Characterization of Porous Anodic Aluminum Oxide Film by Combined Scattering Techniques

Home , Anodizing

A thesis submitted to the Graduate School

of the University of Cincinnati

in partial fulfillment of the requirements for the degree of

Master of Science

at the School of Energy, Environmental, Biological and Medical Engineering

of the College of Engineering and Applied Science

July, 2013

Xueying He

Committee: Dr. Dale. W. Schaefer (Chair)

Dr. Relva C. Buchanan

Dr. Jude O. Iroh ABSTRACT

An approach based on a combination of ultra small angle x-ray scattering (USAXS), neutron reflectivity (NR) and x-ray reflectivity (XRR), is exploited to characterize anodic aluminum oxide film. The oxide film layer structure, the porosity, and porosity evolution was obtained. A combination of XRR and NR on the film yields the density and degree of hydration of the films. Aluminum of 1000 Å thickness was deposited on Si wafers by e-beam evaporation.

Porous anodic aluminum oxide (AAO) films are formed by polarizing at constant voltage up to

20 V noble to the open circuit potential. USAXS shows that the pore size and interpore distance are fixed in the first 10 s after initiation of anodization. Pores then grow linearly in time at constant radius and interpore distance. The average AAO film density of the porous film at the air surface is 2.45 ± 0.20 g/cm3. The density of the “barrier” layer at the metal interface is 3.3 ±

0.2 g/cm3, which indicates that this layer is not as dense as a crystalline aluminum oxide.

Although this study covers limited number of variables relevant to production of AAO films, it does show that scattering methods are powerful tools to determine quantitatively the structural parameters.

Acknowledgement

First and foremost, praise and thanks go to my Savior Jesus Christ for all the blessings and protection all through my life.

I would also like to take this opportunity to express my special gratitude to my advisor Dr.

Dale W. Schaefer for guiding me as his student through this period of my graduate study. His support, guidance and inspiration help me to work through the challenges in my research and coursework study. Through my three years study in the Materials Engineering program in

CEAS, Dr. Relva Buchanan offered me great help in Financial and academic support, and I’ve gained most of my polymer knowledge through the systematic classes offering by Dr. Jude Iroh.

I would like to thank Dr. Buchanan and Dr. Iroh for being on my thesis committee and taking time to review this thesis.

I worked very closely with Dr. Naiping Hu on this research project. I appreciate her for helping me with data analysis, reviewing my papers and being my mentor for this project. Her patience towards me, her working attitude and creativity in research set a great role model for me. I would like to thank all my co-workers, Sandip Argekar, Yan Zhang, Peng Wang and other group members from Dr. Schaefer’s research group. And I would also like to thank Linqian

Feng, Yan Jin, Aniket Vyas, Hui Chen and Meixi Zhang for help in research and friendship in life.

Additionally, I thank Dr. Peng Wang, Dr. Jarek Majewski, and Dr. Jan Ilavsky for assistance in collecting and interpreting the data. The neutron reflectivity data were collected at the SPEAR reflectometer at Los Alamos National Laboratory (LANL). The USAXS data were measured at beam line 15 ID-D at the Advanced Photon Source, Argonne National Laboratory.

SANS data were collected using the LQD instrument at LANL. Lujan Neutron Scattering

Center (LANSCE) is supported by LANL under DOE contract W7405-ENG-36, and by Office of Basic Energy Sciences, U. S. Department of Energy. Use of the Advanced Photon Source was supported by the U. S. Department of Energy, Office of Science, Office of Basic Energy

Sciences, under Contract DE-AC02-06CH11357.

Special thanks go to Shang and Jowen Ko. Thanks for the Ko’s hospitality, their love, gentleness, kindness in the past two years towards me. Felling “adopted” and being loved in a totally new country by someone you never know before is such a blessing for me. Without their help and encouragement, I would hardly overcome the hard times here in the States. The

Koworkers fellowship group brings me lots of tears and laughers. Love and thanks go to each of you, who have become one of the most important parts in my life.

Finally, I would like to thank my parents for their love, understanding and support all over the 25 years of my life. Thanks for bring me into this world and giving me everything they can give.

Without their love and support, I cannot achieve anything.

Contents

ABSTRACT ...... 1 Acknowledgement ...... 3 Contents ...... 5 List of Tables ...... 7 List of Figures ...... 8 1. Introduction ...... 10 1.1 Research Significance and Objectives ...... 10 1.2 Anodization of Aluminum and Porous AAO structure ...... 11 1.2.1 Types of AAO ...... 12 1.2.2 General Structure of Porous AAO layer ...... 14 1.2.3 Pore formation mechanisms ...... 15 1.3 Sealing ...... 16 1.4 Current progresses in porous AAO films research...... 17 1.4.1 Progresses in fabrication of porous AAO films ...... 17 1.4.2 Progresses in characterization of porous AAO films ...... 19 2. Characterization Techniques ...... 23 2.1.1 Scanning Electron Microscope ...... 23 2.2 Ultra Small Angle X-Ray Scattering (USAXS)...... 24 2.2.1 Theory for Small Angle Scattering ...... 24 2.2.2 Data Analysis ...... 25 2.3 Neutron reflectivity and X-ray reflectivity ...... 26 2.3.1 Scattering length density (SLD)...... 27 2.3.2 Critical angle ...... 27 2.3.3 Kiessig fringes ...... 28 2.3.4 Analysis of reflectivity data ...... 28 2.4 Sample Preparation...... 29 2.4.1 AAO prepared for reflectivity study ...... 30 2.4.2 AAO prepared for reflectivity study ...... 31 2.4.3 Characterization of porous AAO films ...... 32 2.5 X-ray reflectivity ...... 32 2.5.1 Neutron reflectivity ...... 33

2.5.2 Ultra Small Angle X-ray Scattering ...... 35 3. Results and discussion ...... 37 3.1 Aluminum Oxide Film structure ...... 37 3.1.1 Native oxide film formed on Al ...... 38 3.1.2 AAO layered structure after anodization ...... 39 3.2 Study of Sealing ...... 41 3.2.1 In situ NR study of cold sealing ...... 41 3.2.2 Sealing study from USAXS ...... 43 3.3 Pore growth study ...... 44 3.3.1 Porosity of the AAO layer from Neutron Reflectivity...... 44 3.3.2 Evolution of Porosity ...... 46 4. Conclusion and future work ...... 54

List of Tables

Table 2.1 Calculated neutron and X-ray SLD values for the materials used in this study (calculated from http://www.ncnr.nist.gov/resources/sldcalc.html) ...... 35 Table 3.1 Modeling neutron SLD of porous AAO layer before and after sealing...... 46 Table 3.2 SAXS fitting data for level one in figure 3.7 and pore length calculation ...... 50

