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DOI: 10.1002/adma.200800700

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Template Synthesis of HierarchicallyStructured Composites**

By Wei Wei and Zhenzhong Yang*

The subtle performance of a virus is closely related to its specifichierarchical structure, which is composed of a rigid shell and transverse,responsive, nanometer-sized channels. Virus-like structured colloidsare of great interest for their potential applications, for example in drug delivery. Adequateknowledge of the structure and composition control of both colloids andmesoporous materials issignificant in the design and synthesis of hierarchically structured colloids to mimic viruses. Somerecent developments in the synthesis of composite colloids and mesoporous materials aresummarized. Template synthesis is a major tool to control both the macroscopic morphologyand microstructures of these composites, in which gel colloids and supramolecular structuresfrom amphiphilic species are used as templates.

1. Introduction

Natural species possesses unique, fascinating functions and

properties, which arise from their hierarchical structures and

well-defined constituent spatial distribution within a specific

morphology. For example, a virus can be regarded as a core

(RNA) / shell (protein) structure, with responsive nanometer-

size channels across the shell, which are used to switch mass

transport on and off (Fig. 1).[1] Synthesis of such hierarchically

structured biomimetic composite colloids is becoming attrac-

tive for potential applications in absorption, drug delivery,

catalysis, etc. Viruses can be simply divided into two basic

topological building blocks: 1) a core/shell colloid and the

corresponding hollow sphere; 2) a mesoporous membrane.

Biomimetic colloids will be synthesized in the near future by

properly assembling the two building blocks. Our recent

[*] Prof. Z. Z. Yang, Dr. W. WeiState Key Laboratory of Polymer Physics and ChemistryInstitute of ChemistryChinese Academy of SciencesBeijing 100080 (P.R. China)E-mail: yangzz@iccas.ac.cn

[**] We gratefully acknowledge NSF of China for continuous financialsupport. Many students in our group are thanked for their greatcontributions. Yang ZZ thanks Prof. Yunfeng Lu of UCLA andProf. Zhibing Hu of UNT for fruitful discussions and collaboration.

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advances in the synthesis of both hollow spheres and

mesostructured materials will be summarized in this article.

From the viewpoint of materials chemistry, template

synthesis is the main tool used to control both the macroscopic

morphology and the microstructure. Both colloids and supra-

molecular structures assembled from amphiphilic species are

commonly used as templates to synthesize hollow spheres and

mesostructured materials. During the synthesis, polymeric gels

with a tunable physicochemical microenvironment play a

significant role, inducing favorable growth in desired sites by

specific interactions.

2. Results and Discussion

2.1. Hierarchically Structured Composite Core/Shell

Colloids and Hollow Spheres

Spheres or capsules with interior compartments (empty or

filled with desired materials) enveloped with a shell have

diverse applications in catalysis, delivery and controlled

release, microcavity resonance, photonic crystals, etc. It is

important to developmethods to control both the structure and

composition of the spheres. A template-free ‘‘one pot’’

approach for inorganic hollow spheres has been proposed,

in which Ostwald ripening under hydrothermal conditions

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Figure 1. The hierarchical structure of a model virus (reproduced fromwith permission from [1]. Copyright MacMillan Publishers 1998.): rigidprotein capsid and transverse nanometer-sized channels, which protrudeand are responsible for switching mass transport on and off. The two basicbuilding blocks of the virus are 1) a hollow sphere with controlled surfacestructure and 2) a responsive mesoporous membrane.

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followed by aggregation of the corresponding nanocrystallites

gives a spherical contour.[2] The Kirkendall effect has also been

employed to hollow out a colloid by an asymmetric diffusion-

Figure 2. Some representative hierarchically structured spheres. a) Silica-strengthened CNT/polyelectrolytecomposite hollow spheres. b) PS/epoxy resin composite spheres. c) Double-shelled titania hollow spheres withpillars protruding out of the surface. d) Ultra-microtomed double-shelled titania hollow spheres.

induced directional matter flow

and consequential vacancy accu-

mulation.[3] Recently, layer-by-

layer (LBL) deposition onto a

colloid template has been exten-

sively used to synthesize hollow

spheres, whose shell composi-

tion can be broadly tuned, ran-

ging from polymeric to inor-

ganic and metallic materials.[4]

Control of their surface micro-

structure has become a major

concern. Modified carbon nano-

tubes (CNTs) are coated onto a

polystyrene (PS) template par-

ticle, and a CNT nonwoven

fabric cage is formed with con-

trolled thickness and pore size

(Fig. 2a).[5] The cage is strength-

ened by introducing inorganic

materials by means of a sol-gel

process. A kind of raspberry-

structured composite sphere is

simply achieved by heterocoa-

gulation of a binary colloidal

mixture of smaller curable

epoxy resin on a larger PS

particle (Fig. 2b).[6] The shell

becomes more robust and inso-

luble after a subsequent coales-

cence of the shell colloids and

crosslinking. Roughness of the

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outer surface can be tuned by controlling the coalescence

degree, which is important to further control wettability, for

example superhydrophobicity, of the particles when they are

used in coatings.

