2. Structure of sp<sup>2</sup> nanocarbons

those particular Raman bands and features that provide useful structural/thermal data about carbon nanotube samples. It will be followed by an overview of graphene and graphene nanoplatelets and their usefulness for gas-sensing applications utilizing Raman spectroscopy.

The one-dimensional graphite allotrope, carbon nanotube, is conceptually described as being a rolled-up graphene sheet, yielding the cylindrical nanomaterials that have diameters of a few nanometers. The multi-walled varieties contain several concentric cylindrical shells. The Raman bands and the variations under various external perturbations of the sp2 graphitic materials cited above include the graphite G-band common to all sp<sup>2</sup> carbons at around 1580 cm�<sup>1</sup> due to the intraplanar bond stretching of the two carbons in the hexagonal lattice unit cell, and the carbon nanotube specific radial breathing mode (RBM)—which arises as a consequence of their cylindrical geometry. Lastly, there is the defect D-Raman band, which arises due to defects, finite size effects, or any other cause of departure from perfect crystalline regularity, and the 2-D band. The characterization topics discussed in connection with the graphitic materials will be the identification of the chirality types present in carbon nanotube samples using the resonant RBM mode of carbon nanotubes and its connection to the interesting quasi 1-dimensional character of their electronic structure. The other properties obtained via Raman spectroscopy discussed will also be the anomalous thermal expansion and thermal

We have also utilized Raman spectroscopy to understand the behavior of vibrational modes associated with graphene following gas exposure. Specifically, we have studied the effects of water vapor and toxic gases (SO2, NO2, NO), via variable humidity levels, gas concentrations,

Functionalized graphene nanoplatelets are comprised of an amorphous mixture of graphene sheets. Their thicknesses range from 6 to 8 nm, and the overall density usually lies between 0.03 and 0.1 g/cc. The oxygen content of the majority of samples normally are <1%, with the remaining carbon content exceeding 99.5 wt % (STREM). The morphology of this amorphous material plays a large and significant role in its enhanced mechanical properties, such as stiffness, strength, and surface hardness. By incorporating a small number of certain atoms that differ in the number of valence electrons into the pure crystal, the doping of graphene

For the thermal conductivity measurements of the carbon allotropes, we have used the G-Raman band and its variation with increased sample temperature through laser heating. The method and the useful information it provides is due to Terekhov et al. [1]. Also, edge defect characterization of graphene nanoplatelets based on Eq. (1) due to Cancado et al. [2] has also been included, where L is the characteristic in-plane crystallite size of the graphene nano flake, λ is the laser wave-

Our research presented here is aimed at extending the knowledge regarding the nature of graphitic nanomaterial-gas sensing interactions and help develop better models for their enhanced understanding, which in turn would make the development and production of more

ð Þ ID=IG �<sup>1</sup> (1)

length, and ID and IG are the intensities of the Raman D-band and G-band, respectively.

<sup>L</sup> <sup>¼</sup> <sup>2</sup>:<sup>4</sup> � <sup>10</sup>�<sup>10</sup> � <sup>λ</sup><sup>4</sup>

conductivity of the sp<sup>2</sup> graphitic materials investigated.

156 Raman Spectroscopy

nanoparticles can lead to an enhancement in conductivity.

effective in situ gas sensors feasible.

exposure times, and thermal loading, on the Raman spectra of graphene.

This introductory section presents a cursory discussion of SWNT and MWNT structures. This will be done by first looking at the unit cell of planar graphene and also graphite since the former material is considered to be the conceptual parent material of all sp2 graphitic materials, including SWNTs through the application of a simple rolling up operation. Since the molecular/electronic and geometric structures are highly dependent on graphene, the majority of a SWNT's structural features are expressed via the lattice vectors a<sup>1</sup> !, and a<sup>2</sup> ! of the graphene unit cell shown in Figure 1.

The two unique Carbon atoms A and B in each unit cell are located respectively at (0, 0) and at 1/3\* a<sup>1</sup> ! <sup>þ</sup> <sup>a</sup><sup>2</sup> ! (Figure 1 adapted from Wong and Akinwande [3]). The progression to the first related graphitic material, three dimensional graphite, is accomplished through the stacking of several layers of 2-dimensional graphene layers, where in the A-B Bernal stacking structure, there are 2\*N atoms per unit cell, N being the number of layers [4]. A major structural factor of graphite that results in the electronic structure of 2-dimensional graphene being a reasonable first order approximation of the former is the average inter-layer spacing of 3.35 Angstroms. This distance is much larger than the nearest neighbor Carbon–Carbon distance of 1.42 Å, hence resulting in much weaker overall attractive interaction between layers compared to intra-planar interactions [5].

Moving now to one of major foci of the chapter single-walled carbon nantoubes (SWNTs), the conceptual operation performed on the single 2-dimensional graphene sheet is "rolling" it up into a cylinder. The diameter distribution of most SWNTs produced by various techniques is dominated by tubes with diameters less than 2 nm, although diameters in the range of 0.7– 10.0 nm are possible [5]. Ignoring the two ends and exploiting the very large length to diameter ratio (~104 –105 ) of SWNTs allows one to safely view these sp2 nanocarbons as quasi 1-dimensional objects [5].

The concept of chirality is essential in the description of SWNT structure. It is defined by the chiral vector, denoted by Ch in Figure 1, and several equivalent interpretations of this structural quantity are usually given. For example one may consider the fact that the chiral vector determines the arrangement of the six-sided carbon hexagons in the curved planar wall of the SWNT [5]. Alternatively, one may also view the chirality of a SWNT in terms of the overall symmetry of the constructed SWNT, specifically whether or not the SWNT has vertical mirror plane reflection

Figure 1. Graphene unit cell.

symmetry across planes containing the tube axis [3]. An additional benefit of the latter viewpoint is its more direct path to the discovery of there being only three overall structural categories of SWNT as shown in Figure 1.

