J. Cent. South Univ. (2018) 25: 729-735
DOI: https://doi.org/10.1007/s11771-018-3777-9
Preparation and growth mechanism of plate-like basic magnesium carbonate by template-mediated/homogeneous precipitation method
CHEN Juan(陈娟), HUANG Zhi-liang(黄志良), CHEN Chang-lian(陈常连),
LI Wen-zhao(李文昭), XU Wei-rong(徐伟荣)
School of Materials Science and Engineering, Wuhan Institute of Technology, Wuhan 430074, China
Central South University Press and Springer-Verlag GmbH Germany, part of Springer Nature 2018
Abstract: In order to expand the application of the basic magnesium carbonate in the field of flame retardant, the plate-like basic magnesium carbonate (Mg5(CO3)4(OH)2.4H2O) was prepared successfully by template-mediated/ homogeneous precipitation method, using magnesium chloride hexahydrate (MgCl2.6H2O) and urea (CO(NH2)2) as reaction materials. Phase and morphology of the product were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM) and atomic force microscope (AFM), respectively. The results showed that well-crystallized plate-like Mg5(CO3)4(OH)2.4H2O can be prepared at the water bath temperature of 100 °C, water bath time of 24 h, the aging time of 5 h after adding organic template agent. The investigation on organic template mediated mechanism shows that the template affects the crystal morphology by changing surface energy of different crystal plane. Through a preliminary study on the growth mechanism of the product, it is found that the generation of the plate-like Mg5(CO3)4(OH)2.4H2O could be explained by two-dimensional nucleation/step growth mechanism.
Key words: plate-like; basic magnesium carbonate; template-mediated/homogeneous precipitation; two-dimensional nucleation; step growth
Cite this article as: CHEN Juan, HUANG Zhi-liang, CHEN Chang-lian, LI Wen-zhao, XU Wei-rong. Preparation and growth mechanism of plate-like basic magnesium carbonate by template-mediated/homogeneous precipitation method [J]. Journal of Central South University, 2018, 25(4): 729–735. DOI: https://doi.org/10.1007/s11771-018-3777-9.
1 Introduction
Basic magnesium carbonate (Mg5(CO3)4(OH)2·4H2O), a kind of important inorganic chemical materials with good fluidity, filling and dispersion, can improve the tensile strength and abrasion resistance of rubber. Therefore, it is widely used in the field of polymer materials as excellent filling agent and reinforcing agent [1]. In addition, Mg5(CO3)4(OH)2.4H2O is green-friendly flame retardant for generating CO2 and H2O without toxic and corrosive substances in the thermal decomposition [2–5]. At present, the crystal morphology of Mg5(CO3)4(OH)2·4H2O used in flame retardant has microtube [6], spherical-like [7], rod-like [8], etc. In terms of the dosage, the plate-like filler is less than other crystal morphology. The plate-like Mg5(CO3)4(OH)2·4H2O shows perfect miscibility with polymer for its low surface polarity and surface energy. It has to be noticed that the mechanical strength of polymer can be enhanced by dispersing platelets of the filler [9, 10]. The advantage of composites containing platelets is mainly the improvement of their “barrier properties” that are supposed to play a significant role in fire resistance [11]. So the exploration of preparation methods of the plate-like Mg5(CO3)4(OH)2·4H2O is of great significance for developing new functional flame retardant materials.
Many methods have been used for control of the morphology of Mg5(CO3)4(OH)2·4H2O such as ultrasonic irradiation [12], carbonation [13], and hydrothermal process [14]. However, the phenomena of the crystal aggregation and self- assembly in such a heterogeneous system result in generating the porous Mg5(CO3)4(OH)2·4H2O. Thus these methods are not suitable to prepare the plate- like Mg5(CO3)4(OH)2·4H2O. In this paper, directed growth of the plate-like Mg5(CO3)4(OH)2·4H2O can be achieved by adding organic template agent sorbitol (C6H14O6). In addition, the method of homogeneous precipitation solves the problem that particles tend to be agglomeration and poor crystallization.
