Tantalum Oxynitride Research Articles

ZnS–CdS–TaON nanocomposites with enhanced stability and photocatalytic hydrogen evolution activity

In recent years, tantalum oxynitride (TaON) semiconductor as one of the most efficient photocatalysts has been studied intensively owing to the appropriate potentials for overall solar water splitting. In this work, ZnS/CdS/γ-TaON composite photocatalysts with wide light response were successfully prepared by nitridation of Ta2O5 at 800 °C in a wet NH3 flowing and subsequently deposition CdS and ZnS quantum dots. The powders were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), UV–vis diffuse reflection spectroscopy (DRS), N2 adsorption–desorption isothermals, X-ray photoelectron spectroscopy (XPS), and so on. The photocatalytic property for hydrogen production was also studied. The results indicated that CdS and ZnS quantum dots with a diameter of 3–8 nm were uniformly dispersed on the surface of γ-TaON, which enlarged the spectral response of the ZnS/CdS/γ-TaON photocatalysts. And the optimized H2 evolution rate of ZnS/CdS/γ-TaON composite (839.6 μmol h−1 g−1) is about 14 times higher than that of CdS/γ-TaON in the absence of any noble-metal cocatalyst and 47 times higher than that of Pt loaded γ-TaON sample. The enhanced photocatalytic activity is attributed to the higher separation efficiency of electrons and holes with the heterogeneous junction between ZnS, CdS, and γ-TaON. Besides, the presence of γ-TaON and ZnS also prevent the photocorrosion of CdS. The results provide a new insight for developing the TaON-based nanocomposite photocatalysts with enhanced stability and excellent photocatalytic H2 production activity.

-TaON thin films: production by reactive magnetron sputtering and the question of non-stoichiometry

In this work we report on the structure, morphology, composition, chemical bonding and optical properties of tantalum oxynitride (Ta–O–N) thin films prepared by reactive direct current magnetron sputtering. The films have been investigated by grazing incidence x-ray diffraction, x-ray reflectivity, grazing incidence small angle x-ray scattering, time-of-flight elastic recoil detection analysis, x-ray photoelectron spectroscopy and spectroscopic ellipsometry. The composition and the partial pressure of the reactive atmosphere (N2 + O2) have been varied in order to find conditions suitable for the -TaON phase production. As prepared thin films are amorphous and annealing at °C is necessary to promote crystallization. We discuss the role of N2 gas on the kinetics of sputtered particles and its influence on the oxidation rate and porosity of the growing film. The anion composition in Ta–O–N films strongly depends on the reactive gas condition during the deposition. We found that the O/N ratio in -TaON films increases with more N2 or O2 gas in the chamber, whereas the corresponding absorption onset and valence band maximum show a blue shift of up to 0.5 eV. These results were related to the non-stoichiometry in -TaOxNy crystallites with x > y .

Tantalum Oxynitride Thin Films: Assessment of the Photocatalytic Efficiency and Antimicrobial Capacity.

Tantalum oxynitride thin films have been deposited by reactive magnetron sputtering, using a fixed proportion reactive gas mixture (85% N2 + 15% O2). To produce the films, the partial pressure of the mixture in the working atmosphere was varied. The characteristics of the produced films were analyzed from three main perspectives and correspondent correlations: the study of the bonding states in the films, the efficiency of photo-degradation, and the antibacterial/antibiofilm capacity of the coatings against Salmonella. X-ray Photoelectron Spectroscopy results suggest that nitride and oxynitride features agree with a constant behavior relative to the tantalum chemistry. The coatings deposited with a higher reactive gas mixture partial pressure exhibit a significantly better antibiofilm capacity. Favorable antibacterial resistance was correlated with the presence of dominant oxynitride contributions. The photocatalytic ability of the deposited films was assessed by measuring the level of degradation of an aqueous solution containing methyl orange, with or without the addition of H2O2, under UV or VIS irradiation. Degradation efficiencies as high as 82% have been obtained, suggesting that tantalum oxynitride films, obtained in certain configurations, are promising materials for the photodegradation of organic pollutants (dyes).

Observation of visible light activated photocatalytic degradation of stearic acid on thin films of tantalum oxynitride synthesized by aerosol assisted chemical vapour deposition.

