Surface-Induced Anisotropic Binding of a Rhenium CO₂-Reduction Catalyst on Rutile TiO₂(110) Surfaces

Vibrational sum frequency generation (SFG) spectroscopy has been utilized to study the spatial orientation and alignment of Re(CO)₃Cl(dcbpy) (dcbpy = 4,4′-dicarboxy-2,2′-bipyridine) (or ReC0A) on the (001) and (110) surfaces of rutile single-crystalline TiO₂. The SFG intensity of the CO stretching modes shows an isotropic distribution on the (001) surface and an anisotropic distribution on the (110) surfaces with respect to the in-plane rotation of the crystal relative to the surface normal (or the incident laser beam plane). By combining these results with ab initio SFG simulations and with modeling of ReC0A-TiO₂ cluster binding structures at the density functional theory level, we reveal that the origin of the optical anisotropy for ReC0A on the TiO₂(110) surface is associated with the binding preference of ReC0A along the [-110] axis. Along this direction, the binding structure is energetically favorable, because of the formation of proper hydrogen bonding between the carboxylate group and passivating water molecules adsorbed on the TiO₂(110) surface. Simulations of dimers of ReC0A molecules binding close together with full nearest-neighbor effects give a structure that reproduces the experimental SFG polar plot. The tilt angle, defined by the bpy ring angle relative to the surface normal, of the catalyst is found to be 26° for one monomer and 18° for the other, which corresponds to an aggregate at high surface coverage.