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SCIENCE
WITHOUT BORDERS
Graduation Final Work
Tutor
Dr. Carlos Maria Müller Jevenois
Physical Chemistry Department
Electro...
THE AUTHOR
Silio Lima de Moura, graduating in Chemistry at the Federal
University of Piauí (UFPI) – Brazil, and University...
Copyright © 2013 by University of Barcelona.
All rights reserved.
No part of this publication may be reproduced or transmi...
Aluminum anodizing: The study of the structure of the alumina layer 1
CONTENTS
ABSTRACT 3
1 INTRODUCTION 4
2 LITERATURE VI...
4.7. MORPHOLOGICAL STUDY 24
4.7.1. Scanning electron microscopy (SEM) 24
4.7.2. Transmission electron microscopy (TEM) 25
...
Aluminum anodizing: The study of the structure of the alumina layer 3
ABSTRACT
This work is focused on the study of the fo...
1 INTRODUCTION
In recent years, studies and investments have intensified in a new area of scientific
knowledge, the called...
Aluminum anodizing: The study of the structure of the alumina layer 5
molecular level. Chemical sensors open the possibili...
2 LITERATURE REVIEW
2.1 Inorganic membranes
Various structured inorganic membranes from metals, inorganic polymers, and ce...
Aluminum anodizing: The study of the structure of the alumina layer 7
slip casting in a porous support. An enormous amount...
This is important to remark the differences between the anodizing process (controlled
oxidation process) and the corrosion...
Aluminum anodizing: The study of the structure of the alumina layer 9
2.3 Anodizing applications
The anodizing process is ...
2.4 Nanomaterials
A new area of research, with a focus on systems formed by structures of dimensions of the
order of nanom...
Aluminum anodizing: The study of the structure of the alumina layer 11
2.5 Anodic alumina: layer formed and thickness
Anod...
2.6 Structures nanoporous anodic alumina
The structure of the porous anodic alumina (Al2O3) has been studied intensively o...
Aluminum anodizing: The study of the structure of the alumina layer 13
pore average size (approximately 30-80 nanometers),...
At the start of anodizing, at random nucleation of the pores on the surface of the Al sheet
takes place. At higher times, ...
Aluminum anodizing: The study of the structure of the alumina layer 15
the manufacture of metallic or semiconductor copies...
3 OBJECTIVES
The work of the completion course presented here was developed in the Electrodeposition
and Corrosion Laborat...
Aluminum anodizing: The study of the structure of the alumina layer 17
4 EXPERIMENTAL METHODOLOGY
To obtain homogeneous an...
copolymer of styrene and butadiene. In a second step is to change the area and geometry of
the parts to fabricate such a p...
Aluminum anodizing: The study of the structure of the alumina layer 19
4.2.4. Electrochemical polishing
The electro polish...
(a) (b)
Figure 6. Experimental systems to; a) electro-polishing; b) stripping acid.
The process is carried out in a jacket...
Aluminum anodizing: The study of the structure of the alumina layer 21
with a ramp of current until 250 mA.cm-2 are reache...
Figure 7. Cell control system anodization.
The anodizing is performed immediately after the pretreatment of the aluminum s...
Aluminum anodizing: The study of the structure of the alumina layer 23
achieve barrier layer thinning (BLT) using a curren...
The sample active area of alumina (working electrode WE) has been reduced to 7.2 ± 0.6 cm2.
The ratio volume of the soluti...
Aluminum anodizing: The study of the structure of the alumina layer 25
4.7.2. Transmission electron microscopy (TEM)
The m...
5 RESULTS AND DISCUSSION
5.1. Mechanism of the aluminum anodizing process
During the anodizing process, various parameters...
Aluminum anodizing: The study of the structure of the alumina layer 27
j A. e , with E
∆!
"
(5.1)
Figure 10. Mechanism of ...
the formation of a new different layer starts – the porous layer. Due to the growth of the cavities,
pores are generated, ...
Aluminum anodizing: The study of the structure of the alumina layer 29
The stripping model schematized in Figure 11 can ex...
Figure 12 – j vs. t associated with the process 1AN and 2AN, in 0.30 M oxalic acid at 20 ºC and E1AN =
E2AN = 45 V.
In all...
Aluminum anodizing: The study of the structure of the alumina layer 31
(a) (b)
Figure 13. On-top SEM image of the alumina ...
5.3. Analysis of the structural parameters of alumina
Regarding the structural parameters of alumina (dint and δcell) have...
Aluminum anodizing: The study of the structure of the alumina layer 33
#$%&&/() *
+ .,-.
/ · 12344 5678 (5.2)
As indicated...
: ; :- . <=. >/?
@. >/A
B (5.3)
The degree of thinning of the barrier layer is directly related to the level of the voltag...
Aluminum anodizing: The study of the structure of the alumina layer 35
It has been detected that the reduction process pro...
Figure 18. Scheme of the processes that occur during the various stages of series of pulses A-C. I = total
current intensi...
Aluminum anodizing: The study of the structure of the alumina layer 37
these nanostructures [47, 48]. A more careful analy...
6 CONCLUSIONS
During the course this work, we have successfully developed a complete anodizing process,
simple, versatile ...
Aluminum anodizing: The study of the structure of the alumina layer 39
REFERENCES
1. Feynman, R. There's Plenty of Room at...
17. Zhao, X. H. Yu Zuo, Jing-mao Zhao, Jin-ping Xiong, Yu-ming Tang. A study on the self-
sealing process of anodic films ...
Aluminum anodizing: The study of the structure of the alumina layer 41
33. N. Birbilis , R.G. Buchheit. Electrochemical Ch...
Aluminum anodizing: The study of the structure of the alumina layer
Aluminum anodizing: The study of the structure of the alumina layer
Aluminum anodizing: The study of the structure of the alumina layer
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Aluminum anodizing: The study of the structure of the alumina layer

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This work is focused on the study of the formation of an alumina (aluminum oxide) layer by double anodizing of aluminum. From this controlled oxidation method, a nanostructured porous layer with hexagonal cells was obtained. Ordered porous structures with 40-52 nm pore diameter were obtained by applying oxalic acid and a voltage-controlled procedure. The morphology of the surface and cross-section of the samples was analyzed by electron microscopy (Scanning and transmission). Finally, using this porous alumina as a template, Ni nanowires were obtained by electrochemical growth of the metal into the pores.

