Compounds and other New Materials

Layered Compounds and Other New Materials. Another active area of research in photoelectrosynthesis involves the study of new semiconductor electrode materials. Layered compounds are particularly interesting since the top of the valence band is comprised of metal orbitals rather than anion 4p orbitals (55). This means that an optical transition near the band gap energy involves only metal to metal transitions (the conduction band is also made of d-like metal orbitals) and does not disrupt a metal-anion bond. This type of optical transition is believed to be less susceptible to photocorrosion than the usual transitions involving anion-like orbitals in the valence band. 

Results with layered chalcogenides (56-60) such as M0S2 (Eg= 

MoSe CEg = 1.4 eV), MoTe2 (Eg = 1.0 eV), WSe2 (Eg = 1.6 eV), and W 2 (Eg = 2.0 eV) show that enhanced resistance to photocorrosion is indeed observed. However, photocorrosion readily occurs at the edges of the layered materials (11 to C-axis), and this presents problems for the planar face also since atomic-size step dislocations in the surface become photocorrosion sites. Much intense research in layered compounds is proceeding at the present 

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