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Yutong Liu - Final Report - Anodized Aluminium Oxide (AAO)

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Yutong Liu - Final Report - Anodized Aluminium Oxide (AAO)

  1. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. Reference [1] Ali Eftekhari, Nanostructured Materials in Electrochemistry, Wiley – Vch, German, 2008. [2] Masuda, H.; Fukuda, K. Science 1995, 268, 1466. [3] Kornelius Nielsch et al, Nano Letters 2002, 2, 677. [4] A. P. Li et al, Journal of Applied Physics 1998, 84, 6023. [5] Kornelius Nielsch et al, Advanced Materials 2000, 8, 582. [6] O. Jessensky et al, Applied Physics Letters 1998, 72 1173. [7] Gao, L. Z. et al, Nature Nanotechnology 2007, 2, 557. [8] Hui Wei, Erkang Wang, Analytic Chemistry 2008, 80, 2250. [9] Bergeman, C. et al, Magn. Mang. Mater. 1999, 194, 45. [10] Morishita, N. et al, Biochem. Biophys. Res. Commun. 2005, 334, 1121. [11] de Vries, I. J. et al, Nature Biotechnol, 2005, 23, 1407 [12] Faquan Yu et al. Biomaterials 2009, 30, 4716. [13] Yanping Liu ; Faquan Yu. Nanotechnology 2011, 22, 1. [14] J.P. Hoar, G. C. Wood, Electrochem. Acta. 1962, 7, 333. [15] F. Mansfeld, M. W. Kendig, J. Electrochem. Soc. 1988, 135, 828. [16] Gamry Instruments, Basics of Electrochemical Impedance Spectroscopy. [17] B. Chang, S. Park, Annual Review of Analytical Chemistry 2010, 3, 207. [18] R. Potucek et al, Journal of ECS, 2006, 153, B304.
  12. 12. 12 [19] Novelcontron Technology, Electrochemical Impedance Spectroscopy EIS. [20] N D Cogger, N J Evans, Solartron Analytical Technical Report 2006, 99. [21] B. Van Der Linden et al, Journal of Applied Electrochemistry 1990, 20, 798. [22] J. A. Gonzaa Lez et al, Journal of Applied Electrochemistry 1999, 29, 229. [23] Assen Girginov et al, The Solid Films 2006, 515, 1548. [24] T. Hasebe, R. S. Alwitt, Journal of ECS 2007, 154, C626. [25] Electrochemical Impedance Spectroscopy EIS www.novocontrol.de/html/intro_eis.htm [26] Adam Heller, Acc. Chem. Res. 1990, 23, 128. [27] Andrzej Lasia, Modern aspects of electrochemistry 2002. [28] L.E.A.N. Lab Anodic Aluminum Oxide Synthesis Procedure. [29] Jessica Ray, Young-Shin Jun et al. Environmental Sci. & Tech. 2012, 46, 13167. [30] Wei Shi et al, Journal of Membrane Science 2008. 325 801-808. [31] Han-Jun Oh, Choong-Soo Chi, Bull. Korean Chem. Soc. 2000, 21, 193. [32] Han-Jun Oh et al, Bull. Korean Chem. Soc. 1999, 20, 1340. [33] Qing Wang et al, J. Phys. Chem. B 2005, 109, 14945.

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