UV activated photocatalysts deposited using chemical vapour deposition have found commercial success as self-cleaning coatings. However, only limited work has been conducted on the use of the more recently discovered visible light activated photocatalysis for this application. Tantalum oxynitride is an established visible light photocatalyst, and in this paper we have investigated the ability of thin films of tantalum oxynitride to photocatalytically degrade a model organic pollutant, stearic acid, and therefore assess the coatings potential for self-cleaning applications. Thin films of tantalum oxide were formed using aerosol assisted chemical vapour deposition (AACVD) of tantalum ethoxide, and then converted into tantalum oxynitride through ammonolysis at temperatures between 550 °C and 750 °C. Investigation of the films using XRD, UV-vis spectroscopy and XAFS identify that amorphous tantalum oxynitride is formed during the ammonolysis, with complete conversion to TaON under conditions of 700 °C for 24 hours. The self-cleaning ability of this film was assessed using stearic acid as the model pollutant, with a degradation rate of 2.5(2) × 1013 molecules per min per cm2 when exposed to a 5-sun solar simulator, equipped with a UV cut-off filter. We therefore conclude that tantalum oxynitride thin films are able to act as self-cleaning coatings through visible light photocatalysis and that films of tantalum oxynitride can be synthesized using a scalable chemical vapour deposition route.

Synthesis of vanadium doped tantalum oxy-nitride for photocatalytic reduction of carbon dioxide under visible light

We successfully used V as dopant to enhance activity of TaON for visible light photocatalytic reduction of CO2 into valuable fuels. We investigated that the used V dopants existed in the TaON lattice and replaced several Ta elements in the lattice leading to decrease in the conduction band minimum and increase in the valence band maximum of the prepared VTaON. Hence, the band gap energy of the prepared VTaON was lower than that of prepared TaON or the prepared VTaON material could absorb significant amount of incident visible light for production of e− and h+ pairs, which participated in reactions with CO2 and H2O to generate CH4, CO, O2 and H2. We also investigated that the optimal V/Ta ratio (or optimal amount of V dopant) for maximum enhancing photocatalytic activity of TaON was 1.5 wt%. The prepared 1.5VTaON visible light photocatalytically converted CO2 with H2O to generate CH4, CO, O2 and H2 with generation rates of 673, 206, 1479 and 67 (µmol·g−1cat·h−1), respectively.

Effect of annealing on structure and properties of Ta–O–N films prepared by high power impulse magnetron sputtering

High power impulse magnetron sputtering of a Ta target in various Ar+O2+N2 gas mixtures was utilized to prepare amorphous tantalum oxynitride (Ta–O–N) films with a finely controlled elemental composition in a wide range. We investigate the effect of film annealing at 900°C in vacuum on structure and properties of the films. We show that the finely tuned elemental composition in combination with the annealing enables the preparation of crystalline Ta–O–N films exhibiting a single TaON phase with a monoclinic lattice structure, refractive index of 2.65 and extinction coefficient of 2.0×102 (both at the wavelength of 550nm), optical band gap width of 2.45eV (suitable for visible light absorption up to 505nm), low electrical resistivity of 0.4Ωcm (indicating enhanced charge transport in the material as compared to the as-deposited counterpart), and appropriate alignment of the band gap with respect to the redox potentials for water splitting. These films are therefore promising candidates for application as visible-light-driven photocatalysts for water splitting.

Enhanced photoelectrochemical oxidation of alkali water over cobalt phosphate (Co-Pi) catalyst-modified ZnLaTaON2 photoanodes

Zinc lanthanum tantalum oxynitride [ZnLaTaON2] powders were synthesized by conventional solid state reaction. ZnLaTaON2 photoelectrodes were prepared by electrophoretic deposition of ZnLaTaON2 suspension in acetone onto ITO substrate. The photoelectrodes of ZnLaTaON2 were established to reveal photoelectrochemical properties for water oxidation reaction. Moreover, a cobalt phosphate (Co-Pi) was loaded on ZnLaTaON2 photoelectrodes via photodeposition method to enhance the photoelectrochemical water oxidation performances. Photocurrent voltage characteristics of the Co-Pi/ZnLaTaON2 photoelectrodes were enhanced with its effect which is more evidenced at lower water oxidation potentials. A relatively stable photocurrent density of 5 mA/cm2 at 1.5 V vs RHE was attained with the support of electron donor in alkaline phosphate solution. Comparatively, in Co-Pi/ZnLaTaON2 photoelectrodes, approximately threefold enhancement was noticed at 1.8 VRHE in assessment with parent photoelectrode. On the other hand, Co-Pi/ZnLaTaON2 photoelectrodes have been shown as an alternative pathway to improve the photoelectrochemical current gain through the PEC water oxidation reaction.

