colloidal crystal templating of three-dimensionally ordered

8/10/2019 Colloidal Crystal Templating of Three-dimensionally Ordered

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5 (2001) 553564Current Opinion in Solid State and Materials Science

Colloidal crystal templating of three-dimensionally orderedmacroporous solids: materials for photonics and beyond

*Andreas Stein , Rick C. SchrodenDepartment of Chemistry,University of Minnesota, 207Pleasant St.SE,Minneapolis,MN 55455,US A

Abstract

This review discusses strategies for the synthesis of three-dimensionally ordered macroporous (3DOM) solids (inverse opals) by

colloidal crystal templating. Compositions of 3DOM structures include simple and ternary oxides, chalcogenides, non-metallic and

metallic elements, hybrid organo-silicates, and polymers. A wide range of 3DOM synthesis techniques, including solgel chemistry,

polymerization, salt-precipitation and chemical conversion, chemical vapor deposition, spray pyrolysis, ion spraying, laser spraying,nanocrystal deposition and sintering, oxide and salt reduction, electrodeposition, electroless deposition, fabrication from core-shell

spheres, and patterning methods, as well as templating using inverse opal molds to produce new opal compositions are reviewed. Potential

uses of 3DOM solids, including photonic crystal, optical, catalytic, and bioglass applications are briefly discussed. 2002 Elsevier

Science Ltd. All rights reserved.

Keywords:Colloidal crystal templating; Three-dimensionally ordered macroporous solids; Photonics; Inverse opals

1. Introduction plate to obtain a porous inverse replica. Fig. 1 shows

electron micrographs of typical 3DOM structures. Pore

Methods for shaping and structuring solids into func- sizes of a few hundred nanometers, together with the order

tional objects have been developed and improved to create of the pore structure, endow 3DOM materials with optical

increasingly more complex features since the fabrication of and photonic crystal properties, which may be utilized inearly tools. Macroscopic features have traditionally been waveguides, low-threshold lasers, and sensors. With highly

attained by physical or mechanical methods, but as fea- accessible surfaces and large pore sizes these materials

tures on nanoscopic length scales have become more may be useful for chromatography, catalysis, and as

important, chemical approaches have made significant bioactive materials. Multiple pore sizes may permit selec-

contributions. To achieve further structural complexity, tive uptake, stabilization, separation, or release of small

physical, chemical, and engineering approaches toward and large guest molecules. Furthermore, the skeletal

materials fabrication must converge. Novel multidisciplin- dimensions can be small enough to produce size-dependent

ary approaches toward the synthesis of hierarchically properties (i.e., nanosize effects). Combinations of these

structured, functional materials are now being developed. properties within a given structure may lead to new

One such class of materials is three-dimensionally ordered multifunctional materials.

macroporous (3DOM) solids. These materials have been The colloidal crystal templating approach is very general

developed in parallel in different research communities, and can be applied to solgel, salt solution, CVD, electro-including chemists, physicists, and engineers. Syntheses chemical, nanocrystalline, and other precursors to produce

and potential applications of 3DOM solids will be the 3DOM insulators, semiconductors, and metals of many

focus of this review. different compositions. The range of 3DOM materials

The general concept of colloidal crystal templating is prepared so far includes simple oxides [111], ternary

simple: form a colloidal crystal of close-packed, uniformly oxides [5,1214], chalcogenides [1517], non-metallic and

sized spheres, fill the interstitial spaces with a fluid metallic elements [1826], alloys [27,28], hybrid organo-

precursor capable of solidification, and remove the tem- silicates [5], and polymers [2938]. Despite the overall

simplicity and generality of colloidal crystal templating,

optimization of chemical processes and precursor/template*Corresponding author. Tel.: 11-612-624-1802; fax: 11-612-626-

interactions must be tailored for each class of precursor to7541.E-mail address:[email protected] (A. Stein). control the structure and substructure of the framework.

1359-0286/02/$ see front matter 2002 Elsevier Science Ltd. All rights reserved.PII: S1359-0286(01)00022-5


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5 (2001) 553564554 A.Stein,R.C. Schroden /Current Opinion in Solid State and Materials Science

The Synthesis section of this review is, therefore, orga-

nized by class of reaction used for synthesis. A brief

discussion of potential applications for 3DOM materials

follows. The reader is also referred to a number of related

reviews that have recently been published

[10,16,22,29,3943].

2. Synthesis

2.1. The colloidal crystal template

The colloidal crystal templates used for the generation

of 3DOM materials are prepared from monodisperse silica

or polymer spheres, including polystyrene (PS) and poly-

(methyl methacrylate) (PMMA). The spheres can be

arranged into close-packed structures by many methods

(for recent reviews, see Refs. [44,45]) including gravity

sedimentation, centrifugation, vertical deposition, tem-

plated deposition, electrophoresis, patterning, and con-

trolled drying, which provide 26 vol% void space for

penetration by fluid precursors. Multilayers or gradient

layers of spheres with different radii have also been grown

[46,47]. Colloidal crystals can either be used directly as

templates, or annealed or sintered to increase their stability

and ensure interconnection.

