Synthesis and Characterization of MoO₃ Thin Films and Powders from a Molybdenum Chloromethoxide

A molybdenum chloromethoxide was prepared by reaction of MoCl 5 with methanol. By water addition, a sol was obtained that was stable on a month time scale. The sols were characterized by UV−vis and Raman spectroscopy, showing that upon water addition hydrolysis takes place but without extensive condensation, explaining the sol stability. The sol was modified by addition of cetyltrimethylammonium bromide to get deposition of thin films onto silicon substrates. Very uniform, polycrystalline α-MoO 3 thin films on silicon or polycrystalline powders could be obtained after heat treatment at 350 °C of the as-deposited film and the evaporated sol, respectively. Fourier transform infrared and X-ray photoelectron spectroscopy studies on the films and powders have shown that, while the films are extensively polymerized after the deposition and chlorine free, the powders evolve with the heating temperature toward the final structure by a stepwise process in which the Mo(IV) and Mo(V) states are progressively oxidized to the final Mo(VI) state. The powders heated at at least 250 °C are chlorine free. X-ray diffraction patterns measured on thin films and powders show that the films are amorphous up to 250 °C, while powders begin to crystallize as α-MoO 3 at such a temperature. These results are supported by thermal analysis on powders. Scanning electron microscopy observations show that the film morphology is initially very smooth while after heating at 500 °C it is constituted by flat micrometric platelets. The powders heated at 500 °C show a lamellar structure.