TMMOB Metalurj i ve Malzeme Mühendisleri Odas ı Eğ i t im MerkeziBildir i ler Kitab ı

56319. Uluslararas ı Metalurj i ve Malzeme Kongresi | IMMC 2018

Synthesis of Hierarchical Calcium Carbonate Particles

Çağatay Mert Oral¹, Batur Ercan¹, Derya Kapusuz²

¹Middle East Technical University, Faculty of Engineering, Department of Metallurgical and Materials Engineering, Ankara Turkey²Gaziantep University, Faculty of Engineering, Department of Metallurgical and Materials Engineering, Gaziantep, Turkey

Abstract

Recently, synthesis of hierarchically structured porous materials gained particular interest for various industrial applications. As a biocompatible and chemically stable material, hierarchical CaCO3 structures formed by ordered aggregation and growth of nanocrystals offer significant advantages for structural and biomedical use compared to their currently-used alternatives. During solution processing, incorporating doping agents to calcium positions or altering CaCO3 supersaturation, new crystals can heterogeneously nucleate on previously formed nuclei, leading to particle growth in a hierarchical manner. In this study, hierarchical CaCO3 microparticles were synthesized at temperatures below 100 °C by an additive-modified precipitation approach. The morphology and crystal structure of the CaCO3 particles were determined by electron microscopy (SEM and TEM) and X-ray diffraction (XRD). By varying additive content and temperature, morphologies including hollow microspheres, crab-like and flower-like particles were successfully obtained.

1. Introduction

Calcium carbonate (CaCO3) is one of the most abundant minerals on earth. It is synthesized by living organisms and it has been used in various biomedical applications including drug delivery and orthopedics [1,2]. CaCO3 has three anhydrous polymorphs, namely vaterite (hexagonal), aragonite (orthorhombic) and calcite (rhombohedral) [3]. Among these polymorphs, vaterite is unstable polymorph and it can readily transform to aragonite or calcite in aqueous environments [4]. Aragonite is the metastable polymorph and generally appears at high temperatures, whereas calcite can easily be obtained at ambient conditions [5].

To utilize CaCO3 particles in different biomedical applications, their physical and chemical properties (i.e.size, morphology, polymorph, etc.) need to be controlled depending on the biomedical application. For this purpose, several variables were proposed in literature. For instance, Trushina et al. [6] used glycerol as an additive to synthesize spherical vaterite particles and successfully decreased average size of the CaCO3

particles from 1.5 μm to 350 nm by solely controlling glycerol concentration. Qi et al. [7] used ethylene glycol and microwave heating to synthesize dagger-like vaterite particles. Trushina et al. [6] also varied temperature of precursor solutions to synthesize cuboidal calcite particles at low temperatures (2-3 °C) while star-like vaterite particles were obtained at medium temperatures (40 °C).

Herein, CaCO3 particle properties were controlled by changing additives and reaction temperatures below 100 °C. Hierarchically structured hollow, crab-like and flower-like particles were obtained without referring to the use of templates and complex procedures. Ultimately, these synthesized CaCO3 particles having distinct physical and chemical properties can be effective candidates in different biomedical applications including but not limited to drug delivery, bone paste and composites for bone regeneration. For this purpose, this study investigated alternate synthesis routes to obtain CaCO3 particles.

2. Experimental Procedure

CaCO3 particles were synthesized via precipitation reaction between calcium acetate (Ca(CH3COO)2) and sodium bicarbonate (NaHCO3). Calcium acetate (4 ml) and sodium bicarbonate (4 ml) solutions were prepared separately and 20 ml ethylene glycol was added onto each solution. Solutions were mixed under magnetic stirring for 15 minutes under 3 different conditions to control CaCO3 particle properties. For synthesis of hollow microspheres (condition 1), sodium dodecyl sulfate (SDS) was added to sodium bicarbonate solution and nitrogen gas was supplied for 15 minutes to the mixture. For synthesis of crab-like particles (condition 2), experiment was performed at 90 °C and urea solution (10 ml) was added to the mixture. For synthesis of flower-like particles (condition 3), precursor solution pH values were adjusted to 13 using NaOH (1M) before they were mixed. Once the stirring step was complete, precipitation reaction further continued for another hour without magnetic stirring for all experimental conditions. To collect synthesized CaCO3 particles, the mixed solution was washed with ethanol and water, followed by drying overnight at 50 °C.

