HAADF-STEM Images Research Articles

Unsupervised Learning for the Automatic Counting of Grains in Nanocrystals and Image Segmentation at the Atomic Resolution.

Identifying the grain distribution and grain boundaries of nanoparticles is important for predicting their properties. Experimental methods for identifying the crystallographic distribution, such as precession electron diffraction, are limited by their probe size. In this study, we developed an unsupervised learning method by applying a Gabor filter to HAADF-STEM images at the atomic level for image segmentation and automatic counting of grains in polycrystalline nanoparticles. The methodology comprises a Gabor filter for feature extraction, non-negative matrix factorization for dimension reduction, and K-means clustering. We set the threshold distance and angle between the clusters required for the number of clusters to converge so as to automatically determine the optimal number of grains. This approach can shed new light on the nature of polycrystalline nanoparticles and their structure-property relationships.

Electron microscopy of seismic waves.

Changes in the surrounding environment, if transmitted to the electron microscope, are frequently perceived as noise that diminishes the quality of the images. However, in fact, 'noises' contain rich information about the environment. This work reports a very rare event where aberration-corrected HAADF-STEM images were acquired during the impact of seismic waves, resulted from a mild earthquake. By analysing these images, we found that the drift and vibration of the sample are detectable and quantifiable. Despite many potential challenges, this work demonstrates the utilisation of electron microscopes in detecting and monitoring seismic waves with high spatial resolution, which may lead to unique applications in the low-frequency regime.

Hierarchically structured nanospherical fibrous silica-supported bimetallic catalysts: An enhanced performance in methane decomposition

Hydrogen production by methane catalytic decomposition is a promising method that also allows the formation of valuable carbon nanomaterials which can be utilized in transportation fuels, chemical synthesis, and fuel cell technology. In this study, bimetallic 3d transition metal (Ni, Fe, Co) catalysts supported on spherical mesoporous nanofibrous silica (KCC1) were successfully synthesized using a hydrothermal method followed by sono co-impregnation. The catalysts were evaluated for their performance in the catalytic decomposition of methane. Comprehensive characterization of the catalysts was conducted utilizing XRD, FTIR, SEM, TEM, STEM, XPS, H2-TPR, and BET techniques. XRD and TEM analyses confirmed the formation of Ni–Fe, Ni–Co, and Co–Fe bimetallic alloys on the nanofibrous silica without compromising its dendrimeric hierarchical structure. STEM HAADF imaging revealed a uniform dispersion of bimetallic nanoparticles within the spherical fibrous framework. Catalytic tests demonstrated that all bimetallic catalysts showed high stability and activity for methane cracking at 700 °C over 180 min of time on stream (TOS). Among them, Fe-based catalysts Ni–Fe/KCC1 and Co–Fe/KCC1 achieved the highest hydrogen yields of 80% and 60%, respectively. Methane conversion was 1.56 and 2.23 times higher in Ni–Fe/KCC1 compared to Co–Fe/KCC1 and Ni–Co/KCC1, respectively. Post-reaction characterization of the spent catalysts was performed using XRD, Raman spectroscopy, SEM, TEM, and TGA. XRD and Raman spectroscopy were used to evaluate the degree of graphitization and crystallinity in the carbon nanotubes (CNTs) produced on the catalysts, enabling a correlation with the catalyst properties. TEM examination was used to investigate the structural morphology and growth methods of the CNTs, including tip or base growth and chain-like or parallel-wall types. TGA results revealed that all bimetallic catalysts exhibited no amorphous carbon during methane cracking reaction.

Comparative study of microstructure and mechanical properties of Mg/B4C composites: Influence of sintering method and temperature

