Inverse opal SiO₂ photonic crystals as structurally-colored pigments with additive primary colors

Porous silica, structured as three-dimensionally ordered macroporous (3DOM) photonic crystal particles, is investigated as a pigment material with structural color. Brightly and uniformly colored 3DOM SiO₂ powders were obtained by templating of a tetraethoxysilane-containing precursor in polymeric colloidal crystals, pyrolysis in an inert atmosphere to maintain a small amount (ca. 5-8 wt%) of residual carbon as a background absorber, and particle size reduction. The color depended on placing optical stop bands in the appropriate range of the visible spectrum. This was achieved through selection of the polymer sphere diameter in the colloidal crystal template and finetuning by the pyrolysis temperature, which controls the extent of condensation and shrinkage of the 3DOM SiO₂ structure. The stop bands shifted to shorter wavelengths, and the chroma of these pigments increased with increasing pyrolysis temperature in the range from 475 to 900 °C. Using this temperature control, it was possible to obtain materials with a variety of colors and chroma using a single template sphere size. In addition, the assortment of spectral colors could be extended by mixing 3DOM SiO₂ pigments with primary colors (red, green, blue) via a unique additive color-mixing mechanism, which distinguishes these pigments from most conventional pigments that rely on subtractive color mixing. These approaches enable the preparation of pigments with multiple colors using a single material composition, providing opportunities to replace potentially harmful dyes and pigments with nontoxic and relatively inert silica.