Helium Isotopes Quantum Sieving through Graphtriyne Membranes

Molecular hydrogen isotope separation by a graphdiyne membrane: a quantum-mechanical study

Graphdiyne (GDY) has emerged as a very promising two-dimensional (2D) membrane for gas separation technologies. One of the most challenging goals is the separation of deuterium (D₂) and tritium (T₂) from a mixture with the most abundant hydrogen isotope, H₂, an achievement that would be of great value for a number of industrial and scientific applications. In this work we study the separation of hydrogen isotopes in their transport through a GDY membrane due to mass-dependent quantum effects that are enhanced by the confinement provided by its intrinsic sub-nanometric pores. A reliable improved Lennard-Jones force field, optimized on accurate ab initio calculations, has been built to describe the molecule-membrane interaction, where the molecule is treated as a pseudoatom. The quantum dynamics of the molecules impacting on the membrane along a complete set of incidence directions have been rigorously addressed by means of wave packet calculations in the 3D space, which have allowed us to obtain transmission probabilities and, in turn, permeances, as the thermal average of the molecular flux per unit pressure. The effect of the different incidence directions on the probabilities is analyzed in detail and it is concluded that restricting the simulations to a perpendicular incidence leads to reasonable results. Moreover, it is found that a simple 1D model-using a zero-point energy-corrected interaction potential-provides an excellent agreement with the 3D probailities for perpendicular incidence conditions. Finally, D₂/H₂ and T₂/H₂ selectivities are found to reach maximum values of about 6 and 21 at ≈50 and 45 K, respectively, a feature due to a balance between zero-point energy and tunneling effects in the transport dynamics. Permeances at these temperatures are below recommended values for practical applications, however, at slightly higher temperatures (77 K) they become acceptable while the selectivities preserve promising values, particularly for the separation of tritium.

In pursuit of accurate interlayer potentials for twisted bilayer graphynes

Recent explorations of twist in bilayer graphene and the discovery of superconducting phases at certain magic angles have laid the groundwork for a new branch in materials science called twistronics. However, theoretical studies on twisted layered materials are impeded due to the computational expense associated with first-principles calculations. Empirical force field approaches that include anisotropic terms to describe interlayer interactions have come to the fore as excellent alternatives to deal with such a stumbling block. Taking a cue from these formulations, herein, we describe our pursuit of capturing the interlayer interactions in bilayer graphynes with atomistic empirical potentials. The choice of the potentials, namely the improved Lennard-Jones potential and Hod's interlayer potential, is motivated by the objective of bringing out the role of anisotropy explicitly. Empirical parameters for both the potentials are calibrated against dispersion-corrected DFT calculations that are performed to incorporate the stacking, sliding and twisting features of the bilayer configurations. Although the isotropic improved Lennard-Jones potential is able to describe the interlayer stacking of graphynes, it is inadequate to account for the interlayer twist properties. The anisotropic Hod's interlayer potential portrays the interlayer twisting energy profiles of the benchmark DFT calculations with a reasonable accuracy. Our potential formulations can bestow impetus to the research on the homo- and hetero-bilayer structures of graphynes and other two-dimensional materials.

Graphene-based composite membranes for isotope separation: challenges and opportunities

Graphene-based membranes have got significant attention in wastewater treatment, desalination, gas separation, pervaporation, fuel cell, energy storage applications due to their supreme properties. Recently, studies have confirmed that graphene based membranes can also use for separation of isotope due to their ideal thickness, large surface area, good affinity, 2D structure etc. Herein, we review the latest groundbreaking progresses in both theoretically and experimentally chemical science and engineering of both nanoporous and lamellar graphene-based membrane for separation of different isotopes. Especially focus will be given on the current issues, engineering hurdles, and limitations of membranes designed for isotope separation. Finally, we offer our experiences on how to overcome these issues, and present an ideas for future improvement and research directions. We hope, this article is provide a timely knowledge and information to scientific communities, and those who are already working in this direction.

Isotopic separation of helium through graphyne membranes: a ring polymer molecular dynamics study

Microscopic-level understanding of the separation mechanism for two-dimensional (2D) membranes is an active area of research due to potential implications of this class of membranes for various technological processes. Helium (He) purification from the natural resources is of particular interest due to the shortfall in its production. In this work, we applied the ring polymer molecular dynamics (RPMD) method to graphdiyne (Gr2) and graphtriyne (Gr3) 2D membranes having variable pore sizes for the separation of He isotopes, and compare for the first time with rigorous quantum calculations. We found that the transmission rate through Gr3 is many orders of magnitude greater than Gr2. The selectivity of either isotope at low temperatures is a consequence of a delicate balance between the zero-point energy effect and tunneling of ⁴He and ³He. In particular, a remarkable tunneling effect is reported on the Gr2 membrane at 10 K, leading to a much larger permeation of the lighter species as compared to the heavier isotope. RPMD provides an efficient approach for studying the separation of He isotopes, taking into account quantum effects of light nuclei motions at low temperatures, which classical methods fail to capture.