Polymorphous Transformations,Gas Sensing Performance Of Fe₂O₃ Nanostructures And Mechanism Discussion

Metal-oxide semiconductor gas sensor has potential advantages in environmental monitoring and breath diagnosis analyzers.Sensitivity,selectivity,stability and quick response and recovery of gas sensors are mainly dependent on the design of the sensing material.Metastable phase nanostructures may display high sensitivity and different selectivity.In this project,Fe₂O₃ nanostructures with different crystal structures and morphology were synthesized.The preparation process and the phase transition mechanism of ?-Fe₂O₃ were discussed.Their gas sensing performance were investigated and the sensing mechanism of ?-Fe₂O₃ was discussed.?-Fe₂O₃ could be prepared by calcining the precursor without IIA metal ion at 1050 °C for 1 h or at 900-1000 °C with the addition of 10 wt% Ba²⁺.The addition of Ba²⁺ would lower the phase transition temperature of Fe₂O₃ and lead to preferred crystal growth.The phase transition was mainly promoted by the free energy G variation accompanied by an increase in particle size.The stable phase was ?-Fe₂O₃,?-Fe₂O₃ and ?-Fe₂O₃ with the increasing of the particle size.Thus,with the increasing of the calcining temperature or the calcining time,the particle size of Fe₂O₃ would increase.Then reconstruction-phase transition would take place.Whereas,the influence of the IIA metal ion was complicated.The addition of the IIA metal ion would promote the growth and the phase transition of the Fe₂O₃ by decreasing the viscosity of silicate.Besides,the selective adsorption of IIA metal ions may change the free energy G of ?-Fe₂O₃ and put off the phase transition of it.Compared with ?-Fe₂O₃ and ?-Fe₂O₃,?-Fe₂O₃ exhibited excellent comprehensive gas sensing performance to ethanol.Besides,nanorods with pure ?-Fe₂O₃ and consisted of ?-Fe₂O₃ and ?-Fe₂O₃ with the same growth orientation exhibited similar selectivity to VOCs.Otherwise,?-Fe₂O₃ nanoparticle exhibited p-type sensing response to ethanol with low concentration,which was resulted from the modification to the energy band at the surface.Thus,it could be inferred that ?-Fe₂O₃ may be the surface-controlled type sensing material the same as ?-Fe₂O₃.