Investigating the effect of single-walled carbon nanotubes chirality on the electrokinetics transport of water and ions: A molecular dynamics study

https://doi.org/10.1016/j.diamond.2020.108105Get rights and content

Highlights

  • Under electric field, (m,m) CNTs transport more water and ions than (m,0) CNTs

  • There is a peak in the flux-voltage diagram for water transport in CNTs; peak for (m,m) happens at lower electric field than for (m,0) (m,m)

  • At a specific electric field, water molecules tend to create an ice-like structure at the entrance of CNTs

Abstract

The effect of carbon nanotubes (CNTs) chirality on the water electrokinetics transport through CNTs while embedded in silicon nanochannels is studied using molecular dynamics simulation. Voltage difference across the CNTs changed from 0 V to 3.2 V that resulted in a shift in water transport rate from ~8 to ~140 H2O/ns, 4 to ~120 H2O/ns, 15 to ~300 H2O/ns and 10 to ~285 H2O/ns for CNT (8,8), CNT (14,0), CNT (16,16) and CNT (28,0), respectively. In comparison to zigzag CNTs, armchair CNTs showed higher flux for all kind of cations (Na+, K+, Zn2+). It has been shown that water flux through CNTs are not linearly dependent on electric field, yet it faces a peak. Interestingly, zigzag CNTs showed a tendency to retard this maximum voltage to higher values of electric field than armchair CNTs. This work highlights the importance of CNTs chirality effect in future CNT-nanofluidic systems design.

Introduction

The ultrafast transport of water through carbon nanotubes has made them a revolutionizing material for next generation microfluidic and nanofluidic systems for myriad of applications [[1], [2], [3], [4]]. The interfacial effects arising from the interaction of water molecules and solid surface of carbon atoms in confined space has made significant transport properties compared to macroscale liquid transports [5,6]. Due to atomic scale closure of liquids and solid interface, physiochemical properties of both materials involved, contribute in characterizing the transport efficacy [7,8]. This open the field for synthesizing unique materials for a specific transport rate in nanofluidic devices for application in various fields [9,10]. Along this avenue, huge research interests have been drawn toward carbon nanotubes owing to their extremely smooth surface that provide a unique energy landscape for enhanced water flow rate which scrupulously represent the flow characteristics of biological nanopore in cells like aquaporin asa biological protein channel [11,12]. It is proved that the fluidic transport in CNTs exceeds the classical hydrodynamics prediction by three or five order of magnitude which is of significant interest in microfluidic and nanofluidic community for developing ultrafast and atomically controllable ultralow fluid volume transport [13,14]. There is a cascade of interwoven effects, in terms of physical and chemical phenomena which influence this high rate of water flux which the most noteworthy ones are: the geometry and structure of confined space, water arrangement and layers, orneriness of water density, number of carbon walls and so on [[15], [16], [17], [18], [19]]. Beside these geometrical contributing factors, there are other imperative physiochemical influences such as temperature fluctuations, water helical motions, friction between water and CNTs wall, viscosity of water, complex hydrogen bonding, ionic concentration, carbon nanotubes defects and so on [[20], [21], [22], [23], [24], [25], [26], [27], [28]].

As far as the contemporary microfluidics and nanofluidic technologies are highly demanded in enhancing the efficiency in miniaturized technologies such as nanotechnology-based water purification [29,30], nano scale energy storage [31,32] and Bio-MEMS/NEMS based biomedical devices [[33], [34], [35]], CNTs integration in micro electromechanical systems (MEMS) materials is an important practice both in computational investigation and fabrications for realization of that goal. Computational analysis at atomic scale of water permeation into carbon nanotubes has brought about meticulous understanding of the phenomena interplaying in nanotube-based nanofluidic. The confront of research in carbon nanotubes community recently have focused on different aspects of carbon nanotubes nanofluidic such as electric field driven flows properties [36], pressure driven flows [37,38], thermally actuated water transport [39], nanotubes functionalization [40], which almost cover all aspects of carbon nanofluidic dynamics. Nevertheless, there are rigorous fabrication-related concerns which hamper the researchers from giving those ideas an experimental sense. Realization of thermal controlling, pressure driven and functionalization of carbon nanotubes is extremely difficult procedure from fabrication point of view and the introduced computational models are too ideal. The pressure driven system needs further nanofluidic system compartment development for precision control over pressure, furthermore the fluid mechanics principles introduce various challenges in front of integration of nanosized channels with microfluidic parts [41]. While the pressure difference and thermal actuation needs further elaborated instrumentations, it seems the electrokinetics nanofluidic might solve those challenges as it provides easier way to manipulate the flow by having only two contact electrodes in fluidic area [42]. The importance of electrokinetics comes to attention when the manipulative force is scaled down to nano-scale where controlling the force with pressure of external flow is extremely difficult as the flow will be somehow in molecular range. The order of manipulative force for pressure difference method sometimes falls into the error bandwidth of control instruments which make the controlling a cumbersome process.

