Abstract
Structural colors originate by constructive interference following reflection and scattering of light from nanostructures with periodicity comparable to visible light wavelengths. Bright and noniridescent structural colorations are highly desirable. Here, we demonstrate that bright noniridescence structural coloration can be easily and rapidly achieved from suspended two-dimensional nanosheets of a clay mineral. We show that brightness is enormously improved by using double clay nanosheets, thus optimizing the clay refractive index that otherwise hampers structural coloration from such systems. Intralayer distances, and thus the structural colors, can be precisely and reproducibly controlled by clay concentration and ionic strength independently, and noniridescence is readily and effortlessly obtained in this system. Embedding such clay-designed nanosheets in recyclable solid matrices could provide tunable vivid coloration and mechanical strength and stability at the same time, thus opening a previously unknown venue for sustainable structural coloration.
INTRODUCTION
Structural colors arise when photonic waves interfere constructively following reflection and scattering from nanostructures with distances comparable to wavelengths of visible light (1). The structural coloration mechanism is fundamentally different from the absorbance of dyes or pigments. With structural colors, the material might be semitransparent, and the color spectrum may be tuned by adjusting the nanostructures. This mechanism, most often in combination with light-absorbing dark pigments, is a major biological coloration mechanism found in nature, featured in birds, marine animals, some mammalian species, insects, and certain plants (2–8).
Bright and noniridescent structural colorations are highly desirable. Several approaches to obtain them are reported in the literature. Gabriel et al. (9) used a lamellar system of solid acids to obtain a nematic phase that can be aligned using a magnetic field and displays a bluish color. More recently, Mouri et al. (10), working on niobate nanosheets, showed structural colors by adjusting the electrolyte concentration. Structural colors can also be obtained using block copolymer micelles, which can have tunable particle sizes and be dried and stored (11). Bioinspired cellulose-based materials have also demonstrated this capacity (11, 12). In addition, as discussed by Barty-King et al. (13), small amounts of biodegradable replacements, such as polystyrene particles, are not necessarily harmful, as long as they are fully functional replacements.
Structural coloration sparks an enormous effort within the industrial sector to incorporate them into everyday products. L’Oreal’s photonic cosmetics and Morphotex are representative bioinspired designs (14). The Lexus LC blue (15) is a spectacular example of scaled-up structural coloration; however, according to Lexus, it takes 8 months to fabricate a sufficient quantity of pigments to cover 300 cars (15). The pigment’s fabrication time and abundance are major and general obstacles for industrial upscaling. Fast and simple preparation together with incorporation of abundant sustainable materials in industrial structural coloration could promote upscaling and help to achieve the goals of circular economy.
One factor of importance for structural coloration is iridescence (3, 5–7). It is related to the quality of periodicity realized in the one-dimensional (1D), 2D, or 3D photonic crystals (1, 7, 16, 17). For the noniridescent colors, some degree of disorder should be present at the nanoscale level (16). One example of this is the feathers of blue birds and the wings of blue butterflies, which were successfully mimicked using colloidal particles
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