NANO CLUSTERS AND NANOCOMPOSITES

 

Clusters are collections of atoms lying between individual atoms/molecules and bulk materials. In some materials, certain collections of atoms are more preferred due to energy minimization and exhibiting stable structures and providing unique properties to the materials. These collections of atom providing stable structures to the materials are called as MAGIC NUMBER. For example, one of the combination of 55 atoms of gold provides stable structure and hence its magic number is 55

What is cluster in nanotechnology?

Clusters are small aggregates of atoms and molecules. Small means really tiny pieces of matter—they are composed of a few to thousands of units and have a diameter of nanometers. Nanoclusters have at least one dimension between 1 and 10 nm and a narrow size distribution. Nanoclusters are composed of up to 100 atoms, but bigger ones containing 1000 or more are called nanoparticles.

What are nanoclusters used for?

Nanoclusters have potential uses in chemical reactors, telecommunications, microelectronics, optical data storage, catalysts magnetic storage, spintronic devices, electroluminescent displays, sensors, biological markers, switches, transducers and many other fields. The florescence silver nanoclusters have been extensively used as biological markers for photodynamic therapy.

What are metal nanoclusters?

Metal nanoclusters (NCs) are composed of a small number of atoms, up to dozens. These nanoclusters can consist of a single element or multiple elements, usually smaller than 2 nm. Compared with their larger counterparts, this nanocluster exhibits attractive electronic, optical and chemical properties. These particles have quasi-continuous energy levels and display intense colors due to surface plasmon resonance. If their dimension is further reduced to the size approaching the Fermi wavelength of electrons, the band structure becomes discrete energy levels. The ultrasmall metal nanoparticles display molecule-like properties and no longer exhibit plasmonic behavior. Metal NCs, such as AuNCs, AgNCs, CuNCs, and PtNCs, exhibit a marked photoluminescence property due to quantum confinement. The most studied among these metal NCs are AuNCs, AgNCs, and CuNCs.

What are gold nanoclusters?

Nanocluster are collective groups composed of a specific number of atoms or molecules held together through a certain interaction mechanism. Gold nanoclusters attract increasing attention due to their potential applications in sensing, catalysis, optoelectronics, and biomedicine.

How are gold nanoclusters made?

In one example, green-emitting gold nanoclusters could be prepared by adding mercaptoundecanoic acid (MUA) into small AuNP solutions prepared by the reduction of HAuCl4 with tetrakis(hydroxymethyl)phosphonium chloride (THPC).

Size 

Nanoclusters: Typically have at least one dimension between 1–10 nanometers (nm).

Quantum dots: Typically have a diameter of 2–10 nm.

Composition 

Nanoclusters: Made up of up to 100 atoms, but larger ones can contain 1000 or more atoms.

Quantum dots: Made of a semiconducting material.

Properties 

Nanoclusters: Have strong fluorescence emission, good photostability, and high conductivity.

Quantum dots: A type of semiconductor nanocrystal.

Nanocomposites

Composite materials are prepared from the combination of two or more different materials with distinct chemical or physical characteristics. The resultant composite exhibits properties which are superior to its constituent materials. 

Nanocomposites are broad range of materials consisting of two or more components, with at least one component having dimensions in the nm range (i.e. between 1 and 100 nm)

Nanocomposites consist of two phases (i.e nanocrystalline phase + matrix phase)

Nanocomposite means nanosized particles (i.e metals, semiconductors, dielectric materials, etc) embedded in different matrix materials (ceramics, glass, polymers, etc).

Nanocomposites differ from traditional composites in the smaller size of the particles in the matrix materials.

Classification: 

nanocomposites can also be classified into three categories based on the matrix material type:

Polymer-matrix Nanocomposites -  These nanocomposites are made from polymer matrix materials such as thermoplastic, thermoset polymers, layered silicates, and polyester

Ceramic-matrix Nanocomposites - These nanocomposites are made from ceramic matrix materials such as Al2O3 or SiC.

Metal-matrix Nanocomposites - These nanocomposites are made from a ductile metal or alloy matrix with nanosized reinforcement material.