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Aluminum anodizing: The study of the structure of the alumina layer

  1. 1. SCIENCE WITHOUT BORDERS Graduation Final Work Tutor Dr. Carlos Maria Müller Jevenois Physical Chemistry Department Electrodeposition and Corrosion Laboratory Aluminum anodizing The study of the structure of the alumina layer Silio Lima de Moura January 2013
  2. 2. THE AUTHOR Silio Lima de Moura, graduating in Chemistry at the Federal University of Piauí (UFPI) – Brazil, and University of Barcelona (UB) – Spain. It acts in the Bioelectrochemistry Laboratory (UFPI), and in the Electrodeposition and Corrosion Laboratory (UB). He was student Scholarship Initiation Industrial Technology - Level A from 2009 to 2011 from the National Counsel of Technological and Scientific Development (CNPq) - Brazil, in the project Interinstitutional Center for Research and New Generation Technologies for Strengthening the Productive Arrangement. He published 04 papers in specialty journals, and more than 20 full texts at conferences. He has experience in the area of electrochemistry with emphasis on bioelectrochemistry, acting on the following topics: nanostructured platforms, nanoporous membranes, metal electrodeposition, corrosion, protons/electrons transfer in biological systems, sensors & biosensors. E-mail: siliosilicio@hotmail.com The world is small for those that think WITHOUT BORDERS. Silio Moura
  3. 3. Copyright © 2013 by University of Barcelona. All rights reserved. No part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopy, recording, or any information storage and retrieval system, you may not decompile, disassemble, reverse engineer, reproduce, modify, create derivative works based upon, transmit, distribute, disseminate, sell, publish or sublicense the work or any part of it, without University of Barcelona prior consent. You may use the work for your own noncommercial and personal use; any other use of the work is strictly prohibited. Your right to use the work may be terminated if you fail to comply with these terms. Permissions may be sought directly from University of Barcelona, in Barcelona, Spain. Phone: +34 93 402 12 01, e-mail: socrates.quimica@ub.edu. INFORMATIONS Moura, S. L. Aluminum anodizing: The study of the structure of the alumina layer/ Silio Lima de Moura, Carlos Maria Müller Jevenois. 1. anodic alumina 2. porous and barrier layer 3. nickel nanowires. 4. Moura, S. L. Digital address: http://www.slideshare.net/ (Use title to search) Printed in the Kingdom of Spain REPORT
  4. 4. Aluminum anodizing: The study of the structure of the alumina layer 1 CONTENTS ABSTRACT 3 1 INTRODUCTION 4 2 LITERATURE VIEW 6 2.1. INORGANIC MEMBRANES 6 2.2.1. Preparation methods 6 2.2. ANODIZING 7 2.3. ANODIZING APPLICATIONS 9 2.4. NANOMATERIALS 10 2.5. ANODIC ALUMINA: LAYER FORMED AND THICKNESS 11 2.6. STRUCTURES NANOPOROUS ANODIC ALUMINA 12 3 OBJECTIVES 16 4 EXPERIMENTAL METHODOLOGY 17 4.1. CONSIDERATIONS ON THE BASE MATERIAL 17 4.2. ALUMINUM SURFACE PREPARATION 18 4.2.1. Degreasing 18 4.2.2. Stripping alkaline 18 4.2.3. Stripping acid 18 4.2.4. Electrochemical polishing 19 4.2.4.1. Stripping acid 21 4.3. ALUMINUM ANODIZING 21 4.4. NANOTEXTURING 22 4.5. PROCESS IMPROVEMENT OF BARRIER LAYER 22 4.6. ELECTRODEPOSITION OF NANOWIRES 23 4.6.1. Nickel nanowires 23
  5. 5. 4.7. MORPHOLOGICAL STUDY 24 4.7.1. Scanning electron microscopy (SEM) 24 4.7.2. Transmission electron microscopy (TEM) 25 5 RESULTS AND DISCUSSION 26 5.1. MECHANISM ALUMINUM ANODIZING PROCESS 26 5.2. TWO-STEPS ANODIZING 28 5.3. ANALYSIS OF THE STRUCTURAL PARAMETERS OF ALUMINA 32 5.4. BARRIER LAYER THINNING 33 5.5. ELECTRODEPOSITION OF NICKEL NANOWIRES 35 6 CONCLUSIONS 38 REFERENCES 39
  6. 6. Aluminum anodizing: The study of the structure of the alumina layer 3 ABSTRACT This work is focused on the study of the formation of an alumina (aluminum oxide) layer by double anodizing of aluminum. From this controlled oxidation method, a nanostructured porous layer with hexagonal cells was obtained. Ordered porous structures with 40-52 nm pore diameter were obtained by applying oxalic acid and a voltage-controlled procedure. The morphology of the surface and cross-section of the samples was analyzed by electron microscopy (Scanning and transmission). Finally, using this porous alumina as a template, Ni nanowires were obtained by electrochemical growth of the metal into the pores. Keywords: anodic alumina, porous and barrier layer, nickel nanowires.
  7. 7. 1 INTRODUCTION In recent years, studies and investments have intensified in a new area of scientific knowledge, the called Nanoscience. The Nanoscience has focused on research of nanostructures of different species that have demonstrated their usefulness in helping to understand problems in areas as medicine, biology and chemistry. A major milestone of Nanoscience was on December 1959, at Caltech, California, when Richard Feynman in his lecture, delivered "There's plenty of room at the bottom" [1]. Feynman believed it is possible condense on the head of a pin, the pages of the 24 volumes of the Encyclopedia Britannica. Aluminum anodizing is an electrochemical technique implemented for many years as an excellent method to improve properties of the metallic substrate and its corrosion resistance. It is also the lower cost method [3] used to obtain membranes usually applied to micro and nanofiltration. In contrast to the organic ones, these inorganic membranes exhibit a unique combination of physical and chemical properties: they can be used at temperatures significantly higher, have better structural stability, and resist chemically unfavorable environments and action of microbiological agents. Recently, this porous alumina structure has been used as a template to obtain 1D nanostructures. Masuda and Fukuda (1995) [2] pioneered in obtaining highly ordered arrays of nanopores in aluminum foils by double anodizing. This technique allows obtaining alumina blades with a hexagonal arrangement in the nanometer range, with highly regular pore-diameters and distances inter-pores. By electrodeposition is possible to fill the membrane pores with transition metals such as Fe, Co and Ni, which enable obtaining magnetic nanowires with high shape anisotropy. Such materials are very promising for perpendicular magnetic recording [4], due the density of nanopores (and then, nanowires), which can be as high as 1011–1012 porous/cm2. This leads to a potential density of 100-500 Gbits/cm2 for these blades. These nanowires also have great potential for application in the development of magnetic field sensors, photonic crystals, optoelectronic devices or chemical and biological sensors at
  8. 8. Aluminum anodizing: The study of the structure of the alumina layer 5 molecular level. Chemical sensors open the possibility of manufacturing and development of new technologies in vivo [5]. In this work, a pulse plating technique proposed by Nielsch and Müller [6] was applied for nanowires deposition. The choice of subject is justified by the theoretical and practical knowledge, acquired throughout the course, of the studies previously conducted by national and international authors.
