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A rapid hydrothermal synthesis route for nanocrystalline SrZrO3 using reactive precursors
Materials Science and Engineering B 119 (2005) 87–93
A rapid hydrothermal synthesis route for nanocrystalline SrZrO3 using reactive precursors Anjali A. Athawale a, ∗ , Asha Chandwadkar b , P. Karandikar b , Malini Bapat a b
a Department of Chemistry, University of Pune, Pune 411007, India Catalysis Division, National Chemical Laboratory, Dr. Homi Bhabha Road, Pune 411008, India
Received 6 October 2004; received in revised form 30 December 2004; accepted 12 January 2005
Abstract Synthesis of nanocrystalline, orthorhombic (space group Pnma) SrZrO3 was achieved using appropriate molar proportions of nitrate salts of strontium and zirconium as precursors. Formation of SrZrO3 (SZ) was facilitated hydrothermally in time period as less as half-an-hour, in aqueous medium with alkaline pH as confirmed by XRD. Sample characterization was performed by FT-IR spectroscopy, powder XRD, SEM, TEM and DSC. The results have been explained by proposing a suitable reaction mechanism based on the step-wise analysis of the reaction intermediates using FT-IR as well as XRD. A double cation hydroxide composite formation is anticipated that under hydrothermal conditions results in the formation of SrZrO3 . Morphology of the samples was determined with the help of SEM and TEM. The particle size distribution as seen from the TEM micrograph lies in the range of 15–25 nm. Heats of reaction for different decomposition steps were obtained by DSC. © 2005 Elsevier B.V. All rights reserved. Keywords: Ceramics; Crystallization; Diffraction; Electron microscopy; Infrared spectroscopy
1. Introduction Alkaline earth zirconates are known for their uses as electrical ceramics, refractory materials, SOFCs and hydrogen/steam sensors due to protonic conduction at higher temperatures when doped with acceptor ions [1]. Conventional solid state reactions as also several other recent reports for synthesis of SrZrO3 require high temperature calcination of precursors as a necessary condition for phase formation resulting in undesirable powder properties alternately consuming excess energy thereby, contributing to increased enthalpy [2]. On the other hand, hydrothermal synthesis for zirconates is advantageous due to its ease of handling, control of morphology, particle size of the powder, level of agglomeration through selection of starting materials, pH, time, temperature and has been corroborated by thermodynamic models [3]. Synthesis at ambient conditions and in aqueous solutions ∗
Corresponding author. Tel.: +91 20 25601229x566; fax: +91 20 25691728. E-mail address: [email protected] (A.A. Athawale). 0921-5107/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2005.01.007
generally requires highly alkaline pH [4–6]. Water, a natural solvent is known to exhibit unique properties under supercritical conditions and this has been suitably exploited to promote dissolution, diffusion, adsorption, reaction rate, nucleation and growth for the synthesis of ceramics [7]. However, the limitations posed by the reported hydrothermal methods for formation of SrZrO3 are: prolonged periods of synthesis from 24–60 h, particle sizes in microns, co-existence of carbonate phase and higher temperatures for phase formation [8,9]. In the present work, we report the synthesis of nanocrystalline (∼15–25 nm) strontium zirconate (SZ) at temperatures ∼160 ◦ C with hydrothermal activation time as low as 30 min in purely aqueous medium using Sr(NO3 )2 and ZrO(NO3 )2 ·xH2 O precursor salts. 2. Experimental All the chemicals used were of A.R. grade. The precursors i.e. Sr(NO3 )2 and ZrO(NO3 )2 ·xH2 O (Loba Chemie, India) were taken in an agate mortar pestle with a concentration of 0.01 M so as to give an equimolar ratio of 1:1. The mixing
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of the powders was facilitated by the addition of a few drops of concentrated (16 N) HNO3 (Qualigens, India). The mixture was heated for ∼7–10 min to expel off nitrates. KOH (Thomas Baker, India) was then added in slight excess (five times in molar proportion) and ground with the dried mass. Few ml of boiled, double distilled water was added intermittently and grinding was continued to form a slurry. The slurry was transferred to a stainless steel Teflon-lined autoclave and subjected to hydrothermal treatment under autogenous pressure. Synthesis was carried out by varying the hydrothermal activation time.
