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Sol–gel synthesis of macroporous barium zirconate monoliths from ionic precursors via a phase separation route
Journal of Physics and Chemistry of Solids 102 (2017) 105–109
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Journal of Physics and Chemistry of Solids journal homepage: www.elsevier.com/locate/jpcs
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Sol–gel synthesis of macroporous barium zirconate monoliths from ionic precursors via a phase separation route Xingzhong Guo, Zichen Wang, Jie Song, Hui Yang
⁎
School of Materials Science and Engineering, Zhejiang University, Hangzhou, 310027, China
A R T I C L E I N F O
A BS T RAC T
Keywords: Barium zirconate Porous monolith Sol–gel Phase separation
Monolithic macroporous barium zirconate derived from ionic precursors has been successfully prepared via a phase separation route in the presence of poly(ethylene oxide) (PEO) and propyleneoxide (PO). Poly(ethylene oxide) (PEO) acts as a phase separation inducer, while propyleneoxide (PO) acts as a gelation accelerant in the sol–gel process. Appropriate choice of poly(ethylene oxide) (PEO) and propyleneoxide (PO) allows the production of continuous macroporous monolithic gel with a porosity of ca. 63% and a macropore size of 1.8 µm. Some BaCl2 recrystallizes in the dried gel, and subsequently tetragonal ZrO2 phase precipitates after heat-treated at 800 °C. The crystalline phase barium zirconate forms after heat treatment at 1100 °C in air, while the macroporous structure is preserved with a slight increase of porosity and a decrease of macropore size.
1. Introduction Barium zirconate (BaZrO3) is one of the excellent structural ceramic materials, and has been widely used as a high-temperature insulation material due to its good thermostability, low coefficient of thermal expansion and high mechanical strength [1–5]. Recently, barium zirconate was also applied in functional ceramic, fuel cells and catalysis fields because of other intrinsic properties, such as high dielectric constant, high proton conductivity with rare-earth doping and capability to generate hydrogen by water catalytic reaction [5–11]. Porous barium zirconate has larger specific surface area and lower density, which are expected to improve its application performances and broaden its application fields. However, there is no literature about preparation of porous barium zirconate materials. The sol–gel process accompanied by phase separation is one of the synthetic methods to fabricate porous monoliths [4,12–15]. This technique provides an easy way to prepare well-defined hierarchical pore structures with continuous macropores in a monolithic shape under low-temperature wet chemical reaction [16–20]. The porous zirconia based monoliths such as macroporous zirconia monoliths, macroporous yttria-stabilized zirconia monoliths and porous La2Zr2O7 monoliths have been successfully synthesized by this technique [21– 23]. So far, there have been adequate theoretical justifications to account for the mechanism of porous monoliths preparations by the sol–gel process accompanied by phase separation. It is promising that more and more macroporous monolithic materials can be fabricated via this technique.
⁎
In this study, we successfully synthesized macroporous barium zirconate with continuous skeletons by means of sol–gel process accompanied by phase separation. The ionic precursors are utilized to synthesize barium zirconate monolith in the presence of poly(ethylene oxide) (PEO) and propyleneoxide (PO). The major analyses were focused on the influence of phase separation inducer and gelation accelerant on the morphologies of the monoliths, and the effect of the heat-treatment on the phase transformations and pore structures. 2. Experimental 2.1. Synthesis procedure Zirconium oxychloride octahydrate (ZrOCl2·8H2O, Aladdin, 99.9%) and barium chloride dihydrate (BaCl2·2H2O, Aladdin, 99.95%) were used as precursors. Poly(ethylene oxide) (PEO, Aladdin) having average molecular weight (Mv) of 3×105 was used as a phaseseparation inducer, and ethylene glycol (EG, Sinopharm Chemical Reagent, 99.5%) was used as complex agent. Formamide (FA, Sinopharm Chemical Reagent, 99.5%) was used to control the drying process, and propyleneoxide (PO, Aldrich, 99%) was used as a gelation accelerant. For all the eight samples, the amounts of ZrOCl2·8H2O, BaCl2· 2H2O, H2O, EtOH, FA and EG are 1.610 g, 1.220 g, 6.6 ml, 3.6 ml, 0.2 ml and 0.4 ml, respectively. The variable reagents of barium zirconate gels prepared in this study are listed in Table 1. The sample gels were prepared as follows: ZrOCl2·8H2O, BaCl2·2H2O and PEO
Corresponding authors. E-mail addresses: [email protected] (X. Guo), [email protected] (H. Yang).
http://dx.doi.org/10.1016/j.jpcs.2016.11.012 Received 14 September 2016; Received in revised form 28 October 2016; Accepted 5 November 2016 Available online 23 November 2016 0022-3697/ © 2016 Elsevier Ltd. All rights reserved.
