A reverse cation-exchange route to hollow PbSe nanospheres evolving from Se/Ag2Se core/shell colloids

Article abstract of DOI:10.1021/jp060305l

A reverse cation-exchange approach for the synthesis of hollow PbSe nanospheres is successfully established. This route involves a new strategy of a stepwise, in-situ template-based evolution from spherical amorphous Se colloids to Se/Ag₂Se core/shell colloids, then to hollow PbSe nanospheres. Se colloids are prepared as the initial product by utilizing the chelation of ethylenediamine to bulk Se. They are converted into Se/Ag₂Se core/shell colloids through the reaction with Ag⁺ in ethylene glycol. During the conversion from Ag₂Se shell to PbSe shell, a small amount of tributylphosphine is crucial as the capping agent. The characterization results, including XRD, SEM, TEM, HRTEM, and EDX, reveal that hollow PbSe nanospheres with polycrystalline and cubic structure are prepared. The corresponding optical band gap is calculated to be 0.56 eV. This conformation is potentially beneficial to the improvement concerning the applications of PbSe nanostructures.

Full text of DOI:10.1021/jp060305l

J. Phys. Chem. B 2006, 110, 9785-9790
9785
A Reverse Cation-Exchange Route to Hollow PbSe Nanospheres Evolving from Se/Ag2Se
Core/Shell Colloids
Wei Zhu,†,‡ Wenzhong Wang,*,† and Jianlin Shi†
State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of
Ceramics, Chinese Academy of Sciences, 1295 Dingxi Road, Shanghai 200050, P. R. China, and
Graduate School of the Chinese Academy of Sciences, 19 Yuquan Road, Beijing 100049, P. R. China
ReceiVed: January 16, 2006; In Final Form: March 30, 2006
A reverse cation-exchange approach for the synthesis of hollow PbSe nanospheres is successfully established.
This route involves a new strategy of a stepwise, in-situ template-based evolution from spherical amorphous
Se colloids to Se/Ag2Se core/shell colloids, then to hollow PbSe nanospheres. Se colloids are prepared as the
initial product by utilizing the chelation of ethylenediamine to bulk Se. They are converted into Se/Ag2Se
core/shell colloids through the reaction with Ag+ in ethylene glycol. During the conversion from Ag2Se shell
to PbSe shell, a small amount of tributylphosphine is crucial as the capping agent. The characterization results,
including XRD, SEM, TEM, HRTEM, and EDX, reveal that hollow PbSe nanospheres with polycrystalline
and cubic structure are prepared. The corresponding optical band gap is calculated to be 0.56 eV. This
conformation is potentially beneficial to the improvement concerning the applications of PbSe nanostructures.
Introduction
reported the first synthesis of highly monodisperse and size-
controlled PbSe colloidal nanocrystals (NCs),12 and then the
For nanomaterials, the morphological retrofit has always been
one of the focusing subjects for the affinitive relationship among
the shape, the property, and the subsequent opportunity of
designing and preparing special building blocks in nanodevice
fabrications.1-3 Compared with other structured nanoparticles
(NPs), hollow nanospheres are anticipated to exhibit enhanced
or novel functionalities due to their higher surface area and the
capability of forming composite structures by embedding
specific particles in the interiors. The breadth of the subject
material has been demonstrated by the fact that it covers and
intertwines with the research of catalysis,4 drug delivery,5
bioencapsulation,6 plasmonics,7 superparamagnetism,8 etc. For
example, Suslick demonstrated that catalytic activity of hollow
MoS2 nanospheres for hydrodesulfurization of triophene is
substantially superior to both MoS2 NPs and conventional
microsized powders.4 By encapsulating iron oxides nanoparticles
in the interiors, Xia synthesized superparamagnetic colloids with
controllable surfaces.9 Therefore, the successful extension of
this conformation into the existing important materials is of
significance for the fundamental research and prospective
applications.
