Fabrication And Properties Of Hierarchical Nanostrutured Metal Oxides/Silicon Carbide Ultrathin Fibers

The gas sensors applied in harsh environments,such as aerospace,nuclear industry,military and so on,must be capable of withstanding the high-temperature,high-frequency,corrosive and/or radiative environments.On the other hand,to face the exhausted chemical fuels,photocatalytic water-splitting to produce hydrogen drived by the solar light has attracted more and more attentions.As the 3rd generation semiconductor,silicon carbide（SiC）has been widely used in various applications due to its high mechanical strength,chemical inertness,thermal stability,antioxidation,high thermal conductivity and fast electron transfer rate.In this paper,macro-meso-microporous SiC ultrathin fibers（MMM-SFs）and mesoporous SiC nanofibers（SiC NFs）with high specific surface areas were fabricated via electrospinning combined with polymer-derived ceramics routes and carbothermal reduction method,respectively.Based on these two kinds of fibers,hierarchical nanostructured TiO₂/SiC and SnO₂/SiC heterojunctions micro-/nanofibers were controllably synthesized by hydrothermal method.The compositions and microstructures of these samples were characterized in detail.And last,we investigated the influences of composition and microstructures on the sensing and photocatalytic performances of these samples as well as corresponding sensing and photocatalytic mechcanism.Firstly,MMM-SFs were prepared via electrospinning combined with polymer-derived ceramics route by using PCS as raw materials.The effects of solvent composition,environmental humidity,PCS concentration and pyrolysis temperature on the porous structures of MMM-SFs were investigated.It was demonstrated that macroporous PCS ultrathin fibers could be successfully prepared by electrospinning the PCS solution with concentration of 1.05-1.35 g ml-1 using xylene and DMF as mixing spinning solvent under 60⁸0% relative humidity atmosphere.After being air-cured,the MMM-SFs can be synthesized by treating the cured PCS fibers at 1550 °C in Ar atmosphere.And meso-microporous SiC fibers（MM-SFs）could be fabricated through the same route except DMF was absence in the solvent.Furthermore,SiC ultrathin fibers with single macroporous pores（M-SFs）were obtained when the pyrolysis temperature was 1300¹400 °C.The macropores on the fiber surface were formed due to the vapor-induced phase separation and the formation of meso-micropores might be ascribed to the gas evaporation from the decomposion of SiOxCy phase at high temperature.The received MMM-SFs were composed of SiC and trace of SiOxCy and SiO₂ phases.The fiber diameters were measured to be 3.7⁴.8 μm and the specific surface areas were 86.1¹28.2 m2 g-1.Thanks to the high surface area,good flexibility,superior thermal stability,chemical inertness and fast mass transport,the synthesized MMM-SFs are not only considered as raw materials for subsequent experiments to construct hierarchical structure,but also can be used itself for high-temperature filtration and heterogeneous catalysts.Mesoporous SiC nanofibers（SiC NFs）were prepared by carbothermal reduction reaction between commercial Si powders and carbon nanofibers,which were obtained from electrospinning polyacrylonitrile/DMF solution followed by air-curing and high temperature carbonization.The carbon contents embedded in the SiC NFs can be well-controlled in the range of 0¹7.2 wt% as well as the fiber diameters in the range of 150⁵00 nm.Aligned SiC NFs were also fabricated by using parallel Al electrodes as collector.The embedded carbon can enhance the visible light response,and more importantly,it can act as the electron transfer mediums to transfer the photoinduced electrons quickly.The additive OH-in the solution can simultaneously transfer the photoexcited holes.This “electron-hole dual transfer” can greatly enhance the separation efficiency of photoinduced electrons and holes.Thus,SiC NFs exhibited a high