A novel n-UV convertible colour-tunable emitting oxynitride phosphor: Realization based on Ce3+ -Tb3+ energy transfer

In the present work, a novel n-UV convertible colour-tunable emitting phosphor was obtained based on the efficient Ce3+ -Tb3+ energy transfer in the Y10 Al2 Si3 O18 N4 host. By properly controlling the ratio of Ce3+ /Tb3+ , the colour hue of the obtained powder covered the blue and green regions, under excitation of 365 nm. The steady-state and dynamic-state luminescence measurement was performed to shed light on the related mechanism, which was justified by the electronic dipole-quadrupole dominating the related energy transfer process. Preliminary studies showed that Y10 Al2 Si3 O18 N4 :Ce3+ ,Tb3+ can be promising as an inorganic phosphor for white LED applications.

Nanoparticulate TiN Loading to Promote Z-Scheme Water Splitting Using a Narrow-Bandgap Nonoxide-Based Photocatalyst Sheet

Some oxide-based particulate photocatalyst sheets exhibit excellent activity during the water-splitting reaction. The replacement of oxide photocatalysts with narrow-bandgap photocatalysts based on nonoxides could provide the higher solar-to-hydrogen energy conversion efficiencies that are required for practical implementation. Unfortunately, the activity of nonoxide-based photocatalyst sheets is low in many cases, indicating the need for strategies to improve the quality of nonoxide photocatalysts and the charge transfer process. In this work, single-crystalline particulate SrTaO2 N is studied as an oxygen evolution photocatalyst for photocatalyst sheets applied to Z-scheme water splitting, in combination with La5 Ti2 Cu0.9 Ag0.1 O7 S5 and Au as the hydrogen evolution photocatalyst and conductive layer, respectively. The loading of SrTaO2 N with CoOx provided increases activity during photocatalytic water oxidation, giving an apparent quantum yield of 15.7% at 420 nm. A photocatalyst sheet incorporating CoOx -loaded SrTaO2 N is also found to promote Z-scheme water splitting under visible light. Notably, the additional loading of nanoparticulate TiN on the CoOx -loaded SrTaO2 N improves the water splitting activity by six times because the TiN promotes electron transfer from the SrTaO2 N particles to the Au layer. This work demonstrates key concepts related to the improvement of nonoxide-based photocatalyst sheets based on facilitating the charge transfer process through appropriate surface modifications.

Ammonia Synthesis over Fe-Supported Catalysts Mediated by Face-Sharing Nitrogen Sites in BaTiO3-x Ny Oxynitride

Nitride and hydride materials have been proposed as active supports for the loading of transition metal catalysts in thermal catalytic ammonia synthesis. However, the contribution of nitrogen or hydride anions in the support to the catalytic activity for supported transition-metal catalysts is not well understood, especially for Fe-based catalysts. Here, we report that hexagonal-BaTiO3-x Ny with nitrogen vacancies at face-sharing sites acts as a more efficient support for Fe catalysts for ammonia synthesis than BaTiO3 or BaTiO3-x Hx at 260 °C to 400 °C. Isotopic experiments, in situ measurements, and a small inverse isotopic effect in ammonia synthesis have revealed that nitrogen molecules are activated at nitrogen vacancies formed at the interface between Fe nanoparticles and the support. Nitrogen vacancies on BaTiO3-x Ny can promote the activity of Fe and Ni catalysts, while electron donation and suppression of hydrogen poisoning by BaTiO3-x Hx are significant in the Ru and Co systems.

