⚗️ Unit 5 – Part B (Nanomaterials)

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

Nanomaterials are materials having components with size less than 100 nm in at least one dimension. When the particle size is reduced to the nanoscale, surface area increases significantly, leading to unique size-dependent properties that differ from bulk materials.

Key Property Changes:

• Melting Point: Nanoparticles possess a lower melting point than bulk materials.
  • Example: Gold nanoparticles (2.5 nm) melt at approximately \(300^{\circ}C\), whereas a bulk Gold slab melts at \(1064^{\circ}C\).
• Optical Properties: Reduction in dimensions leads to unexpected optical properties due to Surface Plasmon Resonance (SPR) and quantum confinement.
  • Example: Gold nanoparticles appear deep red to black in solution, compared to the familiar yellow colour of bulk gold.
• Electrical Properties: Electrical conductivity can change significantly. For instance, the resistivity of nanoparticles may increase due to surface scattering. However, nanostructured materials can also be used to improve battery performance.
  • Example: Nanocrystalline nickel and metal hydrides in batteries require less frequent recharging and last longer than bulk equivalents.
• Mechanical Properties: Nanomaterials often have fewer defects than bulk materials, increasing mechanical strength. Hardness can be 5 times more and strength 3-10 times higher than bulk materials.
  • Example: Nanocrystalline carbides are used in micro drills because they are much stronger, harder, and wear-resistant.
• Magnetic Properties: Bulk ferromagnetic behavior may disappear and transfer to super-paramagnetism when particle size is reduced.
  • Reason: This is due to the huge surface area available at the nanoscale.

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

Carbon Nanotubes (CNTs)

Carbon nanotubes are tubular forms of carbon with diameters ranging from 1–3 nm and lengths of a few nanometers to microns. They are formed when graphite sheets (graphene) are rolled into a cylinder. Ideally, they consist of hexagonal networks of carbon atoms linked by covalent bonds.

Types of CNTs:

1. Single-Walled Nanotubes (SWNTs): Consist of a single tube of graphite, one-atom thick. Based on the orientation of the hexagon lattice, they are classified as:
   • Arm-chair: Hexagon lines parallel to the axis (Metallic).
   • Zig-zag: Carbon bond lines down the center (Semiconducting).
   • Chiral: Exhibits a twist or spiral (Semiconducting).
2. Multi-Walled Nanotubes (MWNTs): Consist of multiple layers of graphite rolled in on themselves (nested tubes). They exhibit both metallic and semiconducting properties.

Properties of CNTs:

• Mechanical: Very strong; they can withstand extreme strain in tension and possess high elastic flexibility.
• Electrical: Highly conducting; can behave as metallic or semiconducting materials depending on their structure.
• Thermal: Very high thermal conductivity.
• Chemical: Chemically inert, making them useful for biosensors.

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

i) Laser Ablation Synthesis

This method utilizes a high-power laser pulse to evaporate material from a target.

• Process: A target material (e.g., graphite with Co/Ni catalyst) is placed in a quartz tube reactor heated to \(\approx 1200^{\circ}C\). An intense pulsed laser beam vaporizes the target.
• Collection: An inert gas (Argon or Helium) is passed through the reactor to sweep the evaporated particles from the furnace to a water-cooled copper collector, where they condense as nanoparticles/nanotubes.
• Advantages: It is an eco-friendly method (no solvents), operates easily, and produces stable products.

ii) Comparison: Molecules vs. Nanoparticles vs. Bulk Materials

Property	Atom/Molecule	Nanoparticles/Cluster	Bulk Material
Size	Few Angstroms (\(10^{-10}\) m)	1 to 100 nm (\(10^{-9}\) m)	Microns to higher
Constituents	One to several atoms	Two to several thousands	Infinite
Electronic Structure	Confined	Confined	Continuous
Structure	Well defined	Well defined	Crystal structure decides
Examples	NaCl, HCl	\((NaCl)_n\)	Gold bar, Silver bar

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

i) Sol-Gel Synthesis

Sol-gel is a "bottom-up" chemical solution deposition method used to produce solid materials (often metal oxides) from small molecules.

