Adsorption and photo-transformation of organic compounds at the TiO2 surface: in situ ATR – FTIR studies

MENDIVE C.; BREDOW T.; BLESA M.A.; BAHNEMANN D.

Recent Research Developments in Physical Chemistry: Surfaces and Interfaces of Nanostructured Systems

Año: 2007; p. 35 - 65

The first part of this overview will focus on research concerned with the characterization of adsorbed species at the surface of TiO2 particles in aqueous solutions by means of the Attenuated Total Reflection – Fourier Transformed Infrared (ATRFTIR) in situ Spectroscopy. Infrared (IR) spectroscopy of thin particle films, deposited on an ATR crystal and immersed in solution, has a high potential for exploring the chemistry of the solid-solution interface. For films with a thickness of a few micrometers, the penetration depth of the evanescent IR wave is sufficient to monitor surface reactions occurring at a high surface area particle film. Simple organic compounds that adsorb at the TiO2 surface, such as carboxylic acids, alcohols and amino acids have been thoroughly investigated. These model compounds are suitable for the study of many reactions, including photosensitization, heterogeneous photo-catalytic oxidation of pollutants, and biocompatibility of prosthetic materials. Phenomena that can be studied include pH and ionic strength dependence of adsorption equilibria and kinetics to validate various models, e.g., surface complexation speciation descriptions. The contribution of every adsorbed surface complex can, in principle, be analyzed and each adsorption constant can be calculated. The second part of this review will discuss photo-transformation reactions monitored in situ at the solid-solution interface. These studies are usually carried out as follows. A layer of TiO2 particles is allowed to reach equilibrium in the dark with the aqueous solution of a certain model compound, and the system is illuminated with UV(A) light. A detailed analysis of the chemical reactions taking place at this interface under UV(A) illumination is then performed employing ATR-FTIR in combination with quantum chemical calculations. Thus, the structures of different surface complexes can be identified and molecular dynamics simulations can be used to compare their predictions with the experimental data. All spectral changes occurring under UV(A) illumination will be discussed in the light of the hypotheses existing in the literature, e.g., the de-aggregation concept, super-hydrophilicity phenomena and photo-induced removal of impurities and will moreover be compared to a new theory involving photodesorption of water molecules from the surface of the TiO2 particles.