The existing theory of crystal growth, such as the screw dislocation growth theory put forward by FRANK [15] and VLS growth theory [16], can well illuminate the growth of crystal under the heterogeneous system without template. The growth direction of this kind of crystal with screw shape is perpendicular to screw dislocation outcrop, which indicates that it is controlled by dynamics and is hard to orient growth [17]. Therefore, the growth mechanism of the plate-like Mg5(CO3)4(OH)2·4H2O through homogeneous precipitation under the action of organic template is also discussed in this paper. Compared to other forms, plate-like Mg5(CO3)4(OH)2·4H2O has the advantages of less usage and better flame retardant effect. As a result, the research on the preparation of plate-like Mg5(CO3)4(OH)2·4H2O can enlarge its application in flame retardants. The study on the induced mechanism has theoretical guiding significance in crystal oriented growth and morphology controlling.
2 Experimental
Magnesium chloride hexahydrate (MgCl2.6H2O), urea (CO(NH2)2), and sorbitol (C6H14O6) were used in this experiment and were of analytical grade. In a typical experiment, a certain quality of MgCl2.6H2O was dissolved in distilled water, dripping with dilute hydrochloric acid to pH=3. After adding CO(NH2)2 (1/4 molar ratio of MgCl2.6H2O) and C6H14O6 (template agent of 5%), the aqueous solution was uniformly mixed and transferred into airtight reaction kettle that was put into a constant temperature water bath pot. The water bath reaction temperature was maintained at 100 °C for 24 h. After taken out, aged for 5 h, cooled to room temperature, filtered by vacuum suction filter method, washed with distilled water as well as ethanol and dried in vacuum oven for 5 h, then the solid products were obtained.
The crystal phases of the products were determined by XD-5A-type powder X-ray diffraction (XRD) with Ni-filtered Cu Ka radiation. The morphology of the product was observed using a JSM-5510LV scanning electron microscope (SEM), and an AJ-III-model atomic force microscope (AFM).
3 Results and discussion
3.1 Crystal structure of Mg5(CO3)4(OH)2·4H2O
XRD diffraction spectrum diagram of the obtained Mg5(CO3)4(OH)2.4H2O is shown in Figure 1. Compared with PDF standard card (PDF 25-513), position and relative signal intensity of each peak are in accordance with the standard diffraction spectrum. There are only diffraction peaks of Mg5(CO3)4(OH)2·4H2O without other impurity phase peaks. Moreover, the sharp peaks of the product show its high crystallinity. The Mg5(CO3)4(OH)2·4H2O belongs to monoclinic system (space group P21/c) with the lattice constants a=10.110
, b=8.940, c=8.380, α=β=90 ° and γ=90.25 °.
By analyzing the crystal phase and component, the authors speculated and drew the crystal structure of Mg5(CO3)4(OH)2·4H2O [18, 19] (Figure 2).
Figure 1 XRD pattern of Mg5(CO3)4(OH)2·4H2O
Figure 2 Crystal structure of Mg5(CO3)4(OH)2·4H2O
3.2 Crystal morphology of Mg5(CO3)4(OH)2·4H2O
SEM images of the product obtained under the experimental conditions are shown in Figures 3(a)–(c). It can be seen from the SEM images that the Mg5(CO3)4(OH)2.4H2O is made from a tiny single crystal in plate shape. According to the crystal habit, the visible crystal faces of plate crystal are a(100) and b(010). In addition, combination of the relationship between crystal faces, it can be inferred the crystal side is z(201). And the crystal plane diagram of the plate-like Mg5(CO3)4(OH)2.4H2O is drew as shown in Figure 3(d).
3.3 Growth mechanism of plate-like Mg5(CO3)4(OH)2·4H2O
3.3.1 Organic template mediated mechanism of plate-like Mg5(CO3)4(OH)2·4H2O
Figure 4 shows SEM image of the product prepared without adding organic template. It can be noted that this product, unsystematic and disorderly, is unlike the product in Figure 3 as the long-shaped plate. The growth rate of the side crystal planes does not appear to be much different. This is two dimensional growth process. Through hybridization of the organic/inorganic ion, C6H14O6 as organic template agent can change the surface energy of the various the crystal planes and further control the growth rate as well as the crystal morphology [20].
The total rate of oriented growth:
(1)
Crystal face (hkl):
(2)
where the critical free energy of crystal nucleus 
△Ga is the activation energy, C is the concentration of the system, C0 is the concentration of crystal nucleus, γ(hkl) is the interfacial energy.
