SERS at Structured Palladium and Platinum Surfaces Mamdouh E. Abdelsalam, ² Sumeet Mahajan, ² Philip N. Bartlett,* ,² Jeremy J. Baumberg, ‡ and Andrea E. Russell ² Contribution from the School of Chemistry, UniVersity of Southampton, Southampton, SO17 1BJ, U.K., and School of Physics and Astronomy, UniVersity of Southampton, Southampton, SO17 1BJ, U.K. Received February 22, 2007; E-mail: pnb@soton.ac.uk Abstract: Palladium and platinum are important catalytic metals, and it would be highly advantageous to be able to use surface enhanced Raman spectroscopy (SERS) to study reactive species and intermediates on their surfaces. In this paper we describe the use of templated electrodeposition through colloidal templates to produce thin (<1 μm) films of palladium and platinum containing close packed hexagonal arrays of uniform sphere segment voids. We show that, even though these films are not rough, when the appropriate film thickness and sphere diameter are employed these surfaces give stable, reproducible surface enhancements for Raman scattering from molecules adsorbed at the metal surface. We report SERS spectra for benzenethiol adsorbed on the structured palladium and platinum surfaces of different thicknesses and void diameters and show that, for 633 nm radiation, enhancements of 1800 and 550 can be obtained for palladium and platinum, respectively. Introduction Surface enhanced Raman spectroscopy (SERS) is a very sensitive technique. It has been reported that molecules adsorbed on an electrochemically roughened silver substrate produce a Raman spectrum that is, in some cases, a million fold more intense than expected. 1-3 Therefore SERS has been widely used to identify molecules adsorbed at metal surfaces and, for a range of analytical applications, 4,5 to study intermediates in electro- chemical reactions 6,7 and to study the structure of the electrode/ electrolyte interface. 8,9 Unfortunately, surface enhancement at roughened metal surfaces is only strong on the coinage metals, Ag, Au, and Cu, and this limitation severely reduces the range of applications of SERS. Although the Pt-group metals, e.g., palladium and platinum, have better surface stability and find much wider application as electrodes and catalysts in electro- chemistry and surface science, they have been commonly considered as non-SERS-active substrates. There have been considerable efforts over the past 20 years to expand the use of SERS to the Pt-group metals, particularly by the groups of Tian, 10-12 Weaver, 13-16 and Pe ´rez. 17,18 Tian’s group have made extensive studies of surface roughen- ing and surface structuring to create SERS active palladium and platinum surfaces using surface roughening by repetitive potential cycling 10,11,19 and by chemical etching. 20 Low quality SERS and resonance Raman spectra have been reported for adsorbates with a large Raman cross section on roughened platinum surfaces 21 and mechanically polished Pt electrodes. 22 The disadvantage of these roughened surfaces is that they have a wide distribution of surface features that vary in shape and size ranging from nanometers to microns. Thus, it is very hard to judge which feature or size of surface geometry causes the major part of the SERS activity. 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