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Effect of surfactants on preparation of nanoscale α-Al2O3 powders by oil-in-water microemulsion
Advanced Powder Technology 24 (2013) 354–358
Contents lists available at SciVerse ScienceDirect
Advanced Powder Technology journal homepage: www.elsevier.com/locate/apt
Original Research Paper
Effect of surfactants on preparation of nanoscale a-Al2O3 powders by oil-in-water microemulsion Jingjing Ma, Bolin Wu ⇑ College of Material Science and Engineering, Guilin University of Technology, Guilin, Guangxi 541004, PR China
a r t i c l e
i n f o
Article history: Received 27 March 2012 Received in revised form 7 August 2012 Accepted 21 August 2012 Available online 27 September 2012 Keywords: Surfactant Oil-in-water Microemulsion a-Al2O3
a b s t r a c t The nanoscale a-Al2O3 powders have been synthesized by the O/W microemulsion system using cyclohexane as the oil phase, ultrapure water as the water phase, OP-10 and alcohol as the surfactant and co-surfactant. It has been found that the nature of surfactants played an important role to regulate the size and morphologies of the a-Al2O3 nanoparticles. Three different nonionic surfactants (OP-10, Triton X-100 and Tween-80) have been used for the preparation of microemulsions. The microemulsions were characterized by Zeta Potential Analyzer. The results showed that the OP-10 system possesses wide and stable microemulsion phase regions. The synthesized a- Al2O3 powders have been comprehensively characterized using scanning electron microscopy (SEM), X-ray diffraction (XRD). The powders calcined at 900–1150 °C showed the presence of alumina phase with crystal structure, The SEM images showed that the powders was an average diameter of 30–100 nm. Ó 2012 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.
1. Introduction The first microemulsion structures termed at that time ‘‘oleophatic hydro-micelle’’ were discovered in 1943 by Hoar and Schulman [1]. A great variety in structure of these single phase microemulsions is known in literature ranging from water-in-oil (W/O) over bicontinuous to oil-in-water (O/W) structures. Microemulsions are thermodynamically stable, clear or translucent micellar solution comprised of oil, water and surfactant [1,2]. The formation and stability of a microemulsion can be affected by various factors, including the nature and molecular weight of surfactant, alcohol chain length and concentration, salinity, and temperature. However, surfactants play an important role in reducing the interfacial tension in microemulsions. They can be selected based on the HLB concept. Surfactants with a low HLB value (3–6) are preferred for the formation of W/O microemulsion, whereas surfactants with high HLB value (8–18) are preferred for O/W microemulsions [3,4]. The internal phase is solubilized by particles ranging between 10 and 100 nm in size and composed of surfactant and co-surfactant molecules. It merits over other vehicles or solvents by both improved stability and solubilization characteristics [5,6]. The O/W microemulsions are composed of submicrometer oil droplets that are dispersed throughout an aqueous continuous phase. The droplets are covered by a shell consisting of a suitable ⇑ Corresponding author. Tel.: +86 773 5897060; fax: +86 773 5895613. E-mail address: [email protected] (B. Wu).
surfactant and a co-surfactant. The surfactant molecules form interface film that separates the oil phase from the aqueous continues phase. The film has a low surface tension in the oil–water mixture. The addition of co-surfactant reduces the interfacial tension further as it locates itself at the oil–water interface and therefore lowers the interfacial free energy which favors the formation of stable microemulsion [7,8]. The microemulsion method is often used to synthesize nanoparticles with specific size and morphology. The advantage of the method is the ability to control the particle size or shape-controlled and chemically clean nanoparticles with narrow size distribution, which are easily incorporated into a variety of substances to form nanocomposites or assembled into higher-order nanostructures. The first application of W/O microemulsion for the synthesis of catalytic nanoparticles was introduced in 1982 and concerns nanoparticles of noble metals [9]. Over the past few decades, microemulsion technique has attracted particular attention because the aqueous phase dispersed into the oil phase can form specifically a transparent microemulsion. The W/O microemulsions have been used to synthesize of SiO2, ZnO, CdSe and TiO2 nanoparticles, etc. [10–21]. Recently, we developed a novel and straight forward approach for the synthesis of inorganic nanoparticles by using O/W microemulsions [22], in contrast to the typically used W/O microemulsion method. The O/W microemulsions structure has several advantages over more the W/O microemulsion: water phase and oil phase ratio is far greater than 1, consumption organic matter less and relatively low cost. The new strategy implies the use of organometallic precursors, dissolved within oil
0921-8831/$ - see front matter Ó 2012 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved. http://dx.doi.org/10.1016/j.apt.2012.08.008
J. Ma, B. Wu / Advanced Powder Technology 24 (2013) 354–358
