Robust
Structurally Colored Coatings Composed of Colloidal
Arrays Prepared by the Cathodic Electrophoretic Deposition Method
with Metal Cation Additives
Structurally colored coatings composed
of colloidal arrays of monodisperse
spherical particles have attracted great attention owing to their
versatile advantages, such as low cost, resistance to fading, and
low impacts on the environment and human health. However, the weak
mechanical stability is considered to be a major obstacle for their
practical applications as colorants. Although several approaches based
on the addition of polymer additives to enhance the adhesion of particles
have been reported, the challenge remains to develop a strategy for
the preparation of structurally colored coatings with extremely high
robustness using a simple process. Here, we have developed a novel
approach to fabricate robust structurally colored coatings by cathodic
electrophoretic deposition. The addition of a metal salt, i.e., Mg(NO3)2, to the coating dispersion allows SiO2 particles to have a positive charge, which enables the electrophoresis
of SiO2 particles toward the cathode. At the cathode, Mg(OH)2 codeposits with SiO2 particles because OH– ions are generated by the decomposition of dissolved
oxygen and NO3– ions. The mechanical
stability of the colloidal arrays obtained by this process is remarkably
improved because Mg(OH)2 facilitates the adhesion of the
particles and substrates. The brilliant structural color is maintained
even after several cycles of the sandpaper abrasion test. We have
also demonstrated the coating on a stainless steel fork. This demonstration
reveals that our approach enables a homogeneous coating on a complicated
surface. Furthermore, the high durability of the coating is clarified
because the coating did not peel off even when the fork was stuck
into a plastic eraser. Therefore, the coating technique developed
here will provide an effective method for the pervasive application
of the structural color as a colorant.