Nanocasting A Versatile Strategy for Creating Nanostructured Porous Materials
DOI:10.1002/adma.200600148
Nanocasting:A Versatile Strategy for Creating Nanostructured Porous Materials**
By An-Hui
Lu and Ferdi Schüth*
1.Introduction
Porous materials are of great interest in various applica-tions,ranging from catalysis,adsorption,sensing,and separa-tion to biotechnology,owing to their high surface area, tunable pore size,adjustable framework,and surface proper-ties.The specific surface areas can reach values of up to several thousand square meters per gram,depending on the material.Many synthetic pathways have been reported for the synthesis of porous materials,either with a disordered pore system or ordered with various structures,which can meet the demands of the target application.[1]In particular,research in the synthesis of ordered porous materials has seen tremen-dous growth since the discovery of the ordered mesoporous silica of the M41S family[2]or related materials,[3]which are synthesized with the help of cooperative surfactant tem-plating.Since these pioneering studies,significant progress has been made in terms of structural,compositional,and morphological control.Several reviews covering synthesis, properties,and applications of mesoporous materials are available.[4–14]
As important as siliceous materials are,both from a funda-
mental and from an applications point of view,the design and synthesis of nonsiliceous materials with controlled composi-
tion and structural ordering are even more important from an academic as well as an industrial perspective,since they have
a much broader application range and their syntheses provide additional https://www.sodocs.net/doc/7f14996028.html,pared to the silicon alkoxides,the hydrolysis and polymerization of transition-metal alkoxides
are more difficult to control precisely.Consequently,the ob-
tained metal oxides usually exhibit very poor structural order-
ing and low thermal stability after removal of the surfactant templates.[15]Only special precursors,such as atranes,allow
these problems to be circumvented in selected cases.[16]More-
over,it is very difficult to obtain mesoporous carbon materials
with an ordered structure via a sol–gel process involving a sur-
factant templating strategy,owing to the complexity of the
carbon-structure evolution.[17,18]Only four recent reports are available concerning the synthesis of ordered mesoporous carbon,mainly as a thin film,by the use of a rigid polymer re-
sorcinol-formaldehyde as the carbon source in a sol–gel pro-
cess.[19–22]As an alternative to cooperative surfactant templat-
ing in solution,the nanocasting pathways developed over the
last five years,which use hard templates to create ordered re-
plicas,provide promising routes for the preparation of meso-structured materials with novel framework compositions.[23]
The first report by Ryoo’s group on this pathway described
the synthesis of mesoporous carbon with an ordered structure,
where the replication of the MCM-48structure led to the formation of a new type of mesoporous carbon material
(CMK-1).[24]In the following,we will discuss the recent devel-
REVIEW
Nanocasting is a powerful method for creating materials that are more
difficult to synthesize by conventional processes.We summarize recent
developments in the synthesis of various structured porous solids,cov-
ering silica,carbon,and other nonsiliceous solids that are created by a
nanocasting pathway.Structure replication on the nanometer length scale allows materials’properties to be manipulated in a controlled manner,such as tunable composition,controllable structure and morphology,and specific functionality.The nanocasting pathway with hard templates opens the door to the design of highly porous solids with multifunctional properties and interesting application perspectives.
–
[*]Prof.F.Schüth,Dr.A.-H.Lu
Max-Planck-Institut für Kohlenforschung
45470Mülheim an der Ruhr(Germany)
E-mail:schueth@mpi-muelheim.mpg.de
[**]The authors thank the Leibniz Program and the FCI for support in
addition to the basic funding provided by the Institute.
opments in the field of nanocasting for the creation of porous materials.The basic principles of nanocasting are introduced,and the various replicated porous materials with their differ-ent framework compositions,structures,and properties will be described.An interesting possibility is the direct creation of additional functionality during the nanocasting process,and this topic will be addressed before we will,in the last part,attempt to highlight the perspectives and possibilities created by this synthetic approach for the generation of structured materials.
2.The Concept of Nanocasting
In a casting process on the macroscopic scale,a rigid mold,made of wax,plaster,metal,or other material,is normally needed.[10]By filling the void of the mold with the material to be cast,or a precursor for it,subsequent optional processing,and final removal of the mold,a replica structure can be ob-tained,which is the negative replica,if the casting process is done only once.If this procedure is conceptually scaled down to the nanometer scale,“nanocasting”would be the most suit-able word to describe this process.Nanocasting is thus the process in which a mold with relevant structures on the length scale of nanometers is filled with another material,and the initial mold is afterwards removed.In order to avoid any mis-understanding,the word “template”is used to describe the concept of a “mold”when the casting process proceeds on the nanometer scale.
The structures and properties of the templates play a crucial role with respect to the properties of the replicated porous materials.Generally,two kinds of templates,defined as hard and soft templates,have been described as molds for nano-casting processes.Nanocasting from soft templates was devel-
oped first,and organic precursor species,often polymers,which allow the formation of liquid crystals,can be used as soft templates.[25]For clarity of terminology,one should keep in mind that not all surfactant-assisted synthesis pathways are nanocasting routes,since many syntheses rely on a coopera-tive assembly between the surfactant and inorganic phase,and they do not replicate a preformed surfactant structure.[5]For instance,in the synthesis of mesoporous silica,the concen-tration of surfactant can be varied from low (even below the critical micelle concentration)to high (fully developed liquid-crystal phase).The synthetic mechanisms range from coopera-tive mechanisms to a true liquid-crystal-phase templating (TLCT)mechanism.However,even in the TLCT mechanism,the organic mesophase can be temporarily destroyed,for in-stance,by the methanol released during the hydrolysis of the silica source,tetramethoxysilane.[26]Consequently,templating via TLCT is often not a real nanocasting process,because the soft templates do not really provide a rigid framework,but rather are nanoreactors.In the confined spaces that are pro-vided by the soft template,the liquid phase structured by the surfactant is solidified by a chemical reaction,for instance a sol–gel reaction or a reductive coupling,thus leading to a mesostructured solid.In contrast,in cases where a hard tem-plate is used,the synthesis indeed corresponds to a direct-casting mechanism,where a relatively precise negative replica of the template is created.[24]Another advantage of using a hard template is the fact that the syntheses are relatively easy to control,since the template structures are fixed.Soft tem-plate structures are often much more flexible,and can be dependent on temperature,solvent,ionic strength,and other parameters,which makes the prediction of the resulting nega-tive replica more difficult.
