Gas-sensing Mechanism of CeO2/Graphene Nanocomposites and Optimization and Regulation of the Performance

氧化鈰/石墨烯氣敏傳感機制及其性能優化調控研究

Student thesis: Doctoral Thesis

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Award date2 Jul 2021

Abstract

Nitrogen dioxide (NO2) is one of the major pollutant in the atmosphere. It seriously endangers human health. Inhaling small amount of NO2 will stimulate the respiratory system, resulting in pharyngeal discomfort, dry cough and other symptoms. If the concentration of NO2 is larger than 50 ppm, it will seriously damage respiratory and cardiovascular system. If the concentration is more than 100 ppm, it will cause death. In addition, NO2 is the major component of smog, contributing to formation of acid rain. Therefore, it is particularly important to detect concentration of NO2 gas in air. Currently, the metal oxides are widly used to detect harmful NO2 gas. While, the high operating temperature limits its application. Therefore, it is urgent to develop and design gas sensors with high stability, high selectivity and rapid response and recovery at room temperature (RT). Because of large specific surface area and high carrier mobility, the graphene has been widely used as room-temperature sensing materials. However, the selectivity is poor and recovery time is very long. In recent years, fabrication of metal oxide and graphene composites can realize rapid response to specific harmful gas at RT. In this thesis, the cerium oxide (CeO2) and graphene composites were successfully prepared by hydrothermal method. The effect of oxygen vacancy, crystal plane, graphene content and size on gas-sensing response are studied. Based on first-principles calculation, it reveals the intrinsic physical mechanism of enhanced NO2 sensing response. The main research contents of this thesis are as follows:

(1) The composites of graphene and CeO2 nanoparticles with size of 3-5 nm were prepared via adjusting the volume ratio of triethylene glycol (TEG) to deionized water (H2O). H2O2 is strong oxidant, reducing concentration of Ce3+ ions. While, ascorbic acid (AA) is reducing agent, increasing concentration of Ce3+ ions. Once the Ce3+ ions appear, the oxygen vacancies (Ov) will be formed. If the concentration of Ce3+ ions is increased to 50.7%, the response to NO2 increases. After that value, the NO2 sensing response decreases. The electronegativity of Ce3+ ions is lower than that of Ce4+ ions. The work function of CeO2 decreases and Fermi level moves up if introduction of Ce3+ ions. Therefore, the schottky barrier height (SBH) decreases, promoting electron transfer and enhancing NO2 sensing response. However, at high concentration of Ce3+ ions, the deep energy level is generated and Fermi level is pinned. Hence, the carrier concentration decreases and SBH increases, resulting in poor NO2-sensing response. Therefore, the proper introduction of oxygen vacancy on material’s surface can effectively enhance NO2-sensing response.

(2) The composites of graphene and CeO2 nanoparticles with different morphology and facet were successfully synthesized by changing volume ratio of ethylene glycol (EG) to H2O. If the volume ratio of EG to H2O is 1:1, the CeO2 nanoparticles show the cubic structure with exposed {100} facet. Otherwise, CeO2 nanoparticles are mainly enclosed by {111} facet. Increasing volume ratio of EG to H2O, the size of CeO2 nanoparticles decreases. Compared with CeO2{111}/graphene composites, the CeO2{100}/graphene composites show the better response to NO2 at RT and minimum detection concentration could reach 1 ppm. The excellent NO2-senisng response of CeO2{100}/graphene composites is mainly attributed to following three points: (a) The surface energy of {100} facet is higher than that of {111} facet. The {100} facet has more dangling bonds, which is conducive to adsorption of NO2 gas; (b) For the CeO2{100}/graphene composites, the electrons are mainly transferred from {100} facet to graphene at interface, forming electron accumulation layer on the graphene. While, for the CeO2{111}/graphene composites, the electrons are mainly transferred from graphene to {111} facet and accumulative layers of electron are formed on {111} facet. Because graphene has high electron mobility, the CeO2{100}/graphene composites are more conducive to electron exchange with NO2 molecule; (c) Based on density of states, CeO2{100}/graphene composites show the metal characteristics. While, the CeO2{111}/graphene composites show the semiconductive characteristics. For the CeO2{100}/graphene composites, the electrons in valence band are more likely to transfer at interface and trapped by NO2 molecules.

(3) Controling the volume ratio of EG to H2O was 1:1, the different contents of graphene and CeO2 composites were prepared. The graphene content is increased from 2 to 10 wt%, CeO2 nanoparticles always present cubic structure with mainly exposed {100} facet. If the graphene content is increased from 2 wt% to 4.67 wt%, the NO2-sensing response enhances. After that value, the NO2-sensing response decreases. Introduction of graphene can effectively increase specific surface area. However, at high content of graphene, the graphene sheets tend to agglomeration and specific surface area is reduced, resulting in poor NO2-sensing response.

(4) The composites of graphene and CeO2 nanoparticles with different size were successfully synthesized by changing hydrothermal temperature. If the temperature is increased from 160 to 220 ℃, the CeO2 nanoparticles are mainly enclosed by {100} facet and particle size is increased from 7.92 to 27.63 nm. Increasing hydrothermal temperature, the concentration of Ce3+ ions and oxygen vacancy (Ov) decreases. It’s surprisingly found the response to NO2 gas is enhanced with increasing the size of CeO2. The effect of crystal plane on NO2-sensing response is superior to that of size. Based on theoretical analysis, the CeO2{100} facet is the polar surface. The electrostatic force between NO2 molecules and CeO2{100} facet occurs through electric dipole-dipole interaction. The fewer surface defects strengthen polarity of {100} facet and thus enhance interaction between {100} facet and NO2 molecules.

    Research areas

  • Gas sensor, CeO2, graphene