Zeolite like metal–organic frameworks (ZMOFs) (1)

REVIEW DOI: 10.1002/adma.200501576 Recent Progress in the Synthesis of Porous Carbon Materials** By Jinwoo Lee, Jaeyun Kim, and Taeghwan Hyeon* In this review, the progress made in the last ten years concerning the synthesis of porous carbon materials is summarized Porous carbon materials with various pore sizes and pore structures have been synthesized using several different routes Microporous activated carbons have been synthesized through the activation process Ordered microporous carbon materials have been synthesized using zeolites as templates Mesoporous carbons with a disordered pore structure have been synthesized using various methods, including catalytic activation using metal species, carbonization of polymer/polymer blends, carbonization of organic aerogels, and template synthesis using silica nanoparticles Ordered mesoporous carbons with various pore structures have been synthesized using mesoporous silica materials such as MCM-48, HMS, SBA-15, MCF, and MSU-X as templates Ordered mesoporous carbons with graphitic pore walls have been synthesized using soft-carbon sources that can be converted to highly ordered graphite at high temperature Hierarchically ordered mesoporous carbon materials have been synthesized using various designed silica templates Some of these mesoporous carbon materials have successfully been used as adsorbents for bulky pollutants, as electrodes for supercapacitors and fuel cells, and as hosts for enzyme immobilization Ordered macroporous carbon materials have been synthesized using colloidal crystals as templates One-dimensional carbon nanostructured materials have been fabricated using anodic aluminum oxide (AAO) as a template Introduction Porous carbon materials have received a great deal of attention due to their many applications.[1] Porous carbon materials have been applied to gas separation, water purification, cata- – [*] Prof T Hyeon, Dr J Lee, J Kim National Creative Research Initiative Center for Oxide Nanocrystalline Materials and School of Chemical and Biological Engineering Seoul National University Seoul 151–744 (Korea) E-mail: thyeon@snu.ac.kr [**] We thank the financial support by the Korean Ministry of Science and Technology through the National Creative Research Initiative Program Adv Mater 2006, 18, 2073–2094 lyst supports, and electrodes for electrochemical double layer capacitors and fuel cells.[2] According to the International Union of Pure and Applied Chemistry (IUPAC) recommendation, porous carbon materials can be classified into three types based on their pore sizes: microporous < nm, nm < mesoporous < 50 nm, and macroporous > 50 nm Porous carbon materials have been synthesized using various methods The following are representative traditional methods 1) Chemical activation, physical activation, and a combination of the physical and chemical activation processes.[3] 2) Catalytic activation of carbon precursors using metal salts or organometallic compounds.[4] 3) Carbonization of polymer blends composed of a carbonizable polymer and a pyrolyzable polymer.[5] 4) Carbonization of a polymer aerogel synthesized under supercritical drying conditions.[6] © 2006 WILEY-VCH Verlag GmbH & Co KGaA, Weinheim 2073 REVIEW J Lee et al./Porous Carbon Materials Although many porous carbon materials have been developed using the above-mentioned methods, the synthesis of uniform porous carbon materials has been very challenging Over the last ten years, many kinds of rigid and designed inorganic templates have been employed in an attempt to synthesize carbons with uniform pore sizes Knox and his co-workers pioneered the template synthesis of porous carbons.[7] Since then, many porous carbon materials with uniform pore sizes having micropores, mesopores, or macropores have been synthesized using various inorganic templates Figure 1a depicts the overall concept of the template procedure, which is essentially the same as that used to fabricate a ceramic jar, but scaled down to the nanometer regime To make a jar, a piece of wood with the desired shape is first carved, and then clay is applied to the surface of the wood Through heating at ca 1000 °C under air, the clay is transformed to ceramic and the wood is simultaneously burnt to generate the empty space inside the jar The general template synthetic procedure for porous carbons is as follows: 1) preparation of the carbon precursor/inorganic template composite, 2) carbonization, and 3) removal of the inorganic template Various inorganic materials, including silica nanoparticles (silica sol), zeolites, anodic alumina membranes, and mesoporous silica materials, have been used as templates Figure 1b to 1d describes the synthesis of microporous, mesoporous, and macroporous carbons using zeolite, mesoporous silica, and synthetic silica opal as templates, respectively Figure 1e shows the synthesis of carbon nanotubes (CNTs) using an anodic alumina membrane template Broadly speaking, the template approaches can be classified into two categories In the first approach, inorganic Taeghwan Hyeon received his B S (1987) and M S (1989) in Chemistry from Seoul National University, Korea He obtained his Ph.D from the University of Illinois at Urbana-Champaign (1996) Since he joined the faculty of the School of Chemical and Biological Engineering of Seoul National University in September 1997, he has focused on the synthesis of uniform-sized nanocrystals and new nanoporous carbon materials and published more than 100 papers in prominent international journals He is currently a Director of National Creative Research Initiative Center for Oxide Nanocrystalline Materials supported by the Korean Ministry of Science and Technology He has received numerous awards, including the Korean Young Scientist Award from the Korean President and DuPont Science and Technology Award He is currently serving as an editorial advisory board member for Advanced Materials, Chemical Communications, and Small Jinwoo Lee was born in Seoul, Korea, in 1974 He received his B.S (1998), M.S (2000), and Ph.D (2003) from the Chemical and Biological Engineering Department of Seoul National University, Korea During his graduate research under the direction of Prof Taeghwan Hyeon, he worked on the synthesis of mesoporous carbon materials using mesostructured silica templates As a postdoctoral researcher he is studying the biological applications of large-pore mesoporous carbons Jaeyun Kim was born in Tongyeong, Korea, in 1978 He received his B.S (2001) and M.S (2003) from the Chemical and Biological Engineering Department of Seoul National University, Korea Since then he has worked on his doctoral thesis studying the synthesis and application of mesoporous carbon and the self-assembly of nanoparticles under the direction of Prof Taeghwan Hyeon 2074 www.advmat.de © 2006 WILEY-VCH Verlag GmbH & Co KGaA, Weinheim Adv Mater 2006, 18, 2073–2094 J Lee et al./Porous Carbon Materials Adv Mater 2006, 18, 2073–2094 © 2006 WILEY-VCH Verlag GmbH & Co KGaA, Weinheim www.advmat.de REVIEW stroms in diameter These MSCs have been applied to various areas including the separation of gas molecules, shape-selective catalysts, and electrodes for electrochemical double-layer capacitors MSCs have advantages over inorganic molecular sieves (zeolites) in terms of their hydrophobicity and high corrosion resistance The most representative synthetic method for the synthesis of MSCs is the pyrolysis of appropriate carbon precursors Miura et al prepared MSCs by pyrolyzing a mixture of coal and organic additives.[8] The carbon materials obtained using organic additives have pore structures different from those of the carbons prepared from coal only By changing the experimental conditions it was possible to finely tune the pore size of the MSCs For example, by changing the carbonization temperature and the mixing ratio of coal, pitch, phenol, and formaldehyde, MSCs having a uniform pore size of around 0.35 nm were synthesized The Miura group also used ion-exchange resins to produce MSCs.[9] Spherical polystyrene based resins with a sulfonic acid group were ionexchanged with several kinds of cations, and the resulting resins were carbonized at between 500 and 900 °C In this way resins having various cations including H+, K+, Na+, Ca2+, Zn2+, Cu2+, Fe2+, Ni2+, and Fe3+ were prepared from the ion-exchange resin When the ion-exchanged resin was carbonized at 900 °C under a nitrogen atmosphere, the MSCs prepared from the resins with di- or trivalent catFigure a) Schematic representation showing the concept of template synthesis ions maintained sharp pore distributions, whereas b) Microporous, c) mesoporous, and d) macroporous carbon materials, and e) carthose prepared from the resins with univalent catbon nanotubes were synthesized using zeolite, mesoporous silica, a synthetic silica ions lost most of their pores The main reason for opal, and an AAO membrane as templates, respectively this drastic difference is that di- or trivalent cations can form ionic crosslinks connecting two or three functional groups in the resins, and these crosslinks act as piltemplates such as silica nanoparticles are embedded in the lars to stabilize the pores during the carbonization process carbon precursor Carbonization followed by the removal of the template generates porous carbon materials with isolated The wide-angle X-ray diffraction (XRD) pattern of the carpores that were occupied by the template species In the secbonized samples revealed the presence of metal-sulfide nanoond approach, a carbon precursor is introduced into the pores particles, which are responsible for the formation of uniformly of the template Carbonization and the subsequent removal of sized micropores Films of these MSCs were fabricated for their applications in gas separation the templates generate porous carbons with interconnected Microporous carbon membranes have been prepared using pore structures In this review, we summarize the recent develvarious polymeric resins.[10] Carbon membranes have been opments in the synthesis of porous carbon materials, focusing on the synthesis of porous carbon materials with uniform pore prepared in two main configurations, that is, unsupported carsizes via the template approaches The main part of the review bon membranes and membranes supported on macroporous is divided into three sections based on the pore sizes: micropomaterials A supported carbon membrane was prepared by rous, mesoporous, and macroporous carbon materials casting 13 wt % polyamic acid in N-methylpyrrolydone (NMP) on a macroporous carbon support.[10a] The resulting polymer was heat-treated through a two-step process involv2 Microporous Carbon Materials ing imidization at 380 °C and subsequent carbonization at 550 °C The gas-permeation experiment showed that the gas 2.1 Disordered Microporous Carbons (Molecular Sieving transport through the MSC membrane occurs according to Carbons) the molecular-sieving mechanism The membrane had selective permeation for O2/N, He/N, CO2/CH4, and CO2/N The Molecular sieving carbons (MSCs) are special forms of actihighest separation factors were achieved at 25 °C A molecuvated carbons that possess uniform micropores of several ang- 2075 REVIEW J Lee et al./Porous Carbon Materials 2076 lar-sieving carbon film with a nanometer-sized a) nickel catalyst was prepared from polyimide-containing nickel nitrate.[11] The combination of the catalytic function with the molecular-sieving property was also investigated The molecular-sieving property of the MSC film with a nickel catalyst was comparable to that of Zeolite 5A It was found that the MSC catalyst carbonized at low temperab) ture (600 °C or 650 °C) showed a high selectivity in the competitive hydrogenation reactions of butene isomers (butene and isobutene) In the narrow nanospace of the MSC with a nickel catalyst, smaller molecules can be more easily hydrogenated compared to larger molecules Considering the relative sizes of butene and isobutene, the hydrogenation of isobutene was much slower than that of butene However, perfect shape selectivity could not be achieved, because of the presence of the cataFigure a) Schematic explaining the overall template synthetic procedure for microlyst particles on the outer surface of the MSC carporous carbons using a zeolite Y template b) High-resolution transmission electron bon matrix Consequently, the elimination of the microscopy (HRTEM) image of the ordered microporous carbon prepared following the procedure reported The inset corresponds to a diffraction pattern taken from this image nickel catalyst particles formed on the outer surReproduced with permission from [18] Copyright 2001 American Chemical Society face of the MSC film is extremely important to achieve perfect shape selectivity Shiflett and Foley reported the fabrication of a stainlessbon precursor.[14] The chemical vapor deposition (CVD) steel-supported MSC membrane via the ultrasonic deposition method was also adopted for the introduction of carbon into of poly(furfuryl alcohol) on stainless-steel tubes and subsethe channels of USY zeolite CVD was carried out by exposquent pyrolysis at 723 K.