Highly Efficient Water Splitting into H2 and O2 over Lanthanum-Doped NaTaO3 Photocatalysts with High Crystallinity and Surface Nanostructure

Abstract

NiO-loaded NaTaO₃ doped with lanthanum showed a high photocatalytic activity for water splitting into H₂ and O₂ in a stoichiometric amount under UV irradiation. The photocatalytic activity of NiO-loaded NaTaO₃ doped with lanthanum was 9 times higher than that of nondoped NiO-loaded NaTaO₃. The maximum apparent quantum yield of the NiO/NaTaO₃:La photocatalyst was 56% at 270 nm. The factors affecting the highly efficient photocatalytic water splitting were examined by using various characterization techniques. Electron microscope observations revealed that the particle sizes of NaTaO₃:La crystals (0.1−0.7 μm) were smaller than that of the nondoped NaTaO₃ crystal (2−3 μm) and that the ordered surface nanostructure with many characteristic steps was created by the lanthanum doping. The small particle size with a high crystallinity was advantageous to an increase in the probability of the reaction of photogenerated electrons and holes with water molecules toward the recombination. Transmission electron microscope observations and extended X-ray absorption fine structure analyses indicated that NiO cocatalysts were loaded on the edge of the nanostep structure of NaTaO₃:La photocatalysts as ultrafine particles. The H₂ evolution proceeded on the ultrafine NiO particles loaded on the edge while the O₂ evolution occurred at the groove of the nanostep structure. Thus, the reaction sites for H₂ evolution were separated from those of O₂ evolution over the ordered nanostep structure. The small particle size and the ordered surface nanostep structure of the NiO/NaTaO₃:La photocatalyst powder contributed to the highly efficient water splitting into H₂ and O₂.