Synthesis and characterization of hollow mesoporous carbon spheres with a highly ordered bicontinuous cubic mesostructure

Synthesis and characterization of hollow mesoporous carbon spheres with a highly ordered bicontinuous cubic mesostructure

Available online at www.sciencedirect.com Microporous and Mesoporous Materials 112 (2008) 597–602 www.elsevier.com/locate/micromeso Synthesis and ch...

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Available online at www.sciencedirect.com

Microporous and Mesoporous Materials 112 (2008) 597–602 www.elsevier.com/locate/micromeso

Synthesis and characterization of hollow mesoporous carbon spheres with a highly ordered bicontinuous cubic mesostructure Yongsheng Li a, Yanqiu Yang a, Jianlin Shi

a,b,*

, Meiling Ruan

b

a

b

Key Laboratory for Ultrafine Materials of Ministry of Education, School of Materials Science and Engineering, East China University of Science and Technology, 130 Meilong Road, Shanghai 200237, China The State Key Laboratory of High Performance Ceramics and Superfine Microstructures, Shanghai Institute of Ceramics, Chinese Academy of Sciences, 1295 Dingxi Road, Shanghai 200250, China Received 27 August 2007; received in revised form 25 October 2007; accepted 28 October 2007 Available online 1 November 2007

Abstract Hollow mesoporous carbon spheres (HMCSs) with a bicontinuous mesostructure were directly replicated from hollow mesoporous aluminosilicate spheres (HMASs) via a simple incipient-wetness impregnation technique. The highly ordered cubic mesostructure and hollow spherical features of HMCSs were demonstrated by XRD, N2 sorption, FESEM, HRTEM and other techniques. The aluminum species incorporated into the wall of HMASs not only determined the hollow morphology of HMASs, but also could catalyze the polymerization of the carbon source in the pore channels during the replicating process, so that the conventional catalyst loading step or acidic catalyst was no longer needed. The hollow mesoporous structure and the spherical morphology of HMASs were faithfully and directly replicated with only one impregnation/carbonization step. Ó 2007 Elsevier Inc. All rights reserved. Keywords: Mesoporous carbon; Hollow sphere; Bicontinuous; Replication

1. Introduction Ordered mesoporous carbon has been the recent research interest since the first report on the synthesis of CMK-1 [1–9]. Its remarkable properties, such as high specific surface area, large pore volume, chemical inertness, and good mechanical stability, make it greatly potential to be used as catalysts, electrodes, sensors, adsorbents, hydrogen storage materials and templating matrixes for fabricating nanostructures [10]. The synthesis process generally involves the infiltration of the carbon precursors into the pores of mesoporous silica templates, subsequent poly* Corresponding author. Address: Key Laboratory for Ultrafine Materials of Ministry of Education, School of Materials Science and Engineering, East China University of Science and Technology, 130 Meilong Road, Shanghai 200237, China. Tel.: +86 21 64252599; fax: +86 21 64250740. E-mail address: [email protected] (J. Shi).

1387-1811/$ - see front matter Ó 2007 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2007.10.042

merization and carbonization of the carbon precursors, and finally the removal of silica templates. Two techniques, i.e. liquid-phase impregnation [1–6] and chemical vapor deposition (CVD) [7,11,12] have been well established for the infiltration of carbon precursors, and many carbon precursors such as sucrose, glucose, xylose, furfuryl alcohol, mesophoase pitch, acenaphthene, benzene have been demonstrated to be suitable for preparing mesoporous carbon materials [13]. For liquid-phase impregnation, catalysts such as sulfuric acid, aluminum, etc, were required to ensure the polymerization and carbonization of carbon precursors. Therefore, in the cases of furfuryl alcohol and phenol resin used as carbon sources, an extra step of implanting aluminum onto mesoporous silica to generate strong acid catalytic sites or acidic catalyst was needed for the polymerization of the carbon source [10]. Compared to mesoporous carbons with other morphologies [14,15], hollow carbon spheres are more interesting owing to their hollow-core volumes and low density, so

