Nanochemistry Questions and Answers

Answer: C

Correct answer: closed cage of pentagons and hexagons

C₀ is twenty hexagons and twelve pentagons arranged like a football. The twelve pentagons are not incidental: Euler’s theorem requires exactly twelve of them to close a hexagonal network into a sphere, however large the cage.

Answer: D

Correct answer: a graphene sheet rolled into a cylinder

The description is exact enough to be useful: the direction in which the sheet is rolled, expressed as the chiral vector, determines whether the tube is metallic or semiconducting. Armchair tubes are metallic; most others are semiconductors with a gap depending on diameter.

Answer: A

The way the hexagonal lattice wraps around the axis decides how the electronic bands fall, so nominally identical tubes may differ entirely in conduction. A synthesis giving a single chirality would transform the electronics application.

Answer: B

Correct answer: tensile strength and stiffness

A nanotube’s covalent carbon-carbon bonds run continuously along its axis, giving a tensile strength of tens of gigapascals and a modulus near a terapascal - far beyond any conventional fibre. The practical difficulty is not the tube but the composite: transferring load into it, and dispersing tubes that would rather bundle together.

Answer: C

Correct answer: bulk material broken down to nanoscale

Top-down carves the small out of the large: milling, lithography, etching. It is straightforward and scales well but gives broad size distributions and surfaces damaged by the process. Bottom-up assembles from atoms or molecules and gives far better control, which is why the two approaches suit different purposes.

Answer: D

Chemical reduction, sol-gel, vapour deposition and self-assembly all assemble the particle from its constituents, which allows the size and shape to be controlled closely. Scaling such routes up while keeping the uniformity is the difficulty.

Answer: A

Correct answer: reduces a coarse powder by mechanical attrition

High-energy ball milling grinds a coarse powder down by repeated impact between the balls and the vessel, which is the definition of a top-down route. It is cheap and produces kilogram quantities, at the cost of a wide size distribution and contamination from the milling media.

Answer: B

Correct answer: hydrolysis and condensation of a precursor into a gel

A metal alkoxide is hydrolysed to hydroxide, which condenses into a network of particles suspended in liquid - the sol - and then into a continuous gel. Drying and calcining give the oxide. The method is bottom-up, works at low temperature, and allows the composition to be controlled in solution before anything solidifies.

Answer: C

The precursor breaks down on the substrate, often at a catalyst particle that seeds the growth, which is how carbon nanotubes and graphene are grown. Substrate temperature and gas composition control the product closely.

Answer: D

Correct answer: prevent the particles from agglomerating

A capping agent adsorbs on the growing surface and keeps particles apart, by electrostatic repulsion between charged layers or by steric hindrance from long molecules. It also terminates growth at a chosen size. Without it the high surface energy drives the particles together within minutes.