Purification of Oleylamine for Materials Synthesis and Spectroscopic

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Methods/Protocols pubs.acs.org/cm

Cite This: Chem. Mater. XXXX, XXX, XXX−XXX

Purification of Oleylamine for Materials Synthesis and Spectroscopic Diagnostics for trans Isomers Dmitry Baranov,§ Michael J. Lynch,† Anna C. Curtis,‡ Alexa R. Carollo, Callum R. Douglass, Alina M. Mateo-Tejada, and David M. Jonas* Department of Chemistry, University of Colorado, 215 UCB, Boulder, Colorado 80309-0215, United States

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S Supporting Information *

ABSTRACT: Oleylamine is widely used in nanomaterial and metal-halide perovskite synthesis. Impurities in commercially available oleylamine reagents are identified, purification procedures are explained, beneficial effects on lead chloride/ oleylamine mixtures are documented, and impacts on materials synthesis are discussed. Suspensions of 0.12 mole fraction PbCl2, PbBr2, and PbI2 in purified, dried, and filtered oleylamine were found to form clear solutions upon heating to 100 °C. The infrared, Raman, and NMR spectra of oleylamine and a common trans impurity present in literature reference IR spectra are presented.

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INTRODUCTION Oleylamine, cis CH3(CH2)7CHCH(CH2)8NH2, is a linear unsaturated primary amine with one double bond that makes it a liquid at 25 °C. Inexpensive commercially available oleylamine reagents are widely used in the synthesis1 of metal,2 semiconductor,3−8 and metal−halide perovskite9−11 nanomaterials because liquid oleylamine can act as a solvent, an electronic surface-passivating ligand, and a long-chain surfactant for stabilizing colloids over a wide temperature range.12 Oleylamine has been called the “dominant coordinating solvent in nanomaterials synthesis”.13 However, the use of oleylamine as a solvent for lead halides has often been limited by the formation of a suspension that ultimately interferes with isolation of the synthesized materials.4,7 When attempting to record Raman spectra of precursor solutions in oleylamine, it was found that oleylamine reagents luminesce in the visible region of the spectrum (see Figure 1). This luminescence can interfere with materials characterization. The purification protocol described here eliminates oleylamine impurity luminescence, increases lead halide solubility or complexation to form clear solutions, slows formation of a suspension even at elevated temperatures, and allows nanocrystals to be selectively precipitated before lead chloride during washing. Vacuum distillation alone changed the shape of the photoluminescence spectrum and reduced the photoluminescence maximum by about a factor of 2. For excitation at 488 nm, photoluminescence can be reduced by a factor of over 25 000 with a three-step purification procedure14 in which the first two steps are adapted from a description in a patent.15 The above procedure did not eliminate photoluminescence for excitation at 458 nm. Briefly, the complete four-step procedure (1) precipitates oleylamine hydrochloride from ethyl ether © XXXX American Chemical Society

with acetonitrile and then (2) neutralizes the oleylamine hydrochloride with aqueous hydroxide before (3) vacuum distillation over metallic sodium and (4) drying of the distillate over a molecular sieve. This multistep procedure eliminates the yellow tinge of the as-received Aldrich reagents (the asreceived Strem reagent was colorless). The oleylamine hydrochloride crystals formed in the initial precipitation with acetonitrile were white or pale pink; the final liquid residue in the distillation flask was colored yellow-orange. The distillate is a clear and colorless oleylamine liquid.

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MATERIALS USED The CAS nomenclature for oleylamine is cis-9-octadecen-1amine (CAS registry number 112-90-3). Materials for oleylamine purification have been selected with a view toward elimination of air, moisture, and luminescent impurities in the final product. At the time of a 2013 review,1 the highest purity oleylamine reagent was 80−90%. Here, “70% technical grade” (oleylamine, prod. no. O7805-500G, 70% technical grade, Sigma-Aldrich, lot no. STBF9554V) has been compared to two higher purity oleylamine reagents: “≥98% primary amine” (oleylamine, prod. no. HT-OA100-1.5KG, ≥98% primary amine, Sigma-Aldrich, lot no. MKBZ7016V), and “minimum 95% amine” (oleylamine, prod. no. 07-1668, min. 95%, Strem, lot no. 21991600). Oleylamine hydrochloride precipitation used ethyl ether (E138-1, anhydrous, BHT stabilized, Certified ACS, 1L, Fisher Chemical or BDH1121-1LPC, stabilized with 1 ppm of BHT, ACS grade, 1L, BDH), hydrochloric acid (H613-05, 36.5%−38%, ACS Reagent grade, Macron ChemReceived: October 2, 2018 Revised: January 17, 2019

