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Why to synthesize vaterite polymorph of calcium carbonate on the cellulose matrix via sonochemistry process?
Ultrasonics Sonochemistry 20 (2013) 1188–1193
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Why to synthesize vaterite polymorph of calcium carbonate on the cellulose matrix via sonochemistry process? Lian-Hua Fu a, Yan-Yan Dong a, Ming-Guo Ma a,⇑, Wen Yue a, Shao-Long Sun a, Run-Cang Sun a,b a b
Beijing Key Laboratory of Lignocellulosic Chemistry, College of Materials Science and Technology, Beijing Forestry University, Beijing 100083, PR China State Key Laboratory of Pulp and Paper Engineering, South China University of Technology, Guangzhou 510640, PR China
a r t i c l e
i n f o
Article history: Received 20 January 2013 Received in revised form 5 March 2013 Accepted 20 March 2013 Available online 4 April 2013 Keywords: Calcium carbonate Vaterite Cellulose Ultrasound Mechanism
a b s t r a c t Vaterite is an important biomedical material due to its features such as high specific surface area, high solubility, high dispersion, and small specific gravity. The purposes of this article were to explore the growth mechanism of vaterite on the cellulose matrix via sonochmistry process. In the work reported herein, the influences of experimental parameters on the polymorph of calcium carbonate were investigated in detail. The calcium carbonate crystals on the cellulose matrix were characterized by means of Xray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), and scanning electron microscopy (SEM). Experimental results revealed that all the reactants, solvent, and synthesis method played an important role in the polymorph of calcium carbonate. The pure phase of vaterite polymorph was obtained using Na2CO3 as reactant in ethylene glycol on the cellulose matrix via sonochmistry process. Based on the experimental results, one can conclude that the synthesis of vaterite polymorph is a system process. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction As a typical biomineral, calcium carbonate (CaCO3) is abundant in both organisms and nature. It is well known that CaCO3 has six polymorphs including vaterite, aragonite, calcite, amorphous, crystalline monohydrate, and hexahydrate CaCO3 [1]. Among of these polymorphs, vaterite is a less stable polymorph of CaCO3, compared with aragonite and calcite. Vaterite is a candidate for practical biomedical applications due to its features such as high specific surface area, high solubility, high dispersion, and small specific gravity [2]. Vaterite microsphere biocomposites can be used to augment bone matrix formation within an in vivo model for impaction bone grafting seeded with human bone marrow stromal cells [3]. Recently, Parakhonskiy et al. [4] reported the porous vaterite particles applied as drug delivery system. Therefore, it is important for us to synthesize the vaterite with various structures and shapes by various methods. There have been reports on the preparation of vaterite with various shapes such as hexagonal plates [5], microspheres [6], nanoparticles [7], core–shell microspheres [8], hollow microspheres [9], and porous particles [4,10] by the strategies such as microemulsion route [5], biomimetic route [6], supercritical carbon dioxide medium [9], electrospinning [11], microwave [12], and so on.
⇑ Corresponding author. Tel.: +86 10 62336592; fax: +86 10 62336903. E-mail address: [email protected] (M.-G. Ma). 1350-4177/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.ultsonch.2013.03.008
More recently, the mixed polymorphs of calcite and vaterite on bacterial cellulose membranes were reported via ultrasonic irradiation method [13]. The sonochemical method as a promising green methodology has received more and more attention due to its characteristics of intense local heating, high pressures, and extremely rapid cooling rates [14–17]. The ultrasound was widely used in organic synthesis such as Knoevenagel condensation, Michael addition, Biginelli reaction, etc. [16,18]. Ultrasonic-assisted method was also a promising route for the conversion of carbohydrates to lower weight molecules [19]. In recent years, ultrasound agitation method has been applied to synthesize the inorganic functional materials including Au/Pt [20], US [21], ZnO [22–24], Co3O4 and Mn3O4 [25], TiO2 [26], PbO2 [27], Mg(OH)2 and MgO [28], and BaCO3 [29], etc. Our research group has developed the ultrasound agitation method for the synthesis of vaterite spheres [30]. Our experiments showed that the ultrasound agitation method do more favors to the synthesis of CaCO3 crystals with pure phase, uniform morphology and size, compared with microwave-assisted method. However, the mechanism of vaterite synthesized in the presence of ultrasound agitation still needs to be explored. The present article focuses on the research of the mechanism of vaterite by the ultrasound agitation method. The effects of experimental parameters on the polymorph of calcium carbonate were researched in detail. These researches maybe favor the synthesis and applications of vaterite, extend the applications of
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ultrasound agitation method, and open value-added application of cellulose. 2. Experimental 2.1. Preparation of vaterite crystals on the cellulose matrix by ultrasound agitation method All chemicals were of analytical grade and used as received without further purification. All experiments were conducted under air atmosphere. The vaterite crystals were synthesized on the cellulose matrix via ultrasound agitation method in ethylene glycol (EG) as previously described by our group [30]. In a typical synthesis, microcrystalline cellulose (0.324 g) was added into EG (40 mL) under vigorous stirring. Then, CaCl2 (0.222 g) and Na2CO3 (0.212 g) were added into the above solution under vigorous stirring. The above solution was subjected to sonication (Xin-Zhi, JY92-2D, Tihorn, 20 kHz, 80 W/cm2) at ambient condition with a high-density ultrasonic probe immersed directly in the solution. During the ultrasonic irradiation, the temperature of the reaction solution rose to around 90 ± 2 °C and the ultrasonic irradiation was kept on at this temperature for 30 min. The product was separated from the solution by centrifugation, washed with deionized water and absolute ethanol several times, and dried in air at 60 °C for further characterization. 2.2. Investigation the influence of reaction parameters on CaCO3 crystals For comparison, sample 2 was fabricated without microcrystalline cellulose, and kept the other conditions the same. Sample 3 was fabricated by using water instead of EG, and kept the other conditions the same. Sample 4 was fabricated by using (NH4)2CO3 instead of Na2CO3, and kept the other conditions the same. Furthermore, the samples were also synthesized in EG and water by oil heating, respectively, and kept the other conditions the same. The detailed experimental parameters for the synthesis of some typical samples are listed in Table 1. 2.3. Characterization X-ray powder diffraction (XRD) was performed in 2h range from 10° to 70° on a Rigaku D/Max 2200-PC diffractometer with Cu Ka radiation (k = 0.15418 nm) and graphite monochromator at ambient temperature. Fourier transform infrared (FTIR) spectroscopy was carried out on Thermo Scientific Nicolet iN10 FTIR Microscope (Thermo Nicolet Corporation, Madison, WI, USA), which was equipped with a liquid nitrogen cooled MCT detector. Dried samples were ground and pelletized with BaF2 and the spectra were recorded in the range of 4000–670 cm 1 at 4 cm 1 resolution and 128 scans/sample. Scanning electron microscopy (SEM) images were observed with a Hitachi 3400N scanning electron microscopy. All samples were Au coated prior to examination by SEM.
