Scanning Electrochemical Microscopy Meets Optical Microscopy: Probing the Local Paths of Charge Transfer Operando in Booster-Microparticles for Flow Batteries.

Understanding the oxidation/reduction dynamics of secondary microparticles formed from agglomerated nanoscale primary particles is crucial for advancing electrochemical energy storage technologies. In this study, the behavior of individual copper hexacyanoferrate (CuHCF) microparticles is explored at both global and local scales combining scanning electrochemical microscopy (SECM), for electrochemical interrogation of a single, but global-scale microparticle, and optical microscopy monitoring to obtain a higher resolution dynamic image of the local electrochemistry within the same particle. Chronoamperometric experiments unveil a multistep oxidation/reduction process with varying dynamics. On the one hand, the global SECM analysis enables quantifying the charge transfer as well as its dynamics at the single microparticle level during the oxidation/reduction cycles by a redox mediator in solution. These conditions allow mimicking the charge storage processes in these particles when they are used as solid boosters in redox flow batteries. On the other hand, optical imaging with sub-particle resolution allows the mapping of local conversion rates and state-of-charge within individual CuHCF particles. These maps reveal that regions of different material loadings exhibit varying charge storage capacities and conversion rates. The findings highlight the significance of porous nanostructures and provide valuable insights for designing more efficient energy storage materials.

Mechanism of formation of Co-Ru nanoalloys: the key role of Ru in the reduction pathway of Co.

The chemical synthesis of alloy nanoparticles requires adequate conditions to enable co-reduction instead of separate reduction of the two metal cations. The mechanism of formation of bimetallic cobalt-ruthenium nanoalloys by reducing metal salts in an alcohol medium was explored to draw general rules to extrapolate to other systems. The relative kinetics of the reduction of both metal cations were studied by UV-visible and in situ Quick-X-ray absorption spectroscopies as well as H2 evolution. The addition of Co(II) ions does not influence the reduction kinetics of Ru(III) but adding Ru(III) to a Co(II) solution promotes the reduction of cobalt cations. Indeed, while CoO is formed when reaching the boiling temperature of the solvent for the monometallic system, a direct reduction of Co is observed at this temperature without formation of the oxide for the bimetallic one. The co-reduction of the metal cations results in the formation of bimetallic nanoplatelets, the size of which can be tuned by changing the Ru content.

Green synthesis of water splitting electrocatalysts: IrO2 nanocages via Pearson's chemistry.

Highly porous iridium oxide structures are particularly well-suited for the preparation of porous catalyst layers needed in proton exchange membrane water electrolyzers. Herein, we report the formation of iridium oxide nanostructured cages, via a water-based process performed at room temperature, using cheap Cu2O cubes as the template. In this synthetic approach, based on Pearson's hard and soft acid-base theory, the replacement of the Cu2O core by an iridium shell is permitted by the difference in hardness/softness of cations and anions of the two reactants Cu2O and IrCl3. Calcination followed by acid leaching allow the removal of residual copper oxide cores and leave IrO2 hierarchical porous structures with outstanding activity toward the oxygen evolution reaction. Fundamental understanding of the reaction steps and identification of the intermediates are permitted by coupling a set of ex situ and in situ techniques including operando time-resolved X-ray absorption spectroscopy during the synthesis.

Plasmonic photocatalysis in aqueous solution: assessing the contribution of thermal effects and evaluating the role of photogenerated ROS.

Plasmon-induced photocatalysis can drive photochemical processes with an unprecedented control of reactivity, using light as sole energy source. Nevertheless, disentangling the relative importance of thermal and non-thermal features upon plasmonic excitation remains a difficult task. In this work we intend to separate the role played by the photogenerated charge carriers from thermal mechanisms in the plasmonic photo-oxidation of a model organic substrate in aqueous solution and using a metal-semiconductor hybrid as model photocatalyst. Accordingly, we present a simple set of experimental procedures and simulations that allow us to discard the thermal dissipation upon plasmonic excitation as the main driving force behind these chemical reactions. Moreover, we also study the photogeneration of reactive oxygen species (ROS), discussing their fundamental role in photo-oxidation reactions and the information they provide regarding the reactivity of the photogenerated electrons and holes.

