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

Synthesis of TiOxNy oxynitrides with a tunable nitrogen content

TiOxNy is a solid solution of cubic TiN and cubic TiO, with the ability to adjust its physical and chemical properties by modifying the O/N ratio, thus making it suitable for various applications. However, the synthesis of TiOxNy remains a significant challenge for inorganic chemists. In this work, we have successfully synthesized TiOxNy nanoparticles using the layered oxide Cs0.68Ti1.83O4 as the precursor and urea as the nitrogen source. The synthesis process was conducted within a temperature range of 800 to 1200 °C, leading to the transformation of the nitrided products from a two-dimensional layered precursor structure to a three-dimensional structure as the temperature increased. By varying the reaction temperature, the O/N ratio can be controlled. The experimental findings demonstrate that the nitrogen content in TiOxNy nanoparticles increases with rising temperature, ranging from TiO0.8N0.2 at 800 °C to TiO0.25N0.75 at 1200 °C. This work highlights the potential of the solid-state method in tailoring the properties of TiOxNy nanoparticles and presents a novel approach for synthesizing oxynitrides.

High-Temperature Synthesis of Ferromagnetic Eu3Ta3(O,N)9 with a Triple Perovskite Structure

Europium tantalum perovskite oxynitrides were prepared by a new high-temperature solid-state synthesis under N2 or N2/H2 gas. The nitrogen stoichiometry was tuned from 0.63 to 1.78 atoms per Eu or Ta atom, starting with appropriate N/O ratios in the mixture of the reactants Eu2O3, EuN and Ta3N5, or Eu2O3 and TaON, which was treated at 1200 °C for 3 h. Two phases were isolated with compositions EuTaO2.37N0.63 and Eu3Ta3O3.66N5.34, showing different crystal structures and magnetic properties. Electron diffraction and Rietveld refinement of synchrotron radiation X-ray diffraction indicated that EuTaO2.37N0.63 is a simple perovskite with cubic Pm3̅m structure and cell parameter a = 4.02043(1) Å, whereas the new compound Eu3Ta3O3.66N5.34 is the first example of a triple perovskite oxynitride and shows space group P4/mmm with crystal parameters a = 3.99610(2), c = 11.96238(9) Å. The tripling of the c-axis in this phase is a consequence of the partial ordering of europium atoms with different charges in two A sites of the perovskite structure with relative ratio 2:1, where the formal oxidation states +3 and +2 are respectively dominant. Magnetic data provide evidence of ferromagnetic ordering developing at low temperatures in both oxynitrides, with saturation magnetization of about 6 μB and 3 μB per Eu ion for EuTaO2.37N0.63 and the triple perovskite Eu3Ta3O3.66N5.34 respectively, and corresponding Curie temperatures of about 7 and 3 K, which is in agreement with the lower proportion of Eu2+ in the latter compound.

Cost-effective preparation of layered tantalum oxynitrides for visible light-driven photocatalysis

Layered oxynitrides are promising materials for visible light photocatalysis. However, the conventional method for the synthesis of oxynitrides using ammonia as a nitrogen source is dangerous. In this work, we successfully synthesized two layered tantalum oxynitrides, K_1.35LaTa₂O_6.65N_0.35 and K_1.4Ca₂Ta₃O_9.6N_0.4, via a topochemical nitridation process using urea as a solid nitrogen source. Employing different characterization methods, we determined the structure and composition of layered oxynitrides. Furthermore, using Pt as a co-catalyst, these two layered oxynitrides showed excellent photocatalytic performances under visible light irradiation. In contrast to ammonia, urea process provides easy access for the synthesis of layered oxynitrides and sheds new light on the design of effective visible light-driven photocatalysts.

Bandgap Tunable Oxynitride LaNb2 O7-x Nx Nanosheets

Bandgap tunable lanthanum niobium oxynitride [LaNb₂ O_7-x N_x ]^(1+x)- nanosheet is prepared by the delamination of a Ruddlesden-Popper phase perovskite oxynitride via ion-exchange and two-step intercalation processes. The lanthanum niobium oxynitride nanosheets have a homogeneous thickness of 1.6 nm and exhibit a variety of chromatic colors depending on the nitridation temperature of the parent-layered oxynitride. The bandgap energy of the nanosheets is determined by ultraviolet photoemission spectroscopy, Mott-Schottky, and photoelectrochemical measurements and is found to be tunable in the range of 2.03-2.63 eV. Furthermore, the oxide/oxynitride superlattice structures are fabricated by face-to-face stacking of 2D crystals using oxynitride [LaNb₂ O_7-x N_x ]^(1+x)- and oxide [Ca₂ Nb₃ O₁₀ ]^- nanosheets as building blocks. Moreover, the superlattices-like restacked oxynitride/oxide nanosheets hybrid exhibits unique proton conductivity and dielectric properties strongly influenced by the oxynitride nanosheets and enhanced photocatalytic activity under visible light irradiation.

