Colloidal CdS

Rossetti R ef al 1984 Size effects In the excited electronic states of small colloidal CdS crystallites J. Chem. Phys. 80 4464... 

When colloids of metal sulfides, selenides, phosphides and arsenides are illxuninated in the presence of air, decomposition takes place. Metal ions migrate into solution and sulfate, selenite, phosphate or arsenate are formed. The process was first investigated in the case of colloidal CdS... 

Figures 6 and 7 show the absorption spectra of colloidal CdS and ZnS at various times of illumination. The two colloids were prepared by adding an NaSH solution to solutions of Cd(C10j2 or Zn(C104)2, respectively, colloidal silicon dioxide (commercially available from Dupont Ludox HS30) being present at 6 x 10 M as stabilizer in both cases. The absorption starts in both cases close to the wavelengths that correspond to the photon energies (515 nm or 2.4 eV for CdS 340 nm or 3.7 eV for ZnS) at which the absorptions begin in the macrocrystalline materials. It is seen that illumination causes not only a decrease in the intensity of the absorption spectrum but also a change in the shape of the spectrum. The onset of light absorption is shifted towards...

Fig. 8. Intensity of fluorescence and rate of photoanodic dissolution of colloidal CdS as function of the concentration of added methyl viologen...

The first observations on the fluorescence of colloidal CdS were made using a colloid stabilized by colloidal silicon dioxide . The fluorescence spectrum consisted of a broad band with the maximum between 580 nm and 650 nm. The reproducibility of this red fluorescence was very poor. In the presence of excess Cd ions the intensity of the fluorescence was increased, which indicates that anion vacancies were centers of luminescence. Aging of the sol for a few weeks in the dark and in the absence of air was accompanied by an increase in fluorescence intensity by a factor of ten and a gradual red shift of the fluorescence band. However, even after this increase, the fluorescence quantum yield was still below 10 . ... 

Nosaka and Fox determined the quantum yield for the reduction of methyl viologen adsorbed on colloidal CdS particles as a function of incident light intensity. Electron transfer from CdS to MV " competes with electron-hole recombination. They derived a bimolecular rate constant of 9 10 cm s for the latter process. 

The earlier studies of aqueous solution of Cd2+ containing mercaptan were found to produce colloidal CdS by sonication [89] under nitrogen atmosphere. The H atoms produced during sonication attacked on molecules possessing a thiol moiety and lead to either H atoms extraction or H2S formation. The proportion of H2 to H2S was highly dependent on the type of mercaptan. Possible reaction mechanism was reported as follows [90],... 

Sostaric JZ, Caruso-Hobson RA, Mulvaney P, Grieser F (1997) Ultrasound-induced formation and dissolution of colloidal CdS. J Chem Soc Faraday Trans 93 1791-1795... 

Colloidal CdS particles 2-7 nm in diameter exhibit a blue shift in their absorption and luminescence characteristics due to quantum confinement effects [45,46]. It is known that particle size has a pronounced effect on semiconductor spectral properties when their size becomes comparable with that of an exciton. This so called quantum size effect occurs when R < as (R = particle radius, ub = Bohr radius see Chapter 4, coinciding with a gradual change in the energy bands of a semiconductor into a set of discrete electronic levels. The observation of a discrete excitonic transition in the absorption and luminescence spectra of such particles, so called Q-particles, requires samples of very narrow size distribution and well-defined crystal structure [47,48]. Semiconductor nanocrystals, or... 

Charge-transfer processes in colloidal CdS-Ti02 and CdS-Agl systems. J Phys Chem 94 6435-6440... 

Fig. 3.5.1 The minimum of the first derivative of UV/visible absorbance spectra of CdS particles as a function of the particle diameter. The data points have been collected from literature sources where particle sizes were determined by EM or XRD. If specific data about the minimum of the first derivative were not expressly provided, they were estimated from spectra supplied. The estimated error in this technique is less than 5 nm. For clarity, only data from groups with the greatest number of data points have been used here. A curve has been fitted to the data using a theoretical relationship between particle diameter and wavelength using the effective mass model (6). Inset Absorbance spectra of colloidal CdS produced by exposure of a CdAr film, or a Cd2+/HMP solution, to H S. The minima of the first derivative (380 nm in film, 494 nm in solution) correspond to particle sizes of approximately 2.5 nm and 6.0 nm, respectively. (From Ref. 5.)...