reaction forming SrZrO3 on hydrothermal treatment. The heat evolved during the continuous formation and decomposition of the by-product KNO3 in the reaction mixture probably assists in driving this reaction. The diffusion of solid particles of the reactants at ambient temperature is short-ranged and aid in yielding nanosized products. The probable reactions involved in the formation of the product are: ZrO(NO3 )2 · xH2 O + Sr(NO3 )2 2HNO3
−→ SrZrO(NO3 )4 + NO2 ↑ + 21 O2 ↑ + (x + 1) H2 O ∆
(1)
3. Characterization 2SrZrO(NO3 )4 + 4KOH Fourier transform-infrared (FT-IR) spectroscopy was recorded in the region 4000–400 cm−1 on a Shimadzu 8400 infrared spectrophotometer using KBr as a reference material. X-ray analysis of the powders was done on a Rigaku miniflex X-ray diffractometer using Ni filter and Cu K␣ radiation. Scanning electron microscopic (SEM) pictures were scanned on a Cambridge Instrument (Stereoscan S120, Leica 440). Transmission electron micrographs (TEM) were taken on a Philips Model CM-200 instrument with an accelerating voltage of 200 kV. DSC curves were recorded using a PerkinElmer differential scanning calorimeter model DSC-7 in the temperature range between 50 and 450 ◦ C in N2 atmosphere. 4. Results and discussions The highly acidic nitrate precursors on thermal treatment result in the formation of a composite along with the simultaneous decomposition of the nitrate anions releasing in situ oxygen. On addition of KOH in slight excess, a highly exothermic reaction occurs resulting in the co-precipitation of a cation composite hydroxide salt with the formation of KNO3 salt as the by-product. The constant formation and precipitation of the molten KNO3 salt in the saturated aqueous reaction mixture assists in the formation of SrZrO3 crystallites together with KNO3 . The Zr4+ ion is relatively large, highly charged and spherical with no stereochemical preference therefore, its compounds exhibit high coordination numbers and a variety of coordination polyhedra. The release of nascent oxygen formed due to decomposition of the nitrate ions aid in Zr–O–Zr bridging. However, on addition of acid, the +4 Zr ion tends to get extensively hydrolyzed in the highly acidic medium and exist as tetranuclear [Zr4 (OH)8 (H2 O)16 ]8+ and octanuclear [Zr8 (OH)20 (H2 O)24 ]12+ complex species which co-exist in equilibrium. Conversely, under basic conditions, the tetranuclear units aggregate as two-dimensional sheets that form a gel or precipitate [10]. In the present case, the nitro and hydroxo-bridges probably co-exist within the zirconyl polymeric gel while Sr2+ cations trapped in this polyhedra give rise to the hydrated zirconates in the highly basic medium (pH 13–14) followed by dehydroxylation
→ Sr2 Zr2 O2 (OH)4 (NO3 )4 + 4KNO3
(2)
Sr2 Zr2 O2 (OH)4 (NO3 )4 Atm. CO2
−→ Sr2 Zr2 O5 (CO3 ) + 4NO2 ↑ +2H2 O ↑
(3)
Sr2 Zr2 O2 (OH)4 (NO3 )4 Hydrothermal activation
−→
SrZrO3 + 2H2 O + 4NO2 ↑ +O2 ↑ (4)
Accordingly, the reaction pathway has been explained on the basis of FT-IR and XRD. The major steps involved being: • Formation of a salt mixture; • Evolution of a composite species and expulsion of the nitrates; • Formation of a cation composite hydroxide on addition of alkali; and • Decomposition of the composite hydroxide complex yielding strontium zirconate on hydrothermal treatment. Scheme 1 depicts the synthesis route of nanocrystalline strontium zirconate. Fig. 1 shows the set of FT-IR spectra
Scheme 1. Flow chart depicting synthesis route of nanosized strontium zirconate.
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Fig. 1. IR spectra of the precursor salts, reaction intermediates and orthorhombic SrZrO3 .
recorded for the precursor salts (Sr and Zr nitrates) together with the product obtained at every stage of the reaction procedure towards synthesis of SrZrO3 . The peaks characteristic of nitrate (NO3 − ) group in the FT-IR (Fig. 1A) are observed at 1438.8 cm−1 (νN O ) and 1377.1 cm−1 in case of Sr(NO3 )2 . The peaks for nitrate group in the zirconyl salt appear at 1554.5 cm−1 , 1515.9 cm−1 , 1384.8 cm−1 and 1280.6 cm−1 (Fig. 1B). Moreover, the hydrated salt shows typical anti-symmetric stretching at 3531.4 cm−1 and HOH bending mode at 1610.5 cm−1 . The addition of concentrated HNO3 at the outset not only assists in conversion of nitrates into a composite but also ensures neutralization of carbonates if present within the precursors (Fig. 1C). The major changes observed in the IR spectra after heating the salt mixture (Fig. 1D) are as follows: Shift in the zirconyl salt (O-NO2 ) peak from 1573.8 cm−1 to 1541.0 cm−1 , appearance of a broad band at 432.0 cm−1 which is formed due to merging of several small peaks in the region from 450–700 cm−1 indicating the formation of ZrO6 octahedra [11] and decrease in the intensity of the
band corresponding to the stretching mode of lattice water at 3535.3 cm−1 . These changes in the IR result as a consequence of loss of water of hydration and nitrogen oxides from the precursor nitrates. After addition of KOH to the reaction mixture, the FT-IR (Fig. 1E) shows a radical transformation in the region between 1300 and 1700 cm−1 . The peaks corresponding to the nitrate groups of SN and ZN appear to have merged into a single large band with maxima at 1479.3 cm−1 and a shoulder at 1580.0 cm−1 . The formation of a composite hydroxide salt is indicated by a single broad band at 3182.3 cm−1 . Appearance of small peaks at 1120.6 and 858.3 cm−1 corresponds to the emergence of carbonate species at this stage. The above product after hydrothermal treatment shows the Zr–O–Zr band at ∼450 cm−1 (Fig. 1F) while on calcination at 900 ◦ C for 2 h (Fig. 1G) results in a much sharper metal–oxygen band at 553 cm−1 with the complete absence of carbonate peak [12]. These observations are found to agree with the XRD patterns of the samples, which are given in Fig. 2(a–c). Distinct crystalline peaks of the cubic phase of
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Fig. 2. XRD of the intermediate samples: (a) precursor salts after heating in acidic media; (b) reactants and mineralizer paste; (c) paste after washing with water prior to hydrothermal treatment.
