Jaime C. Grunlan

Two-Dimensional Nanosheets Produced by Liquid Exfoliation of Layered Materials

Layered transition metal dichalcogenides, such as tungsten disulfide, are exfoliated into atomically thin flakes. If they could be easily exfoliated, layered materials would become a diverse source of two-dimensional crystals whose properties would be useful in applications ranging from electronics to energy storage. We show that layered compounds such as MoS2, WS2, MoSe2, MoTe2, TaSe2, NbSe2, NiTe2, BN, and Bi2Te3 can be efficiently dispersed in common solvents and can be deposited as individual flakes or formed into films. Electron microscopy strongly suggests that the material is exfoliated into individual layers. By blending this material with suspensions of other nanomaterials or polymer solutions, we can prepare hybrid dispersions or composites, which can be cast into films. We show that WS2 and MoS2 effectively reinforce polymers, whereas WS2/carbon nanotube hybrid films have high conductivity, leading to promising thermoelectric properties.

Large-scale exfoliation of inorganic layered compounds in aqueous surfactant solutions.

Ronan J. Smith , Paul J. King , Mustafa Lotya , Christian Wirtz , Umar Khan , Sukanta De , Arlene O’Neill , Georg S. Duesberg , Jaime C. Grunlan , Gregory Moriarty , Jun Chen , Jiazhao Wang , Andrew I. Minett , Valeria Nicolosi , and Jonathan N. Coleman *

Improved Thermoelectric Behavior of Nanotube-Filled Polymer Composites with Poly(3,4-ethylenedioxythiophene) Poly(styrenesulfonate)

The thermoelectric properties of carbon nanotube (CNT)-filled polymer composites can be enhanced by modifying junctions between CNTs using poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS), yielding high electrical conductivities (up to approximately 40000 S/m) without significantly altering thermopower (or Seebeck coefficient). This is because PEDOT:PSS particles are decorated on the surface of CNTs, electrically connecting junctions between CNTs. On the other hand, thermal transport remains comparable to typical polymeric materials due to the dissimilar bonding and vibrational spectra between CNT and PEDOT:PSS. This behavior is very different from that of typical semiconductors whose thermoelectric properties are strongly correlated. The decoupled thermoelectric properties, which is ideal for developing better thermoelectric materials, are believed to be due to thermally disconnected and electrically connected contact junctions between CNTs. Carrier transport at the junction is found to be strongly dependent on the type and concentration of stabilizers. The crucial role of stabilizers was revealed by characterizing transport characteristics of composites synthesized by electrically conducting PEDOT:PSS and insulating gum Arabic (GA) with 1:1-1:4 weight ratios of CNT to stabilizers. The influence of composite synthesis temperature and CNT-type and concentration on thermoelectric properties has also been studied. Single-walled (SW) CNT-filled composites dried at room temperature followed by 80 degrees C exhibited the best thermoelectric performance in this study. The highest thermoelectric figure of merit (ZT) in this study is estimated to be approximately 0.02 at room temperature, which is at least one order of magnitude higher than most polymers and higher than that of bulk Si. Further studies with various polymers and nanoparticles with high thermoelectric performance may result in economical, lightweight, and efficient polymer thermoelectric materials.

Thermoelectric Behavior of Segregated-Network Polymer Nanocomposites

Segregated-network carbon nanotube (CNT)-polymer composites were prepared, and their thermoelectric properties were measured as a function of CNT concentration at room temperature. This study shows that electrical conductivity can be dramatically increased by creating a network of CNTs in the composite, while the thermal conductivity and thermopower remain relatively insensitive to the filler concentration. This behavior results from thermally disconnected, but electrically connected, junctions in the nanotube network, which makes it feasible to tune the properties in favor of a higher thermoelectric figure of merit. With a CNT concentration of 20 wt %, these composites exhibit an electrical conductivity of 4800 S/m, thermal conductivity of 0.34 W/m x K and a thermoelectric figure of merit (ZT) greater than 0.006 at room temperature. This study suggests that polymeric thermoelectrics are possible and provides the basis for further development of lightweight, low-cost, and nontoxic polymer composites for thermoelectric applications in the future.

Super Gas Barrier and Selectivity of Graphene Oxide‐Polymer Multilayer Thin Films

Super gas barrier thin films, fabricated with layer-by-layer assembly of polyethylenimine and graphene oxide, exhibit significantly reduced oxygen and carbon dioxide transmission rates. This thin films nanobrick wall structure also provides high gas selectivity for hydrogen.

Flame retardant behavior of polyelectrolyte-clay thin film assemblies on cotton fabric.

