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Paderborn University in February 2023 Show image information

Paderborn University in February 2023

Photo: Paderborn University, Hannah Brauckhoff

Prof. Dr. Michael Tiemann

Prof. Dr. Michael Tiemann

Inorganic Chemistry - Research Group Tiemann


Chinesisch-Deutsche Technische Fakultät (CDTF)

Coordinator - Professor - Coordinator of the bachelor's "chemistry" program

+49 5251 60-2154
+49 5251 60-3423
Warburger Str. 100
33098 Paderborn
Prof. Dr. Michael Tiemann
01.10.2009 - today

Paderborn University

Michael Tiemann was appointed to the University of Paderborn in 2009 as W2 professor of Inorganic Chemistry. Since 2014 he is a chair holder (W3 professor). He declined a call for a chair at the Clausthal University of Technology (2014). Since 2011, he has also been a visiting professor at the Sino-German Technical Faculty (CDTF) of the Qingdao University of Science and Technology (QUST) in Qingdao (China); on the Paderborn side he coordinates the joint Bachelor's program "Chemistry" with the CDTF.

01.10.2002 - 30.09.2009

University of Giessen

From 2002 to 2009 Michael Tiemann was group leader at the Institute for Inorganic and Analytical Chemistry of the Justus Liebig University Giessen. In 2008, he received his habilitation and took over the Interim Chair of Inorganic Solid State Chemistry.

01.10.2001 - 30.09.2002

Abo Akademi

From 2001 to 2002, Michael Tiemann worked as a postdoctoral research assistant at the Institute of Physical Chemistry of the Åbo Akademi in Turku (Finland).

01.10.1997 - 30.09.2001

University of Hamburg

After studying chemistry (diploma), Michael Tiemann was a scientific coworker at the Institute of Inorganic and Applied Chemistry at the University of Hamburg from 1997 to 2001. He finished his PhD (Dr. rer. nat.) in 2001 at the chair of Prof. Armin Reller in the group of Dr. Michael Fröba (summa cum laude).

Open list in Research Information System


Broadband Mie scattering effects by structural features of setae from the Saharan silver ant Cataglyphis bombycina

B. Schwind, X. Wu, M. Tiemann, H. Fabritius, Journal of the Optical Society of America B (2023)

The Saharan desert ant Cataglyphis bombycina is densely covered with shiny silver setae (hair-like structures). Their appearance was explained by geometric optics and total internal reflection. The setae also increase the emissivity of the ant, as they form an effective medium. This work provides additional data on microstructural details of the setae that are used to simulate the scattering of an individual seta to explain their influence on the optical properties. This is achieved by characterization of their structure using light microscopy and scanning/transmission electron microscopy. How the microstructural features influence scattering is investigated wave-optically within the limits of finite-difference time-domain simulations from the ultraviolet to the mid-infrared spectral range to elucidate the optical effects beyond ray optics and effective medium theory. The results show that Mie scattering plays an important role in protecting the ant from solar radiation and could be relevant for its thermal tolerance.


Stimulation and Enhancement of Near‐Band‐Edge Emission in Zinc Oxide by Distributed Bragg Reflectors

L. Kothe, M. Albert, C. Meier, T. Wagner, M. Tiemann, Advanced Materials Interfaces (2022), 2102357

The free exciton transition (near-band-edge emission, NBE) of ZnO at ≈388 nm can be strongly enhanced and even stimulated by an underlying photonic structure. 1D Photonic crystals, so-called distributed Bragg reflectors, are utilized to suppress the deep-level emission of ZnO (DLE, ≈500–530 nm). The reflector stacks are fabricated in a layer-by-layer procedure by wet-chemical synthesis. They consist of low-ε porous SiO2 layers and high-ε TiO2 layers. Varying the thickness of the SiO2 layers allows tuning the optical bandgap in a wide range between ≈420 and 800 nm. A ZnO layer is deposited on top of the reflector stacks by sol–gel synthesis. The spontaneous photoluminescence (PL) emission of the ZnO film is modulated by the photonic structure. When the optical bandgap of the reflector is in resonance with the deep-level emission of ZnO (DLE, ≈500–530 nm), then this defect-related emission mode is suppressed. Strong NBE emission is observed even when the ZnO layer does not show any NBE emission (due to low crystallinity) in the absence of the photonic structure. With this cost-efficient synthesis method, emitters for, e.g., luminescent gas sensors can be fabricated.

The role of sulfonate groups and hydrogen bonding in the proton conductivity of two coordination networks

A. Javed, F. Steinke, S. Wöhlbrandt, H. Bunzen, N. Stock, M. Tiemann, Beilstein Journal of Nanotechnology (2022), 13, pp. 437-443

<jats:p>The proton conductivity of two coordination networks, [Mg(H<jats:sub>2</jats:sub>O)<jats:sub>2</jats:sub>(H<jats:sub>3</jats:sub>L)]·H<jats:sub>2</jats:sub>O and [Pb<jats:sub>2</jats:sub>(HL)]·H<jats:sub>2</jats:sub>O (H<jats:sub>5</jats:sub>L = (H<jats:sub>2</jats:sub>O<jats:sub>3</jats:sub>PCH<jats:sub>2</jats:sub>)<jats:sub>2</jats:sub>-NCH<jats:sub>2</jats:sub>-C<jats:sub>6</jats:sub>H<jats:sub>4</jats:sub>-SO<jats:sub>3</jats:sub>H), is investigated by AC impedance spectroscopy. Both materials contain the same phosphonato-sulfonate linker molecule, but have clearly different crystal structures, which has a strong effect on proton conductivity. In the Mg-based coordination network, dangling sulfonate groups are part of an extended hydrogen bonding network, facilitating a “proton hopping” with low activation energy; the material shows a moderate proton conductivity. In the Pb-based metal-organic framework, in contrast, no extended hydrogen bonding occurs, as the sulfonate groups coordinate to Pb<jats:sup>2+</jats:sup>, without forming hydrogen bonds; the proton conductivity is much lower in this material.</jats:p>

Challenges in the interpretation of gas core levels for the determination of gas-solid interactions within dielectric porous films by ambient pressure XPS

T. de los Arcos, C. Weinberger, F. Zysk, V. Raj Damerla, S. Kollmann, P. Vieth, M. Tiemann, T. Kühne, G. Grundmeier, Applied Surface Science (2022), 604, 154525

Near ambient pressure XPS in nitrogen atmosphere was utilized to investigate gas-solid interactions within porous SiO2 films ranging from 30 to 75 nm thickness. The films were differentiated in terms of porosity and roughness. The XPS N1s core levels of the N2 gas in presence of the SiO2 samples showed variations in width, binding energy and line shape. The width correlated with the surface charge induced in the dielectric films upon X-ray irradiation. The observed different binding energies observed for the N1s peak can only partly be associated with intrinsic work function differences between the samples, opening the possibility that the effect of physisorption at room temperature could be detected by a shift in the measured binding energy. However, the signals also show an increasing asymmetry with rising surface charge. This might be associated with the formation of vertical electrical gradients within the dielectric porous thin films, which complicates the assignment of binding energy positions to specific surface-related effects. With the support of Monte Carlo and first principles density functional theory calculations, the observed shifts were discussed in terms of the possible formation of transitory dipoles upon N2 physisorption within the porous SiO2 films.

The Structure of Water in Silica Mesopores – Influence of the Pore Wall Polarity

C. Weinberger, F. Zysk, M. Hartmann, N. Kaliannan, W. Keil, T. Kühne, M. Tiemann, Advanced Materials Interfaces (2022), 9(20), 2200245

In the spatial confinement of cylindrical mesopores with diameters of a few nanometers, water molecules experience restrictions in hydrogen bonding. This leads to a different behavior regarding the molecular orientational freedom (‘structure of water') compared to the bulk liquid state. In addition to the pore size, the behavior is also strongly affected by the strength of the pore wall-to-water interactions, that is, the pore wall polarity. In this work, this is studied both experimentally and theoretically. The surface polarity of mesoporous silica (SiO2) is modified by functionalization with trimethylsilyl moieties, resulting in a change from a hydrophilic (pristine) to a hydrophobic pore wall. The mesopore surface is characterized by N2 and H2O sorption experiments. Those results are combined with IR spectroscopy to investigate pore wall-to-water interactions leading to different structures of water in the mesopore. Furthermore, the water's structure is studied theoretically to gain deeper insight into the interfacial interactions. For this purpose, the structure of water is analyzed by pairing densities, coordination, and angular distributions with a novel adaptation of surface-specific sum-frequency generation calculation for pore environments.

Pyrolysis of sucrose-derived hydrochar

M. Wortmann, W. Keil, B. Brockhagen, J. Biedinger, M. Westphal, C. Weinberger, E. Diestelhorst, W. Hachmann, Y. Zhao, M. Tiemann, G. Reiss, B. Hüsgen, C. Schmidt, K. Sattler, N. Frese, Journal of Analytical and Applied Pyrolysis (2022), 161, 105404

The electrochemical properties of carbonaceous materials produced by hydrothermal carbonization, referred to as hydrochar, can be substantially improved by post-carbonization via pyrolysis. Although these materials have been widely studied for a variety of applications, the mechanisms underlying the pyrolysis are yet poorly understood. This study provides a comprehensive temperature-resolved characterization of the chemical composition, morphology and crystallinity of sucrose-derived hydrochar during pyrolysis. Thermogravimetric analysis, differential scanning calorimetry, and elemental analysis have shown that the dry hydrochar loses about 41% of its dry mass due to the exothermic disintegration of oxygen-containing groups until the carbonization is completed at about 850 °C with a total carbon yield of 93%. The carbonization and aromatization of the initially furanic and keto-aliphatic structure were analyzed by 13C solid-state nuclear magnetic resonance spectroscopy, X-ray photoelectron spectroscopy, and Fourier transform infrared spectroscopy. The transition from an amorphous to a nanocrystalline graphitic structure was analyzed using X-ray diffraction and Raman spectroscopy. The pore formation mechanism was examined by helium ion microscopy, transmission electron microscopy, and nitrogen adsorption measurements. The results indicate the formation of oxygen-rich nanoclusters up to 700 °C, which decompose up to 750 °C leaving behind equally sized pores, resulting in a surface area of up to 480 m2/g.

Porous SiO2 coated dielectric metasurface with consistent performance independent of environmental conditions

R. Geromel, C. Weinberger, K. Brormann, M. Tiemann, T. Zentgraf, Optical Materials Express (2022), 12(1), pp. 13-21

With the rapid advances of functional dielectric metasurfaces and their integration on on-chip nanophotonic devices, the necessity of metasurfaces working in different environments, especially in biological applications, arose. However, the metasurfaces’ performance is tied to the unit cell’s efficiency and ultimately the surrounding environment it was designed for, thus reducing its applicability if exposed to altering refractive index media. Here, we report a method to increase a metasurface’s versatility by covering the high-index metasurface with a low index porous SiO2 film, protecting the metasurface from environmental changes while keeping the working efficiency unchanged. We show, that a covered metasurface retains its functionality even when exposed to fluidic environments.


Modeling of gyroidal mesoporous CMK-8 and CMK-9 carbon nanostructures and their X-Ray diffraction patterns

B. Schwind, J. Smått, M. Tiemann, C. Weinberger, Microporous and Mesoporous Materials (2021), 110330

Powder X-ray diffraction (XRD) patterns of ordered mesoporous CMK-8 and CMK-9 carbon materials are simulated by geometric modeling. The materials are amorphous at the atomic length scale but exhibit highly symmetric gyroidal structures at the nanometer scale, corresponding to regular, continuous nanopore systems with cubic symmetry. Their structures lead to characteristic low-angle XRD signatures. We introduce a model based on geometrical considerations to simulate CMK-8 and CMK-9 structures with variable volume fraction of carbon (vs. pore volume, i.e., variable 'pore wall thickness'). In addition, we also simulate carbon materials with variable amounts of guest species (e.g., sulfur) residing in their pores. The corresponding XRD patterns are calculated. The carbon volume fraction turns out to have a significant impact on the relative diffraction peak intensities, especially in case of CMK-9 carbon that features a bimodal porosity. Likewise, the presence of guest species in the pores may also strongly affect the relative peak intensities. Our study suggests that careful evaluation of experimental low-angle XRD patterns of (real) CMK-8 or CMK-9 materials offers an opportunity to obtain detailed information about the nanostructural properties in addition to the mere identification of the pore systems geometry.