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DOI: 10.1021/acs.chemmater.8b04198 Chem. Mater. XXXX, XXX, XXX−XXX

Chemistry of Materials

Methods/Protocols

Figure 1. Top panel: spectra of secondary radiation for 488 nm excitation of as-received (red curve) and purified (blue curve) samples of oleylamine (≥98% primary amine, Aldrich). Bottom panel: Raman spectra collected with 532 nm excitation from three oleylamine reagents after purification. Spectra are vertically offset by 600 counts (70% technical grade) and 1000 counts (≥98% primary amine) for clarity. The dotted vertical line marks a CH2 scissor and asymmetric bend transition at 1442 cm−1 to illustrate calibration precision (0.5 cm−1).

for fatty amines.15 Adaptations of this procedure reduced the reaction scale by over an order of magnitude, replaced t-butyl methyl ether with ethyl ether (which was more readily available), and incorporated anhydrous sodium sulfate as a static drying agent before solvent evaporation. In this work, metallic sodium was used as the distillation drying agent because it was available in the laboratory. When further drying of oleylamine and other amines was necessary (see below), molecular sieves were used. The oleylamine hydrochloride precipitation was performed under air. A total of 50 mL of oleylamine (∼0.15 mol, a clear and colorless or pale yellow liquid, depending on the reagent) was mixed with 160 mL of ethyl ether in a 500 mL roundbottom flask. While stirring, 15.5 mLof concentrated hydrochloric acid (∼1.2 equiv of HCl) was added dropwise using a Pasteur pipet over 10 min. Next, a 1000 mL Erlenmeyer flask was charged with 500 mL of acetonitrile, and the wet amine hydrochloride−ether solution was added to it dropwise while stirring, forming a white cloudy precipitate. The resulting mixture was kept in an ice bath for 30 min, filtered under air, and rinsed four times with 100 mL portions of acetonitrile. A snowy white solid (with a slight hint of pink on 2 out of 5 purification runs) was transferred to a 500 mL round-bottom flask and dried under vacuum (∼1 Torr) for 30 min. In the meantime, 12.3 g of sodium hydroxide (∼2 equiv) was dissolved in 150 mL of deionized water. A total of 200 mL of ethyl ether was added to a dried solid of the amine hydrochloride, forming a white slurry. A magnetic stirring bar was added, and the slurry was placed on a magnetic stirring plate. Next, using a separatory funnel, the sodium hydroxide solution was added dropwise to the amine hydrochloride slurry in ethyl ether over 5−10 min. Upon addition of sodium hydroxide, the white slurry gradually changed appearance to a two-phase mixture (light yellow organic phase on the top).

icals), and acetonitrile (A21-1, Certified ACS, 1L, Fisher Scientific). Sodium hydroxide (S318-500, Certified ACS, Fisher Scientific) in deionized water (Milli-Q grade) was used to recover the amine. Sodium sulfate (S421-500, anhydrous, Certified ACS, Fisher Scientific), sodium metal (prod. no. 483745-100G, cubes, contains mineral oil, 99.9% trace metals basis, Sigma-Aldrich), and 3 Å molecular sieves (4490-04, grade 564, 8-12 mesh, Mallinckrodt Chemicals) were used in succession for drying. Luer lock syringe filters with a 0.2 μm PTFE membrane (VWR 28145-495 25 mm 0.2 μm PTFE Membrane Syringe Filter) were used to filter the dried oleylamine. For preparing solutions in oleylamine, lead(II) chloride (PbCl2) of two purities was ground to a fine powder (Coors 50 mL mortar and matching pestle). Strem 99% PbCl2 was stored and ground in the fume hood and is referred to as regular lead chloride. Alfa-Aesar 99.999% PbCl2, ultra dry, was stored and ground in the glovebox and is referred to as anhydrous. Similarly, anhydrous lead(II) bromide (PbBr2, ultra dry, 99.999% Alfa Aesar) and lead(II) iodide (PbI2, ultra dry, 99.999% Alfa Aesar) were stored and ground in the glovebox for solubility tests. The nitrogen filled glovebox (MBraun UNILab) catalyst was regenerated every 1−2 months such that levels of oxygen did not exceed 5 ppm and water read 0.1 ppm (readout error is specified as ±1.1 ppm).