Journal of Physics and Chemistry of Solids 102 (2017) 105–109
X. Guo et al.
⎛ ⎞ φ φ ∆G=∆H −T ∆S =RT ⎜χ12 φ1 φ2 + 1 ln φ1+ 2 ln φ2⎟ ⎝ ⎠ P2 P1
Table 1 Variable compositions of different samples. Sample
PEO/g
PO/mL
Gelation times
BZ1 BZ2 BZ3 BZ4 BZ5 BZ6 BZ7 BZ8
0 0.03 0.10 0.12 0.10 0.10 0.10 0.10
0.36 0.36 0.36 0.36 0.0 0.20 0.60 0.70
5h 5h 5h 5h Failed gelation 8h 8 min 5 min
(5)
In this system, PEO has weak hydrogen bonds with inorganic oligomers and ethylene glycol (EG) complexing with Ba2+inhibits the connection between PEO and oligomers, so PEO mainly distributes in liquid phase during the phase separation. With the increase of the degree of polymerization (P), the compatibility between oligomers and PEO chain reduces, which causes the decrease of system entropy change (ΔS) and the increase of system Gibbs free energy change (ΔG). When ΔG turns into positive, the system phase separation happens (Eq. (1)) [28,29]. When propylene oxide (PO) is added to the solution, PO captures the dissociated protons and conducts irreversible ring-opening reaction as shown in Eq. (2)[24,30], which drastically increases the pH of the solution from 2 to 4 and induces the gelation transition to form monolithic complex gels. The sol–gel process accompanied by phase separation of Ba-Zr-O system can be summarized in Fig. 1. Fig. 2 shows SEM images of dried barium zirconate gels prepared with various PEO contents. The content of PEO used as a phase separation inducer determines the macroporous morphology of the gel. A dense monolithic gel sample without any clear pore structures is obtained when the PEO is not added (Fig. 2a). When the PEO content is 0.03 g, the as-dried sample shows some independent submicron pores but no continuous macropores and skeletons (Fig. 2b). When the PEO content increases to 0.10 g, continuous structures form with smooth skeletons and homogeneous macropores (Fig. 2c). The skeletons become thicker and the continuous structures are lost when 0.12 g PEO is added (Fig. 2d). As the phase separation inducer, different PEO contents prominently affect the phase separation tendency of the system and the morphology of as-dried samples. Fig. 3 shows SEM images of as-dried barium zirconate gels prepared with various PO contents. The contents of PO used as a gelation accelerant determines macroporous morphology of the gels as well. There is no gelation transition in the system when PO is not added. When the PO content is 0.2 mL, the system becomes gel with course skeletons and no clear continuous structures in 8 h (Fig. 3a). When the PO content is 0.36 mL, the gelation transition takes place in about 5 h to form gels with continuous and homogenous skeletons (Fig. 3b). When the PO content is 0.5 mL, the system gelates in 1 h, and the continuous skeletons become less obviously and the pore diameters become smaller (Fig. 3c). With the further increase of PO content to 0.6 mL, the system gelates in only 5–8 min with the loss of skeletons and the transition of continuous pores into independent micron and nanoscale pores (Fig. 3d). As the gelation accelerant, the moderate PO content and the proper relative content of PEO are two key factors on the formation of interconnected macroporous barium zirconate gels with continuous skeletons.