  9. 9. 2 LITERATURE REVIEW 2.1 Inorganic membranes Various structured inorganic membranes from metals, inorganic polymers, and ceramics, have been proposed for the filtration of liquids and gases. The interest in the use of inorganic membranes has increased since the acquisition of quality consistency, marketing and availability of ceramic membranes with pore size distribution smaller. The inorganic membranes exhibit specific physical and chemical properties, only partially showed or not shown by organic membranes. One example is the ability to withstand high temperatures being steam sterilized and bear the flow of water, maintaining its structural stability without problems of swelling or compaction. In addition to having the ability to withstand even harsher environments chemically and are not subject to microbiological attack [7]. Dense membranes such as palladium, silver, zirconia or other alloys have been shown to be permeable only to certain gases like hydrogen and oxygen and are used industrially as tools for separation. Currently, microporous stainless steel, silver and ceramic membranes as alumina and zirconia are commercially available and have gained acceptance in the process of liquid phase separation, filtration of gases or biological systems [8]. In other study conducted by Gong et al. (2003) [9], capsules with fluorescent molecules were filled into porous alumina and sealed with silicone. This system was placed in a saline solution under stirring and the solution fluorescence, due to the diffusion of molecules trapped in capsules, was monitored over time. This will make it possible to use such a system to control the time in which a drug is released from the capsule into the patient's body [9]. 2.2.1 Preparation methods Several methods for making microporous membrane of alumina have been applied: sol-gel [10], based on the CVD growth [11], anodic oxidation of the metal aluminum, and 'slipcasting' -
  10. 10. Aluminum anodizing: The study of the structure of the alumina layer 7 slip casting in a porous support. An enormous amount of documentation is available about the anodic oxidation of an aluminum substrate in a solution of an aggressive acid [12]. Asymmetric membranes of aluminum oxide that are prepared by this process generally consisted of two regions: one bulky layer of pores with average diameter sizes around 0.1 mm, and a thin selective membrane layer together with the metal fine porosity. The pore diameter depends strongly on the electrolyte used, and 100, 200 and 300 Å respectively when used sulfuric, oxalic and phosphoric acids. The membranes obtained without post-treatment, are not stable over long exposures to water. Calcination or hydrothermal treatment at 35-80 °C, are needed to promote stability to the membrane [7]. 2.2 Anodizing Anodizing is an electrolytic reaction in which an anodic layer is formed on the aluminum surface. This anodic layer composed of aluminum oxide is produced on the metal surface in a controlled and uniform way through the application of a differential current in electrolytic baths, under intense stirring and accurate temperature control [13]. To anodize the aluminum substrate, an electrochemical cell is needed as shown in Figure 1. This cell consists of four key elements: • Anode: electrode where there is the oxidation reactions; • Cathode: electrode where the reduction reactions are produced; • Electrolyte: ionic conductor or conductive solution that involves both electrodes; • Electrical connections between the two electrodes. Figure 1. Electrochemical cell.
  11. 11. This is important to remark the differences between the anodizing process (controlled oxidation process) and the corrosion process (spontaneous process). In the corrosion process the appearance of the cells is the result of potentials at two points of the metal surface – anodic and cathodic area are defined at these points (Figure 2). On the contrary, the anodizing process involves the controlled oxidation of aluminum in a solution by applying a voltage or an anodic current strength to its oxidation. Thus, the metal being oxidized (anode) is connected to the positive pole of a power source, and other metal (cathode), usually resistant to the electrolyte, is connected to the negative pole. Figure 2. Electrochemical reactions occurring during the corrosion of zinc in air-free hydrochloric acid. The anodic growth is due to the migration of ions through the growing oxide layer to react with the metal / components of the electrolyte solution and produces an insoluble compound, forming a surface film to the electrode. Thus, the growth of an oxide porous layer occurs [8]. During the growth of the porous film in aluminum, the process of migration of oxygen anions from the interface oxide/solution, into the aluminum electrode and the formation of hydrogen gas at the cathode will occur according to the following reactions: 2Al 2Al 6 2Al 3O Al O (2.1) 6H 6 3H
  12. 12. Aluminum anodizing: The study of the structure of the alumina layer 9 2.3 Anodizing applications The anodizing process is used for many purposes thanks to the improvement of the properties of the base material that can be obtained related to the formation and post-treatment of the oxide layer: improved weather resistance, surface hardness, water resistance, electrical resistance, abrasion resistance and paintability/ electrolytic coloring can be attained [8]. For certain thicknesses and conditions of coloring, the anodized aluminum has properties suitable for the selective photothermal conversion of solar energy [14]. Given its benefits, the anodizing process has been applied in many industries: among them automotive, construction, aeronautics, electronics, textile, energy or medical that applying components of aluminum and its alloys. In many of these applications, the pores must be sealed: they are placed generally in boiling water where the porous layer is hydrated, making the formation of boehmite [14]. Figure 3 illustrates an application of the commercial anodizing. Figure 3. Extruded aluminum profiles, colored after anodizing. More recently it has been studied to use this process to obtain ceramic membranes, due to specific properties presented by the anodic layer, which are: chemical and thermal resistance, and do not swell and can control growth and shape of pores [7]. These membranes are being studied for use as a template for nanomaterials.
  13. 13. 2.4 Nanomaterials A new area of research, with a focus on systems formed by structures of dimensions of the order of nanometers, became the focus of current science: nanotechnology. The field of technology is between 0.1 and 100 nm (from atomic sizes up to about the wavelength of visible light), a region where material properties are determined and can be controlled. Although the science of atoms and simple molecules, on one hand, and the science of matter, the other, are already well defined, nanotechnology is still in its early stages, because there is much to be understood about the behavior of nanoscale materials. Currently, only simple structures and devices can be created in a controlled manner and reproductive. So many materials and nanoscale phenomena have been studied by scientists worldwide to better understand the fundamentals and the laws of nanotechnology. The synthesis of nanoscale materials fabrication anticipate and control the structure of matter at the molecular level and represents the introduction of a new era, where you could have access to new properties and behavior of materials and devices so ever. For example, nanostructured metals are more ductile than conventional metals and can be used in various types of applications, particularly cold welding [15, 16]. Nanosized systems stand out in areas such as magnetic recording and reading information involving polarized currents and tunneling. Another application is in medicine, which features daily new results related to low-impact procedures, medications with fewer side effects and treatments with less time to recover from research in nanoscale systems. Finally, another important branch of technology is effective in monitoring and control system for the detection and quantification of sources of air pollution. As previously cited, anodic alumina layers are a promising material as a template to obtain nanomaterials to be applied in materials and biomedical applications [17, 18].
  14. 14. Aluminum anodizing: The study of the structure of the alumina layer 11 2.5 Anodic alumina: layer formed and thickness Anodic alumina have a complex structure: a high ordered porous structure consisting of an array of hexagonal cells, with a central pore perpendicular to the surface, on the top and a compact layer (barrier layer) on the bottom in contact with the aluminum surface [19], as can be seen in Figure 4. The oxide layer formed depends on the type of electrolyte used, because the oxide formed may be porous or not. Electrolytes carrying solvent action on the oxide, produce film porous, otherwise compact films are obtained. Examples of electrolytes which attack the oxide are sulfuric, phosphoric, chromic and oxalic acids. Among those that do not attack can cite, boric acid, borates containing electrolytes and certain organic electrolyte as citric acid [12]. The thickness of the compact film is a function of applied voltage, for being a dielectric film - This film may be employed in capacitors as Ta, Nb, Zr, Hf and Sb. On the other hand, the thickness and the geometry of the porous film depend on current density, temperature, voltage and anodizing time. At low temperatures (0-5 °C), the porous layer is more resistant, because at high temperatures (60-75 °C), the film is thinner and less durable than films grown at room temperature [14]. So, the cell size is determined by the applied voltage, whereas the layer thickness is determined by the transferred charge (current * applied time). The performance of the anodic films depends on the size and number density of the pores, parameters that are directly correlated with the correct dissipation of the heat generated in the process. According to Martins (2001) [8], the formation of pores is due to the dissolution of aluminum oxide due to chemical attack by the electrolyte. Chemical processes at the surface of alumina, exposed to our atmosphere, leads to formation of a hydrated alumina, gel type. Al O H O 2AlO OH (2.2) This hydrated alumina is formed on the walls of the pores, reducing the diameter thereof and preventing the conduction of any substance inside thereof.
