Perovskite Oxynitrides Research Articles

Effects of nitrogen vacancy sites of oxynitride support on the catalytic activity for ammonia decomposition

Nitrogen-containing compounds such as imides and amides have been reported as efficient materials that promote ammonia decomposition over nonnoble metal catalysts. However, these compounds decompose in an air atmosphere and become inactive, which leads to difficulty in handling. Here, we focused on perovskite oxynitrides as air-stable and efficient supports for ammonia decomposition catalysts. Ni-loaded oxynitrides exhibited 2.5–18 times greater catalytic activity than did the corresponding oxide-supported Ni catalysts, even without noticeable differences in the Ni particle size and surface area of the supports. The catalytic performance of the Ni-loaded oxynitrides is well correlated with the nitrogen desorption temperature during N2 temperature-programmed desorption, which suggests that the lattice nitrogen in the oxynitride support rather than the Ni surface is the active site for ammonia decomposition. Furthermore, NH3 temperature-programmed surface reactions and density functional theory (DFT) calculations revealed that NH3 molecules are preferentially adsorbed on the nitrogen vacancy sites on the support surface rather than on the Ni surface. Thus, the ammonia decomposition reaction is facilitated by a vacancy-mediated reaction mechanism.

Physically-Based Descriptors of the Nitrogen Reduction Reaction on Transition Metal Nitrides

Due to the high energy intensity and consequent carbon footprint of the Haber-Bosch process for ammonia production, researchers have been intensely studying the electrochemical nitrogen reduction reaction (NRR) as a means to directly produce ammonia using clean energy with lower emissions of carbon by-products. One exciting class of materials for NRR catalysis are transition metal nitrides (TMNs), as they have been shown to have good stability, flexible electronic structures, and decent electrochemical activity [1]. Previous studies using density functional theory (DFT) have explored the energetics of known NRR pathways (associative, dissociative, and Mars-van Krevelen) on the mononitrides of all naturally occurring d-block metals in rock salt and zincblende structures [2]. The most favorable NRR mechanism (determined to be the pathway that contains the smallest maximum potential change across all steps of the reaction) on these materials was shown to be the Mars-van Krevelen mechanism. Uncovering electronic, structural, and dynamic descriptors that correlate well with the energetics of this favorable NRR mechanism on TMNs will be useful in mitigating the need to run full DFT calculations when expanding the search space of potential catalyst candidates, such as ternary transition metal nitrides (TTMNs) and perovskite transition metal oxynitrides (TMONs). Previous literature has shown that the proton adsorption energy is a good descriptor for catalytic NRR activity on TMNs [3], oxygen vacancy content [4] and the oxygen p-band center of bulk perovskites [5] are good descriptors for catalytic oxygen evolution reaction performance, and the metal d-band center of pure transition metal catalysts is a good descriptor for reactivity and adsorption energies [6]. Inspired by these results, this work uses DFT on a set of TMNs to calculate a variety of potential physics-based descriptors, including: metal d and nitrogen p-band centers, hybridization between nitrogen p and metal d orbitals, nitrogen vacancy content, surface Bader charges, and phonon band centers. Since the energetics of the NRR can be modulated by the properties of either the atoms in the bulk or in the top surface layers, we calculate all of our descriptors from atoms in the TMN bulk form as well as from surface and subsurface atoms in the TMN slab form.Our results reveal several correlations between the descriptors and material attributes (such as adsorption energies and vacancy formation energies), which in turn govern the energetics of the NRR on the TMNs. We find that the N2 adsorption energy is negatively correlated with the metal d-band center, the N adsorption energy is positively correlated with the nitrogen p-band center, and both the proton adsorption energy and nitrogen vacancy formation energy are negatively correlated with nitrogen p and metal d orbital hybridization. Interestingly, we also find that for some attributes, the subsurface layer atoms dictate the attribute values and not the surface layer atoms. This suggests that heterogeneous catalysis reactivity may be modulated by atomic properties not purely from the bulk or purely from the surface, but rather from multiple subsurface layers as well.Overall, the insights gained from our descriptor analysis can be used to constrain the search and design space of future potential NRR catalysts, such as TTMNs and TMONs. In doing so, the descriptors can be used in future works in either a high-throughput screening or machine learning framework to allow for the inverse material design of optimal NRR catalysts. These optimal catalysts can in turn lead to the emergence of electrochemical ammonia synthesis in industrial production.References Qiao Z, Johnson D and Djire A. Cell Rep Phys Sci 2021; 2.Abghoui Y and Skúlason E. Catal Today 2017; 286: 69-77.Abghoui Y, Garden AL, Hlynsson VF, Björgvinsdóttir S, Ólafsdóttir H and Skúlason E. Physical Chemistry Chemical Physics 2015; 17: 4909-4918.Marelli E, Gazquez J, Poghosyan E, Müller E, Gawryluk DJ, Pomjakushina E, Sheptyakov D, Piamonteze C, Aegerter D, Schmidt TJ, Medarde M and Fabbri E. Angewandte Chemie - International Edition 2021; 60: 14609-14619.Jacobs R, Hwang J, Shao-Horn Y and Morgan D. Chemistry of Materials 2019; 31: 785-797.Nørskov JK, Abild-Pedersen F, Studt F and Bligaard T. Proc Natl Acad Sci U S A 2011; 108: 937-943.