Besides composition and surface structure control, synchro-

nous control of both cavity size and shell thickness is another

concern in order to precisely tune optical properties and

density. We have proposed a core–gel shell template synthesis

to solve the problem.[7] The shell gel is derived by an inward

sulfonation of a PS colloid. This allows the gel-shell thickness

to be controlled, ranging over the whole particle size, whilst the

PS core size eventually decreases to zero. Many materials

(polymer: polyaniline; inorganic: titania, silica; etc.) can be

induced to grow preferentially within the sulfonated PS (sPS)

gel shell by specific interactions, forming a composite shell,

while the PS core is not infiltrated by the materials. When the

PS core is removed, corresponding hollow spheres are

obtained. The shell thickness and cavity size of the hollow

spheres is thus controlled using different templates with

different gel-shell thickness. On the one hand, because it is a

kind of charged soft matter, sPS can be modulated to

experience an instable fluctuation upon application of an

external electric field, which is then fixated by a fast sol-gel

process, forming a porous titania shell. Alternatively, because

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it is a strong acid, sPS can induce favorable growth of

polyaniline with in situ doping to tune the conductivity.

It should be mentioned that the synthesis remains mainly

based on a core/shell approach. The osmotic pressure

associated with molecular diffusion during removal of the

core templates usually fractures the shell. It is a challenge to

preserve the shell intact. Recently, hollow spheres such as

polyelectrolyte capsules, viral capsids, or lipid vesicles have

been used as templates to synthesize composite hollow spheres.

However, those hollow templates are rather soft and easily

become deformed or even broken during complexation with

other materials. We realize that some commercial polymer

hollow latexes with promising composition and structural

characteristics can be used as robust hollow templates.[8] A

representative hollow sphere is composed of a PS shell

skeleton with transverse, nanometer-sized, polymeric gel

channels connecting the interior polymeric gel to the surface.

