Biomedical Engineering Reference
In-Depth Information
In 1999, we found the optimal composition ratio to synthesize highly
ordered MCM-41 in ammonia medium, in which the concentration of CTAB
is 4.3 wt% [50, 51]. Recently, the MCM-41 single crystals can grow larger to
nearly 10
m via multiple-growth under the same condition, in extremely
diluted ammonia solution. A novel crystal form of hexagonal circular bicone
of MCM-41 crystal has been observed, which is the combination form of
three hexagonal bipyramid and circular bicone forms [52-55]. In fact, there
are still many unknowns about the crystal of MCM-41 and its prior growth
habit. For a 2D mesoporous crystal, as a unit of its structure, rod-like micelles
make it essentially different from traditional single crystals at the atomic
scale of 2D/3D. Especially, certain characteristics of the building blocks are
indefi nite, for example, the d-spacing of the c-axis of the 2D single crystal is
usually uncertain, which makes it possible for a change in crystallography.
Generally, the traditional crystallography describes in detail crystal forms as
the total 47 geometric forms including hexagonal prism forms, hexagonal
bipyramid forms, etc., and combination forms which combine two or more
geometric forms. This includes crystals composed of atoms or ions con-
nected by chemical bonds. However, our study indicates that the morphol-
ogy of a MCM-41 single crystal could be of the combination forms, in which
a novel geometric form is different from those described in the traditional
crystallography, as illustrated in the scheme of MCM-41 in Figure 13.7.
We also reported preparations of the nanosized mesoporous silica par-
ticles with sizes ranging from 10 nm to 60 nm and different pore struc-
tures in extremely dilute solutions [56]. By adjusting the concentrations
of reactants, we can control the scale of mesoporous silica from 10 nm to
60 nm with different morphologies and pore structures. Compared to tra-
ditional methods, surfactant concentrations in our study were extremely
low, 0.2 wt% at most. The morphology transformation of samples was
observed when altering reactants' concentration, and a mechanism, based
on self-assembly of monosilicate combined with surfactant molecular in
extremely dilute solution, was proposed to interpret the formation pro-
cess of nanosized mesoporous materials.
It was suggested that the for-
mation of the mesoporous silica nanoparticles could be attributed to the
deposition of self-assembled silicate micelles.
The synthesis procedures of the typical preparation of MCM-41 mes-
oporous silica (MCM-41 MS) nanoparticles were as follows: (1) 3.5 ml of
NaOH (2M) and 1 g CTAB were added into a certain volume of deionized
water with strong stirring. The reaction temperature was kept about 70
μ
C;
(2) when the solution became homogeneous, 5 ml of TEOS was introduced
to the resulted solution; (3) 2 h later, the reaction was terminated and the
product was fi ltered and washed with deionized water, then dried at a tem-
perature of about 100
°
C.
In the diluted aqueous system, as shown in Figure 13.8, the different
additive factor would lead to special preferred morphology. For exam-
ple, the nanorod as well as nanofi ber of MCM-41 could be synthesized by
°
C for 12 h, then calcinated in air for 4 h at 500
°
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