This present work compares the effect of heating methods including microwave and spark plasma sintering (SPS) process on the microstructure and mechanical properties of Mg–B4C. 5 wt%B4C mixed with Mg powders through high-energy mixer mill for 10 min at ethanol media. After mixing, the green bodies were prepared by uniaxially pressing the mixture. The microwave heating was done at sintering temperatures of 600, 650 and 700 °C without soaking time in graphite bed. While, in the case of SPS, the mixture of the powders was directly inserted into a graphite mold and sintering process was performed at 450 °C with initial and final pressures of 10 and 30 MPa at vacuum condition of 17 Pa, sequentially. The corresponding XRD patterns revealed Mg and B4C crystalline phase alongside the reaction product of MgB2 and also MgO byproduct. The FESEM investigations depict uniform distribution of B4C reinforcements at the Mg matrix. Moreover, increasing microwave sintering temperature led to decreasing pores at the microstructure of the prepared sample. The microstructure of SPSed sample revealed almost full densification compared to that of the microwave processed samples at all the sintering temperatures applied. Also, The XPS results show the presence of Mg, O, C and B elements on the surface of prepared composite, and HRTEM with HAADF-STEM images were obtained to study the interface of matrix and reinforcement. The highest bending strength and Vickers hardness of 116 ± 6 Hv and 231 ± 11 MPa were obtained for SPSed sample as well as for the highest relative density. Although the microwave sample was sintered at 700 °C at low pressure, sintering method demonstrated considerable mechanical properties in comparison to SPSed specimen, indicating a successful preparation method as a low-cost, energy- and time-saving process.

Insights into the Structure and Activity of Bimetallic Au/Cu2O Catalysts during CO2 Electroreduction to C2 products

Despite its promising performance for C2+ product formation in electrochemical reduction of CO2 (CO2RR), Cu-based catalysts exhibit a relatively poor selectivity towards important chemical intermediates such as ethylene and ethanol. Up to 16 different products can typically be formed during CO2RR.1 Cu is the only element that can form C-C bonds at an appreciable rate.2 Therefore, it is critical to better understand the active sites of Cu-based electrocatalysts, as it can lead to better catalyst design with improved selectivity towards desired products. A common approach is to investigate the reactivity of well-defined surface terminations of Cu, which can be achieved by employing shape-controlled (nano)particles.3 Here, we expanded on such studies by evaluating the impact of Au on well-defined Cu2O surfaces for the electrochemical reduction of CO2. This promoter metal was chosen, because it can reduce CO2 to CO, which is widely accepted as the key intermediate for C-C coupling.2,4 In this study, 40 nm Cu2O nanocubes that predominantly expose the (100) facet were synthesized with a ligand-free method. The (100) facet is more active in the production of ethylene at the expense of unwanted methane formation, which takes place predominantly at the (111) facet.3 The surface of the nanocubes was decorated with Au nanoparticles by a galvanic replacement reaction. HAADF-STEM images of the catalysts with various Au loadings are shown in Figure 1a-d. After addition of Au nanoparticles, the shape of the Cu2O nanocubes was preserved. However, the Au nanoparticles were highly dispersed at low loadings (1 mol % Au) whereas Au clusters were observed at higher loadings (10 mol % Au). This result was in line with synchrotron X-ray diffraction (XRD) results, whereby the Au(111) reflection appears for the 10Au/Cu2O sample. Besides, the formation of a Au-Cu alloy and CuO phase was confirmed as well. The highly dispersed nature of the Au nanoparticles at low loadings was further established by Au 4f X-ray photoelectron spectroscopy (XPS), whereby a shift towards higher binding energies was observed as compared to the samples with higher Au loadings. The electrocatalytic performance of the catalysts was assessed over a range of potentials in neutral electrolyte (0.1 M KHCO3) in H-cell configuration. With highly dispersed Au as promotor, we found that the formation of ethylene and ethanol was significantly enhanced (Figure 1e-f). More specifically, the Faradaic effiency towards ethanol increased by roughly 1.5 times by introducing 1 mol % Au to the Cu2O nanocubes. Surprisingly, the 10Au/Cu2O sample displayed the lowest Faradaic efficiency for C2 products. We believe that the dispersion of Au on the Cu2O surface plays an important role in the product formation, whereby CO can be formed at Au sites and diffuse to Cu sites in proximity for further reduction to C2 products (CO spillover). To further study structure-activity relationships, we performed in-situ X-ray absorption spectroscopy (XAS) measurements. Figure 1g shows the normalized Cu K-edge X-ray absorption near-edge structure (XANES) of the Cu2O and Au/Cu2O samples in the final state. The evolution of the Cu(0), Cu(I) and Cu(II) fractions was followed by Multivariate Curve Resolution-Alternating Least Squares (MCR-ALS) analysis. This analysis revealed that the final oxidation state of the catalysts is different. The Cu(I)/Cu(0) ratio shows the following trend 1Au/Cu2O > 5Au/Cu2O > Cu2O > 10Au/Cu2O. Based on these observations, we hypothesize that the oxidation state of Cu affects the product distribution and in particular, the formation of oxygenates. The presence of Cu(I) species and the formation of Cu(0) species during CO2 electroreduction was further confirmed with in-situ XRD measurements. Finally, we performed quasi in-situ XPS measurements to study the reduction of the surface of the catalysts. Acknowledgements This publication is part of the research programme 'Reversible Large-scale Energy Storage' (RELEASE) with project number 17621 which is financed by the Dutch Research Council (NWO).

