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

High surface area TiO 2 electrodes are extensively used in photocatalytic solar cells. However, it remains controversial how molecular catalysts interact with the TiO 2 surfaces and how the electrodes affect the functionality of catalytic adsorbates. Here, we examine the CO 2 reduction catalyst Re I (dcbpy)(CO) 3 Cl (dcbpy = 4,4′-dicarboxy-2,2′-bipyridine) on TiO 2 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 TiO 2 surfaces that could be engineered for selective catalytic performance.