Semiconductor surface shift

Because of the adsorption equilibrium for H+ and OFT ions between the surface of semiconductors and an aqueous (aq) solution, the semiconductor surface attains the point of zero charge (PZC). The flat-band potential U[h of most semiconductors including all oxides and also other compounds such as n- and p-type GaAs, p-type GaP, and n- and p-type InP in an aqueous solution is determined solely by pH and shifts proportionately with pH with a slope of -59 mV/decade, that is, pH, for example,... 

Fig. 10-2. Splitting of Fenni level of electrode, cnsci. into quasi-Fermi levels of electrons, ep, and of holes, pCp, respectively, in a surface layer of photoexcited n-type and p-type semiconductors a shift of quasi-Fermi levels from original Fermi level is greater for minmity charge carriers than for majority charge carriers.

Poly(acrylic acid) (PAA), a MIP film candidate, has been shown to bind to the bare CdSe surface from methanol solution with considerable affinity [13]. Placement of drops of a PAA-methanol solution on the surface of CdSe and evaporation of the solvent leaves a PAA film on the semiconductor surface. Once coated with this PAA film, CdSe shows no change in PL intensity in the presence of amines. Despite the lack of a PL change, the deprotonation of the carboxylic acid could be observed by the shifting of the infrared (IR) carboxylic acid peak to lower frequencies characteristic of the carboxylate anion upon amine binding, as shown in Fig. 5. The reaction chemistry is ... 

The second important difference is that the interface potential is present at the (outer) Helmholtz layer of the semiconductor/soiution interface. The interface potential is produced by surface dipoles of surface bonds as well as surface charges due to ionic adsorption equilibria between the semiconductor surface and the solution. If the interface potential can be regulated by a change in the chemical structure of the semiconductor surface, then the semiconductor band energies can be shifted to match the energy levels of the solution species (oxidant or reductant). This is another advantage of the semiconductor system because this enables improvement of the electron transfer rate at the semiconductor/soiution interface and the energy conversion efficiency. 

We assumed in Fig. 4.2 that no surface charge or surface dipole is present in the semiconductor. In general, however, both surface charges and surface dipoles are present in the semiconductor owing to adsorption equilibria for various ions between the electrolyte and the semiconductor surface as well as formation of polar bonds at the semiconductor surface. Such surface charges and surface dipoles change the potential difference in the (outer) Helmholtz layer and thus cause shifts in the surface band positions, as shown schematically in Fig. 4.3. The shifts can be expressed as changes in 0(0) or in the above equations, with the... 

Figure 4.4 shows the surface band positions of some typical semiconductors in aqueous electrolytes (pH 7), calculated from the experimentally determined C/ft, compared with the redox levels of some important redox reactions. It is known that the U for most semiconductors, such as n- and p-GaAs, n- and p-GaP, n- and p-InP, n-ZnO, n-Ti02, and n-Sn02, in aqueous electrolytes is solely determined by the solution pH and shifts in proportion to pH with a slope of -0.059 V/pH.3,4) This is explained by the adsorption equilibrium for H+ or OH- between the semiconductor surface and the solution, for example,... 

In spite of a great number of investigations aimed at the preparation of photocatalysts and photoelectrodes based on the semiconductors surface-modified with metal nanoparticles, many factors influencing the photoelectrochemical processes under consideration are not yet clearly understood. Among them are the role of electronic surface (interfacial) states and Schottky barriers at semiconductor / metal nanoparticle interface, the relationship between the efficiency of photoinduced processes and the size of metal particles, the mechanism of the modifying action of such nanoparticles, the influence of the concentration of electronic and other defects in a semiconductor matrix on the peculiarities of metal nanophase formation under different conditions of deposition process (in particular, under different shifts of the electrochemical surface potential from its equilibrium value), etc. 

Interpret these facts to favor the view that photoelectrocatalysis occurs because of a rate-controlling reaction at the metal/solution interface, or, alternatively, that it occurs because the metal added to the surface shifts the Fermi level of the semiconductor. (Bockris)... 

A drastic decrease of photovoltage in UHV is obtained by introduction of surface states at the semiconductor surface. Particle bombardement of cleaved (0001) faces leads to preferential sputtering of the chalcogenide. The metal is reduced and new electronic bandgap states are formed at the surface. As a consequence a Fermi level pinning effect occurs which results in a smaller shift of EB due to halogen adsorption and decreased photovoltages and consequently an increased double layer potential drop (Fig. 4). 