Atomic-scale observation of the interfaces between η and γ matrix in ATI718Plus

In polycrystalline nickel-base superalloys, the presence of secondary phase precipitation at grain boundaries significantly influences mechanical properties. Numerous studies have revealed that the Ni3(Al/Ti)0.5Nb0.5 phase precipitated at grain boundaries can optimize stress rupture and reduce notch sensitivity, However, there was limited consideration given to the presence of the secondary phase within the grains. Herein, the atomic structure of η- Ni3(Al/Ti)0.5Nb0.5 phase in grain was investigated using atomic scale STEM-HAADF, and the η/γ interface and interfacial dislocation during tensile and creep process were also discussed. The results indicated that the η/γ interface exhibited different characteristic features in different stages, and interfacial dislocation interaction occurred during the tensile process, resulting in the formation of a parallel array of dislocations in a nano space. However, dislocations within the η phase and stacking faults (SFs) surrounding the interface are evident for the creep process. STEM-HAADF images also revealed that η/γ interfacial dislocation interaction coordinated the deformation of two phases and provided the driving force of misfit lattice, which positively influenced the mechanical properties of 718Plus. The crystal structure of the η phase was reconstructed using VESTA software, and its ordered structure was found to be consistent with the corresponding HAADF images for B = [21¯1¯0] zone axils. This present work provides valuable insights into the atomic-scale coordinated deformation of secondary phases and matrix.

Moiré fringe imaging of heterostructures by scanning transmission electron microscopy

A heterostructured crystalline bilayer specimen is known to produce moiré fringes (MFs) in the conventional transmission electron microscopy (TEM). However, the understanding of how these patterns form in scanning transmission electron microscopy (STEM) remains limited. Here, we extended the double-scattering model to establish the imaging theory of MFs in STEM for a bilayer sample and applied this theory to successfully explain both experimental and simulated STEM images of a perovskite PbZrO3/SrTiO3 system. Our findings demonstrated that the wave vectors of electrons exiting from Layer-1 and their relative positions with the atomic columns of Layer-2 should be taken into account. The atomic column misalignment leads to a faster reduction in the intensity of the secondary scattering beam compared to the single scattering beam as the scattering angle increases. Consequently, the intensity distribution of MFs in the bright field (BF)-STEM can be still described as the product of two single atomic images. However, in high angle annular dark field (HAADF)-STEM, it is approximately described as the superposition of the two images. Our work not only fills a knowledge gap of MFs in incoherent imaging, but also emphasizes the importance of the coherent scattering restricted by the real space when analyzing the HAADF-STEM imaging.

Clogauite, PbBi4Te4S3, a new member of the aleksite series

AbstractClogauite, ideally PbBi4Te4S3 is the new n = 1 member of the aleksite series, PbnBi4Te4Sn+2, where n is the homologue number. Clogauite is named from the type locality, the Clogau gold mine, Dolgellau Gold belt, Gwynedd, North Wales, United Kingdom. The mineral and name have been approved by the Commission on New Minerals, Nomenclature and Classification of the International Mineralogical Association (IMA2023–062). The aleksite series is an accretional homologous series in which each member is derived from the same 5-atom tetradymite archetype. Clogauite crystallises in the trigonal crystal system (space group: P$\bar{3}$m1, #164). Three distinct polytypes of clogauite are recognised, corresponding to identical chemistry but different layer sequences, expressed as (57), (5559) and (557.559), respectively, in reference to the number of atoms in individual layer sequences. These are clogauite-12H, a = 4.277(4) Å, c = 23.46(14) Å, V = 371.598 Å3 and Z = 1; clogauite-24H, a = 4.278(4) Å, c = 46.88(31) Å, V = 743.053 Å3 and Z = 2; and clogauite-36H, a = 4.278(4) Å, c = 70.36(32) Å, V = 1115.283 Å3 and Z = 3. Clogauite is opaque, with a pale grey colour in reflected light. Reflectance is higher than tetradymite or galena. Bireflectance and anisotropy are strong. Structural data were determined from measurement of atomic-scale HAADF STEM imaging showing the internal arrangement of component atoms and characteristic selected area electron diffraction patterns for each polytype. The structures were then further constrained from ab initio total energy calculations and structure relaxation using density functional theory (DFT) using the measured parameters as input data. The relaxed crystal structure for each polytype was modelled to generate crystallographic information files (cif). STEM and electron diffraction simulations based on the crystallographic information data obtained from the DFT calculations show an excellent match to the empirical measurements.

