物理化学学报 >> 2019, Vol. 35 >> Issue (12): 1412-1421.doi: 10.3866/PKU.WHXB201905054

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引入应力的锐钛矿TiO2(001)薄膜的外延生长

王明越,谭世倞,崔雪峰,王兵*()   

  • 收稿日期:2019-05-14 录用日期:2019-05-30 发布日期:2019-06-03
  • 通讯作者: 王兵 E-mail:bwang@ustc.edu.cn
  • 基金资助:
    国家重点研发计划(2016YFA0200603);国家自然科学基金(21573207)

Introducing Strain in Anatase TiO2(001) Films by Epitaxial Growth

Mingyue WANG,Shijing TAN,Xuefeng CUI,Bing WANG*()   

  • Received:2019-05-14 Accepted:2019-05-30 Published:2019-06-03
  • Contact: Bing WANG E-mail:bwang@ustc.edu.cn
  • Supported by:
    The project was supported by the National Key Research and Development Program of China(2016YFA0200603);the National Natural Science Foundation of China(21573207)

摘要:

锐钛矿TiO2(001) (anatase TiO2(001),简记为ATO)表面因其优异的催化活性受到了广泛的关注。理论计算结果表明,ATO表面应力导致的晶格畸变可能会增强该表面的催化活性。因此有必要研究应力对ATO表面结构的影响。本文利用BaTiO3(001) (简记为BTO)与ATO之间存在较大的晶格失配度,将ATO薄膜外延生长在BTO衬底上从而引入应力,研究了存在应力情况下的ATO薄膜的结构特征。实验中,利用脉冲激光沉积方法在Nb掺杂的SrTiO3(001) (简记为STO)单晶衬底上制备了ATO/BTO/STO外延薄膜。X射线衍射(XRD)和扫描透射电子显微术(STEM)结果表明,作为应力引入层的BTO薄膜厚度约为4–6 nm时,能够部分地将应力引入到ATO薄膜中。X射线光电子能谱(XPS)结果显示,ATO薄膜合适的厚度应大于15 nm,从而降低从衬底反向扩散至表面的Sr和Ba原子的浓度;Ti 2p的高分辨XPS谱仅呈现出Ti4+峰,表明ATO表面Ti原子为完全氧化的价态。ATO外延薄膜表面的扫描隧道显微术(STM)图像仍然呈现为(1 × 4)重构的结构,但在(1 × 4)重构的脊上存在明暗交替并具有一定周期性的特征。根据完全氧化的“增氧原子模型”(ad-oxygen model,AOM),脊上观察到的明暗交替特征可归因于表面应力导致的“TiO2”空位缺陷结构。

关键词: 锐钛矿TiO2(001), 薄膜, 扫描隧道显微术, 表面重构, 表面应力

Abstract:

The anatase phase of TiO2 is often considered to have the highest reactivity among TiO2 polymorphs. Since the anatase TiO2(001) surface has a relatively high surface energy, it is expected to be active; however, because of its high surface energy, the surface generally forms a (1 × 4) reconstructed structure. A model named, "ad-molecule" (ADM) model, has been suggested for this (1 × 4) reconstruction, theoretically predicting that the surface retains a high reactivity. However, several recent experimental results have shown that the (1 × 4) reconstructed surface is not as active as expected, leading to a controversy about the actual atomic geometry of the reconstruction. Recent theoretical work suggests that the introduction of strain in the anatase TiO2(001) surface may enhance its reactivity by distorting the surface lattice. Thus, understanding the surface structure under strain may be the key to resolving these existing challenges with this material. Herein, we present a systematic study of the epitaxial growth of anatase TiO2(001) films on BaTiO3(001)/SrTiO3(001) substrates using pulsed laser deposition, characterized using X-ray diffraction (XRD), X-ray photoemission spectroscopy (XPS), scanning transmission electron microscopy (STEM), and scanning tunneling microscopy (STM). A thin layer of BaTiO3(001) was epitaxially grown on several SrTiO3(001) substrates to introduce strain in anatase TiO2(001) films by leveraging the relatively large lattice mismatch between the anatase TiO2(001) and BaTiO3(001). The XRD and STEM results showed that strain was partially introduced in the films when the thickness of the BaTiO3 layer was ~4–6 nm. The XPS results showed that a suitable thickness of the anatase TiO2(001) films was at least 15 nm, inducing a negligible concentration of outwardly diffused Sr and Ba from the substrate to the surface, and minimizing their possible effects on the surface structure. Dominant Ti4+ oxidation state was observed, indicating that the anatase TiO2(001) surface was fully oxidized. The surface structure as characterized by STM showed that the (1 × 4) reconstruction remained as films grew on the SrTiO3(001) substrate. However, ridges in the (1 × 4) reconstructed surface showed additional super-periods typically shown as dim features in the ridges separated by 2–5 lattice distances. Considering the high-resolution STM images and fully oxidized surface, we propose that these dim features may have been caused by "TiO2" vacancies in the ridges. This is consistent with the ad-oxygen model (AOM) for the fully-oxidized (1 × 4) reconstructed surface of anatase TiO2(001). In the AOM model, Ti atoms in the ridges were coordinated fivefold, in contrast with the fourfold coordination in the ADM model. We find direct evidence that the strains introduced in anatase TiO2(001) films can significantly modify ridge structure in the (1 × 4) reconstructed surface, providing key insights into the complicated surface structure, and suggesting important implications for furthering our understanding of the reactivity of this commonly used surface.

Key words: Anatase TiO2(001) surface, Thin film, Scanning tunneling microscopy, Surface reconstruction, Surface strain

MSC2000: 

  • O647