Photocorrosion cathodic

Mishra et al. [198] discussed in an exemplary way the dark and photocorrosion behavior of their SnS-electrodeposited polycrystalline films on the basis of Pourbaix diagrams, by performing photoelectrochemical studies in aqueous electrolytes with various redox couples. Polarization curves for the SnS samples in a Fe(CN) redox electrolyte revealed partial rectification for cathodic current flow in the dark, establishing the SnS as p-type. The incomplete rectification was... 

Photocorrosion can be prevented by adding a redox couple to the electrolyte whose potential is more favourable than the decomposition potential such that the redox reaction occurs preferentially. When n-CdS is used as photoanode in aqueous electrolytes, the electrode is photocorroded since the reaction, CdS -1- 2h - S -1- Cd, occurs readily. By adding NaOH and sodium polysuphide to the electrolyte (Ellis et al, 1976), photocorrosion is prevented. The /S redox couple preferentially scavenges the photoholes. At the anode, sulphide is oxidized to polysulphide (free sulphur) and free sulphur is reduced back at the dark cathode. Similarly n-Si anodes have been stabilized by using a nonaqueous electrolyte containing a ferricinium/ferrocene redox couple (Legg et al, 1977 Chao et al, 1983). Unfortunately, a similar stabilization technique cannot be applied to photoelectrolysis cells. Some examples of electrode... 

KC1, which bathed the CdS film. This system was also investigated by cyclic voltammetry both in the dark and under illumination. Starting at about — 0.9 V, the dark cathodic current exhibited a peak at — 1.15 V due to Cd2+ reduction and then rose to — 1.4 V as a result of hydrogen production. The observed anodic peak at — 0.85 V was attributed to the stripping of cadmium deposits in the lattice (Cd ). Cyclic voltammetry subsequent to illumination resulted in the appearance of cathodic waves at — 1.0 V and — 1.3 V, at the expense of that at — 1.15 V. The anodic peak broadened, as is indicative of photocorrosion. 

Fig. 15. Diagram illustrating the thermodynamic stability of a semiconductor against corrosion and photocorrosion (a) semiconductor is absolutely stable, (b) stable against cathodic decomposition, (c) stable against anodic decomposition, and (d) unstable. [From Gerischer (1977a).]...

Consider now the processes caused by the formation of quasilevels. As was noted above, the shift of Fn relative to F is very small for majority carriers (electrons) and can usually be neglected precisely, this was done in constructing Fig. 16b. But for minority carriers (holes) the shift of Fp can be very large. The shifts of both Fnx F and Fp increase with the growing intensity of semiconductor illumination, so that for a certain illumination intensity Fp may reach the level of the electrochemical potential of anodic decomposition Fdec, p, and Fn—the level of a certain cathodic reaction (for example, reduction of water with hydrogen evolution FHljH20). These reactions start to proceed simultaneously, and their joint action constitutes the process of photocorrosion. 

By analogy, the stability condition for cathodic photocorrosion can be written as ep tdox >... 

Admittedly this scheme is daunting in its complexity and the kinetic implications are as yet unclear. Early studies on p-GaP, p-GaAs and other Group III-V (13-15) semiconductors reported onset of cathodic photocurrents (attributable to HER) only at potentials far removed (ca. 0.6 V) from V[b (see, e.g., Ref [264]). This was attributed to steps 33b and 33h in the above scheme. More recent work [91] has shown that the HER at illuminated p-InP-electrolyte contacts is accompanied by a photocorrosion reaction, leading to indium formation on the semiconductor surface. Representative PEIS data from this study are shown in complex plane format in Figure 28. Note that unlike in the cases above (e.g.. Figure 27), multiple relaxa-... 

Photovoltage spectra are measured at open circuit using chopped light of low intensity. It might appear that an advantage of photovoltage spectroscopy over photocurrent spectroscopy is that no photocorrosion occurs. However, this is not necessarily correct, because anodic photocorrosion in the illuminated areas may be balanced by cathodic reduction of solution species such as oxygen or protons. 

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. 

It is the necessity to prevent photocorrosion that initiated increased interest in photocathodes during last decade since cathodes operate in reductive conditions and therefore... 

It is therefore important to stabilize the photocathodes for operation at the maximum power point where anodic as well as cathodic photocorrosion can occur. Among the approaches presented in Figure 2.11, the protection of the semiconductor surface from the reactive electrolyte interface has been considered as most promising. Interfacial film formation can be achieved in situ by scanning the... 

A well-known oxide that suffers from severe photocorrosion in aqueous environments is ZnO. For p-type oxides, cathodic photoreduction can lead to the formation of metal deposits ... 

This has been observed for, e.g., CU2O. Alternatively, protons can enter the metal oxide lattice when free electrons are available at or near the surface, a process known as intercalation. It is worth mentioning that III-V semiconductors, such as GaAs and GaP, are prone to anodic photocorrosion, but appear to be relatively stable against cathodic photocorrosion in practice [69]. 

Semiconductor-Liquid Junction From Fundamentals to Solar Fuel Generating Structures, Fig. 9 Stability assessment against photocorrosion by relative alignment of quasi-Fermi levels and anodic and cathodic decomposition levels (see text) left p-semicon-ductor note that the decomposition level is located above... 

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