Size-Dependent Response of Hydrothermally Grown SnO₂ for a High-Performance NO₂ Sensor and the Impact of Oxygen

Abstract

A NO 2 sensor with a detection limit down to the ppb level based on pristine SnO 2 has been developed through a facile poly(acrylic acid)-mediated hydrothermal method. SnO 2 particles of solid microsphere, hollow microsphere, and nanosphere morphologies were synthesized, with respective constitutional crystallite of size ∼2 μm in length and 10−20 nm and ∼7 nm in diameter. All sensors show great selectivity to NO 2 . The hollow microsphere sensor exhibits the best performance, with medium specific surface area (SSA), followed by the nanosphere sensor with the largest SSA. This is attributed to the superposition of two opposite effects on sensor response with increased SSA: more adsorption sites and fewer electrons to be taken out with overly small crystallite that may reach complete depletion. O 2 is found to speed up the response and recovery times but reduce the response because O adsorbates facilitate the adsorption/desorption of NO 2 thermodynamically, and the two oxidizing gases compete in harvesting electrons from SnO 2 . The adverse effect of humidity can be minimized by operating the sensor at 110 °C. The response of the hollow microsphere sensor to 50 ppb of NO 2 is 8.8 (R g /R a ) at room temperature, and it increases to 15.1 at 110 °C. These findings are useful for developing other oxidizing gas semiconductor sensors.