Langmuir 2007, 23, 9435-9442
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Control of the Morphology and Particle Size of Boehmite Nanoparticles Synthesized under Hydrothermal Conditions Yannick Mathieu, Be´ne´dicte Lebeau, and Valentin Valtchev* Laboratoire de Mate´ riaux a` Porosite´ Controˆ le´ e, UMR-7016 CNRS, ENSCMu, UniVersite´ de Haute Alsace, 3, rue Alfred Werner, 68093 Mulhouse Cedex, France ReceiVed January 29, 2007. In Final Form: April 27, 2007 The spontaneous nucleation under hydrothermal conditions often leads to aggregation of crystallizing particles, which is an undesired phenomenon when the goal is the preparation of nanocrystals with narrow particle size distribution. The present paper reports on the synthesis of boehmite nanocrystals under hydrothermal conditions. An aqueous aluminum chloride salt solution was first prepared, and the pH was increased to 11 using a 5 M sodium hydroxide solution. The hydrothermal treatment was performed at 160 °C for different periods of time. The system yielded relatively small (15-40 nm) boehmite crystallites aggregated into larger (160 nm) particles. To avoid the aggregation, a biocompatible polymer, sodium polyacrylate (NaPa) 2100, was employed as a size-/morphology-controlling agent. Thus, stable colloidal suspensions of rounded boehmite nanoparticles having a size between 15 and 40 nm were obtained at 160 °C for 24 h. Further, the effect of synthesis time on the morphological features of boehmite synthesized in such a NaPa-containing system was investigated. The increase of the synthesis time from 24 to 168 h resulted in the formation of very long boehmite fibers (1000-2000 nm) with an average diameter of about 10 nm. The boehmite samples were characterized by XRD, DLS, TEM, IR, N2 adsorption, and ζ potential measurements. The colloidal stability of the obtained suspension was also studied.
Introduction The past decade was marked by the extensive development of nanomaterials technology. In this respect, the preparation of inorganic nanoparticles with desired size and morphology is highly desired since these materials exhibit interesting properties for a number of applications ranging from catalysis to electronics or optics. Boehmite is an aluminum oxyhydroxide (γ-AlOOH), which is used as a precursor for many aluminum oxide materials especially for the preparation of ceramic catalysts,1-2 membranes,3 coatings,4-5 adsorbents,6 or materials with photoluminescent properties.7 Due to its biocompatibility, another important area of interest is the use of boehmite as an orthopedic or a dental material.8-9 Boehmite syntheses were generally accomplished using three main routes: (i) hydrolysis of an aluminum alkoxide under ambient conditions, (ii) precipitation of inorganic salts in an aqueous medium, and (iii) hydrothermal synthesis at elevated temperatures. Each of these routes has some drawback. For instance, the relatively high price of aluminum alkoxides makes this route less attractive for industrial applications. Moreover, the toxicity of some of the byproducts is also not in favor of such an approach. On the other hand, the precipitation method under ambient conditions requires a perfect control of several experimental parameters such as pH, temperature, ionic strength, and possible contaminations to avoid the formation of aluminum hydroxides such as gibbsite or bayerite. A small deviation of the synthesis procedure and grade of the reactants may lead to an * Corresponding author. E-mail:
[email protected]. (1) Cruz, A. M. A.; Eon, J. G. Appl. Catal., A 1998, 167, 203. (2) Sermon, P. A.; Bond, G. C. J. Chem. Soc., Faraday Trans. I 1979, 75, 395. (3) Furuta, S.; Katsuki, H.; Takagi, H. J. Mater. Sci. Lett. 1994, 13, 1077. (4) Hwang, K.-T.; Lee, H.-S.; Lee, S.-H.; Chung, K.-C.; Park, S.-S.; Lee, J.-H. J. Eur. Ceram. Soc. 2001, 21, 375. (5) Lu, X.; Zhu, R.; He, Y. Surf. Coat. Technol. 1996, 79, 19. (6) Kirkland, J. J. Anal. Chem. 1963, 35, 1295. (7) Yu, Z. Q.; Wand, C. X.; Gu, X. T.; Li, C. J. Lumin. 2004, 106, 153. (8) Webster, T. J.; Hellenmeyer, E. L.; Price, R. L. Biomaterials 2005, 26, 953. (9) Price, R. L.; Gutwein, L. G.; Kaledin, L.; Tepper, F.; Webster, T. J. J. Biomed. Mater. Res. 2003, 67, 1284.