photocatalytic activity without noble metal as co-catalysts.The results showed that the photocatalytic hydrogen production rate can be as high as 180.2 μmol g-1 h-1 under simulated solar light irradiation in the solution with pH value of 14,when the embedded carbon content is 3.2 wt%.Moreover,the photocatalytic hydrogen production rate is 31.0 μmol g-1 h-1 under visible light（λ >450 nm）irradiation,which is higher than the reported SiC-based photocatalysts.The “electron-hole dual transfer” method in our paper can be also expanded to other co-catalysts free and stable photocatalysts with high catalytic activity.Up to date,the reported TiO₂/SiC composites were prepared by simply combined TiO₂ with SiC semiconductor and their specific surface areas are relatively low.Herein,TiO₂ nanosheets（NSs）and nanorods（NRs）were directly grown on the MMM-SFs to form hierarchical TiO₂ NRs@MMM-SFs and TiO₂ NSs@MMM-SFs heterojunctions with corresponding TiO₂ contents of 77 and 79wt%,respectively.For rutile TiO₂ NRs,it exposed a high percentage of（110）crystal face while the anatase TiO₂ NSs exposed a high percentage of（001）crystal face.The formation of TiO₂ NRs and TiO₂ NSs with different morphologies was attributed to the different thermaldynamical stability and the influences of Cl-and F-on the crystal growth process.The TiO₂ NSs@MMM-SFs gas sensor showed a higher sensing performance than TiO₂ NRs@MMM-SFs.The highest response of TiO₂ NSs@MMM-SFs sensor to acetone at the optimized temperature（450 °C）was 19.2,which were 1.2 and 2.3 times to TiO₂ NRs@MMM-SFs and pure TiO₂ NSs,respectively.In addition,the response of TiO₂ NSs@MMM-SFs sensor was linear to the concentration of acetone.It is worth noting that the TiO₂ NSs@MMM-SFs sensor also exhibited a high selectivity,ultrafast response time（3 s）,reproducibility and low detect limit（<1 ppm）toward acetone.These results indicated that TiO₂ NSs@MMM-SFs was an ideal candidate for high temperature gas sensor.On the other hand,TiO₂ NSs@MMM-SFs showed a high photocatalytic hydrogen production rate of 1206.1 μmol g-1 h-1 under simulated solar light,which is 1.2 and 1.5 times higher than those of TiO₂ NRs@MMM-SFs and pure TiO₂ NSs,respectively.The superior sensing performance and high photocatalytic activity of TiO₂ NSs@MMM-SFs can be contributed to the hierarchical structure,synergy effects of the heterojunctions and the exposed high percentage of highly active（001）crystal face.The hierarchical SnO₂ nanoparticles chains（NPCs）/Si C composite fibers were fabricated by in situ growth of SnO₂ NPCs on the SiC ultrathin fibers via a simple and scaled-up sol-gel-flame method.The nonlinear S-type I-V curves suggested that there were Schottky junctions in the Ag/SnO₂ NPCs-SiC/Ag systems.This kind of SnO₂ NPCs/SiC composite fibers showed a high photoluminescence intensity between 400⁶50 nm,implying its great potential in application in light-emission diodes.We also synthesized hierarchical SnO₂ NSs@SiC NFs by growing ultrathin SnO₂ nanosheets（NSs）on the SiC NFs through hydrothermal method.The mercaptoacetic acid and urea in the solution played a key role in the formation of SnO₂ NSs.This hierarchical sample exhibited an ultrafast response/recovery rate as well as high ethanol selectivity,outstanding reproducibility and long-term stability.For reducing gases,the response time of SnO₂ NSs@SiC NFs sensor were lower than 5 s with corresponding recovery time <15 s.In the case of oxidizing gas,nitric monoxide,the response and recovery times were just 8 and 15% of those of pure SnO₂ NSs.Besides,the sample SnO₂ NSs@SiC NFs showed a high photocatalytic hydrogen production rate of 471.82 μmol g-1 h-1 under simulated solar light,which were 1.25 and 3.03 times than those of pure SiC NFs and SnO₂ NSs,respectively.Such a high sensing performance and photocatalytic activity of SnO₂ NSs@SiC NFs were benefited from the synergy effects of the heterojunctions and the specific hierarchical structure to avoid the aggregation of SnO₂ NSs.