Study On The Structure Modulation And Room-temperature Hydrogen Sensing Property Of The Nb₂O₅ Nanorods And Compositing Materials

Hydrogen energy has been regarded as one of the most promising clean and renewable energy,but the high permeability together with the inflammable and explosive properties of hydrogen gas leads to great safety risks to its production,transportation,storage,and usage process.The application of stable and reliable hydrogen detector to monitor the hydrogen concentration in the environment is the premise to guarantee the hydrogen energy safety.Semiconductor hydrogen sensors have exhibited several advantages including the long lifetime,high sensitivity,low production cost and easy integration.However,the sensors must operate at high temperature,leading to high power consumption,poisoning,and potential safety hazards.The development of high-performance semiconductor hydrogen sensor which can work at room temperature has attracted great attention in the research field.The low-dimensional nanostructures of semiconductor oxides with high specific surface area and outstanding electrical transportation properties have been regarded as the promising host materials for building high-performance room-temperature hydrogen sensors.Nb₂O₅ is a wide-band-gap semiconductor oxide material with outstanding catalytic performance for the dissociation of hydrogen molecules.The Nb₂O₅ low-dimensional nanostructures have exhibited great potential in hydrogen sensor area.Aiming at the problems including the unclear hydrogen sensing mechanism,structure-performance correlation and the unsatisfactory room-temperature hydrogen sensing performance of the Nb₂O₅ and its compositing materials,the Nb₂O₅ nanorods are selected in this thesis as the research object for studying the impact of phase,morphology,and defects on the electrical and room-temperature hydrogen sensing properties of the materials.Different approaches including the modulation of calcining temperature and the introduction of heterojunction interfaces are employed for optimizing the room-temperature hydrogen sensing performance of the products.Moreover,the hydrogen sensing mechanism of the materials are also analyzed.The major research results are listed as following:（1）The hydrogen sensing mechanism of the Nb₂O₅ nanorods and the impact of phase structure on the room-temperature hydrogen sensing performance were studied.The monoclinic Na₂Nb₂O₆·H₂O nanobelts with Sandia octahedral molecular sieve structure and high aspect ratio were prepared through the hydrothermal method.Their ion exchange properties promote the large-scale synthesis of H₂Nb₂O₆·H₂O nanobelts.The Nb₂O₅ single-crystalline nanorods,polycrystalline nanorods and coral-like porous micro/nano-structures,with pseudo-hexagonal（TT）,orthorhombic（T）and monoclinic（H）phase,were obtained after the high-vacuum calcination treatment at 600,800,and 1000 ^oC to the H₂Nb₂O₆·H₂O nanobelts,respectively.The experimental results reveal that the hydrogen sensing behavior of the Nb₂O₅ nanorods can be attributed to contribution from both the redox reaction between hydrogen and pre-adsorbed oxygens,and the surface adsorption of hydrogen.The phase structure exhibit great impact on the electrical and room-temperature hydrogen sensing performance of the Nb₂O₅ Due to the high interplanar distance and the existance of large amount of vacant sites in the orthorhombic Nb₂O₅ lattice that promotes the gas adsorption and distribution process,the orthorhombic Nb₂O₅ exhibited the highest hydrogen response reactivity among the different phases.The room-temperature lowest limit of detection（LOD）of the T-Nb₂O₅ hydrogen sensor prepared by the above methods is down to 15 ppm,with the response time down to 29 s.However,as the gas adsorption and diffusion is restricted to a certain extent by the interstitial Nb⁴⁺defects as well as the polycrystalline grain boundary and nanojunctions,the room-temperature hydrogen sensing performance of the devices still has plenty room for further improvement.（2）The relation between the defects of Nb₂O₅ nanorods and the electrical transportation and hydrogen sensing performance was studied.The experimental results confirm the formation of oxygen vacancies and the interstitial Nb⁴⁺defects after the high-temperature heat treatment in oxygen-deficient atmosphere.Owing to the size effect,the relative content of the oxygen vacancies at the terminal site and bridging sites will be changed,which leads to the conversion on the type of the majority charge carriers along with the acceptor impurity of the interstitial Nb⁴⁺.Such phenomenon can be the major reason for the n-type and p-type semiconductor behavior of the Nb₂O₅ nanorods and ultra-thin nanobelts,respectively.The rapid thermal treatment technique was employed for the preparation of the TT and T-Nb₂O₅ nanorods,which can suppress the formation of the interstitial Nb⁴⁺defects with the conservation of the oxygen vacancies.It can stable the n-type semiconductor property of the nanorods and enhance the gas adsorption and diffusion,thus leading to the improved gas-solid reactivity.Moreover,the rapid heating calcination can also slow down the polycrystallization and aggregate growth of the materials.The highest initial response rate of hydrogen sensing behavior among all the Nb₂O₅material systems was achieve（2.26%/s）in the T-Nb₂O₅ nanorods obtained by such method.The sensor response factor of the device was increased from 1.04 to above 1.7,together with the good cycling stability and outstanding selectivity towards hydrogen.（3）The heter-compositing materials consisting of Nb₂O₅/Mo Se₂ with adjustable response type and specific hydrogen sensing behavior were developed.Few-layer of 2H-Mo Se₂nanosheets with layered two-dimensional（2D）structure were synthesized by hydrothermal process.The semiconductor hydrogen sensors based on the hetero-composites of T-Nb₂O₅/Mo Se₂ were fabricated by ultra-sonic treatment.The electron released during the gas-solid interaction between Nb₂O₅and hydrogen will transfered from the Nb₂O₅ to the Mo Se₂side under the driving-force of the interfacial potential in the compositing system.When the mass fraction of Mo Se₂ is below 20 wt.%in the composites,the Nb₂O₅ serves as the host for charge transportation,which leads to the electron accumulation at the Mo Se₂ side around the interface area and results in the increase of potential height and the broadening of the electron depletion layer.As a result,the device exhibited p-type hydrogen response behavior at room-temperature due to the hydrogen sensing property of Nb₂O₅.With the increase of the Mo Se₂content,the Mo Se₂ will become the host materials for the charge transportation process.The injected electrons in the Mo Se₂ nanosheets from the hydrogen sensing process of the Nb₂O₅nanorods can be rapidly transferred in the transportation network conducted by the Mo Se₂nanosheets.Consequently,the devices exhibited n-type hydrogen response under the assistance of the Nb₂O₅ additives.When the mass fraction of Mo Se₂ was increased 40 wt.%,the sensor resistance was decreased to kΩlevel,together with the room-temperature sensor response to2000 ppm hydrogen reaching up to 1.094.Meanwhile,the room-temperature LOD was decreased to 122 ppm,while a specific n-type room-temperature response to hydrogen,which is different from other interfering gases,is achieved.The results show that the T-Nb₂O₅nanorods are not only a good host material for the construction of semiconductor room temperature hydrogen sensors,but also exhibits excellent sensitization capability for the construction of 2D-based hydrogen sensor,which is an ideal material for the development of high-performance semiconductor room-temperature hydrogen sensors.