Water-dispersible carbon nanotubes (WD-CNTs) have great importance in the fields of biotechnology, microelectronics, and composite materials. Sidewall functionalization is a popular method of enhancing their dispersibility in a solvent, which is usually achieved by strong acidic treatment. But, treatment under such harsh conditions deviates from green chemistry and degrades the structure and valuable properties of CNTs. Alternative safer and easier plasma method is discussed to produce functionalized CNTs (f-CNTs). The f-CNTs remain dispersed in water for more than 1 month owing to the attachment of a large number of carboxyl groups onto their surfaces. The WD-CNTs are applied to produce conductive cotton textile for the next generation textile technologies. Nonconducting cotton textile becomes electroconductive by repeatedly dipping into the f-CNT-ink and drying in air. The f-CNTs uniformly and strongly cover the individual cotton fibers. After several cycle of dipping into the f-CNT-ink, the textile becomes conductive enough to be used as wire in lighting up an LED. As a demonstration of practical use, the textile is shown as a conductive textile heater, where the textile can produce uniformly up to ca. 80°C within ca. 5 min by applying an electric power of ca. 0.1 W/cm2.
- carbon nanotubes
- water dispersibility
- electroconductive cotton
Carbon nanotubes (CNTs) possess a unique place in nanoscience owing to their exceptional electrical, thermal, and mechanical properties . They have found applications in areas diverse as composite materials, energy storage and conversion, sensors, drug delivery, field emission devices, and nanoscale electronic components . Water-dispersible CNTs (WD-CNTs) have great importance in the fields of biotechnology, microelectronics, and composite materials [2–5]. However, the stable dispersion of CNTs in solvent without changing their physical properties is a significant challenge and a prerequisite for their applications [6–8]. Sidewall functionalization is a popular method of enhancing the dispersibility of CNTs, which is achieved usually by oxidizing CNTs by strong acids or oxidative gases [6, 9–10]. But, treatment under such harsh conditions deviates from green chemistry, and effect in the change of the CNT structure [9–11], which markedly degrade their basic properties [6, 12]. To overcome these problems, alternative safer and easier functionalization methods should be considered. In this chapter, we will discuss about different functionalization methods, especially, the method we have developed in our laboratory to functionalize the CNTs to enhance their water dispersibility.
Possible applications of WD-CNTs will also be discussed, where we will demonstrate the applications of the WD-CNTs to produce conductive cotton textile for the next generation textile technologies. Integration of electronics is one of the smart applications of the textile, which covers the applications in high-performance sportswear, wearable displays, new classes of portable power, and embedded health monitoring devices [13–20]. Recently, interests on the preparation of lightweight and flexible electrothermal materials have been increased for the aviation and aerospace industries, microreactor technologies, and different kinetic systems [14–20]. Resistive wires made of metal alloys have been used as the heat source in many appliances, but in those cases flexibility is poor, and the heat is localized at the wires . If flexible cotton textile can be used as a heating element, it would offer a spectrum of advantages over these traditional materials [13, 20–23].
2. Potential applications of CNTs
2.1. In composite technology
Because CNTs have the highest strength to weight ratio of any known material, combining them with other materials into composites can be used to build lightweight spacecraft, windmill blades to increase the amount of electricity generated, stronger bicycle components made by adding CNTs to a matrix of carbon fibers, cables strong enough to be used for the space elevator to drastically reduce the cost of lifting people, and materials into orbit. In addition, new materials combined with nanosensors and nanorobots could improve the performance of spaceships, spacesuits, and the equipment used to explore planets and moons.
2.2. In biotechnology
CNTs can easily penetrate membranes such as cell walls . The long and narrow shape makes them look like miniature needles, so it makes sense that they can function like a needle at the cellular level . Medical researchers are using this property by attaching molecules to CNTs that are attracted to cancer cells to deliver drugs directly to the diseased cells. Another interesting property of CNTs is that their electrical resistance changes significantly when other molecules are attached to the carbon atoms . This property is utilized to develop sensors that can detect chemical vapors such as carbon monoxide or biological molecules . They are also used to improve the healing process for broken bones by providing a CNT scaffold for new bone material to grow on.
2.3. In electronics
CNTs can be used to increase the capabilities of electronics devices while reducing their weight, size, and power consumption, for example display screens on electronics devices or highly dense memory chips with a projected density of one terabyte of memory per square inch or greater. CNT ink is used in inkjet printers for printable electronics devices.