Perovskite BaTaO2 N: From Materials Synthesis to Solar Water Splitting

Barium tantalum oxynitride (BaTaO2 N), as a member of an emerging class of perovskite oxynitrides, is regarded as a promising inorganic material for solar water splitting because of its small band gap, visible light absorption, and suitable band edge potentials for overall water splitting in the absence of an external bias. However, BaTaO2 N still exhibits poor water-splitting performance that is susceptible to its synthetic history, surface states, recombination process, and instability. This review provides a comprehensive summary of previous progress, current advances, existing challenges, and future perspectives of BaTaO2 N for solar water splitting. A particular emphasis is given to highlighting the principles of photoelectrochemical (PEC) water splitting, classic and emerging photocatalysts for oxygen evolution reactions, and the crystal and electronic structures, dielectric, ferroelectric, and piezoelectric properties, synthesis routes, and thin-film fabrication of BaTaO2 N. Various strategies to achieve enhanced water-splitting performance of BaTaO2 N, such as reducing the surface and bulk defect density, engineering the crystal facets, tailoring the particle morphology, size, and porosity, cation doping, creating the solid solutions, forming the heterostructures and heterojunctions, designing the photoelectrochemical cells, and loading suitable cocatalysts are discussed. Also, the avenues for further investigation and the prospects of using BaTaO2 N in solar water splitting are presented.

Stability of the Interface Between LIPON and LCO During TEM Sample Preparation by FIB

An amorphous interphase between lithium phosphorus oxynitride (LIPON) solid electrolyte and lithium cobalt oxide (LCO) has been reported recently in several electron microscopy studies of Li ion thin-film micro-batteries (TFMB), along with its implications to battery operation. However, the origin of the observed interphase at the as-made LIPON/LCO interface remained obscure. In this work, this interface has been characterized comprehensively by scanning electron microscope (SEM) imaging at all steps of the in situ focused ion beam (FIB) lift-out procedure for transmission electron microscopy (TEM) sample preparation. It was found that the interphase is formed during TEM lamella preparation when the portion of LIPON layer contained within the lamella is physically disconnected from the rest of the LIPON layer by FIB. Therefore, it was demonstrated that a disordered interphase can form in LCO at its interface with LIPON during TEM sample preparation by the FIB lift-out procedure and that subtle nature of the interphase formation makes it likely to go unnoticed during the preparation. This interphase was not produced even after galvanostatic charging of a battery with a Li metal anode but inevitably appeared after the FIB lift-out of that sample.

Photoelectrochemical Performance of Strontium Titanium Oxynitride Photo-Activated with Cobalt Phosphate Nanoparticles for Oxidation of Alkaline Water

Photoelectrochemical (PEC) solar water splitting is favourable for transforming solar energy into sustainable hydrogen fuel using semiconductor electrodes. Perovskite-type oxynitrides are attractive photocatalysts for this application due to their visible light absorption features and stability. Herein, strontium titanium oxynitride (STON) containing anion vacancies of SrTi(O,N)_3-δ was prepared via solid phase synthesis and assembled as a photoelectrode by electrophoretic deposition, and their morphological and optical properties and PEC performance for alkaline water oxidation are investigated. Further, cobalt-phosphate (CoPi)-based co-catalyst was photo-deposited over the surface of the STON electrode to boost the PEC efficiency. A photocurrent density of ~138 μA/cm at 1.25 V versus RHE was achieved for CoPi/STON electrodes in presence of a sulfite hole scavenger which is approximately a four-fold enhancement compared to the pristine electrode. The observed PEC enrichment is mainly due to the improved kinetics of oxygen evolution because of the CoPi co-catalyst and the reduced surface recombination of the photogenerated carriers. Moreover, the CoPi modification over perovskite-type oxynitrides provides a new dimension for developing efficient and highly stable photoanodes in solar-assisted water-splitting reactions.

Effects of Film Thickness of ALD-Deposited Al2O3, ZrO2 and HfO2 Nano-Layers on the Corrosion Resistance of Ti(N,O)-Coated Stainless Steel

The goal of this stydy was to explore the potential of the enhanced corrosion resistance of Ti(N,O) cathodic arc evaporation-coated 304L stainless steel using oxide nano-layers deposited by atomic layer deposition (ALD). In this study, we deposited Al₂O₃, ZrO₂, and HfO₂ nanolayers of two different thicknesses by ALD onto Ti(N,O)-coated 304L stainless steel surfaces. XRD, EDS, SEM, surface profilometry, and voltammetry investigations of the anticorrosion properties of the coated samples are reported. The amorphous oxide nanolayers homogeneously deposited on the sample surfaces exhibited lower roughness after corrosion attack compared to the Ti(N,O)-coated stainless steel. The best corrosion resistance was obtained for the thickest oxide layers. All samples coated with thicker oxide nanolayers augmented the corrosion resistance of the Ti(N,O)-coated stainless steel in a saline, acidic, and oxidising environment (0.9% NaCl + 6% H₂O₂, pH = 4), which is of interest for building corrosion-resistant housings for advanced oxidation systems such as cavitation and plasma-related electrochemical dielectric barrier discharge for breaking down persistent organic pollutants in water.

