Water permeation through a subnanometer boron nitride nanotube.

Water-filled nanometer-scale porous structures have gained considerable attention over the past decade due to their enormous promise in substantially improving the performance and efficiency of many applications such as biological/chemical systems,1-3 water purification systems,4 fuel cell devices,5 etc. Single-walled carbon nanotubes (SWCNTs), due to their extraordinary physical and chemical properties, are currently being investigated for a number of the above-mentioned applications. Molecular dynamics (MD) simulations by Hummer and his co-workers6 have indicated that a (6,6) CNT with a diameter of approximately 8 Å can conduct water at 300 K. The wetting behavior of the carbon nanotube was confirmed by experimental study.7 Boron nitride nanotubes (BNNTs) possess many of the superior properties of CNTs such as a high Young’s modulus8 and thermal conductivity,9 but unlike CNTs, BNNTs exhibit high resistance to oxidation10 and a wide band gap regardless of its chirality.11 These exciting properties allow BNNTs to act as complementary materials to CNTs or even replace the CNTs for applications requiring chemical stability, high-temperature resistance, or electrical insulation. There have been, however, no studies on the water conduction properties of BNNTs. In this work, we report that a (5,5) BNNT with a diameter of 6.9 Å and a finite length of 14.2 Å can conduct water, while a CNT with a similar diameter and length has only intermittent filling of water. To gain fundamental insights into the water permeability of BNNTs and to compare the results with those in CNTs, we performed molecular dynamics simulations on a finite length (5,5) BNNT and (5,5) CNT, with a diameter of 6.9 Å and length of 14.2 Å (for longer nanotubes, the filling kinetics could be different as discussed in the case of CNTs12). Both tubes are saturated at the ends with hydrogen atoms. The MD simulation domain consists of the nanotube, water, and a slab. The nanotube is fixed in a slab, as shown in Figure 1. The boron and nitride atoms in the BNNT and carbon atoms in the CNT are modeled as uncharged Lennard-Jones particles. The extended simple point charge (SPC/E) model13 was used in the simulations. The simulations were performed for 40 ns with a 1.0 fs time step using modified GROMACS 3.2.114 with a constant pressure15 of 1 bar and a constant temperature16 of 300 K. The MD simulation was started with an empty (5,5) BNNT. The water from the water reservoir filled the empty (5,5) BNNT within 50 ps of simulation time (Figure 2a). There was a small fluctuation in the number of water molecules occupying the BNNT, but during the simulation time of 40 ns, the BNNT is occupied by approximately five water molecules forming a single-file chain. In addition, water molecules traversed the tube at a rate of about 5.1 molecules/ns. Despite the same size as the BNNT, the initially empty (5,5) CNT was barely filled by water. A few water molecules enter the CNT during the simulation time. The water molecules inside the CNT formed a single-file chain only a few times, but it did not last longer than 1 ns (Figure 3a). Water structure inside the nanotube can be best understood by examining the water density distribution in the tube axial and radial directions (Figures 2b,c and 3b,c). Figure 2b shows the water density averaged within 0.8 Å from the tube axis along the z-axis. The five peaks indicate that there are five favorable locations of water inside the tube. Unlike in (5,5) BNNT, the axial density distribution of water inside the (5,5) CNT (Figure 3b) indicates that the water molecules like to reside at the ends of the nanotube. Following Beckstein et al.,17 we define the openness ω of the nanotube for water conduction based on water density; we assign ω(t) ) 1 (open) when water density in the tube at any instant is greater than 50% of the density Figure 1. Visualization of a boron nitride nanotube in a water bath.