were dissolved in a mixture of H2O and EtOH under vigorous stirring at room temperature, and then EG was added into the mixture liquid. After 5 min, FA was added dropwise, then PO was added into the mixture liquid under 1 min stirring. The container together with the whole mixed solution was sealed and placed in 80 °C drying oven to gelate and age for 24 h. Then EtOH and a mixture of EtOH and TMOS were added into the container successively to exchange the solvent. After solvent exchange, the container was placed in 80 °C drying oven for 4–7 days to obtain dried gel. 2.2. Characterization of materials Porous morphologies of as-prepared samples with different compositions were observed by a SU-70 scanning electron microscope (SEM, HITACHI Corp.). Powder X-ray diffractions (XRD) of the heattreated as-prepared samples performed with Empyrean 200895 diffractometer (PANalytical B.V. Corp.) using Cu-Kα radiation (λ=0.154 nm). The nitrogen adsorption–desorption of as-prepared samples were characterized by an Autosorb-1-C nitrogen adsorption– desorption apparatus (Quantachrome Corp.). Before each nitrogen adsorption–desorption measurement, the as-prepared samples were degassed at 200 °C under vacuum for more than 6 h. Specific surface area and pore size distributions were calculated using the Brunauer– Emmett–Teller (BET) and Barrett–Joyner–Halenda (BJH), respectively. 3. Results and discussion 3.1. Formation of macroporous morphology In the present system, zirconium oxychloride octahydrate (ZrOCl2· 8H2O) and barium chloride dihydrate (BaCl2·2H2O) hydrolyze and polymerize together in the mixture of H2O and EtOH to prepare homogeneous and transparent solutions. The PH of the solution declines to 2 after co-hydrolysis of two ionic precursors. The phase separation and gelation of the solutions can be controlled by the addition of poly(ethylene oxide) (PEO) and propyleneoxide (PO). The phase separation and gelation process can be described by Flory– Huggins formulation and ring-opening reaction as follows [24,25]. The Eqs. (1)–(4) come from Flory–Huggins formulation. χ12 is the interaction parameter, φi and Pi are the volume fraction and the degree of polymericzation of component i (i=1 or 2), respectively, R is the gas constant, and T is the temperature. KB is the Kauri-butanol parameter. As for the system Gibbs free energy change (Eq. (4)), the former term in parenthesis represent the enthalpic contribution (H), and the last two terms the entropic contribution (S) [16,26,27].
∆H =RT (χ12 φ1 φ2 )
χ12 ∝
(δ1 − δ 2 )2 KB T
⎛φ ⎞ φ ∆S =−R ⎜ 1 ln φ1+ 2 ln φ2⎟ ⎝ P1 ⎠ P2
(4)
3.2. Heat-treatment and crystallization
(1)
Fig. 4 shows the XRD patterns of BZ3 sample after heat-treatment at different temperatures. It is noted that the diffraction peaks are weak in as-dried sample and the one heat-treated in 800 °C, indicating low crystallinity. Because the precursor BaCl2·2H2O does not dissolve in EtOH liquid, the BaCl2 concentration in the system nearly reaches its
(2)
(3) 106
Journal of Physics and Chemistry of Solids 102 (2017) 105–109
X. Guo et al.
Fig. 1. The sol–gel process accompanied by phase separation of Ba-Zr-O system..
volume shrinkage appears and the skeletons become dense and thick in the crystalline barium zirconate monoliths compared with the dried gel. Fig. 5 shows the SEM image of BZ3 sample after heat-treatment at 1100 °C. The well-defined cocontinuous macroporous mophology is basically maintained after heat-treatment, indicating that the formation of crystalline phase does not spoil the macroporous morphology of barium zirconate monoliths. There are also some small pores exsiting on the skeleton, which results form the volatilization of some organics. Fig. 6 shows the macropore size distributions of BZ3 sample before and after heat-treatment at 1100 °C, determined by mercury intrusion porosimetry. It is obvious that the BZ3 sample has the macroporous structure with a narrow pore size distribution from 1 to 2 µm nearly, which is caused by spinodal decomposition. The bulk density of as-
aqueous saturation concentration. In the process of gelation, BaCl2 recrystallizes and attached on the gel skeletons, so weak BaCl2 phase can be found in the as-dried sample. Tetragonal ZrO2 phase appears except for BaCl2 when the sample is heat-treated at 800 °C. During heat-treatment at 1100 °C, BaZrO3 occurs due to the solid-phase reaction between ZrO2 and BaCl2, and a few redundant tetragonal ZrO2 transforms to monoclinic ZrO2. With the increase of heattreatment temperature, the diffraction intensity becomes stronger and the diffraction shape becomes shaper, which indicates better crystallinity and larger grain size. As a result, stable BaZrO3 phase forms at 1100 °C with a few monoclinic ZrO2 phase. The variation of macroporous structure and properties of crystalline monoliths after heat-treatment was examined. It is found that 30%
Fig. 2. SEM images of dried barium zirconate gels prepared with various PEO contents; (a)-BZ1, (b)-BZ2, (c)-BZ3, (d)-BZ4.
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Journal of Physics and Chemistry of Solids 102 (2017) 105–109
X. Guo et al.
Fig. 3. SEM images of as-dried barium zirconate gels prepared with various PO contents; (a)-BZ6, (b)-BZ3,(c)-BZ7,(d)-BZ8.
Fig. 6. Macropore size distributions of BZ3 sample before and after being heat-treated at 1100 °C.
