CO₂ preactivation in photoinduced reduction via surface functionalization of TiO₂ nanoparticles

Salicylate and salicylic acid derivatives act as electron donors via charge-transfer complexes when adsorbed on semiconducting surfaces. When photoexcited, charge is injected into the conduction band directly from their highest occupied molecular orbital (HOMO) without needing mediation by the lowest unoccupied molecular orbital (LUMO). In this study, we successfully induce the chemical participation of carbon dioxide in a charge transfer state using 3-aminosalicylic acid (3ASA). We determine the geometry of CO₂ using a combination of ultraviolet-visible spectroscopy (UV-vis), surface enhanced Raman scattering (SERS), ¹³C NMR, and electron paramagnetic resonance (EPR). We find CO₂ binds on Ti sites in a carbonate form and discern via EPR a surface Ti-centered radical in the vicinity of CO ₂, suggesting successful charge transfer from the sensitizer to the neighboring site of CO₂. This study opens the possibility of analyzing the structural and electronic properties of the anchoring sites for CO₂ on semiconducting surfaces and proposes a set of tools and experiments to do so.