List of Figures

Figure 1.1 Schematic diagram of aluminum anodization in electrolyte ...... 12 Figure 1.2 Schematic diagram of AAO structure (a) Barrier type (b) Porous type ...... 13 Figure 1.3 (A) Schematic of the ideal close-packed array of hexagonally arranged pores; (B) SEM image of a typical synthesized AAO film [24]...... 15 Figure 1.4 Schematic diagram of the kinetics of porous oxide growth on aluminum in (a) constant current and (b) constant voltage condition. The stages of porous structure development are also shown [32]...... 16 Figure 1.5 Schematic of the two-step anodization process developed by H. Masuda and K. Fukuda [11, 24]...... 18 Figure 2.1 SEM images of AAO film on 3500 Å Al-coated Si wafers after anodization. [1]The sample is anodized under 5 V in 20 wt% H2SO4 for 30 s (a) and 60 s (b). An average pore diameter of less than 50 Å is observed. Based on the data presented, not all pores are visible. [2] ...... 23 Figure 2.2 SEM images of AAO film on 0.025 mm thick Aluminium foil after anodization. The sample is anodized under 15 V in 20 wt% H2SO4 for 120 s...... 24 Figure 2.4 Ultra small-angle X-ray scattering data (desmeared) for anodic films on Al foil. The fit is for highly correlated spherical pores with a radius-of-gyration of 84 Å...... 26 Figure 2.5 XRR fringes from Al thin film with a thickness of 1254.5 Å on a silicon substrate with an 11.7 Å silicon dioxide layer...... 28 Figure 2.6 Sample preparation and test procedure ...... 30 Figure 2.7 X’ Pert Pro MRD equipment...... 32 Figure 2.8 Schematic diagram of SPEAR...... 33 Figure 2.9 Schematic of USAXS instrument ...... 36 Figure 3.1 Reflectivity data of bare Al coated silicon wafer (a) NR data curves (red) and fitted data curves (purple) (b) NR SLD profile (c) XRR data curves (red) and fitted data curves (purple) (d)XRR SLD profile...... 38 Figure 3.2 (a) XRR data and (b) XRR SLD profiles (c) NR data and (d)NR SLD profiles of the pure Al-coated Si wafers, and the anodized sample. The anodized sample (in red, anodized in sulfuric acid at 15 V for 15 s) is compared with the bare Al sample (in black, 100-nm pure Al coated wafer). A four-layer (on top of Si) model is required to obtain reasonable agreement with the experimental data for the anodic samples...... 40 Figure 3.3 In situ NR data and SLD of sample 1 and 2 with sealing duration of 9.5 hours; (a) anodized in sulfuric acid at 15 V for 15 s (b) anodized in sulfuric acid under 15V for 20 s. (Fitted with a background of 1.5 10-6) (c)Dependence of SLD increase on time...... 42 Figure 3.4 NR data of sample 3 with two-step sealing (The sample was anodized at 15 V for 15 s in sulfuric acid)...... 43 Figure 3.5. USAXS data on through-thickness AAO samples sealed by different methods. The solid lines are the 3-level unified fit...... 44 Figure 3.6 Neutron reflectivity and SLD profiles of the anodized sample1, 2 after 0.5 h. (a)Sample 1 anodized in sulfuric acid at 15 V for 15 s and (b)sample 2 anodized in sulfuric acid at 15 V for 20 s...... 45

Figure 3.8 Unified fit for Smeared USAXS data of aluminum film anodized in 20 wt.% sulfuric acid under 15V for 110 second...... 48 Figure 3.9 Hexagonal structure of porous AAO ...... 49 Figure 3.11 Dependence of pore length on anodization time. The AAO was anodized under 15V in 20% wt sulfuric acid for 120s...... 52

1. Introduction

1.1 Research Significance and Objectives

Anodization of aluminum has been commercially used for corrosion protection for decades. Anodization is an electrochemical method to grow oxide layers on aluminum. Sealing of anodized aluminum as a post treatment is also widely used in various applications ranging from giftware and novelties through automotive trim and bumper systems [1-4]. Anodic aluminum oxide (AAO) has an unusual porous structure [5-11]. The cylindrical pores, perpendicular to the film surface, form hexagonal arrays that can be used as templates for carbon nanotubes, metallic nanowires, and quantum dot arrays [12-15]. Understanding the structure of the porous anodic oxide film is critical to understanding the mechanism of film formation in order to develop better fabrication processes. However, the characterization of

AAO films is mainly based on imaging methods such as scanning or transmission electron microscopy, or atomic force microscopy. These techniques reveal only a small area, which may not be representative of the overall structure or long-range order [15-17]. Also the post treatment of the sample for better microscopy images may result in pore widening. To understand and control the properties of the film, a more complete characterization is desired.

The objectives of this work are to:

1. Develop methods to characterize the chemical composition, pore structure and properties of AAO films using non-destructive techniques.

2. Investigate the effect of fabrication conditions and post treatment on the structure of

AAO films.

In our study, porous AAO films were measured by multiple radiation scattering techniques, including neutron reflectivity (NR), X-ray reflectivity (XR) and ultra-small angle X-ray scattering (USAXS). NR and XR give the film thickness and the scattering length density (SLD) profile normal to the surface. Chemical composition can be determined by comparing data from

NR and XR. USAXS is helpful in investigating the pore form factors and structure factors. Our results show that the combined scattering techniques open an new era of systemically and deeply characterizing the properties of porous AAO films, leading us to further investigate the dynamic formation mechanisms and pore structures under different fabrication conditions, and expanding the available tools for characterizing porous AAO films, and for other nano-porous materials.

1.2 Anodization of Aluminum and Porous AAO structure

Anodization is an electrochemical surface treatment mainly used for aluminum and its alloys [1, 18]. It is a passivation process to increase the thickness of the natural oxide layer formed on the metal to increase the corrosion and wear resistance.

Figure 1.1 shows a simplified apparatus for anodization. The electrochemical cell consists of a two-electrode system, e.g., the platinum (Pt) sheet acting as the counter electrode (the cathode), and Al sheet acting as the working electrode (the anode). Both electrodes are immersed in the electrolyte, mostly acids, such as boric acid [19], sulfuric acid[12, 20], oxalic acid [19-21], and phosphoric acid [22, 23].

Figure 2.1 Schematic diagram of aluminum anodization in electrolyte

1.2.1 Types of AAO

Anodic oxide films can be classified into two types, based on the reactivity of electrolyte with the oxide layer. One is a non-porous barrier film which is dense and has a good wear resistance and behaves as an electrical insulator (Figure 1.2a) [23, 24]. Another is a porous oxide structure (Figure 1.2b) with a high aspect ratio[24]. Barrier oxide film can form on aluminum in several different neutral or basic solutions, such as boric acid or alkali borates, which possess little or no ability to dissolve the oxide layers. It is generally accepted that the thickness of barrier-type alumina is mainly determined by the applied voltage[25], although the difference of electrolytes and anodizing temperature also play a role[26, 27]. The maximum attainable thickness in the barrier-type alumina film was reported to be less than 1 μm, corresponding to breakdown voltages in the range of 500 - 700 V. Dielectric breakdown of the films occurs above the limiting voltage[25]. Oxide films of this type possess unique electrical properties and have been used extensively in electrolytic capacitors and rectifiers[28].

