Abstract
Nanoparticles belong to modern, progressive materials with wide application potential. Its interesting properties come from the high surface to volume ratio and often differ from the properties of bulk or coarse-grained materials with the same chemical composition.
Nanoparticles catalytic, magnetic, electric or optical properties are just examples. In this study we investigated niobium nanoparticles prepared by magnetron sputtering in combination with gas aggregation cluster source operated in DC mode in argon atmosphere.
Studied nanoparticles deposited on silicon substrates were characterized by combination of x-ray scattering and diffraction methods and transmission and scanning electron microscopies with energy-dispersive x-ray spectroscopy. These analytical methods provide the information about the nanoparticles sizes, shapes, their distributions, chemical composition and the real atomic structure.
In order to determine the thermal evolution of nanoparticles microstructure, the in situ high temperature XRD measurement was done in ambient air atmosphere up to 800C. Measured SAXS data for temperatures in range between 50C - 800C are shown in Figure 1.
Two main transition are observed and described. Thin oxygen layer is present on the surface of as prepared niobium nanoparticles, which acts as a protection barrier against further corrosion.
The thickness of this amorphous oxide layer, as determined by fitting the SAXS patterns, is about 0.9 nm. The nanoparticle core is formed by pure niobium bcc phase and its mean radius is 11 nm.
With increasing temperature, the growth of the oxide shell layer at the expense of the size of nanoparticle metallic core occurs. Slightly above 200 C the nanoparticles are fully amorphized which is clear from XRD patterns.
In this temperature range the mean nanoparticle size remains almost same. The results of SAXS measurement are plotted in Figure 2.
Crystalline phase of niobium pentoxide is formed at temperature around 450C. Additionally, we observe one more phase transformation at 625C after which also pronounced increase of nanoparticles sizes starts.
Based on the combination of SAXS and XRD measurement we conclude that the size of original niobium nanoparticles corresponds to the mean crystallite size, i.e. it means one nanoparticle consists of one coherently diffracting domain. At the end of the annealing process, after heating to 800 C and formation of the orthorhombic niobium pentoxide phase, there are more crystallites in one particle.