Topochemical Methods Research Articles

Fabrication and physical properties of Na(Nb1-xSbx)O3 templates for the [001]-texturing of KNN-based ceramics

Na(Nb1-xSbx)O3 [N(N1-xSx)] templates were fabricated to texture the 0.96(Na0.5K0.5)(Nb1-ySby)O3-0.01BaZrO3-0.03(Bi0.5Ag0.5)ZrO3 [KN(N1-ySy)-BZ-BAZ] piezoceramics along the [001] direction. To the best of our knowledge, this is the first study that fabricated these templates. Thus far, NaNbO3 (NN) templates have been utilized to texture KNN-modified piezoceramics; however, they produce many holes in the piezoceramics because of the dissolution of the NN templates during sintering. N(N0.9S0.1) templates with an average size of ∼18 μm were produced by the molten-salt and topochemical methods utilizing Bi2.5Na3.5(Nb0.9Sb0.1)5O18 precursors. Due to their high melting temperature, the N(N0.9S0.1) templates did not dissolve during sintering. Therefore, no holes were present in the KN(N0.95S0.05)-BZ-BAZ piezoceramic textured using the N(N0.9S0.1) templates. The piezoceramic textured using the N(N0.9S0.1) templates showed a large hardness (7.56 ± 0.26 GPa) and an elastic modulus (154 ± 5 GPa) similar to those of the untextured piezoceramic. However, the piezoceramic textured using the NN templates showed reduced hardness (5.94 ± 0.58 GPa) and elastic modulus (127 ± 14 GPa) because of the presence of the holes. The KN(N0.95S0.05)-BZ-BAZ piezoceramics textured using the N(N0.9S0.1) templates provided large d33 (740 pC/N) and kp (0.62) values. Cantilever-type piezoelectric energy harvesters (CPEHs) were produced using the KN(N0.95S0.05)-BZ-BAZ piezoceramic textured with N(N1-xSx) templates and the CPEH produced using the piezoceramic textured by N(N0.9S0.1) templates generated the largest power of 2.92 mW. Therefore, N(N0.9S0.1) is a promising template for the [001] texturing of KNN-based piezoceramics with good mechanical properties.

Topochemical polymerization in microparticles of crystalline triazine-based monomers: Study by conventional and ultra-fast chip calorimetry

The work addresses complex thermal behavior of two triazine-based azide-alkyne monomers: 2-azido-4,6-bis(prop‑2-yn-1-yloxy)-1,3,5-triazine (ABPOT) and 2,4-diazido-6-(prop‑2-yn-1-yloxy)-1,3,5-triazine (DAPOT). Despite the similarity of the two AB2-type structures, the isothermal aging kinetics of DAPOT occurs significantly faster than that of ABPOT. During storage at room temperature, the chemical transformation starts in the crystalline state via topochemical reactions. Calorimetry, along with X-ray diffraction, makes it possible to probe transformations in isolated microcrystals including melting, oligomerization, polymerization and decomposition. The thermal behavior of microparticles can strongly differ from that of the same material in bulk due to enhanced contribution of the monomer evaporation at the particle surface at extremely high heating rates. The isothermal melting caused by the topochemical reaction and subsequent polymerization during long-term storage can be modeled using the correlated two-stage Avrami equation. The Avrami parameter corresponding to the amorphization process is close to unity while that of the polymerization is approximately 0.5. Consequently, both processes can likely be viewed as one-dimensional, with diffusion control dominating the polymerization process. The observed impact of long-term storage on the polymerization capability of the monomers may prove beneficial in synthesizing hyperbranched polymers through "green" topochemical methods.

Factors Effecting and Structural Engineering of Molybdenum Nitride‐Based Electrocatalyst for Overall Water Splitting: A Critical Review

The advancement of spotless hydrogen energy generation is being accelerated by the advancement in nonprecious electrocatalysts for effective hydrogen evolution reaction (HER), oxygen evolution reaction (OER), and overall water splitting. In this review, the main focus is given to the most significant innovations in the surface topography, morphological shape, size distribution, and electrocatalytic functionality of highly engineered molybdenum nitride catalysts, equal to or even better than state‐of‐the‐art electrocatalyst for HER and OER. Different morphological or interfacial engineering techniques, such as in situ electrospinning, topochemical methods, and classic chemical methods for synthesizing nonprecious molybdenum nitride electrocatalysts, are extensively covered. Molybdenum nitrides are discovered the excellent electrocatalyst, and the factors that can improve its electrocatalytic properties, such as morphological tuning, controlled geometrical composition, pH effect, hybrid and electronic structures, the effect of temperature, and effects of active sites, are comprehensively discussed.

