Torben Dankwort

Cubic mesoporous Pd–WO3 loaded graphitic carbon nitride (g-CN) nanohybrids: highly sensitive and temperature dependent VOC sensors

The urgent need for real-time monitoring of toxic/hazardous gases in the immediate indoor environment has attracted much attention owing to the recent advancements in the development of ultra-efficient gas sensors with increased accuracy and portability at around room temperature. In this work, we report on a high performance volatile organic compound (VOC) sensor using a Pd–WO3 loaded ordered mesoporous graphitic carbon nitride (g-CN)-based nanohybrid prepared via a nanocasting strategy on a hard 3D porous silica (KIT-6) template. The nanocasted Pd–WO3/g-CN sensor exhibits highly selective temperature dependent trace detection of important VOCs (formaldehyde, toluene, acetone and ethanol), which are commonly present in the indoor climate. The 3D cubic ordered mesoporous structure of the 2D layered g-CN in the hybrid nanodevice is very advantageous towards improving its sensing response with enhanced linearity, swift response/recovery time, selectivity, reversibility, stability with respect to various VOCs (at their respective optimum temperature) and reusability. The proposed functional hybrid nanomaterial-based sensing strategy offers an effective design for highly sensitive and efficient VOC detection devices, which can operate well at low temperatures also.

Martensite adaption through epitaxial nano transition layers in TiNiCu shape memory alloys

Titanium-rich TiNiCu shape memory thin films with ultralow fatigue have been analysed for their structural features by transmission electron microscopy. The stabilization of austenite (B2) and orthorhombic martensite (B19) variants epitaxially connected to Ti2Cu-type precipitates has been observed and found responsible for the supreme mechanical cycling capability of these compounds. Comprehensive ex situ and in situ cooling/heating experiments have demonstrated the presence of an austenitic nanoscale region in between B19 and Ti2Cu, in which the structure shows a gradual transition from B19 to B2 which is then coupled to the Ti2Cu precipitate. It is proposed that this residual and epitaxial austenite acts as a template for the temperature-induced B2↔B19 phase transition and is also responsible for the high repeatability of the stress-induced transformation. This scenario poses an antithesis to residual martensite found in common high-fatigue shape memory alloys.

Effect of crystallographic compatibility and grain size on the functional fatigue of sputtered TiNiCuCo thin films.

The positive influence of crystallographic compatibility on the thermal transformation stability has been already investigated extensively in the literature. However, its influence on the stability of the shape memory effect or superelasticity used in actual applications is still unresolved. In this investigation sputtered films of a highly compatible TiNiCuCo composition with a transformation matrix middle eigenvalue of 1±0.01 are exposed to thermal as well as to superelastic cycling. In agreement with previous results the thermal transformation of this alloy is with a temperature shift of less than 0.1 K for 40 cycles very stable; on the other hand, superelastic degradation behaviour was found to depend strongly on heat treatment parameters. To reveal the transformation dissimilarities between the differently heat-treated samples, the microstructure has been analysed by transmission electron microscopy, in situ stress polarization microscopy and synchrotron analysis. It is found that good crystallographic stability is not a sufficient criterion to avoid defect generation which guarantees high superelastic stability. For the investigated alloy, a small grain size was identified as the determining factor which increases the yield strength of the composition and decreases the functional degradation during superelastic cycling. This article is part of the themed issue ‘Taking the temperature of phase transitions in cool materials’.

Structural properties of the thermoelectric material CuCrS2 and of deintercalated CuxCrS2 on different length scales: X-ray diffraction, pair distribution function and transmission electron microscopy studies

We report on the structural alterations of the thermoelectric material CuCrS2 introduced by the removal of 1/3 of the Cu+ ions which are located between CrS2 layers. X-ray diffraction (XRD) and pair distribution function (PDF) analyses revealed a newly formed Cu0.66CrS2 phase with monoclinic symmetry and a 3a superstructure. Simultaneously, a distortion of CrS6 octahedra is observed strongly indicating the oxidation of Cr3+ → Cr4+ leading to a Jahn–Teller distortion. The structural features extracted from XRD indicate a pronounced disorder in the cationic sub-lattice at moderate temperatures (400 K). Transmission electron microscopy (TEM) examination elucidates the formation of a second Cu0.66CrS2 phase without the superstructure, caused by incipient Cu+ mobility upon beam irradiation. The synergetic combination of high temperature XRD and TEM investigations unveiled the complete mechanism of the phase transition occurring at 503 K, where a transformation into the spinel CuCr2S4 and stoichiometric CuCrS2 occurs.

Influence of mechanochemical syntheses and compacting methods on the thermoelectric properties of nanostructured AgSnmSbTe2+m (TAST-m)

Tin telluride doped with silver antimony telluride (Tin-Antimony-Silver-Tellurium, TAST-m) was prepared by two mechanochemical methods: mechanical alloying and—as an alternative synthetic route—co-ball-milling, a recently developed way of combining the procedures of mechanical alloying of single elements with the usual ball-milling of polycrystalline compounds. For the two series of products with varying degrees of doping, three different compacting techniques, cold-pressing/annealing (CPA), hot-pressing (HP) and short-term sintering (STS), were applied, and their influence on the thermoelectric properties was investigated. Although short-term sintering seemed to be the most advantageous method in most cases, the influence of the density of an individual pellet on the thermoelectric transport properties needs to be considered. A structural characterization of the different series of nanopowders obtained by the two milling methods was carried out by x-ray diffraction (XRD) and transmission electron microscopy (TEM) studies. The latter also allowed further insight into the microstructure and revealed a homogeneous distribution of silver and antimony in the tin telluride matrix.

