Cation Diffusion Research Articles

Mg2+ Diffusion-Induced Structural and Property Evolution in Epitaxial Fe3O4 Thin Films.

Epitaxial Fe3O4 thin films grown on single crystal MgO(001) present well-defined model systems to study fundamental multivalent ion diffusion and associated phase transition processes in transition-metal-oxide-based cathodes. In this work, we show at an atomic scale the Mg2+ diffusion pathways, kinetics, and reaction products at the Fe3O4/MgO heterostructures under different oxygen partial pressures but with the same thermal annealing conditions. Combining microscopic, optical, and spectroscopic techniques, we demonstrate that an oxygen-rich environment promotes facile Mg2+ incorporation into the Fe2+ sites, leading to the formation of Mg1-xFe2+xO4 spinel structures, where the corresponding portion of the Fe2+ ions are oxidized to Fe3+. Conversely, annealing in vacuum results in the formation of a thin interfacial rocksalt layer (Mg1-yFeyO), which serves as a blocking layer leading to significantly reduced Mg2+ diffusion to the bulk Fe3O4. The observed changes in transport and optical properties as a result of Mg diffusion are interpreted in light of the electronic structures determined by X-ray photoelectron spectroscopy and X-ray absorption spectroscopy. Our results reveal the critical role of available anions in governing cation diffusion in the spinel structures and the need to prevent formation of unwanted reaction intermediates for the promotion of facile cation diffusion.

Development of mesoporous Bi2WO6/g-C3N4 heterojunctions via soft- and hard-template-assisted procedures for accelerated and reinforced photocatalytic reduction of mercuric cations under vis light irradiation

In this study, mesoporous Bi2WO6/g-C3N4 heterojunctions were developed using soft and hard templates [triblock copolymer surfactant (F127) and mesoporous silica (MCM-41), respectively]. The performance of the developed heterojunctions was assessed through the photocatalytic reduction of mercuric cations under Vis light illumination, with HCOOH being adopted to provide sacrificial holes agent. Surface measurements demonstrated that the fabricated specimens acquired large specific surface areas when compared with the neat ingredient. Furthermore, a transmission electron microscopy (TEM) analysis of the developed heterojunctions showed the homogeneous distribution of the spherical Bi2WO6 nanoparticles (NPs) on the surface of g-C3N4 nanosheets. Meanwhile, an accelerated rate (700 μ·mol·g−1·h−1) of photocatalytic mercuric cation reduction with improved efficiency (approximately 100%), compared with those of the pure ingredients [rate of 55 μ·mol·g−1·h−1 and efficiency of 13% for g-C3N4 nanosheets; rate of 95 μ·mol·g−1·h−1 and efficiency of 20% for mesoporous Bi2WO6 NPs], was accomplished via testing of the Bi2WO6/g-C3N4 heterojunction comprising 4 wt% Bi2WO6 after 40 min of illumination. Evidently, the efficiency of the photocatalytic reduction of mercuric cations endorsing the Bi2WO6/g-C3N4 heterojunction comprising 4 wt% Bi2WO6 NPs is 7.7 and 5 times more when compared with those of the neat g-C3N4 nanosheets and mesoporous Bi2WO6 NPs, respectively. The improved performance of the fabricated heterojunctions in the photocatalytic reduction of mercuric cations could be ascribed to i) fast diffusion of the mercuric cations through the mesoporous texture to the active ensembles, ii) greater specific surface area, iii) limited bandgap magnitude, iv) homogenous dispersion of the Bi2WO6 NPs on the surface of the nanosheets, and v) finite particle dimension of the mesoporous Bi2WO6 NPs. The durability and stability of the Bi2WO6/g-C3N4 heterojunctions were confirmed via their recyclability, which was maintained for up to five runs without pronounced activity loss.

RNA-based thermoregulation of a Campylobacter jejuni zinc resistance determinant.

