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

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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.

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