Effect of the size and shape of platinum nanoparticles on their optical absorption spectra in aqueous solutions

The optical spectra of platinum sots in aqueous solutions associated with i nterband transitions and resonance plasmon absorption were calculated. For spherical particles 2-8 nm in diameter, the theoretical spectrum was found to be in close agreement with the experimental spectrum, which peaks at 215 mn [the molar absorption coefficient at this wavelength is (6 +/- 1) X 10( 3) M-1 cm(-1)] and then smoothly decays in the direction of longer waveleng ths. As the particle size increases, the absorption band shifts to the visi ble spectrum and the contribution from light scattering to the overall atte nuation increases. For elongated particles, the absorption band is split du e to the overlapping of the transverse and longitudinal resonance spectra. As the degree of elongation of the particle increases, the band associated with longitudinal resonance shifts to longer wavelengths.