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Synthesis of γ-Al2O3 nanopowders by freeze-drying
Materials Research Bulletin 41 (2006) 1520–1529 www.elsevier.com/locate/matresbu
Synthesis of g-Al2O3 nanopowders by freeze-drying Carolina Tallo´n, Rodrigo Moreno *, Ma Isabel Nieto Instituto de Cera´mica y Vidrio, CSIC, c/Kelsen, no. 5, Cantoblanco 28049 Madrid, Spain Received 11 October 2005; accepted 18 January 2006 Available online 14 February 2006
Abstract This work studies the synthesis of g-Al2O3 nanopowders by a freeze-drying method. Aqueous solutions of Al2(SO4)318H2O were used as precursors to Al concentrations of 0.76, 1.00 and 1.40 M. Homogeneous spherical granules with diameters ranging from 1 to 100 mm have been obtained. These porous granules are constituted by soft agglomerates of nanoparticles with primary particle size lower than 20 nm. The microstructure of the agglomerates largely depends on the freezing kinetics. After drying amorphous aluminium sulphate powder is obtained that decomposes at 825 8C leading to the formation of g-Al2O3. Physicochemical study of the freeze-dried powders is performed through particle size distribution and zeta potential measurements. The characterisation of the powders is evaluated considering the influence of processing parameters such as the salt concentration, the freezing rate and the thermal treatment for the synthesis and the dispersing conditions of the obtained powders. By adjusting the dispersing conditions a minimum particle size <30 nm is measured, thus confirming that granules can be easily dispersed into nanoparticles. # 2006 Elsevier Ltd. All rights reserved. Keywords: A. Ceramics; A. Nanostructures; A. Oxides; B. Chemical synthesis; D. Microstructure
1. Introduction A broad body of work is being performed on nanostructured materials due to their interesting properties and important applications [1–3]. In particular, nanoceramics show remarkable properties that are expected to enhance behavioural properties for different applications including mechanical [4,5], catalytic [6,7], and electronic [8,9] devices. Consequently, major efforts have been recently devoted to the synthesis of nanoparticles with high uniformity and controlled shape and size (<100 nm) [10]. The main synthesis methods are vapour-phase synthesis (CVD, spray and laser pyrolysis, etc.) [10–12], and sol–gel processing [13,14]. Other techniques for producing nanoparticles include high-energy ball milling [15–17], microemulsion processing [18,19] and freeze-drying [20–24]. Freeze-drying basically consists in rapidly freezing an aqueous salt solution containing the desired cation or cations and the further sublimation of ice under vacuum conditions. As a result, porous spherical granules consisting of anhydrous sulphate or nitrate (or other salt) are obtained that need a conventional thermal treatment in order to remove the anion and to grow the oxide nanoparticles [20,25]. This method allows the synthesis of a big variety of powders with accurate control of the composition [26,27]. The elimination of water leads to a porous structure of amorphous salt with a high surface area, which consolidates during the calcination process [23,28]. * Corresponding author. Tel.: +34 91 7355840; fax: +34 91 7355843. E-mail address: [email protected] (R. Moreno). 0025-5408/$ – see front matter # 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.materresbull.2006.01.021
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The effect of freeze-drying parameters (temperature and pressure of the chamber and the annealing of the sample) on the rate of sublimation has been studied [29], the chamber temperature having the largest influence on the sublimation rate. The microstructure of the obtained powder shows agglomerates with both microporosity between nanoparticles (intragranular pores) and macroporosity between the agglomerates (intergranular pores) [30,31]. The packing density of granulated powders is also a function of the freezing rate [32,33]. The most important applications of this method deal with the synthesis of mixed oxides or metal composites [9,34– 37], due to the simplicity of preparing solutions of salts mixtures. However, and in spite of the cost of liquid nitrogen, the interest of nanostructured ceramics, including the important applications of gamma-alumina as catalytic support, justifies the study of methods for producing nanopowders of transition alumina with high surface area. Since nanopowders tend to spontaneously agglomerate, a useful way to obtain them is through a controlled granulation process, in which easily dispersible soft agglomerates are formed intentionally. Consequently, freeze-drying can be considered as a promising process for such purpose [13,20,30]. Nitrates are often used due to their high solubility and low temperature for decomposition, but concentrated solutions of nitrates, especially those of trivalent metals, as Al3+, are very difficult to freeze-dry without backmelting [34,38]. For this reason, the use of sulphate has been also explored [30]. The aim of the present work is to study the influence of different parameters in the morphology, microstructure and crystalline phases of alumina nanopowders obtained by a freeze-drying method. The morphology of granules (size, density and porosity), the size, microstructure and strength of agglomerates, the size of primary nanoparticles, and the phase evolution from g-Al2O3 to a-Al2O3 are studied in terms of freeze-drying parameters, such as the concentration of solution, the rate of freezing (i.e., temperature of freezing) and the temperature of calcinations [20,27,28,31]. The dispersion of the granules in water is studied by controlling deflocculant addition and sonication time. The optimum dispersing conditions are studied in terms of particle size distribution and zeta potential. 