Synthesis, Crystal Structure and Optical Properties of LaNbON2

CeTiO2N oxynitride perovskite: paramagnetic 14N MAS NMR without paramagnetic shifts

14N magic-angle spinning (MAS) nuclear magnetic resonance (NMR) spectra of diamagnetic LaTiO2N perovskite oxynitride and its paramagnetic counterpart CeTiO2N are presented. The latter, to the best of our knowledge, constitutes the first high-resolution 14N MAS NMR spectrum collected from a paramagnetic solid material. The unpaired 4f-electrons in CeTiO2N do not induce a paramagnetic 14N NMR shift. This is remarkable given the direct Ce−N contacts in the structure for which ab initio calculations predict substantial Ce→14N contact shift interaction. The same effect is revealed with 14N MAS NMR for SrWO2N (unpaired 5d-electrons).

Oxygen Evolution Activity of LaNbN2O-Based Photocatalysts Obtained from Nitridation of a Precursor Oxide Structurally Modified by Incorporating Volatile Elements

The perovskite-type oxynitride LaNbN2O is a photocatalyst that can evolve oxygen from aqueous solutions in response to long-wavelength visible light. However, it is challenging to obtain active LaNbN2O because of the facile reduction of Nb5+ during the nitridation of the precursor materials. The present study attempted to synthesize a perovskite-type oxide La0.6Na0.4Zn0.4Nb0.6O3, containing equimolar amounts of La3+ and Nb5+ in addition to volatile Na+ and Zn2+, followed by the nitridation of this oxide to generate LaNbN2O. The obtained oxide was not the intended single-phase material but rather comprised a cuboid perovskite-type oxide similar to La0.5Na0.5Zn0.33Nb0.67O3 along with spherical LaNbO4 particles and other impurities. A brief nitridation was found to form a LaNbN2O-like shell structure having a light absorption onset of approximately 700 nm on the cuboid perovskite-type oxide particles. This LaNbN2O-based photocatalyst, when loaded with a CoOx cocatalyst, exhibited an apparent quantum yield of 1.7% at 420 nm during oxygen evolution reaction from an aqueous AgNO3 solution. This was more than double the values obtained from the nitridation products of LaNbO4 and LaKNaNbO5. The present work demonstrates a new approach to the design of precursor oxides that yield highly active LaNbN2O and suggests opportunities for developing efficient Nb-based perovskite oxynitride photocatalysts.

Visible-light-driven photocatalytic water oxidation over LaNbON2–LaMg2/3Nb1/3O3 solid solutions

Solid solutions (LaNbON₂)_1−x(LaMg_2/3Nb_1/3O₃)_x (0.0 ≤ x ≤ 1.0) show promising photocatalytic activity for water oxidation into oxygen under visible light illumination (λ ≥ 420 nm).

Demonstration of Visible Light-Activated Photocatalytic Self-Cleaning by Thin Films of Perovskite Tantalum and Niobium Oxynitrides

Metal oxynitrides adopting the perovskite structure have been shown to be visible light-activated photocatalysts, and therefore, they have potential as self-cleaning materials where surface organic pollutants can be removed by photomineralization. In this work, we establish a route for the deposition of thin films for seven perovskite oxynitrides, CaTaO₂N, SrTaO₂N, BaTaO₂N, LaTaON₂, EuTaO₂N, SrNbO₂N, and LaNbON₂, on quartz and alumina substrates using dip-coating of a polymer gel to form an amorphous oxide precursor film, followed by ammonolysis. The initially deposited oxide films were annealed at 800 °C, followed by ammonolysis at temperatures from 850 to 1000 °C. The perovskite oxynitride thin films were characterized using XRD and EDX, with band gaps determined using Tauc plots derived from UV-vis spectroscopic data. A cobalt oxide co-catalyst was deposited onto each film by drop casting, and the photocatalytic activity assessed under visible light using dichloroindophenol dye degradation in the presence of a sacrificial oxidant. The light source used was a solar simulator equipped with a 400 nm cut-off filter. The dye degradation test demonstrated photocatalytic activity in all samples except EuTaO₂N and BaTaO₂N. The three most active samples were SrNbO₂N, CaTaO₂N, and SrTaO₂N. The cobalt oxide loading was optimized for these three films and found to be 0.3 μg cm^-2. Further, catalytic tests were conducted using stearic acid degradation, and this found the film of SrNbO₂N with the cobalt oxide co-catalyst to be the most active for complete mineralization of this model pollutant.