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PROCEDURE Typical recommendations for purification of primary amines involve static drying over a variety of solid hygroscopic drying reagents (KOH, LiAlH4, CaH2, etc.) followed by distillation over a drying agent (P2O5, CaH2, CaC2, etc.) or further purification through precipitation of the hydrochloride with subsequent conversion to amine and distillation.16 The patent by Gibson outlines a procedure of the latter type specifically B



Methods/Protocols

with visible electronic absorption transitions,17,18 and solid oleylamide has been reported to have a pale yellow color;15 such absorptions might lead to visible luminescence. C18H36N+ (possibly from linoleamine), C16H36N+, C16H34N+, and similar impurities with varying chain length and saturation account for about 17% of the “purified oleylamine” from the ≥98% primary amine reagent. These shorter chain lengths and less saturated chain impurities lower the ratio of methylene to amine protons in the NMR spectra, but the ratios found here (see Supporting Information) are slightly higher than reported previously.13 Elemental analysis of a purified and dried 70% technical grade oleylamine sample yielded weight % 80.74 C, 13.68 H, and 5.52 N (identical within error to 80.76 C, 13.66 H, 5.55 N for the as-received reagent). Deviation from the calculated 80.80 C, 13.97 H, and 5.24 N can be rationalized based on the variations in chain length and saturation found for the ≥98% primary amine reagent by mass spectrometry. Purification and drying reduced the Cl content measured by ion chromatography from 72 ppm to below the 1 ppm detection limit (Robinson Microlit Laboratories). Other potential impurities considered in as-received oleylamine reagent include peroxides (C18H38NO2+ could arise from an olefinic hydroperoxide formed by the ene reaction with oxygen;19−21 however, hydroperoxides were not detected in unpurified oleylamine, see Supporting Information), their decomposition products, and ammonium carbamates, formed by reaction of the amine with atmospheric carbon dioxide.22−24 Exposure to the atmosphere can convert the entire alkylamine sample into alkylammonium alkylcarbamate (1:1 ratio alkylammonium to alkylcarbamate).24 Ammonium carbamates have been reported to have beneficial effects on the selectivity of zinc sulfide nanoparticle synthesis.22 However, for long-chain n-alkylamines, the formation of carbamates is slow under atmospheric conditions, requiring ∼1000 h for octadecylamine.24 Positive ion mass spectrometry is blind to carbamates, but comparing the ATR-IR (attenuated total reflection-infrared) spectra of as-received oleylamine (≥98% primary amine), purified oleylamine, and oleylammonium oleylcarbamate shows no sign of carbamates and establishes an upper bound that no more than 0.5% of the unpurified oleylamine exists as carbamates (see Supporting Information). These purified oleylamine samples are suitable for Raman spectroscopy (Figure 1). However, the Raman spectra of the samples obtained from commercially available oleylamine reagents have two vibrational peaks in the region of the vinylic CC stretch (∼1760 cm−1) when only one is expected for oleylamine. The intensity ratio of the two peaks varies between 5:1 and 1:1 with the oleylamine source reagent, suggesting an impurity with variable concentration. The two frequencies are consistent with cis and trans vinylic CC stretches.25 The purified Strem minimum 95% amine sample’s Raman spectrum also shows evidence of other impurities at low Raman frequencies. The two major components were identified by comparing the vinylic and allylic regions of the proton NMR spectrum to the proton NMR spectra of high purity (>99%) oleic acid (cis CH3(CH2)7CHCH(CH2)7COOH) and elaidic acid (trans CH3(CH2)7CHCH(CH2)7COOH) (Figure 2). The purified oleylamine reagents have multiplet patterns26,27 consistent with a reagent dependent ratio of cis:trans isomers (Figure 2; see Supporting Information). Unlike the infrared and Raman spectra, where the cis and trans vinylic CC stretch peaks have different line strengths, the ratio of integrals over two

