Investigation On Oxygen Vacancies Modulating Of MoO₃ Nanoribbons And The Room-temperature Enhancing Mechanism For The Hyroden Sensing Properties

Hydrogen energy has been considered as one important member in the family of green energy source,due to its high combustion exotherm,recycle-ability and eco-friendly byproducts.However,the small and light hydrogen molecules are very easy to get leaked,leading to explosions.In order to ensure the safety of the produce and usage of hydrogen energy,it is necessary to monitor the hydrogen leakage with the hydrogen sensors.The semiconductor oxide-based electrical hydrogen sensors have been widely utilized in hydrogen detection by taking advantages of the high sensitivity,good stability and low cost.Nevertheless,the requirement for high work temperature will not only consume enormous electric power energy,but also bury some potential safety loopholes.Therefore,it is urgent to develop new type room-temperature hydrogen sensors.In this dissertation,the MoO₃nanoribbons were firstly prepared by a hydrothermal method.The MoO₃ nanoribbons-based hydrogen sensor was then fabricated to realize the high sensitivity hydrogen detection at room temperature.The detection sensitivity of the MoO₃ nanoribbons-based hydrogen sensor was optimized through the hydrogen annealing and the iron(Fe)element doping.The mechanism for the performance enhancement was analyzed with the aid of first-principles calculation.The main contents of this dessertation are listed as follows:(1)Calculation about hydrogen sensitive properties of perfect single-layer MoO₃ and Fe-doped single-layer MoO_3.Based on density functional theory,the sensitivity of the perfect single-layer MoO₃ and Fe-doped single-layer MoO₃ to hydrogen was simulated.There is no effective active site on a perfect MoO₃(010)monolayer surface,therefore the H₂ and O₂molecules are not allowed to adsorb on the surface.The Fe atom replaces a Mo atom on a perfect monolayer MoO₃(010)surface to form a Fe-doped monolayer MoO₃ structure with a binding energy of-8.09 e V,which loses terminal oxygen atoms at the Fe site and generates oxygen vacancies.The O₂ molecules in the environment can form stable chemisorption at oxygen vacancies perpendicular to the MoO₃(010)surface,and capture the electrons of 0.2 e from the surface of Fe-MoO₃,reducing the density of electron carrier in Fe-MoO₃;Under H₂atmosphere,the chemically adsorbed O₂ would undergo a redox reaction with H₂ to generate H₂O molecules and release the electrons of 1.01 e,increasing the density of electron carrier in Fe-MoO₃.The 2Fe-MoO₃ structure can be formed by replacing two Mo atoms with two Fe atoms in a perfect MoO₃ monolayer,and two oxygen vacancies are generated simultaneously,with an average binding energy of-6.432 e V.2Fe-MoO₃ can form stable chemisorption with two O₂ molecules in the environment and release the electrons of 0.37 e.After the redox reaction with H₂,2Fe-MoO₃ will obtain the electrons of 1.92 e.This work provides significant theoretical proofs for the study of room temperature improvement of MoO₃ materials.(2)The room-temperature hydrogen-sensitive properties of MoO₃ materials after H₂atmosphere annealing.Using Na₂MoO₄·2H₂O as the molybdenum source,orthogonal MoO₃nanoribbons with an average length of 20m were synthesized by hydrothermal method and annealed in H₂ atmosphere subsequently.The annealing temperature in H₂ atmosphere is a key factor affecting the concentration of oxygen vacancies in MoO₃ nanoribbons.The samples annealed at 300? has the maximum concentration of Mo⁵⁺.Compared with the unannealed MoO₃ nanoribbons,the response and the recovery time of the optimal sample towards 1000 ppm H₂ reduced from?21.2 s to?10.9 s and?80.1 s to?30.4 s,respectively.The response sensitivity was increased by about 3 times.Annealing in H₂ atmosphere can reduce Mo⁶⁺to Mo⁵⁺partially in the samples,increasing the content of chemisorption oxygen on the surface of the MoO₃ nanoribbons.The increased oxygen vacancies in MoO₃nanoribbons are responsible for the improving of the room temperature hydrogen sensitivity.(3)Room-temperature hydrogen sensitivity of Fe-doped MoO₃ nanoribbons.Using Fe(NO₃)₃·9H₂O as Fe source,MoO₃ nanoribbon with different Fe concentration were prepared by the hydrothermal method.When the Fe concentration is between 2 at%and 6 at%,the room temperature hydrogen sensitivity of the material improves with the increase of the Fe concentration.However,the replacement of Mo⁶⁺by too much Fe³⁺will deform the lattice of MoO₃,making it difficult for MoO₃ to grow in the[001]direction.The sample size will become smaller and the dispersibility becomes worse,hindering the reactions between the sample and the gas.The sample with the Fe doping concentration of 6 at%exhibited the best response performance to 1000 ppm H₂ at room temperature,with a response sensitivity of?26.3,a response time of?22.3 s and a response time of?34.7 s,respectively.After doping,the concentration of oxygen vacancies in MoO₃ nanoribbons was enhanced due to the different valence state of Fe³⁺and Mo⁶⁺.Furthermore,the increased specific surface area of the doped material is beneficial to improve the room temperature hydrogen sensitivity.