Sophie Carenco

Nanoscaled Metal Borides and Phosphides: Recent Developments and Perspectives

and Perspectives Sophie Carenco,†,‡,§,∥,⊥ David Portehault,*,†,‡,§ Ced́ric Boissier̀e,†,‡,§ Nicolas Meźailles, and Cleḿent Sanchez*,†,‡,§ †Chimie de la Matier̀e Condenseé de Paris, UPMC Univ Paris 06, UMR 7574, Colleg̀e de France, 11 Place Marcelin Berthelot, 75231 Paris Cedex 05, France ‡Chimie de la Matier̀e Condenseé de Paris, CNRS, UMR 77574, Colleg̀e de France, 11 Place Marcellin Berthelot, 75231 Paris Cedex 05, France Chimie de la Matier̀e Condenseé de Paris, Colleg̀e de France, 11 Place Marcellin Berthelot, 75231 Paris Cedex 05, France Laboratory Heteroelements and Coordination, Chemistry Department, Ecole Polytechnique, CNRS-UMR 7653, Palaiseau, France

Size-Dependent Dissociation of Carbon Monoxide on Cobalt Nanoparticles

In situ soft X-ray absorption spectroscopy (XAS) was employed to study the adsorption and dissociation of carbon monoxide molecules on cobalt nanoparticles with sizes ranging from 4 to 15 nm. The majority of CO molecules adsorb molecularly on the surface of the nanoparticles, but some undergo dissociative adsorption, leading to oxide species on the surface of the nanoparticles. We found that the tendency of CO to undergo dissociation depends critically on the size of the Co nanoparticles. Indeed, CO molecules dissociate much more efficiently on the larger nanoparticles (15 nm) than on the smaller particles (4 nm). We further observed a strong increase in the dissociation rate of adsorbed CO upon exposure to hydrogen, clearly demonstrating that the CO dissociation on cobalt nanoparticles is assisted by hydrogen. Our results suggest that the ability of cobalt nanoparticles to dissociate hydrogen is the main parameter determining the reactivity of cobalt nanoparticles in Fischer-Tropsch synthesis.

White phosphorus and metal nanoparticles: a versatile route to metal phosphide nanoparticles

P(4) reaction with metal NPs (In, Pb, Zn) provides an easy access to the corresponding metal phosphide NPs in a soft and stoichiometric reaction. Size-influence on the reactivity is investigated in the case of indium.

Revisiting the molecular roots of a ubiquitously successful synthesis: nickel(0) nanoparticles by reduction of [Ni(acetylacetonate)2].

The widely used preparation of Ni(0) nanoparticles from [Ni(acac)(2)] (acac=acetylacetonate) and oleylamine, often considered to be a thermolysis or a radical reaction, was analyzed anew by using a combination of DFT modeling and designed mechanistic experiments. Firstly, the reaction was followed up by using TGA to evaluate the energy barrier of the limiting step. Secondly, all the byproducts were identified using NMR spectroscopy, mass spectrometry, FTIR, and X-ray crystallography. These methods allowed us to depict both main and side-reaction pathways. Lastly, DFT modeling was utilized to assess the validity of this new scheme by identifying the limiting steps and evaluating the corresponding energy barriers. The oleylamine was shown to reduce the [Ni(acac)(2)] complex not through a one-electron radical mechanism, as often stated, but as an hydride donor through a two-electron chemical reduction route. This finding has strong consequences not only for the design of further nanoparticles syntheses that use long-chain amine as a reactant, but also for advanced understanding of catalytic reactions for which these nanoparticles can be employed.

Structural transitions at the nanoscale: the example of palladium phosphides synthesized from white phosphorus

Stoichiometric reactions of Pd(0) nanoparticles with various amounts of white phosphorus (P4) are an efficient route to convert them into the corresponding Pd phosphides Pd(x)P(y). Formation of crystallized palladium phosphide nanoparticles is a two-step process, which allows exploring in detail the phase transitions of the Pd(x)P(y) system, from amorphous Pd-P nanoparticles (formed in a first step at moderate temperature) to crystallization (at higher temperature). The second temperature was found to be strongly dependent on the Pd/P ratio: PdP2, Pd5P2 and Pd3P stoichiometries form the amorphous phases, but only PdP2 and Pd5P2 could be further crystallized from them. Although it exists as a bulk crystalline material, Pd3P could only be crystallized by starting from the more Pd-rich Pd6P composition. Phase-to-phase transformations from P-poor phosphides (Pd3P and Pd5P2) to the P-rich PdP2 were also demonstrated, and a first Pd-P phase diagram at the nanoscale was tentatively produced.

Carbon monoxide-induced dynamic metal-surface nanostructuring.

Carbon monoxide is a ubiquitous molecule in surface science, materials chemistry, catalysis and nanotechnology. Its interaction with a number of metal surfaces is at the heart of major processes, such as Fischer-Tropsch synthesis or fuel-cell optimization. Recent works, coupling structural and nanoscale in situ analytic tools have highlighted the ability of metal surfaces and nanoparticles to undergo restructuring after exposure to CO under fairly mild conditions, generating nanostructures. This Minireview proposes a brief overview of recent examples of such nanostructuring, which leads to a discussion about the driving force in reversible and non-reversible situations.

