Synthesis of stable ACC using mesoporous silica gel as a support
© Fu et al.; licensee Springer. 2014
Received: 22 June 2014
Accepted: 14 August 2014
Published: 29 August 2014
Stable amorphous calcium carbonate supported by mesoporous silica gel was successfully synthesized. The silica gel support is prepared through the hydrolytic polycondensation of ethyl silicate under suitable conditions. Laser scanning confocal microscopy (LSCM) observations reveal that the morphology of the products is branched with cruciform-like and flower-like structure. Raman spectroscopic analysis and scanning electron microscopy (SEM) observation of the products confirm the combination of stable amorphous calcium carbonate (ACC) nanoparticles and mesoporous silica gel. A possible growth mechanism for the branched structure has been proposed. Results indicate potential application of this work to ACC storage, crystal engineering, biomimetic synthesis, etc.
Amorphous calcium carbonate (ACC) has attracted increasing interest as a result of its potential use in biomimetic and industrial applications. However, it is a transient precursor phase to crystalline modification [1–4], so it is difficult to obtain in vitro. Stabilizing amorphous precusors is one of the major issues in biomineralization studies . Moreover, people had been trying to add process-directing agents during the nucleation stage. Additives such as phosphorproteins , aspartic acid , and ployacrylic acid (PAA)  have been proved to act as stabilizers for ACC. In addition, researchers have also tried other inorganic substances, with the result that spherical ACC accompanied by vaterite or calcite was obtained .
The reason ACC is unstable under ambient conditions is because of its large interfacial energy. Accordingly, it would be interesting to develop a highly porous support material which would make it possible to lower the interfacial energy of ACC during the reaction . With this in mind, silica gel was chosen as the material because of its tunable porosity via hydrolytic polycondensation of liquid precursors such as the silicon alkoxides under controlled conditions . The first synthesis of porous silica was described by Kistler in 1931 . Since that time, silica gels have been used as functional materials with an impressive range of applications . The use of silica gel for CaCO3 single crystal growth has been employed as a means to control the purity and morphology [13, 14]. However, a silica gel-based system for controlling the formation of amorphous CaCO3 has not been studied.
In this work, we used a porous silica gel support to form ACC for the first time. Silica gel is obtained through the hydrolytic polycondensation of ethyl silicate as an additive to a solution of CaCl2 and (NH2)2CO. The morphology of silica gel can be tailored to form a 3D-matrix during hydrolytic polycondensation under suitable conditions , so that support is afforded that lowers the interfacial energy of the ACC. The structure and morphology of the product were characterized by laser scanning confocal microscopy (LSCM), micro-Raman spectroscopy, and scanning electron microscopy (SEM).
The ethyl silicate (ES), calcium chloride dihydrate (CaCl2··2H2O), urea, ethyl alcohol (C2H5OH), and sodium hydroxide (NaOH) used as precursors were of analytical grade and used without further purification. All chemicals were purchased from Sinopharm Chemical Reagent Co., Ltd (Shanghai, China). Deionized water with an electrical conductivity of less than 106 S m-1 was taken from a Mili-Q system.
Four separate silica solutions were prepared by mixing 0.2 mL ethyl silicate, 0.2 mL alcohol, 6.5 mL NaOH (0.1 M), and deionized water in 100-mL plastic beakers and stirring for 1 h. A 0.5 M calcium chloride solution and 2.5 M urea solution were prepared in 30-mL quantities. Subsequently, different amounts (0.5, 1, 1.5, and 2 mL) of the 0.5 M calcium chloride solution and 1.5 mL of the 2.5 M urea solution were added to the plastic beakers. As a result, the concentration of CaCl2 is, respectively, 2.5, 5, 7.5, and 15 mM, in these four mixing solutions. Deionized water was added until the total amount of mixture was 100 mL. After that, 5 mL each of the solutions was transferred to separate Petri dishes, each with a 5 cm × 5 cm slide substrate. Each Petri dish was sealed by parafilm with seven pinholes and then incubated at 60°C until bakeout. The sample on the slide substrate was then subjected to analysis.
Laser scanning confocal microscopy (LSCM) and scanning electron microscopy (Hitachi S-4800 SEM, Hitachi, Ltd., Chiyoda-ku, Japan) were used to observe the morphology of the sample. SEM images were obtained without gold coating in order to avoid spurious results. The Raman scattering spectrum was obtained using the 532-nm line of a Nd, YAG laser as the excitation source. For all measurements with visible excitation, the slits were set at 100 μm and a × 100 objective was used.
Results and discussion
In this work, the possibility of synthesizing stable ACC supported by mesoporous silica gel has been described. These composites are obtained using the reaction of CaCl2 and (NH2)2CO in a silica gel medium that is prepared through the hydrolytic polycondensation of ethyl silicate. LSCM, Raman, and SEM observations show that the morphology of the composites, which are composed of ACC nanoparticles and mesoporous silica gel takes on a branched form with cruciform-like and flower-like structures. The growth mechanism is discussed and a possible self-assembly process for the branched products is proposed. Silica gel with 3D-matrix morphology was successfully fabricated as a support for ACC. As a result, chemical agents with 3D-matrix morphology, such as silica gel, have the potential to significantly improve the utility and integrity of underground reservoirs for ACC storage. Moreover, the results suggest potential applications of the work with important implications in crystal engineering, biomimetic synthesis, etc.
This study was financially supported by Natural Science Foundation of China under grant No. 11134006 and 51321091.
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