Tantalum Oxynitride Research Articles

Supposed Versatile β-TaxOyNz Structures: DFT Studies

In the last decade, tantalum oxynitride material has been proposed as an interesting photo-catalyst for its spectral response extension in the visible light and good efficiency in water splitting process [1]. The electronic and structural properties of crystalline materials based on ß-TaON (baddeleyite-type) have been theoretically investigated (PBE, B3LYP) in the density functional theory (DFT). The effects of Ta-vacancies, together with changes of the stoichiometry ratio Ta:O:N, suggested from experimental literature, are investigated. The electronic properties are valued by changing N and O atom positions in the crystallographic TaON, to reach the most stable form. The ß-form of TaON is an n-type semiconductor when O prevails, while is a p-type semiconductor when N prevails. Consideration was given on the formation of acceptors or donors states related to the energetic positions. A structure with the best stability has also been found when a Ta-vacancy is formed in conjunction to an O increase. A model of the crystal structure that fully meets the experimentally observed properties (optical, structural and stoichiometric ratio) is proposed.

Effect of the rapid thermal annealing on the structure and optical properties of TaOxNy thin films deposited by reactive magnetron sputtering

Tantalum oxynitride thin films are deposited by radio-frequency magnetron sputtering using a pure tantalum target under argon/oxygen/nitrogen gas mixture. The argon flow is kept constant while the oxygen and nitrogen flows are changed simultaneously in a way to keep constant the total flow of these reactive gases. We succeed to deposit TaOxNy films with stoichiometry ranging between those of TaN and Ta2O5. All films are deposited at room temperature without any biasing. A thermal annealing in a RTA furnace was applied to all the films in a nitrogen atmosphere. A phase transition was detected by XRD investigations and SEM scanning from as deposited to annealed films, noticing the crystallisation of all films which depends on the composition of each film.

Preparation of calcium tantalum oxynitride from layered oxide precursors to improve photocatalytic activity for hydrogen evolution under visible light

A new precursor route to prepare calcium tantalum oxynitride (CaTaO2N) was explored in an attempt to improve the photocatalytic activity for H2 evolution from water under visible light. CaTaO2N catalysts were prepared using either a layered metal oxide (e.g., RbCa2Ta3O10 or HCa2Ta3O10) or a bulk-type Ca-Ta oxide prepared by the polymerized complex (PC) method. CaTaO2N samples prepared from the layered oxides had a less-aggregated morphology and a bluer absorption edge than an analog from a PC-derived precursor, in addition to a lower density of anionic defects. Samples prepared from the layered oxide also showed an enhanced photocatalytic activity for H2 evolution from an aqueous methanol solution under visible light (λ>420nm), primarily because of their lower density of anionic defects and the enhanced driving force for surface redox reactions that resulted from their blue-shifted absorption edge.

Controlled Synthesis of Tantalum Oxynitride and Nitride Nanoparticles

Synthesis of monodisperse metal nitride and oxynitride nanoparticles with a controllable chemical composition, size, and a well-defined morphology are useful for applications in fields of catalysis, optoelectronics, and electrochemistry. In this paper a novel method for the controllable synthesis of highly crystalline tantalum oxynitride and nitride nanoparticles with a relatively low polydispersity is presented using urea as the nitrogen source. In the presence of calcium ions as assisting agents, by simply varying the initial urea/Ta molar ratio, both the composition and size of the final product can be tailored to switch from TaON to Ta3 N5. The mechanism proposed is that Ca2+ slows down the release of NH3 from the decomposition of urea, which is indispensable for the controllable synthesis of oxynitride and nitride based nanoparticles.

Formation of Sol–Gel-Derived TaOxNy Photocatalysts

Tantalum oxynitride (TaON) is a visible photocatalyst (λ < 520 nm) that has been investigated for green energy applications, specifically, the solar production of hydrogen gas. Metal oxynitrides are often generated by the ammonolysis of metal oxides at very high temperatures. We have investigated an alternative synthetic procedure to produce gram-scale quantities of photocatalytically active TaOxNy materials using room-temperature sol–gel chemistry. X-ray photoelectron spectroscopy (XPS) results confirm that nitrogen was incorporated into materials with a Ta2O5 crystal structure. Optical characterization reveals that sol–gel-synthesized TaOxNy has a 2.3 eV bandgap, in agreement with bulk TaON. Furthermore, photoluminescence experiments reveal that two distinct material systems are being created during the initial sol phase of the synthesis: one species is primarily metal oxide, while the other is a N-doped species. This observation may lead to synthetic methods that improve the doping content of sol–gel-derived materials.

