🔬 Unit 5 – Part B (Essay Q&A)

1. What are the properties that change from its bulk form to nano size form? Explain each with example.

Properties of materials change drastically when their size is reduced to the nanoscale (1-100 nm). This is primarily due to two factors: high surface-area-to-volume ratio and quantum confinement effects.

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• Optical Properties: [cite: 10] Bulk materials have fixed optical properties. At the nanoscale, these change with size.
  • Example: Bulk gold is metallic and yellow. Gold nanoparticles, however, can appear red, purple, or blue depending on their size. This is due to an effect called Surface Plasmon Resonance (SPR), where the electrons on the nanoparticle's surface collectively oscillate when hit by light.
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• Electrical Properties: [cite: 10] The electrical conductivity of nanomaterials can be very different from their bulk form due to quantum confinement, which changes the electronic band structure.
  • Example: Bulk silicon is a semiconductor. Silicon quantum dots (nanoparticles) can behave more like molecules, with discrete energy levels. Their band gap changes with size, allowing them to emit different colors of light when excited, which is used in QLED displays.
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• Mechanical Properties: [cite: 10] Nanomaterials are often significantly stronger and harder than their bulk counterparts.
  • Example: Bulk ceramics are hard but brittle. Nanostructured ceramics, however, show increased ductility and toughness. Carbon nanotubes are immensely stronger than steel while being much lighter.
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• Magnetic Properties: [cite: 10] Magnetic behavior also changes with size.
  • Example: Bulk iron oxide (rust) is ferromagnetic. Nanoparticles of the same material become superparamagnetic, meaning they are magnetic only in the presence of an external magnetic field and lose their magnetism immediately when the field is removed. This is crucial for applications like MRI contrast agents.

2. Write a note on carbon nanotubes and their properties.

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Carbon Nanotubes (CNTs) are one-dimensional (1D) allotropes of carbon, conceptually similar to a sheet of graphene (a single layer of carbon atoms) rolled up into a seamless cylinder. [cite: 10]

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There are two main types: [cite: 10]

• Single-Walled Carbon Nanotubes (SWNTs): Consist of a single graphitic cylinder (diameter ~1-2 nm).
• Multi-Walled Carbon Nanotubes (MWNTs): Consist of multiple concentric graphitic cylinders nested inside one another (diameter ~2-100 nm).

Properties of Carbon Nanotubes:

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• Mechanical: They are among the strongest and stiffest materials known, with extremely high tensile strength (up to 100 times stronger than steel at a fraction of the weight) and flexibility. [cite: 10]
• Electrical: Their electrical properties are unique and depend on their "chirality" (the angle at which the graphene sheet is rolled up). [cite_start]They can be either metallic (like copper) or semiconducting (like silicon). [cite: 10]
• Thermal: They have very high thermal conductivity, allowing them to efficiently transfer heat.
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• Chemical: They have a large surface area and their outer walls can be chemically modified (functionalized), making them useful as supports for catalysts or sensors. [cite: 10]

3. i) Discuss the laser ablation method of synthesis of nano materials. ii) Compare the properties of molecules, nanoparticles and bulk materials.

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i) Laser Ablation Method: [cite: 10]

Laser ablation (or Pulsed Laser Ablation in Liquid, PLAL) is a "top-down" physical method for synthesizing nanomaterials. The process is as follows:

1. Setup: A solid target of the bulk material (e.g., a piece of gold, silver, or titanium) is placed at the bottom of a beaker filled with a liquid (often water or an organic solvent).
2. Ablation: A high-power, pulsed laser beam is focused onto the surface of the solid target.
3. Plasma Formation: The intense energy from each laser pulse instantly vaporizes (ablates) a tiny portion of the target, creating a high-temperature, high-pressure plasma plume.
4. Condensation: This plasma plume rapidly expands into the surrounding liquid and is immediately cooled (quenched). This rapid cooling prevents the atoms from reorganizing back into the bulk structure and forces them to condense into small clusters, which then grow into nanoparticles.

Advantages: This method is clean as it doesn't require chemical precursors, leading to high-purity nanoparticles. The particle size can be controlled by adjusting laser parameters (wavelength, pulse duration) and the liquid environment.

