Structural analysis and characterization of layer perovskite oxynitrides made from Dion–Jacobson oxide precursors

A three-layer oxynitride Ruddlesden–Popper phase Rb1+xCa2Nb3O10−xNx·yH2O (x=0.7–0.8x=0.7–0.8, y=0.4–0.6y=0.4–0.6) was synthesized by ammonialysis at 800 °C from the Dion–Jacobson phase RbCa2Nb3O10 in the presence of Rb2CO3. Incorporation of nitrogen into the layer perovskite structure was confirmed by XPS, combustion analysis, and MAS NMR. The water content was determined by thermal gravimetric analysis and the rubidium content by ICP-MS. A similar layered perovskite interconversion occurred in the two-layer Dion–Jacobson oxide RbLaNb2O7 to yield Rb1+xLaNb2O7−xNx·yH2O (x=0.7–0.8x=0.7–0.8, y=0.5–1.0y=0.5–1.0). Both compounds were air- and moisture-sensitive, with rapid loss of nitrogen by oxidation and hydrolysis reactions. The structure of the three-layer oxynitride Rb1.7Ca2Nb3O9.3N0.7·0.5H2O was solved in space group P4/mmm   with a=3.887(3)a=3.887(3) and c=18.65(1)Å, by Rietveld refinement of X-ray powder diffraction data. The two-layer oxynitride structure Rb1.8LaNb2O6.3N0.7·1.0H2O was also determined in space group P4/mmm   with a=3.934(2)a=3.934(2) and c=14.697(2)Å. GSAS refinement of synchrotron X-ray powder diffraction data showed that the water molecules were intercalated between a double layer of Rb+ ions in both the two- and three-layer Ruddlesden–Popper structures. Optical band gaps were measured by diffuse reflectance UV-vis for both materials. An indirect band gap of 2.51 eV and a direct band gap of 2.99 eV were found for the three-layer compound, while an indirect band gap of 2.29 eV and a direct band gap of 2.84 eV were measured for the two-layer compound. Photocatalytic activity tests of the three-layer compound under 380 nm pass filtered light with AgNO3 as a sacrificial electron acceptor gave a quantum yield of 0.025% for oxygen evolution.

Scheme for the conversion of a Dion–Jacobson oxide to a Ruddlesden–Popper oxynitride by ammonialysis.Figure optionsDownload full-size imageDownload as PowerPoint slide