Controllable Synthesis of CaCO 3 Micro/Nanocrystals with Different Morphologies in Microemulsion

Transcription

1 CHEM. RES. CHINESE UNIVERSITIES 2010, 26(5), Controllable Synthesis of CaCO 3 Micro/Nanocrystals with Different Morphologies in Microemulsion YU Dong 1,2, CHU Ying 1*, DONG Li-hong 1 and ZHUO Yu-jiang 1 1. Department of Chemistry, Northeast Normal University, Changchun , P. R. China; 2. Elementary Normal College, Dalian University, Dalian , P. R. China Abstract Lens-like vaterite CaCO 3 microrings composed of CaCO 3 nanoparticles were synthesized via a microemulsion-mediated route at room temperature with ethanol and n-hexanol as co-surfactant. This process did not demand any additional energy such as heating or continuous agitation. It was the first time to use ethanol as co-surfactant in the synthesis of micro or nanomaterials. And the ethanol was believed to play an important role in the aggregation fashion of CaCO 3 nanoparticles. Moreover, shuttle-shaped nanorods, hexagonal nanoplates, and rice-like nanoparticles were also fabricated by modulating the growth parameters. Additionally, the introduce of ethanol into microemulsions as co-surfactant may be generalized as a novel green route to control the structure of other functional materials. Keywords CaCO 3 ; Microring; Cosurfactant Article ID (2010) Introduction The precipitate of CaCO 3 particles with different sizes, morphologies, and surface properties has been intensively investigated due to their importance in geo- and bio-sciences and numerous industrial application such as paints, plastics, rubber, or paper [1]. Hence, different approaches, including Langmuir monolayers [2], self-assembled monolayers [3], micelle [4], microemusions [5,6], as well as varied soluble additives [7 10], have been applied to controlling polymorphs, directional aggregation, size, and shape of CaCO 3 crystals. Furthermore, slaking high-purity CaO, Wang s group [11] synthesized cubic CaCO 3 nanoparticles. Among the methods mentioned above, the water-in-oil(w/o) microemulsion has been widely used as an ideal medium to prepare inorganic nanoparticles, which consists of nanometer-scale water droplets surrounded by surfactant that are dispersed in oil. Additionally, the water droplets are thermodynamically stable and can spontaneously self-assemble into well-defined structures [12]. When crystallization reactions are carried out in microemulsions, the resulting mesoscale crystal surface structures and intrinsic morphologies can be simultaneously controlled or directed by the confinement of reagents in the aqueous droplets and the adsorption of surface-active reagents on the growing crystals. Several papers [5,6] on growing CaCO 3 micro/nanocrystals in W/O microemulsions have been reported recently. For example, by adding sodium carbonate-containing NaAOT microemulsions into reverse micelle of Ca(AOT) 2 [AOT=bis(2-ethylhexyl)sulfosuccinate], micrometer-sized doughnutshaped aragonite crystals consisting of densely packed plate-like layers were obtained [5]. In the cetyltrimethylammonium bromide(ctab)/1-pentanol/cyclohexane microemulsion system, CaCO 3 hexagonal nanoplates and nanorods were prepared, respectively [6]. Ring-like micro/nanocrystals have received significant attention because of their unique structural features and new properties generated from ring cavities. Several attempts have been made to prepare micro or nanoscopic ring-like structures over the past few years, such as carbon nanotube rings [13], zinc oxide coils and hexagonal rings [14,15]. Under the direction of double-hydrophilic block copolymer, CaCO 3 microrings have also been prepared by Gao et al. [7]. However, to the best of our knowledge, the fabrication of ring-like CaCO 3 particles by microemulsion method has not been reported up to date. Herein lies a *Corresponding author. chuying@nenu.edu.cn Received November 26, 2009; accepted January 11, Supported by the National Natural Science Foundation of China(No ), the Natural Science Young Foundation of Northeast Normal University, China(No ), and the Analysis and Testing Foundation of Northeast Normal University, China.

