| 1. |
- Baričić, Miran, et al.
(författare)
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Chemical engineering of cationic distribution in spinel ferrite nanoparticles : the effect on the magnetic properties
- 2024
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Ingår i: Physical Chemistry, Chemical Physics - PCCP. - : Royal Society of Chemistry. - 1463-9076 .- 1463-9084. ; 26:7, s. 6325-6334
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Tidskriftsartikel (refereegranskat)abstract
- A set of ∼9 nm CoFe2O4 nanoparticles substituted with Zn2+ and Ni2+ was prepared by thermal decomposition of metallic acetylacetonate precursors to correlate the effects of replacement of Co2+ with the resulting magnetic properties. Due to the distinct selectivity of these cations for the spinel ferrite crystal sites, we show that it is possible to tailor the magnetic anisotropy, saturation magnetization, and interparticle interactions of the nanoparticles during the synthesis stage. This approach unlocks new possibilities for enhancing the performance of spinel ferrite nanoparticles in specific applications. Particularly, our study shows that the replacement of Co2+ by 48% of Zn2+ ions led to an increase in saturation magnetization of approximately 40% from ∼103 A m2 kg−1 to ∼143 A m2 kg−1, whereas the addition of Ni2+ at a similar percentage led to an ∼30% decrease in saturation magnetization to 68–72 A m2 kg−1. The results of calculations based on the two-sublattice Néel model of magnetization match the experimental findings, demonstrating the model's effectiveness in the strategic design of spinel ferrite nanoparticles with targeted magnetic properties through doping/inversion degree engineering.
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| 2. |
- Caselli, Lucrezia, et al.
(författare)
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Interaction of nanoparticles with lipid films : the role of symmetry and shape anisotropy
- 2022
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Ingår i: Physical Chemistry, Chemical Physics - PCCP. - : Royal Society of Chemistry (RSC). - 1463-9076 .- 1463-9084. ; 24:5, s. 2762-2776
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Tidskriftsartikel (refereegranskat)abstract
- The bioactivity, biological fate and cytotoxicity of nanomaterials when they come into contact with living organisms are determined by their interaction with biomacromolecules and biological barriers. In this context, the role of symmetry/shape anisotropy of both the nanomaterials and biological interfaces in their mutual interaction, is a relatively unaddressed issue. Here, we study the interaction of gold nanoparticles (NPs) of different shapes (nanospheres and nanorods) with biomimetic membranes of different morphology, i.e. flat membranes (2D symmetry, representative of the most common plasma membrane geometry), and cubic membranes (3D symmetry, representative of non-lamellar membranes, found in Nature under certain biological conditions). For this purpose we used an ensemble of complementary structural techniques, including Neutron Reflectometry, Grazing Incidence Small-Angle Neutron Scattering, on a nanometer lengthscale and Confocal Laser Scanning Microscopy on a micrometer length scale. We found that the structural stability of the membrane towards NPs is dependent on the topological characteristic of the lipid assembly and of the NPs, where a higher symmetry gave higher stability. In addition, Confocal Laser Scanning Microscopy analyses highlighted that NPs interact with cubic and lamellar phases according to two distinct mechanisms, related to the different structures of the lipid assemblies. This study for the first time systematically addresses the role of NPs shape in the interaction with lipid assemblies with different symmetry. The results will contribute to improve the fundamental knowledge on lipid interfaces and will provide new insights on the biological function of phase transitions as a response strategy to the exposure of NPs.
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| 3. |
- Dürr, Robin N., et al.
