Figure 1

Sequence of STM images of a vanadium oxide decorated Pd(111) surface exposed to reducing and oxidizing ambient conditions. (a) The Pd(111) surface, annealed at 300 °C in vacuum after oxide deposition, 65% covered with two coexisting V-oxide phases: $s\mathrm{\text{−}}{\mathrm{V}}_2{\mathrm{O}}_3$ and $r\mathrm{\text{−}}{\mathrm{V}\mathrm{O}}_2$. (b) After $\sim 1000\mathrm{s}\mathrm{e}\mathrm{c}$ exposure to $1\times {10}^{-9}\mathrm{m}\mathrm{b}\mathrm{a}\mathrm{r}$ ${\mathrm{H}}_2$ at 250 °C. (c) After $\sim 2000\mathrm{s}\mathrm{e}\mathrm{c}$ exposure to $1\times {10}^{-9}\mathrm{m}\mathrm{b}\mathrm{a}\mathrm{r}$ ${\mathrm{H}}_2$ at 250 °C. The surface is now reduced and completely covered by $s\mathrm{\text{−}}{\mathrm{V}}_2{\mathrm{O}}_3$. (d) After $\sim 2000\mathrm{s}\mathrm{e}\mathrm{c}$ exposure of surface (c) to $5\times {10}^{-8}\mathrm{m}\mathrm{b}\mathrm{a}\mathrm{r}$ ${\mathrm{O}}_2$ at 250 °C. The zigzag ${\mathrm{V}}_6{\mathrm{O}}_{14}$ oxide appears. (e) After $\sim 400\mathrm{s}\mathrm{e}\mathrm{c}$ exposure of surface (d) to $5\times {10}^{-8}\mathrm{m}\mathrm{b}\mathrm{a}\mathrm{r}$ ${\mathrm{O}}_2$ at 250 °C, followed by pump-down of the ${\mathrm{O}}_2$. The surface contains zigzag ${\mathrm{V}}_6{\mathrm{O}}_{14}$ and ($4\times 4$) ${\mathrm{V}}_5{\mathrm{O}}_{14}$ oxide phases along with some free Pd areas. (f) Reduction of surface (e) by $\sim 1250\mathrm{s}\mathrm{e}\mathrm{c}$ exposure to $3\times {10}^{-9}\mathrm{m}\mathrm{b}\mathrm{a}\mathrm{r}$ ${\mathrm{H}}_2$ at 250 °C. The surface is again wetted by the reduced $s\mathrm{\text{−}}{\mathrm{V}}_2{\mathrm{O}}_3$ phase.Reuse & Permissions

Figure 2

Atom-resolved STM images of the principal V-oxide “monolayer-type” phases on Pd(111) and corresponding DFT derived structure models. (a) Surface-${\mathrm{V}}_2{\mathrm{O}}_3$ phase. (b) Rectangular-${\mathrm{V}\mathrm{O}}_2$ phase. (c) Zigzag ${\mathrm{V}}_6{\mathrm{O}}_{14}$ phase. (d) ($4\times 4$) ${\mathrm{V}}_5{\mathrm{O}}_{14}$ phase. Red spheres: oxygen; green: vanadium; grey: palladium.Reuse & Permissions

Figure 3

Percentage areas of bare Pd and various V-oxide covered parts of the surface plotted versus the time during a reduction-oxidation-reduction cycle, as evaluated from STM images. The schematic diagram on top of the figure illustrates the morphology changes, (a)–(f) refer to the STM images of Fig. 1.Reuse & Permissions

Figure 4

Phase diagram of V-oxides on Pd(111), as calculated by DFT, as a function of the chemical potential of oxygen ${\mu }_{\mathrm{O}}$ (which is related to the oxygen pressure [19]) and the vanadium coverage. The thick bars indicate the stability regions of the various oxide phases. Points I–IV are discussed in the text; the dotted arrows indicate the coexistence or transformation of oxide phases.Reuse & Permissions