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Morphology and formation mechanism of vaterite particles grown in glycine-containing aqueous solutions
Materials Science and Engineering C 26 (2006) 644 – 647 www.elsevier.com/locate/msec
Morphology and formation mechanism of vaterite particles grown in glycine-containing aqueous solutions Wentao Hou, Qingling Feng * Department of Materials Science and Engineering, Biomaterials Laboratory, Tsinghua University, Beijing 100084, People’s Republic of China Received 1 November 2004; received in revised form 7 August 2005; accepted 7 September 2005 Available online 2 November 2005
Abstract Calcium carbonate crystals were precipitated in glycine-containing aqueous solutions using two methods: CO32 dripping method or diffusion method. By modulating the dripping velocity and glycine (Gly) concentration, we can control the morphologies and proportion of the acquired vaterite particles. Based on the observation, a sheet-growth mechanism was developed to explain the various vaterite morphologies, including spindly, spherical and agglomerate ones. Further discussion proposed that stirring and Gly may both serve as nucleation energy changers, which alternates the nucleation priority of vaterite and calcite. D 2005 Elsevier B.V. All rights reserved. Keywords: Vaterite; Glycine; Calcium carbonate
1. Introduction Calcium carbonate is an ordinary mineral existing in living creatures. There are three polymorphs of calcium carbonate: calcite, aragonite and vaterite (metastable form). Their crystal structures are respectively rhombohedral, orthorhombic and hexagonal [1]. In aqueous system at 25 -C, they have decreasing stabilities and increasing solubility limits. Their solubility constant (K sp) values are respectively 10 8.48, 10 8.34 and 10 7.91 [2]. Vaterite is rarely found in nature but can be easily synthesized in the laboratory at room temperature. It is normally spherical particles of about 1 –10 Am with ovoid structure [3]. Until now, we have known little about the formation mechanism of this metastable form. In this paper we describe a sheet-growth model of vaterite to interpret the morphologies. 2. Methods We selected two methods to precipitate CaCO3 particles: injection method and diffusion method. In injection experi* Corresponding author. Tel.: +86 10 62782770; fax: +86 10 62771160. E-mail address: [email protected] (Q. Feng). 0928-4931/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.msec.2005.09.098
ment, Na2CO3 solutions (25 ml, 0.02 M) were slowly injected into stirring CaCl2 solutions (25 ml, 0.02 M), which contain different concentrations of glycine. The stirring speed was controlled to be around 600 RPM. The concentration magnitude of glycine in the CaCl2 solutions was set as 0, 10 4 M or 10 2 M and the dripping velocity of CaCl2 solutions was 15 ml/min or 2 ml/min. In diffusion experiment, NH4CO3 solid was placed in a closed desiccator to produce CO32 in the Gly-containing CaCl2 solutions for 2 h without stirring. After that, the precipitation was washed and then dispersed in glass slip surface for the investigation of scanning electron microscope (SEM) and X-ray diffraction (XRD). 3. Results and discussion The polymorph and morphology of acquired CaCO3 particles were investigated and partly listed in Table 1. No aragonite was observed. When we mixed the solutions in 2 min, both calcite and vaterite particles formed (Figs. 1a and 2b) and the proportion of vaterite increased with the increase of the Gly concentration. The shape of calcite and vaterite are normally rhombic and spherical respectively. Whereas, when we decrease the injecting velocity to mix them in 12 min, SEM observation showed
W. Hou, Q. Feng / Materials Science and Engineering C 26 (2006) 644 – 647 Table 1 Morphologies of obtained calcium carbonate deposition Dripping Depositing Glycine concentrations velocity time (min) 0 M 10 4 M (ml/min) 15
2
2
12
All rhombic calcite
10
2
M
Calcite and vaterite spheres (less amount)
Calcite and vaterite spheres (about 1:1) All vaterite; spherical or spindly particles (about 1:1)
similar spherical and spindly morphologies of the obtained vaterite particles (Figs. 1b and 2c) whatever glycine concentration was. These results revealed that stirring condition itself was sufficient to induce formation of spindly vaterite when the dripping velocity was slow enough. These results confirmed the work of Hostomsky and Jones [4]. The possible reason is that stirring changed the activation energy of nucleation (DG n) of calcite and vaterite in the aqueous system and upset their nucleation priorities [5]. So, rapidly injection will produce local instantaneous high supersaturation (S = [Ca2+][CO32 ]/K sp) to overcome the nucleation energy barrier of both calcite and vaterite. On the other hand, slow injection will provide a longer time for nucleation and limit the supersaturation value to induce vaterite nuclei but not calcite. In the diffusion method, the control samples are pure rhombic calcite particles. In the present of glycine, depositions were almost all spherical vaterite particles without any spindly ones (Fig. 1c), which implies that the spindly shape occurring in the injection experiments is directly caused by stirring. Some
