Gerhard Sextl

Metal-organic framework luminescence in the yellow gap by codoping of the homoleptic imidazolate ∞(3)[Ba(Im)2] with divalent europium.

The rare case of a metal-triggered broad-band yellow emitter among inorganic-organic hybrid materials was achieved by in situ codoping of the novel imidazolate metal-organic framework ∞(3)[Ba(Im)2] with divalent europium. The emission maximum of this dense framework is in the center of the yellow gap of primary light-emitting diode phosphors. Up to 20% Eu2+ can be added to replace Ba2+ as connectivity centers without causing observable phase segregation. High-resolution energy-dispersive X-ray spectroscopy showed that incorporation of even 30% Eu2+ is possible on an atomic level, with 2-10% Eu2+ giving the peak quantum efficiency (QE = 0.32). The yellow emission can be triggered by two processes: direct excitation of Eu2+ and an antenna effect of the imidazolate linkers. The emission is fully europium-centered, involving 5d → 4f transitions, and depends on the imidazolate surroundings of the metal ions. The framework can be obtained by a solvent-free in situ approach starting from barium metal, europium metal, and a melt of imidazole in a redox reaction. Better homogeneity for the distribution of the luminescence centers was achieved by utilizing the hydrides BaH2 and EuH2 instead of the metals.

Modified Superparamagnetic Nanocomposite Microparticles for Highly Selective HgII or CuII Separation and Recovery from Aqueous Solutions

The synthesis of a reusable, magnetically switchable nanocomposite microparticle, which can be modified to selectively extract and recover Hg(II) or Cu(II) from water, is reported. Superparamagnetic iron oxide (magnetite) nanoparticles act as the magnetic component in this system, and these nanoparticles were synthesized in a continuous way, allowing their large-scale production. A new process was used to create a silica matrix, confining the magnetite nanoparticles using a cheap silica source [sodium silicate (water glass)]. This results in a well-defined, filigree micrometer-sized nanocomposite via a fast, simple, inexpensive, and upscalable process. Hence, because of the ideal size of the resulting microparticles and their comparably large magnetization, particle extraction from fluids by low-cost magnets is achieved.

Homoleptic Lanthanide 1,2,3-Triazolates ∞2–3[Ln(Tz*)3] and Their Diversified Photoluminescence Properties

The series of homoleptic lanthanide 1,2,3-triazolates (∞)(3)[Ln(Tz*)3] (Ln3+ = lanthanide cation, Tz*– = 1,2,3-triazolate anion, C2H2N3(–)) is completed by synthesis of the three-dimensional (3D) frameworks with Ln = La, Ce, Pr, Nd, and Sm, and characterization by X-ray powder diffraction, differential thermal analysis-thermogravimetry (DTA/TG) investigations and molecular vibration analysis. In addition, α-(∞)(2)[Sm(Tz*)3], a two-dimensional polymorph of 3D β-(∞)(3)[Sm(Tz*)3], is presented including the single crystal structure. The 3D lanthanide triazolates form an isotypic series of the formula (∞)(3)[Ln(Tz*)3] ranging from La to Lu, with the exception of Eu, which forms a mixed valent metal organic framework (MOF) of different structure and the constitution (∞)(3)[Eu(Tz*)(6+x)(Tz*H)(2–x)]. The main focus of this work is put on the investigation of the photoluminescence behavior of lanthanide 1,2,3-triazolates (∞)(3)[Ln(Tz*)3] and illuminates that six different luminescence phenomena can be found for one series of isotypic compounds. The luminescence behavior of the majority of these compounds is based on the photoluminescence properties of the organic linker molecules. Differing properties are observed for (∞)(3)[Yb(Tz*)3], which exhibits luminescence properties based on charge transfer transitions between the linker and Yb3+ ions, and for (∞)(3)[Ce(Tz*)3] and (∞)(3)[Tb(Tz*)3], in which the luminescence properties are a combination of the ligand and the lanthanide metal. In addition, strong inner-filter effects are found in the ligand emission bands that are attributed to reabsorption of the emitted light by the trivalent lanthanide ions. Antenna effects of varying efficiency are present indicated by the energy being transferred to the lanthanide ions subsequent to excitation of the ligand. (∞)(3)[Ce(Tz*)3] shows a 5d-4f induced intense blue emission upon excitation with UV light, while (∞)(3)[Tb(Tz*)3] shows emission in the green region of the visible spectrum, which can be identified with 4f-4f-transitions typical for Tb3+ ions.

