Article pubs.acs.org/JPCC
Consecutive Water Transport through Zero-Dimensional Graphene Gates of Single-Walled Carbon Nanohorns Tomonori Ohba* Graduate School of Science, Chiba University, 1-33 Yayoi, Inage, Chiba 263-8522, Japan S Supporting Information *
ABSTRACT: The unique water transport properties in nanospaces are essential for control of various chemical reactions, biochemical activities, and electrochemical systems. Fast water transport has been observed in one-dimensional nanospaces. However, water transport via zero-dimensional nanospaces has not yet been observed. Zero-dimensional nanospaces were obtained by extremely small and thin gate (zero-dimensional gate) insertion on graphene walls of singlewalled carbon nanohorns. The water transport properties were examined by water vapor loading and release via the zerodimensional gates, and molecular dynamics simulation. Although relatively large gates provided considerable adsorption hysteresis by long-term equilibrium, water vapor loading and release via the extremely small gates showed consecutive water loading and release. The molecular dynamics simulation showed consecutive water transport via the gates, probably because of lower energy barriers to water transport in the vicinity of the gates. The zero-dimensional gates rejected water vapor transfer, but admitted condensed water for transfer.
1. INTRODUCTION An understanding of fluid flows in nanoscopic spaces and interfaces enables improved control of molecular systems in nanoscience. Unique fluidic properties in nanospaces and nanointerfaces have been observed, especially those of water in nanospaces caused by anomalous hydrogen bonding.1,2 Water could fill hydrophobic nanospaces, although intermolecular water interactions in nanospaces are shallower than those in the bulk,3,4 and hydrogen bonding of water induces cluster formation.5−7 Cluster formation of water is key for its stabilization in hydrophobic nanospaces.8,9 In hydrophilic nanospaces, water adsorption and capillary condensation behaviors have been observed, because of relatively strong interactions with hydrophilic surfaces.10 Fuel-cell performances depend significantly on the ease of water access to hydrophobic and hydrophilic nanospaces and nanointerfaces.11,12 Water channel proteins (aquaporins) have hydrophobic nanospaces, which facilitate passive water transport into living cells,13,14 i.e., biological activities are controlled by water transport via hydrophobic nanospaces.15,16 An understanding of water transport via nanospaces and nanointerfaces has become one of the most important issues in fundamental and applied sciences. Carbon nanotubes (CNTs) have simple one-dimensional nanospaces and therefore provide an ideal platform for investigating water transport properties.17 Molecular dynamics (MD) simulations of water transport in models of water pumps through CNTs have clarified water transport mechanisms in water channels.18,19 Hummer proposed that rapid water, proton, and ion motions were facilitated by one-dimensional © 2016 American Chemical Society
water wires in hydrophobic CNTs, based on MD simulations.3,20 Fast water transport was experimentally observed using fabricated nanotube arrays.21 These results for fast water transport through nanochannels have led to studies of water transport in various CNTs and graphene.22−24 Charged nanotubes, particularly vibrationally charged nanotubes, accelerate water transport;22,25 fast water transport by vibrations is a result of disruption of hydrogen bonds.26 Water transport in flexible CNTs is impeded by the increased energy barrier.27 This has been confirmed experimentally; fast water transport in narrow CNTs was coincident with a significant decrease in hydrogen bonding.5,7 Anomalous hydrogen-bond formation and ring-like structures in CNTs have also been observed.6,28 The driving forces of water confinement in hydrophobic nanospaces were examined using MD simulations of water in CNTs.23 The factors which drive water confinement, and fast water translations and rotations, are the enthalpy contributions made by a rigid hydrogen-bonded network and by a reduction in the number of hydrogen bonds, respectively. Nonstraight channels in CNTs induce faster water transport, because of the weak and inhomogeneous potential wells of CNTs.8,24 Smooth water transport can be achieved by self-assembled structural changes during the water loading and release processes, even in the straight channels of CNTs.29 Striolo proposed that clusters strongly affect water diffusion, based on MD simulations.30 Direct transmission electron microscopy observations of water Received: March 27, 2016 Revised: April 7, 2016 Published: April 18, 2016 8855