[12] The membrane was successfully ing the zeolite to propylene gas at 700 or 800 °C The resulting applied to gas separation with the following permeances, meamicroporous carbons exhibited high surface areas of over sured in moles per square meter per Pascal per second: nitro2000 m2 g–1 The similar morphology of the resulting micropo–12 –11 –10 gen, 1.8 × 10 ; oxygen, 5.6 × 10 ; helium, 3.3 × 10 ; and hyrous carbon particles and the original zeolite template partidrogen, 6.1 × 10–10 The ideal separation factors as compared cles, observed using scanning electron microscopy (SEM), to that for nitrogen were 30:1, 178:1, and 331:1 for oxygen, hedemonstrated that the carbonization occurred inside the chanlium, and hydrogen, respectively nels of the zeolite template However, the Kyotani group failed to synthesize ordered microporous carbon arrays and the carbon material that they fabricated possessed a consider2.2 Ordered Microporous Carbon Materials Synthesized able amount of mesopores The generation of mesopores reUsing Zeolite Templates sulted from the partial collapse of the carbon framework after the removal of the zeolite template by HF etching The thin To make microporous carbon materials not only with uniwall thickness of the carbon, derived from the small pores of form pores, but also with ordered regular pore arrays, rigid inthe zeolite template (0.74 nm), did not exhibit a sufficiently organic templates are required Zeolites are aluminosilicate high mechanical strength to survive the removal of the temmaterials having ordered and uniform sub-nanometer sized plate Rodriguez-Miraso et al adopted a similar approach to pores Zeolites have been widely used as molecular sieves, solproduce microporous carbon using zeolite Y as a template, id acid catalysts, and catalyst supports, and have also been and they went on to examine the oxidation behavior of the reused as shape-selective catalysts owing to their uniform mosulting porous carbon.[15] [13] lecular-sized pores Because the walls of zeolites have a uniThe Mallouk group synthesized phenol–formaldehyde (PF) form thickness of < nm, zeolites have been used as inorganic polymers by making use of the acidity of the zeolite frametemplates for the synthesis of microporous carbons with uniwork inside various zeolites, for instance, zeolites Y, b, and L, form pore sizes USY zeolite was adopted as the template to and then carbonized the polymer/zeolite composites to obtain prepare a microporous carbon by the Kyotani group porous carbons.[16] Phenol was infiltrated into the narrow Figure 2a shows the overall template synthetic procedure for pores of the zeolite by the vapor-phase infiltration method microporous carbons using a zeolite Y template A carbon These carbons possessed a considerable amount of mesopores, precursor was incorporated into the pores and channels of the which was consistent with the results obtained by Kyotani and zeolite Carbonization followed by the removal of the zeolite co-workers Moreover, the ordered structure of the zeolite template produced microporous carbon materials Poly(acrywas not faithfully transferred to the resulting porous carbons lonitrile) or poly(furfuryl alcohol) was employed as the carLater, the Kyotani group was able to successfully synthesize www.advmat.de © 2006 WILEY-VCH Verlag GmbH & Co KGaA, Weinheim Adv Mater 2006, 18, 2073–2094 J Lee et al./Porous Carbon Materials Adv Mater 2006, 18, 2073–2094 the structural regularity of the corresponding zeolite template The extent of such transferability, however, strongly depended on the kind of zeolite template employed The authors concluded that in order to obtain microporous carbons with high structural regularity the pores in the zeolite template should be sufficiently large (> 0.6–0.7 nm), as well as being three-dimensionally interconnected More recently, the Kyotani group also synthesized a nitrogen-containing microporous carbon with a highly ordered structure by using zeolite Y as a template.[20] The formation of nitrogen-doped carbon in the zeolite channels was achieved by the impregnation of FA and subsequent CVD of acetonitrile The nitrogen-incorporated and ordered microporous carbon exhibited a stronger affinity to H2O adsorption than the nitrogen-free, ordered, microporous carbon materials with similar pore structures, demonstrating the polar and hydrophilic nature of the nitrogen-doped carbon For many industrial applications, such as the selective permeation of gas molecules, the control of the pore size is a critical issue Consequently, we expect more research on the poresize control of ordered microporous carbon materials to be conducted in the future REVIEW uniformly sized and ordered microporous carbon materials using zeolite Y as a template via the two-step carbonization method.[17] The one-step carbonization method did not enable the complete filling of the channels and pores of the zeolite template, and this resulted in the extensive collapse of the carbon framework during the removal of the template In order to prevent this partial collapse of the carbon framework, the additional incorporation of carbon was achieved by a CVD process using propylene gas after the initial carbonization by the heat-treatment of the zeolite/furfuryl alcohol (FA) composite at 700 °C The carbon obtained after the removal of the zeolite template exhibited an ordered zeolite replica structure, as confirmed by the strong (111) reflection of zeolite Y at a 2h angle of 6.26° in the XRD pattern Although ordered microporous carbon materials with a negative replica structure of zeolite Y were obtained by the two-step method, there was still an amorphous (002) peak at the 2h angle of 23° in the XRD pattern, which demonstrated the partial collapse of the carbon framework in the zeolite channels Later, the same research group reported the synthesis of ordered microporous carbon having a rigid framework, but without the amorphous (002) peak, using heat treatment of the carbon/zeolite composite obtained by the above two-step method at 900 °C.[18] The carbon inside the channels seemed to be better carbonized and its structure would be expected to be more rigid and stable as a result Consequently, the long-range ordering of the carbon particles replicated from the zeolite template might be better retained than that of the carbon obtained without this heat treatment at 900 °C The carbon so produced had almost no mesoporosity (its micropore and mesopore volumes were 1.52 cm3 g–1 and 0.05 cm3 g–1, respectively) The (111) peak of the ordered microporous carbon prepared by the additional heat treatment at 900 °C was more intense than that obtained without the additional heat treatment, indicating the presence of a larger amount of highly ordered carbon structure in the microporous carbon The surface area of the ordered microporous carbon was found to be 3600 m2 g–1, which is much higher than that of the carbon prepared without the additional heat treatment (2200 m2 g–1) Although the surface area of some KOH activated carbons is over 3000 m2 g–1, these carbons always suffer from the presence of some mesoporosity and have a broad pore distribution, which is undesirable for many applications, such as gas storage Figure 2b shows the high-resolution transmission electron microscopy (TEM) image of the ordered microporous carbon obtained from zeolite Y Its excellent 3D ordering is clearly demonstrated The internal structure of this ordered microporous carbon was also characterized using 13C solid-state NMR, and was found to consist of a condensed aromatic ring system In a subsequent paper, the same research group extended the two-step replication process to other zeolite systems, in order to make various ordered microporous carbon arrays.[19] The optimum conditions to be used to obtain carbon with the highest long-range ordering varied depending on the zeolite templates that were used When using the simple CVD method, unlike in the case of zeolite Y, the carbons inherited Mesoporous Carbon Materials Over the last decade, there have been significant advances in the synthesis of mesoporous carbon materials.[21] Mesoporous carbon materials are very important for applications involving large molecules, such as adsorbents for dyes, catalyst supports for biomolecules, and electrodes for biosensors 3.1 Mesoporous Carbons with Disordered Pore Structures Catalytic activation using metal ions was employed to synthesize several types of mesoporous carbon materials Yasuda and co-workers synthesized mesoporous activatedcarbon materials by the steam invigoration of pitches mixed with 1–3 wt % of rare-earth metal complexes, such as Ln(C5H5)3 and Ln(acac)3 (where Ln = Sm, Y, Yb or Lu).[22] All of the resulting mesoporous carbons had high mesopore ratios of up to 80 %, surface areas of ca 200 m2 g–1, and pore sizes ranging from 20 nm to 50 nm These mesoporous activated carbons selectively adsorbed large molecules, such as vitamin B12, blue acid 90 dye, dextran, nystatin, and humic acid, reflecting their large mesopore volumes Oya and coworkers synthesized activated-carbon fibers containing a significant fraction of mesopores with sizes of several tens of nanometers from the catalytic activation of a phenol resin mixed with cobalt acetylacetonate.[23] The carbonization of polymer blends composed of two different types of polymers, that is, a carbon precursor polymer and a decomposable polymer that is pyrolyzed to generate pores, produced mesoporous carbon materials Ozaki et al synthesized mesoporous carbons with a pore diameter of © 2006 WILEY-VCH Verlag GmbH & Co KGaA, Weinheim www.advmat.de 2077 REVIEW J Lee et al./Porous Carbon Materials 2078 ca nm from the carbonization of a polymer blend composed of phenolic resin and poly(vinyl butyral).[5b] Later, Oya and co-workers synthesized carbon fibers from the carbonization of a polymer blend composed of a phenol–formaldehyde (PF) polymer embedded in a polyethylene (PE) matrix with a PF/PE weight ratio of 3:7.[5c] A bundle of PF-derived thin carbon fibers smaller than several hundred nanometers in diameter was produced The nanofiber bundle so obtained was easily separated into thin fibers These polymer-blend carbonization methods have been extensively used to synthesize many other mesoporous carbon materials.[5] The carbonization of organic aerogels prepared by the sol– gel technique, followed by supercritical drying, produced porous carbon materials.[6] Silica aerogels having high mesoporosity were prepared by the sol–gel polymerization of silica precursors, followed by supercritical drying.[24] The supercritical drying process relieves the large capillary forces generated during the drying process, and makes it possible to preserve the highly crosslinked and porous structure generated during the sol–gel polymerization Pekala et al synthesized carbon aerogels from the carbonization of organic aerogels based on a resorcinol–formaldehyde (RF) gel.[6] The resulting mesoporous carbon materials had high porosities (> 80 %) and high surface areas (> 400 m2 g–1) Subsequent studies on the poresize control of carbon aerogels were conducted by Tamon et al.[25] The pore radius of the RF aerogels was controlled in the range of 2.5–6.1 nm by changing the molar ratios of resorcinol to sodium carbonate and resorcinol to water Metal species were incorporated into the carbon framework during the preparation of carbon aerogels in order to modify their structure, conductivity, and catalytic activity Titanialoaded carbon aerogels were prepared by adding titanium alkoxide during the sol–gel reaction, and the resulting composite aerogels were used for the combined adsorption and photocatalytic removal of waste water Subsequent heat treatment at high temperature (between 500 and 900 °C) under a He flow generated a highly crystallized, titanium dioxide loaded mesoporous carbon.[26] A ruthenium/carbon aerogel composite was prepared via a novel two-step metal-vapor-impregnation method.[27] The resulting composite had highly dispersed Ru particles attached to the carbon aerogel and was used as the electrode material for supercapacitors Capacitances greater than 250 F g–1 were obtained for the samples with 50 wt % Ru and the capacitance of these composites could be tailored by varying the Ru loading and/or the density of the host carbon aerogel Carbon aerogels with a partially graphitized structure were synthesized by catalytic graphitization using Cr, Fe, Co, and Ni.[28] HRTEM, XRD, and Raman spectroscopy showed the presence of graphitized areas with a 3D stacking order The resulting carbon aerogels had a well-developed mesoporosity along with a graphitic character, which allow them to be used as the electrode materials for supercapacitors and fuel cells The synthesis of mesoporous carbon foams was achieved by Lukens and Stucky using RF gels as the carbon precursor and microemulsion-polymerized polystyrene (PS) microspheres as www.advmat.de the template.