Adlattice

Figure 2.7. STM image (unfiltered) of a Pt single crystal surface consisting mainly of Pt( 111) terraces and covered by a Pt( 111)-(12x 12)-Na adlattice formed via electrochemical Na+ supply (from a P"-Al203 Na+ conductor interfaced with the Pt single crystal)3 36 on a Pt(l 1 l)-(2x2)-0 adlattice. Each sphere on the image corresponds to a Na atom.30,36 Reproduced from ref. 36 by permission of The Electrochemical Society.

Figure 2.8. STM image (unfiltered) of a Pt( 111) surface of a Pt single crystal interfaced with P"-A1203, a Na+ conductor showing different domains of Na coverage. The Pt(l 11 )-(2x2)-0 surface was initially covered by the Pt(ll l)-(2x2)-Na adlattice (domain A) and was intentionally only partly electrochemically cleaned (via positive UWR=1V potential application and Na+ removal into the P"-A1203 lattice) leading to the formation of clean domains (domain B) and of higher Na coverage domains (domain C) corresponding to a (V3 x V3 )-Na adlattice.

As expected, the Pt(l 11) surface is covered under ambient conditions by the well-known Pt(lll)-(2x2)-0 adlattice which corresponds to Oq -0.25 where the superscript Pt denotes that the coverage is based on the total surface Pt atoms. The measured interatomic distance of 5.61 A (Fig. 5.49a) is in excellent agreement with literature for the Pt(lll)-(2x2)-0 adlatice. As manifest by the Fourier transform spectmm (Fig. 5.49b) of the surface image of Fig. 5.49a there exists on the surface a second hexagonally ordered adlattice,... 

Figure 5.49. (a) STM image (unfiltered) of the initially sodium-contaminated Pt(l 1 l)-(2x2)-0 adlattice (b) corresponding Fourier transform spectrum (c) Fourier-filtered STM image of the overlapping Pt(l 1 l)-(2x2)-0 and Pt(111)-(12x12)-Na adlayers (bias Ut = 80 mV, tunelling current I, = 10 nA, total scan size 319 A).78 Reprinted with permission from Elsevier Science. 

Upon application of positive potential (UWr=0.4V) for 10 min the surface is cleaned from sodium and one obtains the image of Fig. 5.50a which shows only the Pt(lll)-(2x2)-0 adlattice. 

Figure 5.54. Effect of sodium coverage on the change AUWR of polycrystalline Pt catalyst potential UWr and on the catalytic rates of CO oxidation (solid lines37) and C2H4 oxidation (dashed lines36). Comparison with the theoretical Na coverage required to form the Pt(l 11)-(12xl2)-Na adlayer 0 is based on the number of surface Pt atoms 09a is based on the number of surface O atoms corresponding to the Pt(l 1 l)-(2x2)-0 adlattice. Reprinted from ref. 78 with permission from Elsevier Science,...

It is worth noting that for both systems the observed AUWr value corresponding to the onset of the formation of the ordered Na adlattice is practically the same, which strongly supports the idea that this AUwr value is characteristic of the chemical potential of this structure. The fact that a small but not negligible Na coverage (0ga < 0.015) preceeds the formation of the ordered Na structure on the surface of polycrystalline Pt samples (Fig. 5.54) may indicate preferential Na adsorption on stepped surfaces before Na adsorption on Pt(lll) starts taking place. 

It should be clear that, as well known from the surface science literature (Chapter 2) and from the XPS studies of Lambert and coworkers with Pt/(3"-A1203 (section 5.8), the Na adatoms on the Pt surface have a strong cationic character, Nas+-5+, where 5+ is coverage dependent but can reach values up to unity. This is particularly true in presence of other coadsorbates, such as O, H20, C02 or NO, leading to formation of surface sodium oxides, hydroxides, carbonates or nitrates, which may form ordered adlattices as discussed in that section. What is important to remember is that the work function change induced by such adlayers is, regardless of the exact nature of the counter ion, dominated by the large ( 5D) dipole moment of the, predominantly cationic, Na adatom. 

Scanning tunneling microscopy, STM ordered adlattices, 264 oxygen adlattices, 261 platinum, 261 sodium adlattices, 262 spillover-backspillover, 259 Self-consistent field, 269 Selectivity definition, 17... 