Quantification of alloy atomic composition sites in 2D ternary MoS2(1-x)Se2x and their role in persistent photoconductivity, enhanced photoresponse and photo-electrocatalysis

Engineering transition metal dichalcogenides-based semiconducting two-dimensional (2D) layered materials for photo(electro)chemical (PEC) hydrogen evolution reaction (HER) by water splitting is an enduring challenge. Here, alloy-assisted photoconductivity and photoresponse from CVD-grown MoS2(1-x)Se2x (MSSE) 2D ternary atomic layered alloy-based photodetector device is presented for the realization of PEC HER. The explicit role of ‘S–Se’ and ‘Se2’ atomic alloy sites including chalcogen-induced vacancy defects on the photoconductivity/photoresponse and PEC HER performance of MSSE 2D alloy is investigated. Alloy formation, atomic site-by-site ‘Se’ composition and atomic structure are characterized using Raman/Photoluminescence (PL) spectroscopy, high-angle annular dark field (HAADF)- scanning transmission electron microscopy (STEM) extensively and supported with Auger Electron Spectroscopy (AES) mapping. Further, the local density and concentration of S–Se, Se2 atomic sites and defects were quantitatively estimated using HAADF-STEM image analysis in correlation with AES and it is found between the range of ∼15–20 % in MSSE alloy. A 10-fold high photoresponsivity in the case of MSSE concerning as-grown MS having fast photocurrent growth time and the prolonged decay time originates from the ‘Se’ and this alloy assisted states to enhance the PEC performance of MSSE alloy. The enhanced PEC HER activity of MSSE alloy was identified in terms of overpotential and current density. In addition, increased density of states as a function of ‘Se’ alloying, shifts in a p-band centre and lowers ΔGH* according to density functional theory calculations, which makes MSSE alloy an efficient HER activity. Further, the PEC stability and presence of the ‘S–Se’ and ‘Se2’ alloying and their role towards HER have been correlated by the spectral line shape analysis of PL and Raman spectra from post-PEC HER catalysts. These experimental and theoretical findings establish the role of chalcogen, and transition metal-based 2D alloy, leading to the design of new PECs of engineered 2D atomic layer interfaces.

Kobellite homologues from the Boliden Au–Cu–(As) deposit, Sweden: jigsaw patterning via nanoscale intergrowths in chessboard structures

Abstract Sulfosalt assemblages in a specimen from the Boliden Au–Cu–(As) deposit in northern Sweden, comprise micrometre to nanometre scale intergrowths of Se-rich izoklakeite and tintinaite with average formulae and calculated homologue number (N) given as: (Cu1.88Fe0.18)2.06(Pb22.92Ag1.47Cd0.01Zn0.01)24.41(Sb13.12Bi8.69)21.8(S50.19Se6.43Te0.12)56.73,N = 3.83, and (Cu1.31Fe0.74)2.05(Pb10.58Ag0.18Cd0.05Zn0.02)10.83(Sb10.2Bi5.23)15.43(S32.22Se2.46)34.7, N = 2.05, respectively. Tintinaite coexists with (Bi, Se)-rich jamesonite. High-angle annular dark field scanning transmission electron microscopy (HAADF STEM) imaging reveals chessboard structures comprising PbS and SnS modules with the number of atoms in the octahedral (M) sites counted as: n1 = 18 and n2 = 8 for tintinaite and n1 = 30 and n2 = 16 for izoklakeite. The homologue number can be calculated using the formula: N = (n1/6)–1 and N = n2/4 for PbS and SnS modules giving NTti = 2 and NIz = 4. A new N = 3 homologue, defined by n = 12 and n = 24 SnS and PbS modules, respectively, is identified as single or double units within areas with intergrowths between kobellite and izoklakeite. HAADF STEM imaging also reveals features attributable to lone electron pair micelles within the Sb-rich kobellite homologues. Atomic-resolution EDS STEM chemical mapping of Pb–Bi–Sb-sulfosalts shows a correlation with crystal structural modularity. The maps also highlight sites in the SnS modules of tintinaite in which Sb > Bi. Coherent nanoscale intergrowths between tintinaite and izoklakeite define jigsaw patterns evolving from chessboard structures and are considered to have formed during co-crystallisation of the two phases. Displacement textures and crosscutting veinlets (a few nm in width) are interpreted as evidence for superimposed syn-metamorphic deformation and are associated with the redistribution of Bi and Se. Imaging and mapping using HAADF STEM techniques is well suited to characterisation of Pb–Sb–Bi-sulfosalt phases, offering largely untapped potential to unravel the evolution of chessboard structures with applications across mineralogy but also extending into allied fields.

