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. NiMoO
4
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,
NiMoO
4
·H
2
O 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 NiMoO
4
·H
2
O. 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 NiMoO
4
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.
Sol–gel synthesis was used in order to obtain nanocrystallites of the SrFe12O19 (SFO) hexaferrite in an efficient and reliable way. By optimizing the initial synthetic conditions, we were able to control the size of the nanoparticles (NPs), at lower annealing temperature. The x-ray powder diffraction, transmission electron microscopy (TEM), and magnetic measurements have demonstrated a significant relation between the morphology, size, and magnetic properties of the nanoscale SFO, revealing a definite dependence on the crystallite size along the c-axis. The obtained NPs appear almost isotropic, in the form of platelets and exhibit similar magnetic performance, in terms of the energy product (BH)MAX, thus, demonstrating the suitability of reducing the annealing temperature without any deterioration in the magnetic properties. Additionally, this work illustrates the feasibility of the sol–gel bottom-up approach to employ magnetic NPs as building-blocks for designing hard/soft exchange-coupled bi-magnetic nanocomposites, combining the high coercivity of a hard phase (SFO) and the high saturation magnetization of a soft phase (CoFe2O4); in this regard, we discuss the tunability of the magnetic anisotropy by symbiotically restricting the growth of both phases.
Magnetic nanocomposites (NCs) are extremely appealing for a wide range of energy-related technological applications, specifically as building blocks for next-generation permanent magnets. The design of such nanostructures requires precise chemical synthesis methods, which will permit the fine-tuning of the magnetic properties. Here we present an in-depth structural, morphological and magnetic characterization of ferrite-based nanostructures obtained through a bottom-up sol−gel approach. The combination of the high coercivity of a hard phase SrFe 12 O 19 (SFO) and the high saturation magnetization of a soft phase, CoFe 2 O 4 (CFO), allowed us to develop exchange-coupled bimagnetic NCs. A symbiotic effect is observed in a SFO/CFO nanocomposite, as the unique oriented growth of SFO prevents grain growth of the CFO, thus restricting the crystallite size of both. Through X-ray powder diffraction (XRPD), transmission electron microscopy (TEM), and magnetic measurements we clarify the relationship between the distribution and size of hard/soft particles, the optimization of interfaces and the obtained uniform magnetic response. This study allowed us to establish the potentiality of hard/soft SFO/CFO nanostructures in current permanent magnet technology.
The specific effects induced by some strong electrolytes or neutral cosolutes on aqueous mixtures of guar gum (GG), sodium alginate (SA) and sodium hyaluronate (SH) were studied through rheology and DSC experiments. The results are discussed in terms of changes in the polymer conformation, structure of the network and hydration properties. This study is also aimed at controlling the viscosity of the aqueous mixturesfor application in green formulations to be used as fracturing fluids for shale gas extraction plants.
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 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.
Magnetic viscosity experiments have been performed in order to investigate the magnetization reversal in Sr nanoferrite particles (nanoscale SrFe12O19) and interacting Sr/Co nanoferrite particles (SrFe12O19–CoFe2O4 nanocomposites). The magnetic viscosity S = d M ( t ) / d l n ( t ), where M ( t ) is the magnetization as a function of time, has been collected. For Sr nanoferrite S shows a maximum close to the coercive field, reflecting the relation between S and the energy barrier distribution. We evidence that magnetic viscosity experiments on Sr nanoferrite and interacting Sr/Co nanoferrite particles provide reliable qualitative results for the different magnetic field sweep rate and saturating field H s a t considered. In addition, the activation volumes extracted from the magnetic viscosity experiments performed at different temperatures on Sr nanoferrite are quantitatively correlated to anisotropy changes.
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