Publications

Abstract While surfactants and polymers have been independently investigated as agents to separate, disperse and stabilize carbon nanotubes (CNTs) in water, mixed polymer/surfactant (P/S) systems have been far less studied for those ends. In this work, we investigated the ability of various types of P/S mixtures to effectively separate multiwalled carbon nanotubes (MWNTs) in water, using rigorously controlled processing conditions. Two types of mixtures were explored: i) nonionic polymer (PVP, polyvinylpyrrolidone) and ionic surfactant (sodium dodecylbenzene sulfonate, SDBS, or cetyltrimethylammonium bromide, CTAB); and ii) ionic polymer (poly(diallyl dimethyl ammonium chloride), PDDA, and sodium polyacrylate, PAS) and nonionic surfactant (TX-100). Detailed, high precision dispersibility curves (concentration of dispersed nanotubes vs. total P/S concentration, at fixed S concentration) are presented for four P/S mixtures (PVP/SDBS, PVP/CTAB, PDDA/TX-100 and PAS/TX-100) and their respective individual components. Quantitative metrics extracted from the dispersibility curves allow for reliable comparisons between the systems. In all P/S mixtures, beneficial (synergistic) effects in nanotube dispersibility are observed compared to the individual components, with the exception of the PDDA/TX-100 one for which a detrimental (antagonistic) effect occurs. Morphological characterization of the as-obtained dispersions by scanning electron microscopy (SEM) shows a significant degree of nanotube separation by the P/S systems. Surface tension and zeta potential measurements provide further information on the interactions at play between the MWNTs and the P/S mixtures, allowing to conceive plausible mechanisms for the synergistic effects observed. P/S association may not only offer conditions for an enhanced dispersibility of CNTs but also expand the types of noncovalent, reversible functionalization required in many applications, such as the development of nanocomposite particles, films and coatings.

Abstract The incorporation of carbon-based nanomaterials into biopolymer matrix, to provide mechanical reinforcement and to obtain electrically conductive bionanocomposites, requires the homogeneous dispersion of the fillers. Herein, it is investigated the influence of surfactant structures on the dispersibility of multiwalled carbon nanotubes (MWNT) within starch matrix. Three different ionic surfactants, sodium dodecyl sulphate (SDS), cetyltrimethylammonium bromide (CTAB) and sodium cholate (SC), are employed to disperse the MWNT. Films with MWNT-SC show better dispersibility and an increase of about 75% of tensile strength and 60% of Young's modulus compared with films using MWNT-SDS and MWNT-CTAB. Nevertheless, MWNT functionalized with CTAB impart the highest values of antioxidant activity (scavenging activity around 30% in 1.5 h) and electrical conductivity (sigma =14.75 S/m) to starch matrix. The properties of starch-based films can be tailored according to the physical adsorption of each surfactant on MWNT surface and/or the interfacial interaction of the surfactant with starch chains.

Abstract Fuel cells are emerging devices as clean and renewable energy sources, provided their efficiency is increased. In this work, we prepared nanocomposites based on multiwalled carbon nanotubes (MWNTs) and transition metal dichalcogenides (TMDs), namely WS2 and MoS2, and evaluated their performance as electrocatalysts for the oxygen evolution reaction (OER) and the oxygen reduction reaction (ORR), relevant to fuel cells. The one- and two-dimensional (1D and 2D) building blocks were initially exfoliated and non-covalently functionalized by surfactants of opposite charge in aqueous media (tetradecyltrimethylammonium bromide, TTAB, for the nanotubes and sodium cholate, SC, for the dichalcogenides), and thereafter, the three-dimensional (3D) MoS2@MWNT and WS2@MWNT composites were assembled via surfactant-mediated electrostatic interactions. The nanocomposites were characterized by scanning electron microscopy (SEM) and structural differences were found. WS2@MWNT and MoS2@MWNT show moderate ORR performance with potential onsets of 0.71 and 0.73 V vs. RHE respectively, and diffusion-limiting current densities of -1.87 and -2.74 mA center dot cm(-2), respectively. Both materials present, however, better tolerance to methanol crossover when compared to Pt/C and good stability. Regarding OER performance, MoS2@MWNT exhibits promising results, with eta(10) and j(max) of 0.55 V and 17.96 mA center dot cm(-2), respectively. The fabrication method presented here is cost-effective, robust and versatile, opening the doors for the optimization of electrocatalysts' performance.