After all sodium hydroxide was added, the two-phase mixture was transferred to a separatory funnel, briefly shaken, and left to separate for several minutes. After that, the aqueous phase was discarded, and the organic phase was washed four times with 50 mL of deionized water. After the last wash, the organic phase was transferred to a 500 mL round-bottom flask and dried with 10 to 15 g of anhydrous sodium sulfate overnight. On the next day, the organic phase was decanted from the sodium sulfate, the ether solvent was removed with a rotary evaporator, and the residual crude amine product was distilled over a 1 cm3 cube of freshly cut metallic sodium under vacuum (60−100 mTorr) at a bath temperature of 190−200 °C. An ∼3/4 mL first fraction was collected in a separate flask before ∼40 mL (second fraction) was collected as purified amine over about 60−90 min, leaving behind 2−3 mL of yellow-orange residue in the distilling flask. The distilled amine, a clear colorless liquid, was stored in the nitrogen filled glovebox at ∼33 °C (temperature inside the glovebox) in foil-wrapped glass vials capped with PTFE-lined screw caps. The purification was also performed on a smaller scale (∼6 mL of the starting amine) by downscaling amounts of HCl and NaOH and approximately downscaling solvent amounts. The yield of the purification procedure was 70−80%. Some modifications to this procedure were used for subsequent work and are enumerated in the Supporting Information. Oleylamine from the above three-step procedure will be referred to as “purified.” Only amines (both purified and as-received) that have been dried over 3 Å molecular sieves will be referred to as “dried.” The sieves were activated by heating at ∼300 °C under nitrogen flow for 4 h. To dry the liquid amine, it was stored over activated sieves [∼20% (v/v), e.g., 1 mL of sieves per 4 mL of amine] for a minimum of 14 h before use. In some cases, amines dried over molecular sieves were filtered with 0.2 μm PTFE syringe filters in order to remove sieve dust.

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TROUBLESHOOTING AND SAFETY A sufficient quantity of metallic sodium is needed to dry the oleylamine reagent. If all metallic sodium is consumed before the distillation is finished, the purified oleylamine distillate will not be dry enough to dissolve lead chloride into a clear solution. Loss of oleylamine during transfers between glassware throughout purification can be minimized by rinsing glassware with small portions of ethyl ether or acetonitrile. Lead halide dust is a lead poisoning inhalation hazard and must be contained inside the glovebox (anhydrous lead halides) or fume hood (regular lead chloride). After purification, glassware was cleaned by rinsing with methanol into a waste container. At the scale given here, the purification procedure generates ∼1.5−2 L of hazardous waste.

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CHARACTERIZATION Positive ion electrospray ionization gently protonates molecules in the sample. The resulting mass spectra (see Supporting Information) of the as-received and purified oleylamine hydrochloride (from Aldrich ≥98% primary amine) indicate that oxygen containing impurities with formulas C18H36NO+, C18H34NO+, and C16H32NO+ (consistent with protonated unsaturated amides) and C18H38NO2+ and C18H34NO2+ (consistent with protonated nitroalkanes and unsaturated variants) have all been reduced from the percent level to below the 0.1% noise floor by purification. Nitroalkanes in amines have been reported to form charge transfer complexes C



Methods/Protocols

Figure 3. Raman spectra of cis-9-octadecen-1-amine and the trans isomeric impurity obtained from the ratio method applied to Raman spectra (532 nm excitation) of purified “70% technical grade” and “≥98% primary amine” oleylamine samples. The peak at 973 cm−1 is assigned to the cis-CH out of plane deformation, the peak at 1266 cm−1 is assigned to the cis-CH symmetric rock, and the peaks at 1656 and 1671 cm−1 are assigned to the cis- and trans-CC stretches, respectively.

in the different reagents to obtain the separate Raman (Figure 3) and infrared (Figure 4) spectra of the cis and trans isomers by the ratio method.30−32 Based on ref 25, vibrational assignments for the infrared and Raman spectra are proposed in ref 14; these assignments support oleylamine and elaidylamine as the major components. Figures 3 and 4 each highlight three differences that are consistent with a comparative study of the IR and Raman spectra of oleic acid and elaidic acid.36 By referencing the cis and trans peak strengths to the integrated CH bending peak strength, these spectra can be used to assess isomeric composition under the assumption that the samples contain only cis and trans isomers of 9-octadecen-1-amine. The resulting cis:trans ratios from the Raman spectra are 1.1:1 for the ≥98% primary amine and 3.7:1 for 70% technical grade (see Supporting Information). Despite the presence of other impurities, these ratios agree with the ratios determined separately from the vinylic and allylic multiplets in NMR (perhaps because unsaturated molecules have stronger Raman scattering37).

