Two-phase approach to high-quality, oil-soluble, near-infrared-emitting PbS quantum dots by using various water-soluble anion precursors

Article abstract of DOI:10.1002/ejic.201100012

Colloidal PbS quantum dots (QDs) with tunable photoemission throughout the near-infrared (NIR) region (ca. 750-1000 nm) were synthesized by a two-phase approach. Here, oil-soluble lead oleate formed by a reaction of lead acetate and oleic acid (OA, the capping agent) in n-decane at 130 °C was used as lead precursor, and water-soluble Na₂S, thioacetamide (TAA), and thiourea, each having a different reactivity, were used as sulfur sources. When an n-decane solution of lead precursor and an aqueous solution of sulfur precursor were mixed at the appointed temperature, oil-soluble, near-infrared-emitting PbS QDs were achieved. In this study, we investigated the influence of the reactivity of water-soluble sulfur sources on the synthesis of PbS QDs. The morphology and crystal structure of the as-prepared PbS QDs were characterized by (high-resolution) transmission electron microscopy (TEM), selected area electron diffraction (SAED), and X-ray diffraction (XRD).

Full text of DOI:10.1002/ejic.201100012

FULL PAPER
DOI: 10.1002/ejic.201100012
Two-Phase Approach to High-Quality, Oil-Soluble, Near-Infrared-Emitting
PbS Quantum Dots by Using Various Water-Soluble Anion Precursors
Dawei Deng,[a,b] Jie Cao,[b] Junfei Xia,[b] Zhiyu Qian,[c] Yueqing Gu,*[b] Zhongze Gu,*[a]
and Walter John Akers[d]
Keywords: Lead sulfide / Quantum dots / Synthesis design / Fluorescence
Colloidal PbS quantum dots (QDs) with tunable photoemis-
sion throughout the near-infrared (NIR) region (ca. 750–
1000 nm) were synthesized by a two-phase approach. Here,
oil-soluble lead oleate formed by a reaction of lead acetate
and oleic acid (OA, the capping agent) in n-decane at 130 °C
was used as lead precursor, and water-soluble Na2S, thio-
acetamide (TAA), and thiourea, each having a different reac-
tivity, were used as sulfur sources. When an n-decane solu-
tion of lead precursor and an aqueous solution of sulfur pre-
cursor were mixed at the appointed temperature, oil-soluble,
near-infrared-emitting PbS QDs were achieved. In this study,
we investigated the influence of the reactivity of water-solu-
ble sulfur sources on the synthesis of PbS QDs. The mor-
phology and crystal structure of the as-prepared PbS QDs
were characterized by (high-resolution) transmission elec-
tron microscopy (TEM), selected area electron diffraction
(SAED), and X-ray diffraction (XRD).
commonly used organic NIR dyes, these quantum dots
show many favorable photophysical properties, such as
fluorescence emission that can be tuned by varying the size
and composition, broad excitation spectra, large Stokes
shifts, and superior photostability.[5] QDs have therefore
been promising alternatives to organic NIR dyes in many
applications.[5,6] Thus, the synthesis of inorganic QDs emit-
ting in the NIR, the optical window for in vivo imaging, is
of particular interest.
Introduction
Colloidal semiconductor quantum dots (QDs, or nano-
crystals) have attracted tremendous attention since the
1990s, owing to their unique size-dependent optical and
electronic properties,[1] as well as their potentials for appli-
cations in light-emitting diodes (LEDs), solar cells, and bio-
logical labeling.[2] To date, a large variety of high-quality
semiconductor quantum dots such as CdSe, CdTe, CdS,
have been synthesized, through traditional organometallic
or aqueous methods.[3,4] Among them, colloidal QDs with
tunable near-infrared (NIR) emission in the range 700–
1000 nm are becoming increasingly attractive in the last five
years, because they can be used for noninvasive in vivo bio-
medical imaging.[5,6] The use of NIR (700–1000 nm) light
for biomedical imaging is grounded in first principles, and
it is best understood in the context of photon propagation
through living tissue and the signal-to-background ratio (in
this spectral region, the autofluorescence and absorption
from most flesh tissues are lowest).[5–7] In comparison to
Over the past decade, great efforts have been invested
into the synthesis of NIR-emitting QDs.[5,6] On the basis
of CdTe (typically, type II CdTe/CdSe),[8,9] CdHgTe,[10] and
HgTe[11] semiconductors, quantum dots emitting above
700 nm in the NIR region were firstly obtained. Unfortu-
nately, these Cd-, Hg-, Te-, and Se-based quantum dots
show high in vitro and in vivo toxicity, since they will
eventually lead to a release of toxic elements (com-
pounds).[12] This represents a major roadblock to the prac-
tical use of QDs and has motivated the development of new
Hg- and Cd-free NIR QDs, based on III–V,[12–14] I–III–
VI2,[15–18] or other semiconductors, such as representative
InAs,[12,13] InP,[14] CuInSe2,[16] CuInS2,[17] and so on.[18]
However, so far, most of the previous approaches are, to
the best of our knowledge, still very complicated and need
high reaction temperatures (greater than ca. 200 °C).[12–18]
Furthermore, there are some other major problems that are
worth mentioning. For InP and InAs nanocrystals,[12–14] the
precursors, namely tris(trimethylsilyl) phosphide and tris-
(trimethylsilyl) arsenide, are extremely expensive, unstable,
and hazardous; the resulting phosphides and arsenides ex-
hibit poor chemical stability in comparison to sulfides,
which is always a concern in technical applications. For Cu-
[a] State Key Laboratory of Bioelectronics, School of Biological
Science and Medical Engineering, Southeast University,
Nanjing 210096, China
E-mail: gu@seu.edu.cn
[b] Department of Biomedical Engineering, School of Life Science
and Technology, China Pharmaceutical University,
Nanjing 210009, China
[c] Department of Biomedical Engineering, School of Automation,
Nanjing University of Aeronautics and Astronautics,
Nanjing 210016, China
E-mail: cpuyueqing@163.com
[d] Department of Radiology, Washington University School of
Medicine,
4525 Scott Ave, St. Louis, USA
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High-Quality, Oil-Soluble, Near-Infrared-Emitting PbS Quantum Dots
InSe2 and CuInS2 nanocrystals, not surprisingly, control of using water-soluble anion precursors, the synthesis steps of
the ternary composition stoichiometric ratio in the nano- the traditional organic-phase methods will be simplified
crystals is an obvious challenge, in comparison to binary greatly. However, this has proved to be a difficulty.[28] A few
semiconductor nanocrystals; to prevent the oxidation of years ago, the group of An succeeded in developing a versa-
copper(I) in nanocrystals and improve the optical proper- tile two-phase approach to synthesize highly luminescent
ties, forming the CuInS2(or CuInSe2)/ZnS core/shell struc- CdS,[29] extremely small CdSe,[30] TiO2,[31] and other nano-
ture is essential, through complex high-temperature pro- crystals,[32] derived from the Brust method.[33] Sub-
cesses.[15–17] Hence, developing a new, facile, rapid, and mild sequently, they further studied the nucleation and growth
approach to synthesize air-stable NIR-emitting QDs re- of CdSe and CdS QDs in a two-phase system using water-
mains an urgent challenge for scientists.
