Origin of the large dispersion of magnetic properties in nanostructured oxides: FexO/Fe3O4 nanoparticles as a case study

The intimate relationship between stoichiometry and physicochemical properties in transition-metal oxides makes them appealing as tunable materials. These features become exacerbated when dealing with nanostructures. However, due to the complexity of nanoscale materials, establishing a distinct relationship between structure-morphology and functionalities is often complicated. In this regard, in the FexO/Fe3O4 system a largely unexplained broad dispersion of magnetic properties has been observed. Here we show, thanks to a comprehensive multi-technique approach, a clear correlation between the magneto-structural properties in large (45 nm) and small (9 nm) FexO/Fe3O4 core/shell nanoparticles that can explain the spread of magnetic behaviors. The results reveal that while the FexO core in the large nanoparticles is antiferromagnetic and has bulk-like stoichiometry and unit-cell parameters, the FexO core in the small particles is highly non-stoichiometric and strained, displaying no significant antiferromagnetism. These results highlight the importance of ample characterization to fully understand the properties of nanostructured metal oxides

Antibacterial effect of hyaluronan/chitosan nanofilm in the initial adhesion of Pseudomonas aeruginosa wild type, and IV pili and LPS mutant strains

Materials coated with nanofilms obtained by polyelectrolytes assembled layer-by-layer are promising as antibacterial surfaces. Nanofilms of hyaluronan/chitosan (HA/CHI) have satisfactory antibacterial effects against human pathogenic bacteria, such as Escherichia coli and Staphylococcus aureus, but are not efficient for Pseudomonas aeruginosa. To better understand the interaction between P. aeruginosa and HA/CHI nanofilms, this work evaluates the role of type IV pili (T4P) and lipopolysaccharide (LPS) structures in the initial adhesion of this opportunistic pathogen on a bioinert silica substrate and a 20 nm-thick HA/CHI nanofilm using two genetically modified strains: Delta pilA and LPS-. Delta pilA cannot twitch and LPS- lacks the O-antigen structures of LPS molecules. Our results indicate that each strain presented a different adhesion on both surfaces according to their particular features. For the silica substrate, the PA14 wild-type strain exhibited motility because formed interconnected rings, as a result of cell motility; however, in the case of Delta pilA strain, nonconfluent aggregates were generated by the lack of twitching motility in cells. For the LPS- strain, bacteria completely covered the silica, demonstrating a significantly higher rate of adhesion and growth when compared to the other strains. The HA/CHI nanofilm produced membrane damage and lysis on all the used strains, confirming its antibacterial effect during the first hours of culture. However, the lack of LPS seemed to protect the bacteria partially from the HA/CHI nanofilm, probably due to their autoaggregative phenotype, preventing the exposure of part of the cells to the nanofilm. Moreover, in the case of PA14 wild-type, cells were able to adhere on top of the lysed bacteria, using them as a new surface. This behavior may explain why this antibacterial material has not been so efficient against P. aeruginosa for longer culture times

Significant Dzyaloshinskii-Moriya interaction at graphene-ferromagnet interfaces due to the Rashba effect.

The possibility of utilizing the rich spin-dependent properties of graphene has attracted much attention in the pursuit of spintronics advances. The promise of high-speed and low-energy-consumption devices motivates the search for layered structures that stabilize chiral spin textures such as topologically protected skyrmions. Here we demonstrate that chiral spin textures are induced at graphene/ferromagnetic metal interfaces. Graphene is a weak spin-orbit coupling material and is generally not expected to induce a sufficient Dzyaloshinskii-Moriya interaction to affect magnetic chirality. We demonstrate that indeed graphene does induce a type of Dzyaloshinskii-Moriya interaction due to the Rashba effect. First-principles calculations and experiments using spin-polarized electron microscopy show that this graphene-induced Dzyaloshinskii-Moriya interaction can have a similar magnitude to that at interfaces with heavy metals. This work paves a path towards two-dimensional-material-based spin-orbitronics

Synthesis and density functional calculations of the new molecule-based magnet precursor [Fe(H2opba-i)(dmso)2]Cl