However, the anodization of aluminum in certain strong acid electrolytes, which could dissolve the oxide layers, leads to the formation of an anodic aluminum oxide film on the surface. This film is comprised of a relatively thick porous outer layer with regularly spaced pores extending from the outer surface toward the aluminum substrate, and a relatively thin nonporous barrier layer adjacent to the aluminum metal/oxide interface (Figure 1.2b). As anodization time increases, the aluminum metal is converted to aluminum oxide at the aluminum metal/oxide interface, and the pores extend further into the film. This porous aluminum oxide typically exhibits a uniform array of hexagonal cells, each cell containing a cylindrical pore [7, 8, 11, 24, 29]. About the porous AAO layer structure, read 1.2.2 part for detail.

With films formed in electrolytes that react appreciably with the oxide, a relatively high, steady current flow and continued film growth has been observed. The amount of oxide formed is generally a function of current and time. In this case, the film is formed with a porous structure due to the reaction between the oxide layer and the electrolyte[5].

(a) (b)

Figure 2.2 Schematic diagram of AAO structure (a) Barrier type (b) Porous type

Figure 1.2 shows the structure of the two types of layers formed on aluminum. Both the barrier and porous layer consist of a native oxide layer formed with aluminum an outer oxide layer grown on top of the native oxide layer by the anodization process.

1.2.2 General Structure of Porous AAO layer

The porous layer can display a well-order structure [28]. The porous AAO grown by anodizing is represented schematically as a close-packed array of hexagonally arranged cells containing a pore in each cell center (Figure 1.3)[28]. Highly ordered nanostructures are characterized by parameters such as a pore diameter, wall thickness, barrier layer thickness and inter-pore distance (cell diameter). The uniform pore diameter, which is easily controllable by altering the anodization conditions, can range from a few nanometers to hundreds of nanometers.

The depth of fine parallel channels can even exceed 100 µm, a characteristic that makes anodic porous alumina one of the most desired nanostructures with a high aspect ratio and high pore density. Parallel growth of controlled dimension pores can proceed throughout the thickness of the anodized material [8].

Adjusting the anodizing parameters can directly affect the features of the pore structure.

The anodization voltage, electrolyte type, concentration and temperature have been recognized as the most critical parameters to control the self-ordering process and the geometry of the resulting pore structures [15, 20, 22, 27, 30, 31]. Using the conventional anodization process the arrangement of the pores is quite disordered. Masuda and Fukuda, however, introduced a two- step approach to anodization, producing highly-ordered pore structures [11]. Using this approach one can easily obtain the highly ordered hexagonal pores in a controllable way. The technical advancement of the past decades make it possible to control a number of surface parameters. For example the size of the pore diameter can be adjusted from 5 nm to 10 μm.

Different electrolytes and anodization conditions combined with chemical etching provide broad scope to engineer AAO with desired pore dimensions and shapes, which expand the range of applications for AAO films.

Figure 2.3 (A) Schematic of the ideal close-packed array of hexagonally arranged pores; (B) SEM image of a typical synthesized AAO film [24]. 1.2.3 Pore formation mechanisms

In the case of porous AAO formation, the following current density profiles are typically observed[32]. Under a constant voltage anodizing, the current density first decreases rapidly (regime 1 in Fig. 1.4). Then, the current passes through minimum (regime 2 in Fig. 1.4), after which, it increases to arrive at a maximum value (regime 3 in Fig. 1.4). Finally, the current density decreases slightly and reaches a steady current density (regime 4 in Fig. 1.4).

The pore formation mechanism is shown schematically in Fig. 1.4, corresponding to the four regimes described above. At the beginning of anodization, the natural oxide layer covers the entire surface of aluminum (stage 1 in Fig. 1.4). The electric field is focused locally on fluctuations of the surface (stage 2 in Fig. 1.4), leading to field-assisted or/and temperature enhanced dissolution in the formed oxide and thus to the growth of pores (stage 3 in Fig. 1.4).

Since some pores stop growing due to competition among the pores, the current decreases again

15 as shown in regime 4 in Fig. 1.4. Finally, the current density reaches steady state. At this stage, pores grow in a stable manner.

Figure 2.4 Schematic diagram of the kinetics of porous oxide growth on aluminum in (a) constant current and (b) constant voltage condition. The stages of porous structure development are also shown [32]. 1.3 Sealing

As mentioned above, there are two types of AAO layers, one is the non-porous barrier layer, and the other one is the porous AAO film. The porous oxide layers have open pores in their surface. This kind of structure provides poor corrosion resistance, the only resistance being provided by the impervious thin barrier layer adjacent to the Al surface. Generally, for the aluminum oxide film, the most effective way to improve the corrosion resistance is to seal the pore structure and form a thick protective barrier between the Al surface and the environment, which is called sealing. Excellent corrosion resistance can be achieved using the thick porous

16 structure in conjunction with a suitable surface treatment and protective protocol. In this process, the anodized AAO is immersed in a solution of boiling water or other solutions such as nickel acetate in order to seal the pores.

1.4 Current progresses in porous AAO films research

Porous anodic aluminum oxide (AAO) films have become one of the most popular materials with potential applications in numerous areas, including molecular separation, catalysis, energy generation and storage, electronics, photonics, sensing, drug delivery, and template synthesis[1, 4, 9, 21, 24, 33, 34]. Over the past decade, significant research effort has explored the properties and emerging applications of AAO films, resulting in more than 2000 publications related to AAO being published in the past five years. The useful and controllable features make porous AAO a highly promising and attractive material for many applications. Here we make a brief summary about the recent progresses in fabrication and characterization of the porous AAO films research.

1.4.1 Progresses in fabrication of porous AAO films

Anodization of aluminum for the protection and decoration of the metal surfaces has been widely used in industry for almost more than one century, since the first patent on the porous anodic aluminum oxide was approved in 1898[35]. Over several decades, people made extensive efforts to study and optimize the anodization conditions to achieve highly-ordered pores with controllable dimensions. It is widely accepted that the anodization voltage, electrolyte type, concentration and temperature are the most critical parameters to control the pore forming process and the dimensions and shapes of the resulting pore structures[19, 24, 26,

27, 36, 37].

In 1995, ordered anodic aluminum oxide (AAO) discovered by H. Masuda and K. Fukuda using two-step replication anodization led to a breakthrough in the synthesis of ordered structures with a very narrow pore size distribution[11]. In their method, after the first anodization step, the resulting porous oxide layer is removed to pre-structure the Al surface and make the propagation of well-defined pores from the top to bottom possible during the second anodization step. The main disadvantage of this anodization process is the slow pore-growth rate (i.e. 2–7 μm/h), which requires around two days to prepare well-ordered AAO structures, see Figure 1.5.