Synthesis of tantalum oxynitride with enhanced visible light efficiency in photoreduction of carbon dioxide

AbstractThe tantalum oxynitride (TaON) content in preparation process was increased with the melamine and tantalum pentoxide (Ta2O5) mass ratios. The Ta2O5 nitridation by melamine (C3H6N6) was occured in at 850 °C for 4 hours with various melamine and tantalum pentoxide mass ratios under a nitrogen gas flow. The photocatalytic performance of prepared samples was evaluated by the photoreduction of carbon dioxide into renewable fuel under visible light irradiation. Results show that the obtained photocatalysts exhibited a significant enhanced photocatalytic activity in comparison with pure Ta2O5. This work may be useful for a facile way to synthesize the highly efficient photocatalyst for application in energy conversion.

Intergrowth between the Oxynitride Perovskite SrTaO2N and the Ruddlesden-Popper Phase Sr2TaO3N.

Strontium tantalum oxynitrides were prepared within the nominal composition range of 1.0 ≤ x ≤ 2.0, where x = Sr/Ta atomic ratio. A gradual structural transition was observed between the perovskite SrTaO2N and the Ruddlesden-Popper phase Sr2TaO3N with increasing SrO content. X-ray diffraction analyses showed that a single-phase perovskite was obtained up to x = 1.1, after which Sr2TaO3N gradually appeared at x ≥ 1.25. High-resolution scanning transmission electron microscopy observations identified the gradual intergrowth of a Ruddlesden-Popper Sr2TaO3N type planar structure interwoven with the perovskite crystal lattice upon increasing x. The crystal lattice at x = 1.4 was highly defective and consisted primarily of perovskite intergrown with a large amount of the Ruddlesden-Popper phase structure. This Ruddlesden-Popper phase layer intergrowth is a characteristic of an oxynitride perovskite rather than the Ruddlesden-Popper defects previously reported in oxide perovskites. Partial substitution of Ta with Sr was also evident in this perovskite lattice. Just below x = 2, a perovskite-type structure was intergrown as defects in the Ruddlesden-Popper Sr2TaO3N. Characterization of Sr2TaO3N in ambient air was challenging due to its moisture sensitivity. Thermal analysis demonstrated that this material was relatively stable up to approximately 1400 °C in comparison with SrTaO2N perovskite, especially under nitrogen. Sr2TaO3N could keep its structure in a sealed tube, and some amount of SrCO3 was observed in XRD after 10 days of exposure to 75% relative humidity under prior ambient conditions. A compact of this material had a relative density of 96% after sintering at 1400 °C under 0.2 MPa of nitrogen, even though a drastic loss of nitrogen was previously reported for a SrTaO2N perovskite under these same conditions. Postammonolysis of the Sr2TaO3N ceramics was not required prior to studying its dielectric behavior. This is in contrast to the SrTaO2N perovskite, which requires postammonolysis to recover its stoichiometric composition and electrical insulating properties.

Characterization of tantalum and tantalum nitride films on Ti6Al4V substrate prepared by filtered cathodic vacuum arc deposition for biomedical applications

This paper explores tantalum and tantalum nitride thin films produced by filtered cathodic vacuum arc deposition method as bio-stable surface treatment for Ti6Al4V titanium alloy. Effect of nitrogen to argon gas ratio on microstructure of the deposited film has been investigated. Corrosion behaviour of the films in simulated biological fluid solution was evaluated by electrochemical impedance spectroscopy. It was found that both the Ta and TaN films enhanced corrosion resistance of the Ti6Al4V substrate with the best protective characteristics achieved by the TaN film deposited at 0.25 N2/Ar gas ratio. The protective characteristic was attributed to the formation of tantalum oxide and oxynitride compound at the surface, as verified by X-ray photoelectron spectroscopy. Increasing N2/Ar gas ratio increased susceptibility to localized corrosion. The corrosion resistance decreased as the thickness of the film increased to 1 μm.

