Explosive Reaction for Barium Niobium Perovskite Oxynitride

Communication pubs.acs.org/IC

Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

Explosive Reaction for Barium Niobium Perovskite Oxynitride Jin Odahara,† Akira Miura,*,‡ Nataly Carolina Rosero-Navarro,‡ and Kiyoharu Tadanaga‡ †

Graduate School of Chemical Sciences and Engineering and ‡Faculty of Engineering, Hokkaido University, Sapporo 060-8628, Japan S Supporting Information *

have reported the synthesis of nitrides and oxynitrides through the reaction of oxides with molten NaNH2 salt below 573 K with the formation of NaOH as a byproduct.10 However, there have only been a few reports on the production of perovskite oxynitrides. Very recently, the reaction of Ba(OH)2, TaCl5, and NaNH2 was found to produce a perovskite oxynitride, BaTaO2N, even at 493 K in 20 h.11 However, there was no description of the explosive reaction. In this work, we report the first explosive reaction for barium niobium perovskite oxynitride. Caution! NbCl5, TaCl5, and NaNH2 powders are highly sensitive to moisture. An explosive reaction occurs suddenly by mixing these powders. Synthesis using a large amount of the starting materials may cause a severe accident. First, Ba(OH)2 and NbCl5 powders were mixed in an argonfilled glovebox. Then, the powder mixture was placed in a Teflonlined autoclave. The total amount of the mixture was less than 250 mg. Two different procedures were performed in the subsequent sequences. One involved adding NaNH2 powder to the mixture and stirring the resulting mixture in the glovebox. The product was washed with ethanol and distilled water in air and collected by filtration. The second procedure involved dispersing the mixture in hexane. NaNH2 powder was added, and the suspension was mixed. The autoclave was sealed tightly, and the mixture was heated at 498 K for 20 h. The product was washed with ethanol and water as in the first procedure. Barium tantalum perovskite oxynitride was synthesized in the same way using TaCl5 instead of NbCl5. The mixing of the starting materials Ba(OH)2, NbCl5, and NaNH2 in an argon atmosphere without hexane caused an explosive reaction (Figure 1a,b and Supporting Movie 1). Typical amounts of Ba(OH)2, NbCl5, and NaNH2 were 0.5, 0.5, and 2.5 mmol, respectively. Black powder was obtained after subsequent washing with distilled water and ethanol (Figure 1c). Figure 2 shows the X-ray diffraction (XRD) patterns of the product before and after washing with the water−ethanol

ABSTRACT: An intense exothermic and explosive reaction between Ba(OH)2, NbCl5, and NaNH2 produced barium niobium perovskite oxynitride in seconds. The addition of hexane reduced the risk of explosion during mixing of the starting materials, and subsequent heat treatment at 498 K in hexane allowed control of this exothermic reaction, leading to formation of the perovskite oxynitride with fewer impurities. The synthesis of barium tantalum perovskite oxynitride under similar reaction conditions was successful.

P

erovskite oxynitrides have attracted great attention because their electronic structures and properties can be tuned not only through cations but also through oxygen and nitrogen anions.1 Therefore, they are advantageous for fabricating functional materials, some of which cannot be accessed with perovskite oxides. For example, the optical band gap of Ca1−xLaxTaO2−xN1+x can be optimized through the oxygen and nitrogen anion contents.2 Nitrogen anions raise the bottom of the valence band and thus reduce the band gap. Optimized electronic structures allow us to design materials, such as pigments and photocatalysts, for water splitting.3 The ordering and local structure of oxygen and nitrogen anions can provide additional freedom for designing electronic structures,4 which can bring about ferroelectricity5 and optical absorption.6 Most of the syntheses of perovskite oxynitrides require large amounts of ammonia gas,7 and control of the morphology, composition, and structure is not simple. Perovskite oxynitrides have often been synthesized by heating oxides under an ammonia flow above 773 K for tens of hours.7b The thermodynamic driving force is the formation of water vapor, which is purged by the ammonia flow. Thus, the synthesis conditions, namely, the starting materials, temperature, time, and flow rates of ammonia and water, should be optimized in order to control the morphology, composition, and structure. The other approach is solid-state synthesis under a high pressure because a high pressure stabilizes nitrogen anions and prevents the formation of nitrogen molecules.7b Thin films of perovskite oxynitrides have been synthesized under high-vacuum conditions.8 The formation of a thermodynamically stable salt as a byproduct is the general strategy to obtain the desired products.9 For example, MoS2 has been synthesized in seconds via the exothermic reaction between MoCl5 and Na2S, with the formation of NaCl as a byproduct.9a Nitrides have been synthesized by such exothermic reactions using choroids as starting materials.9b−d An example of the synthesis of a nitride is that of GaN nanoparticles from GaCl3 and Li3N in a benzene solution at 553 K with the formation of LiCl as a byproduct.9e We © XXXX American Chemical Society

