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Chapter 9

NEWER SYNTHETIC STRATERGIES FOR NANOMATERIALS

P. Sangeetha

1. INTRODUCTION

The emerging science in the world today mainly focuses on nanomaterials and its

applications. Nano is interrelated to chemistry, physics and biology and it has attracted

the attention of all eminent scientists to its side. Nanochemistry can be described as a

special discipline of inorganic or solid state chemistry. It focuses on the synthesis of

nanoparticulate systems. The nanochemist can be considered to work towards this goal

from the atom "up", whereas the nanophysicist tends to operate from the bulk "down". It

is schematically represented in Fig. 9.1

Fig. 9.1 Schematic diagram

1.1 What is Nano?

The term nano is a measurement of size. A nanometre (nm) is a millionth of a millimetre.

By way of illustration, a nanometer is about 1/50,000th the width of a human hair, and a

sheet of normal office paper is about 100,000 nm thick. A nanomaterial or a nanoparticle

is usually considered to be a structure between 0.1 and 100 nm (1/1,000,000 mm). The

origin of the prefix "nano" is from the Greek word "nanos"' meaning dwarf.

Nanotechnology deals with a scale that is over 10,000 times smaller than a millimetre. It

9.2

Newer Synthetic Strategies for Nanomaterials

involves investigating, producing and applying structures that are smaller than 100

nanometres (nm). At the nanoscale, the physical, chemical, and biological properties of

materials differ in fundamental and often valuable ways from the properties of individual

atoms and molecules or bulk matter. Research and development in nanotechnologies is

directed toward understanding and creating improved materials, devices, and systems that

exploit these new properties. These properties have been found to be very useful for an

increasing number of commercial applications, for example: protective coatings, light-

weight materials, self-cleaning clothing, to name but a few. The main classes of

nanoscale structures and tunable properties are summarized in Table 9.1 and 9.2.

Table 9.1 Main classes of nanoscale:

Dimension

3 dimensions < 100nm

Particles, quantum dots, hollow spheres, etc.

2 dimensions < 100nm

Tubes, fibres, wires, platelets, etc.

1 dimension < 100nm

Films, coatings, multilayer, etc.

Phase composition

Single-phase solids

Crystalline, amorphous particles and layers, etc.

Multi-phase solids

Matrix composites, coated particles, etc.

Multi-phase systems

Colloids, aerogels, Ferro fluids, etc.

Manufacturing process

Gas phase reaction

Flame synthesis, condensation, CVD, etc.

Liquid phase reaction Sol-gel, precipitation, hydrothermal processing etc.

Mechanical properties

Ball milling, plastic deformation etc.

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9.3

Table 9.2. Tunable properties by nanoscale surface design and their application potentials

Mechanical properties (e.g., hardness,

Wear protection of machinery and equipment,

scratch resistance, tribology).

mechanical protection of soft materials

(polymers, wood, textiles, etc.)

Wetting properties (e.g. anti-adhesive,

Anti-graffiting, anti-fouling, lotus-effect, self

hydrophobic, hydrophilic).

cleaning surface for textiles and ceramics, etc.

Corrosion protection for machinery and

Thermal and chemical properties (e.g.

heat resistance and insulation, corrosion equipment, heat resistance for turbines and

engines, thermal insulation equipment and

resistance).

building materials etc.

Biological properties (biocompatibility,

Biocompatible implants, medical tools and

anti-infective).

wound dressings etc.

Electronical and magnetic properties

Ultra-thin dielectrics for field-effect transistors,

( e.g. magnetoresistance, dielectric)

magneto-resistive sensors and data memory etc.

Optical properties (e.g. anti-reflection,

Photo and electro-chromic windows, anti-

photo and electro-chromatic)

reflective screens and solar cells etc.

1.2 Nanotechnology:

Nanotechnology is one of the key technologies of the 21st century. Nanotechnology refers

broadly to a field of applied science and technology whose unifying theme is the control

of matter on the atomic and molecular scale as shown in Fig. 9.2, generally 100

nanometers or smaller, and the fabrication of devices with critical dimensions that lie

within this size range.

Fig. 9.2. Schematic representation for atom and molecule

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Newer Synthetic Strategies for Nanomaterials

1.3 Nanomaterials:

It is the study of how materials behave when their dimensions are reduced to the

nanoscale. It can also refer to the materials themselves that are used in nanotechnology.

