# Structural Colors Based on Amorphous Arrays Comprised Solely of Silica Particles
## 1 Introduction  
Generally, the color of a substance is determined by its dyestuff and pigment. However, the colors of these chemicals can degrade after prolonged exposure to ultraviolet radiation, which can lead to environmental pollution and product cost issues. Structural colors are gaining attention as a way to solve the problems above. The structural color is composed of micro-structures with intervals at the wavelengths level of light. This structure causes a constructive interference effect for a specific wavelength of light and destructive interference for the other light wavelengths [1; 2; 3; 4; 5]. Additionally, structural color is generated by the Bragg's diffraction condition, upon occasion, producing a rainbow of colors [6]. Structural colors have physical properties that can overcome the limitations of color expression, which occur when conventional chemically colored artificial pigments are used. Moreover, it has the advantages of environmental friendliness, productivity, low cost, and high sustainability [7; 8; 9; 10]. Structural color expression in colloidal particles is classified into two types: a regular array and an irregular array of particles. Structural color with a regular array of particles changes color depending on the viewing angle. This material is called a photonic crystal. The structural color associated with an irregular array of particles is less dependent on the viewing angle. This substance is called a photonic glass [11; 12; 13]. In particular, photonic glasses without angle dependency have the same optical characteristics in all directions. These characteristics can increase the energy efficiency of solar cells and can be used in paint or cosmetic materials with super-hydrophobicity and high sustainability. Over the past two decades, research on photonic glasses has been actively pursued. [PERSON] et al. [14] fabricated structural color films on glass coverslips using spin-cast spherical polystyrene particles with average diameters of 226 nm and 271 nm. The produced film did not change color with thechange in the viewing angle and it appeared blue-green. Additionally, spherical polystyrene particles and carbon black were mixed to express structural color intuitively, where the carbon black content controlled color saturation. Similarly, in research on carbon particles, [PERSON] et al. [15] attempted to induce an amorphous array of micrometer-sized spherical silica particles by rapid evaporation of the solvent. However, iridescence appeared due to the crystalline arrays. To solve this problem, the authors induced an amorphous array by mixing silica particles and carbon black. This mixture resulted in the fabrication of structural color without angle dependency, with different colors arising depending on the size of the silica particles. In another case, [PERSON] et al. [16] created structural colors by producing monodispersed spherical aggregates using micro-flow-focusing devices. The various colors produced depended on the concentration of colloidal silica, the size of the silica particles, and the amount of magnetite colloidal particles used. In recent years, [PERSON] et al. [17] finely controlled the ratio by mixing spherical polystyrene particles and cuttlefish ink particles to obtain various high purity colors. Additionally, [PERSON] et al. [18] used a centrifuge to replace the solvent of the colloidal particles with ethanol. They then dispersed it on a substrate to form colloidal aggregates with non-iridescent properties. However, the evaporation process is complicated when a variety of structural colors are fabricated under various conditions. Therefore, this calls for a method that can simplify the formation of a photonic glass without using an additional process or particles. In this paper, a simple method for fabricating structural colors using the sol-gel process with low angle-dependence was proposed, which did not require the addition of other particles. Atypical silica was prepared by adding phenol-formaldehyde resin.