Synthesis of Structurally Defined Ta3N5 Particles by Flux-Assisted

Nov 23, 2010 - Synthesis of Structurally Defined Ta3N5 Particles by Flux-Assisted ... Jeongsuk Seo , Tsuyoshi Takata , Mamiko Nakabayashi , Takashi ...
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DOI: 10.1021/cg901025e

Synthesis of Structurally Defined Ta3N5 Particles by Flux-Assisted Nitridation

2011, Vol. 11 33–38

Tsuyoshi Takata,† Daling Lu,‡ and Kazunari Domen*,† †

Department of Chemical System Engineering, School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan, and ‡Chemical Resources Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan Received August 24, 2009; Revised Manuscript Received November 5, 2010

ABSTRACT: The synthesis of a transition metal nitride, Ta3N5, was studied. Morphological changes of Ta3N5 particles, depending on the synthetic method, starting materials, and nitridation conditions, were examined in detail. Flux-assisted nitridation was found to be a facile one-step route to a metal nitride with morphological control of the resulting particles. Scanning electron microscopy (SEM) observation revealed that morphologically defined particles of Ta3N5 were obtained using Zn, NaCl, or Na2CO3 as a flux. Flux-assisted nitridation of TaCl5 with Zn or NaCl produced nearly monodisperse fine particles larger than a few tens of nanometers. The size of these particles could be controlled by variation of the nitridation temperature. Highly crystallized Ta3N5 particles with a rectangular parallelepiped shape were produced from TaCl5-NaCl or Ta2O5-Na2CO3 mixtures by nitridation above 1123 K, because of the annealing effect of the flux.

Introduction Many metal nitrides and oxynitrides are of current interest for their excellent properties as potential advanced ceramics for use in various fields, for example, pigments,1,2 optoelectronics,3,4 photocatalysis/electrolysis,5-10 refractory ceramics,11,12 and wear-resistant coatings.13,14 One outstanding characteristic of metal nitrides is their high hardness,15 which is exceeded only by the carbide and boride groups, and results from their strong covalent bonding nature. This high hardness leads to the advantages of high chemical, thermal, and mechanical durability, but also makes the structure of products and the synthetic process more difficult to control. The most conventional method for the synthesis of metal nitrides is nitridation of metals, metal oxides, or chlorides under flowing NH3 at medium to high temperature, where the morphology of the starting material is not necessarily transcribed due to thermal sintering effects. For most metal oxides, various approaches for structural control have been well-established over a wide size range, from the macro- to the nanoscale. Several molecular routes to synthesize metal nitrides via liquid phase at relatively low temperature by a metathesis reaction between a metal halide and a nitrogen source have been examined.16-20 In most of these studies, however, control of morphology appears to be incomplete, as in the cases of metal oxides or chalcogenides. The reactivity of the examined nitrogen sources was not very controllable, which accounts for the difficulty in morphological control of the nitrided product. In addition, carbon contamination of the products, which would degrade physical performance, could not be avoided due to the use of organic solvents. Development of a controllable synthesis for nitride products with defined morphology is a challenging and novel goal in materials science and technology. Flux-assisted nitridation is sometimes employed in the synthesis of (oxy)nitrides for sufficient mixing of each component. We found that the