Prepration:

Synthetic methods are commonly employed to prepare nanocomposites, such as lamination, soft lithography, solution casting, and spin-coating. Homogenous nanoparticle dispersion is a primary challenge during preparation. Dispersion affects phase interfaces and the final nanocomposite properties.

Different materials, structures, and compositions allow for fine-tuning of nanocomposite properties, such as electrical, mechanical, thermal, acoustical, and magnetic properties. Nanocomposites have given rise to the field of multifunctional materials.

Application Areas

Application areas in the automotive industry include engine covers, intake manifolds, door handles, mirror housings, and timing belt covers in various vehicle types. Applications in other commercial areas include vacuum impellers and blades, mower hoods, mobile phone covers, and power tool housings.

Nanocomposites in Food Packaging

Nanoclays, which are added to,polypropylene or polylactic acid packaging films, prevent the diffusion of oxygen or flavorings and thus prolong the shelf life of foods. Nanosilver has an antimicrobial effect and can be used in plastics composites, for example to manufacture food packaging such as films or containers to protect food from spoilage.

QUANTUM DOTS

 Dear students,

This post deals with Quantum dots, a new class of materials that is neither molecular nor bulk. They have the same structure and atomic composition as bulk materials, but their properties can be tuned using a single parameter, the particle’s size.

What is a quantum dot?

Quantum dots are tiny crystals of semiconducting material. The crystals are so small – just a few nanometers wide (diameter of 2–10 nm) – that their physical size actually confines the electrons in the material to the point that it changes their behaviour. This means that quantum phenomena determine the properties of these tiny crystals – hence the name quantum dots. 

Changing the size of the particle changes its properties in a predictable way. This effect can be used to tune the band gap of the semiconductor so that dots emit different colours of light when illuminated by a single-wavelength source – smaller dots (e.g., 2-3 nm) providing higher energy emissions   will emit blue light, while larger ones (e.g., 5-6 nm) providing lower energy emissions emit at the red end of the visible spectrum. This property is sometimes referred to as “quantum confinement. 

Like other semiconductors, quantum dots tend to be made of combinations of transition metal and metalloid elements. Cadmium selenide and cadmium telluride are among the most regularly used materials.

Quantum Dot Production

There are various methods of producing quantum dots. The most typical is via a colloidal synthesis, which is the process of heating a solution, causing the precursors to decompose to form monomers, which then produce nanocrystals.

Quantum dots produced using this method can consist of compounds including indium arsenide, lead sulfide, lead selenide, and cadmium sulfide. Colloidal synthesis is a popular method as quantum dots can be produced in batches large enough to be potentially used for commercial applications.

Plasma synthesis is another popular technique for the production of quantum dots. This process enables the control of the composition, surface, size, and shape of the quantum dot, and it also reduces the challenges associated with doping.

Not every nanoparticle is a quantum dot. Only some materials (such as semiconductors) will show quantum size effects in their electronic structure when they are made into nanoscale particles.

Working Principle of a Quantum Dot

Within a quantum dot, there are confined valence band holes, conduction band electrons, or excitons. These are the particles that carry the electricity, and because of this confinement, the quantum dot has a distinct energy level. The optical absorption and emission of CdSe quantum dots can be tuned across nearly the entire visible range of the optical spectrum. This is possible because the energy bandgap of CdSe quantum dots varies between 1.8 eV (its bulk value) to 3 eV (in the smallest quantum dots)

Applications:

One of their most widely known uses is in forming the basis QLED television screens recognized as the “Q” in QLED TVs. where dots of different sizes are excited by blue light and then emit pure red and green light to give the three-colour output of the pixels in a TV screen. But they also have uses in biotechnology, catalysis, sensors, solar cells and more. In particular, the Nobel committee highlighted the use of quantum dots in medical devices that are used to map biological tissue because the dots’ fluorescence is brighter and longer-lived than that of other fluorescent tags such as molecular fluorophores. This means they can help to guide surgeons when removing tumours, for example. 

NANO MATERIALS

 

Here is the intro and some properties of nano materials..