Steps involved:

1. Hydrolysis & Polycondensation: A metal precursor (usually a metal alkoxide) is dissolved in water/alcohol. It undergoes hydrolysis and condensation to form a colloidal solution called a Sol.
2. Gelation: The sol gradually evolves into a semi-solid network called a Gel.
3. Aging: The gel is allowed to sit, which increases particle size and reduces shrinkage.
4. Drying: Solvent is removed, accompanied by densification.
5. Calcination/Firing: Thermal treatment is applied to crystallize the nanoparticles and enhance mechanical properties.

ii) Chemical Vapour Deposition (CVD)

CVD involves the conversion of gaseous molecules into solid nanomaterials (tubes, wires, or films).

• Principle: Solid materials are first converted to gaseous molecules, which then deposit onto a substrate.
• Process (Example for CNTs): A substrate containing a catalyst (Ni, Co, Fe) is placed in a furnace. Hydrocarbons (like acetylene) and nitrogen gas are passed into the furnace. At high temperatures (\(1000^{\circ}C\)), the carbon atoms produced by decomposition condense on the cool catalyst surface to form nanotubes.
• Types of Reactors:
  • Hot-wall CVD: The entire reactor is heated by surrounding resistance elements.
  • Cold-wall CVD: Only the substrate is heated (inductively), while the chamber walls are cooled.
• Advantages: Produces highly pure, defect-free nanomaterials and is suitable for mass production.

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

Due to their unique chemical, physical, and mechanical properties, nanomaterials have applications across many sectors.

• Medicine:
  • Nano drugs: Used for cancer and TB therapy.
  • Nano-medibots: Release anti-cancer drugs specifically to tumors.
  • Gold Nanoshells: Convert light to heat to destroy tumors and are used in imaging.
  • Drug Delivery: Targeted delivery using gold nanoparticles prevents damage to healthy organs.
• Agriculture:
  • Nano-formulations: Fertilizers and plant growth regulators from plant extracts.
  • Crop Protection: Nanosensors identify diseases and agrochemical residues.
  • Food Packing: Silver nanoparticles act as antimicrobial agents in packaging.
• Energy:
  • Solar Cells: Nanotech solar cells are cheaper to manufacture. Nanoparticles increase absorption in photovoltaic cells.
  • Batteries: Nanomaterials in batteries (e.g., Li-ion) increase efficiency and storage capacity.
  • Hydrogen Storage: Graphene layers in fuel tanks allow for higher hydrogen storage at lighter weights.
• Electronics:
  • Transistors: Nanowires are used to build transistors without p-n junctions. NOMFETs combine gold nanoparticles with organic molecules.
  • Memory: Integrated memory circuits are more effective using nanomaterials.
• Catalysis:
  • Nanoparticles (like Nano Silver or Gold) are highly effective catalysts due to their huge surface area and enhanced reactivity. They are used in water purification and biodiesel production.

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.

i) Electro-spinning Technique

Electro-spinning produces ultrafine fibers (in nanometers) by charging and ejecting a polymer solution through a spinneret under a high-voltage electric field.

• Components: High voltage power supply, polymer reservoir, conductive needle (emitter), and a conductive collector.
• Process: A polymer solution is held in a capillary. When high voltage is applied, charge accumulates. When electrostatic repulsion overcomes surface tension, the liquid meniscus forms a Taylor Cone. A charged jet is ejected, solvent evaporates, and solid nanofibers are collected.

ii) Nanoclusters, Nanowires, and Nanorods

• Nanoclusters: Fine aggregates of atoms or molecules sized 0.1 to 10 nm.
  • Stability: Clusters of a critical size (Magic Number) are more stable.
  • Properties: Lower melting point than bulk; reactivity decreases with size decrease. Used as catalysts and sensors.
• Nanorods: One-dimensional cylindrical solid materials with an aspect ratio (length/width) less than 20.
  • Application: Display technologies and micro-mechanical switches.
• Nanowires: One-dimensional cylindrical solids with an aspect ratio greater than 20 and diameter 10–100 nm.
  • Properties: Distinct optical and electrical properties; silicon nanowires show photoluminescence.
  • Uses: Enhancing composite strength and in digital computing components (logic gates).

