Showing papers by "Cinzia Casiraghi published in 2016"

Raman Fingerprints of Atomically Precise Graphene Nanoribbons

Abstract: Bottom-up approaches allow the production of ultranarrow and atomically precise graphene nanoribbons (GNRs) with electronic and optical properties controlled by the specific atomic structure. Combining Raman spectroscopy and ab initio simulations, we show that GNR width, edge geometry, and functional groups all influence their Raman spectra. The low-energy spectral region below 1000 cm–1 is particularly sensitive to edge morphology and functionalization, while the D peak dispersion can be used to uniquely fingerprint the presence of GNRs and differentiates them from other sp2 carbon nanostructures.

Poly(ethylene oxide) Functionalized Graphene Nanoribbons with Excellent Solution Processability

Abstract: Structurally well-defined graphene nanoribbons (GNRs) have attracted great interest as next-generation semiconductor materials. The functionalization of GNRs with polymeric side chains, which can widely broaden GNR-related studies on physiochemical properties and potential applications, has remained unexplored. Here, we demonstrate the bottom-up solution synthesis of defect-free GNRs grafted with flexible poly(ethylene oxide) (PEO) chains. The GNR backbones possess an armchair edge structure with a width of 1.0–1.7 nm and mean lengths of 15–60 nm, enabling near-infrared absorption and a low bandgap of 1.3 eV. Remarkably, the PEO grafting renders the GNRs superb dispersibility in common organic solvents, with a record concentration of ∼1 mg mL–1 (for GNR backbone) that is much higher than that (<0.01 mg mL–1) of reported GNRs. Moreover, the PEO-functionalized GNRs can be readily dispersed in water, accompanying with supramolecular helical nanowire formation. Scanning probe microscopy reveals raft-like self...

The influence of few-layer graphene on the gas permeability of the high-free-volume polymer PIM-1.

Abstract: Gas permeability data are presented for mixed matrix membranes (MMMs) of few-layer graphene in the polymer of intrinsic microporosity PIM-1, and the results compared with previously reported data for two other nanofillers in PIM-1: multiwalled carbon nanotubes functionalized with poly(ethylene glycol) (f-MWCNTs) and fused silica. For few-layer graphene, a significant enhancement in permeability is observed at very low graphene content (0.05 vol.%), which may be attributed to the effect of the nanofiller on the packing of the polymer chains. At higher graphene content permeability decreases, as expected for the addition of an impermeable filler. Other nanofillers, reported in the literature, also give rise to enhancements in permeability, but at substantially higher loadings, the highest measured permeabilities being at 1 vol.% for f-MWCNTs and 24 vol.% for fused silica. These results are consistent with the hypothesis that packing of the polymer chains is influenced by the curvature of the nanofiller surface at the nanoscale, with an increasingly pronounced effect on moving from a more-or-less spherical nanoparticle morphology (fused silica) to a cylindrical morphology (f-MWCNT) to a planar morphology (graphene). While the permeability of a high-free-volume polymer such as PIM-1 decreases over time through physical ageing, for the PIM-1/graphene MMMs a significant permeability enhancement was retained after eight months storage.

Raman spectroscopy of highly pressurized graphene membranes

Abstract: Raman spectroscopy is an ideal tool for the characterization of strained graphene. Biaxial strain, in particular, allows for more reliable calculation of the Gruneisen parameters than uniaxial strain. However, the application of biaxial strain is rather difficult to achieve experimentally, so all previous studies reported on graphene subjected to relatively small biaxial strains (0.1%–1%), in contrast to uniaxial strain above 10%. Here, we report a simple fabrication technique to produce pressurized and stable graphene membranes that can support differential pressures up to 14 bar, corresponding to a reversible strain up to ∼2%. We find that the Gruneisen parameters remain constant even for the largest strains achieved, in agreement with the theoretical predictions. However, for strains above 1%, a distinctive broadening of both the G and 2D peaks was observed for biaxial strain. We attribute this to the nanoscale variations of strain in the membrane within an area comparable with the laser spot size.

Self-catalytic membrane photo-reactor made of carbon nitride nanosheets†

Abstract: Solar-driven photo-oxidation is a very attractive and efficient technique for chemical conversion of organic dyes in water into non-hazardous compounds, but it requires a catalyst in order to overcome the barrier of oxidation degradation. In this study we use a membrane photo-reactor (MPR) made of nanosheets of graphitic carbon nitride (g-C3N4), assembled by vacuum filtration. The membrane was characterized by X-Ray Diffraction (XRD), Nuclear Magnetic Resonance (NMR), electron microscopy and IR spectroscopy. Photo-degradation studies show that the membranes are very efficient in the degradation of Sudan orange G, rhodamine 110 and methylene blue. As the catalyst is a porous laminate, the reactant can flow through the pores of the membrane. Because the space between g-C3N4 nanosheets is comparable to the size of the dyes, the probability of the reactants to be close to the catalyst is enhanced, making the reaction very efficient.

Perchlorination of Coronene Enhances its Propensity for Self-Assembly on Graphene

Abstract: Providing a quantitative understanding of the thermodynamics involved in molecular adsorption and self-assembly at a nanostructured carbon material is of fundamental importance and finds outstanding applications in the graphene era. Here, we study the effect of edge perchlorination of coronene, which is a prototypical polyaromatic hydrocarbon, on the binding affinity for the basal planes of graphite. First, by comparing the desorption barrier of hydrogenated versus perchlorinated coronene measured by temperature-programmed desorption, we quantify the enhancement of the strength of physisorption at the single-molecule level though chlorine substitution. Then, by a thermodynamic analysis of the corresponding monolayers based on force-field calculations and statistical mechanics, we show that perchlorination decreases the free energy of self-assembly, not only enthalpically (by enhancing the strength of surface binding), but also entropically (by decreasing the surface concentration). The functional advantage of a chemically modulated 2D self-assembly is demonstrated in the context of the molecule-assisted liquid-phase exfoliation of graphite into graphene.

Substrate dependence of graphene reactivity towards hydrogenation

Abstract: The ability to functionalize graphene with several methods, such as radical reactions, cyclo-additions, hydrogenation, and oxidations, allows this material to be used in a large range of applications. In this framework, it is essential to be able to control the efficiency and stability of the functionalization process—this requires understanding how the graphene reactivity is affected by the environment, including the substrate. In this work we provide an insight on the substrate dependence of graphene reactivity towards hydrogenation by comparing three different substrates: silicon, hexagonal boron nitride (h-BN), and molybdenum disulfide (MoS2). Although MoS2 and h-BN have flatter surfaces than silicon, we found that the H coverage of graphene on h-BN is about half of the H coverage on graphene on both silicon and MoS2. Therefore, graphene shows strongly reduced reactivity towards hydrogenation when placed on h-BN. The difference in hydrogenation reactivity between h-BN and MoS2 may indicate a stronger van der Waals force between graphene and h-BN, compared to MoS2, or may be related to the chemical properties of MoS2, which is a well-known catalyst for hydrogen evolution reactions.