Morphogenesis of self-assembled crystalline materials of calcium carbonate and silica

Abstract

Silica-carbonate biomorphs are self-assembled crystalline materials with a wide range of biomimetic morphologies such as sinuous boundaries and continuous smooth curvatures, which are beyond the constraints of crystallographic symmetry and reminiscent of living organisms. These materials are composed of carbonate nanocrystals in well-defined orientations and exhibit complex structure from macro to the nanoscale. Compared to most of the recent studies of morphologies in biominerals and biomimetic materials, which are mainly attributable to the effect of organic molecules, the crystallization of silica-carbonate biomorphs is a pure inorganic process without the presence of any organic matter and it occurs in moderately alkaline silica rich solution or silica gel matrix (pH range from 11 to 8.5). The phenomenon of pure inorganic system exhibiting life-like morphologies has been discovered and wellstudied through barium carbonate and strontium carbonate. However, in contrast to calcium carbonate, one of the most abundant materials not only in biomineralization but also in sediments and sedimentary rocks on Earth, there is less study on the morphogenesis of such a kind of mineral under alkaline silica-rich conditions. Unlike barium and strontium carbonates, which are well known as witherite and strontianite, calcium carbonate is a polymorphic mineral system composed of three anhydrous crystalline phases, calcite, vaterite and aragonite, two hydrated crystalline phases, monohydrocalcite (CaCO3·H2O) and ikaite (CaCO3·6H2O) and amorphous calcium carbonate (ACC). This difference leads to a much more complicated system of the morphogenesis of calcium carbonate minerals. The aim of this thesis was the characterization and study of the different growth behaviors of polymorphic calcium carbonate, in order to discover the mechanism of the morphogenesis of the calcium carbonate mineral structures in alkaline silica-rich media. Moreover, the thesis was also focused on the crystallization of barium carbonate in alkaline silica-rich media, to study the relation between biomorphic calcium carbonate and biomorphic barium carbonate, aiming to elucidate the key factor of the formation of silica-carbonate biomorphs. In this thesis, the crystallization of the silica-carbonate minerals was carried out mainly by counterdiffusion in silica gel method, where temperature, initial pH, additives and concentrations of the reactants could be easily controlled. The process of the crystallization was followed in situ by optical microscopy in order to study the morphological evolution. Additionally, the study of the crystallization kinetics was performed by the growth rate measurements from the time-lapse video recorded by optical microscopy. Since calcium carbonate is a polymorphic compound, the in situ Raman microspectroscopy, including mapping technique, has been applied to determine the crystalline phase during the growth process. Phase characterization was also done by X-ray diffraction including single crystal diffraction and powder diffraction. Meanwhile, the pH values and the contents of calcium and silica in solution have also been followed by time to describe the local environment of crystals during the growth. A texture study of the crystalline aggregates was performed mainly by means of optical microscopy, field emission scanning electron microscopy and energy dispersive X-ray spectroscopy. In addition, a new technique associated with fluorescence spectroscopy was performed for the pH measurements in solutions, and the results provided the information of the very local pH over the microenvironment of the growing front during the crystallization. On the basis of the results and their further discussion, the PhD thesis is summarized by chapters as below: Chapter 1 is the introduction and the state of the art in silica-carbonate biomorphs, including the formation and the growth behaviors of silica-carbonate biomorphs as well as their mechanism of the formation and the challenging task for future studies. The fundamental background of the crystallization of calcium carbonate and its relative study on biomimetic mineralization is also introduced in this part. Chapter 2 corresponds to the experimental methods used in the thesis and describes the crystallization methods and characterization techniques used to monitor the crystallization process (in situ Raman microspectroscopy, ex situ calcium and silica time-lapse measurements, pH measurements and kinetic study by optical microscopy) and after crystallization (X-ray diffraction, field emission scanning electron microscopy, energy dispersive X-ray spectroscopy and inverted microscopy). Chapter 3 introduces a new route to crystallize and stabilize the monohydrocalcite phase. The crystallization was performed in a silica-rich alkaline solution at room temperature using the counterdiffusion method in the absence of magnesium or any other additive, by the reaction of calcium chloride and sodium carbonate. Phase characterization by X-ray diffraction and the mapping analysis of Raman microspectroscopy demonstrated that monohydrocalcite remained stable for several months. The crystallization process was also monitored by in situ Raman microspectroscopy, and it was found that monohydrocalcite behaved as the initial phase since it was observed at the micron scale, with no phase transformation occurring during the whole process. The further texture study by field emission scanning electron microscopy showed the monohydrocalcite was a polycrystalline material exhibiting a unique onion-like hemispherical multi-layered structure. This work demonstrates that silica plays a key role in the formation and stabilization of the monohydrocalcite phase. The growth behavior