Mechanochemical Synthesis of Perovskite Oxyhydrides: Insights from Shear Modulus.

Perovskite oxyhydrides have attracted recent attention due to their intriguing properties such as ionic conductivity and catalysis, but their repertoire is still restricted compared to perovskite oxynitrides and oxyfluorides. Historically, perovskite oxyhydrides have been prepared mostly by topochemical reactions and high-pressure (HP) reactions, while in this study, we employed a mechanochemical (MC) approach, which enables the synthesis of a series of ABO2H-type oxyhydrides, including those with the tolerance factor (t) much smaller than 1 (e.g., SrScO2H with t = 0.936) which cannot be obtained by HP synthesis. The octahedral tilting, often present in perovskite oxides, does not occur, suggesting that the lack of π-symmetry of the H 1s orbital and the large polarization destabilize tilted low-symmetry structures. Interestingly, SrCrO2H (t = 0.997), previously reported with the HP method, was not achieved with the MC method. A comparative analysis revealed a correlation between the feasibility of MC reactions and the (calculated) shear modulus of the starting reagents (binary oxides and hydrides). Notably, this indicator is not exclusive to oxyhydride perovskites but extends to oxide perovskites (SrMO3). This study demonstrates that MC synthesis offers unique opportunities not only to expand the compositional space in oxyhydrides in various structural types but also to provide a guide for the choice of starting materials for the synthesis of other compounds.

Strain tuned structural, mechanical, electronic, and optical properties of lead-free oxy-nitride SrTaO2N perovskite using first-principles study

Recent advancements in perovskite-based solar energy conversion technologies require materials having enhanced optoelectronic properties and stability. Harnessing density functional theory (DFT), we have investigated here a novel orthorhombic phase of a recently synthesized oxynitride perovskite, SrTaO2N, and its uniaxial strain-tunable electronic and optical properties. Phonon dispersion and formation energy calculations are utilized to determine lattice dynamic stability and exothermic formation feasibility of the structure, correspondingly. The predicted bandgap at the Heyd–Scuseria–Ernzerhof [generalized gradient approximation Perdew–Burke–Ernzerhof (GGA-PBE)] level is ∼2.125 eV (∼1.125 eV), which is highly receptive to uniaxial strains. The bandgap formed in between X and G points with high symmetry at the first Brillouin zone was further dissected using the atomic orbital projected density of states (PDOS). The PDOS showed that the N-pz orbital dominantly contributes to valence band maxima and the Ta-dz2 orbital to conduction band minima. Compressive uniaxial strain widens the bandgap by ∼1.21 times, while tensile uniaxial strain lowers the bandgap by ∼1.1 times from the intrinsic value, suggesting strain switchable bandgap nature in the material. An elastic constant matrix also evaluates the mechanical stability of strained structures, and we found that in the strained structures from −6% to +6%, SrTaO2N is mechanically stable and ductile. Optical absorption reveals an increased absorption coefficient in the visible spectrum. These strain-tuned optoelectronic properties through the DFT approach thus suggest an evident route to a wide range of optoelectronics applications of the SrTaO2N perovskite material.