Synthesis of Three-Layer Perovskite Oxynitride K2Ca2Ta3O9N·2H2O and Photocatalytic Activity for H2 Evolution under Visible Light

Substitution of oxide anions (O^2-) in a metal oxide for nitrogen (N^3-) results in reduction of the band gap, which is attractive in heterogeneous photocatalysis; however, only a handful of two-dimensional layered perovskite oxynitrides have been reported, and thus, the structural effects of layered oxynitrides on photocatalytic activity have not been sufficiently examined. This study reports the synthesis of a Ruddlesden-Popper phase three-layer oxynitride perovskite of K₂Ca₂Ta₃O₉N·2H₂O, and the photocatalytic activity is compared with an analogous two-layer perovskite, K₂LaTa₂O₆N·1.6H₂O. Topochemical ammonolysis reaction of a Dion-Jacobson phase oxide KCa₂Ta₃O₁₀ at 1173 K in the presence of K₂CO₃ resulted in a single-phase layered perovskite, K₂Ca₂Ta₃O₉N·2H₂O, which belongs to the tetragonal P4/mmm space group, as demonstrated by synchrotron X-ray diffraction, scanning transmission electron microscopy measurements, and elemental analysis. The synthesized K₂Ca₂Ta₃O₉N·2H₂O has an absorption edge at around 460 nm, with an estimated band gap of ca. 2.7 eV. K₂Ca₂Ta₃O₉N·2H₂O modified with a Pt cocatalyst generated H₂ from an aqueous solution containing a dissolved NaI as a reversible electron donor under visible light (λ > 400 nm) with no noticeable change in the crystal structure and light absorption properties. However, the H₂ evolution activity of K₂Ca₂Ta₃O₉N·2H₂O was an order of magnitude lower than that of K₂LaTa₂O₆N·1.6H₂O. Femtosecond transient absorption spectroscopy revealed that the lifetime of photogenerated mobile electrons in K₂Ca₂Ta₃O₉N·2H₂O was shorter than that in K₂LaTa₂O₆N·1.6H₂O, which could explain the low photocatalytic activity of K₂Ca₂Ta₃O₉N·2H₂O.

Heteroanionic Materials by Design: Progress Toward Targeted Properties

The burgeoning field of anion engineering in oxide-based compounds aims to tune physical properties by incorporating additional anions of different size, electronegativity, and charge. For example, oxychalcogenides, oxynitrides, oxypnictides, and oxyhalides may display new or enhanced responses not readily predicted from or even absent in the simpler homoanionic (oxide) compounds because of their proximity to the ionocovalent-bonding boundary provided by contrasting polarizabilities of the anions. In addition, multiple anions allow heteroanionic materials to span a more complex atomic structure design palette and interaction space than the homoanionic oxide-only analogs. Here, established atomic and electronic principles for the rational design of properties in heteroanionic materials are contextualized. Also described are synergistic quantum mechanical methods and laboratory experiments guided by these principles to achieve superior properties. Lastly, open challenges in both the synthesis and the understanding and prediction of the electronic, optical, and magnetic properties afforded by anion-engineering principles in heteroanionic materials are reviewed.

Undoped Layered Perovskite Oxynitride Li2 LaTa2 O6 N for Photocatalytic CO2 Reduction with Visible Light

Oxynitrides are promising visible-light-responsive photocatalysts, but their structures are almost confined with three-dimensional (3D) structures such as perovskites. A phase-pure Li2 LaTa2 O6 N with a layered perovskite structure was successfully prepared by thermal ammonolysis of a lithium-rich oxide precursor. Li2 LaTa2 O6 N exhibited high crystallinity and visible-light absorption up to 500 nm. As opposed to well-known 3D oxynitride perovskites, Li2 LaTa2 O6 N supported by a binuclear RuII complex was capable of stably and selectively converting CO2 into formate under visible light (λ>400 nm). Transient absorption spectroscopy indicated that, as compared to 3D oxynitrides, Li2 LaTa2 O6 N possesses a lower density of mid-gap states that work as recombination centers of photogenerated electron/hole pairs, but a higher density of reactive electrons, which is responsible for the higher photocatalytic performance of this layered oxynitride.

Controllable doping of nitrogen and tetravalent niobium affords yellow and black calcium niobate nanosheets for enhanced photocatalytic hydrogen evolution

Yellow (N-doped) and black (N-/Nb⁴⁺-codoped) [Ca₂Nb₃O₁₀]⁻ nanosheets with molecular thickness were fabricated by liquid exfoliation of selectively doped KCa₂Nb₃O₁₀ perovskite.

Hydrothermal synthesis of a doped Mn-Cd-S solid solution as a visible-light-driven photocatalyst for H2 evolution

The effect of metal doping (i.e., with Cr, Fe, Ni, Cu, Zn, Ag and Sn) on the crystal structure of hydrothermally synthesized Mn(1-x)Cd(x) S (where x≈0.1) is studied with the aim of enhancing photocatalytic activity. In contrast to the low-crystalline, undoped solid solution Mn(1-x)Cd(x)S, Ni doping yields a well-crystallized wurtzite-type Mn-Cd-S solid solution, which precipitates as planar hexagonal facets of several hundred nanometers in size, together with much larger grains of α-MnS (>10 μm). By removing inactive α-MnS through sedimentation, a single phase with composition of Ni(0.01)Mn(0.56)Cd(0.43)S is obtained successfully. The Ni doping achieved a threefold higher photocatalytic activity for H(2) evolution from a 0.1 M Na(2)S/0.5 M Na(2)SO(3) solution under visible-light irradiation (λ≥420 nm). The apparent quantum yield of 1 wt % Pt-loaded Ni(0.01)Mn(0.56)Cd(0.43)S measured at λ=420 nm reached 25 %. The enhanced photocatalytic activity is most likely the result of a decreased concentration of defects, responsible for electron-hole recombination, in the active solid-solution phase and a slightly higher bandgap energy (2.4 eV).

Tetragonal-to-orthorhombic structural phase transition at 90 K in the superconductor Fe(1.01)Se

In this Letter we show that superconducting Fe(1.01)Se undergoes a structural transition at 90 K from a tetragonal to an orthorhombic phase but that nonsuperconducting Fe(1.03)Se does not. High resolution electron microscopy at low temperatures further reveals an unexpected additional modulation of the crystal structure of the superconducting phase that involves displacements of the Fe atoms, and that the nonsuperconducting composition shows a different, complex nanometer-scale structural modulation. Finally, we show that magnetism is not the driving force for the phase transition in the superconducting phase.