1. 1
Nanoparticle Formation in Anodized Aluminium Oxide
Nano-pore Structure
Yutong Liu
Abstract
Self-ordered Anodized Aluminum Oxide (AAO) fabricated via 2-step anodizing process has promising
application potential in the area of magnetic storage, solar cells, carbon nanotubes, catalysts and so on. Iron
Oxide Magnetic Nanoparticles has similar intrinsic enzyme mimetic activity with peroxide to natural
peroxidase, which is more efficient, more Robust, stable to Temperature and pH value, reusable and
economy when compared with natural peroxidase. The objective of this research roatation is to investigate
the iron oxide nanoparticle formation in nano-pore structures, which can be divided into 2 parts: Part 1,
Creation of nano-pore structures with Anodized Aluminum Oxide films; Part 2, Investigation of Iron Oxide
nanoparticle formation kinetics in AAO pore structures via Electrochemical Impedance Spectroscopy (EIS).
Keywords: Anodized Alumnium Oxide (AAO), Nanoparticles, Electrochemical Impedance Spectroscopy
(EIS), Electrochemical Equivalent Circuit (EEC).
1 Introduction
Self-Ordered Al structures fabricated via 2-step
anodizing process was first published by Masuda
and Fukuda on Science in 1995. [1,2] Ever since,
new areas of applications have emerged in the
fields of magnetic storage, solar cells, carbon
nanotubes, catalysts and metal nanowires due to
its relatively easy and low cost. [3-6]
In 2007, Yan’s group proved that Iron Oxide
Magnetic Nanoparticles has similar intrinsic
enzyme mimetic activity with Peroxide to Natural
Peroxidase, which was published on Nature
Nanotechnology. [7] Compared to the Natural
Peroxidase, Iron Oxide Magnetic Nanoparticle is
more efficient, more Robust, stable to
Temperature and pH value, reusable and
economy due to its high surface – volume ratio,
inorganic and magnetic structure. So, Iron Oxide
MNPs have promising future in the fields of
Proteins Separation; Drug Targeting &
Separation; Magnetic Biosensor; Magnetic
Resonance Imaging; Wastewater Treatment and
so on. [7-13, 29-30]
Electrochemical Impedance Spectroscopy (EIS)
has been known for more than a century. Hoar &
Wood first proposed an Electrochemical
Equivalent Circuit (EEC) for Aluminum Alloy in
1962. [14] In 1988, Mansfeld and Kendig
proposed an EEC for Anodized Aluminum
Surface. [15] The essential of EIS is the
Electrochemical Equivalent Circuit analysis. [16]
As a test method, EIS bears advantages of fast,
economy and in situ. It also has good
discrimination between underlying compact &
overlying porous oxide layers. These features
make EIS as a useful tool in studying Oxide Film
thickness, corrosion rates, complex
electrochemical reactions and also batteries &
fuel cells. [17-27, 31-33]
2 Theory
2.1 Anodized Aluminum Oxide
Fabrication of AAO nano-pore structure now can
be explained well by Mechanical Stress [1]:
The Volume Expansion Coefficient R generated
by deference between Porous Alumina layer and
Aluminum Substrate explains stress in Alumina.
The expression of R is:
R =
𝑤𝐴𝑙2𝑂3
𝑤𝐴𝑙
*
𝑑𝐴𝑙
𝑓∗𝑑𝐴𝑙2𝑂3

2. 2
Where w means weight, d means density and f
means weight fraction. We can get ordered nano-
pore structure when R is around 1.4.
Figure 1 is Schematic diagram showing current
distribution during pore initiation and
development of nano-pores on AAO, which can
illustrate Mechanical Stress Model [1]:
Figure 1 Scheme of AAO nano-pore structure development
In A, film and current distribution are uniform.
However, some local variations in field strength
can appear on a surface with defects. This non-
uniform current distribution enhances field-
assisted dissolution of oxide and local film
becomes thicker, which is shown in B. In C, the
higher current above metal ridges, along with a
local Joule heating, leads to thicker oxide layer.
