Mesoporous carbons with a disordered pore structure have been synthesized using various methods, including catalytic activation using met-al species, carbonization of polymer/polymer bl
Trang 1DOI: 10.1002/adma.200501576
Recent Progress in the Synthesis of Porous
Carbon Materials**
By Jinwoo Lee, Jaeyun Kim,
and Taeghwan Hyeon*
1 Introduction
Porous carbon materials have received a great deal of
atten-tion due to their many applicaatten-tions.[1]Porous carbon materials
have been applied to gas separation, water purification,
cata-lyst supports, and electrodes for electrochemical double layercapacitors and fuel cells.[2]According to the International Un-ion of Pure and Applied Chemistry (IUPAC) recommenda-tion, porous carbon materials can be classified into three typesbased on their pore sizes: microporous < 2 nm, 2 nm < meso-porous < 50 nm, and macroporous > 50 nm Porous carbonmaterials have been synthesized using various methods Thefollowing are representative traditional methods
1) Chemical activation, physical activation, and a tion of the physical and chemical activation processes.[3]
combina-2) Catalytic activation of carbon precursors using metalsalts or organometallic compounds.[4]
3) Carbonization of polymer blends composed of a nizable polymer and a pyrolyzable polymer.[5]
carbo-4) Carbonization of a polymer aerogel synthesized undersupercritical drying conditions.[6]
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
synthe-sized 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
met-al 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
nanostruc-tured materials have been fabricated using anodic aluminum oxide (AAO) as a template.
–
[*] 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.
Trang 2Although many porous carbon materials have been
devel-oped 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
inor-ganic templates have been employed in an attempt to
synthe-size carbons with uniform pore synthe-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
de-picts 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 andthe wood is simultaneously burnt to generate the empty spaceinside the jar The general template synthetic procedure forporous carbons is as follows: 1) preparation of the carbon pre-cursor/inorganic template composite, 2) carbonization, and3) removal of the inorganic template Various inorganic mate-rials, including silica nanoparticles (silica sol), zeolites, anodicalumina membranes, and mesoporous silica materials, havebeen used as templates Figure 1b to 1d describes the synthe-sis of microporous, mesoporous, and macroporous carbonsusing zeolite, mesoporous silica, and synthetic silica opal astemplates, respectively Figure 1e shows the synthesis of car-bon nanotubes (CNTs) using an anodic alumina membranetemplate Broadly speaking, the template approaches can beclassified 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 Initia- tive 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 Uni- versity, 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 Taegh- wan Hyeon.
Trang 3templates such as silica nanoparticles are embedded in the
carbon precursor Carbonization followed by the removal of
the template generates porous carbon materials with isolated
pores that were occupied by the template species In the
sec-ond approach, a carbon precursor is introduced into the pores
of the template Carbonization and the subsequent removal of
the templates generate porous carbons with interconnected
pore structures In this review, we summarize the recent
devel-opments in the synthesis of porous carbon materials, focusing
on the synthesis of porous carbon materials with uniform pore
sizes via the template approaches The main part of the review
is divided into three sections based on the pore sizes:
micropo-rous, mesopomicropo-rous, and macroporous carbon materials
2 Microporous Carbon Materials
2.1 Disordered Microporous Carbons (Molecular Sieving
Carbons)
Molecular sieving carbons (MSCs) are special forms of
acti-vated carbons that possess uniform micropores of several
ang-stroms in diameter These MSCs have been applied
to various areas including the separation of gasmolecules, shape-selective catalysts, and electrodesfor electrochemical double-layer capacitors MSCshave advantages over inorganic molecular sieves(zeolites) in terms of their hydrophobicity and highcorrosion resistance The most representative syn-thetic method for the synthesis of MSCs is the py-rolysis of appropriate carbon precursors Miura
et al prepared MSCs by pyrolyzing a mixture ofcoal and organic additives.[8]The carbon materialsobtained using organic additives have pore struc-tures different from those of the carbons preparedfrom coal only By changing the experimental con-ditions it was possible to finely tune the pore size
