ИСТИНА

Metal-semiconductor composites attract scientists’ interest as prospective materials with special optical properties. But it was found that nominally identical nanocomposites demonstrate different photocatalytic properties. These properties depend on the nanocomposite band structure, while band structure itself depends on the shape of nanoparticles and quality of the contacts between metals and semiconductors nanoparticles. It is possible to find relationship between optical properties and microstructure of metal-semiconductor nanocomposites if we use in complex TEM, STEM, EDX and EELS. At the same time we guess, that local low-energy EELS data and results of the optical diffuse reflection spectroscopy (DRS) analysis should help us to interpret nanocomposites photocatalytic activity (PCA). Here we study nanocomposites (Ag/ZnO, Ag/TiO2, Au/ZnO, Au/TiO2) with similar morphology of metal nanoparticles. We prepare these materials using three methods of the initial oxides decoration by silver and gold nanoparticles: the Turkevich method, the reduction by the sodium borohydride and the one-step hydrothermal synthesis. All three methods result is different contact between metal and semiconductor nanoparticles. The EEL spectra were collected using transmission electron microscope Libra 200 equipped with monochromator and omega-filter. High energy resolution allows us to measure SPR peak position in the visible range. Moreover, it is possible to collect simultaneously plasmonic peaks and zero-loss peak, which allows to calculate the relative thickness of the sample. Therefore, it is possible to collect low-energy EEL spectra from individual nanoparticles with respect to their thickness. Obtained composites demonstrate different photo catalytic activities. It may be caused by the different contacts in the composites. In the case of slow reduction, it results in heterogeneous nucleation at the matrix surface, therefore, the obtained contact is better. According to the results of optical and EEL spectroscopy, the contact between metal and semiconductor nanoparticles results in the SPR peak position shift. The photo catalytic activity data on nanocomposites with silver demonstrate that the sample with a better contact between particles has a lower PCA because of Ohmic contacts between silver and matrices.