The nanocasting pathway to create nanostructured materi-als involves three main steps:i)formation of the template;
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A.-H.Lu,F.Schüth /Nanocasting:Creating Nanostructured Porous Materials
Ferdi Schüth studied Chemistry and Law at the Westf?lische-Whilhelm Universit?t in Münster,where he received his Ph.D.in Chemistry in 1988and the State Examination in Law in 1989.After post-doctoral work in the group of L.D.Schmidt at the University of Minnesota,he joined the group of K.Unger at the University of Mainz.After Habilitation in 1995he became Profes-sor at the Johann-Wolfgang-Goethe Universit?t Frankfurt.In 1998he was appointed Director at the Max-Planck-Institut für Kohlenforschung,Mülheim.His research interests include porous solids and high-throughput experimentation in
catalysis.
An-Hui Lu studied Chemical Engineering at Taiyuan University of Technology (P .R.China),where he received his B.S.in 1996.In 2001he received his Ph.D.from the Institute of Coal Chemistry,Chinese Academy of Sciences.After post-doctoral work (as Max-Planck research fel-low and Alexander von Humboldt fellow)in the group of F.Schüth at the Max-Planck-Institut für Kohlenforschung,he was promoted to group leader in 2005.His research interests include synthesis and functionalization of nanostructured materials and magnetically separable catalysts,and the use of these materials in heterogeneous catalytic reactions.
ii)the casting step with target precursors,including the con-version of the precursor—which is typically molecular—to a solid;and iii)removal of the template,as shown in Figure1. Inorganic,ordered porous solids are mostly used as the nano-scale hard template in the first step.For instance,zeolites,[27] alumina membranes,[28]ordered mesoporous silica,[24]ordered mesoporous carbon,[10]or,for somewhat larger structure sizes, the assembly structure of colloidal spheres[29]have been em-ployed as a true template to replicate other materials.The pore structures of these parent materials can be transferred to the solid structure of the generated porous materials,while the walls of the parent become the voids of the replica.In order to be able to control the morphology and structural parameters of the replicas,templates with a controllable morphology and structure are highly advantageous.One of the most versatile hard templates is ordered mesoporous silica,since it can be prepared in shapes as diverse as noodle-like,spherical,fibrous,rodlike,and even with chiral morphol-ogies.[30–32]Another requirement for the template is the ability to remove it without affecting the cast.Possibilities are leach-ing with different agents,melting
(although this has not been used
yet,to the best of our knowledge),
or combustion,which is possible
with a carbon template.
The target material is usually not
incorporated in the pore system of
the template as such,but in the form
of a precursor that subsequently has
to be converted to the final material.
This precursor needs to meet several
requirements:As it must enter the
template structure,it must either be
gaseous,highly soluble,or liquid at
moderate conditions,so that it can
be infiltrated into the voids of the
template while achieving sufficiently
high loadings.Conversion to the de-
sired composition should be simple
and with as little volume shrinkage
as possible.Finally,it should not
chemically react with the hard
template.In addition,the templates
should be easily and completely re-moved by chemical(leaching,combustion)or physical(ther-
mal treatment)methods to obtain the true replicas.
3.Nanostructured Porous Materials Created by Nanocasting
3.1.Porous Carbon
3.1.1.Ordered Mesoporous Carbon
If one traces back the history of the hard-templated synthe-
sis of porous carbon,Knox et al.were probably the first to synthesize porous carbon via such a pathway,using silica gel
or porous glass as templates.[33]However,as the templates
had a disordered mesostructure,so had the replicas.The suc-
cessful synthesis of carbon with an ordered pore structure was
first achieved by Ryoo’s group in1999,whereby MCM-48was
used as a template to create a carbon material(CMK-1).[24] Previous attempts to replicate the narrower pore systems of
zeolites had given indications that a replication should be pos-
sible,but the materials lacked the perfection of the CMK-1 structure.[34]Starting with the initial publication from the
Ryoo group,many studies were carried out to synthesize mesoporous carbons with ordered structures.Table1gives a
survey over the types of carbon materials so far generated by nanocasting.
Generally,the synthetic procedure for ordered mesoporous
carbon(OMC)can be described as follows:Mesoporous silica
with a specific structure(as a template)is impregnated with a
carbon precursor(s)(including monomer and polymer)to give
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A.-H.Lu,F.Schüth/Nanocasting:Creating Nanostructured Porous Materials
Table1.Summary of the reported ordered mesoporous carbons(OMCs)generated by nanocasting.
OMC Space group Template Space group Precursor Reference
CMK-1 (SNU-1)I41/a
or lower
MCM-48Ia3d sucrose,phenol resin[24]
CMK-2Unknown
cubic
SBA-1Pm3n sucrose[35] CMK-3p6mm SBA-15p6mm sucrose,[53]
CMK-3 analogue p6mm HMS
MSU-H
SBA-3
MCM-41
p6mm sucrose,phenol resin,
furfuryl alcohol
[36–40]
CMK-4Ia3d MCM-48Ia3d acetylene[35] CMK-5p6mm SBA-15p6mm furfuryl alcohol[45,57,58,
61,63,64] NCC-1p6mm SBA-15p6mm furfuryl alcohol[67]
N-OMC p6mm SBA-15p6mm acrylonitrile,pyrrole[50,51]
G-CMK-3p6mm SBA-15p6mm acenaphthene,benzene,
mesophase pitches,
pyrrole,
poly(vinyl chloride)
[72–76]
OMC(cubic)Ia3d KIT-6,FDU-5Ia3d sucrose,furfuryl alcohol[41,70] OMC(cubic)unknown FDU-12Fm3m sucrose[42]
OMC(cubic)Im3m SBA-16Im3m sucrose,furfuryl alcohol,
acenaphthene [43,44]
Template Composite Replica Figure1.Schematic illustration of the nanocasting pathway.