of the MSCs For example, by changing the ization temperature and the mixing ratio of coal,pitch, phenol, and formaldehyde, MSCs having auniform pore size of around 0.35 nm were synthe-sized The Miura group also used ion-exchange res-ins to produce MSCs.[9] Spherical polystyrenebased resins with a sulfonic acid group were ion-exchanged with several kinds of cations, and the re-sulting resins were carbonized at between 500 and
carbon-900 °C In this way resins having various cations cluding H+, K+, Na+, Ca2+, Zn2+, Cu2+, Fe2+, Ni2+,and Fe3+were prepared from the ion-exchange res-
in-in When the ion-exchanged resin was carbonized
at 900 °C under a nitrogen atmosphere, the MSCsprepared from the resins with di- or trivalent cat-ions maintained sharp pore distributions, whereasthose prepared from the resins with univalent cat-ions lost most of their pores The main reason forthis drastic difference is that di- or trivalent cationscan form ionic crosslinks connecting two or threefunctional groups in the resins, and these crosslinks act as pil-lars to stabilize the pores during the carbonization process
The wide-angle X-ray diffraction (XRD) pattern of the bonized samples revealed the presence of metal-sulfide nano-particles, which are responsible for the formation of uniformlysized micropores Films of these MSCs were fabricated fortheir applications in gas separation
car-Microporous carbon membranes have been prepared usingvarious polymeric resins.[10] Carbon membranes have beenprepared in two main configurations, that is, unsupported car-bon membranes and membranes supported on macroporousmaterials A supported carbon membrane was prepared by
casting 13 wt % polyamic acid in N-methylpyrrolydone
(NMP) on a macroporous carbon support.[10a]The resultingpolymer was heat-treated through a two-step process involv-ing imidization at 380 °C and subsequent carbonization at
550 °C The gas-permeation experiment showed that the gastransport through the MSC membrane occurs according tothe molecular-sieving mechanism The membrane had selec-tive permeation for O2/N, He/N, CO2/CH4, and CO2/N Thehighest separation factors were achieved at 25 °C A molecu-
Figure 1 a) Schematic representation showing the concept of template synthesis.
b) Microporous, c) mesoporous, and d) macroporous carbon materials, and e)
car-bon nanotubes were synthesized using zeolite, mesoporous silica, a synthetic silica
opal, and an AAO membrane as templates, respectively.
Trang 4lar-sieving carbon film with a nanometer-sized
nickel catalyst was prepared from
polyimide-con-taining nickel nitrate.[11] The combination of the
catalytic function with the molecular-sieving
prop-erty 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
tempera-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,
small-er molecules can be more easily hydrogenated
compared to larger molecules Considering the
rel-ative sizes of butene and isobutene, the
hydrogena-tion of isobutene was much slower than that of
bu-tene However, perfect shape selectivity could not
be achieved, because of the presence of the
cata-lyst particles on the outer surface of the MSC
car-bon matrix Consequently, the elimination of the
nickel catalyst particles formed on the outer
sur-face of the MSC film is extremely important to
achieve perfect shape selectivity
Shiflett and Foley reported the fabrication of a
stainless-steel-supported MSC membrane via the ultrasonic deposition
of poly(furfuryl alcohol) on stainless-steel tubes and
subse-quent pyrolysis at 723 K.[12]The membrane was successfully
applied to gas separation with the following permeances,
mea-sured in moles per square meter per Pascal per second:
nitro-gen, 1.8 × 10–12; oxygen, 5.6 × 10–11; helium, 3.3 × 10–10; and
hy-drogen, 6.1 × 10–10 The ideal separation factors as compared
to that for nitrogen were 30:1, 178:1, and 331:1 for oxygen,
he-lium, and hydrogen, respectively
2.2 Ordered Microporous Carbon Materials Synthesized
Using Zeolite Templates
To make microporous carbon materials not only with
uni-form pores, but also with ordered regular pore arrays, rigid
in-organic templates are required Zeolites are aluminosilicate
materials having ordered and uniform sub-nanometer sized
pores Zeolites have been widely used as molecular sieves,
sol-id acsol-id catalysts, and catalyst supports, and have also been
used as shape-selective catalysts owing to their uniform
mo-lecular-sized pores.[13]Because the walls of zeolites have a
uni-form thickness of < 1 nm, zeolites have been used as inorganic
templates for the synthesis of microporous carbons with
uni-form pore sizes USY zeolite was adopted as the template to
prepare a microporous carbon by the Kyotani group
Figure 2a shows the overall template synthetic procedure for
microporous carbons using a zeolite Y template A carbon
precursor was incorporated into the pores and channels of the
zeolite Carbonization followed by the removal of the zeolite
template produced microporous carbon materials
Poly(acry-lonitrile) or poly(furfuryl alcohol) was employed as the
