W. Albert Noyes, Jr. Distinguished Visiting Lectureship - Tianquan “Tim” Lian

Recent reports of light enhanced electrocatalysis on metal electrodes, including H₂ generation and CO₂ reduction, suggest exciting new approaches for solar fuels generation and for dynamic modulation of electrocatalysis. The mechanisms of such processes remain unclear. A commonly proposed mechanism invokes selective excitation of adsorbate vibrational modes and/or excited states, leading to non-thermal chemistry. In this talk, I will discuss two recent studies that are aimed at time-resolved probe of the hot electron induced adsorbate vibrational excitation and decay dynamics on metal electrodes and nanoparticles.

In the first study, we use time-resolved vibrational sum frequency generation (TR-VSFG) spectroscopy to directly measure hot electron induced vibrational dynamics of adsorbates on metal electrodes. For CO chemisorbed on Au electrodes, we show that excitation of the Au electrode with visible light leads to the generation of vibrationally excited CO molecules, which is followed by rapid IVR and vibrational cooling. Detailed analysis of the time-resolved SFG spectra reveals that 1) the formation the excited adsorbates is instrument response time limited (<100fs) and follows the electronic temperature of the hot electron, indicating the direct coupling of hot electrons in the electrodes with adsorbate vibration modes, likely through the selective excitation of the frustrated rotational mode; 2) the extent of adsorbate excitation is dependent on the applied potential, suggesting possible control of hot electron chemistry with applied bias.

In the second study, we use time-resolved surface enhanced Raman spectroscopy (TR-SERS) to investigate selective excitation of adsorbate vibrational modes on metal particles. For 4-nitrothiolphenol on Au nanoparticles, the excitation of the Au plasmon band lead to selective excitation of the nitro symmetric stretching mode with negligible excitation of other modes, including nitro scissoring mode. We further observe that the selectivity of vibrational mode excitation, subsequent intramolecular vibrational energy redistribution (IVR), and photochemistry depend sensitively on the nature of the adsorbate. Ongoing studies are examining the selection rules that govern selective excitation and how the initial nonthermal distribution can compete with IVR to lead to non-thermal photochemistry.