Figure 2. Regions of 1H NMR (300 MHz, cyclohexane-d12) spectra showing vinylic (left panel) and allylic (right panel) regions for >99% elaidic (trans-9-octadecenioc) acid, >99% oleic (cis-9-octadecenoic) acid, and the three purified commercial 9-octadecen-1-amine samples (“98%”: ≥98% primary amine, “70% tech”: 70% technical grade, “min. 95%”: minimum 95% amine). The chemical shift is referenced to ferrocene (δH [Fe(C5H5)2] = 4.04 ppm in C6D12) as the internal standard.33

multiplets in an NMR spectrum directly gives the ratio of the number of protons generating each multiplet.28,29 Using the ratio method from optical spectroscopy,30−32 the partially overlapping cis and trans multiplets in the NMR spectra were separated from the NMR spectra of two reagents with different cis:trans ratios (see Supporting Information). Both the vinylic and allylic multiplets give cis:trans proton integral ratios equal to 1.1:1 (Aldrich ≥98% primary amine), 3.7:1 (Aldrich 70% technical grade), and 5.4:1 (Strem minimum 95% amine). These proton integral ratios are interpreted as cis:trans isomeric ratios. These cis:trans isomeric ratios from NMR are essentially independent of distant functional groups and aliphatic chain length or branching, so they should incorporate impurities with a double bond (which account for most of the impurities). Although elaidylamine (trans-9-octadecen-1amine) seems likely, the double bond has not been precisely located within the aliphatic chain; COSY 2D NMR spectra show that the trans double bond must be at least two carbons away from both the methyl and the amine functional groups. For the Strem minimum 95% amine reagent, NMR spectra of the vinylic protons give a cis:trans ratio of 4.9 ± 0.5 for the asreceived reagent vs 5.4 ± 0.5 after purification. This proves that the trans isomer is not produced by the purification process. Interestingly, the reference infrared spectra of oleylamine34,35 contain both a 966 cm−1 peak assigned here to the trans-CH wag and a 3005 cm−1 peak assigned here to the cis-CH stretch.25 Further, the intensities of these two peaks exhibit anticorrelated variations among the four reference spectra. We have used the varying ratios of cis and trans peaks

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IMPACTS Lead chloride is often dissolved in oleylamine to form synthesis precursors, but the reported solubility has been low (∼67 mg/mL, corresponding to 0.074 mole fraction lead chloride),7 and some reports mention using clear solutions7,8 while others mention suspensions3,6 or gels4 even upon heating to 120 °C. Some of this variation may arise from differences in concentration. The suspensions or gels have been reported to have problematic impacts on recovery of nanocrystals after washing.4,7 Upon heating to 120 °C under a dry nitrogen atmosphere (on a Schlenk line, in a glovebox, or in glass vials that were sealed with PTFE-lined screw caps in the glovebox before being removed for heating), the purified oleylamine dissolves anhydrous lead chloride powder to form a clear and colorless liquid solution at lead chloride mole fractions up to 0.12 (corresponding to 112 g/L or to 0.4 M concentration if it is assumed that the lead chloride and oleylamine volumes are additive upon dissolution) (Figure 5). The clear and colorless solutions form a waxy white solid upon cooling to room temperature but melt to the clear colorless solution upon reheating to 120 °C, even after a few days. D



Methods/Protocols

Figure 4. Differences between the infrared vibrational spectra of cis-9-octadecen-1-amine and the trans- isomeric impurity obtained using the ratio method for FTIR spectra of purified “70% technical grade” and “≥98% primary amine” oleylamine samples. The peak at 966 cm−1 is assigned to the trans-CH wag, and the peak at 3005 cm−1 is assigned to the cis-CH stretch. The strongest C−H stretch bands are gray shaded in the right panel because their line shapes and relative intensities are distorted by overdepletion of the IR beam.

