Thermodynamics driving the strong metal–support interaction: Titanate encapsulation of supported Pd nanocrystals

The strong metal–support interaction (SMSI) is the encapsulation of a supported metal particle by an oxide layer that diffuses from the substrate. This process is usually described as being driven by a reduction in the surface energy of the metal particle and has a significant influence on the catalytic activity of the metal. Here, epitaxial Pd nanocrystals grown in ultrahigh vacuum on $\mathrm{SrTi}{\mathrm{O}}_3\left(001\right)$ and anatase $\mathrm{Ti}{\mathrm{O}}_2\left(001\right)$ substrates are studied by scanning tunneling microscopy. At annealing temperatures above ∼600 ºC, the Pd crystals can become encapsulated by a $\mathrm{Ti}{\mathrm{O}}_x$ monolayer originating from the substrates. For both bare and encapsulated Pd crystals, their height-to-width ratio increases with the crystal height as a mechanism to partially release their interfacial misfit strain with the substrate. However, the rate of this increase is lower for encapsulated crystals, indicating that during the SMSI process the interface between the particle and the oxide is modified to form a lower energy interfacial structure which also results in less strain in the encapsulated particle. The SMSI is found to preferentially occur on larger crystals, driven by the reduction in their elastic strain energy, which scales with the crystal volume. Compared with the traditional view of SMSI our results provide a more complete description of the encapsulation process.

Figure 1

Bare and encapsulated Pd crystals. (a), (b) 3D sketches and 3D-rendered STM images of bare (a) and encapsulated (b) Pd crystals. (c)–(e) Winterbottom constructions of substrate-supported bare (c) and encapsulated (d), (e) Pd crystals. The model in (d) shows a $\mathrm{Ti}{\mathrm{O}}_x$ overlayer (blue) encapsulating the crystal outer surface and the model in (e) also includes a modified interface (green).

Figure 2

STM images of the (a) nanostructured $\mathrm{SrTi}{\mathrm{O}}_3\left(001\right)$ and (b) anatase (001) substrates. The insets are higher-magnification images with rectangular unit cells drawn (a: $V_{\mathrm{s}}=1.9\mathrm{V}, I_{\mathrm{t}}=0.24\mathrm{n}\mathrm{A}$; inset: $V_{\mathrm{s}}=1.8\mathrm{V}, I_{\mathrm{t}}=0.18\mathrm{nA}$; $\mathrm{b}:V_{\mathrm{s}}=1.6\mathrm{V}, I_{\mathrm{t}}=0.23\mathrm{nA}$; inset: $V_{\mathrm{s}}=1.3\mathrm{V}, I_{\mathrm{t}}=0.60\mathrm{nA}$). The crystallographic lattice directions of $\mathrm{SrTi}{\mathrm{O}}_3\left(001\right)$ are labeled.

Figure 3

3D-rendered STM images of Pd crystals. (a) Pd crystals nucleated at 300 ºC and post-annealed at 550 ºC for 2 h on anatase (001) ($V_{\mathrm{s}}=1.6\mathrm{V}, I_{\mathrm{t}}=0.17\mathrm{nA}$). (b) Pd crystals nucleated at 650 ºC and post-annealed at 650 ºC for 1 h on nanostructured $\mathrm{SrTi}{\mathrm{O}}_3\left(001\right)$ ($V_{\mathrm{s}}=2.0\mathrm{V}, I_{\mathrm{t}}=0.30\mathrm{nA}$). Some crystals are encapsulated by a $\mathrm{Ti}{\mathrm{O}}_x$ overlayer, showing a wagon wheel superstructure on the top facet, indicated by arrows. (c) Magnified images of a bare (c.i) and an encapsulated (c.ii) crystal on anatase (001): top view (upper row) and oblique view (bottom row) (c.i: $V_{\mathrm{s}}=2.0\mathrm{V}, I_{\mathrm{t}}=0.17\mathrm{nA}$; e.ii: $V_{\mathrm{s}}=2.6\mathrm{V}, I_{\mathrm{t}}=1.33\mathrm{nA}$).

Figure 4

Aspect ratio of Pd crystals. (a) Top and oblique views of the Wulff shape of a supported Pd crystal, with three dimensions labeled: $l, s$, and $h$. (b) $h/l$ plotted against $h$ for Pd crystals on nanostructured $\mathrm{SrTi}{\mathrm{O}}_3\left(001\right)$ and anatase (001) substrates.

Figure 5

Qualitative sketches of the energy components of a crystal with a constant volume supported on a lattice-mismatched substrate, illustrating the origin of strain-induced crystal heightening.

Figure 6

$\mathrm{\Delta }{\gamma }^{*} \mathit{vs} \mathrm{\Delta }{\gamma }_{111}$ for the flattest crystals $\left(h<2\mathrm{nm}\right)$ on nanostructured $\mathrm{SrTi}{\mathrm{O}}_3\left(001\right)$.

Figure 7

Populations of bare and encapsulated Pd crystals that have been post-annealed at ≥ 600 ºC on nanostructured $\mathrm{SrTi}{\mathrm{O}}_3\left(001\right)$ and anatase (001) substrates, plotted against crystal volume.