Hybrid materials - past, present and future

Hybrid materials represent one of the most growing new material classes at the edge of technological innovations. Unique possibilities to create novel material properties by synergetic combination of inorganic and organic components on the molecular scale makes this materials class interesting for application-oriented research of chemists, physicists, and materials scientists. The modular approach for combination of properties by the selection of the best suited components opens new options for the generation of materials that are able to solve many technological problems. This review will show in selected examples how science and technological driven approaches can help to design better materials for future applications

Solids: Exciting Research Should Be Accessible to the General Public—A Plea for a New Open Access Journal

Solid-state sciences continue to be one of the key pillars of scientific and technological progress in our society [...

Reversible Diels–Alder Reactions with a Fluorescent Dye on the Surface of Magnetite Nanoparticles

Diels–Alder reactions on the surface of nanoparticles allow a thermoreversible functionalization of the nanosized building blocks. We report the synthesis of well-defined magnetite nanoparticles by thermal decomposition reaction and their functionalization with maleimide groups. Attachment of these dienophiles was realized by the synthesis of organophosphonate coupling agents and a partial ligand exchange of the original carboxylic acid groups. The functionalized iron oxide particles allow a covalent surface attachment of a furfuryl-functionalized rhodamine B dye by a Diels–Alder reaction at 60 ◦C. The resulting particles showed the typical fluorescence of rhodamine B. The dye can be cleaved off the particle surface by a retro-Diels–Alder reaction. The study showed that organic functions can be thermoreversibly attached onto inorganic nanoparticles

Influence of alkyl groups on the formation of softenable polysilsesquioxanes

Polysilsesquioxanes (PSQ) containing phenyl groups as substituents on the silicon atom can form transparent, hard and glassy materials at room temperature, which reversibly soften when heated above the glass transition temperature. Further increase in temperature leads to irreversible curing. With this property, polyphenylsilsesquioxanes can be assigned to the so-called melting gels. In contrast to the aromatic systems, polysilsesquioxanes with alkyl groups belonging to this class of materials are not known. To identify structural differences between aryl and alkyl systems, polyalkylsilsesquioxanes (RPSQs with R = methyl, ethyl, propyl, hexyl, octyl, decyl, dodecyl, hexadecyl, and octadecyl) were synthesised by a solvent-free, acid-catalysed hydrolysis and condensation reaction of alkyltrimethoxysilanes followed by thermal treatment at 300 °C under N2 atmosphere. The influence of the alkyl chain length on the structure built and the thermally initiated further condensation reactions were investigated via NMR, FTIR, TGA, DSC, SEC, and XRD. Depending on the alkyl chain length, the formation of highly crosslinked, insoluble systems (Me-PrPSQ), low molecular weight oligosilsesquioxanes in the form of cages (Hex-DecPSQ) to semicrystalline, lamellar layers (Dodec-OctadecPSQ) were detected. A low degree of condensation, inhibition of self-assembly and preferentially intermolecular condensation reactions were found to be crucial factors in the melting gel formation

Perylene polyphenylmethylsiloxanes for optoelectronic applications

The incorporation of fluorescent organic dyes in an encapsulating matrix represents a route to generate stable and processable materials for optoelectronic devices. Here, we present a method to embed perylene dyes into a high refractive index (HRI) polysiloxane matrix applying an allyl functionalized perylene dye and hydrosilylation chemistry. In a first approach, the dye molecules were covalently integrated into the backbone of linear polyphenylmethylsiloxane chains. The fluorescent and liquid polymers were synthesized with molecular weights from 5660 up to 8400 g mol−1. In a second approach, the dye itself was used as a cross‐linking agent between linear polyphenylmethylsiloxane chains. These preformed fluorescent batch polymers are liquids with dye concentrations between 0.025 and 8 wt %. The applied synthetic methods incorporated the dye covalently into the polymer structure and avoided the crystallization of the dye molecules and thus the formation of excimers, which would reduce the optical emission. The resulting products can be easily incorporated into curable commercially available HRI polyphenylmethylsiloxane resins. The formed materials are ideal LED encapsulants with a solid and flexible consistency, a uniform dispersion of the dyes, and adjustable mechanical properties, realized by changing the amount of perylene polymers. Further properties of the obtained materials are thermal stabilities up to 478 °C, quantum yields larger than 0.97, and high photostabilities. Thus, the covalent integration of dyes into polyphenylsiloxane structures represents a possible route for the stabilization of the organic dyes against the extreme irradiance and thermal conditions in LED applications

Long Alkyl Chain Organophosphorus Coupling Agents for in Situ Surface Functionalization by Reactive Milling

Innovative synthetic approaches should be simple and environmentally friendly. Here, we present the surface modification of inorganic submicrometer particles with long alkyl chain organophosphorus coupling agents without the need of a solvent, which makes the technique environmentally friendly. In addition, it is of great benefit to realize two goals in one step: size reduction and, simultaneously, surface functionalization. A top-down approach for the synthesis of metal oxide particles with in situ surface functionalization is used to modify titania with long alkyl chain organophosphorus coupling agents. A high energy planetary ball mill was used to perform reactive milling using titania as inorganic pigment and long alkyl chain organophosphorus coupling agents like dodecyl and octadecyl phosphonic acid. The final products were characterized by IR, NMR and X-ray fluorescence spectroscopy, thermal and elemental analysis as well as by X-ray powder diffraction and scanning electron microscopy. The process entailed a tribochemical phase transformation from the starting material anatase to a high-pressure modification of titania and the thermodynamically more stable rutile depending on the process parameters. Furthermore, the particles show sizes between 100 nm and 300 nm and a degree of surface coverage up to 0.8 mmol phosphonate per gram