car-bon precursor.[14] The chemical vapor deposition (CVD)method was also adopted for the introduction of carbon intothe channels of USY zeolite CVD was carried out by expos-ing the zeolite to propylene gas at 700 or 800 °C The resultingmicroporous carbons exhibited high surface areas of over
2000 m2g–1 The similar morphology of the resulting rous carbon particles and the original zeolite template parti-cles, observed using scanning electron microscopy (SEM),demonstrated that the carbonization occurred inside the chan-nels of the zeolite template However, the Kyotani groupfailed to synthesize ordered microporous carbon arrays andthe carbon material that they fabricated possessed a consider-able amount of mesopores The generation of mesopores re-sulted from the partial collapse of the carbon framework afterthe removal of the zeolite template by HF etching The thinwall thickness of the carbon, derived from the small pores ofthe zeolite template (0.74 nm), did not exhibit a sufficientlyhigh mechanical strength to survive the removal of the tem-plate Rodriguez-Miraso et al adopted a similar approach toproduce microporous carbon using zeolite Y as a template,and they went on to examine the oxidation behavior of the re-sulting porous carbon.[15]
micropo-The Mallouk group synthesized phenol–formaldehyde (PF)polymers by making use of the acidity of the zeolite frame-work inside various zeolites, for instance, zeolites Y, b, and L,and then carbonized the polymer/zeolite composites to obtainporous carbons.[16] Phenol was infiltrated into the narrowpores of the zeolite by the vapor-phase infiltration method.These carbons possessed a considerable amount of mesopores,which was consistent with the results obtained by Kyotani andco-workers Moreover, the ordered structure of the zeolitewas not faithfully transferred to the resulting porous carbons.Later, the Kyotani group was able to successfully synthesize
b)
Figure 2 a) Schematic explaining the overall template synthetic procedure for
micro-porous carbons using a zeolite Y template b) High-resolution transmission electron microscopy (HRTEM) image of the ordered microporous carbon prepared following the procedure reported The inset corresponds to a diffraction pattern taken from this image Reproduced with permission from [18] Copyright 2001 American Chemical Society.
Trang 5uniformly 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
car-bon 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)
com-posite at 700 °C The carbon obtained after the removal of the
zeolite template exhibited an ordered zeolite replica
struc-ture, 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
struc-ture 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
re-search 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
com-posite obtained by the above two-step method at 900 °C.[18]
The carbon inside the channels seemed to be better
carbon-ized 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
vol-umes were 1.52 cm3g–1 and 0.05 cm3g–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,
indicat-ing 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 m2g–1,
which is much higher than that of the carbon prepared
with-out the additional heat treatment (2200 m2g–1) Although the
surface area of some KOH activated carbons is over
3000 m2g–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
mi-croscopy (TEM) image of the ordered microporous carbon
obtained from zeolite Y Its excellent 3D ordering is clearly
demonstrated The internal structure of this ordered
micropo-rous 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
ex-tended the two-step replication process to other zeolite
sys-tems, 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
the structural regularity of the corresponding zeolite template
The extent of such transferability, however, strongly pended on the kind of zeolite template employed Theauthors concluded that in order to obtain microporous car-bons with high structural regularity the pores in the zeolitetemplate should be sufficiently large (> 0.6–0.7 nm), as well asbeing three-dimensionally interconnected More recently, theKyotani group also synthesized a nitrogen-containing micro-porous carbon with a highly ordered structure by using zeo-lite Y as a template.[20]The formation of nitrogen-doped car-bon in the zeolite channels was achieved by the impregnation
de-of FA and subsequent CVD de-of acetonitrile The corporated and ordered microporous carbon exhibited astronger affinity to H2O adsorption than the nitrogen-free, or-dered, microporous carbon materials with similar pore struc-tures, demonstrating the polar and hydrophilic nature of thenitrogen-doped carbon
nitrogen-in-For many industrial applications, such as the selective meation of gas molecules, the control of the pore size is a criti-cal issue Consequently, we expect more research on the pore-size control of ordered microporous carbon materials to beconducted in the future
per-3 Mesoporous Carbon Materials
