- Email: [email protected]
Elaboration of boehmite nano-powders by spray-pyrolysis
Powder Technology 190 (2009) 95–98
Contents lists available at ScienceDirect
Powder Technology 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 / p ow t e c
Elaboration of boehmite nano-powders by spray-pyrolysis J.M.A. Caiut a,b, J. Dexpert-Ghys a,⁎, Y. Kihn a, M. Vérelst a, H. Dexpert a, S.J.L. Ribeiro b, Y. Messaddeq b a b
CEMES-CNRS, 29 rue Jeanne Marvig 31055 Toulouse, France Photonic Materials Lab., Institute of Chemistry, UNESP Araraquara-SP, Brazil
A R T I C L E
I N F O
Available online 1 May 2008 Keywords: Spray-pyrolysis Nano-powders Boehmite Alumina Sol–gel
A B S T R A C T Boehmite (γ-AlOOH) synthesis have been investigated using a spray pyrolysis (SP) device starting from a stable sol of Al-tri-sec-butoxide peptized by nitric acid. Free spherical particles from 100 to 500 nm have been elaborated. Particles sub-structure is made of nano-crystalline boehmite with very small average crystallite size (one crystal cell along the b axis). The nano-crystalline boehmite synthesized by SP at low temperature (200 °C) is spontaneously dispersible in water without any surface treatment. Boehmite powder may be transformed to transition γ-alumina by heat post-treatment. Powders of sub-micrometric and spherical γ-alumina particles may also be synthesized by SP at 700 °C. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Spray pyrolysis (SP) is an aerosol process commonly used to form a wide variety of materials in powder form including metals, metal oxides, ceramics, superconductors, fullerenes. SP offers specific advantages compared to other material processing techniques as gas-to-particle conversion processes, liquid or solid-state processing followed by milling. These advantages are a higher purity of the produced powders, a better uniformity in chemical composition, a narrower size distribution, a better regularity in shape and the synthesis of multi-component materials. Another advantage is the relative simplicity of the process which allows scale-up [1–4]. During the past years, we have built in our group a pilot-scale SP apparatus for the synthesis of sub-micrometric phosphor powders [5,6]. More recently we concentrated on the obtention of nanometric powders [7]. In this paper we report on the synthesis of the γ-AlOOH (boehmite) aluminium oxyhydroxide by the spray-pyrolysis of a precursor sol. Boehmite is the most important precursor of the transition aluminas. Transition aluminas are widely used in the industry of adsorbents and catalysts [8,9]. γ-AlOOH is traditionally prepared by i) solid state decomposition of gybsite, ii) precipitation from acidic or basic aluminium aqueous solutions, iii) sol–gel procedures from aluminium alcoholates. The third method allows the preparation of very pure boehmite, although from more expensive chemicals. A very common procedure was first described in [10,11]. It consists of aluminium alkoxide hydrolysis followed by peptization to a clear sol, the gel formation, and then the gel drying to get hydrated boehmite. Further pyrolysis at higher temperature gives porous alumina. The control of the drying conditions is necessary to control the properties of the ⁎ Corresponding author. E-mail addresses: [email protected] (J.M.A. Caiut), [email protected] (J. Dexpert-Ghys). 0032-5910/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.powtec.2008.04.054
boehmite powder. The names “pseudoboehmite” or “nano crystalline boehmite” are used in contrast to microcrystalline or well crystallized boehmite. The nanocrystalline boehmites exhibit crystallite size less than 10 nm, defects and numerous adsorbed water molecules: these features strongly depend on the preparation conditions [12]. The γ-AlOOH powder synthesis described here is a droplet-toparticle synthesis. As discussed in [13] for the case of titanium oxide, the benefits of the aerosol route (i.e. the control of shape and size of the particles) and the benefits of the alkoxide chemistry (i.e. the compositional homogeneity) are combined in this procedure. The spraypyrolysis process may be decomposed in five steps: i) generation of a spray from a liquid precursor by an appropriate droplet generator, ii) spray transport by an air flow during which solvent evaporation occurs, iii) thermolysis of the precipitated particles at higher temperature, iv) intra-particulate sintering, v) finally, extraction of the particles from the gas flow. We have proposed a global modeling of the first two steps with the operating parameters employed in the pilot-scale set-up [14,15]. The following step: i.e. the thermolysis of the precipitated particles, is very dependant on the chemical system considered (each solid particle is in fact a micro-reactor) and on the kinetics of the reactions in the microreactor versus the total time of presence in the reactor. One advantage of SP is to be flexible: the thermolysis may be conducted at any temperature (from 200 to 1000 °C or more) so that low or high temperature stable phases may be achieved in one step. Post synthesis annealing is more often applied to the SP powders, either in conventional ovens or in activated fluidized bed [16,17]. In the case under consideration here, the sol–gel synthesis takes place at much lower temperature since the conventional sol–gel synthesis of boehmite has been performed between 75 and 95 °C [10,11]. We describe in the following the operating conditions required to obtain the nano-sized boehmite by SP, and some physico-chemical characteristics of these nano-powders in particular their ability to be dispersed in water. We also give the characteristics of the transition