2 No.5 YU Dong et al. 679 microemulsion-mediated method to synthesize lenslike CaCO 3 microrings composed of CaCO 3 nanoparticles by introducing ethanol and n-hexanol as co-surfactant at room temperature. It was the first time to use ethanol as co-surfactant in the synthesis of micro or nanomaterials. And the reaction did not demand any additional energy such as heating, continuous agitation or expensive reagents. Moreover, shuttle-shaped nanorods, hexagonal nanoplates, and rice-like nanoparticles were also fabricated by modulating the reaction parameters. And a reasonable mechanism was put forward for the formation of these CaCO 3 micro/nanocrystals. 2 Experimental the product obtained via a typical experiment. It can be seen that the product displays a lens-like shape and most of them are hollow in the center part. The size of them is uniform and about 1 μm in outer diameter. There are also a few of rhombic particles coexisting with the lens-like microcrystals according to Fig.1(A), which should be of calcite crystalline phase that might be due to secondary nucleation as pointed out by Gao et al. [7]. As shown in the magnified image [Fig.1(B)], the peripheral surfaces of these microcrystals are rough, which indicates that they are superstructures formed by assembly of little nanoparticles with diameters of around 50 nm. All the starting materials were of analytical grade and used without further purification. In a typical synthesis of lens-like microrings, two identical solutions were prepared by dissolving CTAB(0.5 g) into 15 ml of cyclohexane containing 0.75 ml of n-hexanol and 0.4 ml of ethanol respectively used as co-surfactants. The mixed solutions were stirred for 30 min until they became transparent. Next, 0.25 ml of 0.2 mol/l CaCl 2 aqueous solution or 0.25 ml of 0.2 mol/l Na 2 CO 3 aqueous solution was respectively added into the above solutions to make the W 0 =10(W 0 is defined as the molar ratio of water to surfactant). After substantial stirring, the two optically transparent microemulsion solutions were mixed and stirred for another 10 min. The resulting microemulsion solution was then kept static for 24 h. Then, the white precipitate was collected and washed several times with absolute ethanol and distilled water. Finally, the sample was dried at 50 C. Other samples with different morphologies were prepared by modulating W 0 value, reactant concentration or co-surfactant species. The as-prepared powder samples were characterized by X-ray powder diffraction(xrd) on a Rigaku X-ray diffractometer with Cu Kα radiation(λ= nm). The morphology and size of as-obtained product were observed by transmission electron microscopy (TEM) and scanning electron microscopy(sem), which were carried out on a Hitachi H-800 transmission electron microscope and scanning electron microscope(jeol JSM-6700F), respectively. 3 Results and Discussion Fig.1 shows the representative SEM images of Fig.1 SEM images of microrings obtained with W 0 =10, n-hexanol and ethanol as cosurfactants (B) is the enlarged image of (A). In a general microemulsion synthesis, only one kind of co-surfactant, such as n-hexanol or n-pentanol, is used. In the present case, not only n-hexanol but also ethanol was utilized as co-surfactant and it was found that ethanol played a vital role in the formation of the lens-like superstructures. When the fabrication was carried out without ethanol and other conditions were kept constant, shuttle-shaped nanorods with a diameter of nm and a length of nm formed as shown in Fig.2(A). Fig.2(B) is the enlarged TEM image of the shuttle-shaped nanorods, and it can be found that these shuttle-shaped nanorods are also composed of little nanoparticles by their oriented attachment. Otherwise, the influence of the dosage of ethanol on the morphology could not be ignored.

