Carbon nanofibers with radially oriented channels

Carbon 45 (2007) 173–179 www.elsevier.com/locate/carbon

Carbon nanofibers with radially oriented channels Seongyop Lim a,*, Seong-hwa Hong a, Wenming Qiao a, D. Duayne Whitehurst a, Seong-Ho Yoon a,*, Isao Mochida a, Bei An b, Kiyoshi Yokogawa b a

Institute for Materials Chemistry and Engineering, Kyushu University, Fukuoka 816-8580, Japan b AIST, Chugoku Center, Kure, Hiroshima-ken 737-0197, Japan Received 19 April 2006; accepted 12 July 2006 Available online 12 September 2006

Abstract An easy templateless method to synthesize porous carbon nanofibers (CNFs) having radially-oriented mesopores is reported, using selective gasification with nano-sized catalyst particles and hydrogen as the gasification reagent. The gasification generated pores whose size corresponds to that of the catalyst particle used for the drilling. The pores were formed along the graphitic layers, reflecting the structural alignment in the CNFs. The pore size and structure can be controlled by selection of catalyst and gasification conditions. Ó 2006 Elsevier Ltd. All rights reserved.

1. Introduction Mesoporous carbons have attracted much attention to meet many important applications as selective adsorbents, catalyst supports, molecular sieves, and electrode materials [1–5]. They are generally prepared in a manner similar to the lost wax process. Such a process includes incorporation of a carbon precursor with a removable moiety (template), carbonization into solid carbons and then removal of the moiety, leaving a porous solid having pores that are similar in size and shape to the original moiety. The resultant porous carbon can have a very narrow pore size distribution in the mesopore range. However, the preparation of these materials is basically expensive and time consuming, since the template of well-controlled structure is required and should be removed by troublesome processes after all. The structure of CNFs has been recognized on the basis of particular stacks of graphitic layers, to illustrate them as a miraculous single crystal of fibrous and nano-sized

* Corresponding authors. Tel.: +81 92 583 7959; fax: +81 92 583 7897 (S. Lim). E-mail addresses: [email protected] (S. Lim), yoon@cm. kyushu-u.ac.jp (S.-H. Yoon).

0008-6223/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2006.07.009

graphite [6–8]. Recently, Helveg et al. [9] showed through exquisite TEM observation on CNF growth over nickel particles that CNF is formed by repetition of temporal nucleation and growth of graphenes. The present authors found that catalytically grown CNFs consist of assemblages of small sub-structure that are called carbon nano rod [10–12]. The nano rod is considered one of the fundamental building blocks for various forms of CNFs and possibly many other types of carbon materials. Such results would significantly contribute to understanding on the structure of CNFs such as their formation over catalyst particles, their structural changes through various post-treatments, and the relation between structure and properties. Gasification of carbon materials has been widely applied to generate porosity on the carbon surface. Catalytic gasification of graphite has been extensively studied in various environments since 1970s, in order to understand basic reaction schemes of carbon with oxygen, hydrogen, steam and carbon dioxide [14–20]. The catalyst particles were observed to migrate preferably parallel to the h11 20i direction in graphite under controlled atmosphere electron microscopy [14,17–20]. Such a preferential reaction progress appeared to reflect much higher reactivity of the edge of graphitic layer than the basal plane. Hence, the catalytic

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gasification is expected to generate nano-sized pores selectively on a carbon material whose surface consists substantially of the edge of graphitic layers, such as particular CNFs having the layers aligning perpendicular (platelet) or symmetrically angled (herringbone) to the fiber axis. Uniformly-aligned pores can be introduced into the platelet and herringbone CNFs, since the nano rods in such CNFs can define the moving direction of catalyst particles through the gasification. In this study, porous CNFs were synthesized through two steps which include the careful preparation of platelet and herringbone CNFs, and then selective drilling of channels along with the aligned graphitic layers by means of catalytic gasification from the outside of the fiber into its center in a substantially transverse fashion using carefully selected nano-sized catalysts under specific reaction conditions. The as-prepared CNFs are well-defined and thoroughly characterized with SEM, TEM, STM and Raman spectroscopy [10–13]. Formation of the aligned pores was discussed under recognition of the CNF structure which consists of carbon nano rods.