Novel synthetic approaches and morphological design of perovskite-type oxynitrides in powder and ceramic form

Perovskite-type oxynitrides are a new class of inorganic materials that have potential applications as photocatalysts, inorganic pigments and dielectrics. Design of the morphology in conjunction with new synthesis methods is essential for their practical use. In this review, we present the formation of fine particles via a low temperature and ammonia-free synthesis method, the morphology of the oxynitride perovskites obtained using metal carbodiimide, and the toughness of the sintered ceramics in relation to their microstructure.

Synthesis and symmetry of perovskite oxynitride CaW(O,N)3.

Perovskite oxynitrides, in addition to being promising electrocatalysts and photoabsorbers, present an interesting case study in crystal symmetry. Full or partial ordering of the O and N anions affects global symmetry and influences material performance and functionality; however, anion ordering is challenging to detect experimentally. In this work, we synthesize a novel perovskite oxynitride CaW(O,N)3 and characterize its crystal structure using both X-ray and neutron diffraction. Through co-refinement of the diffraction patterns with a range of literature and theory-derived model structures, we demonstrate that CaW(O,N)3 adopts an orthorhombic Pnma average structure and exhibits octahedral distortion with evidence for preferred anion site occupancy. However, through comparison with a large, low-symmetry unit cell, we identify the presence of disorder that is not fully accounted for by the high-symmetry model. We compare CaW(O,N)3 with SrW(O,N)3 to demonstrate the broader presence of such disorder and identify contrasting features in the electronic structures. This work signifies an updated perspective on the inherent crystal symmetry present in perovskite oxynitrides.

High-pressure synthesized perovskite-type CeTaN<sub>2</sub>O and its magnetic and electrical properties

Recently, it has been discovered that the <i>AB</i>(N,O)<sub>3</sub>-type perovskite oxynitrides exhibit excellent dielectric, ferroelectric, and photocatalytic properties, promising for applications in the fields of optoelectronics, energy storage, and communication. Due to the differences in charge, ionic radius, and covalent bonding between N<sup>3–</sup> ion and O<sup>2–</sup> ion, the N substitution for O enhances the <i>B</i>(N,O)<sub>6</sub> octahedron tilting, giving rise to exotic properties and functionalities. However, the current fabrication process for this type of material is rather time-consuming, leading to products with an appreciable quantity of impurities. In this study, using oxide precursors and sodium amide as the nitrogen source, high-purity perovskite-type oxynitride CeTaN<sub>2</sub>O bulk materials are successfully synthesized under high-temperature and high-pressure conditions provided by a cubic-anvil press. The synthesis time decreases to 1 h, achieving rapid production. The lattice structure and physical properties of the obtained samples are comprehensively investigated. X-ray powder diffraction experiments and subsequent Rietveld refinement indicate that the title material shows an orthorhombic crystal structure with the space group of <i>Pnma</i>. The X-ray absorption spectra confirm the charge configuration and the anion composition as Ce<sup>3+</sup>Ta<sup>5+</sup>N<sub>2</sub>O. Magnetization and specific heat measurements reveal that the exchange interactions are mainly antiferromagnetic, with a potential magnetic transition below 2 K. The electrical transport data demonstrate typical semiconductor behaviors, which can be further explained by a three-dimensional variable-range hopping model. Our study paves the way for putting this exotic perovskite oxynitride into practical applications.

Perovskite and layered perovskite oxynitrides for efficient sunlight-driven artificial synthesis

Perovskite and layered perovskite oxynitrides are regarded as promising visible-light-responsive semiconductors for efficient artificial photosynthesis to produce renewable value-added energy resources, including H2, formic acid, and ammonia.

