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Environ Eng Res > Volume 29(6); 2024 > Article
Lee and Kim: Innovative membrane technology for water treatment solutions: Current status and future prospects of carbon nanotube membranes

Abstract

Carbon nanotube (CNT)-based membranes are gaining attention for their unique properties and applications in membrane technology. This review comprehensively explores major types of CNT-based membranes—buckypaper, CNT mixed, and vertically aligned CNT membranes—along with their fabrication methods, pore size control strategies, unique properties, and potential applications. Understanding these aspects will help fully harness the potential of CNT-based membranes in addressing challenges in water treatment and other fields. CNTs offer promise as water treatment membranes due to their ability to enhance performance of existing membranes, overcome trade-offs, exhibit antibacterial properties, and facilitate fast water transport.

Graphical Abstract

1. Introduction

As the phenomenon of water scarcity has become more severe, water treatment technologies have received attention as crucial technologies. Membrane-based water treatment technologies, which can rapidly and efficiently remove particles and ions [1] from wastewater and seawater, have attained prominence [2]. However, this technology has drawbacks, such as high energy consumption and membrane fouling [35]. To address these issues, several researchers have developed a membrane with high permeability and antifouling for water treatment [6] by applying nanotechnologies [79]. In particular, membranes with carbon nanotubes (CNTs) are emerging as next-generation membranes that overcome the trade-off between permeability and rejection rate in existing membranes because of their fast mass transport and unique structure [10].
CNTs are attractive because of their outstanding mechanical [11], electrical [12, 13], and chemical properties [14]. In particular, their high surface area [15], antifouling, and ballistic mass transport are fascinating characteristics for environmental applications. CNTs have a cylindrical hollow structure. This indicates the presence of nano-sized pores inside. Nano-sized pores are an essential factor for water treatment membranes because they allow only water molecules to permeate through the membrane and exclude the permeation of small-sized particles or ions; however, nano-sized pores are difficult to artificially create. Therefore, CNTs with nano-sized pores are attractive materials for next-generation water treatment membranes. The unique flow phenomena in nano-sized pores, such as frictionless flow and alignment of water molecules, enable ballistic mass transport, leading to the realization of next-generation membranes with ultra-high permeability [1618]. These distinctive characteristics inherent to CNTs render them highly appealing and exceptional materials for water treatment membrane applications, demonstrating their excellence as a material of choice.
CNTs reportedly exhibit antimicrobial and anti-fouling properties [10]. When conventional membranes are used for an extended period, microorganisms grow on the surface of the membrane, and contaminants accumulate, causing the performance of the membrane to deteriorate. The unique geometrical shape and hydrophobicity of CNTs contribute to their antimicrobial and anti-fouling properties [1921], which are exceptional characteristics not present in traditional membrane materials. These distinctive features of CNTs, coupled with their absence in conventional materials, have drawn significant attention as advanced materials for next-generation water treatment membranes.
In this review, we outline water treatment membranes using CNTs, which are drawing attention as next-generation materials, and introduce why they exhibit outstanding abilities compared to those exhibited by conventional materials, particularly their fast transport mechanism for water molecules, from a nano-scale perspective. This review provides an extensive exploration of the fabrication methodologies, structural characteristics, pore size control strategies, and inherent properties of CNT-based membranes. Furthermore, it delves into their diverse applications, ranging from water purification to electrochemical desalination, emphasizing their crucial role in addressing global challenges related to clean water access and sustainable energy technologies.

2. Classification of CNT Membranes

Various forms of CNTs have been used in water treatment membranes. They can be classified into two primary categories (Fig. 1). The first category involves CNT mixed membranes, which combine CNTs with existing membrane materials such as polymers —the most common materials for water treatment membranes (Fig. 1a). In this method, CNTs are added as fillers to the polymer matrix. The second category comprises membranes constructed entirely of CNTs. There are two types of membranes within this category: buckypaper and vertically aligned CNT (VA-CNT) membranes. Buckypaper membranes are composed of a random network of intertwined CNTs. This network forms a tight mesh resembling the structure of the polymer membranes created from the nanofibers (Fig. 1b and c). These membranes are the form of flexible and thin film; however, owing to the random network of CNTs, achieving uniform pore sizes and fully utilizing the natural characteristics of CNTs as water treatment membranes is challenging [22]. An additional type of CNT membrane is the VA-CNT membrane, which is prepared by synthesizing CNTs onto a substrate, primarily using the chemical vapor deposition (CVD) method, a vacuum deposition method used to produce high-quality, and high-performance, solid, film and nano materials. In typical CVD, the wafer (substrate) is exposed to one or more volatile precursors, which react and/or decompose on the substrate surface to produce the desired deposit, mainly with hydrocarbon gases [acetylene (C2H2), methane (CH4), etc.]. The CNTs grow vertically on the substrate and are densely packed (density: approximately 1010–1011/cm2), resembling a forest-like structure, often referred to as a CNT forest (Fig. 1d and e). Fabricating VA-CNT membranes is complex, involving the preparation of a substrate (silicon wafer) with a metal catalyst (Fe, Ni, CO, etc.) for CNT synthesis at high temperatures (approximately 700–800 °C) through the thermal CVD or at low temperature through plasma assisted CVD process. This process is sensitive to various parameters and often involves intricate procedures such as microelectromechanical system (MEMS) processes to prepare the substrate. Despite the difficult fabrication process, VA-CNT membranes can fully utilize the advantages of CNTs for water treatment.

