Effects of Moisture and Synthesis-Derived Contaminants on the Mechanical Properties of Graphene Oxide: A Molecular Dynamics Investigation

ReaxFF molecular dynamics of graphene oxide/NaCl aqueous solution interfaces

In this work, the interaction of NaCl aqueous solution with graphene (G), graphene oxide (GO), and graphite oxide (GTO) is studied using the ReaxFF module of Amsterdam Modeling Suite (AMS) software. We consider four models using the NaCl aqueous solution, containing a graphene sheet (G), a single sheet of GO with epoxide and hydroxyl groups on its surface, 4 layers of GO to model GTO, and a bulk NaCl solution as a reference. The structural and dynamical properties of G, GO, and GTO were quantified by analyzing the functional groups, radial distribution functions, density profiles and diffusivities of water and ions. Due to the reactive force field, the systems underwent spontaneous modification of surface functional groups during the first 750 ps after which the structure stabilizes (the energy stabilizes in less than 400 ps). Pristine graphene in contact with the NaCl solution formed hydroxyl groups on the edges, i.e., converted to partially reduced graphene oxide. The epoxy groups (Oe) on the initial GO were rather unstable, leading to a reduction of their number, however, there was an increase in the number of hydroxyl groups (Oh), mainly at the edges. The interactions of NaCl with the carbon-based sheets are rather weak, including GO and GTO which are decorated with numerous functional groups. Diffusion coefficients of water agree with the available data, but discrepancies in Na+ and Cl- diffusivity compared to other references underscore the need for further development in the dynamic parameters of the reactive force field used. In essence, our research provides specific data previously unreported, laying a foundation for advancing water desalination system design. The study's novelty lies in its realistic approach to graphene/graphene oxide modification, comprehensive characterization, and the application of the reactive force field to explore the graphene oxide-NaCl aqueous interface, contributing to the development of a practical membrane system for water desalination.

Origami Metamaterials Enable Low-Stress-Driven Giant Elastocaloric Effect

Elastocaloric materials, capable of achieving reversible thermal changes in response to a uniaxial stress, have attracted considerable attention for applications in advanced thermal management technologies, owing to their environmental friendliness and economic benefits. However, most elastocaloric materials operating on the basis of first/second-order phase transition often exhibit a limited caloric response, field hysteresis, and restricted working temperature ranges. This study resorts to origami engineering for realizing multifunctional metamaterials with exceptional elastocaloric effects at both nano (exemplified by computational simulations for graphene) and meso (demonstrated by experimentation on thermoplastic polyurethane elastomers) scales. The findings uncover that the graphene origami exhibits low-stress-driven reversible and giant elastocaloric effects without a hysteresis loss and with a high elastocaloric strength. These effects are achieved across a wide working temperature range (100-600 K) and are tailorable by fine-tuning the topological parameters and configurational status of the origami metamaterials. We demonstrate the scalability of the origami design strategy for magnifying the elastocaloric effect by the 3D printing of a mesoscale origami-inspired thermoplastic polyurethane metastructure that showcases enhanced elastocaloric performance at room temperature. This study presents the potential for the realization of architected elastocaloric materials through surface functionalization and origami engineering. The findings impart exciting prospects of elastocaloric origami metamaterials as the next generation of multiscale and sustainable thermal management technologies.