Sehmus Ozden

Covalently Interconnected Three- Dimensional Graphene Oxide Solids

The creation of three-dimensionally engineered nanoporous architectures via covalently interconnected nanoscale building blocks remains one of the fundamental challenges in nanotechnology. Here we report the synthesis of ordered, stacked macroscopic three-dimensional (3D) solid scaffolds of graphene oxide (GO) fabricated via chemical cross-linking of two-dimensional GO building blocks. The resulting 3D GO network solids form highly porous interconnected structures, and the controlled reduction of these structures leads to formation of 3D conductive graphene scaffolds. These 3D architectures show promise for potential applications such as gas storage; CO2 gas adsorption measurements carried out under ambient conditions show high sorption capacity, demonstrating the possibility of creating new functional carbon solids starting with two-dimensional carbon layers.

Low-density three-dimensional foam using self-reinforced hybrid two-dimensional atomic layers

Low-density nanostructured foams are often limited in applications due to their low mechanical and thermal stabilities. Here we report an approach of building the structural units of three-dimensional (3D) foams using hybrid two-dimensional (2D) atomic layers made of stacked graphene oxide layers reinforced with conformal hexagonal boron nitride (h-BN) platelets. The ultra-low density (1/400 times density of graphite) 3D porous structures are scalably synthesized using solution processing method. A layered 3D foam structure forms due to presence of h-BN and significant improvements in the mechanical properties are observed for the hybrid foam structures, over a range of temperatures, compared with pristine graphene oxide or reduced graphene oxide foams. It is found that domains of h-BN layers on the graphene oxide framework help to reinforce the 2D structural units, providing the observed improvement in mechanical integrity of the 3D foam structure.

CoMoO4 Nanoparticles Anchored on Reduced Graphene Oxide Nanocomposites as Anodes for Long-Life Lithium-Ion Batteries

A self-assembled CoMoO4 nanoparticles/reduced graphene oxide (CoMoO4NP/rGO), was prepared by a hydrothermal method to grow 3-5 nm sized CoMoO4 particles on reduced graphene oxide sheets and used as an anode material for lithium-ion batteries. The specific capacity of CoMoO4NP/rGO anode can reach up to 920 mAh g(-1) at a current rate of 74 mA g(-1) in the voltage range between 3.0 and 0.001 V, which is close to the theoretical capacity of CoMoO4 (980 mAh g(-1)). The fabricated half cells also show good rate capability and impressive cycling stability with 8.7% capacity loss after 600 cycles under a high current density of 740 mA g(-1). The superior electrochemical performance of the synthesized CoMoO4NP/rGO is attributed to the synergetic chemical coupling effects between the conductive graphene networks and the high lithium-ion storage capability of CoMoO4 nanoparticles.

Field Emission with Ultralow Turn On Voltage from Metal Decorated Carbon Nanotubes

A simple and scalable method of decorating 3D-carbon nanotube (CNT) forest with metal particles has been developed. The results observed in aluminum (Al) decorated CNTs and copper (Cu) decorated CNTs on silicon (Si) and Inconel are compared with undecorated samples. A significant improvement in the field emission characteristics of the cold cathode was observed with ultralow turn on voltage (Eto ∼ 0.1 V/μm) due to decoration of CNTs with metal nanoparticles. Contact resistance between the CNTs and the substrate has also been reduced to a large extent, allowing us to get stable emission for longer duration without any current degradation, thereby providing a possibility of their use in vacuum microelectronic devices.

Enhanced field emission properties from CNT arrays synthesized on Inconel superalloy.

One of the most promising materials for fabricating cold cathodes for next generation high-performance flat panel devices is carbon nanotubes (CNTs). For this purpose, CNTs grown on metallic substrates are used to minimize contact resistance. In this report, we compare properties and field emission performance of CNTs grown via water assisted chemical vapor deposition using Inconel vs silicon (Si) substrates. Carbon nanotube forests grown on Inconel substrates are superior to the ones grown on silicon; low turn-on fields (∼1.5 V/μm), high current operation (∼100 mA/cm(2)) and very high local field amplification factors (up to ∼7300) were demonstrated, and these parameters are most beneficial for use in vacuum microelectronic applications.