INTRODUCTION

The word “nano” derives from the latin word “nanus” and Greek word “𝜈 ́𝛼νς,” Both indicating a person of very low height, i.e. a dwarf.  Nanomaterials are conventionally defined as materials having at least a dimension between 1 and 100 nm.

Common types of nanomaterials include nanofilaments, nanotubes, nanowires, nanocables, nanothin film, dendrimers, quantum dots, composite materials, and other materials besides nanopowder. Because of the unique nanoscale (1–100 nm) size, nanomaterials are different from microscopic atoms molecules and macro-objects in terms of their physical, chemical, electrical, and magnetic properties.

On the basis of reduction in size of materials in different dimensions, nanomaterials are classified into three groups.

Surface effects

Nanomaterials possess a large fraction of surface atom per unit volume. This dramatic increase in the ratio of the surface atom to interior atom in nanomaterials is responsible for great change in physical and other properties of the materials. When the size of the object is reduced to a nanometric range the proportion of surface atom increased leading to substantially more reactive surface sites. Thus nanomaterials possess high surface area over volume ratio, which leads them to interact with the environment more effectively compared to bulk materials. The surface atoms have less coordination that lead them to lesser stability and highly reactive than the interior atoms. A further consequence of this lower stability of atoms or molecules at the surface is the lower melting point of surface layers. 

Properties of nanomaterials:

High surface area and quantum effects at the nanoscale are responsible for different properties of nanomaterials compared to bulk materials. The quantum confinement effects describe electrons in terms of energy levels, valence bands, conduction bands, and electron energy band gaps. The confinement means restricting the motion of randomly moving electrons to specific energy levels (discreteness). When the particle size is reduced to nanoscale, the band gap is increased as compared with the bulk materials, and it leads to wider in the separation between the energy levels. Hence, properties such as melting point, fluorescence, electrical conductivity, magnetic permeability, and chemical reactivity change as a function of the size of the particle.

 Electrical properties :

The electrical conductivity of nanomaterials is generally lower than the bulk materials due to the increase of the band gap energy with a decrease in particle size of the nanomaterials. In bulk metals, the valence and conduction bands overlap, while in metal nanoparticles there is a gap between these bands. The gap observed in metal nanoparticles can be similar in size to that seen in semiconductors (< 2 eV) or even insulators (> 2 eV). This results in a metal becoming a semiconductor.  

 Optical properties: 

The optical properties such as reflection, transmission, absorption, and light emission of

the nanomaterials are completely dependent on their electronic structure.  At nanoscale level, nanoparticles are so small that electrons in them are not as much as free to move as in case of bulk material. Due to this quantum confinement of electrons, nanoparticles react differently with light compared to the bulk material.  Optical properties such as emission and adsorption occur when electron transition occurs between the highest occupied molecular orbital, which is a valence band, and the lowest unoccupied molecular orbital, which is essentially the conduction band. This optical bandgap increases with the decrease in particle size. Tuning the nanosize of a semiconductor means tuning of bandgap leading to light of certain wavelength being emitted. Thus the same material emits different colours depending on its size. 

Example: 

It is well known that gold has the characteristic yellow, brilliant color, but when gold is produced in the nanosized dimension range, its color invariably changes to red. In other words, gold shows

a color depending on the size of the gold particles. This phenomenon has an impressive example in the so-called Lycurgus cup now preserved at the British Museum in London. 

This cup was probably created in Rome during the fourth century and shows a different color changing the light from opaque green to bright red. This is the most famous example of the so-called dichroic glass. Actually, it presents some tiny amount of nanosized gold and silver that gives these unusual optical properties. This happens because at the nanoscale range, the electron cloud located on the surface of a gold nanoparticle has the possibility to resonate with different wavelengths of light depending on their frequency. 

