Download The Role of Organosulfur Compounds in the Growth of PbS

Supporting Information for:
The Role of Organosulfur Compounds in the Growth of PbS Quantum Dots
Martin R. McPhail and Emily A. Weiss*
Dept. of Chemistry, Northwestern University, Evanston, IL 60208-3113
*corresponding author. Email: [email protected]
EXPERIMENTAL METHODS
Dissolution of Sulfur in Hexadecane. We measured the comparative rate of fluorescent
species formation in samples of elemental sulfur heated at 180 °C when hexadecane is used as a
solvent instead of 1-octadecene. Aliquots were taken from both samples beginning at five
minutes following addition of elemental sulfur to the solvent, at which point the sulfur flakes had
completely disappeared into solution for both solvents. Photoluminescence of aliquots diluted in
hexanes was measured from 370 to 600 nm under an excitation wavelength of 360 nm. Figure
S1 plots the integrated PL signal of these aliquots as a function of heating duration. The rate of
the integrated PL signal increase is much slower when sulfur is dissolved in hexadecane instead
of 1-octadecene, and suggests the solvent’s carbon-carbon double-bond is able to accelerate the
formation of the fluorescent species. We take catena-sulfur—extended sulfur chains formed as a
consequence of S8 ring-opening—to be the primary fluorescent species. The residual
fluorescence in sulfur heated in hexadecane may indicate slow proton abstraction by catenasulfur.
S1
Figure S1. Plot of integrated PLE spectra relative to background (λem = 370 – 600 nm; λex =
360 nm) for aliquots of sulfur dissolved in either 1-octadecene or hexadecane and heated at
180°C. We observe minimal evolution of the PLE spectrum when hexadecane is used as a
solvent; the formation of photoluminescent species appears to require an unsaturated solvent.
Pathways for Polymerization of Sulfur and ODE. Scheme S1 briefly summarizes two
reported reaction pathways that can produce organosulfur compounds upon heating sulfur with
an alkene: (1) radical polymerization following homolytic S–S cleavage and (2) ionic
polymerization following nucleophilic attack by an olefin.1-6 Radical polymerization initiates at
temperatures above 158°C when cyclooctasulfur (S8) rings undergo spontaneous homolytic S–S
cleavage and ring opening to form a diradical (Reaction 1). These diradicals can either attack
other S8 rings to form longer sulfur chains or attack vinyl groups to form a polysulfur-alkyl
adduct (Reactions 2,3). Ionic polymerization involves the nucleophilic attack of an olefin on an
S8 ring to form a polysulfonium cation and polysulfide anion, which propagates the reaction by
attacking either other vinyl groups or other S8 rings (Reactions 4-6). Both reactions can also
cause carbon-carbon double-bond migration; for ODE this migration would convert the doublebond from a terminal to an internal position (Reactions 7,8). This migration for sulfur heated in
1-octene at 140 °C has been previously reported.4
S2
Scheme S1. Possible Reaction Pathways for Sulfur and 1-Octadecene
S3
1
H-NMR of ODE Heated at 210°C. We performed a control study of the structural effects
of heating ODE to 210 °C in the absence of elemental sulfur. Heating was done in a roundbottom flask while stirring with a magnetic stir bar under a dry N2 atmosphere. We periodically
removed aliquots and monitored the molecular structure by 1H-NMR spectroscopy in the same
manner as for S/ODE precursor aliquots. 210 °C was chosen as the temperature for this control
study because it was the maximum temperature used in variable temperature studies of sulfur
dissolution in ODE. Figure S2 shows a representative 1H-NMR spectrum of the 90% technical
grade ODE used in the present study without further purification. The primary impurities are
constitutional isomers of ODE, which include alkenes with internal double-bonds (δ = 5.39 ppm)
and terminal alkenes that are disubstituted at the C2 position (δ = 4.69 ppm). Figure S3 shows
the integrated peak intensity of the methyl proton signal, the internal alkene proton signal, and
the terminal C2-disubstituted alkene signal normalized to the intensity of the alkene proton
signal from ODE as a function of ODE heating duration. These normalized proton signals are
constant with respect to ODE heating duration, which indicates that neither double-bond
migration nor double-bond elimination occur when ODE is heated at 210 °C for two hours in the
absence of elemental sulfur.
