Silver nuclei

Post-fixation physical development is simpler in mechanism than direct development. The fundamental reaction is the reduction of silver ions from a solution of silver salt. This reaction is accelerated by the presence of silver nuclei, and the mechanism of the development is the mechanism of this catalytic process. II. 

Fig. 3. Electron micrographs of colloidal silver nuclei after 21 hours physical development. Gold shadowing at a 20° angle was used to indicate the thickness of the crystals.

The autoacceleration of the silver-catalyzed reaction in the early stages is greater than would be expected from a simple increase in catalyst surface by growth of the silver nuclei orginally present. Apparently new nuclei are readily formed during the reaction. A possible mechanism which could lead to this result is ... 

The essential difference between the hydroxylamine reaction and the hydrazine reaction appears to be that silver nuclei are formed in the solution much more readily by hydrazine than by hydroxylamine. At sufficiently low pH and in the absence of copper, hydroxylamine does not readily form nuclei in the solution, and the catalytic reduction of the silver chloride occurs essentially at a solid interface with the silver nuclei. Hydrazine, on the other hand, readily forms nuclei in the solution and an important fraction of the total reaction involves the catalytic reduction of dissolved silver chloride. This would account for the well-known photographic properties of the two agents. Hydroxylamine is a cleanworking developer which, under proper conditions, yields little fog. Hydrazine shows much less selectivity and, although it develops an image, it also yields a relatively high fog density. 

The silver nuclei constitute an invisible latent image, which can be converted to a visible image by chemical action. The silver halide crystals that contain latent image nuclei can be reduced completely to silver by this chemical action (development) or silver ions from solution can be reduced at these nuclei. The amplification factor... 

The coatings generate physically developable silver nuclei when exposed.172 One example is the merocyanine dye (95). 

In another interesting amplification system, an image in silver or other noble metal nuclei is used for catalysis, and a cobalt(III) complex is used as an oxidant in place of a peroxy compound. The amplification permits less silver to be used. An imagewise distribution of silver nuclei is associated with a non-diffusible dye-forming coupler, and the image is amplified by (1) imbibing the system with a solution containing a color developer such as a p-phenylenediamine and (2)... 

On heating, the silver nuclei formed during light exposure catalyze the decomposition of the silver salt, thereby amplifying the image. 

The 7 = 1/2 nuclear spins of " Ag and Ag have made it possible to study organometallic silver complexes by NMR (a tool that has been less successfiilly used with similar copper complexes), although the sensitivity of the silver nuclei is rather low. 

Dubinin et al. [39] confirmed the autocatalytic participation of product silver nuclei in the decomposition of AgjO. Deposition of a thin layer of Ag or Ni on the reactant increased the rate of breakdown at 603 K, but mechanically-mixed metal provided inadequate contact. Lagier et al. [40] also confirmed the catalytic role of the product metal. The above discussion concludes that an interface reaction is rate limiting. Atomic oxygen was not identified [2,4,41] in the products. 

Many kinetic studies of the thermal decomposition of silver oxalate have been reported. Some ar-time data have been satisfactorily described by the cube law during the acceleratory period ascribed to the three-dimensional growth of nuclei. Other results were fitted by the exponential law which was taken as evidence of a chain-branching reaction. Results of both types are mentioned in a report [64] which attempted to resolve some of the differences through consideration of the ionic and photoconductivities of silver oxalate. Conductivity measurements ruled out the growth of discrete silver nuclei by a cationic transport mechanism and this was accepted as evidence that the interface reaction is the more probable. A mobile exciton in the crystal is trapped at an anion vacancy (see barium azide. Chapter 11) and if this is further excited by light absorption before decay, then decomposition yields two molecules of carbon dioxide ... 

Simmons, W., Gonnissen, D., and Hubin, A. 1997. Study of the initial stages of silver electrocrystallisation from silver thiosulphate complexes Part I. Modelling of the silver nuclei formation during the induction period. Journal of Electroanalytical Chemistry 433, 141-151. 

The reduction of the attached silver precursor ions leads to silver nuclei and nanoparticle deposition onto the polystyrene beads (Scheme 2.4). The as-prepared shining reddish-black, silver-coated beads, [R(Ag)°] " "C1, were washed thoroughly with distilled water and dried at room temperature under vacuum. The mechanism of the above synthetic procedure is represented well. 

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