Preparation And Modification Of Lithium Titanate Lanthanum And Their H₂S Gas Sensitivity

Gas sensors provide fast,real-time,and in-situ information on the type and concentration of gases and are important for detecting hazardous and explosive gases in the environment.Among them,due to simple preparation,low cost and excellent performance,semiconductor oxides are widely used in the field of sensing,especially for the detection of flammable,toxic and odorless gases.Hydrogen sulfide（H₂S）is a toxic and flammable gas.As we all know,H₂S can not only corrode equipment and pollute the environment,but also cause great harm to the human body.When it exceeds 250 ppm,it will cause irreversible danger to the human body.Therefore,the detection of H₂S is very important to protect environmental safety and human health.At present,the problems of poor selectivity,high operating temperature,and poor recoverability of hydrogen sulfide gas sensors are widely existed.The researchers prepared the gas-sensing material La_0.5Li_0.5TO₃（LLTO）,a perovskite material with a unique hollow porous spherical structure,which significantly improved its gas sensing performance.Although its recovery,and selectivity is good,there are still problems such as high operating temperature.In this thesis,LLTO based heterojunction gas sensing materials haved been constructed by compounding with semiconductor oxides and noble metal loads,and thus reduce the operating temperature and improve its sensitivity.The specific research contents are as follows:（1）The In₂O₃/LLTO heterojunction materials with different composite ratios were prepared by the solvothermal method,and then characterized and applied to H₂S gas-sens and their gas-sensitive working mechanism of the materials was analyzed.The study found that the minimum response temperature of the composite material is200°C,and the maximum response of the sensor is reached when the composite ratio of In₂O₃ is 7 mol%.At the optimal working temperature of 260°C,the sensitivity to50 ppm hydrogen sulfide reaches 10.95,and the response and recovery time is 2 and 4s,respectively.For his lowest detection limitation of 5 ppm,the sensitivity is 3.86.It also has good stability and selectivity.The enhanced gas sensing performance after the construction of the In₂O₃/LLTO heterojunction is mainly due to the combined effect of the interface effect of n-n homotypic heterojunction and surface oxygen defects.For the heterojunction,the energy banding is enhanced,the depletion layer becomes larger,the barrier height increases,and the resistance increases.When thehydrogen sulfide gas is released,the redox reaction with the surface adsorbed oxygen releases electrons,which reduces the height of the grain boundary barrier,thereby reducing the resistance of the materials.（2）The pristine LLTO was prepared by solvothermal method,and Ni O/LLTO heterojunction composites with different molar ratios were prepared by further impregnation method.The composite samples were characterized and then applied to the H₂S gas sensors,and their gas sensing mechanism was studied according to the band theory.The results reveal that the Ni O/LLTO composite sensors existing the response of n-type semiconductors（from 200°C to 340°C）due to the low Ni O contents within the composite.At the optimal operating temperature of 260°C,the response sensitivity to 50 ppm hydrogen sulfide is 9.35.and the response and recovery time 1～2 and 3 s,respectively.For the Ni O/LLTO p-n heterojunction composites,due to the formation of a depletion layer on the contact surface,more electrons are exchanged between the material and the gas,the adsorption oxygen increases,and the resistance increases.After the hydrogen sulfide is introduced,the reaction causes the thickness of the depletion layer to decrease and the resistance to decrease.（3）Ag and Pd particles are supported onto the LLTO by solvothermal and impregnation methods.At the optimal compounding ratio of 2 mol%,for Ag/LLTO and Pd/LLTO samples,the response and recovery time is nearly the same,the response and recovery time for 50 ppm hydrogen sulfide gas are 1～2 and 2～3 s,respectively.The optimal working temperature for 2 mol%Ag/LLTO material is300℃,and the sensitivity to 50 ppm hydrogen sulfide at this temperature is 3.81.The response of 2 mol%Pd/LLTO material to 50ppm hydrogen sulfide at the optimal working temperature of 240℃is 3.14.That is,both of the Ag/LLTO and Pd/LLTO sensors reduce their working temperature.This is due to the upward bending of the energy band caused by contacting between the nobel metals with LLTO,resulting in high resistances.On the other hand,precious metals reduce the activation energy of chemisorption through catalytic action,and a large number of oxygen defects are formed on the surface,which promotes the reaction proceeds within the gas sensors.