Measurement of the Rate of Water Translocation through Carbon Nanotubes

Abstract

E nhanced water flow through atomic smooth and hydrophobic carbon nanotubes (CNTs) have been demonstrated by both theoretical calculations and experiments. 1À5 There is, however, a great controversy between theory and experiments and even between experiments. The very limited experiments using CNTs membrane demonstrated enormous water flow velocity up to 5 orders of magnitude faster than predicted from conventional fluid-flow theory with three orders of deviation from different sources.1,2 In contrast, molecular dynamics (MD) calculation only gives a rate enhancement of 47À6500 for CNTs with diameters of 4.99À0.81 nm.3,5 One more general debate is whether there exists a clear transition from continuum to subcontinuum transport as the tube diameter shrinks to subnanometer regime. 5 The bottleneck for experimental attempts arises from fabrication of CNTs membrane with well-defined structures and the rational estimation of the available flow area.1 Here we show a single-tube level approach for elucidating such fundamental nanofluidic issues. The unique field effect transistors (FETs) array-based experimental design enables a direct measurement of water flow velocity inside individual CNTs. Our work demonstrates a rate enhancement of 51 to 882 for CNTs with diameters of 1.59 to 0.81 nm, which supports the MD calculation.3,5 Additionally, we achieved the first experimental evidence for the transition from continuum to subcontinuum flow by varying the diameters of CNTs.The key of our approach is to trace the water flow "front" inside an individual millimeter long CNT electrically with a configuration of three FETs in series (Figure 1a,b). The FET1 is used to "in-situ" open the tube end under water droplet by electrical breakdown, 6,7 and the synchronous FET2 and FET3 to detect the water front flowing in based on its influence on the current flow (Figure 2). 8,9 It should be emphasized that opening the tube end under water is a determinant factor for the success of this experimental design. A bias voltage of 0.01 V was applied on FET2 and FET3 (no gate voltage) all the time to detect current change. Simply by measuring the time delay of current signal jumps between FET2 and FET3 with a given interspacing, we can then estimate the average water flow velocity inside the nanotube. The CNT-FETs structure was constructed through directly growing ultralong CNT on SiO 2 /Si substrate with predesigned Pt-pattern (Figure 1b,c). Carbon nanotubes were synthesized by gas flow-directed chemical vapor deposition (CVD) method. 10À12 The catalysts pattern was made on growth substrate using PDMS stamp from the ethanol solution of 0.01 mol/L FeCl 3 . The typical growth conditions are 930À950°C, 3 sccm CH 4 and 5 sccm H 2 . Pt was sputtered and patterned as electrodes on SiO 2 /Si substrate by standard technique of photolithography and magnetron sputtering. The as-grown CNTs were characterized by scanning electron microscopy (SEM) followed by gold-wire wedge bonding, water filling and velocity measurement. A drop of pure water (18.2MΩ ...

Self-assembling subnanometer pores with unusual mass-transport properties

“…We noted recent studies on carbon nanotubes showing that water can spontaneously fill small hydrophobic nanopores 36,37 , especially those with small pore diameters [36][37][38][39][40][41][42] , leading to enhanced mass flow or resonance 40,43 . The observed ion conductance of the hydrophobic channel formed by 1a implies that this sub-nm pore may also be filled with water molecules.…”

Hybrid molecular-continuum simulations of water flow through carbon nanotube membranes of realistic thickness

“…(9) �P = 8µLQ HP πR 4 , Recent experimental work by Qin et al (2011) managed to reduce a source of errors in CNT membrane experiments by performing flow rate measurements of water passing through a single CNT. One of these experimental cases is replicated here in an attempt to evaluate the performance of the SeN-IMM in terms of accuracy and computational efficiency.…”

“…One of these experimental cases is replicated here in an attempt to evaluate the performance of the SeN-IMM in terms of accuracy and computational efficiency. The diameter of the chosen CNT in Qin et al (2011) is D = 1.1 nm, while its length is L = 1 mm. For a CNT of this length an MD simulation would be intractable.…”

“…The same methodology was used as for the previous hybrid simulations, with the only differences being the reduced CNT diameter and the external pressure difference, which is reduced to P = 100 MPa. The flow enhancement values reported by Qin et al (2011) are also calculated under some assumptions. The first is that two van der Waals diameters of carbon (or, equivalently, twice the thickness of the CNT wall) is subtracted from the overall CNT diameter, i.e.…”

Multilayer Two-Dimensional Water Structure Confined in MoS2

“…[1][2][3][4][5][6][7][8][9][10][11] Inside a one-dimensional channel of carbon nanotubes, for example, water molecules could undergo unconventional phase transitions [12][13] and form ice-like structures at room temperatures depending on the channel diameter. Also, a delicate balance between entropy and enthalpy can render these confined water thermodynamically more stable than the bulk water.…”