| SnO2 microspheres were prepared via a solution phase method, which exhibited good response for ethanol as the concentrations lower as 2 ppm; Fe3+-Nb5+ co-doped SnO2 were prepared at 1200 oC by a simple chemical co-precipitation method, the solubility of Fe3+ in SnO2 has been increased up to 33 % by co-doping with Nb5+; Sn4+ doped FeNbO4 were also prepared at 1000 oC, both of orthorhombic and monoclinic phases have been obtained for the samples of SnxFe(1-x)/2Nb(1-x)/2O2 (0≤x≤0.10).The SnO2 microspheres with different sizes were obtained by adjusting the experimental conditions such as reaction temperature, reaction time, concentration of Sn(OH)62- and water content. The nanorods, which grow along the [001] direction, were about 20-100 nm in diameter and from 50 nm to 2μm in length. A mechanism for the growth of the dandelion-like nanostructures was proposed: the microspheres were formed by primary nucleation of SnO2 nanocrystals followed by aggregation into microspheres. Then, the SnO2 nanocrystals ripened and grow further into nanorods, and the dandelion-like structures were formed. Furthermore, the SnO2 microspheres exhibited high sensitivity for ethanol as well as quick response and recovery time due to the novel structures. In the dandelion-like structure, the nanorods dispersed from the center and formed interval space between the nanorods, interval space play a crucial role in reaction efficiencies during gas diffusion, and may play an important role in their short recovery time.Fe3+-Nb5+ co-doped SnO2 were prepared at 1200 oC by a simple chemical co-precipitation method. The Sn1-2xFexNbxO2 solid solutions kept cassiterite structure in the range of 0 < x≤0.33, and their cell parameters decrease with increasing x. While x = 0.40, a second phase with orthorhombic FeNbO4 structure co-exists with the cassiterite phase, and the second phase becomes dominant while x≥0.45. The cassiterite structure is stabilized because of the charge balance due to the substitution of two Sn4+ by one Nb5+ and one Fe3+. The magnetic measurements indicated that low doping ratio sample (x = 0.03) exhibits paramagnetic behavior. A paramagnetic to antiferromagnetic phase transition was observed for the samples with higher doping ratio (x≥0.15). In all of the samples, no ferromagnetic were obtained. According to F-center exchange mechanism, overlap of the F-center electron orbitals with the d-orbitals of the neighboring Fe3+ spins to form Fe3+-□-Fe3+ groups is crucial for ferromagnetic coupling. Doped Fe3+ spins might also exist as isolated paramagnetic spin systems wherever the F-center mediated ferromagnetic coupling is not achieved due to lack of Fe3+ neighbors and/or oxygen vacancies. In addition, the nearest-neighbor Fe3+-Fe3+ directed interaction is antiferromagnetic. In our samples, when the doping ratio is low, the isolated Fe3+ show paramagnetic behavior, with increasing doping concentration, the number of magnetic ions occupying adjacent lattice positions increase, and the antiferromagnetic coupling results in the magnetic moments being antiferromagnetically aligned, leading to a reduction in the average magnetic moment per magnetic ion.Sn4+ doped FeNbO4 were prepared at 1000 oC. Owing to the different doping ratio of Sn4+, the samples presented either orthorhombic or wolframite structures, which means different levels of cation order in the samples. The results indicated that the degree of cation order in the samples depend on the doping ratio of Sn4+. Antiferromagnetic behaviors were observed in all of the samples, and the samples show different magnetic behaviors correlate with the different structures. The results indicated that the degree of cation order in the samples depend on the doping ratio of Sn4+.
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