2. Experimental Reagent grade Al2(SO4)318H2O (Panreac, Spain) was used as Al precursor. Solutions were prepared in deionised water to Al concentrations of 0.76, 1.00 and 1.40 M (saturated solution at room temperature). Droplets were formed with a conventional sprayer and sprayed into the refrigerant. As refrigerant, liquid nitrogen (196 8C) was used, although ethanol cooled with an acetone–ice mixture (5 8C) and a freezing chamber (20 8C) were also tested for comparison purposes. The frozen solution was introduced in the freeze-dryer (CRYODOS-50, Telstar, Spain) for 24 h. The condensator temperature was 50 8C, and the conditions of the storage camera were 20 8C and 0.050 mbar. After drying, considering the differential thermal analysis and thermogravimetry results, DTA-TG (STA 409, Netzsch, Germany), the samples were calcined in open air atmosphere at temperatures ranging from 760 to 1000 8C with soaking times of 1 or 2 h. The thermal treatment had intermediate steps at 400 and 840 8C with dwell times of 15 min and 1 h, respectively. The heating rate was 2 8C/min until 400 8C and 5 8C/min for the other steps. The powders obtained were brushed before the characterization. The particle size distribution of granules was measured by Laser Diffraction (Mastersizer S, Malvern, UK). In order to study the primary particle size and to break the agglomerates, a physicochemical study of the optima dispersing conditions was carried out, using citric acid as dispersant, and considering the effect of the sonication time (Ultrasonication Probe, UP 400S, Hielscher, Germany). The particle size distribution and the zeta potential of nanoparticles were determined by dynamic light scattering and laser Doppler velocimetry, respectively (Zetasizer Nano ZS, Malvern S, UK). The phases were identified by X-ray diffraction, XRD (D5000, Siemens, Germany). The microstructure was observed using transmission electron microscopy, TEM (Hitachi H7100, 125 kV, Japan), scanning electron microscopy, SEM (DSM 950, Zeiss, Germany) and field emission microscopy, FEM (Hitachi S-4700 type I, Japan). The specific surface area was measured by the N2 adsorption, BET method (Monosorb Surface Area Analyser MS-13, Quantachrome Co., USA), and the density by helium pycnometry (Multipycnometer, Quantachrome Co., USA). 3. Results and discussion Fig. 1 shows the size distribution of the granules treated at 900 8C/2 h and their morphology observed by SEM. The freeze-dried powders consisted of spherical porous granules similar to those obtained by different authors
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Fig. 1. Particle size distribution and SEM micrograph of the g-Al2O3 granules calcined at 900 8C/2 h.
[23,24,28,30], the size distribution being a function of the sprayer used. In this work, granules diameter had a wide distribution ranging from 1 to 100 mm. The FEM microstructure of the bulk granules before and after the thermal treatment is shown in Fig. 2, which demonstrates that the morphology is maintained after the thermal treatment. This confirms that the water sublimates during freeze-drying and the salt does not melt during drying and calcinations. The granules are constituted by soft agglomerates of primary nanoparticles as can be seen at high magnification (Fig. 2c). The behaviour of the freeze-dried powders with thermal treatment was studied by DTA-TG, as plotted in Fig. 3. The first strong endothermic peak at 150 8C is associated to the loss of residual water in the salt [22,25], and is accompanied with a weight loss up to 400 8C. A further endothermic reaction with a maximum at 825 8C and the corresponding weight loss are attributed to the decomposition of SO42 into SO3 and SO2 [25]. The formation of Al2O3 occurs at this temperature, but the signal is masked by the sulphate decomposition. Fig. 4 shows the XRD spectra of the as-synthesized powder and after treating at 760 and 800 8C for 1 h. In opposition to observations of other authors [28], no crystallization occurs during freezing, so that an amorphous powder is obtained (Fig. 4a). This powder transforms into anhydrous crystalline aluminium sulphate (ASTM 30-0043) during the thermal treatment (Fig. 4b) and further decomposes leading to the formation of a crypto-crystalline g-alumina (ASTM 79-1558). This reaction is completed at temperatures of 800 8C for 1 h (Fig. 4c). No evidence of sulphur was detected by energy dispersive X-ray analysis (EDX). The transformation of g- to a-alumina is a process that involves a number of metastable transition aluminium oxides between 850 and 1125 8C [38], and it is assumed that the temperature for the total transformation to a-alumina is 1100 8C [20,39,40], although it is possible to enhance the transformation rate by means of a-alumina seeding [40]. In order to establish the temperature range of stability of the g-alumina obtained in this work, the evolution of phases was studied by X-ray diffraction at temperatures ranging from 840 to 1000 8C. The corresponding XRD spectra are plotted in Fig. 5. For short thermal treatments only low crystalline g-alumina was detected, and it was necessary to increase the soaking time of powders treated at 1000 8C to 15 h in order to detect the presence of a-alumina (ASTM 46-1212).
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Fig. 2. Field emission micrographs of the sample: (a) as freeze-dried; (b) and (c) after the temperature treatment (900 8C/1 h) at two different magnifications.