As a narrow-band semiconductor, lead selenide (PbSe) is one
of the most attractive selenides used for IR detectors, photo-
graphic plates, selective and photovoltaic absorbers, and so on.10
The sensitivity of its band gap to the dimension also makes it
useful to investigate the nanosized effects specially. Over the
past few years, much effort has been devoted to the preparation
of nanoscale PbSe in solution system. For example, Zhu and
co-workers illustrated a sonochemical method for the fabrica-
tions of spherical and rectangular PbSe NPs.11 Murray et al.
investigations relevant to their interband and intraband optical
properties were followed.13 Kuno et al. described the productions
of both straight and branched PbSe nanowires, where Au/Bi
core/shell NPs were introduced as seeds, through a seeded
solution approach.14 Owing to PbSe crystallographic texture
(cubic, Fm3m), however, kinetic control is futile for the
evolution to the artificial hollow nanospheres. In comparison,
potential advantages of hollow PbSe nanospheres include the
possibility of higher efficiency and an increased flexibility by
which the resulting spheres can incorporate with other functional
NPs. As a consequence to the fundamental interest and
prospective applications progress, the realization of this structure
becomes naturally attractive, whereas the concerned production
has never been reported to date.
Presently, an improved in-situ template method involving a
cation-exchange reaction is illustrated as an efficient way to
“duplicate” nanostructures without altering the morphology.
Large solubility product (Ksp) difference is a key reference for
the direct replacement between two kinds of cations. The NCs,
in the solution containing appropriate precursor, can spontane-
ously convert into another kind of more stable NCs with much
smaller Ksp. For example, several cations including Cu+, Ag+,
Sb3+, Bi3+ have been used to replace Zn2+ in ZnS nanotubes
to produce sulfides with preservation of the tubular shape.15 This
approach opens up a new access to design and prepare
nanostructures that is difficult to achieve through other general
methods. On the other hand, this brings another challenge. When
the Ksp of goal product is larger than that of the precursor that
used as the template, how to make the reverse process proceed
is uncertain. To facilitate the reverse reaction, generally, the
stronger bond between cation and anion in primal NCs should
be impaired or substituted through special management. Intro-
duction of an organic coupling agent is substantiated in some
cases as an effective strategy. The ligand could complex with
target cations of template and form nearly inert cation-organic
* Corresponding author. Phone: +86-21-5241-5295. Fax: +86-21-5241-
3122. E-mail: wzwang@mail.sic.ac.cn.
† State Key Laboratory of High Performance Ceramics and Superfine
Microstructure, Shanghai Institute of Ceramics, Chinese Academy of
Sciences.
‡ Graduate School of the Chinese Academy of Sciences.
10.1021/jp060305l CCC: $33.50 © 2006 American Chemical Society
Published on Web 05/04/2006

9786 J. Phys. Chem. B, Vol. 110, No. 20, 2006
Zhu et al.
Figure 1. XRD patterns of (a) Ag2Se shells, obtained after the removal
of Se core in the core/shell Se/Ag2Se colloids by hydrazine; (b) hollow
PbSe nanospheres.
ions, such as tributylphosphine (TBP) to Ag+ 16,17 and 12-
crown-4 to Li+.18 Then this allows another kind of cation to
associate with the anions backbone. These explorations signifi-
cantly increase the scope of the methodological application in
the field of nanostructures preparation. Herein, the current work
further advances our understanding about the cation-exchange
reaction in the appliance of complicated structures, by describing
a synthetic approach through which one can achieve hollow
PbSe nanospheres.
Briefly, hollow PbSe nanospheres have been successfully
prepared via a reverse cation-exchange approach. This method
is primarily concerned with the production of amorphous (a-)
Se spheres based on dissolution-reprecipitation of bulk Se in
ethylenediamine, and then the formation of Se/Ag2Se core/shell
structure through the reaction of a-Se with insufficient Ag+ in
ethylene glycol (EG), subsequently the conversion from orthor-
hombic Ag2Se (Ksp ) 2.0 × 10-64) shell to cubic PbSe (Ksp
)
7.94 × 10-43) shell via a reverse cation-exchange process.
