Surface-Enhanced Raman Scattering on Aggregates of Platinum Nanoparticles with Definite Size

Platinum nanoparticles with nominal diameters of 28 to 105 nm were prepared by a multistep seed-mediated growth method and the surface-enhanced Raman scattering (SERS) characteristics of their aggregates were examined. In the film state, a broad but distinct UV-vis absorption band occurred at similar to 330 nm, regardless of the size of the Pt particles, but with higher absorbance from a film assembled with larger Pt particles. Higher Raman intensity was thus observed at shorter excitation wavelengths from adsorbates on a Pt film made of 105 nm sized Pt particles, in agreement with the electromagnetic enhancement mechanism. The enhancement factor (EF), determined to be 1.5 x 10(2) at 514.5 nm excitation, was, however, an order of magnitude smaller than that of our earlier Pt film fabricated with laser-ablated 17 nm sized particles. Consulting the X-ray diffraction analyses, the smaller EF value was attributed to the fact that all Pt particles prepared in this work via a seed-mediated growth method were actually composed of 7.2 nm sized seed particles. Even with lower EF values, we were able to successfully characterize the SERS characteristics of two typical aromatic thiols, 4-nirobenzenethiol (4-NBT) and 4-aminobenzenethiol (4-ABT). Specifically, we found that 4-NBT is barely subjected to photoreaction on a Pt surface, in contrast with that on Ag. We found also that the chemical enhancement mechanism is operating, along with the electromagnetic enhancement mechanism, in the SERS of 4-ABT on Pt to show its b(2)-type bands, similarly to that on Ag.