Table 1 shows the density, the surface area and the primary particle size data of samples obtained from solutions of different concentrations, sprayed into liquid nitrogen, and thermally treated at temperatures ranging from 800 to 1000 8C. Average sizes (dBET) were calculated from the surface area and density measurements, considering that particles are spherical and with the same size, using the equation: dBET ¼
6 rSs
Fig. 3. Differential thermal analysis and thermogravimetric curves of the freeze-dried aluminum sulphate. (Heating rate of 5 8C/min until 700 8C and 2 8C/min until 1200 8C, air atmosphere.)
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Fig. 4. Diffraction patterns of samples: (a) as freeze-dried; (b) treated at 760 8C/1 h; (c) treated at 800 8C/1 h.
The analysis of these results reveals that the primary size of the obtained nanoparticles is lower than 20 nm. However, the nanoparticles arrange into agglomerates, as it can be seen in the TEM pictures of Fig. 6. The sample of 1.4 M treated at 1000 8C/2 h was chosen for the study of the primary particle size and the zeta potential, because of its higher crystallinity (density = 3.6 g/cm3). Freeze-dried granules were suspended in water to
Fig. 5. Diffraction patterns of samples treated at different temperatures and times: (a) 840 8C/1 h; (b) 900 8C/1 h; (c) 900 8C/2 h; (d) 1000 8C/2 h; (e) 1000 8C/15 h.
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Table 1 Characterisation of the g-Al2O3 particles obtained by the freeze-drying method from different concentration solutions and thermal treatments Concentration (M)
T (8C)/t (h)
r (g/cm3)
Ss (m2/g)
dBET (nm)
0.76
800/1 840/1 900/1 900/2 1000/2
2.7 0.3 2.7 0.3 2.8 0.3 3.1 0.3 3.2 0.3
186 13 170 12 153 11 150 11 127 9
12 2.0 13 2.2 14 2.4 14 2.4 15 2.5
1.00
900/1 900/2
3.2 0.3 3.4 0.3
146 10 148 10
13 2.2 12 2.0
1.40
900/1 900/2 1000/2
3.1 0.3 3.0 0.3 3.6 0.4
146 10 154 11 138 10
13 2.2 13 2.2 12 2.0
a concentration of 0.05 wt.% using ultrasonic mixing for 2 min. To improve the stability, citric acid was used as a short-chain surfactant. Fig. 7a plots the particle size distribution measured for suspensions with different contents of citric acid. Without dispersant, the distribution is bimodal and a high value of d50 (704 nm) is obtained, which indicates that particles are bond together. The addition of citric acid allows the break-down of the agglomerates leading to monomodal distributions that reach the minimum mean diameter (82 nm) for a dispersant concentration of 0.1 wt.% on a dry solids basis. At these conditions, the influence of the sonication time was tested, in order to reduce even more the size of agglomerates (Fig. 7b). The measured mean diameter decreases below 30 nm after 3 min of sonication. A longer time produces a rearrangement of the particles, and the d50 increases to 182 nm. It can be concluded that the optimum dispersion conditions are achieved for 0.1 wt.% of citric acid and 3 min sonication. The minimum value measured is in the range of the nanoparticle size observed by TEM and dBET calculated. Consequently, the agglomerates of the freeze-dried granules can be easily destroyed so that nanopowders are obtained. The zeta potential versus pH curves are shown in Fig. 8. For the zeta potential determinations, suspensions of 100 mg/l were prepared using KCl 102 M as inert electrolyte. pH was adjusted by adding HCl and KOH. The isoelectric point (IEP) of the g-alumina produced in this work is by 9, which is in good agreement to the results of Tang et al. [41]. This high value of the IEP is typical of very pure aluminas, as it is the case of the one produced in this work. The addition of citric acid shifts down the IEP to a lower pH of 8.1, as expected for the adsorption of an anionic molecule on the surface of amphoteric particles [42]. The thermal treatment has a major influence on the surface area values due to the decrease of intragranular porosity [31] and the formation of necks in the earlier stages of sintering at 1000 8C, which implies a higher strength of the agglomerates. The density values are below the crystallographic density of g-alumina (3.67 g/cm3) and they show a slight tendency to increase with temperature. This can be explained by the increased crystalline degree of g-alumina, in agreement to the DRX results, Fig. 5, and the micrographs of Fig. 6 (a and b). In opposition to the results of Wang and Lloyd [43], who found amorphous alumina with low values of density (1.9–3.2 g/cm3), we obtained g-alumina at any thermal treatment. It is observed that at temperatures lower than 900 8C, the surface area is higher than those of the samples treated at T 900 8C due to the beginning of the sintering process. The temperature and the solution concentration seem not to have any influence on the size of the obtained nanoparticles, as it is observed in TEM pictures and BET calculations. Panitz et al. [28] have reported that the morphology and size of freeze-dried RbCl powders depended on the solution concentration, but they used a wider range of concentrations (0.33–6.6 M) than that employed in this study (0.76–1.4 M). Obviously, the concentration influences on the efficiency of the process, according to the water content of the solution. In all cases, the sublimed water leaves channels (Fig. 2) which is an evidence of phase separation during the freezing process [30]. Another parameter of great importance in the powder microstructure and properties is the type of refrigerant. A refrigerant with very low freezing rate, ethanol cooled with an acetone–ice mixture (5 8C) was tested and compared to liquid nitrogen (196 8C). When the solution is frozen, hydrates are formed which are not soluble in ice, so that salt and water freeze independently. If the solution is sprayed over a low freezing rate refrigerant, large ice crystals appear. Both the salt and the ice have time enough to grow in different sides of the sample, enhancing the formation of layers of
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Fig. 6. Transmission electron microscopy images of samples calcined at 840 8C/1 h, 900 8C/1 h, and 1000 8C/2 h.
