Rudolf C. Hoffmann

Virus-templated synthesis of ZnO nanostructures and formation of field-effect transistors.

The search for novel methods for the synthesis of nanostructured materials is an important step towards the miniaturization of multifunctional devices, which requires careful and appropriate integration of various materials into a single unit. However, most of the conventional synthesis methods for multicomponent systems involve harsh reaction conditions and thereby introduce limitations in the choice of materials to be combined. For instance, in ceramic synthesis methods, extreme heating and/or pressure are often used, which may be inapplicable to certain components of a device structure. Further factors critical to the miniaturization are the size of the obtained powder particles and their tendency to agglomerate. Hence, the integration of different materials is still a challenging goal and can hardly be achieved by conventional processing. Biomineralization is a process used by organisms to generate composite materials composed of organic and inorganic phases, which often exhibit exceptional properties. [ 1 ] Organic molecules, such as peptides, proteins, or polysaccharides, guide the crystal growth at ambient conditions that eventually determine the morphology and the functional properties of the materials. [ 2 ] The integration of biomolecules as templates or structure-directing agents, on the other hand, offers the opportunity to explore alternative low-temperature methods in the synthesis of bioinorganic hybrid materials with novel tailored functionalities. [ 3 , 4 ] For some applications, however, the adaptation of bionic mineralization approaches to the synthesis of artifi cial composite materials is not possible, since no interactions between the inorganic phase

Synthesis and sensoric response of ZnO decorated carbon nanotubes

ZnO nanoparticles of size 2–10 nm were generated in situ from the single source precursor [2-(methoxyimino)propanoato]zinc(II), ([CH3ONCCH3COO]2Zn·2H2O) onto multiwalled carbon nanotubes (MWCNTs) at low temperature (150 °C). The degree of ZnO coverage on the MWCNTs can be tuned and is dependent upon the ZnO precursor concentration. A plausible growth mechanism based on surface saturation of as-deposited precursor on the MWCNTs has been proposed. The X-ray diffraction (XRD) pattern and transmission electron microscopy (TEM) indicate the nano-crystalline nature of the ZnO particles. Scanning electron microscopy (SEM) and TEM investigations of the ZnO deposition revealed a dense and homogeneous deposition along the complete periphery of the MWCNT. The ZnO/MWCNT nanocomposite hybrid materials were further electronically characterized by micro-Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), ultraviolet-visible spectroscopy (UV-Vis) as well as room temperature photoluminescence (PL). The nanostructured ZnO/MWCNT composite shows a better sensing performance when compared to bare MWCNTs in the detection of low CO levels (20–200 ppm).

Zinc oxide derived from single source precursor chemistry under chimie douce conditions: formation pathway, defect chemistry and possible applications in thin film printing

A series of zinc complexes with oximate ligands is investigated for their suitability as precursors for zinc oxide in inkjet printing. The variation of hydrogen and alkyl groups in the side chains of the oximate framework (R1–ON–C2O2–R2) of the corresponding zinc complexes influences the decomposition temperature, and also important parameters such as solubility and wettability. Detailed investigations of the degradation mechanism reveal their behavior as excellent single source precursors for ZnO under very mild (chimie douce) conditions. Best results for the formation of zinc oxide thin films are obtained with solutions of [2-(methoxyimino)propanato]zinc in methoxyethanol. By calcincation well adherent (tensile strength of 1.95 (±0.95) MPa) nanocrystalline films of zincite are formed. This technique is applied for inkjet printing of ceramic layers on polyethylene-terephthalate thin films. Results of EPR spectroscopy studies on the ZnO nanoparticles are in accord with a core–shell model in which the grain particles of the core consist of vacancy centers which are electronically different from the surrounding shell of the ZnO nanoparticles.

