Shell Nanotubes with a High

J. Phys. Chem. C 2009, 113, 61–68

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Synthesis of Carbon/Carbon Core/Shell Nanotubes with a High Specific Surface Area Yejun Qiu,† Jie Yu,*,† Gang Fang,† Hao Shi,† Xiaosong Zhou,† and Xuedong Bai‡ Department of Materials Science and Engineering, Shenzhen Graduate School, Harbin Institute of Technology, Xili, Shenzhen 518055, China, and Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100080, China ReceiVed: August 5, 2008; ReVised Manuscript ReceiVed: NoVember 1, 2008

Novel carbon/carbon core/shell nanotubes (C/C-NTs) composed of well-crystallized core carbon nanotubes (CNTs) and structurally disordered carbon shells were prepared for obtaining a new type of nanotube material with high specific surface area (SSA). The disordered carbon shells were prepared by coating polyaniline (PANI) layers on the core CNTs through in situ polymerization and subsequent carbonization. Chemical activation was conducted for the C/C-NTs with KOH at 1123 K to prepare activated C/C-NTs (AC/C-NTs). Scanning electron microscopy, transmission electron microscopy (TEM), X-ray diffraction, Raman spectroscopy, and an adsorption analyzer were used to characterize the nanotube samples. The outer carbon shells thermally converted from the PANI layers are much less ordered and thus much more reactive than the CNTs. The AC/C-NTs tend to have much higher SSA than that obtained by directly activating the CNTs, and the obtained SSA for the AC/C-NTs is as high as 2924 m2/g, which is the highest among the existing nanotube materials. TEM shows that the hollow structure and high crystallinity of the core CNTs were well preserved during carbonization and activation. The novel AC/C-NTs may find important applications in many areas such as supercapacitors, catalyst supports, adsorption, and hydrogen storage due to their superhigh SSA and nanotubular structure. The present work provides a way for preparing other core/shell nanotube or nanowire materials with high SSA. 1. Introduction Quasi-one-dimensional core/shell nanostructures including nanowires, nanofibers, and nanotubes have been widely investigated because multiple or enhanced functions can be obtained by integrating different materials together in the form of a core/shell.1-6 For example, PbSe/PbS and PbSe/PbTe core/shell nanowires have lower lattice thermal conductivities in thermoelectric applications,5 CdSe/ZnS core/shell nanorods were prepared for increasing the fluorescence quantum yields and stability,2 and core/shell Ag/SiO2 nanowires and BCN/BN nanotubes were prepared for insulating purpose.1,3 So synthesis of the core/shell nanostructures is an effective way to enhance the multifunctionality of nanomaterials. Carbon nanotubes (CNTs) are regarded as one of the most promising materials for many applications related to nanotechnology ever since its discovery. For many of these applications such as hydrogen storage, catalyst supports, electrodes of supercapacitors, adsorbents, and electromechanical actuators high specific surface area (SSA) is required to obtain excellent performance. For example, it has been proved that when using CNTs as electrodes of supercapacitors the capacitance increases with the SSA of the CNTs,7-10 and the hydrogen storage capacity increases with the SSA of the CNTs.11 The SSA of the CNTs decreases drastically with increasing the number of the walls12 and is generally in the range of 100-400 m2/g.11-15 The highest SSA reported so far is 948 m2/g,16 which is obtained from a mixture of individual single-wall and double-wall CNTs. Considering the high SSA of 2000 to over 3000 m2/g of the conventional activated carbon * Corresponding author. Phone: 86-755-26033478. Fax: 86-75526033504. E-mail: [email protected], [email protected]. † Harbin Institute of Technology. ‡ Institute of Physics, Chinese Academy of Sciences.

