FRACTAL ANALYSIS OF SURFACES OF ZnO NANO- AND MICROSTRUCTURES OBTAINED BY DIFFERENT METHODS

Abstract

Fractal analysis of the surfaces of microstructures, nanorods, microgranules and nanowires based on ZnO was performed. In order to the synthesis of these structures various methods were used, such as: vapor phase synthesis, hydrothermal synthesis, thermal evaporation of zinc metal powder, and vacuum electron sputtering. Microphotographs were obtained using a REMMA-102-02 scanning electron microscope (OJSC SELMI,Ukraine). ZnO microstructures with a developed surface, which were grown from the vapor phase on a silicon (001) substrate, had an element diameter of 2–10 μm and a length of up to 20 μm. ZnO nanorods, obtained by the hydrothermal method on a Si (001) substrate, had the following geometric dimensions: length - up to 1 μm, diameter - 100–300 nm. Granular ZnO microstructures were grown on an n-type silicon substrate by thermal evaporation of zinc metal powder (purity 99.99%) at a temperature of700°C. The diameter of the granules is from 2 to 60 μm. ZnO nanorods were also obtained from the vapor phase. Their diameter is significantly less than 1 μm and length is up to 5 μm. ZnO nanowires were synthesized on a quartz substrate by electron sputtering in vacuum. As can be seen from, nanowires have an even smaller diameter - about 60–130 nm, and a length of up to 5 μm.

The statistical approach to determine some set of parameters that characterize this surface was used to analyze the surface. Free Gwiddion 2.55 software was used for this purpose. In this work for the fractal analysis of structures the method of calculation of cubes which is deduced directly from definition of the fractal dimension based on calculation of blocks was used. The distribution of fluctuations in the height of nanostructures on the surface of the sample was investigated by using the method of analysis of profilograms. The surface area was estimated by the method of simple triangulation. The dependence of the fractal dimensionality and effective surface area on the method of synthesis, type of structure and characteristic sizes of the elements of ZnO micro- and nanostructures was illustrated. The analysis of the obtained results showed that all the studied objects possess the fractal dimensionality ≥ 2.5, that is favorable for the structures that are promising for use in the photocatalytic method of water purification, in sensor and photoelectronic technologies. ZnO nanorods and nanowires, synthesized by the hydrothermal method and electronic spray method in vacuum, respectively, were characterized by the largest values of the fractal dimensionality and the ratio of the surface area to the projection area. Due to the fact that their high values of fractal dimension differ only by 0.034, the hydrothermal synthesis method can be considered the most promising for further synthesis of samples with a developed surface for photocatalysis and other applications, given its simplicity and cheapness and qualitative characteristics of the obtained nanostructures.

Key words: nanostructures, microstructures, fractal dimension, photocatalysis.