Acta Physico-Chimica Sinica ›› 2019, Vol. 35 ›› Issue (12): 1412-1421.doi: 10.3866/PKU.WHXB201905054

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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)

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