The successful preparation of the Rb3[Fe(CN)6] and Cs3[Fe(CN)6] single crystals enabled us to compare the environment of the monovalent cation for all the nonradioactive elements in group IA of the periodic table in A3[Fe(CN)6] single crystals (A+ = H+, Li+, Na+, K+, Rb+, and Cs+). Three different coordination modes between the monovalent cation and [Fe(CN)6]3- complexes were identified and discussed throughout the series. These were then compared to the expected binding modes for all of the cation sites surrounding the Fe(CN)6 units in Prussian blue analogues (PBA). CoFe PBAs were prepared from Co(NO3)2 and A3[Fe(CN)6] aqueous solutions. The resulting powders were studied by X-ray diffraction, infrared spectroscopy, and magnetic measurements. The electronic structure of all the powders and the switching properties of most of them can be explained by the effect of the number of interstitial A+ cations per unit cell on the redox potentials of the transition metal ions. However, the weak or nonexistent switching properties of the powders containing H+ and Rb+ cations suggest that a better knowledge of the location of interstitial cations within the PBA structure is also important for a full understanding of their properties.

Mesoporous silica containing Co species is effective in a wide variety of catalytic processes. Nevertheless, the catalytic efficiency of such materials strongly depends on their preparation. Two model compounds made of SBA-15 type ordered mesoporous silica monoliths containing Co(II) nitrate salt or CoCo Prussian Blue Analog are thermally treated under oxidizing and reducing atmospheres. A detailed quantitative description of the microstructure of the nanocomposites is obtained by combining X-ray diffraction, electron microscopy, UV-visible and X-ray absorption spectroscopies, and magnetic measurements. The complementarity of these techniques, which are able to identify isolated molecular species as well as amorphous or crystalline condensed species and intra- or inter-particle interactions, allows a deep and unprecedented knowledge of the microstructure of such nanocomposites. The impact that this improved description of the microstructure can have in the field of catalysis is then illustrated i) by revisiting the literature on Fischer-Tropsch catalysts in the light of the improved microstructural description, opening up new perspectives for improving the efficiency of these catalysts and ii) by comparing the catalytic activity of two catalysts with very different microstructures but containing the same amount of Co species for the water photooxidation reaction.

The discovery of a photomagnetic effect in a CoFe Prussian blue analog (PBA) has triggered a growing interest for photo-switchable bimetallic cyanide-bridged systems. Nevertheless, in between cyanide-bridged extended coordination polymers and discrete molecules, the photo-switching phenomena are much less well known in nano-sized materials. A photo-induced transformation, specific to the nanometric size, is evidenced by magnetometry and by X-ray absorption spectroscopy at the Co and Fe K-edges in an alkali cation free Prussian blue analog. The nanoparticles before irradiation can be described as having a core-shell structure, the core being made of the well-known fcc-Co-II(HS)Fe-III structure of CoFe PBAs while the shell contains Co-II ions in octahedral geometry and significantly distorted Fe(CN)(6) entities. Irradiation induces a change of the local structures around the transition metal ions, which remain in the same oxidation state, with different behaviors of the Co and Fe sub-lattices.

Most of large-size permanent room-temperature hard magnets contain rare-earth elements. Here, we present the preparation of a large-size rare-earth free hard magnet based on the loading of the ordered mesoporosity of a preformed ordered mesoporous silica monolith obtained by a sol-gel route with the Fe(NO3)(3) salt, followed by a simple calcination in air. The investigation of the influence of the thermal treatment temperature as well as the Fe/Si ratio on the microstructure of the epsilon-Fe2O3/SiO2 nanocomposites reveals well-controlled epsilon-Fe2O3 particles size and size distribution. Their magnetic study allows us i) to specify the size of the particles with an enhanced coercivity and ii) to show a linear correlation between the magnetic coercivity of the materials and the mass percent of these particles in the nanocomposites. A epsilon-Fe2O3 based bulk hard magnet with the highest coercive field of 18 kOe at room temperature was obtained by this synthesis process.

Mesoporous silica monoliths containing dispersed Fe-III sites have been prepared following a direct and simple synthetic method. The iron-containing materials are able to catalyze the epoxidation of cyclooctene by H2O2 under mild conditions. Textural and spectrophotometric analyses reveal that, when the thermal treatment temperature of the materials increases (from 500 degrees C to 1000 degrees C), the pore diameter of the silica matrix decreases while the formation of small oxo-iron(III) clusters is promoted. Among the five materials obtained after treatment at 500, 700, 800, 900 or 1000 degrees C, the one prepared at 700 degrees C exhibits the best catalytic performances. This indicates that a compromise must be found between several parameters, such as the pore size (which decreases with calcination temperature) and the hydrophobicity of the channel surface (which is favored at higher temperature), for an optimal reactivity.

