Coupled Exfoliation and Surface Functionalization of Titanate Monolayer for Bandgap Engineering

Exfoliation of layered compounds generates nanosheets, such as monolayers and few-layers. In general, it is not easy to control surface chemistry of the exfoliated nanosheets. Here, a new approach for surface functionalization of titanate monolayer coupled with exfoliation is reported. The precursor layered composite is prepared by intercalation of long-chain alkyl-ammonium ion, such as stearylammonium ion (C₁₈NH₃ ⁺), in the interlayer space of layered titanate. The layered composite of titanate and C₁₈NH₃ ⁺ (C₁₈TiO₂) is dispersed in the toluene solution containing dihydroxynaphthalene (DHN) for the coupled exfoliation and surface modification. The surface molecule is partially exchanged from the original C₁₈NH₃ ⁺ to the guest DHN. The resultant hydrophobic titanate monolayer with surface modification by the C₁₈NH₃ ⁺ and DHN (DHN/C₁₈TiO₂) shows the visible-light absorption based on the charge-transfer excitation from the highest occupied molecular orbital of the adsorbed DHN to the conduction band of the titanate. The present synthetic approach can be applied to synthesis of a variety of surface-functionalized monolayers and their bandgap engineering.