Tsuchiya Masaru, Bojarczuk Nestor A, Guha Supratik, Ramanathan Shriram
Harvard School of Engineering and Applied Sciences, Harvard University, Cambridge, Massachusetts 02138, USA.
J Chem Phys. 2009 May 7;130(17):174711. doi: 10.1063/1.3126092.
Microstructure evolution and electrical conductivity relaxation kinetics in highly textured and nanocrystalline dense ceria thin films (approximately 65 nm) are reported in this paper. Highly textured films were grown on sapphire c-plane substrates by molecular beam synthesis (MBS) with orientation relationship (111)CeO(2)parallel(0001)Al(2)O(3) and [110]CeO(2)parallel[1210]Al(2)O(3). No significant structural changes were observed in highly textured films even after extensive annealing at high temperature. In contrast to MBS grown films, ceria films grown by electron beam evaporation at room temperature had polycrystalline structure with approximately 10 nm grains, which grew to approximately 30 nm upon annealing at 1173 K. Grain growth kinetics was self-limiting and the out-of-plane orientation was found to be substrate dependent. From conductivity relaxation measurements, oxygen exchange rate in highly textured thin films was found to be much slower than that in polycrystalline films. The response time for highly textured films to changes in P(O(2)) from 1.07x10(-12) to 5.43x10(-10) Pa at 1148 K was 0.65 s, whereas that for polycrystalline films was 0.13 s under identical conditions. From temperature dependent experiments, activation energy for relaxation time was found to be similar, suggesting similar rate-limiting mechanisms in polycrystalline and highly textured films. The results highlight the importance of near-surface defects in controlling kinetics of oxygen incorporation into nanostructured oxides. In a broader context, the results maybe of relevance to designing catalytic surfaces in solid state ionic devices such as fuel cells.
本文报道了高度织构化且纳米晶致密氧化铈薄膜（约65纳米）的微观结构演变和电导率弛豫动力学。通过分子束合成（MBS）在蓝宝石c面衬底上生长出高度织构化的薄膜，其取向关系为(111)CeO₂平行于(0001)Al₂O₃且[110]CeO₂平行于[1210]Al₂O₃。即使在高温下进行长时间退火，高度织构化的薄膜也未观察到明显的结构变化。与MBS生长的薄膜不同，室温下通过电子束蒸发生长的氧化铈薄膜具有约10纳米晶粒的多晶结构，在1173 K退火后晶粒长大至约30纳米。晶粒生长动力学是自限性的，且发现面外取向依赖于衬底。通过电导率弛豫测量发现，高度织构化薄膜中的氧交换速率比多晶薄膜中的慢得多。在1148 K时，高度织构化薄膜对P(O₂)从1.07×10⁻¹²变化到5.43×10⁻¹⁰ Pa的响应时间为0.65秒，而在相同条件下多晶薄膜的响应时间为0.13秒。从温度相关实验中发现弛豫时间的活化能相似，这表明多晶和高度织构化薄膜中的速率限制机制相似。结果突出了近表面缺陷在控制氧掺入纳米结构氧化物动力学方面的重要性。从更广泛的角度来看，这些结果可能与设计诸如燃料电池等固态离子器件中的催化表面相关。
