O. L. Stroyuk

Spectral and luminescent properties of ZnO–SiO2 core–shell nanoparticles with size-selected ZnO cores

Deposition of silica shells onto ZnO nanoparticles (NPs) in dimethyl sulfoxide was found to be an efficient tool for terminating the growth of ZnO NPs during thermal treatment and producing stable core–shell ZnO NPs with core sizes of 3.5–5.8 nm. The core–shell ZnO–SiO2 NPs emit two photoluminescence (PL) bands centred at ∼370 and ∼550 nm originating from the direct radiative electron–hole recombination and defect-mediated electron–hole recombination, respectively. An increase of the ZnO NP size from 3.5 to 5.8 nm is accompanied by a decrease of the intensity of the defect PL band and growth of its radiative life-time from 0.78 to 1.49 μs. FTIR spectroscopy reveals no size dependence of the FTIR-active spectral features of ZnO–SiO2 NPs in the ZnO core size range of 3.5–5.8 nm, while in the Raman spectra a shift of the LO frequency from 577 cm−1 for the 3.5 nm ZnO core to 573 cm−1 for the 5.8 nm core is observed, which can indicate a larger compressive stress in smaller ZnO cores induced by the SiO2 shell. Simultaneous hydrolysis of zinc(II) acetate and tetraethyl orthosilicate also results in the formation of ZnO–SiO2 NPs with the ZnO core size varying from 3.1 to 3.8 nm. However, unlike the case of the SiO2 shell deposition onto the pre-formed ZnO NPs, individual core–shell NPs are not formed but loosely aggregated constellations of ZnO–SiO2 NPs with a size of 20–30 nm are. The variation of the synthetic procedures in the latter method proposed here allows the size of both the ZnO core and SiO2 host particles to be tuned.

Colloidal ZnO nanocrystals in dimethylsulfoxide: a new synthesis, optical, photo- and electroluminescent properties

Stable colloidal solutions of zinc oxide in dimethylsulfoxide were synthesized via interaction between zinc(II) acetate and tetraalkylammonium hydroxides (alkyl-ethyl, propyl, butyl, and pentyl). Colloids of ZnO emit photoluminescence in a broad band with a maximum at 2.3-2.4 eV with quantum yields of up to 9-10% at room temperature and 15-16% at 80 K. The photoluminescence is supposed to originate from the radiative recombination of conduction band electrons with holes captured by deep traps having corresponding states in the band gap 1.0-1.2 eV above the valence band edge. The size of colloidal ZnO nanocrystals depends on the duration and temperature of the post-synthesis treatment and varies in the range of 3-6 nm. Growth of the ZnO nanocrystals can be terminated at any moment of the thermal treatment by freezing the colloidal solution or by addition of tetraethyl orthosilicate which hydrolyses forming core-shell ZnO@SiO2 particles. ZnO nanocrystals introduced into polyethyleneimine films can be used as an active component of an LED emitting at an applied voltage higher than 13 V.

Photochemical Processes Involving Graphene Oxide

Recent research on photochemical processes involving graphene oxide are summarized and analyzed. Such processes include the reduction of this oxide upon photoexcitation both with and without molecular or semiconductor photocatalysts, conversions of various substrates induced by graphene oxide as a photocatalyst (photoinitiator), and photocatalytic reactions, in which graphene oxide and the products of its reduction are cocatalysts. The major features and possible mechanisms of these reactions as well as areas for the further development of basic and applied research in this field of photochemistry of graphene oxide are discussed.

Structured Films of CuxS – Counter Electrodes for Solar Cells Based on FTO/ZnO/CdS Heterostructures and Sulfide/Polysulfide Redox Couple

It was shown that nanostructured films of zinc oxide electrodeposited on the surface of conducting FTO glass can be converted into copper sulfide CuxS without affecting their morphology. The FTO/CuxS films can be used as electrocatalytically active cathodes in solar cells based on a FTO/ZnO/CdS photoanode sensitive to visible light and a sulfide/polysulfide redox couple. By using the FTO/CuxS films in place of platinum foil it is possible to increase the efficiency of light conversion by more than 20 times, from 0.07% to 1.4%.