Characterizing Density and Spatial Distribution of Trap States in Ta3N5 Thin Films for Rational Defect Passivation

Tantalum nitride (Ta₃N₅) has gained significant attention as a potential photoanode material, yet it has been challenged by material quality issues. Defect-induced trap states are detrimental to the performance of any semiconductor material. Beyond influencing the performance of Ta₃N₅ films, defects can also accelerate the degradation in water during desired electrochemical applications. Defect passivation has provided an enormous boost to the development of many semiconductor materials but is currently in its infancy for Ta₃N₅. This is in part due to a lack of experimental understanding regarding the spatial and energetic distribution of trap states throughout Ta₃N₅ thin films. Here, we employ drive-level capacitance profiling (DLCP) to experimentally resolve the spatial and energetic distribution of trap states throughout Ta₃N₅ thin films. The density of deeper energetic traps is found to reach ∼2.5 to 6 × 10²² cm^-3 at the interfaces of neat Ta₃N₅ thin films, over an order of magnitude greater than the bulk. In addition to the spatial profile of deep trap states, we report neat Ta₃N₅ thin films to be highly n-type in nature, owning a free carrier density of ∼9.74 × 10¹⁷ cm^-3. This information, coupled with the present understanding of native oxide layers on Ta₃N₅, has facilitated the rational design of a targeted passivation strategy that simultaneously provides a means for catalyst immobilization. Loading catalyst via silatrane moieties suppresses the density of defects at the surface of Ta₃N₅ thin films by two orders of magnitude, while also reducing the free carrier density of films by over one order of magnitude, effectively dedoping the films to ∼2.40 × 10¹⁶ cm^-3. The surface passivation of Ta₃N₅ films translates to suppressed defect-induced trapping and recombination of photoexcited carriers, as determined through absorption, photoluminescence, and transient photovoltage. This illustrates how developing a deeper understanding of the distribution and influence of defects in Ta₃N₅ thin films has the potential to guide future works and ultimately accelerate the integration and development of high-performance Ta₃N₅ thin film devices.

Oxynitride-Encapsulated Silver Nanowire Transparent Electrode with Enhanced Thermal, Electrical, and Chemical Stability

Silver nanowire (AgNW) networks have been explored as a promising technology for transparent electrodes due to their solution-processability, low-cost implementation, and excellent trade-off between sheet resistance and transparency. However, their large-scale implementation in applications such as solar cells, transparent heaters, and display applications has been hindered by their poor thermal, electrical, and chemical stability. In this work, we present reactive sputtering as a method for fast deposition of metal oxynitrides as an encapsulant layer on AgNWs. Because O₂ cannot be used as a reactive gas in the presence of oxidation-sensitive materials such as Ag, N₂ is used under moderate sputtering base pressures to leverage residual H₂O on the sample and chamber to deposit Al, Ti, and Zr oxynitrides (AlO_xN_y, TiO_xN_y, and ZrO_xN_y) on Ag nanowires on glass and polymer substrates. All encapsulants improve AgNW networks' electrical, thermal, and chemical stability. In particular, AlO_xN_y-encapsulated networks present exceptional chemical stability (negligible increase in resistance over 7 days at 80% relative humidity and 80 °C) and transparency (96% for 20 nm films on AgNWs), while TiO_xN_y demonstrates exceptional thermal and electrical stability (stability up to over temperatures 100 °C more than that of bare AgNW networks, with a maximum areal power density of 1.72 W/cm², and no resistance divergence at up to 20 V), and ZrO_xN_y presents intermediate properties in all metrics. In summary, a novel method of oxynitride deposition, leveraging moderate base pressure reactive sputtering, is demonstrated for AgNW encapsulant deposition, which is compatible with roll-to-roll processes that are operated at commercial scales, and this technique can be extended to arbitrary, vacuum-compatible substrates and device architectures.

