Electrokinetic Transport of Methanol and Lithium Ions Through a 2.25

Jan 6, 2017 - ... of Nanometer-Sized Objects Using Pyramidal-Shaped Nanopores Anal. ..... 9. Biance , A. L. Focused Ion Beam Sculpted Membranes for ...
0 downloads 0 Views 3MB Size
Download PDF
Article pubs.acs.org/JPCC

Electrokinetic Transport of Methanol and Lithium Ions Through a 2.25-nm-Diameter Carbon Nanotube Nanopore Mark D. Ellison,*,† Samuel Menges,† Laura Nebel,† Gabrielle D’Arcangelo,†,§ Anna Kramer,†,§ Lee Drahushuk,‡ Jesse Benck,‡ Steven Shimizu,‡ and Michael S. Strano‡ †

Department of Chemistry, Ursinus College, Collegeville, Pennsylvania 19426, United States Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States

‡

S Supporting Information *

ABSTRACT: The flow enhancement and molecular selectivity of carbon nanotubes make them unique nanopore conduits. In this study, we examine separately the transport of Li+ and methanol through a 2.25 nm-diameter, 200-μm long singlewalled carbon nanotube (SWNT). Threshold voltages of 200 mV and 700 mV were found for Li+ and methanol, respectively, to exhibit pore blocking. As the applied electric field was increased, the pore-blocking currents for Li+ and for methanol were both found to generally increase in the range of 1 to 6 pA. A simple volumetric model for methanol and hydrated Li+ is consistent with these observations. For applied voltages between 200 and 1000 mV, the dwell times for Li+ transport varied from 200 to 1200 ms and scaled linearly with inverse electric field. These results indicate an electrophoretic mobility of 1.6 × 10−7 m2 V−1 s−1, in agreement with previous measurements of alkali metal ions in SWNTs. Conversely, for applied voltages between 700 and 1000 mV, the dwell times for methanol remained relatively constant at an average of 88 ms, consistent with the expected behavior of a neutral molecule. The average velocity of methanol was found to be 2.3 × 10−3 m/s, which is in agreement with an electro-osmotic flow model of neutral molecules through a small-diameter nanopore. By comparing charged and neutral pore blocking species in this way, this approach promises to deconvolve the effects of electrokinetics and electro-osmotic flow in molecularly sized nanopore conduits.

■

INTRODUCTION There has been significant interest in measuring and understanding molecular transport through single, isolated pores of diameters commensurate with molecular size, including pores made from Si/SiN,1−7 SiC,8−11 track-etched polymers,12,13 graphene,14−18 and carbon nanotubes.19−26 It is known that nanopores require small diameters and high stability to be able to detect and selectively transport small ions or molecules27,28 and that flow properties depend on the nanopore’s material and dimensions. Research has already led to demonstrated success in applications such as sensing, separation, and protein and DNA identification.29−33 Although research has been performed on a variety of different pore materials, small-diameter carbon nanopores are of particular interest because they demonstrate exotic transport behavior, including flow enhancement, as well as deviations from the surface flow mechanisms that are characteristic of larger pores (generally those greater than 5 nm). Ionic transport through carbon nanotube nanopores with diameters greater than 5 nm resembles that of ions through Si and other inorganic nanopores because in both cases the flow exhibits separate bulk and surface contributions. However, deviations from this type of flow behavior in nanopores whose interior diameter approaches molecular dimensions indicate a confinement effect that should be further studied. Therefore, there is a strong need for measurements and analysis of carbon nanotube nanopores with © XXXX American Chemical Society

diameters below 5 nm to understand the effects of water confinement. Carbon nanotube nanopores have been used for the study of ion flow in nanoconfined environments,19−22,24 as well as protein and DNA translocation.23 To date, researchers have primarily studied the motion of charged particles through carbon nanotube nanopores, so the motion of neutral molecules, aside from water,34−44 has yet to be studied in detail. Several molecular dynamics simulations have predicted that small neutral molecules such as methanol should pass through the interior of a single-walled carbon nanotube (SWNT),45,46 and it is important to test these predictions experimentally. Furthermore, the flow of small neutral molecules, such as methanol, through nanometer-scale pores under an applied electric field is of particular significance. In fuel cells, for instance, the motion of water by electro-osmotic drag through polymeric membranes plays a crucial role in device performance.47−49 Our carbon nanotube nanopore devices, therefore, provide a well-characterized platform for studying the details of the electrokinetic motion of molecules in nanoconfined environments. In this work, we report experimental evidence for the transport of methanol through a 2.25 nm diameter (RamanReceived: December 1, 2016 Published: January 6, 2017 A

DOI: 10.1021/acs.jpcc.6b12104 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C

Figure 1. (a) Schematic of the device construction and electrode operation. (b) Optical microscope picture of the multiwell device. The specific reservoirs used in these experiments are highlighted in the red rectangle. The ruler at right has markings in millimeters. (c) SEM image of the Si wafer before the PDMS mask was attached. (d) Enlarged section of panel b, showing region of PDMS mask (red rectangle) and SWNTs (light gray vertical lines) (e) Raman spectrum of wider-diameter section SWNT connecting reservoirs showing RBM region (main) and G region (inset). (f) Raman spectrum of narrower-diameter section SWNT connecting reservoirs showing RBM region (main) and G region (inset).

volumetric hydration model for methanol and hydrated Li+ is consistent with these observations. However, whereas the dwell times for Li+ transport varied linearly with inverse electric field, the dwell times for methanol remained invariant with electric field. The Li+ dwell time results indicate that it has a constant and positive electrophoretic mobility, whereas the methanol dwell time data are consistent with those expected for a neutral blocker. The results are compared in terms of electrokinetic and electro-osmotic flow effects in molecularly sized nanopore conduits.

measured, via the radial breathing mode), 200-μm-long SWNT nanopore and compare its kinetics to those of Li+ measured in the same nanopore device. This allows us to compare and contrast the behaviors of a nominally charged blocker (Li+) and an uncharged blocker (CH3OH) to expand our understanding of the transport of small neutral molecules and to better characterize motion in nanoconfined environments. Li+ was chosen for comparison because its transport through SWNTs was investigated in several previous studies.19,21 A very important aspect of our work is its effort to assign a specific tube diameter to the conduit under investigation, a detail not typically provided in previous studies. Transport in carbon nanotubes with diameters of 2 nm or less exhibits highly nonmonotonic trends with diameter,21 underscoring the importance of diameter assignment. In the current work, we found that the Li+ and methanol pore-blocking currents generally increase with applied electric field, as expected for blockers with similar molecular sizes. Indeed, a simple