Figure 3 SEM images (a, b, c) and crystal plane diagram (d) of plate-like Mg5(CO3)4(OH)2·4H2O
Figure 4 SEM image of plate-like Mg5(CO3)4(OH)2·4H2O prepared without adding C6H14O6:
Obviously, U(hkl) depends on U0, △Ga, △G* and T0. Since △Ga is only consistent with the diffusion of the solute, it can be regarded as constant in the process of crystallization [21]. Due to the homogeneous system environment, U0, T0 and ln(C/C0) remains the same in different directions. So the interfacial energy of different crystal face γcl(hkl) is the key factor to determine the value of the U(hkl). The a(100) plane and b(010) plane, the low-index surfaces of the plate-like Mg5(CO3)4(OH)2·4H2O possess the smallest interfacial energy γcl(100) and γcl(010), as well as the smallest critical free energy △G*(100) and △G*(010). Therefore, it can be concluded that U(100) and U(010) are the smallest value. According to the crystal selectivity, finally the revealed crystal plane is a(100) and b(010), which can be proved in previous work [22, 23]. After adding organic template agent, through hybridization of the organic/inorganic ion, C6H14O6 is on the growing point of Mg5(CO3)4(OH)2·4H2O by forming hydrogen bond with hydroxyl on the crystal face a(100), which can reduce the surface energy and decrease ulteriorly the growth rate of the crystal face. The growth process is that crystal grows up along the crystal face possessed the minimum interfacial energy, which is one dimensional oriented growth(Figure 3(c)). Thus the most revealed crystal plane is a(100), and the product is long plate-like Mg5(CO3)4(OH)2·4H2O.
3.3.2 Homogeneous precipitation of Mg5(CO3)4(OH)2·4H2O
As the temperature (25 °C–95 °C) continues to increase, the urea shows wet chemical reaction with releasing the OH– and CO2 gas in the reaction(3). The pressure in the airtight reaction kettle increases because of the CO2 gas generated gradually. Under the condition of high pressure, partial CO2 turns into CO32– by hydrolysis reaction (4) [24].With further reaction, the concentration of OH– and CO32– increases and reaches a certain value, which causes the supersaturated solution. Mg5(CO3)4(OH)2·4H2O generates precipitation and nucleus begins to form (3). At the same time, the temperature is always constant. With the OH– and CO32– slowly releasing, crystallization and release are in a dynamic balance, so mother liquor system saturation almost remains constant. All precipitation and nucleation process is under homogeneous solution system. Therefore, crystal diameter is small enough, even can achieve nanometer level and micro level.
The hydrolysis reaction of urea:
→
(3)
The hydrolysis reaction of CO2:
→
(4)
The precipitation reaction of Mg5(CO3)4(OH)2·4H2O:
→
(5)
3.3.3 Two-dimensional nucleation/step growth mechanism of plate-like Mg5(CO3)4(OH)2·4H2O
Figure 5 shows the SEM images of the products obtained at 12, 24, 36 h of duration. When the reaction time is 12 h, lots of “two-dimensional crystal nuclei” form on the interface (Figure 5(a)). As the reaction continues, a large number of crystal nuclei assemble and grow along the steps (Figure 5(b)). After 36 h, due to the decreasing supersaturation of solution system, the nucleation rate is less than the growth rate of crystal. The crystal surface becomes much smoother gradually and then grows completely (Figure 5(c)). We preliminary refer to this process as “step growth” (Figure 6). At first, crystal nuclei begin to form in supersaturated solution (Figure 6(a)). With the further reaction, Mg5(CO3)4(OH)2·4H2O at site 1 arrives in interface at site 2. Simultaneously, one nearest-neighbor bonds and four next nearest- neighbor bonds would be originated. If the bonding energies of the nearest-neighbor and the next nearest-neighbor molecules are 2φ1 and 2φ2 [25], then the crystallization release energy (ws: thermodynamics driving force) is WS=2φ1+8φ2. Similarly, the energies released at site 3 and at kinking site 4 are calculated according to the formula WS=4φ1+12φ2 and WS=6φ1+12φ2, respectively. Thus, the kinking site 4, the largest the driving force, has the minimum potential energy. That is to say, the kinking site 4 is the most stable site where molecules are absorbed on the interface location. When Mg5(CO3)4(OH)2·4H2O arrive in the kinking site through the diffusion, the crystal will grow gradually. The kink stops growing after sweeping the crystal plane b(010). Finally, the crystal plane a(100), b(010) and the coarser crystal plane
are revealed in the plate-like Mg5(CO3)4(OH)2·4H2O, which can also be proved in AFM image shown in Figure 7.
