Synthetic Conditions and Color Characteristics of Tantalum Oxynitride Prepared via Liquid-NH 3 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.


Introduction
The development of green chemical processes is not only desirable but expected for creating a sustainable society in harmony with nature.Tantalum oxynitrides, such as TaON, have shown promise as nontoxic yellow pigments containing no heavy metals, which are similar to praseodymium yellow and cerium zirconium bismuth based oxide [1].To date, these materials have been prepared via a gas nitriding process, in which tantalum oxide is transformed into the nitride under a flow of NH 3 gas under high temperatures [1].In contrast, the liquid-NH 3 process affords the nitride or oxynitride through vacuum-calcination of a precursor material obtained by the reaction between a metal halide and liquid-NH 3 [2][3][4].Because the liquid-NH 3 process allows for the precise control of the elemental composition of the starting materials, the process is suitable for screening new pigment compositions.In one case, the color of the tantalum oxynitride prepared via the liquid-NH 3 process was different from the color of TaON (Yellow TaON) prepared via the gas nitriding process using a flow of moist NH 3 gas, but was similar to that of green TaON [5].Our aim is to clarify the conditions of the liquid-NH 3 process that control the color of the products, and to understand the color characteristics of the resulting pigments.Because the liquid-NH 3 process involves the circulation of an inert gas within a glove box, the atmosphere is reductive and thus the oxidation number of Ta can be lower than expected.To prevent this, an additional recalcination step in air was carried out after the vacuumcalcinations.

Glove Box Operations for Precursor Synthesis.
To minimize the effects of atmospheric moisture, the reactions were carried out inside a vacuum glove box (UNICO, UN-650F), based on a previously reported synthetic method [2], in which the water vapor concentration was less than 23.8 × 10 −4 wt% (dew point: 223 K) under a N 2 atmosphere.Accordingly, precautions were taken with the commercially available reaction reagents.The bottles were opened inside the glove box, and the reagents were allowed to stand at room temperature (rt) for over 24 hours before use.The precursor for the nitride was prepared in the absence of an oxygen source inside a glove box, as previously reported, as follows.Under a N 2 atmosphere, TaCl 5 (3 mmol, Aldrich, 99.99%) and KCl (30 mmol, Wako Pure Chemical Industries, special grade) as flux were placed in a 100 mL three-necked flask equipped with a glass stirrer, then sealed with a pair of two-way stopcocks and a Teflon plug.The flask was then placed in a low-temperature constant temperature bath (EYELA, PSL-1800; 1-propanol cooling medium, set at 193 K) under a flow of Ar gas (2,000 cm 3 min −1 ).Highpurity NH 3 gas (Sumitomo Seika Chemicals, 99.9%) was flowed at 3,000 cm 3 min −1 over the contents of the flask until 50 cm 3 of liquid NH 3 was condensed.Next, H 2 O (3 mmol) was injected into the flask with a microsyringe.After stirring the cooled mixture for 30 minutes, the flask was warmed to rt, which allowed the unreacted NH 3 to evaporate.The resulting product was dried for 1 hour under vacuum (5 × 10 3 Pa) using a vacuum line.

Vacuum Calcination.
The crude mixture of the precursor (described above), containing both the byproduct NH 4 Cl and flux KCl, was transferred under a N 2 atmosphere into a quartz reaction tube (φ12 × 300 mm) with one end sealed.After inserting quartz wool for the prevention of powder scattering, the reaction tube was sealed using a two-way stopcock with a Teflon plug.The reaction tube was placed in a small muffle furnace (KDF S-70, Denken Co., Ltd.), then heated from rt to 1073 K at rate of 20 K min −1 and held at 1073 K for 10 minutes under vacuum using an oil rotary vacuum pump (SATO SW-150).Immediately after the vacuum/heat treatment, the quartz reaction tube was removed from the electric furnace, and the product was separated from the NH 4 Cl and KCl by washing, filtering, and drying.

