Dual-step reduction of copper and formation mechanism of Cu pseudo-icosahedral microcrystals

W.S. wishes to thank University of St Andrews for a CSC-St Andrews scholarship.Pseudo-icosahedral Cu microcrystals have been synthesized in a solvothermal system containing CuSO4·5H2O as the precursor, polyvinylpyrrolidone (PVP) as a reductant/capping agent, and dimethylformamide as the solvent. The structural and morphological evolutions over the reaction time are investigated, which enable us to establish a novel formation mechanism of pseudo-icosahedral crystals of Cu. The first crystalline phase that appeared in the solution is Cu4SO4(OH)6·H2O in the form of microflakes. The microflakes are reduced and decomposed to Cu2O nanocrystallites, which assemble with PVP into spherulites. The Cu2O crystals are further reduced to Cu nanocrystallites, which aggregate with PVP again into spherical particles. An extraordinary phenomenon is that 20 separated (111) triangular plates form on each particle during surface recrystallization of Cu, and their locations match to the final facets of the pseudo-icosahedron. The plates extend to cover the whole surface of the sphere, forming a pseudo-icosahedral shell, followed by extension of the recrystallization from the surface to the core. This reversed crystal growth process increases the thickness of the plates until all the Cu nanocrystallites in the core are consumed. It is found that, during the surface recrystallization of polycrystalline spheres, the particles preferentially select the (111) planes of the face-centered cubic structure of Cu as the exposed faces because these planes have the minimum surface binding energy. The particles then try to keep as small as possible the specific surface area, and a pseudo-icosahedral shape consisting of 20 (111) plates, which has a specific surface area, about 10% lower than that of an octahedron, formed by eight (111) plates. Accordingly, the overall surface free energy of a pseudo-icosahedron is about 10% lower than that of an octahedron. The formation of tetrahedra as building units of icosahedra, as predicted previously, has not been observed. The formation of ideal icosahedra, the so-called perfect Platonic solid, and the formation of twin defects between neighboring (111) plates are not possible. The newly proposed formation mechanism of pseudo-icosahedra Cu sheds light on the understanding of formation of many other polyhedral crystals.Publisher PDFPeer reviewe

Reversed crystal growth

In the last decade, reversed growth route has been found in many crystal growth processes. In these systems, a single crystal does not develop from a single nucleus. The precursor molecules/ions or nanocrystallites aggregate into some large amorphous or polycrystalline particles. Multiple-nucleation on surface of the amorphous particles or surface re-crystallization of the polycrystalline particles then takes place, forming a single crystal shell with a regular morphology. Finally, the crystallization extends from the surface to the core to form single crystals. This non-classical crystal growth route often results in some special morphologies, such as core-shell structures, hollow single crystals, sandwich structures, etc. This article gives a brief review of the research in the reversed crystal growth and demonstrates that investigation of detailed mechanisms of crystal growth enables us to better understand the formation of many novel morphologies of the crystals. Some unsolved problems are also discussed.Publisher PDFPeer reviewe

Phase transformation of Mg-calcite to aragonite in active-forming hot spring travertines

HFG would like to thank the University of St Andrews for the studentship and Mr Ross Blackley for his help on using the SEM and TEM microscopes. WZZ thanks EPSRC for financial support on FEG-SEM equipment (EP/F019580/1) and a Platform (EP/K015540/1).A travertine specimen collected from the western part of Yunnan Province of China was subjected to microstructural analysis by powder X-ray diffraction, scanning electron microscopy, high resolution transmission electron microscopy and energy dispersive X-ray spectroscopy. A new formation mechanism was proposed whereby polycrystalline rhombohedral particles of magnesium-containing calcite underwent a phase transformation into sheaf-like clusters of aragonite microrods. It is proposed that a high concentration of magnesium ions and embedded biological matter poisoned the growth of calcite and therefore instigated the phase transformation of the core of the rhombohedral calcite particles to an aragonite phase with a higher crystallinity. The single crystalline aragonite microrods with a higher density than the Mg-calcite nanocrystallites grew at the expense of the latter to generate sheaf-like clusters. This newly discovered formation mechanism is expected to enhance previous knowledge on this geologically important phase transformation from a morphology point of view.PostprintPeer reviewe

High Rate Electrochemical Reduction of Carbon Monoxide to Ethylene using Cu-Nanoparticle-Based Gas Diffusion Electrodes

Gas diffusion electrodes (GDEs) with high electrochemically active surface areas (ECSAs) and triple-phase boundaries for efficient gas, electron, and ion transport offer a unique opportunity for high-rate electrochemical CO reduction (COR) in relative to traditional aqueous configurations. Cu-nanoparticle-based GDEs were fabricated by applying a mixture of carbon powders, copper acetate aqueous solution, and Teflon onto a Cu gauze substrate. The catalyst-coated substrate was air-dried, mechanically pressed, and subsequently annealed under forming gas to produce GDEs. Two distinctive types of GDE configurations, a flow-through configuration and a flow-by configuration, were constructed, characterized, and tested to quantitatively evaluate the effects of reactant gas transport on the activity and the selectivity of the GDE materials for COR. In the flow-through configuration, a high partial current density of 50.8 mA cm^(–2) for COR to C_2H_4 was achieved at −0.85 V vs RHE in 10 M KOH at −15 °C, while in the flow-by configuration with the same catalyst materials the partial current density for C_2H_4 generation was limited to <1 mA cm^(–2)

