@inbook{25960, abstract = {{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.}}, author = {{Donato, Nicola and Wagner, Thorsten and Tiemann, Michael and Waitz, Thomas and Kohl, Claus-Dieter and Latino, Mariangela and Neri, Giovanni and Spadaro, Donatella and Malagù, Cesare}}, booktitle = {{Lecture Notes in Electrical Engineering}}, issn = {{1876-1100}}, title = {{{NO2 Sensors with Reduced Power Consumption Based on Mesoporous Indium Oxide}}}, doi = {{10.1007/978-1-4614-0935-9_10}}, year = {{2012}}, } @article{25956, abstract = {{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.}}, author = {{Wagner, Thorsten and Kohl, Claus-Dieter and Morandi, Sara and Malagù, Cesare and Donato, Nicola and Latino, Mariangela and Neri, Giovanni and Tiemann, Michael}}, issn = {{0947-6539}}, journal = {{Chemistry - A European Journal}}, pages = {{8216--8223}}, title = {{{Photoreduction of Mesoporous In2O3: Mechanistic Model and Utility in Gas Sensing}}}, doi = {{10.1002/chem.201103905}}, year = {{2012}}, } @article{25957, abstract = {{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.}}, author = {{Wilke, Timm and Haffer, Stefanie and Tiemann, Michael and Waitz, Thomas}}, issn = {{0944-5846}}, journal = {{CHEMKON}}, pages = {{67--72}}, title = {{{Mesoporöse Silica - Moderne Funktionsmaterialien im Chemieunterricht}}}, doi = {{10.1002/ckon.201210170}}, year = {{2012}}, } @article{25955, abstract = {{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.}}, author = {{Haffer, Stefanie and Weinberger, Christian and Tiemann, Michael}}, issn = {{1434-1948}}, journal = {{European Journal of Inorganic Chemistry}}, pages = {{3283--3288}}, title = {{{Mesoporous Al2O3 by Nanocasting: Relationship between Crystallinity and Mesoscopic Order}}}, doi = {{10.1002/ejic.201200131}}, year = {{2012}}, } @article{25965, abstract = {{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.}}, author = {{Chernikov, Alexej and Horst, Swantje and Waitz, Thomas and Tiemann, Michael and Chatterjee, Sangam}}, issn = {{1932-7447}}, journal = {{The Journal of Physical Chemistry C}}, pages = {{1375--1379}}, title = {{{Photoluminescence Properties of Ordered Mesoporous ZnO}}}, doi = {{10.1021/jp104293e}}, year = {{2011}}, } @article{25962, abstract = {{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).}}, author = {{Wagner, T. and Bauer, M. and Sauerwald, T. and Kohl, C.-D. and Tiemann, Michael}}, issn = {{0040-6090}}, journal = {{Thin Solid Films}}, pages = {{909--912}}, title = {{{X-ray absorption near-edge spectroscopy investigation of the oxidation state of Pd species in nanoporous SnO2 gas sensors for methane detection}}}, doi = {{10.1016/j.tsf.2011.04.187}}, year = {{2011}}, } @article{25964, abstract = {{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.}}, author = {{Wagner, Thorsten and Krotzky, Sören and Weiß, Alexander and Sauerwald, Tilman and Kohl, Claus-Dieter and Roggenbuck, Jan and Tiemann, Michael}}, issn = {{1424-8220}}, journal = {{Sensors}}, pages = {{3135--3144}}, title = {{{A High Temperature Capacitive Humidity Sensor Based on Mesoporous Silica}}}, doi = {{10.3390/s110303135}}, year = {{2011}}, } @article{25963, abstract = {{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.}}, author = {{Smått, J.-H. and Lindén, M. and Wagner, T. and Kohl, C.-D. and Tiemann, Michael}}, issn = {{0925-4005}}, journal = {{Sensors and Actuators B: Chemical}}, pages = {{483--488}}, title = {{{Micrometer-sized nanoporous tin dioxide spheres for gas sensing}}}, doi = {{10.1016/j.snb.2010.12.051}}, year = {{2011}}, } @article{25961, abstract = {{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.}}, author = {{Wagner, T. and Hennemann, J. and Kohl, C.-D. and Tiemann, Michael}}, issn = {{0040-6090}}, journal = {{Thin Solid Films}}, pages = {{918--921}}, title = {{{Photocatalytic ozone sensor based on mesoporous indium oxide: Influence of the relative humidity on the sensing performance}}}, doi = {{10.1016/j.tsf.2011.04.181}}, year = {{2011}}, } @article{25971, abstract = {{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.}}, author = {{Schmidt, Wolfgang and Bussian, Patrick and Lindén, Mika and Amenitsch, Heinz and Agren, Patrik and Tiemann, Michael and Schüth, Ferdi}}, issn = {{0002-7863}}, journal = {{Journal of the American Chemical Society}}, pages = {{6822--6826}}, title = {{{Accessing Ultrashort Reaction Times in Particle Formation with SAXS Experiments: ZnS Precipitation on the Microsecond Time Scale}}}, doi = {{10.1021/ja101519z}}, year = {{2010}}, } @article{25967, abstract = {{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.