# Fewer Dimensions for Higher Thermal Performance: A Review on 2D Nanofluids
## 2 Types of 2D Nanofluids
### Carbon Nanotubes  
Nanofluids containing CNTs are very promising heat transfer operating fluids in several technological fields. For instance, the use of such nanofluids can improve the efficiency of different types of solar thermal energy collectors due to their enhanced light absorption ability and extremely high TC. The CNTs dispersed in base fluids can ameliorate the thermal performance of several types of thermal management equipment, such as heat exchangers, heat pipes, and thermosyphons to be used in a wide spectrum of applications, including, for instance, the cooling of electronics. CNTs are Gr nanosheets rolled into cylindrical tubes with a diameter of less than 1 nm with a half fullerene-capped end. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Reference** & **Authors/Year** & **Concentration** & **Base Fluid** & **Main Findings** & **Remarks** \\ \hline
[66] & Kanti et al., 2022 & Gr from 0.05\% to 1.0\% vol. & Ionic Liquid & \
\begin{tabular}{c} 27.6\% increase in \\ the TC at 0.5\% vol. \\ and at 60 \({}^{\circ}\)C. \\ \end{tabular} & The viscosity of the & ionic nanofluids \\  & Kanti et al., 2022 & 1.0\% vol. & Ionic Liquid &
\begin{tabular}{c} the TC at 0.5\% vol. \\ and at 60 \({}^{\circ}\)C. \\ \end{tabular} & was lower than & that of the ionic \\ \hline
[67] & Huminic et al., 2022 & Go-Si at 0.25\% wt. & Water & \
\begin{tabular}{c} A new correlation \\ was proposed for \\ the viscosity. \\ \end{tabular} &
\begin{tabular}{c} Improved \\ efficiency in \\ laminar and \\ turbulent flows. \\ \end{tabular} \\ \hline
[68] & Ali, 2022 & Gr at from 0.01 to 0.1\% vol. & Water & \
\begin{tabular}{c} 125.7\% increase in \\ the TC. \\ \end{tabular} &
\begin{tabular}{c} 5\% decrease in \\ viscosity. \\ Stable after 45 days \\ with the addition \\ of SDS. \\ \end{tabular} \\ \hline
[69] & Rahman et al., 2022 & Gr functionalized & \begin{tabular}{c} 24.4\% increase in \\ with apple cider \\ vinegar at 0.1\% wt. \\ \end{tabular} & Clove & \
\begin{tabular}{c} The TC of the \\ nanofluids was \\ the TC in reference \\ to water. \\ \end{tabular} &
\begin{tabular}{c} The TC of the \\ nanofluids was \\ slightly lower than \\ that of the \\ conventional Gr \\ nanofluids. \\ \end{tabular} \\ \hline \hline \end{tabular}
\end{table} 
Table 2: _Cont._Considering the number of consistent tubes, CNTs can be divided into single-walled carbon nanotubes (SWCNTs), double-walled carbon nanotubes (DWCNTs), and multi-walled carbon nanotubes (MWCNTs). SWCNTs are composed of only one tube, whereas DWCNTs and MWCNTs comprise of two and three or more tubes, respectively. The fundamental synthesis techniques of CNTs are laser ablation, arc discharge, and CVD. The preparation method based on laser ablation also uses the evaporation of a rod of graphite with a metallic-based catalyst to produce CNTs. The laser ablation synthesis procedure applies high-energy laser irradiation to heat a carbon substrate, leading to the transition from the solid to the gaseous phase, as the nanomaterial is deposited in a cold trap placed within the synthesis chamber. Using arc discharge methodology, doped graphite rods and catalysts are vaporized at temperature values of up to 5000 K in a closed chamber by an electric arc, resulting in a sediment of CNTs. On the other hand, the CVD decomposes carbonaceous gases on catalytic nanoparticles to fabricate CNTs. The employed catalytic nanoparticles are grown during the synthesis or, alternatively, are produced by a separate procedure. Moreover, the major benefit of the CVD is the improved controllability over the operating parameters, such as the carbon supply rate, temperature of growth, size of the catalytic nanoparticles, and nature of the CNTs growing substrate. The nanofluids with CNTs are used in different solar energy systems, such as PV/T collectors and water heating technology (it has been previously demonstrated that nanofluids with CNTs ameliorate the efficiency of the CSP systems in terms of energy and exergy standpoints).