soluble Na2S, thiourea, NaHSe, and selenourea as sulfur
Compared with the materials mentioned above, PbS can and selenium sources.[34] These previous studies have proven
offer notably excellent size tunability across in the NIR re- that the reactivity of water-soluble anion precursors plays
gion, in view of its small band gap (0.41 eV) and large exci- an important role in affecting the nucleation and growth
ton Bohr radius (18 nm).[19] Meanwhile, it also provides a of the oil-soluble CdS and CdSe QDs. To the best of our
chance to produce air-stable, NIR-emitting QDs with inex- knowledge, the use of the two-phase approach to synthesize
pensive and relatively safe synthesis precursors.[19] Here we NIR-emitting PbS QDs has not yet been reported, and the
focus on the controlled synthesis of luminescent colloidal growth rate of PbS QDs is significantly different from those
PbS QDs. In previous reports, most of the synthesis efforts of CdS and CdSe QDs: the growth of PbS QDs is much
have been made on synthesizing irregularly shaped PbS faster, and could even occur at room temperature.[26,27]
nanocrystals;[20] only a limited number of approaches have Thus, it is of great interest to investigate and better under-
been used to form spherical PbS QDs, in aqueous or or- stand what role the activity of anion precursors plays in the
ganic media.[19,21–26] For water-soluble PbS QDs, prepared nucleation and growth processes of PbS QDs in a two-phase
firstly by Kumacheva et al.,[21] lead acetate and sodium sul- system and how to adjust the properties of the products.
fide were used as reactant sources, while thioglycerol and
In this paper, we prepared high-quality, oil-soluble PbS
dithioglycerol were added as capping agents. However, as QDs emitting in the NIR region of 750–1000 nm by a two-
indicated in our recent report,[22] the resulting aqueous PbS phase approach for the first time and explored the influence
QDs exhibited long-wavelength emission ranging from 1000 of the reactivity of water-soluble anion precursors on the
to 1400 nm,[21] relatively poor storage stability, and low nucleation and growth processes of PbS QDs, by using
photoluminescence (PL) quantum efficiency (ca. 10%), al- Na2S, TAA, and thiourea, which differ in reactivity, as sul-
though these water-soluble approaches are very facile.[21–23] fur sources. In addition, using highly reactive Na2S as sul-
For non-water-soluble (or oil-soluble) PbS QDs,[19] the hot- fur source, we investigated further the effects of the heating
injection approach developed by Hines and Scholes seems temperature, the precursor Pb/S molar ratio, the precursor
to be the best to engineer high-quality QD ensembles. This OA/Pb molar ratio, and the storage time on the PbS synthe-
hot-injection method was conducted by heating a mixture sis, as well as studying intensively the role of the reaction
of lead oxide (PbO) and oleic acid (OA, as capping ligands) temperature in controlling the reactivity of water-soluble
in octadecene (ODE) at 150 °C under Ar for one hour fol- anion precursor, by using moderately reactive TAA as sul-
lowed by the injection of a solution of (TMS)2S (sulfur fur source. The progress achieved by this two-phase ap-
source) in ODE at 150 °C. After the hot injection, the QD proach not only increases the quality of these oil-soluble
growth was monitored either at 80–140 °C or at room tem- PbS QDs, especially their emissive properties, up to a level
perature. The resulting oil-soluble PbS QDs exhibited long- comparable to that of CdSe and other II-VI semiconductor
wavelength emission in the range 1000–2000 nm (PL quan- QDs, but also enhances our understanding of the relation
tum yield: ca. 20%), presumably due to the high hot-injec- between the reactivity of water-soluble anion precursors
tion temperature.[19,24] Subsequently, the optical properties and the synthesis of oil-soluble QDs in a two-phase system.
of oil-soluble PbS QDs were further optimized through the
hot injection method and its variants.[19,24–26] Very recently,
the group of Yu developed a new non-injection approach to
Results and Discussion
synthesize NIR-emitting PbS QDs in ODE.[27] These results
represented the best optical quality of the NIR fluorescent
This manuscript addresses our two-phase approach to
PbS QDs in organic media. However, unfortunately, these NIR-emitting, oil-soluble PbS QDs in n-decane, by using
methods are still complicated and involve the dangerous differently reactive water-soluble Na2S and thioacetamide
chemical bis(trimethylsilyl) sulfide [(TMS)2S] or poorly re- (and thiourea) as sulfur precursors. The results and dis-
active elemental S.