Review of infrared spectroscopy techniques for the determination of internal structure in thin SiO2 films

T. de los Arcos, H. Müller, F. Wang, V.R. Damerla, C. Hoppe, C. Weinberger, M. Tiemann, G. Grundmeier, Vibrational Spectroscopy (2021), 103256

A comparison of infrared spectroscopic analytical approaches was made in order to assess their applicability for internal structure characterization of SiO2 thin films. Markers for porosity and/or disorder based on the analysis of the asymmetric stretching absorption band of SiO2 between 900−1350 cm−1 were discussed. The shape of this band, which shows a well-defined LO–TO splitting, depends not only on the inherent characteristics of the film under analysis but also on the particular geometry of the IR experiment and the specific surface selection rules of the substrate. Three types of SiO2 thin films with clearly defined porosity ranging from dense films to mesoporous films were investigated by transmission (at different incidence angles), direct specular reflection (at different angles), and diffuse reflection. Two different types of substrate, metallic and semiconducting, were used. The combined effect of substrate and specific technique in the final shape of the band, was discussed, and the efficacy for their applicability to the determination of porosity in thin SiO2 films was critically evaluated.

Selective Modification of Hierarchical Pores and Surfaces in Nanoporous Materials

M. Tiemann, C. Weinberger, Advanced Materials Interfaces (2021), 2001153

Tailor-made ordered mesoporous materials bear great potential in numerous fields of application where large interfaces are required. However, the inherent surfacechemical properties of conventional materials, such as silica, carbon or organosilica, poses some limitations with respect to their application. Surface manipulation by functionalization with chemically more reactive groups is one way to improve materials for their desired purpose. Another approach is the design of high surface-area composite materials. The surface manipulation, either by functionalization or by introducing guest species, can be performed selectively. This means that when several distinct, i.e. , hierarchical, types of surfaces or pore systems exist in a material, each of them may be chosen for manipulation. Several strategies can be identified to achieve this goal. Molecules or molecule assemblies can be utilized to temporarily protect pores or surfaces (soft protection), while manipulation occurs at the accessible sites. This approach is a recurring motive in this review and can also be applied to rigid template matrices (hard protection). Furthermore, the size of functionalization agents (size protection) and their reactivity/diffusion (kinetic protection) into the pores can also be utilized to achieve selectivity. In addition, challenges in the synthesis and characterization of selectively manipulated ordered mesoporous materials are discussed.

Examination of the evolution of iron oxide nanoparticles in flame spray pyrolysis by tailored in situ particle sampling techniques

R. Tischendorf, M. Simmler, C. Weinberger, M. Bieber, M. Reddemann, F. Fröde, J. Lindner, H. Pitsch, R. Kneer, M. Tiemann, H. Nirschl, H. Schmid, Journal of Aerosol Science (2021), 105722

In this report, a flame spray pyrolysis setup has been examined with various in situ extraction methods of particle samples along the flame axis. First, two precursor formulations leading to the formation of iron oxide nanoparticles were used in a standardized SpraySyn burner system, and the final particle outcome was characterized by a broad range of established powder characterization techniques (TEM/HRTEM, SAXS, XRD, BET). The characterization of the powder products evidenced that mostly homogeneous gas-to-particle conversion takes place when applying an acidic precursor solution, whereas the absence of the acid leads to a dominant droplet-to-particle pathway. Our study indicates that a droplet-to-particle-pathway could be present even when processing the acidic formulation. However, even if a secondary pathway might take place in this case as well, it is not dominant and nearly negligible. Subsequently, the in situ particle structure evolution was investigated for the dominant gas-to-particle pathway, and particles were extracted along the flame axis for online SMPS and offline TEM/HRTEM analysis. Due to the highly reactive conditions within the flame (high temperatures, turbulent flow field, high particle number concentrations), the extraction of representative samples from spray flames is challenging. In order to handle the reactive conditions, two extraction techniques were tailored in this report. To extract an aerosol sample within the flame for SMPS measurement, a Hole in a Tube probe was adjusted. Thus, the mobility particle diameter as well as the corresponding distribution widths were obtained at different heights above the burner along the flame axis. For TEM/HRTEM image analysis, particle samples were collected thermophoretically by means of a tailored shutter system. Since all sampling grids were protected until reaching the flame axis and due to the low sampling time, momentary captures of local particle structures could be extracted precisely. The particle morphologies have clearly shown an evolution from spherical and paired particles in the flame center to fractal and compact agglomerates at later synthesis stages.

TiO2 nanoparticle coatings on glass surfaces for the selective trapping of leukemia cells from peripheral blood

J.A. Garcia Diosa, A. Gonzalez Orive, C. Weinberger, S. Schwiderek, S. Knust, M. Tiemann, G. Grundmeier, A. Keller, R.J. Camargo Amado, Journal of Biomedical Materials Research Part B: Applied Biomaterials (2021), 109, pp. 2142–2153

Photodynamic therapy (PDT) using TiO2 nanoparticles has become an important alternative treatment for different types of cancer due to their high photocatalytic activity and high absorption of UV-A light. To potentiate this treatment, we have coated commercial glass plates with TiO2 nanoparticles prepared by the sol–gel method (TiO2-m), which exhibit a remarkable selectivity for the irreversible trapping of cancer cells. The physicochemical properties of the deposited TiO2-m nanoparticle coatings have been characterized by a number of complementary surface-analytical techniques and their interaction with leukemia and healthy blood cells were investigated. Scanning electron and atomic force microscopy verify the formation of a compact layer of TiO2-m nanoparticles. The particles are predominantly in the anatase phase and have hydroxyl-terminated surfaces as revealed by Raman, X-ray photoelectron, and infrared spectroscopy, as well as X-ray diffraction. We find that lymphoblastic leukemia cells adhere to the TiO2-m coating and undergo amoeboid-like migration, whereas lymphocytic cells show distinctly weaker interactions with the coating. This evidences the potential of this nanomaterial coating to selectively trap cancer cells and renders it a promising candidate for the development of future prototypes of PDT devices for the treatment of leukemia and other types of cancers with non-adherent cells.

New isoreticular phosphonate MOFs based on a tetratopic linker

F. Steinke, A. Javed, S. Wöhlbrandt, M. Tiemann, N. Stock, Dalton Transactions (2021), pp. 13572-13579

The tetratopic linker 1,1,2,2-tetrakis(4-phosphonophenyl)ethylene (H8TPPE) was used to synthesize the three new porous metal–organic frameworks of composition [M2(H2O)2(H2TPPE)]·xH2O (M = Al3+, Ga3+, Fe3+), denoted as M-CAU-53 under hydrothermal reaction conditions, using the corresponding metal nitrates as starting materials. The crystal structures of the compounds were determined ab initio from powder X-ray diffraction data, revealing small structural differences. Proton conductivity measurements were carried out, indicating different conductivity mechanisms. The differences in proton conductivity could be linked to the individual structures. In addition, a thorough characterization via thermogravimetry, elemental analysis, IR-spectroscopy as well as N2- and H2O-sorption is given.


Cellulose Nanocrystal-Templated Tin Dioxide Thin Films for Gas Sensing

A. Ivanova, B. Frka-Petesic, A. Paul, T. Wagner, A.N. Jumabekov, Y. Vilk, J. Weber, J. Schmedt auf der Günne, S. Vignolini, M. Tiemann, D. Fattakhova-Rohlfing, T. Bein, ACS Applied Materials & Interfaces (2020), pp. 12639-12647

Porous tin dioxide is an important low-cost semiconductor applied in electronics, gas sensors, and biosensors. Here, we present a versatile template-assisted synthesis of nanostructured tin dioxide thin films using cellulose nanocrystals (CNCs). We demonstrate that the structural features of CNC-templated tin dioxide films strongly depend on the precursor composition. The precursor properties were studied by using low-temperature nuclear magnetic resonance spectroscopy of tin tetrachloride in solution. We demonstrate that it is possible to optimize the precursor conditions to obtain homogeneous precursor mixtures and therefore highly porous thin films with pore dimensions in the range of 10–20 nm (ABET = 46–64 m2 g–1, measured on powder). Finally, by exploiting the high surface area of the material, we developed a resistive gas sensor based on CNC-templated tin dioxide. The sensor shows high sensitivity to carbon monoxide (CO) in ppm concentrations and low cross-sensitivity to humidity. Most importantly, the sensing kinetics are remarkably fast; both the response to the analyte gas and the signal decay after gas exposure occur within a few seconds, faster than in standard SnO2-based CO sensors. This is attributed to the high gas accessibility of the very thin porous film.

Nanoporous aluminum oxide micropatterns prepared by hydrogel templating

Z. Chen, D. Kuckling, M. Tiemann, Nanotechnology (2020), 31, 445601

Micropatterned nanoporous aluminum oxide arrays are prepared on silicon wafer substrates by using photopolymerized poly(dimethylacrylamide) hydrogels as porogenic matrices. Hydrogel micropatterns are fabricated by spreading the prepolymer mixture on the substrate, followed by UV photopolymerization through a micropatterned mask. The hydrogel is covalently bonded to the substrate surface. Al2O3 is produced by swelling the hydrogel in a saturated aluminum nitrate solution and subsequent thermal conversion/calcination. As a result, micropatterned porous Al2O3 microdots with heights in µm range and large specific surface areas up to 274 m2 g−1 are obtained. Hence, the hydrogel fulfills a dual templating function, namely micropatterning and nanoporosity generation. The impact of varying the photopolymerization time on the properties of the products is studied. Samples are characterized by light and confocal laser scanning microscopy, scanning electron microscopy, energy-dispersive x-ray spectrometry, and Kr physisorption analysis.

Synthesis of Metal Oxide Inverse Opals from Metal Nitrates by PMMA Colloidal Crystal Templating

X. Zhang, C. Weinberger, S. Amrehn, X. Wu, M. Tiemann, T. Wagner, European Journal of Inorganic Chemistry (2020), pp. 3402-3407

Metal oxide inverse opals are interesting for various applications. To achieve highly ordered inverse opal structures, one important issue during the colloidal crystal templating procedure is to form a stable precursor network before the template loses its structural integrity at high temperature. Using poly(methyl methacrylate), PMMA, colloidal crystal templates, it is essential to consider the physical and chemical changes of the precursors induced by the changes of PMMA during the thermal conversion. For a systematic investigation of this matter, we synthesized a variety of metal oxide inverse opals from the respective metal nitrates, including Cr2O3, Ga2O3, Fe2O3, In2O3, CuO, CeO2, and ZnO, to compare the effect of various modifications of precursors on the structural and optical properties. When the nitrate precursors have a lower thermal stability than the PMMA template, we have modified the metal nitrates by chelating or by polyacrylamide gelation to form more stable precursor networks.

Proton Conduction in a Single Crystal of a Phosphonato‐Sulfonate‐Based Coordination Polymer: Mechanistic Insight

A. Javed, T. Wagner, S. Wöhlbrandt, N. Stock, M. Tiemann, ChemPhysChem (2020), pp. 605-609

The proton conduction properties of a phosphonato-sulfonate-based coordination polymer are studied by impedance spectroscopy using a single crystal specimen. Two distinct conduction mechanisms are identified. Water-mediated conductance along the crystal surface occurs by mass transport, as evidenced by a high activation energy (0.54 eV). In addition, intrinsic conduction by proton ′hopping′ through the interior of the crystal with a low activation energy (0.31 eV) is observed. This latter conduction is anisotropic with respect to the crystal structure and seems to occur through a channel along the c axis of the orthorhombic crystal. Proton conduction is assumed to be mediated by sulfonate groups and non-coordinating water molecules that are part of the crystal structure.

