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1994, 3, 10.[3] J. Wei, J. M. Buriak, G. Siuzdak, Nature 1999, 399, 243.[4] R. C. Anderson, R. S. Muller, C. W. Tobias, Sens. Actuators 1990,

A21±A23, 835.[5] M. T. Kelly, J. K. M. Chun, A. B. Bocarsly, Nature 1996, 382, 214.[6] J. Harper, Ph. D. Thesis, University of California, San Diego 1996.[7] A. Motohashi, M. Ruike, M. Kawakami, H. Aoyagi, A. Kinoshita,

A. Satou, Jpn. J. Appl. Phys. 1996, 35, 4253.[8] M. J. Schoning, M. Crott, F. Ronkel, M. Thust, J. W. Schultze,

P. Kordos, H. Luth, Proc. ESSDERC '96 (Bologna, Italy) 1996, p. 907.[9] G. M. O'Halloran, M. Kuhl, P. J. Trimp, P. J. French, Sens. Actuators.

1997, A A61, 415.[10] M. J. Sailor, in Properties of Porous Silicon, Vol. 18 (Ed: L. Canham),

Short Run Press, London 1997, p. 364.[11] M. Thust, M. J. Schoning, S. Frohnhoff, R. Arens-Fischer, Meas. Sci.

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Proc. 1999, 557.[14] A. Janshoff, K.-P. S. Dancil, C. Steinem, D. P. Greiner, V. S.-Y. Lin,

C. Gurtner, K. Motesharei, M. J. Sailor, M. R. Ghadiri, J. Am. Chem.Soc. 1998, 120, 12 108.

[15] N. F. Starodub, L. L. Fedorenko, V. M. Starodub, S. P. Dikij, S. V.Svechnikov, Sens. Actuators B 1996, 35, 44.

[16] R. Herino, in Properties of Porous Silicon, Vol. 18 (Ed: L. Canham),Short Run Press, London 1997, p. 89.

[17] C. L. Curtis, V. V. Doan, G. M. Credo, M. J. Sailor, J. Electrochem.Soc. 1993, 140, 3492.

[18] C. A. Papadopoulos, D. S. Vlachos, J. N. Avaritsiosis, Sens. ActuatorsB 1996, 32, 61.

[19] G. Sberveglleri, W. Hellmich, G. Muller, Microsyst. Technol. 1997, 3,183.

[20] Y. S. E. Chung, L. Keenan (Motorola Inc), U.S. Patent number5 683 569 1997.

[21] B. Rossi, Optics, Addison-Wesley, Reading, MA 1957.[22] Y.-I. Lee, K.-H. Park, J. Lee, C.-S. Lee, H. J. Yoo, C.-J. Kim, Y.-S.

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39, 423.

Development of a New Mesoporous CarbonUsing an HMS Aluminosilicate Template**

By Jinwoo Lee, Songhun Yoon, Seung M. Oh,Chae-Ho Shin, and Taeghwan Hyeon*

Template synthesis of porous materials is one of the mostintensively studied research areas in materials chemistry.Since the development of M41S materials by Mobil Oil re-searchers in 1992,[1,2] many different mesoporous inorganicmaterials have been prepared using various types of organ-

ic templates.[3±6] Porous polymeric and carbon materialshave been synthesized using inorganic templates.[7±9] Manyporous carbons have been extensively applied in separationand purification technology.[10] They are also used as cata-lytic supports, chromatography columns, and electrodematerials for batteries and capacitors.[11±13] These porouscarbons are usually microporous and the production oflarger pore-sized mesoporous carbons has been intensivelypursued for applications in separation of bulky organic ma-terials and electrode materials. Recently, we have devel-oped new preparative methods to produce mesoporous car-bons using inorganic templates such as surfactant-stabilizedsilica sol particles[14] and mesoporous MCM-48.[15] Inparticular, the carbon material produced using the MCM-48 template exhibited interesting electrochemical double-layer capacitance (EDLC) behavior, resulting from regular3D interconnected mesopores. Difficulty in the synthesis ofthe template MCM-48 material, however, would hamperthe extensive application of mesoporous carbon material.Herein we report the synthesis of a new mesoporous car-bon using hexagonal mesoporous silica (HMS) aluminosili-cate as a template. With the knowledge gained from this re-search, we could indirectly elucidate the pore structure ofHMS. We also present preliminary results on the EDLCperformance of the material.

