Highly Dispersed TiO6 Units in a Layered Double Hydroxide for Water?Splitting

A family of photocatalysts for water splitting into hydrogen was prepared by distributing TiO6 units in an MTi-layered double hydroxide matrix (M=Ni, Zn, Mg) that displays largely enhanced photocatalytic activity with an H2-production rate of 31.4 mu mol?h-1 as well as excellent recyclable performance. High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) mapping and XPS measurement reveal that a high dispersion of TiO6 octahedra in the layered doubled hydroxide (LDH) matrix was obtained by the formation of an M2+-O-Ti network, rather different from the aggregation state of TiO6 in the inorganic layered material K2Ti4O9. Both transient absorption and photoluminescence spectra demonstrate that the electronhole recombination process was significantly depressed in the Ti-containing LDH materials relative to bulk Ti oxide, which is attributed to the abundant surface defects that serve as trapping sites for photogenerated electrons verified by positron annihilation and extended X-ray absorption fine structure (EXAFS) techniques. In addition, a theoretical study on the basis of DFT calculations demonstrates that the electronic structure of the TiO6 units was modified by the adjacent MO6 octahedron by means of covalent interactions, with a much decreased bandgap of 2.1 eV, which accounts for its superior water-splitting behavior. Therefore, the dispersion strategy for TiO6 units within a 2D inorganic matrix can be extended to fabricate other oxide or hydroxide catalysts with greatly enhanced performance in photocatalysis and energy conversion.