Synthesis of Vertical MoO2/MoS2 Core–Shell Structures on an Amorphous Substrate via Chemical Vapor Deposition

Vertical MoO2/MoS2 core–shell structures were synthesized on an amorphous surface (SiO2) by chemical vapor deposition at a high heating rate using a configuration in which the vapor phase was confined. The confined reaction configuration was achieved by partially covering the MoO3-containing boat with a substrate, which allowed rapid buildup of the partially reduced MoO3–x crystals in an early stage (below 680 °C). Rapid temperature ramping to 780 °C enabled spontaneous transition of the reaction environment from sulfur-poor to sulfur-rich, which induced a sequential phase transition from MoO3–x to intermediate MoO2 and finally to MoO2/MoS2 core–shell structures. The orthorhombic crystal structure of MoO3–x contributed to the formation of vertical crystals on the amorphous substrate, whereas the nonvolatility of the subsequently formed MoO2 enabled layer-by-layer sulfurization to form MoS2 on the oxide surface with minimal resublimation loss of MoO2. By adjustment of the sulfurization temperature and time...