Preparation of Flowerlike Porous Mg5(CO3)4(OH)2·4H2O by CO2 Bubble Templating

Flowerlike porous basic magnesium carbonate (Mg5(CO3)4(OH)2·4H2O) were obtained successfully using magnesium chloride hexahydrate and ammonium bicarbonate as raw materials. We investigated the effect of reaction time and reaction temperature on the Mg5(CO3)4(OH)2·4H2O preparation. In the first three hours’ reaction, Magnesium carbonate trihydrate (MgCO3·3H2O) whiskers generated, and transferred to the flowerlike porous Mg5(CO3)4(OH)2·4H2O as the reaction time prolonged. During the transformation, the flowerlike Mg5(CO3)4(OH)2·4H2O crystals firstly appeared at the head of the MgCO3·3H2O whisker.


Introduction
The morphology and properties of particles are closely related. To fabricate materials with certain morphology to achieve better performance, there has been increasing interests in controlled synthesis of inorganic microstructure with desirable shape and size [1][2].
Because of the special structure, spherical porous materials have advantages in easily moving, closely packing and occupying the available vacancies, which implies potential applications [3]. In recent years, in particular, spherical porous materials have attracted much attention and many studies have been carried out on the superiority of spherical porous materials. For example, Wang et al. have synthesized spherical porous CaCO 3 microparticle which could be applied as relatively safe drug vehicles. Yu et al have prepared spherical porous LiFePO 4 /C particles which provide better contact with electrolyte and are easier to bind than isolated LiFePO 4 particles [4][5].
This paper presents the formation process of flowerlike porous Mg 5 (CO 3 ) 4 (OH) 2 .4H 2 O. By investigating particles under various reaction times, the shape evolution of these flowerlike porous particles is discussed in detail. CO 2 bubble template was introduced to simulate the formation of flowerlike porous Mg 5 (CO 3 ) 4 (OH) 2 .4H 2 O, and the equation of reaction temperature and the pore size was given by the derivation of Laplace and Clapyeron equations. was implemented by vacuum suction filter method and washed with distilled water as well as ethanol. The washed residue was dried in vacuum oven for 2 h.

Experimental
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). Figure.   2 shows SEM images of the particles synthesized at different reaction time, which indicates an obvious growth process occurring from whisker to flowerlike microsphere with porous structure. As shown in Fig. 2a, a large number of whiskers were obtained by reacting 3 h. Through the phase analysis, the product was determined as MgCO 3 . 3H 2 O (Fig. 1a). The structure and surface of MgCO 3 . 3H 2 O whiskers were unbroken and smooth. In addition, the length was about 140 μm and the aspect ratio was about 25. At the time of reacting 4 h, the sample was still whisker with reduced aspect ratio, but the surface became rough and a small number of leaf-like crystals appeared on the surface of the whisker. When the reaction time was increased to 5 h, the sample not only contained whisker product, but also began to appear flowerlike spherical particles. From a magnifying SEM image, it seemed that "connected flakes" gathered on the head of the whisker (Fig. 2c). When the samples were produced for reacting 6 h, the original whisker shape disappeared completely. All samples turned into the flowerlike porous Mg 5 (CO 3 ) 4 (OH) 2 . 4H 2 O that are assembled by lamella (Fig. 2d). h)

Results and discussion
The conversion reaction giving rise to the Mg 5 (CO 3 ) 4 (OH) 2 . 4H 2 O crystal occurs via the formation of metastable MgCO 3 . 3H 2 O in the synthetic process [6][7][8]. The reaction time has a significant impact on the phase transition process. Reactions of the transition process occur as follows: As shown in Figure. 2d, the surfaces of the flowerlike Mg 5 (CO 3 ) 4 (OH) 2 . 4H 2 O are assembled from a number of lamellae that interconnected to form an open porous structure. Generally, gas bubbles, such as CO 2 bubbles, can form open porous structure [9]. With the increase of reaction time, the NH 4 HCO 3 slowly releases CO 2 bubbles. At the same time, in order to obtain lower surface tension to achieve a relatively stable state, small CO 2 bubbles gather into big spheres of micrometer size and preferentially adhere to the head of whisker (see Figure. 3). This process can be explained by Gibbs function: where G, S and V are Gibbs function, entropy and volume of system, separately; μ B and n B are chemical potential and amount of substance of component B in phase A, respectively; A S is the interfacial area; γ is the interfacial tension. Since the temperature and pressure are constant, equation 3 can be simplified as follows: Under the condition of constant temperature and constant pressure, the decrease of Gibbs function of interface system is a spontaneous process from the thermodynamic view. According to the equation 6, In order to reduce the interfacial Gibbs function to achieve relatively stable state, Many small CO 2 bubbles reduce its surface area by gathering into big CO 2 bubbles , and the head of whisker reduce the surface tension by dissolving and adsorpting CO 2 bubbles.