Microwaves in Nanoparticle Synthesis E-Book

• Herausgeber: Wiley-VCH
• Kategorie: Wissenschaft und neue Technologien
• Sprache: Englisch

Seitenzahl: 574

Veröffentlichungsjahr: 2013

Beschreibung

For the first time, this comprehensive handbook presents the emerging field of microwave technology for the synthesis of nanoparticles. Divided into three parts--fundamentals, methods, and applications--it covers topics including microwave theory, scale-up, microwave plasma synthesis, characterization, and more. This offers both an important volume for academic researchers, and a resource for those in industry exploring the applications of nanoparticles in semiconductors, electronics, catalysis, sensors, and more.

Leseprobe

Table of Contents

Related Titles

Title page

Copyright page

Preface

List of Contributors

1: Introduction to Nanoparticles

1.1 General Introduction to Nanoparticles

1.2 Methods of Nanoparticle Synthesis

1.3 Surface Plasmon Resonance and Coloring

1.4 Control of Size, Shape, and Structure

1.5 Reducing Agent in Nanoparticle Synthesis

1.6 Applications of Metallic Nanoparticles

2: General Features of Microwave Chemistry

2.1 Microwave Heating

2.2 Some Applications of Microwave Heating

2.3 Microwave Chemistry

2.4 Microwave Chemical Reaction Equipment

3: Considerations of Microwave Heating

3.1 General Considerations of Microwave Heating

3.2 Peculiar Microwave Heating

3.3 Relevant Points of Effective Microwave Heating

4: Combined Energy Sources in the Synthesis of Nanomaterials

4.1 Introduction

4.2 Simultaneous Ultrasound/Microwave Treatments

4.3 Sequential Ultrasound and Microwaves

4.4 Conclusions

5: Nanoparticle Synthesis through Microwave Heating

5.1 Introduction

5.2 Microwave Frequency Effects

5.3 Nanoparticle Synthesis under a Microwave Magnetic Field

5.4 Synthesis of Metal Nanoparticles by a Greener Microwave Hydrothermal Method

5.5 Nanoparticle Synthesis with Microwaves under Cooling Conditions

5.6 Positive Aspects of Microwaves' Thermal Distribution in Nanoparticle Synthesis

5.7 Microwave-Assisted Nanoparticle Synthesis in Continuous Flow Apparatuses

6: Microwave-Assisted Solution Synthesis of Nanomaterials

6.1 Introduction

6.2 Synthesis of ZnO Nanocrystals

6.3 Synthesis of α-FeO Nanostructures

6.4 Element-Based Nanostructures and Nanocomposite

6.5 Chalcogenide Nanostructures

6.6 Graphene

6.7 Summary

7: Precisely Controlled Synthesis of Metal Nanoparticles under Microwave Irradiation

7.1 Introduction

7.2 Precise Control of Single Component under Microwave Irradiation

7.3 Precise Control of Multicomponent Structures under Microwave Irradiation

7.4 An Example of Mass Production Oriented to Application

7.5 Conclusion

8: Microwave-Assisted Nonaqueous Routes to Metal Oxide Nanoparticles and Nanostructures

8.1 Introduction

8.2 Nonaqueous Sol–Gel Chemistry

8.3 Polyol Route

8.4 Benzyl Alcohol Route

8.5 Other Mono-Alcohols

8.6 Ionic Liquids

8.7 Nonaqueous Microwave Chemistry beyond Metal Oxides

8.8 Summary and Outlook

9: Input of Microwaves for Nanocrystal Synthesis and Surface Functionalization Focus on Iron Oxide Nanoparticles

9.1 Introduction

9.2 Biomedical Applications of Iron Oxide Nanoparticles

9.3 Nanoparticle Synthesis

9.4 Nanoparticle Surface Functionalization

9.5 Microwave-Assisted Chemistry

9.6 Conclusions

10: Microwave-Assisted Continuous Synthesis of Inorganic Nanomaterials

10.1 Introduction and Overview

10.2 Microwave-Assisted Continuous Synthesis of Inorganic Nanomaterials

10.3 Types of Microwave Apparatus Used in Continuous Synthesis

10.4 Microwave Continuous Synthesis of Molecular Sieve Materials

10.5 Microwave Continuous Synthesis of Metal Oxides and Mixed Metal Oxide Materials

10.6 Microwave Continuous Synthesis of Metallic Nanomaterials

10.7 Conclusions and Outlook

11: Microwave Plasma Synthesis of Nanoparticles: From Theoretical Background and Experimental Realization to Nanoparticles with Special Properties

11.1 Introduction

11.2 Using Microwave Plasmas for Nanoparticle Synthesis

11.3 Experimental Realization of the Microwave Plasma Synthesis

11.4 Influence of Experimental Parameters

11.5 Nanoparticle Properties and Application

11.6 Summary

12: Oxidation, Purification and Functionalization of Carbon Nanotubes under Microwave Irradiation

12.1 Introduction

12.2 Oxidation and Purification

12.3 Functionalization

12.4 Conclusion

Index

Related Titles

Loupy, A., de la Hoz, A. (eds.)

Microwaves in Organic Synthesis

Third, Completely Revised and Enlarged Edition

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ISBN: 978-3-527-33116-1

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Optical Properties of Nanoparticle Systems

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Gubin, S. P. (ed.)

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The Editors

Prof. Satoshi Horikoshi

Sophia University

Dep. of Materials and Life Sciences

7-1 Kioicho, Chiyodaku

Tokyo 102-8554

Japan

Prof. Nick Serpone

Visiting Professor

Universita di Pavia

Dipartimento di Chimica

Gruppo Fotochimico

Via Taramelli 10

Pavia 27100

Italy

The cover picture shows Scanning Electron Micrographs of γ-MnO2 synthesized for 2 h, in lower and higher magnification. Taken from Chapter 10 of this book, Figure 18, with permission.

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A catalogue record for this book is available from the British Library.

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The Deutsche Nationalbibliothek lists this publication in the Deutsche Nationalbibliografie; detailed bibliographic data are available on the Internet at <http://dnb.d-nb.de>.