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aluminas synthesized in one step by SP at higher temperature or after post-synthesis heat treatment. 2. Experimental The precursor solution is prepared following the methodology established by B. Yoldas [10]. Aluminum-tri-sec-butoxide 25.93 g/ 0.1 mol is added to distilled water (300 mL) at 80 °C. After 2 h of stirring, nitric acid is added as peptizing agent, up to 0.07 HNO3/Al3+ (molar). This sol is very stable: over months. Before processing, the sol is diluted to the concentration 0.2 mol/L of Al3+, it is then poured into the vessel over the piezoelectric pellet. The synthesis corresponds to the following equation: 1AlðO–C4 H9 Þ3ðlÞ þ nH2 OðlÞ þ 0:07HNO3ðaqÞ →1AlOOH:0:75H2 OðSÞ − þ 3C4 H9 OHðgÞ þ 0:07H3 Oþ ðaqÞ þ 0:07NO3ðaqÞ þ ðn–2:82ÞH2 OðlÞ : The laboratory scale SP device is schematized in Fig. 1. The spray is generated by ultrasonic vibrations of a piezoelectric pellet (2.4 MHz) immersed in the solution of precursor. The droplets are collected by an air stream (0.3 m3/h) and drawn into a drying area maintained at 100– 120 °C, then a decomposition-densification area where the temperature may be adjusted up to 1200 °C (Tsynt in the following). Collection of the dried powders is achieved by an electrostatic collector. The residence time of the droplets in the hot zone is about 3–4 s. The reactors are made of pyrex or of quartz depending on their working temperature. The flow (vector gas + solvent and decomposition products vapors + solid particles) is then drawn in an electrostatic precipitator consisting of two collection plates inside a quartz tube. A 5 mm metallic wire is maintained at 12 kV. The flow in the precipitator is maintained at about 130 °C. Currently 500 mL of the precursor are sprayed within 4 h, giving about 5 g of hydrated boehmite. Postsynthesis heat treatments for one hour at Tpt are made in an oven under air. The powder morphologies are evidenced by electron microscopy in the SEM mode in a field-emission scanning electron microscope (Jeol JSM 6700F) or in the TEM mode with a Philips-CM12 transmission electron microscope. The obtained powders are characterized by X-ray diffraction (XRD), on a Seifert XRD3000 diffractometer. Crystallite sizes are calculated using the Scherrer equation D = 0.9λ/(FWHM) cos θ with the full width at half maximum measured in (2θ) an expressed in radians. Thermal analysis scans are recorded with a TG-DTA/DSC Setaram, Labsys instrument, the powders put in platinum vessels, under O2 atmosphere, with either 5 or 10 °C/min heating rates. Nitrogen adsorption–desorption curves are measured with a Belsorp-mini (BEL Japan Inc.) between 0 and 99 p/p0 at 77 K. Pre-treatment was performed under vacuum during 24 h at 80 °C. The
Fig. 2. Thermogravimetric analysis of a sample γ-AlOOH obtained at Tsyn = 200 °C.
size distributions of as prepared boehmite dispersed in water are determined by dynamic laser light scattering using a Brookhaven apparatus with the BI-9000 particle sizing software. 3. Results 3.1. Thermal decomposition of the hydrated boehmite (Tsyn = 200 °C) The thermal evolution of the boehmite samples obtained at Tsyn = 200 °C are analyzed by thermo-gravimetry (Fig. 2). An important weight loss, corresponding to the dehydration — dehydroxylation of the samples is observed from 30 to 450 °C, which could be separated in two more or less distinct steps. The first step (30 to 200 °C) corresponds to the dehydration of physisorbed water molecules and of part of the chemisorbed molecules, whereas the second step (200 to 450 °C) is due to the removal of chemisorbed molecules and to the decomposition of boehmite into alumina. From 450 to 1000 °C, the weight loss corresponds to the dehydroxylation of the surface of alumina. The dehydration-dehydroxylation processes can be expressed following the formulas: 30−450˚C
450−1000˚C
AlOOH; nH2 O Y 1=2Al2 O3 ; mH2 O Y 1=2Al2 O3 E E The average formulation for our hydrated boehmites is then: (Al1 − xEux)OOH, (1.2 ± 0.2)H2O. It must be noticed that the water content may be slightly over-estimated because part of the weight loss is due to the removal of residual nitrates (from the synthesis mixture) and of adsorbed carbonates: these species are detected by IR spectroscopy, but are not quantified. 3.2. X-ray diffraction
Fig. 1. Scheme of the spray-pyrolysis set up.
