Walid Baaziz

Design of covalently functionalized carbon nanotubes filled with metal oxide nanoparticles for imaging, therapy, and magnetic manipulation.

Nanocomposites combining multiple functionalities in one single nano-object hold great promise for biomedical applications. In this work, carbon nanotubes (CNTs) were filled with ferrite nanoparticles (NPs) to develop the magnetic manipulation of the nanotubes and their theranostic applications. The challenges were both the filling of CNTs with a high amount of magnetic NPs and their functionalization to form biocompatible water suspensions. We propose here a filling process using CNTs as nanoreactors for high-yield in situ growth of ferrite NPs into the inner carbon cavity. At first, NPs were formed inside the nanotubes by thermal decomposition of an iron stearate precursor. A second filling step was then performed with iron or cobalt stearate precursors to enhance the encapsulation yield and block the formed NPs inside the tubes. Water suspensions were then obtained by addition of amino groups via the covalent functionalization of the external surface of the nanotubes. Microstructural and magnetic characterizations confirmed the confinement of NPs into the anisotropic structure of CNTs making them suitable for magnetic manipulations and MRI detection. Interactions of highly water-dispersible CNTs with tumor cells could be modulated by magnetic fields without toxicity, allowing control of their orientation within the cell and inducing submicron magnetic stirring. The magnetic properties were also used to quantify CNTs cellular uptake by measuring the cell magnetophoretic mobility. Finally, the photothermal ablation of tumor cells could be enhanced by magnetic stimulus, harnessing the hybrid properties of NP loaded-CNTs.

Strain-induced metal–semiconductor transition observed in atomic carbon chains

Carbyne, the sp1-hybridized phase of carbon, is still a missing link in the family of carbon allotropes. While the bulk phases of carbyne remain elusive, the elementary constituents, that is, linear chains of carbon atoms, have already been observed using the electron microscope. Isolated atomic chains are highly interesting one-dimensional conductors that have stimulated considerable theoretical work. Experimental information, however, is still very limited. Here we show electrical measurements and first-principles transport calculations on monoatomic carbon chains. When the 1D system is under strain, the chains are semiconducting corresponding to the polyyne structure with alternating bond lengths. Conversely, when the chain is unstrained, the ohmic behaviour of metallic cumulene with uniform bond lengths is observed. This confirms the recent prediction of a metal–insulator transition that is induced by strain. The key role of the contacting leads explains the rectifying behaviour measured in monoatomic carbon chains in a nonsymmetric contact configuration.

Few layer graphene decorated with homogeneous magnetic Fe3O4 nanoparticles with tunable covering densities

Magnetic iron oxide nanoparticles (NPs) with narrow size distribution (8 ± 2 nm), well defined chemical composition and crystalline structure are synthesized and homogeneously dispersed onto the surface of few-layer graphene (FLG) via a solvothermal decomposition method. The iron oxide NPs are strongly anchored to the graphene surface and confer a magnetic character to the final composite. The metal oxide/support interaction is high enough to avoid the NPs coalescence and/or agglomeration and thus to preserve the NPs size and dispersion after thermal treatment up to 400 °C. The introduced iron oxide NPs on FLG also play a role of nano-spacers to prevent the re-stacking of the graphene sheets upon the drying process. It is expected that such a composite could find use in several application fields such as catalyst support for liquid-phase reactions with easy magnetic separation, in electrochemical energy storage and in waste water treatment. The ability of the synthesized iron oxide NP/FLG composite to adsorb foreign elements (organic pollutants) is demonstrated in the methylene blue (MB) adsorption and its catalytic properties are evaluated in the selective oxidation of H2S.

A 3D insight on the catalytic nanostructuration of few-layer graphene

The catalytic cutting of few-layer graphene is nowadays a hot topic in materials research due to its potential applications in the catalysis field and the graphene nanoribbons fabrication. We show here a 3D analysis of the nanostructuration of few-layer graphene by iron-based nanoparticles under hydrogen flow. The nanoparticles located at the edges or attached to the steps on the FLG sheets create trenches and tunnels with orientations, lengths and morphologies defined by the crystallography and the topography of the carbon substrate. The cross-sectional analysis of the 3D volumes highlights the role of the active nanoparticle identity on the trench size and shape, with emphasis on the topographical stability of the basal planes within the resulting trenches and channels, no matter the obstacle encountered. The actual study gives a deep insight on the impact of nanoparticles morphology and support topography on the 3D character of nanostructures built up by catalytic cutting.