■

EXPERIMENTAL DETAILS The SWNT nanopore fabrication process follows a procedure that has been previously described,19−21 and full details are given in the Supporting Information. Briefly, a catalyst solution was applied to the edge of a rectangular piece of silicon wafer (Figure 1a). Aligned SWNTs were grown via a chemical vapor deposition (CVD) process with CH4 as the carbon feedstock. B

DOI: 10.1021/acs.jpcc.6b12104 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C

Once it was confirmed that the SWNTs remained after the PDMS mask had been removed, Raman spectra of the SWNTs were obtained and are shown in Figure 1, parts e and f. These spectra display the characteristic G peak of graphitic carbon near 1600 cm−1 and the radial breathing modes (RBM) of carbon nanotubes in the range of 80−200 cm−1. The RBM are of particular interest because the diameter, d (in nanometers) can be found from the RBM frequency, ωRBM, using the relation d = 248/ωRBM, which is valid for isolated carbon nanotubes on a silicon dioxide substrate.52 Raman spectra were collected at several locations along the SWNT. At one location, the spectrum (Figure 1e) showed a peak at 96.5 cm−1, which corresponds to a diameter of 2.57 nm. At several other locations, the spectra (Figure 1f) showed a peak at 110 cm−1, which corresponds to a diameter of 2.25 nm. These different spectra indicate that the SWNT changed chirality and diameter during the growth process, which complicates the diameter identification somewhat. Given that more spectra were collected with peaks at 110 cm−1, 2.25 nm appears to be the diameter over much of the length of this segment of SWNT. Additionally, 2.25 nm is a narrower diameter and, as the lower value, would present a size limit for entering ions or molecules. Therefore, we characterize the SWNT connecting the reservoirs as having a diameter of 2.25 nm along most of its length, knowing that, at one point, it widens to 2.57 nm. The nanotube devices were mounted on optical mounting hardware (Newport, Thorlabs) on a nitrogen-cushioned vibration isolation table surrounded by a Faraday cage (Kinetic Systems). This low-noise setup allows pore-blocking events with amplitudes as low as 2 pA to be distinguished, as illustrated in Figure S6. The electrodes used were Ag/AgCl formed by immersing Ag wire in bleach for 20 min. The voltage applied across the SWNT was controlled using a Molecular Devices Axopatch 200B, and the current was collected using a Molecular Devices Digidata 1550 D/A converter. Data collection was performed using Clampex software (Molecular Devices, 2-kHz Bessel low-pass filter, 250-kHz acquisition frequency) while pore-blocking current and dwell times were obtained from the current data with ClampFit software. (Molecular Devices) Before analysis, the data were filtered in the ClampFit software with a Boxcar low-pass filter using 21 smoothing points. The ClampFit software was used to automatically extract the pore-blocking current (PBC, magnitude of current decrease) and the dwell time (length in time of current decrease) of poreblocking events. A square wave model of pore-blocking events is based on the hypothesis that the blocker suddenly disrupts the current through the pore, and the current is quickly reestablished when the blocker exits the pore. Because the software models the events as square waves, a standard protocol was developed in previous studies19−22,53 for classifying poreblocking events based on how closely they conform to the case of an ideal square waveform. This standard categorization system is described in more detail in the Supporting Information. The statistical analysis of the valid pore-blocking current and dwell time data points was performed in Igor (Wavemetrics). Solutions of the different analytes were made by the following steps: Methanol (Aldrich, 99.5%) was dissolved in ultrapure water to a concentration of 1.0 M. Concentrated hydrochloric acid was added in 10-μL increments until the pH was measured to be 3.0 with a pH meter. Separately, lithium chloride (Aldrich, ≥ 99.99%), was dissolved in ultrapure water,

The SWNTs were located and characterized using scanning electron microscopy and Raman spectroscopy. In contrast to our previous work with SWNT nanopores, in these studies a multiwell device (Figure 1b) was created by gluing a polydimethylsiloxane (PDMS, Dow Sylgard) mask with regularly arranged openings to the wafer. Then, the device was briefly exposed to an oxygen plasma, which removed uncovered sections of the SWNTs and is known to produce carboxylic acid functional groups on the ends of the remaining SWNT segments.50 The reservoirs were then thoroughly rinsed with ultrapure water (Millipore, 18.2 MΩ·cm). Several views of the multiwell device are shown in Figure 1. Figure 1a shows a schematic of the construction process, and Figure 1b shows a light microscope image of the device with the PDMS overlayer next to a ruler with millimeter-spaced markings. In experiments using this device, many adjacent reservoir combinations were tested. The combination that reliably gave results were reservoirs separated by a 200 ± 15 μm thick section of PDMS, indicated by the red box. The SWNT length is assumed to be the same as the width of the PDMS. All results from the device reported herein were obtained with this 200 ± 15 μm long SWNT under this section of PDMS. Figure 1c shows a scanning electron microscopy (SEM) image of the silicon wafer prior to the attachment of PDMS. For ease of viewing, the locations of SWNTs are indicated with red lines. An enlarged version of this exact SEM image is shown in Figure S1. Figure 1d shows an enlarged region of the SEM image. The red box indicates the eventual location of the portion of the PDMS layer that separated the two reservoirs. Two SWNTs are observable as light gray vertical lines. It is possible that one or both of these SWNTs connected the reservoirs that were tested in these experiments. Although both of the SWNTs are near the opening of the well, the SWNT on the left appears to be in a region completely covered by the PDMS mask/glue. Therefore, we have determined that the observed transport occurs entirely through the SWNT on the right. We did not observe any evidence of a three-state system in the poreblocking current that is characteristic of two simultaneously active SWNT, as reported previously for a different device.20 An important advantage of the multiwell device over a twowell device is that a single SWNT is divided into 8−10 segments along its length, providing more opportunities to find active sections of the SWNT. This approach increases the probability of finding an active segment of an SWNT. Additionally, it allows for control experiments to be performed on the same device. Adjacent reservoirs that are not connected by an SWNT or are connected by an SWNT that is inactive to ion transport can easily be tested. Several such control experiments were indeed performed, and their results are shown in Figures S6−S8. In addition to SEM, the SWNTs on the Si wafer were characterized using Raman spectroscopy (Horiba LabRAM HR, 532 nm laser). This technique confirms the presence of SWNTs and also provides chirality and diameter information.51,52 Because the transport properties strongly depend on SWNT diameter,21 knowledge of the nanotube diameter is very important for understanding and interpreting results and also makes reproduction of these studies possible. After the complete set of current measurements were completed, the PDMS mask was removed and the Raman spectra for this device were collected. Details of the mask removal are given in the Supporting Information. C

