Redox couples flatband potential

The energy levels in the solution are kept constant, and the applied voltage shifts the bands in the oxide and the silicon. The Gaussian curves in Figure 4b represent the ferrocyanide/ferricyanide redox couple with an excess of ferrocyanide. E° is the standard redox potential of iron cyanide. With this, one can construct (a) to represent conditions with an accumulation layers, (b) with flatbands, where for illustration, we assume no charge in interface states, and (c) with an inversion or deep depletion layer (high anodic... 

Unfortunately, the redox potential of the Pt4 + /3+ couple is not known in literature. Although some stable Ptm compounds have been isolated and characterized (37), the oxidation state III is reached usually only in unstable intermediates of photoaquation reactions (38-40) and on titania surfaces as detected by time resolved diffuse reflectance spectroscopy (41). To estimate the potential of the reductive surface center one has to recall that the injection of an electron into the conduction band of titania (TH) occurs at pH = 7, as confirmed by photocurrent measurements. Therefore, the redox potential of the surface Pt4 + /3+ couple should be equal or more negative than —0.28 V, i.e., the flatband potential of 4.0% H2[PtClal/ TH at pH = 7. From these results a potential energy diagram can be constructed as summarized in Scheme 2 for 4.0% H2[PtCl6]/TH at pH = 7. It includes the experimentally obtained positions of valence and conduction band edges, estimated redox potentials of the excited state of the surface platinum complex and other relevant potentials taken from literature. An important remark which should be made here is concerned with the error of the estimated potentials. Usually they are measured in simplified systems - for instance in the absence of titania - while adsorption at the surface, presence of various redox couples and other parameters can influence their values. Therefore the presented data may be connected with a rather large error. 

As described above the oxidation potential of the platinum(III) species should be equal or more negative than —0.28 V. This is supported by the pH dependence of 4-CP photodegradation in presence of 4.0% H2[PtCl6]/TH (10). The reaction is slowed down in basic media and almost suppressed at pH = 11-12. Since Bahnemann et al. observed a contrary tendency, i.e., a slight increase in 4-CP degradation rate with increasing pH for unmodified P25 (42), our observation may be connected with a too cathodic position of the flatband potential and therefore the electron injection should be much slower in basic suspensions. This supports the estimation of the redox potential of Pt4+ 3+ couple to be at ca. —0.3 to —0.4 V, i.e., around the flatband potential of the catalyst at pH = 7-8. However,... 

The measured potential Vm, and thus jEf and K. can be varied through external polarization. Vm is the applied potential when the electrode is externally polarized and is the open-circuit potential without external polarization. When the semiconductor has no excess charge, there is no space charge region and the bands are not bent. The electrode potential under this condition is called the flatband potential Vn,. The flatband potential is an important quantity for a semiconductor electrode because it connects the energy levels of the carriers in the semiconductor to those of the redox couple in the electrolyte and it connects the paramete s that can be experimentally determined to those derived from solid-state physics and electrochemistry. It can generally be expressed as... 

Flatband potential shift may also result from deposition of metallic and organic species, which act as surface states. Figure 2.33 shows that the flatband potential of an -Si electrode in acetonitrile solution containing redox couples changes with... 

Deposition of a small amount of noble metals such as Cu, Pt, and Au increases the kinetics of redox reactions on silicon electrodes as shown in Fig. 6.3. Deposition of equivalent of 1 to 10 monolayers of Pt on silicon surface results in a shift of about 0.2V of the onset potential for hydrogen evolution to the positive direction. Because the flatband potential does not change with the Pt deposition, the enhanced hydrogen reaction kinetics is due to the catalytic effect of the deposited metal. The energy levels of the deposited metal grains are considered to lie in the middle of the band gap and communicate favorably to the surface states both energetically and spatially. The photovoltage of n-Si coated with sparsely scattered Pt islands has been found to increase proportionally to the inaease in the potential of the redox couple. Noble metal islands effectively collect photoelectrons and thus prevent the oxidation of the silicon surface by the photoelectrons. 

The transition metal chalcogenides such as n-WSe2 are a particular class of electrode materials, and their photoelectrochemical behaviour is of interest from the fundamental point of view. If the basal planar surfaces (perpendicular to the c-axis) with a low density of steps are contacting the electrolyte, these layered materials are relatively stable. Since the corrosion rate is very small, the anodic photocurrent occurs at a high overvoltage with respect to the flatband potential in the dark. As discussed in Section 2.3.1 (Fig. 2.15), the flatband potential t/fb is shifted on illumination because holes accumulate at the surface. On addition of a redox couple such as [Fe(phen)3] ... 

The capacitance of the depleted interface reaches a maximum in the flatband condition. Measuring the flatband potential t/g, of die elech ode on same scale as LJ ofdie redox couple, and knowing the forbidden gap and the offset (AE in Fig. 4.24) of the semiconductor Fermi level from the majority carrier band edge, enables andE p " to be placed on a common scale. 

Concerning the potential dependence of the interfacial current under illumination, it is frequently useful to measure it in the presence of only one species of the redox couple, the reduced species for an anodic and the oxidized species for a cathodic reaction. Taking n-WSe2 as an example, then the current-potential curve under illumination, as measured in an aqueous solution free from any redox system, is presented in Fig. 7.26. The cathodic dark current which occurs cathodic of the flatband potential, is due to H2 formation (conduction band process). The anodic photocurrent which starts... 

---