Chemistry of Carbon Nanotubes

Carbon Nanotubes (CNTs): Preparation, Properties, Structure, Bonding and Uses

NEET • JEE Main • JEE Advanced • GATE • CSIR-NET • IIT-JAM • State PSC • BARC

1. Discovery & Basics

• Discovered by Sumio Iijima (NEC, Japan) in 1991
• Cylindrical tubes of graphene sheet(s)
• Diameter: 0.4–100 nm, Length: up to millimetres
• Strongest and stiffest material ever known

2. Types of Carbon Nanotubes

Type	Description	Conductivity
SWCNT (Single-Walled)	One graphene cylinder Diameter ~0.4–2 nm	Can be metallic or semiconducting (depends on chirality)
DWCNT	Double-walled (special case)	Intermediate properties
MWCNT (Multi-Walled)	2–50 concentric tubes Diameter ~2–100 nm	Always metallic (outer wall conducts)

Structure of SWCNT, DWCNT, and MWCNT

JEE Advanced repeated: SWCNT can be metallic (armchair) or semiconductor (zigzag/chiral).

3. Structure & Chirality (Very High Weightage)

• Rolled-up graphene sheet
• Defined by chiral vector C = n a₁ + m a₂ → denoted as (n,m)
• Armchair: n = m → always metallic
• Zigzag: m = 0 → can be metallic or semiconducting
• Chiral: n ≠ m ≠ 0
• Rule of thumb: If (n–m) is multiple of 3 → metallic, else semiconducting

Structure & Chirality of Carbon Nano Tubes

CSIR-NET/GATE favourite: (10,10) armchair → metallic, (10,0) zigzag → semiconducting.

4. Preparation Methods

Method	Key Features	Exam Point
Arc Discharge	Original Iijima method, He atmosphere	Produces both SWCNT & MWCNT
Laser Ablation	High-quality SWCNT, expensive	Rice University method
HiPco (High-pressure CO)	Fe(CO)5 catalyst, industrial	Best for pure SWCNT
CVD (Chemical Vapour Deposition)	CH4/C2H2 + Fe/Mo catalyst on substrate	Most scalable, used commercially

Most asked: CVD is the commercially preferred method for CNT synthesis.

5. Exceptional Physical Properties

Property	Value	Comparison
Tensile strength	~100–200 GPa	~100× steel
Young’s modulus	~1 TPa	Highest known
Density	1.3–1.4 g/cm³	1/6th of steel
Current density	10⁹ A/cm²	1000× copper
Thermal conductivity	~3500 W/m·K (SWCNT)	Higher than diamond
Electrical conductivity	Metallic SWCNT > copper	Ballistic conduction

NEET/JEE repeated: CNTs have highest strength-to-weight ratio and thermal conductivity among known materials.

6. Chemical Properties & Functionalisation

• Inert sidewalls, reactive tips & defects
• Functionalised by:
  • Oxidation → –COOH groups (HNO₃/H₂SO₄)
  • Fluorination → CNT–F
  • Cycloaddition, polymer wrapping
• Functionalisation improves solubility & processability

7. Applications (Extremely High Weightage)

1. Composites – ultra-strong materials (space elevator rope)
2. Electronics – transistors, interconnects, flexible displays
3. Energy storage – Li-ion batteries, supercapacitors
4. Hydrogen storage (theoretical)
5. Biomedical – drug delivery, cancer therapy
6. Sensors – gas, bio-sensors
7. Conductive films – transparent electrodes
8. Field emission displays

JEE Main 2024 asked: “CNTs are used as anode material in lithium-ion batteries due to high surface area and conductivity.”

8. Quick Comparison of Carbon Allotropes

Allotrope	Dimensionality	Conductivity	Strength
Diamond	3D	Insulator	Hardest
Graphite	2D layers	Anisotropic conductor	Soft
Fullerene	0D	Insulator → superconductor	Soft
Graphene	2D	Best conductor	200× steel
CNT	1D	Metallic or semiconducting	Strongest fibre

Must Read Carbon Nanotubes (CNTs) MCQs asked in NEET ▪ JEE Main & Advanced ▪ JAM ▪ GATE ▪ CSIR-NET ▪ State PSC ▪ DRDO

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