From sulfur-amine solutions to metal sulfide nanocrystals: Peering into the oleylamine-sulfur black box

Article abstract of DOI:10.1021/ja1109997

In this Article, we present our findings on the formation of metal sulfide nanocrystals from sulfur-alkylamine solutions. By pulsed field gradient diffusion NMR along with the standard toolbox of 1D and 2D NMR, we determined that sulfur-amine solutions used as a sulfur precursor exist as alkylammonium polysulfides at low temperatures. Upon heating to temperatures used in nanocrystal synthesis, the polysulfide ions react with excess amine to generate H₂S, which combines with the metal precursor to form metal sulfide. Four different reaction pathways were found, each of which produced H ₂S and the byproducts identified in this Article. Thioamides were identified as an intermediate and were shown to exhibit much more rapid kinetics than sulfur-alkylamine solutions at low temperatures in the synthesis of metal sulfide nanocrystals.

Full text of DOI:10.1021/ja1109997

ARTICLE
pubs.acs.org/JACS
From Sulfur-Amine Solutions to Metal Sulfide Nanocrystals: Peering
into the Oleylamine-Sulfur Black Box
Jordan W. Thomson,† Kaz Nagashima,‡ Peter M. Macdonald,†,‡ and Geoffrey A. Ozin*,†
†Department of Chemistry, University of Toronto, 80 St. George Street, Toronto, Ontario M5S 3H6, Canada
‡Department of Chemical and Physical Sciences, University of Toronto Mississauga, 3359 Mississauga Road North, Mississauga,
Ontario L5L 1C6, Canada
S Supporting Information
b
ABSTRACT: In this Article, we present our findings on the formation
of metal sulfide nanocrystals from sulfur-alkylamine solutions. By
pulsed field gradient diffusion NMR along with the standard toolbox
of 1D and 2D NMR, we determined that sulfur-amine solutions used
as a sulfur precursor exist as alkylammonium polysulfides at low
temperatures. Upon heating to temperatures used in nanocrystal
synthesis, the polysulfide ions react with excess amine to generate H2S, which combines with the metal precursor to form metal
sulfide. Four different reaction pathways were found, each of which produced H2S and the byproducts identified in this Article.
Thioamides were identified as an intermediate and were shown to exhibit much more rapid kinetics than sulfur-alkylamine
solutions at low temperatures in the synthesis of metal sulfide nanocrystals.
’ INTRODUCTION
exceptions include ethylenediamine and benzylamine, for which
crystalline products were obtained and characterized by NMR in
the case of benzylamine.8,9 For simple aliphatic amines where the
solvent is also a reactant, spectroscopy must be performed on
reaction mixtures, making characterization very difficult.
Semiconductor nanocrystals, also known as quantum dots,
show great promise in applications ranging from sensing and
imaging to photovoltaics and optoelectronics.1 While good
control over monodispersity has been achieved for many tech-
nologically relevant compositions, very little is still understood
about the mechanism of precursor reaction and the formation of
reactive species. For instance, the synthesis of II-VI and IV-VI
nanocrystals from trioctylphosphine selenide (TOPSe), a reac-
tion that has been used for 20 years, was only very recently and
unexpectedly shown to require dialkylphosphine impurities
present in very small amounts in commercial trioctylphos-
phine.2,3 Understanding the different molecular species respon-
sible for the formation of nanocrystals is crucial to controlling
size and shape, size distributions, yields, properties, and ulti-
mately utility in functional devices.
While phosphine-based syntheses are commonly used for the
formation of metal selenides and tellurides, the precursor of
choice for metal sulfides is sulfur powder dispersed in long chain
primary alkylamines, most typically oleylamine.4 This precursor
is both cheaper and less environmentally hazardous as compared
to phosphine-based precursors and is tolerant of air and moist-
ure. Its popularity as a precursor has led to many interesting and
novel structures and materials.4 However, its chemistry has been
little studied in comparison to phosphine-based precursors. The
purpose of this work is to uncover the nature of the sulfur-
oleylamine precursor and how it reacts at temperatures typically
used for nanocrystal syntheses.
