Effective Use of Mesoporous Silica Films for the Preparation of Nanostructured Metals

金属ナノ構造体の作製に向けた

メソポーラスシリカ薄膜の有効利用

Thesis was submitted to Waseda University

Yosuke Kanno

Department of Applied Chemistry, Graduate School of Advanced Science and Engineering

February, 2013

This thesis was reviewed and approved by the following

Dr. Kazuyuki Kuroda

Professor

Waseda University

Thesis advisor

Chair of Committee

Dr. Tetsuya Osaka

Professor

Waseda University

Dr. Yoshiyuki Sugahara

Professor

Waseda University

Dr. Takayuki Homma

Professor

Waseda University

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Preface Mesoporous materials attract wide attention because of their possible

application in environment, energy, and health. High specific surface area and huge pore

volume derived from mesopores, which size is in range of 2-50 nm, are valuable for

adsorbents, catalysts, and catalyst supports, etc. Furthermore, various properties of them

are possible to design by controlling their morphology, mesostructure, and composition

of pore wall at macro, meso, and atomic scales, which expands their potential

applications in electronics, optics, and medical science, and so on.

Ordered mesoporous materials are suitable as templates to fabricate other

nanomaterials, e.g. nanostructured metals and semiconductors, compared with

traditional porous materials, such as activated carbons, silica gels, and zeolites, because

of their relatively large pores with uniform size and ordered arrangement of the

mesopores. Replication by introduction of other materials into the mesopores, called

hard templating, can strictly control the nanostructures of the products. In particular,

unique properties of nanostructured metals originated from surface plasmon resonance

and size effect are depend on its size and shape, therefore, precise directing of size and

shape of the products in hard templating are significant. For design of nanostructured

metals with control of their size and shape, nanostructures of the templates should be

easily tuned to the desired structures. Mesoporous silica must be suitable ones in terms

of controllability of its mesostructures. Various morphologies and mesostructures of

mesoporous silica are prepared in corresponding synthesis condition, which supports

novel and facile fabrications of diverse nanostructured metals, indeed. However,

morphology of the templates is powder in many of the papers on the hard templating of

nanostructured metals. Introduction methods are, thus, still quite limited by its

morphology.

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This thesis focus on the mesoporous silica films for preparation of

nanostructured metal in order to explore novel processes for synthesis, and to advance

chemistry in the nanospace. Film morphology is chosen because its size is centimeter

scale, which is much larger than that of granule of powder. Film morphology enables to

apply various introduction method, such as electrodeposition in which the rate and the

position of the deposition can be easily controlled. Electrodeposition of Au and vapor

deposition of Cu using mesoporous silica films as templates are studied in this thesis.

Unidirectional alignment of mesopores in a mesoporous silica film is also studied for its

potential applicability to fabricate unique metal nanostructures.

This thesis is composed of 6 chapters as follows.

Chapter 1 describes the general introduction and the background of this thesis.

Mesoporous silica films, nanostructured metals, and hard templating of nanostructured

metals using mesoporous silica films are overviewed.

Chapter 2 demonstrates the preparation of Au nanowires in mesochannels of

mesoporous silica films by electrodeposition. Au nanowires are prepared using

mesoporous silica films with vertically oriented mesochannels on a conductive substrate

as a template. In contrast, the use of mesoporous silica films with parallel oriented

mesochannels without vertical aligned mesopores causes cleavage of the film, which is

in clear contrast to successful formation of Pt nanowire arrays.

Chapter 3 demonstrates the preparation of Au nanoparticles in spherical

mesopores of mesoporous silica films by electrodeposition. The effect of the difference

of mesostructures on the deposition of Au and optical property of the Au/silica

composite film are discussed.

Chapter 4 demonstrates the preparation of Cu nanopatterns on the mesoporous

silica film by vapor deposition. Stripe and hexagonal patterns are replicated on the Cu

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surface after the peeling of Cu films from mesoporous silica films with mesochannels

and spherical mesopores, respectively.

Chapter 5 demonstrates the alignment control of mesochannels in a

mesoporous silica film on a freshly cleaved mica surface. prepared by an

evaporation-induced self-assembly process, is found to be unidirectional with the

narrowest directional distribution.

Chapter 6 describes general conclusion of this thesis and future prospects.

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Contents Preface Chapter 1: General Introduction Introduction 1.1 Nanostructured metals 2 1.1.1 Introduction to nanostructured metals 2 1.1.2 Synthetic routes of nanostructured metals 2 1.1.2.1 Formation of nanostructured metals without capping agent and template 4 1.1.2.2 Formation of nanostructured metals with capping agent 4 1.1.2.3 Formation of nanostructured metals with template: templating 7 1.1.2.3.1 Soft templating 7 1.1.2.3.2 Hard templating 8

1.2 Mesoporous silica films 10 1.2.1 Introduction to mesoporous silica 10 1.2.2 Feature of mesoporous silica films 13 1.2.3 Preparation of mesoporous silica films 14 1.2.3.1 Evaporation induced self-assembly 15 1.2.3.2 Hydrothermal method 17

1.2.4 Alignment control in mesoporous silica films 18 1.2.5 Mesoporous silica films for applications 21

1.3 Hard templating of metals using mesoporous silica films 23 1.3.1 Features of hard templating of nanostructured metals with mesoporous

silica films 23 1.3.2 Preparation process in hard templating of metals using mesoporous silica

films 24 1.3.2.1 Electrodeposition of metals in mesoporous silica films 24 1.3.2.2 Deposition of metals using reducing agents in mesoporous silica films 27

1.3.3 Significance of hard templating using mesoporous silica films toward application 29

1.4 References 31 Chapter 2: Preparation of Au Nanowire Films by Electrodeposition Using Mesoporous Silica Films as a Template: Vital Effect of Vertically Oriented Mesopores on a Substrate 2.1 Introduction 44

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2.2 Experimental 46 2.2.1 Materials 46 2.2.2 Preparation of mesoporous silica films 47 2.2.3 Electrodeposition of Au using mesoporous silica films as templates 48 2.2.4 Characterization 48

2.3 Results and discussion 49 2.3.1 Mesostructure of the mesoporous silica films coated on ITO substrates 49 2.3.2 Au deposition with cracking of template 55 2.3.3 Au deposition without cracking of template 60 2.3.4 Au nanowires after removal of silica 66

2.4 Conclusion 71 2.5 References 72 Chapter 3: Formation of Au nanostructure by electrodeposition in a mesoporous silica film with interconnected cage-type mesopores 3.1 Introduction 82 3.2 Experimental 83 3.2.1 Materials 83 3.2.2 Preparation of a mesoporous silica film 84 3.2.3 Electrodeposition of Au using a mesoporous silica film 85 3.2.4 Characterization 85

3.3 Results and Discussion 86 3.3.1 Mesostructure of the mesoporous silica films 86 3.3.2 Au deposition using a mesoporous silica film 87 3.3.3 Au nanostructure after removal of silica 96

3.4 Conclusion 98 3.5 References 98 Chapter 4: Formation of Cu Nanopatterns using the surface of Mesoporous Silica Films 4.1 Introduction 102

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4.2 Experimental 103 4.2.1 Materials 103 4.2.2 Preparation of mesoporous silica films 103 4.2.3 Vapor deposition of Cu 104 4.2.4 Characterization 105