Fig. 4. XRD patterns of BZ3 sample before and after being heat-treated at different temperatures.
Fig. 7. N2 adsorption–desorption isotherms of sample BZ3 before and after heat-treated at 1100 °C.
Fig. 5. SEM image of BZ3 sample after being heat-treated at 1100 °C.
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Journal of Physics and Chemistry of Solids 102 (2017) 105–109
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dried BZ3 sample is 0.78 g·cm−3, measured by mercury intrusion porosimetry. After heat-treatment, the median pore diameter decreases from 1.8 to 1.5 µm and the porosity increases slightly from 63–65% with the volatilization of organics. Fig. 7 depicts N2 adsorption–desorption isotherms of sample BZ3 before and after heat-treatment at 1100 °C. The porous barium zirconate gels before heat-treatment exhibits the isotherm of Ⅳ-type according to the IUPAC classification, indicating that the sample has large amounts of mesopores and micropores. After heat-treatment, the porous barium zirconate gels exhibits the isotherm of Ⅲ-type. Under the low and middle pressure, N2 volume adsorption stays in a low amount. Under nearly 1.0 relative pressure, the volume adsorption obviously ascends due to macropores existed in the sample. After calculation, BET surface area reaches 188 m2·g−1 before heat-treatment and plunges to 0.8 m2·g−1 after heat-treatment. The micropores and mesopores have direct links with the sample's BET surface area, but the macropores do not have obvious effect on it, which accounts for the enormous change of the sample's BET surface area before and after heat-treatment. Although the continuous skeletons and interconnected macropores are retained after heat-treatment, the mesopores and micropores on the skeletons are lost due to solid phase reaction, crystal transfer and grain aggregation, which causes the substantial decrease of BET surface area.
References [1] X. Guo, Q. Zhang, X. Ding, Q. Shen, C. Wu, L. Zhang, H. Yang, J. Sol.-Gel Sci. Technol. (2016). [2] M.E. Bjorketun, P.G. Sundell, G. Wahnstrom, Faraday Discuss. 134 (2007) 247–265. [3] K.H. Ryu, S.M. Haile, Solid State Ion. 125 (1999) 355–367. [4] B. Robertz, F. Boschini, R. Cloots, A. Rulmont, Int J. Inorg. Mater. 3 (2001) 1185–1187. [5] C. Aruta, C. Han, S. Zhou, C. Cantoni, N. Yang, A. Tebano, T.L. Lee, C. Schlueter, A. Bongiorno, J. Phys. Chem. C. 120 (2016) 8387–8391. [6] P.P. Khirade, S.D. Birajdar, A.V. Humbe, K.M. Jadhav, J. Electron. Mater. 45 (2016) 3227–3235. [7] H.M. Zhang, A. Suresh, C.B. Carter, B.A. Wilhite, Solid State Ion. 266 (2014) 58–67. [8] T.S. Bjorheim, M. Arrigoni, S.W. Saeed, E. Kotomin, J. Maier, Chem. Mater. 28 (2016) 1363–1368. [9] S. Gopalan, A.V. Virkar, J. Am. Ceram. Soc. 82 (1999) 2887–2899. [10] A. Erb, E. Walker, R. Flukiger, Physica C. 258 (1996) 9–20. [11] T.S. Bjorheim, M. Arrigoni, D. Gryaznov, E. Kotomin, J. Maier, Phys. Chem. Chem. Phys. 17 (2015) 20765–20774. [12] K. Kanie, Y. Seino, M. Matsubara, M. Nakaya, A. Muramatsu, New J. Chem. 38 (2014) 3548–3555. [13] R. Borja-Urby, L.A. Diaz-Torres, I. Garcia-Martinez, D. Bahena-Uribe, G. Casillas, A. Ponce, M. Jose-Yacaman, Catal. Today 250 (2015) 95–101. [14] T. Charoonsuk, W. Vittayakorn, N. Vittayakorn, P. Seeharaj, S. Maensiri, Ceram. Int. 41 (2015) S87–S94. [15] X.Z. Guo, K. Nakanishi, K. Kanamori, Y. Zhu, H. Yang, J. Eur. Ceram. Soc. 34 (2014) 817–823. [16] Y. Tokudome, K. Fujita, K. Nakanishi, K. Miura, K. Hirao, Chem. Mater. 