Some precursors can diffuse inwardly through the channels and

grow preferentially within the interior gel, forming a composite

hollow sphere. The conventional removal of core templates is

avoided. When the exterior polymer shell is removed from the

outside, the interior composite shell is less influenced by the

osmotic pressure, thus giving an intact shell. Additional

precursors can grow within the channels, and composite

hollow spheres with nanometer-sized pillars protruding from

the surface are prepared. If another material is further

deposited onto the composite hollow sphere, double-layered

hollow spheres are prepared (Fig. 2c).[8a] Such hollow spheres

have, for example, greater strength and enhanced photo-

catalytic performance. Many steps are involved in the synthesis

of the double-layered hollow spheres. The problem of how

such double-layered hollow spheres can be synthesized in one

step remains unsolved. Similar to the sulfonation of a PS

colloid, the shell of the PS hollow sphere can be transformed

into a sPS/PS/sPS sandwiched gel, and composite/PS/compo-

site sandwiched hollow spheres are thus derived by favorable

growth of the desiredmaterials within the gel. After removal of

the polymer template, corresponding double-layered hollow

spheres, for example of titania, are obtained (Fig. 2d).[8b] The

shell thickness and the gap can be tuned by controlling the

sulfonation degree, and a single-layered hollow sphere is

eventually achieved. This approach can be extended from

inorganic materials to polymers for example phenolic resinor

even their derivatives to form mesoporous hollow carbon

spheres.[8c] The PS shell of the hollow sphere can be swelled

with monomers, and a diversity of polymer hollow spheres with

an interpenetrating network shell can be generated after

polymerization.[9] Many monomers, for example acrylate,

acrylonitrile, vinylbenzyl chloride, and divinylbenzene, can be

used. For instance, when divinylbenzene is used and cross-

linked within the PS shell, the hollow sphere becomes more

robust and insoluble in solvents. After being completely

sulfonated, the sPS gel hollow spheres preserve their spherical

shape well, forming for some composites, for example,

magnetic/gel hollow spheres.[9a] In contrast, the hollow sphere

made of linear PS disintegrates after being completely

Adv. Mater. 2008, 20, 2965–2969 � 2008 WILEY-VCH Verl

sulfonated. The crosslinked sPS can catalyze itself into carbon

at high temperature, giving a mesoporous hollow carbon

sphere.[9b] Similarly, mesoporous carbon/silica and carbon/

metal composite hollow spheres have been produced (unpub-

lished work), which are attractive for fuel cells and hydrogen

storage. The hollow spheres can be loaded with reagents in the

cavity. Under proper conditions, the reagents diffuse out-

wardly through the channels, accompanied by solidification,

and complex hierarchically structured spheres are formed. In

particular, nanometer-size needles or networks can be formed

on the exterior surface. For example, a composite sphere with

hairy polyaniline (PANi) on the exterior surface is formed by

diffusion-limited polymerization of aniline through the

channels (unpublished work). This hierarchical structure

mimics the lotus leaf papillae.[10] By fixation of such spheres

by crosslinking epoxy resin onto a substrate, a robust

superhydrophobic coating is easily prepared by a facile post-

treatment with the proper organosilanes.

2.2. Mesoporous Materials

Mesoporous materials with high specific surface area and

uniform nanometer-sized pores are useful in many areas. They

are commonly template-synthesized using supramolecular

structures from amphiphilic species, for example surfactants

or block copolymers, whose pore size and microstructure can

be tuned.[11] In view of practical applications, such as in

separation, membrane reactors, and sensors, it is important

that macroscopic morphology control by proper processing can

result in particulates, thin films, and supported membranes.

Aerosol-assisted synthesis of mesoporous materials is effective

at controlling morphology ranging from individual colloids to a

continuous thin film (Fig. 3a).[12] Within an anodic alumina

porous membrane, one-dimensional uniform mesostructured

silica nanotubes or nanofibers have been synthesized, which is

controlled by the membrane pore surface wettability

(Fig. 3b).[13] If a desired material is encapsulated inside the

mesostructured nanotubes, it will be of interest to be able

trigger release through the mesopores. The mesostructure con-

formation is strongly related to an increased pore confine-

ment.[14] Such a mesoporous silica/alumina composite

membrane is useful in separation of small molecules. However,

the support alumina is too fragile to be processed into desired

shapes. Alternatively, a flexible mesoporous silica/polymer

composite membrane can be formed within a porous polymer

(PP)membrane (Fig. 3d).[15] Thesemembranes have some new

properties, such as transparency, uniform nanometer-sized

pores, and enhanced permeability. The composition of the

mesoporous materials can be controlled, thus affecting their

properties.[16] For example, functional materials, such as

thermochromatically responsive conjugated polymers (e.g.,

polydiacetylene, PDA), can be introduced to be used for

sensors (Fig. 3c).[17] The mesostructured material is thermally

responsive after poly(N-isopropylacrylamide) is incorpo-

rated.[18]

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SW. Wei, Z. Z. Yang / Template Synthesis of Hierarchically Structured Composites

Figure 3. Some representative mesostructuredmaterials. a) Mesostructured silica particles. b) Mesostructured silica nanotubes. c) Mesostructured PDA/silica nanocomposite thin film. d) Mesostructured PP/silica composite membrane.

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3. Conclusions and Perspectives

Thanks to sufficient knowledge about the structure and

composition control of both colloids and mesoporous

materials, the synthesis of colloids with hierarchical struc-

tures, for example virus-mimetic colloids, will become

possible. There have been some attempts to achieve

mesoporous hollow silica spheres by coating mesostructured

silica onto a polymer core template. These virus-like hollow

spheres with mesoporous shell are promising in drug delivery

and controlled release.[19] The following questions need to be

answered in the synthesis of virus-mimetic hierarchically

structured colloids: 1) How can responsive pore channels and

protruding pillars be created with the proper chemistry? 2)

How can the desired materials be encapsulated in the desired

shell in one step? 3) How can recognition, targeting, and

biocompatibility be achieved?:

[1] a) T. Douglass, M. Young, Nature 1998, 393, 152. b) T. Douglass, M.

Young, Adv. Mater. 1999, 11, 679.

www.advmat.de � 2008 WILEY-VCH Verlag GmbH &

[2] H.C. Zeng, J. Mater. Chem. 2006, 16, 649.

[3] H.G. Yang, H.C. Zeng, J. Phys. Chem. B 2004, 108, 3492.

[4] a) F. Caruso, R.A. Caruso, H. Mohwald, Science 1998, 282, 1111. b) F.

Caruso, Adv. Mater. 2001, 13, 11.

[5] L.J. Ji, J. Ma, C.G. Zhao, W. Wei, X.C. Wang, M.S. Yang, Y.F. Lu,

Z.Z. Yang, Chem. Commun. 2006, 1206.

[6] X.Y. Liu, Z.W. Niu, H.F. Xu, M.L. Guo, Z.Z. Yang,Macromol. Rapid

Commun. 2005, 26, 1002.

[7] a) Z.Z. Yang, Z.W. Niu, Y.F. Lu, Z.B. Hu, C.C. Han, Angew. Chem.

Int. Ed. 2003, 42, 1943. b) Z.W. Niu, Z.Z. Yang, Z.B. Hu, Y.F. Lu, C.C.

Han, Adv. Funct. Mater. 2003, 13, 949.

[8] a) M. Yang, J. Ma, Z.W. Niu, X. Dong, H.F. Xu, Z.K. Meng, Z.G. Jin,

Y.F. Lu, Z.B. Hu, Z.Z. Yang, Adv. Funct. Mater. 2005, 15, 1523. b) M.

Yang, J. Ma, C.L. Zhang, Z.Z. Yang, Y.F. Lu, Angew. Chem. Int. Ed.

2005, 44, 6727. c) M. Yang, S.J. Ding, Z.K. Meng, J.G. Liu, T. Zhao,

L.Q. Mao, Y. Shi, X.G. Jin, Y.F. Lu, Z.Z. Yang, Macromol. Chem.

Phys. 2006, 207, 1633.

[9] a) S.J. Ding, C.L. Zhang, M. Yang, X.Z. Qu, Y.F. Lu, Z.Z. Yang,

Polymer 2006, 47, 8360. b) S.J. Ding, C.L. Zhang, M. Yang, X.Z.

Qu, J.G. Liu, Y.F. Lu, Z.Z. Yang, Colloid Polym. Sci. 2008,

in press.

[10] L. Feng, S.H. Li, Y.S. Li, H.J. Li, L.J. Zhang, J. Zhai, Y.L. Song, B.Q.

Liu, L. Jiang, D.B. Zhu, Adv. Mater. 2002, 14, 1857.

[11] a) C.T. Kresge, M.E. Leonowicz, W.J. Roth, J.C. Vartuli, J.S. Beck,

Nature 1992, 359, 710. b) D. Zhao, J. Feng, Q. Huo, N. Melosh, G.H.

Fredrickson, B.F. Chmelka, G.D. Stucky, Science 1998, 279, 548. c) A.

Kuperman, S. Nadimi, S. Oliver, G.A. Ozin, J.M. Garces, M.M. Olken,

Co. KGaA, Weinheim Adv. Mater. 2008, 20, 2965–2969

RESEARCH

NEW

SW. Wei, Z. Z. Yang / Template Synthesis of Hierarchically Structured Composites

Nature 1993, 365, 239. d) H. Yang, N. Coombs, G.A. Ozin, Nature

1997, 386, 692.

[12] Y.F. Lu, B.F. McCaughey, D.H. Wang, J.E. Hampsey, N. Doke, Z.Z.

Yang, C.J. Brinker, Adv. Mater. 2003, 16, 884.

[13] Z.L. Yang, Z.W. Niu, X.Y. Cao, Z.Z. Yang, Y.F. Lu, Z.B. Hu, C.C.

Han, Angew. Chem. Int. Ed. 2003, 42, 4201.

[14] D.H. Wang, R. Kou, Z.L. Yang, J.B. He, Z.Z. Yang, Y.F. Lu, Chem.

Commun. 2005, 166.

[15] J. Ma, Z.L. Yang, X.C. Wang, X.Z. Qu, J.G. Liu, Y.F. Lu, Z.B. Hu,

Z.Z. Yang, Polymer 2007, 48, 4305.

Adv. Mater. 2008, 20, 2965–2969 � 2008 WILEY-VCH Verl

[16] Y.F. Lu, Angew. Chem. Int. Ed. 2006, 45, 7664.

[17] a) H.S. Peng, J. Tang, J.B. Pang, D.Y. Chen, L. Yang, H.S. Ashbaugh,

C.J. Brinker, Z.Z. Yang, Y.F. Lu, J. Am. Chem. Soc. 2005, 127, 12782.

b) H.S. Peng, J. Tang, L. Yang, H.S. Ashbaugh, C.J. Brinker, Z.Z.

Yang, Y.F. Lu, J. Am. Chem. Soc. 2006, 128, 5304.

[18] G. Garnweitner, B. Smarsly, R. Assink, W. Ruland, E. Bond, C.J.

Brinker, J. Am. Chem. Soc. 2003, 125, 5626.

[19] a) B. Tan, S.E. Rankin, Langmuir 2005, 21, 8180. b) W.R. Zhao, J.L.

Gu, L.X. Zhang, H.R. Chen, J.L. Shi, J. Am. Chem. Soc. 2005, 127,

8916.

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