Nano-engineering of conducting polymers /

Oxidative interfacial and template polymerization approaches were developed to synthesize conducting polymer nanostructures for possible applications of these materials in charge storage devices like supercapacitors. Resulting nano-structured materials were characterized using scanning electron microscopy (SEM), fourier transform infrared coupled with attenuated total reflectance (FTIR-ATR), cyclic voltammeter, ultraviolet-visible-near infrared (UV-vis-NIR) spectrophotometer, multimeter and Brunaur-Emmett-Teller (BET) analyses. Two different methods were used to obtain desired nano-structured conducting polymers. In the first method, a homogeneous polypyrrole (PPy) nano-network structure was produced. Optimization results indicated that mole ratios of oxidant to monomer should be kept as 4:1 and surfactant to monomer as 3:1, 4:1 and 5:1 for the production of PPy nanofibers in networks. The average size of fibers and beads were measured as ca. 150 and 300 nm, respectively. The electrical conductivity measurements and electrochemical studies with cyclic voltammetry resulted in reasonable electroactivity of these PPy nano-networks. BET analyses resulted in a surface area greater than 500 m2/g. In the latter method, conductive nano-arrays of polypyrrole (PPy) (1), poly(Nmethylpyrrole) (P(NMPy)) (2), poly(thiophene) (PTh) (3) and poly(3,4- ethylenedioxythiophene) (PEDOT) (4) were aligned in one dimension (1D). Reaction conditions; such as, monomer concentrations, types of membranes and solvents for the synthesis of each polymer nanostructure were optimized. SEM images revealed that fiber diameters were in a range of 80-350 nm with 10-30 ´m length for conducting polymer nanotubules aligned unidirectionally. Characterization of P(NMPy) nanotubules with cyclic voltammetry indicated that template-synthesized materials are electroactive as desired

Room Temperature Metastability of Multilayer Graphene Oxide Films

International audienceGraphene oxide has multiple potential applications. The chemistry of graphene oxide and its response to external stimuli such as temperature and light are not well understood and only approximately controlled. This under- standing is crucial to enable future applications of this material. Here, a com- bined experimental and density functional theory study shows that multilayer graphene oxide produced by oxidizing epitaxial graphene via the Hummers method is a metastable material whose structure and chemistry evolve at room temperature with a characteristic relaxation time of about one month. At the quasi-equilibrium, graphene oxide reaches a nearly-stable reduced O/C ratio, and exhibits a structure intensively deprived of epoxide groups and enriched of hydroxyl groups. Our calculations show that the structural and chemical changes are driven by the availability of hydrogen in the oxidized graphitic sheets, which favors the reduction of epoxide groups and the formation of water molecules

Chemical bonding and stability of multilayer graphene oxide layers

International audienceThe chemistry of graphene oxide (GO) and its response to external stimuli such as temperature and light are not well understood and only approximately controlled. This understanding is however crucial to enable future applications of the material that typically are subject to environmental conditions. The nature of the initial GO is also highly dependent on the preparation and the form of the initial carbon material. Here, we consider both standard GO made from oxidizing graphite and layered GO made from oxidizing epitaxial graphene on SiC, and examine their evolution under different stimuli. The effect of the solvent on the thermal evolution of standard GO in vacuum is first investigated. In situ infrared absorption measurements clearly show that the nature of the last solvent in contact with GO prior to deposition on a substrate for vacuum annealing studies substantially affect the chemical evolution of the material as GO is reduced. Second, the stability of GO derived from epitaxial graphene (on SiC) is examined as a function of time. We show that hydrogen, in the form of CH, is present after the Hummers process, and that hydrogen favors the reduction of epoxide groups and the formation of water molecules. Importantly, this transformation can take place at room temperature, albeit slowly (~ one month). Finally, the chemical interaction (e.g. bonding) between GO layers in multilayer samples is examined with diffraction (XRD) methods, spectroscopic (IR, XPS, Raman) techniques, imaging (APF) and first principles modeling

Examining the interlayer interactions formed between reduced graphene oxide and ionic liquids

It is important to understand the electrolyte-electrode interactions for fabricating graphene oxide (GO)- and ionic liquid (IL)-based ultracapacitors. Therefore, we explored how the type and size of the cations in various ILs determine the nature of processed materials. In all cases, the ILs intercalate into the graphitic structure but marked differences are observed during exfoliation via thermal reduction. The combination of a long alkyl chain ammonium-based cation and a large-volume anion leads to strong interactions and defect formation, as evidenced by CO2 production during annealing. In contrast, using the same anions but different cations stabilize the GO functional groups below 400 degrees C

