Photocorrosion under illumination

This catalytic effect of RUO2 has been exploited recently to stabilize small band gap semiconductor particles which from their absorption properties are more suitable for solar energy conversion than Ti02. An undesirable property of these materials is that they undergo photocorrosion under illumination. Holes produced in the valence band migrate to the surface where photocorrosion occurs, l.e.,... 

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. 

The corrosion behavior of semiconductors can, in principle, be described within the framework of the same concepts as for metals (see, for example, Wagner and Traud, 1938), but with due account for specific features in the electrochemical behavior of a solid caused by its semiconducting nature (Gerischer, 1970). One of the main features is photosensitivity related to a change in the free-carrier concentration under illumination. Photosensitivity underlies the phenomenon of photocorrosion. 

The quasithermodynamic approach used for interpreting photocorrosion phenomena is based on the concept that acceleration of an electrode reaction under illumination is due to the formation of the Fermi quasilevels F and Fp, which are shifted relative to the equilibrium position F. 

Figure 7. Current-voltage curves in the dark and under illumination (680-nm light, 1 mV [cm2) of a MoS2 electrode of -c orientation at different states of surface perfection (15) (1) freshly cleaved (2) after 25-min photocorrosion (solar light of AM 1) at 1 VSCE in 1M KCl solution (3) after 115-min photocorrosion, same conditions as (2) electrolyte 1M KCl + 0.05M Kl...

There is also an etched layer of Si on the surface of the anodized Si sample under illumination as illustrated in Fig. 8.45. This etched layer, which unlike that in the dark is required for the surface roughening for the initiation of pores, is mainly due to photocorrosion. As a result of the photoinduced dissolution the top surface of the PS layer recedes with time. The rate of dissolution depends on doping, HF concentration, current density, and illumination intensity. Figure 8.46 shows the variation of the three layers with the amount of charges passed the etched layer on a highly doped sample is thicker than that on a lowly doped material. The thickness of the micro PS decreases while pore diameter and etched layer increase with increasing light intensity. Table 8.5 shows... 

Illumination generates holes within the material of the PS and causes photocorrosion of the PS. Depending on the illumination intensity and time, the PS can be thinned to various extents by the photoinduced corrosion. This corrosion process is responsible for the etched crater between the initial surface and the surface of PS as shown in Fig. 8.45. It is also responsible for the fractal structure of the micro PS formed under illumination due to the different widths of the surface charge layer at which the holes are generated. 

Another interesting cell variation devised by Gerritsen and Ruppel (1984) took advantage of photocorrosion to drive a storage cell. Under illumination, n-CdSe is decomposed and p-CdTe is electroplated the reverse occnrs during cell discharge. 

In addition to band-gap and band-edge positions, some criteria for the selection of a good semiconductor include its chemical and photochemical stability and its environmental impact. Ti02 is the most popular semiconductor because of its resistivity to strong acids and bases and its stability under illumination [15,17,18]. ZnO, although its band-edge positions are very similar to those of Ti02 [30], is less desirable due to photocorrosion induced by self-oxidization. CdS has limited potential for practical use despite its attractive spectral response to solar radiation because CdS decomposes to release environmentally harmful Cd " [15]. 

The strong photocorrosion effect on an electrodeposited CdSe film treated near short-circuit conditions (positive to the flat band potential) in a polysulfide media under intense illumination is shown in Fig. 5.5, as manifested by the formation of numerous, regularly arranged pinholes often reaching the substrate surface [99],... 

Fig. 5.5 SEM surface view and cross section of an electrodeposited, ca. 1 p.m thick, CdSe/li film subjected to accelerated photocorrosion by the apphcation of -0.1 V vs. Pt bias in polysulfide solution under a focused, high-power (1 W cm ) solar illumination for 30 min. The coherence of the as-deposited film morphology is evident. The authors emphasize that, even in this situation, the liquid junction nature prevents the flow of high leakage currents during the process (as it might be the case with a solid junction). (Reprinted from [99], Copyright 2009, with permission from Elsevier)...

Like other non-oxidic semiconductors in aqueous solutions, surface oxidized and photocorrosive InP is a poor photoelectrode for water decomposition [19,27,32,33], To enhance properties several efforts have focused on coupling of the semiconductor with discontinuous noble metal layers of island-like topology. For example, rhodium, ruthenium and platinum thin films, less than 10 nm in thickness, have been electrodeposited onto p-type InP followed by a brief etching treatment to achieve an island-like topology on the surface [27,28]. In combination with a Pt counter electrode, under AM 1.5 illumination of 87 mW/cm the metal (Pt, Rh, Ru) functionalized p-InP photocathodes [27] see a reduction in the threshold voltage for water electrolysis from 1.23 V to 0.64 V, and in aqueous HCl solution a photocurrent density of 24 mA/cm with a photoconversion efficiency of 12% [27]. 

Let us consider in more detail, using the above concepts, how a photocorrosion process occurs under the illumination of a semiconductor. Suppose that electron transitions at the interface between the semiconductor and solution do not take place in darkness in a certain potential range (the semiconductor behaves like an ideally polarizable electrode). This range is confined to the potentials of decomposition of the semiconductor and/or solution. The steady state potential of a semiconductor is usually determined in this case by chemisorption processes (e.g., of oxygen) or, which is the same in the language of the physics of semiconductor surface, by charging of slow surface states. It is these processes that determine the steady state band bending. 

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. 

The preparation of a CdS dispersion by sulfidation of the non-dehydrated Cd2+ loaded sample results in sulfide agglomerates 6-8 nm in diameter (ref. 6), which do not show the detailed structure in the absorption spectrum Compared to the small particles. In this case the photocorrosion is considerably suppressed under the same illumination conditions (Fig. 4a-c). 

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