Superparamagnetic Luminescent MOF@Fe3O4/SiO2 Composite Particles for Signal Augmentation by Magnetic Harvesting as Potential Water Detectors

Herein, we present the generation of a novel complex particle system consisting of superparamagnetic Fe3O4/SiO2 composite microparticle cores, coated with luminescent metal-organic frameworks (MOFs) of the constitution (∞)(2)[Ln2Cl6(bipy)3]·2bipy (bipy = 4,4′-bipyridine) that was achieved by intriguing reaction conditions including mechanochemistry. The novel composites combine the properties of both constituents: superparamagnetism and luminescence. The magnetic properties can be exploited to magnetically collect the particles from dispersions in fluids and, by gathering them at one spot, to augment the luminescence originating from the MOF modification on the particles. The luminescence can be influenced by chemical compounds, e.g., by quenching observed for low concentrations of water. Thus, the new composite systems present an innovative concept of property combination that can be potentially used for the detection of water traces in organic solvents as a magnetically augmentable, luminescent water detector.

Environmental assessment of electrically controlled variable light transmittance devices

A comprehensive benchmark analysis has been performed on five electrically controlled state-of-the-art transmittance modulation devices including their production routes, from ‘cradle-to-gate’. The benchmarks have been modeled employing the GaBi life cycle assessment software tool, which successfully yielded the most important environmental problem areas for the product life cycles of electrochromic and electrotropic light-modulating devices. In terms of the energy demand of processing, all-solid-state technology was found to be less favorable than wet-chemical electrodeposition processes; however, the effect is interestingly overcompensated for by the resource depletion resulting from higher layer thicknesses in the latter case. As opposed to the mineral-glass based benchmarks, a plastic-film based system was particularly favorable, implying that the substrate is a factor with a strong environmental impact in transmittance modulation devices. Eventually, very high impacts were found for tin-doped indium oxide (ITO) and iridium oxide, i.e. a common transparent conductor and anodic electrochromic material, respectively. The results obtained support important current trends such as in-line manufacturing of electrochromic devices, the quest for ITO replacement materials, and, in general, the replacement of energy- and resource-intensive processes (sputter deposition of heavy metal oxides) by less demanding methods.

Nitric acid-stabilized superparamagnetic iron oxide nanoparticles studied with X-rays

Agglomerated superparamagnetic iron oxide nanoparticles can easily and in large scale be precipitated from iron salt solutions. Although the process is well known, it is ambiguously either assumed that magnetite or maghemite is obtained. The first part of our study clarifies this question using X-ray absorption spectroscopy. For further processing of the nanoparticles, i.e., for giving them a surface functionality or incorporating them into composites, it is important to break the agglomerates and individualize the particles at first. This can effectively be done with nitric acid treatment. The influence of this process on the particles chemistry and structure was analyzed in great detail using X-ray diffraction, X-ray absorption, and small-angle X-ray scattering. In contrast to our expectation, no oxidation from magnetite (Fe3O4) to maghemite (γ-Fe2O3) was found; the formal valence of the particles in any case is magnetite (Fe3O4). Instead, an increase in the particles’ surface disorder was discovered from X-ray absorption analyses and high-resolution transmission electron microscopy. The acid treatment roughens and distorts the surface of the nanoparticles which is connected with an increased spin disorder.