HMS has several advantages over MCM-48 from a syn-thetic viewpoint: 1) the use of cheap primary alkyl aminesas the structure-directing agent; 2) a higher silica recoveryyield (>95 %) than MCM-48 (~50 %); 3) a shorter synthesistime (18 h for HMS and 4 days for MCM-48); and 4) no hy-drothermal reaction.[16,17] In the first report on HMS theauthors claimed that the pore structure of the silica materi-al is similar to that of well-known hexagonal mesoporousMCM-41, but with a much smaller scattering domain size.The small domain size enabled formation of textural pores,along with framework pores from the template. In a laterpublication, the same group suggested a wormhole-likepore structure for HMS based on transmission electron mi-croscopy (TEM).[17] The pore structure and the pore con-nectivity of HMS have not yet been elucidated. The poreconnectivity of mesoporous materials is important in cata-lytic and electrochemical applications. By changing the po-larity of the reaction solvent, HMS with dominant frame-work pores has also been synthesized. In our study HMSsilica with predominant framework pores (negligible tex-tural pores) has been utilized as a template.

The synthetic procedure for the synthesis of mesoporouscarbon using HMS as a template is as follows: HMS wasprepared by the reported method using the starting reac-tion mixture in a molar ratio of tetraethylorthosilicate(TEOS)/hexadecylamine/EtOH/H2O = 1:0.25:10:30.[17] Sur-factant was removed by extraction with refluxing ethanol.The resulting HMS was treated with AlCl3 in ethanol atroom temperature to generate strong acid catalytic Al sitesfor the polymerization of phenol and formaldehyde. Thegas adsorption of the Al-HMS template exhibited the pre-

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[*] Prof. T. Hyeon, J. Lee, S. Yoon, Prof. S. M. OhSchool of Chemical Engineering and Institute of Chemical ProcessesSeoul National UniversitySeoul 151-742 (Korea)

Prof. C.-H. ShinDepartment of Chemical EngineeringChungbuk National UniversityCheongju, Chungbuk 360-763 (Korea)

[**] This work was supported by the Korea Research Foundation (NewMaterials Research 1998) and the KOSEF (Basic Research Program#98-05-02-03-01-3).

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dominant framework pores. The resulting aluminum incor-porated HMS silica material (Al-HMS) has a Si/Al ratio of30.4. The surface area and pore diameter of Al-HMS were958 m2/g and 3.2 nm, respectively. Phenol was incorporatedinto the pores of Al-HMS by heating at 90 �C for 12 h un-der reduced pressure. Formaldehyde and the phenol/Al-HMS composite were reacted in an autoclave at 125 �C for5 h to polymerize phenol and formaldehyde inside thepores.[18] The resulting Al-HMS/phenol resin compositewas heated under a constant stream of N2 at a heating rateof 5 �C/min to 700 �C and held there for 7 h to carbonizephenol resin. The dissolution of Al-HMS using 48 % hydro-fluoric acid (aqueous HF) generated a new mesoporouscarbon, designated SNU-2 (Seoul National University).

The ordered mesoporous structure of SNU-2 was investi-gated by X-ray diffraction (XRD) and gas adsorption mea-surements. Trace A in Figure 1 shows the characteristicXRD pattern of the Al-HMS template with a d-spacingvalue of 4.50 nm.[16] A carbon/Al-HMS composite showed

a similar XRD pattern with lower intensity (Fig. 1,trace B), confirming that the aluminosilicate framework ofAl-HMS is retained after the high-temperature carboniza-tion step. The XRD intensity was significantly decreasedbecause the pores of Al-HMS were filled with amorphouscarbon materials. Interestingly, a strong peak at 2y = 2.18from the ordering of mesopores is clearly observed in theXRD pattern of SNU-2 carbon (Fig. 1, trace C), demon-strating that the mesoporous structure was preserved evenafter the removal of the aluminosilicate framework by HFetching. Elemental analysis of SNU-2 revealed a high C/Hmolar ratio of 11.76 (C 91.48 % and H 0.65 %). Thermo-gravimetric analysis (TGA) under an oxygen atmosphererevealed that the maximum silica residue is 3.5 wt.-% (ap-proximately 0.7 mol.-%), confirming that the intense XRDpeak does not result from the HMS silica template. TheXRD result clearly demonstrates that HMS has 3D inter-

connected pores, unlike the originally proposed tubularhexagonal pores (similar to those of MCM-41).

To corroborate our result, we have synthesized carbonmaterial inside the mesopores of Al-MCM-41. The syn-thetic procedure is similar to that of SNU-2. Trace A inFigure 2 shows the characteristic XRD pattern of hex-agonal Al-MCM-41. The Al-MCM-41/carbon compositematerial (Fig. 2, trace B) still retains the hexagonal struc-ture, but with much lower diffraction intensity because ofthe pore filling. The resulting carbon material (designatedas M41-C) after HF etching (Fig. 2, trace C), however,shows no Bragg diffraction in the small-angle region, con-firming the absence of regular mesopores.

