Evolution of Physical and Photocatalytic Properties in the Layered Titanates A2Ti4O9 (A = K, H) and in Nanosheets Derived by Chemical Exfoliation

Abstract

K2Ti4O9 has been known as a photocatalyst for the oxidation of methanol under UV irradiation. Here we study the evolution of morphology, optical, and photocatalytic properties of this titanate as it is converted into H2Ti4O9 and subsequently exfoliated into individual tetrabutylammonium (TBA)-supported [Ti4O9]2- nanosheets. We find that proton exchange and exfoliation are accompanied by a red shift of the optical absorption edge and fluorescence maximum, suggesting a reduction of the bandgap in the series K2Ti4O9 (3.54 eV), H2Ti4O9 (3.25 eV), TBA2Ti4O9 (3.00 eV). Neither compound is active for photochemical water splitting, even after photochemical deposition of platinum nanoparticles. However, in aqueous methanol, all platinated compounds are moderately active for H2 evolution upon bandgap irradiation, and in 0.01 M AgNO3, they all produce moderate quantities of O2. From the onset potentials for photoelectrochemical methanol oxidation, the values for the valence band edges at pH = 7 are deduced to lie be...