Abstract Most applications of nanocarbons, such as carbon nanotubes and graphene, require that they are well-separated and well-dispersed in a liquid phase. Intensive efforts have been put on exfoliating and dispersing nanocarbons in aqueous solvents, typically using amphiphilic dispersants and sonication/centrifugation procedures, alongside a drive to fundamentally understand and rationally optimize these processes. Herein, we employed a robust method to separate and disperse multiwalled carbon nanotubes (MWNTs), and graphene nanoplatelets (GnPs) either from bulk graphite or from pre-formed GnP powders, using rigorously controlled processing conditions. An ionic (sodium cholate) and a nonionic (Triton X-100) surfactant were used as dispersants. Our aim was to determine high-precision dispersibility curves (concentration of dispersed nanomaterial versus initial surfactant concentration) for the different nanocarbon/dispersant systems, characterize morphologically the dispersed particles and compare the mechanisms of exfoliation of 1D and 2D nanocarbons at molecular level. Typically bell-shaped dispersibility curves with a plateau were obtained, and from the latter several quantitative metrics were extracted that permitted reliable comparisons between nanocarbon/surfactant systems. Scanning electron and atomic force microscopies allowed to characterize the suspended particles in the as-obtained dispersions, namely the MWNT bundle width and GnP dimensions (mean lateral size and layer number). Under fixed conditions (in particular, delivered energy per carbon mass), MWNTs are dispersed in much higher yields, by two orders of magnitude, than GnPs. However, and significantly, a master curve for the dispersibility was obtained, implying that common fundamental features underpin the dispersing process, irrespective of nanocarbon (1D or 2D) or surfactant (ionic or nonionic) types.

Abstract The development of composites from 1D and 2D nanocarbon building blocks, namely carbon nanotubes and graphene layers, with enhanced properties or novel functionalities is an emerging challenge in material science. Herein, we developed a colloid-based approach using surfactants and polymers to non-covalently functionalize multiwalled carbon nanotubes (MWNTs) and graphene nanoplatelets (GnPs), and to fabricate GnP@MWNT nanocomposites via an electrostatic-driven assembly process in aqueous solution. In the assembly process, two building methods were used and compared (bulk mixing and adapted layer-by-layer assembly), using surfactant and polymer/surfactant combinations as the dispersants for the initial nanomaterials. After their characterization by scanning electron microscopy, Raman spectroscopy and BET analysis, the nanocomposites were evaluated as electrocatalysts for the oxygen reduction reaction (ORR). Results show that the type of the dispersant (namely the presence of polymer) plays a more relevant role than the specific building method in almost all the ORR parameters. Further, the nanocomposites show selectivity toward the 2-electron pathway oxygen reduction for the electrochemical production of hydrogen peroxide. The development and optimization of further nanocomposite electrocatalysts can be pursued using this type of versatile and robust assembly method.

Abstract Surfactants have been widely employed to debundle, disperse and stabilize carbon nanotubes in aqueous solvents. Yet, a thorough understanding of the dispersing mechanisms at molecular level is still warranted. Herein, we investigated the influence of the molecular structure of gemini surfactants on the dispersibility of multiwalled carbon nanotubes (MWNTs). We used dicationic n-s-n gemini surfactants, varying n and s, the number of alkyl tail and alkyl spacer carbons, respectively; for comparisons, single-tailed surfactant homologues were also studied. Detailed curves of dispersed MWNT concentration vs. surfactant concentration were obtained through a stringently controlled experimental procedure, allowing for molecular insight. The gemini are found to be much more efficient dispersants than their single-tailed homologues, i.e. lower surfactant concentration is needed to attain the maximum dispersed MWNT concentration. In general, the spacer length has a comparatively higher influence on the dispersing efficiency than the tail length. Further, scanning electron microscopy imaging shows a sizeable degree of MWNT debundling by the gemini surfactants in the obtained dispersions. Our observations also point to an adsorption process that does not entail the formation of micelle-like aggregates on the nanotube surface, but rather coverage by individual molecules, among which the ones that seem to be able to adapt best to the nanotube surface provide the highest efficiency. These studies are relevant for the rational design and choice of optimal dispersants for carbon nanomaterials and other similarly water-insoluble materials.

Abstract A fundamental understanding of the mechanisms involved in the surfactant-assisted exfoliation and dispersion of carbon nanotubes (CNTs) in water calls for well-controlled experimental methodologies and reliable comparative metrics. We have assessed the ability of several ionic surfactants to disperse single and multiwalled carbon nanotubes, resorting to a stringently controlled sonication-centrifugation method for the preparation of the dispersions. The CNT concentration was accurately measured for a wide range of surfactant concentration, using combined therrnogravimetric analysis and UV-vis spectroscopy. The obtained dispersibility curves yield several quantitative parameters, which in turn allow for the effects of nanotube morphology and surfactant properties (aromatic rings, chain length, headgroup charge, and cmc) to be assessed and rationalized, both in terms of dispersed indicate that the CNT-surfactant association follows patterns that are markedly different from other equilibrium processes governed by hydrophobicity (such as micellization); in particular, the surfactant concentration needed for maximum dispersibility, c(s,max), and the number of surfactant molecules per unit CNT area at cs,max are shown to depend linearly on chain length. The results further suggest that the presence of micelles in the exfoliation process is not a key factor either for starting CNT dispersibility or attaining its saturation value. nanotube mass and surface area. The data also