[29] Upon pyrolysis under an argon atmosphere, the organic PS microspheres were burnt off generating large mesopores The pore size of the mesoporous carbon foams was roughly two-thirds that of the template Silica materials have been extensively used as templates to synthesize mesoporous carbons The template silica materials were easily removed by treating them with HF or NaOH As described in the Introduction, Knox et al reported the synthesis of spherically shaped mesoporous carbon materials using silica gel and porous glass as templates.[7] The polymerization of the phenol–hexamine mixture within the pores of the silica gel, followed by the pyrolysis of the resulting resin in a nitrogen atmosphere at temperatures below 1000 °C, and subsequent dissolution of the silica template produced the mesoporous carbon materials The further graphitized spherical mesoporous carbons were successfully used as high-performance liquid chromatography (HPLC) column materials Our group synthesized mesoporous carbons using commercial silica sol nanoparticles as templates.[30] The polymerization of resorcinol and formaldehyde in the presence of a silica sol solution (Ludox HS-40 silica sol solution, average particle size ca 12 nm) generated RF gel/silica nanocomposites Carbonization followed by HF etching of the silica sol templates generated porous carbons, designated as silica sol mediated carbon (SMC1), having pore sizes predominantly in the range of 10–100 nm These carbon materials exhibited very high pore volumes of over cm3 g–1 and high surface areas of ca 1000 m2 g–1 Because the aggregated form of the silica nanoparticles acted as templates, the pore size distribution of the resulting carbon was broad, ranging from 10 nm to 100 nm These SMC1 carbon materials exhibited excellent adsorption capacities for bulky dyes[31] and humic acids.[32] In order to prevent the aggregation of the silica nanoparticles during the synthesis, surfactant-stabilized silica nanoparticles were used as the template (Fig 3a).[33] The resulting carbon material, designated as SMC2, exhibited a narrow pore size distribution centered at 12 nm, which matched very well with the particle size of the silica nanoparticle template Figure 3b compares the pore size distribution curves and the corresponding nitrogen adsorption/desorption isotherms of SMC1 and SMC2 carbons, demonstrating that SMC2 has a more uniform pore size distribution as compared to SMC1 When silica nanoparticles with a particle size of nm were used as the template, SMC2 carbon with uniform nm sized pores was produced, demonstrating the excellent template role of the surfactant-stabilized silica nanoparticles Jaroniec and his co-workers reported a colloidal imprinting method to synthesize mesoporous carbons using mesophase pitch as a carbon precursor and silica sol as a template.[34] Using colloidal silica particles with different sizes and adjusting the imprinting conditions such as imprinting time and temperature, they were able to synthesize carbon materials with controlled pore size, surface area, and pore volume.[35] One interesting characteristic of the carbon materials synthesized using mesophase pitch as a carbon precursor was that they had nearly no micropores The same group also reported gra- © 2006 WILEY-VCH Verlag GmbH & Co KGaA, Weinheim Adv Mater 2006, 18, 2073–2094 J Lee et al./Porous Carbon Materials b) Figure a) Synthetic strategy for uniform mesoporous carbons: 1) gelation of RF in the presence of cetyltrimethylammonium bromide (CTAB)stabilized silica particles; 2) carbonization of the RF-gel/silica composite at 850 °C to obtain a carbon–silica composite; 3) HF etching of the silica templates to obtain mesoporous carbons Reproduced with permission from [33] Copyright 1999 Royal Society of Chemistry b) The pore size distributions calculated from the adsorption branch of the nitrogen isotherm by the Barrett–Joyner–Halenda (BJH) method and the corresponding N2 adsorption and desorption isotherms (inset) of mesoporous carbons synthesized using isolated CTAB-stabilized silica particles (solid line) and using silica particle aggregates (dashed line) as templates phitized mesoporous carbon with a high surface area by the colloidal imprinting method via carbonization at 900 °C and subsequent graphitization at 2400 °C.[36] The resulting graphitic mesoporous carbons were successfully used as the stationary phase for reverse-phase liquid chromatography in the separation of alkylbenzenes, such as benzene, ethylbenzene, and propylbenzene.[37] Jang and co-workers synthesized carbon nanocapsules and mesocellular carbon foams by surface-modified colloidal silica-templating methods.[38] Carbon nanocapsules were synthesized using polydivinylbenzene (DVB) as a carbon precursor, poly(methyl methacrylate) (PMMA) as a barrier for the pre- Adv Mater 2006, 18, 2073–2094 vention of intraparticle crosslinking of DVB, and surfactantcoated colloidal silica particles as a template Direct polymerization of DVB on the surface of the silica particles without PMMA, followed by carbonization and dissolution of the silica template, resulted in mesocellular carbon foams Jang and his co-workers also reported the synthesis of mesoporous carbons via vapor deposition polymerization of polyacrylonitrile on the surface of silica particles.[39] Lu et al reported an aerogel-based approach to synthesize spherical mesoporous carbon particles.[40] In the synthesis, an aqueous solution containing sucrose and various silica templates was passed through an atomizer and dispersed into aerogel droplets Solvent evaporation at 400 °C resulted in spherical silica/sucrose nanocomposite particles and the subsequent carbonization and removal of the silica templates generated the spherical porous carbon particles Kyotani and co-workers reported the synthesis of mesoporous carbon through the co-polymerization of FA and tetraethylorthosilicate (TEOS).[41] A nanocomposite of carbon and silica was prepared by using a sol–gel process with TEOS in the presence of FA, followed by the polymerization of FA, and its subsequent carbonization In this synthesis, the silica template and carbon precursor were simultaneously synthesized to produce a silica/carbon precursor nanocomposite Using a similar synthetic procedure, Han et al synthesized mesoporous carbon using inexpensive sucrose and sodium silicate as the carbon precursor and template, respectively.[42] Lu and his co-workers synthesized unimodal and bimodal mesoporous carbons from the sucrose/silica nanocomposites prepared by sol–gel process of TEOS with or without colloidal silica particles in the presence of sucrose.[43] Lu and his coworkers also reported the synthesis of continuous mesoporous carbon thin films by a rapid sol–gel, spin-coating process using sucrose as the carbon precursor and TEOS as the silica precursor.[44] Continuous sucrose/silica nanocomposite thin films were formed by the spin-coating of homogeneous sucrose/silicate/water solutions that were prepared by reacting TEOS in acidic sucrose solutions Carbonization converted the sucrose/ silica thin films into carbon/silica nanocomposite thin films The mesoporous carbon thin films exhibited a high specific surface area of 2603 m2 g–1 and a specific pore volume of 0.21 cm3 g–1 This was the first reported synthesis of continuous mesoporous carbon thin films through a direct and rapid organic/inorganic self-assembly and carbonization process REVIEW a) 3.2 Synthesis of Uniform Mesoporous Carbons Using Mesoporous Silica Templates 3.2.1 Synthesis of Ordered Mesoporous Carbons with Various Pore Structures In 1992, Mobil Corporation researchers reported the synthesis of mesoporous M41S silica materials from the sol–gel polymerization of silica precursors in the presence of a surfactant self-assembly.[45] The pore structure and dimension of the © 2006 WILEY-VCH Verlag GmbH & Co KGaA, Weinheim www.advmat.de 2079 REVIEW J Lee et al./Porous Carbon Materials mesoporous silica materials could be controlled by varying the experimental conditions, such as the ratio of the silica precursor to the surfactant and the chain length of the surfactant The development of the M41S family triggered the synthesis of many mesoporous silica materials having diverse pore structures using various organic structure-directing agents, including neutral amine surfactants,[46,47] alkyl(PEO) surfactants,[48] and triblock copolymers.[49] These mesoporous silica materials have uniform pore sizes and high surface areas Mesoporous silicas with interconnected pore structures have been successfully used as the templates for the synthesis of mesoporous carbon materials Both the Ryoo group[50] and our own group[51] employed MCM-48 (alumino)silica materials as the templates for the fabrication of mesoporous carbon The carbon precursor, sucrose or in situ polymerized phenol resin, was incorporated into the 3D interconnected pores of the MCM-48 template, and subsequent carbonization followed by the removal of the silica template resulted in the generation of mesoporous carbon materials having 3D interconnected pore structures Figure 4a shows the overall template strategy used for the synthesis of ordered mesoporous carbon materials using mesoporous silica templates.[50,51] The phenol-resin/MCM-48 nanocomposite was prepared by the in situ polymerization of phenol and formaldehyde in the pores of the MCM-48 aluminosilicate template The carbonization of the phenol-resin/MCM-48 nanocomposite, followed by the dissolution of the aluminosilicate template using aqueous hydrofluoric acid produced an ordered mesoporous car- a) b) Figure a) Schematic representation of the formation of an ordered mesoporous carbon SNU-1 b) TEM image of a mesoporous SNU-1 carbon Reproduced with permission from [51] Copyright 1999 Royal Society of Chemistry 2080 www.advmat.de bon (SNU-1) The TEM image of SNU-1 carbon showed a regular array of nm sized pores separated by nm thick carbon walls (Fig 4b) Judging by the low-angle XRD pattern, the resulting carbon was not a real negative replica of the MCM-48 silica template, because the replicated carbon underwent a structural transformation during the removal of the silica template It was suggested that the cubic MCM-48 with the Ia3d structure was converted to a new cubic I41/a structure.[52] Using the same template (MCM-48), Ryoo and his coworkers synthesized mesoporous carbon (CMK-1) using sucrose as a carbon precursor.[50] To improve the thermal stability and ordering of the resulting mesoporous carbon materials, Yu and co-workers used silylated MCM-48 as a template and poly(divinylbenzene) as a carbon precursor.[53] The mesoporous carbon synthesized using the silylated MCM-48 silica template showed much better overall structural order compared to that obtained using pure MCM-48 silica, according to the small-angle XRD patterns and TEM images Following the first report on the synthesis of ordered mesoporous carbons using the MCM-48 silica template, various mesoporous carbon materials with different pore structures were synthesized using a variety of different mesoporous silica templates For example, our group used a hexagonal mesoporous silica (HMS)[47] template to synthesize mesoporous SNU-2 carbon.[54] Through this template synthesis, we were able to indirectly elucidate that the HMS silica possesses a wormholelike pore structure rather than the originally proposed MCM-41-like hexagonal 1D channel structure The mesoporous carbon materials synthesized using mesoporous silica templates contain not only mesopores generated from the replica of the templates, but also micropores formed by the carbonization of the precursor For example, SNU-2 carbon exhibited a bimodal pore size distribution curve, with 0.6 nm size micropores generated from the carbonization of the carbon precursor and the other centered at 2.0 nm from the replica of the template Hexagonally ordered mesoporous silica SBA-15 was used as a template for a mesoporous carbon designated as CMK-3.[55] In the original study by the Stucky group, SBA-15 was reported to have a hexagonal tubular pore structure similar to that of MCM-41 However by using SBA-15 silica as the template, Ryoo and his co-workers successfully synthesized an ordered mesoporous carbon in which parallel carbon fibers were interconnected through thin carbon spacers Through this synthesis and further studies on the pore structure, the SBA-15 silica turned out to have complementary pores, which were generated by the penetration of the hydrophilic ethylene oxide groups into the silica framework.[56–58] The ordered structure of the CMK-3 carbon was the exact inverse replica of the SBA-15 silica without the structural transformation during the removal of the silica template CMK-3 type ordered mesoporous carbon was also synthesized by the infiltration of the carbon precursor via adsorption in the vapor phase and using p-toluene sulfonic acid impregnated SBA-15 as a template.[59] A nanopipe-type mesoporous carbon, designated as CMK-5, was also synthesized by Ryoo and co-workers The © 2006 WILEY-VCH Verlag GmbH & Co KGaA, Weinheim Adv Mater 2006, 18, 2073–2094 J Lee et al./Porous Carbon Materials Figure TEM image viewed along the direction of the ordered nanopipe-type carbon and corresponding Fourier diffractogram (inset) Reproduced with permission from [60] Copyright 2001 Nature Publishing Group condition by pyrolyzing poly(furfuryl alcohol) under a vacuum atmosphere, resulting in the formation of high-quality CMK-5 carbon.