Characterization and Application of Nanomaterials (2022) Volume 5 Issue 2 doi:10.24294/can.v5i2.1702 128 Original Research Article Advances in preparation and application of carbon nanotube films Yaxian Wu, Qilin Zhang * School of Mechanical and Automotive Engineering, Anhui Polytechnic University, Wuhu 241000, China. E-mail: hel- lozhangql@163.com ABSTRACT Nanotechnology is recognized as one of the high and new technologies in the 21st century. Carbon nanotubes have been widely used in molecular sieve, drug transport and seawater desalination due to their unique mechanical, electrical, optical and other excellent properties. As the main representative of carbon nanotube macroscopic materials, carbon nanotube film not only retains the microscopic properties of carbon nanotube, but also has good mechanical properties and stable chemical properties. The preparation and application of carbon nanotubes (CNTS) have attracted extensive attention from scholars at home and abroad. In this paper, the research on carbon nanotube films in recent years is reviewed. Based on the preparation of carbon nanotube films, chemical vapor deposition, LB (Lang- muir-Blodgett) film and electrostatic layer-by-layer self-assembly techniques are briefly described. In addition, the ap- plications of carbon nanotubes in biological field, photoelectric nano devices, water treatment, seawater desalination and other fields are also described. Keywords: Carbon Nanotubes; Carbon Nanotube Film; Preparation Method; Application Progress ARTICLE INFO Received: 1 August 2022 Accepted: 19 October 2022 Available online: 27 October 2022 COPYRIGHT Copyright © 2022 Yaxian Wu, et al. EnPress Publisher LLC. This work is li- censed under the Creative Commons At- tribution-NonCommercial 4.0 International License (CC BY-NC 4.0). https://creativecommons.org/licenses/by-nc/ 4.0/ 1. Research background Since carbon nanotubes were discovered by Iijima[1] in 1991, a re- search upsurge has been set off worldwide. Carbon nanotubes have been widely used in molecular sieve, drug transport, particle ex- change, seawater desalination and other aspects due to their unique mechanical, electrical and optical properties, as well as their good per- formance at the nanoscale[2–5]. In 2001, Hummer[6] studied the transpor- tation behavior of water molecules in carbon nanotubes by means of computer simulation, and selected the radius of carbon nanotubes at 0.8 nm, the one-dimensional distribution of water molecules in the carbon nanotubes was observed. It was also found that when the molecular in- teraction potential between water molecules and the pipe changed to a small degree, the pipe would change from empty state to filled state. Holt[7] found through experiments that the inner surface of carbon nanotubes with a diameter of 1 to 2 nm has a long slip length. Under the same external pressure, water flow can reach about one thousand times that predicted by macroscopic (no slip) theory. Carbon nanotube thin film is a two-dimensional carbon nanotube network structure formed by filling freely arranged carbon nanotube arrays through physical or chemical methods[8]. Since the carbon nano- tube arrays suitable for filling are perpendicular to and parallel to the substrate orientation respectively[9,10], the carbon nanotube films can be divided into horizontally aligned carbon nanotube films, vertically 129 aligned carbon nanotube films and hybrid aligned carbon nanotube films. Carbon nanotube films re- tain the original microscopic properties of carbon nanotubes, and have the advantages of low operat- ing pressure, large flux, high retention rate and low cost, so they are widely used in many fields. In this paper, the preparation methods of carbon nanotubes are reviewed, and their applications in biology, nano devices, water treatment and other fields are reviewed. 2. Preparation of carbon nanotube films At present, there are many preparation meth- ods for carbon nanotube films, such as spin coat- ing[11], electrophoresis[12], vacuum filtration[13–16], spraying[17–20] and chemical vapor deposition method, LB (Langmuir-Blodgett) membrane meth- od, electrostatic layer-by-layer self-assembly tech- nology, etc. The last three methods of preparing carbon nanotube films are briefly described below. 