the desired composition.Subsequent polymerization (in some cases,curing or stabilization steps are needed)and carboniza-tion of the precursor in the pore system result in a carbon–sili-ca composite.Finally,a replica mesoporous carbon can be ob-tained after removing the silica template by leaching.One should keep in mind that carbon precursors should be selected such that they have a high carbon yield and do not simply decompose during the carbonization step.This ensures that the pores of the template remain relatively well filled,and that the final product is really a replica of the silica templates.Suitable carbon precursors were found to be sucrose,[24]furfuryl alcohol,[45]phenol resin,[46]mesophase pitch,[47,48]polydivinylbenzene,[49]acrylonitrile,[50]pyrrole,[51]etc.
All available data indicate that the structure of the resulting carbon is indeed determined by the parent template.[52]For instance,CMK-n carbons were templated from different mesoporous silicas,such as CMK-1(I 41/32or lower)from MCM-48,CMK-2from SBA-1,and CMK-3(p 6mm )from SBA-15.[53–55]Nanocasting,however,is not only a method for producing novel materials.The finding of a nanocast carbon-copy (CMK-3)made with SBA-15as a template allows conclusions to be made with respect to the structure of the SBA-15:it incidentally proves that the mesopores in SBA-15are most probably interconnected through the walls via micro-pores,because otherwise the stability of the carbon replica could not be explained.An unconnected packing of carbon rods that would result from the replication of an unconnected honeycomb pore structure should fall apart,and should not retain its ordered arrangement as soon as the template struc-ture is removed.This interpore connectivity of the mesopores in SBA-15must be rather well developed,because it is not only the structure on the nanometer scale that is reproduced:even the morphology of the resulting CMK-3corresponds to the rodlike or noodlelike shapes of the original SBA-15tem-plate.[31]Simultaneous control of pore size and morphology was recently demonstrated for the example of SBA-15repli-cated by a chemical vapor deposition (CVD)process with acetonitrile.[56]The resulting carbon was graphitic,the extent of carbonization dependent on the processing conditions,most importantly on the treatment temperature.
Interestingly,by varying the filling degree of the carbon precursor in the pore system of a mesoporous silica,the struc-ture of the resulting carbon can be varied.If the pore system of the SBA-15is only coated by the carbon precursor instead of completely filling it,a surface-templated mesoporous car-bon,named CMK-5,with an array of hollow carbon tubes is obtained.[45]A transmission electron microscopy (TEM)im-age of CMK-5is presented in Figure 2a.The removal of the silica template then results in two different types of pores in the CMK-5matrix.One type of pore is generated in the inner part of the channels that are not filled with carbon precursor.The other type of pore is obtained from the spaces where the silica walls of the SBA-15template had previously been.Since there are two different mechanisms for pore generation,it should be possible to control the properties of the two pore systems independently.Moreover,due to the fact that the
tubular structure exhibits two—an inner and an outer—sur-faces,CMK-5can reach very high surface areas and large pore volumes,thus making this a potential material for use in adsorption and catalyst-support applications.A first example is described in the initial publication on CMK-5,where it had been used as a support for the anode catalyst in a proton ex-change membrane (PEM)fuel cell.[45]To explore the synthe-sis space of CMK-5-type carbon more comprehensively,sever-al groups have studied its synthesis and have reported their synthetic procedures,such as controlling the polymerization temperature and time,[57–62]introducing the carbon precursor by catalytic CVD,[63]and varying the concentration of furfuryl alcohol.[64]To synthesize bimodal porous carbons,a new method was reported,i.e.,a combination of the nanocasting and imprinting strategies.[65,66]In principle,the pore sizes of the resulting carbons can be tuned by choosing different silica colloid particles and mesoporous silica.
To create mesoporous carbons with larger pore diameters (>4nm),we synthesized NCC-1carbon,which is essentially similar to CMK-5and has a bimodal pore size distribution and high pore volume.[67]The crucial factors for the synthesis of such carbons are an aging temperature of 140°C for the template SBA-15,a relatively low concentration of furfuryl alcohol (25vol %),and a carbonization temperature higher than 750°C.[68]In Figure 3,the TEM images shows that two pore systems can be clearly identified for NCC-1.The iso-therms of such carbons have a pronounced double hysteresis loop,as seen in Figure 3.This demonstrates the existence of a bimodal pore system,with the step at lower relative pressure corresponding to the pores left by the silica template,and the step at higher pressure to the pores in the inner part of the nanotubes.To synthesize porous carbon with larger pores,Hyeon and co-workers reported the synthesis of mesocellular silica foam with uniform and large mesocells (>20nm)by using cellular,foamlike molecular-sieve silica as the tem-plate.[69]
Structures other than the ones initially used have now been https://www.sodocs.net/doc/7f14996028.html,rge-pore ordered silica,KIT-6,with cubic Ia 3d symmetry was synthesized in a triblock-copolymer–butanol mixture.[70]The pore size of this material can be easily tuned from 4to 12nm via hydrothermal https://www.sodocs.net/doc/7f14996028.html,ing this silica as a hard template,rodlike (CMK-8)or tubelike (CMK-9)
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b
a Figure 2.a)TEM image of CMK-5with a tubular structure.b)Schematic depiction of this structure.Reprinted with permission from [45].Copy-right 2001(Nature Publishing Group).
mesoporous carbons,maintaining the same symmetry (Ia 3d )as the parent silica,were synthesized,depending on the car-bon precursor.This is in contrast to the carbon synthesized using MCM-48as the template,where typically the symmetry is reduced upon replication.[35]This maintenance of the origi-nal symmetry was the first indication that the walls in this block-polymer-templated silica also have pores connecting the mesopore systems,as had previously been observed for SBA-15.More direct proof was later obtained by the Terasaki group using electron crystallography.[71]In contrast,however,to the case of SBA-15,where the micropores seem to be basi-cally disordered,the micropores connecting the two enantio-meric pore systems in the large-pore Ia 3d silica are ordered and are located on special flat points on the G-surface,[71]which separates the two pore systems.The carbon copy was found to be an exact replica of the silica pore system.Other syntheses of cubic mesoporous carbons are compiled in Table 1.