droplets in the nanometer scale, and dispersed in a continuous aqueous phase. However, descriptions concerning the preparation of a-Al2O3 powders by the use of O/W microemulsion have been limited. The O/W microemulsions are suitable alternatives as a solvent medium for such reactions as it can minimize the use of organic solvents. This study can encourage application of various types of O/W microemulsions as potential reaction medium for various industrially important reactions. Since the oil required for preparation of O/W type systems, is less, the use of organic solvents is minimized in the reaction system [23,24]. In the present work, a-Al2O3 powders were synthesized through a simple and efficient oil-inwater microemulsion method. The synthesized powders were characterized by X-ray powders diffraction (XRD), scanning electron microscopy (SEM). 2. Experimental 2.1. Materials The materials include: OP-10 (C34H62O11, HLB = 14.5, C34H62O11), Triton X-100 (C34H62O11, HLB = 13.5), Tween-80 (C64H124O26, HLB = 15.0), cyclohexane (C6H12), absolute ethyl alcohol (CH3CH2OH), aluminum chloride crystal (AlCl36H2O) and 5.0 M solution ammonia (NH3H2O), PEG6000 (HOCH2 (CH2OCH2)nCH2OH). All reagents were of AR grade. Ultrapure water was used in synthesis and purification steps. 2.2. The microemulsion composition and design To choose a suitable composition for the microemulsion processing, a partial phase diagram at room temperature for the system consisting of cyclohexane, ultrapure water, mixed surfactant of OP-10 and co-surfactant of alcohol was first established (W1). The Phase diagrams were constructed using a traditional titration method. Fig. 1 shows the oil-in-water microemulsion was prepared at 10 points of different compositions. Appropriate amounts of OP10, cyclohexane, alcohol were weighed into glass beaker and the solution was stirred until a clear solution. Titrating with water containing ethanol drop by drop, the mixing solution was vigorously shaken with the mixer. After equilibrium was reached, the mixtures were checked both visually for clarity, All mixtures produced optically clear solutions at low oil concentrations, forming the stable oil-in-water microemulsion. Similarly, Triton X-100 system (W2) and Tween-80 system (W3) separately according to the method was preparation. The titration was continued in order to observe the presence of a clear region (L1). In Fig. 1 the microemulsion was represented by the L1 area, within which the
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compositions are optically transparent. The microemulsion composition at the point marked L1 consisting of 10% cyclohexane, 60% Tween-80 and absolute ethyl alcohol (the mole ratio of was 1:4) and 30% aqueous phase was chosen for the preparation precursor. 2.3. Preparation of a-Al2O3 powders The oil-in-water microemulsion of the above chosen microemulsion composition was prepared. They consisted of the same percentages of oil, surfactant, co-surfactant and aqueous phases. In this work, a quarternary microemulsion (OP-10/ultrapure water/cyclohexane/ethanol) was selected. The preparation of the alumina precursor was shown in Fig. 2. The two different solutions (O and W) were prepared as follows: The O (the mixed solution of cyclohexane and aluminum chloride crystal, OP-10 and ethanol) was used as oil phase. It was mixed by magnetically stirring until the mixture became transparent and the Al3+ ions were evenly dispersed in the oil phase solution. The W (ultrapure water, ethanol and OP-10) as water phase was mixed. Meanwhile, Fig. 2 shows the hydrophilic group (O) and the lipophilic group (W) were exposed under the action of the surfactant [25,26]. The oil phase was poured into the water phase for emulsification, and obtained a uniform and stable aluminum salt microemulsion. Then, an appropriate amount of (1.0 mol L1) ammonia solution was added dropwise into the microemulsions while they were stirred until pH = 8.5. The Al3+ ions and the OH ions were attracted to each other directional movement and interaction. In order to ensure the uniform distribution of the alumina precursor obtained [27,28]. The samples were performed at 60 °C in constant water bath for 30 min, and added 0.1% PEG6000 for ultrasonic dispersion at 30 °C for 40 min. Finally, the amorphous precursor gels were filtered, washed with ethanol for three times, dried at 80 °C and sintered at 1150 °C for 4 h. The filtrate was recovered by fractionation. 2.4. Characterization Measurements of zeta potential of the microemulsions were performed with a Zeta Potential Analyzer (Model No. ZS90 of Malvern Instruments Ltd. (Malvern, UK). Measurement of the zeta potential of a particle dispersion as a function of any of the above can lead to information in formulating the microemulsion to give maximum stability or in determining the optimum conditions for flocculation of the system. The X-ray diffraction spectrum of the calcined powders was characterized using X’Pert PRO made X-ray diffraction spectrometer using Cu Ka, radiation operated at 40 kV and 40 mA current at a step size 0.0170°/s, X-ray patterns were taking by measuring 2h
Fig. 1. The oil-in-water (O/W) microemulsion systems of oil (O), water (W), and surfactant + co-surfactant (S + C). (W1: OP-10 system, W2: Triton X-100 system, W3: Tween80 system.)