Figure 1. Exothermic reaction between Ba(OH)2, NbCl5, and NaNH2 powders in an argon atmosphere: (a) mixing of starting materials; (b) explosion; (c) product. Received: October 16, 2017

A

DOI: 10.1021/acs.inorgchem.7b02660 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

Inorganic Chemistry

Figure 2. XRD patterns of product synthesized instantly by mixing Ba(OH)2, NbCl5, and NaNH2 without hexane (a) before washing and (b) after washing. The Ba(OH)2/NbCl5/NaNH2 ratio was 1:1:10.

Figure 3. XRD patterns of products obtained with different amounts of hexane: (a) 0.5 mL; (b) 1 mL; (c) 5 mL. The Ba(OH)2/NbCl5/NaNH2 ratio was 0.9:1:10.

solution. The main peaks in the pattern obtained before washing were attributed to BaNbO2N and NaCl. Excess NaNH2 was also detected. After washing, BaNbO2N and BaCO3 were found. The appearance of BaCO3 after washing would be attributed to the reaction of residual Ba(OH)2 with atmospheric CO2. A broad peak was also observed at around 28°, indicating the formation of amorphous phase(s). These reactions were reproducible in terms of the observation of an explosive reaction and the XRD patterns of the products (Figure S1). Thus, the explosive reaction produced a perovskite oxynitride phase in seconds, although impurities were also formed. In order to control the reaction to reduce impurities, it was performed with 1 mL of hexane. Ba(OH)2, NbCl5, and NaNH2 were dispersed in 1 mL of hexane, which prevented the explosive reaction upon mixing. The oxynitrides were produced following subsequent heat treatment at 498 K for 20 h with hexane. The reaction featuring hexane likely involved the explosive reaction because the powder and solution were spattered inside the Teflon liner after heat treatment. The optimal ratio of starting materials for the synthesis of oxynitrides with fewer impurities was found to be 0.9:1:10 (mol) Ba(OH)2/NbCl5/NaNH2 (Figures S2 and S3). Hence, subsequent experiments were performed with this optimal molar ratio in hexane. Figure 3 shows the XRD patterns of the products synthesized in different amounts of hexane. When 0.5 or 1 mL of hexane was used, BaNbO2N was reproducibly produced as the main phase (Figure S4). The use of 1 mL led to the fewest impurities. The crystalline size of BaNbO2N estimated by the peak width was 18−25 nm. On the other hand, the addition of 5 mL led to no BaNbO2N phase; only BaCO3 peaks and a broad peak at around 28° were observed. The scanning electron microscopy (SEM) image and auger electron spectroscopy profile of the product are shown in Figure 4. Powders smaller than a few hundred nanometers in size form aggregates. The Ba/Nb/Na/N ratio of the product was semiquantitatively determined to be 1:0.9:0.1:0.8. No chlorine was detected. The amount of oxygen was not quantified because the oxygen signal was partially attributed to carbon tape. The reaction to produce BaNbO2N from Ba(OH)2, NbCl5, and NaNH2 involved the formation of NaCl. Thus, the reaction can be expressed as follows:

Figure 4. SEM image and auger electron spectroscopy profile of the product synthesized with 1 mL of hexane. The Ba(OH)2/NbCl5/ NaNH2 ratio was 0.9:1:10.

