Honeycomb Texturing of Hierarchical Nanoflowers of WO3 as an Efficient Route to Improve Repeatability and Stability of Room Temperature Vapor Sensor

Hierarchical nanostructures of WO3 was synthesized by a hydrothermal method at 180°C using sodium tungstate dihydrate (Na2WO4.2H2O), sodium bisulfate dihydrate (NaHSO4.2H2O) and ethylene glycol (having different molar concentrations) with Titanium as the substrate. Molar concentrations of ethylene glycol was taken to be 2.241 M, 8.274 M and 15.693 M, which eventually produced, (i) nanoneedles with sparsely distributed nanosphere clusters (S-I) (ii) nanospheres (S-II) (iii) honeycomb textured hierarchical nanoflowers (S-III), respectively. After detailed morphological and structural characterizations, like field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM) and X-ray diffraction (XRD), these different nanostructures were employed as the gas sensing layer targeting the alcohol vapors in the concentration range of 10–400 ppm over a temperature range of 27°C (room temperature) to 200°C. Among the hierarchical nanoforms, S-III offered the best sensing performance in terms of response magnitude (~29.35%/~91.59%), response time (~31 s/~19 s) and recovery time (~42 s/~27 s), for 10/400 ppm ethanol at 27°C. Moreover, improved stability and repeatability were also achieved by the same structure. Such improvement in the sensing performance was duly correlated with the corresponding morphology of the nanostructure employing a semi-quantitative geometrical model.