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Fig. 2. Formation process for the alumina precursor. (O: Oil phase solution; W: Water phase solution) A: cyclohexane and aluminum chloride crystal; B: ultrapure water; C: ethanol and OP-10; D: alumina precursor.
from 5° to 90°. The peak positions and relatively intensities of the powders pattern were identified by comparison with powders diffraction file (PDF) data. Calculation on the diameter of particles was based on Scherrers’ equation:
Dhkl ¼ 0:9k=bhkl cos h
ð1Þ
(Dhkl is the size of the particles, k is the wavelength of the X-ray and bhkl is the width of the base of diffraction line at half its maximum intensity). The ultrasonic bath was used to suspend calcined powders in ethanol. It was used for microstructure evaluation by scanning electron microscopy (SEM).
3. Results and discussion 3.1. Zeta potential of the oil-in-water microemulsions Surfactants play a crucial role in improving stability in microemulsions. In order to investigate the stability of the (OP-10, Triton X-100, Tween-80) three systems, zeta potentials of the microemulsions were measured at the pH = 7.0. As shown in Fig. 3, zeta potential of numerical remained in three types of microemulsion, all in the stable region range. Among them, the OP-10 system (f = 25.4 mV) was more stable. At this point, the OP-10 system particles were relatively stable with a better dispersion, and there was the stability of the wide area in OP-10 system (Fig. 1: O/W microemulsion phase regions: OP-10 system > Triton X-100 system > Tween-80 system), it was suitable for synthesis of nanoparticles. 3.2. Investigation of the phase transformation of the Al2O3 crystallite using XRD Fig. 4 shows the X-ray diffraction patterns of the precursor powders sample slowly calcined for 3–4 h at various temperatures.
Fig. 3. Zeta potential of three systems (OP-10, Triton X-100, Tween-80).
Fig. 4(2), it was found that the powders remain amorphous up to 900 °C. The most intense diffraction peaks were identified as c-Al2O3 major phase together with the presence of weak maximum that could be associated to the monoclinic h-Al2O3 phase. The XRD analysis in Fig. 4(2) shows the most stable phase, aAl2O3 occurred dominantly at 1150 °C. The diffraction pattern is extremely sharp indicating the existence of a highly crystalline material. Usually a-Al2O3 crystallizes around 1200 °C, however, in the present study the lower crystallization temperature of aAl2O3 could be related to the fine crystalline size and higher specific surface area. The observation reported by Hyuk-Joon et al. [29] indicated that completion of the most stable phase, a-Al2O3 occurs at this temperature. Fig. 4(2) shows the split (1 1 3) diffraction peak of the prototype rhombohedral alumina. In order to find out the crystallite size, a slow scan with step size 0.0170°/s of selected diffraction peaks such as (1 1 3) has been recorded and calculated by Scherers’ formula–equation. The average crystallite size was about 35 nm. The c ? d ? h ? r ? a-Al2O3 transformation is reconstructive and proceeds through a nucleation and growth process. Since most of the activation energy is required for the nucleation process, elevated temperatures are needed to nucleate a-Al2O3. Normally, the c-Al2O3?a-Al2O3 transformation temperature is as high as 1200 °C [30]. A high transformation temperature always results in the coarsening of particles and formation of hard agglomerates in the powders. Thus, a reduction in the c-Al2O3?a-Al2O3 transformation temperature was crucial for the processing of highly reactive ultrafine a-Al2O3 powders. 3.3. Microstructure The morphology of the a-Al2O3 powders were observed on the SEM micrograph included in Fig. 5. As shown in Fig. 5A, a small loosely aggregated mass formed in the dispersion process through aggregation of suspended particles. Selection of Fig. 5A local B, as shown in Fig. 5B exhibited much denser agglomeration, The morphology observations showed that the a-Al2O3 particle were regularly spherical in the shape, grain size was more even. It is ideal morphology for the preparation and the development of high performance alumina ceramics; While the powders mainly consisted of much more porous aggregates. The aggregates were reduced or eliminated by adopting proper processes methods. However, Fig. 5A reveals at the powders appear as irregular agglomerates of widely different sizes, the larger ones reaching 100–200 nm, while the smallest size observed was about 35 nm. The actual particle size in the powders agglomerates was revealed, as illustrated in Fig. 5B. This SEM micrograph clearly shows how one of the large powders agglomerates was composed of numerous powders particles having sizes of less than 100 nm.
