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New Science and Technology of Artificial Layered Structures and Devices

Periodic Reporting for period 3 - MATTERDESIGN (New Science and Technology of Artificial Layered Structures and Devices)

Reporting period: 2018-01-01 to 2019-06-30

The extensive and growing library of layered crystals opens up the exciting possibility of exfoliating them into single atomic layers and stacking these atomic layers back together in new sequences to fabricate a new class of artificial van der Waals solids, with novel properties and functionalities. The principal objectives of this project are to study the fundamental physics and chemistry of novel materials, structures and devices synthesised by assembling different two-dimensional (2D) crystals and to exploit their unique properties for diverse applications. This project's primary aim is to explore the new science and technology of extremely narrow 2D capillaries synthesised by assembling various precisely patterned 2D crystals. Molecular transport through nano-capillaries is highly important due to its applicability in filtration and separation technology and also because of the unusual fundamental behaviour arising at the molecular scale. In this project, we are studying the behavior of molecules, atoms and ions confined in the ultra-narrow 2D channels and explore the use of these devices for nanofluidic applications.

Molecular permeation through nano channels has tremondous potential for various applications. For example, perfectly layered graphene oxide (GO) membranes exhibit unusual molecular transport, which is entirely determined by the 2D nature of the capillary formed between the layered GO sheets. In dry conditions, GO membranes show unimpeded water permeation while impermeable to all gases including helium. On the other hand, it behaves as molecular sieves with a cut-off radius around 4.5Å if immersed in water. This unusual property opens many possible applications for GO membranes for water filtration and separation. In this project, we mainly focuses to explore the potential applications of the nanofluidic devices and 2D channels for water filtration, solvent filtration, and gas separation applications.
The application of graphene-based membranes for organic solvent nanofiltration (OSN) received very limited attention so far, despite their potential due to chemical, thermal and mechanical stability. This could be due to the fact that graphene oxide (GO) membranes were previously shown to be completely impermeable to all solvents except for water. Our project changes this wrong perception. Our studies found that ultrathin GO membranes, referred to as highly laminated GO (HLGO) membranes due to their perfect laminar structure, are highly permeable to organic solvents (e.g. alcohol) without sacrificing its atomic-scale sieving properties. This is the first clear-cut experiment showing a great potential of GO membranes for OSN, where solutes molecules are separated from organic solvents. Developing OSN membranes has proven extremely challenging because the conventional polymeric membranes are highly unstable in organic solvents while ceramic inorganic membranes are costly and lack separation efficiency. We explain the ultrafast permeation of organic solvents combined with precise sieving by the presence of randomly distributed pinholes in the membrane which are interconnected by 1 nm wide nanochannels provided by aligned GO sheets. We also show how the transition from high permeability to complete impermeability with respect to organic solvents occurs in GO membranes. With increasing the membrane thickness, the organic solvent permeance decayed exponentially whereas water continues to permeate anomalously fast through graphene nanocapillaries. This behaviour is explained by changing the dominant permeation pathway, from pinholes to graphene capillaries, and allowed us to resolve some long-standing controversies in this research field.

Developing smart membranes with a stable and reversible response to external stimuli is a long-sought objective for scientists and technologists due to its importance in fundamental sciences and its potential for wide-variety of applications ranging from filtration and separation technology to healthcare. Fabrication of smart membranes with precise control of water and other molecules’ permeation has also been on the wish list of manufactures but no even experimental demonstration has been achieved so far. Such membranes would be of particular interest for life-science and healthcare applications, as they can mimic the performance of natural membranes. Ultimately, we can foresee that such technology will be applied for tissue engineering and for the creation of artificial membranes with biological functions. For the first time, we demonstrate electrically-tunable water transport through micrometre-thick graphene oxide (GO) membranes. To achieve electrical control over water permeation, we create conductive filaments in the graphene oxide membranes via controllable electrical breakdown. The electric field that concentrates around these current-carrying filaments ionizes water molecules inside graphene capillaries within the graphene oxide membranes, which impedes water transport. Our work opens up an avenue for developing smart membrane technologies for artificial biological systems, tissue engineering and filtration.
1. For the first time, we demonstrate electrically-tunable water transport through micrometre-thick graphene oxide (GO) membranes. We have found that an electrical breakdown of GO membranes leads to the formation of vertical conductive filaments and the current through them controls the water permeation. We attribute the observed electric field effect to ionisation of molecules around current carrying filaments in graphene capillaries. Developing smart membranes with a stable and reversible response to external stimuli is a long-sought objective for scientists and technologists due to its importance in fundamental sciences and its potential for wide-variety of applications ranging from filtration and separation technology to healthcare. Fabrication of smart membranes with precise control of water and other molecules’ permeation has also been on the wish list of manufactures but no even experimental demonstration has been achieved so far. Such membranes would be of particular interest for life-science and healthcare applications, as they can mimic the performance of natural membranes. Ultimately, we can foresee that such technology will be applied for tissue engineering and for the creation of artificial membranes with biological functions.

2. Developing organic solvent nanofiltration (OSN) membranes has proven extremely challenging because the conventional polymeric membranes are highly unstable in organic solvents while ceramic inorganic membranes lack separation efficiency. The rising interest in OSN technology by both the scientific community and industry is witnessed by the recent increase in the number of published papers and patents in this topic. The application of graphene-based membranes for organic solvent nanofiltration received very limited attention so far, despite their potential due to chemical, thermal and mechanical stability. This could be due to the fact that GO membranes were previously shown to be completely impermeable to all solvents except for water. Our research changed this wrong perception. We show that ultrathin GO membranes, referred to as highly laminated GO (HLGO) membranes due to their perfect laminar structure, are highly permeable to organic solvents without sacrificing sieving properties. This is the first clear-cut experiment showing a great potential of GO membranes for OSN. We explain the ultrafast permeation of organic solvents combined with precise sieving by the presence of randomly distributed pinholes in the membrane which are interconnected by 1 nm wide nanochannels provided by aligned GO sheets.


Expected results - We are currently investigating molecular permeation properties of other 2D materials based membranes and also trying to scale-up the graphene oxide membranes for several applications, including water filtration.
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