Structure-property correlations in molybdenum trioxide thin films and nanoparticles

Molybdenum trioxide films were grown by thermal evaporation of MoO3 powder on silica, silicon, and alumina substrates and their crystal structure; short−range order, stoichiometry, surface morphology, optical, electrical, and gas-sensing properties were studied. Optical band gap decreases from 2.88 eV to 1.88 eV with post−deposition annealing and crystallization of amorphous film into the orthorhombic phase. Refractive indices and thicknesses of amorphous and crystalline films were determined from the analysis of interference fringes in the UV-visible transmittance and reflectance spectra. Nanoparticles of MoO3 were produced by solvo-thermal method and their structural, surface morphology and gas-sensing properties were compared with those of thin films. High−resolution Raman studies were performed on amorphous and crystalline film, bulk and nanopowder samples, these studies revealed that the short-range structure of amorphous MoO3 is similar to that of the orthorhombic phase. The relative intensities of low frequency Raman peaks at 285 cm−1 and 291 cm−1 show correlation with the oxygen concentration in the samples and it is found that Raman spectroscopy can be used to determine the Mo−O stoichiometry. The gas sensing properties of crystalline film and nanoparticles were studied towards H2S and C2H5OH. Thin films show H2S response of 2.8 towards 150 ppm of H2S, and a response of 2.6 for 2000 ppm of C2H5OH. Nanoparticles exhibit higher H2S and C2H5OH response of ~6 and faster response times. Maximum response towards the two gases is in the operating temperature range of 453 K to 500 K and is due to the reaction of H2S and C2H5OH molecules with surface adsorbed O− ions, and there is little or no role of surface redox reactions.