Facile solution-phase synthesis of CuInSe2 nanocrystals with controlled morphologies

Article abstract of DOI:10.1002/ejic.201301069

The morphology of CuInSe₂ nanocrystals plays a key role in their functional properties. Achieving controllable morphology is significant for studying their structures and novel properties. Here, CuInSe₂ nanocrystals with a trigonal-pyramidal shape have successfully been synthesized by a facile solution-phase method. The nanocrystals were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS), etc. The morphological evolution of the CuInSe₂ nanocrystals was illustrated by tuning the injection temperature of oleylamine-selenium (OAm-Se). In addition, CuInSe₂ nanocrystals with spherical and ellipsoidal shapes were also obtained when copper stearate was used as the copper precursor. CuInSe₂ nanocrystals with a trigonal-pyramidal shape were successfully synthesized by a facile solution-phase route. When copper stearate was used as the precursor, a spherical shape was obtained, and this further evolved into an ellipsoid by tuning the concentration of oleylamine. The results demonstrate that precise control of the morphology of CuInSe₂ nanocrystals can be achieved.

Full text of DOI:10.1002/ejic.201301069

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DOI:10.1002/ejic.201301069
Facile Solution-Phase Synthesis of CuInSe2 Nanocrystals
with Controlled Morphologies
Tianyu Bai,[a] Feifei Li,[a] He Huang,[a] Zhaorui Wang,[a]
Zhan Shi,[a] Chunguang Li,*[a] Ge Wang,*[b] and Shouhua Feng[a]
Keywords: Nanoparticles / Copper / Indium / Selenium / Morphology control / Injection temperature / Oleylamine
The morphology of CuInSe2 nanocrystals plays a key role in
their functional properties. Achieving controllable morph-
ology is significant for studying their structures and novel
properties. Here, CuInSe2 nanocrystals with a trigonal-py-
ramidal shape have successfully been synthesized by a facile
solution-phase method. The nanocrystals were characterized
by X-ray diffraction (XRD), transmission electron microscopy
(TEM), and X-ray photoelectron spectroscopy (XPS), etc. The
morphological evolution of the CuInSe2 nanocrystals was il-
lustrated by tuning the injection temperature of oleylamine-
selenium (OAm-Se). In addition, CuInSe2 nanocrystals with
spherical and ellipsoidal shapes were also obtained when
copper stearate was used as the copper precursor.
finement regime (d ϽϽ 2aB; d is the diameter of the nano-
crystals, aB is the exciton Bohr radius) are predicted to be
luminescent emitters in the red and near-infrared region.[6]
So far, many groups have reported CuInSe2 nanocrystals
with excellent shape, dispersion, and quality.[4,7–14] Korgel
et al. synthesized monodisperse CuInSe2 nanocrystals with
trigonal-pyramidal shapes from the solution–phase route
using selenourea as the selenium precursor.[11] Schoen et al.
synthesized CuInSe2 nanowires by vapor–liquid–solid
(VLS) techniques.[12] Guo et al. synthesized CuInSe2 nano-
crystals with nanoring inks for low-cost solar cells, and the
efficiency of the cell is 2.8% under standard AM 1.5 illumi-
nation.[13] Furthermore, wurtzite CuInSe2 nanocrystals
were first synthesized by Brutchey’s group.[4] It is note-
worthy that morphology is a key parameter in the design of
CuInSe2 nanocrystals with controlled functional properties.