Postreaction Treatment: Recalcination in Air.
For the postreaction recalcination in air, a portion of the calcined sample was poured into a porcelain crucible at rt, then heated to 973 K at rate of 10 K min −1 using an electric muffle furnace (KDF S-70, Denken Co., Ltd.).After maintaining the mixture at the elevated temperature, the sample was removed and characterized.
The color (L * a * b * color index) of the synthesized powder samples was measured using a JASCO UV-Vis spectrophotometer (V-650) attached to an ISV-22 integrating sphere.The samples were positioned in a PSH-001 powder sample holder such that the rubber plate section was completely covered.
The binding energies determined by XPS (SHIMADZU ESCA-850) were corrected in reference to the C1s peak for each sample.Ta 2 O 5 (Wako Pure Chemical Industries, special grade) was used as a reference for the Ta 4f peak.

Results and Discussion
The XRD pattern of the green TaON powder (synthesized via the liquid-NH 3 process) before the postreaction recalcination is shown in Figure 1(a).The positions of all diffraction peaks were in good agreement with those of the calculated TaON (Figure 1(c)) [7].No minor peaks assignable to calculated Ta 3 N 5 (Figure 1(d)) [6] were observed.The XRD pattern of the yellow TaON powder (synthesized via the liquid-NH 3 process) after the postreaction recalcination is shown in Figure 1(b).The XRD pattern of the yellow TaON powder gives the same XRD pattern as the green TaON.This result indicates that the crystal phase of the TaON powder samples synthesized via the liquid-NH 3 process is not influenced by the postreaction recalcination at 973 K in air.Diffuse reflectance spectra for the green and yellow TaON powders synthesized via the liquid-NH 3 process are shown in Figures 2(a) and 2(b), respectively.These diffuse reflectance spectra are similar to the reported green and yellow TaON spectra observed by Orhan et al. [5].According to these researchers, the difference in the reflectance spectra between the yellow and green TaON is caused by the difference in composition of the TaON.The yellow TaON has slightly more oxygen atoms than nitrogen atoms (TaO 1.02 N 0.99 ).
XPS spectra for the green and yellow TaON powders synthesized via the liquid-NH 3 process are shown in Figures 3(a) and 3(b), respectively.The green TaON produces a photoelectron signal at lower energy than Ta 2 O 5 (Figure 3(c)) and yellow TaON, which can be explained in terms of the difference in bond polarities of Ta-O and Ta-N as mentioned above, that is, the Ta-N bond is more covalent than the Ta-O bond [10].This suggests that the green TaON has more Ta-N bonds than Ta-O bonds, while the yellow TaON has more Ta-O bonds than Ta-N bonds.We have previously reported that the precursor of TaON synthesized via the liquid-NH3 process consists of a dimer complex [{TaCl 2 (NH 3 )(NH 2 )} 2 (μ − O) 2 ] [4], as described in the following equation: During vacuum calcination (at 973 K) within the liquid-NH 3 process, this precursor can transform to TaON, as described in the following equation: This reaction mechanism implies that the TaON synthesized via the liquid-NH3 process has more Ta-N bonds than Ta-O bonds resulting in the green color.Alternatively, during the postreaction recalcination in air, the Ta-N bonds in the green TaON transform to Ta-O bonds, resulting in a color change from green to yellow.The color change, however, does not result in broadening of the XRD peaks in the yellow TaON (Figure 1

Conclusion
Using a liquid-NH 3 process, we have prepared a TaON powder that can be employed as a nontoxic yellow pigment containing no heavy metals.The preparation involves the vacuum-calcination of a precursor material obtained via reaction between a metal halide, H 2 O and liquid NH 3 .Calcination of the precursor at 1073 K under vacuum was followed by recalcination at 973 K under air.The reaction conditions were adjusted to afford a powder with a color index of L * = 84.20, a * = −2.71,and b * = 44.07.
(b)), which indicates that recalcination in air does not deform the crystal lattice of the green TaON powder samples synthesized by the liquid-NH3 process.From the diffuse reflectance spectrum (Figure 2(b)), L * a * b * data for yellow TaON powders synthesized using the liquid-NH 3 process after postreaction recalcination was calculated to be L * = 84.20, a * = −2.71,and b * = 44.07.For the green TaON, values of L * = 43.82, a * = −1.83,andb * = 3.31 were obtained from the diffuse reflectance spectrum (Figure 2(a)).Such a drastic color change in TaON is different from the case of the small binding energy shift of Ta 4f that has been reported by Maeda et al.[11].