Beside encompassing the privilege of introducing the simplest form of flow induction into carbon nanotubes, electrokinetics transport of aqueous solution provide further developments in nanofluidic based system in terms of utilizing newer aspects of fluids atomic-related anomalous characteristics. Recent studies have unfolded novel characteristics of water molecules inside narrow spaces (. e.g. carbon nanotubes) that controlling them might transform the next generation nanofluidic based devices which are working heavily based on the water molecules flow in confined spaces [[43], [44], [45]].

Along this line, last decade witnessed the surge of many research works in studying water and ions transport through carbon nanotubes, toward focusing more on the electrical filed driven transport properties. However, it is still extremely difficult to achieve a controllable and single direction of water flow through carbon nanotubes in real experimental condition [42]. Headed for discover the physics of such complex flows, various works with different boundary conditions have been conducted. The correlation between water flux and dipole orientation was investigated, and by controlling this interrelation, a noteworthy range of water flux could be controlled [46]. The water flow inside carbon nanotubes has been reported to be largely mediated by hydrogen bonding dynamics inside the tubes and this motivated others to think of controlling the transport by functionalizing CNTs with hydrophilic and hydrophobic agents [42,[46], [47], [48], [49]]. From various studies the dependence of water dipole orientation on external electric filed has been approved which all demonstrated a spontaneous correlation between time dependent dipole course variation and the direction of external electric field [[50], [51], [52], [53]].

Molecular dynamics simulation was utilized to investigate the transport of water under electric filed through a 9.83 nm long carbon nanotube by Joesph and Aluru [54]. In this work a remarkable result was demonstrated in which authors described the flux of water along water dipole orientation. Olga N et al. also discussed the mechanism and physics of water and ions transport through different diameter carbon nanotubes. They correlated ionic current, potential of mean force for ions and water shell around ions to describe the physics behind ions transportation. A strong dependence between the ionic current and carbon nanotube radius were demonstrated. The conditions for successful ion permeation through CNTs were deduced under action of external electric field. They found that ions only have to overcome a barrier about 1.5–2 kcal/mol to permeate through CNTs with radius around 6.8–8.2 A°.

Most recent studies on electric field driven water transport in carbon nanotubes has been oriented in a way to focused more on practical issues in separation science. Manash et al. used electric field incident angle variation to induce various quantity of water and ethanol permeation through carbon nanotubes which resulted in a promising method for separation of water from ethanol by only changing the direction of electric field applied [55]. Electric field was applied to graphene bilayer to separate positive ions form negative ions to produce fresh water [56]. It was observed that by increasing the strength of the applied electric field, ion separation improved noticeably [56]. Li Zhang et al., studied the possibility of controlling water transport through (6,6) BNNTs by spatial charge placements that resulted in an induction of a local electric field on the water molecules [57]. Again, to facilitate water-alcoholic solution separation, electrical field was applied on the system with a mixture of water and alcohol at the gate of carbon nanotube that resulted in a highly ordered structure of water molecules flow [58]. According to the results, the electric field while assisted ordering of water molecules, caused a preference for water to fill the carbon nanotube faster than alcohol which eventually led to separation of water molecules from alcoholic solution [58]. Jiyu Xu et al. investigated the water molecular transport through sub-nanopores via a chemical-reaction-like activated process [59]. This activated water molecules can be controlled by acute manipulation of pressure and temperature in a graphite-like membrane [59]. Kai Xiao et al., used the concept of electrostatic charge effects of membrane proteins in aquaporins to control the energetics of water permeation through carbon nanotubes inside solid-state nanochannels [60]. Functionalized carbon nanotubes were studied in the presence of electric field to discover the effectiveness of corresponding functionalization on the ions transport [[61], [62], [63]]. The ionic current was dramatically improved by functionalizing the nanopore gate by hydrogen and hydroxyl groups. The functionalization effects on water transport inside carbon nanotubes was also reported by panahi et al., [47,49,64], in which they used fluorinated carbon nanotubes under electric field for water desalination and heavy metals removal from water.

