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.