cussion are presented in two main parts. The QD synthesis
As described above, these previous reactions were all car- using highly reactive Na2S as sulfur precursor is presented
ried out either in the aqueous phase or the organic phase, first. Here, various synthesis parameters affecting the qual-
and both nucleation and growth of the QDs happened only ity of the PbS QDs are discussed in detail, in the sequence
in a homogeneous system.[19,21–27] As expected, if we can heating temperature, precursor Pb/S molar ratio, precursor
combine the respective advantages of both approaches OA/Pb molar ratio, and storage time. The QD synthesis
properly, for instance, to synthesize oil-soluble QDs by with moderately reactive thioacetamide (and poorly reactive
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FULL PAPER
thiourea) as sulfur precursor is dealt with next, and here
the influence of the reaction temperature on the nucleation
and growth of PbS QDs is investigated intensively. In ad-
dition, we also present the results of the further characteri-
zation of the morphology and crystal structure of the as-
prepared PbS QDs by means of TEM, SAED, and powder
XRD.
Synthesis of OA-Capped PbS QDs with Highly Reactive
Na2S as Sulfur Source
In this study, we initially used the highly reactive Na2S
as sulfur precursor to synthesize OA-capped PbS QDs by a
two-phase method. The typical synthesis steps are described
in the Experimental Section. When the aqueous solution of
Na2S was added dropwise to the lead oleate precursor solu-
tion whilst stirring, the color of the upper organic phase
changed from clear yellow to dark-brown instantly, indicat-
ing the formation of PbS QDs. Figure 1 shows the absorp-
tion and PL spectra of the as-prepared OA-capped PbS
QDs with an OA/Pb/S molar ratio of 40:10:3. The absorp-
tion shoulder and PL peak are at 595 and 770 nm, respec-
tively. Compared with bulk PbS (the band gap energy for
bulk-phase PbS is 0.41 eV; λmax = 3020 nm), the PL peak
and absorption shoulder are vastly blueshifted. Thus, the
band gap of PbS QDs is engineered in the near-infrared
spectral region of 700–900 nm, which is also evidence for
the strong effect of quantum confinement. Clearly, the pho-
toemission bandwidth is narrow, with full width at half-
maximum (fwhm) on the order of approximately 100 nm.
Here, it is worth noting that this fwhm value is small and
comparable to the best values obtained with other synthesis
routes,[19,21–27] which suggests a narrow size distribution
achieved without any post-synthesis size-selective precipi-
tation (Figure 7Ͻxigr7ϾA). Also, the PL quantum yield
Figure 1. (A) Absorption and PL spectra of OA-capped PbS QDs
using highly reactive Na2S as sulfur precursor, where the precursor
OA/Pb/S molar ratio was set to 40:10:3 ([Pb2+] = 10 mm); (B) evol-
ution of PL spectra of the growing PbS QDs with the reaction
temperature increased from the initial synthesis temperature
(40 °C) to 100 °C.
(QY) of the resulting oil-soluble PbS QDs in n-decane was formation process, which was finished within about one
estimated to be approximately 30% (relative to the NIR dye minute after the complete addition of Na2S solution, as evi-
cypate in 20% aq. DMSO). To the best of our knowledge, denced by an immediate color change from clear yellow to
this PL QY is larger than those previously reported (PL QY dark-brown in the reaction vessel. To promote further
ca. 20%),[24–27] especially those obtained in water solution growth of PbS QDs in this study, the reaction flask was
(PL QY ca. 10%).[21–23]
heated up by a rate of approximately 2 °C/min. Aliquots
In addition to these advantages, the present two-phase were taken at 40, 50, 75, and 100 °C during the heat treat-
approach is more facile and environmentally friendly, be- ment. Each sample aliquot was kept in a vial and cooled
cause of the use of the low-cost, less noxious, and highly down to room temperature. Figure 1B shows the evolution
reactive water-soluble Na2S as sulfur precursor, relative to of the PL spectra of the growing PbS QDs with the reaction
the existing organic routes based on bis(trimethylsilyl) sul- temperature. During such a temperature increase, the PL
fide [(TMS)2S, sulfur source].[19,24,27] As we know, the peak of the resulting PbS QDs redshifts from approximately
chemical (TMS)2S is dangerous, toxic, and expensive. With 770 to about 823 nm, indicating the gradual growth in QD
poorly reactive elemental S,[25] higher reaction temperatures size as the temperature increases; the fwhm increases from
(90–220 °C) are needed, and the resulting PbS QDs exhibit about 105 nm to about 127 nm, which is consistent with
long-wavelength emission in the range approximately 1250– Ostwald ripening, the process by which larger particles
1500 nm. Hence, using water-soluble Na2S as sulfur source grow at the expense of smaller ones; the PL emission inten-
is very favorable for the preparation of high-quality, oil- sity reaches a maximum at 50 °C (the corresponding PL
soluble PbS QDs with tunable emission in the NIR optical quantum yield in n-decane is ca. 40%), and then, it de-
window of approximately 700–1000 nm, which makes these creases gradually. If the growth temperature was further in-
QDs more promising in near-infrared biomedical imaging.
creased to 120 °C, the PL peak could redshift to approxi-
When using highly reactive water-soluble Na2S as sulfur mately 860 nm, whereas the PL emission intensity would
source, a burst of nucleation occurred with a very fast QD become very weak.
