Ultrafast production of ordered mesoporous carbons via microwave ...

Letters to the Editor / Carbon 45 (2007) 2843–2854

2851

Ultrafast production of ordered mesoporous carbons via microwave irradiation Hyung Ik Lee a, Jin Hoe Kim a, Sang Hoon Joo b, Hyuk Chang b, Doyoung Seung b, Oh-Shim Joo c, Dong Jin Suh c, Wha-Seung Ahn d, Chanho Pak b,*, Ji Man Kim a,* a

Department of Chemistry, BK21 School of Chemical Materials Science and SKKU Advanced Institute of Nanotechnology, Sungkyunkwan University, Suwon 440-746, Republic of Korea b Energy and Environment Laboratory, Samsung Advanced Institute of Technology, P. O. Box 111, Suwon 440-600, Republic of Korea c Environment & Process Technology Division, Korea Institute of Science and Technology, Seoul 136-791, Republic of Korea d Department of Chemical Engineering, Inha University, Incheon 402-751, Republic of Korea Received 25 April 2007; accepted 8 August 2007 Available online 1 September 2007

Recently, a new type of ordered mesoporous carbon (OMC) materials has been synthesized via a nano-casting technique using ordered mesoporous silica as the template [1]. Compared with typical porous carbon materials, the OMC materials promise to be suitable as adsorbents, catalyst supports, and materials for advanced electronics applications, due to not only their high porosity and surface area but also their regular and controllable pore system [2]. However, most of the methods for preparing the OMC materials have been based on the conventional thermal process, which is highly time and energy consuming one. Microwave energy has been employed as an advanced heating media for chemical reactions as well as material syntheses, in which it frequently induced intriguing changes in the reaction kinetics and selectivity [3]. One of the most fascinating application areas of microwave irradiation is the syntheses of nano-structured materials such as zeolites and ordered mesoporous silica materials under hydrothermal conditions [3], which results in a remarkable enhancement in the efficiency of sol–gel synthesis, as manifested by the shortened time or lowered temperature required for the synthesis, and the unique or uniform properties of the products. Microwave energy has also been utilized for the synthesis and modification of carbon materials such as carbon nanotubes [4,5]. Herein, a new carbonization method for the ultrafast production of OMC materials, that utilizes the intense heat generated by microwave irradiation, is reported. Compared with the conventional thermal process, the microwave-assisted heating method enables the use of a very short car-

* Corresponding authors. Fax: +82 31 299 4174 (J.M. Kim), +82 31 280 9359 (C. Pak). E-mail addresses: [email protected] (C. Pak), [email protected] (J.M. Kim).

0008-6223/$ - see front matter 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2007.08.031

bonization period over a broad range of synthesis temperatures. Carbon replication was performed by the impregnation of sucrose as the carbon precursor into the pores of the mesoporous silica template (MSU-H) with 2D hexagonal mesostructure [6], the carbonization of the carbon precursors under N2 flow, and subsequent removal of the mesoporous silica template. For comparison, the carbonization was carried out by two kinds of heating methods, viz. the conventional thermal heating (OMC-T) and the microwave-assisted heating methods (mwOMC-T) where T stands for the carbonization temperature (400–900 C). A microwave furnace (Thermwave TW 2.0RS, RMS, see Fig. S1 in Supplementary Material) was used to obtain the mwOMC-T samples. After the carbonization, the mesoporous silica template was removed using a 20% HF solution. Experimental details are given in the Supplementary Material. Fig. 1 shows the X-ray diffraction (XRD) patterns of the OMC-T and mwOMC-T materials. All of the OMC-T and mwOMC-T materials, except for the OMC-400 sample, exhibit XRD patterns with a very intense diffraction peak and one or more weak peaks, which are similar to that of MSU-H (see Fig. S2 in Supplementary Materials) and the characteristics of a 2D hexagonal (P6mm) mesostructure [2,6,7]. Compared with the approximately 12 h required for obtaining the OMC-T materials, it takes only 5, 7, and 10 min for the formation of the mwOMC-400, 600 and 900 materials, respectively. Even though the carbonization period under microwave irradiation is much shorter than that of the conventional thermal process, the structural orders of the mwOMC-T materials are comparable or better than those of the OMC-T materials. More interestingly, the preparation of mwOMC material is possible even at extremely low temperatures of around 400 C under microwave irradiation, whereas the conventional heating does not develop any ordered mesostructure at the

