Optical and XPS evidence for the electrochemical generation of an N-heterocyclic carbene and its CS2 adduct from the ionic liquid [bmim][PF6]

Room temperature ionic liquids continue to be at the forefront of chemistry, covering a broad spectrum of research areas from electrochemistry and energy to catalysis and green chemistry. Therefore, it is of great value to fully understand the chemical and electrochemical reactivity and stability of ionic liquids utilized in these applications. In this context, we have investigated the electrochemical generation of an N-heterocyclic carbene and its CS2 adduct from the ionic liquid [bmim][PF6], and X-ray photoelectron spectroscopy (XPS) proved to be a highly effective spectroscopic tool to study such systems. Initially, the dithiocarboxylate adduct was chemically synthesized as a reference compound starting from both [bmim][PF6] and [bmim][OAc], and characterized by HRMS, and 1H- and 13C-NMR, FTIR, visible and X-ray photoelectron spectroscopy. While a simple mixture of [bmim][PF6] and CS2 revealed no evidence of adduct formation, the application of an electrochemical stimulus led to the formation of the dithiocarboxylate adduct as evidenced optically and through the newly formed S2p peak in the XP spectrum. Further evidence for the electrochemical reduction of [bmim][PF6] to the corresponding N-heterocyclic carbene came from the XPS analysis via the appearance of a new N1s peak in the XP spectrum. © 2017 The Royal Society of Chemistry and the Centre National de la Recherche Scientifique

XPS-evidence for in-situ electrochemically-generated carbene formation

Stable N-heterocyclic carbenes (NHC) are a class of compounds that has attracted a huge amount of interest in the last decade. One way to prepare NHCs is through chemical or electrochemical reduction of 1,3-disubstituted imidazolium cations. We are presenting an in-situ electrochemical X-ray Photoelectron Spectroscopy (XPS) study where electrochemically reduced imidazolium cations lead to production of stable NHC. The electroactive imidazolium species is not only the reactant, but also part of the ionic liquid which serves as the electrolyte, the medium and the electroactive material. This allows us to directly probe the difference between the parent imidazolium ion and the NHC through the use of XPS. The interpretation of the results is supported by both observation of reversible redox peaks in the voltammogram and the density functional theory calculations. © 2017 Elsevier Lt

Raman and X-Ray photoelectron spectroscopic studies of graphene devices for identification of doping

Tunability of electronic properties of graphene is one of the most promising properties to integrate it to high efficiency devices in the field of electronics. Here we demonstrate the substrate induced doping of CVD graphene devices using polymers with different functional groups. Both X-Ray secondary electron cut-off and Raman spectra confirm p-type doping of a PVC-Graphene film when compared to a PMMA-Graphene one. We also systematically analyzed the reversible doping effect of acid-base exposure and UV illumination to further dope/undope the polymer supported graphene devices. The shifts in the Raman 2D band towards lower and then to higher wavenumbers, with sequential exposure to ammonia and hydrochloric acid vapors, suggest n-type doing and restoration of graphene to its original state. Finally, the n-type doping with UV irradiation on half-covered samples was utilized and shown by both XPS and Raman to create two regions with different electronic properties and resistances. These type of controlled and reversible doping routes offer new paths for electronic devices especially towards fabricating graphene p-n junctions. © 2017 Elsevier B.V