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High-Quality, Oil-Soluble, Near-Infrared-Emitting PbS Quantum Dots
Optimization of the Synthesis Conditions
In addition to the Pb/S feed molar ratio described above,
the OA/Pb feed molar ratio was also observed to influence
the PL properties of the resulting PbS QDs. Here, the con-
We explored further the influence of various synthesis pa- centration of Pb2+ ions was fixed at 10 mm and the Pb/S
rameters on the PL properties of the as-prepared PbS QDs. molar ratio was set to 10:3. As shown in Figure 2B, when
The obtained experimental results are presented in detail in tuning the OA/Pb molar ratio from 20:10 to 80:10, the PL
the following sequence: the precursor Pb/S molar ratio, the emission intensity firstly increased then decreased, and it
precursor OA/Pb molar ratio, and the storage time. Fig- reached a maximum at an OA/Pb molar ratio of 40:10.
ure 2A shows the PL spectra of OA-capped PbS QDs ob- Meanwhile, we noted that such increase in the OA/Pb feed
tained by changing the Pb/S feed molar ratio, where the molar ratio only causes a small redshift in the PL emission
concentrations of OA and Pb2+ ions were fixed at 40 mm peak (ca. 15 nm, from ca. 760 to ca. 775 nm). These experi-
and 10 mm, respectively, and only the added amount of mental results demonstrate that the OA/Pb feed molar ratio
Na2S was varied. When the Pb/S molar ratio was tuned plays a more important role in determining the PL emission
from 10:1 to 10:8, the PL emission peak redshifted grad- intensity rather than the PL peak position. Hence, in this
ually from approximately 750 to about 880 nm. The PL in- study, the precursor OA/Pb/S feed molar ratio of 40:10:3 is
tensity reached a maximum at a Pb/S molar ratio of 10:3. optimal for the synthesis of oil-soluble, NIR-emitting PbS
When the amount of Na2S was increased further, the PL QDs.
intensity decreased gradually. The experimental results (i.e.,
In this study, we also explored the storage stability of the
the gradual redshift of the emission peak induced by con- as-synthesized PbS QDs prepared by using Na2S as sulfur
tinuously feeding more Na2S) may indicate that in the pres- source to further evaluate the QD quality. As we know,
ent synthesis system, a section of the newly formed PbS storage stability is important for the potential applications
by the reaction between the successively introduced sulfur of the QDs. Here, the prepared QD solutions were stored
anions and lead ions would deposit on the surface of the at room temperature in the dark. Temporal evolutions of
initially formed small QDs, resulting in an increase in the PL and NIR absorption spectra of PbS QDs during storage
size of the QDs (as we know, the PL emission maximum of are shown in Figure 3. These data show that for the initial
QDs is mainly determined by the particle size of the seven days, the optical properties (PL emission and absorp-
QDs).[1–5] In addition, the Pb-rich facets of PbS QDs were tion) of the QD solution is relatively stable; the PL emission
capped by OA molecules as a result of the coordination intensity will decrease slowly with prolonged storage time.
interaction between the carboxyl groups of OA (ligand) and However, it is worth mentioning that after approximately
Pb cations; it can be argued that low Pb/S feed molar ratios 30 d of storage at room temperature, the PL emission inten-
would lower the density of lead cations existing on the sur- sity of the QD dispersions decreased only by about 20%,
face of the PbS QDs, which would prevent the formation and no obvious redshift in the PL peak was observed.
of a high-quality Pb–OA complex shell on the QD surface Hence, the OA-capped PbS QDs prepared by using Na2S
(as evidenced by our PL data and those in ref.[35]). As a as sulfur source have slightly improved storage stability, as
result, the PL intensity of the as-synthesized PbS QDs de- compared to those obtained by using the dangerous bis(tri-
creased gradually when the Pb/S molar ratio was decreased methylsilyl) sulfide [(TMS)2S] as sulfur source.[19,27] Mean-
to lower than 10:3. In the case of a 1:1 Pb/S feed molar while, this result also confirms further that lead sulfide QDs
ratio, as expected, large particles were formed and precipi- have better chemical stability (air-stable) under ambient
tated out quickly, suggesting that the Pb/S feed molar ratio conditions, in comparison to phosphide and arsenide
needs to be larger than 1:1 to avoid uncontrollable growth. QDs,[12–14] which is important for technical applications.
Figure 2. Photoluminescence spectra of OA-capped PbS QDs prepared by using Na2S as sulfur source, obtained by changing the precursor
Pb/S feed molar ratio (A) or the precursor OA/Pb feed molar ratio (B) ([Pb2+] = 10 mm).
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D. Deng, J. Cao, J. Xia, Z. Qian, Y. Gu, Z. Gu, W. J. Akers
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Figure 3. Photoluminescence (A) and absorption (B) spectra of OA-capped PbS QDs monitored at different time intervals. The PbS QDs
synthesized at 40 °C with Na2S as sulfur source and a precursor molar ratio OA/Pb/S of 40:10:3 ([Pb2+] = 10 mm) are stored at room
temperature in the dark.
