Stop Motion Animation Reveals Formation Mechanism of Hierarchical Structure in Graphene Oxide Papers

Abstract

≈500 nm thick)-that are believed to lend GO papers their extraordinary ductility and mechanical toughness. [ 8 ] Lamellae consist of hundreds of highly aligned nanosheets, whereas superlamellae consist of tens of aligned lamellae. From the inspection of swollen GO papers lamellae are found to be organized into a highly interconnected, branched, lattice-like structure.In a previous study, we suggested that the formation mechanism of GO papers formed by vacuum-assisted fi ltration progresses in two stages whereby a semiordered aggregate (SOA) of loosely packed nanosheets forms and grows due to the fl ow of water through the fi ltration membrane. This is followed by compaction of the SOA, induced by the lowering air/ liquid interface into the fi nal free-standing paper with a layered, hierarchical structure containing preferentially aligned nanosheets and lamellae (groups of tens of nanosheets). [ 9 ] In a subsequent study, we discovered an additional length scale in the hierarchical structure, whereby groups of lamellae form superlamellae. [ 7a ] However, the evidence to support the two-stage paper-formation mechanism was indirect and did not provide any mechanism for the evolution of lamellae or superlamellae. In this study, we provide direct experimental describing the formation mechanism of GO papers as well as the origin of the hierarchical structure of GO papers.Taking inspiration from a process used for the fabrication of GO sponges, foams, and other structures, [ 10 ] we present a novel method that preserves the paper formation process in sequential snapshots akin to stop-motion animation. Several different methods of fl ash freezing (see Experimental Section and Note S2, Supporting Information) were implemented to rapidly arrest the formation of the paper at various stages. The partially formed material is subsequently cleaved from the resulting ice puck, maintained at a low temperature and then lyophilized to remove water without surface tension driven aggregation or collapse. Samples are then imaged by SEM to observe the interconnected GO nanosheet network. It is important to note that by controlling both the direction and rate of ice crystal formation prior to lyophilization, the structure of porous materials can be modifi ed, a process known as ice templating. To reduce the infl uence of ice templating the samples were frozen as rapidly as possible. In addition, the direction of ice crystal formation with respect to the direction of fi ltration was varied (Note S2, Supporting Information). In each of the cases, the structure obtained in the vicinity of the fi lter membrane was consistent, implying that the overall structure of the nanosheet network is preserved by this method.In order to form a time-elapsed picture of the formation mechanism for GO papers, several samples were taken atThe need for materials with favorable properties that can be used across many scientifi c and technological disciplines has motivated a burgeoning interest in graphene and its derivatives over the last decade. In this p...

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