Cuicui Ling

Influence of chemical functionalization on the CO2/N2 separation performance of porous graphene membranes

The separation of CO₂ from a mixture of CO₂ and N₂ using a porous graphene membrane was investigated using molecular dynamics (MD) simulations. The effects of chemical functionalization of the graphene sheet and pore rim on the gas separation performance of porous graphene membranes were examined. It was found that chemical functionalization of the graphene sheet can increase the absorption ability of CO₂, while chemical functionalization of the pore rim can significantly improve the selectivity of CO₂ over N₂. The results show that the porous graphene membrane with all-N modified pore-16 exhibits a higher CO₂ selectivity over N₂ (∼11) due to the enhanced electrostatic interactions compared to the unmodified graphene membrane. This demonstrates the potential use of functionalized porous graphene as single-atom-thick membrane for CO₂ and N₂ separation. We provide an effective way to improve the gas separation performance of porous graphene membranes, which may be useful for designing new concept membranes for other gases.

Tunable Hydrogen Separation in Porous Graphene Membrane: First-Principle and Molecular Dynamic Simulation

First-principle density functional theory (DFT) calculation and molecular dynamic (MD) simulation are employed to investigate the hydrogen purification performance of two-dimensional porous graphene material (PG-ESX). First, the pore size of PG-ES1 (3.2775 Å) is expected to show high selectivity of H2 by DFT calculation. Then MD simulations demonstrate the hydrogen purification process of the PG-ESX membrane. The results indicate that the selectivity of H2 over several other gas molecules that often accompany H2 in industrial steam methane reforming or dehydrogenation of alkanes (such as N2, CO, and CH4) is sensitive to the pore size of the membrane. PG-ES and PG-ES1 membranes both exhibit high selectivity for H2 over other gases, but the permeability of the PG-ES membrane is much lower than the PG-ES1 membrane because of the smaller pore size. The PG-ES2 membrane with bigger pores demonstrates low selectivity for H2 over other gases. Energy barrier and electron density have been used to explain the difference of selectivity and permeability of PG-ESX membranes by DFT calculations. The energy barrier for gas molecules passing through the membrane generally increase with the decreasing of pore sizes or increasing of molecule kinetic diameter, due to the different electron overlap between gas and a membrane. The PG-ES1 membrane is far superior to other carbon membranes and has great potential applications in hydrogen purification, energy clean combustion, and making new concept membrane for gas separation.

Effect of defects on Young's modulus of graphene sheets: a molecular dynamics simulation

The effect of defects including vacancy and Stone–Wales (SW) defects on the Youngs modulus of graphene sheets is investigated using molecular dynamic (MD) simulations. The simulations show that the presence of defects reduces the Youngs modulus of graphene sheets and Youngs modulus decreases with increasing degree of defects. In addition, the vacancy defects bring about a decrease in the Youngs modulus, but their reconstruction is an important factor in stabilizing the modulus. Furthermore, we explore the Youngs modulus of graphene with defects functionalized by hydrogen atoms and find that the hydrogenation of vacancy defects can increase the Youngs modulus of the defective graphene but the hydrogenation of SW defects has the opposite effect.

Highly enhanced sensitivity of hydrogen sensors using novel palladium-decorated graphene nanoribbon film/SiO2/Si structures

Graphene nanoribbons (GNRs) have been of great interest in a wide range of device applications for their novel physical, electronic and spin transport properties. Here we study the hydrogen (H2) sensing properties of GNRs decorated with palladium (Pd) nanoparticles by a one-step chemical modification and reduction process. A novel Pd-GNR film/SiO2/Si structure was fabricated by transferring a Pd-GNR film onto a heavily doped Si substrate with a native oxide layer. It is demonstrated that the Pd-GNR film/SiO2/Si structure exhibits highly enhanced responses to H2 than the pristine Pd-GNR film at parts-per-million (ppm) concentration levels at room temperature. For example, the Pd-GNR film/SiO2/p-Si structure shows an excellent response (ΔR/Rair) of ∼94% to 100 ppm H2 in air. The sensing mechanism was proposed to explain these H2 sensing characteristics based on p–n junction theory. Our findings demonstrate the incorporation of nanoparticles in GNR-based gas sensors through chemical functionalization and present a novel strategy for the application of GNRs in high performance gas sensors.

Release of encapsulated molecules from carbon nanotubes using a displacing method: a MD simulation study

Using molecular dynamics (MD) simulations, we report a displacing method to release encapsulated molecules out of open-ended single-walled carbon nanotubes (SWNTs). The simulations indicated that encapsulated molecules such as water, DNA and indomethacin (IMT) molecules would be expelled from SWNTs by fillers such as C60 molecules and metal nanowires through a displacement process, which is attributed to the much stronger van der Waals interaction between SWNTs and fillers than that between SWNTs and encapsulated molecules. In order to quantitatively reveal the release process, the center of mass (COM) distances between encapsulated molecules (and fillers) and SWNTs were calculated. Furthermore, investigations of various energies indicated that the release process is spontaneous and the van der Waals force is the main drive for the release process. Moreover, some influencing factors such as nanotube diameter and length are discussed as well. This study provides a new method for the release of drugs/genes in the biomedical field, and provides valuable information for designing novel assembled nanoscale devices.

Effect of functional groups on the radial collapse and elasticity of carbon nanotubes under hydrostatic pressure.

