Research On Nanoscale Metal Oxides Heterostrucure Based Gas Sensors

Nowadays,with the rapid increase of economy,people’s living standard has been improved largely.However,the air pollution problem has become serious at the same time,which has been a threat to public health and limited the sustainable development of ecomomy of our country.Semiconducting metal oxide gas sensors have excellent sensing performance and low cost.However,there is great demand for semiconducting metal oxide gas sensors with more excellent performance,such as higher response,lower detection limit,better selectivity and lower working temperature.It is well known that sensing material,as the core part of gas sensor,plays a crucial role in the gas sensing performance.Thus,how to prepare high-performance gas sensing material has been research focus.Constructing heterostructure is regard one of the most promosing method to imorove the gas sensing characteristics.Whereas,more efforts are needed to make strategy more complete.In this dissertation,we prepared four semiconducting metal oxides sensing materials with different type heterostructure,including mixed composite heterostructure,core-shell heterostructure and heterostructures decorated with second-phased particles.The gas sensing performances of these as-prepared samples were tested and analyzed.The main reaearch content of this dissertation is described as follows:1.Multi-shell,stable,porous metal-oxide microspheres（Ni-Co oxides,Co₃O₄and NiO）were synthesized through amorphous-coordination-polymers based self-templated method.Both oxides of Ni and Co showed poor selectivity to xylene,but the composite phase had substantial response(eg.S_xylene/S_ethanol=2.69),and remarkable sensitivity（11.5 to 5 ppm xylene at 255 ^oC）.The short response and recovery times（6 and 9 seconds）,excellent humidity-resistance performance（with coefficient of variation=11.4%）,good cyclability and long-term stability（sensitivity attenuation of⁹.5%after 30 days;stable sensitivity thereafter）all showed that this composite is a competitive solution to the problem of xylene sensing.The sensing performances are evidently due to the high specific surface area and the nano-heterostructure in the composite phase.2.Porous hollow nanostructures,namely Co₃O₄/ZnCo₂O₄ composite hollow nanostructures,were prepared through a self-sacrificing template method.The method included the synthesis of Co/Zn-ZIF@Co-Zn layered double hydroxides precursor and Co₃O₄/ZnCo₂O₄ composite hollow nanostructures obtained through thermal annealing of Co/Zn-ZIF@Co-Zn LDH precursor in air.The gas sensing investigations revealed that the Co₃O₄/ZnCo₂O₄ composite hollow nanostructures-based gas sensor exhibited high response（16.3-100 ppm）and selectivity towards acetone.Besides,enhanced gas sensing properties of Co₃O₄/ZnCo₂O₄ composite hollow nanostructures were observed when compared with Co₃O₄and ZnCo₂O₄ hollow nanostructures.The excellent gas sensing characteristics of Co₃O₄/ZnCo₂O₄ composite hollow nanostructures might be attributed to their high porosity,large specific surface area,and heterostructure between Co₃O₄ and ZnCo₂O₄.3.Hierarchical urchin-like Co₃O₄/CoWO₄ core/shell microspheres were fabricated using a hydrothermal ion exchange approach.Co₃O₄/CoWO₄ microspheres were made using Co₃O₄ nanowires,which were then covered using thin CoWO₄ nanosheets.Capture of cobalt ions from Co₃O₄ nanowires by the tungsten acid radical was responsible for the CoWO₄ nanosheet covering.The gas sensing characteristics showed that urchin-like Co₃O₄/CoWO₄ core/shell microspheres-based sensors showed high response towards acetone vapor,which was larger than that of either Co₃O₄or CoWO₄.A possible gas sensing mechanism for the improved performance of Co₃O₄/CoWO₄ core/shell microspheres-sensor was the formation of heterostructure,which modulates the hole accumulation layer of Co₃O₄.4.Marigold-shaped ZnO nanoflowers were fabricated via a simple precipitation reaction and subsequently catalytically modified with RuO₂ on the surface through an ethylene glycol solvothermal treatment.The experimental results had been proven that a very low content of Ru on the surface of ZnO existed in an oxidized state.However,the gas response of the sensor based on RuO₂-modified ZnO was remarkably improved by 17 times to 100 ppm acetone with the decrease of optimal operating temperature from 219°C to 172°C and reduction in recovery time from79 to 52 s.The sensing enhancement mechanism of surface modification can be attributed to the formation of massive small heterostructure between p-type RuO₂ultrasmall nanoparticles and n-type ZnO as well as the catalytic effect of Ru⁴⁺and a rougher surface.