Fabrication of novel micro–nano carbonous composites based on self-made hollow activated carbon fibers

Applied Surface Science 265 (2013) 352–357

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Fabrication of novel micro–nano carbonous composites based on self-made hollow activated carbon fibers Yuxia Kong a , Tingting Qiu a , Jun Qiu a,b,∗ a b

School of Materials Science and Engineering, Tongji University, Shanghai 201804, PR China Key Laboratory of Advanced Civil Engineering Materials of Education of Ministry, Shanghai 201804, PR China

a r t i c l e

i n f o

Article history: Received 14 October 2012 Received in revised form 31 October 2012 Accepted 3 November 2012 Available online 8 November 2012 Keywords: Template method Hollow carbon fibers Chemical vapor deposition Carbon nanotubes Micro–nano carbonous composites

a b s t r a c t The hollow activated carbon fibers (HACF) were prepared by using commercial polypropylene hollow fiber (PPHF) as the template, and phenol-formaldehyde resin (PF) as carbon precursors. Final HACF was formed through the thermal decomposition and carbonization of PF at 700 ◦ C under the nitrogen atmosphere, and activation at 800 ◦ C with carbon dioxide as the activating agent, consecutively. Then, carbon nanotubes (CNTs) were grown by chemical vapor deposition (CVD) techniques using the as-grown porous HACF as substrate. The growth process was achieved by pyrolyzing ethanol steam at 700 ◦ C using nickel as catalyst. Finally, CNTs was grown successfully on the substrate, and a novel treelike micro–nano carbonous structure CNTs/HACF was fabricated. The as-grown HACF and micro–nano CNTs/HACF were characterized by scanning electron microscope (SEM), transmission electron microscope (TEM), X-ray diffraction (XRD), and thermogravimetric analysis (TG), respectively. Moreover, the formation mechanisms were also discussed. © 2012 Elsevier B.V. All rights reserved.

1. Introduction Activated carbon fibers (ACF) are characterized by high carbon contents, large specific surface areas, abundant micropores and small or narrow aperture. Hollow activated carbon fibers (HACF) are a kind of specific ACF with hollow structures. There are enrich micropores on both the internal and external surfaces of HACF, hence make it has larger specific surface areas, and possess unique advantages of fast adsorption rate, large adsorption capacity and simpler regeneration in the gas–liquid adsorption fields [1–3]. For a long time, most hollow activated carbon fibers (HACF) were prepared based on spinning technologies [2–6], which usually use various kinds of organic polymer fibers including polyacrylonitrile fiber, cellulose fiber, polyethylene fiber, and polyvinyl alcohol fiber, etc. to get hollow fiber strands, and then repass preoxidation, carbonization process to obtain expected carbon fibers with hollow structures. However, the spinning process is very complex, including the proper selection of polymer precursors, the parameters involved in electrospining process, the pre-oxidation, carbonization and activation techniques, etc. In addition, it is difficult to control the pore size distribution and pore connectivity.

∗ Corresponding author at: School of Materials Science and Engineering, Tongji University, Shanghai 201804, PR China. Tel.: +86 21 69582101; fax: +86 21 69582101. E-mail address: [email protected] (J. Qiu). 0169-4332/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2012.11.011

In recent years, alternative controllable and cost – effective HACF synthetic routes – template method has aroused intensive attention, by using the polymer or organic fibers (polypropylene hollow fiber, polyvinyl alcohol fiber, organic silica fiber, etc.) as template, carbon-rich compounds (sucrose, polystyrene, etc.) as carbon precursors. After template materials were decomposed or removed at certain temperatures, hollow structured carbon fiber with ideal regular and uniform morphologies can be obtained. Shi et al. [7] firstly reported the synthesis of hollow carbon fibers (HCFs) by using polypropylene hollow fibers templates with sucrose as carbon precursors. Cheng et al. [8] prepared circular and rectangular opening HCFs by using electrospun silica fibers as template and polystyrene as carbon source. Fatema et al. [9] reported the HCFs fabrication using PVA fibers with the aid of iodine pretreatment. The discovery of carbon nanotubes (CNTs) by Iijima [10] has attracted great attention because of their unique structural, thermal, adsorptive, electronic, and mechanical properties, etc. [11–13]. Chemical vapor deposition (CVD) is a simple and economic technique for synthesizing CNTs at low temperature and ambient pressure with various substrates [14]. There have been many reports about CNTs growth on different carbon matrices, such as carbon fibers [15–17], carbon paper (CP) [18,19], active carbon [20,21], and ordered mesoporous carbon [22]. However, there is still no report about CNTs deposited on the hollow activated carbon fibers so far to the best of our knowledge. In the present work, HACF has been successfully prepared by template method using polypropylene hollow fibers as template, and phenol-formaldehyde resin is firstly used as carbon precursor.