Simultaneously, the enhanced field-assisted
dissolution of oxide tends to flatten the
oxide/metal interface. Consequently, successive
cracking of the film and its rapid healing at the
high local current density occur in D. Finally,
with a consumption of aluminum base and
enhanced progress in the oxide thickness build-
up above the flaw sites, the crack–heal events are
more pronounced and the curvature of the film at
the oxide/metal interface increases, which is E.
Figure 2. SEM image of ideal AAO anodized by Oxalic
Acid.
Figure 2 is SEM image of ideal ordered AAO
sample anodized by Oxalic Acid. [2] We can get
specimen with 100nm inter-pore distance under
40 Volt anodization potential.
We use inter-pore distance to represent pore
diameter so as to eliminate the influence of
barrier thickness. According to former research,
inter-pore distance is proportional to Applied
Voltage, i.e. Anodizing Potential. Additionally,
electrolyte concentration, Solution pH,
Anodizing time and Widening time can also
influence the pores’ diameter. Among them, only
solution pH is negative to pore diameter, all of the
rest have positive influence. [3]
Here are optimal conditions summed up from
several articles [1-6]: Using Oxalic Acid as
electrolyte acid and Applied Voltage as 40 Volt,
we can get Volume Expansion Coefficient R
equal to 1.4 and 10% for corresponding Porosity
of Hexagonal P. Under Optimal conditions, Inter-
pore Distance is 100nm and Inner pore Diameter
is 40nm. These Optimal values are applied to this
research rotation project. [28]
2.2 Iron Oxide Magnetic Nanoparticles
Yan’s group proved that Iron Oxide MNP has
Intrinsic Peroxidase-like activity by demonstrate
4 aspects as following [7]: Firstly, Iron Oxide

3. 3
MNPs has the same color change when catalyze
the reaction with TMB, DAB and OPD. Secondly,
the peroxidase-like activity of Iron Oxide MNPs
is also Size, pH, Temperature and Peroxide
concentration dependent. Thirdly, catalysis by
Iron Oxide MNPs shows typical Michaelis –
Menten Kinetics Curve. Lastly, catalysis by Iron
Oxide MNPs was consistent with a Ping-Pong
Mechanism.
Peroxidase catalyze oxidation of certain
substrates to produce characteristic color with
peroxide, which can be seen in Figure 3. [7] For
instant, Iron Oxide MNP can be used to catalyze
the oxidation of a peroxidase substrate ABTS,
which can be used to detect Peroxide & Glucose.
We can see the reaction speed up with Iron Oxide
MNPs catalysis.
Figure 3 The Fe3O4 MNPs catalyze oxidation of various
peroxidase substrates in the presence of H2O2 to produce
different color reactions.
Nanoparticles are generally considered to be
biologically and chemically Inert. The separating
power of the magnetic properties of nanoparticles
can be combined with the catalytic activity of
metal surface or enzyme conjugate if MNPs are
coated with metal catalyst or conjugated with
enzymes, which refers to dual functional
nanoparticle. Dual functional nanoparticles are
composed of 2 parts: their cores provide a
magnetic function and their shells allow catalysis.
[8-10] The ferrous ions on the nanoparticles’
surface is the key factor to Intrinsic Peroxidase
like activity. [29, 30]
2.3 Electrochemical Impedance Spectroscopy
Electrochemical impedance is usually measured
by applying an AC potential to an
electrochemical cell and then measure the current
through the cell. Assume that a sinusoidal
potential excitation is applied. The response to
this potential will be an AC current signal, which
can be analyzed as a sum of sinusoidal functions,
i.e. a Fourier series. The potential signal is
applied by means of digital-to-analog converter
and the current response is measure by analog-to-
digital converter.