RNA thermometers (RNATs) trigger bacterial virulence factor expression in response to the temperature shift on entering a warm-blooded host. At lower temperatures these secondary structures sequester ribosome-binding sites (RBSs) to prevent translation initiation, whereas at elevated temperatures they “melt” allowing translation. Campylobacter jejuni is the leading bacterial cause of human gastroenteritis worldwide yet little is known about how it interacts with the host including host induced gene regulation. Here we demonstrate that an RNAT regulates a C. jejuni gene, Cj1163c or czcD, encoding a member of the Cation Diffusion Facilitator family. The czcD upstream untranslated region contains a predicted stem loop within the mRNA that sequesters the RBS to inhibit translation at temperatures below 37°C. Mutations that disrupt or enhance predicted secondary structure have significant and predictable effects on temperature regulation. We also show that in an RNAT independent manner, CzcD expression is induced by Zn(II). Mutants lacking czcD are hypersensitive to Zn(II) and also over-accumulate Zn(II) relative to wild-type, all consistent with CzcD functioning as a Zn(II) exporter. Importantly, we demonstrate that C. jejuni Zn(II)-tolerance at 32°C, a temperature at which the RNAT limits CzcD production, is increased by RNAT disruption. Finally we show that czcD inactivation attenuates larval killing in a Galleria infection model and that at 32°C disrupting RNAT secondary structure to allow CzcD production can enhance killing. We hypothesise that CzcD regulation by metals and temperature provides a mechanism for C. jejuni to overcome innate immune system-mediated Zn(II) toxicity in warm-blooded animal hosts.

Effect of alumina added by mechanical mixing and co-doping on the densification mechanisms of zirconia nanoparticles at the initial stage of sintering

The comparative analysis of the mechanical activation and Al2O3 doping effect on the densification behavior of tetragonal 3Y-TZP nanopowders has been carried out. Two types of zirconia nanopowders doped by alumina have been studied. Namely, nanopowders obtained by the mechanical mixing and co-precipitation methods. The behavior of nanopowders at initial stage of sintering was examined by the constant rate of heating technique.It was found that the volume diffusion mechanism, which is realized by sintering the 3Y-TZP nanopowders undoped and doped with a small amount of Al2O3 nanopowders obtained by co-precipitation, ceases to function when the Al2O3 content increases to 2 wt% and the Al2O3 inclusions block the diffusion flux of Zr4+ cations from the volume of ZrO2 particles to the necks. Annealing of the structure defects caused by milling of the 3Y-TZP and Tosoh TZ-3Y powders during preliminary heating leads to the realization of the volume diffusion mechanism instead of the grain boundary diffusion mechanism at the initial sintering stage in milled nanopowders. Therefore, the GBD mechanism in the milled zirconia powders caused by the stressed and defective state of zirconia nanoparticles, and not by size effects.It was shown that the Al2O3 particles added to zirconia nanopowders by mechanical mixing did not change the volume diffusion mechanism at the initial stage of sintering of 3Y-TZP NPs until the Al2O3 concentration did not blocked the diffusion of cation to the necks.It was proposed to separate the effects of mechanical activation of nanopowders from the influence of doping on the diffusion mechanism at the initial stage of sintering of doped zirconia nanopowders. The simultaneous influence of both impacts (mechanical energy and chemical composition) can complicate and confuse reasoning.

Hydration and Diffusion of H+, Li+, Na+, Cs+ Ions in Cation-Exchange Membranes Based on Polyethylene- and Sulfonated-Grafted Polystyrene Studied by NMR Technique and Ionic Conductivity Measurements.