2.4. In environmental issue
CNTs are being used in several applications to improve the environment. These include cleaning up existing pollution, improving manufacturing methods to reduce the generation of new pollution, and making alternative energy sources more cost effective. Inexpensive CNT-based sensor can detects bacteria in drinking water. Because of the small size of CNTs with high surface area, a few gas molecules are sufficient to change the electrical properties of the sensing elements. This allows the detection of a very low concentration of chemical vapors.
2.5. In energy
Use of CNT in solar cells can reduce manufacturing costs as a result of using a low temperature process instead of the high temperature vacuum deposition process typically used to produce conventional cells made with crystalline semiconductor material . They can reduce installation costs by producing flexible rolls instead of rigid crystalline panels, and therefore can be installed as a coating on windows or other building materials as integrated photovoltaic . CNTs can decrease the power needed to run reverse osmosis desalination plants because water molecules pass through CNTs more easily than through other types of nanopores. They are used to make current collecting layer for the cathode in batteries and as electrodes in thermocells that generate electricity from waste heat. Combining CNTs with buckyballs and polymers inexpensive solar cells can be produced by simply painting on a surface. CNT-based supercapacitors do even better than batteries in hybrid cars by significantly reducing the weight of the batteries needed to provide adequate power, increasing the available power, and decreasing the time required to recharge a battery.
2.6. In consumer products
CNT has already found its way into lots of consumer products such as fabric, sporting goods, cleaning products, food, building materials, and skin care. The composite fabric with CNTs allows improvement of fabric properties without a significant increase in weight, thickness, or stiffness.
3. Functionalization of CNTs
CNTs in all their forms are difficult to disperse and dissolve in water or organic media [24, 28]. They are extremely resistant to wetting, which is very important for different applications. A suitable functionalization of the CNTs, i.e., the attachment of chemical functionalities represents a strategy for overcoming these barriers, and thus become an attractive target for synthetic chemists and materials scientists. Functionalization can improve dispersibility  and processibility, and will allow combination of the unique properties of CNTs with those of other types of materials. Chemical bonds might be used to tailor the interaction of the CNTs with other entities, such as a solvent, polymer and biopolymer matrices, and other nanotubes. Functionalized CNTs might have mechanical or electrical properties that are different from those of the unfunctionalized CNTs, and thus may be utilized for fine-tuning the chemistry and physics of CNTs.
Both covalent and noncovalent functionalizations of CNTs are possible [6, 24, 31]. Different possibilities of these are the sidewall functionalization, defect-group functionalization, noncovalent exohedral functionalization with molecules through
3.1. Functionalization to increase water dispersibility
One of the most common functionalization techniques is the oxidative treatment of CNTs by liquid-phase or gas-phase oxidation, introducing carboxylic (−COOH) groups and some other oxygen-bearing functionalities such as hydroxyl, carbonyl, ester, and nitro groups into the tubes. In this process, CNTs are treated by strong acids, such as refluxing in a mixture of sulfuric acid and nitric acid [9, 10], “piranha” solution (sulfuric acid-hydrogen peroxide) , boiling in nitric acid , or treating with oxidative gases, such as ozone [6, 33]. Upon oxidative treatment the introduction of −COOH groups and other oxygen-bearing groups at the end of the tubes and at defect sites is promoted, decorating the tubes with a somewhat indeterminate number of oxygenated functionalities. However, mainly because of the large aspect ratio of CNTs, considerable sidewall functionalization takes place (Figure 2) . However, treatment under such harsh conditions clearly deviates from green chemistry, and results in the opening of the tube tips , shortening of the tubes , and fragmentation of the sidewalls . These markedly degrade their basic properties [6, 35]. Since reactivity is a function of curvature , the oxidative stability also depends on the tubes’ diameter and on the production process responsible for the tubes’ dimensions .