J. Ma, B. Wu / Advanced Powder Technology 24 (2013) 354–358
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Fig. 4. XRD patterns for the samples calcined at different temperature: (1) 900–1100 °C; (2) 1150 °C.
Fig. 5. The SEM micrograph showing the microemulsion derived a-Al2O3 powders. (A: flocculent–like a-Al2O3 powders particle, B: details of A particle.)
4. Conclusions The pseudo-ternary phase diagram of ultrapure water/(OP-10, Triton X-100 and Tween-80)/ethanol/cyclohexane systems were drawn based on experimental data. Through the analysis, the OP10 system has a wide and stable (O/W) microemulsion region. It was suitable for the preparation of inorganic nanoparticles. The nanoscale a-Al2O3 powders have been successfully prepared by oil-in-water microemulsion method. Based on XRD results, the as-sprayed nanoparticles consisted of c-phase and a-phase Al2O3, which can be converted to a-phase by calcinations at 1150 °C for 4 h. The average crystallite size was about 35 nm by Scherers’ formula–equation. The scanning electron microscopy (SEM) images showed that the powders were spherical with an average diameter of 30–100 nm. The granula small uniform of nano a-Al2O3 powder is a kind of fine material of high performance alumina ceramics industry. The thermodynamic stability of microemulsion system can increase yield, and the experimental waste can be used fractionation methods of separation and purification, recycling and reuse, both to reduce costs and protect the environment.
Acknowledgments The authors are thankful to the National Natural Science Foundation of China (Project: 50972028), for sponsoring this research work. Special thanks are due to Professor WU Bo-lin, Laboratory
Technician HU Chang-zheng and Senior Engineer ZHU Wen-feng, for their kind help in experimental instruction, XRD test and SEM test, respectively. References [1] T.P. Hoar, J.H. Schulman, Transparent water-in-oil dispersions: the oleopathic hydro-micelle, Nature 152 (1943) 102. [2] Ganzuo Li, Theory and Application of microemulsion, Petroleum Industry Press, Beijing, China, 1995. pp 47–180. [3] R. Pichot, F. Spyropoulos, I.T. Norton, O/W emulsions stabilised by both low molecular weight surfactants and colloidal particles: the effect of surfactant type and concentration, J. Colloid. Interface. Sci. 352 (2010) 128–135. [4] Andras Nagy, Joseph P. Kennedy, Ping Wang, et al., Extent of coverage of surfaces treated with hydrophobizing microemulsions: a mass spectrometry and contact angle study, Appl. Surf. Sci. 252 (2006) 3751–3759. [5] Hamdan Suhaimi, Dzulkefly Kuang, Salina Sharifudin, et al., Triglyceride microemulsion systems with palm oil, Pertanika J. Sci. Technol. 4 (2) (1996) 173–181. [6] Oliver Lade, Carlos C. Co, Patricia Cotts, et al., Microemulsion polymerization: phase behavior driven mechanistic changes, Colloid. Polym. Sci. 283 (2005) 905–916. [7] Tawatchai Charinpanitkul, Amornsak Chanagul, Joydeep Dutta, et al., Effects of cosurfactant on ZnS nanoparticle synthesis in microemulsion, Sci. Technol. Adv. Mater. 6 (2005) 266–271. [8] A. Hiroshi Araya, Mikio Tomita, Masahiro Hayashi, The novel formulation design of O/W microemulsion for improving the gastrointestinal absorption of poorly water soluble compounds, Int. J. Pharm. 305 (2005) 61–74. [9] Magali Boutonnet, Jerzy Kizling, Per Stenius, et al., The preparation of monodisperse colloidal metal particles from microemulsions, Colloids Surf. 5 (1982) 209–225. [10] Mikaela Wallin, Neil Cruise, Uta Klement, et al., Preparation of Mn, Fe and Co based perovskite catalysts using microemulsions, Colloids Surf. A: Physicochem. Eng. Aspects 238 (2004) 27–35.
