Investigation of cyanide ions adsorbed on platinum and palladium coated silver island films by surface-enhanced Raman spectroscopy

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Abstract

Bimetallic films prepared by vacuum-depositing platinum onto silver island films have been evaluated for surface-enhanced Raman scattering (SERS), using CN as the adsorbate. For platinum overlayers with mean thickness corresponding to 1–2 monolayers the spectrum is dominated by a band consistent with either CN adsorbed at the platinum surface or with Pt(CN)42−. For thinner platinum overlayers, changes in the spectra due to migration of CN from Ag(CN)2 to the surface platinum atoms were observed during the first few minutes of laser illumination. Similar palladium-on-silver films showed a prominent SERS band assigned to surface Pd(CN)42−.

Introduction

Surface-enhanced Raman spectroscopy (SERS) is a well-established technique for studying adsorbates on metal surfaces, but only metals of the Cu, Ag, Au or alkali group show the large enhancements (4–6 orders of magnitude) in the Raman scattering intensity which gives the SERS effect its high sensitivity and surface specificity. Considerable experimental work has been done to extend the range of metal substrates to which it may be applied. Of particular interest are the catalytically active metals of the platinum group, but although there have been several reports of SERS from molecules adsorbed on finely divided platinum surfaces 1, 2, 3, 4, the signals were very weak, with SERS enhancement factors estimated to be ∼100 for electrochemically roughened platinum electrodes [1], and <10 for a platinum colloid [2]. An alternative strategy which is capable of giving considerably more intense SERS signals from such weakly enhancing metals is to borrow the enhancing effect of a closely adjacent roughened silver or gold surface. Thus Weaver et al. have used SERS to study electrochemical reactions at platinum [5]and other transition metal electrodes formed by electrochemically depositing ∼2 atomic layers of these metals onto a roughened gold electrode substrate. In this technique the enhanced field due to the roughened gold penetrates to the transition metal–aqueous interface, where SERS is excited. The optimum transition metal layer thickness of ∼2 atomic layers represents a compromise between minimising the damping of the enhanced fields from the gold underlayer whilst giving good coverage of the gold surface by the transition metal, and with recently developed electrochemical deposition techniques it has been found possible to deposit transition metal films of this thickness on gold which are essentially pinhole-free 6, 7, 8.

As an alternative to electrochemical methods, thermal evaporation in vacuo provides a method both for the preparation of silver or gold island films which serve as the underlying SERS-active substrate, and for the coating of these with a few atomic layers of a transition metal. For some purposes, preparation of SERS-active surfaces in vacuo offers advantages over electrochemical methods since there is no contact of the surfaces with electrolytes during the preparation, but as far as we are aware there have been no previous reports of Raman scattering by bi-metallic island films. In this investigation we therefore evaluate thin platinum and palladium layers deposited onto silver island films as SERS-active surfaces, using cyanide ion as a test adsorbate, since the stretching frequency of this ion is sensitive to the identity of the metal atom to which it is attached.

Section snippets

Experimental

Silver and gold island films of mean thickness 9.0 and 7.0 nm, respectively, were prepared by vacuum-depositing these metals at ∼0.1 nm s−1 onto cleaned glass microscope slides in an Edwards model E306A vacuum coating unit, as described previously [9]. Platinum or palladium was then deposited very slowly on top of the silver and gold films from a tungsten filament evaporation source. Film mean thicknesses and rates of deposition were measured with a quartz crystal film thickness monitor. For

Results and discussion

After exposing gold island films with platinum overlayers of mean thickness 0.1, 0.2, or 0.4 nm (∼0.5–2.0 atomic layers of Pt) to HCN vapour, the SER spectra of the films in the ν(CN) region showed only a band at the wavenumber 2139 cm−1 characteristic of CN adsorbed on gold [9]. This band decreased sharply in intensity with increase in the platinum layer thickness, and for a pure platinum film treated with HCN no SER band could be detected. In contrast to platinum-coated gold films however,

Acknowledgements

We gratefully acknowledge support from a Research Grant from the Engineering and Physical Research Council, UK.

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