Cotton fabric was treated with flame-retardant coatings composed of branched polyethylenimine (BPEI) and sodium montmorillonite (MMT) clay, prepared via layer-by-layer (LbL) assembly. Four coating recipes were created by exposing fabric to aqueous solutions of BPEI (pH 7 or 10) and MMT (0.2 or 1 wt %). BPEI pH 10 produces the thickest films, while 1 wt % MMT gives the highest clay loading. Each coating recipe was evaluated at 5 and 20 bilayers. Thermogravimetric analysis showed that coated fabrics left as much as 13% char after heating to 500 degrees C, nearly 2 orders of magnitude more than uncoated fabric, with less than 4 wt % coming from the coating itself. These coatings also reduced afterglow time in vertical flame tests. Postburn residues of coated fabrics were examined with SEM and revealed that the weave structure and fiber shape in all coated fabrics were preserved. The BPEI pH 7/1 wt % MMT recipe was most effective. Microcombustion calorimeter testing showed that all coated fabrics reduced the total heat release and heat release capacity of the fabric. Fiber count and strength of uncoated and coated fabric are similar. These results demonstrate that LbL assembly is a relatively simple method for imparting flame-retardant behavior to cotton fabric. This work lays the foundation for using these types of thin film assemblies to make a variety of complex substrates (foam, fabrics, etc.) flame resistant.

Light-Weight Flexible Carbon Nanotube Based Organic Composites with Large Thermoelectric Power Factors

Typical organic materials have low thermal conductivities that are best suited to thermoelectrics, but their poor electrical properties with strong adverse correlations have prevented them from being feasible candidates. Our composites, containing single-wall carbon nanotubes, poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) and/or polyvinyl acetate, show thermopowers weakly correlated with electrical conductivities, resulting in large thermoelectric power factors in the in-plane direction of the composites, ∼160 μW/m·K(2) at room temperature, which are orders of magnitude larger than those of typical polymer composites. Furthermore, their high electrical conductivities, ∼10(5) S/m at room temperature, make our composites very promising for various electronic applications. The optimum nanotube concentrations for better power factors were identified to be 60 wt % with 40 wt % polymers. It was noticed that high nanotube concentrations above 60 wt % decreased the electrical conductivity of the composites due to less effective nanotube dispersions. The thermal conductivities of our 60 wt % nanotube composites in the out-of-plane direction were measured to be 0.2-0.4 W/m·K at room temperature. The in-plane thermal conductivity and thermal contact conductance between nanotubes were also theoretically estimated.

Interpretations of Indentation Size Effects

For very shallow indentations in W, Al, Au, and Fe-3wt%Si single crystals, hardness decreased with increasing depth irrespective of increasing or decreasing strain gradients. As such, strain gradient theory appears insufficient to explain the indentation size effect (ISE) at depths less than several hundred nanometers. Present research links the ISE to a ratio between the energy of newly created surface and plastic strain energy dissipation. Also, the contact surface to plastic volume ratio was nearly constant for a range of shallow depths. Based on the above, an analytical model of hardness versus depth provides a satisfactory fit to the experimental data and correlates well with embedded atom simulations. @DOI: 10.1115/1.1469004#

Intumescent All‐Polymer Multilayer Nanocoating Capable of Extinguishing Flame on Fabric

According to the National Fire Protection Association (NFPA), there were an estimated 1.3 million fi res in the United States in 2009, which resulted in 3010 civilian deaths (one every 175 minutes), 17 050 injuries (one every 31 minutes), [ 1 ] and direct property loss estimated at

Clay–Chitosan Nanobrick Walls: Completely Renewable Gas Barrier and Flame-Retardant Nanocoatings

12.5 billion. There were more than 40 000 deaths worldwide from fi re in 2006 and it cost every country an average of 1% of their gross domestic product in property loss, medical services for burn victims, etc. [ 2 ] Firerelated issues continue to drive the development of materials that can reduce fi re risk to save lives and protect property, but any fl ame retardants used to reduce that fi re risk have to meet various safety standards to reduce the deleterious effect on the environment or human health. Textiles in particular require effective anti-fl ammable performance combined with minimal enviornmental impact because they are often washed and fl ame retardant additives can leach out of the fabric and into the environment. [ 3 , 4 ] There are numerous strategies used to make textile fi bers fl ame retardant: surface treatment, fi re-retardant additives or co-monomers in synthetic fi bers, nanocomposite technology, heat-resistant and inherently fi re-retardant fi bers, and fi ber blending. [ 5 ] More recently, layer-by-layer (LbL) assembly has been used as a surface treatment to impart fl ame resistance to cotton fabric by coating each individual fi ber with a claypolymer nanobrick wall. [ 6 ]