Giulia Fornasieri

Control of stoichiometry, size and morphology of inorganic polymers by template assisted coordination chemistry

Recent years have seen the discovery of fascinating electronic properties in Prussian blue analogues and derivatives making them appealing candidates for the realization of molecular devices. Their successful integration into real applications however depends on a further processing step allowing the control of their size, shape and spatial organization at the surface or within a solid matrix. Here, we report an original strategy allowing the controlled precipitation of Prussian blue analogues and derivatives within the well-defined porosity of ordered mesoporous silica monoliths exhibiting various structures. This synthetic route offers great potentials for the study of Prussian blue derived particles as a function of their size, their shape and even their orientation in magnetic or electric fields. Furthermore, the thermal decomposition in oxidative or reductive atmosphere of the confined Prussian blue derivatives also offers appealing perspectives for the elaboration and the study of mixed-oxides and alloys with finely tuned stoichiometry.

Tailor‐made Nanometer‐scale Patterns of Photo‐switchable Prussian Blue Analogues

Molecular materials that exhibit bistability in their physical properties as a function of external stimuli (light, temperature, pressure, magnetic, or electric fi eld, etc.) offer appealing perspectives for the realization of molecular-scale electronic devices [ 1 ] provided that i) a processing step allows the integration of the functional object into real applications and ii) the bistability in the physical properties is retained in the processed compound. In order to assemble and organize functional objects, solids that exhibit well-defi ned pore size, pore shape, and pore organization, and can also be deposited onto various substrates, are particularly suited. Nanostructured oxides elaborated by sol–gel chemistry combined with surfactant micelle templating combine all these capabilities. [ 2 ] In this work, combining the fascinating electronic properties of Prussian blue analogues (PBAs) and the exceptional processing possibilities inherent in nanostructured oxide chemistry, the guided placement of bistable nano-objects onto a substrate is achieved thanks to an appropriate series of chemical processes. The nanocomposite opens new perspectives in fundamental studies of isolated, size and shape-controlled multifunctional nanoparticles and can be seen as a pioneering model of a molecular optical recording medium built from molecules only. The successful elaboration of the nanocomposite shows that soft chemistry routes, which involve a bottom-up approach only, have acquired enough maturity to tackle nanodevice fabrication. The photomagnetic effect in CoFe PBAs was discovered in 1996. [ 3 ] When the stoichiometry of CoFe PBAs of chemical formula C x Co 4 [Fe(CN) 6 ] (8 + x )/3 ⋅ H 2 O (C is an alkali cation) is well tuned, they are composed of Co III (low spin, LS) and Fe II (LS) diamagnetic ions at low temperature. Irradiation in the visible range induces the Co III (LS)Fe II (LS) → Co II (high spin, HS) Fe III (LS) electron transfer. When the light is switched off, the compound is trapped in the Co II (HS)Fe III (LS) metastable state of long lifetime. [ 3–7 ] The initial electronic state is recovered by heating the sample above the relaxation temperature (100–150 K)

Elaboration of Prussian Blue Analogue/Silica Nanocomposites: Towards Tailor-Made Nano-Scale Electronic Devices

The research of new molecular materials able to replace classical solid materials in electronics has attracted growing attention over the past decade. Among these compounds photoswitchable Prussian blue analogues (PBA) are particularly interesting for the elaboration of new optical memories. However these coordination polymers are generally synthesised as insoluble powders that cannot be integrated into a real device. Hence their successful integration into real applications depends on an additional processing step. Nanostructured oxides elaborated by sol-gel chemistry combined with surfactant micelle templating can be used as nanoreactors to confine PBA precipitation and organize the functional nano-objects in the three dimensions of space. In this work we present the elaboration of different CoFe PBA/silica nanocomposites. Our synthetic procedure fully controls the synthesis of PBA in the porosity of the silica matrix from the insertion of the precursors up to the formation of the photomagnetic compound. We present results on systems from the simplest to the most elaborate: from disordered xerogels to ordered nanostructured films passing through mesoporous monoliths.

Chemistry of Cobalt(II) Confined in the Pores of Ordered Silica Monoliths: From the Formation of the Monolith to the CoFe Prussian Blue Analogue Nanocomposite

Recently we conceived of an original strategy that allows the precipitation of Prussian blue analogues (PBAs) in the ordered pores of silica monoliths to lead to photomagnetic CoFe PBA-silica nanocomposites. To determine the critical parameters and fully control the synthesis of the photoactive CoFe PBA in the pores of the silica matrix, X-ray absorption spectroscopy was performed at the cobalt K-edge. This study showed that cobalt cation chemistry is the keystone of the entire process. The local environment and the electronic structure of the cobalt cation undergo several modifications during the formation process: first the incorporation of the cation as an octahedral complex into the ordered block copolymer phase, then the deprotonation by thermohydrolysis to give a fourfold-coordinated deprotonated lowly condensed species and finally the formation of the 3D coordination network of CoFe PBA in acidic conditions through a rapid reprotonation followed by nucleophilic substitution accompanied by the electronic transfer, thus leading to the photomagnetic Co(III)(LS)-Fe(II)(LS) (LS=low spin) pairs.

