Porous Anodic Metal Oxide Films

but pores can also open toward the surface owing to the similar open-circuit oxide dissolution, which is a hrst-order reaction with respect to proton activity and is hindered by incorporated electrolyte anions (Patermarakis and Moussoutzanis, 2002 Patermarakis et al., 2007). 

FIGURE 6.11 Schematic diagram shown migration processes accompanying the formation of porous oxide hlms by anodization of metals. 

FIGURE 6.12 SEM image of the imprints of anodic films of Al surface prepared in oxalic acid baths. (From Patermarakis et al., 2007. J. Solid State Electrochem. 11, 1191-1204, with permission.) 

FIGURE 6.13 Schematic representation of a section parallel to the pore axis of an elongated, columnar cell of a porous anodic alumina film. 

FIGURE 6.14 Variation of the reciprocal capacitance and resistance of anodic oxide film on pure aluminum rod specimen with film formation potential in contact with 0.5 M H2SO4 solution. (From Moon and Pyun, 1998. J. Solid State Electrochem. 2, 156-161, with permission.) 

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Finally, the electrochemistry of porous metal oxides prepared as films from anodic treatment of metal electrodes will also be discussed. Porous metal oxide films on electrodes have applications in a variety of fields, from corrosion protection to batteries and catalysis. 

On the other hand, although tin electrodeposition on metallic substrate deserved significant interest for developing batteries, it is worth to note that the anodic oxidation of a tin foil can produce porous electrodes [78]. Similarly, electrodeposited transition metals can be oxidized to form porous oxide films on flat metal substrate [38]. These porous transition metal oxides—although they are based on conversion reactions—exhibit a considerable pseudocapacitance. 

Anodic Oxidation. The abiUty of tantalum to support a stable, insulating anodic oxide film accounts for the majority of tantalum powder usage (see Thin films). The film is produced or formed by making the metal, usually as a sintered porous pellet, the anode in an electrochemical cell. The electrolyte is most often a dilute aqueous solution of phosphoric acid, although high voltage appHcations often require substitution of some of the water with more aprotic solvents like ethylene glycol or Carbowax (49). The electrolyte temperature is between 60 and 90°C. 

This is comparable to or slightly higher than the values reported for single crystal (11) and polycrystalline Ti02 (12), and much higher than those for the TiC>2 film electrode prepared by other methods such as chemical vapor deposition (13) and oxidation (14) and anodization (15) of Ti metal. The high efficiency of the dip-coated Ti(>2 film may be attributed to the porous nature of the film as described below. 

A schematic view of the cold cathode fabrication process is shown in Fig. 10.18. The cold cathode is fabricated by low pressure chemical vapor deposition (LPCVD) of 1.5 pm of non-doped polysilicon on a silicon wafer or a metallized glass substrate. The topmost micrometer of polysilicon is then anodized (10 mA cnT2, 30 s) in ethanoic HF under illumination. This results in a porous layer with inclusions of larger silicon crystallites, due to faster pore formation along grain boundaries. After anodization the porous layer is oxidized (700 °C, 60 min) and a semi-transparent (10 nm) gold film is deposited as a top electrode. 

Figure 2.15. The formation of an oxide film on an aluminum film by anodic oxidation. Arrows show the path PM or PN of oxygen through the compact oxide film AB (barrier layer). BC is the already formed porous oxide film, M and Hare located in the metal film (Hoar and Mott 1959).

Non-porous amorphous alumina films (several hundred Angstroms thick) can be prepared by the anodic oxidation of clean, high-purity Al. The oxide film is separated by dissolving any unoxidized metal in a mercury chloride solution. The oxide films are then washed in distilled water and collected on suitable electron microscopy grids. They are dried and heated to 800 "C to obtain amorphous AI2O3. High-purity wires of the desired metals can then be vacuum evaporated on to the films in an evaporator. These films can also be prepared using Al-nitrate,... 

The most commonly used hard templates are anodic aluminum oxide (AAO) and track-etched polycarbonate membranes, both of which are porous structured and commercially available. The pore size and thickness of the membranes can be well controlled, which then determine the dimension of the products templated by them. The pores in the AAO films prepared electrochemically from aluminum metals form a regular hexagonal array, with diameters of 200 nm commercially available. Smaller pore diameters down to 5 nm have also been reported (Martin 1995). Without external influences, capillary force is the main driving force for the Ti-precursor species to enter the pores of the templates. When the pore size is very small, electrochemical techniques have been employed to enhance the mass transfer into the nanopores (Limmer et al. 2002). 

Preparation of porous oxide films by anodization of metal electrodes... 

The electrochemical oxidation of the nickel is of special interest since it is a typical passivation metal in which very thin passive oxide films of a few nm thickness on the surface can cover the substrate metals efficiently. The passive oxide layer on the nickel was studied by Sikora and Mac Donald [118] who claimed that the passive film consisted of the inner nickel oxide of a barrier layer and an outer Ni(OH)2 porous or hydrated layer, in which the inner layer behaves as a p-type oxide with a cation vacancy. Oblonsky and Devine measured the surface enhanced Raman spectra of the nickel passivized in a neutral borate solution and estimated the amorphous Ni(OH)2 in the passive potential region and the NiOOH in the higher transpassive region [119]. Further, the passive films formed in the acidic and neutral solutions were assumed as partially hydrated nickel oxide [120,121]. The anodic film formed in the alkaline solution was assumed to be Ni(OH)2 in the... 

Taking into account the stoichiometric ratio of metal and oxygen atoms in A12O3 and WO3 one can calculate that the formed oxide is oxygen-deficient and has excess of W atoms. The oxygen deficiency can be explained by an appearance of oxygen vacancies. Moreover, a part of W atoms can be in unbound states like it takes place for Si atoms in anodized Al-Si alloys [4,5]. The higher concentration of excess W is in the bottom part of the porous anodic film. 

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