Effect of the substrate temperature on the physical properties of molybdenum tri-oxide thin films obtained through the spray pyrolysis technique

Polycrystalline molybdenum tri-oxide thin films were prepared using the spray pyrolysis technique; a 0.1 M solution of ammonium molybdate tetra-hydrated was used as a precursor. The samples were prepared on Corning glass substrates maintained at temperatures ranging between 423 and 673 K. The samples were characterized through micro Raman, X-ray diffraction, optical transmittance and DC electrical conductivity. The species MoO3 (H2O)2 was found in the sample prepared at a substrate temperature of 423 K. As the substrate temperature rises, the water disappears and the samples crystallize into α-MoO3. The optical gap diminishes as the substrate temperature rises. Two electrical transport mechanisms were found: hopping under 200 K and intrinsic conduction over 200 K. The MoO3 films' sensitivity was analyzed for CO and H2O in the temperature range 160 to 360 K; the results indicate that CO and H2O have a reduction character. In all cases, it was found that the sensitivity to CO is lower than that to H2O.

► A low cost technique is used which produces good material.
► Thin films are prepared using ammonium molybdate tetra hydrated.
► The control of the physical properties of the samples could be done.
► A calculation method is proposed to determine the material optical properties.
► The MoO3 thin films prepared by spray pyrolysis could be used as gas sensor.