Pascale Jégou

Spontaneous Grafting of Diazonium Salts: Chemical Mechanism on Metallic Surfaces

The spontaneous reaction of diazonium salts on various substrates has been widely employed since it consists of a simple immersion of the substrate in the diazonium salt solution. As electrochemical processes involving the same diazonium salts, the spontaneous grafting is assumed to give covalently poly(phenylene)-like bonded films. Resistance to solvents and to ultrasonication is commonly accepted as indirect proof of the existence of a covalent bond. However, the most relevant attempts to demonstrate a metal-C interface bond have been obtained by an XPS investigation of spontaneously grafted films on copper. Similarly, our experiments give evidence of such a bond in spontaneously grafted films on nickel substrates in acetonitrile. In the case of gold substrates, the formation of a spontaneous film was unexpected but reported in the literature in parallel to our observations. Even if no interfacial bond was observed, formation of the films was explained by grafting of aryl cations or radicals on the surface arising from dediazoniation, the film growing later by azo coupling, radical addition, or cationic addition on the grafted phenyl layer. Nevertheless, none of these mechanisms fits our experimental results showing the presence of an Au-N bond. In this work, we present a fine spectroscopic analysis of the coatings obtained on gold and nickel substrates that allow us to propose a chemical structure of such films, in particular, their interface with the substrates. After testing the most probable mechanisms, we have concluded in favor of the involvement of two complementary mechanisms which are the direct reaction of diazonium salts with the gold surface that accounts for the observed Au-N interfacial bonds as well as the formation of aryl cations able to graft on the substrate through Au-C linkages.

ABS Polymer Electroless Plating through a One-Step Poly(acrylic acid) Covalent Grafting

A new, efficient, palladium- and chromium-free process for the electroless plating of acrylonitrile-butadiene-styrene (ABS) polymers has been developed. The process is based on the ion-exchange properties of poly(acrylic acid) (PAA) chemically grafted onto ABS via a simple and one-step method that prevents using classical surface conditioning. Hence, ABS electroless plating can be obtained in three steps, namely: (i) the grafting of PAA onto ABS, (ii) the copper Cu(0) seeding of the ABS surface, and (iii) the nickel or copper metallization using commercial-like electroless plating bath. IR, XPS, and SEM were used to characterize each step of the process, and the Cu loading was quantified by atomic absorption spectroscopy. This process successfully compares with the commercial one based on chromic acid etching and palladium-based seed layer, because the final metallic layer showed excellent adhesion with the ABS substrate.

Critical role of surface chemical modifications induced by length shortening on multi-walled carbon nanotubes-induced toxicity

Given the increasing use of carbon nanotubes (CNT) in composite materials and their possible expansion to new areas such as nanomedicine which will both lead to higher human exposure, a better understanding of their potential to cause adverse effects on human health is needed. Like other nanomaterials, the biological reactivity and toxicity of CNT were shown to depend on various physicochemical characteristics, and length has been suggested to play a critical role. We therefore designed a comprehensive study that aimed at comparing the effects on murine macrophages of two samples of multi-walled CNT (MWCNT) specifically synthesized following a similar production process (aerosol-assisted CVD), and used a soft ultrasonic treatment in water to modify the length of one of them. We showed that modification of the length of MWCNT leads, unavoidably, to accompanying structural (i.e. defects) and chemical (i.e. oxidation) modifications that affect both surface and residual catalyst iron nanoparticle content of CNT. The biological response of murine macrophages to the two different MWCNT samples was evaluated in terms of cell viability, pro-inflammatory cytokines secretion and oxidative stress. We showed that structural defects and oxidation both induced by the length reduction process are at least as responsible as the length reduction itself for the enhanced pro-inflammatory and pro-oxidative response observed with short (oxidized) compared to long (pristine) MWCNT. In conclusion, our results stress that surface properties should be considered, alongside the length, as essential parameters in CNT-induced inflammation, especially when dealing with a safe design of CNT, for application in nanomedicine for example.

Metal-free nitrogen-containing carbon nanotubes prepared from triazole and tetrazole derivatives show high electrocatalytic activity towards the oxygen reduction reaction in alkaline media.

High-performance oxygen reduction reaction (ORR) catalysts based on metal-free nitrogen-containing precursors and carbon nanotubes are reported. The investigated systems allow the evaluation of the effect of nitrogen-containing groups towards ORR and the results show that the catalysts are compatible with the conditions encountered in alkaline fuel cells, exhibiting good catalytic activity and stability compared with conventional Pt/C electrocatalyst.

