CdS colloid

When colloidal particles of very small diameter are prepared by carefully regulating the conditions, it was found that their electronic structure changes with size (size quantisation). CdS colloids of very small particle size were found to fluoresce, the colour depending on the particle size. 

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Thin film coatings of nanocrystalline semiconductors, as collections of quantum dots (QD or Q-dot) attached to a solid surface, resemble in many ways semiconductor colloids dispersed in a liquid or solid phase and can be considered as a subsection of the latter category. The first 3D quantum size effect, on small Agl and CdS colloids, was observed and correctly explained, back in 1967 [109]. However, systematic studies in this field only began in the 1980s. 

Studies performed on CdS [282, 283] have revealed the importance of the microstructure, i.e., crystal structure, crystallite size, and geometrical surface area, in both the control of band structure and the concentration and mobility of charges, in relation to the photocatalytic performance of the photocatalyst. It has been shown also that the solubility product of CdS colloids prepared from acetate buffer aqueous solutions of suitable precursors increases from 7.2x 10 for large particles to about 10 for small (< 2.5 nm) particle colloids, this increase invoking a positive shift on the cathodic corrosion potential [284]. 

Already Henglein found with bare CdS colloids that no reaction occurs in Oj-free solutions probably because a blocking sulfur layer was formed . This was also confirmed with catalyst loaded particles . ... 

Most striking is the increase in the fluorescence intensity of a CdS colloid as it undergoes photoanodic dissolution. As the colloidal particles become smaller they fluoresce with a greater quantum yield Very small CdS particles prepared by the methods described in section 5.1, fluoresce with a quantum yield of 3 %... 

An extraordinary way of stabilizing RUO2-coated CdS colloids for H2 generation was chosen by Fendler and co-workers The colloidal particles were generated in situ in surfactant vesicles of dioctadecyldimethylammonium chloride and dihexa-decyl phosphate. Thiophenol as a membrane permeable electron donor acted as a sacrificial additive. Later, a surface active re-usable electron donor (n-C,gH3,)2N — (CHj)—CH2—CHj—SH, Br was incorporated into the vesicles. Its R—SS—R oxidation product could be chemically reduced by NaBH to regenerate the active electron donor. The H2 yields in these systems were only 0.5 %. However, yields up to 10% were later reported for a system in which CdS was incorporated into a polymerizable styrene moiety, (n-C,jH3jC02(CH2)2) N (CH3) (CH2CgH4CH=CH2>, CP, and benzyl alcohol was used as the electron donor. 

Wu YD, Wang LS, Xiao MW, Huang XJ (2008) A novel sonochemical synthesis and nanostructured assembly of polyvinylpyrrolidone-capped CdS colloidal nanoparticles. J Non-Cryst Solid 354(26) 2993-3000... 

Absorption spectra of CdS colloid indicate the formation of quantum sized CdS particles. The particle size increased upon sonication, indicated by the red shift in the onset of absorption. The particle size was highly dependent on the mercaptan used, because of the absorption of the mercaptan on the particle acting as a capping agent and the rate of H2S produced. Study of mercaptan systems revealed that there was also a thermal process responsible for CdS formation. 25% of the total CdS produced sonochemically was formed via a thermal mechanism presumably in the hot shell around the compressed bubble. CdS colloid could be dissolved quite readily by sonicating solutions under air saturated conditions [89] by the following reaction,... 

Hobson RA, Mulvaney P, Grieser F (1994) Formation of Q-state CdS colloids using ultrasound. J Chem Soc Chem Commun 7 823-824... 

A modified SILAR system has been used to grow CdSe in CdS/CdSe core shell semiconductor nanocrystals.12 A cadmium precursor solution, with CdO dissolved with oleic acid in octadecane, was injected onto the substrate, and the Se solution (Se powder dissolved with tributylphosphine in octadecane) was similarly injected. The temperature of the reaction solution was 185 °C. A CdS outer layer in the CdS/CdSe/CdS colloidal quantum wells was deposited by alternating injections of cadmium and sulfur both in octadecane solutions at 230-240 °C. These structures showed high PL quantum yields (20-40%), relatively narrow emission bands, and tunable emission colors from about 520 to 650 nm depending on the number of CdSe monolayers. 

There have been a few experiments related to the effect of illumination of the growth of CdS films. Simple heating of the deposition bath by absorption of the radiation is one obvious factor that can affect the deposition [68]. However, even in this case, other effects occur, since the color of the bath was reported to darken if UV (sunlight) illnmination was employed. Based on previous studies of illuminated CdS colloids when elemental Cd was formed, both as a film and in solution [69], as well as the known tendency of ZnS to undergo reduction to metallic Zn under UV illumination, this darkening may be assumed to be caused by elemental Cd. There are several possible mechanisms that may explain snch an effect re-dnction of the CdS by photogenerated electrons is one possibility. 

In this work the authors summarize their own studies of photoprocesses on CdS colloids with particles of various size. In these studies, attention was given precisely to photocatalytic reactions on CdS, the photocatalytic reactions on TiC>2 were considered concurrently with the reported ones. In most cases photocatalytic reactions on semiconductors are the redox reactions. So of special interest was to study the regularities of reactions of interfacial transfer of photoexcited electron by the pulse photolysis and luminescence quenching methods. Many interesting phenomena were found while studying the model photocatalytic reactions by the method of stationary photolysis, i.e., under the conditions of real photocatalysis. 

In this chapter we consider the feasibility of easily controlled and reproducible synthesis of CdS colloids. To provide control and restrain the growth rate of the CdS nanoparticles, we used the complex salt of a colloid-forming component (Cd2+) instead of its diluted solution actually, in this case the rate of colloid growth may be limited by the decay rate of the initial cadmium complex. 