The X-ray diffractions recorded on powders elaborated at Tsyn = 200, 500 or 700 °C, respectively are displayed on Fig. 3. At Tsyn = 200 °C, and at Tsyn = 500 °C the γ-AlOOH boehmite phase is synthesized. The observed diffraction peaks positions agree well with the data reported for micro-crystalline boehmite (JCPD no. 21-1307). There is a considerable broadening of the diffraction peaks. The coherent lengths (usually understood as the average crystallite sizes) evaluated with the Scherrer formula following three directions are: 1.6 nm perpendicular to the (020) plane, and 3 nm perpendicular to (120) and (031). These are very weak values, characteristics of “nano-crystalline” boehmite. For Tsyn = 500 °C, the peaks are slightly less broad, giving coherent lengths of 2.6 nm (020), 4 nm(120) and 4 nm (031). The
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Fig. 3. X-ray diffraction of the SP samples, (a): Tsyn = 200 °C, (b): Tsyn = 500 °C, (c): Tsyn = 700 °C, (d): Tsyn = 200 °C, Tpt = 300 °C, (e): Tsyn = 200 °C, Tpt = 500 °C, (f): Tsyn = 200 °C, Tpt = 1000 °C.
Fig. 4. SEM images of particles. Left and centre: Tsyn = 200 °C, right: Tsyn = 200 °C + Tpt = 1000 °C.
powder synthesized at 700 °C gives a very badly resolved diffraction, with some peaks characteristic of the γ-alumina. The boehmites synthesized at 200 °C then submitted at post treatments at various temperatures exhibit several phase transformations (Fig. 3 d to f). At 300 °C, γ-AlOOH transforms to a completely amorphous state, then to γ-Al2O3 at 500 °C. Very sharp diffraction peaks of the α-Al2O3 corindon phase are observed at 1000 °C. The boehmite powders prepared by spray-pyrolysis behave as it had been reported previously for boehmite samples from other synthesis methods. After 1 h post-treatment at 300 °C, the boehmite structure has completely disappeared, whereas when the SP is performed at 500 °C, the boehmite structure is well defined. This is because the
residence time in the hot zone of the SP process is very short (few seconds), so that the reactions cannot be completed.
Fig. 5. TEM images of particles. Left: Tsyn = 200 °C, right: Tsyn = 200 °C + Tpt = 1000 °C.
Fig. 6. N2 adsorption – desorption curves.
3.3. Electron microscopy Some images of the powders obtained by scanning or by transmission electron microscopy are gathered in Figs. 4 and 5.
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Fig. 7. Distribution of hydrodynamic sizes for SP boehmite dispersed in water (pH = 7.0).
The boehmite obtained at Tsyn = 200 °C is made of spherical particles. Some particles have a diameter around 2 μm, whereas the major part has a much smaller diameter (estimated from 500 nm to 100 nm). At higher magnification, small platelet like elements around 100 nm or less are observed on the sphere. After post treatment at 1000 °C, the spherical morphology is well maintained, some inter-particulate bridges are observed. The transmission electron microscopy permits to probe the internal core of the particles. After synthesis, the particles are filled: this is evidenced by the contrast between the center of the sphere and its circumference. The image appears darker towards the center because the electrons go through more matter. After post treatment at 1000 °C, the powders are still made essentially of spherical filled particles. The crystallization in the spheres is visible as smaller sub-particles (i.e. the crystallites). 3.4. N2 sorption Characteristic nitrogen absorption-desorption isotherms obtained for SP-boehmite after treatment at 80 °C under vacuum are displayed in Fig. 6. The surface area deduced from these curves is 180 m2/g. 3.5. Water dispersion of the boehmite One very interesting property of the boehmite powders synthesized by SP is that they are spontaneously dispersible in water at room temperature and form very stable suspensions without re-agglomeration and without any surface treatment. The size distribution of particles in the stable sol was analysed by dynamic light scattering. A representative histogram is displayed in Fig. 7. About 1% particles were the isolated crystallites (around 5 nm diameter). 60% particles were aggregates with about 30 nm in diameter, and the remaining 39% particles had 60 to 80 nm size. It is worthy of note that the powders obtained by slow drying the same sol do not form stable dispersion in water. 4. Discussion and conclusion The synthesis, with a spray pyrolysis device, of γ-AlOOH (boehmite) has been investigated starting from a stable sol of Al-trisec-butoxide peptized by nitric acid. Compared to the conventional slow gel formation and gel drying, spray pyrolysis of the sol prevents the long-range polymerization of the aluminate network. Well isolated spherical particles are synthesized. Most particles exhibit size from 100 to 500 nm. The particles sub-structures consist of nanocrystalline boehmite with very small average crystallite size. The
