Synthesis of monodisperse single-crystal Cu2O spheres and their application in generating structural colors

Monodisperse semiconductor single-crystal spheres hold great potential for building photonic crystals owing to their higher refractive index and smoother surface than their corresponding polycrystalline ones. However, the difficulty in obtaining isotropic growth with a slower rate of nucleation and growth makes it challenging to synthesize highly uniform single-crystal spheres with sizes of several hundred nanometers. Herein, we designed a room temperature two-step reductive approach in the presence of both sodium citrate and PVP as capping ligands in a H2O/ethylene glycol (EG) mixed solvent. By this method, real monodisperse single-crystal solid Cu2O spheres were obtained. The key functions of EG, sodium citrate and PVP in the formation of single-crystal Cu2O spheres were investigated. The mechanism of isotropic growth of the monodisperse single-crystal solid Cu2O spheres is discussed in detail. The results indicate that copper citrate complex, PVP, hydrogen bonding and appropriate viscosity are all indispensable for isotropic growth. The diameter of the as-prepared single-crystal solid Cu2O spheres could be tuned from 168 nm to 265 nm by varying the amount of sodium citrate. The SEM and TEM images revealed that the prepared Cu2O spheres had a narrow size distribution and are single-crystal solid spheres. Using Cu2O spheres with different diameters as building blocks, amorphous photonic structures with vivid brown-red, green and pink colors were fabricated by a spray-coating method. Our results provide a method for preparing single-crystal solid semiconductor spheres, unveiling the possibility of using single-crystal spheres as building blocks for creating structural color.