### Preparation of Composites Based on [M(Salen)] and Carbon Nanomaterials
#### 2.2.1 Nanotubes and Graphene as a Promising Basis for Composites  
CNTs and graphene have attracted considerable attention due to their exceptional surface area and impressive electrical, mechanical, thermal, and chemical properties. These materials have diverse applications, such as catalysis, sensor technologies, and energy storage and conversion. Additionally, they offer significant potential for use in solar cells and fuel cells [64; 65; 66; 67]. Table 1 presents a summary of the essential properties of these materials for comparative analysis. CNTs are a type of carbon allotrope, which were discovered by [PERSON] [74]. These 1D, nanoscale, hollow cylinders are formed by rolling up a hexagonal lattice of sp\({}^{2}\)-hybridized carbon atoms without any seams [75; 76; 77; 78]. CNTs measure a few microns in length and have a nanosized outer diameter, resulting in a large aspect ratio. Depending on the number of rolled-up graphene layers, they can be categorized as single-walled (SWCNTs) (outer diameter \(\sim\)1-2 nm) and multiwalled (MWCNTs) (outer diameter \(\sim\)2-50 nm), with interlayer spacing in the range of 0.32-0.35 nm [75; 79]. There are three main nanotube configurations for SWCNTs depending on the chirality vector (\(C_{h}\)), i.e., the direction along which the graphene sheet is rolled up: armchair, zigzag, and chiral (Figure 3a). It is the chirality that predetermines the electrical, mechanical, optical, and other properties of CNTs [75]. Therefore, SWCNTs can exhibit both metallic and semiconductor conductivity depending on chirality, whereas MWCNTs only exhibit metallic conductivity. MWCNTs are able to take on different shapes, including a Russian doll made of several concentric graphene tubes, a scroll-type structure created by wrapping a single graphene sheet, and a bamboo-shaped structure (Figure 3b) [80; 81]. Significantly, the multilayered structure of MWCNTs enables the outer layer not only to \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Material**} & **Specific Surface Area,** & \multirow{2}{*}{**Young’s Modulus, TPa**} & \multicolumn{2}{c}{**Thermal Conductivity**} \\  & **m\({}^{2}\)g\({}^{-1}\)** & & **(at Room Temperature),** & **Refs.** \\  & & & **Wm\({}^{-1}\)K\({}^{-1}\)** & \\ \hline SWCNTs & 400–900 & \(\sim\)1 & 3000 & [68] \\ \hline MWCNTs & 200–400 & 0.3–1 & 3000 & [68] \\ \hline Monolayer graphene & up to 2630 & 1 & 5000 & [69; 70; 71] \\ \hline rGO sheet & up to 713 & 0.25 & 46.1 to 118.7 & [69; 70; 72] \\ \hline Freestanding GO & up to 456 & 0.7 \(\pm\) 0.015 &
\begin{tabular}{c} 72 (oxidation degree of 0.35); \\ 670 (oxidation degree of 0.05) \\ \end{tabular} & [69; 70; 73] \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Comparison of physical properties for some carbon nanomaterials. protect the inner tubes from chemical reactions during environmental interactions but also to manifest superior tensile properties, which was a disadvantage of SWCNTs [80]. CNTs can be obtained by using the following different methods: arc discharge, laser ablation, chemical vapor deposition (CVD), plasma-enhanced CVD, liquid pyrolysis, solid-state pyrolysis, flame pyrolysis, bottom-up organic approach, etc. [80; 82; 83]. Graphene is a 2D allotrope of carbon, first discovered by [PERSON] and [PERSON] in 2004 [84]. It is a single layer of carbon atoms linked by sp\({}^{2}\) hybridized bonds to form a hexagonal two-dimensional crystal lattice (Figure 4a). There are also bilayer graphene and few-layer graphene, which are formed from multiple graphene sheets held together by strong \(\pi\)-\(\pi\) interactions (Figure 4a) [85; 86; 87; 88]. The complexity of the actual structure of graphene sheets is much more complex due to the presence of edge regions [88]. They are intricate chemical compounds consisting mainly of oxygen-containing functional groups, such as carboxylates, carbonyls, hydroxyl, epoxy, alkoxy groups, etc. (Figure 4a) [89; 90].