Distinct Binding of Rhenium Catalysts on Nanostructured and Single-Crystalline TiO2 Surfaces Revealed by Two-Dimensional Sum Frequency Generation Spectroscopy

High surface area TiO2 electrodes are extensively used in photocatalytic solar cells. However, it remains controversial how molecular catalysts interact with the TiO2 surfaces and how the electrodes affect the functionality of catalytic adsorbates. Here, we examine the CO2 reduction catalyst ReI(dcbpy)(CO)3Cl (dcbpy = 4,4′-dicarboxy-2,2′-bipyridine) on TiO2 by probing spectroscopically isolated carbonyl modes that are highly sensitive to the complex electronic structure and solvation environment. Combining interferometric two-dimensional sum frequency generation spectroscopy and ab initio simulations of the spectra, we find distinct spectroscopic and dynamical properties of catalysts bound to single crystals and nanocrystalline surfaces. Different cross-peak patterns resulting from interferences between elements of the fourth-order susceptibility, along with different vibrational relaxation dynamics, reveal distinct vibrational relaxation pathways of the catalysts on different TiO2 surfaces that could be ...