Are nanoparticles thermodynamically more or less stable than the corresponding the bulk material? Explain your answer

They are less thermodynamically stable then bulk material.

Atoms at surfaces have fewer neighbours than atoms in the bulk. Because of this lower coordination and unsatisfied bonds, surface atoms are less stabilized than bulk atoms. The smaller a particle the larger the fraction of atoms at the surface, and the higher the average binding energy per atom.

(ii) Briefly explain the origin of colour in (a) semiconductor nanoparticles and (b) gold and silver nanoparticles.

(a) Chemists may prefer to think of small clusters as molecules. Instead of bands they talk of delocalized molecular orbitals, and the band gap becomes the HOMOâ€“LUMO gap.
It is analogous to the well-known shift of colour of conjugated polyenes with increasing chain length. It can be understood on the basis of the fundamenta lmodel of electrons confined in a spherical square well potential (â€˜particles in a boxâ€™).

The size dependence of the HOMOâ€“LUMO band gap is best seen in the luminescence properties of semiconductor nanoparticles. By adjusting the particle size the fluorescence can be tuned between blue for particles of 1.7 nm diameter and red at 6 nm.

A similar size effect but shifted to higher energy relative to fluorescence applies to the absorbed light. The difference in wavelengths between absorption and fluorescence occurs because after absorption the system relaxes to adapt to the new charge distribution.

(b) Color of gold and silver nanoparticles comes from the surface plasmon band (SPB), a broad absorption band in the visible region around 520 nm.

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