© 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Boschstr. 12, 69469 Weinheim, Germany

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Print ISBN: 978-3-527-33197-0

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Preface

The special optical characteristics imparted by metallic nanoparticles have been used in producing colored glass ever since the 4th century AD, even though the craftsmen were unable to see the nanoparticles and thus explain the true character of metallic colloids. The first scientific evaluation of a colloid (gold) was done by Michael Faraday in 1857; he remarked that colloidal gold sols have properties different from bulk gold (Chapter 1, Table 1.2). The history of nanomaterials dates back to 1959, when Richard P. Feynman, a physicist at Cal Tech, forecasted the advent of nanomaterials. In one of his classes he stated that “there is plenty of room at the bottom” and suggested that scaling down to the nano-level and starting from the bottom-up was the key to future technologies and advances. The remarkable progress in characterizing nanoparticles and unravelling novel physical and chemical properties of nanoparticles has opened the possibility of new materials. Simple preparation methods using various techniques to produce high-quality nanoparticles are now available (Chapter 1, Figure 1.4), one of which is the use of microwave heating that has attracted considerable attention worldwide. Several books have been written mostly on microwave-assisted organic syntheses in the past decade, yet none have dealt specifically with microwaves and inorganic materials except perhaps in the use of microwave radiation in the sintering of ceramics. The latter notwithstanding, research in nanoparticle syntheses with microwaves has seen a remarkable growth in the last several years.

The main purpose of this book is to give an overview of nanoparticle synthesis using the microwave method, with the first chapter providing an introduction to nanoparticles followed by two other chapters that explain some of the fundamentals of microwave heating (Chapters 2 and 3). In the remaining chapters several specialists in the field describe some of the specifics and variations in nanoparticle synthesis. As the data available in the literature were enormous, we had to make the difficult choice of including only the most relevant and up-to-date literature; we apologize to the reader if we missed to include other worthwhile contributions. Prominent in the book are abundant chemical information and some beautiful TEM data that define the structural features of nanoparticles. We are thankful to all the contributors who have answered the call, and also to the Wiley-VCH editorial staff for their thorough and professional assistance. The data presented would not have been possible without the fruitful collaboration of many university and industrial researchers, and not least without the cooperation of students whose names appear in many of the earlier publications. We are indeed very grateful for their effort.

We hope this book becomes a starting point for researchers in other fields to become interested in pursuing microwave chemistry, in general, and microwave-assisted nanoparticle syntheses, in particular.