and carbon fibers,17-20 the SSA of the CNTs is not high and far from the requirements of the related applications. Recently, chemical activation was applied to increase the SSA of the CNTs.9,15,21-23 The chemical activation was conventionally used for fabricating activated carbon or activated carbon fibers with the advantages of low reaction temperature, high reaction rate, and high carbon yield.17 The SSA of the activated CNTs from 511 to 1184 m2/g was obtained depending on the structure of the precursor CNTs and activation conditions.15,21-23 The obtained SSA of the activated CNTs is obviously much lower than that of the activated carbon fibers, which is because the crystallinity of the CNTs synthesized by chemical vapor deposition (CVD) is much better than that of the carbon fibers converted from polymer precursor. It has been demonstrated that activation reaction is strongly dependent on crystallinity of the carbon materials and becomes much more difficult as the structural order increases.23,24 The highest SSA of 1670 m2/g was achieved for the activated CNTs by using poorly organized CNTs synthesized at 723 K as precursors, whereas the SSA decreased to 1220 and 868 m2/g when using the relatively wellcrystallized CNTs synthesized at 773 and 873 K as the precursors,23 respectively. A comparison study about the effects of the crystallinity on the activation behavior indicated that after activation the SSA of the well-crystallized graphite crystals is only 9 m2/g, the SSA of the CVD-synthesized carbon nanofibers is 263 m2/g, and the SSA of the carbon fibers converted from the organic precursor fibers with crystal size of Lc (002) ) 1.7 nm and Lc (002) ) 1.1 nm are 891 and 2541 m2/g,24 respectively. It is worth noting that the diameter of the aforementioned activated carbon fibers is above 7 µm, whereas the diameter of the CNTs is in the range of 10-40 nm. Considering the large difference in diameter, the CNTs may possess much higher SSA than that of 2000-3000 m2/g obtained in the activated carbon

10.1021/jp806971e CCC: $40.75  2009 American Chemical Society Published on Web 12/11/2008

62 J. Phys. Chem. C, Vol. 113, No. 1, 2009 fibers if similar pore structure and density are generated on the CNT surface. In this work, novel carbon/carbon core/shell nanotubes (C/ C-NTs) were prepared aiming at increasing the SSA of the CNTs, where the cores are well-graphitized multiwall CNTs synthesized by CVD and the shells are disordered carbon layers thermally converted from polymer layers. The encapsulating carbon layers are similar to the conventional carbon fibers in structure and, thus, have high activation reactivity similar to the carbon fibers. In order to prepare the C/C-NTs pristine CNTs were first encapsulated with a layer of polymer by in situ polymerization. Subsequently, the encapsulating polymer layers were converted into carbon layers by heat treatment at appropriate high temperature. By chemical reaction with KOH activated C/C-NTs (AC/C-NTs) were obtained from the C/C-NTs, and the SSA as high as 2924 m2/g was achieved. The high crystallinity and hollow structure of the core CNTs were well preserved for the AC/C-NTs. The present novel nanotube materials with core/shell structure may be highly promising in applications as porous adsorbents, catalyst supports, in drug delivery, hydrogen storage, and electrodes due to the high SSA and hollow structure. The present work provides an effective way for preparing other core/shell nanotube or nanowire materials with high SSA. 2. Experimental Section The multiwall CNTs used for preparing the C/C-NTs were produced by CVD, which was carried out in a horizontal reactor consisting of a quartz tube, 30 mm in diameter and 1000 mm in length. An amount of 150 mg of NiO/La2O3 catalyst precursor was placed in a quartz boat, placed in the center of the quartz tube, and then reduced in H2 at 1073 K for 30 min. Subsequently, the reactor temperature was increased to 1273 K in a flow of Ar. Then CH4 at a flow rate of 30 mL/min was introduced into the reactor, and the reaction was maintained at 1273 K for 10 min. After cooling down to room temperature in a flow of Ar the products were collected and washed with acid solution to remove metal particles. The carbon residue was then washed using deionized water until the pH value of the effluent solution was close to 7 and then dried at 473 K. The preparation of the AC/C-NTs includes three steps, i.e., preparation of the encapsulating polymer layers on the CNTs by in situ polymerization, carbonization of the encapsulating polymer layers by heat treatment, and activation of the surface carbon layers by KOH. Polyaniline (PANI) in situ polymerized on the CNT surface from aniline monomer was selected as the precursor of the carbon shells. Before coating PANI the CNTs were acidified by a mixture solution of H2SO4/HNO3 (3:1) for improving the dispersibility in water. The CNT solution was prepared by putting 150 mg of acidified CNTs in 60 mL of water and agitating for 1 h in a supersonic bath. In order to reduce the dissolved oxygen content the CNT solution was bubbled with nitrogen flow for 5 min at 293 K. Subsequently, 5 mL of aniline and 50 mL of HCl solution at 1 M were added into the CNT solution followed by supersonic agitation for another 1 h. In the next step, 100 mL of water solution of ammonium persulfate (APS) at different concentrations was dripped into the above solution at a rate of about 20 mL/h for polymerization of aniline monomer, where APS acted as an initiator. This polymerization reaction was carried out at the temperature of 273-278 K in an ice-water bath under supersonic agitation. After in situ polymerization reaction, the core/shell carbon/polyaniline nanotubes (C/P-NTs) were obtained by filtering, washing with

