*4.2.1 Vacuum arc melting method*

Guler et al. have reported another second generation of HEMs using vacuum arc melting followed by a heat treatment process. The materials were composed of AlCoCrFeNiTix with (x = 0.0, 0.2, 0.4, 0.6, 0.8 and 1.0) stoichiometry [68]. Ti was incorporated in the most studied HEAs (AlCoCrNi). The materials were heated at various temperatures that is 500, 700 and 1000°C. Zhang et al. reported the equimolar NbZrTiCrAl HEAs prepared by the vacuum arc melted. The equimolar HEAs were prepared with oxidation response (treated) vacuum arc melted NbZrTiCrAl HEA at 800–1200°C (**Figure 2**).

The HEAs prepared using this type of method show fine and conformal (uniform) grains, uniform chemical composition and high density [70]. However, it is difficult and tedious to evenly combine five or more metal species into a single nanostructured phase and precisely shape the HEAs into low dimensional (1D, 0D) structures to obtain diverse applications (owing to their intrinsic and thermodynamic instability) using the tradition wet chemical method. To overcome this shortcoming, there have been many approaches developed for the fabrication of HEAs to low dimensional structure, including boot-up and top-down to fabricate HEAs. Some of these methods include the sputtering technique, solvothermal deposition, carbothermal shock method and chemical dealloying method [71]. Recently, HEAs have been prepared successfully by employing the carbothermal shock and chemical dealloying method. The section below will discuss the methods for the fabrication of HEAs.

### *4.2.2 The bottom-up carbothermal shock method*

The carbothermal shock (CTS) method relies on thermally shocking the metal salt covered with carbon nanofibers at around 1730°C, followed by rabid quenching [72–74]. As illustrated in **Figure 3**, this method is a facile throughput to fabricate HEAs on carbon support by rapid heating and ultrafast cooling at about 105– 2000 K.S<sup>1</sup> . The rapid increase of temperature from ambient temperature to high temperature (≥1700°C) and cooling fast to 25°C is the promising solution to dictate the alloying of multicomponent at ultrafast kinetics [76]. This method was first introduced in 2018 by Yao et al. by developing the HEAs nanoparticles with PtPdRhRuCe composition for application in ammonium oxidation [72]. Through optimizing the method parameters namely, the concentration of the metal precursor, the shock duration, substrate and cooling rate a uniform dispersion and narrow particle size distribution can be achieved (**Figures 3** and **4**) [67].
