Role of organosulfur compounds in the growth and final surface chemistry of PbS quantum dots

Article abstract of DOI:10.1021/cm4040819

This paper describes the mechanism by which reaction of sulfur with 1-octadecene (ODE) induces a change in the shape of PbS quantum dots (QDs), synthesized from the S/ODE precursor and lead(II) oleate, from cubic to hexapodal by altering the ligand chemistry of the growing QDs. ¹H NMR and optical spectroscopies indicate that extended heating of sulfur and ODE at 180 °C produces a series of organosulfur compounds with optical transitions in the visible region and that the binding of organosulfur ligands to the growing QD induces a preferential growth at the 100 faces (over the 111 faces) and, therefore, a hexapodal geometry for the particles. The study shows that S/ODE can be made a more reliable precursor by reducing the temperature and duration of the sulfur dissolution step and that any metal sulfide QD synthesis using elemental sulfur heated to high temperatures should take steps to reduce the in situ yield of organosulfur byproducts by avoiding olefinic solvents.

Full text of DOI:10.1021/cm4040819

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Article
The Role of Organosulfur Compounds in the Growth of PbS Quantum Dots
Martin R. McPhail, and Emily A. Weiss
Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/cm4040819 • Publication Date (Web): 15 May 2014
Downloaded from http://pubs.acs.org on May 30, 2014
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The Role of Organosulfur Compounds in the Growth and Final Surface
Chemistry of PbS Quantum Dots
Martin R. McPhail and Emily A. Weiss*
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Dept. of Chemistry, Northwestern University, Evanston, IL 60208-3113
*corresponding author. Email: e-weiss@northwestern.edu
ABSTRACT
This paper describes the mechanism by which reaction of sulfur with octadecene (ODE) induces
a change in the shape of PbS quantum dots (QDs), synthesized from the S/ODE precursor and
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lead oleate, from cubic to hexapodal by altering the ligand chemistry of the growing QDs. H-
NMR and optical spectroscopies indicate that extended heating of sulfur and ODE at 180 °C
produces a series of organosulfur compounds with optical transitions in the visible region, and
that the binding of organosulfur ligands to the growing QD induces a preferential growth at the
<
100> faces (over the <111> faces), and therefore a hexapodal geometry for the particles. The
study shows that S/ODE can be made a more reliable precursor by reducing the temperature and
duration of the sulfur dissolution step, and that any metal sulfide QD synthesis using elemental
sulfur heated to high temperatures should take steps to reduce the in situ yield of organosulfur
byproducts by avoiding olefinic solvents.
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INTRODUCTION
This paper describes the mechanisms by which the composition of the sulfur-1-octadecene
S/ODE) precursor modifies both the shape and surface chemistry of lead sulfide quantum dots
PbS QDs) produced using reaction mixtures of S/ODE and lead oleate. The S/ODE precursor is
(
(
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useful for the scalable synthesis of metal sulfide QDs because it is cheap, air-stable, and highly
reactive with reducing agents. S/ODE has become a primary sulfur source for the CdS QD
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synthesis following the original report of Yu and Peng ; dozens of reports detail its use to
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prepare CdS QDs and core/shell particles.
More recently, S/ODE has become a popular
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precursor for PbS QD synthesis.
Within the numerous reports of the synthesis of metal
sulfide QDs based on S/ODE precursors, the temperature and duration reported for dissolving
sulfur in ODE to prepare the precursor varies widely. While elemental sulfur dissolves in ODE
within a few minutes at 180°C, QD synthetic procedures often describe heating the S/ODE
mixture for longer than one hour, which causes a transition of the solution from colorless to
bright yellow. The reactions within chalcogen-olefin mixtures have been studied extensively for
decades—mostly due to the use of this chemistry in industrially important application such as
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rubber vulcanization and polymer synthesis —but there is no study of the influence of the
reaction products—namely, alkyl-polysulfur adducts—on QD nucleation and growth. The most
closely related work on the role of chalcogen-olefin precursor chemistry on QD synthesis is by
Bullen et al., who have shown that extended heating of the analogous selenium precursor
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Se/ODE lowers CdSe QD yields by 50% due to covalent binding of selenium and ODE. Herein
we describe the mechanism by which reaction of sulfur with ODE induces a change in the shape
of PbS QDs synthesized from the S/ODE precursor from cubic to hexapodal by altering the
ligand chemistry of the growing QD. We monitor the heating-induced structural changes in the
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S/ODE precursor with H-nuclear magnetic resonance spectroscopy ( H NMR), mass
spectrometry, fluorescence spectroscopy, and absorbance spectroscopy. The absolute
concentrations of these adducts are small—less than 4% for the temperature ranges studied—but
still dramatically alter the PbS QD growth process. Understanding the mechanism of PbS QD
growth under these conditions should enable both the reproducible laboratory preparation of high
quality QDs and a commercially viable synthesis of QDs using cost-effective starting materials.
