Orientation of a Series of CO2 Reduction Catalysts on Single Crystal TiO2 Probed by Phase-Sensitive Vibrational Sum Frequency Generation Spectroscopy (PS-VSFG)

We report the average molecular orientation of a series of rhenium bipyridyl CO2 reduction catalysts adsorbed onto a rutile TiO2(001) single crystal surface. The molecular conformation of these catalysts on electrode surfaces is expected to affect their overall catalytic efficiency, since the catalytic Re center must be free to coordinate a CO2 molecule in working systems. Phase sensitive vibrational sum frequency generation spectroscopy (PS-VSFG) and ab initio SFG simulation and conformation search were used to determine the molecular tilt angles of five such complexes, Re(L(n)A)(CO)(3)Cl [L(n)A = 2,2'-bipyridine-4,4'-(CH2)(n)-COOH, n = 0-4] (abbreviated as ReCnA), as a function of the length of their anchoring groups. Results show that both the short and long axes of the catalytic bipyridine ring tilt further toward the TiO2(001) surface as the number of methylene groups in the anchoring groups increases. The increasing tilt angles are shown to correlate with an increasing conformational difference between the two anchoring groups as they lengthen, caused by the free rotation of sigma-bonds between -CH2- groups. This study demonstrates an effective approach to manipulate the spatial orientation of heterogeneous electrocatalysts on TiO2 semiconductor surfaces by varying the length of the molecular linkers.