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ii) Comparison of Molecules, Nanoparticles, and Bulk Materials: [cite: 10]

• Molecules:
  • Size: Sub-nanometer scale (e.g., a water molecule is ~0.3 nm).
  • Properties: Governed strictly by quantum mechanics. Properties are discrete and exact (e.g., specific bond lengths, vibrational frequencies).
  • Surface Area: Concept doesn't apply in the same way, but they are all "surface."
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• Nanoparticles: [cite: 10]
  • Size: 1 - 100 nm.
  • Properties: Governed by a mix of quantum mechanics and classical physics. Properties are size-dependent (e.g., optical, magnetic, electrical properties all change with size).
  • Surface Area: Have an extremely high surface-area-to-volume ratio, making them highly reactive and excellent catalysts.
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• Bulk Materials: [cite: 10]
  • Size: Macroscopic scale (>> 100 nm).
  • Properties: Governed by classical physics. Properties are size-independent and constant (e.g., a gold bar has the same color and melting point regardless of its size).
  • Surface Area: Have a low surface-area-to-volume ratio. Most atoms are on the "inside" (bulk) rather than the "outside" (surface).

4. (i). With a neat sketch, explain Sol-Gel synthesis for producing nanomaterials. (ii) Explain chemical vapour deposition technique of synthesis of nano particles.

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(i) Sol-Gel Synthesis: [cite: 10]

The sol-gel process is a versatile "bottom-up" wet-chemical method used to produce solid materials (like ceramics and glasses) from small molecules. It involves the creation of a "sol" (a colloidal suspension) that evolves into a "gel" (a 3D solid network).

The process generally involves four main steps:

1. Hydrolysis: A chemical precursor, typically a metal alkoxide (e.g., Si(OR)₄), is mixed with water in a solvent (like alcohol). The water hydrolyzes the precursor, replacing the -OR groups with hydroxyl (-OH) groups.
   M-OR + H₂O → M-OH + R-OH
2. Condensation: The hydrolyzed precursor molecules react with each other in a condensation reaction. They form metal-oxygen-metal (M-O-M) bridges, releasing water or alcohol. This process links the molecules together, forming larger particles (the "sol").
   M-OH + HO-M → M-O-M + H₂O
3. Gelation: As condensation continues, the particles link up to form a continuous three-dimensional network that spans the entire liquid volume. This rigid, porous network, which traps the solvent within it, is called the "gel."
4. Drying & Calcination: The wet gel is carefully dried to remove the solvent (forming a "xerogel" or "aerogel"). It is then often heated at high temperatures (calcination) to burn off any remaining organic residues and densify the material, resulting in the final solid nanoparticle-based ceramic or glass.

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(ii) Chemical Vapour Deposition (CVD): [cite: 10]

Chemical Vapour Deposition (CVD) is a "bottom-up" synthesis technique used to produce high-purity, high-performance solid materials, often as thin films or 1D nanostructures like nanotubes and nanowires.

The process involves:

1. Reactants: One or more volatile precursor gases (reactants) are introduced into a reaction chamber.
2. Substrate: Inside the chamber, a heated substrate (the material on which the solid will be deposited) is placed.
3. Reaction: At the high temperature of the substrate, the precursor gases decompose and react with each other on or near the substrate surface.
4. Deposition: The product of this chemical reaction is a solid material, which then deposits onto the substrate, building up a film or growing as nanostructures.
5. Removal: The byproduct gases from the reaction are exhausted from the chamber.

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Example: To grow carbon nanotubes[cite: 10], a substrate coated with catalyst nanoparticles (like iron or nickel) is heated to ~700-900°C. A hydrocarbon gas (like methane, CH₄) is passed over it. The methane decomposes on the hot catalyst, and the carbon atoms assemble into the growing structure of a nanotube.

