Effective Structural Color Generation by Highly Ordered and Stacked Nanoparticle Layers of Titanium Dioxide Modified with Long-Chain Carboxylic Acids

The aim was to easily achieve clearer structural colors than ever before by layering nanoparticles by using the Langmuir-Blodgett method. The outermost surface of titanium dioxide (TiO₂) particles with a particle diameter of 15 nm was modified with long-chain fatty acids to synthesize organo-modified TiO₂. The organo-modified TiO₂ was spread on the air/water interface, and the obtained single-particle layer was laminated with different numbers of layers to obtain stepped multiparticle layers with thicknesses of 400–800 nm. The obtained multiparticle layer exhibited a clear structural color gradation. Compared with previous studies involving the use of triiron tetroxide and nanodiamonds with a particle diameter of 5 nm, TiO₂ exhibited a clear structural color without the need for pretreatment. The organo-modified TiO₂ multiparticle layer formed a highly ordered layer structure demonstrating 11th-order reflection in out-of-plane X-ray diffraction. Additionally, compared to other systems, the modification rate was high and the in-plane packing of the modified chains was highly advanced. However, the hue changed when the samples were annealed at 50 °C for 50 h. A method was subsequently proposed to introduce hydrogen-bonding functional groups into the modified chains to suppress this behavior. The pursuit of ease and stability in the expression of structural colors using nanoparticles will lead to future applications in cosmetic science.