Surface Structural Reconstruction for Optical Response in Iodine-Modified TiO2 Photocatalyst System

We report an alternative mechanism for the optical response of an iodine-modified anatase TiO2 photocatalyst material. Unlike the general realization that the iodine atom provides impurity levels within the optical band gap, we suggest that a distorted surface structure plays a dominant role. Anatase pure TiO2 and iodine-modified TiO2 (1-TiO2) nanocrystals were synthesized by a simple solvothermal method, and I-TiO2 exhibited an extended absorption edge up to 550 nm. Employing iodine K-edge X-ray absorption fine structure (XAFS), we demonstrate that iodine is not incorporated into the nanoparticle interior but exists in the form of IO3- groups at the surface. Furthermore, this IO3- cluster adsorption largely induces a disordered structure, as revealed by Ti 2p(3/2) X-ray photoelectron spectroscopy, high-resolution transmission electron microscopy, and Ti K-edge XAFS data fitting. Density functional theory calculation indicates that this distorted structure can form a midgap state, which should be responsible for its excellent optical property. This finding represents a promising route for improving the optical response of the nanophotocatalyst system via surface treatment.