DOI: 10.1021/acs.jpcc.6b12104 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C

Figure 2. (a) Current traces of 0.1 M LiCl solution at pH = 3.0 in the device at various voltages, showing pore-blocking events over many seconds. (b) Dependence of pore-blocking current on applied voltage for 0.1 M LiCl. No pore blocking was observed below 200 mV, so no PBC data points are displayed below that value. (c) Dependence of dwell time of 0.1 M LiCl on the inverse of applied voltage. The line is the best fit to the data. The linear relationship demonstrates that the Li+ ions have a constant electrophoretic mobility.

possible that proton current could flow around the Li+ as it moves through the tube, and the extent of disruption of the proton current depends on the specific dynamical details of each event. For example, an ion traveling near the center of the tube could have a different PBC than an ion traveling near the wall of the tube. It is also possible that different Li+ ions have different solvation shells when they enter the SWNT or that their solvation shells respond differently to the motion of the ion through the SWNT. Each of these factors would cause variations in the PBC that are independent of applied voltage. On the other hand, the wetting/dewetting of hydrophobic nanopores depends on applied voltage,13 as does the establishment of the organized structure of water molecules within the SWNT, which is necessary for the current flow.43,55 We hypothesize that a combination of all these dynamical factors lead to variations in the PBC that are exhibited in the scatter in the data in Figure 2b. It was also observed that the current remained at the same magnitude but reversed sign when the voltage was switched to the corresponding negative value. The sign of the current indicates that the charge carriers are positively charged, that is, are protons and not hydroxide ions. Dwell time results for Li+ in the device are shown in Figure 2c. The plot of dwell time versus the reciprocal of applied voltage is linear, as expected for an ion with constant electrophoretic mobility. This result is also significant because it strongly suggests that the nanopore in this system is an SWNT. Because our device is constructed with PDMS, it is important to consider the possibility that the nanopore might be a groove or fold in the PDMS. However, Li+ in SWNT nanopores exhibits a dwell time that is inversely proportional to voltage,19,21 whereas Li+ in PDMS nanopores exhibits dwell times that are essentially independent of voltage, attributed to bubble formation through spontaneous dewetting and rupture in previous studies of such systems.54 Thus, the Li+ data (both

and concentrated hydrochloric acid was added in 10-μL increments until the pH was 3.0.

■

RESULTS AND DISCUSSION Because the behavior of alkali metal ions in SWNT nanopores has been well-characterized,19−21 the device was first tested with solutions of 0.1 M LiCl. Solutions were added by micropipette to fill a reservoir on either side of the SWNT. These reservoirs were thoroughly rinsed with ultrapure water between experiments. Traces of current fluctuations at various voltages are shown in Figure 2a. These data show pore-blocking events, which are characterized by a sudden decrease in the current caused by a blocker (ion or molecule) entering the SWNT and disrupting the current flow through the tube. From a brief inspection, these data show two trends: First, the poreblocking current (PBC, i.e., the magnitude of the current decrease) increases with increasing voltage. Second, the dwell time (duration of current decrease) decreases with increasing voltage, as observed and quantified previously.21 The pore-blocking currents and dwell times for Li+ were studied as a function of applied voltage. The PBC results are displayed in Figure 2b. The points show the average PBC and the error bars show the standard deviations. Although the results exhibit significant scatter and the PBC does not steadily increase with increasing voltage, the PBC at the highest voltages is statistically significantly higher than the PBC at the lowest voltages. This overall trend is similar to the previously observed behavior exhibited by Li+ in SWNT19,21 and PDMS54 nanopores. The variation in average PBC as the voltage increases is due to several factors. First, the data reflect variations in dynamical behavior from event to event. Second, the 2.25 nm-diameter nanotube is larger than SWNTs previously studied, and the Li+ does not necessarily block the entire tube cross section, as illustrated in Figure 4d. It is D

DOI: 10.1021/acs.jpcc.6b12104 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C

Figure 3. (a) Raw current data, shown prior to filtering and smoothing for analysis, for 1.0 M methanol at different voltages. (b) Current data for− 1000 mV. Solution was 1 M methanol at pH = 3.0. The upper level is the unblocked high-current state, and the lower level is the blocked low-current state. The current histogram shows a characteristic signature of two states. The view at the right zooms in on a single event. (c) Voltage dependence of pore-blocking current. Error bars show the standard deviations of all events recorded at each voltage. No pore blocking was observed below 700 mV, so no PBC data points are displayed below that value. (d) Voltage dependence of dwell time. Error bars show the standard deviations of all events recorded at each voltage.

Figure 4. Histogram of conductance changes in pS of all events recorded at each voltage for (a) Li+, and (b) methanol. (c) Geometry of a 4coordinate hydrated Li+ ion for estimating its volume. (d) Space-filling model of 4-coordinate hydrated Li+ ion in a 2.24 nm diameter SWNT. (e) Space-filling model of a methanol molecule in a 2.24 nm diameter SWNT.

with the value of 9.6 × 10−8 m2 V−1 s−1 measured in nanotube membranes.57 The mobility of ions through SWNTs is strongly dependent on the diameter of the SWNT,21 which likely accounts for the differences observed here. Current fluctuations observed in methanol solutions in the device are shown in Figure 3a. Under an applied voltage, there is a steady current observed, which decreases when a particle passes through the SWNT and temporarily disrupts that current. Numerous control experiments were performed to rule out other effects as sources of the current fluctuations and to establish methanol as the current blocker. Specifically, experi-

