8 Anodic Oxide Nanostructures: Theories of Anodic Nanostructure Self-Organization Naveen Verma1, Jitender Jindal2,*, Krishan Chander Singh1 and Anuj Mittal1 1
2
Department of Chemistry, Maharshi Dayanand University, Rohtak, India Department of Chemistry, Rao Pahlad Singh Degree College, Balana, India
Abstract This chapter reviews the morphologies, growth kinetics, and theories of growth kinetics of anodic oxide films. Experimentally, it is possible to control the growth conditions of these films to yield several distinct morphologies, including orderly arranged nanoporous and nanotubular films. Fundamental processes that lead to self-ordering of nanoscale features, including interfacial reactions, ionic transport, stress generation, and space charge accumulation, are discussed. Various theories are included to explain the growth mechanism of oxide film. Keywords: Anodic oxide film, anodization, barrier film, porous film, EIS
8.1 Introduction Fabrication and characterization of anodic oxide film on valve metals like aluminium, niobium, tantalum, and zirconium is a research area of keen interest since the last 60 years. In recent years, the nanoporous anodic oxide films become an important ingredient of modern technology. Nanostructured metal oxide materials have shown wide application in the field of dye-sensitized solar cells [1], displays and smart windows [2], biosensors [3], lithium batteries [4], and supercapacitors [5]. Indeed, to fulfill the great promise and expectation of nanomaterials, the analysts
*Corresponding author:
[email protected] Liang Li and Qing Yang (eds.) Advanced Coating Materials, (235–254) © 2019 Scrivener Publishing LLC
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have developed various new manufactured techniques and patterning methods, for example, self assembly [6], lithography [7], or templateand membrane-based synthesis [8]. Among these synthesis methods, the anodizing methodology is a less expensive and sophisticated technique to build the metal oxide nanomaterials of the desired dimensions (size, structure, and morphology) on which the material properties are highly dependent. Anodic metal oxides are applied in different fields, for example, in the prevention of corrosion of metal substrates from their service environment [9], forming capacitor dielectrics [10, 11], templating nanomaterials [12–17], and in numerous different fields, for example, catalysis, optics, and electronic gadgets [18–21]. Some groups of inner transition metals of periodic table such as IV B (Ti, Zr, Hf), V B (V, Nb, Ta), and VI B (Cr, Mo, W) have the following common properties. (i) They are refractory metals, which means their melting points are higher than setting point of iron (melting point, 1539 °C). Their melting point ranges from 1660 °C for titanium to 3410 °C for tungsten. (ii) They are reactive and combine strongly with oxygen, nitrogen, carbon, and other nonmetals. (iii) They exhibit valve action properties, which could be defined as when acting as cathode these metals allow current to pass but when acting as anode they prevent passage of current owing to rapid building up of an insulating anodic film. Due to this property, these are called valve metals. Anodic oxidation of valve metals plate in an acidic aqueous solution is a widely accepted technique for preparing oxide films with high bonding strength and has become an important process for improving the surface chemistry of metals. The surface characteristics of film formed (porous or non porous) is highly influenced by the anodization conditions. The best known porous anodic oxide film has been found to be formed on aluminum commonly called as porous anodic alumina [22–25] and is currently used in industries. It can be used as a perfect template for the synthesis of nanomaterials, for example, nanoparticles, nanowires, and nanotubes. Porous anodic oxide films have also been accomplished on surfaces of numerous different valve metals, for example, titanium [26–28], hafnium [29], niobium [30], tantalum [31], tungsten [32], vanadium [33], and zirconium [34]. A lot of work has been done, but there is still much work will be possible regarding the optimization of the anodization conditions to manage precise control of the growth of the anodic metal oxide in different electrolyte, finally improving and widening their scientific and industrial applications.