Surface Analysis of Perovskite Oxynitride Thin Films as Photoelectrodes for Solar Water Splitting

Perovskite oxynitride semiconductors have attracted huge interest recently as promising photoelectrode materials for photoelectrochemical (PEC) water splitting. Depicted by, the extensive studies of the PEC activity of oxynitride powder-based photoelectrodes and/or deposited thin-film electrodes. High-crystalline-quality, oxynitride thin films grown by physical vapor deposition are ideal model systems to study the fundamental physical and chemical properties of the surface of these materials, including their evolution. In this work, using a combination of high-sensitivity low-energy ion scattering (LEIS) and X-ray photoelectron spectroscopy (XPS), we monitor surface evolution of LaTiO_xN_y (LTON) and CaNbO_xN_y (CNON) thin films before and after the PEC characterizations. The as-prepared epitaxial LTON films show a preferential LaO termination at the surface layers, followed by a Ti-enriched subsurface. Whereas, the polycrystalline CNON thin films exhibit a non-uniform surface, with a mixed surface termination and a significant Ca-segregated subsurface. After the PEC characterizations, additional precipitated LaO species are found on the outer surface of the LTON epitaxial films. However, no significant surface change is observed on the polycrystalline CNON films by LEIS. The XPS analysis shows, an increase of the oxidized Ti and Nb cations (Ti⁴⁺ and Nb⁵⁺) after the PEC reaction in the LTON and CNON films, respectively. The initial drops in photocurrent for the LTON and CNON films are attributed to the changes in the surface chemical status. This work provides insight into the surface characteristics and evolution of LTON and CNON oxynitride thin films as photoelectrodes for PEC applications.

Lu-atom-ordered oxonitridoaluminosilicate Ba0.9Ce0.1LuAl0.2Si3.8N6.9O0.1

A single crystal of Ba_0.9Ce_0.1LuAl_0.2Si_3.8N_6.9O_0.1 (barium cerium lutetium aluminosilicate nitride oxide) was obtained by heating a mixed powder of Ba₃N₂, Si₃N₄, Al, Lu₂O₃, and CeO₂ at 2173 K for 1 h under N₂ gas at 0.85 MPa. X-ray single-crystal structure analysis revealed that the title oxynitride is hexa­gonal and isostructural with BaYbSi₄N₇. (Ba,Ce) and Lu atoms occupy twelvefold and sixfold coordination sites, respectively.

A single crystal of Ba_0.9Ce_0.1LuAl_0.2Si_3.8N_6.9O_0.1 (barium cerium lutetium aluminosilicate nitride oxide) was obtained by heating a mixed powder of Ba₃N₂, Si₃N₄, Al, Lu₂O₃, and CeO₂ at 2173 K for 1 h under N₂ gas at 0.85 MPa. X-ray single-crystal structure analysis revealed that the title oxynitride is hexa­gonal (lattice constants: a = 6.0378 (5) Å, c = 9.8133 (9) Å; space group: P6₃mc) and isostructural with BaYbSi₄N₇. (Ba,Ce) and Lu atoms occupy twelvefold and sixfold coordination sites, respectively.

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.

Luminescence from Si-Implanted SiO₂-Si₃N₄ Nano Bi-Layers for Electrophotonic Integrated Si Light Sources

In this paper, we present structural and luminescence studies of silicon-rich silicon oxide (SRO) and SRO-Si 3 N 4 bi-layers for integration in emitter-waveguide pairs that can be used for photonic lab-on-a-chip sensing applications. The results from bi and mono layers are also compared. Two clearly separated emission bands are respectively attributed to a combination of defect and quantum confinement⁻related emission in the SRO, as well as to defects found in an oxynitride transition zone that forms between the oxide and the nitride films, while ruling out quantum-confinement phenomena in the silicon nitride.