3. Results and discussion Fig. 1 displayed XRD pattern and SEM images of the typical vaterite synthesized using CaCl2 and Na2CO3 as reactants on the cellulose matrix in ethylene glycol (EG) by the ultrasound agitation method for 30 min. XRD pattern was indexed to well-crystallized vaterite with a hexagonal structure (JCPDS 33-0268) (Fig. 1a). The peaks of at 2h = 15.9o and 22.5o (marked with ⁄ in Fig. 1a) were indexed to the crystalline cellulose type I. No other impurities such as polymorphs of CaCO3 such as aragonite and calcite were observed. XRD result demonstrated that the pure phase of vaterite was obtained using cellulose as matrix via ultrasound agitation method. Vaterite spheres with uniform shape and size dispersed on the cellulose matrix were observed with SEM (Fig. 1b–d). This is the first report on the fabrication of the vaterite spheres on the cellulose matrix by ultrasound agitation method. This is a very interesting result. Although a briefly discussion was provided in consideration of current experimental results, the detailed formation mechanism of the vaterite spheres on the cellulose matrix needs to be further explored [30]. For example, why the vaterite polymorph of calcium carbonate was synthesized on the cellulose matrix via sonochemistry process? What experimental parameters determined the synthesis of pure phase vaterite? In view of the experimental procedure, all the ultrasound agitation method, microcrystalline cellulose, solvent, and reactant maybe determine the obtained pure phase vaterite. To investigate the determined experimental parameters on the synthesis of vaterite, the samples were synthesized without microcrystalline cellulose, in water, and using (NH4)2CO3 by ultrasound agitation method, respectively, and kept the other conditions the same. When (NH4)2CO3 instead of Na2CO3 was used as reactant, only the weak (1 0 4) peak of calcite appeared, as shown in Fig. 2a. No vaterite was observed. This result indicated that the reactant had an effect on the phase of calcium carbonate and the Na2CO3 favored the synthesis of vaterite in this system. The sample was also synthesized without microcrystalline cellulose, as shown in Fig. 2b, which was indexed to well-crystallized calcite with a hexagonal structure (JCPDS 47-1743). Vaterite was not also observed. This result further implied that it was important to the synthesis of vaterite in the presence of microcrystalline cellulose. Using water instead of EG, the obtained phase was also indexed to calcite (Fig. 2c). Based on the XRD results in Fig. 2, one can conclude that all the microcrystalline cellulose, solvent, and reactant played an important influence on the synthesis of vaterite. In our previous article, we found that the ultrasound agitation method favored the synthesis of vaterite, compared with microwave-assisted method [30]. To further explore the synthesis method on the vaterite, the samples were also synthesized in EG and water by oil heating, respectively, and kept the other conditions the same. Fig. 3 displayed the XRD patterns of the samples. Although the peaks intensity had obvious difference, both the samples were indexed to calcite and vaterite was not observed. These results further demonstrated that the ultrasound agitation method favored the synthesis of vaterite.
Table 1 Detailed experimental parameters for the synthesis of some typical samples prepared. Sample
Methods
1 2 3 4 5 6 7
Ultrasound Ultrasound Ultrasound Ultrasound Oil heating Oil heating /
agitation agitation agitation agitation
Reaction system
Temperature/°C
Times/min
MCC (0.324 g) + EG (40 mL) + CaCl2 (0.222 g) + Na2CO3 (0.212 g) EG (40 mL) + CaCl2 (0.222 g) + Na2CO3 (0.212 g) MCC (0.324 g) + water (40 mL) + CaCl2 (0.222 g) + Na2CO3 (0.212 g) MCC (0.324 g) + EG (40 mL) + CaCl2 (0.222 g) + (NH4)2CO3 (0.192 g) MCC (0.324 g) + EG (40 mL) + CaCl2 (0.222 g) + Na2CO3 (0.212 g) MCC (0.324 g) + water (40 mL) + CaCl2 (0.222 g) + Na2CO3 (0.212 g) MCC (0.324 g) + EG (40 mL) + CaCl2 (0.222 g) + Na2CO3 (0.212 g)
90 ± 2 90 ± 2 90 ± 2 90 ± 2 90 ± 2 90 ± 2 Room temperature
30 30 30 30 30 30 10–30
10
∗
Cellulose: ∗
∗
20
30
40
(224)
(304)
(118)
(300)
(110)
Intensity (a.u.)
(a)
(112)
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(114)
1190
50
60
2 θ ( degree )
70
∗
∗
Cellulose: *
110 113 202 018 116
Intensity (a.u.)
104
Fig. 1. (a) XRD pattern and (b–d) SEM images of the vaterite crystals on the cellulose matrix by ultrasound agitation method.
(c) (b) (a)
10
20
30
40
50
2θ (degree)
60
70
Cellulose: *
110 113 202 018 116
Intensity (a.u.)
104
Fig. 2. XRD patterns of samples prepared by ultrasound agitation method: (a) using (NH4)2CO3 instead of Na2CO3; (b) without MCC; (c) using water instead of EG.
∗ ∗
(a) (b)
10
20
30
40
50
2θ (degree)
60
70
Fig. 3. XRD patterns of samples synthesized by oil heating at 90 °C for 30 min using different solvents: (a) EG; (b) water.
