Wrinkling and Periodic Folding of Graphene Oxide Monolayers by Langmuir-Blodgett Compression.

Crumples, wrinkles, and other three-dimensional topographical features in graphene oxide (GO) have been of recent interest as these features have improved material performance for a variety of applications. However, wrinkling of monolayer GO films has yet to be reported. Herein, we demonstrate wrinkling and folding of monolayer GO using the Langmuir-Blodgett technique for the first time. First, cetyltrimethylammonium bromide (CTAB) and GO are deposited on the air-water interface and uniaxially compressed to form a monolayer. CTAB enhances in-plane rigidity of the monolayer through hydrophobic tail aggregation, preventing GO-GO in-plane sliding behavior. Overcompression of the GO monolayer results in the out-of-plane periodic nanoscale wrinkling and in turn generates folds that are stable during deposition onto a substrate and GO chemical reduction. Furthermore, we investigate one potential application of this material by constructing a 3D electrode of the stacked nanofolded GO-CTAB layers that exhibits superior volumetric capacitance compared to commercial devices and comparable volumetric capacitance compared to high-performing recently reported devices. The high volumetric capacitance is attributed to the electrolyte-accessible channels generated by the nanofolds which are similar in size to the hydrated ions.

Controlling the Roughness of Langmuir-Blodgett Monolayers.

Controlling the surface roughness of thin films with nanoscale precision is of significant interest for the rational design of surface coatings. Although wrinkling and buckling of Langmuir monolayers under compression has been demonstrated for several years, there is currently no method to precisely control this behavior during compression and thereby modify the surface roughness of deposited films. Here, we combine conventional Langmuir phase analysis with a novel dynamic viscoelasticity measurement to simply and accurately observe the jamming transition of monolayers of silica spheres, graphene oxide, and surfactant. By overcompressing beyond this point, the surface roughness of the deposited monolayer can be precisely controlled. This technique could be used to tune the surface properties of a variety of materials from lipids to nanoparticles.

Fabrication and morphology tuning of graphene oxide nanoscrolls.

Here we report the synthesis of graphene oxide nanoscrolls (GONS) with tunable dimensions via low and high frequency ultrasound solution processing techniques. GONS can be visualized as a graphene oxide (GO) sheet rolled into a spiral-wound structure and represent an alternative to traditional carbon nano-morphologies. The scrolling process is initiated by the ultrasound treatment which provides the scrolling activation energy for the formation of GONS. The GO and GONS dimensions are observed to be a function of ultrasound frequency, power density, and irradiation time. Ultrasonication increases GO and GONS C-C bonding likely due to in situ thermal reduction at the cavitating bubble-water interface. The GO area and GONS length are governed by two mechanisms; rapid oxygen defect site cleavage and slow cavitation mediated scission. Structural characterization indicates that GONS with tube and cone geometries can be formed with both narrow and wide dimensions in an industrial-scale time window. This work paves the way for GONS implementation for a variety of applications such as adsorptive and capacitive processes.

Semiquantitative Performance and Mechanism Evaluation of Carbon Nanomaterials as Cathode Coatings for Microbial Fouling Reduction.

In this study, we examine bacterial attachment and survival on a titanium (Ti) cathode coated with various carbon nanomaterials (CNM): pristine carbon nanotubes (CNT), oxidized carbon nanotubes (O-CNT), oxidized-annealed carbon nanotubes (OA-CNT), carbon black (CB), and reduced graphene oxide (rGO). The carbon nanomaterials were dispersed in an isopropyl alcohol-Nafion solution and were then used to dip-coat a Ti substrate. Pseudomonas fluorescens was selected as the representative bacterium for environmental biofouling. Experiments in the absence of an electric potential indicate that increased nanoscale surface roughness and decreased hydrophobicity of the CNM coating decreased bacterial adhesion. The loss of bacterial viability on the noncharged CNM coatings ranged from 22% for CB to 67% for OA-CNT and was dependent on the CNM dimensions and surface chemistry. For electrochemical experiments, the total density and percentage of inactivation of the adherent bacteria were analyzed semiquantitatively as functions of electrode potential, current density, and hydrogen peroxide generation. Electrode potential and hydrogen peroxide generation were the dominant factors with regard to short-term (3-h) bacterial attachment and inactivation, respectively. Extended-time electrochemical experiments (12 h) indicated that in all cases, the density of total deposited bacteria increased almost linearly with time and that the rate of bacterial adhesion was decreased 8- to 10-fold when an electric potential was applied. In summary, this study provides a fundamental rationale for the selection of CNM as cathode coatings and electric potential to reduce microbial fouling.

Controlling self-assembly of reduced graphene oxide at the air-water interface: quantitative evidence for long-range attractive and many-body interactions.

Industrial-scale applications of two-dimensional materials are currently limited due to lack of a cost-effective and controlled synthesis method for large-area monolayer films. Self-assembly at fluid interfaces is one promising method. Here, we present a quantitative analysis of the forces governing reduced graphene oxide (rGO) assembly at the air-water interface using two unique approaches: area-based radial distribution functions and a theoretical Derjaguin-Landau-Verwey-Overbeek (DLVO) interaction potential for disks interacting edge-to-edge. rGO aggregates at the air-water interface when the subphase ionic strength results in a Debye screening length equal to the rGO thickness (∼1 mM NaCl), which is consistent with the DLVO interaction potential. At lower ionic strengths, area-based radial distribution functions indicate that rGO-rGO interactions at the air-water interface are dominated by long-range (tens of microns) attractive and many-body repulsive forces. The attractive forces are electrostatic in nature; that is, the force is weakened by minor increases in ionic strength. A quantitative understanding of rGO-rGO interactions at the air-water interface may allow for rational synthesis of large-area atomically thin films that have potential for planar electronics and membranes.

Graphene-based environmental barriers.

Many environmental technologies rely on containment by engineered barriers that inhibit the release or transport of toxicants. Graphene is a new, atomically thin, two-dimensional sheet material, whose aspect ratio, chemical resistance, flexibility, and impermeability make it a promising candidate for inclusion in a next generation of engineered barriers. Here we show that ultrathin graphene oxide (GO) films can serve as effective barriers for both liquid and vapor permeants. First, GO deposition on porous substrates is shown to block convective flow at much lower mass loadings than other carbon nanomaterials, and can achieve hydraulic conductivities of 5 × 10(-12) cm/s or lower. Second we show that ultrathin GO films of only 20-nm thickness coated on polyethylene films reduce their vapor permeability by 90% using elemental mercury as a model vapor toxicant. The barrier performance of GO in this thin-film configuration is much better than the Nielsen model limit, which describes ideal behavior of flake-like fillers uniformly imbedded in a polymer. The Hg barrier performance of GO films is found to be sensitive to residual water in the films, which is consistent with molecular dynamics (MD) simulations that show lateral diffusion of Hg atoms in graphene interlayer spaces that have been expanded by hydration.