Nanochemistry Questions and Answers

Answer: A

Correct answer: surface energy

A large fraction of a nanoparticle’s atoms are at its surface, each with unsatisfied bonds, so the total surface energy is very high. Merging reduces the total surface area and therefore the energy, which makes agglomeration thermodynamically favourable - a capping agent supplies the kinetic barrier that prevents it.

Answer: B

Non-covalent interactions such as hydrogen bonding and hydrophobic association drive the units into the arrangement of lowest energy without external direction. Designing the units so the desired structure is that minimum is the whole art of it.

Answer: C

Electrons passing through a thin specimen form an image at a resolution far below a nanometre, so individual particles and even lattice planes can be seen. Sample preparation and the small volume examined are its limitations.

Answer: D

Smaller crystallites diffract over a wider range of angles, so the breadth of a peak is inversely related to the size. It gives the crystallite rather than the particle size, which differ if a particle is polycrystalline.

Answer: A

Smaller particles diffuse faster and the scattered intensity fluctuates more rapidly, which yields the hydrodynamic diameter. That diameter includes any adsorbed layer, so it exceeds the size an electron micrograph shows.

Answer: B

Correct answer: adsorbed capping and solvation layer

Dynamic light scattering measures how fast the particle diffuses, and what diffuses is the particle together with everything moving with it - the capping molecules and the associated solvent. Electron microscopy sees only the dense core, so the two figures should differ, and a much larger hydrodynamic diameter is a sign of aggregation.

Answer: C

Correct answer: grown or begun to aggregate

The plasmon resonance depends on particle size and on the coupling between neighbouring particles, and both growth and aggregation shift it to longer wavelength. This is why an aggregating gold colloid turns from red through purple to grey, and why the shift is used as a sensitive optical signal in assays.

Answer: D

Correct answer: far greater surface area

Catalysis happens at surfaces, and dividing a given mass into smaller particles multiplies the surface area available. Dividing a one-centimetre cube into one-nanometre cubes increases the area ten million times. The greater proportion of edge and corner sites adds a further, qualitative advantage.

Answer: A

Correct answer: absorb ultraviolet light while appearing transparent

Titanium dioxide has a band gap in the ultraviolet, so it absorbs there while transmitting visible light. As a coarse powder it scatters visible light strongly and looks white; reduced to well below the wavelength of visible light, the scattering falls away and the film appears transparent while still blocking ultraviolet.

Answer: B

A few per cent of nanoscale clay or nanotube presents more interface than a much larger loading of conventional filler, and the matrix near an interface behaves differently from the bulk. Achieving true dispersion rather than agglomeration is what decides whether the benefit appears.