The first reported examples of nanocast carbons—and also most of the other nanocast carbon materials described in the literature—have amorphous carbon frameworks.However,a graphitic carbon structure on the atomic scale is highly desir-able for some possible applications.For instance,the electron-ic conductivity is much higher for graphitic carbon than for the amorphous material.Ryoo’s group was the first to synthe-
size porous carbon with graphitic framework structures (Fig.4)through in situ conversion of an aromatic com-pound—acenaphthene—to mesophase pitch inside the silica templates.[72]Later,Pinnavaia’s group also synthesized a
CMK-3-type carbon with graphitic walls.[73]The obtained pure-carbon replica shows high electronic conductivity com-pared to normal CMK-3carbon with amorphous https://www.sodocs.net/doc/7f14996028.html,ing mesophase pitch as the carbon precursor,Zhao’s group prepared OMC replicas with ordered graphitized struc-tures (2D hexagonal (p 6mm )or 3D bicontinuous cubic (Ia 3d )structures)via a one-step melting-impregnation method.[74]These carbon types show relatively low surface areas of 350m 2g –1and pore volumes of 0.4cm 3g –1,owing to their gra-phitized frameworks.Better textural properties (surface areas up to 1560m 2g –1)were achieved when polypyrrole was used as the carbon precursor,as this allows the creation of graphitic structures.[51]The pyrrole is oxidatively polymerized with FeCl 3,the amount of which can be used to control the loading before pyrolysis.However,one should bear in mind that the presence of nitrogen in the precursor will lead to nitrogen-containing carbon materials,and not to pure carbons (see below).Fuertes and Centeno reported the synthesis of meso-porous carbon with improved graphitization using mesopo-rous silica as the template,pyrrole as the carbon precursor,and FeCl 3as both an oxidant and catalyst for the graphitiza-tion of an amorphous carbon.[75]As in other graphitic meso-porous carbons,the electrical conductivity was markedly improved compared to the nongraphitic case (0.14S cm –1compared to 0.003S cm –1).In addition,Fuertes and Alvarez reported that mesoporous carbon with well-developed gra-phitic order and a surface area of 260m 2g –1can be prepared using poly(vinyl chloride)as the carbon precursor.The poly-(vinyl chloride)was infiltrated into the pores of mesoporous silica,followed by carbonization,removal of the template,and graphitization of this carbon at 2300°C.[76]However,even though mesoporosity with pores in the size range of the ordered material was retained,the long-range order had collapsed in samples treated under such harsh conditions.In addition,graphitic porous carbons with a wide variety of textural properties were obtained by using a silica xerogel as
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A.-H.Lu,F.Schüth /Nanocasting:Creating Nanostructured Porous Materials
p/p 0
V a d (c m 3g ?1 S T P )
10 nm
100 nm
500 nm Figure 3.Nitrogen sorption isotherm (inset pore size distribution;V ad :volume,D :pore diameter,p /p 0:partial pressure,STP:standard tempera-ture and pressure)and TEM images of the mesoporous carbon NCC-1.Reprinted from [67].
5 nm
30 nm Figure 4.TEM images of CMK-3-type graphitic carbon (left)and its photomagnification (right),and the corresponding electron diffraction pattern (inset).Reprinted from [72].
the template,a phenolic resin as the carbon precursor,and metals (Fe,Ni,or Mn)as the catalyst.[77]
3.1.2.Ordered Mesoporous Carbon with Additional Framework Components
OMCs with other frameworks,i.e.,nitrogen-containing ske-letons,also attract research interest.[50,51]One might find many new properties and applications for such carbons,owing to their special chemical and physical features.OMCs con-taining nitrogen groups have been synthesized,using mesopo-rous silica SBA-15as the template,polyacrylonitrile (PAN)as the carbon source,and 2,2′-azobisisobutyronitrile as the initi-ator.[50]A series of steps,i.e.,stabilization,carbonization,and removal of the silica template,leads to the formation of PAN-based ordered mesoporous carbon.By changing the sta-bilization and carbonization temperatures,PAN-based OMCs with either monomodal or bimodal pore size distributions can be prepared.In the case of PAN-based OMCs with a bimodal pore size distribution,the connectivity between adjacent pores was improved owing to the randomly distributed,in-completely coated pore walls.By combining the pore connec-tivity and the surface functionality,such PAN-based OMCs could become promising materials for use as adsorbents and catalysts.Interestingly,as is known for other PAN-based but disordered carbon materials,the nitrogen content and the type of nitrogen species present in the material can be controlled by the treatment temperature.[78]On increasing the treatment temperature,the nitrogen content strongly de-creases,and the major species changes from a pyridine-like nitrogen species to pyridinium ions.An alternative approach to nitrogen-containing carbons is provided by using pyrrole vapor as the precursor.[51]Since the polymerization proceeds oxidatively,the loading with polypyrrole can be rationally determined by controlling the amount of pre-impregnated Fe III species,which acts as the oxidant to induce the formation of radical cations (C 4NH 5+.).
