Encapsulation Nanotechnologies E-Book

• Herausgeber: John Wiley & Sons
• Kategorie: Fachliteratur
• Sprache: Englisch

Seitenzahl: 616

Veröffentlichungsjahr: 2013

Beschreibung

This unique and comprehensive book covers all the recent physical, chemical, and mechanical advancements in encapsulation nanotechnologies. Encapsulation is prevalent in the evolutionary processes of nature, where nature protects the materials from the environment by engulfing them in a suitable shell. These natural processes are well known and have been adopted and applied in the pharmaceutical, food, agricultural, and cosmetics industries. In recent years, because of the increased understanding of the material properties and behaviors at nanoscale, research in the encapsulation field has also moved to the generation of nanocapsules, nanocontainers, and other nano devices. One such example is the generation of self-healing nanocontainers holding corrosion inhibitors that can be used in anti-corrosion coatings. The processes used to generate such capsules have also undergone significant developments. Various technologies based on chemical, physical, and physico-chemical synthesis methods have been developed and applied successfully to generate encapsulated materials. Because of the increasing potential and value of the new nanotechnologies and products being used in a large number of commercial processes, the need for compiling one comprehensive volume comprising the recent technological advancements is also correspondingly timely and significant. This volume not only introduces the subject of encapsulation and nanotechnologies to scientists new to the field, but also serves as a reference for experts already working in this area. Encapsulation Nanotechnologies details in part: * The copper encapsulation of carbon nanotubes * Various aspects of the application of fluid-bed technology for the coating and encapsulation processes * The use of the electrospinning technique for encapsulation * The concept of microencapsulation by interfacial polymerization * Overviews of encapsulation technologies for organic thin-film transistors (OTFTs), polymer capsule technology, the use of supercritical fluids (such as carbon dioxide), iCVD process for large-scale applications in hybrid gas barriers Readership Encapsulation Nanotechnologiesis of prime interest to a wide range of materials scientists and engineers, both in industry and academia.

Leseprobe

Contents

Cover

Half Title page

Title page

Copyright page

Preface

List of Contributors

Chapter 1: Copper Encapsulation of Multi-Walled Carbon Nanotubes

1.1 Introduction

1.2 Preparation of Copper Encapsulated CNTs

References

Chapter 2: Novel Nanocomposites: Intercalation of Ionically Conductive Polymers into Molybdic Acid

2.1 Introduction

2.2 Experimental

2.3 Intercalation into Molybdic Acid

2.4 Preparation of Polymer-Lithium Complexes

2.5 Instrumentation

2.6 Results and Discussion

2.7 Conclusions

Acknowledgements

References

Chapter 3: Fluid-Bed Technology for Encapsulation and Coating Purposes

3.1 Introduction

3.2 Principles of Fluidization

3.3 Classification of Powders

3.4 Fluidized Bed Coaters

3.5 Fluid-Bed Coating and Encapsulation Processes

3.6 The Design, Optimization and Scale-Up of the Coating Process and the Apparatus

3.7 Numerical Modeling of Fluid-Bed Coating

References

Chapter 4: Use of Electrospinning for Encapsulation

4.1 Introduction

4.2 Electrospun Structures for the Encapsulation of Bioactive Substances in the Food Area

4.3 Electrospun Encapsulation Structures for Biomedical Applications

4.4 Other Uses of Electrospinning for Encapsulation

4.5 Outlook and Conclusions

References

Chapter 5: Microencapsulation by Interfacial Polymerization

5.1 Introduction

5.2 Generalities

5.3 Encapsulation by Heterophase Polymerization

5.4 Microencapsulation by Polyaddition & Polycondensation Interfacial

5.5 Microencapsulation by In Situ Polymerization

5.6 Conclusion

References

Chapter 6: Encapsulation of Silica Particles by a Thin Shell of Poly(Methyl) Methacrylate

6.1 Introduction

6.2 Synthesis of Silica (Nano)Particles and Their Surface Modification

6.3 Encapsulation of Silica Particles in a Thin PMMA Shell

6.4 Summary

References

Chapter 7: Organic Thin-Film Transistors with Solution-Processed Encapsulation

7.1 Introduction

7.2 Environment-Induced Degradations of OTFTs

7.3 Encapsulation of OTFTs

7.4 Summary and Outlook

References

Chapter 8: Tunable Encapsulation Property of Amphiphilic Polymer Based on Hyperbranched Polyethylenimine

8.1 Introduction

8.2 Synthesis of PEI-CAMs

8.3 Unimolecularity versus Aggregate of PEI-CAMs

8.4 Host–Guest Chemistry of PEI-CAMs

8.5 Charge Selective Encapsulation and Separation

8.6 Recognition and Separation of Anionic-Anionic Mixtures by Core Engineering of a CAM

8.7 Modulation of the Guest Release of a CAM

8.8 Concluding Remarks

Acknowledgements

References

Chapter 9: Polymer Layers by Initiated CVD for Thin Film Gas Barrier Encapsulation

9.1 Introduction

9.2 Initiated CVD Polymerization

9.3 Coating by Initiated CVD

9.4 Advantages of iCVD in Hybrid Multilayer Gas Barriers

9.5 Specific Requirements for the Use in Hybrid Multilayers

9.6 Multilayer Gas Barriers Containing Polymers by iCVD

9.7 Upscaling and Utilization

References

Chapter 10: Polymeric Hollow Particles for Encapsulation of Chemical Molecules

10.1 Introduction

10.2 Colloidosome Approach

10.3 Internal Phase Separation/Precipitation Approach

10.4 Self-Assembly of Amphiphilic Copolymers (Copolymer Vesicles)

10.5 Layer-by-Layer (L-b-L) Deposition

10.6 Unimolecular Micelles Approach

10.7 Heterophase Polymerization

10.8 Key Design Features for Applications of Hollow Polymer Particles

10.9 Conclusions

References

Chapter 11: Protic Ionic Liquids Confinement in Macro, Meso and Microporous Materials for Proton Conduction

11.1 Introduction

11.2 Structure and Properties of Materials for Proton Conduction

11.3 Encapsulation Procedures and Proton Conduction Performance

11.4 New Activities and Development Trends

References

Chapter 12: Encapsulation Methods with Supercritical Carbon Dioxide: Basis and Applications

12.1 Introduction

12.2 Supercritical Fluids – Properties

12.3 Particle Engineering and Encapsulation with Supercritical Fluids

References

Index

Encapsulation Nanotechnologies

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Library of Congress Cataloging-in-Publication Data:

ISBN 978-1-118-34455-2

Preface

The encapsulation process is prevalent in the evolutionary processes of nature, where nature protects the materials from the environment by engulfing them in a suitable shell. These natural processes are well known and have been applied to numerous processes in the pharmaceutical, food, agricultural, and cosmetics industries. Thus, this allows one to combine the properties of the various components along with the time point of combination, if the release from such capsules can be controlled.

