Synthesis and self-assembly of triangular Cu2-xSe nanocrystals

Article abstract of DOI:10.1039/c4ra05228e

High quality triangular Cu_2-xSe nanocrystals were successfully synthesized through a green phosphine free route by the non-injection one-pot colloidal approach with Se-octadecene (ODE) as a precursor. CuSt₂ was used as reactant and oleylamine (OAM) was used as reaction solvent. By using this new non-injection method, triangle shaped Cu_2-xSe nanocrystals with sizes ranging from 2.8 to 12 nm were achieved by simply controlling the reaction time. Transmission electron microscopy (TEM) and high resolution TEM (HRTEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and energy dispersive X-ray (EDX) measurements were used to characterize as-synthesized nanocrystals. Cubic berzelianite crystal phase and narrow size distributions were demonstrated by the results. Additionally, the narrow size distribution promoted as-synthesized Cu_2-xSe nanocrystals self-assembled into ordered superstructures with the assistance of OAM.

Full text of DOI:10.1039/c4ra05228e

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Synthesis and self-assembly of triangular Cu2ꢀxSe
nanocrystals
Cite this: RSC Adv., 2014, 4, 39547
b
c
b
a
Guimin Tian, Taotao Zhao, Jinzhong Niu, Huaibin Shen and Lin Song Lia
*
*
High quality triangular Cu2ꢀxSe nanocrystals were successfully synthesized through a green phosphine free
route by the non-injection one-pot colloidal approach with Se–octadecene (ODE) as a precursor. CuSt2
was used as reactant and oleylamine (OAM) was used as reaction solvent. By using this new non-
injection method, triangle shaped Cu2ꢀxSe nanocrystals with sizes ranging from 2.8 to 12 nm were
achieved by simply controlling the reaction time. Transmission electron microscopy (TEM) and high
resolution TEM (HRTEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and energy
dispersive X-ray (EDX) measurements were used to characterize as-synthesized nanocrystals. Cubic
berzelianite crystal phase and narrow size distributions were demonstrated by the results. Additionally,
the narrow size distribution promoted as-synthesized Cu2ꢀxSe nanocrystals self-assembled into ordered
superstructures with the assistance of OAM.
Received 2nd June 2014
Accepted 19th August 2014
DOI: 10.1039/c4ra05228e
www.rsc.org/advances
used imidazoline-2-selenone as the Se precursor and 17 nm
sized nanoplates were obtained.21 Manna group reported the
1. Introduction
preparation of Cu2ꢀxSe nanocrystals using octadecene (ODE) as
the solvent.22 The hot-injection method was needed to produce
nanocrystals at elevated temperatures in these reports. So it is
still challenging to synthesize size-, shape-, and assembly-
controllable copper selenide nanocrystals by facile green route
and energy saving methods.
Colloidal semiconductor nanocrystals have been enthusiasti-
cally exploited not only because of the quantum size effects
resulting in unique size-dependent properties, but also for their
potential wide-spread applications in biomedical diagnosis,
light emitting diodes (LED), lasers, and solar cells, etc.1–16 In
various semiconductors, copper selenide has gained much
attention as a promising material in solar cell applications
because it has both a direct bandgap of 2.2 eV and an indirect
bandgap of 1.0–1.4 eV, very suitable in light harvest. And more,
copper selenide is low-cost, p-type semiconductor and free of
heavy metals.
Recently, great progress has been made in the synthesis of
copper selenide based nanocrystals and their application in
solar cells.17–24 Malik et al. reported the preparation of CuSe
nanoparticles with a size of about 16 nm by the thermolysis in
trioctylphosphine oxide (TOPO) using Cu(Se2CNEt2)2 as a
single-source precursor.18 Prieto group reported the synthesis of
Cu2Se nanocrystals using trioctylphosphine (TOP) as a solvent
via simultaneous injection of TOP–Se and TOP–Cu.19 In these
reports, relative expensive and dangerous TOPO and TOP were
used. In 2009, Deng et al. introduced the synthesis of two
dimensional Cu2ꢀxSe nanosheets, they used paraffin oil as the
solvent, the Cu precursor was injected into Se precursor at 250
ꢁC, phosphine contained reagents were avoided.20 Choi et al.
