Design of nitrogen-doped layered tantalates for non-sacrificial and selective hydrogen evolution from water under visible light

Nitrogen doping into a series of layered tantalates (ALaTa₂O₇, where A = Li, Na, K, Rb, or Cs) was attempted in order to produce materials capable of catalyzing non-sacrificial and endergonic water reduction under visible light. Heating of KLaTa₂O₇ and RbLaTa₂O₇ in an NH₃ stream at 1073 K led to successful nitrogen doping, accompanied by a significant shift in the absorption edge of the material toward the visible-light region, while similar treatment of the other tantalates resulted in the collapse of the layered structure or partial anion substitution at the surface. Although the NH₃ heating of a conventional RbLaTa₂O₇ precursor prepared with nearly stoichiometric Rb (Rb/La = 1.2) resulted in the formation of impurities such as Ta₃N₅ and amorphous tantalum nitrides, the use of a Rb-rich precursor prepared with excess Rb (Rb/La = 2.4) effectively suppressed this impurity formation. The Rb⁺ cations in the prepared pure nitrogen-doped sample were exchanged with H⁺ to facilitate the intercalation of water, and a cationic Pt precursor was then selectively introduced into the interlayers and photocatalytically reduced to Pt metal particles. The internally platinized H⁺/RbLaTa₂O_7-xN_y showed stable H₂ evolution in the presence of I^- as an electron donor under visible light, accompanied by the generation of I₃ ^-. Although the externally platinized H⁺/RbLaTa₂O_7-xN_y sample and other bulk-type photocatalysts such as Ta₃N₅ generated H₂ in the presence of a sacrificial electron donor, H₂ evolution was negligible in the presence of I^-. The stable H₂ evolution over the internally platinized H⁺/RbLaTa₂O_7-xN_y sample is due to the suppressed backward reduction of I₃ ^- to I^- at selective reduction sites in the interlayer spaces, which are accessible only to cationic species and water.