Synthesis, crystal structure, and magnetic properties of oxynitride perovskites SrMn0.2M0.8O2.6N0.4 (M = Nb, Ta)

Complex perovskites SrMn0.2Nb0.8O2.6N0.4 and SrMn0.2Ta0.8O2.6N0.4 were synthesized; these are rare examples of octahedral Mn2+ in oxynitride perovskites. Joint Rietveld refinement of neutron and X-ray data revealed that SrMn0.2Nb0.8O2.6N0.4 and SrMn0.2Ta0.8O2.6N0.4 had orthorhombic symmetries, in contrast with those of analogous perovskites SrM'0.2M0.8O3-xNx (M' = Li, Na, Mg; M = Nb, Ta), which are all tetragonal. Both SrMn0.2Nb0.8O2.6N0.4 and SrMn0.2Ta0.8O2.6N0.4 exhibited paramagnetic behavior with effective magnetic moments of 5.60μB and 5.94μB, respectively, consistent with a high-spin Mn2+ (d5, S = 5/2) state. The Weiss constants were -24.7 K for SrMn0.2Nb0.8O2.6N0.4 and -15.4 K for SrMn0.2Ta0.8O2.6N0.4, indicating the presence of weak antiferromagnetic spin-spin interactions. The band gaps of SrMn0.2Nb0.8O2.6N0.4 and SrMn0.2Ta0.8O2.6N0.4 were determined to be 1.75 eV and 2.2 eV, respectively, suggesting that the Mn 3d electrons were essentially localized.

Band Gap Adjustment in Perovskite-type Eu1−x Ca x TiO3 via Ammonolysis

Perovskite-type oxynitrides AB(O,N)3 are potential candidates for photoelectrode materials in solar water splitting. A drawback of these materials is their low sintering tendency resulting in low electrical conductivities. Typically, they are prepared by ammonia treatment of insulating, wide band gap oxides. In this study, we propose an approach starting from small band gap oxides Eu1−x Ca x TiO3− δ and then widen the band gaps in a controlled way by ammonolysis and partial Ca2+ substitution. Both together induced a distortion of the octahedral network and dilution of the Eu4f and N2p levels in the valence band. The effect is the stronger the more Ca2+ is present. Within the series of samples, Eu0.4Ca0.6Ti(O,N)3 had the most suitable optical band gap (EG ≈ 2.2 eV) for water oxidation. However, its higher Eu content compared to Eu0.1Ca0.9Ti(O,N)3 slowed down the charge carrier dynamics due to enhanced trapping and recombination as expressed by large accumulation (τ on) and decay (τ off) times of the photovoltage of up to 109 s and 486 s, respectively. In contrast, the highly Ca2+-substituted samples (x ≥ 0.7) were more prone to formation of TiN and oxygen vacancies also leading to Ti3+ donor levels below the conduction band. Therefore, a precise control of the ammonolysis temperature is essential, since even small amounts of TiN can suppress the photovoltage generation by fast recombination processes. Water oxidation tests on Eu0.4Ca0.6Ti(O,N)3 revealed a formation of 7.5 μmol O2 from 50 mg powder together with significant photocorrosion of the bare material. Combining crystal structure, chemical composition, and optical and electronical band gap data, a first simplified model of the electronical band structure of Eu1−x Ca x Ti(O,N)3 could be proposed.

A study on the thermal conversion of scheelite-type ABO4 into perovskite-type AB(O,N)3

Scheelite-type SrMO₄ oxides (M = Mo, W) convert upon thermal treatment under an ammonia atmosphere into an intermediate scheelite-type oxynitride phase SrMO_4−yN_y at 600 °C, and subsequently rearrange into perovskite-type oxynitrides SrMO_3−xN_x at higher temperatures.

A hybrid density functional theory study of the anion distribution and applied electronic properties of the LaTiO2N semiconductor photocatalyst

The LaTiO₂N structure is theoretically obtained as aperiodic stacks of primitive cells.

Transformation of a layered perovskite to a defect perovskite via cooperative Li-insertion and O/N substitution

Defect perovskite oxynitrides containing lithium on the octahedral sites, i.e., Sr(1-x)(Li(x)Ta(1-x))(O(3.5-3x-3y)N(2y)), were produced by the ammonolytic heating of a layered perovskite, Sr2Ta2O7, with Li2CO3. The above phase evolution involved the thermal diffusion of Li(+) into the Sr2Ta2O7 lattice and concurrent 3O(2-) → 2N(3-) substitution. The concentrations of Li, N, and anion vacancies in the product oxynitride could be controlled by adjusting the Li2CO3 to Sr2Ta2O7 ratio in the reactant mixture. This study identified three different compositions, Sr(0.92)Li(0.08)Ta(0.92)O(1.91)N(0.89), Sr(0.83)Li(0.17)Ta(0.83)O(1.88)N(0.74) and Sr(0.73)Li(0.27)Ta(0.73)O(1.82)N(0.58), where an increase in the Li composition accompanied decreases in the a- and c-parameters of the tetragonal cell, along with an increase in the band gap. Neutron diffraction and solid state (7)Li nuclear magnetic resonance spectroscopy of Sr(1-x)(Li(x)Ta(1-x))(O(3.5-3x-3y)N(2y)) showed that Li occupied the octahedral site, and these oxynitrides exhibited enhanced bond covalency compared to similar oxides.