Impact of Ionic Liquids on the Exfoliation of Graphite Oxide

The fabrication of high performance, graphene-based electrochemical energy storage devices, such as ultracapacitors, depends on the reduction of graphite oxide (GO) and its interaction with ionic liquids (ILs), which may be used as the conductive electrolyte. To explore the physical and chemical interactions between ILs and thermally reduced GO (TRG) as a function of annealing temperature, three ILs with ammonium based structures were selected to differentiate the role of their anions and cations in the exfoliation process. Intercalation was accompanied by either covalent or noncovalent bonding, as supported by thermogravimetric analysis (TGA) and infrared (IR) absorption spectroscopy performed in situ during thermal annealing and by X-ray diffraction (XRD) analysis. Upon IL intercalation, covalent bonding between the IL and TRG prevented exfoliation, while noncovalently physisorbed ILs were readily removed and therefore facilitated exfoliation of the reduced GO. The anion and cation moieties of the ILs in GO–IL intercalation compounds investigated were found to affect the decomposition temperature as well as the degree of thermal stabilization. Indeed, the ammonium-based cations bearing long alkyl carbon chains did not functionalize the TRG and therefore promoted both sheet expansion and thermal exfoliation. The solvent-dependency of these properties was also investigated by forming GO–IL intercalation compounds from both deionized (DI) water and propylene carbonate (PC). In contrast to DI water, PC was found to decrease the thermal decomposition temperature of GO by about 100 °C in the presence of intercalated ILs, thus enabling highly efficient oxygen removal in GO–IL intercalation compounds

Graphitization of Graphene Oxide with Ethanol during Thermal Reduction

As an atypical reductant, ethanol has recently been considered for reducing the oxygen concentration and restoring the graphitic structure in multilayered graphene oxide (GO) during annealing [Su et al., <i>ACS Nano</i> <b>2010</b>, <i>4</i>, 5285]. The reaction mechanism is, however, still not well understood, hindering progress in the use of GO. By combining density functional theory (DFT), ab initio molecular dynamics (MD) calculations, and in situ infrared absorption spectroscopy, the thermal evolution of both carbonyl and ether groups in multilayered GO is shown to vary dramatically upon individual intercalation of methanol, ethanol, and water molecules. In hydrated GO, bare etch holes are generated by thermal annealing, as evidenced by the evolution of carbon dioxide (CO₂) molecules. In contrast, the replacement of water by methanol or ethanol prevents defect enlargement during annealing. Furthermore, ethanol is found to repair the etch holes by facilitating the formation of new hexagonal carbon rings. Ab initio MD simulations map out the likely reaction pathways that are subsequently verified by DFT total energy calculations. The elucidation of the mechanism of etch hole healing in GO suggests new ways to tailor the structural and electronic properties of reduced GO (rGO) and graphene for a variety of applications requiring defect engineering

Spectroscopic Evaluation of Out-of-Plane Surface Vibration Bands from Surface Functionalization of Graphite Oxide by Fluorination

The fluorination of graphite/graphite oxide (GO) and their derivatives has been widely investigated for how fluorine interacts with sp2/sp 3 carbon; however, the mechanism of this interaction has not yet been elucidated. Fluorination of GO (FGO) at either 10 or 15 psi for 24 h, produced two new absorption bands at ???743 cm-1 and 482 cm -1, and are attributed to the presence of out-of-plane surface fluorine bonds in FGO (absent in fluorographite - FG). IR studies confirmed the stability of the formed C-F bonds and defect formation due to the introduction of oxyfluorinated species into the graphitic carbon through fluorination of epoxides. Fluorination of GO resulted in ???4-5 times more fluorine incorporation in bulk as compared to FG. (4.57 vs. 0.8 at.% and 6.64 vs. 1.4 at.% at 10 and 15 psi, respectively). PXRD analyses also showed that the interlayer spacing of FGO expanded in the presence of intercalated C-F species and a defect formation was observed with the evidence of increase of the I D/IG ratio from Raman spectra. To this end, understanding the origin of surface C-F bonds and structural changes in FGO therefore leads to new applications such as implementation of FGO for sensing, nano-electronics and energy storage. &amp;#169; 2014 Elsevier Ltd. All rights reserved.open0

Substitutional Growth of Methylammonium Lead Iodide Perovskites in Alcohols

Methylammonium lead iodide (MAPbI(3)) perovskites are organic-inorganic semiconductors with long carrier diffusion lengths serving as the light-harvesting component in optoelectronics. Through a substitutional growth of MAPbI(3) catalyzed by polar protic alcohols, evidence is shown for their substrate-and annealing-free production and use of toxic solvents and high temperature is prevented. The resulting variable-sized crystals ((approximate to 100 nm-10 mu m) are found to be tetragonally single-phased in alcohols and precipitated as powders that are metallic-lead-free. A comparatively low MAPbI(3) yield in toluene supports the role of alcohol polarity and the type of solvent (protic vs aprotic). The theoretical calculations suggest that overall Gibbs free energy in alcohols is lowered due to their catalytic impact. Based on this alcohol-catalyzed approach, MAPbI(3) is obtained, which is chemically stable in air up to approximate to 1.5 months and thermally stable (&lt;= 300 degrees C). This method is amendable to large-scale manufacturing and ultimately can lead to energy-efficient, low-cost, and stable devices