A fluorinated carbon with an ordered mesoporous structure was synthesized by reacting OMC obtained from nanocasting with fluorine at room temperature.[79]Strictly speaking,this fluorinated carbon framework was synthesized by post-treat-ment with fluorine gas,rather than directly from nanocasting;since this is an interesting surface functionality,the material should nevertheless be mentioned here.On increasing the reaction temperature,the color of the fluorinated carbon gradually changed from black to dark brown to gray to white.The mesostructure of the carbon gradually degrades during the treatment,because the fluorine reacts with unsaturated carbon atoms to form sp 3-hybridized carbon atoms,which inevitably leads to an increase in bond length as well as the dimension of carbon framework.The fluorinated carbon could have potential applications in electrochemistry or in batteries.
Ordered carbons that possess order on longer scales cannot be prepared by casting from surfactant-templated silica,since
the templates are no longer accessible.If repeat distances of the order of several tens of nanometers in the carbons are desired,then colloidal-crystal templating has to be chosen.Colloidal crystals consisting of silica spheres have thus been used as templates to prepare ordered macroporous carbons with 3D structures and lower degrees of graphitization by CVD at 750–850°C.[80,81]Highly graphitized,ordered nanopo-rous carbon (Fig.5)was achieved by graphitization (2500°C)of the pitch-based carbon obtained by using silica colloidal crystals as template.[82]This carbon has spherical pores on the borderline between mesopores and macropores,i.e.,40–
100nm,indicating that it retains some of the features of the template.However,the disadvantage of colloidal-crystal templating to date is the fact that a much lower diversity of template structure is accessible,since,at least for extended colloidal crystals,they mostly have densely packed structures.
3.1.3.Monolithic Carbon
From the viewpoint of many practical applications,mono-lithic carbons are easier to handle than powdered materials.The shaping of carbon materials is often rather difficult,ow-ing to the elastic properties of the grains,and,therefore,direct shaping of the materials during their generation can be highly advantageous.In general,carbon monoliths are fabricated by extrusion or,directly,by wet chemistry (sol–gel process).[1,83]However,fine tuning of the pore structure of the carbon monoliths is difficult to achieve using these pathways,and they also have the disadvantage of needing binders and addi-tives in the cases of extrusion,or supercritical drying/freeze-drying during the sol–gel process.[84,85]In comparison,the na-nocasting pathway from shaped precursors provides an oppor-tunity to create monolithic carbons with an ordered or hier-archical structure,and the pore sizes are tailorable to some extent,if the integrity of the template can be maintained in the cast carbon.[86–88]
Monolithic mesoporous carbon with a bicontinuous cubic structure (Ia 3d symmetry)was prepared by using mesoporous silica monoliths as the hard template (Fig.6).[85]This mono-lithic carbon shows a uniform pore size of 4.6nm and a sur-face area of 1530m 2g –1,and was described as a promising
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Figure 5.TEM images of ordered nanoporous carbon graphitized under argon at 2500°C.Reprinted with permission from [82].Copyright 2005(American Chemical Society).
electrode for electrochemical double-layer capacitors.The difficulty in preparing such carbon monoliths is,in fact,the preparation of the noncracked silica monoliths needed as tem-plates.Owing to the stresses involved in the preparation, cracking of the structures can occur at different stages,the most sensitive ones being the drying stage,due to capillary forces,and the calcination stage,due to the temperature-in-duced stress and/or pressure developing inside the monoliths during the decomposition and combustion of the organic ma-
terial in the pore system.Thus,the preparation of monolithic mesoporous carbon with an ordered mesostructure is still a great challenge.
Related to the synthesis proposed by Nakanishi,[89]silica monoliths with a hierarchical structure containing macropores and mesopores can be prepared by adding poly(ethylene gly-col)and/or hexadecyltrimethylammonium bromide as a poro-gen.[90]Using such silica monoliths as templates and furfuryl alcohol or sucrose as a carbon precursor,carbon monoliths with well-developed porosity are accessible.[88–93]Interest-ingly,the pore system of the nanocast carbon monoliths can be varied to three-or four-modal porosity by varying the load-ing amount of furfuryl alcohol in the one-step impregnation. Regardless of the loading with the carbon precursor,the ob-tained carbon monolith is a positive replica of the silica mono-lith on the micrometer scale,and a negative replica on the nanometer scale,as shown in https://www.sodocs.net/doc/7f14996028.html,bined volume and surface templating,together with the controlled synthesis of the starting silica monoliths used as the scaffold,provides a flexible means of pore-size control on several length scales simultaneously.[87,88,91]
3.2.Metal Oxides,Metals,and Other Inorganic Materials
3.2.1.Ordered Silica as the Template
As discussed in the Introduction,using a surfactant as a template to synthesize metal oxides often leads to the loss of the ordered structure after removal of the https://www.sodocs.net/doc/7f14996028.html,ing mesoporous silica as a hard template to create an ordered metal oxide is an alternative method that helps to circumvent the problems in templating with surfactants.The synthesis of a wide variety of materials by this technique is a very interest-
ing topic,both for fundamental research,concerning the ba-
sics of a replication on the nanometer length scale,and for the production of materials with desired properties.The require-
ments for such a casting process to be successful have already
been addressed in the Introduction.
The surface functionality of the silica template seems to be
crucial for obtaining a high-quality replica structure.This is probably due to the wetting behavior of the parent material,
since only if the precursor—and also the primary products of
the precursor conversion towards the final materials—has a favorable interaction with the wall of the template will a fully coherent material be formed.After all,one should bear in
mind that a high interfacial area is created in the nanocasting process.Even if the interfacial energies are only of the order
of some hundred joules per square meter,then energy contri-
butions of some tens of kilojoules per mole will result,if the interface area is1000m2g–1and the molar mass is above
100g mol–1,as is frequently found for oxides.Thus,in many
cases,unmodified silica was found to be unsuitable for the replication of certain materials.
