Effect of solution concentration on physicochemical and gas sensing properties of sprayed WO3 thin films

Sub-stoichiometric tungsten trioxide (WO₃) thin films are deposited onto the glass substrates by spray pyrolysis technique using ammonium metatungstate. Effect of solution concentration on structural, morphological, optical, electrical and NO₂ sensing properties of WO₃ thin films is studied. Films are polycrystalline with monoclinic crystal structure and sub-stoichiometric as observed form the XRD and XPS studies, respectively. The SEM and AFM images show micro grained structure and surface roughness increases with increase in solution concentration. The PL studies revealed that the majority of the defects are the oxygen vacancies. From XPS and PL studies it is observed that, oxygen vacancies decrease with increase in solution concentration. The dielectric constant of the films as a function of frequency is in concurrence with resistivity measurements. Films show reproducible and reversible gas response at various operating temperatures and gas concentrations. Highest sensor response (38%) towards 200 ppm NO₂ concentration is observed for the film with 15 mM solution concentration at moderate operating temperature (200 °C). Pd sensitization enhanced gas response to 68% and improved kinetics of the sensor. Films are highly selective towards NO₂ as compared with the various gases such as SO₂, LPG, NH₃ and H₂S.