PBC and dwell time) strongly suggest that the nanopore connecting the tested reservoirs is an SWNT. Using the slope of the fit in Figure 2c, the average electrophoretic mobility of Li+ is determined to be 1.6 × 10−7 m2 V−1 s−1. This is slightly lower than the average Li+ mobility of 8 × 10−6 m2 V−1 s−1 previously found in singleSWNT nanopores19 and of 1 × 10−5 m2 V−1 s−1 found in multiple-SWNT20 nanopores. However, it is higher than the cation’s mobility in bulk water and the theoretical mobility through a 3 nm-diameter SWNT56 (both ∼10−8 m2 V−1 s−1). It is also slightly higher than but in fairly reasonable agreement E

DOI: 10.1021/acs.jpcc.6b12104 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C

attribute the smaller conductance changes to the large 2.25 nm diameter of the SWNT in these experiments. We hypothesize that the disruption of the current through the SWNT depends on the volume of the object traversing through the nanotube, so the ratio of the average PBC for Li+ to that of methanol would be expected to be equal to the ratio of the volume of hydrated Li+ to that of methanol. Specifically, Mähler and Persson found that 4-coordinate, solvated lithium ions have an average Li−O distance of about 1.9 Å.58 A sketch of this complex ion is shown in Figure 4c. Using that distance, the molecular volumes of 4-coordinate solvated Li+ and methanol were computed using the computational program Spartan ’16, (Wavefunction, Inc.) which uses van der Waals radii to calculate volumes. Full details of these volume calculations are given in the Supporting Information. The volume ratio for Li+ solvated with four water molecules to the volume of methanol alone and methanol with one solvating water is 2.0 and 1.4, respectively. (See Supporting Information for details of volume calculation.) As an estimate, these values are in good agreement with the observed ratio of 1.8 for the average conductance change of Li+ to that of methanol. Thus, this simple model of molecular volume predicts that Li+ and methanol should have similar magnitudes of conductance change, with Li+ being somewhat higher, in approximate quantitative agreement with the conductance change data. Parts d and e of Figure 4 show space-filling models of a 4-coordinate solvated lithium ion and a methanol molecule, respectively, inside a 2.24 nm SWNT. This approximate model may underestimate the size of solvated Li+ because it does not include a possible second, outer solvation shell. A second solvation shell would increase the effective volume, but removal of an outer, more loosely held solvation shell is probable upon entry into the SWNT.59,60 Additionally, the inner solvation shell is likely to be disrupted and undergo dynamic changes not captured by this static model. Finally, it is difficult to know how many water molecules would solvate a methanol molecule as it enters a SWNT nanopore. The fact that this model provides reasonable agreement with the conductance changes suggests that the solvation numbers of four and zero or one for lithium and methanol, respectively, are good approximations for the average behavior of these particles in the SWNT. Therefore, this simple model is effective for understanding the conductance changes of pore blocking in SWNTs. An important result of this work is that the dwell times of methanol molecules were found to be independent of applied voltage as expected for an approximately neutral blocker. The methanol molecules should have a neutral charge and therefore not exhibit a velocity in the applied electric field. If the motion of methanol through the SWNT is assumed to be due to diffusion, using the average dwell time and the length of the SWNT as the mean-square displacement would give a diffusion coefficient of D = 0.020 cm2/s for methanol. This is significantly (4 orders of magnitude) greater than the experimentally measured value of 7.1 × 10−6 cm2/s for diffusion of methanol in bulk water61 and the computed selfdiffusion constant of 10−7 cm2/s for methanol in an SWNT.46 The dwell times, then, indicate motion of neutral particles whose velocity is too high to be explained by diffusion. Based on the measured dwell times, we calculated the average velocity of methanol through the SWNTs. For our device, the length of the PDMS barrier was measured to be 200 ± 15 μm. With the average dwell time of 87.7 ms, the average

ments with ultrapure water in the reservoirs, ultrapure water with the pH adjusted to 3.0, methanol solutions in reservoirs in the same device that were not connected by an SWNT all did not demonstrate any current fluctuations like those in Figure 3a. Representative current traces from these control experiments are presented in the Supporting Information (Figures S6−S8). The results of these control experiments support our hypothesis of a proton current through the SWNT that is blocked when a methanol molecule enters the nanotube. These results suggest that the methanol molecules enter and transport through the SWNT. We hypothesize that methanol molecules randomly diffuse into the region near the pore mouth. Once in that space, the molecules can collide with protons that are moving toward the pore mouth, pushing the methanol in that direction. Methanol molecules will also be attracted to the protons through their oxygen atoms, so a combination of collisions and this attraction can move molecules into and through the SWNT. The pore-blocking current (PBC) and dwell time were studied as functions of applied voltage. The PBC results for the device are displayed in Figure 3b. In Figure 3c, the average PBC at each voltage is displayed, and the error bars show the standard deviations of the distributions of measured values. As was found with Li+, the PBC generally increases with increasing voltage, indicating that the current carriers are electrically charged. This is in agreement with previous studies in which hydrogen ions were identified as the primary current carriers.19,21 We therefore conclude that hydrogen ions are the current carriers in the methanol solutions used in this research. As shown in Figure 3c, no pore blocking by methanol is apparent below 700 mV applied voltage. This observation of a threshold voltage is in good agreement with results of molecular dynamics simulations for methanol entering SWNT nanopores with carboxylic acid groups at the entrance.46 Hydrogen bonding between the methanol molecules and carboxylic acid groups results in an activation barrier for entrance of the methanol into the SWNT. Below 700 mV, the electroosmotic flow of the protons does not impart enough force to separate the methanol molecules from the carboxylic acid moieties. Consequently, the methanol molecules do not enter the SWNT, and pore blocking is not observed. Panel d of Figure 3 shows the average dwell time of methanol at different applied voltages. The data do not show a trend, indicating a fairly constant velocity of methanol molecules through the SWNT. The data at 800 mV (Figure 3a) showed several long events and many short events. This disparity led to the large error bars at 800 mV. The longer events are valid and should not be excluded from the data set, so they were included in the analysis. They are, however, the reason for the large error bars at 800 mV in Figure 3d. Panels a and b of Figure 4 display histograms of all of the conductance changes, ΔG, measured for all voltages for Li+ and methanol, respectively. The average conductance change for methanol is 3.5 ± 1.8 pS, which is somewhat smaller than those found in previous work.21 It is, however, of the same order of magnitude as the 6.2 ± 2.4 pS conductance change observed for Li+ in the same device. These results for methanol and Li+ are consistent with previous studies of the diameter dependence of conductance changes. Both experimental results and a theoretical model find that conductance change should reach a maximum for SWNTs with a diameter of about 1.6 nm and steadily decline as the diameter increases.21 Therefore, we F

