Manganese-cobalt geomimetic materials for supercapacitor electrode.

A manganese-cobalt asbolane material synthesized by low-temperature cationic exchange from birnessite in cobalt nitrate solution has been comprehensively characterized and tested for the first time as a massive (with high active mass loading) positive electrode material for to asymmetric aqueous supercapacitors. The structure of this Mn-rich material, which is homologous to the natural asbolanes well known by mineralogists, consists of MnO2-type slabs with partial substitution of Co3+ for Mn; the slabs alternate with Co(OH)2 islands located in the interlayer spacing. This structural arrangement was confirmed through in-depth electronic transmission microscopy analyses, which reveal two interlocking hexagonal sublattices with distinct a lattice-cell parameters but identical c parameters. The electrochemical performance of this geomimetic phase in alkaline electrolytes is highly promising, with specific capacitance of up to 180 F g-1 at moderate current densities and 94 F g-1 at 10 A g-1. Investigation into the charge storage mechanisms indicates effective synergy between the pseudocapacitive properties of the MnO2 slabs and the Co(OH)2 islands, in which protonic conduction is suspected to play a key role. Additionally, long-term cycling and calendar aging tests suggest that the interlayer cobalt gradually migrates to the metal oxide layer upon cycling while maintaining excellent energy storage performance. This study clearly underscores the value of exploring geomimetic minerals as potential electrode materials for energy storage applications.

Induced circular dichroism from helicoidal nano substrates to porphyrins: the role of chiral self-assembly.

Because the combination of chiral and magnetic properties is becoming more and more attractive for magneto-chiral phenomena, we here aim at exploring the induction of chirality to achiral magnetic molecules as a strategy for the preparation of magneto-chiral objects. To this end, we have associated free base- and metallo-porphyrins with silica nano helices, using a variety of elaboration methods, and have studied them mainly by electronic natural circular dichroism (NCD) and magnetic circular dichroism (MCD) spectroscopies. While electrostatic or covalent surface grafting uniformly yielded very low induced CD (ICD) for the four assayed porphyrins, a moderate response was observed when the porphyrins were incorporated into the interior of the double-walled helices, likely due to the association of the molecules with the chirally-organized gemini surfactant. A generally stronger, but more variable, ICD was observed when the molecules were drop casted onto the helices immobilised on a quartz plate, likely due to the different capacities of the porphyrins to aggregate into chiral assemblies. Electronic spectroscopy, electron microscopy and IR spectroscopy were used to interpret the patterns of aggregation and their influence on ICD and MCD. No enhancement of MCD was observed as a result of association with the nanohelices except in the case of the free base, 5,10,15,20-tetra-(4-sulfonatophenyl)porphyrin (TPPS). This nanocomposite demonstrated a large ICD in the Soret region and a large MCD in the Q-region due to J-aggregation. However, no induced MChD was observed, possibly due to the spectral mismatch between the ICD and MCD peaks.

From the Irreversible Transformation of VO2 to V2O5 Electrochromic Films.

Since its discovery, electrochromism, known as the modulation of optical properties under an applied voltage, has attracted strong interest from the scientific community and has proved to be of significant utility in various applications. Although vanadium dioxide (VO2) has been a candidate for extensive research for its thermochromic properties, its intrinsic electrochromism has scarcely been reported so far. In this study, multi-electrochromism is described for VO2 thick films. Indeed, a VO2 opaque film, doctor bladed from homemade monoclinic VO2 powder, shows a pronounced color modulation from orange to green and blue associated with an amorphization-recrystallization phenomenon upon cycling in a lithium-based electrolyte. The strong memory effect allows us to follow the coloration mechanism by combining various ex situ and in situ characterizations addressing both structural and electronic aspects. Upon cycling, the multichromism of VO2 finds its origin in the transformation of VO2 into orange V2O5 upon oxidation, while in reduction, the blue lithiated state illustrates a mixed vanadium oxidation state.

Silica-Supported Phosphine-Gold Complexes as an Efficient Catalytic System for a Dearomative Spirocyclization.