Synthesis of OA-Capped PbS QDs by Using Moderately
Reactive Thioacetamide as Sulfur Source
the (initial) reaction temperatures were set at 60, 65, and
70 °C. The selection of a suitable reaction temperature is
based on the thermal decomposition temperature of TAA
(about 55 °C in water). The results presented in Figure 4
In this study, highly luminescent, NIR-emitting, oil-soluble are as follows:
PbS QDs (PL peak: from ca. 750 to ca. 880 nm) with a
(1) Using moderately reactive TAA as sulfur precursor
narrow size distribution were prepared by using Na2S as will slow down the nucleation rate of the PbS QDs, relative
sulfur source via a two-phase approach at an n-decane/ to highly reactive Na2S. As shown in Figure 4A, at the
water interface. Sodium sulfide, as a highly reactive sulfur lower reaction temperature (60 °C), the decomposition rate
source, will result in a rapid nucleation event. In this case, of TAA is very slow. Under normal pressure, it takes about
a larger number of smaller PbS particles are achieved. As 35 min for a color change from pale yellow to pale brown
proved in previous studies on the two-phase syntheses of to appear. In the following 15–20 min (from 35 min to 50–
CdS and CdSe QDs by An et al.,[34] the reactivity of anion 55 min), the residual TAA decomposes continually to re-
precursors plays an important and complicated role in af- lease sulfur anions. As we know, the released sulfur anions
fecting the nucleation and growth of the QDs. Here, we could react with Pb cations to form new crystal nuclei of
noted that there is a significant difference between the syn- PbS or feed the growth of the formed QDs. Here, the PL
thesis of PbS and those of CdS and CdSe with respect to peak redshifts from approximately 835 nm to about 895 nm,
their influence on the QD growth rate. The growth of PbS but the PL spectra do not broaden with reaction time (PL
QDs is much faster, and can even occur at room tempera- bandwidth ca. 130 nm), which suggests that the decomposi-
ture, relative to CdS or CdSe QDs.[26,27] Thus, the role that tion of TAA mainly feeds the QD growth rather than form-
the activity of the anion precursor plays in the nucleation ing new crystal nuclei, according to the QD growth theory
and growth processes of PbS QDs in our two-phase system reported by Peng et al.[36] On prolonging the reaction time
and how it can be controlled is the next question to investi- further (Ͼ50–55 min), the TAA is consumed eventually. At
gate.
this time, the growth of QDs is in conformity with the
The reactivity of an anion precursor depends mainly on Ostwald ripening mechanism (the smaller particles dissolve,
its nature. In this case, we used thioacetamide (TAA) and feeding growth of the bigger nanocrystals), resulting in the
thiourea as sulfur sources (see the Experimental Section for broadening of the size distribution. The corresponding PL
more details) to understand and reveal the influence of the bandwidth broadens from approximately 130 nm to about
reactivity of the S precursors on the nucleation and growth 150 nm.
of PbS QDs. As compared to highly reactive sodium sulfide,
(2) When the reaction temperature is increased to 65 °C,
TAA and thiourea are moderately reactive and poorly reac- the decomposition rate of TAA and the resulting nucleation
tive, respectively, according to the report by An and co- and growth rates of PbS QDs all increase. As displayed in
workers.[34] In our two-phase system, when using poorly re- Figure 4B, at 65 °C, the time needed for the appearance of
active thiourea as sulfur source, the resulting PbS QDs a color change from pale yellow to pale brown is shortened
show very poor near-infrared fluorescence. Therefore, in to 25 min. Approximately in the next 9 min (from 25 to
this work, we study the influence of the reactivity of TAA 34 min), the residual TAA decomposes rapidly and is con-
(moderately reactive) on the synthesis of PbS QDs. Besides sumed eventually. As a result, the PbS QDs grow fast. The
the nature of the precursor, the reaction temperature may PL peak redshifts significantly from 800 to 890 nm within
also play an important role in adjusting the reactivity of approximately 9 min. Meanwhile, the appearance of double
anion precursors. So in this part, we further study the effect peak structures in the PL spectra suggests that, due to the
of the reaction temperature on the reactivity of TAA. The high decomposition rate of TAA and the high growth rate
experimental results obtained are shown in Figure 4, where of QDs at 65 °C, some new crystal nuclei of PbS are formed
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High-Quality, Oil-Soluble, Near-Infrared-Emitting PbS Quantum Dots
Figure 4. Temporal evolution of the PL spectra of OA-capped PbS QDs prepared with TAA as sulfur source. Quantum dots grown at
60 °C (A), 65 to 75 °C (B), and 70 °C (C). Here, the precursor molar ratio of OA/Pb/S is 40:10:3 ([Pb2+] = 10 mm).
besides feeding the growth of the formed QDs,[37] which process with the growth process to different degrees. At
results in an accompanying spectral broadening (the PL 60 °C, a slow nucleation (because of a slow decomposition
bandwidth broadens from ca. 140 to ca. 160 nm). Then, we of TAA) does not lead to polydisperse nanocrystals, since
further increased the reaction temperature from 65 °C to the growth is also slow,[34] and the PL bandwidth is stable
75 °C at a rate of approximately 1 °C/min. During such a at approximately 130 nm before the TAA is exhausted. On
temperature increase, the PL peak of the resulting PbS QDs increasing the reaction temperature to 65 °C, the growth
redshifted rapidly from approximately 890 to about 980 nm, rate of PbS QDs might improve more than the decomposi-
and the fwhm (or PL bandwidth) increased further from tion rate of TAA and the nucleation rate. As a result, the PL
approximately 160 to about 180 nm upon the action of spectra display double peak structures (the PL bandwidth is
Ostwald ripening; the corresponding PL emission intensity broadened to ca. 160 nm), presumably due to the appear-
decreased gradually. It should be mentioned that, at 65 °C ance of two main QD populations of slightly different (and
or 60 °C, the as-synthesized PbS QDs showed strong NIR approximate) average sizes,[37] caused by the single nucle-
fluorescence emission. PL QYs for the most favorable prod- ation and the fast growth of QDs. When the TAA was
ucts were determined to be of the order of approximately exhausted, the PbS QDs grew with further redshifting and
35% relative to the standard NIR dye cypate.
broadening of the PL spectra upon the action of Ostwald
(3) As shown in Figure 4C, the decomposition rate of ripening. At 70 °C, the decomposition rate of TAA and the
TAA (i.e., the rate of release of the sulfur monomer) and nucleation and growth rates of PbS QDs all increase greatly.