5. Discuss in details the applications of Nanoparticles in various fields

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The unique properties of nanoparticles allow them to be used in a wide range of fields: [cite: 10]

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• Medicine: [cite: 10]
  • Targeted Drug Delivery: Nanoparticles (like liposomes or polymeric nanoparticles) can be loaded with drugs and functionalized with molecules that specifically target diseased cells (e.g., cancer cells), delivering a high dose of the drug only where it's needed and reducing side effects.
  • Medical Imaging: Gold nanoparticles and superparamagnetic iron oxide nanoparticles are used as contrast agents in CT scans and MRIs, respectively, to provide clearer images of tissues and tumors. Quantum dots are used as fluorescent labels in biological research.
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• Electronics: [cite: 10]
  • Displays: Quantum dots (QDs) are used in QLED TVs to produce brighter, more vibrant, and purer colors.
  • Components: Carbon nanotubes are being explored as potential replacements for silicon in transistors to create smaller, faster, and more power-efficient chips. Silver nanowires are used to create flexible, transparent conductive films for touchscreens.
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• Energy: [cite: 10]
  • Solar Cells: Nanoparticles are used to create "quantum dot solar cells" or to enhance the light-absorbing layer of traditional cells, increasing their efficiency.
  • Batteries: Nanomaterials (like nano-silicon or carbon nanotubes) are used in the electrodes of lithium-ion batteries to increase their power density (faster charging) and energy capacity (longer life).
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• Catalysis: [cite: 10]
  • Due to their extremely high surface-area-to-volume ratio, nanoparticles are incredibly efficient catalysts. For example, platinum and palladium nanoparticles are used in catalytic converters in cars to break down toxic exhaust gases.
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• Agriculture: [cite: 10]
  • Nanosensors: To detect pathogens, pesticides, or nutrient levels in soil.
  • Controlled Release: Nano-encapsulation of fertilizers or pesticides allows them to be released slowly and only when needed, reducing environmental runoff.

6. (i). Write in detail about the preparation of nanomaterial by Electro spinning technique. (ii). What are nanoclusters and nanowires nanorods? Explain their properties and applications.

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(i) Electrospinning Technique: [cite: 10]

Electrospinning is a method used to produce ultrafine fibers, typically with diameters in the nanometer range (nanofibers). These nanofibers are collected as a non-woven mat.

The process works as follows:

1. Polymer Solution: A polymer (e.g., polycaprolactone, PCL) is dissolved in a volatile solvent to create a viscous polymer solution. This solution is loaded into a syringe.
2. High Voltage: The syringe is connected to a high-voltage power supply (typically 5-30 kV). A metallic needle at the tip of the syringe acts as one electrode, and a grounded collector plate (usually a piece of metal foil) acts as the other electrode, placed several centimeters away.
3. Taylor Cone Formation: When the voltage is applied, the polymer solution at the tip of the needle becomes charged. This charge creates an electrostatic force that pulls the droplet towards the collector, counteracting its surface tension. The droplet deforms into a conical shape known as the Taylor cone.
4. Jet Ejection & Stretching: At a critical voltage, a fine jet of the polymer solution erupts from the tip of the Taylor cone and accelerates rapidly toward the collector.
5. Solvent Evaporation: As the jet travels through the air, the solvent rapidly evaporates. Simultaneously, the jet undergoes a "whipping instability" where it bends and stretches dramatically, thinning the fiber to the nanoscale.
6. Fiber Collection: A solid, dry nanofiber is deposited on the collector plate, forming a non-woven mat.

Applications: Electrospun mats are used in tissue engineering (as scaffolds for cell growth), filtration (high-efficiency air filters), and protective clothing.

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(ii) Nanoclusters, Nanorods, and Nanowires: [cite: 10]

