High-angle Annular Dark-field Scanning Transmission Electron Microscopy Research Articles

Statistical 3D morphology characterization of vaterite microspheres produced by engineered Escherichia coli

Hollow vaterite microspheres are important materials for biomedical applications such as drug delivery and regenerative medicine owing to their biocompatibility, high specific surface area, and ability to encapsulate a large number of bioactive molecules and compounds. We demonstrated that hollow vaterite microspheres are produced by an Escherichia coli strain engineered with a urease gene cluster from the ureolytic bacteria Sporosarcina pasteurii in the presence of bovine serum albumin. We characterized the 3D nanoscale morphology of five biogenic hollow vaterite microspheres using 3D high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) tomography. Using automated high-throughput HAADF-STEM imaging across several sample tilt orientations, we show that the microspheres evolved from a smaller more ellipsoidal shape to a larger more spherical shape while the internal hollow core increased in size and remained relatively spherical, indicating that the microspheres produced by this engineered strain likely do not contain the bacteria. The statistical 3D morphology information demonstrates the potential for using biogenic calcium carbonate mineralization to produce hollow vaterite microspheres with controlled morphologies. Statement of significanceThe nanoscale 3D structures of biomaterials determine their physical, chemical, and biological properties, however significant efforts are required to obtain a statistical understanding of the internal 3D morphology of materials without damaging the structures. In this study, we developed a non-destructive, automated technique that allows us to understand the nanoscale 3D morphology of many unique hollow vaterite microspheres beyond the spectroscopy methods that lack local information and microscopy methods that cannot interrogate the full 3D structure. The method allowed us to quantitatively correlate the external diameters and aspect ratios of vaterite microspheres with their hollow internal structures at the nanoscale. This work demonstrates the opportunity to use automated transmission electron microscopy to characterize nanoscale 3D morphologies of many biomaterials and validate the chemical and biological functionality of these materials.

Structural analysis of η14 phase and its forming mechanism in Al-Zn-Mg-Cu alloy

The structural characteristic and formation mechanism of a latest reported η14 phase formed in Al-Zn-Mg-Cu alloy were systematically studied by high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM). By detail structural analysis, the orientation relationship (OR) of the η14 and Al matrix is modified as: (112¯0)η14//(11¯2¯)Al, [1¯103]η14//[1¯11¯]Al and [1¯100]η14//[3¯1¯1¯]Al. Structural analysis demonstrated that the η14 phase displays the C14 Laves stacking structure, however, the Zn1 and Zn2 columns composed the R'-units are only half filled by Zn. This phenomenon is mainly due to the Zn1–2 columns composed the R'-units and Zn3 column composed the R-units formed with different processes, leading to uneven intensity among these columns for the η14 phase. The formation mechanism of the η14 phase does not shows any relevance with the faulted layers reported in previous work. In contrast, The η14 phase is suggested to evolved from a metastable η14’ phase with OR of (0001)η14’//(021¯)Al and [112¯0]η14’//[11¯2¯]Al, formed by regularly replacements of Zn and Mg at the (021¯)Al habit planes The special habit plane of η14’ phase contributes to the unusual OR of η14 phase with Al matrix. Our findings extend the understanding of the precipitates formation and evolution appearing in Al-Zn-Mg-Cu alloys.

The γ” Phase in Mg-RE-TM Alloys: A Review on the Structure and Stability of the γ” Phase and Its Effect on Mechanical Properties

In magnesium–rare earth–transition metal (Mg-RE-TM) alloys, the γ” phase (with a hexagonal structure with the space group P6¯2m) is a critical strengthening phase that can significantly improve their mechanical properties. However, compared to other phases in Mg-RE-TM alloys, research on the γ” phase is less documented, and an understanding of the γ” phase is not well established. As a result, different models of the structure of the γ” phase have been proposed. In this review, we summarize these structural models and find that the γ” phase is different from the Guinier–Preston (G.P.) zone, as revealed via Cs-corrected high-angle annular dark field-scanning transmission electron microscopy (HAADF-STEM) and density functional theory (DFT) calculations. In addition, we briefly summarize the stability of the γ” phase and its effect on the mechanical properties of Mg-RE-TM alloys.

Study on the Microstructure of Mg-4Zn-4Sn-1Mn-xAl As-Cast Alloys.

