Nonstoichiometric cadmium sulfide nanoparticles ([Cd 2+ ]/[S 2-] ) 3) in 2-propanol were surface-modified with Cu 2+ ions. Addition of copper(II) perchlorate to CdS nanoparticles leads to binding of copper ions onto the surface of the semiconductor, accompanied by rapid reduction of Cu 2+ to Cu + , as confirmed by EPR and absorption spectra. Copper(II) perchlorate also quenches the recombination luminescence of CdS nanoparticles effectively. The quenching data obey a static interaction model, which confirms the binding of copper ions onto CdS. The latter was confirmed also by ultrafiltration and ICP spectroscopy. Copper ions bound onto the surface of CdS lead to formation of a new, red-shifted, luminescence band. The maximum of the new band is at 14 700 cm -1 compared to that of the original band at 17 900 cm -1 . It is suggested that, at low copper ion concentrations, copper ions bound onto the surface of CdS nanoparticles exist as isolated Cu + ions. They create a new energy level in the bandgap at about 1.2 eV below the conduction band, which is responsible for the new emission band (14 700 cm -1 ). Higher copper(II) perchlorate concentrations give rise to the formation of ultrasmall particles of CuxS (x ) 1-2) on the surface of CdS, which eventually lead to precipitation. Both isolated Cu + ions and ultrasmall particles of CuxS quench the original recombination luminescence of CdS nanoparticles by facilitating e -/h + nonradiative annihilation. The presence of copper ions bound onto the surface of CdS nanoparticles retard both the photocorrosion of the latter and the photodecomposition of copper(II) tetraphenylporphyrin induced by CdS nanoparticles. Appropriate mechanisms are presented.
when the inhomogeneous broadening due to the particle size distribution is reduced.The photocatalytic polymerization of styrene is the most probable candidate for a photocatalytic reaction. The existence of styrene monomer was indispensable to the observation of the photoirradiation effect. In addition, the photoirradiation effect was enhanced by increasing the styrene monomer concentration. Photocatalytic polymerization of styrene would create a polystyrene coating on the surface of the CdS particle. The polymer layer prevents the particles from growing further. In spite of our efforts to obtain evidence for the formation of polystyrene by using such methods as FT-IR analysis, no trace of polystyrene has yet been detected. This is probably due to the low concentration of polystyrene on the surface. This model describing the polymerization of styrene on the surface of the growing particles thus remains hypothetical at present. It was initially thought that o-dicyanobenzene was necessary as an electron acceptor or mediator for generating active radicals for subsequent reactions or by simply making holes more reactive by removing electrons. However, the experimental results on the contribution of each component in the solution revealed that o-dicyanobenzene was not indispensable as long as enough styrene monomer is in the solution.As the irradiating light intensity increased, the wavelength of the absorption onset shifted to shorter wavelength, approaching 440 nm, as shown in Figure 1. With 0.21 W/cm2 intensity light, the onset wavelength almost coincided with the shortest wavelength (440 nm) of the irradiating light. This indicates that photoirradiation is effective for preventing the CdS particles from growing over a size corresponding to the shortest wavelength of the irradiating light.The photoirradiation effect is expected to become more effective when the CdS formation rate is small enough so that the photocatalytic reaction can take place before the growing particles have become larger than the size corresponding to the irradiation light wavelength. In fact, decreasing the H2S concentration in the H2S and He gas mixture from 0.25 to 0.02 vol % resulted in a shift of the absorption onset wavelength to shorter wavelengths.We also investigated the possibility that previously-grown particles undergo photobleaching, giving rise to smaller particles. When a previously-prepared CdS colloidal solution was irradiated without the presence of H2S, almost no change in the optical absorbance was observed. Thus, photobleaching does not occur easily. It was found by us that counteranions play an important role in photobleaching. We found that different cadmium compounds gave different results for the photoirradiation effect. When Cd(N03)2.4H20 was used instead of Cd(C104)2.6H20, no photoirradiation effect was observed. Furthermore, the colloidal solution of CdS derived from the nitrate suffered from photobleaching. That is, a decrease in the absorbance of the solution over the entire spectral range was observed when irradi...
The rates of pyrolysis of n-hexane, 2-methyl pentane, and 3-methyl pentane decrease with increasi~lg S / V ratio in the pressure range of 15 to 90 mm a t temperatures around 520 "C. The partial pressure of nitric oxide required for maximum inhibition which appears to be independent of the hydrocarbon pressure and is approximately constant for all isomers, namely 12-14 mm, for hydrocarbon ranging from 50 to 100 111111, iricreases slightly with increasing S / V ratio. Nitric oxide affects the product distribution rnarl~edly. The simultaneous increase in the yield of C2H.i and decrease in the yield of C~H G with increasing S O pressure can be attributed to the set of competitive reactions CzI-1, + NO 4 C2H.I + HNO C?H; + RE1 + C?I-IG + R.Similarly, the slight decrease in the yield of C3Ms with increasing NO pressure can be accounted for by the competitionIncreased S / V ratios favor the formation of C~H G in both the ~~ninhibited and the inhibited pyrolyses, but has no effect on the yields of such products as CaHG, l-c.~Hs, and i-C.,I-Is. T h e analytical balance beco~lles less co~llplete a t higher NO pressures for the inhibited decomposition, which can be attributed to the formation of some nitrogen-containing compounds undetectable by the analytical nlethods used. bIechanisms are proposed accou~iti~ig clualitatively for the products of the uninhibited and the inhibited pyrolyses.
A pulse radiolysis investigation of aqueous eosin (S) has shown that the initial products are semireduced eosin (R) [eaq~+ S -R (k ^5 X 109 if-1 sec-1) ], semioxidized eosin (X)[OH + S -* OH-+ X (k = (1.4 ± 0.4) X 1010 M-1 sec-1)], and a long-lived "red product" attributed to the H and OH ring-addition products. In deaerated solutions the dye radicals decay predominantly by the back reaction R + X-*-2S(fc = 8X 108 M-1 sec-1), with permanent bleaching due to R + R -* S + leuco base (k = (4.2 ± 0.9) X 106 M-1 sec-1) and X + X -* products (k = (2.0 ± 0.5) X 10s M~l sec-1). Measurements made in the presence of H2O2, to convert eaq-to OH, and OH scavengers led to the results S + CH3ÓHOH R + CH3CHO (jfc = (1.1 ± 0.2) X 109 M-1 sec-1) and S + C02--> R + CO2 (k = (2.5 ± 0.5) X 10s M~1 34sec-1). The results are compared with previous flash photolysis measurements and predictions of the Debye equation for encounter-limited reactions.
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