For instance,for the production of porous chromium oxide
single crystals it was necessary to use aminopropyltriethoxysi-
lane-functionalized SBA-15as the template and H2Cr2O7as
the chromium precursor,which was chemically adsorbed. Subsequent thermal treatment and template removal by an aqueous hydrofluoric acid(HF)solution led to the desired product.[94]In this synthesis,calcination temperatures exceed-
ing350°C were needed in order to create the crystalline
phase.This material is believed to have potential as a catalyst
with high activity,owing to its relatively large surface area
(58m2g–1)and possible shape-selective properties.Zhao and
co-workers have demonstrated that microwave-digested3D
REVIEW
A.-H.Lu,F.Schüth/Nanocasting:Creating Nanostructured Porous Materials
Figure6.Photographs of monolithic mesoporous silica(left,2.1cm di-ameter;0.3cm thick)and mesoporous carbon(right,1.7cm diameter;
0.2cm thick).Reprinted with permission from[85].Copyright2002(Roy-al Society of
Chemistry).
Figure7.Scanning electron microscopy(SEM)images(upper),photo-
graph(lower,left),and TEM image(lower,right)of silica and carbon
monoliths.
mesoporous silica can be used as a hard template to fabricate various ordered crystalline gyroidal metal oxides,such as In 2O 3,Co 3O 4,Mn 2O 3,and CeO 2.[95–97]The microwave diges-tion treatment probably leads to an enhanced number of sila-nols,which provides the surface functionality needed for the incorporation of the precursor species.A representative TEM image of cubic In 2O 3is shown in Figure 8.These ordered met-al oxides have large surface areas of 50–140m 2g –1and exhibit single crystallinity in larger domains.Recently in our group,
nanocast ordered mesoporous Co 3O 4with the spinel structure was synthesized,wherein vinyl-functionalized large-pore Ia 3d silica was used as the template and Co(NO 3)2was the cobalt precursor.[98]Although replication of neat silica was mean-while also found to be possible,the vinyl-functionalized pre-cursor species provided an easier processing method to the cast cobalt oxide.It is believed that the vinyl groups in the sili-ca play a role in bonding the Co 2+ions in the pores.This mate-rial is antiferromagnetic,shows a weak ferromagnetic transi-tion at low temperatures,and a negative exchange bias that is found in ferromagnetic/antiferromagnetic coupled systems.This process seems to be extendable to other metal oxides,and has recently been used to produce ordered mesoporous ferromagnetic CoFe 2O 4.It is also possible to synthesize the hexagonally ordered Co 3O 4by casting from SBA-15(the product also having interesting magnetic behavior).[99]
In addition,metal sulfide nanowires,such as CdS,ZnS,and In 2S 3,with ordered mesostructure were synthesized by a
nanocasting process using mesoporous SBA-15as a hard tem-plate.[100]The incorporation of metal and sulfur precursors can be carried out by one-step impregnation.These nanowires are polycrystalline and are discussed due to their potential application in optical and electronic devices.
However,the use of silica as the hard template is limited,because it can only be leached under strongly alkaline condi-tions or by HF.However,such a leaching process is not com-patible with many different oxides,since they are also at-tacked by these reagents.Carbon templates provide an interesting alternative to silica,since carbon can be removed by combustion or treatment with other highly reactive gases.
3.2.2.Carbon as the Template
Porous carbons can be used as templates to generate high-surface-area inorganic solids,owing to their special properties,such as high porosity,high thermal stability,and easy removal by combustion.[10,101]In principle,an ordered mesoporous car-bon,such as CMK-3,could be used as a template to construct other compositions with ordered mesoporous structures.The realization of this idea was reported almost simultaneously by two independent groups.[102,103]Tetraethoxysilane (a silicon source)can be conveniently infiltrated into the pore system of CMK-3-type carbon,and hydrolysis can be initiated by treat-ment with a solution of HCl.A repeated impregnation proce-dure is necessary in order to achieve the desired loading.After inducing silanol condensation to an as-complete-as-pos-sible extent by thermal treatment at 700°C under a nitrogen flow,the composites are calcined at 550°C in air,which removes the carbon,producing a white powder,designated as NCS-1.Figure 9shows the nanocasting procedure and the corresponding TEM images of the template and final https://www.sodocs.net/doc/7f14996028.html,bining the X-ray diffraction (XRD)and nitrogen sorp-tion analyses,it can be verified that NCS-1does replicate the ordered structure of CMK-3and exhibits strong structural similarities to SBA-15,even if there are differences in detail.[104]Another example is the synthesis of a new silica mesostructure,HUM-1,by nanocasting from ordered meso-porous carbon that had been obtained via replication of the MCM-48silica.The structure of the final silica had a different symmetry than the parent silica.[105,106]While this multistep route is inefficient for the production of mesoporous silicates because they are more easily accessible by the cooperative pathway,it is a viable method for the synthesis of other com-positions that are more difficult to template directly with sur-factants.Roggenbuck and Tiemann have demonstrated the flexibility of this type of carbon templating for the production of magnesium https://www.sodocs.net/doc/7f14996028.html,ing CMK-3carbon as a template,obtained via a nanocasting pathway,they synthesized mesopo-rous magnesium oxide with hexagonal p 6mm symmetry by repeated nanocasting from the carbon.[107]One should note that the synthesis of basic oxides,such as MgO,is difficult fol-lowing the solution-mediated pathway using surfactants,since the solubilities of these more basic oxides are typically rather
R E V I E W
A.-H.Lu,F.Schüth /Nanocasting:Creating Nanostructured Porous Materials
Figure 8.TEM images of the cubic Ia 3d mesostructured In 2O 3framework along the a)[100],b)[111],and c)[311]directions.d)TEM image of the cubic (possibly I 432)mesostructured In 2O 3framework along the [111]di-rection.Reprinted with permission from [95].Copyright 2003(American Chemical Society).
high under the synthetic conditions used.Subsequently,using
tri(methylamino)borazine as the boron nitride source and
CMK-3as the template,ordered mesoporous boron nitride with a specific surface area of 500m 2g –1was synthesized,
where the carbon template was removed by a high-tempera-ture ammonia treatment.[108]
We have attempted to use CMK-3as a template to synthesize mesoporous alumina.However,
such alumina does not maintain the ordered structure,as is
the case in NCS-1,even though the resulting alumina is meso-porous.[109]
This is mainly due to the complicated phase transi-tion of alumina during the calcination process.Recently,
porous metal oxides,such as Al 2O 3,TiO 2,ZrO 2,V 2O 5,etc.,
with high thermal stabilities as well as crystalline frameworks
were prepared via the above nanocasting technique.[110]
How-ever,characterization of the materials showed that,although
some of the structural features of the parent templates could
be transferred to the casts,the degree of structural order was
appreciably lower than in the parent materials.
