Spontaneous clustering of exfoliated two-dimensional materials at the air-water interface

[Display omitted] •Two-dimensional nanomaterial clusters can form spontaneously at the air–liquid interface.•Clusters transition from high to low density as dispersion concentration is reduced.•Universal concentration effect is applicable to nanosheets regardless of hydrophobicity and morphology.•Cluster structure largely impacts the quality of the densified Langmuir films.•Two-stage clustering mechanism is used to explain the clustering behavior. Coating approaches which trap nanoparticles at an interface have become popular for depositing single-layer films from nanoparticle dispersions. Past efforts concluded that concentration and aspect ratio dominate the impact on aggregation state of nanospheres and nanorods at an interface. Although few works have explored the clustering behaviour of atomically thin, two-dimensional materials, we hypothesize that nanosheet concentration is the dominant factor leading to a particular cluster structure and that this local structure impacts the quality of densified Langmuir films. We systematically studied cluster structures and Langmuir film morphologies of three different nanosheets, namely chemically exfoliated molybdenum disulfide, graphene oxide and reduced graphene oxide. We observe cluster structure transitions from island-like domains to more linear networks in all materials as dispersion concentration is reduced. Despite differences in material properties and morphologies, we obtained the same overall correlation between sheet number density (A/V) in the spreading dispersion and cluster fractal structure (df) is observed, with reduced graphene oxide sheets showing a slight delay upon transitioning into a lower-density cluster. Regardless of assembly method, we found that cluster structure impacts the attainable density of transferred Langmuir films. A two-stage clustering mechanism is supported by by considering the spreading profile of solvents and an analysis of interparticle forces at the air–water interface.