Most recently, Alan Sam et al., have studied the effect of single walled carbon chirality nanotubes on water transport. They concluded that in same size, carbon nanotubes with (m,m) chirality transport water with higher velocity than all other chirality vectors which has resulted in higher water flux [65]. This research team investigated the electrostatic interaction and chirality effects in carbon nanotubes and boron nitride nanotubes with both zigzag and armchair configurations. They found that the friction coefficient of water transporting inside zigzag boron nitride nanotubes is much larger than that in zigzag carbon nanotubes with the same diameters [66]. By means of molecular dynamics simulation, Zhao Wang predicted the effect of chirality on the conduction of benzene molecules along the surface of carbon nanotubes subjected to a thermal gradient [67]. The effects of the chiral vector of a carbon nanotube on the performance characteristics of the NEAS (Nanofluidic Energy Absorption System) has been investigated using molecular dynamics simulation by Ganjiani and Hossein Nezhad [68]. They analyzed six CNTs with different diameters for each type of armchair, zigzag and chiral, and several chiral CNTs with different chiral vectors [68]. Lei Yang and Yanjie Guo studied the effect of graphene chirality index on the water transport properties [69]. In terms of energy adsorption in carbon nanotubes, it was found that chiral nanotubes are better candidates in comparison to armchair and zigzag.

For water desalination purposes, chirality effect was also studied for similar diameter in which a low activation energy has been deduced for water when is moving in armchair carbon nanotube in comparison to zigzag nanotubes. Nanodroplet in carbon nanotubes was investigated under action of thermal differences in different carbon nanotubes in which nanodroplets were moving faster in armchair nanochannels while their transport time increased as they were entered into the zigzag and chiral CNTs [70]. A free-energy calculation was performed to investigate the water transport difference in armchair and zigzag CNTs [65]. The outcome for this research indicated that the energy barriers in form of water molecules when facing both CNTs structures is the main reason for such difference [65]. These studies clearly demonstrate the influence of carbon nanotube atoms configurations on the fluidic transports. This structural dependence is of key importance for enhancing the potential application of CNTs in water purification, energy harvesting and ion field effective transistors (ISFET) based biosensors [71].

While numerous studies have been conducted on the flow of water in CNTs and boron nitride nanotubes (BNNTs), the study of chirality effects in electrical field driven flows is missing which consider both zigzag and armchair architecture. Due to paramount importance of electrokinetics of water flow in nanochannels for next generation biosensing devices we have embarked on this important study to analyze the effect of CNTs chirality on electrokinetics transport performance. Besides the water desalination purpose that has been the main focus of this work, electrokinetics effects (water driven by electric field) is significantly important phenomena in other subjects such as biosensors which are operating based on ionic current conduction (e.g. DNA sequencers [72]), nanoenergy systems [73,74] and novel batteries [75]). For this reason, we have initiated this study to study the chiral effects on water transport in carbon nanotubes under action of electric field in silicon membrane as this is a common material that have been used frequently in micro/nano electro mechanical systems (MEMS/NEMS) with high capability for integration with complementary metal oxide semiconductor (CMOS) devices for further placement in microfluidic and nanofluidic chips. Carbon nanotubes can bring higher efficiency in designing these systems. Previous works did comprehensive simulations for studying chirality effect on water transport, but the effect of electrical field in parallel has not been reported, especially in silicon nanochannel. In current study, we have investigated the transport differences between armchair and zigzag carbon nanotubes for two radii (0.54 nm and 1.08 nm) and chirality of (8,8), (14,0), (16,16) and (28,0) while embedded in silicon nanochannel to study the proficiency of these nanotubes for conduction of water and ions through silicon membranes.

Section snippets

Modelling of system

In this research, two types of carbon nanotubes were examined for understanding the transport properties of water flow inside nanopore. To do so, two sets of 3 nm long CNTs with chirality vector of [ (8,8), (14,0)] and [(16,16) and (28,0)] with diameter of approximately 1 nm and 2 nm have been used in state-of-the-art molecular dynamics simulation. These CNTs were embedded in silicon membranes that are placed at two ends of carbon nanotubes to effectively apply the influence of carbon nanotube

Water/ ions flux and ionic current analysis

Molecular dynamics simulation was performed to investigate the transport of water molecules and ions through carbon nanotubes with different carbon arrangements while are embedded in silicon nanochannels under a horizontal electric field. All simulations were performed for 24 ns and an electrical field was applied along z direction corresponding to the CNTs longitudinal axis. The electrical field were calculated based on Eq. (5), where Lz is approximately 15 nm. The electrical field were

Conclusion

In this research, the effect of chirality of carbon nanotubes on the water and ions transport has been scrutinized under action of electric field using molecular dynamics simulation. A considerable difference between water flux were determined and simulation studies showed, water flux in (m,m) were more than (m,0) More importantly, in this study by means of state of the art molecular dynamics simulation we could define the maximum possible water flux in the carbon nanotubes which happens in a

CRediT authorship contribution statement

Abbas Panahi: Conceptualization, Methodology, Software, Writing - original draft, Formal analysis, Validation. Pooria Sadeghi: Data curation, Software, Visualization. Amir Akhlaghi: Visualization, Investigation. Mohammad Hossein Sabour: Supervision, Resources, Writing - review & editing.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgment

We are thankful to Faculty of New Science and Technology of University of Tehran for providing authors with high-speed computational facility.

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