Study On Sensing Performance Optimization Of Gas Sensors Based On Multiple Metal Oxide Semiconductors

A gas sensor is an important device for the detection of information about environmental gases.It shows great application prospect and research value in many fields such as industry,agriculture,medicine and so on.The advanced,efficient semiconductor gas sensor is closely related to the progress of nanotechnology and the innovation of nanomaterials.It is very necessary to design new sensitive materials,and study systematically the internal relationship between sensing performances and the material nano-structure and components.This can not only improve the sensing performances,but also promote the miniaturization and intelligent development of gas sensors.The work aims at the advantages of multiple metal oxides with variable cation valence states,abundant defects,and controllable nano-structure and surface morphology.Different sensing materials?stannate,ferrites and cobaltates?were synthesized by various liquid-phase methods and also applied to gas sensors to detect volatile organic compounds?VOCs?.From the aspect of the morphology and chemical composition,the sensing performances?sensitivity,response/recovery speed and selectivity?of sensors based on multiple metal oxides were enhanced through the methods of morphology control,stoichiometric ratio optimization,surface modification and heterojunction construction.In addition,the superiority and sensing mechanism of multiple metal oxides as sensitive materials were also proposed.The main research contents in this work are summarized as follows:1?The first part is about the controllable preparation of stannate and its composites,and the study of gas sensing performances.This work studies in the following areas:?1?Multishelled hollow ZnSnO₃ cubes with variable shell numbers of 0-3 shells were synthesized by an alkaline etching route.The formaldehyde sensing results show that the sensors the based on the ZnSnO₃ multishelled cubes revealed about 3.5,2.4,and 1.5 times enhancement in response at 220°C compared to solid cubes,single-shelled cubes and double-shelled cubes,respectively.Moreover,the sensors also exhibited fast response time?1 s?.?2?The Mn3O4 nanowires were decorated with Zn2SnO4 nanorods,Zn2SnO4 nanocubes and Zn2SnO4nanooctahedrons to prepare different Mn₃O₄/Zn₂SnO₄ heterogeneous sensing materials.The sensors based on composites constituted of Mn3O4 nanowires and Zn2SnO4 nanooctahedrons showed the highest responses.At 220°C,the response to200 ppm acetone is 8.3 and the long-term stability is great.Through the modification of Zn2SnO4 octahedrons,the oxygen adsorption capacity of the sensing material is effectively markedly enhanced,leading to a significant increase in the sensitivity of the sensors.2?The second part is about the controllable preparation of ferrites and its composites,and the study of gas sensing performances.This work studies in the following areas:?1?NiFe₂O₄ core-shell nanospheres were prepared via a hard template synthesis method.These materials possess porous structure and a large effective specific surface area with less nanoparticle aggregation.The NiFe2O4 based gas sensors showed great acetone sensing performances.The response to 100 ppm acetone is 10.6 and the response/recovery time is 1s/7s at 280°C.?2?On the basis of the above work,one-dimensional?1D??-Fe₂O₃/NiFe₂O₄ heterojunction materials were synthesized by using metal-organic frameworks?MOFs?MIL-88 as sacrificial templates.The materials obtain hollow tube-shaped morphology and great structural stability.The sensors based on?-Fe₂O₃/NiFe₂O₄ hollow nanotubes received an enhancement of acetone sensing performances compared with pure?-Fe₂O₃ and NiFe₂O₄.The response of 23 to 100 ppm acetone for the?-Fe₂O₃/NiFe₂O₄ composites was 5.4 and 1.6 times higher than pure?-Fe2O3 and NiFe2O4.The sensor also showed good selectivity.?3?Pure?-Fe₂O₃ and a series of?-Fe₂O₃/LaFeO₃ hollow spheres were prepared.From the structural characterization results,the morphology of composite materials will change obviously by tailoring the composition of?-Fe₂O₃and LaFeO₃.With increasing of the LaFeO₃ content,the shell surface of the hollow spheres possesses fewer pore structures and becomes denser.The sensor based on porous?-Fe₂O₃/LaFeO₃ hollow spheres obtained the response of 10.1 and fast response/recovery speed when it used to detect 100 ppm ethanol.The response and recovery time is 1 s and 5 s,respectively.The great sensing performances to ethanol result from the synergistic effects of material composition and morphology.By the the construction of n-p heterojunction,the depletion layer can be formed in the material interfaces.When the target gas reacted with oxygen surface oxygen species,carrier concentration changed significantly.In addition,the porous structure of the materials can promote ethanol gas diffusion.Thus the sensor showed not only a higher sensitivity but also fast response speed.3?The third part is about the controllable preparation of ferrites and its composites,and the study of gas sensing performances.This work studies in the following areas:?1?The effect of cation?Mn,Ni and Zn?substitution of cobalt-based spinel oxides in tetrahedrally/octahedrally coordinated positions on the gas-sensing performances was investigated.The solvothermal method and subsequent thermal treatment were used for the synthesis of MCo2O4?M=Mn,Ni and Zn?multishelled hollow twin spheres.The results showed that sensors based on normal spinel ZnCo₂O₄ exhibited higer sensitivity to acetone?190°C?and formaldehyde?170°C?.However,sensing performances of MnCo₂O₄ and NiCo₂O₄ with inverse spinel structure were unsatisfactory.This is because normal spinel ZnCo2O4 possesses more abundant Co³⁺at the octahedrally coordinated position,which can significantly improve the ability of gas adsorption.?2?In order to further improve the gas sensitivity of ZnCo₂O₄,the bimetal cation stoichiometric ratio was studied.ZnxCo3-xO4 hollow cubes were prepared using ZIFs as templates.The Zn/Co atomic ratios for Zn_xCo_3-xO₄ materials were 0.27,0.37 and 0.48.The gas sensor based on ZnxCo3-xO4 with Zn/Co atomic ratio of 0.37 obtained the highest response to acetone.The response to 200 ppm acetone is 35.6 and the detection limit is as low as 0.5 ppm.In addition,the detecting signal is obvious even in a high humidity environment.?3?For inverse spinel NiCo₂O₄,the structural construction and adopting appropriate catalyst materials were used to overcome limitations of low sensitivity.The response and selectivity to acetone for sensors based on PdO-Ni O/NiCo₂O₄ truncated nanocages were improved compared to NiO/NiCo2O4 solid nanocubes and NiO/NiCo2O4 truncated nanocages without PdO decorating.The enhanced gas sensing performance is attributed to the synergistic effects of structure feature and efficient catalytic activity.Based on the above study on the material systems?stannate,ferrite and cobaltates?,the relationship between the gas sensing performances and surface morphology,crystal structure and material components was investigated systematically.In addition,the gas sensing mechanism of multiple metal oxide semiconductors has been deeply studied.This work provides the experimental and theoretical basis for the construction of high-performance metal oxide semiconductor based gas sensors.