Dopant-Dependent SFG Response of Rhenium CO₂ Reduction Catalysts Chemisorbed on SrTiO₃ (100) Single Crystals

Monolayers of fac-Re(4,4′-dicarboxyl-2,2′-bipyridine)(CO)₃Cl (ReC0A) complexes, covalently bound to TiO₂-terminated SrTiO₃ (100) (STO) single crystals, were studied by both vibrational Sum Frequency Generation (SFG) spectroscopy and Density Functional Theory (DFT). Homodyne-detected SFG results show that the SFG spectral shape of ReC0A on Nb-doped STO surfaces is almost opposite to that of ReC0A on undoped or Fe-doped STO surfaces, which we assign to a phase change of the SFG response of STO induced by the Nb rather than a change in orientation of the molecule. DFT calculations show that the doping with Nb populates the conduction band of the system, giving a metallic nature to the otherwise insulator STO substrate. This change in electronic structures of the substrate leads to a big nonresonant SFG response and thus interference with the spectra. The orientation of the ReC0A on all three substrates was found to be tilted by 45-50°, with a bidentate binding geometry and the Cl atom facing the surface. The reported results indicate that a molecular probe can be used to obtain information about the underlying material.