S4
Figure S2. (a) Reference 1H-NMR spectrum of the 90% technical grade 1-octadecene used in
this study. The distinguishable proton signals are indicated with capital letters along with the
residual solvent peak of CDCl3. The primary impurity is indicated by the star, which
corresponds to a disubstituted terminal carbon-carbon double-bond. (b) Expanded view of the
vinyllic proton region. The inset shows the region associated with internal carbon-carbon
double-bonds, which is flanked by signals from the terminal carbon-carbon double bond
protons.
S5
Figure S3. The stability of 90% technical grade 1-octadecene to heating at 210°C is shown
by 1H-NMR spectra taken at various intervals. Plotted are the total number of methyl (blue
triangles), 2,2-disubstituted terminal vinyl (red circles), and internal vinyl (green squares)
groups normalized to the number of terminal vinyl groups based on the integrated 1HNMRsignal.
COSY NMR of S/ODE Precursor. We performed 1H–1H Correlation Spectroscopy (COSY)
NMR on a sample of sulfur heated in ODE for two hours at 180°C. Figure S4 shows the
spectrum acquired under a double quantum filtering pulse sequence; correlation peaks
corresponding to organosulfur species have been highlighted. These correlation peaks comprise
groups of coupled protons corresponding to three groupings of organosulfur species: (A)
Coupling between protons alpha to sulfur and vinyl protons indicates an RHC=CH–CH2–Sx
structure, which is consistent with addition of sulfur to a terminal carbon-carbon double-bond
followed by double-bond migration from a terminal to an internal position. (B) Coupling
between protons alpha to sulfur and a secondary carbon indicates an R–CH2–CH2–Sx structure,
which is consistent with the complete opening of a double-bond following sulfur addition. (C)
Coupling between protons alpha to sulfur and a tertiary carbon indicates an (R)2–CH–CH2–Sx
structure, which is consistent with either the opening of a double-bond from the branched
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structural impurity in ODE (Figure S2) or the addition of sulfur to ODE’s double-bond followed
by addition of a carbanion to open the double-bond. The branched impurity only makes up
approximately 10% of the molecules in the technical grade ODE used for this study, but the
branched organosulfur product (C) makes up approximately 25% of the integrated organosulfur
proton signal observed between δ = 2.5 and 3.5 ppm. This discrepancy may indicate either: (1)
the rate of sulfur addition to the branched alkene is faster than for the unbranched alkene or (2)
carbanions are formed in sufficient quantities in situ to create a variety of branched alkane
species via addition of these carbanions to ODE.
Figure S4. Plot of the Double Quantum-Filtered NMR 1H-1H Correlation Spectrum of sulfur
heated in ODE under nitrogen at 180°C for two hours. Important signals along the diagonal
corresponding to either ODE or organosulfur species are indicated with corresponding
structural identifiers. Three correlation peaks corresponding to distinct organosulfur
structures are highlighted and lettered: (A) RHC=CHCH2Sx (B) RCH2CH2Sx (C) (R)2CH2CH2Sx. “R” indicates a straight-chain alkane and bolding indicates coupled proton for each
correlation peak.
S7
MALDI-TOF Mass Spectrometry of S/ODE Precursor. We performed matrix-assisted
laser desorption/ionization-time of flight mass spectrometry on aliquots of S/ODE precursor
removed at 5 and 120 minutes after addition of elemental sulfur to ODE at 180°C (Figure S5).
We observed the molecular ion of the matrix compound, α-cyano-4-hydroxycinnamic acid (mw
= 189.17 g/mol) at the mass-to-charge ratio m/z = 189 Da in both spectra. We used the CHCA
molecular ion peak to calibrate the instrument; we performed no post-acquisition corrections to
the spectra.A molecular ion peak for ODE (mw = 252.48 g/mol) is not observed, but we do
observe two principal fragment peaks at m/z = 211 Da and m/z = 233 Da corresponding to the
loss of CH2–CH=CH3 and CH3 fragments, respectively. Two important peaks at m/z = 282 Da
and 530 Da appear after heating the S/ODE precursor at 180°C for 120 minutes (arrows in
Figure S5b). Based on the S/ODE reaction mechanism in Scheme S1, the 282 Da corresponds to
a single sulfur atom bonded to a single ODE molecule, C17H34–CH2–S, while the 530 Da
corresponds to a single sulfur atom bridging two ODE molecules, C17H34–CH2–S–CH2–C17H34.
