Interfacial Charge Transfer Transitions in Colloidal TiO₂ Nanoparticles Functionalized with Salicylic acid and 5-Aminosalicylic acid: A Comparative Photoelectron Spectroscopy and DFT Study

Interfacial charge transfer (ICT) complexes between wide-band-gap metal oxides and small colorless organic molecules have promising use in a variety of applications. The possibility to control optical properties and alignment of energy levels by a proper combination of inorganic and organic parts of ICT complexes is the main advantage of such materials. We used vacuum-ultraviolet electron imaging photoelectron spectroscopy (VUV PES) to determine the ionization potentials of isolated TiO₂ nanoparticles surface-modified with salicylic acid (SA) and 5-aminosalicylic acid (5-ASA). The VUV PES measurements indicated a shift of the photoemission onset from 7.2 eV, observed for pristine TiO₂ nanoparticles, to 6.5 and 5.9 eV for the surface-modified ones, with SA and 5-ASA, respectively. These results are consistent with the red shift of the absorption onset observed upon surface modification of TiO₂ colloids. To obtain a detailed description of the optical properties of surface-modified TiO₂ with SA and 5-ASA, density functional theory (DFT) calculations were performed with periodic boundary conditions (PBCs) and with a finite-size TiO₂ cluster. The PBC model provides a qualitatively good description of electronic states of oxide surfaces functionalized with small organic molecules, while an excellent agreement between experimentally determined and calculated band gap values using a finite-size model was found.