Potential of TaO&lt;sub&gt;X&lt;/sub&gt;N&lt;sub&gt;Y&lt;/sub&gt; Thin Films Deposited by Reactive Sputtering as Antireflective Coatings: Composition and Optical Properties

Tantalum oxynitride thin films are deposited by radio-frequency magnetron sputtering using a pure tantalum target under argon/oxygen/nitrogen gas mixture. The argon flow is kept constant while the oxygen and nitrogen flows are changed simultaneously in a way to keep constant the total flow of these reactive gases. We succeed to deposit TaOxNyfilms with stoichiometry ranging between those of TaN and Ta2O5. All films are deposited at room temperature and are amorphous. Spectroscopic ellipsometry and UV-visible spectrometry investigations show a direct relation between the optical properties and the stoichiometry of the films. In particular, the results show a variation of the refractive index from pure tantalum nitride-like films (3.76) to tantalum pentoxyde-like films (2.1), which confirms the possibility to deposit graded antireflective coatings with tantalum oxynitride.

Effect of reactive gases flow ratios on the microstructure and electrical resistivity of Ta–N–O thin films by reactive co-sputtering

A combination of the beneficial properties of tantalum oxide and tantalum nitride may result in a new and functional tantalum oxynitride (Ta–O–N). In this paper, Ta–O–N thin films were fabricated by using reactive magnetron sputtering at different reactive gas ratios. The ratio of reactive gases to total flow gases was controlled from 6% to 30%. The microstructure, composition, chemical bonding, morphology and resistivity of the Ta–O–N thin films were characterized. Increasing reactive gas flow ratio will result in the increase of the O/(O + N) ratio in Ta–O–N from 0.22 to 0.79. All films were quasi-amorphous structures in this study. The higher reactive gas flow ratio led to more disorder or amorphous microstructure with a broader diffraction peak. The position of the highest intensity peak about 36° shifts downward with increasing O/(O + N) ratio because of the sufficient Ta–O phase formation. The FTIR absorption bands at 500–800 cm − 1 corresponding to Ta–O–Ta and Ta–O stretching vibration modes were observed . A weak absorption band at 800–1000 cm − 1 was the tantalum suboxides. The resistivity increased from 288 μΩcm (conducting) to 11,540 Ωcm (semiconducting–insulating) with increasing O/(O + N) ratio. The oxygen content dominates the microstructure formation and resistivity of the Ta–O–N system compared to nitrogen.

Structures and photocatalytic behavior of tantalum-oxynitride thin films

Tantalum oxynitride films were created by direct nitridation/oxidation during rapid thermal annealing at temperatures 450–700 °C. Instead of during deposition, this post process may be proved to be an alternative way to make transition metallic oxynitride films. With sufficient supply of oxygen flow (≥ 30 sccm), TaO xN y was formed as examined from X-ray diffraction (XRD) analysis. This oxynitride film has a broad optical absorption over the range of visible light and sufficient photocatalytic function. For optical absorption, the films' transmittance and reflectance were measured by a UV–VIS–NIR spectrophotometer with wavelengths ranging from 300 to 900 nm. The broad visible light absorption is associated with the formation of band gap in TaO xN y film, which was examined by the theoretical calculations combining the Beer–Lambert law and Tauc formula. Lastly, the photocatalysis of TaO xN y was gauged by the photodegradation test which measured the reduction of light absorbance affected by the decomposition of methylene blue (C 16H 18N 3SCl.3H 2O) on TaO xN y under visible light irradiation.

ChemInform Abstract: First‐Principles Study of the Electronic, Optical Properties and Lattice Dynamics of Tantalum Oxynitride.