Despite the success in the synthesis of CuInSe2 nanocrys-
tals, tuning the morphology of CuInSe2 nanocrystals is still
rare. Here, we have a great interest in the trigonal-pyr-
amidal CuInSe2 nanocrystals reported by Korgel et al.[11]
Inspired by their report, we speculate that if we can slow
Introduction
Semiconductor nanocrystals have been the object of in-
tensive scientific and technological interest.[1] They exhibit
outstanding optical, electronic, thermal, and magnetic
properties that are different from the bulk materials because
of their quantum-confinement effects and size-dependent
photoemission characteristics.[2] In recent years, the I-III-
VI family of semiconductor nanocrystals, which consists of
I (Cu, Ag), III (Ga, In), and VI (S, Se, Te) have been re-
garded as promising solar-cell and biolabeling materials be-
cause of their low toxicity, optimal band gaps that are well
matched with the solar spectrum, large light absorption co-
efficient, large Stokes shift, and emission wavelengths in the
near-infrared region.[3] Among the I-III-VI family, CuInSe2
exhibits thermodynamic stabilization at room temperature,
a direct band gap of 1.03–1.10 eV (in the bulk phase) and
a Bohr excitation radius of 10.6 nm as previously reported.
It has been considered as an absorbed layer material of so-
lar cells with excellent performance.[4] Furthermore,
CuInSe2 nanocrystals, which may produce large numbers of
P–N junctions, are expected to be able to greatly improve
the conversion efficiencies of thin-film solar cells.[5] In ad-
dition, CuInSe2 nanocrystals in the strong quantum con-
¯
down the growth rate along the [112] direction, precise con-
trol of the shape of the CuInSe2 nanocrystals may be
achieved.
In this paper, we report a facile route to synthesizing
CuInSe2 nanocrystals with a trigonal-pyramidal shape.
Cuprous chloride, indium stearate, and OAm-Se were used
as copper, indium, and selenium precursors, respectively.
Monodisperse chalcopyrite CuInSe2 nanocrystals with a
trigonal-pyramidal shape were obtained. Subsequently, the
shape of CuInSe2 nanocrystals gradually evolved into a cut-
away octahedron by tuning the injection temperature from
210 °C to 130 °C. In addition, we also attempted to use cop-
per stearate instead of cuprous chloride. Spherical CuInSe2
[a] State Key Laboratory of Inorganic Synthesis and Preparative
Chemistry, College of Chemistry, Jilin University
Qianjin Street 2699, ChangChun, Jilin 130012, P. R. China
E-mail: cglee@jlu.edu.cn
http://synlab.jlu.edu.cn/
[b] School of Materials Science & Engineering, University of
Science & Technology Beijing,
Beijing 10083, P. R. China
E-mail: gewang@mater.ustb.edu.cn
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201301069.
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nanocrystals were obtained, and the shape could be evolved analysis further confirms the local element composition of
into ellipsoidal by changing the volume of oleylamine CuInSe2 nanocrystals (Figure S3).
(OAm) from 1 mL to 3 mL.
Results and Discussion
CuInSe2 nanocrystals shown in Figure 1a were produced
by injecting OAm-Se into a preheated mixture of OAm.
Cuprous chloride and indium stearate were maintained at
210 °C, while the reaction temperature was also maintained
at 210 °C. Upon injection, the color of the reactive system
turned brown rapidly, indicating that nucleation had oc-
curred, and the nanocrystals began to grow. The reaction
stopped after 5 min. Figure 1a shows the TEM image of the
CuInSe2 nanocrystals with a triangular shape. The nano-
crystals have an average length of 13.5 nm on two sides and
14.5 nm on the other side (Figure S1). The three major
rings of the electron diffraction (ED) pattern shown in Fig-
ure 1b correspond to the (112), (204)/(220), (116)/(312) re-
flection direction of the as-synthesized CuInSe2 nanocrys-
tals. The high-resolution TEM (HRTEM) image of an indi-
vidual nanoparticle, as shown in Figure 1c, displays high
crystallinity for CuInSe2 nanocrystals. The distance be-
tween the adjacent atomic lattice fringe is about 3.34 Å and
2.04 Å, which is consistent with the (112) and (220) planes,
respectively, of chalcopyrite CuInSe2. Both the electron dif-
fraction (ED) patterns and the HRTEM images confirm
that the particle has a single-crystalline feature. Phase struc-
tures of the as-synthesized CuInSe2 nanocrystals were in-
vestigated by XRD. Figure 1d shows the XRD pattern of
Figure 1. (a) TEM images of CuInSe2 nanocrystals. (b) ED pat-
terns of the as-synthesized CuInSe2 nanocrystals. (c) HRTEM
images of CuInSe2 nanocrystals. (d) XRD patterns for the CuInSe2
nanocrystals. Literature values for the peak positions and inten-
sities for bulk chalcopyrite (tetragonal) CuInSe2 samples are indi-
cated by the vertical bars.