Qiu et al.

Figure 1. SEM images of the pristine CNTs (a), C/P-NTs (b), C/CNTs (c), and AC/C-NTs (d).

ammonia-water, methanol, and distilled water in sequence, and drying at 393 K for 24 h. Conversion of the C/P-NTs into the C/C-NTs was accomplished in two steps, namely, stabilization and carbonization. Stabilization was carried out at 543 K in air for 1 h. Carbonization was carried out at 1123 K in N2 atmosphere for 2 h with the heating rate of 2 K/min and N2 flow rate of 200 mL/min. Chemical activation of the C/C-NTs were performed at 1123 K by using KOH as an activating agent in N2 with a flow rate of 200 mL/min. The mass ratio of C/C-NTs/KOH is 1:4, and the duration of activation reaction is 1.5 h. All the above heat treatments were carried out in a high-temperature tube furnace. After cooling down, the activated samples were washed with 0.5 M HCl solution and then leached by distilled water for several times until the pH value remained constant. Finally, the filtered products were dried for 24 h at 393 K, and the final products are the AC/C-NTs. X-ray diffraction (XRD) was recorded with an XD-2 diffractometer made by Beijing Purkinje General Instrument Co., Ltd. The morphology observation of the samples was performed with a Hitachi S-4700 scanning electron microscope (SEM). Transmission electron microscopy (TEM) imaging was performed at 200 kV in a JEOL 20l0F field emission type highresolution TEM. A Renishaw RM-1000 micro-Raman spectrometer was used to measure the Raman spectra of the samples. All Raman spectra were measured at room temperature with the 514.53 nm line of an argon laser. The laser beam was focused at the sample through a microscope with a spot size of approximately 2 µm and a power of 20 mW. Pore structure of the samples was characterized by physical adsorption of N2 at 77 K (Micromeritics ASAP 2010). The samples of CNTs, C/C-NTs, and AC/C-NTs were outgassed at 573 K, whereas PANI and C/P-NTs were outgassed at 453 K under nitrogen flow for 4 h prior to measurement. The SSA was obtained by the BET (Brunauer-Emmett-Teller) method.25 The micropore volume, micropore surface area, and external surface area were obtained using the t-plot method. The pore size distribution was calculated by the density functional theory (DFT) method. The total pore volumes were estimated to be the liquid volume of adsorption (N2) at a relative pressure of 0.99. 3. Results and Discussion The SEM images of the pristine CNTs and the core/shell nanotubes prepared at different stage are shown in Figure 1.