In this study, the microstructure of the Mg-4Zn-4Sn-1Mn-xAl (x = 0, 0.3 wt.%, denoted as ZTM441 and ZTM441-0.3Al) as-cast alloys was investigated using scanning electron microscopy (SEM), focused-ion/electron-beam (FIB) micromachining, transmission electron microscopy (TEM), and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM). The analysis results revealed that the microstructure of the ZTM441 and ZTM441-0.3Al as-cast alloys both mainly consist of the α-Mg matrix, skeleton-shaped MgZn2 eutectic texture, block-shaped Mg2Sn, and Zn/Sn-rich nanoscale precipitate bands along the grain boundary and the interdendrite. Nanoscale α-Mn dispersoids formed in the grain in the ZTM441 alloy, while no α-Mn formed in the ZTM441-0.3Al alloy instead of nanoscale Al3Mn2 particles. In the ZTM441 as-cast alloy, part of the Zn element is dissolved into the α-Mn phase, and part of the Mn element is dissolved into the MgZn2 phase, but in the ZTM441-0.3Al alloy, there are no such characteristics of mutual solubility. Zn and Mn elements are easy to combine in ZTM441 as-cast alloy, while Al and Mn are easy to combine in ZTM441-0.3Al as-cast alloy. The Mg-Zn phases have not only MgZn2-type crystal structure but also Mg4Zn7- and Mg149Zn-type crystal structure in the ZTM441-0.3Al as-cast alloy. The addition of Al changes the combination of Mn and Zn, promotes the formation of Al3Mn2, and the growth of the grain.

Advanced approach of bulk (111) 3C-SiC epitaxial growth

3C-SiC films grown on (111) Si substrates exhibit poor crystal quality and experience wafer cracks and bowing preventing access to bulk growth. This work reports innovative Chemical Vapor Deposition (CVD) growth methodology on 4 in. Si substrates which allowed the growth of 230 mm thick layer of (111) 3C-SiC through the melting of the Si substrate in the CVD chamber and the adoption of the resulting free standing 3C-SiC for the growth of bulk (111) 3C-SiC layer under high N fluxes. From the molten KOH etching and subsequent SEM investigation it has been ascertained that with a N2 flux of 1600 sccm there is a significant reduction in the concentration of stacking faults (SFs) from (7.16 ± 0.04) × 103 cm−1 to (0.4 ± 0.3) × 103 cm−1. This reduction is consistent with the cross section m-PL response displaying steep and uniform increase in the intensity of the band-edge signal a factor 10 higher on the surface with respect to the equal (100) 3C-SiC grown thickness. Furthermore, the emission attributed to point defects is considerably lower in (111) 3C-SiC. From Scanning Transmission Electron Microscopy (STEM) investigation it appears evident how the typical mechanism valid in (100) growths consisting in the mutual closure of SFs coming from opposing {111} planes that give rise to Lomer and l-shaped dislocations is replaced by a different panorama of evolution. Indeed, it is shown how the SFs shred but do not interrupt each other during growth. Furthermore, dropping in the number of atomic planes composing SFs layers appears to be a key phenomenon leading to both the shrinkage of the number of SF atomic layers as well as to the SF self-closure. High Angle Annular Dark Field-Scanning Transmission Electron Microscopy (HAADF-STEM) attested how the crystal tends to smooth out the lattice mismatch until the SF is suppressed. Because of the foregoing, the mechanisms of evolution of the defects in (111) 3C-SiC revealed in this study, demonstrates how the growth parameters must be matched with the kinetics of the defects in order to endorse (111) 3C-SiC adoption in high performing devices.

Nucleation and evolution mechanism of precipitates during the initial aging stage in Al-Zn-Mg-Cu alloy

The evolution process and nucleation mechanism of the precipitates during the initial aging stage in Al-Zn-Mg-Cu alloy were studied using the combination of the Cs-corrected high-angle annular dark field-scanning transmission electron microscopy (HAADF-STEM) and first-principles calculations. A new nucleation mode of η' phase with the thickness of 7 at. layers was found, which is parallel to {111}Al planes and consists of 5-layer transition structures and 2 transition layers at the interface. The precipitate is the initial aged η' phase with the simplest structure. Its interfaces are coherent with the Al matrix and meet the stacking periodicity, which is more stable than ordinary η' phases. Besides, the interfaces have a tendency to segregate heavy elements. The presence of segregating atoms enhances the interface interaction and makes it more stable. These two aspects improve the stability of the nano-scale 7-layer particle and maintain the aging strengthening effect of Al alloy.

