Gas Sensor Properties Of NiO Based Heterojunction

In recent years,with the widespread use of different types of chemicals,the air pollution caused by toxic and harmful volatile organic compounds（VOCs）has become increasingly serious,posing a serious threat to human and ecological health.Therefore,rapid detection and real-time monitoring of harmful gases in the air play a crucial role.Semiconductor metal oxide gas sensors have low manufacturing costs and meet application requirements,making them widely used in the field of gas monitoring.Due to the low sensitivity and high required operating temperature of single component semiconductor metal oxide sensing materials,their application in the sensing field is greatly limited.Therefore,there is an urgent need to develop semiconductor metal oxide gas sensors with high sensitivity,low operating temperature,and good stability.NiO,as a typical p-type semiconductor metal oxide,,has extensive applications in catalysis,electrochemistry,environmental protection,and other fields.Due to its unique chemical properties,NiO has a catalytic oxidation effect on benzene based VOCs,making it highly selective for benzene based VOCs.It is a highly promising gas sensing material for benzene based VOCs.However,due to its sensitive mechanism,its gas sensing performance is usually poor,and its application in the sensing field is extremely limited.Therefore,it is necessary to further optimize and regulate the composition and structure of NiO,improve its sensitivity,and promote its application in benzene based VOCs monitoring,which is expected to promote the development of benzene based VOCs gas sensing materials.In view of this,based on the hydroxide precursor method,Co₃O₄ and Cr₂O₃ with catalytic oxidation were introduced to composite with NiO,and the gas sensing performance of NiO was significantly improved by virtue of the Electronic effect,structural effect and catalytic effect of heterojunction,thus two NiO based gas sensing materials with excellent gas sensing performance for benzene VOCs were constructed.The main research content and conclusions of this paper are as follows:1.Flower like Ni Co hydroxide precursors with Co/Ni molar ratios of5%,10%,and 15%were synthesized using a one-step hydrothermal method.Flower like Co₃O₄ NiO p-p heterojunctions with Co/Ni molar ratios of 5%,10%,and 15%were prepared by high-temperature calcination.The composition and structure of the prepared flower shaped Co₃O₄ NiO p-p heterojunction were characterized in detail using analysis techniques such as XRD,SEM,BET,HRTEM,EDS,XPS,etc.The analysis results indicate that high-temperature calcination transforms the precursor of flower shaped Ni Co hydroxide into a flower shaped Co₃O₄ NiO p-p heterojunction assembled by bent nanosheets,and Co₃O4 is uniformly distributed on NiO nanosheets;The specific surface area of the flower shaped Co₃O₄ NiO p-p heterojunction（10%Co₃O₄ NiO）shows a trend of first increasing and then decreasing with the increase of Co/Ni molar ratio.The specific surface area of the flower shaped Co₃O₄ NiO p-p heterojunction（10%Co₃O₄ NiO）with a Co/Ni molar ratio of 10%is the highest,reaching 164.2 m²/g;The interaction between Co₃O₄ and NiO nanoparticles forms a p-p heterojunction,and there is electron transfer behavior between Co₃O₄ and NiO nanoparticles.The response of Co₃O₄NiO p-p heterojunction to xylene gas is dependent on the Co/Ni molar ratio and calcination temperature.The 10%Co₃O₄NiO sample prepared at 300has the highest gas sensing response,with a response value of 110 to 50ppm xylene gas at the optimal working temperature of 260,which is 7times higher than the response value of pure NiO（15）;In addition,the 10%Co₃O₄ NiO p-p heterojunction exhibits a fast response recovery rate,with a response/recovery time of 47/43 seconds,and good selectivity for xylene gas.Its response values are about 44 times,8 times,11 times,and 3 times that of methanol（2.5）,ethanol（13）,acetone（10）,and xylene（30）at the same concentration;Meanwhile,the 10%Co₃O₄ NiO p-p heterojunction exhibits a good response concentration linear relationship and good stability for low concentration xylene gas.It is found that the formation of Co₃O₄-NiO p-p heterojunction can provide more high active site,promote the adsorption and activation of O₂ and form more active oxygen species,accelerate the reaction between active oxygen and xylene gas,and thus significantly improve the gas sensing performance of the materials.2.Flower like Ni（OH）₂ was prepared by hydrothermal method,and then Cr（OH）₃ was deposited on the surface of Ni（OH）₂ nano sheet by single displacement reaction.Cr（OH）₃/Ni（OH）₂ precursors with Ni/Cr molar ratios of 5%,10%and 15%were synthesized.Flower like Cr₂O₃-NiO p-p heterostructures with different Ni/Cr molar ratios were obtained after calcination.The composition and structure of the prepared flower ball shaped Cr₂O₃-NiO p-p heterojunction were analyzed using analysis techniques such as XRD,SEM,BET,HRTEM,EDS,XPS,etc.The specific surface area of 10%Cr₂O₃-NiO reached 176.9 m²/g,and Cr₂O₃was uniformly distributed on NiO nanosheets.Cr₂O₃ and NiO nanoparticles contacted each other to form a p-p heterojunction.The response value of 10%Cr₂O₃-NiO to xylene reached 148,which is 8 times that of NiO（19）,and it has good selectivity for xylene gas.Its response value is about 11 times,12 times,30 times,9 times,and 3 times that of methanol（13）,ethanol（12）,acetone（4.8）,formaldehyde（17）,and toluene（53）at the same concentration.The response/recovery time of the 10%Cr₂O₃-NiO p-p heterojunction is 61/92 s,respectively,and it exhibits good response concentration linear relationship,repeatability,and stability for low concentration xylene gas.The formation of Cr₂O₃-NiO p-p heterostructure improves the specific surface area and the number of defect sites,optimizes the electronic structure of NiO,promotes the adsorption and activation of O₂ and xylene gas,and thus greatly improves the gas sensing performance of the material.