David Škoda

Ag-Cu colloid synthesis: bimetallic nanoparticle characterisation and thermal treatment

The Ag-Cu bimetallic colloidal nanoparticles (NPs) were prepared by solvothermal synthesis from metalloorganic precursors in a mixture of organic solvents. The nanoparticles were characterized by dynamic light scattering (DLS) and small angle X-ray scattering (SAXS). The properties of metallic core and organic shell of the nanoparticles were studied by direct inlet probe mass spectrometry (DIP/MS), Knudsen effusion mass spectrometry (KEMS), double-pulse laser-induced breakdown spectroscopy (DPLIBS), and differential scanning calorimetry (DSC). The transmission electron microscopy (TEM) and scanning electron microscopy (SEM) were used for particle characterization before and after thermal analysis. The experiment yielded results that were for AgCu nanoparticles for the first time. The detected liquidus temperature has been compared with the prediction obtained from calculation of the phase diagram of Ag-Cu nanoalloy. The experimental results show that of near-eutectic composition AgCu nanoparticles possess the fcc crystal lattice. Surprisingly, spinodal decomposition was not observed inside the AgCu nanoparticles at temperatures up to 230°C. The depression of the eutectic AgCu melting point was calculated but not observed. The eutectic AgCu microparticles are formed before melting.

Non-aqueous template-assisted synthesis of mesoporous nanocrystalline silicon orthophosphate

The first synthesis of mesoporous nanocrystalline silicon orthophosphate Si5P6O25 is presented. The synthetic procedure is based on the non-hydrolytic sol–gel reaction in the presence of Pluronic P123 template and subsequent calcination in air. The condensation of silicon acetate, Si(OAc)4, and tris(trimethylsilyl)phosphate, OP(OSiMe3)3 (TTP), in non-aqueous solvents driven by elimination of trimethylsilyl acetate provides a homogeneous network with a high content of Si–O–P bonds and SiO6 moieties. After burning out the template, mesoporous silicon orthophosphate was obtained with surface areas up to 128 m2 g−1 and pore sizes around 20 nm. The nanocrystalline Si5P6O25 phase forms relatively easily (500 °C, 4 h) in comparison with other synthetic routes. All samples were characterized by SEM, TEM, elemental analysis, TGA, nitrogen adsorption, SAXS, 1H, 13C, 29Si, and 31P solid-state NMR spectroscopy, and powder XRD. These xerogels showed superior catalytic activity and selectivity in methylstyrene dimerization.

Novel non-hydrolytic templated sol–gel synthesis of mesoporous aluminosilicates and their use as aminolysis catalysts

A novel non-hydrolytic sol–gel (NHSG) synthesis of mesoporous aluminosilicate xerogels is presented. The polycondensation between silicon acetate, Si(OAc)4, and tris(dimethylamido)alane, Al(NMe2)3, leads to homogeneous aluminosilicate xerogels containing Si–O–Al linkages through dimethylacetamide elimination. The addition of Pluronic P123 and F127 templates provides stiff gels that are, after calcination at 500 °C, converted to stable mesoporous xerogels with a high surface area (>600 m2 g−1) and wormhole-type pores (d = 5.9 nm). The xerogels exhibit high catalytic activity in aminolysis of styrene oxide (82% conversion) with the turnover frequency up to 100.

Control of micro/mesoporosity in non-hydrolytic hybrid silicophosphate xerogels

Non-hydrolytic sol–gel reactions of acetoxysilanes with trimethylsilyl esters of phosphoric and phosphonic acids produce cross-linked matrices containing homogeneous dispersions of silicon and phosphoryl groups connected together by networks of Si–O–P(O) linkages. These polycondensation reactions proceed cleanly and under mild conditions for a wide variety of precursor silanes RnSi(OAc)4−n (R = alkyl, aryl; n = 1, 2) and phosphoryl compounds RP(O)(OSiMe3)2 (R = alkyl, aryl) to provide hybrid xerogels, the final properties of which are a sensitive function of the organic substituents and the Si : P ratio of the precursors. The reactions of bridged acetoxysilanes (AcO)3Si–X–Si(OAc)3 and phosphoryl reagents (Me3SiO)2P(O)–X–P(O)(OSiMe3)2 have also been investigated and found to produce gels that exhibit large surface areas (up to 700 m2 g−1). The presence of SiO6 structural units in bridged-phosphoryl xerogels is related to their microporosity while the absence of such moieties in bridged-acetoxysilane networks is congruent with significant mesoporosity. Several important parameters are identified which can be used to tailor the properties of these hybrid matrices such that gels with specific polarity, porosity and surface area can be targeted at the time of synthesis.