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that they are expected to show high performances when used in catalysis, controlled drug delivery, sensing and storage, etc. Yoon et al. [16] have reported the preparation of carbon capsules with hollow-core/mesoporous shell structures or mesocellular carbon foams by employing submicrometer-sized solid core/mesoporous shell silica composite spheres or mesocellular silica foams as templates. Recently, Xia and Mokaya [17] reported the synthesis of hollow mesoporous carbon spheres with one-dimensional pore channels by using conventional particulate mesoporous silica as hard templates through a CVD method. However, the direct replication of hollow mesoporous carbon spheres (HMCSs) from hollow mesoporous silica spheres has not been found. Besides, for various potential applications including controlled drug release and adsorption of bulky pollutants, hollow carbon spheres with well-interconnected channels will be necessary. Herein, we report the direct synthesis of HMCSs with highly ordered, three-dimensional cubic mesostructured channels in the shells by employing simple incipient-wetness impregnation with furfuryl alcohol as carbon precursor and cubic hollow mesoporous aluminosilicate spheres (HMASs) as hard template. To the best of our knowledge, this is the first report on the direct nanocasting preparation of hollow carbon spheres with a bicontinuous cubic structure.

was converted into carbon inside the mesopores in the shells of HMASs by pyrolysis at 1273 K under N2 flow with a heating rate of 2.5 K/min. HMCSs were then recovered by removing silica framework using a 5 wt% aqueous solution of HF and drying at 373 K overnight. 2.2. Characterization methods

2. Experimental

XRD patterns were recorded on a Rigaku D/MAX2550 diffractometer equipped with CuKa radiation at 40 kV and 40 mA. The N2 adsorption–desorption isotherms were measured at 77 K on a Micromeritics Tristar 3000 analyzer. The surface area was calculated using the Brunauer–Emmett–Teller (BET) method. The pore size distribution was obtained using the Barrett–Joyner–Halenda (BJH) model. FE SEM was performed using a JSM6700F field emission scanning electron microscope at 10.0 kV. TEM and HRTEM images were obtained on a JEOL JEM-2100 F microscope operating at 200 kV. Samples for HRTEM measurements were suspended in ethanol and supported on a carbon coated copper grid. Raman spectrum was recorded on a inVia + Reflex Raman Microscope. 27Al MAS NMR spectrum was measured on a Bruker AVANCE-500 NMR spectrometer. The NMR spectrum was recorded at room temperature. The probe head was 4 mm MAS BB-1H. The pulse program was zg and the rotating rate was set as 4000 rpm. The numbers for scanning were 5515.

2.1. Preparation of HMASs and HMCSs

3. Results and discussion

HMASs were synthesized following the procedure reported previously [18,19]. The typical synthesis is described as follows. Firstly, the precursor sol was prepared by adding tetrapropylammonium hydroxide (TPAOH) and NaOH to a mixed aqueous solution of Al2(SO4)3 Æ 18H2O and tetraethyl orthosilicate (TEOS) under stirring. To control the formation of HMASs, the temperature was kept under 291 K. After aging for 20 h at the same temperature, the resulting sol was dropped into an aqueous solution of cetyltrimethylammonium bromide (CTAB) under vigorous stirring. Stirring was continued for another 2 h. The molar composition of the resultant gel was 1Al2O3:100SiO2:20Na2O:16TPAOH:15CTAB:16600H2O. Then, the gel was sealed in Teflon-lined autoclaves and heated at 408 K for 12–48 h. The solid product was recovered by filtration and dried in an oven at 373 K overnight. The as-synthesized material was then calcined in air at 823 K for 10 h to remove the templates. For the synthesis of HMCSs, incipient-wetness impregnation was employed to introduce carbon source into the template. Briefly, 1.0 g calcined HMASs were impregnated with 2.2 g furfuryl alcohol, and the resulting mixture was dried at 333 K for 2 h, followed by drying under vacuum at 333 K overnight to generate the polymerization of furfuryl alcohol, which was in-situ catalyzed by aluminosilicate template. Subsequently, the polymerized furfuryl alcohol

In the synthesis of HMASs, it was found that the Al content in the precursor sol determined the mesostructure and morphology of HMASs [20]. Without Al source added in the precursor sol, the mesophase of the resultant product was a mixture of p6mm and Ia3d symmetries. When the Al2O3/SiO2 ratio was increased to 1/100 or 1.5/100, the mesostructure had a typical Ia3d symmetry. With the Al2O3/SiO2 ratio being increased to 2/100, the

Fig. 1.