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EXPERIMENTAL METHODS
Materials. Sulfur flakes (99.998%), lead(II) oxide (99.999%), lead(II) acetate trihydrate
(
99.999%), oleylamine (70% tech. grade), 1-octadecene (90% tech. grade), oleic acid (90% tech.
grade), hexadecane (99%), biphenyl (99.5%), 1-decanethiol (96%), and chloroform-d (99.8
atom% D) were purchased from Aldrich. Reagent grade acetone, chloroform, and hexanes were
purchased from VWR International. All reagents and solvents were used as-received without
further purification.
Preparation of the S/ODE Precursor. We prepared sulfur precursors for PbS QD synthesis
by dissolving elemental sulfur in 1-octadecene solvent. We heated 100 ml ODE in a round-
bottom flask to 102 °C and stirred for 1 hr under a flow of dry N gas to dry. The temperature
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was then raised to the temperature desired for the study (180 °C to 210 °C) and 240 mg
elemental sulfur flakes was swiftly added through the flask sidearm under positive N pressure.
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Sulfur flakes completely dissolved within two to five minutes after solid addition to form an
optically clear solution with a light yellow tint, which would become colorless upon cooling to
room temperature. Continued heating produced a slow change in coloration to darker shades of
yellow, and this coloration would persist after cooling. For heating duration studies, aliquots
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were removed at specified intervals, transferred to a glass vial, and cooled using a room
temperature water bath. All precursors could be effectively used for PbS QD synthesis more than
two weeks following preparation when stored under ambient conditions.
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Synthesis and Purification of PbS Quantum Dots. We prepared PbS QDs using a
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procedure modified from the one reported by Li et al. 357 mg lead(II) oxide, 8.5 ml oleic acid,
and 56 ml 1-octadecene were added to a round-bottom flask and heated to 210 °C while stirring
and continuously flowing dry, N gas through the vessel. Heating took approximately 1 hour,
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during which time the lead(II) oxide reacted with oleic acid to form a colorless, optically clear
solution of lead(II) oleate. Immediately prior to reaction with the sulfur precursor, 8 ml of
oleylamine was injected into the lead(II) oleate solution and the temperature allowed to rise back
to 210 °C. Once the solution reached the reaction temperature of 210 °C, 8 ml of the S/ODE
precursor solution was swiftly injected into the reaction vessel. The reaction solution began
turning brown within 15 seconds following S/ODE injection. The reaction mixture was quenched
one minute after S/ODE injection by pouring the reaction mixture into a jar of chloroform chilled
in an ice water bath. We recovered the QDs by centrifuging the reaction mixture and then
washed the QDs once by making them up in chloroform and crashing by adding acetone to this
solution in a 3:1 (v:v) ratio of acetone:chloroform. The PbS QD product was a black precipitate
that could be dispersed in either chloroform or hexanes to produce a black, optically clear
colloidal QD solution.
Spectroscopic Characterization of the S/ODE Precursor. We performed ground state
absorption spectroscopy of S/ODE precursor aliquots using an Agilent Technologies Cary 5000
UV-vis-NIR spectrophotometer. We acquired absorption spectra using an optical glass cuvette
with a 2mm pathlength. We performed photoluminescence (PL) spectroscopy using a Horiba
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Jobin Yvon Fluorolog 3 Spectrofluorometer, with emission and excitation slit widths of 5 nm
and an excitation wavelength of 360 nm. All PL spectra were acquired in a 1-cm pathlength
quartz cuvette; S/ODE precusors were diluted to 1% by volume in hexanes for all PL studies.