7. What are the 12 Principles of Green Chemistry?

Green Chemistry focuses on designing chemical products and processes that reduce or eliminate the use and generation of hazardous substances. The 12 principles are:

1. Prevention: It is better to prevent waste than to treat or clean up waste after it has been created.
2. Atom Economy: Synthetic methods should be designed to maximize the incorporation of all materials used in the process into the final product.
3. Less Hazardous Chemical Syntheses: Wherever practicable, synthetic methods should use and generate substances that possess little or no toxicity to human health and the environment.
4. Designing Safer Chemicals: Chemical products should be designed to affect their desired function while minimizing their toxicity.
5. Safer Solvents and Auxiliaries: The use of auxiliary substances (e.g., solvents, separation agents) should be made unnecessary wherever possible and innocuous when used.
6. Design for Energy Efficiency: Energy requirements should be recognized for their environmental and economic impacts and should be minimized (e.g., conducting reactions 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 because 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 the end 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 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

i) Microwave Synthesis

This technique uses microwave radiation (electromagnetic waves) to heat the reaction mixture directly.

• Principle: Microwaves interact directly with the molecules (dipolar polarization and ionic conduction), causing rapid and uniform internal heating. Unlike conventional heating (conduction/convection), the vessel walls remain cool.
• Advantages:
  • Faster Reaction: Reaction times reduced from hours to minutes.
  • Uniform Heating: No "hot spots" or overheating at the walls.
  • Energy Efficient: Direct energy transfer reduces wastage.
  • Higher Yields: Fewer side reactions and cleaner products.

ii) Ultrasound Synthesis (Sonochemistry)

This technique uses high-frequency sound waves (ultrasound) to drive chemical reactions.

• Principle (Acoustic Cavitation): When ultrasound passes through a liquid, it creates cycles of compression and expansion. This forms tiny bubbles (cavities) that grow and collapse violently.
  • The collapse generates extremely high local temperatures (~5000 K) and pressures (~1000 atm) for a microsecond.
  • This energy breaks chemical bonds and generates reactive free radicals.
• Advantages: Increases reaction rates, allows reactions to occur under milder conditions, and reduces the need for hazardous solvents.

iii) Photocatalysis

This technique uses light (photons) and a catalyst to accelerate a chemical reaction.

• Principle: A semiconductor photocatalyst (like Titanium Dioxide, TiO₂) absorbs light energy.
  • This excites an electron from the valence band to the conduction band, creating an "electron-hole" pair.
  • These charge carriers generate free radicals (like •OH) on the surface, which can degrade organic pollutants or drive synthesis.
• Advantages: Uses renewable energy (solar light), operates at room temperature, and is excellent for degrading pollutants in water/air.

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

i) Pharmaceutical Industry

• Safer Drugs: Developing new drug synthesis routes that produce less waste.
• Example (Ibuprofen): The traditional synthesis of Ibuprofen was a 6-step process with 40% atom economy. The Green Synthesis is a 3-step process with nearly 77-99% atom economy, drastically reducing waste.

ii) Agriculture

• Biopesticides: Developing pesticides from natural sources (like Neem) that are effective against pests but harmless to humans and wildlife.
• Slow-release Fertilizers: Designing fertilizers that release nutrients slowly to prevent runoff and water pollution.

iii) Polymer Industry (Bioplastics)

• Biodegradable Plastics: Replacing petroleum-based plastics with plastics made from renewable resources like corn starch or sugarcane (e.g., PLA - Polylactic Acid).
• Advantage: These plastics decompose naturally, reducing landfill waste and ocean pollution.

iv) Energy Sector

• Green Fuels: Production of Biodiesel from vegetable oils and Bioethanol from plant starch.
• Battery Technology: Developing safer electrolytes and recyclable battery materials to reduce toxic electronic waste.