In consideration of the above mentioned results, one can conclude that all the ultrasound agitation method, microcrystalline cellulose, solvent, and reactant were the key parameters on the synthesis of vaterite. Therefore, all the parameters worked together to induce the pure phase of vaterite in this experimental system. However, how these experimental parameters induced the synthesis of pure phase vaterite? Next we discuss the formation mechanism of vaterite. One of the critical factors responsible for the phase determination of the crystals is the reaction solvent during the synthesis processes. It is well known that the solvent had an important effect on the products in the synthesis of materials. EG was widely used in the materials science, which has many characteristics such as high viscosity, high cohesive energy, and high dielectric constant. In the literature, vaterite was obtained in the water/EG/sodium dodecyl sulfate system by microwave-assisted method and the mixed phase of vaterite and calcite were obtained with longer heating time [12]. The diffusion of the Ca2+ and CO32 ions and fast nucleation were restrained due to the high viscosity of EG. With the elevation of temperature during the reaction producer, the viscosity of the EG rapidly decreased and induced the synthesis of calcium carbonate with uniform shape and pure phase. Tremel et al. [7] reported the synthesis of vaterite nanoparticles of 20–60 nm size by heating a dispersion of calcium bicarbonate (CaHCO3) in EG at 40–100 °C for 30 min. The nucleation and growth rapidly completed using water as solvent. The mixed polymorphs of calcite and vaterite were obtained in the presence of aqueous solutions under ultrasonic irradiation [13]. Moreover, there are a few reports about the synthesis of monodisperse particles in the polyol solvent [31,32]. The EG favored the synthesis of vaterite spheres with uniform shape and size. Cellulose is the most abundant renewable material and natural polysaccharide found on the earth, which consists of b (1–4) – linked glucose repeating units. Cellulose was widely used as matrix in the synthesis of organic–inorganic (nano) composites [33,34]. Most of all, cellulose was only used as substrate and a component in (nano) composites. It is well known that there are numerous intermolecular and intramolecular hydrogen bonds in the structure of cellulose. Moreover, EG molecules can also form hydrogen-bonded networks similar in nature to those of water. It is
1191
(224)
(300)
(304) (118)
(114)
(110) (112)
∗Calcite
(e)
Intensity (a.u.)
∗(104)
10
(d) (c) (b) (a) 20
30
40
50
2θ (degree)
60
70
Fig. 4. XRD patterns of samples synthesized by ultrasound agitation method open 2 s for different close times: (a) 1 s; (b) 3 s; (c) 4 s; (d) 5 s; (e) 6 s.
(224)
(300)
∗202
∗018 ∗116 (304)
∗110 ∗113
(118)
∗Calcite ∗∗Cellulose (114)
∗∗
(110) (112)
Intensity (a.u.)
assumed that the cellulose and EG formed networks structure by hydrogen bonding and the vaterite spheres were bound to the networks, which restrained the transformation of vaterite to calcite. Na2CO3 is a relatively strong base and (NH4)2CO3 is a weak base. As we all know, the pH of the solution had an influence on the nucleation and growth of products. This may explain the different polymorph of calcium carbonate obtained using different CO32 source. Sommerdijk et al. [35] investigated the formation of hexagonal plate-like vaterite crystals via an amorphous precursor phase through a solid state transformation in the presence of NH4+ ions. Therefore, this result only indicated that Na2CO3 favored the synthesis of pure phase vaterite in this system. In the literature, Stoica-Guzun et al. [13] investigated the influence of ultrasound upon CaCO3 on bacterial cellulose membranes and synthesized mixed polymorphs of calcite and vaterite. The mixed polymorphs of calcite and vaterite were also observed by microwave-assisted method [30]. As mentioned above, the calcite was obtained by oil heating (Fig. 3). All these results demonstrated that the synthesis strategies influenced the polymorphs of calcium carbonate and the ultrasound method favored the synthesis of vaterite in our experimental system. However, how to the ultrasound affect the fabrication of vaterite? The sonication can not always open during the reaction procedure so as to protect the instrument. The sonication opened 2 s and closed 2 s in the synthesis of typical sample. To explore the influences of ultrasound on the products, a list of detail experimental were conducted (the total reaction time was 30 min). As shown in Fig. 4, all the samples are indexed to the vaterite (majority phase). When the sonication opened 2 s and closed 1 s, a weak peak of (1 0 4) in calcite was clearly observed (Fig. 4a). When the sonication opened 2 s and closed 2 and 3 s, the samples were indexed to pure phase vaterite (Figs. 1a and 4b). When the closed time of sonication increased to 4 and 5 s, a peak of (1 0 4) in calcite appeared (Fig. 4c and d). Increased the closed time of sonication to 6 s, the peak intensity of (1 0 4) in calcite obviously increased (Fig. 4e). From Fig. 4, one can see that the open and close time of sonication had an effect on the polymorph of calcium carbonate. To further identify this influence of ultrasound on the phase of products, the samples were also fabricated using different open and close time of sonication, as shown Fig. 5. When the sonication opened 3 s and closed 3 s (the total reaction time was still 30 min), mixed polymorphs of calcite and vaterite were clearly observed (Fig. 5a). When the closed time of sonication increased to 4 and 5 s, and kept the opened time the same, the pure phase of calcite was observed and no vaterite was obtained (Fig. 5b and c). From Figs. 4 and 5, one can clearly observed the process from vaterite
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(a) (b) (c)
10
20
30
40
50
2θ (degree)
60
70
Fig. 5. XRD patterns of samples synthesized by ultrasound agitation method open 3 s for different close times: (a) 3 s; (b) 4 s; (c) 5 s.
