Introduction
According to [1], carbon nanotubes belong to a carbon molecular family that is different from that of graphite and diamond referred to as fullerene. These structures, cylindrical in shape, formed by the carbon atoms fall into two major categories. The categories are single-walled nanotubes and the other category is multi-walled carbon nanotubes (SWCNTs and MWCNTS). Each of the two major categories is associated with its own benefits as well as disadvantages for various applications in which they are used. However, there can be consideration of the multi-walled carbon nanotubes at even some other level where bucky papers and double-walled carbon nanotubes can be looked at as this paper is going to do.
In considering the single-walled carbon nanotubes, these are basically single layers of the carbon atoms “that are rolled into a seamless tube capped at each end by half-spherical fullerene structures” [1] (page 1). These ones are different from the multi-walled carbon nanotubes in a way that all the atoms that form them make a single network that is bound in a covalent manner. Based on this, the single-walled carbon nanotubes are given more unique properties (optical and electronic properties).
There has been a rapid evolution of the physics of carbon nanotubes in a research field beginning from the time they were discovered by Iijima in the year 1991 in the multiwall form and two years thereafter when they were discovered as single-walled tubes [2]. From that time, studies in various fields (both experimental and theoretical) like electronics, mechanics as well as optics have put focus on the basic physical properties as well as the possible application of the nanotubes. In the course of the decade that has passed, progress has been realized to a substantial level in all study
fields concerning the nanotubes and currently, there has been appearance of the real applications on the market [2]. The unique properties that the carbon nanotubes possess have made them to be potential materials that can be used in various applications which include sensors, catalysts and also energy storage and conversion. However, one problem of using this class of materials is their lack of solubility in many common solvents such as water.
This paper is going to look at the advantages and applications of the various forms of the carbon nanotubes including the single-walled nanotubes, the double-walled-nanotubes, the multi-walled nanotubes and the buckypapers. But before this, there is going to be consideration of the solubility of the carbon nanotubes especially in water as a common solvent since this is of great importance in making these materials to be more effective in their application.
Solubility of carbon nanotubes in water
Increasing the level of solubility of carbon nanotubes in water serves as a facilitation of chemical modification of carbon nanotubes as well as their purification and separation from impurities that are insoluble. A discussion was carried out by [3] of a new polymer wrapping technique to bring up the level of carbon nanotubes solubility. Among the water-soluble polymers that were employed for this purpose are poly vinyl pyrrolidone or PVP. There was testing of the polymer adhesion strength by employing the technique of field flow fractionation and the results gave an indication of strong uniform wrapping of the polymer on to the carbon nanotubes.
Quantifying the amount of polymer in solution was realized through employing NMR spectroscopy and there was obtaining of the total concentration of polymer by employing the absorption spectroscopy techniques. The concentration difference provides the quantity of polymer wrapped around the nanotubes. There was consideration of those factors linked to the wrapping process (thermodynamic factors) and there was suggestion of the polymer helical wrapping around the carbon nanotubes as the potential occurring mechanism [3].
According to [4], making nanotubes soluble serves as a facilitation of their separation and purification. The existing techniques of nanotubes solubilization involve utilization of synthetic polymers [5]. A limitation in the use of these kinds of polymers is that these polymers are not very much biocompatible [4]. There can be use of natural polymers in wrapping nanotubes because of the biocompatibility they have.
Consideration is made by [6] of the utilization of starch among other natural substances such as glucosamine as well as gum Arabic to make the carbon nanotubes soluble. [6] points out that, in specific terms, dissolving of carbon nanotubes failed in the case where they were in contact with an aqueous solution of starch. But on the other hand, they dissolved in the case where they were in contact with an aqueous solution of starch-iodine complex.
There was attribution of these results to the fact that there was combination of the starch amylase component with the iodine molecules to make a helix that wraps around the carbon nanotubes. It was discovered that the linear component of starch which is amylase was the major constituent which assists in the dissolving of the carbon nanotubes as on the other hand, the branched constituent which is amylopectin brings up the level of solubilizing ability of starch that is wrapped around the carbon nanotubes [4].
There can be precipitation of nanotubes from solution by adding saliva to the mixture. The amylase found in the saliva assists in the breaking of the amylase chains and carries out the precipitation of the tubes from solution. The electrical properties as well as the mechanical properties possessed by individual carbon nanotubes are very much above those of the ropes. Isolation of individual tubes from ropes has been carried out using gum Arabic.
There is knowledge about carbon nanotubes having a higher affinity for amine groups. Those compounds that have high solubility in water and have amine groups could play a great role in increasing carbon nanotubes solubility. According to [4], among the compounds that have undergone tests with the carbon nanotubes is glucosamine. [6] points out that it is clear that the potential applications of the carbon nanotubes are in the area of targeted drug delivery in which nanotubes having antibodies grafted on them can be utilized to aim and destroy tumor cells.
Advantages and Applications of carbon nanotubes
Under this section, advantages and applications of the carbon nanotubes are going to be looked at. The various forms of carbon nanotubes are going to be considered. The carbon nanotubes forms to be considered include; the single-walled carbon nanotubes, the multi-walled carbon nanotubes, the buckypapers and the double-walled carbon nanotubes.
