| According to the "Action Plan(2016-2030)for Energy Technology Revolution Innovation" promulgated by the National Development and Reform Commission and the Energy Administration,hydrogen energy and fuel cell technology is one of the key innovation tasks.However,hydrogen is colorless,odorless,flammable and explosive,safe storage and transportation of hydrogen energy is an urgent problem to be solved at present.With the deepening of hydrogen energy research and the increasing application,it is particularly important to develop gas sensors which are extremely sensitive to hydrogen.At present,the most widely studied type of sensors is based on n-type metal oxide semiconductor(MOS,such as TiO2,SnO2,ZnO and CeO2,etc.)gas sensors.TiO2 has become one of the best hydrogen sensitive materials because of its low cost,simple fabrication,high sensitivity and good physical and chemical stability.Meanwhile,due to its abundant surface lattice oxygen vacancy and low redox potential between Ce3+ and Ce4+CeO2 has become a hydrogen sensitive material widely concerned by researchers.However,the high operating temperature and the slower recovery time limit the further application of n-type MOS sensors.Therefore,in order to reduce the response temperature of hydrogen sensor and shorten the recovery time,this paper studies the modification of precious metal,microstructure design and the construction of semiconductor heterostructure.The details are as follows:(1)Considering the strong hydrogen absorption capacity of Pd,we prepared different mass ratios of Pd modified anatase TiO2 nanoparticles by hydrolys-hydrothermal method and applied them to hydrogen sensors.The study found that the Pd modified TiO2 nanoparticles sensors exhibit an n-type response to hydrogen over a wide temperature range of 260～440 ℃.Under the optimum conditions,the 1 wt%Pd-Ti02 sensor has a maximum response of 6.87 to 1000 ppm hydrogen at 380 ℃,the response time and recovery time are both 1～2 s,it can also provide fast response and recovery.The main reasons for the enhancement of hydrogen sensitivity of Pd modified TiO2 nanoparticles are as follows:the specific surface area of Pd modified samples were larger,providing more active sites for gases and reaction media;Schottky barrier is formed at the interface between Pd and Ti02,thus the reaction in the hydrogen sensing process is promoted;and the existence of Pd reduces the activation energy.(2)Pd source is expensive and difficult to be widely used,while silver has a low cost and is a good catalyst with excellent conductivity.Based on this,in order to further improve their hydrogen sensing performance and reduce costs,we use Ag instead of Pd to modify anatase Ti02 and take advantage of the unique two-dimensional structure of TiO2 nanosheets.Ag nanoparticles@TiO2 nanosheets with different Ag composite ratios were prepared by one-step hydrothermal method and applied to hydrogen sensors.It is found that Ag@TiO2 sensors have n-type response to hydrogen in the wide temperature range of 280～440 0C,Under the optimum conditions,the response time of 1 wt%Ag@TiO2 sensor to 1000 ppm hydrogen at 340 ℃ is 1～2 s,the recovery time is about 4 s,and the maximum response is 14.4,which is about twice as much as that of Pd-TiO2 nanoparticles(6.87).The enhanced hydrogen sensitivity of Ag@TiO2 sensors are mainly due to the following reasons:the interface between Ag and TiO2 is a reverse barrier layer,which can transfer electrons rapidly;Ag nanoparticles as catalyst can reduce activation energy and promote the adsorption of gas molecules;and the surfaces of the Ag@TiO2 synthesized by hydrothermal method are rich in oxygen vacancy defects,which can promote the hydrogen sensing reaction.(3)Aim to solving the disadvantage of the low carrier concentration of CeO2,we introduce TiO2 into the system,we prepared TiO2@CeO2 nanorods with different Ti/Ce ratios by the hydrothermal method,and then Pd modified Ti02@Ce02 nanorods were further obtained by chemical precipitation,and finally applied the samples to the hydrogen sensors.The results show that the Pd modified TiO2@CeO2 nanorod sensors exhibit good low temperature sensing performance for hydrogen.At the optimum response temperature of 180 ℃,the response of TCP-2 sensor to 500 ppm hydrogen is 3.81,the response/recovery time is about 5/12 s,respectively.The response of TCP-1 to hydrogen is p-type response,and TCP-2,TCP-3 and TCP-4 exhibit an n-type response to hydrogen.The low temperature response of Pd modified TiO2@CeO2 sensors to hydrogen is due to the combination of the strong adsorption and dissociation effect of Pd on hydrogen,the interfacial effect of TiO2-CeO2 n-n heterostructure,and the strong adsorption of oxygen on the surface of TiO2-CeO2. |
