The surface chemistry of metal oxide nanocrystals : a solution NMR study

Ceramic nanocrystals (NCs) are of general interest because of their potential applications in catalysis, gas sensing, LED’s, and other areas. However, the formed particles need to be processed in order to be used in a specific application. Gaining knowledge about the surface chemistry of the NCs is therefore of special importance and NMR provides us with the tools to address this due to the extensive T2 relaxation of bound ligands. HfO2 NCs are solvothermally synthesized in benzyl alcohol with microwave heating. The surface of the obtained charge stabilized NCs can subsequently be modified with fatty acids and oleylamine to allow solubility in nonpolar solvents.1 During this process, clusters are broken up and the constituent particles are obtained. We present here a detailed study of the fundamental acid/base processes during the surface modification, using 1D proton, NOESY and DOSY NMR. We demonstrate that there is a crucial difference in surface chemistry between metal oxide NCs and the more widely studied chalcogenide NCs, such as PbS and CdSe.2 The electronegative property of oxygen allows for protons to be accommodated at the surface of HfO2 NCs which was shown with the help of exchange experiments with deuterated acid. The binding event of a carboxylic acid on a metal oxide NC is thus a dissociative process, see scheme 1. Subsequently, we show the practical implications of these findings. First, we show that ligand exchange reactions, which were considered impossible, do take place at the metal oxide NC surface. We examined the exchange of X-type ligands for L-type ligands by performing titration experiments which were followed by 1D proton and DOSY NMR. Secondly, we demonstrate that acid catalyzed organic reactions could proceed in the presence of the HfO2 NCs due to the surface attached protons. As an example of such catalytic process, we monitored in situ the esterification of ethanol and oleic acid with 1D proton NMR and characterized the final compound with HMBC, HSQC and COSY. In conclusion, metal oxide NCs hold protons on the surface, in contrast to metal selenides or sulfides. This property allows unexpected catalytic activity and ligand exchange to take place. 1. De Roo, J.; De Keukeleere, K.; Feys, J.; Lommens, P.; Hens, Z.; Van Driessche, I. J. Nanopart. Res. 2013, 15, 1778. 2. De Roo, J.; Van den Broeck, F.; De Keukeleere, K.; Martins, J. C.; Van Driessche, I.; Hens, Z. J. Am. Chem. Soc. 2014, 136, 9650-9657