What is a nuclear shell?
Preparation method of core shell:
1. The continuous reduction process is as follows: firstly, one metal salt is returned to the original form of "seed" M1 (i.e., nucleus), and then another metal M2 atom is deposited and attached to the surface of the formed M1 metal seed by a process similar to "seed growth", so as to form the core shell structure of m1-m2. Due to two successive reduction processes, it is called "continuous reduction method", also known as "crystal seed growth process", which is a typical strategy of nucleation first and then shell coating. In recent years, continuous reduction has been widely used to synthesize bimetallic nanomaterials with various core-shell structures.
2. Co-reduction method is also known as simultaneous reduction method: namely, there are two kinds of metal salt precursors in the system at the same time. During the reduction process, metal species with higher REDOX potential deposit nucleation first, while metal species with lower REDOX potential deposit nucleation and shell structure on their surface after reduction. This is a typical strategy for forming a core-shell structure in one go. The deposition sequence of the metal during co-reduction can be changed by adding specific complexing agents (surfactants). For example, due to the higher potential of Pd reduction, pd-ag particles are generally formed in the co-reduction of Ag and Pd, while after the addition of ammonia, the opposite ag-pd particles are formed due to the stronger complexation between NH3 and Pd. Due to the limitation of the inherent properties of metal, coreduction method is suitable for the synthesis of core-shell nanometer alloy. Using ethylene glycol as solvent and reductant, Harpeness and Gedanken successfully synthesized au-pd core-shell nanoparticles by using microwave assisted reflux and reducing liquid phase Au3+ and Pd2+.
3, Other auxiliary means: in addition, there are microemulsion synthesis, solvent metal atom dispersion (SMAD), and so on. In general, the liquid phase chemical reduction is a complex reaction system, which involves many factors (temperature, pH, solvent, metal precursor, reducing agent, complexing agent/protective agent types and the impurity ions, etc.), any changes in factors to a certain extent influence on product, such as the average particle size, morphology, dispersion, etc. Some scholars have done a lot of useful work in understanding the effects of synthesis conditions. The intrinsic properties of metals are important factors affecting the formation of bimetallic nanoparticles, especially the microstructure. In the formulation of the synthesis scheme, various influencing factors need to be considered comprehensively to obtain the desired results.