Two similar proposed steps in the “solubilization” of sulfur
and liquid amines are found in the literature. Note that sulfur
exists as S8 rings at its standard state.10 In reaction 1 shown in
Scheme 1, proposed by Davis and Nakshbendi,6 direct nucleo-
philic attack by nitrogen on S8 occurs causing ring-opening,
thereby forming an open chain alkylammonium N-polythioa-
mine salt with an N-S bond. The second, proposed by Levi11
and shown in reaction 2, results in the formation of hydrogen
sulfide and an N,N0-polythiobisamine with two N-S bonds.
Levi11 claimed the isolation of the tetraalkylpolythiodiamine
from secondary amines and sulfur, but characterized the products
only with elemental analysis. Hodgson et al.7 have ascribed the
ESR spectrum of sulfur and primary/secondary amines to the
homolytic scission of S-S bonds in N,N0-polythiobisamines.
While reaction 2 directly forms H2S, Davis and Naskshbendi6
propose H2S formation from the reaction of sulfur with the
methylene protons R to nitrogen, which forms a thioamide
(reaction 3). In both papers, H2S is proposed to react with S8
to form alkylammonium polysulfides as shown in reaction 4.
Alkylammonium polysulfides have been isolated in the case of
ethylenediamine and benzylamine and have been proposed to
produce solution conductivity in other amines.6,8,9
In this Article, we will study the reaction between sulfur and
alkylamine by performing NMR experiments directly on the
Previous work on sulfur-amine solutions has suffered from the
inability to isolate pure compounds from reaction mixtures.5-7
Attempts to crystallize components typically resulted in deposition
of oil, which decomposed upon distillation attempts.8 Notable
Received: December 7, 2010
Published: March 08, 2011
r
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Scheme 1. Proposed Reactions of Sulfur with Amine from
the Literature
taken to minimize the radiation damping from the strong samples.15 All
spectra were recorded at a sample temperature of 25 °C. PFG diffusion
measurements were conducted using a stimulated-echo sequence.16
Slice-selection with sinc pulse and gradient17 was used to excite the
sample nuclei only in the central region of the RF coil to mitigate
radiation damping and to compensate the inhomogenieity of the field
gradient. Two methods of 1H-13C correlation 2D NMR experiments,
gradient-selected heteronuclear single quantum coherence (gHSQC)18
and heteronuclearmultiple bond coherence (gHMBC),19 were conducted,
using default pulse sequences and parameters of the spectrometer.
NMR Assignment of Products. Octanethioamide: 1H NMR (500
MHz) R1CH2C(S)NH2 3.60 ppm (t); 13C NMR (125 MHz) R1CH2C-
(S)NH2 204 ppm, R1CH2C(S)NH2 45.6 ppm. Octanamide: 1H NMR
(500 MHz) R1CH2C(O)NH2 3.69 ppm (t); 13C NMR (125 MHz)
R1CH2C(O)NH2 186 ppm, R1CH2C(O)NH2 47.0 ppm. N0-Octyloc-
tanamidine: 1H NMR (500 MHz) R1CH2N0 3.15 ppm (t); 13C NMR
(125 MHz) RN0dCR1(NH2) 149.6, R1CH2N0 44.5 ppm. N0-Octyl-2-
thioketooctanamidine: 1H NMR (500 MHz) R2CH2C(S) 5.08 ppm (t),
R1CH2N0 3.45 ppm (t); 13C NMR (125 MHz) R2CH2C(S) 190 ppm,
R2CH2C(S)CdN0R(NH2) 160 ppm, R2CH2C(S) 75.5 ppm, R1CH2N0
51.9 ppm. Note: Characterization of resonances other than those shown
was not possible due to signal overlap. All spectra were internally
referenced to the CH3 reference value of 0.887 ppm for the 1H NMR
and 14.10 ppm for the 13C NMR.
reaction mixtures and without the addition of deuterated sol-
vents. The use of pulsed field gradient (PFG) NMR allows us to
study diffusion behavior,12 and the extension, diffusion ordered
spectroscopy (DOSY), correlates the diffusion coefficient with
the chemical shift.13,14 By investigating the translational diffusion
behavior, we can indirectly probe the nature of the sulfur species.