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H NMR. We acquired all H-NMR spectra of S/ODE precursor samples using a Bruker
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Avance III 500 MHz spectrometer and all H-NMR spectra of PbS QDs using an Agilent DD2
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00MHz spectrometer with 99.8 atom % CDCl as a solvent. We prepared NMR samples of
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S/ODE precursor by adding 50 μl of the raw precursor to 1.0 ml of CDCl with 19 mM biphenyl
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as an internal concentration standard. Prior to acquiring H-NMR spectra of the PbS QD
samples, we performed additional purification of the samples to remove organics not bound to
the QD. We removed solvent from the QD dispersion by rotary evaporation and washed the
recovered QD powder once with 5 ml of acetone followed by rotary evaporation to remove
residual acetone. We made up the final samples in 1.0 ml CDCl . All signals from residual
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ODE—used as solvent in the QD synthesis—were gone following a single washing of the QD
powder with acetone. The standard VnmrJ s2pul pulse sequence was used for all samples, except
for the PbS QD samples, for which we used a two second overhead delay. Chemical shifts of all
spectra were referenced to the residual CHCl singlet at 7.26 ppm.
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TEM. We performed transmission electron microscopy (TEM) on PbS QD samples using an
Hitachi H-8100 TEM operated at 200 kV emission voltage. We deposited samples from
chloroform onto ultrathin carbon on holey carbon/Cu TEM grids, 400 mesh (Ted Pella).
MALDI-TOF. We acquired matrix-assisted laser-desorption ionization time-of-flight mass
spectra of aliquots of elemental sulfur heated in 1-octadecene at 180 °C removed at 5 and 120
minutes following sulfur addition using a Bruker Apex III MALDI. We used α-cyano-4-
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hydroxycinnamic acid as the matrix material loaded to approximately 20% by mass with analyte.
We acquired mass spectra in reflectron TOF mode and positive polarity.
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RESULTS AND DISCUSSION
The Heating Time for the S/ODE Precursor Influences the Size and Shape of PbS QDs.
We separately prepared sulfur and lead precursors by heating elemental sulfur in ODE and
heating lead(II) oxide in the presence of oleic acid to form solutions of sulfur in ODE and
lead(II) oleate, respectively. We then combined the lead and sulfur precursors at 210 °C under
nitrogen in the presence of oleylamine to reduce sulfur in situ and form PbS. Over the course of
heating for two hours at a temperature of 180 °C, the color of the S/ODE solution changes
continuously from colorless to bright yellow. The shape and size of the PbS QDs produced from
the S/ODE precursor depends on how long it is heated. For example, PbS particles prepared
using a S/ODE precursor heated for two minutes at 180 °C were cubic with an average edge
length of 11 ± 1 nm, whereas particles prepared using a precursor heated for two hours at 180 °C
were hexapodal with an average tip-to-tip diameter of 27 ± 7 nm. The estimated hexapod
volumes are between five and fifteen times larger than the mean volume for the cubes. The TEM
images in Figure 1a-d show this transition from cubic to hexapodal QD shape as a function of
the duration of S/ODE precursor heating. To understand how this seemingly small change in the
precursor preparation produces such a drastic effect on the final QD shape, we analyzed the
chemical changes occurring in the S/ODE precursor solution when exposed to elevated
temperatures for various lengths of time.
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Figure 1. Transmission electron micrographs
of purified PbS QDs synthesized using
aliquots of sulfur heated in 1-octadecene at
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80 °C under N for either a) 5 minutes, b) 30
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minutes, c) 60 minutes, or d) 90 minutes. The
scalebar represents 50 nm for the main
images and 10 nm for insets. These QDs
show a shape transition from cubic to
hexapodal as the heating duration of the
S/ODE precursor is increased.
