Surface-enhanced Raman scattering and the preparation of copper substrates for catalytic studies

Abstract

Copper surfaces producing enhancement of the Raman scattering cross-section for adsorbed molecules comparable to the largest enhancements observed on silver have been prepared. These substrates are suitable for investigations of reactant intermediates in heterogeneous catalytic reactions, since they exhibit strong enhancement under atmospheric conditions. Roughening of the Cu surface involves etching with 2 mol dm^–3 HNO₃ or a combination of sandblasting and etching. The dye Nile Blue was adsorbed to test the Raman activity. The enhanced Raman spectrum of the molecule is analysed, and the dependence of the enhancement on etch time is investigated. For dye-laser excitation at 662 nm the observed surface enhancement is of the order of 10³–10⁴ for Nile Blue, which has an electronic absorption resonance, and of the order of 10⁵–10⁶ for p-nitrobenzoic acid, which is non-absorbing. Electron micrographs of the Cu surfaces show that with both roughening procedures maximum enhancement coincides with the creation of surface roughness features on the 10–100 nm scale, which corroborates present views on the size dependence of the electromagnetic enhancement contribution.