Zinc Chalcogenides

Efficient photoelectrochemical decomposition of ZnSe electrodes has been observed in aqueous (indifferent) electrolytes of various pHs, despite the wide band gap of the semiconductor [119, 120]. On the other hand, ZnSe has been found to exhibit better dark electrochemical stability compared to the GdX compounds. Large dark potential ranges of stability (at least 3 V) were determined for I-doped ZnSe electrodes in aqueous media of pH 0, 6.3, and 14, by Gautron et al. [121], who presented also a detailed discussion of the flat band potential behavior on the basis of the Gartner model. Interestingly, a Nernstian pH dependence was found for 

C/pB estimated by both electrical (Mott-Schottky) and optical (photocurrent voltammetry) methods in the media studied, for (11 l)-oriented ZnSe electrode surfaces. A different variation was observed for the (110) orientation at pH 6. At pH 0, for both (110) or (11 l)-oriented electrode surface, the flat band potential value was -1.65 V (SHE) and the measured potential stability range (no detected current) was -0.35 to +2.65 V (SHE). A comparison of band levels with the other II-VI compounds as well as decomposition levels of ZnSe is given in Fig. 5.6. 

Having investigated the electrochemical behavior of ZnSe, and in view of the well-known blue luminescence of the compound, the previous authors extended their work to study electroluminescence from I-doped n-ZnSe crystals under anodic polarization in aqueous media containing metal ions such as Cu(II) and Sn(II) [123]. 

It was observed in other works that in sulfide electrolyte, decomposition of ZnSe was still obtained stable PECs could be constructed though from singlecrystal, n-type, Al-doped ZnSe electrodes and aqueous diselenide or ditelluride electrolytes [124]. Long-term experiments in these electrolytes were accompanied by little electrode weight loss, while relatively constant photocurrents and lack of surface damage were obtained, as well as competitive electrolyte oxidation. Photoluminescence and electroluminescence from the n-ZnSe Al electrodes were investigated. 

Photoelectrochemical studies with ternary chalcogenide systems containing zinc as one of the components have been published however, such investigations on bulk or thin film binary ZnS and ZnTe electrodes are practically absent from the literature or may be found fragmentary in electrosynthesis-oriented works. ZnTe has been studied as a possible candidate for a photocathode in the photoelectrochemical production of hydrogen. Related information will be given in the relevant section. 

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Equilibria considerations on solution-grown zinc chalcogenide compounds have been put forward by Chaparro [28] who examined the chemical and electrochemical reactivity of solutions appropriate for deposition of ZnS, ZnSe, ZnTe (and the oxide ZnO) in order to explain the results of recipes normally used for the growth of such thin films. The author compared different reaction possibilities and analyzed the composition of solutions containing zinc cations, ammonia, hydrazine, chalcogen anions, and dissolved oxygen, at 25 °C, by means of thermodynamic diagrams, applicable for concentrations usually employed in most studies. 

Zinc chalcogenide thin hlms have been grown by ECALE using zinc sulphate as metal source and sodium sulphide and sodium selenite as chalcogenide precursors.145-148 The formation of the hrst layers of ZnS on (lll)Au has been analyzed by STM and XPS.145 HRSEM images showed that the him surface was very hat, even at an atomic level. On the other hand, thicker ZnS hlms were formed of well-separated crystal nuclei. The stoichiometry of a thicker ZnS him showed a slight excess of sulphur, with a Zn S ratio of 1 1.2. The band gap of a thicker him (deposition time 12 h) was 3.60eV.147... 

Reasonable results have also been obtained for Zn Mdssbauer isomer shifts in zinc chalcogenides from LMTO band calculations (Svane and Antoncik, 1986) and for Sb compounds (Ravenak et al., 1983) using the DYM-Aot method. The overall conclusion must be that the time is ripe for the systematic calculation of Fe isomer shift and electric-field-gradient values in Fe-0, Fe-S, and other polyhedra using ab initio Hartree-Fock-Roothaan or DVM-Aa methods. 

Svane, A., and E. Antoncik (1986). Theoretical investigation of the Zn Moss-bauer isomer shifts in zinc chalcogenides. Phys. Rev. B33, 7462-73. 

Dec et al. 1993, Wu 1998). The NMR spectra of all the zinc chalcogenides (ZnS, ZnSe and ZnTe) have been determined, showing that static Zn lineshapes of the hexagonal forms contain a CSA contribution (Bastow and Stuart 1988). 

There are only a few measurements on non-oxidic substrates. On polar zinc chalcogenides a6T behaves like on oxidic substrates [24,51]. 

M. Haher, W.E. Herder, O. Lutz, A. NoUe, Zinc-67 and sulfur-33 nuclear magnetic shielding in the zinc chalcogenides. Solid State Commun. 33 (1980) 1051-1053. 

Nasar A, Shamsuddin M (1992) Investigations of the thermodynamic properties of zinc chalcogenides. Theimochim Acta 205 157-169... 

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