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

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 (CeO2-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.

Ammonia MQuant test stripes for semiquantitative Nessler determinations were obtained from Sigma Aldrich. Interference with OLA and dodecanenitrile was tested from saturated solution of the respective organics in water.

Synthesis of CeO 2 NCs
The syntheses of metal oxides were adapted from a reported synthesis of CeO2. 1 In a typical synthesis Ce(NO3)3·6 H2O (434.8 mg, 1 mmol), 6 mmol of amine and 4mL of n-octadecane were mixed in a three neck flask. The mixture was degassed at room temperature and at 80°C for 30 minutes each. At 80°C a brown suspension forms. The flask is backfilled with argon and the temperature is increased to 300°C with a ramp of 15°C per minute. At 200°C the suspended solids dissolve completely, forming a clear solution and at about 240°C the reaction mixture turns dark brown. The reaction mixture is kept at 300°C for 60 minutes and afterwards it is cooled down in air until 160°C when 2 mL of toluene were injected. The mixture is precipitated with 25 ml acetone and centrifuged for 6 minutes at 6500rpm. For washing the particles, they are resuspended and precipitated with toluene (5 ml) and acetone (25 ml) and centrifuged 3 minutes at 3500 rpm. This washing procedure is repeated twice, and the particles were stored in 5 mL toluene.
Note: the reaction proceeds with strong gas evolution. It is therefore recommended to work with big flaks (for a standard synthesis 50 mL or bigger). In small flasks strong bubbling can cause the solution to rise though the condenser creating big temperatures differences in the reaction mixture.

Ligand stripping
For the characterization of bound ligands, NCs were stripped by adding 10 μL of pure trifluoroacetic acid to a solution of 50 mg of NCs in 0.5 mL CDCl3. The particles precipitated, and the mixture was put in the ultrasonic bath for 30 min and subsequently dried under vacuum. CDCl3 (0.6 mL) was added and after thorough mixing, the precipitate was filtered. The supernatant was measured in NMR.

Pair Distribution Function Analysis
Total scattering X-ray PDF experiments were conducted at room temperature using Malvern Panalytical Empyrean Nano Edition lab source PDF diffractometer with Ag-Kα source (0.56 Å and 22.1keV). Samples were prepared in a 0.2 mm glass capillary. Data collection was carried out with 1D focusing X-ray mirror/slit system and a Galipix 3D hybrid pixel detector attached to 85mm radius reduction interface using Data collector software. The collected data was processed after proper background subtraction using Highscore Plus. 4 Qmin of 0.4 Å -1 and Qmax of 20.6 Å -1 was used to generate G(r). Diffpy-CMI was used for modeling and fitting. 5 Figure S3: X-ray PDF analysis of CeO1.74. The figure shows the single-phase fit to the data, using the cubic crystal structure of ceria (Fm-3m). Due to the small x-ray scattering factor of oxygen, we cannot use PDF to refine the oxygen vacancies. Therefore, we use the oxygen occupancy (0.875) as derived from XPS. Figure S4: X-ray PDF analysis of CeO1.74. The figure shows the dual phase fit to the data, using the cubic crystal structure of ceria (Fm-3m). The fit is clearly improved (lower Rw) compared to the single-phase fit. We conclude the sample is quite polydisperse with a broad range of crystallite sizes (from 2 to 8 nm).

X-ray photoelectron spectroscopy analysis
We performed XPS measurements on different samples to determine the ratio Ce (III) /Ce (IV) in the samples along with the presence of N, C and O. Since ceria gets reduced progressively under vacuum: We kept the samples similar times under vacuum (between 21 and 23 hs).
We analyzed particles prepared with hexadecylamine and particles prepared with dioctadecylamine. Moreover, we exposed the particles to air and analyzed them one week later to determine if the Ce (III) was oxidized changing the composition of the sample, and similar results were obtained.

Calculation of ligand density:
Assuming that the mass loss in the TGA experiment corresponds to loss of hexadecanoic acid (palmitic acid, Mw = 256.43 g/mol), and using the average NC size from PDF (see further), 6.5 nm, a ligand coverage of 3.3 acid molecules per nm 2 can be calculated.
First the volume and surface area of a single nanocrystal are calculated.    Figure S9: A) 1 H NMR spectra of ligands stripped from CeO2-X nanocrystals in prepared with hexadecylamine (blue), protonated hexadecylamine (red) and protonated stearic acid (black) references. B) HSQC (black) and HMBC (red) spectra of the stripped ligand indicating the chemical shifts of the α and β carbon atoms, and C) COSY spectra of the stripped ligand.