Ba(OH)2 + NbCl5 + 5NaNH 2 → BaNbO2 N + 5NaCl + 4NH3

The enthalpy of formation of BaNbO2N at 0 K calculated using density functional theory is −1102 kJ/mol. Considering the reported enthalpies of other compounds at 298 K (Table S1),12 the thermodynamic driving force of this reaction would be the formation of perovskite oxynitride and NaCl salt. The enthalpy of the above reaction is estimated to be −981 kJ/mol. Therefore, this is a highly exothermic reaction, which is presumably the cause of the explosion even during mixing at B

DOI: 10.1021/acs.inorgchem.7b02660 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

Inorganic Chemistry

produces barium niobium perovskite oxynitride from Ba(OH)2, NbCl5, and NaNH2 was found to be highly exothermic and instantly produced perovskite oxynitrides with NaCl as a byproduct even at room temperature in an argon atmosphere. The risk of explosion can be reduced by adding hexane, which reduces the contact area between the powders and manages the heat generated in this exothermic reaction. Similar reactions were conducted for tantalum perovskite oxynitrides. Although a safety issue for practical usage remains a further challenge, this interesting reaction shows that a long-term heat treatment is not an essential requirement for the synthesis of perovskite oxynitrides and suggests the potential for the rapid syntheses of various perovskite oxynitrides.

room temperature in an inert atmosphere. Moreover, the increase in the entropy of this reaction by the generation of NH3 gas is thermodynamically favorable. As a result, perovskite oxynitrides can be formed instantly in this highly exothermic reaction. In order to scale these reactions up safely, it is essential to measure the temperature and pressure in advance. Although the actual reaction temperatures were not measured, hexane allowed the reaction to occur more homogeneously by managing the temperature. Dispersing the starting materials in hexane prevented the explosive reaction by reducing the contact area between the powders upon mixing at room temperature. During the heating of this mixture, hexane could homogeneously distribute the heat generated by the reaction, which would be useful for the synthesis with fewer impurities. When 0.5 or 1 mL of hexane was used, the perovskite oxynitride was formed, but perovskite oxynitrides did not form when 5 mL of hexane was used. The heat generated in the above synthesis reaction for BaNbO2N was calculated to be 490 J; this calculation was performed for 0.5 mmol of Ba(OH)2 and NbCl5, the values of which are close to the experimental values of 0.45 and 0.50, respectively. Assuming that 1 mL of hexane, which has a heat capacity of 195 J/mol·K,12 was used, the maximum temperature of the reaction would be ca. 798 K; the heating temperature was 498 K, and the maximum increase in the temperature was derived from the reaction energy of 490 J and the heat capacity of 195 J/ mol·K. On the other hand, under the same assumption, the maximum temperature can be reduced to 558 K by using 5 mL of hexane. Therefore, the addition of appropreciate amounts of hexane can reduce the risk of explosion during mixing by reducing the contact areas of the starting powders and can manage the heat of the exothermic reaction that produces perovskite oxynitrides. It is likely that sodium was incorporated into the synthesized perovskite phase, according to auger electron spectroscopy. When the amount of Ba(OH)2 was stoichiometric or in excess, more BaCO3 was formed as an impurity. NaNH2 is in excess with respect to Ba and Nb, suggesting the formation of a Ba/Na/Nb/ O/N perovskite oxynitride. The lattice parameter of perovskite oxynitrides is 0.4147(2) nm and is independent of the synthesis conditions with or without hexane. This value is slightly larger than that reported for BaNbO2N [0.41283(1) nm13 and 0.412829(2) nm14], indicating the substitution of Na+ (0.102 nm15) into Nb5+ sites (0.064 nm15). According to semiquantitative measurements and the increased lattice parameter, the synthesized perovskite oxynitride is Ba(Na0.1,Nb0.9)O2.1N0.8. Although quantitative analyses are essential for determining the detailed structure and composition, the amount of the product limits such analyses; the typical mass of the product was 20−70 mg. We also synthesized BaTaO2N under explosive reactions similar to those for the niobium oxynitride, which were not mentioned in the previous work.11 The reaction between Ba(OH)2, TaCl5, and NaNH2 without hexane caused an explosive reaction and gave snuff powders as the products (Figure S5a). With 1 mL of hexane, BaTaO2N was synthesized as the main phase with fewer impurities (Figure S5b). The XRD peaks were indexed as BaTaO2N with a lattice parameter of 0.41512(8) nm, which is close to the reported value of 0.41128(1) nm.13 The results suggest that this exothermic reaction can potentially be extended to the syntheses of other perovskite oxynitrides. In summary, we investigated an explosive reaction for the synthesis of niobium perovskite oxynitride. The reaction that

