The team's research fits naturally into the general theme of structured surface materials considering micrometric and/or nanometric scales.

Studies are conducted with a view to maximizing performance (desired properties, durability, reliability, cost and ecological constraints), chemical selectivity of compounds with respect to targets (surfaces, polymer matrices, etc.) and control of structure at the molecular scale.

Surface functionalization: from micro to nano, structuring in monolayers or hierarchical assembly

Molecular self-assembly in monolayers on metal or oxide surfaces

• Adsorption of organothiols and organoselenols on oxidizable metal surfaces

Self-assembly of organothiols and organoselenols is a recent field, interesting from a fundamental point of view and offering numerous advantages in a wide range of applications, and is taking a significant place in nanotechnology projects. Numerous studies carried out on noble metals, and gold in particular, have demonstrated the potential of this type of assembly. Using a flexible method and an appropriate selection of reaction media and surface-reactive molecules, these molecules chemisorb and self-organize on a surface to form a monolayer. This monolayer has the particularity of inducing changes in surface properties (wettability, friction, corrosion, electron transfer...).

Extending it to oxidizable metals is a particularly interesting challenge, hence CES's interest in following this as yet largely unexplored direction. Because gold and silver surfaces are not very oxidizable, thiol adsorption on such surfaces is easy and spontaneous. The case is different and very difficult for oxidizable metals such as copper, nickel, tin, zinc and many others. The alkanethiol molecule in contact with an oxide is transformed into an oxidized species (sulfonate and sulfinate...) and does not give rise to chemisorption and the formation of organized monolayers. Very early on, CES was able to show that by combining electrochemical activation of the surface with adsorption in solution, a self-assembled monolayer could be formed with an organization dependent on the substrate structure and adsorption conditions, as well as the nature of the adsorbate. The nature of the substrate is no longer a detrimental parameter to either adsorption or organization, as long as it is free of its oxide layer.

We have thus demonstrated, by controlling the oxidation state of the active metal, that significant improvements in corrosion resistance or lubrication can be achieved on various oxidizable metals such as iron, copper or nickel.

This model study has enabled us to extend our research to the adsorption of a variety of original bifunctional molecules which attribute other properties to the surface. In addition to nickel, we have been interested in the development of monolayers on metals that are even more reactive with the atmosphere, such as cobalt, zinc, tin and various types of alloys (Ag-Ni, Cu-Ni..). This research is the subject of various thesis and dissertation projects.

• Adsorption of organosilanes and phosphonic acids on metal oxides

The CES is also interested in molecular adsorption on oxide surfaces, among those studied, titanium, aluminum and tin. The oxide layer is either formed naturally or enhanced by chemical or electrochemical oxidation. Suitable molecules in this case are organosilanes and organophosphonic acids. Alongside the study of the surface chemistry of model and commercial molecules, the CES synthesizes, independently or with collaborations with organic chemists, new molecules in order to realize hierarchical structures (see below) on its oxidized surfaces.

Chemical functionalization of carbon nanowires and nanotubes

Building on its skills and expertise in the molecular functionalization of monolayer and multilayer surfaces, CES has recently been focusing on transferring this expertise to nanomaterials such as carbon nanotubes (synthesized by CVD in Prof. B. Nagy's NMR laboratory), carbon nanotubes (synthesized by CVD in Prof. B. Nagy's NMR laboratory) and nanotubes (synthesized by CVD in Prof. B. Nagy's NMR laboratory). B. Nagy's NMR laboratory), metal nanowires (gold, silver, nickel and copper) and hybrid nanowires (produced using the template method developed by Prof. Ch. Martin at the University of Florida). CES has developed an original method for functionalizing CNTs with organosilanes. This method offers a wide choice of functions grafted onto the CNT wall. In this way, we can modulate the affinity of CNTs to one polymer rather than another, or improve dispersion in a particular liquid.

The use of CNTs functionalized by this method has enabled us to achieve exceptional dispersion in polymers such as polydimethylsiloxane.

From model surfaces to biomaterials

The expertise that CES has developed in surface functionalization on the one hand, and spectroscopic and electrochemical surface analysis on the other, has earned it collaboration solicitations from several international academic groups and industrialists. Over the past two years, a number of fundamental and applied research projects have been launched at CES in the field of biomaterials, some of which are listed below:

• Study of biomaterial corrosion in physiological environments and development of new biocompatible corrosion inhibitors. This research is conducted in collaboration with Cardiatis.
• Surface treatment to improve the visibility of stents in Magnetic Resonance Imaging. This research, funded and supported by the DGTRE, involves collaboration with Cardiatis and the Centre Hospitalier Universitaire de Genève (neuroradiology department).
• Improvement of bone growth.
• Elaboration of nanobiosensors by combining self-assembly chemistry, with electrochemical methods involved in both fabrication and readout of sensor response. The project is being carried out in collaboration with French researchers at the University of Orsay and the CNRS in Thiais.

From monolayer modifications to hierarchical structures

It involves the anchoring and elaboration of thin or ultra-thin films on substrates of various kinds. The strategy is to graft molecularly organized organic films (molecules, oligomers, polymers), inorganic films (metals, metal oxides, SiO2) or composites (composites of polymers and nanoparticles such as carbon nanotubes or metal nanowires) in multilayers onto the substrates mentioned below.