Over the last decade, there have been significant advances
in the synthesis of mesoporous carbon materials.[21] porous carbon materials are very important for applicationsinvolving large molecules, such as adsorbents for dyes, catalystsupports for biomolecules, and electrodes for biosensors
Meso-3.1 Mesoporous Carbons with Disordered Pore Structures
Catalytic activation using metal ions was employed tosynthesize several types of mesoporous carbon materials
Yasuda and co-workers synthesized mesoporous carbon materials by the steam invigoration of pitches mixedwith 1–3 wt % of rare-earth metal complexes, such asLn(C5H5)3and Ln(acac)3 (where Ln = Sm, Y, Yb or Lu).[22]
activated-All of the resulting mesoporous carbons had high mesoporeratios of up to 80 %, surface areas of ca 200 m2g–1, and poresizes ranging from 20 nm to 50 nm These mesoporous acti-vated carbons selectively adsorbed large molecules, such as vi-tamin B12, blue acid 90 dye, dextran, nystatin, and humicacid, reflecting their large mesopore volumes Oya and co-workers synthesized activated-carbon fibers containing a sig-nificant fraction of mesopores with sizes of several tens ofnanometers from the catalytic activation of a phenol resinmixed with cobalt acetylacetonate.[23]
The carbonization of polymer blends composed of two ferent types of polymers, that is, a carbon precursor polymerand a decomposable polymer that is pyrolyzed to generatepores, produced mesoporous carbon materials Ozaki et al
dif-synthesized mesoporous carbons with a pore diameter of
Trang 6ca 4 nm from the carbonization of a polymer blend
com-posed of phenolic resin and poly(vinyl butyral).[5b]Later, Oya
and co-workers synthesized carbon fibers from the
carboniza-tion 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
di-ameter was produced The nanofiber bundle so obtained was
easily separated into thin fibers These polymer-blend
carbon-ization 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
meso-porosity were prepared by the sol–gel polymerization of silica
precursors, followed by supercritical drying.[24]The
supercriti-cal 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
mesopo-rous carbon materials had high porosities (> 80 %) and high
surface areas (> 400 m2g–1) Subsequent studies on the
pore-size 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
resor-cinol to sodium carbonate and resorresor-cinol 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
Titania-loaded carbon aerogels were prepared by adding titanium
alk-oxide during the sol–gel reaction, and the resulting composite
aerogels were used for the combined adsorption and
photo-catalytic 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
compos-ite was prepared via a novel two-step
metal-vapor-impregna-tion 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
the template.[29]Upon pyrolysis under an argon atmosphere,the organic PS microspheres were burnt off generating largemesopores The pore size of the mesoporous carbon foamswas roughly two-thirds that of the template
Silica materials have been extensively used as templates tosynthesize mesoporous carbons The template silica materialswere easily removed by treating them with HF or NaOH Asdescribed in the Introduction, Knox et al reported the syn-thesis of spherically shaped mesoporous carbon materialsusing silica gel and porous glass as templates.[7]The polymer-ization of the phenol–hexamine mixture within the pores ofthe silica gel, followed by the pyrolysis of the resulting resin in
a nitrogen atmosphere at temperatures below 1000 °C, andsubsequent dissolution of the silica template produced themesoporous carbon materials The further graphitized spheri-cal mesoporous carbons were successfully used as high-perfor-mance liquid chromatography (HPLC) column materials.Our group synthesized mesoporous carbons using commer-cial silica sol nanoparticles as templates.[30] The polymeriza-tion of resorcinol and formaldehyde in the presence of a silicasol solution (Ludox HS-40 silica sol solution, average particlesize ca 12 nm) generated RF gel/silica nanocomposites Car-bonization followed by HF etching of the silica sol templatesgenerated porous carbons, designated as silica sol mediatedcarbon (SMC1), having pore sizes predominantly in the range
of 10–100 nm These carbon materials exhibited very highpore volumes of over 4 cm3g–1 and high surface areas of
ca 1000 m2g–1 Because the aggregated form of the silicananoparticles acted as templates, the pore size distribution ofthe resulting carbon was broad, ranging from 10 nm to