undesired aluminum hydroxide polymorph. On the contrary, the formation of boehmite under hydrothermal conditions can be observed in a very wide range of synthesis conditions. Consequently, this is the method usually used when a fine control of boehmite crystallinity is required.10-13 We have employed this approach in our efforts to obtain boehmite particles with controlled size and morphology. An overview of the earlier studies, including the size, morphology, and specific surface area of boehmite particles is summarized in Table 1. Bugosh et al.14 synthesized fiberlike boehmite with a length of 100-200 nm. The procedure includes the addition of Al powder to an aqueous solution of AlCl3 with an Al/Cl ratio of 2/3. The obtained aluminum chloride solution was then diluted and heated at 160 °C for 40 h. This method was further studied by Brusasco et al.15 who found that a hydrothermal treatment of at least 40 h was necessary to produce a fibrous boehmite at 160 °C. Sterte and Ottersted who studied the boehmite formation in the temperature range 110-160 °C observed that both the crystallinity and fiber length increased with the temperature.16 The synthetic approach published by Bugosh et al.14 was also used under stirring conditions (5.75 rpm), which substantially decreased the synthesis time.17 Thus, fibrous boehmite was obtained in the temperature range 140-160 °C for a 20-h hydrothermal treatment. The authors reported also a substantial change in crystalline morphology; in particular, the increase of the temperature to 220 °C led to the disappearance (10) Sa´nchez-Valente, J.; Bokhimi, X.; Herna´ndez, F. Langmuir 2003, 19, 3583. (11) Bokhimi, X.; Toledo-Antonio, J. A.; Guzma´n-Castillo, M. L.; Mar-Mar, B.; Herna´ndez-Beltra´n, F.; Navarrete, J. J. Solid State Chem. 2001, 161, 319. (12) Bokhimi, X.; Toledo-Antonio, J. A.; Guzma´n-Castillo, M. L.; Herna´ndezBeltra´n, F. J. Solid State Chem. 2001, 159, 32. (13) Bokhimi, X.; Sa´nchez-Valente, J.; Pedraza, F. J. Solid State Chem. 2002, 166, 182. (14) Bugosh, J. J. Phys. Chem. 1961, 65, 1789. (15) Brusasco, R.; Gnassi, J.; Tatian, C.; Baglio, J.; Dwight, K.; Wold, A. Mater. Res. Bull. 1984, 19, 1489. (16) Sterte, J. P.; Otterstedt, J. E. Mater. Res. Bull. 1986, 21, 1159. (17) Buining, P. A.; Pathmamanoharan, C.; Bosboom, M.; Jansen, J. B.; Lekkerkerker, H. J. Am. Ceram. Soc. 1990, 73, 2386.
10.1021/la700233q CCC: $37.00 © 2007 American Chemical Society Published on Web 08/03/2007
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Table 1. Morphological Type, Size, Specific Surface Area, pH Conditions, and Corresponding Literature Reference of the Earlier Work Devoted to Boehmite Synthesis under Hydrothermal Conditionsa
a
pH
morphology
dimensions (nm)
specific surface area (m2‚g-1)
initial
final
ref
fibers fibers fibers fibers fibers fibers plates plates fibers fibers fibers plates nanotubes fibers fibers and plates plates
100 < L < 200 (D ) 5) / 870 < L < 930 255 < L < 358 137 and 306 100 < L < 500 L < 100 L < 100 300 < L < 600 100 < L < 300 L < 50 L < 40 30 < L < 70 (3 < D < 4) 30 < L < 70 (5 < D < 6) / 25-50
275 / 136-272 / / / / / / 321.6-376.2 / / 137.5 >400 / 100