Nano/micro-patterning of anatase TiO2 thin film from an aqueous solution by site-selective elimination method

Abstract We proposed a novel method to fabricate nano/micro-scaled patterns of thin films and successfully fabricated patterns of anatase TiO2 thin films in an aqueous solution at 50 8C. The patterned self-assembled monolayer (SAM) having octadecyltrichlorosilane (OTS) regions and silanol regions was immersed in a solution containing a Ti precursor and subjected to ultrasonication for several hours. The difference in adhesion of thin films on substrates was employed for the site-selective elimination method. Heterogeneously nucleated TiO2 and homogeneously nucleated TiO2 particles adhering to the OTS–SAM could be easily eliminated from the substrate by ultrasonication, whereas those on silanol groups maintained their adhesion during the immersion period. TiO2 can form chemical bonds such as Ti–O–Si with silanol groups, but cannot form them with octadecyl groups, resulting in the difference in adhesion, which is the essence of the site-selectivity of this method. The site-selective elimination method can be applied to fabricate nano/micro-scaled patterns in the solution by the immersion of the substrate that has regions on which depositions adhere strongly and regions on which depositions adhere weakly, enabling elimination by treatment such as ultrasonication.

A “Clickable” Hybrid Nanocluster of Cubic Symmetry

In biological systems, multivalency is a common strategy to enhance the affinity and specificity of ligand–receptor interactions. Precise clustering of multiple synthetic ligands in the space around suitable scaffolds to maximize their biological effects has attracted considerable attention. Compact scaffolds with the highest symmetry and number of ligation sites, which allows high functional density in a defined homogenous environment to be created, are most appealing. In five platonic solids, convex regular polyhedra, would offer the utmost number of symmetry elements. Especially, the organic molecular equivalents (Td tetrahedrane, Oh cubane, Ih dodecahedrane; Figure 1) [3] would allow a specific ligand to be positioned in a defined spatial vicinity of three neighboring ligands within a convex geometry, but unfortunately none of these structures is accessible in a functional form for ligand coupling. However, cube-octameric silsesquioxanes (COSS), inorganic–organic hybrid nanoclusters (RSiO1.5)8 with a silicalike core decorated with an organic group at each silicon apex (Figure 1), share a unique eight-fold symmetry and rigid globular architecture with cubane. Cube-shaped COSS clusters can be produced with remarkable ease to carry various aliphatic, alkene, or aryl substituents that project into each octant in Cartesian space from a well-defined nanosized core, offering a formidable opportunity to display eight ligands in a spherical environment that can be finetuned by varying the organic spacer arms. Thus, silsesquioxane derivatives have recently attracted a broad range of technological applications, including that of nanostructured polymers and composites, to which the inorganic core is expected to contribute special properties. Being regarded as biocompatible, COSS have recently been conjugated with peptide and glycoside moieties for exploratory biological studies. A demanding challenge for the application of high-density COSS-type scaffolds toward the display of complex ligands, however, is the requirement of complete and uniform conjugation at each apex to avoid the persistence of under-functionalized or regioisomeric reaction intermediates that are difficult to remove during product purification. This necessity limits the allowable coupling methods to very high yielding reactions only. In fact, biorelevant conjugates have been prepared from octa(aminopropyl) COSS by standard amide coupling with only low yields of 20–61 % or by photoinduced radical addition of thiols to octavinyl COSS in 66–73 % yield. Clearly, more effective and selective ligation processes (“click chemistry”) are desirable for flexible ligand attachments to the COSS core. Especially, the Huisgen-type 1,3-dipolar cycloaddition of organic azides and alkynes has gained prominent popularity recently in both its thermal and Cu-catalyzed versions, because it proved to be a highly [a] D. Heyl, Prof. Dr. W.-D. Fessner Clemens-Schçpf-Institut f r Organische Chemie und Biochemie Technische Universit t Darmstadt Petersenstr. 22, 64287 Darmstadt (Germany) Fax: (+49) 6151-166636 E-mail : fessner@tu-darmstadt.de [b] Dr. E. Rikowski, Dr. R. C. Hoffmann, Prof. Dr. J. J. Schneider Eduard-Zintl-Institut f r Anorganische Chemie und Physikalische Chemie Technische Universit t Darmstadt Petersenstr. 19, 64287 Darmstadt (Germany) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201000488. Figure 1. Top: (CH)4, (CH)8, and (CH)20 as organic chemical realization of Platonic solids. Bottom: The structure of R8Si8O12 cube-octameric silsesquioxanes (COSS) and cubic symmetry elements.