The origin of the high electrochemical activity of pseudo-amorphous iridium oxides.

Combining high activity and stability, iridium oxide remains the gold standard material for the oxygen evolution reaction in acidic medium for green hydrogen production. The reasons for the higher electroactivity of amorphous iridium oxides compared to their crystalline counterpart is still the matter of an intense debate in the literature and, a comprehensive understanding is needed to optimize its use and allow for the development of water electrolysis. By producing iridium-based mixed oxides using aerosol, we are able to decouple the electronic processes from the structural transformation, i.e. Ir oxidation from IrO2 crystallization, occurring upon calcination. Full characterization using in situ and ex situ X-ray absorption spectroscopy, X-ray photoelectron spectroscopy, X-ray diffraction and transmission electron microscopy allows to unambiguously attribute their high electrochemical activity to structural features and rules out the iridium oxidation state as a critical parameter. This study indicates that short-range ordering, corresponding to sub-2nm crystal size for our samples, drives the activity independently of the initial oxidation state and composition of the calcined iridium oxides.

Introducing cobalt as a potential plasmonic candidate combining optical and magnetic functionalities within the same nanostructure.

The control of magnetic properties at the nanoscale is a current topic of intense research. It was shown that combining both magnetic and plasmonic nanoparticles (NPs) led to the improvement of their magneto-optical signal. In this context, common strategies consist of the design of bimetallic NPs. However, the understanding of the physics leading to the coupling between magnetic and plasmonic NPs is lacking, preventing any significant progress for the development of future photonic devices. In this article, we propose to focus our attention on an efficient and commonly used magnetic metal, cobalt, and evaluate its plasmonic properties at the nanoscale through the use of NP regular arrays, as a potential candidate combining both optical and magnetic functionalities within the same metal. We show that such NPs display plasmonic properties within a large spectral range from the UV to the NIR spectral range, with efficient quality factors, when the inter-particle distance is properly selected. These as-fabricated simple materials could find applications in integrated photonic devices for telecommunications.

On the importance of the crystalline surface structure on the catalytic activity and stability of tailored unsupported cobalt nanoparticles for the solvent-free acceptor-less alcohol dehydrogenation.

Unsupported nanoparticles are now recognized as model catalysts to evaluate the intrinsic activity of metal particles, irrespectively of that of the support. Co nanoparticles with different morphologies, rods, diabolos and cubes have been prepared by the polyol process and tested for the acceptorless catalytic dehydrogenation of alcohols under solvent-free conditions. Rods crystallize with the pure hcp structure, diabolos with a mixture of hcp and fcc phases, while the cubes crystallize in a complex mixture of hcp, fcc and ε-Co phases. All the cobalt particles are found to be highly selective towards the oxidation of a model secondary alcohol, octan-2-ol, into the corresponding ketone while no significant activity is found with octan-1-ol. Our results show the strong influence of particle shape on the activity and catalytic stability of the catalysts: Co nanorods display the highest conversion (85%), selectivity (95%) and recyclability compared to Co diabolos and Co cubes. We correlate the nanorods excellent stability with a strong binding of carboxylate ligands on their {1 1 2¯ 0} facets, preserving their crystalline superficial structure, as evidenced by phase modulation infrared reflection absorption spectroscopy.

Enhanced Magnetic Behavior of Cobalt Nano-Rods Elaborated by the Polyol Process Assisted with an External Magnetic Field.

Cobalt nano-rods with the hexagonal close-packed (hcp) structure were prepared by reduction of the long-chain carboxylate Co (II) precursor in polyol. The application of an external magnetic field (µ0H = 1.25 T) during the nucleation and growth steps resulted in a noticeable modification of the mean aspect ratio (length/diameter) of the particles. The particle morphology was also modified as the nano-rods did not exhibit conical heads at their extremities anymore, which are observed for particles prepared without application of an external magnetic field. Besides, the stacking faults density along the c axis of the hcp structure in the cobalt nano-rods has been found to decrease with the increase in the applied magnetic field. The coercive field of randomly oriented nano-rods increased with the aspect ratio, showing the highest value (i.e., 5.8 kOe at 300 K) for the cobalt nano-rods obtained under the highest applied magnetic field. For partially oriented Co nano-rods in toluene solution, the magnetic properties were significantly enhanced with a coercive field of 7.2 kOe at 140 K, while the magnetization saturation reached 92% of the bulk. The MR/MS value was about 0.8, indicating a good orientation of the anisotropic particles relative to each other, making them suitable for the preparation of permanent magnets via a bottom-up approach.