salt without the characteristic channels due to the sublimation of ice, as it can be observed in Fig. 9. For high freezing rate (nitrogen), salt and water freeze together, and channels from sublimation of ice appear in the granules (Fig. 2). To ensure that the powder microstructure is related to the freezing kinetics rather than the refrigerant nature, a third test was performed consisting in freezing a thin layer of solution in a chamber previously cooled at 20 8C. The chamber involves a slower freezing rate than the other refrigerants because the sample has not been sprayed, although it reaches an intermediate temperature. In this case, the powder appearance is similar to that of powders frozen in ethanol, shown in Fig. 9, so the microstructure is only a function of the freezing rate and no influence of the nature of the refrigerant is observed.
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Fig. 7. Particle size distributions for: (a) different citric acid contents for samples treated with 2 min sonication; (b) different sonication times for samples dispersed with 0.1 wt.% of citric acid.
Fig. 8. Zeta potential vs. pH curves for nanoparticles with and without citric acid.
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Fig. 9. Field emission micrograph of the sample frozen in ethanol cooled with an acetone–ice mixture.
4. Conclusions The freeze-drying method was used to obtain nanoparticles of g-Al2O3 from a solution of Al2(SO4)318H2O. The primary particles are spherical and have uniform size lower than 20 nm and values of the specific area surface of 120– 180 m2/g. They are forming spherical agglomerates with large porosity due to the channels originated by the sublimation of ice. The microstructure and characteristics of the obtained particles depend on different parameters associated to the process. The temperature and time of the calcination determine the crystalline degree of the g-Al2O3 and have a great influence on the specific surface area as a consequence of the interparticle porosity decrease. However, the salt concentration has an effect over the yield of the process and the macroporosity of the spheres. The microstructure is mainly influenced by the freezing rate that is related to the temperature of the refrigerant selected to spray over. Granules are easily dispersed in water with citric acid and sonication leading to a measured average size of 30 nm. The isoelectric point near pH 9 demonstrates that a high-purity powder is obtained. Acknowledgements This work has been supported by CICYT (Spain, contract No. MAT2003-836). C. Tallo´n acknowledges CSIC and ESF for the concession of an I3P-BPD2004 grant. References [1] A.J. Burggraaf, A.J.A. Winnubist, H. Verweij, in: P. Dura´n, J.F. Ferna´ndez (Eds.), Third ECERS Proceedings, Faenza Editrice Iberica S.L., Castello´n de la Plana, Spain, 1993, pp. 561–576. [2] M.N. Rittner, Am. Ceram. Bull. 81 (3) (2002) 33. [3] A. Arora, Adv. Eng. Mater. 6 (4) (2004) 244. [4] C.C. Koch, Nanostructured Science and Technology, chap. 6, www.wtec.org/loyola/nano/06_01.htm. [5] M. Poorteman, P. Descamps, F. Cambier, M. Plisnier, V. Canonne, J.C. Descamps, J. Eur. Ceram. Soc. 23 (13) (2003) 2361. [6] D.M. Cox, Nanostructured Science and Technology, chap. 4, www.wtec.org/loyola/nano/06_01.htm. [7] E. Di Bartolomeo, N. Kaabbuathong, A. D’Epifanio, M.A. Grilli, E. Traversa, H. Aono, Y. Sadaoka, J. Eur. Ceram. Soc. 24 (6) (2004) 1187. [8] J. Maier, J. Eur. Ceram. Soc. 24 (6) (2004) 1251. [9] D. Perez-Coll, P. Nun˜ez, J.R. Frade, J.C.C. Abrantes, Electrochim. Acta 48 (11) (2003) 1551. [10] M.T. Swihart, Curr. Opinion Coll. Interf. Sci. 8 (1) (2003) 127. [11] N. Kieda, Key Eng. Mater. 264–268 (2004) 3. [12] K. Okuyama, I.W. Lenggoro, Chem. Eng. Sci. 58 (2003) 537. [13] W.M. Zeng, L. Gao, J.K. Guo, Nanostr. Mater. 10 (4) (1998) 543. [14] C.J. Brinker, G.W. Scherner, Sol–gel Science: The Physics and Chemistry of Sol–Gel Processing, Academic Press Inc., New York, 1990. [15] C.C. Koch, Ann. Rev. Mater. Sci. 19 (1989) 121.
