Photoanodic decomposition

A few other applications of dithiolenes make use of their redox properties. Kumar et a/.219 proposed the use of dithiolenes as photosensitizers. Umezawa et al.22<> coated a Pt cathode with (Et4N)Ni(mnt)2 and saw a modest degree (1.4 x 10 4 %) of light conversion upon irradiation. On the opposite side, Bradley et al.721 used dithiolenes to stabilize n-type Si anodes against photoanodic decomposition. 

N-type semiconductors can be used as photoanodes in electrochemical cells Q., 2, 3), but photoanodic decomposition of the photoelectrode often competes with the desired anodic process (1 4 5). When photoanodic decomposition of the electrode does compete, the utility of the photoelectrochemical device is limited by the photoelectrode decomposition. In a number of instances redox additives, A, have proven to be photooxidized at n-type semiconductors with essentially 100% current efficiency (1, 2, 3, 6>, ], 8, 9). Research in this laboratory has shown that immobilization of A onto the photoanode surface may be an approach to stabilization of the photoanode when the desired chemistry is photooxidation of a solution species B, where oxidation of B is not able to directly compete with the anodic decomposition of the "naked" (non-derivatized) photoanode (10, 11, 12). Photoanodes derivatized with a redox reagent A can effect oxidation of solution species B according to the sequence represented by equations (1) - (3) (10-15). 

Lin M. S., Hung N. and Wrighton M. S. (1982), Interface energetics for n-type semiconducting strontium titanate and titanium dioxide contacting liquid electrolyte solutions and competitive photoanodic decomposition in non-aqueous solutions , J. Electroanal. Chem. 135, 121-143. 

Mechanism 1 involves two different electron transfer steps, so that the meaning of kt, needs to be clarified. Both mechanisms involve an adsorbed hydrogen intermediate, and this may have consequences for the potential distribution across the interface and hence the band bending. The formation of adsorbed intermediates is a common feature of multi-electron transfer reactions. Other examples are encountered in the photodecomposition of compound semiconductors, for example the photoanodic decomposition of n-CdS ... 

The sol-gel process to prepare SIO2 glass fibers and T102 films has been reviewed. It has been known that the hydrolysis conditions such as molar ratio of water to alkoxide and reaction temperature are critical to the desired forms of the gel product (fiber, film or bulk). Some properties of the resultant products have been examined. Especially, Ti02 films have been attempted to use as a photoanode for decomposition of water, and their photoelectrochemical properties are described in comparison with the results previously obtained for single crystal and polycrystalline TiC>2, and are discussed in terms of the microstructure of the film. 

One additional problem at semiconductor/liquid electrolyte interfaces is the redox decomposition of the semiconductor itself.(24) Upon Illumination to create e- - h+ pairs, for example, all n-type semiconductor photoanodes are thermodynamically unstable with respect to anodic decomposition when immersed in the liquid electrolyte. This means that the oxidizing power of the photogenerated oxidizing equivalents (h+,s) is sufficiently great that the semiconductor can be destroyed. This thermodynamic instability 1s obviously a practical concern for photoanodes, since the kinetics for the anodic decomposition are often quite good. Indeed, no non-oxide n-type semiconductor has been demonstrated to be capable of evolving O2 from H2O (without surface modification), the anodic decomposition always dominates as in equations (6) and (7) for... 

This chapter considers photoanodes comprised of metal oxide semiconductors, which are of relatively low cost and relatively greater stability than their non-oxide counterparts. In 1972 Fujishima and Honda [1] first used a crystal wafer of n-type Ti02 (rutile) as a photoanode. A photoelectrochemical cell was constructed for the decomposition of water in which the Ti02 photoanode was connected with a Ft cathode through an external circuit. With illumination of the Ti02 current flowed from the Ft electrode to the... 

Fig. 7.7 Percentage of photogenerated holes that contribute to anodic decomposition versus O2 evolution from naked (no catalyst), Pt-coated, and polymer-Pt coated n-CdS photoanode in 0.5 M Na2S04 solution (pH = 8.6) [14].

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... 

All these results can be explained in terms of the model proposed above (cf. Fig. 11). Namely, with ferrous oxalate having a standard redox potential E° (Ox/R) of —0.2 V (SCE), which is a little more negative than the E of the surface trapped hole located ca. 0.5 V above E , the surface trapped hole is effectively quenched by the rapid reduction, and the photoanodic current flows without decomposition. With ferrocyanide, having an E(0x/R) of 0.2 V (SCE), which is more positive than the E of the surface trapped hole, the surface trapped holes are accumulated to the extent that the surface potential created will level it down to the E(0x/R) of the redox couple. At this point, the rates of nu-cleophillic attack of H2O and OH to the surface trapped holes are still low and the electrode decomposition is prevented. 

It has been pointed out by some authors CL, 2) that for a semiconductor having a thermodynamic decomposition potential, E in between Ec and E , a redox couple with a standard redox potential, E°, more negative than E is needed in order to operate the photoanode without decomposition. Then, the maximum photovoltage attainable is Us - Ej, which is often much lower than Eg -A-x. For GaP, this is only 0.8 V (4) (Fig. 11). S... 

From the results it is concluded that the equilibrium depends on the electrolyte composition and has an influence on the surface concentration of the mobile and immobile decomposition intermediates. Hence, the kinetics of the photooxidation of TMPD and the decomposition at illuminated nGaAs photoanodes are dependent on the electrolyte composition. 

Measurements of the stabilization ratio s were performed on the GaAs photoanode in aqueous medium with 0.25 mol.dnr3 and with 4 mol.dm 3 LiCl, in three water + methanol mixtures with 18, 48 and 80 mol % methanol respectively, and in two water + acetonitrile mixtures with 13 and 42 mol % acetonitrile. The stabilization ratio s was measured as a function of the photocurrent density i and the concentration c of dissolved TMPD. The measurements were performed at a constant electrode potential V corresponding to high band bending, so that surface recombination can be neglected. All experiments were performed in acid medium as required for the solubility of TMPD and decomposition products of GaAs. 

The kinetic laws (eqn, (3), (4) and (5)), which were found to describe the competition between the photoanodic oxidation of TMPD and that of GaAs, can be interpreted on the basis of a reaction mechanism comprising the following electrochemical steps. Capture of a valence band hole h+ in a surface bond of the semiconductor (GaAs)surf leads to the formation of an intermediate Xb In all the following decomposition steps a mobile species (M) participates, which can be an intermediate Xj or a valence band hole. In this way intermediates of a higher degree of oxidation (i = 2,.,5) and products are formed which 90 into solution ... 

BaSnCh 3.0 Estimated to be stable toward photoanod-ic decomposition over a 0.4-14 pH range. 65... 

The photoelecfrochemical cell for wafer decomposition (Figure 3) involves two electrodes immersed in an aqueous electrolyte, of which one is a photocatalysf exposed to light (photoanode in Figure 3). 

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