19 (2007) 3393–3398. [17] X.Z. Guo, W.J. Zhu, X.B. Cai, S.X. Liu, H. Yang, Mater. Des. 83 (2015) 314–319. [18] X.Z. Guo, R. Wang, H. Yu, Y. Zhu, K. Nakanishi, K. Kanamori, H. Yang, Dalton Trans. 44 (2015) 13592–13601. [19] K. Kanamori, M. Aizawa, K. Nakanishi, T. Hanada, Adv. Mater. 19 (2007) (1589O_). [20] K. Nakanishi, N. Tanaka, Acc. Chem. Res. 40 (2007) 863–873. [21] M.D. Gonçalves, R. Muccillo, Ceram. Int. 40 (2014) 911–917. [22] J. Konishi, K. Fujita, S. Oiwa, K. Nakanishi, K. Hirao, Chem. Mater. 20 (2008) 2165–2173. [23] S.X. Wang, W. Li, S. Wang, Z.H. Chen, J. Eur. Ceram. Soc. 35 (2015) 105–112. [24] A.E. Gash, T.M. Tillotson, J.H. Satcher, J.F. Poco, L.W. Hrubesh, R.L. Simpson, Chem. Mater. 13 (2001) 999–1007. [25] X.Z. Guo, G.S. Hao, Y. Xie, W.W. Cai, H. Yang, J. Sol.-Gel Sci. Technol. 76 (2015) 651–657. [26] K. Nakanishi, J. Porous Mater. 4 (1997) 67–112. [27] T. Amatani, K. Nakanishi, K. Hirao, T. Kodaira, Chem. Mater. 17 (2005) 2114–2119. [28] K. Nakanishi, R. Takahashi, T. Nagakane, K. Kitayama, N. Koheiya, H. Shikata, N. Soga, J. Sol.-Gel Sci. Technol. 17 (2000) 191–210. [29] X.Z. Guo, X.B. Cai, J. Song, Y. Zhu, K. Nakanishi, K. Kanamori, H. Yang, New J. Chem. 38 (2014) 5832–5839. [30] H. Yu, Y. Zhu, H. Yang, K. Nakanishi, K. Kanamori, X.Z. Guo, Dalton Trans. 43 (2014) 12648–12656.
4. Conclusions Monolithic macroporous barium zirconate materials are successfully prepared by sol–gel process accompanied by phase separation. The morphologies and structures of monolithic macroporous barium zirconate materials are closely connected with poly(ethylene oxide) (PEO) as phase separation inducer and propyleneoxide (PO) as gelation accelerant. It has been proved that continuous macroporous samples with interconnected skeletons can be synthesized by controlling the content of PEO as 0.1 g and the content of PO as 0.36 mL. Heattreatment develops the solid reaction to synthesize barium zirconate and retains the continuous macroporous structure. Acknowledgements This work is supported by the National Natural Science Foundation of China (51372225).
109
Contents lists available at ScienceDirect
Journal of Physics and Chemistry of Solids journal homepage: www.elsevier.com/locate/jpcs
crossmark
Sol–gel synthesis of macroporous barium zirconate monoliths from ionic precursors via a phase separation route Xingzhong Guo, Zichen Wang, Jie Song, Hui Yang
⁎
School of Materials Science and Engineering, Zhejiang University, Hangzhou, 310027, China
A R T I C L E I N F O
A BS T RAC T
Keywords: Barium zirconate Porous monolith Sol–gel Phase separation
Monolithic macroporous barium zirconate derived from ionic precursors has been successfully prepared via a phase separation route in the presence of poly(ethylene oxide) (PEO) and propyleneoxide (PO). Poly(ethylene oxide) (PEO) acts as a phase separation inducer, while propyleneoxide (PO) acts as a gelation accelerant in the sol–gel process. Appropriate choice of poly(ethylene oxide) (PEO) and propyleneoxide (PO) allows the production of continuous macroporous monolithic gel with a porosity of ca. 63% and a macropore size of 1.8 µm. Some BaCl2 recrystallizes in the dried gel, and subsequently tetragonal ZrO2 phase precipitates after heat-treated at 800 °C. The crystalline phase barium zirconate forms after heat treatment at 1100 °C in air, while the macroporous structure is preserved with a slight increase of porosity and a decrease of macropore size.
