| In recent years,due to the increasing use of fossil fuels in transportation and industry,environment pollution in China has become an unavoidable reality.The research of International Energy Conservation and Environmental Protection Association(IEEPA)show that nitrogen oxides(NOx gases),as a byproduct of fossil fuel combustion,are the culprit of air pollution.NOx gases are poisonous environmental pollutants,which can directly lead to haze days,photochemical smog,acid rain,and ozone damage.In addition to the air pollution,the huge amounts of NOx released into the atmosphere everyday also cause fatal problems to humans,animals and plants.For public health,environmental and industrial safety,research and development of rugged,reliable,highly sensitive and capable of real-time monitoring NO,sensors become more and more strategically important.There is a consensus in academia that the level of NO in exhaled gases of asthma patients is significantly increased.The analysis of exhaled gases to detect human diseases such as lung cancer,asthma and diabetes is an emerging field of medical diagnostic,which is non-invasive and compared to traditional blood analysis methods is a very good cost-saving alternative.For example,the mean exhaled NO concentration of healthy person was 6.7-16.2 ppb,however,an asthma patient suffer an increased concentration in the range of 34.7-51.1 ppb.Therefore,the person with an exhaled NO concentration of higher than 30 ppb may be suffering from asthma caused by respiratory inflammation,especially for young children.Therefore,rapid and accurate detection of NO content also has important medical significance.Sensing materials are the most critical factor in gas sensors that determines the gas-sensing performance.Metal oxides semiconductors are known for years as efficient gas sensing materials in many applications including detection of toxic and combustible gases and breath analysis for medical diagnosis,which is the primary choice as sensitive materials for low cost,low power consumption,high sensitivity and fast response NOx sensors.The response of the semiconducting metal oxide gas sensor depends to a large extent on the morphology of the materials and device structures.Relevant literature analysis shows that WO3 is the most promising material for NO,detection,taking into account the advantages of its high sensitivity,excellent selectivity,lower detection limit and lower operating temperature.The mean purpose of this work is to improve the performance of NOx sensors by innovating the combination of WO3 and other metal oxides,innovating structures of sensor devices,and preparing the innovative structures of WO3.The major results are as follows:(1)Five micrometers thick SnO2 bottom layer and 8 ?m thick WO3 top layer were consecutively printed onto the Al2O3 substrate with Pt electrodes,and decoration with Pd nanoparticles was also applied to the WO3 top layer,thus sensors with the structure of Pt/SnO2/WO3(Pd)were fabricated.The NO sensing properties were characterized in the temperature range of 250 to 500 ?.When exposed to 200 ppm NO at 400 ?,the sensor achieved a response of 38,and the response and recovery time were only 20 s and 37.5 s,respectively.The WO3 top layer with Pd decoration ensured a good response to NO gas,and the SnO2 bottom layer accelerated the electron transport,therefore,enhancing the response and recovery speed dramatically.(2)Single crystalline WO3 nanowires were directly grown on FTO substrates by a hydrothermal approach.After calcination at 400 ?,the nanowires assumed an orthorhombic structure and their diameters were mostly in the range of 15-20 nm.The sensor based on the WO3 nanowires exhibited a markedly high response and fast response-recovery characteristics to NO gas at 300 ?.XPS and DRIFT studies demonstrated that the chemisorption of NO and the oxidation of NO by defect-like oxygen anions on the nanowire surfaces were key to the sensing performance.(3)Hierarchical flowerlike WO3 nanostructures,assembled from single crystalline WO3 nanoflakes,were prepared on patterned FTO substrates via a solvothermal technique.The flowerlike nanostructures provide large area of active surfaces,and the interconnected nanoflakes are continuous paths for the electrical conduction.With the addition of PdCl2 in the precursor solution,the surface of the WO3 nanoflakes were etched and became rough and porous,which is advantageous for gas sensing.A response of 203 to 20 ppm NO was achieved at 200 ? by the flowerlike WO3 with 0.1 mmol PdCl2 addition in the precursor solution,while the sample with 0.01 mmol PdCl2 addition always shows fast response and recovery speed(e.g.27 s/23 s at 200 ?).Grow metal particles on the surface metal oxide semiconductor is an important technology,can be used in many areas such as catalysis and sensing.Here we report a facile method for growth of noble-metal particles on WO3 through an in situ redox reaction between weakly reductive WO3 surface and oxidative metal salts in aqueous solution.The surface of WO3 nanostructure was reduced by ascorbic acid in advance through a facile water bath method,which greatly guarantee the nanostructures not be damaged.No any additional conditions limit the implementation of this method,making it a general way to develop metal-nanoparticle-loaded semiconductor composites. |
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