# Fewer Dimensions for Higher Thermal Performance: A Review on 2D Nanofluids
## 2 Types of 2D Nanofluids
### Graphene/Graphene Oxide
* rGO, which is a GO prepared through chemical, thermal, microwave, photo-thermal, and bacterial green methods for reducing the content of oxygen. The use of Gr nanoparticles is an attractive option, given that they are less costly, and their production is simpler when compared with that of Gr nanosheets or CNTs. Furthermore, a homogeneous dispersion of Gr nanoplatelets is easier to obtain than those of Grand CNTs since during dispersion, the single-layered Gr may be curled and the CNTs may be entangled. Moreover, Gr nanoparticles exhibit a much lower surface area compared with Gr nanosheets, but their surface area is still much greater than that of CNTs. It should be noted that the TC of Gr nanoparticles decreases as the number of layers increases [35]. Sadri et al. [36] synthesized covalently functionalized Gr nanoparticle colloidal suspensions that had enhanced stability and environmental benevolence. Gr nanoparticles were functionalized using clove buds through the one-pot method. Then, the clove-treated Gr nanoparticles were incorporated into the base fluid to synthesize the nanofluid using a two-step method. The hydrophilic functionalized groups were confirmed by UV-VIS spectroscopy and their presence resulted in good stability even two months after the as-prepared condition of the nanofluids. Dhar et al. [37] produced Gr nanofluids with the addition of Gr nanosheets, which were synthesized using a two-step preparation method. The oxidation process of the graphite nanopowder into GO nanosheets was performed using the modified Hummers method, which was followed by GO reduction into rGO. Ghozatloo et al. [38] and Park et al. [39] synthesized Gr nanoflakes. They used the CVD technique to create Gr nanosheets on copper foil by catalytic decomposition in a quartz tube furnace. Additionally, Safaei et al. [40] evaluated the heat transfer characteristics, pressure drop, and required pumping power for using an aqueous nanofluid with Gr/silver nanoparticles flowing in a conduit with a rectangular section. It was verified that the heat transfer features were enhanced with an increasing Reynolds number and concentration of the nanoparticles. However, when using the developed nanofluids, an increase in the required initial pumping power was also observed. Yarmand et al. [41] investigated the attributes of Gr nanoparticles and platinum dispersed in water. The prepared nanofluids had improved stability and any noticeable sedimentation was not observed after 22 days. The TC of the nanofluids showed a near 18% enhancement when compared with the TC of the water alone at 40 \({}^{\circ}\)C and at 0.1% wt. Since the synthesis of Gr is complex and costly, efforts have been made to develop cheaper procedures to synthesize and use Gr derivatives, such as GO. GO can be synthesized by exfoliating the GO into layered plates through ultrasonication or vigorous stirring and can be incorporated into base fluids, forming GO nanofluids. Vincely and Natarajan [42] conducted experiments using a solar flat plate collector and a GO nanofluid as the heat transfer fluid. The nanofluid was prepared via ultrasonication of GO in water. The enhancement of the collector efficiency was 7.3% at a flow rate of 0.0167 kg/s and a weight fraction of 0.02% wt. compared with that of the water alone. In addition, Yu et al. [43] determined the TC of the nanofluids composed by GO nanosheets and different base fluids. At a weight fraction of 5% wt., the enhancements of the TC were 30.2%, 62.3%, and 76.8% for water, propyl glycol, and liquid paraffin as base fluids, respectively. Owing to the enhanced thermal stability, charge transportation, TC, mechanical strength, and optical properties, rGO and its nanofluids can be used in the development of different types of innovative nanomaterials and thermal fluids in various applications, including solar energy conversion and harvesting systems, filtration, chemical batteries, and nanoscale electrodes. In this sense, Lin et al.