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SCYA1101: Engineering Chemistry UNIT 4 : Carbon Materials for Health, Stealth and Energy
UNIT 4
CARBON MATERIALS FOR HEALTH, STEALTH AND
ENERGY
Introduction to Carbon Materials: Fullerenes: Production, Properties and Applications – Van Der Waal’s Solids: Structure of Graphene, Graphene Oxide and Reduced Graphene Oxide – Mechanical and Electrical Properties of Graphene – Graphene Based Energy Storage Devices for Space Applications – Carbon Nanotubes: Singlewalled and Multiwalled CNTs – Synthesis of CNTs by Thermal CVD and Laser Ablation Method – Electrical and Mechanical Properties of CNTs – Applications of CNTs.
4.1 INTRODUCTION TO CARBON MATERIALS
1. What are the allotropes of carbon?
Graphite, diamond, fullerenes and carbon nanotubes.
2. Classify carbon nanomaterials based on dimensions.
Zero-dimensional carbon nanomaterials: Fullerenes
One-dimensional carbon nanomaterials: Carbon nanotubes
Two-dimensional carbon nanomaterials: Graphene
3. Differentiate graphene, fullerene and carbon nanotubes.
Graphene is a single atomic plane of graphite. It has two-dimensional planar sheet of sp^2 hybridized carbon atoms with a carbon-to-carbon bond length of 0.142 nm.
Fullerenes are zero dimensional nanomaterials. Fullerene, C 60 is a single, stable molecule containing 60 carbon atoms. It is of about 1nm in diameter.
Carbon nanotubes are one dimensional cylindrical nanomaterial represented by folding of graphene sheet.
4.2 FULLERENES
1. Define fullerenes and fullerene C 60?
Definition of fullerenes: Fullerenes are closed hollow cages made of sp^2 hybridized carbon atoms. The surface of fullerenes consists of 12 pentagons and a predictable number of hexagons that depends on the total number of carbon atoms present in it.
Definition of fullerene C 60 : Fullerene C 60 is a single, stable molecule consisting of 60 carbon atoms. The shape is the same as that of a soccer ball. It is of about 1 nm in diameter.
SCYA1101: Engineering Chemistry UNIT 4 : Carbon Materials for Health, Stealth and Energy
2. Explain the synthesis of fullerene C 60 by Kratschmer-Huffman electric arc technique.
Synthesis of Fullerenes by Kratschmer-Huffman Electric Arc Technique
The Kratschmer-Huffman method (1990) is a straight forward, low-cost method used for producing large quantities of fullerene containing carbon soot (dust).
In Kratschmer-Huffman method, large quantities of C 60 are obtained by producing an arc between two graphite rods to burn in a helium atmosphere and extracting the carbon condensate so formed using a suitable organic solvent.
To vacuum
Gas inlet Graphite electrodes
Flubby carbon condensate, soot
The following are the conditions required for C 60 fullerene.
Electrodes Pure graphite rods Diameter of electrodes 6 mm Gap between the electrodes 1 mm Current 50 to 120Amperes Voltage 20 to 25V Inert gas pressure Helium atmosphere with 100 to 200 Torr Temperature rises during arc discharge
3000˚C
The apparatus consists of two graphite electrodes (6 mm in diameter) maintained at a constant distance (1 mm) from each other by a motor inside a vacuum vessel.
When a sufficient intensity of current 50 – 120 amperes with a DC potential of 20– 25 volts is applied to the electrodes in presence of inert helium pressure of 100– 200 Torr, an arc current mostly carried by electrons is produced with a plasma temperature of 3000°C at the of tip of anode.
Since the tip of the anode reaches higher temperatures, it causes the vaporization of the anode material and produces fullerene containing fluffy condensate known as soot.
The fullerenes containing soot are then extracted by solvation in a small amount of toluene.
SCYA1101: Engineering Chemistry UNIT 4 : Carbon Materials for Health, Stealth and Energy
- Biological Properties
The structural morphology makes C 60 a very useful radical scavenger.
Because of its spherical shape and electron-deficient character with 30 number of C=C bonds, Fullerene C 60 reacts easily with all kinds of free radicals and used as an antioxidant in cosmetics and biological systems.
4. Explain the various applications of fullerenes.
S. No. Role as / in Applications
- Radical scavenger Fullerenes react readily with free radicals because of the presence of large amounts of conjugated double bonds can act as the most efficient radical scavenger.
This property makes use of C 60 molecules to act as antioxidant.
- Photovoltaics Fullerenes find applications in photovoltaic devices, organic electronics and photo catalysis because of its electro donor-acceptor properties.