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[11] Yuvaraj Haldorai, Van Hoa Nguyen, Jae-Jin Shim, Synthesis of polyaniline/QCdSe composite via ultrasonically assisted dynamic inverse emulsion polymerization, Colloid Polym. Sci. 289 (2011) 849–854. [12] C.E. Zubieta, J.F.A. Soltero-Martínez, C.V. Luengo, P.C. Schulz, Preparation, characterization and photoactivity of TiO2 obtained by a reverse microemulsion route, Powder Technol. 212 (2011) 410–417. [13] Víctor M. Ovando-Medina, René D. Peralta, Eduardo Mendizábal, et al., Synthesis of polypyrrole nanoparticles by oil-in-water microemulsion polymerization with narrow size distribution, Colloid Polym. Sci. 289 (2011) 759–765. [14] Masahiro Kishida, Toshiaki Hanaoka, Won Young Kim, et al., Size control of rhodium particles of silica-supported catalysts using water-in-oil microemulsion, Appl. Surf. Sci. 121/122 (1997) 347–350. [15] M.A.P. Almeida, L.S. Cavalcante, J.A. Varela, M. Siu Li, E. Longo, Effect of different surfactants on the shape, growth and photoluminescence behavior of MnWO4 crystals synthesized by the microwave-hydrothermal method, Adv. Powder Technol. 23 (2012) 124–128. [16] Chaopeng Fu, Haihui Zhou, Ding Xie, et al., Electrodeposition of gold nanoparticles from ionic liquid microemulsion, Colloid. Polym. Sci. 288 (2010) 1097–1103. [17] T. Nomura, T. Mori, H. Arima, Y. Konishi, Shape and size control of barium chromate nanoparticles using reverse micelle, Adv. Powder Technol. 20 (2009) 101–105. [18] Jun Oshitani, Shiho Takashina, Mikio Yoshida, Kuniaki Gotoh, Phase behavior and size variation of Na-AOT-based W/O microemulsions by increasing NaOH concentration in the water pool, Adv. Powder Technol. 20 (2009) 554–557. [19] J. Chandradass, Baekil Nam, Ki Hyeon Kim, Fine tuning of gadolinium doped ceria electrolyte nanoparticles via reverse microemulsion process, Colloids Surf. A: Physicochem. Eng. Aspects 348 (2009) 130–136. [20] J. Chandradass, Arvind H. Jadhav, Hern Kim, Surfactant modified MgFe2O4 nanopowders by reverse micelle processing: effect of water to surfactant ratio
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Contents lists available at SciVerse ScienceDirect
Advanced Powder Technology journal homepage: www.elsevier.com/locate/apt
Original Research Paper
Effect of surfactants on preparation of nanoscale a-Al2O3 powders by oil-in-water microemulsion Jingjing Ma, Bolin Wu ⇑ College of Material Science and Engineering, Guilin University of Technology, Guilin, Guangxi 541004, PR China
a r t i c l e
i n f o
Article history: Received 27 March 2012 Received in revised form 7 August 2012 Accepted 21 August 2012 Available online 27 September 2012 Keywords: Surfactant Oil-in-water Microemulsion a-Al2O3
a b s t r a c t The nanoscale a-Al2O3 powders have been synthesized by the O/W microemulsion system using cyclohexane as the oil phase, ultrapure water as the water phase, OP-10 and alcohol as the surfactant and co-surfactant. It has been found that the nature of surfactants played an important role to regulate the size and morphologies of the a-Al2O3 nanoparticles. Three different nonionic surfactants (OP-10, Triton X-100 and Tween-80) have been used for the preparation of microemulsions. The microemulsions were characterized by Zeta Potential Analyzer. The results showed that the OP-10 system possesses wide and stable microemulsion phase regions. The synthesized a- Al2O3 powders have been comprehensively characterized using scanning electron microscopy (SEM), X-ray diffraction (XRD). The powders calcined at 900–1150 °C showed the presence of alumina phase with crystal structure, The SEM images showed that the powders was an average diameter of 30–100 nm. Ó 2012 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.