crystal structure of γ-AlOOH first described in [18] is orthorhombic. The unit cell consists of double layers of aluminium-centered distorted octahedra AlO4(OH)2. The hydroxyls (OH groups) are at the outer surface of the double layers and interact to hold the layers together. The sequence of hydrogen-bonded layers expands following the b axis. The crystallite size we have measured in the b crystallographic direction is equivalent to only one cell: then a great number of the OH are in fact at the surface of the crystals. As the OH groups are adsorption sites for water molecules, this may explain why our nanocrystalline boehmite is so much hydrated: AlOOH, (1.18 ± 0.22)H2O. The water molecules at the surface of the nano crystals probably stabilize the nano particules (or small aggregates of nano particles), to form stable dispersion in water. In summary, the spray pyrolysis of a sol is an easy process to synthesize shape controlled spherical sub-micrometric particles of γAlOOH (boehmite). Moreover these sub-micronic particles may be easily destroyed in water to give a stable suspension of nanometric (30–80 nm) particles. When performed at higher temperature, the spray pyrolysis of the same sol gives sub-micrometric particles of transition (γ-) alumina. References [1] A. Gurav, T. Kodas, T. Pluym, Y. Xiong, Aerosol processing of materials, Aerosol Science and Technology 19 (1993) 411–452. [2] S.E. Pratsinis, S. Vemury, Particle formation in gases, Powder Technology 88 (1996) 267–273. [3] G.L. Messing, S.C. Zhang, G.V. Jayanthi, Ceramic powder synthesis by spray pyrolysis, Journal American Ceramic Society 76 (11) (1993) 2707–2726. [4] S. Alavi, B. Caussat, J.P. Couderc, J. Dexpert-Ghys, N. Joffin, D. Neumeyer, M. Verelst, Spray pyrolysis synthesis of submicronic particles. Possibilities and limits, Advances in Science and Technology A 30 (2003) 417–424. [5] N. Joffin, J. Dexpert-Ghys, M. Verelst, G. Baret, A. Garcia, The influence of microstructure on luminescent properties of Y2O3:Eu prepared by spray pyrolysis, Journal of Luminescence 113 (2005) 249–257. [6] N. Joffin, B. Caillier, J. Dexpert-Ghys, M. Verelst, G. Baret, A. Garcia, P. Guillot, J. Galy, R. Mauricot, S. Schamm, Elaboration by spray pyrolysis and characterization in the VUV range of phosphor particles with spherical shape and micronic size, Journal of Physics D 38 (2005) 3261–3268. [7] C. Rossignol, M. Verelst, J. Dexpert-Ghys, S. Rul, Synthesis of undoped ZnO nanoparticles by spray pyrolysis, Advances in Science and Technology 45 (2006) 237–241. [8] Aluminium oxide, activated, in: A. Pearson (Ed.), Kirk-Othmer Encyclopedia of Chemical Technology, vol. 2, J. Wiley, New York, 1994, p. 291. [9] Aluminium oxide, hydrated, in: C. Misra (Ed.), Kirk-Othmer Encyclopedia of Chemical Technology, vol. 2, J. Wiley, New York, 1994, p. 317. [10] B.E. Yoldas, Alumina sol preparation from alkoxides, Ceramic Bulletin 54 (3) (1975) 289–290. [11] B.E. Yoldas, Alumina gels that form porous transparent Al2O3, Journal of Materials Science 10 (1975) 1856–1860. [12] M. Nguefack, S. Rossignol, A.F. Popa, C. Kappenstein, Thermal evolution and model of intermediate boehmite, Physical Chemistry Chemical Physics 5 (2003) 4279–4289. [13] P.P. Ahonen, U. Tapper, E.I. Kaupinnen, J.C. Joubert, J.L. Deschanvres, Aerosol synthesis of Ti–O powders via in-droplet hydrolysis of titanium alkoxide, Materials Science and Engineering A 315 (2001) 113–121. [14] N. Reuge, J. Dexpert-Ghys, M. Verelst, B. Caussat, Y2O3:Eu micronic particles synthesized by spray pyrolysis: global modeling and optimization of the evaporation stage, Chemical Engineering and Processing 47 (2008) 731–743. [15] N. Reuge, J. Dexpert-Ghys, M. Verelst, H. Dexpert, B. Caussat, Modelling of Spray Pyrolysis. Why are the synthesized Y2O3 microparticles hollow? AIChE Journal 54 (2008) 394–405. [16] S. Alavi, N. Joffin, M. Verelst, B. Caussat, Crystallization of Y2O3 micronic powders by different techniques of fluidization at high temperatures, Chemical Engineering Journal 125 (2006) 25–33. [17] S. Alavi, J. Dexpert-Ghys, B. Caussat, High temperature annealing of micrometric Zn2 SiO4 :Mn phosphor powders in fluidized bed, Mat. Res. Bull. (2008), doi:10.1016/j.materresbull.2007.10.019. [18] W.O. Milligan, J.L. Mac Atee, Crystal structure of γ-AlOOH and γ-ScOOH, The Journal of Physical Chemistry 60 (1956) 273–277.
