Dark current Decomposition

Interestingly, the anodic dark current at n-Ge electrodes increases considerably upon addition of the oxidized species of a redox system, for instance Ce" ", to the electrolyte, as shown in Fig. 8.4 [7]. The cathodic current is due to the reduction of Ce. The latter process occurs also via the valence band (see Chapter 7), i.e. since electrons are transferred from the valence band to Ce", holes are injected into the Ge electrode. Under cathodic polarization these holes drift into the bulk of the semiconductor where they recombine with the electrons (majority carriers) and the latter finally carry the cathodic current. In the case of anodic polarization, however, the injected holes remain at the interface and are consumed for the anodic decomposition of germanium, as illustrated in the insert of Fig. 8.4. Accordingly, the cathodic and anodic current should be compensated to zero. Since, however, the anodic current is increased upon addition of the redox system there is obviously a current multiplication involved, similarly to the case of two-step redox processes (see Section 7.6). Thus, in step (e) (Fig. 8.1) electrons are injected into the conduction band. This experimental result is a very nice proof of the analytical result presented by Brattain and Garrett [3]. 

Fig. 10-32. Polarization curves of cell reaction for photoelectrolytic decomposition of water at a photoexdted n-type anode and at a photoezdted p-type cathode solid curve n-SC s anodic polarization curve of oxygen evolution at photoexdted n Qpe anode (Fermi level versus current curve) dashed curve n-SC = anodic polarization curve of oxygen evolution at dark p>type anode of the same semiconductor as photoexdted n-type anode (equivalent to the curve of current versus quasi-Fermi level of interfadal holes in photoezdted n-type anode) solid curve p-SC = cathodic polarization curve of hydrogen evolution at photoexdted p-type cathode (Fermi level versus current curve) dashed curve n-8Cr = cathodic polarization curve of hydrogen evolution at dark n-type electrode of the same semiconductor as photoezdted p-type cathode (equivalent to the curve of current versus quasi-Fermi level of interfadal electrons in photoexdted p-type cathode) > > = flat band potential of n-type (p-type) electrode nn.sc (v p sc) = inverse overvoltage for generation of photoexdted electrons (holes) in a p-type (n-type) electrode.

Thirty grams of sodium. pentacyanonitrosylferrate (II) 2-hydrate (No. 74) are covered with 120ml of ice-cold water in a 250ml suction flask with a gas-inlet tube and a thermometer. The flask is cooled in an ice-salt bath while a steady stream of ammonia (3 bubbles/sec) is led in under the hood. Csre must be taken that the temperature does not rise above 20°C during this time because decomposition would occur the optimum range is 8-12°C. When no more gas is absorbed at this temperature, (indicated when the level of the liquid fails to rise in the inlet tube as the current of gas is interrupted), the dark yellow-brown solution is allowed to stand at 0°C for about 2 days. The amber-colored crystalline product separates with attendant evolution of gas,... 

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... 

Fig. 18. Current-potential curve for a rotating (1000 rpm) n-GaAs-electrode in the dark in 6 M HCl with 0.76 mM Cu ". Dashed curves are the partial currents of anodic decomposition (] ) and of Cu -reduction (jrea), as determined by a rotating ring-disk electrode [93]...

To summarize, the semiconduetor/electrolyte interfaee presents two types of currents in the dark this is a current of majority carriers whereas the photocurrent is a current of minority carriers. The same reactions can be monitored at n- and p-type electrodes but under different conditions. Hole accumulation corresponds to corrosion, since holes are trapped in surface bonds. Electron accumulation is generally not destructive for the surface unless cathodic reduction leads to decomposition. The band diagrams of Fig. 5 indicate that a downward shift of the flat band potential is expected at an illuminated n-type electrode. At negative bias, conversely, the shift is upward since electrons are accumulated in a thin surface layer (metallic-like behavior). 

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