MECHANISIM OF PLASMON-DRIVEN PHOTOCHEMICAL REACTIONS

Understanding a photocatalytic reaction condition that selectively leads to a desired product on metal surfaces is a longstanding research problem in heterogeneous catalysis. Here, using plasmon enhanced N-demethylation of methylene blue (MB) as model reaction, we show that high degree of product selectivity can be achieved by switching the mechanism from charge transfer to adsorbate electronic excitation and vice versa using surface ligands. In the presence of cetyl trimethyl ammonium bromide (CTAB) surface ligand on gold nanoparticles, MB is selectively transformed to thionine at 633 nm excitation wavelength that overlaps with the electronic transition of the adsorbate. At resonant excitation wavelength, the mechanism involves near-field enhanced intramolecular electronic excitation of the MB adsorbate, and this mechanism is favored by the presence of CTAB that appears to increase the rate of adsorbate excitation by orienting the molecular dipole along the driving surface field and to prolong the lifetime of the excited state by slowing down adsorbate-to-metal energy transfer. On the other hand, vi when MB is directly adsorbed on the nanoparticles, the mechanism involves electron transfer that may lead to formation of anionic complex that likely includes MB, oxygen, and water molecules. In situ surface enhanced Raman scattering spectra suggest that the complex remains stable at long excitation wavelengths (808 and 785 nm), while at shorter wavelengths (671, 633 and 561 nm), it undergoes non-selective N-demethylation, yielding partial N-demethylation derivatives in addition to thionine. These experimental observations underscore the importance of adsorption condition in determining the mechanism of plasmon enhanced photocatalytic reactions. Moreover, it has been shown that hot electrons could transfer to the oxygen on the surface of plasmonic nanoparticles and then activated oxygen could drive the chemical reaction. Therefore, detecting activated oxygen on the s