Experimental Section
In a typical procedure, 18 mg bulk gray Se was first dissolved
into 10 mL ethylenediamine to form a yellowish solution after
being stirred for about 4 h at room temperature. Then, this
solution was gradually added into 10 mL deionized water under
agitation, which resulted in a brick-red opaque, indicating the
formation of a-Se that mainly existed as spherical colloids.19,20
The temperature of the mixture also rose as a response to the
exothermic reaction. After the mixture was quenched with ice
water for about 5 min, a-Se was readily collected by centrifuga-
tion, washed by deionized water for several times and dried at
3 °C.
The preparation of the Se/Ag2Se core/shell structure was
conducted by re-dispersing 6 mg a-Se colloids homogeneously
in 5 mL EG, then introducing them into 15 mL EG solution of
AgNO3. The mole ratio of Ag+ and Se was set as 3:2. This
reaction was allowed to proceed at 3 °C for 4 h and the
suspension turned from red to brown gently. Subsequently, the
suspension was diluted with 30 mL H2O and the colloids were
separated by centrifugation, washed with ethanol and dried in
air.
Figure 2. (a) SEM image of spherical a-Se colloids, the inset is a
TEM micrograph of an individual colloid; (b) TEM of the Se/Ag2Se
core/shell colloids, where the thickness of the shell is measured to be
20 nm; (c) TEM of Ag2Se shells, the inset is a SEM image of an
individual shell.
3H2O (the molar ratio of Pb2+:Ag+ was about 23:1). After 0.05
mL TBP was added in, this suspension was kept at 55 °C in a
reflux system for 15 h to ensure the entire reaction. Then the
product was collected by centrifugation and washed three times
with methanol. After the as-prepared products were immersed
in 5 M hydrazine for 10 min, hollow PbSe nanospheres were
obtained.
The X-ray diffraction (XRD) patterns were recorded with a
Japan Rigaku Rotaflex diffractometer equipped with a rotating
anode and using Cu KR radiation over the range of 20° e 2θ
e 80°. The scanning electron microscope (SEM) characteriza-
tions were performed on a JEOL JSM-6700F field emission
scanning electron microscope. The transmission electron mi-
croscope (TEM) analyses were performed by a JEOL JEM-
2100F field emission electron microscope. The optical diffuse
reflectance spectra were conducted on a UV-Vis-NIR scan-
ning spectrophotometer (UV-3101PC, Shimadzu) using an
integrating sphere accessory.
To prepare hollow PbSe nanospheres, these as-obtained core/
shell colloids were suspended in 20 mL methanol that containing
0.5 g poly(vinylpyrrolidone) (PVP) and 1 g Pb(CH3COO)2‚

Reverse Cation-Exchange Route to Hollow PbSeNanospheres
J. Phys. Chem. B, Vol. 110, No. 20, 2006 9787
Figure 3. The images of hollow PbSe nanospheres prepared through the reverse cation-exchange process. (a) TEM micrograph at low magnification;
(b) TEM micrograph at high magnification; (c) TEM of an individual PbSe shell, the inset is the corresponding SAED pattern; (d) HRTEM image
corresponding to the local region of the shell; (e) SEM image of the hollow PbSe nanospheres.
Results and Discussion
Utilizing the chelation of ethylenediamine to bulk Se, in our
synthesis, spherical a-Se colloids are first synthesized through
a dissolution-reprecipitation procedure. Then they are converted
into Se/Ag2Se core/shell colloids, which are used as the in-situ
template in the following cation-exchange reaction. Figure 2
reveals the electronic microscopic images of the stepwise
products, including a-Se colloids (Figure 2a), Se/Ag2Se core/
shell colloids (Figure 2b), and Ag2Se shells (Figure 2c), to
demonstrate the whole evolution process of the in-situ template.
From the SEM image shown in Figure 2a, it is obvious that the
initial product (a-Se) consists of spherical colloids with diam-
eters in range of 150∼300 nm. Due to the low glass transition
temperature of a-Se (32 °C),21 a small amount of assemblies
also form through the limbic attachment between the adjacent
colloids (as marked by the arrow). TEM of an individual colloid,
which represents the round characteristic of the sample, is
inserted. Via the reaction with insufficient Ag+ in EG, these
colloids are coated by thin Ag2Se layers and Se/Ag2Se core/
The compositional conversion of the shell structure from Ag2-
Se to PbSe, resulting from the cation-exchange reaction, is first
confirmed by the use of wide-angle X-ray diffraction (XRD).