A catalytically quite interesting material is ZnO,since it is a
major component in catalysts for methanol synthesis and
methanol steam reforming.Direct synthesis of mesostruc-tured ZnO by solution techniques is very difficult,and so far
only thin films have been obtained.Synthesis of ZnO by the
nanocasting technique from carbon is not straightforward
either,since ZnO is relatively easily to reduce,and the carbon
in contact with ZnO at high temperatures could lead to inter-mediate reduction.Recently,Polarz et al.succeeded in
synthesizing an ordered mesoporous ZnO by replication from
a PAN-based CMK-3-type material.TEM analysis revealed
the ordered structure,and surface areas came close to
200m 2g –1.
[111]
A substantial fraction of the publications on ordered meso-porous materials is devoted to the control of morphology,i.e.,
the synthesis of films,spheres,fibers,etc.This interest has also
led to similar work following nanocasting strategies.Monodis-perse mesoporous inorganic spheres have been synthesized by
the nanocasting route by using mesoporous carbon spheres,
which were nanocast from mesoporous silica spheres,as the
template.[112]Single-component (ZrO 2,TiO 2,and
2O 3)and multicomponent mesoporous oxides 3O y and Ti 2ZrO y ),and metal phosphates
and AlP)can be prepared with surface areas
from 100to 400m 2g –1.Another approach preparing inorganic materials with mesoporosity a nanocasting pathway is the use of car-bon aerogels as templates.Although their porosity
is not ordered,they can be synthesized with adjust-able pore size distributions that are rather narrow
compared to many other materials.Such aerogels have been employed to prepare porous MgO,[113]glassy Al 2O 3,[114]and monolithic zeolite sam-ples [115]via the nanocasting process.The successful synthesis of inorganic porous materials verifies that
carbon aerogel is an alternative template.Since the pore structure of carbon aerogel can be varied to some extent during the synthesis,it is expected that
inorganic porous materials with many other compositions will be prepared in the near future.3.2.3.Colloidal Crystals as the Template Macroporous solids have potential applications as optical crystals,catalysts,supports,sensors,and porous electrodes or electrolytes.The synthesis of ordered macroporous colloidal crystals via the replication of ordered array structures of poly-styrene or silica lattices has received much attention in phys-ics,chemistry,and materials science.Covering the progress achieved in this field would exceed by far the scope of this review;however,the processes used are in fact nanocasting steps.More details can be found in the reviews by Velev and Lenhoff [116]and Stein.[117]Here we only wish to highlight the principle that such syn-theses can also be considered and generalized as nanocasting processes.As seen from Figure 10,colloidal crystals are first formed by packing uniform spheres into 3D or 2D arrays.Then the interstitial space of the colloid crystals is filled with liquid precursor that is subsequently converted into a solid skeleton.Removal of the spheres leads to the generation of a solid skeleton in the location of the former interstitial spaces and interconnected voids where the spheres were originally located.Periodic porous solids with various compositions,in-cluding silicates and organosilicates,metal oxides,metals,metal chalcogenides,and carbon allotropes,have been pre-pared by nanocasting from colloidal crystals.The resulting materials could be interesting for various applications in pho-tonics.[116,117]3.3.Functionalized Porous Solids by Nanocasting As discussed above,many new materials have been created by the nanocasting strategy.Removal of the template leads to the negative replica of the primary template.However,the REVIEW
A.-H.Lu,F.Schüth /Nanocasting:Creating Nanostructured Porous Materials
SBA-15
CMK -3
NCS-1
Figure 9.Illustration of the nanocasting procedure for NCS-1and the corresponding
TEM images of SBA-15,CMK-3,and NCS-1.Reprinted from [104].
nanocasting pathway can be modified to also provide the pos-sibility for functionalization of the inner or outer surface of porous materials in a controlled manner.If the pores of such a material can be filled reversibly,then one has the equivalent of the protection-group strategy used in organic synthesis for the functionalization of porous solids.A pore could thus be filled with a blocking agent,then the material could be modi-fied in the unblocked part,and finally the blocking agent could be removed to make the unmodified part of the surface accessible again.
Ryoo’s group has confirmed that the surface of ordered nanoporous carbon (CMK-3)can be nanocast by an organic polymer,for example polystyrene.[118]The resultant materials,exhibiting surface properties of the polymers,as well as the electrical conductivity of the carbon framework,could pro-vide new possibilities for advanced applications.Such a strat-egy can furthermore be extended to other inorganic tem-plates,such as mesoporous silicas.[119]
Most of the as-made porous carbon materials have particle sizes in the sub-micrometer range.These carbons are notor-iously difficult to separate from solution,and thus magnetic separation is an attractive alternative to filtration or centrifu-gation and has therefore been high on the wish list in catalysis for a long time.[120]We have established a series of consecutive manipulation steps to fabricate magnetically separable or-dered mesoporous carbons,on which magnetic nanoparticles were selectively deposited on the outer surface of the car-bons—the pore system was left blocked during the modifica-tion,and the nanoparticles were protected by a nanometer-thick carbon layer.[121]The overall synthetic strategy and typi-cal TEM images of such a magnetically separable carbon are presented in Figures 11and 12,respectively.Such magnetic nanocomposites have very high surface areas,large pore vol-umes,and uniform pore sizes.Applications of this porous car-bon as magnetically separable adsorbents and catalysts have been demonstrated.However,one may also envisage other applications;for example,as a magnetically directable drug carrier.If a drug could be loaded onto the porous carbon,one
could possibly accumulate the magnetic particles in the target area in the organism,and then induce release of the drug by magnetic heating (in an alternating magnetic field)of the par-ticles.[122]
To deposit magnetic nanoparticles spatially on the outer surface of mesoporous silica while simultaneously keeping the
R E V I E
W
A.-H.Lu,F.Schüth /Nanocasting:Creating Nanostructured Porous Materials
Figure 10.General synthesis scheme for or-dered macroporous solids and the correspond-ing SEM images for a polystyrene/silica system prepared with tetraethylorthosilicate.Reprinted with permission from [117].Copyright 2001(Elsevier).