3. CNT Mixed Membranes

A CNT mixed membrane is a type of composite membrane that improves the membrane performance by uniformly dispersing CNTs into the existing membrane matrix. This is a type of membrane that primarily aims to improve the performance of existing membrane materials, such as polymers (Fig. 2a and d), which are most widely used in water treatment membranes. Therefore, CNTs play a secondary role in mixed membranes. The primary roles of CNTs in membranes can be summarized in several approaches. First, CNTs exhibit excellent mechanical properties. In particular, CNTs exhibit excellent anisotropic properties with high mechanical strength and stiffness in the axial direction. Therefore, a uniform distribution of CNTs into conventional membrane materials can enhance the mechanical strength of the membrane [23, 24]. In particular, they can be expected to reinforce the durability and stiffness of the membrane. Accordingly, CNTs can improve the long-term stability of the membrane. Reverse osmosis membranes generally operate at pressures of 15 bar or above. A high operating pressure causes compression of the membrane [23, 24] and, ultimately, a decrease in the water flux. Embedding CNT relieves the decrease in water flux by protecting against compression owing to the improved mechanical strength of the membrane [25] (Fig. 2e and f). In addition, embedding CNT can also improve chemical stability and prevent membrane damage by chorine [26]. Therefore, it can alleviate the decrease in the salt rejection rate due to damage over time (Fig. 2g), which is typically observed in conventional polymeric membranes. Second, CNTs exhibit inherent hydrophobicity. In addition, the surfaces of CNTs are atomically highly smooth. Therefore, water can pass through a membrane at a very high speed without friction on the CNT surface. CNTs form rapid pathways for water within the membrane matrix, enabling high-speed passage [27, 28]. This enhances the water permeability of the membrane (Fig. 1a). Third, CNTs have a cylindrical hollow structure with nanometer-scale voids inside, forming nano-scale pores within the matrix and increasing the porosity of the membrane [2935], resulting in an increase in membrane permeability. Fourth, CNTs have antibacterial properties that can improve the fouling resistance of membranes. Microorganisms, such as bacteria, in water can multiply on the surface of the membrane, forming a thick layer called the cake layer and reducing the permeability of the membrane. Therefore, the development of membranes with antibacterial properties and fouling resistance has been required in recent years. CNTs not only prevent the adhesion of bacteria but also cause physical damage by destroying the bacterial cell membrane (Fig. 2h–k), thus inactivating bacteria by interfering with their reproduction [36]. In addition, by interfering with the adhesion of contaminants [37, 38], they can prevent the formation of bacterial and contaminant layers on the surface, thereby preventing the decrease in water permeability. Because CNT mixed membranes can improve the performance of conventional membranes, they have been actively researched as a commercialization technology.
CNT mixed membranes are primarily composed of polymer-based composite materials. Conventional polymer membranes are synthesized using various methods, such as interfacial polymerization [39, 40], a type of step-growth polymerization in which polymerization occurs at the interface between two immiscible phases (generally two liquids), resulting in a polymer that is constrained to the interface (Fig. S1a) and phase inversion [39, 4143], a chemical phenomenon exploited in the fabrication of artificial membranes. It is performed by removing the solvent from a liquid-polymer solution, leaving a porous, solid membrane (Fig. S1b). Interfacial polymerization is the most widely used method for forming a thin film-like active layer on the surface of a porous support. There are two approaches to synthesizing CNT mixed membranes [4446]. First, each reactive monomer is dispersed in two immiscible solutions, organic such as n-Hexane and aqueous, and the monomers such as m-Phenylenediamine (MPD), trimesoyl chloride (TMC) diffuse and react at the interface of the two solutions to form a polymer active layer (polyamide) for solute rejection on the support (polysulfone, etc.) [25]. At this time, the CNTs are dispersed in either of the two solutions and incorporated into the active layer when the monomer diffuses to the interface and diffuses together to form a polymeric active layer (Fig. S1c). Second, surface-modified CNT is coated onto the porous support layer, and interfacial polymerization is conducted to form a polymeric active layer on the CNT layer. Therefore, the CNT layer is added as an intermediate layer between the support and active layers [47] (Fig. S1d). For the CNTs to be homogeneously distributed in the active layer (in the first approach) and to obtain a well-coated CNT layer on the support layer (in the second approach), it is necessary for them to be uniformly dispersed in an organic or aqueous solution. For this purpose, various methods of CNT surface modification have been reported [45, 4850]. Oxidation with acid or air introduces various functional groups, such as hydroxyl and carboxyl groups, onto the surface of the CNTs to strengthen their binding to polar organic solvents [51]. Consequently, the structure of the active layer, including its hydrophilicity, porosity, and roughness, is modified by strengthening its affinity for polymer matrices [48]. In addition to oxidation, the chemical modification of CNT surfaces via covalent functionalization, aryl diazonium salts, or cycloaddition reactions has been reported [5256]. However, these methods change the intrinsic properties of the CNTs. Non-covalent functionalization can enhance compatibility with polymers without changing the intrinsic properties of CNTs. However, the non-covalent interactions between the CNTs and polymer matrices are relatively weak (Fig. S2).
The low permeability of conventional membranes comprised of polymer materials is a problem that must be addressed. CNT mixed membranes are an efficient method to enhance permeability without degrading the rejection rate of the existing membranes. However, there are limitations, such as the need to disperse an appropriate amount of CNTs evenly in the polymer active layer without agglomeration and to enhance compatibility with the polymer. Nonetheless, it represents the closest form of a CNT membrane for water treatment toward commercialization and is being actively researched. In fact, several membranes are commercially available.

