Surface chemistry is a key enabler for colloidal nanocrystal applications. Here, we report on our recent investigations on the surface chemistry of group 4 (Ti, Zr, Hf) metal oxides synthesized in either benzyl alcohol or tri-n-octylphosphine oxide (TOPO). ZrO2 and HfO2 NCs, synthesized in benzyl alcohol, are capped with HCl. Exchange with carboxylate ligands is not possible unless a basic environment is provided by amines. After purification, only carboxylates are retrieved on the surface. As a crucial difference from semiconductor quantum dots, the negative charge of the carboxylate is not balanced by excess surface cations but by surface adsorbed protons. As such, carboxylic acids dissociate on metal oxide nanocrystal surfaces. Furthermore, we show that this new NC(X)2 binding motif alters ligand exchange strategies and provides solutions in nano-catalysis. Recently, we turned our attention to TiO2, ZrO2 and HfO2 nanocrystals synthesized in tri-n-octylphosphine oxide (TOPO). Using liquid and solid state 1H and 31P nuclear magnetic resonance spectroscopy and X-ray fluorescence spectroscopy, we show that the nanocrystal surface is capped by several derivatives of TOPO. In the 31P NMR spectrum, di-n-octylphosphinate (δ = 57 ppm) and P,P’-(di-n-octyl) pyrophosphonate (δ = 20 ppm) are found coordinated to the nanocrystal. In addition, hydrogen chloride associates with the metal oxide nanocrystal surface and protonates TOPO. The resulting hydroxyl-tri-n-octylphosphonium, [HO-PR3]+, is tightly associated with the nanocrystal surface (δ(31P) = 73 ppm) due to electrostatic interactions and hydrogen bonding. To simplify the complex surface composition, we exchange the original surface species for carboxylate or phosphonate ligands. Our results contrast with the classically envisaged L-type binding motif of TOPO to surface metal ions. The generality of this stabilization mode and its relevance to catalysis is discussed.

Surface chemistry of group 4 metal oxide nanocrystals

255th National Meeting (2018)

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