Silicon carbides are basilic ceramics with proper bandgaps (2.4–3.3 eV) and unique optical properties. SiC@C monocrystal nanocapsules with different morphologies, sizes, and crystal types were synthesized via the fast and facile direct current (DC) arc discharge plasma method. The influence of Ar atmosphere on the formation of nanocrystal SiC polytypes was investigated by optical emission spectroscopy (OES) diagnoses on the arc discharge plasma. Boltzmann's plot was used to estimate the temperatures of plasma containing different Ar concentrations as 10, 582 K (in 2 × 10⁴ Pa of Ar partial pressure) and 14, 523 K (in 4 × 10⁴ Pa of Ar partial pressure). It was found that higher energy state of plasma favors the ionization of carbon atoms and promotes the formation of α-SiC, while β-SiC is generally coexistent. Heat-treatment in air was applied to remove the carbon species in as-prepared SiC nanopowders. Thus, the intrinsic characters of SiC polytypes reappeared in the ultraviolet–visible (UV–vis) light absorbance. It was experimentally revealed that the direct bandgap of SiC is 5.72 eV, the indirect bandgap of β-SiC (3C) is 3.13 eV, and the indirect bandgap of α-SiC (6H) is 3.32 eV; visible quantum confinement effect is predicted for these polytypic SiC nanocrystals.

Carbon-coated SiC@C nanocapsules (NCs) with a hexagonal platelet-like morphology were fabricated by a simple direct current (DC) arc-discharge plasma method. The SiC@C NCs were monocrystalline, 120–150 nm in size, and approximately 50 nm thick. The formation of the as-prepared SiC@C NCs included nucleation of truncated octahedral SiC seeds and subsequent anisotropic growth of the seeds into hexagonal nanoplatelets in a carbon-rich atmosphere. The disordered carbon layers on the SiC@C NCs were converted into SiO₂ shells of SiC@SiO₂ NCs by heat treatment at 650 ℃ in air, during which the shape and inherent characteristics of the crystalline SiC core were obtained. The interface evolution from carbon to SiO₂ shells endowed the SiC@SiO₂ NCs with enhanced photocatalytic activity due to the hydrophilic and transparent nature of the SiO₂ shell, as well as to the photosensitive SiC nanocrystals. The band gap of the nanostructured SiC core was determined to be 2.70 eV. The SiC@SiO₂ NCs degraded approximately 95% of methylene blue in 160 min under visible light irradiation.