Small excitation (1-10mV) rather than big one is
employed to avoid harmonic and ensure linearity
of the system. Analogous to Ohm’s law,
Impedance can be expressed in terms of a
magnitude Z0 and a phase shift∅. And according
to Euler’s relationship, the impedance is
represented as a complex number [14]:
𝑍 =
𝐸𝑡
𝐼𝑡
=
𝐸0𝑠𝑖𝑛(𝜔𝑡)
𝐼0sin⁡( 𝜔𝑡 + ∅)
= 𝑍0
sin⁡( 𝜔𝑡)
sin⁡( 𝜔𝑡 + ∅)
𝑍(𝜔) = 𝑍0 exp(𝑗∅) = 𝑍0(𝑐𝑜𝑠∅ + 𝑗𝑠𝑖𝑛∅)
Nyquist Plot is the most useful means in EIS data
presentation. Nyquist Diagram can be obtained
by plotting the real part on X-axis and imaginary
part on Y-axis, which is shown on left. In Nyquist
Plot the impedance can be represented as a vector
of length Z and the angle between this vector and
the X-axis, commonly called phase angle ∅ .
Figure 4 shows a typical Nyquist Plot [14]:
Figure 4 Typical example of Nyquist Plot.

4. 4
EIS data are generally analyzed in terms of an
equivalent circuit model. A model need to be
determined in which impedance matches the
measured data. The type of electrical components
in the model control the shape of impedance
spectrum. The model's parameters control the
size of each feature in the spectrum. The Circuit
Elements include Resistor, Capacitor & Inductor,
which is illustrated in Table 1 [14-17]:
Element I-V Impendence
Frequency
Influence
Shift
Phase
Resistor E=IR Z=R No ∅=0
Inductor E=L*di/dt Z=j𝜔L Positive ∅=-90
Capacitor I=C*dE/dt Z=1/j𝜔C Negative ∅=90
Table 1 Elementary Circuit elements and relative
parameters.
3 Experimental
3.1 Research Design
AAO was fabricated firstly, and then Iron
Hydroxide and Iron Oxide Magnetic
nanoparticles were formed in the AAO for the
first time. After this, these samples were tested
via EIS technique and data was analyzed. At last,
Electrochemical Equivalent Circuit was modeled
and parameters were calculate. At first time 12
AAO samples with Iron Hydroxide Nanoparticles
inside, thickness T=1um, inner Diameter
Dinner=80nm were fabricated. At Second time,
12 AAO samples with Iron Hydroxide NPs and
12 samples with Iron Oxide were fabricated, both
of them have parameter of T=2um Dinner=80nm.
0, 20, 40, 60 minute were recorded as time point
for all the samples.
3.2 Anodized Aluminum Oxide Fabrication
There are 4 steps in AAO fabrication procedure:
Electro-Polishing [1-6, 28], 1st
Anodization,
Electro-Etching and 2nd
Anodization. An
additional step – pore widening – is usually
employed to obtain goal diameter. For Electro-
polishing, 166ml Perchloric Acid and 834ml
Ethanol were mixed after refrigeration, chiller
was kept at 4 Celsius Degree and Voltage 15 Volt.
For 1st
and 2nd
Anodization, 0.3M Oxalic aqueous
solution was used as electrolyte and Chiller was
kept at 8 Celsius Degree and Voltage at 40 Volt.
The Anodization Current – Time Data was
recorded by .csv format in computer. Phosphoric
Acid and Chromic Acid aqueous solution were
employed for etching and the chiller temperature
was set as 60 Celsius Degree. 40nm Inner
Diameter specimens were obtained under above
condition according to the optimal fabrication
conditions. For pore widening, Ammonium
Hydroxide aqueous solution was applied. The
rate of anodization, etching and pore widening is
72nm/min, 108nm/min and 2.5nm/min
respectively. So it is easy to calculate that etching
time is 2/3 of 1st
anodization time. The time took
by electro-polishing, 1st
anodization, electro-
etching, 2nd
anodization and pore widening was 5
minutes, 4 hours, 6 hours, 15/30 minutes, and 16
minutes respectively. Figure 5 shows the AAO
samples of different stages.
Figure 5 AAO samples in different stages.
3.3 Nanoparticle Formation
For Iron Hydroxide NP formation [29, 30]:
0.0202g Fe(NO)3 was dissolved in 50 mL DI
water as solution A, while 0.0425g NaNO3 was
dissolved in 45 mL DI water as solution B. And
Iron hydroxide nanoparticles were obtained by
adding 5 mL solution A into solution B.