2852


Fig. 1. X-ray diffraction patterns of the ordered mesoporous carbon materials obtained by the (a) conventional thermal process and (b) microwave heating method.

same temperature. After the carbonization, the colors of the OMC-400 and mwOMC-400 thus obtained are obviously different: the former is dark brown and the latter is

black, indicating that the carbon precursor for the mwOMC-400 material is relatively more carbonized by the microwave irradiation than the OMC-400 material is by the conventional thermal treatment. Therefore, it is reasonable to suppose that the carbon precursors in the mesoporous silica template are transformed into a partially carbonized species, and subsequently, the composite material, containing a kind of carbon intermediate which may be active to microwaves [8], seems to be self-heated to a higher temperature than the detectable temperature during the process. It should be mentioned here that due to the design of the microwave device (Fig. S1), the thermocouple measures the temperature of the atmosphere surrounding the sample, but not the sample’s temperature directly. The SEM and TEM images of the mwOMC-400 material shown in Fig. 2 clearly demonstrate that the material exhibits uniform particle morphologies as well as a highly ordered mesostructure, even though it is obtained using microwave irradiation for only 5 min at 400 C. The SEM image shows the overall particle morphology of the mwOMC-400 material to be very similar to that of the mesoporous silica template, and the inverse mesostructure of the silica template is obtained after its removal, as shown in the TEM image. The SEM images of the other OMCT and mwOMC-T materials are similar to those of mwOMC-400 (see Fig. S3 in Supplementary Material). Fig. 3 shows the N2 adsorption–desorption isotherms and corresponding pore size distribution curves (BJH method) for the OMC-T and mwOMC-T materials. All of the N2

Fig. 2. SEM images of (a) mesoporous silica template and (b) mwOMC-400, and TEM images of (c) mesoporous silica template and (d) mwOMC-400 (scale bars for (a) and (b) are 100 nm and for (c) and (d) are 50 nm).


2853

Fig. 3. (a, c) N2 adsorption–desorption isotherms for the OMC-T and mwOMC-T materials, and (b, d) the corresponding pore size distribution curves obtained from the N2 adsorption branches using the BJH method.

sorption isotherms in Fig. 3a and c, except for that of the OMC-400 sample, are type IV isotherms with hysteresis loops, which coincides with the data reported elsewhere [7,9]. Well defined steps in the adsorption–desorption curves appear between relative pressures, p/p0, of 0.5–0.7, indicating the presence of a narrow distribution of mesopores. This is also confirmed by the data shown in Fig. 3b and d. Gas adsorption data along with the results obtained from XRD demonstrate that the OMC materials

possess an ordered hexagonal mesostructure, coming from the replication of the silica template. The physical properties of the OMC materials are listed in Table 1. In addition to the ultrafast carbonization less than 10 min, the microwave-assisted synthesis of OMC materials endows them with some unique advantages. As shown in Table 1, the oxygen contents of the mwOMC-400 sample is above 10 at.%, indicating that the highly ordered mesoporous carbon materials with a controlled oxygen content

Table 1 Physico-chemical properties of OMC-T and mwOMC-T materials Materials

a0 (nm)a

SBET (m2 g 1)b

Pore size (nm)c

Vtot (cm3 g 1)

