Effect of Process Conditions on the Structure and Optical Properties of MoO₃ Produced by Vapor Transport Deposition

Abstract—

MoO₃ samples have been prepared by vapor transport deposition under different process conditions: at two distinct hot-zone temperatures in a tube furnace (800 and 1100°C) and with different additives to argon as a major carrier gas: O₂, H₂O vapor, or N₂O gas. According to X-ray diffraction, electron-microscopic, and optical characterization results, the layered structure of microcrystals and band gap of MoO₃ are sensitive to not only vapor transport deposition conditions (synthesis temperature and vapor phase composition) but also mechanical treatment (grinding). At a synthesis temperature of 800°C, the distortion of the MoO₃ crystal lattice by incorporated H₂O molecules causes MoO₃ to transform from the major, orthorhombic α-phase (space group Pbnm) into a monoclinic phase (P2₁/n), which is accompanied by a reduction in band gap from 2.85 to 2.68 eV. At a higher synthesis temperature of 1100°C, neither hydrogen or oxygen impurities originating from water (H₂O) vapor nor nitrogen or oxygen impurities originating from N₂O change the layered orthorhombic structure of the microcrystals, but the addition of N₂O to the vapor transport medium reduces the band gap of MoO₃ to 2.51 eV. Mechanical treatment (grinding) of the MoO₃ microcrystals synthesized at the higher temperature (1100°C) with the addition of water vapor or N₂O to argon carrier gas produces a monoclinic phase (space group P2₁/n) in addition to the major, stable, orthorhombic phase (Pbnm). The MoO₃ microcrystals synthesized at a temperature of 800°C are more resistant to mechanical treatment: after grinding, they consist of only one phase: orthorhombic (Pbnm) in the case of synthesis in an argon–oxygen vapor transport medium or monoclinic (P2₁/n) in the case of the addition of water vapor to argon as a major carrier gas.