FORMATION OF PLATELET STRUCTURE CARBON NANO- AND MICRO-FIBERS BY TEMPLATE METHOD

Transcription

1 FORMATION OF PLATELET STRUCTURE CARBON NANO- AND MICRO-FIBERS BY TEMPLATE METHOD Hidetaka Konno 1, Shin ya Sato 1, Hiroki Habazaki 1, Michio Inagaki 2 1 Graduate School of Engineering, Hokkaido University, Sapporo , Japan 2 Aichi Institute of Technology, Yakusa, Toyota , Japan Corresponding author address: ko@eng.hokudai.ac.jp Introduction Porous anodic oxide films on aluminum are useful templates for formation of nano- and micro-structured carbons [1-3]. By vapor phase deposition into these templates, carbon nanotubes of concentric structure were formed [1,2], while by impregnation of mesophase pitch into the templates, platelet structure naonofibers were obtained [3]. We have been studying to utilize polymer powders for carbon coating on ceramics powder [4] and aluminum [5] by liquid phase carbonization. In the latter work, we pointed out that pitch-like materials formed by the thermal decomposition of poly(vinyl alcohol) (PVA) entered the miropores of anodic oxide films on aluminum [5]. Thus, in the present work formation of carbon nano- and micro-fibers by liquid phase carbonization of PVA and poly(vinyl chloride) (PVC) was investigated using aluminum based templates. Experimental The following five templates were used. I. Porous alumina membrane filter (Whatman ANODISC TM, pore diameter 200 nm, thickness 50 µm). II. Anodic aluminum oxide film formed on % Al in 0.3 mol L -1 (COOH) 2 at 40 V for 2 h: the film was peeled from the substrate and its barrier layer was dissolved in a H 3 PO 4 solution (pore diameter 80 nm, thickness 10 µm). III. Anodic aluminum oxide film formed on % Al in 0.3 mol L -1 H 2 SO 4 at 25 V for 1 h: pores were widened by dipping in 5% H 3 PO 4 for 30 min but not peeled from the Al substrate (pore diameter 40 nm, thickness 10 µm). IV. Tunnel-etched Al foil (Nippon Chemi-Con, no oxide film, pore diameter >1 µm, thickness 80 µm). V. Tunnel-etched Al foil anodized in 0.5 mol L -1 H 3 BO mol L -1 Na 2 B 4 O 7 with 100 A m -2 up to 300 V (pore diameter µm, thickness 80 µm). The 0.1 g of PVC powder (degree of polymerization 700) or 0.2 g of PVA powder (degree of polymerization 1700) per I cm 2 of template was put in an alumina boat and heated in Ar at 400 K h -1 to 300 C and kept for 30 min, then heated up to 600 C and

2 kept for 1 h. Templates were dissolved in a 10 mass% NaOH solution. Thus formed precursor was heat-treated at 1500 C for 1 h in Ar. The products were examined by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and Xray diffraction (XRD). Results and Discussion Similar to the products from mesophase pitch and porous alumina membrane filter [3], carbon nanofibers having a diameter of the template were obtained with template I by liquid phase carbonization of PVA and PVC, as shown in Figure 1. The structure, however, was different depending on the polymers: PVC produced solid fibers, while PVA hollow ones. We must not jump to conclusions but one of the reasons may be a Figure 1. SEM images of the fibers formed with PVC (left) and PVA (right) using template I, after 1500ÞC treatment. Bars are 1 µm. (c) 1 µm 100 nm 5 nm Figure 2. a SEM image, a TEM image, and (c) a lattice image of the fibers formed with PVC and template II, after 1500ÞC treatment. An arrow shows fiber axis.

