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Synthesis of Ca(OH)2 Nanoparticles from Diols Barbara Salvadori and Luigi Dei* Department of Chemistry and Consortium CSGI, University of Florence via Gino Capponi, 9 I-50121 Firenze, Italy Received November 16, 2000. In Final Form: January 19, 2001 The aim of this project was to study the preparation and characterization of nanosized Ca(OH)2 particles. Synthesis of Ca(OH)2 particles was performed at a high temperature, and diols were employed as the reaction media. The size and shape of the particles were found to be dependent on different experimental factors, such as reaction temperature, concentration of the reactants, molar ratio, and aging time. Several syntheses were carried out using different parameters. The higher solubility of Ca(OH)2 in diols than in water made the synthesis of the nanoparticles particularly difficult. The diols used (1,2-ethanediol and 1,2-propanediol) remained adsorbed onto the nanoparticles, which caused aggregation, forming micronsized agglomerates. Their removal, with subsequent dispersion of the nanosized units, was achieved by peptization with 2-propanol in an ultrasonic bath. The nanoparticles were characterized by X-ray diffraction analysis, transmission electron microscopy, and Fourier transform infrared spectroscopy. Short aging times produced very small particles sized ca. 30-60 nm. For all the other syntheses carried out, the particle size was in the range of 50-150 nm depending on the molar ratio of the reactants.
Introduction The research on synthesis procedures to prepare nanosized particles of Ca(OH)2 presents interesting implications in both fundamental and applied science. However, the literature on the preparation of moderately watersoluble inorganic nanomaterials is relatively rare,1 compared to the studies concerning nanoparticles of insoluble compounds (sulfides, oxides, metals, etc.).2-7 Nanoparticles of Ca(OH)2 can be used as a new consolidant material for wall paintings and carbonatic stone conservation.8,9 Several papers report that the precipitation of metal hydroxides from corresponding salt solutions is strongly affected by a variety of parameters such as reaction temperature, concentration of reacting species, and aging time.10-15 In particular, it has been shown15 that temperatures above 100 °C promote the formation of nanoscaled particles in nonaqueous media. Also, some studies report the significant effect of organic solvents on the shape and size of the particles obtained by precipitation.16,17 Recently, the synthesis of spherical nanoparticles of * To whom correspondence should be addressed. Fax: + 39055240865. E-mail:
[email protected]. http://apple.csgi.unifi.it. (1) Rees, G. D.; Evans-Gowing, R.; Hammond, S. J.; Robinson, B. H. Langmuir 1999, 15, 1993. (2) Kurihara, K.; Kizling, J.; Stenius, P.; Fendler, J. H. J. Am. Chem. Soc. 1983, 105, 2574. (3) Lisiecki, I.; Pileni, M. P. J. Phys. Chem. 1995, 99, 5077. (4) Bagwe, R. P.; Khilar, K. C. Langmuir 1997, 13, 6432. (5) Bowers, C. R.; Pietrass, T.; Barash, E.; Pines, A.; Grubbs, R. K.; Alivisators, A. P. J. Phys. Chem. 1994, 98, 9400. (6) Chhabbra, V.; Pillai, V.; Mishra, B. K.; Morrone, A.; Shah, D. A. Langmuir 1995, 11, 3307. (7) Vogel, R.; Hoyer, P.; Weller, H. J. Phys. Chem. 1994, 98, 3183. (8) Giorgi, R.; Dei, L.; Baglioni, P. Stud. Conserv. 2000, 45, 154. (9) Ambrosi, M.; Dei, L.; Giorgi, R.; Neto, C.; Baglioni, P. Langmuir, to be submitted. (10) Wilhemy, D. M.; Matijevic, E. J. Chem. Soc., Faraday Trans. 1 1984, 80, 563. (11) Hsu, P.; Ronnquist, L.; Matijevic, E. Langmuir 1988, 4, 31. (12) Matijevic, E.; Scheiner, P. J. Colloid Interface Sci. 1978, 63 (1), 509. (13) Hamada, S.; Kudo, Y.; Minagawa, K. Bull. Chem. Soc. Jpn. 1990, 63, 102. (14) Sugimoto, T.; Matijevic, E. J. Colloid Interface Sci. 1980, 74, 227. (15) Yura, K.; Fredrikson, K. C.; Matijevic, E. Colloids Surf., A 1990, 50, 281.