- Water Purification Functionalizing C 60 with hydrophilic moieties can produce a material that can inactivate pathogenic microorganism via photo catalytic processes.
Therefore, fullerenes have a potential to act as adsorbents for removing organic compounds.
- Hydrogen Storage Fullerenes can be used as ideal storage molecules for hydrogen.
Fullerenes have the ability to retain a maximum of 6.1% of hydrogen due to their chemistry and cage molecular structure.
- Energy Materials A hybrid system of fullerene and graphene molecules has a specific capacitance of 135 Fg-^1 when used as electrode material for super capacitors. But, the specific capacitance of pure graphene electrode is only 102 Fg-^1.
4.3 CARBON NANOTUBES
1. What are carbon nanotubes?
Carbon nanotubes (CNTs) are one dimensional cylindrical nanomaterial represented by folding of graphene sheet.
CNTs exhibit unusual strength and unique electrical properties, and are efficient conductors of heat.
SCYA1101: Engineering Chemistry UNIT 4 : Carbon Materials for Health, Stealth and Energy
2. Differentiate single-walled carbon nanotube and multi-walled carbon nanotubes.
If a nanotube has a single cylinder of graphene sheet, it is known as SWNT. It is characterized with a diameter of 1- 2 nm and a length of 1- 100 nm.
If a nanotube has a number of concentric rings of graphene sheets, it is known as MWNT. It is characterized with an inner diameter of 1- 2 nm, an outer diameter of 2 - 20 nm with inter tubular distance of 0.34 nm and a length of 1- 100 μm.
3. How are CNTs classified on the basis of chirality (or) helicity?
Chirality (or) Helicity of carbon nanotubes refers to the rolling of hexagonal chains with respect to the tube axis.
Helicity results three different types of carbon nanotubes. (i) Armchair tube (Metallic conducting) (ii) Zig-zag tube (Semiconducting) (iii) Chiral tube (Semiconducting)
4. What are the unique properties of CNTs?
CNTs are 100 times stronger and about six times lighter than steel.
CNTs can conduct electricity like copper and acts as semiconductor like silicon.
CNTs can transport heat 10 times higher than silver.
5. Discuss the thermal CVD method and laser ablation methods of synthesis of nanomaterials. 1. Chemical Vapour Deposition (CVD) Method
Substrate
Vacuum Pump
C 2 H 2
Furnace^ N 2
Quartz Tube
Thermal CVD can be used to synthesis both SWCNTs as well as MWCNTs.
A thermal CVD reactor is simple and inexpensive to construct. It consists of a quartz tube enclosed in a furnace.
The substrate material may be silica, mica, quartz, or alumina and coated with nanoparticle catalysts like Fe, Co and Ni.
SCYA1101: Engineering Chemistry UNIT 4 : Carbon Materials for Health, Stealth and Energy
The high intensity laser vapourises graphite and produces carbon molecules and atoms.
The argon gas sweeps theses carbon atoms from the high temperature zone to the colder copper collector, where they condense and grow as carbon nanotubes.
6. Discuss in detail the electrical and mechanical properties of carbon nanotubes. 1. Electrical properties of CNTs
Carbon nanotubes (CNTs) can be both metallic or semiconducting, depending upon the helicity (chirality) and diameter of the nanotubes.
Chirality refers to the rolling of graphene sheet with respect to tube axis.
Rolling of C-atoms in graphene sheet results three types of CNTs.
✓ Rolling of graphene sheet in vertical direction with respect to tube axis results zig- zag NTs. ✓ Rolling of graphene sheet in horizontal direction with respect to tube axis results arm chair CNTs. ✓ Rolling of graphene sheet in diagonal results chiral CNTs.
( 0,0 ) ( 1,0 ) ( 2,0 ) ( 3,0 ) ( 4,0 ) ( 5,0 )
(0,1) ( 1,1 ) (2,1) (^) (3,1) ( 4,1 ) (5,1)
(0,2) (1,2) (^) ( 2,2 ) (3,2) (4,2)
(0,3) (1,3) (2,3) ( 3,3 ) (^) (4,3)
(0,4) (1,4)^ (2,4)^ (3,4) ( 4,4 ) a 1
a 2
( 6,0 )
( 5,2 )
(5,3)
(5,4) (^) (6,4)
( 6,3 ) (7,3)
(6,2) (7,2)
(6,1)
( 7,0 ) ( 8,0 )
(7,1) (8,1)
(8,2 )
Metallic conducting (^) Semiconducting
Tube axis
n axis
m axis
The general rules to predict their electrical conductivity are:
Metallic properties would dominate when n = m (or)
𝑛 −𝑚 3 =^ integer.
