The influence of thermal and visible light activation modes on the NO2 response of WO3 nanofibers prepared by electrospinning

The paper reports on the influence of visible light at different wavelengths (red, λ = 630 nm; green, λ = 570 nm; purple-blue, λ = 430 nm), light irradiance conditions (from 30 to 770 μW/cm2) and operating temperatures (from 25 °C to 100 °C) on the electrical response of WO3 electrospun nanofibers (NFs) to 100-400 ppb NO2 gas in dry air. WO3 NFs were prepared by mixing a W-O sol-gel transparent solution (WCl6 in ethanol) with a polymeric solution made of polyvinylpyrrolidone (PVP) and dimethylformamide (DMF). Electrospun NFs were annealed between 300 °C and 500 °C and the their microstructures features investigated by SEM and XRD. Room temperature (25 °C) gas responses of the 450 °C annealed NFs have shown that, beside a slight reduction of the relative gas response (RRs), a decrease of the light wavelength (toward the purple-blue) and an increase of its irradiance, greatly improve the base line recovery and the response time with respect to dark conditions. At operating temperatures ranging from 25 °C to 100 °C, sensor relative responses in dark always resulted to be higher as respect to the ones displayed under purple-blue light illumination. The combined action of purple-blue light with an irradiance of 770 μW/cm2 and of mild operating temperature of 75 °C, relative responses (RRs) of 12.4 and base line recovery percentages (RPs) of 97% were attained at 400 ppb NO2. The capability to tune the response of WO3 NFs to NO2 by combining light and mild thermal gas sensors activations is addressed and discussed also considering the involved response mechanisms.