(författare)
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Clearing Up Discrepancies in 2D and 3D Nickel Molybdate Hydrate Structures
- 2024
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Ingår i: Inorganic Chemistry. - : American Chemical Society (ACS). - 0020-1669 .- 1520-510X. ; 63:5
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Tidskriftsartikel (refereegranskat)abstract
- When electrocatalysts are prepared, modification of the morphology is a common strategy to enhance their electrocatalytic performance. In this work, we have examined and characterized nanorods (3D) and nanosheets (2D) of nickel molybdate hydrates, which previously have been treated as the same material with just a variation in morphology. We thoroughly investigated the materials and report that they contain fundamentally different compounds with different crystal structures, chemical compositions, and chemical stabilities. The 3D nanorod structure exhibits the chemical formula NiMoO4·0.6H2O and crystallizes in a triclinic system, whereas the 2D nanosheet structures can be rationalized with Ni3MoO5–0.5x(OH)x·(2.3 – 0.5x)H2O, with a mixed valence of both Ni and Mo, which enables a layered crystal structure. The difference in structure and composition is supported by X-ray photoelectron spectroscopy, ion beam analysis, thermogravimetric analysis, X-ray diffraction, electron diffraction, infrared spectroscopy, Raman spectroscopy, and magnetic measurements. The previously proposed crystal structure for the nickel molybdate hydrate nanorods from the literature needs to be reconsidered and is here refined by ab initio molecular dynamics on a quantum mechanical level using density functional theory calculations to reproduce the experimental findings. Because the material is frequently studied as an electrocatalyst or catalyst precursor and both structures can appear in the same synthesis, a clear distinction between the two compounds is necessary to assess the underlying structure-to-function relationship and targeted electrocatalytic properties.
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| 4. |
- Dürr, Robin N., et al.
(författare)
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From NiMoO4 to γ-NiOOH : Detecting the Active Catalyst Phase by Time Resolved in Situ and Operando Raman Spectroscopy
- 2021
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Ingår i: ACS Nano. - : American Chemical Society (ACS). - 1936-0851 .- 1936-086X. ; 15:8, s. 13504-13515
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Tidskriftsartikel (refereegranskat)abstract
- Water electrolysis powered by renewable energies is a promising technology to produce sustainable fossil free fuels. The development and evaluation of effective catalysts are here imperative; however, due to the inclusion of elements with different redox properties and reactivity, these materials undergo dynamical changes and phase transformations during the reaction conditions. NiMoO4 is currently investigated among other metal oxides as a promising noble metal free catalyst for the oxygen evolution reaction. Here we show that at applied bias, NiMoO4·H2O transforms into γ-NiOOH. Time resolved operando Raman spectroscopy is utilized to follow the potential dependent phase transformation and is collaborated with elemental analysis of the electrolyte, confirming that molybdenum leaches out from the as-synthesized NiMoO4·H2O. Molybdenum leaching increases the surface coverage of exposed nickel sites, and this in combination with the formation of γ-NiOOH enlarges the amount of active sites of the catalyst, leading to high current densities. Additionally, we discovered different NiMoO4 nanostructures, nanoflowers, and nanorods, for which the relative ratio can be influenced by the heating ramp during the synthesis. With selective molybdenum etching we were able to assign the varying X-ray diffraction (XRD) pattern as well as Raman vibrations unambiguously to the two nanostructures, which were revealed to exhibit different stabilities in alkaline media by time-resolved in situ and operando Raman spectroscopy. We advocate that a similar approach can beneficially be applied to many other catalysts, unveiling their structural integrity, characterize the dynamic surface reformulation, and resolve any ambiguities in interpretations of the active catalyst phase.
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| 5. |
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| 6. |
- Maltoni, Pierfrancesco
(författare)
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Complex correlations between microstructure and magnetic behavior in hard nanomagnets : A study of the structural and magnetic properties of SrFe12O19 nanocrystallites
- 2022
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Magnetic materials at the nanoscale are of huge scientific and technological interest, as theirmagnetic properties display remarkably different behavior compared to bulk-size. Among na-nostructured materials, M-type hexaferrite SrFe12O19 (SFO) nanoparticles (NPs) has becomeextremely appealing as promising candidate material for the renewal of permanent magnets ap-plications. The conventional method for preparing hexaferrites is high temperature solid statereactions, which produce large crystallites, therefore other approaches are needed for a more ac-curate investigation. These include bottom-up methods such as co-precipitation, hydrothermalsynthesis, and sol–gel auto-combustion. In particular, the latter one was thoroughly studied inthe last 2 years, since it is a novel method, with a unique combination of the chemical sol–gelprocess and the combustion process, whose advantage is the production of nanocrystalline pow-ders. Here our aim is to improve the potentiality of SFO in the application for rare-earth-freepermanent magnets.To begin, the synthesis conditions were optimized to decrease the anneal-ing temperature, without any deterioration in the magnetic performance. The investigation ofthe relationship between the structural/morphological features of the nanocrystallites and the re-sulting magnetic properties was carried out by X-ray powder diffraction (XRPD), transmissionelectron microscopy (TEM), and various characterizations of the magnetic properties by dif-ferent magnetometers. Specifically, a size reduction of such particles yielded the formation ofsingle-domain state below a critical value, which results in an increase of magnetic anisotropyand thus coercivity. However, many factors, e.g., size, shape and interphase conditions, affectthe energy barrier distribution, and deserve careful consideration, as consequently influence themagnetization reversal mode.