/ / / 3.8-4.4 / 8.65 11.05 / 8 / 2-6 10 / 0.47 and PEO/Al < 0.47, respectively. On the contrary, without PEO surfactant, particles with no regular shape were obtained. Another group
employed Pluronic 84 and obtained lathlike nanolayers of boehmite by hydrothermal treatment at 100 °C,26 whereas without Pluronic a wormhole-to-spongelike structure was obtained. Kuang et al.27 reported the synthesis of boehmite nanotubes with a length of 30-70 nm by employing the cetyltrimethylammonium bromide (C16TMABr) surfactant. All these studies showed that the morphology of boehmite can be controlled by using surface active organic additives and fine-tuning of the synthesis parameters. In the present study we employed a biocompatible polymer agent to control the growth of boehmite, that is, sodium polyacrylate (NaPa) 2100, which has already been used in the synthesis of organic and inorganic crystals.28-32 For instance, the presence of NaPa during the formation of L-asparagine provided needlelike crystals instead of the typical prismatic ones.32 The present investigation is based on the synthetic approach reported in ref 33, which was further developed by the employment of NaPa as a morphology-/size-controlling agent to obtain boehmite nanoparticles with controlled morphology and narrow particle size distribution. The study of the boehmite morphology evolution as a function of the synthesis time was also part of the present work. The obtained nanoparticles are considered for medical applications, in particular to increase the number of bone-forming cells. Indeed, it was found that nanosized aluminum oxides could control the osteoblast behavior in human bones.9 Another application may be devoted to the preparation of thin γ-Al2O3 films by high-temperature transformation of boehmite nanoparticles.3
(18) Buining, P. A.; Pathmamanoharan, C.; Philipse, A. P.; Lekkerkerker, H. N. W. Chem. Eng. Sci. 1993, 48, 411. (19) Buining, P. A.; Philipse, A. P.; Lekkerkerker, H. N. W. Langmuir 1994, 10, 2106. (20) Buining, P. A.; Veldhuizen, Y. S. J.; Pathmamanoharan, C.; Lekkerkerker, H. N. W. Colloids Surf. 1992, 84, 47. (21) Music´, S.; Dragcˇevic´, }.; Popovic´, S.; Vdovic´, N. Mater. Lett. 1994, 18, 309. (22) Tsuchida, T. J. Eur. Ceram. Soc. 2000, 20, 1759. (23) Kaya, C.; He, J. Y.; Gu, X.; Butler, E. G. Microporous Mesoporous Mater. 2002, 54, 37. (24) Zhu, H. Y.; Riches, J. D.; Barry, J. C. Chem. Mater. 2002, 14, 2086. (25) Zhu, H. Y.; Gao, X. P.; Song, D. Y.; Ringer, S. P.; Xi, Y. X.; Frost, R. L. Microporous Mesoporous Mater. 2005, 85, 226.
(26) Zhang, Z. R.; Pinnavaia, T. J. J. Am. Chem. Soc. 2002, 124, 12994. (27) Kuang, D. B.; Fang, Y. P.; Liu, H. Q.; Frommen, C.; Fenske, D. J. Mater. Chem. 2003, 13, 660. (28) Bigi, A.; Boanini, E.; Borghi, M.; Cojazzi, G.; Panzavolta, S.; Roveri, N. J. Inorg. Biochem. 1999, 75, 145. (29) Bigi, A.; Boanini, E.; Botter, R.; Panzavolta, S.; Rubini, K. Biomaterials 2002, 23, 1849. (30) Bigi, A.; Boanini, E.; Falini, G.; Panzavolta, S.; Roveri, N. J. Inorg. Biochem. 2000, 78, 227. (31) Ahmadi, T. S.; Wang, Z. L.; Henglein, A.; El-Sayed, M. A. Chem. Mater. 1996, 8, 1161. (32) Cooper, S. J. Cryst. Eng. Commun. 2001, 56. (33) Music´, S.; Dragcˇevic´, }.; Popovic´, S.; Vdovic´, N. Mater. Chem. Phys. 1999, 59, 12.
