Materials Letters 64 (2010) 1630–1633
Contents lists available at ScienceDirect
Materials Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a t l e t
Emulsion-derived urchin-shaped hydroxy sodalite particles D. Kundu, B. Dey, M.K. Naskar, M. Chatterjee ⁎ Sol–Gel Division, Central Glass and Ceramic Research Institute (CSIR), Kolkata 700 032, India
a r t i c l e
i n f o
Article history: Received 1 February 2010 Accepted 5 April 2010 Available online 24 April 2010 Keywords: HS particles Emulsion technique Surfactant Characterization methods Microstructure
a b s t r a c t Urchin-shaped hydroxy sodalite (HS) particles composed of numerous nanorods of 400–500 nm in length with hexagonal cross-section of 50–100 nm in diameter were synthesized following non-ionic surfactant-stabilized water-in-oil (w/o) emulsions at a considerably low temperature of 900C with a short duration, 10 h. The nonionic surfactant, i.e. sorbitan monooleate (Span 80) of hydrophilic–lipophilic balance (HLB) value of 4.3 was found to be suitable for the preparation of emulsions. Crystalline phases and microstructures of the synthesized particles were studied by X-ray diffraction (XRD) and field emission scanning electron microscopy (FESEM) respectively. A relatively lower synthesis temperature (80 °C), under the same reaction conditions, resulted in the formation of thread-ball-like particles of HS along with a little amount of cubic NaA zeolite particles while flower-like HS particles were obtained at 100 °C. A tentative mechanism for the formation of HS particles of different morphologies was proposed. The HS particles find important use as seed crystals for the preparation of ceramic membranes in the separation technology and catalysis in various reactions. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Hydroxy sodalite (HS) is crystalline and hydrophilic in nature [1,2]. HS, belonging to the group of clathrasils, is made of a cubic array of β-cages and exhibits similar structure of sodalite. With sixmembered oxygen aperture, HS has a kinetic diameter of 2.65 Å [1], which provides access to small molecules e.g. helium (∼ 2.6 Å), water (∼2.7 Å), and ammonia (∼2.5 Å), making it a potential membrane material in separation technology. In addition, HS powder finds applications in semiconductors, hydrogen storage, hydrogen separation, catalysts and pigment occlusion [3]. Several methods are available for the synthesis of HS particles by solution phase techniques with or without structure directing agents [2–6]. However, in the present investigation, to the best of our knowledge, we report first time the synthesis of urchin-shaped HS particles at a considerably low temperature of 90 ± 1 °C and short duration for 10 h following a simple non-ionic surfactant assisted emulsion technique. In this communication, the formation mechanism of HS at different temperatures is also reported. Further, in the above method, we could tailor the particle morphology. 2. Experimental section
reagent, were used as starting materials for the synthesis of HS particles. A mix solution of sodium aluminate and sodium silicate was used as the water phase (w) of the w/o emulsion [7,8]. Molar ratio of the precursor solution was 50 Na2O:Al2O3:5 SiO2:1000 H2O. The support solvent was composed of n-heptane (oil phase ‘o’ of the w/o emulsion) and the non-ionic surfactant i.e., Span 80 (1 vol.% with respect to n-heptane). The w/o emulsion, obtained by adding the precursor solution to the support solvent in 1:4 volume ratio, was heated at 90 °C for 10 h in Teflon-lined stainless steel autoclave [7]. After synthesis, the as-prepared particles were collected and washed with methanol followed by drying at 70 °C for 2 h. Following the same procedure, separate experiments were performed at 80 and 100 °C for 10 h each. Fig. 1 shows schematically the synthesis of HS particles by the Span 80-stabilized emulsion technique. In the Span 80 molecule, the hydrophilic sorbitan group acts as the polar ‘head’ and the hydrophobic oleic acid group acts as the non-polar ‘tail’. 2.2. Characterization Crystal phases in the particles were identified by XRD (Model: Philips, 1730, Philips, Almedo, The Netherlands) using Ni-filtered Cukα radiation (λ = 0.15418 nm). The morphology of the particles was examined by FESEM (Model: Zeiss, SupraTM 35VP, Oberkochen, Germany).
2.1. Synthesis of HS particles 3. Results and discussion Sodium hydroxide (NaOH), sodium metasilicate nonahydrate, Na2SiO3.9H2O and aluminium metal foil, all of analytical grade ⁎ Corresponding author. Tel.: + 91 33 24838086; fax: + 91 33 24730957. E-mail address:
[email protected] (M. Chatterjee). 0167-577X/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2010.04.015
Fig. 2 shows the XRD patterns of emulsion-derived powders obtained at 80–100 °C for 10 h each. Fig. 2a reveals that a significant amount of HS crystals (JCPDS File No. 11-401) was formed along with a small amount of NaA particles (JCPDS File No. 39-222) at 80 °C.
D. Kundu et al. / Materials Letters 64 (2010) 1630–1633
1631
Fig. 1. A schematic representation for the synthesis of HS particles from the Span 80-stabilized w/o emulsions.
Fig. 2. XRD patterns of emulsion-derived HS particles crystallized at (a) 80°; (b) 90° and (c) 100 °C for 10 h ageing time at each temperature.