Y. Kong et al. / Applied Surface Science 265 (2013) 352–357

What more important, novel micro–nano CNTs/HACF composites are obtained using HACF as the substrate through CVD technology. 2. Experimental 2.1. Preparation process of the micro–nano carbonous composites CNTs/HACF (i) Preparation of hollow activated carbon fibers (HACF) A polypropylene hollow fiber (PPHF) (400 ␮m I.D., 30–40 ␮m wall thickness, 0.2 ␮m pore size, from Hangzhou Kaihong Membrane Technology Co., Ltd., China) was used as the template. Firstly, the pristine PPHF were cut into short fibers with about 3 cm long, and purified with alcohol for 24 h. Subsequently, purified PPHF was immersed into phenol-formaldehyde resin (PF) solution (PPHF:PF:ethanol = 0.25 g:5 g:25 mL) for 24 h, and gradually dried at 50 ◦ C, 80 ◦ C, 120 ◦ C, and 160 ◦ C for 2 h, respectively. Then, the obtained PPHF–PF was placed in the tube furnace at the heating rate of 2.5 ◦ C/min, via carbonized at 700 ◦ C for 60 min under nitrogen (N2 ) atmosphere and activated at 800 ◦ C for 60 min using carbon dioxide (CO2 ) as activating agent, respectively, resulting in the product HACF. (ii) Fabrication of the micro–nano carbonous composites CNTs/HACF based on HACF (i) The above prepared HACF was placed into nickel nitrate solution (0.1 mol/L) for 5 h at ambient temperature to ensure nickel ions were adsorbed onto surface sites. A thermal reduction process was carried out at 400 ◦ C for 60 min under the atmosphere of reductive hydrogen (H2 ) and shielding gas N2 with the chemical composition of vol. N2 :H2 = 3:1, thus nickel ions was reduced to elements. Then, a CVD technique was used to grow CNTs on the nickel (Ni) coated HACF with ethanol (C2 H5 OH) vapor as the carbon source, and N2 as the carrier gas. The vapor deposition process was carried out in the quartz tube of tube furnace at 700 ◦ C for a growth period of 50 min, in order to form the resultant CNTs/HACF hybrid micro–nano carbonous composites. Finally, the as-grown samples were immersed into nitric acid (5 mol/L) at 80 ◦ C for 6 h in order to remove the catalyst particles. 2.2. Microstructure characterization of the micro–nano carbonous composites CNTs/HACF The morphological features of the as grown HACF and CNTs/HACF micro–nano carbonous composites were analyzed on scanning electron microscope (Quanta 200F, FEG, USA) and transmission electron microscope (JEM-2100F, JEOL, Japan). The structure of HACF and CNTs/HACF were investigated by X-ray diffraction using a high power X-ray diffractometer (D8 Advance, Bruker, German) with Cu Ka radiation ( = 0.15418 nm). The pyrolysis characteristics of HACF and CNTs/HACF were performed by thermal gravimetric analysis with a thermal analyzer (STA449C, Netzsch, Germany) under nitrogen atmosphere at the heating rate of 10 ◦ C/min. 3. Results and discussion 3.1. Morphologies and structures of the HACF and CNTs/HACF The morphologies, surface structures, and dimension of HACF and CNTs/HACF hybrids were characterized by SEM at different magnifications and view angles. Fig. 1(a) and (b) displays the crosssectional images of the obtained HACF at different magnifications. The hollow structure indicates that thermal decomposition of PPHF does not result in the collapse of the fibers. The external and internal