Ce-doping promotes the electrochemical performance of NaNi0.5Mn0.5O2

Layered sodium ion cathode material is increasingly popular due to the superiority of low-cost and excellent rate performance. However, it usually suffers from poor capacity retention arising from microcracks during multiple-phase transitions. Herein, a Ce-doped O3-type cathode material NaNi0.5Mn0.5O2(NNM) was synthesized by high shear mixer-assisted precipitation of oxalate precursors. Using the characterizations of XRD, XPS, HRTEM, GITT, etc., the results indicate that Ce dopants can adjust the initial phase structure as well as postpone the phase transition in such O3-type material. The optimal NNM-0.06Ce shows a superior initial discharge capacity of 128.3 mAh g−1 and a retention of 85.96% after 100 cycles (1.5–4.0 V, 1 C, 25 °C), as well as superior rate performance of 107 mAh g−1 at 5 C. HAADF-STEM images disclose that the doped Ce atoms are probably located in the sodium layer of the cathode material, moreover, DFT calculations reveal that the Ce dopant is prone to occupy the Na site and cause charge rising of Ni and Mn. In-situ XRD tests indicate that the earlier formed O1 phase induced by Ce dopants can postpone the phase transition and increase the reversibility of the cathode, therefore improving the electrochemical performance of cathode materials.

Shuttling Active Elements in and out of Ultrathin Nanowires: Toward Rational Design of Multicomponent Catalysts.

One-dimensional nanostructures, with a high ratio of surface-bulk atoms, find applications as active catalysts. Here, we report tunability in ultrathin single-crystalline AuPdPt nanowires by modifying synthesis conditions and postsynthetic treatment in a controlled ambient atmosphere. The surface microstructure modification of these nanostructures has been analyzed by integrating the results of three crucial techniques including Z-contrast HAADF-STEM imaging, X-ray photoelectron spectroscopy, and electrochemically active surface area from cyclic voltammograms.

High-Entropy Alloys as Oxygen Reduction Reaction Electrocatalysts for Proton Exchange Membrane Fuel Cells Application

High entropy alloys (HEA) are complex multi-element materials. Owing to their multi-elemental nature, they can finetune surface adsorption energy and active sites which make them appealing to various catalytic reactions. HEI is a subclass of HEA that consist of an ordered alloy structure rather than a random alloy structure. The order-ness may induce more stable structures and uniform active sites, which in turn enhance the durability and activity under catalytic conditions. Here, we have synthesized a HEI(Pt40Ni20Co20Mn10Ir10) supported on porous carbon via a seed-mediated impregnation method. The intermetallic characteristics with periodic trends can be seen in the HAADF-STEM image (Fig. 1a). Based on the atomic arrangement, the HEI resembled a L12 structure. The corresponding EDX mapping the catalyst also showed the presence of all alloying species in the nanoparticles. The catalyst performance of the catalysts was evaluated in a 3-electrode liquid cell set-up with catalyst coated glassy carbon rotating disk electrode, Ag/AgCl and Pt mesh as working, reference and counter electrode, respectively. The catalyst’s mass activity (MA) was calculated based on the result obtained from ORR polarization test conducted under O2 saturated 0.1M HClO4. The initial mass activities for PtCo and HEA in ORR were 0.81 and 1.23 A mg-1 Pt, respectively (Fig. 1b), while that of Pt/C was only 0.26 A mg-1 Pt. A subsequent accelerated degradation test (ADT) was conducted under Ar-saturated 0.1M HClO4 with 0.6-1.0 V vs RHE square-wave cycling (3s hold at each potential) for 30k cycles. MA was measured after 10k and 30k cycles. HEA retained a MA of 0.48 A mg-1 Pt after the ADT, above the DOE target (0.44 A mg-1 Pt). We also compared our HEI samples synthesized with different annealing temperatures and duration (Fig. 1c), and it showed that 800◦C with 20 mins temperature hold showed the highest MA and electrochemical surface area (ECSA).Extended X-ray absorption spectroscopy (EXAFS) for the catalyst has also been conducted to further elucidate the structural-performance relationship of HEI (Fig.1d). From Pt L3 edge EXAFS fitting, the fitted R space showed that the Pt-Pt bond length was around 2.704 ± 0.004 Å, which was same as that of the PtCo (Fig. 1e). Therefore, it is believed that both HEI and PtCo has the same strain effect but HEI possessed stronger ligand effect than PtCo due to its unique Pt-M coordination, hence resulting in superior ORR performance over PtCo and Pt. Figure 1