addition of a flux in the synthesis of nitrides resulted in the production of morphologically characteristic particles, and such effect has yet been examined in detail. In this study, novel flux-assisted synthetic routes to transition metal nitride, Ta3N5, particles and their structural characteristics such as dispersion, size, and crystallinity were examined in detail. Experimental Section Ta3N5 was prepared by ammonolysis of Ta2O5 or TaCl5 by heating with a flux under dry ammonia flow. A mixture of a Ta source and a flux was employed as the starting material. Mixing with the flux was performed by mechanical grinding of the powders in an agate mortar for Ta2O5, or by simply shaking the powders in a closed bottle for TaCl5 to prevent spontaneous hydrolysis of TaCl5 because of its high moisture sensitivity. The mixture was loaded on an alumina boat and inserted into the center of a horizontal alumina tube-furnace with an inner diameter of 24 mm. The samples were heated to a temperature of 973-1173 K with a constant ammonia flow rate of 100 or 200 mL min-1. A highly purified ammonia (4N-grade) purchased from Sumitomo Seika Chemicals Co. was directly used for ammonolysis. The nitridation conditions, precursor, flux material, temperature, time, etc., are summarized in Table 1. The crystal structures of the products were identified by X-ray diffraction (XRD) (Rigaku, RINT Ultima3, CuKR radiation, λ = 154.06 pm). The specific surface areas of the products were measured by Brunauer-Emmett-Teller(BET) method with N2 adsorption (BEL Japan Inc., BELSORP-mini). The oxygen and nitrogen contents in the products were measured by an oxygen/nitrogen combustion analyzer (Horiba, EMGA-620W). The surface morphologies of the products were observed using a field-emission scanning electron microscope (FE-SEM, Hitachi, S-4700) equipped with an energy dispersive X-ray analysis system (EDX, Horiba, E-max7000). High-resolution transmission electron microscopy (HR-TEM) and selected area electron diffraction (SAED) were measured using a field-emission transmission electron microscope (FE-TEM, JEOL, JEM-2010F).

Results and Discussion

*To whom correspondence should be addressed. Tel.: þ81-3-5841-1148. Fax: þ81-3-5841-8838. E-mail: [email protected].

Synthesis of Ta3N5 by Thermal Ammonolysis of Ta2O5 and TaCl5 (Entry 1, 2). Figure 1 shows XRD patterns of Ta2O5 and TaCl5 subjected to nitridation under various conditions. Nitridation was examined under 100 mL min-1 of NH3 flow at 1123 and 1173 K for Ta2O5, and 973 and 1073 K for TaCl5.

r 2010 American Chemical Society

Published on Web 11/23/2010

pubs.acs.org/crystal

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Figure 2. SEM images of a precursor and nitrided products. (a) Ta2O5, (b) nitrided product from Ta2O5 (1123 K-20 h), (c) low- and (d) high-magnification images of nitrided product from TaCl5 (1073 K-10 h).

Figure 1. XRD patterns of nitrided products prepared from Ta2O5 and TaCl5 by thermal ammonolysis under various conditions. (a) Ta2O5 (1173 K-10 h), (b) Ta2O5 (1173 K-20 h), (c) Ta2O5 (1123 K-10 h), (d) Ta2O5 (1123 K-20 h), (e) TaCl5 (973 K-10 h), and (f) TaCl5 (1073 K-10 h). Table 1. Summary of the Variously Examined Synthetic Conditions starting materials (mmol) entry

Ta-source

flux

1 2 3 4 5 6 7

Ta2O5(2) TaCl5(2) Ta2O5(2) Ta3N5(1) TaCl5(2) TaCl5(2) Ta2O5(1.5, 2)

none none NaCl(4) NaCl(6) NaCl(4,þ4) Zn(6) Na2CO3(1.5, 2)

nitridation condition temperature (K) duration (h) 1123, 1173 973, 1073 1123 1123 1023-1173 1123 1123, 1173

10, 20 10 10 10 10, þ10 10 40-80

When Ta2O5 powder was heated for 10 h at 1173 K, a bright red powder was obtained. The formation of a single phase of Ta3N5 was confirmed from the XRD pattern (a) in comparison to the reference data.21 The composition of this sample measured was Ta3N4.632O0.146. The imperfect nitridation is likely due to the trace of water content in ammonia as an impurity. This result also implies a nitrogen-defective compound. Then a small amount of Ta4N5 phase22 was found to coexist after another 10 h of nitridation, although the crystal structure of Ta3N5 was largely maintained as seen from the XRD pattern (b). In practice, the lower portion of the sample placed on the alumina boat turned black due to the formation of lower-valence-state Ta species. This is likely due to the progress of partial reduction of the parent valence of Ta5þ by the hydrogen released from ammonia by thermal decomposition. A small amount of monoclinic BaddeleyiteTaON,23 a metastable phase appearing during the structural transformation from Ta2O5 to Ta3N5, was detected in the XRD pattern (c) for the sample nitriding Ta2O5 at 1123 K for 10 h, implying that there was insufficient nitridation under these conditions. The TaON phase disappeared, leading to a single phase of Ta3N5 during a prolonged nitridation of 20 h (pattern d). As seen from XRD patterns (e) and (f), a single