First-principles calculations of the electronic, optical properties and lattice dynamics of tantalum oxynitride are performed with the density functional theory plane-wave pseudopotential method. The analysis of the electronic structure shows a covalent nature in Ta−N bonds and Ta−O bonds. The hybridization of anion 2p and Ta 5d states results in enhanced dispersion of the valence band, raising the top of the valence band and leading to the visible-light response in TaON. It has a high dielectric constant, and the anisotropy is displayed obviously in the lower energy region. Our calculation indicated that TaON has excellent dielectric properties along [010] direction. Various optical properties, including the reflectivity, absorption coefficient, refractive index, and the energy-loss spectrum are derived from the complex dielectric function. We also present phonon dispersion relation, zone-center optical mode frequency, density of phonon states, and some thermodynamic properties. The experimental IR modes...

First-Principles Study of the Electronic, Optical Properties and Lattice Dynamics of Tantalum Oxynitride

First-principles calculations of the electronic, optical properties and lattice dynamics of tantalum oxynitride are performed with the density functional theory plane-wave pseudopotential method. The analysis of the electronic structure shows a covalent nature in Ta-N bonds and Ta-O bonds. The hybridization of anion 2p and Ta 5d states results in enhanced dispersion of the valence band, raising the top of the valence band and leading to the visible-light response in TaON. It has a high dielectric constant, and the anisotropy is displayed obviously in the lower energy region. Our calculation indicated that TaON has excellent dielectric properties along [010] direction. Various optical properties, including the reflectivity, absorption coefficient, refractive index, and the energy-loss spectrum are derived from the complex dielectric function. We also present phonon dispersion relation, zone-center optical mode frequency, density of phonon states, and some thermodynamic properties. The experimental IR modes (B(u) at 808 cm(-1) and A(u) at 863 cm(-1)) are reproduced well and assigned to a combination of stretching and bending vibrations for the Ta-N bond and Ta-O bond. The thermodynamic properties of TaON, such as heat capacity and Debye temperature, which were important parameters for the measurement of crystal physical properties, were first given for reference. Our investigations provide useful information for the potential application of this material.

Synthetic Conditions and Color Characteristics of Tantalum Nitride Prepared by Liquid NH3 Process

Tantalum nitrides and oxynitrides, such as Ta3N5 and TaON, respectively, have exhibited promising properties as red and yellow pigments that do not contain toxic heavy metals. We have developed a process for preparing nitrides or oxynitrides that involves the vacuum-calcination of a precursor material obtained by reaction between a metal halide and liquid NH3. Herein, we describe the synthetic conditions of the liquid NH3 process that affects the color, and thus the color characteristics of resulting pigments. Reaction and post-reaction treatment conditions were adjusted to attain the desired red color ; specifically, a powder with a color index of L*=a*=33.11 and b*=30.53 was obtained by the liquid NH3 process using 0.5 eq of NH2OH-HCl (relative to TaCl5) as the oxygen source, and calcination of the precursor at 973 K under vacuum, followed by recalcination at 673 K for 60 min under air.

Photocatalytic Overall Water Splitting under Visible Light Using ATaO2N (A = Ca, Sr, Ba) and WO3 in a IO3−/I− Shuttle Redox Mediated System

Three mixed tantalum oxynitrides, ATaO2N (A = Ca, Sr, Ba), prepared by heating amorphous A2Ta2O7 under NH3, are applied in a two-step photoexcitation water-splitting system as a photocatalyst for H2 evolution. Platinum-loaded CaTaO2N and BaTaO2N are demonstrated to be active for H2 evolution in the presence of methanol or I− as an electron donor, whereas SrTaO2N is unable to produce H2 stably in the presence of I− due to self-oxidative decomposition accompanied by N2 production. The combination of Pt−CaTaO2N or Pt−BaTaO2N with Pt−WO3 achieves overall water splitting under visible irradiation. Pt−BaTaO2N is also demonstrated to be photoactive at wavelengths up to 660 nm, representing the first example of an overall water-splitting system in which visible light at wavelengths longer than 600 nm is effectively utilized for H2 evolution.

