Size-selected agglomerates of Sn O₂ nanoparticles as gas sensors

The effect of nanoparticle structure on gas sensing performance is investigated. Size-selected nanostructured Sn O₂ agglomerate particles for gas sensors were made by scalable flame spray pyrolysis. These particles were polydisperse (up to 12 μm in diameter) and consisted of primary particles of 10 nm in grain and crystal size as measured by transmission electron microscopy, x-ray diffraction, and Berner low pressure impactor (BLPI). The effect of agglomerate size on thermal stability and sensing of ethanol vapor (4-100 ppm) and CO (4-50 ppm) was investigated by selecting nearly monodisperse fractions of these agglomerates by the BLPI. Sensor layers made with these size-fractionated agglomerates exhibited higher thermal stability and dramatically enhanced sensitivity for both analytes than layers made with polydisperse agglomerates. This is attributed to their aggregate (or hard agglomerate) structure exhibiting small sinter necks between their constituent primary particles of tin dioxide that had also a narrow size distribution as expected for particles generated in flames. Upon further sintering of these optimally sized, nanostructured agglomerates, grain and neck growth degraded their superior sensitivity, supporting the proposed mechanism of their enhanced sensitivity: optimal primary particle necking.