# Fewer Dimensions for Higher Thermal Performance: A Review on 2D Nanofluids
## 2 Types of 2D Nanofluids
### Hybrid
Nonetheless, the majority of the published studies state that the specific heat decreased with the inclusion of nanoparticles, but a few unexpected results were also reported. It appears that the specific heat of the base fluid and the nanoparticles affects the final specific heat of the nanofluids, and the interfacial solid/liquid free energy is altered with the modification of the included nanoparticles. Due to the greater surface area of the nanoparticles, their surface free energy has a greater impact on the capacity of the system, affecting the specific heat of the composite nanomaterials. In the study conducted by Sundar et al. [119] GO nanosheets with a homogeneous dispersion of spherical Co\({}_{3}\)O\({}_{4}\) nanoparticles were prepared using in situ growth and co-precipitation. The reduction of aqueous cobalt chloride in the presence of GO with sodium borohydrate led to the production of hybrid GO/Co\({}_{3}\)O\({}_{4}\) nanoparticles. The hybrid nanofluids were synthesized by scattering the GO/Co\({}_{3}\)O\({}_{4}\) nanoparticles in water, ethylene glycol, and ethylene glycol/water base fluids. The results revealed that the TC increase in the aqueous nanofluid was 19.1% and the ethylene glycol-based nanofluid was 11.9% at 0.2% vol. at 60 \({}^{\circ}\)C compared with the corresponding base fluids. The viscosity enhancement of the aqueous nanofluid was 1.7-times higher and that of the ethylene glycol-based nanofluid was 1.42-times higher at 0.2% vol. at 60 \({}^{\circ}\)C. In the ethylene glycol-based nanofluids, the TC increase associated with the GO/Co\({}_{3}\)O\({}_{4}\) nanoparticles was more pronounced compared with those of other GO nanofluids. This is due to the synergistic thermal properties of the hybrid nanoparticles. The increase in the viscosity of the GO/Co\({}_{3}\)O\({}_{4}\) nanofluid was similar to that of the GO nanofluids. Table 6 summarizes the experimental works on 2D hybrid nanofluids. In sum, the already published research on nanofluids are mostly devoted to the employment of mono nanofluids in certain heat transfer purposes, such as the heat exchangers field of actuation. There appears to be a shortage of works on the heat transfer capability provided by the hybrid nanofluids in affordable and sustainable solutions. Further studies are required to fully understand and explore the performant thermal characteristics of the hybrid nanofluids in areas including the cooling of electronics, borehole heat exchangers, and PV/T systems. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Reference** & **Authors/Year** & **Concentration** & **Base Fluid** & **Main Findings** & **Remarks** \\ \hline \hline \multirow{9}{*}{[120]} & \multirow{9}{*}{Ahammed et al., 2016} & Gr–Al2O3 at 0.1\% vol. & \multirow{9}{*}{Water} & Increase in the & \multirow{9}{*}{Increase in the \( \begin{array}{c}\text{cooling capacity}\\ \text{and coefficient of}\\ \text{performance of a}\\ \text{thermoelectric}\\ \text{cooler by}\\ \text{nearly 31.8\%}\text{.}\end{array} \)} & \multirow{9}{*}{ \begin{tabular}{c} \text{Gr-Al2O3 at \\ 0.1\% vol. \\ \end{tabular} } & \multirow{9}{*}{Water} & Increase in the \( \begin{array}{c}\text{cooling capacity}\\ \text{and coefficient of}\\ \text{performance of a}\\ \text{themoelectric}\\ \text{cooler by}\\ \text{nearly 31.8\%}\text{.}\end{array} \) \\  & & & & 20.3\%. & \\ \cline{1-1}  & & & & 18\% at 0.