27

Al MAS NMR spectrum of HMASs.

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Fig. 2. Small-angle XRD patterns of (a) calcined HMASs and (b) HMCSs replica.

mesostructure became obviously disordered. The morphology of HMASs changed with the change of Al2O3/SiO2 ratio as well, as confirmed by the HRTEM images. It has been concluded that hollow spheres with Ia3d symmetry could only be obtained at the Al2O3/SiO2 ratio around 1/100. Therefore, HMASs sample prepared at Al2O3/ SiO2 = 1/100 was chosen as template for the replication of HMCSs. The distribution and content of Al species in the wall of HMASs (Al2O3/SiO2 = 1/100) were analyzed by

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Fig. 3. N2 adsorption–desorption isotherms and pore size distribution curves (inset) of (a) HMASs and (b) HMCSs replica.

Table 1 Pore structure parameters of HMASs and the replica HMCSs obtained from the desorption branch of N2 sorption isotherms at 77 K Sample

S a /m2 g

HMASs HMCSs

1285 1809

a b c

1

d b /nm

V c/cm3 g

2.5 2.1

0.86 1.01

BET surface area. BJH pore diameter. Total pore volume.

Fig. 4. TEM images of (a) HMASs, (b) HMCSs, (c) crushed HMCS and (d) SEM image of HMCSs.

1

600 27

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Al MAS NMR and EDS. According to the 27Al MAS NMR spectrum shown in Fig. 1, it can be concluded that aluminum has been incorporated into the mesoporous wall in a tetra-coordinated position with high symmetry (signal at 56.34 ppm). The Al2O3/SiO2 ratio was calculated to be around 1.78/100 from the EDS results, indicating that not all Si source had been incorporated into the wall of HMASs. Fig. 2 presents the small angle XRD patterns of HMASs and the HMCSs replica. The host template shows highly ordered cubic Ia3d structure, as indicated by the wellresolved (2 1 1), (2 2 0) and higher-order reflections. The XRD pattern of HMCSs also shows intense and distinct reflections. The additional peak (1 1 0), locating at 2h below 2°, indicates a structure transformation from Ia3d to lower I4132 symmetry, which is consistent with the results in the literature [1,21–23]. The reason for such a transformation may be due to the displacement of two non-interconnecting pore networks in the host template by pyrolyzed carbon after the dissolution of host silica. The nitrogen adsorption–desorption isotherms and pore size distribution curves of HMASs and HMCSs are shown in Fig. 3, respectively. It can be found that HMCSs has higher adsorptive capacity than that of HMASs, and both HMASs and HMCSs have narrow pore size distributions. The BET specific surface areas and pore parameters of

the samples determined from the desorption branch are summarized in Table 1. The BET surface area of HMCSs is as high as 1809 m2/g, and the average pore diameter is 2.1 nm, which is consistent with the wall thickness of HMASs. The hysteresis loops are closer to H4 type, which is consistent with the results for hollow spheres with walls composed of ordered mesoporous silica reported by Kooyman et al. [24]. These suggest that both HMASs and HMCSs have large mesopores embedded in a matrix with pores of much smaller size [25]. Based on the amount of HMASs used, the yield of HMCSs was calculated to be around 70%. Combined with the lower framework density and highly ordered mesostructure, it is reasonable for HMCSs to have higher specific surface area than HMASs. Fig. 4 shows the TEM and SEM images of HMASs and HMCSs. It is clear (Fig. 4a) that HMASs are hollow spheres with their diameters in a range of 300–500 nm [18,19]. In Fig. 4b, it is observed that HMCSs sample has approximately the same morphology and similar spherical size as HMASs. The hollow feature of HMCSs is more obviously exhibited from the crushed sphere shown in Fig. 4c and d. These results demonstrate that HMCSs are faithful replications of HMASs, and are structurally and morphologically stable during heating and etching treatment. The highly ordered mesostructure of HMCSs is confirmed in Fig. 5. Interestingly, ordered HMCSs with both