Nanostructure investigation of the layered ternary compound Ni3–x Sn1–y Te2

The structure of Ni3−x Sn1–y Te2 is characterized by layered structural motifs related to an average NiAs/Ni2In-type. Order/disorder phenomena were analyzed via a detailed nanostructure investigation including electron diffraction and high resolution transmission electron microscopy (HRTEM) in conjunction with image simulation. Dependent on the stoichiometry, commensurate and incommensurate satellite reflections with respect to the parent NiAs structure were observed in Fourier transform and electron diffraction pattern as a result of occupational modulation of Te and Sn atoms. For the commensurate case a triplication of the c-lattice parameter is evident as a result of Sn–Te–Te stacking. Further, HRTEM micrographs indicate additional ordering phenomena along the c* direction depending on Ni/vacancy ordering which was rationalized by an alternating filling of van der Waals gaps with Ni. Also morphological defects in bright field images were observed. HRTEM investigations prove that these morphological defects are of structural nature, i.e. they are based on domains shifted relative to each other (antiphase boundaries).

Au–TiO2-Loaded Cubic g-C3N4 Nanohybrids for Photocatalytic and Volatile Organic Amine Sensing Applications

A green and efficient approach for efficient nanohybrid photocatalysts in extending the light response to the visible spectrum is a hot research topic in sustainable energy technologies. In this work, novel Au-TiO2@m-CN nanocomposite was synthesized using hard template of cubic ordered mesoporous KIT-6 via the nanocasting process. The as-prepared Au-TiO2@m-CN nanohybrids exhibit enhanced photocatalytic activities with improved stability and reusability using methyl orange dye. The enhanced photocatalytic performance is a result of the conjugated effect of catalytic active Au and TiO2 nanoparticles supported on highly efficient visible light sensitizer, graphitic carbon nitride (m-CN or g-C3N4), and ordered mesoporous morphology. Besides, the sensing performance of Au-TiO2@m-CN nanohybrids was also tested for the detection of amine gases, wherein a significant response was reported for triethylamine at low operating temperatures. This study reveals a simple and scalable methodology to design and develop next generation of layered mesoporous materials for multifunctional applications.

Underwater Leidenfrost nanochemistry for creation of size-tailored zinc peroxide cancer nanotherapeutics

The dynamic underwater chemistry seen in nature is inspiring for the next generation of eco-friendly nanochemistry. In this context, green synthesis of size-tailored nanoparticles in a facile and scalable manner via a dynamic process is an interesting challenge. Simulating the volcano-induced dynamic chemistry of the deep ocean, here we demonstrate the Leidenfrost dynamic chemistry occurring in an underwater overheated confined zone as a new tool for customized creation of nanoclusters of zinc peroxide. The hydrodynamic nature of the phenomenon ensures eruption of the nanoclusters towards a much colder region, giving rise to growth of monodisperse, size-tailored nanoclusters. Such nanoparticles are investigated in terms of their cytotoxicity on suspension and adherent cells to prove their applicability as cancer nanotherapeutics. Our research can pave the way for employment of the dynamic green nanochemistry in facile, scalable fabrication of size-tailored nanoparticles for biomedical applications.

Effect of preparation procedure and nanostructuring on the thermoelectric properties of the lead telluride-based material system AgPbmBiTe2+ m (BLST-m)

We report on the preparation and thermoelectric properties of the quaternary system AgPbmBiTe2+m (Bismuth-Lead-Silver-Tellurium, BLST-m) that were nanostructured by mechanical alloying. Nanopowders of various compositions were compacted by three different methods: cold pressing/annealing, hot pressing, and short term sintering. The products are compared with respect to microstructure and sample density. The thermoelectric properties were measured: thermal conductivity in the temperature range from 300 K to 800 K and electrical conductivity and Seebeck coefficient between 100 K and 800 K. The compacting method and the composition had a substantial impact on carrier concentration and mobility as well as on the thermoelectric parameters. Room temperature Hall measurements yielded carrier concentrations in the order of 1019 cm−3, slightly increasing with increasing content of the additive silver bismuth telluride to the lead telluride base. ZT values close to the ones of bulk samples were achieved. X-ray diff...

Real Structure and Structural Changes of Functional Tellurides

Nowadays, functional tellurides are widely used as bulkand nanomaterials for thermoelectric power generators and phase-change based applications, e.g. optical data storage. The function and performance of the materials strongly depend on their unique nanostructural properties and their structural evolution upon operation. Both features can be fully characterized in-situ, ex-situ and on a broad range of length scales by combining diverse characterization techniques with transmission electron microscopy. Consequently, essential information about real-structure property relations and fatigue mechanisms can be determined enabling first steps to a knowledge-based tailoring of the materials.