Elsevier

Carbon

Volume 173, March 2021, Pages 410-418
Carbon

Research Article
Shear failure in supported two-dimensional nanosheet van der Waals thin films

https://doi.org/10.1016/j.carbon.2020.10.079Get rights and content

Abstract

Liquid-phase deposition of exfoliated 2D nanosheets is the basis for emerging technologies that include writable electronic inks, molecular barriers, selective membranes, and protective coatings against fouling or corrosion. These nanosheet thin films have complex internal structures that are discontinuous assemblies of irregularly tiled micron-scale sheets held together by van der Waals (vdW) forces. On stiff substrates, nanosheet vdW films are stable to many common stresses, but can fail by internal delamination under shear stress associated with handling or abrasion. This “re-exfoliation” pathway is an intrinsic feature of stacked vdW films and can limit nanosheet-based technologies. Here we investigate the shear stability of graphene oxide and MoSe2 nanosheet vdW films through lap shear experiments on polymer-nanosheet-polymer laminates. These sandwich laminate structures fail in mixed cohesive and interfacial mode with critical shear forces from 40 to 140 kPa and fracture energies ranging from 0.2 to 6 J/m2. Surprisingly these energies are higher than delamination energies reported for smooth peeling of ordered stacks of continuous 2D sheets, which we propose is due to energy dissipation and chaotic crack motion during nanosheet film disassembly at the crack tip. Experiment results also show that film thickness plays a key role in determining critical shear force (maximum load before failure) and dissipated energy for different nanosheet vdW films. Using a mechanical model with an edge crack in the thin nanosheet film, we propose a shear-to-tensile failure mode transition to explain a maximum in critical shear force for graphene oxide films but not MoSe2 films. This transition reflects a weakening of the substrate confinement effect and increasing rotational deformation near the film edge as the film thickness increases. For graphene oxide, the critical shear force can be increased by electrostatic cross-linking achieved through interlayer incorporation of metal cations. These results have important implications for the stability of functional devices that employ 2D nanosheet coatings.

Introduction

Liquid phase deposition of two-dimensional (2D) nanosheets is used to create films, coatings, or papers with diverse applications that include molecular barriers [[1], [2], [3]], selective membranes [4], wearable sensors [5,6], electronic devices [5] and protective coatings with anticorrosive, antifouling or antibacterial function [6,12]. Nanosheet films are complex structures formed by random tiling and stacking of individual nanosheets of finite size, usually on the micron- or submicron-scale. The resulting films do not possess a continuous 2D lattice, but are van der Waals (vdW) assemblies held together by weak non-covalent forces both in the Z-direction and between adjacent nanosheets in the X–Y film plane.

Nanosheet films are often supported on substrates, most commonly on one-side (as a coating), but sometimes on two-sides in the form of imbedded “sandwich” laminates. In these cases the substrate(s) carry mechanical load and can provide sufficient device stability against most types of mechanical stress during use. One exception where the substrate cannot provide mechanical stability is shear stress, which may be introduced by rubbing or abrasion for one-sided coatings, or shear or peel loadings on laminate architectures. In these cases, 2D nanosheet films may fail internally by cleavage along vdW gaps in a manner analogous to the original synthesis of nanosheets by exfoliation of layered crystals. While much attention has been paid to the mechanical properties of 2D materials [7,8], this “re-exfoliation” pathway for shear failure of vdW nanosheet films has not been studied to our knowledge.

The limited relevant data in the literature suggest that stacked nanosheet films will be highly shear sensitive. Interlayer adhesion energies in 2D layered crystals are calculated to be of order 50–500 mJ/m2 [[9], [10], [11]]. Adhesion energies have been measured at 86 mJ/m2 for bilayer graphene [10] and 220 mJ/m2 for bilayer molybdenum disulfide [12]. These energies are much lower than those for polymer adhesives, which are engineered to create trans-interface chain entanglement and/or covalent bonding that increase adhesion energy [13,14]. Where shear failure limits nanosheet film technologies, it may be addressed by addition of polymer, metal or ceramic to form a composite, but some of the characteristic behaviors of the neat 2D films can be lost, especially those behaviors related to the well-defined interlayer regions of interest in nano-fluidics and intercalative energy storage. The interface between the nanosheet film and the substrate can be engineered to prevent interfacial/adhesive failure, but even so the internal cohesive failure of neat vdW films would remain as a fundamental limitation on overall strength.

This paper examines the shear stability and failure modes of supported nanosheet vdW films/coatings using graphene oxide (GO) and molybdenum diselenide (MoSe2) as model materials. Two-sided polymer-nanosheet-polymer laminates are used to create uniform shear loadings and lap shear experiments are adopted as the primary tool for mechanical characterization. The failure modes are identified and the critical shear forces and fracture energies determined as a function of film thickness, and compared to other types of coatings or adhesive films. A mechanical model of the fracture process was developed using finite-element methods. The model predicts a transition between a sliding failure mode and tensile-type failure mode as film thickness increases, and also explains the contrasting behavior of GO and MoSe2 films based on the large (GO) and small (MoSe2) lateral dimensions of the constituent nanosheets.

Section snippets

Film structures and lap shear experiments

Graphene oxide and MoSe2 nanosheets were synthesized to serve as model materials with large differences in lateral size and aspect ratio (Fig. 1). Lateral dimensions by SEM image analysis (Fig. 1b) are <0.4 μm for MoSe2 and 0.5–2 μm for GO. Lateral size (L) distributions by image analysis (see Fig. 1) give volume-weighted average lateral sizes (∑L2.d.L/∑L2.d = ∑L3/∑L2) of 1.0 μm (GO) and 0.36 μm (MoSe2) and mean aspect ratios of 1430 (GO), 550 (MoSe2). The significant difference in aspect ratio

Conclusions

This work shows that supported, neat (matrix free) nanosheet films are sensitive to shear forces and can fail by either delamination from the substrate or internal delamination. The latter failure mode is a type of “mechanical re-exfoliation” with similarities to the original synthesis of the constituent nanosheets from bulk layered crystals, and is an intrinsic limitation of these vdW assemblies. Mechanical modelling predicts the nanosheet films fail internally by either a sliding or

Methods

Materials. Materials. Iron Chloride, Aluminum Nitrate and Sodium Tetraborate Decahydrate were purchased from Sigma-Aldrich. Laboratory nitrile gloves were purchased from Kimberly-Clark Professional. Clear polyester substrates were purchased from Coveme. All water was deionized (18.2 MΩ, mill-Q pore). All reagents were used as received without further purification.

Nanosheet synthesis and film formation. GO suspensions were prepared by a modified Hummer’s method as described previously [Yang

Author statement

C.J.C and Y.X. designed and performed the experiments, interpreted and analyzed the data. R.H.H. C.J.C and Y.X. wrote the paper. D.L and H.G. developed the mechanical modeling and wrote the paper.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

We acknowledge financial support from National Institute of Environmental Health Sciences Superfund Research Program P42 ES013660 through a project on barrier films for personal protective equipment, and a seed grant from the Institute at Brown for Environment and Society (IBES) on protective barrier technologies. The modeling part of the work was also supported by a graduate fellowship to D.L. from the China Scholarship Council (CSC).

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