Carbon-Assisted Isothermal Close Space Vapor Transport Growth of Mo₄O₁₁ and MoO₂ Micro/Nanostructures by in Situ Formation of the Mo(CO)₆ Transport Agent

In this article, we present a novel Isothermal Close Space Vapor Transport growth method to obtain isolated nano- and microstructures of molybdenum oxide compound crystals onto the surface of SiO₂/Si substrates. They include laying rhombic platelets and high aspect ratio wires of MoO₂ as well as 3D Mo₄O₁₁ hillocks. Contrary to previous results in which MoO₂ was grown from a source of MoO₃ in a H₂ reductive flow, in the present work N₂ carrier gas was used instead. Since the reduction of MoO₃ is not expected in a N₂ atmosphere, we discuss the influence of the graphite crucible on the in situ formation of the volatile species Mo(CO)₆. Experiments using a quartz crucible instead of graphite as well as thermodynamic calculations, based on Gibbs free energy analysis, corroborate such an influence. The rhombic platelets were modified by annealing in air, and their evolution was followed by Raman studies as a function of temperature. X-ray diffraction, Raman spectroscopy, atomic force and scanning electron microscopies, and photoluminescence techniques were used to characterize the crystal orientation, phase, and morphology of the obtained structures.