Unique changes occur in the electronic structure and chemical properties of solids when

their dimensions are reduced to the nanoscale, including development of discrete

electronic structure, quantum confinement, structural strain and distortion, and altered

interfacial/surface reactivity. These changes translate into new physical and chemical

behavior which is not observed in the `bulk' form of the material.

Given that processes at the gas/solid and liquid/solid interfaces are largely controlled

by local phenomena (e.g., the availability, reactivity and density of surface sites), changes

in the local electronic and physical structures of a solid brought about by

`nanostructuring' which should have a tremendous impact on heterogeneous chemistry

(catalysis). Examples: Au for selective oxidation, arene and alkene hydrogenation by Ir,

Rh, photocatalytic activity of TiO2 and unique reactivity of oxide nanoparticles.

2. NEWER SYNTHETIC STRATEGIES TO BUILD PROTEIN

BASED NANOMATERIALS:

Recent progress in nanotechnology has yielded new device components with

unprecedented capabilities. However, the small size of these building blocks makes it

difficult to position them into functional assemblies using existing patterning techniques.

(a)

(b)

Fig. 9.3 (a) Arrys of chromophores for light harvesting and (b) Spherical carriers for

PET imaging applications

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9.5

3. SYNTHESIS AND CHARACTERIZATION OF INORGANIC

NANOMATERIALS

As one solution to this problem, the protein shells of two viruses are converted into

scaffolds that can position nanoscale objects with excellent spatial resolution. In one case,

this strategy has been used to synthesize arrays of fluorescent molecules, providing

efficient mimics of the light harvesting system present in photosynthetic organisms. In a

second research area, well-defined core/shell materials have been prepared for

applications in diagnostic imaging. The cornerstone of these efforts has been a series of

new synthetic reactions that can modify biomolecules (Fig. 9.3) with high site-selectivity

and yield. The synthetic route will focus on the development of the methods and the

applications of the new materials that have been built through their use.

The study of nanoscale inorganic materials is an exciting and rapidly growing area of

research which offers multiple opportunities for innovation and creativity. Our research

focuses primarily on the development of new synthetic approaches to nanoparticles with

tunable properties. One aspect of our work is synthesis of new precursors as well as the

application of established molecular precursors to synthesize nanometer size metal alloy,

metal phosphide, metal oxide, and metal chalcogenide materials. Some new wet-

chemistry procedures to produce size- and morphology-controlled nanoparticles are:

Single-source precursors to heterometallic oxide materials

Single-source precursors to phosphide nanoparticles

Single-source precursors to metal chalcogenide materials

Synthesis of nanoalloys from organometallic precursors

Shape control syntheses of iron and manganese oxides

3.1 Single-Source Precursors to Heterometallic Oxide Materials

The development of efficient photocatalytic systems has been of vital interest since these

can contribute to the reduction of pollution and solve some energy-related problems.

However the synthesis of environmentally-friendly binary oxides of desired morphology

still remains a challenge. In the synthesis described so far, the problems encountered

were phase inhomogenities and the inclusion of carbon originating from the ligands.

Developing new heterometallic (single-source) precursors to synthesize AMO3 (A = Li,

Na, K, Rb; M = Nb, Ta) and MBiO4 (M=V, Nb, Ta) nanoparticles, which are known as

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Newer Synthetic Strategies for Nanomaterials

stable and efficient photocatalysts. The precursors to be synthesized are composed of

salicylate and alkoxide complexes that contain the metal elements in the desired ratios. A

number of heterometallic complexes were synthesized (Fig. 9.4) and their utility as

single-source precursors was tested. Pyrolysis and wet-chemistry routes were

successfully used to produce mixed oxide of micron and nano-sized particles.

Fig. 9.4 Bi2Ta2(sal)4(Hsal)4(OEt)2 (left) and SEM image hydrolysis product (right)

(sal = O2CC6H4O, Hsal = O2CC6H4OH) [12]

3.2 Single-Source Precursors to Phosphide Nanoparticles

Magnetic materials can be made in a variety of forms including nanoparticles, thin

molecular films, and bulk materials.

Fig. 9.5. Iron phosphide nanorods [13]

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9.7

Improving the current technologies that utilize magnetic compounds depends upon the

ability to produce pure materials; of concern in the synthesis of these materials is the

ability to produce materials with a controlled stoichiometry. While most studies of

magnetic nanoparticles have been on magnetic metals, alloys, and oxides, the focus of

this research is on binary magnetic materials composed of transition metals and p-block

elements.

