@inproceedings{5736879,
  abstract     = {{Ceramic nanocrystals (NCs) are of general interest because of their possible applications in catalysis, gas sensing, LED’s, nanocomposites, etc. However after synthesis, the formed NCs need to be properly processed in order for them to be used in applications. Hence the preliminary importance to gain knowledge over the surface chemistry of the NCs. In this contribution, we focused on hafnium oxide as a model system and subsequently validated the theory with other oxides.
HfO2 NCs are solvothermally synthesized in benzyl alcohol with microwave heating. The surface of the thus obtained charge stabilized NCs can be subsequently modified with fatty acids and oleylamine to allow solubility in nonpolar solvents. Concomitantly, clusters are broken up, yielding the constituting individual particles. We present here a detailed study of the fundamental (acid/base) processes during the surface modification. 
We also demonstrate that there is a crucial difference in surface chemistry between these metal oxide NCs and the more widely studied metal sulfide and selenide nanocrystals, such as PbS and CdSe. The electronegative property of oxygen allows for protons to be accommodated at the surface of HfO2 NCs and one can see the binding event of a carboxylic acid on a metal oxide NC as a dissociative process. This is in contrast to the case of other chalcogenide NCs where only the carboxylate binds to the surface metal ions and the proton is removed during the binding event. 
Subsequently, we show the practical implications of our fundamental findings. Ligand exchange reactions which before were considered impossible do take place at the metal oxide NC surface, e.g., exchange of X-type ligands for L-type ligands. Even more, due to the surface protons, acid catalyzed organic reactions could proceed in the presence of the HfO2 NCs. This catalytic process was properly investigated for the esterification of ethanol with oleic acid. 
In a second application, all inorganic nanocomposites were synthesized. HfO2 NCs were distributed in a matrix of a High Temperature Superconductor, YBa2Cu3O7-x. In order to do so, the NC surface was modified via ligand exchange to transfer the particles from apolar to polar solutions while maintaining impeccable colloidal stability. The transfer is necessary since the YBa2Cu3O7-x precursor solution is methanol based. The resulting ink (NCs + precursor solution) was deposited via spin coating and after epitaxial film growth the superconducting nanocomposite was retrieved and its superconducting properties were evaluated. 
In conclusion, metal oxide NCs hold protons on the surface, in contrast to metal selenides or sulfides. This characteristic allowed for unexpected catalytic activities and ligand exchanges.}},
  author       = {{De Roo, Jonathan and Van den Broeck, Freya and De Keukeleere, Katrien and Martins, José and Van Driessche, Isabel and Hens, Zeger}},
  booktitle    = {{Belgian Ceramic Society, Annual meeting, Abstracts}},
  language     = {{eng}},
  location     = {{Ghent, Belgium}},
  publisher    = {{Belgian Ceramic Society (BCerS)}},
  title        = {{The surface chemistry of metal oxide nanocrystals: theory and applications}},
  year         = {{2014}},
}