UCTEA Chamber of Metallurgical & Materials Engineers’s Training Center Proceedings Book

564 IMMC 2018 | 19th International Metallurgy & Materials Congress

3. Results and Discussion

Morphology of the synthesized CaCO3 particles were characterized using SEM and TEM, and the particle polymorphs were investigated using XRD. As shown in Figure 1, microspheres were synthesized using incorporation of SDS and nitrogen gas (condition 1). Significant number of the particles in this sample had hollow internal structures, which could be explained by incomplete circumferential particle growth surrounding the nitrogen bubbles during CaCO3 synthesis.

Figure 1. SEM micrograph of hollow CaCO3microspheres.

In order to further characterize the internal structure of these particles, TEM analysis was completed. Morphological investigations exhibited in Figure 2 also confirmed hollow structure of these particles. XRD spectra of the particles (Figure 6a) showed characteristic peaks of calcite at 29.40° (104), 35.90° (110), 39.50° (113) and vaterite at 24.92° (110), 26.99° (112) and 32.78° (114) [8]. Results suggest initial vaterite crystal nucleation, followed by aggregation into larger particles, where calcite transformation was suppressed by the combined effects of SDS and nitrogen gas incorporation. Most probably, vaterite crystals initially nucleated, followed by adsorption of negatively charged SDS groups onto the crystal surfaces, which helped isolate the cations and suppressed vaterite to calcite transformation. It can be speculated that gas bubble surfaces decreased the surface free energy required for nucleation, promoting the attachment of vaterite crystals to each other on the bubbles and finally forming a hierarchical hollow structure. In biomedical applications, the hollow interior of the particles could potentially be used to upload and deliver load drug payload at the desired biological site.

Figure 2. TEM micrograph of a hollow CaCO3microsphere.

As discussed previously, synthesis of pure vaterite is challenging due to the stable nature of calcite under ambient conditions. This study shows the potential use of SDS/nitrogen gas combinational approach to suppress the vaterite-to-calcite transformation at ambient conditions, while promoting the formation of hierarchical hollow CaCO3 particles.

When experimental conditions were altered to use urea at 90 °C (instead of using SDS and nitrogen gas bubbling), crab-like particles were formed, as shown in Figure 3 (condition 2). In this study, crab-like CaCO3 particlemorphology was -for the first time- introduced to literature. Owing to the branched morphology of the microparticles with arms having submicron feature size, these particles could effectively interlock with a polymeric matrix, leading to superior mechanical strength and wear resistance in biomedical composites.

Figure 3. SEM micrograph of crab-like CaCO3 particles.

5 μm

1 μm

5 μm

TMMOB Metalurj i ve Malzeme Mühendisleri Odas ı Eğ i t im MerkeziBildir i ler Kitab ı

56519. Uluslararas ı Metalurj i ve Malzeme Kongresi | IMMC 2018

TEM analysis was completed to characterize the internal structure of these particles. Figure 4 showed solid structure in the arms of these crab-like particles.

Figure 4. TEM micrograph of a crab-like CaCO3 particleobtained from the tip of its arm.

XRD analysis shown in Figure 6b indicated that these particles had characteristic aragonite peaks at 26.3° (111) and 45.9° (221), along with low-intensity calcite peaks [2]. Calcite polymorph generally has a characteristic cuboidal morphology and SEM investigations showed only a few cuboidal particles in Figure 3. Therefore, crab-like particles predominantly consisted of aragonite polymorph.