High Specific Surface Area Core/Shell Nanotubes The diameter of the CNTs is in the range of 30-50 nm (Figure 1a). Many tube ends can be observed in the SEM image because we used short CNTs in this work in order to improve the dispersibility. Different from the pure CNTs, the surface of the C/P-NTs becomes rough and the diameter increases to the range of 100-120 nm (Figure 1b). From the diameter increase it is known that the thickness of the encapsulating PANI layers on the CNTs are over 30 nm. We found that the in situ polymerization behavior of the aniline monomer on the CNT surface is closely related to the APS/CNTs mass ratio and the diameter of the C/P-NTs increases with increasing the APS/CNTs ratio. The sample presented in this paper is prepared at the APS/CNTs ratio of 60:1 unless shown specially. The preparation of the C/P-NTs at the APS/CNTs ratios of 10:1, 20:1, and 120:1 were also investigated. We found that at the ratio of 10:1 and 20:1 the diameters of the C/P-NTs are 45-65 and 70-80 nm, respectively. However, with further increasing the APS/CNTs ratio to 120:1 the PANI agglomerated in the solution rather than in situ polymerizing on the CNT surface. The diameter of the C/P-NTs can be controlled by changing the APS/CNTs ratio. Polymer encapsulation has been investigated for surface modification and exploiting new properties of the CNTs.26-30 In situ polymerization is an effective method for polymer encapsulation of the CNTs. In this process the CNTs act as substrates for monomer deposition and polymerization. It has been demonstrated that the interaction between PANI and CNT occurs during the in situ polymerization.31,32 The strong interactions between aromatic rings of the PANI backbone and the CNT surface caused by π-π interactions33,34 promote PANI to adsorb and crystallize on the CNT surface and thus form a uniform PANI layer around the CNTs. Generally, supersonic agitation is required during polymer encapsulation because it greatly improves the dispersion of the CNTs in solution and, thus, increases the probability that the polymerization occurs on the CNT surface and the growth rate of the polymer layers.29,35 In the present work, we also found that the CNTs were much better encapsulated with PANI under supersonic agitation than without supersonic agitation. So we applied supersonic agitation all through the polymerization reaction. The reaction rate of polymerization is another important factor affecting the deposition of PANI on the CNT surface, where high polymerization rate is unfavorable because in this case aniline monomer had not enough time to select a reaction site and therefore agglomerated into large particles in the solution separately rather than coating on the CNT surface.29 So the initiator APS was fed at low enough rates in the present work in order to increase the probability that the polymerization reaction occurred on the CNT surface. In the case of excessive aniline monomer, the PANI product increases with increasing the quantity of APS initiator,36 and correspondingly, the diameter of the C/P-NTs increases. On the other hand, excessive APS causes the aniline monomer to polymerize solely and form agglomerated PANI away from the CNTs or form too much PANI leading to agglomerating of the C/P-NTs. Under appropriate conditions, the core/shell C/P-NTs with the PANI layers well coated on the core CNTs can be prepared in a controllable way. Figure 1c shows the SEM images of the C/C-NTs carbonized at 1123 K. Obviously, the fibrous morphology of the nanotubes was retained during carbonization. The diameter of the C/CNTs is in the range of 75-100 nm, smaller than that of the C/PNTs. This is caused by shrinkage of the PANI layers during the carbonization process due to release of non-carbon elements such as H, N, and O, which have been reported by many

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Figure 2. XRD patterns of the pristine CNTs (a), PANI (b), C/P-NTs (c), C/C-NTs (d), and AC/C-NTs (e).