A reaction cell with sample laser heating for in situ soft X‐ray absorption spectroscopy studies under environmental conditions

A miniature (1 ml volume) reaction cell with transparent X-ray windows and laser heating of the sample has been designed to conduct X-ray absorption spectroscopy studies of materials in the presence of gases at atmospheric pressures. Heating by laser solves the problems associated with the presence of reactive gases interacting with hot filaments used in resistive heating methods. It also facilitates collection of a small total electron yield signal by eliminating interference with heating current leakage and ground loops. The excellent operation of the cell is demonstrated with examples of CO and H2 Fischer-Tropsch reactions on Co nanoparticles.

The core contribution of transmission electron microscopy to functional nanomaterials engineering

Research on nanomaterials and nanostructured materials is burgeoning because their numerous and versatile applications contribute to solve societal needs in the domain of medicine, energy, environment and STICs. Optimizing their properties requires in-depth analysis of their structural, morphological and chemical features at the nanoscale. In a transmission electron microscope (TEM), combining tomography with electron energy loss spectroscopy and high-magnification imaging in high-angle annular dark-field mode provides access to all features of the same object. Today, TEM experiments in three dimensions are paramount to solve tough structural problems associated with nanoscale matter. This approach allowed a thorough morphological description of silica fibers. Moreover, quantitative analysis of the mesoporous network of binary metal oxide prepared by template-assisted spray-drying was performed, and the homogeneity of amino functionalized metal-organic frameworks was assessed. Besides, the morphology and internal structure of metal phosphide nanoparticles was deciphered, providing a milestone for understanding phase segregation at the nanoscale. By extrapolating to larger classes of materials, from soft matter to hard metals and/or ceramics, this approach allows probing small volumes and uncovering materials characteristics and properties at two or three dimensions. Altogether, this feature article aims at providing (nano)materials scientists with a representative set of examples that illustrates the capabilities of modern TEM and tomography, which can be transposed to their own research.

Towards nanoscaled gold phosphides: surface passivation and growth of composite nanostructures

Gold phosphide (Au2P3) is known as a crystalline metastable phase at the macroscale. In this paper, the formation of Au2P3 nanostructures is investigated. Here, white phosphorus (P4) is used as a soluble phosphorus donor and reacted on 16 nm gold nanoparticles, in a strategy similar to the one previously used for the production of various metal phosphide nanoparticles including Ni2P, Pd5P4, PdP2, Cu3P and InP. Moderate temperature (250 °C for up to 6 h) gives a reaction limited to the surface of the nanoparticles, while the unreacted P4 stays in solution. This surface modification is then optimized by reducing the stoichiometry of P4 to Au : P = 10 : 1 and lowering the temperature to 110 °C. Interestingly, this surface modification shields the plasmon band against ligand exchange with thiols, providing more robust nanoparticles. The reaction is then conducted under harsh conditions (320 °C for 3 h) to produce crystalline Au2P3. This triggered the aggregation of the starting nanoparticles into larger nanostructures such as nanowires. Moreover, the formation of composite Au2P3–Au nanostructures is observed, where the gold phosphide domains are systematically larger than the unreacted gold nanoparticles. This suggests that gold is particularly reluctant to form gold phosphide, which relates to the metastable character of this phase.

In Situ Solid–Gas Reactivity of Nanoscaled Metal Borides from Molten Salt Synthesis

Metal borides have mostly been studied as bulk materials. The nanoscale provides new opportunities to investigate the properties of these materials, e.g., nanoscale hardening and surface reactivity. Metal borides are often considered stable solids because of their covalent character, but little is known on their behavior under a reactive atmosphere, especially reductive gases. We use molten salt synthesis at 750 °C to provide cobalt monoboride (CoB) nanocrystals embedded in an amorphous layer of cobalt(II) and partially oxidized boron as a model platform to study morphological, chemical, and structural evolutions of the boride and the superficial layer exposed to argon, dihydrogen (H2), and a mixture of H2 and carbon dioxide (CO2) through a multiscale in situ approach: environmental transmission electron microscopy, synchrotron-based near-ambient-pressure X-ray photoelectron spectroscopy, and near-edge X-ray absorption spectroscopy. Although the material is stable under argon, H2 triggers at 400 °C decomposition of CoB, leading to cobalt(0) nanoparticles. We then show that H2 activates CoB for the catalysis of CO2 methanation. A similar decomposition process is also observed on NiB nanocrystals under oxidizing conditions at 300 °C. Our work highlights the instability under reactive atmospheres of nanocrystalline cobalt and nickel borides obtained from molten salt synthesis. Therefore, we question the general stability of metal borides with distinct compositions under such conditions. These results shed light on the actual species in metal boride catalysis and provide the framework for future applications of metal borides in their stability domains.