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Synthesis of g-Al2O3 nanopowders by freeze-drying Carolina Tallo´n, Rodrigo Moreno *, Ma Isabel Nieto Instituto de Cera´mica y Vidrio, CSIC, c/Kelsen, no. 5, Cantoblanco 28049 Madrid, Spain Received 11 October 2005; accepted 18 January 2006 Available online 14 February 2006
Abstract This work studies the synthesis of g-Al2O3 nanopowders by a freeze-drying method. Aqueous solutions of Al2(SO4)318H2O were used as precursors to Al concentrations of 0.76, 1.00 and 1.40 M. Homogeneous spherical granules with diameters ranging from 1 to 100 mm have been obtained. These porous granules are constituted by soft agglomerates of nanoparticles with primary particle size lower than 20 nm. The microstructure of the agglomerates largely depends on the freezing kinetics. After drying amorphous aluminium sulphate powder is obtained that decomposes at 825 8C leading to the formation of g-Al2O3. Physicochemical study of the freeze-dried powders is performed through particle size distribution and zeta potential measurements. The characterisation of the powders is evaluated considering the influence of processing parameters such as the salt concentration, the freezing rate and the thermal treatment for the synthesis and the dispersing conditions of the obtained powders. By adjusting the dispersing conditions a minimum particle size <30 nm is measured, thus confirming that granules can be easily dispersed into nanoparticles. # 2006 Elsevier Ltd. All rights reserved. Keywords: A. Ceramics; A. Nanostructures; A. Oxides; B. Chemical synthesis; D. Microstructure
1. Introduction A broad body of work is being performed on nanostructured materials due to their interesting properties and important applications [1–3]. In particular, nanoceramics show remarkable properties that are expected to enhance behavioural properties for different applications including mechanical [4,5], catalytic [6,7], and electronic [8,9] devices. Consequently, major efforts have been recently devoted to the synthesis of nanoparticles with high uniformity and controlled shape and size (<100 nm) [10]. The main synthesis methods are vapour-phase synthesis (CVD, spray and laser pyrolysis, etc.) [10–12], and sol–gel processing [13,14]. Other techniques for producing nanoparticles include high-energy ball milling [15–17], microemulsion processing [18,19] and freeze-drying [20–24]. Freeze-drying basically consists in rapidly freezing an aqueous salt solution containing the desired cation or cations and the further sublimation of ice under vacuum conditions. As a result, porous spherical granules consisting of anhydrous sulphate or nitrate (or other salt) are obtained that need a conventional thermal treatment in order to remove the anion and to grow the oxide nanoparticles [20,25]. This method allows the synthesis of a big variety of powders with accurate control of the composition [26,27]. The elimination of water leads to a porous structure of amorphous salt with a high surface area, which consolidates during the calcination process [23,28]. * Corresponding author. Tel.: +34 91 7355840; fax: +34 91 7355843. E-mail address: [email protected] (R. Moreno). 0025-5408/$ – see front matter # 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.materresbull.2006.01.021
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The effect of freeze-drying parameters (temperature and pressure of the chamber and the annealing of the sample) on the rate of sublimation has been studied [29], the chamber temperature having the largest influence on the sublimation rate. The microstructure of the obtained powder shows agglomerates with both microporosity between nanoparticles (intragranular pores) and macroporosity between the agglomerates (intergranular pores) [30,31]. The packing density of granulated powders is also a function of the freezing rate [32,33]. The most important applications of this method deal with the synthesis of mixed oxides or metal composites [9,34– 37], due to the simplicity of preparing solutions of salts mixtures. However, and in spite of the cost of liquid nitrogen, the interest of nanostructured ceramics, including the important applications of gamma-alumina as catalytic support, justifies the study of methods for producing nanopowders of transition alumina with high surface area. Since nanopowders tend to spontaneously agglomerate, a useful way to obtain them is through a controlled granulation process, in which easily dispersible soft agglomerates are formed intentionally. Consequently, freeze-drying can be considered as a promising process for such purpose [13,20,30]. Nitrates are often used due to their high solubility and low temperature for decomposition, but concentrated solutions of nitrates, especially those of trivalent metals, as Al3+, are very difficult to freeze-dry without backmelting [34,38]. For this reason, the use of sulphate has been also explored [30]. The aim of the present work is to study the influence of different parameters in the morphology, microstructure and crystalline phases of alumina nanopowders obtained by a freeze-drying method. The morphology of granules (size, density and porosity), the size, microstructure and strength of agglomerates, the size of primary nanoparticles, and the phase evolution from g-Al2O3 to a-Al2O3 are studied in terms of freeze-drying parameters, such as the concentration of solution, the rate of freezing (i.e., temperature of freezing) and the temperature of calcinations [20,27,28,31]. The dispersion of the granules in water is studied by controlling deflocculant addition and sonication time. The optimum dispersing conditions are studied in terms of particle size distribution and zeta potential. 