Adsorption Studies of p‑Aminobenzoic Acid on the Anatase TiO₂(101) Surface

The adsorption of p-aminobenzoic acid (pABA) on the anatase TiO₂(101) surface has been investigated using synchrotron radiation photoelectron spectroscopy, near edge X-ray absorption fine structure (NEXAFS) spectroscopy, and density functional theory (DFT). Photoelectron spectroscopy indicates that the molecule is adsorbed in a bidentate mode through the carboxyl group following deprotonation. NEXAFS spectroscopy and DFT calculations of the adsorption structures indicate the ordering of a monolayer of the amino acid on the surface with the plane of the ring in an almost upright orientation. The adsorption of pABA on nanoparticulate TiO₂ leads to a red shift of the optical absorption relative to bare TiO₂ nanoparticles. DFT and valence band photoelectron spectroscopy suggest that the shift is attributed to the presence of the highest occupied molecular orbitals in the TiO₂ band gap region and the presence of new molecularly derived states near the foot of the TiO₂ conduction band.