Tantalum Species Research Articles

Improvement of photocatalytic activity of tantalum nitride by ammonothermal treatment at high pressure

Tantalum nitride (Ta3N5) was treated under ammonothermal conditions with the ammonia pressure of 10 to 100MPa at 823K for 24h to enhance the photocatalytic activity for hydrogen evolution from aqueous methanol solution under visible light irradiation. The rates of hydrogen evolution for all treated Ta3N5 were faster than those for untreated Ta3N5. Treated Ta3N5 showed higher enhanced activity for hydrogen evolution when treated with high-pressure ammonia at more than 50MPa. The photocatalytic activity for hydrogen evolution of treated Ta3N5, related to the amount of defects, is due to reduced tantalum species. The enhancement in the activity is brought about by a decrease in surface defects, which contributes to improved electron transfer from the surface of Ta3N5 to the loaded Pt co-catalyst.

XAFS studies of Ni, Ta, and Nb chlorides in the ionic liquid 1-ethyl-3-methyl imidazolium chloride/aluminum chloride

The structures of anhydrous nickel, niobium, and tantalum chlorides have been investigated in situ in acidic and basic ionic liquids (ILs) of 1-methyl-3-ethylimidazolium chloride (EMIC)/AlCl3 with X-ray absorption spectroscopy (XAS). The coordination of NiCl2 changes from tetrahedral in basic solution to octahedral in acidic solution. The NiCl2 is a strong Lewis acid in that it can induce the AlCl3 to share its chlorides in the highly acidic IL, forming a structure with six near Cl− ions and eight further distant Al ions which share the chloride ions surrounding the Ni2+. When Nb2Cl10, a dimer, is added to the acidic or basic solution, the dimer breaks apart and forms two species. In the acid solution, two trigonal bipyramids are formed with five equal chloride distances, while in the basic solution, a square pyramid with four chlorides forming a square base and one shorter axial chloride bond. Ta2Cl10 is also a dimer and divides into half in the acidic solution and forms two trigonal bipyramids. In the basic solution, the dimer breaks apart but the species formed is sufficiently acidic that it attracts two additional chloride ions and forms a seven coordinated tantalum species.

Nanoparticulate precursor route to fine particles of TaON and ZrO2–TaON solid solution and their photocatalytic activity for hydrogen evolution under visible light

Abstract Preparation of fine TaON particles is attempted to improve the photocatalytic activity for H2 evolution from aqueous methanol solution under visible light (λ > 420 nm). TaON is synthesized by nitriding Ta2O5 powder at 1123 K for 15 h under a flow of NH3. When nanoparticulate Ta2O5 with a primary particle size of 30–50 nm is used as the precursor, fine particles of TaON (FP-TaON) with 50–80 nm in size is obtained, but the primary particles aggregate to form larger secondary particles. Nitridation of Zr–Ta mixed oxide (Zr/Ta = 0.1) nanoparticles results in the production of a solid solution of monoclinic-ZrO2 and TaON exhibiting 30–50 nm primary particle size with less aggregation and higher activity, as compared to FP-TaON. Suppression of surface defects (reduced tantalum species) produced during nitridation from Ta2O5 to TaON is shown to be essential for achieving efficient H2 evolution using TaON-based photocatalysts.

Growth of multioxide planar film with the nanoscale inner structure via anodizing Al/Ta layers on Si

An Al/Ta bilayer specimen prepared by a successive sputter-deposition of a 150-nm tantalum layer and a 180-nm aluminium layer onto a silicon wafer is anodically processed in a sequence of steps in oxalic acid electrolytes, at voltages of up to 53 V, which generates a 260-nm alumina film with well-ordered nanoporous structure. Further potentiodynamic reanodizing the specimen to 220 V causes the simultaneous growth of a 65-nm tantalum oxide layer beneath the alumina film and an array of oxide ‘nanocolumns’ (∼50 mn wide, ∼80 nm apart, ∼7 × 10 9 cm −2 population density) penetrating the alumina pores and reaching precisely to the top of the alumina film. The complete filling of the alumina pores is assisted by the high Pilling–Bedworth ratio for Ta/Ta 2O 5 and a substantially increased transport number for tantalum species (0.4), which is an average value of all migrating tantalum ions with different oxidation states. The nanocolumns are shown to be composed of a unique, regular mixture of Ta 2O 5 (dominating amount), suboxides TaO 2 and TaO x (0.5 < x < 1), Al 2O 3, metallic Ta and Al aggregates, tantalum diboride (TaB 2) and oxidized boron from the electrolyte. The ionic transport processes determining the self-organized growth of these planar oxide nanostructures are considered and described conceptually.

