Nanomaterials
Nanoparticles of metals such as gold and tin melt at temperatures below the bulk melting point when their size falls to a few nanometers. Which is the main cause of this melting point depression?
- AMore atoms sit at the surface, and the surface energy raises the free energy of the solid, so the liquid becomes stable at a lower temperature
- BAt a few nm, the quantum size effect widens the band gap and weakens metallic bonding, so the atoms move more freely and the particle melts more easily
- CThe smaller the particle, the larger the heat of fusion per atom, so melting proceeds all at once from the surface to the interior with less heating
Show answer and explanation
Answer: A. More atoms sit at the surface, and the surface energy raises the free energy of the solid, so the liquid becomes stable at a lower temperature
Surface atoms have lost some of their bonds, so a surface energy term is added to the free energy of the particle. The smaller the particle, the larger the fraction of surface atoms and the more the free energy of the solid is pushed up, so the temperature at which the free energies of solid and liquid become equal, the melting point, falls (the Gibbs–Thomson effect). Considering the solid–liquid interfacial energy, the melting point depression is roughly inversely proportional to particle size. The quantum size effect concerns electron energy levels and is not the main cause of melting point depression in metals. Also, the heat of fusion actually tends to decrease in nanoparticles, so the reasoning that “a larger heat of fusion makes melting easier” runs backward.