# Fewer Dimensions for Higher Thermal Performance: A Review on 2D Nanofluids
## 2 Types of 2D Nanofluids
### Graphene/Graphene Oxide  
Gr, GO, and rGO nanofluids possess very high heat transfer properties and they can be used in almost all heat transfer practical cases, including those related to solar thermal energy conversion and storage, PV/T systems, cooling of electronics, and thermal management solutions such as heat sinks and thermosyphons. Nevertheless, the synthesis of these nanofluids is not inexpensive and the use of Gr family nanofluids entails some important limitations that should be tackled in the future. These and other processing influencing factors, together with the improved thermophysical properties and field of application for Gr nanofluids, will be addressed in this extensive sub-section of this review. Gr is a carbon material originating from graphite in bulk state. It possesses the morphology of a two-dimensional, one-atom thick nanosheet (monolayer) and lattice of hexagonally arranged sp\({}^{2}\) bonded carbon atoms. Its transverse sizes range from several nanometers to the macroscale. Gr was initially synthesized through the pioneering work of Novoselov et al. [33] using mechanical exfoliation by Scotch tape of the bulk graphite. Gr was categorized according to its structural arrangement, ranging from the 0D quantum dots, 1D nanofibers and nanoribbons, and 2D nanosheets, nanoplatelets, and rippled/wrinkled nanomesh. Regarding the 2D Gr nanosheets, a few proper production methods are commonly employed, including the laser-induced graphene production technique, exfoliation by mechanical means, silicon carbide sublimation, graphite fluid exfoliation, and CVD. These fabrication procedures synthesize solid Gr, excepting fluid exfoliation that produces Gr under the form of a colloidal suspension. The mechanical exfoliation method was the first approach to obtain Gr using Scotch tape to peel off a bulk pyrolytic graphite substrate, producing thin films of monolayer Gr attached to the tape. There are other, less common types of mechanical exfoliation, such as ball milling of graphite nanoparticles [34]. The laser-inducement route is usually conducted in ambient conditions through the application of a CO\({}_{2}\) pulsed laser onto a carbon substrate. Such methodology combines bulk Gr fabrication and patterning in a single operating step without using any wet chemicals. Liquid exfoliation involves the application of a peeling force created by a horn ultrasonicator to separate the Gr nanosheets from the bulk graphite immersed in a, for instance, N-methyl-2-pyrrolidone solvent, but aqueous solutions can also be employed if a surfactant is included. The yield from the liquid exfoliation procedure is comparatively low and, consequently, it should be conducted at the centrifugation stage to obtain a significant monolayer and enough to create several layers of Gr flakes in the final dispersion. Moreover, the CVD technique uses hydrocarbon gases to induce the growth of Gr on high carbon solubility metallic substrates (e.g., nickel) by carbon diffusion and segregation. The improved features of Gr are derived mainly from the 2p orbitals that form the \(\pi\)-bonds, which are hybridized together to generate the \(\pi\)-\(\pi\) bands. Those bands are delocalized over the carbon platelet and produce Gr. Accordingly, Gr has excellent stiffness, enhanced TC, and great mobility of the charge carriers. Gr can be arranged according to the following possible configurations and nanomaterials:  
* Multi-layer Gr, which is a two-dimensional carbon nanomaterial composed of a small number of well-defined, stacked Gr layers. * Gr quantum dots, which contain one or few layers of Gr nanosheets mainly used in photoluminescence situations. In general, Gr quantum dots usually have transverse sizes of less than 10 nm. * Gr nanoplatelets, containing a two-dimensional honeycomb Gr lattice. * GO, which is a Gr modified chemically and produced by exfoliation and oxidation together with the general modification of the basal plane by oxidation. The GO reveals a monolayer material with an elevated content of oxygen.