The absorption spectra of the as-synthesized triangular
CuInSe2 nanocrystals with a triangular shape and the stan- CuInSe2 nanocrystals in a toluene solution are shown in
dard data for chalcopyrite-phase CuInSe2 as well. All dif- Figure 2. The band gap of the triangular CuInSe2 nano-
fraction peaks are in good agreement with the bulk CuInSe2 crystals was determined to be 1.14Ϯ0.02 eV, which is basi-
(JCPDS 40-1487). The major diffraction peaks can be inde- cally in agreement with the reported 1.04–1.10 eV for bulk
xed as (112), (204)/(220), (116)/(312), (008)/(400), and (316)/ chalcopyrite CuInSe2.[3a,4] Compared with the bulk chalco-
(332). The average crystalline sizes are estimated as 14.3 nm pyrite CuInSe2, whose absorption onsets at about 1200 nm,
by the Scherrer equation from the XRD diffraction peaks, the absorption band edge of the as-synthesized CuInSe2
which are consistent with the results from TEM images. nanocrystals does not display a blueshift because of quan-
XPS was used to confirm the composition and valence tum confinement, suggesting that the as-synthesized nano-
states of the CuInSe2 nanocrystals with a trigonal-pyrami- particles are not small enough.[13]
dal shape (Figure S2). All the measurements were carried
The CuInSe2 nanocrystals observed by scanning electron
out with reference to the C1s binding energy (BE) microscopy (SEM) (Figure S4) have a trigonal-pyramidal
(284.6 eV) as an internal standard. Figure S2a shows the shape. We believe the morphology of our as-synthesized
photoelectron spectra of the as-prepared products over a CuInSe2 nanocrystals is similar to that in the report by
wide range of binding energies. It is very clear that Cu, In, Korgel and co-workers.[11] According to their demonstra-
Se, C, O are the only prominent elements present in the tion, the trigonal-pyramidal CuInSe2 nanocrystals are
¯ ¯
sample. Detailed XPS spectra of Cu2p3/2, Cu2p1/2, In3d5/ bound by three {114} and one (112) surface facets. In ad-
¯
2, In3d3/2, and Se3d binding energies for nanocrystals are dition, an opposing (112) surface is correspondingly un-
¯
shown in Figure S2b–d, respectively. The BE values of stable and leads to fast growth in the [112] direction. We
¯
Cu2p3/2 and Cu2p1/2 are 931.8 eV and 951.8 eV, respec- thought that if the fast growth rate along the [112] direction
tively, for our sample, in good agreement with Cu+ that has is restrained, the shape of the nanocrystals could be
been reported elsewhere.[15] Furthermore, Cu2+ with a bind- changed. The traditional method to control the shape of
ing energy of 942 eV has not been observed. Therefore, we nanocrystals is usually to tune the amount of the surfactant
can confirm that only monovalent copper exists in the directly, but we achieved this purpose by tuning the injec-
CuInSe2 nanocrystals.[16] The peak of In3d located at tion temperature of OAm-Se, while keeping the other pa-
444.5 eV and 451.95 eV, and the Se3d peaks located at rameters fixed. Figure 3a–c shows the TEM images with an
54.1 eV indicate In3+ and Se2–, respectively.[9] X-ray (EDX) injection temperature of 190 °C, 160 °C, and 130 °C, respec-
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Figure 2. UV/Vis absorption spectrum of the as-synthesized Cu-
InSe2 nanocrystals. The band gap of the nanocrystals is approxi-
mated by using the direct band gap method plotting the absorbance
squared vs. energy, and extrapolating to zero as shown in the inset.