A crystal supporting preadsorbed Cl adlattices formed with HC1 gas, as described previously, was immersed in water for one minute to investigate the stability of the Cl adlayers. Subsequent examination of the emersed (removed) crystal evidenced a partial loss of chlorine, about 1/3, and uptake of oxygen on both Cu(lll) and Cu(llO). There was no detectable oxygen uptake on the Cu(100) surface. 

Ordered deposits were formed at -0.3 V on all three low-index planes of Au precoated with the Se adlattices listed above. The clearest LEED patterns were observed for deposits formed on Au(l 11), which happens to be the plane for which ordered Se structures were seen most infrequently with LEED. The Au(lOO) surface, which evidenced the sequence of Se atomic layers diagrammed in Fig. 55, showed the least tendency to... 

Atomic structures of several adlayers of Cd deposited underpotentially on Au(lll) surface in H2SO4 solution have been visualized applying in situ STM [418]. Three ordered adlattices have been observed, all of which had a long-range linear morphology and were rotated by 30° with respect to the substrate lattice directions. The same system has been studied later... 

Gichuhi etal. [439] have used Au(lll) electrode covered with the initial under-potentially deposited Cd layer. When H2S was electrolyzed at this surface, applying underpotential, an adlattice of the S—S interatomic spacing equal to 0.34 nm was obtained. The second monolayer of Cd and S had the same structure as the first CdS monolayer, showing that these two CdS monolayers were epitaxial. However, the third deposited monolayer of CdS exhibited interatomic spacing as observed for the bulk CdS. A direct fabrication of monodispersed, ultrasmall nanocrystals from the SAMs at Au(lll) substrate has also been described [440]. Reconstruction of CdS monolayers has been studied by Demir and Shannon [441]. 

If an adlattice is oriented in an oblique angle with respect to the substrate, e.g., 0° < cp < 30° for a surface lattice with hexagonal symmetry, two mirror-... 

Fig.4 Examples for adsorbate lattice structures on (100) and (111) fcc-surfaces. The choices of unit vectors for adlattice and substrate lattice are indicated. Adatoms are shown in grey. Instead of the four-fold and three-fold coordination sites for the adsorbates, the same adlayer periodicities can result for sites with different coordination e.g., on top or bridge sites...

If a rotation operation must be included in order to bring two domains on a surface to coincide, we speak of rotational domains . If, by no means, a regular adlattice can be brought into a periodic relation with the underlying substrate lattice, the superstructure is incommensurate. In this case, the lateral interactions are so strong that the substrate registry cannot govern the lateral order. 

A LEED pattern is the projection of the reciprocal lattice. The unit cells of molecular adlattices are usually larger than the atomic substrate lattice, which results in smaller reciprocal vectors and smaller distances between the superstructure spots of the reciprocal adlattice. 

Chiral supramolecular architectures are sometimes formed by molecules that stay achiral as a single entity. Hence, chirality arises just because of close-packed self-assembly on the surface. The single pentane molecule in its linear configuration remains achiral. For the close-packed monolayer, a rectangular unit cell has been identified by neutron diffraction. In addition, a tilt of the molecular axis with respect to the adlattice vectors would make the whole layer chiral [33]. For a particular mirror domain, the tilt angle i// can be either turned clockwise or counterclockwise (Fig. 12). 

Fig. 15 a M-[7]H forms at 95% of the saturated monolayer CW-rotated pinwheels (top, left) while CCW-rotated pinwheels are observed via STM for P-[7H] (top, right) [44]. At full ML opposite tilt angles of cloverleaf clusters with respect to the adlattice are observed (bottom). Images 10 nm x 10 nm. Reprinted with permission from Wiley, b Model for the M-[7]H cloverleaf structure obtained from MMC. Minimal repulsion is achieved for certain relative azimuthal orientations... 

Another example is the adsorption of prochiral 1,2,4-benzene tricarboxylic acid on Cu(100). As determined via X-ray absorption studies, this molecule has a tilted local geometry on the surface. The adsorbate complex is therefore chiral [70]. The long range ordered adlattice, however, has p(3 x 3) periodicity i.e., the molecules are strictly aligned by the quadratic substrate mesh. 

---