Thermodynamic modelling for the long-period stacking ordered phase in Mg–Y–Al system: Construction and application

An adequate thermodynamic model is required for describing the long-period stacking ordered (LPSO) phase because previously developed models cannot account for the accurate atomic occupancy of the LPSO structure. In this study, atomic structures of the equilibrated 18R and 10H phases in Mg–Y–Al alloys were systematically investigated via atomic-resolution high-angle annular dark-field scanning transmission electron microscopy (HAADF-TEM) and first-principles calculations. Three structures (α-type, β-type, and γ-type) with different numbers of Mg layers (one, two, and three, respectively) sandwiched between L12 structures were present in the 18R phase. Meanwhile, δ-type (without Mg layers), ε-type (with one Mg layer), and ζ-type (with two Mg layers) structures were detected in the 10H phase. The presence of the interstitial atoms in the L12 structure was confirmed by TEM and theoretical calculations. Based on the obtained results, a new thermodynamic model for the LPSO phase was constructed and successfully applied to establish the thermodynamic description of the Mg–Y–Al system. The CALPHAD-type calculation data obtained using the proposed thermodynamic description were in good agreement with the experimental results. The findings of this study can help elucidate the formation mechanism of LPSO phases and guide Mg–Y–Al alloy design.

CO rich syngas production from catalytic CO2 gasification-reforming of biomass components on Ni/CeO2

The advancement of biomass utilization technology is vital for addressing global climate change and the depletion of fossil resources. CO rich syngas production from the CO2 gasification-reforming of biomass components (cellulose, xylan, lignin, and starch) was investigated by a two-stage fixed bed reactor. The four biomass components were heated from room temperature to 600 °C in a CO2 atmosphere, among which lignin exhibited the highest residue fraction (51.6 wt.%) and the lowest gas yield (5.7 mmol gbiomass−1), whereas cellulose demonstrated the highest gas yield from CO2 gasification-reforming (14.6 mmol gbiomass−1). Nanorod CeO2 supported Ni catalysts were used to enhance volatile (tar) CO2 reforming reactions, and the gas yield increased by 104.3 % for cellulose, 103.3 % for xylan, 149.3 % for lignin, and 128.3 % for starch compared with noncatalytic CO2 gasification-reforming. Furthermore, the structure of fresh and used catalysts were characterized by X-ray diffraction (XRD), N2 adsorption/desorption isotherm, scanning electron microscope (SEM), high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), H2-temperature programmed reduction (H2-TPR), and thermogravimetry (TGA). For the CO2 gasification-reforming of cellulose, the optimum CO yield of 27.6 mmol gbiomass−1 was achieved at a gasification/reforming temperature of 600/700 °C and a biomass/catalyst ratio of 6.25 using 2 %Ni/CeO2 catalyst. The stability tests of catalysts showed an approximately linear decrease in CO yield with the increase of the number of cycles, and catalyst deactivation resulted from Ni sintering and coke deposition.

Modulating the Coordination Environment of Carbon-Dot-Supported Fe Single-Atom Nanozymes for Enhanced Tumor Therapy.

Herein, carbon dot (CD)-supported Fe single-atom nanozymes with high content of pyrrolic N and ultrasmall size (ph-CDs-Fe SAzyme) are fabricated by a phenanthroline-mediated ligand-assisted strategy. Compared with phenanthroline-free nanozymes (CDs-Fe SAzyme), ph-CDs-Fe SAzyme exhibit higher peroxidase (POD)-like activity due to their structure similar to that of ferriporphyrin in natural POD. Aberration-corrected high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) and X-ray absorption fine structure spectroscopy (XAFS) analyses show that metal Fe is dispersed in ph-CDs-Fe SAzyme as single atoms. Steady-state kinetic studies show that the maximum velocity (Vmax ) and turnover number (kcat ) of H2 O2 homolyticcleavage catalyzed by ph-CDs-Fe SAzyme are 3.0 and 6.2 more than those of the reaction catalyzed by CDs-Fe SAzyme. Density functional theory (DFT) calculations show that the energy barrier of the reaction catalyzed by ph-CDs-Fe SAzyme is lower than that catalyzed by CDs-Fe SAzyme. Antitumor efficacy experiments show that ph-CDs-Fe SAzyme can efficiently inhibit the growth of tumor cells both in vitro and in vivo by synergistic chemodynamic and photothermal effects. Here a new paradigm is provided for the development of efficient antitumor therapeutic approaches based on SAzyme with POD-like activity.

