Thickness Optimization of Highly Porous Flame-Aerosol Deposited WO₃ Films for NO₂ Sensing at ppb

Nitrogen dioxide (NO₂) is a major air pollutant resulting in respiratory problems, from wheezing, coughing, to even asthma. Low-cost sensors based on WO₃ nanoparticles are promising due to their distinct selectivity to detect NO₂ at the ppb level. Here, we revealed that controlling the thickness of highly porous (97%) WO₃ films between 0.5 and 12.3 μm altered the NO₂ sensitivity by more than an order of magnitude. Therefore, films of WO₃ nanoparticles (20 nm in diameter by N₂ adsorption) with mixed γ- and ε-phase were deposited by single-step flame spray pyrolysis without affecting crystal size, phase composition, and film porosity. That way, sensitivity and selectivity effects were associated unambiguously to thickness, which was not possible yet with other sensor fabrication methods. At the optimum thickness (3.1 μm) and 125 °C, NO₂ concentrations were detected down to 3 ppb at 50% relative humidity (RH), and outstanding NO₂ selectivity to CO, methanol, ethanol, NH₃ (all > 10⁵), H₂, CH₄, acetone (all > 10⁴), formaldehyde (>10³), and H₂S (835) was achieved. Such thickness-optimized and porous WO₃ films have strong potential for integration into low-power devices for distributed NO₂ air quality monitoring.