Nanochemistry Questions and Answers

Answer: A

Correct answer: 4

With \( E \propto 1/d^2 \), halving the diameter multiplies the confinement energy by \[ \left(\frac{1}{1/2}\right)^2 = 4. \] This is why quantum dot emission is so sensitive to size: a modest change in diameter moves the band gap enough to shift the colour visibly, which is the basis of tunable dot phosphors.

Answer: B

Correct answer: 1-Q, 2-P, 3-R

The dimensionality counts the directions in which carriers are free to move. A quantum dot confines them in all three, so it is zero-dimensional and has discrete atom-like levels. A nanotube leaves one direction free and is one-dimensional. Graphene leaves two free and is two-dimensional.

Answer: C

The three give different sizes for the same sample, because each measures a different thing: the crystallite, the diffusing body with its coating, and the visible outline. Quoting a size without naming the method is meaningless.

Answer: D

The salt and the stabiliser go in first, reduction generates the atoms, nuclei form and grow under the capping agent's control, and the product is then cleaned up. Adding the reductant to an unstabilised solution gives agglomerates.

Answer: A

Correct answer: 1 and 3 only are correct

Statements 1 and 3 are correct: the ratio goes as \( 1/d \), and a smaller dot has a wider gap and so emits at a shorter wavelength. Statement 2 has the sign wrong - the melting point is depressed, not raised, because surface atoms are less tightly bound. Gold nanoparticles a few nanometres across melt hundreds of degrees below bulk gold.

Answer: B

Correct answer: high-energy ball milling

Milling breaks a coarse powder down and is therefore top-down. Chemical reduction of a salt, the sol-gel process and chemical vapour deposition all build the material up from atoms or molecules, which is what bottom-up means - and which is why all three give better control of size than milling does.

Answer: C

Correct answer: their depressed melting point

A lower melting point is a genuine nanoscale effect but it works against catalytic use rather than for it: it is exactly why nanoparticle catalysts sinter and lose activity at high temperature. The three real reasons are the large area per unit mass, the unsatisfied surface bonds, and the high proportion of edge and corner atoms.

Answer: D

Correct answer: sintering of the particles into larger ones

Nanoparticles are thermodynamically driven to coalesce, and heat supplies the mobility to do it. Larger particles have a smaller surface to volume ratio and fewer edge and corner sites, so the activity falls even though the metal is chemically unchanged. Supports and spacers exist largely to keep particles apart and delay this.

Answer: A

Correct answer: the particles have aggregated and settled

A gold colloid is red because of the plasmon resonance of well-separated particles. With no capping agent the surface energy drives them together, the resonance shifts and broadens as they couple, and the aggregates eventually grow large enough to scatter and settle - giving grey. Dissolution would leave a clear solution rather than a grey one.

Answer: B

The smaller the particle the larger the proportion of those weakly bound surface atoms, so less energy is needed to disrupt the whole lattice. The depression can amount to hundreds of degrees for a particle a few nanometres across.