Facet engineering of LaNbON2 transformed from LaKNaNbO5 for enhanced photocatalytic O2 evolution

Abstract

LaNbON₂ absorbs visible light up to approximately 750 nm, one of the longest λ_max values among Nb-based oxynitrides, but its photocatalytic water oxidation activity remains negligible owing partly to the exposure of inactive facets during thermal nitridation. Herein, LaNbON₂ with exposed metastable {010} facets was prepared by nitriding plate-like LaKNaNbO₅. During nitridation, the transformation of LaKNaNbO₅ into LaNbON₂ along the [010] direction occurred owing to their matching crystal structures, resulting in {010} facets exposed on the surface of plate-like LaNbON₂. DFT calculations revealed that the {010} facets have a higher surface energy than the {100} facets of conventional LaNbON₂. The exposure of such metastable {010} facets appears to have improved electron transfer from the interior of LaNbON₂ to its surface along the [010] direction. After loading a CoO_x cocatalyst, plate-like LaNbON₂ exhibited enhanced photocatalytic O₂ evolution activity with an apparent quantum yield (AQY) of 0.82% while conventional LaNbON₂ was almost inactive. This is probably due to the small effective mass of electrons along the [010] direction and the low defect densities in LaNbON₂ crystals after a mild nitridation. This work provides a novel transformation strategy yielding LaNbON₂ with low defect densities and exposed metastable facets, thus exhibiting unique photocatalytic activity.