  15. 15. 2.6 Structures nanoporous anodic alumina The structure of the porous anodic alumina (Al2O3) has been studied intensively over the past five decades [8]. Anodizing of pure aluminum resulted in membranes of porous alumina with nano-pores with controllable geometry and distribution (Figure 4). The structure of anodic alumina membranes is arranged as a closed container of hexagonal cells, each containing a central cylindrical portion that is perpendicular to sheet surface underlying Al. The pores extend down to the alumina barrier layer. This thin non-porous layer has a hemispherical geometry and its thickness is different that the pore wall. For alumina structures, the pore diameter is between one third and one-half of the cell size [20]. (a) (b) Figure 4. (a) Image of the structure of the anodic layer on the aluminum substrate and (b) aspect of the surface of the anodic layer obtained on pure Al. Using acidic solutions it is possible to attack the walls of the pore and consequently modify (increase the diameter) or widen the pores. One of the most important advantages of anodic alumina membranes is the possibility of modify in a large extend the pore diameter and cell size modifying the experimental parameters [16]. The cell diameter of the alumina structures is proportional to the anodizing voltage and can be adjusted in the range of approximately 25-420 nm (the size of the corresponding pore is in the range of approximately 10-200 nm) [21]. Typically, three kinds of acidic solutions are used for anodizing alumina membranes: sulfuric, phosphoric or oxalic solutions. The sulfuric acid solution is suitable for the preparation of alumina membranes with small pores that are in the range of approximately 10-30 nanometers. A solution of oxalic acid is suitable for the preparation of alumina membranes with
  16. 16. Aluminum anodizing: The study of the structure of the alumina layer 13 pore average size (approximately 30-80 nanometers), whereas large pore membranes (greater than 80 nanometers) can be prepared in phosphoric acid solutions. The proper adjustment of the pore diameter and the size of the cells allow us to obtain alumina membranes of small and average pore size [21, 22]. Mainly, the following chemical processes dominate the formation (growth) of the alumina membrane into the pores [23]. (1) The formation of Al ions at the interface of metal/oxide and its distribution in the oxide layer near the interface. Al Al 3 (2.3) (2) The electrolysis of water (a reaction of breakdown of the water molecule) occurs at the bottom of the pore near the interface of the electrolyte/oxide. 2H O 2O 4H (2.4) (3) Due to the high electric field produced at the barrier layer, the ions of O migrate through this layer from the oxide/electrolyte interface to the oxide/metal, and react with Al ions, forming the Al O : 2Al 3O Al O (2.5) (4) There is a dissolution of the oxide at the interface electrolyte/oxide: Al O 6H 2Al 3H O (2.6) In the process, there is an equilibrium between the dissolution at the oxide interface electrolyte/oxide and oxide formation at the interface oxide/metal. This balance is critical to the formation of porous alumina structures, therefore maintains the barrier layer thickness constant in the entire process of anodizing and allows the steady state propagation of the porous layer.. The electric field enhanced dissolution is the driving force of the formation of the porous alumina. This mechanism is not produced when anodize in neutral solutions. The formation of highly ordered hexagonal arrangements of the membranes of pore alumina is a high-organization process during anodizing of the Al [24]. It is suggested that the repulsive forces between neighboring pores in the metal/oxide interface promote the formation of hexagonal ordered pore arrangements.
  17. 17. At the start of anodizing, at random nucleation of the pores on the surface of the Al sheet takes place. At higher times, when the pores grow, the pores become high organized and regular pore arrangements can be obtained. Typically, the regularity of the pore and arrangement increases with anodizing time. Consequently, in a one-step process of conventional anodizing, a relatively long time anodizing will result in alumina membranes with pores more disordered on the surface and more regular at the bottom of the layer near the barrier layer [25]. An anodizing process of two steps was proposed by Masuda and Satoh (1996) [29], and successfully performed the manufacturing of alumina membranes with regular pore everywhere of the membrane. In this process, after an initial anodizing, the alumina layer is removed from the Al foil, leaving a highly ordered concave mold on the surface of Al [26]. A second anodization is then performed on this sheet-shaped Al surface, resulting alumina layers with regular arrangements of pores on both sides of the membranes. Some pre-treatment of the Al sheet are required before anodizing, such as electrochemical polishing and annealing, because the control of its surface roughness is essential for assure the homogeneity and order of the pore structure [27]. It should be mentioned that the arrangement of the columnar pore is difficult to be attained for areas larger than individual grains - areas free from defects are typically only from several square microns. The size of these areas increases with anodizing time but it seems to be a limitation of this improvement. This difficulty in obtaining long-range order in the self-organized porous layer of the alumina limits their applications [27, 28]. In 1997, Masuda et al. [29], proposed a pre-texturing process, using a mold of well-distributed nanostructures of silicon nitride, to bookmark a sheet of aluminum prior to anodizing. By anodizing this pre-textured Al sheet, alumina membranes with long-range ordered arrangements (mm2 size) can be obtained. In the last two decades, many types of nano-wire and nano-tube were prepared using anodic alumina membranes as templates, which include metals, semiconductors, carbon, polymers, and other materials. The provisions of the nano-wire and nano-tube with on-demand dimensions can be released of mold by simply removing the alumina membrane using solutions of acid or alkali [30, 31]. In addition, the alumina membranes can be used also as the original in
  18. 18. Aluminum anodizing: The study of the structure of the alumina layer 15 the manufacture of metallic or semiconductor copies, some of which have interesting chemical and physical properties. Our interest is focused on the use of the anodized aluminum structure (aluminum substrate + alumina barrier layer + alumina porous layer) as the direct template. Due to the high resistivity of the thin barrier layer, the conductivity of the whole system is very poor and some pre- treatment of the anodic layer must be made before fill some metal on the pores by electrochemical techniques [32].
  19. 19. 3 OBJECTIVES The work of the completion course presented here was developed in the Electrodeposition and Corrosion Laboratory (Electrodep) of Physical Chemistry Department of the University of Barcelona. This research work focuses on the study of engineering of porous alumina structures and their subsequent use in the synthesis of nanomaterials: nickel nanowires. Some parameters of analysis and optimization of the different stages of preparation of membranes were previously studied by researchers of Electrodep, and were used as standard in this work. • Study of double anodizing process for obtaining alumina structure ordered in bath of oxalic acid. • Analyze the influence of the most important parameters of the anodization process. Extraction of conclusions about the mechanism of formation of the anodized layer of alumina. • Characterize the structure and morphology of the alumina layer by electron microscopy. • Synthesis of nickel nanowires via electrochemical deposition and characterization using morphological observation technique.