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b02660. Chemicals and characterization details, XRD patterns for optimizing experimental conditions and checking the reproducibility of BaNbO2N and BaTaO2N, and enthalpies of formation (PDF) Movie of an explosive reaction (AVI)

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (A.M.). ORCID

Akira Miura: 0000-0003-0388-9696 Nataly Carolina Rosero-Navarro: 0000-0001-6838-2875 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS Auger electron spectroscopy analyses were conducted at the Laboratory of XPS Analysis, Hokkaido University, supported by the “Nanotechnology Platform” Program of the Ministry of Education, Culture, Sports, Science and Technology, Japan. The experiments were partially supported by KAKENHI Grants 17H04950 and 17H03382 and the Nippon Sheet Glass Foundation for Materials Science and Engineering.

■

REFERENCES

(1) Fuertes, A. Chemistry and applications of oxynitride perovskites. J. Mater. Chem. 2012, 22, 3293. (2) Jansen, M.; Letschert, H. P. Inorganic yellow-red pigments without toxic metals. Nature 2000, 404, 980−982. (3) (a) Kasahara, A.; Nukumizu, K.; Hitoki, G.; Takata, T.; Kondo, J. N.; Hara, M.; Kobayashi, H.; Domen, K. Photoreactions on LaTiO2N under Visible Light Irradiation. J. Phys. Chem. A 2002, 106, 6750−6753. (b) Maeda, K.; Domen, K. Water oxidation using a particulate BaZrO3BaTaO2N solid-solution photocatalyst that operates under a wide range of visible light. Angew. Chem., Int. Ed. 2012, 51, 9865−9. (4) Yang, M.; Oró-Solé, J.; Rodgers, J. A.; Jorge, A. B.; Fuertes, A.; Attfield, J. P. Anion order in perovskite oxynitrides. Nat. Chem. 2011, 3, 47−52. (5) Kikkawa, S.; Sun, S.; Masubuchi, Y.; Nagamine, Y.; Shibahara, T. Ferroelectric Response Induced incis-Type Anion Ordered SrTaO2N Oxynitride Perovskite. Chem. Mater. 2016, 28 (5), 1312−1317. (6) Ziani, A.; Le Paven, C.; Le Gendre, L.; Marlec, F.; Benzerga, R.; Tessier, F.; Cheviré, F.; Hedhili, M. N.; Garcia-Esparza, A. T.; Melissen, S.; Sautet, P.; Le Bahers, T.; Takanabe, K. Photophysical Properties of C

DOI: 10.1021/acs.inorgchem.7b02660 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