In recent years, owing to the increased understanding of the material properties and behaviors at nanoscale, research in the encapsulation field has also moved to the generation of nanocapsules, nanocontainers, etc. One such example is the generation of self-healing nanocontainers containing corrosion inhibitors which can be used in anti-corrosion coatings. The processes used to generate such capsules have also undergone significant developments. Various technologies based on chemical, physical and physic-chemical synthesis methods have been developed and applied successfully to generate encapsulated materials.

Owing to the high potential of the developed technologies and products in a large number of commercial processes, it is of significance to compile the recent technological advancements in a comprehensive volume. This volume not only introduces the subject of encapsulation to readers new to the field, but also serves as a reference for experts working in this area.

Chapter 1 details the copper encapsulation of carbon nanotubes. Since copper is a good electrical and thermal conductor and has a low binding energy to carbon, its encapsulation into CNTs would lead to many interesting practical applications. Chapter 2 describes the intercalation of ionically conductive polymers into the layers of molybdic acid. The resulting intercalation compounds were characterized by powder X-ray diffraction (XRD), thermogravimetric analysis (TGA), Fourier-transform infrared spectroscopy (FTIR), and ac impedance spectroscopy. Chapter 3 discusses various aspects of the application of fluid-bed technology for the coating and encapsulation processes. Particular attention has been paid to the principles of the fluidization technique, the miscellaneous fluid-bed coating processes and various coaters configurations with special emphasis on fine powder coating, dry coating and encapsulation. Chapter 4 demonstrates the use of the electrospinning technique for encapsulation. The electrospinning technique, consisting of the application of an electrical voltage to a polymeric solution to generate fiber or capsule-like morphologies, has tremendous potential for the development of encapsulation structures of interest in a number of areas such as biomedicine, food technology, bioremediation, energy storage, etc. Chapter 5 details the concept of microencapsulation by interfacial polymerization. Interfacial polymerization, including polycondensation, polyaddition, in situ polymerization as well as other heterophase polymerization processes, is defined by the formation of the capsules shell at or on a droplet or particles by polymerization of reactive monomers. Chapter 6 summarizes the main contributions from the literature for the preparation of a specific example of such hybrid materials, core-shell particles composed of an inner silica core and a poly(methyl methacrylate) outer shell. Chapter 7 provides an overview of recent progress in encapsulation technologies for organic thin-film transistors (OTFTs). General mechanisms of environment-induced degradation to OTFTs is reviewed, along with a discussion on the general requirements of encapsulation. Chapter 8 demonstrates that the derivatives of a hyperbranched polymer (mainly hyperbranched polyethylenimine (PEI)) can encapsulate a variety of guest species, and the encapsulating system shows a rather high guest selectivity, in which a specific interaction is absent or very weak. Chapter 9 presents a description of the initiated chemical vapor deposition (iCVD) process, concentrating on aspects like molecular weight of the deposited polymer, which is important for stability, and deposition rate. Both aspects, molecular weight and deposition rate, are essential for large-scale application in hybrid gas barriers. Chapter 10 provides an overview of the current status of polymer capsule technology, with a specific focus on preparation methods and their areas of application. The preparation of polymer capsules and their general features for applications are addressed. Chapter 11 demonstrates the potentialities of encapsulated ionic liquids (IL) within porous moieties in the proton exchange membranes field. One approach relies on the IL immobilization in large pore zeolites, which are further deployed as inorganic fillers to the polymer casting solution. Chapter 12 reviews the encapsulation and co-precipitation processes based on the use of supercritical fluids, i.e., carbon dioxide. These processes are classified according to the role of the carbon dioxide (solvent, antisolvent, solute or reaction medium). The focus is set on the process mechanisms description, as well as the evolution of different techniques for overcoming the challenges set according to the physical properties of the different processed materials.

Vikas MITTALAbu Dhabi

List of Contributors

Alvise Benedetti is a professor of physical chemistry at the Department of Molecular Sciences and Nanosystems, Università Ca’Foscari Venezia, Italy. He is the author of more than 120 papers published in international journals. His research has focused on physical-chemical studies, mostly from a structural point of view, of amorphous, partially crystalline and polycrystalline systems containing nanostructured phases both in surface and/or in bulk systems.

Rabin Bissessur received his PhD from Michigan State University in 1994, and is currently professor of chemistry at the University of Prince Edward Island. His research interests include the development of nanocomposites for lithium rechargeable batteries and carbon capture. He has co-authored 38 refereed articles, and 4 book chapters.

María José Cocero founded the High Pressure Process Group at the University of Valladolid (Spain) in 1998. Since then, she has published more than 200 papers on natural bioactive compounds extraction and formulation, supercritical water oxidation for the treatment of highly contaminated wastewater and biorefinary applications, among other topics.

Douglas C. Dahn received his PhD from the University of British Columbia in 1985, and is currently associate professor of physics at the University of Prince Edward Island. He has co-authored 22 refereed articles and two patents on topics including scanning probe microscopy and condensed matter and materials physics.

Adela Eguizábal received her diploma in chemical engineering in 2008 and the MSc degree in chemical engineering in 2010 from the University of Zaragoza, Spain. She is currently a research associate in the Department of Chemical and Environmental Engineering and also works in the Nanoscience Institute of Aragon (INA). Her research nterests are focused on composites based on microporous materials, high temperature PEMFCs and polymer based microsystems prepared by soft litography for preconcentration and reaction applications.

Maria José Fabra is a post-doctoral researcher at the Novel Materials and Nanotechnology Group of IATA-CSIC. She has published thirty papers, one book and eight book chapters. The main research interests are the development of new biodegradable packaging materials and the encapsulation of functional and bioactive compounds.

Isidora Freris is a post-doctoral researcher at the Department of MolecularSciences and Nanosystems, Università Ca’Foscari Venezia, Italy. She obtained her PhD in chemistry from Monash University, Australia in 2007. Her current research is focused on the development of nanostructured hybrid materials, particularly organic-inorganic core-shell hybrids and luminescent materials via sol-gel processing. She has co-authored 9 scientific publications.

Yu Fu received his doctorate degree in materials science and engineering from the National Taiwan University in 2011 and is now general manager of Yu Crystal Encapsulation Co.

Blakney Hopkins worked on an honours project under the supervision of Professor Rabin Bissessur. She graduated with a BSc (Honors) degree in chemistry in 2010 from the University of Prince Edward Island.

José M. Lagaron, PhD, is Founder and Group Leader of the Novel Materials and Nanotechnology Group of the IATA-CSIC (Valencia, Spain) and is part-time professor of materials science at the Universitat Jaume I. He has published more than one hundred peer-reviewed papers and has fourteen patent applications in nanotechnology applied to polymers.