  20. 20. Aluminum anodizing: The study of the structure of the alumina layer 17 4 EXPERIMENTAL METHODOLOGY To obtain homogeneous and ordered porous alumina layers, the procedure of double anodizing of aluminum alloy was used in the present work. To do this, oxalic acid solutions and mild anodizing conditions (auto-ordered) were chosen. 4.1. Considerations on the base material The metal used in this work is a commercial alloy aluminum laminate 0.7 mm thick provided by Alu-Stoke, SA ( PURALTOK-H24 99.5) which denotes EN AW-1050 by European standards CEN. This type of aluminum has a relatively high purity, compared to other aluminum alloys (99.5% Al minimum). The table 1 lists the most common elements of these types of alloys, including iron stands (up 0.40%) and silicon (up 0.25%). Morphological analysis by scanning electron microscopy coupled with an energy dispersive spectrometer (SEM-EDS) shows that these elements are not found homogeneously distributed in aluminum, but forming intermetallic compounds of the type Fe-Al-Si, Al-Si Al -Fe [33,34] with a size between 1 and 5 microns. This fact marks a key difference with respect to aluminum of high purity (>99.999%) commonly used in the manufacture of alumina membranes. Indeed, in this work we plan to use this aluminum alloy as substrate to reduce significantly the cost of the process. The control of the area of the samples used is an important point to get reproducibility in the experiments, since some of the steps and electrochemical tests involve the application of a controlled current density. A well fixed, even in the case of geometric area, will determine and control the applied current density. Therefore, the area of the samples has been measured with a meter with an accuracy in longitudinal ± 0.5 mm. To study the first anodizing process samples consisting of aluminum strips of 27 mm x 125 mm has been prepared, with a total active area of 14.6 ± 0.5 cm2 (includes the two faces) defined by a mask resin resistant to the aggressive chemical solutions. The trade name of this resin is Turko Mask Yellow 522 and consists of a
  21. 21. copolymer of styrene and butadiene. In a second step is to change the area and geometry of the parts to fabricate such a power at one time, three samples to be anodized. The piece consists of three parallel stripes, each 6.5 mm x 55 mm with a total exposed area of 21 ± 1 cm2 (includes the two faces), which subsequently able to separate and manipulate independently. Al Si Fe Cu Mn Mg Zn Ti >99.5 0.25 0.40 0.05 0.05 0.05 0.07 0.05 Table 1. Maximum weight percentage composition of an aluminum alloy EN AW-1050. 4.2. Aluminum surface preparation The purpose of the following treatments is to get a very low surface roughness (of the order of nanometers) and high reproducibility, by minimizing the number of surface defects. 4.2.1. Degreasing A slightly alkaline industrial degreasing (Metex T5-40A), specific for aluminum, was used. The solution is prepared with deionized water. The Al sample was introduced into the vigorously stirred solution and heated to 55 °C for 3 minutes. Degreasing finalized, the piece was washed thoroughly with deionized water. 4.2.2. Alkaline stripping The aluminum surface is chemically attacked with a NaOH and sodium gluconate solution at 55 °C. Both the sodium gluconate and sodium hydroxide are quality PRS and deionized water was used to prepare the solution. A piece of aluminum is pickled for 2 minutes under intensive stirring, rinsed with deionized water and finally with water from the system Milli- Q®. 4.2.3. Acid stripping It is performed in a solution of nitric acid (HNO3) prepared from concentrated nitric acid (PA - HNO3 65%) and water quality system Milli-Q® at room temperature and with gentle shaking for 2 minutes, rinsed with deionized water and with water from the Milli-Q® system and finally air dried.
  22. 22. Aluminum anodizing: The study of the structure of the alumina layer 19 4.2.4. Electrochemical polishing The electro polished (electrochemical polishing or electrolytic polishing) is a process that is used to decrease the surface roughness. The process relies on the application of an electric anodic current to the aluminum, to dissolve accurately its surface. In this case, the bath chemistry conditions do not allow the formation of aluminum oxide, but forms a viscous layer on the surface from the products of dissolution of the aluminum substrate, which will slowly spreading in the electrolyte. This layer is responsible for the leveling effect of this process because its high electrical resistance is combined with the fact that its thickness is not constant – higher in the valleys than on the “peaks” of the surface. Thus, inversely, the current on the peaks must be higher than in the valleys, giving rise to their preferential dissolution and the smoothing of the surface (Figure 5). The temperature, the hydrodynamic conditions and the composition of the bath has a large influence on the properties of the viscous layer showing that an accurate control of the experimental parameters is needed to optimize the process. An electrochemical hot polishing solution of concentrated acids (H3PO4: H2SO4 60:40) containing a little amount of aluminum sulfate, 0.65% Al2(SO4)3 is used. The products are quality PA (85% H3PO4, H2SO4 95-98% and Al2(SO4)3·18H2O). Figure 5. Schematic representation of the principle of the electro-polishing. The procedure of preparation of solution has an important effect on the reproducibility of the process. The applied protocol was: mixing of the acids and heating to about 100-120 °C; addition of aluminum sulfate and heating and stirring until complete dissolution of the solid; return to room temperature and transfer to the electro polishing cell.
  23. 23. (a) (b) Figure 6. Experimental systems to; a) electro-polishing; b) stripping acid. The process is carried out in a jacketed cell containing 1.5 L of dissolution (Figure 6a). The cathodes are of two parallel graphite bars (3 cm x 1 cm x 6 cm) at a distance of 6 cm with a total active area of 100 cm2. The aluminum sample is introduced in the center, at 3 cm from each cathode. The relationship between the surface area of the cathode and the anode is from 5 to 7 to prevent a collapse of cathodes capacity for acceptance of electric current. The solution is heated to 77.5 °C during the process although it may reach up to 82 °C. The ratio volume of the solution/anodic area is found between 70 and 100. This parameter is significant, and must be so high, because the oxidation of aluminum is highly exothermic. Nevertheless, the system must have a good ability to eliminate heat generated during the reaction to prevent overheating of the electrolyte and lose control of the process. A controlled agitation by air is used in our experimental device to minimize this problem. Four vents placed at the base of the cell, generating air bubbles by relatively small output porous glass. The amount of bubbles is controlled through an adjustable air pump. The outlets are situated in such a position that the sample is wrapped symmetrically by four columns of bubbles. The airflow creates sufficient surface agitation and helps to remove some of the heat generated, since air is injected cold. Hydrodynamics is one of the most important factors in order to have a controlled process. The oxidation of aluminum surface is forced by applying a constant anodic current density (190-210 mA.cm-2) over 10 minutes. After the first minute, the voltage is stabilized to 19 and 22 V, depending on the intensity of agitation. This voltage is taken as an indicator that the process works properly, since it is considered that practically corresponds to the voltage drop in the viscous layer. Furthermore, to facilitate the initial formation of the layer, the process starts
  24. 24. Aluminum anodizing: The study of the structure of the alumina layer 21 with a ramp of current until 250 mA.cm-2 are reached, at a rate of 8 mA.cm-2.s-1. The voltage drop increases accordingly as the layer thickness increases. The value of the stabilized potential can be associated with thickness of viscous layer. Once finalized the process of electro polishing, the piece is washed with abundant water from the system Milli-Q®. 4.2.4.1. Acid stripping It is performed to remove the low amount of oxides present on the surface after electro polishing. 3.5% H3PO4 (v/v) and 2% CrO3 (w/w) solution, prepared from 85% H3PO4, CrO3 PA reagents, and water system Milli-Q®, placed at 55 °C and stirred vigorously (Figure 6b) was used. After rinsed in water of the system Milli-Q®, the polished sample goes immediately to the anodizing cell. 4.3. Aluminum anodizing The anodizing cell is a jacketed vessel with a capacity of 1.5 L of solution (Figure 7). The cell is coated with a material that insulates it thermally to maintain better the temperature control system. The cathode is a lead cylinder of 6 cm in diameter and 8 cm high. A piece of aluminum (anode) is placed inside. The total geometric cathode active area is 150 cm2, with a ratio among the areas of the cathode and anode between 7.5 and 10. The ratio between volume solution / anode area is the same as defined in electro polishing. The solution is vigorously stirred by air to assure the rapid removal of heat from the aluminum surface. Two outputs porous glass are placed in the base of the cell to achieve small air bubbles. The control of electric current and voltage is performed by two multimeters (HP 34401A). One is connected in series between the anode and the positive pole of the power supply to control the current intensity, and the other in parallel to the electrode circuit for voltage control. The monitoring of parameters is performed by a computer using the program IntuiLink from HP. Temperature is controlled with a cryostat, which allows working at a stable temperature of ±0.2 °C between -10 and 30 °C. The power supply used (Grelco GE2501DVG) lets you apply up to 250 VDC and 1 A, both with potentiostatic or galvanostatic control.
