Study On The Controllable Preparation And Room-Temperature Hydrogen Sensing Performance Of SnO₂ Nano Flowers

Hydrogen gas is not only a novel type of clean and renewable energy carrier,but also a kind of industial reducing agent and coolant,which has been widely used in medical,chemical industry,aerospace and integrated circuit manufacturing fields,etc.Due to its small molecular volume,wide flammable range?4%to 75%?and low ignition energy?0.02 mJ?,hydrogen is prone to leakage and deflagration during its production,application,storage and transportation processes,which could result in serious personnel and property loss.Because hydrogen is colorless,odorless,and odorless,it is necessary to use the fast,sensitive,stable and reliable hydrogen sensors for monitoring the concentration of hydrogen in the environment in real time,to ensure the safe operation of hydrogen production,hydrogen transportation,and use of hydrogen sites.The metal oxide semiconductor nanomaterials with one-dimensional nanostructures have the advantages of long life-time,rapid hydrogen response,largespecific surface area,and simple synthesis method,which have been regarded as one of the most outstanding hydrogen sensing materials.Among them,SnO₂ is a kind of n-type semiconductor gas sensing material with outstanding sensing performance,which has been widely used in gas sensing.In this dissertation,in order to solve the problems of slow response,high working temperature and poor stability of traditional SnO₂ hydrogen sensor,tetragonal SnO₂ nanoflowers were synthesized by hydrothermal method.The impacts of hydrothermal process on the phase and morphology of the products were studied.The SnO₂ nanoflower-based semiconductor resistive hydrogen sensors were assembled with microfabrication technology.The hydrogen response of the sensors at room temperature were characterized and analyzed.On this basis,the SnO₂ nanoflowers were surface-decorated by the noble metal nanoparticles such as Ag,Pd and Pt,respectively,through the photochemical reduction method.The impact of the surface decoration on the room-temperature hydrogen sensitivity of SnO₂ nanoflowers was analyzed.Moreover,sensing mechanisms of the SnO₂ nanoflowers were also discussed.The main results are listed as following:1.A flower-like micro-nano structure composed of tetragonal rutile SnO₂ nanorods was prepared by hydrothermal method.The impact of hydrothermal temperature,time and raw material ratio on the phase and morphology of the product were studied.The results show that with the increase of hydrothermal temperature and reaction time,the aspect ratio of the nanorods in the product continuously increases.When the hydrothermal temperature was 200 ?,the hydrothermal time was 72 h,and the molar ratio of raw material?NaOH and SnC14�5H2O?was 11:1,the SnO₂ nanoflower with particle size of 3 ?m and good dispersibility,which was consisted of nanorods with diameters of 300 nm,could be obtained.2.The SnO₂ nanoflowers obtained by the hydrothermal method were used to assemble the semiconductor resistive hydrogen sensors.The effects of vacuum annealing on the valence states of the materials and the electrical properties of the devices were analyzed.The results show that vacuum annealing is beneficial to increase the oxygen vacancies content in the SnO₂ nanoflowers.Moreover,the electrical conductivity of the sensitive components decreases significantly after annealing.At the operating voltage of 5 V,the resistance of the devices after annealing??35 k??is significantly higher than that of the unannealed samples??8 k??.Since the electrons introduced by the oxygen vacancy donor action are bound by the oxygen adsorbed on the surface,the higher resistance value of the device can be attributed to the increase in the probability of scattering of electrons in the material after the increase of the oxygen vacancy content,which results in the decrease of the carrier mobility.3.The room-temperature hydrogen sensing results of the devices show that the hydrogen sensors before and after annealing exhibit good n-type resistance response to hydrogen in the environment.After vacuum annealing,the response sensitivity and response rate of the device to 1000 ppm hydrogen at room temperature increased from 27.1%and 0.34%to 80.2%and 1.15%/s,respectively.The response time decreased from 71 s to 62 s,but its recovery rate declined.The above phenomenon can be attributed to the increase of the adsorbed oxygen content on the SnO₂ surface after vacuum annealing,which increases the reaction rate of hydrogen with the nanorod surface while increasing the effective reaction sites.In addition,the annealed hydrogen sensor has good selectivity to hydrogen,and its sensitivity to the same concentration of C2H5OH,CO,and CH4 gases is less than 50%of the hydrogen sensitivity.4.The surface of the SnO₂ nanoflowers were decorated by the nobel metal particles including Ag,Pd and Pt,respectively,through the photochemical reduction method.The analysis shows that Ag exists on the surface of SnO₂ in the two states of Ag and AgO,while Pd exists on the surface of SnO₂ as elementary substance,and Pt exists on the surface of SnO₂ in the states of Pt,PtO and PtO₂.In addition,the adsorbed oxygen content of the surface-modified SnO₂ nanoflowers was higher than that of pure SnO₂ nanoflowers.The results of performance testing of the obtained hydrogen sensor show that the Pt-modified SnO₂ nanoflower has the best room temperature hydrogensensitivity.At room temperature,the element hydrogen sensor has 99.1%response-sensitivity to 1000 ppm of hydrogen.The response time and recovery time are 4.4 s and 142.6 s,respectively.Moreover,the response recovery rates are 20.4%/s and 0.63%/s,respectively.In addition,the device also has good room-temperature cycling stability.