Investigations On Preparation And Ionic Transport Of Sub-nanopore Membrane Based On Swift Heavy Ion Track

Sub-nanoporous membranes are a class of membrane materials with channel characteristic sizes at the sub-nanoscale.Due to their extremely space confined natures,the ionic transport properties in such membrane channels are completely different from those in the corresponding nanopore materials and bulk solutions.The sub-nanochannel size is comparable to that of the hydrated ions in aqueous solution results in the entering of hydrated ions in sub-nanochannel requiring external driving force to help to strip off the polarized water molecules around them,with the energy penalty depending on the relative size between the hydrated ions and the sub-nanopore and the ionic hydration free energy.The entering ions will fill the sub-nanochannel,blocking the passage of other ions side by side and making the ions form an ordered single-file chain to move together along the channel.If the sub-nanochannel walls are charged,it will not only possess charge selectivity far beyond that of the charged nanochannel but may also lead to ion Coulomb blockade.The charge sites on the channel wall will bind with the counter ions to form ion pairs in the charged sub-nanochannel that will block the passage of other ions until they are destroyed by a certain external electric field.The combination of ion dehydration,single-file transport and Coulomb blockade leads to unique ion transport behavior in the sub-nanochannel.In addition,the ionic hydrated size and hydration energy-dependent entry rate at the entrance of the sub-nanochannel and the influence of the channel wall charges on ion conduction will be the key to the selective ion transport in sub-nanochannel membranes.Inspired by the biological ion channels,we aimed to obtain porous membrane materials with fast ion permeability and high ionic selectivity by introducing charged sub-nanometer transmembrane channels in membranes that are analog to the selectivity filters.Different approaches based on conventional swift heavy ion track polymer membranes are employed to achieve this goal.Firstly,the negatively charged sub-nanochannels are introduced in PC membranes by combining fast heavy ion irradiation,UV sensitization and pulsed electric field etching in an electrolyte to treat the membranes,and the channel size is tuned at the sub-nanoscale via adjusting the energy loss of incident ion and UV sensitization time.Secondly,the composite nanopore systems are prepared in graphene/PET composite membranes using fast heavy ion irradiation and asymmetric chemical etching.Meanwhile,the sub-nanoporous graphene/PET nanopore membranes with different conical channel tip sizes can be obtained by adjusting the chemical etching time.Further,the nanoporous PC membranes are fabricated using fast heavy ion irradiation and chemical etching,and the membranes are chemically gold-plated.The gold nanotube PC membranes with sub-nanometer inner diameters can be prepared by significantly prolonging the gold plating time.After that,the ion transport properties of these three sub-nanoporous membranes are systematically studied,respectively.It is shown that ionic transport in the sub-nanoporous PC membranes exhibits a voltage-activated behavior associated with ion dehydration and is related to the membrane channel size,ion hydration diameter,ion concentration and solution p H.The ionic conductance of the membrane also shows a negative correlation with ion hydration size and hydration energy,demonstrating its distinct conductance for different ions.Electrodialysis separation experiments show that such sub-nanoporous PC membranes possess excellent ionic sieving performance.As for the membrane with channel sizes close to 1 nm,its Li⁺/Mg²⁺separation ratio can still reach 40 and the Li⁺/La³⁺separation ratio is as high as 3000.Meanwhile,it is found that there are three different transport behaviors for the ionic transport of graphene/PET composite nanoporous membrane:rectification enhancement,rectification reversal and voltage activation.Finite element simulations indicate that these phenomena are related to the coupling between graphene sub-nanopores and PET conical channels.In addition,the modulation phenomena of ionic transport and diffusion in this composite nanochannel membrane by the gate voltage on graphene are also observed.Finally,the study of gold nanotube PC membrane with tiny inner diameters reveals that the regulation of membrane voltage on ionic transport is extremely different from the reported regulation in the gold nanotube membrane with large inner diameters.The ionic conductance of the membrane varies oscillating with monotonic changes of the membrane voltage and has extreme conductance values at specific voltages.These sub-nanochannel membranes based on swift heavy ion track will have promising applications in ion separation,seawater desalination,nanofluidic devices,DNA sequencing,etc.Additionally,there are many shortcomings in this dissertation.For instance,in the study of sub-nanoporous PC membranes,despite such membranes have high ion separation ratios,the ion permeability is still too low to further commercial applications;in the research on graphene/PET composite nanochannel membranes,the study of this composite system is not deep enough and lacks the specific effects of gate voltage on graphene pore and surface chemistry as well as its separation properties to different ions,etc.;in the study of gold nanotube membrane,there is a lack of direct evidence that the inner diameters of the prepared gold nanotubes are in sub-nanometer scale and the studies on the ionic transport and separation of the membrane.To this end,I will improve these shortcomings in the subsequent scientific research work.