About 50% of the atoms in crystals a few nanometer in diameter are surface atoms, a number highlighting the importance of surfaces in nanocrystal science. In the case of colloidal nanocrystals, attempts to elucidate their surface chemistry quickly followed the development of synthesis procedures. A general picture that applies to a wide range of II-VI and III-V semiconductors was however only established in the last 5 years, where it was found that the ligand coordination is close linked to the nanocrystal stoichiometry.1 Ligands classified as X-type, such as carboxylates, phosphonates or thiolates proved to bind tightly to cation-rich nanocrystals, with an overall ligand:excess cation stoichiometry that makes for charge-neutral nanocrystals.2 Ligands classified as L-type, such as amines or phosphines were typically weakly binding to stoichiometric nanocrystals.3 In this contribution, we extend this prevailing picture by analyzing binding of amines to CuInS2 and of carboxylates to HfO2 and ZrO2 nanocrystals.We use solution 1H NMR in combination with elemental analysis to investigate the surface chemistry of CuInS2 nanocrystals synthesized in the presence of amines. Opposite from known results on various nanocrystals (CdSe, CdTe, PbS, ZnO), we find that as-synthesized nanocrystals are capped by tightly bound amine ligands that show no indication of self-exchange at room temperature. Nevertheless, using in-situ heating experiments in a solution NMR spectrometer, we demonstrate the amine are involved in an adsorption/desorption equilibrium, opposite from typical X-type systems such as CdSe/carboxylate. We thus conclude that amines bind to the CuInS2 nanocrystals as L-type ligands, a result in line with the stoichiometric Cu:In:S ratios in these nanocrystals. These findings are used to rationalize the outcome of different ligand exchange reactions, where we found that amines could be replaced by thiols but not by carboxylic acids.We synthesized HfO2 nanocrystals from HfCl4 using a surfactant-free solvothermal process in benzylalcohol and found that the resulting nanocrystals could be transferred to apolar media using a mixture of carboxylic acids and amines. Using solution 1H NMR and elemental analysis, we studied the details of the transfer reaction and the surface chemistry of the resulting sterically stabilized nanocrystals. As synthesized nanocrystals are charge-stabilized by protons, with chloride acting as the counterion. Treatment with only carboxylic acids does not lead to any binding of ligands to the HfO2 surface. On the other hand, we find that the addition of amines provides the basic environment in which carboxylic acid can dissociate and replace chloride. This results in stable apolar dispersions of HfO2 nanocrystals, sterically stabilized by carboxylate ligands. However, titrations with deuterated carboxylic acid show that the charge on the carboxylate ligands is balanced by co-adsorbed protons. Hence, opposite from the X-type/non-stoichiometric nanocrystals picture prevailing in literature, one should look at HfO2/carboxylate nanocrystals as systems where carboxylic acids are dissociatively adsorbed to bind to the nanocrystals. Since similar results were obtained with ZrO2 NCs, this could be a general picture for the surface chemistry of metal oxide nanocrystals with important consequences on the chemistry of ligand exchange reactions.   1.             (a) Hens, Z.; Martins, J. C., A Solution NMR Toolbox for Characterizing the Surface Chemistry of Colloidal Nanocrystals. Chemistry of Materials 2013,  25 (8), 1211-1221; (b) Anderson, N. C.; Hendricks, M. P.; Choi, J. J.; Owen, J. S., Ligand Exchange and the Stoichiometry of Metal Chalcogenide Nanocrystals: Spectroscopic Observation of Facile Metal-Carboxylate Displacement and Binding. Journal of the American Chemical Society 2013,  135 (49), 18536-18548.2.             (a) Owen, J. S.; Park, J.; Trudeau, P. E.; Alivisatos, A. P., Reaction chemistry and ligand exchange at cadmium-selenide nanocrystal surfaces. Journal of the American Chemical Society 2008,  130 (37), 12279-+; (b) Fritzinger, B.; Capek, R. K.; Lambert, K.; Martins, J. C.; Hens, Z., Utilizing Self-Exchange To Address the Binding of Carboxylic Acid Ligands to CdSe Quantum Dots. Journal of the American Chemical Society 2010,  132 (29), 10195-10201.3.             Fritzinger, B.; Moreels, I.; Koole, R.; Lommens, P.; Hens, Z.; Martins, J. C., In Situ Observation of Rapid Ligand Exchange in Colloidal Nanocrystal Suspensions Using Transfer NOE Nuclear Magnetic Resonance Spectroscopy. Journal of the American Chemical Society 2009,  131 (8), 3024.

Surface chemistry of colloidal nanocrystals � from semiconductors to metal oxides

248th National Meeting (2014)

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