Using DFT calculation, we investigate the effects of halogens on the interactions between rutile TiO2 (110) and noble metal atoms (Au, Ag, Cu, Pt, and Pd). Fluorine, chlorine, and bromine atoms occupy the oxygen defect sites of TiO2, decreasing the stability of noble metal atoms on the surface. This decrease occurs because the halogens inhibit electron transfer from TiO2 to the noble metal atoms; the electron transfer from reduced TiO2 to the noble metal atom stabilizes the noble metal atom adsorption. In contrast, iodine strengthens the interactions between TiO2 and some noble metal atoms, namely Ag and Cu. This stabilization occurs because of the covalent interaction between iodine-doped TiO2 and the noble metal atom. Therefore, the stabilization is explained well by chemical hardness. This result suggests that iodine doping of a TiO2 surface would be an effective method for the preparation of highly stabilized noble metal clusters. (C) 2017 Elsevier B.V. All rights reserved.