Selenosulfate

Selenosulfate reacts with ethyleneknine ia the same way as thiosulfate to give 2-ainiQoethaiieselenosulfuric acid. However, the reaction of ethyleneimine using selenous acid does not yield a stable product (137). 

In a cychc method selenium is dissolved in hot sodium sulfite solution to form sodium selenosulfate [25468-09-1]. 

Other inorganic selenium compounds include sodium selenocyanate [4768-87-0], NaSeCN, which is prepared by melting together selenium and sodium cyanide selenocyanogen [27151 -67-3] (SeCN)2 sodium selenosulfate [25468-09-1], Na2SeS02, which is prepared by dissolving selenium in aqueous sodium sulfite (acidification decomposes this compound) and selenate alums, eg, Al2(Se0 2 I SeO [13530-59-1]. 

Yochelis, S. and Hodes, G. (2004). Nanocrystalline CdSe Formation by Direct Reaction between Cd Ions and Selenosulfate Solution. Chem. mater., 16,2740-2744. 

Elemental sulfur dissolves in boiling aqueous sodium sulfite solutions with the formation of sodium thiosulfate (Na2S203). The reaction proceeds quantitatively if sulfur and excess sodium sulfite are boiled for some time in weakly alkaline solutions. In the cold, however, practically no reaction occurs. Alternatively, thiosulfate can be produced quantitatively in solution phase by using organic solvents to first dissolve sulfur and then accomplish the reaction with aqueous sulfite. In a parallel reaction, elemental selenium dissolves in alkaline sulfite solution to produce selenosulfate, SeSO ... 

Selenosulfate is an analogue of thiosulfate wherein one of the S atoms is replaced by a Se atom. Thiosulfate and selenosulfate anions are known to have tetrahedral structure as constituting the S and Se analogues, respectively, of the sulfate anion. The isomeric thioselenate anion SSeO " is not produced by the reaction of sulfur with selenite nor is the selenoselenate ion 86203 formed from selenium and selenite. Actually, SSeOj may be produced as a metal salt by boiling an aqueous solution of selenite with sulfur, but in aqueous solution thioselenates are not stable and isomerize to selenosulfates. 

Colloidal sulfur (selenium) can be produced readily from the disproportionation of aqueous thiosulfate (selenosulfate) with dilute acids ... 

A method to circumvent the problem of chalcogen excess in the solid is to employ low oxidation state precursors in solution, so that the above collateral reactions will not be in favor thermodynamically. Complexation strategies have been used for this purpose [1, 2]. The most established procedure utilizes thiosulfate or selenosulfate ions in aqueous alkaline solutions, as sulfur and selenium precursors, respectively (there is no analogue telluro-complex). The mechanism of deposition in such solutions has been demonstrated primarily from the viewpoint of chemical rather than electrochemical processes (see Sect. 3.3.1). Facts about the (electro)chemistry of thiosulfate will be addressed in following sections for sulfide compounds (mainly CdS). Well documented is the specific redox and solution chemistry involved in the formulation of selenosulfate plating baths and related deposition results [11, 12]. It is convenient to consider some elements of this chemistry in the present section. 

In alkaline media, the selenosulfate ions disproportionate to sulfate and highly reactive selenide ions ... 

The value of this method lies in the fact that formation of elemental selenium is unlikely to occur since the high-valency species such as Se(IV) that could oxidize the selenide ions are absent from solution. The SeSO and SOj ions (or their protonated forms) do not oxidize Se , while any free Se that may be formed would redissolve in sulfite giving selenosulfate again, since the latter is prepared by dissolving Se in excess sulfite. 

In the case of electrochemical deposition, several mechanisms have been proposed to account for the formation of the end-product film, the difference among them consisting in the assumed electrochemical step. This may be the reduction of selenosulfate (3.3), inducing deposition of the metal (3.4) ... 

Alternatively, formation of a metal-selenosulfate complex may be assumed to take place in the solution (3.7)) ... 

This complex may be reduced either directly to the metal chalcogenide or to the zero-valent metal, which then reacts chemically with selenosulfate adsorbed at the cathode to form MSe. 

The detailed mechanism dictating the regulation of the process depends on the specific nature of the system, i.e., on the particular compound to be deposited, complexing agent, solution pH, film thickness, potential, etc. For example, in the case of the Cd-Se system, electroreduction of selenosulfate occurs at more positive potentials for either EDTA-ammonia- or NTA-complexed cadmium [13], whereas for ZnSe, the potential required for the reduction of selenosulfate is already reducing for zinc, implying thus a different mechanism. The metal complex has to be adequately stable and should not interfere with selenosulfate reduction. In these terms. 

Alkaline aqueous media containing selenosulfate ions (SeS03 ) and complexes of Cd with EDTA or nitrilotriacetate (NTA) have been successfully utilized to accomplish cathodic electrodeposition of nearly stoichiometric, wurtzite CdSe [62-64]. Similar results have been reported for seleno-cyanate (SeCN ) alkaline baths... 

A quantitative analysis of the kinetics of CdSe deposition from selenosulfate, Cd(II)-EDTA baths in terms of a mechanism involving nucleation and electrode kinetics has been given by Kutzmutz et al. [65], Note also that selenosulfate-containing baths have been used for the anodic selenization of vacuum-deposited metal films in order to synthesize CdSe and other binary selenide semiconductor thin films such as CuSe and InSe [66],... 

Utilizing low-oxidation selenium precursors appears to be particularly suited for obtaining single-phase ZnSe deposits. Results have been presented of ZnSe electrosynthesis from alkaline selenosulfate solutions of complexed Zn(II) [108]. 

The redox behavior of the SeSO -Zn-EDTA system has been discussed on the basis of Pourbaix and solubility diagrams [11], Different complexes and substrates have been employed in order to optimize the electrodeposited thin films. By the selenosulfate method it is generally possible to grow ZnSe with an almost stoichiometric composition however, issues of low faradaic efficiency as well as crystallinity and compactiveness of the product, remain to be solved. Interestingly, in most reports of photoelectrochemically characterized ZnSe electrodeposits, the semiconductor film was found to be p-type under all preparation conditions (ZnSe is normally n-type unless deliberately doped p-type). 

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]. 

Yochelis S, Hodes G (2004) Nanocrystalhne CdSe formation by direct reaction between Cd ions and selenosulfate solution. Chem Mater 16 2740-2744... 

SkyUas-Kazacos M, Miller B (1980) Electrodeposition of CdSe films from selenosulfate solution. J Electrochem Soc 127 2378-2381... 

Bouroushian M, Kosanovic T, Spyrellis N (2005) Aspects of ZnSe electrosynthesis from selenite and selenosulfate aqueous solutions. J Solid State Electrochem 9 55-60... 

Kosanovic T, Bouroushian M, Spyrellis N (2005) Soft growth of the ZnSe compound from alkaline selenosulfate solutions. Mater Chem Phys 90 148-154... 

While most mechanistic investigations have been carried out on CdS, other semiconductors, in particular CdSe, have also been studied with regard to the deposition mechanism. Kainthla et al., in their study of the formation of CdSe films from ammoniacal solutions containing sodium selenosulfate, noted that when a visible precipitate of Cd(OH)2 was present in their solutions (obtained by adding... 

Selenium forms thio"-type compounds, such as SeS03 by reaction of selenium and sulfur tnoxide, SeS032 (selenosulfates) by reaction of selenium and sulfites, SeSy (selenosulfides) by reaction of selenium with sulfides, as well as diselenides, Se2i, and polyselenides, Set2-. 

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