*2.1. Functionalization and Preparation Process*

Since the raw materials of GNPs and Gr are hydrophobic and cannot dissolve in polar solvents like H2O, a suitable way to make PEG@GNPs and PEG@TGr hydrophilic is to present the covalent functionalization via acid treatment. The process will dope the surface of GNPs and Gr with -OH- and -COOH. In a typical experiment [24], the chemical reactions were performed by dispersing GNPs (1 g) and Gr (1 g) in the acid medium of AlCl<sup>3</sup> (18.54 g) and HCl (10 mL), followed by 1 h microwave radiation. Then, the solution was separated at 11,500 rpm and filtered through a polycarbonate filter (0.45 µm) before sequential washing with DMF, THF, diluted HCl, and enough DI-water to eliminate unreacted AlCl<sup>3</sup> and PEG overnight at 60 ◦C. Furthermore, the dry aluminum oxide (Al2O3-NPs = 50 nm) and silicon dioxide (SiO2-NPs = 50 nm) were ultrasonicated for 1 h to avoid agglomeration/settlement. Nanomaterials were mixed with DW by an ultrasonic probe (Sonics Vibra-Cell, VC 750, Sonics & Materials Inc., Newtown, CT, USA) with an output power of (750 W) and a power supply of (20 kHz) frequency. The production process and nanofluid preparation method were shown in Figure 1 [25].
