Selenite formation

Cathodic electrodeposition of microcrystalline cadmium-zinc selenide (Cdi i Zn i Se CZS) films has been reported from selenite and selenosulfate baths [125, 126]. When applied for CZS, the typical electrocrystallization process from acidic solutions involves the underpotential reduction of at least one of the metal ion species (the less noble zinc). However, the direct formation of the alloy in this manner is problematic, basically due to a large difference between the redox potentials of and Cd " couples [127]. In solutions containing both zinc and cadmium ions, Cd will deposit preferentially because of its more positive potential, thus leading to free CdSe phase. This is true even if the cations are complexed since the stability constants of cadmium and zinc with various complexants are similar. Notwithstanding, films electrodeposited from typical solutions have been used to study the molar fraction dependence of the CZS band gap energy in the light of photoelectrochemical measurements, along with considerations within the virtual crystal approximation [128]. 

Epitaxial effects are not limited to single-crystalline substrates. The possibility for substrate-induced epitaxial development in the difficult case of ZnSe (cf. conventional electrodeposition) has been established also by using strongly textured, albeit polycrystalline, zinc blende (111) CdSe electrolytic films to sustain monolithic growth of ZnSe in typical acidic selenite baths [16]. Investigation of the structural relations in this all-electrodeposited ZnSe/CdSe bilayer revealed that more than 30-fold intensification of the (111) ZnSe XRD orientation can be obtained on the textured (111) CdSe films, compared to polycrystalline metal substrates (Fig. 4.2). The inherent problems of deposition from the Se(IV) bath, i.e., formation of... 

Alanyahoglu M, Demir U, Shannon C (2004) Electrochemical formation of Se atomic layers on Au(lll) surfaces the role of adsorbed selenate and selenite. J Electroanal Chem 561 21-27... 

Figure 3 Gradient separation of anions using suppressed conductivity detection. Column 0.4 x 15 cm AS5A, 5 p latex-coated resin (Dionex). Eluent 750 pM NaOH, 0-5 min., then to 85 mM NaOH in 30 min. Flow 1 ml/min. 1 fluoride, 2 a-hydrox-ybutyrate, 3 acetate, 4 glycolate, 5 butyrate, 6 gluconate, 7 a-hydroxyvalerate, 8 formate, 9 valerate, 10 pyruvate, 11 monochloroacetate, 12 bromate, 13 chloride, 14 galacturonate, 15 nitrite, 16 glucuronate, 17 dichloroacetate, 18 trifluoroacetate, 19 phosphite, 20 selenite, 21 bromide, 22 nitrate, 23 sulfate, 24 oxalate, 25 selenate, 26 a-ketoglutarate, 27 fumarate, 28 phthalate, 29 oxalacetate, 30 phosphate, 31 arsenate, 32 chromate, 33 citrate, 34 isocitrate, 35 ds-aconitate, 36 trans-aconitate. (Reproduced with permission of Elsevier Science from Rocklin, R. D., Pohl, C. A., and Schibler, J. A., /. Chromatogr., 411, 107, 1987.)...

For Elg2Se03 three modifications have been described.133 They all contain dumbbell-shaped EIg22+ ions that are linked by the selenite ions. A common feature of all crystal structures is the formation of channels, which obviously incorporate the lone electron pairs of the selenite ions. 

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... 

The methodology most practiced is referred to here as codeposition, where a single solution contains precursors for all the elements being deposited and is reduced at a fixed potential or current density. The earliest report appears to be that by Gobrecht et al., which was published in 1963 [45]. Two anodes were used in the study, one of Se and one of Cd (or Ag), to form selenite and cadmium ions, respectively. CdSe was then formed by co-reduction of both species at the cathode. Reports of the formation of GaP in 1968 [46] and ZnSe in 1975 [47] via codeposition were subsequently published, and both involved molten salt electrolysis. 

In the case of CdSe formation using the codeposition methodology, a problem was encountered early on and studied by Skyllas-Kazacos and Miller [122]. It concerned the formation of selenide ions and their reaction with the selenite starting material to form elemental Se ... 

Andreesen JR, Ljungdahl LG. 1973. Formate dehydrogenase of Clostridium ther-moaceticum incorporation of selenium-75, and the effects of selenite, molybdate, and tnngstate on the enzyme. J Bacteriol 116 867-73. 

Pinsent J. 1954. The need for selenite and molybdate in the formation of formic dehydrogenase by members of the coli-aerogenes group of bacteria. Biochem J 57 10-16. 

Selenium, in the form of selenate or selenite, is toxic to D. desulfuricans (Tomei et al. 1995) and Wolinella succinogenes (Tomei et al. 1992) at elevated levels. At sublethal levels of 0.1-1. OmM selenite or 10 mM selenate, minimal levels of growth is observed with both D. desulfuricans and Wolinella succinogenes. With both selenate and selenite, colloidal elemental selenium (Se°) is produced inside the cell and released into the culture fluid after cell death. This reduction of Se(VI) and Se(IV) by these anaerobes is not coupled to growth and proceeds by mechanisms that have not yet been identified. Selenite and selenate reduction with formation of elemental selenium by these nonrespiratory processes serve to detoxify the environment for future bacteria and may be important for the geochemical cycle of selenium. 

The kinetics of reaction of Fe " aq, of FeOH +aq, and of Fe2(OH)2 " "aq with variously proto-nated forms of phosphate, phosphite, hypophosphite, sulfate, and selenite have been investigated, mainly at 283 K. The formation mechanism from the dimer is somewhat complicated, e.g., by formation of mononuclear complexes, probably via /i-hydroxo-/r-oxoanion di-iron intermediates, after the initial 4 complexation step. ... 

Davis, C.D. and Uthus, E.O. (2002) Dietary selenite and azadeoxycytidine treatments affect dimefhylhydrazine-induced aberrant crypt formation in rat colon and DNA mefhylation in HT-29 cells. The Journal of Nutrition, 132, 292-297. 

A few sulfite, arsenate, selenite and selenate compounds were reported355 357 but should be reinvestigated. V02+ forms a deep purple complex with phosphotungstic acid,589 in contrast with the yellow complex with Vv. Spectrophotometrically, the formation constant is 1.3 x 105. The kinetics with 12-tungstovanadophosphates were analogous to those with 12-molybdotungstophosphates.590... 

The selenites are readily attacked by micro-organisms with the formation of a red substance, probably metallic selenium.13 The... 

In comparative studies with C. humicola (90), Cox and Alexander reported that phosphate will inhibit (CH3)3As formation from arsenite, arsenate, or methylarsonate but not from cacodylate. Selenite, selenate, and... 

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