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

• Pd sensitized WO3 thin films based moderate operating temperature (200 °C) selective NO2 sensor is synthesized.
• Effect of solution concentration on structural and morphological and NO2 sensing properties is investigated.
• XPS studies show presence of W in different chemical states.
• The results indicate that physicochemical properties and sensor response strongly depend on the solution concentration.
• Various defects present in WO3 thin films corresponding to NUV, blue, green and red transitions.

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.

Typical transient gas response of WO3 thin film for various NO2 concentrations.Figure optionsDownload as PowerPoint slide