[61] Several other research groups synthesized similar nanopipe-type ordered mesoporous carbon materials The Schüth group synthesized ordered mesoporous carbon, denoted as NCC-1, whose structure was similar to that of CMK-5, using hydrothermally treated SBA-15 silica as the template.[62] Previously, the Zhao group showed that hydrothermal treatment at 140 °C of the silica template induced the formation of mesotunnels between the main mesopores of SBA-15.[63] FA was wetted on the inner pore surface of the hydrothermally treated SBA-15 aluminosilicate, and subsequent polymerization using the acidic Al sites on the template generated poly(furfuryl alcohol), which was used as the carbon precursor The Schüth group also synthesized ordered nitrogen-doped mesoporous carbons using SBA-15 as the template and poly(acrylonitrile) as the carbon/nitrogen source.[64] The same group also used a conducting polymer, polypyrrole, as the carbon source to synthesize CMK-3 type mesoporous carbon.[65] The SBA-15 silica template was first impregnated with ferric chloride, which served as the oxidant for the vaporphase oxidative polymerization of pyrrole vapor at room temperature The resulting materials had an ordered structure, high surface area, and large pore volume Nanopipe-type hexagonally ordered mesoporous carbons were also prepared through the catalytic chemical vapor deposition (CCVD) Adv Mater 2006, 18, 2073–2094 method using cobalt metal incorporated SBA-15 as the templates.[66] The cobalt/SBA-15 silica was prepared by dispersing ethylenediamine-functionalized SBA-15 silica in water containing cobalt ions, followed by thermal treatment Increasing the deposition time resulted in the generation of highly hexagonally ordered nanopipe-type mesoporous carbon.[66] Ordered mesoporous carbon CMK-3 with a hollow spherical particle shape was synthesized by CVD.[67] SBA-15 silica was employed as the template and styrene as the carbon source In most templating processes, the morphology of the mesoporous carbon materials is very similar to that of the template However, during this high-temperature CVD process, the carbon precursor that was initially deposited in the outer pores of the template seemed to block the internal pores The subsequent carbonization and removal of the template generated hollow, spherical carbon with a mesoporous shell structure Following the first report on MCM-48 silica, much effort has been made to synthesize cubic Ia3d mesoporous silica with large pores for use as a catalyst for large-sized molecules However, ordered mesoporous carbon with an Ia3d structure could not be obtained using a cubic Ia3d structured MCM-48 silica template, because of the disconnectivity between the enantiomerically paired channels.[50,51] Later, three research groups independently reported the synthesis of cubic Ia3d structured mesoporous silica with very large pores using a P123 triblock copolymer ((EO)20(PO)70(EO)20) as the template,[68–70] and the successful replication to highly ordered mesoporous carbons.[69–71] The Zhao group synthesized largepore 3D bicontinuous cubic Ia3d mesoporous silica by a solvent-evaporation method using P123 triblock copolymer as the template and a small amount of 3-mercaptopropyltrimethoxysilane (MPTS) and trimethylbenzene as additives.[68] A mesoporous silica material with a monolithic form was used as the template for the synthesis of Ia3d cubic structured mesoporous carbon.[69] The Ryoo group synthesized Ia3d cubic mesoporous silica by hydrothermal treatment using butanol as a structure modifier.[70] Using the cubic mesoporous silica as a template, they were able to synthesize not only unimodal mesoporous carbon, but also tubular bimodal Ia3d ordered mesoporous carbon, by the controlled polymerization of FA inside the pores In contrast to the mesoporous carbon synthesized using the MCM-48 silica template, the mesoporous carbons obtained using the cubic mesoporous silica template retained the bicontinuous Ia3d structures of the template The authors claimed that the bridges between the channel-like enantiomeric pore systems of the cubic mesoporous silica template connected the carbon rods in the channels.[69] The control of the pore size of the mesoporous carbons was not easy to accomplish through the template approach, because it was difficult to control the thickness of the wall during the synthesis of the mesoporous silicas Ryoo and coworkers were the first to report the successful control of the pore size of ordered mesoporous carbons They employed mixed surfactants (cetyltrimethylammonium bromide (C16TAB) and polyoxyethylene hexadecylether-type surfac- © 2006 WILEY-VCH Verlag GmbH & Co KGaA, Weinheim www.advmat.de REVIEW hexagonally ordered arrays of carbon nanotubules were obtained from the partial wetting of poly(furfuryl alcohol) onto the SBA-15 silica channels and subsequent carbonization.[60] The ordered nanoporous carbon was rigidly interconnected by the carbon spacers that were formed inside the complementary pores between the adjacent cylinders, forming a highly ordered hexagonal array The pore size distribution curve exhibited bimodal pores, corresponding to the inside diameter of the carbon cylinders (5.9 nm) and the pores formed between the adjacent cylinders (4.2 nm), respectively The TEM image, shown in Figure 5, shows an ordered array of carbon tubules with diameters of ca nm In a subsequent paper, the Ryoo and Jaroniec groups optimized the synthetic 2081 REVIEW J Lee et al./Porous Carbon Materials tants (C16EO8)) in the acidic synthesis of hexagonal mesoporous silica By decreasing the C16TAB/C16EO8 ratio, the wall thickness in the mesoporous silica was increased systematically from 1.4 nm to 2.2 nm.[72] The resulting hexagonal mesoporous sieves were used as templates for the synthesis of ordered mesoporous carbons, which allowed the size of the pores in the carbon products to be controlled in the range of 2.2 to 3.3 nm By adjusting the thickness of the silica wall, the pore diameters of the resulting carbon materials were able to be successfully controlled.[72] a) 3.2.2 Mesoporous Carbons with Ultralarge Mesopores For applications involving large-sized molecules, such as biosensors using protein-incorporated carbons, mesoporous carbons having well-interconnected pores with a diameter of ca 10 nm are necessary Although many mesoporous carbons can be synthesized using different mesoporous silica templates, as described above, the resulting pore sizes are generally less than 10 nm, because the pore size of the replicated mesoporous carbon is generally determined by the wall thickness of the silica template Even in the case of the nanopipe-type mesoporous carbons, the inner pore diameter is smaller than nm To synthesize mesoporous carbon materials with uniform pore sizes of > 10 nm, our group employed mesocellular silica foam,[73] synthesized by the Stucky group, as the template.[74] The synthetic scheme used for the mesocellular carbon foam is shown in Figure 6a Phenol was incorporated into the complementary pores of the mesocellular aluminosilicate foam (AlMCF) The subsequent polymerization with formaldehyde generated a phenol-resin/ AlMCF nanocomposite Carbonization followed by the removal of the template produced mesocellular carbon foam The key to the success of the synthesis was that the phenol was only incorporated partially, since it could only fill the complementary pores of the MCF template Phenol vapor could be incorporated into the complementary pores at low vapor pressure, whereas it could not infiltrate into the main cells of the AlMCF template, because a very high vapor pressure was required for it to be incorporated into the large mesocellular pores When we used MCF aluminosilicate with a main cell diameter of 27 nm and window size of 11 nm as the template, we obtained a mesocellular carbon foam with a main cell diameter of 27 nm and window size of 14 nm Small mesopores with a pore size of 3.5 nm were also generated from the replication of the wall of the silica template Spherical cells with a diameter of ca 27 nm are evident in the TEM image of the carbon material (Fig 6b) Subsequently, the Tatsumi group synthesized a mesocellular carbon foam with a main cell size of 24 nm and window size of 18 nm using two successive impregnations of sucrose and subsequent carbonization.[75] The mesoporous carbon obtained had closed hollow spherical pores, while the carbon obtained by the single-step impregnation of sucrose had open mesocellular pores 2082 www.advmat.de b) Figure a) Schematic illustration for the synthesis of a mesocellular carbon foam Reproduced with permission from [74] Copyright 2001 American Chemical Society b) TEM image of a mesocellular carbon foam 3.2.3 Mesoporous Carbons with Graphitic Pore Walls Given their good electrical conductivity and uniform and large pores, mesoporous carbon materials with good graphitic characteristics could find many important applications, including electrodes for electrochemical double-layer capacitors, fuel cells, and biosensors It is well known that it is extremely difficult to synthesize carbon materials with both a high surface area and good graphitic crystallinity To achieve such a goal, the Ryoo group synthesized ordered mesoporous carbons with graphitic pore walls (CMK-3G) through the in situ conversion of aromatic compounds to a mesophase pitch inside the SBA-15 silica template by carbonization under high pressure using an autoclave.[76] The carbon frameworks were composed of discoid graphene sheets, which self-aligned perpendicularly to the template walls during the synthesis CMK-3G carbon exhibited much better mechanical strength than the CMK-3 synthesized using sucrose or FA as the carbon precursor The discoid alignment of the graphitic frame- © 2006 WILEY-VCH Verlag GmbH & Co KGaA, Weinheim Adv Mater 2006, 18, 2073–2094 J Lee et al./Porous Carbon Materials 3.2.4 Cost-Effective and Direct Synthesis of Mesoporous Carbons The cost of synthesizing templated mesoporous carbons is largely dependent on the production cost of the mesoporous silica templates, because they are sacrificed in the final step of Adv Mater 2006, 18, 2073–2094 the synthesis The Pinnavaia group developed a very economical route to synthesize mesoporous MSU silica materials via the sol–gel reaction of sodium silicate under near-neutral conditions.[81,84] The cost of synthesizing MSU-H silica is much lower than that of the similarly structured SBA-15 silica, given that a very small amount of acid and inexpensive sodium silicate are used By adding trimethylbenzene (TMB) to the synthesis solution, mesocellular silica foams, which were denoted as MSU-F and had a similar pore structure to that of MCF silica, could also be synthesized.[81] The Pinnavaia group used MSU-H silica as the template to synthesize hexagonally ordered mesoporous carbons, denoted as C-MSU-H.[85] The pore structure of C-MSU-H was very similar to that of the CMK-3 carbon synthesized using an SBA-15 silica template Our group reported the synthesis of mesocellular carbon foams using inexpensive MSU-F silica as the inorganic template.[86] The cellular pore structure of C-nano-MSU-F was very similar to that of mesocellular carbon foams synthesized using the MCF-silica template However, the C-nano-MSU-F was composed of individual particles with sizes of a few hundred nanometers, in contrast to several micrometer-sized particles of the MCF-carbon This small individual particle size is highly desirable for the facile access of molecules into the framework pores The procedure employed to synthesize mesoporous carbons using mesostructured silica templates is rather complex and time consuming The general synthetic procedure for ordered mesoporous carbons using a mesostructured silica template is as follows: 1) the preparation of the mesostructured silica/surfactant composite, which often takes about 2–3 days; 2) the removal of the surfactant by calcination or solvent extraction; 3) the generation of the catalytic sites inside the walls of the mesostructure for the polymerization and, if necessary, the re-calcination; 4) the incorporation of the polymeric carbon precursor, for example, phenol, FA, or sucrose, into the pores of the mesoporous silica template; 5) the polymerization of the polymeric carbon precursor; 6) carbonization; and, finally, 7) the removal of the silica template with HF or NaOH solution This long and complicated multistep template synthesis limits the application of mesoporous carbons, despite their many desirable and unique characteristics A short and facile synthetic procedure needs to be developed in order for the extensive applications of these mesoporous carbons Recently, much effort has been made to find a way of directly synthesizing uniform pore-sized mesoporous carbon materials Sayari and co-workers reported a simple and direct preparation route to synthesize uniform microporous carbon materials by the direct carbonization of cyclodextrin-templated silica mesophase materials.