(Invited) Interfacial Functionalization of Carbon Nanotubes: From Effective Charge Propagation and Storage to Enhancement of Electrocatalytic and Bioelectrocatalytic Properties

Of particular interest to the preparation of advanced materials is synthesis and characterization of carbon nanostructures (particularly nanotubes) or noble metal nanoparticles, their stabilization (e.g. through self-assembly), as well as organization into two-dimensional arrays and three-dimensional network films through controlled fabrication (e.g. sequential attraction). Thus functionalized nanosized materials with well-defined composition, structure and thickness can be formed. The interfaces can also be highly functionalized, and they can exhibit specific catalytic or unique electronic, charge storage, optical and sensing properties. We explore here the ability of inorganic structures to stabilize and derivatize carbon nanotubes. Among inorganic systems, polyoxometallates of molybdenum and tungsten are attractive since they can not only adsorb irreversibly on solid surfaces but also exhibit reversible stepwise multi-electron transfer reactions. The concept of the layer-by-layer formation of hybrid assemblies composed of anionic polyoxometallate-protected carbon nanotubes (or metal nanoparticles) and ultra-thin films of positively charged conducting polymers (e.g. PEDOT) or polycationic metal oxo species (e.g. zirconyl or vanadyl)  will be described and discussed here. The resulting novel composite materials have been fabricated as thin or moderately thick (µm level) films on electrode surfaces. As evidenced from STM and scanning electron microscopy, their morphology is still granular but the structure is fairly dense. Further, they are characterized by fast dynamics of charge propagation. Obviously, this research is of importance to the construction of effectively operating charge storage devices (capacitors), charge mediators (e.g. in bioelectrochemistry), molecular electronic systems and electrocatalysis. In the latter case, polyoxometallates can also be applied to stabilize and link Pt-Ru, Pt-Sn and various alloyed Pt-based nanoparticles. It is apparent from diagnostic cyclic voltammetric, rotating disk voltammetric  and chronoamperometric measurements that such systems exhibit attractive properties towards electroreduction of oxygen or carbon dioxide as well as oxidation of small organic molecules (ethanol, methanol or dimethyl ether). In addition to the fact that introduction of carbon nanotubes improves charge distribution at the electrocatalytic interface, further addition of metal oxide nanospecies can generate –OH groups at low potentials that induce oxidation of passivating CO adsorbates (e.g. on Pt); they can potentially break C-H or C-O bonds, as well as they can possibly weaken C-C bonds during ethanol oxidation (e.g. through changes of the electronic properties of Pt).