researchers.37-39 Figure 1d shows the SEM image of the AC/ C-NTs activated by KOH at 1123 K. The fibrous morphology was also well maintained during activation. The diameter decreased to 70-85 nm, and the surface seems to become rough due to activation. It is indicated that serious etching occurred during activation and considerable carbon materials were consumed by the activation reaction. During activation the carbon materials reacted with KOH and evolved CO and CO2 by some intermediate reaction steps,23 generating micropores and decreasing the nanotube diameters. Figure 2 shows the XRD patterns of the pristine CNTs, pure PANI, C/P-NTs, C/C-NTs, and AC/C-NTs. For the XRD pattern of the pristine CNTs an intense and sharp peak at 26.1° and a weak peak at 42.8° were observed (Figure 2a), which correspond to the diffraction of graphitic (002) and (100), respectively. The small peak width indicates that the CNTs are well crystallized. Two broad bands centered at 2θ ∼ 20° and 2θ ∼ 25° can be observed for the pure PANI (Figure 2b), which originates from the periodicity parallel and perpendicular to the polymer chain,40,41 respectively. Emergence of the two broad bands shows that the PANI is partially crystallized. The XRD pattern of the C/P-NTs shows the features of both the PANI and CNTs (Figure 2c), which indicates that the sample is a hybrid of the CNTs and PANI. The peak at 25.7° is the overlap of the CNTs (002) peak at 26.1° and the peak of PANI at 25°. For the XRD pattern of the C/C-NTs (Figure 2d), only the peaks characteristic of the graphitic carbon were observed with the graphitic (002) peak at 25.7° while the PANI peaks disappeared, indicating that the encapsulating PANI layers have been converted into carbon layers by high-temperature carbonization. However, the peak width of the C/CNTs increased greatly and the position shifted downward in comparison with that of the CNTs (Figure 2a), which shows that the carbon layers converted from the PANI layers are much less graphitized than that of the CNTs. Similar to the C/C-NTs, the XRD pattern of the AC/C-NTs only exhibits the characteristic peaks of the graphitic structure also, but the (002) peak sharpens greatly and shifts up to 25.9°, indicating that crystallinity improvement occurred for the encapsulating carbon layers. This is mainly because of the annealing effect that occurred during high-temperature activation. However, the crystallinity of the AC/C-NTs is still inferior to the CNTs. Figure 3 shows the Raman spectra of the CNTs, C/P-NTs, C/C-NTs, and AC/C-NTs. The spectrum of the CNTs presents two peaks characteristic of the graphite structure at approximately 1341 and 1567 cm-1 (Figure 3a), respectively. The peak at 1567 cm-1 can be identified as the G peak of crystalline graphite arising from the zone-center E2g mode, and the peak at 1340 cm-1 as the D peak assigned to the A1g zone-edge

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Figure 3. Raman spectra of the pristine CNTs (a), C/P-NTs (b), C/CNTs (c), and AC/C-NTs (d).

TABLE 1: Peak Parameters of Raman Spectra in Figure 3 Obtained by Curve Fitting CNTs C/CNTs AC/CNTs

peak

position (cm-1)

fwhm (cm-1)

D G D G D G

1341 1567 1350 1578 1345 1573

81 58 271 88 249 78

ID/IG 0.76 3.03 2.42

phonon induced by the disorder due to finite crystalline size.29,42,43 After encapsulating with PANI, the features of the PANI dominate the Raman spectrum of the C/P-NTs. The peaks overlap each other, and the discernible bands can be approximately ascribed to C-H bending of quinoid/benzenoid rings (1177 cm-1),27,44,45 C-N stretching deformation (1238 cm-1),44-46 electronic absorption of free charge carriers (1400 cm-1),27 CdN stretching in quinoid ring (1465 cm-1),27,44,45 N-H bending (1499 cm-1),27 CdC stretching of the quinoid ring (1590 cm-1),44-46 and C-C stretching of the benzenoid ring (1624 cm-1),27,44 respectively. The bands at 1345 and 1557 cm-1 may be mainly related to the CNTs with the former partially contributed by C-N+• stretching28-30 and the latter by C-C stretching of structural units in PANI.44 Considering the fibrous morphology of the C/P-NTs (Figure 1b), the PANIdominated Raman spectrum confirms that the CNTs were wrapped in the PANI layers. After carbonization and activation the Raman spectra of the C/C-NTs and AC/C-NTs only exhibit the characteristic peaks of carbon materials, indicating that the encapsulating PANI layers were completely converted into carbon. The peak positions, full width at half-maximum (fwhm), and the integrated area ratio of the D to G peak, ID/IG, for the CNTs, C/C-NTs, and AC/C-NTs obtained by curve fitting are listed in Table 1. The fwhm, ID/IG ratio, and peak positions are closely related to the crystallinity of the carbon materials. It has been demonstrated that the fwhm and the ID/IG ratio increase and the peak positions shift upward as a result of the increase in the degree of structural disorder.47-49 As indicated in Table 1, the degree of the structural order decreases for the C/C-NTs in comparison with the pristine CNTs. This is because both the core CNTs and the shell carbon layers contribute to the Raman spectrum of the C/CNTs and the shell carbon layers thermally converted from PANI are generally much poorer in crystallinity than the graphitic CNTs. An improvement in crystallinity was observed for the AC/C-

