2D MATERIAL NANOFLUIDICS AND CRYOGENICS LAB

The significant difference in salt concentration at the seawater and river water interface is a clean source of enormous osmotic power of ∼ 2.4 TW. This power is much larger than that solar and wind power produced together as of 2021. However, in the osmotic power generation field, reaching the industrial benchmark has been challenging because of the need for capillaries close to the sizes of ions and molecules. Here, we fabricated well-controlled ‘along-the-capillary’ membranes of Na-vermiculite with a capillary size of ∼ 5 Å. They exhibit 1600 times enhanced conductivity compared to commonly studied ‘across-the-capillary’ membranes. Interestingly, they show a very high cation selectivity of 0.83 for NaCl solutions, which resulted in large power densities of 9.6 W/m2 and 12.2 W/m2 at concentration gradients of 50 and 1000, respectively, at 296 K, for an unusually large membrane length of 100 μm. The power density shows an exponential increase with temperature, reaching 65.1 W/m2 for a concentration gradient of 50 at 333 K. This markedly differs from the classical behavior and indicates the role of ion (de)hydration in enhancing power density, opening new possibilities for exploiting such membranes for energy harvesting applications.

2D materials-based membranes have emerged as a promising alternative to polymer-based membranes in desalination and dye separation applications. With their exceptional permeation properties, 2D membranes outperform traditional polymers, offering higher water flux rates and improved efficiency. These membranes exhibit tunable properties, allowing customization for specific separation needs, and their atomic smoothness minimizes fouling, ensuring prolonged operational life. To enhance ion rejection and water flux, researchers propose using 2D membranes with high surface charge and sterically excluded sub-nm fluidic structures. These advancements in membrane technology hold great potential for addressing water scarcity and improving industrial separation processes. However, further research is needed to ensure their practical implementation on a larger scale.

The pursuit of cutting-edge technologies for desalination, chemical separation, and sensing has brought us to the forefront of nanoscience, where the fabrication of fluidic channels with dimensions comparable to that of molecules is of paramount importance. However, the realization of such channels, with precisely controlled dimensions at the Angstrom scale, presents an extraordinary challenge, demanding ingenuity and perseverance from researchers.

In this study, we fabricated highly crystalline vermiculite membranes with interlayer spacing ranging from 2.6 Å - 5.5 Å. Generally, the pristine lithium-intercalated vermiculite membranes are highly unstable in water and tend to disintegrate within a few minutes. We used various salts to enhance the nanosheet-cation interaction that imparted stability to our vermiculite membranes in water. The hydrophilicity of the membrane was also modulated by these cations, and Na-V membranes displayed superior dye rejection efficiency of > 99% with water permeability of 5400 L m-2 h-1 bar-1  at a low differential pressure of 0.9 bar. Additionally, our membranes also displayed a rejection efficiency of 95% for NaCl ions.  Our study also showcased the tunability of interlayer spaces with intercalants and pressure. The ion transport study revealed a continuous exchange of cations, which was demonstrated by the decreasing conductance of the K-V membrane in the presence of AlCl3 solution, and was verified through XRD analysis for all other salts. Our highly confined channels exhibit sub-linear ionic conductance related to hydration sizes, steric exclusion, K+ mobility enhancement, and conductance saturation at concentrations ≤10 mM, paving the way for exciting new applications in nanofluidics. (arXiv preprint: arXiv:2303.12463v1 )

A novel technique based on high-pressure spray has recently been explored for large-scale exfoliation of two-dimensional materials. We have successfully exfoliated MoS2 into 4-7 layers. It was found that the nanosheets' concentration and their lateral size depend on the exfoliation cycles, applied pressure and surfactant concentration. The estimated exfoliation yield was 7.25%, with a nanosheet concentration of 1.45 mg/ml. The exfoliated MoS2 nanosheets provide a very high H2 evolution rate of 30.13 mmol g-1 h-1 under ambient laboratory conditions. The nanosheets prepared by this method are stable in acidic medium and are free from surfactants. The nanosheets were found to be stable in solvents for periods of up to six months. This technique is useful in producing ink at a large scale. MoS2 nanosheets are semiconducting and can find applications in printing electrodes of capacitors and batteries, inkjet devices and flexible and wearable electronic devices. Our method is an industrial-scale and eco-friendly approach to produce highly pure nanosheets from bulk two-dimensional materials. (arXiv preprint: arXiv:2210.13813v1) 

In this work, we have created controllable water transport channels in graphite crystal with the help of an electric field and potassium chloride (KCl) ions, which allows only water to move through the crystal and blocks the movement of any salt ions. This is the first such method that could controllably manipulate the graphite interlayer spaces using an aqueous solution without damaging its structural integrity. The solvents used in this method are environment-friendly and non-toxic. Selective transport of molecules and ions is commonly observed in biological systems. Mimicking these biological channels could result in highly efficient filtration systems. 

This research is inspired by the trees’ natural intake of water that uses the capillary effect and can be impactful in providing affordable and energy-efficient drinking water solutions compared to energy-intense RO technologies. Natural graphite does not allow any water molecules or ions to pass through it because there is not enough space for the movement of these molecules. This issue was solved by using an electric field and inserting potassium chloride (KCl) ions in it, which create just enough space inside the graphite crystal and provide a stable structure for easy passage of water molecules, at the same time hindering the movement of any salt ions. This technique can also be useful in designing filters for gas purification, proton exchange in a fuel cell, chemical separation, recovery of precious metal from waste etc. It can also be suitable for dehumidification applications as expanded graphite has high water evaporation rates.