DOI: 10.1021/acs.jpcc.6b03142 J. Phys. Chem. C 2016, 120, 8855−8862
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303 K, measured using a volumetric adsorption apparatus (Autosorb-1; Quantachrome Co., Boynton Beach, FL, USA). The nanopore structures were evaluated by αS and Dubinin− Radushkevich analysis of the N2 adsorption isotherms at 77 K.45−47 Water vapor adsorption isotherms were measured at 303 K using an in-house-built volumetric adsorption apparatus; the results were confirmed by comparison with those obtained using a gravimetric adsorption apparatus. 2.2. Simulation Methods. Molecular dynamics simulations were performed to investigate water penetration through 0D graphene gates of diameter 0.3, 0.4, and 0.7 nm, using the Verlet leapfrog algorithm. The schematic image is shown in Figure S1. Numerical integration of the equation of motion was performed with a time step of 0.5 fs and cutoff length of 45.0 nm. Water trajectories were calculated for 3.0 ns and the calculation time-step was 0.5 fs. The molecular coordinates were output every 0.5 ps. The unit cell was 2.459 × 100.0 × 100.0 nm3 with a three-dimensional periodic boundary condition. The number of water molecules was 230, which corresponded to a fractional filling of 0.5 of the internal nanopores of a carbon nanotube (CNT). The initial configuration was obtained from grand canonical Monte Carlo simulations in 1 × 106 steps. The CNT was armchair type and of diameter 3.0 nm; it consisted of 1200 carbon atoms. CNTs with 0.3, 0.4, and 0.7 nm graphene gates consisted of 1174, 1128, and 1074 carbon atoms, respectively. The corresponding length of a CNT was 2.459 nm, which was equal to the unit cell size in the CNT direction. Water molecules in a CNT in the initial configurations were thus released only via gates on a CNT. The molecular coordinates were output every 0.5 ps and the total calculation time was 3.0 ns. The temperature was controlled at 300 ± 20 K using the weak heat-bath-coupling method, as shown in Figure S2a. The intermolecular potential of water was represented by the five site transferable interaction potentials, and was a combination of the Lennard-Jones 12−6 potential and Coulombic interactions, as follows:48
in CNTs showed water cluster formation during water loading.31 Graphene is a member of the carbon family and has attracted considerable attention, because of its potential applications. Graphene provides the thinnest sheet and ideal graphene prevents molecular penetration. However, molecules can penetrate through graphene after graphene gate generation.32−35 Kim and co-workers reported the selective penetration of various gases using gate-generated multilayer graphene oxide.32 Zhu and co-workers reported desalination of aqueous solution using graphyne membrane.36 O’Hern and coworkers fabricated graphene with nanoscopic gates for molecular transport.33,34 The gate sizes, approximately 0.4 ± 0.2 nm, were controlled by ion bombardment, showing selective penetration of ions.33 Single-walled carbon nanohorns (CNHs) have single graphene walls and the internal nanopores of as-synthesized CNHs were closed for any molecules.37 The graphene gates of CNHs were strictly controlled by heating in an O2 atmosphere and had high gas separation abilities.38,39 CNHs also have high potentials for various applications of drug delivery, biosensing, catalytic supports, and storage, reviewed by Zhu and Xu.40 Controlled gate size of CNHs is available for those applications. Molecular separations with graphene gates were also examined using MD simulations.35,38,41 The molecular gates on graphene act as zero-dimensional (0D) and molecular penetration nanospaces. The structure of 0D water was determined using fullerene.42 MD simulations showed that 0D graphene gates provided molecular penetration better than that in one-dimensional CNTs.43 MD simulation showed that penetration by water via large graphene gates of diameter 2.75 nm was faster than that via CNTs of a similar size, but water penetration via narrow 0D graphene gates of diameter 0.75 nm was slower than that via CNTs of similar size.43 However, facilitation of water vapor penetration via 0D graphene gates has been observed.44 Therefore, there remains a lack of knowledge regarding water properties in 0D nanospaces. CNHs have advantages in evaluating 0D water dynamic properties, because strictly controlled 0D graphene gates can be obtained by heating, as mentioned above. In this study, 0D water properties were examined using strictly controlled CNH graphene gates and MD simulations.