1. Introduction The first microemulsion structures termed at that time ‘‘oleophatic hydro-micelle’’ were discovered in 1943 by Hoar and Schulman [1]. A great variety in structure of these single phase microemulsions is known in literature ranging from water-in-oil (W/O) over bicontinuous to oil-in-water (O/W) structures. Microemulsions are thermodynamically stable, clear or translucent micellar solution comprised of oil, water and surfactant [1,2]. The formation and stability of a microemulsion can be affected by various factors, including the nature and molecular weight of surfactant, alcohol chain length and concentration, salinity, and temperature. However, surfactants play an important role in reducing the interfacial tension in microemulsions. They can be selected based on the HLB concept. Surfactants with a low HLB value (3–6) are preferred for the formation of W/O microemulsion, whereas surfactants with high HLB value (8–18) are preferred for O/W microemulsions [3,4]. The internal phase is solubilized by particles ranging between 10 and 100 nm in size and composed of surfactant and co-surfactant molecules. It merits over other vehicles or solvents by both improved stability and solubilization characteristics [5,6]. The O/W microemulsions are composed of submicrometer oil droplets that are dispersed throughout an aqueous continuous phase. The droplets are covered by a shell consisting of a suitable ⇑ Corresponding author. Tel.: +86 773 5897060; fax: +86 773 5895613. E-mail address: [email protected] (B. Wu).
surfactant and a co-surfactant. The surfactant molecules form interface film that separates the oil phase from the aqueous continues phase. The film has a low surface tension in the oil–water mixture. The addition of co-surfactant reduces the interfacial tension further as it locates itself at the oil–water interface and therefore lowers the interfacial free energy which favors the formation of stable microemulsion [7,8]. The microemulsion method is often used to synthesize nanoparticles with specific size and morphology. The advantage of the method is the ability to control the particle size or shape-controlled and chemically clean nanoparticles with narrow size distribution, which are easily incorporated into a variety of substances to form nanocomposites or assembled into higher-order nanostructures. The first application of W/O microemulsion for the synthesis of catalytic nanoparticles was introduced in 1982 and concerns nanoparticles of noble metals [9]. Over the past few decades, microemulsion technique has attracted particular attention because the aqueous phase dispersed into the oil phase can form specifically a transparent microemulsion. The W/O microemulsions have been used to synthesize of SiO2, ZnO, CdSe and TiO2 nanoparticles, etc. [10–21]. Recently, we developed a novel and straight forward approach for the synthesis of inorganic nanoparticles by using O/W microemulsions [22], in contrast to the typically used W/O microemulsion method. The O/W microemulsions structure has several advantages over more the W/O microemulsion: water phase and oil phase ratio is far greater than 1, consumption organic matter less and relatively low cost. The new strategy implies the use of organometallic precursors, dissolved within oil
0921-8831/$ - see front matter Ó 2012 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved. http://dx.doi.org/10.1016/j.apt.2012.08.008
J. Ma, B. Wu / Advanced Powder Technology 24 (2013) 354–358
droplets in the nanometer scale, and dispersed in a continuous aqueous phase. However, descriptions concerning the preparation of a-Al2O3 powders by the use of O/W microemulsion have been limited. The O/W microemulsions are suitable alternatives as a solvent medium for such reactions as it can minimize the use of organic solvents. This study can encourage application of various types of O/W microemulsions as potential reaction medium for various industrially important reactions. Since the oil required for preparation of O/W type systems, is less, the use of organic solvents is minimized in the reaction system [23,24]. In the present work, a-Al2O3 powders were synthesized through a simple and efficient oil-inwater microemulsion method. The synthesized powders were characterized by X-ray powders diffraction (XRD), scanning electron microscopy (SEM). 2. Experimental 2.1. Materials The materials include: OP-10 (C34H62O11, HLB = 14.5, C34H62O11), Triton X-100 (C34H62O11, HLB = 13.5), Tween-80 (C64H124O26, HLB = 15.0), cyclohexane (C6H12), absolute ethyl alcohol (CH3CH2OH), aluminum chloride crystal (AlCl36H2O) and 5.0 M solution ammonia (NH3H2O), PEG6000 (HOCH2 (CH2OCH2)nCH2OH). All reagents were of AR grade. Ultrapure water was used in synthesis and purification steps. 2.2. The microemulsion composition and design To choose a suitable composition for the microemulsion processing, a partial phase diagram at room temperature for the system consisting of cyclohexane, ultrapure water, mixed surfactant of OP-10 and co-surfactant of alcohol was first established (W1). The Phase diagrams were constructed using a traditional titration method. Fig. 1 shows the oil-in-water microemulsion was prepared at 10 points of different compositions. Appropriate amounts of OP10, cyclohexane, alcohol were weighed into glass beaker and the solution was stirred until a clear solution. Titrating with water containing ethanol drop by drop, the mixing solution was vigorously shaken with the mixer. After equilibrium was reached, the mixtures were checked both visually for clarity, All mixtures produced optically clear solutions at low oil concentrations, forming the stable oil-in-water microemulsion. Similarly, Triton X-100 system (W2) and Tween-80 system (W3) separately according to the method was preparation. The titration was continued in order to observe the presence of a clear region (L1). In Fig. 1 the microemulsion was represented by the L1 area, within which the