This research has been able to develop thanks to experience and mastery of the modification of noble (Au, Pt, Ag) and oxidizable (Ni, Cu, Zn, Al and Ti) metal substrates by bifunctional molecular connectors (X-spacer-Y) where -X is a reactive group (-SH, -S-S-, -SiR3, -PO(OH)2,...) chosen to graft preferentially to the substrate surface ( see above).

The -Y function is a chemical function conferring a property (or properties) of use (lubrication, anti-wear, corrosion inhibition, ...) to the surface thus modified in the case of a monolayer and promoting the attachment of subsequent coatings in the case of a hierarchical system. Successive coatings can be deposited by direct application (painting, spin-coating, etc.) of polymer films, electroplating of metals or conductive polymers, or co-deposition of metals and particles, both organic and inorganic. Some non-exhaustive examples of hierarchical systems are briefly described below.

• Hierarchical system in molecular bilayers, chemical grafting
• Hierarchical system in molecular bilayers and lubricant

This system was developed as part of a European project whose aim was to develop a treatment for the porous gold and oxidizable metal surfaces making up low-level electrical connectors. The aim is to improve the connector's properties of corrosion resistance, lubrication and wear resistance, while maintaining its electrical conduction properties. The connector's durability and efficiency are highly dependent on its surface properties. CES has developed a bilayer surface treatment (figure 5a) consisting of a molecularly fixed layer chemisorbed by thiolate bonds on the substrate and a mobile layer, the lubricant. Optimization of the organothiol structure increased the organization and coverage of the monolayer. The manufacturing process developed enabled the bilayer system to be built up in a single step (figure 5b), with the mobile layer made up of lubricant and containing a quantity of organothiol with self-healing properties thanks to the reservoir effect. The CES has designed original molecular lubricants, which have been produced in collaboration with partners including Solvay-Solexis. The elaborate surface treatment achieved the objectives of very high corrosion resistance (figure 5c image of treated and untreated connector after Belcorre test), high wear resistance (figure 5d), high lubrication without altering electrical properties.

• Hierarchical system of molecular and polymer bilayers

Chemically grafted coupling agents are used to enable the electrodeposition of conductive polymers on oxidizable metals. The role of these bifunctional molecules is manifold: they inhibit the electrochemical oxidation of the metal, thereby promoting the oxidation of the monomer in solution and consequently surface nucleation and electropolymerization. Careful selection of the terminal function of the grafted molecule will also enable grafting of the polymer and improved adhesion of the polymer to the susbstrate.

• Hierarchical system in polymer and composite bilayers

The first layer is a polyacrylonitrile (PAN) electrochemically grafted onto a copper electrode. The second layer is a composite film (functionalized CNTs dispersed in chemical PAN) deposited by immersion. The chemical compatibility between the two layers promotes adhesion of the composite to the surface, as demonstrated by the tape test. This type of bilayer system is being studied with a view to application in supercapacitors, but this can only be achieved by studying and controlling various properties and parameters such as adhesion, the percolation rate of CNTs in PAN, their good dispersion...Figure 6: (a) schematic diagram of the bilayer system (electrochemical PAN + (PAN + functionalized CNTs) composite), (b) photos of the electrode and tape after the adhesion test.

Electrochemical development of anti-corrosion coatings

The development of new, environmentally-friendly coatings that protect metals against corrosion is another focus of the laboratory's research. The approach pursued by the CES, with the support of industrialists, is to combine different coatings combining electrodeposited metal alloys and layers of hybrid materials formed by soft chemistry. The aim is to use this combination to achieve the performance provided by coatings such as those based on toxic hexavalent chromium, which will be banned in the near future despite their effectiveness.

Development and application of local electrochemical analysis

At a time when the materials being developed are becoming ever smaller (microscopic, nanoscopic), the use of suitable techniques to characterize these materials is necessary and requires a good deal of expertise. The CES laboratory uses surface spectroscopic techniques adapted to the elemental, chemical and vibrational characterization of surfaces. All these methods provide important information for understanding the surface of materials, but are limited to their passive states. Electrochemical methods offer a complementary approach, since they can be used to stress the material under selected electrolyte conditions, to measure its ability to oxidize or reduce, and to determine its reactivity. These techniques provide a characterization of the active and reactive states. Classical electrochemical methods (potentiometry, voltammetry, electrochemical impedance spectroscopy), which we use routinely in our group, give global information (in current or potential) on the macroscopic surface brought into contact with the electrolyte.

We are now oriented towards the use of local electrochemical techniques namely the scanning Kelvin microprobe (SKP) and the scanning vibrating electrode (SVET).

The former, limited to measuring local corrosion potential, the latter, operating in the presence of electrolyte, enables measurement of the reactivity of any reactive surface.

The SKP and SVET are local electrochemical methods of characterization, one of our prospects is to develop local electrochemical fabrication by scanning electrochemical microscopy (SECM).

Promotrice (PI)

Zineb Mekhalif is a member of the NISM.