2.1 Chemical vapor deposition Chemical vapor deposition (CVD) is a method for preparing carbon nanotube films. This method is characterized by convenient operation, low cost and high productivity. The pore size of carbon nanotube films can be controlled by separating fluid or gas[21], which is suitable for large-scale production and widely used by researchers. Holt[22] used chemical vapor deposition meth- od to fill gaps in multi-walled carbon nanotube ar- rays with Si3N4 to synthesize carbon nanotube films, which have high water flux. Yu[23] prepared high-density vertical array carbon nanotube films by chemical vapor deposition in order to solve the problems of low porosity and low permeability efficiency. In order to significantly improve the growth efficiency of single-walled carbon nano- tubes, Hata [24] add appropriate oxidants in the prep- aration process by chemical vapor deposition, and it was found in the experiment that single-walled carbon nanotubes arrays with consistent aperture, high density and high purity could be grown in 10 min. Haque[25], through chemical vapor deposition, uses pulse laser to melt diamond and form compo- site carbon nanotube film, which is applied in such aspects as wear-resistant coating, thermal manage- ment of integrated circuits, electrochemistry, field emission equipment and electric field shielding in MEMS. 2.2 Langmuir-Blodgett (LB) membrane method LB film technology is one of the traditional film making technologies. The method is to transfer monolayer molecules to the surface of solid matrix to form a film. If multilayer films preparations are wanted, layer by layer transfer is required[26,27]. At present, LB film technology has been widely used, but its mechanical stability and thermal stabil- ity are problematic. Therefore, researchers have adopted various physical or chemical methods to improve the mechanical carrying capacity of LB film and improve its stability. LB film technology requires specific film forming equipment and strict operation, which makes it difficult to produce large area films. The stability of LB films depends on the film van der Waals forces and hydrogen bond energies on the surface of medium and substrate, molecules within and between layers. Venet[28] used LB film method to prepare films containing single-walled carbon nanotubes (SWCNTs), and studied the morphology of the films by atomic force microscopy. Under low applied voltage, pure SWCNTs layers showed rela- tively high ohmic conductivity, while mixed LB films showed nonlinear current-voltage behavior. Song[29] prepared orderly and organized carbon nanotube composite films by LB film method at room temperature, which has potential application prospect in many fields. Figure 1 is the process of preparing carbon nanotube film by Kim et al.[30] using LB film method. Figure 1(a) is the induced orientation obtained by barrier compression, Figure 1(b) is the horizontally deposited film, and Figure 1(c) is the vertically deposited film. By using this method, Kim et al. prepared carbon nanotube films with good spreading performance, and the latter had a higher degree of tube orientation than the for- mer[30]. 130 Figure 1. Preparation of carbon nanotubes by LB film method. 2.3 Electrostatic layer-by-layer self-assembly technology Electrostatic layer-by-layer self-assembly is a polyelectrolyte with an opposite charge that can be used in an aqueous solution to prepare multiple lay- ers on a deposited substrate thin film technology. Compared with LB technology, this method has the advantages of simple operation, fast film forming speed, no special equipment is required, and the film made is stable. At present, the driving force of self-assembly has changed greatly from electrostat- ic force diffusion to hydrogen bond and charge in- teraction, which provides experimental conditions for the preparation of composite carbon nanotube films such as nanoparticles and carbon nanotubes. Figure 2. Preparation of carbon nanotube films by electrostatic layer-by-layer self-assembly method. The method of preparing carbon nanotube film by electrostatic self-assembly technology[31] is shown in Figure 2. It is continuously adsorbed al- ternately on the substrate surface with positive and negative charges, so as to obtain carbon nanotube film of appropriate thickness. Lee[32] used the lay- er-by-layer self-assembly technology to prepare the surface functionalized multi-walled carbon nano- tube films with positive and negative charges. The thickness and morphology were controlled by pH value, which showed higher conductivity compared with single-walled carbon nanotube composites. Rivadulla[33] first used the coating technology to wrap the carbon nanotubes dispersed in water be- fore film formation, which was convenient to con- trol the transmission characteristics independent of the substrate. Then, the electrostatic layer-by-layer self-assembly technology was used to prepare the carbon nanotubes film, which could improve the conductivity and thermoelectric power. The above three methods of preparing carbon nanotube films are commonly used by researchers in the experimental process, which can prepare carbon nanotube films with stable performance. 