4.3 Results and discussion 105 4.3.1 Mesostructures of the mesoporous silica films 105

4.3.2 Chemical etching of the mesoporous silica films 107 4.3.3 Surface nanostructure of Cu films 110

4.4 Conclusion 114 4.5 References 115 Chapter 5: Facile Unidirectional Alignment of Mesochannels in a Mesoporous Silica Film on a Freshly Cleaved Mica Surface 5.1 Introduction 118 5.2 Experimental 119 5.2.1 Materials 119 5.2.2 Preparation of mesoporous silica films on a mica surfaces 120

5.2.2.1 EISA process 120 5.2.2.2 Hydrothermal deposition process 120

5.2.3 Characterization 121

5.3 Results and discussion 122 5.3.1 In-plane alignment of mesopores in the film 122 5.3.2 Effect of the synthesis process on alignment control 125 5.3.3 Mechanism of alignment control 127

5.4 Conclusion 129 5.5 References 129 Chapter 6: Conclusions 6.1 Conclusions 134

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6.2 Future prospects 136 List of Achievements 139 Acknowledgements 145

Chapter 1 General Introduction

Chapter 1

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1.1 Nanostructured metals1-5

1.1.1 Introduction to nanostructured metals

Nanostructured metals, such as many type of nanoparticle and its arrays, are

attractive materials in various fields, such as optics, magnetics, (electro-) catalysis,

conversion and storage of energy, diagnosis and therapy, and so on. Their nanostructures

enhance features of metals and develop emerging properties which have been never

shown in bulk metals. These unique properties have been driving many researches on

the fabrication and the utilization of diverse nanostructured metals with various

compositions, structures, and morphologies.

Many of the studies on the fabrication of nanostructured metals have exerted

massive efforts to extend the variety of nanostructured metals in terms of their

composition, structure, and morphology because their combination decides the property.

Novel strategies and processes to prepare nanostructured metals have been invented,

which expand their functionality. On the other hand, it is important industrially to

explore effective combination in specific applications. In order to utilize in huge

applicable area, a lot of nanostructured metals have been synthesized. These materials

should be precisely control the composition, the structure, and the morphology at each

level to satisfy the demands. Varied fashions to prepare nanostructured metals are

requisite for this multi-level design.

1.1.2 Synthetic routes of nanostructured metals

Synthetic routes are highly significant in materials chemistry, of course, in the

preparation of nanostructured metals, also. Products walk through synthetic pathways,

and then, there is something persisting as a vestige of the process in the product.

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Different processes produce different materials; even if they are just similar to each

other, they did not completely same. Because the vestige remains somewhere in the

composition, the structure, and the morphology, selection of the synthetic pathways are

essential to achieve to construct desired design at multi-level in preparation of

nanostructured metals.

Various fabrication techniques of nanostructured metals can be broadly divided

into top-down and bottom-up approaches.6 The former starts with a bulk or thin film of

metals and removes selective parts to fabricate nanostructures, like sculpture. The latter

employs smaller building blocks, such as atoms, molecules, clusters, and colloids, as

starting materials. For example, traditional lithography and micromachining techniques

belong to top-down approach. On the other hand, almost all chemical fabrication

processes reside in the bottom-up approach. The interest of this thesis focused on the

bottom-up approach because this approach enables to prepare large amount of relatively

small nanostructured metals briefly and rapidly and is possible to shift the practical

technology dramatically.

The bottom-up approach in the fabrication of nanostructured metals can be

classified into three types: preparation without capping agent and template, with

capping agent, and with template. Here, the terms of capping agent and template mean

stabilizer additive capping the surface of nanostructured metals and material with

decided nanostructures to determine nanostructures of metals.

In contrast to the first method, the second and third methods employ a capping

agent and a template respectively in order to direct nanostructures of metals more

precisely in terms of the size and the shape.

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1.1.2.1 Formation of nanostructured metals without capping agent and template

The preparation of nanostructured metals without capping agent and template

is quite simple. Nanostructures are controlled physically and almost kinetically. For

example, Bi and Sb nanopowders on chamber walls and mica substrates were prepared

by thermal evaporation in 1930.7-9 Prepared powders were described as "extremely fine"

and "loosely packed" crystalline deposits. For another example, the gas condensation

method can produce metal nanoparticles more systematically. In the method, the

nanoparticles form through following three steps: 1) the evaporation producing a

supersaturated vapor, 2) nucleation, and 3) particle growth by coalescence and

coagulation.

Prepared nanoparticles have isotropic shape and bare surface in the formation

without capping agent and template, which is important for the application requiring

active surface, such as catalyst.

The nucleation rate and the growth rate during formation of the products must

be carefully controlled in order to obtain fine nanoparticles. Also, it is difficult to

fabricate anisotropic shape of nanoparticles, e.g. nanorods, because of the formation

mechanism. Additionally, the agglomeration and coalescence of the particles must be

avoided once they have been synthesized, which suggest that the nanoparticles cannot

be dispersed or located in high density without any aggregation.

1.1.2.2 Formation of nanostructured metals with capping agent

Capping agent, as which organic molecules with amine, ammonium, thiol, or

carboxyl groups are frequently selected, adsorbs on defined crystallographic planes of

metals in the preparation of nanostructured metals with capping agent.9-17 Adsorbed

Chapter 1

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molecules reduce the surface energy of nanoparticles and suppress the crystal growth of

the adsorbed planes, resulting in the formation of stable tiny nanoparticles with an

isotropic or anisotropic shape.

One of the well-known processes is citrate reduction of hydrogen

tetrachloroaurate in water.18 It leads to Au nanoparticles of ca. 20 nm. Here, citrate acts

as a reducing agent and capping agent.

Capping agent enables to prepare tiny nanoparticles without aggregation. Pd

nanoparticles of 2-4 nm in size were prepared using phenanthroline derivatives as a

capping agent. 19 The tiny nanoparticles showed unique magnetic property at very low

temperatures.

Also, anisotropic nanoparticles can be fabricated under coexistence of capping

agent. Yang et al. reported high yield synthesis of regular polyhedrons of Au by a

refined polyol process.20 The term polyol is short for polyalcohol, e.g. ethylene or

propylene glycol. In the traditional method, the polyol acts as solvent, reducing agent,

and capping agent. The group added polyvinylpyrrolidone as another capping agent in

this system, resulting in the particle shape can be tuned. The polyol method is a useful

technique for the synthesis of other metals (Ni, Pt, Pd, and so on), nanocrystalline alloys,

and bimetallic clusters. As another example, Murphy et al. proposed and developed a

seed-mediated growth process to fabricate multiple shapes of metal nanoparticles by

wet chemical approach.10, 13, 21, 22 In this process, first, metal salts were rapidly reduced

with a strong reducing agent in water to produce ~4 nm seed particles. Subsequently,

slow reduction of newly added metal salt with a weak reducing agent in the presence of

capping agent, which leads to the controlled formation of nanorods of specified aspect

ratio and can also yield other shapes of nanoparticles (stars, tetrapods, blocks, cubes,

Chapter 1

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etc.).

Metal nanoparticles prepared using capping agents have various shape and size.