There is anticipation among researchers about the applications of nanotubes in a number of areas of importance. One of the applications is their application as field emitters in the flat-panel display. This is an application that will possibly turn out to be available as product in the soonest time possible ahead of any other application. According to [1], demonstration was carried out by Samsung in the year 1999 of a working display prototype and it was anticipated that this company would introduce a product in the course of the year 2004. [1] points out that “in Samsung’s display, the small, rod-shaped nanotubes provide sharp conductive points that allow a field-emission display to work more efficiently than today’s TV screens and computer monitors” (page 4).
The liquid-crystal displays and the plasma displays could in the end be displaced by the single-walled carbon nanotubes in big flat panels for the reason that the CNT panels should not be very much complex and should be less costly to make [7].
Since the CNTs are quite strong, there exists some consideration for them because of the mechanical properties they have. They are stronger than steel by one hundred times at 1/6 the weight [8]. Therefore, the single-walled carbon nanotubes “may offer reinforcing elements for composite materials that would have exceptionally, and possibly, superior thermal characteristics”[1] (page 4).
Another possible application of the single-walled carbon nanotubes can be traced in ultra miniaturized electronics. [1] observes that such companies as IBM have put in place research programs meant to carry out investigation of the way they would make use of the CNTs for the coming generations of “nonsilicon microchip circuitry”, which would turn out to be about 1/100 or less the size of the existing versions that are seen to be most advanced.
More so, researchers in this field have commenced on carrying out the exploration of the possible medical applications [9]. These researchers have come up with various forms of soluble nanotubes by making use of starches, as well as proteins and DNA which have served as outer wrappings [10].
Considering multi-walled carbon nanotubes, their structure is composed of “multiple layers of graphite superimposed and rolled in on themselves to form a tubular shape” ( [11] page 5). The multi-walled carbon nanotubes have some interesting properties. These properties are the mechanical strength they have, electrical conductivity and thermal conductivity [12]. These properties, when combined, offer a great chance for these materials to be used in various applications that are of great benefit.
Since the multi-walled carbon nanotubes have high conductivity and a natural trend to form ropes and also have a high aspect ratio, they turn out to be perfect material in offering intrinsically lengthy conductive pathways and they can still carry out this at ultra-low loadings. The additive lower loading provides various benefits like processability that is improved or greater mechanical properties maintenance of the initial polymer.
It is because of this reason that utilization of CNTs for conductive applications as well as antistatic applications in polymers has been realized in the commercial field and expanding in such parts of the economy as the automotive industry as well as the electronic industry. Basing on applications like these ones, CNTs can offer competition basing on a performance of price to additives that are greatly conductive like carbon black.
Another form of carbon nanotubes are the buckypapers. These are “thin films of nano-particulate membranes” [13]. Confirmation has been carried out by the research team from the “High Performance Materials Institute” of the buckypapers as consisting of useful mechanical properties as well as thermal and electrical properties and this is for the reason of them having nanoscale dimension and the structural network that is of its own kind [14].
Buckypapers are made of carbon nanotubes which are greatly strong materials whose diameter is 0.00002 percent of the diameter of a hair of a human being. In relative terms, this material (buckypapers) is a new one whose production is supposed to be carried out in a more efficient manner so as to realize production of bulk-nanostructured complex having dispersion value that can be desired and in-plane arrangement of nanotubes [15].
These materials have the physical as well as thermal and electrical properties that are exceptional which can be utilized for various applications. These applications include miniaturization of the electrical connections, de-icing, and smart-materials among other applications. According to [16], “the potential that the buckypapers have in a variety of applications, including the development of aerospace structures, the production of more-effective body armor, armored vehicles and the construction of next-generation computer displays ([16], Para 3).
Other than the single-walled carbon nanotubes, buckypapers and the multi-walled carbon nanotubes, another form of the carbon nanotubes is the double-walled carbon nanotubes. These have been synthesized and are associated with some advantages and problems in their application. The problem that is associated with this kind of material is that their production also leads to the production of the single-walled and multi-walled carbon nanotubes that are unwanted. However, this kind of nanotubes offer benefits when they are used in transparent conductors since they are transparent optically and electrically conductive. As on the one hand there is increasing demand for energy devices that are energy efficient, on the other hand there is a rise in demand for transparent conductive films [17].
Conclusion
Carbon nanotubes have several advantages and applications which make them to be very much useful. The advantages and applications of these materials result from the unique properties they have. Despite the fact that there may be a problem in the dissolving of the carbon nanotubes in most of the common solvents such as water, the researchers have come up with some means that ensures their solubility in these solvents. Making carbon nanotubes to be soluble in such solvents as water serves as facilitation for their separation and purification and this makes them to be more effective in their applications.
The existing techniques of nanotubes solubilization involve utilization of synthetic polymers. Various forms of the carbon nanotubes have been considered which include the single-walled carbon nanotubes, the multi-walled carbon nanotubes, buckypapers and the double-walled carbon nanotubes. These materials have unique electrical, mechanical and optical properties that make them important in various applications.
The properties they possess have triggered research and many researchers have come up to investigate more and more probable applications of these materials. These researches are of great significance and will help in identifying even more applications of the carbon nanotubes. This will play a great role in achieving solutions in regard to cutting down costs and use of high quality materials in various applications.
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