In(OH)3 has been achieved at high temperatures by using 1,2-ethanediol (bp 195 °C at atmospheric pressure) as the reaction medium.18 The present work concerns the preparation and the characterization of nanosized crystals of Ca(OH)2 by hydrolyzing calcium chloride solutions in diols (1,2ethanediol or 1,2-propanediol) by addition of aqueous sodium hydroxide at elevated temperatures. Many syntheses were set up by adjusting the critical parameters, namely, temperature, mole ratio of reactants, diol type, and aging time. The syntheses were followed by peptization of the particles’ agglomerates, according to a procedure reported in the literature.18 The particles obtained were characterized, determining the chemical composition (Fourier transform infrared (FTIR) spectroscopy), the crystallinity (X-ray diffraction (XRD)), and the shape/size characteristics (transmission electron microscopy (TEM)). Experimental Section A. Materials. Calcium chloride dihydrate, 1,2-ethanediol (ED), 1,2-propanediol (PD), sodium hydroxide, and 2-propanol pro analysi products were supplied by Merck (Darmstadt, Germany) and used without further purification. Water was purified by a Millipore Organex system (R g 18 MΩ cm). B. Synthesis of the Particles and Peptization Procedure. CaCl2‚2H2O (7.35 g) was solubilized in 50 cm3 of ED or PD, by heating the reactor at the selected temperature in an oil bath. Thereafter, 16.7 cm3 of a 6 mol dm-3 aqueous NaOH solution was added dropwise to the Ca2+-containing solution, aging the system at the same temperature under stirring for some minutes (16) Hamada, S.; Matijevic, E. J. Chem. Soc., Faraday Trans. 1 1982, 78, 2147. (17) Matijevic, E.; Cimas, S. Colloid Polym. Sci. 1987, 265, 155. (18) Pe´rez-Maqueda, L. A.; Wang, L.; Matijevic, E. Langmuir 1998, 14, 4397. (19) Mellor, J. W. A Comprehensive Treatise of Inorganic and Theoretical Chemistry; Longmans: London, 1937; Vol. III, pp 673-687. (20) Tekaia-Elhsissen, K.; Delahange-Vidal, A.; Nowogrocki, G.; Figlarz, M. C. R. Acad. Sci. Paris 1989, 309, 349. (21) Farmer, V. C. The Infrared Spectra of Minerals; Mineralogical Society: London, 1974; pp 137-182. (22) Arai, Y. Chemistry of Powder Production; Scarlett, B., Ed.; Powder Technology Series; Chapman & Hall: London, 1996; Chapter 4.
10.1021/la0015967 CCC: $20.00 © 2001 American Chemical Society Published on Web 03/24/2001
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Table 1. Experimental Conditions for the Hydrolysis of the CaCl2/Diol System with Aqueous NaOH Solution and Results synthesis no.
solvent
T (°C)
NaOH/mol dm-3
CaCl2/mol dm-3
NaOH/CaCl2
aging time/min
1 2 3 4 5 6 7 8 9
ED PD ED ED PD ED ED ED ED
150 150 150 150 150 175 175 175 115
1.50 1.50 0.70 0.70 0.70 0.17 0.18 0.18 0.70
0.75 0.75 0.50 0.50 0.50 0.10 0.14 0.14 0.50
2.0 2.0 1.4 1.4 1.4 1.7 1.2 1.2 1.4
40 40 5 40 40 40 5 40 40
(aging time). The particles were separated from the supernatant dispersion by hot filtration under vacuum. During the filtration, it was necessary to keep the temperature high, as the solubility of Ca(OH)2 will increase with decreasing temperature.19 To study the effects of different experimental conditions on the resulting particles, several syntheses were performed, changing one condition each time while the other parameters remained constant. The parameters investigated were the following: (a) type of diol: ED or PD; (b) aging time of the solution; (c) reaction temperature: 115, 150, 175 °C; (d) concentration of Ca2+ in the diol: 0.10, 0.14, 0.50, 0.75 mol dm-3; (e) concentration of added NaOH: 0.18, 0.70, 1.50 mol dm-3; (f) molar ratio of NaOH/CaCl2 in the range of 1.2-2.0. After filtration, the micron-sized particle agglomerates were peptized,18 by washing with water or 2-propanol in an ultrasonic bath; they were then separated by centrifugation at 8000 rpm for 10 min. The liquid phase still containing some particles was peptized again by addition of 2-propanol and immersion in the ultrasonic bath. The entire procedure was repeated up to five times. C. Physicochemical Characterization. The chemical composition of the obtained materials was ascertained by FTIR spectroscopy with a BioRad FTS-40 spectrometer. The crystallinity was checked by XRD using an X-Rays Diffractometer Philips PW 1050/37 equipped with a Co KR (λ ) 1.78 Å) source. About 1 mg of the dried Ca(OH)2 nanoparticles was put as randomly oriented powder onto a Plexiglas sample container, and the XRD patterns were recorded at a scan rate of 2° min-1. The morphology and size of the particles were studied by TEM microscopy using a Philips EM201C apparatus operating at 80 kV. The samples were placed onto carbon-coated copper grids supplied from Taab Chemicals & Equipment for Microscopy Ltd, U.K.
