Selenid-Ion

The synthetic approach is very simple and does not require any special set up. In a typical room temperature reaction, 1.0 mL aqueous solution of cadmium chloride was added to 20 mL aqueous solution of soluble starch in a 50 mL one-necked round-bottom flask with constant stirring at room temperature. The pH of the solution was adjusted from 6 to 11 using 0.1 M ammonia solution. This was followed by a slow addition of 1.0 mL colourless selenide ion stock solution. The mixture was further stirred for 2 h and aged for 18 h. The resultant solution was filtered and extracted with acetone to obtain a red precipitate of CdSe nanoaprticles. The precipitate was washed several times and dried at room temperature to give a material which readily dispersed in water. The same procedure was repeated for the synthesis of PVA and PVP - capped CdSe nanoparticles by replacing the starch solution with the PVA and PVP polymers while the synthesis of elongated nanoparticles was achieved by changing the Cd Se precursor ratio from 1 1 to 1 2. The synthesis of polymer capped ZnSe nanoparticles also follows the same procedure except that ZnCb solution was used instead of CdCb solution. 

As discussed earlier the whole process is a redox reaction. Selenium is reduced using sodium borohydride to give selenide ions. In the above reaction, the metal ion reacts with the polymer (PVP or PVA) solution to form the polymer-metal ion solution. Addition of the selenide ion solution to the polymer-metal ion solutions resulted in instantaneous change in the colour of the solutions from colourless to orange (PVA) and orange red (PVP). This indicates the formation of CdSe nanoparticles. The addition of the selenide solution to the polymer - metal ion solution resulted in gradual release of selenide ion (Se -) upon hydrolytic decomposition in alkaline media (equation 4). The released selenide ions then react with metal ion to form seed particles (nucleation). 

H2Se (hydrogen selenide, colorless), HSe (acid telluride ion, colorless), Se (selenide ion, colorless), H2Se03 (selenous acid, colorless), HSeOJ (acid selenite ion, colorless), SeO (selenite ion, colorless), H2Se04 (selenic acid, colorless), HSeOJ (acid selenate ion, colorless), SeO " (selenate ion, colorless). 

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. 

A selenium centered cube is found in Cu8(p8-Se)[Se2P(OPr )2]6, in which each face of the cube is bridged by a diselenophosphato ligand and a selenide ion, Se2, is encapsulated within the cube.435,436... 

Such a linear relationship between and pH has also been observed with compound semiconductors [Pleskov, 1980 Morrison, 1980]. Figure 5-55 shows experimental observations for several compound semiconductors in which the flat band potential depends linearly on pH, except for WSc2 whose flat band potential does not depend on pH but depends on the concentration of hydrated selenide ions in aqueous solution. 

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 ... 

Gobet and Matijevic (17) produced monodisperse submicrometer-size particles of cadmium selenide (CdSe) and lead selenide (PbSe) by reversible release of selenide ions from selenourea in solutions of the corresponding metal salts. The equilibrium between selenourea and selenide ions is written as follows ... 

In one stndy of PbSe deposition from a citrate-complexed selenourea solution containing hydrazine, the rate was proportional to the pH and to the sele-nonrea concentrations bnt independent of the Pb and citrate concentrations [65], This was explained by a rate-determining step involving decomposition of sele-nonrea at the (catalytic) PbSe snrface by hydroxide. It is noteworthy that the Pb concentration was typically an order of magnitude less than that of selenourea. Therefore the independence of the rate on Pb (or citrate, which determines the concentration of free Pb ) concentration, suggests that formation of selenide ion, and not a complex-decomposition mechanism, occurs. 

Before going into details of the various aspects of specific CdSe depositions, and although it is not intended to deal with mechanistic aspects here (they have been considered already in Chap. 3), it bears mentioning that, although in contrast to CdS, the complex-decomposition mechanism has not been discussed with respect to CdSe deposition, it is still possible that this mechanism does occur in some, or even many, cases. If there is no evidence specifically in favor of this mechanism in general, there is also none against it. This point is stressed here since, in the literature on CdSe (and selenides in general), it is automatically assumed that the reaction proceeds via free selenide ions. 

For low selenosulphate concentrations, only the small crystals were formed, even in thicker films, and this was rationalized by the lower steady-state selenide concentration, which would favor cluster growth over ion-by-ion formation (the product of free lead and selenide ions needs to be larger than the solubility product of PbSe for ion-by-ion deposition to occur). An important difference between the citrate depositions and the NTA or hydroxide ones is that, even in the ion-by-ion citrate deposition, some low concentration of colloidal hydrated oxide was present, due to the relatively low complexing strength of citrate. The pH of the hydroxide baths (> 13) was much higher than that of the citrate or NT A baths (10.8). 

Electrochemical methods can be applied to the determination of the composition of solid phases as well as mixtures of solids [224-228], The first situation is illustrated in Fig. 4.1, where cathodic voltammograms of CuS, CuSe, and a solid phase of composition CuSeoASo.e reported by Meyer et al. [227] are shown. This last can be described as a solid solution formally regarded as a copper sulfide, in which 40% of sulfide ions have been replaced by selenide ions. The new phase produces a voltammetric peak at a potential intermediate between those for CuS and CuSe. 

What type of reagent is the selenide ion in this reaction Give a reason for your choice. [3]... 

In a rare example of the use of iodonium salts for heteroatom-heteroatom bond formation, diaryliodonium halides were employed with sodium 0,0-diethyl phosphoroselenolate for a one-pot synthesis of diaryl diselenides (Scheme 9) [27]. These transformations probably occur via arylation of the phosphoroselenolate salt with the diaryliodonium ions, hydrolysis of the resulting aryl phosphoroselenolates with sodium hydroxide, and air oxidation of the arene-selenide ions thus produced. 

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