The combination of metal catalyst and inorganic silica frameworks provides a greener approach to recyclable catalysis. In this study, three phosphine-gold chloride complexes have been successfully covalently grafted onto chiral silica nanohelices. The resulting 3D ensembles showed chiroptical properties that allowed the monitoring of the supported ligands. The heterogeneous gold chloride catalysts in cooperation with silver triflate exhibited high reactivity in various reactions, especially in the spirocyclization of aryl alkynoate esters, for which a catalytic loading of 0.05 mol % could be employed. The heterogeneous catalysts could be easily recovered and recycled seven or eight times without any loss of efficiency. By adding more silver triflate, 25 cycles with full conversion were achieved owing to a complex catalytic system based on silica and metallic species.

Probing Co- and Fe-doped LaMO3 (M = Ga, Al) perovskites as thermal sensors.

The synthesis of a Co-doped or Fe-doped La(Ga,Al)O3 perovskite via the Pechini process aimed to achieve a color change induced by temperature and associated with spin crossover (SCO). In Fe-doped samples, iron was shown to be in the high-spin state, whereas SCO from the low-spin to the high-spin configuration was detected in Co-doped compounds when the temperature increased. Fe-doped compounds clearly adopted the high-spin configuration even down to 4 K on the basis of Mössbauer spectroscopic analysis. The original SCO phenomenon in the Co-doped compounds LaGa1-xCoxO3 (0 < x < 0.1) was evidenced and discussed on the basis of in situ X-ray diffraction analysis and UV-vis spectroscopy. This SCO is progressive as a function of temperature and occurs over a broad range of temperatures between roughly 300 °C and 600 °C. The determination of a crystal field strength of about 2 eV and a Racah parameter B of about 500 cm-1 for Co3+ (3d6) ions show that these values allow the occurrence of SCO. Hence, this study shows the possibility of using LaGa1-xCoxO3 compounds as thermal sensors at low Co contents (x = 0.02). The competition between steric and electronic effects in LaGaO3 in which Co3+ is stabilized in the LS state shows that electronic effects with the creation of M-O covalent bonds are predominant and contribute to the stabilization of a high crystal field around Co3+ (LS) although its ionic radius is smaller in comparison with that of Ga3+.

Spray-Drying to Get Spin-Crossover Materials.

Spin-crossover (SCO) triazole-based coordination polymers can be synthesized by micelle techniques, which almost always lead to rod-shaped nanoparticles. In order to notably reach new morphologies, we explore here the potentiality of the spray-drying (SD) method to get SCO materials. Three SCO coordination polymers and a mononuclear complex are investigated. In all cases, the SD method obtains particles definitely showing SCO. The features of the latter are yet always different from those of the referenced materials, in the sense that SCO is more gradual and incomplete, in adequacy with the poor crystallinity of the powders obtained by SD. In the case of coordination polymers, the particles are preferentially spherical. Indications of possible polymorphism and/or new materials induced by the use of the SD method are evidenced. In the case of the mononuclear complex, the SD method has allowed reproducing, in a quick and easy way, the well-known bulk compound. This exploratory work demonstrates the relevance of the concept and opens the way to a systematic scrutiny of all the experimental parameters to tune the size, morphology, and properties of the SD-synthesized SCO particles.

Anisotropic Metal Deposition on TiO2 Particles by Electric-Field-Induced Charge Separation.

Deposition of metals on TiO2 semiconductor particles (M-TiO2 ) results in hybrid Janus objects combining the properties of both materials. One of the techniques proposed to generate Janus particles is bipolar electrochemistry (BPE). The concept can be applied in a straightforward way for the site-selective modification of conducting particles, but is much less obvious to use for semiconductors. Herein we report the bulk synthesis of anisotropic M-TiO2 particles based on the synergy of BPE and photochemistry, allowing the intrinsic limitations, when they are used separately, to be overcome. When applying electric fields during irradiation, electrons and holes can be efficiently separated, thus breaking the symmetry of particles by modifying them selectively and in a wireless way on one side with either gold or platinum. Such hybrid materials are an important first step towards high-performance designer catalyst particles, for example for photosplitting of water.