Semi conducting properties would dominate when
𝑛 −𝑚 3 ≠^ integer.
Hence, (7,1) and (5,2) tubes would be metallic whereas (8,0) and (6,1) tubes would be semiconducting and, the arm chair tube (5,5) tube would always be metallic conducting due to the absence any defects.
SCYA1101: Engineering Chemistry UNIT 4 : Carbon Materials for Health, Stealth and Energy
In metallic state, the conductivity of CNTs is very high.
It is estimated that CNTs can carry billion amperes of current per square centimeter. The copper wire fails at one million amperes of current per square centimeter because resistive heating which melts the wire.
The chiral and zigzag tubes show a very small band gap due to the replacement of hexagonal carbon ring with a pentagonal or heptagonal ring. This kind of defects can severely affect the electrical conductivity of nanotubes.
It has been observed from a plot energy gap (Eg) versus 1/Diameter that, as the diameter of the one-dimensional carbon tube increases, its band gap energy decreases.
E
ne
rg
y^
ga
p,
E
g
0 1 2 3 4 5
2
4
6
(^0). 8
- 0
1 /Diameter, (nm)
2. Mechanical Properties of CNTs
S. No. Properties Description
- Tensile strength It is a measure the amount of stress needed to pull a material.
CNTs have high tensile strength of 63 GPa while steels have only 1-2 GPa.
It means that CNTs are 50-60 times stronger than steel.
There are two things accounts for greater strength of CNTs.
✓ The strength is provided by the interlocking of carbon-to-carbon covalent bonds
✓ Each CNT is one large molecule. It means that, it does not have any weak spots like grain boundaries, dislocation etc.
SCYA1101: Engineering Chemistry UNIT 4 : Carbon Materials for Health, Stealth and Energy
4.4 VAN-DER WAALS SOLIDS
4.4.1 Graphene
1. Define graphene.
According to the definition provided by IUPAC, graphene is ‘a single atomic plane of graphite’.
2. Write a short note on the structure of graphene.
Graphene is a two-dimensional planar sheet of sp^2 hybridized carbon atoms, which are arranged into a honeycomb lattice with a carbon-to-carbon bond length of 0.142 nm.
In graphene, carbon orbitals 2s, 2px, 2py form the hybrid orbital sp^2 with three major lobes at 120°. The remaining orbital, pz, is oriented perpendicular to the graphene's plane.
3. Mention any two important mechanical properties of graphene.
Strength : Defect-free, monolayer graphene is considered as the strongest material with an intrinsic strength of 130 GPa.
Stiffness : It has been experimentally observed that graphene has a Young's modulus of 1.0 ± 0.1 TPa with an effective thickness of 0.335 nm.
4. Explain the electronic properties of graphene.
The graphene electrons act very much like photons in their mobility due to their lack of mass. These free massless mobile electrons act as charge carriers and are able to travel sub-micro-meter distances without scattering. This phenomenon is known as ballistic transport.
Tests have shown that the electronic mobility of graphene is very high (above 15, cm^2 V−1^ s−1) with the potential producing up to 2,00,000 cm^2 V−1^ s−1.
5. Write any two applications of graphene.
Graphene can be used as a potential candidate as electrode materials for energy storage applications because of strong inter-sheet Van der Waals force of attraction.
Due to their excellent electron-transport properties and extremely high carrier mobility, graphene is used with transition-metal dichalcogenides and black phosphorus for making solar cell electrodes that are inexpensive, lightweight and flexible.
SCYA1101: Engineering Chemistry UNIT 4 : Carbon Materials for Health, Stealth and Energy
6. Discuss the structure, synthesis and properties of graphene.
Definition of Graphene
According to the definition provided by IUPAC, graphene is ‘a single atomic plane of graphite’.
Structure of Graphene
Graphene is a two-dimensional planar sheet of sp^2 hybridized carbon atoms, which are arranged into a honeycomb lattice with a carbon-to-carbon bond length of 0. nm.
In graphene, carbon orbitals 2s, 2px, 2py form the hybrid orbital sp^2 with three major lobes at 120°. The remaining orbital, pz, is oriented perpendicular to the graphene's plane.
Synthesis of Graphene
S. No. Name of Method Description
- Mechanical Exfoliation (Top-down method)
Mechanical exfoliation is a top-down fabrication method, graphite (graphene precursor) is split into layer by layer forming graphene sheets by using ‘the scotch tape’.