  25. 25. Figure 7. Cell control system anodization. The anodizing is performed immediately after the pretreatment of the aluminum surface in a solution of oxalic acid which is prepared with (COOH)2·2H2O PA and water system Milli-Q®. Finished the anodization, the coupon is rinsed with water system Milli-Q®, air dried and stored in a desiccator with silica gel in vacuum. 4.4. Nanotexturing It consists in creating a pattern on the aluminum surface that serves as a nucleation in the pores of the second anodizing process. For this purpose a solution of chromium (VI) in phosphoric acid, 3.5% H3PO4 (v/v) 2% CrO3 (w/w), at 55 °C allows a selective attack of alumina (Figure 6b). The kinetics of dissolution of the oxide in the bath is strongly higher than that of aluminum. Thus, if the attack time is optimized, it is possible to dissolve completely the alumina layer without affecting the structure of the aluminum surface. For the different samples, an attack time of 10 to 30 minutes was applied, depending on the thickness of the alumina layer. 4.5. Process of the reduction of the barrier layer Taking into account that electrodeposition into anodized aluminum is only possible if the high electrical resistance of the barrier layer is reduced, the crash of the barrier layer is performed. This process can be doing using the same experimental setup as anodizing (Figure 7), but under different conditions. Variable galvanostatic signals were applied in this work to
  26. 26. Aluminum anodizing: The study of the structure of the alumina layer 23 achieve barrier layer thinning (BLT) using a current source 075-2 ES (Delta Elektronika) computer controlled using LabView software. Signal to be applied is defined by the current density and the duration of the first anodizing step, the bath temperature. 4.6. Electrodeposition of Nanowires 4.6.1. Nickel nanowires In the electrodeposition of nickel a Watts bath (240gL-1 Ni2SO4.6H2O, 40 gL-1 NiCl2.6H2O, 30 gL-1 H3BO3, PA grade reagent and water and Milli-Q® system) was used .The pH of preparation (3.0) is in principle not compatible with the layer of alumina. Anyway, any effect was not detected enlargement of pores, at least during the process of electrodeposition (less than two hours). The deposition is performed using a pulse plating technique, proposed by Nielsch and Müller (2000) [6]. This signal is optimized by alumina structures prepared according to the methodology described previously. At the end, the signal consists of a series of three pulses, each with a particular function and necessary for a homogeneous growth of nanowires into the pores (Figure 8). Figure 8. Pulsed signal applied to the electrodeposition of nickel in Watts bath. A – Cathodic galvanostatic pulse of 8 ms – - 70 mAcm-2 B – Anodic potentiostatic pulse of 2 ms – + 1 V C – Step open circuit of 1 s The 200 mL electrodeposition cell (Figure 9) contains Watts bath, a cylindrical titanium / iridium oxide mesh (Magneto Special Anodes B. V) as auxiliary electrode (EC), and a reference electrode (RE) of Ag/AgCl/NaCl 1 M with Eref = +0222 V vs. SHE (standard hydrogen electrode).
  27. 27. The sample active area of alumina (working electrode WE) has been reduced to 7.2 ± 0.6 cm2. The ratio volume of the solution / anode area is found between 25 and 30 and the ratio between the areas of cathode and anode is 15-20. The solution is stirred gently with a magnetic core. The deposition is carried out at room temperature (25 °C). The signal pulse is applied by a potentiostat VSP (Bio-Logic) with current amplifier coupled according to the protocol and "Electric power surge with Potential Limitation 3" (GCPL3) (Ec-Lab v9.54) computer program to control the potentiostat. Figure 9. Cell control system electrodeposition. 4.7. Study of the morphology 4.7.1. Electron microscopy scanning (SEM) The morphology of the alumina layer at different stages of the process has been studied by scanning electron microscope field emission (FE-SEM) (Hitachi H-4100FE). The equipment consists of a tube issuing field effect, cold cathode, with a resolution of 1.5 nm, reaching 200,000 magnification. Due to the lack of electrical conductivity of samples of alumina, the deposition of a thin layer of carbon (less than a tenth of a nanometer) is needed - by arc evaporation of graphite.
  28. 28. Aluminum anodizing: The study of the structure of the alumina layer 25 4.7.2. Transmission electron microscopy (TEM) The morphology of nickel nanowires was analyzed in a transmission electron microscope of high resolution Hitachi 800MT. Nanowires are first released from alumina matrix with a solution of chromium oxide, which selectively removes the alumina without affecting the nanowires of nickel. The composition of the solution is 3.5% H3PO4 (v/v) 2% CrO3 (w/w) and the attack was carried out at room temperature for 24 hours. The use of ultrasound facilitates the dispersion of the nanowires, which were separated from the solution by centrifugation, washed with Milli-Q® and stored in isopropanol until the moment of observation.
  29. 29. 5 RESULTS AND DISCUSSION 5.1. Mechanism of the aluminum anodizing process During the anodizing process, various parameters change every moment, and it is extremely important to promote uptake of data for further analysis of the same and thereafter generate a method that meets the needs of particular situations. In the present work, the need was first to obtain one anode layer that had a high pore density and a very small size thereof. Two parameters strongly influence the morphology of the anodic layer: the voltage and current intensity during the process. However, as mentioned in the objectives the anodizing experiments were optimized previously and used as a standard for this work. As previously indicated, the structure of the anodic alumina layer is result of a complex process, intrinsically related to the growth of the double anodic layer on the aluminum surface. This process is developed in several steps, as shown in Figure 10: I) forming the barrier layer; II) Break of the barrier layer and pore nucleation; III) Growth of the pores and forming the porous layer. Initially, no resistive layer is present on the aluminum surface. The high voltage applied and the low resistance of the solution does not restrict the passage of electric current, reaching the power a maximum value that depends on the equipment used (industrially to avoid this peak using voltage ramps rather than directly apply). Immediately aluminum oxidizes and is formed a first layer aluminum oxide, which has a large electrical resistance. The electron mobility through the layer is practically non-existent and the current depends only on the mobility of oxide ion (cations Al in sense aluminum-electrolyte and anions O /OH in reverse). Previous studies have proved that its thickness and the applied voltage - the electric field supported, determine the ion current density through the layer of alumina. The relationship is exponential according to the equation 5.1.
