Characterization of siloxane adsorbates covalently attached to TiO ₂

Siloxanes with the general formula R-(CH₂)_n-Si-(OR') ₃ form durable bonds with inorganic materials upon hydrolysis of labile -OR' groups, and serve as robust coupling agents between organic and inorganic materials. In the field of dye-sensitized solar cells, functionalization of TiO₂ thin-films with siloxane adsorbates has been shown to be useful as a surface-passivation technique that hinders recombination processes and improves the overall efficiency of light-to-electricity conversion. However, the attachment of siloxane adsorbates on TiO₂ surfaces still remains poorly understood at the molecular level. In this paper, we report the characterization of 3-(triethoxysilyl) propionitrile (TPS) adsorbates, covalently attached onto TiO₂ surfaces. We combine synthetic methods based on chemical vapor deposition, Fourier transform (FT) infrared (IR) spectroscopy and electronic structure calculations based on density functional theory (DFT). We predict that trifunctional siloxanes form only 2 covalent bonds, in a 'bridge' mode with adjacent Ti⁴⁺ ions on the TiO₂ surface, leaving 'dangling' alkoxy groups on the surface adsorbates. Our findings are supported by the observation of a prominent fingerprint band at 1000-1100 cm^-1, assigned to Si-O-C stretching modes, and by calculations of binding enthalpies at the DFT B3LYP/(LACVP/6-31G**) level of theory indicating that the 'bridge' binding (ΔH_b= -55 kcal mol^-1) is more stable than 'tripod' motifs (ΔH_b= -45 kcal mol^-1) where siloxanes form 3 covalent bonds with the TiO₂ surface. The alkoxysiloxane groups are robust under heat and water treatment and are expected to be particularly relevant for analytical methods since they could be exploited for immobilizing other functionalities onto the TiO₂ surfaces.