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Abstract
Two-dimensional (2D) materials can be produced using ball milling with the help of liquid surfactants or solid exfoliation agents, as ball milling of bulk precursor materials usually produces nanosized particles because of high-energy impacts. Post-milling treatment is thus needed to purify the nanosheets. We show here that nanosheets of graphene, BN, and MoS2 can be produced by ball milling of their bulk crystals in the presence of ammonia or a hydrocarbon ethylene gas and the obtained nanosheets remain flat and maintain their single-crystalline structure with low defects density even after a long period of time; post-milling treatment is not needed. This study does not just demonstrate production of nanosheets using ball milling, but reveals surprising indestructible behaviour of 2D nanomaterials in ammonia or hydrocarbon gas under the high-energy impacts; in other milling atmospheres such as air, nitrogen or argon the same milling treatment produces nanosized particles. A systematic study reveals chemisorption of ammonia and hydrocarbon gases and chemical reactions occurring at defect sites, which heal the defects by saturating the dangling bonds. Density functional theory was used to understand the mechanism of mechanochemical reactions. Ball milling in ammonia or hydrocarbon is promising for mass-production of pure nanosheets.
Highlights
Physics and Mechanical Engineering, Queensland University of Technology, Brisbane, QLD 4001, Australia. 3A
The intensity of the diffraction peaks decreases as milling time increases and the peaks broaden as the result of graphite exfoliation, the X-ray diffraction (XRD) patterns clearly suggest that NH3 gas slows down or prevents disordering of graphite structure under high-energy impacts
Nanosheets of graphene, BN, and MoS2 were protected by NH3, C2H4 and CH4 gases under high-energy ball milling, while amorphous or highly disordered nanoparticles were produced in Ar, N2, and O2 under the same milling conditions. 2D nanomaterials become indestructible under high-energy impacts in certain gases due to high absorption of the ammonia and hydrocarbon gases and the mechanochemical reaction of reactive gases with dangling bonds formed during milling and chemisorption of reactive species, terminating bonds and preventing the cross-linking of layers due to the formation of bridging bonds
Summary
Under violent ball impacts (Figure S1), materials normally suffer from severe fracturing and plastic deformation until complete loss of the original crystalline structure[9]. The XRD, SEM and TEM results confirm that the high-energy ball milling in NH3 gas exfoliates graphite particles into thin nanosheets without destroying their in-plane structure. The stronger peaks of pyrrolic nitrogen/amine and graphitic nitrogen suggest that NH3 molecules are decomposed, producing amine and nitrogen groups on carbon The influence of this functionalization on mechanical strength of nanosheets was investigated theoretically using Density Functional Theory (DFT) on a graphene sheet in different gases (N2 and NH3). SEM and TEM analyses found that most nanosheets remain flat and don’t have severe plastic deformation (folding or twist) or cross links between layers These gas molecules are chemisorbed on defects and edges, saturating dangling bonds and preventing cross-linking of graphene or BN layers[12] and further damage. The nanosheets can be used as solid lubricants, additive to polymers, battery electrodes and many other applications where large amounts of multilayer 2D flakes or nanosheets are needed
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