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

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

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

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

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

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

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

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

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

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

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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