Degraded creep resistance induced by static precipitation strengthening in high-pressure die casting Mg–Al–Sm alloy

Relationship between precipitation strengthening and creep resistance improvement has been an important topic for the widespread applications of magnesium alloys. Generally, static precipitation strengthening through thermal stable precipitates would generate satisfactory creep resistance. However, an opposite example is presented in this work and we propose that the size of precipitates plays a crucial role in controlling the operative creep mechanisms. In addition, the precipitate components along with their crystal structures in the crept Mg–4Al–3Sm–0.4Mn samples with/without pre-aging were thoroughly studied using Cs aberration-corrected high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM). Previous aging generates a large density of fine precipitates (< ∼5 nm) homogeneously distributing in Mg matrix and exhibiting satisfactory strengthening effect. However, the number density of precipitate strings consisting of several or even dozens of relatively coarse precipitates (∼10 nm) was significantly decreased at the same time. As revealed in this work, the relatively coarse particles in Mg matrix are much more efficient than the fine precipitates in promoting dislocation climb. Therefore, the rate-controlling mechanisms are transferred from dislocation climb to dislocation slip after previous aging, thus leading to degradation of creep resistance. Moreover, there are mainly five types of precipitates/clusters, namely β″-(Al, Mg)3Sm, Al5Sm3, ordered Al–Sm cluster, ordered Al–Mn cluster and ordered/unordered AlMnSm clusters. The crystal structures of the former two precipitates were discussed and the formation mechanisms of the precipitates/clusters were revealed.

Investigating the influence of Ru structures and supports on hydrogenolysis of polyethylene plastic waste

The Ru dispersion of Ru/SBA-15 was controlled by varying the triethanolamine (TEA)/Ru molar ratios to examine the influence of Ru structures on the hydrogenolysis of low-density polyethylene (LDPE) under moderate conditions (3 MPa hydrogen and 250 °C). Among the different Ru catalysts tested, Ru/SBA-15 with a TEA/Ru molar ratio of 10 (Ru/SBA-15 (10)) exhibited the best activity in LDPE hydrogenolysis. Analyses, including X-ray diffraction (XRD), CO chemisorption, CO Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), and High-angle annular dark-field-scanning transmission electron microscopy (HAADF-STEM) showed that the optimal amount of TEA (TEA/Ru = 10) increased the Ru dispersion (92%) and ultimately improved the catalytic activity of LDPE hydrogenolysis. The addition of TEA led to the formation of a [Ru(triethanolamine)2]3+ complex, which facilitated the introduction of Ru species into the confined space in SBA-15. The highly dispersed Ru species confined within SBA-15 in Ru/SBA-15 (10), along with the inherent properties of SBA-15, results in increased hydrogen coverage on the Ru species. This increase in hydrogen coverage leads to a higher liquid yield in LDPE hydrogenolysis compared to Ru/SiO2 (10). Additionally, we successfully employed Ru/SBA-15 (10) in the hydrogenolysis of postconsumer polyolefin waste, including commercial LDPE and LDPE bottles, producing valuable chemicals, such as liquid fuel and wax, in high yields of 65–70%.

Localized Exciton Anatomy and BandGap Energy Modulation in 1D MoS2 Nanostructures

This study presents an in‐depth investigation of the electronic properties and bandgap energy distribution in 1D molybdenum disulfide (1D‐MoS2) nanostructures. Through a combination of high‐angle annular dark‐field scanning transmission electron microscopy (HAADF‐STEM) and electron energy‐loss spectroscopy (EELS), it reveals significant differences between 1D‐MoS2 nanostructures and their 2D counterparts, shedding light on their localized exciton behavior and their bandgap energy modulation within the nanostructures. Excitonic peaks at around 2 and 3 eV appear localized at the ends or along the sides of the 1D‐MoS2 nanostructures, while the plasmonic resonance at 8.3 eV retains its inner‐region localization. It demonstrates the spatial dependence of the bandgap energy, with the central region exhibiting a bandgap of approximately 1.2 eV, consistent with bulk MoS2, while regions characterized by curvature‐induced local strain fields exhibit instead a noticeable reduction. The findings provide valuable insights into the intricate relationship between excitonic behavior and bandgap sensitivity in 1D‐MoS2 nanostructures, streamlining the design and optimization of nanophotonic and optoelectronic devices.

