Cluster hydroxide clusters

Reaction (2.12) occnrs because K p for CdS (10 Table 1.1) is much smaller than that for Cd(OH)2 (2 X 10 ). Another way of looking at this is that the free energy of formation of CdS is more negative than that of Cd(OH)2. It has also been suggested that the hydroxide cluster can act as a catalyst for thiourea decomposition. In this case, sulphide formation will occur preferentially at the surface of the hydroxide rather than nucleate separately in the solution. Such a course is logical based on the previous discussion of the effect of metal cations on the equilibrium of thiourea decomposition. 

The chalcogenide precursors possess many talents. Apart from forming the chalcogenide ions, they also form complexes with metal ions. As noted at the beginning of this section, and ignoring the distinction between ion-by-ion and hydroxide cluster mechanisms treated previonsly, CD processes can be divided according to two basic mechanisms participation of free snlphide ions (the... 

Probably the least-known aspect of the CD process is what determines the nucle-ation on the substrate and the subsequent film growth. In considering this aspect, we will treat the ion-by-ion and hydroxide cluster mechanisms separately, although there will be many features in common. The principles discussed should be the same for both the free chalcogenide and the complex-decomposition mechanisms. 

The basic features of the ion-by-ion and hydroxide cluster film-forming mechanisms are shown schematically in Figures 2.1 and 2.2, respectively. Film formation involving complex decomposition will proceed in a similar manner (Fig. 2.3 shows this for a molecule-by-molecule deposition). 

For the cluster mechanism, while growth and termination can be similarly explained, the induction period is less obvious. The hydroxide cluster can start to adsorb on the substrate immediately after immersion of the substrate in the deposition solution, yet experiments have shown that film growth often does not occur for some time. While the reason for this is not clear, it may be connected with the... 

Fig. 2.9 Transmission spectra of CD CdSe films deposited at various temperatures from CdS04/NTA/Na2SeS03 baths. All samples deposited by hydroxide cluster mechanism except 80°C HC (high complex), which proceeded via the ion-by-ion mechanism. The effective bandgap can be approximated by the wavelength (photon energy) a little shorter (higher) than the absorption onset. A second absorption knee, ca. 0.4 eV to higher energy of the initial onset, seen clearly in the 41 °C and 80°C samples, is due to a transition from the spin-orbit valence level to the lowest conduction level and is commonly observed in these films.

They based this modification on the known adsorbance of OH on glass and on the common occurrence of transition metal mixed water-ammonia complexes with coordination number of 4. Parallel stractural studies of the deposited CdS showed textured growth, supporting a mechanism whereby alternate Cd and S species were involved, in an ion-by-ion process. Such a growth suggests adsorption of a molecular hydroxy-ammine species rather than a cluster. In fact, the mechanism of Ortega-Borges and Lincot also does not differentiate between a hydroxide cluster and molecule. 

All these results show that Cd(OH)2 colloids do adsorb on a substrate (either under conditions where Cd(OH)2 is present in solution or, according to the studies of Rieke and Bentjen and Ortega-Borges and Lincot [48], even when it is not present in solution but under solution conditions close to solid hydroxide formation). The induction period when no deposition is seen in the hydroxide-cluster deposition therefore is understood to mean that a fast and nongrowing Cd(OH)2 adsorption has occurred, which is too fast and/or too httle to measure by the experimental methods used to make the kinetic curves, and that only when the hydroxide starts to convert into the chalcogenide, by reaction of the slowly formed chalcogenide ion with the hydroxide, does real film formation proceed. 

The factors that determine crystal size, a topic of particular relevance to this chapter, have been discussed to some extent in Section 3.4. There are two main factors that generally affect crystal size for any particular material the deposition mechanism and the deposition temperature. The hydroxide cluster mechanism is expected to give a crystal size similar to that of the original hydroxide cluster (with some growth possible as deposition proceeds). That size depends mainly on temperature, both because higher temperatures allow more grain growth and, possibly more important, lower temperatures kinetically stabilize very small nuclei in solution that are thermodynamically unstable. For example, in the hydroxide cluster mechanism, where crystal size is believed to be controlled mainly by the size of the Cd(OH)2 colloids, the relevant equilibria are... 

CN, OSO2CF3, OH, BF4, OMe, and L = H2O, DMSO, DMF, OPPh3, OAsPhs, NH3 MeCN, pyridine, PR3. The stability of [M6Xi2] + compounds toward hydrolysis decreases in the order Ta > Nb, Cl > Br, and with increasing oxidation states. Moisture- and oxygen-sensitive alkoxides of Me units have been obtained by substitution reactions their hydrolysis offers a clean route to cluster hydroxides. ... 

Figure6.32 Structure of the cationic cluster complex [Eu4(p.3-0H)4(nic)6(H20)8]2 + (nic= nicoti-nate). Hydrogen atoms are removed for clarity [57]. (Redrawn from X. Kong et al., Hydrolytic synthesis and structural characterization of lanthanide hydroxide clusters supported by nicotinic acid, Inorganic Chemistry, 48, 3268-3273, 2009.)...

The octanuclear lanthanide hydroxide cluster core was first identified in Er8( X4-0)( X3-OH)i2(THD)s (THD = 2,2,6,6-tetramethyUieptane-3,5-dionate) [25, 87]. It is a triangulated dodecahedron with an interstitial oxo group each of its triangular faces is capped by a IX3-OH group. The same cluster core has also been found in [Eu8(p-6-0)((i,3-0H)i2( X2-0Tf)i6(0Tf)2] (Eigure 6.40) [99] and [Eu8( X4-0)((i,3-0H)i2(DME)8(Se3)(Se4)3(Se5)2] [105]. That the same cluster core is obtained with different ancillary ligands suggests the prevalence of this motif in the lanthanide hydroxide coordination chemistry. 

Zheng, Z. (2003) AssembUng lanthanide hydroxide clusters. Chemtracts, 16, 1-12. 

Andrews, P.C., Beck, T., Forsyth, C.M., Fraser, B.H., Junk, PC., Massi, M. et al. (2007) Templated assembly of a p-g-COj" dodecanuclear lanthanum dibenzoylmethanide hydroxide cluster with concomitant formation of phenylglyoxylate. Dalton Transactions, 5651-5654. 

Experiments were also conducted to see whether that the alkynyl groups are perfect substitutes for alkoxyl groups towards hydrolysis. Indeed, when 4 is hydrolyzed, it leads to the same butyltin oxide-hydroxide cluster with 12 tin atoms as when butyltin triisopropoxide is used. Moreover, with the more bulky triisopropylphenyl group linked to the tin, two unusual oxide-hydroxide clusters, one with 10 tin atoms and the other with six, are produced, depending on the experimental conditions. ... 

In the present coprecipitation process, the interaction between hydroxide clusters of Mn and Zr would suppress each other to grow into large crystallites. This is a possible reason for the small particle size and thus the large surface area of the MnOx-ZrOa system. 

The line corresponds to the maximum value of the total concentration of cadmium ions in solution without precipitation of the hydroxide. If the experimental value is higher, then cadmium hydroxide clusters will be formed in the solution which can be converted into sulfide clusters (chapter 4). This must be taken into account in fixing the metal concentration in the CBD bath. It is possible to modify the solubility limit by changing the strength of the ligand or its concentration as also shown in figure 6. 

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