Phase Composition of the Cathodic Products Obtained in Alkali Chloride Melts Containing Potassium Monooxyfluoride Complexes of Tantalum

The electrochemical behavior of molten salts systems where LiCl, NaCl, NaCl-KCl, KCl and CsCl were used as solvent for K2TaF7 and K3TaOF6 was investigated. In particular, the influence of the electrolysis parameters, such as concentration of tantalum species in electrolyte, temperature and current density on the cathodic products phase composition was determined by X-ray diffraction and scanning electron microscopy. Several phases generally crystallized at the cathode simultaneously. Among identified phases were metallic tantalum in cubic (α) and tetragonal (β) crystal modifications, tantalum oxide bronzes (hexagonal, tetragonal, cubic) and TaO. The most characteristic phases for the CsCl melt were two isostructural compounds with pyrochlor type crystal lattice CsTa52+zO5+yF1- y and CsTa5-x2+zO5+yF1-y. The former deposited in the form of transparent dielectric crystals and the later - in the form of metal-like conductive ones. This latter compound seems to be synthesized for the first time.

Electrode and chemical reactions during electrodeposition of tantalum products in CsCl melt

This work was accomplished in the context of systematic examination of a role played by electrolyte cation composition in electroreduction processes. The electrochemical behaviour of molten salts systems where CsCl was used as solvent for potassium and caesium oxyfluorotantalates was investigated. In particular, the influence of the electrolysis parameters, such as concentration of tantalum species in electrolyte, temperature and current density on the cathodic products phase composition was determined by X-ray diffraction and scanning electron microscopy. Several phases generally crystallized at the cathode simultaneously. Among identified phases were metallic tantalum in cubic (α) and tetragonal (β) crystal modifications and TaO. But the most characteristic phases for the melt under study were two isostructural compounds with pyrochlor type crystal lattice CsTa 2+ z 5O 5+ y F 1− y and CsTa 2+ z 5− x O 5+ y F 1− y . The former deposited in the form of transparent dielectric crystals and the later – in the form of metal-like conductive ones. This latter compound seems to be synthesized for the first time. The peculiarities of its structure were described. Electrochemical redox processes in melts containing potassium oxyfluorotantalates dissolved in CsCl were studied by cyclic voltammetry. Three discharge peaks were detected on voltammograms in the temperature range 700–850 °C. They were associated with the discharge of TaOF 6 3− and TaO 2F 4 3− complexes and alkali metal cations.

Characterization techniques employed in the study of niobium and tantalum-containing materials

This paper summarizes the techniques applied for the characterization of niobium and tantalum species in solid materials and described in the literature in the last two decades. The idea of this paper is not only the exhibition of new and developed techniques but also pointing at the papers in which readers can find the particular results of the applied methods.

Electrodeposition processes of tantalum(V) species in molten fluorides containing oxide ions

The influence of the molar ratio O–Ta noted R o on the electrodeposition of tantalum in molten fluorides at 800 °C is examined. After additions of both K 2TaF 7 and Na 2O, we observed: (i) tantalum deposit is obtained only by reduction of TaF 7 2−; (ii) the oxide ions react with TaF 7 2− to give oxifluoride species, identified first as TaOF 5 2− and for higher oxide content, as TaO 2F 4 3−; (iii) the electroreduction of TaOF 5 2− leads to the formation of an unstable bivalent tantalum species (TaO); (iv) the conversion of TaF 7 2− to oxifluoride is complete when R o is 1. Titration of Ta by ICP and oxygen by a carbothermal-reduction technique show that for R o>1, insoluble compounds Na 3TaO 4 and NaTaO 3 are formed. For R o>3, these species seem to be solubilised. Electrodeposition runs on copper cathodes show that pure and coherent Ta coatings are obtained for a lower oxide content ( R o<0.5). For higher R o, the faradic efficiencies are low. The coatings are powdery and include the presence of Na 3TaO 4, NaTaO 3 and traces of Ta metal, arising from the decomposition of TaO.