Four additional peaks occur at m/z = 312, 347, 378, and 416 Da of varying intensity. We
interpret these peaks as corresponding to ODE molecules with integer numbers of between two
and five sulfur atoms bound. The simplest bonding model of these sulfur atoms is as a chain of
variable length bound to the end of an ODE molecule, C17H34–CH2–Sx.. The analogous structure
for the bridged compound is C17H34–CH2–Sx–CH2–C17H34, but we only able to observe the x = 2
peak (m/z = 567 Da) before these higher molecular weight peaks are indistinguishable from
noise.
S8
Figure S5. MALDI-TOF mass spectra are shown for S/ODE precursors heated at 180°C for
(a) 5 minutes and (b) 120 minutes following solid sulfur addition. The primary ion peak
located at 211 Da (star) corresponds to fragmentation of ODE at the C3-C4 bond. Two
important peaks indicating mercaptan-alkyl adduct formation are indicated by arrows: (1) the
282 Da ion corresponding to an ODE molecule with one sulfur atom added and (2) the 530 Da
ion corresponding to two ODE molecules bridged by a sulfur atom.
S9
Measurement of Sulfur Loss as H2S.
Under conditions where sulfur can be reduced, there
is the possibility of hydrogen sulfide production. In the sulfur and 1-octadecene precursor
system, either the presence of trace water or repeated cycles of cleavage of polysulfur by vinyl
groups followed by hydrogen abstraction by polysulfide can produce hydrogen sulfide. The
evolution of hydrogen sulfide gas leaches sulfur from the precursor solution and renders it less
reactive for nanoparticle synthesis. To assess the degree of sulfur loss via hydrogen sulfide
evolution, we bubbled N2 gas through the S/ODE solution as it was heated at 210°C and trapped
the eluting H2S by passing the eluent through an absorption train of lead(II) acetate in water
(Figure S6a). H2S reacts with the lead(II) acetate in good yield (Keq = × M) to produce the
highly insoluble PbS (Ksp = 9×10-29), which we collected and used to determine sulfur yield via
gravimetric analysis.7-8 Figure S6b shows a plot of the cumulative percentage conversion of
sulfur in the heated S/ODE solution to evolved H2S. Over the first two hours of heating, which
includes the timespan precursor aliquots for the reactions of Figure 1 were taken, only 0.5% of
the sulfur is lost as H2S; continued heating shows only 1.2% yield of H2S after 24hrs. This
percentage of sulfur loss is within the measurement error for weighing the elemental sulfur solid,
and we have not observed this scale of variation in the initial mass of added sulfur to cause
variations in the PbS synthesis like those seen in Figure 1. We therefore conclude that while
some sulfur is lost during the precursor heating process as H2S, this loss is not responsible for the
observed differences in the synthesized PbS particles.
S10
Figure S6. (a) Illustration of the H2S gas collection apparatus. N2 bubbled through the heated
sulfur in 1-octadecene solution carries away evolving H2S, and a reflux condenser collects
any organic vapor before it reaches the absorption train. As the carrier gas bubbles through
the Pb(OAc)2 solution in the adsorption train vials, H2S is converted to insoluble PbS. These
vials are removable to allow for aliquot collection. (b) Cumulative percent conversion of
sulfur to H2S at 210°C as determined by the mass of collected PbS.
Measurement of S/ODE Reactivity with ODE. Many standard methods of PbS QD
synthesis use oleylamine to reduce elemental sulfur in situ to hydrogen sulfide.9-10 The
mechanism of oleylamine reduction of sulfur is complex11, and recent work has shown the
reduction mechanism to be a nontrivial process in the formation of metal sulfide clusters.12
Polymerization between ODE and sulfur produces carbon-sulfur bonds and a range of higher
molecular weight species, and these changes could inhibit the reducing action of the oleylamine.