SN and ZN are seen in the precursor salt mixture on heating with HNO3 (Fig. 2a). This indicates that at this stage the salts exist as a composite, ionic dissociation being suppressed. Due to highly acidic pH, carbonates would tend to get neutralized at this stage. Also, the loss of nitrate evolved on heating produces an oxidizing effect that tends to bring about structural changes in the ZN salt (rearrangement). Fig. 2b shows the diffraction pattern pertaining to KOH added heated precursor salt mixture. Intense reflections matching those of oxycarbonate as well as hydroxynitrate are observed. The d-values of the former are well in agreement as reported by Gangadevi et al. [13]. Additionally, low intensity peaks assignable to KNO3 are observed too. Its formation has been confirmed by qualitative ‘brown ring test’. Fig. 2c shows a number of peaks of strontium oxycarbonate and nitrates. The product obtained after hydrothermal treatment appears distinctly as the major phase in the XRD (Fig. 3a–d). Further, hydrothermal treatment was carried out for the reaction mixture in a teflon-lined autoclave placed in an electric heating system for different time intervals with a minimum time period of half-an-hour to 8 h. The formation of SrZrO3 could be characterized by the 2θ values matching with the
orthorhombic phase for the XRD patterns obtained for each of the samples (Fig. 3). The XRDs of all samples obtained at different time periods confirm the presence of orthorhombic phase (Pnma) with lattice constants being a = 0.5814 nm, b = 0.8196 nm and c = 0.5792 nm. The particle size as calculated from the Debye formula lies between 20 and 32 nm. It is observed that the particle size does not change significantly with hydrothermal time of processing (Table 1) but longer time of hydrothermal treatment brings about increase in carbonate formation. Further, annealing of SrZrO3 (2 h hydrothermal treatment) at 400 ◦ C for 4 h does not seem to affect the crystal structure as no significant change is observed in the XRD given in Fig. 4a and b.The intensity of peaks related to the carbonate impuTable 1 Hydrothermal activation time vs. calculated particle size in nm Time (h)
Particle size calculated from d0 0 2 (nm)
0.5 1 2 8
27 29 32 25
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Fig. 3. XRDs obtained after hydrothermal treatment at various time intervals (a) 0.5 h (b) 1 h (c) 2 h and (d) 8 h.
rity were found to decrease. However, the sample is seen to undergo a phase transition directly from orthorhombic space group Pnma to the tetragonal space group (I4/mcm) on annealing at 900 ◦ C [14] for 4 h with the crystallinity getting scarcely affected (Fig. 4c).
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The role of KOH and KNO3 in the formation of SrZrO3 could be ascertained by carrying out two sets of experiments. The first set i.e. the KOH washed intermediate when subjected to hydrothermal treatment did not yield the desired product while, the second set of experiments involving the hydrothermal treatment of the KOH added reaction mixture at different time intervals showed distinct phase of orthorhombic SrZrO3 . The formation of the co-produced KNO3 seems to lower the overall reaction temperature which co-relates to lower crystallinity of the product SrZrO3 . If the reaction enthalpy is insufficient, then spread of the reaction to intervening layer is impeded and propagation through the molten flux fails. The mineralizer not only serves to maintain a highly alkaline pH to facilitate hydrothermal synthesis but also assists in co-producing KNO3 that lowers the heat of the main reaction thus enabling formation of the required phase [15]. The exact mechanism is still under study but the exclusive formation of the intermediate Sr2 Zr2 O5 CO3 can be ruled out since the reported temperature required for the formation of the product would have been no less than 600 ◦ C [16] which cannot be achieved in a teflon-lined autoclave with autogeneous pressures ranging up to 5 bar. Similar method of hydrothermal treatment using halide precursor salts resulted in the formation of crystalline SrCO3 in large quantity along with poor yields of SrZrO3 . Use of nitrate precursor salts is thus found to be most suitable and was used in our earlier work on BaZrO3 synthesis that has been reported [17]. Scanning electron micrographs were obtained both before as well as after hydrothermal treatment as shown in Fig. 5(a–f).