The standard toolbox of 1D and 2D NMR experiments, along
with diffusion data, can then establish what byproducts are
formed during a nanocrystal reaction and determine the active
species in the formation of metal sulfide nanocrystals.
The results of our study show that, at low temperatures,
sulfur-alkylamine solutions employed as a sulfur precursor exist
mainly as alkylammonium polysulfides. Upon heating to nano-
crystal growth temperatures, the polysulfide ions react with
excess alkylamine to liberate H2S, which under appropriate
conditions combines with the metal precursor to form metal
sulfide nanocrystals. The byproducts formed in the initial reac-
tion react further, forming H2S. We believe this work will lead to
a better understanding of metal sulfide nanocrystal reactions and
should allow for improvements in control over nanocrystal size,
shape, and yields.
’ RESULTS AND DISCUSSION
To understand which reaction pathway(s) occur in the
synthesis of metal sulfide nanocrystals, we first studied a model
sulfur-in-amine solution typically injected into the metal pre-
cursor. This solution is generally formed with sonication and
mild heating (<80 °C) before being injected into the hot metal
precursor solution/slurry. To minimize variables, most especially
in diffusion measurements, we chose octylamine, which is
commercially available at 99% purity and is a liquid at room
temperature, making it a good model for technical grade
oleylamine (70% pure).
’ EXPERIMENTAL SECTION
Figure 1 (left) shows a stack plot of the 1H NMR spectra of
octylamine with increasing concentrations of S. As mentioned
previously, the NMR sample contains only octylamine and sulfur.
The sulfur-to-octylamine molar ratio of 0.5 (thereafter referred
to as 0.5 S:OA ratio) is the same as a typical composition used in
the nanocrystal synthesis using oleylamine and was close to the
solubility limit of sulfur in octylamine at room temperature. The
majority of chemical shifts remain largely unchanged as the sulfur
concentration increases from 0 to 0.5 S:OA ratio. The N-H
protons, however, show a large drift from 1.17 ppm in neat
octylamine to 3.39 ppm in the most concentrated solution. When
the chemical shift is plotted against S:OA ratio, a linear relation-
ship is observed, as shown in Figure 1 (right). This trend suggests
an increase in the degree of amine protonation. In both the 1H
and the 13C NMR spectra, only one major species is observed.
Minor components can be observed at approximately 50 ppm in
the 13C NMR spectrum (Figure S1 in the Supporting Infor-
mation).
Figure 2 shows the diffusion coefficient of octylamine as a
function of S:OA ratio. The two diffusion coefficients, as
determined by the methyl and the amine resonances, almost
overlapped. This indicates that the N-H protons diffuse asso-
ciating with the backbone, although they are expected to be more
or less labile and actually do not show a clear splitting due to
scalar coupling. A linear decrease is observed as sulfur concen-
tration increases, in an analogous fashion to the chemical shift.
Materials. Sulfur (Aldrich, reagent grade) and octylamine (Aldrich,
99%) were used as received without further purification.
