Protonated Oxide, Nitrided, and Reoxidized K₂La₂Ti₃O₁₀ Crystals: Visible-Light-Induced Photocatalytic Water Oxidation and Fabrication of Their Nanosheets

Protonated lanthanum titanium oxide H₂La₂Ti₃O₁₀ and oxynitride H₂La₂Ti₃O_10-3/2xN_x crystals were synthesized from the oxide, nitrided, and reoxidized layered K₂La₂Ti₃O₁₀ crystals prepared by solid-state reaction through proton exchange. Here, we investigated the holding time of nitridation of oxide K₂La₂Ti₃O₁₀ crystals influencing their crystal structure, shape, and absorption wavelength and band gap energy. The XRD and SEM results confirmed that the crystal structure and plate-like shape of the parent oxide were maintained after nitridation at 800 °C for 10 h, and the color of crystals was changed from white to dark green. However, no clear absorption edges were observed in the UV-vis diffuse reflectance spectra of the nitrided crystals due mainly to the reduced titanium species (Ti³⁺), which act as the recombination center of the photogenerated charge carriers. To decrease the amount of the reduced titanium species, the nitrided crystals were further reoxidized at 400 °C for 6 h. After partial reoxidation, the absorption intensity in the longer wavelength region was reduced, and the absorption edges appeared at about 449-460 nm. The photocatalytic activity for the water oxidation half-reaction was evaluated only for the protonated samples. The protonated reoxidized K₂La₂Ti₃O₁₀ crystals showed the O₂ evolution rate of 180 nmol·h^-1 (for the photocatalytic water oxidation) under visible-light irradiation, and the unexpected photocatalytic decomposition of N₂O adsorbed onto the photocatalyst surfaces was observed for the protonated oxide and protonated nitrided layered K₂La₂Ti₃O₁₀ crystals. Furthermore, lanthanum titanium oxide [La₂Ti₃O₁₀]^2- and oxynitride [La₂Ti₃O_10-3/2xN_x]^2- nanosheets were successfully fabricated by proton exchange and mechanical exfoliation (sonication) of the oxide, nitrided, and reoxidized K₂La₂Ti₃O₁₀ crystals. The TEM results revealed that the lateral sizes of the fabricated nanosheets grown along the «001» direction are 270-620 nm. Apparently, the colloidal suspensions of the fabricated nanosheets showed a Tyndall effect, implying their good dispersion and stability for several weeks in water.