The surface chemistry of the sphere templates influences

framework formation. Strong wetting interactions aid

penetration and formation of a continuous network. Strong

interactions with dilute precursors typically lead to films

around the spheres, whose thickness can be increased by

multiple exposures [11]. Framework solidification can be

affected by catalytic groups, such as adventitious car-boxylic acid groups on PS spheres which catalyze hy-

drolysis and condensation of solgel precursors [5], and

thiol-groups on silica spheres which serve as anchors for

nanocrystalline metals that catalyze further metal deposi-

tion [19].

Porous structures are produced by removal of the

templates. With polymers, this is often carried out by

calcination simultaneously with conversion of the pre-

cursor to a solid in the desired phase. If solidification is

feasible at low temperatures spheres can also be extracted

with appropriate solvents, such as toluene or tetrahydro-

furan/ acetone mixtures. Photo-degradation of polymer

templates has also been noted [38]. Silica sphere templates

are removed by dissolution in aqueous HF solutions.

Fig. 1. (A) Scanning electron micrograph (SEM) of a polystyrene (PS)2.2. Solgel chemistry

colloidal crystal. (B) Low magnification SEM of a single particle of

3DOM titania. (C) High magnification SEM of 3DOM silica showing

close-packed macropores, which are interconnected through smaller Alkoxide precursors and alkoxide/ metal acetate mix-windows. The white regions are walls of the first layer, the gray regions tures have been employed to create 3DOM metal oxides byare walls of the second layer, and the black regions are voids. (D) solgel chemistry (Fig. 2). Precursors are used directly [2]Transmission electron micrograph (TEM) of 3DOM silica, which has

or diluted in alcohol, in which case repeated filling may beamorphous walls. The dark regions are the silica walls. (E) TEM of

necessary to form an interconnected network [4]. As layers3DOM nickel, which has nanocrystalline walls. Small grains (appearingas dark spots) fuse together to form the 3DOM skeleton. build up on the sphere surfaces, they can cause pore


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5 (2001) 553564 555A.Stein,R.C. Schroden /Current Opinion in Solid State and Materials Science

whereas cubic alumina formed at 10008C (Blanford CF,

Carter CB, Stein A, unpublished results). An a-alumina

phase was prepared from an aluminum nitrate/ ammonium

hydroxide solution after calcination at 11008C [49].

In addition to the polymer sphere colloidal crystals used

above, silica templates have been investigated. 3DOM

titania films were formed by filling silica sphere opals on a

quartz substrate with an alcoholic/ aqueous solution oftitanium ethoxide [48]. The material was calcined at

temperatures up to 10508C to effect densification of the

anatase walls, causing significant shrinkage, similar to

observations with polymer-based templates.

Multiple levels of porosity can be attained for macro-

porous silicates by employing dual templating methods.

3DOM silica prepared from alkoxide precursors contains

amorphous walls with a wide distribution of mesopores

from 2 to 40 nm [5]. Addition of cetyltrimethylammonium

hydroxide surfactant to the synthesis gel for 3DOM silica,Fig. 2. Schematic of the preparation of ordered macroporous metal followed by calcination, produced a structure with macro-oxides by solgel chemistry. Latex colloidal crystal templates are pores (a few hundred nm) and uniform mesopores (,4infiltrated with solgel precursors, dried, then calcined to remove the nm), and increased the surface area from 200 to over 1300template and cure the walls of the macroporous solid. Adapted from Ref.

2m / g [5]. Other surfactants, including amphiphilic tri-[2].block-copolymers, have also been employed as secondary

templates [5052]. Dual templating methods have also

blockage, resulting in small openings at the intersections of yielded macroporous zeolites [53]. In this case, tetra-

the solid [4,9,11]. Precursors may be prehydrolyzed before propylammonium hydroxide [54] and PS were used to

infiltration [1,3], or hydrolysis can be induced by moisture produce 3DOM materials with thin (20220 nm) silicalite

within the template or atmosphere [2,5]. Hydrolysis and walls and 0.5 nm micropores around larger spheroidal

condensation can be catalyzed by acidic surface groups on voids. This material combines the advantages of facile

the spheres [2,5], by added acid or base, or by cationic transport through the macropores, selectivity of the zeolitic

surfactants on the polymer sphere surfaces [1,3]. The final micropores, and short diffusion paths for potential guest

void dimensions are often smaller than the original sphere species.

