有序二氧化铈薄膜的制备与热稳定性研究

Synthesis and Thermal Stability of Ordered CeO_2 Thin Films

利用低能电子衍射、同步辐射光电子能谱和X射线光电子谱原位、系统地研究Ru（0001）单晶表面外延生长CeO2（111）薄膜表面形貌、电子结构、化学元素组成和热稳定性。在超高真空条件下和氧气氛（1.33×10^-5Pa）中,在Ru（0001）单晶衬底表面上热蒸发金属Ce,反应生成的CeO2薄膜呈外延生长。结果表明,高覆盖度条件下,生成的CeO2（111）薄膜是连续的,且完全覆盖衬底,只有1.4×1.4结构的衍射花样。在4.5 nm完全氧化的CeO2（111）薄膜价带谱中,3～8 eV之间展宽的谱峰归属于O 2p能级,其中,4.1和6.5 eV位置的2个谱峰分别对应于O 2p轨道与Ce 4f0和Ce 5d轨道形成的杂化态峰。当样品表面升温到700℃时,4.1 eV处的Ce^4＋（4f0）峰略微减弱,并且在1.6 eV处出现了Ce^3＋（4f1）峰,说明样品表面已经有部分Ce4＋转化成Ce3＋,发生了还原反应。若进一步提升样品的退火温度,则CeO2的还原程度也越高。

The ordered CeO2（ 111） thin films were synthesized by molecular beam epitaxy（ MBE） at 1. 33 × 10^-5Pa of oxygen on Ru（ 0001） substrate. The surface microstructures,electronic structures,and thermal stability of the CeO2（ 111） films were characterized in-situ with lowenergy electron diffraction（ LEED）,synchrotron radiation photoemission spectroscopy（ SRPES）,and X-ray photoelectron spectroscopy. The results show that the Ce coverage and annealing temperature significantly affect the microstructures and stoichiometry of the CeO2（ 111） films. For instance,at a high Ce coverage,continuous CeO2（ 111） films formed,as shown by the sharp hexagonal（ 1. 4 × 1. 4） LEED pattern. For a 4. 5 nm fully oxidized CeO2films,the valence band stretched from 3 to 8 eV,possibly because of emission from the O2p valence orbitals. The two peaks at 4. 1 and 6. 5 eV could be O2p orbitals hybridized with Ce4f0and Ce5d orbitals,respectively. At 700℃,the Ce4f0hybridized peak at 4. 1 eV was slightly reduced,accompanied with the emergedpeak at 1. 6 eV originated from emission from Ce4f1orbital of the Ce3 ＋oxide,indicating that some Ce^4 ＋were reduced to Ce^3 ＋. Moreover,we found that a higher temperatureresulted in stronger CeO2reduction.