Experimental Section Synthesis. Sodium polyacrylate 2100 (Fluka) and aluminum chloride hexahydrate (Avocado) were used without further purification. A typical synthesis procedure includes the dissolution of 9 g
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Table 2. Relation between the NaPa/Al Molar Ratio, pH, and NaPa Solubility in the NaPa-Containing System after 24 h Aging NaPa/Al molar ratio
pH
NaPa solubility
0.57 0.43 0.24 0.05
5.7 5.5 4.8 3
soluble soluble insoluble insoluble
of NaPa 2100 in 75 mL of a 0.1 M aqueous solution of aluminum chloride (NaPa/Al molar ratio ) 0.57). The resulting mixture was mechanically stirred at room temperature for 24 h. A 5 M sodium hydroxide solution was added to the AlCl3 + NaPa mixture within 5 min to increase the pH to 11, and the mixture was stirred for 10 more min. Finally, the mixture was transferred in a PTFE-lined stainless steel autoclave and heated under stirred conditions (15 rpm) at 160 °C. The synthesis time was varied from 17 to 168 h. The solid was recovered after filtration (200 nm, Whatmann), washed three times with 500 mL distilled water, and dried overnight at 80 °C. Characterization Techniques. A Hanna HI991001 pH meter was employed for the pH measurements with a combined glass electrode operating in the pH range 1-14. Powder diffraction patterns were recorded on a STOE STADI-P diffractometer equipped with a linear position-sensitive detector (6° 2θ) in Debye-Scherrer geometry and employing Ge monochromated Cu KR1 radiation (λ ) 1.5406 Å). Thermal (TG/DTA) analyses were performed under air on a Setaram Labsys thermoanalyzer with a heating rate of 5 °C/min up to 1000 °C. Nitrogen adsorption measurements were carried out on a Micromeritics ASAP2010 surface area analyzer after outgassing the samples for at least 15 h at 150 °C. Specific surface areas and pore size distributions were determined by the BET and BJH methods, respectively.34 IR spectroscopy was carried out using a BRUKER Equinox 55 FRA 106/S spectrometer in the wave number range from 400 to 5000 cm-1, on samples prepared by the KBr method. Particle size analysis was performed by dynamic light scattering (DLS) with a Malvern ZetaSizer Nano ZS instrument. An aliquot of the colloidal suspension was collected for the analysis, and the measurement was repeated five times with an interval of 180 s between each measurement. Particle size distributions were calculated using a non-negative least-square (NNLS) algorithm. Zeta potential measurements were also performed with the Malvern ZetaSizer Nano ZS instrument using a folded capillary cell (DTS1060). TEM images were collected on a Philips CM200 microscope equipped with a LaB6 filament. The accelerating voltage was 200 kV. The samples were prepared by depositing several drops of a diluted boehmite suspension onto Cu grids coated with a thin (5 nm) holey carbon film. The size of the boehmite particles (L) was evaluated by the Scherrer equation using the diffraction lines having Miller indices (020), (120), and (051).35
Results Effect of the Sodium Polyacrylate. The salting-out effect resulting in the decrease of polymer solubility in the presence of inorganic salts is a well-known phenomenon.36 Therefore, a series of preliminary experiments was performed with a NaPa/ Al ratio ranging from 0.05 to 0.57 in order to determine the low solubility area for the employed polymer (Table 2). The pKa of sodium polyacrylate 2100 is in the range 4.5-5. For pH values below the pKa, in our case NaPa/Al < 0.43, the polymer NaPa is not fully ionized and contains several hydroxyl groups, thus resulting in a lower degree of solubility. These experiments showed that a NaPa/Al molar ratio of 0.57 was reasonable and allowed the study to be performed without extraneous effects. (34) Brunauer, S.; Emmett, P. H.; Teller, E. J. Am. Chem. Soc. 1938, 60, 309. (35) Guinier, A. Theorie et Technique de la Radiocristallographie, 3rd ed.; Dunod: Paris, 1964; p 482. (36) Ruckenstein, E.; Shulgin, I. L. AdV. Colloid Interface Sci. 2006, 123126, 97.
Figure 1. Particle size distribution of the NaPa-free (a) and NaPacontaining samples (b) synthesized at 160 °C for 24 h.
