Solvents are widely used in crystallization, but their effects on the shape development of crystals are under debate. Here, we report a view on how solvents play their role by considering the viscosity of solvents. We synthesize silver particles in a mixture of alcohol and water. The viscosity of the solvent is changed by varying the volume ratio of alcohol in water. With the variation of viscosity, diverse morphologies of silver particles are synthesized. Small cubic crystals are formed at low viscosity, while hierarchical flower-like particles are formed at high viscosity. Two alcohols are employed, namely ethanol and propanol. No matter which alcohol is employed, the results are similar. Due to the limitation of mass transfer at high viscosity, the particles synthesized in such solvent are smaller and stabilized by the solvent. When the solution containing these particles are dropped onto metal substrates, these tiny particles firstly aggregate, which is followed by classic crystallization, forming flower-like hierarchical structures. These findings show the importance of the viscosity of solvents in shaping particles, which is underestimated previously.

Rational synthesis of materials is a long-term challenging issue due to the poor understanding on the formation mechanism of material structure and the limited capability in controlling nanoscale crystallization. The emergent in situ electron microscope provides an insight to this issue. By employing an in situ scanning electron microscope, silver crystallization is investigated in real time, in which a reversible crystallization is observed. To disclose this reversible crystallization, the radicals generated by the irradiation of electron beam are calculated. It is found that the concentrations of radicals are spatiotemporally variable in the liquid cell due to the diffusion and reaction of radicals. The fluctuation of the reductive hydrated electrons and the oxidative hydroxyl radicals in the cell leads to the alternative dominance of the reduction and oxidation reactions. The reduction leads to the growth of silver crystals while the oxidation leads to their dissolution, which results in the reversible silver crystallization. A regulation of radical distribution by electron dose rates leads to the formation of diverse silver structures, confirming the dominant role of local chemical concentration in the structure evolution of materials.