⎡⎛ σij ⎞12 ⎛ σij ⎞6 ⎤ ϕij(r ) = 4εij⎢⎜ ⎟ − ⎜ ⎟ ⎥ + ⎝r ⎠⎦ ⎣⎝ r ⎠
4
4
∑∑ i
j
1 qiqj 4πε0 rij
Here, εij and σij are the water potential-well depth (80.5 K) and the effective diameter (0.312 nm), respectively. The absolute value of the partial charge of water qi is 0.241 C. Ewald summation was applied to the long-range Coulombic interactions between water molecules. The interaction between a water atom and a carbon atom was represented by the Lennard-Jones 12−6 potential. The potential-well depth and effective diameter of a carbon atom were 28.0 K and 0.34 nm, respectively, obtained from the interaction between a gas molecule and a carbon.49,50 We assumed that the carbon atom was neutral and that the interaction between water and the CNT came only from the sum of the Lennard-Jones interactions. Lorentz−Berthelot mixing rules and Ewald summation were applied to the potential parameters between water and carbon, and long-range coulomb interactions between water molecules, respectively. Water was released from a CNT by applying a weak external field of 0.01 nN in the radial direction to the CNT (Figure S2b). The self-diffusion coefficients in the Einstein relationship were obtained from the slope of the mean-square displacement averaged over the trajectories of individual water molecule in three dimensions for the last 0.05 ns.
2. EXPERIMENTAL AND SIMULATED PROCEDURES 2.1. Experimental Procedures. As-prepared carbon nanohorns (CNHs) were oxidized in an O2 atmosphere (100 mL min−1) at 673 K for 15, 30, 60, or 540 min; the heated CNH samples were denoted by CNH15, CNH30, CNH60, and CNH540, respectively, and as-prepared CNH was denoted by CNH0. Thermogravimetric and differential thermal analyses (DTG-60AH, Shimadzu Co., Kyoto, Japan) were performed to determine the conditions for gate opening on CNHs. The thermogravimetric measurements were performed after pretreatment under a dry O2 gas flow at ambient temperature for more than 2 h at a heating rate of 5 K min−1. The weight changes of the CNHs at 673 K in an O2 atmosphere were measured after heating to 673 K at a heating rate of 5 K min−1. Transmission electron microscopy of CNHs was performed at 120 kV (JEM-2100F; JEOL Co., Tokyo, Japan). Surface functional groups were identified using X-ray photoelectron spectroscopy with Mg Kα radiation at 10 kV and 10 mA (JPS9010MX, JEOL Co.). The CNH gate sizes were determined from N2 adsorption isotherms at 77 K, N2, O2, CO2, and CH4 adsorption isotherms at 273 K, and SF6 adsorption isotherms at 8856
DOI: 10.1021/acs.jpcc.6b03142 J. Phys. Chem. C 2016, 120, 8855−8862
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3. RESULTS AND DISCUSSION Figure 1 shows the weight changes of as-prepared CNHs with increasing temperature and time at a constant temperature, i.e.,
CNH30, and CNH540, respectively. The oxygen/carbon ratios for typical graphites such as Madagascar graphite, acetylene black, and other carbon blacks are 0.03−0.07; therefore, the CNHs are still hydrophobic after oxidation. Thus, the CNH assembled structures did not change much when heated for less than 60 min. The treated CNHs have internal and interstitial nanopores, but the internal pores of the as-prepared CNHs are closed to all molecules, i.e., the internal nanopores are closed, but are opened by oxidation.39,56 The average internal tube diameter and intertube distance were 2.9 and 0.7 nm, respectively, based on structural and simulated evaluations.57,58 The N2 adsorption isotherms of the CNHs at 77 K, shown in Figure 3a, also
Figure 1. Gate preparation by slight carbon removal from CNHs. Thermogravimetric and differential thermal analyses of original CNH in O2 atmosphere (a) and weight change of original CNH on heating at 673 K (b).