Figure 1 shows the XRD patterns of both the template Ag2Se
shell and hollow PbSe nanospheres. In Figure 1a all the
diffraction peaks are in good agreement with the corresponding
literature data of orthorhombic Ag2Se, with lattice parameters
a ) 4.333 Å, b ) 7.062 Å, and c ) 7.764 Å (JCPDS 24-1041).
These Ag2Se crystals would be transformed into PbSe during
the cation-exchange process. Figure 1b displays the typical XRD
pattern of the as-prepared PbSe nanospheres. As represented in
the micrograph, the diffraction peaks can be indexed as pure
cubic phase of PbSe with lattice parameters a ) 6.124 Å
(JCPDS 06-0354). The evident intensity of the peaks reveals
the fine crystallinity of the final product. In comparison to Figure
1a, moreover, the peaks from Ag2Se are not detected, implying
the efficient and complete replacement of Ag+ by Pb2+
.

9788 J. Phys. Chem. B, Vol. 110, No. 20, 2006
Zhu et al.
Figure 4. EDX spectra of (a) Ag2Se shells; (b) PbSe shells.
shell structures form. Figure 2b is the typical TEM micrograph
of the complicated colloids, in which a deep contrast along the
colloid edges is observed. The thickness of the surrounding Ag2-
Se shell is estimated to be approximately 20 nm. These Ag2Se
layers in the in-situ template would be converted into PbSe shells
in the following cation-exchange step. To closely examine the
template conformation, Figure 2c displays the TEM of the Ag2-
Se shells obtained after the removal of Se core with hydrazine.
The obvious contrast implies the formation of the shell structure.
And as shown in the SEM image (inset of Figure 2c), which
reveals an individual Ag2Se particle containing a gap in the
surface, the hollow characteristic is apparent.
For the conversion to hollow PbSe nanospheres, the Se/Ag2-
Se core/shell colloids (Figure 2b) are used as the in-situ template
instead of the Ag2Se shells (Figure 2c). The reservation of Se
core is expected as substrate to prevent the collapse of the shell
structure during the cation-exchange reaction. Figure 3 repre-
sents the morphologies of the final PbSe product. As shown in
Figure 3a and b, hollow PbSe nanospheres with diameters in
range of 150∼300 nm are achieved. Both the TEM images show
a uniform shell thickness without collapse of the shell structure
during the cation-exchange preparation. The thickness of the
shell is characterized to be around 20 nm, which is in accordance
with the value of the initial Ag2Se layer in the template. Clearer
structure is displayed in Figure 3c that illustrates an individual
hollow PbSe nanosphere. The corresponding select area electron
diffraction (SAED) pattern (inset of Figure 3c) approves that
the PbSe sphere is polycrystalline. Further investigations of the
shell structure come from the high-resolution transmission
electron microscope (HRTEM) analyses (Figure 3d). The legible
crystal lattice implies the crystal perfection on the shell. And
the observed interplanar spacing marked representatively in
Figure 3d is about 0.353 nm, corresponding to the (111) plane
of cubic PbSe. Figure 3e is the typical SEM image that displays
the exterior morphology of the PbSe spheres. As depicted in
the micrograph, most of the PbSe nanospheres have a compact
and sealed surface. The similarities in morphology and dimen-
sions of hollow Ag2Se and PbSe nanospheres demonstrate the
success of the reverse cation-exchange approach in duplicating
nanostructures.
Figure 5. Diffuse reflectance spectra of (a) Se/Ag2Se core/shell colloids
and (b) hollow PbSe nanospheres, the insets are the corresponding
curves plotted as normalized (F(R)*hν)2 versus hν (eV).
exchange reaction, as shown in Figure 4b, the peaks from Pb
and Se are observed and Ag is not detected in the sample. The
atomic ratio of Pb and Se is computationally 1:1. Both these
two results are in well accordance with the XRD analyses
mentioned above.