Samples prepared in the NaPa-free and NaPa-containing systems will be called NaPa-free and NaPa-containing samples, respectively. The first experiments showed that NaPa influences the crystallization of boehmite without changing the nature of the crystallizing material. The NaPa-free and NaPa-containing samples exhibited the characteristic X-ray diffraction patterns of boehmite.37 However, the presence of broader and less intense peaks in the XRD pattern of the NaPa-containing sample suggested lower crystallinity or much smaller crystallites. This effect is certainly due to the polymer agent since all other parameters were similar. It is worth mentioning that a similar effect of NaPa was observed in the synthesis of octacalcium phosphate28-30 and cubic platinum31 nanoparticles. DLS analysis showed a bimodal particle size distribution in both the NaPa-free and NaPa-containing samples. The peaks corresponding to the two particle classes in the NaPa-free sample are centered at about 60 and 300 nm (Figure 1a). Again two populations (Figure 1b) with peaks shifted to smaller values (15-40 and 160 nm) were observed for the NaPa-containing sample. The number distribution analysis revealed that the number of smaller particles, which we considered as primary particles, was much higher than that of the larger ones corresponding to the strong peaks in Figure 1. These data clearly show that the particles synthesized in the NaPa-containing system are smaller. Nitrogen adsorption measurements were in agreement with the DLS data showing a much higher SBET for the particles resulting from the NaPa-containing system. Thus, the NaPa-free and NaPacontaining systems yielded materials with 105 m2‚g-1 and 155 m2‚g-1, respectively. Further, the two samples were studied by TEM, and a substantial difference in the size of the boehmite crystals was observed (Figure 2). It was also found that, besides the difference in crystal size, the morphologies of the two materials differed. The NaPa-free system yielded platelike crystals with well-developed crystal faces (Figure 2a), whereas rounded particles without typical features for crystalline materials were produced from the NaPa-containing system (Figure 2b). Contrast differences in the rounded particles revealed their inhomogeneous nature. Obviously they are built up of small disordered domains, which explains the lower crystallinity evidenced by the XRD analysis. This conclusion matches well the recorded higher specific surface area for the NaPa-containing sample. The interpretation of the TEM and DLS results showed also that the flocculation was much more abundant in the NaPa-free sample. In other words, larger aggregates with a broader particle size distribution were formed when the synthesis system did not contain sodium polyacrylate. (37) Tettenhorst, R.; Hofmann, D. A. Clays Clay Miner. 1980, 28, 373.
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Figure 3. IR spectra of the NaPa-free (a) and NaPa-containing samples (b) synthesized at 160 °C for 24 h and sodium polyacrylate 2100 (c).
Figure 2. TEM micrographs of the NaPa-free (a) and NaPacontaining samples (b) synthesized at 160 °C for 24 h.
The boehmite samples were further studied by IR spectroscopy. The vibration bands of the NaPa-free and NaPa-containing samples along with those of sodium polyacrylate 2100 are shown in Figure 3. For the two boehmite samples, the bands in the range 500-750 cm-1 and at about 1060 cm-1 (Figure 3a) are characteristic of octahedrally coordinated aluminum and AlOH bending vibrations, respectively. Other bands, which are characteristic of the water vibration (1635 cm-1) and of the stretching vibrations of OH groups (3200-3700 cm-1) are also observed. Moreover, the spectrum of the NaPa-containing sample exhibits additional bands in the range 1400-1600 cm-1 (Figure 3b). Inspection of the NaPa spectrum shows that the most intense bands corresponding to the polymer are in this range (Figure 3c). Thus, the spectrum of the NaPa-containing sample comprises characteristic bands at about 1565 and 1410 cm-1, corresponding to the stretching and bending vibrations of R-COO-, and at 1455 cm-1 which is attributed to the CH2 bending band. These data clearly show that the sample contains some polymer agent, which was not eliminated during the purification procedure. Nevertheless, new vibration bands that might be attributed to the NaPaboehmite interactions were not found. The NaPa amount adsorbed on the boehmite particles was evaluated by TG analysis. For the NaPa-free sample, two distinct weight losses can be seen (Figure 4a). The first one of about 2.8% occurring between about 25 and 190 °C is coupled with a weak endothermic signal which is attributed to the elimination of physisorbed water. The second one (∼15.7 wt %) takes place in the range 300-550 °C with a maximum at 470 °C. It corresponds to the dehydroxylation of boehmite (2 γ-AlOOH f γ-Al2O3 + H2O) and its transformation into γ-Al2O3. The XRD
Figure 4. TG/DTA curves of the NaPa-free (a) and NaPa-containing samples (b) synthesized at 160 °C for 24 h.
