A. L. Stroyuk

Photoinitiation of buthylmethacrylate polymerization by colloidal semiconductor nanoparticles

Abstract Nanoparticles of zinc and iron oxides as well as cadmium sulfide of different size were synthesized in 2-propanol, their band gaps and allowed bands positions were determined. The foregoing nanoparticles turned out to be efficient photoinitiators of butylmethacrylate polymerization. On the basis of the kinetic parameters of the process a free radical mechanism of the butylmethacrylate photopolymerization induced by investigated nanosized semiconductors was inferred. An increase in the specific rate of the butylmethacrylate photopolymerization with the decrease in the average CdS nanoparticles diameter was explained in terms of the theory of quantum confinement effects in semiconductors and was attributed to an increase in the probability of monomer reduction by photogenerated electrons of CdS nanocrystals conduction band with the diminishing of crystals size. It was shown that a number of xanthene dyes—fluorescein and its halogenated derivatives are efficient sensitizers of ZnO nanoparticles to the visible light. Butylmethacrylate photopolymerization rates grow at the presence of ZnO nanoparticles and sensitizers symbately with an increase in singlet and triplet-excited states energy of dyes studied. Such relation indicates the determining role of the process of electron transfer from photoexcited sensitizers to the ZnO nanoparticles conduction band in the initiation stage of polymerization. Flash photolysis was used for the investigation of the nature and decay characteristics of short-lived intermediates generated in systems consisting of ZnO nanoparticles, sensitizers and monomer. Schemes of the mechanism of the butylmethacrylate photopolymerization induced by investigated nanosized semiconductors were proposed.

Resonant Raman scattering study of CdSe nanocrystals passivated with CdS and ZnS

CdSe nanocrystals (NCs) were obtained from cadmium sulfate and sodium selenosulfate in aqueous gelatin solutions. A near-bandgap emission of CdSe NCs was noticeably enhanced after passivation with CdS or ZnS. Resonant Raman scattering spectra of the passivated NCs revealed new peaks attributed to the formation of the sulfide shells around CdSe cores. The peaks observed for the CdSe/CdS core–shell NCs near 280 cm−1 were attributed to LO vibrations within a thin CdS passivating layer. Observation of the peak in the same frequency range for CdSe/ZnS is discussed within an assumption of alloying at the core–shell interface. Notable changes in the Raman spectra at different excitation wavelengths and shell parameters were attributed to the resonant and size-selective nature of the Raman process.

Nonresonant Surface-Enhanced Raman Scattering of ZnO Quantum Dots with Au and Ag Nanoparticles

Pronounced 10(4)-fold enhancement of Raman scattering has been obtained for ZnO nanocrystals on substrates coated with 50 nm Ag nanoparticles under nonresonant excitation with a commercial red-emitting laser. This makes feasible beyond 10(-18) mole detection of ZnO nanocrystals with a commercial setup using a 0.1 mW continuous wave laser and can be purposefully used in analytical applications where conjugated nanocrystals serve as Raman markers. For Au-coated surfaces the enhancement is much lower and the heating effects in the course of Raman experiments are pronounced.

Optical study of CdS- and ZnS-passivated CdSe nanocrystals in gelatin films

CdSe nanocrystals were synthesized in aqueous solutions using gelatin as a stabilizer. An appreciable improvement of photoluminescence efficiency of nanocrystal-containing polymeric films was achieved by passivation of asformed nanocrystals in solutions with CdS and ZnS. The passivation-induced variation of the photoluminescence efficiency and its maximum position was found to be dependent on the total volume of the passivating material and the sequence of the reagents involved. The photoluminescence of both unpassivated and passivated nanocrystals was noticeably (∼200 meV) red-shifted from the first absorption maxima. A possible origin of the emission observed is discussed.

Spectral, Optical, and Photocatalytic Characteristics of Quantum-Sized Particles of CdTe

Methods were developed for the synthesis of quantum-sized particles of CdTe, stabilized in aqueous solutions of thioglycolic acid, using H2Te or NaHTe produced in situ by electrochemical reduction of tellurium. The spectral and optical characteristics of the synthesized CdTe nanoparticles were studied. It was found that they have photocatalytic activity in the reduction of methylviologen with sodium sulfite.

Interplay of factors affecting Raman scattering in cadmium chalcogenide nanocrystals in dielectric media

Micro-Raman spectra of CdS and CdSe nanocrystals embedded in different dielectric media (borosilicate glass, gelatin, polyacrylamide, polyvinyl alcohol) are studied. The observed Raman features are discussed with the account of confinement-related selection rules relaxation, scattering by surface phonons, and host matrix pressure. For nanocrystals grown in borosilicate glass a pronounced resonant dependence of surface phonon frequency on the excitation wavelength is observed. The effect of the excitation wavelength is much stronger than that of the nanocrystal size or the dielectric matrix type.

Spectral and Luminescent Characteristics of Products from Exfoliation of Graphitic Carbon Nitride Produced at Various Temperatures

It was shown that graphitic carbon nitride (CN) produced at 500–725 °C can be exfoliated in aqueous solutions of tetraethylammonium hydroxide with the formation of stable colloids characterized by photoluminescence (PL) in the visible region of the spectrum. It was established that increase of temperature leads to decrease of the intensity of PL until it disappears completely at T > 675 °C. In the case of exfoliated CN the intensity of PL increases sharply with increase of temperature. This may be due to decrease in the size of the colloidal particles of CN, to the dynamics of internal conversion of electronic excitation in colloidal CN, and to the removal of peroxy groups from the core of the bulk CN in the course of exfoliation.

Optical Characteristics of Colloidal Nanoparticles of CdS Stabilized with Sodium Polyphosphate and Their Behavior during Pulse Photoexcitation

The optical and photophysical characteristics of quantum-dimensional particles of CdS stabilized in water (the oscillator force of the first exciton transition, the forbidden band width, the conduction and valence band potentials) were determined. It was established that pulse excitation of CdS nanoparticles gives rise to nonstationary bleaching of the colloid, due to the accumulation of excess electrons in the conduction band. Their amount and their consumption rate are determined by the nature and concentration of the electron donors and acceptors present in the system.

Effect of the Method of Preparation of ZnO/CdS and TiO2/CdS Film Nanoheterostructures on Their Photoelectrochemical Properties

A study was carried out on the photoelectric properties of ITO/TiO2/CdS and ITO/ZnO/CdS nanostructurized films obtained by the SILAR method and the photocatalytic reduction of sulfur by ethanol in the presence of Cd(II) salts. The nanostructures obtained by the latter method display much higher photocurrent density (by a factor of 2 in the case of systems derived from ZnO and a factor of 5 in systems derived from TiO2) upon irradiation with white light (λ > 400 nm) in aqueous solutions of sodium sulfide than the heterostructures obtained by the traditional SILAR method.

Optical studies of CdSe/HgSe and CdSe/Ag2Se core/shell nanoparticles embedded in gelatin

CdSe/HgSe and CdSe/Ag2Se core–shell nanoparticles are obtained by colloidal synthesis from aqueous solutions in the presence of gelatin. Optical absorption, luminescence, and Raman spectra of the nanoparticles obtained are measured. The variation of the optical spectra of CdSe/HgSe and CdSe/Ag2Se core–shell nanoparticles with the shell thickness is discussed. Sharp non-monotonous variation of the photoluminescence spectra at low shell coverage is observed.