1. Introduction Barium zirconate (BaZrO3) is one of the excellent structural ceramic materials, and has been widely used as a high-temperature insulation material due to its good thermostability, low coefficient of thermal expansion and high mechanical strength [1–5]. Recently, barium zirconate was also applied in functional ceramic, fuel cells and catalysis fields because of other intrinsic properties, such as high dielectric constant, high proton conductivity with rare-earth doping and capability to generate hydrogen by water catalytic reaction [5–11]. Porous barium zirconate has larger specific surface area and lower density, which are expected to improve its application performances and broaden its application fields. However, there is no literature about preparation of porous barium zirconate materials. The sol–gel process accompanied by phase separation is one of the synthetic methods to fabricate porous monoliths [4,12–15]. This technique provides an easy way to prepare well-defined hierarchical pore structures with continuous macropores in a monolithic shape under low-temperature wet chemical reaction [16–20]. The porous zirconia based monoliths such as macroporous zirconia monoliths, macroporous yttria-stabilized zirconia monoliths and porous La2Zr2O7 monoliths have been successfully synthesized by this technique [21– 23]. So far, there have been adequate theoretical justifications to account for the mechanism of porous monoliths preparations by the sol–gel process accompanied by phase separation. It is promising that more and more macroporous monolithic materials can be fabricated via this technique.
⁎
In this study, we successfully synthesized macroporous barium zirconate with continuous skeletons by means of sol–gel process accompanied by phase separation. The ionic precursors are utilized to synthesize barium zirconate monolith in the presence of poly(ethylene oxide) (PEO) and propyleneoxide (PO). The major analyses were focused on the influence of phase separation inducer and gelation accelerant on the morphologies of the monoliths, and the effect of the heat-treatment on the phase transformations and pore structures. 2. Experimental 2.1. Synthesis procedure Zirconium oxychloride octahydrate (ZrOCl2·8H2O, Aladdin, 99.9%) and barium chloride dihydrate (BaCl2·2H2O, Aladdin, 99.95%) were used as precursors. Poly(ethylene oxide) (PEO, Aladdin) having average molecular weight (Mv) of 3×105 was used as a phaseseparation inducer, and ethylene glycol (EG, Sinopharm Chemical Reagent, 99.5%) was used as complex agent. Formamide (FA, Sinopharm Chemical Reagent, 99.5%) was used to control the drying process, and propyleneoxide (PO, Aldrich, 99%) was used as a gelation accelerant. For all the eight samples, the amounts of ZrOCl2·8H2O, BaCl2· 2H2O, H2O, EtOH, FA and EG are 1.610 g, 1.220 g, 6.6 ml, 3.6 ml, 0.2 ml and 0.4 ml, respectively. The variable reagents of barium zirconate gels prepared in this study are listed in Table 1. The sample gels were prepared as follows: ZrOCl2·8H2O, BaCl2·2H2O and PEO
Corresponding authors. E-mail addresses: [email protected] (X. Guo), [email protected] (H. Yang).
http://dx.doi.org/10.1016/j.jpcs.2016.11.012 Received 14 September 2016; Received in revised form 28 October 2016; Accepted 5 November 2016 Available online 23 November 2016 0022-3697/ © 2016 Elsevier Ltd. All rights reserved.
Journal of Physics and Chemistry of Solids 102 (2017) 105–109
X. Guo et al.
⎛ ⎞ φ φ ∆G=∆H −T ∆S =RT ⎜χ12 φ1 φ2 + 1 ln φ1+ 2 ln φ2⎟ ⎝ ⎠ P2 P1
Table 1 Variable compositions of different samples. Sample
PEO/g
PO/mL
Gelation times
BZ1 BZ2 BZ3 BZ4 BZ5 BZ6 BZ7 BZ8
0 0.03 0.10 0.12 0.10 0.10 0.10 0.10
0.36 0.36 0.36 0.36 0.0 0.20 0.60 0.70
5h 5h 5h 5h Failed gelation 8h 8 min 5 min
(5)
In this system, PEO has weak hydrogen bonds with inorganic oligomers and ethylene glycol (EG) complexing with Ba2+inhibits the connection between PEO and oligomers, so PEO mainly distributes in liquid phase during the phase separation. With the increase of the degree of polymerization (P), the compatibility between oligomers and PEO chain reduces, which causes the decrease of system entropy change (ΔS) and the increase of system Gibbs free energy change (ΔG). When ΔG turns into positive, the system phase separation happens (Eq. (1)) [28,29]. When propylene oxide (PO) is added to the solution, PO captures the dissociated protons and conducts irreversible ring-opening reaction as shown in Eq. (2)[24,30], which drastically increases the pH of the solution from 2 to 4 and induces the gelation transition to form