MSU-H OMC-400 OMC-600 OMC-900 mwOMC-400 mwOMC-600 mwOMC-900

12.4 – 12.3 11.3 12.1 12.1 12.1

477 618 1542 1337 1108 1400 1384

10.3 – 4.6 4.0 4.3 4.7 5.0

1.29 0.41 1.66 1.60 1.02 1.49 1.95

a b c d e f

d

Oxygen (at.%)e

Oxygen (wt%)e

Resistance (mX cm 2)f

– 18.7 4.9 2.2 10.2 4.7 1.8

– 28.9 7.4 2.9 15.7 7.2 2.5

– 5.7 · 108 1.5 · 107 1.1 · 102 1.0 · 1010 3.3 · 106 4.6 · 10

Unit cell parameter calculated from XRD data using the equation a0 = 2 · 3 1/2d100. Surface area calculated by the BET method. Mesopore sizes obtained from the N2 adsorption branches using the BJH method. Total pore volume obtained from the N2 sorption isotherm at p/p0 = 0.99. Oxygen contents measured by elemental analysis. Sheet resistances obtained by the four-point probe method at a pressure of 150.8 kgf cm 2.

2854


that may alter the surface properties of materials can be easily prepared by using the microwave-assisted carbonization. This is probably because the samples are self-heated to a higher temperature than the detectable temperature during the process as discussed previously. Table 1 also shows that the mwOMC-T materials exhibit much lower sheet resistances than those of the OMC-T materials at the same pressure, which are obtained by pressing the samples and subsequent measurement by the four-point probe method [10]. In summary, highly ordered mesoporous carbon materials have been obtained by instant carbonization within 10 min using the microwave-assisted method. The energy efficient microwave-assisted carbonization process has various advantages, such as allowing for energy saving production, low temperature preparation even at 400 C, controllable oxygen amount, i.e., a wide range of hydrophilicity without post-activation, and the enhancement of the conductivity of the carbon materials. These ordered mesoporous carbon materials, prepared by microwave-assisted synthesis, should have great potential for use in applications such as selective adsorbents, catalyst supports and so on, by taking advantage of their unique physico-chemical properties. Acknowledgement This work was supported by a grant (M102KP01001507K1601-01512) from Carbon Dioxide Reduction & Sequestration Center, one of the 21st Century Frontier Programs funded by the Ministry of Science and Technology of Korean government.

Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.carbon.2007. 08.031. References [1] Ryoo R, Joo SH, Jun S. Synthesis of highly ordered carbon molecular sieves via template-mediated structural transformation. J Phys Chem B 1999;103:7743–6. [2] Lu AH, Schu¨th F. Nanocasting: a versatile strategy for creating nanostructured porous materials. Adv Mater 2006;18:1793–805. [3] Tompsett GA, Conner WC, Yngvesson KS. Microwave synthesis of nanoporous materials. ChemPhysChem 2006;7:296–319. [4] Hong EH, Lee KH, Oh SH, Park CG. Synthesis of carbon nanotubes using microwave radiation. Adv Funct Mater 2003;13:961–6. [5] Wang Y, Iqbal Z, Mitra S. Microwave-induced rapid chemical functionalization of single-walled carbon nanotubes. Carbon 2005;43: 1015–20. [6] Kim SS, Pauly TR, Pinnavaia RJ. Non-ionic surfactant assembly of ordered, very large pore molecular sieve silicas from water soluble silicates. Chem Commun 2000:1661–2. [7] Kresege CT, Leonowicz ME, Roth WJ, Vartuli JC, Beck JS. Ordered mesoporous molecular sieves synthesized by a liquid-crystal template mechanism. Nature 1992;359:710–2. [8] Gallis KW, Landry CC. Rapid calcination of nanostructured silicate composites by microwave irradiation. Adv Mater 2001;13:23–6. [9] Kang M, Yi SH, Lee HI, Yie JE, Kim JM. Reversible replication between ordered mesoporous silica and mesoporous carbon. Chem Commun 2002:1944–5. [10] Choi M, Ryoo R. Ordered nanoporous polymer-carbon composites. Nature Mater 2003;2:473–6.