The surface modification of CNTs can be carried out through a wide range of plasma processes, which provide a cost effective and environmentally friendly alternative to other processes, related to environmental issues  and biomedical applications . Compared with other chemical modification methods, the plasma-induced functionalization presents interesting properties, is solvent-free and time efficient process. Moreover, this treatment allows the grafting of a wide range of different functional groups depending on the plasma parameters such as power, gas used, duration of treatment, and pressure . In addition, the amount of functional groups can also be tailored. This is important since having saturation of these groups on the surface can alter the electronic conductivity of CNTs. The most common plasma treatment of CNTs is the low pressure RF cold plasma, which is successfully used to bind oxygen , hydrogen , and fluorine groups . It has been observed that a complete purification of CNTs can also be reached out after their treatment in glow discharges (RF or MW) . However, it was also observed that the average diameter of CNTs decreases with treatment duration. Therefore, the nature of the plasma gas is important, because oxygenated ions or radicals are very reactive in the etching processes. However, destruction of CNT sidewalls is also observed for other less reactive plasma gas such as CF4 or Ar [45, 46]. Moreover, it was shown that UV photons promote the defunctionalization of moieties grafted on the CNTs . A probable solution could be the reduction of the power supplied to the plasma or duration of the treatment to limit destruction of the sidewalls. In this work, RF plasma is used to functionalization the CNTs using a parallel plate capacitively coupled reactor.
4. Methodology of the research
Here, an environmentally friendly approach to functionalizing CNTs has been described, which is developed to attach −COOH groups onto their surfaces, and carried out under a wet condition using citric acid solution in RF (13.56 MHz) oxygen plasma [48, 49]. CNTs are first pretreated supersonically in ethanol. Then they are wetted with citric acid solution and subsequently treated using oxygen plasma including citric acid and water. This method is safer than the methods available in the literature, as no hazardous reagents are used here. The surfaces of the CNTs are chemically functionalized with −COOH groups, and they can be easily dispersed in water. To achieve the main objective of avoiding the destruction of the structure, which could change the valuable properties of CNTs, the functionalize conditions are optimized in each step. The functionalized CNTs can be used as a multifunctional coating material in improved electronic applications , in energy storage devices , as well as in the pharmaceutical industry, particularly in the area of drug delivery or as components of biosensors [4, 51]. They are also highly suitable as a filler component for water-soluble polymer composites .
4.1. Functionalization of CNTs
A flow chart of the functionalization process is shown in Figure 3 and the setup of the plasma reactor, indicating the dissociation of oxygen, water, and citric acid molecules is shown schematically in Figure 4. 20−30 mg of CNT powder (Sigma-Aldrich, outer diameter = 10−30 nm, inner diameter = 3−10 nm, length = 1−10 µm, purity >90%) is added to 20 mL of pure ethanol (Wako Pure Chemicals Co., purity >95%) and sonicated at room temperature using a supersonic homogenizer (Sonics Vibra cell, VC 130, Sonic & Materials Inc.,
The suspension is dried under reduced pressure and soaked in 0.15 mole (5 ml) of citric acid (Wako Pure Chemicals Co., assay >98%) solution for more than 24 h. The CNTs in the solution are then placed on the lower electrode (SUS, 50 mm
The dispersibility and dispersion stability of CNTs are primarily observed by mixing 20 mg of CNTs in 10 mL of pure water with bath sonication for 2 min, then keeping the mixture undisturbed for more than 20 days. To confirm the dispersion stability of the pristine, and plasma-treated CNTs (hereinafter denoted as
FT-IR spectroscopy is used to identify the chemical groups attached onto the CNTs. Approximately 0.5 mg of the dried sample is dispersed in 1.0 mL of propanol, and the mixture is uniformly coated on a CaF2 substrate (Sigma Koki Co., 20 mm diameter and 1 mm thickness), dried, and measured using an FT-IR spectrometer (Shimadzu Co., 8700, 100 scans averaged). The spectra in this thesis are presented after baseline correction.