Carbon nanotube channels selectively filled with monodispersed Fe3−xO4 nanoparticles

Magnetic iron oxide (Fe3−xO4) nanoparticles (NPs) with high density and narrow size distribution were selectively cast inside or at the outer surface of the channels of carbon nanotubes (CNTs) by performing thermal decomposition of an organic iron precursor in liquid-phase medium in the presence of adequately pre-treated CNTs. The size of the Fe3−xO4 NPs generated by the synthesis is around 13 ± 3 nm and no agglomeration was observed thanks to the presence of oleic acid playing the role of surfactant in the synthesis medium. The selective filling of the Fe3−xO4 NPs inside the CNT channel was confirmed by X-ray diffraction, transmission electron microscopy (also in tomography mode) and X-ray photoelectron spectroscopy. Such NPs were found to be less sensitive towards oxidation compared to the same NPs synthesized without CNTs according to the powder X-ray diffraction and they display thus a higher saturation magnetization (65 emu g−1). Such a composite was found to be superparamagnetic at room temperature. Due to its new “magnetic state”, it could efficiently be employed in applications, which need both magnetic and electric properties, especially in catalysis in liquid-phase medium where the catalyst–product separation is facilitated by the magnetic properties of the catalyst.

Microscopy investigations of the microstructural change and thermal response of cobalt-based nanoparticles confined inside a carbon nanotube medium

Faceted cobalt–cobalt oxide based nanoparticles (NPs) with high density and narrow size distribution (50 ± 5 nm) were selectively cast inside the channels of multi-walled carbon nanotubes (CNTs) through the controlled thermal decomposition of cobalt stearate in the presence of oleic acid as surfactant. The total loading of the NPs is ca. 60 wt% due to the confinement effect of CNTs, which play the role of “nanoreactors” for the cobalt complex filling and its further decomposition. The cast Co-based NPs consist of a Co–CoO core–shell structure with a relatively high contribution of the metallic phase, thanks to the confinement effect which prevents excessive oxidation. The Co–CoO NPs were transformed into highly porous cobalt aggregates constituted by small cobalt clusters (5 nm) after reduction with a high effective surface area. These cobalt clusters display an extremely high resistance towards oxidation, thanks to the confined medium. The evolution of the Co-based NP microstructure of the as-synthesized and the reduced composites, as a function of temperature, was evaluated by in situ heating the systems inside the TEM. On the as-synthesized sample with a mixture of Co and CoO the heating process leads to a rapid shape modification and coalescence of the NPs at a temperature of 400 °C along with the formation of a cobalt carbide interface. In contrast, the reduced sample displays a much higher sintering resistance upon annealing under the same conditions as cobalt NPs with a mean diameter of 10 nm, still present at an annealing temperature up to 800 °C. The work reported here underlines the benefit of the confinement effect to improve the oxidative and sintering resistance of cast metal NPs and also in the synthesis of a new porous structure containing small metal NPs which could be of great interest for catalytic and magnetic applications.

Design and Fabrication of Highly Reducible PtCo Particles Supported on Graphene-Coated ZnO

Cobalt particles dispersed on an oxide support form the basis of many important heterogeneous catalysts. Strong interactions between cobalt and the support may lead to irreducible cobalt oxide formation, which is detrimental for the catalytic performance. Therefore, several strategies have been proposed to enhance cobalt reducibility, such as alloying with Pt or utilization of nonoxide supports. In this work, we fabricate bimetallic PtCo supported on graphene-coated ZnO with enhanced cobalt reducibility. By employing a model/planar catalyst formulation, we show that the surface reduction of cobalt oxide is substantially enhanced by the presence of the graphene support as compared to bare ZnO. Stimulated by these findings, we synthesized a realistic powder catalyst consisting of PtCo particles grafted on graphene-coated ZnO support. We found that the addition of graphene coating enhances the surface reducibility of cobalt, fully supporting the results obtained on the model system. Our study demonstrates that realistic catalysts with designed properties can be developed on the basis of insights gained from model catalytic formulation.