Unzipping Carbon Nanotubes at High Impact

The way nanostructures behave and mechanically respond to high impact collision is a topic of intrigue. For anisotropic nanostructures, such as carbon nanotubes, this response will be complicated based on the impact geometry. Here we report the result of hypervelocity impact of nanotubes against solid targets and show that impact produces a large number of defects in the nanotubes, as well as rapid atom evaporation, leading to their unzipping along the nanotube axis. Fully atomistic reactive molecular dynamics simulations are used to gain further insights of the pathways and deformation and fracture mechanisms of nanotubes under high energy mechanical impact. Carbon nanotubes have been unzipped into graphene nanoribbons before using chemical treatments but here the instability of nanotubes against defect formation, fracture, and unzipping is revealed purely through mechanical impact.

Density Variant Carbon Nanotube Interconnected Solids

DOI: 10.1002/adma.201404995 from the individual dimensions of the CNTs, the interconnection also affects the density. Detailed microscopy and spectroscopy studies have been performed to explain the structure, properties, and other functional properties. In order to understand the structure, the top, bottom, and lateral sides of the 3D nanotube blocks were imaged using scanning electron microscopy (SEM). Figure 1 b,c shows the top and side view of the 3D-CNT solid blocks. SEM images show uniformly fl at top surfaces as well as the vertical alignment of the nanotubes. The top surface reveals the aligned CNTs are nicely spaced and form a microporous structure giving rise to the solid block. The side view shows aligned millimeter length CNTs exhibiting both buckling and bending due to their large height and network structure. In order to understand the in-depth morphology of the blocks, we sectioned the block at different planes and imaged using SEM as shown in Figure 2 . The 3D architectures entirely consist of entangled CNTs with different orientations producing spatially varying morphologies (Figure 2 a,e). The morphology for the intermolecular junctions reveals junctions of Y-type, X-type, multibranched, and ring-like confi gurations (Figure 2 b,c,d,f, respectively). Highly branched 3D-CNT network junctions are shown in Figure 2 d. Recent reports have shown that these kinds of multiterminal junctions are formed due to the presence of topological defects in the hexagonal carbon lattice within the junction region. [ 2,16,18 ] Solid blocks also contain solder-like junctions between CNT bundles (Figure 2 g,h). The nanoscale and solder-like junctions could be expected to improve mechanical and physical properties of the material. [ 2,19–21 ]

3D Macroporous Solids from Chemically Cross‐linked Carbon Nanotubes

Suzuki reaction for covalently interconnected 3D carbon nanotube (CNT) architectures is reported. The synthesis of 3D macroscopic solids made of CNTs covalently connected via Suzuki cross-coupling, a well-known carbon-carbon covalent bond forming reaction in organic chemistry, is scalable. The resulting solid has a highly porous, interconnected structure of chemically cross-linked CNTs. Its use for the removal of oil from contaminated water is demonstrated.

Strain Rate Dependent Shear Plasticity in Graphite Oxide

Graphene oxide film is made of stacked graphene layers with chemical functionalities, and we report that plasticity in the film can be engineered by strain rate tuning. The deformation behavior and plasticity of such functionalized layered systems is dominated by shear slip between individual layers and interaction between functional groups. Stress-strain behavior and theoretical models suggest that the deformation is strongly strain rate dependent and undergoes brittle to ductile transition with decreasing strain rate.

Ambient solid-state mechano-chemical reactions between functionalized carbon nanotubes

Carbon nanotubes can be chemically modified by attaching various functionalities to their surfaces, although harsh chemical treatments can lead to their break-up into graphene nanostructures. On the other hand, direct coupling between functionalities bound on individual nanotubes could lead to, as yet unexplored, spontaneous chemical reactions. Here we report an ambient mechano-chemical reaction between two varieties of nanotubes, carrying predominantly carboxyl and hydroxyl functionalities, respectively, facilitated by simple mechanical grinding of the reactants. The purely solid-state reaction between the chemically differentiated nanotube species produces condensation products and unzipping of nanotubes due to local energy release, as confirmed by spectroscopic measurements, thermal analysis and molecular dynamic simulations.