One-Pot Seed-Mediated Growth of Co Nanoparticles by the Polyol Process: Unraveling the Heterogeneous Nucleation.

The one-step seed-mediated synthesis is widely used for the preparation of ferromagnetic metal nanoparticles (NPs) since it offers a good control of particle morphology. Nevertheless, this approach suffers from a lack of mechanistic studies because of the difficulties of following in real time the heterogeneous nucleation and predicting structure effects with seeds that are generated in situ. Here, we propose a complete scheme of the heteronucleation process involved in one-pot seed-mediated syntheses of cobalt nanoparticles in liquid polyols, relying on geometrical phase analysis (GPA) of high-resolution high-angle annular dark field (HAADF)-STEM images and in situ measurements of the molecular hydrogen evolution. Cobalt particles of different shapes (rods, platelets, or hourglass-like particles) were grown by reducing cobalt carboxylate in liquid polyols in the presence of iridium or ruthenium chloride as the nucleating agent. A reaction scheme was established by monitoring the H2 evolution resulting from the decomposition of metal hydrides, formed in situ by ß-elimination of metal alkoxides, and from the polyol dehydrogenation, catalytically activated by the metal particles. This is a very good probe for both the noble metal nucleation and the heterogeneous nucleation of cobalt, showing a good separation of these two steps. Ir and Ru seeds with a size in the range 1-2 nm were found exactly in the center of the cobalt particles, whatever the cobalt particle shape, and high-resolution images revealed an epitaxial growth of the hcp Co on fcc Ir or hcp Ru seeds. The microstructure analysis around the seeds made evident two different ways of relaxing the lattice mismatch between the seeds and the cobalt, with the presence of dislocations around the Ir seeds and compression zones of the cobalt lattice near the Ru seeds. The relationship between the nature of the nucleating agent, the reaction steps, and the microstructure is discussed.

Optical microalgal biosensors for aqueous contaminants using organically doped silica as cellular hosts.

Optical biosensors for the detection of toxic species in aqueous media were developed via the encapsulation of microalgae in sol-gel matrices. In a first step, the effect of cadmium(II), lead(II), and anthracene on the chlorophyll a fluorescence intensity of Anabaena flos-aquae, Chlorella vulgaris, and Euglena gracilis microalgae in suspension was studied. Complementary ATP-metry measurements demonstrated a direct relationship between optical response and pollutant toxicity, in a cell- and dose-dependent manner. In a second step, microalgae were successfully encapsulated in silicate-colloidal silica nanocomposite matrices. However, a complete loss of cell response to pollutant addition was observed, despite the preservation of cell viability. Introduction of a low amount (5 mol%) of amine- or ethyl-bearing silanes in the matrix formulation allowed the recovery of the sensing capacity of the immobilized microalgae, without impacting on the response time (30 s). Porosimetry and 29Si solid-state NMR spectroscopy showed that the organic moieties are fully integrated into the inorganic network, tuning the ability of the target pollutant to diffuse and reach the encapsulated algae. This versatile strategy could be useful for the easy and fast assessment of contamination levels in polluted waters. Graphical Abstract Microalgal biosensors for aqueous contaminants using organically doped silica as cellular hosts.

Control of the anisotropic shape of cobalt nanorods in the liquid phase: from experiment to theory and back.