Visible-Light-Induced Photocatalytic Oxidation of Polycyclic Aromatic Hydrocarbons over Tantalum Oxynitride Photocatalysts

The photooxidations of five typical polycyclic aromatic hydrocarbons (PAHs) were investigated by using tantalum oxynitride and Pt-tantalum oxynitride as visible light-driven photocatalysts. The electron paramagnetic resonance spin-trap technique and hydrogen peroxide test strip were used to monitor active species formed in these photocatalytic systems. Moreover, the participations of HO*, O2(-*) anions, and holes were further examined by adding their scavengers t-butanol, benzoquinone, and iodine anions, respectively. The reaction intermediates were analyzed by gas chromatography-mass spectrometer (GC-MS). The results show that tantalum oxynitride exhibits good photocatalytic activity for the PAHs photodegradation and the activity is greatly promoted by loading cocatalyst Pt. After 6 h visible light irradiation, phenanthrene, anthracene, benzo[a]anthracene, and acenaphthene can be completely oxidized over Pt-tantalum oxynitride. Under UV light irradiation, the photodegradation rate of PHE over Pt-tantalum oxynitride is 8 times faster than that over titanium dioxide (P25, 80% anatase, 20% rutile). Oxygen plays a crucial role on the photooxidations of PAHs. t-Butanol and benzoquinone almost have no effect on PAHs photodegradations, which indicate that HO* and O2(-*) anions play a negligible role on the photodegradations of PAHs. However, the presence of iodide anions significantly inhibits these degradation reactions, implying the crucial effect of holes on the photocatalytic systems. The PAHs degradations could therefore be attributed to the formation of holes in these systems. Based on the GC-MS analysis, the possible photooxidation pathways of PAHs were also proposed.

Synthetic Conditions and Color Characteristics of Tantalum Oxynitride Prepared via Liquid‐NH3 Process

Tantalum oxynitrides, such as TaON, exhibit promising color properties and can be employed as nontoxic yellow pigments containing no heavy metals. We have developed a process for preparing nitrides or oxynitrides involving the vacuum‐calcination of a precursor material obtained via reaction between a metal halide and liquid NH3. Herein, we describe the synthetic conditions of the liquid‐NH3 process that affect the color, and thus the color characteristics, of the resulting pigments. Reaction and postreaction treatment conditions were adjusted to obtain the desired yellow color. The liquid‐NH3 process was performed using 1.0 eq of H2O (relative to TaCl5) as the oxygen source and 30.0 eq of KCl (relative to TaCl5) as flux. Calcination of the precursor at 1073 K under vacuum was followed by recalcination from room temperature to 973 K at rate of 10 K min−1 under air. A powder with a color index of L∗ = 84.20, a∗ = −2.71, and b∗ = 44.07 was obtained.

산소결핍 탄탈륨 산화물을 활용한 탄탈륨 산질화물 및 질화물 합성

Colored tantalum oxy-nitride (TaON) and tantalum nitride (<TEX>$Ta_{3}N_{5}$</TEX>) were synthesized by ammonolysis. Oxygen deficient tantalum oxides (<TEX>$TaO_{1.7}$</TEX>) were produced by a titration process, using a tantalum chloride (<TEX>$TaCl_5$</TEX>) precursor. The stirring speed and the amount of <TEX>$NH_{4}OH$</TEX> were important factors for controling the crystallinity of tantalum oxides. The high crystallinity of tantalum oxides improved the degree of nitridation which was related to the color value. Synthesized powders were characterized by XRD, SEM, TEM and Colorimeter.

Surface Modification of TaON with Monoclinic ZrO2 to Produce a Composite Photocatalyst with Enhanced Hydrogen Evolution Activity under Visible Light

Abstract A composite material consisting of tantalum oxynitride (TaON) and monoclinic zirconium oxide (m-ZrO2) is prepared by a surface modification method as a photocatalyst with enhanced activity for H2 evolution from water under visible light (λ&amp;gt;420 nm). The composite is prepared by loading particulate Ta2O5 with nanoparticulate m-ZrO2 followed by nitridation at 1123 K for 15 h under NH3 flow. The activity of the m-ZrO2/TaON composite for H2 evolution from aqueous methanol solution is higher than that of either m-ZrO2 or TaON when loaded with nanoparticulate ruthenium as a H2 evolution site. The highest activity is obtained using a composite prepared with a Zr/Ta molar ratio of 0.1. The optimized material also provides elevated activity when used as an H2 evolution photocatalyst in Z-scheme overall water splitting in combination with Pt-loaded WO3 as an O2 evolution photocatalyst and an IO3−/I− shuttle redox mediator. The improvement in activity is attributed to the suppression of surface defect formation by inhibiting tantalum reduction during nitridation.