to calcite with increasing open and close time of sonication. Based on the results of Figs. 4 and 5, the open and close time of sonication played an important role in the polymorph of calcium carbonate. Only when the sonication opened 2 s and closed 2 s or 3 s, the pure phase vaterite was obtained. Calcite is the thermodynamically stable phase of CaCO3. When long open time of sonication was used, the calcite was synthesized due to the high temperature and high pressure during the sonochemical process. When long close time of sonication was used, the experimental condition was the similar to that without ultrasound because of the heating and cooling rates in excess of 1010 K s 14. As discussion in the literatures, three regions including the inside collapsing bubbles, the interfacial region between the cavitation bubbles and the bulk solution, and the bulk solution existed during sonochemical process [36–38]. The temperature of the inside collapsing bubbles and interfacial region was measured to be about 5000 K and 1900 K, respectively [37]. Moreover, very high pressure about 1000 atm existed in the inside collapsing bubbles. High intensity ultrasound provides unusual effects (chemical effect and physical effect) because of acoustic cavitation. Physical effect of ultrasound was caused by high-speed jets or shock waves derived from bubble collapse. Based on the above discussion, it seems that the chemical effect is more important in this system. Gasphase chemistry occurred inside collapsing bubbles and solutionphase chemistry occurred outside the bubbles. It is suggested that the reaction took place at the inside or outside collapsing bubbles in this study. Vaterite is a less stable polymorph of CaCO3; meanwhile calcite is a thermal stable polymorph of CaCO3. It is suggested that the reaction took place at very short time, the nucleation and growth of vaterite completed at short open and close time of sonication. The phase of calcite was obtained at high temperature with long reaction open time of sonication. When long close time of sonication was used, the nucleation and growth of CaCO3 were consistent with the reaction without ultrasound. With increasing open and close time of sonication, the vaterite transformed to calcite. Therefore, the appropriate open and close time of sonication is important for the formation of pure phase vaterite. The process of acoustic cavitation included the formation, growth, and implosion of bubbles with high intensity ultrasound. The size of bubbles increased to critical size with increasing ultrasound time. It is well known that the synthesis of particles involved the nucleation, growth, ripening, or agglomeration processes. When the reaction took place at the inside or outside collapsing bubbles, the collapsing bubbles was used as template for the obtained particles. Therefore, the size of collapsing bubbles
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Fig. 6. SEM images of samples prepared by ultrasound agitation method: (a–c) without MCC; (d–f) using water instead of EG; (g–i) using (NH4)2CO3 instead of Na2CO3.
Fig. 7. SEM images of cellulose/CaCO3 nanocomposites prepared (a and b) in EG and (c and d) in water by oil heating.
restricted the growth size of the particles. In addition, EG and cellulose stabilized the vaterite nuclei and restricted their transformation from vaterite to calcite by hydrogen bonding due to its high cohesive energy.
The morphologies of the CaCO3 crystals were investigated with SEM, which were synthesized without microcrystalline cellulose, in water, and using (NH4)2CO3 by ultrasound agitation method, respectively, and kept the other conditions the same. When the
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sample was synthesized without microcrystalline cellulose, the cube shape with uniform size was observed (Fig. 6a–c). Cube is the typical shape of calcite. Using water instead of EG, big cube and small cube shape were obtained (Fig. 6d–f). When (NH4)2CO3 instead of Na2CO3 was used as reactant, no obvious irregular shape of CaCO3 was observed on the cellulose matrix (Fig. 6g–i). XRD pattern indicated only the weak (1 0 4) peak of calcite (Fig. 2a), implying the limited CaCO3 concentration and poor crystallinity of CaCO3 on the cellulose matrix. When the sample was synthesized in EG by oil heating, big cube and irregular cube were observed, as shown in Fig. 7a and b. When the sample was synthesized in water by oil heating, small particles were obtained on the cellulose matrix (Fig. 7c and d). All the samples displayed different shapes, compared with vaterite spheres (Fig. 1b–d). Nucleation and growth are two important steps during the precipitation reactions. In general, if the number of nucleation was more than that of growth, the nucleation was key step and particles with small size were obtained. When growth was the key step, the particles with big size will be obtained. During the synthesis of typical vaterite, when the reaction time was 10, 20, and 30 min, the conversion rate of CaCO3 nanoparticles was 56.3%, 60.1%, and 83.3%, respectively. These results indicated that growth was the key step. In addition, as discussion above, the collapsing bubbles was used as template for the synthesis of pure phase vaterite. When the bubble is unperturbed by a surface, then the rapid rebound from its minimum radius is spherical [39]. The preparation of protein microspheres was reported by high intensity ultrasound [40]. Therefore, vaterite spheres with big size were obtained. 4. Conclusions In summary, the influences of synthetic parameters including the ultrasound agitation method, microcrystalline cellulose, solvent, and reactant on the phase and shape of vaterite were investigated. In view of the experimental results, all the experimental parameters work together to induce the synthesis of pure phase vaterite spheres. The possible mechanism of the fabrication of pure phase vaterite spheres was discussed in detail, which may guide the direct synthesis of vaterite and open new applications of vaterite in the future. Acknowledgments Financial supported by Beijing Nova Program (Z121103002512030), Program for New Century Excellent Talents in University (NCET-11-0586), and the National Natural Science Foundation of China (31070511) is gratefully acknowledged. References [1] K.M. McGrath, Probing material formation in the presence of organic and biological molecules, Adv. Mater. 13 (2001) 989–992. [2] K. Naka, Y. Tanaka, Y. Chujo, Effect of anionic starburst dendrimers on the crystallization of CaCO3 in aqueous solution: size control of spherical vaterite particles, Langmuir 18 (2002) 3655–3658. [3] D.W. Green, B.J.R.F. Bolland, J.M. Kanczler, S.A. Lanham, D. Walsh, S. Mann, et al., Augmentation of skeletal tissue formation in impaction bone grafting using vaterite microsphere biocomposites, Biomaterials 30 (2009) 1918–1927. [4] B. Parakhonskiy, A. Haase, F. Piccoli, I. Caola, F. Tessarolo, T. Bukreeva, et al., Porous vaterite particles as drug delivery system: synthesis, encapsulation, and controlled release, Eur. Biophys. J. Biophys. Lett. 40 (2011) 230. [5] J. Ahmed, A.K. Ganguli, Controlled growth of nanocrystalline rods, hexagonal plates and spherical particles of the vaterite form of calcium carbonate, CrystEngComm 11 (2009) 927–932. [6] S. Kim, C.B. Park, Dopamine-induced mineralization of calcium carbonate vaterite microspheres, Langmuir 26 (2010) 14730–14736. [7] T. Schuler, W. Tremel, Versatile wet-chemical synthesis of non-agglomerated CaCO3 vaterite nanoparticles, Chem. Commun. 47 (2011) 5208–5210. [8] M. Yang, X.Q. Jin, Q.A. Huang, Facile synthesis of vaterite core-shell microspheres, Colloids Surf. A-Physicochem. Eng. Asp. 374 (2011) 102–107.