Composite materials combining multiple luminescent MOFs and superparamagnetic microparticles for ratiometric water detection

Smart optical composite materials suitable for ratiometric sensing applications and harvesting options have been developed. The hybrid materials consist of core/shell particles with Fe3O4/SiO2 as the core and different luminescent lanthanide-containing metal–organic frameworks (MOFs) as the shell. The magnetic properties enable collection of microparticles via an external magnetic field and, thus, a strong signal augmentation of the luminescence signal. Thereby, MOF luminescence functions as a read-out signal of the sensing that can be influenced by different chemical compounds, e.g. by quenching with low concentrations of water. The combination of MOFs, which contain different luminescence centers combined with a different sensitivity towards water, results in a system that can be exploited as a ratiometric sensor. We have utilized the MOFs 3∞[Eu2(BDC)3]·2H2O·2DMF (BDC2− = benzene dicarboxylate) and 2∞[Ln2Cl6(bipy)3]·2bipy (Ln = Eu, Tb; bipy = 4,4′-bipyridine) for functionalization of the microparticles, resulting in a color-tuned yellow-emitting mixed-MOF composite system together with a Fe3O4/SiO2 core. Interaction with water decreases the luminescence unequally for both luminescence centers, which enables a quantitative determination of the water content by analysis of the ratio of the Tb3+ and Eu3+ luminescence bands. This process is supported by possible harvesting via superparamagnetism of the composite. Altogether, high sensitivity with a detection limit of 0.3% (20 μg) is achieved, equal to Karl-Fischer titration but also suitable for an “on-the-fly” analysis.

Immobilization of ionic liquids within micro- and mesoporous materials

Abstract Room temperature imidazolium-based ionic liquid (1-ethyl-3-methylimidazolium tetrafluoroborate) was immobilized within high-silica zeolite Beta and mesoporous silica (SBA-15) samples using methanol as solvent. The resultant samples were then subjected to Soxhlet extraction in order to remove the ionic liquids which are loosely bound on the external surfaces. All the samples (before and after Soxhlet extraction) were analyzed by TG-DTA, 13 C MAS-NMR and N 2 -adsorption measurements. Upon immobilization of ionic liquid, the BET surface areas of the host materials decreased significantly. Nevertheless, the ionic liquid immobilized host materials regained their surface areas after Soxhlet extraction. The amount of immobilized ionic liquid was estimated from TG measurements to be 0.6 wt.% (zeolite Beta) and 1.3 wt.% (SBA-15), respectively.

Smart Optical Composite Materials: Dispersions of Metal-Organic Framework@Superparamagnetic Microrods for Switchable Isotropic-Anisotropic Optical Properties

A smart optical composite material with dynamic isotropic and anisotropic optical properties by combination of luminescence and high reflectivity was developed. This combination enables switching between luminescence and angle-dependent reflectivity by changing the applied wavelength of light. The composite is formed as anisotropic core/shell particles by coating superparamagnetic iron oxide-silica microrods with a layer of the luminescent metal-organic framework (MOF) 3∞[Eu2(BDC)3]·2DMF·2H2O (BDC2- = 1,4-benzenedicarboxylate). The composite particles can be rotated by an external magnet. Their anisotropic shape causes changes in the reflectivity and diffraction of light depending on the orientation of the composite particle. These rotation-dependent optical properties are complemented by an isotropic luminescence resulting from the MOF shell. If illuminated by UV light, the particles exhibit isotropic luminescence while the same sample shows anisotropic optical properties when illuminated with visible light. In addition to direct switching, the optical properties can be tailored continuously between isotropic red emission and anisotropic reflection of light if the illuminating light is tuned through fractions of both UV and visible light. The integration and control of light emission modes within a homogeneous particle dispersion marks a smart optical material, addressing fundamental directions for research on switchable multifunctional materials. The material can function as an optic compass or could be used as an optic shutter that can be switched by a magnetic field, e.g., for an intensity control for waveguides in the visible range.

Detection of cell-stack inhomogeneities via mechanical SOC and SOH measurement

Common lithium-ion battery cells consist of a cathode material, such as a Nickel-Manganese-Cobalt NMC and a graphite anode. Expansion of the graphite anode due to lithium intercalation while charging leads to macroscopic expansion of the entire cell, which is directly related to the state-of-charge. Therefore, conventional rigid-battery-module bracing causes pressure forces to build up periodically in the cell stack due to charging and discharging steps. In homogeneous cell stacks, the measured-pressure curve is a multiple of a single cells measured pressure force curve. One cell, perhaps prematurely aged (i.e. diverging state-of-health), disturbs the module-pressure force curve. Due to the mechanical coupling of the cells in a module, a single sensor can monitor this.