2.2. Drilling pores in the CNFs A Ni precursor as drilling catalyst was dispersed on the CNF prepared by the incipient wetness with aqueous solution of nickel nitrate (Wako Chemical, Japan) at the ratio Ni:nanofiber = 5:100 (w/w) and then drying the composite at 423 K in a vacuum oven for 2 h. Drilling of the Ni–CNF composition was performed 873–1173 K in a He/H2 mixture. Iron nitrate was also applied as a drilling catalyst precursor.

2.3. Characterization of CNFs as prepared and drilled The structures of pristine and modified CNFs were examined using SEM (scanning electron microscope; JSM-6700F, JEOL) and HR-TEM (high resolution transmission electron microscope; JEM-2100F, JEOL). For SEM, a powdered sample was well dispersed over a piece of carbon tape sticking to a cylindrical sample holder of copper. For TEM, a very small quantity of the sample, which was finely dispersed in n-butanol, was dropped onto a microgrid (a copper grid coated with amorphous carbon) supplied by JEOL, Co. Multi-point BET surface area of CNFs produced was measured with nitrogen adsorption–desorption isotherm using a surface area analyzer (Sorptomatic 1990, FISONS Instruments). Prior to this measurement, the samples were degassed at 423 K for 2 h. The pore distribution was calculated from the desorption isotherm by the BJH method.

2. Experimental

3. Results and discussion

2.1. Synthesis of CNFs

3.1. Characteristics of as-prepared CNFs

Two types of CNFs were used for preparation of these porous materials: herringbone CNF was synthesized from a C2H4/H2 mixture over a copper–nickel catalyst (Cu–Ni (2/8 w/w)) at 853 K [12,13], and platelet CNF from a CO/H2 mixture over an iron catalyst [11,12]. The unsupported Cu–Ni and iron catalysts were prepared by the precipitation of the copper and nickel carbonates from corresponding metal nitrate solutions using ammonium bicarbonate, followed by calcination, and reduction as described in Ref. [12]. The apparatus used for the synthesis of CNF has been described elsewhere [12]. Powdered catalysts in a silica tray were treated in a 10% H2/He mixture for 2 h at a prescribed reaction temperature in a conventional horizontal furnace, before introduction of the reactant gases such as a C2H4/H2 mixture or CO/H2 mixture (4/1 v/v, total flow rate 200 ml/min), where the gas flow to the reactor was precisely controlled by mass flow controllers. The synthesized CNFs were treated in 10 wt% HCl until the metal content was less than 0.5 wt%. The HCl-treated CNFs were used for the further step.

As-prepared herringbone CNFs consist of stacking of graphitic layers symmetrically angled to the fiber axis with the average diameter of 180 nm (100–500 nm). Platelet CNF, which shows quite high graphitization degree close to the graphite [10,11], is characterized by the graphene alignment perpendicular to the fiber axis with the shape of a ribbon of which the longer width was around 80–350 nm. The two fibers were found to consist of carbon nano rods as a sub-structure as described in details by our previous papers [10–12]. 3.2. Drilling pores in CNFs Fig. 1 shows TEM images of a herringbone CNF which was drilled with nickel and hydrogen (Fig. 1). The porous

Fig. 1. TEM images of herringbone CNF drilled with Ni (5 wt% loading) and He/H2 (4/1 v/v) at 1123 K for 3 h (62% wt loss).