Fig. 4. XRD of SrZrO3 (a) annealed at 900 ◦ C for 4 h; (b) annealed at 400 ◦ C for 4 h; and (c) as-synthesized sample.
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Fig. 5. SEM images of the intermediates and product obtained at different time periods of hydrothermal treatment. (i) Alkaline precursor salt paste prior to hydrothermal treatment at (a) 4000× and (b) 10,000×, (ii) SrZrO3 after 0.5 h of hydrothermal treatment at (c) 1000× and (d) 4000×, (iii) SrZrO3 after 8.0 h of hydrothermal treatment at (e) 1000× and (f) 2500×.
Fig. 5a depicts the SEM of the paste showing a large number of homogeneously distributed agglomerates. At higher magnification (Fig. 5b) one can observe a number of protuberances covering the agglomerates, which represent the presence of crystalline KNO3 which is known to be formed as a by-product during the synthesis of SrZrO3 . Conversely, the micrographs of SrZrO3 observed on posthydrothermal treatment for a period of half-an-hour (Fig. 5c) shows the formation of clusters with size being ∼1–2 m, more clearly observed at higher magnification (Fig. 5d). Further, the product obtained at higher time span of hydrothermal treatment of 8 h (Fig. 5e and f) reveals the presence of uniformly globular, finer agglomerates with a lower degree of size dispersity. The TEM of the as synthesized, dry powder was taken by dispersing it in alcohol and suspending few drops of it on a 400-mesh carbon coated copper grid. The photograph was recorded after obtaining a relatively good dispersion. Fig. 6 reveals that the particle size of the sample varies between 15 and 25 nm. As observed from the micrograph, spherical nature of the particles in the ag-
glomerates for the 2 h hydrothermally-treated sample can be made out. Fig. 7 shows the DSC for the reaction intermediates obtained at each stage of the reaction together with that of the final product. A broad decomposition exotherm correlating to structural change within the reactants is observed (Fig. 7a) at a temperature of ∼44 ◦ C with high heat of reaction being −215.93 J/g. At elevated temperatures of ∼160 ◦ C an endothermic peak with H = 27.19 J/g is observed denoting a decrease in the order of the system followed by an endothermal reaction starting at ∼203 ◦ C and ending at ∼224 ◦ C. The high heat of reaction is determined to be 147.8 J/g which could be a consequence of re-crystallization of the precursors in acidic media. Further, the DSC of the KOH added sample was run. Radical changes are observed in the calorigram (Fig. 7b). An exothermic peak with maxima at 55 ◦ C is seen which is followed by a broad endothermal relaxation peak. Within this broad curve a sharp but well-defined endothermic peak at 134 ◦ C with H = 28.23 J/g is seen. Between the temperature range of 160–300 ◦ C, a broad but shallow crystallization
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exotherm is observed with the melting heat of the order of 32.42 J/g. The DSC of the hydrothermally synthesized (Fig. 7c) final product as well as the calcined (900 ◦ C for 2 h) powder show similar nature exhibiting stable nature of the powder run from room temperature to 450 ◦ C (Fig. 7d).
5. Conclusions Nanoparticles of SrZrO3 were synthesized hydrothermally using reactive precursors. The advantages of this process being low temperature and reduced synthesis time with small particle size of the powder. In the present work, an attempt has been made to study the reaction pathway leading to the rapid synthesis of the refractory ceramic.
Acknowledgements
Fig. 6. Transmission electron microscopic image of SrZrO3 obtained after 2 h of hydrothermal treatment.
We are grateful to ISRO for funding the research work, MACS Research Institute, Pune for the SEM and IIT–Powai (Mumbai) for the TEM facility.
References
Fig. 7. DSC of: (a) reactant precursors in acidic media; (b) alkaline paste of salts; (c) hydrothermally synthesized SrZrO3 ; (d) calcined sample of SrZrO3 (at 900 ◦ C).