phase of Ta3N5 was obtained even after 10 h of nitridation at 973 or 1073 K when TaCl5 was employed as a precursor. This indicates that the formation of Ta3N5 proceeds under milder nitridation conditions using TaCl5 as a precursor than with Ta2O5, most likely because of the facile dissociation of the Ta-Cl bond. The surface morphology of each product was observed by SEM. Figure 2a,b shows SEM images of Ta2O5 with and without nitridation at 1123 K for 20 h, respectively. Welldispersed crystalline particles of sub-micrometer size were observed from the Ta2O5 precursor. Structural transformation from Ta2O5 to Ta3N5 particles was pseudomorphic as observed in the previous study.24 The external size and geometry of the parent particle essentially remained unchanged during nitridation, while a mesoscale porous structure appeared in each nitrided particle, as can be observed in Figure 2b. Lattice shrinkage took place with the structural transformation during nitridation because the average coordination number of Ta was reduced due to the different valence of O2- and N3-, which led to the generation of void space. Moreover, the porous structure and particle outline remained essentially unchanged, even after heating at higher temperatures and/or prolonged nitridation, indicating that sintering of the nitrided sample did not occur. In Figure 2c,d, SEM images of Ta3N5 prepared from TaCl5 by ammonolysis at 1073 K are presented. Large masses, several tens of micrometers in size, were observed in the low magnification image (c). These masses were comprised of fine crystallites several tens of nanometers in size, as observed in the high magnification image (d). Flux-Assisted Synthesis of Ta3N5 (Entry 3-6). The observed structural features of nitrided particles in the former section reflect aspects of the high hardness of Ta3N5. Therefore, the synthesis method was further improved in order to facilitate morphological control by softening Ta3N5 with the aid of a flux. Synthesis of Ta3N5 was examined from various combinations of a Ta-source and a flux, as listed in Table 1. As seen from XRD pattern (a) in Figure 3, a NaTaO3 phase25 appeared in addition to the Ta3N5 phase when nitrided from a mixture of Ta2O5 and NaCl. Generally, NaCl is chemically less reactive, which is why NaCl is often employed as a flux material, and so the formation of NaTaO3 is an unexpected result. The peak intensity ratio of NaTaO3 to Ta3N5 decreased

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Figure 3. XRD patterns of nitrided products prepared by various flux methods. (a) Ta2O5-NaCl (1123 K-10 h), (b) Ta3N5-NaCl (1123 K-10 h), (c) TaCl5-NaCl (1073 K-10 h), and (d) TaCl5-Zn (1073 K-10 h).