3.3 Single-Source Precursors to Metal Chalcogenide Materials

The initial research will involve the synthesis of iron phosphide nanomaterials from

known heterometallic precursors; there are a few binary phases of iron phosphide that

possess magnetic properties (FeP possesses antiferromagnetic behavior, while Fe3P and

Fe2P exhibit ferromagnetic properties). Fig. 9.5 shows iron phosphide nanorods

synthesized, utilizing a heterometallic iron-phosphorus cluster.

50 nm

Fig. 9.6. Nanoparticles of PbS and Bi2S3 [14]

The

design of chalcogenide materials in discrete forms is a major problem for modern

solid-state chemists and materials scientists. Many of these chalcogenide materials have

potential applications in electronic devices. It is mainly focused on the design, syntheses,

and decomposition of new molecular precursors to such chalcogenide materials in forms,

such as rods, wires, and more complex shapes that have been unattainable with

conventional solid-state methods. Developing the chemistry of metal complexes with

chalcogen containing ligands as molecular precursors to a number of metal chalcogenide

materials, including Bi2S3, Bi2Se3, PbS, PbSe, CdS, CdSe (Fig. 9.6). A major emphasis

of the synthesis is to control the organization of the materials at the nanoscale and to find

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Newer Synthetic Strategies for Nanomaterials

a relationship between the nature of the precursors and the quality of the nanomaterials

obtained. To accomplish this, an interdisciplinary approach, with experience in synthetic

and materials chemistry, as well as crystallography, will provide a plethora of

opportunities to investigate these interesting problems in nanoscience. Various methods

are being developed to synthesize, re shape and study the assembly characteristics of

chalcogenide metal nanoparticles.

3.4 Synthesis of Nanoalloys from Organometallic Precursors

Another research in the area of nanoscale materials concerns the synthesis of alloy and

intermetallic nanoparticles from the corresponding organometallic precursors. The

preparation of a series of Bi-Ru, Bi-Pd, and Bi-Pt intermetallic nanocrystals using wet-

chemistry procedures has been achieved (Fig. 9.7). These methods are based on the

relatively low-temperature reaction between two organometallic precursors in the

presence or absence of stabilizing agents. The most advantageous points of these

procedures consist in overcoming the need of chemical reducing agents and high

temperature decomposition. In addition, regular spherical agglomerates are formed and

their size could be tuned by varying temperature and surfactants. The efficiency of this

procedure is shown by the ability of the particles to be dried and redispersed, while

maintaining their morphology. These nano-alloys have recently attracted attention

because of their activity and selectivity toward numerous catalytic reactions such as

dehalogenation, oxidation of alcohols and aldehydes, as well as for applications in fuel

cells.

Fig. 9.7 Bi-Pd nanoparticles prepared at two different temperatures [13]

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9.9

3.5 Shape Control Synthesis of Iron and Manganese oxides

Bulk iron oxides and manganese oxides have many technical applications due to their

magnetic and catalytic properties. These properties can be enhanced by tuning particle

sizes within the nanometer scale. Magnetic nanoparticles (NPs), for example, have

potential applications in information storage, medical imaging, drug delivery, and water

remediation. Many applications, such as heterogeneous catalysis, are enhanced by the

high surface area that nanoparticles possess. However, enhancement of physical

properties is not only limited to size effects. There are a number of publications on

nanoparticles with shape-dependent magnetic, electronic, and optical properties. Most of

these studies are on NPs such as spheres, rods, wires, belts, and disks, which can be

classified as `traditional' shapes. The synthesis, the physical properties, and the growth

mechanisms of `traditional' shapes, including iron and manganese oxides, have been

widely investigated and are well-understood. However, the formation mechanism and

physical properties of non-traditional multi-branched NPs such as tetrapods, hexapods,

tripods, stars, and dumbbells, are not completely understood and have led to

investigations on shape controlled synthesis and to the development of non-traditional

growth mechanism.

Fig. 9.8. MnO (A and B) and FeO (C andD) NPs [13]

The synthesis of new cross-shaped, hexapod, and concave-faces cubic nanoparticles

(NPs) of MnO, FeO, and Fe1-xMnxO have been reported. The main goals of these studies

are :

1. To develop simple and inexpensive synthesis to grow nanoparticles with new shapes.

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Newer Synthetic Strategies for Nanomaterials

2. To study the growth mechanisms by observation of the nanoparticle morphologies as

a function of the reaction parameters: surfactant ratio, water concentration, time,

temperature, and precursor.

