Long-Range Surface Plasmon Enhanced Raman Spectroscopy at Catalytic Platinum Electrodes

Abstract

Surface enhanced Raman scattering (SERS) is recognized as a powerful method for in-situ observation of electrode/electrolyte interfaces, due to the high chemical sensitivity and surface selectivity. However, the applicability of this technique is limited by the nature of surface plasmon resonances at the interface. Although platinum group metals (PGMs) are one of the most interesting materials for electrocatalysis, surface plasmon polaritons are largely damped at these metal surfaces by their localized d-electrons near the Fermi level. Thus, the plasmon enhanced Raman spectroscopy is normally useless for studying such catalytic surfaces. So far, the signal enhancement factors reported at PGM surfaces are in the order of 102 while those at coinage metal surfaces can reach the order of 106 to 1010. In order to gain more intensity at highly damping PGM surfaces, the intensity-borrowing method is a common method; for example, when a large number of Au nanoparticles are deposited on a PGM surface, Raman scattering signals are largely enhanced by the sphere-plane type coupled plasmon modes [1,2]. However, such a nanostructure may influence mass transport, selectivity, and reactivity of electrochemical reactions. For spectro-electrochemistry, it is necessary to reduce the plasmon damping without suffering from such a nanoscale effect. In this work, the attenuation of surface plasmons at highly damping Pt surface is reduced using a symmetric slab mode, which is a strongly coupled surface plasmons excited at both interfaces of a thin metal film [3]. The prolonged propagation of surface plasmons in this double-interface configuration leads to a substantial enhancement of local fields even at lossy Pt surface. The theoretical calculation indicates that the enhancement factor of Raman scattering intensity can reach the order of 104 at Pt/solution interfaces in this coupled plasmon mode. We experimentally demonstrate that Raman scattering signals are significantly enhanced at Pt surface under electrochemical conditions. This strategy provides us a novel surface-specific spectroscopic method for the molecular-scale investigation of electrocatalytic reactions.[1] J. Hu, N. Hoshi, K. Uosaki, K. Ikeda, Nano Lett., 15, 7982-7986 (2015).[2] J. Hu, M. Tanabe, J. Sato, K. Uosaki, K. Ikeda, J. Am. Chem. Soc.,136, 10299-10307 (2014).[3] S. Ikegaya, K. Motobayashi, K. Ikeda, J. Raman Spectrosc. DOI: 10.1002/jrs.5925 (2020)