diameters (typically 1530%) due to a large volume loss The solgel process is also amenable to the synthesis ofduring the solgel process as alcohol is lost. hybrid organicinorganic macroporous silicates. 3DOM

Examples of oxides prepared by solgel methods in- silicates with organic surface functional groups or organic

clude those of Si [1,3,5], Ti [2,4,5,9,11,48], Zr [5], Al linkers within a silicate framework can be prepared from

[2,5,6,49], W, Fe, and Sb [5,6]. Additional elements can be organosiloxanes RSi(OR9) or bis(organosiloxanes)3

incorporated either by subsequent doping (e.g., incorpora- (R9O) SiRSi(OR9) , in a manner similar to mesoporous3 3

tion of the spinel Co TiO into 3DOM titania [7]), or by sieves [55], but using colloidal crystals rather than surfac-2 4

employing mixed metal precursors, such as mixed alkox- tants as templates [5]. Examples include macroporous

ides (e.g., to prepare yttria-stabilized zirconia [5]) or silicates with vinyl- or cyanoethyl-surface groups [5], and

alkoxide/ acetate mixtures (e.g., titanium butoxide mixed macroporous organosilicates, in which Si framework atoms

with barium acetate or lead acetate to produce macro- are linked by CH CH groups (Melde BJ, Skugrud K,2 2

porous BaTiO [14] or PbTiO [13]). Stein A, unpublished results). Solvent extraction methods3 3

For a given composition, the processing temperature are usually required for template removal to prevent loss of

during template removal influences the crystallographic organic functional groups (exception: phenyl groups [56]).

phase of the 3DOM walls. In nanocrystalline materials, if Organic functionalities have also been directly incorpo-

grains grow to dimensions comparable to the macropore rated in hierarchical meso/macroporous structures by

size, the periodicity is reduced and eventually lost. Thus, to using dual templating (surfactant and PS) with the addition

maintain an ordered 3DOM structure, grain growth must of a dye-functionalized precursor to the synthesis gel [57].

be carefully controlled by choice of reaction temperature

and use of chemical additives [10]. For example, amor- 2.3. Polymerization

phous titania formed from the alkoxide upon solvent

extraction of the template, whereas anatase formed after Polymerization of organic precursors within silica or

calcination at 4508C [5]. Similarly, amorphous alumina polymer colloidal crystal templates affords mesoporous

walls were obtained by calcination at 4508C [2,5,6], and macroporous polymers. In typical syntheses the sphere


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arrays are filled with a liquid monomer, which is polymer- saturated salt solution [4]. Templated precipitation of metal

ized and/ or crosslinked by thermal treatment, exposure to salts (acetates, oxalates, oxides) within a polymer colloidal

UV light, or catalysis. In this manner, 3DOM compositions crystal and subsequent chemical conversion provides a

including poly(divinylbenzene) (PDVB), poly- route to macroporous metal oxides, metals, and metal

(ethyleneglycol dimethacrylate) (PEDMA), polyurethane, carbonates [8,10,20]. For metal oxides, an annealed PS or

poly(acrylate-methacrylate), PMMA, poly(methyl acrylate) PMMA colloidal crystal is immersed in a metal salt

(PMA), PS, poly(p-phenylenevinylene) (PPV), epoxy, and solution. If the metal salt has a low melting point (e.g.,

epoxy resin have been prepared [3034,36,38]. The me- certain metal acetate hydrates) it is converted to a metalchanical and physical properties of the porous products oxalate by treatment with oxalic acid solution. Upon

depend on the material composition and processing con- calcination, ordered metal oxides or carbonates (Co O ,3 4

ditions. For example, mesoporous PDVB replicas were Cr O , Fe O , MgO, Mn O , NiO, ZnO, CaCO ) are2 3 2 3 2 3 3

rigid, while the PEDMA replicas were flexible [31]. In obtained in an oxidizing atmosphere, and metals (Ni and

mixtures of PDVB and PEDMA the final pore size Co) are obtained in an inert atmosphere [60]. Macroporous

depends on the ratio of the monomers. The window size in Ni prepared in this manner has a surface area exceeding2

3DOM polyurethane was controlled by adjusting the 200 m /g, and ignites if exposed to air rapidly [60].

polymerization temperature [33]. 3DOM alloys have been prepared by the same method

Instead of in situ polymerization, it is possible to using mixed precursors [27].

prepare 3DOM polymers by filling colloidal crystals with Grain sizes of 3DOM materials, which affect the order,

solutions of preformed polymers. For example, a ferroelec- surface area, mechanical, and optical properties, can be

tric 3DOM polymer structure was obtained by infiltrating controlled by thermal and chemical methods. For Fe O ,2 3

poly(vinylidene fluoride-trifluoroethylene) copolymer solu- the oxalate-based synthesis produced materials with small-

tions (in cyclohexanone) into silica opals at 1551608C er grains and more ordered products than the solgel