Figure 5 SEM image of plate-like Mg5(CO3)4(OH)2·4H2O at different reaction time:
Figure 6 Step growth model of plate-like Mg5(CO3)4(OH)2·4H2O
Figure 7 AFM image of plate-like Mg5(CO3)4(OH)2·4H2O
4 Conclusions
1) With MgCl2·6H2O and CO(NH2)2 as raw material, adding C6H14O6 as template agent, the plate-like Mg5(CO3)4(OH)2·4H2O with high crystallinity and purity was prepared successfully by homogeneous precipitation method.
2) The organic template mediated mechanism has been discussed in this paper. C6H14O6 as organic template affects the surface energy and changing the growth rate of different crystal plane, which can effectively control the oriented growth and the crystal morphology.
3) The wet chemistry reaction of urea guarantees the dynamic balance between the crystallization and the release of OH– and CO32–, thus saturation of the liquid system almost remains unchanged. Therefore, under the homogeneous system, the crystal can grow stably and avoid agglomeration.
References
[1] QING Li, Yi Ding, Yu Gui-hua, Li Cun, Li Fan-qing, Qian Yi-tai. Fabrication of light-emitting porous hydromagnesite with rosette-like architecture [J]. Solid State Communications, 2003, 125(2): 117–120.
[2] Botha A, Strydom C A. Preparation of a magnesium hydroxy carbonate from magnesium hydroxide [J]. Hydrometallurgy, 2001, 62(3):175–183.
[3] LIU Xin-wei, FENG Ya-li, LI Hao-ran. Preparation of basic magnesium carbonate and its thermal decomposition kinetics in air [J]. Journal of Central South University of Technology, 2011, 18: 1865–1870.
[4] CHEN Xiao-lang, YU Jie, GUO Shao-yun, LU Sheng-jun, LUO Zhu, HE Min. Surface modification of magnesium hydroxide and its application in flame retardant polypropylene composites [J]. Journal of Materials Science, 2009, 44: 1324–1332.
[5] Sener A A, Demirhan E. The investigation of using magnesium hydroxide as a flame retardant in the cable insulation material by cross-linked polyethylene [J]. Materials & Design, 2009,44(5): 1324–1332.
[6] Mitsuhashi K, Tagami N, Tanabe K, Suzuki S, Iwanaga S, Ohkubo T, Sakai H, Koishi M, Abe M. Synthesis and properties of a microtube photocatalyst with photoactive inner surface and inert outer surface [J]. Journal of Photochemistry and Photobiology A: Chemistry, 2007,185(2): 133–139.
[7] Zhang Zhi-ping, Zheng Ya-jun, Zhang Ji-xiu, Zhang Qing. Synthesis and shape evolution of monodisperse basic magnesium carbonate microspheres [J]. Crystal Growth & Design,2007,7(2): 337–342.
[8] Hao Zhi-hua, Pan Jie, Du Fang-lin. Synthesis of basic magnesium carbonate microrods with a surface of “house of cards” structure [J]. Materials Letters, 2009, 63(12): 985–988.
[9] Henrist C, Mathieu J P, Vogels C, RULMONT A, CLOOTS R. Morphological study of magnesium hydroxide nanoparticles precipitated in dilute aqueous solution [J]. Journal of Crystal Growth, 2003,249(1, 2): 321–330.
[10] Krishnamoorti R, Yurekli K. Rheology of polymer layered silicate nanocomposites [J]. Macromolecules, 2001,6(5): 464–470.
[11] Alexandre1 M, Beyer G, Henrist C, Cloots R, Rulmont A, 
R, Dubois P. Preparation and properties of layered silicate nanocomposites based on ethylene vinyl acetate copolymers [J]. Macromolecular Rapid Communications, 2001,6(5): 464–470.
[12] Ohkubo T, Suzuki S, Mitsuhashi K. Preparation of petaloid microspheres of basic magnesium carbonate [J]. Langmuir the ACS Journal of Surfaces & Colloids, 2007,23(11): 5872–5874.