4. Buckypaper Membranes

Buckypaper membranes are the easiest to fabricate and are the simplest form of CNT membranes. To fabricate the buckypaper membranes, it is essential to synthesize a well-dispersed CNT solution. When a well-dispersed CNT solution is subjected to vacuum filtration (Fig. 3a), the CNTs are randomly entangled with each other by van der Waals forces and form a network (Fig. 3b and c), forming a thin film with a porous structure similar to that of non-woven paper [57, 58]. Polymeric membranes with non-woven paper-like structures have also been widely used (Fig. 3d and e). The key process is to create a well-dispersed solution of CNTs to create a buckypaper membrane that is well-entangled with each other without CNT agglomeration. In general, the CNTs are dispersed in organic solvents. Because CNTs are synthesized using metal catalysts, such as Fe, Ni, and Co, during the synthesis step, they inherently contain metal impurities. During the synthesis process, CNTs are formed through the crystallization of double bonds between carbon atoms. However, carbons that do not participate in this crystallization process produce carbonaceous impurities, such as amorphous carbon, which are deposited on the CNT wall. As these impurities reduce the dispersibility of the CNTs, they cause agglomeration (bundling) between the CNTs after filtration (Fig. 3f), preventing the formation of a porous network. Therefore, impurities such as metal catalysts and carbonaceous impurities generated during the synthesis process should be removed through purification, and a homogeneous suspension should be obtained using sonication or shear mixing methods to uniformly disperse the purified CNTs in the solvent [5961]. Chemical oxidation is the most conventional method for purifying CNTs. In particular, carbon impurities such as amorphous carbon can be removed by acid oxidation or heat treatment in an oxidizing atmosphere such as air. In general, carbonaceous impurities can be removed by moderate heat treatment in nitric acid or air, and metal impurities can be removed by acid treatment such as hydrochloric acid [6265]. Purification through these oxidative treatments creates structural defects in the CNTs, resulting in chemical modifications that generate functional groups. They are primarily functionalized with hydrophilic groups, such as carboxyl and hydroxyl groups, to improve their dispersibility in polar solvents [66, 67]. However, the inherent properties of CNTs can also be altered. Buckypaper membranes constructed from hydrophilic functional group-modified CNTs exhibit hydrophilic properties. Conversely, buckypaper membranes with enhanced hydrophobicity obtained through the silanization, the attachment of an organosilyl group to some chemical species, of CNTs with alkoxysilane moieties have also been reported [68]. In addition, polymers such as polystyrene are reportedly covalently attached to the buckypaper surface by photopolymerization to enhance the hydrophobicity of the buckypaper surface [68]. The attachment of functional groups to the surface of CNTs by electrochemical methods using aryl diazonium salts has also been reported [6971].
As the buckypaper membrane comprises a network of randomly entangled CNTs, it is a major disadvantage that uniformly sized pores cannot be expected. The pore size distribution of the buckypaper membrane is very wide [72] (Fig. S3a). Buckypaper membranes fabricated with multi-walled CNTs have been reported to have pores of approximately 20–40 nm [73, 74]. The pore size of buckypaper membranes is influenced by the type of CNTs and their dispersibility in the solution. To render the pore size more uniform, the average pore size can be controlled by mixing CNTs with different diameters [22] or by controlling the pore size through the CNT length [73]. In addition, a method to improve the porosity by dispersing CNT and polymer beads together has also been reported [75] (Fig. S3b and c).
Reportedly, membrane distillation can best utilize the inherent hydrophobicity of CNTs. Membrane distillation is currently being investigated as an alternative to reverse osmosis and other desalination technologies [76] (Fig. S4a). In this method, liquids from hot seawater or brackish water in the feed cannot pass through air gaps or empty spaces inside the membrane. However, owing to the difference in the partial vapor pressure between the hot feed and cold permeate, water vapor can pass through the membrane and condense in the cold permeate to produce fresh water. Therefore, membrane distillation requires a hydrophobic, porous membrane that is impermeable to liquid and permeable to water vapor. CNTs have inherent hydrophobic properties, and CNT buckypaper membranes have a porous structure suitable for membrane distillation. Water vapor permeabilities as high as 3.3 × 10−12 kg/(m s Pa) have been reported for CNT buckypaper membranes [77, 78]. Hydrophobic CNT bucky paper membranes can also be used for the separation of oil/water and water-in-oil emulsion [68, 79].
Hydrophilic buckypaper membranes can be used as ultrafiltration membranes, given their reported pore sizes. Removal rates of 80% for spherical polystyrene particles of 100 nm diameter and 98% for particles of 500 nm diameter have been reported [22]. In addition, given the porous structure of buckypaper, it is difficult to expect nano-filtration-level capabilities for removing multivalent ions and small solutes. As previously mentioned, the functionalization of CNTs by chemical oxidation generally results in the formation of various hydrophilic functional groups, such as carboxylic, carbonyl, and hydroxyl groups, on their surfaces. These hydrophilic functional groups can be used to adsorb and remove heavy metal ions. The oxidation of multi-walled CNTs with nitric acid to form oxygen-containing functional groups, which can adsorb and remove nickel (II) [80] (Fig. S4b), and the chemical modification of CNTs with hydrophilic functional groups to adsorb and remove lead (II) and copper (II) have been reported [81, 82]. Hydrophilic CNT buckypaper has been reported to separate humic acid solutions by allowing only the permeation of water [83].
In addition, CNT buckypaper can be used as an electrode for capacitive deionization (CDI), an electrochemical desalination technology [8488]. This technology is operated in the same manner as that of a battery, in which brine of low salinity (<5000 mg/L) is passed between two electrodes. When an electric field is applied to the two electrodes, sodium and chloride ions are removed by electrochemical adsorption on the anode and cathode, respectively. Electrodes used in this technology must be electrically conductive and porous. Therefore, the buckypaper comprised of conductive CNTs has suitable properties for use as an electrode in CDI [89] (Fig. S4c).
Buckypaper can be used as a high-efficiency particulate air filter to remove fine particles with diameters of 100–500 nm and can filter powdered organic dyes and condensed lead fumes [90].
A major cause of membrane degradation in water treatment is biofouling, in which microorganisms multiply on the membrane surface and form a thick layer that reduces permeability. Therefore, there is a need to develop membranes with antibacterial properties. CNT buckypaper membranes have been reported to not only filter micro-scale microorganisms but also inactivate microorganisms by damaging their cell membranes [9193].
As previously mentioned, CNT buckypaper membranes have several advantages, such as easy fabrication, a high specific surface area, and a porous structure. However, there are certain challenges to expanding the value of CNT buckypaper membranes. The loss of the intrinsic properties of CNTs during chemical treatment and dispersion using sonication is a major limitation. A large amount of CNTs is required for the scale-up of buckypaper, which increases production costs. In addition, vacuum filtration is not suitable for producing buckypaper with a large area and uniform specifications. CNT bucky-paper membranes by vacuum filtration, for instance, have a wide distribution in pore sizes, which is the most important factor in conventional membranes for water treatment. This results in a wide distribution of removable particle sizes. Therefore, it is necessary to develop appropriate coating technologies that enable the fabrication of large-area buckypaper with uniform specifications. The mechanical properties of buckypaper are determined by the interactions between the CNTs. The introduction of heterogeneous materials such as polymers to strengthen the interaction degrades the electrical and chemical properties. Therefore, it is necessary to develop a method to strengthen the interactions between CNTs without introducing heterogeneous materials. In addition, achieving uniform pore sizes remains a challenge owing to the wide pore size distribution. Nonetheless, these membranes are useful in membrane distillation, oil–water separation, and heavy metal ion adsorption, and more.