Zaban and co-workers reported the use of chemical redox titrations to measure the potential of sensitizers bound to Ti02 [136], An unexpected result from these studies is that redox couples that are not pH sensitive in fluid solution become pH dependent when bound to the semiconductor surface. The magnitude of the pH-induced shift varied from 21 to 53 mV per pH unit depending on the physical location of the sensitizer. Sensitizers inside the semiconductor double layer track the 59 mV pH shift of the semiconductor. When sensitzers were outside the double layer, their potential was almost independent of the semiconductor. This finding has important implications for the determination of interfacial energetics for dye sensitization and interfacial electron transfer studies [136]. 

In general, the absorption spectra of sensitizers bound to colloidal semiconductor films closely resemble those measured in fluid solution. In some cases small spectral shifts have been observed and attributed to Stark effects, acid-base chemistry or stabilization of the sensitizer excited states by the semiconductor surface. However, the effects are small, typically a few nanometers in the visible region. 

A shift in the band edge position also explains the observed dependence of the hole injection rate on the electrode polarization. Fig. 11 exempliHes this by the total current-potential behavior of a (111) n-GaP electrode in alpine Fe(CN) solutions (pH = 13), together with the partial current due to the injection of holes (revealed by rotating ring-disk experiments, see ref. [73]). Also at p-GaP, it was shown that the hole injection rate is lower with anodic polarization than with cathodic polarization. The potential-dependent position of the band edges is ascribed to a potential-dependent accumulation of positive charges (holes, surface decomposition intermediates,. ..) at the semiconductor surface [62, 73]. 

The theme of photosensitizing semiconductor electrodes introduced in Section 57.3.2.5(iii) may be developed with an example from ruthenium—bipyridyl chemistry. The sequence (40) is well known. The effectiveness of the photosensitization should be increased by the covalent attachment of the tris(bipyridyl)ruthenium(II) entity to the semiconductor surface, for example to Sn02. This has been achieved using the versatile halosilane chemistry shown in equation (41). The coimter anion was PFg . Cyclic voltammetry showed that the behaviour of the sjretems Sn02/aqueous [Ru(bipy)3] " and Sn02(Alm)/electroly te were very similar but with a -1-0.05 V shift in E°. The coated electrode gives a photocurrent with a red shift of 10 nm which is twice as large as for the non-coated electrode. Unfortunately the current falls off due to promotion of the hydrolysis of the Aim. 

Metal islands or nanoparticles deposited on a semiconductor surface undergo Fermi level equilibration following charging with photogenerated electrons. The effect of Fermi level equilibration is predominantly seen when the metal deposits consist of small islands or small particles. Unlike the ohmic contact observed in bulk metals, the nanoparticles retain the charge before transferring them to the redox species. During extended UV-photolysis, electron-capture by metal islands of Ag, Au and Cu becomes inhibited as their Fermi level shifts close to the conduction band of the semiconductor. Pt on the other hand acts as an electron sink and fails to achieve Fermi level equilibration. 

The solution-phase chemistry of the electrolyte is an important feature of the cell that can dramatically influence photoeffects. The redox potential of the photoelectrode couple determines equilibrium band bending. As noted above, a photoanode system allied with a more positive potential redox couple exhibits greater band bending (assuming no shift in band edges—this assumption is often valid for redox species that do not specifically interact with the semiconductor surface), which in turn leads to a higher photovoltage and more efficient carrier separation under normal experimental conditions. 

For a certain illumination intensity, the hole quasilevel Fp at the semiconductor surface can reach the level of an anodic reaction (reaction of semiconductor decomposition in Fig. 9). In turn, the electron quasilevel F can reach, due to a shift of the Fermi level, the level of a cathodic reaction (reaction of hydrogen evolution from water in Fig. 9). Thus, both these reactions proceed simultaneously, which leads eventually to photocorrosion. Hence, nonequilibrium electrons and holes generated in a corroding semiconductor under its illumination are consumed in this case to accelerate the corresponding partial reactions. 

Considerations similar to those presented above show that illumination of a semiconductor leads to a shift of both the Fermi level and the quasi-levels of holes and electrons, and both the forward and reverse reactions, proceeding according to Eq. (1), are accelerated. In other words, the result of illumination is, above all, the efficient increase of the exchange current in the redox couple but this is not the only result. If a semiconductor under illumination is an electrode in an electrochemical cell and is connected through a load resistor with an auxiliary electrode, the cathodic and anodic reactions become spatially separated, as in the case of water photoelectrolysis (Fig. 11) considered above. The reaction with the minority carriers involved proceeds on the semiconductor surface, and that with the majority carriers involved, on the auxiliary electrode. Thus, the illumination of a semiconductor electrode gives rise to an electric current in the external circuit, so that some power can be drawn from the load resistor. In other words, the energy of light is converted into electricity. This is the way a photoelectrochemical cell, called the liquid junction solar cell, operates. 

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