Atomic-Scale 3D Structure of a Supported Pd Nanoparticle Revealed by Electron Tomography with Convolution Neural Network-Based Image Inpainting.

Electron tomography based on scanning transmission electron microscopy (STEM) is used to analyze 3D structures of metal nanoparticles on the atomic scale. However, in the case of supported metal nanoparticle catalysts, the supporting material may interfere with the 3D reconstruction of metal nanoparticles. In this study, a deep learning-based image inpainting method is applied to high-angle annular dark field (HAADF)-STEM images of a supported metal nanoparticle to predict and remove the background image of the support. The inpainting method can separate an 11nm Pd nanoparticle from the θ-Al2O3 support in HAADF-STEM images of the θ-Al2O3-supported Pd catalyst. 3D reconstruction of the extracted images of the Pd nanoparticle reveals that the Pd nanoparticle adopts a deformed structure of the cuboctahedron model particle, resulting in high index surfaces, which account for the high catalytic activity for methane combustion. Using the xyz coordinate of each Pd atom, the local Pd-Pd bond distance and its variance in a real supported Pd nanoparticle are visualized, showing large strain and disorder at the Pd-Al2O3 interface. The results demonstrate that 3D atomic-scale analysis enables atomic structure-based understanding and design of supported metal catalysts.

Preparation of Co-C/N flower-like single-atom catalysts via TCPP coordination confinement for enhanced activation of peroxymonosulfate

The design and preparation of highly efficient advanced oxidation processes (AOPs) catalysts is a crucial research area in the field of water pollution control. Herein, a single-atom AOP catalyst Co-N/C material (TPM-650) has been synthesized through the pyrolysis of the flower-shaped Co-TCPP MOF precursor, which is designed based on coordination confinement with TCPP. HAADF-STEM image analysis confirms the uniform distribution of single Co atoms throughout the TPM-650 material, which served as the highly efficient catalytic centers and can achieve 99.99% catalytic degradation of MG dyes in 24 min with only trace dosage ([TPM-650] = 1 mg/L, pseudo-first-order kinetic model, Kobs=0.302 min−1) and low leaching of Co ions (0.348 μg/L). Under optimal condition, the degradation of MG exhibits a high turnover number (dTON) of 68.5 mmol h−1 gcat−1 and a significant reduction in total organic carbon (TOC) by 58.7%. Radical quenching experiments and EPR measurements demonstrated that various reactive oxygen species (•SO4−, •OH, 1O2, MIV(O)/MV(O)) contributes to accelerating MG degradation. The enhanced efficiency and stability were attributed to the ultrathin layer supported single-atom Co-N/C structure, high mesopore volume, and synergy between the catalyst and peroxymonsulfate (PMS) with a synergy index of 16.5. This study not only reports a highly efficient and stable advanced oxidation catalyst Co-N/C, but also confirms that the nitrogen-coordinated Co sites are the primary active sites responsible for PMS activation; thus providing valuable insights into designing efficient transition metal-based AOP catalyst via coordination method for environmental remediation.