[87] During the preparation of the cyclodextrin/silica mesophase materials, sulfuric acid was used instead of hydrochloric acid, because it catalyzes the carbonization of cyclodextrin The pore size of the resulting carbon was less than nm, i.e., it was microporous Moriguchi and co-workers reported the direct synthesis of a mesoporous carbon material by the in situ polymerization of divinylben- © 2006 WILEY-VCH Verlag GmbH & Co KGaA, Weinheim www.advmat.de REVIEW works was consistent with the general tendency of the edgeon anchoring of polycyclic aromatic hydrocarbons in a mesophase pitch on the silica surface Mokaya and co-workers synthesized nitrogen-doped mesoporous carbons with graphitic pore walls via CVD of acetonitrile.[77] Pyrolysis/carbonization in the temperature range of 950–1100 °C was found to be suitable for the fabrication of well-ordered mesoporous carbon These nitrogen-doped, ordered mesoporous carbon materials had a macroscopic spherical morphology, which was similar to that of the other mesoporous carbons synthesized via CVD methods Later the Mokaya group generalized the CVD method and synthesized many mesoporous nitrogen-doped carbon materials using various mesoporous silica templates including SBA-12, SBA-15, MCM-48, HMS, and MCM-41.[78] The carbon materials prepared at high CVD temperatures of > 1000 °C exhibited high graphitic properties Fuertes and co-workers synthesized graphitic mesoporous carbons by the simple impregnation of poly(vinyl chloride) and subsequent carbonization.[79] These carbons had a good electrical conductivity of 0.3 S cm–1, which is two orders of magnitude higher that that of non-graphitized carbon By heating them at a high temperature of > 2600 °C, the graphite crystallite size (Lc) of the mesoporous carbons was increased to 19.4 nm, while preserving the high Brunauer–Emmett–Teller (BET) surface area of 260 m2 g–1 The Fuertes group also fabricated an ordered mesoporous graphitic carbon material using iron-impregnated polypyrrole as a carbon source and SBA-15 as a template.[80] FeCl3 was used not only as an oxidant for the polymerization but also as a catalyst which promotes the formation of a graphitic structure during the carbonization step When used as electrode materials for electrochemical double-layer capacitors (EDLCs), graphitic carbon showed a superior performance to other non-graphitic mesoporous carbons at high current densities This superior electrode performance seemed to be derived from highly accessible pores and the high conductivity of the graphitic framework The Pinnavaia group synthesized ordered graphitic mesoporous carbon materials with high electrical conductivity using MSU-H silica[81] as the template and aromatic precursors, such as naphthalene, anthracene, and pyrene, as the carbon sources.[82] Zhao and co-workers used a melt-impregnation method using a cheap mesophase pitch to synthesize mesostructured graphitic carbon materials.[83] The pore walls are composed of domains with the (002) crystallographic plane perpendicular to the long axis of the carbon nanorods They also used Fe2O3 nanoparticle-loaded mesoporous silica to obtain graphite carbon nanofiber bundles 2083 REVIEW J Lee et al./Porous Carbon Materials 2084 zene (DVB) in the hydrophobic phase of a hexagonally arrayed micelle/silicate nanocomposite and its subsequent carbonization and HF treatment.[88] However, the ordered structure of the DVB/surfactant/silicate composite was not transferred to the resulting carbon materials after the removal of the silica The carbon structure achieved had wormholelike mesopores with diameters of ca nm The Inagaki group[89a] and later the Ozin[89b] and Stein groups[89c] synthesized organic-group-incorporated mesoporous silica materials from the sol–gel reactions of organosilane precursors in the presence of surfactant self-assembly templates The Lu group reported the direct synthesis of mesoporous carbon from the carbonization of a phenylene-incorporated mesostructured silica/surfactant nanocomposite followed by the removal of the silica.[90] The synthesis of the organosilica/surfactant was achieved by co-assembling octadecyltrimethylammonium bromide (OTAB) with 1,4-bis(triethoxysilyl)benzene (BTE) After its carbonization at 900 °C, the molecular ordering present in the mesoporous walls disappeared, but an ordered mesoporous silica/ carbon composite was nevertheless obtained The benzene molecules present in the walls of the ordered mesostructured materials were converted to carbonaceous materials After the HF etching, a mesoporous carbon with a high surface area of 850 m2 g–1, a pore volume of 0.5 cm3 g–1, and an average pore size of 2.5 nm was produced The hydrogen-adsorption capacity of the mesoporous carbon was comparable to those of activated charcoal and activated carbon fiber Our group synthesized uniform mesoporous carbons with various pore structures by the carbonization of P123 triblock copolymer/ phenol-resin/silica nanocomposites.[91] The composites were simply prepared from the sol–gel polymerization of silica in the presence of a Pluronic P123 triblock copolymer and phenol The pore size of the mesoporous carbons was controlled by varying the ratio of phenol to the P123 triblock copolymer Yu and his co-workers reported the direct preparation of mesoporous carbons using as-synthesized MCM-48.[92] The mechanical strength and thermal stability were improved compared to the mesoporous carbons prepared using calcined mesostructured silica as a template However, in this process, an additional carbon precursor, poly(divinylbenzene) was synthesized inside the empty space of the silica/surfactant composite after the preparation of the mesostructured MCM-48 Our group successfully synthesized ordered mesoporous carbon and mesocellular carbon foam with large pores via the carbonization of mesostructured silica/surfactant nanocomposites using the surfactant as the carbon precursor.[93] The synthesis was achieved by treating the as-synthesized silica (MCF, SBA-15)/triblock copolymer nanocomposite with sulfuric acid to crosslink the triblock copolymers Carbonization followed by the removal of the silica resulted in the generation of ordered mesoporous carbons Without the treatment with H2SO4, ordered mesoporous carbon could not be synthesized Pinnavaia and co-workers reported the synthesis of carbon nanotubes using P123 surfactant inside mesoporous silica, but they could not obtain ordered arrays of carbon nanotubes.[94] www.advmat.de Dai and co-workers reported the preparation of highly ordered and well-oriented mesoporous carbon thin films through the carbonization of a nanostructured resorcinol-formaldehyde resin and self-assembled block copolymer nanocomposite.[95] The resorcinol monomers were pre-organized into a well-ordered polystyrene-block-poly(4-vinylpyridine) (PS-P4VP) self assembled nanostructured film through spincoating followed by solvent annealing Resorcinol and formaldehyde were in situ polymerized by exposing the film to formaldehyde gas Through the carbonization under an N2 atmosphere, a hexagonal carbon channel array was synthesized by sacrificing the block-copolymer template The self-assembly of the PS-P4VP/resorcinol mixture was essentially driven by the hydrogen bonding interaction between resorcinol and the P4VP block The mesopores were oriented perpendicular to the film surface The pore diameter was ca 34 nm and the wall thickness was ca 9.0 nm Very recently, Nishiyama and co-workers reported the fabrication of mesoporous carbon thin films, designated as COU, with an ordered channel structure via the direct carbonization of an organic–organic nanocomposite.[96] The synthetic procedure is described in Figure 7a The key to the success of their synthetic procedure was the formation of a periodically ordered organic–organic nanocomposite composed of thermosetting polymeric carbon precursors, RF and triethyl orthoacetate (EOA), and the use of a thermally decomposable surfactant, triblock copolymer Pluronic F127 The reaction mixture containing the organic–organic nanocomposite was spin-cast on a silicon substrate, and the resulting film was heat a) b) Figure a) Schematic illustration of the direct synthetic route for an ordered mesoporous carbon (COU): 1) self-assembly of a carbon precursor and a surfactant and 2) the removal of the surfactant by direct carbonization to obtain COU Reproduced with permission from [96] Copyright 2005 Royal Society of Chemistry b) SEM image of mesoporous COU carbons prepared at the carbonization temperatures of 600 °C treated stepwise, first at 90 °C for h in air for the polymerization of resorcinol with formaldehyde, then under a nitrogen atmosphere at 400 °C for h, and finally at 600 and 800 °C for © 2006 WILEY-VCH Verlag GmbH & Co KGaA, Weinheim Adv Mater 2006, 18, 2073–2094 J Lee et al./Porous Carbon Materials Figure a) TEM and b) HRTEM image of C-FDU-15 calcined at 1400 °C c) TEM and d) HRTEM image of C-FDU-16 calcined at 1400 °C Reproduced from [98] Adv Mater 2006, 18, 2073–2094 rials by heating at temperatures > 700 °C Notably, the carbon materials were stable even up to 1400 °C under a nitrogen atmosphere, demonstrating the ultrahigh thermal stability The thermal stability might be derived from the synthesis of nanoparticulate polymer precursors before the assembly with triblock copolymers The mesostructured metal oxides assembled with preformed nanoparticles were known to be more thermally stable than those prepared from molecular inorganic precursors.[99] REVIEW h each for the purpose of carbonization The X-ray diffraction pattern revealed a sharp reflection peak at a 2h angle of 0.9–1.3°, demonstrating the periodically ordered structure of the carbon film The field-emission (FE)SEM image in Figure 7b clearly shows the hexagonally arranged pores with a lattice spacing and pore diameter of 7.5 nm and 6.2 nm, prepared at the carbonization temperatures of 400 and 600 °C, respectively Zhao and co-workers synthesized an ordered mesoporous polymer and carbon denoted FDU-14 and C-FDU-14 in powder form through the direct assembly of a Pluronic triblock copolymer and resol (phenol/formaldehyde).[97] Cooperative assembly between resols and hydrophilic PEO blocks of P123 resulted in a resol-block-copolymer mesophase in the dilute basic solution The method was very similar to that of ordered mesoporous silica (SBA-15, MCM-48, etc.) and the pore structure of the resulting polymer and carbon was of Ia3d symmetry The carbonization of an FDU-14 polymer material was conducted at 700 °C under nitrogen The pore size of the resulting carbon, C-FDU-14 was 2.7 nm, similar to that of CMK-3 synthesized using SBA-15 as the template In a subsequent paper, the Zhao group synthesized ordered mesoporous polymer and carbon materials with 2D hexagonal (p6m), 3D caged cubic (Im3m), and lamellar frameworks by simply adjusting the mass ratio of the polymer precursors and amphiphilic surfactants.[98] Figure shows TEM and HRTEM images of C-FDU-15 and C-FDU-16 carbons with hexagonal (p6m) and cubic (Im3m) structures, respectively They first synthesized a soluble low-molecular-weight polymer of resol and mixed the resol with a Pluronic surfactant to make highly ordered mesostructured materials The surfactant was removed by heating at 350 °C The ordered mesoporous polymers could be converted to ordered mesoporous carbon mate- 3.2.5 Synthesis of Hierarchically Ordered Mesoporous Carbon Materials Recently, various mesoporous carbons having hierarchical structures were synthesized using hierarchically ordered mesoporous silica materials as templates Bimodal mesoporous silicas, denoted as Meso-nano-S, were developed by our research group through a simple and low-cost synthetic procedure.[100] Compared to the previous synthesis of bimodal mesoporous silica materials, our synthetic procedure is much simpler and more cost effective, because sodium silicate is polymerized under neutral conditions, followed by hydrothermal treatment The hydrothermal treatment is critical to produce bimodal mesoporous silica materials Bimodal mesoporous silica is comprised of 30–50 nm sized particles and 3D wormholelike nm sized pores The large ca 20 nm sized mesopores were derived from interparticle voids The 3D interconnected wormholelike pore structure of Meso-nano-S silica made it possible for it to be used as the template for producing porous carbon To make hierarchical mesoporous carbon using Meso-nano-S as the template, it was found to be important to control the amount of carbon precursor, so that it would be selectively incorporated into the framework pores of the Meso-nano-S silica template The carbon precursor, phenol, was first adsorbed into the small pores, because capillary condensation into small pores occurs at low pressure The Meso-nano-C carbon so produced was composed of 30–50 nm sized particles having 3D wormholelike pores, and was the exact negative replica of the Meso-nano-S silica template.