irreversibly formed a cloudy white suspension. In the same way, anhydrous PbI2 (0.12 mole fraction, or 185 mg PbI2 per mL of oleylamine solvent) dissolved upon heating to form a clear yellow liquid solution. Upon removal from heat, the iodide solutions irreversibly became cloudy yellow suspensions. Continuous heating of lead iodide solutions for ∼10 min after dissolution also irreversibly lead to a cloudy yellow suspension. Only purified, dried, and filtered oleylamine (70% technical grade) was used for these tests, and mole fractions above 0.12 were not tested to determine solubility limits. While lead bromide in oleylamine remains a clear and colorless liquid solution at 30 °C, the lead chloride solutions undergo a phase transition. As opposed to the freezing point depression expected for ideal solutions,39 the apparently elevated freezing temperature suggests a reaction such as the formation of soluble lead chloride−oleylamine coordination complexes40,41 or polymers42 (hereafter simply complexes). On this hypothesis, several observations in oleylamine, octadecylamine, and octylamine can be explained by a phase diagram for complexes in amine solution and solid complexes. Such phase diagrams show the liquidus temperature as a function of composition; upon equilibrium cooling to the liquidus temperature, solid complexes phase separate and lower the mole fraction of complexes in amine solution. Clear and colorless solutions of anhydrous lead chloride in octylamine (dried over molecular sieves) have liquidus temperatures that increase with lead chloride mole fraction (from 5 °C at 0.016 mole fraction to 18 °C at 0.03 mole fraction and 31 °C at 0.06 mole fraction). Only under the slowest cooling at 31 °C was phase separation observed as the solid initially formed (the solid sinks under gravity). When held at temperatures near the 31 °C liquidus temperature for days, the PbCl2-rich waxy solid from 0.06 mole fraction lead chloride in octylamine phase separates further to yield a PbCl2depleted solution (these statements infer lead chloride concentration from ion chromatography elemental analyses for chlorine by Robertson Microlit Laboratories). Assuming the amines behave similarly, the apparent freezing temperature of 38−40 °C for clear and colorless solutions of 0.12 mole fraction lead chloride in oleylamine is actually a nonequilibrium liquidus temperature. Though more phases might be involved, these rough observations are consistent with nonequilibrium freezing on a three-component phase diagram39 in which the liquidus temperature increases with lead chloride mole fraction up to the solubility limit (i.e., there

Figure 5. Dissolution of 0.12 mole fraction regular and anhydrous lead chloride reagent in as-received and purified oleylamine after 10 min of heating at 120 °C while stirring on a magnetic stir plate: (a) regular PbCl2 + as-received oleylamine, (b) regular PbCl2 + purified oleylamine, (c) anhydrous PbCl2 + as-received oleylamine, and (d) anhydrous PbCl2 + purified oleylamine (white magnetic stirbar is visible). The oleylamine is Aldrich, 70% technical grade. Mixtures a−c are suspensions; d is a solution.

In glass vials with PTFE-lined screw caps, these clear solutions irreversibly form a white suspension after cumulative heating for 40 min at 120 °C. Isolation of the suspended material might require a heated centrifuge; it could be lead chloride, the waxy precipitate reported in ref 38 (we observed a similar precipitate in aged lead sulfide (PbS) quantum dots made by the lead chloride synthesis before we began purifying the oleylamine) or some other reaction product. Investigation revealed that purification is not necessary or sufficient to dissolve lead chloride, but rigorous drying of both lead chloride and oleylamine is essential. Drying unpurified 70% technical grade oleylamine over 3 Å molecular sieves overnight allowed heating of a 0.12 mole fraction lead chloride oleylamine solution at 120 °C for 90 min before this white suspension formed. This is long enough to carry out most syntheses. Drying purified oleylamine over molecular sieves allowed heating at 120 °C for over 8 h without formation of a white suspension. Like anhydrous lead chloride, anhydrous lead bromide and lead iodide can be dissolved in purified, dried, and filtered oleylamine (70% technical grade) to form clear solutions upon heating and stirring at 100 °C in the glovebox. Anhydrous PbBr2 (0.12 mole fraction, or 147 mg PbBr2 per mL of oleylamine solvent) formed a clear and colorless solution. Upon removal from heat, the lead bromide solution remained a clear and colorless liquid solution for more than 72 h at the 30 °C temperature of the glovebox. Two days later, it had E



