Generation of maghemite nanocrystals from iron–sulfur centres

Iron oxide nano-crystals 0.1–1.1 μm in diameter were generated on sulfur-doped amorphous carbon surfaces by electron beam irradiation of the novel 13e− high-spin complex [Fe(4-methyl-1,2-benzenedithiolate)2][NHEt3] encapsulated in a triblock copolymer. Possible relevance to iron nano-mineralization from Fe–S ferredoxin proteins and iron dysregulation in neurological disorders is discussed. Graphical abstract: Generation of maghemite nanocrystals from iron–sulfur centres Iron is an essential element for mammals and, amongst many other functions, plays an important role in the human brain.1 Recent research has indicated a strong association between iron dysregulation and Alzheimer's disease (AD), although it is unknown how the chemical and magnetic state of iron is linked to AD pathogenesis.2–4 Reports from Collingwood et al., and Dobson et al., for example, have shown the presence of iron oxide as the mixed oxidation state mineral, magnetite (Fe3O4) in AD tissue, a possible source of redox-active iron, but it remains unclear how this kind of iron mineral forms in the tissue.5,6 These unsolved and important questions have led us to consider how atomic resolution microscopy might provide new insight into nanoscale iron mineralization. Recently we reported methodology for studies of the nano-mineralisation of osmium, gold, ruthenium and iridium from their respective 1,2-dicarba-closo-dodecarborane-1,2-dithiolate complexes encapsulated in polymer micelles upon electron beam irradiation.7–9 Here we report the synthesis and characterization of the novel 13e− iron(iii) complex [Fe(4-methyl-1,2-benzenedithiolate)2][NHEt3] (1), containing Fe–S bonds analogous to those in the ubiquitous iron–sulfur ferredoxin proteins. Importantly, recent research has indicated a strong relationship between neurodegenerative disorders and defective Fe–S clusters.10,11 We have characterized complex 1 using Mössbauer, Raman and far-infra red spectroscopy, and investigated the generation of iron nanocrystals from 1 encapsulated in a poly(ethylene glycol)-block-poly(propylene glycol)-block-poly(ethylene glycol) polymer (Scheme 1) by electron beam irradiation, and used electron energy-loss spectra (EELS) to identify the oxidation state of iron and its coordination environment in the nanocrystals

Characterization of Ordering in A-Site Deficient Perovskite Ca1-xLa2x/3TiO3 Using STEM/EELS

The vacancy ordering behavior of an A-site deficient perovskite system, Ca1-xLa2x/3TiO3, was studied using atomic resolution scanning transmission electron microscopy (STEM) in conjunction with electron energy-loss spectroscopy (EELS), with the aim of determining the role of A-site composition changes. At low La content (x = 0.2), adopting Pbnm symmetry, there was no indication of long-range ordering. Domains, with clear boundaries, were observed in bright-field (BF) imaging, but were not immediately visible in the corresponding high-angle annular dark-field (HAADF) image. These boundaries, with the aid of displacement maps from A-site cations in the HAADF signal, are shown to be tilt boundaries. At the La-rich end of the composition (x = 0.9), adopting Cmmm symmetry, long-range ordering of vacancies and La3+ ions was observed, with alternating La-rich and La-poor layers on (001)p planes, creating a double perovskite lattice along the c axis. These highly ordered domains can be found isolated within a random distribution of vacancies/La3+, or within a large population, encompassing a large volume. In regions with a high number density of double perovskite domains, these highly ordered domains were separated by twin boundaries, with 90° or 180° lattice rotations across boundaries. The occurrence and characteristics of these ordered structures are discussed and compared with similar perovskite systems

Design, identification and evolution of surface ruthenium (II/III) single-site for CO activation

Ru(II) compounds are widely used in catalysis, photocatalysis and medical applications. They are usually obtained in reductive environment as molecular O 2 can oxidize Ru(II) to Ru(III) and Ru(IV). Here we report the design, identification and evolution of an air‐stable surface ‐[bipy‐Ru(II)(CO) 2 Cl 2 ] site that is covalently mounted onto a polyphenylene framework. Such Ru(II) site was obtained by reduction of ‐[bipy‐Ru(III)Cl 4 ] ‐ with simultaneous ligand exchange from Cl ‐ to CO. This structural evolution was witnessed by a combination of in situ X‐ray and infrared spectroscopy studies. The ‐[bipy‐Ru(II)(CO) 2 Cl 2 ] site enables oxidation of CO with a turnover frequency of 0.73 × 10 ‐2 s ‐1 at 462 K, while the Ru(III) site is completely inert. This work contributes to the studies of structure‐activity relationship by demonstrating a practical control over both geometric and electronic structures of single‐site catalysts at molecular level, which can be further applied in other single site catalyst researches

Design, identification, and evolution of a surface ruthenium(II/III) single site for CO activation

RuII compounds are widely used in catalysis, photocatalysis, and medical applications. They are usually obtained in a reductive environment as molecular O2 can oxidize RuII to RuIII and RuIV. Here we report the design, identification and evolution of an air-stable surface [bipy-RuII(CO)2Cl2] site that is covalently mounted onto a polyphenylene framework. Such a RuII site was obtained by reduction of [bipy-RuIIICl4]− with simultaneous ligand exchange from Cl− to CO. This structural evolution was witnessed by a combination of in situ X-ray and infrared spectroscopy studies. The [bipy-RuII(CO)2Cl2] site enables oxidation of CO with a turnover frequency of 0.73×10−2 s−1 at 462 K, while the RuIII site is completely inert. This work contributes to the study of structure–activity relationship by demonstrating a practical control over both geometric and electronic structures of single-site catalysts at molecular level

Stabilized tilted-octahedra halide perovskites inhibit local formation of performance-limiting phases.