Amparo López-Rubio PhD, is a research scientist and project leader in the encapsulation area within the Novel Materials and Nanotechnology Group of the Institute of Agrochemistry and Food Technology (IATA) of the Spanish Council for Scientific Research (CSIC). She has published more than forty five papers in peer-reviewed international journals on the subjects of food technology, nanotechnology, packaging and biopackaging.

Ángel Martín is a senior researcher at the University of Valladolid (Spain). During his PhD and postdoctoral visits to the universities of Delft (The Netherlands) and Bochum (Germany), he specialized in the development of new materials by supercritical fluids techniques for a wide portfolio of applications, from energy to pharmaceutics.

Boateng Onwona-Agyeman received his PhD from Saga University in Japan. He is currently a senior lecturer at the Department of Materials Science and Engineering, University of Ghana. His current research includes the development of nanoporous structured materials and thin films for solar cell applications and has 30 publications in peer-reviewed journals.

Jong Myung Park is currently a professor in the fields of polymer materials and coatings in the Graduate Institute of Ferrous Technology at Pohang University of Science and Technology (POSTECH), Korea. He received his PhD in polymer science and engineering from Lehigh University, U.S.A. in 1990. He has more than 25 years of industrial R&D experience in the fields of organic coatings and corrosion protection/surface treatments and holds more than 90 patents and published more than 50 scientific papers. His research interests include (1) polymer synthesis and characterization, (2) morphology control for polymeric and hybrid particles, (3) functional coatings for corrosion control/self-healing and (4) functional nano-materials for bio- and energy-related areas.

Rocío Pérez-Masiá has an Msc in food science and engineering and is a PhD student in the Novel Materials and Nanotechnology group at IATA-CSIC. She is focused in the development of new materials through the electrospinning technology for active packaging applications and has coauthored 4 publications in the field of food packaging.

Mariá Pilar Pina received her diploma in chemistry in 1994 and her PhD in chemistry in 1998 from the University of Zaragoza, Spain. She is currently associate professor with tenure at the Chemical Engineering and Environmental Department at the University of Zaragoza and active member of the Nanoscience Institute of Aragon (INA). Her research activities are mainly focused on microfabrication using zeolite membranes as structural layers for reaction, separation and sensing applications; development of polymer based microsystems by soft lithography and modified with nanostructured materials; and chemical sensors for gas detection at trace level.

Jatindra Kumar Rath Manager Utrecht University lab at HTC, Eindhoven, Netherlands, has over 25 years of experience in thin films silicon solar cells and has published more than 250 papers. His main research interests are transient and dusty plasmas in PECVD, nanocrystalline silicon and quantum dots, multijunction and heterojunction solar cells and solar cells on nano-textured surfaces and plastics.

Soraya Rodríguez-Rojo is assistant lecturer at the University of Valladolid, Spain where she earned her doctorate in chemical engineering (2008) on the hydrodynamics of supercritical fluidized bed for microparticle coating. During her two post-doctoral research fellowships (ETH-Zurich, Switzerland, and ITQB-IBET, Portugal), she has specialized in the formulation of nutraceutical compounds.

Fabien Salaün is a professor assistant at ENSAIT/GEMTEX, France. His research interests focus on polymer synthesis, encapsulation, and functional coatings for textile applications. His obtained his PhD in 2004 from the University of Lille 1, France. He has published more than 20 articles in refereed journals and 3 book sections in these research fields.

Toshifumi Satoh received his PhD from Hokkaido University in 1996. He subsequently joined the faculty of this university and was promoted to a professor in 2013. He has 177 publications to his credit, including research papers, reviews, and book chapters. His current fields of interest are branched polymers and unimolecular micelles

Ruud Schropp received his PhD in science from the University of Groningen in 1987. After that he worked in R&D at Glasstech Solar, Inc. in Colorado, USA on solar cells. In 1989 he joined Utrecht University and in 2000, he was appointed Full Professor in “Physics of Devices”. In 2012 he joined the Energy Research Center of the Netherlands (ECN), while continuing professorship in thin film photovoltaics at Eindhoven University of Technology.

Diederick Spee studied physics at Utrecht University where he completed his master thesis on back contacted HIT solar cells in 2009. Currently, he is completing his PhD research on the hot wire chemical vapor deposition of flexible thin film organic/inorganic multilayer moisture barriers.

Yong Sun received his PhD from Kyushu Institute of Technology in Japan. He is currently an associate professor at the Department of Applied Science for Integrated System Engineering, Kyushu Institute of Technology. His research area includes the electromagnetic and acoustic properties of nano-materials and semiconductors.

Roman Grzegorz Szafran is an assistant professor at the Wroclaw University of Technology, Faculty of Chemistry, Department of Chemical Engineering, Poland. He is a specialist in the field of chemical engineering, fluid-bed and spout-fluid bed systems, coating, CFD modeling and microengineering. He is the author of 44 publications and 9 patent applications.

Feng-Yu Tsai received his doctorate degree in materials science from the University of Rochester in 2002 and is an associate professor of materials science and engineering at the National Taiwan University. His research interests include atomic and molecular layer deposition, nanotechnologies, and organic and flexible electronics including light-emitting diodes, organic photovoltaics, and thin-film transistors. Dr. Tsai has more than 50 publications and patents in related fields.

Decheng Wan received his PhD from Fudan University in 1999. He joined the faculty of Tongji University in 2006 and was promoted to a professor in 2011. He has (co)authored 68 research papers, reviews, and book chapters. His research interest is macromolecular synthesis and supramolecular chemistry.

Chapter 1

Copper Encapsulation of Multi-Walled Carbon Nanotubes

Yong Sun1 and Boateng Onwona-Agyeman2

1Dept. of Applied Science for Integrated System Engineering, Kyushu Institute of Technology, Tobata-ku, Kitakyushu-city, Japan

2Graduate School of Bioresource and Bioenvironmental Sciences, Department of Agro-environmental Sciences, Kyushu University, Hakozaki, Higashi-ku, Fukuoka-city, Japan

Abstract

Properties of hollow carbon nanotubes (CNTs) could be modified by introducing foreign materials into the interior. Different materials used to fill CNTs include water molecules, DNA segments, metals and many others. Among CNTs filled with different materials, metal-filled CNTs show great potential in numerous applications, such as data storage nanotechnology, due to their small size. In addition, the carbon sheets of CNTs provide an effective layer against oxidation and therefore ensure long-term stability of the encapsulated metals. Since copper is a good electrical and thermal conductor and has a low binding energy to carbon, its encapsulation into CNTs would lead to many interesting practical applications. Typically, CNTs filled with Cu are useful for the fabrication of ultra-low resistance nanoscale electronic devices. Recently, bamboo-like tapered CNTs with only copper located at the tip region were also found to be useful for tube spot welding using current-induced Joule heating. The Cu impregnation in the hollow inner region of CNTs can be attained in situ during the CNT growth by incorporating metals or metal precursors along with the carbon source. The other fabrication method is to fill copper into the prepared CNTs by various means, such as electrodeposition, wet chemistry, capillary suction and plasma irradiation.