3.1. Thermo fisher scientific DXR smart Raman spectrometer

order to provide the Stokes-shifted Raman bands.

3.2. Renishaw inVia Raman spectrometer

final detection of the Raman spectrum.

3.3. Ventacon heated cell

variation of 0.1C.

The primary instrument used to record the majority of the Raman spectra was a DXR SmartRaman spectrometer (that uses 780, 532, and 455 nm laser sources). The first wavelength (780 nm) was used for the bulk of the recorded spectra and utilized a high brightness laser of the single mode diode (as does the 532 nm light source), while the 455 nm source is a diodepumped solid state laser. This instrument employs the 180-degree backscattering geometry, full range grating and triplet spectrograph, coupled with automated entrance slit selections in

Raman Spectroscopy of Graphitic Nanomaterials http://dx.doi.org/10.5772/intechopen.72769 159

The Renishaw inVia Raman spectrometer uses a 532-nm laser source and was used to obtain the Stokes spectra of the graphene and functionalized Nanoplatelets samples. It consists of a microscope to shine light on the sample and collecting the scattered light, filtering all the light except for the tiny fraction that has been Raman scattered, together with a diffraction grating for splitting the Raman scattered light into component wavelengths, and a CCD camera for

The Raman spectral data in this study of the two different SWNT samples were obtained under thermal loading from room temperature to 200C in steps of 10C. Both powdered samples were heated externally via a Ventacon™ model H4–200 heat cell that is diagrammed in Figure 2. The first SWNT sample was produced by Unidym™ Carbon Nanotubes. The Hipco technique was used in the production of this sample, which involves the nucleation of SWNTs on Fe(CO)5 catalyst material using high pressure CO, followed by various quality control methods (Misra et al., 2013) [6]. According to manufacturer specifications, the diameters and lengths of the nanotubes in this sample ranged between 0.8 to 1.2 nm, and 100 to 1000 nm, respectively. The sample data also claimed a purity level of only 8% residual Fe catalyst by weight present.

Information about the method of production or purity levels of the second SWNT sample was not available. In the data sets for both samples each point in the ωRBM vs. temperature plots is the mean value from two separate Raman collections. The standard error of each data point obtained from both Raman spectra collected at each temperature is also displayed for both samples. The spectra recorded at each pre-set temperature were obtained with a temperature

Figure 3 is a diagram of the components of the sample cell and how it is placed underneath the Renishaw Raman spectrometer. The cell contains apertures that connect to the center of the cell, where the sample is placed and sealed through means of a glass disk and an O-ring. In addition, the cell has an aperture to place a thermocouple to read the temperature of the cell and another one where we place a voltage-induced heating cylinder. The gas flow comes from the gas cylinder into the rotameter and then through a series of tubing to the cell. These tubes

3.4. Aluminum disk cell for graphene gas exposure Raman spectroscopy

Chiral SWNTs are formed such that the orientation of the carbon hexagons on the tube surface do not allow the sides of the tube across a vertical mirror plane to be superimposed on one another. The remaining two subcategories achiral SWNTs, armchair and zig-zag SWNTs however, do allow for such reflection symmetry based on the arrangement of the Carbon hexagons along the cylinder walls. The names armchair and zig-zag refer to the circular cross-sections of each of these achiral SWNT types shown by the bold lines in Figure 1. We conclude this section dealing with the structural properties of primarily SWNTs with the actual construction of a SWNT, starting just the two SWNT graphene lattice vectors, a<sup>1</sup> ! and a<sup>2</sup> !, which have the following Cartesian components <sup>a</sup> ffiffi 3 p <sup>2</sup> ; <sup>a</sup> 2 h i, and <sup>a</sup> ffiffi 3 p <sup>2</sup> ; � <sup>a</sup> 2 h i respectively.

Relying on the example of Figure 1 which demonstrates the formation of a (3, 3) armchair nanotube, the planar unit cell is formed by rolling the gray shaded region along the chiral vector such that points C and D coincide respectively with points D and B. After performing the previous conceptual rolling operation the pertinent quantity that defines the SWNT unit

cell in the resultant nanotube is the translational vector T ! as shown in the right hand portion of Figure 1. As its name suggests this vector is the shortest vector that is perpendicular to the chiral vector, and represents the axial component of the SWNT unit cell that is repeated in this same direction. This formalism of SWNT construction that begins from the planar graphene lattice provides the readily obvious interpretation for the magnitude of the chiral vector, namely its magnitude equaling the nanotube circumference given by the expression, C ! <sup>h</sup> <sup>¼</sup> <sup>a</sup> ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi <sup>m</sup><sup>2</sup> <sup>þ</sup> <sup>n</sup><sup>2</sup> <sup>þ</sup> nm <sup>p</sup> where a <sup>≈</sup> 2.46 Å is the graphene hexagonal lattice constant equal to ffiffiffi 3 p times the nearest neighbor C-C distance of 1.42 Å. The remaining structural parameter in Figure 1, the angle q, is the chiral angle, conventionally chosen to be the angle between the chiral vector and the a<sup>1</sup> ! lattice vector. This angle ranges from 0� ≤ q ≤ 30� with the lower bound corresponding to zig-zag SWNTs and the upper bound corresponding to armchair SWNTs. The integers, n and m simply refer to the number of a<sup>1</sup> ! and a<sup>2</sup> ! lattice vectors used in the construction of the chiral vector usually with the convention of n ≥ m. For zig-zag SWNTs m = 0, and in the case of armchair SWNTs both chiral indices are identical [3].
