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

Protonated lanthanum titanium oxide H 2 La 2 Ti 3 O 10 and oxynitride H 2 La 2 Ti 3 O 10–3/2 x N x crystals were synthesized from the oxide, nitrided, and reoxidized layered K 2 La 2 Ti 3 O 10 crystals prepared by solid-state reaction through proton exchange. Here, we investigated the holding time of nitridation of oxide K 2 La 2 Ti 3 O 10 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 (Ti 3+ ), 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 K 2 La 2 Ti 3 O 10 crystals showed the O 2 evolution rate of 180 nmol·h –1 (for the photocatalytic water oxidation) under visible-light irradiation, and the unexpected photocatalytic decomposition of N 2 O adsorbed onto the photocatalyst surfaces was observed for the protonated oxide and protonated nitrided layered K 2 La 2 Ti 3 O 10 crystals. Furthermore, lanthanum titanium oxide [La 2 Ti 3 O 10 ] 2– and oxynitride [La 2 Ti 3 O 10–3/2 x N x ] 2– nanosheets were successfully fabricated by proton exchange and mechanical exfoliation (sonication) of the oxide, nitrided, and reoxidized K 2 La 2 Ti 3 O 10 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.