  30. 30. Aluminum anodizing: The study of the structure of the alumina layer 27 j A. e , with E ∆! " (5.1) Figure 10. Mechanism of formation of the alumina layer by aluminum anodizing process and j vs. time associated with the process, when working with voltage control. A and β are constants that depend on the temperature and E is the electric field which is generated through the layer. The electric field is inversely proportional to the layer thickness, so that, increased thickness produces a very rapid fall of current. If there are no changes, the current will continue decreasing exponentially to achieve virtually zero. Otherwise, in the anodizing aluminum process is observed that after a certain time the current recovers itself, increasing until it stabilizes. Here is another active process, the localized dissolution of oxide assisted by the electric field. Since at the beginning of the process, the electric field distribution on the surface is not homogeneous, the existence of roughness at the nanometer and micrometer scale (surface defects) generates an irregular growth of the barrier layer. When growing this layer, the electric field is increased considerably at these points leading to the formation of millions of wells that penetrate the layer and allow the process not saturate. Thus, 0 700 1400 2100 2800 3500 0 10 20 II IIIIII 0 50 100 150 200 4 5 6 7 8 9 j/mAcm -2 Time / s j/mAcm -2 Time / s IIII IV
  31. 31. the formation of a new different layer starts – the porous layer. Due to the growth of the cavities, pores are generated, which are propagated perpendicularly to the surface. After some time, a pseudo-equilibrium is established and a stationary plateau is attained in the experimental curve (Voltage or current). At these conditions, the thickness of the barrier layer remains constant. With time, the thickness of the porous layer can attain high values and hinder the diffusion of ionic species (the conductive fillers by dissolving and within the pores). There is then a slow and gradual decline of the current. Finally, long-term processes must take into account the effect of chemical dissolution of oxide by the action of the electrolyte employed, which is produced homogenously over the entire surface. When the growth rate of the porous layer (bottom) is equal to the rate of its dissolution (top), the maximum thickness of the layer that can be obtained by this anodizing process is reached. Since the dissolution rate depends on the attack power of the electrolyte, the maximum thickness is largely determined by the temperature of the bath and the concentration of the electrolyte. 5.2. Two-step anodizing It is known that two-step anodizing can improve the structure of the porous AAO layer [35–38]. With this process, the aluminum alloy sheets are anodized twice, although the intermediate forming alumina layer must be stripped (referred to as ST). The aim of the stripping process is to prepare the aluminum surface for a new anodic oxidation. The composition of the solution, the bath temperature and the application time must be accurately defined to obtain a very smooth surface. In this step only the AAO layer must be etched, without modifying the aluminum surface. If the application time becomes prolonged, then aluminum surface will be damaged. If the application time is too short, structured alumina remains on the surface. In addition, although it might prove interesting to employ a high temperature, one must remember that the process cannot be implemented too quickly, or the control time will be jeopardized. The temperature that best balanced stripping rate and time was 55 °C.
  32. 32. Aluminum anodizing: The study of the structure of the alumina layer 29 The stripping model schematized in Figure 11 can explain these results. The alumina pores were initially filled with solution. Stripping then began over the inner pore alumina, ending on the pore walls. Thus, the diameter of pores initially increased without affecting the thickness of the layer. As the pore walls starts to dissolve, thickness decreased; when completely dissolved, the structure collapsed. At that particular moment, oxides rests remained across the surface (visible to the naked eye) and it was impossible to measure the thickness. During the process, brightness increased as the surface changed from ceramic to metallic and extra time was needed to reach stationary values. This nanotexturing will serve for the growth of the second layer of anodic alumina. Figure 11. Schematic diagram of the stripping process. Arrows shows the evolution of the dissolution of the alumina cell, which goes from the pore (the hydrated aluminum hydroxide — the darker gray circles) to the cell walls and vertices (the amorphous aluminum oxide — the lighter gray circles). Figure 12 shows that when the same bath, voltage, and time conditions were used, the initial steps in the anodizing process barrier layer formation and pore nucleation were different in the 1AN and 2AN processes. However, the same stationary j values were attained. Moreover, the differences observed after short times were enhanced by increasing the anodizing voltage. These differences between 1AN and 2AN were related to the formation of a nanoimprinted surface on aluminum after the stripping step. SEM images show that a hexagonal pattern was left on the aluminum surface after 1AN and ST, Figure 13b.
  33. 33. Figure 12 – j vs. t associated with the process 1AN and 2AN, in 0.30 M oxalic acid at 20 ºC and E1AN = E2AN = 45 V. In all cases (1AN and 2AN), the current density initially decreased because of the formation of the barrier alumina layer. After the breakdown of the barrier layer, nucleation and the growth of pores took place and the current increased, a minimum current value appeared until stationary conditions were attained. The time and the current density of this minimum value jmin, the stationary current-density plateau, and the structure and geometry of the porous layer were directly related to the applied voltage. For instance, higher applied voltages led to higher stationary currents, as well as higher jstatinary–jmin and more uniform and ordered pore distributions. In these experiments, ordering was achieved by domains. Defects were accumulated at the edges of these domains [39, 40]. However, single-step anodizing was not sufficient to produce a homogeneous ordered structure throughout the entire layer. In one-step samples, the structure of the alumina porous layer became ordered along the anodizing process. Thus, the SEM micrographs of the porous alumina layer formed at the end of the process showed no ordered structures (Figure 13a). 0 50 100 150 200 2 4 6 8 10 j/mAcm -2 Time / s 1AN 2AN
  34. 34. Aluminum anodizing: The study of the structure of the alumina layer 31 (a) (b) Figure 13. On-top SEM image of the alumina layer after (a) single-step anodizing process at 45 V and (b) the aluminum surface after 1AN + ST. In order to enhance the ordering and homogeneity of the porous layer, a second anodizing step was applied. To undertake this two-step process, the first anodizing layer has to be chemically etched before applying the second layer (ST step). Therefore, this nanoimprinted surface induced the formation of a more ordered and homogeneous layer from the beginning of the 2AN process. By selecting the appropriate experimental parameters, homogeneous and ordered porous AAO layers were obtained for AA1050 (Figure 14). Figure 14. On-top SEM image of the alumina layer after a two-step anodizing process at 45 V.
  35. 35. 5.3. Analysis of the structural parameters of alumina Regarding the structural parameters of alumina (dint and δcell) have shown that are determined solely by the value of the voltage established at the interface of the aluminum in both operating modes, a process known voltage control [38, 39, 41, 42], but little studied in a current control process [43]. Figure 15 show the schematic representation of the porous anodic alumina (PAA) layer. Figure 15. Schematic representation of the porous anodic alumina (PAA) layer. Different cell parameters of the resulting alumina were estimated by statistical calculations. The parameter that is commonly used to characterize the porous AAO structure is the interpore distance. However, this was difficult to measure in some cases. Therefore, measurements of pore and cell density and diameter (δpore, δcell, dpore, and dcell) were used. Using the program Digital Micrograph 3.7.0, the pore number and diameter were calculated from SEM images with an area of 4.25 µm2. A Gaussian adjustment was applied to obtain the average pore diameter from the distribution graph (pore number vs. pore diameter). Sigmoidal adjustment was applied to calculate the density of pores from the accumulative graph (accumulative pore number vs. pore diameter) and the image area. As the ratio of pores/number of cells was greater than 1 in some cases, another parameter was defined to characterize the porous structure. This was the pore/cell density ratio. The cell diameter was calculated by assuming that the cells were circular with an area equal to π · (dcell/2)2, and the cell density was inversely proportional to the cell area. The following equation, including the unity conversion agreement, was used
  36. 36. Aluminum anodizing: The study of the structure of the alumina layer 33 #$%&&/() * + .,-. / · 12344 5678 (5.2) As indicated previously, when the same anodizing voltage was applied in 1AN and 2AN, the short time processes changed but the stationary current was very similar in the two steps. In these experiments, the ordering and size of the hexagonal cells and the diameter of the pores in the layer obtained in the second anodizing process were controlled by voltage. The geometric parameters of the porous AAO layer obtained in a single-step and two- step anodizing process and the aluminum pattern remaining after stripping the AAO layer obtained in a single-step anodizing process. E1AN = E2AN (V) dcell (nm) dpore (nm) δpore (µm-2) Thickness (µm) (1AN-2AN) 45 131 ± 9 46 ± 6 76 ± 11 10.30 – 9.39 Table 2. The geometric parameters of the porous AAO layer obtained in a single-step and two-step anodizing process. 5.4. Barrier layer thinning The aim of this process is to minimize the electrical resistance of the barrier layer to allow the system Al/AAO to work as an electrode, condition needed to apply electrochemical techniques to grow nanostructures into the pores of the alumina layer. The procedure is based on proportionality between the size of the hexagonal cell of the alumina layer and thickness of the barrier layer vs. the applied voltage. As a general rule, the same electrolyte than anodizing is used but working at low temperatures (5 ⁰C) to minimize the phenomena of enlargement of the pores during the process, one consequence of the chemical attack of the acid medium to the pore walls of hydrated alumina. The signal is characterized by a set of parameters but the current density and duration of the initial step control the rest of parameters. When continuous exponential decline of the applied potential is applied (Figure 16), the initial current density and the total duration of the process are the main parameters..