DOI: 10.1021/acs.jpcc.6b12104 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C velocity of methanol molecules through this SWNT is 2.3 × 10−3 ± 1.7 × 10−4 m/s. In contrast to methanol, alkali metal ions exhibit different pore-blocking behavior. In agreement with previous work,19,21 solutions of LiCl exhibited PBCs that are directly proportional to applied voltage and dwell times that are inversely proportional to applied voltage. A dwell time that is inversely proportional to the voltage is expected for a charged particle that has a constant electrophoretic mobility. The fact that these control experiments showed expected behavior for pore-blocking by Li+ ions strongly suggests that the methanol results were not caused by experimental artifacts or electronic noise. Electro-osmotic flow is known to be responsible for water’s motion accompanying protons through membranes in fuel cells. To our knowledge, only one model for electo-osmotic flow through carbon nanotubes has been developed. Miller, Young, and Martin used the Helmholtz−Smoluchowski equation to model the electro-osmotic flow through multiwalled carbon nanotubes (MWNTs) that were ∼120 nm wide.62 Their MWNTs were much larger in diameter than the SWNTs in our research, so the applicability of their model to our system is questionable. On the other hand, Berg and Ladipo developed an analytical solution to the Poisson− Boltzmann equation for cylindrical channels in polymer membranes.63 Although their channel walls are of different material, the size of their channels, ∼ 3 nm, is much closer to our SWNT diameter. Moreover, they developed their model to describe the electro-osmotic flow of molecules from a proton current through a cylindrical channel, which is quite similar to our system. Because of these similarities, we compared their model’s predictions to our results. Specifically, Berg and Ladipo derived an equation for the average velocity of molecules in an electro-osmotic flow u̅ ≈

c0qER2 4μ

Supporting Information.) This small surface charge agrees with our measured velocities because the interaction between methanol and SWNT walls is through weak van der Waals forces45,46,66 and not the charge−charge interaction assumed by the model. Additionally, the surface charge of 0.04 C/m2 is very similar to the surface charge used in simulations of ion motion through MWNTs.67 It is also consistent with molecular dynamics simulations that find that small charges on SWNTs facilitate filling the nanotube with water, but large charges hinder internal transport.34 Finally, it is also similar to the Mulliken charges computed to be induced by acetone molecules inside a SWNT.68 This suggests that the water molecules and protons in the nanotube induce a slight polarization of the electron cloud of the inner wall of the nanotube, which affects the motion of the methanol molecules through the SWNT. This good agreement between our results and the model of Berg and Ladipo, then, suggests further exploration of the model’s utility for understanding electroosmotic flow through SWNT nanopores. Comparison of the average methanol velocity in SWNTs to other results is difficult because this work is one of the first studies of the motion of neutral molecules through individual nanotubes. A study by Majumder, Chopra, and Hinds of pressure-induced liquid flow through carbon nanotube membranes found flow velocities of 26.1 cm/s (high pressure) and 10.9 cm/s (low pressure) for water and 4.5 cm/s (high pressure) for ethanol.25 These values are higher than our experimental value of 0.23 cm/s for methanol. However, the pressure-driven flow in those experiments is expected to be faster because the electric field force on a positively charged ion applied across the 2.25-nm diameter of this SWNT corresponds to a pressure of about 200 Pa. The effective pressure on an uncharged methanol molecule is expected to be significantly lower. In contrast, the pressures used by Majumder et al. ranged from 103 to 105 Pa. Additionally, the nanotubes in the membranes of Majumder et al. had larger diameters of ∼7 nm as compared to 2.25 nm in this work. Hinds et al. also found that caffeine molecules would traverse through carbon nanotube membranes with velocities up to 0.036 cm/s.69 This velocity is somewhat less than our result for methanol, with an important difference being that the largerdiameter nanotubes in their membranes are necessary to accommodate the large caffeine molecules. Additionally, we expect the velocity of methanol to be greater because of its lower mass and the observed threshold voltage. Hinds et al. observed caffeine translocation through their nanotube membranes even with no applied voltage,69 indicating that thermal energy is sufficient to overcome any activation energy barrier to caffeine entering the pores in their membranes. In contrast, the observed threshold voltage for methanol in our SWNT nanopore indicates that the methanol molecules must have sufficient kinetic energy to be able to enter and move through the SWNT.

(1)

where c0 is the concentration of charge carriers along the cylindrical axis of the channel, q is the charge of the proton (charge carrier), E is the electric field gradient (V/m), R is the radius of the channel, and μ is the viscosity of the solution. As shown in eq 2, c0 depends on the surface charge of the channel, the channel radius, the dielectric constant of the solution, and the temperature. c0 =

8σs 2 2

q R σs/εε0kBT − 4qR

(2)

Specifically, σs is the surface charge density in C/m , q is the charge of an electron, R is the radius of the nanopore in m, ε is the dielectric constant, ε0 is the permittivity of vacuum, kB is the Boltzmann constant, and T is the temperature of the system. The dielectric constant of a 1.0 M methanol solution is ε = 77,64 and its viscosity is μ = 9 mP.65 Additionally, we used R = 1.125 × 10−10 m and E = 1 V/200 μm = 5000 V/m. We find that using eq 2 with a surface charge of 0.04 C/m2, the model estimates the electro-osmotic flow velocity of methanol molecules to be 2.4 × 10−3 m/s, which compares quite well with our experimentally determined average methanol velocity of 2.3 × 10−3 m/s. This is indeed very good agreement, considering the different channel material for which Berg and Ladipo developed their model. Additionally, for a 2.25 nm diameter SWNT, 0.04 C/m2 corresponds to about 0.013 electrons per C atom. (Complete calculations are shown in the 2