5. VA-CNT Membranes

5.1. Classification of VA-CNT Membranes

The VA CNT membranes reported thus far can be classified into three types: open-ended, outer wall, and template-synthesized membranes. Fig. 4 shows schematic illustrations of the VA-CNT membranes: an open-ended membrane (Fig. 4a) and an outer wall membrane (Fig. 4b). Open-ended membranes, the most reported type of membrane, use the inner empty space of VA-CNTs as pores. To fabricate this type of membrane, the outer space among VA-CNTs must be filled with materials such as polymers or inorganic substances that are impermeable to gases and liquids (first step: outer gap filling), and then both closed-end caps of the VA-CNTs must be opened by removing the caps with plasma or mechanical methods (second step: opening of the end caps) [16, 9496]. These two steps–gap filling and opening of the end caps–are key processes, in which both ends of a CNT are opened using the mechanical method [95] and plasma [97]. The properties of the open-ended membrane are determined based on the properties of the VA-CNTs. The density and pore diameter of the membrane are the density and inner diameter of the VA-CNTs, respectively. The pore densities of the open-ended membranes reported thus far are approximately 1010 #/cm2, and membranes with pore diameters of 2–10 nm have also been reported [1618]. The rejection performance of open-ended membranes has been reported to be comparable to that of ultrafiltration membranes or nano-filtration membranes, and their permeability has been reported to be more than 10-fold higher than that of commercial membranes with similar pore diameters [98]. This superior performance is due to the high density and pore tortuosity of the membrane compared to those of conventional membranes, as well as unique nanofluidic phenomena, such as frictionless flow and water molecule alignment inside the inner pores of the CNTs, as previously mentioned. However, because the VA-CNTs used in open-ended membranes are synthesized by chemical vapor deposition, it is difficult to control their density, diameter, and tortuosity after synthesis. Therefore, it is impossible to control the properties of the membrane, and the fabrication process is relatively complicated.
The outer wall membrane is a type of membrane that uses the outer space existing between VA-CNTs as the pores of the membrane [98101]. The space between VA-CNTs is tens of nanometers, giving it a pore size comparable to that of an ultrafiltration membrane. However, they exhibit pore diameters that are relatively larger than those of the open-ended membranes. An advantage of the outer wall membrane is that the synthesized VA-CNTs are used without any post-processing, eliminating the need for a complex fabrication process similar to that for open-ended membranes. In addition, the size of the outer space between the VA-CNTs (i.e., the pore size and density of the outer wall membrane) can be controlled through densification. Therefore, densification, to increase the number per unit area, or density, of a nanomaterial such as CNTs, is the most crucial process in the fabrication of outer wall membranes and can overcome the disadvantages of outer wall membranes, such as pore diameters larger than those of open-ended membranes. Capillary densification [99, 102] and mechanical densification [98, 103] have been introduced for the densification process (Fig. S5). Capillary densification is the most widely known method, in which the capillary force generated between the VA-CNTs attracts the CNTs when the liquid absorbed between the VA-CNTs is evaporated. The method using capillary force can bring the VA-CNTs into close proximity to each other, which can reduce the pore size of the membrane from 28 nm to 6 nm and increase the pore density from 1010 #/cm2 to 1012 #/cm2 [99]. However, the degree of densification cannot be controlled because the capillary force cannot be controlled. Therefore, through capillary densification, the pore size can be reduced to the greatest extent possible, and the density can be increased to the greatest extent possible; however, it cannot be controlled. In addition, densification can reduce the alignment of VA-CNTs. In contrast, mechanical densification is a simple method that applies a mechanical force perpendicular to the axis of the VA-CNTs to densify them, and the degree of densification can be controlled by the magnitude of the applied force (Fig. S5b, S6a and b). The mechanical densification method was first proposed by the Wardle group [103], and the first application of this method to an outer wall membrane for water treatment was reported by Lee et al. [98]. As mentioned previously, mechanical densification can control the degree of densification. Lee et al. [98] defined a densification factor (Df) to quantitatively express the degree of mechanical densification. The densification factor is the area ratio (A/A0) of the VA-CNTs before and after densification. Df = 1 indicates no densification, and Df = 10 indicates that the area of the VA-CNTs is reduced by 1/10 after densification. Table 1 lists the characteristics and performance of the densified outer wall membranes. The pore densities of the outer wall membranes with densification factors Df = 1, 3, 6, and 10 were 8.14, 26.4, 50.8, and 83.3 × 1010 #/cm2, respectively (Fig. S6a and c), and the pore density of the densified outer wall membrane with Df = 10 was 10-fold higher than that of the non-densified membrane with Df = 1. The pore diameter was reduced from 37.8 nm to 6.7 nm (Fig. S6d). In