Selective occupancy of Fe substitution enhance the room-temperature ferromagnetism of (FexNi1-x)2P nanocrystals: Experimental and theoretical studies

Understanding the heteroatoms occupancy in metal phosphides is significant to regulate their properties. (FexNi1-x)2P (0 ≤ x ≤ 0.5) nanocrystals were prepared by thermal decomposition. Effects of Fe substitution on structure, morphology and magnetic properties were studied. As the Fe content increases, (FexNi1-x)2P maintain hexagonal structure with morphologies changing from sphere to irregular sheet. Atomic HAADF-STEM image reveals that the atom configuration matches well with [001] crystallographic orientation. (FexNi1-x)2P nanocrystals show room-temperature ferromagnetism. The Ms values and magnetocrystalline anisotropy are enhanced by increasing Fe substitution. The Ms value of (Fe0·5Ni0.5)2P reaches 2.092 emu/g, 19.5 times larger than Ni2P. XPS and Mössbauer spectra show that Fe2+/Fe3+ selectively occupy tetrahedral and pyramidal positions due to the difference of ionic radius, resulting in enhanced room-temperature ferromagnetism. First-principles calculations show that Fe substitution can induce ferromagnetic ordering in Ni2P, and Fe substitution in pyramidal positions induces larger magnetic moment, further confirming the experimental results.

Copper-Bearing Magnetite and Delafossite in Copper Smelter Slags

The cooling paths and kinetics in the system Cu-Fe-O are investigated by the empirical micro- and nanoscale analysis of slags from the flash furnace smelter at Olympic Dam, South Australia. We aim to constrain the exsolution mechanism of delafossite (Cu1+Fe3+O2) from a spinel solid solution (magnetite, Fe3O4) and understand why cuprospinel (CuFe2O4) is never observed, even though, as a species isostructural with magnetite, it might be expected to form. Flash furnace slags produced in the direct-to-blister copper smelter at Olympic Dam contain four Cu-bearing phases: Cu-bearing magnetite, delafossite, metallic copper, and cuprite. Delafossite coexists with magnetite as rims and lamellar exsolutions, as well as bladed aggregates, associated with cuprite within Si-rich glass. The empirical compositions of magnetite and rim delafossite are (Fe2+6.89Cu2+0.86Co0.13Mg0.15Si0.02)8.05 (Fe3+15.52Al0.41Ti0.01Cr0.01)15.95O32, and (Cu1+0.993Co0.002Mg0.002)0.997(Fe3+0.957Al0.027Ti0.005Si0.004)0.993O2, respectively. The measured Cu content of magnetite represents a combination of a solid solution (~6 mol.% cuprospinel endmember) and exsolved delafossite lamellae. Atomic-resolution high-angle annular dark field scanning transmission electron microscope (HAADF STEM) imaging shows epitaxial relationships between delafossite lamellae and host magnetite. Defects promoting the formation of copper nanoparticles towards the lamellae margins suggest rapid kinetics. Dynamic crystallization under locally induced stress in a supercooled system (glass) is recognized from misorientation lamellae in delafossite formed outside magnetite grains. The observations are concordant with crystallization during the cooling of molten slag from 1300 °C to <1080 °C. Melt separation through an immiscibility gap below the solvus in the system Cu-Fe-O is invoked to form the two distinct delafossite associations: (i) melt-1 from which magnetite + delafossite form; and (ii) melt-2 from which delafossite + cuprite form. Such a path also corroborates the published data explaining the lack of cuprospinel as a discrete phase in the slag. Delafossite rims form on magnetite at a peritectic temperature of ~1150 °C via a reaction between the magnetite and copper incorporated in the oxide/Si-rich melt. The confirmation of such a reaction is supported by the observed misfit orientation (~10°) between the rim delafossite and magnetite. HAADF STEM imaging represents a hitherto underutilized tool for understanding pyrometallurgical processes, and offers a direct visualization of phase relationships at the smallest scale that can complement both experimental approaches and theoretical studies based on thermodynamic modelling.

Experimental and first-principles characterization of heterogeneous nucleation of η on the E(Al18Mg3Cr2) phase in the 7075 Al alloys

In this work, the heterogeneous nucleation process in the early ageing period is systematically analyzed by means of electron microscope images and phase transition crystallography calculations with a near-row matching model, which illustrate the phenomenon that the surface of the E phase directly nucleates the equilibrium phase η instead of following the ageing sequence. It is found that the structures of E phase and η phase at the interface have high similarity, and atom matching can be realized in several crystal directions and crystal planes. Through a combination of experimental observation and first-principles, HAADF-STEM images from three different E/η interfaces are characterized in detail, and they are found to belong to three different views of the same orientation relation (OR), while the segregation behavior at such E/η interfaces is calculated. By calculating and analyzing interfacial properties at the atomic level, this basic data will be useful for more in-depth analyses in the future, as well as facilitating the design and updating of materials.