(slightly above the boiling point of the solvent and above synthesis [6,8]. In situ transmission electron microscopy

the melting point of the copolymer) followed by solvent (TEM) demonstrated the effect of temperature on the

evaporation [37]. A similar method was applied to the structure of 3DOM Fe O prepared from iron oxalate2 3

synthesis of a macroporous conducting polymer [35]. A precursors [10]. When heated to 6508C grains grew 50%

macroporous conductive glassy carbon was synthesized by within 15 min, and more than doubled in size by 7508C,

filling the voids of an artificial opal with a phenolic resin, while maintaining an ordered structure. Chemical control

curing, and removing the silica spheres with HF [18]. of grain size has been demonstrated with 3DOM zirconia

Pyrolysis at temperatures up to 10008C converted the samples. Average grain sizes of 1030 nm were obtained

framework to glassy carbon. with zirconium n-propoxide precursors [2], compared to

A completely different approach to synthesize macro- only a few nm for zirconium oxalate precursors with

porous polymers involves polystyrene/ poly(2-hydroxy- sulfuric acid as an additive [10,61].ethyl methacrylate) (PS/polyHEMA) sphere colloidal crys-

tals containing PS-rich cores and polyHEMA-rich shells 2.5. Chemical vapor deposition

[58]. The colloidal crystal arrays were exposed to styrene

or toluene vapor (good solvents for PS but poor solvents Various chemical vapor deposition (CVD) techniques

for polyHEMA) to extract PS from the structure, producing have been employed to create 3DOM structures involving

ordered porous polymer nets. group 14 elements. Graphitic carbon was obtained using

An interesting synthesis of ordered macroporous poly- CVD with propylene around silica spheres, with period-

mers uses thermocapillary convection for templating, icity, pore size, and degree of carbonization dependent on

instead of preformed templates [59]. Hexagonally ordered temperature [18]. These properties affected electrical con-

porous structures are formed when dilute PS solutions in a ductivity, magnetoconductance, and optical spectra of the

volatile solvent are cast on a flat support and exposed to materials [62]. A dielectric diamond film was prepared on

moist air flowing across the surface. The velocity of a macroporous carbon skeleton by seeding opals with

airflow controlled pore dimensions in the range from 0.2 to nanocrystalline diamond, which acted as nucleation sites

20 mm. for further diamond formation, and depositing carbon from

a hydrogen/methane plasma [18].

2.4. Salt-precipitation and chemical conversion 3DOM Si with a refractive index contrast high enough

to obtain a complete photonic bandgap was prepared by

Precipitation and crystallization reactions within a col- exposing a sintered silica opal to disilane [23]. The filling

loidal crystal template provide an alternative to condensa- factor increased with deposition temperature between 250

tion or polymerization processes. This procedure is less and 3508C with complete filling by Si nanoparticles

sensitive to atmospheric humidity and permits formation of achieved at higher temperatures. Annealing at 6008C

ordered structures for compositions difficult to prepare by produced a connected silicon skeleton that maintained its

solgel methods. 3DOM NaCl can be prepared from a periodicity after template removal with HF.


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Room temperature CVD, which allows for the use of

PMMA opal templates, was used to prepare thin films

(2040 monolayers) of macroporous SnS [63,64]. The2

colloidal crystal was exposed to SnCl vapor and H S gas4 2

at ambient temperature and pressure to produce inter-

connected SnS nanoparticles that remained stable during2

PMMA extraction with THF. Due to the relatively loose

structure of nanoparticles within the walls, the refractiveindex was only ca. half that of bulk SnS (n 53.2).

2 bulk

2.6. Spraying techniques

Various spraying techniques, including spray pyrolysis

[65], ion spraying [66], and laser spraying [66], have been

successfully used in the preparation of periodic macro-

porous structures. These techniques are most suitable for

thin film preparation, since penetration is limited with

spraying. 2D macroporous titania (anatase) was synthes-

ized by using spray pyrolysis to deposit titanyl acetylaceto-

nate in ethanol solution on an array of silica spheres on a

flat glass support, annealing, and removing silica by HF

[65]. Ion spraying was employed for the preparation of 2D

macroporous Au [66]. Pulsed laser deposition was used to

create macroporous silicon films with a thin layer of

surface oxide [66].