the subsequent nucleation and growth rate of PbS QDs at The rapid decomposition of TAA could lead to the rapid
70 °C are much faster relative to those at 60 °C and 65 °C. formation of large crystal nuclei, and the formed PbS QDs
After only approximately 6–7 min, we could observe a color exhibit relatively unfavorable optical properties.
change from pale yellow to brown, which might indicate
We further explored the storage stability of the resulting
that at 70 °C, the TAA (sulfur source) is rapidly consumed. PbS QDs prepared by using TAA as sulfur source: the QDs
In the next 5 min (from 7 to 12 min), the PL peak redshifted synthesized at 65 °C were stored in the synthesis medium at
fast from 895 to 975 nm; the corresponding PL emission room temperature. During the storage, the PL and absorp-
bandwidth broadened rapidly from 130 to 180 nm, as a re- tion spectra were measured at time intervals spanning up
sult of the interaction of the rapid release of sulfur mono- to 30 d, and the results are shown in Figure 5. The initial
mer and the “Ostwald ripening” effect. In addition, at the wide PL spectrum (bandwidth, ca. 160 nm) clearly indicates
higher reaction temperature (70 °C), the initially formed that the size distribution of the QD sample is broad. With
crystal nuclei of PbS have a large particle size, and the re- increasing storage time, the PL spectrum sharpens and red-
sulting PbS QDs show relatively poor NIR fluorescence shifts gradually, suggesting a spontaneous narrowing of the
(PL QY ca. 10%).
particle size distribution and an accompanied increase in
The experimental results described above fully confirm average particle size. In the case of PbS nanocrystals, the
that the reactivity of water-soluble sulfur precursors plays self-focusing of the particle size after the reaction is not
an important and complicated role in affecting the nuclea- uncommon, which has been reported by Hines and
tion and growth of the oil-soluble PbS QDs in a two-phase Scholes.[19] They attribute the size focusing to the digestive
system; by controlling the reaction temperature, the decom- ripening of PbS nanocrystals. Digestive ripening occurs
position rate of TAA can also be adjusted. In particular, when larger particles break apart and smaller particles in-
with increasing reaction temperature, the decomposition crease in size until a uniform size distribution is achieved.
rate of TAA and the nucleation and growth rates of PbS Meanwhile, the spectral feature blueshifts, and the average
QDs all increase, but such changes are not uniform. At particle size decreases. Hence, digestive ripening is different
lower reaction temperatures (60–65 °C), the decomposition from Ostwald ripening, which commonly results in the de-
rate of TAA and the nucleation and growth rates of QDs focusing of the size distribution and an accompanying red-
all are low, which results in the overlap of the nucleation shift in the PL emission spectra.
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or ethanol, and it is also easy to isolate them by centrifuga-
tion and decantation; the obtained purified QD samples
(that can be redispersed in n-decane) will show more favor-
able storage stability than the corresponding QD solution,
regardless of whether they were obtained by using Na2S or
TAA as sulfur source.
When using poorly reactive thiourea as sulfur source, the
resulting PbS QDs will exhibit very poor near-infrared fluo-
rescence (data not shown), and the reaction has a poor re-
producibility. Each time, the size and PL properties of PbS
QDs might be different at the same temperature and reac-
tion time. These experimental results are greatly different
from those of the CdS or CdSe QDs, where poorly reactive
thiourea was found to be more favorable for the formation
of QDs.[34] This difference might be attributed to the high
decomposition temperature of thiourea and the high
growth rate of PbS QDs. Specifically, the decomposition
temperature of thiourea in water is high, close to 100 °C.
At this reaction temperature, the growth rate of PbS QDs
is very fast (as compared to CdS QDs,[34] the growth of PbS
QDs is much faster, it could even occur at room tempera-
ture), and the high reaction temperature is also unfavorable
for preparing highly luminescent PbS QDs as shown in Fig-
ure 1B. Therefore, it is difficult to improve the size control-
lability and PL properties of PbS QDs by changing the re-
action parameters and using poorly reactive thiourea as sul-
fur source. These experimental results enhance our under-
standing of the relation between the reactivity of water-sol-
uble anion precursors and the QD synthesis in a two-phase
Figure 5. PL (A) and absorption (B) spectra of a single n-decane
solution of OA-capped PbS QDs measured at different time inter-
vals. The PbS QDs synthesized at 65 °C by using TAA as sulfur
source are stored at room temperature in the dark; the precursor
molar ratio of OA/Pb/S was 40:10:3 ([Pb2+] = 10 mm).
In our experiments, we observed the sharpening and red- system: the selection of water-soluble anion precursors for
shifting of the PL spectrum simultaneously, as shown in the QD synthesis depends on their reactivity and the rate
Figure 5A. Therefore, in this case, the focusing of the par- of growth of the QDs. That is to say, if the growth rate of
ticle size could not be rationally attributed to the digestive QDs is high (as in the present study), we should select
ripening of PbS nanocrystals, as observed by Hines et al.[19] highly reactive water-soluble anion precursors; otherwise,
By analyzing the experimental results, we consider that, in the situation is the converse (as in the report of An
this two-phase system, with moderately reactive TAA as et al.[34]).
sulfur source, at 65 °C, the decomposition of sulfur precur-
sor and the nucleation and growth of the PbS QDs occur
simultaneously at the initial stage, resulting in a broad par-
ticle size distribution (Figure 4B). When the as-prepared
relatively polydisperse PbS QDs (confirmed by TEM analy-
Morphological and Structural Characterizations of As-
Synthesized OA-Capped PbS QDs
sis) are further stored in the synthesis medium at room tem-
In this study, transmission electron microscopy (TEM)
perature, the smaller particles begin to dissolve gradually, and powder XRD were employed to characterize the mor-
feeding the growth of the bigger nanocrystals. However, un- phology and crystal structure of oil-soluble PbS QDs syn-
like those occurring generally at high reaction temperature, thesized by a two-phase approach, using highly reactive
this process is very slow at room temperature and tends to water-soluble Na2S and moderately reactive thioacetamide
be thermodynamically stable. As a result, the size distribu- (TAA) as sulfur sources. The experimental results are
tion of the PbS QDs becomes narrower, and this is ac- shown in Figures 6 and 7, respectively. As presented in Fig-
companied by gradual redshifts of the optical spectra.
