The Controllable Synthesis, Tunable Structure And Gas-sensing Properties Of Iron Oxides

In recent years,with the rapid development of industrialization and technology,there are also some negative effects:exhaust gas,effluent and waste are increased gradually.So it is urgent and necessary to monitor all kinds of harmful and poisonous gases in real time.Owing to their high sensor response,low fabrication cost,simple manufacturing process and good process compatibility,gas sensors based on semiconducting metal oxides have been widely used in many fields such as gas monitoring and fire alarms.Among them,hematite??-Fe₂O₃?is an environmentally friendly n-type semiconductor and has been extensively investigated in catalysts,gas sensors,optical devices,lithium-ion batteries and electromagnetic devices.However,the response of sensor based traditional?-Fe₂O₃ semiconductor no longer satisfies the practical application.In this paper,mesoporous iron oxide nanowires??-Fe₂O₃NWs?were synthesized via nanocasting using ordered silicon dioxide SBA-15 as hard templates and the gas-sensing properties of the?-Fe₂O₃ sensor was enhanced by changing their size,morphology and doping metal elements In.Then the effects of calcination temperature,size,morphology and doping on the gas-sensing performance of?-Fe₂O₃ nanowires sensors were investigated.The main contents are as follows:?1?The dispersed and uniform hematite nanowires??-Fe₂O₃ NWs-70??-Fe₂O₃NWs-100??-Fe₂O₃ NWs-130?were synthesized using SBA-15-70,SBA-15-100 and SBA-15-130 as hard templates by the nanocasting method,and the diameter of?-Fe₂O₃ NWs-70,?-Fe₂O₃ NWs-100??-Fe₂O₃ NWs-130 was about 4,6 and 8 nm,respectively.The results showed that with the diameter decreased,the Eg was increased.The gas-sensing performance of?-Fe₂O₃ NWs-70 was slightly better than that of?-Fe₂O₃ NWs-100 and NWs-130,which was 31.38 to 100 ppm ethanol at300?.With the diameter decreased,the gas-sensing performance was better.The response and recovery time of?-Fe₂O₃ NWs to ethanol was fast,and that of?-Fe₂O₃NWs-70 was 7 s and 2 s,while that of?-Fe₂O₃ NWs-100 and NWs-130 were both 7 s and 3 s.Besides,the selectivity to ethanol was also relatively poor.?2?Iron oxide nanowires were synthesized by hard-template method and changed the calcination temperature with?550,650 and 750??.Then the effects of the calcination temperature on the gas-sensing properties of mesoporous?-Fe₂O₃ NWs were investigated.The results indicated that the sensitivity of?-Fe₂O₃ NWs-650 was the highest with the value of 41.63 to 100 ppm ethanol at 300?and 650?was the best calcination temperature.The response and recovery times of?-Fe₂O₃ NWs-550,NWs-650 and NWs-750 were only 6 s,8 s,8 s and 1 s,1 s,1 s.Besides,the selectivity of sensors to ethanol was relatively poor.?3?Iron oxide nanowires were synthesized by hard-template method and?-Fe₂O₃ DNWs and BNWs were successfully separated through concentration technique.Both samples with the same diameter presented the different interwires distance.The results indicated that?-Fe₂O₃ BNWs and DNWs all exhibited the excellent gas-sensing performance.The sensitivity of?-Fe₂O₃ BNWs was the higher with the value of 36.06 to 100 ppm ethanol at 300?,while that of?-Fe₂O₃ BNWs was 26.11.The?-Fe₂O₃ BNWs and DNWs presented the rapid response and recovery times?7 s and 1 s?,but poor selectivity to ethanol.?4?Iron oxide nanowires and indium-doped?-Fe₂O₃ NWs were synthesized via nanocasting using mesoporous silica SBA-15 as hard templates.The results indicated that the In-doped?-Fe₂O₃ not only showed significantly improved gas-sensing properties as compared to undoped?-Fe₂O₃,but also greatly improved the selectivity to ethanol gas.The sensor fabricated from the 3 mol%In-doped?-Fe₂O₃ with mesoporous nanowires exhibited excellent sensing properties to ethanol at the optimum temperature of 300?and the highest sensitivity to 100 ppm ethanol gas was 47.4.The response and recovery times of 3 mol%In-doped?-Fe₂O₃ were 8 s and1 s.In addition,the selectivity of 3 mol%In-doped?-Fe₂O₃ to ethanol was outstanding.