Influence of\nSodium Ions and Carbon Black on the Formation\nand Structural Color of Photonic Balls by Silica Particles

In this study, photonic ballsspherical aggregates\nof submicrometer-sized\nsilica particles with uniform particle sizewere investigated\nas structural colored materials. The structural color of these photonic\nballs is influenced by the ordered arrangement of the silica particles.\nThe research focused on how the addition of electrolytes, specifically\nNaCl, affects the formation of photonic balls to achieve the desired\nstructural color. Without NaCl, the photonic balls formed onion-shaped\ncolloidal crystals. At NaCl concentrations above 0.006 mol/L, the\nparticles aggregated into short-range ordered structures. When the\nconcentration exceeded 0.05 mol/L, the aggregates lost their spherical\nshape. The study also explored the addition of carbon black (CB),\na water-dispersible material due to its surface charge. The findings\nrevealed that NaCl induced the phase separation between the charged\nsilica particles and CB, resulting in Janus-shaped photonic ballsone\nside exhibiting structural color and the other side appearing black\ndue to the presence of CB. Changing the silica particle size altered\nthe hues of these Janus-shaped photonic balls, though they appeared\nuniformly colored to the naked eye. While this study did not specifically\nexamine the applications of Janus-shaped photonic balls composed of\nsilica particles and CB, CB is known for its ability to absorb near-infrared\nradiation and convert it into heat as well as its conductive properties.\nSilica, on the other hand, has a low thermal conductivity and acts\nas an electrical insulator. The structurally colored Janus-shaped\nphotonic balls created in this study may serve as pigments in applications\nrequiring anisotropic heat generation and electrical conduction. Additionally,\nthe study’s findings suggest the potential for creating various\ntypes of Janus-shaped photonic balls from materials with differing\ndensities.