Mesoporous SnO2–SiO2 and Sn–silica–carbon nanocomposites by novel non-hydrolytic templated sol–gel synthesis

A novel non-hydrolytic sol–gel (NHSG) synthesis of mesoporous tin silicate xerogels is presented. The polycondensation between silicon tetraacetate, Si(OAc)4, and tetrakis(diethylamido)tin, Sn(NEt2)4, resulting in acetamide elimination leads to tin silicate xerogels containing Si–O–Sn linkages. The addition of Pluronic P123 or F127 templates provides homogeneous stiff gels that are, after template removal by calcination at 500 °C in air, converted to stable mesoporous silica xerogels with large surface areas (476 m2 g−1) and dispersed SnO2 nanoparticles (6–7 nm). Heat treatment of the as-prepared tin silicate gels in an inert N2 atmosphere leads to reduction and transformation to Sn nanoparticles (70–150 nm) embedded in a silica–carbon matrix. The composition and morphology of the xerogels, volatile reaction byproducts, and thermal transformations were followed by elemental analyses, IR spectroscopy, thermal analysis TG-DSC, nitrogen adsorption measurements, solid-state NMR spectroscopy, DRUV-vis spectroscopy, electron microscopy, and HT powder XRD. The SnO2–SiO2 xerogels were tested as potential catalysts for aminolysis of styrene oxide with aniline and for the Meerwein–Ponndorf–Verley reduction of 4-tert-butylcyclohexanone. The resulting reaction systems displayed good activity and selectivity.

Sonochemical synthesis of Gd3+ doped CoFe2O4 spinel ferrite nanoparticles and its physical properties

In this work, a facile and green method for gadolinium doped cobalt ferrite (CoFe2-xGdxO4; x=0.00, 0.05, 0.10, 0.15, 0.20) nanoparticles by using ultrasonic irradiation was reported. The impact of Gd3+ substitution on the structural, magnetic, dielectric and electrical properties of cobalt ferrite nanoparticles was evaluated. The sonochemically synthesized spinel ferrite nanoparticles were characterized by X-ray Diffraction (XRD), scanning electron microscopy (SEM), Raman spectroscopy, Fourier transform infrared (FTIR) spectroscopy, X-ray photoelectron spectroscopy (XPS), vibrating sample magnetometer (VSM). X-ray diffraction (XRD) study confirmed the formation of single phase spinel ferrite of CoFe2-xGdxO4 nanoparticles. XRD results also revealed that ultrasonic irradiation seems to be favourable to achieve highly crystalline single crystal phase gadolinium doped cobalt ferrite nanoparticles without any post annealing process. Fourier Transform Infrared and Raman Spectra confirmed the formation of spinel ferrite crystal structure. X-ray photoelectron spectroscopy revealed the impact of Gd3+ substitution in CoFe2O4 nanoparticles on cation distribution at the tetrahedral and octahedral site in spinel ferrite crystal system. The electrical properties showed that the Gd3+ doped cobalt ferrite (CoFe2-xGdxO4; x=0.20) exhibit enhanced dielectric constant (277 at 100Hz) and ac conductivity (20.2×10-9S/cm at 100Hz). The modulus spectroscopy demonstrated the impact of Gd3+ substitution in cobalt ferrite nanoparticles on grain boundary relaxation time, capacitance and resistance. Magnetic property measurement revealed that the coercivity decreases with Gd3+ substitution from 234.32Oe (x=0.00) to 12.60Oe (x=0.05) and further increases from 12.60Oe (x=0.05) to 68.62Oe (x=0.20). Moreover, saturation magnetization decreases with Gd3+ substitution from 40.19emu/g (x=0.00) to 21.58emu/g (x=0.20). This work demonstrates that the grain size and cation distribution in Gd3+ doped cobalt ferrite nanoparticles synthesized by sonochemical method, is effective in controlling the structural, magnetic, and electrical properties, and can be find very promising applications.