645
other reports [6] suggested that aspartate could bind on the surface steps to terminate the growth of microcrystals and form porous vaterite crystals. Here we do not analyze surface adsorption of glycine as it only affects morphologies. We propose that glycine may change the activation energy of nucleation in aqueous solutions, like the function of stirring as discussed above. Without any additives, free energy of nucleation (DG n) of calcite is normally lower than that of vaterite. Continually increasing supersaturation will first cause nucleation of calcite. In the presence of Gly, DG n of calcite becomes relatively higher than that of vaterite. So vaterite nuclei form first in the solution. Once they form, ions may accumulate to enlarge the crystal size because 3-dimensional nucleation is relatively difficult in thermodynamics comparing with surface growth. Vaterite spheres are usually described as aggregated ‘‘snowballs’’ of oviods [1] and this detailed structure was also showed in Fig. 1d. However, based on the different morphologies of vaterite observed in the experiment (Fig. 3a), our growth model suggests that it is the growing edges of vaterite sheets [3] (Fig. 3b). Firstly, microsheets of vaterite formed in the solution with its surface perpendicular to c-axis. This direction can be determined referring to the hexagonal shapes in Fig. 3a. Secondly, 2-dimensional nucleation on the ab face and step edge growth of the edge will enlarge the particles. Under stirring conditions, the ion concentration is uniform everywhere in the solution. As [Ca2+] is decreasing with time, both step edge growth velocity and density of 2-dimensional nucleation decreased. Finally the particle exhibited a spindly
Fig. 1. SEM morphologies of vaterite crystals obtained by the [CO23 ] injecting method. (a) [Gly] = 10 3 M, dripping velocity 15 ml/min, C: calcite, V: vaterite; (b) [Gly] = 10 3 M, dripping velocity 2 ml/min; sample without Gly showed similar result; (c) [Gly] = 10 3 M, diffusion method, almost all vaterite; (d) detailed structure of vaterite, composed of oviods with a size about 0.1 Am.
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W. Hou, Q. Feng / Materials Science and Engineering C 26 (2006) 644 – 647
c{104}
a
b
shape (Fig. 3b), because step edge growth (a direction) is normally faster than 2-dimensional nucleation (h direction). Without stirring, diffusion of ions would restrict the relatively higher growth velocity of a position and induce spherical shape. Another reason of spherical particles might be the multinuclei agglomeration which more likely to occur in the condition of stirring (Fig. 3c). Two or three or more nuclei may agglomerate and after fully growing up they will show a sphere-like shape.
x-ray intensity
4. Conclusions
20
22
24
26
v{112}
v{114}
28
34
30
32
36
c
38
40
42
44
46
48
2θ Fig. 2. XRD spectrum of (a) rhombohedral calcite particles; (b) a sample same as Fig. 1a, vaterite and calcite; (c) a sample same as Fig. 1b, spindly vaterite. c{hkl} represents hkl face of calcite and v{hkl} represents hkl face of vaterite.
1. Stirring condition can directly induce formation of vaterite and most particles have spindly morphologies. Glycine can induce spherical vaterite particles in static aqueous solution. 2. Stirring and glycine may both serve as energy changers, which elevate the nucleation priority of vaterite. Further growth would occur on the existing nuclei and do not change its polymorph. 3. Vaterite particles have possibly sheets structure instead of oviods structure. Spindly shape is caused by different sheetgrowth velocity of a/b direction and c-axis direction. Some agglomerate morphologies can be seen.
Fig. 3. (a) The evidence of hexagonal figure of vaterite suggests that their c-axis is perpendicular to the sheet structure; (b) schematic representation of formation of spindly and spherical vaterite particles; (c) some vaterite particles composed of multinuclear sheets or spindles.
W. Hou, Q. Feng / Materials Science and Engineering C 26 (2006) 644 – 647
Acknowledgements This work is supported by the National Natural Science Foundation of China, Grant 50272035. References [1] Nora H. de Leeuw, Stephen C. Parker, J. Phys. Chem. B 102 (1998) 2914. [2] L. Niel, Geochim. Cosmochim. Acta 46 (1982) 1011.
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[3] Moreton Moore, Mineral. Mag. 50 (1986) 332. [4] J. Hostomsky, A.J. Jones, J. Phys., D, Appl. Phys. 24 (1991) 165. [5] S. Mann, D.D. Archibald, J.M. Didymus, T. Douglas, B.R. Heywood, F.C. Meldrum, N.J. Reeves, Science 261 (1993) 5126 1286. [6] Tong Hua, Ma Wentao, Wang Leilei, Wan Peng, Hu Jiming, Cao Lianxin, Biomaterials 25 (2004) 3923.
