Graphene-based 2-dimensional Nanocapillaries And Their Transportation Properties

Graphene has attracted enormous interests since first discovered in 2004,and leads to intense research on other 2-dimensional materials,including hexagonal boron nitride?h BN?and transition metal dichalcogenides?TMDCs?.It is just in recent years,our quest to help make seawater drinkable leads to rapid development of membrane science.As a derivative of graphene,graphene oxide?GO?has been widely investigated because it can be easily prepared and readily dispersed,also because of its low cost.GO membranes can be prepared through facile vacuum filtration method,showing better filtration and separation properties than the state-of-the-art commercial polymer membranes and better chemical stability.However,it's been reported that submicrometer-thick GO membranes prohibit the permeance of organic solvents,and the mechanism remains unclear.On the other hand,channels fabricated using standard lithography techniques and conventional materials are limited in size by intrinsic roughness of materials'surfaces,which hinders the precise measurements at the atomic-size and makes it difficult to disentangle various mechanisms that contributes to ion transport though them.In this work,we construct graphene-based 2D nanochannels with different architectures for fluidic transportation studies,and clarify the transport mechanisms of fluid or ions inside these channels by direct experiments.?1?Large-sized graphene oxide suspension is prepared using modified Hummers method,along with proper multi-level ultrasonication and centrifugation processes.The as-prepared GO flakes are of large size?²0?m?and narrow size distribution.Ultrathin GO membranes with highly laminated structures are then prepared using vacuum filtration method.It is shown that 8 nm highly laminated graphene oxide?HLGO?membranes could reject ions with hydrated radius over 4.5?,exhibiting precise molecular sieving properties.Meanwhile,this HLGO membrane allows ultrafast permeation of water and a variety of organic solvents.And the fluid permeances display linear denpendence with the inverse viscosities of the solvents,indicating a viscous flow inside the GO membrane.Further exploration demonstrates that HLGO membranes on different substrates are capable of rejecting dye molecules,of which the molecular weight ranging from 249 to 1017 g/mol,dissolved in organic solvents with high permeance,revealing its potential application in organic solvent nanofiltration?OSN?.?2?Organic solvents intercalation experiments exclude transportation through GO interlayer as a main transportation route for solvent to pass through.Together with the thickness-dependence permeances experiments,two different pathways are proposed for molecular transport.At lower thickness,water and organic solvents could rapidly reach the pinholes on the membrane surfaces,but the transportation is limited by how the molecules transport through one pinehole to the next,which is a probability-based event.Thus the membrane permeance decays exponentially with thickness increasing.But water has a second pathway-the graphene nanocapillary network,which allows the transport and thus a linear dependence with membrane thickness is observed.The proposed mechanism is further examined and proved by pervaporation experiments and solvents parameters analysis.Furthermore,partially reduced Mg²⁺crosslinked membranes with better performances are presented based on the proposed mechanism.?3?Graphene-based nanocapillaries with atomically smooth surfaces and angstrom-size precision are fabricated using nanofabrication technique;it is assembled by van der Waals atttractions between 2D crystals.On this basis,transport behaviors of ions with different sizes are explored in angstrom-size channels with smooth walls.The effects of channel amounts and surface charge density on ion transport properties are illustrated.It is found that ions with hydrated diameter larger than the slits height can still permeate through,by partially shred or flatten their hydration shells like a soft ball,albeit with reduced mobility.But no detectable permeance of the ions with diameter after dehydration still larger than channel height is observed,that is,channels show rejection to such ions.?4?Capillary condensation is observed under atomic-size confinement,which is comparable to the size of water molecules.Weakly hydrophobic graphene and hydrophilic mica nanoslits are constructed to study capillary condensation in channels with different sizes.Experimental results are compared with the theoretical basis of capillary condensation-the Kelvin equation,to test its accessibility at the atomic scale.It is found that the Kevin equation is not applicable for slits with weakly-hydrophobic graphene surfaces.Since water structure near hydrophobic surfaces is liquid-vapor like because of hydrogen bond lossing.But hydyophilic mica slits experimental results show less deviation from the Kelvin equation,because water structure on hydrophilic surface remains the same as that of the bulk.It is the water structure discrepancies that lead to the different behavious of capillary condensation in graphene and mica nanoslits.