Reduction of Ln2Ti2O7 Layered Perovskites: A Survey of the Anionic Lattice, Electronic Features, and Potentials

The reduction of the layered perovskites Ln(2)Ti(2)O(7) (LnTO, with lanthanide Ln = La, Pr, or Nd) was studied with the aim of shifting the ultraviolet (UV) photocalytic activity for water splitting in the visible range by Ti3+ donor doping. For all phases, after reduction by CaH2, the absorbance is extended beyond the UV visible region, giving rise to a gaplike edge in the mid infrared at similar to 0.4 eV with a dark coloration of the samples. When the precursor with Ln = La was reduced under a high-temperature H2 flow, we found a progressive nanotexturation down to 300 nm, which is responsible for a degree of Ti3+ segregation at the surface. Magnetic measurements, thermal analysis, and powder neutron diffraction reveal that the samples reduced by both routes have a similar amount of anion vacancy with S = 0.27 (in La2Ti2O7-delta). It represents a limited topotactic reduction stage, prior to the reconstructive reduction into La5Ti3.8+5O17 observed under more severe reducing conditions. For the sample reduced by CaH2, a minor amount of hydride appears to be incorporated (similar to 0.02 H per FU), with Ti3+center dot center dot center dot H- bonding observed by hyperfine sublevel correlation spectroscopy electron paramagnetic resonance and density functional theory calculations. Preliminary electrocatalysis tests show a promising anodic activity for water splitting hydrogen evolution with a voltage onset as low as 0.6 V versus the reversible hydrogen electrode.