon Alloys”. References and Notes (1) Martin, C. R. Science 1994, 266, 1961. (2) Lakshmi, B. B.; Dorhout. P. K.; Martin, C. R. Chem. Mater. 1997, 9, 857. (3) Hoyer, P.; Baba, N.; Masuda, H. Appl. Phys. Lett. 1995, 66, 2700. (4) Hoyer, P. Langmuir 1996, 12, 1411. (5) Routkevitch, D.; Bigioni, T.; M. Moskovits, M.; Xu, J. M. J. Phys. Chem. 1996, 100, 14037. (6) Davydov, D. N.; Haruyama, J.; Routkevitch, D.; Statt, B. W.; Ellis, D.; Moskovits, M.; Xu, J. M. Phys. ReV. B: Condens. Matter. 1998, 57, 13550.

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