of monohydrocalcite in silica-rich alkaline water solutions is studied in Chapter 4. During the crystallization, the evolution of both the pH and the content of calcium and silica species in solution were followed either by in situ (pH) or ex situ (calcium and silica) time-lapse analysis. The growth of monohydrocalcite particles occurred by different mechanisms that were related to the pH and the rate of pH change with time. The initial peanut-like crystal converted into an onion-like multilayered texture, which was built up by the alternation of loose layers and compact layers as a result of different levels of silica incorporation, according to the characterization of field emission scanning electron microscopy and energy dispersive X-ray spectroscopy. Then a final silica-rich skin covered the hemisphere and inhibited the further growth of monohydrocalcite. When the silica skin failed to cover the whole surface of the hemisphere, a bulge of monohydrocalcite grew from the uncovered area until a new silica skin inhibited its growth except from a small uncovered area from which a new bulge formed. The iteration of this mechanism created particles with caterpillar-like morphology. These results showed that silica played a key role in the morphogenesis and texture of monohydrocalcite crystallization due to the coupled interaction between the reverse solubility of silica and carbonate versus pH. Chapter 5 deals with the effect of temperature on the growth of biomorphic and nonbiomorphic monohydrocalcite. Monohydrocalcite was crystallized under alkaline silicarich solution at different temperatures and the results showed a strong effect of temperature on morphogenesis of monohydrocalcite. Unlike the onion-like layered hemispheres of monohydrocalcite obtained at room temperature, monohydrocalcite with typical features of biomorphs such as continuous smooth curvature and twisted ribbon formed at two selected temperatures (45 and 60 ºC). The textural study conducted on different crystals showed that the biomorphic monohydrocalcite exhibited a multilayered structure similar to the hemispherical aggregates at room temperature and also the coupled co-precipitated silica layer which affected the morphologies of monohydrocalcite. However, considering the hexagonal structure of the monohydrocalcite, the later result was quite different from any other silica-carbonate biomorphs reported before, concerning the orthorhombic lattices of aragonite, witherite and strontianite. Moreover, the morphological variety in one single crystalline phase (monohydrocalcite) suggests that the morphogenesis of silica-carbonate biomorphs is not directly linked to the crystalline lattice. Further study in details was focused on the building blocks of the aggregates at nanoscale and the evolution of the crystalline domain during growth. Compared to the non-biomorphic monohydrocalcite formed at room temperature, the results showed that the biomorphic monohydrocalcite consisted of not only the similar building blocks in the inner part but also other bigger elongated rod-like building blocks on the surface. The analysis of crystalline domain by X-ray power diffraction also indicated that the size of the crystalline domain of biomorphic monohydrocalcite was always bigger than the non-biomorphic one, pointing out that building block was the key factor on the formation of silica-carbonate biomorphs. In addition to that and considering other crystalline phases formed in the area near monohydrocalcite crystals, the morphological variety of calcite by increasing the experimental temperature also exhibited an evolution of the building blocks at the nanoscale. This fact also supports the hypothesis of the effect of building blocks on morphogenesis in such single crystalline phase. A pathway to produce silica-aragonite biomorphs in silica gel under ambient conditions without any additive is explained in Chapter 6. The crystallization was performed by counterdiffusion method in silica gel and aragonite crystals with typical features of biomorphs such as continuous smooth curvatures were obtained. However, aragonite never appeared as the only crystalline phase in the product, while rhombohedral calcite was the first crystalline phase in silica gel. Within a particular area of the gel, biomorphic aragonite and sheaf-of-wheat calcite formed together. Parallel experiments at higher temperatures (45, 60 and 80 °C) revealed a higher quantity of aragonite biomorphs when increasing the experimental temperature. The study of textures suggested that the difference of the building blocks of aragonite and calcite was the key determinant of the morphological variation observed within the same microenvironment. The building blocks of biomorphic aragonite were elongated needle-like nanocrystals, similarly to the one of biomorphic barium carbonate, while the one of sheaf-of-wheat calcite was found to be spherical nanocrystals. In addition, the building block of diabolo-like calcite which formed at 45 °C has also been studied in this chapter. Chapter 7 introduces a novel method to in situ measure the local pH at the micron scale during the crystallization of biomorphic barium carbonates in solution. The measurement was performed by fluorescence spectroscopy, which allows detecting the change of intensity of the fluorescence from a pH sensitive dye (acridine orange), and converts the intensity of the signal to pH values. The experiment of the crystallization was done by mixing barium chloride solution with alkaline silica sol containing small amounts of acridine orange. Then, the crystallization started by the diffusion of the atmospheric carbon dioxide while the pH was monitored. Chapter 8 contains the conclusions achieved in this thesis on the morphogenesis of polymorphic calcium carbonate minerals in alkaline silica-rich media.