Graphene-based environmental barriers

Abstract

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.

Figure 1

Custom glass diffusion cell developed for measurement of Hg-vapor permeability through polymer and polymer/graphene sheet materials. A narrow gap (2mm) below the film and forced convection above are features needed to minimize gas-phase mass transport barriers and isolate the test film resistance.

Figure 2

Demonstration of graphene oxide as a hydraulic sealant. A. Pressure drop during forced filtration through nanochannel alumina filters (20 nm pore size) for graphene oxide compared to carbon black material) and carbon nanotubes (2D material), which form three-dimensional porous filter cakes; B. SEMs showing structure of deposited films or porous filter cakes; C. Concentration dependence of sealant effect for GO and Nylon filters. Label “blocking” refers to a control experiment in which the syringe was capped; D. Unification of panel C. data by renormalizing X-axis as the total mass of GO deposited (M = C^·V_filtration)

Figure 3

Measured Hg-vapor permeability coefficients (solid markers) for common commercial polymer sheet materials. Literature values for small gases in the same materials included for comparison

Figure 4

Effectiveness of thin graphene oxide films as Hg-vapor barrier enhancers on 50 μm polyester. The film configuration is seen to be much more effective than the common configuration with GO uniformly mixed (compounded) with the polymer (Nielsen model; Cussler model^–). The data lie between the Nielsen ideal imbedding limit and the limit for random overlapping disks whose centers of mass deposit on the film by a Poisson process. For the modeling, the atomic GO thickness was taken as 1 nm and the lateral dimension 1 μm. Image shows polyester film treated with GO film (right) and untreated (left).

Figure 5

Effect of drying conditions on the interlayer spacing and Hg permeability of GO films. The permeability of films dried at room temperature at 15 % relative humidity for 48 hours (left), and at approximately 70°C with zero relative humidity for 48 hours (right). The inserted X-ray diffraction (XRD) spectra show that these two drying conditions lead to interlayer spacings of 0.82 nm and 0.73 nm respectively. These GO films have an areal density of 35 mg/m² and approximately 25 layers.

Figure 6

Molecular dynamics (MD) simulations of Hg-atom diffusion in graphene interlayer spaces containing water. The diffusivity of a tracer Hg-atom is seen to decrease with decreasing interlayer spacing, and Hg becomes immobile below 7 Å spacing. Inset shows a graphene interlayer region with with 8 Å spacing, and one mercury atom (blue) in water (red-grey beads). The dashed line gives the known diffusivity of Hg in bulk water.

Figure 7

Results from the Nielsen model modified for excluded volume and using the Hg diffusivity values obtained by MD simulation in Fig. 6. The curves are predictions for 25 nm GO films with various values of GO aspect ratio, α. The triangles give the measured results from Fig. 5. These results support the hypothesis that the higher Hg permeability in wet films is due to the increase in interlayer spacing associated with water pillaring, which allows lateral diffusion of Hg atoms through the expanded and hydrated interlayer spaces. More complete drying reduces interlayer spacing and restricts lateral Hg diffusion, which leads to lower permeability.