addition, the pore tortuosity was closer to a straight line with a higher densification factor (Fig. S6e). The permeability and particle removal performance of mechanically densified outer wall membranes exhibit interesting results. For traditional membranes, there is a trade-off between permeability and rejection rate. To rephrase, reducing the pore size to remove fine particles accompanies a reduction in permeability. This trade-off relationship is a major factor limiting the performance improvement of traditional membranes. Fig. S6f and g shows the water permeability and particle rejection rate of the densified outer wall membrane. For the outer wall membrane, the reduction in pore size through mechanical densification did not result in a decrease in permeability. In contrast, the permeability increased compared with that before densification, and the rejection rate for fine particles increased. This result is because, as mechanical densification is performed, pore size decreases while pore density and tortuosity increase. Consequently, the outer wall membrane shows a tendency to overcome this trade-off. Another reason for overcoming this trade-off relationship is the increase in water permeability due to structural changes in the pores of the outer wall membrane [98]. In the outer wall membrane, the pore walls are the outer walls of the VA-CNTs. Before densification, the pores in the outer wall membrane are surrounded by an outer wall and are not closed; that is, they can be considered as an open channel structure. Before densification, the porosity of the outer wall membrane is greater than 96%. However, at the maximum densification (Df = 10), the pores in the outer wall membrane become closed channels that are entirely surrounded by the outer wall of the CNT. The channels surrounded by the outer walls can be considered to have the same pore structure as an open-ended membrane, which utilizes the inner space of the VA-CNTs. Therefore, the fast mass transport phenomenon that occurs in the pores of the open-ended membrane may also occur in the pores of the densely packed outer wall membrane, which can contribute to an increase in water permeability. The fact that the water permeation rate increases with the degree of densification supports the reason for the enhanced permeability (Table 1).
Based on the above results, the permeability of the outer wall membrane can be increased by increasing the permeation rate (velocity) of the water molecules near the wall surface [98]. This can be achieved by a simple post-treatment of the outer wall membrane. The outer walls of VA-CNTs synthesized by chemical vapor deposition have amorphous carbon deposited on them [104]. These amorphous carbons hinder the frictionless flow of water molecules around the outer wall. Therefore, by removing these amorphous carbons, the enhanced slip behavior of the water molecules around the outer wall can lead to an increase in water permeability. Amorphous carbons have a lower oxidation temperature than sp2-bonded carbon atoms in the CNT wall [105, 106]; therefore, they can be removed without damaging the outer wall by simple heat treatment [98]. After heat treatment under optimized heat treatment conditions, the permeability of the outer wall membrane with Df = 10 was more than doubled (5,800 → 13,200 LMH-bar). The increase in slip length from 12 to 29 μm before and after amorphous carbon removal, respectively, supports the increase in permeability due to the enhanced frictionless flow of water molecules near the outer wall after the removal of amorphous carbon [98] (Table 1).
To date, two types of vertically arranged CNT membranes have been reported. Since the first proposed open-ended membrane was reported, the advent of outer wall membranes has increased the potential for the utilization of VA-CNT membranes.
In contrast, VA-CNT membranes, including open-ended and outer wall membranes, have distinct advantages. Open-ended membranes utilize the inner space of VA-CNTs as pores, whereas outer wall membranes utilize the external space between the CNTs. VA-CNT membranes exhibit high water permeability, which is attributed to the frictionless flow and alignment of water molecules within the nanotubes. Moreover, they are resistant to fouling, which renders them promising candidates for water treatment and filtration applications.
However, despite its advantages, the specification of VA CNT membranes is determined by the VA CNT synthesis process. The synthesis of VA CNTs is mainly carried out in a high-temperature CVD process and is synthesized on a silicon substrate with a metal catalyst. As a result of this synthesis process, the area of the VA CNT arrays is determined by the size of the CVD chamber and substrate. This makes it difficult to fabricate large-area VA CNT membranes. In the case of open-ended membranes, the pore density and size cannot be controlled because the density of the VA CNT arrays synthesized on the silicon substrate and the size of the internal pores become the pore density and size of the membrane. In addition, the rather complicated fabrication process such as infiltration and end cap opening process for open-ended membranes is a challenge that needs to be solved. On the other hand, the outer wall membrane does not require infiltration and opening processes, so the manufacturing process is simple, and the density and pore size can be controlled through densification as described earlier. However, the pore size is larger than that of open-ended membranes that use the internal pores of CNTs.