Figure 2.5 Schematic of the two-step anodization process developed by H. Masuda and K. Fukuda [11, 24].

Generally, aqueous solutions of sulfuric (H2SO4) at 25 V, oxalic (H2C2O4) at 40 V and phosphoric (H3PO4) acid at 195 V are the most commonly used electrolytes and voltages for preparation of AAO by conventional anodization process, called “mild” anodization[24].

Several studies have reported on the self-ordering mechanism of AAO in other electrolytes (e.g.,

18 aqueous solutions of citric, maleic, malonic, tartaric and sulfamic acids), but they achieved poor pore ordering.

A new approach, so-called “hard” anodization (HA), was introduced. Under these conditions, the pore-growth rate is considerably higher (50–100 μm/h)[24]. These two anodization strategies, using different electrolytes and anodization conditions combined with chemical etching, provide broad scope to engineer AAO with desired pore dimensions and shapes. To fabricate AAO with complex pore geometries, several electrochemical approaches have been successfully implemented, including periodic changing of anodization conditions (voltage or current) with or without replacement of the acid electrolyte. The structural engineering of AAO pores, combined with other modification methods, provides enormous scope to improve the properties of AAO for its wider applications.

1.4.2 Progresses in characterization of porous AAO films

Although ex situ techniques such as scanning and transmission electron microscopy (SEM,

TEM) can provide some information about porosity, chemical composition et al, these techniques involve the transfer of films to a high vacuum environment and require some other sample preparation treatment, which may alter the original film structures[6, 7, 24, 38]. For these reasons, in situ approaches, including scanning tunneling or atomic force microscopies

(STM, AFM), would be desirable, even though the STM/AFM techniques may not be particularly useful for the investigation of highly porous, possibly rough (at the 1-10 nm scale) surfaces[39].

Both small-angle X-ray scattering (SAXS) and small-angle neutron scattering (SANS) have been used to characterize the structure of AAO films under conditions where a highly ordered hexagonal array of pores is observed[16]. These non-destructive methods are ideal to

19 determine all the properties of AAO films. However, these scattering methods are not well appreciated for now. Here we developed a combined of scattering techniques to systemically investigate the features of AAO films,

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2. Characterization Techniques

2.1.1 Scanning Electron Microscope

The scanning electron microscope (SEM) uses a focused beam of high-energy electrons to investigate surface morphology, chemical composition, and crystalline structure, etc.

The SEM results were obtained by using the FEI XL30 ESEM in the Advanced Materials

Characterization Center in University of Cincinnati. The electron acceleration voltage is 15kV.

Samples were mounted and coated with 2–3 nm of gold and platinum using a sputtering coater to minimize the artifacts caused by sample charging.

The porous structure formed under a lower voltage in our experiment result in a disordered porous structure shown in figure 2.1 and 2.2 respectively on Al-coated wafer and Al foil.

Figure 2.3 SEM images of AAO film on 3500 Å Al-coated Si wafers after anodization. [1]The sample is anodized under 5 V in 20 wt% H2SO4 for 30 s (a) and 60 s (b). An average pore diameter of less than 50 Å is observed. Based on the data presented, not all pores are visible. [2]

Figure 2.4 SEM images of AAO film on 0.025 mm thick Aluminium foil after anodization. The sample is anodized under 15 V in 20 wt% H2SO4 for 120 s. 2.2 Ultra Small Angle X-Ray Scattering (USAXS)

2.2.1 Theory for Small Angle Scattering

Small angle scattering is a powerful tool to investigate the hierarchal structure of materials.

All the scattering processes follow the same reciprocity law, which means small feature size is characterized at large angles and large feature size is characterized at small angles.

Small-angle X-ray scattering (SAXS) experiments can generally be represented as shown in Figure 2.3: the sample is exposed to an incident radiation beam, which is scattered due to scattering length density fluctuations within the sample. For x-ray, the scattering length density is the electron density multiplied by the scattering length of the electron.

All scattering data were collected as the scattering cross section or intensity, , versus the modulus of the momentum transfer wave vector, , which is defined as

( ) (2.1)

where is the wavelength of the radiation.

Figure 2.5 Schematic of SAXS procedure

SAXS measurements typically are concerned with scattering angles < 6°. USAXS focused on scattering angle even smaller, which reveals the structure on even larger scales ranging from nanometer to the magnitude of several millimeters.

2.2.2 Data Analysis

After background subtraction, a fitting procedure is needed to get the appropriate model to interpret the data. The fitting method applied here is unified fitting [1], described in the equation

2.2,

( ( )) ( ) ( ) ∑ ( ) ( ) √ (2.2)

where represents different structure levels, and refers to the smallest size structure.

For each structural level , is the Guinier prefactor, is the radius of gyration, and is a constant prefactor specific to the type of power law scattering.

Figure 2.6 Ultra small-angle X-ray scattering data (desmeared) for anodic films on Al foil. The fit is for highly correlated spherical pores with a radius-of-gyration of 84 Å.

2.3 Neutron reflectivity and X-ray reflectivity

The specular reflection of X-rays and neutrons from surfaces offers a reliable and valuable method to examine the nature of surfaces and interfaces [2]. The development of the technique of X-ray reflectivity (XRR) and neutron reflectivity (NR) measurement along with the improvement in sample preparation regarding smoothness accounts for the rapid growth of this technique [3].

In many ways, neutron scattering plays a complementary role to X-ray scattering in the characterization of material structure. Neutron scattering cross section varies seemingly randomly among elements, where X-ray atomic scattering factor increases smoothly with atomic number. So in X-ray scattering, scattering from heavy atoms can dominate the scattering from the rest of the materials, which makes it difficult to see the other lighter atoms. In this case, neutron scattering has its advantages. For example, it is almost impossible to determine the

26 position and composition of hydrogen because hydrogen is almost invisible to X-ray. This difficulty can be avoided by using neutron scattering, especially when the hydrogen atoms in the system have been replaced by deuterium. The large difference in the scattering cross section between isotopes of the same element, ex. hydrogen and deuterium, can help us to make the molecules selectively visible to neutron beams. [2].

2.3.1 Scattering length density (SLD)

Scattering length density is a measure of the power of a material to scatter radiation

(neutron or X-ray). The neutron SLD is defined by Equation (2.3) [2]:

∑ (2.3)

where s the coherent scattering length of the atom, is density, is molecular weight, is Avogadro’s number.

For X-ray, the SLD is proportional to average electron density and can be calculated by

Equation 2.4[2]:

∑ (2.4)

where is the atomic number and, is the bound coherent scattering length of an electron. For a given material, the SLD is determined by chemical composition and mass density. SLD of air can be taken as zero due to its low density.