Functional Nanoporous Materials

C. Weinberger, M. Tiemann, Nanomaterials (2020), 699

This Special Issue on “Functional Nanoporous Materials” in the MDPI journal nanomaterials features seven original papers ...

Long- and Short-Term Tensile Strength and Morphology of Joined Beta-Nucleated Polypropylene Parts

A. Wübbeke, V. Schöppner, A. Paul, M. Tiemann, L. Austermeier, M. Fitze, M. Chen, F. Jakob, H. Heim, T. Wu, T. Niendorf, M. Röhricht, M. Schmidt, in: SPE ANTEC 2020: The Virtual Edition 5 , 2020

This paper presents the results of static short-term and long-term tensile tests for beta-nucleated joined polypropylene samples by the hot plate welding process. In the present study different dimensionless joining displacements are accounted for. The results show that high short-term tensile strength does not directly transfer to high long-term tensile strength. The morphology of the weld seam in the joined samples is examined by means of transmitted and reflected light microscopy. For the dimensionless joining displacements of 0.75 and 0.95, stretched spherulites are obtained. X-Ray diffraction can be used as a tool for qualitative and quantitative analysis and eventually for differentiation of samples of various joining displacements.

Chemical and Morphological Transition of Poly(acrylonitrile)/Poly(vinylidene Fluoride) Blend Nanofibers during Oxidative Stabilization and Incipient Carbonization

M. Wortmann, N. Frese, A. Mamun, M. Trabelsi, W. Keil, B. Büker, A. Javed, M. Tiemann, E. Moritzer, A. Ehrmann, A. Hütten, C. Schmidt, A. Gölzhäuser, B. Hüsgen, L. Sabantina, Nanomaterials (2020), 1210

Thermally stabilized and subsequently carbonized nanofibers are a promising material for many technical applications in fields such as tissue engineering or energy storage. They can be obtained from a variety of different polymer precursors via electrospinning. While some methods have been tested for post-carbonization doping of nanofibers with the desired ingredients, very little is known about carbonization of blend nanofibers from two or more polymeric precursors. In this paper, we report on the preparation, thermal treatment and resulting properties of poly(acrylonitrile) (PAN)/poly(vinylidene fluoride) (PVDF) blend nanofibers produced by wire-based electrospinning of binary polymer solutions. Using a wide variety of spectroscopic, microscopic and thermal characterization methods, the chemical and morphological transition during oxidative stabilization (280 °C) and incipient carbonization (500 °C) was thoroughly investigated. Both PAN and PVDF precursor polymers were detected and analyzed qualitatively and quantitatively during all stages of thermal treatment. Compared to pure PAN nanofibers, the blend nanofibers showed increased fiber diameters, strong reduction of undesired morphological changes during oxidative stabilization and increased conductivity after carbonization.

Nano-architectural complexity of zinc oxide nanowall hollow microspheres and their structural properties

K. Engelkemeier, J. Lindner, J. Bürger, K. Vaupel, M. Hartmann, M. Tiemann, K. Hoyer, M. Schaper, Nanotechnology (2020), 31, pp. 095701

Zinc oxide (ZnO) hollow spheres with defined morphology and micro-/nanostructure are prepared by a hydrothermal synthesis approach. The materials possess fine-leaved structures at their particle surface (nanowall hollow micro spheres). Morphology control is achieved by citric acid used as an additive in variable relative quantities during the synthesis. The structure formation is studied by various time-dependent ex situ methods, such as scanning electron microscopy, x-ray diffraction, and Raman spectroscopy. The fine-leaved surface structure is characterized by high-resolution transmission electron microscopy techniques (HRTEM, STEM), using a high-angle annular dark field detector, as well as by differential phase contrast analysis. In-depth structural characterization of the nanowalls by drop-by-drop ex situ FE-SEM analysis provides insight into possible structure formation mechanisms. Further investigation addresses the thermal stability of the particle morphology and the enhancement of the surface-to-volume ratio by heat treatment (examined by N2 physisorption).

Humidity-Mediated Anisotropic Proton Conductivity through the 1D Channels of Co-MOF-74

A. Javed, I. Strauss, H. Bunzen, J. Caro, M. Tiemann, Nanomaterials (2020), 1263

Large Co-MOF-74 crystals of a few hundred micrometers were prepared by solvothermal synthesis, and their structure and morphology were characterized by scanning electron microscopy (SEM), IR, and Raman spectroscopy. The hydrothermal stability of the material up to 60 °C at 93% relative humidity was verified by temperature-dependent XRD. Proton conductivity was studied by impedance spectroscopy, using a single crystal. By varying the relative humidity (70–95%), temperature (21–60 °C), and orientation of the crystal relative to the electrical potential, it was found that proton conduction occurs predominantly through the linear, unidirectional (1D) micropore channels of Co-MOF-74, and that water molecules inside the channels are responsible for the proton mobility by a Grotthuss-type mechanism.

Selected Aspects for the Assessment of Laser Transmission Welding

V. Schöppner, A. Wübbeke, F.. Schriegel, A.. Paul, M. Tiemann, B.. Geißler, M.. Schmidt, A.. Magnier, T.. Niendorf, Joining Plastics (2020), pp. 30-35

In diesem Artikel werden das Scherzugverhalten und der morphologische Zustand von konturgeschweißtem Polypropylen (PP) mit einem Massenanteil von 0,2% Ruß untersucht. Dabei zeigen die Ergebnisse ...


Anisotropic Water-Mediated Proton Conductivity in Large Iron(II) Metal–Organic Framework Single Crystals for Proton-Exchange Membrane Fuel Cells

H. Bunzen, A. Javed, D. Klawinski, A. Lamp, M. Grzywa, A. Kalytta-Mewes, M. Tiemann, H.K. von Nidda, T. Wagner, D. Volkmer, ACS Applied Nano Materials (2019), pp. 291-298

Herein we present a new proton-conducting iron(II) metal–organic framework (MOF) of an unusual structure formed by chains of alternating bistriazolate-p-benzoquinone anions and iron(II) cations with four axially coordinated water molecules. These chains assemble via π–π stacking between the aromatic units to form a three-dimensional grid-like network with channel pores filled with water molecules. The material was structurally characterized by single-crystal XRD analysis, and its water and thermal stability was investigated. The proton conductivity was studied by impedance measurements on needle-like single crystals. A simple but efficient measurement setup consisting of interdigital electrodes was used. The influence of the crystal orientation, temperature, and humidity was investigated. The iron(II)-MOF showed the highest proton conductivity of 3.3·10–3 S cm–1 at 22 °C and 94% relative humidity. Contrary to most known structures, the conductivity in this material is controlled by chemical properties of the pore system rather than by grain boundaries. The presented material is the starting point for further tailoring the proton-conducting properties, independent of morphological features which could find potential applications as membrane materials in proton-exchange membrane fuel cells.

Copper Oxide/Silica Nanocomposites for Selective and Stable H2S Gas Detection

A. Paul, C. Weinberger, M. Tiemann, T. Wagner, ACS Applied Nano Materials (2019), pp. 3335-3338

A composite material of copper oxide (CuO) dispersed in the nanopores of KIT-6 silica (SiO2) is used as a dosimetric sensor for the detection of hydrogen sulfide (H2S) gas in low parts per milion concentrations. The sensor principle is based on the reversible chemical conversion of CuO to CuS, which guarantees a high selectivity, and on the corresponding percolation-induced change in electronic conductance.

Langzeitfestigkeit von Schweißungen aus PP unter Berücksichtigung der Morphologie

V. Schöppner, A. Wübbeke, A. Paul, M. Tiemann, F. Fitze, L. Austermeier, M. Chen, F. Jakob, H. Heim, T. Wu, T. Niendorf, M. Röhricht, M. Schmidt, in: Werkstoffwoche (2019), 2019

Langzeitfestigkeit von Schweißungen aus PP unter Berücksichtigung der Morphologie

Water adsorption and capillary bridge formation on silica micro-particle layers modified with perfluorinated organosilane monolayers

I. Giner, B. Torun, Y. Han, B. Duderija, D. Meinderink, A.G. Orive, M.T. de los Arcos de Pedro, C. Weinberger, M. Tiemann, H. Schmid, G. Grundmeier, Applied Surface Science (2019), pp. 873-879

Monodisperse micron-sized silica particle monolayers deposited onto plasma-grown SiOx-ultra-thin films have been used as reference systems to investigate wetting, water adsorption and capillary bridge formation as a function of silica surface functionalization. 1H,1H, 2H,2H perfluorooctyltriethoxysil (FOTS) monolayers, have been deposited on the respective surfaces by means of chemical vapor deposition resulting in macroscopically low energy surfaces. X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared (FTIR) reflection absorption spectroscopy confirmed the monolayer formation. Water adsorption isotherms were studied by a combination of in-situ FTIR reflection spectroscopy and quartz crystal microbalance (QCM) while macroscopic wetting was analysed by contact angle measurements. The comparative data evaluation indicates that the macroscopic wetting behaviour was changed as expected, however, that water nanodroplets formed both at intrinsic defects of the FOTS monolayer and at the FOTS/SiOx interface. Capillary bridges of liquid water are dominantly formed in the confined particle contact areas and between surface asperities on the particles. The comparison of wetting, adsorption and capillary bridge formation shows that the hydrophobization of porous materials by organosilane monolayers leads to the formation of morphology dependent nanoscopic defects that act as sites for preferential capillary bridge formation.

Straightforward Immobilization of Phosphonic Acids and Phosphoric Acid Esters on Mesoporous Silica and Their Application in an Asymmetric Aldol Reaction

C. Weinberger, T. Heckel, P. Schnippering, M. Schmitz, A. Guo, W. Keil, H.C. Marsmann, C. Schmidt, M. Tiemann, R. Wilhelm, Nanomaterials (2019), 249

<jats:p>The combined benefits of moisture-stable phosphonic acids and mesoporous silica materials (SBA-15 and MCM-41) as large-surface-area solid supports offer new opportunities for several applications, such as catalysis or drug delivery. We present a comprehensive study of a straightforward synthesis method via direct immobilization of several phosphonic acids and phosphoric acid esters on various mesoporous silicas in a Dean–Stark apparatus with toluene as the solvent. Due to the utilization of azeotropic distillation, there was no need to dry phosphonic acids, phosphoric acid esters, solvents, or silicas prior to synthesis. In addition to modeling phosphonic acids, immobilization of the important biomolecule adenosine monophosphate (AMP) on the porous supports was also investigated. Due to the high surface area of the mesoporous silicas, a possible catalytic application based on immobilization of an organocatalyst for an asymmetric aldol reaction is discussed.</jats:p>

Phase Transitions of Ice in Aqueous Salt Solutions within Nanometer-Sized Pores

E. Jantsch, C. Weinberger, M. Tiemann, T. Koop, The Journal of Physical Chemistry C (2019), pp. 24566-24574

We examined the effect of CaCl2 and LiCl on ice melting in mesoporous silica (MCM-41 and SBA-15 silica). For that purpose, we determined the ice melting temperature in pores of various size (pore radii between 1.9 and 11.1 nm) in water and aqueous solutions up to high total solute molality (up to about 12 mol kg–1) using differential scanning calorimetry. We found that both electrolytes reduce the ice melting temperature within the pores. An exception is the melting of ice in the smallest pores, which does not seem to be affected by the presence of solutes, most likely owing to an exclusion of the ions from entering the pores. For all other pores, we observed that the ice melting temperature decreases as a function of pore size and electrolyte concentration. Using thermodynamic considerations as well as additional experimental data we developed a parametrization that can be used to predict the ice melting point as a function of pore size and total solute molality. For that purpose, we extended a formulation of the effective water activity of aqueous solutions under mechanical pressure toward its application in confinement and tested this new parametrization on literature data.