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compositions are optically transparent. The microemulsion composition at the point marked L1 consisting of 10% cyclohexane, 60% Tween-80 and absolute ethyl alcohol (the mole ratio of was 1:4) and 30% aqueous phase was chosen for the preparation precursor. 2.3. Preparation of a-Al2O3 powders The oil-in-water microemulsion of the above chosen microemulsion composition was prepared. They consisted of the same percentages of oil, surfactant, co-surfactant and aqueous phases. In this work, a quarternary microemulsion (OP-10/ultrapure water/cyclohexane/ethanol) was selected. The preparation of the alumina precursor was shown in Fig. 2. The two different solutions (O and W) were prepared as follows: The O (the mixed solution of cyclohexane and aluminum chloride crystal, OP-10 and ethanol) was used as oil phase. It was mixed by magnetically stirring until the mixture became transparent and the Al3+ ions were evenly dispersed in the oil phase solution. The W (ultrapure water, ethanol and OP-10) as water phase was mixed. Meanwhile, Fig. 2 shows the hydrophilic group (O) and the lipophilic group (W) were exposed under the action of the surfactant [25,26]. The oil phase was poured into the water phase for emulsification, and obtained a uniform and stable aluminum salt microemulsion. Then, an appropriate amount of (1.0 mol L1) ammonia solution was added dropwise into the microemulsions while they were stirred until pH = 8.5. The Al3+ ions and the OH ions were attracted to each other directional movement and interaction. In order to ensure the uniform distribution of the alumina precursor obtained [27,28]. The samples were performed at 60 °C in constant water bath for 30 min, and added 0.1% PEG6000 for ultrasonic dispersion at 30 °C for 40 min. Finally, the amorphous precursor gels were filtered, washed with ethanol for three times, dried at 80 °C and sintered at 1150 °C for 4 h. The filtrate was recovered by fractionation. 2.4. Characterization Measurements of zeta potential of the microemulsions were performed with a Zeta Potential Analyzer (Model No. ZS90 of Malvern Instruments Ltd. (Malvern, UK). Measurement of the zeta potential of a particle dispersion as a function of any of the above can lead to information in formulating the microemulsion to give maximum stability or in determining the optimum conditions for flocculation of the system. The X-ray diffraction spectrum of the calcined powders was characterized using X’Pert PRO made X-ray diffraction spectrometer using Cu Ka, radiation operated at 40 kV and 40 mA current at a step size 0.0170°/s, X-ray patterns were taking by measuring 2h
Fig. 1. The oil-in-water (O/W) microemulsion systems of oil (O), water (W), and surfactant + co-surfactant (S + C). (W1: OP-10 system, W2: Triton X-100 system, W3: Tween80 system.)
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Fig. 2. Formation process for the alumina precursor. (O: Oil phase solution; W: Water phase solution) A: cyclohexane and aluminum chloride crystal; B: ultrapure water; C: ethanol and OP-10; D: alumina precursor.
from 5° to 90°. The peak positions and relatively intensities of the powders pattern were identified by comparison with powders diffraction file (PDF) data. Calculation on the diameter of particles was based on Scherrers’ equation:
Dhkl ¼ 0:9k=bhkl cos h
ð1Þ
(Dhkl is the size of the particles, k is the wavelength of the X-ray and bhkl is the width of the base of diffraction line at half its maximum intensity). The ultrasonic bath was used to suspend calcined powders in ethanol. It was used for microstructure evaluation by scanning electron microscopy (SEM).
3. Results and discussion 3.1. Zeta potential of the oil-in-water microemulsions Surfactants play a crucial role in improving stability in microemulsions. In order to investigate the stability of the (OP-10, Triton X-100, Tween-80) three systems, zeta potentials of the microemulsions were measured at the pH = 7.0. As shown in Fig. 3, zeta potential of numerical remained in three types of microemulsion, all in the stable region range. Among them, the OP-10 system (f = 25.4 mV) was more stable. At this point, the OP-10 system particles were relatively stable with a better dispersion, and there was the stability of the wide area in OP-10 system (Fig. 1: O/W microemulsion phase regions: OP-10 system > Triton X-100 system > Tween-80 system), it was suitable for synthesis of nanoparticles. 3.2. Investigation of the phase transformation of the Al2O3 crystallite using XRD Fig. 4 shows the X-ray diffraction patterns of the precursor powders sample slowly calcined for 3–4 h at various temperatures.
