Graphene aerogels as efficient adsorbers of water pollutants and their effect of drying methods.

Environmental accidents highlight the need for the development of efficient materials that can be employed to eliminate pollutants including crude oil and its derivatives, as well as toxic organic solvents. In recent years, a wide variety of advanced materials has been investigated to assist in the purification process of environmentally compromised regions, with the principal contestants being graphene-based structures. This study describes the synthesis of graphene aerogels with two methods and determines their efficiency as adsorbents of several water pollutants. The main difference between the two synthesis routes is the use of freeze-drying in the first case, and ambient pressure drying in the latter. Raman spectroscopy, Scanning Electron Microscopy (SEM), X-ray diffraction (XRD), X-ray Photoelectron Spectroscopy (XPS) and contact angle measurements are employed here for the characterisation of the samples. The as-prepared aerogels have been found to act as photocatalysts of aqueous dye solutions like methylene blue and Orange G, while they were also evaluated as adsorbents of organic solvents (acetone, ethanol and methanol), and, oils like pump oil, castor oil, silicone oil, as well. The results presented here show that the freeze-drying approach provides materials with better adsorption efficiency for the most of the examined pollutants, however, the energy and cost-saving advantages of ambient-pressure-drying could offset the adsorption advantages of the former case.

Graphene nanoplatelets and other 2D-materials as protective means against the fading of coloured inks, dyes and paints.

The present work demonstrates the ability of graphene nanoplatelets (GNPs) and other two-dimensional materials (2DMs) like tungsten disulfide (WS2), molybdenum disulfide (MoS2) and hexagonal boron nitride (hBN) to act as protective barriers against the fading of architectural paints and also inks/paints used in art. The results present a new approach for improving the lightfastness of colours of artworks and painted indoor/outdoor wall surfaces taking advantage of the remarkable properties of 2DMs. As shown herein, commercial inks and architectural paints of different colours doped with graphene nanoplatelets (GNPs), graphene oxide (GO), reduced graphene oxide (rGO) and other 2DMs, exhibit a superior resistance to fading under ultraviolet radiation or even under exposure to visible light. A spectroscopic study on these inks and dyes reveals that the peaks which are characteristic of the colour pigments are less affected from aging/fading when the GNPs and the other 2DMs are present. The protection mechanism for the GNPs and the other 2DMs differs. For GNPs, mainly their high surface area which leads to free radicals scavenging (especially hydroxyl radicals), and secondarily their UV absorption, are responsible for their protection effects, while for GO, a transition to rGO structures and consequently to 'smart' paints can be observed after the performed aging routes. In this way, the paint gets improved by time preventing or slowing its own fading and decolorization. For the other 2DMs, the transition-metal dichalcogenides performed better than hBN, even though they all absorb in the UV region. This can be ascribed to the facts that the formers also absorb in the visible, while hBN does not, while most importantly, they can trap reactive oxygen species (ROS) and corrosive gases in their structure as opposed to hBN. By conducting colorimetric measurements, we have discovered that the lifetime of the as-developed 2DM-doped inks and paints can be extended by up to ∼40%.

Preventing colour fading in artworks with graphene veils.

Modern and contemporary art materials are generally prone to irreversible colour changes upon exposure to light and oxidizing agents. Graphene can be produced in thin large sheets, blocks ultraviolet light, and is impermeable to oxygen, moisture and corrosive agents; therefore, it has the potential to be used as a transparent layer for the protection of art objects in museums, during storage and transportation. Here we show that a single-layer or multilayer graphene veil, produced by chemical vapour deposition, can be deposited over artworks to protect them efficiently against colour fading, with a protection factor of up to 70%. We also show that this process is reversible since the graphene protective layer can be removed using a soft rubber eraser without causing any damage to the artwork. We have also explored a complementary contactless graphene-based route for colour protection that is based on the deposition of graphene on picture framing glass for use when the direct application of graphene is not feasible due to surface roughness or artwork fragility. Overall, the present results are a proof of concept of the potential use of graphene as an effective and removable protective advanced material to prevent colour fading in artworks.