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• Nanoclusters: [cite: 10]
  • Definition: Zero-dimensional (0D) nanomaterials. They are small aggregates of atoms or molecules, typically containing from a few up to several hundred units.
  • Properties: They represent an intermediate state between individual atoms and nanoparticles. Their properties (e.g., electronic, magnetic) are highly discrete and depend on the exact number of atoms in the cluster.
  • Applications: Primarily used in catalysis, where their high surface area and unique electronic structure make them highly reactive.
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• Nanorods:
  • Definition: One-dimensional (1D) nanomaterials. They are solid, rod-shaped materials with both their diameter and length on the nanoscale, but the length is greater than the width.
  • Properties: They have an "aspect ratio" (length/width) that is typically low (e.g., 3-10). Their properties are anisotropic (different along their length vs. their width).
  • Applications: Used in solar cells (to improve charge transport), sensors, and LED lighting.
• Nanowires:
  • Definition: One-dimensional (1D) nanomaterials, similar to nanorods but with a much larger aspect ratio. Their diameter is at the nanoscale, but their length can be macroscopic (microns or even millimeters).
  • Properties: They have a very high aspect ratio (can be >1000). They are often flexible and exhibit unique electrical and optical properties (e.g., acting as waveguides for light).
  • Applications: Used as components in nano-electronics (as interconnects or transistors), highly sensitive chemical/biological sensors, and in battery electrodes.

7. What are the 12 Principles of Green Chemistry?

The 12 Principles of Green Chemistry, developed by Paul Anastas and John Warner, provide a framework for designing chemical products and processes that are safer for human health and the environment. [cite: 10]

1. Prevent Waste: It is better to prevent waste than to treat or clean it up after it is created.
2. Atom Economy: Synthetic methods should be designed to maximize the incorporation of all materials used in the process into the final product (i.e., minimize waste atoms).
3. Less Hazardous Chemical Syntheses: Wherever practicable, synthetic methods should be designed to use and generate substances that possess little or no toxicity.
4. Designing Safer Chemicals: Chemical products should be designed to preserve efficacy of function while reducing toxicity.
5. Safer Solvents and Auxiliaries: The use of auxiliary substances (e.g., solvents, separation agents) should be made unnecessary wherever possible and, when used, innocuous. [cite: 10]
6. Design for Energy Efficiency: Energy requirements should be recognized for their environmental and economic impacts and should be minimized. Synthetic methods should be conducted at ambient temperature and pressure.
7. Use of Renewable Feedstocks: A raw material or feedstock should be renewable rather than depleting whenever technically and economically practicable.
8. Reduce Derivatives: Unnecessary derivatization (use of blocking groups, protection/deprotection) should be minimized or avoided, as such steps require additional reagents and can generate waste.
9. Catalysis: Catalytic reagents (as selective as possible) are superior to stoichiometric reagents.
10. Design for Degradation: Chemical products should be designed so that at theDnd of their function they break down into innocuous degradation products and do not persist in the environment.
11. Real-time Analysis for Pollution Prevention: Analytical methodologies need to be further developed to allow for real-time, in-process monitoring and control prior to the formation of hazardous substances.
12. Inherently Safer Chemistry for Accident Prevention: Substances and the form of a substance used in a chemical process should be chosen to minimize the potential for chemical accidents, including releases, explosions, and fires.

8. Discuss in detail about Techniques involved in Green synthesis. i)Microwave ii)Ultrasound iii)Photocatalysis

Green synthesis techniques aim to make chemical reactions more efficient, faster, and less energy-intensive, while reducing or eliminating the use of hazardous substances. [cite: 10]