The shape and size can be controlled by the selection of the kind of capping agent and

its concentration mainly. Capping molecules are densely adsorbed on the specific

surfaces, resulting in the very small active surface are remained. Therefore, the particles

show good stability in the suspension.

If you want to prepare the metal nanoparticles for catalyst using capping agent,

choice of capping agent must be sufficiently cared. For this demand, several kinds of

functional polymers should be adopted because of unsaturated adsorption of the

polymer, that is, there is a room for the substance to be adsorbed even after the

polymers adsorbed. Nanostructures of deposited metals are often difficult to be

estimated before the synthesis because the structure is decided by the balance of crystal

nature of metals and adsorption strength of capping molecule on specific surfaces.

Figure 1.1 TEM images and absorption spectra and appearance of Au nanorods

synthesized by seed-mediated growth

Chapter 1

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1.1.2.3 Formation of nanostructured metals with template: templating

Template possesses decided nanostructures which determine nanostructures of

metals. The template confines growth of metal nanoparticles, result in the nanostructure

of metals reflects the nanostructures of the template. The nanostructures of the metals

should be reverse structure of the template.

The templating employs soft or hard templates. For example, (reversed)

micelles and microemulsions belong to the soft template, and anodic porous metal

oxides and mesoporous materials reside in the hard template. Although the definition of

the terms soft and hard is quite obscure, difference between soft and hard template

seems to be relative rigidity. Hard template consists of mainly covalent bonded

materials. In contrast, soft template contains many other bonding, e.g. hydrophobic

bonding. This difference can contribute to the strength of confinement effect.

1.1.2.3.1 Soft templating

In the soft templating, self-assembled nanostructures of organic molecules are

generally employed as templates. Self-assembly of organic molecules must occur under

the existence of metal precursor, otherwise the growth of metals are not regulated by the

template structures.

One of the most successful studies on soft-templating of metals is preparation

of mesoporous metals reported by Yamauchi et al.23 Lyotropic liquid crystals (LLC)

composed of metal salts and surfactant are utilized as template. Metal salts in formed

LLC were reduced by reducing agent or through the electroreduction, which leads to

form mesoporous metals. They proposed evaporation-induced direct templating (EDIT)

method for preparation of LLC.24-31 In EDIT method, volatile solvent is added to the

Chapter 1

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traditional LLC precursors which are consisting of surfactant, metal salts, and water.

And then, the volatile evaporation induces the formation of LLC. This method was

adoptable to various metals, mesostructures, and morphologies. In particular, low

viscosity of precursor solution was important for morphology control. Hierarchical

structural design could be achieved by combination of hard-templating.

Soft templating is broadly adoptable approach. The nanostructure of the metals

is controlled by mainly the choice of surfactants. Morphological design is achieved by

the selection of reduction methods. However, there is a limit to fabricate metal

nanostructures. LLC including some kind of metal species may be difficult to form.

Indeed, LLC did not form when hydrogen tetrachloroaurate was used as Au precursor.

This difficulty may come from stability of self-assembled nanostructures and

distribution of the nature of metal precursor.

1.1.2.3.2 Hard templating

In the hard templating, porous nanostructures with highly cross-linked pore

walls of inorganic materials are generally employed as templates. Hard template must

be premade in principle, and fabrication of template is mostly achieved by other

nanofabrication method. For example, mesoporous materials are generally prepared by

soft templating. Thus, this procedure is consisted of three steps32: 1) fabrication of hard

template corresponding to desired nanostructured metals, 2) infiltration of metal

precursor selectively into the pores of template, 3) removal of template with appropriate

etchant. Anodic aluminum oxide (AAO) 33-37 or mesoporous oxides 38-52 have been

commonly used as hard templates.

Anodic aluminum oxide is formed by anodization of high purity aluminum

Chapter 1

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under certain carefully controlled conditions. The nanopores can organize into a highly

ordered 2D array of uniform straight pores with the pore diameter, the period, and the

array size being tunable. Electrodeposition and evaporation are frequently employed as

deposition method of metals into the nanopores of AAO.33-37 Because the shape of

nanopores is straight, the possible nanostructures of metals are limited to nanospheres,

nanorods, nanowires, and nanotubes. Although it is reported that Y-junction nanowires

can be prepared through two step anodization technique, 3D arrays of nanoparticles and

nanonetworks are very difficult to form.

On the other hand, mesoporous materials are fabricated by soft templating with

amphiphile molecules because the porous structures can be tuned variously. Ordered

mesoporous materials have periodically arranged uniform mesopores (2-50 nm).

Diversity of the mesostructures are expand the possible nanostructures of metals to

nanospheres, nanorods, nanowires, 2D arrays of nanowires, 3D arrays of nanospheres,

and nanonetworks. In the mesochannels, there can be possible many types of the shape

of nanostructured metals: nanospheres, nanorods, nanowires, nanotubes, and their arrays.

Actual formed nanostructures of metals are affected by reduction procedures, surface

property of pore walls, and the amount of incorporated metal precursors. It is noted that

Figure 1.2 formation process of mesoporous silica through soft templating

Surfactants

Silicate species

Mesoporous silica Mesostructured hybrid

Chapter 1

10

many of previous researches32, 53-61 adopted mesoporous silica powders as a template.

Therefore, the morphology of products is limited to be powder which contributes to

(electro-) catalysis enough. If you prepare other morphology of metals, at least same

morphology of template should be prepared.

1.2 Mesoporous silica films

1.2.1 Introduction to mesoporous silica

Mesoporous silica is a representation of mesoporous materials which having

pore sizes between 2 - 50 nm. Mesostructure and morphology is freely tunable in

mesoporous silica relative to other mesoporous materials. This flexibility is

accomplished by various synthetic conditions (the kind of synthetic processes, organic

templates, silica sources, additive, pH, temperature, and humidity etc.) which can be

applied because solidification of silicate species is enough slow to form nanostructured

hybrids in wide pH range.

Mesoporous silica was firstly reported by Yanagisawa et al. at Waseda

University62, which was dawn of mesoporous materials. The products of the

intercalation of cationic surfactants (alkyl-trimethylammonium, CnH2n+1(CH3)3N+,

CnTMA) within a layered polysilicate kanemite were converted to mesoporous silica

after the removal of the surfactants. The mesoporous silica is called KSW-1 (Kanemite

Sheet from Waseda-1) afterward. The mesoporous silica with various ordered

mesostructures prepared by using CnTMA and some silica sources under alkaline

conditions reported by Mobil group.63, 64 Obtained mesoporous silica is named as

MCM-41(Mobil Composition of Matter No. 41) with 2D-hexagonal mesostructure, and

MCM-48 with cubic structure and MCM-50 with lamellar, respectively. These reports

Chapter 1

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have triggered the development of researches on mesoporous materials, and the research

area has been expanding.

In contrast to the disordered mesostructure of KSW-1, Inagaki et al. prepared

the highly ordered mesoporous silica FSM-16 (Folded sheet mesoporous materials-16)

from kanemite.65 The folding of the silicate layers of kanemite induced by the

intercalation of the CnTMA is assumed as the formation mechanism, which is corrected

in later. FSM-16 was formed through the fragmentation of silicate layers in the

corrected formation mechanism. Although the folding mechanism was contradicted in

FSM-16, it was realized in the reports by Kimura et al.66 The ordered mesoporous silica

named as KSW-2 with a 2D orthorhombic structure is formed by the folding of silicate

layers during acetic acid treatment. Furthermore, silylation techniques could produce

KSW-2 retaining the crystalline nature of kanemite.67 It is interesting approach to

mesoporous silica with crystalline, which is useful as catalyst. A pore expansion of

KSW-2 was also achieved,68 which can contribute to a unique catalyst of KSW-2.

