Anodic oxides illumination

The electrochemistry of single-crystal and polycrystalline pyrite electrodes in acidic and alkaline aqueous solutions has been investigated extensively. Emphasis has been laid on the complex anodic oxidation process of pyrite and its products, which appears to proceed via an autocatalytic pathway [160]. A number of investigations and reviews have been published on this subject [161]. Electrochemical corrosion has been observed in the dark on single crystals and, more drastically, on polycrystalline pyrite [162]. Overall, the electrochemical path for the corrosion of n-EeS2 pyrite in water under illumination has been described as a 15 h" reaction ... 

If the same experiment is performed with an n-type Si electrode under identical illumination intensity the anodic photocurrent is found to be larger than for the p-type electrode under cathodic conditions. This increase is small (about 10%) for current densities in excess of JPS. Figure 3.2 shows that in this anodic regime injected electrons are also detected at p-type electrodes. This allows us to interpret the 10% increase in photocurrent observed at n-type electrodes as electron injection during anodic oxide formation and dissolution. 

If an oxide-free, hydrogen-terminated, n-type electrode is anodized under illumination in an electrolyte free of HF (for example HC1), a quantum efficiency of close to 2 is observed for the initial contact of the electrode to the electrolyte. During the oxidation of the first hydrogenated monolayer the quantum efficiency decreases to 1 and remains at that value during the formation of the first few nanometers of anodic oxide, as indicated by filled triangles in Fig. 4.13. For a further increase of oxide thickness the quantum efficiency decreases to values significantly below 1 [Chl4]. 

Fig. 5.2 Thickness of anodic oxides formed potentiostatically on (100) Si in 3% NH4OH, as a function of applied potential for various anodization times. Illumination was provided for n-type Si. After [Bal4].

Fig. 34. Dependence of the current (in fiA) of anodic oxidation of water with oxygen evolution at a Ti02 electrode on the potential in an 0.2 M solution of KC1 —under illumination with UV light, —under irradiation with a 4.2-MeV electron beam. [From Krotova et al. (1981).]...

Electrochemical reactions at metal electrodes can occur at their redox potential if the reaction system is reversible. In cases of semiconductor electrodes, however, different situations are often observed. For example, oxidation reactions at an illuminated n-type semiconductor electrode commence to occur at around the flat-band potential Ef j irrespective of the redox potential of the reaction Ergdox Efb is negative of Ere 0 (1 2,3). Therefore, it is difficult to control the selectivity of the electrochemical reaction by controlling the electrode potential, and more than one kind of electrochemical reactions often occur competitively. The present study was conducted to investigate factors which affect the competition of the anodic oxidation of halide ions X on illuminated ZnO electrodes and the anodic decomposition of the electrode itself. These reactions are given by Eqs 1 and 2, respectively ... 

The anodic oxidation of iodide, bromide and chloride ions at illuminated ZnO electrodes, which occurs in competition with the anodic decomposition of the electrode itself, was studied as functions of halide ion concentration, illumination intensity and solution pH in order to investigate factors which affect the degree of competition. The reactivity of halide ions, obtained under fixed conditions, was in the order of I >Br >Cl, reflecting the importance of the redox potential in determining the reactivity. 

An investigation of the anodic oxidation of mesitylene in nitrate-ion based electrolytes but with aprotic solvents revealed little more to illuminate the mechanistic picture (Nyberg, 1971d). Again, a very pronounced shift of the voltammetric curve was observed upon addition of the substrate when platinum was the anode material, whereas on graphite a small shift toward less positive potentials was noted. Product distributions are shown below eqn (67). The forma-... 

This model has been proved experimentally by studying the competition of the anodic decomposition reaction and the oxidation of Cu at p-GaAs in the dark and at n-GaAs under illumination [93]. This is a suitable redox system, because reduction and oxidation occur via the valence band, and because the anodic oxidation of Cu proceeds independently from the corrosion. Accordingly, the total current is given by... 

Iron-thiazine photogalvanic cells use the photoredox reactions of Fe(II) with thiazine dyes, represented for thionine by Reactions 1, 2, 3, 4, and 5, to convert photon energy into chemical potential. The spontaneous ground state reactions represented by Reactions 6, 7, 8, and 9 also occur in homogeneous solution during illumination. Photogalvanic action results when homogeneous Reactions 7, 8, and 9 are replaced by anodic oxidation of and TH2 coupled with cathodic reduction of... 

Electrodeposition of Te on n-Si is irreversible. Te nanoparticles deposited on n-Si are stable in the dark. The Shottky barrier on n-Si/Te interface prevents their anodic oxidation. The anodic current on n-Si/Te electrode appears only under illumination (Fig. 3a). This photocurrent is considerably higher when compared with the bare n-Si electrode and proportional to Te amount on Si surface. 

Similar to the formation of porous aluminum oxide a passivation - dissolution mechanism can be used to form nanopo-rous structures on InP. If (OOl)n-InP is polarized anodically under illumination in HCl solutions, nanoscaled pores are formed [117]. For potentials up to 1.2 V vs. SGE the main reaction is uniform anodic dissolution. Above this potential porous InP with a surface oxide is formed. The overpotential and anodizing time influence pore diameter (110-250 nm), wall thickness (16-50 nm) and pore length... 

The photoproduction and subsequent separation of electron-hole pairs in the depletion layer cause the Fermi level in the semiconductor to return toward its original position before the semiconductor-electrolyte junction was established, i.e., under illumination the semiconductor potential is driven toward its flat-band potential. Under open circuit conditions between an illuminated semiconductor electrode and a metal counter electrode, the photovoltage produced between the electrodes is equal to the difference between the Fermi level in the semiconductor and the redox potential of the electrolyte. Under close circuit conditions, the Fermi level in the system is equalized and no photovoltage exists between the two electrodes. However, a net charge flow does exist. Photogenerated minority carriers in the semiconductor are swept to the surface where they are subsequently injected into the electrolyte to drive a redox reaction. For n-type semiconductors, minority holes are injected to produce an anodic oxidation reaction, while for p-type semiconductors, minority electrons are injected to produce a cathodic reduction reaction. The photo-generated majority carriers in both cases are swept toward the semiconductor bulk, where they subsequently leave the semiconductor via an ohmic contact, traverse an external circuit to the counter electrode, and are then injected at the counter electrode to drive a redox reaction inverse to that occurring at the semiconductor electrode. 

The cell, however, requires a separator since sulphurous acid would otherwise reduce to sulphur at the cathode. There are also problems at the anode since, with presently available electrode materials, the oxidation of sulphurous acid to sulphuric acid requires an overpotential of several hundred millivolts. Hence, the advantage compared with direct water electrolysis is yet to be realized. Currently, the most promising examples of photochemically assisted water electrolysis are photovoltaic ceils which use a semiconductor electrode, e.g. it is possible to construct a cel) with an anode made from an optically transparent material covered with a thin layer of an n-type semiconductor, an aqueous electrolyte and a cathode which has a low overpotential for hydrogen evolution. When the anode js illuminated, photons of energy greater than the energy gap of the semiconductor will excite electrons from the filled levels to the conduction band leaving a hole in the filled band ... 

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