Binary Au/MWCNT and Ternary Au/ZnO/MWCNT Nanocomposites: Synthesis, Characterisation and Catalytic Performance

Gold nanoparticles of 10-24 and 5-8 nm in size were obtained by chemical citrate reduction and UV photoreduction, respectively, on acid-treated multiwalled carbon nanotubes (MWCNTs) and on ZnO/MWCNT composites. The shape and size of the deposited Au nanoparticles were found to be dependent upon the synthetic method used. Single-crystalline, hexagonal gold particles were produced in the case of UV photoreduction on ZnO/MWCNT, whereas spherical Au particles were deposited on MWCNT when the chemical citrate reduction method was used. In the UV photoreduction route, n-doped ZnO serves as the e(-) donor, whereas the solvent is the hole trap. All materials were fully characterised by UV/Vis spectroscopy, scanning electron microscopy, transmission electron microscopy, X-ray photoelectron spectroscopy, Raman spectroscopy and BET surface analysis. The catalytic activity of the composites was studied for the selective hydrogenation of alpha,beta-unsaturated carbonyl compound 3,7-dimethyl-2,6-octadienal (citral). The Au/ZnO/MWCNT composite favours the formation of unsaturated alcohols (selectivity=50% at a citral conversion of 20%) due to the presence of single-crystalline, hexagonal gold particles, whereas saturated aldehyde formation is favoured in the case of the Au/MWCNT nanocomposite that contains spherical gold particles.

Molecular based, chimie douce approach to 0D and 1D indium oxide nanostructures. Evaluation of their sensing properties towards CO and H2

The synthesis of a new molecular In(III) precursor complex, the generation of nanoscaled In2O3 (bixybyte structure) in 0D and 1D morphology, and the sensoric behavior of the 0D and 1D In2O3 nanostructures towards reducing gas atmospheres is studied. The indium precursor complex can be converted to a ceramic green body under mild reaction temperatures (160 °C), followed by conversion into crystalline indiumoxide at higher temperatures (350 °C). Geometric confinement by endotemplating of the molecular In precursor in track-etched polycarbonate templates yields polycrystalline indium oxide nanotubes in high yields. Controlled conversion of the molecular precursor without geometric confinement gives nanoparticulate crystalline In2O3. Both morphologically different In2O3 nanomaterials are sensitive to reducing gas conditions, however they show distinct differences towards reducing H2 and CO gas atmospheres.

Deposition of ceramic materials from aqueous solution induced by organic templates

Besides conventional high-temperature sintering processes for the preparation of ceramics, synthesis routes like chemical and physical vapor phase deposition or molecular beam epitaxy as well as sol-gel and polymer thermolysis carried out at decreased temperatures gain in significance. Such a trend is especially pronounced in the case of the deposition of oxide-based solids from aqueous solutions induced by organic templates. This approach is inspired by biomineralization that in general involves the formation of well-structured and complex-shaped organic/inorganic composites by the deposition of an inorganic solid on an organic matrix that consists of biomolecules like proteins. It occurs at ambient conditions with respect to temperature, pressure and atmosphere. The imitation of these processes by technical means is the deposition of thin oxide films from aqueous solutions in the presence of organic self-assembled monolayers. Besides general features of this technique like low synthesis temperatures, processing costs and equipment expenditure as well as the line-of-sight deposition suitable for coating complex shaped and/or temperature-sensitive substrates the characteristics of the films and their mechanisms of formation are discussed by way of the oxide systems TiO 2 , ZrO 2 and ZnO.