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Ultrasonics Sonochemistry journal homepage: www.elsevier.com/locate/ultson
Why to synthesize vaterite polymorph of calcium carbonate on the cellulose matrix via sonochemistry process? Lian-Hua Fu a, Yan-Yan Dong a, Ming-Guo Ma a,⇑, Wen Yue a, Shao-Long Sun a, Run-Cang Sun a,b a b
Beijing Key Laboratory of Lignocellulosic Chemistry, College of Materials Science and Technology, Beijing Forestry University, Beijing 100083, PR China State Key Laboratory of Pulp and Paper Engineering, South China University of Technology, Guangzhou 510640, PR China
a r t i c l e
i n f o
Article history: Received 20 January 2013 Received in revised form 5 March 2013 Accepted 20 March 2013 Available online 4 April 2013 Keywords: Calcium carbonate Vaterite Cellulose Ultrasound Mechanism
a b s t r a c t Vaterite is an important biomedical material due to its features such as high specific surface area, high solubility, high dispersion, and small specific gravity. The purposes of this article were to explore the growth mechanism of vaterite on the cellulose matrix via sonochmistry process. In the work reported herein, the influences of experimental parameters on the polymorph of calcium carbonate were investigated in detail. The calcium carbonate crystals on the cellulose matrix were characterized by means of Xray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), and scanning electron microscopy (SEM). Experimental results revealed that all the reactants, solvent, and synthesis method played an important role in the polymorph of calcium carbonate. The pure phase of vaterite polymorph was obtained using Na2CO3 as reactant in ethylene glycol on the cellulose matrix via sonochmistry process. Based on the experimental results, one can conclude that the synthesis of vaterite polymorph is a system process. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction As a typical biomineral, calcium carbonate (CaCO3) is abundant in both organisms and nature. It is well known that CaCO3 has six polymorphs including vaterite, aragonite, calcite, amorphous, crystalline monohydrate, and hexahydrate CaCO3 [1]. Among of these polymorphs, vaterite is a less stable polymorph of CaCO3, compared with aragonite and calcite. Vaterite is a candidate for practical biomedical applications due to its features such as high specific surface area, high solubility, high dispersion, and small specific gravity [2]. Vaterite microsphere biocomposites can be used to augment bone matrix formation within an in vivo model for impaction bone grafting seeded with human bone marrow stromal cells [3]. Recently, Parakhonskiy et al. [4] reported the porous vaterite particles applied as drug delivery system. Therefore, it is important for us to synthesize the vaterite with various structures and shapes by various methods. There have been reports on the preparation of vaterite with various shapes such as hexagonal plates [5], microspheres [6], nanoparticles [7], core–shell microspheres [8], hollow microspheres [9], and porous particles [4,10] by the strategies such as microemulsion route [5], biomimetic route [6], supercritical carbon dioxide medium [9], electrospinning [11], microwave [12], and so on.
⇑ Corresponding author. Tel.: +86 10 62336592; fax: +86 10 62336903. E-mail address: [email protected] (M.-G. Ma). 1350-4177/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.ultsonch.2013.03.008
More recently, the mixed polymorphs of calcite and vaterite on bacterial cellulose membranes were reported via ultrasonic irradiation method [13]. The sonochemical method as a promising green methodology has received more and more attention due to its characteristics of intense local heating, high pressures, and extremely rapid cooling rates [14–17]. The ultrasound was widely used in organic synthesis such as Knoevenagel condensation, Michael addition, Biginelli reaction, etc. [16,18]. Ultrasonic-assisted method was also a promising route for the conversion of carbohydrates to lower weight molecules [19]. In recent years, ultrasound agitation method has been applied to synthesize the inorganic functional materials including Au/Pt [20], US [21], ZnO [22–24], Co3O4 and Mn3O4 [25], TiO2 [26], PbO2 [27], Mg(OH)2 and MgO [28], and BaCO3 [29], etc. Our research group has developed the ultrasound agitation method for the synthesis of vaterite spheres [30]. Our experiments showed that the ultrasound agitation method do more favors to the synthesis of CaCO3 crystals with pure phase, uniform morphology and size, compared with microwave-assisted method. However, the mechanism of vaterite synthesized in the presence of ultrasound agitation still needs to be explored. The present article focuses on the research of the mechanism of vaterite by the ultrasound agitation method. The effects of experimental parameters on the polymorph of calcium carbonate were researched in detail. These researches maybe favor the synthesis and applications of vaterite, extend the applications of
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ultrasound agitation method, and open value-added application of cellulose. 2. Experimental 2.1. Preparation of vaterite crystals on the cellulose matrix by ultrasound agitation method All chemicals were of analytical grade and used as received without further purification. All experiments were conducted under air atmosphere. The vaterite crystals were synthesized on the cellulose matrix via ultrasound agitation method in ethylene glycol (EG) as previously described by our group [30]. In a typical synthesis, microcrystalline cellulose (0.324 g) was added into EG (40 mL) under vigorous stirring. Then, CaCl2 (0.222 g) and Na2CO3 (0.212 g) were added into the above solution under vigorous stirring. The above solution was subjected to sonication (Xin-Zhi, JY92-2D, Tihorn, 20 kHz, 80 W/cm2) at ambient condition with a high-density ultrasonic probe immersed directly in the solution. During the ultrasonic irradiation, the temperature of the reaction solution rose to around 90 ± 2 °C and the ultrasonic irradiation was kept on at this temperature for 30 min. The product was separated from the solution by centrifugation, washed with deionized water and absolute ethanol several times, and dried in air at 60 °C for further characterization. 2.2. Investigation the influence of reaction parameters on CaCO3 crystals For comparison, sample 2 was fabricated without microcrystalline cellulose, and kept the other conditions the same. Sample 3 was fabricated by using water instead of EG, and kept the other conditions the same. Sample 4 was fabricated by using (NH4)2CO3 instead of Na2CO3, and kept the other conditions the same. Furthermore, the samples were also synthesized in EG and water by oil heating, respectively, and kept the other conditions the same. The detailed experimental parameters for the synthesis of some typical samples are listed in Table 1. 2.3. Characterization X-ray powder diffraction (XRD) was performed in 2h range from 10° to 70° on a Rigaku D/Max 2200-PC diffractometer with Cu Ka radiation (k = 0.15418 nm) and graphite monochromator at ambient temperature. Fourier transform infrared (FTIR) spectroscopy was carried out on Thermo Scientific Nicolet iN10 FTIR Microscope (Thermo Nicolet Corporation, Madison, WI, USA), which was equipped with a liquid nitrogen cooled MCT detector. Dried samples were ground and pelletized with BaF2 and the spectra were recorded in the range of 4000–670 cm 1 at 4 cm 1 resolution and 128 scans/sample. Scanning electron microscopy (SEM) images were observed with a Hitachi 3400N scanning electron microscopy. All samples were Au coated prior to examination by SEM.