■

CONCLUSIONS In conclusion, we have demonstrated the motion of single neutral molecules through a SWNT nanopore. The dwell time of the methanol molecules is independent of the applied voltage, as would be expected for a neutral molecule. The blocking of current through the SWNT is related to the molecular volume of the blocker, and the velocity of the methanol molecules is reasonably well described by a model based on analytical solution of the Poisson−Boltzmann G

DOI: 10.1021/acs.jpcc.6b12104 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C

Membrane at High Temperature. Adv. Mater. Res. 2007, 26−28, 271−4. (12) Powell, M. R.; Sullivan, M.; Vlassiouk, I.; Constantin, D.; Sudre, O.; Martens, C. C.; Eisenberg, R. S.; Siwy, Z. S. NanoprecipitationAssisted Ion Current Oscillations. Nat. Nanotechnol. 2008, 3, 51−7. (13) Powell, M. R.; Cleary, L.; Davenport, M.; Shea, K. J.; Siwy, Z. S. Electric-Field-Induced Wetting and Dewetting in Single Hydrophobic Nanopores. Nat. Nanotechnol. 2011, 6, 798−802. (14) Garaj, S.; Hubbard, W.; Reina, A.; Kong, J.; Branton, D.; Golovchenko, J. A. Graphene as a Subnanometre Trans-Electrode Membrane. Nature 2010, 467, 190−193. (15) Siwy, Z. S.; Davenport, M. Nanopores: Graphene Opens up to DNA. Nat. Nanotechnol. 2010, 5, 697−698. (16) Schneider, G. F.; Xu, Q.; Hage, S.; Luik, S.; Spoor, J. N. H.; Malladi, S.; Zandbergen, H.; Dekker, C. Tailoring the Hydrophobicity of Graphene for Its Use as Nanopores for DNA Translocation. Nat. Commun. 2013, 4, 2619. (17) Schneider, G. F.; Kowalczyk, S. W.; Calado, V. E.; Pandraud, G.; Zandbergen, H. W.; Vandersypen, L. M. K.; Dekker, C. DNA Translocation through Graphene Nanopores. Nano Lett. 2010, 10, 3163−3167. (18) Merchant, C. A.; et al. DNA Translocation through Graphene Nanopores. Nano Lett. 2010, 10, 2915−2921. (19) Lee, C. Y.; Choi, W.; Han, J.-H.; Strano, M. S. Coherence Resonance in a Single-Walled Carbon Nanotube Ion Channel. Science 2010, 329, 1320−4. (20) Choi, W.; Lee, C. Y.; Ham, M.-H.; Shimizu, S.; Strano, M. S. Dynamics of Simultaneous, Single Ion Transport through Two SingleWalled Carbon Nanotubes: Observation of a Three-State System. J. Am. Chem. Soc. 2011, 133, 203−205. (21) Choi, W.; Ulissi, Z.; Shimizu, S.; Bellisario, D.; Ellison, M. D.; Strano, M. S. Diameter-Dependent Ion Transport through the Interior of Isolated Single-Walled Carbon Nanotubes. Nat. Commun. 2013, 4, 2397. (22) Ulissi, Z. W.; Shimizu, S.; Lee, C. Y.; Strano, M. S. Carbon Nanotubes as Molecular Conduits: Advances and Challenges for Transport through Isolated Sub-2 Nm Pores. J. Phys. Chem. Lett. 2011, 2, 2892−2896. (23) Liu, H.; He, J.; Tang, J.; Liu, H.; Pang, P.; Cao, D.; Krstic, P.; Joseph, S.; Lindsay, S.; Nuckolls, C. Translocation of Single-Stranded DNA through Single-Walled Carbon Nanotubes. Science 2010, 327, 64−67. (24) Pang, P.; He, J.; Park, J. H.; Krstic, P. S.; Lindsay, S. Origin of Giant Ionic Currents in Carbon Nanotube Channels. ACS Nano 2011, 5, 7277−83. (25) Majumder, M.; Chopra, N.; Hinds, B. J. Mass Transport through Carbon Nanotube Membranes in Three Different Regimes: Ionic Diffusion and Gas and Liquid Flow. ACS Nano 2011, 5, 3867− 77. (26) Majumder, M.; Zhan, X.; Andrews, R.; Hinds, B. J. Voltage Gated Carbon Nanotube Membranes. Langmuir 2007, 23, 8624− 8631. (27) Fornasiero, F.; Park, H. G.; Holt, J. K.; Stadermann, M.; Grigoropoulos, C. P.; Noy, A.; Bakajin, O. Ion Exclusion by Sub-2-Nm Carbon Nanotube Pores. Proc. Natl. Acad. Sci. U. S. A. 2008, 105, 17250−17255. (28) MacKinnon, R. Potassium Channels and the Atomic Basis of Selective Ion Conduction (Nobel Lecture). Angew. Chem., Int. Ed. 2004, 43, 4265−4277. (29) Thomas, J. M.; Raja, R. Nanopore and Nanoparticle Catalysts. Chem. Rec. 2001, 1, 448−466. (30) Keyser, U. F. Controlling Molecular Transport through Nanopores. J. R. Soc., Interface 2011, 8, 1369−78. (31) Mulero, R.; Prabhu, A. S.; Freedman, K. J.; Kim, M. J. Nanopore-Based Devices for Bioanalytical Applications. JALA 2010, 15, 243−252. (32) Robertson, J. W. F.; Rodrigues, C. G.; Stanford, V. M.; Rubinson, K. A.; Krasilnikov, O. V.; Kasianowicz, J. J. Single-Molecule

equation. An increased understanding of the motion of small neutral molecules through SWNT nanopore offers the potential to construct single-molecule reactors or single-molecule sensing devices.

■

ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b12104. Details of device fabrication, SEM images, Raman spectra, methanol current data, pore-blocking analysis and event types, control experiments, geometry of hydrated Li+ ion, calculating the average methanol velocity, and calculation of surface charge on SWNT (PDF)

■

AUTHOR INFORMATION

Corresponding Author

*(M.D.E.) E-mail: [email protected]. Present Address §

Student at Agnes Irwin High School, Rosemont, PA 19010

Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was supported by National Science Foundation Awards CHE-1306349 and CHE-1306529. The authors thank Dr. Daichi Kozawa for collecting AFM images and Erica Ellison for critical proofreading of the manuscript.

