Preparation Of WO₃·xH₂O Micro-flowers And WO₃/GO Composite Materials And Their Gas-sensing Properties

NO₂ gas is highly toxic and harmful which provide great harm to the environment and human health and therefore,monitoring its low-level concentration is highly required.Metal oxide semiconductor(MOS)based gas sensors have many advantages such as high sensitivity,simple structure,low cost and so on,thus it is widely used in daily production and life.The metal oxide WO₃ offers high sensitivity and high selectivity to NO₂ gas,so it become a gas sensitive material with more research value and potential in NO₂ gas sensor.In this paper,we have synthesized WO₃·xH₂O and WO₃/GO composite materials under different process conditions and studied their NO₂ gas sensing performance and sensing mechanism.The obtained results are analyzed as follows:(1)A simple hydrothermal method used to synthesize WO₃·xH₂O(a mixture of hexagonal phase structure WO₃ and WO₃·0.33H₂O)nanostructured materials,and the influence of the p H values(0.5,0.8,1.1,1.4,1.7,and 2.0)to WO₃·xH₂O product morphology was discussed.As the p H value increased from 0.5 to 2.0,the morphology of WO₃·xH₂O product changed from free particles to micro-flowers and finally to nanorod bundles.At p H=1.1,the 3D WO₃·xH₂O micro-flower structure was synthesized which composed of many nanoplates with different growth directions with loose and porous characteristics.Its outer diameter was 1?2?m,specific surface area was 16.1 m²/g,and average pore diameter was about 10.9 nm respectively.The WO₃·xH₂O micro-flower gas sensor test results shown sensor's characteristics such as optimal operating temperature of 105?,response of 22.9 toward 2 ppm of NO₂ gas,response/recovery time of 105 s/144 s,lowest detection limit is as low as 125 ppb,and sensor exhibits excellent repeatability and long-term stability,respectively.The gas sensing mechanism of WO₃·xH₂O micro-flower gas sensor was based on the adsorption,desorption and chemistry of O₂,NO₂ gas reaction.(2)A simple hydrothermal method was used to synthesize WO₃·xH₂O micro-flowers with an average diameter of 3-5?m.The annealing temperature(unannealed,300,400,500,and 600?)was investigated for the assembly of nanoribbons.The results shown that as the annealing temperature increased from 300 ? to 600 ?,the nanoribbons with regular shapes transformed into coarse columnar grains,the WO₃·xH₂O mixture micro-flower structure transformed into pure hexagonal WO₃ structure and finally into pure monoclinic WO₃ structure with average diameter of 5-7?m.After annealing at 400 ?,the hexagonal phase WO₃ gas sensor shown best gas sensing performance such as response value as high as 25.36 to 2 ppm NO₂ gas at optimal working temperature of?110 ?,response and recovery time of 8 min and 14 min,minimum detection limit as low as 125 ppb,and good repeatability and long-term stability,respectively.(3)A simple hydrothermal method combined with annealing(400 ?)process was used to synthesize a three-dimensional,disc-shaped hexagonal phase WO₃ nanorod bundle with a uniform and regular shape with diameter of about 4 to 6?m.The WO₃/GO nanocomposite materials(referred as WO₃,WO₃-0.25GO,WO₃-0.5GO,WO₃-0.75GO)obtained by combining the above nanorod bundles with GO solutions of different amounts(0,0.25,0.5,0.75,1 m L),and the corresponding gas sensor was constructed.The gas sensing results shown that the WO₃-0.75GO composite gas sensor revealed the best gas sensing performance compared with WO₃gas sensor at optimal working temperature which reduced from 130 ? to 100 ?.The sensor shown the sensing response improved by 5.13 times(3.429?17.6),response and recovery time shortened by 2.75 times(33 s?12 s)and 2 times(134 s?67 s),and showed good repeatability and long-term stability towards NO₂ gas,respectively.The WO₃/GO composite material shown enhanced in gas sensing performance which is analyzed as follows:first,the formation of p-n heterojunction between WO₃ and GO will accelerate the electron transport speed and improve the conductivity of the composite material;second,the C-O-W chemical bond forming between WO₃ and GO will lead to acceleration of electron transport and chemical reaction;and third,the larger specific surface area of GO will increase the adsorption sites on the surface of WO₃/GO composite material.(4)The recombination between WO₃ and GO materials does play an important role in improving the gas-sensing performance of WO₃ materials to NO₂ gas.