Selective Growth and Dissolution of Ni on a PdAu Bimetallic Surface by In Situ STM: Determining the Relative Adsorbate-Substrate Interaction Energy

By studying the electrochemical growth and dissolution of a Ni monolayer on a PdAu bimetallic surface with in situ STM, we demonstrate that Ni grows preferentially on Au(111) in a wide potential range. In contrast, a Ni monolayer covering the entire bimetallic surface can be selectively dissolved from Pd islands. We show that the Ni-substrate interactions play a key role in this selectivity, and we estimate the binding energy of Ni to Pd to be 80 meV smaller than that of Ni to Au.

Figure 1

(a) STM image of the bare bimetallic $\mathrm{Pd}(0.5\mathrm{ML})/\mathrm{Au}(111)$ surface recorded at $-0.6\mathrm{V}$. Dotted lines are used as a landmark to localize Pd islands. (b)–(c) STM images ($151\times 151{\mathrm{nm}}^2$), recorded at $U=-1.07\mathrm{V}$, illustrating selective growth of Ni on the same surface spot at $U_{\mathrm{dep}}=-1.44\mathrm{V}$ for 0.6 s (b) and $U_{\mathrm{dep}}=-1.34\mathrm{V}$ for 2 s (c). White arrows point to the same position on the surface. (d) Zoom ($32\times 32{\mathrm{nm}}^2$) of the $\mathrm{Ni}/\mathrm{Au}$ morphology showing the moiré structure.

Figure 2

(a) STM image ($100\times 100{\mathrm{nm}}^2$) of the complete Ni monolayer deposited on $\mathrm{Pd}(0.5\mathrm{ML})/\mathrm{Au}(111)$ at $U_{\mathrm{dep}}=-1.24\mathrm{V}$ for 20 s and recorded at $U=-1.07\mathrm{V}$. (b) Zoom ($40\times 40{\mathrm{nm}}^2$) of the boundary region between the Au substrate and Pd islands covered by a Ni layer. The period of the hexagonal moiré structure on $\mathrm{Ni}/\mathrm{Au}$ is identical to that on $\mathrm{Ni}/\mathrm{Pd}$ (21–22 Å).

Figure 3

Variation of Ni coverage on $\mathrm{Pd}({\theta }_{\mathrm{Ni}/\mathrm{Pd}})$ and $\mathrm{Au}({\theta }_{\mathrm{Ni}/\mathrm{Au}})$ vs the average Ni thickness $t_{\mathrm{Ni}}$. Data are derived from quantitative STM image analysis recorded during Ni deposition (black filled symbols) and dissolution of a Ni monolayer (gray filled symbols) from a bimetallic $\mathrm{Pd}(0.5\mathrm{ML})/\mathrm{Au}(111)$ surface. Open symbols refer to macroscopic electrochemical measurements on monometallic surfaces (see Ref. 14).

Figure 4

Series of STM images ($155\times 155{\mathrm{nm}}^2$) showing the dissolution of a full Ni monolayer (${\theta }_{\mathrm{Ni}/\mathrm{Pd}}={\theta }_{\mathrm{Ni}/\mathrm{Au}}\sim 1$) which was deposited on $\mathrm{Pd}(0.5\mathrm{ML})/\mathrm{Au}(111)$ at $U_{\mathrm{dep}}=-1.24\mathrm{V}$ for 20 s. Images were recorded (a) at $U=-1.07\mathrm{V}$ and successively at $U=-1.02\mathrm{V}$ after (b) 4, (c) 17, and (d) 38 min. Complete selective Ni dissolution from the Pd islands occurred in 55 min.