2.3.2 Critical angle

The critical angle in reflectometry corresponds to the critical scattering vector (fig.

2.3), ( )), at which the radiation begins to penetrate into the film. At the reflected

intensity of the specularly reflected beam drops dramatically. and the SLD of the film are related by Equation 2.5.

√ (2.5)

Figure 2.7 XRR fringes from Al thin film with a thickness of 1254.5 Å on a silicon substrate with an 11.7 Å silicon dioxide layer. 2.3.3 Kiessig fringes

For a single layer film of thickness d, the interference of the beams reflected at the two interfaces results in an oscillation superimposed on a monotonic decay of the reflectivity with q.

The oscillations are called “Kiessig fringes,” named after H. Kiessig, who first observed such fringes with metal films in 1931. The thickness of the film can be estimated from the spacing

(fig. 2.3), between successive maxima or minima,

(2.6)

2.3.4 Analysis of reflectivity data

The raw reflectivity data are reduced to a normalized reflectance defined as the ratio of the measured specularly reflected flux at a given angle of incidence ( ) to the flux of the direct

beam ( ). The specular angle , is reduced to ( ).

Normalization allows the data to be interpreted independent of the incident beam wavelength and flux. Because the peak intensities of the Kiessig fringes decay rapidly, the data are typically plotted as the logarithm of the reflectivity as a function of q(see Fig. 2.5).

The procedures for fitting specular reflectivity data are well-established. Typical approaches require constructing a model SLD-density profile according to the SLD parameters known from the material composition and density, discretizing the model profile with different layers of materials defined by their SLD, thickness and roughness, changing the different parameters accordingly until a satisfactory fit with the experimental reflectivity curve is obtained.

2.4 Sample Preparation

A series of porous AAO samples were prepared for reflectivity study and small angle scattering analysis.

The sample preparation and test method is explained in the following text and figure 2.4.

Figure 2.8 Sample preparation and test procedure

2.4.1 AAO prepared for reflectivity study

NR and XRR require smooth substrates. So the AAO was prepared using aluminum- coated silicon wafers.

2.4.1.1 Silicon wafer cleaning

The wafer is 3-in-diameter single crystal (111) obtained from Wafer World Inc. (West

Palm Beach, FL, USA).

A standard Piranha clean procedure was applied to clean each wafer. The Piranha Clean solution is a mixture of H2SO4 (98%) from Pharmco-Aaper, Inc. (Brookfield, CT. USA) and

H2O2 from Fisher Scientific, (Fair Lawn, NJ. USA) in a volume ration of 3:1.

[4][40][40][40][40] [40][40][40][40](Kern) Each wafer was cleaned in Piranha cleaning solution for 10-20 minutes, and rinsed with DI water for three times. Then the wafer is rinsed

30 with 200 proof absolute ethanol from Pharmco-Aaper, Inc. (Brookfield, CT. USA). After the rinse is finished, remove the wafer and blow them dry with the N2 gun. Piranha is highly reactive, hot and corrosive, and attacks organic materials. This procedure must be therefore conducted carefully in the fume hood with full personal protective equipment.

2.4.1.2 Al-coated silicon wafers

The silicon wafer was coated by Al using electron beam evaporation, which is performed at Advanced Materials Characterization Center at the University of Cincinnati. The instrument is Temescal FC1800 e-Beam Evaporator from AircoTemescal, (Berkeley, CA. USA.)

Electron beam evaporation is a physical vapor deposition method. In the evaporation process, a block of the material is heated by the electron beam in a vacuum chamber, and evaporated freely in the chamber, the vapor deposits on the wafer forming a smooth, uniform coating.

2.4.1.3 Anodization of Al-coated silicon wafer

After the wafer is coated with aluminum, anodization is conducted on the wafer. The Al substrate is anodized under constant potential in aqueous sulfuric acid (20% wt) using a power supply. The cell is a two-electrode system consisting of graphite acting as the counter electrode and Al as the working electrode. The morphology of porous aluminum oxide is characterized by

SEM (the FEI XL30 ESEM). The thickness and the composition of aluminum oxide formed at different potentials are studied by XRR and NR.

2.4.2 AAO prepared for reflectivity study

Sample used for USAXS study is prepared from aluminum foil (99.45%), 0.025 mm thick,

30×30 cm, purchased from Alfa Aesar Inc. (Ward Hill, MA, USA.) The anodization follows same procedure described in 2.4.1.3.

After anodization, sealing was performed on the foil samples for the purpose of studying effect of sealing on morphology of the AAO. Sealing was done in hot water (100 °C), hot nickel acetate solution (5g/L), and cold nickel acetate solution (5g/L dilute and 50g/L saturated). The anodized Al foil immersed into different sealing agents for 30 minutes, and air dried overnight following the sealing process.

2.4.3 Characterization of porous AAO films

2.5 X-ray reflectivity

The x-ray reflectivity (XRR) measurements were performed at X'Pert PRO Materials

Research Diffractometer (MRD) (figure 2.5), at the Advanced Materials Characterization

Center (AMCC), University of Cincinnati, Cincinnati, OH, USA. Cu-kα X-rays (λ= 1.54 Å) were employed.

Figure 2.9 X’ Pert Pro MRD equipment.

2.5.1 Neutron reflectivity

The neutron reflectivity measurements were performed at Surface Profile Analysis

Reflectometer (SPEAR), Manuel Lujan Jr. Neutron Scattering Center (LANSCE), Los Alamos

National Laboratory, Los Alamos, NM, USA. The schematic diagram of SPEAR is shown in

Figure 2.8. The neutrons are moderated by partially coupled liquid hydrogen moderator at 20 K.

A linear 3He position-sensitive detector is utilized. The moderator-to-detector distance is 12.4 m.

Figure 2.10 Schematic diagram of SPEAR. SPEAR is a time-of-flight (TOF) neutron reflectometer that is ideally suited to study thin

(5 Å - 3000 Å) organic and inorganic layers in a variety of different environments.

The instrument uses an unpolarized neutron beam to study solid/solid, solid/liquid, solid/gas, and liquid/gas interfaces. SPEAR is utilizes two choppers to define a typical wavelength range of 1.5 Å to 16 Å. The underlying principle is that the speed of the neutron varies as the inverse of the wavelength, which is directly related the time taken by the neutron to travel from the source to the detector. With this polychromiatic beam, a range of momentum transfer vectors (qz) can be measured without altering the angle of the incident beam.

An important feature of SPEAR’s design is that the beam is inclined to the horizon at 0.9°.

This inclination allows for reflectivity measurements from liquid/air interfaces. Using a position sensitive detector, the TOF and reflected position of individual neutrons can be measured and converted to a wavelength and reflection angel. With this arrangement, good statistics can be obtained down to a minimum reflectivity of about 5×10-7 in 3-4 hours.

