Surface high-index facets

Bismuth has attracted significant interest as a Pt/C modifier for formic acid electrooxidation [21, 24, 26, 27]. A wide range of stable and well-characterized electrode surfaces modified by irreversible Bi adatom adsorption on Pt have been reported in the literature for a range of Bi coverages 6). Chen et al. have explored Bi adatom decoration on 81 nm tetrahexahedral Pt nanoparticles that while composed of (100) and (110) facets that are the least active for formic acid electrooxidation, they are boimd by 730 and vicinal high-index facets that are extremely active [18]. They have measured current densities of 10 mA cm for Bi coverages up to 0.9 at 0.4 V in 0.25 M formic acid and 0.5 M H2SO4 solution see Fig. 4.4. They also showed steady-state activity at 0.3 V of 2.8 mA cm after 1 min vs. 0.0003 mA cm for the non-modified Pt baseline. 

Figure 4.47. Effect of Pt surface reshaping by square wave voltammetry on the electrocatalytic activity toward formic acid and ethanol oxidation, a) Formic acid oxidation at 0.25 V vs. SCE in 0.25 M HCOOH - 0.5 M H2SO4. b) Ethanol oxidation at 0.3 V vs. SCE in 0.1 M EtOFl - 0.1 M F1C104 295 K [220]. (Reproduced with permission from Tian N, Zhou Z-Y, Sun S-G, Ding Y, Wang ZL. Synthesis of tetrahexahedral platinum nanocrystals with high-index facets and high electro-oxidation activity. Science 2007 316 732-5.)...

Fig. 47. Work function of the (110) facet of a tungsten field emitter at 300 K as a function of silver exposure. The authors interpret this data as follows Below an exposure treshold of about one ML, Ag adatoms migrate to the high-index facets that surround the W(llO) plane, and the emission properties are indistinguishable from those of the bare substrate. At 1 ML the adatoms invade the (110) plane, modifying the surface density of electron states and bringing about an abrupt increase in electron emission. Above 2 ML the additionally evaporated Ag atoms migrate again to the surrounding facets. From [95D].

For Sb and A1 on HOPG, and for Si and Ge on crystalline SiNx/Si(lll), most grown-up crystallites have definite polar (also azimuthal for some) orientation alignment with the substrate. In contrast, for Ge on graphite, and Si and Ge on amorphous SiNx/Si(001), the orientation of the nanocrystals seems completely random, and high-index facets are observed quite often. Surface reconstructions not formed on bulk crystals are observed on some facets. These observations reflect the unique capacity of nanoscale facets to accommodate certain surface superstructures that are not observable on a macroscopic scale. 

However, this assumption is not necessarily justified. Even for a well-faceted nanoparticle there are a number of nonequivalent adsorption sites. For example, in addition to the low-index facets, the palladium nanoparticle exhibits edges and interface sites as well as defects (steps, kinks) that are not present on a Pd(l 1 1) or Pd(lOO) surface. The overall catalytic performance will depend on the contributions of the various sites, and the activities of these sites may differ strongly from each other. Of course, one can argue that stepped/kinked high-index single-crystal surfaces (Fig. 2) would be better models (64,65), but this approach still does not mimic the complex situation on a metal nanoparticle. For example, the diffusion-coupled interplay of molecules adsorbed on different facets of a nanoparticle (66) or the size-dependent electronic structure of a metal nanoparticle cannot be represented by a single crystal with dimensions of centimeters (67). It is also shown below that some properties are merely determined by the finite size or volume of nanoparticles (68). Consequently, the properties of a metal nanoparticle are not simply a superposition of the properties of its individual surface facets. 

In addition to those Pt nanoparticles bound by low-indexed surfaces, Tian et al. synthesized tetrahexahedral platinum nanoparticles by using an electrochemical method [7]. Here, a square-wave potential function was used to treat Pt nanospheres supported on glassy carbon, and this led to the formation of Pt nanocrystals bound by high-indexed 730, 210 and/or 520 facets. These particles proved to be catalytically active due to their high density of atomic steps and kinks [98]. 

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