Anodic Oxide Nanostructures 237
8.2 Anodization An anodic oxide film can be grown on certain metals like aluminium, niobium, tantalum, zirconium, titanium by an electrochemical procedure called anodization. The procedure is called “anodization” because the part to be treated forms the anode terminal of the preparing electrical circuit. Anodization can be used to build the thickness of the characteristic oxide layer on the surface of metal parts, which can increase corrosion and wear resistance and give better adhesion to paint and primers than bare metals. Anodic films are mainly much stronger and more adherent than most kind of paint and metal plating, making them less likely to split and peel. It has been widely applied to protect metal alloys but is not a useful treatment for iron or carbon steel because these metals exfoliate when oxidized; that is, the iron oxide (otherwise called rust) pieces off, constantly exposing the underlying metal to corrosion. In the process of anodization, a small piece of metal foil is used as anode by connecting it to the positive terminal of a DC power supply. The cathode is connected with the negative terminal of the DC power supply. The cathode can be a plate of carbon, platinum, lead, nickel, and stainless steel, that is, any electronic conductor that is inert in the anodizing bath. When the circuit is shut, electrons are withdrawn from the metal at the positive terminal, permitting ions at the metal surface to react with water to form oxide layer on the metal. The electrons return to the bath at the cathode where they react with hydrogen ions to make hydrogen gas. A bath electrolyte is selected in which the oxide is insoluble or at least dissolves at a lower rate than its deposition and eventually an adherent oxide layer grows. The bath composition is the primary determinant of whether the film will be a barrier or porous. Barrier oxide grows in nearneutral solutions in which the metal oxide is hardly soluble. Porous oxide grows in acidic electrolyte in which oxide not only is deposited but also dissolves. Thus, depending on the electrolyte used, two types of anodic metal oxide layers could be produced; barrier type films with uniform thickness in a near-neutral electrolyte and porous anodic oxide films containing dense nanoscale pores in an acidic or alkaline electrolyte [35, 36].
8.3 Barrier-Type Anodic Metal Oxide Films When metals are anodized in the electrolytes where the formed oxide film is completely insoluble, nonporous barrier-type films are achieved. It has
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been generally accepted that during the formation of nonporous anodic metal oxide in a near-neutral electrolyte, metal oxide simultaneously grows at both the electrolyte/oxide and oxide/metal interfaces. Oxygencontaining anions O−2/OH− mainly coming from dissociation of water at the electrolyte/oxide interface move inward to react with metal at the oxide/ metal interface to form oxide. At the same time, metal cations move outward from the metal surface to react with water at the electrolyte/oxide interface to form oxide but as the oxide is insoluble in a near-neutral electrolyte so a barrier layer forms. This type of nonporous barrier layer can normally increase the resistance to corrosion and wear of metals. Barrier oxide layer protects the surface of metal for further oxidation by its service environment and is an excellent electrical insulator.
8.4
Porous-Type Anodic Metal Oxide Films
When metals are anodized in acidic solutions, due to the solubility of metal oxide in the electrolyte, instead of nonporous uniform barrier oxide, a duplex structure with a barrier layer close to the metal surface, and a porous layer grown on top, could be formed. Examples of this type of electrolyte are numerous; the most commonly used being sulphuric acid (H2SO4), phosphoric acid (H3PO4), chromic acid (H2CrO4), and oxalic acid (H2C2O4.2H2O). Because of the high affinity of aluminum surfaces for oxygen, the metal is always covered with a highly resistant oxide film; the improvement of this natural oxide film to produce an anodic oxide film, which has attractive finish, excellent corrosion resistance, and possesses other commercially important qualities, is the aim of the anodizing industry these days. Different from the barrier-type anodic films, the thickness of the poroustype anodic alumina films mainly depends upon the applied current density/voltage and anodization time. A high voltage is not needed to grow a thick porous-type oxide film as in the case of barrier-type oxide film. For porous films, apart from the current density and anodization time, electrolyte used and temperature are also an important criterion in determining the film thickness. At low temperature, that is, 0–5 °C, the porous film formed is thick and compact. At high temperature, that is, 60–75 °C, the porous film formed is thin and nonprotective. From this, it is clear that the temperature plays an important part not only in the formation of the porous films but also in its subsequent existence.
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Due to outstanding thermal, chemical, and mechanical stability, highly ordered anodic aluminum oxide has been widely used as a template in synthesis of nanomaterials in recent years [37–39].
