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Z.H. Su et al. / Solid State Communications 149 (2009) 1782–1785
a
b
Fig. 3. Side view of the PAA sample after electrochemical anodization oxide and chemical etching. (a) and (b) show the different morphologies of the PAA membrane with
the nanowires, respectively.
Fig. 1(a) shows the morphology of the PAA membrane closed
to aluminum substrate after electrochemical anodic oxidation at
8
0 V for half an hour. The nano-pores in PAA membrane are
disordered after one-step anodic oxidation, which is consistent
with former reports [3]. However, some nanowires adhere to the
PAA membrane, which are sparse and disordered. Their length
is about several microns and the diameter is around decades
nanometers. The detail is shown in fig (b).
Fig. 2(a) presents the morphology on the PAA membrane
surfaces after electrochemical anodic oxidation at 80 V for half an
hour.
When electrochemical and chemical etching time was pro-
longed, different morphologies on the PAA membrane were ob-
served (shown in Fig. 2(b), (c), (d)). In these figures, there are a
lot of disordered stripes on the surface of the sample that are
composed of thousands of grass-like (shown in the zoom-in fig-
ures of Fig. 2(a) and (b)) or noodle-like nanowires (shown in the
zoom-in figures Fig. 2(c) and (d)). The roots of the nanowires were
connected with PAA surface and their twigs were bound together
with each other. The difference in morphologies of PAA mem-
brane surfaces is mainly due to the differences in the length of
the nanowires, which differs from several hundred nanometers to
decades of microns. Their diameter is about decades of nanome-
ters. As is known, nano-materials are inclined to gather together.
If the nanowires become longer, their tips can assemble more eas-
ily. It can form different macroscopical morphologies on the PAA
surfaces.
So from Figs. 1 and 2, we suppose that the nanowires in Fig. 1 are
not connected with the side of PAA membrane directly. When the
PAA membrane was taken out from the solution, some nanowires
fall into the solution during the reaction, and adhere to the other
side of the PAA membrane closed to aluminum substrate.
Fig. 3 is the side view of the PAA sample after the electrochemi-
cal anodic oxidation and the chemical etching. In Fig. 3(a), the PAA
membrane has been synthesized and some short nanowires have
been obtained. The tips of the nanowires have begun to assemble.
The nanowires are longer, and the border between the PAA and the
nanowires can be clearly observed in Fig. 3(b).
Fig. 4. SEAD of the nanowire.
the detailed morphology of nanowire-rich area. Compared to the
stripes, thousands of the nanowires, with decades of nanometers
in diameter and a few of microns in length, have disordered
orientation. There is a kind of interesting nanostructure in the
interstitial part between the nanowire-rich areas (shown in
Fig. 5(b)). It is like a tube or the straight-cut section of a
tube, which can also be named ‘‘alumina nanotube’’ or ‘‘alumina
nanospatulate structure’’. The bottom of the nanotube (half-
tubular nanostructure) is connected with the PAA membrane, the
diameter of whose pore is about 150 nm. Combining Fig. 5(a), (b),
we suppose that the nanowire in Fig. 5(a) and the nanotube (half-
tubular nanostructure) in Fig. 5(b) are the twig and the culm of the
nanowire, respectively.
From the above experimental results shown in Fig. 5, the
reasonable synthetic process of the amorphous Al2O3 nanowires
was described as follows:
(
1) PAA membrane is synthesized by electrochemical anodic
oxidation (shown in Fig. 6(a)).
(
2) When the anodic material is oxided continuously, the pore
goes deep into the anodic material vertically. At the same time,
the acid begins to etch the synthesized PAA by electrochemical
and chemical reaction. The walls between the nanopores
are gradually becoming thinner and thinner. (shown in
Fig. 6(b), (c)).
(3) The walls of nanopores are etched totally. But the pieces among
the three neighboring pores can survive through this process.
The porous structure becomes the nanowire connected with
the alumina substrate. The diameter of the top part of the
nanowire is much smaller than that of the lower part. (shown
in Fig. 6(d)).
(4) With increasing of the reacting time, the depth of the pores in
PAA increases and the etching along the radial direction goes
on. So the length of the nanowires becomes longer and longer.
Fig. 4 is the SEAD of nanowires, which shows that the nanowire
is amorphous alumina.
However, what is the mechanism that rules the process from
PAA membrane to thousands of nanowires? To answer this
question, PAA membrane was synthesized by electrochemical
etching with the voltage of 80 V for 20 min in 5%wt oxalic acid
solution and was successfully separated from aluminum substrate.
Then it was put into the 5% oxalic acid solution for 15 h. The
morphology of the surface contacted with oxalic acid is given
in Fig. 5. In Fig. 5, the stripes are grass-like and have relatively
ordered orientation. According to the fine nanostructure, their
morphology can be classified into two areas: the nanowire-rich
area and the interstitial (nanotube-rich) area. Fig. 5(a) shows
4. Conclusion
We directly got the disordered pattern in macrostructure
as well as the relatively ordered nanowires in microstructure
Su