}}, author = {{Waitz, T. and Becker, B. and Wagner, T. and Sauerwald, T. and Kohl, C.-D. and Tiemann, Michael}}, issn = {{0925-4005}}, journal = {{Sensors and Actuators B: Chemical}}, pages = {{788--793}}, title = {{{Ordered nanoporous SnO2 gas sensors with high thermal stability}}}, doi = {{10.1016/j.snb.2010.08.001}}, year = {{2010}}, } @article{25972, abstract = {{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.}}, author = {{Haffer, Stefanie and Waitz, Thomas and Tiemann, Michael}}, issn = {{1932-7447}}, journal = {{The Journal of Physical Chemistry C}}, pages = {{2075--2081}}, title = {{{Mesoporous In2O3 with Regular Morphology by Nanocasting: A Simple Relation between Defined Particle Shape and Growth Mechanism}}}, doi = {{10.1021/jp910336f}}, year = {{2010}}, } @article{25968, abstract = {{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.}}, author = {{Haffer, Stefanie and Tiemann, Michael and Fröba, Michael}}, issn = {{0947-6539}}, journal = {{Chemistry - A European Journal}}, pages = {{10447--10452}}, title = {{{Periodic Mesoporous Organosilica (PMO) Materials with Uniform Spherical Core-Shell Structure}}}, doi = {{10.1002/chem.201000643}}, year = {{2010}}, } @inbook{25973, abstract = {{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...}}, author = {{Tiemann, Michael}}, booktitle = {{Nanostructured Materials}}, editor = {{Korotcenkov, Ghenadii}}, pages = {{291 -- 310}}, publisher = {{Momentum Press}}, title = {{{Ordered Mesoporous Films and Membranes: Synthesis, Properties and Applications in Gas Sensors}}}, volume = {{2}}, year = {{2010}}, } @article{25975, abstract = {{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.}}, author = {{Waitz, Thomas and Wagner, Thorsten and Sauerwald, Tilman and Kohl, Claus-Dieter and Tiemann, Michael}}, issn = {{1616-301X}}, journal = {{Advanced Functional Materials}}, pages = {{653--661}}, title = {{{Ordered Mesoporous In2O3: Synthesis by Structure Replication and Application as a Methane Gas Sensor}}}, doi = {{10.1002/adfm.200801458}}, year = {{2009}}, } @article{25977, abstract = {{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.}}, author = {{Schwalm, Michael and Horst, Swantje and Chernikov, Alexej and Rühle, Wolfgang W. and Lautenschläger, Stephan and Klar, Peter J. and Meyer, Bruno K. and Waitz, Thomas and Tiemann, Michael and Chatterjee, Sangam}}, issn = {{1610-1634}}, journal = {{physica status solidi (c)}}, pages = {{542--545}}, title = {{{Time-resolved photoluminescence study of mesoporous ZnO nanostructures}}}, doi = {{10.1002/pssc.200880315}}, year = {{2009}}, } @article{25974, abstract = {{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.}}, author = {{Wagner, T. and Sauerwald, T. and Kohl, C.-D. and Waitz, T. and Weidmann, C. and Tiemann, Michael}}, issn = {{0040-6090}}, journal = {{Thin Solid Films}}, pages = {{6170--6175}}, title = {{{Gas sensor based on ordered mesoporous In2O3}}}, doi = {{10.1016/j.tsf.2009.04.013}}, year = {{2009}}, } @article{25976, abstract = {{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.}}, author = {{Waitz, Thomas and Tiemann, Michael}}, issn = {{0944-5846}}, journal = {{CHEMKON}}, pages = {{183--186}}, title = {{{Halbleitende Metalloxide als Gassensoren im Chemieunterricht}}}, doi = {{10.1002/ckon.200910099}}, year = {{2009}}, } @article{25980, abstract = {{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.}}, author = {{Tsoncheva, Tanya and Rosenholm, Jessica and Linden, Mika and Kleitz, Freddy and Tiemann, Michael and Ivanova, Ljubomira and Dimitrov, Momtchil and Paneva, Daniela and Mitov, Ivan and Minchev, Christo}}, issn = {{1387-1811}}, journal = {{Microporous and Mesoporous Materials}}, pages = {{327--337}}, title = {{{Critical evaluation of the state of iron oxide nanoparticles on different mesoporous silicas prepared by an impregnation method}}}, doi = {{10.1016/j.micromeso.2007.10.005}}, year = {{2008}}, } @article{25979, abstract = {{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.}}, author = {{Roggenbuck, Jan and Waitz, Thomas and Tiemann, Michael}}, issn = {{1387-1811}}, journal = {{Microporous and Mesoporous Materials}}, pages = {{575--582}}, title = {{{Synthesis of mesoporous metal oxides by structure replication: Strategies of impregnating porous matrices with metal salts}}}, doi = {{10.1016/j.micromeso.2007.12.018}}, year = {{2008}}, }