- Chemical Vapour Deposition (CVD) Method (Bottom-up method)
Graphene and few-layer graphene can be grown by chemical vapor deposition (CVD) method.
CVD is a bottom-up approach in which the carbon containing precursor in gaseous form (methane or ethylene) is passed over the catalytic surface (Cu or Ni or Pt).
The direct decomposition of the precursor releases carbon atoms in a controlled manner. Further, these carbon atoms align themselves and form graphene on copper.
SCYA1101: Engineering Chemistry UNIT 4 : Carbon Materials for Health, Stealth and Energy
- Ballistic Transport of Graphene’s Electrons
The graphene electrons act very much like photons in their mobility due to their lack of mass.
These free massless mobile electrons act as charge carriers and are able to travel sub-micro-meter distances without scattering. This phenomenon is known as ballistic transport.
Tests have shown that the electronic mobility of graphene is very high (above 15,000 cm^2 V−1^ s−1) with the potential producing up to 2,00,000 cm^2 V−1^ s−1.
7. Explain the graphene-based energy storage devices for space applications.
S. No. Role of Graphene as Description
- Electrode Materials Graphene can be used as a potential candidate as electrode materials for energy storage applications because of strong inter-sheet Van der Waals force of attraction.
Further, the large surface area, micro-porosity and, good electrical conductivity of graphene improves the energy density, capacity and charge rate in rechargeable batteries.
- Barrier Materials Graphene has the ability to suppress the growth of dendrites (branch-like filament deposits) on the electrodes that can penetrate the barrier between the two halves of the battery and potentially cause electrical shorts, overheating and fires.
- Flexible and Inexpensive Solar Cell Electrodes
Due to their excellent electron-transport properties and extremely high carrier mobility, graphene is used with transition-metal dichalcogenides and black phosphorus for making solar cell electrodes that are inexpensive, lightweight and flexible.
- Supercapacitors Graphene with 96% carbon content can be used for fabricating supercapacitors with high energy density and power density that can be used in spacecraft, aircraft, and other commercial applications.
- Robotic and Human Martian Exploration Missions
Graphene based energy storage device with extraordinary capacity, energy density with 10, charge/discharge cycles is used for NASA robotic and human Martian exploration missions.
SCYA1101: Engineering Chemistry UNIT 4 : Carbon Materials for Health, Stealth and Energy
4.4.2 Graphene Oxide (GO)
1. Define graphene oxide.
Graphene oxide comprises of a large number of oxygen functionalities including carboxyl (COOH), hydroxyl (OH) and epoxide groups.
2. Write a short note on the structure of graphene oxide.
Graphene oxide (GO) has a similar hexagonal carbon structure like graphene but it is enriched with reactive oxygen functional groups on the basal plane with carboxyl- functionalized edges.
According to L–K model (Lerf and Klinowski), the hydroxyl and epoxy groups are randomly distributed on the graphene oxide single layer, while the carboxyl and carbonyl groups are introduced at the edge of the single layer.
3. Explain the mechanical properties of graphene oxide.
Monolayer graphene oxide has a lower effective Young’s modulus (~ 23.4 GPa when a thickness of 0.7 nm is used) as compared to the “pristine” graphene (Young’s modulus of ~1.0 TPa) and the ultimate breaking strength of ~130 GPa. The lower Young’s modulus of GO is due to the defects in the graphitic structure formed during oxidation.
4. Explain the electronic properties of graphene oxide.
Graphene oxide is very much electrically insulated due to its disrupted sp^2 bonding interactions.
However, the electrical conductivity of graphene oxide can be improved by restoring the π bonding network through a rapid reaction known as reduction.
5. Write any two applications of graphene oxide.
Biomedical Applications : The functional groups on the surface of graphene oxide allow interaction with organic and inorganic molecules by covalent, non-covalent (π-π or hydrophobic) and/or ionic interactions. This enables the use of GO for drug delivery applications.
Electronic Devices: Graphene oxide can be deposited as transparent conductive films on any substrate. Such coatings could be used in flexible electronics, solar cells, liquid crystal devices, chemical sensors, and touch screen devices.
SCYA1101: Engineering Chemistry UNIT 4 : Carbon Materials for Health, Stealth and Energy
Properties of Graphene Oxide
S. No. Properties Description
- Mechanical Properties Monolayer graphene oxide has a lower effective Young’s modulus (~ 23.4 GPa when a thickness of 0.7 nm is used) as compared to the “pristine” graphene (Young’s modulus of ~1.0 TPa) and the ultimate breaking strength of ~130 GPa.