The magnetic properties of nanoparticle assemblies strongly depend on the structural and morphological characteristics of the individual nanoparticles as well as on their organization within the assembly. Here, we present the synthesis of cobalt and/or iron oxide nanoparticles within the ordered mesoporosity of a silica monolith by two different synthesis pathways (using either Prussian blue analogues or nitrate salts as a precursor). We describe the influence of the nature of the metal ion and of the synthesis pathway on the morphology of the nanoparticles. With respect to these observations, we present and discuss the temperature-dependent magnetic behaviors of the final nanocomposites.

Using reverse emulsion systems, we were able to trigger mineralization confined at an oil-water interface. In this process, the alcoxide silica precursor is dissolved in the oil continuous phase of the emulsion and diffuses through the bulk to the interface where it starts to hydrolyze and condense as soon as a certain concentration threshold is attained. The process takes place only in the presence of a water soluble surfactant inside the droplet. This surfactant leads to the presence of a controlled mesoporosity inside the silica shells. The obtained objects could be used in different encapsulation applications.

Ni@SBA-15 monoliths with up to 5 wt.% of Ni were successfully synthetized by means of an original and easy one-pot sol-gel method. Transmission Electron Microscopy (TEM), X-ray Photoelectron Spectroscopy (XPS), Temperature-Programmed Reduction (TPR), Pair Distribution Function (PDF) and X-Ray Diffraction (XRD) were used for the structural characterization of the samples. After H-2-reduction, those solids exhibited small Ni degrees particles (between 1-3 nm) highly dispersed (one of the highest dispersion reported in the literature to date for 5 wt.% Ni/Silica materials) in strong interaction with the silica support. Scanning Transmission Electron Microscopy in the High Angle Annular Dark Field (STEM/HAADF) mode, chemical mapping by Energy Dispersive X-Ray (EDX) spectroscopy and electron tomography in STEM-HAADF mode highlighted the presence of Ni particles homogeneously distributed, especially in the mesopores. Such confined Ni nanoparticles were shown to be very selective and stable in the dry reforming of methane.

5 nm nanocrystals of CoFe Prussian blue analogue totally exempt from any alkali cation were prepared. Their photomagnetic properties were compared to those of the corresponding powder made of 150 nm particles as well as to those of 5 nm nanocrystals of CoFe PBAs embedded in comparable silica matrices, made of CoIIFeIII pairs and prepared under various conditions. The photomagnetic investigation of the nanoparticles exempt of any alkali cation clearly shows that they are transformed by light and the comparison of their photomagnetic properties to those of the powder made of particles of bigger size with the same chemical composition suggests that the species involved in the switching properties are surface species. Furthermore, the comparison of the magnetic properties of nanoparticles prepared under various conditions also suggests that the aggregation state of the nanoparticles in the porous channels of the silica matrix, by modulating inter-particle interactions, plays a predominant role in the magnetic properties of the nanoparticles assemblies.

The size of CoFe Prussian blue analogue nanoparticles containing either rubidium or cesium cations was controlled by their formation through a nucleation process inside the calibrated pores of an ordered mesoporous silica matrix. The corresponding references were synthesized in powder form using the same ratio between the metal species in the reaction solution. The obtained samples were characterized via infrared (IR) spectroscopy, X-ray diffraction, and magnetometry to understand the differences arising from the size reduction and the nature of the inserted alkali cation. We were able to show that the nature of the cation has a significant influence on the photoswitching properties at the macroscale in the powders. However, on the nanoscale, the influence of the surface contribution becomes more prominent and cooperative effects disappear, resulting in a new role of the alkali cation in the photomagnetic properties. (C) 2019 Academie des sciences. Published by Elsevier Masson SAS. All rights reserved.

The calcination of (nano) Prussian blue analogues is now a fully controlled and understood route to synthesize Co–Fe spinel oxides.

The epsilon-Fe2O3 polymorph of iron oxide, which has outstanding physical properties, is successfully stabilized inside mesoporous silica particles with tailored shapes. Using mesoporous silica particles with three different morphologies of rod, platelet and donut, we obtain epsilon-Fe2O3/mesoporous silica nanocomposites. Iron oxide is loaded inside the porosity using a two steps impregnation cycle: solvent-free impregnation followed by oxidation under 1000 degrees C. The amount of loaded iron oxide can be enhanced using two successive impregnation cycles. We fully characterise these nanocomposite particles with a wide panel of techniques to establish the exact amount of loaded iron inside the porosity, the nature of the iron oxide phase and the size of the nanocrystals. As a result, we conclude that epsilon-Fe2O3 nanocrystals can be confined and stabilized in all types of morphologies, even inside the donut morphology which possesses closed mesopores.

