Characterization of raw materials and self-organized Bi2O3–Ag eutectic by X-ray diffraction, scanning electron microscopy, and X-ray photoelectron spectroscopy

Bi2O3-Ag raw materials were prepared using different synthetic routes: mortar and pestle and ball-milling. They were later used as starting materials for the micro-pulling down method. After the micro-pulling down, the Bi2O3-Ag composites were annealed under vacuum, H-2 and air atmospheres. All samples were characterized by the following methods: X-ray diffraction (XRD), scanning electron microscopy (SEM/EDX) and X-ray photoelectron spectroscopy (XPS). The SEM/EDS and XPS analyses showed that the raw materials prepared by ball-milling had a much more homogenous distribution of silver. XPS investigations of all Bi2O3-Ag composites showed that Bi ions are mainly in the 3(+) oxidation state but additionally some may exist in the Bi-0 (metallic) and defected Bi(3-x)+ valence states. In the case of Bi2O3-Ag composites grown from ball-milled raw materials and annealed in H-2, XPS studies clearly showed the presence of metallic silver nanoparticles, Ag-0. Besides the nanoparticles, bulk metallic silver Ag-0 and two oxidation states for silver Ag2O and AgO for all examined eutectic were found. The estimated band gap, based on XPS was about 2.8 eV. In the case of samples obtained from ball-milled raw materials, additional states near the Fermi level were found which could be related to partial transformation of Bi2O3 into Bi-0.