Preparation Of Zinc Oxide Nanomaterials And Their Application In NO_x Sensing At Room Temperature

Nitrogen oxides(NO_x)are typically toxic and harmful gases commonly produced from fossil fuel combustion and vehicle exhaust.Brief exposure to low amounts of NO_xwill irritate the nose and throat;when large amounts of NO_x are inhaled,it will cause lung inflammation,edema,cancer,and even death.Therefore,developing gas sensors with high sensitivity and selectivity for NO_x is essential for environmental safety and human health.So far,the most effective NOx sensing materials are metal oxide semiconductors such as Sn O₂,In₂O₃,Zn O,and WO₃.Still,the drawbacks such as high operating temperature,slow response rate,and high cost of NO_x gas sensors prepared based on metal oxides limit their further applications.This paper aims to achieve the sensitive detection of NO_x at room temperature.Using Zn O as the NO_x sensing material,the simple and rapid water bath method,carbon microsphere-assisted impregnation method,and hydrothermal method are used to achieve the NO_x detection at room temperature by modulating the morphology of Zn O nanomaterials,and the NO_x sensing mechanism is investigated.The details of the study are as follows:1.Porous Zn O nanomaterials were prepared by water bath heating method using urea and ammonia as base sources and used as sensitive materials for gas sensors to detect NO₂ at room temperature.The experimental results show that the microscopic morphology of the Zn O nanomaterials obtained with different base sources is different.The Zn O nanomaterials with the sheet-like structure were obtained when urea was used as the base source.The Zn O nanomaterials with cluster structure were obtained when ammonia was used as the base source.The gas-sensitive test results show that both Zn O nanomaterials respond completely to NO₂ at room temperature.In particular,the porous Zn O nanosheets synthesized with urea as the base source showed a response of 1.87 to0.5 ppm NO₂ at room temperature,which was higher than the response of the Zn O nanoclusters to 0.5 ppm NO₂(1.48).This may be due to the porous structure of the Zn O nanosheets providing an effective gas diffusion channel for NO₂,thus increasing the possibility of gas contact with the material surface.In addition,the higher oxygen vacancy(O_V)content of the Zn O nanomaterials in the sheet structure can also be found based on the XPS results,which is also one of the reasons for their higher gas-sensitive performance.2.Snowflake-like Zn O nanomaterials were prepared by a combination of zinc salt impregnation and high-temperature calcination using carbon microspheres as a sacrificial template to achieve room-temperature detection of NO₂.It was shown that the ethanol concentration has a significant effect on the sensing performance of Zn O nanomaterials:the snowflake-like Zn O nanomaterials(Zn O-25)obtained by using 25%aqueous ethanol solution as the impregnation solution achieved a response of 7.18 for3 ppm NO₂ with a relative standard deviation of 1.63%for the response over 8 days,exhibiting good NO₂ sensing performance at room temperature.This is mainly due to the fact that the snowflake-like Zn O is assembled by a large number of nanoclusters,which makes it rich in channels and large specific surface area,which facilitates the rapid diffusion of the target gas.The XPS analysis results indicate that the snowflake-like Zn O nanomaterials contain a large amount of chemisorbed oxygen,which can promote the surface reaction of NO₂.In addition,the residual carbon not removed during calcination can effectively improve the electrical properties of Zn O and achieve a highly selective response to NO₂ at room temperature.3.Porous spherical Zn O nanomaterials were synthesized by a simple hydrothermal combined with a high-temperature calcination method and used for the room-temperature detection of NO.The results show that the Zn O nanomaterials(Zn O-O)synthesized with the participation of oxalic acid have a richer pore structure,which is beneficial to the rapid diffusion of the target gas.The introduction of oxalic acid in the synthesis of Zn O precursors led to the release of more CO₂ linear gas molecules during calcination,forming pore channels that facilitate the rapid diffusion of NO,which is also a linear molecule,while increasing the content of oxygen vacancies(O_V)on the surface of Zn O nanomaterials,thus exhibiting superior selectivity and rapid response and recovery behaviors for NO at room temperature.Zn O-O showed a better response at room temperature for 5 ppm NO reached 4.31,which was 2.65 times higher than that of the un-oxalate product(Zn O-C).The response and recovery time of Zn O-O to 0.5-8ppm NO were within 0.5 min.