Advanced Characterisation of Carbon Nanotubes

I have chosen to use a Carbon nanotubes (CNT) as an example material for my analysis of both electronic devices and nanomaterials. Carbon nanotubes any attractive physio-chemical properties such as high mechanical strength, excellent thermal, and electrical conductivity and this has made Carbon CNT an important material for the broad-spectrum applications in the fields of optoelectronics, nanorobotics, electrochemical catalysis, microarray chips, and sensors as memory, semiconductor components and transparent conducting films for touch screens, displays, solar cells, sensors and other devices.

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The material is best described as carbon sheets rolled in a tube-like network to form either a seamless cylinder, known as single-walled carbon nanotubes (SWCNTs), or many cylinders stacked one inside the other, known as multi-walled carbon nanotubes (MWCNTs) (see figures a and b). Conceptually, a SWCNT is a one-atom-thick layer of graphite, called graphene, wrapped into a seamless cylinder with either open or closed ends. As their name implies, MWCNTs consist of multiple concentric layers of graphene that form a tube shape.  The lengths of these tubes generally range from several hundred nanometers (nm) to several micrometers (µm), though tubes in the millimeter range have also been reported. Their diameter is dependent on the number of walls and is usually less than 100 nm.
On further analysis of all the characterization techniques that we have covered this semester it has been clear that some characterization techniques could be in theory used to analyze carbon nanotubes but there are practical and experimental constraints that make them unsuitable. Characterization techniques such as Transmission electron microscopy (TEM) would accurately give information regarding lattice structures, sidewalls, diameters, lengths, impurities, and defects. However, sample preparation is extensive and troublesome and drying may cause ambiguous results. Another factor is that TEM is based on the transmission of electrons through a sample. As a result, the darker the area is, the thicker the sample is on that point. In the case of tangled MWCNTs, it is very hard to differentiate between dark areas where the thickness is because the nanotubes are tangled or to the presence of other micro-sized materials.
Scanning tunneling microscopy (STM), is another option that would accurately show the morphology of the sample, but it would not provide any element specific information, so I also would rule it out for this application. While in scanning electron microscopy (SEM), only the surface of the sample is analysed, and the structures caged within the MWCNTs will not be imaged. Energy dispersive X-ray spectroscopy (EDS/EDX), would provide a quantitative estimate of CNTs’ metal contents or impurities however errors can occur when a large-scale CNT characterization is required. Many elements such as Titanium and Vanadium, Magnesium and Iron in CNTs also show overlapping peaks, for these reasons I would also rule these techniques out.
X-ray photoelectron spectroscopy (XPS) would provide surface composition and, in principle, information about CNT functional groups. But it requires relatively large amounts of sample (~5 mg) and peak-fitting is often ambiguous and very hard to interpret. This method is also not valid for assessing CNT purity.
Methods such as thermogravimetric analysis (TGA) would provide very accurate chemical information but would not give any information on the crystal structure of the sample, this is critical as nanomaterials can exist in structurally different but chemically identical forms. These techniques also do not provide an unambiguous separation between the content of SWCNTs and carbonaceous impurities and data interpretation is often troublesome with this material.
Among the aforementioned techniques, morphological probes mainly unveil local CNT features however XRD possesses unique advantages as it simultaneously reveals most of the physiochemical aspects of CNTs both at the local and global scales. Since bulk CNT is a combination of many structural orientation, lattices, and crystals; its complete characterization needs a global probe and XRD is because of this reason in my opinion a good option for characterisation of CNT’s.
As we know in XRD an x-ray beam is forced onto the sample and the interactions of these incident X-rays with the sample atomic planes create diffracted, transmitted, refracted, scattered and adsorbed beams according to Bragg’s law. The degree of diffracted X-rays depends on arranging the material’s atomic planes within the crystal lattices. Usually, a detector is used to detect diffracted X-rays followed by their processing and counting to give rise diffracted or pattern beams. Conversion of diffracted patterns into d-spacing allows recognition of an unknown sample. Materials are then identified by comparing the diffracted pattern beams with many reference patterns which could possibly be found in the Joint Committee on Powder Diffraction Standards or JCPDS library or another database of diffraction patterns.
Although CNT is considered as a noncrystalline material, its periodic structure results in distinct X-ray diffraction peaks. The carbon atoms in CNTs act as 3D optical diffractors that scatter light at different, but specific angles. From the diffracted angles, it would be possible to extract information on aligning graphene sheets of CNT from the position and intensity of the diffracted beam. XRD pattern of CNTs has shown some distinct similarities to those of graphite probably because of their similar intrinsic graphene properties (see figure 1). The atomic carbon structure in graphene has random orientations and translations rather than graphite sheet which has piled up sequential carbon atoms. Therefore, there is no lattice atom plane existing especially (001) and it makes the diffraction pattern peaks specific and different from other.
In conclusion, XRD diffraction pattern positions and intensities of the CNT sample are taken and analyzed against previously known data to characterize all physio-chemical features of nanotube structures such as lattice structure, ropes identification, crystallinity, domain size, bundle characterization etc which is not easily done on any of the other characterization techniques mentioned previously.
For my example of a surface/coating analysis and contaminants, I have chosen to look at the process of how a plasma electrolytic oxidation (PEO) creates a nanostructured coating on the surface layer of a NiTi alloy and how I would go about analysing and characterizing the composition of the surface.
PEO is an electrochemical surface treatment process for generating oxide coatings on metals. The sample is charged in a bath containing an electrode and a generally concentrated orthophosphoric acid with an addition of copper II nitrate. This process is similar to anodizing however, it employs higher potentials so that discharges occur, and the resulting plasma modifies the structure of the oxide layer. This process is used to grow thick (tens or hundreds of micrometers), largely crystalline, oxide coatings which are then used to increase the hardness, corrosion resistant and mechanical properties of the sample. The coating is a chemical conversion of the substrate metal into its oxide which grows both inwards and outwards from the original metal surface. As it grows inward into the substrate, it has excellent adhesion to the substrate metal. On this basis, I would characterize it as both a bulk analysis as well as a surface analysis. The analysis methods I would use would include scanning electron microscopy (SEM) in conjunction with the energy-dispersive X-ray spectroscopy (EDS) and, X-ray photoelectron spectroscopy (XPS).