The main particularities of sulfonate groups hydration, water molecule and alkaline metal cation translation mobility as well as ionic conductivity were revealed by NMR and impedance spectroscopy techniques. Cation-exchange membranes MSC based on cross-linked sulfonated polystyrene (PS) grafted on polyethylene with ion-exchange capacity of 2.5 mg-eq/g were investigated. Alkaline metal cation hydration numbers (h) calculated from temperature dependences of 1H chemical shift of water molecule for membranes equilibrated with water vapor at RH = 95% are 5, 6, and 4 for Li+, Na+, and Cs+ ions, respectively. These values are close to h for equimolar aqueous salt solutions. Water molecules and counter ions Li+, Na+, and Cs+ diffusion coefficients were measured by pulsed field gradient NMR on the 1H, 7Li, 23Na, and 133Cs nuclei. For membranes as well as for aqueous chloride solutions, cation diffusion coefficients increased in the following sequence: Li+ < Na+ < Cs+. Cation and water molecule diffusion activation energies in temperature range from 20 °C to 80 °C were close to each other (about 20 kJ/mol). The cation conductivity of MSC membranes is in the same sequence, Li+ < Na+ < Cs+ << H+. The conductivity values calculated from the NMR diffusion coefficients with the use of the Nernst–Einstein equation are essentially higher than experimentally determined coefficients. The reason for this discrepancy is the heterogeneity of membrane pore and channel system. Ionic conductivity is limited by cation transfer in narrow channels, whereas the diffusion coefficient characterizes ion mobility in wide pores first of all.

Anti-sintering non-stoichiometric nickel ferrite for highly efficient and thermal-stable thermochemical CO2 splitting

The thermochemical splitting of CO2 into CO based on redox reaction demonstrates great promising for clean energy production. However, the redox material inevitably suffers from high-temperature sintering, leading to an increased ion diffusion barrier that deactivates the bulk phase. In this study, a distinct strategy is demonstrated to neutralize the adverse effect of sintering by engineering a non-stoichiometric nickel ferrite for the CO2 splitting. It maintains a very high CO yield over multiple consecutive cycles, performing better than state-of-the-art redox materials. Extensive characterizations and first principles calculations reveal that the bulk cation diffusion boosts the splitting reaction. Such cation diffusion mode in the non-stoichiometric ferrite provides a more accessible diffusion path for the bulk reaction, leading to accelerated CO2 splitting kinetics. Meanwhile, high stability is demonstrated for the non-stoichiometric nickel ferrite with 100% of the initial reactivity maintaining often 20 redox cycles. Our findings guide to design newly anti-sintering redox materials toward highly efficient energy conversion and production.

Toxic trace element resistance genes and systems identified using the shotgun metagenomics approach in an Iranian mine soil.

This study aimed to identify the microbial communities, resistance genes, and resistance systems in an Iranian mine soil polluted with toxic trace elements (TTE). The polluted soil samples were collected from a mining area and compared against non-polluted (control) collected soils from the vicinity of the mine. The soil total DNA was extracted and sequenced, and bioinformatic analysis of the assembled metagenomes was conducted to identify soil microbial biodiversity, TTE resistance genes, and resistance systems. The results of the employed shotgun approach indicated that the relative abundance of Proteobacteria, Firmicutes, Bacteroidetes, and Deinococcus-Thermus was significantly higher in the TTE-polluted soils compared with those in the control soils, while the relative abundance of Actinobacteria and Acidobacteria was significantly lower in the polluted soils. The high concentration of TTE increased the ratio of archaea to bacteria and decreased the alpha diversity in the polluted soils compared with the control soils. Canonical correspondence analysis (CCA) demonstrated that heavy metal pollution was the major driving factor in shaping microbial communities compared with any other soil characteristics. In the identified heavy metal resistome (HV-resistome) of TTE-polluted soils, major functional pathways were carbohydrates metabolism, stress response, amino acid and derivative metabolism, clustering-based subsystems, iron acquisition and metabolism, cell wall synthesis and capsulation, and membrane transportation. Ten TTE resistance systems were identified in the HV-resistome of TTE-polluted soils, dominated by "P-type ATPases," "cation diffusion facilitators," and "heavy metal efflux-resistance nodulation cell division (HME-RND)." Most of the resistance genes (69%) involved in resistance systems are affiliated to cell wall, outer membrane, periplasm, and cytoplasmic membrane. The finding of this study provides insight into the microbial community in Iranian TTE-polluted soils and their resistance genes and systems.