study of the materials subjected to TG analysis confirmed that γ-Al2O3 was formed. A larger loss (5.1 wt %) was recorded for the NaPa-containing sample (Figure 4b) in the low-temperature range (25-150 °C), suggesting a higher hydrophilicity in respect to the organic-free material. The second weight loss (ca. 8 wt %) coupled with an exothermic peak in the range 280-400 °C is related to the NaPa thermal decomposition. The last weight loss (ca. 12 wt %) is assigned to the dehydroxylation of boehmite. It is difficult, however, to evaluate the exact weight losses related to boehmite dehydroxylation and polymer combustion since the two weight losses overlap. Crystal Growth Kinetics of Boehmite in a NaPa-Containing System. The comparison between the conventional route and the sodium polyacrylate-modified synthesis of boehmite clearly
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Figure 5. XRD patterns of the NaPa-containing samples synthesized for 17 (a), 24 (b), 48 (c), 96 (d), and 168 h (e) heating.
Figure 6. Particle size distribution of the NaPa-containing samples synthesized at 160 °C for 17 (a), 24 (b), and 48 h (c). Table 3. Particle Size and Morphology of the NaPa-Containing Samples Synthesized at 160 °C for 17, 24, and 48 h particle size (nm) time (h)
primary particles
agglomerates
morphology
17 24 48
25-40 15-40 10-30
160-190 160-180 160-190
rounded plates rounded plates rounded plates + fibers
shows that the polymer agent has a great impact on the crystal size, morphology, and aggregation of the AlOOH particles. In a further step, the effect of crystallization time on the characteristics of boehmite synthesized in a NaPa-containing system was studied. Again hydrothermal treatment was performed at 160 °C with a ratio NaPa/Al ) 0.57 for 17, 24, 48, 96, and 168 h, respectively. The pH of the suspension was measured before and after hydrothermal treatment. The final pH varied between 8.5 and 8.0, and a general trend toward a lower pH with the increase of synthesis duration was observed. It should be mentioned that the final pH of NaPa-free systems was much higher, that is about 12.5. All syntheses yielded pure boehmitetype materials. Relatively large diffraction lines suggesting small crystallites and/or imperfect crystal ordering were observed (Figure 5). The increase of synthesis duration slightly improved the boehmite crystallinity as evidenced by the peak widths at half-maximum. The particle size distribution for the NaPa-containing samples synthesized at 160 °C for 17, 24, and 48 h is shown in Figure 6 and summarized in Table 3. As can be seen, each sample comprises two particle classes. Smaller particles that we consider as primary particles range between 10 and 60 nm. This population is less than 10% of the total number of particles. Larger particles
Figure 7. TEM micrographs of the NaPa-containing samples synthesized at 160 °C for 17 (a) and 168 h (b). (Inset) High magnification TEM image of boehmite fibers obtained after 168 h.
which make up the predominant population in each sample are actually aggregated primary particles. Longer lasting syntheses provided fibrous particles for which the DLS technique is not applicable. The evolution of the boehmite morphology in the NaPacontaining system as a function of synthesis time is shown in Figure 7. A representative electron micrograph of rounded boehmite particles synthesized for 17 h is shown in Figure 7a.
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Figure 8. Nitrogen adsorption-desorption isotherms for the NaPa-containing samples synthesized for 24 (a), 48 (b), 96 (c), and 168 h (d). (Inset) Corresponding BJH pore size curves. Table 4. Crystallite Size L (nm) in Different Directions Evaluated from the Width at Half-Maximum Intensity of the [020], [120], and [051] Diffraction Lines for the NaPa-Free and NaPa-Containing Samples for Different Periods of Time L (nm) sample NaPa-free NaPa-containing
time (h)
[020]
[120]
[051]
24 17 24 48 96 168
16.2 7.2 9.2 8.2 9.1 10.2
30.0 11.2 11.2 12.8 14.0 14.7
30.5 12.4 13.4 12.6 15.3 16.7