3. Application of carbon nanotube films The carbon nanotube membrane retains the original excellent properties, while obtaining many outstanding properties, which can potentially im- prove the fouling resistance, hydrophilicity and permeability of the membrane. Membrane separa- tion technology can have important applications in such situations as harmful diffusion of methanol in fuel cells, retention of valuable proteins in bio- pharmaceutical processing, rejection of organics and microorganisms in water treatment, and salt penetration in desalination processes. Therefore, carbon nanotube membrane has become a research hotspot at home and abroad, and is widely used in biology, medicine, water treatment and other fields. 3.1 Biological field In 2001, De Groot[34] found that the flow of water molecules in biological systems is one-dimensional, similar to the flow of water mol- 131 ecules in carbon nanotubes discovered by Hum- mer[6] (Figure 3). Since then, researchers have in- clined to use carbon nanotubes to mimic alterna- tive biological channel proteins, which are simpler and easier to study. In nature, most biological mac- romolecules are electrically charged, so carbon nanotube membranes with better biological activity, conductivity and catalytic properties can be pre- pared with carbon nanotube membranes or nano- particles modified by biological macromolecules. Figure 3. Flow of water molecules in carbon nanotubes. Yang[35] used the layer-by-layer self-assembly method to prepare ultra-thin carbon nanotube films with positive and negative charges, which can de- tect hydrogen peroxide H2O2 at low potential. Moreover, such nanotube films have high sensitivity. Cholesterol enzyme is added on the surface of the film, and the original biological activity can be maintained on the surface of the film, which can be used for cholesterol detection. Du[36] uses electro- static self-assembly technology to assemble carbon nanotubes and DNA aptamers into a film (Figure 3), which is characterized by high sensitivity, good sta- bility and high molecular selective permeability, and can be used to detect targeted substances thrombin and dissolved alcohol. The application of carbon nanotube film as a biosensor is also a mainstream direction at present. Biosensors use enzymes, antibodies, receptors or catalysis as biosensitive materials as recognition elements, convert their concentration into electrical signals, and use carbon nanotubes as modified ma- terials for detection instruments[37]. The carbon nanotube-polymer composite prepared by adding cyclodextrin is insoluble in water, and the stability and electrical conductivity of carbon nanotubes are greatly improved due to the presence of cyclodex- trin, and it has catalytic activity; and the addition of ethyl phthalate cholinesterase can obtain carbon nanotube composite films with high sensitivity for biosensors[38]. Wee[39] added dispersed carbon nanotubes into enzyme solution to prepare phe- nol biosensor, which greatly improved its analysis and detection ability and sensitivity in water envi- ronment. 3.2 Field of optoelectronic nano devices Carbon nanotubes have been widely welcomed in the field of optoelectronic nanodevices due to their low preparation cost, good electrical conduc- tivity, good chemical stability and large specific surface area. As a new type of energy storage device, su- percapacitors (or electrochemical capacitors) have long cycle life, high power density and high energy density. It fills the gap in energy and power of tradi- tional capacitors and batteries. Wu[40] made use of the excellent performance of carbon nanotube array and put carbon nanotube array with weak mechani- cal strength into neutral gel electrolyte to prepare flexible composite carbon nanotube film and obtain flexible solid supercapacitor. Kaempgen[41] used single-wall carbon nanotube films to prepare high-performance supercapacitors, as shown in Figure 4(a), which can be used as