A density-functional study on the stability of anatase-type phases in the system Mg–Ta–O–N

Magnesium-doped tantalum oxynitrides, which were prepared by ammonolysis of amorphous mixed oxides, have been investigated using quantum-theoretical methods. For small magnesium amounts (5 cat%), density-functional total-energy calculations indicate an anatase-type structure consisting of stretched, corner-sharing TaO 3 N 3 octahedra with a tetrahedrally distorted equatorial plane. The calculated structural parameters are in excellent agreement with those obtained using X-ray powder diffraction and synchrotron radiation. Additionally, the quantum-chemical results show a clear preference for an ordered anionic distribution (space group I 4 1 md , no. 109) of the host lattice, which is locally disturbed around Mg 2 + . For thermodynamical reasons, the excess oxygen anions, which replace nitrogen on account of the lower charge of the dopant cation, segregate next to magnesium, thus forming local MgO “domains”. For higher magnesium contents ( ⩾ 10 % ) , minor phases of rutile-type structure have to be expected, which is in good agreement with experimental data.

Characterization of tantalum oxy-nitrides deposited by ECR sputtering

We have investigated the electrical characteristics of tantalum oxy-nitride (TaON) films deposited using electron cyclotron resonance (ECR) plasma sputtering. A pure tantalum metal target was used as raw material combined with gases of oxygen and nitrogen. The electrical properties have been measured using metal–insulator–metal (MIM) structures of Al/TaON/Ru/Si or metal–insulator–semiconductor (MIS) structure of Al/TaON/Si. By controlling the oxygen gas flow in a moderate low gas flow rate at fixed nitrogen gas flow, TaON films have been stably obtained with the refractive indices of over 2.5 at 632.8 nm wavelength, and the dielectric constants of over 30. However, the leakage currents have increased with an increase in the dielectric constants. To improve the leakage current, we have investigated the periodical deposition process, in which the ECR plasma irradiation was additionally introduced after thin TaON film was deposited. The breakdown strength of about 1 MV/cm was obtained by the measurement using MIM structure. By the estimation of the C–V characteristics of the silicon–MIS structure using TaON, the dielectric constant of 34 was obtained for a TaON thickness of 12 nm.

Defect migration in fluorite-type tantalum oxynitrides: A first-principles study

In this paper we present results of our theoretical studies targeted at anion mobility in solid M–Ta–O–N systems. Periodic supercell calculations at density-functional level have been performed to investigate the local structures of N-doped tantalum oxides and their anion diffusion mechanisms. The migration pathways and activation barriers were calculated using the nudged elastic band method with the climbing-image enhancement. We show that the defect migration is mainly caused by the diffusion of oxygen anions. The activation energy can be lowered by increasing the defect concentration, and it is, to a large extent, depending on the dopant size.

First‐principles and molecular‐dynamics study of structure and bonding in perovskite‐type oxynitrides ABO2N (A = Ca, Sr, Ba; B = Ta, Nb)

A series of perovskite-type phases of alkaline-earth-based tantalum and niobium oxynitrides has been studied using both first-principles electronic-structure calculations and molecular-dynamics simulations, in particular by investigating different structural arrangements and anion distributions in terms of total-energy calculations. The structural properties are explained on the basis of COHP chemical bonding analyses and semiempirical molecular orbital calculations. We provide theoretical proof for the surprising result that the local site symmetries of these phases are lower than cubic because density-functional calculations clearly show that all crystallographic unit cells are better described as being orthorhombic with space group Pmc2(1) to optimize metal-nitrogen bonding; nonetheless, there is no contradiction with a macroscopic cubic description of the structures of BaTaO(2)N and BaNbO(2)N adopting space group Pm3m. Additionally, we find that the anionic sublattice is ordered in all compounds studied over a wide temperature range.