Surface reactivity of non-hydrolytic silicophosphate xerogels: a simple method to create Brønsted or Lewis acid sites on porous supports

Non-hydrolytic sol-gel reactions of silicon acetates with trimethylsilyl (TMS) esters of phosphoric and phosphonic acids produce cross-linked matrices containing homogeneous dispersions of silicate and phosphoryl groups connected together by networks of Si-O-P(QO) linkages. The condensation degrees reach 80 to 90%. Residual organic groups (10 to 20%) were reacted with a variety of compounds (H2O, Me3SiOSiMe3, POCl3, SiCl4, AlMe3, Al(NMe2)(3), and AlCl3) in order to enrich the surface of these porous matrices with Bronsted (RP-OH) and Lewis (tetracoordinated Al) acid functional groups. The differences in the reactivity of RSi-OAc and RP-OSiMe3 groups were utilized for the selective modification at the silicon and phosphorus atoms. The reaction procedures were optimized and significantly porous silicophosphate materials with a high content of either hydroxyl groups or four-coordinated aluminium species were obtained. The activity and selectivity of prepared samples as catalysts for the dimerization of a-methylstyrene were tested. Excellent activities and moderate to very high selectivities were achieved suggesting the potential use of silicophosphate xerogels in heterogeneous catalysis.

Comparison of the Degree of Consolidation of Historic Plaster Layers by Means of Lime Hydrate Nanosuspensions

Historic masonry is usually coated with multiple plaster layers, exposed to long-term extreme loading (climatic effects, elevated moisture levels, salt crystallization and recrystallization, biological effects, etc.). As a consequence of the above negative cyclic effects, degradation processes are triggered off leading to a loss in plaster cohesion with the substrate masonry and successive falling-off of surface layers. The strengthening and rehabilitation of plasters with degraded binder components, impaired structure, insufficient adhesion of individual layers of plaster to the masonry surface, with elevated salt contents in the pore system and surface crusts represent a demanding task in terms of the stabilization of such degraded plasters and coatings. Consolidation (i.e. enhancement of mechanical characteristics) belongs to the basic objectives of the restoration of historic plasters, its principle is to return binder into the material structure, fill in existing hollow spaces, heal cracks, etc. Degraded lime plasters are strengthened with consolidating agents applied in the form of solutions, e.g. lime water, organosilicates or lime nanosuspensions, etc. The lime nanosuspension is composed of lime hydrate nanoparticles, dispersed in an alcoholic medium. Due to the deposition of calcium carbonate in the degraded material, the bonds are re-strengthened and the material consolidated. The article presents partial results of theoretical and experimental research addressing potential consolidation of historic plaster layers with lime nanosuspensions prepared by the synthesis of Ca(OH)2, which were successively modified and enriched with other components.

NANOTECHNOLOGY IN THE CULTURAL HERITAGE - INFLUENCE OF NANOSPENSIONS ADOPTED BY NANOPARTICLES OF TiO2 FOR CLEANING THE SURFACE OF HISTORICAL PLASTERS

The continuous development of nanostructure and the study of physico chemical processes in the nanometer range lead to new methods that can slow down the degradation processes of a work of art, or even restore damage caused, for example, by an inappropriate restoration process. The use of nanosuspensions based on calcium hydroxide is probably the most widespread application of nanomaterials in heritage care, especially in the field of hardening of lime building materials (plaster, limestone, etc.). In combination with titanium dioxide, it should be a successful suspension in the fight against biological agents, surface contamination or in the protection of UVresistant building materials.