For Iron Oxide MNP formation [7-10]: 0.0095g
FeCl3 along with 0.5 mL 0.1mM NH4OH were
dissolved in 49.5 mL DI water as solution A,

5. 5
while 0.0065g FeCl2 along with 0.5 mL 0.1mM
NH4OH were dissolved in 49 mL DI water as
solution B. And Iron oxide nanoparticles were
obtained by adding 0.5 mL solution A into
solution B.
The sample of AAO was cut into 6nm × 9nm
specimens to fit into nanoparticle formation cell.
The inner wall of cell was covered by Kapton film
to prevent nanoparticles formation on the cell
wall, which can be seen in Figure 6. The mixture
solution of A and B was added into cell
immediately after the mixing so as to investigate
the dynamics of nanoparticle formation in the
AAO nano-pores.
Figure 6 AAO Samples in cells with Kapton film.
3.4 Electrochemical Impedance Spectroscopy
A three – electrode electrochemical cell was setup
for EIS Analysis. And then the electrochemical
equivalent circuit model was determined
according to the previous existing models and
elements parameters of EEC were calculated via
EIS Lab software.
Figure 7 and 8 show how to set up a typical three
electrode electrochemical cell for impedance
measurement. In addition to the two parallel
electrodes (denoted as Counter and Working
electrode), a third voltage reference electrode was
placed close to the polarization layer and
measures the voltage difference of the
polarization double layer capacity to the working
electrode. This applies for the electrochemical
cell only for the counter electrode feeding current
into the electrolyte. In this research, working
electrode is AAO sample with nanoparticles
inside, reference electrode is Silver / Silver
Chloride reference and Counter Electrode is
Platinum. [25]
Figure 7 Seheme of a typical three electrode
electrochemical cell
Figure 8 Typical three electrode electrochemical cell
AAO specimens with nanoparticles inside were
prepared to attach with Copper so as to act as
working electrode in the EIS cell. 2 different
methods were employed in AAO preparation:
First time, top surface of AAO was scratched to
contact Aluminum with Copper, which may
introduce cracks on the Alumina surface; Second
time, Sodium Hydroxide instead of Scratching
was employed and operation was taken on the
bottom instead of top surface. The bottom surface
was sealed by epoxy. Figure 9 and 10 are samples
prepared by 2 methods.

6. 6
Figure 9 Samples prepared by mechanical stretching.
Figure 10 Samples prepared by Sodium Hydroxide etching.
4 Results
4.1 Single Sample
Sample 004 was participated with Fe(OH)3 Nano-
particle and its top surface was scratched to
contact Copper with under Aluminum. The
thickness and inner Diameter were T=1um
Dinner=80nm. Figure 11 shows the Nyquist
Impedance plot of sample 004. From which we
can tell that both Imaginary Impedance and Real
Impedance Imaginary increase with time
increasing. The fastest accumulating time period
of nanoparticles occurred between 20 to 40
minutes. The shape of the Nyquist plot is similar
to Mixed Kinetic & Charge transfer control
Randle Cell.
Figure 11 Nyquist Impedance of Sample 004.
Sample 007 was participated with Fe(OH)3 Nano-
particle and its bottom surface was etched by
Sodium Hydroxide to contact Copper with
Aluminum beneath Alumina. The thickness and
inner diameter were T=2um Dinner=80nm. Figure
12 shows the Nyquist Impedance plot of sample
007. From which we can tell that both Imaginary
Impedance and Real Impedance Imaginary
increase with time increasing. The fastest
accumulating time period of nanoparticles
occurred between 40 to 60 minutes. The low
frequency part is missing due to the noise.
Figure 12 Nyquist Impedance of Sample 007.
Sample 012 was participated with Fe3O4 Nano-
particle and its bottom surface was etched by
Sodium Hydroxide to contact Copper with
Aluminum beneath Alumina. The thickness and
inner diameter were T=2um Dinner=80nm. Figure
13 shows the Nyquist Impedance plot of sample
012. From which we can tell that both Imaginary
Impedance and Real Impedance Imaginary
increase with time increasing. The fastest

7. 7
accumulating time period of nanoparticles
occurred between 20 to 40 minutes. The low
frequency part is missing due to the noise.