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Colorimetric Detection of the Kinetics of the Reaction between Sulfur and ODE. The
S/ODE precursor consists of elemental sulfur (i.e. cyclooctasulfur, S ) dissolved in 1-octadecene
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by heating under nitrogen. While heating at 180°C, solutions of S/ODE develop a bright yellow
color that persists after the solution is cooled to room temperature. The absorbance of aliquots
taken at early heating times is predominantely in the ultraviolet region, but later aliquots show
appreciable absorbance intensity over 400-500 nm and the emergence of a series of broad
absorption peaks between 320 and 400 nm (Figure 2a) consistent with the observed color
change. Photoluminescence (PL) spectra of aliquots of S/ODE precursor in hexanes, excited at
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60 nm, show a very broad emission feature at 430 nm that grows in over time (Figure 2b). The
broad absorbance and emission features indicate the heating produces a structurally disperse set
of fluorescent species that are chemically stable for more than thirty days at ambient conditions.
We do not believe the colorimetric changes we observe are due to the formation of solvated
sulfur, because when we heat elemental sulfur in hexadecane instead of ODE, the dissolution rate
is very similar, but the yield of fluorescent species (λem = 430 nm) is dramatically lower in the
hexadecane solutions than in the ODE solutions (Figure S1). This experiment reveals that the
carbon-carbon double bond in ODE accelerates the formation of the stable fluorescent species.
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Figure 2. a) Ground state absorption spectra
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of S/ODE aliquots normalized to absorbance
at 300nm show a shift of the absorption
intensity from ultraviolet to visible
wavelengths as the heating duration increases.
b) Photoluminescence spectrum of S/ODE
aliquots diluted in hexanes for a series of
heating times between six and 150 minutes,
and excited at 360 nm. A large, broad
emission feature grows in as the S/ODE
solution is heated. The sharp peak at 402nm
is due to the hexanes Raman mode. c) Plot of
the change in PL signal, integrated from λ =
em
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70 to λem = 600 nm, of aliquots taken from
an S/ODE solution heated at various
temperatures, and excited at 360 nm. Linear
fits of the initial rates are shown as dashed
lines. d) Plot of the natural log of the initial
PL signal growth rate versus reciprocal
temperature. Error bars indicate fitting error
for the linear fits in (c). The Arrhenius-type
behavior indicates a 92 kJ/mol activation
barrier to formation of the photoluminescent
species.
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Figure 2c plots the change in PL signal integrated from λem = 370 to λem = 600 nm as a
function of the heating time for the S/ODE solutions, for four heating temperatures: 180, 190,
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00, and 210 °C. PL is defined as the integrated PL for the aliquot acquired at the earliest time
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point; we subtract PL from the integrated PL at later times to yield “PL-PL ” in Figure 2C. This
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process subtracts out the Raman feature of hexanes at 402 nm. The PL intensity increases
linearly with time for all temperatures, and only the PL of the samples heated at 210 °C show
signs of saturation over the studied time range. Figure 2d shows a plot of the natural logarithm
of the initial PL growth rate, obtained from linear fits of the data in Figure 2c, versus reciprocal
temperature, and a linear fit of this data. The Arrhenius analysis yields an activation energy for
the formation of the photoluminescent species of 92 kJ/mol. This activation energy is similar to
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the value measured for the polymerization of S initiated by thiol in the presence of base.
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The kinetics of the colorimetric changes in S/ODE require a mechanism that forms a
structurally disperse set of air-stable species, is thermally irreversible, and is accelerated by the
presence of carbon-carbon double bonds. Sulfur melts undergo a discontinuous change in their
viscosity at 158 °C—known as the “lambda transition”—during which the cyclooctasulfur rings
that comprise elemental sulfur begin to homolytically cleave sulfur-sulfur bonds and polymerize
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to produce catena-sulfur, which is composed of variable-length polysulfur chains. Catena-
sulfur exhibits vibrationally-broadened fluorescence, but the lambda transition is generally
thermoreversible. Given the acceleration of the formation of fluorescent species by the presence
of a carbon-carbon double bond, we hypothesize from the colorimetric studies that the actual
fluorescent species is a polysulfur-alkyl adduct formed when sulfur reacts with the double-bond
of ODE. A polysulfur-alkyl adduct would retain the fluorescence of catena-sulfur while trapping
sulfur in a stable, polymeric form via covalent bonding to an alkane.