8.5 Theories or Models of Growth Kinetics of Anodic Oxide Films and Fundamental Equations for High-Field Ionic Conductivity The process of anodization involves submerging two electrodes in a conducting electrolyte and applying a bias between the two electrodes. The positive terminal of electrode is the anode and negative terminal electrode is the cathode. Oxidation reactions dominate at the anode, while reduction reactions dominate at the cathode. When the anode is metal foil, the applied bias generates an intense electric field across the oxide and causes the oxide to grow as the metal and oxygen atoms migrate under the influence of the electric field. This process of ionic conduction in solids has been studied for many years, but it is still debate. The ionic transport or ionic conduction mechanism is responsible for growth of anodic metal oxide films, which has been studied for more than 50 years. The mechanism of high-field ionic conduction depends upon the postulate that the ionic current produced by a given electrostatic field in the oxide depends on the film formed. The ionic conductivity depends upon the electric field strength across the oxide film. Electric field strength is highly enough to prevent the movement of cations against the field strength in high-field ionic conduction, but movement of cations against the field is negligible in low-field ionic conduction. In the following theories, the range of electric field strength lies between 106 to 107 V/cm in high-field ionic conduction mechanism. These fields are sufficient enough to prevent ionic motion against the field direction. The following models are used to study the mechanism of high-field ionic conduction in oxide films.
8.5.1 Guntherschulze and Betz Model At the time of oxide arrangement, a high field over the film is created and metal cations transfer through the film. Upon film thickening, the electric field diminishes until cation transfer gets to be deficient and the film growth stops. The current at low field strength is accepted to be an electronic one. As indicated by Guntherschulz
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and Betz [40], the current under high field conditions takes the straightforward form:
i
e
E
or
i
exp E
(8.1)
In equation (8.1), i represents ionic current density; E denotes electric field strength; α is the jump probability of a cation interstitial. α and β are calculated by the following equations:
0.24ekT / a2
o
e
o / kT
3ae /8kT
(8.2) (8.3)
In equations (8.2) and (8.3); e is the electron charge, k is Boltzmann’s constant, T is the absolute temperature, a is one half of the jump distance, ψo is the minimum potential barrier, and is the time of vibration. Guntherschulz and Betz equation evidences that the current will decay exponential with the film thickness, and the validity of this mathematical statement has been confirmed at various current densities and for different natural corrosive arrangements. In an electrochemical reaction, charge transfer is controlled by the step, which has the highest potential energy regarding the initial state, that is, the rate-determining step. Following rate-deciding step is utilized to look at the reaction environment: (i) Ion transfer over the metal/metal oxide interface. (ii) Ion transfer over the oxide bulk. (iii) Ion transfer over the metal oxide/electrolyte interface producing a solvated ion. Theoretical justification of the exponential relationship proposed by Guntherschulz and Betz has been based upon a combination of (i) and (ii) as the rate-determining step.
8.5.2 Cabrera and Mott Model The kinetics of low temperature oxidation of metal nanoparticles acquires of practical importance with the rapidly developing nanotechnology.