TEM image in Figure 4 could potentially indicate initial formation of small aragonite particles, followed by their preferential attachment on a specific crystallographic direction to decrease the surface free energy during growth. During synthesis of crab-like particles, urea could decompose into ammonia and carbonate groups, leading to simultaneous increase in carbonate concentration and pH [9]. We can speculate that simultaneous increase in pH with carbonate groups varied the supersaturation rate and suppressed the aragonite-to-calcite transformation in this study.

In the third synthesis condition, pH was adjusted to 13 at room temperature without incorporation of urea, SDS or gas bubbling, flower-like CaCO3 particles were obtained, as shown in Figure 5. In this case, pH was increased by NaOH addition, leading to increase in OH- concentration while carbonate concentration remained the same. XRD spectra of flower-like particles (Figure 6c) showed only calcite peaks. As discussed previously, calcite is the most stable polymorph of CaCO3 and it typically exhibits a cuboidal morphology. These particles had larger size compared to hollow microspheres and crab-like particles.

This was due to uncontrolled particle growth observed at high pH solutions.

Figure 5. SEM micrograph of flower-like CaCO3particles.

CaCO3 particles sythesized in this study had distinct and separate physical properties, including their morphology, polymorph, size and structure. For biomedical applications, this could offer an advantage where desired CaCO3 particle properties can be attained depending on the application requirements. In a follow-up research, biocompatibility of these particles will be assessed for their use in orthopedic and bone-tissue engineering applications.

Figure 6. XRD spectra of a) hollow microsphere, b) crab-like and c) flower-like CaCO3 particles. Red lines are JCPDS references for vaterite (#033-0268), aragonite (#005-0453) and calcite (#005-0586).

4. Conclusion

In this study, SDS, urea, nitrogen gas, temperature and pH were used as variables to alter CaCO3 particle

5 μm

200nm

UCTEA Chamber of Metallurgical & Materials Engineers’s Training Center Proceedings Book

566 IMMC 2018 | 19th International Metallurgy & Materials Congress

properties. By adding SDS and nitrogen gas to precursor solutions, hollow microspheres were synthesized. When synthesis temperature increased to 90 °C and urea was incorporated into the system, crab-like particles were obtained. In addition, precursor solution pH was increased to 13 where flower-like calcite particles could be synthesized. Among these morphologies, crab-like aragonite was synthesized for the first time in literature. Furthermore, CaCO3 particles obtained in this study had distinct properties in terms of their morphologies, polymorphs and sizes; allowing selection of desired CaCO3 particle properties depending on the application requirements. In the future, these CaCO3 particles will be tested for their biological properties.

Acknowledgements

The authors would like to thank Serkan Yılmaz for TEM images and Middle East Technical University Research Funds (BAP-03-08-2016-009) for providing financial support.

References

[1] M. Ni, B. D. Ratner, Surface and Interface Analysis, 40 (2008), 1356-1361. [2] R. M. Santos, P. Ceulemans, T. Gerven, Chemical Engineering Research and Design, 90 (2012), 715-725. [3] A. C. Tas, Applied Surface Science, 330 (2015), 262-269.[4] K. Sawada, Pure and Applied Chemistry, 69 (1997), 921-928.[5] Y. Chen, X. Ji, X. J. Wang, Journal of Crystal Growth, 312 (2010), 3191-3197. [6] D. B. Trushina, T. V. Bukreeva, M. N. Antipina, Crystal Growth and Design, 16 (2016), 1311-1319. [7] R. Qi, Y. Zhu, Journal of Physical Chemistry B, 110 (2006), 8302-8306. [8] J. A. Juhasz-Bortuzzo, B. Myszka, R. Silva, A. R. Boccaccini, Crystal Growth and Design, 17 (2017), 2351-2356.[9] L. Wang, I. Sondi, E. Matijevic, Journal of Colloid and Interface Science, 218 (1999), 545-553.