2.7. Nanocrystal deposition and sintering

To minimize shrinking and cracking of 3DOM struc-

tures during template removal, methods using preformed

nanoparticle precursors have been developed (Fig. 3). The

smaller nanoparticles are codeposited with colloidal Fig. 3. Schematic of the formation of ordered macroporous solids from

nanocrystalline precursors. Latex or silica spheres and preformedspheres, filling the interstitial spaces. Because thenanoparticles settle into a colloidal crystal of spheres filled withnanoparticles are already in their desired phase, shrinkagenanoparticles. The nanoparticles are fused together by heating and the

of the pores is limited to 510%. Products prepared by thistemplate is removed to produce the macroporous solid.

method range from insulators [67] to semiconductors [12]

and metals [21]. Thin films (1020 mm) of macroporous

titania prepared from nanocrystalline titania and PS

spheres, contained ordered domains up to 503100 mm bulk CdSe (12398C). Even before sintering, the semi-

[6870]. The degree of order depended on the drying rate conductor framework remained self-supporting upon tem-

and orientation of the flat substrate during deposition, with plate removal due to van der Waals interactions between

best quality for vertical deposition and worst for horizontal the nanoparticles.

[67]. Macroporous gold has been prepared from colloidal gold

Hierarchical pore structures can be created by assem- particles (1525 nm) by various methods [21,24,26]. In

bling pre-synthesized zeolitic nanocrystallites in polymer one approach, macroporous gold flakes were obtained by

templates [71,72]. For example, silicalite has been pre- depositing colloidal gold particles within a latex sphere

pared in colloidal particles (3080 nm) and infiltrated in colloidal crystal by filtration. By calcination, smooth gold

PS colloidal crystal templates. After calcination a sample surfaces were obtained, whereas extraction produced walls

containing zeolite micropores, templated macropores, and with interconnected grains. In another method, highly

mesopores between intercrystalline voids was obtained. ordered products were obtained by dispensing a liquid

In the preparation of 3DOM CdSe from nanocrystals, a mixture of latex and gold nanoparticles between two glass

size dependent melting point was observed [12]. CdSe slides and dragging one slide across the other. In another

nanocrystals were slowly infiltrated into silica opals over technique, a colloidal crystal containing gold particles

12 months. Sintering at 6008508C and HF etching nucleated outward from the center of a concave meniscus

produced CdSe inverted opals. Melting of the nanoparti- in an enclosed cell, eventually filling the entire cell to form

cles occurred at 6508C, much below the melting point of thin films of macroporous gold.


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2.8. Oxide and salt reduction

A range of syntheses exist for preparing 3DOM metals

and other elemental compositions in their reduced states.

One approach involves the reduction of 3DOM metal

oxides in a hydrogen atmosphere, e.g., reduction of NiO to

Ni [20]. During reduction the wall grains tend to grow,

resulting in a lower surface area than the starting oxide.Nonetheless, the periodic structure is maintained. The

surface areas of samples prepared by this method are lower

than those of 3DOM metals synthesized directly from

oxalate precursors (see above).

3DOM Ge was prepared from the oxide by hydrogen

reduction using a silica opal template that was not removed

until the final reduced product formed [28]. First, a

tetramethoxygermane precursor was infiltrated into the

opal to form GeO . During subsequent reduction in2

hydrogen at 5508C, grain growth resulted in poor connec-

tivity between the grains. The GeO and Ge formation2

processes were therefore repeated several times to maxi-

mize filling and connectivity. After template removal by

HF etching the 3DOM Ge product exhibited a high

dielectric contrast, an important property for photonic

crystal applications (see later).

A related method involved reduction of salts within a

colloidal crystal template [73]. Silica colloidal crystals

were filled with aqueous solutions of H PtCl ?6H O or2 6 2

AuCl , dried, and reduced in hydrogen at 1508C. The3

filling and reduction steps were repeated to maximize

filling. Template removal produced mesoporous or macro-Fig. 4. Schematic of the general procedure for the synthesis of macro-porous Au and Pt. The Au sample was not as ordered asporous solids by electrodeposition. Colloidal crystals are assembled ontothe porous Pt, presumably due to less wetting of the silica

an electrode and immersed in an electrobath, along with a countersurface. electrode. The interstitial space of the colloidal crystal fills uponapplication of a potential. Template removal produces the macroporous

2.9. Electrodeposition solid.

Electrochemical methods have been employed to syn-

thesize macroporous metals, alloys, semiconductors, and Gold deposited potentiostatically from HAuCl solution4

conducting polymers (Fig. 4). In electrodeposition re- produced ordered structures with silica colloidal crystals;

actions, the colloidal crystal templates are deposited on a however, only small gold flakes with a few macropores

conducting substrate (e.g., ITO-coated or gold-coated were obtained with PS templates [25]. Gold structures

glass), and a counterelectrode (e.g., Pt) is placed above the prepared by constant current electrodeposition using com-

sample. Alternately, a conducting film (e.g., Cu) is de- mercial gold plating solutions and a silica opal template

posited on one side of millimeter-sized opal pieces [74]. collapsed after template etching [74]. Thin films of ordered