January 2013

Satoshi Horikoshi

Nick Serpone

List of Contributors

Farah Benyettou

UMR 7244 CNRS, University of Paris 13

CSPBAT Laboratory

74 rue Marcel Cachin, 93017 Bobigny

France

Luisa Boffa

Università di Torino

Dipartimento di Scienza e Tecnologia del Farmaco

via P. Giuria 9

10125 Torino

Italy

Zhi Chen

Tokyo Institute of Technology

Department of Applied Chemistry

Graduate School of Science and Engineering

2-12-1 Ookayama

Meguro, Tokyo 152-8552

Japan

Giancarlo Cravotto

Università di Torino

Dipartimento di Scienza e Tecnologia del Farmaco

via P. Giuria 9

10125 Torino

Italy

Caroline de Montferrand

UMR 7244 CNRS, University of Paris 13

CSPBAT Laboratory

74 rue Marcel Cachin, 93017 Bobigny

France

Davide Garella

Università di Torino

Dipartimento di Scienza e Tecnologia del Farmaco

via P. Giuria 9

10125 Torino

Italy

Frederic Geinguenaud

UMR 7244 CNRS, University of Paris 13

CSPBAT Laboratory

74 rue Marcel Cachin, 93017 Bobigny

France

Erwann Guenin

UMR 7244 CNRS, University of Paris 13

CSPBAT Laboratory

74 rue Marcel Cachin, 93017 Bobigny

France

Satoshi Horikoshi

Sophia University

Faculty of Science and Technology

Department of Materials and Life Sciences

7-1 Kioicho

Chiyodaku, Tokyo 102-8554

Japan

Xianluo Hu

Huazhong University of Science and Technology

College of Materials Science and Engineering

State Key Laboratory of Material Processing and Die & Mould Technology

Wuhan 430074

China

Hui Huang

University of Connecticut

Department of Chemistry

55 North Eagleville Road

Storrs, CT 06269-3060

USA

Yoann Lalatonne

UMR 7244 CNRS, University of Paris 13

CSPBAT Laboratory

74 rue Marcel Cachin, 93017 Bobigny

France

Irena Milosevic

UMR 7244 CNRS, University of Paris 13

CSPBAT Laboratory

74 rue Marcel Cachin, 93017 Bobigny

France

Dai Mochizuki

Tokyo Institute of Technology

Department of Applied Chemistry

Graduate School of Science and Engineering

2-12-1 Ookayama

Meguro, Tokyo 152-8552

Japan

Laurence Motte

UMR 7244 CNRS, University of Paris 13

CSPBAT Laboratory

74 rue Marcel Cachin, 93017 Bobigny

France

Markus Niederberger

ETH Zürich

Laboratory for Multifunctional Materials

Department of Materials

Wolfgang-Pauli-Strasse 10

8093 Zürich

Switzerland

Naftali N. Opembe

University of Connecticut

Department of Chemistry

55 North Eagleville Road

Storrs, CT 06269-3060

USA

Nick Serpone

Universita di Pavia

Dipartimento di Chimica

Gruppo Fotochimico

Via Taramelli 10

Pavia 27100

Italy

Steven L. Suib

University of Connecticut

Department of Chemistry

55 North Eagleville Road

Storrs, CT 06269-3060

USA

Silvia Tagliapietra

Università di Torino

Dipartimento di Scienza e Tecnologia del Farmaco

via P. Giuria 9

10125 Torino

Italy

Dorothée Vinga Szabó

Karlsruhe Institute of Technology

Institute for Applied Materials – Materials Process Technology

Hermann-von-Helmholtz-Platz 1

76344 Eggenstein-Leopoldshafen

Germany

Yuji Wada

Tokyo Institute of Technology

Department of Applied Chemistry

Graduate School of Science and Engineering

2-12-1 Ookayama

Meguro, Tokyo 152-8552

Japan

Jimmy C. Yu

The Chinese University of Hong Kong

Department of Chemistry

Shatin, New Territories, Hong Kong

China

1

Introduction to Nanoparticles

Satoshi Horikoshi and Nick Serpone

1.2 Methods of Nanoparticle Synthesis

Various preparation techniques for nanoparticles (nanomaterials) are summarized in Figure 1.4. Two approaches have been known in the preparation of ultrafine particles from ancient times. The first is the breakdown (top-down) method by which an external force is applied to a solid that leads to its break-up into smaller particles. The second is the build-up (bottom-up) method that produces nanoparticles starting from atoms of gas or liquids based on atomic transformations or molecular condensations.

Figure 1.4 Typical synthetic methods for nanoparticles for the top-down and bottom-up approaches.

The top-down method is the method of breaking up a solid substance; it can be sub-divided into dry and wet grinding. A characteristic of particles in grain refining processes is that their surface energy increases, which causes the aggregation of particles to increase also. In the dry grinding method the solid substance is ground as a result of a shock, a compression, or by friction, using such popular methods as a jet mill, a hammer mill, a shearing mill, a roller mill, a shock shearing mill, a ball mill, and a tumbling mill. Since condensation of small particles also takes place simultaneously with pulverization, it is difficult to obtain particle sizes of less than 3 μm by grain refining. On the other hand, wet grinding of a solid substrate is carried out using a tumbling ball mill, or a vibratory ball mill, a planetary ball mill, a centrifugal fluid mill, an agitating beads mill, a flow conduit beads mill, an annular gap beads mill, or a wet jet mill. Compared with the dry method, the wet process is suitable for preventing the condensation of the nanoparticles so formed, and thus it is possible to obtain highly dispersed nanoparticles. Other than the above, the mechanochemical method and the mechanical alloying method are also known top-down methods.

The bottom-up approach is roughly divided into gaseous phase methods and liquid phase methods. For the former, the chemical vapor deposition method (CVD) involves a chemical reaction, whereas the physical vapor deposition method (PVD) uses cooling of the evaporated material. Although the gaseous phase methods minimize the occurrence of organic impurities in the particles compared to the liquid phase methods, they necessitate the use of complicated vacuum equipment whose disadvantages are the high costs involved and low productivity. The CVD procedure can produce ultrafine particles of less than 1 μm by the chemical reaction occurring in the gaseous phase. The manufacture of nanoparticles of 10 to 100 nm is possible by careful control of the reaction. Performing the high temperature chemical reaction in the CVD method requires heat sources such as a chemical flame, a plasma process, a laser, or an electric furnace. In the PVD method, the solid material or liquid material is evaporated and the resulting vapor is then cooled rapidly, yielding the desired nanoparticles. To achieve evaporation of the materials one can use an arc discharge method. The simple thermal decomposition method has been particularly fruitful in the production of metal oxide or other types of particles and has been used extensively as a preferred synthetic method in the industrial world.

For many years, liquid phase methods have been the major preparation methods of nanoparticles; they can be sub-divided into liquid/liquid methods, and sedimentation methods. Chemical reduction of metal ions is a typical example of a liquid/liquid method, whose principal advantage is the facile fabrication of particles of various shapes, such as nanorods, nanowires, nanoprisms, nanoplates, and hollow nanoparticles. With the chemical reduction method it is possible to fine-tune the form (shape) and size of the nanoparticles by changing the reducing agent, the dispersing agent, the reaction time and the temperature. The chemical reduction method carries out chemical reduction of the metal ions to their 0 oxidation states (i.e., Mn+ → M0); the process uses non-complicated equipment or instruments, and can yield large quantities of nanoparticles at a low cost in a short time. Of particular interest in this regard is the use of microwave radiation as the heat source that can produce high quality nanoparticles in a short time period. Besides the chemical reduction method which adds a reducing agent (direct reduction method), other reduction methods are known, such as photoreduction using gamma rays, ultrasonic waves, and liquid plasma which can be used to prepare nanoparticles. These methods that do not use a chemical reducing substance have the attractive feature that no extraneous impurities are added to the nanoparticles. Other than these methods, spray drying, spray pyrolysis, solvothermal synthesis, and the supercritical method are also known.

The general technique in the sedimentation method is a sol–gel process, which has been used extensively for the fabrication of metal oxide nanoparticles. This procedure transforms a solution of a metal alkoxide into a sol by hydrolysis, followed by polycondensation to a gel. Several books are available that provide details of the sol–gel process (see e.g., [11]). The wet process (liquid phase method) guarantees a high dispersivity of nanoparticles compared to the dry method. However, if the resulting nanoparticles are dried, aggregation of the particles soon follows. In this case, re-dispersion can be carried out according to the process used in the solid phase method.

Although various techniques have been summarized in Figure 1.4, there are some features to consider that are common to all the methods. That is, the synthesis of nanoparticles requires the use of a device or process that fulfills the following conditions:

control of particle size, size distribution, shape, crystal structure and composition distributionimprovement of the purity of nanoparticles (lower impurities)control of aggregationstabilization of physical properties, structures and reactantshigher reproducibilityhigher mass production, scale-up and lower costs

1.3 Surface Plasmon Resonance and Coloring

The physical phenomenon of surface plasmon resonance (SPR) was reported long ago by Wood who could detect sub-monomolecular coverage [12]. Not only did Wood discover the plasmon resonance phenomenon, but also found that it changed with the composition of the liquid in touch with the metal surface. Although he speculated on how the light, grating and the metal interacted with each other, a clear rationalization of the phenomenon was not provided. He observed a pattern of “anomalous” dark and light bands in the refracted light when he shone polarized light on a mirror with a diffraction grating on its surface. The first theoretical treatment of these anomalies was put forward by Rayleigh in 1907 [13]. Rayleigh's “dynamical theory of the grating” was based on an expansion of the scattered electromagnetic field in terms of outgoing waves only. With this assumption, he found that the scattered field was singular at wavelengths for which one of the spectral orders emerged from the grating at the grazing angle. He then observed that these wavelengths, which have come to be called the Rayleigh wavelengths, λR, correspond to the Wood anomalies. Further refinements were made by Fano [14], but a complete explanation of the phenomenon was not possible until 1968 when Otto [15], and in the same year Kretschmann and Raether [16], reported the excitation of the surface plasmon band. Surface plasmon resonance has also been similarly researched in solid state physics in recent years in application studies, especially in such applied research as biosensing, solar cells, and super high-density recording. Details on surface plasmon resonance from the point of view of solid state physics have been given by Schasfoort and Tudos [17].

