Fig. 4

Reversible changes in the vanadium oxide surface layer imaged by TEM. a High-resolution TEM image of a 2 monolayer VO_x/TiO₂ particle indicating the areas on the (001) and (101) surfaces cropped from the subsequent EW phase images and shown throughout the reduction-oxidation cycle. The crystalline contrast of the outermost layer on the (001) surface, indicated by black arrows, was greatly reduced under the reducing conditions (300 °C, vacuum), but reappeared in the exact same position when O₂ was introduced again (300 °C, 1 mbar O₂). No such change was visible at the (101) surface. Corresponding images are also shown for a 0.5 monolayer VO_x/TiO₂ particle b and for a pure TiO₂ particle c, neither of which undergoes a similar change in the structure of the outermost layer. The bright arrows in b indicate single surface atomic rows that shifted between equivalent anatase positions. The lattice fringes outside the particle surface in c stems from the larger, underlying particle outlined in the overview image, which superimpose contrast in the image over the free surfaces. The dashed box at the (001) surface in b is shown at higher magnification together with a simulated EW phase overlaid with an anatase structure model. The same pattern is also indicated at the (101) surface to show their relative orientation. Scale bar for the overview images, 10 nm. Scale bar for the EW phase images, 1 nm