Effects of shear strain on HZO ferroelectric orthorhombic phases

The stabilities of hafnium and zirconium oxide ferroelectric orthorhombic phases, oIII-phase (Pca21) and oIV-phase (Pmn21), under shear strain are investigated theoretically by atomic modeling with density functional theory calculations. The results indicate that oIV-phase serves as a buffer state preventing oIII-phase from transforming into m-phase (P21/c) under structural distortion caused by shear strain and meanwhile preserves the remanent polarization to some certain degree. Shear strain will also induce the reduction in coercive field of the HZO film due to the distortion of oIII-phase or phase transition into oIV-phase. It is very possible to identify oIV-phase with careful analysis of high-resolution transmission electron microscopy and high-angle annular dark-field STEM images at the region having a crystal tilt angle smaller than 86°.

Transmission Electron Microscopy Peeled Surface Defect of Perovskite Quantum Dots to Improve Crystal Structure.

Transmission electron microscopy (TEM) is an excellent characterization method to analyze the size, morphology, crystalline state, and microstructure of perovskite quantum dots (PeQDs). Nevertheless, the electron beam of TEM as an illumination source provides high energy, which causes morphological variation (fusion and melting) and recession of the crystalline structure in low radiolysis tolerance specimens. Hence, a novel and facile strategy is proposed: electron beam peel [PbBr6]4- octahedron defects from the surface of QDs to optimize the crystal structure. TEM and high-angle annular dark-field scanning TEM (HAADF) tests indicate that the [PbBr6]4- octahedron would be peeled from the surface of QDs when QDs samples were irradiated under high-power irradiation, and then a clear image would be obtained. To avoid interference from a protective film of "carbon deposits" on the surface of the sample when using high resolution TEM, amorphous carbon film (15-20 nm) was deposited on the surface of QDs film and then characterized by TEM and HAADF. The detection consequences showed that the defection of PbBr2 on the surface of QDs will gradually disappear with the extension of radiation time, which further verifies the conjecture.

Microstructural evolution of Mg–14Gd–0.4Zr alloy during compressive creep

The present work reports the creep behavior and microstructural evolution of the sand-cast Mg–14Gd–0.4Zr alloy (wt.%) prepared by the differential pressure casting machine. Their compressive creep tests at 250 °C were performed under various applied stresses (i.e., 60, 80 and 100 MPa). Among them, the sand-cast Mg–14Gd–0.4Zr samples examined under 250 °C/80 MPa for 39 and 95 h, respectively, were chosen to systemically analyze their creep mechanisms using high-angle annular dark field-scanning transmission electron microscopy (HAADF-STEM). The obtained results showed that the enhancement of creep resistance can be mainly attributed to the coherent β' and β'F phases with an alternate distribution, effectively impeding the basal <a> dislocations movement. However, with the creep time increasing, the fine β'+β'F precipitate chains coarsened and transformed to semi-coherent β1 phase and even to large incoherent β phase (surrounded by precipitate-free areas) in grain interiors. The precipitate-free zones (PFZs) at grain boundaries (GBs) were formed, and they could expand during creep deformation. Apart from the main cross-slip of basal and prismatic <a> dislocations, <c + a> type dislocations were activated and tended to distribute near the GBs. The aforementioned phenomena induced the stress concentrations, consequently leading to the increment of the creep strain.

Sonication strategy for anchoring single metal atom oxides (W, Cu, Co) on CeO2-rGO for boosting electrocatalytic oxygen evolution reaction

In the field of electrocatalysis, single-atomic-layer tungsten, copper, and cobalt oxide on CeO2, ethylene diamine (ED) and reduced graphene oxide (rGO) supported materials shows tremendous potential. Despite the enormous interest in single metal atom oxide (SMAO) catalysts, it is still very difficult to directly convert readily available bulk metal oxide into single atom oxide. It is crucial and tough to create high performance materials for the oxygen evolution reaction (OER) in an alkaline environment. Herein, a single tungsten, copper and cobalt atom oxide (SMAO) anchored on the CeO2 atomic layer and overall components deposited on the rGO (rGO-ED-CeO2-WCuCo) is prepared through a one-pot sonication technique. The presence of SMAO is identified by high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) imaging. The electrocatalytic performance of final rGO-ED-CeO2-WCuCo-30 nanocomposite for the OER in 1 M KOH electrolyte is evidenced by providing low overpotential of 283 mV at 10 mA cm−2. The Tafel slope for OER using rGO-ED-CeO2-WCuCo-30 electrocatalysts is 57.03 mV dec−1. The electrocatalytic activity of rGO-ED-CeO2-WCuCo-30 nanocomposites for OER was noticeably increased when compared to bare CeO2 nanorods (401 mV), rGO-ED-CeO2-WCo-30 (345 mV), rGO-ED-CeO2-WCu-30 (340 mV) and rGO-ED-CeO2-WCuCo-20 (321 mV) samples.