In this section we present a simple outline of the relation between surface plasmon resonance and the color of nanoparticles. In solid state physics, the plasmon represents the collective oscillation of a free charge in a metal, and may be considered as a kind of plasma wave. The positive electrical charge in the metal is fixed and the free electron is free to move around it. An applied external electric field, as from a light source, causes the free electrons at the surface of the metal to vibrate collectively, giving rise to surface plasmons.

Since electrons are also particles with an electric charge, when they vibrate they also generate an electric field, and when the electric field from the vibration of free electrons and the applied external electric field (e.g., electromagnetic waves) resonate the resulting phenomenon is referred to as a surface plasmon resonance that takes place at the surface of the metal. However, if light irradiates a solution that contains dispersed metal nanoparticles smaller than the wavelength of light, then depending on the electric field of light, the deviation produces a free electron at the surface of the metal. As a result, the weak or thick portions of the electric field appear on the nanoparticle surface (Figure 1.5) and can be considered as a kind of polarization. Such localized plasmon resonance is called localized surface plasmon resonance (LSPR).

Figure 1.5 Mechanism of a localized surface plasmon resonance.

The LSPR is typically concentrated in a very narrow region on the surface of a nanoparticle. The electric field distribution on the nanoparticle surface caused by LSPR can be visualized using an electromagnetic field analysis software with a finite element method (Comsol Multiphysics 4.2a). The LSPR distribution on the surface of a 20-nm (diameter) Au nanoparticle is shown in Figure 1.6a. When visible light at 520 nm, which corresponds to the maximum position of the LSPR band in the Au nanoparticle, is used to irradiate the nanoparticle, the electric field generated is concentrated on the right and left side of the Au nanoparticle, perpendicular to the incident light direction. On the other hand, the electric field generated in two adjacent Au nanoparticles is concentrated within the gap between the two particles. Figure 1.6b illustrates two particles with a 4-nm gap, while Figure 1.6c displays two particles separated by a gap of 1 nm. As the gap becomes narrower and narrower, the density of the generated electric field gets bigger and bigger.

Figure 1.6 Images of the electric field distribution on the Au nanoparticle surface (20 nm diameter) under visible light irradiation visualized with Comsol Multiphysics 4.2a: (a) single Au nanoparticle, (b) two Au nanoparticles with 4 nm gap, and (c) two Au nanoparticles with 1 nm gap.

The wavelength corresponding to the LSPR depends on the kind of metal, the shape of the metal nanoparticle, and the extent of aggregation of the metallic nanoparticles. Moreover, the surface plasma vibration also changes with the dielectric constant and the quality of the carrier fluid. The plasma oscillations in the metal occur mainly in the ultraviolet (UV) region. However, in the case of Au, Ag, and Cu, the plasma shifts nearer to the visible light domain with the band due to electrons in the s atomic orbitals. For example, the wavelength of the surface plasmon resonance band maximum of a spherical Au nanoparticle is 520–550 nm. If a colloidal Au nanoparticle solution is now irradiated with visible light at these wavelengths (520–550 nm), the visible light corresponding to the green color is absorbed and the particles now display a red purple color, which is the complementary color to green. In a colloidal Ag nanoparticle solution which has a plasmon resonance band maximum near 400 nm, the blue color of the visible light is absorbed and the Ag particles now take on a yellow color, the complementary color to blue.

1.4 Control of Size, Shape, and Structure

1.4.1 Size Control of Nanoparticles

The physical and chemical properties of nanomaterials depend not only on their composition but also on the particle size [18] and shape [19]. Accordingly, a high quality synthesis protocol must first of all provide control over particle size and shape. For example, if the diameter of an Au nanosphere is made to increase, the surface plasmon resonance will be gradually shifted from 530 nm to the longer wavelength side (see Figure 1.7) [20]. Thus, if nanoparticles differ in size, their optical characteristics will also change significantly.

Figure 1.7 Visible-light spectra of Au nanospheres with various particle sizes. From ref. [20]. Copyright 2006 by the American Chemical Society.