Methods/Protocols

and trans 9-octadecen-1-amine can be separated by gas chromatography under conditions that lead to the formation of stearylamine.46 Here, it is demonstrated that three asreceived commercial oleylamine reagents contain significant fractions (ranging from 14% to 43%) of elaidylamine or a similar trans isomer. Elaidylamine can be detected through the ∼966 cm−1 vibrational mode in the infrared spectrum (which contaminates the four reference infrared spectra for oleylamine)34,35 or the vinylic and allylic regions of the proton NMR spectrum. It is known that elaidic acid packs more densely than oleic acid in Langmuir−Blodgett films.47,48 This suggests that the variable fraction of elaidylamine in commercial oleylamine reagents may have an impact on nanomaterial surface coverage, passivation, and colloidal stabilization during synthesis. Both isomers bind to PbS nanocrystals (see Supporting Information). Packing differences between elaidylamine, oleylamine, and their mixtures could cause batch sensitive results when oleylamine is used as a liquid template (e.g., in the synthesis of gold nanowires49) or as a ligand. Higher isomeric purity oleylamine (or elaidylamine) may be available through the synthesis from oleic acid (or elaidic acid) patented by Gibson.15 Purified and rigorously dried oleylamine can dissolve high concentrations of lead halides to form clear solutions. This high solubility may widen the parameter space for materials synthesis by eliminating the need to add oleic acid in order to dissolve lead halides.11,50 A 0.12 mole fraction solution of lead halide in oleylamine corresponds to slightly less than 8 oleylamine molecules per lead halide formula unit. Aliphatic primary amines are L-type ligands;51 typical lead coordination numbers of 6 or more52 suggest that at least two amines coordinate to each lead atom in the coordination complexes or polymers implicated by the phase diagram for lead chloride (two amines might allow a coordination number of 6 through coordination similar to the polymer in ref 42). An NMR study on PbS quantum dots53 reports that amine ligands exchange rapidly on a ∼5 ms time scale; this suggests amine coordination to lead in lead halide solutions may be similarly dynamic.

could be only one well-defined lead chloride amine complex, but an accurate phase diagram could indicate more). The most obvious impact of drying and removing other impurities from the oleylamine reagents on material synthesis is that anhydrous lead halide solutions in purified and dried oleylamine replace potentially heterogeneous reactions in suspensions3 with homogeneous reactions in solution. A second impact could arise from removal of catalytic impurities. By following a modified (see next paragraph) literature synthesis of chloride-passivated PbS quantum dots,5 purified oleylamine and as-received oleylamine were found to behave similarly, but purified oleylamine can be used to prevent the formation of a white suspension/precipitate that interferes with isolation of the nanocrystals and has limited the use of this synthesis. Using lead chloride in oleylamine precursors for PbS synthesis has been reported to produce air stable quantum dots that have small size dispersion and have high photoluminescence quantum yields.5 The literature procedure5 was modified by replacing methanol with dry acetonitrile as the nonsolvent for precipitating the quantum dots. Acetonitrile was chosen as nonsolvent because it can be readily dried to sub-5 ppm water content with molecular sieves,43 has a polarity similar to the methanol used in the published synthesis, has been reported not to strip X-type ligands off the semiconductor nanocrystal surface, and has little effect on the nanocrystal photoluminescence quantum yield.44 These two changes (purified oleylamine solvent and dry acetonitrile nonsolvent) allow selective precipitation of lead sulfide nanocrystals before lead chloride during nanocrystal washing. In the modified synthesis of PbS quantum dots briefly presented here, a sulfur−oleylamine solution (2 mL, 6 mg/mL sulfur concentration at 33 °C) was injected into a concentrated solution of lead chloride in oleylamine at 90 °C (0.84 g of PbCl2 in 7.5 mL oleylamine, ∼0.12 molar fraction of lead chloride prepared at 120 °C) under air-free conditions. After injection, the lead chloride mole fraction in oleylamine drops to 0.09. Quenching the synthesis with ∼12 mL anhydrous toluene yields colloidal quantum dots in a 55% (v/v) solution of toluene and ∼0.09 mole fraction (∼0.3 M) lead chloride in oleylamine. The key is that oleylamine capped PbS quantum dots of ∼5 nm diameter precipitate out from this solution at 21% (v/v) acetonitrile (dark chocolate brown precipitate) at 34 °C. In contrast, a clear 55% (v/v) solution of anhydrous toluene and 0.09 mole fraction lead chloride in purified and dried oleylamine starts to form a white precipitate around 34% (v/v) acetonitrile at 34 °C (sulfur remains in solution under similar conditions). When washing nanocrystals, adding acetonitrile nonsolvent too rapidly precipitates a light brown (milky coffee) mixture of quantum dots and this white precipitate. Dropwise addition of anhydrous acetonitrile nonsolvent can selectively precipitate nanocrystals while the residual lead chloride remains in solution. As measured by the ∼35 meV red-edge HWHM for PbS quantum dots with their 1S-1S absorbance maximum at 0.93 eV, the above synthesis yielded a narrow 1S-1S line width comparable to that reported in ref 5 (see Supporting Information).