The Coordination Chemistry of cis-3, 5-Diaminopiperidine and Substituted Derivatives

An efficient and convenient method for the preparation of cis-3,5-diaminopiperidine (dapi) has been established and the coordination chemistry of this ligand with CoII, CoIII, NiII, CuII, ZnII, and CdII has been investigated in the solid state and in aqueous solution. Potentiometric measurements revealed a generally high stability for the bis complexes of the divalent cations with maximum stability for NiII (log beta2 = 21.2, beta2 = [M(dapi)2][M](-1)[dapi](-2), 25 degrees C, mu = 0.1 mol dm(-3)). Cyclic voltammetry established quasi-reversible formation of [Ni(dapi)2]3+ with a redox potential of 0.91 V (versus NHE) for the Ni(II/III) couple. [Co(dapi)2]3+ was prepared by aerial oxidation of the corresponding CoII precursor. The two isomers trans-[Co(dapi)2]3+ (1(3+), 26%) and cis-[Co(dapi)2]3+ (2(3+), 74%), have been separated and isolated as solid Cl- and CF3SO3- salts. In a non-aqueous medium 1(3+) and 2(3+) reacted with paraformaldehyde and NEt3 to give the methylidene-imino derivatives 3(3+) and 4(3+), in which the two piperidine rings are bridged by two or one N-CH2-O-CH2-N bridges, respectively. Crystal structure analyses were performed for H3dapi[ZnCl4]Cl, 1Cl3 x 2H2O, 2Cl3 x H2O, 3[ZnCl4]Cl, 4[ZnCl4]Cl, [Ni(dapi)2]Cl2 x H2O, [Cu(dapi)2](NO3)2, [Cu(dapi)Cl2], [(dapi)ClCd-(mu2-Cl)2-CdCl(dapi)], and [Co(dapi)(NO2)(CO3)]. The stability of [M(II)(dapi)]2+ and [M(II)(dapi)2]2+ complexes in aqueous solution, particularly the remarkably high tendency of [M(dapi)]2+ to undergo coordinative disproportionation is discussed in terms of the specific steric requirements of this ligand. Molecular mechanics calculations have been performed to analyze the different types of strain in these complexes. A variety of alkylated derivatives of dapi have been prepared by reductive alkylation with formaldehyde, benzaldehyde, salicylaldehyde, and pyridine-2-carbaldehyde. The NiII complexes of the pentadentate N3,N5-bis(2-pyridinylmethyl)-cis-3,5-diaminopiperidine (py2dapi) and the hexadentate N3,N5,1-tris(2-pyridinylmethyl)-cis-3,5-diaminopiperidine (py3dapi) have been isolated as crystalline ClO4- salts [Ni(py2dapi)Cl]ClO4 and [Ni(py3dapi)](ClO4)2 x H2O and characterized by crystal structure analyses.

Growth of thin ZnO films from aqueous solutions in the presence of PMAA-graft-PEO copolymers

The deposition of homogeneous and adherent ZnO films on Si wafers by thermohydrolysis of zinc salts in aqueous solution is reported. Using a graft copolymer (polymethacrylic acid partially grafted with polyethyleneoxide side chains) homogeneous films of nanosized particles on Si wafers with self-assembled monolayers (SAMs) were obtained. The addition of the polyelectrolyte is necessary to suppress the formation of undesired, larger zincite crystals, which form inevitably otherwise. Films grown at different growth rates were characterized by atomic force microscopy (AFM) and scanning electron microscopy (SEM). Films that were obtained at high growth rates exhibit canyon-like morphology, whereas low growth rates yield uniform films with low surface roughness. According to X-ray Photoelectron Spectroscopy (XPS) a part of the polymer remains attached to the precipitated particles and can be removed by pyrolysis at about 723 K.