Thermally Prepared Oxide Films

Because of its importance as a dimensionally stable anode material (DSA) for the evolution of chlorine and oxygen, RuOj in pure form, or mixed with the oxides of other metals, including Ta, Ti and Ir (72), has received considerable attention. IrOj has also been investigated, but to a lesser extent, presumably due to its greater cost. Several books and review articles have been devoted to the physical, chemical, and electrochemical properties of these and related oxides (73). 

Films are prepared by thermal decomposition and oxidation of soluble precursor materials. For RuOj, the most common precursor is RuCIs xHjO, dissolved in water or an alcohol, such as isopropanol. The solution is applied to a valve-metal substrate, the most common material being titanium, by painting, dipping, or spraying. The chloride can then be decomposed and oxidized in air or oxygen to RuOj at relatively low temperatures (300°C - 500°C). The properties of the oxide, e.g., surface area, depend significantly on the preparation temperature. 

Obviously, if multiple ionization processes are possible, then more charge can be accommodated on the surface. It Is therefore unnecessary to postulate b ulk reaction of protons with RuOj. Other evidence that points to the same conclusion may briefly be summarized as follows  

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Thermally prepared thin films of RUO2 and IrO2 on titanium have been tested as cathodes for use in water electrolysis cells (Kondintsev and Trasatti, 1994 Burke and Naser, 2005). The mechanism of the HER at oxide electrodes, however, should differ significantly from that studied at metal electrodes (Burke et al., 2007). 

The long-term stability of devices based on as-prepared PS films is generally poor. Under emission conditions the degradation is measured usually in minutes or hours. The stability of devices based on chemically [Kol8], anodically [Gell] or thermally oxidized PS films [It4] is usually better however the standard lifetime requirement for microelectronic devices (> 105 h) has not been met. 

Polycrystalline oxide materials, both undoped and doped, have been extensively examined for use as photoanodes. Ti02 electrodes have been prepared by thermal oxidation of a Ti plate in an electric furnace in air at 300-800°C (15-60 min) and in a flame at 1300°C (20 min) [27-30]. XRD analysis of thermally oxidized samples indicates the formation of metallic sub-oxide interstitial compounds, i.e. TiOo+x (x < 0.33) or Ti20i y (0 < y < 0.33) and Ti30 together with rutile Ti02 [27]. The characteristic reflection of metallic titanium decreases in intensity after prolonged oxidation (60 min) at 800° C indicating the presence of a fairly thick oxide layer (10-15 pm). Oxidation at 900°C leads to poor adhesion of the oxide film... 

Few data are available on the hydrolysis of simple metal alkoxides of these elements. Alkoxides of alkaline and alkaline earth metals are mostly used as precursors for the preparation of complex oxides or solid oxide solutions. Commercial production of pure magnesium oxide by hydrolysis of Mg(OMe)2 with formation of transparent gel has been described [715], as well as hydrolysis of Mg(OC5H11i)2 with the following thermal treatment to produce a fine MgO powderthat sinters at low temperatures [1766]. Solutions prepared by dissolving magnesium in methoxyethanol are by far the most convenient precursors for preparation of magnesium oxide films. 

IR spectra measurements as well as variation of the film thickness, shrinkage, and refractive index demonstrated substantial differences in the mechanisms of thermal decomposition of films prepared from the exclusively metal alkoxide precursor and from the metal alkoxides modified by 2-ethylhexanoic acid. These differences affect the evolution of film microstructure and thus determine the different dielectric properties of the obtained films. The dielectric permittivity of the films prepared from metal alkoxide solutions was relatively low (about 100) and showed weak dependence ofthe bias field. This fact may be explained by the early formation of metal-oxide network (mostly in the... 

A promising alternative is surface textured doped zinc oxide films. ZnO films can offer excellent transparency and are highly resistant to hydrogen plasmas [78]. Textured ZnO films have been prepared by several deposition techniques. Examples are boron doped zinc oxide (ZnO B) prepared by low-pressure chemical vapor deposition (LPCVD) ([79,80], see also Chap. 6) or ZnO films deposited by expanding thermal plasma CVD [81], Quite recently, ZnO films for back contacts of solar modules have been developed using chemical bath deposition [82]. 

The various stages of the preparation and thermal treatments of supported metal catalysts arc very schematically illustrated in Fig. 3. A very similar presentation was earlier given by van Delft et al. [16]. Typically, the support is impregnated with a metal salt (sec Section A.2.2.1.1) which serves as the metal precursor and should be well dispersed. Small metal particles may be formed by either direct reduction under mild conditions or by reduction after an intermediate oxidation step. Mild oxidation will lead to thin oxide films spread out on the support or to small oxide particles, where particles and film may also coexist. More severe treatments in oxidative atmospheres can lead to the... 

The electrocatalytic behavior of olefins was studied by Zanta et al. (2000) at thermally prepared ruthenium-titanium- and iridium-titanium-dioxide-coated anodes. The aliphatic olefins were shown to be inactive in the region before oxygen evolution, while aromatic ones showed one or two oxidation peaks, and the catalytic activity seemed to be the same for both substrates. However, as for platinum anodes, voltammetric studies and FTIR analyses have also shown the formation of a polymeric film that blocks the surface of the electrode and decreases its activity. 

The technologically important dimensionally stable anodes (DSA) are thermally prepared mixed oxide films supported on an inert substrate, usually Ti, which contain Ru02 as the catalytically active component. These anodes exhibit high performance in the industrial generation of chlorine. 

Only a few investigations of 02 evolution on thermally prepared Ni oxide films have been carried out [242, 334]. Miles et al. [242] observed an average... 

Because thermally formed RUO2 films on Ti electrode supports are usually prepared from RuClj, the question of the possibility of Cl being incorporated in the oxide structure has long been considered (386, 388), and analyses for Cl and O have been carried out (388, 389). General agreement exists that some Cl" remains in the oxide lattice. Cl content tends to decrease toward the external surface of RUO2 while O increases Ru(VI) states near the surface (as RuOj) have been suggested on the basis of photoelectron spectroscopy (390, 597). 

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