Other Metal Chalcogenides

The other compounds that have been examined are compiled in Table 14. These materials have been studied both as photoanodes (Entries 2 4) and in powder form (Entries 1, 5 10). As with their oxide counterparts, the chalcogenide family is also rich in solid solution chemistries and Entries 6, 7, 9 and 10 in Table 14 exemplify this trend. These materials are further discussed in Section 12 of this Chapter. 

Interest in d-band semiconductors derives from the fact that holes in d bands are not equivalent to broken chemical bonds and thus do not constitute a primary pathway for photocorrosion as in the case of semiconductors (e.g., CdS) with p-orbital derived valence bands.33,541,542 Thus M0S2,541,542 WS2,541,542 PtS2,543 RuS2 33,532-534 

Entry number Metal chalcogenide(s) Band gap energy eV Comments Reference (s) 

2 n-CdSe 1.75 Used in bipolar configuration with CoS. Polymer-coated samples also examined. 155, 205, 206 

4 n-MSe2 (M = Mo, W) -1.0 Used in conjunction with p-InP photocathodes in p-n photoelectrolysis cells. 204 

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The participation of Cd(OH)2 in the deposition of CdS (and other metal chalcogenides) has been demonstrated or suggested on many occasions. Kitaev et al. presented a theoretical thermodynamic treatment of the Cd " /ammonia/ thiourea system to show when Cd(OH)2 should be present as a solid phase in the deposition solution [36]. A graphical representation of this analysis is shown in Eigure 3.1. This graph is based on two equilibria the solubility product of Cd(OH)2 and the stability constant of the ammonia (ammine) complex of Cd. Consider first the former ... 

Hodes G. (1983), Photoelectrochemistry of cadmium and other metal chalcogenides in polysulfide electrolytes , Ch. 13 in Energy Resources through Photochemistry and Catalysis, Gratzel M. (ed.). Academic Press, New York, pp. 421-465... 

A novel CD technique used metallic A1 to reduce Se. Elemental Se and metallic A1 foil, together with ZnCE, were dissolved in NaOH solution and heated to 80°C in an autoclave with substrates of Teflon or alumina [145]. (Metal chalcogenide films have been chemically deposited onto Teflon in a number of reports.) It is notable that the films deposited onto alumina were reported to be 1.8 p,m thick— much thicker than ZnSe CD from other solutions. The films were sphalerite ZnSe... 

Considering that homogeneous precipitation of metal chalcogenides (mainly sulphides) by reaction between metal ions and dissolved chalcogen is well established, the main difference between this deposition and similar reactions seems to be that the products adhere to a substrate to give a visible fdm (in this case) rather than only precipitate. Whether this is connected with the redissolu-tion/redeposition process that occurs with the Sn-S system or has some other explanation is important. If the former, it may be limited to only those systems that behave similarly. Otherwise it is not unreasonable to expect that other metal sulphides and selenides (possibly also tellurides, although tellurium tends to be much less soluble, if at all, in such solvents) may be deposited as films in this manner. 

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