The polyol process is one of the few methods allowing the preparation of metal nanoparticles in solution. Hexagonal close packed monocrystalline Co nanorods are easily obtained in basic 1,2-butanediol at 448 K after a few minutes using a Co(II) dicarboxylate precursor. By using a combined experimental and theoretical approach, this study aims at a better understanding of the growth of anisotropic cobalt ferromagnetic nanoparticles by the polyol process. The growth of Co nanorods along the c axis of the hexagonal system was clearly evidenced by transmission electron microscopy, while the mean diameter was found to be almost constant at about 15 nm. Powder X-ray diffraction data showed that metallic cobalt was generated at the expense of a non-reduced solid lamellar intermediate phase which can be considered as a carboxylate ligand reservoir. Density functional theory calculations combined with a thermodynamic approach unambiguously showed that the main parameter governing the shape of the objects is the chemical potential of the carboxylate ligand: the crystal habit was deeply modified from rods to platelets when increasing the concentration of the ligand, i.e. its chemical potential. The approach presented in this study could be extended to a large number of particle types and growth conditions, where ligands play a key role in determining the particle shape.

Preparation of materials in the presence of hydrogen peroxide: from discrete or "zero-dimensional" objects to bulk materials.

Based on selected examples from the literature, this perspective aims to provide a short overview of synthetic methods using hydrogen peroxide and/or peroxidic species for the elaboration of discrete or zero-dimensional species, as well as mono-, bi- and tri-dimensional materials. There are several advantages in using peroxidic species: no foreign ion or organic ligand is introduced in the reaction medium, they are relatively cheap compared to costly alkoxides, for example, and in certain favorable cases these methods allow the selective formation of a given oxide polymorph. The materials prepared are used in several important technological applications such as electrochromism, gas sensing and electrochemistry but, in this review, special emphasis is placed on oxidation catalysis. Indeed, the deposition of peroxo species on or into oxide supports generally leads to catalytic materials that display higher activities related to a better dispersion of the active metal species in the host matrix. This review also focuses on the use of hydrogen peroxide for the recycling of toxic metal-containing spent materials such as those found in batteries for portable electronic devices.

FTIR and XPS study of Pt nanoparticle functionalization and interaction with alumina.

Platinum nanoparticles with a mean size of 1.7 nm were synthesized by reduction in sodium acetate solution in 1,2-ethanediol. The particles were then functionalized with dodecylamine, dodecanethiol, and omega-mercapto-undecanoic acid (MUDA). Fourier transform infrared (FTIR) spectroscopy and X-ray photoelectron spectroscopy (XPS) showed important variations of the particle surface state with functionalization whereas their structure differs only slightly. Platinum-to-sulfur charge transfer inferred from XPS of thiol-coated particles enabled the identification of the formation of Pt (delta+)-S (delta-) bonds. The native carbon monoxide (CO) at the surface of the particles was a very efficient probe for following the functionalization of the particles by FTIR. The red shift of nu(CO) accounts for the nature of the ligands at the surface of the particles and also for their degree of functionalization. Immobilization on alumina substrates of particles functionalized with MUDA was realized by immersion in colloidal solutions. Free molecules, isolated particles, and aggregates of particles interconnected by hydrogen bonds at the surface of alumina were evidenced by FTIR. With successive washings, the energy variation of the CO stretch of carbon monoxide and of carboxylic acid groups and the relative intensity nu(CH2)/nu(CO) showed that the free molecules are eliminated first, followed by aggregates and less-functionalized particles. Particles presenting a high degree of functionalization by MUDA remain and interact strongly with alumina.

Formation of assembled silver nanowires by reduction of silver thiolate in polyol/toluene medium.

Reduction of silver nitrate in polyol/toluene biphasic medium containing dodecanethiol led to organised silver nanowires, results of an interaction between silver nanoparticles and the layered phase AgSC12H25.

Novel Distorted Pentagonal-Pyramidal Coordination of Anionic Oxodiperoxo Molybdenum and Tungsten Complexes.

Model compounds for silica surfaces modified with metal complexes are provided by the mononuclear anionic diperoxo species 1 and the dinuclear complex anions 2 (M = Mo, W). They were obtained in fair to nearly quantitative yields by the reactions of Ph3 SiOH and [Ph2 Si(OH)]2 O, respectively, with an aqueous solution of [MO(O2 )2 (H2 O)2 ].