673 K, and thermal analysis of the as-prepared CNHs. The thermogravimetric and differential thermal curves in Figure 1a suggest that the as-prepared CNHs were significantly oxidized at around 780 K in an O2 atmosphere and burned out above 850 K. The slight decrease below 450 K indicates desorption of adsorbed gases, mainly water vapor. A heating temperature of 673 K was chosen to open the gates on the CNH graphene walls, as 673 K is the temperature just before a significant weight decrease and provided moderate exothermic combustion reactions. The weight changes of as-prepared CNHs in an O2 atmosphere at 673 K are shown in Figure 1b. The weight decreases in 15, 30, 60, and 540 min were only 2%, 3%, 4%, and 20%, respectively. The structures of CNH15, CNH30, and CNH60 should therefore be similar to that of CNH0. The Raman spectra of the CNHs in the G and D band ranges show that the graphitic structures of the CNHs were less ordered than those of other nanocarbon families.51−53 During oxidation of as-prepared CNH (CNH0), the G bands of the oxidized CNHs broadened and shifted to higher frequencies (blue shift), and the D bands grew. The reason for the blue shift is that defect creation caused the appearance of another band, G′, at 1620 cm−1.54,55 Disordered structures were created on the CNH graphene walls, as shown elsewhere.38 The oxygen functional groups created by oxidation were investigated using X-ray photoelectron microscopy (Figure 2a and b). The C 1s peaks in Figure 2a show that the CNH graphene frameworks were maintained. The oxygen/carbon ratios calculated from the C 1s and O 1s peak areas were 0.03, 0.05, and 0.07 for CNH0,
Figure 3. Nanopore structures of CNHs. (a) N2 adsorption isotherms of CNHs at 77 K, and nanopore volumes (b), specific surface areas, (c) and nanopore diameters (d).
supposed that the internal nanopores of CNH0 were closed to all molecules, but oxidation permitted adsorption of N2 molecules in the internal nanopores. The amount adsorbed by CNH540 was lower. The reduction in the amount of adsorbed N2 for CNH540 is caused by removal of the tips of CNH particles, as shown later. However, the adsorption isotherm of CNH540 corresponded to those of the other opened CNHs after correction for the burned-out rate of 20% obtained from the TG analysis. N2 was adsorbed in both the internal and interstitial nanopores of CNH30 and CNH60, whereas N2 adsorption was observed only in the interstitial nanopores of CNH0. The differences between the amounts of N2 adsorbed by CNH0 and CNH15, CNH30, or CNH60 were the result of adsorption in the internal nanopores of the CNHs or in other words, N2 penetration through the 0D gates. There were few differences between the adsorption isotherms of CNH30 and CNH60, showing that N2 molecules can penetrate their 0D graphene gates. In contrast, the amount adsorbed by CNH15 was significantly smaller than those adsorbed by CNH30, CNH60, and CNH540. This shows that N2 partly penetrated through the 0D graphene gates on CNH15. The nanopore volumes, specific surface areas, and nanopore diameters were evaluated from the N2 adsorption isotherms at 77 K; the results are shown in Figure 3b−d. The nanopore volumes were 0.14, 0.21, 0.58, and 0.58 mL g−1 for CNH0, CNH15, CNH30, and CNH60, respectively, and the internal
Figure 2. X-ray photoelectron spectroscopic C 1s (a) and O 1s (b) peaks. 8857
DOI: 10.1021/acs.jpcc.6b03142 J. Phys. Chem. C 2016, 120, 8855−8862
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Figure 4. Transmission electron microscopy images of CNHs. (a) CNH0, (b) CNH30, and (c) CNH540. Top and bottom images were obtained at low and high magnifications, respectively.
