I am working on the synthesis of 2-D transition metal nano structure by using hydro-thermal route. I am confused about the aging process and also want to learn about the post-synthesis aging? is that affects the size and the structure of nano structure material?
Aging in nanomaterials, leads to aggregation or agglomeration. As you are doing hydrothermal synthesis then synthesis at higher temperature combined with shorter reaction time could give you narrower size distribution of nanoparticles (higher nucleation rate and shorter growth time). Further, you can adjust the temperature, pH, ratio of metal ion, capping agent for post-aging treatment.
You should read this article:
Article Effect of aging on the properties of TiO2 nanoparticle
Article Influence of Aging and Environment on Nanoparticle Chemistry...
When the ceria NPs are synthesized by adding peroxide to the initial solution, the oxidizing agent starts both the oxidation of Ce3+ to Ce4+ and the nucleation and/growth of NPs. It is important to account for the chemical changes in the ceria system during and after the synthesis stages to understand the observed variations in the band edge values of ceria.The formation of perhydroxyl species during the reaction of Ce4+ with hydrogen peroxide has been confirmed by electron spin resonance spectroscopy. The hydroperoxy radical is known to disproportionate in the oxidation and reduction of solutions with cations of multiple valence states.
Our experiments are consistent with these observations indicating that ceria NPs can be stable in either Ce3+ or Ce4+ oxidation state in aqueous media under suitable conditions. Hence, Ce3+ is formed from the Ce4+ oxidation state in the presence of the hydroperoxy species, following the chemical reaction,
Ce4+ + HO2 @ Ce3+ + H+ + O2
Hydrogen peroxide converts the Ce3+ to Ce4+ immediately after it is added to the Ce3+ solution. The Ce4+ ions quickly bond with the available hydroxyl radicals and nucleate the hydrated ceria particles to form a stable suspension at a pH of ∼3.5. After this stage, the solution will become depleted of the Ce3+ ions and will be rich in Ce4+(in the particles) and HO2 radicals. Once the solution is depleted of the oxidizing species, cerium will revert back to its stable oxidation state in aqueous medium, that is, Ce3+ oxidation state. So, we propose that the oxidation state cycling capability of ceria NPs is determined by immediate environment of the particle. This implies that the Ce4+ ions can be reduced and regenerated as the environment changes. Such a mechanism has been suggested in the oxidation of ROS to protect retinal cells.
Our observations indicate that the formation of Ce4+ by reaction with H2O2 is much faster than the reduction by hydroperoxyl species. One reason may be that the reduction reaction involves solid NPs, whereas the initial oxidation reaction involves ions in solution to form solid NPs. Once the NPs have formed, the process of reduction is reversible, and oxidation of the NPs can be induced by the addition of oxidizing species like H2O2.
When the addition of H2O2 to a NP suspension that had been aged for 3 weeks (and contained predominantly Ce3+ NPs) regenerates the Ce4+ oxidation state. However, the process of reoxidation is slower than the initial oxidation reaction. Irrespective of the nature of the NPs − that is, individual particles, small and large agglomerates − we have observed reversible oxidation state changes in various aqueous media.
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