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SEM would provide data in relation to the porosity of the surface layer as evident in the sample given in figure 2. The EDS would give initial bulk chemical analysis data. As seen in figures 3 and 5, the amount of copper nitrate added during the PEO process greatly influences the chemical composition of the surface layer that forms. Additionally, the amount of phosphorus, as detected by the data provided by the EDS in figure 5 in comparison to Figure 3, suggests that the coating is composed of oxides/hydroxides and phosphates. In this case, one should emphasize that the coating obtained on NiTi alloy (figure 5) after the PEO process can be considered as a biocompatible one. (Rokosz, K et al. 2017)
Because by the EDS measurements the signals of a matrix cannot be separated from the signals of coatings, additional quantitive data, i.e. coming from the X-ray Photoelectron Spectroscopy (XPS), would be needed in order to better determine the exact elemental composition of the surface. From the data shown by the XPS spectra, it is clear that the PEO coating is composed mainly of phosphorus and oxygen with a small amount of copper, titanium, and nickel. In order to better determine the chemical composition of the PEO coating, the high-resolution X-ray photoelectron spectroscopy (XPS) measurements were performed, which are given in the sample Fig. 9 taken from Rokosz, K et al. (2017). This figure shows the binding energies needed throughout the oxidation process on the surface material and also more importantly they give more accurate data in relation to the peak intensities of the elements present which for a better understanding of the oxide coatings composition on the surface of the titanium.
 The information given by the sample high resolution XPS data it shows the presence of carbon and nickel in the surface of the sample (as shown in table 4) which would be contaminants in this sample materials application. So, the amount of copper nitrate added to the PEO process would then be changed in order to give results similar to those shown in table 6 where no nickel or carbon contaminants are present on the surface of the material. Another method that is often used to access surface contaminants is TOF sims however XPS is more relevant for this material.
As we have learned throughout this module there are several different characterization techniques available to analyze a given material and after looking at all of these in detail I felt the best characterization techniques for this particular material were SEM with an EDS detector and then a further XPS process, as this would give me the porosity of the coatings surface, the chemical composition and the percentage of the contaminants on the surface of the material.
References

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Habazaki, H., Tsunekawa, S., Tsuji, E. and Nakayama, T., 2012. Formation and characterization of wear-resistant PEO coatings formed on β-titanium alloy at different electrolyte temperatures. Applied Surface Science, 259, pp.711-718.
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Rokosz, K., Hryniewicz, T. and Raaen, S., 2017. SEM, EDS and XPS analysis of nanostructured coating formed on NiTi biomaterial alloy by Plasma Electrolytic Oxidation (PEO). Tehnički vjesnik, 24(1), pp.193-198.
Rokosz, K., Hryniewicz, T., Pietrzak, K. and Dudek, Ł., 2017. Development and SEM/EDS characterisation of porous coatings enriched in magnesium and copper obtained on titanium by PEO with ramp voltage. World Scientific News, 80, pp.29-42.