Magnetic properties:

Compared to bulk materials nanomaterials show a variety of unusual magnetic behavior due to the surface or interface effects. The magnetic properties of a magnet are described by its magnetisation curve (called hysteresis curve which is the variation of intensity of magnetisation with applied magnetic field). These curves show the property like whether a magnet can be used as permanent magnet or a electromagnet etc…. The size of the material can change the property of a magnet by changing the magnetisation curves. Thus nanostructuring of bulk magnetic materials leads to changes in the curves which can produce soft or hard magnets with improved properties. Thus the nanomaterials may become superparamagnetic, even though their corresponding bulk materials are not magnetic. For example, Fe3O4 nanoparticles showed superparamagnetic-like behavior, even though bulk iron oxide (Fe3O4) is ferromagnetic. Exceptional surface energy and the flipped orientation of the spin electrons of the Fe3O4 nanoparticles are responsible for this phenomenon.  Superparamagnetic nanoparticles are not magnetic when located in a zero magnetic field, but they quickly become magnetized after an external magnetic field is applied. When they are below the superparamagnetic diameter, the nanoparticles can revert quickly to a nonmagnetized state after an external magnet is removed

 Mechanical properties 

 Mechanical properties of nanomaterials may reach the theoretical strength, which are one or two orders of magnitude higher than that of single crystals in the bulk form. The enhancement in mechanical strength is simply due to the reduced probability of defects. Carbon nanotubes are 100 times stronger than steel but six times lighter. 

HALOALKANES AND HALOARENES

 Dear Students,

In this post, I have shared the theoretic basis of haloalkanes and haloarenes in Q and A format. Thisi is useful for your examination purpose.

Haloalkanes and haloarenes are compounds in which one or more hydrogen atoms in a hydrocarbon have been replaced with halogen atoms. The difference between haloalkanes and haloarenes is that haloalkanes are derived from alkanes while haloarenes are derived from aromatic hydrocarbons. Haloalkanes are commonly referred to as alkyl halides whereas haloarenes are commonly referred to as aryl halides.

In haloalkanes, the halogen atoms are attached to sp3 hybridized carbon atoms. In haloarenes, the halogen atoms are attached to sp2 hybridized carbon atom. The difference in the hybridization state of the carbon atom in C-X (carbon-halogen) bond is responsible for the different reactivates and properties of these compounds.

You can download the study material from the below link.

Haloalkanes-Haloarenes

LAWS OF THERMODYNAMICS

 

First law of thermodynamics:

The first law of thermodynamics states that heat is a form of energy, and  that heat energy cannot be created or destroyed.  It can, however, be transferred from one location to another location or converted to one from to other forms of energy. 

Whenever a system goes through any change due to the interaction of heat, work and internal energy, it is followed by numerous energy transfers and conversions. However, during these transfers, there is no net change in the total energy. Thus, according to first law of thermodynamics the energy of the universe remains the same.  This is known as the principle of conservation of energy. Any gain in energy by the system will correspond to a loss in energy by the surroundings, or any loss in energy by the system will correspond to a gain in energy by the surroundings.

ΔE = q + w

 ΔE -  Change in internal energy 

ΔE =  Efinal - Einitial

A positive value of ΔE results when Efinal > Einitial, indicating that the system has gained energy from its surroundings. A negative value of ΔE results when Efinal < Einitial, indicating that the system has lost energy to its surroundings. 

Endothermic and Exothermic Processes:

When a process occurs in which the system absorbs heat, the process is called endothermic (endo- means “into”). During an endothermic process, such as the melting of ice, heat flows into the system from its surroundings. If we, as part of the surroundings, touch a container in which ice is melting, the container feels cold to us because heat has passed from our hand to the container. A process in which the system loses heat is called exothermic (exo- means “out of”). During an exothermic process, such as the combustion of gasoline, heat exits or flows out of the system into the surroundings.

To understand the meaning of the first law, we can take the common example of a heat engine. In a heat engine, the thermal energy is converted into mechanical energy. Heat engines are mostly categorized as open systems. The basic working principle of a heat engine is that it makes use of the different relationships between heat, pressure and volume of a gas. A heat engine is a system which converts heat into work by taking heat from the reservoir (hot body) to carry out some work. There is a discharge of some heat to the sink (cold body). In this system, there will also be some waste in the form of heat. 

A very basic diagram of the heat engine is given below:

THERMODYNAMICS

 

The word ‘thermodynamics’ is derived from two words: thermo and dynamics. ‘Thermo’ stands for heat while ‘dynamics’ is used in connection with a mechanical motion which involves ‘work’. 