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H-NMR Measurement of S/ODE Reaction Products. The kinetics of colorimetric
changes of S/ODE mixtures heated to 180 °C suggest a reaction between sulfur and ODE that
forms fluorescent polysulfur-alkyl adducts. Previous studies of the polymerization reactions of
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sulfur with alkenes
show that elemental sulfur undergoes addition chemistry with alkenes at
elevated temperatures via either radical or ionic mechanisms (see Scheme S1 in the Supporting
Information). Based on these mechanisms, the principal products of sulfur addition to ODE are a
constitutionally disperse set of compounds with a polysulfur chain bonded to either the C1 or C2
position; in the formation of these products, the carbon-carbon double-bond either undergoes an
addition reaction with sulfur or migrates to an internal position. Distinguishing between the
mechanisms of S/ODE polymerization is outside the scope of the current study; we instead focus
on the structure and yield of organosulfur products of the S/ODE precursor preparation on QD
growth.
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H-NMR spectra of aliquots taken from S/ODE solutions heated at 180 °C provide clear
evidence of the structural changes associated with the reaction of sulfur and ODE. After 90
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minutes of heating sulfur in ODE at 180°C, a series of new H signals appear between 2.5 and
3.5 ppm (Figure 3a). Figure 3b shows the assignments of these features to the protons on
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carbons bonded to sulfur; assignments are based on H- H Correlation Spectroscopy (Figure
S4). Notable are –S–CH – groups bound to an internal carbon-carbon double-bond (product 1 in
2
Figure 3b), which are a characteristic sign of the migration of ODE’s terminal carbon-carbon
double bond to an internal double bond following reaction with S . The distinctive peak at 2.51
8
ppm corresponds to –S–CH – bound to a tertiary carbon (product 3 in Figure 3b).
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Figure 3. a) Region of the H-NMR
spectrum of S/ODE aliquots heated at 180°C
for 5 minutes and 90 minutes that contain
signals from protons on carbons directly
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bonded
corresponding to the numbered regions in
a). These compounds may also take on a
to
sulfur.
b)
Structures
(
bridged form, where two hydrocarbon
chains are linked by a variable-length sulfur
chain (S ), without measurable change in the
x
1
H NMR spectrum. c) The change in
1
integrated peak area of H-NMR peaks in
(a) with time, for a reaction mixture of
elemental sulfur and ODE heated at 180C.
The concentrations of all three products
shown in (b) increase with increased S/ODE
heating duration. d) The change in
1
integrated peak area of H-NMR features
corresponding to protons on internal (red)
and terminal (blue) double bonds within the
S/ODE reaction mixture after heating at
1
80C for a series of times. With increasing
S/ODE heating time, the concentration of
species with terminal double bonds
decreases due to reduction of sulfur
accompanied by either elimination of the
double bond to form compounds 2 and 3, or,
to a lesser degree, migration of the double
bond to form compound 1 (red circles in d
and yellow triangles in c).
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Figure 3c shows that the integrated peak area corresponding to all organosulfur compounds
(
1, 2, and 3) increases with S/ODE heating time. The majority products are unsaturated, straight-
chain alkanes bonded to a polysulfur chain. Branched and double-bond migration products have
a lower yield.
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H-NMR spectra of our samples show both an elimination of terminal double-bonds and a
formation of internal double-bonds in S/ODE aliquots heated at 180 °C (Figure 3d); over a
period of 90 minutes, the integrated peak areas associated with terminal double-bonds (δ = 5.81
ppm; δ = 4.99 ppm; δ = 4.93 ppm) all decrease by ~10% while those associated with internal
double-bonds (initially present due to isomeric impurities in technical grade ODE, Figure S2) (δ
=
5.41 ppm) increase by ~80%. Approximately ten terminal double-bonds are eliminated for
every internal double-bond formed, so there is a net loss of vinyl functional groups. Following
coordination of elemental sulfur to ODE’s terminal double bond to form either a cation or radical
intermediate (Reactions 3 and 4, Scheme S1), the intermediate can produce either: (1) a 2,3-
unsaturated product following proton loss (Reactions 5 and 6, Scheme S1) or (2) a 1,2-
disubstituted product following attack by a nucleophile. The overall loss of carbon-carbon
double-bonds indicates the proton loss required for double-bond migration occurs more slowly
than the nucleophilic attack required for addition to the double-bond. This result is consistent
with the higher yield of saturated products (product 2 in Figure 3b) than 2,3-unsaturated
products (product 1 in Figure 3b).
