Copper-platinum monolayers

Kolb D M, Kdtz R and Yamamoto K 1979 Copper monolayer formation on platinum single crystal surfaces Optical and... 

A well researched and popular class of monolayers is based on the strong adsorption of thiols (R - SH), disulfides (R - S - S - R) and sulfides (R - S - R) onto metal surfaces. Although thiols, disulfides, and sulfides strongly align with a number of different metals Hke gold, silver, platinum, or copper, gold is usually the substrate of choice because of its inert properties and the formation of a well-defined crystal structure. 

It was demonstrated that the change in thickness of these layers depends on the physicochemical properties of water in these thin water layers. It is reported that on iron surfaces, the number of adsorbed water layers is about 15 at RH 55% and 90 at 100%. Similar values are obtained for Copper and Zinc however significant differences are reported for Platinum, gold, aluminum and silver. These monolayers have been calculated only in presence of water (without oxygen) where the corrosion process is very slow and, consequently, in conditions far from the reality. 

Chemical analysis of solids and solutions indicate that in all cases metallic ruthenium, platinum, and gold are deposited on copper. Ruthenium, deposit is restricted to approximately 0.33 of the copper surface atoms, demonstrating that the redox reaction between Cu and Ru3+ can occur only on some special copper sites [11]. With platinum or gold, for the highest amount of modifier introduced (M"+/Cu(s) > 100), a deposit larger than a monolayer is obtained, indicating that all accessible copper atoms and subsurface copper atoms are involved in the redox reaction [13]. 

Hexadecanethiol monolayers also self-assemble on mercury surfaces and provide an extremely low defect density . Alkanethiols can likewise be assembled on GaAs (100) surfaces and can act as a useful semiconductor. Alkaneni-triles bind side-on (= x ) to gold and copper surfaces. The infrared signal in the range of 2000-2500 cm Ms replaced by a 1570-1630 cm band which is close to x -coordinated nitrile on platinum. Molecular dynamics calculations for —SH and —SCF13 on gold produced two chemisorption modes very close in energy . 

Fig. 15 Correlation between shifts in surface core-level binding energy (crossed bars) and the shifts in CO desorption temperature (empty bars). The properties of the platinum, nickel and copper monolayers are compared with the corresponding values of the pure metals. Reprinted from ref. [15].

At higher surface coverages (more than Vi but less than 1), anions can be entirely displaced by copper adatoms from the surface or both form a two-layer structure in which anions are adsorbed on both the platinum and the copper sites. The final step is the total filling of the copper monolayer to form a bilayer phase with a disordered anion ad-layer on the topmost of Cu-Pt(lll) [106] or an ordered (2 x 2) bilayer of copper-halide structure on Pt(100) [104], The same physical models can be used in the case of bromide and chloride with little differences between the anion distances with a surface structure like that of a honeycomb ad-layer. The situation accounting for iodine adsorption is very different because of its large atomic radius and specific adsorption on noble metals. 

C and n-octadecane-1,2-H at a concentration of 0.35 molar in stearic acid and studied the coadsorption of stearic acid and octadecane on polished surfaces of silver, platinum, copper and iron. The films were prepared by retraction from the melt at 40 C (at room temperature the mixture was solid). The proportion of n-octadecane in the film was assayed by differential extraction with cyclohexane. The results of the investigation adequately demonstrate that n-octadecane coadsorbs with stearic acid but not necessarily as a mixed oriented monolayer. Some of the data indicate that more than a single layer is present on the surface. Thus the structure of the long-chain material on the surface may be open to conjecture, but that each constituent adsorbs and in what relative amount is directly determined by radioactive assay. 

This rather simple experiment demonstrates quite clearly the poisoning of the oxygen reduction process by copper. Incidentally, Cu(II) is a rather common impurity in distilled water and mineral acids, and this experiment demonstrates that underpotential deposition of a monolayer of copper from solutions containing as little as 1 ppm Cu can drastically affect the behavior of a platinum electrode. Adsorption of small amounts of other impurities (i.e., organic molecules) can also have an effect on solid-electrode behavior. Thus, electrochemical experiments often require making great efforts to establish and maintain solution purity. 

Blodgett monolayers of stearic acid-l-C and stearic acid-9, 10-H had been previously prepared on silver, platinum, copper, and iron and counted in order to convert counting rate data to monolayers of stearic acid. To calculate surface coverage by n-octadecane it was assumed that vertically oriented n-octadecane molecules would occupy the same cross-sectional area as vertically oriented stearic acid molecules. 

In the case of powder bulk catalysts, Cu Raney was modified by direct redox reaction between reduced copper and the salt of a noble metal M (Ru, Pt and Au) [5, 7]. Typically the Cu-M bimetallic catalysts were obtained by mixing a freshly prepared Cu Raney with an aqueous solution of the noble metal salt. When the amount of M is in excess compared to the number of copper surface atoms, it appears that ruthenium deposition is restricted to approximately 1/3 of the copper surface atoms. For platinum and gold, a deposit larger than a monolayer is obtained, indicating that subsurface copper atoms are involved in direct redox reaction. This result is explained by the lower potential difference between copper and rathenium compared to those of copper and platinum or gold [7j. However, the reactions involved in the direct redox reaction may not be as simple as indicated in Section 9.2. A typical time distribution of ion concentrations in solution during the preparation of Cu-Pt is shown in Fig. 9.1. It can be observed that platinum ions disappear very rapidly from the solution while at the same time copper... 

Enormous developments in the area of soluble noble metal clusters protected with monolayers are discussed. Mass spectrometry has been the principal tool with which cluster growth has been examined. The composition and chemistry of clusters have been examined extensively by mass spectrometry. Besides gold, silver, platinum, copper and iron clusters have been examined. Clusters have also been examined by tandem mass spectrometry and the importance of ligands in understanding closed shell electronic structure is understood from such studies. Protein protected noble metal clusters belong to a new group in this family of materials. Naked metal clusters bearing the same core composition as that of monolayer protected clusters is another class in this area, which have been discovered by laser desorption ionization from protein templates. 

---