The lower Young’s modulus is due to the defects in the graphitic structure formed during oxidation.
- Electronic Properties Graphene oxide is very much electrically insulated due to its disrupted sp^2 bonding interactions.
However, the electrical conductivity of graphene oxide can be improved by restoring the π bonding network through a rapid reaction known as reduction.
- Optical Properties The optical transparency greater than 90% can be achieved with graphene oxide films less than 5 layers thick. Each graphene layer reduces transparency by about 2%.
Transparent conductive inks and films can be made by combining GO with carbon nanotubes or silver nanowires.
- Physical Properties Graphene oxide has high solubility in many solvents and polymers, which enables the solution processing properties of graphene oxide.
It can be patterned and reduced with a laser, even using a DVD writer in a PC.
Applications of Graphene Oxide
S. No. Role of Graphene Oxide as
Description
- Drug Delivery The functional groups on the surface of graphene oxide allow interaction with organic and inorganic molecules by covalent, non-covalent (π-π or hydrophobic) and/or ionic interactions. This enables the use of GO for drug delivery applications.
- GO-based Electronic Devices
Graphene oxide can be deposited as transparent conductive films on any substrate. Such coatings could be used in flexible electronics, solar cells, liquid crystal devices, chemical sensors, and touch screen devices.
SCYA1101: Engineering Chemistry UNIT 4 : Carbon Materials for Health, Stealth and Energy
- Water Filter Permeation of water through the membrane was attributed to swelling of GO structures, which enables a water penetration path between individual GO layers. The interlayer distance of dried graphite oxide is 6.35 Å, but in liquid water it is increased to 11.6 Å. Therefore, GO could be used as membrane for water filter.
Further, GO can also be used as cation exchange membrane if water containing the dissolved salts like KCl, HCl, CaCl 2 , MgCl 2 , and BaCl 2.
4.4.3 Reduced Graphene Oxide (rGO)
1. Define reduced graphene oxide.
The graphene oxide containing less of hydroxyl and carboxyl groups at the basal plane at the edges is known as reduced graphene oxide.
2. Write a short note on the structure of reduced graphene oxide.
The graphene oxide containing less of hydroxyl and carboxyl groups at the basal plane at the edges is known as reduced graphene oxide. The loss of oxygen functional groups in graphene oxide results in the less hydrophilic reduced graphene oxide (rGO).
3. Write the percentage elemental composition of reduced graphene oxide.
Reduced graphene oxide is composed of four elements: The oxygen content varies from 13 – 22%. The hydrogen content ranges from 0 – 1%. The nitrogen content ranges from 0 – 1%. The carbon content varies from 77 – 87%.
4. What are the unique properties of reduced graphene oxide?
The electrical conductivity of reduced graphene oxide is S cm−1^ and high mobility of 320 cm^2 V−1^ s−1. rGO sheets show strong mechanical strength with Young’s modulus of ~1.0 TPa and breaking strength of ~130 GPa which is similar to the graphene. The BET surface area of reduced graphene oxide is 500 m^2 /g. The density of reduced graphene oxide is 1.91 g/cm^3.
SCYA1101: Engineering Chemistry UNIT 4 : Carbon Materials for Health, Stealth and Energy
S. No. Properties Description
- Mechanical Properties rGO sheets show strong mechanical strength, with Young’s modulus of ~1.0 TPa and breaking strength of ~130 GPa which is similar to the graphene.
- Electronic Properties One of the most important effects of GO reduction process is the increase of electric conductivity up to 6300 S cm−^1 and high mobility of 320 cm^2 V−^1 s−^1.
- Physical Properties As opposed to GO, reduced graphene oxide acquires a hydrophobic behavior due to the increased C/O ratio of the structure.
- Chemical Properties The surface area of rGO also increases during the reduction process. The BET surface area of reduced graphene oxide is 500 m^2 /g.
Applications of reduced graphene oxide
S. No. Role of Reduced Graphene Oxide as
Description
- Electronic Devices rGO has been used for fabricating field effect transistors, chemical sensors and biosensors.
rGO has been used as transparent electrodes in the visible range of light for light emitting diodes (LEDs) and solar cell devices.
rGO has also been used as a hole transport layer in OLEDs.
- Energy Storage rGO nanocomposites have been used for high- capacity energy storage in lithium-ion batteries.
High surface area GO can be synthesized by using microwaves for exfoliation and can be reduced. The high surface area rGO is used for energy storage material in supercapacitors.
