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.

Shedding light on azopolymer brush dynamics by fluorescence correlation spectroscopy.

Understanding the response to illumination at a molecular level as well as characterising polymer brush dynamics are key features that guide the engineering of new light-stimuli responsive materials. Here, we report on the use of a confocal microscopy technique that was exploited to discern how a single molecular event such as the photoinduced isomerisation of azobenzene can affect an entire polymeric material at a macroscopic level leading to photodriven mass-migration. For this reason, a set of polymer brushes, containing azobenzene (Disperse Red 1, DR) on the side chains of poly(methacrylic acid), was synthesised and the influence of DR on the polymer brush dynamics was investigated for the first time by Fluorescence Correlation Spectroscopy (FCS). Briefly, two dynamics were observed, a short one coming from the isomerisation of DR and a long one related to the brush main chain. Interestingly, photoinduced polymer aggregation in the confocal volume was observed.

Thermoresponsive PNIPAAm hydrogel scaffolds with encapsulated AuNPs show high analyte-trapping ability and tailored plasmonic properties for high sensing efficiency.

The fabrication of a scaffold able to control the positioning of AuNPs and to trap and concentrate target molecules inside them is a promising idea for a large variety of sensing applications. In this work, we designed and fabricated a scaffold of already-prepared 20 nm AuNPs encapsulated in a PNIPAAm hydrogel and utilizing surface enhanced Raman spectroscopy (SERS), we used it as a sensor with remarkably low limits of detection. In fact, as the target is trapped inside the hydrogel, the following takes place: (a) the concentration of the target increases dramatically and (b) the localization of the AuNPs and thus of the hotspots (areas with extremely high SERS enhancement factors) work synergistically, improving the sensing ability of the scaffold. The SERS enhancement ability of our scaffolds was checked with adenine, 2-naphthalenethiol and melamine molecules; the trapping efficiency was investigated for the melamine and a partition coefficient of k = 5 × 105 was found. Finally, by focusing on a single PNIPAAm hydrogel with encapsulated AuNPs, we managed to detect 10-6 M or rather 108 molecules of melamine trapped inside the scaffold.

Molecular orientation of poly(ethylene terephthalate) and poly(butylene terephthalate) probed by polarized Raman spectra: a parallel study.

Poly(ethylene terephthalate) (PET) and poly(butylene terephthalate) (PBT) samples uniaxially drawn above Tg and beyond the yield point exhibit significant differences in their molecular orientation behavior as probed by polarized Raman spectra. The quasi-amorphous PET samples, drawn close to the Tg, manifest considerable molecular orientation development; however, when drawn above Tg + 30 degrees C, they exhibit significant molecular orientation relaxation. The semi-crystalline PBT samples maintain prominent molecular orientation even when drawn 110 degrees C above Tg. The drawing process, in PET samples, when resulting in molecular orientation, is accompanied by a gauche-trans transformation of the glycol linkage and a concurrent initiation of crystallinity development. In PBT specimens, it gives rise to a coexistence of alpha- and beta-type crystalline phases. Phase alpha is predominant at high draw temperatures, i.e., Tg + 110 degrees C, while phase beta dominates at low draw temperatures, i.e., Tg + 10 degrees C. PBT samples, with beta-phase predominance, left at relevant draw temperatures without stress, exhibit a beta-alpha phase change though no molecular orientation relaxation occurs. A note is made of the fact that complete molecular orientation analysis of PBT segments utilizing the depol method gives more reliable results than the simplified analysis assuming a cylindrical tensor for the 1614 cm(-1) symmetric stretch of the para-disubstituted benzene ring of PBT. In this context, segments of PBT specimens rich in alpha-phase exhibit higher molecular orientation than those with beta-phase predominance.