Crystal structures of Ca4+x Y3-x Si7O15+x N5-x (0 ≤ x ≤ 1) comprising of an isolated [Si7(O,N)19] unit

Single crystals of the solid solution series Ca_4+x Y_3-x Si₇O_15+x N_5-x were obtained by a solid-state reaction method using a flux for x = 0, 0.5 and 1, resulting in Ca₄Y₃Si₇O₁₅N₅ (tetra-calcium triyttrium hepta-silicon oxynitride), Ca_4.5Y_2.5Si₇O_15.5N_4.5 and Ca₅Y₂Si₇O₁₆N₄ (penta-calcium diyttrium hepta-silicon oxynitride). Single-crystal X-ray analysis revealed that the three compounds are isotypic and belong to space-group type P6₃/m. Ca²⁺ and Y³⁺ cations are distributed over two crystallographic sites (site symmetry .. and 1) in a disordered manner. The corresponding (Ca,Y)-centred polyhedra are connected by edge-sharing, resulting in the formation of a layer structure extending parallel to (001). Three [Si(O,N)]₄ tetra-hedra (two with point group symmetry m.., one with 3.. and half-occupancy) are condensed into an isolated [Si₇(O,N)₁₉] unit, in which an [Si(O,N)]₄ tetra-hedron is located at the center of a 12-membered oxynitride ring with composition [Si₆O₁₅N₃]. The present compounds are the first to have such an [Si₇(O,N)₁₉] unit in their structures.

Anion-Substitution-Induced Nonrigid Variation of Band Structure in SrNbO3- xN x (0 ≤ x ≤ 1) Epitaxial Thin Films

Pervoskite oxynitrides exhibit rich functionalities such as colossal magnetoresistance and high photocatalytic activity. The wide tunability of physical properties by the N/O ratio makes perovskite oxynitrides promising as optical and electrical materials. However, composition-dependent variation of the band structure, especially under partially substituted composition, is not yet well understood. In this study, we quantitatively analyzed the composition-dependent variation of band structure of a series of SrNbO_{3- x}N _x (0 ≤ x ≤ 1.02) epitaxial thin films. Electrical conductivity decreased along with the increase of N content x as a result of an increase in Nb valence from 4+ to 5+. Optical measurements revealed that the N 2p band is formed at a critical composition between 0.07 < x < 0.38, which induces charge-transfer transition (CTT) in the visible-light region. These variations in the band structure were explained by first-principles calculations. However, the CTT energy slightly increased at higher N contents (i.e., lower carrier density) on contrary to the expectation based on the rigid-band-like shift of the Fermi level, which suggests a complex combination of the following band-shifting effects induced by N-substitution: whereas (1) reduction of the Burstein-Moss effect causes CTT energy reduction, (2) enhancement of hybridization between Nb 4d and N 2p orbitals and/or (3) suppression of many-body effects enlarge the band gap energy at larger N content. The band structure variation in perovskite oxynitride as presently elucidated would be a guidepost for future material design.

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.

Combinatorial Discovery of Lanthanum-Tantalum Oxynitride Solar Light Absorbers with Dilute Nitrogen for Solar Fuel Applications

Oxynitrides with the photoelectrochemical stability of oxides and desirable band energetics of nitrides comprise a promising class of materials for solar photochemistry. Challenges in synthesizing a wide variety of oxynitride materials has limited exploration of this class of functional materials, which we address using a reactive cosputtering combined with rapid thermal processing method to synthesize multi-cation-multi-anion libraries. We demonstrate the synthesis of a La_xTa_1-xO_yN_z thin film composition spread library and its characterization by both traditional thin film materials characterization and custom combinatorial optical spectroscopy and X-ray absorption near edge spectroscopy (XANES) techniques, ultimately establishing structure-chemistry-property relationships. We observe that over a substantial La-Ta composition range the thin films crystallize in the same perovskite LaTaON₂ structure with significant variation of anion chemistry. The relative invariance in optical band gap demonstrates a remarkable decoupling of composition and band energetics so that the composition can be optimized while retaining the desirable 2 eV band gap energy. We also demonstrate the intercalation of diatomic nitrogen into the La₃TaO₇ structure, which gives rise to a direct-allowed optical transition at 2.2 eV, less than half the value of the oxide's band gap. These findings motivate further exploration of the visible light response of this material that is predicted to be stable over a wide range of electrochemical potential.