nanopore volumes of the CNHs were 0.44 mL g−1. Only 16% of the 0D gates of CNH15 permitted penetration by N2 molecules. These results indicate that N2 molecules can be adsorbed in the internal nanopores of CNHs after opening treatment, except in the case of CNH15. The reduction in the amount of adsorbed N2 for CNH540 is caused by removal of the tips of CNH particles. Transmission electron microscopy images of CNH0, CNH30, and CNH540 in Figure 4 show that the CNH particles were aggregated and had interstitial and internal nanopores, in agreement with previous reports.37,38 The diameter of the assembled CNH particles was approximately 100 nm. The structures of the assembled CNH particles were similar before and after oxidation, because only a small percentage of CNHs were removed by oxidation of CNH15, CNH30, and CNH60. Although the assembled structure of CNH540 was similar to those of the others, the CNH particle tips were removed by oxidation and the assembled diameter was approximately 80 nm. Partial oxidation of CNH0 formed nanoscale gates on the graphene walls of CNH0, and molecules penetrated into the internal nanopores via the graphene gates, although the internal nanopores of CNH0 were closed to adsorbed molecules.38,56 The graphene gate sizes for CNH15, CNH30, and CNH60 were estimated using a multimolecular probe method; it was assumed that the graphene gates of CNH0 and CNH540 prevented and permitted penetration of any molecules, respectively. Figure 5a shows the penetration rates of molecules through the graphene gates of CNH15, CNH30, and CNH60, evaluated from O2, CO2, N2, CH4, and SF6 adsorption isotherms.38 The molecular penetration rates into the internal nanopores of the CNHs via graphene gates were calculated under the assumption that the CNH0 and CNH540 graphene gates respectively prevented and permitted molecular penetration into the internal nanopores (Figure 5a). The molecular penetration rate is equivalent to that through graphene gates that are larger than molecules. The size distribution of the graphene gates can therefore be obtained by Gaussian fitting of the data. The graphene gate sizes of CNH15, CNH30, and
Figure 5. Graphene gate sizes for CNHs. (a) Penetration rates for graphene gates of NH15, NH30, and NH60 and (b) graphene gate size distributions. Reprinted with permission from ref 38. Copyright 2014 American Chemical Society.
CNH60 were 0.31 ± 0.02, 0.37 ± 0.02, and >0.5 nm, respectively, as shown in Figure 5b; they were determined using a multimolecular probe method; the details have been given in a previous paper.38 The length of graphene gates was equivalent to graphene thickness of 0.34 nm. The water vapor adsorption isotherms at 303 K for CNH30 and CNH60 agree well with each other (Figure 6a), and these amounts correspond to the nanopore volumes evaluated from the N2 adsorption isotherms at 77 K. The adsorption uptake shifted to slightly higher pressure and adsorption of a large amount of water vapor was also observed for CNH15, despite the considerable decrease in the amount of N2 adsorbed. Water (molecular size 0.31 nm) could therefore penetrate through the 0.31 nm gates of CNH15, through which 16% of N2 (molecular size 0.33 nm) penetrated, although the measurement temperatures were different. The interstitial and internal nanopore volumes evaluated from the N2 and water vapor adsorption isotherms agreed with each other, except in the case of NH15, as shown in Figure 6b. The density of adsorbed water in the internal nanopores of CNH15 was extremely high when the nanopore volume based on N2 adsorption was adopted as the nanopore space (Figure 6c), whereas the nanopore volume given by extrapolation of the water vapor adsorption isotherms to the saturated vapor pressure gave reasonable densities 8858
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(Figure 6d). The extremely high density obtained using the N2 nanopore volume is a result that the N2 molecules could fill only 16% of the internal nanopores of CNH15, as mentioned above. The water vapor loading and release isotherms for the internal nanopores of CNH30 and CNH60 show that water vapor was preferentially adsorbed at high pressure and then condensed in the nanopores, but little condensed water was released above a relative pressure of 0.6. This adsorption hysteresis of water vapor has also been observed for hydrophobic nanopores.51,59,60 The water vapor loading and release for CNH15 were anomalous: 1, the adsorption of water vapor shifted to higher pressure; 2, adsorption hysteresis disappeared; and 3, the amounts of water released were smaller than the loaded amounts. On the other hand, molecular penetrations of simple molecules such as N2, O2, CO2, CH4, and SF6 via the 0D gates were determined by gate size, and the adsorption hysteresis and threshold pressure shift of adsorption were rarely observed from the preceding study.38 The higher adsorption uptake for CNH15 indicates that the 0.31 nm gates acted as barriers to water vapor penetration, because water and the gate are similar sizes. Conversely, no penetration barriers were expected for the 0.37 nm CNH30 gates and the CNH60 gates, which were larger than 0.5 nm. The stabilized energy of water assembly in the water loading and release processes suggests that water loading and release are both in quasi-equilibrium states, i.e., long-term equilibrium states;60 therefore, the penetration barrier to water loading induced the shift in the adsorption uptake. Although the CNHs have the same nanopore structures except for the gates, for CNH15, the adsorption hysteresis of water vapor disappeared. The 0D graphene gates rejected water vapor loading, but accepted condensed water release. It can be concluded that the 0D graphene gates made the water elastic. Water clusters were
Figure 6. Water vapor adsorption by CNHs. (a) Water vapor adsorption isotherms of CNHs at 303 K. Nanopore volumes evaluated from extrapolation of water vapor desorption isotherms to P/P0 = 1.0 were 0.10, 0.63, 0.62, and 0.64 mL g−1 for CNH0, CNH15, CNH30, and CNH60, respectively. (b) Internal and interstitial nanopore volumes (blue and red bars, respectively) evaluated from N2 and water vapor adsorption isotherms. (c) Adsorbed water densities in internal nanopores of CNHs, calculated from nanopore volumes evaluated from N2 adsorption isotherms at 77 K. (d) Water densities in internal nanopores evaluated from water vapor adsorption isotherms: ● CNH15, ■ CNH30, and ⧫ CNH60. Filled and open symbols represent adsorption and desorption courses, respectively.
