GaP-electrolyte interface

GaP/electrolyte interface. The electrolyte is 0.15M HN03 and the current density flowing through the interface is 20 mA/cm2. The low-energy limit of the spectrum is determined by the photomultiplier sensitivity, (b) Strongly cathodically biased p-GaP/electrolyte interface. Hot electrons are created by tunneling from valence to conduction bands. These may decay radioactively to fill empty states created by cation injection or drive other redox reactions. 

Figure 8. Frequency of Imaginary impedance maximum as a function of bias potential of GaP electrolyte interface containing CO2 under illumination.

The impedance data for the GaP-electrolyte interface can be represented by the equivalent circuit discussed for the CdTe electrode. 

Zinc sulfide, with its wide band gap of 3.66 eV, has been considered as an excellent electroluminescent (EL) material. The electroluminescence of ZnS has been used as a probe for unraveling the energetics at the ZnS/electrolyte interface and for possible application to display devices. Fan and Bard [127] examined the effect of temperature on EL of Al-doped self-activated ZnS single crystals in a persulfate-butyronitrile solution, as well as the time-resolved photoluminescence (PL) of the compound. Further [128], they investigated the PL and EL from single-crystal Mn-doped ZnS (ZnS Mn) centered at 580 nm. The PL was quenched by surface modification with U-treated poly(vinylferrocene). The effect of pH and temperature on the EL of ZnS Mn in aqueous and butyronitrile solutions upon reduction of per-oxydisulfate ion was also studied. EL of polycrystalline chemical vapor deposited (CVD) ZnS doped with Al, Cu-Al, and Mn was also observed with peaks at 430, 475, and 565 nm, respectively. High EL efficiency, comparable to that of singlecrystal ZnS, was found for the doped CVD polycrystalline ZnS. In all cases, the EL efficiency was about 0.2-0.3%. 

Non-situ and ex situ studies can provide important information for understanding the properties of metal/electrolyte interfaces. The applicability of these methods for fundamental studies of electrochemistry seems to be firmly established. The main differences between common electrochemical and UHV experiments are the temperature gap (ca. 300 vs. 150 K) and the difference in electrolyte concentration (very high concentrations in UHV experiments). In this respect, experimental research on double-layer properties in frozen electrolytes can be treated as a link between in situ experiments. The measurements of the work functions... 

Semiconductor - Electrolyte Interlace The electric field in the space charge region that may develop at the semiconductor electrolyte interface can help to separate photogenerated e /h 1 couples, effectively suppressing recombination. When a semiconductor is brought into contact with an electrolyte, the electrochemical potential of the semiconductor (corresponding to the Fermi level, Ey of the solid [50]) and of the redox couple (A/A ) in solution equilibrate. When an n-type semiconductor is considered, before contact the Ey of the solid is in the band gap, near the conduction band edge. After contact and equilibration the Ey will... 

While desorption is operative to some extent at most Semico nductor/electrolyte interfaces, in some cases surface passivation can occur. Here charge transfer across the interface is used to establish covalent bonds with electrolyte species, which results in changes in surface composition. This is typified by the well-known oxide film growth on n-GaP and n-GaAs surfaces in aqueous solutions. In these cases, however, the passivation process can be competed with effectively by the use of high concentrations of other redox species such as the polychalcogenides. 

Thus hole or electron transfer can follow a number of pathways across the semiconductor/electrolyte interface. First, one can have direct oxidative or reductive charge transfer to solution species resulting in desired product formation. Second, one can have direct charge transfer resulting in surface modification, such as oxide film growth on GaP or CdS in aqueous PECs. Finally, one can have photoemission of electrons or holes directly into the electrolyte. All of these processes provide some information about the electronic structure of the interface. 

Experiments using p-Si in non-aqueous electrolytes (5.6) indicate that the reduction of redox species with redox potentials more negative than the conduction band edge could apparently beachieved. However, further analysis showed that these results are not caused by hot electron injection, but by the unpinning of the semiconductor band edges at the semiconductor-electrolyte interface this unpinning effect is caused by the creation of an inversion layer at the p-Si surface. This is an important effect, especially for small band gap semiconductors, that has received little attention 0n Sabbatical leave from the Weizmann Institute of Science, Rehovot, Israel. 

As mentioned earlier, photochemical diodes489 can be either of the Schottky type, involving a metal and a semiconductor, or a p n junction type, involving two semi conductors (which can be the same, i.e., a homojunction or different, a heterojunc tion). Only the latter type is considered in this Section involving two irradiated semi conductor/ electrolyte interfaces. Thus n Ti02 and p-GaP crystal wafers were bonded together (through the rear Ohmic contacts) with conductive Ag epoxy cement.489 The resultant heterotype p n photochemical diode was suspended in an acidic aqueous... 

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