Surface Modification of ZrO₂ Nanoparticles with TEOS to Prepare Transparent ZrO₂@SiO₂-PDMS Nanocomposite Films with Adjustable Refractive Indices

Here, highly transparent nanocomposite films with an adjustable refractive index were fabricated through stable dispersion of ZrO₂ (n = 2.16) nanoparticles (NPs) subjected to surface modification with SiO₂ (n = 1.46) in polydimethylsiloxane (PDMS) (n = 1.42) using the Stöber method. ZrO₂ NPs (13.7 nm) were synthesized using conventional hydrothermal synthesis, and their surface modification with SiO₂ (ZrO₂@SiO₂ NPs) was controlled by varying the reaction time (3-54 h). The surface modification of the NPs was characterized using Fourier-transform infrared spectroscopy, dynamic light scattering, X-ray photoelectron spectroscopy, scanning electron microscopy, transmission electron microscopy, and ellipsometry. The surface modification was monitored, and the effective layer thickness of SiO₂ varied from 0.1 nm to 4.2 nm. The effective refractive index of the ZrO₂@SiO₂ NPs at λ = 633 nm was gradually reduced from 2.16 to 1.63. The 100 nm nanocomposite film was prepared by spin-coating the dispersion of ZrO₂@SiO₂ NPs in PDMS on the coverslip. The nanocomposite film prepared using ZrO₂@SiO₂ NPs with a reaction time of 18 h (ZrO₂@SiO₂-18h-PDMS) exhibited excellent optical transparency (T_average = 91.1%), close to the transparency of the coverslip (T_average = 91.4%) in the visible range, and an adjustable refractive index (n = 1.42-1.60) as the NP content in the film increased from 0 to 50.0 wt%.