# Fewer Dimensions for Higher Thermal Performance: A Review on 2D Nanofluids
## 1 Introduction
Two-dimensional nanomaterials can be a suitable option as nanofillers in traditional heat transfer fluids as they present a very high heat transfer area [10]. A common synthesis technique for these layered nanomaterials is exfoliation, where the single layers of the material are separated from each other by chemical or mechanical procedures [11]. It is worth mentioning that even though the exfoliation process can be performed mechanically on a small scale, liquid phase procedures are required for various ends, such as nanoelectronics, micro-electromechanical systems, nanoelectromechanical systems, and sensors, among others. These 2D materials can also be manufactured by chemical growth of the individual layers, which is the most common technique to produce Gr nanosheets [12], using chemical vapor deposition (CVD) onto the surface of catalysts, such as copper or silica, through heating at temperature values up to 1200 \({}^{\circ}\)C and passing a carbon-containing gas (e.g., methane) over the catalyst. Coleman et al. [2] demonstrated that it is possible to synthesize 2D materials, such as MoS\({}_{2}\), WS\({}_{2}\), and h-BN, using wet exfoliation procedures. The exfoliation process of 2D insulators, such as h-BN, would decrease its residual bulk conductance [13] and promote the surface-driven effects. This novel class of materials and fluids represent a broadened, unexploited part of 2D systems with superior thermophysical characteristics, high specific surface areas of relevance for thermal management, and thermal energy storage purposes. Furthermore, Gr nanofluids can be potentially investigated and utilized as a promising thermal fluid in many applications, including concentrated solar power (CSP) and PV/T (photovoltaic/thermal) systems due the following advantages:  
* Facile preparation methods [14]. * Lower induced erosion, clumping, and corrosivity [15]. * Larger surface area to volume ratios [16]. * Very high TC values. * Superior heat transfer coefficient (HTC) and rate. * Enhanced stability in reference to conventional nanoparticles. These benefits offer the 2D nanomaterials and corresponding nanofluids the possibility to perform better in comparison with conventional nanomaterials and nanofluids. Figure 2 presents a schematic qualitative comparison of the main thermophysical characteristics and cost effectiveness to be considered in heat transfer enhancement applications of the main 2D nanofluids. Figure 2: Schematic diagram of the qualitative comparison of the thermophysical properties of the main 2D nanofluids. Moreover, hybrid nanofluids can be characterized by the incorporation of binary, ternary, and tetra [17] hybrid nanoparticles in base fluids and are very high TC heat transfer fluids when compared, for instance, with some mono-dispersed nanofluids. They exist as metal hybrid nanofluids, metal oxide hybrid nanofluids, and carbon-based hybrid nanofluids, among others. The hybrid nanofluids have composite nanoparticles and nanoparticle-decorated 2D nanostructures and can be synthesized according to various physical and chemical methodologies. The physical methodologies include atom beam splintering, laser induced heating, and ion implantation. Among the chemical synthesis routes, which are facile and inexpensive, there is the sol-gel process, sonochemical synthesis, hydrothermal synthesis, co-precipitation, and seed growth. The hybrid nanofluids are commonly employed to enhance the operating fluid thermophysical characteristics in solar thermal energy conversion and harvesting systems and equipment, such as flat plate collectors. Recent experimental works on innovative hybrid nanofluids [18; 19] focused on the extremely high thermal transport capability of hybrid nanofluids and modified hybrid nanofluids flowing in rigid channels or in dilating/squeezing channels. Figure 3 presents the main potential applications of 2D nanofluids.