  When the particle size of a metal is reduced, it is expected that an energy
gap will open due to the quantum size effect. However, the energy gap in
platinum (Pt) metal nanoparticles has not been observed directly by nuclear
magnetic resonance (NMR). To investigate the particle size dependence of the
electronic state of Pt nanoparticles, we performed 195Pt NMR experiments on
thiol-capped Pt nanoparticles with three different average diameters of less
than 3 nm. For the nanoparticles with a diameter of 2.8 nm, we observed usual
metallic behavior with a smaller density of states than that of the bulk Pt. In
contrast, the temperature dependence of 1/T1T in nanoparticles less than 2.5 nm
in diameter is an activation-energy form above 150 K, which is semiconducting
behavior with an energy gap of the order of 2000 K. The significant decrease in
1/T1T by more than two orders of magnitude in the smaller Pt nanoparticles
compared to the bulk Pt is attributable to the disappearance of the density of
states at the Fermi energy, which is consistent with the opening of an energy
gap. These results indicate a metal-insulator transition below 2.5 nm in
diameter is present in our thiol-capped Pt nanoparticle samples. The effect of
the thiol capping on the electronic structure suggested by the experimental
results is also discussed.