Some of the morphologies obtained are shown in Fig. 9.8

A motivation in nanoscience is to try to understand how materials behave when

sample sizes are close to atomic dimensions. Fig. 9.8 for example shows a picture of

nanofibrils that are 10 to 100 times smaller in diameter than conventional textile fibers. In

comparison to a human hair which is ca. 80,000 nm in diameter, the nanofibers are 1,000

times smaller in diameter. When the characteristic length scale of the microstructure is in

the 1- 100 nm range, it becomes comparable with the critical length scales of physical

phenomena, resulting in the so-called "size and shape effects." This leads to unique

properties and the opportunity to use such nanostructured materials in novel applications

and devices. Phenomena occurring on this length scale are of interest to physicists,

chemists, biologists, electrical and mechanical engineers, and computer scientists,

making research in nanotechnology a frontier activity in materials science.

Fig. 9.9 A picture of nanofibrils shown with a human hair for reference [17]

On tracking the nano evolution, it has been stated that no matter what the market

outcomes in the near or long term, nanoscience will never be an industry unto itself but a

science of many avenues of application, and possibility that could redefine the direction

of several industries. This insight allows one to recognize that nanotechnology is not "a

Synthetic Strategies in Chemistry

9.11

technology" but "a set of technologies," yielding a set of technical breakthroughs that

will sweep into many different markets. Within such a framework, the world of

nanotechnology may be divided into three broad categories: nanostructured materials,

nanotools, and nanodevices.

4. Nanotools

These include fabrication techniques; analysis and metrology instruments; and software

for nanotechnology research and development. They are used in lithography, chemical

vapor deposition (CVD), 3-D printing, and nanofluidics. Nanofluidics, the study of

nanoscale fluid behavior, for example, the study of dynamics of droplets adsorbed onto

surfaces under shearing, is mostly used in areas such as medical diagnostics and

biosensors.

5. Nanostructured Materials

Nanocrystalline Materials

Fullerenes/ Carbon Nanotubes

Dendrimers (Organic Nanoparticles)

Polyhedral Silsesquioxanes (Inorganic-Organic Hybrid Nanoparticles)

Nano-Intermediates

Nanocomposites

Nanostructured (NsM) materials are materials with a microstructure the characteristic

length scale of which is on the order of a few (typically 1-100) nanometers. The

microstructure refers to the chemical composition, the arrangement of the atoms, and the

size of a solid in one, two, or three dimensions. Effects controlling the properties of

nanostructured materials include size effects (where critical length scales of physical

phenomena become comparable with the characteristic size of the building blocks of the

microstructure), changes of the dimensionality of the system, changes of the atomic

structure, and alloying of components (e.g., elements) that are not miscible in the solid

and/or the molten state.

The synthesis, characterization and processing of nanostructured materials are part of

an emerging and rapidly growing field. Research and developement in this field

emphasizes scientific discoveries in the generation of materials with controlled

microstructural characteristics, research on their processing into bulk materials with

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Newer Synthetic Strategies for Nanomaterials

engineered properties and technological functions, and introduction of new device

concepts and manufacturing methods.

Nanostructured materials may be grouped under nanoparticles (the building blocks),

nano-intermediates, and nanocomposites. They may be in or far away from

thermodynamic equilibrium. For example, nanostructured materials consisting of

nanometer-sized crystallites of Au or NaCl with different crystallographic orientations

and/or chemical compositions vary greatly from their thermodynamic equilibrium.

Nanostructured

materials

synthesized

supramolecular

chemistry

yielding

nanoassemblies are examples of those in thermodynamic equilibrium. In the subsequent

paragraphs, the various classes of nanoparticles that serve as the building blocks of

nanomaterials and devices will be discussed. They include nanocrystalline materials such

as ceramic, metal and metal oxide nanoparticles; fullerenes, nanotubes and related

structures; nanofibers and wires, and precise organic as well as hybrid organic-inorganic

nanoarchitechtures such as dendrimers and polyhedral silsesquioxanes, respectively.

