Bandgap narrowing of titanium oxide nanosheets: 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-Ti0.91O2 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.