Photocatalytic Hydrogen Evolution Under Visible Light Illumination in Systems Based on Graphitic Carbon Nitride

The state of the art in the area of photocatalytic systems derived from graphitic carbon nitride (g-CN) for hydrogen evolution from aqueous solutions of electron-donating substrates as well as water splitting has been reviewed. The discussion is focused on various types of g-CN, namely, pristine g-CN, metal- and nonmetal-doped g-CN materials, g-CN modified by organic compounds, porous, exfoliated, dye-sensitized g-CN, spatially-organized structures and hybrid g-CN materials with various additives. The prospects for future research in this area are discussed.

Photoelectrochemical Solar Cells with Semiconductor Nanoparticles and Liquid Electrolytes: a Review

The principles for the function of solar cells made with semiconductors and liquid electrolytes and methods for the preparation of components for these cells are surveyed. Methods for enhancing the efficiency of light conversion by creating intermediate barrier layers in the photoanodes, band design of nanoscale semiconductor sensitizers, morphology of wide-bandgap oxide transport layers, as well as the composition and structure of counter electrodes were discussed. Special attention was given to an analysis of current trends in the development of such cells, including the use of low-toxicity and accessible semiconductor and carbon nanomaterials as well as new methods for the formation of nanostructured electrodes.

Photoelectrochemical Properties of Titanium Dioxide Nanoheterostructures with Low-Dimensional Cadmium Selenide Particles

Feasibility was demonstrated for the use of low-dimensional CdSe@CdS particles with core@shell structure and 1.8-2.0 nm mean core diameter as a component in the FTO/TiO2/CdSe@CdS photoanode in photoelectrochemical solar cells with aqueous polysulfide electrolyte and copper sulfide-based counter-electrode. An average light conversion efficiency of 6.3% was achieved (at excitation intensity of 30 mW/cm2) in cells with the FTO/TiO2/CdSe@CdS photoanode and an FTO/TiO2/Cu2S counter-electrode produced by sulfidation of copper particles photochemically deposited onto the titanium dioxide surface.

Semiconductor-Based Photocatalytic Systems for the Reductive Conversion of CO 2 and N 2

Semiconductor-based photocatalytic systems aimed at the reduction of carbon dioxide and dinitrogen are continuously studied for more than 30 years (Gratzel in Energy resources through photochemistry and catalysis. Academic Press, Inc., New York, 1983). A gradual shift from micro- to nanocrystalline semiconductor photocatalysts, which is, probably, the main trend in modern semiconductor photocatalysis/photoelectrochemistry, allowed to achieve attractively high quantum efficiencies of the CO2 and N2 conversion as well as to apply a potent array of spectral methods for the elucidation of mechanistic aspects of these important photoreactions. A decrease of the photocatalyst crystal size to a few nanometers allows not only to intensify the photocatalytic synthetic reactions but also to engineer the surface and band structure of the nano-photocatalysts to direct the reactions toward desirable products.

Effect of Post-Synthesis Heat Treatment of ZnO Nanoparticles in DMF on Their Size and Spectral and Luminescent Properties

A method is proposed for the formation of ZnO nanoparticles (NP) in dimethylformamide, which permits variation of the mean NP diameter from 3.6-3.7 to 6.0 nm by selecting the suitable duration and temperature of the post-synthesis heat treatment. The ZnO nanoparticles in DMF display characteristic photoluminescence, emitting in a broad band with maximum at 2.24-2.25 eV, with a quantum yield up to 13%, and a mean radiative lifetime of ~2 μs. Giant enhancement of Raman scattering on the surface phonons of the ZnO NP is observed upon the photoexcitation of these NP with an island-like silver film deposited on their surface.