Reaction of Sulfur and Octylamine. Sulfur-octylamine solutions
were prepared by pipetting 1.50 mL of octylamine into a 20 mL
scintillation vial (low temperature solution) or round-bottom flask
(high temperature solution). Sulfur powder was then added to the vial
(flask) to prepare solutions with sulfur-to-octylamine (S:OA) ratios
ranging from 0.01 to 0.5. Specifically, in the preparation of the 0.5 S:OA
low temperature precursor solution, 145.3 mg of sulfur powder was
added to 1.50 mL of octylamine in a scintillation vial. The mixture was
sonicated for 10 min and heated at 80 °C for 10 min. This procedure was
repeated three times until a clear red solution was formed. The sample
was then directly pipetted into a 5 mm NMR tube for analysis. For the
samples heated at 130 °C, an identical mixture of sulfur and octylamine
was prepared and purged with N2 at room temperature while stirring for
10 min in a flask fitted with a condenser. The flask was then immersed
in an oil bath at 130 °C for 2 h. The solution was cooled to room
temperature under N2 before being pipetted directly into a 5 mm NMR
tube for analysis. Neat octylamine was subjected to the same procedure
as a control and shown to remain unchanged.
NMR Spectroscopy. All NMR spectra were recorded on a Varian
Unity 500 MHz NMR spectrometer using a Varian 5 mm switchable
broadband liquids probe. The NMR samples were not diluted with
deuterium solvent to preserve the original condition. Deuterium field
lock was turned off during the whole measurement, and the static field
was shimmed using a 1H NMR gradient shimming macro. Caution was
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Figure 1. (Left) A stack plot of the 1H NMR spectra of octylamine with an increasing concentration of sulfur (S:OA ratio) from bottom to top. The
black rectangles denote the N-H resonance. (Right) 1H NMR chemical shift of the N-H resonance of octylamine (and octylammonium polysulfides)
plotted against S:OA ratio.
the N-H protons in N-polythioamines/N,N0-polythiobisamines
are chemically and magnetically inequivalent, they would likely
be labile and exchange rapidly with protons supplied by octyla-
mine, octylammonium molecules, and others, leading to the
observation of one average resonance.
Although the majority of sulfur exists in the form of octylam-
monium polysulfides, the question of where the hydrogen sulfide
came from is still unclear. At the low temperatures used in the
formation of the unheated solution, it is clear from the 13C NMR
that thioamide formation as in reaction 3 does not occur to an
appreciable extent. It appears that only a small amount of N,N0-
polythiobisamine is formed, which produces enough H2S to form
octylammonium polysulfides. The small degree of curvature in the
PFG NMR intensity profiles may be due to the presence of this species.
In an attempt to better understand the precursor reactivity at
temperatures employed in nanocrystal synthesis, we heated the
Figure 2. Diffusion coefficients of octylamine, as determined by the
methyl and the amine resonances, as a function of S:OA ratio.
precursor solution containing octylammonium polysulfides (0.5
S:OA ratio) for 2 h at 130 °C. Figure 3 (left) shows the 13C NMR
The PFG NMR intensity profiles (Figure S2 in the Supporting
Information) for each concentration of sulfur decayed single-
exponentially and identically for both the methyl and the N-H
resonances. The 0.5 S:OA sample showed a small degree of
multiexponentiality, probably due to the increase of slowly
diffusing components with increases in sulfur contents.
These results are most consistent with the large majority of
sulfur existing as octylammonium polysulfides. The apparent
diffusion coefficient decreases as the amount of alkylammonium
polysulfide increases. Observed in this case is an average diffusion
coefficient as the molecular exchange between octylammonium
and octylamine molecules is expected to be very rapid due to very
low activation energy of ammonium ion formation. The same
reasoning explains the apparent existence of only one species in
the 1H and 13C NMR spectra, as well as the increase in the degree
of amine protonation caused by an increasing fraction of
alkylammonium ions (cf., Figure 1).
spectrum after heating, which clearly indicates the formation of
new species substantially deshielded as compared to the octyla-
1
mine-octylammonium resonances. The H NMR spectrum,
Figure 3 (right), also shows a series of new resonances. The new
products still represent a very small percentage of the overall
solution (∼1% by resonance integration as compared to octyla-
mine R-CH2 resonance), and the solution still contains a large
amount of unreacted octylamine and octylammonium polysul-
fides. As a control, neat octylamine was heated to 130 °C under
N2 gas for 2 h and did not show any new resonances in the 13
C
and 1H NMR spectra. The significant downfield drift of the N-
H proton resonance, from 3.39 ppm before heating to 3.83
ppm after heating, is attributable to the production of H2S. The
presence of H2S could be confirmed by placing PbO-coated
paper in the headspace of the flask, which immediately turned
black due to the formation of PbS.