In this chapter we will introduce three general copper encapsulation methods; electric arc discharge, chemical vapor deposition and laser ablation. The mechanism of the encapsulation will also be discussed.

Keywords: Multi-walled carbon nanotube, copper, encapsulation, electric arc discharge, chemical vapor deposition, laser ablation, vapor-liquid-solid model, tip growth, root growth

1.1 Introduction

Since its discovery in the 1990s [1–4], carbon nanotubes (CNTs), including single-wall carbon nanotube (SWNT) and multi-wall carbon nanotubes (MWCNTs), have attracted great industrial and academic interest. Due to their superior mechanical, thermal, electrical and optical properties, CNTs are expected to replace many classic components in the near future [5–7]. It has also been practically shown that they possess extremely good mechanical properties and remarkable electrical transport properties, therefore enabling their potential use in nanoelectronic devices, energy storage, field emission displays, chemical and biological sensors and other technological fields [8, 9]. Since CNTs possess hollow cylindrical structures they could be used as containers of atoms and small molecules [10–12], and also can be used for hydrogen storage [13].

Different materials used to fill CNTs include water molecules [14], DNA segments [15], metals [16, 17] and many others [18, 19]. Among CNTs filled with different materials, metal-filled CNTs show great potential in numerous applications, such as data storage nanotechnology, due to their small size. In addition, the carbon sheets of CNTs provide an effective layer against oxidation and therefore ensure long-term stability of the encapsulated metals. One such example is the filling of iron in CNTs demonstrated by Borowiak-Palen et al. [19]. The encapsulation of iron in CNTs is suitable for use as magnetic field sensors due to the ferromagnetic behavior of the system at room temperatures. Also, CNTs filled with ferromagnetic fillers can be used in controlling the heating of tumor tissues [20].

Since copper is a good electrical and thermal conductor and has a low binding energy to carbon, its encapsulation into CNTs would lead to many interesting practical applications. Recently, bamboo-like tapered CNTs with only copper located at the tip region were found to be useful for tube spot welding using current-induced Joule heating inside a transmission electron microscope (TEM) [21].

1.2 Preparation of Copper Encapsulated CNTs

Modification of CNTs provides an effective strategy to expand, improve or change their properties and functions giving way to many promising applications [22–24]. Cu impregnation in the hollow inner region of CNTs can be attained in situ during the CNT growth by incorporating metals or metal precursors along with the carbon source. Among the metals, copper shows the highest thermal and electrical conductivity apart from silver, and exhibits a low binding energy towards carbon about 0.1 eV [25]. Therefore copper-filled CNTs show potential applications as mentioned above. Various studies have been conducted in the filling of different materials into CNTs [26–29]. For the preparation of copper-filled CNTs we will discuss three general methods; electric arc discharge, chemical vapor deposition (CVD) and laser ablation.

1.2.1 Arc Discharge

The arc discharge method is a common and easy way of producing CNTs. It is a technique that produces a complex mixture of components and sometimes requires further purification to separate the CNTs from the soot and other residual materials. The method creates CNTs through arc vaporization of two carbon rods placed end to end, separated by a small gap, for example, 1 mm in a chamber filled with an inert gas at low pressure. A direct current (DC) of 50–100A, driven by a potential difference creates a high temperature discharge between the carbon rod electrodes. The discharge vaporizes the surface of one of the carbon electrodes and forms a deposit of materials on the surface of the other electrode. The evaporated carbon atoms coagulate to form carbon nanoparticles including fullerenes. A part of the evaporated carbon is deposited on the adjacent cathode (at lower temperature) and MWCNTs grow there.

A. Setlur and coworkers [30] have prepared large quantities of CNTs filled with pure copper by using hydrogen arc. In their method, the interaction of small copper clusters with polycyclic aromatic hydrocarbons (PAH) was shown to form CNTs and encapsulated copper nanowires. The DC arc chamber used in this method was filled with hydrogen to the operating pressure range of several hundred Torr. Two graphite rods of approximately 10 mm in diameter were used as electrodes. A 6 mm diameter hole is made 20 mm deep into the anode and a copper rod is inserted. The arc was generated by a DC supply (100 A, 200 V) and its stability maintained by adjusting the electrode spacing. Materials produced by the arcs were examined by TEM. The authors observed the following; 1. the deposits produced by the hydrogen arcs with the copper composite anodes differ greatly from arcs operated with pure graphite anodes, 2. the rod used as the cathode is covered with a leafy growth. For the 100 and 500 Torr cases, the leaves appear to have small copper particles deposited on them, indicating that the temperature around the deposited rod is less than 1083°C, the melting point of copper. For 500 Torr case, the leaves have a rubbery texture while at 100 Torr the leaves are generally harder. The leaves produced in 500 Torr of hydrogen contain carbon nanotubes, many of which are filled with copper. For the 100 Torr, the leaves produced are less and consist of graphitic sheets and copper particles. From these observations, the authors proposed that the PAH molecules produced by the arc interact with copper clusters to form nuclei for nanotube growth. Once the interaction occurs between the PAH molecules and the copper clusters, growth proceeds by the addition of atoms, chains and rings. Figure 1.1a is a low magnification image of a portion of the soot produced at 500 Torr of hydrogen, consisting of long hollow carbon nanotubes. Figure 1.1b shows a copper rich region of the soot, which has copper-filled nanotubes, copper nanocrystals and larger copper crystals. It is estimated that, in these regions about 80–90% of the nanotubes are completely filled with copper. The selected area electron diffraction (SAED) pattern (inset) shows the presence of crystalline copper (111) and graphitic layers. Figure 1.2 shows a high resolution transmission electron micrograph (HRTEM) of the filled nanotube. The copper in the nanotubes is polycrystalline with twins occurring in some tubes. It was estimated through TEM observations that 20–30% of the nanotubes were filled with copper. To explain the observation of both filled and unfilled nanotubes, the authors proposed the following model as shown schematically in Figure 1.3. Small copper clusters produced by the arc must either coagulate with other copper clusters or interact with PAH molecules as shown in Figures 1.3(a), (b) and (c) to reduce their energy. The authors proposed that in these experiments, the PAH molecules produced by the arc resembling small graphitic sheets interact with copper clusters similarly to graphite to form nuclei for nanotube growth in Figure 1.3(c). It is evident from their results that copper and PAH molecules interact to form nuclei for nanotube growth. In a copper rich region, Figure 1.1(b), there is copper available to fill the nanotubes as seen in Figure 1.3(d). In a copper poor region, Figure 1.1(a), there is not enough copper to fill the nanotubes as they grow, resulting in the empty nanotubes shown in Figure 1.3(e).

