Anodic oxidation semiconductor anodes

This chapter considers the fabrication of oxide semiconductor photoanode materials possessing tubular-form geometries and their application to water photoelectrolysis due to their demonstrated excellent photo-conversion efficiencies particular emphasis is given in this chapter to highly-ordered Ti02 nanotube arrays made by anodic oxidation of titanium in fluoride based electrolytes. Since photoconversion efficiencies are intricately tied to surface area and architectural features, the ability to fabricate nanotube arrays of different pore size, length, wall thickness, and composition are considered, with fabrication and crystallization variables discussed in relationship to a nanotube-array growth model. 

Of particular interest in this context has been the finding that the Kolbe reaction, the anodic oxidation of carboxylic acids (Equation 1) (2), can be made to occur at n-type oxide semiconductor photoanodes to the virtual exclusion of oxygen formation (3,4,5). 

The response of oxide semiconductor photoelectrodes (as anodes) to solar radiation can be enhanced by chemisorbed dyes. The dye must be selected such that its ground state red-ox potential lies within the band gap of the semiconductor while that of the excited state lies above the conduction band edge (see Gratzel,... 

In closing this Section, comparative studies on binary oxide semiconductors are available62,65,353,383 including one study383 where the electron affinities of several metal oxides (used as anodes in photoelectrolysis cells) were calculated from the atomic electronegativity values of the constituent elements. These electron affinity estimates were correlated with the Vih values measured for the same oxides in aqueous media.383... 

An alternative reductive quenching mechanism for the supersensitizer sensitization of metal oxide semiconductors has been explored in considerable detail by Kirsch-De Mesmaeker and co-workers [78-82]. These researchers found that unlike Ru(bpy)3 +, sensitizers such as Ru(tap)3 + did not inject electrons into the semiconductor from the excited state, yet an anodic photocurrent was observed when an electron donor such as hydroquinone (H2Q) was present in the electrolyte. The sensitization mechanism they proposed is distinctly different from trapping the oxidized sensitizer described above, and is more similar to that proposed for photogalvanic cells. In the presence of H2Q, excited state intermolecular electron transfer yields Ru(tap)3+. The photogenerated Ru(tap)3+ is a strong reductant (-0.76 V vs. SCE) that injects an electron into the Sn02 conduction band which is measured as a photocurrent (Figure 14). 

Anodic oxidation is used commercially in the production of metal-oxide semiconductor (MOS) devices on GaAs The production of AsjOj/GBjOj films... 

Keywords Anodization Catalysts Metal oxide semiconductors Photoelectrochemical hydrogen production... 

In the processing of integrated circuits, silicon dioxide (SiOa) can be used as a mask during ion implantation or diffusion of impurity into silicon, for passivation, for protection of the device surface, as interlayers for multilevel metallization, or as the active insulating material — the gate oxide film in metal-oxide-semiconductor (MOS) devices [1, 2], At the present time, several methods have been developed for the formation of Si02 layers, including thermal and chemical oxidation, anodization in electrolyte solutions, and various chemical vapor deposition (CVD) techniques [2, 3],... 

Electrochemical oxidation of (Hg,Cd)Te has several shortcomings. Various reports have indicated a lack of thermal stability of the oxide (2)(10-13). Also, the oxide-semiconductor interface and oxide near the interface has relatively poor interface quality as compared to thermally oxidized silicon (14-18). Anodic oxidation at an elevated temperature has been used in one instance (9), and was reported to have increased stabiiity over room temperature anodic oxides. This idea has not been further explored. 

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