1H-13C correlation 2D NMR experiments, gHSQC18 and
gHMBC,19 were conducted to determine the structural connec-
tivity of the new products in the mixture. The spectra are shown
in the Supporting Information (Figures S3 and S4). Very high
signal strength is recorded for octylamine/octylammonium
polysulfide; however, connectivity could still be observed for
the weaker resonances.
N-Polythioamines/N,N0-polythiobisamines, if present, would
exchange much more slowly with unprotonated octylamine as
the N-S bonds would need to be broken and re-formed, which
likely would have a significant energy barrier. If appreciable
quantities of N-polythioamines/N,N0-polythiobisamines were
present, we would expect to see multiple components in the
form of curvature in the PFG NMR intensity profiles and
Figure 4 shows the DOSY plot for the heated sulfur-
additional resonances in the H and 13C NMR spectra. While
octylamine solution. DOSY13 is perfectly suited for resolving
1
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Figure 3. (Left) 13C NMR spectrum and (right) 1H NMR spectrum for the 0.5 S:OA solution heated at 130 °C for 2 h. Note: The strong resonances
were cut off at the top for clarity.
Scheme 2. Reaction Pathways in Sulfur-Octylamine Solu-
tion at 130 °C and New Product Identities (R = C8H17, R1 =
C7H15, R2 = C6H13)
Figure 4. 1H DOSY plot for the 0.5 S:OA sample heated at 130 °C for 2
h. The software designed by Nilsson14 was used for the processing with
single-exponential fitting.
the overlapping resonances observed in complex mixtures like
this sample. It is clear that there are two distinct diffusion
coefficient ranges. The most intense and fastest diffusing species
(bottom of DOSY plot) corresponds to octylamine/octylammo-
nium polysulfide. The second extreme is the slow diffusing
species (top of DOSY plot), which corresponds to the new
species produced after heating.
On the basis of the data presented, we can deduce that there
are four new species produced at 130 °C. Scheme 2 shows the
new products and the possible reaction pathways.
If the unheated sulfur-octylamine solution contains mostly
octylamine and octylammonium polysulfides, and if products 5-
8 were not present in significant concentrations before heating, it
follows that the polysulfide ions represent the reactive sulfur
precursor. Open chain polysulfide ions have previously shown
greater reactivity than S8 rings.20
Reaction 5 shows the formation of octanethioamide, which is
the same as reaction 3 proposed by Davis and Nakshbendi.6 At
130 °C, it is possible that a radical polysulfide ion could be the
reactant, as the open chain S-S bonds are fairly weak and radicals
have been detected in sulfur-amine solutions at room
temperature.7 In the presence of trace water, the thioamide will
be hydrolyzed, releasing hydrogen sulfide as in reaction 6.
Both octanamide and octanethioamide are characterized by a
distinct quaternary 13C NMR resonance at 186 and 204 ppm,
respectively, slightly downfield from aliphatic amides/thioa-
mides. The diffusion coefficient as determined from the DOSY
plot is about one-half of that of octylamine. It is plausible that
in octylamine, which is relatively nonpolar, these species form
hydrogen-bonded dimers because the N-H protons are good
H-bond donors and the O/S atoms are good H-bond acceptors.
The downfield shift of both the R and the β carbon atoms relative to
typical aliphatic (thio)amides is also consistent with this.
Excess octylamine molecules are expected to attack octanethioa-
mide in a fashion similar to water. Reaction 7 shows the formation of
N0-octyloctanamidine. As in reactions 5 and 6, the formation of the
amidine releases H2S. This reaction has been shown to occur at low
temperatures in the presence of Lewis acid.21 The observation of a
quaternary carbon at 150 ppm in the 13C NMR is consistent with
this assignment. The DOSY plot shows the roughly one-half slower
diffusion than octylamine. This is expected, as the amidine is
essentially a dimer of two octylamine molecules.