[1] H. Iwahara, T. Yajima, T. Hibino, K. Ozaki, H. Suzuki, Solid State Ionics 61 (1993) 65. [2] E.K. Keler, A.K. Kuzestor, Zh. Prikl Khim. 34 (1961) 2146. [3] M.M. Lencka, E. Nielsen, A. Anderko, R.E. Riman, Chem. Mater. 9 (1997) 1116. [4] M. Leoni, P. Nanni, V. Buscaglia, J. Mater. Sci. Lett. 15 (1996) 1302. [5] P.P. Phule, D.C. Grundy, Mater. Sci. Eng. B. 23 (1994) 29. [6] M. Makarova, P.E. Kazin, D.D. Zaitsev, N.S. Eremina, Yu.D. Tret’yakov, M. Jansen, Inorg. Mater. 39 (5) (2003) 514. [7] A. Rabenau, Angew. Chem. Int. Ed. Engl. 24 (1985) 1026. [8] W. Zheng, W. Pang, G. Meng, Solid State Ionics 108 (1998) 37. [9] M.C.B. Lopez, G. Fourlaris, B. Rand, F.I. Riley, J. Am. Ceram. Soc. 82 (1999) 1777. [10] F.A. Cotton, G. Wilkinson, Advanced Inorganic Chemistry, 5th ed., Wiley, Chichester, 1988. [11] C.H. Perry, D.J. McCarthy, Phys. Rev. A 138 (1965) 1537. [12] C. Chen, W. Zhu, T. Yu, X. Chen, X. Yao, R. Gopalkrishnan, Surf. Coat. Technol. 167 (2003) 245–248. [13] T. Gangadevi, M. Subbarao, T.R.N. Kutty, Ind. J. Chem. 19A (1980) 309–312. [14] B.J. Kennedy, C.J. Howard, B.C. Chakoumakos, Phys. Rev. B 59 (1999) 4023. [15] I.P. Parkin, Chem. Soc. Rev. 25 (1996) 199–207. [16] V.B. Reddy, P.N. Mehrotra, Thermochim. Acta 31 (1979) 349– 355. [17] A.A. Athawale, M. Bapat, J. Meta. Nano. Mater. 23 (2005) 3– 6.
A rapid hydrothermal synthesis route for nanocrystalline SrZrO3 using reactive precursors Anjali A. Athawale a, ∗ , Asha Chandwadkar b , P. Karandikar b , Malini Bapat a b
a Department of Chemistry, University of Pune, Pune 411007, India Catalysis Division, National Chemical Laboratory, Dr. Homi Bhabha Road, Pune 411008, India
Received 6 October 2004; received in revised form 30 December 2004; accepted 12 January 2005
Abstract Synthesis of nanocrystalline, orthorhombic (space group Pnma) SrZrO3 was achieved using appropriate molar proportions of nitrate salts of strontium and zirconium as precursors. Formation of SrZrO3 (SZ) was facilitated hydrothermally in time period as less as half-an-hour, in aqueous medium with alkaline pH as confirmed by XRD. Sample characterization was performed by FT-IR spectroscopy, powder XRD, SEM, TEM and DSC. The results have been explained by proposing a suitable reaction mechanism based on the step-wise analysis of the reaction intermediates using FT-IR as well as XRD. A double cation hydroxide composite formation is anticipated that under hydrothermal conditions results in the formation of SrZrO3 . Morphology of the samples was determined with the help of SEM and TEM. The particle size distribution as seen from the TEM micrograph lies in the range of 15–25 nm. Heats of reaction for different decomposition steps were obtained by DSC. © 2005 Elsevier B.V. All rights reserved. Keywords: Ceramics; Crystallization; Diffraction; Electron microscopy; Infrared spectroscopy
1. Introduction Alkaline earth zirconates are known for their uses as electrical ceramics, refractory materials, SOFCs and hydrogen/steam sensors due to protonic conduction at higher temperatures when doped with acceptor ions [1]. Conventional solid state reactions as also several other recent reports for synthesis of SrZrO3 require high temperature calcination of precursors as a necessary condition for phase formation resulting in undesirable powder properties alternately consuming excess energy thereby, contributing to increased enthalpy [2]. On the other hand, hydrothermal synthesis for zirconates is advantageous due to its ease of handling, control of morphology, particle size of the powder, level of agglomeration through selection of starting materials, pH, time, temperature and has been corroborated by thermodynamic models [3]. Synthesis at ambient conditions and in aqueous solutions ∗
Corresponding author. Tel.: +91 20 25601229x566; fax: +91 20 25691728. E-mail address: [email protected] (A.A. Athawale). 0921-5107/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2005.01.007
generally requires highly alkaline pH [4–6]. Water, a natural solvent is known to exhibit unique properties under supercritical conditions and this has been suitably exploited to promote dissolution, diffusion, adsorption, reaction rate, nucleation and growth for the synthesis of ceramics [7]. However, the limitations posed by the reported hydrothermal methods for formation of SrZrO3 are: prolonged periods of synthesis from 24–60 h, particle sizes in microns, co-existence of carbonate phase and higher temperatures for phase formation [8,9]. In the present work, we report the synthesis of nanocrystalline (∼15–25 nm) strontium zirconate (SZ) at temperatures ∼160 ◦ C with hydrothermal activation time as low as 30 min in purely aqueous medium using Sr(NO3 )2 and ZrO(NO3 )2 ·xH2 O precursor salts. 2. Experimental All the chemicals used were of A.R. grade. The precursors i.e. Sr(NO3 )2 and ZrO(NO3 )2 ·xH2 O (Loba Chemie, India) were taken in an agate mortar pestle with a concentration of 0.01 M so as to give an equimolar ratio of 1:1. The mixing
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of the powders was facilitated by the addition of a few drops of concentrated (16 N) HNO3 (Qualigens, India). The mixture was heated for ∼7–10 min to expel off nitrates. KOH (Thomas Baker, India) was then added in slight excess (five times in molar proportion) and ground with the dried mass. Few ml of boiled, double distilled water was added intermittently and grinding was continued to form a slurry. The slurry was transferred to a stainless steel Teflon-lined autoclave and subjected to hydrothermal treatment under autogenous pressure. Synthesis was carried out by varying the hydrothermal activation time.