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| 7. |
- Maltoni, Pierfrancesco, et al.
(författare)
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Complex correlations between microstructure and magnetic behavior in SrFe12O19 hexaferrite nanoparticles
- 2021
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Ingår i: Scientific Reports. - : Springer Nature. - 2045-2322. ; 11:1
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Tidskriftsartikel (refereegranskat)abstract
- The magnetic properties of SrFe12O19 (SFO) hard hexaferrites are governed by the complex relation to its microstructure, determining their relevance for permanent magnets ' applications. A set of SFO nanoparticles obtained by sol-gel self-combustion synthesis was selected for an in-depth structural X-Rays powder diffraction (XRPD) characterization by means of G(L) line-profile analysis. The obtained crystallites ' size distribution reveal a clear dependence of the size along the [001] direction on the synthesis approach, resulting in the formation of platelet-like crystallites. In addition, the size of the SFO nanoparticles was determined by transmission electron microscopy (TEM) analysis and the average number of crystallites within a particle was estimated. These results have been evaluated to illustrate the formation of single-domain state below a critical value, and the activation volume was derived from time dependent magnetization measurements, aiming to clarify the reversal magnetization process of hard magnetic materials.
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| 8. |
- Maltoni, Pierfrancesco, 1994-
(författare)
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Design, Synthesis And Characterization Of Magnetic Ferrite Nanostructures : Toward Novel Permanent Magnets
- 2023
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Magnetic oxide nanoparticles (NPs) may interact with each-other for example via dipolar or superexchange interactions, depending whether they are in direct contact. These interparticle interactions yield both ferro- and/or antiferromagnetic coupling and modify the energy barrier of the magnetic particle, depending upon the strength of the coupling and orientation of the particles. The corresponding perturbation of the magnetic order can readily be investigated by measuring the dc-magnetization or/and the ac-susceptibility of the samples as a function of the temperature or magnetic field. Furthermore, remanence plots, First Order Reversal Curves (FORCs) analysis and magnetic relaxation measurements are ideal methods to investigate reversal mechanisms and magnetic interactions. As we show in this thesis for a set of reference nanoparticle systems comprising magnetically hard/soft ferrites, the strong interaction regime leads to interesting phenomena, including collective dynamics, exchange bias-like hysteresis loop shifts, and interface-mediated exchange-coupling of hard and soft phases. The strength of the interparticle interactions has been investigated for a set of dense assemblies of equally sized magnetically soft maghemite NPs coated with different fractions of oleic acid layer, and compared to the dilute case of silica-coated NPs. When hard exchange-biased Co-doped maghemite NPs are mixed with unbiased soft particles with equal size, we observe that dipolar interactions yield a horizontal magnetic hysteresis loop shift. We observe that the measured hysteresis bias of this system is larger than that of the exchange bias of the unmixed Co-doped particles, and assign the extra contribution to have dipolar origin ("dipolar bias"). A more complex scenario is reported for hard/soft nanostructured powder systems of Sr and Co ferrite whose morphology (epitaxial texture or lacking coherence) strongly alter the existing interparticle magnetic interactions, and in turn the reversal process of magnetization: the magnetic coherence length scales have been estimated and thus limit for rigid coupling uncovered. Doping strategies by chemical substitution with diamagnetic cations have been also investigated, to tailor the hard/soft properties: eventually, the plasma sintered compacted ferrite composites exhibit a larger energy product compared to the single phased components, establishing a strategy to produce permanent magnets with large coercivity. We believe that our studies provide new and useful knowledge into the role of magnetic interactions at the nanoscale.
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