Colloid Approach to the Sustainable Top-Down Synthesis of Layered Materials

The successful future of 2D materials, which are crucial for accelerating technology development and societal requirements, depends on their efficient preparation in an economical and ecological way. Herein, we present a significant advance in the top-down exfoliation and dispersion method via an aqua colloid approach. We demonstrate that a broad family of natural oil-in-water emulsification agents with an elevated hydrophilic/lipophilic balance acts in the exfoliation of layered materials and the formation of their concentrated colloids. The concentration exceeds 45 g/L for exfoliated few-layered graphene sheets possessing a micrometer size. The exfoliation of carbon nanofibers provides one of the best known unsupported and N-undoped metal-free catalysts to date in the selective dehydrogenation of ethylbenzene to styrene. Other examples include aqua colloids of exfoliated/dispersed nitrides, carbides, or nanodiamonds.

Thermal behavior of Pd@SiO2 nanostructures in various gas environments: a combined 3D and in situ TEM approach

The thermal stability of core-shell Pd@SiO2 nanostructures was for the first time monitored by using in situ Environmental Transmission Electron Microscopy (E-TEM) at atmospheric pressure coupled with Electron Tomography (ET) on the same particles. The core Pd particles, with octahedral or icosahedral original shapes, were followed during thermal heating under gas at atmospheric pressure. In the first step, their morphology/faceting evolution was investigated in a reductive H2 environment up to 400 °C by electron tomography performed on the same particles before and after the in situ treatment. As a result, we observed the formation of small Pd particles inside the silica shell due to the thermally activated diffusion from the core particle. A strong dependence of the shape and faceting transformations on the initial structure of the particles was evidenced. The octahedral monocrystalline NPs were found to be less stable than the icosahedral ones; in the first case, the Pd diffusion from the core towards the silica external surface led to a progressive decrease of the particle size. The icosahedral polycrystalline NPs do not exhibit a morphology/faceting change, as in this case the atom diffusion within the particle is favored against diffusion towards the silica shell, due to a high amount of crystallographic defects in the particles. In the second part, the Pd@SiO2 NPs behavior at high temperatures (up to 1000 °C) was investigated under reductive or oxidative conditions; it was found to be strongly related to the thermal evolution of the silica shell: (1) under H2, the silica is densified and loses its porous structure leading to a final state with Pd core NPs encapsulated in the shell; (2) under air, the silica porosity is maintained and the increase of the temperature leads to an enhancement of the diffusion mechanism from the core towards the external surface of the silica; as a result, at 850 °C all the Pd atoms are expelled outside the silica shell.

Effect of environment medium and the in situ formation of precursor on the composition-shape of iron oxide nanoparticles synthesized by thermal decomposition method

In this work, we investigate the effect of the reaction environment and the in situ formation of an iron precursor on the synthesis of iron oxide nanoparticles (IONPs) through thermal decomposition. Spherical IONPs with a spinel Fe3−xO4 structure were synthesized reproducibly through the thermal decomposition of iron stearate (Fe(stearate)2) assisted by oleic acid in octyl ether or eicosene solvents under air. Under similar conditions and by adjusting only the reaction environment, namely under argon, core–shell FeO@Fe3−xO4 NPs with the same sizes were obtained. On the other hand, the thermal decomposition of iron oleate (Fe(oleate)3) in the presence of a mixture of oleic acid and sodium oleate, under air and argon, leads to the formation of cubic-shaped NPs with a similar FeO@Fe3−xO4 structure. Cubic NPs with a homogenous Fe3−xO4 composition were synthesized from the iron oleate formed in situ from FeCl3·6H2O and sodium oleate reactants, which was immediately decomposed. The composition and shape dependence on the experimental conditions, i.e. atmosphere and iron complex, is discussed. The synthesized NPs were characterized by combining several techniques including transmission electron microscopy (TEM), scanning-TEM, X-ray diffraction (XRD) and 57Fe Mossbauer spectrometry. The correlation between the crystalline composition and the magnetic properties was investigated by carrying out magnetization measurements as a function of the applied field and temperature. The spherical and cubic-shaped NPs with a core–shell structure display exchange bias coupling due to the interaction between the antiferromagnetic (AFM) core and the ferrimagnetic (FiM) layer at the surface, while the Fe3−xO4 NPs exhibit a saturation magnetisation lower than that for the bulk counterpart due to the oxidation effects and the presence of a spin canted layer at their surface.