An eruption chronometer based on experimentally determined H-Li and H-Na diffusion in quartz applied to the Bishop Tuff

The diffusion of hydrogen in natural hydrothermal quartz crystals was studied between 657–956°C at atmospheric pressure and various oxygen fugacity (fO2) conditions. Single crystals of OH-bearing quartz were dehydrated in the presence of Li or Na-enriched powders to induce Li-H or Na-H exchange, with the resulting diffusion profiles measured by both Fourier transform infrared (FTIR) spectroscopy and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). Diffusion parallel to [0001], i.e. the crystallographic c-axis, is described by:log10⁡D(m2s−1)=−6.5±0.3+−100.4±5.2kJmol−12.303RT where log10D is the base 10 logarithm of the diffusion coefficient, R is the gas constant and T is the temperature in kelvins, and uncertainties are 1 σ. Diffusion is not affected by (fO2) in this system. Diffusion perpendicular to [0001] is consistently slower, but quantitative constraints cannot be obtained from our data given experimental limitations. This diffusivity is primarily associated with H+-Li+ (or H+-Na+, Na+-Li+) exchange, where the H+ and Li+ are charge-balanced by tetrahedrally-coordinated Al3+. A faster mechanism may also exist where the monovalent cations are charge balanced by excess oxygen. The final defect population, as imaged by FTIR spectroscopy and LA-ICP-MS, likely results from a combination of diffusion and inter-site rearrangement of the monovalent cations. Regardless of such complexities, the determined Arrhenius relationship should be applicable for natural volcanic quartz crystals from the Bishop Tuff, California, wherein H+ is associated with Al3+, and H loss from quartz preceding and/or accompanying the eruption is charge-balanced by Li-gain, without Al movement. Li and H profiles from an example quartz crystal suggest that it experienced eruption/cooling timescales of just 16 minutes to 2.4 hours, showing the considerable promise of using frozen H diffusion profiles in quartz to extract timescales and thus elucidate the last moments of such explosive silicic eruptions.

Electromechanical properties of Nafion/carbon nanotube composites enhanced by black phosphorus

ABSTRACT Ionic polymer-Carbon nanotube composite (IPCC) actuators based on black phosphorus (BP)-Nafion composite membranes were fabricated. BP-Nafion composite was used as the ionic matrix with functional carbon nanotube as electrodes. BP presents as a promising material owing to its high charge mobility using in energy storage materials. It was used in Nafion composite improving the electromechanical properties of BP-Nafion composite. Significantly the interaction between BP and lithium ion was strong which restricted the back diffusion of cation during action and thus enhanced the deformation of actuator without back-relaxation. In addition, layer crystal structure of BP protected the inter-lamellar liquid from evaporation thus ensured the durability. The result IPCC actuator exhibited improvement actuation with blocking force of 2 wt.% BP-Nafion composite of 33.6 gf g−1 for 30s under an electrical field of 2 V, 2.7 times that of neat Nafion. Actuation behavior under AC field could be repeatedly played hundreds of cycles without obvious decrease of blocking force for BP-Nafion composite-based IPCC.

Factors Defining the Intercalation Electrochemistry of CaFe2O4-Type Manganese Oxides

Oxides with the CaFe2O4-type structure have been predicted as being suitable hosts for reactions of intercalation of light cations such as Li and Mg because of their favorable cationic diffusion. A...

Correlations between the ionic conductivity and cation size in complex borohydrides

The ionic conduction in the metal borohydrides is often linked to the energy barriers of BH4− reorientation and cation diffusion. However, the ionic conduction is a complex phenomenon, and limited reports are available to establish straightforward correlations with experimental trends in alkali metal borohydrides. This communication reported the correlations between ionic conductivity and cationic size in complex borohydrides. The ionic conductivities of LiBH4, NaBH4, and KBH4 were found to decrease monotonically with increasing cationic radius. This evidences that borohydrides follow trends observed for other ionic conductors and provide further ground toward designing better borohydride-based electrolytes.