[100] Fuertes synthesized spherical-shaped mesoporous carbon materials with controlled particle diameters ranging from 10 nm to 10 lm by using corresponding mesoporous silica spheres synthesized at various synthetic conditions.[101] A modified pitch-based colloidal imprinting method was used to synthesize bimodal mesoporous carbons.[102] Co-imprinting of hexagonally ordered SBA-15 particles and 13 nm sized colloidal silica particles in pitch followed by carbonization and removal of the silica templates generated bimodal mesoporous carbons Two different sizes of mesopores (i.e., 3–4 nm and ca 13 nm) were derived from the wall thickness of the SBA-15 silica and the colloidal silica particles, respectively When the carbon synthesis was performed without using colloidal silica particles, unimodal mesoporous carbon particles with 3–4 nm sized mesopores were produced.[103] Very recently, our research group reported a simple and cost-effective synthesis of a mesocellular silica foam with © 2006 WILEY-VCH Verlag GmbH & Co KGaA, Weinheim www.advmat.de 2085 REVIEW J Lee et al./Porous Carbon Materials ordered SBA-15-type mesoporous walls, designated as hierarchical mesocellular mesoporous silica (HMMS), using a Pluronic triblock copolymer as a single structure-directing agent.[104] Using the HMMS as the silica template and in situ generated phenol–formaldehyde gel as a carbon precursor, we were able to synthesize a hierarchical mesocellular mesoporous carbon (HMMC).[104] The TEM image of the HMMC (Fig 9a) showed that the ca 40 nm sized cellular pores of HMMS were well preserved and that small mesopores were also present These small mesopores were generated by the replication of the 13 nm sized mesopores of the HMMS silica a) directing agent Using FA as a carbon source, hierarchical porous monolithic carbon containing wormholelike mesopores and macropores was successfully replicated.[105] It is possible to fabricate the carbon materials in the monolithic form by using monolith-shaped mesoporous silica as the template The macroporosity of the silica template was preserved in the carbon materials, which indicates that the carbon source, FA, only infiltrated into the mesopores Yoon et al synthesized hollow core/mesoporous shell (HCMS) carbon using solid core/mesoporous shell (SCMS) structured silica as a template.[106] The SCMS template was synthesized by a two-step process involving the synthesis of a nonporous solid silica core by the Stöber method and the formation of the mesoporous silica shell by the sol–gel reaction of TEOS with octadecyltrimethoxysilane (C18TMS) on the surface of the solid silica spheres The synthetic scheme for HCMS is presented in Figure 10a The selective deposition of phenol into the mesopores of SCMS is important to preserve the spherical morphology of the template silica The diameter of the hollow core and the thickness of the mesoporous shell could be controlled by using appropriate SCMS silica templates The HCMS carbon had very uniform hollow cores and b) Intensity (arbi units) a) q[nm ] Figure a) TEM image of an HMMC carbon b) Small-angle X-ray scattering pattern of an HMMC carbon showing the regularity of the large cells and small ordered pores Reproduced from [104] The N2 isotherms of HMMC exhibited two major capillary condensation steps, resulting from the large ca 40 nm sized cellular pores (P/P0 ≈ 0.9) and small ordered 4.74 nm sized mesopores (P/P0 ≈ 0.6) derived from the dissolution of the silicate walls The BET surface area and single point total pore volume of HMMC were 853 m2 g–1 and 1.54 cm3 g–1, respectively The SAXS pattern of HMMC showed two sets of scattering peaks, revealing that the regularity of both the large cellular pores and small mesopores of HMMS was preserved during the replication (Fig 9b) Linden and co-workers prepared a hierarchical silica monolith possessing fully interconnected uniform mesopores and macropores, by using a mixture of poly(ethylene glycol) and alkyltrimethylammonium bromide (CnTAB) as the structure- 2086 www.advmat.de b) Figure 10 a) Schematic illustration for the synthesis of hollow core/mesoporous shell (HCMS) carbon capsules b) TEM and SEM images of HCMS carbon capsules Reproduced from [106] © 2006 WILEY-VCH Verlag GmbH & Co KGaA, Weinheim Adv Mater 2006, 18, 2073–2094 J Lee et al./Porous Carbon Materials 3.2.6 Synthesis of Ordered Mesoporous Materials Using Ordered Mesoporous Carbons as Templates Ordered mesoporous carbon (OMC) materials generated from ordered mesoporous silica templates were used as second-generation templates to synthesize ordered mesoporous metal oxides.[21d] The rigidity of OMC made it possible to use it as the template for mesoporous metal oxides In addition, the OMC could be easily removed by simple calcination The Schüth and Kim groups independently reported the synthesis of SBA-15-like mesoporous silica using CMK-3 carbon as the template.[110,111] The mesopores of CMK-3 were filled with the silica precursor, which was either TEOS or sodium silicate, and subsequent sol–gel polymerization was performed using HCl as a catalyst Further aging and heat treatment designed to remove the CMK-3 carbon-generated hexagonal mesoporous silica, which was similar to SBA-15 silica This approach was then extended to the synthesis of other mesoporous metal oxide materials The conventional synthetic approach to the production of mesoporous silica using surfactant self-assembly templates could not be easily extended to the synthesis of non-siliceous metal oxides Therefore, to overcome this drawback, a two-step template synthetic route to monodisperse mesoporous inorganic spheres with various compositions and a controllable crystalline phase was developed.[112] Using spherical-shaped mesoporous carbons as templates, various kinds of mesoporous metal oxides and phosphides, including TiO2, ZrO2, Al2O3, Ti2Si3Oy, Adv Mater 2006, 18, 2073–2094 Ti2ZrOy, ZrP, and AlP, were synthesized The resulting mesoporous metal oxides and phosphides had a high surface area and large pore volume Highly ordered mesoporous magnesium oxide (MgO) with a high surface area was successfully synthesized using CMK-3 as a template.[113] This was achieved by repetitive impregnation of magnesium nitrate and its subsequent thermal conversion to MgO The resulting materials preserved their structural order upon heating at 800 °C, as confirmed by the XRD pattern, which is essential for the basic property of MgO to be activated The desorption of chemisorbed CO2 showed that the degree of basicity was comparable to that of MgO supported on SBA-15 silica Xia and Mokaya synthesized hollow spheres and shells of metal oxides using hollow mesoporous carbon spheres as templates.[114] Ordered mesoporous carbon CMK-3 with a hollow spherical particle shape was synthesized by CVD.[67] Metal alkoxide was infiltrated into the pores of hollow spherical carbons, and subsequent removal of the carbon template by calcining at 500–600 °C generated metal oxides with predominantly hollow spherical morphology The synthesized metal oxides include alumina (c-Al2O3; surface area of 212 m2 g–1), titania (anatase; 100 m2 g–1), mixed MgO–Al2O3, (322 m2 g–1), and binary MgTiO3 (154 m2 g–1) The Yu group synthesized a new ordered mesostructured silica material using MCM-48-templated carbon as a sacrificial template.[115] The MCM-48-templated carbon was not a negative replica of MCM-48, rather it possessed a new ordered structure with I41/a symmetry.[52] The regenerated ordered mesoporous silica was distinctly different from MCM-48 silica materials and was a previously unreported new structured mesoporous silica Boron nitride has good thermal conductivity and chemical durability Ordered mesoporous boron nitride was prepared using tri(methylamino)borazine as a boron nitride source and CMK-3 carbon as a template.[116] The CMK-3 template was successfully removed by heating at 1000 °C under ammonia atmosphere The ordered structure of CMK-3 was successfully transferred to the boron nitride structures after the removal of the CMK-3 carbon template REVIEW mesoporous shells, as shown in Figure 10b The Yu group synthesized silicalite-1 zeolite core/mesoporous silica shell (ZCMC) structures from the sol–gel reaction of TEOS with C18TMS on the surface of solid pseudohexagonal prismaticshaped silicalite-1 zeolite particles.[107] The ZCMS particles with their bimodal microporous core/mesoporous shell structure were utilized as the template to fabricate a carbon replica structure Interestingly, the pore-replication process took place only through the mesopores in the shell, and not through the micropores, due to the smallness of the micropores in the zeolite core, resulting in the production of hollow core/mesoporous shell (HCMS) carbon with a pseudohexagonal prismatic shape Using a similar synthetic procedure, our group synthesized nanorattles, each composed of a gold nanoparticle encapsulated in a hollow mesoporous carbon sphere, using a silica template with solid silica cores each containing a gold nanoparticle and mesoporous shell.[108] When the carbonization step was skipped, nanorattles with a mesoporous poly(divinylbenzene) shell were also fabricated Zhao and co-workers synthesized mesoporous carbon rods using rod-shaped SBA-15 silica as the template.[109] SBA-15 silica rods with lengths of 1–2 lm were synthesized by adding KCl salt to the hydrothermal synthesis mixture for SBA-15 silica These SBA-15 silica rods were used as the template for the synthesis of rod-shaped mesoporous carbon with a CMK-3 type structure 3.2.7 Applications and Functionalization of Ordered Mesoporous Carbons Ordered mesoporous carbons denoted as SNU-1 and SNU-2 were successfully employed as the electrodes for EDLCs.[51,54,117] EDLCs are considered as promising highpower energy sources for digital communication devices and electric vehicles, due to their high power density and good cyclability EDLCs utilize electric double layers formed at the interface of the electrode/electrolyte, where electric charges are accumulated on the electrode surface and ions of opposite charge are arranged on the electrolyte side of the interface EDLC electrode materials should have a large surface area for effective charge accumulation and an appropriate pore structure for good electrolyte wetting and rapid ionic motion To satisfy these requirements, it is desirable to synthesize me- © 2006 WILEY-VCH Verlag GmbH & Co KGaA, Weinheim www.advmat.de 2087 REVIEW J Lee et al./Porous Carbon Materials 2088 soporous carbons with 3D interconnected pore structures Consequently, the above-mentioned mesoporous carbons having 3D interconnected pores, SNU-1 and SNU-2, were successfully employed as the electrode materials for EDLCs The EDLC performance of SNU-1 and SNU-2 was compared to that of the most popularly applied activated carbon, MSC-25 The cyclic voltammograms (CVs) obtained in an organic electrolyte (1 M NEt4BF4 in propylene carbonate) showed that SNU-1 exhibited a more ideal capacitor behavior than MSC-25, with a steep current change at the switching potentials (0.0 and 3.0 V), resulting in a more rectangular-shaped I–V (current–potential) curve The slow changes at the switching potentials in the cyclic voltammograms of the MSC-25 electrode seemed to stem from the slow reorganization of the double-layer, owing to the slow ionic motions in the micropores The steep change in the CV of the SNU-1 electrode in turn reflected the dominance of the regular interconnected mesopores among the electrochemically usable pores The capacitance properties of CMK-3 carbon have also been investigated In this case, a rectangular-shaped CV was observed even when the scan rate was increased to 50 mV s–1, which was similar to the results obtained for SNU-1 and SNU-2 After the 100th cycle, the capacity decreased to 20 % of that of the first scan.[118] CMK-5 with its nanopipe-type hexagonal structure supports the high dispersion of platinum nanoparticles, exceeding that of other common microporous carbon materials (such as carbon black, charcoal, and activated carbon fibers) The diameter of the platinum clusters was able to be kept below nm and the high dispersion of these metal clusters enabled them to have promising electrocatalytic activity for oxygen reduction, which could prove to be practically relevant for low-temperature fuel cells.[60] A high specific-energy capacity of about 1100 mAh g–1 (Li3C6) for lithium storage in the hexagonally ordered mesoporous carbon CMK-3 was reported.[119] After the first cycle, the discharge and charge remained at a reversible capacity level (LixC6, x = 2.3 to 3.0) with good cycle performance The average loss per cycle (v) was smaller than that of other mesoporous carbons The authors concluded that CMK-3 had the potential to be used in lithium rechargeable batteries, considering the large absolute value of the reversible capacity and the small loss per cycle The adsorption of methane gas into CMK-3 was also investigated.