Morphology and formation mechanism of vaterite particles grown in glycine-containing aqueous solutions Wentao Hou, Qingling Feng * Department of Materials Science and Engineering, Biomaterials Laboratory, Tsinghua University, Beijing 100084, People’s Republic of China Received 1 November 2004; received in revised form 7 August 2005; accepted 7 September 2005 Available online 2 November 2005
Abstract Calcium carbonate crystals were precipitated in glycine-containing aqueous solutions using two methods: CO32 dripping method or diffusion method. By modulating the dripping velocity and glycine (Gly) concentration, we can control the morphologies and proportion of the acquired vaterite particles. Based on the observation, a sheet-growth mechanism was developed to explain the various vaterite morphologies, including spindly, spherical and agglomerate ones. Further discussion proposed that stirring and Gly may both serve as nucleation energy changers, which alternates the nucleation priority of vaterite and calcite. D 2005 Elsevier B.V. All rights reserved. Keywords: Vaterite; Glycine; Calcium carbonate
1. Introduction Calcium carbonate is an ordinary mineral existing in living creatures. There are three polymorphs of calcium carbonate: calcite, aragonite and vaterite (metastable form). Their crystal structures are respectively rhombohedral, orthorhombic and hexagonal [1]. In aqueous system at 25 -C, they have decreasing stabilities and increasing solubility limits. Their solubility constant (K sp) values are respectively 10 8.48, 10 8.34 and 10 7.91 [2]. Vaterite is rarely found in nature but can be easily synthesized in the laboratory at room temperature. It is normally spherical particles of about 1 –10 Am with ovoid structure [3]. Until now, we have known little about the formation mechanism of this metastable form. In this paper we describe a sheet-growth model of vaterite to interpret the morphologies. 2. Methods We selected two methods to precipitate CaCO3 particles: injection method and diffusion method. In injection experi* Corresponding author. Tel.: +86 10 62782770; fax: +86 10 62771160. E-mail address: [email protected] (Q. Feng). 0928-4931/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.msec.2005.09.098
ment, Na2CO3 solutions (25 ml, 0.02 M) were slowly injected into stirring CaCl2 solutions (25 ml, 0.02 M), which contain different concentrations of glycine. The stirring speed was controlled to be around 600 RPM. The concentration magnitude of glycine in the CaCl2 solutions was set as 0, 10 4 M or 10 2 M and the dripping velocity of CaCl2 solutions was 15 ml/min or 2 ml/min. In diffusion experiment, NH4CO3 solid was placed in a closed desiccator to produce CO32 in the Gly-containing CaCl2 solutions for 2 h without stirring. After that, the precipitation was washed and then dispersed in glass slip surface for the investigation of scanning electron microscope (SEM) and X-ray diffraction (XRD). 3. Results and discussion The polymorph and morphology of acquired CaCO3 particles were investigated and partly listed in Table 1. No aragonite was observed. When we mixed the solutions in 2 min, both calcite and vaterite particles formed (Figs. 1a and 2b) and the proportion of vaterite increased with the increase of the Gly concentration. The shape of calcite and vaterite are normally rhombic and spherical respectively. Whereas, when we decrease the injecting velocity to mix them in 12 min, SEM observation showed
W. Hou, Q. Feng / Materials Science and Engineering C 26 (2006) 644 – 647 Table 1 Morphologies of obtained calcium carbonate deposition Dripping Depositing Glycine concentrations velocity time (min) 0 M 10 4 M (ml/min) 15
2
2
12
All rhombic calcite
10
2
M
Calcite and vaterite spheres (less amount)
Calcite and vaterite spheres (about 1:1) All vaterite; spherical or spindly particles (about 1:1)
similar spherical and spindly morphologies of the obtained vaterite particles (Figs. 1b and 2c) whatever glycine concentration was. These results revealed that stirring condition itself was sufficient to induce formation of spindly vaterite when the dripping velocity was slow enough. These results confirmed the work of Hostomsky and Jones [4]. The possible reason is that stirring changed the activation energy of nucleation (DG n) of calcite and vaterite in the aqueous system and upset their nucleation priorities [5]. So, rapidly injection will produce local instantaneous high supersaturation (S = [Ca2+][CO32 ]/K sp) to overcome the nucleation energy barrier of both calcite and vaterite. On the other hand, slow injection will provide a longer time for nucleation and limit the supersaturation value to induce vaterite nuclei but not calcite. In the diffusion method, the control samples are pure rhombic calcite particles. In the present of glycine, depositions were almost all spherical vaterite particles without any spindly ones (Fig. 1c), which implies that the spindly shape occurring in the injection experiments is directly caused by stirring. Some