Fig. 3. Zeta potential of three systems (OP-10, Triton X-100, Tween-80).
Fig. 4(2), it was found that the powders remain amorphous up to 900 °C. The most intense diffraction peaks were identified as c-Al2O3 major phase together with the presence of weak maximum that could be associated to the monoclinic h-Al2O3 phase. The XRD analysis in Fig. 4(2) shows the most stable phase, aAl2O3 occurred dominantly at 1150 °C. The diffraction pattern is extremely sharp indicating the existence of a highly crystalline material. Usually a-Al2O3 crystallizes around 1200 °C, however, in the present study the lower crystallization temperature of aAl2O3 could be related to the fine crystalline size and higher specific surface area. The observation reported by Hyuk-Joon et al. [29] indicated that completion of the most stable phase, a-Al2O3 occurs at this temperature. Fig. 4(2) shows the split (1 1 3) diffraction peak of the prototype rhombohedral alumina. In order to find out the crystallite size, a slow scan with step size 0.0170°/s of selected diffraction peaks such as (1 1 3) has been recorded and calculated by Scherers’ formula–equation. The average crystallite size was about 35 nm. The c ? d ? h ? r ? a-Al2O3 transformation is reconstructive and proceeds through a nucleation and growth process. Since most of the activation energy is required for the nucleation process, elevated temperatures are needed to nucleate a-Al2O3. Normally, the c-Al2O3?a-Al2O3 transformation temperature is as high as 1200 °C [30]. A high transformation temperature always results in the coarsening of particles and formation of hard agglomerates in the powders. Thus, a reduction in the c-Al2O3?a-Al2O3 transformation temperature was crucial for the processing of highly reactive ultrafine a-Al2O3 powders. 3.3. Microstructure The morphology of the a-Al2O3 powders were observed on the SEM micrograph included in Fig. 5. As shown in Fig. 5A, a small loosely aggregated mass formed in the dispersion process through aggregation of suspended particles. Selection of Fig. 5A local B, as shown in Fig. 5B exhibited much denser agglomeration, The morphology observations showed that the a-Al2O3 particle were regularly spherical in the shape, grain size was more even. It is ideal morphology for the preparation and the development of high performance alumina ceramics; While the powders mainly consisted of much more porous aggregates. The aggregates were reduced or eliminated by adopting proper processes methods. However, Fig. 5A reveals at the powders appear as irregular agglomerates of widely different sizes, the larger ones reaching 100–200 nm, while the smallest size observed was about 35 nm. The actual particle size in the powders agglomerates was revealed, as illustrated in Fig. 5B. This SEM micrograph clearly shows how one of the large powders agglomerates was composed of numerous powders particles having sizes of less than 100 nm.
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Fig. 4. XRD patterns for the samples calcined at different temperature: (1) 900–1100 °C; (2) 1150 °C.
Fig. 5. The SEM micrograph showing the microemulsion derived a-Al2O3 powders. (A: flocculent–like a-Al2O3 powders particle, B: details of A particle.)
4. Conclusions The pseudo-ternary phase diagram of ultrapure water/(OP-10, Triton X-100 and Tween-80)/ethanol/cyclohexane systems were drawn based on experimental data. Through the analysis, the OP10 system has a wide and stable (O/W) microemulsion region. It was suitable for the preparation of inorganic nanoparticles. The nanoscale a-Al2O3 powders have been successfully prepared by oil-in-water microemulsion method. Based on XRD results, the as-sprayed nanoparticles consisted of c-phase and a-phase Al2O3, which can be converted to a-phase by calcinations at 1150 °C for 4 h. The average crystallite size was about 35 nm by Scherers’ formula–equation. The scanning electron microscopy (SEM) images showed that the powders were spherical with an average diameter of 30–100 nm. The granula small uniform of nano a-Al2O3 powder is a kind of fine material of high performance alumina ceramics industry. The thermodynamic stability of microemulsion system can increase yield, and the experimental waste can be used fractionation methods of separation and purification, recycling and reuse, both to reduce costs and protect the environment.
Acknowledgments The authors are thankful to the National Natural Science Foundation of China (Project: 50972028), for sponsoring this research work. Special thanks are due to Professor WU Bo-lin, Laboratory
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