reaction forming SrZrO3 on hydrothermal treatment. The heat evolved during the continuous formation and decomposition of the by-product KNO3 in the reaction mixture probably assists in driving this reaction. The diffusion of solid particles of the reactants at ambient temperature is short-ranged and aid in yielding nanosized products. The probable reactions involved in the formation of the product are: ZrO(NO3 )2 · xH2 O + Sr(NO3 )2 2HNO3
−→ SrZrO(NO3 )4 + NO2 ↑ + 21 O2 ↑ + (x + 1) H2 O ∆
(1)
3. Characterization 2SrZrO(NO3 )4 + 4KOH Fourier transform-infrared (FT-IR) spectroscopy was recorded in the region 4000–400 cm−1 on a Shimadzu 8400 infrared spectrophotometer using KBr as a reference material. X-ray analysis of the powders was done on a Rigaku miniflex X-ray diffractometer using Ni filter and Cu K␣ radiation. Scanning electron microscopic (SEM) pictures were scanned on a Cambridge Instrument (Stereoscan S120, Leica 440). Transmission electron micrographs (TEM) were taken on a Philips Model CM-200 instrument with an accelerating voltage of 200 kV. DSC curves were recorded using a PerkinElmer differential scanning calorimeter model DSC-7 in the temperature range between 50 and 450 ◦ C in N2 atmosphere. 4. Results and discussions The highly acidic nitrate precursors on thermal treatment result in the formation of a composite along with the simultaneous decomposition of the nitrate anions releasing in situ oxygen. On addition of KOH in slight excess, a highly exothermic reaction occurs resulting in the co-precipitation of a cation composite hydroxide salt with the formation of KNO3 salt as the by-product. The constant formation and precipitation of the molten KNO3 salt in the saturated aqueous reaction mixture assists in the formation of SrZrO3 crystallites together with KNO3 . The Zr4+ ion is relatively large, highly charged and spherical with no stereochemical preference therefore, its compounds exhibit high coordination numbers and a variety of coordination polyhedra. The release of nascent oxygen formed due to decomposition of the nitrate ions aid in Zr–O–Zr bridging. However, on addition of acid, the +4 Zr ion tends to get extensively hydrolyzed in the highly acidic medium and exist as tetranuclear [Zr4 (OH)8 (H2 O)16 ]8+ and octanuclear [Zr8 (OH)20 (H2 O)24 ]12+ complex species which co-exist in equilibrium. Conversely, under basic conditions, the tetranuclear units aggregate as two-dimensional sheets that form a gel or precipitate [10]. In the present case, the nitro and hydroxo-bridges probably co-exist within the zirconyl polymeric gel while Sr2+ cations trapped in this polyhedra give rise to the hydrated zirconates in the highly basic medium (pH 13–14) followed by dehydroxylation
→ Sr2 Zr2 O2 (OH)4 (NO3 )4 + 4KNO3
(2)
Sr2 Zr2 O2 (OH)4 (NO3 )4 Atm. CO2
−→ Sr2 Zr2 O5 (CO3 ) + 4NO2 ↑ +2H2 O ↑
(3)
Sr2 Zr2 O2 (OH)4 (NO3 )4 Hydrothermal activation
−→
SrZrO3 + 2H2 O + 4NO2 ↑ +O2 ↑ (4)
Accordingly, the reaction pathway has been explained on the basis of FT-IR and XRD. The major steps involved being: • Formation of a salt mixture; • Evolution of a composite species and expulsion of the nitrates; • Formation of a cation composite hydroxide on addition of alkali; and • Decomposition of the composite hydroxide complex yielding strontium zirconate on hydrothermal treatment. Scheme 1 depicts the synthesis route of nanocrystalline strontium zirconate. Fig. 1 shows the set of FT-IR spectra
Scheme 1. Flow chart depicting synthesis route of nanosized strontium zirconate.
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89
Fig. 1. IR spectra of the precursor salts, reaction intermediates and orthorhombic SrZrO3 .