[120] The amount of adsorbed methane gas was 81.35 mg g–1 (= 117.33 mL(STP) g–1) at the high pressure of 35 kg cm–2 at 298 K This result was much higher than that of zeolite, but much lower than that of microporous coordination polymers with an open framework CMK-3 carbons supporting palladium and platinum were used as the catalyst for the hydrogenation of nitrobenzene and ethylanthraquinone.[121] The hydrogenation activity of Pd/ CMK-1 and Pt/CMK-1 was superior to that of Pd/activated carbon and Pt/activated carbon, because of the high dispersion of the palladium species inside the ordered mesopores having a high surface area Ordered mesoporous carbons at various loading levels have larger hydrogen uptakes than Pd- www.advmat.de or Pt-loaded activated carbons MnO2 nanoparticles were incorporated into the pores of ordered mesoporous CMK-3 carbon via a sonochemical method.[122] In the analysis of these MnO2/CMK-3 materials, CMK-3 with a 20 wt % loading of MnO2 demonstrated improved discharge performance, owing to the nanometer-sized MnO2 formed within the CMK-3 Metal nanoparticles could be directly inserted into carbon rods by the pyrolysis of the metal and carbon precursors in the ordered mesoporous silica.[123] The growth and aggregation of the metal nanoparticles were hindered inside the confined mesoporous channels, resulting in the formation of highly dispersed nanoparticles Pt nanoclusters studded in the microporous carbon nanorods were synthesized using direct conversion methods The size of the Pt nanoclusters was smaller than the channel size of the SBA-15 silica, while the size of Pt synthesized by conventional impregnation on CMK-3 was larger than the channel size of SBA-15 The Pt nanoclusters were accessible to CO, which indicated that they were exposed to the gas via the micropores in the carbon rods The directly synthesized Pt/C nanocomposite showed an excellent performance in direct methanol fuel cells Mesoporous carbons with embedded cobalt nanoparticles were also synthesized using a similar approach.[124] An ordered nanostructured, tin-based oxide/carbon composite (ONTC) was prepared for use as the negative electrode material for lithium-ion batteries ONTC was prepared by filling tin-based oxides into the mesopores of hexagonally ordered mesoporous CMK-3 Nitrogen-adsorption experiments indicated that tinbased oxide materials were fully deposited into the internal pores of CMK-3 carbon The presence of interconnected carbon frameworks prevented aggregation of the tin species, which resulted in a much better cycle performance for the use as negative electrodes for lithium-ion batteries than those of nanometer-sized tin-based oxides.[125] Ordered mesoporous carbons were used as adsorbents for biomolecules Hartmann and co-workers studied the adsorption of cytochrome c on the hexagonally ordered mesoporous CMK-3 carbon.[126] The adsorbed amount increased near the isoelectric point of cytochrome c because there is no repulsive interaction between CMK-3 and cytochrome c A high adsorption capacity of 18.5 lmol was observed for CMK-3, which was significantly higher than the reported value using mesoporous silica materials The Hartmann group also studied adsorption of vitamin E on the ordered mesoporous carbons using various concentrations of a vitamin E solution in different solvents, such as n-heptane (nonpolar) and n-butanol.[127] The nonpolar solvent was more suitable for high loadings of vitamin E, which is because of the weak interaction between the solvent and vitamin E A highly sensitive and fast glucose biosensor was fabricated by simply immobilizing glucose oxidase (GOx) in a mesocellular carbon foam, MSU-F-C.[128] GOx with molecular dimensions of 5.2 nm × 6.0 nm × 7.7 nm was immobilized in MSU-F-C by a simple adsorption method and the loading amount was 40 wt % Even though the total pore volume of MSU-F-C is only 1.5-fold higher than that of CMK-3 (4 nm © 2006 WILEY-VCH Verlag GmbH & Co KGaA, Weinheim Adv Mater 2006, 18, 2073–2094 J Lee et al./Porous Carbon Materials Adv Mater 2006, 18, 2073–2094 particles were pulled away from the electrode there was no current Thus, the Mag-MCF-C/CLEA-GOx particles could be switched alternately in the on and off states by positioning the magnetic field REVIEW sized pores with hexagonal symmetry), MSU-F-C showed as much as a 50 times higher GOx loading than CMK-3, which was because of the large uniform pores (> 20 nm) of MSU-F-C The glucose biosensor fabricated with MSU-F-C/ GOx showed a much higher sensitivity (order of magnitude) and faster response than those using polymer matrices Fluorination has been used to modify the properties of graphite, activated carbons, and carbon nanotubes, and these fluorinated carbons have found applications as lubricants or as cathode materials in lithium batteries Dai and co-workers synthesized ordered fluorinated mesoporous carbons by fluorination of the corresponding mesoporous carbons using diluted fluorine gas.[129] The high-temperature fluorination induced the collapse of the ordered structure of the mesoporous carbon In contrast, the fluorinated carbon prepared at room temperature preserved the ordered structure of the pristine ordered mesoporous carbon Ordered mesoporous carbon nitride (CN) was synthesized using a mixture of ethylene diamine and carbon tetrachloride.[130] The physical characteristics of ordered mesoporous CN were very similar to those of CMK-3 carbon synthesized using a SBA-15 silica template The atomic environment of C and N in the walls was similar to other nonporous carbon nitride materials The Schüth group synthesized a magnetically separable, ordered mesoporous carbon, denoted Co-OMC, by the deposition of superparamagnetic nanoparticles on the surface of mesoporous carbon, followed by capping with a carbon material.[131] Co-OMC on which rhodamine 6G (Rh6G, C28H31N2O3Cl) was adsorbed was successfully separated using a magnet Palladium-loaded Co-OMC was used as a magnetically separable catalyst for hydrogenation This catalyst could be recycled after the magnetic separation The synthetic procedure for Co-OMC was rather complex A short and simple synthetic procedure for magnetically separable, ordered mesoporous carbon was developed by our group.[132] Poly(pyrrole) and the residual Fe2+ ions, which remained after their use as the polymerization catalyst and were located in the mesoporous channels, were converted to carbon materials containing superparamagnetic nanoparticles in the carbon rods Our group developed a magnetic mesocellular carbon foam, denoted Mag-MCF-C, with large interconnected cellular pores of > 20 nm.[133] Magnetic nanoparticles were simply generated from impregnated iron salts during the carbonization at high temperature Mag-MCF-C had many desirable characteristics for the preparation of immobilized magneto– bio–electrocatalysis.[134] GOx (6.0 nm × 5.2 nm × 7.7 nm) molecules were incorporated into the large cellular pores and crosslinked by treating with glutaraldehyde, which resulted in the generation of stable crosslinked enzyme aggregates (CLEA) As a result of crosslinking, Mag-MCF-C/CLEAGOx maintained more than 90 % of its initial activity after 12 washings as well as after continuous incubation with vigorous shaking for 22 days When Mag-CLEA/CLEA-GOx particles were brought into contact with the electrode by an applied magnetic field, an increased anodic current was observed On the contrary, when the Mag-MCF-C/CLEA-GOx Synthesis of Macroporous Carbon Materials 4.1 Synthesis of Macroporous Carbon Materials Using Silica Particles as Templates Spherical submicrometer-sized silica particles have been used as templates for the synthesis of macroporous carbon materials with core/shell and hollow structures The pore size of the resulting macroporous carbon materials could be easily controlled by varying the particle size of the silica spheres Zakhidov et al synthesized various macroporous carbon materials using synthetic silica opals, which were made by the self-assembly of uniform-sized silica spheres, known as colloidal crystals, as templates.[135] Macroporous carbon materials with glassy carbon, graphitic carbon, and diamond were synthesized by the infiltration of a phenol resin, the CVD of propylene gas, and plasma-enhanced CVD, respectively, followed by the carbonization The removal of the silica template by HF etching generated macroporous carbons with inverse opal structures (Fig 11) In some cases, before the infiltration of the carbon precursors, sintering was performed to create necks between the silica spheres, which provided interconnections between the spherical pores in the resulting carbons Since the first report on the synthesis of macroporous carbon using colloidal templating, similar macroporous carbon materials have been synthesized using simple and Figure 11 SEM image of a graphitic macroporous carbon synthesized using 200 nm sized silica opal templates Reproduced with permission from [135] Copyright 1998 American Association for the Advancement of Science cost-effective methods involving the carbonization of an aqueous solution of sucrose or phenol resin.[136] These macroporous carbons exhibited large pore volumes and high surface areas For example, macroporous carbon synthesized using phenol resin as the carbon precursor showed closepacked uniform spherical pores with a diameter of 62 nm, a total pore volume of 1.68 cm3 g–1, and a BET surface area of 750 m2 g–1 © 2006 WILEY-VCH Verlag GmbH & Co KGaA, Weinheim www.advmat.de 2089 REVIEW J Lee et al./Porous Carbon Materials 2090 Yu and co-workers reported the synthesis of 3D ordered macroporous carbon materials with different morphologies.[137] The process of controlling the morphology of the carbon materials was achieved by altering the acid catalyst sites used for the polymerization of the carbon precursor, i.e., a mixture of phenol and formaldehyde The complete filling of the interstitial pores of sintered colloidal silica crystals was achieved by the infiltration of a phenol–formaldehyde mixture, and the subsequent infiltration of sulfuric acid, followed by polymerization The carbonization and removal of the silica template generated ordered macroporous carbon materials with solid carbon walls Using Al-impregnated silica particles as the template, the polymerization of a phenol resin occurred selectively on the surface of the silica particles, resulting in surface-templated macroporous carbon The resulting macroporous carbons showed a high loading of the Pt–Ru catalyst nanoparticles The specific activity of the Pt–Ru alloy catalyst supported on the macroporous carbon for methanol oxidation was much higher than those of the commercial E-TEK and Vulcan XC-72 supported Pt–Ru catalysts generally used for methanol oxidation This improved catalytic activity seemed to be due to the high surface area of the ordered macroporous carbon, which provided for the high catalyst dispersion, and the 3D interconnected uniform macropores, which allowed for the efficient diffusion of the fuel and product The same group further controlled the pore size of the macroporous carbon materials in the range of 10–100 nm by using silica spheres with various particle sizes, and conducted detailed studies on their use as the electrodes in fuel cells.[138] The Yu and Jaroniec groups reported the preparation of highly graphitized ordered mesoporous carbons using commercial mesophase pitch as a carbon precursor and silica colloidal crystals as templates.[139] The synthesis of the graphitized ordered nanoporous carbon was carried out by the incorporation of mesophase pitch dissolved in quinoline in the interstitial space of the silica templates under a static vacuum After carbonization and the removal of the silica template, the resulting carbon was further heated at the high temperature of 2500 °C for 30 under an argon atmosphere to generate highly graphitized carbon Interconnected, ordered spherical pores and relatively large graphite crystallites (interlayer spacing of ca 0.33 nm) in the carbon pore walls were clearly observed in the TEM image The XRD patterns of the carbon sample after the graphitization at 2500 °C showed a sharp (002) peak and other reflections characteristic of a graphitic structure The Raman spectra of the graphitized carbon showed a strong G-band signal at 1588 cm–1 and a weak D-band at 1356 cm–1, which was consistent with the XRD data showing the highly graphitic characteristics of the macroporous carbon materials The Baumann group reported the synthesis of ordered macroporous carbons incorporating various metal nanoparticles (Co, Ni, and Cu) using polystyrene (PS) microspheres as a template and a metal-doped hydrogel, which was derived from the base-catalyzed polymerization of formaldehyde with the potassium salt of 2,4-dihydroxybenzoic acid, as a carbon www.advmat.de precursor.[140] Metal ions, such as Co2+, Ni2+, and Cu2+, were exchanged with the K+ ion in the K+-doped hydrogel These metal ions were reduced to metal nanoparticles during the carbonization step 4.2 Synthesis of 1D Carbon Nanostructures Using Anodic Aluminum Oxide (AAO) Templates Anodic aluminum oxide (AAO) films were prepared by the electrochemical anodization of aluminum metal in electrolyte cells.