Metastable Kitaev Magnets.

Nearly two decades ago, Alexei Kitaev proposed a model for spin- particles with bond-directional interactions on a two-dimensional honeycomb lattice which had the potential to host a quantum spin-liquid ground state. This work initiated numerous investigations to design and synthesize materials that would physically realize the Kitaev Hamiltonian. The first generation of such materials, such as NaIrO, -LiIrO, and -RuCl, revealed the presence of non-Kitaev interactions such as the Heisenberg and off-diagonal exchange. Both physical pressure and chemical doping were used to tune the relative strength of the Kitaev and competing interactions; however, little progress was made towards achieving a purely Kitaev system. Here, we review the recent breakthrough in modifying Kitaev magnets via topochemical methods that has led to the second generation of Kitaev materials. We show how structural modifications due to the topotactic exchange reactions can alter the magnetic interactions in favor of a quantum spin-liquid phase.

Fluorination and reduction of CaCrO3 by topochemical methods.

Topochemical reactions between CaCrO3 and polyvinylidene difluoride yield the new fluorinated phase CaCrO2.5F0.5, which was characterized by powder synchrotron X-ray diffraction, X-ray photoemission spectroscopy, and magnetic susceptibility measurements. The reaction proceeds via reduced oxide intermediates, CaCrO2.67 and CaCrO2.5, in which CrO6 octahedral and CrO4 tetrahedral layers are stacked in a different manner along the c axis of CaCrO3. These two intermediate phases can be selectively synthesized by the carbothermal reduction with g-C3N4. Both CaCrO3 and CaCrO2.5F0.5 adopt the same orthorhombic space group, Pbnm; however, the fluorinated phase has decreased Cr-O-Cr bond angles as compared to the parent compound in both the ab plane and along the c-direction, which indicates an increased orthorhombic distortion due to the fluorination. While the oxygen vacancies are ordered in both intermediate phases, CaCrO2.67 and CaCrO2.5, a site preference for fluorine in the oxyfluoride phase cannot be confirmed. CaCrO3 and CaCrO2.5F0.5 undergo antiferromagnetic phase transitions involving spin canting, where the fluorination causes the transition temperature to increase from 90 K to 110 K, as a result of the competition between the increased octahedral tilting and the enhancement of superexchange interactions involving Cr3+ ions in the CaCrO2.5F0.5 structure.

Unconventional Conjugated Polymers Derived from a Common Set of trans-Enediyne Monomers

This account describes our recent efforts in the design and synthesis of several series of unconventional conjugated polymers derived from a common set of trans-enediyne (tEDY) monomers. The journey started with a failed attempt, through acyclic diene metathesis of triene monomers, to prepare soluble polyacetylenes (PAs) having cross-conjugated side-groups on alternate double bonds along the main chain. At this seemingly dead end of the project, we found that the tEDY intermediates leading to triene monomers could undergo alkyne metathesis to generate soluble polydiacetylenes (PDAs). Such acyclic enediyne metathesis represents the first example of a solution synthesis of PDAs, in contrast to the conventional topochemical methods. By applying Glaser–Hay-type reaction conditions with selected tEDY monomers, polytriacetylenes were smoothly obtained; these possessed aromatic units directly attached to the polymer main chains, which significantly affected the electronic properties of the polymers. Furthermore, through hydroboration of the tEDY monomers, ‘boron-doped’ polyacetylenes (BDPAs) were prepared that can be considered as PAs with every fourth double bond replaced with a boron atom. These BDPAs represent the first boron main-chain conjugated polymers lacking aromatic units along the backbone, potentially enhancing electronic delocalization along the main chain.

