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 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 TiO2 nanocrystals (Degussa, P25).

Improved Niobate Nanoscroll Photocatalysts for Partial Water Splitting

“…Besides, there are synthetic obstacles for the preparation of defined multicomponent nanostructures. As part of our ongoing search for a nanoscale water-splitting photocatalyst, [3][4][5][6][7][8][9][10][11][12][13] we describe here scalable syntheses of catalysts that contain up to three separate nanoparticle components for light absorption, water reduction, and oxidation. The light absorber is a niobate nanoscroll derived by chemical exfoliation from the known water-splitting photocatalyst K 4 Nb 6 O 17 .…”

Strategies for designing metal oxide nanostructures

“…In detail, 0.025-1.0 g of HxTi2−xO4·H2O was shaken at 150 rpm with 100 mL of TBA aqueous solution to form a stable colloidal suspensions, which was translucent or opalescent in appearance, depending on the titanate content. Thereafter, 2D TiO2 nanosheets were synthesized by exfoliating different types of layered host materials such as Cs0.67Ti1.83O4, H1.8Bi0.2CaNaNb3O10, Li0.5TiO2, K2Ti4O9, and H2Ti4O9 [167][168][169][170][171]. Fig.…”

Temperature-controlled reversible exfoliation-stacking of titanate nanosheets in an aqueous solution containing tetraalkylammonium ions

“…17,18 The interlayer distance (0.94 nm) calculated by the d-spacing (1.69 nm) and the nanosheet thickness was nearly equal to the ionic size of TBA + (~ 1.0 nm). 11 That is, the TBA + was interstratified between the negatively charged titanate layers during the drying. The crystallite size at 25 o C was estimated to be ca.…”