tively. It is interesting to note that the shape of the CuInSe2
nanocrystals gradually evolved into hexagons. According to
Pang and co-workers,[17] injection and growth temperatures
are crucial factors in tuning the balance between nucleation
and growth. For our synthesis system, when the injection
temperature is decreased, the initial nucleation rate is slower
so that the amount of crystal nuclei is less. If the amount
of crystal nuclei is less, a relatively high concentration of
OAm would bind to the crystal faces, and further suppress
¯
the growth rate along the [112] direction. A similar phe-
nomenon, which we have speculated on, also happened for
¯ ¯ ¯ ¯ ¯ ¯ ¯
the growth along three other directions [114], [114], [114].
And then the CuInSe2 nanocrystals with a cutaway octahe-
dral shape (Figure S5) appeared when the injection tem- Figure 3. TEM images of CuInSe2 nanocrystals with the injection
temperature of Se/OAm at (a) 190 °C, (b) 160 °C, (c) 130 °C. All
scale bars are 50 nm.
perature reached 130 °C, as shown in Figure 3c. In order to
further testify the relationship between injection tempera-
ture and the amount of nucleation, we prolonged the reac-
tion time when the injection temperature was 210 °C (Fig-
ure S6), but the size of the nanocrystals exhibited almost
no change. This indicates that there were few residual in Figure S9. When we increased the volume of OAm to
monomers for the growth of the nanocrystals, and most 3 mL, the shape of the CuInSe2 nanocrystals changed to
were used for the nucleation of nanocrystallites. Herein, we ellipsoidal as shown in Figure 4b. The HRTEM image of
can control the shape of the as-synthesized nanocrystals by an individual spherical and ellipsoidal shape displays high
tuning the injection temperature of OAm-Se.
crystallinity for CuInSe2 nanocrystals (Figure 4c–d). The
When the Cu precursor was changed to copper stearate, distance between the adjacent atomic lattice fringe is about
OAm-Se was injected into the preheated mixture at 130 °C, 3.36 Å and 3.35 Å, which is consistent with the (112) planes
while maintaining the reaction temperature at 210 °C. After of the chalcopyrite CuInSe2. We consider that the divalent
1 h, spherical CuInSe2 nanocrystals were prepared, as copper atom was reduced by OAm, which is similar to that
shown in Figure 4a. The phase structures of the as-prepared reported by Xie and co-workers where OAm is used as the
products were investigated by XRD (Figure S7). The three reducing agent instead of ethylenediamine.[8] In addition,
major peaks are in good agreement with the bulk chalco- we believe that the high concentration of OAm can bind to
pyrite CuInSe2 crystal phase (JCPDS 40-1487). But there certain crystal faces more strongly so as to block the growth
are some other small diffraction peaks in the XRD patterns, of them as a result of the difference in the growth rate for
and the same phenomenon also occurs in the ED patterns different crystal faces. In Figure 4, we can clearly observe
(Figure S8). We speculate that they could be attributed to the shape of the CuInSe2 nanocrystals that have changed
CuSe (JCPDS 49-1457) and wurtzite CuInSe2 as reported.[4] from spherical to ellipsoidal. Consequently, it is easy to
The average diameter of the nanocrystals calculated by the understand that the OAm plays an important role not only
Scherrer equation from the XRD diffraction peaks was in assisting the formation of the CuInSe2 nanocrystals but
24.8 nm, which basically agrees with the average size shown also in controlling the nanocrystal morphologies.
