Preparation and Transport Performances of High-Density, Aligned Carbon Nanotube Membranes
© Zhang et al. 2015
Received: 3 March 2015
Accepted: 2 June 2015
Published: 19 June 2015
We report a simple and effective method for the preparation of high-density and aligned carbon nanotube (CNT) membranes. The CNT arrays were prepared by water-assisted chemical vapor deposition (CVD) and were subsequently pushed over and stacked into dense membranes by mechanical rolling. It was demonstrated that various gases and liquids, including H2, He, N2, O2, Ar, water, ethanol, hexane, and kerosene, could effectively pass through the aligned carbon nanotube membranes. The membranes exhibited different selections on different gases, indicating that there was a separation potential for the gas mixtures. The selectivities (H2 relative to other gases) of H2/He, H2/N2, H2/O2, and H2/Ar were found to be lower than that of the ideal Knudsen model. For pure water, the permeability was measured to be 3.23 ± 0.05 ml·min−1·cm−2 at 1 atm, indicating that the CNT membranes were promising for applications in liquid filtration and separation.
In the past decade, vertically aligned carbon nanotube (VACNT) membranes where carbon nanotubes are sealed in a polymer or inorganic matrix have been developed for gas and liquid transport applications [1–7]. It is well known that those composite membranes suffer from a trade-off between selectivity and permeability. In some cases, they are even susceptible to fouling or exhibit low chemical resistance. Hinds et al.  first fabricated multi-walled VACNT membranes with an inner diameter of 6–7 nm embedded in a rigid polystyrene matrix. They demonstrated that liquid transporting through the composite membrane was several orders of magnitude faster than that predicted by the classical hydrodynamics theory owing to the smooth CNT walls. Holt et al.  adopted a micro-fabrication method to produce membranes in which the double-walled CNTs were used as the only pores to span through a silicon nitride matrix deposited by chemical vapor deposition (CVD). They found that the measured gas flow was more than one order of magnitude larger than that predicted from the Knudsen diffusion model. In spite of their smaller pore sizes, the gas and water permeabilities of those nanotube-based membranes were several orders of magnitude greater than those of the commercial polycarbonate membranes. Kim et al.  reported the results of gas mixture transporting through the single-walled CNT membrane with an average pore size of 1.2 nm. The aligned single-walled CNTs were filtrated with a poly(tetrafluoroethylene) filter and the spaces among the CNTs were then sealed with polysulfone polymer by spin coating. They confirmed that non-Knudsen transport could occur in the aligned CNT membranes and found that the permeabilities of CO2 and CH4 passing through the membrane with additional polymer coating were lower than those predicted from the Knudsen diffusion model. The reduction in permeability was found to be proportional to the transport resistance offered by the additional polymer layer.
The previously reported composite membranes had low CNT porosity since their fractions of CNT permeation areas were only 0.079–2.7 %. Although the fluxes passing through the individual nanotubes were high, the fluxes of membrane areas were limited because of the low porosity. In addition, the fabrication processes of the composite CNT membranes were expensive and complicated. In order to achieve more efficient and cost-effective purification, advanced membrane technologies with controlled and novel pore architectures have to be developed. In contrast to the above studies, which used CNT pores as transport pathways, Srivastava et al.  made a CNT filter from high-density and vertically aligned CNT forests without a filler. Because their nanotubes were mostly blocked by catalyst particles, transport was in the interstitial spaces, which were approximately 20–30 nm across. Yu et al.  fabricated a freestanding VACNT membrane with high packing density by shrinking VACNT arrays, and found that gas permeances based on total membrane area were 1–4 orders of magnitude higher than VACNT membranes in the literature, which highlights the potential of high-density CNT membranes in mass transport.
In this work, a facile and effective method is developed to prepare high-density, aligned, and freestanding CNT membranes by mechanically rolling and densifying VACNT arrays. The membrane structure is characterized and transport performances of some gases and liquids across the membrane are investigated. Compared with the buckypaper membrane , this large area and aligned CNT membrane, which employ the narrow spacing among aligned CNTs as mass transport pathway, have more ordered pore structure.
Growth of VACNTs was conducted by a water-assisted CVD technique by using Fe(1.4 nm)/Al2O3(40 nm)/Si as the catalyst [9, 10]. High-purity ethylene (99.99 %) was used as carbon source and Ar/H2 (99.999 %) were used as carrier gases with a total flow rate of 650 sccm. During the growth process, a controlled amount of water vapor was employed as catalyst preserver and enhancer and was supplied by passing a portion of the carrier gas Ar through a water bubbler [11–14]. Typically, VACNT array was grown at 815 °C with ethylene (100 sccm) under a water concentration of 100–200 ppm for 10 min .
Two glass slides were used as the simple tool to fabricate CNT membrane. Firstly, the as-grown CNT array was fixed by gluing the Si substrate on a glass slide. Secondly, another slide was put on top of the CNT array and used as a guide for the subsequent shear pressing from a roller. During the shear pressing, CNTs were forced down to one direction. Then, the aligned CNT membranes were peeled from the substrate by ultrasonication in deionized water. After drying in vacuum at 60 °C for 4 h, freestanding and aligned CNT membranes were obtained.