with further nitridation. This suggests a possible nitrida tion route to Ta3N5 via NaTaO3 by elimination of Na2O. Post NaCl treatment was also examined. Ta3N5 prepared by ammonolysis of Ta2O5 at 1173 K was subsequently treated by NaCl-flux in NH3 flow. In this case, as seen in Figure 3b, the product was a single phase of Ta3N5, and no significant difference in the crystal structure was found between the XRD patterns before and after NaCl treatment. When a mixture of TaCl5 and NaCl was employed as a precursor, a single phase of Ta3N5 was obtained without any formation of Na-Ta-N ternary phases according to the XRD pattern in Figure 3c. In this case, almost all the added NaCl was excluded from the sample due to volatilization during the nitridation treatment. When a higher amount of NaCl, such as an Na/Ta molar ratio of 5, was added, the sample became dark red to black, probably due to the formation of N defects with a large amount of NaCl residue. This is because excess soaking of product in molten NaCl reduced the accessibility of N-sources from the gas phase. Therefore, the amount of NaCl addition was determined empirically, so as to allow a suitable exposure to NH3 while still providing an adequate soak in molten NaCl. After the NaCl-residue was rinsed with distilled water, neither Na nor Cl was detected by EDX, confirming that the flux material did not contaminate the product. Zn, with a melting point of 692.5 K, was also examined as a flux. In this case as well, only Ta3N5 phase was detected (Figure 3, XRD pattern d). No residual Zn was detected by EDX. The widths of the diffraction lines from this sample were broader than in the other cases, indicating a smaller crystallite size or insufficient crystallization of the resulting products. Notable changes in the morphology of the particles after flux-treatment were recognized by SEM observation. Figure 4a shows an SEM image of a mixture of Ta2O5 and NaCl nitrided at 1123 K for 10 h. The morphology of each particle was not homogeneous, probably because two phases, Ta3N5 and NaTaO3, coexisted in the sample. Crystalline particles were observed to have smooth surfaces, but the porous structure discussed above (see Figure 2b) was not produced so much according to the image, indicating a different formation route to Ta3N5 from that of direct ammonolysis of

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Figure 4. SEM images of samples prepared by various flux-assisted nitridation methods. (a) Ta2O5-NaCl (1123 K-10 h), (b) Ta3N5NaCl (1123 K-10 h), (c) TaCl5-NaCl (1073 K-10 h), and (d) TaCl5-Zn (1073 K-10 h).

Ta2O5. Figure 4b shows an SEM image of Ta3N5 with a post NaCl treatment. The porous structure that existed in the precursor of Ta3N5 largely disappeared, and a smooth surface appeared after NaCl treatment despite a lack of any clear changes in composition or crystal structure. This suggests that a reconstruction of Ta3N5 particles occurs during NaCl treatment via the liquid phase. Since NaCl is a chemically inert substance, the major role of NaCl is not a reactant but a solvent. NaCl melts and works as a polar solvent at the nitridation temperature, where dissolution and recrystallization steps are repeated, leading to the reconstruction of crystalline Ta3N5 particles. Figure 4c shows an SEM image of Ta3N5 prepared from a TaCl5-NaCl mixture by nitridation at 1073 K for 10 h. In contrast to the case of direct nitridation of TaCl5, no large sintered mass of Ta3N5 was observed in this image. Interestingly, the size of the resulting Ta3N5 particles was about 40 nm, and each particle was wellisolated from the others, despite the high temperature heating. Although agglomerates remained in parts of the sample, repeating the procedure of heating the sample under NH3 flow along with additional NaCl led to decreased agglomeration, probably due to a dissolution effect of the flux. The product obtained using Zn flux was also composed of monodisperse fine particles, which were somewhat smaller than those prepared from the TaCl5-NaCl mixture. Molten Zn is a nonpolar solvent and has an affinity for less-ionic crystals. Melting and boiling points of Zn are 692.5 and 1180 K, respectively, and removal of Zn from the sample should occur in the early stages of nitridation because of its high vapor pressure at the nitridation temperature. Therefore, the effect of Zn flux should occur early in the nitridation process. We believe that the molten Zn coats the nitrided crystallite at the nucleation stage and stabilizes the surface, much like a surfactant would, and thus the aggregation to a secondary particle is inhibited. Nearly monodisperse fine particles were successfully obtained by a very simple process, without requiring complicated procedures such as confinement in a porous template. Most of the added flux was removed from the sample during nitridation due to volatilization for both NaCl and Zn addition. Nevertheless, the nitrided particles did not undergo thermally driven aggregation and sintering. This phenomenon

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Figure 5. TEM (a, b) and SEM (c, d) images of Ta3N5 prepared from TaCl5-Na(þK)Cl mixture at various nitridation temperatures for 20 h. (a) 1023 K, (b) 1073 K, (c) 1123 K, (d) 1173 K.