Figure 5a and b show the optical diffuse reflection spectra
of both the Se/Ag2Se core/shell colloids and the hollow PbSe
nanospheres in order to resolve the excitonic or interband
(valence-conduction band) transitions, which allows us to
calculate their band gaps. The spectral envelopes are clearly
the summations of a number of subspectra. The estimates of
the optical band gap (Eg) can be obtained using the following
equation (eq 1) for a semiconductor:
R(V) ) A(h′V/2 - Eg)m/2
(1)
where A is a constant, h′ ) h/2π, R is the absorption coefficient,
and m is equal to 1 for a direct allowed transition. Since R is
proportional to F(R), the Kubelka-Munk function, the energy
intercept of a plot of (F(R)*hν)2 versus hν yields Eg for a direct
allowed transition when the linear region is extrapolated to zero
ordinate.22 From the spectra (insets of a and b), the band gaps
of the Se/Ag2Se colloids and PbSe nanospheres are calculated
to be 0.69 and 0.56 eV respectively. Both the values of the
band gap energies are larger than those of the reported values
for bulk materials (0.07∼0.15 eV for Ag2Se and 0.29 eV for
PbSe).23,24 The increase in the band gaps of the as-prepared
To further confirm the componential transformation of the
hollow nanospheres, energy dispersion X-ray (EDX) spectra of
the template shell and the final product are respectively recorded
in Figure 4. Figure 4a displays that the chemical component of
the template shell consists of Ag and Se. After the cation-

Reverse Cation-Exchange Route to Hollow PbSeNanospheres
J. Phys. Chem. B, Vol. 110, No. 20, 2006 9789
process about the hollow PbSe nanospheres is schematically
illustrated in Figure 6. And it’s worth noting that, to facilitate
the entire conversion from Ag2Se to PbSe, large excess amounts
of Pb2+ is necessary. When the mole ratio of Pb2+ to Ag+ is
adjusted to 10, even after 30 h of reaction, Ag2Se peaks are
also detected in the XRD pattern besides PbSe.
Conclusion
In summary, we have prepared pure hollow PbSe nanospheres
with a cubic structure through a reverse cation-exchange
approach, in which Se/Ag2Se core/shell colloids are used as the
in-situ template. The dimensions of the as-obtained PbSe
spheres, including diameter and thickness, are similar to those
of the initial Ag2Se layer in the template. In view of the
extensive applications of PbSe such as IR and photography, this
conformation is anticipated to enhance the efficiency or
introduce novel functionality. Furthermore, the current work
demonstrates the availability of the reverse cation-exchange
approach in the field of duplicating nanostructures, especially
for some particular morphologies. This strategy is also suggested
to be practicable to other functional selenides or tellurides.
Figure 6. Schematic illustration of the whole evolution process about
the hollow PbSe nanospheres.
materials are indicative of size quantization effects that lead to
a series of discrete states in the conduction and valence
bands.25-27
Within the stepwise evolution process, a-Se colloids are
prepared as the starting template via the chelation of ethylene-
diamine to bulk gray Se. Ethylenediamine could dissolve the
gray Se powder to produce a homogeneous solution based on
the formation of complex polyanions.28-30 However, the high
metastability of the complex ions makes the system labile and
extraordinary sensitive to the environment in the solution. When
the solution is mixed with deionized water, the equilibrium is
destroyed and a large quantity of Se molecules forms. Spherical
a-Se colloids are of preference resulting from the rapid ag-
gregation of the molecules. And the average diameter of the
colloids is determined by the amount of the as-prepared Se
molecules that depends on the initial concentration of Se in the
ethylenediamine solution, as the other experimental parameters
are kept unchanged. Subsequently, these colloids are converted
into the Se/Ag2Se core/shell structures by storing the EG
suspension of a-Se and insufficient Ag+ for the desired time.
EG is a solvent possessing reducibility.31,32 In the dispersion,
the Ag+ cations are reduced by EG to Ag atoms that further
react with a-Se to generate spherical colloids consisting of a-Se
cores and Ag2Se shells as seen in eq 2:
Acknowledgment. Financial support from Chinese Academy
of Sciences and Shanghai Institute of Ceramics under the
program for Recruiting Outstanding Overseas Chinese (Hundred
Talents Program) is gratefully acknowledged.
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serve as the in-situ template to synthesize hollow PbSe spheres.
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