Fig. 2. X-ray diffraction (XRD) patterns of Al-MCM-41 template (trace A),Al-MCM-41/carbon composite (trace B), and replica carbon M41-C(trace C). Patterns were obtained with a Rigaku D/Max-3C diffractometerequipped with a rotating anode and Cu Ka radiation (l = 0.15418 nm).

The gas adsorption study of SNU-2 presents more com-pelling evidence for the formation of regular mesoporesfrom the 3D interconnected porous nature of HMS. Volu-metric gas adsorption measurements have been conductedusing argon at liquid argon temperature. The pore size dis-tribution (PSD) was computed by the adsorption isothermanalysis proposed by Horvath and Kawazoe.[19] SNU-2 car-bon exhibited a bimodal pore size distribution, one peakfrom micropores centered at 0.6 nm and the other from thetemplating effect of mesoporous Al-HMS centered at2.0 nm (Fig. 3a). Argon adsorption and desorption curves(Fig. 4) exhibited the characteristics of mixed micro- andmesoporous carbons.[20] In contrast, carbon made from theetching of Al-MCM-41/carbon composite is microporouswith a peak at 0.6 nm, which results from the carbonizationof phenol resin (Fig. 3b). Similar microporous carbon wasalso synthesized from the carbonization of phenol resinwithout using a silica template. The total pore volume ofSNU-2, measured at P/P0 = 0.99 using liquid argon with aconversion factor of 1.280 ´ 10±3, was 0.69 cm3/g. The spe-cific surface area of SNU-2, calculated using an argon cross-section of 0.142 nm2, was found to be 1056 m2/g, which iscomparable to commercially available activated carbons.

Fig. 1. X-ray diffraction (XRD) patterns of Al-HMS template (trace A),Al-HMS/carbon composite (trace B), and mesoporous SNU-2 carbon(trace C). Patterns were obtained with a Rigaku D/Max-3C diffractometerequipped with a rotating anode and Cu Ka radiation (l = 0.15418 nm).

Cyclic voltammetry (CV) measurements have been con-ducted in order to test the EDLC performance of SNU-2.EDLCs find new promising applications as pulse powersources for digital communication devices and electric ve-hicles.[21,22] The popularity of these devices results fromtheir high energy and power density and longer life cycle.EDLCs utilize the electrochemical double layer formed atthe interface of electrode and electrolyte to store electriccharge. Conducting materials with high surface areas, suchas activated carbons or activated carbon fibers, have beencommonly applied as the electrode materials. CVs havealso been taken of the most popularly applied activatedcarbon, MSC-25, as a reference (Kansai Coke and Chemi-

cals, Brunauer±Emmett±Teller (BET) surface area of1935 m2/g). The cyclic voltammograms of SNU-2 andMSC-25, in 2 M H2SO4(aq) electrolyte solution at a scanrate of 10 mV/s, are shown in Figure 5a. SNU-2 (solid line)exhibits a more ideal capacitor behavior than MSC-25(dotted line) with a steeper current change at the switching

potentials of 0.0 and 0.7 V, resulting in a more rectangular-shaped (mirror image) I±V curve. In an ideal capacitor,energy must be retrievable as discharge over the same po-tential range as that required to store the energy on charg-ing, which is reflected in rectangular-shape cyclic voltam-mograms. The steepness in the current change at theswitching potential is determined by the resistance and ca-pacitance (RC) time constant. The approximate time con-stant of SNU-2 is much lower than that of MSC-25, pre-dominantly resulting from the lower resistance of SNU-2.The higher time constant and the resulting slow change atthe switching potentials in the CV of the MSC-25 electrodewould result from the slow ionic motions in micropores.The steep changes in the CV of the SNU-2 electrode reflectthe dominance of regular mesopores among the electro-chemically usable pores.

Achieving rectangular-shaped cyclic voltammogramsover a wide range of scan rates is the ultimate goal inEDLC. This behavior is important for practical applica-tions with respect to high energy and power density. SNU-2carbon is much closer to this behavior than MSC-25. When

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Fig. 3. Pore size distributions of carbons computed by the analysis of adsorp-tion isotherm proposed by Horvath and Kawazoe: a) SNU-2 and b) M41-C.

Fig. 4. Argon adsorption and desorption isotherms of mesoporous SNU-2carbon. The volumetric gas adsorption measurements were conducted usingargon at liquid argon temperature.