may be due to the thermal stability of the nitrided product, even in such fine particles of several tens of nanometers, which originates from the strong covalency of the Ta-N bond. Monodisperse fine particles could feasibly allow the formation of a suspension, which would be useful in a wet-coating process, which could in turn enable a variety of new applications. Synthesis from a TaCl5-NaCl Mixture (Entry 5). The results obtained from the flux methods clearly indicate the possibility of structural control of product particle size, dispersion, and crystallinity, and therefore the flux-assisted synthesis of Ta3N5 was further examined. The dependence of the Ta3N5 particle size on the nitridation temperature was examined, starting from a combination of TaCl5 and NaCl over a temperature range of 1023-1173 K. Since the melting point of NaCl is about 1073 K, a KCl-NaCl eutectic mixture with a K/Na molar ratio of 3:7 was employed for the nitridation at 1023 K in order to keep the melting point of the flux below the nitridation temperature. In this case, nitridation was carried out for 20 h with an intermediate addition of flux and regrinding. This extended process provided a longer soak in molten NaCl, in order to enable a more complete flux-assisted morphological change. Figure 5 shows TEM and SEM images of Ta3N5 particles prepared in this manner at various temperatures. The particle sizes were mostly 10-30 nm, and approximately 20-40 nm for samples prepared at 1023 and 1073 K, respectively. Two types of distinguishable morphologies were recognized in the particles obtained at 1123 K, as seen in Figure 5c. At this temperature, the crystals grew preferentially toward one crystal axis, forming rod-like particles in some portions of the sample, while other parts remained as particles several tens of nanometers in size, with a similar shape and size to samples prepared at 1073 K. Mazumder et al. also reported a solvothermal method of producing rodlike Ta3N5 particles.26 Such anisotropy in the particles is thought to appear when crystal growth proceeds by a molecular route via the liquid phase. The synthesis of Ta3N5 by chemical transport via the gas phase also produced anisotropic crystalline particles.27 In this case, anisotropy appeared more clearly in the product to grow with needle-like shape. Most of the particles were grown to several hundred nanometers and tended to possess a rectangular parallelepiped shape with smooth crystal faces after nitridation at

Takata et al.

Figure 6. XRD patterns of nitrided samples prepared from various amounts of Ta2O5-Na2CO3 mixture under various nitridation conditions. (a) 2 mmol (1173 K-40 h), (b) 2 mmol (1123 K-60 h), (c) 2 mmol (1123 K-80 h), and (d) 1.5 mmol (1123 K-60 h).

1173 K. The particle size increased with increasing the nitridation temperature in the presence of flux. In the present study, nitridation temperature was the most decisive factor affecting the particle size. However, the particle size essentially remained unchanged, although the sample prepared with NaCl-flux was subsequently nitrided at a higher temperature without the addition of NaCl. This implies that the crystal growth producing the observed rectangular parallelepiped particles was due to an annealing effect of the NaClflux. The increase of particle size with increasing nitridation temperature in the flux-assisted nitridation method is likely due to enhancement of the growth rate relative to the elution rate. The observed anisotropy of the particles reflects the crystal system of Ta3N5, orthorhombic but that is very close to tetragonal (a, b, c = 3.886, 10.212, 10.262 A˚). This crystal habit in the resulting particles of Ta3N5 suggests that annealing was promoted by the flux in this synthetic method. Although the particles were not uniform in size in the samples prepared at each nitridation temperature, narrower particle size distributions were obtained at 1023 and 1073 K. Further examination is necessary for the production of homogeneous size particles by optimizing the factors other than nitridation temperature, such as the state of the precursor, the nitridation duration, the amount of flux added, and the repeat time of the procedure. Synthesis from a Ta2O5-Na2CO3 Mixture (Entry 7). The route to Ta3N5 via NaTaO3 suggested by the experiment of entry 3 was also examined in detail. A mixture of Ta2O5 þ Na2CO3 (2 þ 2 mmol) was selected as a typical combination of starting materials for the facile formation of NaTaO3. Nitridation was conducted at 1123 or 1173 K under NH3 flow of 200 mL min-1 with intermediate grindings every 20 h. XRD patterns of the products subjected to nitridation under various conditions are given in Figure 6. Nitridation was complete by 40 h at 1173 K, resulting in a single phase of Ta3N5, as seen from the XRD pattern in Figure 6a. After nitridation for 60 h at 1123 K, the diffraction peaks of NaTaO3 as an intermediate phase were identified from the XRD pattern in Figure 6b in addition to those of Ta3N5. The NaTaO3 phase disappeared after another 20 h of nitridation (Figure 6,

