This set of Nanotechnology Multiple Choice Questions & Answers (MCQs) focuses on “Carbon Nanotubes Synthesis – Set 2”.
1. What is the correct range of pressure applied in HiPCO method of CNT synthesis?
a) 10 – 30 atm
b) 30 – 50 atm
c) 50 – 70 atm
d) 70 – 90 atm
View Answer
Explanation: HiPCO – High Pressure Carbon Monoxide method of CNT production is executed at a very high pressure of about 30 – 50 atm. It also operates at a high temperature range of about 800 – 1000°C. Here, carbon monoxide is used as a precursor while iron or nickel carbonyl is implemented as catalyst for CNT formation.
2. Which expression from the given options correctly indicates the forest height in super growth CVD?
a) H(t) = βτ0(1-e-t/τ0)
b) H(t) = β(1-e τ0)
c) H(t) = (β/τ0)(1-e)
d) H(t) = β2(τ0-e-t)
View Answer
Explanation: Super – growth CVD is the water – assisted chemical vapour deposition method. In this process, the CVD reactor is supplied with water in order to enhance the activity and lifetime of the catalyst. The tall vertically aligned nanotube arrays (VANTAs) formed during this process are known as forests. The height of these forests can be expressed as;
H(t) = βτ0 (1-e-t/τ0)
where β = initial growth rate; τ0 = characteristic catalyst lifetime.
3. Low temperature CVD yields MWCNTs while high temperature CVD gives SWCNTs.
a) True
b) False
View Answer
Explanation: It has been observed that low-temperature CVD (600 – 900°C) yields MWCNTs, while high-temperature CVD (900-1200°C) favours SWCNTs growth. Hence, MWCNTs are easier to grow from most of the hydrocarbons while synthesis of SWCNTs, owing to higher energy of formation, is faced with difficulties. SWCNTs grow from selected hydrocarbons.
4. Find out a disadvantage of the CVD method used for CNT production.
a) Consistent alignment of nanotubes
b) Evaporation from solid source is not involved
c) Large sized growth area tend to wrap and shrink
d) Both SWCNT and MWCNT can be produced using CVD
View Answer
Explanation: The CVD method is the most effective procedure for CNT synthesis. However, there are certain downsides to this process too. First, the sizes of growth area being arbitrary large sized areas tend to shrink, crack or wrap. Second, the catalyst support needs to be removed using an acid that can often destroy the original structure of the carbon nanotubes.
5. Strong catalyst-substrate interactions lead to tip-growth mechanism for CNT development.
a) True
b) False
View Answer
Explanation: When the catalyst – substrate interaction is strong that is if the metal has an obtuse contact angle with the substrate, base – growth mechanism is followed for CNT development. While weak catalyst – substrate interaction that is if the metal has an acute contact angle with the substrate, tip-growth mechanism is followed.
6. Which among the following is not used as a precursor of CNT?
a) Xylene
b) Ethanol
c) Nitric acid
d) Methane
View Answer
Explanation: CNT precursors that are commonly used are methane, ethylene, benzene, xylene, carbon monoxide etc. However, ethanol has recently become the most popular CNT precursor since ethanol-growth CNTs have shown very high purity that is almost free from amorphous carbon.
7. Which among the following is not an advantage of the Arc discharge method for CNT production?
a) Abundance of CNTs produced
b) Random size of the tubes formed
c) High quality product
d) CNTs formed have fewer structural defects
View Answer
Explanation: Arc discharge method for synthesis of CNT has multiple advantages. These include i) High quality of CNT production, ii) Simple procedure, iii) Inexpensive method of synthesis, iv) Abundance of CNT with fewer structural defects.
8. Choose the incorrect statement from the following options with respect to Laser Ablation Method.
a) Pulsed laser beam is introduced and focused onto the graphite target
b) Cobalt and nickel fail to act as catalytic nucleation sites
c) Target is vaporized at a high temperature of about 1200°C
d) The SWCNTs produced are conveyed by the buffer gas to the collection point
View Answer
Explanation: In laser ablation method, CNTs are made by laser evaporation. A quartz tube containing graphite is heated in a furnace, in the presence of argon gas. An intense pulsed laser vaporizes the graphite target. This vaporized carbon is carried away by the inert gas and condensed in the cold copper collector as nanotubes. Composite of graphite and metal catalysts such as cobalt and nickel have very high quality yield.
9. The below reaction takes place in which method of CNT formation?
Li2CO3 ⟶ Li2O + CNTs + O2
a) Laser Ablation
b) n-hexane pyrolysis
c) Liquid electrolysis method
d) Super-growth CVD
View Answer
Explanation: Liquid electrolysis method for CNT production has similar mechanism to that of CVD. Here, molten carbonates are electrolysed in order to synthesize MWCNTs. The metals that attach to the cathode after getting reduced from metal ions act as a nucleation point for the growing CNTs. The reaction occurring at the cathode is
Li2CO3 ⟶ Li2O + CNTs + O2
10. Identify the correct statement from the given options.
a) Transition metals like iron, cobalt and nickel only can act as CNT catalysts
b) CNT growth in the absence of a metal is impossible
c) A particular catalyst can function differently on different support materials
d) Zeolite substrates having catalyst in its nanopores have resulted in poor yields of CNTs
View Answer
Explanation: For growing CNTs several types of catalysts and substrates are used. Interestingly, a particular catalyst can work differently on different support materials. For an effective CNT growth, the catalyst-substrate interaction should be favourable.
11. Solid organometallocenes are poor catalysts for CNT production.
a) True
b) False
View Answer
Explanation: Research has revealed that solid organometallocenes such as cobaltocene, ferrocene, nickelocene can be widely used as catalyst for CNT synthesis. This is because these substances can liberate metal nanoparticles insitu that catalyzes the hydrocarbon decomposition more efficiently.
Sanfoundry Global Education & Learning Series – Nanotechnology.
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