Influence of Support Material on the Structural Evolution of Copper during Electrochemical CO2 Reduction

The electrochemical CO2 reduction reaction (CO2RR) provides the combined benefit of reducing anthropogenic CO2 emissions while supplying a wide variety of valuable chemical feedstock chemicals.[1] While many transition metals are CO2RR active catalysts, only copper can form products requiring more than two electrons in the electrochemical conversion of CO2. This includes a multitude of high-value-added products such as ethylene, ethanol, or propanol.[1-3] The selectivity of the CO2RR is strongly influenced by the structure and morphology of the copper catalysts. Consequently, structural properties such as copper nanoparticle size, display an influence on CO2RR product selectivity.[4] This creates a substantial problem as copper is highly susceptible to reorganization during the electrochemical CO2RR, which inhibits the retention of apparently favorable surface structures during the CO2RR.[5] As a consequence, structure-selectivity relations can neither be reliably described nor completely understood. This problem is exacerbated by a lack of understanding of the underlying reasons that drive reorganization and therefore degradation of the catalyst. At reductive potentials, as used in the electrochemical CO2RR, coalescence and subsequent agglomeration are assumed to be the dominant degradation mechanism, while oxidative potentials generally trigger support corrosion and dissolution. Strategies for stabilization to retain activity and selectivity are therefore essential. Nevertheless their importance in the electrochemical CO2RR, stabilization methods are largely missing. For other reactions, such as the oxygen reduction reaction, various studies showed an increase in overall stability by containing the catalysts in a confined space.[6-8] This work presents the stabilizing effect of three different carbon support materials with unique morphologies on copper nanoparticles (CuNPs). Through the use of a non-porous Vulcan, a Ketjenblack with a diverse pore size distribution and highly defined mesoporous graphitic spheres (MGS) with a unimodal pore size distribution, the influence on structural characteristics of the carbon material on the stabilization of the CuNPs can be presented. The representative copper catalysts were prepared by colloidal synthesis or incipient wetness impregnation and evaluated through micro-flow cell measurements with online gas chromatography (GC) analytics and pre-/post catalytic high angle annular dark field scanning transmission electron microscopy (HAADF-STEM), visualizing changes in the catalyst structure and morphology after the reaction. We demonstrate the successful stabilization of CuNPs by pore confinement against particle coalescence and subsequent agglomeration for up to 20 h of electrochemical CO2RR conditions, with GC analysis providing great insight into the influence of particle size, morphology, and location of the CuNPs on CO2RR product selectivity. Intriguingly, a high share of CO2RR products was observed for stabilized < 4 nm CuNPs putting the established particle size effect for the CO2RR, where the hydrogen evolution reaction was predominantly observed on nanoparticles below 15 nm, into question.[4] These results present new ways of stabilizing CuNPs and highlight the need for post-catalytic evaluation of the catalysts' morphological state to precisely derive performance-structure indicators.Work accepted and soon to be published in ChemElectroChemdoi.org/10.1002/celc.202200924Literature:[1] S. Nitopi, E. Bertheussen, S. B. Scott, X. Liu, A. K. Engstfeld, S. Horch, B. Seger, I. E. L. Stephens, K. Chan, C. Hahn, J. K. Nørskov, T. F. Jaramollo, I. Chorkendorff, Chem Rev. 2019, 119, 7610–7672.[2] Y. Hori, Modern Aspects of Electrochemistry No. 42, Springer, 2008.[3] K. P. Kuhl, E. R. Cave, D. N. Abram, T. F. Jaramillo, Energy Environ. Sci. 2012, 5, 7050–7059.[4] R. Reske, H. Mistry, F. Behafarid, B. Roldan Cuenya, P. Strasser, J. Am. Chem. Soc. 2014, 136, 6978–6986.[5] W. T. Osowiecki, J. J. Nussbaum, G. A. Kamat, G. Katsoukis, M. Ledendecker, H. Frei, A. T. Bell, A. P. Alivisatos, ACS Appl. Energy Mater. 2019, 2, 7744–7749.[6] Y. Li, F. Cui, M. B. Ross, D. Kim, Y. Sun, P. Yang, Nano Lett. 2017, 1312–1317.[7] Y. T. Guntern, J. R. Pankhurst, J. Vavra, M. Mensi, V. Mantella, P. Schouwink, R. Buonsanti, Angew. Chem. 2019, 131, 12762–12769.[8] C. Galeano, J. C. Meier, V. Peinecke, H. Bongard, I. Katsounaros, A. A. Topalov, A. Lu, K. J. J. Mayrhofer, F. Schüth, J. Am. Chem. Soc. 2012, 134, 20457 – 20465. Figure 1