The metal binding site composition of the cation diffusion facilitator protein MamM cytoplasmic domain impacts its metal responsivity

The cation diffusion facilitator (CDF) is a conserved family of divalent d-block metal cation transporters that extrude these cations selectively from the cytoplasm. CDF proteins are composed of two domains: the transmembrane domain, through which the cations are transported, and a regulatory cytoplasmic C-terminal domain (CTD). It was recently shown that the CTD of the CDF protein MamM from magnetotactic bacteria has a role in metal selectivity, as binding of different metal cations exhibits distinctive affinities and conformations. It is yet unclear whether the composition of the CTD binding sites can impact metal selectivity and if we can manipulate the CTD to response to other non-native metals in CDF proteins. Here we performed a mutational study of the model protein MamM CTD, where we exchanged the native metal binding residues with different metal-binding amino acids. Using X-ray crystallography and Trp-fluorescence spectrometry, we studied the impact of these mutations on the CTD conformation in the presence of non-native metals. Our results reveal that the incorporation of such mutations alters the domain response to metals in vitro, as mutant forms of the CTD bind metals differently in terms of the composition of the binding sites and the CTD conformation. Therefore, the results demonstrate the direct influence of the CTD binding site composition on CDF proteins structure and hence, function, and constitute a first step for rational design of MamM for transporting different metals in vivo.

Reasons for the Formation and Properties of Soliton-Like Charge Waves in Membrane Systems When Using Overlimiting Current Modes.

The study of ion transport in membrane systems in overlimiting current modes is an important problem of physical chemistry and has an important application value. The influence of the space charge on the transport of salt ions under overlimiting current modes was first studied in the work of Rubinstein and Shtilman and later in the works of many authors. The purpose of this research is to study, using the method of mathematical modeling, the reasons of formation and properties of the local maximum (minimum) space charge in membrane systems under overlimiting current conditions. It is shown that, in the diffusion layer of the cation-exchange membrane (CEM), the local maximum of the space charge appears due to the limited capacity (exchange capacity) of the membrane at a given potential jump, i.e., the local maximum of space charge appears due to the presence of a local minimum of space charge at the surface of the CEM. The local maximum of the space charge moves as a single soliton-like wave into the depth of the solution. Unlike real solitons, this charged wave changes its size and shape, albeit quite slowly. In the section of the desalination channel, the situation is completely different. First, the space charge of the anion-exchange membrane (AEM) has a negative value, so we should be talking about the local minimum (or the maximum of the absolute value of the charge). However, this is an insignificant clarification. Secondly, the space charge waves of different signs begin to interact, which leads to a new effect, namely the effect of the breakdown of the space charge. The dependence of the local maximum on the input parameters—the cation diffusion coefficient, the growth rate of the potential jump, and the initial and boundary concentrations—is studied.

An idealised approach of geometry and topology to the diffusion of cations in honeycomb layered oxide frameworks

Honeycomb layered oxides are a novel class of nanostructured materials comprising alkali or coinage metal atoms intercalated into transition metal slabs. The intricate honeycomb architecture and layered framework endows this family of oxides with a tessellation of features such as exquisite electrochemistry, unique topology and fascinating electromagnetic phenomena. Despite having innumerable functionalities, these materials remain highly underutilised as their underlying atomistic mechanisms are vastly unexplored. Therefore, in a bid to provide a more in-depth perspective, we propose an idealised diffusion model of the charged alkali cations (such as lithium, sodium or potassium) in the two-dimensional (2D) honeycomb layers within the multi-layered crystal of honeycomb layered oxide frameworks. This model not only explains the correlation between the excitation of cationic vacancies (by applied electromagnetic fields) and the Gaussian curvature deformation of the 2D surface, but also takes into consideration, the quantum properties of the cations and their inter-layer mixing through quantum tunnelling. Through this work, we offer a novel theoretical framework for the study of multi-layered materials with 2D cationic diffusion currents, as well as providing pedagogical insights into the role of topological phase transitions in these materials in relation to Brownian motion and quantum geometry.