2.5.1.1 Test Procedure

The sealing effect on film structure was studied with NR. NR data were obtained from the as-prepared film, the film after exposure to D2O, and during the immersion in D2O as an in-situ study. A liquid cell was used to carry out the in-situ experiment.

2.5.1.2 Data Acquisition and Analysis

Neutron reflectivity, R(q), which is defined as the intensity ratio between reflected and

incident neutron beams is measured as a function of the scattering vector, ( ), where

is the angle of incidence (and reflectance) and is the neutron wavelength. As introduced above, SPEAR is TOF reflectometer. q is varied by measuring intensity for a range of different wavelengths at a fixed angle of incidence. The wavelength range is 1.5 to 16 Å. To obtain a sufficient range of q, the reflectivity curves were merged from three angles of incidence. The q- dependence of the reflectivity depends on the neutron scattering length density (SLD) profile normal to the substrate. The neutron SLD is a function of the density and atomic composition, expressed as

∑ , where s the coherent scattering length of the atom, is

density, is molecular weight, is Avogadro’s number. The calculated neutron and X-ray

SLD values for the materials used in this study are listed in table 2.1.

Table 2.1 Calculated neutron and X-ray SLD values for the materials used in this study (calculated from http://www.ncnr.nist.gov/resources/sldcalc.html)

Density 6 -2 X-ray SLD (Cu) Material 3 10 ×Neutron SLD(A ) (g/cm ) 105×Real(A-2) 107×Imaginary(A-2) Si 2.33 2.07 2 4.64

SiO2 2.2 3.47 1.89 2.45 Al 3.7 2.08 2.24 2.22

Al2O3(crystalline) 3.9 5.6 3.28 3.82

Al2O3 (Amorphous) 3.0 4.31 2.52 2.94

D2O 1.1 6.33 0.936 0.298 H2O 1 -0.56 0.946 0.301

2.5.2 Ultra Small Angle X-ray Scattering

Ultra small angle x-ray scattering (USAXS) measurements were carried out at synchrotron

Beamline 15ID-D (figure 2.7) at ChemMatCARS, Advanced Photon Source, Argonne National

Laboratory (Argonne, IL). At this beamline, a Bonse Hart camera collects small-angle scattering

data over a q range from 0.0001 to 1 Å -1, and stability and reliability over extended running

periods, the operational configurations include a one-dimensional collimated (slit-smeared)

USAXS, two-dimensional SAXS, and USAXS imaging. This instrument allows one to resolve

objects with sizes from 10 nm to approximately 2 mm. Data points were distributed in

approximately logarithmic scale across the q. The x-ray energy used was 18 keV. USAXS data

were corrected for all instrumental effects, including slit smearing. Data analysis was done with

the Irena software package.

Figure 2.11 Schematic of USAXS instrument Reference 1. Beaucage, G. and Schaefer, D. W. (1994). Structural studies of complex systems using small- angle scattering: a unified Guinier/power-law approach. Journal of Non-Crystalline Solids, 172 174 (1994), 797- 805. 2. Ryong-Joon, R.(2002). Methods of X-ray And Neutron Scattering In Polymer Science ed. Topics In Polymer Science. New York, US: Oxford University Press. 3. Russell, T. P.(1996). On the reflectivity of Polymers: Neutrons and X-rays. Physica B: Physics of Condensed Matter, 221(1-4), 267-283. 4. Kern, W. (1993). Handbook of semiconductor wafer cleaning technology science, technology, and applications. Park Ridge, New Jersey: Noyes Publications.

3. Results and discussion

3.1 Aluminum Oxide Film structure

As described in chapter 2, reflectivity experiments can quantitatively determine the composition, thickness and roughness of a multi-layer material. To study the AAO structure, both XRR and NR were used. In this section, the Al coated wafer was studied before anodization and after anodization to determine the composition and porosity of AAO film.

In order to determine the hydration and the porosity of the AAO film, the following

strategies have been employed. From equation 2.3 ( ∑ ), and equation

2.4 ( ∑ ), the SLD of neutron and X-ray of the same layer can be

combined to calculate the hydration of a specific layer when the composition of the layer is assumed as Al2O3 H2O

∑ (3.1) ∑

Assuming that the oxide layer is Al2O3 H2O, then

∑

So we can determine easily.

Also for a porous material, the SLD can be calculated as equation 3.2.

( ) (3.2)

In equation 2.3 and 2.4, if the neutron and X-ray SLD of Al2O3 is known or calculated

from the fitting, then we can calculate the density of the material from equation 3.2, with

known.

3.1.1 Native oxide film formed on Al

Al coated silicon wafer was studied before anodization to determine the native oxide film composition. Figure 3.1 shows the SLD profile of the same sample from XRR and NR data.

(a) (b)

Figure 2.1 Reflectivity data of bare Al coated silicon wafer (a) NR data curves (red) and fitted data curves (purple) (b) NR SLD profile (c) XRR data curves (red) and fitted data curves (purple) (d)XRR SLD profile.

From the SLD profile, we can get the thickness of the native oxide formed on Al is 35 Å ±

20 Å. The SLD we obtained from XRR is (26.2±0.5) ×10-6 Å-2, from NR is (4.53±0.35) ×10-6

-2 Å . Both values are lower than the SLD of crystalline Al2O3, which indicates either the oxide is amorphous or it’s hydrated, because the scattering length of water is very low due to the

38 negative scattering length of hydrogen. The composition of the layer is assumed as Al2O3

H2O,

( ) ( )

If the degree of hydration of the film is zero, which means the oxide layer is not hydrated, then

( )

By comparing ( ) ( ), 0.08, which indicates the

film is not hydrated.

In order to calculate the density of the film, from equation 2.3, ∑ ,

for Al2O3, So density

( )

From the above calculation, we conclude that the native oxide formed on Al is amorphous Al2O3, and not hydrated, with a density of 3.1 g/cm3.

3.1.2 AAO layered structure after anodization

Figure 3.2 shows the AAO layer formed after anodization. The sample (Al-coated silicon wafer) was anodized in sulfuric acid at 15 V for 15 s at room temperature. The Al has been transformed into a porous layer with a thickness of 120 nm, which is given in the SLD profile in figure 3.2.