3. Results and discussion Fig. 1 displayed XRD pattern and SEM images of the typical vaterite synthesized using CaCl2 and Na2CO3 as reactants on the cellulose matrix in ethylene glycol (EG) by the ultrasound agitation method for 30 min. XRD pattern was indexed to well-crystallized vaterite with a hexagonal structure (JCPDS 33-0268) (Fig. 1a). The peaks of at 2h = 15.9o and 22.5o (marked with ⁄ in Fig. 1a) were indexed to the crystalline cellulose type I. No other impurities such as polymorphs of CaCO3 such as aragonite and calcite were observed. XRD result demonstrated that the pure phase of vaterite was obtained using cellulose as matrix via ultrasound agitation method. Vaterite spheres with uniform shape and size dispersed on the cellulose matrix were observed with SEM (Fig. 1b–d). This is the first report on the fabrication of the vaterite spheres on the cellulose matrix by ultrasound agitation method. This is a very interesting result. Although a briefly discussion was provided in consideration of current experimental results, the detailed formation mechanism of the vaterite spheres on the cellulose matrix needs to be further explored [30]. For example, why the vaterite polymorph of calcium carbonate was synthesized on the cellulose matrix via sonochemistry process? What experimental parameters determined the synthesis of pure phase vaterite? In view of the experimental procedure, all the ultrasound agitation method, microcrystalline cellulose, solvent, and reactant maybe determine the obtained pure phase vaterite. To investigate the determined experimental parameters on the synthesis of vaterite, the samples were synthesized without microcrystalline cellulose, in water, and using (NH4)2CO3 by ultrasound agitation method, respectively, and kept the other conditions the same. When (NH4)2CO3 instead of Na2CO3 was used as reactant, only the weak (1 0 4) peak of calcite appeared, as shown in Fig. 2a. No vaterite was observed. This result indicated that the reactant had an effect on the phase of calcium carbonate and the Na2CO3 favored the synthesis of vaterite in this system. The sample was also synthesized without microcrystalline cellulose, as shown in Fig. 2b, which was indexed to well-crystallized calcite with a hexagonal structure (JCPDS 47-1743). Vaterite was not also observed. This result further implied that it was important to the synthesis of vaterite in the presence of microcrystalline cellulose. Using water instead of EG, the obtained phase was also indexed to calcite (Fig. 2c). Based on the XRD results in Fig. 2, one can conclude that all the microcrystalline cellulose, solvent, and reactant played an important influence on the synthesis of vaterite. In our previous article, we found that the ultrasound agitation method favored the synthesis of vaterite, compared with microwave-assisted method [30]. To further explore the synthesis method on the vaterite, the samples were also synthesized in EG and water by oil heating, respectively, and kept the other conditions the same. Fig. 3 displayed the XRD patterns of the samples. Although the peaks intensity had obvious difference, both the samples were indexed to calcite and vaterite was not observed. These results further demonstrated that the ultrasound agitation method favored the synthesis of vaterite.
Table 1 Detailed experimental parameters for the synthesis of some typical samples prepared. Sample
Methods
1 2 3 4 5 6 7
Ultrasound Ultrasound Ultrasound Ultrasound Oil heating Oil heating /
agitation agitation agitation agitation
Reaction system
Temperature/°C
Times/min
MCC (0.324 g) + EG (40 mL) + CaCl2 (0.222 g) + Na2CO3 (0.212 g) EG (40 mL) + CaCl2 (0.222 g) + Na2CO3 (0.212 g) MCC (0.324 g) + water (40 mL) + CaCl2 (0.222 g) + Na2CO3 (0.212 g) MCC (0.324 g) + EG (40 mL) + CaCl2 (0.222 g) + (NH4)2CO3 (0.192 g) MCC (0.324 g) + EG (40 mL) + CaCl2 (0.222 g) + Na2CO3 (0.212 g) MCC (0.324 g) + water (40 mL) + CaCl2 (0.222 g) + Na2CO3 (0.212 g) MCC (0.324 g) + EG (40 mL) + CaCl2 (0.222 g) + Na2CO3 (0.212 g)
90 ± 2 90 ± 2 90 ± 2 90 ± 2 90 ± 2 90 ± 2 Room temperature
30 30 30 30 30 30 10–30
10
∗
Cellulose: ∗
∗
20
30
40
(224)
(304)
(118)
(300)
(110)
Intensity (a.u.)
(a)
(112)
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(114)
1190
50
60
2 θ ( degree )
70
∗
∗
Cellulose: *
110 113 202 018 116
Intensity (a.u.)
104
Fig. 1. (a) XRD pattern and (b–d) SEM images of the vaterite crystals on the cellulose matrix by ultrasound agitation method.
(c) (b) (a)
10
20
30
40
50
2θ (degree)
60
70
Cellulose: *
110 113 202 018 116
Intensity (a.u.)
104
Fig. 2. XRD patterns of samples prepared by ultrasound agitation method: (a) using (NH4)2CO3 instead of Na2CO3; (b) without MCC; (c) using water instead of EG.
∗ ∗
(a) (b)
10
20
30
40
50
2θ (degree)
60
70
Fig. 3. XRD patterns of samples synthesized by oil heating at 90 °C for 30 min using different solvents: (a) EG; (b) water.
