Organic acids assisted hydrothermal synthesis of WO3 nanoplates and their gas sensing properties

Tungsten oxide is amongst the most widely used materials in electro-, photo-chromic applications. Recently tungsten oxide has been employed as sensing layer for detection of hazardous gases. In this work, we report synthesis of WO₃ nanoparticles via a facile hydrothermal method using sodium tungstate and different organic acids (viz. citric acid, oxalic acid, malonic acid, and (L+) tartaric acid). We have investigated the effect/role of organic acid on the morphology and gas sensing properties. The X-ray diffraction (XRD) studies confirmed that citric acid and oxalic acid assisted routes give monoclinic structure (m-WO₃) while malonic acid and (L+) tartaric acid give hexagonal structure (h-WO₃). The nanoplate-like morphology was revealed by Scanning electron microscopy (SEM) analysis. The thick film of WO₃ powder was deposited by using a screen printing technique. The gas response of thick films fired at 400°C/2h was studied. The change in the gas response of WO₃ nanoplates for various concentrations and operating temperatures were studied for NO_X, acetone, ethanol and ammonia vapors. In general, we observed response towards acetone, ethanol and ammonia vapors at higher operating temperature. However, the citric acid, oxalic acid and tartaric acid assisted WO₃ exhibited good response for NO_X at lower operating temperature (from 80°C-200°C). The gas response studies revealed that WO₃ synthesized by citric acid assisted route exhibits highest sensitivity (S=77%) at 130°C towards NO_X gas.