5.2. CNT Wall Membranes

The CNT wall membrane recently proposed by Lee et al. [82] is a novel type of VA-CNT membrane that uses the entire space in the VA-CNTs as membrane pores and can be considered a combination of an open-ended membrane and a densified outer wall membrane [98]. Fig. 5 shows a schematic diagram of the CNT wall membrane. After the synthesized VA-CNTs are mechanically densified to form an outer wall membrane, both ends of the VA-CNTs, that is, both surfaces of the outer wall membrane, are opened by removing the end caps with oxygen plasma. Thus, both the outer spaces between the VA-CNTs (pores in the outer wall membrane) and the inner pores (pores in the open-ended membrane) can be used as pores in the membrane. Therefore, the most crucial processes in the fabrication of CNT wall membranes are the densification and opening of the end caps. The oxygen plasma treatment introduces hydrophilic functional groups at the ends of the CNTs in addition to the removal of the end caps. After the end caps of CNT are removed, defects are formed at the ends of the carbon nanotube, and oxygen reacts with these defects to form oxygen functional groups. This indicates that the surface of the membrane changes from hydrophobic to hydrophilic. CNTs are inherently hydrophobic. Recent simulation studies have predicted that water molecules require a pressure of over 120 bar to pass through the pore entrance of a hydrophobic CNT and over 1000 bar to pass through the exit [107]. This indicates that a very high amount of energy is required for water molecules to pass through the surface of the hydrophobic CNT membrane, and energy losses are inevitable at the entrance and exit of the membrane. CNT wall membranes are treated with oxygen plasma to render the surface of the membrane hydrophilic, while the interior (CNT walls) remains hydrophobic, allowing water molecules to flow frictionlessly. This membrane structure is optimal for water molecules to move rapidly without energy loss.
As previously mentioned, the CNT wall membrane utilizes both the internal and external spaces of VA-CNTs as membrane pores, resulting in a twenty-fold increase in pore density compared to that of the open-ended membrane and a two-fold increase compared to that of the outer wall membrane with Df = 10. Fig. 5b and c shows the comparative water permeabilities of a commercial membrane, an open-ended membrane reported to date, a densely packed outer wall membrane, and a CNT wall membrane. The permeability reached approximately 30,000 LMH bar, which is the highest permeability of any membrane with a similar pore size reported to date, including the VA-CNT membranes. The high permeability of the CNT wall membrane is not only due to the increased pore density but also to the membrane structure optimized for water permeation owing to the hydrophilic modified surface. This is supported by the fact that the water permeation rate is higher than that of the heat-treated outer wall membrane with a Df = 10 (Table 1).
The CNT wall membrane can remove dextran particles with a size of 12 kDa (Stokes diameter: 5.9 nm) and shows the same rejection performance as that of an outer wall membrane with a Df = 10, an open-end membrane, and a commercial ultrafiltration membrane.

