Almost Complete Removal of Ceria Particles Down to 10 nm Size from Silicon Dioxide Surfaces

It has been very difficult to remove smaller ceria particles from silicon dioxide surfaces. One of the likely reasons is that the smaller ceria particles with a higher surface concentration of Ce3+ are more strongly coupled with silicon dioxide surfaces via strong Ce-O-Si bonding. Here, we show that some equimolar solutions containing H2O2 and either NH4OH or KOH, aided by ultrasonic cleaning, and at high pH can almost completely remove even 10 nm sized ceria particles from silicon dioxide film surfaces. The particles on the oxide surface before and after cleaning were imaged using atomic force microscopy and counted. The optimal composition of H2O2-NH4OH solutions was identified by investigating ten solutions with different compositions using an augmented simplex-centroid method along with four H2O2-KOH solutions. Equimolar mixtures of H2O2 and NH4OH or KOH with the highest possible perhydroxyl ion concentration [HO2−] showed ∼99% cleaning efficiency for 10 nm ceria particles. We propose that the strong Ce-O-Si bonding can be ruptured effectively by the nucleophile HO2−, formed by the decomposition of H2O2 in the high pH environment, and show that [HO2−] is highest in equimolar solutions. The cleaning efficiencies for 10 nm, 30 nm and 90 nm particles correlate very nicely with [HO2−].