In optical applications of nanoparticles, simplification of the size distribution of the particles becomes a very important factor. Therefore, it is important to fabricate nanoparticles with a single target size in mind. Generally, in order to prepare monodispersed nanoparticles, it is imperative that the nanoparticles grow very slowly after the rapid generation of the seed particles [21]. If the size of the nanoparticles decreases (i.e., increase in specific surface area), then the increase in the surface energy of such nanoparticles will facilitate their aggregation. Consequently, after their growth to the desired optimal size, it will be necessary to stabilize the particulate surface by addition of a dispersing agent. The historical use of dispersing agents in nanoparticle syntheses is not new; for example, Ag colloids protected by citrate were reported by Lea way back in 1889 [22]. However, where the concentration of nanoparticles is unusually high, the decentralized stabilization will fall, because the protective action of the organic substrate (citrate) is no longer strong enough to prevent aggregation. Thus, several studies of dispersing agents that maintain a high dispersivity of the nanoparticles, and also at various concentrations, have been reported. According to the hard and soft acids and bases (HSAB) rule [23], Ag+, Au+, Pd2+, Pt2+ are classified as soft acids in the Lewis sense (SA), and substrates possessing the thiol (R–SH) and the phospine (P–R3) functional groups, classified as soft bases, have proven to be suitable dispersing agents [24]. Early research that examined organic thiol molecules as possible dispersing agents was reported by Brust and coworkers [25]. As shown in Figure 1.8, if 1-dodecanethiol is used as the dispersing agent in Au nanoparticle synthesis the 1-dodecanethiol molecule can form a monomolecular layer on the Au nanoparticle surface, and firmly stabilize the dispersed Au nanoparticles. This particular paper has been the third most cited article in the journal Chemical Communications ever since 1965 [26], and so can easily be said to have had a significant impact in the chemistry field. Moreover, an increase in the size of the nanoparticles can be achieved by the change in alkyl chain length from 1-dodecanethiol to alkyl chains of octane, decane and hexadecane, as well as with a decrease in steric hindrance. For this reason, the 1-dodecanethiol dispersing agent is also used to control the particle size. Development of polymers as the dispersing agents has also been studied. In this case, the protection capability is determined by the affinity of the nanoparticle surface and by the molecular weight of the polymer.

Figure 1.8 Mechanism for the capping of metal nanoparticles with 1-dodecanethiol. From ref. [26]. Copyright 1994 by the Royal Society of Chemistry.

It is important to realize that the physical properties of a nanoparticle can change with the aggregation ratio, even though the colloidal solution may contain nanoparticles of identical size [27]. The images and the characteristics of the state of aggregation of nanoparticles are depicted in Figure 1.9. In the dispersed random structure shown in Figure 1.9a, the dynamical physical properties and the optical properties are significant. On the other hand, the electronic proper­ties are displayed by the fractal structure shown in Figure 1.9b, and ion and electronic transport properties appear in the structure orientation of Figure 1.9c. The optical properties appear in the close-packed structure of the nanoparticles (Figure 1.9d), whereas the discrete structures or otherwise orderly structures of Figure 1.9e and f display dynamical physical properties, magnetism, optical, and electronic properties.

Figure 1.9 Typical group diagram of a nanoparticle; (a) Random structure; (b) fractal structure (c) structural alignment; (d) close-packed structure; (e) ordered structure (dispersion); (f) ordered structure (dense). From ref. [27].

Methods to separate out particles of a given target size from a colloidal solution which contains nanoparticles of various sizes are known. They are (i) separation by precipitation, (ii) centrifugal separation, (iii) gel filtration column, and (iv) gel electrophoresis. As a feature of each screening method, the precipitation separation is suitable for a large distribution of colloid nanoparticles in the solution. The centrifugal separation and the gel filtration column are well suited for solutions of colloidal nanoparticles with a narrow size distribution. Gel electrophoresis is a suitable method to separate nanoparticles taking advantage of the difference in charge density of the particles, and is suitable for separating particles with a small cluster size. In fact, a combination of these various methods might prove advantageous. However, a problem with sorting the various sized nanoparticles using these methods is that only a fraction of the nanoparticles of a given size may be collected, and then only in small quantities. The digestive ripening method and high temperature melting technique have been proposed to resolve this problem [28].

1.4.2 Shape Control of Nanoparticles

The shape of nanoparticles is an important factor that determines the nature of the surface plasmon resonance band just as the size of the nanoparticles did (see Figure 1.7). Absorption spectra in the visible spectral region of various Au rod-shaped nanoparticles (i.e., nanorods) with changes in the aspect ratio (length of long side and short side) are shown in Figure 1.10 [29]. The diameters of the Au nanorods espousing a pillar form and used in this experiment ranged from 5 to 20 nm and the lengths from 20 to 150 nm. It is worth noting that the change in the ratio of a nanorod is related to the size ratio of a crystal face. An increase in the size ratio (aspect ratio) shifts the maximal absorption band to longer wavelengths. Therefore, the physical composition of the nanorods can easily change their spectroscopic features, such that various studies have been required to understand these characteristics.

Figure 1.10 Visible-light spectra of Au rod-shaped nanoparticles with various aspect ratios (long side to short side in the rod-shaped nanoparticle). From ref. [29]. Copyright 2006 by the American Chemical Society.

The preparation of Au nanorods using surfactants has been reported by Yu and coworkers [29]. Gold nanorods were synthesized using an Au anode under ultrasonic irradiation with a template consisting of the cationic surfactant hexadecyltrimethylammonium bromide (CTAB). Au exfoliates as a cluster from the electrode and is molded into the shape of a rod through the interaction with the CTAB micelle (at concentrations above the cmc). In the growth mechanism of nanorods, the CTAB dispersing agent is selectively adsorbed onto the {100} and {110} crystal faces of the Au nanoparticles. For this reason, the {111} crystal face grows and a rod-like metal nanoparticle is generated as a result. The use of CTAB as the dispersing agent subsequently quickly led to reports on nanoparticle research [30, 31]. Nanoparticles of various forms and shapes have been prepared using the adsorption characteristics of a dispersing agent. Chen and coworkers reported an unusual composition of branched Au nanoparticles (see Figure 1.11) using high concentrations of CTAB [32]. Evidently, the molecular association of a surfactant as a dispersing agent determines the various shape features of metallic nanoparticles.

Figure 1.11 (a) TEM image of a regular tripod nanocrystal and (b) high-resolution image of the pod end as marked by a white frame in panel (a); (c) diagram showing the crystal planes and pod directions. The lower row of panels exhibits the particles developed at various stages: (d) embryo of a triangular shape, (e) monopod, (f) V-shaped bipod, and (g) Y-shaped tripod. From ref. [32]. Copyright 2003 by the American Chemical Society.

The physical aspects of Au nanorods prepared using a hard template, such as mesoporous alumina, are similar to those when using a soft template like CTAB. In an early report that made use of a hard template, the Au nanorods were synthesized in the inner fine pores of mesoporous alumina [33]. In the initial stage, the nanosize porous alumina electrode is produced and the metal is then electrochemically deposited sequentially in the fine pores, which provide a firm mold; the diameter of the short axis of the Au nanorod which grows inside the fine pores is regulated by the size of the pores. Subsequently, the alumina mold is dissolved and removed; the so-formed Au nanorods are then taken out of the template.