2. Experimental Reagent grade Al2(SO4)318H2O (Panreac, Spain) was used as Al precursor. Solutions were prepared in deionised water to Al concentrations of 0.76, 1.00 and 1.40 M (saturated solution at room temperature). Droplets were formed with a conventional sprayer and sprayed into the refrigerant. As refrigerant, liquid nitrogen (196 8C) was used, although ethanol cooled with an acetone–ice mixture (5 8C) and a freezing chamber (20 8C) were also tested for comparison purposes. The frozen solution was introduced in the freeze-dryer (CRYODOS-50, Telstar, Spain) for 24 h. The condensator temperature was 50 8C, and the conditions of the storage camera were 20 8C and 0.050 mbar. After drying, considering the differential thermal analysis and thermogravimetry results, DTA-TG (STA 409, Netzsch, Germany), the samples were calcined in open air atmosphere at temperatures ranging from 760 to 1000 8C with soaking times of 1 or 2 h. The thermal treatment had intermediate steps at 400 and 840 8C with dwell times of 15 min and 1 h, respectively. The heating rate was 2 8C/min until 400 8C and 5 8C/min for the other steps. The powders obtained were brushed before the characterization. The particle size distribution of granules was measured by Laser Diffraction (Mastersizer S, Malvern, UK). In order to study the primary particle size and to break the agglomerates, a physicochemical study of the optima dispersing conditions was carried out, using citric acid as dispersant, and considering the effect of the sonication time (Ultrasonication Probe, UP 400S, Hielscher, Germany). The particle size distribution and the zeta potential of nanoparticles were determined by dynamic light scattering and laser Doppler velocimetry, respectively (Zetasizer Nano ZS, Malvern S, UK). The phases were identified by X-ray diffraction, XRD (D5000, Siemens, Germany). The microstructure was observed using transmission electron microscopy, TEM (Hitachi H7100, 125 kV, Japan), scanning electron microscopy, SEM (DSM 950, Zeiss, Germany) and field emission microscopy, FEM (Hitachi S-4700 type I, Japan). The specific surface area was measured by the N2 adsorption, BET method (Monosorb Surface Area Analyser MS-13, Quantachrome Co., USA), and the density by helium pycnometry (Multipycnometer, Quantachrome Co., USA). 3. Results and discussion Fig. 1 shows the size distribution of the granules treated at 900 8C/2 h and their morphology observed by SEM. The freeze-dried powders consisted of spherical porous granules similar to those obtained by different authors
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Fig. 1. Particle size distribution and SEM micrograph of the g-Al2O3 granules calcined at 900 8C/2 h.
[23,24,28,30], the size distribution being a function of the sprayer used. In this work, granules diameter had a wide distribution ranging from 1 to 100 mm. The FEM microstructure of the bulk granules before and after the thermal treatment is shown in Fig. 2, which demonstrates that the morphology is maintained after the thermal treatment. This confirms that the water sublimates during freeze-drying and the salt does not melt during drying and calcinations. The granules are constituted by soft agglomerates of primary nanoparticles as can be seen at high magnification (Fig. 2c). The behaviour of the freeze-dried powders with thermal treatment was studied by DTA-TG, as plotted in Fig. 3. The first strong endothermic peak at 150 8C is associated to the loss of residual water in the salt [22,25], and is accompanied with a weight loss up to 400 8C. A further endothermic reaction with a maximum at 825 8C and the corresponding weight loss are attributed to the decomposition of SO42 into SO3 and SO2 [25]. The formation of Al2O3 occurs at this temperature, but the signal is masked by the sulphate decomposition. Fig. 4 shows the XRD spectra of the as-synthesized powder and after treating at 760 and 800 8C for 1 h. In opposition to observations of other authors [28], no crystallization occurs during freezing, so that an amorphous powder is obtained (Fig. 4a). This powder transforms into anhydrous crystalline aluminium sulphate (ASTM 30-0043) during the thermal treatment (Fig. 4b) and further decomposes leading to the formation of a crypto-crystalline g-alumina (ASTM 79-1558). This reaction is completed at temperatures of 800 8C for 1 h (Fig. 4c). No evidence of sulphur was detected by energy dispersive X-ray analysis (EDX). The transformation of g- to a-alumina is a process that involves a number of metastable transition aluminium oxides between 850 and 1125 8C [38], and it is assumed that the temperature for the total transformation to a-alumina is 1100 8C [20,39,40], although it is possible to enhance the transformation rate by means of a-alumina seeding [40]. In order to establish the temperature range of stability of the g-alumina obtained in this work, the evolution of phases was studied by X-ray diffraction at temperatures ranging from 840 to 1000 8C. The corresponding XRD spectra are plotted in Fig. 5. For short thermal treatments only low crystalline g-alumina was detected, and it was necessary to increase the soaking time of powders treated at 1000 8C to 15 h in order to detect the presence of a-alumina (ASTM 46-1212).
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Fig. 2. Field emission micrographs of the sample: (a) as freeze-dried; (b) and (c) after the temperature treatment (900 8C/1 h) at two different magnifications.
