Gold crystallites

Figure C2.17.13 presents a model ealeulation of the absorption of gold nanoerystals, using the formalism outlined above. The qualitative result is that, as metal eolloids beeome smaller, the primary absorption peak shifts to lower energy, and broadens signifieantly. The peak shifts predieted are small, of the order of 0.1 eV for a 2 nm gold crystallite. In eontrast, the peak widths are far more sensitive to size. This simple theory, and its variations, have been successful at explaining many experimental observations, espeeially for elusters greater than 3 nm in size...

Si/Si02- Gold crystallites have been imaged with (111) orientation on the surface of Ce02 [8]. Similarly, pyramids of wiistite FeO growing at the surface of spinel ZnCrFe04 crystals under the influence of the electron beam are formed. 

In 1910, Kolbe was the first to prove that dichroic nanocomposite samples based on gold contained the metal indeed in its zero-valence state. Such affirmation was confirmed a few years later by X-ray scattering. In particular, it was shown that zero-valence silver and gold were present in the respective nanocomposites made with oriented ramie fibers, and the ring-like interference patterns of the metal crystallites showed that the individual primary crystallites were not oriented (32). Based on Scherrer s equation, which was developed just in this period, the average particle diameter of silver and gold crystallites was determined in fibers of ramie, hemp, bamboo, silk, wool, viscose, and cellulose acetate to be between 5 and 14 nm (33). 

In order to study the stability of gold colloids in solutions of poorly interacting polymers, we chose block copolymers of styrene and ethyleneoxide (PS-b-PEO). Polyethyleneoxide does not interact strongly with transition metal surfaces and is thus a rather poor stabilizer for colloidal gold (5). It can, however, complex lithium cations and can also be protonated (9). In this way, PEO can bind compounds like LiAuCU and HAuCU, and we attempted to load inverse PS-b-PEO micelles by these gold salts. Subsequent reduction was expected to yield small gold crystallites entrapped in the micellar cores. 

After supporting these sols on activated carbon, however, the obtained particle size depends on the capability of the protective agent to maintain the particle dimension. The obtained three catalysts, having different characteristics, are summarized in Table 3. As it is shown, mean size of gold nanoparticle obtained by TEM measurement did not always match with X-ray powder diffraction (XRPD) data. This result is not surprising as TEM measurements represent particle sizes, whereas from X-ray diffraction (XRD) it is possible to obtain crystallite dimensions that do not necessarily coincide with the size of... 

In contrast to platinum, palladium nickel, and gold, with silver only crystallites with an approximately circular plan shape are seen (S3) and there is no worthwhile evidence of angularity the likely general shape (S3, 34) is that of a flatish curved dome. 

A schematic view of the cold cathode fabrication process is shown in Fig. 10.18. The cold cathode is fabricated by low pressure chemical vapor deposition (LPCVD) of 1.5 pm of non-doped polysilicon on a silicon wafer or a metallized glass substrate. The topmost micrometer of polysilicon is then anodized (10 mA cnT2, 30 s) in ethanoic HF under illumination. This results in a porous layer with inclusions of larger silicon crystallites, due to faster pore formation along grain boundaries. After anodization the porous layer is oxidized (700 °C, 60 min) and a semi-transparent (10 nm) gold film is deposited as a top electrode. 

The easiest model to treat theoretically is the sphere, and many colloidal systems do, in fact, contain spherical or nearly spherical particles. Emulsions, latexes, liquid aerosols, etc., contain spherical particles. Certain protein molecules are approximately spherical. The crystallite particles in dispersions such as gold and silver iodide sols are sufficiently symmetrical to behave like spheres. 

SEM pictures show that a very thin layer of small clusters/crystallites forms first on the gold surface, followed by crystalline agglomerates that finally leads to a 500-1000 nm thick silicon layer (Figure 6.12). 

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