• i) Microwave-Assisted Synthesis:
  • Principle: This technique uses microwave energy to heat the reaction mixture. Unlike conventional heating (like an oil bath), which transfers heat slowly via conduction and convection, microwaves heat the reactants *directly* and *volumetrically*.
  • Mechanism: Microwave radiation interacts with polar molecules (like water or ethanol) and ions, causing them to rapidly oscillate. This rapid agitation creates intense internal heating (dielectric heating) that is fast, uniform, and highly efficient.
  • Green Advantages:
    • Speed: Reactions that take hours or days conventionally can often be completed in minutes.
    • Energy Efficiency: Heats only the reactants, not the entire apparatus, saving significant energy.
    • Higher Yields: The rapid heating can lead to higher product yields and fewer side products.
• ii) Ultrasound-Assisted Synthesis (Sonochemistry):
  • Principle: This technique uses high-frequency sound waves (ultrasound, >20 kHz) to drive chemical reactions in a liquid medium.
  • Mechanism: The ultrasound waves cause acoustic cavitation: the formation, rapid growth, and violent collapse of microscopic bubbles in the liquid. When these bubbles implode, they generate immense localized "hot spots" with temperatures of ~5000°C, pressures of >1000 atm, and cooling rates of >10⁹ K/s.
  • Green Advantages:
    • Enhanced Rates: This intense energy breaks chemical bonds, creates highly reactive radicals, and improves mass transfer, dramatically accelerating reaction rates.
    • Ambient Conditions: Allows reactions to occur at low *bulk* temperatures and pressures (even though the "hot spots" are extreme).
    • Phase Transfer: The shockwaves created by cavitation can emulsify immiscible liquids, often eliminating the need for phase-transfer catalysts.
• iii) Photocatalysis:
  • Principle: This process uses light (photo) and a semiconductor catalyst (e.g., Titanium Dioxide, TiO₂) to initiate and drive chemical reactions.
  • Mechanism:
    1. The semiconductor catalyst absorbs a photon of light (e.g., UV or visible) with energy greater than its band gap.
    2. This excites an electron (e⁻) from the valence band to the conduction band, leaving behind a positive "hole" (h⁺).
    3. This electron-hole pair is highly reactive. The hole can oxidize nearby molecules (like water) to create hydroxyl radicals (•OH), which are extremely powerful, non-selective oxidants. The electron can reduce other molecules (like oxygen).
    4. These reactive species then attack and break down pollutants or synthesize new chemicals.
  • Green Advantages:
    • Energy Source: Uses light, a clean and renewable energy source (especially if sunlight is used).
    • Ambient Conditions: Most photocatalytic reactions occur at room temperature and pressure.
    • Waste Treatment: Extremely effective at destroying persistent organic pollutants in water and air, converting them into harmless CO₂ and H₂O.

9. Discuss in details the applications of Green Chemistry in various fields.

Green Chemistry principles are applied across many industries to reduce environmental impact and improve sustainability. [cite: 10]

• Pharmaceutical Industry:
  • Problem: Pharmaceutical synthesis is known for being multi-step and generating enormous amounts of waste (often >100 kg of waste per 1 kg of product).
  • Green Solution: Redesigning syntheses to improve atom economy and reduce steps.
    • Example: The synthesis of Ibuprofen. The traditional process had 6 steps and low atom economy (<40%). A greener, catalyzed process (BHC process) was developed that has only 3 steps and a high atom economy (~80%), generating far less waste.
    • Example: Using biocatalysis (enzymes) to replace traditional chemical catalysts. Enzymes work in water, are highly selective, and non-toxic.
• Polymer Industry:
  • Problem: Reliance on petroleum-based feedstocks and the creation of non-biodegradable plastics.
  • Green Solution:
    • Renewable Feedstocks: Developing bioplastics like Polylactic Acid (PLA)[cite: 10], which is made from corn starch or sugarcane (a renewable resource) and is biodegradable.
    • Safer Catalysts: Replacing toxic heavy-metal catalysts (like those used in PVC production) with safer, more efficient alternatives.
    • Using CO₂: Developing polymers that use captured Carbon Dioxide (a greenhouse gas) as a feedstock, effectively sequestering it in a useful product.
• Agriculture:
  • Problem: Traditional pesticides are often toxic to non-target species (like bees, birds, and humans) and persist in the environment, contaminating water and soil.
  • Green Solution:
    • Designing Safer Pesticides: Creating pesticides that are highly specific to the target pest and are designed to biodegrade quickly into harmless substances.
    • Pheromones: Using insect sex pheromones to trap pests or disrupt their mating, providing pest control without using any toxic chemicals.
• Energy Sector:
  • Problem: Heavy reliance on fossil fuels and the use of toxic materials in energy technologies.
  • Green Solution:
    • Biofuels: Developing efficient processes to convert non-food biomass (like algae, switchgrass, or wood waste) into liquid fuels like ethanol and biodiesel.
    • Safer Solar Cells: Researching and developing new photovoltaic materials that avoid the use of scarce or toxic elements like cadmium (used in CdTe cells).
    • Greener Solvents: Using "green solvents" like supercritical CO₂ or ionic liquids in processes like battery manufacturing or biofuel extraction, replacing volatile organic compounds (VOCs). [cite: 10]