Development of a reactor for the in situ monitoring of 2D materials growth on liquid metal catalysts, using synchrotron x-ray scattering, Raman spectroscopy, and optical microscopy.

Liquid metal catalysts (LMCats) (e.g., molten copper) can provide a new mass-production method for two-dimensional materials (2DMs) (e.g., graphene) with significantly higher quality and speed and lower energy and material consumption. To reach such technological excellence, the physicochemical properties of LMCats and the growth mechanisms of 2DMs on LMCats should be investigated. Here, we report the development of a chemical vapor deposition (CVD) reactor which allows the investigation of ongoing chemical reactions on the surface of a molten metal at elevated temperatures and under reactive conditions. The surface of the molten metal is monitored simultaneously using synchrotron x-ray scattering, Raman spectroscopy, and optical microscopy, thereby providing complementary information about the atomic structure and chemical state of the surface. To enable in situ characterization on a molten substrate at high temperatures (e.g., ∼1370 K for copper), the optical and x-ray windows need to be protected from the evaporating LMCat, reaction products, and intense heat. This has been achieved by creating specific gas-flow patterns inside the reactor. The optimized design of the reactor has been achieved using multiphysics COMSOL simulations, which take into account the heat transfer, fluid dynamics, and transport of LMCat vapor inside the reactor. The setup has been successfully tested and is currently used to investigate the CVD growth of graphene on the surface of molten copper under pressures ranging from medium vacuum up to atmospheric pressure.

Stress transfer at the nanoscale on graphene ribbons of regular geometry.

The knowledge of the mechanism of stress transfer from a polymer matrix to a 2-dimensional nano-inclusion such as a graphene flake is of paramount importance for the design and the production of effective nanocomposites. For efficient reinforcement the shape of the inclusion must be accurately controlled since the axial stress transfer from matrix to the inclusion is affected by the axial-shear coupling observed upon loading of a flake of irregular geometry. Herein, we study true axial phenomena on regular- exfoliated-graphene micro-ribbons which are perfectly aligned to the loading direction. We exploit the strain sensitivity of vibrational wave numbers in order to map point-by-point the strain built up along the length of graphene. By considering the balance of shear-to-axial forces, we identify the shear stress at the interface and develop a universal inverse-length parameter that governs the stress transfer process at the nanoscale. An important parameter that has come out of this approach is the prediction and measurement of the transfer length that is required for efficient stress in these systems.

Stress-transfer from polymer substrates to monolayer and few-layer graphenes.

In the present study, the stress transfer mechanism in graphene-polymer systems under tension is examined experimentally using the technique of laser Raman microscopy. We discuss in detail the effect of graphene edge geometry, lateral size and thickness which need to be taken under consideration when using graphene as a protective layer. The systems examined were composed of graphene flakes with a large length (over ∼50 microns) and a thickness of one to three layers simply deposited onto PMMA substrates which were then loaded to a tension of ∼1.60% strain. The stress transfer profiles were found to be linear while the results show that large lateral sizes of over twenty microns are needed in order to provide effective reinforcement at levels of strain higher than 1%. Moreover, the stress built up has been found to be quite sensitive to both edge shape and geometry of the loaded flakes. Finally, the transfer lengths were found to increase with the increase of graphene layers. The outcomes of the present study provide crucial insight into the issue of stress transfer from polymers to graphene nano-inclusions as a function of edge geometry, lateral size and thickness in a number of applications.

Compressive response and buckling of graphene nanoribbons.

