Detrimental Effect and Neutralization of in Situ Produced Water on Zirconia Nanoparticles Obtained by a Nonaqueous Sol–Gel Method

In this work, the phase purity and size of zirconia nanocrystals samples were studied in terms of zirconium concentration, added water content, and subsequent use of a post solvothermal treatment. The progressive tetragonal-to-monoclinic transformation of zirconia sample was observed to be strongly related to the water content of the alcoholic medium. But more surprisingly, it has been shown that even under initially anhydrous conditions and for particle size below 5 nm, the phase purity of the samples was deteriorated by a side-reaction of alcohol dehydration catalyzed by the surface of the nanoparticles (NPs). Since the phase transformation is essentially driven by the water content of the reaction mixture, we have shown that it was possible to recover an excellent phase purity without the help of any usual dopants by adding a strong alkaline desiccating agent. Provided that a sufficient sodium to zirconium ratio was ensured, the formation of the monoclinic phase was not observed whatever the zirconium precursor concentration. The effectiveness of this cure was related to the ability of sodium metal to generate reactive alkoxide able to neutralize water and to catalyze an alternative sol-gel mechanism leading to the formation of the t-ZrO2 NPs.