(2)
(1)
Figure 11.Illustration of the synthesis procedure of magnetically separ-able carbon:a)ordered mesoporous silica SBA-15;b)carbon/SBA-15composite;c)(b)with surface-deposited cobalt nanoparticles;d)pro-tected cobalt nanoparticles on (c);e)magnetically ordered mesoporous carbon;f)Pd on (e).1)Carbonization of the carbon precursor in SBA-15;2)incorporation of cobalt nanoparticles on (b);3)coating of carbon on cobalt nanoparticles;4)dissolution of silica to create pore system;5)loading of Pd in pores to introduce catalytic function.Reprinted from [121].
Figure 12.TEM images of magnetically separable carbon at a)low and b)high magnifications.Reprinted from [121].
pore system open,we have developed and used the strategy of reversible polymer protection of the silica pore system, which was briefly mentioned in the introductory paragraph to this section.[123]This also creates the possibility of functiona-lizing the inner surface of the mesoporous silica.
CMK-5has a bimodal pore system,the pores of which are generated at different stages,i.e.,during the carbonization and silica-removal steps.This provides the possibility for inde-pendently modifying the two pore systems in order to create a material with specific physical and chemical properties.Re-cently,we have chemically modified the surfaces of the inner and outer sides of the CMK-5tubes selectively by nitric acid oxidation under moderate conditions followed by esterifica-tion or alkylation.The results will be reported in detail in the near future.
An alternative possibility to keep the pore system open is the modification of the pore walls by introducing foreign atoms,molecules,or nanoclusters.This strategy has been widely used for the synthesis of organosilica materials.We have synthesized,by the nanocasting pathway,Pd supported on ordered mesoporous carbon,as revealed by EDX analysis, where highly temperature-stable,molecular-level dispersed Pd clusters(below the detection limit of approximately1nm) are uniformly embedded in the carbon walls.[124]Although this material was pyrolyzed at temperatures up to750°C,no visi-ble Pd clusters were formed in the carbon walls,as revealed by high-resolution TEM.The catalytic activity of Pd-OMC was tested in the oxidation of an alcohol(to an aldehyde) using supercritical CO2as the reaction medium.Alcohols,in-cluding benzyl alcohol,1-phenylethanol,and cinnamyl alco-hol,were used as the substrates.Under the conditions used, the selectivity to the corresponding aldehyde was in all cases higher than99%.No acid was detected as a reaction product, indicating that the catalyst is highly selective for the conver-sion of the alcohols to the corresponding aldehydes.
Another development in the field of nanocast silica is the homogeneous incorporation of metallic nanoparticles(Pd,Pt, Ru)into the solid-silica skeleton by nanocasting from a3D array of polystyrene spheres.Cyclodextrin as an additive in this process plays two roles:inclusion of the metallic nanopar-ticles and homogeneous dispersion in the silica sol through hydrogen bonding.[125]The resulting silica material shows a bimodal pore structure,which might be interesting for cataly-sis because it allows fast mass transfer owing to the larger pores,while also maintaining the smaller pore system.
4.Conclusion and Outlook
Nanostructured porous materials created by the nanocast-ing strategy,especially using hard templates,can be produced as monoliths or powders with ordered or disordered struc-tures,which mainly depend on the structures of the primary templates.The range of the templates applied in nanocasting already extends from silica to carbon,the latter being easily removed by simple https://www.sodocs.net/doc/7f14996028.html,ing the nanocasting strat-egy,one can create negative—or,after repeated nanocasting, positive—replicas that can preserve the fine structural details
of the template.It has been demonstrated in principle that
this technique allows the generation of porous materials on
the nanometer scale with partly variable textural parameters.
The systematic exploration of the nanocasting pathway will
add a new dimension to the fabrication of many porous inor-
ganic materials by using different hard templates.In particu-
lar,in the synthesis of multicomponent metal oxides,sulfides,
or alloys,the nanocasting strategy is probably the most prom-
ising approach—as compared to the commonly employed surfactant-assisted strategies—for the synthesis of ordered
porous solids.Many compositions will not be accessible in mesostructured form by techniques other than nanocasting.In addition,the mostly expensive alkoxides used in solution-
based processes can be replaced by metal nitrates,chlorides, sulfides,or acetates as raw materials,thus also avoiding the difficulties in the control of the hydrolysis rate of the alkox-
ides.However,if ordered mesoporous carbon is used as the template,one still needs to synthesize the carbon template in
the first synthetic step,which is a major drawback.
There are still a number of remaining challenges:For many
target compositions,the chemistry of the target material is not compatible with the conditions of the template-removal pro-
cess,be it leaching or combustion.Increasing the loading of
the template with as much precursor as possible is also a chal-
lenge.This is necessary to ensure a rigid structure,thus avoid-
ing collapse of the pore system after removal of the mold.Ba-
sically,a very high concentration of the precursor solution,or,
if at all possible,the neat precursor,is highly recommended. Although the replication of non-silica materials does not seem
to be as straightforward as for silica,it can be foreseen that
these obstacles will be overcome in the coming years by con-
tinued research effort in this rapidly developing field.It is al-
most certain that the nanocasting pathway will be extended to
many other compositions that are not accessible by solution-
based methods,and that this technique will become a stan-
dard tool in the tool box for the synthesis of ordered porous materials.
Received:January23,2006
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