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 Ti 3+ donor doping. For all phases, after reduction by CaH 2, the absorbance is extended beyond the UV–visible region, giving rise to a gaplike edge in the mid-infrared at ∼0.4 eV with a dark coloration of the samples. When the precursor with Ln = La was reduced under a high-temperature H 2 flow, we found a progressive nanotexturation down to 300 nm, which is responsible for a degree of Ti 3+ 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 δ = 0.27 (in La 2 Ti 2 O 7−δ ). It represents a limited topotactic reduction stage, prior to the reconstructive reduction into La 5 Ti 3.8+ 5 O 17 observed under more severe reducing conditions. For the sample reduced by CaH 2, a minor amount of hydride appears to be incorporated (∼0.02 H per FU), with Ti 3+ ···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.