particle size/nm 60-150 50-120 30-60 40-80 60-90
>200
solutions. The different experimental conditions are reported in Table 1. The concentration of NaOH and CaCl2 and their molar ratio had a crucial influence on the synthesis of Ca(OH)2 nanoparticles. The product is relatively soluble in the reaction medium (ED or PD), and this forced us to use suitable concentrations of the reactants. When the concentrations of NaOH and CaCl2 were below 0.2 mol dm-3, no particles were obtained (syntheses 6, 7, and 8) even with an aging time of more than 40 min. The chemical composition of the resulting nanomaterials was checked by FTIR spectroscopy. The spectrum of the material from synthesis 1 (data not reported), collected on the particles immediately after filtration, without any washing (peptization) showed the bands typical of ED, indicating that the Ca(OH)2 particles were contaminated by the diol. No spectral features typical of metal-glycolate formation were observed,20 indicating that the particles remained aggregated by simple ED adsorption without formation of a Ca-ED complex. The peak at 3644 cm-1 relative to the O-H stretching of the solid Ca(OH)221 was
Results and Discussion It has been found that the experimental conditions greatly affected the morphology and the particle size, prepared by hydrolysis of CaCl2 in diol with aqueous NaOH
Figure 1. FTIR spectrum of the material obtained from synthesis 1 (Table 1) after washing five times to remove the excess adsorbed diol.
Figure 2. TEM micrographs of the material obtained from (a) ED (synthesis 1, Table 1) after three peptizations and (b) PD (synthesis 2, Table 1) after five peptizations.
Synthesis of Ca(OH)2 Nanoparticles from Diols
Figure 3. TEM micrographs of the material obtained from (a) PD (synthesis 5, Table 1) after three peptizations and (b) ED (synthesis 3, Table 1) after one peptization.
readily detectable. The solid was washed in 2-propanol or water and immersed in an ultrasonic bath several times to remove the adsorbed diol and peptize the particles.18 The entire procedure yielded distinct nanoparticles of different morphology and size, depending on the conditions used for the synthesis. Peptization removed all the adsorbed ED, as deduced by Figure 1, where the typical FTIR bands of ED completely disappeared. Figure 1 shows also that partial carbonatation of Ca(OH)2 occurred during the peptization procedure, as confirmed by the bands of CaCO3 at 1460 and 874 cm-1. Identical results were achieved with PD as the reaction medium. Because Ca(OH)2 is moderately soluble in water, 2-propanol was selected as a peptizing agent instead of water in order to have high yields of the peptized nanomaterial. Nanosized flat hexagonal particles, sized 60-150 nm, were obtained by peptizing the sample, from syntheses 1 and 2, with 2-propanol. Only partial deagglomeration occurred after three washings (Figure 2a), and distinct particles were obtained after further peptization (Figure 2b). Figure 2a still shows some halos surrounding the particles ascribable to diol retention. The hexagonal shape, evidenced in Figure 2b, implies crystallinity, because Ca(OH)2 crystals belong to the hexagonal system. Moreover, it has been shown that very thin hexagonal platelets are obtained at high temperature and with an elevated supersaturation degree.22 Ca(OH)2 particles prepared under nonstoichiometric conditions in ED and PD (syntheses 4 and 5) were peptized by the same method, yielding also flat hexagonal particles of about 40-80 nm. Only one peptization produced large, ill-defined aggregates, confirming that several peptization procedures are necessary to achieve single nanosized
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Figure 4. XRD patterns of the material obtained from PD (synthesis 5, Table 1) after (a) five peptizations in water, (b) a further five peptizations in 2-propanol, and (c) one further peptization in water.
