| In the 21st century,hydrogen gas（H2）has attracted extensive attention in the field of new energy due to its high calorific value,pollution-free and renewable advantages.However,there exists great danger in the preparation,storage,transportation and use process of hydrogen energy because of the low explosion limit（4%）,colorless,odorless,and high diffusibility properties of H2 gas.Therefore,the research on sensitive and safe H2 gas sensors is of great significance.Compared to catalyst or resistance type H2 gas sensors,the optical type H2 gas sensor has lower operation temperature and stronger anti-electromagnetic interference ability.Besides,substitution of electrical signal by optical signal can also reduce the electrical contact in the sensor device,thus reducing the potential hazards of H2 explosion caused by electric sparks.Tungsten trioxide（WO3）is a chemically stable n-type semiconductor oxide.After loaded with catalyst Pt or Pd,WO3 film can exhibit a reversible change of optical transmittance when the atmosphere is switched between H2 gas and air at room temperature.Therefore,WO3can be used as a hydrogen sensitive material for optical hydrogen sensors.However,there exist two important problems to be solved in the current WO3 gasochromic film:（1）the room temperature response time still needs to be shortened.Specifically,the room temperature response time of the dense or nanorod structured WO3 film is 2～10 min,and the response time of the nanowire structured WO3 film is～20 s;（2）the reports on the composition regulation of WO3 based gasochromic films is few,and the influence mechanism of the added components on the gasochromic properties is also lack of in-depth research.In view of the above problems of WO3 gasochromic film,ordered porous WO3 films were prepared by the colloidal template method from the perspectives of micro/nano structure design and composition regulation,and its gasochromic properties were studied in detail.Based on the study of ordered porous WO3 films,the ordered porous（WO3）1-x（Zn O）x composite films,hierarchical WO3 films and hierarchical Zn O/WO3 composite films were prepared.The microstructure and gasochromic properties of WO3 based films with different structure or composition were systematically studied,and the influence mechanism of Zn O addition and hierarchical structure design on gasochromic properties were studied.The main research contents and conclusions are as follows:（1）Preparation and gasochromic properties of ordered porous WO3 films with different pore diameters.By choosing polystyrene（PS）microspheres with different diameters,porous WO3 films with pore diameters of～400 nm,～800 nm and～1100 nm were prepared by the colloidal template method.The porous structure of WO3 film is uniform,single-layer and ordered.The specific surface area of porous WO3 films decreases from 18.2 m2/g to 13.9 m2/g as the pore diameter increases from～400 nm to～1100 nm.By contrast,the specific surface area of dense WO3 film（3.2 m2/g）is only one-fifth to one-third of that of the porous WO3 films.The results show that the optimal magnetron sputtering time of Pt is 30 s.Among three porous films,the porous WO3 film with a pore diameter of～400 nm exhibits the shortest response time to 4%H2 gas at room temperature.The time to complete the 80%transmittance variation(t80%)is 66.3 s and the transmittance variation at 1500 nm wavelength(ΔT1500 nm)is 28.6%.The response time of porous WO3 film with a pore diameter of～400 nm is only one-sixth of that of dense WO3 film(t80%=411.5 s,ΔT1500nm=13.6%).These results demonstrate that the higher specific surface area can increase the loading sites of catalyst and the adsorption sites of H2 gas on the surface of WO3 film,thus shorten the response time.（2）Preparation and gasochromic properties of ordered porous（WO3）1-x（Zn O）x composite films.Ordered porous（WO3）1-x（Zn O）x composite films（x=0,0.05,0.10,0.15 and 0.20,molar ratio）with a pore diameter of～400 nm were prepared by colloidal template method and sol-gel method.The results show that when the content of Zn O is x=0～0.10,the pore wall of porous films is smooth and compact and Zn element is evenly distributed in the porous skeleton.When the content of Zn O increases to x=0.20,the porous framework becomes rough,and Zn element is unevenly distributed in the porous skeleton with obvious aggregation.XRD results show that the addition of Zn O can refine the grain size of WO3 in the films.The average grain size of WO3 phase in porous（WO3）0.90（Zn O）0.10 composite film is 25.7 nm,which is about half of that in pure WO3 film（58.8 nm）.The gasochromic properties test results show that when the Zn O content increase,the response time of porous（WO3）1-x（Zn O）x composite film first decreases and then increases,and the transmittance variation decreases slightly.Porous（WO3）0.90（Zn O）0.10 film exhibit the best gasochromic properties(t80%=23.0 s,ΔT1500nm=23.2%),and its response time is only one-third of that of pure WO3 film(t80%=66.3 s,ΔT1500nm=28.6%).（3）Preparation and gasochromic properties of WO3 films with hierarchical structure.WO3films with hierarchical structure were successfully prepared by the combination of colloidal template method and solvothermal method.The morphology of hierarchical WO3 films exhibits the ordered porous WO3 structure and large numbers of slender WO3 nanowires on the pore walls.By selecting templates with different polystyrene microsphere diameters,the pore size of porous layer in hierarchical film can be effectively adjusted.The specific surface area test results show that the specific surface area of hierarchical WO3 films obtained by solvothermal treatment for 12 h is 2～3 times of that of porous WO3 films.In the gasochromic properties test,the hierarchical films prepared by the solvothermal treatment of porous WO3 film with a pore diameter of～400 nm for 12 h exhibits the best gasochromic properties(t80%=24.8 s,ΔT1500nm=65.1%),which is significantly superior to that of the porous WO3 film with no hierarchical structure and a pore diameter of～400 nm(t80%=66.3 s,ΔT1500nm=28.6%).（4）Design,preparation and gasochromic properties of Zn O/WO3 composite films with hierarchical structure.Based on the research of hierarchical WO3 films,Zn O/WO3 composite films with hierarchical structure were obtained by the solvothermal treatment of the porous Zn O film with WO3 seed layer.The Zn O/WO3 films possess ordered porous Zn O structure and slender WO3 nanowire structure.The results show that the Zn O/WO3 film obtained by the solvothermal treatment of porous Zn O film with a pore diameter of～1100 nm for 12 h exhibited the best gasochromic properties.The response time(t80%)of optimal Zn O/WO3 composite film is 11.2 s,andΔT1500nm is 61.8%.The open porous structure and the WO3 nanowire structure with high aspect ratio can synergistically promote the diffusion of H2 gas and H atoms,thus improving the gasochromic properties of Zn O/WO3 composite films.The transmittance variation of Zn O/WO3 composite films decreases with the decrease of H2 concentration,implying the ability to detect different H2 concentrations. |