monolithic complex gels. The sol–gel process accompanied by phase separation of Ba-Zr-O system can be summarized in Fig. 1. Fig. 2 shows SEM images of dried barium zirconate gels prepared with various PEO contents. The content of PEO used as a phase separation inducer determines the macroporous morphology of the gel. A dense monolithic gel sample without any clear pore structures is obtained when the PEO is not added (Fig. 2a). When the PEO content is 0.03 g, the as-dried sample shows some independent submicron pores but no continuous macropores and skeletons (Fig. 2b). When the PEO content increases to 0.10 g, continuous structures form with smooth skeletons and homogeneous macropores (Fig. 2c). The skeletons become thicker and the continuous structures are lost when 0.12 g PEO is added (Fig. 2d). As the phase separation inducer, different PEO contents prominently affect the phase separation tendency of the system and the morphology of as-dried samples. Fig. 3 shows SEM images of as-dried barium zirconate gels prepared with various PO contents. The contents of PO used as a gelation accelerant determines macroporous morphology of the gels as well. There is no gelation transition in the system when PO is not added. When the PO content is 0.2 mL, the system becomes gel with course skeletons and no clear continuous structures in 8 h (Fig. 3a). When the PO content is 0.36 mL, the gelation transition takes place in about 5 h to form gels with continuous and homogenous skeletons (Fig. 3b). When the PO content is 0.5 mL, the system gelates in 1 h, and the continuous skeletons become less obviously and the pore diameters become smaller (Fig. 3c). With the further increase of PO content to 0.6 mL, the system gelates in only 5–8 min with the loss of skeletons and the transition of continuous pores into independent micron and nanoscale pores (Fig. 3d). As the gelation accelerant, the moderate PO content and the proper relative content of PEO are two key factors on the formation of interconnected macroporous barium zirconate gels with continuous skeletons.
were dissolved in a mixture of H2O and EtOH under vigorous stirring at room temperature, and then EG was added into the mixture liquid. After 5 min, FA was added dropwise, then PO was added into the mixture liquid under 1 min stirring. The container together with the whole mixed solution was sealed and placed in 80 °C drying oven to gelate and age for 24 h. Then EtOH and a mixture of EtOH and TMOS were added into the container successively to exchange the solvent. After solvent exchange, the container was placed in 80 °C drying oven for 4–7 days to obtain dried gel. 2.2. Characterization of materials Porous morphologies of as-prepared samples with different compositions were observed by a SU-70 scanning electron microscope (SEM, HITACHI Corp.). Powder X-ray diffractions (XRD) of the heattreated as-prepared samples performed with Empyrean 200895 diffractometer (PANalytical B.V. Corp.) using Cu-Kα radiation (λ=0.154 nm). The nitrogen adsorption–desorption of as-prepared samples were characterized by an Autosorb-1-C nitrogen adsorption– desorption apparatus (Quantachrome Corp.). Before each nitrogen adsorption–desorption measurement, the as-prepared samples were degassed at 200 °C under vacuum for more than 6 h. Specific surface area and pore size distributions were calculated using the Brunauer– Emmett–Teller (BET) and Barrett–Joyner–Halenda (BJH), respectively. 3. Results and discussion 3.1. Formation of macroporous morphology In the present system, zirconium oxychloride octahydrate (ZrOCl2· 8H2O) and barium chloride dihydrate (BaCl2·2H2O) hydrolyze and polymerize together in the mixture of H2O and EtOH to prepare homogeneous and transparent solutions. The PH of the solution declines to 2 after co-hydrolysis of two ionic precursors. The phase separation and gelation of the solutions can be controlled by the addition of poly(ethylene oxide) (PEO) and propyleneoxide (PO). The phase separation and gelation process can be described by Flory– Huggins formulation and ring-opening reaction as follows [24,25]. The Eqs. (1)–(4) come from Flory–Huggins formulation. χ12 is the interaction parameter, φi and Pi are the volume fraction and the degree of polymericzation of component i (i=1 or 2), respectively, R is the gas constant, and T is the temperature. KB is the Kauri-butanol parameter. As for the system Gibbs free energy change (Eq. (4)), the former term in parenthesis represent the enthalpic contribution (H), and the last two terms the entropic contribution (S) [16,26,27].
