Nanochemistry Questions and Answers

Answer: A

Correct answer: 1 to 100 nanometres

The upper bound is a convention rather than a physical boundary, but it is chosen because the size-dependent effects that make nanomaterials interesting - quantum confinement, the dominance of surface atoms, plasmon resonance - become appreciable below roughly a hundred nanometres. A nanometre is a thousand-millionth of a metre, about four atoms across.

Answer: B

A particle a few nanometres across may have a third of its atoms in the surface layer, and those atoms have unsatisfied bonds and different energies from those inside. Reactivity, melting point and colour all follow from that.

Answer: C

Correct answer: rises in inverse proportion to the diameter

For a sphere, \[ \frac{S}{V} = \frac{4\pi r^2}{\frac{4}{3}\pi r^3} = \frac{3}{r} = \frac{6}{d}. \] Area falls as the square of the size and volume as the cube, so their ratio goes as the inverse of the first power. This single relation underlies nanoscale catalysis, the tendency to agglomerate, and the depression of the melting point.

Answer: D

Correct answer: wavelength of its charge carriers

When the particle becomes comparable with the de Broglie wavelength of its electrons and holes - the exciton Bohr radius - the carriers are squeezed and their energy levels become discrete and size-dependent. This is why a quantum dot’s band gap, and so its emission colour, can be tuned simply by changing its diameter.

Answer: A

Tighter confinement raises the gap, so a smaller dot emits bluer light and a larger one redder, from the very same material. Tuning the colour by size rather than by chemistry is the property that made such dots useful in displays.

Answer: B

Surface atoms are less strongly bound than interior ones, and the smaller the particle the larger their share, so less energy is needed to disrupt the lattice. Gold particles a few nanometres across melt hundreds of degrees below bulk gold.

Answer: C

Correct answer: surface atoms are coordinatively unsaturated

An atom in the bulk is surrounded on all sides and has nothing left to bond with. An atom at a corner or an edge of a small particle has neighbours missing, so it retains reactivity, and in a two-nanometre particle a large fraction of the atoms are in that position. The gold is chemically the same; it is the coordination that has changed.

Answer: D

The conduction electrons oscillate collectively at a frequency in the visible, absorbing green light and leaving the solution red. The position of the band shifts with particle size and shape, which makes it a useful diagnostic.

Answer: A

Correct answer: one dimension outside the nanoscale

A nanowire is confined in two dimensions - its cross-section - and extended in the third, so it is a one-dimensional nanomaterial. The convention counts the dimensions that are NOT nanoscale: a quantum dot has none and is zero-dimensional, a nanowire one, and a sheet such as graphene two.

Answer: B

Correct answer: one atom thick and extended in the other two directions

Graphene is a single layer of carbon atoms in a hexagonal lattice, and it is two-dimensional because its thickness is a single atom rather than a nanoscale distance - electrons are free to move in the plane and confined completely across it. Two stacked sheets would be bilayer graphene, whose electronic behaviour is already different.