NTs, which is obviously caused by the annealing effects during the activation process. The Raman results are in good agreement with those of the XRD patterns. Figure 4a presents the TEM image of the pristine CNTs. It is indicated that the pristine CNTs have a bamboo-like structure and the tube walls are relatively thick. The diameter of the CNTs falls in the range estimated by the SEM images. The TEM image of the C/P-NT is shown in Figure 4b. Because the C/P-NT is too thick the structure of the core CNT cannot be clearly observed. But the boundary between the CNT and PANI layer can be discerned. The diameter of the C/P-NT was measured to be 130 nm with the PANI layer to be about 35 nm in thickness. After carbonization, the boundaries between the outer carbon layers and the core CNTs were not observed (Figure 4c) probably due to the large diameters of the C/C-NTs. However, the hollow structure was clearly observed, suggesting that the CNTs were encapsulated in the carbon shells. The thickness of the outer carbon layers is not clear because the boundaries cannot be distinguished. Figure 4d shows the TEM image of the AC/C-NTs. The core/shell structure with the core CNTs encapsulated inside the carbon shells converted from the PANI was clearly observed. The brightness difference between the core and the shell indicates that their structures are different. Generally, the thickness of the shells is uniform and in the range of 20-30 nm. Chemical activation of carbon materials by KOH is an effective approach for producing a highly porous surface and obtaining superhigh SSA.23 In the present work, a superhigh SSA of 2924 m2/g was achieved for the AC/C-NTs prepared at the APS/CNTs ratio of 60:1. Figure 5 shows the N2 adsorption/ desorption isotherms and pore size distribution of different samples. It is clearly observed that the absorption capacity increased greatly for the AC/C-NTs in comparison with the pristine CNTs and C/C-NTs (Figure 5, parts a and b). Micropores (2 nm), and macropores (>50 nm) are all present in the pristine CNTs, C/C-NTs, and AC/CNTs (Figure 5, parts c and d). The abrupt increase of adsorption volume at very low pressures corresponds to the contribution of the micropores mainly while the subsequent gradual increase with increasing the relative pressure p/p0 is resulted from the presence of the mesopores. However, the adsorption capacity of the AC/C-NTs increased continuously in a large range with p/p0, whereas that of the CNTs and C/C-NTs experienced a nearly saturated stage after the initial leap, which was caused by the presence of much more abundant mesopores and their wider range distribution in size in the AC/C-NTs (Figure 5, parts c and d). Pure PANI and the C/P-NTs have almost similar adsorption/desorption behavior with little N2 adsorption of the micropores (Figure 5, parts a and c), which also indicates that the CNTs were well encapsulated by PANI. Contrary to the samples before activation, where the pore width ranges mainly over 10 nm (Figure 5c), the pore width of the AC/C-NTs is mainly in the range below 10 nm (Figure 5d). The adsorption capacity of the AC/C-NTs increases with increasing the APS/ CNTs ratio first and then decreases after reaching a maximum at the APS/CNTs ratio of 60:1 (Figure 5b). Adsorption/ desorption hysteresis loops were observed more or less for all the samples (Figure 5, parts a and b), which are caused by the well-known capillary condensation. Interestingly, the adsorption/ desorption isotherms of the AC/C-NTs prepared at the APS/ CNTs ratios of 20:1, 60:1, and 120:1 show two hysteresis loops in the relative pressure range of about 0.40-0.80 and 0.86-0.99, respectively. The AC/C-NTs prepared at different APS/CNTs ratios have almost similar pore size distribution, which mainly

High Specific Surface Area Core/Shell Nanotubes

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Figure 4. TEM images of the pristine CNTs (a), C/P-NTs (b), C/C-NTs (c), and AC/C-NTs (d).

Figure 5. N2 adsorption/desorption isotherms of the pristine CNTs, PANI, C/P-NTs, and C/C-NTs (a) and AC/C-NTs prepared at different APS/ CNTs ratio (10:1, 20:1, 60:1, 120:1) (b) and their corresponding pore size distribution (c and d).

includes three groups of