Enhanced Trapping Efficiency in Acid-Treated Silica Nanostructures

The color of the photoluminescence (PL) of silica nanostructures changes remarkably from blue to bluish green, with the addition of mineral acids. This remarkable observation is rationalized in light of an excited-state protonation of some of the trap states, as well as electrostatic assistance provided by the H<sub>3</sub>O<sup>+</sup> ions adsorbed at the surface. A combination of these two leads to more efficient trapping. The enhanced trapping is manifested in the time-resolved PL parameters and spectra. A fast decay of PL is observed in the higher-energy region of the spectrum, while a rise, with an almost equal time constant, is observed at the lower-energy end. Time-resolved area-normalized emission spectra are dominated by a structured spectrum in the high-energy region at shorter times and a broad, red shifted spectrum at longer times.