Nowadays, mesoporous silica is generally prepared through the cooperative

self-assembly method, which completely differs from the folding sheet mechanism. In

the method, mesoporous silica is formed through the simultaneous self-assembly of

surfactants and condensation of silicate species. Charge matching of surfactant and

silicate species69 under a synthesis condition is essential in the system. If they do not

interact, surfactants recuse to form mesostructured hybrids and only amorphous silica

without mesostructures should deposit.

For example, previously described MCM series63, 64 is prepared through the

mechanism with the interaction shown as S+ I- (S: surfactants, I: silicate species).

Negatively charged silica species electrostatically interact with cationic surfactant

Chapter 1

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because of the synthesis under the basic condition. Direct interaction of silicate species

with cationic surfactants is avoided under the acidic condition (pH < 2) due to the

positively charged silica species under the condition, therefore, one can hardly imagine

that the mesoporous silica is able to prepare under the synthetic system. However, a

hexagonal mesoporous silica can be prepared under the condition.70 The formation is

explained by halide anion mediated interaction, described as S+ X- I+ (S: surfactants, X:

halide anions, I: silicate species).

In the case of nonionic surfactants such as alkyl-polyethylene oxide (CnEOm),

the interaction under neutral condition is hydrogen bonds which described as S0 I0 and

under strongly acidic condition is halide anion mediated electrostatic ones between

protonated ethylene oxides moiety of surfactants and positively charged silicate species

which described as (S0 H+)(X- I+) (S: surfactants, H: proton, X: halide anions, I: silicate

species). For example, the mesoporous silica MSU-1 (Michigan State University-1) was

synthesized in the former condition71, and SBA series (Santa Barbara Amorphous,

which reported by the University of California, Santa Barbara) was prepared in the latter

condition.72 In particular, SBA-15 with 2D-hexagonal structure is preferred in the

various researches probably due to its facile synthesis method with high reproducibility

and well known structural properties. Because polyethylene oxide moieties stated in the

silica walls before calcination, the mesoporous silica has micropores.

Using of anionic surfactants as a template needs a special strategy because of

difficulty of the interaction between the negatively charged silica species and the

anionic surfactant under the basic condition. Mesoporous silica can be prepared in this

system under existence of a co-structure directing agent (CSDA), which is a molecule

with a silicate moiety and a cationic group, through interaction shown as S- N+ - I0 (S:

Chapter 1

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surfactants, N: cationic part in CSDA, I: silicate species). Mesoporous silica AMS series

(Anionic surfactant templated mesoporous silicas)73, 74 have been prepared according to

the concept. A mesoporous silica with chirality firstly reported using the chiral anionic

surfactant.75 Thus, this method highly advanced the preparation of mesoporous silica.

In concentrated surfactants systems, the mesostructure of the product is

controlled by the formed lyotropic liquid crystals. Ordered mesoporous silica with

various mesostructures (lamellar, 2D-hexagonal, and cubic) was successfully

prepared.76 The surfactant concentration and temperature directly affect the

mesostructures; therefore, these factors are very important in the case.

As described above, many types of mesoporous silica have prepared under

various conditions. The feature of mesoporous silica in the synthesis seems to be

expressed in following three points: 1) Hydrolysis and condensation of silicate species

can be control even in the water, which enables to apply the hydrothermal synthesis. 2)

The synthesis is allowed in wide pH range due to the isoelectric point; therefore, various

surfactants can act as a template, which contributes to diversity of the mesostructure. 3)

Crystallization hardly proceeds in calcination at ca. 500 oC. If the pore walls easily

crystalize in calcination, finely tuned mesostructures may be deformed.

1.2.2 Features of mesoporous silica films

Morphology control of mesoporous silica is significant for its application. In

particular, mesoporous silica films are desired in optics because of their high

transparency or low refractive index. Mesoporous silica films are also useful as hosts

for preparing a film consisting of nanostructured hybrids of dyes, photofunctional

polymers, nanosized metal and semiconductor, and so on. On the other hand, low

Chapter 1

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dielectric constant of mesoporous silica is attractive in the electronic devices; therefore,

homogeneous mesoporous silica films on a substrate are researched as an electronic

material. Features of mesoporous silica films for these applications are mainly listed

below.

Optics: transparency – avoidance of light scattering, low refractive index – for

anti-reflecting coating, porosity – accommodating photofunctional materials in the

mesopores, light and heat resistance – for application using strong light, absence of any

strong (photo-) catalytic activity – keeping guest species away from the decompose.

Electronics: homogeneity, low dielectric constants – for insulating layers, porosity –

utilizing space filled by air, heat resistance – for electronic devices such as integrated

circuits.

Properties of mesoporous silica films are dominated by their mesostructure

including the pore shape, the pore size, arrangement and connectivity of the mesopores,

alignment of the mesopores, etc. Therefore, control of the mesostructures should be paid

enough attention.

1.2.3 Preparation of mesoporous silica films

A fabrication process of mesoporous silica films has much influence on their

structures. Mesoporous silica films are generally fabricated by two pathways: EISA

(evaporation induced self-assembly) method and hydrothermal method.

The EISA method is mainly adopted in order to prepare highly homogeneous

mesoporous silica films with a smooth surface rapidly although the process is sensitive

to a surrounding environment especially at a coating.

In contrast, the hydrothermal method enables to prepare thermally stable

Chapter 1

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mesoporous silica films with high reproducibility. However, a surface of the

mesoporous silica films tends to be rough due to the growing mechanism in

hydrothermal synthesis, which frequently leads to enhance a light scattering. In addition,

a substrate must be chosen carefully because of the synthetic condition of low pH and

high temperature. These two methods with different characters should be adopted for

use.

1.2.3.1 Evaporation induced self-assembly

The EISA (evaporation induced self-assembly) method, found by Ogawa77,

employs a sol consisting of a silica source, a surfactant, an acid, and water in an alcohol.

Normally, the sol is aged with stirring before the coating in order to promote hydrolysis

and condensation reaction of the silicate species enough. During the coating, such as

spin coating or dip coating, the concentration of the surfactant in the aged sol rise over

the critical micelle concentration, resulting in the formation of micelles in the liquid

film on the substrate. The concentration of silicate species also rises simultaneously,

which leads to advance condensation reaction of them. Thus, the solvent evaporation

induces the formation of micelles and the reaction of silicate species at the same time,

which produces mesostructured films composed of surfactant micelles surrounded by

silica walls. Therefore, the mesostructure is kinetically controlled in the EISA method.

Mesostructured silica films with highly ordered lamellar mesostructure were

reported firstly by EISA method.77 The composition of a precursor solution directly

controlled the mesostructure of the products. The report revealed that the interlayer

space and the thickness of silica layer could be controlled by a length of the alkyl chain

of surfactant and a ratio of silicate species/surfactant, respectively. Mesostructures e.g.