In consideration of the above mentioned results, one can conclude that all the ultrasound agitation method, microcrystalline cellulose, solvent, and reactant were the key parameters on the synthesis of vaterite. Therefore, all the parameters worked together to induce the pure phase of vaterite in this experimental system. However, how these experimental parameters induced the synthesis of pure phase vaterite? Next we discuss the formation mechanism of vaterite. One of the critical factors responsible for the phase determination of the crystals is the reaction solvent during the synthesis processes. It is well known that the solvent had an important effect on the products in the synthesis of materials. EG was widely used in the materials science, which has many characteristics such as high viscosity, high cohesive energy, and high dielectric constant. In the literature, vaterite was obtained in the water/EG/sodium dodecyl sulfate system by microwave-assisted method and the mixed phase of vaterite and calcite were obtained with longer heating time [12]. The diffusion of the Ca2+ and CO32 ions and fast nucleation were restrained due to the high viscosity of EG. With the elevation of temperature during the reaction producer, the viscosity of the EG rapidly decreased and induced the synthesis of calcium carbonate with uniform shape and pure phase. Tremel et al. [7] reported the synthesis of vaterite nanoparticles of 20–60 nm size by heating a dispersion of calcium bicarbonate (CaHCO3) in EG at 40–100 °C for 30 min. The nucleation and growth rapidly completed using water as solvent. The mixed polymorphs of calcite and vaterite were obtained in the presence of aqueous solutions under ultrasonic irradiation [13]. Moreover, there are a few reports about the synthesis of monodisperse particles in the polyol solvent [31,32]. The EG favored the synthesis of vaterite spheres with uniform shape and size. Cellulose is the most abundant renewable material and natural polysaccharide found on the earth, which consists of b (1–4) – linked glucose repeating units. Cellulose was widely used as matrix in the synthesis of organic–inorganic (nano) composites [33,34]. Most of all, cellulose was only used as substrate and a component in (nano) composites. It is well known that there are numerous intermolecular and intramolecular hydrogen bonds in the structure of cellulose. Moreover, EG molecules can also form hydrogen-bonded networks similar in nature to those of water. It is
1191
(224)
(300)
(304) (118)
(114)
(110) (112)
∗Calcite
(e)
Intensity (a.u.)
∗(104)
10
(d) (c) (b) (a) 20
30
40
50
2θ (degree)
60
70
Fig. 4. XRD patterns of samples synthesized by ultrasound agitation method open 2 s for different close times: (a) 1 s; (b) 3 s; (c) 4 s; (d) 5 s; (e) 6 s.
(224)
(300)
∗202
∗018 ∗116 (304)
∗110 ∗113
(118)
∗Calcite ∗∗Cellulose (114)
∗∗
(110) (112)
Intensity (a.u.)
assumed that the cellulose and EG formed networks structure by hydrogen bonding and the vaterite spheres were bound to the networks, which restrained the transformation of vaterite to calcite. Na2CO3 is a relatively strong base and (NH4)2CO3 is a weak base. As we all know, the pH of the solution had an influence on the nucleation and growth of products. This may explain the different polymorph of calcium carbonate obtained using different CO32 source. Sommerdijk et al. [35] investigated the formation of hexagonal plate-like vaterite crystals via an amorphous precursor phase through a solid state transformation in the presence of NH4+ ions. Therefore, this result only indicated that Na2CO3 favored the synthesis of pure phase vaterite in this system. In the literature, Stoica-Guzun et al. [13] investigated the influence of ultrasound upon CaCO3 on bacterial cellulose membranes and synthesized mixed polymorphs of calcite and vaterite. The mixed polymorphs of calcite and vaterite were also observed by microwave-assisted method [30]. As mentioned above, the calcite was obtained by oil heating (Fig. 3). All these results demonstrated that the synthesis strategies influenced the polymorphs of calcium carbonate and the ultrasound method favored the synthesis of vaterite in our experimental system. However, how to the ultrasound affect the fabrication of vaterite? The sonication can not always open during the reaction procedure so as to protect the instrument. The sonication opened 2 s and closed 2 s in the synthesis of typical sample. To explore the influences of ultrasound on the products, a list of detail experimental were conducted (the total reaction time was 30 min). As shown in Fig. 4, all the samples are indexed to the vaterite (majority phase). When the sonication opened 2 s and closed 1 s, a weak peak of (1 0 4) in calcite was clearly observed (Fig. 4a). When the sonication opened 2 s and closed 2 and 3 s, the samples were indexed to pure phase vaterite (Figs. 1a and 4b). When the closed time of sonication increased to 4 and 5 s, a peak of (1 0 4) in calcite appeared (Fig. 4c and d). Increased the closed time of sonication to 6 s, the peak intensity of (1 0 4) in calcite obviously increased (Fig. 4e). From Fig. 4, one can see that the open and close time of sonication had an effect on the polymorph of calcium carbonate. To further identify this influence of ultrasound on the phase of products, the samples were also fabricated using different open and close time of sonication, as shown Fig. 5. When the sonication opened 3 s and closed 3 s (the total reaction time was still 30 min), mixed polymorphs of calcite and vaterite were clearly observed (Fig. 5a). When the closed time of sonication increased to 4 and 5 s, and kept the opened time the same, the pure phase of calcite was observed and no vaterite was obtained (Fig. 5b and c). From Figs. 4 and 5, one can clearly observed the process from vaterite
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(a) (b) (c)
10
20
30
40
50
2θ (degree)
60
70
Fig. 5. XRD patterns of samples synthesized by ultrasound agitation method open 3 s for different close times: (a) 3 s; (b) 4 s; (c) 5 s.
