Study On Gas Sensor Based On MOFs-derived Semiconductor With Spinel Structure

Gas sensor can identify and detect gas information in the external environment,which plays an important role in air quality monitoring,food quality assessment,exhaled breath analysis and smart farming.In order to achieve reliable detection of target gases in complex gas environments,it is necessary to further develop gas sensors with high sensitivity,low detection limit and high selectivity.Metal-organic frameworks（MOFs）,bridged by metal nodes and organic ligands,are organic-inorganic hybrid materials with high porosity and large specific surface area.By adjusting the composition and ratio of metal ions in MOFs as well as their assembly with organic ligands,the design of gas-sensitive materials such as monometallic,bimetallic,polymetallic oxide,and composite metal oxide structures can be realized.Among various metal oxides,metal oxides with spinel structure（AB₂O₄）exhibit promising physicochemical properties with regulable composition and abundant internal defect states,which have great potential as sensitive materials in the field of gas sensing.MOFs-derived spinel oxide semiconductors can inherit the morphological and structural superiority of MOFs precursors,and possess diverse crystal structure,various kinds of multivalent cations and facile control over defect states.This study focuses on MOFs-derived spinel oxide semiconductor materials as the research subject.Their gas sensing performances（sensitivity,detection limit and selectivity）are effectively enhanced through strategies of crystal structure and semiconductor type regulation,cationic doping,and heterogeneous composite material construction.Meanwhile,the influence of lattice site occupation of metal cation and heterostructure interface in spinel oxide on the gas-sensitive reaction are investigated.Furthermore,the applications of MOFs-derived spinel oxide semiconductors based gas sensors are further explored in air quality monitoring,meat quality assessment,and human exhaled breath analysis.The main research contents of this study include:（1）The occupation behavior of different metal cations at lattice sites in spinel oxide semiconductors affects the type of conductivity and the reaction process to different gases.In order to investigate the effect of A-site metal cations substitution in spinel ferrite on the selectivity and sensitivity to target gases,bimetallic spinel ferrites with different metal ions（MFe₂O₄,M=Co,Ni,Cu and Zn）were designed utilizing Prussian blue（PB）with the coordination network of Fe²⁺-C≡N-Fe³⁺,as templates.It was found that NiFe₂O₄ and Co Fe₂O₄ with inverse spinel structure exhibit p-type semiconductor type,while Cu Fe₂O₄ with inverse spinel structure and Zn Fe₂O₄ with normal spinel structure are n-type semiconductors.Among them,p-type NiFe₂O₄ and n-type Cu Fe₂O₄with inverse spinel structure based gas sensors exhibited high response and selectivity towards C₃H₆O and H₂S,and displayed the detection limits as low as 1 ppm（C₃H₆O）and 0.5 ppm（H₂S）,respectively.As analyzed by X-ray photoelectron spectroscopy（XPS）,the gas-sensitive reaction between NiFe₂O₄ and C₃H₆O were accompanied by the reversible chemical valence change of nickel ions on the surface,whereas the phase-change for a small number of Cu Fe₂O₄ on the surface were caused during the gas-sensitive reaction process between Cu Fe₂O₄ and H₂S.Therefore,the two sensors exhibited different selectivities to target gases.Furthermore,NiFe₂O₄ and Cu Fe₂O₄based sensors were employed in detecting trace gas concentrations in human exhaled breath analysis and food quality monitoring applications respectively.