An interesting feature of the above method is the fabrication of nanorods of multiple layers of different metals such as Au–Ag–Au. Therefore, a nanoparticle with various features can be synthesized. As for the plural layer-type nanorod, applied research can lead to a nanosize system that might be considered a nanosized bar code [34].

1.4.3 Structure Control of Nanoparticles

Nanoparticles that are composed of two or more metals differ in their catalytic, magnetic, and optical characteristics from nanoparticles that consist of a single metal. Such nanoparticles can be sub-divided into three kinds of structures: (i) the alloy structure that exists randomly in a crystal (Figure 1.12a); (ii) the core–shell structure in which the metal at the center differs from the peripheral metal (Figure 1.12b); and (iii) the twinned hemisphere structure wherein two sorts of hemispheres are joined. The latter heterojunction structure facilitates phase separation (Figure 1.12c). Nanostructures consisting of complex metal nanoparticles tend to hide the various new features.

Figure 1.12 Schematic images of bimetal nanoparticles: alloy structure (a), core–shell structure (b), and heterojunction structure (c) of complex metal nanoparticles.

The core–shell structure is comparatively easy to fabricate in complex metal nanoparticles with effective functional control, which has led to several studies and reports in the literature. For instance, although the color of an Au nanoparticle liquid dispersion is purplish red (the purple of Cassius) and that of an Ag nanoparticle liquid dispersion appears yellow, whenever Au forms the core and Ag the shell the structure then takes an orange color. Moreover, if a structured matter has magnetic properties, such as magnetite nanoparticles, then the magnetic metal particles could be used to form the structure's core, such that the structure will now be embodied with both magnetic and optical characteristics.

Synthetic methods of preparing core–shell nanoparticles are roughly divided into two categories: (i) involving a simultaneous reduction reaction and (ii) involving a sequential one-electron reduction reaction. As an example of the simultaneous reduction reaction, consider the core being made up of Pt nanoparticles and the shell composed of Pd nanoparticles [35]. A unique method that uses differences in the oxidation potentials of Ag and Au has also been reported [36]. Here, a silver nanoparticle is added to an HAuCl4 solution, following which an oxidation–reduction reaction takes place (Eq. 1.1) wherein gold is deposited on the surface of a Ag nanoparticle yielding the core–shell structure.

(1.1)

The development of this method has led to the fabrication of Au nanotubes by first making the pentagonal prismatic Ag nanowires to use as the template (Figure 1.13) [37].

Figure 1.13 SEM image of gold nanotubes that had been broken through sonication to show their cross-sections. The gold nanotubes were prepared by reacting silver nanowires with an aqueous HAuCl4 solution. From ref. [37]. Copyright 2003 by the American Chemical Society.

1.5 Reducing Agent in Nanoparticle Synthesis

Gold hydrosols have been synthesized through reduction of a gold chloride solution under an atmosphere of phosphorus; the method succeeded in controlling the size of Ag nanoparticles [8]. Decades later, Turkevich and coworkers examined the mechanism of metal salt reduction with citric acid [38]. The latter acid has been used widely as a reducing agent to fabricate various metal nanoparticles. The reduction of hexachloroplatinic acid with sodium borohydride has also been investigated, together with hydroxylamine hydrochloride, dimethylaminoborane, sodium citrate, hydrazine hydrate, sodium formate, boranetrimethylamine complex, sodium borohydride and formaldehyde as the reducing agents; the various features of the resulting nanoparticles were examined [39].

More recently, the polyol method using ethylene glycol has become quite popular. Ethylene glycol can be both the reaction solvent and the reducing agent in the synthesis of nanoparticles. What made ethylene glycol attractive was its polar nature, which is useful in dissolving the metal salt and can also play the role of a dispersing agent, and since it has a high boiling point (198 °C) it is suitable for the preparation of the base metal. On the other hand, the disadvantage of this polyol is the high boiling point making removal of the solvent difficult.

1.6 Applications of Metallic Nanoparticles

The various characteristics of different nanoparticles relative to bulk metals are summarized below.

Optical function: The surface absorption plasmon of Au and Ag can express various colors by changing the size of the particle, the form or shape of the particle, and the rate of condensation. A new paint that has the durability of an inorganic pigment and the vivid color of an organic substrate can be made. Nanoparticles smaller than the wavelength of light can be used to make high-penetration conductivity materials (there is little absorption, dispersion, and reflection).

Catalyst function: Reaction efficiencies can be enhanced since the specific surface area of such nanoparticles is large compared with existing particles; to the extent that the surface terrace is regular at the atomic level, a hyperactive catalyst with high selectivity can be made: for example, Au nanoparticles.

Thermal function: When the particle diameter is small (less than 10 nm), the melting point is also lower than a bulk metal. Electronic wiring can be made with nanoparticles that have a low boiling point, for example, a polymer.

Electrical function: Since superconductivity transition temperature rises so that particle diameter is small (less than 1 nm), it can be used to make high-temperature superconductivity material.

Mechanical function: Since the mechanical characteristics improve, mechanical strength can be sharply raised by mixing the nanoparticles with metals or ceramics.

Magnetic function: The attractive force of a magnetic metal increases on reduction of the particle diameter, such that soft-magnetic materials can be made in the form of an alloy of nanoparticles. Moreover, a permanent magnet can be made if the nanoparticles are smaller than the magnetic domain made to magnetize.

