Pulse-front propagation and interaction during the growth of CdS nanoparticles in a gel

We studied the spatiotemporal dynamics of a new system consisting of sulfide ions (outer electrolyte) diffusing into an organic gel (gelatin) containing mercaptoethanol-capped cadmium ions (inner electrolyte). The product, cadmium sulfide, exhibits a faint yellow transparent propagating front starting at the gel-outer electrolyte interface. When subjected to UV light, this system reveals fluorescing CdS nuclei localized spatially in a narrow region, called pulse, that leads the front and propagates down the tube. We show that the pulse consists of CdS nanoclusters of an average size of about 4 nm, whereas the trailing front consists of 6-8 nm cubic-phase CdS crystallites. The width of the pulse remains constant in time, t, at about 2 mm and independent of the outer concentration S0. It was found that the speed of the pulse fluctuates as the concentration of the capping agent is varied, with fastest pulses attained at a concentration of 40 mM for two different outer concentrations of sulfide ions. The origin of the yellow fluorescence of the pulse originates from emission from surface states. This dynamical system was then theoretically studied using a competitive particle growth model. The resulting evolution equations were solved numerically, and the results were compared to the experimental findings. It was shown that the model agrees in many aspects with the experiment. The densities of small particles and large particles rho were shown to evolve like a pulse and a front, respectively. The front was shown to extend "diffusively" as t1/2, as found experimentally. The distance traveled by the pulse xpeak was shown to increase with outer concentration S0 and obeys a concentration power law xpeak approximately S(0)1/4. The width w of the pulse also obeys a time power law w approximately ta with a crossover between early times (a=1/3) and intermediate times (a=0). This system would enable us to study the early time dynamics of Liesegang systems.