Herein, we report
a facile, “green”, phosphine-free,
economic, and non-injection one-pot method, which has been
proved to be highly successful not only on precise size- and
shape-control but also on the assembly-controlled synthesis of
Cu2ꢀxSe nanocrystals. With the use of CuSt2, oleic acid (OA),
oleylamine (OAM), and selenium–octadecene (Se–ODE) as
reagents to conduct the synthesis, triangular Cu2ꢀxSe nano-
crystals were obtained and they self-assembled into ordered
superstructures. Toxic, pyrophoric, and expensive alkylphos-
phine contained precursors are avoided, and the non-injection
one-pot method facilitate the conduct of experiments. The
mechanisms of size/shape control and assemble behavior are
discussed in detail.
2. Experimental section
2.1 Chemicals
1-Octadecene (ODE, 90%), oleylamine (OAM, 70%) and sele-
nium (Se, 99.99%, powder) were purchased from Aldrich, stea-
ric acid (SA, 98%) was purchased from Alfa. CuCl2, sodium
stearate, hexanes (analytical grade), and methanol (analytical
grade) were obtained from Beijing Chemical Reagent Ltd.,
China. All chemicals were used as received without any further
purication.
aKey Laboratory for Special Functional Materials, Henan University, Kaifeng 475004,
P. R. China. E-mail: shenhuaibin@henu.edu.cn
bCollege of Science, Henan Institute of Engineering, Zhengzhou, 451191, P. R. China.
E-mail: niujinzhong@gmail.com
cCollege of Foreign Language, Henan University, Kaifeng 475004, P. R. China
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It has been known that the key to synthesize monodisperse
NCs is to balance the reactivity of the precursors by their
ligands, solvent matrix composition, and reaction tempera-
tures, etc.28,29 In our synthesis of copper selenide, copper stea-
rate was chosen as the precursor for Cu and stearic acid as the
so acid, the binding between Stꢀ with Cu is weaker than that of
traditional Cu precursors like CuI, CuCl etc., this will improve
the reactivity of Cu. OAM was used to further activate the CuSt2.
ODE–Se was chosen as the Se precursor because its high reac-
tivity and it is phosphine-free.7,30 Additionally, to further
improve and achieve a better balance of the reactivities between
Cu and Se, the reaction temperature was increased to promote
the reactions.
Fig. 1 shows the temporal evolution of TEM images of
Cu2ꢀxSe nanocrystals and corresponding size distribution
histograms. 3 minutes aer the reaction temperature reached
220 ꢁC, small irregular nanocrystals were formed with size of
about 2.8 ꢂ 1.8 nm, the size distribution was not very narrow, as
shown in Fig. 1A and A1. Another 2 minutes aer the formation
of nanocrystals in Fig. 1A, 3.7 ꢂ 1.3 nm sized nanocrystals were
obtained, and the size distribution was narrower than that in
Fig. 1A, but the shape of as-prepared nanocrystals was still not
clear. When the reaction time reached 10 min, triangle shaped
Cu2ꢀxSe nanocrystals were formed and the size distribution
continuously gets narrower with an average size of about 5.4 ꢂ
1.4 nm. Aer that, the triangle shape of Cu2ꢀxSe nanocrystals
was kept till the end of the reaction, and the average sizes were
8.4 ꢂ 1.4 nm (Fig. 1D), 10.8 ꢂ 1.3 nm (Fig. 1E), and 12 ꢂ 1 nm
(Fig. 1F) when the reaction times were 30 min, 60 min, and 120
min, respectively. And more, the triangular Cu2ꢀxSe nano-
crystals self-assembled into ordered structures as shown in
Fig. 1D–F. Fig. 1G shows the HRTEM image of Cu2ꢀxSe nano-
crystals in Fig. 1F, it reveals the well-resolved lattice fringes,
demonstrating the highly crystalline nature of the as-prepared
nanocrystals. The distance between two adjacent planes is
0.206 nm, which is consistent with the lattice parameters of
(220) plane of cubic structured Cu2ꢀxSe (JCPDS no. 06-0680). As
shown in Fig. 1H, the SAED pattern showed several distinct
rings and the calculated d-spacings matched well with inter-
planar distances of cubic Cu2ꢀxSe.