Contents lists available at ScienceDirect
Powder Technology 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 / p ow t e c
Elaboration of boehmite nano-powders by spray-pyrolysis J.M.A. Caiut a,b, J. Dexpert-Ghys a,⁎, Y. Kihn a, M. Vérelst a, H. Dexpert a, S.J.L. Ribeiro b, Y. Messaddeq b a b
CEMES-CNRS, 29 rue Jeanne Marvig 31055 Toulouse, France Photonic Materials Lab., Institute of Chemistry, UNESP Araraquara-SP, Brazil
A R T I C L E
I N F O
Available online 1 May 2008 Keywords: Spray-pyrolysis Nano-powders Boehmite Alumina Sol–gel
A B S T R A C T Boehmite (γ-AlOOH) synthesis have been investigated using a spray pyrolysis (SP) device starting from a stable sol of Al-tri-sec-butoxide peptized by nitric acid. Free spherical particles from 100 to 500 nm have been elaborated. Particles sub-structure is made of nano-crystalline boehmite with very small average crystallite size (one crystal cell along the b axis). The nano-crystalline boehmite synthesized by SP at low temperature (200 °C) is spontaneously dispersible in water without any surface treatment. Boehmite powder may be transformed to transition γ-alumina by heat post-treatment. Powders of sub-micrometric and spherical γ-alumina particles may also be synthesized by SP at 700 °C. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Spray pyrolysis (SP) is an aerosol process commonly used to form a wide variety of materials in powder form including metals, metal oxides, ceramics, superconductors, fullerenes. SP offers specific advantages compared to other material processing techniques as gas-to-particle conversion processes, liquid or solid-state processing followed by milling. These advantages are a higher purity of the produced powders, a better uniformity in chemical composition, a narrower size distribution, a better regularity in shape and the synthesis of multi-component materials. Another advantage is the relative simplicity of the process which allows scale-up [1–4]. During the past years, we have built in our group a pilot-scale SP apparatus for the synthesis of sub-micrometric phosphor powders [5,6]. More recently we concentrated on the obtention of nanometric powders [7]. In this paper we report on the synthesis of the γ-AlOOH (boehmite) aluminium oxyhydroxide by the spray-pyrolysis of a precursor sol. Boehmite is the most important precursor of the transition aluminas. Transition aluminas are widely used in the industry of adsorbents and catalysts [8,9]. γ-AlOOH is traditionally prepared by i) solid state decomposition of gybsite, ii) precipitation from acidic or basic aluminium aqueous solutions, iii) sol–gel procedures from aluminium alcoholates. The third method allows the preparation of very pure boehmite, although from more expensive chemicals. A very common procedure was first described in [10,11]. It consists of aluminium alkoxide hydrolysis followed by peptization to a clear sol, the gel formation, and then the gel drying to get hydrated boehmite. Further pyrolysis at higher temperature gives porous alumina. The control of the drying conditions is necessary to control the properties of the ⁎ Corresponding author. E-mail addresses: [email protected] (J.M.A. Caiut), [email protected] (J. Dexpert-Ghys). 0032-5910/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.powtec.2008.04.054
boehmite powder. The names “pseudoboehmite” or “nano crystalline boehmite” are used in contrast to microcrystalline or well crystallized boehmite. The nanocrystalline boehmites exhibit crystallite size less than 10 nm, defects and numerous adsorbed water molecules: these features strongly depend on the preparation conditions [12]. The γ-AlOOH powder synthesis described here is a droplet-toparticle synthesis. As discussed in [13] for the case of titanium oxide, the benefits of the aerosol route (i.e. the control of shape and size of the particles) and the benefits of the alkoxide chemistry (i.e. the compositional homogeneity) are combined in this procedure. The spraypyrolysis process may be decomposed in five steps: i) generation of a spray from a liquid precursor by an appropriate droplet generator, ii) spray transport by an air flow during which solvent evaporation occurs, iii) thermolysis of the precipitated particles at higher temperature, iv) intra-particulate sintering, v) finally, extraction of the particles from the gas flow. We have proposed a global modeling of the first two steps with the operating parameters employed in the pilot-scale set-up [14,15]. The following step: i.e. the thermolysis of the precipitated particles, is very dependant on the chemical system considered (each solid particle is in fact a micro-reactor) and on the kinetics of the reactions in the microreactor versus the total time of presence in the reactor. One advantage of SP is to be flexible: the thermolysis may be conducted at any temperature (from 200 to 1000 °C or more) so that low or high temperature stable phases may be achieved in one step. Post synthesis annealing is more often applied to the SP powders, either in conventional ovens or in activated fluidized bed [16,17]. In the case under consideration here, the sol–gel synthesis takes place at much lower temperature since the conventional sol–gel synthesis of boehmite has been performed between 75 and 95 °C [10,11]. We describe in the following the operating conditions required to obtain the nano-sized boehmite by SP, and some physico-chemical characteristics of these nano-powders in particular their ability to be dispersed in water. We also give the characteristics of the transition