Improved Topotactic Reactions for Maximizing Organic Coverage of Methyl Germanane

The topotactic transformation of Zintl phases such as CaGe2 into organic-terminated germanium graphane analogues using haloalkanes is a powerful route for generating new 2D optoelectronic and spintronic building blocks. However, uniform ligand coverage is necessary for optimizing the properties and stability of these single-atom-thick frameworks. Here, we compare the effectiveness of different topochemical methods to maximize methyl-termination in GeCH3. We show that a previously developed CH3I/H2O phase transfer route produces a small percentage of partially oxidized germanane. The partially oxidized termination is readily removed upon HCl treatment, which leads to Ge–Cl termination, but rapidly reoxidizes after exposure to the ambient atmosphere. We then show that a one-pot route with CH3I in distilled CH3CN solvent and at least six equivalents of H2O results in no oxidation. The GeCH3 prepared from this one-pot route also has an increased −CH3/–H ratio of termination from ∼90:10 to ∼95:5, is air-stable, has greater thermal stability, has a sharper absorption onset, and has more narrow band edge photoluminescence, all of which are signatures of a less defective semiconductor.

Polarization Microscopy and Topochemistry of the Cocoon ofLithurgus chrysurus(Hymenoptera: Megachilidae)

AbstractThe cocoon of Lithurgus chrysurus Fonscolobe, a univoltine species belonging to the megachilid subfamily Lithurginae, was studied using polarization microscopy and topochemical methods. The aim of the study was to establish the composition and structure of the cocoon wall of this bee in comparison with reported data for other species of this subfamily. The cocoon was found to be composed by macromolecularly oriented, positively birefringent silk protein layers that acquire a complex and thicker multilayered distribution at its rear zone. Although the composition and distribution of the silk threads in this species' cocoon was considered similar to the previously described patterns for other lithurgine bees, the absence of a mucous layer, typically produced by Malpighian tubules, differed from reported data for other species of the same subfamily.

Chemical characterization of environmental and industrial particulate samples†

The characterization of particles, especially aerosol particles, is of great importance to many scientific fields. A relevant brief overview is given. A rigorous scheme of sampling and in-depth characterization of particulate samples has been developed in the authors’ laboratories and by collaborative groups, including investigations by the following techniques: total reflection X-ray fluorescence spectrometry for quantitative bulk characterization; solid-state speciation by valence band X-ray spectrometry using electron microprobe or Mossbauer spectrometry (only bulk particle characterization possible); scanning electron microscopy and electron probe microanalysis for automatic semiquantitative single particle characterization of particles ≥0.5 µm; transmission electron microscopy for semiquantitative single particle characterization of particles with diameters down to 10 nm; high resolution-scanning electron microscopy, which is also capable of characterizing particles morphologically and qualitatively down to 10 nm in diameter; secondary ion mass spectrometry for the study of trace elemental distributions and isotopic ratios in particles with diameters above 1 µm. It is the aim of this paper to show the advantages and characteristics of this scheme of analysis to match today′s requirements for topochemical methods of analysis. For this purpose a short overview of these methods for particle characterization is also presented.

Formation of nuclear microarchitecture in the preimplantation bovine embryo at the onset of transcription: Implications for biotechnology

Fine-structural and topochemical methods have revealed a highly ordered sequence of nuclear events in blastomeres of the early bovine embryo at the time when the embryo gains the capacity for the transcription of RNA. At the 8-cell stage, the DNA, in earlier stages localized at the periphery of the nucleus, is equally distributed throughout the nucleoplasm. At the same time, both nucleolar and extranucleolar RNA synthesis is resumed. The time sequence of this process is reflected by the ultrastructural morphology of nucleologenesis, during which the nucleolus precursor body (NPB) is transformed into a functional nucleolus as a result of the stepwise penetration by DNA and by changes in the extranucleolar chromatin. The most remarkable feature of differentiation is the chromatin organization in distinct blocks on the borderline of which the synthesis and maturation of extranucleolar RNA takes place. This is demonstrated by the localization of perichromatin fibrils and by detection of a class of small nuclear ribonucleoproteins (snRNPs) known to be involved in RNA splicing in that specific region. The conspicuous morphological characteristics of bovine embryonic nucleologenesis can be used as landmarks for the evaluation of the degree of nuclear differentiation under various experimental conditions. In embryos derived from different biotechnological approaches deviations from the normal pattern of differentiation in the nuclear microarchitecture were detected: (I) The condensed chromatin in 8-cell stages formed more prominent blocks in nuclei from embryos obtained by in vitro procedures than in those grown in vivo. (II) In embryos produced in vitro from oocytes isolated from different size categories of antral follicles the degree of extranucleolar chromatin condensation often did not reach that of ‘normal’ embryos. This finding was connected with a less intense RNA synthesis as detected by autoradiography. The NPB development was altered since its association with the perinuclear chromatin was less developed. (III) In embryos reconstructed after fusion of a blastomere with an enucleated oocyte, the development of the NPB structure as well as the RNA synthesis were arrested for three consecutive cleavages. (IV) The morphological features and in particular the autoradiographic data can be interpreted as the visualization of a consecutive switch-on of genes probably inversely related to the totipotency of a given nucleus and can thus be helpful in the assessment of nuclear transfer efficiency. Collectively, these findings suggest that embryos resulting from various biotechnological procedures can display distinct ultrastructural deviations at the level of nuclear function that could contribute to their lower developmental capacity.

