Lewis-Acid Sites of TiO₂ Surface for Adsorption of Organic Dye Having Pyridyl Group as Anchoring Unit

Adsorption of an organic dye having a pyridyl group as an anchoring unit (NI4) is studied on nanocrystalline TiO₂ surface by being compared with a similar dye having a carboxyl anchor (NI2). Adsorption of both dyes followed a Langmuir isotherm, and analysis of the isotherms showed that an adsorption equilibrium constant of NI4 is (1.1 ± 0.1) × 10³ M^–1, much smaller than (0.6 ± 0.05) × 10⁵ M^–1 for NI2, and amounts of adsorbed dyes at saturation are similar to each other: [(1.5 ± 0.2) and (1.7 ± 0.3)] × 10¹⁴ cm^–2 for NI2 and NI4, respectively. Coadsorption experiments with 4-carboxy TEMPO (4CT) and either NI2 and NI4 revealed that the adsorption sites of NI2 and NI4 are different from each other. This was supported by measurements of average nearest-neighbor interspin distances of 4CT radicals coadsorbed on TiO₂ by a spin-probe ESR technique. The above findings and elaborate FT-IR studies demonstrated that NI2 and NI4 adsorb, respectively, on Brønsted- and Lewis-acid sites of TiO₂ surface. The number of Lewis-acid sites evaluated from the temperature-programmed desorption experiments of TiO₂ was in good agreement with the amount of NI4 adsorbed on TiO₂ at saturation, providing a further confirmation for Lewis-acid sites acting as a predominant adsorption site of NI4.