Figure 13 Nyquist Impedance of Sample 012.
4.2 Samples with different thickness
Differences between sample 004 and 007 are
thickness and preparation method. As for the
Nyquist Plot, because of the low frequency area
missing of sample 007, we focus on the high
frequency part. Figure 14 shows the Nyquist
Impedance plot of sample 004 & 007. From
which we can tell that both Imaginary Impedance
and Real Impedance Imaginary increase with
both time and thickness increasing for these 2
samples. And the fastest accumulating time
period of nanoparticles was delayed from 20 – 40
minutes to 40 – 60 minutes due to the increasing
thickness.
Figure 14 Nyquist Impedance of Samples with different
thickness.
4.3 Samples with different nanoparticles
Differences between sample 007 and 012 is type
of the nanoparitcles in the nano-pores. As for the
Nyquist Plot, because of the low frequency area
missing of both sample, we focus on the high
frequency part. Figure 15 shows the Nyquist
Impedance plot of sample 007 & 012. From
which we can tell that both Imaginary Impedance
and Real Impedance Imaginary increase with
time increasing for these 2 samples. The samples
with different nanoparticles inside had the same
magnitude of impedance. And the fastest
accumulating time period of nanoparticles was
different. Fe(OH)3 occurred between 40 – 60
minutes and Fe3O4 occurred between 20 – 40
minutes.
Figure 15 Nyquist Impedance of Samples with different
nanoparticles inside.
5 Analysis
5.1 Model determination
The Simplified Randles was employed in
Electrochemical Equivalent Circuit Anaylsis first.
In addition to being a most common model in its
own right, the Simplified Randles Cell is also the
starting point for other more complex models. [17]
This model includes a solution resistance, a
double layer capacitor and a charge transfer
resistance (or polarization resistance). The double
layer capacitance is in parallel with the charge
transfer resistance. The equivalent circuit for a
Simplified Randles Cell is shown in Figure 16.
[14] Figure 17 is the Nyquist Plot for a typical

8. 8
implified Randles cell. [14] The Nyquist Plot for
a Simplified Randles cell is always a semicircle.
However, it is too simple to be employed since
the shape of the plot does not fit the smaples well.
Figure 16 EEC for a Simplified Randles Cell.
Figure 17 Nyquist Plot for a typical implified Randles cell.
Another model, which is just for the AAO
barriers, was employed. Figure 18 shows the
equivalent circuit of AAO, where s represents
solution, b represents barrier (underlying
compact layer), w represents wall (overlying
porous layer) and sp represents the solution in the
pores, which can be neglected when it is far
smaller than Rs. [18] Figure 19 shows the Ferric
Oxide formation in the nano-pore structure.
There are three RC elements (as can be seen by
the three hemispheres forming) increase with
Ferric Oxide precipitation. However, the
thickness of sample fitting this model was 5 um
and the samples in this research were 1 and 2 um.
So, this model is too complex to be used.
Figure 18 EEC for AAO barriers model.
Figure 19 Nyquist Plot for Ferric Oxide nanoparticles
formed in AAO nano-pores.
According to the diagram and discussion above,
the mixed control circuit should be the best and
simplest model to describe existing data. This
model’s formal name is Kinetic & Charge
Transfer Mixed Control Randles Cell. [14, 18-20]
This model can be obtained via adding a Warburg
Impedance to the simplified Randles Cell Model,
which characterize transfer process. Figure 20 is
the circuit model and Figure 21 shows the
Nyquist Impedance Plot of this model. [14] In this

9. 9
diagram, the left part is the Kinetic Control
Region and the right part is the Mass Transfer
Control Region. However, there is still no simple
element to model a Warburg impedance, it is not
possible to construct a dummy cell that models
the Randles Cell. So just solution resistance,
Charge Transfer Resistance & Double Layer
Capacity are analyzed.
