Study On Synthesis And Gas Sensing Properties Of Zn₂SnO₄ Based Composites

Air pollution is one of the most concerned problems to human today.It is necessary and of great significance to detect the toxic,harmful,flammable and explosive gases in the air accurately,rapidly and in real time.During the last decades,much work has been devoted on the fabrication of metal oxide semiconductors based gas sensors because of their advantages of easy manufacturing process,low cost,high sensitivity,good stability and easy to achieve online detection.With the development of economy and improvement of technology,higher requirements are put forward for the gas sensing performances in the fields of environmental monitoring,production safety,agricultural production and medical diagnosis.Exploring and developing novel sensing materials plays the key role in constructing high-performance gas sensors.Binary metal oxides have the disadvantages of single component,limited active sites and small adjustment space of gas sensing properties,while ternary metal oxide possess the feature of microstructure and composition flexibility.Utilizing that characteristic,this paper starts from the design and synthesis of sensitive materials,aiming at the advantages of good chemical and thermal stability of Zn₂SnO₄,as well as the flexible regulation of synthesis parameters on product structure and composition,Zn₂SnO₄ based products with different morphologies and compositions were synthesized and applied for volatile organic compounds（VOCs）detection.The morphology regulation and heterostructure constructing of sensing material is achieved just through control the synthesis parameters,so as to improve the sensing performances.At the same time,the sensing performance optimization strategy of bimetallic modification was applied to Zn₂SnO₄based composites,which provides a new idea for the construction of ternary metal oxide-based gas sensors with high performance.The specific research contents of this work are summarized as follows:（1）Based on the effect of hydrothermal reaction temperature and reaction time on the morphology and structure of products,Zn₂SnO₄ with the morphologies of octahedrons,nanosheets and nanoflowers were successfully synthesized and applied in ethanol detection.Among them,the construction of nanoflowers is conducive to the improvement of gas sensing performances,then,Ni O/Zn₂SnO₄ composites were prepared by decorating p-type Ni O on the as-prepared Zn₂SnO₄ nanoflowers though solution impregnation.The gas sensing testing results reveal that the Ni O/Zn₂SnO₄composites exhibit excellent sensing performances to ethanol.Compared with Zn₂SnO₄,the optimum operating temperature of Ni O/Zn₂SnO₄ based sensor is decreased from255°C to 230°C,the response towards 100 ppm of ethanol reaches 46.5,which is nearly twice higher than that before decorating,the response and recovery time is 3s and 16 s,respectively.Moreover,the Ni O/Zn₂SnO₄ based sensor possesses great selectivity and stability.The nanoflowers structure composed of nanosheets,the promotion effect of charge transfer at interface of the p-n type heterojunctions formed between Ni O and Zn₂SnO₄ as well as the catalysis synergetic effect of Ni O can effectively improve the gas sensing properties of Ni O/Zn₂SnO₄ composites,it provides a theoretical basis and feasible research ideas for the next experimental design.（2）The composite sensing materials usually have the drawbacks of complex preparation process and poor crystallinity when prepared through two-step synthetic method,and that would lead to poor long-term stability or recovery characteristics of gas sensors,which is not conducive to large-scale applications.When employing hydrothermal method to synthesize Zn₂SnO₄,the amount of Na OH which could tune the p H value of precursor solution and act as mineralizer would play a key role in the transformation of product composition.Therefore,Zn O/Zn₂SnO₄ nanocomposites were facilely synthesized through one-step hydrothermal method by changing the amount of Na OH appropriately in synthesis process and the as-prepared sample took on a homogeneous hexagonal lamellar nanostructure.Gas sensing properties of the sample were tested and the results demonstrate that the Zn O/Zn₂SnO₄ composites possess excellent sensing performances to formaldehyde.At 160°C,the Zn O/Zn₂SnO₄composites-based sensor exhibits a high response of 22.5 towards 100 ppm of formaldehyde and a detection limit as low as 0.5 ppm.Compared with pristine Zn₂SnO₄and Zn O,Zn O/Zn₂SnO₄ composites show higher sensitivity（increased by 120%and80%）,better selectivity and lower detection limit.The n-n type heterojunctions between Zn O and Zn₂SnO₄ can effectively promote the electron transfer process at the interface,and that can markedly improve the selectivity and sensitivity of the Zn O/Zn₂SnO₄ based sensor to formaldehyde.（3）Composite flower-like SnO₂/Zn₂SnO₄ composed of SnO₂ nanoneedles and Zn₂SnO₄ nanosheets were synthesized by one-step gradient-temperature hydrothermal method,the effects of hydrothermal reaction temperature and reaction time on the composition and structure of the product as well as the formation mechanism of the composite flower-like nanostructures were discussed.The sensing properties of the as-prepared sample were improved by composition and morphology tailoring simultaneously.The SnO₂/Zn₂SnO₄ composites show excellent sensing performances to acetone,at 255°C,it displays a response of 23.2 to 100 ppm of acetone,a detection limit of 0.2 ppm,good selectivity and stability.The composite flower-like nanostructure of SnO₂/Zn₂SnO₄ which is beneficial for gas diffusion and provides more active sites for gas reaction is one reason for the improved sensing performances to acetone.At the same time,the superior sensing properties were also attributed to the effect of n-n type heterojunctions as well as the synergetic effect between SnO₂ and Zn₂SnO₄.（4）Bimetallic modification is applied for the gas sensing performances enhancement of SnO₂/Zn₂SnO₄ composites sensing materials.SnO₂/Zn₂SnO₄microspheres were firstly prepared as support material,then novel metal decorated Au-SnO₂/Zn₂SnO₄,Pd-SnO₂/Zn₂SnO₄ and Pd Au-SnO₂/Zn₂SnO₄ were obtained by ascorbic acid（AA）reduction.The gas sensing data results indicate that Pd Au-SnO₂/Zn₂SnO₄microspheres show the best sensing performances to acetone.The response to 10 ppm of acetone reaches 10.4 at 255°C with a detection limit of 0.2 ppm,which is beneficial for detection of acetone in low concentration.Noble metal modification can significantly improve the sensing performances of SnO₂/Zn₂SnO₄ based gas sensors,compared with single novel metal decoration（Au or Pd）,the enhanced sensing performance of bimetallic decoration（Pd Au）may be ascribed to the combined action of chemical sensitization and electronic sensitization as well as the synergistic and catalysis effect of Pd Au bimetallic.This paper is based on the design and synthesis of sensing materials.Utilizing the flexible regulation effect of synthesis parameters to microstructure and composition,ternary metal oxide Zn₂SnO₄ based composites were synthesized and the gas sensing properties of the samples were investigated in detail,based on that work,the relationship between gas sensing properties and microstructures as well as composition of sensing materials was proposed systematically,providing experimental basis and new research ideas for the construction of high-performance ternary metal oxide-based gas sensors.