■

REFERENCES

(1) Smeets, R. M.; Keyser, U. F.; Dekker, N. H.; Dekker, C. Noise in Solid-State Nanopores. Proc. Natl. Acad. Sci. U. S. A. 2008, 105, 417− 21. (2) Arjmandi, N.; Van Roy, W.; Lagae, L.; Borghs, G. Measuring the Electric Charge and Zeta Potential of Nanometer-Sized Objects Using Pyramidal-Shaped Nanopores. Anal. Chem. 2012, 84, 8490−6. (3) Smeets, R. M.; Keyser, U. F.; Krapf, D.; Wu, M. Y.; Dekker, N. H.; Dekker, C. Salt Dependence of Ion Transport and DNA Translocation through Solid-State Nanopores. Nano Lett. 2006, 6, 89−95. (4) Kowalczyk, S. W.; Dekker, C. Measurement of the Docking Time of a DNA Molecule onto a Solid-State Nanopore. Nano Lett. 2012, 12, 4159−63. (5) Langecker, M.; Pedone, D.; Simmel, F. C.; Rant, U. Electrophoretic Time-of-Flight Measurements of Single DNA Molecules with Two Stacked Nanopores. Nano Lett. 2011, 11, 5002−7. (6) Zhao, Q.; Comer, J.; Dimitrov, V.; Yemenicioglu, S.; Aksimentiev, A.; Timp, G. Stretching and Unzipping Nucleic Acid Hairpins Using a Synthetic Nanopore. Nucleic Acids Res. 2008, 36, 1532−41. (7) Zhao, Q.; Sigalov, G.; Dimitrov, V.; Dorvel, B.; Mirsaidov, U.; Sligar, S.; Aksimentiev, A.; Timp, G. Detecting Snps Using a Synthetic Nanopore. Nano Lett. 2007, 7, 1680−5. (8) Look, D. C.; Fang, Z. Q.; Soloviev, S.; Sudarshan, T. S.; Boeckl, J. J. Anomalous Capture and Emission from Internal Surfaces of Semiconductor Voids: Nanopores in Sic. Phys. Rev. B: Condens. Matter Mater. Phys. 2004, 69, 195205. (9) Biance, A. L.; et al. Focused Ion Beam Sculpted Membranes for Nanoscience Tooling. Microelectron. Eng. 2006, 83, 1474−1477. (10) Gogotsi, Y.; Nikitin, A.; Ye, H.; Zhou, W.; Fischer, J. E.; Yi, B.; Foley, H. C.; Barsoum, M. W. Nanoporous Carbide-Derived Carbon with Tunable Pore Size. Nat. Mater. 2003, 2, 591−594. (11) Kim, Y.; Kim, E. B.; Kim, S. R.; Suh, M. H.; Choi, D. J.; Kwon, W. T. Hydrogen Separation Characteristics of Sic Nanoporous H

DOI: 10.1021/acs.jpcc.6b12104 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C Mass Spectrometry in Solution Using a Solitary Nanopore. Proc. Natl. Acad. Sci. U. S. A. 2007, 104, 8207−8211. (33) Xie, P.; Xiong, Q.; Fang, Y.; Qing, Q.; Lieber, C. M. Local Electrical Potential Detection of DNA by Nanowire-Nanopore Sensors. Nat. Nanotechnol. 2012, 7, 119−125. (34) Alexiadis, A.; Kassinos, S. Molecular Simulation of Water in Carbon Nanotubes. Chem. Rev. 2008, 108, 5014−34. (35) Cao, D.; Pang, P.; He, J.; Luo, T.; Park, J. H.; Krstic, P. S.; Nuckolls, C.; Tang, J.; Lindsay, S. Electronic Sensitivity of Carbon Nanotubes to Internal Water Wetting. ACS Nano 2011, 5, 3113−3119. (36) Dellago, C.; Naor, M. M.; Hummer, G. Proton Transport through Water-Filled Carbon Nanotubes. Phys. Rev. Lett. 2003, 90, 105902. (37) Kalra, A.; Garde, S.; Hummer, G. Osmotic Water Transport through Carbon Nanotube Membranes. Proc. Natl. Acad. Sci. U. S. A. 2003, 100, 10175−10180. (38) Kolesnikov, A. I.; Zanotti, J.-M.; Loong, C.-K.; Thiyagarajan, P.; Moravsky, A. P.; Loutfy, R. O.; Burnham, C. J. Anomalously Soft Dynamics of Water in a Nanotube: A Revelation of Nanoscale Confinement. Phys. Rev. Lett. 2004, 93, 035503. (39) Maniwa, Y.; Kataura, H.; Abe, M.; Suzuki, S.; Achiba, Y.; Kira, H.; Matsuda, K. Phase Transition in Confined Water inside Carbon Nanotubes. J. Phys. Soc. Jpn. 2002, 71, 2863. (40) Maniwa, Y.; Kataura, H.; Abe, M.; Udaka, A.; Suzuki, S.; Achiba, Y.; Kira, H.; Matsuda, K.; Kadowaki, H.; Okabe, Y. Ordered Water inside Carbon Nanotubes: Formation of Pentagonal to Octagonal IceNanotubes. Chem. Phys. Lett. 2005, 401, 534−8. (41) Marti, J.; Gordillo, M. C. Time-Dependent Properties of Liquid Water Isotopes Adsorbed in Carbon Nanotubes. J. Chem. Phys. 2001, 114, 10486−92. (42) Pascal, T. A.; Goddard, W. A., III; Jung, Y. Entropy and the Driving Force for the Filling of Carbon Nanotubes with Water. Proc. Natl. Acad. Sci. U. S. A. 2011, 108, 11794. (43) Su, J.; Guo, H. Control of Unidirectional Transport of SingleFile Water Molecules through Carbon Nanotubes in an Electric Field. ACS Nano 2011, 5, 351−9. (44) Takaiwa, D.; Hatano, I.; Koga, K.; Tanaka, H. Phase Diagram of Water in Carbon Nanotubes. Proc. Natl. Acad. Sci. U. S. A. 2008, 105, 39−43. (45) Chaban, V. V.; Kalugin, O. N. Structure and Dynamics in Methanol and Its Lithium Ion Solution Confined by Carbon Nanotubes. J. Mol. Liq. 2009, 145, 145−51. (46) Zheng, J.; Lennon, E. M.; Tsao, H.-K.; Sheng, Y.-J.; Jiang, S. Transport of a Liquid Water and Methanol Mixture through Carbon Nanotubes under a Chemical Potential Gradient. J. Chem. Phys. 2005, 122, 214702−7. (47) Eikerling, M.; Kharkats, Y. I.; Kornyshev, A. A.; Volfkovich, Y. M. Phenomenological Theory of Electro-Osmotic Effect and Water Management in Polymer Electrolyte Proton-Conducting Membranes. J. Electrochem. Soc. 1998, 145, 2684−2699. (48) Ren, X.; Henderson, W.; Gottesfeld, S. Electro-Osmotic Drag of Water in Ionomeric Membranes: New Measurements Employing a Direct Methanol Fuel Cell. J. Electrochem. Soc. 1997, 144, L267. (49) Cheah, M. J.; Kevrekidis, I. G.; Benziger, J. Effect of Interfacial Water Transport Resistance on Coupled Proton and Water Transport across Nafion. J. Phys. Chem. B 2011, 115, 10239−10250. (50) Wong, S. S.; Joselevich, E.; Woolley, A. T.; Cheung, C. L.; Lieber, C. M. Covalently Functionalized Nanotubes as NanometreSized Probes in Chemistry and Biology. Nature 1998, 394, 52−55. (51) Dresselhaus, M. S.; Dresselhaus, G.; Saito, R.; Jorio, A. Raman Spectroscopy of Carbon Nanotubes. Phys. Rep. 2005, 409, 47−99. (52) Jorio, A.; Saito, R.; Hafner, J. H.; Lieber, C. M.; Hunter, M.; McClure, T.; Dresselhaus, G.; Dresselhaus, M. S. Structural (N,M) Determination of Isolated Single-Wall Carbon Nanotubes by Resonant Raman Scattering. Phys. Rev. Lett. 2001, 86, 1118−1121. (53) Shimizu, S.; Agrawal, K. V.; O’Mahony, M.; Drahushuk, L. W.; Manohar, N.; Myerson, A. S.; Strano, M. S. Understanding and Analyzing Freezing-Point Transitions of Confined Fluids within Nanopores. Langmuir 2015, 31, 10113−10118.