electrodes and charge collection devices, greatly improving the performance of printed electronic devices. Carbon nanotube films can be used as elec- trodes in transparent scalable capacitor arrays and as pressure and strain sensors[42]. As shown in Fig- ure 4(b), Dharap[43] developed a carbon nanotube 132 film for strain sensor by using the characteristic that carbon nanotubes change their electrical properties when subjected to strain, and attached the insulated carbon nanotube film to brass to measure its strain. Wan[44] made a flexible nanogenerator with P (VDF TrFE) nanofiber and PDMS /MWCNT composite film. As shown in Figure 4(c), the film can work under the hybrid mechanism of triboelectric and piezoelectric. When the pressure is 5 N, the output peak voltage can reach 25 V, the power is 98.56 μ W and the power density is 1.98 MW·cm−3. The films prepared by Chen[45] with superaligned carbon nanotube arrays can be used to prepare transparent conductive films, as shown in Figure 4(d), which are comparable to the sheet resistance and conduc- tivity values of indium tin oxide (ITO) films. The carbon nanotube film prepared by Liu[46] is applied to the infrared sensor. As shown in Figure 4(e), working in voltage mode can not only reduce the low-frequency noise, but also improve the sig- nal-to-noise ratio of the detector. Figure 4. Practical application in the field of optoelectronic nano devices. 3.3 Water treatment field 3.3.1 Drinking water treatment Due to the rapid increase of population, envi- ronmental pollution is becoming more and more serious, leading to a series of natural disasters such as greenhouse effect and sea level rise. It is a diffi- cult problem for people in many areas to drink safe and healthy drinking water. Unhealthy drinking water can cause physical diseases, and even people may lose their lives. Therefore, researchers are committed to studying how to effectively remove pollutants in water and ensure the health of drinking water. Membrane water treatment is expected to play an increasingly important role in drinking water treatment, brackish water and seawater desalination, wastewater treatment and reuse, etc. Water soften- ing[47], desalination[48], removal of organic matter in water and so on are common drinking water treat- ment methods, all of which need the help of carbon nanotube film. Ion exchange is often used in water softening treatment to reduce the hardness of water. In order to achieve water softening, other polyva- lent ions in water need to be replaced by Na+. However, this method has the disadvantage that ion exchange has no effect on Na+. Chemical method is often used for desalting treatment. In this process, new reaction substances need to be added, but products harmful to human body will be produced in the reaction process. Biodegradation is a method to remove organic matter from water, but this method not only has no obvious removal effect, but also has high requirements on the type and toler- ance of microorganisms. For the removal of metal ions in drinking wa- ter, Dipankar[49] reported that water rich in sodium ions can be separated with charged polyamide 133 membrane. The results show that when the pH of the solution is reasonably acidic or alkaline, the flux decreases and the rejection rate increases. When the ionic strength remains constant, the flux and rejec- tion rate decrease with the increase of pH, which is suitable for tap water treatment. Ahmad[50] prepared two types of carbon nanotube membranes, and ana- lyzed and compared the performance of nanofiltra- tion membranes in NaCl and MgSO4 aqueous solu- tion under the operating pressure of 500 kPa and 1,000 kPa and the temperature of 25 ℃, and drew the following conclusions. (1) Asymmetric polyeth- ersulfone (PES) carbon nanotube membranes were prepared by the method of phase inversion in- duced by immersion deposition technology, and two different non-solvents were selected, and it was found that the performance of the membrane con- taining water mixture as the non-solvent was better than that of pure water as the non-solvent. (2) The composite polyamide carbon nanotube film was prepared by interfacial polymerization of 1,3-phe- nylenediamine (PDA) and TMC, and it was found that the film had high water softening ability. Car- bon nanotube film removes metal ions from wa- ter by adsorption. The carbon nanotube film pre- pared by Liu[51] has a strong adsorption capacity for Cu2+, and the maximum adsorption capacity is 4.14 