100 nm These SMC1 carbon materials exhibited excellent sorption capacities for bulky dyes[31]and humic acids.[32]In or-der to prevent the aggregation of the silica nanoparticles dur-ing the synthesis, surfactant-stabilized silica nanoparticleswere used as the template (Fig 3a).[33]The resulting carbonmaterial, designated as SMC2, exhibited a narrow pore sizedistribution centered at 12 nm, which matched very well withthe particle size of the silica nanoparticle template Figure 3bcompares the pore size distribution curves and the corre-sponding nitrogen adsorption/desorption isotherms of SMC1and SMC2 carbons, demonstrating that SMC2 has a more uni-form pore size distribution as compared to SMC1 When silicananoparticles with a particle size of 8 nm were used as thetemplate, SMC2 carbon with uniform 8 nm sized pores wasproduced, demonstrating the excellent template role of thesurfactant-stabilized silica nanoparticles
ad-Jaroniec and his co-workers reported a colloidal imprintingmethod to synthesize mesoporous carbons using mesophasepitch as a carbon precursor and silica sol as a template.[34]Using colloidal silica particles with different sizes and adjust-ing the imprinting conditions such as imprinting time and tem-perature, they were able to synthesize carbon materials withcontrolled pore size, surface area, and pore volume.[35]One in-teresting characteristic of the carbon materials synthesizedusing mesophase pitch as a carbon precursor was that theyhad nearly no micropores The same group also reported gra-
Trang 7phitized 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
graphit-ic mesoporous carbons were successfully used as the
station-ary phase for reverse-phase liquid chromatography in the
sep-aration of alkylbenzenes, such as benzene, ethylbenzene, and
propylbenzene.[37]
Jang and co-workers synthesized carbon nanocapsules and
mesocellular carbon foams by surface-modified colloidal
sili-ca-templating methods.[38]Carbon nanocapsules were
synthe-sized using polydivinylbenzene (DVB) as a carbon precursor,
poly(methyl methacrylate) (PMMA) as a barrier for the
pre-vention of intraparticle crosslinking of DVB, and coated colloidal silica particles as a template Direct polymer-ization of DVB on the surface of the silica particles withoutPMMA, followed by carbonization and dissolution of the sili-
surfactant-ca template, resulted in mesocellular surfactant-carbon foams Jang andhis co-workers also reported the synthesis of mesoporous car-bons via vapor deposition polymerization of polyacrylonitrile
on the surface of silica particles.[39]
Lu et al reported an aerogel-based approach to synthesizespherical mesoporous carbon particles.[40]In the synthesis, anaqueous solution containing sucrose and various silica tem-plates was passed through an atomizer and dispersed intoaerogel droplets Solvent evaporation at 400 °C resulted inspherical silica/sucrose nanocomposite particles and the sub-sequent carbonization and removal of the silica templatesgenerated the spherical porous carbon particles
Kyotani and co-workers reported the synthesis of porous carbon through the co-polymerization of FA and tetra-ethylorthosilicate (TEOS).[41]A nanocomposite of carbon andsilica was prepared by using a sol–gel process with TEOS inthe presence of FA, followed by the polymerization of FA,and its subsequent carbonization In this synthesis, the silicatemplate and carbon precursor were simultaneously synthe-sized to produce a silica/carbon precursor nanocomposite
meso-Using a similar synthetic procedure, Han et al synthesizedmesoporous carbon using inexpensive sucrose and sodium sili-cate as the carbon precursor and template, respectively.[42]Luand his co-workers synthesized unimodal and bimodal meso-porous carbons from the sucrose/silica nanocomposites pre-pared by sol–gel process of TEOS with or without colloidalsilica particles in the presence of sucrose.[43]Lu and his co-workers also reported the synthesis of continuous mesoporouscarbon thin films by a rapid sol–gel, spin-coating process usingsucrose as the carbon precursor and TEOS as the silica pre-cursor.[44]Continuous sucrose/silica nanocomposite thin filmswere formed by the spin-coating of homogeneous sucrose/sili-cate/water solutions that were prepared by reacting TEOS inacidic sucrose solutions Carbonization converted the sucrose/
silica thin films into carbon/silica nanocomposite thin films
The mesoporous carbon thin films exhibited a high specificsurface area of 2603 m2g–1 and a specific pore volume of0.21 cm3g–1 This was the first reported synthesis of continu-ous mesoporous carbon thin films through a direct and rapidorganic/inorganic self-assembly and carbonization process
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 thesis of mesoporous M41S silica materials from the sol–gelpolymerization of silica precursors in the presence of a surfac-tant self-assembly.[45]The pore structure and dimension of the
syn-a)
b)
Figure 3 a) Synthetic strategy for uniform mesoporous carbons: 1)
gela-tion 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
iso-therm by the Barrett–Joyner–Halenda (BJH) method and the
correspond-ing N 2 adsorption and desorption isotherms (inset) of mesoporous
car-bons synthesized using isolated CTAB-stabilized silica particles (solid
line) and using silica particle aggregates (dashed line) as templates.
