Size effect and gas sensing characteristics of nanocrystalline xSnO₂-(1-x)α-Fe₂O₃ ethanol sensors

Non-equilibrium nanocrystalline xSnO₂-(1-x)α-Fe₂O₃ powders have been prepared using the mechanical alloying technique. The thick film screen printing technology is then employed to fabricate these ethanol gas sensors. Their particle size and structural properties are systematically characterized using X-ray diffraction (XRD) and transmission electron microscopy (TEM). The gas sensing characteristics are also measured. Based on the experimental results, we have observed that particle size of the powders is drastically milled down to about 10 nm after 24 h of high-energy milling. A very high gas sensitivity value of 845 for 1000 ppm of ethanol gas in air has been obtained. Our proposed new structural model for these non-equilibrium nanocrystalline xSnO₂-(1-x)α-Fe₂O₃ materials explains both the lattice expansion of these high energy mechanically alloyed powders as well as the charge neutrality in terms of additional oxygen dangling bonds at the nano-sized particle surfaces. It is those enormous oxygen-dangling bonds at the particle surfaces that give rise to the high gas sensitivity. The sensors are found to be 32.5 times more selective to the ethanol gas compared to CO and H₂ gases.