Bandgap narrowing of titanium oxidenanosheets: homogeneous doping of molecular iodine for improved photoreactivity

Abstract

We developed a new strategy for homogeneous doping of iodine molecules (I₂) to achieve bandgap narrowing of semiconducting metal oxide nanosheets. This was realized by electrostatic interactions between nonpolar I₂ molecules and negatively charged exfoliated metal oxide nanosheets, mediated by the partial polarizability of the molecular iodine. Flocculated titania nanosheets doped with I₂ molecules (I₂-Ti_0.91O₂ sheets) were explored as a model system to demonstrate the effectiveness of this strategy. We observed an extension of the intrinsic absorption edge into the visible light region through a shifting of the valence band maximum at high doping levels of I₂ molecules. Importantly, at the same time the geometric structure of the host retained its integrity. The experimental together with first-principles calculations reveal the molecular nature of adsorbed iodine atoms and imply that the mechanism of electronic structure modulation in the titania layers changes depending on the concentration of I₂ molecules. This simple and efficient strategy may be suitable for other charged metal oxide nanosheets with large bandgaps to facilitate the design of new functional materials.