recorded for the precursor salts (Sr and Zr nitrates) together with the product obtained at every stage of the reaction procedure towards synthesis of SrZrO3 . The peaks characteristic of nitrate (NO3 − ) group in the FT-IR (Fig. 1A) are observed at 1438.8 cm−1 (νN O ) and 1377.1 cm−1 in case of Sr(NO3 )2 . The peaks for nitrate group in the zirconyl salt appear at 1554.5 cm−1 , 1515.9 cm−1 , 1384.8 cm−1 and 1280.6 cm−1 (Fig. 1B). Moreover, the hydrated salt shows typical anti-symmetric stretching at 3531.4 cm−1 and HOH bending mode at 1610.5 cm−1 . The addition of concentrated HNO3 at the outset not only assists in conversion of nitrates into a composite but also ensures neutralization of carbonates if present within the precursors (Fig. 1C). The major changes observed in the IR spectra after heating the salt mixture (Fig. 1D) are as follows: Shift in the zirconyl salt (O-NO2 ) peak from 1573.8 cm−1 to 1541.0 cm−1 , appearance of a broad band at 432.0 cm−1 which is formed due to merging of several small peaks in the region from 450–700 cm−1 indicating the formation of ZrO6 octahedra [11] and decrease in the intensity of the
band corresponding to the stretching mode of lattice water at 3535.3 cm−1 . These changes in the IR result as a consequence of loss of water of hydration and nitrogen oxides from the precursor nitrates. After addition of KOH to the reaction mixture, the FT-IR (Fig. 1E) shows a radical transformation in the region between 1300 and 1700 cm−1 . The peaks corresponding to the nitrate groups of SN and ZN appear to have merged into a single large band with maxima at 1479.3 cm−1 and a shoulder at 1580.0 cm−1 . The formation of a composite hydroxide salt is indicated by a single broad band at 3182.3 cm−1 . Appearance of small peaks at 1120.6 and 858.3 cm−1 corresponds to the emergence of carbonate species at this stage. The above product after hydrothermal treatment shows the Zr–O–Zr band at ∼450 cm−1 (Fig. 1F) while on calcination at 900 ◦ C for 2 h (Fig. 1G) results in a much sharper metal–oxygen band at 553 cm−1 with the complete absence of carbonate peak [12]. These observations are found to agree with the XRD patterns of the samples, which are given in Fig. 2(a–c). Distinct crystalline peaks of the cubic phase of
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Fig. 2. XRD of the intermediate samples: (a) precursor salts after heating in acidic media; (b) reactants and mineralizer paste; (c) paste after washing with water prior to hydrothermal treatment.
SN and ZN are seen in the precursor salt mixture on heating with HNO3 (Fig. 2a). This indicates that at this stage the salts exist as a composite, ionic dissociation being suppressed. Due to highly acidic pH, carbonates would tend to get neutralized at this stage. Also, the loss of nitrate evolved on heating produces an oxidizing effect that tends to bring about structural changes in the ZN salt (rearrangement). Fig. 2b shows the diffraction pattern pertaining to KOH added heated precursor salt mixture. Intense reflections matching those of oxycarbonate as well as hydroxynitrate are observed. The d-values of the former are well in agreement as reported by Gangadevi et al. [13]. Additionally, low intensity peaks assignable to KNO3 are observed too. Its formation has been confirmed by qualitative ‘brown ring test’. Fig. 2c shows a number of peaks of strontium oxycarbonate and nitrates. The product obtained after hydrothermal treatment appears distinctly as the major phase in the XRD (Fig. 3a–d). Further, hydrothermal treatment was carried out for the reaction mixture in a teflon-lined autoclave placed in an electric heating system for different time intervals with a minimum time period of half-an-hour to 8 h. The formation of SrZrO3 could be characterized by the 2θ values matching with the
orthorhombic phase for the XRD patterns obtained for each of the samples (Fig. 3). The XRDs of all samples obtained at different time periods confirm the presence of orthorhombic phase (Pnma) with lattice constants being a = 0.5814 nm, b = 0.8196 nm and c = 0.5792 nm. The particle size as calculated from the Debye formula lies between 20 and 32 nm. It is observed that the particle size does not change significantly with hydrothermal time of processing (Table 1) but longer time of hydrothermal treatment brings about increase in carbonate formation. Further, annealing of SrZrO3 (2 h hydrothermal treatment) at 400 ◦ C for 4 h does not seem to affect the crystal structure as no significant change is observed in the XRD given in Fig. 4a and b.The intensity of peaks related to the carbonate impuTable 1 Hydrothermal activation time vs. calculated particle size in nm Time (h)
Particle size calculated from d0 0 2 (nm)
0.5 1 2 8
27 29 32 25
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Fig. 3. XRDs obtained after hydrothermal treatment at various time intervals (a) 0.5 h (b) 1 h (c) 2 h and (d) 8 h.
rity were found to decrease. However, the sample is seen to undergo a phase transition directly from orthorhombic space group Pnma to the tetragonal space group (I4/mcm) on annealing at 900 ◦ C [14] for 4 h with the crystallinity getting scarcely affected (Fig. 4c).