Aqueous pools of reversed micelles have been fruitfully employed for the in situ generation of semiconductor particles. The first publication in this area described the formation of CdS in sodium bis(2-ethylhexyl)sulfosuccinate (AOT) aggregates in isooctane [611]. The preparation involved the addition of aqueous CdCl or Cd(N03)2 to isooctane solutions of AOT. Exposure to controlled ammeters of the CdS particles formed. Irradiation of degassed, AOT-reversed-micelle-entrapped, platinized CdS by visible light (450-W Xenon lamp X > 350 nm) in the presence of thiophenol (PhSH) resulted in sustained hydrogen formation. Sacrificial electron transfer occurred from thiophenol to positive holes in the colloidal CdS and, consequently, diminished undesirable electron-hole recombinations (Fig. 101) [611]. 

Porous membrane Colloidal CdS particles converted to Xerogels and dried at about 10 3 Torr at 30-40 °C to produce crack-free, optically transparent membranes... 

Reversed micelle-entrapped, colloidal CdS showed the characteristic weak fluorescence emission (Figure 2), previously observed in homogeneous solutions (16-19). However, the maximum emission intensity corresponded to full band gap emission (approximately 500 nm) and was not red-shifted as observed in homogeneous solution (17). This discrepancy might arise from the mode of prep>aration (H S instead of Na S), or from the specific effect of surfactant aggregates. 7 lternatively, tras can be the result of a size... 

Figure 2. (Top) Stern-Volmer plots for the quenching of the fluorescence of colloi fcl CdS in AOT-entrapped water pools in isooctane by RMV + (0), MV2+ 4Q), and PhSH (0) (Bottom) Absorption and emission spectra of colloidal CdS in AOT entrapped water pools in isooctane. The shoulder observed at 400 nm is due to a spectrometer artifact.

To be active in H--production, a catalyst had to be incorporated in the system and depositecr on the semiconductor surface. Platinization was carried out by adding aqueous K PtCl solutions to the reversed micelle entrapped, colloidal CdS and irradfktingft by a 450 W Xenon lamp under Ar bubbling for 30 minutes. Platinization was monitored absorption spectrop ho to metrically. 

Figure 7 illustrates the mechanism of photosensitized H formation from PhSH in rhodium-coated colloidal CdS in nonpolymerized rationic vesicles. The proposed position of the CdS particle (partially buried in the vesicle bilayer) is supported by the following observations, the last two made specifically in DHP vesicles ... 

Figure 5. An idealized mechanism of photoinduced electron transfer from CdS conduction band to methylviologen (MV +)( resulting in formation of methylviologen radical cation (MV,+). The colloidal CdS particle as represented, was generated at the inside surface of the DHP vesicle. Its exact location is based on fluorescence quenching experiments (Figure 5). Inserts oscilloscope trace showing the formation of MV by the absorbance change at 396 nm, after a laser pulse at 355 nm.

In an attempt to overcome the problem of accumulation of the oxidized electron donor, we have incorporated a recyclable surface-active electron donor in DODAC vesicles (12). This electron donor contains a sulfide moiety which dimerizes upon light-induced oxidation. Simultaneously, hydrogen is evolved via vesicle-stabilized, catalyst-coated, colloidal CdS particles. The dimer could be chemically reduced for additional hydrogen formation. Figure 9 is an idealized view of this cyclic process (12). 

Figure 9. An idealized model for the cyclic oxidation-reduction process using thiol-functionalized surfactant, incorporated into DODAC vesicles together with Rh-coated colloidal CdS, for sustained hydrogen generation under visible light irradiation in one part of the cycle.

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