Stability testing of silver nanodisc suspensions for solar applications

In solar and optical applications, nanofluids can be exposed to intense light, high temperature, and variable pH levels. Over time these forces can destabilize particle suspensions by altering particle morphology, breaking functional groups, and inducing agglomeration. Since UV exposure has been a critical unknown for nanoparticle suspension stability in solar and optical applications, a modified ISO standard UV accelerated lifetime test method was developed and applied herein. Aqueous formulations of protective silica shells over silver nanodiscs (relative to unprotected silver) were investigated as indicative non-spherical nanoparticles that might be expected to survive these perturbations. As such, the dispersion stability of these suspensions was investigated before and after exposure to elevated temperature, UV light, and pH levels. Dispersion stability was determined using a few techniques - changes in spectral transmission, zeta potential, and particle size distribution via nanoparticle tracking analysis. A protective shell of silica deposited via a modified Stober (using tetraethylorthosilicate - TEOS) method was found to provide stability against temperature, UV, and pH exposure, whereas uncoated silver nanoparticles or those with a shell grown using a sodium silicate silica source were considered relatively unstable. The TEOS silica shells also exhibited a beneficial UV curing effect, which can be explained by increased crosslinking throughout UV exposure.