Efforts to stabilize photoactive formamidinium (FA)–based halide perovskites for perovskite photovoltaics have focused on the growth of cubic formamidinium lead iodide (α-FAPbI3) phases by empirically alloying with cesium, methylammonium (MA) cations, or both. We show that such stabilized FA-rich perovskites are noncubic and exhibit ~2° octahedral tilting at room temperature. This tilting, resolvable only with the use of local nanostructure characterization techniques, imparts phase stability by frustrating transitions from photoactive to hexagonal phases. Although the bulk phase appears stable when examined macroscopically, heterogeneous cation distributions allow microscopically unstable regions to form; we found that these transitioned to hexagonal polytypes, leading to local trap-assisted performance losses and photoinstabilities. Using surface-bound ethylenediaminetetraacetic acid, we engineered an octahedral tilt into pure α-FAPbI3 thin films without any cation alloying. The templated photoactive FAPbI3 film was extremely stable against thermal, environmental, and light stressors

Adsorption and activation of molecular oxygen over atomic copper(I/II) site on ceria

Supported atomic metal sites have discrete molecular orbitals. Precise control over the energies of these sites is key to achieving novel reaction pathways with superior selectivity. Here, we achieve selective oxygen (O2) activation by utilising a framework of cerium (Ce) cations to reduce the energy of 3d orbitals of isolated copper (Cu) sites. Operando X-ray absorption spectroscopy, electron paramagnetic resonance and density-functional theory simulations are used to demonstrate that a [Cu(I)O2]3− site selectively adsorbs molecular O2, forming a rarely reported electrophilic η2-O2 species at 298 K. Assisted by neighbouring Ce(III) cations, η2-O2 is finally reduced to two O2−, that create two Cu–O–Ce oxo-bridges at 453 K. The isolated Cu(I)/(II) sites are ten times more active in CO oxidation than CuO clusters, showing a turnover frequency of 0.028 ± 0.003 s−1 at 373 K and 0.01 bar PCO. The unique electronic structure of [Cu(I)O2]3− site suggests its potential in selective oxidation

TEM analysis of the microstructure in TiF3-catalyzed and pure MgH2 during the hydrogen storage cycling

We utilized transmission electron microscopy (TEM) analysis, with a cryogenically cooled sample stage, to detail the microstructure of partially transformed pure and titanium fluoride-catalyzed magnesium hydride powder during hydrogenation cycling. The TiF3-catalyzed MgH2 powder demonstrated excellent hydrogen storage kinetics at various temperatures, whereas the uncatalyzed MgH2 showed significant degradation in both kinetics and capacity. TEM analysis on the partially hydrogen absorbed and partially desorbed pure Mg(MgH2) revealed a large fraction of particles that were either not transformed at all or were completely transformed. On the other hand, in the MgH2+TiF3 system it was much easier to identify regions with both the hydride and the metal phase coexisting in the same particle. This enabled us to establish the metal hydride orientation relationship (OR) during hydrogen absorption. The OR was determined to be (1 1 0)MgH2 || ( 121 1 0 121)Mg and [ 121 1 1]MgH2 || [0 1 121 1]Mg. During absorption the number density of the hydride nuclei does not show a dramatic increase due the presence of TiF3. Conversely, during desorption the TiF3 catalyst substantially increases the number of the newly formed Mg crystallites, which display a strong texture correlation with respect to the parent MgH2 phase. Titanium fluoride also promotes extensive twinning in the hydride phase.Peer reviewed: YesNRC publication: Ye

Hydrogen storage in bulk Mg-Ti and Mg-stainless steel multilayer composites synthesized via accumulative roll-bonding (ARB)

We have tested the hydrogen storage cycling behavior of bulk centimeter-scale magnesium - titanium and magnesium - stainless steel multilayer composites synthesized by accumulative roll-bonding (ARB). Roll-bonding of either titanium or stainless steel with magnesium allows the reversible hydrogen sorption of the resulting composite at 350\ub0C. Identically roll-bonded pure magnesium can hardly be absorbed at this temperature. In the composites, the kinetics of the first cycle of absorption (also called "activation") improves with increased number of fold and roll (FR) operations. With increasing FR operations the distribution of the Ti phase is progressively refined, and the shape of the absorption curve no longer remains sigmoidal. Increasing the loading amount of the second phase also accelerates the kinetics. This holds true up to a threshold limit. Microscopy analysis performed on 1-2 wt.% hydrogen absorbed composites demonstrates that MgH 2 formed exclusively on various heterogeneous nucleation sites. During activation, MgH 2 nucleation occurred at the Mg-hard phase interfaces. During the subsequent absorption cycles, heterogeneous nucleation primarily occurred in the vicinity of "internal" free surfaces such as cracks. \ua9 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.Peer reviewed: YesNRC publication: Ye