Hybrid Materials with Controlled Precipitation Pattern of Nanoparticles
Authors: Itatani Masaki , Fang Qing , Unoura Kei , Nabika Hideki 
Published: February 2020

Hybrid materials with ordered patterns are important to control their functionality. Such ordered structure can be spontaneously formed in nature by self-organization under far from equilibrium, where energies or substances within the system is always dissipated (dS / dt ≠ 0). Although formation mechanism of self-organized patterns in nature are complex and hard to control, development of various materials that mimic self-organization in nature (e.g. bio-mimetics and bio-inspired) have been developed remarkably in recent years. Formation of various patterned structures with useful function by self-organization lead to further development of material chemistry based on mimic of these structures.
In the present study, we focused on Liesegang pattern found by R. E. Liesegang in 1896. Liesegang pattern has gained a lot of interests due to several features; simple procedure of the pattern formation, diversity of the pattern morphology and no selection of materials. In fact, Liesegang pattern can be formed by contact between a gel containing an inner electrolyte and an aqueous solution containing an outer electrolyte. Thus, the formed patters consisted of precipitates which is a reaction product between inner and outer electrolytes, has spatial periodicity with geometric series formulated by spacing law (xn

+1/xn
= 1+p), where xn

+1, xn
, and p are the position of two consecutive (n+1st and n
th) bands measured from the junction point of the electrolytes and spacing coefficient, respectively. Therefore, Liesegang pattern can be applied for patterning technologies because periodic patterns are easily formed. However, control of the periodicity in Liesegang pattern has not been achieved for a long time. Therefore, it is necessary to control periodicity to apply patterning technologies in micro-nanotechnologies.
For this purpose, we examined the influence of experimental parameters on the geometry of pattern formation. We focused on the nucleation rate as one of the parameter, whose effect on the pattern formation has not been clarified yet. In this study, we applied the strategy of surface chemistry to control the nucleation rate. Liesegang pattern formation was discussed based on the nucleation and growth model, where the particle is produced through the production of nuclei (solid state) by nucleation of a certain precursors (molecular state) and the growth of nuclei. To occur the nucleation process, it is necessary to bring the energy state in this system unstable to exceed the energy barrier (ΔG). To achieve this process, the concentration of the precursor should exceed the nucleation threshold (C*). Therefore, ΔG governs the nucleation rate and can be considered C* in the model. can be reduced by the coordination of an appropriate ligand for the decrease of surface free energy. Cysteine was used as the ligand for Agn nuclei and/or Ag nanoparticles in the present system to reduce their surface free energy and thus ΔG.
In the system without cysteine, the periodic concentric ring pattern (Liesegang pattern) with the interring distance of about 100−200 μm was formed (Figure 1a), where the interring distance increased further away from the edge of agarose stamp. On the other hand, in the system with cysteine, the periodic concentric ring pattern (Liesegang pattern) with the interring spacing of below 100 μm was formed (Figure 1b), meaning that the ring periodicity was almost half that of the pattern formed without cysteine. Thus, the addition of cysteine that is corresponding to the decrease of induced the Liesegang pattern with the finer periodic structures. To prove this relationship between the decrease ofΔG and the finer structures, we used reaction-diffusion simulation with lowered C*. As a result, Liesegang pattern became finer with the lowering C*. Furthermore, to demonstrate the effect of the lowering and C* on a miniaturization of Liesegang pattern in more detail, we compared the spacing coefficient as a measure of the periodicity of patterns. As a result, both lowering and C* gave smaller spacing coefficient (i.e., finer periodic patterns) (Figure 2). Therefore, it was suggested that the lowering induced the lowering C*. Then, the smaller spacing coefficient at lowered C* was discussed in terms of a slower rate of Ostwald ripening. Similar control of C* through surface chemistry can be applied to various precipitation systems to control the periodicity in these systems. In his study, we propose that the approach based on the surface chemistry enable us to control the periodicity of Liesegang patterns by the controlling C*.

Figure 1