Perovskite-Type Oxynitride Nanofibers Performing Photocatalytic Oxygen and Hydrogen Generation

(111)-layered perovskites such as Ba5Ta4O15 or Ba5Nb4O15 are known for their promising activity in hydrogen evolution and overall water splitting.[1] Their activity can be further enhanced by the preparation of Ba5Ta4O15-Ba3Ta5O15-BaTa2O6 composites.[2,3] However these materials can only absorb UV-light, which prevents them to be used in solar-light induced photocatalysis. Therefore, the preparation of visible-light absorbing photocatalysts is very important for future application in solar-light induced hydrogen evolution and overall water splitting.Perovskite-type oxynitrides AB(O,N)3 are one group of materials which can absorb visible light, i.e. BaNbO2N with an absorption edge up to 740 nm.[4-7] These perovskite-type oxynitrides can be quite easily prepared by ammonolysis of (111)-layered perovskites, due to their structural similarity of 6-fold coordinated B cations, corner-shared BO6 octahedra, and the 12-fold oxygen coordinated A cations in both crystal structures.[8] However, this conversion often results in micrometer-sized particles due to the high temperatures used in the ammonolysis process, thus resulting in small surface areas.[6,7] Therefore, the nanostructuring of such perovskite-type oxynitrides can be of interest, due to the higher surface area and shorter diffusion pathways of the minority charge carriers. One possibility to nanostructure photocatalysts and enhance the photocatalytic activity is the electrospinning of nanofibers. Hildebrandt et al. and Bloesser et al. reported the preparation of highly crystalline Ba5Nb4O15, Ba5Ta4O15, and Ba5Nb2Ta2O15 (111)-layered perovskite nanofibers and the tailoring of the nanofiber diameter of Ba5Ta4O15 and Ba5Ta4O15-Ba3Ta5O15 nanofibers by electrospinning.[9-11]Herein, we present a novel approach for the preparation of nanostructured complex perovskite oxynitrides.[12] Ba5Nb4O15 (111)-layered perovskite nanofibers with tailored nanofiber diameters were electrospun and converted via ammonolysis. Most importantly, the nanofiber morphology was maintained during ammonolysis, and the nanofiber diameter could be adjusted as well. A thorough characterization, including Rietveld refinement, revealed the formation of a novel BaNbO2N-Ba2NbO3N perovskite oxynitride composite. UV-vis measurements further supported the results of a BaNbO2N-Ba2NbO3N composite formation and revealed a reduction of the band edge by more than 2 eV from 3.9 eV for Ba5Nb4O15 to 1.7 eV for the converted nanofiber composites. A diameter-dependent hydrogen and oxygen evolution activity of the BaNbO2N-Ba2NbO3N composite nanofibers after deposition of Pt or CoNbO4, respectively, with an optimum nanofiber diameter will be presented.[1] Y. Miseki et al. Energy Environ. Sci. 2019, 2, 306 [2] J. Soldat, et al. Chem. Sci. 2014, 5, 3746[3] A. Hofmann et al. J. Phys. Energy 2021, 3, 014002[4] T. Hisatomi et al., Energy Environ. Sci., 2013, 6, 3595[5] J. Seo et al., J. Mater. C hem. A, 2019, 7, 493[6] M. Hojamberdiev et al., J. Mater. Chem. A, 2016, 4, 12807[7] T. Yamada et al., J. Phys. Chem. C, 2018, 122, 8037[8] S. Ramaraj et al. J. Energy Chem. 2022, 68, 529[9] N. C. Hildebrandt et al., Small, 2015, 11, 2051 [10] A. Bloesser et al., J. Mater. Chem. A, 2018, 6, 1971[11] A. Bloesser, R. Marschall, ACS Appl. Energy Mater. 2018, 1, 2520[12] A. Hofmann et al., Adv. Mater. Interfaces, 2021, 8, 2100813

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.

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.

Realizing efficient photocatalytic water splitting over Mg-modified BaNbO2N under visible light illumination

With a light absorption up to 740 nm, BaNbO2N is promising for solar water splitting but normally owns a low activity due to a high defect concentration. Here, Mg is adopted as a dopant to modify the BaNbO2N. The presence of Mg not only reduces the defect concentration and enhances surface hydrophilicity but also tunes the bandgap as well as the band edge positions. These modifications greatly promote the charge separation and transfer of BaNbO2N, boosting the photocatalytic activities for O2-evolution reactions. An apparent quantum efficiency of 1.65 % at 420 ± 20 nm has been attained for Mg modified BaNbO2N. Overall water splitting at a stable gas-evolution rate (∼ 2.0 μmol·h−1 for H2 and ∼ 1.0 μmol·h−1 for O2) has been realized by integrating Mg-modified BaNbO2N into a Z-scheme system. These findings justify Mg as an effective dopant to modulate the photocatalytic behavior of Nb-based perovskite oxynitrides.

