Atomic-Scale Observation of Oxygen Vacancy-Induced Step Reconstruction in WO₃

Abstract

As the most common type of defects in WO 3 , surface oxygen deficiencies have been widely investigated in terms of their structure as well as physicochemical properties. In general, oxygen vacancy on the surface may lead to the reconstruction of the surface structure due to the need for surface energy minimization, especially for the area with high oxygen vacancy concentration. However, such a phenomenon has not been directly demonstrated or observed from the experiment. In the paper, we observed such surface reconstruction in the step area or high-index facets in WO 3 using the spherical aberration-corrected scanning transmission electron microscope (Cs-STEM) with atomic-scale resolution. This surface reconstruction results in a structure of 5-fold symmetry pentagonal columns, which is first reported. According to the DFT calculation, the formation energy of oxygen vacancy in the step edge is much lower than the smooth surface. Furthermore, the formation energy of oxygen vacancy in the higher index (110) facet is lower than the low index ( 100) and ( 010) facets. Our experimental results support these findings, i.e., that the reconstruction is pronounced in the step or high-index facets with a high concentration of oxygen vacancy. Therefore, the high concentration of oxygen vacancy in the step or high-index facet area contributes to the formation of pentagonal column structures. This work could provide a novel insight into the correlation between intrinsic defect and surface structure formation in these materials.

Recent Progress in the Synthesis and Potential Applications of Two‐Dimensional Tungsten (Sub)oxides

“…With sufficiently low reduction, single oxygen vacancies can be produced. [43] In this case, oxygen atoms are removed from the surface, forming oxygen vacancies. To minimize the surface energy, single PCs appear on the surface, stabilizing the structure.…”

“…With sufficiently low reduction, single oxygen vacancies can be produced [43] . In this case, oxygen atoms are removed from the surface, forming oxygen vacancies.…”

Spatial Control of Oxygen Vacancy Concentration in Monoclinic WO3 Photoanodes for Enhanced Solar Water Splitting

“…However, the O atoms could not be imaged because of the small atomic number of oxygen. [ 23 ] It is very hard to observe O atoms directly even with state‐of‐the‐art Cs‐STEM instruments. For example, Kim et al could quantify the concentration of OV defects in TiO 2 NRs by comparing the image brightness of the Ti−O and Ti−Ti columns.…”

A facile strategy for the growth of high-quality tungsten disulfide crystals mediated by oxygen-deficient oxide precursors

“…W 18 O 49 (WO 2.72 ) and W 20 O 58 (WO 2.9 ) are the most thermodynamically stable phases and exhibit metallic characteristics as the Fermi level moves up in the conduction band to the parent WO 3 . 32,33 Therefore, it is likely that the aforementioned phases either co-exist in the PLD-grown films or metastable oxides present in the PLD precursors transform into the most thermodynamically stable phases during the CVD process. Nanostructures of WO 2.72 were synthesized by various approaches, such as heating a W foil at a high temperature (1600 °C), 34 photochemical reduction of carbon dioxide by visible light, 35 thermal evaporation of WO 3 powder, electron-beam induced synthesis.…”

“…In particular, oxygen vacancies are a signature of the PLD process 31 and can cause an atomic re-arrangement of the WO 3 crystal. 32 Yet, the detailed mechanism of surface reconstruction in sub-stoichiometric tungsten oxide is beyond the scope of the work. A wide range of oxides can form between WO 3 and WO 2 .…”