NANOMATERIALS, THEIR PROPERTIES, SYNTHESIS AND APPLICATION

NANOMATERIALS, THEIR PROPERTIES, SYNTHESIS AND APPLICATION

1    INTRODUCTION TO NANOMATERIALS

Nanomaterials are those materials in which one dimension is atleast less than 100 nanometers. The diameter of human hair is 100,000 times larger than a nanopartical. Because of their size nanomaterials have unique magnetic, optical, electrical and other properties. Due these properties nanomaterials have potential applications in medicine, electronics and other fields.

Fig.1: Nanomaterials ( carbon nanotubes)

•  Evolution of science and nanomaterials

Nanotechnology and nano-science are dependent upon nanoparticals. Nanotechnology and nano- science has been evolved as a vast area of development and research activity. It can revolutionize the methods of creating products and materials and broad range of functionalities can be retrieved. It has significant commercial value which will be increased in future.

3.      Classification of Nanomaterials:

The size of nanomaterials is extremely small with at least one dimension of 100 nm or smaller. Common types of nanomaterials are

Ø  Zero Dimensional eg. gold, palladium with size 1-50nm

Ø  One Dimensional eg. surface films.

Ø  Two Dimensional eg. strands or fibres

Ø  Three Dimensional eg. particles

Nanomaterials can exist as nanotubes, quantum dots, dendrimers and fullerenes.

Nanomaterials have been characterized by an ultra fine grain size to 50 nm. Nanomaterials can be adopted into different modulations which are defined by Richard W. Siegel as zero dimension (cluster assemblies and filaments, atomic clusters), one dimension (multilayers), two dimension (buried layers or ultrafine-grained overlayers), and three dimension (equiaxed nanometer sized grains)

4.     Causes of so much interest in nanomaterials

Nanomaterials have got significant importance in recent years because of their extra-ordinary electrical, mechanical, magnetic and optical properties. Some useful examples are as follows:

Ø  Nanophase ceramics are of special interest due to their extended ductility at elevated temperatures in comparison to the coarse-grained ceramics.

Ø  Different non-linear optical properties have been achieved by using nanostructured semiconductor. Quantum confinement effects have also been achieved by using nanostructured semiconductors which may results special properties, like the luminescence effect in silicon powders and  infrared optoelectronic devices by using silicon germanium quantum dots . Nanostructured semiconductors also have applications in solar cells as window layers.

Ø  Various gas tight materials, porous coatings and dense parts have been produced by using nanosized metallic powders.

Ø  Magnetic nanocomposites are now widely using for mechanical

force transfer (ferrofluids), magnetic refrigeration and for high density information storage.

Ø  Nanostructured metal clusters particular impact in catalytic applications. They have also been used as precursors for novel type of heterogeneous catalysts and also offer substantial utilization including selectivity, activity and lifetime in electrocatalysis and chemical transformations. Nanoscale metal particles having chiral modifiers on their surface have been used in Enantioselective catalysis.

Ø   Nanostructured metal-oxide thin films have been received much attention for the realization of gas sensors (NOx, CO2, CO, CH4 and aromatic hydrocarbons) with greater selectivity and sensitivity. Nanostructured metal-oxide eg. MnO have found uses for rechargeable batteries for consumer goods and cars.

Ø   Polymer based composites with a major percentage of inorganic particles having a high dielectric constant are important materials for photonic band gap structure.

5.      SYNTHESIS AND PROCESSING OF NANOMATERIAL

Two basic synthetic approaches can be used for naomaterial synthesis, i.e. either

Ø  top down approach’ in which bulk solid is dissociate into smaller particals until their constituents of only a few aroms.

Ø  bottom up approach’ in which atoms are assembled together.

Fig.2: Schematic representation of synthetic methods of nanomaterials

It is a field of interdisciplinary work confining chemistry, physics and engineering including medicine 

6.     Methods of creating nanostructures:

There are many various methods  of synthesizing nanostructures. Some of them are given below:

Mechanical grinding:

Mechanical grinding is done by ‘top down’ method of creating nanomaterials. In this method coarser-grained structure is decomposed by severe plastic deformation. This is one of the best method for creating nanocrystalline materials and it is relatively simple and inexpensive method and materials of all classes can be synthesized by this method The major advantage often quoted is the possibility for easily scaling up to tonnage quantities of

material for various applications. But it has two major problems

1. Contamination may occur from milling media and atmosphere

2. Powder consolidation may occur without coarsening microcrystalline material

Therefore this method is dismissed for some material due to above mentioned problems.

Fig.3: Schematic illustration of principle of mechanical milling

 Wet Chemical Synthesis of Nanomaterials

In principle the wet chemical synthesis of nanomaterials can be classified into two major groups:

1. The top down approach: where single crystals are chiseled in an aqueous solution for creating nanomaterials, For example, electrochemical etching is used for synthesizing porous silicon.

