Periodic Reporting for period 1 - GraFludicDevices (Realization of water permeation kinetics in two-dimensional nanocapillaries to develop desalination and energy harvesting membranes)
Periodo di rendicontazione: 2019-04-01 al 2021-03-31
Often, enhanced water and ion transport in nano-conduits made of carbon nanomaterials is attributed to the slippage effects. For example, despite the presence of defects and oxygen functionalities in monolayer graphene oxide (GO) sheets, the reported fast water permeation in GO membranes is usually attributed to the slip enhanced flow of water molecules in pristine graphene capillaries exist in interlayer nanochannels. However, an extensive and systematic direct experimental demonstration is still required to validate and establish the slippage effects on water/ion transport in nanochannels made of carbon nanomaterials. This project addresses this issue to obtain a complete mechanistic understanding of molecular transport through two-dimensional (2D) carbon nanochannels which will be significant step-forward for rational design of GO and GO-like layer-by-layer membranes for energy and environmental applications.
Research objectives of this Marie - Curie Fellowship project are as follows:
1) Fabrication of single and multi nanochannel devices using van der Waals (vdW) assembly of different 2D layered crystals including graphite and hexagonal boron nitride (hBN)
2) Optimization of fluorescence-based flow measurement method and, performing experiments on nanofluidic devices with single and multi-channel devices for exploration of slip-enhanced flow in 2D capillaries
3) Fabrication of nanochannel devices with functional channel wall surface for rationalization of slip and surface charge effects on molecular transport and, for harvesting osmotic energy from salinity gradients
Research objectives of this Marie - Curie Fellowship project are as follows:
1) Fabrication of single and multi nanochannel devices using van der Waals (vdW) assembly of different 2D layered crystals including graphite and hexagonal boron nitride (hBN)
2) Optimization of fluorescence-based flow measurement method and, performing experiments on nanofluidic devices with single and multi-channel devices for exploration of slip-enhanced flow in 2D capillaries
3) Fabrication of nanochannel devices with functional channel wall surface for rationalization of slip and surface charge effects on molecular transport and, for harvesting osmotic energy from salinity gradients
Using vdW technique, we have first fabricated the nanochannel devices which comprise the top, bottom and space layers. Typically, a nanochannel device contains a single or multiple (50 to 100) channels with channel height varied from 5 to 15 nm, length of 10 µm and width of 200 nm. During this project, electrical conductivity and streaming measurements were performed to investigate the electrokinetic properties of the nanochannel devices made of graphite and hBN. Further, we have performed fluorescence-based flow measurements to establish a completely novel and first direct experimental demonstration of nanojet emerging out of 2D nanochannels. In a standard fluorescence-based flow experiment, we have fixed the nanochannel device between two reservoirs where one reservoir contains salt solution and the other reservoir contains the corresponding cation sensitive fluorophore. In our experiments we have used calcium sensitive Fluo-4ff Penta Potassium salt as a fluorophore. We have further analyzed the experimental results and extracted the slip lengths on graphitic surfaces.
Recent simulation studies have shown high mobility for the physisorbed hydroxyl ions leading to mobile surface charge on the pristine carbon surfaces. We have performed electrical conductivity and streaming measurements at different pH along with diffusio-osmotic (DO) measurements for understanding the effect of mobile surface charge contribution to the electrokinetics in the pristine graphitic nanochannels. Our experiments on pristine graphitic nanochannel devices have shown enhanced conductivity at each salt concentration. We have further developed a generalized theoretical fame work to rationalize the slip and surface charge effects on enhanced ionic transport in 2D nanochannels. Our experiments together with detailed theoretical analysis have shown the concentration dependent surface charge on pristine graphitic surfaces in the order of mC/m2 which can be attributed to the physisorbed hydroxyl ions on pristine graphitic surfaces.