Gas Responsive Nanoswitch: Copper Oxide Composite for Highly Selective H2S Detection

A. Paul, B. Schwind, C. Weinberger, M. Tiemann, T. Wagner, Advanced Functional Materials (2019), 1904505

A nanocomposite material based on copper(II) oxide (CuO) and its utilization as a highly selective and stable gas-responsive electrical switch for hydrogen sulphide (H2S) detection is presented. The material can be applied as a sensitive layer for H2S monitoring, e.g., in biogas gas plants. CuO nanoparticles are embedded in a rigid, nanoporous silica (SiO2) matrix to form an electrical percolating network of low conducting CuO and, upon exposure to H2S, highly conducting copper(II) sulphide (CuS) particles. By steric hindrance due to the silica pore walls, the structure of the network is maintained even though the reversible reaction of CuO to CuS is accompanied by significant volume expansion. The conducting state of the percolating network can be controlled by a variety of parameters, such as temperature, electrode layout, and network topology of the porous silica matrix. The latter means that this new type of sensing material has a structure-encoded detection limit for H2S, which offers new application opportunities. The fabrication process of the mesoporous CuO@SiO2 composite as well as the sensor design and characteristics are described in detail. In addition, theoretical modeling of the percolation effect by Monte-Carlo simulations yields deeper insight into the underlying percolation mechanism and the observed response characteristics.


Graphene oxide as flexibilizer for epoxy amine resins

A. Wolk, M. Rosenthal, J. Weiß, M. Voigt, J. Wesendahl, M. Hartmann, G. Grundmeier, R. Wilhelm, G. Meschut, M. Tiemann, W. Bremser, Progress in Organic Coatings (2018), pp. 280-289

Different types of reduced graphene oxide and graphene oxide particles have been studied regarding their influence on the curing behaviour of epoxy-amine resins. Especially the specific surface area of reduced graphene oxide was selectively influenced by controlled drying of the material. The different types of reduced graphene oxide particles were used to produce epoxy-amine composites that significantly change their curing behaviour and mechanical properties. A variety of surface areas and compositions were prepared by combination of a fast heating rate and different drying methods. The combination of freeze drying with a fast heating rate leads to a large specific surface area of 680 m2/g. The morphologies of the particles were observed by scanning electron microscope and the BET surface area was measured with nitrogen-physisorption. The exfoliation quality was measured by XRD. The generated graphene oxide and thermally reduced graphene oxide particles were mixed with epoxy-amine resin. The curing behaviour was studied with rheological and differential scanning calorimetry (DSC) measurements. We observed that different surface functionalities lowers the Glass transition temperature and the gel time of an epoxy-amine curing system. In addition, we found that generated graphene oxide acts as flexibilizer. An increase of the deformation from 2.5 mm to 3.1 mm was measured by Erichsen Cupping Test.

Selective pore filling of mesoporous CMK-5 carbon studied by XRD: Comparison between theoretical simulations and experimental results

C. Weinberger, M. Hartmann, S. Ren, T. Sandberg, J. Smått, M. Tiemann, Microporous and Mesoporous Materials (2018), pp. 24-31

It is possible to infiltrate a guest species selectively in one pore system of bimodal mesoporous CMK-5 carbon by an optimized nanocasting procedure. The selective filling has a drastic impact on the low-angle X-ray diffraction pattern of this novel class of materials. The structures of CMK-5, CMK-5 composite materials (sulfur and SnO2 as guest species), and CMK-3 carbon were simulated to investigate the influence of the pore filling with different guest species on the diffraction pattern and compared with experimental results. Additionally, the impact of structural defects is taken into account. The nature of the guest species strongly influences the relative intensity of the diffraction peaks. It turns out that the diffraction patterns of sulfur-carbon composite materials are nearly identical as those of CMK-3 carbon, which is attributed to a similar electron density of carbon and sulfur. Thus, sulfur is an ideal guest species to investigate the selective pore filling in CMK-5 carbon.

Porous Aluminum Oxide and Magnesium Oxide Films Using Organic Hydrogels as Structure Matrices

Z. Chen, D. Kuckling, M. Tiemann, Nanomaterials (2018), 186

We describe the synthesis of mesoporous Al2O3 and MgO layers on silicon wafer substrates by using poly(dimethylacrylamide) hydrogels as porogenic matrices. Hydrogel films are prepared by spreading the polymer through spin-coating, followed by photo-cross-linking and anchoring to the substrate surface. The metal oxides are obtained by swelling the hydrogels in the respective metal nitrate solutions and subsequent thermal conversion. Combustion of the hydrogel results in mesoporous metal oxide layers with thicknesses in the μm range and high specific surface areas up to 558 m2∙g−1. Materials are characterized by SEM, FIB ablation, EDX, and Kr physisorption porosimetry.

Bimodal Mesoporous CMK-5 Carbon: Selective Pore Filling with Sulfur and SnO2 for Lithium Battery Electrodes

C. Weinberger, S. Ren, M. Hartmann, T. Wagner, Karaman, J.M. Rosenholm, M. Tiemann, ACS Applied Nano Materials (2018), pp. 455-462

Ordered mesoporous CMK-5 carbon exhibits two distinct pore systems that can be modified individually. This work demonstrates how one of the pore systems can be selectively filled with elemental sulfur, while the other pore system remains empty. The resulting sulfur–carbon composite material with high residual porosity can be used as the cathode material in lithium–sulfur battery cells. We present a systematic investigation of the loading of CMK-5 carbon with variable relative amounts of sulfur and compare the results to the preparation of SnO2 (as well as TiO2, Mn2O3/Mn3O4, NiO) nanoparticle-loaded CMK-5 carbon.

Hydrogels as Porogens for Nanoporous Inorganic Materials

C. Weinberger, D. Kuckling, M. Tiemann, Gels (2018), 83

Organic polymer-hydrogels are known to be capable of directing the nucleation and growth of inorganic materials, such as silica, metal oxides, apatite or metal chalcogenides. This approach can be exploited in the synthesis of materials that exhibit defined nanoporosity. When the organic polymer-based hydrogel is incorporated in the inorganic product, a composite is formed from which the organic component may be selectively removed, yielding nanopores in the inorganic product. Such porogenic impact resembles the concept of using soft or hard templates for porous materials. This micro-review provides a survey of select examples from the literature.


Kinetics of ozone decomposition in porous In2O3 monoliths

D. Klawinski, C. Weinberger, D. Klaus, J. Smått, M. Tiemann, T. Wagner, Physical Chemistry Chemical Physics (2017), pp. 10326-10332

<p>We determine ozone decomposition on indium oxide by utilizing the gas transducing properties of hierarchically porous monoliths.</p>

Photo-Cross-Linked Polydimethylacrylamide Hydrogels as Porogens for Mesoporous Alumina

C. Weinberger, Z. Chen, W. Birnbaum, D. Kuckling, M. Tiemann, European Journal of Inorganic Chemistry (2017), pp. 1026-1031

Dimethylacrylamide-based hydrogels were utilized as porogenic matrices in the synthesis of mesoporous aluminum oxide (γ-Al2O3) with specific BET surface areas up to 360 m2 g–1. Polymers with molecular mass in the range 12000–35000 g mol–1 were synthesized from dimethylacrylamide and various comonomers by free-radical polymerization. Photo-cross-linking of the polymers and impregnation with aluminum nitrate [Al(NO3)3] was carried out in a single step, followed by formation of Al(OH)3/AlO(OH) and subsequent calcination. Calcination led to the formation of mesoporous Al2O3 and simultaneous combustion of the hydrogel. The structural properties of the products were characterized by powder XRD, N2 physisorption analysis, Hg intrusion porosimetry, and thermogravimetric analysis.

Organic Polymers as Porogenic Structure Matrices for Mesoporous Alumina and Magnesia

Z. Chen, C. Weinberger, M. Tiemann, D. Kuckling, Processes (2017), 70

Dimethylacrylamide-based hydrogels were utilized as porogenic matrices in the synthesis of mesoporous aluminum oxide (γ-Al2O3) with specific BET surface areas up to 360 m2 g–1. Polymers with molecular mass in the range 12000–35000 g mol–1 were synthesized from dimethylacrylamide and various comonomers by free-radical polymerization. Photo-cross-linking of the polymers and impregnation with aluminum nitrate [Al(NO3)3] was carried out in a single step, followed by formation of Al(OH)3/AlO(OH) and subsequent calcination. Calcination led to the formation of mesoporous Al2O3 and simultaneous combustion of the hydrogel. The structural properties of the products were characterized by powder XRD, N2 physisorption analysis, Hg intrusion porosimetry, and thermogravimetric analysis.


Selective surface modification in bimodal mesoporous CMK-5 carbon

C. Weinberger, X. Cao, M. Tiemann, Journal of Materials Chemistry A (2016), pp. 18426-18431

Ordered, bimodal mesoporous CMK-5 carbon is prepared by using mesoporous SBA-15 silica as a structural mold. The carbon material is chemically modified by oxidative treatment with acidic persulfate solution. This leads to the creation of oxygen-containing functionalities at the pore walls of the carbon (up to 13 wt% oxygen), as confirmed by IR spectroscopy. The oxidative treatment is carried out before removal of the silica mold which ensures that only one of the two distinct modes of mesopores (namely, the intra-tubular pores) is affected; the other mode (inter-tubular pores) is protected from oxidation by the presence of the silica mold. This is proven by water vapor physisorption analysis. The oxidatively treated (intra-tubular) pores are significantly more polar and, hence, better wettable than the untreated (inter-tubular) pores.

Assessment of the density of (meso)porous materials from standard volumetric physisorption data

C. Weinberger, S. Vetter, M. Tiemann, T. Wagner, Microporous and Mesoporous Materials (2016), pp. 53-57

Characterization and application of (meso)porous materials often require information about the density of the respective samples. For example, the BET surface area is, by definition, normalized to the sample mass; hence, any comparison between samples of different composition needs to take into account their respective densities. Literature data on the densities of porous materials are scarce. Frequently, only bulk-phase densities are available which sometimes differ from those of porous samples, especially for amorphous systems, such as silica or carbon. The apparent density, i.e. the density of the sample excluding the gas-accessible pore volume, is typically determined by helium gas pycnometry utilizing specialized pycnometers. We demonstrate how to obtain the same data from standard N2 physisorption measurements as part of the regular measurement routine. We evaluate the method by reference measurements utilizing a non-porous reference sample (glass rod) to confirm the validity of the method. Then we present results on apparent density measurements of several mesoporous silica materials (MCM-41, MCM-48, SBA-15, KIT-6), mesoporous carbon (CMK-3, -5, -8, -9), and a variety of mesoporous metal oxides obtained by nanocasting.

Screening of mixed-linker CAU-10 MOF materials for humidity sensing by impedance spectroscopy

A. Weiss, N. Reimer, N. Stock, M. Tiemann, T. Wagner, Microporous and Mesoporous Materials (2016), pp. 39-43

The sorption properties of mixed-linker CAU-10 type metal organic frameworks (MOFs), [Al(OH)(1,3-BDC-X)n(1,3-BDC-SO3H)m] with 1,3-BDC = 1,3-benzenedicarboxyliate, X = H, NO2 or OH, 0.76 ≤ n ≤ 0.89 and 0.11 ≤ m ≤ 0.24, can be varied by surface modification through variation of the respective linker molecules. It is thus possible to design surface-modified CAU-10 type MOFs with variable affinity and accessibility of the pores for water vapour. When used as a dielectric in a capacitor, the MOF material will change its permittivity depending on the amount of physisorbed water; this is the working principle of capacitive humidity sensors. Three different mixed-linker compounds with CAU-10 structure are compared regarding their water sorption and impedance characteristics. A setup was developed allowing the characterization of the MOF samples under exposure to different relative humidity values in air by impedance spectroscopy. Interpretation of the results by means of standard models shows that the MOFs are qualified for functional layers of capacitive humidity sensors. Since the prepared MOFs are more temperature-stable than many commonly used polymers they offer the potential to build a new generation of high-temperature (up to 350 °C) humidity sensors.


