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.

One of the major limitations yet to the global implementation of polymer electrolyte membrane fuel cells (PEMFCs) is the cathode catalyst. The development of efficient platinum-free catalysts is the key issue to solve the problem of slow kinetics of the oxygen reduction reaction (ORR) and high cost. We report a promising catalyst for ORR prepared through the annealing treatment under inert conditions of the cobalt-benzotriazole (Co-BTA) complex supported on carbon nanotubes (CNTs). The N-rich benzotriazole precursor was chosen based on its ability to complex Co(II) ions and generate under annealing highly reactive radicals able to tune the physicochemical properties of CNTs. X-Ray photoelectron spectroscopy (XPS) was used to follow the surface structure changes and highlight the active electrocatalytic sites towards the ORR. To achieve further evaluation of the catalysts in acidic medium, voltamperometry, rotating disk electrode (RDE), rotating ring-disk electrode (RRDE) and half-cell measurements were performed. The resulting catalysts (Co/N/CNTs) all show catalytic activity towards the ORR, the most active one resulting from annealing at 700 °C. The overall electron transfer number for the catalyzed ORR was determined to be ∼3.7 with no change upon the catalyst loading, suggesting that the ORR was dominated by a 4e(-) transfer process. The results indicate a promising alternative cathode catalyst for ORR in fuel cells, although its performance is still lower (overpotential around 110 mV evaluated by RDE and RRDE) than the reference Pt/C catalyst.

The attempt to decorate carbon nanotubes with organic molecules as a powerful means to form new functional materials has attracted broad attention in the scientific community. Here, we report the functionalization of single-walled carbon nanotubes (SWNTs) with zinc porphyrins (ZnP) using very mild conditions to afford a series of SWNTs-ZnP (1 and 2) electron donor-acceptor conjugates. Owing to the presence of either one or two ZnP, introduced via "click chemistry", different absorption cross sections were realized. Important in this context is that the covalent linkages between SWNT and ZnP were corroborated by monitoring the diagnostic signature of the nitrogen atoms as part of the formed triazole ring by X-ray photoelectron spectroscopy (XPS). The resulting SWNTs-ZnP 1 and 2 were fully characterized. This characterization was complemented by a full-fledged investigation of their electrochemical and photophysical properties. In particular, appreciably strong electronic coupling between the photo- and electroactive constituents (i.e., SWNT and ZnP) led to rapid excited-state deactivation of ZnP via charge transfer to the nanotubes. Here, the different absorption cross sections throughout the visible part of the solar spectrum turned out to be valuable in enhancing the overall light-harvesting features. Upon photoexcitation, for both SWNTs-ZnP 1 and 2, radical ion pair states (i.e., reduced SWNT and oxidized ZnP) are formed. The charge-separated states decay to regenerate the singlet ground state with lifetimes of 820 and 200 ps for 1 and 2, respectively.