Composition and structure of the froth of the spittlebug, Deois SP.

The nature of the froth which surrounds spittlebug nymphs during development was studied with topochemical methods, light and electron microscopies, and gel electrophoresis. The composition and structure of the froth were found to be complex, at least ten polypeptides (most of them glycopeptides), acid proteoglycans and calcium being detected. Sinuous filamentary elements of a possibly structural glycoprotein nature were found ultrastructurally. The distribution of an acid proteoglycan-glycoprotein complex at the ultrastructural level resembles that of vertebrate extracellular matrices and of some vertebrate and invertebrate muci reported in the literature. The stereo-arrangement of the proteins in the froth endows it with anisotropic properties, such as birefringence and linear dichroism. The froth of spittlebugs in its whole structure and composition is not properly comparable to salivary and Malpighian tubule silks of other insect groups. This certainly provides it with special properties adequate to the performance of its functions.

ChemInform Abstract: PREPARATIVE SOLID STATE CHEMISTRY. RECENT DEVELOPMENTS

A survey of recent developments in preparative solid state chemistry shows that, with a knowledge of structural chemistry and reactivity patterns of solids, it is possible to synthesize a variety of new solids possessing novel structures. A distinction is made between synthesis ofnew solids and synthesis of solids bynew methods. Three new routes to solid state synthesis are recognized: the precursor method, and topochemical methods involving redox and ion-exchange reactions. The low-temperature topochemical methods enable synthesis of metastable phases that are inaccessible by the high temperature route. Several illustrative examples of solid state synthesis from the recent literature are presented.

Preparative solid state chemistry. Recent developments

A survey of recent developments in preparative solid state chemistry shows that, with a knowledge of structural chemistry and reactivity patterns of solids, it is possible to synthesize a variety of new solids possessing novel structures. A distinction is made between synthesis ofnew solids and synthesis of solids bynew methods. Three new routes to solid state synthesis are recognized: the precursor method, and topochemical methods involving redox and ion-exchange reactions. The low-temperature topochemical methods enable synthesis of metastable phases that are inaccessible by the high temperature route. Several illustrative examples of solid state synthesis from the recent literature are presented.

Quantitative surface analysis of coated hard metals with SIMS

In the case of the development of highly wear resistant coatings on hard metals the application of powerful topochemical methods was mandatory to receive quantitative information on the distribution of boron in Al-B-mixed oxide layers and of oxygen in Ti(C,N,O)-layers. These elements are contained in layers of a few μm thickness in concentrations of 0.0X–X%. After evaluation of different surface techniques the capabilities of SIMS using the ion microprobe CAMECA IMS 3f for this problem were investigated systematically. The result was that quantitative distribution analysis of B in Al-B-mixed oxides using depth profiling and O in Ti(C,N,O) using microanalysis with step scanning over angle lapped specimens can be performed with an accuracy of about ± 25 rel.%. Spatial resolution is a few nm for depth profiles and several μm for step-scanning profiles (corresponding to about 20–50 nm on the depth scale). The high detection power of SIMS for depth profiling and microanalysis makes this method superior to other surface techniques. Additional compound specific information about the structure of such CVD-coatings can be obtained from the molecular ions. The measurements indicate that SIMS is a very powerful technique for compound-specific microanalysis at high lateral resolutions.

Corrosion of nickel in sodium sulphate-sodium chloride melts. II. Potential-time and topochemical studies

The corrosion behaviour of Ni in molten Na2SO4, NaCl, and in mixtures of these salts at 900° C, has been studied by electrochemical and topochemical methods. Microprobe analysis showed deep S penetration which causes embrittlement of the specimens by intergranular attack.