Table 1 shows the density, the surface area and the primary particle size data of samples obtained from solutions of different concentrations, sprayed into liquid nitrogen, and thermally treated at temperatures ranging from 800 to 1000 8C. Average sizes (dBET) were calculated from the surface area and density measurements, considering that particles are spherical and with the same size, using the equation: dBET ¼
6 rSs
Fig. 3. Differential thermal analysis and thermogravimetric curves of the freeze-dried aluminum sulphate. (Heating rate of 5 8C/min until 700 8C and 2 8C/min until 1200 8C, air atmosphere.)
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Fig. 4. Diffraction patterns of samples: (a) as freeze-dried; (b) treated at 760 8C/1 h; (c) treated at 800 8C/1 h.
The analysis of these results reveals that the primary size of the obtained nanoparticles is lower than 20 nm. However, the nanoparticles arrange into agglomerates, as it can be seen in the TEM pictures of Fig. 6. The sample of 1.4 M treated at 1000 8C/2 h was chosen for the study of the primary particle size and the zeta potential, because of its higher crystallinity (density = 3.6 g/cm3). Freeze-dried granules were suspended in water to
Fig. 5. Diffraction patterns of samples treated at different temperatures and times: (a) 840 8C/1 h; (b) 900 8C/1 h; (c) 900 8C/2 h; (d) 1000 8C/2 h; (e) 1000 8C/15 h.
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Table 1 Characterisation of the g-Al2O3 particles obtained by the freeze-drying method from different concentration solutions and thermal treatments Concentration (M)
T (8C)/t (h)
r (g/cm3)
Ss (m2/g)
dBET (nm)
0.76
800/1 840/1 900/1 900/2 1000/2
2.7 0.3 2.7 0.3 2.8 0.3 3.1 0.3 3.2 0.3
186 13 170 12 153 11 150 11 127 9
12 2.0 13 2.2 14 2.4 14 2.4 15 2.5
1.00
900/1 900/2
3.2 0.3 3.4 0.3
146 10 148 10
13 2.2 12 2.0
1.40
900/1 900/2 1000/2
3.1 0.3 3.0 0.3 3.6 0.4
146 10 154 11 138 10
13 2.2 13 2.2 12 2.0
a concentration of 0.05 wt.% using ultrasonic mixing for 2 min. To improve the stability, citric acid was used as a short-chain surfactant. Fig. 7a plots the particle size distribution measured for suspensions with different contents of citric acid. Without dispersant, the distribution is bimodal and a high value of d50 (704 nm) is obtained, which indicates that particles are bond together. The addition of citric acid allows the break-down of the agglomerates leading to monomodal distributions that reach the minimum mean diameter (82 nm) for a dispersant concentration of 0.1 wt.% on a dry solids basis. At these conditions, the influence of the sonication time was tested, in order to reduce even more the size of agglomerates (Fig. 7b). The measured mean diameter decreases below 30 nm after 3 min of sonication. A longer time produces a rearrangement of the particles, and the d50 increases to 182 nm. It can be concluded that the optimum dispersion conditions are achieved for 0.1 wt.% of citric acid and 3 min sonication. The minimum value measured is in the range of the nanoparticle size observed by TEM and dBET calculated. Consequently, the agglomerates of the freeze-dried granules can be easily destroyed so that nanopowders are obtained. The zeta potential versus pH curves are shown in Fig. 8. For the zeta potential determinations, suspensions of 100 mg/l were prepared using KCl 102 M as inert electrolyte. pH was adjusted by adding HCl and KOH. The isoelectric point (IEP) of the g-alumina produced in this work is by 9, which is in good agreement to the results of Tang et al. [41]. This high value of the IEP is typical of very pure aluminas, as it is the case of the one produced in this work. The addition of citric acid shifts down the IEP to a lower pH of 8.1, as expected for the adsorption of an anionic molecule on the surface of amphoteric particles [42]. The thermal treatment has a major influence on the surface area values due to the decrease of intragranular porosity [31] and the formation of necks in the earlier stages of sintering at 1000 8C, which implies a higher strength of the agglomerates. The density values are below the crystallographic density of g-alumina (3.67 g/cm3) and they show a slight tendency to increase with temperature. This can be explained by the increased crystalline degree of g-alumina, in agreement to the DRX results, Fig. 5, and the micrographs of Fig. 6 (a and b). In opposition to the results of Wang and Lloyd [43], who found amorphous alumina with low values of density (1.9–3.2 g/cm3), we obtained g-alumina at any thermal treatment. It is observed that at temperatures lower than 900 8C, the surface area is higher than those of the samples treated at T 900 8C due to the beginning of the sintering process. The temperature and the solution concentration seem not to have any influence on the size of the obtained nanoparticles, as it is observed in TEM pictures and BET calculations. Panitz et al. [28] have reported that the morphology and size of freeze-dried RbCl powders depended on the solution concentration, but they used a wider range of concentrations (0.33–6.6 M) than that employed in this study (0.76–1.4 M). Obviously, the concentration influences on the efficiency of the process, according to the water content of the solution. In all cases, the sublimed water leaves channels (Fig. 2) which is an evidence of phase separation during the freezing process [30]. Another parameter of great importance in the powder microstructure and properties is the type of refrigerant. A refrigerant with very low freezing rate, ethanol cooled with an acetone–ice mixture (5 8C) was tested and compared to liquid nitrogen (196 8C). When the solution is frozen, hydrates are formed which are not soluble in ice, so that salt and water freeze independently. If the solution is sprayed over a low freezing rate refrigerant, large ice crystals appear. Both the salt and the ice have time enough to grow in different sides of the sample, enhancing the formation of layers of
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Fig. 6. Transmission electron microscopy images of samples calcined at 840 8C/1 h, 900 8C/1 h, and 1000 8C/2 h.