3 680 CHEM. RES. CHINESE UNIVERSITIES Vol.26 When the volume of ethanol as-used was increased to 0.75 ml, the main product was a mixture of nanoplates and nanobelts as shown in Fig.2(C). Further increase of the dosage of ethanol to 1 ml, nanobelts became the main product. In contrast, when the amount of ethanol was decreased to 0.2 ml, the product was a mixture of nanorods and nanoparticles[fig.2(d)]. Therefore, an appropriate amount of ethanol played a crucial role in the formation of lens-like CaCO 3 microrings and its excessive or deficient amounts did not favor the growth of the microrings. Therefore, ethanol played a very important role in the aggregating fashion of the nanoparticles, but the exact affecting mechanism is still unclear and more work about it is under way. Fig.2 TEM images of the products with W 0 =10 (A) and (B) Shuttle-shaped CaCO 3 nanorods obtained with only n-hexanol as cosurfactant; (C) product obtained with 0.75 ml of ethanol; (D) product obtained with 0.2 ml of ethanol; (B) is the enlarged image of (A). As mentioned in the introduction section, a similar lens-like morphology of CaCO 3 with a size of more than 10 μm has been reported by Gao et al. [7], which was prepared under the direction of double-hydrophilic block copolymer. We proposed that the formation mechanism of CaCO 3 lens-like microrings similar to that put forward by Gao et al. [7], and to verify our speculation, the time-dependent experiments were carried out and the corresponding TEM images are presented in Fig.3. As shown in Fig.3(A), the product of the reaction for 2 h was primarily disc-like particles with a diameter about 1 μm and the center of solid. When the aging time was prolonged to 6 h, a few of micro-discs with small holes in their centers were found[fig.3(b)]. Further prolonging the aging time to 12 h, more and more micro-discs with holes in the center appeared as shown in Fig.3(C). After aging for 24 h, the discs with hollow centers were the majority of the product(fig.1) with the corresponding TEM image shown in Fig.3(D). What should be pointed out is that the micro-discs in the TEM images should be lens-like in the SEM images. Continuously increasing the aging time to 48 h, we obtained ring-like particles as shown in Fig.3(E). Combining the above experimental results, we are inclined to describe the formation process of the lens-like CaCO 3 microrings as follows. At first, the introduction of ethanol made the size of micelle very small in which tiny CaCO 3 nanoparticles formed. These initially formed tiny nanoparticles were surrounded by the molecules of surfactant and cosufactants. To decrease the energy of the system, they aggregated into the congeries of surfactant CaCO 3 hybrid nanocrystals. And then these hybrid nanocrystals further grew into lens-like particles. The hybrid core part of the lens was composed of primary nanocrystals with a large amount of attached surfactant, which would have a great tendency to dissolve for the Ostwald ripening. As a result, the center part became hollow eventually and micro-ring formed at last. The XRD patterns of lens-like particles obtained with different aging time are shown in Fig.3(F). It can

4 No.5 YU Dong et al. 681 Fig.3 TEM images of lens-like CaCO 3 particles harvested at different reaction intervals[(a) (E)] and XRD patterns of lens-like porticles at different aging time(f) Aging time/h: (A) 2; (B) 6; (C) 12; (D) 24; (E) 48; (F) a. 0; b. 6; c. 12; d. 24. be observed that the dominant phase is vaterite but a little amount of calcite phase is present. The peak of calcite phase should be resulted from those coexisting rhombohedral crystals that were the dissolverecrystalization product during the formation of hollow center because the intensity of the peak was enhanced with the prolonging of reaction time. From the SEM investigation, we estimated that lens-like vaterite CaCO 3 nanostructures observed in our experiment were about 90% of the population and the calcite phase with rhombohedral shape was about 10%. A sketch map for the formation of the single hollow lens is illustrated in Fig.4. Fig.4 Graphic presentation of the formation mechanism for lens-like CaCO 3 microrings Without the assistance of ethanol, the initially formed nanoparticles would attach with each other in a distinct way of end-to-end, which led to the generation