Xuan Tuan Le

Covalent grafting onto self-adhesive surfaces based on aryldiazonium salt seed layers

The chemistry of aryldiazonium salts has been thoroughly used in recent years to graft in a very simple and robust way ultrathin polyphenylene-like films on a broad range of surfaces. We show here that the same chemistry can be used to obtain “self-adhesive surfaces”. This target was reached in a simple way by coating various surfaces with chemisorbed organic films containing active aryldiazonium salts. These “self-adhesive surfaces” are then put into contact with various species (molecules, polymers, nanoparticles, nanotubes, graphene flakes, etc.) that react either spontaneously or under activation with the immobilized aryldiazonium salts. Our self-adhesive surfaces were synthesized following a simple aqueous two-step protocol based on p-phenylenediamine diazotisation. The first diazotisation step results in the robust grafting of thin polyaminophenylene (PAP) layers onto the surface. The second diazotisation step changed the grafted PAP film into a “poly-aryldiazonium polymer” (PDP) film. The covalent grafting between those self-adhesive surfaces and the target species was achieved by direct contact or by immersion of the self-adhesive surfaces in solution. We present in this preliminary work the grafting of multi-wall carbon nanotubes (MWCNTs), flakes of highly oriented pyrolytic graphite (HOPG), various organic compounds and copper nanoparticles. We also tested these immobilized aryldiazonium salts as electropolymerization initiators for the grafting-to process.

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.

Surface Homogeneity of Anion Exchange Membranes: A Chronopotentiometric Study in the Overlimiting Current Range

Electrotransport of sodium chloride near and through the ASV anion exchange membrane was first investigated. Chronopotentiometric and current-voltage characteristics results have shown that the ASV membrane acts as a totally conducting plane with respect to the transport of NaCl electrolyte. SEM and AFM images contributed to confirm the overall homogeneous surface of the membrane. Further chronopotentiometric studies of the membrane were evaluated in the presence of different alkaline chloride solutions in order to explore the influence of alkali co-ions on the transport phenomena. Membrane characterization led to determine the transport number of chloride counterion in the membrane. It is reported in this work that chronopotentiometry using the Sand equation toward the homogeneous ion exchange membrane is a simple and efficient method for determination of the diffusion coefficient of the electrolytes in the bulk solution. Discussions on the transport properties of the electrolyte solutions in relation with the hydrated ion sizes allowed us to verify the diffusion coefficient of the electrolytes determined by means of chronopotentiometric method.

Covalent Grafting of Chitosan onto Stainless Steel through Aryldiazonium Self-Adhesive Layers

Although the conventional methods for strong attachment of chitosan onto stainless steel require many steps in different solvents, it has been demonstrated in this work that covalent grafting of chitosan on a steel surface can be easily achieved through the formation of a self-adhesive surface based on aryldiazonium seed layers. Initially, a polyaminophenyl layer is grafted on a stainless steel surface by means of the one-step GraftFast(TM) process (diazonium induced anchoring process). The grafted aminophenyl groups are then converted to an aryldiazonium seed layer by simply dipping the substrate in a sodium nitrite acidic solution. That diazonium-rich grafted layer can be used as a self-adhesive surface for subsequent spontaneous coating of chitosan onto the steel surface. X-ray photoelectron and impedance electrochemical spectroscopies were used to characterize the pristine and modified steel samples. As evidenced from impedance and linear polarization results, the primary polyaminophenyl layer characterized by a high charge transfer resistance contributed to better protection against corrosion of the resulting chitosan-coated steel in sulfuric acid medium.

On the chemical grafting of titanium nitride by diazonium chemistry

Current research directions with the aim of extending the applications of titanium nitride (TiN) in areas of microelectronics, electrocatalysis, biosensors etc. require identifying new and efficient methods to modify this durable material with desired organic functionalities. We have clearly demonstrated in this work that diazonium chemistry can be considered for surface modification of titanium nitride. Indeed, a near-monolayer of aminophenylene has been reported to be spontaneously grafted onto the TiN surface by simple immersion of the substrates into an acidic solution of the corresponding diazonium cations. X-ray photoelectron spectroscopy measurements strongly suggested a covalent coating of aminophenyl groups on titanium nitride. Surface functionalization with aminophenylene layers was also investigated in presence of hypophosphorous acid and iron powder. Effect of these homogeneous and heterogeneous reducing agents with respect to the formation of aryl layers at different thicknesses was discussed in detail on the basis of conventional hemolytic dediazoniation mechanism in combination with the XPS results.

Pulse potential deposition of thick polyvinylpyridine-like film on the surface of titanium nitride

Electrografting based on the reduction of diazonium salts has been conventionally performed at the laboratory scale with cyclic voltammetry using a typical three-electrode electrochemical system. However, this promising coating technique still needs simplification for industrial feasibility. In this work, we report that pulse potential deposition, using an only two-electrode system, is a powerful tool for the grafting through diazonium chemistry. Importantly, this method allows the covalent attachment of a 135 nm thick polyvinylpyridine-like polymeric film on a titanium nitride wafer of industrial dimensions (200 mm diameter) using an acidic solution of 4-nitrobenzenediazonium and vinylpyridine monomer. Success in grafting suitable polymer films with well-controlled thickness on real engineering materials, such as titanium nitride, opens the door for many novel applications in micro-electromechanical systems (MEMS).