2. The bottom up approach: which consists of precipitation, sol-gel method etc. where a colloidal solution is formed by mixing the materials of desired precursors in a controlled fashion.

 Sol-gel process

The sol-gel method forms the basis of evolution of inorganic network by forming colloidal suspension (sol) and by forming continuous liquid phase (gel) through gelation of sol. Usually metals or metalloids having reactive ligands are used as precursors to synthesize these colloids. Dispersible oxide is formed by processing the starting material and then forming the sol in water or dilute acid. Gel is formed by removal of water from sol and the particle shape and size is controlled by sol/gel transition. The oxide is produced by calcination of sol.

Sol-gel processing involves the hydrolysis and condensing the alkoxide-based precursors eg. tetraethyl orthosilicate (TEOS). The reaction is as fellows:

H₂O + MOR  → ROH + MOH (hydrolysis)

ROM + MOH → ROH + M-O-M (condensation)

where MOR refers to metal alkoxide

Sol-gel method of creating nanomaterials is much popular among chemists and is extensively employed to synthesize oxide materials. The sol-gel method can be descibed by a series of steps.

Fig.4: Schematic illustration of sol-gel process

1. Preparation of various rational solutions of the solvated metal precursor or alkoxide.

2. Gelation results from the plycondensation reaction of an oxide- or alcohol- bridged network (gel) resulting in a considerable increase in the solution’s viscosity.

3. The next step is aging of the gel referred as Syneresis which is done by polycondensation reactions and continue for gel transforms into a solid mass, followed by contracting the gel network and exclusion of solvent from gel pores. Ostwald ripening and phase transformations may happen contemporary with syneresis. The process of gel aging may occur in 7 days or more and is essential to prevent the cracks in gels.

4. Aging is followed by drying of the gel, which is done by removing the water and volatile liquids from the gel network. This process consists of four definite steps: (i) the rate period, (ii) the hypercritical point, (iii) the first falling rate period, (iv) the second falling rate period.

5. Dehydration, which is followed by removal of surface- bound M-OH groups and preventing the gel from rehydration. This is done by calcination of monolith accompanied at temperatures exceed from 8000C.

6. The last step is decomposition and densification of the gels accompanied at high temperatures exceed from 8000C

The main advantage of this process is preparing the non-metallic inorganic materials like ceramic materials, glasses, glass ceramics at very low temperature in comparison to high temperature process which involves the firing ceramics or melting glass.

 Flame assisted ultrasonic spray pyrolysis

This process involves the nebulization of precursors and then flame is used for burning the unwanted components to obtain required material such as ZrO₂ has been prepared by this process using precursor of Zr(CH₂CH₂O)₄

The main idea of  this process is low pressure combustion flame synthesis which extends the pressure range for using in gas phase synthesis and thus minimize the agglomeration. Low pressure flames have been considerably utilized by aerosol scientists for studying the particle formation in the flame.

7.     Properties of Nanomaterials:

Nanomaterials have intermediate structural characteristics between atoms and bulk materials. They exhibit properties significantly different from atom and bulk material. This is mainly due to

Ø  Large surface area to volume ratio

Ø  High surface energy

Ø  Reduced imperfections

Ø  Spatial confinement

Some of the extra-ordinary important properties of nanomaterials are given below:

Optical properties

One of the most interesting and useful features of nanomaterials is their optical properties. The important applications of optical properties of nanomaterials consists of optical detector, sensor, laser, maging, display, phosphor, solar cell, photoelectrochemistry, photocatalysis and biomedicine.

Their optical properties mainly based on features such as size, surface, shape, and other different characteristics which includes interaction and doping with the surrounding environment or nanostructures. Similarly, shape can have considerable influence on optical characteristics of metal nanostructures. Fig. () differentiates between the optical properties of semiconductor nanoparticles and metals. A simple change in size of CdSe semiconductor nanomaterial can alter the opticals properties of nanomaterials. The optical properties of nanomaterials can be changed dramatically by adding an anisotropy to the nanomaterials such as growth of nanorods.

 Electrical Properties

Electrical properties of nanomaterials include electrical conductivity in nanorods and nantubes, photoconductivity of nanorods, carbon nanotubes, electrical conductivity of nanocomposites. One fascinating method which can be adopted to exhibit the steps in conductance is measurement of the electrical current at a constant applied voltage and the mechanical thinning of a nanowire.

   Mechanical Properties

“Mechanical Properties of Nanomaterials” deals with ceramic and bulk metallic materials, effect of porosity, effect of grain size, filled polymer composites, superplasticity, particlefilled polymers, carbon nanotube-based composites, polymer-based nanocomposites filled with platelets. Because of their mechanical properties, nanomaterials have achieved much industrial importance.