mg·g−1. Figure 5(a) shows the process of elec- tro-adsorption and electro-desorption of carbon nanotube film. Wang[52] studied the adsorption ca- pacity of carbon nanotube film for Pb2+. The con- centration of Pb2+ in the original solution was 3 μg·mL-1, and the concentration after adsorption was 0.31 μg·mL-1, the removal rate could reach 89%. The MnO2/CNT composite membrane prepared by Zeng[53] can remove 97.73% of antimony when the pH value of the solution is 2.00 and the temperature is 298 K. Yu[54] also prepared Fe2O3/CNT composite membrane. When the pH value is 7.00 and the temperature is 298 K, the antimony removal rate can reach 99.97%. The comparison shows that Fe2O3/CNT membrane is superior to MnO2/CNT membrane in antimony removal performance. Drinking water is polluted not only by metal ions, but also by endocrine disrupting chemicals (EDCs), pesticide residues and chemical residues. The increase of pollutants will increase the risk of cancer, and the quality of drinking water is very important to human health. Therefore, it is urgent to remove pollutants from drinking water. Gu[55] pre- pared superhydrophobic polymer/carbon nanotube hybrid membrane with polystyrene hydrophobic polymer and carbon nanotubes, which can remove organic solvents in water, and the separation effi- ciency of water-in-oil emulsion is 99.94%, and the flux is high, which can reach 0.05 L·m−2·h−1·pa−1. Figure 5(b) is a photo of the separation of water and toluene emulsion by carbon nanotube hybrid membrane. Majewska[56] studied the transport and separation performance of DS-Ge composite poly- sulfone/polyamide membrane for atrazine. When the concentration was 20 g·m −3, the highest rejec- tion rate could reach 80%. RGO-CNT hybrid mem- brane prepared by vacuum filtration method is used to purify drinking water, which has good intercep- tion, contamination resistance and permeability[57]. Perfluorooctane sulfonate (PFOS) is a persistent pollutant in water environment, and PFOS in drinking water can be removed by using hybrid carbon nanotube membrane[58]. Similarly, there are some hormone pollutants in the water source. Car- bon nanotube membrane was used to separate sev- eral hormones and antibiotics in the solution, and its adsorption rate for hormones and antibiotics was studied. It was found that the membrane had a good adsorption capacity for tetracycline, and its adsorp- tion rate could reach 80%[59]. 3.3.2 Wastewater treatment In order to save costs and reduce the links of wastewater treatment, many factories directly dis- charge sewage containing pollutants into clean riv- ers, resulting in serious river water pollution treat- ment and bad environment, thus threatening human health. Traditional wastewater treatment methods may not meet the standard of clean water quality, and membrane water separation technology can easily solve this problem. Water separation technology can effectively treat industrial wastewater, which can replace tradi- tional wastewater treatment methods. Carbon nanotube membrane is considered as one of the best choices to remove polluted wastewater[60]. Akban[61] used carbon nanotube membrane to treat dye 134 Figure 5. Removing impurities in drinking water. Figure 6. Separation of dye wastewater by carbon nanotube membrane. wastewater, and studied the effects of concentration, pH value and salt on the flux and rejection rate. It was found that the rejection rate of cationic dyes by concentration and pH value exceeded 95%, but the flux would be reduced with the increase of salt concentration. Rahdi[62] used carbon nanotube membrane to filter refractory dye wastewater in textile industry. Under the working condition of 500 kPa, the rejection rates of green and blue dyes were 95.2% and 93.8%, respectively. Han[63] prepared high-flux carbon nanotube films from graphene and multi-walled carbon nanotubes, with water flux of 1.13 × 10−4 L·m−2·h−1·Pa−1, high dye rejection rate (direct yellow is greater than 99%, methyl orange is greater than 96%), and salt ion rejection rate in Na2SO4 solution is 83.5% and in NaCl is 51.4%. The antifouling performance of sodium alginate (SA) and humic acid (HA) is strong. Figure 6(a) is a digital photo of G-CNTm film, and Figure 6(b) is a comparison before and after removal of dye by methyl orange solution. Oil industry will inevitably produce oily wastewater, which is discharged into nature, caus- ing a series of ecological problems. Oil-water sepa- ration is to separate the oil phase from the