Typical carbon-sulfur bond dissociation energies are approximately 50% greater than sulfursulfur bond dissociation energies and their formation could trap sulfur in bonds inaccessible to
oleylamine reduction. Incorporation of sulfur chains into higher molecular weight hydrocarbon
polymers could limit oleylamine’s access to sulfur via steric factors. Lowering either the yield or
initial rate of sulfur reduction by oleylamine would lower the number of initial PbS clusters
formed during the nucleation phase and decrease particle yield. Hypothetically, limited sulfur
S11
availability during the QD growth phase could also affect QD shape by retarding growth of
sulfur-enriched <111> faces relative to stoichiometric <100> faces. Despite the likely
complicated nature of the in situ sulfur reduction and reaction with lead, the measurement of
hydrogen sulfide gas evolution should serve as a useful metric for the reducing activity of
oleylamine. Figure S7 shows the amount of evolved hydrogen sulfide gas produced by an
S/ODE precursor solution injected into a 210 °C solution of oleylamine in ODE at the same
concentrations used for PbS synthesis. The H2S evolution rate is similar whether the precursor is
prepared by heating sulfur in ODE at 180 °C for 5 minutes or 1 hour. Times longer than 20
minutes show deviations in H2S yield, but QD syntheses typically cease within the first 10
minutes where the two curves show good overlap. We conclude that the reducing activity of the
oleylamine is not significantly hindered by the presence of S/ODE polymerization products
formed during the precursor preparation.
The observed availability of sulfur in S/ODE precursors despite formation of alkyl-polysulfur
adducts contrasts with results for Se/ODE published previously where selenium-ODE coupling
reactions lowered yields of CdSe quantum dot reactions.13 Selenium and sulfur take
approximately 2 hours and 5 minutes, respectively, to dissolve in ODE at 180°C, and Se-Se bond
enthalpies are ~30% lower than S-S bond enthalpies. Alkyl-polyselenium adducts in Se/ODE
should form faster and with greater yield than alkyl-polysulfur adducts in S/ODE; the Se/ODE
precursors would more likely exhibit heating-induced changes in reactivity.
S12
Figure S7. Plot of the percent sulfur conversion in an S/ODE precursor solution to hydrogen
sulfide gas by oleylamine at 210°C. We determined the mass of evolved H2S gas by
gravimetric analysis of PbS produced when the eluted gas passes through an aqueous lead(II)
acetate absorption train (see Fig. 6a diagram). Preparation of the S/ODE precursor using a
longer heating time does not have a measurable effect on the rate of H2S gas formation at
early times.
Effect of Residual Water on PbS QD Synthesis. We performed a control PbS synthesis
using an S/ODE precursor heating at 180°C for five minutes to assess the effect of residual water
on the size and shape of the final PbS QDs. Two identical PbS QD syntheses were run, with the
exception that for one reaction the solution of lead(II) oleate and oleylamine in ODE were placed
under vacuum for 30 minutes at 80°C to remove residual water. Figure S8 shows TEM images
of these two batches of QDs without (a) and with (b) the additional step to remove residual
water. Both syntheses produced cubic PbS QDs of similar sizes; we conclude that residual water
does not have a large effect on the synthesis as performed.
S13
Figure S8. TEM images and histograms of edge length of PbS QDs prepared as reported in
the main text (a) and with an additional step to remove trace water by pulling vacuum on the
solution of lead(II) oleate, oleylamine, and 1-octadecene for 30 minutes at 80°C (b). Average
values of edge length are all determined by a Gaussiuan curve fit to the plot of frequency
counts. The scalebar represents 50 nm and 10 nm for main and inset images, respectively.