Alkyl and Alkylidene Tantalum−Lithium Complexes Supported by an Anionic Triazacyclononane Ligand

A new iPr2-tacn--supported alkyl tantalum species, (Me3SiCH2)2(RN)Ta(μ-CH2SiMe3)(μ-η1:η3-iPr2-tacn)Li (R = 2,6-iPr2C6H3; 2), was synthesized and found to undergo α-H abstraction under gentle heating to form the alkylidene (Me3SiCH2)(RN)Ta(μ-CHSiMe3)(μ-η1:η3-iPr2-tacn)Li (3). The syntheses and crystal structures of 2 and 3 and their precursor ((iPr2-tacn)TaCl2(NR) (1)) are described, as well as kinetic data for the first-order thermolysis reaction producing 3.

Versatility of silica used as a ligand: effect of thermal treatments of silica on the nature of silica-supported alkyl tantalum species

The tris(neopentyl)neopentylidene tantalum complex reacts with the silanol groups of silica dehydroxylated at temperatures ranging from 300 to 700°C to form well-defined surface organometallic species. For a silica dehydroxylated at 300°C, the amount of available silanols allows the formation of species 3 linked by two covalent bonds to silica, while the dehydroxylation at 700°C leads to the formation of species 2 with only one covalent bond to silica. Dehydroxylation thus constitutes a way to control the hapticity of silica towards organometallic complexes.

Niobium and tantalum derivatives with bidentate nitrogen ligands as potential precursors to nitrides

Early transition metal derivatives with N-donors have attracted interest as precursors of nitrides, the latter being used as diffusion barriers for the ULSI technology. Potential precursors to niobium and tantalum nitrides have been synthesized in good yields and characterized. They are based on bidentate nitrogen donors namely amidinates, benzamidinates and diamido ligands. These have been reacted with niobium or tantalum derivatives namely penta and tetrachlorides, TaCl 3 (NEt 2 ) 2 . Disubstituted niobium(IV) and tantalum(V) species such as for instance NbCl 2 [RNCR'NR 2 ] 2 [R= Cy, N(SiMe 3 ) 2 ; R = SiMe 3 , R' = p-tolyl] have been obtained and structurally characterized, The more rigid o-dimethylphenylene diamido ligand lead to a volatile, air stable disubstituted Nb(V) species NbCl{N 2 (SiMe 3 ) 2 C 5 H 3 } 2 . However, π-stacking as shown by X-ray studies are responsible of quite low sublimation rates. Thus, volatility could be reached for some species, their properties, high solubility due to the trimethylsilylligands, their stability in solution is more suited for applications in wet MOCVD than conventional one.

Electrochemical Behaviour of Dissolved Niobium, Molybdenum and Tantalum Species in Molten Chloroaluminates

Electrochemical Behaviour of Dissolved Niobium, Molybdenum and Tantalum Species in Molten Chloroaluminates

A hydridotantalum(V)-carborane analogue of Schwartz's reagent: synthesis and reactivity

The dichlorotantalum metallacarborane CpCl 2Ta(Et 2C 2B 4H 4) ( 1) on treatment with LiAlH 4 in THF gave a hydridotantalum dimer [CpTa(H)(Et 2C 2B 4H 4)] 2( μ-Cl) 2 ( 2), which was found to undergo alkyne insertion with p-tolyl acetylene to generate exclusively trans-CpCl( p-MeC 6H 4-CHCH)Ta(Et 2C 2B 4H 4) ( 3); the reaction of the latter complex with anhydrous HCl afforded p-tolylstyrene and 1. In contrast, reaction of 2 with diphenyl acetylene or methyl phenyl acetylene gave hydridotantalum–alkyne complexes, which are apparently the first examples of alkyne π-coordination to a formal d 0 metal. Treatment of 2 with styrene produced the alkyl–tantalum species CpTaCl(CH 2CH 2Ph)(Et 2C 2B 4H 4) ( 5), but no reactivity was observed between 2 and the sterically hindered olefins cis-stilbene, trans-stilbene, and cyclohexene. Addition of anhydrous HCl to 5 generates 1 and styrene via an apparent β-elimination mechanism.

