Liquid phase exfoliation of two dimensional crystals

THESIS 1014

Hydrogen evolution across nano-Schottky junctions at carbon supported MoS 2 catalysts in biphasic liquid systems

The activities of a series of MoS 2-based hydrogen evolution catalysts were studied by biphasic reactions monitored by UV/Vis spectroscopy. Carbon supported MoS 2 catalysts performed best due to an abundance of catalytic edge sites and strong electronic coupling of catalyst to suppor

Preparation of high concentration dispersions of exfoliated MoS 2 with increased flake size

Solvent exfoliation of inorganic layered compounds is likely to be important for a range of applications. However, this method generally gives dispersions of small nanosheets at low concentrations. Here we describe methods, based on sonication of powdered MoS2 in the solvent N-methyl-pyrrolidone, to prepare dispersions with significantly increased lateral nanosheet size and dispersed concentration. We find the concentration to scale linearly with starting MoS2 mass allowing the definition of a yield. This yield can be increased to ~40% by controlling the sonication time, resulting in concentrations as high as 40 mg/ml. We find the nanosheet size to increase initially with sonication time reaching ~700 nm (for a concentration of ~7.5 mg/ml). At longer sonication times the nanosheets size falls off due to sonication induced scission. The nanosheets produced by such methods are relatively thin and have no observable defects. We can separate the dispersed nanosheets by size using controlled centrifugation. This allows us to produce dispersions with mean flake size of up to ~2? m. However, such large flakes are noticeably thicker than the standard nanosheets. We demonstrate that such nanosheets can be mixed with polymers to form composites. While standard nanosheets result in no improvement in composite mechanical properties, addition of size-selected nanosheets results in significant improvements in composite modulus and strength

Improving the mechanical properties of graphene oxide based materials by covalent attachment of polymer chains

We report on the modification of graphene oxide (GO) with polyvinylalcohol (PVA) leading to the mechanical improvement of GO based materials. First, GO was covalently functionalised with PVA by esterification of carboxylic groups on GO with hydroxyl groups of PVA resulting in functionalised f-(PVA)GO. This was carried out for PVA of six different molecular weights. This functionalised graphene oxide could be formed into a paper-like material by vacuum filtration. Papers prepared from f-(PVA)GO showed significant increases in mechanical properties compared to those prepared with GO or with simple mixtures of GO and PVA. The best performance was achieved for PVA functional groups with molecular weights between 50 and 150 kg/mol. Improvements in Young?s moduli of 60% and tensile strength of 400% were observed relative to GO-only paper. The improved mechanical properties are attributed to enhanced inter-flake stress transfer due to the covalently bonded PVA. Second, functionalised f-(PVA)GO was used as filler in PVA-based composites. The application of a pre-selection method allowed the use of only the largest functionalised f-(PVA)GO flakes. This resulted in substantially reinforced PVA-f-(PVA)GO composites. Both modulus and strength increased by 40% relative to the pure polymer for f-(PVA)GO loadings below 0.3 vol.%

Size selection of dispersed, exfoliated graphene flakes by controlled centrifugation

Liquid exfoliation of graphene generally results in flakes with lateral size of one micron or less on average, too small for many applications. In this paper we describe a method to separate an existing dispersion with mean flake length of ~1 ?m into fractions, each with different mean flake size. The initial dispersion is centrifuged at a high centrifugation rate, separating small flakes in the supernatant from large flakes in the sediment. Redispersion of the sediment, followed by successive centrifugation, separation and redispersion cycles can be used to separate the flakes by size so long as the centrifugation rate is decreased with each cycle. This procedure results in a range of dispersions with mean flake length varying from 1 ?m for the highest final centrifugation rate to 3.5 ?m for the sample whose final centrifugation rate was 500 rpm

Development of stiff, strong, yet tough composites by the addition of solvent exfoliated graphene to polyurethane

We have prepared graphene dispersions, stabilised by polyurethane in tetrahydrofuran and dimethylformamide. These dispersions can be drop-cast to produce free-standing composite films. The graphene mass fraction is determined by the concentration of dispersed graphene and can be controllably varied from 0% to 90%. Raman spectroscopy and Helium ion microscopy show the graphene to well-dispersed and well-exfoliated in the composites, even at mass fractions of 55%. On addition of graphene, the Young?s modulus and stress at 3% strain increase by ?100, saturating at 1 GPa and 25 MPa respectively for mass fractions above 50wt%. While the ultimate tensile strength does not vary significantly with graphene content, the strain at break and toughness degrade heavily on graphene addition. Both these properties fall by ?1000 as the graphene content is increased to 90wt%. However, the rate of increase of Young?s modulus and stress at 3% strain with mass fraction is greater than the rate of decrease of ductility and toughness. This makes it possible to prepare composites with high modulus, stress at low strain and ultimate tensile strength as well as relatively high toughness and ductility. This could lead to new materials that are stiff, strong and tough

Trinity Walton Club: What is its potential for promoting interest in STEM?

Concerns are growing in many countries, including Ireland, regarding an inadequate number of graduates to meet workforce needs in science, technology, engineering and mathematics (STEM) fields. These graduate deficiencies are the result of low student uptake of STEM subjects and courses at post-primary and tertiary level education. Low uptake is partly a consequence of negative student attitudes towards STEM. Many students are losing interest in STEM at an early age due to an inability to see any relevance in their everyday lives. In light of such concerns, a Saturday afternoon club entitled “Trinity Walton Club” (TWC) was established in Trinity College Dublin with the purpose of uniting like-minded students to express, shape, inform and grow their interest in STEM. This club attempted to ‘bring STEM to life’ through thought provoking content, real world problems, contextualised analogies and projects. This paper describes the background to the TWC, reviews the literature around promoting student interest in STEM and examines initial feedback from participants in the pilot year of the club. The findings of this preliminary study indicate that the TWC has the potential to promote interest in STEM. Many of the recommendations from the literature review about promoting interest in STEM were referred to by participants in their responses

DMF-exfoliated graphene for electrochemical NADH detection

The electrochemical detection of NADH is of considerable interest because it is required as a cofactor in a large number of dehydrogenase-based biosensors. However, the presence of oxygenated functionalities on the electrode often causes fouling due to the adsorption of the oxidised form, NAD+. Here we report an electroanalytical NADH sensor based on DMF-exfoliated graphene . The latter is shown to have a very low oxygen content, facilitating the exceptionally stable and sensitive detection of this important analyte

Electrochemical ascorbic acid sensor based on DMF-exfoliated graphene

This paper describes the electron transfer properties of graphene nano-sheets (GNSs) immobilised on pyrolysed photoresist film (PPF) electrodes. The former are produced by the dispersion and exfoliation of graphite in dimethylformamide, and they are characterised using transmission electron microscopy, scanning electron microscopy and Raman spectroscopy. Cyclic voltammetry and electrochemical impedance spectroscopy are used to quantify the effect of the GNSs on electrochemical surface area and on electron transfer kinetics. Compelling evidence is reported in relation to the importance of edge-plane sites and defects in the promotion of electron transfer at carbon nanostructures. A novel ascorbic acid (vitamin C) sensor is presented based on the PPF/GNS system, which is effective in the range 0.4 to 6.0 mM, with a 0.12 mM detection limit. The selectivity of the sensor is demonstrated using a commercially available vitamin C supplement. This is the first report of the electrochemical properties of graphene nano-sheets produced using liquid-phase exfoliation, and it will serve as an important benchmark in the development of inexpensive graphene-based electrodes with high surface area and electro-catalytic activity