The above-bandgap illumination of colloidal ZnO nanoparticles (NPs) in ethanol solutions is found to lead to reversible shifts of the absorption and photoluminescence (PL) excitation spectra, indicating charging of the nanoparticles with electrons. A rapid drop of deep-level PL intensity at the early stage of illumination is observed simultaneously with the splitting-off and growth of a new red-shifted near-band-edge PL band. Such a splitting of the near-bandgap PL band under illumination is observed for the first time and corroborates with the previous assumptions about the behavior of the NP ensemble emission upon gradual NPs’ charging with electrons. The possible relation between the new PL band and photoinduced charging of NPs with excess electrons is discussed on the basis of the dependence of the PL spectrum evolution and absorption band shift relaxation on the NP size and controllable access of oxygen during illumination.

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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.

Cathodic photocorrosion occurs upon stationary photoirradiation of ethanolic ZnO sols. The photocorrosion rate increases with decreasing particle size. This effect due to the appearance of quantum-sized effects in ZnO nanocrystals results from an increase in the reduction potential of the semiconductor upon irradiation, which may exceed the Fermi level for cathodic decomposition of zinc oxide nanoparticles.Quantum-sized effects in nanodimensional semiconductor particles play a very significant role in the behavior of these materials, in particular, in their photochemical and photocatalytic properties [1,2]. This effect has been attributed to a change in the energy characteristics of the charges photogenerated in the semiconductor upon a decrease in crystal size in the range comparable to the exciton delocalization region (1-10 nm) [1,3]. The increase in the absolute potentials of the conductance band electrons and valence band holes of the semiconductor nanoparticle with decreasing particle size may lead to a significant increase in activity relative to photochemical and photocatalytic processes. Special interest is found when the size-dependent change in the energy characteristics of the photogenerated charge carriers facilitates redox reactions, which are thermodynamically hindered for the given semiconductor in its usual state. An example of such a process is found in the cathodic photocorrosion of zinc oxide nanoparticles. In the microcrystalline state, zinc oxide is resistant to reductive photodissolution [4]. EXPERIMENTALThe method of preparation of the colloidal suspensions of zinc oxide in ethanol was similar on the whole to the reported procedure for preparing . Samples of analytical-grade zinc acetate, chemically-pure-grade sodium hydroxide, previously dried calcium oxide, and doubly-distilled ethanol were used as the starting reagents. The suspensions were irradiated in glass cells with optical pathlength 1.0 cm using a DRSh-1000 mercury lamp with λ = 310-370 nm. The cell was deaerated by bubbling an argon stream. A 5.0-cm-thick glass cell filled with water was placed between the working cell and the light source during the irradiation to prevent heating.The width of the forbidden band of ZnO nanoparticles (E g ) was defined as the energy corresponding to the wavelength at the x-intercept of the tangent to the long-wavelength edge of the absorption band in the electronic spectra of the colloidal suspensions. The mean width of the ZnO nanoparticles was evaluated using the approximation of the effective masses of the charge carriers [5,8] and the following expression 378 0040-5760/04/4006-0378