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herringbone CNF was obtained by drilling of the herringbone CNF with Ni catalyst (5 wt% loading) and hydrogen at 1123 K. Pores with 2–20 nm diameters were formed quite uniformly along the graphene alignment with little structural change of intact parts (Fig. 1b). The alignment of channels reflects correctly the herringbone texture of CNF. The pores have thin walls which consist of several sheets of graphenes. The fiber sustained the original

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fibrous form despite of severe gasification (weight loss 40–60%). When the platelet-type CNF was drilled by using nickel and hydrogen, a platelet-type pore alignment was introduced in the fiber (Fig. 2). The pore entry can be found in the TEM observation (indicated by arrows in Fig. 2b), clearly exhibiting the formation of pores by catalytic action. Catalyst particles in the fiber (dotted circles in

Fig. 2. TEM images of platelet CNFs drilled with Ni (5 wt% loading) and He/H2 (4/1 v/v) at 1123 K for 3 h (40% wt loss).

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CNF as prepared CNF Fe-H2 modified Fe3O4 (311)

Fe3O4 (220)

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2θ Fig. 3. TEM images of herringbone CNFs drilled with Fe (5 wt% loading) and He/H2 (4/1 v/v) at 1123 K for 3 h (64% wt loss) (a and b), and comparison of X-ray diffraction profiles of as prepared and Fe-drilled herringbone CNFs (c).

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Fig. 2b) also indicate that the width of channel corresponds to the size of the drilling catalysts.

a

600

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Adsorption, cm /g

3.3. Modification of pore walls

3.4. Surface area and pore size distribution Fig. 4 shows the nitrogen adsorption–desorption isotherms and the pore size distributions (the BJH method) of the present catalyst-drilled herringbone CNFs, compared with as-prepared one and its KOH-activated one [13]. The BET specific surface areas (0.05–0.30 P/P0) of as-prepared CNF, Ni-drilled one, Fe-drilled one, and KOH-activated one were 80, 122, 184, and 704 m2/g (0.07, 0.17, 0.31, and 0.61 cm3/g pore volume at P/P0 0.98), respectively. The adsorption–desorption isotherms of the porous CNFs showed hysteresis curves at P/P0 0.48–0.53, which mean development of mesopores. The histeresis shape indicates that average mesopores are not a cylindrical form, but a slit-shaped or plate-like form [24]. Drilling of the herringbone CNF with nano-sized catalysts of Ni and Fe introduced predominantly mesopores into the fiber, while micropores were significantly formed by KOH activation [13].

Herringbone CNF Drilled by Ni and H2

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Pore width (A) Fig. 4. Nitrogen adsorption–desorption isotherms (a) and the pore size distribution calculated by the BJH method (b) of as-prepared and modified herringbone CNFs: herringbone CNFs as prepared (square), drilled by nickel and hydrogen (circle), drilled by iron and hydrogen (triangle), and activated with KOH (inverse triangle).

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80 Wt remaining (wt%)

When an iron catalyst was used in hydrogen condition, a different pore structure was found to be formed. Fig. 3a and b show TEM images of a porous CNF prepared by drilling it with iron (5 wt% loading) and hydrogen at 1123 K. The texture of the fiber appears to originate from herringbone structure, while some parts of the pore wall are extremely transformed (Fig. 3a). The graphitic layers of pore walls aligned well (arrows in Fig. 3b), suggesting the local catalytic graphitization: rearrangement of carbon structure around the catalyst during the gasification of solid carbon [21–23]. The enhanced degree of graphitization was confirmed by X-ray diffraction profiles of CNFs as prepared and after drilling with iron and hydrogen as shown in Fig. 3c. Drilling by iron catalysts generated big pores with more graphitized walls comparing to nickel catalysts. Gasification of solid carbons with hydrogen is promoted on the surface of catalyst particles, liberating methane [14]. Drilling reaction, which is the catalytic gasification, depends strongly on the catalyst and the drilling conditions such as temperature and reacting gas. Most of transition metals such as nickel, iron, manganese, and platinum can be used as the drilling catalyst, and other reactive gases such as O2, CO2, SO2, NO, NO2, and H2O can be applied as oxidation agents [14]. Hence, reactions at interface of catalyst and carbon can make different pore structure, depending on the nature of catalyst.