5.3. Reverse Osmosis Membrane with VA-CNT

As a unique example of utilizing VA-CNTs, their use as a support layer for a reverse osmosis membrane has been reported [108]. A polyamide layer, which is widely used as an active layer in conventional reverse osmosis membranes, was grown directly on the top surface of the VA-CNT (Fig. 6a–e). It provides a substantially higher number of pores available for water flow, including a higher number of straight pores that create a shorter effective path to transport water (Fig. 6f), thereby resulting in more rapid water flow in the supporting layer compared to that in conventional polymeric support. This results in improved permeability without sacrificing the salt rejection rate. The results demonstrate that the pore structure of the support layer, such as the porosity [109] and tortuosity, is a crucial design element for achieving high-performance water treatment membranes with high permeability.

5.4. Antifouling

Membrane fouling is a major factor in the degradation of membrane performance and is directly linked to the operating costs of membrane plants. Therefore, fouling-resistant membranes are being actively developed. Recently, CNTs were reported to possess fouling resistance properties that inhibit biofilm formation [1921]. The antifouling resistances of the outer wall membranes and CNT wall membranes have been studied by Lee et al. [98]. Fig. 7a shows that the density of P. aeruginosa attached to the surfaces of the outer and the CNT wall membranes after the first 24 h was significantly lower than that of the commercial polysulfone membrane. After 72 h, a thick biofilm formed by P. aeruginosa growth was observed on the surface of the commercial membrane. However, minimal biofilm was observed on the surfaces of the outer and CNT wall membranes. Based on this result, it can be summarized that the resistance of the outer and the CNT wall membranes to membrane fouling occurs on two fronts: first, in resisting the surface adhesion of Pseudomonas aeruginosa, and second, in inhibiting biofilm formation resulting from Pseudomonas aeruginosa reproduction. The antibacterial properties of CNTs have been reported to physically destroy bacterial cells and inhibit biofilm formation owing to their high aspect ratio geometry [1921] (Fig. 7b). The resistance to surface adhesion was first reported in VA-CNT membranes and is attributed to the unusual porous structure of the VA-CNTs [98]. The porosity of the Df = 10 outer wall membrane is 67%, and that of the CNT wall membrane is 94%. This high porosity indicates that there is a significantly smaller solid area to which bacteria can attach. Therefore, the resistance to bacterial growth and adhesion of the two types of CNT membranes is a unique antimicrobial effect that conventional commercial membranes and other CNT membranes do not possess.

5.5. Fast Mass Transport Phenomena through VA-CNTs

The high permeability of VA-CNT membranes is attributed to the unique nanoflow phenomenon that occurs within the pores of the CNTs [18, 110112]. Fig. 8 shows a schematic representation of the fast transport phenomenon occurring within the CNTs. Water molecules can move unimpeded by friction from the walls and pass through the tubes in an aligned state, which allows them to pass through the CNTs at high speeds without colliding with each other.

5.5.1. Frictionless flow

CNTs are inherently hydrophobic. This indicates that there are no attractive forces between the water molecules and the surface of the CNT. Furthermore, the CNT wall has an atomically smooth surface, and water molecules around the wall can travel through the tube at very high speeds without encountering any friction from the wall [113]. The permeation rate of water molecules by frictionless flow in the tube has been predicted to be more than 2000-fold higher than that calculated by the traditional continuum theory [16, 17] and has recently been experimentally demonstrated [17, 98]. The slip length is a quantitative measure of this frictionless flow. The slip length is physically defined as the distance that water molecules around the wall surface of a CNT can travel without friction from the wall surface or the velocity of the water molecules (Fig. 8a, left). Two methods have been proposed to calculate slip length: the direct and indirect methods [114]. The direct method is based on molecular dynamics, while the indirect method is based on the ratio of the theoretically calculated flow rate (or flow velocity) with the boundary condition as the non-slip condition(=friction condition) to the experimentally measured flow rate (or velocity under frictionless flow conditions) [16, 98]. Equation 1 is an indirect method for calculating the slip length when the flow in a VA-CNT membrane is assumed to be a Hagen–Poiseuille flow.
(1)
Q(λ)QNS=1+4λa
The measured flow rate (Q(λ)) or velocity (V(λ)) for the frictionless flow condition uses the experimentally measured flow rate of the VA-CNT membrane, and the flow rate (QNS) for the friction condition uses the theoretical flow rate or velocity calculated by the Hagen–Poiseuille equation [QHP=(πd4Δp)/(128 ηL)] from the Stokes equation [16, 98], where a is the pore radius, and λ is the slip length. The slip length can be calculated easily using Equation 1. The slip lengths of the VA-CNT membranes are shown in Fig. 8b.

