Switching a Plasmon-Driven Reaction Mechanism from Charge Transfer to Adsorbate Electronic Excitation Using Surface Ligands

H. Kookhaee, T. E. Tesema and T. G. Habteyes

J. Phys. Chem. C. 124, 22711 (2020)
Understanding photocatalytic reaction conditions 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 a high degree of product selectivity can be achieved by switching the mechanism from charge transfer (CT) to adsorbate electronic excitation (AEE). In the presence of a cetyl trimethyl ammonium bromide (CTAB) surface ligand on gold nanoparticles, MB is selectively transformed to thionine at a 633 nm excitation wavelength that overlaps with the electronic transition of the adsorbate. The AEE 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, when MB is directly adsorbed on the nanoparticles, the mechanism involves electron transfer that may lead to the formation of an anionic complex. 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 may undergo nonselective N-demethylation, yielding partially demethylated derivatives in addition to thionine. These experimental observations underscore the importance of adsorption condition in determining the mechanism of plasmon-enhanced photocatalytic reactions.