Engelbert Redel

Microwave Irradiation for the Facile Synthesis of Transition-Metal Nanoparticles (NPs) in Ionic Liquids (ILs) from Metal–Carbonyl Precursors and Ru-, Rh-, and Ir-NP/IL Dispersions as Biphasic Liquid–Liquid Hydrogenation Nanocatalysts for Cyclohexene

Stable chromium, molybdenum, tungsten, manganese, rhenium, ruthenium, osmium, cobalt, rhodium, and iridium metal nanoparticles (M-NPs) have been reproducibly obtained by facile, rapid (3 min), and energy-saving 10 W microwave irradiation (MWI) under an argon atmosphere from their metal-carbonyl precursors [M(x)(CO)(y)] in the ionic liquid (IL) 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIm][BF(4)]). This MWI synthesis is compared to UV-photolytic (1000 W, 15 min) or conventional thermal decomposition (180-250 degrees C, 6-12 h) of [M(x)(CO)(y)] in ILs. The MWI-obtained nanoparticles have a very small (<5 nm) and uniform size and are prepared without any additional stabilizers or capping molecules as long-term stable M-NP/IL dispersions (characterization by transmission electron microscopy (TEM), transmission electron diffraction (TED), and dynamic light scattering (DLS)). The ruthenium, rhodium, or iridium nanoparticle/IL dispersions are highly active and easily recyclable catalysts for the biphasic liquid-liquid hydrogenation of cyclohexene to cyclohexane with activities of up to 522 (mol product) (mol Ru)(-1) h(-1) and 884 (mol product) (mol Rh)(-1) h(-1) and give almost quantitative conversion within 2 h at 10 bar H(2) and 90 degrees C. Catalyst poisoning experiments with CS(2) (0.05 equiv per Ru) suggest a heterogeneous surface catalysis of Ru-NPs.

Photoinduced Charge-Carrier Generation in Epitaxial MOF Thin Films: High Efficiency as a Result of an Indirect Electronic Band Gap?

For inorganic semiconductors crystalline order leads to a band structure which gives rise to drastic differences to the disordered material. An example is the presence of an indirect band gap. For organic semiconductors such effects are typically not considered, since the bands are normally flat, and the band-gap therefore is direct. Herein we show results from electronic structure calculations demonstrating that ordered arrays of porphyrins reveal a small dispersion of occupied and unoccupied bands leading to the formation of a small indirect band gap. We demonstrate herein that such ordered structures can be fabricated by liquid-phase epitaxy and that the corresponding crystalline organic semiconductors exhibit superior photophysical properties, including large charge-carrier mobility and an unusually large charge-carrier generation efficiency. We have fabricated a prototype organic photovoltaic device based on this novel material exhibiting a remarkable efficiency.

Synthesis, Stabilization, Functionalization and, DFT Calculations of Gold Nanoparticles in Fluorous Phases (PTFE and Ionic Liquids)

Gold nanoparticles (Au-NPs) were reproducibly obtained by thermal, photolytic, or microwave-assisted decomposition/reduction under argon from Au(CO)Cl or KAuCl(4) in the presence of n-butylimidazol dispersed in the ionic liquids (ILs) BMIm(+)BF(4)(-), BMIm(+)OTf(-), or BtMA(+)NTf(2)(-) (BMIm(+) = n-butylmethylimidazolium, BtMA(+) = n-butyltrimethylammonium, OTf(-) = (-)O(3)SCF(3), NTf(2)(-) = (-)N(O(2)SCF(3))(2)). The ultra small and uniform nanoparticles of about 1-2 nm diameter were produced in BMIm(+)BF(4)(-) and increased in size with the molecular volume of the ionic liquid anion used in BMIm(+)OTf(-) and BtMA(+)NTf(2)(-). Under argon the Au-NP/IL dispersion is stable without any additional stabilizers or capping molecules. From the ionic liquids, the gold nanoparticles can be functionalized with organic thiol ligands, transferred, and stabilized in different polar and nonpolar organic solvents. Au-NPs can also be brought onto and stabilized by interaction with a polytetrafluoroethylene (PTFE, Teflon) surface. Density functional theory (DFT) calculations favor interactions between IL anions instead of IL cations. This suggests a AuF interaction and anionic Au(n) stabilization in fluorine-containing ILs. The (19)F NMR signal in BMIm(+)BF(4)(-) shows a small Au-NP concentration-dependent shift. Characterization of the dispersed and deposited gold nanoparticles was done by transmission electron microscopy (TEM/HRTEM), transmission electron diffraction (TED), dynamic light scattering (DLS), UV/Vis absorbance spectroscopy, scanning electron microscopy (SEM), electron spin resonance (ESR), and electron probe micro analyses (EPM, SEM/EDX).

On the dielectric and optical properties of surface-anchored metal-organic frameworks: A study on epitaxially grown thin films

We determine the optical constants of two highly porous, crystalline metal-organic frameworks (MOFs). Since it is problematic to determine the optical constants for the standard powder modification of these porous solids, we instead use surface-anchored metal-organic frameworks (SURMOFs). These MOF thin films are grown using liquid phase epitaxy (LPE) on modified silicon substrates. The produced SURMOF thin films exhibit good optical properties; these porous coatings are smooth as well as crack-free, they do not scatter visible light, and they have a homogenous interference color over the entire sample. Therefore, spectroscopic ellipsometry (SE) can be used in a straightforward fashion to determine the corresponding SURMOF optical properties. After careful removal of the solvent molecules used in the fabrication process as well as the residual water adsorbed in the voids of this highly porous solid, we determine an optical constant of n = 1.39 at a wavelength of 750 nm for HKUST-1 (stands for Hong Kong Un...