Trang 8mesoporous silica materials could be controlled by varying
the experimental conditions, such as the ratio of the silica
pre-cursor 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,
in-cluding neutral amine surfactants,[46,47] alkyl(PEO)
surfac-tants,[48]and triblock copolymers.[49]These mesoporous silica
materials have uniform pore sizes and high surface areas
Me-soporous 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
materi-als 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
fol-lowed by the removal of the silica template resulted in the
generation of mesoporous carbon materials having 3D
inter-connected pore structures Figure 4a shows the overall
tem-plate 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
carbon-ization of the phenol-resin/MCM-48 nanocomposite, followed
by the dissolution of the aluminosilicate template using
aque-ous hydrofluoric acid produced an ordered mesoporaque-ous
car-bon (SNU-1) The TEM image of SNU-1 carcar-bon showed aregular array of 2 nm sized pores separated by 2 nm thick car-bon walls (Fig 4b) Judging by the low-angle XRD pattern,the resulting carbon was not a real negative replica of theMCM-48 silica template, because the replicated carbon under-went a structural transformation during the removal of the sil-ica template It was suggested that the cubic MCM-48 with
the Ia3d structure was converted to a new cubic I41/a
struc-ture.[52]Using the same template (MCM-48), Ryoo and his workers synthesized mesoporous carbon (CMK-1) usingsucrose as a carbon precursor.[50]To improve the thermal sta-bility and ordering of the resulting mesoporous carbon mate-rials, Yu and co-workers used silylated MCM-48 as a templateand poly(divinylbenzene) as a carbon precursor.[53]The meso-porous carbon synthesized using the silylated MCM-48 silicatemplate showed much better overall structural order com-pared to that obtained using pure MCM-48 silica, according
co-to the small-angle XRD patterns and TEM images
Following the first report on the synthesis of ordered porous carbons using the MCM-48 silica template, variousmesoporous carbon materials with different pore structureswere synthesized using a variety of different mesoporous silicatemplates For example, our group used a hexagonal mesopo-rous silica (HMS)[47] template to synthesize mesoporousSNU-2 carbon.[54] Through this template synthesis, we wereable to indirectly elucidate that the HMS silica possesses awormholelike pore structure rather than the originally pro-posed MCM-41-like hexagonal 1D channel structure The me-soporous carbon materials synthesized using mesoporous sili-
meso-ca templates contain not only mesopores generated from thereplica of the templates, but also micropores formed by thecarbonization of the precursor For example, SNU-2 carbonexhibited a bimodal pore size distribution curve, with 0.6 nmsize micropores generated from the carbonization of the car-bon precursor and the other centered at 2.0 nm from the rep-lica of the template
Hexagonally ordered mesoporous silica SBA-15 was used
as a template for a mesoporous carbon designated asCMK-3.[55]In the original study by the Stucky group, SBA-15was reported to have a hexagonal tubular pore structure simi-lar to that of MCM-41 However by using SBA-15 silica as thetemplate, Ryoo and his co-workers successfully synthesized
an ordered mesoporous carbon in which parallel carbon fiberswere interconnected through thin carbon spacers Throughthis synthesis and further studies on the pore structure, theSBA-15 silica turned out to have complementary pores, whichwere generated by the penetration of the hydrophilic ethyleneoxide groups into the silica framework.[56–58] The orderedstructure of the CMK-3 carbon was the exact inverse replica ofthe SBA-15 silica without the structural transformation duringthe removal of the silica template CMK-3 type ordered meso-porous carbon was also synthesized by the infiltration of thecarbon 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 asCMK-5, was also synthesized by Ryoo and co-workers The
a)
b)
Figure 4 a) Schematic representation of the formation of an ordered
me-soporous carbon SNU-1 b) TEM image of a meme-soporous SNU-1 carbon.
Reproduced with permission from [51] Copyright 1999 Royal Society of
Chemistry.