Fig. 5. Cyclic voltammograms of SNU-2 (solid line) and MSC-25 (dottedline) carbons in aqueous electrolyte with a scan rate of 10 mV/s (a) and20 mV/s (b). The three-electrode system was used: carbon materials as aworking electrode, Pt metal as a counter electrode, and SCE (Standard Ca-lonel electrode) as a reference electrode. The electrolyte was 2 M H2SO4(aq)

solution. CV was performed at the potential range from 0 V to 0.7 V (versusSCE reference electrode).

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cyclic voltammograms were taken for these two carbons byvarying the scan rate from 5 mV/s to 50 mV/s, the SNU-2carbon kept the rectangular-shape up to a scan rate of20 mV/s (Fig. 5b, solid line). In contrast, the MSC-25 car-bon showed a deformed cyclic voltammogram at a scanrate of 10 mV/s and a completely collapsed one at a scanrate of 20 mV/s (Fig. 5b, dotted line). A detailed discussionon the electrochemical studies of the material will be pre-sented in a forthcoming paper.

In conclusion, we have made a new high surface area me-soporous carbon using Al-HMS as a template. From this re-search we discovered that the pores of HMS are 3D inter-connected, unlike the originally proposed disorderedhexagonal structure. The EDLC performance of the carbonmaterial was superior to the commercially available carbonMSC-25 due to improved mesoporosity. The CV of the me-soporous carbon showed ideal rectangular shapes at a highscan rate of 20 mV/s.

Received: August 9, 1999Final version: November 22, 1999

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Circularly Polarized Electroluminescence fromLiquid-Crystalline Chiral Polyfluorenes**

By Masao Oda, Heinz-Georg Nothofer, Günter Lieser,Ullrich Scherf,* Stefan C. J. Meskers, and Dieter Neher*

Control of the polarization of light is essential for opticaldata processing and display devices. While the use of lin-early-polarized light is already established in commercialphotonic devices such as liquid-crystal (LC) displays, thepotential applications of circularly polarized light have, toa large extent, not yet been employed. Several reportshave, however, shown the potential use of circularly polar-ized light for photonic devices in, for example, optical datastorage[1] and as backlights for LC displays.[2] While wide-band reflective polarizers might act as passive componentsin such devices,[3] the direct generation of circularly polar-ized light would be far more favorable in terms of energyefficiency and production costs.

Circularly polarized electroluminescence (CPEL) wasfirst demonstrated in a chiral-substituted poly(p-phenylenevinylene) (PPV) derivative.[4] These devices emitted lightwith the right-handed emission intensity, IR, slightly largerthan the left-handed intensity, IL. The degree of circularpolarization, characterized by the dissymmetry factor inelectroluminescence gEL = 2(IL±IR)/(IL+IR) was, however,very low (about 1.3´10±3). In such chiral-substituted conju-gated polymers the chiroptical properties are believed toresult mainly from interchain exciton coupling within chiralaggregates, and the dissymmetry factors, gPL, of circularlypolarized photoluminescence (CPPL) barely exceeded1 %.[5] In accordance with this interpretation, Langmuir±Blodgett assemblies of chiral substituted poly(p-phenyl-ene)s, in which the formation of three-dimensional chiralaggregates in not possible, exhibited only very weak opticalactivity.[6]

An alternative approach to generate circularly polarizedphotoluminescence is to add a luminescent chromophoreto a chiral nematic or cholesteric liquid crystal as first pro-

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±

[*] Dr. U. Scherf, Dr. M. Oda, Dr. H.-G. Nothofer, Dr. G. Lieser,D. NeherMax-Planck-Institut für PolymerforschungAckermannweg 10, D-55128 Mainz (Germany)

Dr. S. C. J. MeskersLaboratory of Macromolecular and Organic ChemistryEindhoven University of TechnologyPO Box 513, NL-5600 MB, Eindhoven (The Netherlands)

Dr. D. NeherInstitut für Physik, Universität PotsdamAm Neuen Palais 10, D-14469 Potsdam (Germany)

[**] We thank Prof. E. W. Meijer (University of Eindhoven) for the oppor-tunity to measure CPPL and CPEL in his laboratories. We also thankProf. W. Knoll, Prof. K. Müllen, Prof. G. Wegner (all MPI in Mainz),and Dr. A. Yasuda (Sony International Europe) for generous supportand fruitful discussions. Financial support was in part by Sony Interna-tional Europe (Germany). MO's stay in Mainz has been supported bya DAAD fellowship.