5.1 Nanocrystalline Materials

Ceramics, metals, and metal oxide nanoparticles fall in this category. In the last two

decades a class of materials with a nanometer-sized microstructure have been

synthesized and studied. These materials are assembled from nanometer-sized building

blocks, mostly crystallites. The building blocks may differ in their atomic structure,

crystallographic orientation, or chemical composition. In cases where the building

blocks are crystallites, incoherent or coherent interfaces may be formed between them,

depending on the atomic structure, the crystallographic orientation, and the chemical

composition of adjacent crystallites. In other words, materials assembled of nanometer-

sized building blocks are microstructurally heterogeneous, consisting of the building

blocks (e.g. crystallites) and the regions between adjacent building blocks (e.g. grain

boundaries). It is this inherently heterogeneous structure on a nanometer scale that is

crucial for many of their properties and distinguishes them from glasses, gels that are

microstructurally homogeneous.

Grain boundaries make up a major portion of the material at nanoscales, and strongly

affect properties and processing. The properties of NsM deviate from those of single

crystals (or coarsegrained polycrystals) and glasses with the same average chemical

Synthetic Strategies in Chemistry

9.13

composition. This deviation results from the reduced size and dimensionality of the

nanometer-sized crystallites as well as from the numerous interfaces between adjacent

crystallites. An attempt is made to summarize the basic physical concepts and the

microstructural features of equilibrium and non-equilibrium NsM. Nanocrystallites of

bulk inorganic solids have been shown to exhibit size dependent properties, such as

lower melting points, higher energy gaps, and nonthermodynamic structures. In

comparison to macro-scale powders, increased ductility has been observed in

nanopowders of metal alloys. In addition, quantum effects from boundary values

become significant leading to such phenomena as quantum dots lasers.

One of the primary applications of metals in chemistry is their use as heterogeneous

catalysts in a variety of reactions. In general, heterogeneous catalyst activity is surface

dependent. Due to their vastly increased surface area over macro-scale materials,

nanometals and oxides are ultra-high activity catalysts. They are also used as desirable

starting materials for a variety of reactions. Nanometals and oxides are also widely used

in the formation of nanocomposites. Aside from their synthetic utility, they have many

useful and unique magnetic, electric, and optical properties.

5.2 a. Fullerenes:

The discovery of fullerenes in 1985 by Curl, Kroto, and Smalley culminated in their

Nobel Prize in 1996. Fullerenes, or Buckminsterfullerenes, are named after Buckminster

Fuller the architect and designer of the geodesic dome and are sometimes called bucky

balls. The names derive from the basic shape that defines fullerenes; an elongated sphere

of carbon atoms formed by interconnecting six-member rings and twelve isolated five-

member rings forming hexagonal and pentagonal faces. The first isolated and

characterized fullerene, C60, contains 20 hexagonal faces and 12 pentagonal faces just

like a soccer ball and possesses perfect icosahedral symmetry.

Fullerene chemistry continues to be an exciting field generating many articles with

promising new applications every year. Magnetic nanoparticles (nanomagnetic

materials) show great potential for high-density magnetic storage media. Recent work

has shown that C60 dispersed into ferromagnetic materials such as iron, cobalt, or cobalt

iron alloy can form thin films with promising magnetic properties. A number of

organometallic-fullerene compounds have recently been synthesized. Of particular note

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Newer Synthetic Strategies for Nanomaterials

are a ferrocene-like C60 derivative and pair of fullerenes bridged by a rhodium cluster.

Some fullerene derivatives even exhibit superconducting character. There has been a

report of a fullerene containing, superconducting field-effect device with a Tc as high as

117 K.

5.2 b. Carbon Nanotubes:

Carbon nanotubes (CNTs) are hollow cylinders of carbon atoms. Their appearance is

that of rolled tubes of graphite such that their walls are hexagonal carbon rings and are

often formed in large bundles. The ends of CNTs are domed structures of six-membered

rings capped by a five-membered ring. Generally speaking, there are two types of CNTs:

single-walled carbon nanotubes (SWNTs) and multi-walled carbon nanotubes

(MWNTs) (Fig.10). As their names imply, SWNTs consist of a single, cylindrical

graphene layer, where as MWNTs consist of multiple graphene layers telescoped about

one another. Carbon nanotubes (CNTs) were first isolated and characterized by Ijima in

1991. Since then numerous research articles have been published, and new applications

for CNTs have been proposed every year. The unique physical and chemical properties

of CNTs, such as structural rigidity and flexibility continue to generate considerable

interest. Additionally, CNTs are extremely strong, about 100 times stronger (stress

resistant) than steel at one-sixth the weight. CNTs can also act as either conductors or

semiconductors depending on their chirality, possess an intrinsic superconductivity, are

ideal thermal conductors, and can also behave as field emitters.

