Elsevier

Journal of Crystal Growth

Volume 312, Issue 3, 15 January 2010, Pages 402-412
Journal of Crystal Growth

The epitaxial role of silica groups in promoting the formation of silica/carbonate biomorphs: A first hypothesis

https://doi.org/10.1016/j.jcrysgro.2009.11.004Get rights and content

Abstract

This paper deals with the influence of silicate species on the morphogenesis of BaCO3 (witherite), one of the main component of “silica/carbonate biomorphs”. The size of barium carbonate crystals decreases progressively and significantly with the increasing amount of Na-metasilicate (Na-MTS) in the crystallization environment. When the Na-MTS amount in the aqueous mother solution is less than 500 ppm, single well-shaped micrometric BaCO3 crystals are obtained. Higher Na-MTS concentrations (4000–5000 ppm) produce polycrystalline structures built by pseudo-hexagonal nano-rods. X-ray powder diffraction (XRPD) diagrams show the decrease of crystal size with Na-MTS concentration and point out as well that silicate groups can be slightly absorbed into the barium carbonate lattice. The last part of the paper is devoted to the first finding, in laboratory, of aragonite polycrystalline structures belonging to the family of “silica/carbonate biomorphs”.

Introduction

Biomimetic syntheses of minerals exhibiting complex morphologies, which are structured through the self-assembly of nano-building blocks, have attracted a considerable attention in the recent years. Notably, the influence of the organic matrix has been taken into account in order to explain the dramatic morphological changes that mineral crystals undergo when growing from pure inorganic or from mixed inorganic–organic additive crystallization. To this end different organic additives have been used for controlling the growth morphology of carbonates [1].

Moreover, in non-biological solutions containing poly-(acrylic, glutamic, aspartic) acids, calcium carbonates grew as a film on various surfaces [2], [3], [4], [5]. Recently, double hydrophilic block polymers (DHBCs) have been also developed as crystal growth modifiers [6], [7], [8], knowing that these polyelectrolytes have a remarkable influence on the morphology of a huge number of inorganic materials [9], [10], [11]. However, biomimetic morphologies are not only attributable to organic and complex molecules. As a matter of fact, the precipitation of orthorhombic alkaline earth carbonates such as the witherite (BaCO3) and the strontianite (SrCO3) in basic silica-rich environment results in the formation of poly-nano-crystalline aggregates exhibiting non-crystallographic shapes like helicoidal filaments or very thin sheets describable by the equation of a cardioid. These objects, named “silica biomorphs” [12], [13] because their morphology resembles that of primitive organisms, originate from carbonate precipitation in Na-MTS gels and/or stagnant aqueous solutions [14].

Our investigation is mainly devoted to understand the influence of a specific inorganic additive such as NaSiO3·9H2O, the sodium metasilicate (Na-MTS hereinafter), on the morphogenesis of these biomimetic nano-aggregates. In the first part of the present work we will focus the role of silicate species (as derived from Na-MTS hydrolysis) on the crystallization medium, being already known that the observed morphologies vary according to other variables such as pH, temperature and fluid-dynamic conditions [14], [15], [16], [17], [18], [19], [20]. From literature [9] and our recent investigation [14] it comes out that the mean size of BaCO3 crystals changes significantly with the concentration of silicate species either in gel or in aqueous mother solution; this was proved through SEM and TEM observations and clearly confirmed by X-ray powder diffraction (XRPD) patterns. Further, we will use the information resulting from the observed witherite/α-quartz 3D-epitaxy [21] to justify the inverse and not yet observed α-quartz/witherite 2D-epitaxy; this to quantitatively explain how the nucleation rate of witherite dramatically increases, from pure aqueous solutions to the doped ones, with the increasing MTS concentration.

In the second part of the paper, we describe the polycrystalline architectures of aragonite (CaCO3) we obtained under laboratory conditions. This is a new finding, since the precipitation of calcium carbonate has not produced, until now, objects similar to the usual witherite and strontianite biomimetic aggregates.

Strictly speaking, Voinescu et al. [22], [23] reported the formation of silica-calcium carbonate biomorphs showing “coralline” self-assembled shapes, these synthetic aggregates being built by aragonite nano-crystals and an amorphous silica matrix. Nevertheless, the thermal conditions for precipitation were sharply different from those used to obtain the usual silica biomorphs [12], [14], since the authors operated far from the room temperature (80 °C). Further Imai et al. [24] grew coral-like morphologies (with sheet-like subunits), at ambient temperature in silica gels at high pH, and with the help of added seed material.

Hence, we shall outline that our considerations on the role exerted by the epitaxy cannot be confined to the witherite/silica interface but can be usefully transferred to the understanding of other silica biomorphs.