Figure 7. Consecutive water release mechanism. Superposition of successive water snapshots via 0D gates at 0, 200, and 500 ps. Blue and red spheres represent oxygen and hydrogen atoms, respectively (a−c). Black balls and bars represent CNTs. Water radial distribution functions from CNT center for 0.3 nm gates (d), 0.4 nm gates (e), and 0.7 nm gates (f). Water amounts in CNTs as a function of time (g) and self-diffusion coefficients (h) for 0.3 nm gates (blue curves), 0.4 nm gates (red curves), and 0.7 nm gates (green curves). Solid and open symbols represent water self-diffusion coefficients in the vicinity of gates and in CNTs, respectively. 8859
DOI: 10.1021/acs.jpcc.6b03142 J. Phys. Chem. C 2016, 120, 8855−8862
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The Journal of Physical Chemistry C formed in the internal nanopores in water loading and the layer form was observed in water release in a previous study.44 Water vapor was introduced into the internal nanopores via the 0D gates and could subsequently form clusters. However, the mechanism of desorption for CNH15 might be different, as discussed later. The significant water release for CNH15, i.e., lower densities in water release than in water loading, occurs without apparent adsorption hysteresis, whereas those for CNH0 show significant adsorption hysteresis (Figure 6d). This suggests that water in the internal nanopores was promptly released via the 0D gates of CNH15 with involvement of water adsorbed in the interstitial nanopores. The anomalous water release behaviors were investigated by evaluating water transport via the 0D gates using MD simulations (Figure 7). An ideal CNT model was used instead of a CNH model, because the 0D gate size dependence of water penetration could be clearly observed by using ideal structure of a CNT. Water in a CNT was released via the 0D gates by weak external fields (Figure S2b). Water left the internal nanopores in a straight line in the vicinity of the 0.3 nm gates, whereas for the 0.4 and 0.7 nm gates, water stagnated in the vicinity of the gates (Figure 7a−c). Water formed layer-like structures in the CNTs with preferential adsorption in the inner parts of internal nanopores and water at the interface with a CNT was preferentially released via the 0D gates (Figure 7d−f and Figure S3). Those snapshots suggested that water was quickly and individually removed from the internal nanopores when water approached to the 0D gates despite weak external field in Figure S2. The decrease in the amounts of water in the internal nanopores seen in Figure 7g indicate that the water release rates were fastest for the 0.3 nm gates. Figure 7h shows the selfdiffusion coefficients of water in the internal nanopores of the CNTs and in the vicinity of the 0D gates. The self-diffusion coefficients above 2 ns are not shown here for avoiding statistical errors. The self-diffusion coefficients in the CNTs were (1−3) × 10−9 m2 s−1 above 1 ns and less dependent on the 0D gate size. In contrast, those in the vicinity of the 0D gates were apparently different from each other; the smaller the 0D gates, the higher the self-diffusion coefficient: 18, 14, and 9 × 10−9 m2 s−1 for the 0.3, 0.4, and 0.7 nm gates, respectively. Water transport was therefore faster via the extremely narrow 0D gates than that via the wider 0D gates, supporting the consecutive water loading and release observed for CNT15 (0.3 nm 0D gates) in the experimental adsorption isotherm in Figure 6d. In the water loading process, internal water penetrated via the 0D gates promoted further water penetration by reducing an energy barrier via the 0D gates.44 The reduction of the energy barrier is due to the contribution of hydrogen bonding between water in a 0D gate and internal water. When the gate size became large enough for the water molecule size, hydrogen bonding of water in a 0D gate was also significant rather than the energy barrier.61 In the water release process, water penetrating via the 0D gates was quickly removed, and thus, promotion of water penetration could not be expected. This is the reason for nonconsecutive water loading and release via the 0D gates except for the 0.3 nm 0D gates. Why were consecutive water loading and release observed in the 0.3 nm 0D gates? Why was water release via the narrower 0D gates faster than those via the other gates? These mechanisms were investigated by evaluating the stabilization energies of water transport via the 0D gates using potential calculations, as shown in Figure 8. The deepest potential wells, 7.2−7.5 kJ mol−1, were observed
Figure 8. Water transport properties via 0D gates. CNT models (left) and potential-well depths of water (right). The abscissas and ordinates are the distances R from the CNT centers (depicted by red lines on the left) and the distance from the positions along CNTs, respectively. Stabilization energies from 0.0 to −8.0 kJ mol−1 are colored red-toblue.