Cation diffusion in polycrystalline thin films of monoclinic HfO2 deposited by atomic layer deposition

Though present in small amounts and migrating at low rates, intrinsic cation defects play a central role in governing the operational lifetime of oxide-ion conducting materials through slow degradation processes such as interdiffusion, kinetic demixing, grain growth, and creep. In this study, a new experimental approach to characterizing the behavior of such slow-moving, minority defects is presented. Diffusion is probed in samples with a constant cation-defect concentration well above the equilibrium values. This approach is applied to monoclinic hafnium dioxide, m-HfO2. To this end, nanocrystalline thin films of m-HfO2 were prepared by atomic layer deposition. Diffusion experiments with ZrO2 as a diffusion source were performed in the temperature range 1173 ≤ T/K ≤ 1323 in air. The Zr diffusion profiles obtained subsequently by secondary ion mass spectrometry exhibited the following two features: the first feature was attributed to slow bulk diffusion and the second was attributed to combined fast grain-boundary diffusion and slow bulk diffusion. The activation enthalpy of Zr diffusion in bulk HfO2 was found to be (2.1 ± 0.2) eV. This result is consistent with the density-functional-theory calculations of hafnium-vacancy migration in m-HfO2, which yield values of ∼2 eV for a specific path. The activation enthalpy of the grain-boundary diffusion of (2.1 ± 0.3) eV is equal to that for bulk diffusion. This behavior is interpreted in terms of enhanced cation diffusion along space-charge layers.

Phosphine-Induced Phase Transition in Copper Sulfide Nanoparticles Prior to Initiation of a Cation Exchange Reaction.

Cation exchange reactions of colloidal copper sulfide nanoparticles are widely used to produce derivative nanoparticles having unique compositions, metastable crystal structures, and complex heterostructures. The copper sulfide crystal structure plays a key role in the mechanism by which cation exchange occurs and the product that forms. Here, we show that digenite copper sulfide nanoparticles undergo a spontaneous phase transition to tetragonal chalcocite in situ, prior to the onset of cation exchange. Room-temperature sonication of digenite (Cu1.8S) in trioctylphosphine, a Lewis base that drives cation exchange, extracts sulfur to produce tetragonal chalcocite (Cu2S). The subtle structural differences between digenite and tetragonal chalcocite are believed to influence the accessibility of cation diffusion channels and concomitantly the mechanism of cation exchange. Structural relationships in nanocrystal cation exchange are therefore dynamic, and intermediates generated in situ must be considered.

Comprehensive study on the microstructure evolution and oxidation resistance performance of NiCoCrAlYTa coating during isothermal oxidation at High temperature

The microstructural evolution and high temperature oxidation resistance performance of NiCoCrAlYTa coating were investigated via isothermal oxidation at 1100℃ and 1150℃ for up to 200 h. The results reveal that the microstructure of oxides scale formed at higher temperature gradually evolve from a dense alumina layer to a mixed oxide layer, consisting of α-Al2O3 and considerable amount of NiAl2O4 spinel, NiO as well as abundant amount of (Y, Ta)-enriched oxides, which not only caused the formation of defects, such as voids and cracks, but also developed a rumpling of coating surface, accelerating the aggregation and growth of cracks. And serious internal oxidation was also detected as the oxidation time increased, which was discussed and explained from the aspect of interdiffusion behavior. At high temperature of 1100℃ and 1150℃, the oxidation kinetics curve deviated from parabolic law, indicating the oxidation resistance property of NiCoCrAlYTa coating compromised under these severe conditions. Based on the result of calculated diffusion activation energy, the failure mechanism of NiCoCrAlYTa coating is largely dependent on the exposure temperature. During the growth of oxides scale, the external diffusion of cations is the mainly process, accompanied with the internal diffusion of oxygen to a certain degree. The results could provide comprehensive understanding of the microstructural evolution during oxidation, which was essential for controlling oxides scale growth and improving the durability of NiCoCrAlYTa coating.