  37. 37. : ; :- . <=. >/? @. >/A B (5.3) The degree of thinning of the barrier layer is directly related to the level of the voltage attained in the last step. Shorter steps result in less de-structuring of the barrier layer, and a smaller decrease in the electrical resistance is produced. We note that the evolution of the voltage with time is slightly dependent on the step length (Figure 16), which suggests that the rate of BLT is really related to the initial applied current density. Figure 16. Applied current density vs. time for curves obtained for BLT signal. The circles show the anodizing voltages where the pore density of the PAA should be double than the previous one (decreasing voltage). Taking into account the previous results, optimum galvanostatic BLT occurs when each step is twice as long as the previous one, and the applied current density is half that of the previous step. Using this procedure a stationary voltage plateau is only reached during the last step. In these conditions, the barrier layer thins during all the intermediate steps and homogeneous thinning is achieved in all the pores at the end of the process. However, a more effective thinning process is achieved using an initial current density of half the current density registered during anodizing at the temperature of BLT. Therefore, anodizing and BLT must be carried out at different temperatures, but the initially applied current density should then be half the anodizing current obtained at the temperature of the BLT process. In general, lower temperatures are preferred, to minimize the chemical etching of the oxide layer. 0 500 1000 1500 2000 0 10 20 30 40 50 0.09 0.18 0.37 0.75 1.50 Time/s E/V Potential Current j/mA.cm -2
  38. 38. Aluminum anodizing: The study of the structure of the alumina layer 35 It has been detected that the reduction process produces a porous structure branched at the base of anodized pores (Figure 17), already described but little studied [6, 44–46]. This structure can be analyzed by observing the nanowires obtained after electrodeposition (Figure 20). This phenomenon is caused by the change in the density pore of the layer (size of the hexagonal cell) due to the change of voltage. The significant potential fall at each current step may cause the unfolding of the pores. The observations indicate that a correlation between the number of splits of the pores (four under the conditions tested) and the steps of the registered voltage. Figure 17. SEM image of the bottom end of a self-ordered PAA modified by BLT signal. 5.5. Electrodeposition of nickel nanowires Nickel nanowires were obtained via electrodeposition in a structure with hexagonal arrangement of porous anodic alumina layer (PAA), applying pulse plating techniques. Figure 18 shows the mechanism of metal electrodeposition by combined pulse. The pulse A (cathodic galvanostatic pulse of 8 ms - -70 mAcm-2) forces the deposition of nickel cations near the interface metal-dissolution. The pulse B (anodic potentiostatic pulse of 2 ms - +1 V) facilitates the discharge of the electrical double layer, to improve the effect of the next cathodic pulse. When removing the load during the pulse B (detecting a positive capacitive current), the diffusion of nickel at the interface during step C (Step open circuit of 1 s), is facilitated, minimizing the hydrogen evolution during the pulse A.
  39. 39. Figure 18. Scheme of the processes that occur during the various stages of series of pulses A-C. I = total current intensity; Ic=Intensity capacitive current; Ewe=Potential working electrode; Eoc=open circuit potential. Figure 19 shows an image of scanning electron microscopy (SEM) of nickel nanowires in the PAA and after removal of PAA. We can clearly see the multinanoelectrodes and several packages of nanowires with a cylindrical structure and high aspect ratio, which leads to a strong isotropy of shape. (a) (b) Figure 19. (a) Multinanoelectrodes on alumina matrix and (b) nickel nanowires released from alumina matrix. As mentioned in section 5.4, about the ramifications of the nanowires, the number of branches matches the number of voltage steps produced in the BLT process (Figure 20a. The electron diffraction measurements obtained with the electronic microscope TEM show that the nanowires branched areas are polycrystalline, with nanocrystalline, while the central growth of the nanowire is polycrystalline (Figure 20b) such as other authors observed
  40. 40. Aluminum anodizing: The study of the structure of the alumina layer 37 these nanostructures [47, 48]. A more careful analysis shows that nickel has the typical FCC structure. (a) (b) Figure 20. TEM image of (a) the bottom end and (b) length of a Ni nanowire obtained by pulse electrodeposition in a Watts bath (1800 pulses) on a self-ordered PAA modified by BLT signal. In Figure 20b, we have an image obtained by transmission electron microscopy (TEM), proving that these nanowires fabricated with this type pores anodic alumina porous are able to reproduce with fidelity and reproducibility. We can see the great precision with which the nanostructure follows the shape of the pore. It is noteworthy that in a previous work presented by W. Lee et. al. [49] has been shown the possibility to modulate the pore diameter changes of porosity by controlling the applied voltage, thus obtaining nanowires with different diameters. The length of the structure varies with different signals analyzed when the charge applied is changed in the process, a state which opens the possibility to control the drawing of the ramified structure. In the case of application in the synthesis of nanostructures, interests minimize the effect that the branched structures may have on the properties of the nanowires. For this purpose, we selected the conditions that create the minor structure.
  41. 41. 6 CONCLUSIONS During the course this work, we have successfully developed a complete anodizing process, simple, versatile and economical, and including a complete pretreatment process. The protocol established was very versatile for the manufacture of controlled layers of porous alumina suitable for synthesis of nanowires. We have demonstrated the feasibility of using a low-purity aluminum (AA1050 Al>99.5%) for production of nanostructures. It has been shown that the phenomenon of self-ordered growth also occurs in this kind of aluminum when a double anodizing process is applied. We have optimized the pretreatment of the aluminum surface. The protocol implemented can achieve good control and reproducibility of the process, giving anodic layers with better quality. The adjustment of surface roughness (electro polishing) and the amount of metal oxides on the metallic surface (stripping acid) are tools that have enabled us to achieve this level of control. The double anodizing process was performed successfully with voltage control. We analyze the phenomenon of nanotexturing of the aluminum surface after first anodizing. It has been proved that, with the nanotexturing it is possible to nanoimprint the aluminum surface prior to the second anodizing, generating a growth of highly ordered pores. If this process is not performed, the regularity pore is not maintained. It has been demonstrated that the system Al/AAO can be used as an electrode to deposit a metal directly into the porous structure, with a prior enhancement of the barrier layer. Through this system, using a suitable pulsed signal we obtained a homogeneous growth of nickel nanowires within the pores of alumina. The knowledge obtained will be useful for the development of new nanomaterials in the laboratory.
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