Morphology and band gap variations of oxynitride LaTaON2 depending on the ammonolysis temperature and precursor

Polycrystalline LaTaON2 samples were prepared by the ammonolysis of LaTaO4, where the oxide preparation temperatures (700, 900, and 1,400 °C) and ammonolysis temperatures (850, 950, and 1,000 °C) were varied to control the specific surface area and porosity. The resulting LaTaON2 samples had a surface area of 15–20 m2/g and a pore volume of 0.045–0.059 cm3/g, as determined from the nitrogen adsorption–desorption isotherms. In particular, the ammonolysis of crystalline LaTaO4 (prepared at 1,400 °C) can be described as a condensation from layered perovskite to three-dimensional perovskite, where mesoporous LaTaON2 were produced with pore diameters of 5–50 nm and particle sizes of ≈1 μm. Diffuse-reflectance measurements demonstrated that these ammonolysis parameters also affected the optical band gap energy of LaTaON2, which varied from 1.93 to 2.07 eV. Increasing the LaTaO4 preparation temperature and/or ammonolysis temperature tended to decrease the band gap of LaTaON2.