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CONCLUSIONS Commercial oleylamine reagents were found to contain up to 43% of a trans isomer, shorter chain and unsaturated amines, and luminescent impurities incorporating oxygen. Multistep purification reduces the luminescent impurities sufficiently for Raman spectroscopy. Infrared, NMR, and Raman spectra of the separate oleylamine and elaidylamine isomers are presented for use in quantifying isomeric composition. Rigorous drying allows oleylamine to dissolve anhydrous lead halides at elevated temperatures and mole fractions up to 0.12. For PbCl2, this delays formation of a suspension by several hours at 120 °C. For PbBr2, the solutions remain clear and colorless for more than 72 h at 30 °C. For PbI2, the solution was clear and yellow while heated for ∼10 min before irreversibly forming a suspension. When solutions of anhydrous lead chloride in dried and purified oleylamine are used in a synthesis of chloride passivated PbS quantum dots, the addition of anhydrous acetonitrile as a nonsolvent can selectively precipitate nanocrystals before precipitating excess lead chloride.

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DISCUSSION It has been argued that some syntheses of oleylamine might cause saturation or isomerization.15 Olefin cis−trans isomerizations typically have activation energies of more than 100 kcal/mol but can be driven photochemically or promoted by radical catalysis.45 It has been reported that derivatives of cis F



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ASSOCIATED CONTENT

S Supporting Information *

Additional notes on the experimental procedure for purification of commercial oleylamine reagents; experimental details of 1H NMR, FTIR, elemental analysis, mass spectrometry, Raman spectroscopy, test for peroxides, ATR-IR test for carbamates, precipitation tests for lead chloride and sulfur, and lead sulfide quantum dot synthesis; absorption spectra of PbS nanocrystals, NMR spectra of 9-octadecen-1-amine ligands bound to PbS nanocrystals, tables of NMR proton integrals; mass spectra; tables of mass spectra peaks; UV−visible absorption spectra of oleylamine reagents before and after purification; tables of elemental analyses; explanation and illustration of the ratio method; FTIR spectra of oleic acid, elaidic acid, and oleylamine samples; and 1H NMR spectra of oleic acid, elaidic acid, and oleylamine samples (PDF)

AUTHOR INFORMATION

Corresponding Author

*(D.M.J.). E-mail: [email protected]. Phone: +1-303492-3818. ORCID

Dmitry Baranov: 0000-0001-6439-8132 David M. Jonas: 0000-0002-1085-8161 Present Addresses §

(D.B.) Italian Institute of Technology, Genoa 16163, Italy. (M.J.L.) Department of Chemistry, University of Minnesota, Minneapolis, Minnesota 55455, United States. ‡ (A.C.C.) Department of Chemistry, Radford University, Radford, Virginia 24142, United States. †

Notes

The authors declare no competing financial interest.

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REFERENCES

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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.8b04198.

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Methods/Protocols

ACKNOWLEDGMENTS

We thank J. Curtis Beimborn II and Prof. Mathias Weber (CU Boulder/JILA) for generous assistance with Raman experiments performed using 532 nm excitation. We thank Tessa Myren and Prof. Oana Luca (CU Boulder) for assistance with the carbon dioxide reaction. This material is based upon work of D.B., A.C.C., and D.M.J. supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences, and Biosciences, under Award Number DE-FG02-07ER15912. Work of M.J.L. was supported by the National Science Foundation under Grant No. CHE-1405050. Work of A.M.M.-T. was supported by the National Science Foundation under Grant No. CHE1800523. Work of A.R.C. was supported by the National Science Foundation Graduate Research Fellowship Program under Grant No. DGE 1144083. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the National Science Foundation. Work of C.R.D. was supported by the Air Force Office of Scientific Research under AFOSR Award No. FA9550-18-1-0211. G



Methods/Protocols

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Purification of Oleylamine for Materials Synthesis and Spectroscopic

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