（2）Based on the previous work,this work continues to investigate the effect of occupation in homolattice,species and ratio of polymetallic cations in spinel ferrites on gas-sensitive reactions.Fe-MIL-101 consists of Fe（III）trimers and terephthalic acid ligands,and iron ions can be multifunctionally substituted by different metal ions.Therefore,a series of Co/Ni/Fe trimetallic oxides were synthesized by using MIL-101as self-sacrificing templates.Ni²⁺and Co²⁺co-doped in spinel ferrite occupied the octahedral interstices,causing Co/Ni/Fe trimetallic oxides to exhibit inverse spinel structure.Doping of Co²⁺and Ni²⁺in spinel ferrite not only effectively reduced the initial resistance of the sensor but also altered its gas sensing ability towards acetone.The Co/Ni/Fe trimetallic oxides(the feed ratio of Co²⁺:Ni²⁺was 4:1)based sensor displayed the higher sensitivity to acetone with a detection limit as low as 1 ppm.This is mainly attributed to the occurrence of charge conduction between Ni²⁺and Co²⁺in the octahedron sites during the gas-sensitive reaction process,and the richer Ni³⁺and Co³⁺active sites as well as more adsorbed oxygen on the surface can effectively enhance the gas sensing ability.Furthermore,Co/Ni/Fe trimetallic oxides-based gas sensors were employed for detecting breath markers in diabetes diagnosis.（3）The heterostructures formed by spinel oxides and other structural metal oxides can not only modulate energy band structure and carrier distribution of spinel oxides semiconductor,but also affect the gas sensing performance of spinel oxide-based gas sensors.By utilizing the features of Fe-MIL-53 that the Fe O₆ octahedrons can be partially replaced by NiO₆ octahedrons,MIL-53-derived NiFe₂O₄,NiO as well as the composite metal oxides NiFe₂O₄/NiO were synthesized.Because of the introdution of NiO,the sensitivity of NiFe₂O₄/NiO towards acetone significantly improved by 15 and8.7 times compared to that of pristine NiO and NiFe₂O₄ sensors respectively.The p-p heterojunction between NiO and NiFe₂O₄ can generate additional electron transport channels and is favorable for more free electrons to take part in the gas-sensing reaction process,which effectively improves the gas sensing performance of spinel oxide semiconductors.Furthermore,the NiFe₂O₄/NiO sensor exhibited high sensitivity（3）and selectivity for as low as 500 ppb acetone gas even in high humidity environments（80%RH）,making it highly promising for human exhaled breath analysis.（4）Based on the previous work,this work further investigates the effect of the proportion of spinel oxides and ternary metal oxides on gas sensing performance and the role of variable valence cations in the gas sensing reaction process for the heterogeneous oxides.Due to the feasibility of ZIF-67 modified by metal ions,a series of Co₃O₄/β-Co Mo O₄ heterogeneous hollow nanocages were designed using ZIF-67 as templates.The influence of the Co/Mo ratio on material morphology,composition,oxygen vacancy concentration,and gas sensitivity were investigated.With introducingβ-Co Mo O₄ into spinel oxide Co₃O₄,the Co₃O₄/β-Co Mo O₄ hollow nanocage based gas sensor exhibited exceptional sensitivity（22）to H₂S with an ultra-low detection limit（10 ppb）,surpassing that of Co₃O₄ sensor by 4.4 times.Furthermore,a sensor array consisting of four Co₃O₄/β-Co Mo O₄ sensors was constructed,which can realize highly selective recognition and detection to H₂S.The enhanced gas sensing performance can be attributed to the formation of heterogeneous interfaces,multi-valence molybdenum ions,and unique hollow structure.Additionally,a H₂S sensing and alarm platform based on the Co₃O₄/β-Co Mo O₄ sensor was designed for air quality monitoring.This platform provided warning alerts regarding excessive hydrogen sulfide concentrations（1 ppm）in polluted air environments to ensure user safety.In order to develop and high-performance gas sensors,this work systematically investigated the intrinsic relationship between the gas sensing performance of MOFs-derived spinel oxide semiconductor and its crystal structure and chemical composition.Furthermore,the gas sensing mechanism of spinel oxide semiconductor gas sensors was studied.The potential applications of gas sensors in various scenarios were also explored.This work not only provides insights for constructing high-performance gas sensors but also offers possibilities for detecting trace gases in practical applications.