The mechanical properties of nanomaterials can be greatly improved by filling polymers with nanorods or nanoparticles and nanotubes. These properties are significantly depend upon the type of filler and the method of filling. Composite which consists of a polymer matrix and defoliated phyllosilicates show exceptional mechanical and thermal properties.

   Magnetic properties

Some materials are non-magnetic at bulk but becomes magnetic at nano size such as bulk Pt and gold are non-magnetic but becomes magnetic at nano size. The surface atoms of nanomaterials are different from bulk atoms due to modifying by interaction with other chemical specie which is done by capping the nanomaterial.

This fact allows us to alter the physical properties of nanomaterials by capping them with suitable molecules. This phenomenon makes it possible to convert non-ferromagnetic bulk material into ferromagnetic at nano size.

8.     Important Applications of nanomaterials

Nanomaterials having vast range of applications in the field of fuel cells, electronics, batteries, food industry, medicines, and agriculture etc. It is obvious that nanomaterials cleave their conventional counterparts due to their superior physical, chemical and mechanical properties and of their extraordinary formability.

 Fuel cells:

A fuel cell is an electrochemical cell which converts the chemical energy into electrical energy. The working of fuel cell depends upon the electrodes. By modifying the physical structure and by utilizing the more active electro catalyst, the working of fuel cell electrode can be optimized.

 Carbon nanotubes - Microbial fuel cell

Microbial fuel cell is an instrument in which bacteria absorb water-soluble waste such as starch, sugar and alcohols and generate electricity in addition to clean water.

Carbon nanotubes can be used to built microbial fuel cells due to their good mechanical properties, chemical stability and high surface area. Because of high electrode surface area for growth medium and three dimensional architectures, bacteria can easily grow, proliferate and become immobilized. Carbon nanotubes and multi walled carbon nanotubes offers biocompatibility for various eukaryotic cells.

Fig.5: Schematic diagram of microbial fuel cell

       Catalysis

Nanomaterial counterparts have higher surface area, nano-catalysts offers extraordinary surface activity.

Nano-aluminium provides such high reaction rates that it is used as a solid-fuel in rocket propulsion. Furthermore surface activity assists catalyst accelerating or retarding reaction rates and are used in operating rate-controlling steps.

      Phosphors for High-Definition TV

The size of the pixel determine the resolution of  a monitor, a television. The material which is used for the formation of pixels are phosphors which shows fluorescence when a beam of electrons strike with them inside cathode ray tube. By reducing the size of phosphors, the resolution can be improved. The resolution of televisions and monitors can be greatly improved by using nanocrystalline zinc sulphide, cadmium sulphide, zinc sulphide and lead telluride. The utilization of nanophosphors help greatly in reducing the cost of personal computers and high definition televisions.

      Computer Chips of Next-Generation:

The microelectronics industry now emphasizes miniaturization in which size of circuits such as resistors, transistors and capacitors are reduced. However there are many technological limitations in addition to advancements such as poor dissipation of several amount of heat produced by these microprocessors and short mean time to failures etc.. These barriers can be greatly solved by nanomaterials providing high purity material having durable, long-lasting interconnections between dfferent components of microprocessors and having better thermal conductivity.

    Junctionless transistors by nanowires:

The tiny size of transistors reduces the sizes of electronic devices but it is very challenging to alter the doping concentration of a material over distances smaller than 10nm. Now researchers have successfully make the junctionless transistors which have ideal electrical properties. It could comparatively work faster and utilize less power than conventional transistors. The devices are made of a silicon nanowire in which flow of current is controlled by a silicon gate which is partitioned from nanowire by thin insulating layer. The whole silicon nanowire is n-doped, which makes it excellent conductor. The silicon gate is p-doped which can deplete the number of electrons in nanowire region under gate. It operates at faster rate and utilizes less energy and free from current leakage.

Fig.6: Schematic diagram of microbial fuel cell

Elimination of Pollutants

The grain boundaries of nanomaterials are larger than their grain size. So nanomaterials are extremely active in terms of their physical, chemical and mechanical properties. Because of their potential chemical activity, nanomaterials are using as catalysts for reacting with toxic and noxious gases such as oxides of nitrogen and carbon monoxide in catalytic converters of automobiles and other power generation equipments for prevention of environmental pollution.

  Sun-screen lotion

Skin-burns and cancer are the leading effects of prolonged UV exposure. Nano-TiO₂ are used to prepare sun-screen lotions which provides potential sun protection factor without stickiness. They protect the skin without penetrating into the skin. Moreover, they have transparency, thus natural skin colour is retained.

      Sensors

Sensors are the highly active substances which can response to a minute change in specie’s concentration which have to be detected. Sensors having engineered monolayers of nanomaterials on their surfaces have specific functionality and are used in sensing.