Bottom-Up, Wet Chemical Technique for the Continuous Synthesis of Inorganic Nanoparticles

Continuous wet chemical approaches for the production of inorganic nanoparticles are important for large scale production of nanoparticles. Here we describe a bottom-up, wet chemical method applying a microjet reactor. This technique allows the separation between nucleation and growth in a continuous reactor environment. Zinc oxide (ZnO), magnetite (Fe3O4), as well as brushite (CaHPO4·2H2O), particles with a small particle size distribution can be obtained continuously by using the rapid mixing of two precursor solutions and the fast removal of the nuclei from the reaction environment. The final particles were characterized by FT-IR, TGA, DLS, XRD and SEM techniques. Systematic studies on the influence of the different process parameters, such as flow rate and process temperature, show that the particle size can be influenced. Zinc oxide was obtained with particle sizes between 44 nm and 102 nm. The obtained magnetite particles have particle sizes in the range of 46 nm to 132 nm. Brushite behaves differently; the obtained particles were shaped like small plates with edge lengths between 100 nm and 500 nm

Hybrid inorganic-organic fluorescent silica nanoparticles—influence of dye binding modes on dye leaching

Silica nanoparticles with embedded fluorescent dyes represent an important class of markers for example in biological imaging. We systematically studied the various incorporation mechanisms of fluorescent xanthene dyes in 30–40 nm silica nanoparticles. An important parameter was the interaction of the dye with the matrix material, either by weak electrostatic or strong covalent interactions, which also has implications on the stability of fluorescence and brightness of the dyes. Factors that can influence leaching of dyes such as the position of the dyes in particles and the intensity of the particle-dye interaction were investigated by using the solvatochromic effect of xanthene dyes and by stationary fluorescence anisotropy measurements. We compared uranine and rhodamine B, which were physically embedded, with modified fluorescein isothiocyanate and rhodamine B isothiocyanate, which were covalently bound to the silica matrix within a usual Stöber synthesis. Systematic leaching studies of time spans up to 4 days revealed that covalent bonding of dyes like fluorescein isothiocyanate or rhodamine B isothiocyanate is necessary for fluorescence stability, since dyes bound by physical interaction tend to leach out of porous silica networks. Coverage of silica particles with hydrophobic protection layers of alkyltrialkoxysilanes or hydrophilic polyethylene glycol (PEG) groups resulted in a better retention of physisorbed dyes and provides the possibility to adapt the particles to the polarity of the medium. Best results were archived with PEG groups, but even small trimethylsilyl (TMS) groups already reduce leaching

Mechanochemical Induced Structure Transformations in Lithium Titanates: A Detailed PXRD and 6Li MAS NMR Study

Lithium titanates are used in various applications, such as anode materials for lithium intercalation (Li4Ti5O12) or breeding materials in fusion reactors (Li2TiO3). Here, we report the formation of nano-crystalline lithium titanates by a mechanochemical approach and present a deeper insight into their structural characteristics by X-ray diffraction (XRD) and solid-state NMR spectroscopy. The compounds were synthesized in a high-energy planetary ball mill with varying milling parameters and different grinding tools. NaCl type Li2TiO3 (α-Li2TiO3) was formed by dry milling of lithium hydroxide with titania (rutile or anatase) and by a milling induced structure transformation of monoclinic β-Li2TiO3 or spinel type Li4Ti5O12. Heating of mechanochemical prepared α-Li2TiO3 induces a phase transformation to the monoclinic phase similar to hydrothermal reaction products, but a higher thermal stability was observed for the mechanochemical formed product. Microstructure and crystallographic structure were characterized by XRD via Rietveld analysis. Detailed phase analysis shows the formation of the cubic phase from the various educts. A set of two lattice parameters for α-Li2TiO3 was refined, depending on the presence of OH− during the milling process. An average crystallite size of less than 15 nm was observed for the mechanochemical generated products. The local Li environment detected by 6Li NMR revealed Li defects in the form of tetrahedral instead of octahedral site occupation. Subsequent adjustment of the structural model for Rietveld refinement leads to better fits, supporting this interpretatio

Mechanochemical Induced Structure Transformations in Lithium Titanates: A Detailed PXRD and 6Li MAS NMR Study

Lithium titanates are used in various applications, such as anode materials for lithium intercalation (Li4Ti5O12) or breeding materials in fusion reactors (Li2TiO3). Here, we report the formation of nano-crystalline lithium titanates by a mechanochemical approach and present a deeper insight into their structural characteristics by X-ray diffraction (XRD) and solid-state NMR spectroscopy. The compounds were synthesized in a high-energy planetary ball mill with varying milling parameters and different grinding tools. NaCl type Li2TiO3 (α-Li2TiO3) was formed by dry milling of lithium hydroxide with titania (rutile or anatase) and by a milling induced structure transformation of monoclinic β-Li2TiO3 or spinel type Li4Ti5O12. Heating of mechanochemical prepared α-Li2TiO3 induces a phase transformation to the monoclinic phase similar to hydrothermal reaction products, but a higher thermal stability was observed for the mechanochemical formed product. Microstructure and crystallographic structure were characterized by XRD via Rietveld analysis. Detailed phase analysis shows the formation of the cubic phase from the various educts. A set of two lattice parameters for α-Li2TiO3 was refined, depending on the presence of OH− during the milling process. An average crystallite size of less than 15 nm was observed for the mechanochemical generated products. The local Li environment detected by 6Li NMR revealed Li defects in the form of tetrahedral instead of octahedral site occupation. Subsequent adjustment of the structural model for Rietveld refinement leads to better fits, supporting this interpretatio