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Figure 7. SEM images of Ta3N5 prepared from a Ta2O5-Na2CO3 mixture at (a) 1123 K for 80 h and at (b) 1173 K for 40 h.

pattern c). Thus, the NaTaO3 phase is not a byproduct but an intermediate phase. The progress of nitridation was different depending on the amount of feedstock, as can be seen by comparing the XRD patterns in Figure 6b-d. No NaTaO3 phase remained after nitriding a 1.5 mmol Na2CO3-Ta2O5 mixture for 60 h, whereas mixed phases of Ta3N5 and NaTaO3 were observed from a 2 mmol starting mixture at this nitridation duration. The time required to complete nitridation leading to a single phase of Ta3N5 appears to be linearly dependent on the amount of feedstock. This is reasonable, considering the reaction scheme of Ta3N5 formation. The scheme for the formation of Ta3N5 via NaTaO3 can be denoted as follows: Na2 CO3 þ Ta2 O5 f 2NaTaO3 þ CO2

ð1Þ

NaTaO3 a 1=2Na2 O þ 1=2Ta2 O5

ð2Þ

Ta2 O5 þ 10=3NH3 a 2=3Ta3 N5 þ 5H2 O

ð3Þ

NaTaO3 is produced in the initial stage according to reaction 1. To the best of our knowledge, there has been no report of a crystal phase of quaternary oxynitride comprising Na-Ta-O-N, and in practice no intermediate phase other than NaTaO3 was detected in this route. Therefore, the formation of Ta3N5 essentially occurs by the nitridation of Ta2O5 dissociated from NaTaO3, eliminating Na2O. The step for the elimination of Na2O is the slowest; thus no Ta2O5 or TaON phase was detectable. The equilibrium of eqs 2 and 3 inclines to the left side due to the dominance of backward reactions. Reduction of the partial pressures of H2O and Na2O is necessary for the progress of Ta3N5-formation, according to Le Chatelier’s Principle, and the role of NH3 flow is conceivably the removal of the two oxygen sources in addition to feeding the nitrogen source, which would displace the equilibrium. This interpretation accounts for the dependence of nitridation progress on the amount of feedstock. The measured composition of the sample prepared from Ta2O5-Na2CO3 mixture by nitridation at 1173 K for 40 h by EDX and O/N analysis was Ta3N4.904O0.092. This is rather close to the stoichiometry compared to that of the sample examined in entry 1. This is evidence that Na2O species was completely removed from the product during the nitridation. In Figure 7, SEM images of Ta3N5 prepared from Na2CO3-Ta2O5 mixture are presented. The particles in samples prepared at 1123 and 1173 K tended to grow into a rectangular parallelepiped shape with smooth crystal faces, as observed in the synthetic route from a TaCl5-NaCl mixture. No porous structure was observed for this sample. No clear difference in particle size or morphology was observed between the two nitridation temperatures of 1123 and 1173 K. This indicates that annealing and crystal growth were well-promoted by

Figure 8. Bright field (a) and high resolution electron microscopic (b) images with a SAED pattern (inset) of Ta3N5 prepared from a Ta2O5-Na2CO3 mixture at 1123 K for 80 h.

