Gas-sensing Properties Of Low-dimensional Nanostructured Metal Oxides

Recently, the metal oxide semiconductor based gas sensors have attracted much more attention. And their performances are primarily affected by the morphology, dimension, specific surface area, grain size, pore size of inter-/intra-materials, and agglomeration of sensing materials. This work mainly explored the synthesis and gas-sensing enhancement mechanisms of low-dimensional nanostructured metal oxides, which can be divided into the following three parts（corresponds to the new nanostructures, heterostructures, and doping enhancement, respectively） :（1） Porous ultrathin Ni O nanosheets have been synthesized by a simplified chemical bath deposition（CBD） method and calcined at different temperatures. Results showed that the nanopores formed with phase transition at calcination process. The pure cubic Ni O phase, with an average grain size of 4.67 nm and a thickness less than 5 nm, formed at 450 °C for 2 h in air. Moreover, the Ni O nanosheets synthesized here showed an enhanced response to ethanol at a low operating temperature of 200 °C. Similar ly, porous Co3O4 nanonetworks（NNWs）, converted from precursor Co OOH nanosheets, have also been synthesized via a controllable chemical reaction route and followed by thermal treatment at 400 °C. We can find that both of the adding K2S2O8 and the amount of aqueous ammonia play key roles in the formation process of Co OOH nanosheets. Gas-sensing results indicated that the NNWs based sensor exhibited a high response（11.79/100 ppm） to toluene at 150 °C, with a low detectable limit of 1 ppm. Overall, these enhanced gas responses were mainly due to their unique porous neck-connected networks.（2） In₂O₃/α-Fe₂O₃（IFO） heterostructure nanotubes have been successfully prepared through a facile single-capillary electrospinning method. The morphology and phase composition of IFO composites could be controlled by the additive amount of indium nitrate in precursor solutions and the calcination temperatures. It should be noted the interesting fact that the limited solid-solubility of In2O3 in α-Fe2O3 determines the formation of the IFO heterostructure at 500 °C. The IFO nanotubes based sensors showed enhancement gas response compared to pure α-Fe2O3 sensor, among them, the IFO-0.1（In/Fe = 1:9, molar ratio） sensors showed the highest response to ethanol at 225 °C（21.41/100 ppm）, which can be due to the ultrafine In2O3 grains and crucial existence of interface between In2O3 and α-Fe2O3. Additionally, Sn O2/α-Fe2O3（SFO） nanotubes with different Sn/Fe molar ratios have also been synthesized by the same route. The changes in morphology, grain size, phase structure, and gas-sensing properties of SFO-x（x Sn O2/0.5（1?x）α-Fe2O3） composite nanotubes were greatly affected by Sn O2 contents. The surface segregation and phase separation effects were suggested as the growth mechanism. And these special architectures expectedly made the sensors, based on SFO-x（0.025 ≤ x ≤ 0.2） nanotubes, significantly enhancing sensitivity and selectivity in ethanol detection. Among them, SFO-0.05 sensors performed the best sensing performance, with a high response（27.45） and good selectivity to 100 ppm ethanol, at a relatively low operating temperature of 200 °C. These sensing enhancements may attribute to the ultrafine grain size and doping effect, especially, the heterostructures between two oxides.（3） At the first part of this chapter, we have synthesized Ca-doped α-Fe2O3 nanotubes by electrospinning. Results showed that the structural and optical properties of the as-prepared α-Fe2O3 nanotubes were significantly affected by doping contents（1?15 mol%）. With increasing Ca doping content, the grain size of α-Fe2O3 nanotubes decreased monotonously（named ―grain refining effect‖）. This may be due to the low calcination temperature and a large mismatch between the radii of Ca2+ and Fe3+ ions. Moreover, gas-sensing measurements showed that the Ca-doped α-Fe2O3 nanotube based sensors exhibited enhanced gas-sensing properties toward both ethanol and acetone. At an optimal operating temperature of 200 °C, 7 mol% Ca-doped sensors present the highest response value to ethanol（26.8/100 ppm） and acetone（24.9/100 ppm） with a short response/recovery time. Furthermore, a possible gas-sensing mechanism was proposed, which suggested the grain refining effect of Ca dopants plays a dominant role in improving the sensor performances. Similarly, Mg-doped In2O3（Mg-In2O3） nanotubes have also been prepared by a facile electrospinning method. The results showed that the Mg dopants could dissolve into the lattice of In2O3. Despite the role of substitutional Mg in In2O3 as acceptors, the doped sensors have shown n-type conductivity in H2 S and ethanol, respectively. Mg-doped sensors exhibited a high sensitivity and selectivity toward H2 S at 150 °C, which could be ascribed to their high oxygen vacancy concentrations introduced by Mg acceptors. Importantly, the Mg-doped sensors also showed higher responses to high-concentration ethanol（500?5000 ppm） at 250 °C than those of primary In2O3 at 200 °C, and a high response value remained even the operating temperature increased to 300 °C. This ethanol sensing enhancement could be attributed to the oxygen vacancies generated at high temperature could overcompensate the Mg acceptors.