Surface-modified CAU-10 MOF materials as humidity sensors: impedance spectroscopic study on water uptake

A. Weiss, N. Reimer, N. Stock, M. Tiemann, T. Wagner, Physical Chemistry Chemical Physics (2015), pp. 21634-21642

Metal–organic frameworks (MOFs) are crystalline microporous materials with tunable chemical and physical properties. By combining various metal clusters with different interconnecting organic linkers, the pore structure, crystallinity, as well as the surface properties can be modified. In the present work, modification of the organic linker molecules is utilized to synthesize CAU-10 type MOFs with variable affinity of the pore surface to water. In principle, this should influence the accessibility of the pores for water vapor and therefore offer a tool to control its sorption properties. For a deeper understanding we studied the water sorption characteristics and compared the results to the conductive and dielectric properties studied by impedance spectroscopy. Spectra in a wide frequency range from 1 mHz to 1 MHz were recorded. Data analysis is performed using the Havriliak–Negami model. The MOFs are also tested as sensitive layers for capacitive humidity sensing by correlating the change in permittivity of the materials with the amount of physisorbed water. Such an MOF-based sensor was tested with respect to environmental monitoring and compared to a commonly used commercial humidity sensor.

Synthesis of Mesoporous Metal Oxides by Structure Replication: Thermal Analysis of Metal Nitrates in Porous Carbon Matrices

C. Weinberger, J. Roggenbuck, J. Hanss, M. Tiemann, Nanomaterials (2015), pp. 1431-1441

A variety of metal nitrates were filled into the pores of an ordered mesoporous CMK-3 carbon matrix by solution-based impregnation. Thermal conversion of the metal nitrates into the respective metal oxides, and subsequent removal of the carbon matrix by thermal combustion, provides a versatile means to prepare mesoporous metal oxides (so-called nanocasting). This study aims to monitor the thermally induced processes by thermogravimetric analysis (TGA), coupled with mass ion detection (MS). The highly dispersed metal nitrates in the pores of the carbon matrix tend to react to the respective metal oxides at lower temperature than reported in the literature for pure, i.e., carbon-free, metal nitrates. The subsequent thermal combustion of the CMK-3 carbon matrix also occurs at lower temperature, which is explained by a catalytic effect of the metal oxides present in the pores. This catalytic effect is particularly strong for oxides of redox active metals, such as transition group VII and VIII metals (Mn, Fe, Co, Ni), Cu, and Ce.

Light-activated resistive ozone sensing at room temperature utilizing nanoporous In2O3 particles: Influence of particle size

D. Klaus, D. Klawinski, S. Amrehn, M. Tiemann, T. Wagner, Sensors and Actuators B: Chemical (2015), pp. 181-185

Ordered mesoporous In2O3 particles of variable size synthesized by the nanocasting method are used for preparation of resistive gas-sensing layers. Light activation by a LED (blue light, 460 nm) permits room-temperature ozone sensing. Apart from differences in base-line resistance in sensing layers containing small (diameter approx. 170 nm) or large particles (approx. 870 nm), differences in the response amplitude and response time constant are also observed. Signal stabilization is achieved faster for small particles. In addition, sensors show a particle size-dependent reaction threshold for low ozone concentration. Larger particles show negligible response to 50 ppb ozone whereas a significant response is observed for the small-particle sensors. A simple model based on geometrical properties and formation of depletion layers explaining the observed behavior is presented.

Nanostructured Co3O4 as a CO gas sensor: Temperature-dependent behavior

S. Vetter, S. Haffer, T. Wagner, M. Tiemann, Sensors and Actuators B: Chemical (2015), pp. 133-138

Cobalt oxide spinel (Co3O4) with an ordered nanostructure is used as a resistive gas sensor for carbon monoxide (CO) in low ppm concentrations. The operating temperature has a strong impact on the concentration-dependent sensing behavior. At lower temperature (473 K) the sensor response is governed mainly by surface coverage with CO and/or CO2, whereas at higher temperature (563 K) oxygen diffusion in the crystal lattice of Co3O4 strongly affects the sensing behavior.


Nanoporous Materials: Synthesis Concepts and Model Experiments for School Chemistry Education

T. Wilke, S. Haffer, C. Weinberger, M. Tiemann, T. Wagner, T. Waitz, Journal of Nano Education (2014), pp. 117-123

Nanoporous Materials, like carbons, silica and semiconducting metal oxides, play a major role in recent scientific research, especially in the fields of energy storage, catalysis, material separation and sensor technology. Thus, our aim is to focus on simple synthesis concepts for these materials, such as soft matter templating or nanocasting, which can be easily introduced by means of appropriate models in school chemistry education or school laboratories. In addition to facile and realizable syntheses in school, several experiments concerning catalysis and gas sensing will be presented, too. By these experiments the characteristics of nanoporous materials can be obviously demonstrated and additionally, these experiments can serve as a starting point for further experiments that could easily be developed by students themselves, particularly in relation to environmental issues.

Fructose and Urea as Precursors for N-/O-Modified Mesoporous Carbon with Enhanced Sorption Capacity for Heavy Metal Ions

C. Weinberger, S. Haffer, T. Wagner, M. Tiemann, European Journal of Inorganic Chemistry (2014), pp. 2787-2792

Ordered mesoporous carbon with a high heteroatom (N, O) content was prepared by nanocasting from a melt of a eutectic mixture of fructose and urea (60/40 wt.-%; melting temperature ca. 65 °C). These precursor compounds are cheap and environmentally friendly. The material possesses enhanced pore-wall surface polarity as compared to that of mesoporous carbon prepared by the same technique without urea. This was verified by water sorption analysis. As a result, the heteroatom-modified material shows higher sorption capacity for the uptake of heavy metal ions (Cu2+) from aqueous solution, which may be interesting for potential application in wastewater cleaning.

Synthesis of mesoporous alumina through photo cross-linked poly(dimethylacrylamide) hydrogels

W. Birnbaum, C. Weinberger, V. Schill, S. Haffer, M. Tiemann, D. Kuckling, Colloid and Polymer Science (2014), pp. 3055-3060

Catalysis plays a central role in many fields of life, e.g., in biochemical processes, to reduce energy costs and resources in chemical industry and to decrease or even avoid environmental pollution and in energy management. Porous alumina (Al2O3) is an essential material in various applications, especially as a support material for catalysts. It is often prepared by nanocasting using porous carbon materials that serve as rigid structure matrices. In this work, an alternative way to synthesize mesoporous Al2O3 by using hydrogels as porogenic material is presented. Hydrogels can easily be patterned by light and used to imprint their structure onto alumina opening a new approach to fabricate patterned Al2O3. The hydrogels used in this work are based on poly(dimethylacrylamide) and were photo-chemically cross-linked. Followed by a nanocasting process, mesoporous alumina samples were synthesized and characterized by N2 physisorption and X-ray diffraction. The cross-linker amount in the polymer network was varied and the influence on the properties of the Al2O3 is analyzed.

A synthesis concept for a nanostructured CoFe2O4/BaTiO3 composite: Towards multiferroics

S. Haffer, C. Lüder, T. Walther, R. Köferstein, S.G. Ebbinghaus, M. Tiemann, Microporous and Mesoporous Materials (2014), pp. 300-304

The synthesis of a periodically ordered, nanostructured composite consisting of CoFe2O4 and BaTiO3 is presented. In a first step, mesoporous CoFe2O4 is prepared by the structure replication method (nanocasting) using mesoporous KIT-6 silica as a structural mold. Subsequently, BaTiO3 is created inside the pores of CoFe2O4 by the citrate route, resulting in a well-ordered composite material of both phases. The two components are known for their distinct ferroic properties, namely ferrimagnetism (CoFe2O4) and ferroelectricity (BaTiO3), respectively. Therefore, this proof of synthesis concept offers new perspectives in the fabrication of composite materials with multiferroic properties.

One-step synthesis of multi-modal pore systems in mesoporous In2O3: A detailed study

D. Klaus, S. Amrehn, M. Tiemann, T. Wagner, Microporous and Mesoporous Materials (2014), pp. 133-139

Ordered mesoporous silica phases (e.g. KIT-6, SBA-15) are used as structure matrices for negative replica structures of mesoporous In2O3. We present a detailed study on how the controlled synthesis of mono-, bi- and trimodal pore systems in the products is accomplished by systematic variation of the procedure of infiltrating a precursor species (In(NO3)3) into the pores of the silica matrix and subsequent thermal conversion into In2O3. Melt impregnation and conversion in a closed reactor facilitates a one-step casting process for ordered mesoporous indium oxide (In2O3). We present a model based on variation of the pore filling.

New Sensing Model of (Mesoporous) In2O3

T. Wagner, N. Donato, M. Tiemann, in: Springer Series on Chemical Sensors and Biosensors, 2014

Recently indium oxide (In2O3) attracted attention as a material for sensing layers in semiconducting gas sensors. Compared to frequently investigated materials like tin dioxide (SnO2), tungsten trioxide (WO3), or gallium oxide (Ga2O3) indium oxide offers some unique properties. The most prominent one is its selectivity to oxidizing gases such as ozone (O3) or nitrogen dioxide (NO2) at low operating temperatures (<150°C). Combined with the photoreduction properties of nanocast, porous In2O3 highly selective sensing layers with a fast response can be prepared. In some cases even room temperature measurements are possible; therefore this material allows for designing low-power sensors without the need for special sensor substrates (e.g., μ-hotplates). Detailed analysis of the sensing mechanism reveals that known sensing models are not able to describe the observed effects. Therefore a new sensing model for ordered nanoporous In2O3 is presented which will be applicable for nonstructured material too.

Arduino-Based Shield for Resistive Gas Sensor Array Characterization Under UV Light Exposure

D. Aloisio, N. Donato, G. Neri, M. Latino, T. Wagner, M. Tiemann, P.P. Capra, in: Lecture Notes in Electrical Engineering, 2014

In this paper, the development and validation of a shield prototype for resistive sensor array characterization with Arduino UNO, a platform based on ATmega328 microcontroller provided by ATMEL, is reported. The resistance variation of the sensor can be evaluated by properly choosing the capacitance value and by measuring the period (frequency) of a custom inverter-based oscillator. The GUI and the developed firmware are able to perform the real-time monitoring of the sensor responses. The developed shield is able to measure the response of up to six sensors under UV radiation by means of LED devices. First results carried out with resistive sensors based on mesoporous In2O3-based material under UV light exposure are reported.

Fructose as a Precursor for Mesoporous Carbon: Straightforward Solvent-Free Synthesis by Nanocasting

C. Weinberger, S. Haffer, T. Wagner, M. Tiemann, in: ACS Symposium Series, 2014

Due to their unique properties, ordered mesoporous carbon (OMC) materials prepared by nanocasting have raised great attention in recent years. Their synthesis usually comprises multiple cycles of impregnating a porous structure matrix with an aqueous solution of a suitable precursor, such as sucrose or other, often hazardous, compound. We present a more straightforward variation of this method by using fructose as the precursor compound. By using a solvent-free melt of the precursor, the impregnation requires only a single step. After carbonization by thermal decomposition and removal of the mesoporous silica structure matrix (SBA-15), ordered mesoporous carbon with one (CMK-3) or two (CMK-5) pore modes in two-dimensional, hexagonal symmetry (p6mm) is obtained.


UV light-enhanced NO2 sensing by mesoporous In2O3: Interpretation of results by a new sensing model

T. Wagner, C. Kohl, C. Malagù, N. Donato, M. Latino, G. Neri, M. Tiemann, Sensors and Actuators B: Chemical (2013), pp. 488-494

The light-enhanced NO2 sensing behavior of mesoporous In2O3 is measured and interpreted by means of a new sensing model. The model aims at explaining (i) the drop in electronic resistance of n-type semiconducting In2O3 under UV light exposure, (ii) the light-enhanced reaction to oxidizing gases, and (iii) the faster reaction and regeneration in mesoporous In2O3 as compared to non-porous material. Contrary to the conventional double Schottky model the dominating factor for the change in resistance is a change of oxygen vacancy donor states (0.18 eV below the conduction band) in the bulk phase due to photoreduction, instead of chemisorption. For the faster reaction and regeneration we propose an explanation based on enhanced oxygen diffusion in the In2O3 crystal lattice, specifically dominant in the mesoporous structure. The response of ordered mesoporous In2O3 to NO2 is stronger than in case of unstructured bulk material (with an average grain size of ca. 40 nm). The reaction is significantly accelerated by illuminating the samples with UV light. However, the response of the mesoporous material is weaker in the illuminated case.