A closer look at these particles reveals that they are built of smaller platelike crystals. The general appearance of the 48-h sample is similar, although some particles and fiberlike crystals can be seen. The TEM study showed that after 96 h heating the solid does not contain any rounded particles, which are substituted by long fibers. The fiberlike morphology of boehmite was not changed after 168 h hydrothermal treatment (Figure 7b). The length of the fibers varied between 1000 and 3000 nm, and the cross section was about 10 nm. This morphological evolution is related to the Ostwald ripening in the system where boehmite fibers grow at the expense of the less stable platelike crystallites. Changes in crystal morphology resulted in a decrease of the specific surface area of boehmite from 155 m2‚g-1 (24 h, rounded plates) to 103 m2‚g-1 (168 h, fibers). It should be mentioned that boehmite is a nonporous material; therefore, these relatively high values are related to the nanometric size of the crystallites. The hysteresis loops observed in the adsorption/desorption isotherms revealed, however, that the synthesized materials possess textural porosity (Figure 8), which is typical of particles with platelike morphology (Figure 7a). According to the BJH pore-size analysis, the textural pores are in the range 25-65 Å (Figure 8, inset). The adsorption/desorption curves of the fiberlike materials obtained from the syntheses longer than 48 h also comprise hysteresis loops. The latter suggest that the boehmite fibers are not single crystals. The presence of 50-200 Å pores (Figure 8, inset) in the fibers may be due to the alignment of nanosized domains, which is in agreement with the low XRD crystallinity of the materials (Figure 5). The nitrogen adsorption measurements were used to calculate the size of the NaPa-containing samples, assuming platelike morphology. Thus, for the 24-h sample a thickness of 4.3 nm was calculated, which matches well the thickness observed by TEM of about 5 nm for the platelike crystallites (Figure 2b). For the fiberlike particles synthesized for 168 h a cross section of 13 nm was found. Again the calculated value is in a good agreement with the TEM study (ca. 10 nm) (Figure 7b).
Figure 9. Zeta potential of the NaPa-free (a) and NaPa-containing samples (b) synthesized at 160 °C for 24 h.
Stable colloidal suspensions were obtained after purification of the synthesized solids. The colloidal stability depended on the particle size and morphology. Thus, the colloidal suspensions of rounded particles obtained for 17-48 h hydrothermal treatment were stable more than a year under ambient conditions.
Discussion The inhibiting role of sodium polyacrylate in the formation of some compounds has already been mentioned in the Introductory section.28-32 However, the interactions between the growing crystal and NaPa that allow the control of the growth were not clearly identified in the earlier studies. We have tried to shed light on the boehmite-NaPa interactions that lead to smaller and less aggregated particles and upon increase of the crystallization time for long boehmite fibers. Complementary analyses of the boehmite samples clearly show that NaPa inhibits the crystal growth process. Thus, for a similar crystallization time (24 h), boehmite crystallites synthesized with NaPa were smaller in size. A point that might bring some clues about the role of NaPa is whether the growth in a specific crystallographic direction is inhibited. The Scherrer equation was employed to calculate the size of the boehmite crystallites obtained in a conventional and a NaPa-containing system (Table 4). The calculated values confirmed that larger crystals were obtained in the NaPa-free system. The size differences between the NaPa-free and NaPacontaining samples synthesized at 160 °C for 24 h in the considered crystallographic directions varied between 7 and 15 nm. However, it is not clear whether the observed differences are due to preferential blocking by the polymer of a crystal face or to the limited growth in the corresponding direction as a characteristic feature of boehmite under the employed synthesis conditions. For instance, the difference between the NaPa-free
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Figure 10. XRD patterns of thin NaPa-boehmite films obtained by evaporation of a suspension of the samples synthesized at 160 °C for 48 (a), 96 (b), and 168 h (c).
Figure 11. Schematic illustration of different stages of boehmite formation in the presence of NaPa.