645
other reports [6] suggested that aspartate could bind on the surface steps to terminate the growth of microcrystals and form porous vaterite crystals. Here we do not analyze surface adsorption of glycine as it only affects morphologies. We propose that glycine may change the activation energy of nucleation in aqueous solutions, like the function of stirring as discussed above. Without any additives, free energy of nucleation (DG n) of calcite is normally lower than that of vaterite. Continually increasing supersaturation will first cause nucleation of calcite. In the presence of Gly, DG n of calcite becomes relatively higher than that of vaterite. So vaterite nuclei form first in the solution. Once they form, ions may accumulate to enlarge the crystal size because 3-dimensional nucleation is relatively difficult in thermodynamics comparing with surface growth. Vaterite spheres are usually described as aggregated ‘‘snowballs’’ of oviods [1] and this detailed structure was also showed in Fig. 1d. However, based on the different morphologies of vaterite observed in the experiment (Fig. 3a), our growth model suggests that it is the growing edges of vaterite sheets [3] (Fig. 3b). Firstly, microsheets of vaterite formed in the solution with its surface perpendicular to c-axis. This direction can be determined referring to the hexagonal shapes in Fig. 3a. Secondly, 2-dimensional nucleation on the ab face and step edge growth of the edge will enlarge the particles. Under stirring conditions, the ion concentration is uniform everywhere in the solution. As [Ca2+] is decreasing with time, both step edge growth velocity and density of 2-dimensional nucleation decreased. Finally the particle exhibited a spindly
Fig. 1. SEM morphologies of vaterite crystals obtained by the [CO23 ] injecting method. (a) [Gly] = 10 3 M, dripping velocity 15 ml/min, C: calcite, V: vaterite; (b) [Gly] = 10 3 M, dripping velocity 2 ml/min; sample without Gly showed similar result; (c) [Gly] = 10 3 M, diffusion method, almost all vaterite; (d) detailed structure of vaterite, composed of oviods with a size about 0.1 Am.
646
W. Hou, Q. Feng / Materials Science and Engineering C 26 (2006) 644 – 647
c{104}
a
b
shape (Fig. 3b), because step edge growth (a direction) is normally faster than 2-dimensional nucleation (h direction). Without stirring, diffusion of ions would restrict the relatively higher growth velocity of a position and induce spherical shape. Another reason of spherical particles might be the multinuclei agglomeration which more likely to occur in the condition of stirring (Fig. 3c). Two or three or more nuclei may agglomerate and after fully growing up they will show a sphere-like shape.
x-ray intensity
4. Conclusions
20
22
24
26
v{112}
v{114}
28
34
30
32
36
c
38
40
42
44
46
48
2θ Fig. 2. XRD spectrum of (a) rhombohedral calcite particles; (b) a sample same as Fig. 1a, vaterite and calcite; (c) a sample same as Fig. 1b, spindly vaterite. c{hkl} represents hkl face of calcite and v{hkl} represents hkl face of vaterite.
1. Stirring condition can directly induce formation of vaterite and most particles have spindly morphologies. Glycine can induce spherical vaterite particles in static aqueous solution. 2. Stirring and glycine may both serve as energy changers, which elevate the nucleation priority of vaterite. Further growth would occur on the existing nuclei and do not change its polymorph. 3. Vaterite particles have possibly sheets structure instead of oviods structure. Spindly shape is caused by different sheetgrowth velocity of a/b direction and c-axis direction. Some agglomerate morphologies can be seen.
Fig. 3. (a) The evidence of hexagonal figure of vaterite suggests that their c-axis is perpendicular to the sheet structure; (b) schematic representation of formation of spindly and spherical vaterite particles; (c) some vaterite particles composed of multinuclear sheets or spindles.
W. Hou, Q. Feng / Materials Science and Engineering C 26 (2006) 644 – 647
Acknowledgements This work is supported by the National Natural Science Foundation of China, Grant 50272035. References [1] Nora H. de Leeuw, Stephen C. Parker, J. Phys. Chem. B 102 (1998) 2914. [2] L. Niel, Geochim. Cosmochim. Acta 46 (1982) 1011.
647
[3] Moreton Moore, Mineral. Mag. 50 (1986) 332. [4] J. Hostomsky, A.J. Jones, J. Phys., D, Appl. Phys. 24 (1991) 165. [5] S. Mann, D.D. Archibald, J.M. Didymus, T. Douglas, B.R. Heywood, F.C. Meldrum, N.J. Reeves, Science 261 (1993) 5126 1286. [6] Tong Hua, Ma Wentao, Wang Leilei, Wan Peng, Hu Jiming, Cao Lianxin, Biomaterials 25 (2004) 3923.