Nanostructure-Related Magnetic Properties of Various Mesoporous Cobalt Oxide and Cobalt Ferrite Spinel Phases

S. Haffer, T. Walther, R. Köferstein, S.G. Ebbinghaus, M. Tiemann, The Journal of Physical Chemistry C (2013), pp. 24471-24478

Nanostructure-related magnetic properties are investigated systematically for various mesoporous cobalt oxide (Co3O4) and cobalt ferrite (CoFe2O4) spinel phases. Synthesis of the materials by nanocasting offers the opportunity to obtain materials which are different from each other with respect to both specific surface area and crystallite size. As a result, the respective contributions of two types of interfaces, namely, “solid–gas” and “solid–solid” interfaces, to the magnetic ordering can be distinguished. Structural characterization of the porous materials by X-ray diffraction, N2 physisorption, and electron microscopy as well as investigation of the magnetic behavior (field-dependent magnetization and temperature-dependent susceptibility) are presented.

Mesoporous materials as gas sensors

T. Wagner, S. Haffer, C. Weinberger, D. Klaus, M. Tiemann, Chem. Soc. Rev. (2013), pp. 4036-4053

Ordered mesoporous materials have great potential in the field of gas sensing. Today various template-assisted synthesis methods facilitate the preparation of silica (SiO2) as well as numerous metal oxides with well-defined, uniform and regular pore systems. The unique nanostructural properties of such materials are particularly useful for their application as active layers in gas sensors based on various operating principles, such as capacitive, resistive, or optical sensing. This review summarizes the basic aspects of materials synthesis, discusses some structural properties relevant in gas sensing, and gives an overview of the literature on ordered mesoporous gas sensors.


Photoreduction of Mesoporous In2O3: Mechanistic Model and Utility in Gas Sensing

T. Wagner, C. Kohl, S. Morandi, C. Malagù, N. Donato, M. Latino, G. Neri, M. Tiemann, Chemistry - A European Journal (2012), pp. 8216-8223

A model is proposed for the drop in electronic resistance of n-type semiconducting indium oxide (In2O3) upon illumination with light (350 nm, 3.5 eV) as well as for the (light-enhanced) sensitivity of In2O3 to oxidizing gases. Essential features of the model are photoreduction and a rate-limiting oxygen-diffusion step. Ordered, mesoporous In2O3 with a high specific surface area serves as a versatile system for experimental studies. Analytical techniques comprise conductivity measurements under a controlled atmosphere (synthetic air, pure N2) and temperature-resolved in-situ Fourier transform infrared (FTIR) spectroscopy. IR measurements reveal that oxygen vacancies form a donor level 0.18 eV below the conduction band.

Mesoporöse Silica - Moderne Funktionsmaterialien im Chemieunterricht

T. Wilke, S. Haffer, M. Tiemann, T. Waitz, CHEMKON (2012), pp. 67-72

Poröse Funktionsmaterialien wie halbleitende Metalloxide, Kohlenstoff-Formen oder auch Silica werden aktuell von verschiedenen Wissenschaftsdisziplinen intensiv für Bereiche der Energiespeicherung, Sensorik, Katalyse und Stofftrennung erforscht. Im Beitrag werden schwerpunktmäßig geordnet-mesoporöse Silica-Materialien behandelt, die seit etwa 20 Jahren synthetisch zugänglich sind. Neben den Grundlagen der Herstellung über ein Templat-Verfahren werden im Beitrag auch drei Experimente vorgestellt, die im Chemieunterricht oder Schülerlabor durchgeführt werden können. Zudem wird gezeigt, dass sich verschiedene Aspekte aus dem Kompetenzbereich Fachwissen mit Hilfe des Themas „Mesoporöse Silica“ miteinander vernetzen lassen.

Mesoporous Al2O3 by Nanocasting: Relationship between Crystallinity and Mesoscopic Order

S. Haffer, C. Weinberger, M. Tiemann, European Journal of Inorganic Chemistry (2012), pp. 3283-3288

Crystalline, mesoporous alumina (Al2O3) materials with specific surface areas up to 400 m2 g–1 have been synthesized by means of structure replication (nanocasting) using CMK-8 carbon as a structure matrix. A crucial step during this synthesis procedure is the conversion of aluminum nitrate into aluminum hydroxide by treatment with ammonia vapor. The impact of this step was investigated in some detail. Prolonged vapor treatment has a positive impact on the crystallinity of the final Al2O3 products but at the same time leads to loss of mesoscopic structural order and porosity.

NO2 Sensors with Reduced Power Consumption Based on Mesoporous Indium Oxide

N. Donato, T. Wagner, M. Tiemann, T. Waitz, C. Kohl, M. Latino, G. Neri, D. Spadaro, C. Malagù, in: Lecture Notes in Electrical Engineering, 2012

We report on sensing properties of ordered mesoporous nanostructures of In2O3 synthesized by nanocasting procedure towards NO2. The nanostructured material shows improved recover times and higher responses compared to non nanostructured material at low operating temperatures (100–150°C) thus allowing the use for low power NO2 sensors. These properties may be related to fast oxygen in and out propagation facilitated by an enhanced surface accessibility of the nanostructure.


X-ray absorption near-edge spectroscopy investigation of the oxidation state of Pd species in nanoporous SnO2 gas sensors for methane detection

T. Wagner, M. Bauer, T. Sauerwald, C. Kohl, M. Tiemann, Thin Solid Films (2011), pp. 909-912

We report the correlation of the aging of Pd-doped SnO2 methane sensors with the change of the oxidation state of Pd. Mesoporous SnO2 doped with palladium species was prepared and exposed to different gas mixtures at high temperature (600 °C) to simulate long term usage. After each exposure step a fraction of the sample was cooled down to “freeze” the current oxidation state of Pd which was then analyzed by X-ray Absorption Near-Edge Spectroscopy (XANES) using the 'white line' (i.e. the absorption peak corresponding to the transition from the 2p3/2 core level to unoccupied 4 d states) intensity of the L(III) edge as a probe for the oxidation state. The Pd oxidation state correlates with the response of the resistive SnO2 sensor to methane gas, as determined by measuring the gas response to different concentrations of methane. Samples treated with 5000 ppm methane in air show a significant reduction of Pd(II) to Pd(0), depending clearly on the carrier gas (synthetic air, pure nitrogen) and on the temperature (600 °C vs. 300 °C).

A High Temperature Capacitive Humidity Sensor Based on Mesoporous Silica

T. Wagner, S. Krotzky, A. Weiß, T. Sauerwald, C. Kohl, J. Roggenbuck, M. Tiemann, Sensors (2011), pp. 3135-3144

Capacitive sensors are the most commonly used devices for the detection of humidity because they are inexpensive and the detection mechanism is very specific for humidity. However, especially for industrial processes, there is a lack of dielectrics that are stable at high temperature (>200 °C) and under harsh conditions. We present a capacitive sensor based on mesoporous silica as the dielectric in a simple sensor design based on pressed silica pellets. Investigation of the structural stability of the porous silica under simulated operating conditions as well as the influence of the pellet production will be shown. Impedance measurements demonstrate the utility of the sensor at both low (90 °C) and high (up to 210 °C) operating temperatures.

Micrometer-sized nanoporous tin dioxide spheres for gas sensing

J. Smått, M. Lindén, T. Wagner, C. Kohl, M. Tiemann, Sensors and Actuators B: Chemical (2011), pp. 483-488

We report the synthesis of mesoporous tin dioxide (SnO2) materials with well-defined particle morphology. The products consist of uniform spheres with a diameter of 5 μm. The spheres are hierarchically porous with two distinct pore modes of 5.0 nm and 52 nm, respectively. This special porosity is the result of a synthesis procedure which involves a ‘hard templating’ (nanocasting) process. The product forms an approximately homogeneous monolayer of spheres on a sensor substrate and shows promising response to methane gas with low cross-sensitivity to water. The structural properties and gas-sensing performance are compared with a mesoporous SnO2 material without defined morphology, prepared by a ‘soft templating’ procedure.

Photocatalytic ozone sensor based on mesoporous indium oxide: Influence of the relative humidity on the sensing performance

T. Wagner, J. Hennemann, C. Kohl, M. Tiemann, Thin Solid Films (2011), pp. 918-921

Mesoporous In2O3, synthesized by a nanocasting procedure, is used as a resistive gas sensor for ozone in very low concentrations (from 20 ppb to 2.4 ppm) at room temperature. Its sensing performance is substantially increased by illumination with blue light (460 nm, 2.7 eV). For low ozone concentrations the sensor response increases with increasing humidity. However, higher humidity also results in the occurrence of a saturation of the response at lower ozone concentrations; this is rationalized by assuming a poisoning of surface active sites by hydroxyl groups.

Photoluminescence Properties of Ordered Mesoporous ZnO

A. Chernikov, S. Horst, T. Waitz, M. Tiemann, S. Chatterjee, The Journal of Physical Chemistry C (2011), pp. 1375-1379

Nanoporous ZnO powders with high surface-to-mass ratios (SMR) between 15 and 70 m2 g−1 are synthesized, structurally characterized, and studied by time-resolved photoluminescence (PL). A strong dependence of the recombination dynamics and spectral width on SMR is observed at T = 10 K, and pronounced disorder-induced effects are found in the temperature dependence. Both the thermally induced shift of the PL maximum and the spectrally integrated PL intensity are interpreted by appropriate theoretical models. This consistent quantitative analysis of the experimental data yields a characteristic energy of 15 meV for the disorder scale in the nanoporous ZnO sample with an intermediate SMR.


Accessing Ultrashort Reaction Times in Particle Formation with SAXS Experiments: ZnS Precipitation on the Microsecond Time Scale

W. Schmidt, P. Bussian, M. Lindén, H. Amenitsch, P. Agren, M. Tiemann, F. Schüth, Journal of the American Chemical Society (2010), pp. 6822-6826

Precipitation of zinc sulfide particles is a very rapid process, and monitoring of the particle growth is experimentally very demanding. Applying a liquid jet flow cell, we were able to follow zinc sulfide particle formation on time scales down to 10−5 s. The flow cell was designed in such a way that data acquisition on the microsecond time scale was possible under steady-state conditions along a liquid jet (tubular reactor concept), allowing SAXS data accumulation over a time scale of minutes. We were able to monitor the growth of zinc sulfide particles and found experimental evidence for very rapid particle aggregation processes within the liquid jet. Under the experimental conditions the particle growth is controlled by mass transfer: i.e., the diffusion of the hydrogen sulfide into the liquid jet.

Ordered nanoporous SnO2 gas sensors with high thermal stability

T. Waitz, B. Becker, T. Wagner, T. Sauerwald, C. Kohl, M. Tiemann, Sensors and Actuators B: Chemical (2010), pp. 788-793

We report the structural characterization and gas sensing properties of mesoporous SnO2 synthesized by structure replication (nanocasting) from ordered mesoporous KIT-6 silica. The products show a high thermal stability with no structural loss up to 600 °C and only minor decrease in specific surface area by 18% at 800 °C, as proven by powder X-ray diffraction (PXRD), transmission electron microscopy (TEM), and nitrogen physisorption. In particular, the samples turn out to be much more stable than porous SnO2 materials prepared by sol–gel-based synthesis procedures for comparison. The thermal stability facilitates the utilization of the materials as sensors for combustible gases which react at high temperatures; test measurements reveal promising responses to methane (CH4) as an example.