[141] AAO films generally have hexagonally arranged honeycomb structures with a uniform pore size and a regular pore-to-pore distance The lengths and diameters of the channels in the AAO film could be easily controlled by changing the oxidation time and current density These AAO films have been used as the templates for the fabrication of various tubular materials Using AAO films as templates and various carbon precursors, carbon nanotubes have been successfully synthesized The general synthetic scheme for the preparation of carbon nanotubes using AAO templates is shown in Figure 12 The Martin group reported the synthesis of carbon nanotubes by depositing polyacrylonitrile in the channels of the AAO template, followed by carbonization.[142] The Kyotani group achieved similar results using propylene as the carbon precursor.[143] The thermal decomposition of propylene in the uniform straight channels of the anodic oxide films results Figure 12 Synthetic scheme for the preparation of carbon nanotubes using AAO templates Reproduced with permission from [143b] Copyright 1996 American Chemical Society in carbon deposition on the channel walls The diameters of the carbon nanotubes could be easily controlled by changing the diameters of the AAO templates Figure 13 shows representative SEM images of the carbon nanotubes synthesized using AAO templates with different diameters of 30 nm (Fig 13a and b) and 230 nm (Fig 13c and d) Furthermore, it was also possible to control the wall thickness of the carbon nanotubes by varying the carbon deposition time In the case of the AAO template method, however, the resulting carbon nanotubes had a lower crystallinity than those grown via the CVD method To synthesize highly crystalline carbon nanotubes, the Martin group performed the CVD process using an AAO template incorporating a metal catalyst.[144] A Ni catalyst was deposited in the form of a film by © 2006 WILEY-VCH Verlag GmbH & Co KGaA, Weinheim Adv Mater 2006, 18, 2073–2094 J Lee et al./Porous Carbon Materials immersing the AAO template in an organometallic nickel solution, followed by the evaporation of the solvent and subsequent heat treatment at 400 °C under an argon atmosphere The decomposition of ethylene or pyrene at 545 °C and subsequent removal of the AAO template in a NaOH solution generated 1D carbon structures Increasing the decomposition time of the carbon precursors resulted in the generation of carbon nanofibers instead of carbon nanotubes Additional heat treatment of the carbon nanofiber/AAO composite at 500 °C for 36 h converted the carbon nanofibers into highly ordered graphite nanofibers The Haslett[145] and Xu groups[146] reported the synthesis of arrays of aligned multiwalled carbon nanotubes by pyrolyzing acetylene in an AAO template containing catalytic cobalt nanoparticles at the bottom The resulting carbon nanotubes had a slightly larger d-spacing (d002) than that of graphite The AAO template successfully oriented the growth of the carbon nanotubes The carbon nanotubes synthesized using the above method had the potential to be used for electron field emission, since they showed a turn-on field of 1.9–2.1 V lm–1 and a field enhancement factor of 3360–5200.[147] The Martin group reported the fabrication of hierarchical tube-in-tube carbon nanotubes using an AAO template, by employing a combination of conventional CVD and the catalytic CVD method involving metal nanoparticles.[148] Firstly, they used the CVD method to deposit carbon nanotubes inside an AAO membrane A CNT/AAO composite was then impregnated with an ethanol solution containing Fe(NO3)3 and reduced under H2 flow at 550 °C to produce Fe nanoparticles inside the as-synthesized carbon tubules After exposing the Fe nanoparticle deposited carbon tubule/alumina membranes to ethylene gas for 30 min, highly graphitic carbon nanotubes with a smaller diameter were generated inside the Adv Mater 2006, 18, 2073–2094 © 2006 WILEY-VCH Verlag GmbH & Co KGaA, Weinheim www.advmat.de REVIEW Figure 13 SEM images of carbon nanotubes synthesized using AAO templates with diameters of 30 nm (a,b) and 230 nm (c,d), respectively Reproduced with permission from [143b] Copyright 1996 American Chemical Society initially synthesized larger carbon nanotubes, forming a hierarchical tube-in-tube nanostructured carbon material The hierarchical tube-in-tube membrane was used as an electrode for Li-ion intercalation The cyclic voltammograms indicate that the carbon tubule membrane was reversibly intercalated with Li+ ions, and exhibited twice the intercalation capacity of the carbon nanotubes synthesized using a single CVD process Both the outer and the inner tubules were electrochemically active for the intercalation of lithium ions, suggesting the possible use of this membrane in lithium-ion batteries The membranes could also be filled with nanoparticles of electrocatalytic metals and alloys, and these catalyst-loaded carbon nanostructured materials could be used to electrocatalyze the reduction of O2 and the oxidation of methanol, which are very important processes for low-temperature fuel cells The Xu group reported the controlled growth of Y-junction carbon nanotubes using a specially designed Y-branched AAO template.[149] The channel diameter of the AAO template was proportional to the anodizing voltage By reducing the anodizing voltage from 50 V to 35 V in the conventional process of preparing the AAO template, a Y-branched AAO template was synthesized The diameters of the stems and branches were 40 nm and 28 nm, respectively Cobalt catalyst nanoparticles were electrochemically deposited at the bottom of the AAO template The catalytic pyrolysis of acetylene at 650 °C and the subsequent removal of the template generated Y-junction carbon nanotubes Using a similar method, the Sui group synthesized multibranched carbon nanotubes by the catalytic pyrolysis of acetylene using a multi-branched AAO template.[150] The multibranched AAO template was synthesized under the conditions of an abrupt increase in the anodizing voltage from 30 V to 60 V and subsequent slow decrease to 30 V It is known that the doping of group III or group V elements imparts semiconductor characteristics to carbon nanotubes.[151] Very recently, the Kyotani group reported the synthesis of double coaxial carbon nanotubes with an N-doped inner wall and B-doped outer wall using a two-step template method.[152] First, the CVD of acetonitrile (CH3CN) on the AAO template at 800 °C for h generated an N-doped CNT/AAO composite After annealing under an N2 atmosphere at 950 °C for h, a second CVD process was conducted on the N-doped CNT/AAO composite, using benzene and BCl3 as the carbon and boron sources, respectively Removing the AAO template with HF etching resulted in the formation of double coaxial carbon nanotubes with an N-doped inner wall and B-doped outer wall Using a similar method, carbon nanotubes with an undoped inner wall/N-doped outer wall structure or an undoped inner wall/ B-doped outer wall structure were synthesized Although the crystallinity of the N-doped carbon nanotubes was lower than that of the undoped ones, the conductivity of the former was higher than that of the latter, due to the doping effect 2091 REVIEW J Lee et al./Porous Carbon Materials Conclusions and Outlook The recent progress made in the synthesis of various porous carbon materials was reviewed in this article By using appropriate synthetic procedures, porous carbon materials with various pore dimensions and pore structures were synthesized These synthetic methods can be divided into two categories: activation processes and template methods Although activation processes have frequently been employed for the synthesis of porous carbon materials because of their simplicity and scalability, in general, porous carbon materials with non-uniform pore sizes and isolated non-interconnected pores are produced Many porous carbon materials having a variety of pore sizes and pore structures were synthesized using various kinds of designed templates These porous carbon materials exhibit uniform pore sizes, high surface areas, and large pore volumes These desirable characteristics have led these porous carbon materials to be extensively applied in various technological areas, including electrodes for batteries, fuel cells, and supercapacitors, and as hosts for the immobilization of biomolecules for biosensors For many biotechnological applications, mesoporous carbon materials having large interconnected pores with diameters of > 10 nm need to be synthesized For the electrodes of electrochemical devices such as fuel cells, porous carbon materials with highly graphitic structures are needed, and accomplishing this has been very challenging The synthesis and application of various porous carbon materials having hierarchical structures is expected in the future Simple and economical template synthetic procedures should be developed for the broad application of these synthesized porous carbons Received: July 29, 2005 Final version: January 19, 2006 Published online: August 1, 2006 – [1] [2] [3] [4] [5] 2092 a) F Rodriguez-Reinoso, in Introduction in Carbon Technology (Eds: H Marsh, E A Heintz, F Rodriguez-Reinoso), Universidad de Alicante, Secretariado de publicationes, Alicante, Spain 1997, p 35 b) S Subramoney, Adv Mater 1998, 10, 1157 a) J W Patrick, in Porosity in Carbons: Characterization and Applications, Edward Arnold, London, UK 1995 b) K Kinoshita, in 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The Yu group synthesized silicalite-1 zeolite core/mesoporous silica shell (ZCMC) structures from the sol–gel reaction of TEOS with C18TMS on the surface of solid pseudohexagonal prismaticshaped silicalite-1 zeolite particles.[107] The ZCMS particles with their bimodal microporous core/mesoporous shell structure were utilized... However, the ordered structure of the DVB/surfactant/silicate composite was not transferred to the resulting carbon materials after the removal of the silica The carbon structure achieved had wormholelike mesopores with diameters of ca 2 nm The Inagaki group[89a] and later the Ozin[89b] and Stein groups[89c] synthesized organic-group-incorporated mesoporous silica materials from the sol–gel reactions... first adsorbed into the small pores, because capillary condensation into small pores occurs at low pressure The Meso-nano-C carbon so produced was composed of 30–50 nm sized particles having 3D wormholelike pores, and was the exact negative replica of the Meso-nano-S silica template.[100] Fuertes synthesized spherical-shaped mesoporous carbon materials with controlled particle diameters ranging from... small mesopores were generated by the replication of the 13 nm sized mesopores of the HMMS silica a) directing agent Using FA as a carbon source, hierarchical porous monolithic carbon containing wormholelike mesopores and macropores was successfully replicated.[105] It is possible to fabricate the carbon materials in the monolithic form by using monolith-shaped mesoporous silica as the template The macroporosity... possible to use it as the template for mesoporous metal oxides In addition, the OMC could be easily removed by simple calcination The Schüth and Kim groups independently reported the synthesis of SBA-15 -like mesoporous silica using CMK-3 carbon as the template.[110,111] The mesopores of CMK-3 were filled with the silica precursor, which was either TEOS or sodium silicate, and subsequent sol–gel polymerization... synthesized using SBA-15 as the template In a subsequent paper, the Zhao group synthesized ordered mesoporous polymer and carbon materials with 2D hexagonal (p6m), 3D caged cubic (Im3m), and lamellar frameworks by simply adjusting the mass ratio of the polymer precursors and amphiphilic surfactants.[98] Figure 8 shows TEM and HRTEM images of C-FDU-15 and C-FDU-16 carbons with hexagonal (p6m) and cubic... a carbon replica structure Interestingly, the pore-replication process took place only through the mesopores in the shell, and not through the micropores, due to the smallness of the micropores in the zeolite core, resulting in the production of hollow core/mesoporous shell (HCMS) carbon with a pseudohexagonal prismatic shape Using a similar synthetic procedure, our group synthesized nanorattles, each... gas into CMK-3 was also investigated.[120] The amount of adsorbed methane gas was 81.35 mg g–1 (= 117.33 mL(STP) g–1) at the high pressure of 35 kg cm–2 at 298 K This result was much higher than that of zeolite, but much lower than that of microporous coordination polymers with an open framework CMK-3 carbons supporting palladium and platinum were used as the catalyst for the hydrogenation of nitrobenzene... hexagonally ordered mesoporous CMK-3 Nitrogen-adsorption experiments indicated that tinbased oxide materials were fully deposited into the internal pores of CMK-3 carbon The presence of interconnected carbon frameworks prevented aggregation of the tin species, which resulted in a much better cycle performance for the use as negative electrodes for lithium-ion batteries than those of nanometer-sized tin-based