Plasmon-mediated chemical transformation from alkane to alkene on a silver nanoparticle array under 532 nm excitation

Surface-enhanced Raman scattering (SERS) spectra of organic compounds, para-methylthiophenol (p-MT), decylamine and 1-butanethiol (1-BT), were measured using a two-dimensional silver nanoparticle array at 532 nm excitation. For p-MT, it was observed that Raman peaks grew at 1580 and 1690 cm-1, which were never observed in the normal Raman spectrum, indicating oxidation from the methyl to carboxyl group. For both decylamine and 1-BT, an intensive SERS peak grew at 1580 cm-1. We measured the time-resolved SERS spectra of 1-BT at the laser intensity of 185 W mm-2 and confirmed that the spectral shapes changed as the total exposure increased. Another SERS peak was also observed at 3050 cm-1 for decylamine and 1-BT. From these results, it was considered that unsaturated bonds were formed in the alkanes, meaning that alkenes were produced from alkanes. Additionally, the SERS spectrum revealed that the chemically transformed alkane possesses a methyl group. The result indicates that dehydrogenation preferentially occurs at the secondary carbons, which is consistent with the stability of radicals on carbon atoms. The laser intensity threshold for plasmon-mediated chemical transformation was experimentally observed to be 2.7 and 40 W mm-2 for p-MT and 1-BT, respectively. The higher laser intensity is necessary for oxidation of alkanes compared with aromatic compounds, which is consistent with the chemical stability of organic compounds.