Prior work on the side products of CdS QD synthesis involving reduction of sulfur with ODE
10
showed thiophenes to be a primary side-product of this reaction. Any such compound would
produce peaks between 6.7 and 7.2 ppm, but we observe no such signals in either the S/ODE
precursor samples tested or the crude PbS QD product prior to purification.
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If we assume each ODE molecule only forms one carbon-sulfur bond, 3% of the original
ODE molecules have bound sulfur after 90 minutes of heating at 180 °C. Matrix-assisted laser-
desorption time-of-flight mass spectroscopy of S/ODE shows the formation of compounds at m/z
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=
282 and 530 during heating. The lighter fragment is consistent with a single sulfur atom bound
to a single ODE molecule, and the heavier fragment is consistent with two ODE molecules
bridged by a sulfur atom (Figure S5). Higher molecular weight fragments were not observed, so
the principal S/ODE reaction products of interest are either: (1) C H –CH –S or (2) C H –
17
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2
x
17 34
CH –S –CH –C H , where x is an integer. ODE heated for two hours at 210 °C without sulfur
2
x
2
17 34
present showed no change in integrated vinyl or aliphatic proton signals (Figure S3).
1
H-NMR confirms the presence of products of types 1, 2, and 3 in Figure 3b from reaction of
elemental sulfur with ODE. The reaction of sulfur with ODE therefore produces multiple new
stable species that able to coordinate to the QD surface.
Mechanism by which S/ODE Polymerization Influences the Growth of PbS QDs. For all
temperatures studied, elemental sulfur flakes readily dissolved within five minutes of addition to
hot ODE. In contrast, the S/ODE reactions progress over a timescale of hours (Figures 2, 3). We
therefore conclude that the dependence of the growth of PbS QDs on the heating time of the
S/ODE precursor (Figure 1) is not attributable to incomplete dissolution of sulfur at short
heating durations. Three possible mechanisms by which ODE reduction of sulfur affects the PbS
QD synthesis are: (1) loss of sulfur as H S gas, (2) decreased oleylamine reduction activity, and
2
(3) coordination of polymerization products to the growing QD surface. Mechanisms (1) and (2)
render the S/ODE solution a less effective source of amine-reducible sulfur and mechanism (3)
alters the QD growth process via in situ generation of organosulfur ligands.
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We tested mechanisms (1) and (2) by quantifying the evolved H S gas both during the
2
dissolution of sulfur in ODE to form the S/ODE precursor, and during the reaction of S/ODE
with oleylamine (Figures S6 and S7). Even when heated at 210°C, the S/ODE solution only
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loses 0.5% of its total sulfur content as H S gas over the first two hours of heating. The yield of
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H S produced by reacting S/ODE with oleylamine is not measurably affected by the heating
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duration of the S/ODE precursor prior to reaction. We conclude that for the S/ODE preparation
temperatures and durations employed in this study, neither the amount of available sulfur nor its
reactivity with oleylamine significantly affect the yield of the reduced sulfur species that form
PbS. We also checked whether the QD synthesis was affected at all by trace water by applying
vacuum to the lead(II) oleate and oleylamine in ODE solution at 80°C, but we observed no
change in PbS QD shape or size following more rigorous water exclusion (Figure S8).
Elucidating water’s complex role in QD growth, which may include altering the number of
bound oleate ligands and participating in the reduction of elemental sulfur by amines, is outside
the role of the current study. As such we have not attempted to quantify the amount of remaining
water; the application of vacuum to the precursor solution shows the reaction is insensitive to the
variations in water concentration commonly encountered in laboratory QD synthesis.
If the in situ creation of polysulfur-alkyl adducts affects the PbS QDs’ shape by binding to
the growing QD (mechanism 3), we should observe changes in the types of ligands present on
the final, purified particles. We prepared PbS QDs according to a published method by injecting
S/ODE precursor into a solution of lead (II) oleate in ODE at 210 °C with oleylamine as an in
17
situ sulfur reducing agent. We repeated the PbS QD synthesis using S/ODE precursors
prepared by heating sulfur in ODE for 5, 30, 60, and 90 min at 180 °C and washed the recovered
QD powder with acetone to remove residual ODE. All QD workup was performed in CHCl3.