to calcite with increasing open and close time of sonication. Based on the results of Figs. 4 and 5, the open and close time of sonication played an important role in the polymorph of calcium carbonate. Only when the sonication opened 2 s and closed 2 s or 3 s, the pure phase vaterite was obtained. Calcite is the thermodynamically stable phase of CaCO3. When long open time of sonication was used, the calcite was synthesized due to the high temperature and high pressure during the sonochemical process. When long close time of sonication was used, the experimental condition was the similar to that without ultrasound because of the heating and cooling rates in excess of 1010 K s 14. As discussion in the literatures, three regions including the inside collapsing bubbles, the interfacial region between the cavitation bubbles and the bulk solution, and the bulk solution existed during sonochemical process [36–38]. The temperature of the inside collapsing bubbles and interfacial region was measured to be about 5000 K and 1900 K, respectively [37]. Moreover, very high pressure about 1000 atm existed in the inside collapsing bubbles. High intensity ultrasound provides unusual effects (chemical effect and physical effect) because of acoustic cavitation. Physical effect of ultrasound was caused by high-speed jets or shock waves derived from bubble collapse. Based on the above discussion, it seems that the chemical effect is more important in this system. Gasphase chemistry occurred inside collapsing bubbles and solutionphase chemistry occurred outside the bubbles. It is suggested that the reaction took place at the inside or outside collapsing bubbles in this study. Vaterite is a less stable polymorph of CaCO3; meanwhile calcite is a thermal stable polymorph of CaCO3. It is suggested that the reaction took place at very short time, the nucleation and growth of vaterite completed at short open and close time of sonication. The phase of calcite was obtained at high temperature with long reaction open time of sonication. When long close time of sonication was used, the nucleation and growth of CaCO3 were consistent with the reaction without ultrasound. With increasing open and close time of sonication, the vaterite transformed to calcite. Therefore, the appropriate open and close time of sonication is important for the formation of pure phase vaterite. The process of acoustic cavitation included the formation, growth, and implosion of bubbles with high intensity ultrasound. The size of bubbles increased to critical size with increasing ultrasound time. It is well known that the synthesis of particles involved the nucleation, growth, ripening, or agglomeration processes. When the reaction took place at the inside or outside collapsing bubbles, the collapsing bubbles was used as template for the obtained particles. Therefore, the size of collapsing bubbles
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Fig. 6. SEM images of samples prepared by ultrasound agitation method: (a–c) without MCC; (d–f) using water instead of EG; (g–i) using (NH4)2CO3 instead of Na2CO3.
Fig. 7. SEM images of cellulose/CaCO3 nanocomposites prepared (a and b) in EG and (c and d) in water by oil heating.
restricted the growth size of the particles. In addition, EG and cellulose stabilized the vaterite nuclei and restricted their transformation from vaterite to calcite by hydrogen bonding due to its high cohesive energy.
The morphologies of the CaCO3 crystals were investigated with SEM, which were synthesized without microcrystalline cellulose, in water, and using (NH4)2CO3 by ultrasound agitation method, respectively, and kept the other conditions the same. When the
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sample was synthesized without microcrystalline cellulose, the cube shape with uniform size was observed (Fig. 6a–c). Cube is the typical shape of calcite. Using water instead of EG, big cube and small cube shape were obtained (Fig. 6d–f). When (NH4)2CO3 instead of Na2CO3 was used as reactant, no obvious irregular shape of CaCO3 was observed on the cellulose matrix (Fig. 6g–i). XRD pattern indicated only the weak (1 0 4) peak of calcite (Fig. 2a), implying the limited CaCO3 concentration and poor crystallinity of CaCO3 on the cellulose matrix. When the sample was synthesized in EG by oil heating, big cube and irregular cube were observed, as shown in Fig. 7a and b. When the sample was synthesized in water by oil heating, small particles were obtained on the cellulose matrix (Fig. 7c and d). All the samples displayed different shapes, compared with vaterite spheres (Fig. 1b–d). Nucleation and growth are two important steps during the precipitation reactions. In general, if the number of nucleation was more than that of growth, the nucleation was key step and particles with small size were obtained. When growth was the key step, the particles with big size will be obtained. During the synthesis of typical vaterite, when the reaction time was 10, 20, and 30 min, the conversion rate of CaCO3 nanoparticles was 56.3%, 60.1%, and 83.3%, respectively. These results indicated that growth was the key step. In addition, as discussion above, the collapsing bubbles was used as template for the synthesis of pure phase vaterite. When the bubble is unperturbed by a surface, then the rapid rebound from its minimum radius is spherical [39]. The preparation of protein microspheres was reported by high intensity ultrasound [40]. Therefore, vaterite spheres with big size were obtained. 4. Conclusions In summary, the influences of synthetic parameters including the ultrasound agitation method, microcrystalline cellulose, solvent, and reactant on the phase and shape of vaterite were investigated. In view of the experimental results, all the experimental parameters work together to induce the synthesis of pure phase vaterite spheres. The possible mechanism of the fabrication of pure phase vaterite spheres was discussed in detail, which may guide the direct synthesis of vaterite and open new applications of vaterite in the future. Acknowledgments Financial supported by Beijing Nova Program (Z121103002512030), Program for New Century Excellent Talents in University (NCET-11-0586), and the National Natural Science Foundation of China (31070511) is gratefully acknowledged. References [1] K.M. McGrath, Probing material formation in the presence of organic and biological molecules, Adv. Mater. 13 (2001) 989–992. [2] K. Naka, Y. Tanaka, Y. Chujo, Effect of anionic starburst dendrimers on the crystallization of CaCO3 in aqueous solution: size control of spherical vaterite particles, Langmuir 18 (2002) 3655–3658. [3] D.W. Green, B.J.R.F. Bolland, J.M. Kanczler, S.A. Lanham, D. Walsh, S. Mann, et al., Augmentation of skeletal tissue formation in impaction bone grafting using vaterite microsphere biocomposites, Biomaterials 30 (2009) 1918–1927. [4] B. Parakhonskiy, A. Haase, F. Piccoli, I. Caola, F. Tessarolo, T. Bukreeva, et al., Porous vaterite particles as drug delivery system: synthesis, encapsulation, and controlled release, Eur. Biophys. J. Biophys. Lett. 40 (2011) 230. [5] J. Ahmed, A.K. Ganguli, Controlled growth of nanocrystalline rods, hexagonal plates and spherical particles of the vaterite form of calcium carbonate, CrystEngComm 11 (2009) 927–932. [6] S. Kim, C.B. Park, Dopamine-induced mineralization of calcium carbonate vaterite microspheres, Langmuir 26 (2010) 14730–14736. [7] T. Schuler, W. Tremel, Versatile wet-chemical synthesis of non-agglomerated CaCO3 vaterite nanoparticles, Chem. Commun. 47 (2011) 5208–5210. [8] M. Yang, X.Q. Jin, Q.A. Huang, Facile synthesis of vaterite core-shell microspheres, Colloids Surf. A-Physicochem. Eng. Asp. 374 (2011) 102–107.
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