Electrochemical impedance and cyclic voltammetry study of the influence of oxide on the redox chemistry of niobium and tantalum in chloride melts

The redox process of penta- and tetravalent niobium and tantalum species dissolved in CsClNaCl eutectic melts (65-35 mol%) have been investigated by electrochemical impedance spectroscopy and cyclic voltammetry. In particular, the influence of oxide has been studied. A strong correlation between the structures of the cyclic voltammograms and the potential dependence of the Warburg diffusion impedance is observed, which is determined by the concentrations of the redox species under consideration. Without any oxide added to melts containing TaCl5 or NbF72− the TavTaIV and NbVNbIV redox process, respectively, is observed in the cyclic voltammograms. This is also the case after equilibrating the melts with the respective metals. Plots of the Warburg coefficients as a function of the electrode potential exhibit minima near the equilibrium potentials. This confirms the presence of both redox species. Addition of oxide, added as Na2O, strongly influences the redox structures of TaVTaIV and NbVNbIV in the volammograms and the minima of the Warburg coefficient curves. When the structure is completely suppressed, for example, when TaV is titrated with an equivalent amount of oxide, the corresponding minima disappear. From the impedance spectra additional informations have been obtained: Indications of electronic conduction, most pronounced for tantalum, and an adsorption process at the electrode in NbIV systems.

Synthesis and Reactivity of Tantalum Alkylidene Complexes Containing the C,N,N‘-Chelating Aryldiamine Ligand [C6H4(CH2N(Me)CH2CH2NMe2)-2]- (CNN). X-ray Structures of [TaCl2(CH-t-Bu)(CNN)], [Ta{(CH2)3-1,3}(CNN)(O-t-Bu)2], and [Ta(CNN)(O-t-Bu)2(H2CCH2)

The potentially C,N,N‘-chelating anionic aryldiamine ligand [C6H4(CH2N(Me)CH2CH2NMe2)-2]- (CNN) has been employed in the preparation of six-coordinate Ta(V) alkylidene complexes. The new dichloro alkylidene complex [TaCl2(CH-t-Bu)(CNN)], 2, prepared from [TaCl3(CH-t-Bu)(THF)2] and [Li(CNN)]2, 1, has been obtained as a mixture of three diastereoisomers (2a−c) in 51% yield. The molecular structure of the major isomer 2a, determined by X-ray methods, shows this to be a tetragonal bipyramidal complex in which the metal coordination sphere is comprised of the C,N,N‘ facially bonded CNN, two cis-positioned chlorides in the equatorial plane, and a neopentylidene group and the NMe2 nitrogen donor mutually trans-positioned at the apices. The dialkoxide complexes [Ta(CHR)(CNN)(O-t-Bu)2] (R = t-Bu (3a), CMe2Ph (3b)) have been obtained by transmetallation of [TaCl(CHR)(O-t-Bu)2(PMe3)] with 1. The neopentylidene complex 3a was isolated as an analytically pure orange solid in 47% yield, whereas the neophylidene complex 3b was isolated as an orange oil of ±90% purity. Complex 3b reacts in pentane with ethene at room temperature to afford the tantalacyclobutane complex [Ta{(CH2)3-1,3}(CNN)(O-t-Bu)2], 4, which was isolated as a white solid in 81% yield. An X-ray diffraction study of 4 shows it to be a heptacoordinate pentagonal bipyramidal tantalum species in which two mutually trans alkoxide groups are in the apical positions; the meridional ligation is comprised of the C,N,N‘-coordination of CNN and the σ-bonded carbon atoms of the metallacyle. Reaction of 3b in hexane at 69 °C with ethene gives a mixture of the new tantalum alkene complex [Ta(CNN)(O-t-Bu)2(H2CCH2)], 5, and 4 in a 5:2 ratio. Complex 5 can be isolated from this mixture as a yellow solid by recrystallization from diethyl ether at −30 °C. The X-ray molecular structure of 5 shows it to be a pentagonal bipyramidal complex in which the array of atoms directly bonded to tantalum is like that in 4. However, in 5, the meridional ligation is comprised of the C,N,N‘-coordination of CNN and the two C atoms of the ethene molecule. Complexes 3b and 4 initiate ring-opening metathesis polymerization with strained cyclic olefins; dicyclopentadiene produces cross-linked polymers, whereas norbornene produces polycyclopentamers with approximately 90% trans-vinylene bonds. In contrast, none of the isomers of alkylidene complex 2 show any reactivity toward either ethene or norbornene.