Broadly speaking, thermodynamics is a branch of science that deals with heat, work and temperature, and their relation to energy, radiation and physical properties of matter. It explains how thermal energy (heat energy) is converted to other forms of energy and how matter is affected by this process.

Different Branches of Thermodynamics:

• Classical Thermodynamics
• Statistical Thermodynamics
• Chemical Thermodynamics
• Equilibrium Thermodynamics

In classical thermodynamics, temperature and pressure are taken into consideration, which helps us to calculate other properties and predict the characteristics of the matter undergoing the process.

Chemical thermodynamics is the study of how work and heat relate to each other in chemical reactions

and in changes of states.

THERMODYNAMIC TERMS AND BASIC CONCEPTS: 

To study the basic concepts of thermodynamics it is important to study a few terms and definitions which must be understood clearly.

Thermodynamic Systems:

A system is that part of the universe which is under thermodynamic study and the rest of the

universe is surroundings. The real or imaginary surface separating the system from the

surroundings is called the boundary

There are three types of systems:

Open System –

 In an open system, the mass and energy both may be transferred between the system and surroundings. A steam turbine is an example of an open system.

Closed System – 

In the closed system, the transfer of energy takes place but the transfer of mass doesn’t take place. Refrigerator, compression of gas in the piston assembly, etc., are examples of closed systems.

Isolated System – 

An isolated system cannot exchange energy and mass with its surroundings. The universe is considered an isolated system.

THERMODYNAMIC PROCESS:

The change of thermodynamic state from one condition to another condition ( change in temperature or pressure) is called thermodynamic process. One example of a thermodynamic process is increasing the temperature of a fluid while maintaining a constant pressure.

REVERSIBLE PROCESS:

A reversible process for a system is defined as a process that, once having taken place, can be reversed, and in so doing leaves no change in either the system or surroundings. In reality, there are no truly reversible processes. One way to make real processes approximate reversible process is to carry out the process in a series of small or infinitesimal steps.

For example, transferring heat across a temperature difference of 0.00001 °F "appears" to be more reversible than for transferring heat across a temperature difference of 100 °F. Therefore, by cooling or heating the system in a number of infinitesimally small steps, we can approximate a reversible process.

IRREVERSIBLE PROCESS

An irreversible process is a process that cannot return both the system and the surroundings to their original conditions. That is, the system and the surroundings would not return to their original conditions if the process was reversed. 

For example, an automobile engine does not give back the fuel it took to drive up a hill as it coasts back down the hill.

Isothermal Process

An isothermal process is a thermodynamic process that takes place at a constant temperature. It means that an isothermal process occurs in a system where the temperature remains constant. However, to keep the temperature of the system constant, heat must be transferred into the system or shifted out of the system.

In simple terms, in the isothermal process: T = constant. This implies, the change in temperature will be zero i.e., ΔT=0

Examples:

Melting and Evaporation, etc. 

Working in the refrigerator is an isothermal process. The temperature of the surroundings changes irrespective of changes in the internal temperature of the refrigerator. Excess heat is removed and transmitted to the surrounding.

Adiabatic Process:

An adiabatic process is a thermodynamic process that can take place without any heat transfer between a system and its surrounding. Here, neither heat nor energy is not transferred into or out of the system. Therefore, in an adiabatic process, the only way the energy transfer takes place between a system and its surroundings is the work. 

Examples:

1. Release of air from a pneumatic tyre.

 A pneumatic tire consists of an outer rubber layer, an inner chamber that contains compressed air and a tread pattern. This type of tire works by utilizing the air-filled chamber to provide cushioning and absorb shocks. Under a load, the tire compresses the air inside, distributing the weight evenly and reducing the impact on surfaces. The design allows for a smooth, stable ride, while the tread pattern on the tire provides grip and traction. The tire's flexibility and the pressurized air inside enable it to adapt to different terrains and surfaces. When a tyre bursts, the sudden release of the high-pressure air inside the tyre causes the air to rapidly expand. This rapid expansion of the gas occurs without any heat transfer to or from the surroundings, as the process happens too quickly for significant heat exchange to take place.