Effect of alkali-metal cations on synthesis of stoichiometric layered perovskite oxynitrides

Layered perovskite oxynitrides, which contain a stoichiometric lattice N content, are promising visible-light-driven photocatalysts. However, their synthesis remains a challenge for inorganic chemists. Herein, we report the successful synthesis of layered perovskite oxynitrides A2A'2Ta3O9N with different cations at A/A′-sites (A = Na, K, Rb, Cs; A' = Ca, Sr, Ba) via thermal ammonolysis from layered Dion–Jacobson phase oxide precursors (AA'2Ta3O10). We present the first experimental study of layered oxynitride formation across the A and A′ cation size series, revealing that the A-site alkali-metal cation rather than the A′-site alkaline-earth metal cation is critical to the successful synthesis of the stoichiometric layered oxynitrides. Nitrided products with A = Na, K, or Rb exhibited structural transitions, while no structural transition was observed for Cs. Interestingly, nitrided products with A = Rb and K exhibited the same structural transition behavior, i.e., alkali-metal cations entered the interlayer to form an oxynitride with a composition A2A'2Ta3O9N·nH2O. By contrast, with A = Na, it formed an oxynitride that differs from the Ruddlesden–Popper structure but has the same composition of A2A'2Ta3O9N·nH2O. Our finding suggests that, as the cation size decreases from Cs to Na, stoichiometric A2A'2Ta3O9N compositions become more favorable than compounds with A-sites. This work provides a new perspective for the future design and synthesis of novel layered oxynitrides.

Defect and migration energies of oxygen vacancies in ABO2N (A – Ba, Ca, Sr and B – Ta, Nb) perovskite oxynitrides

ABO2N (A = Ca, Ba, Sr & B = Ta, Nb) perovskite oxynitrides are gaining attention as they have band gaps (Eg ∼ 1.8–2 eV) in the visible region. However, these systems are reported to have defects which significantly impact their catalytic behavior. Hence it is essential to understand the defect chemistries of ABO2N systems. In this work, the Mott-Littleton (M-L) method is used to calculate the defect and migration energies associated with oxygen vacancies. For this study, Buckingham potential along with core-shell model has been used for describing the defects in the systems. It is shown that BaTaO2N, BaNbO2N, SrTaO2N and SrNbO2N have low defect energy for oxygen vacancy. Hence, these systems can be explored for solid oxide fuel cell and supercapacitor applications. This is since the presence of oxygen vacancies is reported to improve the ionic conductivity in materials. But for rapid ion/vacancy migration, a low migration barrier is also essential. Hence investigations on oxygen migration energetics are pertinent. The migration energy calculations on ABO2N systems identifies SrTaO2N to be most favorable for applications involving ionic conductivity (e.g. SOFC, gas sensors, supercapacitors). Overall, the work presented here provides specific quasi-chemical pointers relevant for functional applications using oxynitrides.

Mild nitridation of perovskite manganite: Synthesis, structure and magnetism

The nitrogen lightly doped perovskite manganite La0.5Ba0.5MnO(3-δ)-xNx was synthesized following a mild nitridation strategy by optimizing annealing temperature, annealing time and NH3/Ar flow rate ratio. Neutron powder diffraction (NPD) characterization evidences an N-doping triggered structural phase transition from pseudo-cubic Pm-3m to tetragonal P4/mmm and further confirms a preferable substitution at O1 and O2 sites closing to the Ba-rich layers. N 1s K-edge X-ray absorption near edge spectrum (XANES) with total fluorescence yield (TFY) mode, X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared (FT-IR) spectroscopy collectively convince the Mn–N bonded state. The O/N atomic ratio is determined to be 2.26/0.22 by XPS analysis closing to the value of 2.25/0.18 from NPD refinement. Magnetic property characterization reveals that both long-range Mn3+-O-Mn4+ ferromagnetic coupling and high Curie temperature (TC) are maintained upon N-doping. Besides, significant spin frozen with robust domain pinning is identified below TC which can be ascribed to the short-range Mn–N–Mn clusters and/or oxygen defects. This work expands the connotation of transition-metal based perovskite oxynitride and may inspire the development of photosensitive semiconductor device with magnetic response.

