Carbon surface nucleation

We already touched on some aspects of carbonate surface chemistry e.g., in Chapter 3.4. We have already illustrated some of the factors that affect surface charge and the point of zero charge, pHpzc, in Chapter 3.5, and have discussed certain elementary aspects of CaC03 nucleation in Chapter 6.5 and of coprecipitation (and solid solution formation) in Chapter 6.7. 

Calcium phosphate precipitation may also be involved in the fixation of phosphate fertilizer in soils. Studies of the uptake of phosphate on calcium carbonate surfaces at low phosphate concentrations typical of those in soils, reveal that the threshold concentration for the precipitation of the calcium phosphate phases from solution is considerably increased in the pH range 8.5 -9.0 (3). It was concluded that the presence of carbonate ion from the calcite inhibits the nucleation of calcium phosphate phases under these conditions. A recent study of the seeded crystal growth of calcite from metastable supersaturated solutions of calcium carbonate, has shown that the presence of orthophosphate ion at a concentration as low as 10-6 mol L" and a pH of 8.5 has a remarkable inhibiting influence on the rate of crystallization (4). A seeded growth study of the influence of carbonate on hydroxyapatite crystallization has also shown an appreciable inhibiting influence of carbonate ion.(5). 

The foregoing discussion serves to show that disordered carbon structures are oxidized more readily than well-ordered graphite planes and that dislocations and active sites provide nucleation points for attack of the carbon crystallite. Another factor that must be considered is that dispersed electrocatalysts, such as platinum, on the carbon surface are not benign. The electrocatalysts interact with the carbon causing local oxidation or corrosion, i.e., the platinum catalyzes the corrosion of the carbon itself. In the presence of oxygen, which is the condition under which the electrocatalyst will operate, reduction intermediates from the oxygen (e.g., HOj) can have an accelerated corrosion effect. 

One mechanism for surface area loss is crystallite migration, for which Kinoshita et al.66 concluded that the mechanism of surface area loss was two-dimensional Ostwald ripening by means of ad-atom migration on the carbon surface. Nevertheless, trap sites for the migrating ad-atoms on the surface of the carbon can produce nucleation points for generation of... 

Results indicate that deposition is facilitated on active (copper, NijCr) compared to inert substrates. This is attributed to an easier decomposition of the precursor on copper or NijCr rather than on glass or carbon surfaces. Hydrogen can be dissociatively adsorbed on NijCr substrates enhancing the decomposition of NiCp. On copper substrates, the dissociative adsorption of hydrogen is not easy. In that case, the enhanced nucleation is certainly due to 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]. 

This general scheme of surface nucleation processes, as described above, may be adequate only for nucleation on a perfect substrate. It is well known that in many important practical situations, nucleation occurs at defect sites on the substrate surface. In addition, the interactions of gas-phase species with the substrate surfece in diamond CVD may lead to surface carbon atoms of different chemical bonding states and structures, for example, sp, sp, or sp bonded carbon, amorphous carbon, diamond-like carbon or carbon in carbides. These factors further increase the difficulty in understanding surface nucleation processes of diamond in CVD. 

Hofmann S, Sharma R, Ducati C, Du G, Mattevi C, Cepek C, et al. In-situ observations of catalyst dynamics during surface-bound carbon nanotube nucleation. Nano Lett 2007 7 602-8. 

Lopez and Wilkes [73] have reported that the low temperature of crystallization and low-molecular-weight PPS favored surface nucleation, and the nucleating ability of carbon fibers was dependent on the polarity of the carbon surface. Also, an inverse relationship was observed between the value of polar component of surface energy and the nucleating efficiency [72]. 

Krishnan A, Dujardin E, Treacy MMJ, Hugdahl J, Lynum S, Ebbesen TW. Graphitic cmies and the nucleation of curved carbon surfaces. Nature. 1997 388 451. 

Qualitative examples abound. Perfect crystals of sodium carbonate, sulfate, or phosphate may be kept for years without efflorescing, although if scratched, they begin to do so immediately. Too strongly heated or burned lime or plaster of Paris takes up the first traces of water only with difficulty. Reactions of this type tend to be autocat-alytic. The initial rate is slow, due to the absence of the necessary linear interface, but the rate accelerates as more and more product is formed. See Refs. 147-153 for other examples. Ruckenstein [154] has discussed a kinetic model based on nucleation theory. There is certainly evidence that patches of product may be present, as in the oxidation of Mo(lOO) surfaces [155], and that surface defects are important [156]. There may be catalysis thus reaction VII-27 is catalyzed by water vapor [157]. A topotactic reaction is one where the product or products retain the external crystalline shape of the reactant crystal [158]. More often, however, there is a complicated morphology with pitting, cracking, and pore formation, as with calcium carbonate [159]. 

Scaling is not always related to temperature. Calcium carbonate and calcium sulfate scaling occur on unheated surfaces when their solubiUties are exceeded in the bulk water. Metallic surfaces are ideal sites for crystal nucleation because of their rough surfaces and the low velocities adjacent to the surface. Corrosion cells on the metal surface produce areas of high pH, which promote the precipitation of many cooling water salts. Once formed, scale deposits initiate additional nucleation, and crystal growth proceeds at an accelerated rate. 

A new, low-pressure, plasma-assisted proeess for synthesising diamonds has been found by Roy et al [83,84]. An intimate mixture of various forms of carbon with one of many metals (e.g., Au, Ag, Fe, Cu, Ni) is exposed to a microwave plasma derived from pure hydrogen at temperatures ranging from 600-1000 °C. Roy et al postulate a mechanism in which a solid solution of atomic hydrogen and the metal. Me, facilitates dissolution of carbon to form molten droplets of Me -Cj,-H. Diamonds nucleate at the surface of the droplets as the temperature is reduced. 

Fig. 13. Flypothetical growth process of SW tubes from a metal/carbon alloy particle (a) segregation of carbon toward the surface, (b) nucleation of SW tubes on the particle surface and, (c) growth of the SW tubes.

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