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Figure 4. (a) Representative H-NMR
spectrum of purified PbS QDs in CDCl3
synthesized using sulfur heated in ODE
for 30 min; a zoom-in on the vinyl region
is shown in the inset. The relevant signals
associated with protons on internal
carbon-carbon double-bonds and terminal
methyl groups are indicated along with
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residual acetone and H O peaks. The
2
broad, intense peak at 1.27 ppm comes
from protons on alkyl carbons at least one
carbon removed from any functional
groups. (b) Vinyl and organosulfur (inset)
1
regions of H NMR spectra of samples of
PbS QDs synthesized using S/ODE
precursors prepared by heating at 180°C
for 5, 30, 60, or 90 minutes. The intensity
of all spectra have been scaled to make
their intensities at 5.34 ppm equal to more
easily compare their linewidths. All
samples except for the five minute sample
include a broad signal from protons on
the carbon alpha to the sulfur at 2.77 ppm
(
inset). (c) Ratio of signal from CH -R
3
protons relative to CH=CH protons
(determined by integrated peak areas) for
PbS QDs synthesized using a sulfur
precursor prepared by heating sulfur in
ODE for the indicated duration. The
dashed line indicates the expected CH3-
R:CH=CH signal ratio if the only ligand
bound to the QDs were oleate.
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Figure 4a shows a representative H-NMR spectrum of purified PbS QDs; the peaks
corresponding to hydrogens on an internal carbon-carbon double bond (δ = 5.34 ppm) and on a
terminal methyl group (δ = 0.88 ppm) are highlighted. The vinyl peak at 5.34 ppm is broader
than that of free oleic acid, which is a clear doublet of triplets, and the linewidth of this signal
varies from sample to sample, so we know that at least a portion of the oleate in the sample is
bound to the QD at any given time. We also observe a weak feature at 2.77 ppm (Figure 4b,
inset) that, based on comparison with spectra of 1-decanethiolate bound to PbS QDs (Figure
S9), we assign to an organosulfur species undergoing exchange with the QD surface. The signal
at 2.77 ppm is most intense for QDs prepared using a S/ODE precursor heated at 180°C for 90
minutes and indistinguishable from noise for QDs prepared using the S/ODE precursor heated
for 5 minutes. We do not attempt to analyze this signal quantitatively because the integration of
signals from these ligands becomes more unreliable as the binding constant of the ligand
increases and the signal broadens.
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Figure 4c plots the ratio of integrated peak area associated with protons on CH -R groups (δ
3
=
0.88 ppm) to the integrated peak area associated with protons on R-CH=CH-R groups (δ =
5.34 ppm). The ratio of methyl-to-vinyl protons increases with S/ODE precursor heating
2+
duration. There are two conclusions we draw from this data. (i) Oleate, which binds to Pb on
the surface of the QDs, cannot be the only ligand binding to the QD surface. Oleate contains one
1
CH -R and one R-CH=CH-R group, and we should observe their integrated H-NMR signal ratio
3
to be 1.5:1 (as indicated by the dotted line in Figure 4c) if oleate is the only ligand bound to the
QD surface. At least one other structurally distinct ligand must be present to account for this
discrepancy. We believe that the additional ligands are organosulfur compounds of types 1, 2,
and 3 (Figure 3b) produced during the S/ODE reaction; these compounds remain on the QD
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surface after purification. (ii) The ratio of organosulfur molecules to oleate ligands in the
sample, as indicated by the proton signal ratio in Figure 4c, increases with the heating duration
of the S/ODE precursor. The change in shape of the QDs (from cubic and hexapodal, Figure 1)
is correlated with the molar ratio of organosulfur species to oleate.
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Previous studies of anisotropic PbS nanoparticle synthesis have shown that ligand chemistry
can control PbS QD shape by controlling the differential rates of growth along <100> and <111>
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crystal faces
. Preferential growth along <100> faces leads to hexapods while preferential
growth along <111> faces leads to cubes or octapods. Density functional calculations of the
binding isotherms of oleate ions to PbSe <100> and <111> faces show these faces to be in a
competitive equilibrium based on the concentration of oleate ions in solution, and <100> faces
lose coverage more rapidly than <111> faces as the oleate ion concentration decreases. These
calculations predict a transition from cubic to octahedral QD growth as oleate ion concentration
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is decreased, which is consistent with experiment.