Fig. 9.10. Carbon nanotubes

Synthetic Strategies in Chemistry

9.15

Currently, the physical properties are still being discovered and disputed. What makes it

so difficult is that nanotubes have a very broad range of electonic, thermal, and

structural properties that change depending on the different kinds of nanotube (defined

by its diameter, length, and chirality, or twist). To make things more interesting, besides

having a single cylindrical wall (SWNTs), nanotubes can have multiple walls

(MWNTs)--cylinders inside the other cylinders.

(i) Properties of Carbon Nanotubes: With graphene tubes parallel to the filament axis,

nanotubes would inherit several important properties of `intra-plane' graphite. This

imparts a very unique combination of properties on this material, namely:

High aspect ratio structures with diameters in nanometers, lengths in

microns

High mechanical strength (tensile strength 60GPa) and modulus (Young's

modulus 1TPa)

-- High electrical conductivity (10-6 ohm m typically), and for well

crystallized nanotubes ballistic transport is observed

High thermal conductivity (1750-58 00 W/mK)

-- Being covalently bonded, as electrical conductors they do not suffer

from electro-migration

or atomic diffusion and thus can carry high

current densities (107 -109 A/cm2 )

Single wall nanotubes can be metallic or semi-conducting

Chemically inert, not attacked by strong acids or alkali

Collectively, nanotubes can exhibit extremely high surface area

(ii) CarbonNnanotubes Developed by Arry Group

Arry has done a lot of research and development work in the carbon nanotubes synthesis

and application, and developed a scalable CVD method to produce high purity single-

walled carbon nanotubes (SWNTs) and multi-walled carbon nanotubes (MWNTs) with

various diameters and narrow diameter distribution. The technology for producing 5kg

20-40 nm MWNTs per day was evaluated as an advanced technology in the world by

CAS scientists. They can produce 1.5kg SWCNTs per day and 10kg MWNTs with 8-

15nm in diameter per day. The production facilities can be enlarged easily. In Fig.11, the

proportion of usage as conceived by Array group is shown.

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Newer Synthetic Strategies for Nanomaterials

Fig. 9.11. Illustration given by Array group [15]

Nanotechnology develops the basis for increasingly smaller data memories with

increasingly larger storage capacity, for highly efficient filters for sewage treatment, for

photovoltaic windows, for materials which can be used to build ultra-light engines and

body parts in the automobile industry or for artificial joints which are better tolerated by

the human body due to organic nano-surfaces. In the future, as nanoscale molecular self-

assembly becomes a commercial reality, nanotech will move into conventional

manufacturing. While nanotechnology offers opportunities for society, it also involves

profound social and environmental risks, not only because it is an enabling technology

to the biotech industry, but also because it involves atomic manipulation and will make

possible the fusing of the biological world with the mechanical world. There is a critical

need to evaluate the social implications of all nanotechnologies; in the meantime, the

Arry Group believes that a moratorium should be placed on research involving

molecular self-assembly and self-replication.

(iii) Carbon Nanotube-Based Nanodevices

Carbon nanotubes are a hot research area at the moment. The excitement has been fueled

by experimental breakthroughs that have led to realistic possibilities of using them

commercially. Applications could include field emission-based flat panel displays, novel

semiconducting devices, chemical sensors, and ultra-sensitive electromechanical

sensors. The utility of carbon nanotubes for molecular electronics or computers, first

predicted by theory and simulations, is now being explored through experiments to

Synthetic Strategies in Chemistry

9.17

fabricate and conceptualize new devices based on simulations. Carbon nanotubes are

now the top candidate to replace silicon when current chip features cannot be made any

smaller in 10-15 year's time. Calculations show that nanotubes can have metallic or

variable semiconducting properties with energy gaps ranging from a few meV to a few

tenths of an eV. Experiments probing the density of states confirm these predictions.

Conductivity measurements on single nanotubes have shown rectification effects for

some nanotubes and ohmic conductance for others. These properties suggest that

nanotubes could lead to a new generation of electronic devices. Simulations to

investigate the interaction of water molecules with a nanotube tip revealed an atomistic

understanding of the interaction, which is critical in designing commercial-quality flat

panel displays around carbon nanotubes. Their use as ultra-sensitive electromechanical

sensors has also been explored.

