Development of NaxWO3 thin films for optical gas sensing applications

Monitoring and measuring gases based on their distinct optical absorption is essential in assessing both industrial processes and environmental dynamics. Optical-based sensors provide faster response, minimal drift and high specificity. Among the various sensing materials, near-infra-red (NIR, 780-2500nm) absorbed nanomaterials, including alkali-doped tungsten bronze nanoparticles and oxygen-deficient tungsten oxides, are attractive. Because of intense absorption in the NIR wavelength range based on sub-band energy levels and transparency in the visible wavelength range, these nanomaterials are promising options for optical gas sensing. This thesis explores the synthesis and characterization of sensing properties of nanostructured tungsten oxide (WO3), oxygen-deficient tungsten oxide (WO3-x) and sodium-doped tungsten bronze (NaxWO3) with varying concentrations of dopant (x= 0.1, 0.33 and 0.55). The aim of doping different concentrations is to understand the effect of dopant amounts on optical properties. These nanostructured oxides were synthesized via two different methods, thin films through sol-gel technique and nanoparticles via a thermal decomposition with oleylamine. Thin films were prepared by spin coating on silicon and fused silica substrates, followed by thermal treatment at 550°C. The prepared samples were characterized using techniques such as SEM, XRD, FT-IR, ellipsometry and UV-Vis-NIR spectroscopy. From XRD, the successful crystallization of the films was confirmed, while UV-VIS-NIR spectroscopy of NaxWO3 nanoparticles showed a broad absorption peak centered at 980nm that confirms the presence of a surface plasmon resonance peak in the NIR region. Furthermore, the hydrogen gas-sensing properties of these thin films were investigated with and without platinum coating. Gas response was measured at different operating temperatures (24oC, 100oC and 200oC) at a concentration of 5% hydrogen in Argon.