2.2 Synthesis of copper stearate (CuSt2)
In a typical synthesis, sodium stearate (20 mmol) was dissolved
in 80 g of methanol and heated to 50–60 C until it became a
clear solution, CuCl2 solution of 10 mmol in 10 g methanol was
then added dropwise with vigorous stirring and precipitation of
CuSt2 occulated slowly. The precipitates were washed repeat-
edly with methanol three times and then dried under vacuum.
ꢁ
2.3 Stock solution for Se–ODE precursor
The Se–ODE precursor was made by degassing Se (0.1578 g, 2
mmol) and 20 mL of ODE in a 50 mL three-necked ask, before
it was heated to 220 ꢁC for 180 min under nitrogen. During this
procedure, the color of the precursor changed from transparent
to orange, then red, and nally turned yellow.
2.4 Synthesis of triangular Cu2ꢀxSe nanocrystals
In a typical synthesis of monodisperse triangular Cu2ꢀxSe
nanocrystals, 0.2 mmol CuSt2, 1.2 mmol SA, 1.2 mmol OAM, 2
mL Se–ODE, and 4 mL of ODE were added into a three-neck
ask at room temperature, and the resulting reaction mixture
was heated to 220 ꢁC under a nitrogen ow and kept at this
temperature for 90 min. The Cu2ꢀxSe nanocrystals can be
separated from the solution aer adding a large amount of
acetone followed by centrifugation. The precipitates, which can
be well redispersed in hexanes, were used for subsequent
characterization.
2.5 Characterization
Transmission electron microscopy (TEM) and high resolution
transmission electron microscopy (HRTEM) observations were
performed using a JEOL JEM-2010 microscope with an accel-
erating voltage of 200 kV. X-ray diffraction (XRD) studies of
Cu2ꢀxSe nanocrystals were carried out at room temperature
with an X-ray diffractometer (Philips X0Pert Pro) using Cu-Ka
radiation (l ¼ 0.154 nm). X-ray photoelectron spectrum (XPS)
measurements were performed using an AXIS ULTRA X-ray
photoelectron spectroscope with monochromatized Al Ka
radiation, an anode voltage of 15 kV, and an emission current of
3 mA.
The structural characteristics of the Cu2ꢀxSe nanocrystals
were determined through X-ray diffraction (XRD) characteriza-
tions. Fig. 2 shows the XRD patterns of the Cu2ꢀxSe NCs
obtained at different reaction times. The diffraction peaks in
3. Results and discussion
Copper selenide can be formed in many stoichiometries and Fig. 2 locate at 2 theta ¼ 26.7, 44.6, 52.9 degrees, corresponding
phases depending on the synthesis conditions.25 Generally to the (111), (220), and (311) planes. All of the peaks match well
speaking, if the Se precursor is very reactive and the amount of with the standard prole of cubic berzelianite phase (JCPDS no.