Figure 1.1 TEM images of the soot formed in 500 Torr of hydrogen with a copper composite anode. (a) Long hollow nanotubes. (b) A copper rich region of the soot, the inset is SAED pattern with (111) twin of the copper encapsulation and the (0002) graphite layer.

Figure 1.2 HRTEM image of a copper-filled nanotube with a diameter of about 10 nm.

Figure 1.3 Schematic diagram of Cu/PAH interactions and nanotube growth, (a) PAH/Cu from hydrogen arc. (b) Coagulation of copper clusters to reduce surface energy, (c) Interaction of copper clusters with PAH molecules, which forms the nucleus for nanotube growth, (d) A copper rich region of the leaves containing large copper clusters, copper-filled nanotubes, and copper-encapsulated nanoparticles. (e) A region of the leaves or soot that contains hollow nanotubes.

Z. Wang and coworkers [31] have also reported a simple arc-discharge method for in situ synthesis of copper-filled CNTs with coal as carbon precursor. The experiment was carried out in a DC arc discharge reactor in an argon gas ambient. A high purity graphite tube filled with a mixture of coal (anthracite) and CuO powder, particle size less than 150, was used as the anode while the cathode was made of high purity graphite rod. The weight ratio of CuO to coal in the mixture was 1:9. The arc discharge was carried out with a direct current of 70 A and voltage of 20 V in argon ambient at 80-90 kPa. After the discharge, the deposits on the cathode were collected and examined using TEM. Figure 1.4(a) shows a low magnification TEM image of the as-prepared sample showing the complete synthesis of copper encapsulated CNTs of several tens micrometers long. They also observed that in some CNTs, there were several distorted defects such as kinks and curls in which the distance between the defects varies from hundreds of nanometers to several micrometers. The HRTEM image in Figure 1.4(b) shows that the CNTs are completely filled with copper with a diameter of about 30-80 nm and the aspect ratio of the copper filled CNTs is about 200-360. Repeated experiments indicated that on average more than 40-50% of the as-prepared CNTs are filled with copper as can be seen in Figure 1.5. The SAED patterns of the distorted nanowires are shown in the insets of Figures 1.5(b) and (d). The observed diffractions consist of regular arrays of sharp spots together with short arc due to the (002) diffraction of hexagonal graphite indicating the presence of well-developed monocrystalline structure in the copper nanowires. These diffraction patterns are in good agreement with typical diffraction pattern of a face-centered cubic (fee) copper along the (011) zone axis. They also showed a HRTEM image of a 30 nm diameter copper-filled CNT in Figure 1.6 in which monocrystals have been observed in long-range order as well as the outside coating consisting of well-oriented graphite layers (about 20 layers with a separation of about 0.34 nm). They concluded that these results show that the encapsulated material inside the CNTs was pure copper consisting of several long monocrystals. A one-step synthesis by J. Ding and colleagues [32] was used to prepare pure copper nanowire in carbon nanotubes with different structures by DC arc discharge. The as-prepared copper encapsulated CNTs (Cu@CNTs) exhibited three different structures, including well-filled Cu@CNT nanocables, symmetrically trifurcate Cu@CNT nanocables and twice capsulated Cu@CNT nanocables. The DC arc discharge system they used consisted of anode-cathode assembly installed in a stainless steel cylindrical chamber capped at both ends. The cathode was a highly pure graphite rod and the anode 65 mm long with outer and inner diameters of 10 and 6 mm before arcing respectively. The anode, being hollow was packed with metal copper powders. The experiment was carried out in a helium/hydrogen ambient (volume ratio: 1:1) at total pressure of 400 Torr, and the arc discharge was created by a current of 120 A. The gap between the cathode and anode was kept at 2 mm by fixing the consumed anode and advancing the cathode manually. The crystal structures of the deposited powder were studied using X-ray diffraction (XRD), and the microstructure and surface morphology were characterized by scanning electron microscope (SEM) and HRTEM. Figure 1.7 shows XRD pattern of the samples with peaks corresponding to graphite, copper carbide and copper respectively. No diffraction peaks corresponding to other phases can be observed. They observed that the diffraction signals were from CNTs, small amounts of copper carbide and copper clusters. In order to study the microstructure and morphology of the samples of the encapsulated copper, TEM images were taken. Figure 1.8(a) shows the CNTs nanostructures consisting of unfilled, partially-filled and well-filled nanotubes marked with U, P and W respectively. It indicates that over 85% of CNTs can be filled with copper and the CNTs walls will still remain intact. The inset of Figure 1.8(a) shows that well-filled CNTs can be as long as over one micrometer. A HRTEM image of a single Cu@CNT is shown in Figure 1.8(b) indicating well-filled CNTs with oriented graphite layers and encapsulated copper crystals inside the MWCNTs. The inset shows the d-spacing of the graphite layers (about 6 layers) and the encapsulated copper crystals with (111) atomic plane. Figure 1.8(c) shows a terminal morphology of an individual Cu@CNT. The hollow structure of the CNT, the tube’s wall and the copper nanowire level inside can be seen. Also, the filled section of the Cu@CNT has a larger diameter compared with the hollow one. They attributed this to the continuous incorporation of the copper clusters into the MWCNTs during growth. Figure 1.8(d) reveals a trifurcate Cu@CNT nanocable and well-filled copper nanocables. The inset of Figure 1.8(d) indicates non-crystal copper nanoparticles embedded into the branched spot, which may be due to the formation of interfacial copper carbide that favors infiltration of liquid copper nanoparticles into CNTs and resides at defect sites. Figure 1.8(e) shows that copper nanocrystalline grains encapsulated in CNTs are found to exist inside copper CNT nanocables, implying that the growth process of copper CNT nanocables is very delicate. Copper carbide will possibly be formed at interface between the twice encapsulated CNTs [33]. The authors explained the growth mechanism of the copper encapsulated CNT by using the vapor-liquid-solid (VLS) model [34]. As shown in Figure 1.9, first the carbon atoms that are evaporated from graphite or decomposed from carbon-rich gases (hydrocarbons resulting from interaction of carbon and hydrogen buffer gas during arc discharge) dissolve into liquid copper clusters as shown in Figure 1.9(a). Copper then catalyzes the decomposition of the carbon precursor and leads to the supersaturated carbon precipitation to grow the CNTs shown in Figure 1.9(b). Later, part of the liquid copper is sucked into the hollow of the CNT shown in Figure 1.9(c). The authors attributed this sucking to capillary attraction, and also the low solubility of copper with carbon is responsible for the outer walls of CNTs being free from copper clusters.

Figure 1.4 TEM images of Cu-filled CNTs prepared with coal as carbon source: (a) image of super-long Cu nanowires; (b) an image of three Cu-filled CNTs from (a), indicated by white arrow.