Thermodynamic Calculation of Equilibrium Size of CdS Colloidal Particles in the Presence of Cadmium Components... 

Upon the formation of CdS colloids, at the point when the colloidal particles stop their growth and the system reaches the quasiequilibrium state, the total amount of cadmium is distributed over CdS particles and the aqueous phase of the solution, where it appears both as activated Cd2+ ion and as the compounds with complexing admixtures. One may assume the thermodynamic equilibrium is reached between the different possible forms of cadmium occurrence. Assuming the presence of only one complexing agent L, which participates in a stepwise complexing, we may describe the above equilibrium by a system of chemical equations ... 

Typically, CdS colloids were prepared by the following method. 10 ml of solution with 10 3 M CdCl2, 2-10 3 M stabilizing surface active substance, and the required amount of complexing admixtures was prepared in a 100 ml vessel at room temperature. 10 ml of 2-10 3 M Na2S was added to the solution under vigorous stirring. The color of the produced colloidal CdS solution depended on the type and amount of admixtures and varied from yellow to colorless. The produced colloidal solutions were stable, in some cases for more than six months. 

The kinetic curves exhibit fast decay at the initial part and very long tails after that. Figs. 2.6 and 2.7 present the relative rate W of electron concentration decay vs. the relative concentration of electrons in the sample calculated at different % values for the kinetic curves from Fig. 2.4. For CdS colloid with 2R 10 nm, the dependence of W on n is fitted by a function ... 

The assumption on the electric charge effect of excess electrons on the rate constant of their interfacial transfer is supported by an evident similarity of these semiconductor colloidal systems with metal colloids, for which effect of the charge of electrons captured by the particle is well known and agrees with the microelectrode theory . Moreover, kinetic curves similar to those we found for CdS colloids were observed previously for silver colloids in ref. [17], where the particles charge q was shown to decrease by the law... 

Fig. 2.9. Photobleaching spectra of a CdS colloid obtained in the excess of Cd2+ ions with the...

Fig. 2.10. Photobleaching relaxation kinetic curves at the bandedge of absorption spectrum of a CdS colloid obtained in the excess of Cd2+ ions with the particles of various sizes. [CdS] = 10 M, [DCH] = 210 3 M, [TG] = 5-10 3 M. Illumination at X < 360 nm (UFS-1), Cell length 1 is 10 cm, T = 20°C.

Studies on luminescence of CdS colloids provide useful knowledge on the energy and nature of recombination sites of charge carriers in the colloidal particles. The regularities of the colloid photoluminescence quenching provide the information on the dynamics of electrons and holes in semiconductor particles as well as on the kinetics of interfacial electron transfer. Of a particular interest are studies on the luminescence of colloidal solutions of the so-called Q-semiconductors, their properties depending on the size of semiconductor particles due to the quantum size effects. 

In most cases, quenching of luminescence of CdS colloids is determined by the reactions of interfacial electron transfer involving either electron or hole. So, study of this process is a convenient method for establishing the regularities of key steps of redox photocatalytic reactions over CdS. In addition, using of various luminescence quenchers (anions, cations, and neutral molecules) allows to reveal the nature of electron capture sites at the CdS surface. 

Fig. 2.16 displays experimental data on the luminescence quenching of CdS colloidal particles of four different size in the Stem-Folmer s coordinates. The data correspond to different luminescence bands. This figure presents also the curves approximating the experimental points to equation (2.19). From these curves, the values of the both parameters, KadS and A, may be extracted. One may see from Fig. 2.16 that the luminescence quenching of CdS by methylviologen differs for different bands and depends on the size of colloidal particles. In particular, the methylviologen adsorption constant rises as the size of the CdS colloidal particles increases. It follows from Fig. 2.17 that the obtained values of rate constant Kads decrease exponentially with an increase of the value opposite to the colloidal particle diameter. 

Fig. 2.16. The Stem-Folmer s presentation of the dependence of the luminescence quantum yields of the Q-CdS colloidal solutions with the particles of different size on the methylviologen concentration. The size of the particles a) 2R 20 A, b) 2R = 23 A, c) 2R = 44 A. The CdS concentration is 510-4 M, T = 23°C. Dimensionality of K, is M 1.

Fig. 2.17. The calculated constant of methylviologen adsorption on the CdS colloids vs. the size of the colloidal particles. The value is obtained by approximation of experimental data on the luminescence quenching to Eq. (2.19). Dimensionality of is M 1.

Fig. 2.19. Changes in the luminescence spectrum of Q-CdS colloidal solutions under the addition of PWi2 (a) Successive addition to give the concentrations 2-10-5,10"4,4-10-4 M (b) successive addition of PW 12 to the point of 2-KT4 M, followed by the addition of H2S04 to give 2.5 10-4,5-10-4,7.5-10"4 M. CdS concentration is 510-4 M, Xexe = 350 nm, T = 23°C.

To prove the above statement on the determining effect of electric charge of both the CdS colloidal particle surface and the quencher molecule on the adsorption of these molecules from aqueous solution, we have modified the surface of colloidal CdS during its preparation. The most efficient method of such modification consists in changing the surface charge of the colloidal particle via the preparation of nonstoichiometric colloid. In this case, the surface charge is determined by the charge of excessive ion (either S 2 or Cd2+). 

Substitution of the lattice cadmium ions in a CdS colloidal particle by the ions of another metal is often accompanied by the formation of the so-called coated particles CdS/MexSy. Such particles are readily produced via the substitution of cadmium ions by other ions if only their sulfide are less soluble compared to the cadmium sulfide. Our studies on the luminescence properties of such particles and regularities of their luminescence... 

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