Robust Structurally Colored Coatings Composed of Colloidal Arrays Prepared by the Cathodic Electrophoretic Deposition Method with Metal Cation Additives

Abstract

Structurally colored coatings composed of colloidal arrays of monodisperse spherical particles have attracted great attention owing to their versatile advantages, such as lowcost, 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 using cathodic electrophoretic deposition (EPD). 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 OHions are generated by the decomposition of dissolved oxygen and NO3ions. 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 structural color as a colorant.

High Heat Resistance of the Structural Coloration of Colloidal Arrays with Inorganic Black Additives

“…Uniform coating films composed of SiO 2 particles were obtained via cathodic EPD using Mg 2+ for 5 min under a 30 V applied voltage. All of the coatings exhibited high robustness attributed to the codeposition of the Mg­(OH) 2 binder Figure depicts photographs and scanning electron microscopy (SEM) images of coatings prepared from SiO 2 particles and various black additives.…”

“…Both CB and Fe 3 O 4 were difficult to find in the SEM images because of their small sizes and small amounts. , All of the films remained colloidal amorphous, regardless of the type and size of the black additives. As reported previously, amorphous arrangement was induced by cathodic EPD using Mg 2+ , and comparable colored coatings containing various black additives had no fine structure differences.…”

“…The coating sols of EPD were prepared according to the literature . Briefly, 0.70 g of monodispersed spherical SiO 2 particles (diameters: 200, 260, and 300 nm) was dispersed in 2-propanol (80 mL) by ultrasonic irradiation.…”

“…The color contrast generated from colloidal amorphous arrays is usually very low because of strong incoherent light scattering across the entire visible-wavelength range. To overcome this, black additives that absorb white light are essential to obtain brilliant structural colors. − Carbon black (CB) nanoparticles, − graphene, polydopamine particles, melanin, cuttlefish inks, and iron­(II,III) oxide (Fe 3 O 4 ) nanoparticles − are commonly used as black additives for structurally colored colloidal arrays. However, these additives have poor thermal stability, making it difficult to use structurally colored colloidal arrays in place of inorganic pigments.…”

“…Therefore, colloidal amorphous arrays were employed for the evaluation in this study, although their coloration saturation is low. To evaluate uniform colloidal amorphous arrays of SiO 2 particles, coating films prepared via the electrophoretic deposition (EPD) method ,, were examined. Calcium manganese-based oxide (CCMO), chromium–iron–cobalt–nickel oxide (CFCNO), and lanthanum manganite (LMO) were synthesized and investigated as metal oxide black additives.…”