Figure 1.5 TEM images of the Cu-filled CNTs. (a) and (c): Magnified images and (b) and (d): SAED patterns showing CNTs filled with Cu nano wires.

Figure 1.6 HRTEM image of one Cu-filled carbon nanotube.

Figure 1.7 XRD pattern of the sample.

Figure 1.8 (a) TEM image of the sample, (b) HRTEM image of single Cu@CNT, (c) image of terminal single Cu@CNT, (d) a trifurcate Cu@CNT, (e) HRTEM image of twice encapsulated Cu@CNT.

Figure 1.9 Schematic diagram of the growth mechanism of Cu@CNT: (a) carbon atom diffusion within the molten copper particles, (b) initial growth of the MWCNTs, (c) part of copper clusters is pulled into the MWCNTs, and (d) formation of partially-filled Cu@CNTs.

Copper encapsulated nanoparticles were synthesized by a modified arc plasma method using methane as carbon source by C. Hao et al. [35]. In the arc reactor, two electrodes for the DC arc discharge were fixed with a 5 mm gap between them. The upper electrode was tungsten and the lower electrode (anode) was graphite crucible packed with copper metal rod. The chamber was filled with helium and methane gas with a pressure of 100 kPa after it was vacuumed to Pa. The discharge current was 80 A and the arc plasma was ignited using a high frequency initiator in the chamber. The copper rods were melted and evaporated by the generated high temperature. The evaporated copper and carbon were deposited on the inner walls of the chamber. The morphologies and size of encapsulated copper particles were determined by SEM and HRTEM. Also, the phase and crystal structure of the particles were characterized by XRD. Figure 1.10 shows the XRD patterns of pure copper nanoparticles (helium/methane =1:2) and copper encapsulated nanoparticles respectively. In Figure 1.10(a) it can be seen that the position of peaks for pure copper nanoparticles is consistent with the reflection lines of fcc copper. In addition to the copper peaks, three peaks due to carbon were also observed in the XRD pattern of the copper encapsulated nanoparticles as shown in Figure 1.10(b). To identify the existence of core-shell structure, SEM and HRTEM images weré observed. Figure 1.11(a) shows a typical SEM image of the copper encapsulated nanoparticles and Figure 1.11(b) shows the TEM image. Figure 1.11(c) shows the HRTEM micrograph of copper encapsulated nanoparticles. The authors observed that the copper nanoparticles were covered with 3–5 nm carbon layers. The SAED shown in the inset of Figure 1.11(c) indicates that the core is composed of fcc copper. The diameter of the core-shell copper/carbon nanoparticles was about 30 nm. Figure 1.11(d) shows a HRTEM image of the carbon shell. It can be observed that the shells outside the core are not amorphous but ordered graphitic carbon. The interlayer spacing of these graphitic planes is about 0.34 nm. The authors observed that as temperature was increased, the decomposition of hydrocarbons occurred resulting in the formation of copper and carbon vapor. The carbon then dissolves in the copper particles reducing the vapor energy of the copper. The final formation of the product was determined by the cooling rate and the solubility of carbon in copper. The authors also observed the formation of copper nanoparticles under helium and hydrogen atmosphere as shown in the image in Figure 1.12(a). The copper particles were spherical in shape and the average particle size is about 40 nm. Under helium and methane ambient, copper encapsulated CNTs were produced and it was noted that a change in the volume ratio of helium/methane affects the morphologies and size of the copper encapsulated particles. When the ratio of helium/methane is 1, part of the copper nanoparticles clinched to each other as shown in Figure 1.12(b) and the average size is about 40–50 nm. When the ratio is decreased to 1/2, the particle size becomes smaller as can be seen in Figure 1.11(b). The authors also observed that compared with pure copper nanoparticles, the size of the copper encapsulated nanoparticles is evenly distributed. They attributed this to the presence of the carbon shells which limits the growth of the particles and inhibits aggregation of copper particles.

Figure 1.10 XRD patterns of (A) pure copper nanoparticles and (B) copper encapsulated carbon nanoparticles.

Figure 1.11 (a) SEM and (b) TEM image of copper encapsulated carbon nanoparticles, (c) HRTEM image of copper encapsulated carbon nanoparticles (inset: electron beam diφφraction of the Cu core), (d) HRTEM photograph of graphitic carbon on the surface of the copper core.

1.2.2 Chemical Vapor Deposition

Chemical vapor deposition of hydrocarbons over metal catalyst is a classic method that has been used to produce various carbon materials such as carbon fiber, diamond-like carbon and recently graphene. Large amounts of CNTs can be formed by CVD of acetylene over metals supported on silica, zeolite or alumina. Currently, the formation of CNTs directly on the metal plate by CVD has attracted much attention because the metal plate can act as the substrate and a catalyst for the growth of the CNTs at the same time. An advantage of the method is that infiltration of unwanted materials into the CNTs can be avoided.

From their experiment, J. Lin et al. [36] found that CNTs could be produced in high yield by catalytic decomposition of methane using a copper catalyst, especially when CuSO4 was used with γ-alumina as the support material. The preparation of the CNTs was carried out using a fixed bed quartz tube reactor. The CuSO4/Al2O3 catalyst was prepared by impregnating the γ-alumina with an aqueous solution of CuSO4.5H2O. The catalyst was then loaded into the reactor and treated with helium gas at different temperatures (600-1100°C) after which a mixture of methane/helium in a ration 3/1 was allowed to flow into the reactor at required temperatures for the synthesis of the CNTs. Figure 1.13 shows the catalytic activity of 5 wt.% copper sulfate/alumina at different temperatures for the synthesis of CNTs. From the graph, the growing of CNTs starts at about 600°C and reaches a maximum at about 800°C, and then drops to zero when grown in temperatures that exceed 1000°C. Figure 1.14 shows the SEM and TEM images of CNTs at 800°C for 1 hour on copper nitrate/alumina and copper sulfate/alumina catalysts. The authors observed that the formation rate of CNTs on copper catalyst drops to a barely detectable level when copper nitrate was used as the copper precursor instead of copper sulfate, and also, the CNTs appeared to be the fish-bone type of carbon nanotube shown in Figure 1.14(b). The TEM image in Figure 1.14(c) suggests that the growth of CNTs on copper catalyst is consistent with a typical tip-growth mechanism where carbon diffusion is followed by precipitation at the rear of the metal particles to form the body of the CNTs. The authors then treated the copper/sulfate catalyst with a stream of helium gas at temperatures between 800 and 1100°C for 2 hours and analyzed the structure by XRD. As shown in Figure 1.15 a broad gamma alumina diffraction pattern is observed at 800°C. An alpha alumina diffraction pattern develops gradually when the temperatures are higher than 800°C, and gamma alumina diffraction pattern fades away completely when temperatures are higher than 1000°C. From the XRD data, the authors postulated that the behavior of copper for catalyzing the CNT synthesis may correlate with the interactions between the copper and gamma alumina. It is well known that copper and gamma alumina forms a mixed oxide with a spinel-type structure. However, high temperature treatment converts the gamma alumina to alpha alumina and destroys the active spinel structure. Therefore a decrease of catalytic activity for the CNT synthesis is observed.