Nanoparticle Films and Photonic Crystal Multilayers from Colloidally Stable, Size-Controllable Zinc and Iron Oxide Nanoparticles

We report a facile sol-gel synthesis of colloidally stable Fe(2)O(3) and ZnO nanoparticles in alcoholic solvents, ROH, where R = methyl, ethyl, n-propyl, isopropyl, and tert-butyl. We show that nanoparticles of ZnO (4-42) nm and Fe(2)O(3) (4-38 nm) monotonically increase in size upon increasing the alkyl chain length and branching of the alcohol solvent. These colloidally stable and size-controllable metal oxide nanoparticles enable the formation of high optical quality films and photonic crystal multilayers whose component layer thickness, refractive index, porosity, and surface area are found to scale with the nature of the alcohol. Utility of these colloidally stable nanoparticles is demonstrated by preparation of one-dimensional porous photonic crystals comprising ncZnO/ncWO(3) and ncFe(2)O(3)/ncWO(3) multilayers whose photonic stop band can be tuned by tailoring nanoparticle size. Myriad applications can be envisaged for these nanoparticle films in, for example, heterogeneous catalysis, photocatalysis, electrocatalysis, chemical sensors, and solar cells.

Photon Upconversion at Crystalline Organic-Organic Heterojunctions

Triplet transfer across a surface-anchored metal-organic-framework heterojunction is demonstrated by the observation of triplet-triplet annihilation photon -upconversion in a sensitizer-emitter heterostructure. Upconversion thresholds under 1 mW cm-2 are achieved. In the broader context, the double-electron-exchange mechanism of triplet transfer indicates that the heterojunction quality is sufficient for electrons to move between layers in this solution-processed crystalline heterostructure.

Electrochromic Bragg Mirror: ECBM

IO N We describe herein the fi rst example of an electrochromic Bragg mirror (ECBM), combining nanoporous multilayers made of NiO and WO 3 nanoparticles. Because NiO and WO 3 are complementary in their coloration effects (e.g. cathodic coloration for WO 3 and anodic coloration for NiO) [ 1 ] and their corresponding change in refractive index, tunability can be achieved by combining these electrochromic components in a 1D Bragg mirror tandem arrangement. The high nanoporosity of this ECBM allows protons and electrons to be quickly shuttled into and out of the multilayers, altering the mix of intervalence charge transfer optical effects within the layers and Bragg diffraction effects between the layers. A proper choice of electrolyte guarantees cycling of the optical properties with negligible degradation. Electrochromic [ 2 , 3 ] devices change their electronic structure and color via electrically-induced storage of ions and electrons in the material, which can be reversed by applying an opposing electrical bias. In comparison, photonic crystals change their color by alterations in the dimension and/or refractive index of the photonic lattice, which can also be changed electrically and reversibly. A prominent example for reversible color changes emanating from alterations of the geometrical structure are voltage-driven, swellable and shrinkable 3D inverse opals built from cross-linked polyferrocenylsilane. [ 3–5 ] This effect is known as the electrophotonic effect. [ 4 , 5 ] Electrochromic materials present themselves as a good alternative, as the coloration usually comes with a change in the refractive index. In the fi eld of electrochromic devices, tungsten trioxide W(VI)O 3 and nickel oxide Ni(II)O are the inorganic material archetypes, and function according to Equations 1–2 . [ 6–8 ]

Green Nanochemistry: Metal Oxide Nanoparticles and Porous Thin Films from Bare Metal Powders

A universal, simple, robust, widely applicable and cost-effective aqueous process is described for a controlled oxidative dissolution process of micrometer-sized metal powders to form high-purity aqueous dispersions of colloidally stable 3-8 nm metal oxide nanoparticles. Their utilization for making single and multilayer optically transparent high-surface-area nanoporous films is demonstrated. This facile synthesis is anticipated to find numerous applications in materials science, engineering, and nanomedicine.

A new class of epitaxial porphyrin metal–organic framework thin films with extremely high photocarrier generation efficiency: promising materials for all-solid-state solar cells

We demonstrate the fabrication of a new class of epitaxial porphyrin metal–organic framework thin films whose photophysical properties can be tuned by the introduction of electron-donating diphenylamine (DPA) groups into the porphyrin skeleton. The attachment of DPA groups results in strongly improved absorption characteristics, yielding the highest photocarrier generation efficiency reported so far. DFT calculations identify a shift of the charge localization pattern in the VBM (lowest unoccupied molecular orbital), confirming that the introduction of the DPA groups is the main reason for the shift of the optical absorption spectrum and the improved photocurrent generation.

Hierarchical nanoparticle bragg mirrors: tandem and gradient architectures.

To manipulate electrons in semiconductor electronic and optical devices, the usual approach is through materials composition, electronic bandgap, doping, and interface engineering. More advanced strategies for handling electrons in semiconductor devices include composition-controlled heterostructures and gradient structures. By analogy to the manipulation of electrons in semiconductor crystals by electronic bandgaps, photons in photonic crystals can be managed using photonic bandgaps. In this context, the simplest photonic crystal is the Bragg mirror, a periodic dielectric construct whose photonic bandgap is engineered through variations of the optical thickness of its constituent layers. Traditionally the materials comprising these periodic dielectric layers are nonporous, and they have mainly been used in the field of optical and photonic devices. More recently these Bragg mirrors have been made porous by building the layers from nanoparticles with functionality and utility that exploit their internal voids. These structures are emerging in the area of photonic color-coded chemical sensing and controlled chemical release. Herein, a strategy for enhancing the functionality and potential utility of nanoparticle Bragg mirrors by making the constituent dielectric layers aperiodic and porous is described. It is exemplified by prototypical tandem and gradient structures that are fully characterized with regards to their structure, porosity, and optical and photonic properties.