Size-dependent disproportionation (in ~ 2–20 nm regime) and hybrid Bond Valence derived interatomic potentials for BaTaO2N

Interatomic potentials for complex materials (like ceramic systems) are important for realistic molecular dynamics (MD) simulations. Such simulations are relevant for understanding equilibrium, transport and dynamical properties of materials, especially in the nanoregime. Here we derive a hybrid interatomic potential (based on bond valence (BV) derived Morse and Coulomb terms), for modeling a complex ceramic, barium tantalum oxynitride (BaTaO2N). This material has been chosen due to its relevance for capacitive and photoactive applications. However, the material presents processing challenges such as the emergence of non-stoichiometric phases during processing, demonstrating complex processing–property correlations. This makes MD investigations of this material both scientifically and technologically relevant. The BV based hybrid potential presented here has been used for simulating sintering of BaTaO2N nanoparticles (~ 2–20 nm) under different conditions (using the relevant canonical ensemble). Notably, we show that sintering of particles of diameter 10 nm in size results in the formation of a cluster of tantalum and oxygen atoms at the interface of the BaTaO2N particles. This is in agreement with the experimental reports. The results presented here suggest that the potential proposed can be used to explore dynamical properties of BaTaO2N and related systems. This work will also open avenues for development of nanoscience-enabled aid-free sintering approaches to this and related materials.

Boosting photocatalytic water oxidation reactions over strontium tantalum oxynitride by structural laminations

Perovskite oxynitrides often own a poor photocatalytic activity under normal conditions, being incommensurate to their strong visible light absorbance. This is particularly true for SrTaO2N which undergoes self-oxidative decompositions even under protection of a hole scavenger. In this work, we laminate the crystal structure of SrTaO2N by inserting extra layers of SrO to form a Ruddlesden-Popper (RP) compound Sr2TaO3N. This structural modification not only improves the light absorption of SrTaO2N but also effectively suppresses the defect formation such as Ta4+ species etc. More importantly, Sr2TaO3N is able to drive photocatalytic water oxidation reactions under visible light illumination (λ ≥ 420 nm) without the aid of a cocatalyst and self-oxidative decompositions found for SrTaO2N are largely inhibited. Further analysis suggests that the presence of extra SrO layers positively shifts the valence band edge and stabilizes N species in the structure according to Pauling’s second rule. Theoretical calculations indicate that Sr2TaO3N has typical 2D charge transportation properties which are associated with the structural laminations. Its conduction band minimum (CBM) and valence band maximum (VBM) are found to be located within TaN2O2 square planes which favors efficient charge transportations.

Zinc Tantalum Oxynitride (ZnTaO₂N) Photoanode Modified with Cobalt Phosphate Layers for the Photoelectrochemical Oxidation of Alkali Water.

Photoanodes fabricated by the electrophoretic deposition of a thermally prepared zinc tantalum oxynitride (ZnTaO2N) catalyst onto indium tin oxide (ITO) substrates show photoactivation for the oxygen evolution reaction (OER) in alkaline solutions. The photoactivity of the OER is further boosted by the photodeposition of cobalt phosphate (CoPi) layers onto the surface of the ZnTaO2N photoanodes. Structural, morphological, and photoelectrochemical (PEC) properties of the modified ZnTaO2N photoanodes are studied using X-ray diffraction (XRD), scanning electron microscopy (SEM), ultraviolet visible (UV−Vis) diffuse reflectance spectroscopy, and electrochemical techniques. The presence of the CoPi layer significantly improved the PEC performance of water oxidation in an alkaline sulphate solution. The photocurrent-voltage behavior of the CoPi-modified ZnTaO2N anodes was improved, with the influence being more prominent at lower oxidation potentials. A stable photocurrent density of about 2.3 mA·cm−2 at 1.23 V vs. RHE was attained upon visible light illumination. Relative to the ZnTaO2N photoanodes, an almost three-fold photocurrent increase was achieved at the CoPi/ZnTaO2N photoelectrode. Perovskite-based oxynitrides are modified using an oxygen-evolution co-catalyst of CoPi, and provide a new dimension for enhancing the photoactivity of oxygen evolution in solar-assisted water-splitting reactions.