Any organosulfur species produced by the
S/ODE reaction would presumably bind through the sulfur in a binding motif similar to a thiol.
3
Like oleate, thiols preferentially bind to <111> faces because they adopt a μ -Pb -SR bridging
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mode where they can bind to three lead atoms; thiol addition favors <100> face growth. In fact,
1
-dodecanethiol has been used to synthesize PbS hexapods both via the thermal decomposition
29-30
of a molecular PbS precursor and via the direct reaction of the thiol with lead(II) acetate.
The partitioning of any ligand between the <100> and <111> faces is controlled by: (i) the
relative binding energy of the ligand on the faces and (ii) the concentration of the ligand, with
lower concentrations favoring coverage of the face with the higher binding energy. The
concentration of organosulfur compounds produced during S/ODE heating is less than 10% of
the concentration of oleate molecules, so the adducts should preferentially bind to <111> binding
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sites. These organosulfur compounds also may have a higher molecular weight than the oleate
ligands because they are the result of a polymerization process with 1-octadecene. This
combination of a binding preference for <111> faces and high molecular weight makes these
compounds highly effective at retarding growth at the <111> faces, such that preferential growth
of the <100> faces results in formation of QDs with octahedral symmetry (i.e. hexapods). This
prediction is consistent with the observed results: as the heating duration of the S/ODE precursor
is extended and more of the organosulfur products are formed, the shape of QDs changes from
cubic (i.e. primarily <111>-face growth) to hexapodal (i.e. primarily <100>-face growth).
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CONCLUSIONS
We have described the variation of PbS QD shape from cubic to hexapodal caused by
increasing the heating time of the S/ODE precursor from five minutes to two hours at 180°C,
which is a typical temperature and range of heating durations found in published PbS QD
1
syntheses. H-NMR spectra indicate that extended heating of S/ODE causes sulfur and ODE to
react and produce a series of organosulfur products. This reaction does not measurably affect the
reactivity of the S/ODE precursor with lead oleate; however, NMR spectra of purified PbS QDs
indicated that the organosulfur products of this reaction bind to the QD surface, even after the
QDs are purified by precipitation and washing with acetone. The creation of organosulfur ligands
during S/ODE heating provides a mechanism for QD shape control: the binding of these ligands
to the growing QD induces a preferential growth at the <100> faces, and therefore a hexapodal
geometry for the particles.
Techniques to produce PbS particles of various shapes – often using S/ODE precursors – are
17, 23, 25-26, 29-38
the focus of numerous reports.
Since the timescale of reaction between sulfur and
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ODE is significantly longer than the timescale of sulfur dissolution—hours instead of minutes—
S/ODE can be made a much more reliable precursor by reducing the temperature and duration of
the sulfur dissolution step. Additionally, any metal sulfide QD synthesis using elemental sulfur
heated to high temperatures should take steps to reduce the in situ yield of organosulfur
byproducts by avoiding olefinic solvents.
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Our present work on the S/ODE precursor has focused solely on the effects of organosulfur
compounds on the growth stage of QD synthesis. Further work will investigate the role of these
compounds in the much faster nucleation process and its influence on the QD product.
Supporting Information Available. Additional experimental methods, pathways for
1
polymerization of sulfur and ODE, additional H NMR spectra of ODE and crude PbS product,
COSY NMR and MALDI-TOF spectra of the S/ODE precursor, measurement of sulfur loss as
H S, measurement of S/ODE reactivity with ODE, effect of residual water on the synthesis, thiol
2
titration of PbS QDs, and Figures S1-S10. This information is available free of charge via the
Internet at http://pubs.acs.org/.
Acknowledgements. This project was funded by the National Science Foundation through
the Northwestern Materials Research Science and Engineering Center (grant DMR-1121262).
The authors acknowledge the EPIC facility of the NUANCE Center at Northwestern University
where electron microscopy studies were performed and the IMSERC facility at Northwestern
University for the use of nuclear magnetic resonance and mass spectrometry facilities.
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