of shuttle-like nanorods composed of the nanoparticles. But the interaction between the two particles was weak van der waals force, which was susceptible to be broken by outer mechanic force. This speculation was verified by the experiment carried out under continuously stirring with otherwise identical conditions, and only quasi-rhombohedral nanocrystals with a size about 100 nm were obtained as shown in Fig.5(A). When the reaction was performed by reflux, a large scale of rice-like nanoparticles formed as shown in Fig.5(B), which suggested that thermal energy could also break the van der waals force. The amount of surfactant also played an important role in the morphology of the product. When the preparation was carried out with W 0 =15 and other conditions were the same as those for shuttle-like nanorods, the products were lots of hexagonal nanoplates as shown in Fig.5(C), and most of them were regular ones. The diameters of these nanoplates were ranged from 1 to 2 μm. Similar hexagonal CaCO 3 nanoplates have been synthesized in water/ctab/ 1-pentanol/cyclohexane microemulsions by Liu et al. [6]. The XRD[Fig.5(D)] pattern of these nanoplates indicates that they were vaterite phase, which is also consistent with the result in the literature [6]. The peak belonging to calcite phase in Fig.5(D) should also be resulted from a little impurity coexisting with the nanoplates. It is known that vaterite is the typical metastable crystallization product of CaCO 3 [16], and the formation of vaterite with hexagonal symmetry is uncommon and slow vapor diffusion through the

5 682 CHEM. RES. CHINESE UNIVERSITIES Vol.26 Fig.5 TEM images[(a) (C)] and XRD patterne(d) of CaCO 3 nanoparticles (A) W 0 =10, only n-hexanol as cosurfactant under continuously stirring; (B) reflux; (C) W 0 =15, only n-hexanol as cosurfactant. thermal decomposition of ammonium carbonate [16] or crystallization in the presence of compressed Langmuir monolayers of octadecylamine [17]. It is believed [1] Dalas E., Klepetsanis P., Koutsoukos P. G., Langmuir, 1999, 15, 8322 References [2] Heywood B. R., Mann S., Adv. Mater., 1994, 6, 9 that the microemulsion system and the surfactant [3] Xu G., Yao N., Aksay A. I., Groves T. J., J. Am. Chem. Soc., 1998, CTAB play a vital role in the formation of the vaterite nanoplates. As is pointed out by Liu et al. [6] 120, 11977, the microemulsion can regulate hexagonal vaterite growth [5] Thachepan S., Li M., Davis S., Mann S., Chem. Mater., 2006, 18, [4] Qi L., Li J., Ma J., Adv. Mater., 2002, 14, 300 by confinement and adsorption of surfactant, media [6] Liu D., Yates M. Z., Langmuir, 2006, 22, 5566 ting crystal growth kinetics and lowering the free [7] Gao Y., Yu S., Cong H., Jiang J., Xu A. W., Dong W. F., Cölfen H., J. energy of the exposed crystal faces. And the positively Phys. Chem. B, 2006, 110, 6432 charged CTAB can be selectively adsorbed on the negatively charged[001] face of vaterite CaCO 3, which [9] Xu A., Antonietti M., Cölfen H., Fang Y. D., Adv. Funct. Mater., [8] Guo X., Yu S., Cai G., Angew. Chem. Int. Ed., 2006, 45, 3977 inhibits the growth along [001] direction and results in 2006, 16, 903 [10] Yan G., Huang J., Zhang J., Qian C. J., Mater. Res. Bull., 2008, 43, the formation of hexagonal nanoplates The amount of CaCl 2 and Na 2 CO 3 was also investigated. When 0.5 ml of CaCl 2 and Na 2 CO 3 was Lett., 2006, 60, 1515 [11] Bala H., Ding X., Guo Y., Deng Y., Wang C., Li M., Wang Z., Mater. used, which was twice as much as the typical one, [12] Pileni P. M., Langmuir, 2001, 17, 7476 [13] Sano M., Kamino A., Okamura J., Shinkai S., Science, 2001, 293, bigger irregular particles were the only product. Continuous increase of the volume to 1 ml, reaction sys [14] Kong X. Y., Ding Y., Yang R., Wang Z., Science, 2004, 303, 1348 tem turned turbid, which meant the system was not [15] Li F., Ding Y., Gao P. X., Xin X. Q., Wang Z. L., Angew. Chem. Int. microemulsion anymore. On the other hand, a too little amount(such as 0.1 ml) resulted in too little pro- [16] Gehrke N., Cölfen H., Pinna N., Antonietti M., Nassif N., Cryst. Ed., 2004, 43, 5238 Growth Des., 2005, 5, 1317 duct so that it was hardly to find the product on the [17] Heywood B. R., Rajam S., Mann S., J. Chem. Soc., Faraday Trans., bottom of the container, so the further characterization 1991, 87, 735 was not carried.