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aluminas synthesized in one step by SP at higher temperature or after post-synthesis heat treatment. 2. Experimental The precursor solution is prepared following the methodology established by B. Yoldas [10]. Aluminum-tri-sec-butoxide 25.93 g/ 0.1 mol is added to distilled water (300 mL) at 80 °C. After 2 h of stirring, nitric acid is added as peptizing agent, up to 0.07 HNO3/Al3+ (molar). This sol is very stable: over months. Before processing, the sol is diluted to the concentration 0.2 mol/L of Al3+, it is then poured into the vessel over the piezoelectric pellet. The synthesis corresponds to the following equation: 1AlðO–C4 H9 Þ3ðlÞ þ nH2 OðlÞ þ 0:07HNO3ðaqÞ →1AlOOH:0:75H2 OðSÞ − þ 3C4 H9 OHðgÞ þ 0:07H3 Oþ ðaqÞ þ 0:07NO3ðaqÞ þ ðn–2:82ÞH2 OðlÞ : The laboratory scale SP device is schematized in Fig. 1. The spray is generated by ultrasonic vibrations of a piezoelectric pellet (2.4 MHz) immersed in the solution of precursor. The droplets are collected by an air stream (0.3 m3/h) and drawn into a drying area maintained at 100– 120 °C, then a decomposition-densification area where the temperature may be adjusted up to 1200 °C (Tsynt in the following). Collection of the dried powders is achieved by an electrostatic collector. The residence time of the droplets in the hot zone is about 3–4 s. The reactors are made of pyrex or of quartz depending on their working temperature. The flow (vector gas + solvent and decomposition products vapors + solid particles) is then drawn in an electrostatic precipitator consisting of two collection plates inside a quartz tube. A 5 mm metallic wire is maintained at 12 kV. The flow in the precipitator is maintained at about 130 °C. Currently 500 mL of the precursor are sprayed within 4 h, giving about 5 g of hydrated boehmite. Postsynthesis heat treatments for one hour at Tpt are made in an oven under air. The powder morphologies are evidenced by electron microscopy in the SEM mode in a field-emission scanning electron microscope (Jeol JSM 6700F) or in the TEM mode with a Philips-CM12 transmission electron microscope. The obtained powders are characterized by X-ray diffraction (XRD), on a Seifert XRD3000 diffractometer. Crystallite sizes are calculated using the Scherrer equation D = 0.9λ/(FWHM) cos θ with the full width at half maximum measured in (2θ) an expressed in radians. Thermal analysis scans are recorded with a TG-DTA/DSC Setaram, Labsys instrument, the powders put in platinum vessels, under O2 atmosphere, with either 5 or 10 °C/min heating rates. Nitrogen adsorption–desorption curves are measured with a Belsorp-mini (BEL Japan Inc.) between 0 and 99 p/p0 at 77 K. Pre-treatment was performed under vacuum during 24 h at 80 °C. The
Fig. 2. Thermogravimetric analysis of a sample γ-AlOOH obtained at Tsyn = 200 °C.
size distributions of as prepared boehmite dispersed in water are determined by dynamic laser light scattering using a Brookhaven apparatus with the BI-9000 particle sizing software. 3. Results 3.1. Thermal decomposition of the hydrated boehmite (Tsyn = 200 °C) The thermal evolution of the boehmite samples obtained at Tsyn = 200 °C are analyzed by thermo-gravimetry (Fig. 2). An important weight loss, corresponding to the dehydration — dehydroxylation of the samples is observed from 30 to 450 °C, which could be separated in two more or less distinct steps. The first step (30 to 200 °C) corresponds to the dehydration of physisorbed water molecules and of part of the chemisorbed molecules, whereas the second step (200 to 450 °C) is due to the removal of chemisorbed molecules and to the decomposition of boehmite into alumina. From 450 to 1000 °C, the weight loss corresponds to the dehydroxylation of the surface of alumina. The dehydration-dehydroxylation processes can be expressed following the formulas: 30−450˚C
450−1000˚C
AlOOH; nH2 O Y 1=2Al2 O3 ; mH2 O Y 1=2Al2 O3 E E The average formulation for our hydrated boehmites is then: (Al1 − xEux)OOH, (1.2 ± 0.2)H2O. It must be noticed that the water content may be slightly over-estimated because part of the weight loss is due to the removal of residual nitrates (from the synthesis mixture) and of adsorbed carbonates: these species are detected by IR spectroscopy, but are not quantified. 3.2. X-ray diffraction
Fig. 1. Scheme of the spray-pyrolysis set up.