Figure 2.2 (a) XRR data and (b) XRR SLD profiles (c) NR data and (d)NR SLD profiles of the pure Al- coated Si wafers, and the anodized sample. The anodized sample (in red, anodized in sulfuric acid at 15 V for 15 s) is compared with the bare Al sample (in black, 100-nm pure Al coated wafer). A four-layer (on top of Si) model is required to obtain reasonable agreement with the experimental data for the anodic samples. With same method described in 3.1.1, the composition, hydration and density of the porous AAO layer is being calculated. The SLD we obtained from XRR is (23.2 ± 0.5) ×10-6 Å-

2 -6 -2 , from NR is (3.7 ± 0.35) ×10 Å . The composition of the layer is assumed as Al2O3 H2O,

which gives x = 0.29

With a chemical composition of Al2O3·0.29H2O, the AAO molecular weight can be calculated, M=107. With NR and XRR scattering length and SLD known, the density of the

3 Al2O3·0.29 H2O we calculated is 2.8 ± 0.14 g/cm . 3.2 Study of Sealing

3.2.1 In situ NR study of cold sealing

One step sealing: The anodized Al-coated Si wafer sample was mounted in the liquid cell and tracked for 9 h for the in situ sealing study. Two Al-coated wafers were anodized in sulfuric acid under 15 V for 15 s and 20 s. The dry samples were put in the liquid cell filled with Nickel

Acetate solution (50 g/L NiAc D2O). The data were collected over 9 hours. Four snapshots were collected at 0, 3, 6, 9 hours. From figure 3.3, the porous AAO layer does not swell because the thickness of AAO layer is not changed. After the initial increase at 0.5 h, the SLD then changes slowly. Also the thickness and SLD of the dense Al2O3 remain the same, which indicates the

D2O has not penetrated this layer. However, the SLD of AAO monotonically increases as the pores fill with D2O. This increase can be attributed to the penetration of solution into the walls of the AAO porous film.

(a)

(b)

(c)

Figure 1.3 In situ NR data and SLD of sample 1 and 2 with sealing duration of 9.5 hours; (a) anodized in sulfuric acid at 15 V for 15 s (b) anodized in sulfuric acid under 15V for 20 s. (Fitted with a background -6 of 1.5 10 ) (c)Dependence of SLD increase on time.

Two-step sealing: In order to eliminate and compare the effect of D2O filling and penetration of AAO, a two-step sealing was studied. Sample 3 was anodized at 15 V for 15 s in

20% wt sulfuric acid. First, the sample was put in liquid cell and immersed in D2O and examined with NR after immersion for 2 hours，then D2O was expelled from the cell, and the

42 cell was rinsed by NiAc solution several times, then NiAc solution was injected, and the sample was examined with NR again. The result of this experiment is shown in Figure 3.4.

Figure 1.4 NR data of sample 3 with two-step sealing (The sample was anodized at 15 V for 15 s in sulfuric acid).

From Figure 3.4, the SLD increases dramatically after immersion in D2O, but no change is seen after immersion in NiAc solution. Assume after D2O immersion, the D2O is completely replaced by NiAc solution. Both NR and SLD profiles overlap for these two procedures. With immersion of 12 hours, swelling occurs as revealed by the increasing SLD and thickness (green line). From figure 3.3 and 3.4, the results of both two-step sealing and one-step sealing are very similar, indicating the increase of SLD is primarily due to D2O penetration. These results will be furthered analyzed in Chapter 3.3.1 for porosity study.

3.2.2 Sealing study from USAXS

We also investigated the sealing strategy (cold nickel acetate, hot nickel acetate, and hot water) with USAXS. With the exception of hot NiAc sealing, there is little change in the

USAXS data after sealing (Figure 3.5). The pores are not filled with sealant. In the case of hot

NiAc sealing, however, there is only a faint signature of the original pore structure.

Figure 1.5. USAXS data on through-thickness AAO samples sealed by different methods. The solid lines are the 3-level unified fit. 3.3 Pore growth study

3.3.1 Porosity of the AAO layer from Neutron Reflectivity

To calculate the porosity, we assume after samples were exposed in liquid cell with NiAc, the pores of AAO are immediately filled with the solution, due to the high SLD of D2O, there’s a big increase of SLD of the AAO layer. Assume that the SLD of the AAO layer is composed by the SLD of D2O filled in the pores, and the skeleton SLD, which is the porous AAO. After a long time fully immersion, D2O penetrates into the micro pores in the walls and reaching to a steady state. Figure 3.6 is the NR data and SLD change for sample 1 and 2 during the first 0.5 hours of in situ one step sealing.

(a)

(b)

Figure 1.6 Neutron reflectivity and SLD profiles of the anodized sample1, 2 after 0.5 h. (a)Sample 1 anodized in sulfuric acid at 15 V for 15 s and (b)sample 2 anodized in sulfuric acid at 15 V for 20 s. Assume the porosity of the sample will not change during sealing. However, due to the filling and D2O penetration of the walls, the calculated pore volumes will increase till it reaches to a final steady state, which can be attributed as the porosity. The SLD of AAO layer before and after sealing in NiAc solution are calculated using equation 3.2.

( ) ,

(3.3)

Table 3.1.1 Modeling neutron SLD of porous AAO layer before and after sealing

106×Neutron SLD (A-2) Long-time immersion(9 Sample Dry 0.5h hours for sample 1 and 2, 12 hours for sample 3) 1(one step sealing, anodized for 15 s) 3.67 ± 0.05 4.65 ± 0.21 5.67 ± 0.21 2(one step sealing, anodized for 20 s) 3.72 ± 0.05 4.91 ± 0.30 5.87 ± 0.20 3 (two step sealing, anodized for 15s) 3.70 ± 0.03 4.97 ± 0.22 5.74 ± 0.13

From equation 3.3 and table 3.1, we can calculate the porosity, at full hydration of AAO film, and after 12 hour immersion of

D2O or NiAc, .

The porosity is the same for anodization after 15 s and 20 s, which indicates after formation, then only the depth of the pores increases, not the width.

3.3.2 Evolution of Porosity

The porosity of the AAO film was also studied on aluminum foil (2×2 inches, 0.001 inch in thickness). The aluminum foil is anodized in 20 wt% sulfuric acid under 15 V from 10 s to

120 s at a 10 s intervals. The evolution of porous structure is revealed by the USAXS data revealed in figure 3.7. The USAXS profile indicates that the intensity of the peak increases as anodizing time increases due to pore formation and evolution.

Figure 1.7 USAXS profile as a function of anodizing time for Al foils in 20 wt% sulfuric acid under 15V.

The analysis of USAXS data is done with unified fit in Irena package for Igor, using a three level model [1]. Using code imbedded in the Irena package form Argonne National

Laboratory, the parameters for a correlated pore structure were extracted [2]. [41] A representative unified fit of the AAO for all three levels are shown in Figure 3.8. The sample is anodized in 20 wt% sulfuric acid under 15 V for 110 s. From fig. 3.8, the level one is fitted with correlation parameters ζ of 261 Å, which takes account of the interference between the pores.

All USAXS data in figure 3.7 are fitted with the same approach.

Figure 1.8 Unified fit for Smeared USAXS data of aluminum film anodized in 20 wt.% sulfuric acid under 15V for 110 second. From the SEM pictures we get, there is no highly ordered structure, however, the SAXS profile shows a very similar pattern as an ordered structure [3]. A method is developed to characterize the AAO structure [4]. The AAO structure can be modeled as a hexagonal array of pores in each hexagon, shown in figure 3.9. The correlation parameter ζ is interpreted as interpore distance, lp is the pore diameter and ls is the wall thickness.

