# Fewer Dimensions for Higher Thermal Performance: A Review on 2D Nanofluids
## 2 Types of 2D Nanofluids
### Tungsten Disulfide  
Tungsten disulfide nanostructures can be synthesized using different methods. These methods are presented in Figure 5.  
Figure 5: Main tungsten disulfide preparation methods. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Reference** & **Authors/Year** & **Concentration** & **Base Fluid** & **Main Findings** & **Remarks** \\ \hline \multirow{2}{*}{[91]} & \multirow{2}{*}{Hammed et al., 2019} & \multirow{2}{*}{MWCNTs at 0.1\% wt.} & \multirow{2}{*}{Kapok seed oil} & \multirow{2}{*}{Approximate 6.2\% increase in the TC.} & TC accurately predicted by the Khanafer and Vafai model. \\  & & & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 3: _Cont._WS\({}_{2}\) nanofluids are promising in heat transfer applications within the CSP technological field. WS\({}_{2}\) nanosheets are well-defined and highly stable, and do not significantly alter certain characteristics such as the surface tension, friction coefficient, viscosity, and Reynolds number of heat transfer systems. The incorporation of WS\({}_{2}\) may improve the TC of synthetic oil base fluids by approximately 30%. Additionally, the authors Martinez-Merino et al. [92] prepared 2D WS\({}_{2}\) nanosheet-based nanofluids with the oil commonly used as heat transfer fluid in CSP stations. The surfactants cetyl-trimethylammonium CTAB and polyethylene glycol PEG-200 were added to the nanofluids to improve the exfoliation procedure and the stability over time of the colloidal suspensions. The experimental results showed that an increase in the ultrasonication duration and the amount of the surfactant enhanced the stability over time of the nanofluids prepared with the surfactant CTAB. Furthermore, in the nanofluids synthesized with the surfactant PEG-200, the increase in the surfactant amount was the main factor contributing to the high concentration of stable nanomaterial suspended in the thermal oil. The size of the agglomerates in the stable nanofluids prepared with the addition of CTAB was between 200 nm and 250 nm, whereas in the PEG-200 nanofluids, the same measurement was approximately 180 nm. Furthermore, the viscosity of the oil base fluid increased to a maximum of only 1.5% with the nanosheets of WS\({}_{2}\) and the surfactant. This minor increase would not result in considerable pressure-drop concerns in the circuits of the CSP plants. Moreover, the nanofluid with a concentration of 0.75% wt. of surfactant PEG-200 with four hours of ultrasonication (Case A) and the 0.01% wt. CTAB with eight hours of sonication (Case B) showed enhancements of 33% and 29% TC, respectively, compared with that of the thermal oil itself. Furthermore, the nanofluid in Case A was found to have a specific heat value 6.5% higher than that of the heat transfer fluid alone. The improvement in the specific heat and TC resulted in an enhancement in the HTC of 21% in the Case A nanofluid and of 17% in the Case B nanofluid when compared with that of the oil itself. On the other hand, a numerical analysis demonstrated that the efficiency of the CSP facilities was enhanced by up to 31% or 18% if the Case B or Case A nanofluids, respectively, are used compared with the commonly employed thermal oil. The predicted efficiency for a volumetric collector working with 2D WS\({}_{2}\) nanofluids is a slightly lower than the efficiency of a surface collector operating with nanofluids; however, such efficiency is higher than that achieved using the surface collector with the thermal oil commonly used at the present time. All in all, the authors believe that the WS\({}_{2}\)-developed nanofluids are a promising alternative to the thermal oil commonly used in solar energy plants and would enable the adoption of volumetric collectors, which are less costly than traditional surface collectors. In the work conducted by Shah et al.