Study On The Controllable Synthesis And Hydrogen Gas-sensitive Properties Of WO_x/nitrogen-doped Graphene Nanocomposites

The safe and efficient use of hydrogen energy is of strategic importance to promote the optimization and transformation and upgrading of energy structure,reduce the dependence on fossil energy and help the overall green transformation of economy and society.Since hydrogen is colorless,odorless,chemically active,flammable and explosive,the safety and cost of hydrogen storage and transportation have limited the application of hydrogen energy.For this reason,researchers have been working on the development of sensitive,portable and low-cost hydrogen sensors.Among the many semiconductor gas-sensitive materials with hydrogen detectors,metal oxides have become a class of hydrogen gas-sensitive materials that researchers have focused on because of their semiconductor energy band properties and controlled microscopic morphology.Among the many metal oxide semiconductor gas sensors,WO_x-based gas-sensitive sensors have high sensitivity,fast response and long-term stability for H₂.However,pure WO_x materials also have disadvantages such as high device operating temperature and suboptimal response rate.It is well known that the response of the sensor is affected by the specific surface area,so in addition to changing the WO_x morphology to increase the specific surface area,other materials can also be loaded to provide additional adsorption sites for WO_x-based sensors to increase the adsorption of the target gas.It is found that nitrogen-doped graphene（NG）materials not only have large specific surface area and high conductivity,but also high electron density of doped N atoms can provide more active sites for gas adsorption,so compounding NG with WO_x materials has become a research focus to improve the performance of WO_x-based gas-sensitive elements,and for this research direction,the following works have been systematically carried out in this paper:（1）reduction method to synthesize sea urchin-like WO_2.7(E-WO_2.7)nanomaterials by reduction,and three composites,NG/WO_3,NG/WO_2.7,and NG/E-WO_2.7,were prepared by hydrothermal synthesis;（2）the structures of the three composites were analyzed at the atomic scale by advanced electron microscopy characterization means such as scanning electron microscopy,transmission electron microscopy,and scanning transmission electron microscopy;（3）the structures of pure WO_x-based sensors and NG/WOx-based sensors and NG/WOx-based sensors were tested for their gas-sensitive properties to reducing gases such as H₂;（4）the energy band structure changes of the WO_2.7 system before and after loading were calculated by energy band theory,which revealed the NG/WO_2.7 gas-sensitive mechanism.The following conclusions were drawn from the above studies.（1）The prepared WO_2.7(E-WO_2.7)nanomaterials exhibit a homogeneous sea urchin shape.（2）Electron microscopic characterization of the NG/WO_x composites confirmed that the structure is a core-shell-like type with NG covering WO_x,in addition to a small amount of NG fragments embedded in the surface defects of the WO_x material.（3）The response/recovery time of the NG/WO_x composite gas-sensitive element was greatly reduced in the gas-sensitive test.In particular,the sensitivity of the NG/WO_2.7-based gas-sensitive element to H₂ is much higher than that of the other two composites,and its optimum operating temperature drops to about 85°C.For the H₂concentration of 30 ppm,the response value can reach 1.81,and the response/recovery time can be shortened to 8s/35s.（4）The theoretical analysis proves that by establishing the metal-n-type semiconductor metal oxide contact mechanism,it is shown that the energy band bending and the formation of charge carrier region at the interface between NG and WO_2.7 material,and the change of the forbidden band width and the increase of Fermi energy level caused by this NG loading are the main reasons for the enhanced sensitivity of NG/WO_2.7 composites to H₂.（5）The shortened response/recovery time of the three composites can be attributed to the N-C bond in the nitrogen-doped graphene structure,and more defective structures can accelerate the rapid transfer of electrons,carriers and holes along the N-C-O-W bond inside the composites,which effectively improves the electrical conductivity of the composites and thus speeds up the adsorption-desorption process of gases on the composites.