Estudo sobre a substituição de ligantes de superfície em nanocristais de ZrO2

Abstract

To produce metal oxide nanocrystals with narrow size distribution, specific crystalline phase, and desired morphology, the solvothermal chemical synthesis route emerges as a successful alternative, but it is necessary to follow very specific synthesis protocols. Required to control growth and to stabilize nanocrystals, species from the reaction medium chemically bond to the surface. Replacing surface binders originating from synthesis with new binders means rationalizing applications and stable dispersion in different solvents. The functionalization of zirconia nanocrystals has been explored in order to optimize the properties of materials in nanocomposites and also to provide stable colloidal dispersion in polar and non-polar solvents. In the literature, different reactional approaches were used to modify the surface, allowing the functionalization of ZrO2 nanocrystals with different surface ligands and enabling new applications. In this sense, the objective of this study was to advance the understanding of the functionalization of ZrO2 nanocrystals produced using benzyl alcohol as a solvent, to evaluate the influence of different reaction methods on the surface composition when replacing the covering binders and to verify the colloidal stability in solvent nonpolar organic. Infrared spectroscopy and thermal analysis were used to study the benzoate and butanoate ligands, derived from benzyl alcohol, butanol and butoxide from the zirconium precursor. Additionally, different reactional approaches were explored for the exchange of ligands from the synthesis of nanocrystals by oleate ligands, and were based on protocols reported in the literature, in which all were able to functionalize zirconia nanocrystals with oleic acid. It was identified by FTIR that the thermal treatment is essential for the removal of the original ligands and promotes the isomerization of the oleate (from cis to trans). The excess of the precursor of the ligand species and a longer reaction time also improves the colloidal stability and help in a greater substitution of the ligands. Pretreatment with ultrasound and a shorter reaction time also promoted functionalization with oleic acid, but they have low colloidal stability in toluene and a significant fraction of native ligands from the production of nanocrystals remain on the surface. Furthermore, washing ZrO2 nanocrystals with acetone before functionalization is detrimental to colloidal stability in nonpolar solvent. Thus, this work contributes to the understanding of surface ligand exchange processes and to the reproducibility of the functionalization of ZrO2 nanocrystals, revealing the different compositions of surface ligands for the different substitution reaction methods and allowing the obtainment of colloidal dispersions stable.