Sub-ppm Methane Sensing by Spark-Ablation-Synthesized Nano-SnO ₂

Methane is considered one of the cleanest energy sources as it produces fewer pollutants upon burning compared to other fossil fuels. Its accidental release during extraction, transportation, and use, however, poses significant environmental and safety risks, warranting advanced sensing technologies to monitor its concentration in ambient air. Here, we prepare nanoparticle-based materials for sensing methane at concentrations that are highly relevant in the atmospheric environment. The nanoparticle (NP) building blocks of the sensing materials are produced by spark-ablating and simultaneously quenching Sn electrodes with a N₂ flow at atmospheric pressure. The resulting Sn NPs are subsequently collected and oxidized to SnO₂ by thermal annealing in ambient air before doctor blading them onto substrates with interdigitated electrodes. The synthesized materials were characterized by X-ray diffraction and photoelectron spectroscopy, Brunauer-Emmett-Teller analysis, as well as atomic force, transmission, and scanning electron microscopy. The results show that our sensing materials can quantify methane concentrations down to 0.2 ppm, having a signal-to-noise ratio of 58 and a theoretical limit of detection of ca. 7 ppb. What is more, they maintain excellent robustness across a relative humidity range of 20-80% and exhibit a high cycling stability and repeatability; features that render them superior compared to other metal oxide semiconducting materials reported in the literature so far. Based on our measurements, we also offer new insight into how the NP synthesis process can affect sensor sensitivity, demonstrating a correlation between spark-ablation energy and NP size, which in turn determines the crystal size, the specific surface area, as well as the fraction of adsorbed oxygen on the surface of the sensing material, and consequently its interaction with the target gas. Combined with the simplicity of their preparation, these sensing materials hold great potential for a wide range of environmental and industrial applications.