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diameters of the HACF are about 420 and 330 ␮m, respectively, and the wall thickness of HCF is about 90 ␮m. Fig. 1(b) fully demonstrates that the HACF consists of compact structures and high porosity on the surfaces and cross-sections. Compared with the HCF synthesized in Ref. [7], at first, the carbon source used here is phenolic resin, which can form high crosslinked structure after thermal decomposition than sucrose precursor. Then, we adds an physical activated process in preparing the HACF, to open the inherent pores or etch out new micropores in the surface of hollow carbon fibers through carbon dioxide reacted with carbon atoms at high temperature 800 ◦ C. Therefore, the obtained HACF in this work has higher structure stability and porosity. Fig. 1(c) and (d) presents the cross-sectional SEM images of the CNTs/HACF through CVD process, which apparently shown that plenty of CNTs are grown on the external, internal and fracture surfaces of the substrate HACF, and a novel micro–nano carbon composite structure is successfully fabricated. The magnified longitudinal SEM images of CNTs grown on the external surface of HACF are displayed in Fig. 1(e) and (f), showing a large number of dense CNTs grows on the substrate. In addition, Fig. 1(f) clearly shows that most CNTs grown toward the same direction which perpendicular to the external surface of HACF, which results in the formation of tree-like structures, that is, the self-made HACF acquired by template method acts as the “trunk”, and the grown CNTs obtained by CVD technique as the “branch”. After acid treatment, most of the catalysts have been removed, as shown in Fig. 1(g) and (h). It can be clearly seen that some gaps appeared on the tops of carbon nanotubes, which means that nitric acid had washed the nickel catalyst at the tip of the carbon nanotubes, but did not bring obvious damage to the morphology and structure of the as-grown CNTs and their distribution on the substrate HACF. The contrasted bright field image of as synthesized CNTs is shown in TEM images of Fig. 2. Fig. 2(a) shows the presence of some undesired carbon impurities and uncompleted grown tubes along with CNTs. But it still clearly presents the Ni catalyst position and morphology and structure of as-grown nanotubes, which is beneficial to know the carbon nanotubes formation process and principle. In addition, from Fig. 2(a) and (b) photos, we can drawn that the outer diameter and wall thickness of obtained CNTs are uniformly in the range of 70–90 nm, and 20–30 nm, respectively. XRD was applied to investigate the crystal structure of as-grown samples. Fig. 3 shows the X-ray diffraction patterns of the (a) asgrown HACF, (b) CNTs/HACF, and (c) acid treatment CNTs/HACF. The diffraction pattern of the HACF shows a high narrow diffraction peak at 2 = 22.5◦ , corresponding to the C(002) reflection of a turbostratic carbon structure [23] of HACF. After the growth of CNTs on the HACF, C(002) reflection become stronger, demonstrated that the structure order degree of the as-grown micro–nano CNTs/HACF composite has increased significantly than HACF substrate. In addition to the peaks of carbons, the diffraction peaks induced by the existence of catalyst particles Ni(111) were also observed at 2 = 44.4◦ . However, after the cleaning of 5 mol/L nitric acid, both the intensity and the FWHM (full width at half maximum) of the Ni peaks decreased obviously, indicating the removing of catalyst particles Ni. Moreover, the C(002) diffraction is getting further stronger, which also proved the removing of Ni in the interior of CNTs or on the surface of HACF enhanced the order degree of whole carbon structure. TGA analysis was used to study the thermal stability of the as-grown HACF (Fig. 4(a)) and CNTs/HACF (Fig. 4(b)) structures. Fig. 4(a) shows that there is a small weight loss about 5% occurred at 50–120 ◦ C due to the volatile of water and air, etc. impurities, after which a slowly weight loss is observed in the temperature range from 120 to 350 ◦ C, which maybe caused by the pyrolysis of residual phenolic resin that did not completely carbonization in

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Fig. 1. SEM images of as-grown HACF and micro–nano composite CNTs/HACF. (a and b) HACF; (c and d) cross sectional images of CNTs/HACF; (e and f) longitudinal images of CNTs/HACF; (g and h) acid treated CNTs/HACF.

synthesizing the HACF. In addition, the weight loss in 600–800 ◦ C is also mainly attributed to the further carbonization of HACF at higher temperature, as some gas decomposed from phenolic resin was not completely carbonized at the stage in preparing HACF. In Fig. 4(b), a puny of weight loss occurred in the temperature range 120–250 ◦ C perhaps because of the decomposition of some unreduced nickel nitrate at hydrogen reduction stage in the process of growing CNTs. Furthermore, the slight weight loss presented from 440 ◦ C to 750 ◦ C may be attributed to the burn of

amorphous carbon and the defects on the as-grown CNTs, due to the gas decomposed form HACF at high temperatures. From the TGA curves, we can found that the remaining weight for HACF and CNTs/HACF are 87% and 94%, respectively at 800 ◦ C, which demonstrate the good thermal stability of micro–nano CNTs/HACF composite structures, and the nano phase CNTs improves the overall graphite structural integrity of the carbon hybrids. Therefore, the conclusion is in corresponding with SEM and XRD results.

