Silica nanoparticles (SNPs) are commonly used to coat materials in order to produce structural colors as well as other beneficial surface modification effects (Gaillou et al.
2008). Structural colors have unique features compared with traditional dyes (Baumberg and Snoswell
2009), which makes producing artificial structural colors desirable (Braun
2011; Fudouzi and Xia
2003). Colloidal crystallization (Gao et al.
2016a,
b; van Blaaderen, Ruel and Wiltzius
1997; Park, Qin and Xia
1998; Lee et al.
2013; Pieranski
1983; Xia et al.
2000) is a frequently applied method when producing artificial structural colors. Synthesizing silica nanoparticles (SNPs) is an established method (Santamaría Razo et al.
2008) for the production of structural color. There are several standard published methods that outline the production of silica nanoparticles, which are illustrated in the disquisition of Tiler in 1979 (Iler and Iler
1979). The most popular method uses four chemicals: ammonia, distilled water, ethanol, and tetraethyl orthosilicate (TEOS) to synthesize the SNPs (Stöber, Fink and Bohn
1968; Giesche
1994). Ammonia is used as the catalyst, with the hydrolysis and condensation occurring between the tetraethyl orthosilicate (TEOS) and distilled water (Gao et al.
2016a,
b; Stöber, Fink and Bohn
1968; Giesche
1994; Gao
2016). In 1956, Kolbe discovered the chemical reaction theory underpinning this method. Since his discovery, substantial research has been focused on this area (Kolbe
1956). In 1968, the study of this reaction system had a breakthrough (Stöber, Fink and Bohn
1968). Stöber, Fink, and Bohn proposed a chemical technique to control the particle size of the silica which was in the micron size range (Stöber, Fink and Bohn
1968; Galisteo-López et al.
2011). This technique is now referred to as the Stöber or SFB method (Stöber, Fink and Bohn
1968). Although the SNPs can be efficiently synthesized using the Stӧber method, the limitation of this method is that the precise prediction of final particle diameter is difficult to achieve (Gao et al.
2016a,
b; Galisteo-López et al.
2011). On the basis of the Stӧber method, several researchers (Iler and Iler
1979; Gao
2016) have published experiments to attempt to predict the diameter of the final particles (Gao
2016). However, the difference between the predicted diameter and the experimental diameter still shows considerable variation (Gao
2016), and their methods cannot be applied to accurately predict the final particle size. In 2016, based on the Stӧber method, Gao et al. proposed the solvent varying technique (SVT) and provided a supporting equation that allowed the user to produce a batch of SNPs with a particular target diameter by only varying the initial amount of ethanol in the solution (Gao et al.
2016a,
b,
2017a,
b). Gao et al. synthesized uniform SNPs by mixing 8 ml ammonia hydroxide, 3 ml distilled water, and a certain calculated initial volume of ethanol (Gao et al.
2016a,
b; Gao
2016). Once the solution was heated to a temperature of 60 °C, 6 ml TEOS was added into the mixture. Using the SVT, SNPs with target diameters of between 207 and 350 nm have been prepared and these particles can be used to produce tunable structural colors by a process of natural gravity sedimentation (Gao et al.
2016a,
b).