Section snippets

Material and methods

Here we will focus our attention on silica biomorphs grown from solution because the morphological variations with concentration of silicate species and time are more evident that in silica gel. In the first run, witherite crystals were grown from pure aqueous solutions by mixing two solutions of BaCl2·2H2O (0.1 M) and NaHCO3 (0.01 M) with 1:2 volume ratio, following a suggestion given by Blount [25] to obtain high sized crystals. It is worth remembering that such initial ratio varies during

Evolution of the biomorphic forms as a function of the crystallization time

Biomorphic aggregates occurred, at the solution/atmosphere interface, when a Na-MTS solution (5000 ppm) was mixed to a BaCl2 solution (0.1 M). The crystallization products were extracted from the reaction vessels at different time intervals (5, 24, 48 and 96 h), in such a way that each extraction is associated to different crystallization times of the same initial solution. The first precipitates (after 5 h) were formed by planar-like aggregates connected each other. After one day, some minute

The consequences of adsorption/absorption of silica (Na-MTS) on witherite crystals

XRPD patterns were analyzed employing the Rietveld's whole profile fitting method based on structure refinement, using GSAS and MAUD software, as mentioned above.

From the results drawn in Table 1 it ensues that, when going from pure to doped solution:

  • -

    the a0 lattice parameter of witherite decreases (per cent) by 0.192 (500 ppm), 0.214 (1000 ppm), 0.889 (4000 ppm), 0.504 (5000 ppm);

  • -

    b0 increases by 0.159, 0.198, 0.754 and 0.513, respectively;

  • -

    c0 increases by 0.159, 0.141, 0.152 and 0.130;

  • -

First finding of silica/aragonite (CaCO3) biomorphs

The method for obtaining these polycrystalline aggregates was already described in our previous works when dealing with Ba and Sr biomorphs [14]. Silica sol was prepared from Na-MTS solution (10%) by adding hydrochloric acid solution (1 M). After controlling pH (pH ranging 10–10.5), silica sol transformed into gel in a glass tube for 24 h at room temperature. Successively, solutions containing various amounts of calcium chloride (0.1–1–1.5–2 M) were poured onto the silica gel for about one month.

The architecture of silica biomorphs by chemical attack.

The chemical nature of the silica biomorphs (BaCO3, SrCO3 and CaCO3 aragonite) can be ascertained to some degree by selective dissolution in acidic or alkaline solutions. During the acidic dissolution (usually done in a dilute solution of HCl) the carbonate material dissolves, while silica is not affected. On the contrary, the basic solution (NaOH 1 M) selectively dissolves the silicate species, leaving unaltered the carbonate portion.

After the basic dissolution, the XRPD patterns of the

Conclusion

The ratio between the percentage of (SiO4)4− groups and the exceeding percentage (with respect to the equilibrium) of barium ions in the growth solution, heavily affects both the nucleation frequency and size of BaCO3 (witherite) crystals. The polycrystalline nano-structures become to form, as non-crystallographic dendrites when the ratio Ba2+/SiO44− in the growth solution exceeds the value of 40, while the well-shaped cardioids and the helicoidal filaments occur exclusively when the amounts of

References (43)

  • H. Cölfen

    Colloid Interface Sci.

    (2003)
  • L.A. Gower et al.

    J. Crystal Growth

    (1998)
  • L.B. Gower et al.

    J. Crystal Growth

    (2000)
  • S.H. Yu et al.

    Nat. Mater.

    (2005)
  • T. Terada et al.

    J. Crystal Growth

    (2003)
  • J.M. García-Ruiz et al.

    J. Crystal Growth

    (1981)
  • J.M. García-Ruiz

    J. Crystal Growth

    (1985)
  • S.T. Hyde et al.

    Physica A

    (2004)
  • A.E. Voinescu et al.

    J. Crystal Growth

    (2007)
  • H. Imai et al.

    J. Crystal Growth

    (2002)
  • I. Sethmann et al.

    J. Inorg. Biochem.

    (2006)
  • T. Kato et al.

    Chem. Lett.

    (1999)
  • A. Sugawara et al.

    Chem. Commun.

    (2000)
  • M. Sedla´k et al.

    Macromol. Chem. Phys.

    (1998)
  • H. Cölfen et al.

    Langmuir

    (1998)
  • H. Cölfen

    Macromol. Rapid Commun.

    (2001)
  • S.F. Chen et al.

    Adv. Mater.

    (2005)
  • T.X. Wang et al.

    Angew. Chem.

    (2006)
  • J.M. García-Ruiz et al.

    Espan. Hist. Nat.

    Secc. Geol.

    (1979)
  • E. Bittarello, D. Aquilano, Eur. J. Mineral. 19 (3) (2007)...
  • J.M. García-Ruiz

    Geology

    (1998)
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