Sintering of a UO2-PuO2 freeze-granulated powder under reducing conditions

A freeze-granulated powder made of UO2 and PuO2, containing 15 mol% of Pu/(U + Pu), was sintered under reducing conditions (oxygen potential of −468 kJ/mol at 1700 °C). Constructing the “grain size versus relative density” trajectory, using the constant rates of heating and the master sintering curves approaches and calculating the diffusion coefficients by exploiting the results of the sintering runs enabled to propose that densification was probably controlled by grain boundary diffusion and grain growth by the grain boundaries. An activation energy around 525 kJ/mol was obtained for densification, which was close to what was reported for grain boundary diffusion of plutonium cations in U0.55Pu0.45O2-x polycrystalline materials. The sintered microstructure appeared homogeneous regarding the plutonium and uranium cations spatial distribution. By combining the master sintering curve approach for anisothermal and isothermal conditions, it was possible to predict the evolution of the relative density over time for any type of thermal path.

Highly Crystalline Hollow Toroidal Copper Phosphosulfide via Anion Exchange: A Versatile Cation Exchange Nanoplatform.

Postmodification of nanocrystals through cation exchange has been very successful in diversifying nanomaterial compositions while retaining the structural motif. Copper compound nanoparticles are particularly useful as templates because of inherent defects serving as effective cation diffusion routes and excellent cation mobility. Therefore, the development of shape-controlled multianion systems, such as copper phosphosulfide, can potentially lead to the formation of diverse metal phosphosulfide nanomaterials that have otherwise inaccessible compositions and structures. However, there is, to the best of our knowledge, no report on the shape-controlled synthesis of copper phosphosulfide nanoparticles because the introduction of the second anion to the metal compound might destroy the nanoparticle morphology and crystallinity due to the required high energy for anion diffusion and mixing. Herein, we report that it is feasible to transfer the structural motif of copper sulfide to copper phosphosulfide using tris(diethylamino)phosphine. The anion-mixed copper phosphosulfide in the form of a hollow toroid could provide a pathway to previously inaccessible phases and morphologies. We verified the versatility of a copper phosphosulfide hollow toroid as a cation-exchange template by the successful synthesis of cobalt, nickel, indium, and cadmium phosphosulfides as well as bimetallic cobalt-nickel phosphosulfide (Co2-xNixP1-ySy) with a retained structural motif.

Competition between thermodynamics, kinetics and growth mode in the early-stage oxidation of an equimolar CoCrFeNi alloy

We demonstrate that the early-stage oxidation of a CoCrFeNi multi-principal element alloy depends upon a competition between kinetic and thermodynamic factors involving the relative diffusion rate of cations and oxygen, and how this couples to inward or outward oxide growth. The microstructures of oxide layers formed at temperatures from 500 to 800 °C for 0.5 h, as well as their chemical compositions, were investigated by transmission electron microscopy. A triple layer microstructure with an outer Fe-rich spinel oxide, an intermediate Cr-rich corundum structure, and a Ni-rich (Fe, Co, and Cr depleted) dealloyed region at the metal/oxide interface was observed. The dominant oxygen transport in corundum at 800 °C and below led to an inward growth of the corundum phase; the spinel oxide growth was dominated by cation diffusion, so it grew outward. The chromium was sequestered in the corundum layer, thereby favoring the formation of the chromium-free, Fe-rich spinel oxides with Co and Ni dopants. Since nickel cannot readily diffuse through corundum, it tends to remain in the alloy phase leading to the Ni-rich dealloyed region at the metal/oxide interface. Beyond the microstructure results, we exploit secondary electron image contrast to show the doping nature of the oxides, a p-type spinel and a n-type corundum growing on the metal surface.