Ligand Conversion in Nanocrystal Synthesis: The Oxidation of Alkylamines to Fatty Acids by Nitrate

Abstract

Ligands are a fundamental part of nanocrystals. They control and direct nanocrystal syntheses and provide colloidal stability. Bound ligands also affect the nanocrystals’ chemical reactivity and electronic structure. Surface chemistry is thus crucial to understand nanocrystal properties and functionality. Here, we investigate the synthesis of metal oxide nanocrystals (CeO 2- x , ZnO, and NiO) from metal nitrate precursors, in the presence of oleylamine ligands. Surprisingly, the nanocrystals are capped exclusively with a fatty acid instead of oleylamine. Analysis of the reaction mixtures with nuclear magnetic resonance spectroscopy revealed several reaction byproducts and intermediates that are common to the decomposition of Ce, Zn, Ni, and Zr nitrate precursors. Our evidence supports the oxidation of alkylamine and formation of a carboxylic acid, thus unraveling this counterintuitive surface chemistry.

Activating Molybdenum Carbide Nanoparticle Catalysts under Mild Conditions Using Thermally Labile Ligands

“…The first attempt to remove native oleylamine ligands on the MoC 1– x nanoparticle surface involved a treatment with trifluoroacetic acid (TFA), which has successfully removed long-chain insulating ligands on the surface of colloidal nanocrystals and is a common technique for ligand removal in general, as aforementioned. − However, after the addition of 500 μL of TFA (i.e., 50× molar excess compared to previous reports) to a colloidal suspension of MoC 1– x nanoparticles and allowing the suspension to stir rapidly overnight, no nanoparticles precipitated out of solution, which would be expected for effective ligand stripping. After isolating the TFA-treated MoC 1– x nanoparticles, TGA and FT-IR spectroscopy (Figures S1 and S2, respectively) were performed and compared to the as-prepared OAm-MoC 1– x nanoparticles.…”

Chemical Considerations for Colloidal Nanocrystal Synthesis

“…We established in the previous section that oleylamine is oxidized to aldimine by nitrate. We recently found that the reaction does not stop there but can proceed all the way to carboxylic acid . As a model system, we investigated the synthesis of CeO 2 nanocrystals, from cerium nitrate and oleylamine. The metal nitrate first forms a complex with oleylamine, which subsequently decomposes, producing a myriad of by-products; see Scheme .…”

The Chemistry of Cu3N and Cu3PdN Nanocrystals**

“…The carboxylate is assigned to a surface bound oleate ligand and its presence is also detected by FTIR (broad signal at 1577 cm −1 , see Figure S15). Oleate is most likely formed by oxidation of oleylamine by nitrate, as we reported earlier in the synthesis of cerium oxide nanocrystals [15b] …”