Effect of organic vapors and potential-dependent Raman scattering of 2,6-dimethylphenylisocyanide on platinum nanoaggregates

Abstract

The surface-enhanced Raman scattering characteristics of 2,6-dimethylphenylisocyanide (2,6-DMPI) on Pt nanoaggregates, in ambient and electrochemical environments and in the presence of organic vapors, were examined and compared with those on Au nanoaggregates. Due to the exclusive adsorption via the isocyanide group, the NC stretching band was very susceptible to the measurement conditions although the ring associated bands showed negligible peak shifts. In ambient conditions, the peak shift of the NC stretching vibration on Pt (29 cm⁻¹) was one half of that on Au (61 cm⁻¹), suggesting that the electron donation capability of the isocyanide group to Au was greater than that to Pt. In the electrochemical environment, the NC stretching peak varied linearly with slopes of ∼42 and ∼36 cm⁻¹ V⁻¹ on Pt and Au, respectively. On the other hand, the NC stretching bands of 2,6-DMPI on Pt red-shifted by as much as 15 and 41 cm⁻¹, in the presence of acetone and ammonia, respectively, corresponding to the lowering of the surface potential of Pt nanoaggregates from +0.2 to −0.2 and −0.8 V, respectively. On Au nanoaggregates, however, acetone appeared to increase the surface potential of Au from +0.2 to +0.3 V, although ammonia decreased the surface potential from +0.2 to −0.4 V. Acetone must then act as an electron donor when interacting with Pt while it serves as an electron acceptor when interacting with Au, in agreement with an ab initio quantum mechanical calculation.