State Key Laboratory for Mechanical Behavior of Materials , Xi'an Jiaotong University , Xi'an 710049 , Shaanxi , China.
College of Physics and Information Technology , Shaanxi Normal University , Xi'an 710062 , Shaanxi , China.
Anal Chem. 2018 Aug 21;90(16):9821-9829. doi: 10.1021/acs.analchem.8b01768. Epub 2018 Aug 1.
Oxygen vacancies (O) as the active sites have significant influences on the gas sensing performance of metal oxides, and self-doping of Ce in CeO might promote the formation of oxygen vacancies. In this work, hydrothermal process is adopted to fabricate the composites of graphene and CeO nanoparticles, and the influences of oxygen vacancies as well as Ce ions on the sensing response to NO are studied. It is found that the sensitivity of the composites to NO increases gradually, as the proportion of Ce relative to all of the cerium ions is increased from 14.6% to 50.7% but decreases after that value. First-principles calculations illustrate that CeO becomes metallic at the Ce proportion of <50.7%, the chemical potential of electrons on surface decreases, and the Fermi level shifts upward due to the existence of low-electronegativity Ce ions, resulting in reduced Schottky barrier height (SBH) at the CeO/graphene interface, enhanced interfacial charge transfer, and high gas sensing performance. However, deep energy level will be induced at the Ce proportion of >50.7%, and the Fermi level is pinned at the interface. As a result, the density of free electrons is reduced, leading to increased SBH and poor gas sensing response. It demonstrates that an appropriate concentration of oxygen vacancies in CeO is needed to enhance the gas sensing performance to NO.
氧空位（O）作为活性位对金属氧化物的气体传感性能有重要影响，而 Ce 在 CeO 中的自掺杂可能会促进氧空位的形成。在这项工作中，采用水热法制备了石墨烯和 CeO 纳米粒子的复合材料，并研究了氧空位和 Ce 离子对其对 NO 传感响应的影响。结果发现，随着 Ce 相对于所有铈离子的比例从 14.6%增加到 50.7%，复合材料对 NO 的灵敏度逐渐增加，但超过该值后则降低。第一性原理计算表明，CeO 在 Ce 比例<50.7%时变为金属，表面电子化学势降低，由于低电负性 Ce 离子的存在，费米能级向上移动，导致 CeO/石墨烯界面处的肖特基势垒高度（SBH）降低，界面电荷转移增强，气体传感性能提高。然而，Ce 比例>50.7%时会诱导深能级，费米能级被钉扎在界面处。结果是自由电子的密度降低，导致 SBH 增加，气体传感响应变差。这表明需要在 CeO 中存在适当浓度的氧空位来增强对 NO 的气体传感性能。