We report on a comparative study of 5.5 nm (embedded in an ordered mesoporous silica matrix) and 100 nm (free) (photo)magnetic CoFe Prussian blue analogue (PBA) particles. Co and Fe K-edge X-ray absorption spectroscopy, X-ray diffraction, infrared spectroscopy, and magnetic measurements point out a core-shell structure of the particles in their ground states. In the 5.5 nm particles, the 11.5 Å thick shell is made of Fe(CN)6 entities and CoII-NC-FeIII linkages departing from the geometry usually encountered in PBA, whatever the oxidation state (CoIIFeIII or CoIIIFeII) of the CoFe pairs in the core. In the photomagnetic particles, the photomagnetic effect in the core of the particles is due to the same photoinduced CoIII(LS)FeII → CoII(HS)FeIII electron transfer whatever the size of the particles. The shell of the nanoparticles exhibits a peculiar photoinduced structural rearrangement, and the nanoparticles in their photoexcited state exhibit a superparamagnetic behavior.

Prussian blue analog (PBA) nanoparticles confined in the ordered mesoporosity of silica monoliths with two-dimensional hexagonal structure are used as precursors and transformed into metal oxide or metal alloy by thermal treatment under oxidizing or reducing atmosphere. X-ray diffraction and transmission electron microscopy show that, after appropriate thermal treatment, the ordered silica monoliths contain spherical nanocrystals of mixed oxide and metal alloy aligned along the cylindrical pore axis. Their size (4 nm) is controlled by the diameter of the cylindrical pores of the silica monolith and their aggregation state by the structure of the porosity and the synthesis route. Furthermore, the chemical composition of the starting PBA precisely determines that of the oxide or alloy nanocrystals. The magnetic properties of the monoliths, governed by strong interparticle interactions, are very sensitive to chemical composition, size and aggregation state of the nanoparticles. The multi-scale control of the nanocomposite enables producing a light macroscopic permanent magnet, which is attracted to a NdBFe magnet at room temperature.

Prussian blue analogues (PBAs) are key compounds in the field of molecular magnetism and photomagnetism. In parallel to the strong development of this class of compounds, many efforts have been devoted to produce PBAs in various shapes and sizes. In this microreview we focus on works reporting the use of porous metal oxide nanostructures as hard templates for controlling the size, shape and solid-state organization of magnetic and photomagnetic PBA nanoparticles. The oxide matrices that have so far been developed and their characteristics are presented first. Then, the approaches that have been successfully used to control PBA formation within the pores of the matrices are described, as well as the morphology and environment of the functional nanoparticles. Lastly, the magnetic and photomagnetic properties of the nanocomposites are reviewed.

Integration of coordination polymers and metal-organic frameworks into real applications requires a processing step at the nanoscale. However, their synthesis in the form of nanoparticles with controlled size, shape and organization remains a challenge faced by diverse scientific communities. Ordered mesoporous silica monolith with 2D-hexagonal structure of the mesopores is used to form ferromagnetic NiFe Prussian blue analog (PBA) nanoparticles with anisotropic shape. The PBA nanowires formed within the nanochannels of the monolith are made of single chains of nanocrystals. Furthermore, when using an ordered mesoporous silica monolith instead of the more frequently used mesoporous powders a parallel organization of the pores is achieved over whole macroscopic fragments of the monolith. A 1 x 1 x 1 mm(3) fragment of monolith exhibits a remarkably strong anisotropy in its magnetic properties, arising from unprecedented magnetic dipolar interaction along the chains of coordination nanocrystals, which is spread to the macroscopic scale thanks to the long-range organization of the pores. As various confined chemistries can be developed within the ordered porosity of such monolith, this tool opens up new opportunities for the development of original nanostructured materials exhibiting anisotropy in their properties at the macroscopic scale.

The cover picture shows our “factory” to produce nanoparticles: the ordered porosity of silica monoliths. The pores are very versatile nanoreactors that can synthesize a large variety of compounds. In this work, we used these monoliths as a useful platform to study the effect of size reduction on the magnetic and photomagnetic properties of CoFe Prussian blue analogue nanoparticles. On the nanoscale, the properties of matter generally depend on several parameters (size, shape, surrounding medium, etc.). The monoliths have been designed for disentangling these different effects on the physical properties. The background of our cover picture represents a factory recognizable by its big chimney. Our monoliths, and more precisely their cylindrical pores (one of them is schematized in the picture), can be considered to be our “factories” that produce nanoparticles with controlled properties. Details are discussed in the article by A. Bleuzen et al. on page 1303 ff (DOI: 10.1002/ejic.201601196). For more on the story behind the cover research, see the Cover Profile (DOI: 10.1002/ejic.201700162).

Invited for the cover of this issue is the group of Anne Bleuzen at Institut de Chimie Moléculaire et des Matériaux d'Orsay, Université Paris-Saclay, France. The cover image shows the versatility of the chemistry that can be developed in the ordered pores of silica monoliths.