ure 6A, the PbS QDs (λem = 800 nm) obtained by using
Here, it should be mentioned that generally, the PL emis- highly reactive Na2S as sulfur source appear as well-sepa-
sion intensity of the PbS QDs decreased gradually during rated nearly spherical particles with good monodispersity
the storage, as presented in Figure 5A. After storage for (the average size is ca. 4 nm). Figure 6B is a typical TEM
approximately 30 d, the corresponding photoemission in- image of the PbS QDs (λem = 808 nm) grown at 65 °C ob-
tensity was reduced by about 50%, suggesting a relatively tained by using TAA as sulfur source, showing that these
poor storage stability compared to that of the PbS QDs QDs have a slightly poorer size uniformity (and monodis-
obtained by using Na2S as sulfur source. However, in our persity), as compared to the PbS QDs obtained with Na2S
experiments, we noted that it is easy to precipitate the as- as sulfur source (generally, the particle size of PbS QDs
synthesized PbS QDs in n-decane solution with methanol produced from thioacetamide is larger than that obtained
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Eur. J. Inorg. Chem. 2011, 2422–2432

High-Quality, Oil-Soluble, Near-Infrared-Emitting PbS Quantum Dots
from Na2S, because thioacetamide produces long-wave- also prove the QDs to be crystalline. The three lattice spac-
length-emitting PbS). The inset HRTEM images (bottom) ings calculated from the diffraction rings are 0.34, 0.21, and
reveal that the as-prepared OA-capped PbS QDs obtained 0.18 nm, corresponding to the reported d spacings for lat-
from two kinds of differently reactive sulfur sources are all tice planes (111), (220), and (311) of cubic PbS. Figure 7
well-crystallized. The lattice spacing, as labeled in the shows the typical powder XRD patterns of the two OA-
HRTEM images, is approximately 0.29 nm, corresponding capped PbS QD samples (shown in Figure 6A and B) sup-
to the (200) plane of the cubic structure of PbS.[25–27] The ported on a glass slide. The XRD patterns also indicate that
selected area electron diffraction (SAED) patterns (top) these oil-soluble PbS QDs have a cubic crystal structure. It
is clear that the diffraction peaks are broader than those in
previous reports,[21–25] implying a smaller QD size. Thus,
the experimental results from XRD measurement are rather
consistent with HRTEM imaging and SAED analysis, con-
firming the uniform cubic structure and the extremely small
size of the produced PbS QDs.
Conclusions
A facile and low-cost two-phase approach under mild
conditions has been developed to prepare high-quality
hydrophobic PbS QDs that offer size-tunable fluorescence
emission in the wide NIR range of approximately 750–
1000 nm, by using differently reactive water-soluble Na2S
and TAA as sulfur sources. (1) Using highly reactive Na2S
as sulfur precursor, small PbS QDs with a narrow size dis-
tribution, which show strong photoluminescence (PL QY
up to 40%), are prepared by our two-phase approach; the
PL peak position can be tuned between 750 and 880 nm.
Moreover, systematic investigation has been performed on
the synthesis parameters affecting the formation of PbS
QDs, including the reaction temperature, the precursor Pb/
S molar ratio, the precursor OA/Pb molar ratio, and the
storage stability. In general, these factors play crucial roles
leading to high-quality PbS QDs. (2) Moderately reactive
TAA was used further as sulfur precursor to explore the
Figure 6. (A) Typical TEM image of OA-capped PbS QDs (λem
=
influence of the reactivity of water-soluble sulfur sources on
the synthesis of oil-soluble NIR-emitting PbS QDs in our
two-phase system. It was found that the decomposition of
TAA and the accompanying nucleation and growth of PbS
QDs strongly depends on the applied synthesis temperature.
A reaction temperature between 60 and 65 °C is favorable
for the formation of highly luminescent PbS QDs (PL QY
up to 35%) that show long-wavelength fluorescence emis-
sion (the PL peak position can be tuned from 800 to
980 nm). Hence, these experimental results confirm fully
that the reactivity of the anion precursors plays an impor-
tant and unique role in determining the nucleation and
growth of QDs in the two-phase synthesis system. In ad-
dition, we characterized the morphology and crystal struc-
ture of the resulting PbS QDs by (high-resolution) TEM
and SAED, and powder XRD. The obtained experimental
results confirm fully that the produced oil-soluble NIR-
emitting PbS QDs have a cubic crystal structure and ex-
tremely small size.
800 nm) with an OA/Pb/S ratio of 40:10:3 obtained by using highly
reactive Na2S as sulfur source. (B) Typical TEM image of the PbS
QDs (λem = 808 nm) grown at 65 °C with a OA/Pb/S ratio of
40:10:3 by using moderately reactive thioacetamide as sulfur
source. Insets are the corresponding SAED pattern (top) and high-
resolution TEM image (bottom) of the PbS QDs.
Currently, our efforts are directed to two main investi-
gations. One is to place these oil-soluble PbS QDs into the
hydrophobic cores of nanoscale micelles (potential drug
carriers) for evaluating their in vivo tumor-specific tar-
Figure 7. Typical powder XRD patterns of the two OA-capped PbS
QD samples (shown in panels A and B of Figure 6) supported on
a glass slide. The diffraction peaks are indexed.