Growth can occur either galvanostatically or potentiostati- macroporous Pt, Pd, and Co have been prepared with PS22cally [15,17]. Electrodeposition affords good control over templates by electrochemical reduction of [PtCl ] ,

622

the degree of filling, wall thickness, and window size. [PdCl ] , or [Co(Ac) ] in aqueous solution [76]. 3DOM4 2

Macroporous metal chalcogenide preparations include Ni has been prepared by electrodeposition from a plating

the galvanostatic deposition of CdS (from DMF or DMSO bath containing NiSO ?6H O, H BO , and NH Cl [66], or4 2 3 3 4

solutions of sulfur and CdCl ) in silica opals, and poten- from a commercial plating solution [74]. A macroporous2

tiostatic growth of CdS (from CdCl and Na S O ) and SnCo alloy was synthesized from a plating bath containing2 2 2 3

CdSe (from CdSO , H SO , and SeO ) in PS colloidal SnCl , CoCl , and K P O ?3H O [66].4 2 4 2 2 2 4 2 7 2

crystals [15,17]. Nearly complete filling was achieved, and Electrochemical polymerizations have produced electri-

shrinkage was below 2% for silica and 8% for PS cally conducting macroporous poly(pyrrole), poly(aniline),

templates after removal. The macroporous semiconductor and poly(bithiophene) around PS or silica templates

ZnO can also be prepared by electrodeposition [75]. [77,78]. Shrinkage was observed for poly(pyrrole) and


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poly(aniline) samples during drying, but less so for poly-

(bithiophene) which formed collections of hollow polymer

spheres rather than a bicontinuous 3DOM structure [77].

The preparation of 3DOM conductors has been motivated

by the desire to increase their performance in redox

reactions by improving mass transfer kinetics between

liquid and solid phases.

2.10. Electroless deposition

3DOM metals can be synthesized by electroless deposi-

tion, i.e., without an electric circuit, by using templates

modified with catalysts for metal plating. This approach

has been used for the synthesis of 3DOM Ag, Au, Co, Cu,

Ni, and Pt [19]. Thiol-functionalized colloidal silica on

glass slides is immersed into a solution of gold nanocryst-

als (which act as catalysts), annealed, then immersed into

metal plating baths. Macroporous films are obtained after

HF etching. 3DOM Ni formed free-standing films, whereas

Ag and Au films fragmented.

Another electroless approach, oxidation polymerization,

has been employed for the synthesis of 3DOM poly-

(aniline) [79]. Aqueous aniline hydrochloride was infil-

trated into a wet PS colloidal crystal template where in situ

oxidation by K S O , followed by template removal in2 2 8

THF, yielded porous poly(aniline). In situ polymerization

is particularly useful for the preparation of macroporous

conducting polymers, since they tend to have low solu-

bility in common solvents making them difficult to intro-

duce directly into a colloidal crystal array.

2.11. Fabrication from coreshell spheres

Using coreshell spheres as templates allows good

control of the 3DOM wall thickness [80]. Macroporous

titania was produced with this method by coating PS

spheres with polyelectrolyte layers, packing by centrifuga-

tion, infiltration with titanium isopropoxide and dryingFig. 5. Schematic of the fabrication of macroporous silicates with zeolitic(multiple cycles), and calcination. The wall thicknesswalls by the coreshell building block approach. Latex spheres areincreased with the number of polyelectrolyte layers. Whenprimed with a polyelectrolyte, then layers of oppositely charged zeolitePS spheres coated with layers of polyelectrolyte and silicananoparticles (silicalite) and polyelectrolyte are alternately deposited until

nanoparticles were used as a template, macroporous TiO /2 a desired wall thickness is achieved. Centrifugation produces a close-

SiO was obtained. This approach also lends itself to the packed structure, and calcination removes the organic components,2leaving a macroporous solid. Adapted from Ref. [81].formation of macroporous silicates with zeolitic walls (Fig.

5) [81]. Alternate deposition of negatively charged zeolite

nanoparticles (silicalite) and positively charged polyelec-

trolyte layers (poly(diallyldimethylammonium chloride)), this approach, PDMS stamps with micrometer-scale pat-

produced materials with continuous networks of relatively terns are placed on a Si substrate, creating accessible

ordered macropores, random mesoporosity between the channels which can be filled with latex spheres. These

zeolite grains, and the microporosity of silicalite. arrays are infiltrated with suitable solgel precursors, and

after sufficient drying and condensation time, the mold is

2.12. Patterning removed, and organic components are eliminated by

calcination. One particularly interesting example involved

Designed features beyond macropores can be produced solgels containing block-copolymers for the creation of

by combining sphere templating with micromolding using patterned structures with mesoporous walls around macro-

polydimethylsiloxane (PDMS) stamps (Fig. 6) [82,83]. In pores.