[13] Mitsuhashi K, Tagami N, Tanabe K, Ohkubo T, Sakai H, KOISHI M, Masahiko A. Synthesis of microtubes with a surface of “house of cards” structure via needlelike particles and control of their pore size [J]. Langmuir the ACS Journal of Surfaces & Colloids, 2005,21(8): 3659–3663.
[14] YAN Cheng-lin, XUE Dong-feng. Novel self-assembled MgO nanosheet and its precursors [J]. Journal of Physical Chemistry B, 2005,109(25):12358–12361.
[15] FRANK F C. The influence of dislocations on crystal growth [J]. Discussions of the Faraday Society, 1949,5(5): 48–54.
[16] WAGNER R S, ELLIS W C. Vapor-liquid-solid mechanism of single crystal growth [J]. Applied Physics Letters. 1964, 4(5): 89–90.
[17] BLASS T, FONSECA I, LEONI G, MORANDOTTI M. Dynamics for systems of screw dislocations [J]. Siam Journal on Applied Mathematics, 2015, 75(2): 393–419.
[18] CHEN Juan, HUANG Zhi-liang, CHEN Chang-lian, LI Wen-zhao, XU Wei-rong. Preparation of basic magnesium carbonate whisker by homogeneous precipitation method and growth mechanism [J]. Journal of Wuhan Institute of Technology, 2015, 37(12): 16–20.
[19] AKAO M, MARUMO F, IWAI S. The crystal structure of hydromagnesite [J]. Structural Science, Crystal Engineering and Materials, 1974, 30(11): 2670–2672.
[20] WANG Pei-pei, LI Cai-hong, GONG Hai-yan, WANG Hong-qiang, LIU Jin-rong. Morphology control and growth mechanism of magnesium hydroxide nanoparticles via a simple wet precipitation method [J]. Ceramics International, 2011, 37: 3365–3370.
[21] Mittemeijer E J. Fundamentals of materials science [M]. Berlin: Springer, 2010: 24.
[22] Cheng Xiao-kun, Huang Zhi-liang, LI Jian-qiu, Liu Yu, Chen Chang-lian, Chi Ru-an, Hu Yue-hua. Self- assembled growth and pore size control of the bubble- template porous carbonated hydroxyapatite microsphere [J]. Crystal Growth & Design, 2010,10(3): 1180–1188.
[23] HE Jian-qiu, HUANG Zhi-liang. Controlled synthesis and morphological evolution of dendritic porous microspheres of calcium phosphates [J]. Journal of Porous Materials, 2008,16(6): 683–689.
[24] Okazaki M, Funazukuri T. Decomposition of urea in sub- and supercritical water with/without additives [J]. Journal of Materials Science, 2008,43(7): 2316–2322.
[25] CHEN Chang-lian, LI Jian-qiu, HUANG Zhi-liang, CHENG Xiao-kun, YU Jun, WANG Han, CHI Ru-an, HU Yu-hua. Phase transformation process and step growth mechanism of hydroxyapatite whiskers under constant impulsion system [J]. Journal of Crystal Growth, 2011,327(1): 154–160.
(Edited by HE Yun-bin)
中文导读
模板诱导/均相沉淀法制备板片状碱式碳酸镁及其生长机理的研究
摘要：为了扩大碱式碳酸镁在阻燃领域的应用，通过模板诱导/均相沉淀法，以六水氯化镁和尿素作为原料，成功合成了碱式碳酸镁晶体。分别采用XRD、SEM、AFM对所得样品进行物相和形貌表征。结果显示，添加有机模板剂，在水浴温度100 °C条件下，反应24 h，陈化5 h后可得到结晶良好的板片状碱式碳酸镁。对有机模板诱导机理的研究显示，模板剂通过改变不同晶面的表面能影响晶体形貌。初步探究了板片状碱式碳酸镁的生长，提出了“二维成核/台阶生长”机理。
关键词：板片状；碱式碳酸镁；模板诱导/均相沉淀；二维成核；台阶生长
Foundation item: Project(51374155) supported by the National Natural Science Foundation of China; Project(2014BCB034) supported by the Hubei Province Key Technology R&D Program, China; Project(2014CFB796) supported by the Natural Science Foundation of Hubei Province of China
Received date: 2016-05-30; Accepted date: 2017-05-10
Corresponding author: HUANG Zhi-liang, PhD, Professor; Tel: +86–13971523197; E-mail: hzl6455@126.com
