Nucleation and Growth of Metals

In the case of co-deposition nucleation and growth of metal nanoparticles are influenced by the process of a polymer matrix formation from deposited low-molecular fragments (TFE) of PTFE. At the initial stages the deposited... 

Some years ago we developed a new method allowing to study in situ and in real time the nucleation and growth of metal clusters on an oxide single crystals [48]. This method is a variant of the well-known... 

Few other methods allow to study in situ the nucleation and growth of metal clusters on oxide surfaces. Modern diffraction methods are able to follow the mean distance between neighbouring clusters and the mean cluster size. If the clusters are well distributed on the surface small diffraction peaks appear close to the specular. The reciprocal distance between these two peaks gives the average distance between the clusters. The width of the specular peak gives the mean cluster size. By GISAXS it has been possible to follow the nucleation and growth of metal on MgO(l 00) [29]. By SPA-LEED the same information can be also obtained but with less accuracy [67]. 

With respect to the Pd/Al203 model catalysts described below, STM was used to examine the structure of the AI2O3 support and the nucleation and growth of metal deposits (e.g.. References (34,63,73,101,215) and references cited therein), providing information about the size, shape, and height of palladium nanoparticles. In some cases, even atomically resolved images of individual palladium nanoparticles were obtained (206). 

XPS or AES is extensively used not only to indicate the cleanliness of the sample before transfer, but also to indicate the presence of adsorbates and their oxidation states following electrochemical experiments and transfer back into the UHV environment. In the case of model platinum-based electrocatalysts, the electron spectroscopies have been used to estimate the coverage of the adsorbate metal atoms or the alloy composition. In the case of alloys, or the nucleation and growth of metal adsorbate structures, the techniques give only the mean concentrations averaged over a depth determined by the inelastic mean free path of the emitted electrons. Adsorption and reaction at surfaces often depend on the... 

Physical characteristics of a support, namely porosity and specific surface area, have long been understood to play a key role in stabilizing active components of the catalysts in dispersed state. Explicitly or implicitly, they reflect topological properties of the carbon surface, namely the nature and quantity of (1) traps (potential wells for atoms and metal particles), which behave as sites for nucleation and growth of metal crystallites and (2) hindrances (potential barriers) for migration of these atoms and particles [4,5]. An increase in the specific surface area and the micropore volume results, as a rule, in a decrease in the size of supported metal particles. Formal kinetic equations of sintering of supported catalysts always take into consideration these characteristics of a support [6]. 

We have seen above (Sect. 2.3), how many different defect sites exist even on the surface a simple oxide like MgO. Each of these sites can react in a different way with an adsorbed species and is a potential candidate for the nucleation and growth of metal particles. The complexity of the problem is increased by the fact that the concentration of the defects is usually low making their detection by integral surface sensitive spectroscopies very difficult. A microscopic view of the metal-oxide interface and a detailed analysis of the sites where the deposited metal atoms or clusters are bound become essential in order to rationalize the observed phenomena and to design new materials with known concentrations of a given type of defects. 

Hermes S, Schroder E, Chehnowski R, WoU C, Eischer RA. Selective nucleation and growth of metal-organic open framework thin films on patterned COOH/CEj-terminated self-assembled monolayers on Au(lll). J Am Chem Soc 2005 127(40) 13744-13745. 

Dispersions of metallic nanoparticles can be obtained by two main methods (i) mechanic subdivision of metallic aggregates (physical method) or (ii) nucleation and growth of metallic atoms (chemical method). The physical method yields dispersions where the particle size distribution is very broad. Traditional colloids are typically larger (>10nm) and not reproducibly prepared, giving irreproducible catalytic activity. Chemical methods such as the reduction of metal salts is the most convenient way to control the size of the particles. Today, the key goal in the metal colloid area is the development of reproducible nanoparticle (or modem nanocluster) syntheses in opposition to traditional colloids. As previously reported, nanoclusters should be or have at least (i) specific size (1-10 nm), (ii) well-defined surface composition, (iii) reproducible synthesis and properties, and (iv) be isolable and redissolvable ( bottleable )- ... 

---