# Structural Colors Based on Amorphous Arrays Comprised Solely of Silica Particles
## 3 Experimental Results and Discussion  
### Spherical Silica Nanoparticles  
Figure 3 shows the XRD patterns for the products synthesized by the sol-gel process. Each pattern shows a peak in the range of 15-30\({}^{\circ}\). No additional peaks were found in other regions. The trend for these XRD patterns was the same as that for the silica obtained by the sol-gel process reported by [PERSON] et al. [20]. The measured pattern (88%) was consistent with the SiO\({}_{2}\) pattern reported in the crystallography open database (COD, no. 96-901-3492) [21]. We could confirm that the material produced using this method was SiO\({}_{2}\).  
Figure 4 presents the SEM images of the silica powder. Figure 4a shows a silica powder synthesized without the phenol-formaldehyde resin, and the silica particles show uniform spherical shapes and sizes. On the other hand, the silica powders in Figure 4b-d are synthesized with phenol-formaldehyde resin. Spherical or atypical particles were formed. Unlike Figure 4a, some necks are observed between the nanoparticles. This result was due to the phenol-formaldehyde resin.  
Figure 4: Scanning electron microscopy (SEM) photographs of the silica powders: (**a**) No added phenol-formaldehyde resin (Case 1); (**b**) 0.027 g of added phenol-formaldehyde resin (Case 2); (**c**) 0.030 g of added phenol-formaldehyde resin (Case 3); (**d**) 0.033 g of added phenol-formaldehyde resin (Case 4).  
Figure 3: X-ray diffraction (XRD) patterns of the silica powders. Case 1: No added phenol-formaldehyde resin; Case 2: 0.027 g of added phenol-formaldehyde resin; Case 3: 0.030 g of added phenol-formaldehyde resin; Case 4: 0.033 g of added phenol-formaldehyde resin.  
Figure 5 indicates the particle size distribution of the manufactured silica powders. Each silica powder was mixed with DI water at a concentration of 0.01 g/mL to determine the size of the particles. Subsequently, the mixture was ground in an ultrasonicator for 15 min. Next, the size of the silica particles was measured using a nano-micro particle size analyzer. The average size of the spherical silica nanoparticles without the phenol-formaldehyde resin was 435 nm (Case 1). Meanwhile, the average size of nanoparticles with the phenol-formaldehyde resin was 359 nm, 375 nm, and 502 nm, respectively (Case 2-4). Consequently, the average size of the silica nanoparticles increased with the increasing phenol-formaldehyde resin content.  
Figure 6 shows photographs of the manufactured silica powders. The silica powder of Figure 6a shows an ivory white color typical of silica. The reason for this color was that light scattering off a single particle passes through the surrounding particles, causing multiple scatterings such that the intensity of the scattered light in all wavelengths is the same. Figure 6b-d shows the various structural colors depending on the sizes of the spherical silica nanoparticles. The spherical silica nanoparticles show a blue color at 359 nm, a green color at 375 nm, and red color at 502 nm. Owing to particle aggregation by neck formation and the presence of atypical silica in Figure 4b-d, silica powders show structural color. Aggregated particles suppress multiple scatterings and increase the size of the specific wavelength [14; 18].  
Figure 5: Particle sizes of synthesized silica distributions. No added phenol-formaldehyde resin (Case 1); 0.027 g of added phenol-formaldehyde resin (Case 2); 0.030 g of added phenol-formaldehyde resin (Case 3); 0.033 g of added phenol-formaldehyde resin (Case 4).  
Figure 6: Photographs of the silica powders: (**a**) No added phenol-formaldehyde resin (Case 1); (**b**) 0.027 g of added phenol-formaldehyde resin (Case 2); (**c**) 0.030 g of added phenol-formaldehyde resin (Case 3); (**d**) 0.033 g of added phenol-formaldehyde resin (Case 4).