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The hypothesis of oxidation in the thin films was initially formulated by Cabrera and Mott. Cabrera and Mott’s model accepts a high field exists over the film and can be allowed to as high field model. This model is in light of the electron transport either by tunneling or by the thermionic emission from the metal into the oxide conduction band and ionic dispersion. Cabrera and Mott introduced logarithmic decay rate for the current as in mathematical statement. This equation is once in a while taken as the key expression for high-field ionic conduction. It was additionally considered as rate-determining step in ionic movement. Cabrera and Mott discussed the growth of very thin films formed both by anodic and by atmospheric oxidation, the electric field necessary being produced by the applied potential and absorbed oxygen, respectively. In this way, a steady-state current or thickness is never accomplished. Cabrera and Mott [41] examined that the electric field being delivered by the applied potential and consumed oxygen during the growth of thin film by anodic and atmospheric oxidation individually. At low temperature (particularly a function of the melt temperature of a specific metal), the growth rate of an oxide film is a component of the rate of ion dispersion through the developing film, which is an element of the electric field produced by ionic absorbates (for example, oxygen) on the surface and the bulk metal. The electric field alone is sufficient to drive oxide growth obeying logarithmic rate. At increased temperatures, the role of thermal energy turns out to be progressively the predominant component driving ion diffusion, especially as films turn out to be progressively thicker and the rate of development gets to be parabolic. A few essential assumptions were made: (i) The transfer of an ion over the metal/oxide interface is the rate-deciding step in oxide development. (ii) The transfer of a particle through the oxide is quick because of the bringing down of diffusion barriers by applied electric field. (iii) The field is adequately high to guarantee that insignificant measures of ions are moving against the electric field bearing. The hypothesis of Cabrera and Mott is valid for thin films, but there are a few expansions for thicker oxide film. The first assumption of Cabrera and Mott model is that electrons can pass openly from the metal to the oxide surface to ionize oxygen atoms. This establishes a uniform field
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Compenent parts 0 Vaccum level
Electron energy
Ø (5.2eV)
Ef
Conduction band
3d 4s
ΔØ
Eb
Ef
Eg (3.7eV) ΔØ
2p
3d Valence band
x Metal
Oxide
Absorbed O2 (surface)
Metal
Oxide
O2 (surface)
Figure 8.1 The shift of the oxide taken from [42, 43].
within oxide, which leads to a shift in the Fermi level of the oxide as demonstrated in Figure 8.1. The so called Mott potential Eb can be calculated as
Eb
1/e(
M
ox
)
This potential drives the moderate ionic transport over the oxide film. The electrons continue to cross the film to maintain zero electrical current. The electrons are assumed to pass through the film via tunneling within the Cabrera–Mott model. This assumption restricts the model to thin films. To extend the model to thicker films, one can assume electron transport via thermionic emission or through semiconducting oxides.
8.5.3 Verwey’s High Field Model Verwey’s HFM depicts the growth of passive oxide film on aluminum in which the rate-determining step is assumed to be the migration of aluminum interstitials through the oxide film. As indicated by Verwey [44], the restricting component is the rate at which ions move starting with one interstitial position then onto the next. He considered the energy barrier for ion movement through the oxide bulk, thought to be in a condition of electrical deficiency of bias, as the rate-determining step. Verwey model is appropriate for thick oxide film where electrical deficiency of bias exists in the bulk oxide.
Anodic Oxide Nanostructures 243
Verwey recommended that the difference was because of defects in the oxygen cross section, which made the ionic movement less difficult.
8.5.4
Young Model
In his model, Young [45, 46] considers both vacancies and interstitials to be mobile species. Young considers that vacancies can form at the metal/oxide interface and in the oxide itself via Frenkel defects. He considers the mobility of oxygen ions in the film to make the model more comprehensive. The information of Young [47] demonstrated that the Tafel slope is dependent on field. This dependence was accounted for quantitatively, over a range of temperatures, by an exact empirical equation of the form
i i0 exp
Q
E kT
E2
(8.4)
where Q, α, and are positive constants. The key result of Young’s model is that the concentration of ions can change with field strength (which is dependent on film thickness), subsequently leading to the formation of a space charge layer at one of the interfaces. This space charge layer isolates the oxide from the effects of the metal/oxide interface. Young model results that the energy can be controlled by two mechanisms: (i) Kinetics is controlled by the reaction rates at the metal/film interface when the film is thin and the space charge effects are negligible. (ii) Kinetics is controlled by the migration of the mobile species in the oxide for thicker films when the space charge isolates the effects of the metal/film interface. The Young model behaves similar to the Cabrera–Mott model for thin films because of the rate-determining step being the injection of metal ions at the metal/film interface. The key difference is that Young accounts for both metal vacancy and oxygen migration in the film. As the film grows, the rate-determining step becomes the migration of ions through the film. Although metal vacancy and oxygen migration are accounted for, the current is attributed to the motion of metal ions. The
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other defects merely contribute to the formation of a space charge that changes the rate-determining mechanism.