Size-Controlled Synthesis of IrO2 nanoparticles at High Temperatures for the Oxygen Evolution Reaction

Polymer electrolyte membrane (PEM) electrolysis is considered to play a vital role in the sustainable energy transition. The efficient generation of hydrogen is largely influenced by the slow rate of the anodic oxygen evolution reaction (OER). Iridium oxide represents one of the most promising catalysts for the electrochemical oxidation of water in an acidic environment. Under harsh operating conditions at the anode, iridium oxide is found to be among the most dissolution-resistant catalysts while offering acceptable OER activity. However, iridium’s limited availability dictates high costs centralizing the research in direction of reducing noble metal content while maintaining favorable electrochemical properties.[1] Designing nanostructured catalyst with an increased surface-to-volume ratio improves the application-oriented mass-specific activity.[2] Hydrous iridium oxide is known for superior OER activity, but for a successful application, drastic dissolution of the catalyst must be addressed by stabilization. This can be achieved by heat treatment to temperatures ≥400ºC with the formation of crystalline order. However, managing to avoid agglomeration of nanoparticles at high temperatures is not trivial, thus, temperature studies on electrochemical stability and activity on similar particle sizes are missing.[3]. Here, we demonstrate how nanoparticles below 10 nm can be obtained at high preparation temperatures up to 800 °C with unprecedented control over particle size and morphology. A detailed understanding of the structural evolution during heating was obtained by in-situ scanning transmission electron microscopy (in-situ STEM) with locally resolved nanoparticles, high spatial resolution, and chemical specificity. Additionally, changes in surface properties at different temperatures were tracked ex-situ by X-ray photoelectron spectroscopy (XPS), the crystal structure was investigated by X-ray diffraction analysis (XRD), size and morphology were characterized by high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM). The OER activities of synthesized iridium oxide nanoparticles were measured in half-cell measurements at forced convection. The stability was carefully studied by operando flow cell measurements that were coupled to online inductively coupled plasma mass spectrometry.[4] The iridium oxide catalyst calcined at the lowest temperature resulted in outstanding mass-specific activity outperforming the reference iridium oxide catalyst by a factor of 40. By gradual increase in calcination temperatures up to 800 °C, we observe improvement in the durability of the synthesized catalysts, being comparable to the reference catalyst, yet still with notable improvement in catalytic activity. This is the first report to synthesize iridium oxide nanoparticles at high temperatures with preserved size and morphology not exceeding 10 nm and allows for the determination of activity and durability of similarly sized nanostructures.[5] Literature:[1] M. Bernt et al. Chemie Ingenieur Technik 2020, 92, 31-39.[2] T. Reier, et al. ACS Catalysis 2012, 2, 1765-1772.[3] Y. Lee et al. The Journal of Physical Chemistry Letters 2012, 3, 399-404.[4] S. Geiger et al. Nature Catalysis 2018, 1, 508-515.[5] M.Malinovic et al. Advanced Energy Materials 2022, Manuscript submitted for publication.

Multicomponent Mixed Ionic and Electronic Conducting (MIEC) Air Electrodes and Their Heterointerface for Effective Application in Reversible Solid Oxide Cell (R-SOC): Efficacy of Stoichiometric and Off-Stoichiometric Compositions