Diazonium-induced anchoring process: an application to improve the monovalent selectivity of cation exchange membranes

An efficient and one-step chemical process (diazonium-induced anchoring process) to graft covalently a thin polyaniline-like layer on the surface of the Selemion CMV commercial cation exchange membrane is reported. SEM, IR and XPS techniques were used to characterize the obtained polymer film. The ability of such a surface modification layer to improve the membrane selectivity for hydrogen ions was confirmed by means of electrodialysis test. In contact with a mixed solution of sulfuric acid and metallic divalent salts, the protonation reaction of the polyaniline-like layer creates positive charges, thus leading to an electrical repulsion barrier which may reduce the penetration of divalent cations with respect to hydrogen ions. The ion exchange capacity, the membrane conductivity as well as the competitive transport of nickel and proton ions inside the modified membrane are discussed in detail in comparison with those of the bare membrane.

Tunable grafting of functional polymers onto carbon nanotubes using diazonium chemistry in aqueous media

In this work, we present an efficient grafting of functional polymers onto multi-walled carbon nanotubes using a simple and versatile chemical process carried out in open air and in aqueous media. The method involves in situreduction of substituted (–COOH, –NH2, –F…) aryl diazonium salts which yields poly(phenylene)-like coatings covalently grafted on the nanotube surface. Tuning the concentration of in situ generated diazonium was found to be an efficient way to control the total amount of grafted polymer. Functionalized nanotube samples were studied by complementary techniques such as electron microscopy, thermogravimetric analysis and X-ray photoelectron spectroscopy.

A new approach to grafting a monolayer of oriented Mn12 nanomagnets on silicon

The functionalisation of a Si(100) silicon wafer allows for the oriented grafting of a monolayer of Mn12 nanomagnets using a two-step procedure.

Formation of linear and hyperbranched porphyrin polymers on carbon nanotubes via a CuAAC “grafting from” approach

Carbon nanotube porphyrin polymers have been synthesised via the click chemistry “grafting from” approach; the hybrids were fully characterised and their electrochemical and optical properties were examined.

Ultrahigh vacuum deposition of CdSe nanocrystals on surfaces by pulse injection

The fabrication of thin films of colloidal semiconductor nanocrystals is attracting much attention due to their exceptional optoelectronic properties. This requires the development of new methods for depositing nanocrystals under well-controlled conditions. Here, we report the use of the pulse injection method to deposit CdSe nanocrystals under ultrahigh vacuum (UHV) on clean and well-ordered surfaces. The deposition of nanocrystals has been tested by x-ray photoelectron spectroscopy (XPS) and near edge x-ray absorption fine structure spectroscopy. Special attention has been paid to the preparation of very pure solutions of CdSe nanocrystals using cadmium stearate, trioctylphosphine oxide (TOPO) and the TOP/Se adduct for the nanocrystals synthesis followed by dissolution in pentane. It has been found that CdSe nanocrystals adsorb with similar sticking coefficients on graphite, hydrogenated silicon (100) and hydrogenated diamond (100) surfaces. Furthermore, the XPS analysis has revealed that the surface of the CdSe nanocrystal is Cd rich, which has important consequences for the optical and chemical properties. This ability to deposit semiconductor nanocrystals under UHV conditions on clean and well-ordered surfaces opens up new perspectives for studying in a reliable manner all their chemical, electronic and optical properties.

Towards organic film passivation of germanium wafers using diazonium salts: Mechanism and ambient stability

Germanium is well-known for its good electronic properties, but also for the poor passivation quality of its natural or thermally-grown oxide layer. The robust passivation of Ge surfaces is thus a crucial step on the way to its integration in electronics at nanoscale. Before passivation, the natural oxide layer must be removed from the surface. Different methods were investigated and compared. The surfaces were analysed by X-ray photoelectron spectroscopy and by atomic force microscopy to determine the impact of the etching on the roughness. Oxide-free Cl-terminated surfaces were chosen and functionalized with various arenediazonium salts. The grafting is a fast reaction (less than 30 min) and proceeds in soft conditions (room temperature in acetonitrile solutions). The functionalization was found to proceed even at low temperature (−18 °C). The modified surfaces were mainly studied by XPS and AFM. The morphology of the organic layer was influenced by the temperature and duration of the grafting as well as by the presence of substituents on the aromatic ring. We demonstrated that smooth oxide-free Ge surfaces could be prepared by a cyclic oxidation- rinsing- etching procedure and could be functionalized with various arenediazonium salts. The stability of the organic layer was compared to the stability of a self-assembled monolayer of alkanethiolates. The absolute coverage was also determined. This constitutes a first step towards Ge crystal passivation by organic thin films.