Ivana Veljković

Solid state synthesis of extra phase-pure Li4Ti5O12 spinel

Extra phase-pure Li4Ti5O12 spinel with particle sizes less than 500 nm was synthesized by solid state reaction of mechanochemicaly activated mixture of nano anatase and Li2CO3 for a very short annealing time, 4 h at 800 °C. Structural and microstructural properties, the mechanism of solid state reaction between anatase and Li2CO3 as well as thermal stability of prepared spinel were investigated using XRPD, SEM and TG/DSC analysis. The mechanism of reaction implies decomposition of Li2CO3 below 250 oC, formation of monoclinic Li2TiO3 as intermediate product between 400 and 600 °C and its transformation to Li4Ti5O12 between 600–800 oC. The spinel structure is stable up to 1000 oC when it is decomposed due to Li2O evaporation.

Investigation of surface defect states in CeO2-y nanocrystals by Scanning−tunneling microscopy/spectroscopy and ellipsometry

Synthesis process strongly influences the nanocrystalline CeO2-y defective structure. The presence of surface defects, in the form of oxygen vacancies in different charge states (F centers), can change the electronic properties of ceria nanocrystals. Nanocrystalline CeO2-y samples were synthesized using three different methods (precipitation, self-propagating room temperature, and hydrothermal synthesis). Raman spectroscopy was used to identify the presence of oxygen vacancies which presumably were formed at the nanoparticle surface. The defect concentration depended on the crystallite size of differently prepared CeO2-y samples. Scanning tunneling microscopy/spectroscopy and ellipsometry were employed to investigate the electronic band structure of defective CeO2-y nanocrystals. Scanning tunneling spectroscopy measurements demonstrated that inside the band gap of CeO2-y nanocrystals, besides the filled 4 f states, appeared additional states which were related to occupied and empty F center defect states. From the ellipsometric measurements, using the critical points model, the energy positions of different F centers states and the values of the reduced band gap energies were determined. The analysis of obtained data pointed out that depending on the synthesis method, different types of F centers (F+ and F0) can be formed in the CeO2-y nanocrystals. The formation of different F center defect states inside the ceria gap have a strong impact on the electrical, optical, and magnetic properties of ceria nanocrystals.

Mechanochemistry of titanium oxides

Mechanochemistry represents an alternative route in synthesis of nanomaterials. Mechanochemical routes are attractive because of their simplicity, flexibility, and ability to prepare materials by solid state reactions at room temperature. The aim of this work is the mechanochemical synthesis of nanostructured titanium oxides of different composition starting from mixtures of Ti and TiO2, TiO and TiO2 or Ti2O3 and TiO2. Emphasis is on the Magneli phases Ti4O7 and Ti5O9 because their mixture is commercially known as EBONEX material. The materials prepared were characterized by XRPD, TG/DTA analysis, SEM and optical microscopy. Titanium monoxide and several Magneli oxides, Ti4O7, Ti5O9 and Ti6O11, are successfully prepared. The results are very interesting because the EBONEX materials were prepared at lower than usual temperature, which would decrease the effective cost of production.