S14
1-Decanethiol Titration of PbS QDs. We performed a control experiment where we added
100, 500, 1000, and 5000 molar equivalents of 1-decanethiol per PbS QD as estimated from NIR
band-edge absorption spectroscopy. These were stirred for 18 hours under N2 and then the 1HNMR spectrum acquired in CDCl3. All QDs used were from the same batch prepared using a
S/ODE precursor prepared by heating at 180°C for 5 minutes. The region for aliphatic protons
alpha to sulfur of NMR spectra for 100 and 5000 molar equivalent samples as well as samples of
the pure dots and pure 1-decanethiol are shown in Figure S9. The pure 1-decanethiol sample was
exposed to identical atmospheric conditions as the QD samples. The quartet of 1-decanethiol
(2.53 ppm), which corresponds to the carbon alpha to the SH group, is completely absent from
the spectra of thiols added to QDs. A new quintet appears at 2.50 ppm that we ascribe to carbon
alpha to the protonated thiol, CH3(CH2)8CH2SH2+. This protonation is reasonable considering
there is an excess of oleic acid as well as presumably hydronium ions produced by the
deprotonation of oleic acid during its binding to the QD surface. An additional, broader triplet
appears at 2.77 ppm, which we ascribe to the thiolate CH3(CH2)8CH2S in exchange with the QD
surface. Increasing the thiol concentration does not dramatically increase the signal from either
of these features, and a new triplet centered at 2.68 ppm appears instead. This triplet may
indicate either a free thiolate or one that is in rapid exchange with the QD surface. The total
integrated intensity from these signals is less than the predicted amount based on the alphacarbon signal in the pure 1-decanethiol control. We conclude there must be additional alphacarbon protons of thiol molecules that achieve a tight-binding motif with the QD surface. With
such close proximity to the QD surface, the alpha-carbon signal is likely broadened to such a
degree that it becomes indistinguishable from noise. We attempted removing excess oleate from
QD solutions prior to thiol exchange to reduce the yield of acid-base side products and improve
S15
the yield of bound thiol, but exposing these 10nm or larger diameter particles to methanol made
them insoluble in organic solvents.
Figure S9. 1H-NMR spectra of QDs with 5000 (blue) or 100 (green) molar equivalents of 1decanethiol (DT) added and stirred for 18 hours. The red and black traces show reference
spectra of the pure (oleate-capped) QD and decanethiol samples exposed to the same
ambient conditions. The quartet of the alpha-carbon of thiol appears at 2.53 ppm and the
quintet of the alpha-carbon of sulfonium ions appears at 2.50 ppm. No traces of disulfide
formation are observed for the pure DT sample. The triplets centered at 2.68 and 2.77 ppm in
QD + DT samples are taken to be thiolates in fast and slow exchange, respectively, with the
QD surface. Signals from more tightly bound thiolates are likely too broadened to be
distinguished.
1
H-NMR of Crude Lead Sulfide QD Product. We perfomed 1H-NMR in CDCl3 of the
crude QD product prepared using a S/ODE precusor heated for 5 minutes at 180°C to investigate
if any major organic side-products are formed during the QD synthesis. Figure S10 shows the
1
H-NMR spectrum between 1.9 and 4.0 ppm with an expanded view of the 2.4 to 3.6 ppm region
inset. Not shown are the residual chloroform solvent at 7.26 ppm, the double-bond peaks from
ODE (5.81, 4.99, and 4.93 ppm) and oleaic acid/oleylamine (5.35 ppm), and the aliphatic
methylene and methyl regions that provide limited stuctural information. All peaks not arising
from ODE, oleic acid, oleylamine, or resdual solvent can be ascribed to the condensation sideproduct between oleic acid and oleylamine. These peaks are indicated in Figure S10. Notably
S16
absent are any signals from protons on carbons alpha to a sulfur. These organosulfur signals
should appear between 2.5 and 3.5 ppm, and a particularly strong, unobscured signal at 2.51 ppm
should be present if a high yield of reduction of sulfur by ODE occurred during the PbS growth
process. The number of double-bonds are conserved during the QD growth process: the ratio of
integrated methyl:vinyl proton signal is 1.52 for the crude product versus the expected ratio of
1.50 for no loss of double-bonds.
Figure S10. 1H-NMR spectrum of the crude PbS QD reaction mixture highlighting the region
where aliphatic protons alpha to sulfur have their characteristic signals (which are absent
here). These QDs were synthesized with an S/ODE precursor prepared by heating elemental
sulfur and ODE at 180°C for five minutes. All peaks in this region can be attributed to either
oleic acid, oleylamine, or their condensation product; no organosulfur side-products of the
PbS QD reaction are observable.
S17
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