Se precursor is sufficient, CuSe would be synthesized as the nal 06-0680), this result is consistent with the analysis based on
product. If the Se precursor is not very reactive or the amount is HRTEM images. In addition, XRD characterizations can also be
not sufficient, Cu2Se will be synthesized eventually. Cu2ꢀxSe is used to determine the size evolution of nanocrystals as a func-
the general formula as the synthesis product in most experi- tion of time by applying the Scherrer equation: Deff ¼ Kl/
ments. And the crystal structures of copper selenide are also B cos(qB) where Deff is the effective diameter determined by
variable, including cubic berzelianite, tetragonal umangite, XRD, K is the Scherrer constant related to the shape of crystal-
hexagonal klockmannite, and orthorhombic athabascaite, all lites. l is the X-ray wavelength, B is the full width at half-
the phases have been identied as p-type semiconducting maximum of the diffraction peak, and qB is the half angle of
materials because of the copper vacancies within the the diffraction peak on the 2q scale. From the bottom to the top,
lattice.17,26,27
the mean crystal sizes are 2.6, 3.9, 5.5, 8.4, 11.1, and 12.3 nm,
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Fig. 2 XRD pattern (Cu2ꢀxSe: 06-0680) of triangular Cu2ꢀxSe nano-
crystals synthesized at the temperature of 220 ꢁC with different
reaction time.
To obtain information about the chemical compositions, X-
ray photoelectron spectroscopy (XPS) and energy dispersive X-
ray (EDX) spectra measurements were performed. As shown in
Fig. 3A, quantitative energy dispersive spectroscopy (EDS)
analysis shows that the atom ratio of Cu/Se is close to 1.6 : 1
which indicates that the composition of the as-synthesized
products is non-stoichiometric Cu2ꢀxSe. The XPS survey
spectra of the synthesized Cu2ꢀxSe NCs identied the presence
of Cu, Se, O and C as shown in Fig. 3B. High-resolution spectra
of Cu 2p, and Se 3d were measured to determine the oxidation
states of the constituent elements. Fig. 3C shows the Cu 2p core
splitting into Cu 2p3/2 (930.1 eV) and Cu 2p1/2 (950.0 eV) peaks
with a peak separation of 19.9 eV, in agreement with those
reported in the literature for the Cu2ꢀxSe nanocrystals. In
addition, the Cu 2p3/2 satellite peak of Cu2+ (942 eV) did appear
Fig. 1 TEM images of triangular Cu2ꢀxSe nanocrystals synthesized at
temperature of 220 ꢁC with reaction time of 3 min (A), 5 min (B), 10 min
(C), 30 min (D), 60 min (E), 120 min (F) and corresponding size distri-
bution histograms A1, B1, C1, D1, E1, and F1. (G) HRTEM image of (F);
(H) SAED pattern of (F).
respectively, calculated from (111) reection by the Scherrer
equation. The calculated values are in agreement with that
determined by statistical analysis based on TEM images.
Fig. 3 (A) EDS spectrum of triangular Cu2ꢀxSe nanocrystals which is
obtained in Fig. 1F. (B–D) The survey and high-resolution XPS spec-
trum of the synthesized triangular Cu2ꢀxSe which is obtained in Fig. 1F.
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2.8 to 12 nm were obtained by simply control the reaction time.
Cubic berzelianite crystal phase and narrow size distributions
were demonstrated by the characterization results. The cubic
berzelianite phase and chemical compositions for Cu/Se of
1.6 : 1 were demonstrated. With the assistance of OAM, as-
synthesized narrow size distributed Cu2ꢀxSe nanocrystals self-
assembled into ordered superstructures.
Fig. 4 (A)–(C) TEM images and corresponding FFT patterns of trian-
gular Cu2ꢀxSe nanocrystals with different assemble behaviors; inset:
corresponding Schemes of the stacking of triangular Cu2ꢀxSe
nanocrystals.
Acknowledgements
This work was nancially supported by the research project of
the National Natural Science Foundation of China (21201055),
Natural Science Research Program of Henan Educational
Committee, China (12A430003), and Doctoral Foundation of
Henan Institute of Engineering (D2012019 and D2010011).
in the spectrum which indicated the presence of +2 valence
state in the Cu2ꢀxSe nanocrystals. All that suggesting that the
copper valence state in the Cu2ꢀxSe nanocrystals is compound
of +1 and +2. Fig. 3D shows the Se 3d5/2 peak located at 52.5 eV,
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