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Finally, reaction 8 shows the formation of an R-thioketoami-
dine. While not having literature precedent as with the thioa-
mide, it is not unexpected. N-H groups activate the
R-methylene protons of octylamine to thioamide formation.
This likely occurs due to the resonance stabilization of the
relatively weak CdS bond by the nitrogen lone pair in the
thioamide, making the resulting product stable. Similarly,
the CdS bond can be stabilized through delocalization into
the amidine group. For this reason, we expect similar reactivity of
the amidine with polysulfide ions.
thioacetamide showed a new resonance at 155 ppm, indicating
the formation of an amidine (Figure S5). This reaction provides
additional evidence for the occurrence of reaction 7. Further
investigation is underway to optimize this reaction.
’ CONCLUSION
We have shown that sulfur-amine solutions widely used as a
“black box” precursor in metal sulfide nanocrystal synthesis exist
as alkylammonium polysulfides at low temperature. Upon heat-
ing, H2S is in situ produced, forming thioamides and other
byproducts. The H2S can then react with metal salts to form
metal sulfide nanocrystals. Furthermore, we have shown that
thioamides can be used as a sulfur precursor and have revealed its
much more rapid kinetics as compared to sulfur in oleylamine.
This work should lead to further optimization of metal sulfide
nanocrystal syntheses using sulfur-amine solutions as well as a
better understanding of their formation mechanisms.
N0-Octyl-2-thioketooctanamidine is characterized by two
quaternary 13C NMR resonances at 160 and 190 ppm. The
HMBC spectrum (Figure S4 in the Supporting Information)
suggests that these carbons are adjacent. The 160 ppm resonance
is attributed to the amidine carbon, while the 190 ppm resonance
is likely due to the thioketo group. Two more 13C resonances
were characterized as adjacent to the thioketo and amidine
carbons, respectively, and are listed in the Experimental Section.
The DOSY plot shows a diffusion coefficient consistent with the
dimeric nature of the R-thioketoamidine. Attempts to provide
further evidence for the formation of this product through mass
spectrometry of reaction mixtures is complicated by (i) ion
suppression in electrospray ionization-mass spectrometry (ESI-
MS), which is caused by the large concentration of octylamine
and octylammonium molecules and (ii) reaction of polysulfide
ions at high sampling temperatures (>300 °C) required in typical
mass spectrometers. Separation with HPLC is complicated by
the poor solubility of the products in the solvents typically
required. While our NMR data indicate the formation of the
R-thioketoamidine, more work may be needed to more con-
fidently comfirm its presence.
’ ASSOCIATED CONTENT
Supporting Information. 1H and 13C NMR spectra of
S
b
sulfur-octylamine, PFG NMR intensity profiles, H-13C cor-
1
relation 2D NMR spectra, and the thioacetamide-octylamine
13C NMR spectrum. This material is available free of charge via
the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
gozin@chem.utoronto.ca
Reactions 5-8 each produce H2S, which likely exists as
octylammonium hydrosulfide after partial neutralization with
octylamine, consistent with the increased chemical shift of the
N-H protons in the heated sample. We therefore postulate that
the synthesis of metal sulfide nanocrystals proceeds by the
reaction of in situ-produced octylammonium hydrosulfide with
the metal precursor salt. The resulting product from this reaction
is metal sulfide nanocrystals and the octylammonium salt (e.g.,
octylammonium chloride/citrate, etc.).
’ ACKNOWLEDGMENT
G.A.O. is the Government of Canada Research Chair in
Materials Chemistry and Nanochemistry. We thank NSERC
and the University of Toronto for strong and sustained funding.
J.W.T. is grateful for an NSERC doctoral scholarship.
’ REFERENCES
Our findings have important implications for the reaction
kinetics in metal sulfide nanocrystal syntheses. We speculate that
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