Chapter 1

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lamellar or 2D-hexagonal can be also controlled.78 This film could be used as a host

material, that is, the hydrophobic alkyl layer in the hybrid films accommodated pyrene

molecules.79 Mesoporous silica films with cubic and 3D-hexagonal mesostructure were

reported by Lu et al. by dip coating.80 These mesostructures were proved to be

transformed from lamellar structure. These reports showed many important concepts in

mesoporous silica films.

A formation process of a mesostructure in a film is very important information

to control it, however this process is frequently difficult to characterize because of the

fast formation of the mesostructure in EISA method relative to hydrothermal method.

Moreover, there is a preferred orientation of the mesostructure on a substrate, which

complicate the characterization. Grazing-incidence small-angle X-ray scattering

(GISAXS) is a powerful tool to obtain the mesostructural information of the films

directly in a short time.

A formation of 2D-hexagonal mesostructure using CTAB (Cetyl trimethyl

ammonium bromide, C16TMABr) proceeded through the below path, which was

clarified by GISAXS.81

Isotropic phase → lamellar phase → corrected micellar phase → hexagonal mesophase

→hexagonal mesophase shrunk perpendicular to the substrate

A 2D-hexagonal mesostructure using block copolymer F127 was transformed from a

wormlike structure by rearrangement of cylindrical micelles.82 A cubic (Pm 3 n)

mesostructure was formed through a lamellar phase and a hexagonal phase as

intermediate structures.83

Accordingly, many of mesostructures forms through other mesostructures

during the evaporation of volatiles components; therefore, the both conditions of

Chapter 1

17

precursor solution and surrounding environment should be carefully controlled for

precisely design of desired mesostructures in EISA method.

1.2.3.2 Hydrothermal method

The hydrothermal method employs an aqueous solution containing of a silica

source, a surfactant, and an acid, which is different from EISA method. Mesoporous

silica films are formed slowly through generation of the mesostructured nuclei and its

growth. This growing mechanism leads to a rough surface and many defects of the film.

The films are prepared at the interface of substrate/mother liquor in a closed vessel;

therefore, the synthesis condition of this method is independent on any other

environmental factors. The substrate must be enough thermally and chemically durable,

otherwise a harsh condition damages it seriously.

The mesoporous silica films prepared on a cleaved mica surface in an aqueous

solution with strong acidity, which firstly reported the preparation of a mesoporous

silica film by the method.84 The 2D-hexagonal mesoporous silica domains were

triaxially aligned due to the atomic arrangement of mica surface, which was the first

study on the alignment of mesopores.

The mesoporous silica film with 2D-hexagonal mesostructure was successfully

prepared on mica, graphite, and silica glass substrates.85 This paper proved that

mesoporous silica films could be coated on both hydrophilic and hydrophobic surfaces

in hydrothermal method.

Of course, a mesostructure of a mesoporous silica film can be tuned by the type

of surfactants. A mesoporous silica film with 3D-hexagonal mesostructure, which

consists of cage-type mesopores, was fabricated by hydrothermal method using the

Chapter 1

18

dicationic surfactant 18-3-1 (C18H37N(CH3)2(CH2)3N(CH3)3Br2).86

Although there are some drawbacks such as a slow growth of a film and a

restriction in the substrate, hydrothermal method has different advantage from EISA

method.

1.2.4 Alignment control in mesoporous silica films

A mesostructure in the film can be controlled in fabrication processes by many

factors (a type of surfactants, silicate species/surfactant ratio, humidity, temperature, pH,

and so on), as shown above. However, the long-range ordering of mesostructure is not

designed in ~centimeter scale. For example, mesochannels in the film of SBA-15 with

2D-hexagonal mesostructure are lying on the substrate and in-plane alignment of these

mesochannels is random in general. Especially, in a mesoporous silica film with

2D-hexagonal mesostructure, strong anisotropy emerges by alignment control of

mesochannels in macroscopic scale. Therefore, the alignment control of mesochannels

has been researched energetically.

There are two types of alignment control of mesochannels: in-plane alignment

and vertical alignment. Because control of in-plane alignment enhances in-plane

anisotropy, mesoporous silica films with in-plane aligned mesochannels are useful for

unique optical applications. In contrast, vertical aligned mesochannels enhances

diffusion from the film surface to substrate surface. Therefore, the films with vertically

aligned mesochannels are valuable for electrochemical applications.

These alignment controls in mesoporous silica films are achieved by mainly

employing external fields87-96, confined spaces97-101, and substrates with surface

anisotropies84, 85, 103-113, as listed in table 1.1. The anisotropic external fields, such as a

Chapter 1

19

Figure 1.3 Alignment of mesochannels in mesoporous silica films

flow field, a magnetic field, and electric field, induce the anisotropic alignment of

mesochannels. 87-96 This method is applicable to both in-plane and vertical alignment

control easily. Mesochannels are forced to align in the confined spaces within

submicron scale in order to become thermodynamically stable. 97-101 This approach is

related to top-down technologies, which will contribute to micro/nano-electronics. The

atomic periodicity at the substrate surface affects the alignment of mesopores on the

substrate, likely epitaxial growth.84, 85, 103-113 Specific interaction between the surface

and the (hemi-) micelles is a key to align mesochannels in the technique. On the other

hand, the substrate for an orientation control of liquid crystals can be applied for

alignment of mesochannels. For example, rubbing-treated polyimide film102, 104-110 is

popular in liquid crystal display. Alignment of mesochannels, which is achieved by

these methods, adds interesting anisotropic functions to mesoporous silica films.

In-plane alignment Vertical alignment

Chapter 1

20

Table 1.1 List of the researches on alignment control in mesoporous silica film

Chapter 1

21

1.2.5 Mesoporous silica films for applications

There are two types of applications of mesoporous silica films. First one takes

advantages of their intrinsic properties, such as low-k films. Second one utilizes

mesoporous silica films as a unique host with confined nanospace. Mesoporous silica

films are attractive in both two types of applications.

Low dielectric constant is one of the most interested intrinsic properties of

mesoporous silica films in electronics. A mesoporous silica film with high mechanical

strength were reported to show quite low dielectric constant (k = 1.5-1.7).118 Although

porous structures were believed to be weak mechanically, a post treatment of

mesostructured materials could reinforce the structure. Mechanical strength is an

important property for electronic devices.

Thin films with low refractive index are also demanded in optics. Mesoporous

silica films were reported to be a promising candidate of it because the film composed

of silica and air which shows quite low refractive index.119 Calcination at relatively high

temperature (850 oC) contributed to decreasing silanol groups, resulting in the

mesoporous silica films with low refractive index (1.32) are thermally stable. Absence

of silanol groups seems to decrease the amount of adsorbed water successfully.

Mesoporous silica films are suitable for a host for optical use due to their high

transparency. Moreover, ordered mesostructures should produce homogeneous hybrids

easily by accommodation of photofunctional guests. For example, photochromic films

with fast optical response were fabricated by incorporation of photochromic dyes

(spiropyran and spirooxazine derivatives) into mesopores of mesoporous silica film.120

The film is thought to be promising candidate of optical shutters and light modulators.