The dispersibility of the CNTs is also observed by a TEM (JEOL JEM-1400 Plus, acceleration voltage of 120 kV), and their structural quality is measured by a Raman spectrometer (JASCO Co., NR-1800,
4.3. Preparation of the cotton nano-composites
To obtain the cotton composite,
5. Results and discussions
5.1. Functionalization of CNTs
The enhancement of dispersion stability is confirmed from visual observation of the mixer of CNTs in pure water and measuring the settling speed for a dispersion using the absorbance data of the UV-visible spectroscopy as shown in Figures 6(a) and (b), respectively . Attachment of the functional groups onto the CNTs is studied by a Fourier transform infrared (FT-IR) spectrometer (Shimadzu Co., 8700, 100 scans averaged). The FT-IR spectra of the
The improved dispersion of the
On the basis of the above results, the basic functionalization scheme of the CNTs by the citric-acid-assisted oxygen plasma treatment is summarized in Figure 8. CNTs are long, web-like, and remain strongly aggregated. When they are dispersed in ethanol by the supersonic treatment, ethanol molecules enter the aggregated parts of the CNTs and weaken the attractive forces between them. When the sonicated CNTs are placed in the citric acid solution, the citrate and hydronium ions attack their weak parts. During the plasma reaction, oxygen, water, and citric acid molecules or ions are fragmented to generate oxygen containing ions, radicals, and CO or CO2, which react with the defect sites [56–58]. The CO and CO2 are oxidized to form −COOH groups and attach to the CNT surfaces . Also, the attached −OH groups are further oxidized to form −COOH groups [58, 60]. These functional groups enable the CNTs to readily disperse in water due to hydrogen bonds formed between the carboxylic acid groups and water molecules . The negatively charged surfaces of the CNTs repel each other and prevent them from coagulating. The polar interactions of the functional groups with the water molecules reduce the settling speed of the
Hussain et al. functionalized CNTs by H2O plasma treatment under controlled environment. Through the controlled functionalization process the electrochemical properties of the CNTs were modified, expanding the range of potential applications of the
5.2. Properties of the cotton nanocomposites
SEM images of the
After dipping in the
Thermal conductivity of the
5.2.1 The textile as a low powered flexible heater
The above results strongly suggest that the
When an electric power of approximately 0.1 W/cm2 is applied to the textile its temperature increases uniformly to ca. 84°C, which is shown in the IR image of Figure 12(c). The white color indicates the area over which the temperature reaches more than 90% of the maximum temperature. Temperature homogeneity of this textile as heating element is better than those made with only stainless steel yarns, because in the latter case, heat is produced and localized only at the conductive yarns [21, 70]. Uniform distribution and dissipation of heat allow the heating element to be located in close proximity to the heated area in order to maximize warmth/heat production/output, to minimize response time, and to eliminate hot spots. The samples with four layers show a temperature increase of ca. 40°C (ca. 0.1 W/cm2) within 0.5 min. The heat releasing ability is also high for the coated textile, which is observed from the exponential decrease of the temperature to the room temperature within few minutes. The temperature of the water could be raised more than 80°C inserting the textile heater into it.
The flexibility of the textile as heating element is studied by measuring the changes in the temperatures within 20 cycles of bending over a time frame of 80 min. It is observed that with repeated bending, the current conducting through the heating textile does not change significantly, and the decrease in temperature is very small as shown in Figure 12(d). These indicate that the flexibility of the textile is high, which along with the temperature homogeneity suggests the use of the
Mattana et al.  employed nanoscale modification of natural cotton fibers with conformal coatings of gold nanoparticles, deposition of thin layers of the conductive polymer poly(3,4-ethylenedioxithiophene) and a combination of these two processes to obtain conductive cotton from plain cotton yarns. The electrical and mechanical properties of these yarns are improved to be successfully used as conductors, in order to bias electronic devices. They demonstrated the possibility of realizing a fully textile circuit, including passive and active elements, and paves the way for a future complete integration between electronics and textiles. Kotov groups in their communication showed the coating of cotton yarn with CNTs and polyelectrolytes . Their method provides a fast, simple, robust, low-cost, and readily scalable process for making e-textiles. Even though the cotton yarn became slightly harder after being coated with SWNTs, it is still very flexible and soft, both of which are important for the wearability of electronic fabric.
It was informed that CNTs have toxic effects. Isolated CNTs in human body would make damage to the organic cells. In order to avoid these effects the textile can be covered with some additional fabric or by other means before using it in different applications, by which direct contact to the human body or diffusion into the air could be avoided.
Functionalization of CNTs is very important to realize their applications in modern technological advancements. A safe method has been developed to functionalize CNTs. In the sequence of treatments, CNTs are pretreated in pure ethanol using a supersonic homogenizer, wetted using citric acid solution, and plasma treated using RF oxygen plasma. By the plasma reaction in the presence of water vapor, O2 and citric acid, plasma species interact with them to create many kinds of ions and radicals. They attack the CNT surfaces and activate a large number of sites to enhance the attachment of −COOH groups onto their surfaces. These attached groups significantly enhance the dispersion stability of the CNTs in water. Therefore, we are able to produce highly stable dispersed
Nonconducting cotton textile becomes electroconductive by repeatedly dipping into the stable
This study was supported by the Promotion of Nano-Biotechnology Research to Support Aging, Welfare Society from Ministry of Education, Culture, Sports, Science & Technology, Japan. We used the TEM and TG/DTA at Research Institute of Green Science & Technology, Shizuoka University. We would like to thank Dr. C. Sawatari of Shizuoka University for her sincere help to do the washing test.