Photocatalytic Research On Mixed Metal Oxynitride Semiconductor Materials With Wide Visible Absorption

Perovskite-type LaTiO2N and LaNbON2 semiconductor materials with wide visible absorption have important roles in improving the utilization efficiency of solar energy.However,the photocatalytic applications of them are not perfect enough,especially for LaNbON2 semiconductor material,there was less report in the field of photocatalytic water splitting.Pure perovskite-type LaTiO2N and LaNbON2 photocatalysts were prepared successfully,after a follow-up activity optimization?annealing treatment?,firstly achieving photocatalytic oxidation of water of LaTiO2N and LaNbON2 prepared via themal ammonalysis of La2TiO5 and La3NbO7,respectively.At first,perovskite-type LaTiO₂N photocatalyst has been synthesized via themal ammonalysis of La2TiO5,which was prepared via NaCl-KCl mixed flux synthesis.Though LaTiO2N derived from La2TiO5 appears defect-free in UV-Vis-NIR and electron paramagnetic resonance spectroscopies,its performance is much lower than conventional LaTiO2N with defects.It is shown by Mott-Schottky analysis that La2TiO5-derived LaTiO2N has a significantly lower donor density.This can result in insufficient build-in electric field for the separation of photo-generated electrons and holes.At the same time,the lower donor density is also consistent with the smaller difference between the Fermi level and valence band maximum,which accounts for a lower oxidative power of the holes.In light of this discovery,the donor density was substantially increased by introducing anion vacancies via annealing in Ar,resulting in improved performance.The CoOx-assisted La2TiO5-derived LaTiO₂N annealed at 713?h as a high oxygen evolution and a high quantum efficiency?25%?at 450.Besides,La₃NbO₇,with richer basic oxide and lower positive Mulliken charge on Nb?V?than LaNbO4,was prepared via NaCl-KCl mixed flux synthesis and converted to perovskite-type LaNbON₂ via thermal ammonolysis.Removal of secondary product La₂O₃from ammonolysis of La3NbO7 gives rise to additional nanopores in La NbON2 besides the pores resulted from nitridation.This makes a bigger specific surface area?23m²/g?than conventional LaNbON₂ prepared from LaNbO₄?11m²/g?.Moreover,UV-Vis spectrum of LaNbON₂ from La₃NbO₇ reveals much lower defect absorption than LaNbON₂ from LaNbO₄;thermogravimetric analysis shows that the mass-gain stage attributed to oxidation of low-valent Nb is present in the curve of LaNbON₂ from LaNbO₄ while absent in that of LaNbON2 from La3NbO7.This indicates that the formation of low-valent Nb defects in LaNbON₂ is inhibited when using La₃NbO₇ as precursor.However,perovskite-type oxynitride LaNbON2 has a wide visible-light absorption band.Significantly lower defect density in LaNbON₂ prepared via nitriding La₃NbO₇ was revealed by UV-Vis spectroscopy comparing with that via conventionally nitriding LaNbO₄,but it does not make a detectable photocatalytic oxygen evolution.By a post-annealing treatment,anion vacancies were introduced in LaNbON₂,and the oxidative power of holes in valence band was enhanced,indicated by valence band XPS.The annealed LaNbON₂-f.317 photocatalyst acquires water oxidation ability,but LaNbON₂-f.114 still lacks oxidation ability.This,however,can be improved by combining CoOx as a co-catalyst.