Surface Raman excitation and enhancement profiles for chromate, molybdate, and tungstate on colloidal silver

Abstract

The surface-enhanced Raman scattering (SERS) excitation profiles of chromate, molybdate, and tungstate ions on colloidal silver were obtained by using excitation wavelengths between 457.9 and 676.4 nm. The intensity of the strongest SERS band between 800 and 900 cm/sup -1/, assignable to nu/sub s/(M-O), was in each case referenced to the 1020-cm/sup -1/ band of the internal standard, methanol, in silver hydrosols. These relative SERS intensities were compared against similar intensity ratios for each oxo anion in solution and then appropriately scaled for the concentration of MO/sub 4//sup 2 -/ in solution and on the surface of the silver particles in the sols. Peak SERS enhancements occurred at 560 nm for CrO/sub 4//sup 2 -/ and at 600 nm for both MoO/sub 4//sup 2 -/ and WO/sub 4//sup 2 -/. The latter peak positions matched the secondary absorption band maxima that originate from aggregates of silver particles in the sols. The numerical values of the SERS enhancements were 5 x 10/sup 4/-l x 10/sup 5/ for CrO/sub 4//sup 2 -/, 9 x 10/sup 5/ for MoO/sub 4//sup 2 -/, and about 2 x 10/sup 5/ for WO/sub 4//sup 2 -/ on colloidal silver. The somewhat lower SERS enhancements for themore » chromophoric adsorbate, CrO/sub 4//sup 2 -/, which adsorbs maximally at 370 nm in aqueous solution, are attributed to a lower degree of aggregation for the sols with added CrO/sub 4//sup 2 -/, rather than to partial quenching of its resonant excited state on the heavy metal (silver) surface. 34 references, 4 figures, 5 tables.« less