Figure 1.13 Relation between the catalytic activity of 5% Cu SO4/Al2O3 and temperature during the synthesis of carbon soot in CH4/He (3/1).

Figure 1.14 SEM images of CNTs on (a) 5% Cu(NO3)2/Al2O3, (b) 5% Cu SO4/Al2O3, and (c) HRTEM image of CNTs on 5% CuSO4/Al2O3.

Figure 1.15 XRD patterns of 5% CuSO4/Al2 O3 pre-treated at (a) 800°C, (b) 1000°C, (c) 1100°C in He for 2 hr, and (d) α-Al2O3.

J. Zhu et al. [37] have studied the synthesis of bamboo-like CNTs on copper foil by the CVD using ethanol. They investigated the effects of temperature (700–1000°C) on the growth of the CNTs, as well as the structural and morphology of the resultant CNTs. In their experiment, a 50 µm thick pure copper foil was placed on an alumina tube and inserted into a horizontal tube furnace. The system was sealed and evacuated to 10-2 Torr before admitting Ar gas and raising the temperature. When the target temperature was attained, the Ar gas was redirected into an ethanol bath before entering the system. After a certain time, the system was allowed to cool down and the copper foil was collected and its surface analyzed. They observed that the size and yield of the CNTs increased with temperature, those prepared at 700°C had a copper droplet tip and those at 800–900°C had a copper nanoparticle inside. On the other hand, an amorphous carbon film consisting of porous and non-porous layer was deposited on the copper foil and CNTs were grown from this layer. They therefore concluded that a carbon film first is deposited on the surface of the copper foil while the surface layer of the copper foil partially melted and migrated across the carbon film where the CNTs formed. Figure 1.16(a) is an SEM image of the cross section of a film of the sample prepared at 900°C for 30 minutes. A high magnification image is also shown in the inset of the Figure. The film consists of two layers (A and B). The thickness of the top layer was about 50 nm (A layer) while the bottom layer was about 760 nm. From its large thickness, the authors concluded that the thick carbon film was not graphene. The EDS results indicated that this black film was a carbon film with about 3.5 at.% of copper shown in Figure 1.16(e). On the surface of the carbon film, clusters of nanostructures were found. The nanostructures were curled with a smooth geometry as shown in the SEM image in Figure 1.16(b). They were extracted and examined by TEM and were confirmed to be CNTs in Figure 1.16(f). The diameter and length of the CNTs were determined to be about 130 nm and several microns respectively. In the TEM images shown in Figure 1.16(c) some particles with a size of about 12 nm (indicated by the dark arrows) were found inside the CNTs near the tips. The particles were not copper oxide as no oxygen signal was found. These CNTs were further examined by HRTEM as shown in Figure 1.16(d) and these CNTs had a bamboo-like structure with complete knots. The authors also studied the structure of the CNTs and their copper catalyst prepared at different temperatures. Figure 1.17 shows the TEM images of the CNTs collected from samples prepared at 700, 800 and 1000°C for 30 minutes. Their diameter increased from 40 to 210 nm when temperature was increased from 700 to 1000°C, while their length extended from about 500 nm to 2–3 µm. These results and others are listed in Table 1.1. The CNTs prepared at 700°C were short and straight as seen in Figure 1.17(a), but those at higher temperatures were long and curly as shown in Figures 1.17(b) and (c). All the CNTs appeared multi-walled with a bamboo-like structure. Copper droplet tips were found in CNTs prepared at 700°C as shown in Figure 1.17(a). The authors suggested that their growth followed the VLS model [34].

Figure 1.16 (a) SEM image of the CNTs/carbon film of the sample prepared at 900°C for 30 min and magnified image of the carbon film (inset). (b) SEM and (c) TEM images of the CNTs from the same sample. (d) HRTEM image of the CNT. EDS spectra: (e) and (f) from arrowed regions in (a) and (c), respectively.

Figure 1.17 TEM images of samples prepared for 30 min at (a) 700, (b) 800, and (c) 1000°C.

Table 1.1 Diameters and lengths of the CNTs, and the I(D)/I(G) ratios of the samples prepared at different temperatures for 30 min.

Copper nanoparticles were found inside the CNTs prepared at 800 and 900°C near their tips as indicated by the dark arrows in Figures 1.17(b) and 1.16(c). The copper particle found in the sample prepared at 700°C was about 100 nm in size in Figure 1.17(a) located at the tip of the CNT. Those found inside the CNT tip prepared at 800 and 900°C were about 12 nm, and no copper particles were found at the tips or inside CNTs Table 1.1 Diameters and lengths of the CNTs, and the I(D)/I(G) ratios of the samples prepared at different temperatures for 30 min.

prepared at 1000°C. The authors observed the decrease and disappearance of the copper particles with increasing temperature, and suggested that the copper atoms would be more likely to be mobile and form smaller particles when the temperature is close to its melting point of 1083°C. The HRTEM images of the graphite layers in the walls of the CNTs prepared at 900°C indicated that they were well oriented as shown in Figure 1.16(d), but the CNTs prepared at other temperatures showed low graphitization. Therefore, 900°C was chosen as the optimum temperature for preparing samples with high yield, small diameter and high graphitization. The authors illustrated the growth of the CNTs in their work using schematic diagrams in Figure 1.18. When incoming ethanol vapor is in contact with hot copper foil shown in Figure 1.18(a), carbon from the decomposed ethanol was deposited on the surface of the copper foil to form a film as shown in Figure 1.18(b). With increasing duration, the carbon film became thicker and copper was found to diffuse into the carbon film and parts of the copper nanoparticles accumulated on the upper surface as shown in Figure 1.18(c), leaving pits in the substrate. The copper particles on the surface of the carbon film acted as the catalyst for the growth of the CNTs via the tip mode VLS process as indicated in Figure 1.18(d). At the same time, the bottom surface of the carbon film interacted with the copper foil and became porous as shown in Figure 1.18(e).

Figure 1.18 Schematic diagrams to illustrate the formation of the CNTs on the copper foil.