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were obtained in cases with weak interactions between

precursors and the polymer template (e.g., for TiO /2

PMMA), while hollow spheres resulted if the polymer

template was wetted strongly by the precursors (e.g., for

TiO /PS). Multi-composition shells could be generated by2

sequential deposition of different precursors. Distortion of

the polymer template permitted fabrication of ellipsoidal

colloids. Non-polymeric 3DOM templates have also beenemployed, including 3DOM NiO for electrochemical tem-

plating of gold sphere arrays [86], and macroporous

conductive carbon for templating low melting metals (Bi,

Pb, Sb, Te) near their melting points [86].

3. Materials properties and potential applications

3.1. Optical applications

Dielectric 3DOM structures are promising materials for

photonic crystals materials with foreseeable applica-

tions involving the control of photons (waveguides, mi-

crocavity lasers, inhibitors of light emission) [39]. In order

to obtain the complete photonic bandgaps desired for these

applications various requirements, including sub-microme-

ter dimensions, low solid fractions, high refractive index

contrast (ca. 3) [87], optical transparency, and specific 3D

periodicity must be met. Control of dimensions and low

solid fractions are relatively easily met with colloidal

crystal templating. A more difficult aspect involves the

choice of wall material. 3DOM titania has a transparent

window in the visible, but in the form of anatase it exhibits

a broad stop band, not a full bandgap [4,9,11,48,8890].Fig. 6. Schematic of a patterning method to form hierarchically struc-

tured macroporous solids. Colloidal crystal growth on a substrate is The optical stop band width and peak attenuation aredirected by a patterned PDMS stamp. Addition of a solgel precursor, controllable by the number of layers and the dielectricfollowed by stamp removal and calcination yields the patterned macro- contrast between walls and voids [34]. Complete bandgapsporous solid. Adapted from Ref. [82].

are possible with 3DOM Si [23] or Ge [28], but absorp-

tions in the visible spectrum limit applications to the

2.13. Inverse opal templating near-IR region. Other materials with sufficiently high

refractive index in bulk form (e.g., SnS ) often exhibit2

3DOM materials (or inverse opals), generated by much lower refractive indices in 3DOM solids [63].

templating with opal structures, may in turn be used as Perhaps the most difficult requirement to achieve syn-

templates to form another opal replica. Using this templat- thetically is better control of the periodicity with limited or

ing procedure a mesoporous polymer mold, that had been controlled defects. Numerical simulations have shown that

prepared from silica colloidal crystals, was used to produce 2% fluctuations in lattice constant can close a photonic

small silica replica particles [31]. The order of the silica bandgap even for high refractive index contrasts [91,92].

replicas was limited, possibly due to the relatively small Most of these challenges can be addressed by process

pore sizes in the mold. A more ordered replica opal optimization, and chemical syntheses of photonic crystals

composed of mesoporous silica was obtained by infiltrating are likely to remain viable competitors to lithographic

a 3DOM PS mold with a solgel precursor for mesoporous techniques.

sieves, followed by PS removal by solvent extraction or Several optical properties with less stringent structural

calcination [84]. This method permits assembly of ordered requirements have been investigated. One involves

mesoporous sieve particles with controlled diameters and changes in color (stop band position and intensity) as a

dispersities. A similar approach was used to form spherical 3DOM material is filled with fluids of varying refractive

and ellipsoidal colloids of a wide range of compositions index [61]. The stop band position varies nearly linearly

(TiO , ZrO , Al O , polypyrrole, PV, CdS, AgCl, Au, Ni) with refractive index of the penetrating fluid, and can be2 2 2 3

with narrow size distributions [85]. Solid colloidal crystals controlled by adjusting the pore spacing, and the thickness,


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density, and composition of the walls. If luminescent dyes 3.3. Bioglass applicationsare incorporated in the structure [57,93], or the 3DOMstructure exhibits intrinsic luminescence [94], overlaps Bioactive glasses are materials that can bond to bonebetween stop bands and emission bands can be used to through a hydroxy apatite layer that spontaneously depositssuppress the luminescence in a controlled manner [95]. on their surface in a body environment. They may beWhen optically birefringent nematic liquid crystals are surface reactive or bioresorbable and can be used as boneinfiltrated in 3DOM Si, it is possible to tune the photonic graft substitutes. Conventionally, these materials are pre-

bandgaps by aligning the liquid crystals in an electric field pared from the melt of glass components or by solgel[96]. A promising new application of thin macroporous methods [99]. Recently, the colloidal crystal templatinggold films involves surface-enhanced Raman spectroscopy method was adapted to the synthesis of bioactive glass[24,26]. The periodicity of the structure appeared to play structures with uniform macropores (3DOM CaO/ SiO )