8.5.5 Dignam Model Dignam [48] proposed another hypothesis of ionic conduction of anodic oxide films on valve metals. The amorphous oxide film is generally made out of little crystalline units. Ion exchange from such units to another is strongly influenced by the distribution of particle inside of these units. Transient ionic conduction phenomenon and amorphous charging current arise as a result of the slow adjustment of the polarization of the medium to new conditions. Two mechanisms for the rate of change of the polarization are employed, one including thermally initiated ion migration and the other being related with the large energies dispersed during the ion transfer event. The dependence of ion current on the effective field strength for steady-state behavior is given by
is
i0 exp[ {
* s
E
(1 E
* s
)C )} / kT ]
(8.5)
where μ*s = (1 + δχs) μ*; io, Φ, μs and C are the four parameters to characterize the steady-state behavior, io is the preexponential element, Φ is an activation energy, μ*s is a effective charge activation distance product and a parameter C, to allow for nonlinear dependence of the net activation energy on field strength. Dignam [49, 50] required two extra parameters, that is, another effective charge activation distance product and a constant B related to crosssectional area of a polymeric unit. The reciprocal of Tafel slope for steady state would along these lines be
Bs
* s
kT
1
2 s* E C
(8.6)
The main objection of Dignam’s theory is the unreasonably large amount of slow polarization that must be postulated to account for the large increase in current in the constant field transients.
8.5.6
Dewald Model: (Dual Barrier Control with Space Charge)
Dewald [51, 52] examined the possibility of an increase in the activation distance as resulting from change in the rate-determining energy barrier from the metal/ oxide interface to the oxide bulk.
Anodic Oxide Nanostructures 245
On the off chance that the electric field present at the metal oxide interface is taken to be equivalent to that in the oxide bulk, the potential energy of the activated state at either position in the oxide relies on the viable bringing down of this energy by the ion moving with the field, which thus depends on the charge on the mobile ion and the activation separation at every obstruction. Let this field meet E, and let the potential energy of the activated state at the metal oxide interface and in the oxide bulk at zero field equal W and U individually. In this manner, the viable bringing down of every barrier is given by W-qaE and U-qa E where a and a are the activation separations at every energy barrier separately and q is the charge conveyed by the mobile ion. Henceforth, if W and U are comparable, a and a are not equivalent, then the effective field at every area will shift in an alternate way with the field. It is clear that if U-qa E >W-qaE, an aggregation of charge will happen at the second barrier; that is, a space charge (δ) will be set up. Dewald has shown that when space charge is high, that is, δ >> 1, for a tenfold increment in oxide thickness, the electric field strength increases 7%. For intermediate space charge, when δ ≈ 1, 2% increase in the electric field strength would be hypothetically expected, again a tenfold increment in oxide thickness. Dewald has additionally demonstrated that under intermediate space charge impact, the plot of the electric field strength versus log ionic current density would not be relied upon to be precisely straight. This hypothesis is able to predict a temperature-independent Tafel slope for a wide range of experimental data. It was assumed that when an ion traverses the film, it may very well experience deactivating collisions with the lattice and requires reactivation before proceeding. As a first approximation, deactivation occurs immediately after each activation. Dewald has demonstrated that the field (E) as a component of thickness (x) in the oxide is given by
E ( x ) E0
1
ln(1
n0 x )
(8.7)
where β = aq/kT
4 q/ where a is the bulk half distance; b is the entrance half distance; ε is the dielectric constant of the oxide; no is the number of mobile ion cm−3 at x = 0, and Eo is the field due to surface charge. The second term in the above equation is the space charge contribution.
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The constant Eo in the above equation is given directly by Cabrera and Mott in the form
E0
i0 kT ln bq N s sq
bq
(8.8)
where Ns is the number of ions/cm2 of the metal surface, νs is their vibrational frequency normal to the barrier, and Φ is the potential barrier opposing entry into the oxide as the rate-determining step. The field (E) due to Dewald is given by
E
E0
1
1
1
ln(1
) 1
(8.9)
)/
(8.10)
where n0 x and the Tafel slope by
T
kT 1 bq
a 1 ln(1 b
The second term in equation (8.10) is space charge contribution to the average field. δ is the dimensionless quantity and gives the importance of space charge. If δ