Inorganic Chemistry SrTaO2N Thin Films and Influence of Anion Ordering: A Joint Theoretical and Experimental Investigation. Chem. Mater. 2017, 29, 3989−3998. (7) (a) Clarke, S. J.; Guinot, B. P.; Michie, C. W.; Calmont, M. J. C.; Rosseinsky, M. J. Oxynitride Perovskites: Synthesis and Structures of LaZrO2N, NdTiO2N, and LaTiO2N and Comparison with Oxide Perovskites. Chem. Mater. 2002, 14, 288−294. (b) Masubuchi, Y.; Sun, S. K.; Kikkawa, S. Processing of dielectric oxynitride perovskites for powders, ceramics, compacts and thin films. Dalton Trans. 2015, 44, 10570−10581. (c) Mikita, R.; Aharen, T.; Yamamoto, T.; Takeiri, F.; Ya, T.; Yoshimune, W.; Fujita, K.; Yoshida, S.; Tanaka, K.; Batuk, D.; Abakumov, A. M.; Brown, C. M.; Kobayashi, Y.; Kageyama, H. Topochemical Nitridation with Anion Vacancy-Assisted N(3‑)/O(2‑) Exchange. J. Am. Chem. Soc. 2016, 138, 3211−7. (8) Oka, D.; Hirose, Y.; Matsui, F.; Kamisaka, H.; Oguchi, T.; Maejima, N.; Nishikawa, H.; Muro, T.; Hayashi, K.; Hasegawa, T. Strain Engineering for Anion Arrangement in Perovskite Oxynitrides. ACS Nano 2017, 11, 3860−3866. (9) (a) Wiley, J. B.; Kaner, R. B. Rapid Solid-State Precursor Synthesis of Materials. Science 1992, 255, 1093−1097. (b) Gillan, E. G.; Kaner, R. B. Synthesis of Refractory Ceramics via Rapid Metathesis Reactions between Solid-State Precursors. Chem. Mater. 1996, 8, 333−343. (c) Mazumder, B.; Hector, A. L. Synthesis and applications of nanocrystalline nitride materials. J. Mater. Chem. 2009, 19, 4673− 4686. (d) Fitzmaurice, J. C.; Hector, A.; Rowley, A. T.; Parkin, I. P. Rapid, low energy synthesis of lanthanide nitrides. Polyhedron 1994, 13 (2), 235−240. (e) Xie, Y.; Qian, Y.; Wang, W.; Zhang, S.; Zhang, Y. A Benzene-Thermal Synthetic Route to Nanocrystalline GaN. Science 1996, 272, 1926−1927. (10) (a) Miura, A.; Takei, T.; Kumada, N. Synthesis of Wurtzite-Type InN Crystals by Low-Temperature Nitridation of LiInO2 Using NaNH2 Flux. Cryst. Growth Des. 2012, 12, 4545−4547. (b) Miura, A.; Takei, T.; Kumada, N. Low-Temperature Nitridation of Manganese and Iron Oxides Using NaNH2 Molten Salt. Inorg. Chem. 2013, 52, 11787− 11791. (c) Miura, A.; Takei, T.; Kumada, N. Synthesis of Cu3N from CuO and NaNH2. J. Asian Ceram. Soc. 2014, 2 (4), 326−328. (d) Miura, A.; Rosero-Navarro, C.; Masubuchi, Y.; Higuchi, M.; Kikkawa, S.; Tadanaga, K. Nitrogen-Rich Manganese Oxynitrides with Enhanced Catalytic Activity in the Oxygen Reduction Reaction. Angew. Chem., Int. Ed. 2016, 55, 7963−7. (e) Miura, A. Low-temperature synthesis and rational design of nitrides and oxynitrides for novel functional material development. J. Ceram. Soc. Jpn. 2017, 125 (7), 552−558. (11) Setsuda, Y.; Maruyama, Y.; Izawa, C.; Watanabe, T. Lowtemperature synthesis of BaTaO2N via the flux method using NaNH2. Chem. Lett. 2017, 46, 987−989. (12) CRC Handbook of Chemistry and Physics; CRC Press: Boca Raton, FL, 2002; Vol. 83, pp 5-5−5-50. (13) Pors, F.; Marchand, R.; Laurent, Y.; Bacher, P.; Roult, G. Etude structurale des perovskites oxyazotees BaTaO2N et BaNbO2N: Structural study of BaTaO2N and BaNbO2N oxynitrided perovskites. Mater. Res. Bull. 1988, 23, 1447−1450. (14) Fujii, K.; Shimada, K.; Yashima, M. Crystal-structure and electrondensity analyses of the perovskitetype oxynitrides BaNbO2N and SrNbO2N through synchrotron X-ray powder diffraction. J. Ceram. Soc. Jpn. 2017, 125, 808−810. (15) Shannon, R. D. Revised effective ionic radii and systematic studies of interatomic distances in halides and chalcogenides. Acta Crystallogr., Sect. A: Cryst. Phys., Diffr., Theor. Gen. Crystallogr. 1976, 32, 751−767.

D

DOI: 10.1021/acs.inorgchem.7b02660 Inorg. Chem. XXXX, XXX, XXX−XXX