5.3 Dendrimers (Organic Nanoparticles)

In recent years, a new structural class of macromolecules, the dendritic polymers, has

attracted the attention of the scientific community. These nanometer sized, polymeric

systems are hyperbranched materials having compact hydrodynamic volumes in solution

and high, surface, functional group content. They may be water-soluble but, because of

their compact dimensions, they do not have the usual rheological thickening properties

that many polymers have in solution. Dendrimers, the most regular members of the

class, are synthesized by step-wise convergent or divergent methods to give distinct

stages or generations. Dendrimers are defined by their three components: a central core,

an interior dendritic structure (the branches), and an exterior surface (the end groups).

Over 50 compositionally different families of these nanoscale macromolecules, with

over 200 end-group modifications, have been reported.They are characterized by nearly

spherical structures, nanometer sizes, large numbers of reactive end group

functionalities, shielded interior voids, and low systemic toxicity. This unique

combination of properties makes them ideal candidates for nanotechnology applications

in both biological and materials sciences. The state of reports in the current literature has

been directed toward their applications in a broad range of fields, including materials

engineering, industrial, pharmaceutical, and biomedical applications. Specifically,

nanoscale catalysts, novel lithographic materials, rheology modifiers, and targeted drug

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Newer Synthetic Strategies for Nanomaterials

delivery systems, MRI contrast agents, and bioadhesives represent some of the potential

applications.

5.4 Polyhedral Silsesquioxanes (Inorganic-Organic Hybrid Nanoparticles)

Hybrid inorganic-organic composites are an emerging class of new materials that

hold significant promise. Materials are being designed with the good physical properties

of ceramics and the excellent choice of functional group chemical reactivity associated

with organic chemistry. New silicon-containing organic polymers, in general, and

polysilsesquioxanes, in particular, have generated a great deal of interest because of

their potential replacement for and compatibility with currently employed, silicon-based

inorganics in the electronics, photonics, and other materials technologies. Hydrolytic

condensation of trifunctional silanes yields network polymers or polyhedral clusters.

Hence they are known by the "not quite on the tip of the tongue" name silsesquioxanes.

Each silicon atom is bound to an average of one and a half (sesqui) oxygen atoms and to

one hydrocarbon group. Typical functional groups that may be hydrolyzed or condensed

include alkoxy- or chlorosilanes, silanols, and silanolates. Synthetic methodologies that

combine pH control of hydrolysis/condensation kinetics, surfactant-mediated polymer

growth, and molecular templating mechanisms have been employed to control molecular

scale regularity as well as external morphology in the resulting inorganic/organic

hybrids (from transparent nanocomposites, to mesoporous networks, to highly porous

and periodic organosilica crystallites) all of which have the silsesquioxane

stoichiometry. These inorganic-organic hybrids offer a unique set of physical, chemical,

and size dependent properties that could not be realized from just ceramics or organic

polymers alone. Silsesquioxanes are therefore depicted as bridging the property space

between these two component classes of materials. Many of these silsesquioxane hybrid

materials also exhibit an enhancement in properties such as solubility, thermal and

thermomechanical

stability,

mechanical

toughness,

optical

transparency,

gas

permeability, dielectric constant, and fire retardancy, to name just a few.

5.5 Nano-Intermediates

Nanostructured films, dispersions, high surface area materials, and supramolecular

assemblies are the high utility intermediates to many products with improved properties

such as solar cells and batteries, sensors, catalysts, coatings, and drug delivery systems.

Synthetic Strategies in Chemistry

9.19

They have been fabricated using various techniques. Nanoparticles are obvious building

blocks of nanosystems but, require special techniques such as self-assembly to properly

align the nanoparticles. Recent developments have lead to air resistant, room

temperature systems for nanotemplates with features as small as 67 nm. More

traditionally, electron-beam systems are used to fabricate devices down to 40 nm.

5.6 Nanocomposites

Nanocomposites are materials with a nanoscale structure that improve the macroscopic

properties of products. Typically, nanocomposites are clay, polymer or carbon, or a

combination of these materials with nanoparticle building blocks. Nanocomposites,

materials with nanoscale separation of phases can generally be divided into two types:

multilayer structures and inorganic/organic composites. Multilayer structures are

typically formed by gas phase deposition or from the self-assembly of monolayers.

Inorganic/organic composites can be formed by sol-gel techniques, bridging between

clusters (as in silsequioxanes), or by coating nanoparticles, in polymer layers for

example. Nanocomposites can greatly enhance the properties of materials. For example,

ppm level impurities can result in the formation of nanoscale aluminide secondary

phases in aluminum alloys, increasing their strength and corrosion resistance. Magnetic

multilayered materials are one of the most important aspects of nanocomposites as they

have led to significant advances in storage media.