A fluorescein attached mesoporous silica film showed very fast response (few seconds)

Chapter 1

22

to pH of a solution.121 The film can be used as optical solid-state pH sensors.

Furthermore, the mesoporous silica films with in-plane aligned mesopores

provide confined space with strong anisotropy in macroscopic scale. Accommodated

guest species in the space are forced to be aligned, which leads to appearance of the

hidden anisotropic properties.

The spontaneous laser emission was successfully achieved by the aligned dye

rhodamine 6G in uniaxially aligned mesochannels.122 Isolated dyes in the mesochannels

emitted without concentration quenching even in a high content of the dyes.

A mesoporous silica film with an aligned cyanine dyes absorbed polarized

lights in an anisotropic manner.123 The absorption maximum was observed when

incident lights were polarized along the mesochannels, which proved that the dyes

aligned by confined effect of the mesochannels.

The mesoporous silica film with aligned mesochannels can accommodate not

only dyes but also photofunctional polymers. When a semiconducting polymer

MEH-PPV was incorporated into the mesopores, the polymer chain became elongated

and isolated in the mesopore.124, 125 The composite film containing aligned polymers

shows the polarized absorption and fluorescence. Moreover, the confinement leads to

polarized, low-threshold amplified spontaneous emission from the aligned polymer

chains.125

Mesoporous films with vertically aligned mesochannels are valuable for

electrochemical applications because of high permeability derived from vertical aligned

mesochannels. Highly methylated mesoporous silica thin films were fabricated in

precise tuned hydrophobicity, which enable to control permeability of aqueous phase

solutes.126 The films might be applicable to functional electrodes with size selective

Chapter 1

23

separation.

Thus, mesoporous silica films can be widely applicable in optics, electronics,

electrochemistry, and so on. Because possible applications have been reporting and

exploring, demands for the films may exceed our expectation in new research fields,

such as cell biology, in the future.

1.3 Hard templating of metals using mesoporous silica films

1.3.1 Features of hard templating of nanostructured metals with mesoporous

silica films

Hard templating using mesoporous silica films are sophisticated method to

prepare designed nanostructured metals. Mesoporous silica is suitable to direct

nanostructure of metals because of uniform sized mesopores with a specific

arrangement. The diversity of mesostructures assures versatility of this method. Film

morphology of the mesoporous silica enables to apply many types of reduction, such as

electrodeposition.

In the hard templating, nanostructured metals are deposited in the mesopores of

the film. Prepared mesoporous silica films with nanostructured metals can be applicable

in optical use because of transparency of the mesoporous silica films, which is quite

different from a case of mesoporous silica powders. Removal of silica matrix converts

nanostructured composite film of silica and metals into nanostructured metals. When

nanostructured metals are highly connected, nanostructured metals inherit film

morphology of mesoporous silica even after the removal. The film consisting of

nanostructured metals are attractive in electrochemical applications due to its

electrochemical activity with high specific surface.

Chapter 1

24

A nanostructure of metals deposited in the mesopores are mainly controlled by

reduction method and mesostructure of the film. Therefore, these two factors should be

chosen carefully in order to prepare designed nanostructured metals.

1.3.2 Preparation process in hard templating of metals using mesoporous silica

films

Generally, reduction of metal ions in the mesopores is employed as a pathway

to control metal nanostructures in the hard templating using mesoporous silica. There

are a lot of available options of the reduction method in hard templating using

mesoporous silica films. Several reduction methods require specific morphology of

templates. For example, electrodeposition can be applied to a film on a conductive

substrate. Of course, the reduction techniques independent of template morphologies

can be chosen. Therefore, mesoporous silica films are valuable as a template.

1.3.2.1 Electrodeposition of metals in mesoporous silica films

Reduction of metal ions continuously proceeds from a mesoporous silica

film/conductive substrate interface toward a film surface in electrodeposition using

mesoporous silica films on conductive substrates. As a result, mesoporous silica films

accommodating interconnected nanostructured metal are formed on the substrates,

which completely differ from dispersed nanostructured metals prepared by an

electroless deposition using reducing agents. The mesostructure and the film

morphology are retained generally after the removal of the silica using alkaline solution

or hydrofluoric acid due to the interconnected structure derived from electrodeposition.

Template films must be without cracks, grain boundaries, and large defects. If

Chapter 1

25

there are these structural disorders, metals mainly deposit there, not in the mesopores.

Therefore, templates are generally prepared through EISA method. When mesoporous

silica films with 2D hexagonal mesostructure are used as a template for the

electrodeposition, nonionic surfactants are suitable template to prepare the films

because they form both mesopores and micropores that connect adjacent mesopores. It

shall not be applied when mesostructure of the templates consists of 3D connected

cage-type mesopores, which has windows between two adjacent pores.

Nanowires and nanonetworks of Pd were electrodeposited in mesoporous silica

films with 2D-hexagonal and cubic mesostructures, which is the first report of

electrodeposition of a metal using a mesoporous silica film.127 A nonionic block

copolymer P123 was used as an organic template to prepare the mesoporous silica films.

The nanowires reflected diameters and arrangement of mesochannels of the template

film, which confirmed by TEM. The nanostructured Pd inherited film morphology of

the template after the silica removal. In the same procedure, 3D nanowire networks

were prepared using mesoporous silica films with a cubic mesostructure. The obtained

film consisting of nanowires can be applied to electrodes, catalysts, and so on.

The preparation of the thin film consisting of Pt nanowires using mesoporous

silica thin film with 2D-hexagonal structure was reported by Wu et al.128 Cross section

of Pt nanowires was ellipsoidal, reflecting the shape of mesochannels distorted by

shrinkage of the film. After the dissolution of silica matrix, arrangement of Pt nanowires

corresponded to S-, Y-, and swirling-shaped structures in a template.

The mesoporous silica film with double-gyroid mesostructure was replicated

with Pt.129 The deposited Pt is filled in both continuous mesochannels and retains both

long-range and short-range order after removal of the original template.

Chapter 1

26

Replication of mesoporous silica SKU series (Sungkyunkwan university) with

various metals by electrodeposition are reported by Kwon group. Pt nanodot and

nanowire arrays are grown in SKU-l (rhombohedral R 3 m, shrunk (111) orientated Im 3

m).130 The Pt replicas show almost single crystalline because they grew on the tips of

deposited replica by reduction of the metal ions. Morphologies of the nanostrustures can

be tuned by conditions of the electrodeposition kinetics. The 3D networks of Pt

nanorods strongly enhanced the electromagnetic field near the nanostructure due to hot

spots, they applicable to SERS active substrate.131 Also, Pt replicas were fabricated

using SKU-4 (curved lamellar) and SKU-5 (centered rectangular, shrunk 2D-hexagonal)

as a template.132 Of course, the method can be simply applied to fabricate nanostructure

with other metal compositions. Co and Au nanowires were successfully prepared in

SKU-l and SKU-2 (wormlike mesostructure).

Enhancing mass diffusion and enlarging electrochemically active surfaces,

hierarchical structure of metals is desired in electrochemical application. Pt nanowire

films with hierarchical porous structures were fabricated by electrodeposition using

mesoporous silica film containing silica nanoparticle several tenth of nanometer in

size.133 The diameter of nanoparticles and the sort of surfactant can independently

control bimodal pore structure of the nanowire films at each level.