Mesoporous In2O3 with Regular Morphology by Nanocasting: A Simple Relation between Defined Particle Shape and Growth Mechanism

S. Haffer, T. Waitz, M. Tiemann, The Journal of Physical Chemistry C (2010), pp. 2075-2081

In2O3 with ordered, uniform mesoporosity is prepared by nanocasting, using various porous silica phases (KIT-6, SBA-15) as structure matrices. The In2O3 particles exhibit well-defined morphologies (spherical or ellipsoidal, depending on the choice of silica matrix) and quite uniform sizes in the range of a few hundred nanometers. The regular morphology of the In2O3 particles is not associated with the morphological properties of the silica matrices. Instead, it is the result of the growth mechanism of In2O3 inside the silica pores; this mechanism is investigated in some detail. Hence, the nanocasting method offers a versatile and simple way of creating mesoporous In2O3 with regular morphology; this will be beneficial for many applications that require well-defined morphological properties, such as gas sensing or catalysis.

Periodic Mesoporous Organosilica (PMO) Materials with Uniform Spherical Core-Shell Structure

S. Haffer, M. Tiemann, M. Fröba, Chemistry - A European Journal (2010), pp. 10447-10452

We report the synthesis of monodisperse, spherical periodic mesoporous organosilica (PMO) materials. The particles have diameters between about 350 and 550 nm. They exhibit a regular core-shell structure with a solid, non-porous silica core and a mesoporous PMO shell with a thickness of approximately 75 nm and uniform pores of about 1.7 nm. The synthesis of the core and the shell is carried out in a one-pot, two-stage synthesis and can be accomplished at temperatures between 25 and 100 °C. Higher synthesis temperatures lead to substantial shrinking of the solid core, generating an empty void between core and shell. This leads to interesting cavitation phenomena in the nitrogen physisorption analysis at 77.4 K.

Ordered Mesoporous Films and Membranes: Synthesis, Properties and Applications in Gas Sensors

M. Tiemann, in: Nanostructured Materials, Momentum Press, 2010, pp. 291 - 310

Chemical sensors are integral to the automation of myriad industrial processes, as well as everyday monitoring of such activities as public safety, engine performance, medical therapeutics, and many more...


Gas sensor based on ordered mesoporous In2O3

T. Wagner, T. Sauerwald, C. Kohl, T. Waitz, C. Weidmann, M. Tiemann, Thin Solid Films (2009), pp. 6170-6175

We present the preparation of a semiconductor gas sensor based on ordered mesoporous In2O3. The In2O3 was synthesized by structure replication procedure from cubic KIT-6 silica. A detailed analysis of the morphology of the mesoporous powders as well as of the prepared sensing layer will be shown. Unique properties arise from the synthesis method of structure replication such as well defined porosity in the mesoporous regime and nanocrystallites with high thermal stability up to 450 °C. These properties are useful for the application in semiconducting gas sensors. Test measurements show sensitivity to methane gas in concentrations relevant for explosion prevention.

Halbleitende Metalloxide als Gassensoren im Chemieunterricht

T. Waitz, M. Tiemann, CHEMKON (2009), pp. 183-186

Halbleitende Metalloxid-Gassensoren werden sowohl im industriellen Bereich als auch im Haushalt zur Luftüberwachung verwendet. Die Funktionsweise basiert auf einer reversiblen Änderung des Sensorwiderstandes in Anwesenheit sowohl oxidierender als auch reduzierender Gase, die mit einfachen Mitteln gemessen werden kann. In diesem Artikel wird vorgestellt, wie sich die bei der Gasdetektion ablaufenden Vorgänge mit Hilfe des Ionosorptionsmodells in Kombination mit einem einfachen Festkörperbändermodell deuten lassen. Abschließend werden einfache, im Chemieunterricht zu realisierende Experimente mit Gassensoren präsentiert.

Ordered Mesoporous In2O3: Synthesis by Structure Replication and Application as a Methane Gas Sensor

T. Waitz, T. Wagner, T. Sauerwald, C. Kohl, M. Tiemann, Advanced Functional Materials (2009), pp. 653-661

The synthesis and characterization of ordered mesoporous In2O3 materials by structure replication from hexagonal mesoporous SBA-15 silica and cubic KIT-6 silica is presented. Variation of the synthesis parameters allows for different pore sizes and pore wall thicknesses in the products. The In2O3 samples turn out to be stable up to temperatures between 450 °C and 650 °C; such high thermal stability is necessary for their application as gas sensors. Test measurements show a high sensitivity to methane gas in concentrations relevant for explosion prevention. The sensitivity is shown to be correlated not only with the surface-to-volume ratio, but also with the nanoscopic structural properties of the materials.

Time-resolved photoluminescence study of mesoporous ZnO nanostructures

M. Schwalm, S. Horst, A. Chernikov, W.W. Rühle, S. Lautenschläger, P.J. Klar, B.K. Meyer, T. Waitz, M. Tiemann, S. Chatterjee, physica status solidi (c) (2009), pp. 542-545

We report a systematic study of the photoluminescence properties of ZnO nanostructures. In particular, mesoporous ZnO powders of varying surface-to-mass ratio are investigated and compared to a bulk reference. At low temperatures the emission from higher-energy states is very pronounced and even dominant for samples with high surface-to-mass ratio.


Critical evaluation of the state of iron oxide nanoparticles on different mesoporous silicas prepared by an impregnation method

T. Tsoncheva, J. Rosenholm, M. Linden, F. Kleitz, M. Tiemann, L. Ivanova, M. Dimitrov, D. Paneva, I. Mitov, C. Minchev, Microporous and Mesoporous Materials (2008), pp. 327-337

Mesoporous SBA-15 (space group p6mm), KIT-6 (Ia3d) and KIT-5 (Fm3m) silicas, exhibiting different 2-D and 3-D channel- or cage-like pore structure and pore dimensions have been used as supports for iron oxide nanoparticles. The iron modification of the silica was performed according to a frequently used impregnation technique from aqueous iron nitrate solution. The materials were characterized by nitrogen physisorption, X-ray diffraction, TEM–EDX, Moessbauer spectroscopy, and temperature-programmed reduction (TPR) and tested in the catalytic decomposition of methanol. It is established that the location and dispersion of iron oxide nanoparticles are affected by the pore topology of the support. The most homogeneously dispersed iron oxide nanoparticles are observed using silica host matrix exhibiting a 3-D channel-like structure and pore diameters about 7 nm, and the thus-obtained composites exhibit high catalytic activity and selectivity in methanol decomposition to CO and hydrogen. For all the samples, characterized with a low mesopore volume and small pore diameters/pore entrances, the formation of larger iron oxide particles, mainly located on the outer surface, is observed. Inhomogeneously dispersed iron oxide particles with a large fraction of isolated, strongly interacting with the support, iron species, and possessing low catalytic activity and usually high selectivity to methane, are found for the silicas with relatively larger pores/pore entrances.

Synthesis of mesoporous metal oxides by structure replication: Strategies of impregnating porous matrices with metal salts

J. Roggenbuck, T. Waitz, M. Tiemann, Microporous and Mesoporous Materials (2008), pp. 575-582

Various measures to optimise the impregnation of mesoporous CMK-3 carbon and SBA-15 silica matrices with metal nitrates for the synthesis of mesoporous metal oxides by structure replication are investigated. The effect of surface modification of the matrix pores, the choice of a solvent with suitable polarity, and the concentration of the metal nitrate solution are studied in detail. The efficiency of pore loading is monitored by nitrogen physisorption measurements. The creation of polar functions at the pore surface of CMK-3 carbon is shown to increase the impregnation efficiency substantially while maximizing the pore wall polarity of SBA-15 silica by increasing the amount of free silanol groups does not have any significant impact. The choice of a less polar solvent (THF instead of water) has a positive effect on the wettability of CMK-3 carbon in the first impregnation cycle but turns out to be disadvantageous in the second cycle; a similar trend is observed for variation of the metal salt concentration.

Iron oxide nanoparticles supported on mesoporous MgO and CeO2: A comparative physicochemical and catalytic study

T. Tsoncheva, J. Roggenbuck, M. Tiemann, L. Ivanova, D. Paneva, I. Mitov, C. Minchev, Microporous and Mesoporous Materials (2008), pp. 339-346

Iron (III) containing nanoparticles with superparamagnetic behaviour are prepared via deposition on various mesoporous supports (MgO, CeO2 and SBA-15). XRD, TEM-EDX, N2 physisorption, FTIR, and Moessbauer spectroscopy are used for their characterization. The reductive properties and catalytic behaviour in methanol decomposition of the materials are also studied. Depending on the chemical nature of the support, the predominant formation of: isolated iron species, strongly interacting with the support (for SBA-15), mixture of hematite and binary MgFe2O4 nanoparticles (for MgO) or almost homogeneously dispersed hematite particles (for CeO2) are observed. The state of iron species strongly affects their catalytic properties. The favorable effect of the support mesoporosity on the catalytic activity is most pronounced for the iron modified CeO2.

New mesoporous metal oxides as gas sensors

T. Waitz, T. Wagner, C. Kohl, M. Tiemann, in: Zeolites and related materials: Trends, targets and challenges, Proceedings of the 4th International FEZA Conference, 2008

Nanoporous semiconducting metal oxide materials (In2O3, WO3) with uniform pore systems, large specific surface areas (80 m2 g−1), and pore-wall crystallinity were prepared by structure replication, using KIT-6 silica as a template. The products show improved properties as gas sensors (for methane or butanone) as compared to non-porous samples. In addition, porous WO3 samples with comparable porosity (prepared by conventional ‘soft templating’, using pluronic P123), which are amorphous on the atomic length scale show poorer gas sensitivity, indicating that crystallinity is a crucial factor in the sensing process.

Ripening Effects in ZnS Nanoparticle Growth

M. Tiemann, F. Marlow, J. Hartikainen, Weiss, M. Lindén, The Journal of Physical Chemistry C (2008), pp. 1463-1467

The growth of ZnS nanoparticles during precipitation from aqueous solution is studied by in situ stopped-flow UV absorption spectroscopy at temperatures between 283 and 323 K. Particle growth is marked by substantial ripening during the first 60 ms. Kinetic data suggest that a ripening mechanism by coalescence (oriented attachment) is predominant over Ostwald ripening, although the latter seems to coexist to a significant degree at early temporal stages, predominantly at low temperatures.

Repeated Templating

M. Tiemann, Chemistry of Materials (2008), pp. 961-971

In recent years, a lot of research activity has focused on the synthesis of new ordered porous materials by utilization of porous matrices as templates. Since the matrices are themselves created by templating procedures, the entire process can be envisaged as “repeated templating”. This review describes recent conceptual developments in the field of structure replication and summarizes the large number of publications on new functional materials prepared by this method.

Mesoporous Ceria by Structure Replication from Various Porous Matrices

J. ROGGENBUCK, M. Tiemann, in: Nanoporous Materials, 2008

Mesoporous ceria was synthesized by using both CMK-3 carbon and SBA-15 silica as structure matrices. All products exhibit uniform mesopores with diameters of 5-6 nm in a two-dimensional periodic arrangement in addition to varying amounts of interparticle porosity. The gas sensing properties (methane detection) of ordered mesoporous ceria was compared to a non-porous sample.


Mesoporous CeO2: Synthesis by nanocasting, characterisation and catalytic properties

J. Roggenbuck, H. Schäfer, T. Tsoncheva, C. Minchev, J. Hanss, M. Tiemann, Microporous and Mesoporous Materials (2007), pp. 335-341

Mesoporous CeO2 was synthesised by using CMK-3 carbon as a structure matrix. Nitrogen physisorption, powder X-ray diffraction, transmission electron microscopy (TEM), selected-area electron diffraction (SAED), energy-dispersive X-ray (EDX), X-ray absorption near-edge structure (XANES), and thermal (TG/MS) analysis were used for their characterisation. Methanol decomposition to hydrogen, CO, and methane was used as a catalytic test reaction. The obtained products exhibit uniform pores with a diameter of ca. 5 nm in a two-dimensional hexagonal periodic arrangement, as well as interparticle porosity, broadly distributed around ca. 35 nm; the specific surface area is 148 m2 g−1. The pore walls are polycrystalline. The polycrystalline nature and high surface-to-volume ratio of the products is reflected in an increased signal intensity in X-ray absorption spectroscopy. The synthesis of CeO2 from Ce(NO3)3 within the pores of the carbon matrix and the subsequent thermal combustion of the carbon is monitored by thermal analysis. Catalytic tests reveal that the activity of the mesoporous products in methanol decomposition are substantially higher than for a non-porous sample.