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The role of KOH and KNO3 in the formation of SrZrO3 could be ascertained by carrying out two sets of experiments. The first set i.e. the KOH washed intermediate when subjected to hydrothermal treatment did not yield the desired product while, the second set of experiments involving the hydrothermal treatment of the KOH added reaction mixture at different time intervals showed distinct phase of orthorhombic SrZrO3 . The formation of the co-produced KNO3 seems to lower the overall reaction temperature which co-relates to lower crystallinity of the product SrZrO3 . If the reaction enthalpy is insufficient, then spread of the reaction to intervening layer is impeded and propagation through the molten flux fails. The mineralizer not only serves to maintain a highly alkaline pH to facilitate hydrothermal synthesis but also assists in co-producing KNO3 that lowers the heat of the main reaction thus enabling formation of the required phase [15]. The exact mechanism is still under study but the exclusive formation of the intermediate Sr2 Zr2 O5 CO3 can be ruled out since the reported temperature required for the formation of the product would have been no less than 600 ◦ C [16] which cannot be achieved in a teflon-lined autoclave with autogeneous pressures ranging up to 5 bar. Similar method of hydrothermal treatment using halide precursor salts resulted in the formation of crystalline SrCO3 in large quantity along with poor yields of SrZrO3 . Use of nitrate precursor salts is thus found to be most suitable and was used in our earlier work on BaZrO3 synthesis that has been reported [17]. Scanning electron micrographs were obtained both before as well as after hydrothermal treatment as shown in Fig. 5(a–f).
Fig. 4. XRD of SrZrO3 (a) annealed at 900 ◦ C for 4 h; (b) annealed at 400 ◦ C for 4 h; and (c) as-synthesized sample.
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Fig. 5. SEM images of the intermediates and product obtained at different time periods of hydrothermal treatment. (i) Alkaline precursor salt paste prior to hydrothermal treatment at (a) 4000× and (b) 10,000×, (ii) SrZrO3 after 0.5 h of hydrothermal treatment at (c) 1000× and (d) 4000×, (iii) SrZrO3 after 8.0 h of hydrothermal treatment at (e) 1000× and (f) 2500×.
Fig. 5a depicts the SEM of the paste showing a large number of homogeneously distributed agglomerates. At higher magnification (Fig. 5b) one can observe a number of protuberances covering the agglomerates, which represent the presence of crystalline KNO3 which is known to be formed as a by-product during the synthesis of SrZrO3 . Conversely, the micrographs of SrZrO3 observed on posthydrothermal treatment for a period of half-an-hour (Fig. 5c) shows the formation of clusters with size being ∼1–2 m, more clearly observed at higher magnification (Fig. 5d). Further, the product obtained at higher time span of hydrothermal treatment of 8 h (Fig. 5e and f) reveals the presence of uniformly globular, finer agglomerates with a lower degree of size dispersity. The TEM of the as synthesized, dry powder was taken by dispersing it in alcohol and suspending few drops of it on a 400-mesh carbon coated copper grid. The photograph was recorded after obtaining a relatively good dispersion. Fig. 6 reveals that the particle size of the sample varies between 15 and 25 nm. As observed from the micrograph, spherical nature of the particles in the ag-
glomerates for the 2 h hydrothermally-treated sample can be made out. Fig. 7 shows the DSC for the reaction intermediates obtained at each stage of the reaction together with that of the final product. A broad decomposition exotherm correlating to structural change within the reactants is observed (Fig. 7a) at a temperature of ∼44 ◦ C with high heat of reaction being −215.93 J/g. At elevated temperatures of ∼160 ◦ C an endothermic peak with H = 27.19 J/g is observed denoting a decrease in the order of the system followed by an endothermal reaction starting at ∼203 ◦ C and ending at ∼224 ◦ C. The high heat of reaction is determined to be 147.8 J/g which could be a consequence of re-crystallization of the precursors in acidic media. Further, the DSC of the KOH added sample was run. Radical changes are observed in the calorigram (Fig. 7b). An exothermic peak with maxima at 55 ◦ C is seen which is followed by a broad endothermal relaxation peak. Within this broad curve a sharp but well-defined endothermic peak at 134 ◦ C with H = 28.23 J/g is seen. Between the temperature range of 160–300 ◦ C, a broad but shallow crystallization
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exotherm is observed with the melting heat of the order of 32.42 J/g. The DSC of the hydrothermally synthesized (Fig. 7c) final product as well as the calcined (900 ◦ C for 2 h) powder show similar nature exhibiting stable nature of the powder run from room temperature to 450 ◦ C (Fig. 7d).
5. Conclusions Nanoparticles of SrZrO3 were synthesized hydrothermally using reactive precursors. The advantages of this process being low temperature and reduced synthesis time with small particle size of the powder. In the present work, an attempt has been made to study the reaction pathway leading to the rapid synthesis of the refractory ceramic.
Acknowledgements
Fig. 6. Transmission electron microscopic image of SrZrO3 obtained after 2 h of hydrothermal treatment.
We are grateful to ISRO for funding the research work, MACS Research Institute, Pune for the SEM and IIT–Powai (Mumbai) for the TEM facility.
References
Fig. 7. DSC of: (a) reactant precursors in acidic media; (b) alkaline paste of salts; (c) hydrothermally synthesized SrZrO3 ; (d) calcined sample of SrZrO3 (at 900 ◦ C).
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