The laboratory of synthesis and chemical analysis is dedicated to the preparation of new materials and development of novel synthetic methods for nanoparticles and nanostructured composites. Moreover, it is also a main source of materials studied within the Department of Magnetics and Superconductors. All relevant chemical analyses of studied samples are provided here as well.
Molten salt synthesis
Magnetic nanoparticles based on manganites of the general composition Ln1-xAexMnO3, where Ln stands for a rare earth metal and Ae for an alkali earth metal, are prepared here by the so-called molten salt synthesis, i.e., by growth of nanoparticles in flux of alkali metal nitrites or nitrates. In contrast to traditional sol-gel procedures, highly dispersed nanoparticles of better defined shape and relatively narrow size distribution are obtained. The resulting nanoparticles are not grown together and do not form sintering bridges that originate in sol-gel products due to the final thermal treatment of a solid precursor. Therefore, mechanical processing of the as-synthesized nanoparticles is not necessary. Moreover, the synthesis in an agitated flux provides homogeneity of the reaction mixture and it enables growth of nanoparticles in an isotropic environment.
Synthesis of nanoparticles by thermal decomposition of organic metal complexes
Controlled thermal decomposition of organic metal complexes is an advanced method for the synthesis of oxide nanoparticles, namely nanoparticles of spinel phases (e.g. doped magnetite and ferrites) as well as manganese oxides, etc. Nanoparticles originate from the decomposition of suitable precursors (e.g. metal acetylacetonates or oleates) at the presence of surfactants (oleic acid, oleylamine, hexadecane-1,2-diol) in high-boiling solvents (octadecene or dibenzylether). The precise control of reaction conditions enables to prepare monodisperse and highly crystalline nanoparticles with the size of 5-25 nm. Importantly, ferrite nanoparticles prepared by this method exhibit bulk-like magnetic properties.
A pressure vessel DAB-2 (Berghof) with exchangeable teflon inserts, allowing hydrothermal synthesis at temperatures up to 250 °C and pressures up to 200 bar, is at our disposal. The synthesis is carried out in a heating chamber which prevents any temperature gradients, while homogeneity of the reaction mixture is achieved by magnetic stirring. We employ hydrothermal synthesis for preparation of magnetic nanoparticles based on iron and manganese oxides (e.g. ferrites of the general composition Mn1-xZnxFe2O4 a Co1-xZnxFe2O4). Smaller nanoparticles can be prepared either by surfactant-assisted procedure, as surfactants efficiently suppress the crystal growth, or by suitable choice of reaction conditions.
We usually employ sol-gel procedures, e.g., the citrate method (Pechini process) for synthesis of both nanocrystalline and bulk samples of various oxides (perovskite manganites, cobaltites, layer structures NaxCoO2, etc.). Such preparation is based on the gradual transition of a solution to a homogenous gel with high content of organic components. Then the gel is dried, thermally decomposed, and thus a reactive mixture of fine particles is formed. By using this precursor, single phase oxides can be achieved through annealing at relatively low temperatures which allows to prepare even nanocrystalline phases. However, further mechanical processing of such raw nanocrystalline products is inevitable to obtain individual nanoparticles that can be dispersed.
Coprecipitation, ceramic method, and thermal processing
Some samples, e.g., larger nanoparticles of Co-Zn ferrites are prepared by the coprecipitation method and subsequent thermal treatment. Further, we also use the traditional ceramic route to prepare large amounts of bulk samples of certain oxides.
Our laboratory is equipped with various furnaces that enable thermal annealing at temperatures up to 1600 °C in different atmospheres (inert gas, oxygen, atmospheres with a controlled partial pressure of oxygen).
Coating of nanoparticles with silica, titania, hybrid organically modified silica, and other materials
Often, we perform coating of magnetic nanoparticles with silica (amorphous hydrated silicon dioxide), eventually by hybrid organically modified silica layers that enable further organic functionalization. These procedures are based on suitable stabilization of magnetic cores followed by deposition of silica shell that originates through the hydrolysis of tetetraethoxysilane, eventually other alkoxysilanes, and subsequent polycondensation of silicic species. Recently, we have developed a new procedure for efficient coating of nanoparticles by amorphous titania which forms a continuous shell around magnetic cores.
Decoration and synthesis of gold nanoshells
Further, preparation of ultrafine gold nanoparticles is carried out in our laboratory. The gold nanoparticles are used for the decoration of various surfaces and mainly for decoration of larger silica-coated particles. In particular, magnetic cores coated with primary silica shell and further decorated with gold nanoparticles are prepared. These complex particles are used for examples as intermediate products in the preparation of continuous gold nanoshells that are, eventually, further organically functionalized (e.g. by thiolated cyclodextrins for specific analytical applications).
Organic synthesis and organic functionalization
The development of complex magnetic nanoparticles for applications in biological research and medicine belongs to important topics of our laboratory. Thus, we perform also organic synthesis and preparative chromatography to prepare and purify various organic compounds for specific functionalization of nanoparticles. For illustration, the synthesis of various fluorescent silanes or complex molecules wit mPEG/PEG (methoxypoly(ethylene glycole)/poly(ethylene glycole)) chains is carried out. The latter compounds are used for the synthesis of organic corona on silica-coated magnetic particles.
Mechanical processing of samples
Certain samples of nanoparticles are obtained by mechanical treatment of nanocrystalline phases that were prepared by sol-gel or coprecipitation procedures. Our laboratory is equipped with a rolling machine and a mixer mill both of which are used for this purpose. In contrast, well-sintered bulk samples are achieved by isostatic cold pressing followed by thermal treatment. The sintered products are usually cut by a diamond wire saw to get final samples for various measurements, eventually the samples are also polished before certain analysis (e.g. scanning electron microscopy).
Classic chemical analysis is carried out in our laboratory to determine the content of metals in starting materials, which enables to prepare products with an accurate chemical composition. The analysis of macrocomponents is performed via titrations (chelatometry) or via gravimetry (e.g. the determination of strontium). We also analyse the actual oxygen stoichiometry of perovskite manganites by a reverse redox titration (cerimetry). Further, the chemical analysis of dilute suspensions of various nanoparticles (silica-coated manganites, ferrites, etc.) is crucial for our studies as well. Concentration of a suspension is precisely determined after mineral decomposition of the sample via atomic absorption spectroscopy (in cooperation with another laboratory).
Contact person: Ondřej Kaman