3 (c) 1 µm 100 nm 5 nm Figure 3. a SEM image, a TEM image, and (c) a lattice image of the fibers formed with PVA and template II, after 1500ÞC treatment. An arrow shows fiber axis. difference in viscosity of the decomposition products around 300 C. The results were similar with the anodic aluminum oxide film (template II), which has a smaller pore diameter and the same filter-type shape, as shown in Figures 2 and 3. The fibers formed from PVC are solid (Figure 2) and the lattice image clearly shows that the formed fibers have the platelet structure (Figure 2(c)), the same with that formed from mesophase pitch [3]. The fibers from PVA are hollow (Figure 3) and the hexagonal carbon layers are not exactly perpendicular to the fiber axis (Figure 3(c)), but still they have the platelet structure. So far, filter type templates have been used but preparation of templates is complicated and takes time. So we attempted to use anodic oxide films without peering from the substrate, that is, template III has pores with a very large aspect ratio and one end closed. With this template and PVC, nanofibers of about 40 Figure 4. a TEM image of the fibers formed with PVC and template III, and a selected area electron diffraction pattern, and a lattice image, both after 1500ÞC treatment. An arrow shows fiber axis.

4 nm in diameter were obtained as shown in Figure 4. It is obvious from a selected area electron diffraction pattern of the circled part in and a lattice image that these fibers have regularly stacked platelet structure, though they are turbostratic carbons by XRD. The fiber length was several micrometers and shorter than the pore depth. The results with template III imply that the pitch-like materials from PVC enter to pores by capillary action. This leads to drastic retrenchment of the process. On the application of platelet structure carbon fibers to energy storage, such as electrodes of lithium ion batteries and electric double layer capacitors, nano-size is not always important. Accordingly, use of tunnel-etched Al foil (template IV) was considered because it is commercially available and not expensive as membrane filters (template I). The pore wall of as received foil is not covered with oxide film. With this template and PVC powder, several to a few tens micrometer microfibers were obtained but the orientation of the hexagonal carbon layers was parallel to the fiber axis as shown in Figure 5. The orientation was unequivocally confirmed after 3000 C treatment of this fiber. This is inconsistent with the reports that alumina and Al metal similarly favor open forms through edge-on anchoring of discotic mesopahse pitch [3,6]. After the pore wall was covered with oxide film by anodic oxidation (template V), however, the hexagonal carbon layers were inclined at shallow angles to the fiber axis, as shown in Figure 5. There is a possibility that the platelet structure will be developed more by further heat treatment. The results were the same with PVA. At present, we have no lucid explanations about the reasons but have to recognize that oxide template is indispensable to develop platelet structure, at least by liquid phase carbonization of polymers. Figure 5. Lattice images of the fibers formed with PVC using template IV and template V, after 1500ÞC treatment. Arrows show fiber axis. Conclusions It was demonstrated that carbon nanofibers of several tens nanometers in diameter were possible to be formed with Al based porous templates by the liquid phase

5 carbonization of polymer powders. As formed porous anodic oxide films are useful as templates instead of through hole types. The fibers had platelet structure, when the pore wall of template was aluminum oxide. References [1] Kyotani T, Tsai L, Tomita A, Formation of ultrafine carbon tubes by using an anodic aluminum oxide film as a template. Chem Mater 1995;7: [2] Kyotani T, Tsai L, Tomita A, Preparation of ultrafine carbon tubes in nanochannels of an anodic aluminum oxide film. Chem Mater 1996;8: [3] Jian K, Shim H-S, Schwartzman A, Crawford GP, Hurt RH, Orthogonal carbon nanofibers by template-mediated assembly of discotic mesophase pitch. Adv Mater 2003;15: [4] Inagaki M, Miura H, Konno H, A new process for carbon coating of ceramic particles using poly(vinyl chloride), J Euro Ceram Soc 1998;18: [5] Konno H, Oyamada K, Inagaki M, Formation of carbonaceous coatings on aluminum using poly(vinyl alcohol), J Europ Ceram Soc 2000;20: [6] Jian K, Shim H-S, Tuhus-Dubrow D, Bernstein S, Woodward C, Pfeffer M, Steingart D, Gournay T, Sachsmann S, Crawford GP, Hurt RH, Liquid crystal durface anchoring of mesophase pitch, Carbon 2003;41: