hollow micro-/nanostructures: synthesis and … micro... · hollow micro-/nanostructures: synthesis...

REVIE

W

DOI: 10.1002/adma.200800854

Hollow Micro-/Nanostructures: Synthesisand Applications**

By Xiong Wen (David) Lou,* Lynden A. Archer,* and

Zichao Yang

Hollow micro-/nanostructures are of great interest in many current and emerging areas oftechnology. Perhaps the best-known example of the former is the use of fly-ash hollow particlesgenerated from coal power plants as partial replacement for Portland cement, to produceconcrete with enhanced strength and durability. This review is devoted to the progress made in thelast decade in synthesis and applications of hollow micro-/nanostructures. We present acomprehensive overview of synthetic strategies for hollow structures. These strategies are broadlycategorized into four themes, which include well-established approaches, such as conventionalhard-templating and soft-templating methods, as well as newly emerging methods based onsacrificial templating and template-free synthesis. Success in each has inspiredmultiple variationsthat continue to drive the rapid evolution of the field. The Review therefore focuses on thefundamentals of each process, pointing out advantages and disadvantages where appropriate.Strategies for generating more complex hollow structures, such as rattle-type and nonsphericalhollow structures, are also discussed. Applications of hollow structures in lithium batteries,catalysis and sensing, and biomedical applications are reviewed.

1. Introduction

Suspensions of particles in liquids are the basis for an

astounding array of materials processes and applications

of technological and scientific importance. Even elements of

an abbreviated list – blood, ceramics, cosmetics, detergents,

inks and paints, milk and associated foods – have played a

pivotal role in our life. The advent of nanostructured

composite materials has extended the impact of particles by

[*] Dr. X. W. Lou, Prof. L. A. Archer, Z. C. YangSchool of Chemical and Biomolecular EngineeringCornell UniversityIthaca, NY 14853-5201 (USA)E-mail: [email protected]; [email protected]

Dr. X. W. LouSchool of Chemical and Biomedical EngineeringNanyang Technological UniversityNanyang Avenue, 637457 (Singapore)

[**] The authors are grateful to the National Science Foundation (DMR0404278) and to the KAUST-Cornell (KAUST-CU) Center for Energyand Sustainability for financial support.

Adv. Mater. 2008, 20, 3987–4019 � 2008 WILEY-VCH Verlag G

marrying their functionality with the facile processibility of

synthetic polymers. In many cases, the particles play the role of

fillers or rheological modifiers and their influence can be

quantified entirely in terms of gross features, such as size,

density, volume fraction, and shape. In a growing number of

applications, including catalysis, cosmetics, drug and gene

delivery, hydrogen production and storage, photonics, photo-

voltaics, and rechargeable batteries, the chemical make-up and

distribution of matter within the particles play important roles

in determining function. For example, the large fraction of void

space in hollow structures has been successfully used to

encapsulate and control release of sensitive materials such as

drugs, cosmetics, and DNA. Likewise, the void space in hollow

particles has been used to modulate refractive index, lower

density, increase active area for catalysis, improve the

particles’ ability to withstand cyclic changes in volume, and

to expand the array of imaging markers suitable for early

detection of cancer.

Before 1998, most hollow particles were of spherical shape

and were synthesized using methods suitable for controlling

structure on the macro- and microscale, such as spray-drying

mbH & Co. KGaA, Weinheim 3987

REVIE

WX. W. Lou et al. / Hollow Micro-/Nanostructures: Synthesis and Applications

3988

and gas-blowing.[1] However, even as early as the 1970s and

1980s, works by Matijevic and others demonstrated core/shell

type colloids, primarily in the context of surface functionaliza-

tion.[2] These efforts culminated in the simple sol–gel-based

approaches for coating Au andAg nanoparticles with silica,[3,4]

and in 1998 with Caruso’s seminal paper on colloidal

templating synthesis of hollow spheres.[5] Both approaches

heralded a new, more versatile, synthesis paradigm for hollow

structures based on hard-templating methods. Because these

methods can be used to fabricate hollow structures from

templates of essentially any size, shape, and chemistry, they

have dramatically expanded the range of hollow particles

available for applications. Indeed, starting around 2001, there

has been a large increase in research activity focused on

synthesis of hollow micro-/nanostructures based on templates,

hard and soft, utilizing shells of a wide range of chemistries.

These advances have in turn catalyzed applications and

fundamental research on nanostructured materials in a wide

range of fields, such as biomedical engineering, chemical

catalysis, energy storage, and photonics.

This Review focuses primarily on developments in

synthesis and applications of hollow micro-/nanostructures

with sizes in the range of around 10 nm to 10mm,

emphasizing progress during the last decade. Reviews of

specialized elements of the literature, for example, synthesis

of core/shell structures[6] or synthesis of hollow particles

based on specific approaches, such as the Kirkendall effect,[7]

Ostwald ripening,[8,9] and layer-by-layer (LBL) assembly,[10]

are already available. Our Review reaches more broadly and

attempts to capture all aspects of the field, from systematic

synthetic approaches to major applications. This is an

important undertaking because it comes at a time when a

Xiong Wen (David) Lou was born

and M.Eng. (2004) in chemical en

received his Ph.D. in chemical e

A. Archer) in 2008. He is currently a

Engineering at Nanyang Technol

include nanostructured materials

sensitized solar cells, and bionano

Lynden A. Archer received his B.Sc

California in 1989, and obtained his

He is currently theMarjorie L. Hart

of Engineering at Cornell Univers

author or co-author of over 100 scie

physics, electrophoresis and biop

applications.

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

large number of single-step or so-called one-pot synthetic

approaches are becoming available for large-scale synthesis

of hollow nanostructures. It also comes at a time when

applications of hollow nanostructures are receiving increasing

commercial attention. We nonetheless recognize that the

broad scope of the Review means that all works in every sub-

area cannot be treated in detail. Instead, we have focused on

the major works in each area and in so doing realize that

some excellent papers reporting new, even novel, approaches

are omitted. Additionally, to keep the Review down to a

manageable level, the Review excludes porous structures

(e.g., MCM-41[11]) with both ordered and disordered pores,

and nanotubes. Hollow structures arising spontaneously in

novel ‘‘molecular-cage’’ chemistries, such as C60 fuller-

enes,[12] and inorganic fullerene (IF)-like particles pioneered

by Tenne,[13,14] are not considered in this Review.

The Review is organized as follows. In the first section we

systematically survey the synthetic approaches for hollow

structures, including hard templating, sacrificial templating,

soft templating, and template-free methods. Next we review

the synthesis of rattle-type hollow structures. The final

synthesis section focuses on non-spherical hollow structures.

We close with a brief survey of key applications of hollow

nanostructures in lithium ion batteries, catalysis and sensing,

and biomedical applications.

2. Synthetic Approaches to Hollow Structures

In this section, we present a comprehensive overview of

synthetic approaches for hollow structures. We broadly divide

these approaches into four categories: (1) conventional hard

in Zhejiang, PR China, in 1978. He received his B.Eng. (2002)

gineering from the National University of Singapore, and he

ngineering from Cornell University (with Professor Lynden

n Assistant Professor at the School of Chemical and Biomedical

ogical University (Singapore). His current research interests

, photocatalysis, electrocatalysis, lithium-ion batteries, dye-

technology.

. degree in chemical engineering from the University of Southern

Ph.D. in chemical engineering from StanfordUniversity in 1993.

Chair in Chemical and Biomolecular Engineering in the College

ity. He is a fellow of the American Physical Society and is the

ntific papers and 1 book. His research interests include polymer

hysics of DNA, and nanostructured materials for energy

Co. KGaA, Weinheim Adv. Mater. 2008, 20, 3987–4019

REVIE


1 32 4

cheme 1. Schematic illustration of a conventional hard templating pro-ess for hollow sphere synthesis.
Sc
templating synthesis, (2) sacrificial templating synthesis, (3)

soft templating synthesis, and (4) template-free methods.

2.1. Conventional Hard Templates

Preparation of hollow structures by templating against hard

particles is conceptually straightforward. In general, it involves

the four major steps illustrated in Scheme 1: (1) preparation of

hard templates; (2) functionalization/modification of template

surface to achieve favorable surface properties; (3) coating the

templates with designed materials or their precursors by

various approaches, possibly with post-treatment to form

compact shells; and (4) selective removal of the templates to

obtain hollow structures. The most commonly employed hard

templates include nearly monodisperse silica particles and

polymer latex colloids. These templates are advantageous for

several reasons including their narrow size distribution, ready

availability in relatively large amounts, availability in a wide

range of sizes from commercial sources, and simplicity of their

synthesis using well-known formulations. Other colloidal

systems, such as carbon nanospheres and nanoparticles of

metals and metal oxides, have also been used as templates for

preparation of hollow structures.

Step 4 is in principle the simplest. It typically entails selective

etching of the template in appropriate solvents or calcination

of the template at high temperatures. Using either approach,

special care is required to prevent collapse of the shells during

template removal. For example, when using organic solvent to

Scheme 2. Schematic illustration of procedures for preparing inorganic and hybrid hollowspheres using the layer-by-layer (LBL) technique based on PS colloidal templates. Reproducedwith permission from [5]. Copyright 1998 American Association for the Advancement ofScience.

dissolve template latex particles, swelling of the

polymer can cause rupture of the hollow

structure, causing the shell to manifest uncon-

trolled shape change. Step 3 is generally

regarded as the most challenging because it

requires robust methods for efficiently precipi-

tating shell materials on substrates with sizes in

the nano-/micrometer range. Incompatibility

between the template surface and shell material

can be overcome by functionalizing/modifying

the template with special functional groups or

characteristics, for example electrostatic

charges, before applying subsequent coating/

deposition of shell materials. In some cases,

functional groups are introduced during the

synthesis of the template particles. Step 2 and

Step 3 together therefore constitute a coating

scheme to form compact shells on any arbitrary

template particles. Unsurprisingly, the most

Adv. Mater. 2008, 20, 3987–4019 � 2008 WILEY-VCH Verl

exciting research in the field has focused on developing flexible

coating schemes suitable for a broad range of materials. This

effort has led to considerable progress during the past decade.

Although organic polymer hollow structures are also often

prepared using different hard templates,[15,16] in the following,

we will focus on the major shell-forming strategies for

inorganic or hybrid materials.

2.1.1. Layer-by-Layer Assembly

LBL assembly refers to sequential deposition of oppositely

charged polymer species on substrates mediated by electro-

static interactions. Combining this approach with colloidal

templating, Caruso et al. were the first to extend the LBL

technique to prepare hollow inorganic silica and hybrid

capsules through electrostatic assembly of negatively charged

silica nanoparticles and positively charged polymer.[5]

Scheme 2 illustrates the basic procedure using polystyrene

(PS) latex particles as templates. The assembly cycle can be

repeated to form multilayer structure with well-defined wall

thickness. The versatility of the LBL method is one of its main

strengths. It also facilitates excellent capsule size control and

uniformity of the shell. Polymer capsules derived from this

route enable the encapsulation of diverse components (e.g.,

DNA[17]) and are currently of special interest in bio-

applications.[18] Apart from various polymer capsules,[10] the

LBL technique has also been extensively applied to prepare

hollow structures of a wide range of inorganic or composite

materials,[19] including zeolite/SiO2,[20] TiO2,

[21,22] SnO2,[23]

Au,[24] magnetic Fe3O4,[25] carbon nanotubes (CNTs),[26] and

others.[21] In addition to nanoparticles, two-dimensional

nanosheets have also been assembled using the LBL technique

to give ultrathin nanoshells. Sasaki and co-workers, for

example, have prepared hollow nanoshells of Mn2O3 and

layered double hydroxide (LDH) through LBL assembly using

exfoliated MnO2[27] and LDH[28] nanosheets as building

blocks, respectively. They also prepared composite hollow

shells based on positively charged polyoxocations of aluminum

ag GmbH & Co. KGaA, Weinheim www.advmat.de 3989

REVIE


Figure 1. Scanning electron microscopy (SEM) image of anatase TiO2

shells obtained after calcination of the titania-coated PS spheres at 600 8Cin air, inset is a transmission electron microscopy (TEM) image. Repro-duced with permission from [31]. Copyright 2001 American ChemicalSociety.

3990

Keggin ions and exfoliated unilamellar crystallites of

Ti0.91O2.[29] Furthermore, while most works involve LBL

assembly of preformed nanoparticles, the polyelectrolyte (PE)

multilayers on templating colloids can be exploited as

nanoreactors for in situ formation of inorganic shells by sol–

gel processes. For example, metal oxide (e.g., LiNbO3) hollow

spheres have been prepared by infiltration and subsequent

hydrolysis and condensation of the precursor (LiNb(OC2H5)6)

in the PE shell, followed by calcination.[30]

Despite its versatility and growing popularity, the LBL

method suffers from three key shortcomings. First, the method

is difficult to use for preparation of smaller hollow structures

with sizes <200 nm. Second, the LBL assembly procedure

becomes quite tedious when many layers are required. Finally,

inorganic and hybrid hollow structures prepared from this

method generally lack the mechanical robustness of particles

prepared using other approaches. As-prepared polymer

capsules are also only stable when kept in solution, once

dried they tend to collapse irreversibly.

2.1.2. Direct Chemical Deposition

Chemical deposition is also a commonly used shell-forming

strategy. It involves precipitation of the shell materials or

precursors on the template particles through various chemical

or physical interactions with the template. The deposition

process is typically followed by a post-treatment (usually

calcination) step to obtain compact shells.

Metal oxide (SiO2, TiO2, ZrO2) hollow spheres have been

prepared by controlled hydrolysis of their metal alkoxide

precursors in the presence of template particles, followed by

removal of the templates. Control of the hydrolysis rate and

heterocoagulation is crucial for the success of the process. This

synthesis demands strict control of the reaction conditions to

obtain smooth shell coatings. The requirements are especially

stringent in the case of TiO2. Smooth and uniform titania shells

on polymeric templates have nonetheless been achieved by

several groups.[31,32] Imhof carried out the coating in ethanol at

room temperature by hydrolysis of titanium tetraisopropoxide

using cationic polystyrene (PS) spheres as templates stabilized

by polyvinylpyrrolidone (PVP).[31] The slight negative charge

on the hydrolyzed titania species ensures their rapid deposition

on the positively charged PS spheres, although the hydrolysis

of titanium alkoxide is rather fast.[31,33] As-synthesized titania

is amorphous, and post-calcination at high temperatures is

usually employed to simultaneously crystallize the particles

and burn off the polymer templates. Shrinkage during the

crystallization process and, possibly, escape of gases produced

by pyrolysis of the polymer templates, often lead to defects

such as holes on surfaces of the final hollow particles (see

Fig. 1). As a result, titania hollow structures derived from this

route are generally quite fragile. Uniform coatings of titania on

anionic PS spheres have also been reported using ammonia-

catalyzed hydrolysis of titanium tetrabutoxide in mixed

solvents.[34,35] A particularly fruitful application of this

approach is found in the synthesis of hollow SiO2 nanos-

tructures. Uniform deposition of SiO2 from ammonia-cata-


lyzed hydrolysis of tetraethoxysilane (TEOS) is known to be

relatively easy, particularly on inorganic templates.[36] As a

result, smooth SiO2 coating on particles of numerous materials

has been reported. Silica hollow spheres, for example, have

been prepared using surface-functionalized PS latex tem-

plates.[37–39] Clay/silica hybrid hollow capsules with ultrathin

walls have been prepared through deposition of negatively

charged clays on cationic latex templates followed by sealing

with silica.[40] Similarly, silica/titania hybrid hollow spheres can

be prepared by sequential deposition.[41] Zirconia hollow

spheres have been synthesized by hydrolysis of zirconium

butoxide using silica templates.[42]

Hollow spheres of many materials have been synthesized by

chemical bath deposition. ZnS can be deposited on both silica

and PS templates under acidic conditions with zinc nitrate and

thioacetamide as precursors.[43,44] Braun and co-workers used

a double templating route to obtain ZnS hollow spheres with

ordered mesoporous shells, where a lyotropic liquid crystal

templates ZnS shell formation on the surface of a silica or PS

template.[45] Bimetallic Au/Pt hollow spheres have been

prepared by chemical reduction/precipitation of Pt on Au

seeded silica templates followed by removal of silica in HF

solution.[46] Chen and co-workers prepared Ni1–xPtx alloy

hollow spheres by chemical reduction of Ni2þ adsorbed on the

surface of poly(styrene-co-methacrylic acid) (PSA) templates

and subsequent alloying with Pt via replacement reaction.[47]

Mesoporous Fe3O4 hollow spheres have been prepared by

chemical adsorption/precipitation of Fe2þ on carboxyl-func-

tionalized PS templates at 80 8C in the presence of ethylene

glycol (EG).[48]

In other syntheses, precursor metal cations are first

precipitated on the template surface in the form of hydroxide

or other intermediate phases through controlled hydrolysis

under basic conditions. These phases can be easily transformed

to metal oxide or metallic phases by calcination in air or in a

reducing atmosphere, respectively. This so-called controlled

precipitation route has been pioneered by Matijevic for


REVIE


Figure 2. A) TEM image of monodisperse silica@SnO2 core/shellparticles. B) SnO2 hollow colloids obtained by complete HF etching ofsilica@SnO2 core/shell particles annealed at 550 8C for 8 h. C) Double-shelled SnO2 hollow colloids with a silica core. D) SnO2 hollow colloidsobtained from double-shelled silica@SnO2 core/shell particles. Repro-duced with permission from [57].

Figure 3. TEM and SEM (inset) images of Pd hollow spheres. Reproducedwith permission from [58].

preparation of uniform size colloids as well as coated and

hollow particles.[2] Wang et al. reported controlled precipita-

tion of Ni(OH)2 nanoflakelets on PSA templates, subsequent

calcination resulted in hollow NiO with a hierarchical

structure.[49] When silica templates are used, only nickel

hydrosilicate (Ni3Si2O5(OH)4) is formed due to the strong

interactions between silica and nickel species.[50,51] After silica

removal and reduction with H2 at 450–500 8C, metallic Ni[50]or

Ni-silica[51] hollow spheres can be obtained. Using a similar

approach, Awaga and co-workers coated PS templates with

cobalt or iron hydroxides (possibly more complex phases).

Controlled calcination in either air or H2–N2 at 350–500 8C,was subsequently performed to prepare hollow spheres of

different phases (ccp-Co, hcp-Co, Co3O4, a-Fe, Fe3O4,

a-Fe2O3).[52,53]

More recently, Xu and co-workers have reported synthesis

of uniform hollow nanospheres of NiO, a-Fe2O3, ZnO, CuO

and Ga2O3 using carbonaceous polysaccharide nanospheres as

hard templates by controlled precipitation followed by

calcination.[54] The key to this controlled precipitation route

appears to involve preferred heterogeneous precipitation on

the template surface induced by slow release of hydroxyl

anions from thermal decomposition of urea. If copious amount

of base is introduced into the system, homogeneous nucleation

and precipitation is inevitable. Under this condition, uniform

coating of templates can only be achieved by other interac-

tions, for example, electrostatic attraction between the formed

particles and the templates,[55] or the metal cations are

exclusively entrapped chemically or physically near the

template surface.[56] With the latter approach, Choi et al.

prepared porous hematite and silica capsules with a con-

trollable surface morphology.[56] The Fe2þ ions are first

adsorbed into the polymer multilayers coated on melamine-

formaldehyde (MF) templates through LBL assembly. After

several cycles of washing, the adsorbed ions are precipitated

within the multilayers by treating with NaOH solution to form

goethite. This decoupled adsorption-precipitation process is

advantageous for preparation of discrete hollow particles.

Lou et al. have recently demonstrated a shell-by-shell (SBS)

templating strategy for preparation of nearly monodisperse

SnO2 hollow colloids.[57] The SBS method is based on direct

hydrothermal deposition of coatings with the desired chemistry

on silica templates. The synthesis requires no prior surface

modification, and as much presumably benefits from the

compatibility between the silica template and polycrystalline

SnO2. As shown in Figure 2A, very uniform and smooth SnO2

shells on silica templates can be achieved in a discrete state.

After post-annealing at 550 8C, these single-shelled SnO2 walls

are robust enough to sustain template removal by HF etching

(see Fig. 2B). Significantly, the shell thickness can be controlled

by a second hydrothermal deposition step, forming double

shells (see Fig. 2C). Due to improved structural integrity of the

double-walls, the spherical shells can be preserved without

collapse upon template removal (see Fig. 2D). In this way, the

SBS method enables synthesis of high-quality SnO2 hollow

colloids with control in both cavity size and shell thickness.


2.1.3. Chemical Adsorption on Surface Layer

Hollow spheres have also been prepared by many groups

using a direct adsorption-calcination route. In this method, pre-

treatment of the template particles is usually required. Hyeon

and co-workers have prepared Pd hollow spheres as shown in

Figure 3.[58] The synthesis involves three major steps:

functionalization of silica templates with –SH groups, adsorp-

tion of Pd precursor onto functionalized template surface, and

Pd shell formation by CO reduction generated during

calcination. This approach is typical for hard-templating

fabrication of hollow spheres. However, formation of a


REVIE


Figure 5. a,b) TEM images of titania hollow spheres obtained after calci-nation at 450 8C in air. c) SEM and TEM (inset) images of double-shelledFe3O4 hollow spheres. d) SEM and TEM (inset) images of double-shelledpolyaniline hollow spheres. Reproduced with permission from [64].

3992

complete shell after template removal is quite uncommon

because the adsorbed precursor amount is rather limited on the

monolayer functionalized surface. From Figure 3, the resulting

hollow spheres with very thin walls and rough surfaces appear

to be aggregated. Li and co-workers prepared hollow spheres

of Ga2O3 by adsorption of metal cations into the surface layer

of hydrophilic carbon (carbonaceous polysaccharide) spheres

with copious –OH groups, followed by calcination in air.[59]

Despite significant shrinkage, ca. 60%, in size during

calcination, Ga2O3 hollow spheres with smooth surfaces and

a wall thickness as low as 16 nm can be obtained (see Fig. 4).

Upon further reaction with NH3 at 850 8C, the corresponding

GaN hollow spheres can also be obtained. This simple method

has been extended by the same authors, as well as others, to

prepare hollow spheres of a wide range of metal oxides[60–62]

and Pt hollow capsules,[63] but the quality appears less

satisfactory.

Yang and co-workers have described an interesting

approach to prepare inorganic hollow spheres via preferential

adsorption of inorganic precursors into the sulfonated shell

layer of PS templates.[64,65] The core/shell PS templates are

obtained by an inward sulfonation of PS particles with

concentrated sulfuric acid. The sulfonation process produces

hydrophilic shells with sulfonic acid ðSO�3 H

þÞ groups randomly

attached to PS chains. Because these groups are capable of

adsorption of or complexation with a large variety of species,

such as metal cations, TiO2 sols from alkoxides, and even

organic monomers (aniline),[64] the method is quite versatile.

The sulfonation time determines the thickness of the

sulfonated shell, which in turn sets the thickness of the as-

derived inorganic hollow spheres. The authors first demon-

strated the concept with preparation of TiO2 hollow spheres

using tetrabutyl titanate as precursor.[65] After removal of PS

templates by calcination, as-prepared TiO2 hollow spheres

appear to have rough surface and some degree of rupture. As

an extension, double-shelled hollow spheres of TiO2, Fe3O4,

and polyaniline have also been prepared by templating against

both exterior and interior surfaces of hollow PS templates (see

Fig. 5).[64,66] The same approach has been applied to prepare

hollow spheres of other inorganic materials (SnO2, In2O3, Sn-

doped In2O3, BaTiO3, SrTiO3).[67,68]

Figure 4. TEM images of Ga2O3 hollow spheres. Reproduced with per-mission from [59].


2.1.4. Nanocasting from Mesoporous Shells

Hollow spheres with mesoporous shells (HSMS) can be

prepared using spherical templates with solid core and

mesoporous shell (SCMS) structure. Scheme 3 illustrates the

general approach. This method requires that the shell materials

(usually precursors) efficiently impregnate into the mesopor-

ous shells. SCMS silica templates are most commonly used,

which can be facilely prepared by incorporating a suitable

porogen agent (e.g., n-octadecyltrimethoxysilane)[70] during

the conventional synthesis of silica particles by the Stober’s

method.[71] Carbon HSMS have been prepared by infiltrating

and carbonizing different polymers such as phenol resin and

poly(divinylbenzene) inside the mesopores of SCMS silica

template.[72–74] However, the obtained carbon HSMS are

largely interconnected (see Fig. 6A) since the method cannot

ensure exclusive infiltration of polymer carbon precursors into

the mesopores. To overcome this problem, Ikeda et al. recently

reported an elegant solution approach for selective adsorption

of glucose-derived carbon precursor (polysaccharide) onto

amino-functionalized SCMS silica.[75] Discrete carbon HSMS

can be obtained using this method (see Fig. 6B). Fuertes and

co-workers also prepared discrete HSMS of silicon oxycarbide

and derived silica (see Fig. 6C) by direct infiltration of short-

chain polycarbomethylsilane into SCMS silica,[69] and carbon

HSMS decorated with ferrite nanoparticles.[76] Binary SCMS

Scheme 3. Schematic illustration of a nanocasting process for preparinghollow spheres with mesoporous shells. Reproduced with permissionfrom [69]. Copyright 2007 American Chemical Society.


REVIE


Figure 6. A) TEM and SEM (inset) images of carbon HSMS. Reproduced with permission from [72].B) TEM image of carbon HSMS (scale bar is 200 nm). Reproduced with permission from [75]. C)TEM image of silicon oxycarbide (SOC) HSMS. Reproduced with permission from [69].

Scheme 4. Schematic illustration of procedure for preparing hollowspheres by templating against colloidal crystals. Reproduced with per-mission from [80].

silica@ZrO2 templates have also been used for preparation of

carbon HSMS.[77] In this case, however, a substantial fraction

of the hollow structures are in a collapsed state because the

templating mesoporous ZrO2 shell is too thin. Taking the

nanocasting one step further from carbon HSMS, HSMS of

silica[78] and various metal oxides[79] have also been prepared.

2.1.5. Templating against Colloidal Crystals and the Lost-Wax

Approach

If a sol–gel precursor is infiltrated into a physically stable

colloidal crystal, one would generally expect to obtain a 3D

porous replica (inverse opal). By templating their respective

sol–gel precursors against quasi-stable arrays of PS beads, Xia

and co-workers first demonstrated that this approach can be

used to provide a novel route for preparation of discrete TiO2

and SnO2 hollow spheres.[80] As illustrated in Scheme 4, the

procedure involves infiltration of sol–gel precursor solution

into dried PS arrays in a confined assembly cell with some

empty space. Electrostatic repulsion between the charged PS

particles ensures that they remain separated and are

surrounded by the sol–gel precursor solution. Once exposed

to moisture in ambient air, the sol–gel precursor hydrolyzes

quickly to form the corresponding metal oxide sols, which

precipitate on the ‘‘sticky’’ surface of PS particles. As solvent

evaporates, the precipitate forms a uniform coating on the PS

template. Keeping other conditions unchanged, the shell

thickness is mainly determined by the concentration of sol–gel

precursor. The success of this method evidently requires good

control of the interfacial properties of both the polymer beads

and sol–gel precursor, as well as of the hydrolysis rate of the

precursor. The authors successfully extended the method to

prepare TiO2 hollow spheres with the interior surface

functionalized with Ag nanoparticles by using Ag nanoparti-

cles derivatized PS as templates.[81] Combining the approach

with electroless plating, Chen et al. showed that it can be used

to prepare high-quality 2D and 3D arrays of Ag hollow

spheres.[82] Unlike previous coating schemes, this novel

method ensures uniform coating by physically limiting the

amount of sol–gel shell precursor locally available to each

Adv. Mater. 2008, 20, 3987–4019 � 2008 WILEY-VCH Verlag GmbH & Co. KGaA, We

template particle, although in theory

formation of interconnected hollow par-

ticles is highly possible.[83,84]

Based on a similar concept of limiting

precursor in a cavity, Jiang et al. demon-

strated an elegant, general nanoscale

‘‘lost-wax’’ method for general prepara-

tion of hollow spheres and their corre-

sponding colloidal crystals.[85] The

method involves two replication steps.

In the first step, a PS inverse opal is

prepared from a silica colloidal crystal.

The sol–gel precursor solution is subse-

quently infiltrated into the interconnected

voids of the macroporous polymer. As in

the previous approach, hydrolysis with

moisture and solvent evaporation cause

the sols to adhere to the inner ‘‘sticky’’ polymer surfaces to

form uniform shells. This infiltration process can be repeated to

obtain hollow spheres (TiO2 as an example) with controlled

shell thickness (see Figure 7). Macroporous templates of other

materials with very different surface properties (e.g., silica and

carbon) have also been employed,[86,87] which further expand

the diversity of hollow structures that can be prepared by the

lost-wax approach.[88] Despite its elegance and versatility,

shortcomings of the lost-wax approach include the relatively

large number of steps required, the low efficiency of each step,

and the stringent controls required in each step to secure the

desired structures. As such, the method is not easily applied to

inheim www.advmat.de 3993

REVIE


Figure 7. Titania hollow colloids replicated from PS templates with shell thickness of A)about 15.4 nm obtained after coating two times, and B) about 31.4 nm after coating seventimes. Reproduced with permission from [85]. Copyright 2001 American Association for theAdvancement of Science.

3994

mass-produce hollow structures in quantities suitable for

commercial applications.

2.1.6. Other Strategies and Templates

Potentially generic coating schemes based on chemical vapor

deposition (CVD),[89,90] atomic layer deposition (ALD),[91,92]

and even physical deposition (e.g., e-beam evaporation), have

been used to prepare hollow nanostructures from hard

templates. For example, Ras et al. have recently applied

ALD to prepare Al2O3 hollow nanospheres employing self-

assembled block copolymer nanospheres as templates.[93]

While silica and latex particles are the most commonly used

templates in hard templating synthesis, numerous other

particles have also been explored, including noble metals,[94,95]

quantum dots,[96] CaCO3,[97,98] and biomolecules.[99,100] Addi-

tionally, many novel templating methods based on other shell-

forming strategies have been reported for preparing hollow

spheres of different materials. In particular, preparation

of hollow particles by sonochemical deposition has been

demonstrated in several works. In this method, the extreme

and transient local conditions (ca. 5000K, ca. 1000 atm

(1 atm¼ 1.013� 105 Pa)) created upon collapse of cavitation

microbubbles generated by ultrasound are utilized for room-

temperature synthesis and deposition of nanoparticles on

surface of template particles.[101] Uniform shells of ZnS with

thickness of 70–80 nm have been sonochemically coated on

carboxyl-modified PS templates in an aqueous bath contain-

ing zinc acetate and thioacetamide.[102] Upon calcination at

400 8C, complete ZnS hollow spheres can be obtained.

Wang et al. reported sonochemical preparation of bimetallic

FePt or FePt/ZnS hollow shells using silica templates.[103]

Moreover, Suslick and co-workers have prepared hollow

nanospheres of MoO3 and MoS2 using small silica nano-

particles as template, and hematite using carbon nanoparti-

cles as template.[104,105]

In a few special cases, core/shell particles made of two

materials (or precursors) have been synthesized using one-step

www.advmat.de � 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinhei

approaches based on hydrothermal method or

pyrolysis.[106,107] After selectively removing the

cores, hollow particles can be obtained. In other

words, preparation of templates and formation

of shells are combined into one step compared

to conventional hard templating. In addition to

high synthetic efficiency, such methods are very

attractive in at least two aspects. First, there is

no need for a separate shell-forming step, which

as discussed earlier is most challenging in hard

templating syntheses. Second, hollow particles

derived from these methods are generally more

robust because of optimal self-organization of

shell materials on the cores. Controlling the

particle size and shell thickness are nonetheless

quite challenging. Brinker and co-worker

reported an interesting aerosol-assisted self-

assembly method to create novel NaCl/meso-

porous silica core/shell particles.[108] In the

procedure, an atomizer is employed to generate aerosol

droplets from a homogenous solution containing silica

precursor and cetyltrimethylammonium bromide (CTAB) as

a structure-directing agent. As the solvent evaporates, a radial

concentration gradient develops, and self-assembly is accord-

ingly radially directed around the NaCl nucleus formed in the

droplet center. Because the NaCl core can be easily dissolved

in water, the template removal step is particularly straightfor-

ward. Noble-metal (Ag, Au) nanoparticles covered with thin

metal oxide (TiO2, ZrO2) shells have been prepared by a one-

step method.[109,110] Upon leaching out noble metal cores,

metal oxide nanobubbles can be produced.[111,112]

2.2. Sacrificial Templates

Broadly speaking, the word ‘‘template’’ implies the object so

used is transitory and hence ‘‘sacrificial’’. Here we wish to

define sacrificial templates in a stricter sense. As used herein,

the key feature of a ‘‘sacrificial template’’ is that the template

itself is involved as a reactant in the synthetic process of the

shell material (or its intermediate). Analogous to conventional

hard templates, sacrificial templates directly determine the

shape and approximate cavity size of the resultant hollow

structures, but the template simultaneously plays the role of

structure-directing scaffold and precursor for the shell. As a

result, the sacrificial template is consumed partially or

completely during the shell-forming process. In this regard,

sacrificial template synthesis is inherently advantageous

because in general it requires no additional surface functio-

nalization and shell formation is guaranteed by chemical

reaction. The process is therefore typically more efficient,

especially when the sacrificial template is completely con-

sumed during the shell-formation reaction. These attractive

characteristics have motivated synthesis of an increasingly

broad array of hollow structures using an equally diverse range

m Adv. Mater. 2008, 20, 3987–4019

REVIE


Figure 8. Evolution of CoSe hollow nanocrystals with time at 455K. From(A) to (F): 0 s, 10 s, 20 s, 1min, 2min, and 30min. Reproduced withpermission from [115].

of sacrificial templates. A number of special formation

mechanisms such as the Kirkendall effect and galvanic

replacement have been identified that can be used to

understand the fundamentals of hollow particle synthesis by

sacrificial templates.

2.2.1. The Kirkendall Effect

The Kirkendall effect as originally conceived provides a

mechanism for void formation near interfaces due to different

interdiffusion rates in a bulk diffusion couple.[113] The net flow

of mass in one direction is balanced by a flux of vacancies,

which may condense into voids preferably around the inter-

face. When applied to spherical particles on the nanoscale, the

phenomenon becomes more complicated owing to the

dominant roles of curvature and surface energetics.[114] It

was first applied byYin et al. to explain the formation of hollow

compound nanocrystals of cobalt oxide and chalcogenides

about 10–20 nm in size.[115] The authors nicely demonstrated

the concept by reacting presynthesized Co nanocrystals with

sulfur, oxygen, and selenium. In the sulfidation example, the

Co nanocrystals are first covered with a very thin layer of cobalt

sulfide, further reaction between Co and S (the growth of

cobalt sulfide shell) requires diffusion of S or Co through the

previously formed compound shell, as there can be diffusion of

only one dominant species, since co-existence of both in the

shell is unlikely. In the Co@S system, the dominant diffusion

happens to be the outward diffusion of Co, which generates a

single void in each nanoparticle. Unfortunately, this reaction is

so fast that it prevents direct observation of hollowing process,

but the void space is expected to develop preferably around the

interface because of its high energy and defect density. In the

Co@Se system, however, the evolution of hollowing process is

sufficiently slow that it can be observed (see Fig. 8). Indeed,

initially void space is observed to develop between the Co core

and the CoSe shell. Similar evolution of structure was also

observed in a recent study by Peng and Sun,[116] where

amorphous Fe@Fe3O4 core/shell nanoparticles were trans-

formed to crystalline Fe3O4 hollow nanoparticles upon heating

and controlled oxidation in the presence of the oxygen-transfer

agent trimethylamine N-oxide. A key question then is how

mass is transported from the core to the shell, since there is

some void space between them. It is contended that the

Figure 9. TEM images (insets: high-resolution TEM images) of wires of hollow nanocrystals:A) Co3S4, B) CoTe, and C) CoSe2. Reproduced with permission from [123].

transport proceeds via formation of bridges and

surface spreading,[115] both of which are more

likely on the nanoscale owing to dominance of

surface effects, particularly surface diffu-

sion.[117] Xia and co-workers however reported

strong evidence in favor of formation of single

voids even in the center of 200 nm Pb spherical

colloids during the initial stage of reaction with

sulfur vapor,[118] which can hardly be explained

by the Kirkendall effect.

Yin’s work has inspired several studies that

attempt to clarify the physics of the nanoscale

Kirkendall effect.[114,117,119] However, these

studies are still largely phenomenological or


based on highly idealized models. For example, the observa-

tion that the shell is multicrystalline and microporous as

probed by small molecules from the catalytic study,[115] might

suggest that there is an outward flow of Co through the shell by

surface diffusion, instead of bulk diffusion. Wang et al.[120] and

Nakamura et al.[121,122] have systematically studied the

formation of oxide hollow nanoparticles through oxidation

of metal nanoparticles (Zn, Al, Fe, Cu), where however the

outward diffusion of metal was explained based on the

Caberra-Mott theory, that the metal cations diffuse through a

thin oxide layer at the initial oxidation stage driven by

electrostatic fields. Moreover, the authors have determined

some critical sizes of metal nanocrystals in order to form

hollow oxide particles. However questions about the exact

mechanism remain, and more mechanistic studies are required

to understand this intriguing process.

Based on the Kirkendall process, many other hollow

nanoparticles have been synthesized with most involving

metal nanocrystals as the core. Gao et al. for example

demonstrated the formation of wires of hollow nanocrystals of

cobalt chalcogenides (see Fig. 9),[123] which was produced by

reacting selenium, sulfur, or tellurium with preassembled 1D

necklace-like structure of 20 nm Co nanocrystals. The

assembly of Co nanocrystals is induced by the strong magnetic

dipoles associated with large Co nanocrystals (20 nm). Such


REVIE


Figure 10. SEM images of slightly truncated Ag nanocubes synthesized bythe polyol process (A), after reacting with 0.3mL (B) and 1.5mL (C) ofaqueous HAuCl4 solution (1mM). Scale bars are 100 nm in (B) and (C).Reproduced with permission from [130]. Copyright 2002 American Associ-ation for the Advancement of Science.

Figure 11. TEM images of Pt hollow nanospheres. Reproduced withpermission from [134].

3996

assembly is not observed for 6 nm Co nanocrystals owing to

weaker dipole interaction. Chiang et al. recently prepared Ni2P

and Co2P hollow nanoparticles by a one-pot solution reaction

under carefully controlled conditions.[124] The process involves

the reaction of first-formed metal nanocrystals with trioctyl-

phosphine (P source). Tan et al. reported the formation of

hollow Ag2Se,[125] in which 50 nm spherical Ag nanocrystals

were reacted with Se generated from photo-dissociation of

CSe2 on the surface. However, a hollow structure was not

observed for smaller Ag nanocrystals. Although the authors

attributed the formation mechanism to the Kirkendall effect,

the high porosity of the shells suggests that a normal chemical

process cannot be entirely ruled out.

We close this section by pointing out that in contrast to the

usual negative implications of Kirkendall void formation in

technology, the nanoscale Kirkendall effect has been

embraced by several groups as a general method for

synthesizing hollow nanostructures. However, except for a

small number of well-defined systems, attributing the forma-

tion of hollow structures to the Kirkendall effect is in many

cases speculative. Indeed, in situations where the hollow

structures are of sub-micro- to micrometer dimensions and

the synthesis is performed in (for example, aqueous)

solution, it is likely that the shell is porous, and the interior

void is filled with solution. Under these conditions, normal

solution transport (Ostwald ripening) would appear to be the

predominant shell-growth process, and it is clearly inap-

propriate to discuss the formation mechanism based on

vacancy flux and in turn the Kirkendall process. We

therefore recommend the use of the generic term, ‘‘sacrificial

templating process’’ for systems insufficiently well-defined to

follow the Kirkendall process.

2.2.2. Galvanic Replacement

As another special class of sacrificial template synthesis,

galvanic replacement reactions have been widely employed for

general preparation of metal hollow nanostructures in a variety

of shapes and sizes. In a typical reaction, the salt of a more

noble metal (B) is reduced with preformed nanocrystals of a

less noble metal (A), resulting in deposition of B on the surface

of A. Upon complete consumption of metal A, hollow

structures of metal B can be obtained under controlled

conditions. The shape and cavity size of the derived hollow

structure are then largely determined by the sacrificial

nanocrystals of A.

Xia’s group and others have prepared various metal (Au, Pt,

Pd) hollow nanostructures in both aqueous and organic

solutions through galvanic replacement reactions with Ag

nanocrystals of sizes ranging from <10 nm to several hundred

nanometers.[126–129] The Ag nanocrystals in a wide range of

sizes and shapes are synthesized by the versatile polyol process

in the presence of a capping reagent such as PVP. In this

process, ethylene glycol serves as both a reducing agent and

solvent. Taking the preparation of Au hollow nanostructures in

aqueous solution as an example, the galvanic replacement

reaction can be described as follows, 3Ag(s)þAuCl4�(aq)!


Au(s)þ 3Agþ(aq)þ 4Cl�(aq). This synthesis was nicely

demonstrated with the generation of Au nanoboxes using

Ag nanocubes as sacrificial templates (see Fig. 10).[130] Despite

the simplicity of the chemistry, the formation mechanism of

hollow structures can be extremely complicated, depending on

the reaction system.[131] The formation of the hollow cavity is

generally attributed to the 3:1 stoichiometric relationship

between Ag and Au in the reaction. However, this might not

be a necessary requirement. After initial deposition of Au on

the surface of Ag nanocubes forming a thin shell, the further

growth of Au shell is likely to be outward as evidenced by

the overall size increase of the Au nanoboxes (ca. 20%)

compared to that of the templating Ag nanocubes.[130] It

might therefore be postulated that the electrons flow from

the Ag template to the exterior surface of the Au shell

where further growth of Au takes place, accompanied by the

dissolution of the Ag template inside the shell. Further

evidence could come from the preparation of Ag nanoshells

using bivalent Co nanocrystals as sacrificial templates,[132] in

which two equivalents of Ag are produced upon consump-

tion of one equivalent of Co.

Cobalt nanocrystals have also been extensively used as

sacrificial templates for preparation of noble-metal hollow

particles through galvanic replacement reactions.[133] Liang

et al. have prepared hollow nanospheres of Pt,[134] Au,[135] and

bimetallic AuPt,[136] using Co nanoparticles as sacrificial

templates and citrate as a capping agent. The Pt hollow

nanospheres (see Fig. 11), consisting of ca. 2 nm Pt nanopar-


REVIE


ticles, appear to be largely porous and rough owing to their

relatively small overall size of ca. 24 nm. Depending on the

concentration of citrate, either 1D tubelike or discrete hollow

nanospheres of AuPt were obtained. The formation of a 1D

structure was again attributed to the magnetic dipole

interaction of the templating Co nanocrystals. Based on this

idea, an external magnetic field can be purposely applied to

induce alignment of the Co nanocrystals, and in turn the noble-

metal hollow nanoparticles. Zeng et al. have recently prepared

necklace-like chains of noble metal (Au, Pt, Pd) hollow

nanoparticles with PVP as the capping agent.[137] Interestingly,

these replacement reactions can sometimes lead to alloy or

bimetallic hollow nanospheres.[126,138] Vasquez et al. reported

formation of CoPt alloy (instead of Pt) hollow nanospheres

also with PVP as the capping agent.[139] This was explained by

recognizing that the presence of excess reducing agent in the

solution guarantees that the Co2þ cations released from initial

oxidation of Co, together with Pt4þ, are reduced back to CoPt.

2.2.3. Conventional Sacrificial Templates

Amorphous Se colloids have been employed as sacrificial

templates for preparation of hollow nanospheres of Ag2Se and

other derived semiconductors.[140–143] The conversion, carried

out in ethylene glycol, takes advantage of the high reactivity of

Se toward freshly generated Ag to form Se@Ag2Se core/shell

particles, with the shell thickness controlled by the relative

amounts of reactants. The unreacted Se core can be selectively

dissolved in hydrazine solution. Moreover, Se@Ag2Se core/

shell particles can be facilely converted to Se@MSe (M¼Zn,

Cd, Pb) by simply performing cations exchange with respective

metal cations. After removal of Se core, hollow nanospheres of

ZnSe, CdSe and PbSe can be obtained. Zhu and co-workers

recently reported preparation of CdX (X¼Te, Se, S) hollow

nanospheres utilizing Cd(OH)Cl nanoparticles as sacrificial

templates,[144] and ZnX (X¼ S, Se) hollow spheres based on

sonochemical anionic exchange of ZnO nanospheres.[145]

Starting with ZnSe hollow microspheres, Li and co-workers

synthesized a series of chalcogenide and oxide semiconductor

hollow microspheres by performing both anionic and cations

exchange reactions with a solution or gas phase conversion

process.[146] Huang et al. reported synthesis of Cu2O hollow

nanospheres from Cu nanospheres through halide-induced

corrosion oxidation with the aid of surfactants.[147] Liu et al.

reported fabrication of ZnO hollow microspheres with a

dandelion-like morphology (see Fig. 12).[148] The synthesis is

Figure 12. SEM images of ZnO dandelions. Reproduced with permissionfrom [148]. Copyright 2004 American Chemical Society.


based on oxidative dissolution of Zn micropowders, which

serve as the template for nucleation and growth of the ZnO

shell.

High-temperature gas–solid reactions have also been

devised to prepare ceramic hollow particles where the solid

particles serve as the sacrificial templates. Silicon carbide (SiC)

hollow spheres have been synthesized in this manner by

reacting carbon nanospheres with thermally generated Si or

SiO vapor at around 1300 8C.[149,150] The shell thickness can be

controlled by varying the reaction time, and the unreacted

inner carbon cores can be easily removed by combustion in air.

Ma et al. reported synthesis of polycrystalline aluminum

nitride (AlN) hollow nanospheres through the reaction of Al

nanopowder with a CH4–NH3 gas mixture at around

1000 8C.[151]

It is important to point out that sacrificial templates are not

restricted to solid particles; they can sometimes be liquid

droplets. Qian and co-workers synthesized metal carbide (TiC,

VC, ZrC) hollow spheres by co-reduction and then reaction of

metal source (TiCl4,VCl4, ZrCl4), and carbon source (C4Cl6,

C6Cl6) on the surface of metallic Na droplets at 500–600 8C,where Na droplets play important roles as both templates and

reductant.[152,153] Yin et al. reported synthesis of GaN hollow

nanospheres through interfacial chemical reaction between

liquid Ga droplets and NH3 gas in a carefully controlled

temperature gradient zone.[154] Xie et al. have recently

prepared Li2NH hollow nanospheres through reaction

between Li droplets and NH3 gas.[155] The formation

mechanism of the hollow particles was ascribed to the

Kirkendall effect by the authors. While all of the above

syntheses involve liquid droplet-gas reactions, a special case is

liquid droplet in solution,[156] which will shortly be discussed in

more detail. Huang et al. described a very interesting approach

for preparation of CdS hollow spheres, which takes advantage

of the unusual reaction between ethylenediamine (en) and

carbon disulfide.[157] CS2 is insoluble in water, and therefore

exists as droplets in aqueous media. The strong interfacial

reaction between en and CS2 produces H2S gas, which in turn

reacts with Cd2þ in the form of Cd(en)22þ forming CdS shells

around the droplets.

2.3. Soft Templates

Templating against hard (solid) templates is arguably the

most effective, and certainly the most common, method for

synthesizing hollow micro-/nanostructures. However, hard

templates have several intrinsic disadvantages, which range

from the inherent difficulty of achieving high product yields

from the multistep synthetic process to the lack of structural

robustness of the shells upon template removal. Additionally,

key applications of hollow structures, such as drug and

therapeutic delivery, require facile access to the hollow

interior space. With the hard templates, refilling the hollow

interior with functional species or in situ encapsulation of

guest molecules during formation of the shells, though


REVIE


Figure 13. A) SEM, and B,C) TEM images of g-AlO(OH) hollow nano-spheres. Reproduced with permission from [160]. Copyright 2007 AmericanChemical Society.

3998

possible, is very challenging. These difficulties have prompted

interest in simpler synthetic approaches for producing hollow

shells that permit easy encapsulation and release of guest

species. Among these approaches, templating against soft

(liquid or gaseous) templates has attracted the greatest

attention and significant progress has been made in the past

decade. In this section, we will survey commonly used soft

templates, including emulsion droplets, surfactant and other

supramolecular micelles, polymer aggregates/vesicles, and gas

bubbles, and discuss their application to synthesis of hollow

structures.

2.3.1. Emulsion Droplets

When two immiscible liquids are mixed together through

mechanical agitation (e.g., shaking, stirring), liquid droplets of

one phase can be dispersed in the other continuous phase,

forming an emulsion. In general, emulsions are thermodynami-

cally unstable, thus surfactants or amphiphilic polymers, which

self-assemble at the interface between the droplets and conti-

nuous phase, are required to increase kinetic stability.[158–161]

Oil-in-water (O/W) or water-in-oil (W/O) emulsions are most

commonly employed. It should be noted that emulsion

polymerization, where the droplets act as microreactors, has

long been used to synthesize solid and hollow polymer

spheres.[162,163] Analogously, the basic idea here is to deposit

the shell materials exclusively around the interface between the

emulsion droplets and the continuous phase. The precursor of

shell materials can initially exist in either the continuous phase or

the droplets or both phases, depending on the chemistry chosen.

Sol–gel processes for metal alkoxides are commonly

employed in emulsion templating to synthesize hollow spheres

of metal oxides such as silica and TiO2.[164–171] However, such

derived hollow spheres typically have sizes in the micrometer

range and possess broad size distributions.[172,173] Nakashima

etal.preparedTiO2hollowmicrospheres(3–20mm)bytemplating

against toluene microdroplets in an ionic liquid; 1-butyl-3-

methylimidazolium hexaflurophosphate ([C4mim]PF6).[174] The

precursor, Ti(OBu)4, is dissolved in the toluene phase. Upon

emulsification with [C4mim]PF6 by vigorous stirring, Ti(OBu)4hydrolyzes selectively at the interface with the trace amount of

water adsorbed in the [C4mim]PF6phase.Thismethodalsoallows

incorporation of functional nanoparticles and organic molecules.

Li et al. synthesized hollow spheres of aluminosilicate with

mesoporous shells by combining emulsiondroplet templating and

surfactant templating.[161] TheO/Wemulsion is formedby adding

TEOS into aqueous Al2(SO4)3 solution. The surfactant (CTAB)

molecules are responsible for stabilization of the droplets and

templating themesoporous shell formation.[161,172] In general, it is

very challenging to obtain uniform emulsion droplets with a small

size <100nm because of droplet coalescence and Ostwald

ripening.[175]Most recently, Feldmannand co-worker synthesized

boehmite (g-AlO(OH)) hollow nanospheres through hydrolysis

of Al(sec-OC4H9)3 via aW/O (n-dodecane) emulsion templating

route.[160] The emulsion droplets are stabilized by CTAB. As a

result, the as-prepared boehmite hollow nanospheres with size of

about 30nmarequiteuniformandnonagglomerated (seeFig. 13).


The authors also applied (n-dodecane)O/Wemulsion templating

tosynthesizegoldhollownanospheres.[159]However, theobserved

morphology is less well-defined, presumably due to rapid

deformation of thin gold nanostructures under electron beam

irradiation.

Despite the general difficulty in preparing monodisperse

hollow spheres by emulsion templating, several reports

describe how the method can be used for preparation of

monodisperse silica hollow spheres. Hah et al., for example,

reported a novel two-step sol–gel process for preparation of

monodisperse organosilica hollow spheres with controllable

size and shell thickness.[176] The intriguing mechanism by

which the synthesis progresses was recently further studied by

Wang et al. to involve O/W emulsion templating, where the

growth of shell through hydrolysis and condensation of

phenyltrimethoxysilane (PTMS) proceeds inward from the

interface to the interior of PTMS droplets.[177] As shown in

Figure 14, Zoldesi et al. synthesizedmonodisperse silica hollow

particles (spheres, capsules, balloons) by templating against

low-molecular-weight polydimethylsiloxane (PDMS) silicone

O/W emulsion droplets with diameters in the range of 0.6–2

mm.[178,179] After the silica shell is formed by the hydrolysis and

condensation of TEOS, the liquid PDMS cores can be easily

removed by solvent extraction.[180]

In addition to sol–gel processes, many other synthetic

approaches, such as hydrothermal methods[158] and g-irradia-

tion,[181–184] have been employed in combination with emul-

sion templating to synthesize hollow particles of various

crystalline materials, including metals,[185,186] oxides[187] and

sulfides,[157,188,189] and inorganic/polymer composites.[183,184]

Bao et al. synthesized Ni hollow spheres (300–450 nm) using

NiSO4 and NaH2PO2 as precursor and reducing agent

respectively.[185] The synthesis is carried out in a cyclohex-

ane–water–polyglycol emulsion system, where the polyglycol is

localized at the O/W interface. Although both NiSO4 and

NaH2PO2 are dissolved in the water phase, the deposition of Ni

takes place preferably at the interface because of the strong

affinity between polyglycol and Ni2þ. Wang et al. reported


REVIE


Figure 14. TEM images of silica hollow particles with different morphoo-gies: A,B) hollow microspheres, C,D) microcapsules, E,F) microballoons.Reproduced with permission from [178].

photochemical synthesis of Pt nanoshells by templating against

lipoporphyrin-stabilized benzene droplets in water.[186] The

lipoporphyrin-tin also serves as a photocatalyst for initial even

deposition of small Pt seeds onto the surface of emulsion

droplets. Magnetic nanoparticles can be incorporated into the

Pt nanoshells if they are initially dispersed in the benzene

droplets. Hu et al. described a complex emulsion system for

synthesis of high-quality TiO2/CdS composite hollow

spheres.[190] The emulsion system is made by mixing one

N,N-dimethylformamide (DMF)/water solution containing

cadmium acetate dehydrate, thiourea, and 1-thioglycerol, with

the other butanol solution containing Ti(OBu)4 and acetyla-

cetone. The shell structure initially formed at the interface

immediately after mixing is further strengthened by refluxing

at 140 8C for 3 h.

The liquid character and deformability of emulsion droplets

provide important advantages for hollow particle synth-

esis.[178,191] First, compared to solid templates, the liquid cores

can be easily removed by low-stress-generating processes such

as gentle evaporation or dissolution in common solvents such

as ethanol after the shell formation. Second, the high

deformability of the droplets should allow them to accom-

modate larger levels of shrinkage, which reduces cracking and

pulverization during post-treatments such as drying and

calcination. More importantly, templating against liquid

droplets allows facile and efficient introduction of functional

liquid cores or species like therapeutic and DNA molecules


inside the capsule,[160,167,169,170,173,174] which is particularly

attractive in drug delivery and pharmaceutical applications. In

practice, controlling the size, uniformity, and interfacial

reactions in emulsion droplets remains a serious challenge.

A related procedure employing self-assembled supramolecular

micelles/vesicles as soft templates can overcome some of these

challenges and as a result has been attracting growing

attention.

2.3.2. Supramolecular Micelles/Vesicles

Amphiphilic molecules such as surfactants can be made to

self-assemble into micelles or vesicles with different structures

when their concentration in solution exceeds some critical

values.[192–195] For example, at concentrations above

ca. 8.2mM at 25 8C (the so-called critical micelle concentration

(CMC)) in an aqueous solution, sodium dedecyl sulfate (SDS)

will spontaneously form spherical micelles, each containing

about 50 molecules arranged in a regular radial pattern. At

concentrations well above the CMC the micelle phase

transforms into vesicles comprised of a closed bilayer structure.

In general, the structure and stability of micelles or vesicles are

affected by many factors such as the solvent polarity, pH value,

and ionic strength of the solution. This sensitivity of micelle/

vesicle structure to easily controlled synthesis conditions can

in principle provide exquisite control over template morphol-

ogy and structure. In practice, however, it means that the

exact geometric and structural characteristics of micelles/

vesicles in a particular synthesis are rarely, if ever, known. It

is in fact quite common in the literature for the terms

‘‘micelle’’ and ‘‘vesicle’’ to be used interchangeably, even

though the phases are thermodynamically well delineated.

Thus, unless independent information about the phase

morphology of surfactant templates in the reaction media

are provided, it is safe to conclude that any vesicular

templating mechanisms (especially for synthesis of non-silica

materials) are best regarded as proposals or as a posteriori

rationalizations based on the structures produced. We believe

that to fully harness the diversity of hollow particle structures

and geometries possible with micelle and vesicle templating,

fundamental studies of the phase behavior of surfactants in

typical particle synthesis media are required. In situ scattering

studies that probe the evolution of particle morphology

during the synthesis can also shed needed light on these

templating processes.

Even in the absence of detailed understanding of micelle

and vesicle templating processes, hollow particles based on a

wide range of inorganic material chemistries have been

prepared using both types of templates. Silica is nonetheless

by far the most popular inorganic material used in synthesis of

hollow particles via soft templating.[196–203] This can be

understood in the following terms. First, the sol–gel chemistries

for hydrolysis and condensation of silicon alkoxides in aqueous

solution can be well controlled. Second, it benefits from the

enormous development in the field of surfactant directed

synthesis of ordered mesoporous silica,[196] for example,

MCM-41.[11] Furthermore, growth of silica and other biocom-


REVIE


Figure 15. TEM images of silica with multilamellar vesicular morphology. The arrows in (C) point tounilamellar vesicles. Reproduced with permission from [196]. Copyright 1998 American Associationfor the Advancement of Science.

Figure 16. TEM images of Cu2O hollow spheres with A) single-shells, B)double-shells, C) triple-shells, and D) quadruple-shells. Reproduced withpermission from [208].

4000

patible mineral materials on self-assembled supramolecular

vesicles is directly relevant to fundamentally important

processes like bio-mineralization[204] (e.g., formation of

diatoms) and bio-applicatioins.[205] Pinnavaia and co-workers

prepared ultrastable silica vesicles with the shells constructed

of lamellar sheets (see Fig. 15).[196] The method is based on

supramolecular assembly through hydrogen bonding between

neutral gemini surfactants of the type CnH2nþ1NH(CH2)2NH2

and silica precursors derived from TEOS. Hubert et al.

reported the first successful deposition of silica on unilamellar

surfactant vesicles of dioctadecyldimethylammonium bromide

(DODAB) by mild hydrolysis and condensation of silicon

alkoxides.[197] Lootens et al. prepared facetted hollow silica

particles by directed growth of silica on equilibrium icosahed-

rally facetted catanionic vesicles.[199] The wall of these micelles

is constructed of surfactant bilayers, made from cetyltrimethy-

lammonium hydroxide (CTAH) and myristic acid. Recently,

Lu and co-workers reported a vesicle–liquid-crystal dual

templating method for producing hollow spheres of periodic

mesoporous organosilica (PMO) with tunable wall thick-

ness.[201]

Micelles or vesicles have also been utilized as soft templates

to synthesize hollow spheres of other materials, such as

carbon,[206] metal oxides,[207–212] as well as somemetals.[213–216]

Xu et al. have recently prepared novel multishelled Cu2O

hollow spheres with single-crystalline shell structure via a

simple CTAB vesicle templating route in aqueous solution (see

Fig. 16).[208] Depending on the concentration of CTAB in the

range of 0.1–0.15M, the Cu2O hollow spheres are found to be

dominantly single-, double-, triple-, or quadruple-shelled in the

product. This was ascribed by the authors to the formation of

multilamellar vesicles of CTAB molecules. After synthesis of

sub-micrometer Ni hollow spheres in an emulsion system,[185]

Xu and co-workers also reported synthesis of nanometer-sized

Ni hollow spheres via the redox reaction of nickel dodecyl-

sulfate (Ni(DS)2) with NaH2PO2 in a Ni2þ-DS micellar

system.[216] Quaternary ammonium ions with short alkyl

chains do not form vesicles in the absence of other surfactants.

However, tetrabutylammonium bromide (Bu4NBr) is found to

assemble rapidly into vesicles in water when mixed with some

metal compounds such as PdCl2, K[AuCl4], AgNO3, K2[PtCl4],

or mixtures of these compounds. Such metal–compound-

induced vesicles can be efficiently utilized as both soft

www.advmat.de � 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

templates and in situ metal sources to

direct growth of multimetallic hollow

spheres, as recently demonstrated by

Zhang and Li.[213]

2.3.3. Polymer Aggregates/Vesicles

As pointed out in the previous section,

the structure and stability of surfactant

micelles/vesicles are sensitive to many

parameters, such as pH, temperature,

concentration, solvent, additives, and

ionic strength. While it might seem that

this provides many handles to manipulate

particle characteristics (e.g., size, shape, shell thickness, and

morphology), in reality this sensitivity makes it difficult to

control the synthesis. As a result, widespread use of surfactant

phases to template hollow particles remains challenging.

Surprisingly, in some cases these difficulties can be resolved

using more complex systems, such as aggregates/vesicles of

surfactant–polymer mixtures or even charged polymers. The

interaction between polyelectrolytes and surfactants has

received great attention because of the variety of applications

in which these systems can be used.[217] Specifically, it has been

found that the systems of polyelectrolytes and surfactants of

opposite charges generally show strong interactions, whereas

the systems of uncharged polymers and ionic surfactants or

polyelectrolytes and surfactants of the same charges show

relatively weak or no interaction. When the concentration

reaches some critical value known as ‘‘critical aggregation

concentration’’ or CAC, which is analogous to CMC, the

Adv. Mater. 2008, 20, 3987–4019

REVIE


Figure 17. SEM images of Ag hollow spheres formed in mixed PEO-b-PMAA-SDS solution. The scale bars are 5mm, 1mm in (A) and (B),respectively. Reproduced with permission from [218].

Figure 18. SEM and confocal microscopy (inset, note that the originalimage is in color) images of Au/SiO2 hollow microspheres. Reproducedwith permission from [235]. Copyright 2004 American Chemical Society.

surfactant-polymer or surfactant-free polymer systems tend to

aggregate to form micelles or vesicles.

Qi and co-workers have applied the complex poly(ethylene

oxide)-block-poly(methacrylic acid)-sodium dodecylsulfate

(PEO-b-PMAA-SDS) micelles as soft template to prepare

CaCO3 and Ag (see Fig. 17) hollow microspheres.[218,219] It is

known that PEO can interact with anionic SDS to form PEO–

SDS complex micelles with EO groups intermingling in the

head group region of the SDS micelles. Therefore, core/shell

micelles are formed in the PEO-b-PMMA-SDS system with a

core of PEO segments solubilized in the head group region of

the SDS micelles, and a corona of PMMA segments. The

anionic PMMA corona is beneficial for arresting cationic metal

ions. Yeh et al. prepared hollow silica spheres with

mesostructured shells using a vesicle template of CTAB-

SDS-P123 (P123 is EO20PO70EO20).[220] Ding et al. prepared

Fe3O4–polymer hybrid hollow nanospheres by polymerization

and cross-linking of acrylic acid (AA) in the shell of micelles

self-assembled from cationic chitosan and anionic AA

preloaded with poly(vinyl alcohol) stabilized Fe3O4 nanopar-

ticles.[221,222] After removing the polymer at 550 8C under

argon, relatively uniform Fe3O4 hollow nanospheres (ca. 22 nm

in diameter) can be obtained.

Surfactant-free polymer aggregates have also been widely

used to direct the formation of hollow spheres. Poly(ethylene

glycol) (PEG) micelles have been used as soft templates for

synthesis of Cu2O/PEG composite hollow spheres,[223] and Co

and CoPd hollow nanospheres.[224,225] Cu2O hollow spheres

have also been synthesized with glucose as the reducing agent

and gelatin micelles as soft templates.[226] In another work,

gelatin also plays important roles as inhibitor of the direct

attack of NH2OH to CuO surfaces and as soft templates for

aggregation of the growing Cu2O into hollow nanospheres.[227]


Li et al. prepared In(OH)3 (In2O3 after calcination) hollow

spheres by templating against vesicles formed in situ from

polymerization of formamide and resorcinol under hydro-

thermal conditions.[228] V2O5 hollow spheres composed of

nanorod building blocks have been prepared using PVP

micelles.[229] Mo et al. prepared ZnO hollow hemispheres

composed of nanorods and hollow spheres constructed from

nanosheets in solutions of poly(sodium 4-styrenesulfonate)

(PSS) and poly(acrylic acid-co-maleic acid) sodium salt

(PAAM), respectively,[230] in which the formation mechanism

may involve the template effect of the aggregated polymer

chains. Dai et al. have recently reported synthesis of CdS

hollow nanospheres with size about 25 nm from Cd(NO3)2 and

Na2S in an aqueous solution of polyglycol,[231] which is

speculated to result from the spherical aggregates induced by

the interaction of polyglycol chains with metal ions.[232]

Recently, hollow silica spheres with tunable sizes and wall

thickness have been synthesized using colloidal aggregates of

PAA as templates in the Stober method.[233]

Formation of hollow structures from self-assembly of

preformed nanoparticles is scientifically challenging. However,

if one can take advantage of the unique self-assembly or

aggregation characteristics of specially designed polymers,

multiple nanoparticles can be spontaneously assembled into

hollow structures. Stucky and co-workers reported that silica

and gold nanoparticles (ca. 10 nm) can be cooperatively

assembled with poly(lysine)-poly(cysteine) diblock copolypep-

tide into robust hollow microspheres.[234] The cysteine and

lysine blocks interact with gold through Au–S bonding and

silica nanoparticles through electrostatic interaction, respec-

tively, such that the walls of these as-formed hollow spheres

consist of a distinct layer of gold surrounded by an outer layer

of silica. Following-up studies appear to conclusively show that

use of copolypeptide is not necessary. Citrate-stabilized

nanoparticles (gold or core/shell CdSe-CdS quantum dots)

can also be assembled into vesicles with homopolymer

polypeptide like poly-L-lysine (PLL).[235,236] Upon addition


REVIE


4002

of silica sols, similar stable Au/SiO2 hollow microspheres are

generated as shown in Figure 18. The formation mechanism is

however theorized to proceed through flocculation, in which

charge-driven aggregation of citrate-stabilized nanoparticles

by PLL provides the critical first step. Wong and co-workers

later showed that these cationic polyelectrolyte chains undergo

counterion condensation in certain salt solutions to form

polymer aggregates ionically crosslinked by multivalent anions

like ethylenediaminetetraacetate (EDTA) and phos-

phate.[237,238] These polymer aggregates then serve as soft

templates for generating microcapsules from various nano-

particles like silica and tin oxide.[237,238] Besides the ease of

synthesis under mild conditions, the other advantage is that

water-soluble compounds, such as enzyme and dye molecules,

can be facilely encapsulated by adding a solution of the

compounds to the polymer aggregates prior to addition of

nanoparticle sols.[237,239]

In a selective solvent, block copolymers will self-assemble to

form micellar or vesicular structures.[195] In a direct analogy to

molecular surfactants, these structures can be used as soft

templates for the formation of inorganic shells around the

surface.[240] For example, poly(ethylene oxide)-poly(propy-

lene oxide)-poly(ethylene oxide) (PEO-PPO-PEO) triblock

copolymers, which form lamellar vesicles with the hydrophobic

PPO blocks shielded from the aqueous phase by hydrophilic

PEO blocks protruding out from the vesicle wall on both

sides,[241] are commonly used for preparation of hollow

particles. The complex micelles formed from P123 and metal

acetate have been utilized by Qi and co-workers as soft

templates to prepare CdS and ZnS hollow spheres.[242,243] The

as-prepared hollow particles are largely interconnected. Li

et al. reported a one-step solvothermal route to prepare

superparamagnetic firrite/polymer hybrid hollow spheres.[244]

This synthesis involves formation of ferrite nanoparticles and

simultaneous self-assembly of nanoparticles and PEO-PPO-

PEO into hollow spheres. Polymer hollow nanospheres can

also be derived from the self-assembled core/shell polymer

micelles.[245,246] The procedure usually involves the cross-

linking of the shell followed by selective degradation of the

core.

2.3.4. Gas Bubbles

Gas bubbles dispersed in a liquid host can be used to create

stable emulsions and foams, which have recently emerged as

Scheme 5. Schematic illustration of the gas-bubble templating process forpreparing hollow spheres. Reproduced with permission from [247].


promising soft templates for synthesis of an increasing number

of hollow particles. In general, the process can be conceptua-

lized in three steps as illustrated in Scheme 5: formation of fine

nanoparticles and gas bubbles, attachment of fine nanoparti-

cles on the gas/liquid interface, and further aggregation of

nanoparticles forming compact shells around the gas bub-

bles.[247,248] It is known that the attachment of solid particles to

gas bubbles is a complex process, which is affected by many

factors, such as particle surface properties, particle size,

electrostatic interactions, and hydrodynamic conditions.[249]

Han et al. prepared CaCO3 hollow spheres by blowing a

mixed gas (CO2 and N2) into a solution of CaCl2 and

ammonia,[248] while the majority of such works involve use of

in situ generated gas bubbles. ZnSe hollow spheres have been

synthesized under hydrothermal conditions with hydrazine as

the reducing agent, in which the resultant N2 gas bubbles are

proposed to play the role of soft templates.[247,250] Gu et al.

prepared ZnS hollow nanospheres via aggregation of small

nanoparticles around the evolved H2S bubbles produced from

decomposition of thioacetamide.[251] Xie and co-workers have

synthesized hollow spheres of TiO2, Co2P, and VOOH (see

Fig. 19) by templating against gas bubbles of O2,[253] PH3 and

CH4,[254] and N2,

[252] respectively. Recently, chains of Co

hollow sub-microspheres have been synthesized via aggrega-

tion of PVP-stabilized primary nanoparticles presumably

assisted by the gas bubbles liberated from the reaction.[255]

Gas bubble templating has also been suggested as a means for

preparing hollow spheres of sulfides (ZnS,[256] CuS[257]), oxides

(SnO2,[258] TiO2,

[259] Fe3O4[260]). It should be pointed out that

the gas bubble templating mechanism remains highly spec-

ulative, especially in systems involving highly soluble gases

(CO2, NH3) resulting from decomposition of organic mole-

cules such as urea and thiourea.[257,258,260]

Another special type of gas bubble templating is utilized in

sonochemical synthesis of hollow particles, where the gas

bubbles generated from the collapse of the acoustic cavita-

tion[261] are generally believed to serve as the soft templates for

shell formation.[101] It has been suggested that cavitation

microbubbles created by sonication of a surfactant-containing

medium are particularly advantageous because they can be

stabilized in a similar manner to normal liquid-liquid

emulsions.[101] Electrostatic LBL assembly of polyelectrolyte

multilayers (polyallylamine, poly(styrene sulfonate)) was

Figure 19. a) SEM, and b) TEM images of VOOH hollow dandelions.Reproduced with permission from [252].


REVIE


successfully accomplished on the surface of air-containing

microbubbles made by ultrasonic agitation of a mixture of

Span/Tween block co-polymers.[262] This ultrasound-assisted

method has been applied for synthesizing submicrometer

hollow silica spheres with mesoporous walls by the application

of ultrasound to an aqueous solution containing surfactant and

silica precursor (TEOS).[198,263]

Ultrasonic synthesis appears very effective for production of

hollow particles such as CdSe[264] by ultrasonically induced

reactions between initial precursors at the gas/liquid interface

of the cavitation microbubble. Wang et al. reported synthesis

of PbS hollow nanospheres with diameters of 80–250 nm by a

surfactant-assisted sonochemical route from Pb(Ac)2, thioa-

cetamide, and sodium dodecylbenzenesulfonate (SDBS).[265]

The ultrasound wave promotes assembly of SDBSmolecules at

the gas/liquid interface of the microbubble to form template

structures, which directly determine the diameter of the final

PbS spheres. CdSe hollow spheres with sizes of 100–200 nm

have previously been synthesized by a SDS-assisted sono-

chemical approach.[266] Cai et al. have recently reported an

interesting CTAB-assisted sonochemical method for synthesis

of biocompatible nearly amorphous calcium phosphate hollow

nanospheres (see Fig. 20) with average diameter of about

145 nm.[205] Importantly, the method also enables facile

encapsulation of drug molecules. The observation that both

CTAB and ultrasound are essential for the formation of hollow

nanospheres suggests the validity of the templating mechanism

against surfactant-stabilized gas cavitation microbubbles,

which although was not realized by the authors.

2.4. Template-Free Methods

It should be clear from the preceding sections that as a group

templating methods have proven very effective and versatile

for synthesizing a wide array of hollow structures. Disadvan-

tages related to high cost and tedious synthetic procedures

Figure 20. TEM and SEM (inset) images of calcium phosphate hollownanospheres. Reproduced with permission from [205]. Copyright 2007American Chemical Society.


have impeded scale-up of many of these methods for large-

scale applications. For instance, the template-removal step is in

general indispensable when hard templates are used, which not

only significantly complicates the process but also detrimen-

tally affects the quality (e.g., high impurity levels and inevitable

shell collapse) of the as-derived hollow particles. Ideally, one

would prefer a one-step template-free method for controlled

preparation of hollow structures in a wide range of sizes. In this

sense, it is highly desirable to explore other simpler and more

efficient synthetic strategies for hollow structure synthesis.

Recently, one-step self-templated methods based on novel

mechanisms, such as inside-out Ostwald ripening (a sponta-

neous process, first described by Wilhelm Ostwald in

1896,[267,268] refers to the growth of large precipitates at the

expense of smaller precipitates caused by energetic factors),

have been successfully applied to synthesize hollow structures

of a wide range of materials. Lou et al., for example, reported a

simple one-pot template-free synthesis of polycrystalline SnO2

hollow nanostructures.[269] The synthesis is performed in an

ethanol-water mixed solvent using potassium stannate

(K2SnO3 � 3H2O) as the precursor. As shown in Figure 21,

Figure 21. A) Schematic illustration of the proposed inside-out Ostwaldripening mechanism. Typical TEM images of SnO2 particles obtained withB) 6 h of reaction, and D) 24 h of reaction. C) Amorphous SnO2 nano-spheres obtained by aging at room temperature. E, F) TEM and SEMimages of interconnected core/shell SnO2 nanostructures. The scale barsin (B–D) are 500 nm. Reproduced with permission from [269].


REVIE


4004

discrete spherical hollow or interconnected hollow core/shell

SnO2 structures can be prepared by simply adjusting the

synthetic conditions (e.g., ethanol fraction in the mixed

solvent). A simple plausible mechanism, inside-out Ostwald

ripening, was proposed to account for the template-free for-

mation of hollow SnO2 structures as illustrated in Figure 21A.

At the initial stage of reaction, amorphous solid nanospheres

are formed by hydrolyzation of stannate. With time, the

surface layer of the nanospheres crystallizes first due to contact

with the surrounding solution. As a result, the materials inside

the solid spheres have a strong tendency to dissolve, which

provides the driving force for the spontaneous inside-

out Ostwald ripening. This dissolution process could initiate

at regions either near the surface or around the center of

the solid spheres, presumably depending on the packing

of primary nanoparticles and ripening characteristics, to

produce hollow core/shell particles or hollow spheres.

The proposed mechanism is supported by several experi-

mental observations. First, time-dependent experiments indi-

cate that after 6 h of reaction the product consists entirely of

solid nanospheres (see Fig. 21B), while after 24h hollow

nanospheres are produced (see Fig. 21D), with a similar size

distribution as their solid precursors. Moreover, hollow spheres

can also be prepared by hydrothermally treating the suspension

of amorphous solid spheres (see Fig. 21C). Finally, if the hollow

interior space is indeed created by the spontaneous evacuation

of the interior through the shell, it should follow that the as-

formed SnO2 shellmust be highly porous since there is generally

no opening observed. N2 sorption measurements confirm that

this highly mesoporous structure leads to a very high Brunauer–

Emmett–Teller (BET) specific area of ca. 110 m2 g�1 (note that

the bulk density of SnO2 is ca. 6.95 g cm�3).[270]

A similar inside-out evacuation process may explain early

syntheses of semiconductor (CdS, ZnS)[271] and titania[272]

core/shell hollow structures. The formation of initial core/shell

hollow structures is however generally attributed to a two-step

precipitation process:[272,273] precipitation of solid cores and

subsequent precipitation of the outer shell onto the ionic-

species-stabilized cores. Yang et al. reported a one-pot

synthesis of anatase TiO2 hollow spheres with diameters of

0.2–1.0 mm by hydrothermally heating a TiF4 solution (1.33–

2.67mM),[274] in which Ostwald ripening was employed to

account for the hollowing effect. In general, reducing the

overall surface energies provides the driving force for

Ostwald ripening within an ensemble of particles. But with

respect to hollowing, no apparent driving force can be easily

identified. Based on an illustrative drawing, it was argued

that for solid spheres composed of numerous small particles,

the small particles in the inner cores have high surface

energies compared to those in the outer surfaces, because

they can also be visualized as smaller spheres having a higher

curvature.

Although fundamental evidence in support of this mechan-

ism is lacking, it has been shown to be applicable in many

systems, including TiO2,[275–277] Cu2O,[278,279] ZnS and

Co3O4,[280] Sb2S3,

[281] Ti1–xSnxO2,[282] Fe3O4,

[283] and


ZnO.[284] Interestingly, in the synthesis of Cu2O hollow

structures,[278,279] it has been shown that the formation

mechanism includes four different steps: generation of primary

nanocrystallites of CuO; spherical aggregate of the primary

CuO; reductive conversion of CuO to Cu2O; and hollowing

process via inside-out Ostwald ripening. Composite hollow

spheres can be synthesized via this route as long as the

components’ chemical properties are similar. For example, Li

et al. have simply extended the above synthesis of TiO2 hollow

spheres to synthesize high-quality Sn-doped TiO2 (i.e., Ti1–

xSnxO2) hollow spheres starting with SnF4 and TiF4.[282] Unlike

TiO2 hollow spheres, however the product also contains

significant fraction of fluorine. Hollow spheres comprised of

nanosheets can also been formed via inside-out Ostwald

ripening.[285] Time-dependent experiments suggest that there

in general exist three stages of growth.[285,286] In the first stage,

the precipitated crystallites assemble together to form loose

aggregates. With increasing reaction time, the aggregates

continuously grow in size and density to form spheres with

dense cores. Finally, interior cavities are gradually generated

via inside-out Ostwald ripening.Wang et al. prepared Ni(OH)2hollow microspheres with b-Ni(OH)2 nanosheets as the in situ

formed building units in a strong alkaline solution of

glycine.[286] The Ni(OH)2 hollow spheres can be easily

converted to NiO without morphological change by thermal

decomposition at 600 8C.The mechanism for this self-templated formation of hollow

spheres has recently been further elaborated in several works

by Mann and co-workers.[287–289] The term ‘‘localized Ostwald

ripening’’ is used to describe the preferential dissolution of the

particle interior. In essence, the concept is identical to the one

proposed for the formation of SnO2 hollow structures.[269]

Namely, the chemically induced self-transformation is char-

acterized by the initial kinetically favored deposition of

amorphous solid spheres. With increasing reaction time, the

surface layer first transforms to a thermodynamically more

stable form (i.e., it crystallizes), as the supersaturation falls in

the surrounding solution. Thus an ultrathin shell of less-soluble

crystalline phase is formed on the amorphous solid spheres. As

a result, the amorphous core will have a strong tendency to

dissolve and diffuse out through the shell because it remains

out of equilibrium with the surrounding solution. However,

unlike the case of SnO2,[269] the transformation from initial

amorphous solid spheres to crystalline hollow spheres is not

generally observed. In most cases, the hollowing process

actually takes place in crystalline particles, for example, in the

above cases of Cu2O[279] and TiO2.

[274] Therefore, crystal-

lization can not be the universal driving force for such

spontaneous inside-outOstwald ripening. In fact, the structural

evolution from solid, to core/shell hollow, to complete hollow

spheres has been observed for amorphous TiO2,[277] which

transform to highly crystalline anatase phase only upon

calcination at 550 8C, and CaCO3.[290] In the latter, the

anhydrous amorphous core is less stable compared to the first

hydrated surface layer, promoting preferential dissolution of

the core.


REVIE


The process of spontaneous dissolution of the particle

interiors can be further rationalized by considering the

stabilization effect of the surface layer by surfactants or simple

inorganic anions in the solution. As a result, the interior

materials will have relatively high surface energy and thus

dissolve preferentially. This selective dissolution of interior is

best demonstrated in syntheses of iron oxide hollow struc-

tures.[291–296] Simple inorganic anions such as phosphate and

sulfate are known to adsorb strongly on surface of iron

oxides.[291,292] Since the exterior surface of the particles are

protected by the adsorbed species, it is comprehensible that the

dissolution will occur preferably in the interior. Other self-

templated approaches for synthesis of hollow structures are

based on similar ideas (see Sec. 4). For example, Hu et al.

synthesized TiO2 microcapsules with tunable size and wall

thickness by heating sol–gel derived TiO2 microspheres with

poly(acrylic acid) (PAA) in a diethylene glycol (DEG)

solution.[297] The dissolution of surface layer of TiO2

nanoparticles is prevented by crosslinking with PAA. As a

result, the interior TiO2 is selectively removed by forming

soluble titanium glycolate.

There are many other template-free methods that appear to

involve non-conventional mechanisms.[298,299] As an example,

amorphous iron oxide nanospheres are transformed into

hollow nanospheres upon exposure to high energy electron

beam.[300] Tartaj et al. reported an aerosol pyrolysis method for

direct synthesis of silica coated g-Fe2O3 hollow spheres with

sizes of 50–250 nm (see Fig. 22) from a methanol solution

containing iron ammonium citrate and TEOS.[299] As illu-

strated in Figure 22C, during the first stage the rapid

evaporation of the methanol solvent favors the surface

precipitation (i.e., formation of hollow spheres) of the

components. The low solubility of the iron ammonia citrate

in methanol, compared to that of TEOS, promotes the initial

precipitation of the iron salt solid shell. With the continuous

Figure 22. A,B) TEM images of g -Fe2O3/silica composite hollow nano-spheres prepared by aerosol pyrolysis. C) Schematic illustration of theformation mechanism. Reproduced with permission from [299].


shrinkage of the aerosol particles, iron salt and TEOS enrich at

the surface, which eventually decompose to produce silica

coated g-Fe2O3 hollow spheres. There are still numerous other

template-free methods, sometimes called self-assembly or self-

aggregate approaches, that involve less-understood mechan-

isms.[301–304] Although with low versatility, once developed,

such self-aggregate methods might be able to produce high-

quality hollow particles of important materials in a surprisingly

efficient way.[302] As an example, Mao et al. have recently

prepared high-quality V2O3 hollow spheres with controlled

sizes of 0.2–1.1mm by simply heating vanadium(IV) acetyla-

cetone dissolved in DMF at 210 8C for 18 h.[305] Moreover,

these low-valent V2O3 hollow spheres can be thermally

converted to V2O5 hollow spheres.

3. Rattle-Type Hollow Structures

Rattle-type hollow structures refer to hollow shells with a

solid particle core and interstitial hollow space in between.

Often denoted as A@B, the core (A) and the shell (B) are

typically made from different materials, such as, metal@silica,

metal@carbon, metal@oxide, metal@polymer, oxide@carbon,

oxide@silica, silica@oxide. There are, however, a few reports

of so-called nano-rattles in which the core and shell aremade of

the same material. Owing to their unique structure, rattle-type

nano-architectures are receiving increasing attention because

of their many potential applications, for example, as

nanoreactors. They are also particularly relevant to the

nanocontainer type of functionalities for biomedical applica-

tions, that is, controlled loading and release of functional

I (a)

(b)

II

III (a) (b)

(b)

IV (a)

V

Scheme 6. Schematic illustration of strategies for preparing rattle-typehollow structures.


REVIE


Figure 23. A) Backscattering SEM and B) TEM images of Au@polymernano-rattles with shell thickness of about 22 nm. Reproduced with per-mission from [306]. Copyright 2003 American Chemical Society.

4006

species. Currently, most of the research efforts in this area are

directed to developing new synthetic approaches for different

materials. The major synthetic strategies for rattle-type

architecture are illustrated in Scheme 6.

3.1. Bottom-up Fabrication of Nanorattles

The most straightforward synthetic strategy is the so-called

bottom-up approach (see Scheme 6.Ia), where the core and

shell are fabricated in an inside-to-outside order. In this

approach, the core particles (e.g., gold) are first synthesized,

followed by consecutive coating with a spacer layer made of a

different material (e.g., silica) that can usually be removed by

simple methods like dissolution or calcination, and an outer

layer made of desired material (e.g., polymer or metal oxide).

In the last step, the spacer layer is removed to produce

nanorattles. Xia and co-workers first applied this idea to

prepare Au@polymer nanorattles, that is, polymer hollow

spherical colloids with movable Au cores (see Fig. 23).[306] The

procedure consists of three major steps: conformal coating of

Au nanoparticles with uniform shells of silica (the spacer

layer), formation of poly(benzyl methacrylate) shells on

Au@silica colloids by a well-known method called atom

transfer radical polymerization (ATRP), and lastly selective

dissolution of the sandwiched silica spacer layer in aqueous HF

solution. This concept has been utilized by others to produce

nanorattles with various combinations of core and shell

materials like Au@polymer,[307] and Au@ZrO2 as high-

temperature-stable catalyst.[308] Zhang et al. prepared silica@-

TiO2 nanorattles by using PS as the spacer layer where the

Figure 24. TEM images of A) Au@silica core/shell particles, B) double-shelcolloids, and C) Au@SnO2 nanorattles after HF etching. Reproduced with p


TiO2 shell is achieved through adsorption of tetrabutyl titanate

(TBT) into sulfonated PS.[309] Lou et al. prepared Au@SnO2

based on hydrothermal deposition of SnO2 on Au@silica (see

Fig. 24).[57] Interestingly, double-shelled SnO2 can be obtained

and the silica spacer layer is largely dissolved in situ during the

hydrothermal deposition under basic conditions. Apart from

polymer and inorganic materials, self-assembled surfactant

layers can also serve as the spacer layer. One such example is

described by Han and co-workers, in which Au@silica

nanorattles are synthesized by depositing silica on gelatin-

wrapped Au nanoparticles.[310] As discussed in Section 2.1.4,

hollow spheres with mesoporous shells (HSMS) can be

prepared by nanocasting particles with solid core mesoporous

shell (SCMS). Combining this bottom-up approach with the

nanocasting method for preparation of HSMS discussed in

Section 2.1.4, a series of rattle-type structures with meso-

porous shells have been obtained, including Au@polymer,

Au@carbon, Au@silica, and Pt@carbon.[78,311,312]

Lou et al. have extended this bottom-up approach to

prepare rattle-type hollow spheres encapsulating multiple

nanoparticles.[313] As illustrated in Scheme 6.Ib, the procedure

for preparing multicore Au(or Pt)@silica nanorattles consists

of three major steps: in situ growth of Au (or Pt) nanoparticles

on the surface of amino-functionalized PS nanospheres, silica

coating of PS@Au(or Pt), and removal of PS templates by

calcination at 450 8C. Figure 25 depicts the particles obtained

after each step. It is clear that uniform silica shells

functionalized with many Au nanoparticles in their interior

space are obtained. The size of Au nanoparticles inside the

shell increases after calcination, ranging from 10 to 50 nm,

presumably due to the fusion of the adjacent Au nanoparticles

during thermal treatment. Based on a conceptually similar

approach, Choi et al. have recently reported a versatile method

to prepare a-Fe2O3 (or silica, or a-Fe2O3/silica) capsules

loaded with size-controlled monometallic (Au, Pt, Ag) or

bimetallic (AuPt) cores.[314] The method involves several

steps. First, metal nanoparticles are consecutively synthesized

within the polyelectrolyte multilayers (PEMs) coated on

melamine-formaldehyde (MF) templates by LBL technique

(see Sec. 2.1.1). After loading the metallic nanoparticles, the

PEMs particles are encapsulated with a dense jagged layer of

goethite (a-FeOOH) (or silica formed by the modified Stober

method) grown from hydrolysis of adsorbed Fe2þ. Lastly,

led Au@SnO2 hollowermission from [57].

Co. KGaA, Weinheim

Au@a-Fe2O3 nanorattles with burlike

morphology are obtained by calcination

at 700 8C. This last step not only elim-

inates all the organic components but also

leads to formation of a single large

metallic core through thermal agglomera-

tion.

3.2. Top-Down Fabrication of

Nanorattles

The other conceptually feasible strat-

egy to fabricate rattletype particle archi-

Adv. Mater. 2008, 20, 3987–4019

REVIE


Figure 25. TEM images of A) Au-decorated PS nanospheres, B)PS@Au@silica colloids, and C) multicore hollow silica spheres afterburning off PS templates. D) Backscattering SEM (inset, normal SEM)image ofmulticore hollow silica spheres. All scale bars in (A–C) are 500 nm.Reproduced with permission from [313].

tectures is the so-called top-down approach illustrated in

Scheme 6.II. A key feature of this approach is that the shells are

constructed first, followed by formation of the cores. Usually

two types of reactants diffuse sequentially into the cavity to

react thus forming the cores, for example, reduction of metal

salts to formmetal nanoparticles. Using this method, Hah et al.

prepared Cu@silica nanorattles by reduction of Cu2þ with

hydrazine inside the silica shells.[315] Since the reduction is

carried out in a bulk Cu(NO3)2 solution containing silica shells,

Cu particles are dominantly formed outside the silica shells.

These Cu aggregates are reported to settle down on the bottom

of the container, hence allowing easy separation. By repeating

this soaking–reduction–separation cycle, the size of the Cu

cores inside the silica shells can be increased. Cheng et al.

prepared Ag@polypyrrole-chitosan (PPy-CS) nanorattles by

photoreduction of Agþwith CS.[316] In this case, only the silver

salt is required to diffuse into the PPy-CS nanoreactors since

one component of the shells can serve as the reducing agent.

Moreover, the amount of AgNO3 loaded into the nanoreactors

can be easily controlled by adjusting the pH-sensitive

permeability of the polymer shells. Other reports include

Au@silica[317] and oxides@carbon.[318] Despite its conceptual

simplicity, this approach is often associated with disadvantages

such as low efficiency and tedious procedures, which arise


mainly from the difficulty in ensuring that the reaction takes

place exclusively inside the shells.

3.3. Other Methods for Nanorattle Fabrication

Rattle-type architectures can also be constructed from other

less conventional approaches. For example, starting with core/

shell particles as illustrated in Scheme 6.III, interior hollow

space can be created either by partial dissolution of the cores as

in the examples of silica@SnO2[57] and Fe2O3@silica[319] or

reducing the shell thickness from inside as for Au@TiO2.[276] In

addition, some of the mechanisms discussed in Section 2 for

generating hollow structures can also be employed to produce

rattle-type structures, for example, inside-out Ostwald ripen-

ing (see Scheme 6.IVa),[269] and Kirkendall or sacrificial

templating processes (see Scheme 6.IVb).[115,116,320] Rattle-

type structures obtained by inside-out Ostwald ripening are

made of only one material, for example, SnO2,[269]

TiO2,[272,275,277] ZnO,[284] ZnS, and Co3O4.

[280]

Besides the synthetic approaches discussed above, new

reports appear frequently, which describe novel methods for

facile preparation of functional nanorattles (e.g., Sn@car-

bon[321]). As an example, Ng et al. have recently reported a

novel method for producing Pt (ca. 2.9 nm)@carbon nanorattles

by photoirradiating a deaerated aqueous suspension of TiO2

nanoparticles, phenol, and a small amount of Pt precursor with a

UV light source, followed by carbonization at 700 8C under

vacuum and subsequent dissolution of TiO2 in HF solution.[322]

In the process, TiO2 nanoparticles function as not only the

photcatalyst for formation a phenolic polymer and Pt nano-

particles but also the physical template. Note that this method is

much more efficient compared to the bottom-up approach

involving small Pt nanoparticles of several nanometers as the

cores, which is also demonstrated by the same group.[312] In view

of the general difficulty in fabricatingnanorattles involving small

nanoparticles as the cores, a conceptually versatile conversion

route (see Scheme 6.V) is proposed to produce many more

functional nanorattles. For example, if well-defined core/shell or

rattle-type particleswith reactive cores (e.g., Co@carbon) canbe

synthesized by some novel method, desirable noble metallic

(e.g., Ag, Au, Pt, Pd) or better multimetallic nanoparticles can

be synthesized exclusively inside the shells, addingmore realism

to the nanoreactor concept.

4. Non-Spherical Hollow Structures

Compared to spherical counterparts, the synthesis of hollow

particles with well-defined non-spherical shapes (tubelike

structures are excluded from this discussion) remains a

significant challenge to materials scientists. This situation is

readily traced to the fact that the synthetic approaches for

spherical hollow structures as discussed in Section 2 do not

generally apply for synthesis of non-spherical hollow struc-

tures. For example, soft templates, such as surfactant micelles/


REVIE


Figure 27. A) TEM and B) SEM images of double-walled SnO2 nano-cocoons with movable a-Fe2O3 spindles. Reproduced with permissionfrom [328].

4008

vesicles and emulsion droplets, in general do not assume well-

defined non-spherical (polyhedral) shapes in order to minimize

the interfacial energy. Even with hard templates, preparation

of non-spherical hollow structures introduces additional

challenges. These range from the difficulty in forming a

uniform coating around surfaces with large variation in

curvature to the paucity of non-spherical templates available

for the synthesis. Because of these difficulties, reports on

synthesis of non-spherical hollow structures are relatively few,

and there is no established general method.

4.1. Hard Templating Synthesis of Non-Spherical Hollow

Structures

Non-spherical hollow structures have been prepared by

templating against shaped colloids made of polymers or

inorganic materials. Polymer beads can be converted into

ellipsoidal particles by viscoelastic deformation at an elevated

temperature.[323] Similarly, macroporous polymers (inverse

opal) can be compressed or extended to form unusual void

shapes.[85] Such oval-shaped polymer beads or voids have been

employed as templates for preparation of eggshell-like TiO2

hollow particles.[85,324] Hollow polypyrrole-chitosan (PPy-CS)

hollow nanostructures with different shapes (sphere, cube and

plate) and a wide range of sizes can be facilely fabricated using

shaped AgBr nanoparticles as templates.[325] Hollow octahe-

dral polyaniline (PANI) has been prepared by templating

against octahedral Cu2O crystals,[326] which can be removed by

reacting with ammonium persulfate (APS) in an acid solution

to form a soluble Cu2þ salt.

Hematite (a-Fe2O3) is one of a very small number of

inorganic materials that can be facilely synthesized in large

amounts with a wide range of sizes and shapes. Lou et al. have

recently demonstrated a templating scheme based on mono-

disperse non-spherical hematite colloids that provides a

general route for preparation of non-spherical anatase TiO2

hollow particles (see Fig. 26).[327] The polycrystalline anatase

TiO2 is hydrothermally deposited on hematite particles by a

PVPmediated protocol in an ethanol-dominant, ethanol/water

Figure 26. TEM images of six types of a-Fe2O3@TiO2 core/shell particles (TiO2 hollow particles after removal of a-Fe2O3 cores (B1–B6), respectively. AllReproduced with permission from [327].


mixed solvent. The presence of PVP is critical for uniform

deposition of TiO2 and forming discrete hematite/TiO2 core/

shell particles, while interconnected chainlike structures are

obtained in the absence of PVP. Hematite templates can be

selectively etched out in dilute HCl solution to produce non-

spherical TiO2 hollow particles. Lou et al. have also reported

synthesis of double-walled SnO2 ‘‘nanococoons’’ (see Fig. 27)

by facile deposition of polycrystalline SnO2 on oval-shaped

silica-based templates derived from hematite nanospindles.[328]

Themodified Stober’s process is employed to coat the hematite

spindles with a layer of silica to produce the oval-shaped

hematite/silica core/shell particles. Importantly, the silica

coating step not only reduces the surface curvature but also

modifies the surface properties, which are beneficial to

subsequent hydrothermal deposition of polycrystalline SnO2

forming uniform shells. Hematite nanospindles have also been

used as templates to produce ellipsoidal hollow capsules of

silica and polymer.[329]

Notwithstanding these successes of multistep conven-

tional hard templating, it is desirable to develop simpler

templating strategies for synthesis of non-spherical hollow

particles. Lou et al., for example, have recently demonstrated

a novel one-step approach for preparation of octahedral

silica nanocages (see Fig. 28).[330] The synthetic strategy is

described as follows. A solution of H2PtCl6 is added to a

mixture of ethanol/H2O/ammonia containing tetraethy-

lorthosilicate (TEOS) and aminopropyltrimethoxysilane

A1–A6) and their correspondingunlabeled scale bars are 200 nm.

Co. KGaA, Weinheim

(APTMS), to instantaneously

form a yellow (NH4)2PtCl6precipitate. This is followed

by in situ surface functionaliza-

tion with amino groups from

APTMS, which plays an impor-

tant role for subsequent deposi-

tion of silica on the (NH4)2PtCl6quasi-template. This ‘‘quasi-

template’’ method has several

important advantages over con-

ventional template strategies.

First, the quasi-templates though

insoluble in the original ethanol/

H2O reaction mixture are easily

removed by washing with water.

Adv. Mater. 2008, 20, 3987–4019

REVIE


Figure 28. a–d) TEM and e) SEM images of octahedral silica nanocagesafter intensive washing with water to remove Pt salt quasi-templates. Thescale bar in (d) is 2mm, all others are 500 nm. Reproduced with permissionfrom [330].

Figure 29. TEM images of CuxS nanocages with a) cubic or b) octahedralmorphology, insets are selected-area electron diffraction patterns. Repro-duced with permission from [331].

Moreover, the template is generated in situ, which renders

presynthesized hard templates unnecessary, thus allowing

structures with a wider range of sizes, and of high purity to be

readily synthesized. Furthermore, the quasi-template itself is the

precursor to noble metal nanoparticles, which can yield

controlled Pt-functionalized products by simple calcination.

4.2. Sacrificial Templating Synthesis of Non-Spherical

Hollow Structures

Sacrificial templates with non-spherical shapes provide

another versatile route for synthesizing non-spherical hollow

particles. Shape-controlledCu2Ocrystals are extensively usedas

sacrificial templates for preparation of copper sulfide hollow

structureswithvariousmorphologies. Jiaoetal.havesynthesized

well-defined non-spherical (cubic, octahedral, and flower-like)

copper sulfide mesocages with single-crystalline shells (see

Fig. 29).[331] The procedure involves the growth ofCu2O crystals

with different morphologies, the formation of Cu2O/CuxS core/


shell structures in Na2S aqueous solution, and then the

dissolution of unreacted Cu2O cores in ammonia solution. By

changing the reaction atmosphere from N2 to air, the composi-

tions of the obtained single-crystallinemesocages can be altered

from Cu2S to Cu7S4 phase. Cao et al. obtained Cu7S4 nanocages

with a high symmetric 18-facet polyhedral morphology by

reacting cubic Cu2O nanocrystals with another sulfur source

(thiourea) at 90 8C.[332] Unlike Jiao’s work above, the Cu2O

couldbe completely converted toCu7S4with prolonged reaction

time of 6 h or longer and the formation of nanocages is simply

ascribed to the Kirkendall effect.[115] However, in another work

of reacting octahedralCu2Owith thiourea at 90 oC toobtainCuS

nanocages,Ostwald ripening is shown tobe themainmechanism

for the formation of hollow interiors.[333] Other shaped crystals

have also been used as sacrificial templates to create non-

spherical hollow structures such as CdS, CuO andMoS2.[334–338]

Qi and co-workers prepared single- or double-walled rhombo-

dodecahedral silver microcages by reducing precursor Ag3PO4

crystals with ascorbic acid, hydrazine orNaBH4.[339] Xia’s group

and others have prepared a wide range of polyhedral metallic

nanoboxes (see Fig. 10) by galvanic replacement reactions

between shaped Ag nanocrystals and more noble metal

salts.[128,130,340]

4.3. Soft-Templating Synthesis of Non-Spherical Hollow

Structures

Soft-templating strategies for synthesizing non-spherical

hollow structures have been reported in a small number of

articles. Under well-controlled conditions, faceted silica hollow

structures can be synthesized by directed growth of silica on

equilibrium icosahedrally facetted catanionic vesicles made

from cetyltrimethylammonium hydroxide/myristic acid surfac-

tant bilayers.[199] For systems involving single-crystal struc-

tures, the intrinsic crystal structure and crystal growth

behavior, together with the templating effect, may play an

important role during growth of shaped polyhedral hollow

structures, that is, template-assisted crystallization. For

example, single-crystal NiO hollow octahedra are formed

through carbothermal treatment of NiCl2/carbon spheres at

460 8C in air.[341] Wang et al. applied a solvothermal method

to synthesize single-crystal PbTe nanoboxes with sizes of

80–180 nm,[342] in which the strong interaction between PEG


REVIE


Figure 30. a) TEM and b,c) SEM images of Co3O4 nanoboxes. Reproducedwith permission from [344].

4010

and Pb2þ was speculated to help formation of shaped non-

spherical micelles during reaction. Single-crystal Cu2O hollow

nanocubes have been synthesized in a Brij 56 based W/O

emulsion in the presence of ethylene glycol (EG).[343] He et al.

have prepared cubic Co3O4 nanoboxes of about 300 nm in size

(see Fig. 30), whose walls are constructed from compactly

assembled cubic Co3O4 nanocrystals.[344] The intriguing

formation mechanism was clearly identified to involve the

formation of cubic template crystals made mainly of the

surfactant (SDBS) molecules and subsequent growth of Co3O4

nanocrystals on the template surface. Geng et al. have

synthesized PbWO4 hollow nanospindles via a surfactant

(P123)-mediated sonochemical route,[345] where the complex

micellar aggregates formed from P123 and lead acetate around

the cavitation bubbles are thought to serve as non-spherical

soft templates. Yang et al. have recently reported the synthesis

of a siliceous structure with an unusual silkworm cocoonlike

morphology using CTAB and perfluorooctanoic acid as

cotemplates.[346] A dual-templating mechanism involving

delicate interplay of vesicular and liquid crystal templating

was proposed to account for the formation of such complex

structures.

4.4. Template-Free Synthesis of Non-Spherical Hollow

Structures

Template-free methods have also been widely reported for

effective construction of non-spherical hollow structures. As


discussed before, these methods are principally of two types:

self-templated[269] and self-aggregation approaches. The latter

is not as well understood as the former as it usually involves a

less obvious formation mechanism. For example, SnO2 hollow

octahedra, in a significant fraction, are observed in the

precipitate after hydrothermally heating a water/2-propanol

solution containing SnF2 and ethylenediamine at 180 8C,[347]

during which some unidentified intermediate crystals might act

as the quasi-templates. The former, in essence, involves a two-

step process, the formation of shaped particles and the

subsequent inside-out evacuation with the shell stabilized or

protected by species like surfactant molecules. Qi and co-

workers have synthesized octahedral Cu2O nanocages of ca.

230 nm in edge size with single-crystalline shells by reducing

the copper tartrate complex (Fehling’s solution) with glucose

at 75 8C in the presence of PdCl2.[348] Mechanistic investigation

indicates that the octahedral Cu2O nanocages are formed by a

two-step process, the formation of octahedral solid nanocrys-

tals by the Pd0-catalyzed reduction of the copper tartrate

complex with glucose, followed by the subsequent hollowing of

the nano-octahedra by Pd0-catalyzed, oxygen-engaged oxida-

tive etching. Based on a similar oxidative corrosion mechan-

ism, single-crystal Pd nanoboxes with edge size of about 48 nm

have been synthesized by Xia and co-workers via the polyol

process in the presence of PVP using Na2PdCl4 as the

precursor.[349]

In other systems that do not undergo changes in oxidation

states, the hollowing process is believed to proceed by a

mechanism analogous to inside-out Ostwald ripening.[269]

Cu2O hollow nanocubes have been synthesized through

reductive self-assembly of CuO nanocrystals followed by

inside-out Ostwald ripening.[278] Wang et al. have recently

demonstrated an extremely simple route that transforms

preformed silver solid nanostructures to corresponding hollow

ones with unaltered morphologies.[350] The method involves

the modification of the solid silver nanoscrystals with dithiol

molecules, and subsequent ultrasound-triggered formation of

thin layer of sulfide and selective dissolution of the interior and

re-deposition on the shell. Hematite spindles are observed to

undergo similar selective dissolution of the interiors under

hydrothermal or solvothermal conditions with the exterior

protected by adsorbed anions such as phosphate and

sulfate.[291,292,351]

5. Applications of Hollow Structures

Hollow micro-/nanostructures possess characteristics such

as low density, high surface-to-volume ratio, and low

coefficients of thermal expansion and refractive index that

make them attractive for applications ranging from catalyst

support, antireflection surface coatings, and rechargeable

batteries. Their capacity for encapsulating sensitive materials

such as therapeutics, fluorescent markers, and field-responsive

agents has been exploited by many groups for drug delivery

and biomedical imaging. The enormous development in hollow


REVIE


particle synthesis has greatly advanced our ability to tune their

mechanical, optical, electrical, chemical, and other properties.

These advances have in turn catalyzed exploration in a growing

list of applications, which on their own arguably deserve a

separate dedicated review. Here we provide a brief review of

what appears to be the most well-developed applications of

hollow micro-/nanostructures. Specifically, we focus on

applications in lithium-ion batteries, catalysis and sensing,

and biomedical applications.

Figure 31. Cycling performance of a) SnO2 hollow nanospheres, b) poorlydefined SnO2 hollow spheres, and c) SnO2 nanoparticles. Reproduced withpermission from [269].

5.1. Lithium-Ion Batteries

Because of their high energy density and low self-discharge

rates, compared to other types of batteries, lithium-ion

batteries (LIBs) are presently the dominant power source

for portable consumer electronics. LIBs are also under active

investigation worldwide as power sources in electric and hybrid

automobiles.[352,353] Nonetheless, even optimistic projections

conclude that their performance is approaching limits set by

the current electrode materials. The major challenges in

designing next-generation lithium-ion batteries include the

need to increase their energy density, cycling life and charge/

discharge rate capability.[352] The limited gravimetric capacity

of graphitic carbon (<372mA h g�1), the most commonly used

anode material, has motivated intense research for alternative

anode materials with higher capacities at low potentials. As a

result, many materials have been suggested and tested as

alternative anodes. Unlike the classical Li insertion–deinser-

tion processes, they are generally based on two different

reaction processes: Li-metal alloying/dealloying for metals

(e.g., Si, Sn/SnO2, Al, Sb),[354–356] and reversible formation–

decomposition of Li2O nanomatrix for transition metal oxide

(e.g., CoO, NiO, FeO).[357]

Despite these successes, the practical use of these materials

is still hindered by the generally high irreversible capacity

losses during the first several charge–discharge cycles and by

gradual breakdown of the electrode over multiple charge–

discharge cycles (i.e., capacity fading). While the former is

intrinsic for most anode materials, the latter is generally

thought to result from large volume changes (e.g., >200% for

tin) that accompany Li insertion–deinsertion. Specifically,

large cyclic stresses induced by these volume changes in the

active particles are believed to catalyze their mechanical

failure, promote agglomeration of the finer resultant struc-

tures, resulting in a steady capacity loss. The irreversible loss of

capacity in metal oxide anodes can in some cases be offset by

pre-lithiating the anode using stabilized lithium metal powder

(SLMP) technologies.[358] To resolve the second problem, that

is, rapid capacity fading over extended cycling, many studies

have focused on electrodes comprised of hollow nanostruc-

tures. The large void space of these structures has been

speculated to enhance their capacity retention by reversibly

accommodating large volume changes (i.e., to enhance

‘‘breathability’’).


The improvement of the electrochemical performance

brought about by the employment of hollow micro-/nanos-

tructures can in reality be explained in terms of multiple

factors. First, the cavities in the hollow structure may provide

extra space for the storage of lithium ions, which is beneficial

for enhancing specific capacity of the battery. Second, the

hollow structure is often associated with larger surface area and

reduced effective diffusion distance for lithium ions, leading to

better rate capabilities. Third, the void space in hollow

structures buffers against the local volume change during

lithium insertion–deinsertion and is able to alleviate the

problem of pulverization and aggregation of the electrode

material, hence improving the cycling performance.

Lou et al. have reported high initial reversible charge

capacity of 1140mA h g�1 and improved cycling performance

for hollow SnO2 nanospheres (see Fig. 21).[269] The ultrahigh

lithium storage capacity is speculated to result from the

ability of the nanopores in the shells and interior microcavities

of hollow nanospheres to enhance Li storage. As shown in

Figure 31, the cycling performance is substantially improved

compared to that of pristine SnO2 nanoparticles (several

nanometers in size) and to previous SnO2 hollow spheres

synthesized from a templating method. Specially, the capacity

of SnO2 hollow nanospheres is comparable to the theoretical

capacity of SnO2 (790mA h g�1) after more than 30 cycles; and

much higher than the theoretical capacity of graphite after

more than 40 cylces. Cycling performance of these mesoporous

SnO2 hollow nanospheres has recently been shown to improve

by infiltration with carbon.[270] The role played by carbon is

two-fold: providing a physical buffering layer for the large

volume change (cushion effect) and increasing the electrical

conductivity of particle-particle contacts. SnO2/carbon com-

posite anode materials with stable lithium storage capacity

retention for hundreds of cycles have very recently been

demonstrated using the same approach.[57,359]

Silicon (Si) has the highest known theoretical lithium

storage capacity of 4200mA h g�1, which makes it very

attractive as a negative electrode material for LIBs. However,

Si nanoparticles-based anodes are often found to have less

satisfactory cycle life compared to other materials (e.g., Sn-

based anodes),[360] again due to the ultralarge (>300%)

volume changes during Li–Si alloying/dealloying processes.


REVIE


Figure 32. Cycling behavior and Coulombic efficiency of nestlike Sihollow nanospheres at the constant current density of 2000mA g�1. Filledcircles: discharge capacity; open triangles: charge capacity; filled triangles:Coulombic efficiency. Inset is a TEM image. Reproduced with permissionfrom [361].

Figure 33. Electrochemical performance of TiO2 hollow particles.a) Charge–discharge voltage profiles at a 0.2 C rate for representativecycles (1st, 2nd, 5th, 15th, 25th, 35th). b) Cycling performance at a0.2 C rate. c) Cycling performance at various C rates (& discharge; &charge). Reproduced with permission from [327].

4012

Ma et al. have recently modified a previously reported method

for Si nanoparticle synthesis to prepare nestlike Si hollow

nanospheres (see Fig. 32 inset),[361] which exhibit much

improved cycle life and rate capability. As shown in Figure

32, the nestlike Si hollow nanospheres displayed an initial

discharge capacity of 3052mA h g�1 at a current density of

2000mA g�1. After extended cycling up to 48 cycles, these

nestlike Si hollow nanospheres are able to retain a capacity of

1095mA h g�1. Furthermore, except for first several cycles, the

Coulombic efficiency is nearly 100%. These observations

clearly verify the advantages of the open nestlike hollow

morphology.

Titania (anatase in particular) is a promising anode material

for high-power lithium batteries because of its exceptionally

fast Li insertion–extraction kinetics. The electrochemical Li

insertion–extraction process can be described as follows:

TiO2þ xLiþþ xe�$LixTiO2. From early studies, the max-

imum value of the insertion coefficient x has been determined

to be about 0.5, which corresponds to a theoretical capacity of

167mA h g�1. Lou et al. have recently reported lithium storage

properties of TiO2 hollow particles (see Fig. 26).[327] Figure 33a

shows the discharge–charge voltage profiles for some repre-

sentative cycles at a 0.2C rate (one Li per formula unit in 5 h)

with a voltage window of 1–3V. Interestingly, the first

discharge capacity is found to be 408mA h g�1, corresponding

to a nominal insertion coefficient of x¼ 1.2. Figure 33b shows

the cycling performance, that is, capacity retention versus cycle

number. Despite the relatively large irreversible loss (48.3%)

in the first cycle, the discharge–charge capacity difference

becomes insignificant in the course of first several cycles; and

thereafter the Coulombic efficiency is nearly 100%. Remark-

ably, the TiO2 hollow particles have excellent capacity

retention over extended cycling, and they are able to deliver

a reversible Li storage capacity of 172mA h g�1 after 35 cycles.

The rate capability is also studied as shown in Figure 33c. These

results indicate that TiO2 hollow particles manifest good

capacity retention at different rates, and are able to deliver a

specific charge capacity of 112mA h g�1 even at a high rate of


2C (i.e., charge or discharge of TiO2 in 10min). Furthermore, a

capacity of 169mA h g�1 can be resumed if the rate is lowered

to 0.2C. The same group also synthesized needlelike Co3O4

nanotubes using a self-supported topotactic transformation

approach and evaluated their lithium storage properties.[362] A

very high reversible charge capacity of 950mA h g�1 is

observed in the first cycle, which can be nearly completely

retained after 30 cycles. This level of capacity retention is

believed to originate from the robustness of the tubular wall of

the active hollow structures.

Improved lithium storage properties have also been

reported in many other hollow nanomaterials including

SnO2/carbon,[89,209,363–365] Sn/carbon,[304,321] Sb,[366]

a-Fe2O3,[294] and a-MnO2.

[367] In particular, hollow structures

of vanadium oxides have been suggested for use as positive

electrode materials.[229,252] Xie and co-workers have shown

that VOOH hollow dandelions (see Fig. 19) manifest excellent

reversible lithium storage properties compared to their solid


REVIE


microparticle counterparts in the voltage window of 3.6–

1.5V.[252] Specially, at a current density of 20mA g�1 these

hollow dandelions can deliver a highly reversible discharge

capacity of 124.7mA h g�1 after 50 cycles, compared to

65.4mA h g�1 for solid microparticles of the same chemistry.

5.2. Catalysis and Sensing

Progress in synthesis of hollow particles has provided

opportunities for their widespread use in catalysis. One of the

early works by Kim et al.[58] utilized hollow palladium

nanospheres (see Fig. 3) as heterogeneous catalysts for Suzuki

coupling reactions. These Pd hollow spheres manifested high

catalytic activity (�90% yield) even after seven recycles, and

little leaching of Pd was observed. In the last two years there

has been additional work performed on this theme. Li et al.[225]

used PdCo bimetallic hollow nanospheres to catalyze Sonoga-

shira reaction in aqueous media with high yields. Ni1–xPtxhollow spheres were demonstrated to exhibit good catalytic

activities for the hydrolysis and thermolysis reactions of

NH3BH3 to release H2.[47] The observed high activities are

most readily attributed to the high surface areas (e.g., 105 m2

g�1 for Ni0.88Pt0.12) of the materials, compared to dense

spheres of similar size, because their open hollow structure

enables both the outer and inner surfaces of the catalyst to

come into contact with the reactants, yielding added benefits

for the catalytic process.

Rattle-type hollow particles with functional cores are

frequently used as catalysts. Ikeda et al.[312] used Pt@carbon

nanorattles for heterogeneous hydrogenation of olefins,

achieving significantly higher yield and good recyclability

compared to other Pt catalysts. The carbon shell is believed to

stabilize the nanoparticle core through prevention of their

coalescence and provide void space for the organic transfor-

mation on the Pt nanoparticle, thus enhancing the catalytic

performance. Similarly, Au@ZrO2 nanorattles have been used

as model high-temperature-stable catalysts for CO oxida-

tion,[308] where the Au nanoparticles are effectively separated

(i.e., catalyst growth is prevented) but still highly accessible to

gas molecules.

Hollow nanoparticle catalysts are playing an increasingly

important role in electrocatalytic and photocatalytic reactions.

There are a number of reports on utilizing hollow metal/metal

oxide nanoparticles in the electroxidation of methanol/

ethanol/formic acid. For example, Liang et al.[134] have shown

that Pt hollow nanospheres (see Fig. 11) exhibit twice the

catalytic activity for methanol oxidation of solid Pt nano-

spheres with roughly the same size. Other

groups[72,73,133,214,368] also reported that hollow particles (or

noble metal catalysts supported on hollow structures)

frequently showed enhanced electrocatalytic performance

in these electroxidation reactions, which could be because

of the larger electrochemical surface area and the ability of

the empty core domain to accommodate larger number of


guest molecules or guests with large size. Hollow TiO2

nanospheres have been reported by several groups to exhibit

high activities and as such have been targeted as the active

electrode component in high-efficiency dye-sensitized solar

cells.[84,369,370] Metal oxides hollow spheres (e.g., TiO2) have

also been widely investigated for photocatalytic degradation of

organic pollutants.[228,253,277,281,303,371] a-Fe2O3 hollow nano-

spheres, for example, have been shown to yield improved

photocatalytic performance over a-Fe2O3 nanocrystals in

oxidation of salicylic acid.[293]

The high surface areas of hollow micro-/nanostructured

materials are also advantageous for chemical sensing.

Chemical sensors are widely used for industrial process control

and are experiencing growing use in security applications.

Current research has mainly focused on metal oxide hollow

structures for conductometric sensing of gases. Sensing with

these materials is performed through measurement of changes

in electrical conductance produced by adsorption-desorption

of a targeted analyte on the oxide surface. Martinez et al.[23]

reported significantly higher sensitivity for hollow Sb-doped tin

oxide (Sb:SnO2) microsphere films over conventional SnO2

films made via the CVD method and Sb:SnO2 microporous

nanoparticle films in the sensing of methanol. This enhance-

ment may be attributed to the hierarchical porous structure,

that is, the high porosity and large degree of mesoporosity of

the nanoparticle shell could facilitate the diffusion of analytes

into the microporous structure. Other groups have also

reported use of metal oxide (e.g., SnO2, In2O3, Cu2O) hollow

nano-/microstructures for gas sensing.[211,228,288,372]

5.3. Biomedical Applications

The applications of nanoparticles (e.g., nanotubes) in a

variety of biomedical applications, such as drug delivery, cell

imaging, and sensing of biomolecules, have been treated in

several recent excellent Review articles.[373–375] Here we focus

on hollow nanospheres for biomedical applications, including

drug/gene delivery, imaging/diagnostics, and therapeutic uses.

Silica is a popular material for drug/gene delivery, because

of its non-toxicity, biocompatibility, and well-established

bioconjugation methods using silane chemistry.[97,376,377]

Porous hollow silica nanoparticles have been used as a carrier

to study the controlled release behavior of a model

drug.[97,376,378] The drug was loaded into the cavity and on

the surface of the hollow nanoparticles, and typical sustained

release pattern was observed without any burst effect.

Additional functionalities could be added to the silica hollow

particles to obtain products with special properties. For

example, Zhu et al. achieved stimuli-responsive controlled

drug release from hollow silica spheres with mesoporous walls

by capping the hollow silica spheres with polyelectrolyte

multilayers.[379] Hollow nanoparticles have also been used for

delivering genetic material to cells. Sokolova et al. reported

synthesis of multishell calcium phophate nanoparticles loaded

with DNA.[380] The transfection efficiency of these particles


REVIE


4014

was shown to be significantly higher than that of the simple

DNA-coated calcium phosphate nanoparticles. In principle,

different layers are able to deliver different agents sequen-

tially, offering additional flexibility for the delivery system. Cai

et al. have recently used biocompatible calcium phosphate

hollow nanospheres (see Fig. 20) as drug carrier.[205]

Importantly, it was demonstrated that drug molecules can

be stably encapsulated during the synthesis of hollow spheres

and the release can be triggered by ultrasound.

Hollow inorganic nanoparticles such as gold or magnetite

nanoshells could be used as imaging and diagnosis agents

because of their unique optical/magnetic properties. Chen

et al.[381] reported the use of gold nanocages (hollow

nanostructures with porous walls) as optical imaging contrast

agent for optical coherence tomography (OCT). The gold

nanocages are synthesized through galvanic replacement

reaction between silver templates and chloroauric acid.[382]

These Au nanocages can be tuned to strongly absorb in the

near-infrared (NIR) region where optical transmission through

tissue is optimal. By targeting cancer cells with these particles

and irradiating with infrared light, a local irreversible ablation

can be generated that can provide a therapeutic effect on the

targeted cancer cells.[382,383] Such photothermal therapy is less

invasive than chemotherapy or surgery and holds strong

promise as a new form of cancer treatment. Hirsch et al.[384]

have previously demonstrated similar photothermal therapeu-

tics both in vitro and in vivo using Au nanoshells supported on

dielectric silica cores. After exposure to low doses of NIR light,

human breast carcinoma cells and solid tumors treated with Au

nanoshells were found to have undergone irreversible photo-

thermally induced damage. Loo et al.[385] further engineered

immunotargeted Au nanoshells to both scatter light in the NIR

enabling optical molecular cancer imaging and to absorb light,

allowing selective destruction of targeted carcinoma cells

through photothermal therapy.

6. Conclusions and Outlook

In the past decade, considerable progress has been made in

synthesis and applications of hollow micro-/nanostructures.

The synthetic strategies for hollow structures can be broadly

categorized into four groups: (1) conventional hard templat-

ing; (2) sacrificial templating; (3) soft templating; and (4)

template-free approaches. Templating against hard (solid)

templates is arguably the most effective, and certainly the

most common, method for synthesizing hollow micro-/

nanostructures. Extension of this method to sacrificial

template-based syntheses is particularly promising because

they generally require no additional surface functionalization

and shell formation is guaranteed by chemical reaction.

Overall, hard template-based synthetic approaches suffer

from several intrinsic disadvantages, which range from the

inherent difficulty of achieving high product yields from the

multistep synthetic process to the lack of structural robustness

of the shells upon template removal. These difficulties can be


partly overcome by use of soft (liquid or gaseous) templates.

Compared to solid templates, the soft templates can be more

easily removed or the syntheses are essentially template-free

if involving gaseous templates. Additionally, soft templates

allow facile and efficient encapsulation of functional species

like therapeutic and DNA molecules. More recently,

template-free methods based on novel mechanisms (e.g.,

inside-out Ostwald ripening) have been developed for

preparing hollow structures of many materials in a wide

range of sizes.

The successes in synthesis of hollow structures have

provided opportunities to tune their mechanical, optical,

electrical, chemical, and other properties. These advances have

in turn catalyzed exploration in a growing list of applications.

However it should be noted that high-quality (e.g., non-

agglomerated, uniform, controlled size) hollow particles will be

required in many cases for both fundamental research and

practical applications. A survey of the literature in the field

shows that methods for producing such high quality hollow

particles are still very limited, and that even if these methods

can be found, they are suitable for a small number of materials.

In addition, many of the methods for synthesis of both

templates and hollow structures are based on solution

synthesis, in which the concentration of precursors is usually

very low, typically in the millimolar range. Scaling-up these

syntheses to produce commercial-scale quantities for applica-

tions is expected to introduce significant challenges for size,

shape, and shell thickness control. This aspect of hollow

structure synthesis, though still in its infancy, offers exciting

opportunities for newcomers to the field.

Received: March 28, 2008Revised: May 10, 2008

Published online: September 25, 2008

[1] Hollow and Solid Spheres andMicrospheres: Science and Technology

Associated with Their Fabrication and Applications, (Eds: D. L.

Wilcox, M. Berg, T. Bernat, D. Kellerman, J. K. Cochran Jr.),Mater.

Res. Soc. Symp. Proc. 1995, Vol. 372, MRS Pittsburgh, PA.

[2] E. Matijevic, Chem. Mater. 1993, 5, 412.

[3] T. Ung, L. M. Liz-Marzan, P. Mulvaney, Langmuir 1998, 14, 3740.

[4] L. M. LizMarzan, M. Giersig, P. Mulvaney, Langmuir 1996, 12, 4329.

[5] F. Caruso, R. A. Caruso, H. Mohwald, Science 1998, 282, 1111.

[6] F. Caruso, Adv. Mater. 2001, 13, 11.

[7] H. J. Fan, U. Gosele, M. Zacharias, Small 2007, 3, 1660.

[8] H. C. Zeng, Curr. Nanosci. 2007, 3, 177.

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

[10] F. Caruso, Chem. Eur. J. 2000, 6, 413.

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

Nature 1992, 359, 710.

[12] H. W. Kroto, J. R. Heath, S. C. Obrien, R. F. Curl, R. E. Smalley,

Nature 1985, 318, 162.

[13] R. Tenne, Angew. Chem. Int. Ed. 2003, 42, 5124.

[14] R. Tenne, Nat. Nanotechnol. 2006, 1, 103.

[15] X. L. Xu, S. A. Asher, J. Am. Chem. Soc. 2004, 126, 7940.

[16] G. J. Guan, Z. P. Zhang, Z. Y.Wang, B. H. Liu, D.M. Gao, C. G. Xie,

Adv. Mater. 2007, 19, 2370.

[17] A. N. Zelikin, Q. Li, F. Caruso,Angew. Chem. Int. Ed. 2006, 45, 7743.


REVIE


[18] X. Y. Liu, C. Y. Gao, J. C. Shen, H. Mohwald, Macromol. Biosci.

2005, 5, 1209.

[19] R. A. Caruso, A. Susha, F. Caruso, Chem. Mater. 2001, 13, 400.

[20] K. H. Rhodes, S. A. Davis, F. Caruso, B. J. Zhang, S. Mann, Chem.

Mater. 2000, 12, 2832.

[21] G. C. Chen, C. Y. Kuo, S. Y. Lu, J. Am. Ceram. Soc. 2005, 88, 277.

[22] F. Caruso, X. Y. Shi, R. A. Caruso, A. Susha, Adv. Mater. 2001,

13, 740.

[23] C. J. Martinez, B. Hockey, C. B. Montgomery, S. Semancik, Lang-

muir 2005, 21, 7937.

[24] Z. J. Liang, A. Susha, F. Caruso, Chem. Mater. 2003, 15, 3176.

[25] F. Caruso, M. Spasova, A. Susha, M. Giersig, R. A. Caruso, Chem.

Mater. 2001, 13, 109.

[26] M. A. Correa-Duarte, A. Kosiorek, W. Kandulski, M. Giersig, L. M.

Liz-Marzan, Chem. Mater. 2005, 17, 3268.

[27] L. Z. Wang, Y. Ebina, K. Takada, T. Sasaki, Chem. Commun. 2004,

1074.

[28] L. Li, R. Z. Ma, N. Iyi, Y. Ebina, K. Takada, T. Sasaki, Chem.

Commun. 2006, 3125.

[29] L. Z. Wang, Y. Ebina, K. Takada, T. Sasaki, J. Phys. Chem. B 2004,

108, 4283.

[30] D. Y. Wang, F. Caruso, Chem. Mater. 2002, 14, 1909.

[31] A. Imhof, Langmuir 2001, 17, 3579.

[32] S. Eiden, G. Maret, J. Colloid Interface Sci. 2002, 250, 281.

[33] T. H. Kim, K. H. Lee, Y. K. Kwon, J. Colloid Interface Sci. 2006, 304,

370.

[34] P. Wang, D. Chen, F. Q. Tang, Langmuir 2006, 22, 4832.

[35] X. J. Cheng, M. Chen, L. M. Wu, G. X. Gu, Langmuir 2006, 22, 3858.

[36] C.Graf, D. L. J. Vossen, A. Imhof, A. van Blaaderen,Langmuir 2003,

19, 6693.

[37] Y. Lu, J. McLellan, Y. N. Xia, Langmuir 2004, 20, 3464.

[38] M. Chen, L. M. Wu, S. X. Zhou, B. You, Adv. Mater. 2006, 18, 801.

[39] J. Cornelissen, E. F. Connor, H. C. Kim, V. Y. Lee, T. Magibitang, P.

M. Rice, W. Volksen, L. K. Sundberg, R. D. Miller, Chem. Commun.

2003, 1010.

[40] B. Z. Putlitz, K. Landfester, H. Fischer, M. Antonietti, Adv. Mater.

2001, 13, 500.

[41] X. F. Song, L. Gao, J. Phys. Chem. C 2007, 111, 8180.

[42] P.M.Arnal, C.Weidenthaler, F. Schuth,Chem.Mater. 2006, 18, 2733.

[43] K. P. Velikov, A. van Blaaderen, Langmuir 2001, 17, 4779.

[44] I. D. Hosein, C. M. Liddell, Langmuir 2007, 23, 2892.

[45] A.Wolosiuk, O. Armagan, P. V. Braun, J. Am. Chem. Soc. 2005, 127,

16356.

[46] L. H. Lu, G. Y. Sun, S. Q. Xi, H. S.Wang, H. J. Zhang, T. D.Wang, X.

H. Zhou, Langmuir 2003, 19, 3074.

[47] F. Y. Cheng, H. Ma, Y. M. Li, J. Chen, Inorg. Chem. 2007, 46, 788.

[48] Z. B. Huang, F. Q. Tang, J. Colloid Interface Sci. 2005, 281, 432.

[49] D. B. Wang, C. X. Song, Z. S. Hu, X. Fu, J. Phys. Chem. B 2005, 109,

1125.

[50] J. Wang, Y. J. Zhu, Y. J. Wu, C. J. Wu, D. F. Xu, Z. Zhang, Mod.

Phys. Lett. B 2006, 20, 549.

[51] P. Jin, Q. W. Chen, L. Q. Hao, R. F. Tian, L. X. Zhang, L. Wang, J.

Phys. Chem. B 2004, 108, 6311.

[52] H. Yoshikawa, K. Hayashida, Y. Kozuka, A. Horiguchi, K. Awaga, S.

Bandow, S. Iijima, Appl. Phys. Lett. 2004, 85, 5287.

[53] M. Ohnishi, Y. Kozuka, Q. L. Ye, H. Yoshikawa, K. Awaga, R.

Matsuno, M. Kobayashi, A. Takahara, T. Yokoyama, S. Bandow, S.

Iijima, J. Mater. Chem. 2006, 16, 3215.

[54] H. S. Qian, G. F. Lin, Y. X. Zhang, P. Gunawan, R. Xu, Nanotech-

nology 2007, 18.

[55] G. Q. Li, C. Y. Liu, Y. Liu, J. Am. Ceram. Soc. 2007, 90, 2667.

[56] W. S. Choi, H. Y. Koo, Z. Zhongbin, Y. D. Li, D. Y. Kim,Adv. Funct.

Mater. 2007, 17, 1743.

[57] X. W. Lou, C. Yuan, L. A. Archer, Small 2007, 3, 261.


[58] S. W. Kim, M. Kim, W. Y. Lee, T. Hyeon, J. Am. Chem. Soc. 2002,

124, 7642.

[59] X. M. Sun, Y. D. Li, Angew. Chem. Int. Ed. 2004, 43, 3827.

[60] X. M. Sun, J. F. Liu, Y. D. Li, Chem. Eur. J. 2006, 12, 2039.

[61] X. L. Li, T. J. Lou, X. M. Sun, Y. D. Li, Inorg. Chem. 2004, 43, 5442.

[62] M. M. Titirici, M. Antonietti, A. Thomas, Chem. Mater. 2006, 18,

3808.

[63] R. Z. Yang, H. Li, X. P. Qiu, L. Q. Chen,Chem. Eur. J. 2006, 12, 4083.

[64] M. Yang, J. Ma, C. L. Zhang, Z. Z. Yang, Y. F. Lu,Angew. Chem. Int.

Ed. 2005, 44, 6727.

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

Int. Ed. 2003, 42, 1943.

[66] 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.

[67] F. Xu, W. Wei, C. L. Zhang, S. J. Ding, X. Z. Qu, J. G. Liu, Y. F. Lu,

Z. Z. Yang, Chem. Asian J. 2007, 2, 828.

[68] H. F. Xu, S. J. Ding, W. Wei, C. L. Zhang, X. Z. Qu, J. G. Liu, Z. Z.

Yang, Colloid Polym. Sci. 2007, 285, 1101.

[69] F. J. Suarez, M. Sevilla, S. Alvarez, T. Valdes-Solis, A. B. Fuertes,

Chem. Mater. 2007, 19, 3096.

[70] G. Buchel, K. K. Unger, A. Matsumoto, K. Tsutsumi, Adv. Mater.

1998, 10, 1036.

[71] W. Stober, A. Fink, E. Bohn, J. Colloid Interface Sci. 1968, 26, 62.

[72] S. B. Yoon, K. Sohn, J. Y. Kim, C. H. Shin, J. S. Yu, T. Hyeon, Adv.

Mater. 2002, 14, 19.

[73] G. S. Chai, S. B. Yoon, J. H. Kim, J. S. Yu, Chem. Commun. 2004,

2766.

[74] B. Fang, J. H. Kim, C. Lee, J. S. Yu, J. Phys. Chem. C 2008, 112, 639.

[75] S. Ikeda, K. Tachi, Y. H. Ng, Y. Ikoma, T. Sakata, H. Mori, T.

Harada, M. Matsumura, Chem. Mater. 2007, 19, 4335.

[76] A. B. Fuertes, T. Valdes-Solis, M. Sevilla, P. Tartaj, J. Phys. Chem. C

2008, 112, 3648.

[77] P. M. Arnal, F. Schuth, F. Kleitz, Chem. Commun. 2006, 1203.

[78] J. Y. Kim, S. B. Yoon, J. S. Yu, Chem. Commun. 2003, 790.

[79] Y. D. Xia, R. Mokaya, J. Mater. Chem. 2005, 15, 3126.

[80] Z. Y. Zhong, Y. D. Yin, B. Gates, Y. N. Xia, Adv. Mater. 2000,

12, 206.

[81] Y. Yin, Y. Lu, B. Gates, Y. Xia, Chem. Mater. 2001, 13, 1146.

[82] Z. Chen, P. Zhan, Z. L.Wang, J. H. Zhang, W. Y. Zhang, N. B. Ming,

C. T. Chan, P. Sheng, Adv. Mater. 2004, 16, 417.

[83] F. Q. Sun, J. C. Yu, Angew. Chem. Int. Ed. 2007, 46, 773.

[84] Y. Z. Li, T. Kunitake, S. Fujikawa, J. Phys. Chem. B 2006, 110, 13000.

[85] P. Jiang, J. F. Bertone, V. L. Colvin, Science 2001, 291, 453.

[86] Z. C. Zhou, Q. F. Yan, F. B. Su, X. S. Zhao, J. Mater. Chem. 2005, 15,

2569.

[87] Z. B. Lei, J. M. Li, Y. X. Ke, Y. G. Zhang, H. C. Zhang, F. Q. Li, J. Y.

Xing, J. Mater. Chem. 2001, 11, 2930.

[88] L. Zhi, J. J. Wang, G. L. Cui, M. Kastler, B. Schmaltz, U. Kolb, U.

Jonas, K. Mullen, Adv. Mater. 2007, 19, 1849.

[89] Y. Wang, F. B. Su, J. Y. Lee, X. S. Zhao, Chem. Mater. 2006, 18,

1347.

[90] F. B. Su, X. S. Zhao, Y.Wang, L. K. Wang, J. Y. Lee, J. Mater. Chem.

2006, 16, 4413.

[91] J. Hwang, B. D. Min, J. S. Lee, K. Keem, K. Cho, M. Y. Sung, M. S.

Lee, S. Kim, Adv. Mater. 2004, 16, 422.

[92] M. Knez, K. Nielsch, L. Niinisto, Adv. Mater. 2007, 19, 3425.

[93] R. H. A. Ras, M. Kemell, J. de Wit, M. Ritala, G. ten Brinke, M.

Leskela, O. Ikkala, Adv. Mater. 2007, 19, 102.

[94] S. J. Kim, C. S. Ah, D. J. Jang, Adv. Mater. 2007, 19, 1064.

[95] J. Yang, J. Y. Lee, H. P. Too, S. Valiyaveettil, J. Phys. Chem. B 2006,

110, 125.

[96] M. Darbandi, R. Thomann, T. Nann, Chem. Mater. 2007, 19, 1700.

[97] J. F. Chen, H. M. Ding, J. X. Wang, L. Shao, Biomaterials 2004,

25, 723.


REVIE


4016

[98] L. D. Gao, L. L. Luo, J. F. Chen, L. Shao, Chem. Lett. 2005, 138.

[99] J. W. Kim, S. H. Choi, P. T. Lillehei, S. H. Chu, G. C. King, G. D.

Watt, Chem. Commun. 2005, 4101.

[100] Y. J. Song, R. M. Garcia, R. M. Dorin, H. R. Wang, Y. Qiu, J. A.

Shelnutt, Angew. Chem. Int. Ed. 2006, 45, 8126.

[101] D. G. Shchukin, H. Mohwald, Phys. Chem. Chem. Phys. 2006, 8,

3496.

[102] M. L. Breen, A. D. Dinsmore, R. H. Pink, S. B. Qadri, B. R. Ratna,

Langmuir 2001, 17, 903.

[103] J. Wang, K. P. Loh, Y. L. Zhong, M. Lin, J. Ding, Y. L. Foo, Chem.

Mater. 2007, 19, 2566.

[104] J. H. Bang, K. S. Suslick, J. Am. Chem. Soc. 2007, 129, 2242.

[105] N. A. Dhas, K. S. Suslick, J. Am. Chem. Soc. 2005, 127, 2368.

[106] J. N. Wang, L. Zhang, J. J. Niu, F. Yu, Z. M. Sheng, Y. Z. Zhao, H.

Chang, C. Pak, Chem. Mater. 2007, 19, 453.

[107] J. C. Yu, X. L. Hu, Q. Li, Z. Zheng, Y. M. Xu,Chem. Eur. J. 2006, 12,

548.

[108] X. M. Jiang, C. J. Brinker, J. Am. Chem. Soc. 2006, 128, 4512.

[109] R. T. Tom, A. S. Nair, N. Singh, M. Aslam, C. L. Nagendra, R. Philip,

K. Vijayamohanan, T. Pradeep, Langmuir 2003, 19, 3439.

[110] I. Pastoriza-Santos, D. S. Koktysh, A. A.Mamedov,M.Giersig, N. A.

Kotov, L. M. Liz-Marzan, Langmuir 2000, 16, 2731.

[111] A. S. Nair, R. T. Tom, V. Suryanarayanan, T. Pradeep, J. Mater.

Chem. 2003, 13, 297.

[112] D. S. Koktysh, X. R. Liang, B. G. Yun, I. Pastoriza-Santos, R. L.

Matts, M. Giersig, C. Serra-Rodriguez, L. M. Liz-Marzan, N. A.

Kotov, Adv. Funct. Mater. 2002, 12, 255.

[113] A. D. Smigelskas, E. O. Kirkendall, Trans. Am. Inst. Min. Metall. Pet.

Eng. 1947, 171, 130.

[114] K. N. Tu, U. Gosele, Appl. Phys. Lett. 2005, 86, 093111.

[115] Y. D. Yin, R. M. Rioux, C. K. Erdonmez, S. Hughes, G. A. Somorjai,

A. P. Alivisatos, Science 2004, 304, 711.

[116] S. Peng, S. H. Sun, Angew. Chem. Int. Ed. 2007, 46, 4155.

[117] H. J. Fan, M. Knez, R. Scholz, D. Hesse, K. Nielsch, M. Zacharias, U.

Gosele, Nano Lett. 2007, 7, 993.

[118] Y. L. Wang, L. Cai, Y. N. Xia, Adv. Mater. 2005, 17, 473.

[119] Y. D. Yin, C. K. Erdonmez, A. Cabot, S. Hughes, A. P. Alivisatos,

Adv. Funct. Mater. 2006, 16, 1389.

[120] C.M.Wang, D. R. Baer, L. E. Thomas, J. E. Amonette, J. Antony, Y.

Qiang, G. Duscher, J. Appl. Phys. 2005, 98.

[121] R. Nakamura, D. Tokozakura, H. Nakajima, J. G. Lee, H. Mori, J.

Appl. Phys. 2007, 101.

[122] R. Nakamura, J. G. Lee, D. Tokozakura, H. Mori, H. Nakajima,

Mater. Lett. 2007, 61, 1060.

[123] J. H. Gao, B. Zhang, X. X. Zhang, B. Xu,Angew. Chem. Int. Ed. 2006,

45, 1220.

[124] R. K. Chiang, R. T. Chiang, Inorg. Chem. 2007, 46, 369.

[125] H. Tan, S. P. Li, W. Y. Fan, J. Phys. Chem. B 2006, 110,

15812.

[126] J. N. Gao, X. L. Ren, D. Chen, F. Q. Tang, J. Ren, Scr. Mater. 2007,

57, 687.

[127] Y. G. Sun, B. T. Mayers, Y. N. Xia, Nano Lett. 2002, 2, 481.

[128] Y. D. Yin, C. Erdonmez, S. Aloni, A. P. Alivisatos, J. Am. Chem. Soc.

2006, 128, 12671.

[129] P. R. Selvakannan, M. Sastry, Chem. Commun. 2005, 1684.

[130] Y. G. Sun, Y. N. Xia, Science 2002, 298, 2176.

[131] Y. G. Sun, Y. N. Xia, J. Am. Chem. Soc. 2004, 126, 3892.

[132] M. Chen, L. Gao, Inorg. Chem. 2006, 45, 5145.

[133] S. J. Guo, Y. X. Fang, S. J. Dong, E. K. Wang, J. Phys. Chem. C 2007,

111, 17104.

[134] H. P. Liang, H. M. Zhang, J. S. Hu, Y. G. Guo, L. J. Wan, C. L. Bai,

Angew. Chem. Int. Ed. 2004, 43, 1540.

[135] H. P. Liang, L. J. Wan, C. L. Bai, L. Jiang, J. Phys. Chem. B 2005, 109,

7795.


[136] H. P. Liang, Y. G. Guo, H. M. Zhang, J. S. Hu, L. J. Wan, C. L. Bai,

Chem. Commun. 2004, 1496.

[137] J. Zeng, J. L. Huang, W. Lu, X. P. Wang, B. Wang, S. Y. Zhang, J. G.

Hou, Adv. Mater. 2007, 19, 2172.

[138] J. Huang, L. He, Y. H. Leng, W. Zhang, X. G. Li, C. P. Chen, Y. Liu,

Nanotechnology 2007, 18.

[139] Y. Vasquez, A. K. Sra, R. E. Schaak, J. Am. Chem. Soc. 2005, 127,

12504.

[140] U. Y. Jeong, Y. N. Xia, Angew. Chem. Int. Ed. 2005, 44, 3099.

[141] P. H. C. Camargo, Y. H. Lee, U. Jeong, Z. Q. Zou, Y. N. Xia,

Langmuir 2007, 23, 2985.

[142] U. Jeong, J. U. Kim, Y. N. Xia, Nano Lett. 2005, 5, 937.

[143] W. Zhu, W. Z. Wang, J. L. Shi, J. Phys. Chem. B 2006, 110, 9785.

[144] J. J. Miao, L. P. Jiang, C. Liu, J. M. Zhu, J. J. Zhu, Inorg. Chem. 2007,

46, 5673.

[145] J. Geng, B. Liu, L. Xu, F. N. Hu, J. J. Zhu, Langmuir 2007, 23, 10286.

[146] Q. Peng, S. Xu, Z. B. Zhuang, X. Wang, Y. D. Li, Small 2005, 1, 216.

[147] C. C. Huang, J. R. Hwu, W. C. Su, D. B. Shieh, Y. Tzeng, C. S. Yeh,

Chem. Eur. J. 2006, 12, 3805.

[148] B. Liu, H. C. Zeng, J. Am. Chem. Soc. 2004, 126, 16744.

[149] Y. Zhang, E. W. Shi, Z. Z. Chen, X. B. Li, B. Xiao, J. Mater. Chem.

2006, 16, 4141.

[150] Z. Y. Liu, L. J. Ci, N. Y. Jin-Phillipp, M. Ruhle, J. Phys. Chem. C

2007, 111, 12517.

[151] Y. W. Ma, K. F. Huo, Q. Wu, Y. N. Lu, Y. M. Hu, Z. Hu, Y. Chen, J.

Mater. Chem. 2006, 16, 2834.

[152] C. Li, X. G. Yang, B. J. Yang, Y. Yan, Y. T. Qian, Eur. J. Inorg.

Chem. 2003, 3534.

[153] G. Z. Shen, D. Chen, Y. F. Liu, K. B. Tang, Y. T. Qian, J. Cryst.

Growth 2004, 262, 277.

[154] L. W. Yin, Y. Bando, M. S. Li, D. Golberg, Small 2005, 1, 1094.

[155] L. Xie, J. Zheng, Y. Liu, Y. Li, X. G. Li, Chem. Mater. 2008, 20, 282.

[156] S. M. Wan, F. Guo, L. Shi, Y. Y. Peng, X. Z. Liu, Y. G. Zhang, Y. T.

Qian, J. Mater. Chem. 2004, 14, 2489.

[157] J. X. Huang, Y. Xie, B. Li, Y. Liu, Y. T. Qian, S. Y. Zhang, Adv.

Mater. 2000, 12, 808.

[158] Y. H. Ni, A. Tao, G. Z. Hu, X. F. Cao, X. W. Wei, Z. S. Yang,

Nanotechnology 2006, 17, 5013.

[159] C. Zimmermann, C. Feldmann,M.Wanner, D. Gerthsen, Small 2007,

3, 1347.

[160] D. H. M. Buchold, C. Feldmann, Nano Lett. 2007, 7, 3489.

[161] Y. S. Li, J. L. Shi, Z. L. Hua, H. R. Chen,M. L. Ruan, D. S. Yan,Nano

Lett. 2003, 3, 609.

[162] C. J. McDonald, K. J. Bouck, A. B. Chaput, C. J. Stevens, Macro-

molecules 2000, 33, 1593.

[163] J. Han, G. P. Song, R. Guo, Adv. Mater. 2006, 18, 3140.

[164] A. M. Collins, C. Spickermann, S. Mann, J. Mater. Chem. 2003, 13,

1112.

[165] M. Fujiwara, K. Shiokawa, Y. Tanaka, Y. Nakahara, Chem. Mater.

2004, 16, 5420.

[166] W. J. Li, X. X. Sha, W. J. Dong, Z. C. Wang, Chem. Commun. 2002,

2434.

[167] T. Miyao, K. Minoshima, S. Naito, J. Mater. Chem. 2005, 15, 2268.

[168] J. H. Park, C. Oh, S. I. Shin, S. K. Moon, S. G. Oh, J. Colloid Interface

Sci. 2003, 266, 107.

[169] A. V. Jovanovic, R. S. Underhill, T. L. Bucholz, R. S. Duran, Chem.

Mater. 2005, 17, 3375.

[170] T. J. Joncheray, P. Audebert, E. Schwartz, A. V. Jovanovic, O. Ishaq,

J. L. Chavez, R. Pansu, R. S. Duran, Langmuir 2006, 22, 8684.

[171] W. J. Li, M. O. Coppens, Chem. Mater. 2005, 17, 2241.

[172] C. E. Fowler, D. Khushalani, S. Mann, Chem. Commun. 2001,

2028.

[173] C. E. Fowler, D. Khushalani, S. Mann, J. Mater. Chem. 2001, 11,

1968.


REVIE


[174] T. Nakashima, N. Kimizuka, J. Am. Chem. Soc. 2003, 125, 6386.

[175] A. Imhof, D. J. Pine, J. Colloid Interface Sci. 1997, 192, 368.

[176] H. J. Hah, J. S. Kim, B. J. Jeon, S. M. Koo, Y. E. Lee, Chem.

Commun. 2003, 1712.

[177] Q. B. Wang, L. Yan, H. Yan, Chem. Commun. 2007, 2339.

[178] C. I. Zoldesi, A. Imhof, Adv. Mater. 2005, 17, 924.

[179] C. I. Zoldesi, C. A. van Walree, A. Imhof, Langmuir 2006, 22,

4343.

[180] H. G. Wang, P. Chen, X. M. Zheng, J. Mater. Chem. 2004, 14, 1648.

[181] Y. Hu, J. Chen, W. Chen, X. Lin, X. Li, Adv. Mater. 2003, 15, 726.

[182] Y. Hu, J. F. Chen, X. Z. Jin, W. M. Chen, Mater. Lett. 2005, 59, 234.

[183] D. Z. Wu, X. W. Ge, Z. C. Zhang, M. Z. Wang, S. L. Zhang,

Langmuir 2004, 20, 5192.

[184] S. Yang, H. R. Liu, J. Mater. Chem. 2006, 16, 4480.

[185] J. C. Bao, Y. Y. Liang, Z. Xu, L. Si, Adv. Mater. 2003, 15, 1832.

[186] H. R. Wang, Y. J. Song, C. J. Medforth, J. A. Shelnutt, J. Am. Chem.

Soc. 2006, 128, 9284.

[187] H.M. Chen, J. H. He, C. B. Zhang, H. He, J. Phys. Chem. C 2007, 111,

18033.

[188] L. H. Dong, Y. Chu, Y. P. Zhang, Y. Liu, F. Y. Yang, J. Colloid

Interface Sci. 2007, 308, 258.

[189] X. L. Yu, C. B. Cao, H. S. Zhu, Q. S. Li, C. L. Liu, Q. H. Gong, Adv.

Funct. Mater. 2007, 17, 1397.

[190] J. S. Hu, Y. G. Guo, H. P. Liang, L. J. Wan, C. L. Bai, Y. G. Wang, J.

Phys. Chem. B 2004, 108, 9734.

[191] A. Imhof, D. J. Pine, Nature 1997, 389, 948.

[192] J. H. Fendler, Acc. Chem. Res. 1980, 13, 7.

[193] J. H. Fendler, Science 1984, 223, 888.

[194] C. A. Bunton, F. Nome, F. H. Quina, L. S. Romsted,Acc. Chem. Res.

1991, 24, 357.

[195] Y. J. Li, X. F. Li, Y. L. Li, H. B. Liu, S. Wang, H. Y. Gan, J. B. Li,

N. Wang, X. R. He, D. B. Zhu, Angew. Chem. Int. Ed. 2006, 45,

3639.

[196] S. S. Kim, W. Z. Zhang, T. J. Pinnavaia, Science 1998, 282, 1302.

[197] D. H. W. Hubert, M. Jung, P. M. Frederik, P. H. H. Bomans, J.

Meuldijk, A. L. German, Adv. Mater. 2000, 12, 1286.

[198] R. K. Rana, Y. Mastai, A. Gedanken, Adv. Mater. 2002, 14, 1414.

[199] D. Lootens, C. Vautrin, H. Van Damme, T. Zemb, J. Mater. Chem.

2003, 13, 2072.

[200] H. P. Hentze, S. R. Raghavan, C. A. McKelvey, E. W. Kaler,

Langmuir 2003, 19, 1069.

[201] H. Djojoputro, X. F. Zhou, S. Z. Qiao, L. Z. Wang, C. Z. Yu, G. Q.

Lu, J. Am. Chem. Soc. 2006, 128, 6320.

[202] J. G.Wang, Q. Xiao, H. J. Zhou, P. C. Sun, Z. Y. Yuan, B. H. Li, D. T.

Ding, A. C. Shi, T. H. Chen, Adv. Mater. 2006, 18, 3284.

[203] L. X. Zhang, P. C. Li, X. H. Liu, L. W. Du, E. Wang, Adv. Mater.

2007, 19, 4279.

[204] P. T. Tanev, T. J. Pinnavaia, Science 1996, 271, 1267.

[205] Y. R. Cai, H. H. Pan, X. R. Xu, Q. H. Hu, L. Li, R. K. Tang, Chem.

Mater. 2007, 19, 3081.

[206] X. M. Sun, Y. D. Li, J. Colloid Interface Sci. 2005, 291, 7.

[207] T. He, D. R. Chen, X. L. Jiao, Y. Y. Xu, Y. X. Gu,Langmuir 2004, 20,

8404.

[208] H. L. Xu, W. Z. Wang, Angew. Chem. Int. Ed. 2007, 46, 1489.

[209] Q. R. Zhao, Y. Xie, T. Dong, Z. G. Zhang, J. Phys. Chem. C 2007,

111, 11598.

[210] H. P. Cong, S. H. Yu, Adv. Funct. Mater. 2007, 17, 1814.

[211] Q. R. Zhao, Y. Gao, X. Bai, C. Z. Wu, Y. Xie, Eur. J. Inorg. Chem.

2006, 1643.

[212] D. B. Yu, X. Q. Sun, J. W. Zou, Z. R. Wang, F. Wang, K. Tang, J.

Phys. Chem. B 2006, 110, 21667.

[213] X. J. Zhang, D. Li, Angew. Chem. Int. Ed. 2006, 45, 5971.

[214] G. Chen, D. G. Xia, Z. R. Nie, Z. Y. Wang, L. Wang, L. Zhang, J. J.

Zhang, Chem. Mater. 2007, 19, 1840.


[215] Y. Liu, Y. Chu, Y. J. Zhuo, L. H. Dong, L. L. Li, M. Y. Li,Adv. Funct.

Mater. 2007, 17, 933.

[216] Q. Liu, H. J. Liu, M. Han, J. M. Zhu, Y. Y. Liang, Z. Xu, Y. Song,

Adv. Mater. 2005, 17, 1995.

[217] E. B. Abuin, J. C. Scaiano, J. Am. Chem. Soc. 1984, 106, 6274.

[218] D. B. Zhang, L. M. Qi, J. M. Ma, H. M. Cheng, Adv. Mater. 2002, 14,

1499.

[219] L. M. Qi, J. Li, J. M. Ma, Adv. Mater. 2002, 14, 300.

[220] Y. Q. Yeh, B. C. Chen, H. P. Lin, C. Y. Tang, Langmuir 2006, 22, 6.

[221] Y. Hu, X. Q. Jiang, Y. Ding, Q. Chen, C. Z. Yang, Adv. Mater. 2004,

16, 933.

[222] Y. Ding, Y. Hu, X. Q. Jiang, L. Y. Zhang, C. Z. Yang,Angew. Chem.

Int. Ed. 2004, 43, 6369.

[223] Y. Y. Xu, D. R. Chen, X. L. Jiao, K. Y. Xue, J. Phys. Chem. C 2007,

111, 16284.

[224] P. Zhou, Y. G. Li, P. P. Sun, J. H. Zhou, J. C. Bao, Chem. Commun.

2007, 1418.

[225] Y. G. Li, P. Zhou, Z. H. Dai, Z. X. Hu, P. P. Sun, J. C. Bao, New J.

Chem. 2006, 30, 832.

[226] L. S. Xu, X.H. Chen, Y. R.Wu, C. S. Chen,W.H. Li,W.Y. Pan, Y.G.

Wang, Nanotechnology 2006, 17, 1501.

[227] M. Yang, J. J. Zhu, J. Cryst. Growth 2003, 256, 134.

[228] B. X. Li, Y. Xie, M. Jing, G. X. Rong, Y. C. Tang, G. Z. Zhang,

Langmuir 2006, 22, 9380.

[229] A. M. Cao, J. S. Hu, H. P. Liang, L. J. Wan, Angew. Chem. Int. Ed.

2005, 44, 4391.

[230] M. Mo, J. C. Yu, L. Z. Zhang, S. K. A. Li, Adv. Mater. 2005, 17, 756.

[231] Z. H. Dai, J. Zhang, J. C. Bao, X. H. Huang, X. Y. Mo, J. Mater.

Chem. 2007, 17, 1087.

[232] X. L. Gou, F. Y. Cheng, Y. H. Shi, L. Zhang, S. J. Peng, J. Chen, P.W.

Shen, J. Am. Chem. Soc. 2006, 128, 7222.

[233] Y. Wan, S. H. Yu, J. Phys. Chem. C 2008, 112, 3641.

[234] M. S. Wong, J. N. Cha, K. S. Choi, T. J. Deming, G. D. Stucky, Nano

Lett. 2002, 2, 583.

[235] V. S. Murthy, J. N. Cha, G. D. Stucky, M. S. Wong, J. Am. Chem. Soc.

2004, 126, 5292.

[236] J. N. Cha, H. Birkedal, L. E. Euliss, M. H. Bartl, M. S. Wong, T. J.

Deming, G. D. Stucky, J. Am. Chem. Soc. 2003, 125, 8285.

[237] R. K. Rana, V. S. Murthy, J. Yu, M. S. Wong, Adv. Mater. 2005, 17,

1145.

[238] J. Yu, V. S. Murthy, R. K. Rana, M. S. Wong, Chem. Commun. 2006,

1097.

[239] J. Yu, M. A. Yaseen, B. Anvari, M. S. Wong, Chem. Mater. 2007, 19,

1277.

[240] T. B. Liu, Y. Xie, B. Chu, Langmuir 2000, 16, 9015.

[241] Q. Y. Sun, P. J. Kooyman, J. G. Grossmann, P. H. H. Bomans, P. M.

Frederik, P. C. M. M. Magusin, T. P. M. Beelen, R. A. van Santen, N.

A. J. M. Sommerdijk, Adv. Mater. 2003, 15, 1097.

[242] Y. R. Ma, L. M. Qi, J. M. Ma, H. M. Cheng, Langmuir 2003, 19, 4040.

[243] Y. Ma, L. Qi, J. Ma, H. Cheng, W. Shen, Langmuir 2003, 19, 9079.

[244] X. H. Li, D. H. Zhang, J. S. Chen, J. Am. Chem. Soc. 2006, 128,

8382.

[245] H. Y. Huang, E. E. Remsen, T. Kowalewski, K. L. Wooley, J. Am.

Chem. Soc. 1999, 121, 3805.

[246] X. Y. Liu, M. Jiang, S. Yang, M. Q. Chen, D. Y. Chen, C. Yang, K.

Wu, Angew. Chem. Int. Ed. 2002, 41, 2950.

[247] Q. Peng, Y. J. Dong, Y. D. Li, Angew. Chem. Int. Ed. 2003, 42,

3027.

[248] Y. S. Han, G. Hadiko, M. Fuji, M. Takahashi, Chem. Lett. 2005, 152.

[249] X. Fan, Z. Zhang, G. Li, N. A. Rowson, Chem. Eng. Sci. 2004, 59,

2639.

[250] C. L. Jiang, W. Q. Zhang, G. F. Zou, W. C. Yu, Y. T. Qian,


[251] F. Gu, C. Z. Li, S. F. Wang, M. K. Lu, Langmuir 2006, 22, 1329.


REVIE


4018

[252] C. Z. Wu, Y. Xie, L. Y. Lei, S. Q. Hu, C. Z. OuYang, Adv. Mater.

2006, 18, 1727.

[253] X. X. Li, Y. J. Xiong, Z. Q. Li, Y. Xie, Inorg. Chem. 2006, 45, 3493.

[254] H. W. Hou, Q. Peng, S. Y. Zhang, Q. X. Guo, Y. Xie, Eur. J. Inorg.

Chem. 2005, 2625.

[255] L. Guo, F. Liang, X. G. Wen, S. H. Yang, L. He, W. Z. Zheng, C. P.

Chen, Q. P. Zhong, Adv. Funct. Mater. 2007, 17, 425.

[256] H. Zhang, S. Y. Zhang, S. Pan, G. P. Li, J. G. Hou, Nanotechnology

2004, 15, 945.

[257] X. Y. Chen, Z. H. Wang, X. Wang, R. Zhang, X. Y. Liu, W. J. Lin, Y.

T. Qian, J. Cryst. Growth 2004, 263, 570.

[258] H. Z.Wang, J. B. Liang, H. Fan, B. J. Xi, M. F. Zhang, S. L. Xiong, Y.

C. Zhu, Y. T. Qian, J. Solid State Chem. 2008, 181, 122.

[259] Y. J. Kim, S. Y. Chai, W. Lee, Langmuir 2007, 23, 9567.

[260] X. Y. Chen, Z. J. Zhang, X. X. Li, C. W. Shi, Chem. Phys. Lett. 2006,

422, 294.

[261] K. S. Suslick, Science 1990, 247, 1439.

[262] D. G. Shchukin, K. Kohler, H. Mohwald, G. B. Sukhorukov, Angew.

Chem. Int. Ed. 2005, 44, 3310.

[263] E. Prouzet, F. Cot, C. Boissiere, P. J. Kooyman, A. Larbot, J. Mater.

Chem. 2002, 12, 1553.

[264] J. J. Zhu, S. Xu, H. Wang, J. M. Zhu, H. Y. Chen, Adv. Mater. 2003,

15, 156.

[265] S. F. Wang, F. Gu, M. K. Lu, Langmuir 2006, 22, 398.

[266] X. W. Zheng, Y. Xie, L. Y. Zhu, X. C. Jiang, A. H. Yan, Ultrason.

Sonochem. 2002, 9, 311.

[267] W. Ostwald, Z. Phys. Chem. 1900, 34, 495.

[268] W. Ostwald, Lehrbuch der Allgemeinen Chemie, Vol. 2, Part 1,

Engelmann, Leipzig, Germany 1896.

[269] X. W. Lou, Y. Wang, C. Yuan, J. Y. Lee, L. A. Archer, Adv. Mater

2006, 18, 2325.

[270] X. W. Lou, D. Deng, J. Y. Lee, L. A. Archer, unpublished.

[271] Y. Xie, J. X. Huang, B. Li, Y. Liu, Y. T. Qian, Adv. Mater. 2000, 12,

1523.

[272] C. W. Guo, Y. Cao, S. H. Xie, W. L. Dai, K. N. Fan,Chem. Commun.

2003, 700.

[273] W. S. Wang, L. Zhen, C. Y. Xu, B. Y. Zhang, W. Z. Shao, J. Phys.

Chem. B 2006, 110, 23154.

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

[275] H. G. Yang, H. C. Zeng, Angew. Chem. Int. Ed. 2004, 43, 5206.

[276] J. Li, H. C. Zeng, Angew. Chem. Int. Ed. 2005, 44, 4342.

[277] H. X. Li, Z. F. Bian, J. Zhu, D. Q. Zhang, G. S. Li, Y. N. Huo, H. Li,

Y. F. Lu, J. Am. Chem. Soc. 2007, 129, 8406.

[278] J. J. Teo, Y. Chang, H. C. Zeng, Langmuir 2006, 22, 7369.

[279] Y. Chang, J. J. Teo, H. C. Zeng, Langmuir 2005, 21, 1074.

[280] B. Liu, H. C. Zeng, Small 2005, 1, 566.

[281] X. B. Cao, L. Gu, L. Zhuge, W. J. Gao, W. C. Wang, S. F. Wu, Adv.

Funct. Mater. 2006, 16, 896.

[282] J. Li, H. C. Zeng, J. Am. Chem. Soc. 2007, 129, 15839.

[283] L. P. Zhu, H.M. Xiao,W. D. Zhang, G. Yang, S. Y. Fu,Cryst. Growth

Des. 2008, 8, 957.

[284] Y. F. Zhu, D. H. Fan, W. Z. Shen, J. Phys. Chem. C 2007, 111, 18629.

[285] C. Luo, D. F. Xue, Langmuir 2006, 22, 9914.

[286] Y. Wang, Q. S. Zhu, H. G. Zhang, Chem. Commun. 2005, 5231.

[287] J. G. Yu, H. T. Guo, S. A. Davis, S. Mann, Adv. Funct. Mater. 2006,

16, 2035.

[288] H. G. Yu, J. G. Yu, S. W. Liu, S. Mann, Chem. Mater. 2007, 19, 4327.

[289] J. G. Yu, H. G. Yu, H. T. Guo, M. Li, S. Mann, Small 2008, 4, 87.

[290] A. W. Xu, Q. Yu, W. F. Dong, M. Antonietti, H. Colfen,Adv. Mater.

2005, 17, 2217.

[291] C. J. Jia, L. D. Sun, Z. G. Yan, L. P. You, F. Luo, X. D. Han, Y. C.

Pang, Z. Zhang, C. H. Yan, Angew. Chem. Int. Ed. 2005, 44, 4328.

[292] J. Lu, D. R. Chen, X. L. Jiao, J. Colloid Interface Sci. 2006, 303,

437.


[293] L. L. Li, Y. Chu, Y. Liu, L. H. Dong, J. Phys. Chem. C 2007, 111,

2123.

[294] S. Y. Zeng, K. B. Tang, T. W. Li, Z. H. Liang, D. Wang, Y. K. Wang,

W. W. Zhou, J. Phys. Chem. C 2007, 111, 10217.

[295] B. D. Mao, Z. H. Kang, E. B. Wang, C. G. Tian, Z. M. Zhang, C. L.

Wang, Y. L. Song, M. Y. Li, J. Solid State Chem. 2007, 180, 489.

[296] B. P. Jia, L. Gao, J. Phys. Chem. C. 2008, 112, 666.

[297] Y. X. Hu, J. P. Ge, Y. G. Sun, T. R. Zhang, Y. D. Yin, Nano Lett.

2007, 7, 1832.

[298] S. H. Im, U. Y. Jeong, Y. N. Xia, Nat. Mater. 2005, 4, 671.

[299] P. Tartaj, T. Gonzalez-Carreno, C. J. Serna, Adv. Mater. 2001, 13,

1620.

[300] A.H. Latham,M. J.Wilson, P. Schiffer,M. E.Williams, J. Am. Chem.

Soc. 2006, 128, 12632.

[301] H. L. Zhu, K. H. Yao, H. Zhang, D. R. Yang, J. Phys. Chem. B 2005,

109, 20676.

[302] H. J. Liu, Y. H. Ni, M. Han, Q. Liu, Z. Xu, J. N. Hong, X. Ma,


[303] W. W. Wang, Y. J. Zhu, L. X. Yang, Adv. Funct. Mater. 2007, 17, 59.

[304] G. L. Cui, Y. S. Hu, L. J. Zhi, D. Q. Wu, I. Lieberwirth, J. Maier, K.

Mullen, Small 2007, 3, 2066.

[305] L. J. Mao, C. Y. Liu, J. Li, J. Mater. Chem. 2008, 18, 1640.

[306] K. Kamata, Y. Lu, Y. N. Xia, J. Am. Chem. Soc. 2003, 125, 2384.

[307] G. Y. Liu, H. F. Ji, X. L. Yang, Y. M.Wang,Langmuir 2008, 24, 1019.

[308] P. M. Arnal, M. Comotti, F. Schuth, Angew. Chem. Int. Ed. 2006, 45,

8224.

[309] K. Zhang, X. H. Zhang, H. T. Chen, X. Chen, L. L. Zheng, J. H.

Zhang, B. Yang, Langmuir 2004, 20, 11312.

[310] S. H. Liu, Z. H. Zhang, M. Y. Han, Adv. Mater. 2005, 17, 1862.

[311] M. Kim, K. Sohn, H. Bin Na, T. Hyeon, Nano Lett. 2002, 2, 1383.

[312] S. Ikeda, S. Ishino, T. Harada, N. Okamoto, T. Sakata, H. Mori, S.

Kuwabata, T. Torimoto,M.Matsumura,Angew. Chem. Int. Ed. 2006,

45, 7063.

[313] X. W. Lou, C. L. Yuan, E. Rhoades, Q. Zhang, L. A. Archer, Adv.

Funct. Mater. 2006, 16, 1679.

[314] W. S. Choi, H. Y. Koo, D. Y. Kim, Adv. Mater. 2007, 19, 451.

[315] H. J. Hah, J. I. Um, S. H. Han, S. M. Koo, Chem. Commun. 2004,

1012.

[316] D. M. Cheng, X. D. Zhou, H. B. Xia, H. S. O. Chan, Chem. Mater.

2005, 17, 3578.

[317] S. Cavaliere-Jaricot, M. Darbandi, T. Nann, Chem. Commun. 2007,

2031.

[318] A. B. Fuertes, M. Sevilla, T. Valdes-Solis, P. Tartaj, Chem. Mater.

2007, 19, 5418.

[319] D. K. Yi, S. S. Lee, G. C. Papaefthymiou, J. Y. Ying, Chem. Mater.

2006, 18, 614.

[320] H. M. Chen, R. S. Liu, K. Asakura, J. F. Lee, L. Y. Jang, S. F. Hu, J.

Phys. Chem. B 2006, 110, 19162.

[321] K. T. Lee, Y. S. Jung, S. M. Oh, J. Am. Chem. Soc. 2003, 125, 5652.

[322] Y. H. Ng, S. Ikeda, T. Harada, S. Higashida, T. Sakata, H. Mori, M.

Matsumura, Adv. Mater. 2007, 19, 597.

[323] K. M. Keville, E. I. Franses, J. M. Caruthers, J. Colloid Interface Sci.

1991, 144, 103.

[324] Y. Lu, Y. D. Yin, Y. N. Xia, Adv. Mater. 2001, 13, 271.

[325] D. M. Cheng, H. B. Xia, H. S. O. Chan, Nanotechnology 2006, 17,

1661.

[326] Z. M. Zhang, J. Sui, L. J. Zhang, M. X. Wan, Y. Wei, L. M. Yu, Adv.

Mater. 2005, 17, 2854.

[327] X. W. Lou, L. A. Archer, Adv. Mater. 2008, 20, 1853.

[328] X. W. Lou, C. Yuan, L. A. Archer, Adv. Mater. 2007, 19, 3328.

[329] L. Y. Hao, C. L. Zhu, W. Q. Jiang, C. N. Chen, Y. Hu, Z. Y. Chen, J.

Mater. Chem. 2004, 14, 2929.

[330] X. W. Lou, C. Yuan, Q. Zhang, L. A. Archer, Angew. Chem. Int. Ed.

2006, 45, 3825.


REVIE


[331] S. H. Jiao, L. F. Xu, K. Jiang, D. S. Xu, Adv. Mater. 2006, 18, 1174.

[332] H. L. Cao, X. F. Qian, C. Wang, X. D. Ma, J. Yin, Z. K. Zhu, J. Am.

Chem. Soc. 2005, 127, 16024.

[333] H. L. Xu, W. Z. Wang, W. Zhu, L. Zhou, Nanotechnology 2006, 17,

3649.

[334] Y. N. Tan, J. Yang, J. Y. Lee, D. I. C. Wang, J. Phys. Chem. C 2007,

111, 14084.

[335] Q. Gong, X. F. Qian, P. L. Zhou, X. B. Yu, W. M. Du, S. H. Xu, J.

Phys. Chem. C 2007, 111, 1935.

[336] L. N. Ye, C. Z. Wu, W. Guo, Y. Xie, Chem. Commun. 2006,

4738.

[337] C. H. B. Ng, W. Y. Fan, J. Phys. Chem. C 2007, 111, 9166.

[338] Y. Wang, J. Y. Lee, J. Nanopart. Res. 2006, 8, 1053.

[339] J. H. Yang, L. M. Qi, C. H. Lu, J. M.Ma, H.M. Cheng,Angew. Chem.

Int. Ed. 2005, 44, 598.

[340] J. Y. Chen, J. M. McLellan, A. Siekkinen, Y. J. Xiong, Z. Y. Li, Y. N.

Xia, J. Am. Chem. Soc. 2006, 128, 14776.

[341] X. Wang, L. J. Yu, P. Hu, F. L. Yuan, Cryst. Growth Des. 2007, 7,

2415.

[342] W. Z. Wang, B. Poudel, D. Z. Wang, Z. F. Ren,Adv. Mater. 2005, 17,

2110.

[343] Q. D. Chen, X. H. Shen, H. C. Gao, J. Colloid Interface Sci. 2007, 312,

272.

[344] T. He, D. R. Chen, X. L. Jiao, Y. L. Wang, Adv. Mater. 2006, 18,

1078.

[345] J. Geng, J. J. Zhu, D. J. Lu, H. Y. Chen, Inorg. Chem. 2006, 45, 8403.

[346] S. Yang, X. F. Zhou, P. Yuan, M. H. Yu, S. G. Xie, J. Zou, G. Q. Lu,

C. Z. Yu, Angew. Chem. Int. Ed. 2007, 46, 8579.

[347] H. G. Yang, H. C. Zeng, Angew. Chem. Int. Ed. 2004, 43, 5930.

[348] C. H. Lu, L. M. Qi, J. H. Yang, X. Y. Wang, D. Y. Zhang, J. L. Xie, J.

M. Ma, Adv. Mater. 2005, 17, 2562.

[349] Y. J. Xiong, B. Wiley, J. Y. Chen, Z. Y. Li, Y. D. Yin, Y. N. Xia,

Angew. Chem. Int. Ed. 2005, 44, 7913.

[350] Y. H.Wang, P. L. Chen, M. H. Liu,Nanotechnology 2008, 19, 045607.

[351] C. J. Jia, L. D. Sun, Z. G. Yan, Y. C. Pang, L. P. You, C. H. Yan, J.

Phys. Chem. C 2007, 111, 13022.

[352] J. M. Tarascon, M. Armand, Nature 2001, 414, 359.

[353] A. S. Arico, P. Bruce, B. Scrosati, J. M. Tarascon,W. Van Schalkwijk,

Nat. Mater. 2005, 4, 366.

[354] M.Winter, J. O. Besenhard,M. E. Spahr, P. Novak,Adv.Mater. 1998,

10, 725.

[355] J. L. Tirado, Mat. Sci. Eng. R 2003, 40, 103.

[356] D. Larcher, S. Beattie, M. Morcrette, K. Edstroem, J. C. Jumas, J. M.

Tarascon, J. Mater. Chem. 2007, 17, 3759.

[357] P. Poizot, S. Laruelle, S. Grugeon, L. Dupont, J. M. Tarascon,Nature

2000, 407, 496.

[358] C. R. Jarvis, M. J. Lain, M. V. Yakovleva, Y. Gao, J. Power Sources

2006, 162, 800.

[359] X. W. Lou, L. A. Archer, unpublished.


[360] U. Kasavajjula, C. S. Wang, A. J. Appleby, J. Power Sources 2007,

163, 1003.

[361] H. Ma, F. Y. Cheng, J. Chen, J. Z. Zhao, C. S. Li, Z. L. Tao, J. Liang,

Adv. Mater. 2007, 19, 4067.

[362] X. W. Lou, D. Deng, J. Y. Lee, J. Feng, L. A. Archer, Adv. Mater.

2008, 20, 258.

[363] S. J. Han, B. C. Jang, T. Kim, S. M. Oh, T. Hyeon,Adv. Funct. Mater.

2005, 15, 1845.

[364] H. X. Yang, J. F. Qian, Z. X. Chen, X. P. Ai, Y. L. Cao, J. Phys. Chem.

C 2007, 111, 14067.

[365] D. Deng, J. Y. Lee, Chem. Mater. 2008, 20, 1841.

[366] H. Kim, J. Cho, Chem. Mater. 2008, 20, 1679.

[367] B. X. Li, G. X. Rong, Y. Xie, L. F. Huang, C. Q. Feng, Inorg. Chem.

2006, 45, 6404.

[368] J. Zhao, W. X. Chen, Y. F. Zheng, X. Li, J. Power Sources 2006, 162,

168.

[369] Y. Kondo, H. Yoshikawa, K. Awaga, M. Murayama, T. Mori, K.

Sunada, S. Bandow, S. Iijima, Langmuir 2008, 24, 547.

[370] H. J. Koo, Y. J. Kim, Y. H. Lee, W. I. Lee, K. Kim, N. G. Park, Adv.

Mater. 2008, 20, 195.

[371] Z. Y. Liu, D. D. Sun, P. Guo, J. O. Leckie, Chem. Eur. J. 2007, 13,

1851.

[372] H. G. Zhang, Q. S. Zhu, Y. Zhang, Y. Wang, L. Zhao, B. Yu, Adv.

Funct. Mater. 2007, 17, 2766.

[373] C. R. Martin, P. Kohli, Nat. Rev. Drug Discov. 2003, 2, 29.

[374] Z. P. Xu, Q. H. Zeng, G. Q. Lu, A. B. Yu, Chem. Eng. Sci. 2006, 61,

1027.

[375] V. Sokolova, M. Epple, Angew. Chem. Int. Ed. 2008, 47, 1382.

[376] Z. Z. Li, L. X. Wen, L. Shao, J. F. Chen, J. Controlled Release 2004,

98, 245.

[377] N. E. Botterhuis, Q. Y. Sun, P. C. M. M. Magusin, R. A. van Santen,

N. A. J. M. Sommerdijk, Chem. Eur. J. 2006, 12, 1448.

[378] J. Zhou, W. Wu, D. Caruntu, M. H. Yu, A. Martin, J. F. Chen, C. J.

O’Connor, W. L. Zhou, J. Phys. Chem. C 2007, 111, 17473.

[379] Y. F. Zhu, J. L. Shi, W. H. Shen, X. P. Dong, J. W. Feng, M. L. Ruan,

Y. S. Li, Angew. Chem. Int. Ed. 2005, 44, 5083.

[380] V. V. Sokolova, I. Radtke, R. Heumann, M. Epple, Biomaterials

2006, 27, 3147.

[381] J. Chen, F. Saeki, B. J. Wiley, H. Cang, M. J. Cobb, Z. Y. Li, L. Au,

H. Zhang, M. B. Kimmey, X. D. Li, Y. Xia, Nano Lett. 2005, 5,

473.

[382] S. Skrabalak, J. Chen, L. Au, X. Lu, X. Li, Y. Xia, Adv. Mater. 2007,

19, 3177.

[383] J. Y. Chen, D. L. Wang, J. F. Xi, L. Au, A. Siekkinen, A. Warsen, Z.

Y. Li, H. Zhang, Y. N. Xia, X. D. Li, Nano Lett. 2007, 7, 1318.

[384] L. R. Hirsch, R. J. Stafford, J. A. Bankson, S. R. Sershen, B. Rivera,

R. E. Price, J. D. Hazle, N. J. Halas, J. L. West, Proc. Natl. Acad. Sci.

USA 2003, 100, 13549.

[385] C. Loo, A. Lowery, N. Halas, J. West, R. Drezek, Nano Lett. 2005, 5,

709.


hollow micro-/nanostructures: synthesis and … micro... · hollow micro-/nanostructures: synthesis...

Documents