Occurrence state and enrichment mechanism of rhenium in molybdenite from Merlin Deposit, Australia

Rhenium (Re) is one of the critical metals, and molybdenite (MoS2) is the main host mineral for Re. Previous studies have shown that the distribution of Re in molybdenite is highly heterogeneous, and the occurrence state and enrichment mechanism of Re in molybdenite is not clear. The molybdenite of Merlin deposit has typical Re rich characteristics, which is an ideal object to study the occurrence state and enrichment mechanism of Re in molybdenite. Based on the geological research of the deposit, this study systematically investigated the Re-rich molybdenite of the Merlin deposit using LA-ICP-MS and STEM techniques. The LA-ICP-MS results showed that the content of Re in molybdenite ranged from 67 to 2726 ppm, and the distribution of Re content in molybdenite was extremely uneven and irregular. The STEM results showed that there was no independent Re mineral in molybdenite, and Re atoms were found to substitute molybdenum (Mo) atoms in situ in the Re-rich area of molybdenite. The substitution mechanism of Re was clarified at the atomic scale in a visual form, and a corresponding rigid sphere stacking model was established. Mo atoms are sandwiched between two layers of S atoms, while Re atoms enter the molybdenite lattice and substitute Mo atoms in situ, thus changing the lattice structure of the original molybdenite matrix. Rare lattice defects such as five-fold twinning were identified in the Re-rich area of molybdenite in high-resolution HAADF STEM images, which are related to plastic deformation caused by the low stacking fault energy and stress of hexagonal molybdenite. It is speculated that lattice defects such as five-fold twinning is an important mechanism for the enrichment of Re in molybdenite. The magmatic-hydrothermal fluids released from the intrusion of igneous rocks may have provided the main source of ore-forming materials for the Merlin deposit. The ore-forming fluids, which are rich in Mo4+ and Re4+, underwent redox reactions with carbonaceous shale, resulting in the decrease in temperature and oxygen fugacity, and the increase in S2- concentration. This ultimately led to the precipitation and enrichment of Re4+ and Mo4+ to form the Re-rich molybdenite in the Merlin deposit.

Cadmium immobilization during nitrate-reducing Fe(II) oxidation by Acidovorax sp. BoFeN1: Contribution of bacterial cells and secondary minerals

Microbially nitrate-reducing Fe(II) oxidation (NRFO) is widespread in anoxic environments at circumneutral pH, in which Fe(II) oxidation is considered to be mainly catalyzed by nitrite via chemodenitrification after microbial nitrate reduction. Cadmium (Cd) is toxic to organisms, and its availability is strongly affected by iron (oxyhydr)oxides and/or bacteria. However, it remains unclear how NRFO affects the fate of Cd with environmentally relevant concentrations, and the relative contributions of microbes and secondary minerals to Cd immobilization during NRFO. Here, Acidovorax sp. BoFeN1, a typical NRFO bacterium, was used to investigate the Cd immobilization during NRFO at pH 7.0 with 0.1, 0.5, and 1.0 mg/L Cd, respectively. Higher initial Cd concentration resulted in a higher amount of Cd immobilized in precipitates, but showed stronger inhibition on nitrate reduction, acetate metabolism, and cell growth. The presence of Cd barely influenced the Fe(II) oxidation during NRFO or chemodenitrification, but retarded the mineral transformation to secondary crystalline minerals, i.e., lepidocrocite and goethite. The extracellular polymeric substances deposited in the bacteria-mineral precipitates were rich in tyrosine-like and tryptophan-like proteins with negatively charged amino and carboxyl groups. The STEM-HAADF images with EDX mapping demonstrated that Cd was strongly co-localized with Fe on the cell surface and in the periplasm. Cadmium immobilization experiments with bacterial cells or during chemodenitrification indicated that the bacterial cells and secondary minerals formed during chemodenitrification are comparably important for Cd immobilization in the Cd0.1 treatment, while the secondary minerals played a predominant role in Cd immobilization in the Cd0.5 and Cd1.0 treatments. These results suggested that Cd was likely associated with the secondary iron minerals, which precipitated with the proteins on the cell surface and in the periplasm. Our findings provide a comprehensive understanding of the roles of Cd, bacterial cells, and secondary minerals formed via chemodenitrification, as well as their interactions during the complicated NRFO process at circumneutral pH.