Trang 9hexagonally ordered arrays of carbon nanotubules were
ob-tained 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
comple-mentary pores between the adjacent cylinders, forming a
highly ordered hexagonal array The pore size distribution
curve exhibited bimodal pores, corresponding to the inside
di-ameter 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 6 nm In a subsequent
paper, the Ryoo and Jaroniec groups optimized the synthetic
condition by pyrolyzing poly(furfuryl alcohol) under a
vacu-um 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
hydro-thermal 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
hy-drothermally treated SBA-15 aluminosilicate, and subsequent
polymerization using the acidic Al sites on the template
gen-erated poly(furfuryl alcohol), which was used as the carbon
precursor The Schüth group also synthesized ordered
nitro-gen-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
car-bon.[65]The SBA-15 silica template was first impregnated with
ferric chloride, which served as the oxidant for the
vapor-phase oxidative polymerization of pyrrole vapor at room
tem-perature The resulting materials had an ordered structure,
high surface area, and large pore volume Nanopipe-type
hex-agonally ordered mesoporous carbons were also prepared
through the catalytic chemical vapor deposition (CCVD)
method using cobalt metal incorporated SBA-15 as the plates.[66]The cobalt/SBA-15 silica was prepared by dispersingethylenediamine-functionalized SBA-15 silica in water con-taining cobalt ions, followed by thermal treatment Increasingthe deposition time resulted in the generation of highly hexag-onally ordered nanopipe-type mesoporous carbon.[66]
tem-Ordered mesoporous carbon CMK-3 with a hollow cal particle shape was synthesized by CVD.[67] SBA-15 silicawas employed as the template and styrene as the carbonsource In most templating processes, the morphology of themesoporous carbon materials is very similar to that of thetemplate However, during this high-temperature CVD pro-cess, the carbon precursor that was initially deposited in theouter pores of the template seemed to block the internalpores The subsequent carbonization and removal of the tem-plate generated hollow, spherical carbon with a mesoporousshell structure
spheri-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 antiomerically paired channels.[50,51] Later, three research
en-groups independently reported the synthesis of cubic Ia3d
structured mesoporous silica with very large pores using aP123 triblock copolymer ((EO)20(PO)70(EO)20) as the tem-plate,[68–70] and the successful replication to highly orderedmesoporous carbons.[69–71]The Zhao group synthesized large-
pore 3D bicontinuous cubic Ia3d mesoporous silica by a
sol-vent-evaporation method using P123 triblock copolymer asthe template and a small amount of 3-mercaptopropyltrimeth-oxysilane (MPTS) and trimethylbenzene as additives.[68] Amesoporous silica material with a monolithic form was used
as the template for the synthesis of Ia3d cubic structured
me-soporous 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 FAinside the pores In contrast to the mesoporous carbon synthe-sized using the MCM-48 silica template, the mesoporous car-bons obtained using the cubic mesoporous silica template re-
tained the bicontinuous Ia3d structures of the template The
authors claimed that the bridges between the channel-like antiomeric pore systems of the cubic mesoporous silica tem-plate connected the carbon rods in the channels.[69]
en-The control of the pore size of the mesoporous carbons wasnot easy to accomplish through the template approach, be-cause it was difficult to control the thickness of the wall dur-ing the synthesis of the mesoporous silicas Ryoo and co-workers were the first to report the successful control of thepore size of ordered mesoporous carbons They employedmixed surfactants (cetyltrimethylammonium bromide(C16TAB) and polyoxyethylene hexadecylether-type surfac-
Figure 5 TEM image viewed along the direction of the ordered
nano-pipe-type carbon and corresponding Fourier diffractogram (inset)
Repro-duced with permission from [60] Copyright 2001 Nature Publishing
Group.
Trang 10tants (C16EO8)) in the acidic synthesis of hexagonal
mesopo-rous silica By decreasing the C16TAB/C16EO8ratio, the wall
thickness in the mesoporous silica was increased
systematical-ly from 1.4 nm to 2.2 nm.[72]The resulting hexagonal
mesopo-rous 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]
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
car-bons 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
repli-cated 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 5 nm To synthesize mesoporous carbon
ma-terials with uniform pore sizes of > 10 nm, our group
em-ployed 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
mesocel-lular aluminosilicate foam (AlMCF) The subsequent
poly-merization with formaldehyde generated a phenol-resin/
AlMCF nanocomposite Carbonization followed by the
re-moval 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
gener-ated 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)
Subse-quently, the Tatsumi group synthesized a mesocellular
car-bon 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
ob-tained had closed hollow spherical pores, while the carbon
obtained by the single-step impregnation of sucrose had
open mesocellular pores
3.2.3 Mesoporous Carbons with Graphitic Pore Walls
Given their good electrical conductivity and uniform andlarge pores, mesoporous carbon materials with good graphiticcharacteristics could find many important applications, includ-ing electrodes for electrochemical double-layer capacitors,fuel cells, and biosensors It is well known that it is extremelydifficult to synthesize carbon materials with both a high sur-face area and good graphitic crystallinity To achieve such agoal, the Ryoo group synthesized ordered mesoporous car-bons with graphitic pore walls (CMK-3G) through the in situconversion of aromatic compounds to a mesophase pitch in-side the SBA-15 silica template by carbonization under highpressure using an autoclave.[76]The carbon frameworks werecomposed of discoid graphene sheets, which self-aligned per-pendicularly to the template walls during the synthesis.CMK-3G carbon exhibited much better mechanical strengththan the CMK-3 synthesized using sucrose or FA as the car-bon precursor The discoid alignment of the graphitic frame-
b)
Figure 6 a) Schematic illustration for the synthesis of a mesocellular
car-bon foam Reproduced with permission from [74] Copyright 2001 can Chemical Society b) TEM image of a mesocellular carbon foam.