1.6.1 Application of Nanoparticles in Paints

One of the most interesting aspects of metal nanoparticles is that their optical properties depend strongly upon the particle size and shape. Bulk Au looks yellowish in reflected light, but thin Au films look blue in transmission. This characteristic blue color steadily changes to orange, through several tones of purple and red, as the particle size is reduced down to ∼3 nm. The nanoparticles attracted attention as color materials and the possibility of their use has been examined in various fields. Figure 1.14 illustrates the photograph of a car to which was applied a “clear colored coating” containing gold nanoparticles on a base coating containing red pearl mica [40]. Spraying with the clear colored coating containing the nanoparticles increased the depth of the red background even more, and since the car is in the shade there is almost no diffuse reflection. The red color becomes a feature of paints containing nanoparticles. Paints that contain nanoparticles cannot be removed as easily as can classical paint. However, because of high costs, paints with nanoparticles are used only in limited applications.

Figure 1.14 Photograph of a car to which was applied a clear coating containing gold nanoparticles on a red colored base coating. From ref. [40].

Metal nanoparticles have also been used in enamel color paints in pottery. Conventional enamel color has used paints with mixed transition metals in the pulverization (glass frit) of glass. If, instead of transition metal paints, Au nanoparticles were used, then high quality red paint could be made with high transparency. Research into iron oxide nanoparticles in paints has also been carried out.

1.6.2 Application in Chemical Catalysis

Ni, Pd, Ag, and Pt have been used as typical metal catalysts in chemical reactions. However, the dissociative adsorption of hydrogen or oxygen molecules cannot be carried out on an Au smooth surface and at a temperature of less than 200 °C [41]. Therefore, such a gold material is inactive as a catalyst in hydrogenation and oxidation reactions. However, when Au nanoparticles are used, they work effectively as catalysts, as discovered by Haruta [42]. The ratio of the corner to the edge of an Au nanoparticle of several nanometers in size becomes large compared with Au particles of larger size. Thus, both the adsorption and the catalytic characteristics of the Au surface increase. In an icosahedron consisting of 2054 Au atoms, the percentage of Au atoms exposed on the surface is 15% in Au nanoparticles of 4.9 nm size (external diameters), whereas the surface exposure of an Au atom reaches 52% in 2.7 nm sized nanoparticles (309 Au atoms). If the number of Au atoms in a nanoparticle was to become even smaller, the number of atoms that constitutes the whole, as well as the electrons, will become limited since the electronic structure of the nanoparticle would then become discontinuous [43]. This state takes on a cluster structure and a quantum size effect shows up in the physical properties.

Generally, metals that form clusters of such a small size tend to be unstable to the atmosphere. However, Au clusters are stable and so Au can be used as a catalyst. The catalytic action is rapid as the size of the Au nanoparticle is small in catalyzed oxidation reactions. For example, in order to oxidize CO with a Pt catalyst, a temperature of not less than 100 °C is needed. However, with an Au nanoparticle as the catalyst, reaction can occur even at temperatures below 0 °C (see Figure 1.15).

Figure 1.15 Particle dependence of Au catalyst and Pt catalyst in the CO oxidation reaction (TOF: turnover frequency).

These effects are the result of changes in the so-called surface plasmon resonance [44], which is observed at the frequency at which conduction electrons oscillate in response to the alternating electric field of the incident electromagnetic radiation. However, only metals with free electrons (essentially Au, Ag, Cu, and the alkali metals) possess plasmon resonances in the visible spectral region, which give rise to such intense colors for these metals. Elongated nanoparticles (ellipsoids and nanorods) display two distinct plasmon bands related to transverse and longitudinal electron oscillations.

1.6.3 Application of Nanoparticles in Micro-wiring

Metal nanoparticle paste is used for circuit pattern formation of a printed wired board in the electronic industry [45]. The melting point of metal nanoparticles decreases relative to bulk metals, so that circuit formation impossible on polymer base material is attainable using a conventional electric conduction paste. Furthermore, whenever particles at the nanoscale are used, the wiring width is thin to a nano level. Formation of nanoparticle wiring can use an ink-jet method, a method that is both inexpensive and requires shorter times than vacuum evaporation and photolithographic methods that are typically used. Generally, Au is used to make the metal nanoparticle paste. However, it is expensive, so that substitution of Cu nanoparticles has been proposed. Cu nanoparticles tend to be oxidized so that the process requires the presence of anti-oxidants.

1.6.4 Application of Nanoparticles in Medical Treatments

Just as the surface plasmon resonance is seen in a metal nanoparticle, an increase in the quantity of nanoparticles raises the scattering intensity. Taking advantage of this feature, the application to specific molecule recognition in a living body tissue is expected (see e.g., refs. [46, 47]). For example, by covering the cancer cell surface it becomes possible to distinguish a healthy cell from a cancer cell by the presence of antibodies joined to the Au nanoparticle. Although the Au nanoparticle junction with the antibody is nicely distributed in the healthy cell (Figure 1.16a), when a cancer cell exists the antibodies are concentrated mostly at the Au nanoparticle (Figure 1.16b). The imaging at various wavelengths is performed by a change in the shape of the nanoparticle [48]. Moreover, if a protein and a functional molecule were joined to the Au nanoparticle, it could also be used for imaging cells other than cancer cells. In addition, to the extent that Au nanorods also display a plasmon resonance in the near-infrared domain, and that such Au nanorods congregate about the circumference of an abnormal cell, it becomes possible to treat cancer using a near-infrared laser.

Figure 1.16 Molecular-specific imaging of cancer cells using Au nanoparticle/anti-EGFR conjugates. (a) Dispersed Au nanoparticle in healthy cells, (b) Concentrated Au nanoparticles about a cancer cell. From ref. [48]. Copyright 2008 by the American Chemical Society.

References

1 For details about NNI see the website: www.nano.gov.

2 For details about the British Museum see the website: http://www.britishmuseum.org/explore/highlight_image.aspx?image=k741.jpg&retpage=20945.

3 Daniel, M.-C., and Astruc, D. (2004) Chem. Rev., 104, 293–346.

4 Kunckel, J. (1676) Ars Vitraria Experimentals oder Vollkommene Glasmacherkunst, Frankfurt, Germany.

5 Helcher, H.H. (1718) Aurum Potabile Oder Gold Tinstur, J. Herbord Klossen, Breslau and Leipzig, Germany.

6 (1769) Dictionaire de Chymie, Lacombe, Paris.