(54) Shimizu, S.; Ellison, M. D.; Aziz, K.; Wang, Q. H.; Ulissi, Z.; Gunther, Z.; Bellisario, D.; Strano, M. S. Stochastic Pore Blocking and Gating in Pdms Nanopores from Vapor-Liquid Phase Transitions. J. Phys. Chem. C 2013, 117, 9641−9651. (55) Hassan, S. A.; Hummer, G.; Lee, Y.-S. Effects of Electric Fields on Proton Transport through Water Chains. J. Chem. Phys. 2006, 124, 204510−204510. (56) Chen, Y.; Ni, Z.; Wang, G.; Xu, D.; Li, D. Electroosmotic Flow in Nanotubes with High Surface Charge Densities. Nano Lett. 2008, 8, 42−48. (57) Wu, J.; Gerstandt, K.; Zhang, H.; Liu, J.; Hinds, B. J. Electrophoretically Induced Aqueous Flow through Single-Walled Carbon Nanotube Membranes. Nat. Nanotechnol. 2012, 7, 133−139. (58) Mähler, J.; Persson, I. A Study of the Hydration of the Alkali Metal Ions in Aqueous Solution. Inorg. Chem. 2012, 51, 425−438. (59) Goldsmith, J.; Martens, C. C. Molecular Dynamics Simulation of Salt Rejection in Model Surface-Modified Nanopores. J. Phys. Chem. Lett. 2010, 1, 528−535. (60) Samoylova, O. N.; Calixte, E. I.; Shuford, K. L. Molecular Dynamics Simulations of Ion Transport in Carbon Nanotube Channels. J. Phys. Chem. C 2015, 119, 1659−1666. (61) Derlacki, Z. J.; Easteal, A. J.; Edge, A. V. J.; Woolf, L. A.; Roksandic, Z. Diffusion Coefficients of Methanol and Water and the Mutual Diffusion Coefficient in Methanol-Water Solutions at 278 and 298 K. J. Phys. Chem. 1985, 89, 5318−5322. (62) Miller, S. A.; Young, V. Y.; Martin, C. R. Electroosmotic Flow in Template-Prepared Carbon Nanotube Membranes. J. Am. Chem. Soc. 2001, 123, 12335−12342. (63) Berg, P.; Ladipo, K. Exact Solution of an Electro-Osmotic Flow Problem in a Cylindrical Channel of Polymer Electrolyte Membranes. Proc. R. Soc. London, Ser. A 2009, 465, 2663−2679. (64) Akerlof, G. Dielectric Constants of Some Organic SolventWater Mixtures at Various Temperatures. J. Am. Chem. Soc. 1932, 54, 4125−4139. (65) Mikhail, S. Z.; Kimel, W. R. Densities and Viscosities of Methanol-Water Mixtures. J. Chem. Eng. Data 1961, 6, 533−537. (66) Ellison, M. D.; Morris, S. T.; Sender, M. R.; Padgett, N. E.; Brigham, J. Infrared and Computational Studies of the Adsorption of Methanol and Ethanol on Single-Walled Carbon Nanotubes. J. Phys. Chem. C 2007, 111, 18127−18134. (67) Scruggs, N. R.; Robertson, J. W. F.; Kasianowicz, J. J.; Migler, K. B. Rectification of the Ionic Current through Carbon Nanotubes by Electrostatic Assembly of Polyelectrolytes. Nano Lett. 2009, 9, 3853− 3859. (68) Kazachkin, D. V.; Nishimura, Y.; Witek, H. A.; Irle, S.; Borguet, E. Dramatic Reduction of Ir Vibrational Cross Sections of Molecules Encapsulated in Carbon Nanotubes. J. Am. Chem. Soc. 2011, 133, 8191−8. (69) Wu, J.; Gerstandt, K.; Majumder, M.; Zhan, X.; Hinds, B. J. Highly Efficient Electroosmotic Flow through Functionalized Carbon Nanotube Membranes. Nanoscale 2011, 3, 3321−3328.

I

DOI: 10.1021/acs.jpcc.6b12104 J. Phys. Chem. C XXXX, XXX, XXX−XXX