Online proceedings - EDA Publishing Association

11-13
May, 2011, Aix-en-Provence, France
Assume there are N nanochannels in the applied AAO
membrane. The total number of particles passing through the
N
AAO membrane during time interval t is n . The flux passing
through the AAO membrane can be estimated as,
N N N
c
c
ni (- Di At
i
) ( DiAi)
c
i1 i1 x x
i1
(14)
j - D
x
N N N
( A) t ( A)
t A
i1
i i i
i1 i1 i1
The diffusion coefficient of the AAO membrane can be
calculated to be,
N
D ( DA)/
A
N
(15)
i i i
i1 i1
For those uniformly distributed nanochannels in an AAO
membrane, the diffusion coefficient for each nanochannels can
be assumed to be similar. It can be derived from Eq. (15) that
the diffusion coefficient of the AAO membrane is close to that
of the individual nanochannel. It is thus feasible using an AAO
membrane to replace a nanochannel for the diffusion
coefficient measurement.
The AAO templates were prepared using the well known
anodizing process. Aluminum foils (99.9995% pure; 175 m
thick) were cleansed using ethanol and then acetone, followed
by annealing at 400C for 3 hours in a vacuum.
Electropolishing was then carried out using a 1:4 perchloric
acid and anhydrous ethanol solution as the electrolyte, under a
constant voltage of 20 V at 40 C for 2 minutes to further polish
the surfaces of the foil. A =10 mm home-made Teflon fixture
was used for the anodization. The anodization process was
conducted using a 0.3 M oxalic acid solution as the etchant
under 90 V of applied voltage at 0C for 2 hours. The remaining
aluminum beneath the barrier layer was dissolved in an aqueous
CuCl 2 HCl solution that was prepared by dissolving 13.45 g of
CuCl 2 powder in 100 ml of 35 wt% hydrochloric acid solution.
The sample was then immersed in a 30 wt% phosphoric acid
solution at room temperature for 80 min to process the barrier
layer by purging and pore widening.
2.3 Experimental apparatus
An electrochemical bath that could hold an AAO membrane
to separate vessels with different ion concentrations was built
(Figure 3). The size of each vessel is 2 2 2 cm 3 . A CH263A
electrochemical analyzer (CH Instruments) integrated with a
Wheatstone bridge circuit having R 1 =R 2 =R 3 =200 was used
for the conductance measurement. The electrolyte used was
potassium chloride (KCl) with initial concentrations of 1M and
0.25 M in vessel 1 and vessel 2, respectively.
i
Figure 3. Electrochemical bath that could hold an AAO thin film to separate
vessels with different ion concentrations
III. RESULTS AND DISCUSSIONS
Figure 4 is a SEM image of an AAO membrane. The
nanopore diameter is around 80 nm and the thickness is 60 m.
The rate of coverage of the nanopores is estimated around
84.2%. Since the diameter of the AAO membrane is 1 cm, the
pore area can be calculated to be 0.6613cm 2 . Three membranes
were fabricated.
Figure 4. SEM image of an AAO membrane
Figure 5 shows the diffusion induced i-t curves for various
AAO membranes. The numerals denote the sample number.
The ion diffusion induced currents reach their steady-state
conditions in less than 200 sec. In general, 80-nm diameter
pores should allow both the cation (K + , 0.137 nm) and anion
(Cl - , 0.181 nm) to penetrate simultaneously. Therefore, the KCl
electrolysis may not be suitable for the measurement
experiments. However, it was reported that there are 4.3 K + ions
and 0.067 Cl - ions respectively on average flow in a nano
channel due to the electric osmosis inside the nanochannel [33].
It is reasonable to assume that K + ions contribute most of the
diffusion current.
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