MgTaO2N Photocatalysts: Perovskite versus Ilmenite Structure. A Theoretical Investigation

We discuss the applicability of magnesium tantalum oxynitride (in the ABO2N stoichiometry) as photocatalyst by calculating its fundamental properties from first principles, fully analyzing structural factors and material polymorphism. We find that, because of their favorable band edge nature, most of the configurations of the perovskite-type MgTaO2N are suitable for water splitting, while ilmenite-type MgTaO2N, the structurally expected more favored polymorph here investigated, is mostly unsuitable for the overall water splitting. Our results also show the strong correlation between the conduction band structures and octahedral tilting, i.e., the more tilted the octahedra, the larger the overlap between Ta 5d bands and anion 2p bands, with subsequent band gap opening and dispersion reduction. Therefore, in the perovskite-type MgTaO2N material design for photocatalytic applications, reduced (if not totally suppressed) octahedral tilting (i.e., via A-site doping) is highly desirable in order to improve the solar-to-energy performance of the material.

Examination of various characteristics for sputtered tantalum oxide-nitride thin films deposited at various oxygen flowrates

ABSTRACTTantalum oxynitride thin films were prepared by reactive sputtering. The argon and nitrogen flow rate were kept stable whereas oxygen flow rate was incremented periodically. The effect of oxygen flow rate on various properties of tantalum oxynitride thin films is reported in this research paper. XRD patterns of tantalum oxynitride thin films displayed peaks commonly as for nano-crystalline materials. Surface topography observed to be smooth and exhibited smaller grain structure. Wettability test showed promising results for hydrophobicity. Wear test was done on uncoated and coated tantalum oxynitride thin films on 10 mm diameter cylindrical pins of brass and mild steel.

A numerical analysis and experimental demonstration of a low degradation conductive bridge resistive memory device

This study investigates a low degradation metal-ion conductive bridge RAM (CBRAM) structure. The structure is based on placing a diffusion blocking layer (DBL) between the device's top electrode (TE) and the resistive switching layer (RSL), unlike conventional CBRAMs, where the TE serves as a supply reservoir for metallic species diffusing into the RSL to form a conductive filament (CF) and is kept in direct contact with the RSL. The properties of a conventional CBRAM structure (Cu/HfO2/TiN), having a Cu TE, 10 nm HfO2 RSL, and a TiN bottom electrode, are compared with a 2 nm TaN DBL incorporating structure (Cu/TaN/HfO2/TiN) for 103 programming and erase simulation cycles. The low and high resistive state values for each cycle are calculated and the analysis reveals that adding the DBL yields lower degradation. In addition, the 2D distribution plots of oxygen vacancies, O ions, and Cu species within the RSL indicate that oxidation occurring in the DBL-RSL interface results in the formation of a sub-stoichiometric tantalum oxynitride with higher blocking capabilities that suppresses further Cu insertion beyond an initial CF formation phase, as well as CF lateral widening during cycling. The higher endurance of the structure with DBL may thus be attributed to the relatively low amount of Cu migrating into the RSL during the initial CF formation. Furthermore, this isomorphic CF displays similar cycling behavior to neural ionic channels. The results of numerical analysis show a good match to experimental measurements of similar device structures as well.

Inhibiting competing reactions of iodate/iodide redox mediators by surface modification of photocatalysts to enable Z-scheme overall water splitting

Construction of photocatalytic Z-scheme overall water splitting (OWS) using iodate/iodide (IO3−/I−) redox mediator commonly confronts challenges of competing reactions and slow reaction kinetics. Here we address the aforementioned key issues using surface modification strategy. Visible-light-responsive tantalum nitride (Ta3N5) and tantalum oxynitride (TaON) are employed as the O2- and H2-evolving photocatalysts, respectively. It is found that the inhibition of competing reactions and promotion of half reactions are indispensable for the successful construction of Z-scheme OWS system, both of which are directly related to the surface property of the photocatalysts. Specifically, the magnesia modification on the surface of Ta3N5 is demonstrated to be effective not only in suppressing the adsorption and oxidation of I− ions (a competing reaction of water oxidation), but also in promoting the charge separation and O2-evolving rate via meliorative dispersion of loaded iridium cocatalyst. And surface coating of Cr2O3 nanolayer on the platinum cocatalyst loaded on the TaON-based photocatalyst is available to inhibit the reduction of IO3− ions (a competing reaction of proton reduction) and improve the H2-evolving rate. Only after both inhibition of competing reactions and promotion of H2- and O2-evolving reactions under the assistance of surface modification, the visible-light-driven Z-scheme OWS system can be achieved successfully with the mixture of IO3− and I− ions as a redox mediator. This work sheds light on the availability and importance of surface modification strategy in fabricating Z-scheme OWS system.