Control of Elemental Distribution in the Nanoscale Solid-State Reaction That Produces (Ga1-xZnx)(N1-xOx) Nanocrystals

Solid-state chemical transformations at the nanoscale can be a powerful tool for achieving compositional complexity in nanomaterials. It is desirable to understand the mechanisms of such reactions and characterize the local-level composition of the resulting materials. Here, we examine how reaction temperature controls the elemental distribution in (Ga_1-xZn_x)(N_1-xO_x) nanocrystals (NCs) synthesized via the solid-state nitridation of a mixture of nanoscale ZnO and ZnGa₂O₄ NCs. (Ga_1-xZn_x)(N_1-xO_x) is a visible-light absorbing semiconductor that is of interest for applications in solar photochemistry. We couple elemental mapping using energy-dispersive X-ray spectroscopy in a scanning transmission electron microscope (STEM-EDS) with colocation analysis to study the elemental distribution and the degree of homogeneity in the (Ga_1-xZn_x)(N_1-xO_x) samples synthesized at temperatures ranging from 650 to 900 °C with varying ensemble compositions (i.e., x values). Over this range of temperatures, the elemental distribution ranges from highly heterogeneous at 650 °C, consisting of a mixture of larger particles with Ga and N enrichment near the surface and very small NCs, to uniform particles with evenly distributed constituent elements for most compositions at 800 °C and above. We propose a mechanism for the formation of the (Ga_1-xZn_x)(N_1-xO_x) NCs in the solid state that involves phase transformation of cubic spinel ZnGa₂O₄ to wurtzite (Ga_1-xZn_x)(N_1-xO_x) and diffusion of the elements along with nitrogen incorporation. The temperature-dependence of nitrogen incorporation, bulk diffusion, and vacancy-assisted diffusion processes determines the elemental distribution at each synthesis temperature. Finally, we discuss how the visible band gap of (Ga_1-xZn_x)(N_1-xO_x) NCs varies with composition and elemental distribution.

The impact of anion ordering on octahedra distortion and phase transitions in SrTaO2N and BaTaO2N

In this work we synthesized BaTaO₂N and SrTaO₂N using a two-step high-temperature solid-state reaction method and analysed the structural distortions, relative to the ideal cubic perovskite structure, according to group theory. From a complete distortion analysis/refinement using high-resolution neutron diffraction data in the temperature range 8 to 613 K, we identified tetragonal structures for BaTaO₂N [P4/mmm (No. 123)] and SrTaO₂N [I4/mcm (No. 140)]. In contrast to an anion-disordered cubic perovskite (Pm \overline{3}m No. 221) with Ta at the cell center, both systems show a site preference for oxygen anions in the two opposite corners (along the c axis) of the Ta-O/N octahedra rather than the four square corners in the ab plane (Γ₃⁺ occupancy distortion), which induces a tetragonal elongation of the unit cell with the c axis being longer than the a axis. A further Ta-O/N octahedra displacement [R₅^-(a,0,0), rotation about the c axis] distortion was observed in SrTaO₂N. This distortion mode is accompanied by an increased unit-cell distortion that decreases as the temperature increases. Ultimately a second-order phase transition caused by the loss of the R₅^-(a,0,0) mode was observed at 400-450 K.

Strain Engineering for Anion Arrangement in Perovskite Oxynitrides

Mixed-anion perovskites such as oxynitrides, oxyfluorides, and oxyhydrides have flexibility in their anion arrangements, which potentially enables functional material design based on coordination chemistry. However, difficulty in the control of the anion arrangement has prevented the realization of this concept. In this study, we demonstrate strain engineering of the anion arrangement in epitaxial thin films of the Ca_1-xSr_xTaO₂N perovskite oxynitrides. Under compressive epitaxial strain, the axial sites in TaO₄N₂ octahedra tend to be occupied by nitrogen rather than oxygen, which was revealed by N and O K-edge linearly polarized X-ray absorption near-edge structure (LP-XANES) and scanning transmission electron microscopy combined with electron energy loss spectroscopy. Furthermore, detailed analysis of the LP-XANES indicated that the high occupancy of nitrogen at the axial sites is due to the partial formation of a metastable trans-type anion configuration. These results are expected to serve as a guide for the material design of mixed-anion compounds based on their anion arrangements.

