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Chemistry Letters Vol.36, No.7 (2007)
Structural Characteristic of Outermost Surface of Cubic Mesoporous Silica Film
Takashi Inoue,1 Itaru Gunjishima,1 Yoko Kumai,1 Shinji Inagaki,1;2 and Atsuto Okamotoꢀ1
1Toyota Central R&D Labs, Inc., Nagakute, Aichi-gun, Aichi 480-1192
2JST, CREST, 4-1-8 Hon-Chou, Kawaguchi 332-0012
(Received April 9, 2007; CL-070382; E-mail: okamoto@mosk.tytlabs.co.jp)
The outermost surface structure of a calcined cubic mesopo-
tion as follows: TMOS:C18TMACl:HCl:EtOH:H2O = 1.00:
0.17:0.002:27.5:24.4. After aging of the sol solution for 40
min at room temperature, a MPS film was fabricated using a con-
stant pull-up rate of 2 cm/min on a silicon (Si) substrate by a dip
coating method from the sol solution. Details of the preparation
method have been described elsewhere.4 The film was finally
calcined in air at 400 ꢁC for 4 h. The XRD pattern of the calcined
film is shown in Figure S1.16 The pattern can be assigned to the
cubic phase SBA-1, even though relative intensities between the
peaks were quite different from those of powder samples.9–11
The overwhelming intensity of the 210 and 420 reflections
relative to the neighbors indicates that predominant plane paral-
lel to the substrate is regarded as {210} crystallographic plane
(inset in Figure S1).16 The outermost surface morphology of
the film without coating was investigated by FESEM (JEOL
JSM-890 and Hitachi S-5500). The film exhibits a characteristic
arrangement of pores with diameters of ca. 3 to 4 nm over a wide
area of the outermost surface, as shown in Figure 1. Moreover, a
cross-sectional SEM image of the calcined film showed a single
domain structure in the direction of thickness although some
defects and imperfections in the pore-arrangement structure
were observed (Figure S2).16 These mean that the domain size
is at least a few hundred nanometers. It is interesting to note
that there are two kinds of pore arrays on the outermost surface:
one is identified as a ‘‘straight line’’ connecting each pore; the
other is a periodic array consisting of a unit with both three
and four pores perpendicular to the ‘‘straight line’’ (Figure 2a).
In contrast, a domain with {100} plane was also detected
on the outermost surface, as shown in Figure 2b. The lattice
constants of before and after calcination were estimated at
a ¼ 8:85 and 7.68 nm by XRD, respectively. In contrast, the
lattice constant estimated by FESEM image (sides of a square
rous silica film was observed for the first time by high-resolution
field emission scanning microscopy, which showed a periodic
pore arrangement with inherent cage openings and one-dimen-
sional grooves on the surface. The grooves are formed to be
the interconnected A-cages associated along the h100i direction
on the {210} crystallographic plane of SBA-1.
Since their discovery,1,2 mesoporous silica (MPS) materials
have attracted much attention for their potential applications
such as molecular separation membrane,3 catalyst supports,4,5
and templates for the fabrication of nanomaterials.6 On the
other hand, the number of investigations into quantum wire
and quantum-dot device have been increasing,7,8 and an idea
using the MPS material as a template for fabrication of nano-
structures was proposed.4,5 It is necessary for that purpose to
clarify both the outermost surface and internal structure of
MPS materials, especially for thin films.9–12 A number of
investigations into the internal structure of MPS materials
have been reported so far.9–14 However, structural studies on
the outermost surface by scanning electron microscopy (SEM)
are rather few,11,15 for instance, Wu and co-workers revealed
inherent external structures on the top surfaces of three kinds
of mesoporous silica thin films through direct high resolution
SEM observation.5 On the other hand, Sakamoto and co-workers
reported that powdered cubic MPS materials, so-called SBA-1
and SBA-6, have a highly ordered dual micro- and mesoscale
pore structure by X-ray diffraction (XRD) analysis and high-res-
olution transmission electron microscopy (HRTEM).13 Howev-
er, to my knowledge, the outermost surface structure of cubic
MPS is still unclear. In addition, when the outermost surface
of the MPS is applied to nanoelectronics devices, not powder
but thin film of MPS on a substrate is suitable for the actual
device manufacture process. Therefore, understanding of the
outermost surface structure, as in SBA-1, is the most essential
and important issue. Here, we report a direct field emission
SEM (FESEM) observation on the outermost surface of calcined
cubic MPS thin film. The film exhibits a periodic pore arrange-
ment with inherent cage openings and unique one-dimensional
(1D) grooves on the outermost surface, and the size of some cage
openings is larger than crystallographically expected one.
The cubic MPS film was fabricated by sol–gel method as
follows. Octadecyltrimethylammonium chloride (C18TMACl)
and tetramethoxysilane (TMOS) were used as a surfactant and
a silica source, respectively. C18TMACl (1.45 g) was dissolved
in a mixture of water (10 mL), ethanol (40 mL) and 2 M HCl
(20 mL). The solution was then added to a hydrolyzed TMOS
solution (TMOS: 3.81 g, 0.012 M HCl: 1.0 mL) to yield a sol-
solution. To avoid the charge-up problem, the typical molar
ratio of precursor solution was modified to a thin film specifica-
50 nm
Figure 1. FESEM image of the typical outermost surface of
calcined MPS film. The location of ‘‘straight lines’’ is shown
by arrows.
Copyright ꢀ 2007 The Chemical Society of Japan
Inoue, Takashi