We examine the mechanical response of single layer graphene nanoribbons (GNR) under constant compressive loads through molecular dynamics simulations. Compressive stress-strain curves are presented for GNRs of various lengths and widths. The dependence of GNR's buckling resistance on its size, aspect ratio, and chiral angle is discussed and approximate corresponding relations are provided. A single master curve describing the dependence of the critical buckling stress of GNRs on their aspect ratio is presented. Our findings were compared to the continuum elasticity theories for wide plates and wide columns. In the large width limit, the response of the GNRs agrees with the predictions of the wide plates theory and thus, with that of wide graphenes. In the small width limit, the behavior of graphene nanoribbons deviates from that of periodic graphenes due to various edge related effects which govern the stiffness and the stability of the graphene membranes, but it qualitatively agrees with the theory of wide columns. In order to assess the effect of thermal fluctuations on the critical buckling stress a wide range of temperatures is examined. The findings of the current study could provide important insights regarding the feasibility and the evaluation of the performance of graphene-based devices.

Wrinkling formation in simply-supported graphenes under tension and compression loadings.

Wrinkles in supported graphenes can be formed either by uniaxial compression or uniaxial tension beyond a certain critical load depending on the mode of loading. In the first case, the wrinkling direction is normal to the compression axis whereas in tension, wrinkles of the same pattern are formed parallel to the loading direction due to Poisson's (lateral) contraction. Herein we show by direct AFM observations that in simply-supported graphenes such instabilities appear as periodic wrinkles over existing stochastic undulations caused by the underlying-substrate-roughness. The critical strain for the generation of these wrinkles in both tension and compression is less than 1% which particularly for the former is far lower than the predicted tensile strain to fracture of suspended graphene estimated at ∼30%. Based on these findings, a constitutive model that provides the critical tensile strain for induced buckling in the lateral direction is proposed that depends only on the graphene-support interaction and not on the nature of the substrate. Understanding the wrinkling failure of graphenes under strain is of paramount importance as it leads to new threshold limits beyond which the physical-mechanical properties of graphene are impaired.

High volume fraction carbon nanotube-epoxy composites.

A versatile processing technique for fabricating epoxy nanocomposites with a high weight fraction of oxidized multi-walled carbon nanotubes is presented. Thin carbon nanotube based preforms were prepared through an oxidation-filtration protocol and then immersed in a pre-polymerized epoxy/curing agent solution in acetone. By adjusting the conditions for the oxidation of carbon nanotubes and the epoxy concentration in the as-prepared solution, high loading of graphitic nanostructures was obtained. Tensile tests indicated that the elastic modulus and strength of certain composites prepared by in situ polymerization as above were improved by 100% and 60%, respectively, compared to neat epoxy. In addition, the composite sheets showed comparable electrical conductivity values to the neat carbon nanotube paper. These results suggest that targeted chemical modification of the carbon nanotube surface is an effective way to enhance the electrical and mechanical properties of carbon nanotube-polymer composites.

Single-walled carbon nanotubes decorated with a pyrene-fluorenevinylene conjugate.

Single-walled carbon nanotubes are noncovalently functionalized using a pyrene-fluorenevinylene dye and the resulting nanohybrids are isolated from the free molecules. The tubes modified by means of this noncovalent approach show enhanced solubility in organic media. The structure and morphology of this hybrid material are fully characterized using absorption, infrared and Raman spectroscopies as well as atomic force and scanning electron microscopies. Steady state fluorescence measurements reveal that significant quenching of the pyrene derivative excited state takes place through an energy transfer mechanism.

Determination of molecular changes in soft tissues under strain using laser Raman microscopy.

The paper presents a non-contact technique to examine the molecular changes in a collagen fibre subjected to in vitro axial tension. Laser Raman microscopy was employed to monitor the vibrational changes in specific assignments of the Raman spectrum of collagen. Results were presented in the form of Raman wavenumber shift as a function of applied tensile strain. Two distinct responses were observed depending on whether the vibrations were axial to, or normal to, the collagen backbone. The former response produced a decrease in wavenumber values, indicating tension, whereas the latter produced an increase, indicating compression. The rate of wavenumber shift with applied strain was non-linear in form, with a marked increase at higher levels of applied strain, for example, a strain 4% in the case of axial vibrations. This technique can prove to be a powerful tool for examining deformation at the molecular level in collagenous tissues.