Formation mechanism of CaCO3 spherulites in myostracum layer of limpet sahells

The authors would like to thank EPSRC for financial support on FEG-SEM equipment (EP/F019580/1) and FEI Scios dualbeam microscope (EP/L017008/01).CaCO3 spherulites were found in the myostracum layer of common limpet shells collected from East Sands, St Andrews, Scotland. Their microstructures were revealed by using powder X- ray diffraction, scanning electron microscopy, high resolution transmission electron microscopy, and energy dispersive X-ray microanalysis. The formation mechanisms of these spherulites and their morphology evolution were postulated. It was proposed that spherical particles of an inorganic and biological composite formed first. In the centre of each spherical particle was developed a double-layer disk of vaterite crystal sandwiching a biological sheet. The disk crystal supplies a relatively strong mirror symmetric dipole filed, guiding the orientations of the nanocrystallites and the arrangement of mesorods, and therefore, determining the final morphology of the spherulite.Publisher PDFPeer reviewe

Reversed crystal growth of calcite in naturally occurring travertine crust

WZZ thanks EPSRC for 510 financial support on FEG-SEM equipment (EP/F019580/1) and a Platform (EP/K015540/1).A microstructural investigation by electron microscopy on a travertine specimen collected from Munigou National Park, Sichuan Province, China revealed evidence of a non-classical reversed crystal growth route previously only discovered in synthetic materials. Examination of the travertine specimen suggests that the presence of organic matter initiates the oriented aggregation of calcite nanocrystallites. Surface re-crystallisation of the aggregates leads to a single crystalline rhombohedral shell with a polycrystalline core. This core-shell structure carries a strong resemblance to synthetic calcite prepared in the presence of chitosan, where the growth of calcite was found to follow the so-called reversed crystal growth process. It is proposed that the similar roles of biomolecules in naturally occurring travertine and chitosan in the synthetic system are based on their isoelectric points and the polymerizable property of long chain chemical structures. This study is important so that the structural similarities between naturally occurring biominerals and biomimetic materials can be further understood.Publisher PDFPeer reviewe

Hierarchical structured graphene/metal oxide/porous carbon composites as anode materials for lithium-ion batteries

This work was financially supported by the Fundamental Research Funds for the Central Universities, and National Natural Science Foundation of China (21101014 and 21273022).As a novel anode material for lithium-ion batteries, CeO2 displays imperceptible volumetric and morphological changes during the lithium insertion and extraction processes, and thereby exhibits good cycling stability. However, the low theoretical capacity and poor electronic conductivity of CeO2 hinder its practical application. In contrast, Co3O4 possesses high theoretical capacity, but undergoes huge volume change during cycling. To overcome these issues, CeO2 and Co3O4 nanoparticles are formed inside the pores of CMK-3 and display various electrochemical behaviors due to the different morphological structures of CeO2 and Co3O4 within CMK-3. Moreover, the graphene/metal oxide/CMK-3 composites with a hierarchical structure are then prepared and exhibit better electrochemical performances than metal oxides with or without CMK-3. This novel synthesis strategy is hopefully employed in the electrode materials design for Li-ion batteries or other energy conversion and storage devices.PostprintPeer reviewe

Structure and cleavage of monosodium urate monohydrate crystals

The structural study of monosodium urate monohydrate, as the principal component in gout stones, reveals that a simple and biocompatible way to breakdown the crystals into polymerised molecules at pH of 7.4 (the acidity of normal human blood) is to peel off them along the [001] direction by sonication.PostprintPeer reviewe

Multivalent cation crosslinking suppresses highly energetic graphene oxide’s flammability

The authors acknowledges National Science Foundation- Experimental Program to Stimulate Competitive Research (NSF-EPSCoR) for partial support, Prof. S. Yu’s lab for the micro-Raman experiments, and Dr. Jingyi Chen’s lab for the TGA study.Graphene oxide (GO), a common intermediate for making graphene-like materials from graphite, was recently found to possess an explosive fire-hazard that can jeopardize the GO’s large-scale production and wide applications. This work reports a simple and facile method to cross-link the GO with Al3+ cations, in one step, into a freestanding flexible membrane. This inorganic membrane resists in-air burning on an open-flame, at which non-cross-linked GO was burnt out within ~5 seconds. All characterization data suggested that the in-situ “epoxy ring opening” reactions on GO surface facilitated the cross-linking, which elucidated a new mechanism for the generalized inorganic polymerization. With the much improved thermal- and water-stabilities, the cross-linked GO-film can help to advance high-temperature fuel-cells, electronic packaging, etc. as one of the long-sought inorganic polymers known to date.PostprintPeer reviewe

Experimental and theoretical investigations of Cs+ adsorption on crown ethers modified magnetic adsorbent

This work was financially supported by the NSFC (No: U1607105 and No: 51574286). The Scientific and Technological Funding in Qinghai Province (No: 2018-GX-101, No: 2018-ZJ-722 and No: 2019-HZ-808). “Light of West China” Program and Youth Innovation Promotion Association of CAS (No: 2015141 and 2016377).Carboxyl Fe3O4 nanoparticles (Fe3O4@R-COOH) modified with 18-Crown-6 ether functional groups have been prepared via an amidation reaction and used as bifunctional adsorbent for Cs+. The adsorbent has a superparamagnetic property, allowing an easy recycling, and a high capacity of Cs+ adsorption on the crown ether. The adsorption isotherms and kinetic behaviors agree well with the Langmuir and the pseudo-second-order models. The material exhibits a high selectivity for Cs+ in the solution with co-existing cations (NH4+, Rb+, K+, Na+ and Li+). A theoretical calculation according to density functional theory (DFT) is used to estimate the structure of Cs+ adsorption on crown ether, demonstrating an exothermic process and showing a good agreement with the experimental observations. The adsorption behavior is affected not only by the size of macrocyclic crown ethers, but also by the chelating symmetry and the binding energy. The newly developed adsorbent has a potential application for removing cesium out of wastewater and salt lakes.PostprintPeer reviewe