ИСТИНА

Over the past decades, titania has been the object of close attention of scientists all over the world [1, 2]. Titania is a wide-gap semiconductor with a band gap varying from 3.2 to 3.4 eV, and continuous illumination by UV radiation must be used for reactive electrons and holes generation [1, 2]. The way to improve the photocatalytic characteristics of titania is doping with ions of metals or nonmetals and to prolong the lifetime of photoexcited carriers (electrons and holes) by their spatial separation in nanoheterostructures, thus prevent recom-bination. In this work we have obtained nanoheterostructures effectively absorbing light in the visible spectral region and accumulating photoexcited charge carriers, providing rather long catalytic activity of the produced structures after illumination is turned off. At the same time we have seriously checked out the behavior of the used nanoheterostructures after adsorbtion of organic dye molecules on their surface. Organic dye molecules adsorption is involved in a standard method for the photocatalytic activity measurements. Nanocrystalline nitrogen-doped titanium dioxide and TiO2−MoO3, TiO2−V2O5, TiO2−WO3, TiO2−MoO3−WO3, and TiO2−MoO3−V2O5 nanoheterostructures were prepared by the method described in detail in [2]. The electron paramagnetic resonance (EPR) spectra were recorded by ELEXSYS-E500 spectrometer (Bruker, Germany). Fluorescence and dif-fuse reflection spectra were recorded using an LS-55 Perkin Elmer spectrometer (spectral range from 250 to 900 nm). The photocatalytic activity (oxidizing power) of the samples was investigated with the reaction of the photodegradation of rhodamine dye. The dye was ap-plied to the sample surface from aqueous and ethanol solutions. To obtain the visible spectral range (from 450 to 750 nm) for the photocatalysis study YG-16 glass filter was used. Chang-es in the surface concentration of the dye were monitored according to the diffuse reflectance R value at a wavelength of the maximum light absorption by the adsorbed dye. The diffuse reflectance was recalculated to a value proportional to the surface concentration using the Kubelka–Munk approach [3]. Nanoparticles average diameter in the nanoheterostructures was approximately 10 nm according our XRD investigatrion and the specific surface area of all samples was in the range of 100-110 m2/g. Paramagnetic centers of N•, Ti3+, Мо5+, V 4+, and W 5+ were detected and characterized in the samples under investigation. The defect concentrations calculated by us had the following values: 1.3∙1018 g–1 (TiO2−WO3), 2.1∙1018 g–1 (TiO2−V2O5), 2.2∙1018 g–1 (TiO2−MoO3), 1.6∙1019 g–1 (TiO2−MoO3−V2O5), 1.4∙1019 g–1 (TiO2−MoO3−WO3). It is revealed that nanoheterostructures consisting of several metal oxides have high photocatalytic activity in the visible spectral region and the ability to accumulate photogenerated charge carriers. As a result, catalytic reactions in the samples take place even after illumination is turned off and thus do not require continuous illumination. These results correlate with the data obtained by EPR. It is found that samples with high radical concentra-tion have a high light absorbance and photocatalysis rate in visible range. The energy position of the different radicals in the band gap of nanooxides was determined using our original EPR method with illumination of samples in situ. We have performed for the first time a comparative study of the luminescence of dye molecules adsorbed on nanoheterostructures surface and these structures catalytic activity. We have shown that under illumination of nanoheterostructures with the adsorbed rhodamine dye at the dye absorption wavelength (500 nm), photocatalytic reactions are mainly determined by the light absorption by the nanostructures themselves, and not by the energy transfer from the dye. Since the strong quenching of the initial fluorescence is observed on the surface of all studied nanoheterostructures, it can be stated that the nonradiative energy transfer from dye molecules associated with such quenching occurs, but does not lead to a significant additional acceleration of photocatalysis. This important result shows that high photocatalytic activity of the material is the primary criterion for creating energy-efficient photocatalysts.