Reversible solid oxide cell (R-SOC) has triggered research interest from the past few years for its reversible applicability for power-to-fuel (P2F) in electrolyser cell (SOEC) to generate hydrogen by splitting of high temperature steam and fuel-to-power (F2P) in fuel cell (SOFC) mode of operation to produce power by oxidation of fuel with >80% conversion efficiency [1]. Primarily, R-SOC consists of a dense electrolyte conventionally yttria stabilized zirconia (YSZ), bridged between two porous electrodes, e.g., strontium doped lanthanum manganite (LSM) as air electrode and Nickel (Ni)-YSZ cermet as fuel electrode. Significant research has been devoted to carry out the long-term stability (≥ 2000 h) degradation in endurance test primarily due to major challenges like highly humid environment in the fuel electrode, microstructural flaws and polarization losses under high operational temperature. In this context, mixed ionic and electronically conducting (MIEC) oxide-based La-Sr-Co-Fe-O (LSCF) and Ba-Sr-Co-Fe-O (BSCF) air electrodes has drawn significant attention for the effective oxygen reduction (ORR) and oxygen evolution (OER) reaction in the context of SOFC and SOEC operational mode [2,3].An attempt has been made in this present research work to establish the effect of off-stoichiometry in A-site lattice over the stoichiometric pristine compositions of La/Ba-Sr-Co-Fe-O (LSCF/BSCF)-based multicomponent MIEC-oxides and their associated heterostructure with optimised off-stoichiometric compositions that governs the electrochemical redox phenomena in conjunction with the functionally engineered Gd-doped ceria-based interlayer and buffer layer. Vacancy driven oxygen reduction (ORR) and oxide ion oxidation (OER) kinetics as a function of aliovalent charge compensation mechanism associated with the defect chemistry has also been investigated for these A-site non-stoichiometric compositions in the range of 1-0.92 for La/Ba(1-x)SrxCo1-yFeyO3-δ [x=0.4 and y=0.2]. A comparative study between the performance of the off-stoichiometric heterostructure synthesised using the optimised pristine compositions with that of the stoichiometric heterointerface has also been evaluated for their efficiency as catalyst for OER/ORR in reversible SOC mode of operation. The increasing trend of oxygen non-stoichiometry (δ) upon the transition from A-site stoichiometry to non-stoichiometric compositions reflected the formation of higher aliovalent cation of ‘Co’ to maintain the charge neutrality and thereby accelerates the associated oxygen redox kinetics. An optimum δ-value of 0.33 and 0.37 are found for LSCF and BSCF-compositions respectively having A-site non-stoichiometry of 0.94. Current density of 0.45 A.cm-2 at an applied voltage of 1.5 V @800℃ is obtained for stoichiometric composition of LSCF wherein 0.5 A.cm-2 is found for the off-stoichiometric batch corresponding to the hydrogen flux of 0.21Nl.h-1.cm-2. Such augmentation in performance may be related to the formation of higher lattice defect which eases the path for oxide ion oxidation. Formation of the interpenetrating heterointerface network at the air electrodes and electrode/electrolyte interfaces in the morphologically engineered cells having gradation in porosity has further been confirmed from Transmission Electron Microscopy (TEM) studies involving the high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and the STEM-EDS elemental spectrum mapping images of the cross-section of the sintered cells and clinically corelated with SOC performances. Orientation of the crystal lattice planes in the two types of grains across the heterojunction has also been determined from the High-Resolution Transmission Electron Microscopy (HRTEM) images. Further study to envisage the presumed hypothesis, based on the rate determining steps involving the defect lattice formation and probable mechanism for selective ORR/OER application, will also be described through hole and/or oxygen vacancy formalism using XPS and density functional theory (DFT) for such hetero-interpenetrating air electrodes.

Supported gold nanoparticles prepared from NHC-Au complex precursors as reusable heterogeneous catalysts

Gold nanoparticles supported on SiO2, Al2O3, TiO2, and carbon (Maxsorb, Ketjen black and Shirasagi New Gold) were prepared by impregnation and solid grinding methods using N-heterocyclic carbene (NHC)-Au complexes as precursors. A total of nine NHC-Au complex precursors were tested by impregnation and solid grinding method, and [Au(OH)(IPr)] (IPr = 1,3-bis(2,6-diisopropylphenyl)1,3-dihydro-2H-imidazol-2-ylidene) complex led to (1–2 nm) particle sizes by both methods. The prepared catalysts were characterized by XRD (X-ray diffraction), HAADF-STEM (high-angle annular dark-field scanning transmission electron microscopy), in situ XAFS (X-ray absorption fine structure), etc. In addition, [Au(OH)(IPr)] and its impregnated [Au(OH)(IPr)]/SiO2 were subjected to TG-DTA (thermogravimetry analysis-differential thermal analysis) and in situ XAFS analysis under calcination conditions, respectively. Moreover, the obtained supported Au nanoparticles (NPs) were effective as catalysts for low-temperature CO oxidation, intramolecular cyclization of alkynyl carboxylic acids, and isomerization of allylic esters. Their results suggested that the reduction of [Au(OH)(IPr)] proceeds rapidly from around 300°C, and the residual ligands should be attached to the surface of the Au NPs and protect them from drastic agglutination. These results reveal that properly designed NHC-Au complexes can facilitate the preparation of supported Au NP catalysts with small particle sizes and high activities.