PURIFICATION TECHNIQUE - I

 

Dear students,

It is known that organic compounds can be derived from natural sources and these are mixed with other substances and are impure. Similarly compounds prepared in the laboratory  are also mixed with other by products formed during the course of the reaction. To purify such compounds, various methods are adopted. Some of the common methods are as follows

Crystallization

Sublimation

Distillation

Extraction

Chromatography

In this post we are about to discuss two methods of purification. They are as follows.

Solvent Extraction: 

Extraction means drawing (pulling out) a compound out of a mixture using a solvent. It means organic compounds have a "choice" of two solvents that they can dissolve in. It requires two solvents that are not miscible in each other. Usually one of the solvents is water. The other solvent is a liquid that does not dissolve very well in water, such as diethyl ether.

The process of removing organic substance from its aqueous solution by mixing with suitable organic solvent is termed solvent extraction. It is also called as liquid-liquid extraction and partitioning. It is an equilibrium process and is a convenient technique can be carried out using separating funnel.

Principle of Solvent Extraction

It is based on the fact that different substances have different solubility in different solvents.

Procedure:

The aqueous solution of the given solute (organic substance) is taken in a separating funnel. It is mixed with the organic solvent. The funnel is closed, and its contents are shaken well. It is then allowed to remain undisturbed for some time. Water and organic solvent will form separate layers, and the  solute will be transferred from aqueous layer to the organic layer. In the funnel, the solvent forms the upper layer while the water forms the lower layer. The two layers can be recovered by opening the stop cock and collecting them in separate bakers. On evaporating the organic solvent, the solute can be recovered.

It is important to note that extraction is more efficient (i.e., higher purification) when the process is repeated. For example, benzoic acid can be extracted from its water solution using benzene. 

It is based on the distribution law, also called the Nernst’s distribution law. It states that if a solute X distributes itself between two immiscible solvents A and B, at constant temperature and X is in the same molecular condition in both solvents, then

The constant 𝐾D (or simply K) is called the distribution coefficient or Partition coefficient or Distribution ratio. 

Applications:

It is used in perfumes, vegetable oil, biodiesel processing, etc.

It is used to segregate hazardous contaminants from sludge and sediments.

In the petrochemical industry, it is used to separate and purify different components of crude oil.

It is used for the separation and purification of organic compounds.

It removes oil and grease that  spilled into water. This helps clean up environmental pollution.

Crystallization:

It is a purification technique used to separate solids from a solution. In this method, the impurities are dissolved in a suitable solvent and then filtered to remove impurities.

Principle: 

An impure organic solid is completely dissolved in a small amount of solvent by boiling. The hot solution is allowed to cool slowly. The impurities remain in the solution (called the "mother liquor") while the organic substance comes out as pure crystals . It is then filtered and dried.

Procedure:

Step 1: Take some water in a beaker

Step 2: Add organic substance (impure) and stir it

Step 3: Now heat the solution

Step 4: Add more impure substance.

Step 5: After some time there will be a point at which no more substance can be dissolved in water. This stage is the saturation point, and the solution is referred to as a saturated solution

Step 6: Now filter the substance with the help of a filter paper

Step 7: Collect the filtrate in a glass bowl and cool it slowly

Step 8: You will observe that some fine crystals are formed in the bowl

Step 9: The process of filtration can separate these crystals.

Selection of solvent:

(a) It dissolves more of the substance at higher temperature than at room temperature 

(b) The impurities are either insoluble or dissolve in solution (in the mother liquor) upon crystallization, 

(c) which is not highly inflammable and 

(d) which does not react chemically with the compound to be crystallized. 

The most commonly used solvents for crystallization are: water, alcohol, ether, chloroform, carbon- tetrachloride, acetone, benzene, petroleum ether etc. 

Fractional crystallization: 

The process of separation of different components of a mixture by repeated crystallizations is called fractional crystallization.

Examples:

(a) Sugar having an impurity of common salt can be crystallized from hot ethanol since sugar

dissolves in hot ethanol but common salt does not.

(b) A mixture of benzoic acid and naphthalene can be separated from hot water in which

benzoic acid dissolves but naphthalene does not.