Na2CO3, even at 1123 K, which is different from the case of NaCl-flux. The crystalline state was investigated by TEM on a particle scale. Figure 8a shows a bright-field image of Ta3N5 prepared from Na2CO3-Ta2O5 by nitridation at 1123 K. SAED was taken from the encircled region along the [110] direction. A clear spot pattern assignable to the orthorhombic phase of Ta3N5 was obtained due to the single-crystalline particle. In the HR-TEM image, lattice images running across the particle were clearly observed, indicating that the crystallinity was enhanced by flux-assisted annealing. Since Na2O is a highly reactive and strongly alkaline substance, the Na2O eliminated from NaTaO3 should recombine with Ta2O5 or Ta3N5, as denoted in eqs 2 and 3. The crystallization mechanism should be different between the cases to employ Na2CO3 and NaCl as fluxes. Na2CO3 added as a flux reacts with Ta2O5 to generate NaTaO3, and no liquid phase exists. This is the distinctive difference from the case of NaCl-flux. Therefore, the formation of crystalline particles did not proceed based on the dissolution-recrystallization step via the liquid phase like the case of NaCl-addition. It is considered that the combination of Na and Ta-species, that is, the formation of NaTaO3, resulted in the softening of the material. This would promote thermal diffusion of atoms, leading to the formation of the well-annealed crystalline particles. Therefore, the porous structure existed in the Ta3N5 particles prepared by direct ammonolysis of Ta2O5 collapsed with an addition of Na2CO3flux. Combination and dissociation steps between Na2OH2O and Ta3N5 should occur reversibly during nitridation, during which annealing of product by molten Na2O also occurred. When the Ta3N5 sample prepared from Ta2O5

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Table 2. Summary of the Variously Examined Synthetic Routes and the Features of Products starting materials entry Ta-source 1 2 5 5 6 7

Ta2O5 TaCl5 TaCl5 TaCl5 TaCl5 Ta2O5

flux

nitridation temperature (K)

feature of product

specific surface area (cm2 g-1)

none none NaCl NaCl Zn Na2CO3

1123 1073 1023, 1073 1123, 1273 1073 1173

porous particle aggregation of fine crystallites monodisperse fine particle well-crystallized particles, rectangular parallelepiped shape monodisperse fine particle well-crystallized particles, rectangular parallelepiped shape

6.0 20 18, 10 4.0, 1.2 9.1 1.7

and Na2CO3 was subjected to additional nitridation for a few tens of hours at 1173 K, the Ta4N5 phase remained undetectable, unlike the sample directly nitrided from Ta2O5. The enhanced stability of Ta3N5 prepared with added Na2CO3 probably resulted from its more complete crystallization.

References (1) (2) (3) (4)

Conclusions

(5)

Major results for the formation of different types of Ta3N5 particles via various synthetic routes were summarized in Table 2. Direct thermal ammonolysis of TaCl5 or Ta2O5 resulted in sintered or pseudomorphous Ta3N5 particles, respectively. However, the addition of Zn, NaCl, or Na2CO3 as a flux for the synthesis of Ta3N5 resulted in the formation of morphologically defined particles. Using a flux method of nitriding TaCl5 along with NaCl, nearly monodisperse fine particles of Ta3N5 above a few tens of nanometers in size were obtained, with the particle size controllable by variation of the nitridation temperature. A route to obtain fine particles of Ta3N5 was accomplished using a simple procedure. When Na2CO3 was employed as a flux, formation of Ta3N5 proceeded via NaTaO3. Ta3N5 was produced by nitriding NaTaO3, eliminating Na2O, which promoted annealing of the products, resulting in the formation of well-crystallized particles. Each resulting particle was essentially comprised of a single crystal domain. It was demonstrated that the addition of flux allows the structural control of the resulting particles by promoting the diffusion of atoms in the nitrided materials.

(6) (7) (8) (9) (10) (11) (12) (13) (14) (15) (16) (17) (18) (19) (20) (21)

Acknowledgment. This work was financially supported by Research and Development in a New Interdisciplinary Field Based on Nanotechnology and Materials, Science Program of the Ministry of Education and Culture, Sports, Science, and Technology (MEXT) of Japan, and a Grant-inAid for Young Scientists (B) No. 20760526, Japan Society for the Promotion of Science (JSPS).

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