A new precursor of molecule-based magnetic systems, [Fe(H2opba-i)(dmso)2]Cl (1), with opba = ortho-phenylenebis(oxamato) in an iminoalcohol tautomeric form, was obtained as a product from the reaction between H4opba and FeCl3. Data from elemental analysis, IR and Mössbauer spectroscopies and magnetic measurements indicate that this precursor is composed of a mixture of trans (83%) and cis (17%) isomers. The chiMT value at 298K (2.1 emu K mol-1) corresponds to FeIII with spin state (S) between 3/2 and 5/2. Theoretical calculations (PBE/DZVP2) of trans- and cis-[Fe(H2opba-i)(dmso)2 ]+ show that both isomers have spin S = 1/2 in the ground state and S = 3/2 for the trans and S = 5/2 for the cis in the first excited state. The combination of these results leads to chiMT values of 0.375 and 2.3 emu K mol-1, at low and high temperature respectively, which are in accordance with the experimental data for 1.Um precursor inédito de sistemas magnéticos moleculares, [Fe(H2opba-i)(dmso)2]Cl (1), onde opba = orto-fenilenobis(oxamato) na forma tautomérica iminoálcool, foi obtido como produto da reação entre H4opba e FeCl3. Os dados de análise elementar, espectroscopias de absorção no infravermelho e Mössbauer e medidas magnéticas indicam que este precursor é constituído por uma mistura de isômeros trans (83%) e cis (17%). O valor de chiMT a 298 K (2,1 emu K mol-1) corresponde ao FeIII com estado de spin (S) entre 3/2 e 5/2. Cálculos teóricos (PBE/DZVP2) de trans- e cis-[Fe(H2opba-i)(dmso)2 ]+ mostram que ambos os isômeros têm spin S = 1/2 no estado fundamental e S = 3/2 para o trans e S = 5/2 para o cis no primeiro estado excitado. A combinação destes resultados leva a valores de chiMT de 0,375 emu K mol-1 e 2,3 emu K mol-1 a baixas e altas temperaturas, respectivamente, os quais são concordantes com os dados experimentais para 1.15341539Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES

Catalytic oxidation of aqueous sulfide in the presence of ferrites (MFe2O4, M = Fe, Cu, Co)

In this work, the spinel structured iron oxides, MFe2O4 (M = Cu2+ and Co2+), magnetite (Fe3O4) and maghemite (γ-Fe2O3) were investigated as catalyst for the sulfide oxidation in aqueous medium. XRD, Mössbauer, BET, TPR, SEM/EDS and XPS analyses suggest the formation of the spinel phase with surface areas varying from 75 to 105 m2 g-1 and the metal ions Cu2+ and Co2+ located in the octahedral and tetrahedral sites. It was observed that the presence of Co and especially Cu strongly increased the catalytic activity for the oxidation of sulfide to polysulfides, i.e. Sx 2- (x = 1-4) and other S-O species. The strong effect of Cu2+ is discussed in terms of thermodynamically favorable processes comprising a strong surface interaction of S2--Cu2+, an electron transfer from sulfide to produce Cu1+ followed by an electron transfer from Cu1+ to Fe3+ bulk.Fil: Cunha, Igor T.. Universidade Federal de Minas Gerais; BrasilFil: Teixeira, Ivo F.. University of Oxford; Reino UnidoFil: Albuquerque, Adriana S.. Centro de Desenvolvimento da Tecnologia Nuclear; BrasilFil: Ardisson, José D.. Centro de Desenvolvimento da Tecnologia Nuclear; BrasilFil: Macedo, Waldemar A. A.. Centro de Desenvolvimento da Tecnologia Nuclear; BrasilFil: Oliveira, Henrique S.. Universidade Federal de Minas Gerais; BrasilFil: Tristão, Juliana C.. Universidade Federal de Viçosa; BrasilFil: Sapag, Manuel Karim. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - San Luis. Instituto de Física Aplicada ; Argentina. Universidad Nacional de San Luis; ArgentinaFil: Lago, Rochel M.. Universidade Federal de Minas Gerais; Brasi