Pt nanowires with bumpy surfaces were prepared by a combination method of

hard templating and soft templating.134 Electrodeposition of Pt in mesochannels under

the coexistence of nonionic surfactants produced the unique nanowires. After silica

matrix was removed, the nanowires showed enhanced catalytic activity for methanol

oxidation reaction because of both high surface area and rich atomic steps with a

concave surface.

Chapter 1

27

It is proved that the electrodeposition using mesoporous silica films is

adoptable to designed metal nanostructures as shown above. Mesostructures of the

metal replicas are easily controlled by the template structures.

Many of the previous researches seem to focus on Pt nanostructures probably

because Pt is chemically stable and the deposition of it can be control relatively;

although other metal nanostructures show unique properties in optics, magnetics, and

electrochemistry. The studies on various types of metal nanostructured films will widely

expand their applicable area.

1.3.2.2 Deposition of metals using reducing agents in mesoporous silica films

Deposition of metals using reducing agents, such as hydrogen reduction, can be

employed to fabricate nanostructured metals in mesoporous silica films whether or not

the substrate has conductivity. Generally, deposition using reducing agents proceeds

two-step approach. First, mesoporous silica films are impregnated with metal precursor

solutions in order to fill the mesopores with the precursor. Second, the precursors are

reduced to metals in the mesopores. This protocol is called impregnation-reduction

method. In contrast to electrodeposition, reduction of metal ions in impregnation-

reduction method is possible to arise anywhere in (even out of) mesoporous silica film;

therefore, nanostructured metals tend to grow discretely. As a result, mesoporous silica

films accommodating isolated metal nanoparticles or microparticles of nanostructured

metal are formed. If you want to incorporate metals densely in mesoporous silica films,

both incorporation of metal ions and reduction of them must be repeated. Deposition on

outer surfaces should be always cared in this method because reduction of metal ions

may arise even out of mesoporous silica film.

Chapter 1

28

Ag nanoparticles in a mesoporous silica film with 2D-hexagonal mesostructure

were successfully formed by hydrogen reduction of Ag(NH3)2+ in tubular mesopores.135

The films appeared yellowish color derived from localized surface plasmon resonance

of Ag nanoparticles in visible region.

Fabrication of Au, Pt, and Pd nanoparticles in regular size using the

mesoporous silica film with 2D-hexagonal mesostructure were reported by Fukuoka et

al.136, 137 They employed hydrogen reduction or UV photoreduction. Isolated

nanoparticles were obtained after the silica was dissolved, which indicates that the

nanoparticles discretely grew in the mesoporous silica film. Highly ordered Pt nanodot

arrays with a cubic arrangement (Pm 3 n) were successfully prepared using mesoporous

silica films with the cubic structure by UV photoreduction, also.138 the nanoparticle

arrays were thermally stable up to 500°C due to the silica walls.

Surface modification of mesopores with amine groups is a smart strategy to

incorporate highly dispersed nanoparticles and nanowires of Au.139, 140 The amine

groups on the surface of the mesopores strongly catch hydrogen tetrachloroaurate(III)

via a neutralization reaction, resulting in Au precursors introduced in the mesopores.

This strategy can be broadly adopted in other metals with a little adjustment.

The impregnation-deposition method is not suitable for the preparation

continuous nanostructured metals because the amount of incorporated metal precursor

in mesopores is quite limited and diffusion of the metal precursor is restricted in order

to avoid deposition on outer surfaces. The electroless deposition with a seed-growth

mechanism was achieved incorporation of Ni and Au nanowires with high density. 141,

142 Mesoporous silica films containing Pd nanoparticles were chosen as a template

because Pd nanoparticle worked as a catalyst for subsequent electroless deposition. The

Chapter 1

29

nanowires were grown from the Pd seeds in electroless deposition baths. This method

seems to be complicated. Simple fabrication of continuous nanostructures such as

nanonetworks remains to be quite difficult in the approach using reducing agents.

1.3.3 Significance of hard templating using mesoporous silica films toward

application

Nanostructured metals fabricated in hard templating using mesoporous silica

films can inherit features of the films; for example, a uniform size, an ordered

arrangement, connectivity, and so on. Because these features directly lead to their

properties, hard templating using mesoporous silica films can design the character of

nanostructured metals for specific applications. Moreover, mesoporous silica films

accommodating nanostructured metals are utilized as-is in optical and magnetic use

because mesoporous silica films do not interfere optically and magnetically.

The mesoporous silica films with highly dispersed Au nanoparticles showed

ultrafast nonlinear optical response and enhanced third-order nonlinear optical

susceptibility. These excellent optical properties were derived from high content,

uniform size, and high dispersion of the incorporated gold nanoparticles. The properties

were also influenced by local field effect which comes from the structure of the

nanostructured metal embedded in a silica matrix.

Fe nanowire arrays were synthesized by photodecomposition of precursor

molecules in the mesochannels and further crystallization in hydrogen flow.141 The

nanowires in the composite film showed very high level of arrangement due to their

parallel orientation, which suggested their potential application high-density data

storages.

Chapter 1

30

Co nanowires with 3D networks showed characteristic isotropic magnetism,

which was different from nonporous films and 2D nanowire arrays.133 Such room

temperature ferromagnetic nanowire networks with short diameter and enhanced

coercivities showed potential for high density information storage applications.

Unique optical anisotropy appears in aligned nanorods or nanowires of metals.

Alignment control of metal nanowires can be achieved by replicating mesoporous silica

films with aligned mesochannels. Uniaxially aligned Pt nanowires were prepared using

a mesoporous silica films with aligned mesochannels.142 The Pt nanowires connected

each other through the Pt deposited in micropores of the mesoporous silica. After the

silica removal, aligned Pt nanowire films showed optical anisotropy macroscopically

derived from surface plasmon resonance of Pt, although periodicity of the arrangement

of nanowires disappeared.

Aligned chain-like and relatively large spheroidal Au nanoparticles were

obtained in the mesoporous silica film with aligned mesochannels by a reduction using

ascorbic acid and a photoreduction, respectively.143 The wavelength of the extinction

band for polarization effects, which is anisotropic optical characteristics, was dependent

on the morphology of the deposited Au nanoparticles. The results revealed that the

mesoporous silica films with aligned Au spheroids could be an excellent alternative to

the conventional organic polarizers because of high durability, the large extinction

coefficient, and the tunable extinction wavelength.

Ni nanowires are deposited by hydrogen reduction in SBA-15 films with

uniaxially aligned mesochannels.144 The composite films show macroscopically

electronic anisotropy depending on parallel or perpendicular to the nanowires.

These previous reports reveal that hard templating using mesoporous silica

Chapter 1

31

films is significant toward application because of its versatility and adjustability. It is

also suggested that both the mesostructures of template films and the reduction methods

are critical to control the nanostructures of metals toward specific application.

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Chapter 2

Preparation of Au Nanowire Films by Electrodeposition Using

Mesoporous Silica Films with 2D hexagonal mesostructure as a

Template

Chapter 2

44

2.1 Introduction

Nanostructured metals are promising because of their high catalytic activities,

high electric conductivities, and unique optical properties. In particular, nanostructured

Au absorbs visible and near infrared lights and generates strong near-field lights.

