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

Abstract

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.

Theoretical Single-Droplet Model for Particle Formation in Flame Spray Pyrolysis

“…Flame spray pyrolysis (FSP) has emerged as a promising technology for industrial production of single-component and multicomponent metal-oxide nanomaterials. − The generated nanoparticles have wide applications including catalysts, , optical materials, sensors, , energy storage materials, , and biotechnology, etc. Different from the conventional vapor-fed aerosol flame synthesis (VAFS) method, − FSP can utilize less-volatile precursors, which significantly lower the cost and extend the versatility in elements.…”

Nanoplasmonic NO2 Sensor with a Sub-10 Parts per Billion Limit of Detection in Urban Air

“…To put this result into perspective, we first note that an LoD of 3 ppb is on par with the best thin-film WO 3 -based MOS-type NO 2 sensors reported in the literature. 9 , 33 However, as the key distinctive feature and a step beyond this state of the art, our sensors exhibit this low ppb LoD in an environment where all molecular species are mixed (and not where the sensor is exposed sequentially to them 9 , 33 ), thereby truly emulating a real urban air environment.…”

Thin Film and Nanostructured Pd-Based Materials for Optical H2 Sensors: A Review

“…This is in contrast to continuous thinner films (40 nm) of very similar composition (Pd 85 Au 15 ), which exhibit hysteresis [44]. Despite that other aerosol-based methods (e.g., flame synthesis [20,[71][72][73][74][75][76]) can be used to produce Pd-based nanoparticulate sensors, so far only spark ablation has been used to produce ANP-based materials for optical H 2 sensors.…”