water Figure 7. Separation of water-in-oil emulsion by independent ultra-thin SWCNT membrane. phase by using the hydrophobic, oleophobic and hydrophilic properties of carbon nanotubes. Ah- mad[64] studied the permeation efficiency of several carbon nanotube membranes in oily wastewater and the efficiency of wastewater treatment, and found that the permeation flux would increase with the increase of applied pressure and temperature. Shi[65] used carbon nanotube membranes to separate five nano-scale water-in-oil, and the flux was 2 to 3 or- ders of magnitude higher than that of commercial nanofiltration membranes, and the oil purity after separation could reach 99.9%. Figure 7 is a sche- matic diagram of the separation of water-in-oil 135 emulsion by single-walled carbon nanotube film, in which oil can selectively penetrate into the sin- gle-walled carbon nanotube film. Selby[66] uses carbon nanotubes/polysulfone/ polyvinyl alcohol composite membrane to separate oily wastewater. Experiments show that when the oil concentration is less than 10 mg·L−1, the oil retention rate can reach 95%. 3.3.3 Desalination of seawater Carbon nanotube film can allow water to flow, adsorb chemical and biological pollutants and sep- arate particles in seawater. Carbon nanotube film can not only filter Na+ and Cl- but also filter water, which can remove bacteria from water and heavy hydrocarbons from petroleum. The properties of carbon nanotube film and its preparation technology play a decisive role in seawater desalination[67]. In order to get the best permeation effect of seawater desalination, reverse osmosis technology and elec- troosmosis technology are the best choices. Amt[68] used MD simulation to study the mo- lecular transport of water through the nanopores of carbon nanotubes. The water flow in these simula- tions is driven by the permeation gradient between two self-assembled carbon nanotube membranes, as shown in Figure 8(a), this membrane separates wa- ter from saline solution and can be used for sea- water desalination. In Figure 8(b), the cation -π binds Na+ at the entrance of the (6,6) nanotube, and the upper left corner of Figure 8(b) can al- so block Na+ at the (8,8) carbon nanotube channel, the upper right corner of Figure 8(b), indicating that cation -π plays a decisive role in seawater fil- tration[69]. Farzadeh[70] studied the effect of func- tional nano-porous boron nitride nanosheets (BNNS) membrane on separating salt from seawater by mo- lecular dynamics simulation. When the membrane was placed in aqueous solution containing Na+ and Cl−, salt could be separated out. When the pressure is 30–100 MPa, the permeability and rejection of the membrane are extremely high. Tofiguhy[71] used carbon nanotube membrane to desalinate seawater, the experiment found that after 6 days, the desalina- tion rejection rate was still 1,320 mg·g−1, which indicated that using carbon nanotube membrane to desalinate seawater was economically feasible. Takizawa[72] used multi-walled carbon nanotubes (MWCNTs)- polyamide (PA) nanocomposite mem- brane technology to desalt seawater. Because of the smooth surface of MWCNTs-PA membrane and the formation of water layer at the interface, the scale inhibition performance of the membrane is very good, which is beneficial to water treatment. Sin- gle-layer transverse flow carbon nanotube film (TFCM) is used as an alternative material for effec- tive desalination. For seawater desalination, harm- ful algae are in a suspended state, preventing water circulation and blocking the filter. Wang[73] used low-pollution carbon nanocomposite membrane to separate algae from seawater, under low pressure, the average permeation efficiency can reach 99%, and the rejection rate and anti-pollution rate are also very good. Figure 8. Application of carbon nanotube film in seawater desalination. 4. Conclusion and prospect Nanotechnology is the core technology in the 21st century. With the deepening of research, the preparation and application of carbon nanotube film has become an important aspect of nanotechnology. The particularity of carbon nanotube film determines that it has a good application prospect in biology, 136 optoelectronic nano-devices, environment, water treatment and so on. However, due to the errors in the preparation process of carbon nanotube films, and the actual situation is more complicated than the experimental environment, it has certain limitations. 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