Grain size effects on gas sensitivity of porous SnO2-based elements

Effects of grain size on gas sensitivity (S) are investigated by using porous sintered SnO2 elements fabricated with pure SnO2, foreign oxide-stabilized SnO2, or impurity-doped SnO2. When the SnO2 crystallite size (D) is controlled to a size in the range 5–32 nm, S for H2, CO and i-C4H10 is found to increase steeply as D decreases to be comparable with or less than 2L (≈ 6 nm) is both pure and stabilized elements, where L is the depth of the space-charge layer. However, S for ethyl alcohol gas is found to be also affected by surface acid-base properties, being greatly promoted by basic oxides. It is found that the control of L by doping impurities (Al3+ or Sb5+) into the SnO2 lattice results in great changes in S even when D is the same. Thus Al-doped SnO2 shows high sensitivity with increasing L even at D above 20 nm, while Sb-doped SnO2 is insensitive in the whole D region. A model for the grain-size effects is proposed, in which the transducer function is operated by a mechanism of grain control, neck control or grain-boundary control, depending on D.