We have also developed a method to fabricate nanochannel devices with functionalized channel wall surface. The nanochannel device made in this method contains only two graphite crystals i.e. top and bottom layers and, the nanochannel and passage for the molecules to enter the nanochannel were made on bottom layer graphite crystal using electron beam induced etching (EBIE). These devices and the pristine graphitic nanochannel devices with moderate confinement (5 to 15 nm) were used to further disentangle surface effects from the bulk effects for the ionic transport and, characterize the surface properties such as surface charge and slip length. The nanochannel devices made using EBIE have shown potential to produce a single pore power density in the order of kW/m2 under salinity gradients. This work is currently under review in Nat. Mater.
Recent simulation studies have shown high mobility for the physisorbed hydroxyl ions leading to mobile surface charge on the pristine carbon surfaces. We have performed electrical conductivity and streaming measurements at different pH along with diffusio-osmotic (DO) measurements for understanding the effect of mobile surface charge contribution to the electrokinetics in the pristine graphitic nanochannels. Our experiments on pristine graphitic nanochannel devices have shown enhanced conductivity at each salt concentration. We have further developed a generalized theoretical fame work to rationalize the slip and surface charge effects on enhanced ionic transport in 2D nanochannels. Our experiments together with detailed theoretical analysis have shown the concentration dependent surface charge on pristine graphitic surfaces in the order of mC/m2 which can be attributed to the physisorbed hydroxyl ions on pristine graphitic surfaces.
We have also developed a method to fabricate nanochannel devices with functionalized channel wall surface. The nanochannel device made in this method contains only two graphite crystals i.e. top and bottom layers and, the nanochannel and passage for the molecules to enter the nanochannel were made on bottom layer graphite crystal using electron beam induced etching (EBIE). These devices and the pristine graphitic nanochannel devices with moderate confinement (5 to 15 nm) were used to further disentangle surface effects from the bulk effects for the ionic transport and, characterize the surface properties such as surface charge and slip length. The nanochannel devices made using EBIE have shown potential to produce a single pore power density in the order of kW/m2 under salinity gradients. This work is currently under review in Nat. Mater.
We have developed a novel technique of fluorescence-based flow measurements beyond the existing simple pervaporation and electrical measurements for characterizing the nanofluidic transport through 2D nanochannel devices. Applicability of this measurement technique is tested on different multichannel devices to a single channel device. Using this measurement technique, experimentally for the first time, we should be able to assess how water flow and ionic transport through nanochannels couple to electronic properties of the confining materials. Indeed, the full potential of this measurement technique will be realized with preforming the fluorescence-based flow measurements using 2D capillary devices with angstrom-scale capillaries. However, this project paves the path for exploration of nanoscale hydrodynamics from 2D capillaries which crucial for developing membranes for desalination and water purification.
In addition, the new route of making nanochannel devices with functionalized channel wall surface establishes a comprehensive platform to thoroughly investigate the nanofluidic transport on both pristine and functionalized carbon surfaces. The developed theoretical transport framework from the electrokinetic and DO measurements using the nanochannel devices with both pristine and functionalized channel wall surfaces allowed us to demonstrate the barely explored salinity dependence of slip length and, the coupled effects of surface charge and slippage on epi-osmotic mobility. Besides these, the nanochannel devices with functionalized channel wall surface have shown potential to produce single pore power density up to 100 kW/m2 under salinity gradients, establishing the use of highly electrified carbon surfaces to fabricate nanofluidic devices and activated carbon-based membrane at large-scale for producing blue energy as a ‘clean alternate’ beyond solar and wind technologies.
In addition, the new route of making nanochannel devices with functionalized channel wall surface establishes a comprehensive platform to thoroughly investigate the nanofluidic transport on both pristine and functionalized carbon surfaces. The developed theoretical transport framework from the electrokinetic and DO measurements using the nanochannel devices with both pristine and functionalized channel wall surfaces allowed us to demonstrate the barely explored salinity dependence of slip length and, the coupled effects of surface charge and slippage on epi-osmotic mobility. Besides these, the nanochannel devices with functionalized channel wall surface have shown potential to produce single pore power density up to 100 kW/m2 under salinity gradients, establishing the use of highly electrified carbon surfaces to fabricate nanofluidic devices and activated carbon-based membrane at large-scale for producing blue energy as a ‘clean alternate’ beyond solar and wind technologies.