However, with increase in synthesis temperature to 90 °C, a single phase HS particles was formed (Fig. 2b), which at least persisted up to 100 °C (Fig. 2c). Fig. 3 shows the FESEM images of powders crystallized at different temperatures. It was observed that at 80 °C, thread-ball-like HS particles (Fig. 3a) [9] along with some NaA crystals of typical cubic morphology were formed. The thread-like particles entangled with each other (shown in the inset of Fig. 3a) forming a big particle. The particles were hollow as shown by the oval mark in Fig. 3a. Fig. 3b indicates the formation of urchin-shaped particles [10] at 90 °C. The particles were composed of a large number of nanorods of 400– 500 nm in length with hexagonal cross-section of 50–100 nm in diameter [11,12], projecting outward from the centre of the particles as indicated by the high magnification image of Fig. 3b. However, with increase in temperature up to 100 °C, flower-like particles of HS were obtained (Fig. 3c). The petal-shaped particles self-assembled together to form flower-like particles (shown in the inset of Fig. 3c). It seems that with an increase in temperature from 80 to 90 °C, the growth of small thread-like crystals gave rise to ripened nanorods of longer dimensions in urchin-shaped particles which were disrupted at 100 °C. Formation of thread-ball-like and urchin-shaped particles was similar to that observed during the synthesis of hollow zeolite analcime and sodalite in presence of organic ligands [11,13]. A tentative mechanism was proposed for the formation of HS particles (shown schematically in Fig. 4). In the emulsion, the surfactant (Span 80) molecules with polar ‘head’ of hydrophilic sorbitan group and non-polar ‘tail’ of hydrophobic oleic acid group (Fig. 1) formed an aqueous pool with the precursor solution. The hydrophilic head group of the surfactants interacted with the aluminosilicate species of the aqueous pool. As the reaction progressed, the particles started growing at the surface of the aqueous pool through the reverse surface growth mechanism [11]. At a certain reaction condition, the growth of the crystals became restricted to a particular crystallographic axis, giving rise to a dramatic increase in their aspect ratio. Thus, the preferential growth along a certain direction formed rod-shaped particles which self-assembled each other forming a big particle. Under the present reaction condition in w/o type emulsion medium, the morphology of the particles was very much sensitive to temperatures. The thread-ball-like particles obtained at 80 °C were readily transformed to urchin-shaped particles at 90 °C. It is worth noting that both thread-ball and urchin-shaped particles crystallized at 80o and 90 °C respectively though reverse surface growth mechanism
1632
D. Kundu et al. / Materials Letters 64 (2010) 1630–1633
Fig. 3. FESEM images of the emulsion-derived HS particles crystallized at (a) 80°; (b) 90° and (c) 100 °C for 10 h ageing time at each temperature.
Fig. 4. A schematic representation for the formation mechanism of HS particles by Span 80-stabilized w/o emulsions.
D. Kundu et al. / Materials Letters 64 (2010) 1630–1633
were hollow [11]. However, the formation of flower-like particles at 100 °C was believed to be due to the weak interaction between the aluminosilicate species and surfactant head groups which is caused by increased thermal vibration of surfactant molecules [12]. Thus, it prevented the formation of large aggregates as well as surface crystallization and facilitated the formation of flower-like particles via normal crystal growth [9] along all possible directions.
4. Conclusion Urchin-shaped hydroxy sodalite (HS) particles were successfully synthesized following non-ionic surfactant-stabilized w/o emulsions at a considerably low temperature of 90 °C and short duration for 10 h. The Span 80, a non-ionic surfactant with an HLB value of 4.3 was found to be highly suitable for the preparation of emulsions. The present method can be very much effective in tailoring the morphology of the particles, by using other surfactants with their different HLB values. The HS particles find applications in semiconductors, hydrogen storage and separation, and as seed crystals for the preparation of ceramic membranes in separation technology.
1633
Acknowledgments The authors thank the Director of this institute for his kind permission to publish this paper. The financial support from CSIR, New Delhi in the project no. SIP 0023 is also thankfully acknowledged. References [1] Breck DW. Zeolite Molecular Sieves: Structure, Chemistry and Use. New York: Wiley; 1974. [2] Khajavi S, Kapteijn F, Jancen JC. J Membrane Sci 2007;299:63–72. [3] Fan W, Morozuma M, Kimura R, Yokoi T, Okubo T. Langmuir 2008;24:6952–8. [4] Hong SB, Camblor MA, Davis ME. J Am Chem Soc 1997;119:761–70. [5] Li D, Yao J, Wang H, Hao N, Zhao D, Ratinac KR, et al. Microporous Mesoporous Mater 2007;106:262–7. [6] Yao J, Wang H, Ratinac KR, Ringer SP. Chem Mater 2006;18:1394–6. [7] Das A, Das N, Naskar MK, Kundu D, Chatterjee M, Maiti HS. Ceramics Int 2009;35: 1799–806. [8] Devi PS, Chatterjee M, Ganguli D. Mater Lett 2002;55:205–10. [9] Greer H, Wheatley PS, Ashbrook SE, Morris RE, Zhow W. J Am Chem Soc 2009;131: 17966–92. [10] Zhang W, Li X, Qu Z, Zhao Q, Chen G. Mater Lett 2010;64:71–3. [11] Chen X, Qiao M, Xie S, Fan K, Zhou W, He H. J Am Chem Soc 2007;129:13305–12. [12] Ishikawa Y, Shimizu Y, Saaki T, Koshizaki N. J Colloid Interface Sci 2006;300:612–5. [13] Han L, Yao J, Li D, Ho J, Zhang X, Kong CH, et al. J Mater Chem 2008;18:3337–41.