## 5 Noble Nanofluids
### Hybrid Nanofluids
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Authors** & **Nanoparticles** & **Concentration of the Nanoparticles** & **Base Fluid** & **Main Findings** & **Observations** & **Reference** \\ \hline \hline \multirow{4}{*}{[PERSON] et al.} & \multirow{4}{*}{Gold-Silver} & \multirow{4}{*}{0.00025\% vol.} & \multirow{4}{*}{Water} & Highest solar & The efficiency of the system using bimetallic & \multirow{4}{*}{[100]} \\  & & & & photothermal & & nanoparticles was higher \\  & & & & conversion efficiency of & & than that using only gold \\  & & & & around 41.4\% & & 37.8\% with a 40\% decrease & \\  & & & & & in the gold consumption & \\ \hline \multirow{4}{*}{[PERSON] et al.} & Gold-Silver/ & \multirow{4}{*}{5\% wt.} & \multirow{4}{*}{Ethylene} & The photothermal & \multirow{4}{*}{\begin{tabular}{} \end{tabular} } & Considerable broadband & \multirow{4}{*}{[100]} \\  & nitrogen-doped & & & conversion efficiency & & absorption in the visible \\  & graphitic & & Glycol & increased by up to & & and near-infrared \\  & polyhedrons & & & approximately 74.4\% & & spectrum at lower \\ \hline \multirow{4}{*}{Sanchéz-Ramirez} & Gold-Palladium & \multirow{4}{*}{—} & \multirow{4}{*}{Water} & The maximum & \multirow{4}{*}{\begin{tabular}{} \end{tabular} } & \multirow{4}{*}{[101]} \\  & with Au/Pd = 12/1, & & & & & \\  & 5/1, 1/1, 1/5 & & & & & \\  & molar ratios & & & gold/palladium & & \\  & & & molar ratio & & & \\ \hline \multirow{4}{*}{Gutierres} & Gold-Silver with & \multirow{4}{*}{—} & \multirow{4}{*}{—} & \multirow{4}{*}{\
\begin{tabular}{} \end{tabular} } & The TD was sensible to & \multirow{4}{*}{[103]} \\  & core-shell structure & & & & & \\  & at Au/Ag = 3/1, & & & & & \\  & 1/1, 1/3, and 1/6 & & & & & \\  & molar ratios & & & & & \\ \hline \multirow{4}{*}{[PERSON] et al.} & Gold-Silver & \multirow{4}{*}{0\% vol. to 1.0\% vol.} & \multirow{4}{*}{Water} & Increase in the TC of 7\% & The TC and viscosity & \multirow{4}{*}{[104]} \\  & spherical with & & & at 0.6\% vol. and the & & & \\  & 30 nm & & & viscosity has nearly & concentration of & \\  & & & & attained 6\% & & nanoparticles \\ \hline \multirow{4}{*}{[PERSON] et al.} & Silver-Copper with & \multirow{4}{*}{0\% vol. to 1.0\% vol.} & \multirow{4}{*}{Water} & Increase in the TC of 8\% & \multirow{4}{*}{
\begin{tabular}{} \end{tabular} } & The TC and viscosity & \multirow{4}{*}{[104]} \\  & 65 nm & & & & \\  & 65 nm & & & & \\  & & & & & \\  & & & & concentration & \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Main findings on the bimetallic nanofluids. promising option to be applied as nanocoolant. Since graphene and silver have the highest thermal conductivity among the carbon materials and metallic materials, respectively, it is reasonable to consider that a hybrid material including both could be a better solution. Nonetheless, the fundamental challenge associated with the development of such enhanced nanomaterials was to ameliorate the stability over time of the graphene and silver dispersions in water. To mitigate such limitation, the authors functionalized the graphene oxide using the 2,4-diaminohydrolypyrimidine that significatively improved the stability of the silver nanoparticles decorated functionalized reduced graphene oxide. The dimensions of the silver nanoparticles decorated over the graphene were between 10 nm and larger silver lumps, adopting distinct preparation methodologies. The Raman spectroscopic characterization showed the SERS property of the produced nanomaterials and, consequently, these ones are very suitable to be applied in sensors. The SERS property increased with increasing size of the silver nanoparticles and there was a rapid increase when their size decreased from 50 nm to 10 nm. The results showed that the hybrid nanomaterial having the smallest silver nanoparticles exhibited the peak thermal conductivity enhancement of 24% at 0.25% wt. The improved stability of the dispersion was due to the existing functionality over the graphene sheet that promoted the molecule-to-base fluid and molecule-to-molecule interactions.