No abstract

We investigate the separation of helium isotopes by quantum tunneling through graphene nanopores, recently proposed as an alternative to conventional methods for 3He production. We propose here a novel defective nanopore created by removing two pentagon rings of a Stone–Thrower–Wales (STW) defect, which significantly decreases the helium tunneling barrier by 50–75%. The barrier height is fine-tuned by adjusting the effective pore size, which is achieved by pore rim passivation using an appropriate functionalizing atom. This fine-tuning leads to positive deviation in the tunneling probability of 3He compared to that of 4He in the low-energy region, and thereby to high selectivity and transmission of the former isotope. It is found that fluorine-passivated nanopores restrict helium atom penetration because of their highly reduced pore size. Defective nanopores in nitrogen- and oxygen-passivated structures exhibit relatively high transmission values of 10–3 for the oxygen variant and improved selectivity value of 669 for the nitrogen variant. It is demonstrated that defective nanopores passivated on both sides with oxygen are the most attractive for 3He/4He separation on the basis of their much higher flux values while still providing good selectivity.

A pyrolysis assisted method was applied for the synthesis of defect controlled carbon nanotubes (CNTs) by varying different growth temperatures. The fabricated resistive devices containing a random network of CNTs were tested for oxygen sensing under standard room-temperature and pressure conditions. Nanotubes grown at moderate growth temperatures (870 °C), when exposed to different concentrations of oxygen, displayed a higher sensitivity (3.6%), with fast response and recovery times of about 60 and 180 s, respectively, compared to nanotubes grown at higher and lower temperatures. A room-temperature oxygen detection concentration as low as 0.3% is achieved. The fast response and recovery of CNTs are explained in terms of physisorption of oxygen molecules at (i) carboxyl functional sites and (ii) graphitic carbon sites (pristine CNT) rather than chemisorption at (iii) defected sites. Interestingly, the density functional theory simulated interaction energies (Eads) of oxygen molecules with defected CNTs (-3.381 eV) and pristine CNTs (-0.753 eV) are higher than that of the carboxyl functional sites (-0.551 eV) and are well correlated with the observed sensing response and recovery times of CNT sensors. Our results show that the carboxyl sites provide lower activation energy or shorter time for desorption of oxygen molecules to yield higher response and fast recovery of the CNT sensors.

There is now increasing recognition of the potential of graphene membranes for gas separation, with the application to CO 2 capture being one of specific interest; however, the co-adsorption of H 2 O which saturates flue-gas remains a major impediment. Towards enhancing hydrophobic characteristics of graphene while increasing specificity to CO 2 , we investigate here the adsorption characteristics of CO 2 and H 2 O on four different kinds of graphene sheetnamely, hydrogen-terminated and fluorineterminated pristine sheets, and the corresponding Stone-Thrower-Wales (STW) defect-incorporated sheets using density functional theory methods. Our results reveal that fluorine termination enhances hydrophobicity and favours the adsorption of CO 2 , while reducing that of H 2 O, in comparison to hydrogen termination. On the other hand, H 2 O adsorption affinity is increased on introducing the Stone-Thrower-Wales defect in both H-terminated and F-terminated sheets, while for CO 2 the affinity change is more marginal, evidenced from the change in height of the adsorbed molecule above the surface, and of the adsorption energy. The Henry law constant for H 2 O is reduced by 54% on F-termination, for both pristine and defective H-terminated graphene sheets, while for CO 2 it is increased by 12% and reduced by 18% respectively, on F-termination of the two sheets; indicating the pristine F-terminated sheet as the preferred option. From the density of states analysis, the Fermi level shows a 0.7 eV shift towards the valence band for fluorine termination in both pristine and STW sheets, but is not influenced by CO 2 and H 2 O adsorption. Fluorine termination is shown to have a significant effect on the valence band, and offers a convenient route for tuning the electronic structure of graphene.