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3.5. Dependency of activation yield on carbon surface Fig. 5 shows the extent of drilling with nickel (5 wt% loading) and hydrogen on three types of CNFs and a

Fig. 5. The extent of drilling with nickel (5 wt% loading) and hydrogen at 1123 K depending on the structure of carbon: herringbone CNF (circle, solid line), platelet CNF (circle, dotted line), tubular CNF (square, dashed line), and carbon black (triangle, dashed line).

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carbon black. Herringbone CNF was most active to the catalytic gasification, the yield being about 30% after 5 h. On platelet CNF, the gasification appeared to start after 1 h, and then to follow similar trend with that on the herringbone CNF. On the other hand, it was found that the weight loss of a tubular CNF (prepared as described in the reference) [12] or a carbon black (#3050B, Mitsubishi Chemical) by drilling with nickel and hydrogen was much lower than that of herringbone or platelet CNFs. The drilling catalyst particles were observed under TEM to be severely aggregated on the surface of the tubular CNF or carbon black (Fig. 6), almost no contribution being suggested to form pores. These results reflect the differences in the gasification reactivity of corresponding carbon surfaces and the catalyst deposition onto the carbon surfaces. The edge of graphenes can be more reactive than the basal plane. The exposed end of the nano rods must be very reactive on the surface of platelet and herringbone CNFs. 3.6. Mechanism of pore formation The key to the preparation of the present unique materials lies in a prior discovery of the detailed microscopic structures of catalytically grown CNFs which consist of

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carbon nano rods, or nano-sized carbon clusters as structural building blocks [11,12]. The catalysts may be envisioned as drilling channels within the CNF structures through the most reactive regions (the edge of graphenes) of the CNFs, since carbon nanotube and carbon black, of which the exterior surface is covered by the basal plane of graphenes, were not vulnerable to the drilling under the same experimental conditions with herringbone or platelet CNFs. Fig. 7 exhibits STM images of the surface of nickeldrilled herringbone CNFs, which showed the pores drilled by the catalytic reaction of this study. Catalysts deposit on the exposed end of nano rods, then react with and remove selectively the reactive carbon moieties (nano rods) by gasification reactions and the nano-sized catalysts drill deeper and deeper into the interior of the CNF. The unique orientation of the pores, which follows the alignment of graphitic layers, originates from selective consumption of carbon nano rods by the catalysts. The pore wall, which is contacting with the drilling catalyst, can be modified during gasification as in the case of Fe and hydrogen drilling (Fig. 3). The deposition of the catalyst particles on the exterior of the nano rods prior to the gasification reaction is critical to make particular mesopores on the surface. In order to obtain appropriate pore system for purposed applications, the drilling catalyst, reactive gases, and drilling conditions such as temperature should be carefully

Fig. 6. TEM images of tubular CNF (a and b) and carbon black (c and d) drilled by nickel (5 wt% loaded) and hydrogen at 1123 K for 3 h. Many sintered nickel particles were observed on the surface of both carbons, not contributing to drilling pores.

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Fig. 7. Scanning electron microscopic images of the surface of herringbone CNFs drilled by nickel and hydrogen, and a proposed pore model with the stacking of carbon nano rods and their selective gasification.

selected. Also, sintering of catalysts under drilling conditions as well as carbon modification during drilling should be taken into account. 4. Conclusion Selective catalytic reaction of carbon and hydrogen was applied to create the desired pore size distribution and pore volumes. The present porous CNF is a typical product derived from the nano-structure recognition by using well-known catalytic gasification of carbon. Hence, this method can be a powerful tool to synthesize mesoporous carbons with the original frame maintained, especially making an aligned pore structure in the CNFs. The pores of the present materials have a particular shape of channel with nano-sized width and depth, which appeared to be formed by removing corresponding nano rods. The pore formation reflects the nano-structural alignment within CNFs. Catalyst dispersion on CNFs and particle sintering under the drilling conditions are critical factors to obtain appropriate surface area and pore system. The present materials are expected to be properly controlled for purposed applications such as selective adsorption, chromatography, catalyst supports for organic reactions, and electrochemical applications.

[2]

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