Eur. J. Inorg. Chem. 2011, 2422–2432
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FULL PAPER
PbS QDs are prepared. A scheme in Figure 8B which illustrates the
proposed mechanism for the formation of NIR-emitting oil-soluble
PbS QDs by using water-soluble Na2S, TAA (and thiourea) as sul-
fur sources with different reactivity. Both nucleation and growth of
QDs occur at the interface of the two liquid phases (i.e., n-decane
and water).[29,30]
geting, by using the NIR imaging system constructed in our
lab;[7b,7c] the other is to seek a simple and efficient pro-
cedure for transferring oil-soluble PbS QDs into water.
These studies are all of great importance for future techni-
cal applications of oil-soluble PbS QDs. (Caution: Although
the toxicity of PbS QDs may be lower than those of Cd-,
Hg-, Te-, and Se-based ones, in vivo biomedical experiments
of these QDs should be limited to mice or other animals.)
Preparation of the Pb Precursor Solution: A typical reaction was
performed by placing lead acetate (38 mg), oleic acid (0.13 mL),
and n-decane (10 mL) in a continuously stirred flask and then heat-
ing the mixture at 130 °C for 20 min until an optically clear precur-
sor solution of lead oleate was obtained. Finally, the solution was
cooled to the desired temperature (for example, 40 °C for the syn-
thesis with Na2S as sulfur precursor). This reaction was run under
a nitrogen atmosphere to prevent oxidation of the oxygen-sensitive
oleic acid.
Experimental Section
Chemicals: Lead(II) acetate trihydrate (99+%), oleic acid (99%), n-
decane (99%), Na2S·9H2O (98+%), thioacetamide (TAA, 99.5%),
and thiourea (99%) were of analytical grade and used as received.
The water used in all experiments had a resistivity higher than
18.2 MΩ·cm.
Synthesis of PbS QDs with Na2S as Sulfur Source: When the above
lead oleate precursor solution was cooled to 40 °C, an aqueous
solution (5 mL, 6 mm) of Na2S prepared freshly was added drop-
wise to the reaction flask whilst stirring. Immediately, the upper
organic phase turned from pale yellow to dark-brown within about
one minute, which indicates that the formation of PbS QDs oc-
curred rapidly in a two-phase synthesis system when using highly
reactive Na2S as sulfur source. Subsequently, the system was stirred
further at 40 °C for 5 min. Finally, the solution was cooled to room
temperature for UV/Vis absorbance and photoluminescence (PL)
measurements without any size sorting. In this part, we investigated
further the effects of the temperature, the precursor Pb/S molar
ratio, the precursor OA/Pb molar ratio, and the storage time on
the synthesis of PbS.
Synthesis of OA-Capped PbS QDs by Using Various Water-Soluble
Anion Precursors: The typical two-phase synthetic approach to oil-
soluble near-infrared-emitting PbS QDs can be divided into two
steps (see Figure 8A): first, the Pb-precursor solution is prepared;
then, the n-decane solution of lead precursor is mixed with an
aqueous solution of sulfur precursor, such as Na2S, thioacetamide
(TAA), or thiourea, at the desired temperature. Thus, NIR-emitting
Synthesis of PbS QDs with Thioacetamide as Sulfur Source: When
the freshly prepared lead oleate precursor solution was cooled to
60 °C (or 65 °C or 70 °C), a solution of moderately reactive thioac-
etamide (5 mL, 6 mm) in water was injected into the reaction flask
whilst stirring. Next, the reaction mixture was kept at 60 °C (or
65 °C or 70 °C) for promoting the nucleation and growth of PbS
QDs. Aliquots were taken at different time intervals, and UV/Vis
absorbance and PL spectra were recorded for each aliquot. In this
part, we investigated the influence of the reaction temperature on
the reactivity (or decomposition) of TAA and the resultant nucle-
ation and growth of PbS QDs.
Characterization: An S2000 eight-channel optical fiber spectropho-
tometer (Ocean Optics corporation, America) and a broadband
light source (λex ≈ 610 nm) (X-Cite Series 120Q, Lumen Dynamics
Group Inc., Canada) were utilized for the detection of fluorescence
spectra. A 754-PC UV/Vis spectrophotometer (JingHua technolog-
ical instrument corporation, Shanghai, China) was used for the
measurement of UV/Vis spectra. All optical measurements were
performed at room temperature. PL quantum yields (QY) of PbS
QDs in n-decane were calculated by comparing their integrated
emission to that of a solution of cypate in 20% aq. DMSO (the
absorption and PL emission peaks of cypate are at 790 nm and
810 nm, respectively; the PL QY is 12%) supplied by Samuel Achi-
lefu (Department of radiology, Washington University at St. Louis,
USA).[38] A JEM-2100 transmittance electron microscope (JEOL,
Japan) operated at 200 kV was used to evaluate the morphology
and crystal structure of quantum dots. Powder XRD measurements
were carried out using a Philips X’Pert PRO X-ray diffractometer
(λ = 1.54178 Å). XRD samples were prepared by concentrating a
drop of the obtained concentrated QD ethanol suspension on a
glass plate.
Figure 8. (A) Schematic presentation of the two-phase synthetic
route to oil-soluble PbS QDs by using water-soluble Na2S, thio-
acetamide (TAA), and thiourea as sulfur sources: preparation of
the Pb precursor solution; synthesis of oil-soluble PbS QDs by
using the water-soluble sulfur source. (B) A proposed mechanism
for forming oil-soluble PbS QDs by using various water-soluble
sulfur sources. Both nucleation and growth of QDs only occur at
the interface of the two liquid phases (i.e., n-decane and water).
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High-Quality, Oil-Soluble, Near-Infrared-Emitting PbS Quantum Dots
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Acknowledgments
This work was financially supported by the National Natural
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