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

Abstract

K 2 Ti 4 O 9 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 H 2 Ti 4 O 9 and subsequently exfoliated into individual tetrabutylammonium (TBA)-supported [Ti 4 O 9 ] 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 K 2 Ti 4 O 9 (3.54 eV), H 2 Ti 4 O 9 (3.25 eV), TBA 2 Ti 4 O 9 (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 H 2 evolution upon bandgap irradiation, and in 0.01 M AgNO 3, they all produce moderate quantities of O 2 . From the onset potentials for photoelectrochemical methanol oxidation, the values for the valence band edges at pH = 7 are deduced to lie between −0.23 and −0.53 V (NHE) for the nonplatinated compounds, and at +0.08 V and −0.30 V for the platinated compounds. This Pt-induced decrease of negative charge on the titanates is likely due to Fermi level equilibration of metal and semiconductor. Its effect can also be seen in a shift of the onset potentials for electrochemical water oxidation, as measured by cyclic voltammetry. Transient absorption data reveal that photogenerated electrons become trapped in mid band gap states, from which they decay exponentially with a time-constant of 43.67 ± 0.28 ms, much slower than observed for 68 ± 1 ns for TiO 2 nanocrystals (Degussa, P25).