S10
Structural elucidation of the products found in the reaction mixture Identification of the secondary aldimine 6 To verify the identity of the aldimine 6 we hydrolyzed the reaction mixture. 200 μL of reaction mixture were mixed with a 400 μL H2SO4 (conc.) and 100 μL H2O in a closed vial and heated for 90 minutes at 130 °C under strong stirring (>1000 rpm). The hydrolysed reaction mixture was extracted with 1 mL of ethyl acetate and dried under vacuum. Figure S13: 1 H NMR spectra of the reaction mixture of a synthesis of CeO2-x with hexadecylamine completion of a synthesis with hexadecylamine (black), hydrolyzed reaction mixture (red) and decanal (blue) as a reference. After hydrolysis the resonance A at 7.61 ppm (Nimine-H ; 6) disappears and a new one at 9.76 ppm corresponding to the aldehyde 7 appears. 10.00 9.00 8.00 7.00 6.00

H  (ppm)
We synthetized a N-hexadecyldecylimine to use as a reference. Briefly, decanal (190 μL, 1 mmol) and hexadecylamine (300,5 mg, 1.24 mmol) were mixed in round bottom flask connected to a vacuum line. The mixture was stirred under vacuum at 100 °C for 45 minutes to remove water formed during the condensation. Afterwards the reaction was cooled down in air and characterized by FTIR-ATR and NMR without further purification. Figure S14: FTIR spectra of the reagents and products of the synthesis of N-hexadecyl decylimine. Hexadecylamine (blue), decanal red) and N-hexadecyldecylimine (black). The ν(C=O) stretching shifts from 1725 cm -1 (aldehyde) to 1668 cm -1 (imine) proving that the reaction took place.

Identification of ammonia
The gas mixtures in the headspace of the reactions (NiO, ZnO, ZrO2 and CeO2) was bubbled into water and the escaping gas still turns universal indicator paper blue, indicating the presence of a highly volatile basic gas in the mixture. The gas evolved during the heat-up of a normal synthesis was collected in acidulated water (pH = 2) and the pH was measured as the reaction proceeded rising steadily to 10.5.
Finally, to confirm that the evolved gas is ammonia we measured the concentration of ammonia in the aqueous solution with the Nessler method using commercial test stripes and found a concentration above 400 ppm. The test-kit showed no interference with neither the dodecanenitrile nor hexadecylamine, both scarcely soluble in water.

S17
Identification of the terminal alkene 2 Figure S21: Magnification of the assigned 1 H NMR resonances assigned to the terminal alkene 2.    Acid free CeO2-x particles were synthesized replacing oleylamine by dioctadecylamine in the procedure described in page S3 and using the same molar ratios. Given the low solubility of dioctadecylamine in polar solvents it coprecipitates with the nanocrystals in the purification procedure. Filtration and multiple (>10) purification steps are required to clean the nanocrystals.

Decomposition of other metal nitrates
The synthesis of other metal oxides was adapted from the synthesis of CeO2-x. 1 mmol of metal nitrate hydrate was mixed with 6 mmol of hexadecylamine (1.45 g) in 4 ml of octadecane. The mixture was heated up to ~4 0 °C to melt the organic reagents before degassing under vacuum at 50°C and 100°C for 30 minutes each. After this time the nitrates solubilize completely. Afterwards the reaction mixtures were heated under argon up to evident color change (NiO: blue to green, ZnO: colorless to white) and finally cooled down in a water bath. Washing was done with several precipitation/redispersion steps using toluene as solvent and acetone as antisolvent. Centrifugation of the products in toluene was also done to separate the insoluble fraction (size selection).
A synthesis was also performed with zirconium (IV) oxynitrate hydrate, although it produced no colloidally stable nanocrystals, an insoluble ZrO2 precipitate was recovered.
As a control experiment, we performed a heat up of NaNO3 (1 mmol) with oleylamine (6 mmol) in 1octadecene. No byproducts were observed in this reaction, although this might be attributed to the low solubility of NaNO3.