The Power of Small Things

            I never forget the moment that I saw the carbon nanotube (CNT) for the first time. It was the first day that I could be in the lab for some instrumental trainings. I was so bored when we talked about UV-Vis and IR because I already knew a lot about those instruments. So, I did not pay attention until we moved on to the next instrument, which was thermogravimetric analyzer (TGA). The instrument was not that interesting, however, what made me paying attention was the samples that was used for the training. I had never seen any samples like that in the lab before. They were small and looked like tiny plastic tubes with the length of about 1 cm. So, I was wondering about those tubes until the instructor told us they were CNT incorporated into polymer. She also told us that when incorporating into polymers, nanomaterials such as CNT can enhance the strength, thermal stability, and barrier properties of the polymer coatings. I was so excited about this type of material, so, I spent days after to do research and learn more about them. Unfortunately, my project was not related to CNT, however, I was so happy to know and learn about this amazing material.

            So, what are nanomaterials? Nanomaterials are usually considered to be materials with at least one nano-sized dimension. There are many types of nanomaterials such as carbon-based or metal-based nanomaterials. However, for this blog, I only focus on CNT by introducing about its structure, properties and one application.

CNT is a type of fullerene, one of the carbon allotropes. The CNTs were discovered in 1991 by a Japanese physicist, Sumio Iijima. Iijima focused on using transmission electron microscopy to study the cathodic soot after the arc-evaporation. Arc-evaporation is the method that uses a high current, low voltage electric arc to vaporize the material. Surprisingly, the cathodic soot contained a new graphitic structure which were long and hallow fibers that were also known as buckytubes or CNTs. Thus, nanotube has cylindrical structure capped at one or both ends with a half fullerene. The chemical bonding of nanotubes involves entirely sp²-hybridized carbon atoms which make it even stronger than diamond, sp³-hypridized carbon. CNT also has nano-sized diameter and micro-centimeter sized length, and due to the length and diameter ratio, CNTs are considered 1-D materials. 

Basic structures of a single-walled, b double-walled, and c multi-walled CNTs

Moreover, depending on the number of rolled-up graphene sheets, carbon nanotubes are also classified to single-walled, double-walled, and multiwalled carbon nanotubes. The structure of single-walled can be conceptualized by wrapping a one-atom-thick layer of graphene into a cylinder. Multiwalled consists of two to 100 or more rolled-up layers of graphene. Double-walled is considered as a special type of multiwalled wherein only two rolled up graphene sheets are present.

There are many ways to synthesize CNTs including arc discharge​, and chemical vapor deposition methods. Arc discharge method is the most common and easiest way to produce CNTs. 

Arc Discharge Schematic for CNT synthesis

A sharp carbon rod is placed in a system and biased to many thousand volts. The voltage is then discharged from the rod, rapidly heating the rod and vaporizing some of the carbons. As the carbon vapor cools down, CNTs are produced. 

Chemical Vapor Deposition Schematic for CNT synthesis

For the chemical vapor deposition method, a vaporized hydrocarbon compound, usually methane or ethane, is injected into a high temperature zone in a furnace. The hot zone contains a thin film of metal catalyst, which can be iron, nickel or cobalt, that has either separated or been pre-patterned into nanoscale islands of the metal. These nanoscale islands are used to catalyze the growth of the carbon nanotubes.  

With its unique structure of sp² hybridized C-C bonds and repeated hexagonal patterns, CNT has many properties such as remarkable strength and high electronical, thermal conductivity, and elasticity. Thus, CNTs are usually used in coating application. In the research conducted in 2016, Harb et. al. asserted that the addition of single-walled CNTs improved the scratch resistance, adhesion, wear resistance, and thermal stability of poly(methyl methacrylate)-siloxane-silica coatings[1]. Moreover, using a very simple method, superhydrophobic films can be made from the flexible CNT/polymer coatings[2]. Those films can be coated onto glass, metals or polymers for their anti-corrosion and self-cleaning benefit2. Corrosion resistance occurs since the presence of a layer between the substrate and the solution inhibits the movement of the corrosive ions[3]. Moreover, self-cleaning benefit is the result of exhibiting high water contact angles and low sliding angles of the materials. Furthermore, when coating these superhydrophobic films on steel, this will “open up possibilities for many new applications in the areas of heat transfer, solar panels, transport of fluids, nonwetting and nonfouling surfaces, temperature resilient coatings, composites, water-walking robots, and naval applications[4]”. Therefore, CNT is a very useful material.

That was the story about my first time seeing that amazing material. So, I hope after reading this blog, you can know more about nanomaterials, especially CNT. To continue studying about nanomaterials, I want to pursue a graduate degree in chemical engineering. With the degree and what I will learn in graduate school, I will have a chance to learn more about this wonderful tiny material.

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[1] https://pubs-acs-org.dml.regis.edu/doi/pdf/10.1021/acsami.6b04780

[2] https://pubs-acs-org.dml.regis.edu/doi/pdf/10.1021/jp1047985

[3] https://pubs-acs-org.dml.regis.edu/doi/pdf/10.1021/acs.iecr.7b04887

[4] https://pubs-acs-org.dml.regis.edu/doi/pdf/10.1021/la9001187