7 Fulhame, M. (1794) An Essay on Combustion with a View to a New Art of Dying and Painting, J. Cooper, London, UK.

8 Faraday, M. (1857) Philos. Trans. R. Soc. London, 147, 145–181.

9 Jin, R., Cao, Y., Mirkin, C.A., Kelly, K.L., Schatz, G.C., and Zheng, J.G. (2001) Science, 294, 1901–1903.

10 For the details about The New York Times article of February 22, 2005 see: http://www.nytimes.com/imagepages/2005/02/21/science/20050222_NANO1_GRAPHIC.html.

11 See e.g. (a) Brinker, C.J. and Scherer, G.W. (1989) Sol-Gel Science: The Physics and Chemistry of Sol-Gel Processing, Elsevier Science, Amsterdam; (b) Hench, L.L., and West, J.K. (1990) Chem. Rev., 90, 33–72.

12 (a) Wood, R.W. (1902) Proc. Phys. Soc. London, 18, 269–275; (b) Wood, R.W. (1902) Philos. Mag., 4, 396–402; Wood, R.W. (1912) Philys. Mag., 23, 310–317.

13 Rayleigh, L. (1907) Proc. R. Soc. London Ser. A, 79, 399–416.

14 Fano, U. (1941) J. Opt. Soc. Am., 31, 213–222.

15 Otto, A. (1968) Z. Phys., 216, 398–410.

16 Kretschmann, E., and Raether, H. (1968) Z. Naturforsch. Teil A, 23, 2135–2136.

17 see e.g. Schasfoort, R.B.M. and Tudos, A.J. (2008) Handbook of Surface Plasmon Resonance, The Royal Society of Chemistry Publishing, Cambridge, UK.

18 Henglein, A. (1989) Chem. Rev., 89, 1861–1873.

19 Burda, C., Chen, X., Narayanan, R., and El-Sayed, M.A. (2005) Chem. Rev., 105, 1025–1102.

20 Liz-Marzán, L.M. (2006) Langmuir, 22, 32–41.

21 Sugimoto, T. (2000) Fine Particles: Synthesis, Characterization, and Mechanisms of Growth, Surfactant Sci. Series Vol. 92, Marcel Dekker Inc., New York.

22 Lea, M.C. (1889) Am. J. Sci., 37, 476–491.

23 Pearson, R.G. (1963) J. Am. Chem. Soc., 85, 3533–3539.

24 Prasad, B.L.V., Stoeva, S.I., Sorensen, C.M., and Klabunde, K.J. (2003) Chem. Mater., 15, 935–942.

25 Brust, M., Walker, M., Bethell, D., Schiffrin, D.J., and Whyman, R. (1994) Chem. Commun., 801–802.

26 See http://www.rsc.org/Publishing/Journals/cc/News/Top40MostCitedArticles.asp.

27 Yamaguchi, Y. (2008) Kagakukougaku, 72, 344–348.

28 Stoeva, S., Klabunde, K.J., Sorensen, C.M., and Dragieva, I. (2002) J. Am. Chem. Soc., 124, 2305–2311.

29 Yu, Y.-Y., Chang, S.-S., Lee, C.-L., and Wang, C.R.C. (1997) J. Phys. Chem. B, 101, 6661–6664.

30 (a) Jana, N.R., Gearheart, L., and Murphy, C.J. (2001) J. Phys. Chem. B, 105, 4065–4067; (b) Jana, N.R., Gearheart, L., and Murphy, C.J. (2001) Chem. Mater., 13, 2313–2322.

31 Nikoobakht, B., and El-Sayed, M.A. (2001) Langmuir, 17, 6368–6374.

32 Chen, S., Wang, Z.L., Ballato, J., Foulger, S.H., and Carroll, D.L. (2003) J. Am. Chem. Soc., 125, 16186–16187.

33 van der Zande, B.M.I., Böhmer, M.R., Fokkink, L.G.J., and Schöneberger, C. (1997) J. Phys. Chem. B, 101, 852–854.

34 Nicewarner-Peña, S.R., Freeman, G.P., Reiss, B.D., He, L., Peña, D.J., Walton, I.D., Cromer, R., Keating, C.D., and Natan, M.J. (2001) Science, 294, 137–141.

35 Toshima, N., Yonezawa, T., and Kushihashi, K. (1993) J. Chem. Soc. Faraday Trans., 89, 2537–2543.

36 Sun, Y., Mayers, B., Herricks, T., and Xia, Y. (2003) Nano Lett., 3, 955–960.

37 Sun, Y., and Xia, Y. (2002) Science, 298, 2176–2179.

38 Turkevich, J., Stevenson, P.C., and Hillier, J. (1951) Discuss. Faraday Soc., 11, 55–75.

39 Rheenen, P.R.V., McKelvy, M.J., and Glaunsinger, W.S. (1987) J. Solid State Chem., 67, 151–169.

40 Iwakoshi, A. (2008) Techno-cosmos, 21, 32–38.

41 Saliba, N., Parker, D.H., and Koel, B.E. (1998) Surf. Sci., 410, 270–282.

42 Haruta, M. (2003) Chem. Rec., 3, 75–87.

43 Haruta, M. (2008) J. Vac. Soc. Jpn., 51, 721–726.

44 Kreibig, U., and Vollmer, M. (1995) Optical Properties of Metal Clusters, Springer-Verlag, Berlin.

45 Kawazome, M., Kim, K., Hatamura, M., and Suganuma, K. (2006/2007) Funsai, 50, 27–31.

46 Bendayan, M. (1989) Colloidal Gold, Volume 1: Principles, Methods, and Applications (ed. M.A. Hayat), Academic press, Inc., London, pp. 34–88 (Chapter 3).

47 Wang, L., Li, J., Song, S., Li, D., and Fan, C. (2009) J. Phys. D Appl. Phys., 42, 203001–203012.

48 Jain, P.K., Huang, X., El-Sayed, I.H., and El-Sayed, M.A. (2008) Acc. Chem. Res., 41, 1578–1586.