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 NOX, 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 NOX 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 NOX gas.