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Fig. 2. TEM images of CNTs grown on the substrate HACF. (a) The grown CNTs accompanying carbonaceous impurities (Ni remained at the tip of tube); (b) A single CNTs (Ni is embedded in the middle channel).

3.2. Mechanism analysis of the as-grown micro–nano carbon composites The formation process of CNTs/HACF micro–nano carbonous composites based on self-made hollow activated carbon fibers (HACF) is described in Fig. 5. Specific principle analysis for the fabrication of HACF and CNTs/HACF can be explained as follows, respectively.

Fig. 3. X-ray diffraction patterns of HACF and micro–nano CNTs/HACF composites. (a) HACF; (b) CNTs/HACF; (c) acid treated CNTs/HACF.

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Temperature (ºC) Fig. 4. Thermogravimetric analysis of the as-grown HACF and CNTs/HACF. (a) HACF; (b) CNTs/HACF.

(i) As reported in the literature [7], PPHF possesses interwoven fibrous frameworks and the interconnected structural pores, which is beneficial for the PF–ethanol solutions being adsorbed and hold in the walls of PPHF through enough time soaking. The function of the gradually 50–160 ◦ C heat treatment for the PPHF–PF mixture is making ethanol volatile and slowing down the PF curing process, just in case generate bubbles on the surface of the fibers when heating up fast, which aims to acquire final samples with integrate morphologies and structures. During the carbonation, PF decomposed out micro molecules H2 O, CO2 , etc., and crosslinked three-dimensional network structure was fabricated by the residual carbon from PF while the template PPHF was burned off. Then, CO2 was used as the physical activation agent to open the closing holes occurred in the carbonization process or etches out new micropores in the surface of hollow carbon fibers. Finally, the hollow activated carbon fiber was obtained. (ii) As we know, the nucleation and growth process of CNTs is complicated, there is long-running argument [14,24,25] about the growth mechanism of CNTs through CVD techniques. In the present work, self-made HACF has abundant porous network structures and graphite crystallite activity. Nickel catalyst particles can be adsorbed on the internal, external, and section surfaces of it, which is the important physical basis for CNTs growth. The simulation growth process schematic diagram is shown in Fig. 6. At the temperature 700 ◦ C, the carbon atoms generated from ethanol decomposition dissolved into the bulk of the Ni particle quickly, and form a substoichiometric nickel carbide at the early stage (as depicted in stage I). Then, the liquid phase nickel carbide may fastly break away from the substrate with more injected carbon deposited above it. The escape velocity is so fast that carbon deposition are very difficult formed in the bottom, thus generate a hollow pipe between the substrate and bolted nickel carbide, as shown in stage II.

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Fig. 5. Formation procedure of the HACF and CNTs/HACF.

Fig. 6. The tip-growth models schematic diagrams for the CNTs grown on the substrate HACF.

Gradually, more carbon atoms spread in nickel catalyst surfaces, or passed through the surface of fusion nickel get into the contact opening of carbon nanotubes and nickel, to realize the growth of CNTs (stage III). In the growing process, nickel catalyst always remained at the end of CNTs, and migrated along with the growth of carbon nanotubes. However, as the catalyst reaction activity gradually reduced, decomposed activated carbon atoms from ethanol at the unit time were also dropped off, thus reduced the growth rate of CNTs, until they stop growing (stage IV). Therefore, the growth of the CNTs in the present work belongs to the tip growth mechanism, which can be demonstrated by the transmission electron microscope result of Fig. 2(a). In addition, sometimes, the constant carbon dissolution would bring the depression of the freezing point of liquid-like metals. Therefore, the metal particle may be squeezed out by the strong pressure buildup due to the formation of graphite layers at the internal surface of the formed CNTs, and then the fresh surface of the squeezed-out nickel particles would then further decompose the ethanol and continue promote the growth of CNTs, resulting in the nanotubes with Ni enclosed in the middle of the channel, as shown in Fig. 2(b). 4. Conclusions In summary, hollow pipe and porous HACF with solid carbon net framework structure was successfully prepared by template

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