The X-ray diffractions recorded on powders elaborated at Tsyn = 200, 500 or 700 °C, respectively are displayed on Fig. 3. At Tsyn = 200 °C, and at Tsyn = 500 °C the γ-AlOOH boehmite phase is synthesized. The observed diffraction peaks positions agree well with the data reported for micro-crystalline boehmite (JCPD no. 21-1307). There is a considerable broadening of the diffraction peaks. The coherent lengths (usually understood as the average crystallite sizes) evaluated with the Scherrer formula following three directions are: 1.6 nm perpendicular to the (020) plane, and 3 nm perpendicular to (120) and (031). These are very weak values, characteristics of “nano-crystalline” boehmite. For Tsyn = 500 °C, the peaks are slightly less broad, giving coherent lengths of 2.6 nm (020), 4 nm(120) and 4 nm (031). The
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Fig. 3. X-ray diffraction of the SP samples, (a): Tsyn = 200 °C, (b): Tsyn = 500 °C, (c): Tsyn = 700 °C, (d): Tsyn = 200 °C, Tpt = 300 °C, (e): Tsyn = 200 °C, Tpt = 500 °C, (f): Tsyn = 200 °C, Tpt = 1000 °C.
Fig. 4. SEM images of particles. Left and centre: Tsyn = 200 °C, right: Tsyn = 200 °C + Tpt = 1000 °C.
powder synthesized at 700 °C gives a very badly resolved diffraction, with some peaks characteristic of the γ-alumina. The boehmites synthesized at 200 °C then submitted at post treatments at various temperatures exhibit several phase transformations (Fig. 3 d to f). At 300 °C, γ-AlOOH transforms to a completely amorphous state, then to γ-Al2O3 at 500 °C. Very sharp diffraction peaks of the α-Al2O3 corindon phase are observed at 1000 °C. The boehmite powders prepared by spray-pyrolysis behave as it had been reported previously for boehmite samples from other synthesis methods. After 1 h post-treatment at 300 °C, the boehmite structure has completely disappeared, whereas when the SP is performed at 500 °C, the boehmite structure is well defined. This is because the
residence time in the hot zone of the SP process is very short (few seconds), so that the reactions cannot be completed.
Fig. 5. TEM images of particles. Left: Tsyn = 200 °C, right: Tsyn = 200 °C + Tpt = 1000 °C.
Fig. 6. N2 adsorption – desorption curves.
3.3. Electron microscopy Some images of the powders obtained by scanning or by transmission electron microscopy are gathered in Figs. 4 and 5.
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Fig. 7. Distribution of hydrodynamic sizes for SP boehmite dispersed in water (pH = 7.0).
The boehmite obtained at Tsyn = 200 °C is made of spherical particles. Some particles have a diameter around 2 μm, whereas the major part has a much smaller diameter (estimated from 500 nm to 100 nm). At higher magnification, small platelet like elements around 100 nm or less are observed on the sphere. After post treatment at 1000 °C, the spherical morphology is well maintained, some inter-particulate bridges are observed. The transmission electron microscopy permits to probe the internal core of the particles. After synthesis, the particles are filled: this is evidenced by the contrast between the center of the sphere and its circumference. The image appears darker towards the center because the electrons go through more matter. After post treatment at 1000 °C, the powders are still made essentially of spherical filled particles. The crystallization in the spheres is visible as smaller sub-particles (i.e. the crystallites). 3.4. N2 sorption Characteristic nitrogen absorption-desorption isotherms obtained for SP-boehmite after treatment at 80 °C under vacuum are displayed in Fig. 6. The surface area deduced from these curves is 180 m2/g. 3.5. Water dispersion of the boehmite One very interesting property of the boehmite powders synthesized by SP is that they are spontaneously dispersible in water at room temperature and form very stable suspensions without re-agglomeration and without any surface treatment. The size distribution of particles in the stable sol was analysed by dynamic light scattering. A representative histogram is displayed in Fig. 7. About 1% particles were the isolated crystallites (around 5 nm diameter). 60% particles were aggregates with about 30 nm in diameter, and the remaining 39% particles had 60 to 80 nm size. It is worthy of note that the powders obtained by slow drying the same sol do not form stable dispersion in water. 4. Discussion and conclusion The synthesis, with a spray pyrolysis device, of γ-AlOOH (boehmite) has been investigated starting from a stable sol of Al-trisec-butoxide peptized by nitric acid. Compared to the conventional slow gel formation and gel drying, spray pyrolysis of the sol prevents the long-range polymerization of the aluminate network. Well isolated spherical particles are synthesized. Most particles exhibit size from 100 to 500 nm. The particles sub-structures consist of nanocrystalline boehmite with very small average crystallite size. The
