Optical absorption of nanoscale colloidal silver: Aggregate band and adsorbate-silver surface band

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Abstract
The absorption band (band A) due to the aggregate effect of nanoscale silver particles in silver sol prepared by a modification of the procedure of the original silver sol used in surface-enhanced Raman scattering (SERS) was observed near 435 nm without the presence of any foreign molecules. This band eventually stabilized at 435 nm and was not significantly shifted by added molecules and chlorions, which differs from the properties of the well-known absorption band of the original silver sol appearing in the longer wavelength region upon addition of foreign molecules. A new absorption band (band B) appeared on the longer wavelength side of the 435 nm aggregate band with the addition of pyridine to the modified silver sol. This band can be associated with chemisorption of molecules on colloidal silver and probably to the charge–transfer transition band between silver particles and adsorbed molecules. The effects of aggregate and chemisorption are clearly separated and identified to band A and band B, respectively.

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