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Monolayer nanoporous graphene represents an ideal membrane for molecular separations, but its practical realization is impeded by leakage through defects in the ultrathin graphene. Here, we report a multiscale leakage–sealing process that exploits the nonpolar nature and impermeability of pristine graphene to selectively block defects, resulting in a centimeter-scale membrane that can separate two fluid reservoirs by an atomically thin layer of graphene. After introducing subnanometer pores in graphene, the membrane exhibited rejection of multivalent ions and small molecules and water flux consistent with prior molecular dynamics simulations. The results indicate the feasibility of constructing defect-tolerant monolayer graphene membranes for nanofiltration, desalination, and other separation processes., Samsung (Firm) (Fellowship), United States. Dept. of Energy. Office of Basic Energy Sciences (Award number DE-SC0008059), King Fahd University of Petroleum and Minerals (Center for Clean Water and Clean Energy at MIT and KFUPM, project number R10-CW-09)

Thesis: S.M., Massachusetts Institute of Technology, Department of Mechanical Engineering, 2015., Cataloged from PDF version of thesis., Includes bibliographical references (pages 62-65)., Graphene, an atomically thin planar lattice of sp2 bonded carbon atoms with high strength and impermeability, has drawn attention as a promising next generation high flux separation membrane. Molecular dynamics simulations predicted graphene's potential for exhibiting both high flux and selectivity as water desalination and gas separation membranes. Measurement of diffusive transport of water and molecules demonstrated the feasibility of harnessing graphene as a nanofiltration membrane with high selectivity. However, experimental investigation on convective flow of water and ions/molecules in liquid phase across nanopores in graphene has been confined due to difficulties in fabricating large area defect-free graphene membranes and complexity in experiment design considerations for convectively driven flow. In this thesis, I present experimental methodologies to measure water and molecular transport driven by osmotic pressure gradient across large area graphene membranes. Measured water flux and salt/organic molecules selectivity consistent with predictions from molecular dynamics simulations and continuum models. Combined with multi-scale graphene defect sealing process, this work shows that forward osmosis presents a facile and reliable platform for measuring transport of water and filtration of ions/molecules across nanopores introduced to centimeter scale single-layer graphene membrane., by Doojoon Jang., S.M.