Preparation and Mechanism of Novel Bioceramics with Controllable Morphology and Crystal Growth - Advanced Bioactive Inorganic Materials for Bone Regeneration and Drug Delivery - page 154

of the HAp nanorods was attributed to the W/O nanoreactors and the soft
template of the surfactants, and the high crystallization of the products was
attributed to the hydrothermal treatment.
The HAp nanowires with uniform width of 60 nm and length up to 1 µm
have been synthesized by Wang, Lai, et al. (2006) via a solvothermal process.
The results revealed that the formation of an amorphous nuclear-surfac-
tant complex and the electrostatic field within the reverse micelles main-
tained the unidirectional, irreversible fusion of reverse micelles, leading to
the growth of nanowires in one direction. In addition, the stable conditions
without intense shearing stress where it was favorable for the formation of
long and uniform HAp nanowires. Furthermore, the ratio of length to diam-
eter increased apparently with the decrease of the initial pH values due to
the interaction between surfactant CTAB molecules and reactant ions. The
higher pH resulted in weakening interactions between the amino group of
CTAB and the reactant PO 4 3- , leading to crystal nucleating in the bulk water
phase without the growth-guiding of surfactant molecules (Wei et al. 2006).
The hydrothermal temperature and time, the additives and additive con-
centration, and the pH value of the solution may also play important roles
with the size and morphology of the products. Wang, Chen, et al. (2006)
hydrothermal synthesized HAp nanocrystals using CTAB as a template.
TEM micrographs revealed the HAp particles ranged from spheroid to long
fibers. The obtained smallest average diameter of spherical particles, ∼27 nm,
were achieved at 90°C, pH = 13 for 20 h. The biggest, longest fibers, more than
1125 nm and about 60 nm in width, were obtained at 150°C, pH = 9 for 20
h. In addition, the crystal size increased apparently with the increase of the
hydrothermal temperatures. The morphology of the obtained particles pre-
sented a certain rule with the temperature at different pH values. A probable
mechanism involved the interaction of the tetrahedral CTAB cation struc-
ture, the tetrahedral PO 4 3− structure, and the OH − ions at different pH values.
The homogeneous precipitation method under hydrothermal treatment
has been widely used to control the morphologies of HAp using EDTA,
Na 2 EDTA, citrate (CA), urea, and amine as the chelated reagents or homo-
geneous precipitation reagents. Seo and Lee (2008) prepared HAp whiskers
with a width of 0.05 µm and a length up to 3 µm through directly refluxing
the typically shaped HAp powders using EDTA via the dissolution-reprecip-
itation process, in which the amorphous reprecipitates were initially formed
in the Ca(EDTA) 2- -PO 4 3- mixed solution and have continuously grown into
long HAp whiskers. Martins et al. (2008) observed the role of the existing
CA species as calcium chelates under different pH conditions on the frame-
work of particle nucleation and growth mechanisms. They suggested that
the anisotropy of surface charge distribution, arising from the adsorption
of citrate species onto preferential crystal facets, has a strong effect on the
formation of particular particle shapes by oriented aggregation of the pri-
mary particles. In higher pH conditions, the adsorption of more negatively
charged species was thought to increase the negative charge on particle shell,