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three-neck flask, which was connected to a Schlenk line and de-
gassed at room temperature for a while followed by purging with
Ar under magnetic stirring. Subsequently, the mixture was heated
to 130 °C under Ar for 30 min. When the initial colorless solution
turned bright-yellow, the mixture was continuously heated to
210 °C. An OAm-Se (1 mL) stock solution was injected rapidly into
the hot mixture and the temperature kept at 210 °C; the reaction
solution turned dark-brown instantly. After 5 min, the products
were cooled to room temperature to yield CuInSe2 nanocrystals.
The products were precipitated by the addition of methanol fol-
lowed by centrifugation at 5000 rpm for 5 min, and the precipitate
was redispersed in a toluene solution.
Synthesis of CuInSe2 Nanocrystals with a Spherical Shape: In a typ-
ical experimental synthesis, copper stearate (31.5 mg, 0.05 mmol)
was used instead of cuprous chloride. OAm (1 mL) and ODE
(3 mL) were used instead of OAm (4 mL). The injection tempera-
ture of the OAm-Se (1 mL) stock solution was 130 °C, and the
reaction time was 1 h. All the other conditions were the same as
those in the above description.
Materials Characterization: An XRD analysis was performed with
a Rigaku D/max-2500 diffractometer with a graphite monochro-
mator by using Cu-Kα radiation operating at 200 mA and 40 kV.
XRD data were collected over the range of 20–80° (2θ) with a step
interval of 0.02° and a preset time of 1.6 s per step at room tem-
perature. TEM and HRTEM images were performed with a FEI
Tecnai G2 S-Twin with a field emission gun operating at 200 kV.
Images were acquired digitally with a Gantan multiple CCD cam-
era. EDX spectra were obtained with a JEOL JSM 6700F instru-
ment. The surface of the CuInSe2 nanocrystals was characterized
by XPS, recorded with an ESCALAB 250 X-ray photoelectron
spectrometer by using Mg-Kα X-rays as the excitation source. SEM
was performed with a FEI Helios 600i. UV/Vis/NIR absorption
spectra were measured with a Shimadzu UV-3600 spectrophotome-
ter.
Figure 4. TEM images of CuInSe2 nanocrystals synthesized with
(a) 1 mL of OAm, and (b) 3 mL of OAm, and their typical
HRTEM images in (c) and (d), respectively.
Conclusions
Monodisperse chalcopyrite CuInSe2 nanocrystals with a
trigonal-pyramidal shape were synthesized by a facile hot-
injection method. The morphology could be tuned by
changing the injection temperature. The reason for the mor-
phological evolution could be that the injection temperature
affects the crystal nucleus, and further affects the effect of
OAm on the growth rate along the different directions,
which realizes the shape evolution. In addition, CuInSe2
nanocrystals with a spherical shape were also obtained
when copper stearate was used as the copper precursor. Be-
cause of the binding effect of OAm, an ellipsoidal shape
was also obtained when increasing the concentration of
OAm.
Supporting Information (see footnote on the first page of this arti-
cle): XPS, EDX spectra, SEM, TEM images and XRD profile.
Acknowledgments
This work was supported by the Foundation of the National Natu-
ral Science Foundation of China (No. 21371069 and 21301068), the
Specialized Research Fund for the Doctoral Program of Higher
Education (No. 20110061110015), the National High Technology
Research and Development Program (863 program) of China (No.
2013AA031702), and the Special Program of China Postdoctoral
Science Foundation (no. 2012T50288).
Experimental Section
Materials: Oleylamine (OAm, 70%) and selenium powder (Se,
99.99%) were purchased from Sigma–Aldrich. Copper(II) nitrate
trihydrate (99.0–102.0%), indium(III) nitrate hydrate (99.5%), and
oleic acid (OA) were obtained from Sinopharm Chemical Reagent
Co., Ltd. 1-Octadecene (ODE, 90%) was purchased from Alfa Ae-
sar. Copper(I) chloride (AR) was purchased from the Beijing
Chemical Reagents Company. Sodium stearate was purchased from
the Shanghai Chemical Reagents Company. All chemicals were
used as received without further treatment.
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