2

an important role in enhancing the Raman signal of probe [100]. When immersed in simulated body fluids (resem-molecules, as the signal intensity with a macroporous film bling the inorganic composition of human blood), apatitewas twice that of an unstructured film. growth occurred more rapidly for 3DOM bioactive glasses

than for the corresponding control samples without macro-3.2. Catalytic applications pores. Amorphous calcium phosphate clusters formed on

the 3DOM walls within 3 h and hydroxy apatite aggregatesSimilar to other silicates, 3DOM silica can be used as a replaced much of the original structure after 4 days. The

support for catalytically active species. A recent study excellent bioactivity was attributed to the highly accessiblecompared the influence of pore structure on the activity of surface of the 3DOM materials.various silica samples modified with transition metal-sub-

stituted polyoxometalates (TMSP) [97]. Clusters of theII 52

type [Co (H O)PW O ] and 4. Conclusions and outlook2 11 39II 102

[SiW O hCo (H O)j ] were chemically anchored to9 37 2 3

amine-functionalized macroporous (350450 nm), The development of the new field of colloidal crystalmesoporous (3060 A), and non-porous silica surfaces. templating has occurred very rapidly over the last few

Whereas the mesoporous silica support had the highest years. The field is driven not only by the aesthetics of the

surface area, partially restricted access of the clusters to the resulting 3DOM structures and the versatility of the

mesopore channels limited the cluster loading. The more method, but also by the exciting properties that 3DOM

open macroporous structure supported more TMSP clusters materials promise to exhibit for the benefit of several key

per amine anchoring group. Because the clusters were technologies. Studies of physical and chemical properties

attached datively to the surface, they were retained in of 3DOM solids are just emerging, as control over the

catalytic reactions involving the epoxidation of cyclohex- quality of the materials is being improved. This review hasene to cyclohexene oxide, with comparable conversion and described many techniques for the preparation of macro-

reaction rates on all three supports. porous solids of numerous compositions. Often a particular

Redox active g-decatungstosilicate clusters (g-SiW O ) composition can be obtained by more than one route, but10 36

were incorporated into the wall structures of 3DOM silica the phase, nanostructure, and properties may vary with the

by a direct synthesis approach [98]. The clusters were production method.

reacted with TEOS, with or without addition of 1,2- In order to tailor better materials, it will be necessary to

bis(triethoxysilyl)ethane as a linking group, and the mix- gain greater understanding of several issues. Much re-

ture was molded by a polymer colloidal crystal, using the search is currently aimed at advances in colloidal crys-

methods described above. The linking group connected the tallization techniques, including defect minimization and

clusters covalently to the silica support. When the polymer control. So far, only a limited number of template com-

spheres were removed by extraction, the intact polyox- positions (mainly silica, PS, PMMA) have been employed.

ometalate clusters remained attached to the hybrid 3DOM New template compositions may be beneficial for easier

structures and were highly dispersed throughout the walls. template removal, or possibly for providing additional

The materials exhibited catalytic activity for the epoxida- functionality. Advances will be possible with a better

tion of cyclooctene. understanding of interactions between the templates and

3DOM a-alumina was used as a support for metallic precursors. Functional groups on the sphere surfaces

silver and was tested as a catalyst for the conversion of influence wetting, penetration, and solidification to allow

ethylene to ethylene oxide [49]. The productivity (mass of more uniform products. This chemistry influences sample

ethylene oxide produced per hour per gram of catalyst) shrinkage during template removal and grain size, which in

was significantly greater for this macroporous support than turn affects the smoothness, density, effective refractive

for a commercial silver-modified a-alumina catalyst, while index, reactive surface area of the wall, and mechanical

selectivity and conversion were slightly higher for the properties. As these synthetic issues are addressed, 3DOM

commercial material. materials will move closer to desired applications.


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chemical synthesis of periodic macroporous NiO and metallic Ni.AcknowledgementsAdv Mater 1999;11:10036.

[21] Velev OD, Tessier PM, Lenhoff AM, Kaler EW. A class of porousPortions of the work described here were funded by 3M,

metallic nanostructures. Nature 1999;401:548.Dupont, the David & Lucile Packard Foundation, the [22] Kulinowski KM, Jiang P,Vaswani H, ColvinVL. Porous metals fromMcKnight Foundation, the NSF (DMR-9701507) and the colloidal templates. Adv Mater 2000;12:8338.

[23] Blanco A, Chomski E, Grabtchak S, Ibisate M, John S, Leonard SW,MRSEC Program of the NSF under Award Number DMR-Lopez C, Meseguer F, Miguez H, Mondia JP, Ozin GA, Toader O,9809364.van Driel HM. Large-scale synthesis of a silicon photonic crystal

with a complete three-dimensional bandgap near 1.5 micrometres.

Nature 2000;405:43740.

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