5.5.2. Water molecule alignment

When water passes through confined channels at the nanometer scale, such as the spaces inside CNTs, water molecules have unique structures that are not observable in bulk water [115]. The state of the water molecules in the tube is primarily based on the results of the molecular simulations [115]. Water molecules in the tube are aligned in one dimension through strong hydrogen bonding (Fig. 8a, right). In this aligned state, there are no collisions between the water molecules and the CNT walls; therefore, water molecules can pass through the tube at high speeds without losing their energy. In addition, it has been reported that the non-polar CNT wall repels water, causing a difference between the density of water molecules near the wall and that of water molecules in the center of the tube. In addition, the water molecules near the wall form a vapor layer such that the water molecules can move through the tube at a high speed in an aligned state without colliding with the wall. This result indicates that the high water permeability of VA-CNT membranes is due to the fast transport of water, which cannot be predicted using classical theoretical models.

6. Conclusions

CNT-based membranes, including buckypaper, VA-CNT, and CNT mixed membranes, exhibit remarkable potential for various applications within the domain of membrane-based separation technologies. These membrane architectures leverage the outstanding properties of CNTs, such as their high surface area, mechanical strength, and hydrophobicity, and offer an array of advantages for specific separation applications. These membranes offer exceptional promise in membrane technology owing to their unique properties, such as their hydrophobicity, high permeability, and resistance to fouling. The fascinating nanoflow phenomena within the CNTs, characterized by frictionless flow and water molecule alignment, underscore their outstanding transport capabilities. Elucidating and harnessing these properties are vital for advancing membrane technology to enhance efficiency and sustainability.
Buckypaper membranes, which mimic non-woven paper-like structures, have shown promise for diverse applications, including membrane distillation, oil–water separation, and heavy metal ion adsorption. However, the challenges related to achieving uniform pore sizes within these membranes must be addressed, highlighting the need for further research and development.
The classification of VA-CNT membranes into open-ended, outer wall, and CNT wall membranes presents innovative approaches for overcoming the challenges of uniform pore sizes. Open-ended membranes offer precise control over pore density and size, showing the potential for a wide array of applications. Outer wall membranes, which leverage the external space between VA-CNTs, also demonstrate significant potential for achieving high-density pore configurations. The recent advancement of CNT wall membranes, which combine both open-ended and outer wall membranes, has resulted in a substantial increase in pore density and resistance to fouling, rendering them highly attractive.
In addition, the incorporation of CNTs into mixed membranes is a notable advancement. This approach further enhances their versatility, enabling the fusion of CNT advantages with those of diverse materials to achieve improved performance. These mixed membranes, which combine CNTs with various polymeric materials, present a synergistic approach that combines the advantages of CNTs with the structural support and functionality of polymers. This strategy extends the range of applications and enhances the overall performance of the membranes.
The unique flow behavior and properties of CNTs described above can be utilized to add functionality to membranes for water treatment. For example, electrically conductive water treatment membranes could be utilized for electrically controlled anti-fouling and controlled permeation of conductive particles. excellent mechanical and chemical properties of CNT can be utilized to develop membranes for extreme environments, and as a flexible and mesoporous material, they have potential applications in catalysis, sorption, gas sensing, optics, photovoltaics and macromolecule separation.
In the area of carbon nanotube (CNT)-based membrane technology, future research efforts can be focused on innovating CNT structures and functionalized composites to increase efficiency and selectivity. This will require exploring scalable fabrication techniques and integration strategies for large-scale production and commercialization of CNT membranes. In addition, there is a surge of interest in multifunctional membranes that can expand their applications by integrating catalytic nanoparticles and reactive polymers. As the field evolves, It expects to see the emergence of dynamically adaptive CNT membranes that can adapt to different conditions or stimuli, enhancing their versatility and efficiency. At the same time, energy efficiency remains a focus, with research being directed towards pressure-driven processes, low-energy regeneration methods, and integration with renewable energy sources. In addition, tailoring CNT membranes for specific applications through property optimization is expected to lead to breakthroughs in a variety of fields. Advanced characterization techniques, including microscopy, spectroscopy, and modeling, play a key role in elucidating structure-property relationships, which can lead to improvements in CNT membrane design and functionality. Collaborative efforts in this direction are expected to revolutionize the future landscape of CNT-based membrane technology.

Supplementary Information

Acknowledgments

This work was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (NRF-2021R1I1A3040360), by the Technology development Program(RS-2023-00322197) funded by the Ministry of SMEs and Startups(MSS, Korea) and by a research grant from the Waste to Energy Recycling Human Resource Development Project of the Korean Ministry of Environment (ME).

Notes

Conflict-of-Interest Statement

The authors have no conflicts of interest to declare.

Author Contributions

B.L. (Postdoctoral Researcher) wrote the paper and drafted the manuscript. C.K. (Associate Professor) critically reviewed and revised the finalized manuscript. All authors commented on the manuscript.

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