Photoelectrochemical activation

In their pioneering work on the formation of photoelectrochemically active metal sulfides by oxidation of the parent metal electrode. Miller and Heller [29] reported the anodic formation of polycrystalline Bi2S3 on a bismuth metal electrode in a sodium polysulfide cell, wherein this electrode was used in situ as photoanode. When a Bi metal electrode is anodized in aqueous sulfide solutions a surface film is formed by the reaction... 

Torimoto T, Takabayashi S, Mori H, Kuwabata S (2002) Photoelectrochemical activities of ultrathin lead sulfide films prepared by electrochemical atomic layer epitaxy. J Electroanal Chem 522 33-39... 

Bolts JM, Wrighton MS (1978) Chemically derivatized n-type semiconducting germanium photoelectrodes. Persistent attachment and photoelectrochemical activity of ferrocene derivatives. J Am Chem Soc 100 5257-5262... 

A number of photochemically or photoelectrochemically activated transition-metal complexes have also been used, both for oxidation and reduction of the nicotinamide cofactors. Among these complexes is the aforementioned Cp Rh(bpy)-complex 9 [52, 53]. For details of these systems or other regeneration procedures using special dyes, the reader is referred to other reviews on coenzyme regeneration [17, 21-23]. 

Reference 91 involved an acid bath (although pH was not given) using thioacetamide as the source of S. The terminal film thickness was greater for lower-temperature deposition films thicker than 3 p,m were obtained at 8°C. In contrast to the acid thiosulphate bath, the use of EDTA decreased the resistivity, as did deposition at lower temperatures. Photoelectrochemical activity was found for these films (see Chap. 9). 

Furthermore, photoelectrochemical activation of rutile or anatase can thus be envisioned as a means for oxygen activation. 

It has recently been recognized that crystal structure and particle size can also influence photoelectrochemical activity. For example, titanium dioxide crystals exist in the anatase phase in samples which have been calcined at temperatures below 500 °C, as rutile at calcination temperatures above 600 °C, and as a mixture of the two phases at intermediate temperature ranges. When a range of such samples were examined for photocatalytic oxidation of 2-propanol and reduction of silver sulfate, anatase samples were found to be active for both systems, with increased efficiency observed with crystal growth. The activity for alcohol oxidation, but not silver ion reduction, was observed when the catalyst was partially covered with platinum black. On rutile, comparable activity was observed for Ag, but the activity towards alcohol oxidation was negligibly small . Photoinduced activity could also be correlated with particle size. 

Coupling at nitrogen could also be observed in the photoelectrochemical activation of aryl amines. With toluidines, for example, azo products are formed, Eq. (21)... 

Carbon-carbon coupling of radicals observed in the photo-Kolbe reaction could also be observed with other surface generated radicals. Kisch and coworkers have shown, for example, that cyclic allylic ethers undergo alpha deprotonation under photoelectrochemical activation, producing radicals that can be oxygenated, Eq. (29). On colloidal zinc sulfide, hydrogen evolution accompanies the photocatalytic... 

In fact, the surface may mediate the requisite chemistry of the initially formed radical cation so that different products can be observed from the same intermediate when generated photoelectrochemically or by other means. The radical cation of diphenyl-ethylene, for example, gives completely different products upon photoelectrochemical activation 2 than upon electrochemical oxidation at a metal electrode or by single electron transfer in homogeneous solution, Eq. (31) . Surface control of... 

It was concluded that by combining a hematite nanorod electrode with a suitable water oxidation catalysts, for instance platinum or ruthenium dioxide, the photoelectrochemical activity for direct water splitting applications should increase by a factor of 20. 

The photoelectrochemical activity inherent in thin films of aggregated cyanine dyes permits them to act as the spectral sensitizers of wide bandgap semiconductors [69]. It is seen from Fig. 4.14 that the photoelectrochemical behaviour of semiconductor/dye film heterojunctions fabricated by deposition of 200 nm-thick films of cyanine dyes on the surface of TiC>2 and WO3 electrodes, bears close similarity to that of semiconductor electrodes sensitized by the adsorption of dye aggregates. Thus, both anodic and cathodic photocurrents can be generated under actinic illumination, the efficiency of the photoanodic and photocathodic processes and the potential at which photocurrent changes its direction being dependent on dye and semiconductor substrate [69]. 

Figure 19.4 shows a schematic representation of a photoelectrochemical cell. Photo-generated electrons are driven through the external circuit as a result of the potential difference across the cell. Hence, oxidation and reduction reactions occur at different electrodes of the device. Photoelectrochemically active semiconductor devices have been prepared from colloidal ZnO, Ti02, Sn02, and WO3 [95]. In a photoelectrochemical cell the photocurrent observed provides a direct measurement of the reaction rate. 

Intramolecular cycloadditions can also be induced by photoelectrochemical activation. Norbomadiene, for example, can be cyclized upon irradiation of ZnO, ZnS, CdS, or Ge semiconductors [179]. 

The term photoelectrochemistry is in general applied to all phenomena where photon absorption is accompanied by electrochemical processes. The usual requirements for photoelectrochemical activity are (1) the semiconductor character of the electrode... 

---