5.6 a. Polymer-Clay Nanocomposites

The large industrial demand for polymers has lead to an equally large interest in

polymer composites to enhance their properties. Clay-polymer nanocomposites are

among the most successful nanotechnological materials today. This is because they can

simultaneously improve material properties without significant tradeoffs. Recent efforts

have focused upon polymer-layered silica nanocomposites and other polymer/clay

composites. These materials have improved mechanical properties without the large

loading require by traditional particulate fillers. Increased mechanical stability in

polymer-clay nanocomposites also contributes to an increased heat deflection

temperature. These composites have a large reduction gas and liquid permeability and

solvent uptake. Traditional polymer composites often have a marked reduction in optical

clarity; however, nanoparticles cause little scattering in the optical spectrum and very

9.20

Newer Synthetic Strategies for Nanomaterials

little UV scattering.Although flame retardant additives to polymers typically reduce

their mechanical properties, polymer-clay nanocomposites have enhanced barrier and

mechanical properties and are less flammable. Compression-injection molding, melt-

intercalation, and coextrusion of the polymer with ceramic nanopowders can form

nanocomposites. Often no solvent or mechanical shear is needed to promote

intercalation.

6. Novel Materials at the Nanoscale Functional Nanomaterials

Morphology-controlled functional nanomaterials have unique chemical, mechanical,

electrical, optical, magnetic or biological properties that are distinctly different form their

macroscopic

analogs

and

provide

new

diverse

opportunities

for

promising

nanotechnologies.

Fig. 9.12 Quantum dots: Colour tuning [2]

6.1 Nanomaterials Size and Composition-Tunable Quantum DotsConducting

Development of a scalable synthetic strategy is necessary to address the quantum dots'

colour-tuning and stability issues. Alloyed ZnCdS/Se nanocrystals have unique

composition-tunable optical properties including:-High luminescence/stability -Increased

narrow luminescence spectral-width. These size and composition-tunable quantum dots

have applications in the optoelectronic and biomedical sectors. (Fig 9.12)

Synthetic Strategies in Chemistry

9.21

7. ZnO/TiO2 Nanorods or Nanoarrays and Silica-Coated Metal Nanocrystals

Fig. 9.13. Nanoarrays of ZnO/TiO2 [16]

Researchers have focused on large-scale growth of well-aligned ZnO nanorods on

selected substrates. Their applications are short-wavelength optoelectronic devices, solar

energy conversion, transparent conducting coating materials and sensors. Researchers try

to find other functional nanomaterials with diverse morphologies such as titania nanorods

and silica coated nano crystals.(Fig. 9.13)

8. Polyhedral Oligomeric Silsesquioxanes (POSS)

Fig. 9.14. POSS Material

9.22

Newer Synthetic Strategies for Nanomaterials

Unique nanostructured material, hybrid inorganic and organic compositions at nanoscale,

development and characterization of POSS-modified epoxy and POSS (Fig. 9.14)

containing high performance polymers were synthesized to enhance its thermal and

mechanical properties.

Advantages:

1. Numerous potential applications such as microelectronics, photonics, aerospace, and

coatings.

2. POSS materials are found to be compatible with mostly all thermoplastics and

thermosets.

3. POSS materials possess size controllable, processable and tunable properties.

8.a Conducting Polymer Nanofibers via Electrospinning :

The major work in this area is to develop a novel methodology for synthesising

conducting polymers with high molecular weight and excellent solubility. It is also

necessary to fabricate conducting polymer nanofibers via electrospinning process.

Investigating size/quantum confinement effect on the optical, electrochemical and

conducting properties of the nanofibers and finally to explore the possible applications of

the conducting polymer nanofibers as OLED semissive layer, sensors and molecular

electronics.

CONCLUSION:

The impact of understanding self-organizing behavior, and of finding ways to further

direct assembly to make exotic nanoscale properties useful at the macroscale, clearly will

be enormous. There are general rules of controlled synthesis and directed assembly to be

discovered, and the systematic application of these will result in the addition of many

different nanostructured materials. Each and every success in the synthesis of

nanomaterials will make available a new subset of engineering materials, and it is well

known from centuries of experience that the discovery and development of new synthetic

stratergies for nanomaterials always have been the source of new technology.

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Synthetic Strategies in Chemistry

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