and NaPa-containing samples obtained at 160 °C for 24 h in [020] is much lower in respect to [120] and [051]. On the other hand, the reference crystal is also less developed in this direction. Therefore, the observed differences do not provide strong evidence for a polymer interaction with a particular crystal face. The increase of the synthesis time up to 168 h did not change substantially the size of the crystalline domains. On the other hand, the presence of polymer changed the surface properties of the boehmite particles. Thus, ζ potential measurements (Figure 9) revealed that at pH ) 8.5 NaPa-containing boehmite particles possess a negative surface charge (ζ ) -45.5 mV), whereas NaPa-free boehmite crystallites are positively charged (ζ ) 25.7 mV). The strong negative charge of NaPa-containing boehmite prevents the coalescence between the particles. Indeed, the DLS data showed smaller primary particles and aggregates. It is worth mentioning that the degree of polydispersity was always lower for the NaPa-containing samples. The calculations based on the Scherrer equation showed that the increase of synthesis time in NaPa-containing media slightly changed the size of the crystalline domains. Thus, boehmite materials synthesized between 17 and 168 h possess crystalline domains of a fairly similar size (Table 4). For instance, the size of the domains in the 020 direction was 7 and 10 nm, respectively. Nevertheless, dramatic changes in crystalline morphology were observed (Figure 7). The isometric particles dominating the products up to 48 h of hydrothermal treatment were substituted
by long (1000-3000 nm) fibers with diameter of about 10 nm. The XRD study on oriented boehmite samples prepared by evaporation of a few drops of colloidal suspension on a flat holder was performed, and the results are presented in Figure 10. With the increase of the synthesis time from 48 to 96 and 168 h the intensity of the (0k0) reflections increases. It is worth mentioning that the first fibers were observed after 48 h, and their content increased gradually with synthesis time. The increase of fiber content in the solid was coupled with the increase of the (0k0) reflections in the oriented XRD samples. This is a strong proof that the fiber length is along the b direction of the boehmite crystals. It is worth recalling that both, broad XRD peaks and a relatively high surface area of fibrous boehmite show that the fibers are not single crystals. This is supported also by the textural porosity demonstrated by the BJH analysis of the desorption isotherm. Hence, the synthesized fibrous boehmite samples are built of small crystalline domains which are oriented, thus suggesting domain-domain interactions and some spatial organization. The boehmite crystal structure is built of a-c oriented sheets of octahedral aluminum units, and the b-axis is perpendicular to these sheets.37 The latter are connected via hydrogen bonds, which we consider as the most likely sites for interaction with the NaPa polymer. As it was previously mentioned, after hydrothermal treatment, the final pH of the NaPa-containing system decreased to 8-8.5 in contrast with the pH of the NaPa-free system which was around 12. Consequently,
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the decrease of the final pH may be explained by the following reaction: which induces the release of protons due to the
coordination between the aluminic species and carboxylate groups of the NaPa polymer as it was already observed by Bouyer and al.38 with lanthanum hydroxide nanoparticles. However, 27Al NMR experiments on the AlCl3 + NaPa precursors solutions will have to be performed to determine the exact nature of the aluminic species. In addition to the interaction between the polymer and aluminic species suggested by the change of pH in the system, the polymer might play an indirect role in the growth of boehmite particles. Indeed, we have observed that the viscosity of the NaPa-containing system is higher, which may be due to cross-linking of the polymer at 160 °C. The formation of the network of sodium polyacrylate in the reaction media would definitely decrease the mobility of the reactive species and thus influence the growth of boehmite particles. This assumption is supported by the smaller size of the crystallites in the NaPacontaining system. A simple model of the reaction media during the different stages of boehmite formation in a NaPa-containing system is presented in Figure 11. (38) Bouyer, F.; Ge´rardin, C.; Fajula, F.; Putaux, J. -L.; Chopin, T. Colloids Surf., A 2003, 217, 179.
Mathieu et al.
Conclusion A comparative study of boehmite formation under hydrothermal conditions from conventional and sodium polyacrylatemodified systems was performed. Our results demonstrated that sodium polyacrylate interacts with the precursor particles to modify the surface properties and prevent abundant aggregation. In addition, the cross-linkage of the polymer during hydrothermal treatment at 160 °C led to changes in the synthesis media and therefore in the transport of the precursor that influenced the crystal growth process. Thus, stable colloidal suspensions of nanosized boehmite were prepared. The polymer agent led also to substantial changes in the boehmite characteristics. Smaller (15-40 nm) boehmite crystallites with respect to the conventional synthesis (60-80 nm) were obtained. They did not possess distinct crystalline features and formed rounded aggregates. These particles dominated the product up to a 48-h hydrothermal treatment. The increase of the synthesis time led to the formation of large (1000-3000 nm) fibers built of oriented crystalline domains. An important feature of the synthesized materials was a relatively high surface area (103-155 m2‚g-1) and textural porosity with a narrow pore size distribution. These results might be used for the preparation of boehmite materials with tailored characteristics, in particular desired crystal size and morphology. Acknowledgment. Financial support by Sanofi Pasteur (Marcy l’Etoile, France) is greatly acknowledged. LA700233Q
