Biomedical Engineering Reference
In-Depth Information
Fig. 6.13
TEM images of amorphous (
a
) and crystalline (
b
) HP-CaCO
3
films. Insets show the
corresponding SAED patterns. (Reproduced from [
90
], Copyright © 2010, American Chemical
Society)
precipitation of ACC microdomains. The PAA was introduced to obtain the unstable
crystalline polymorph of vaterite at an air-water interface. The so-called Kitano
method was used to prepare highly supersaturated CaCO
3
solutions. Moreover,
due to the possible polymorph selection, the mineral is precipitated out of vaterite
instead of the most stable calcite polymorph. The air-water interface in the presence
of PAA is certainly one of the simplest biomimetic systems, which can be used for
easy deposition on a positively charged substrate for further use.
The strategy of colloidal lithography can also be used to obtain the honeycomb-
patterned thin films of ACC and mosaic calcite at the solution surface. The
honeycomb-patterned ACC (HP-ACC) thin films were obtained by a polymer-
induced ACC coating of the monolayer colloidal crystal (MCC) template at the
air/water interface (Fig.
6.13
)[
90
]. Moreover, the honeycomb-patterned, mosaic
calcite thin films composed of single-crystalline calcite plates could form when the
amorphous films is heated at 673 K. In a typical experimental procedure, a piece
of MCC film with an area of 1 cm
2
that was deposited on a silicon substrate and
then was transferred on to the surface of the CaCl
2
solution and PAA. The vessel
holding both the precursor solution, and the floating MCC film was covered with
Parafilm punched with one small aperture and put inside a closed desiccator with
2gof(NH
4
)
2
CO
3
powder located at its bottom. The coating process lasted for
6 h at 298 K. The MCC template was then removed by dissolving the product in
dichloromethane to obtain intact HP-ACC thin films. When the sample was heated
at 673 K in air and then the mosaic calcite thin films composed of single-crystalline
calcite plates was obtained. The very high temperature was applied to enhance
the crystallization process of the ACC. Therefore, the controlled crystallization of
ACC with retained patterns and shapes represents an alternative route to patterned
crystalline thin films.
Moreover, Xu et al
.
observed two modes of transformation of ACC in air into
ordered structures solid-solid phase and dissolution-recrystallization transition by
using the large-area ACC films [
91
]. They found that oriented large-area calcite
films could be prepared by controlling the transformation of an ACC film by