Fig. 5. (a) TEM image of a representative HMCS, (b) and (c) HRTEM images of HMCS recorded along the (1 1 1) direction of I4132 space group from the rectangular area in (a), and along the (1 1 0) direction of Ia3d space group, respectively (insets in b and c are the corresponding ED patterns).

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I4132 (Fig. 5b) and Ia3d (Fig. 5c) symmetries are found, which indicates that Ia3d symmetry transforms partially into I4132 symmetry during the silica removing process. This is also in accordance with the XRD results. From the HRTEM image shown in Fig. 6A, it is worthy to note that some layered structures and fragments can be identified in small localized areas at the very edge of the spheres, which is different from the shell of amorphous carbon reported by Parmentier et al. [26]. This means that the HMCSs may contain partially graphitized pore framework [27]. It is well known that carbons obtained from furfuryl

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alcohol are non-graphitizable materials, so further characterization by such as XRD analysis at the wide-angle range and Raman spectroscopy (RS) are needed to specify the nature of HMCSs. Fig. 6B and C shows the RS spectrum and the wide-angle XRD pattern of HMCSs, respectively. RS is considered to be a solid method for studying carbon phase and the vibration at 1580 cm 1 (G-band) due to the interplane sp2 C–C stretching is the characteristic feature of ordered graphite carbon. An obvious band at 1584 cm 1 (Fig. 6B) can be observed for HMCSs, revealing the possible partially graphitized structure. Another band at around 1340 cm 1 (D-band) can also be observed, which is associated with the vibration of carbon atom with dangling bonds in plane terminations of the disordered graphite and related to the defects and disorders in structures in HMCSs. These suggest that HMCSs probably contain small area graphite sheets with a certain graphitization degree. The XRD pattern at wide-angle range (2h: 20– 50°, Fig. 6C) shows two broad diffraction peaks, which can be respectively indexed to (0 0 2) and a superposition of both (1 0 0) and (1 0 1) reflections for typical graphite carbons. However, no peaks at 53° and 78° corresponding to the (0 0 4) and (1 1 0) reflections can be found, implying that HMCSs are not as fully crystallized into graphite as the graphitic porous carbons prepared with polyaromatic compound or mesophase pitches as the carbon precursors [27– 29]. Combined with the results of HRTEM and RS, it is concluded that there exists some graphitic layers and fragments formed in localized areas of HMCSs. In terms of the highly ordered cubic mesostructure, hollow spherical morphology and nontoxic property, it is believed that HMCSs material is a potential candidate in drug storage-release system, catalysis and other fields. 4. Conclusions HMCSs with highly ordered cubic meso-channels in the shell have been fabricated for the first time by using furfuryl alcohol as carbon source and HMASs as hard template. Aluminum present in the wall of HMASs could in-situ catalyze the polymerization of furfuryl alcohol in the mesopore channels, which allows a direct and faithful replication of the hollow mesoporous structure. Such a hollow mesoporous carbon replica possesses chemical stability under strong alkaline or acidic environment, as well as its rather high specific surface area, large void volume and highly ordered 3D interconnected mesostructure. All these features make HMCSs potentially applicable in catalysis, controlled drug delivery, and templating processes for other hollow mesoporous materials. The further investigation is under progress. Acknowledgments

Fig. 6. (A) HRTEM image of the HMCS sphere near the out surface, (B) Raman spectrum and (C) wide-angle XRD pattern of HMCSs.

This work was financially supported by the National Natural Science Foundation of China (20501015, 20633090) and Shanghai Nano-tech project (0652nm014).

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