Trang 11Ameri-works was consistent with the general tendency of the
edge-on anchoring of polycyclic aromatic hydrocarbedge-ons in a
meso-phase pitch on the silica surface
Mokaya and co-workers synthesized nitrogen-doped
meso-porous carbons with graphitic pore walls via CVD of
acetoni-trile.[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,
or-dered mesoporous carbon materials had a macroscopic
spher-ical morphology, which was similar to that of the other
meso-porous carbons synthesized via CVD methods Later the
Mokaya group generalized the CVD method and synthesized
many mesoporous nitrogen-doped carbon materials using
var-ious mesoporous silica templates including SBA-12, SBA-15,
MCM-48, HMS, and MCM-41.[78]The carbon materials
pre-pared 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–Tel-ler (BET) surface area of 260 m2g–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]FeCl3was 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
car-bons at high current densities This superior electrode
perfor-mance seemed to be derived from highly accessible pores and
the high conductivity of the graphitic framework
The Pinnavaia group synthesized ordered graphitic
mesopo-rous 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
car-bon nanofiber bundles
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
the synthesis The Pinnavaia group developed a very cal route to synthesize mesoporous MSU silica materials viathe sol–gel reaction of sodium silicate under near-neutral con-ditions.[81,84] The cost of synthesizing MSU-H silica is muchlower than that of the similarly structured SBA-15 silica, giventhat a very small amount of acid and inexpensive sodium sili-cate are used By adding trimethylbenzene (TMB) to the syn-thesis solution, mesocellular silica foams, which were denoted
economi-as MSU-F and had a similar pore structure to that of MCF
sili-ca, could also be synthesized.[81] The Pinnavaia group usedMSU-H silica as the template to synthesize hexagonally or-dered mesoporous carbons, denoted as C-MSU-H.[85] Thepore structure of C-MSU-H was very similar to that of theCMK-3 carbon synthesized using an SBA-15 silica template
Our group reported the synthesis of mesocellular carbonfoams using inexpensive MSU-F silica as the inorganic tem-plate.[86] The cellular pore structure of C-nano-MSU-F wasvery similar to that of mesocellular carbon foams synthesizedusing the MCF-silica template However, the C-nano-MSU-Fwas composed of individual particles with sizes of a few hun-dred nanometers, in contrast to several micrometer-sized par-ticles of the MCF-carbon This small individual particle size ishighly desirable for the facile access of molecules into theframework pores
The procedure employed to synthesize mesoporous bons using mesostructured silica templates is rather complexand time consuming The general synthetic procedure for or-dered mesoporous carbons using a mesostructured silica tem-plate is as follows: 1) the preparation of the mesostructuredsilica/surfactant composite, which often takes about 2–3 days;
car-2) the removal of the surfactant by calcination or solvent traction; 3) the generation of the catalytic sites inside thewalls of the mesostructure for the polymerization and, if nec-essary, the re-calcination; 4) the incorporation of the poly-meric carbon precursor, for example, phenol, FA, or sucrose,into the pores of the mesoporous silica template; 5) the poly-merization of the polymeric carbon precursor; 6) carboniza-tion; and, finally, 7) the removal of the silica template with
ex-HF or NaOH solution This long and complicated multisteptemplate synthesis limits the application of mesoporous car-bons, 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 mesoporouscarbons
Recently, much effort has been made to find a way of rectly synthesizing uniform pore-sized mesoporous carbonmaterials Sayari and co-workers reported a simple and directpreparation route to synthesize uniform microporous carbonmaterials by the direct carbonization of cyclodextrin-tem-plated silica mesophase materials.[87]During the preparation
di-of the cyclodextrin/silica mesophase materials, sulfuric acidwas used instead of hydrochloric acid, because it catalyzes thecarbonization of cyclodextrin The pore size of the resultingcarbon was less than 2 nm, i.e., it was microporous Moriguchiand co-workers reported the direct synthesis of a mesoporouscarbon material by the in situ polymerization of divinylben-