# Fewer Dimensions for Higher Thermal Performance: A Review on 2D Nanofluids
## 1 Introduction  
Introductory studies on 1D nanostructures received immediate attention from researchers soon after their introduction by Iijima [1] on carbon nanotubes (CNTs). Following this, various types of 0D and 1D nanostructures were investigated. The recent advances in layered nanomaterials allow the industrial production of different 2D nanomaterials, where their atoms are combined in flat layers that can be stacked on top of each other. Current 2D nanomaterials are of great relevance in the nanotechnologies field of work and research [2], given that their properties, especially in heat transfer practical situations, hold greaterpotential compared with traditional nanoparticles [3]. Recently, 2D nanomaterials and nanofluids have become the focus of the scientific community because of their improved thermophysical properties [4]. These innovative nanomaterials and nanofluids include graphene (Gr), graphene oxide (GO), reduced graphene oxide (rGO), CNTs, fullerenes, transition metal dichalcogenides, graphitic carbon nitride, hexagonal boron nitride (h-BN), tungsten disulfide (WS\({}_{2}\)), and molybdenum disulfide (MoS\({}_{2}\)), MXene, among others, dispersed in adequate base fluids. In this kind of nanomaterial, the heat transfer, photons, and charge carriers will be limited in the 2D plane, leading to intense modifications in the electrical and optical features of such nanomaterials [5]. These materials will perform better than the traditional nanoparticles and corresponding nanofluids due to their large surface area to volume ratio [6]. Apart from three-dimensional nanomaterials, there are two-dimensional (2D), one-dimensional (1D), and zero-dimensional (0D) nanomaterials. The latter are those materials in which all three dimensions are beyond the micro/nanoscale under the form of core-shell nanoparticles, composite nanoparticles, and carbon and graphene quantum dots. One-dimensional nanomaterials are those in which two dimensions are at nanoscale and the remaining dimension is expressed (in most cases) in millimeters. Examples of such nanomaterials are CNTs, carbon nanowires, and graphene fibers. Finally, 2D nanomaterials possess one of their dimensions at nanoscale and the other two dimensions at a different scale. Examples of 2D nanomaterials are graphene nanomesh, nanosheets, and nanowires. In terms of thermal characteristics, such as thermal conductivity (TC) and heat transfer coefficient and rate, these are generally improved during the transfer from the OD nanomaterials to the 2D nanomaterials. However, other factors also influence predominantly the heat transfer capability of these materials, such as the chemical composition, surface-to-mass and volume ratios, and charge carrier mobility, among other factors. For instance, and considering only the heat transfer performance, carbon or graphene quantum dots (0D) are widely used in solar cells and solar light harvesting systems. Those nanomaterials have an improved absorption ability in the ultra-violet region, complementing the solar light harvesting in the visible region of the systems and enhancing their overall efficiencies. There are not used in search of very high TC values, for instance. On the other hand, 1D nanomaterials, such as carbon nanotubes, can achieve a TC of approximately 6000 W\(\cdot\)m\({}^{-1}\cdot\)K\({}^{-1}\). Furthermore, 2D nanomaterials, such as graphene nanoplatelets, can reach 6500 W\(\cdot\)m\({}^{-1}\cdot\)K\({}^{-1}\)[7] of TC, and graphene oxide nanosheets can achieve TC values of nearly 6800 W\(\cdot\)m\({}^{-1}\cdot\)K\({}^{-1}\)[8], enhancing the heat transfer capability to its fullest. Figure 1 shows the materials and their possible forms in 0D, 1D, and 2D nanomaterials. Figure 1: Types of 0D, 1D, and 2D nanomaterials. One of the most common types of layered nanomaterial is Gr, and it has been widely studied for its superior characteristics and uses in different fields [9].