crystal structure of γ-AlOOH first described in [18] is orthorhombic. The unit cell consists of double layers of aluminium-centered distorted octahedra AlO4(OH)2. The hydroxyls (OH groups) are at the outer surface of the double layers and interact to hold the layers together. The sequence of hydrogen-bonded layers expands following the b axis. The crystallite size we have measured in the b crystallographic direction is equivalent to only one cell: then a great number of the OH are in fact at the surface of the crystals. As the OH groups are adsorption sites for water molecules, this may explain why our nanocrystalline boehmite is so much hydrated: AlOOH, (1.18 ± 0.22)H2O. The water molecules at the surface of the nano crystals probably stabilize the nano particules (or small aggregates of nano particles), to form stable dispersion in water. In summary, the spray pyrolysis of a sol is an easy process to synthesize shape controlled spherical sub-micrometric particles of γAlOOH (boehmite). Moreover these sub-micronic particles may be easily destroyed in water to give a stable suspension of nanometric (30–80 nm) particles. When performed at higher temperature, the spray pyrolysis of the same sol gives sub-micrometric particles of transition (γ-) alumina. References [1] A. Gurav, T. Kodas, T. Pluym, Y. Xiong, Aerosol processing of materials, Aerosol Science and Technology 19 (1993) 411–452. [2] S.E. Pratsinis, S. Vemury, Particle formation in gases, Powder Technology 88 (1996) 267–273. [3] G.L. Messing, S.C. Zhang, G.V. Jayanthi, Ceramic powder synthesis by spray pyrolysis, Journal American Ceramic Society 76 (11) (1993) 2707–2726. [4] S. Alavi, B. Caussat, J.P. Couderc, J. Dexpert-Ghys, N. Joffin, D. Neumeyer, M. Verelst, Spray pyrolysis synthesis of submicronic particles. Possibilities and limits, Advances in Science and Technology A 30 (2003) 417–424. [5] N. Joffin, J. Dexpert-Ghys, M. Verelst, G. Baret, A. Garcia, The influence of microstructure on luminescent properties of Y2O3:Eu prepared by spray pyrolysis, Journal of Luminescence 113 (2005) 249–257. [6] N. Joffin, B. Caillier, J. Dexpert-Ghys, M. Verelst, G. Baret, A. Garcia, P. Guillot, J. Galy, R. Mauricot, S. Schamm, Elaboration by spray pyrolysis and characterization in the VUV range of phosphor particles with spherical shape and micronic size, Journal of Physics D 38 (2005) 3261–3268. [7] C. Rossignol, M. Verelst, J. Dexpert-Ghys, S. Rul, Synthesis of undoped ZnO nanoparticles by spray pyrolysis, Advances in Science and Technology 45 (2006) 237–241. [8] Aluminium oxide, activated, in: A. Pearson (Ed.), Kirk-Othmer Encyclopedia of Chemical Technology, vol. 2, J. Wiley, New York, 1994, p. 291. [9] Aluminium oxide, hydrated, in: C. Misra (Ed.), Kirk-Othmer Encyclopedia of Chemical Technology, vol. 2, J. Wiley, New York, 1994, p. 317. [10] B.E. Yoldas, Alumina sol preparation from alkoxides, Ceramic Bulletin 54 (3) (1975) 289–290. [11] B.E. Yoldas, Alumina gels that form porous transparent Al2O3, Journal of Materials Science 10 (1975) 1856–1860. [12] M. Nguefack, S. Rossignol, A.F. Popa, C. Kappenstein, Thermal evolution and model of intermediate boehmite, Physical Chemistry Chemical Physics 5 (2003) 4279–4289. [13] P.P. Ahonen, U. Tapper, E.I. Kaupinnen, J.C. Joubert, J.L. Deschanvres, Aerosol synthesis of Ti–O powders via in-droplet hydrolysis of titanium alkoxide, Materials Science and Engineering A 315 (2001) 113–121. [14] N. Reuge, J. Dexpert-Ghys, M. Verelst, B. Caussat, Y2O3:Eu micronic particles synthesized by spray pyrolysis: global modeling and optimization of the evaporation stage, Chemical Engineering and Processing 47 (2008) 731–743. [15] N. Reuge, J. Dexpert-Ghys, M. Verelst, H. Dexpert, B. Caussat, Modelling of Spray Pyrolysis. Why are the synthesized Y2O3 microparticles hollow? AIChE Journal 54 (2008) 394–405. [16] S. Alavi, N. Joffin, M. Verelst, B. Caussat, Crystallization of Y2O3 micronic powders by different techniques of fluidization at high temperatures, Chemical Engineering Journal 125 (2006) 25–33. [17] S. Alavi, J. Dexpert-Ghys, B. Caussat, High temperature annealing of micrometric Zn2 SiO4 :Mn phosphor powders in fluidized bed, Mat. Res. Bull. (2008), doi:10.1016/j.materresbull.2007.10.019. [18] W.O. Milligan, J.L. Mac Atee, Crystal structure of γ-AlOOH and γ-ScOOH, The Journal of Physical Chemistry 60 (1956) 273–277.