Figure 1.9 Hexagonal structure of porous AAO Though the sample is not highly ordered as this hexagonal model, we use this model as an approximate approach to analyze the USAXS data. For a hollow cylinder shell structure, the radius of gyration Rg is the same as the radius of the inner cylinder, which is the pore radius, so the pore diameter lp has the following equation,

(3.4)

The correlation length, ζ, is approximately the inter-pore distance, so from Figure 4.9, the wall thickness, ls, can be calculated as follows,

(3.5)

From equation 2.2, the Guinier prefactor G has the equation below,

(3.6) where V is the volume of pores, ∆ρ is the average scattering length density of Al2O3, and Nv is the number density of the pores. From this equation, the pore length, can be calculated as follows,

(3.8)

Where area is √ , and H is the thickness of the sample after anodization.

( ) (3.9)

Substitute Nv and V in equation 3.6 with equation 3.8 and 3.9, the pore length can be

calculated as

√ ( ) ( ) (3.10)

Table 3.2 SAXS fitting data for level one in figure 3.7 and pore length calculation

B Normalized Thickness of Calculated pore Sample R (Å) G(cm-1) (cm- P (Å) Packing factor Pore g Sample(mm) length(mm) 1Å-4) length(mm) Pure 1.56e- 7165.81 2.77e+7 3.35 N/A N/A N/A N/A N/A Al 05 AAO 72.13 41.76 0.25 0.48 262.52 4.77 0.0264 0.000018 0.003 (10 s) AAO 1.34e- 73.29 112.19 4 262.27 8 0.0284 0.000029 0.004 (20 s) 05 AAO 3.07e- 76.21 304.06 4 264.77 8 0.0284 0.000045 0.006 (30 s) 05 AAO 6.12e- 72.65 699.82 4 273.00 8 0.0272 0.000076 0.011 (40 s) 05 AAO 4.13e- 81.23 588.58 4 272.56 8 0.0272 0.000056 0.008 (50 s) 05 AAO 7.80e- 73.87 1375.60 4 270.23 8 0.0284 0.000104 0.015 (60 s) 05 AAO 6.97e- 75.67 1764.70 4 277.87 8 0.0274 0.000114 0.017 (70 s) 05 AAO 9.99e- 74.37 2736.91 4 275.40 8 0.0276 0.000146 0.021 (80 s) 05 AAO 8.70e- 74.49 2146.50 4 271.72 8 0.0276 0.000127 0.018 (90 s) 05 AAO 1.03e- 75.83 2205.92 4 273.91 8 0.028 0.000126 0.018 (100 s) 04 AAO 1.25e- 75.92 3914.3 4 272.52 8 0.0288 0.000169 0.023 (110 s) 04 AAO 1.89e- 71.14 5187.3 4 264.36 8 0.0284 0.000295 0.030 (120 s) 04

The Unified fitting data for level one is listed in Table 3.2. Based on the collected data and equation 3.5, 3.6, the pore length is also calculated and listed above. Because of the discrepancy in calculated pore length and the thickness of the sample, a normalization of pore length has been done. Figure 3.10 shows the pore length dependence on anodization time. From Figure

3.10, the pore structure developed within 10 seconds of anodization, and then only grew in depth. The pore growth in vertical direction followed a linear relation.

Figure 1.10 Dependence of interpore distance, wall thickness on anodization time. The AAO was anodized under 15V in 20% wt sulfuric acid for 120s.

Figure 1.11 Dependence of pore length on anodization time. The AAO was anodized under 15V in 20% wt sulfuric acid for 120s. The porosity can also be calculated from the model in figure 3.9. The porosity of a hexagonal structure is given by

( ) (3.8) √

φ is the porosity of the AAO film, rp is the radius of the pores, ζ is the inter-pore distance[5].

Using the average pore radius (75 Å) and inter-pore distance value (270 Å), we can get the porosity φ is 27.6%, which agrees with the result calculated from NR experiment in chapter

3.1.1 where the porosity gotten is 32.6% after a 12 hours fully immersion.

Reference 1. Beaucage, G. and Schaefer, D. W. (1994). Structural studies of complex systems using small- angle scattering: a unified Guinier/power-law approach. Journal of Non-Crystalline Solids, 172-174 , 797- 805. 2. Ilavsky, Jan, and Peter R. Jemian. "Irena: tool suite for modeling and analysis of small-angle scattering." Journal of Applied Crystallography 42.2 (2009): 347-353.

3. Dore, J. C., Benfïeld, R. E., Grandjean, D., Schmid, G., Kröll, M., & Bolloc’h, D. L. (2002). Structural studies of mesoporous alumina membranes by small angle X-ray scattering. Studies in Surface Science and Catalysis, 144, 163-170. 4. Naiping Hu, Xuecheng Dong, Xueying He, Sandip Argekar, Yan Zhang, James F. Browning and Dale W. Schaefer. Interfacial morphology of low-voltage anodic aluminium oxide. J. Appl. Cryst. (2013). 46. In press. 5. Nielsch, Kornelius, et al. "Self-ordering regimes of porous alumina: the 10 porosity rule." Nano letters. 2.7 (2002): 677-680.

4. Conclusion and future work

In this study, thin porous AAO suitable for NR and XRR study was successfully fabricated. An in-situ NR study and XRR study on AAO film were first successfully applied to determine the layer structure of AAO film. The observed AAO films show a very thin “barrier” layer at the metal interface and a much thicker porous layer at the air surface. This structure is consistent with the preponderance of the literature, although the thickness we observe is quite small, only 3-5 nm, compared to previous reports using imaging. We also find the barrier layer is only slightly denser than the porous outer layer, indicating the dense layer is not of crystalline structure.

The in-plane structure was determined by USAXS. A new method has been proposed to and successfully applied to analyze the USAXS data. Although the hexagonal structure often reported in the literature is not evident in our SEM images, the reflectivity profile attained from

USAXS show similar trend with existing results. USAXS shows that the pore diameter and interpore spacing are fixed in the first 10 s of anodizing, after which the pores linearly penetrate the aluminum with constant pore diameter and wall distance.

The cold NiAc sealing method does not have any effect on the aluminum film at in-situ situation, and the increasing SLD in sealing procedure is mainly due to D2O penetration into the pores of the sample.

Although this study covers limited number of variables relevant to production of AAO films, it does show that scattering methods are powerful tools to determine quantitative structural parameters.

In order to further study the sealing effect, electrochemical methods should be explored, by comparing the corrosion protection of the sample, more in detail the effect of sealing

54 methods are able to be analyzed and revealed. The model used for porosity study is based on a highly ordered hexagonal structure; better model needs to be developed for a more accurate study on porosity of structure not highly ordered.