*3.3. Carbon-Based Nanofluids Fabrication*

As was explained previously in Section 2, carbon allotropes, whether ND, graphene, or CNT, have their own production routes and final product form. For instance, it was shown that both NDs and CNTs could only be produced in the form of dried particles, whereas graphene can be fabricated as dried sheets or as part of a dispersion. Therefore, depending on the type of nanoscaled carbon allotrope and base fluid desired for synthesizing the nanofluid, the production process can be constrained by only the two-step method or the manufacturer can be left with the freedom of selecting any of the two approaches. In general, the two-step method is the only approach that can be employed for fabricating dispersions containing NDs or CNTs, while both one- and two-step routes can be used for producing graphene nanofluids. Nevertheless, the majority of the studies have shown the adaptation of the two-step method for producing graphene nanofluids, which can be justified by the difficulties associated with the single-step route of fabrication and the limitations in the type of base fluid that can be used (see Section 3.1) [145,146]. Some of the research work published on fabricating NDs, graphene, and CNTs nanofluids using the two-step method are listed in Table 2. Note that the single-step graphene nanofluid production was excluded from Table 2 because it is precisely the same as liquid-phase exfoliation of graphene; thus, the reader can find further information's within the sources provided previously in Section 2.2 and the work published by Texter [147]. Nevertheless, it is worth mentioning that the common base fluids used in the graphene suspension one-step (or liquid-phase exfoliation) approach are n-methyl-2-pyrrolidone (NMP), γbutyrolactone (GBL), n,n-dimethylacetamide (DMAC), n,n-dimethylformamide (DMF), dimethylsulfoxide (DMSO), ortho-dichlorobenzene (ODCB), acetonitrile (ACN), and water with the aid of surfactant [148].


**Table 2.** Published work on nanodiamond, graphene, and carbon nanotubes nanofluids produced using the two-step approach.



Note: \* and ˆ refers to graphene sheet thickness and CNTs outer diameter, respectively.
