# Fewer Dimensions for Higher Thermal Performance: A Review on 2D Nanofluids
## 2 Types of 2D Nanofluids
### Graphene/Graphene Oxide
[44] synthesized an innovative surfactant composed of two-dimensional charged zirconium phosphate nanoplatelets and applied it in the dispersion of rGO in a base fluid. The obtained results show that rGO nanofluids were altered from non-homogeneous, non-Newtonian nanofluids into homogeneous Newtonian nanofluids after exfoliation with the charged zirconium nanoplatelets. Zubir et al. [45] utilized an rGO nanofluid for improving the thermal performance of a closed duct structure. The authors produced the rGO using the reduction process with tannic acid from the chemically exfoliated GO. The heat transfer enhancement of the nanofluid was higher than the TC enhancement because of the considerable effects of the nanoparticles and turbulent induced flow features. Schlierf et al. [46] employed pyrene derivatives for stabilizing rGO dispersions due to the forces of repulsion and the \(\pi\)-\(\pi\) interaction between functionalized pyrene derivatives and the \(\pi\) systems of rGO. The chemical reduction of GO possesses the beneficial features of being a simple and inexpensive method, resulting in a greater yield of Gr with proper scalability for large-scale applications. The Gr family of nanofluids can be synthesized using the one-step route or two-step route. In the one-step route, the nanofluids are directly synthesized and are produced directly through chemical processes. In the two-step route, the Gr nanosheets are initially manufactured in nanopowder form by physical or chemical methods such as grinding, laser ablation, or the sol-gel method, and then are dispersed in a base fluid. Fan et al. [47] oxidized graphite nanoflakes using an ameliorated Hummer method. A mixture of phosphoric acid and sulfuric acid was incorporated into a mixture of Gr flakes and potassium permanganate. After vigorous stirring, the mixture was air cooled. When the mixture exhibited a yellow coloration after the addition of hydrogen peroxide, the deposit was centrifuged, and the remaining material was washed. The GO sheets were manufactured after air drying. Lee et al. [48] synthesized nanofluids with GO nanosheets dispersed in water. The investigation team prepared the GO nanosheets through CVD. The pH and zeta potential of the developed nanofluids were \(3.58\) and \(-31.5\) mV, respectively, indicating a reasonable stability. Yu et al. [49] proposed an easy method for preparing ethylene glycol-based nanofluids, including Gr and GO nanosheets. The TC of the ethylene glycol increased up to \(86\%\) with the incorporation of the nanosheets. The TC of the GO nanofluid and Gr nanofluid were \(4.9\) W/mK and \(6.8\) W/mK, respectively. Lee and Rhee [50] produced ethylene glycol nanofluids incorporating Gr nanoplatelets, which exhibited similar TC enhancement at a weight fraction of \(2\%\) wt. and at temperature values between \(10\)\({}^{\circ}\)C and \(90\)\({}^{\circ}\)C. The TC of the nanofluids was much greater than that provided by the Hamilton-Crosser model [51], which may be due of the greater surface area and two-dimensional nanostructure of the Gr nanoplatelets. Akhavan-Zanjani et al. [52] measured the TC and viscosity of a nanofluid composed of Gr nanosheets dispersed in water. The experimental results revealed a considerable increase in the TC and a moderate increase in the viscosity with the incorporation of the Gr nanosheets. The maximum increases were of \(4.95\%\) and \(10.30\%\) for the viscosity and TC, respectively. Moreover, the authors Liu et al. [53] investigated the TC, specific heat capacity, viscosity, and density of the nanofluids based on Gr nanosheets suspended in an ionic base fluid. The TC of the nanofluid at \(0.06\) wt.% showed an enhancement within \(15.2\)-\(22.9\%\) in the base fluid itself in cases where the temperature values were raised from \(25\)\({}^{\circ}\)C to \(200\)\({}^{\circ}\)C. The viscosity of the prepared nanofluid was considerably decreased with increasing temperature.