Synthesis of silica nanofibers for visible light scattering applications

Abstract

Silica nanoparticles have been widely studied for their various morphologies and broad optical applications. Among them, curly silica nanofibers (NFs) have attracted increasing interest due to their random morphologies and strong visible light scattering abilities. However, their synthesis, typically via the water-in-oil emulsion method, still lacks a comprehensive understanding of the underlying growth mechanism.

In this thesis, silica NFs with randomly curly morphologies are synthesized via the water-in-oil emulsion method. NFs sprout from small aqueous droplets stabilized by sodium citrate in 1-pentanol, where the hydrolysis and condensation of precursors drive silica growth. Due to their sufficiently small size, both the aqueous droplets and the growing silica undergo Brownian motion. However, these motions are non-synchronized because of differences in size and density, making the droplets more sensitive to Brownian motion than the silica. Consequently, the silica growth follows the stochastic trajectories of the droplets, ultimately leading to randomly curly morphologies of NFs. This mechanism also explains the observed negative correlation between NF curvature and diameter, enabling morphological control through temperature adjustment. The resulting NFs are assembled into porous network films that exhibit strong visible light scattering and high whiteness, with broadband reflectance exceeding 0.8, outperforming both commercial papers and films composed of conventional nanospheres or nanorods. In addition, these NFs are embedded into polymer matrixes to fabricate films with strain-induced tunable haze. Upon the application and release of strain, the formation and disappearance of internal cavities within the film change the refractive index contrast, thereby modulating visible light scattering and haze. Finally, silica fluorescent nanofibers (FNFs) are synthesized, which can be excited by ultraviolet A (UVA) light. Taking advantage of high whiteness of NFs when dry, which transforms to transparency when wet, along with the fluorescent properties of FNFs, a two-factor authentication (2FA) optical security system is constructed for encryption application. This system responds to liquid and UVA light in sequence, thereby requiring two independent and sequential keys for decryption, providing a relatively secure and advanced strategy for information protection application.

In summary, this thesis offers insights into the synthesis of randomly curly silica NFs, reveals the Brownian motion-driven mechanism underlying their formation and morphological control, studies their visible light scattering properties including whiteness and haze, and demonstrates their functional application in optical security. Together, these findings provide a theoretical foundation and valuable reference for future research in this field.