Enhancing Electrocatalytic Performance of Bifunctional Cobalt-Manganese-Oxynitride Nanocatalysts on Graphene

We report the synthesis and characterization of graphenesupported cobalt-manganese-oxynitride nanocatalysts (CoMnON/G) as bifunctional electrocatalysts for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). A nitriding treatment of spinel compound CoMnO increased the ORR activity considerably, and the most active material catalyzed the ORR with only a 30 mV half-wave potential difference from the commercial carbon-supported platinum (Pt/C) in alkaline media. In addition to high activity, the catalyst also exhibited an intrinsic stability that outperformed Pt/C. An appropriately designed nitridation thus facilitates new directions for developing active and durable non-precious-metal oxynitride electocatalysts.

Selective Formic Acid Production via CO2 Reduction with Visible Light Using a Hybrid of a Perovskite Tantalum Oxynitride and a Binuclear Ruthenium(II) Complex

A hybrid material consisting of CaTaO2N (a perovskite oxynitride semiconductor having a band gap of 2.5 eV) and a binuclear Ru(II) complex photocatalytically produced HCOOH via CO2 reduction with high selectivity (>99%) under visible light (λ>400 nm). Results of photocatalytic reactions, spectroscopic measurements, and electron microscopy observations indicated that the reaction was driven according to a two-step photoexcitation of CaTaO2N and the Ru photosensitizer unit, where Ag nanoparticles loaded on CaTaO2N with optimal distribution mediated interfacial electron transfer due to reductive quenching.

Recent progress in oxynitride photocatalysts for visible-light-driven water splitting

Photocatalytic water splitting into hydrogen and oxygen is a method to directly convert light energy into storable chemical energy, and has received considerable attention for use in large-scale solar energy utilization. Particulate semiconductors are generally used as photocatalysts, and semiconductor properties such as bandgap, band positions, and photocarrier mobility can heavily impact photocatalytic performance. The design of active photocatalysts has been performed with the consideration of such semiconductor properties. Photocatalysts have a catalytic aspect in addition to a semiconductor one. The ability to control surface redox reactions in order to efficiently produce targeted reactants is also important for photocatalysts. Over the past few decades, various photocatalysts for water splitting have been developed, and a recent main concern has been the development of visible-light sensitive photocatalysts for water splitting. This review introduces the study of water-splitting photocatalysts, with a focus on recent progress in visible-light induced overall water splitting on oxynitride photocatalysts. Various strategies for designing efficient photocatalysts for water splitting are also discussed herein.

Local analysis of Eu2+ emission in CaAlSiN3

We have investigated the local luminescence properties of Eu-doped CaAlSiN3 by using low-energy electron beam (e-beam) techniques. The particles yield broad emission centered at 655 nm with a shoulder at higher wavelength under light excitation, and a broad band around 643 nm with a tail at 540 nm under e-beam excitation. Using cathodoluminescence (CL) in a scanning electron microscope (SEM), we have observed small and large particles, which, although with different compositions, exhibit Eu2+-related emissions at 645 and 635 nm, respectively. Local CL measurements reveal that the Eu2+ emission may actually consist of several bands. In addition to the red broad band, regularly spaced sharp peaks have been occasionally observed. These luminescence variations may originate from a variation in the composition inside CaAlSiN3.

Photoluminescence properties of β-SiAlON:Yb2+, a novel green-emitting phosphor for white light-emitting diodes

We have synthesized Yb²⁺-activated Si_6-z Al _z O _z N_8-z (0.05⩽z⩽2.3, 0.03 mol% ⩽Yb²⁺⩽0.7 mol%) green phosphors by solid-state reaction at 1900 °C for 2 h under a nitrogen pressure of 1.0 MPa. Phase purity, photoluminescence and its thermal quenching were investigated. A single phase was obtained for all values of z and Yb²⁺ concentration. A distinct emission band was observed at 540 nm originating from the 5d-4f electronic transition in Yb²⁺ under 480 nm excitation. The photoluminescence properties mainly depended on the Yb²⁺ concentration and chemical composition of the matrix. The resultant phosphor showed high thermal stability, that is, the emission intensity at 150 °C was about 82% of that measured at room temperature. The experimental results indicate that β-SiAlON:Yb²⁺ is a potential green phosphor for white light-emitting diodes (LEDs), which use blue LEDs as the primary light source.