The effect of functional groups on the radial collapse and elasticity of a single-walled carbon nanotube (SWNT) under hydrostatic pressure was investigated using molecular dynamics and molecular mechanics simulations. It is found that the radial collapse and elasticity of the chemically modified SWNTs strongly depend on the polarity of the functional groups and the degree of functionalization. The results show that the fluorine modified SWNT (F-SWNT), on which 2.5-5.0% of the atoms are attached to -F groups, can sustain the original elasticity of the intrinsic SWNT, and the pressure needed to collapse the F-SWNT increases by 11.3-21.8%. Functional groups such as hydroxyl groups, amino groups and carboxylic groups can increase the pressure needed to collapse the modified SWNTs, but decrease their radial elasticity. Therefore, the F-SWNTs, due to the higher collapse pressure, are ideal fillers for nanocomposites for high load mechanical support.

Radial collapse of carbon nanotubes without and with Stone–Wales defects under hydrostatic pressure

The effects of carbon nanotube (CNT) chirality, Stone–Wales (SW) defects and defect orientation on the radial collapse and elasticity of single-walled CNTs (SWNTs) were investigated using molecular mechanics and molecular dynamics (MD) simulations. It is found that the collapse pressure (Pc) of the armchair SWNT is 13.75 times higher than that of the zigzag SWNT. Moreover, the armchair SWNT with SW defects is easier to collapse compared to the intrinsic armchair SWNT, while the zigzag SWNT with SW defects is more difficult to collapse compared to the intrinsic zigzag SWNT; the SW2 defect makes Pc of SWNT (10, 10) decrease by 11.0%, while the SW4 defect makes Pc of SWNT (17, 0) increase by 100.0%. We introduce a model for SWNTs deformed in the radial direction according to the projection of the C–C bond along the bending direction. The model is validated for defect-free SWNTs and is then used to study the radial collapse of SWNTs with SW defects. The effect of chirality and SW defect on the radial collapse of SWNTs can be understood by the model. The strong sensitivity of radial collapse of SWNTs to chirality and SW defect can provide some guidance for high load structural applications of SWNTs.

Fullerene filling modulates carbon nanotube radial elasticity and resistance to high pressure

The high pressure behavior of carbon nanotubes (CNTs) filled with fullerenes (C60@CNTs) is investigated systematically using molecular mechanics and molecular dynamics simulations. It is shown that the C60 filling can increase the transition pressure (Pc) of intrinsic CNTs and optimize the radial elasticity of CNTs. The C60 filling increases the Pc of CNT(17, 0) by a factor of ∼25, and the Pc of CNT(10, 10) by a factor of ∼5. An inelastic CNT(17, 0) can be transformed into a superelastic CNT(17, 0) by filling C60 into CNTs. Moreover, C60@CNTs with larger diameters (21.76 A > d > 13.56 A) show the better radial elasticity compared with intrinsic CNTs. These characteristics can make C60@CNTs possess potential applications in pressure sensors, electromechanical oscillators, nanotube memory etc. In addition, C60@CNTs with larger diameters (21.76 A > d > 13.56 A) undergo two structure transitions under high pressure, which is well in agreement with the experimental results. The Lennard–Jones potential can describe the interaction between C60 and CNT well and explain radial collapse and recovery properties of C60@CNT completely, which can provide theoretical guidance for experimental results.

Effects of Sulfur Doping and Humidity on CO2 Capture by Graphite Split Pore: A Theoretical Study

By use of grand canonical Monte Carlo calculations, we study the effects of sulfur doping and humidity on the performance of graphite split pore as an adsorbent for CO2 capture. It is demonstrated that S doping can greatly enhance pure CO2 uptake by graphite split pore. For example, S-graphite split pore with 33.12% sulfur shows a 39.85% rise in pure CO2 uptake (51.001 mmol/mol) compared with pristine graphite split pore at 300 K and 1 bar. More importantly, it is found that S-graphite split pore can still maintain much higher CO2 uptake than that by pristine graphite split pore in the presence of water. Especially, uptake by 33.12% sulfur-doped S-graphite split pore is 51.963 mmol of CO2/mol in the presence of water, which is 44.34% higher than that by pristine graphite split pore at 300 K and 1 bar. In addition, CO2/N2 selectivity of S-graphite split pore increases with increasing S content, resulting from stronger interactions between CO2 and S-graphite split pore. Moreover, by use of density functional theory calculations, we demonstrate that S doping can enhance adsorption energy between CO2 molecules and S-graphene surface at different humidities and furthermore enhance CO2 uptake by S-graphite split pore. Our results indicate that S-graphite split pore is a promising adsorbent material for humid CO2 capture.

Superior Selective CO2 Adsorption of C3N Pores: GCMC and DFT Simulations

Development of high-performance sorbents is extremely significant for CO2 capture due to its increasing atmospheric concentration and impact on environmental degradation. In this work, we develop a new model of C3N pores based on GCMC calculations to describe its CO2 adsorption capacity and selectivity. Remarkably, it exhibits an outstanding CO2 adsorption capacity and selectivity. For example, at 0.15 bar it shows exceptionally high CO2 uptakes of 3.99 and 2.07 mmol/g with good CO2/CO, CO2/H2, and CO2/CH4 selectivity at 300 and 350 K, separately. More importantly, this adsorbent also shows better water stability. Specifically, its CO2 uptakes are 3.80 and 5.91 mmol/g for and 0.15 and 1 bar at 300 K with a higher water content. Furthermore, DFT calculations demonstrate that the strong interactions between C3N pores and CO2 molecules contribute to its impressive CO2 uptake and selectivity, indicating that C3N pores can be an extremely promising candidate for CO2 capture.