A novel marker approach for the determination of transport numbers during the growth of anodic oxide films on tantalum

Abstract From knowledge of the immobility of incorporated silicon species in anodic films on tantalum, developed by high-field ionic conduction, a procedure has been developed for their use as a marker to determine the mobilities of species contributing to film growth on tantalum and other species incorporated from the electrolyte. In the example given, sequential anodizing, firstly in silicate electrolyte and subsequently in sulphuric acid, have been used to determine the transport number of tantalum cations, 0.24, which is an excellent agreement with the value determined by Rutherford back-scattering spectroscopy and chemical sectioning.

The effect of phosphorus on the passivation behavior of Ni-10Ta-P alloys in 12 M HCl

The corrosion behavior of Ni-10Ta-P alloys in 12 M HCl solution was investigated. An increase in the alloy phosphorus content improves the corrosion resistance of the Ni- 10Ta-P alloys. Ni-10Ta-15P and Ni-10Ta-20P alloys with sufficient phosphorus additions are spontaneously passivated due to the formation of a stable passive film in which tantalum ions are accumulated, comprising more than 70% of cations just after immersion. The composition and thickness of the film on these alloys are almost unchanged during prolonged immersion. By contrast, Ni-10Ta-5P and Ni-10Ta-10P alloys with insufficient phosphorus additions are not passivated spontaneously and continue to dissolve actively. Thus, a tantalum-enriched film cannot be formed for initial short periods of immersion. A large amount of dissolution of Ni-10Ta-5P and Ni-10Ta-10P alloys results in accumulation of tantalum ions on the alloy surface, and tantalum species precipitate and are concentrated in corrosion products on the alloy surface. Accordingly, the concentration of tantalum ions in the corrosion product film is increased slowly to more than 90% of cations, which is significantly higher than that in the stable passive film formed on the alloys with high phosphorus additions.

Electrochemical Studies of Tantalum in Fluorochloroaluminate Melts at 200–450°C

The electrochemistry of tantalum(V) species in sodium fluorochloroaluminate melts (10 mole percent ) has been investigated in the temperature range of 200 to 450°C using cyclic, normal pulse, and square wave voltammetries, exhaustive electrolysis, Raman and electronic spectroscopies, and x‐ray diffraction methods. The electrochemical behavior of tantalum(V) is strongly dependent on temperature. Three main reduction waves are observed at a temperature of 300°C or higher. The first and second reduction waves merge into one wave at temperatures below 300°C The first reduction wave is associated with the reduction of tantalum(V) to tantalum(IV) species followed by a dimerization reaction which occurs very slowly at lower temperatures. The second reduction wave is believed to be the reduction of the tantalum(IV) dimer,, to a tantalum(III) species (probably ). The tantalum(III) species decomposes resulting in the formation of the cluster, . The last reduction wave is assigned to the reduction of the trivalent tantalum species to a divalent tantalum species, which is highly unstable and decomposes to form the tantalum cluster, , and metallic tantalum. The clusters are slowly reduced to metallic tantalum.

Electrochemical behavior of titanium implanted with nickel and tantalum ions

Electrochemical behavior of titanium implanted with nickel and tantalum ions and a combination of the two has been investigated as a function of fluences. The polarization curves of the implanted titanium were potentiodynamically measured in the active and passive regions in a boiling 10 wt.% sulfuric acid solution. Electrochemical measurements revealed that nickel ion implantation significantly promoted the passivation of titanium with an increase of fluence and the corrosion potentials resided in the passive region at fluences above 1 × 10 16 ions/cm 2. Tantalum ion implantation was effective in reducing the anodic current densities in the active and passive regions as fluences increased. This effect was reduced at fluences above 7 × 10 16 ions/cm 2. At such fluences, the concentration of the implanted tantalum species decreased due to the etching effect of sputtering. On the other hand, the polarization curves of the co-implanted titanium exhibited more stable passive behavior with considerably low current densities. It was concluded that an excellent corrosion resistance of titanium was achieved by complementary effects of nickel and tantalum ion implantations.

Vaporisation sous pression atmospherique du systeme oxyde de niobium-oxyde de tantale

High temperature ( T ≈ 2900 K) selective vaporization of the niobium oxide from the niobium oxide-tantalum oxide system can lead to separation of niobium and tantalum species. Vaporization selectivity appears as a function of the molar concentration in the condensed phase. Moreover, the vaporization is reactive; the interface oxygen partial pressure generated by either an external or an internal source acts on both the partial vapour pressures and the oxidation state of the condensed niobium. This study is based on a thermodynamic model and high temperature experiments.