Q. Zhang and colleagues [38] have prepared carbon nanotubes with totally hollow channels and totally filled copper nanowires using methane decomposition on copper microgrid as a catalyst at 1173 K. The aim of their work is to develop a method to prepare CNTs with totally hollow channels and to in situ fill copper into the nanotubes. They reported that by using this method, the filling ratio of the copper can be up to 50% of the totally hollow channels of the CNTs. They also reported that the encapsulated copper species can form continuous single crystalline copper nanowires (8–10 μm) by tailoring the copper catalyst and the composition of feed gas. In their method, nanometer-sized copper species were placed on a microgrid, usually used as the sample carrier for the HRTEM observation. The microgrid with the nanosized copper was inserted into a horizontal quartz tube reactor. Then, a mixture of hydrogen, methane and argon with flow rates of 100, 100 and 400 ml/min, respectively was fed into the reactor and decomposed to synthesize the CNTs at 1173 K at atmospheric pressure. The products were directly deposited on the microgrid. After 30 minutes, the power was switched off and the reactor was cooled to the ambient condition. The carbon-based product on the microgrid was directly characterized by HRTEM. As shown in Figure 1.19(a), the relatively low magnification TEM image shows that there are large amounts of CNTs growing along the edge of the microgrid. Most of the CNTs are straight and 6–10 μm long as in Figures 1.19(b) and (c), indicating their relatively uniform growth rate from the microgrid. There are about 70% CNTs with tips free of metal catalyst and 30% CNTs with large metal particles at the tip as shown in Figures 1.19(b) and (d). Also most CNTs have a larger tip as compared with its diameter (Figure 1.20) which the authors could not explain. Regardless of their different growth modes, most CNTs are totally hollow or are totally filled by copper as in Figures 1.19(c)(d) and Figure 1.21. The authors compared the CNTs grown on pure copper catalyst and then on iron-copper catalyst. Grown on the iron-copper catalyst by the intentional addition of iron to the copper catalyst, the CNTs obtained were bamboo-like with nearly no hollow channels as shown in Figure 1.22(a). The EDS characterization indicated the iron in the iron-copper alloy catalyst has an atomic ratio of 1.3% as shown in Figure 1.22(b), and catalyst responsible for the filling of CNTs with hollow channel is pure copper as seen in Figure 1.23. The authors argued that the pure state of the copper is important for controlling the carbon supply rate as compared with iron, cobalt and nickel. Also, copper has a weak interaction with carbon and it is difficult to form copper carbide. Therefore, the bulk phase of the particles must maintain a pure metallic state and not be in a carbide state as shown in the HRTEM image (Figure 1.24) and confirmed by the low atomic ratio of copper to carbon in the CNT tip by EDS (Figure 1.23[c]). The authors also investigated the presence of hydrogen in the feed gas on the formation of CNTs with totally hollow channels. They found out that, it is necessary to add hydrogen into the feed gas to increase the purity of copper catalyst for the analysis of desirable CNTs with totally hollow channels.

Figure 1.19 (a) TEM image of CNTs on the edge of microgrid; (b) TEM image of 70% CNTs following base growth mode; (c) image of the relatively straight CNTs filled or unfilled with copper in (b); (d) TEM image of CNTs with totally hollow core and large copper tips.

Figure 1.20 Image of sintered copper nanoparticles on the microgrid and the CNTs following the tip growth mode or base growth mode.

Figure 1.21 TEM image of large amount of Cu@CNTs and CNTs with hollow channel, without amorphous carbon on the outer wall.

Figure 1.22 (a) TEM image of bamboo-like CNTs prepared from copper catalyst containing 3% iron; (b) EDS pattern of copper-iron nanoparticles in (a).

Figure 1.23 (a) Copper nanoparticle for the growth of CNTs in tip growth mode, with thin carbon layer smaller than 2 nm; (b) the thin carbon layer formed at the carbon-copper interface; (c) EDS of the copper tip with high purity in (b).

Figure 1.24 HRTEM image of the nanotubes filled with copper. The upper insert shows the detailed copper lattice and carbon layer; the lower insert is the SAED pattern of the single crystalline copper nanowires with fcc structure.

Figure 1.25 XRD pattern of the sample purified by acid treatment.

Figure 1.26 (a) FE-SEM images of Cu nanoneedles; (b) filled CNTs at the tips.

Figure 1.27 TEM images of the samples with different growth time: (a) 1 min, (b) 2 min, (c) 5 min, and (d) 30 min. The inset in (b) is the magnified image of the rectangular region.

Figure 1.28 (a) HRTEM image and the corresponding SAED pattern (inset) of a Cu@CNT after acid treatment. (b) The magnified image of the rectangular region in (a). The big arrows indicate the growth direction of the CNTs.

Figure 1.29 Typical EDS spectrum of Cu-filled CNTs after acid treatment.

Figure 1.30 Schematic diagrams of the growth of a Cu-nanoneedle-filled CNT.

1.2.3 Laser Ablation

The laser ablation method involves the ablation of a carbon target containing a small amount of catalyst materials such as copper, nickel and cobalt with intense laser pulses. During the CNT preparation, a flow of inert gas is passed through the growth chamber to collect the grown CNTs. Carbon nanotubes filled completely with polycrystalline copper were synthesized by laser vaporization of copper and graphite under high-pressure argon gas ambient by F. Kokai and coworkers [40]. The authors noted that depending on the argon gas pressure (100–900 kPa) and the copper content (1–40 at.%) in graphite targets for laser vaporization, various products with different morphologies were observed by SEM and TEM. In their work, copper powder (100 µm grain size) was mixed with graphite powder (4 µm grain size) with the copper content ranging from 1–40 at.%. The mixed powder was pressed into copper/graphite pellets (10-mm diameter, 2-mm thickness). A continuous wave Nd:YAG laser (500 W peak power) was used for the vaporization of copper/graphite pellet targets at room temperature. The laser beam was focused on the target through a quartz window in a stainless-steel chamber filled with argon gas at pressures ranging from 100 to 900 kPa. Figure 1.31 shows typical SEM images of the deposits obtained at argon gas pressures of 100, 500 and 900 kPa. Sphere-like particles with diameters of 30–130 nm were dominant at the lower pressure region of argon (100–300 kPa). When they increased the argon gas pressure to 900 kPa, the particle size was increased and the deposits were found to contain straight one-dimensional structures in addition to the sphere-like particles as shown in Figures 1.31(b) and (c). TEM observations indicated the sphere-like particles obtained at lower argon gas pressures were composed of amorphous carbon (AC) and single-wall carbon nanohorn (SWNH) particles. Figures 1.32(a) and (b) are typical TEM images of AC and SWNH particles revealing diameters of 80–150 nm including copper-filled CNTs. Figure 1.32(c) shows a TEM image observed near a core copper particle of a SWNH particle. Figure 1.33 is a typical TEM image of deposits obtained at an argon pressure of 900 kPa, consisting of 1D structures and sphere-like particles. Figures 1.34(a), (b) and (c) are examples of HRTEM images of the 1D structures consisting of copper nanowires with diameters of 18–20 nm surrounded by one, two or three graphitic layers. No unfilled or intermittently-filled CNTs were observed and all grown CNTs were found to contain copper from their roots to their tips in TEM observations of more than 300 copper-filled CNTs. A HRTEM image focusing on the part of a copper nanowire and its corresponding SAED is shown in Figure 1.35