Figure 20 EEC for Kinetic & Charge Transfer Mixed
Control Randles Cell.
Figure 21 Nyquist Plot a typical Kinetic & Charge Transfer
Mixed Control Randles Cell.
5.2 Parameter calculation
Kinetic & Charge Transfer Mixed Control
Randles Cell was simulated via EIS Lab Software.
Parameters of this model is given, which can be
seen from Table 2 – 4: According to the data
obtained from the computer, the solution
resistance is quite flat, which can be considered
as a constant.
Time / min 0 20 40 60
004 R / Ohm 69.53 66.23 60.1 85.2
007 R / Ohm 85.56 82.05 72.31 91.67
012 R / Ohm 78.53 73.44 76.62 91.57
Table 2 Solution Resistance at different time points.
Time / min 0 20 40 60
004 R / Ohm 3818 8905 23655 195800
007 R / Ohm 5821 23301 81657 134370
012 R / Ohm 801 5034 78200 77770
Table 3 Charge Transfer Resistance at different time points.
Time / min 0 20 40 60
004 C / uF 39.23 32.21 21.56 18.74
007 C / uF 72.48 67.13 39 25.22
012 C / uF 52.79 44.54 25.49 20.16
Table 4 Double Layer Capacity at different time points.
As for the Charge Transfer Resistance & Double
Layer Capacity, there are hundreds of models to
describe different Rct and Cdl. Among them,
Adam Heller’s Relation is a promising one:
Resistant goes exponent with time while Product
of Capacity and time is a constant. [26, 27] Adam
Heller’s Relation was applied in fitting. Charge
Transfer Resistance fitted Adam Heller’s
Relation well. The correlation coefficient is over
0.9, which is shown in Figure 22 – 24.

10. 10
Figure 22 – 24 Fitting diagram of Adam Heller’s Relation
of Charge Transfer Resistance.
However, Adam Heller’s relation does not fit
with Double Layer Capacity very well. 3 order
polynomial relation fitted capacity quite well but
physical meaning was missing, which is shown in
Figure 25 – 27. More time points are desired to
make more precise measurement.
Figure 25 – 27 Fitting diagram of Adam Heller’s Relation
of Double Layer Capacity.
5.3 Existing Error
Several problems were faced and need to be fixed
in the future during the research:
Firstly, the Data repeatability is quite low. Noise
always existed in low frequency region.
According to previous research, this noise is
universal for under 10 Hz order, which is really

11. 11
hard to avoid. It may be improved by setting up
cell and prepare samples carefully.
Second problem is about determining the
Electrochemical Equivalent Circuit Model.
Thousands of models exist and we also need
adequate time points to calculate parameters with
higher accuracy.
6 Conclusion
In this research rotation, self-ordered Anodized
Aluminum Oxide nano-pore structure samples
with Iron Hydroxide / Iron Oxide Nanoparticles
inside were fabricated via 2-step method.
Electrochemical Impedance Spectroscopy was
employed to measure the formation of
nanoparticles in the nano-pores at different time
point so as to analyze the dynamics of
nanoparticle formation. Electrochemical
Equivalent Circuit Model type was analyzed and
parameters were calculated.
According to the Nyquist Plot, both imaginary
impedance and real impedance increase with both
time and thickness increase. The fastest
accumulating time period is influenced by oxide
thickness and nanoparticle type. Fe(OH)3 and
Fe3O4 nanoparticles have same impedance
magnitude order, however, Fe3O4 nanoparticles
form faster than Fe(OH)3 in AAO nano-pore
structure. Mechanical stretching method in EIS
sample preparation can show more low frequency
information in Nyquist Plot.
With the assist of EIS Lab Software and analysis
of existing EEC model, Kinetic & Charge
Transfer Mixed Control Randles Cell model was
chosen to describe our cell. And Adam Heller
Relation was employed to fit Resistance and
Capacity. Solution resistance is a constant,
Charge Transfer Resistance goes exponent with
time and double layer capacity is a polynomial of
time. More time points are needed to ensure the
conclusion in the future.
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