Synthesis and photocatalytic properties of mesoporous tantalum oxynitride

The effect of the conditions of ammonolysis of mesoporous magnesiothermic tantalum powders with the specific surface area of 56–63 m2 g–1 on phase composition and specific surface area of the products is studied in the temperature range of 400–900°C. The ammonolysis products are shown to be X-ray amorphous at 400–600°C. At the heating rate of 20 K/min, the nitrogen content in these products is 5.4–7.3% and the specific surface area is 42–35 m2 g–1, while at heating rate of 8 K/min these parameters are 2.4–5.4% and 56–49 m2 g–1, respectively. The crystalline phase of tantalum oxynitride TaON is formed at a temperature of 700°C and above. The resulting materials exhibit a high photocatalytic activity in the reactions of ferroin and methylene blue degradation under irradiation with visible light (λ ≥ 420 nm).

Preparation of tantalum oxynitride thin film photocatalysts by reactive magnetron sputtering deposition under high substrate temperature

This work spotlights the formation behavior of visible light-responsive tantalum oxynitride (TaON) thin film photocatalysts under high substrate temperature in radiofrequency reactive magnetron sputtering deposition. The results emanating from the optimization of the sputtering conditions demonstrated that sputtered N atoms with high kinetic energy generated by controlling target–substrate distances and total pressures in the sputtering chamber were necessary to grow TaON phase even under N2-rich atmosphere. Based on these findings, TaON thin film photocatalysts were successfully synthesized by single-step sputtering under a high substrate temperature of 1073 K before heat treatment. The optimal thickness of TaON thin film photocatalysts was extrapolated to be 450 nm by photoelectrochemical measurements under visible light irradiation (λ > 450 nm), in which distinct photocurrents corresponding to water oxidation were observed. Moreover, the photoelectrochemical activity was able to be improved by postsynthetic heat treatment in gaseous NH3 and loading with IrO2 nanocolloids as cocatalysts. This finding would be because the thin film photocatalyst after heat treatment in NH3 under appropriate conditions possessed better crystallinity and moderate donor density. The optimized TaON thin film photocatalysts with IrO2 nanocolloids also exhibited photocatalytic activity for H2 evolution from aqueous medium containing methanol as a sacrificial electron donor under visible light irradiation (λ > 450 nm).

On the structural, morphological and electrical properties of tantalum oxy nitride thin films by varying oxygen percentage in reactive gases plasma

Tantalum oxy nitride (Ta-ON) thin films possess unique properties due to combined beneficial aspects of tantalum nitride and tantalum oxide. In the present work, Ta-ON thin films are synthesized using energetic ions and electrons beams emanated from the hot and decaying plasma in dense plasma focus (DPF) device. Oxygen percentage in the reactive gas admixture [O/(O+N)]% is varied from 10 to 60 for thin film synthesis. Influence of oxygen percentage on structural, morphological, compositional and electrical properties of thin films is also analyzed. Increase of oxygen percentage in the gas admixture (upto 40%) resulted in transformation of thin film from tantalum nitride to tantalum oxy nitride; however; higher oxygen percentages (≥ 50%) caused amorphization in synthesized thin film. Morphological analysis revealed that pure nitrogen environment yields the granular structure of film in nano-meter range whereas escalation of grains is observed with the increase in oxygen percentage. Cross sectional SEM showed better film growth rate at lower oxygen percentages. Compositional profiles exhibited improvement of oxygen content in thin film by increasing oxygen percentage in the admixture. The electrical resistivity of films shifted from conducting (for 0% oxygen) to semiconducting-insulating (for 60% oxygen) material range for maximum oxygen percentage.