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 Šthick 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.

Weak Ferromagnetic Interaction at the Surface of the Ferrimagnetic Rb2Co4[Fe(CN)6]3.3·11H2O Photoexcited State.

CoFe Prussian blue analogues (PBAs) are well-known for their magnetic bistability tuned by external stimuli. The photoswitching properties are due to the electron transfer from CoLSIII-NC-FeLSII to CoHSII-NC-FeLSIII linkage, accompanied by the spin change of the Co ions (HS stands for high spin and LS for low spin). In this work, we investigated 100 nm particles of the Rb2Co4[Fe(CN)6]3.3·11H2O PBA (named RbCoFe). The photoexcited state of the PBA was reached by red laser excitation (λ = 635 nm) and observed by X-ray absorption spectroscopy and X-ray magnetic circular dichroism (XMCD) that are element-specific probes. The XMCD measurements at the Co and Fe L2,3 edges, probing the magnetic 3d orbitals, have provided a direct evidence of the antiferromagnetic interaction between the CoHSII and the FeLSIII ions belonging to the core of the particles, thus confirming the previously published, though indirect XMCD measurements at K edges. Because of the surface sensitivity of XMCD at the L2,3 edges, the magnetic properties of the particle surface were also revealed. Surface CoHSII-FeLSIII pairs exhibit a weak ferromagnetic interaction. Thus, the magnetic structure of the photomagnetic RbCoFe 100 nm particles can be described as a ferrimagnetic core surrounded by a ferromagnetic shell. This finding brings new insights into the understanding of the complex magnetic properties of photoexcited RbCoFe and shows that the surface can have different magnetic behavior than the core. This should impact the nature of magnetic coupling in nanoparticles of CoFe PBA, where surface effect will dominate.

Co(2+)@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.

In situ site-selective transition metal K-edge XAS: a powerful probe of the transformation of mixed-valence compounds.

We present herein the first in situ site-selective XAS experiment performed on a proof-of-principle transformation of a mixed-valence compound: the calcination of the K0.1Co(II)4[Co(III)(CN)6]2.7·20H2O Prussian Blue analogue (containing Co(2+) and Co(3+) ions in two different Oh sites) into Co3O4 (containing Co(2+) ions in a Td site and Co(3+) in an Oh site). By recording the Co K-edge X-ray absorption spectra using a spectrometer aligned at the Co Kß1,3 emission line, the evolution of each species was singly monitored from 20 °C up to the oxide formation. The experimental spectrum of the Co(2+)(Td) and Co(3+) (Oh) species in Co3O4 is reported for the first time. Our results demonstrate the possibilities offered by site-selective XAS for the investigation of chemical transformations and the study of materials under working conditions whenever the chemical element of interest is present in several states and/or sites.