Study On The Synthesis, Modification And Gas Sensing Properties Of Low-dimensional α-Fe₂O₃ Nanomaterials

In the 21 century,the fast development of industry in sources exploitation, petrochemistry,aviation and spaceflight,not only has promoted the advancement of the society,but also brought the hidden danger to the safety of human and the environment.With the increase of the people's environmental protection consciousness,the necessity and urgency of the detection of combustible and noxious gases has become the most important thing in recent years.Gas sensors based on metal oxide semiconductors have the advantages of high-sensitivity,nice-selectivity as well as easy miniaturization and automation.They can be applied in a wide range of analytical tasks,such as industrial control and environmental monitoring.Due to the excellent properties of magnetism,catalysis and gas-sensitivity, hematite was widely used in the fields of catalysts,pigments,film materials and gas sensors.As is known,α-Fe₂O₃ is a typical n-type semiconductor with 2.2 eV in E_g. The sensors based onα-Fe₂O₃ have been intensively investigated because of its good stability,lower cost,and easy availability.Although much work has been done on the improvement of its gas-sensing properties,the application ofα-Fe₂O₃ gas sensors is still considerably limitted because of some shortcomings,such as selectivity, durability,and resistance to environmental influences such as higher temperature, humidity etc.Due to fact that the sensing mechanism of theα-Fe₂O₃-based gas sensors belongs to the surface-controlled type,the gas-sensing properties ofα-Fe₂O₃- based sensors are coherent with its surface area and structure closely.Hereby,surface modification by proper additives or dopants is one of the efficient ways to improve the response and selectivity of the gas sensing materials for particular applications.In addition,nano-crystalline particles,possessing a large surface area,might be favorable for improving the sensitivity of gas-sensing materials.Hence,nano-technic is one of the new research directions to develop novel gas sensing materials. According to the above-mentioned methods,a series of investigations including the preparation of sensing materials,the surface modification of gas sensors and the application of nano-technic have been performed in this dissertation.In chapter 3,0-Dα-Fe₂O₃ nanoparticles have been obtained by two different preparation methods:chemical precipitation method and solid-state grinding method. Especially,the solid-state grinding method has the advantages of convenient, low-cost and environment friendly.Furthermore,the relationship between the sensitivities of the sensors based on 0-Dα-Fe₂O₃ nanoparticles and the calcination temperature,operation temperature and gas concentration were investigated systematically.The noble metals,well known as active catalysts,have been confirmed to possess the promoting effect on many semiconductor gas sensors.So,in chapter 4, the Au,Ag,Pt and Pd dopedα-Fe₂O₃ nanoparticles were prepared by coprecipitation method and their gas-sensing performance were investigated systematically.In all of these materials,the Ag/α-Fe₂O₃ gas-sensing materials prepared by the solid-state grinding method have been reported for the first time.The gas-sensing measurement results demonstrated that the dopedα-Fe₂O₃ gas sensors presented much better sensing properties than the undoped one and showed excellent selectivity to H₂S at relatively low operation temperature.Hence,the as-preparedα-Fe₂O₃ doped with noble metals would be an ideal candidate for the application in H₂S sensors.A possible H₂S-sensing mechanism of theseα-Fe₂O₃ based gas sensors was proposed according to the XPS analysis results.Because it's specific physical properties and interesting applications in nano-devices,one dimensional(1-D) materials,such as nanorods and nanowires,has become a potential study field in nanoscience and nanotechnology in the last few years.In chapter 5,1-D porousα-Fe₂O₃ nanorods were synthesized by a simple hydrolysis route and a hydrothermal method,respectively.Both the two methods are convenient,environment friendly,inexpensive and efficient.Moreover,α-Fe₂O₃ hollow spheres and nano-sized flower-likeα-Fe₂O₃ were also successfully obtained by the hydrothermal method.The gas-sensing measurement results demonstrated that these nano-structureα-Fe₂O₃ sensors presented much higher response and lower operation temperature than the 0-Dα-Fe₂O₃ nanoparticles and showed excellent selectivity to ethanol vapor.Hence,these two facile methods are fresh and feasible routes to prepare ethanol-sensors based on 1-Dα-Fe₂O₃ nanomaterials.In order to improve the selectivity of the metal oxide sensor,the p-n,n-p type semiconductor materials have attracted considerable attention in recent years.In this dissertation(chapter 6),the p-n type semiconductor CuO/α-Fe₂O₃ was prepared by the deposition-precipitation method.The gas-sensing measurement results suggested that CuO/α-Fe₂O₃ change from n-type to p-type semiconductor with the increase of CuO loading amount,and the CuO/α-Fe₂O₃ sensor exhibited excellent sensitivity and selectivity to CO at relative low temperature.The results provide valuable information to develop new CO gas sensors based onα-Fe₂O₃.Furthermore,we suggested a possible CO-sensing mechanism by the catalytic activity tests of CO oxidation on CuO/α-Fe₂O₃ catalyst for the first time.