Preparation of thick dense ceramic films of dielectric BaTaO2N through electrophoretic deposition

Sintering of the ferroelectric perovskite oxynitride, BaTaO2N is still a big challenge, because nitrogen is partially released from the crystal lattice during high-temperature sintering. Post-sintering annealing under an NH3 flow is essential in order to compensate for this nitrogen loss and to recover the electrically insulating properties of the ceramic. However, fully densified bulk ceramics are nitrided only on their outer surface, while their interior remains semiconducting. In this work, thick ceramic films of the perovskite oxynitride with a high relative density close to 90% were fabricated using electrophoretic deposition and high-temperature sintering. The nitrogen loss was fully recovered during annealing under an NH3 flow, owing to the thickness of the sintered films being less than 100 μm. After annealing, the BaTaO2N ceramics consisted of 200 nm-sized particles with a dense microstructure. This is the first report of the fabrication of BaTaO2N ceramics with a relative density of 89%. The dielectric constant and dielectric loss were approximately 320 and 0.05 at 1 MHz, respectively.

Bandgap Tunable Oxynitride LaNb2 O7-x Nx Nanosheets.

Bandgap tunable lanthanum niobium oxynitride [LaNb2 O7-x Nx ](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 1.6nm 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 2.03-2.63eV. Furthermore, the oxide/oxynitride superlattice structures are fabricated by face-to-face stacking of 2D crystals using oxynitride [LaNb2 O7-x Nx ](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.

Conversion of La2Ti2O7 to LaTiO2N via ammonolysis: a first-principles investigation.

Perovskite oxynitrides are, due to their reduced band gap compared to oxides, promising materials for photocatalytic applications. They are most commonly synthesized from {110} layered Carpy-Galy (A2B2O7) perovskites via thermal ammonolysis, i.e. the exposure to a flow of ammonia at elevated temperature. The conversion of the layered oxide to the non-layered oxynitride must involve a complex combination of nitrogen incorporation, oxygen removal and ultimately structural transition by elimination of the interlayer shear plane. Despite the process being commonly used, little is known about the microscopic mechanisms and hence factors that could ease the conversion. Here we aim to derive such insights via density functional theory calculations of the defect chemistry of the oxide and the oxynitride as well as the oxide's surface chemistry. Our results point to the crucial role of surface oxygen vacancies in forming clusters of NH3 decomposition products and in incorporating N, most favorably substitutionally at the anion site. N then spontaneously diffuses away from the surface, more easily parallel to the surface and in interlayer regions, while diffusion perpendicular to the interlayer plane is somewhat slower. Once incorporation and diffusion lead to a local N concentration of about 70% of the stoichiometric oxynitride composition, the nitridated oxide spontaneously transforms to a nitrogen-deficient oxynitride. Since anion vacancies are crucial for the nitrogen incorporation and diffusion as well as the transformation process, their concentration in the precursor oxide is a relevant tuning parameter to optimize the oxynitride's synthesis and properties.

Interlayer modification and single-layer exfoliation of the Ruddlesden–Popper perovskite oxynitride K2LaTa2O6N to improve photocatalytic H2 evolution activity

Ethylamine-intercalated HxK2−xLaTa2O6N, further modified with a Pt cocatalyst, exhibited 60 times height photocatalytic activity for H2 evolution under visible light, as compared with the parent layered material.

Photocatalytic water oxidation over LaWO0.6N2.4 mesoporous single crystals under visible and near-infrared light illumination

Narrow-bandgap perovskite oxynitrides emerge as a promising class of inorganic photocatalysts to store solar energy into chemical fuels. However, conventional synthetic routes generally introduce a high defect concentration to these...