Adsorption chalcogenides

Thus, it was established that adsorption of metal hydroxide species on the surface of the substrate provides a nucleation layer which is chemically converted to the metal chalcogenide. The forming metal chalcogenide layer acts then as a catalytic surface for subsequent anion and cation adsorption. 

Pathan, H. M. Lockhande, C. D. 2004. Deposition of metal chalcogenide thin films by successive ionic layer adsorption and reaction (SILAR) method. Bull. Mater. Sci. 27 85-111. 

Kanniainen, T. 2001. Studies of zinc and lead chalcogenide thin films grown by SILAR (successive ionic layer adsorption and reaction) technique. Ph.D. thesis. University of Helsinki, Helsinki, Finland. 

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. 

For the ion-by-ion reaction, nucleation is generally slower and the density of nuclei smaller. Additionally, growth occurs (ideally) only at a solid surface therefore nucleation is confined to two dimensions, in contrast to three dimensions for the cluster mechanism. The crystal growth may terminate when adjacent crystals touch each other or by some other termination mechanism, e.g., adsorption of a surface-active species. These factors should be valid regardless of whether the mechanism proceeds via free chalcogenide ions or by a complex-decomposition mechanism. 

In considering photoactivity on metal oxide and metal chalcogenide semiconductor surfaces, we must be aware that multiple sites for adsorption are accessible. On titanium dioxide, for example, there exist acidic, basic, and surface defect sites for adsorption. Adsorption isotherms will differ at each site, so that selective activation on a particular material may indeed depend on photocatalyst preparation, since this may in turn Influence the relative fraction of each type of adsorption site. The number of basic sites can be determined by titration but the total number of acidic sites is difficult to establish because of competitive water adsorption. A rough ratio of acidic to basic binding sites on several commercially available titania samples has been shown by combined surface ir and chemical titration methods to be about 2.4, with a combined acid/base site concentration of about 0.5 mmol/g . 

NCs is indispensable. In the case of cadmium chalcogenide NCs, the concentration of a colloidal solution can be determined in good approximation by means of UV-vis absorption spectroscopy thanks to tabulated relationships between the excitonic peak, the NC size, and the molar absorption coefficient.96 An advanced approach for shell growth derived from chemical bath deposition techniques and aiming at the precise control of the shell thickness is the so-called SILAR (successive ion layer adsorption and reaction) method.97 It is based on the formation of one monolayer at a time by alternating the injections of cationic and anionic precursors and has been applied first for the synthesis of CdSe/CdS CS NCs. Monodispersity of the samples was maintained for CdS shell thicknesses of up to five monolayers on 3.5 nm core CdSe NCs, as reflected by the narrow PL linewidths obtained in the range of 23 to 26 nm FWHM. 

A drastic decrease of photovoltage in UHV is obtained by introduction of surface states at the semiconductor surface. Particle bombardement of cleaved (0001) faces leads to preferential sputtering of the chalcogenide. The metal is reduced and new electronic bandgap states are formed at the surface. As a consequence a Fermi level pinning effect occurs which results in a smaller shift of EB due to halogen adsorption and decreased photovoltages and consequently an increased double layer potential drop (Fig. 4). 

Most common metal oxide and metal chalcogenide semiconductors have valence-band edges that lie positive of the oxidation potentials of most organic functional groups, and thermodynamics will thus favor photocatalytic oxidation. For efficient processes to take place, an easily oxidizable donor is required, but a whole range of substrates have been shown to be useful for this application. For example, a Japanese group has shown that this purpose is served not only by pure compounds, but even by wastes such as polyvinyl chloride, algae, protein, dead insects, and animal excrement, which function as electron sources [104]. Thus, synthetic utility is attained only if this wide reactivity is controlled. In practice, selectivity is best controlled by the adsorption and oxidation potential effects [105],... 

Little is known about the properties and potential applications of the open-framework chalcogenides, though Jiang et al. [102,103] have recently described the adsorption and spectroscopic properties of some of the layered tin sulfides belonging to this family of materials. 

The practical use of these calculations is limited, however, because the kinetics of a reaction can play an important role. This becomes quite obvious for layer compounds such as M0S2. The kinetics may be controlled by adsorption, surface chemistry, surface structure and crystal orientation. According to Fig. 8.15, pEdecomp is close to the conduction band, i.e. M0S2 is rather easily oxidized. In the case of a flat basal surface, it has been observed with several transition metal chalcogenides that the photocurrent onset at n-electrodes occurs with high overvoltages accompanied by a shift of Gfb.(see Section 5.3). Since this is caused by an accumulation of holes at the surface the hole transfer is kinetically inhibited. 

J. K. Lorenz, T. F. Kuech, A. B. EUis, Cad-mimn Selenide Photolmninescence as a Probe for the Surfece Adsorption of Di-alkyl Chalcogenides, Langmuir 1998, 14, 1680-1683. 

Recently it has been reported that Ru addition to PtCo/C results in catalysts with improved tolerance to methanol under typical ORR reaction conditions i.e. potentials more positive than 0.7 V NHE and O2 saturated acid electrolyte. This is because under these reaction conditions, upper oxide Ru species are stable and hinder methanol adsorption. Another interesting alternative is the Ru-based chalcogenides. In particular Ru Sey-based electrocatalysts have received a great deal of attention because of their high tolerance to methanol, even if their performance as electrocatalysts for the ORR is inferior to Pt/C by =40%. ... 

The absorption of the chalcogenide colloidal solution was verified by its adsorption onto a thin film of Ti02 as a function of time. A follow-up of this process was done by UV-VIS absorption. Figure 22 shows the evolution of the spectra. As expected, increasing the adsorption time of the colloids leads to an increase of the absorption in the whole visible region. It should be noted that the adsorption of the RuxSey nanoparticles is very fast, in fact, in 1 minute the adsorption is nearly the same as that measured after 51 rrunutes. The stmctureless spectra can be attributed to the fact that... 

In addition to the adsorption studies of carbonyl, phosphine oxide, and sulfide adsorbates noted in Ref. 10, the studies of interaction of anionic S donors such as dithiocarbamates [17], anionic S-containing species (MoS and Et2NCS ) [18], thiolates [19], and polysulfides [20] with cadmium chalcogenides have been characterized by Wrighton and coworkers [17-20]. This chapter will focus on surface modification of cadmium chalcogenides with thiols, dithiols, sulfides, and disulfides. 

Adsorption of dialkyl chalcogenide compounds, which are candidate chemical vapor deposition precursor molecules, has been examined on CdSe substrates by Ellis and coworkers [21]. The compounds studied in these experiments include two families of dialkyl chalcogenides (CH3)2E (E = S, Se, Te) and R2S (R = CH3, C2H5, n-C Hy, and f-C4H9. The elec-... 

Figure 17.5. Plot showing adsorption isotherms of CO2 and H2 at 273 K on a C0M0S4 aerogel with a surface area of 340 m /g. The chalcogenide aerogel shows a selectivity of CO2 over H2 of nearly 16 times. Reproduced with permission from Nat. Chem., 1 217-224 (2009). Copyright 2009 Macmillan Publishers Ltd.

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