It is widely known that chemical properties of nanomaterials are structure- and composition-dependent. Through the direct control of synthetic methods and experimental conditions, material-specific features, like constituents, dimensionalities, and arrangement of the atoms within the materials, can be engineered, and the manipulation of desired chemical properties can be achieved. Compared with traditional materials preparation and processing methods, a new electrothermal strategy can achieve selective Joule heating of the reactants to a high temperature (>3000 K) with rapid heating and cooling rates (>104 K s-1), enabling the kinetic control of chemical reactions and the formation of hitherto hard-to-access metastable nanomaterials. To this end, my thesis covers research on direct electrothermal methods to synthesize ~30 different thermodynamically metastable nanomaterials, followed by the exploration of phase transformations and evolution using experiments and simulations. These nanomaterials show new mechanical, optical, and electronic properties, distinct from their more thermodynamically stable counterparts. This thesis begins with the syntheses of carbonaceous nanomaterials using the rapid high temperature electrothermal method. In Chapter 1, 8 different types of heteroatom-doped turbostratic graphene were synthesized and compared with commercial graphite and undoped turbostratic graphene, demonstrating that doped turbostratic graphene had better dispersibility and electrocatalytic oxygen reduction performance. In Chapter 2, the detailed spectroscopic and microscopic studies were provided, related to the formation of fluorinated nanodiamonds and subsequent reaction time-dependent phase evolution to fluorinated graphene and concentric carbon. Non-carbonaceous nanomaterials can also be synthesized or processed, demonstrating the universality of the electrothermal method. The preparation of turbostratic boron nitride, various boron-carbon-nitrogen ternary compounds and 1T-phase transition metal dichalcogenides were illustrated in Chapter 3 and Chapter 4. The rapid Joule heating methods to recycle the anode, cathode, and their mixtures were further discussed in Chapters 5-7. In addition, the utilization of above nanomaterials as the additives to fabricate the coating for modifying the surface wettability was demonstrated in Chapter 8. The artificial coating of the reactive metals and current collectors to improve the electrochemical stability in the rechargeable metal batteries were discussed in Chapter 9 and Chapter 10, reflecting the promising applications of these nanomaterials for surface modification.