Alignment under Magnetic Field of Mixed Fe2O3/SiO2 Colloidal Mesoporous Particles Induced by Shape Anisotropy

When using the bottom-up approach with anisotropic building-blocks, an important goal is to find simple methods to elaborate nanocomposite materials with a truly macroscopic anisotropy. Here, micrometer size colloidal mesoporous particles with a highly anisotropic rod-like shape (aspect ratio ≈ 10) have been fabricated from silica (SiO2 ) and iron oxide (Fe2 O3 ). When dispersed in a solvent, these particles can be easily oriented using a magnetic field (≈200 mT). A macroscopic orientation of the particles is achieved, with their long axis parallel to the field, due to the shape anisotropy of the magnetic component of the particles. The iron oxide nanocrystals are confined inside the porosity and they form columns in the nanochannels. Two different polymorphs of Fe2 O3 iron oxide have been stabilized, the superparamagnetic γ-phase and the rarest multiferroic ε-phase. The phase transformation between these two polymorphs occurs around 900 °C. Because growth occurs under confinement, a preferred crystallographic orientation of iron oxide is obtained, and structural relationships between the two polymorphs are revealed. These findings open completely new possibilities for the design of macroscopically oriented mesoporous nanocomposites, using such strongly anisotropic Fe2 O3 /silica particles. Moreover, in the case of the ε-phase, nanocomposites with original anisotropic magnetic properties are in view.

Co2+@Mesoporous Silica Monoliths: Tailor‐Made Nanoreactors for Confined Soft Chemistry

Mesoporous silica monoliths with various ordered nanostructures containing transition metal M(2+) cations in variable amounts were elaborated and studied. A phase diagram depicting the different phases as a function of the M(2+) salt/tetramethyl orthosilicate (TMOS) and surfactant P123/TMOS ratios was established. Thermal treatment resulted in mesoporous monoliths containing isolated, accessible M(2+) species or condensed metal oxides, hydroxides, and salts, depending on the strength of the interactions between the metal species and the ethylene oxide units of P123. The ordered mesoporosity of the monoliths containing accessible M(2+) ions was used as a nanoreactor for the elaboration of various transition metal compounds (Prussian blue analogues, Hofmann compounds, metal-organic frameworks), and this opens the way to the elaboration of a large range of nanoparticles of multifunctional materials.

A chemical model of intermediate states implied in the switching properties of CoFe Prussian blue analogues: how a cell parameter lengthening can cause a crystal field parameter increase

A series of CoFe Prussian blue analogues of chemical formula Rb2Co4−xZnx[Fe(CN)6]3.3·11H2O (x = 0, 1, 1.95 and 2.7) has been synthesized along which the MII/CoIII ions ratio at the Co site has been tuned. The long range order and the electronic structure of the Co ions have been investigated by combined powder X-ray diffraction and X-ray absorption spectroscopy (XAS) measurements. The cell parameter of the face-centered cubic structure lengthens as the MII/CoIII ions ratio increases without phase demixing. The study of the electronic structure of the Co ions by XAS shows that the coordination polyhedra of the CoII(HS) ions play an important role in the flexibility of the cubic structure. The variation of the cell parameter in the series of compounds is accompanied by the variation of the CoII–NC bond angle which allows the expansion or contraction of the cubic structure accompanying the electronic switch without phase demixing. Due to this structural re-arrangement, a lengthening of the cell parameter unusually produces an increase of the Co ion crystal field. Such a re-arrangement occurs in the course of the photo-induced electron transfer.

Titanium Oxo-Clusters: Vesatile Nano-Objects for the Design of Hybrid Compounds

The description of three titanium oxo-clusters and their use as inorganic components of hybrid organic-inorganic materials are reported. The first approach consists to add titanium oxo-clusters, [Ti 6 O 4 (C 6 H 5 COO) 8 (OPr n ) 8 ], in an ORMOCER⊗ based hybrid medium. Nano-sized titanium oxo-clusters combined with the chemical nature of the components, allow the tuning of the optical properties, especially the refractive index. The second approach consists to associate functionalized titanium oxo-clusters to elaborate hybrid materials with perfectly defined inorganic domains. The more relevant example of titanium oxo-cluster to build hybrid networks from nano-building blocks is the oxo-cluster [Ti 16 O 16 (OEt) 32 ]. Indeed, the nature and the number of functional groups at the surface of these metallic oxo-clusters can be tuned in order to generate cross-linking agents of organic polymers. The studies of the structure-property relationships of the resulting nanocomposites have been investigated. Finally the structure of a purely carboxylate oxo-clusters is briefly described. This new family of stable oxo-clusters opens the way for the production of original hybrid compounds.

Evidence of the Core–Shell Structure of (Photo)magnetic CoFe Prussian Blue Analogue Nanoparticles and Peculiar Behavior of the Surface Species

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.