Synthesis of tungsten oxide thin films and investigation of gas sensing properties

Abstract

Nanostructured materials are considered as good candidates for gas sensing applications due to their large surface area-to-volume ratio and the size effect. Tungsten trioxide (WO3) is a typical n-type semiconductor material with a band gap of 2.5–2.8 eV. It has received much interest for applications inphotocatalysis, electrochromic devices, solar energy conversion and gas sensors due to excellent catalytic, optical and dielectric properties, good physical and chemical stability [1]. Gas sensors based on tungsten (VI) oxide are sensitive to a variety of gases such as NO2, O3, H2, H2S, NH3 and organic vapours. NOx is one of the major toxic pollutants in air, which causes respiratory diseases such as bronchitis, emphysema and heart diseases. In particular, tungsten oxide showed superior sensitivity and selectivity in detecting NO2 gas [2]. In this work tungsten oxide thin films (WO3) where synthesized on glass substrate using sol-gel approach with different additives and annealing temperatures. Firstly peroxytungsten acid (PTA) was synthesized as precursors using sodium tungstate dyhidrate (Na2WO4 x 2H2O), nitric acid (HNO3) and hydrogen peroxide (H2O2). Synthesized PTA powder was dissolved in water and ethanol solution and white color sol-gel solution was obtained. Another group of samples was prepared by the same synthesis scheme adding 4% oxalic acid to final sol-gel. Third group of samples was prepared adding PEG 300. After coating procedure samples were annealed at 500 °C, 400°C and 300°C for 2 h with heating rate of 5 °C/min. The coatings were characterized using X-ray diffraction (XRD) analysis, scanning electron microscopy (SEM) and gas sensing properties were analyzed in gas chamber. XRD results showed that crystallinity of phases depends on annealing temperature. After annealing at 300°C crystallinity of tungsten oxide is very low. However after annealing at 400°C and 500°C crystaline phases of tungsten oxide were obtained. Morphology of coatings was analyzed using SEM. It was found to depend on additives and also annealing temperature. 200-400nm sized particles were observed in the coating synthesized using PTA+EtOH sol-gel. Coating synthesized with oxalic acid looks porous with 1μm agglomerates. Coatings with PEG look smooth and without pores. Gas sensing properties were analyzed using different gases (volatile organic compounds, NO2 and H2S). Obtained results depend on many factors including annealing temperatures, different additives and also measurement factors.