Microscopic Regulation And Heterogeneous Structure Construction Of In₂O₃ Nanomaterials And Their Gas Sensing Properties

Toxic,hazardous and flammable gases from both indoor and outdoor sources pose a serious threat to the natural environment and human health.Monitoring them by scientific means can effectively prevent the hazards caused by their leakage.Therefore,the importance of gas sensors is gradually coming to the fore.Among the various types of gas sensors,semiconductor gas sensors are widely used in the field of gas sensing because of their small size and low cost.As the sensitive material is the core of the sensor,the demand for its response value,selectivity,and lower detection limit has been increasing with the development of the sensing field.Metal oxide semiconductor materials（MOS）are extremely important functional materials for semiconductor sensors,and factors such as surface morphology,grain size,and surface chemical state can have an impact on their gas-sensitive properties.Therefore,the gas-sensitive properties of MOS can be enhanced by modulating their dimensionality,morphology,crystal structure,and Fermi energy level.Among them,indium oxide（In₂O₃）,as an n-type metal oxide semiconductor material with abundant oxygen vacancies on its surface and more free electrons in its conduction band,can provide reactive sites for sensitive gases and is one of the most promising sensitive materials.However,the disadvantages of In₂O₃,such as high operating temperature,unsatisfactory lower detection limit,poor selectivity and long response/recovery time,limit its application in practice.According to the high chemical stability of In₂O₃,the gas-sensitive performance such as response value,selectivity and lower detection limit of In₂O₃-based sensors can be effectively improved by the synergistic modulation of its microstructure and energy band structure.In this paper,the gas-sensitive performance of In₂O₃semiconductor nanomaterials is regulated through the gradual improvement of morphology and crystal structure.On the basis of microstructure modulation,different nanomaterials with different properties are selected to construct heterojunctions for different requirements of sensor performance to achieve high response values,low power consumption,low lower detection limits and fast response/recovery rates for formaldehyde,NO₂and triethylamine gases,respectively.The details of the study are as follows:1.In₂O₃nanospheres with small-sized grains were prepared by the solvent thermal method using polyol solvent to inhibit grain growth.Moreover,g-C₃N₄ with unique layered structure was selected for surface modification,which effectively improved the response value of In₂O₃sensitive materials.The gas-sensitive test results show that the response value of g-C₃N₄-In₂O₃-based sensor is up to 1402 for 100 ppm formaldehyde gas at the optimal operating temperature,which is twice as high as the response value of the unmodified In₂O₃-based sensor.At the same time,the g-C₃N₄-In₂O₃-based sensor exhibits a short response time（1 s）and excellent long-term stability.The improved gas-sensitive performance is attributed to the modification of the In₂O₃surface by g-C₃N₄to form heterojunctions,which achieves the modulation of the Fermi energy level of the sensitive material and thus promotes the adsorption and ionization of oxygen molecules.Meanwhile,the N-In bond formed in the g-C₃N₄-In₂O₃composite facilitates the rapid carrier transfer,thus realizing the fast detection of formaldehyde gas.In addition,the increase of specific surface area and surface oxygen vacancy content also contributed to the enhancement of response values of sensitive materials.2.The cavity hexagonal In₂O₃nanotubes were prepared by using the metal-organic framework template method.It was also surface modified by acidic oxide Zn WO₄ to improve the selectivity of the sensitive material to triethylamine gas.Gas sensitivity tests showed that the response value of the Zn WO₄-In₂O₃-based sensor was 221.5 for100 ppm triethylamine gas at the optimum operating temperature（100℃）,which was three times higher than that of the undoped In₂O₃-based sensor.Also,the Zn WO₄-In₂O₃-based sensor exhibited a fast response rate（3 s）and a low lower detection limit（1 ppm）.The improved sensor performance is attributed to the built-in electric field formed between Zn WO₄and In₂O₃,which promotes carrier separation and transfer,thus increasing the response value of the sensitive material.In addition,the close interaction between the basic triethylamine molecule with lone pair of electrons and the acidic Zn WO₄surface with a high oxidation state(W⁶⁺)in the lattice effectively improves the selectivity of the sensitive material for triethylamine gas.3.In order to enrich the response sites of the sensitive material and shorten the response/recovery time of the sensor,hollow structured In₂O₃ nanospheres were prepared.And Sn S₂ with high edge activity and multiple chemically active defects was selected to construct Sn S₂-In₂O₃heterojunctions with 2D/3D structures.The selectivity and response value of the sensitive material to NO₂gas were effectively improved and the response/recovery speed was accelerated.Gas sensitivity tests showed that the Sn S₂-In₂O₃-based sensor exhibited a short response time（5 s）and recovery time（129 s）with a high response value of 64.9 for 1 ppm NO₂gas at the optimal operating temperature（70℃）.Taking advantage of the porous hollow structure and the structure of the two-dimensional nanosheets,more transport channels are provided for NO₂gas,thus accelerating the response/recovery of the sensor.With the advantage of heterojunction,the interfacial charge transport was promoted,which further improved the electrical properties and adsorption capacity of the material.In addition,the strong adsorption of Sn S₂with NO₂molecules exists,which effectively improves the selectivity for NO₂gas.4.To prepare high-performance low-power gas sensors.A hierarchical structure of In₂O₃with a mixed crystalline phase was prepared using a nonionic surfactant.The homogeneous junction between the two crystalline phases accelerates the separation and transfer of carriers,resulting in a low-power In₂O₃-based gas sensor.On this basis,Cd In₂S₄with large specific surface area and suitable band gap is selected for surface modification to further improve the response value and selectivity of the sensitive material.Gas sensitivity tests showed that the response value of the Cd In₂S₄-In₂O₃-based sensor was 29.8 for 10 ppm triethylamine gas at the optimum operating temperature（60℃）,which was 2.9 times higher than that of the unmodified In₂O₃-based sensor,and exhibited a fast response/recovery speed（1s/88 s）.The improved gas-sensitive performance of the Cd In₂S₄-In₂O₃composites was attributed to the combination of the hierarchical structure with the two-dimensional material,which enriched the response sites of the sensitive material and accelerated the adsorption/desorption rate of triethylamine molecules on the surface of the sensitive material.In addition,the formation of heterojunction between mixed crystalline phases In₂O₃and Cd In₂S₄leads to the increase of oxygen vacancy content,which effectively regulates the position of Fermi energy level at the interface and promotes the adsorption and ionization of oxygen molecules,thus enhancing the sensitivity performance of the composites.Meanwhile,the sulfur vacancies as Lewis acidic sites effectively enhance the adsorption of triethylamine gas by the sensitive material,thus effectively improving the selectivity.