Therefore, it is applicable to sensors by utilizing surface enhanced Raman scattering

(SERS),1-3 colored materials,4-7 biomarkers for diagnosis and therapy,8, 9 nanophotonic

materials,10, 11 and so on. Moreover, well-designed Au nanostructures have recently been

attractive as metamaterials.12-16 These applications require the precise structural control

at nano-length scale which strongly influences the properties.

One of the major chemical fabrication processes17 of nanostructured Au is

templating.18-45 The templating method employs soft- and/or hard-templates with some

nanostructures (for example, liquid crystals as a soft-template and mesoporous material

as a hard-template) in order to direct precisely the size and shape of Au nanostructures.

A soft-templating strategy18-21 has been successful to produce various nanomaterials

with controlled morphologies by the selection of organic molecules, though the

resulting shapes or morphologies are rarely predictable. Moreover, the soft templating

needs to control the formation of composites consisting of self-assembled templates and

some inorganic sources. This control is sometimes difficult because the self-assembly of

amphiphilic molecules is affected by coexisting species. For example, it was reported

that mesoporous Pt-Au alloys with various mixed ratios by soft templating,22 though a

lyotropic liquid crystal phase does not form in the range of high concentration of Au

source. Even if a template is formed, self-assembled templates are sometimes fragile to

be replicated. Therefore, Au nanostructures are difficult to be fabricated through the soft

templating, which clarifies the limit of this approach.

Chapter 2

45

In contrast, the hard-templating method employs a rigid and premade template,

which allows fabrication of Au nanomaterials.23-45 Anodic aluminum oxide (AAO)23-26

or mesoporous oxides27-45 have been commonly used as hard-templates. In particular,

mesoporous silica with various shapes and mesostructures has been prepared under

different synthetic conditions. Therefore, it should be possible to produce their

replicated Au nanostructure. However, while nanostructured Au replicas with powder

morphology were prepared by using mesoporous silica powders,27-37 the fabrication of

nanostructured Au films remains to be challenging though they should be applicable

and promising to optical films or sensing devices.

One of the smart strategies is the electroplating using mesoporous films which

has been developed for other kind of metals.46-50 I expect that Au ions are reduced in

mesoporous films on conductive substrate surfaces by electrodeposition, in which the

deposition behavior is different from those of other reduction methods,38-41 and that Au

nanostructures should retain the film morphology after the template removal.

Electroplating methods using mesoporous titania films were reported by a couple of

groups.42,43 However, the mesostructures of these films are almost limited to disordered

ones because of the restriction of pH of the precursor solutions in order to avoid the

dissolution of conductive substrates. Therefore, mesoporous silica films are preferred as

a template because of the controllability of the mesostructure. The electrochemical

depositions of Au using both mesoporous silica films with disordered structure44 and

mesoporous silica formed in the channels of AAO45 were reported. The reason why

controlled deposition of Au was successful in those cases is probably due to the

presence of a sort of vertical connection of deposition path from conductive substrates.

However, as far as I know, none of the studies have succeeded in incorporating Au into

Chapter 2

46

the mesochannels with an ordered 2D-hexagonal arrangement. Let us emphasize that the

highly ordered 2D-hexagonal structure is one of the most difficult structures to simply

replicate because of its low accessibility through the pore walls of mesoporous films.

Parallel orientation of mesochannels on substrates lowers the accessibility even though

inter-connecting micropores are present.51-53 Though various metal depositions were

reported for mesoporous silica, I have found preliminarily that deposition of Au using a

mesoporous silica film with highly ordered 2D-hexagonal structure as a template

produces not nanowires but microparticles.

Here I described the fabrication of well-ordered Au nanowire films by

electrodeposition method using mesoporous silica films on a conductive substrate and

by introduction of vertically oriented mesochannels at the film/substrate interface. This

chapter demonstrates for the first time that the nanostructured Au film retains the

arrangement of nanowires and morphology after the template removal.

2.2 Experimental

2.2.1 Materials

Indium-tin oxide (ITO) substrates (surface resistivity: < 15 Ohms per square

cm) were purchased from Geomatec Co. In this study, I selected the substrates because

of their sufficiently high conductivity as a working electrode. A substrate was cleaned

by ultra-sonication in acetone, 2-propanol, and ethanol for 5 min each, respectively.

Tetraethoxysilane (TEOS) with > 99.0% purity was purchased from Kishida Chemical

Co. Triblock copolymer EO20PO70EO20 (P123) was purchased from Aldrich Co.

Hydrochloric acid (0.01 M), sodium hydroxide aqueous solution (1 M) and hydrogen

hexachloroplatinate (IV) hexahydrate were purchased from Wako Chemical Co. Ethanol

Chapter 2

47

with > 99.5 % purity was purchased from Junsei Chemical Co. Hydrogen

tetrachloroaurate (III) tetrahydrate was purchased from Kanto Chemical Co. Rhodamine

B was purchased from Tokyo Chemical Industry Co. All reagents were used without

further purification.

2.2.2 Preparation of mesoporous silica films

Mesoporous silica films were prepared on an ITO substrate by

evaporation-induced self-assembly (EISA) method, according to previous reports.54-56

In this paper, I used block copolymer P123 as a template of mesopores. Mesoporous

silica generally needs a template, such as surfactant micelles or block copolymers, in

order to form mesopores. Rod-like micelles are formed in a silicate matrix during

spin-coating and these micelles, covered with silica walls, were converted to

mesochannels after calcination. Initially 5.70 mL of TEOS was mixed with 7.59 mL of

ethanol and 2.70 mL of hydrochloric acid (0.01 M) and the mixture was stirred for 20

min at room temperature. Then, 1.38 g of P123 dissolved in ethanol (5.06 mL) was

added into the mixture and the resulting mixture was further stirred for 3.5 h at room

temperature to prepare a precursor solution. On the other hand, an ITO substrate was

partially covered with an adhesive tape in order to fix the deposition area of mesoporous

silica film. The precursor solution obtained above was spin-coated on the masked ITO

substrate at 3000 or 4000 rpm in a controlled humidity (25 oC, 40 %RH) and the formed

films were subsequently dried at an ambient condition for 1 day. After drying, obtained

films were calcined at 400 oC for 4 h in air in order to remove the organic fractions.

Chapter 2

48

2.2.3 Electrodeposition of Au using mesoporous silica films as

templates

Au nanostructures were fabricated by galvanostatic electroplating (2.0

mA·cm-2) in 0.05 M hydrogen tetrachloroaurate (III) aqueous solution for 40 s using a

HZ-5000 electrochemical measurement system (Hokuto Denko Co.). An ITO substrate

covered with a mesoporous silica film (working electrode), a Pt wire (counter electrode),

and a Ag/AgCl electrode (reference electrode) were employed. The formed Au/silica

composite film was kept in an aqueous sodium hydroxide solution (1 M) for 1 day to

remove the silica matrix. After washing with pure water and drying, a Au nanostructure

was obtained.

2.2.4 Characterization

The film morphology was microscopically observed using an Olympus BX-51

microscopy operated under transmitted-light illumination. The structural information of

the films was obtained by grazing incident small angle X-ray scattering (GI-SAXS),

wide angle X-ray diffraction (XRD), transmission electron microscopy (TEM), and

high-resolution scanning electron