salt without the characteristic channels due to the sublimation of ice, as it can be observed in Fig. 9. For high freezing rate (nitrogen), salt and water freeze together, and channels from sublimation of ice appear in the granules (Fig. 2). To ensure that the powder microstructure is related to the freezing kinetics rather than the refrigerant nature, a third test was performed consisting in freezing a thin layer of solution in a chamber previously cooled at 20 8C. The chamber involves a slower freezing rate than the other refrigerants because the sample has not been sprayed, although it reaches an intermediate temperature. In this case, the powder appearance is similar to that of powders frozen in ethanol, shown in Fig. 9, so the microstructure is only a function of the freezing rate and no influence of the nature of the refrigerant is observed.
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Fig. 7. Particle size distributions for: (a) different citric acid contents for samples treated with 2 min sonication; (b) different sonication times for samples dispersed with 0.1 wt.% of citric acid.
Fig. 8. Zeta potential vs. pH curves for nanoparticles with and without citric acid.
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Fig. 9. Field emission micrograph of the sample frozen in ethanol cooled with an acetone–ice mixture.
4. Conclusions The freeze-drying method was used to obtain nanoparticles of g-Al2O3 from a solution of Al2(SO4)318H2O. The primary particles are spherical and have uniform size lower than 20 nm and values of the specific area surface of 120– 180 m2/g. They are forming spherical agglomerates with large porosity due to the channels originated by the sublimation of ice. The microstructure and characteristics of the obtained particles depend on different parameters associated to the process. The temperature and time of the calcination determine the crystalline degree of the g-Al2O3 and have a great influence on the specific surface area as a consequence of the interparticle porosity decrease. However, the salt concentration has an effect over the yield of the process and the macroporosity of the spheres. The microstructure is mainly influenced by the freezing rate that is related to the temperature of the refrigerant selected to spray over. Granules are easily dispersed in water with citric acid and sonication leading to a measured average size of 30 nm. The isoelectric point near pH 9 demonstrates that a high-purity powder is obtained. Acknowledgements This work has been supported by CICYT (Spain, contract No. MAT2003-836). C. Tallo´n acknowledges CSIC and ESF for the concession of an I3P-BPD2004 grant. References [1] A.J. Burggraaf, A.J.A. Winnubist, H. Verweij, in: P. Dura´n, J.F. Ferna´ndez (Eds.), Third ECERS Proceedings, Faenza Editrice Iberica S.L., Castello´n de la Plana, Spain, 1993, pp. 561–576. [2] M.N. Rittner, Am. Ceram. Bull. 81 (3) (2002) 33. [3] A. Arora, Adv. Eng. Mater. 6 (4) (2004) 244. [4] C.C. Koch, Nanostructured Science and Technology, chap. 6, www.wtec.org/loyola/nano/06_01.htm. [5] M. Poorteman, P. Descamps, F. Cambier, M. Plisnier, V. Canonne, J.C. Descamps, J. Eur. Ceram. Soc. 23 (13) (2003) 2361. [6] D.M. Cox, Nanostructured Science and Technology, chap. 4, www.wtec.org/loyola/nano/06_01.htm. [7] E. Di Bartolomeo, N. Kaabbuathong, A. D’Epifanio, M.A. Grilli, E. Traversa, H. Aono, Y. Sadaoka, J. Eur. Ceram. Soc. 24 (6) (2004) 1187. [8] J. Maier, J. Eur. Ceram. Soc. 24 (6) (2004) 1251. [9] D. Perez-Coll, P. Nun˜ez, J.R. Frade, J.C.C. Abrantes, Electrochim. Acta 48 (11) (2003) 1551. [10] M.T. Swihart, Curr. Opinion Coll. Interf. Sci. 8 (1) (2003) 127. [11] N. Kieda, Key Eng. Mater. 264–268 (2004) 3. [12] K. Okuyama, I.W. Lenggoro, Chem. Eng. Sci. 58 (2003) 537. [13] W.M. Zeng, L. Gao, J.K. Guo, Nanostr. Mater. 10 (4) (1998) 543. [14] C.J. Brinker, G.W. Scherner, Sol–gel Science: The Physics and Chemistry of Sol–Gel Processing, Academic Press Inc., New York, 1990. [15] C.C. Koch, Ann. Rev. Mater. Sci. 19 (1989) 121.
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