The humidity dependence of the gas‐sensing characteristics in SnO2‐based sensors, one of the greatest obstacles in gas‐sensor applications, is reduced to a negligible level by NiO doping. In a dry atmosphere, undoped hierarchical SnO2 nanostructures prepared by the self‐assembly of crystalline nanosheets show a high CO response and a rapid response speed. However, the gas response, response/recovery speeds, and resistance in air are deteriorated or changed significantly in a humid atmosphere. When hierarchical SnO2 nanostructures are doped with 0.64–1.27 wt% NiO, all of the gas‐sensing characteristics remain similar, even after changing the atmosphere from a dry to wet one. According to diffuse‐reflectance Fourier transform IR measurements, it is found that the most of the water‐driven species are predominantly absorbed not by the SnO2 but by the NiO, and thus the electrochemical interaction between the humidity and the SnO2 sensor surface is totally blocked. NiO‐doped hierarchical SnO2 sensors exhibit an exceptionally fast response speed (1.6 s), a fast recovery speed (2.8 s) and a superior gas response (Ra/Rg = 2.8 at 50 ppm CO (Ra: resistance in air, Rg: resistance in gas)) even in a 25% r.h. atmosphere. The doping of hierarchical SnO2 nanostructures with NiO is a very‐promising approach to reduce the dependence of the gas‐sensing characteristics on humidity without sacrificing the high gas response, the ultrafast response and the ultrafast recovery.

Ultra-fast responding and recovering C(2)H(5)OH sensors were prepared using nanoscale SnO(2) hollow spheres with NiO-functionalized inner walls. The exceptional ultra-fast recovery characteristics were attributed to the catalytic surface reaction assisted by NiO at the inner shell.

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Magnetite (Fe 3 O 4 ) hollow spheres were prepared by solvothermal reaction of ethanol solution containing Fe-acetate and L-lysine, and were subsequently transformed into hematite (Fe 2 O 3 ) hollow spheres with nanoscale (20-30 nm) thin shells by heat treatment at 500 C for 2 h. Both the as-prepared and heat-treated hollow spheres contained another small sphere within each shell, which was attributed to the following solvothermal self-assembly reactions: (1) the nucleation of Fe 3 O 4 spheres, (2) lysine capping on the nuclei, (3) the growth of lysine-capped particles by cross-linking between lysine molecules, and (4) the formation of Fe shell layers by the interaction between Fe ions and outer lysine molecules. In the assembly reaction, L-lysine with amino and carboxyl radicals played the key role. The heat-treated Fe 2 O 3 hollow spheres showed significantly enhanced C 2 H 5 OH sensing characteristics and promising Li-ion intercalation behaviors. Experimental PreparationThe Fe 3 O 4 hollow spheres were prepared by solvothermal selfassembly reaction. In 50 ml of anhydrous ethanol was dissolved 0.0870 g of iron(II) acetate (Fe(CH 3 CHOO) 2 , 99.995%, Sigma-Aldrich Co.). After adding 0.0146 g of L(+)-lysine (C