∆H =RT (χ12 φ1 φ2 )
χ12 ∝
(δ1 − δ 2 )2 KB T
⎛φ ⎞ φ ∆S =−R ⎜ 1 ln φ1+ 2 ln φ2⎟ ⎝ P1 ⎠ P2
(4)
3.2. Heat-treatment and crystallization
(1)
Fig. 4 shows the XRD patterns of BZ3 sample after heat-treatment at different temperatures. It is noted that the diffraction peaks are weak in as-dried sample and the one heat-treated in 800 °C, indicating low crystallinity. Because the precursor BaCl2·2H2O does not dissolve in EtOH liquid, the BaCl2 concentration in the system nearly reaches its
(2)
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Fig. 1. The sol–gel process accompanied by phase separation of Ba-Zr-O system..
volume shrinkage appears and the skeletons become dense and thick in the crystalline barium zirconate monoliths compared with the dried gel. Fig. 5 shows the SEM image of BZ3 sample after heat-treatment at 1100 °C. The well-defined cocontinuous macroporous mophology is basically maintained after heat-treatment, indicating that the formation of crystalline phase does not spoil the macroporous morphology of barium zirconate monoliths. There are also some small pores exsiting on the skeleton, which results form the volatilization of some organics. Fig. 6 shows the macropore size distributions of BZ3 sample before and after heat-treatment at 1100 °C, determined by mercury intrusion porosimetry. It is obvious that the BZ3 sample has the macroporous structure with a narrow pore size distribution from 1 to 2 µm nearly, which is caused by spinodal decomposition. The bulk density of as-
aqueous saturation concentration. In the process of gelation, BaCl2 recrystallizes and attached on the gel skeletons, so weak BaCl2 phase can be found in the as-dried sample. Tetragonal ZrO2 phase appears except for BaCl2 when the sample is heat-treated at 800 °C. During heat-treatment at 1100 °C, BaZrO3 occurs due to the solid-phase reaction between ZrO2 and BaCl2, and a few redundant tetragonal ZrO2 transforms to monoclinic ZrO2. With the increase of heattreatment temperature, the diffraction intensity becomes stronger and the diffraction shape becomes shaper, which indicates better crystallinity and larger grain size. As a result, stable BaZrO3 phase forms at 1100 °C with a few monoclinic ZrO2 phase. The variation of macroporous structure and properties of crystalline monoliths after heat-treatment was examined. It is found that 30%
Fig. 2. SEM images of dried barium zirconate gels prepared with various PEO contents; (a)-BZ1, (b)-BZ2, (c)-BZ3, (d)-BZ4.
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Fig. 3. SEM images of as-dried barium zirconate gels prepared with various PO contents; (a)-BZ6, (b)-BZ3,(c)-BZ7,(d)-BZ8.
Fig. 6. Macropore size distributions of BZ3 sample before and after being heat-treated at 1100 °C.
Fig. 4. XRD patterns of BZ3 sample before and after being heat-treated at different temperatures.
Fig. 7. N2 adsorption–desorption isotherms of sample BZ3 before and after heat-treated at 1100 °C.
Fig. 5. SEM image of BZ3 sample after being heat-treated at 1100 °C.
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dried BZ3 sample is 0.78 g·cm−3, measured by mercury intrusion porosimetry. After heat-treatment, the median pore diameter decreases from 1.8 to 1.5 µm and the porosity increases slightly from 63–65% with the volatilization of organics. Fig. 7 depicts N2 adsorption–desorption isotherms of sample BZ3 before and after heat-treatment at 1100 °C. The porous barium zirconate gels before heat-treatment exhibits the isotherm of Ⅳ-type according to the IUPAC classification, indicating that the sample has large amounts of mesopores and micropores. After heat-treatment, the porous barium zirconate gels exhibits the isotherm of Ⅲ-type. Under the low and middle pressure, N2 volume adsorption stays in a low amount. Under nearly 1.0 relative pressure, the volume adsorption obviously ascends due to macropores existed in the sample. After calculation, BET surface area reaches 188 m2·g−1 before heat-treatment and plunges to 0.8 m2·g−1 after heat-treatment. The micropores and mesopores have direct links with the sample's BET surface area, but the macropores do not have obvious effect on it, which accounts for the enormous change of the sample's BET surface area before and after heat-treatment. Although the continuous skeletons and interconnected macropores are retained after heat-treatment, the mesopores and micropores on the skeletons are lost due to solid phase reaction, crystal transfer and grain aggregation, which causes the substantial decrease of BET surface area.
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4. Conclusions Monolithic macroporous barium zirconate materials are successfully prepared by sol–gel process accompanied by phase separation. The morphologies and structures of monolithic macroporous barium zirconate materials are closely connected with poly(ethylene oxide) (PEO) as phase separation inducer and propyleneoxide (PO) as gelation accelerant. It has been proved that continuous macroporous samples with interconnected skeletons can be synthesized by controlling the content of PEO as 0.1 g and the content of PO as 0.36 mL. Heattreatment develops the solid reaction to synthesize barium zirconate and retains the continuous macroporous structure. Acknowledgements This work is supported by the National Natural Science Foundation of China (51372225).
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