Crystalline ZnO with an enhanced surface area obtained by nanocasting

T. Waitz, M. Tiemann, P.J. Klar, J. Sann, J. Stehr, B.K. Meyer, Applied Physics Letters (2007), 123108

The authors report the synthesis of nanoporous ZnO, which exhibits a periodically ordered, uniform pore system with crystalline pore walls. The crystalline structure is investigated by x-ray diffraction, transmission electron microscopy, and selected area electron diffraction. The large specific surface area and the uniformity of the pore system are confirmed by nitrogen physisorption. Raman spectroscopy along with low-temperature photoluminescence measurements confirms the high degree of crystallinity and gives insight into defects participating in the radiative recombination processes. The authors thank Günter Koch for recording the TEM images and Marie-Luise Wolff for valuable help in the laboratory one of the authors (M.T.) thanks Michael Fröba for the continuous support.

In-situ X-ray diffraction study on the formation of a periodic mesoporous organosilica material

M. Tiemann, C.V. Teixeira, M. Cornelius, J. Morell, H. Amenitsch, M. Lindén, M. Fröba, in: Recent Progress in Mesostructured Materials - Proceedings of the 5th International Mesostructured Materials Symposium (IMMS2006), Shanghai, P.R. China, August 5-7, 2006, 2007

In-situ X-ray diffraction study on the formation of a periodic mesoporous organosilica material...

Ordered mesoporous ZnO for gas sensing

T. Wagner, T. Waitz, J. Roggenbuck, M. Fröba, C. Kohl, M. Tiemann, Thin Solid Films (2007), pp. 8360-8363

We report on the synthesis and the gas-sensing properties (CO and NO2 detection) of mesoporous zinc oxide. A two-step structure replication method for the synthesis is employed. In the first step mesoporous SBA-15 silica is prepared by the utilization of self-organization of amphiphilic organic agents. This mesoporous silica is used as the structure matrix for synthesizing mesoporous carbon CMK-3, which, in turn, is employed for yet another replication step, using zinc nitrate as the precursor. The resulting material is characterized by X-ray diffraction and nitrogen physisorption and its gas-sensing properties are compared with a non-porous ZnO sample.

Porous Metal Oxides as Gas Sensors

M. Tiemann, Chemistry - A European Journal (2007), pp. 8376-8388

Semiconducting metal oxides are frequently used as gas-sensing materials. Apart from large surface-to-volume ratios, well-defined and uniform pore structures are particularly desired for improved sensing performance. This article addresses the role of some key structural aspects in porous gas sensors, such as grain size and agglomeration, pore size or crack-free film morphology. New synthesis concepts, for example, the utilisation of rigid matrices for structure replication, allow to control these parameters independently, providing the opportunity to create self-diagnostic sensors with enhanced sensitivity and reproducible selectivity.

Gas-sensing properties of ordered mesoporous Co3O4 synthesized by replication of SBA-15 silica

T. Wagner, J. Roggenbuck, C. Kohl, M. Fröba, M. Tiemann, in: Recent Progress in Mesostructured Materials - Proceedings of the 5th International Mesostructured Materials Symposium (IMMS2006), Shanghai, P.R. China, August 5-7, 2006, 2007

Gas-sensing properties of ordered mesoporous Co3O4 synthesized by replication of SBA-15 silica...


Early Stages of ZnS Growth Studied by Stopped-Flow UV Absorption Spectroscopy:  Effects of Educt Concentrations on the Nanoparticle Formation

M. Tiemann, F. Marlow, F. Brieler, M. Lindén, The Journal of Physical Chemistry B (2006), pp. 23142-23147

The growth of ZnS nanoparticles by precipitation from supersaturated aqueous solution is studied by stopped-flow UV absorption spectroscopy. The average size, size distribution, and concentration of the particles are monitored within the sub-second time regime with a resolution of 1.28 ms. Particle growth at these early stages is governed by pronounced ripening. The UV absorption data strongly suggest that growth occurs by preferential adsorption of HS- anions relative to Zn2+ or ZnOH+ cations. Correspondingly, the initial sulfide concentration has a much more pronounced influence on the growth kinetics than the initial zinc concentration. These findings are verified by ζ-potential measurements which confirm that the particle surfaces are negatively charged under near-neutral pH conditions.

Gas Sensing Properties of Ordered Mesoporous SnO2

T. Wagner, C. Kohl, M. Fröba, M. Tiemann, Sensors (2006), pp. 318-323

We report on the synthesis and CO gas-sensing properties of mesoporous tin(IV) oxides (SnO2). For the synthesis cetyltrimethylammonium bromide (CTABr) was used as a structure-directing agent; the resulting SnO2 powders were applied as films to commercially available sensor substrates by drop coating. Nitrogen physisorption shows specific surface areas up to 160 m2·g-1 and mean pore diameters of about 4 nm, as verified by TEM. The film conductance was measured in dependence on the CO concentration in humid synthetic air at a constant temperature of 300 °C. The sensors show a high sensitivity at low CO concentrations and turn out to be largely insensitive towards changes in the relative humidity. We compare the materials with commercially available SnO2-based sensors.

Synthesis of Mesoporous Magnesium Oxide by CMK-3 Carbon Structure Replication

J. Roggenbuck, G. Koch, M. Tiemann, Chemistry of Materials (2006), pp. 4151-4156

Periodically ordered mesoporous magnesium oxide was synthesized in a double replication procedure. Mesoporous SBA-15 silica and CMK-3 carbon were successively used as hard structure matrixes. The carbon pore system was infiltrated with Mg(NO3)2, which was then converted to MgO at 573 K; the carbon matrix was finally removed by thermal combustion. The structure of the mesoporous MgO corresponds to that of the original SBA-15 silica. The products consist of hexagonally arranged cylindrical mesopores and crystalline pore walls. The efficiency of the replication series was studied by variation of the infiltration method and comprehensive pore size analysis of all involved mesoporous materials. The in situ formation of MgO inside the CMK-3 carbon pore system was monitored by thermal analysis. Postsynthetic treatment of the products at 823 K in a vacuum prior to removal of the carbon matrix was found to improve the crystallinity but to diminish the periodic order of the pore system.


Early Stages of ZnS Nanoparticle Growth Studied by In-Situ Stopped-Flow UV Absorption Spectroscopy

M. Tiemann, Weiß, J. Hartikainen, F. Marlow, M. Lindén, ChemPhysChem (2005), pp. 2113-2119

The early stages of ZnS nanoparticle growth from supersaturated solution are investigated in situ by stopped-flow UV absorption spectroscopy with a time resolution of 1.28 ms. A model for data analysis is suggested which makes it possible to study both the average particle radius and the concentration. The average radii lie in the sub-nanometer range. During the first 40 ms, growth is predominantly governed by ripening.

Ordered Mesoporous Magnesium Oxide with High Thermal Stability Synthesized by Exotemplating Using CMK-3 Carbon

J. Roggenbuck, M. Tiemann, Journal of the American Chemical Society (2005), pp. 1096-1097

Periodically ordered mesoporous magnesium oxides were synthesized by utilization of mesoporous CMK-3 carbon as exotemplate. The products exhibit high thermal stability and basic properties, which makes them promising for application in heterogeneous basic catalysis.


In situ Synchrotron SAXS/XRD Study on the Formation of Ordered Mesoscopic Hybrid Materials with Crystal-Like Walls

J. Morell, C.V. Teixeira, M. Cornelius, V. Rebbin, M. Tiemann, H. Amenitsch, M. Fröba, M. Lindén, Chemistry of Materials (2004), pp. 5564-5566

In situ Synchrotron SAXS/XRD Study on the Formation of Ordered Mesoscopic Hybrid Materials with Crystal-Like Walls...


Recent Advances in the Synthesis of Mesostructured Aluminum Phosphates

M. Tiemann, M. Fröba, in: Host‐Guest‐Systems Based on Nanoporous Crystals, 2003

Recent Advances in the Synthesis of Mesostructured Aluminium Phosphates...


In-Situ SAXS Studies on the Formation of Silicate/Surfactant Mesophases with Solubilized Benzene under Acidic Conditions

M. Tiemann, V. Goletto, R. Blum, F. Babonneau, H. Amenitsch, M. Lindén, Langmuir (2002), pp. 10053-10057

The formation of mesoscopically ordered silica/surfactant composites under acidic synthesis conditions is studied by time-resolved in-situ small-angle X-ray scattering (SAXS) using synchrotron radiation. Benzene is used a an additive which acts as a weak swelling agent although most of the benzene molecules are found to reside near the surfactant/silicate interface region rather than in the micelle cores. With increasing relative amounts of benzene, the curvature of the micellar aggregates decreases, which finally leads to a transition from (hexagonally) rodlike to lamellar; a mechanism for this change in curvature is suggested.

Mesoporous aluminophosphates from a single-source precursor

M. Tiemann, M. Fröba, Chemical Communications (2002), pp. 406-407

Mesoporous aluminophosphates with a strict ratio of Al∶P = 1∶1 have been synthesised from a single-source molecular precursor.

Phase Behaviour of the System Triethanolamine-Decanoic Acid-Water

S. Backlund, R. Friman, S. Karlsson, M. Tiemann, Tenside Surfactants Detergents (2002), 39(5), pp. 136-140

Phase Behaviour of the System Triethanolamine-Decanoic Acid-Water...


Mesoporous Aluminophosphate Molecular Sieves Synthesized under Nonaqueous Conditions

M. Tiemann, M. Schulz, C. Jäger, M. Fröba, Chemistry of Materials (2001), pp. 2885-2891

Long-chain n-alkylamine surfactants have been used as structure-directing agents in the synthesis of mesoporous aluminophosphates by a highly cooperative formation mechanism in an alcoholic system. Small amounts of water in the synthesis mixture play a significant role in the hydrolysis of the aluminum precursor (Al[OiPr]3) and are important for the quality of the mesostructured products. The materials exhibit disordered mesostructures, the stability and structural order of which can be improved by a postsynthetic thermal treatment. The products are then stable enough for the removal of the surfactant molecules by acidic solvent extraction, yielding surface areas up to 690 m2/g.

Mesostructured Aluminophosphates Synthesized with Supramolecular Structure Directors

M. Tiemann, M. Fröba, Chemistry of Materials (2001), pp. 3211-3217

Over the past few years a growing number of scientists have dedicated their attention to the synthesis of mesostructured aluminophosphates following the concept of supramolecular structure direction. Their research has produced a large variety of synthetic approaches and yielded materials with various structural properties. Today a substantial number of publications have accumulated; this short review provides a general summary of the literature with the aim to display the opportunities for future work.


Nonaqueous Synthesis of Mesostructured Aluminophosphate/Surfactant Composites:  Synthesis, Characterization, and In-Situ SAXS Studies

M. Tiemann, M. Fröba, G. Rapp, S.S. Funari, Chemistry of Materials (2000), pp. 1342-1348

Mesostructured aluminophosphate/dodecyl phosphate composite materials were synthesized under aqueous and alcoholic conditions. The syntheses were monitored by temperature- and time-resolved in-situ small-angle X-ray scattering (SAXS). In the aqueous synthesis, a lamellar mesostructure is formed within the first few minutes of the reaction; this structure maintains a constant d spacing independent of the reaction time and temperature. The alcoholic synthesis at low temperature yields a mixture of a lamellar and a supposedly inverted hexagonal mesostructure. SAXS investigations show that these two phases evolve competitively. The lamellar structure is favored by higher temperatures and/or longer synthesis times; above ∼70 °C it is formed exclusively. Mixtures of both phases can be isolated as solid materials, but thermal analysis shows that the inverted hexagonal product transforms into the lamellar phase at ∼35−43 °C. The alcoholic synthesis is a highly cooperative reaction; the pure surfactant/alcohol systems are not lyotropic as long as the inorganic reactants are absent. In comparison, the surfactant/water system with the same surfactant concentrations as employed for the aqueous syntheses is lyotropic with a lamellar structure.

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