units.18 Three peptizations were enough for isolating a single hexagonal crystal (see Figure 3a) with a side of 40 nm approximately. A particularly interesting feature is reported in Figure 3b. The decrease of the aging time (5 min of synthesis 3 against 40 min of the other syntheses) caused two main effects: (i) decrease of the average nanoparticle dimensions and (ii) change of the apparent shape from hexagonal to almost spherical. Figure 3b shows these very small particles (ca. 30-60 nm) detected by TEM microscopy. Finally, the washing of particles obtained at a lower temperature (synthesis 9) yielded irregular aggregates. TEM results shown in the previous figures are similar to those found18 for In(OH)3 (Figure 9 in ref 18). This means that the synthesis in diols can be very useful even when the nanomaterial’s solubility is quite high. It could be interesting to check if this method is also suitable for CaSO4‚2H2O nanoparticle productionsanother moderately water-soluble compoundsand compare the results with those achieved by the synthesis in water-in-oil microemulsions.1 The XRD patterns of the nanoparticles obtained under different conditions, after a first peptization by water, show well-defined peaks at d ) 4.90 Å (2θ ) 21°), d ) 3.11 Å (2θ ) 33°), d ) 2.63 Å (2θ ) 40°), d ) 1.93 Å (2θ ) 55°), d ) 1.80 Å (2θ ) 60°), and d ) 1.69 Å (2θ ) 64°), typical of Ca(OH)2 (Figure 4, top). This indicated crystallinity of the synthesized solid. An interesting feature is the peak at d ) 3.03 Å (2θ ) 34°) resulting from the presence of CaCO3, as a consequence of carbonatation during sample handling. After peptization by 2-propanol, the ratio I{001}/
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Figure 5. Simplified model of alignment of Ca(OH)2 hexagonal platelets by 2-propanol adsorption.
I{hkl} was strongly increased (Figure 4, middle), indicating preferential alignment23 of the hexagonal crystals along the basal face (the {001} plane is that of the base of the hexagons). Therefore, 2-propanol destroyed the random orientation of the crystals promoting preferential alignment. To check this hypothesis, the sample used for the XRD measurements (Figure 4, top and middle) was peptized, once again, using water. The intensity of the peaks reverted to the original aspect (Figure 4, bottom), supporting the idea that 2-propanol was physisorbed onto the crystals24 causing the piling. Finally, Figure 5 shows a simplified model of alignment of Ca(OH)2 hexagonal platelets by 2-propanol adsorption. Conclusions This study showed that the method developed in a recent paper18 produced nanosized particles of Ca(OH)2. According to this method, diols are used as reaction media and the size of the synthesized particles is in the micron range. (23) Bloss, F. D. Crystallography and Crystal Chemistry - An Introduction; Holt, Rinehart & Winston Inc.: New York, 1971; pp 493494. (24) Arai, Y. Powder Sci. Eng. 1989, 21 (2), 57.
Salvadori and Dei
Indeed, these micron-sized particles consist of agglomerates of nanometric units that can be altered to form nanoparticles by peptization in suitable solvents. In the present study, we succeeded in synthesizing Ca(OH)2 nanoparticles in ED and PD at high temperatures using both water and 2-propanol as peptizing agents. 2-Propanol was shown to be more effective as a peptizer, because the too-high solubility of Ca(OH)2 in water strongly decreases the nanoparticle yield. The obtained nanoparticles presented different shapes (hexagonal or spherical) and sizes depending on the experimental conditions. The most important parameter seemed to be the molar ratio of the reactants. Nonstoichiometric values (especially [NaOH/CaCl2] ) 1.4) produced units of 50-100 nm, whereas larger particles (60150 nm) were obtained under stoichiometric conditions. The shape of the units was considerably affected by the aging time; 40 min produced hexagonal particles, whereas a very short aging time (5 min) yielded very small, apparently spherical, units. The XRD analysis of the original micron-sized material and of the nanosized particles showed well-defined peaks typical of crystalline Ca(OH)2. Furthermore, the 2-propanol used for peptization was shown to be adsorbed on the basal face of hexagonal particles, making their piling possible. Acknowledgment. The authors express their gratitude to Professor P. Baglioni for invaluable comments and discussions and to Drs. M. Ambrosi, M. A. Cameron, and E. Guarini for very useful suggestions. Thanks are due to Mr. Pierluigi Parrini and to Professor Paola Bonazzi, Dipartimento di Scienza della Terra, Universita` degli Studi di Firenze, for useful comments on XRD measurements. Financial support from CNR “Progetto Finalizzato Beni Culturali”, MURST, University of Florence (Fondi d’Ateneo ex-60%), and Consorzio CSGI, Italy, is gratefully acknowledged. LA0015967