The freestanding CNT membrane was first sealed between two pieces of aluminum adhesive tapes with pre-punched holes (3 mm in diameter) . Then the membrane was mounted in the gas line of a permeation testing apparatus, which was purged with the target gas for several times to avoid any possible impurities [16–18]. Finally, pure H2, He, N2, Ar, O2, or CO2 (99.999 %) were introduced to the upstream side of the membrane [19–22] for permeation measurements. A pressure or flow controller (MKS 250E) was connected to the upstream and downstream sides of the composite membrane to control the relative gas pressures by automatically tuning the gas feeding rates. The permeabilities at a variety of pressures (10–100 Torr) were measured using a mass-flow meter connected at the downstream side.
The transport properties of liquid (water, ethanol, hexane, and kerosene) were measured in a liquid-collecting device, and a permeate sample was weighed every 1 h to determine the flux [23–25]. The pressures were 10–100 Torr on the permeate side. All the measurements were carried out at room temperature.
Results and Discussion
where P Kn is the Knudsen permeation (mol m−2 s−1 Pa−1), ε p is the porosity, τ is the tortuosity, Φ is the inner diameter of CNT (m), L is the layer thickness (m), M is the molecular mass (kg mol−1) of the gas molecule, and T is the absolute temperature (K).
The permeances of the five gases range between 1.0 × 10−5 and 2.5 × 10−5 mol m−2 s−1 Pa−1, and enhancement factors ranged between 30 and 60, indicating a much higher transport rate than Knudsen diffusion. The selectivities (H2 relative to other gases) were lower than Knudsen for dense CNT membranes, as shown in Fig. 3c. These results clearly indicate that the high permeances through these dense CNT membranes are not Knudsen . Surface diffusion and viscous flow may play important roles in the transport of gas molecules through the aligned CNT membranes. From the perspective of separations, lower flux with higher separation for surface diffusion and higher flux with lower separation is obtained for viscous flow. The low gas selectivities indicate the presence of viscous flow in the membrane. Although the average pore diameter is less than 10 nm, the CNT membranes may have a few large pores that favor high molecule weight gases permeation. Furthermore, the smooth surface of the CNTs may also play a role in the selectivity derivation of gas molecules from the Knudsen diffusion.
Pressure-driven liquid transport through the CNT membranes was measured in a pressure flow membrane transport device . Briefly, the membrane was assembled in the flow cell, and the nitrogen provided the required pressure to drive the liquid through the membrane. Transport behavior of water, ethanol, hexane, and kerosene were measured, and all the measurements were carried out at a constant temperature. For measurements of different liquids on the same membrane, the sample is dried in air for 12 h. The weight of the liquid permeating through the membrane was measured at intervals of 1 h over a period of 12 h. And then the volume of permeated liquid was determined based on the weight difference.
where Q HP is the volumetric flow rate (ml min−1 cm−2 atm−1), Δp is the pressure drop (Torr), d is the membrane diameter (nm), μ is the dynamic viscosity, and L is the membrane thickness (μm).
Parameters of the high-density and aligned CNT membranes
Membrane pore size (nm)
Thickness L (μm)
Dynamic viscosity μ (Pa·s)
CNT areal density (cm−2)
Membrane area (cm2)
0.307~1.675 × 10−3
7.065 × 10−2
Permeability of different liquids passing through the CNT membranesa
Polycarbonate membrane 
Pore size (nm)
~6 × 10−3
(1.2~4.6) × 10−1
6.3 × 10−4
In summary, we have demonstrated a simple and effective method to prepare high-density and aligned CNT membranes, which have advantages over other CNT composite membranes. The average spacing between CNT membranes was ~10 nm after rolling. Remarkably, the mechanical rolling did not destroy the aligned structure of CNTs or introduce other defects, and the membranes’ aligned structure remained unchanged. The CNT membranes show significantly high flow rates for the transports of various gases and liquids including H2, He, N2, O2, Ar, water, ethanol, hexane, and kerosene. The gas permeability of the high-density and aligned CNT membrane is much higher than the Knudsen permeability and is scaled with respect to the molecular weight of the gases with an exponent lower than that predicted by the Knudsen diffusion model. Moreover, it was found that different samples with the same preparation conditions kept a good consistency in the permeances of gases that the flow rate increased with increasing pressure drop. This phenomenon of deviation confirms the existence of a non-Knudsen transport and a thermally activated diffusion process. The membranes exhibited different selections on different gases, indicating that there was a separation potential for the gas mixtures. The selectivities of H2/He, H2/N2, H2/O2, and H2/Ar were found to be lower than that of the ideal Knudsen model. For pure water, the permeability was measured to be 3.23 ± 0.05 ml·min−1·cm−2 at 1 atm, indicating that the CNT membranes were promising for applications in liquid filtration and separation. In addition, the CNT membranes are found to have excellent filtration performance in nano-gold solution.
The authors gratefully acknowledge the financial support provided by NSFC (51072118,51272157), the 973 program (2010CB234609), STCSM(10231201103), the Hujiang Foundation of China (B14006), and the Innovation Fund Project For Graduate Student of Shanghai (JWCXSL1201). GPZ is grateful for the support provided by the Science and Technology Innovation Commission of Shenzhen, China.
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