on the CNT interfaces in the internal nanopores. The adsorption potentials, 4.9−5.6 kJ mol−1, on the external CNT interfaces were shallower than those on the internal CNT interfaces, because of the curvature effect. Water vapor was therefore preferentially adsorbed in the internal nanopores rather than on the external interfaces. The potential-well depths at the centers of the 0.3, 0.4, and 0.7 nm gates were 7.3, 2.7, and 1.1 kJ mol−1, respectively. The penetration pathway of water via the 0.3 nm gates therefore provided considerable stability for water, giving a stabilized energy equivalent to that in the internal nanopores. In contrast, the water in the vicinity of the 0.4 and 0.7 nm gates was 5−6 kJ mol−1 less stable than that in the internal nanopores, which is significant compared with the kinetic energy of 2.5 kJ mol−1. It is concluded that water transport without an energy barrier via the 0.3 nm gates involves fast and consecutive water release, whereas transport via the 0.4 and 0.7 nm gates was considerably restricted. In summary, consecutive water transport via extremely narrow CNH 0D gates was observed and involved water vapor loading and release, whereas the water loading and release processes in hydrophobic nanopores typically showed significant adsorption hysteresis. CNHs have internal nanopores of size 2−3 nm and adsorb water vapor in the internal nanopores via wide 0D gates, giving adsorption hysteresis, i.e., nonconsecutive water vapor loading and release. The nonconsecutive processes changed to consecutive for extremely narrow 0D gates, which are similar in size to water molecules. Nonconsecutive water release via wide 0D gates is a result of water transfer during long-term equilibrium, whereas consecutive water transfer via the extremely narrow 0D gates is considered to occur during short-term equilibrium. MD simulation of water release from a CNT indicated fast water 8860
DOI: 10.1021/acs.jpcc.6b03142 J. Phys. Chem. C 2016, 120, 8855−8862
Article
The Journal of Physical Chemistry C
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transfer via the extremely narrow 0D gates without stability loss in the vicinity of the gate. The reduction in the energy barrier at the extremely narrow 0D gates supports anomalous consecutive water loading and release, involving restriction of water vapor transport via the extremely narrow 0D gates, with admission of condensed water transport via the gates. Strict control of the gate size is therefore crucial for management of water properties. The remarkable water transport achieved via 0D gates could provide novel nanoscale controllable chemical reactions.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b03142. Details of experimental and computational procedures, gate structure analysis, water vapor adsorption and desorption, and snapshots of molecular dynamics simulations (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Phone: +81-43-290-2779. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS I thank Dr. M. Yudasaka from the Nanotube Research Center, Advanced Industrial Science and Technology, Japan, and Prof. S. Iijima from Meijo University, Japan for supplying NHs. This research was supported by the Japan Society for the Promotion of Science KAKENHI Grant Numbers 26706001 and 15K12261, and research fellowships from the Kurita Water and Environment Foundation, and the Futaba Electronics Memorial Foundation.
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DOI: 10.1021/acs.jpcc.6b03142 J. Phys. Chem. C 2016, 120, 8855−8862
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DOI: 10.1021/acs.jpcc.6b03142 J. Phys. Chem. C 2016, 120, 8855−8862
