P-type Cu7Te5 single-crystalline nanocuboids: Size-controlled synthesis and large-scale self-assembly

Article abstract of DOI:10.1039/c4ce00670d

Cu₇Te₅ single-crystalline nanocuboids have been synthesized by a low-temperature solvothermal strategy, using environmentally friendly chemicals. The size, from nanometer to sub-micrometer, was controlled by adjusting the dodecanethiol (DDT) concentration. Through DDT capping, which induced appropriate van der Waals interactions, the alignment of the nanocuboids led to large-scale self-assembly of superstructures or supracrystals. The UV-vis-NIR spectra of the as-prepared nanocuboid colloid exhibited potential LSPR properties. Measurement of the Mott-Schottky impedance potential curve confirmed the p-type conductivity of the as-prepared Cu₇Te₅ nanocuboids. This journal is

Full text of DOI:10.1039/c4ce00670d

CrystEngComm
View Article Online
View Journal | View Issue
COMMUNICATION
P-type Cu7Te5 single-crystalline nanocuboids:
size-controlled synthesis and large-scale
self-assembly†
Cite this: CrystEngComm, 2014, 16,
9441
Received 31st March 2014,
Accepted 13th May 2014
*
Baosong Dai, Qian Zhao, Jing Gui, Jiatao Zhang and Hesun Zhu
DOI: 10.1039/c4ce00670d
www.rsc.org/crystengcomm
Cu7Te5 single-crystalline nanocuboids have been synthesized by a
low-temperature solvothermal strategy, using environmentally
friendly chemicals. The size, from nanometer to sub-micrometer,
was controlled by adjusting the dodecanethiol (DDT) concentration.
Through DDT capping, which induced appropriate van der Waals
interactions, the alignment of the nanocuboids led to large-scale
self-assembly of superstructures or supracrystals. The UV-vis-NIR
spectra of the as-prepared nanocuboid colloid exhibited potential
LSPR properties. Measurement of the Mott–Schottky impedance
potential curve confirmed the p-type conductivity of the
as-prepared Cu7Te5 nanocuboids.
from nanometer to micrometer, is still required in order to
control their optical and electronic properties. There are few
reports on the synthesis and properties of Rickardite Cu7Te5
NCs.3–12 In this paper, without the addition of special precur-
sors, we use the environmentally friendly common chemicals,
copper stearate, trioctylphosphine telluride (TOP-Te) and
oleic acid (OA) in toluene solvent to synthesize Cu7Te5 nano-
cuboids with flexible size control using a low-temperature
solvothermal method.
Monodisperse NCs can self-assemble into superstructures
under the appropriate conditions of temperature, solvent and
coating agents.13 These self-assemblies can exhibit tunable
physical and chemical properties, but also lay the foundation
for assembly-based optoelectronic nanodevices. The self-
assembly of nanocubes by dipole–dipole interactions which
enables face-to-face alignment has been researched by the
Yang, Xia, and Fang groups, among others.11–16 Research on
the self-assembly of nanocuboids has only obtained limited
progress because of their flexible anisotropic rectangular
shapes.17 In particular, not only the shape and size of the
NCs, but also the uniform nanogaps between the NCs, can
distinctly affect LSPR properties. Here, by this new synthesis
strategy, good monodispersity, large-scale monolayers, multi-
layer self-assembly and different assembling modes of Cu7Te5
nanocuboids have been realized. Confirmation of the p-type
conductivity and the LSPR properties of the Cu7Te5 nano-
cuboids have also been investigated.
1. Introduction
Copper telluride (Cu2−xTe) nanocrystals (NCs) have composi-
tion-, size- and shape-dependent properties, such as near-
infrared (NIR) localized surface plasmon resonance (LSPR),
thermoelectric and ionic conductivity properties, which
enable their potential application in surface-enhanced
Raman scattering (SERS), photothermal conversion, solar
cells, and electroconductive electrodes.1–3 They are usually
p-type semiconductors because of the presence of copper
vacancies. Copper vacancies not only determine the charge
transport properties, but also provide composition-dependent
LSPR properties.4,5 By adjusting the Cu : Te ratio, copper
tellurides (Cu2−xTe) can exist in a wide range of compositions
and phases. Thus, well-defined composition, shape and uni-
form size control are important in controlling the LSPR and
transport properties. Many groups, such as the Tuan, Cabot,
Feldmann, and Yu groups, have studied the synthesis and
properties of copper telluride nanostructures.3–7 However, a
facile strategy for more flexible size control of Cu2−xTe NCs,
2. Experimental section
2.1 Copper stearate (CuSt2) preparation
All chemicals were used without further processing. CuSt2
was synthesized according to the literature.18 Typically,
2.5 mmol (0.4262 g) CuCl2·2H2O and 5 mmol (1.5323 g) of
sodium stearate were dissolved in a mixture of 10 mL
distilled water, 70 mL hexane and 50 mL methanol at 60 °C
for 4 h. The upper organic layer containing CuSt2 was
Research Center of Materials Science, School of Materials Science & Engineering,
Beijing Institute of Technology, Beijing 100081, PR China. E-mail: zhangjt@bit.edu.cn;
Fax: +86 1068918065; Tel: +86 10 68918065
† Electronic supplementary information (ESI) available: Fig. S1–S3 and the
analysis of the largest exposed faces of the crystal. See DOI: 10.1039/c4ce00670d
This journal is © The Royal Society of Chemistry 2014
CrystEngComm, 2014, 16, 9441–9445 | 9441

View Article Online
Communication
CrystEngComm
decanted and washed several times. CuSt2 in solid form was
finally obtained after total evaporation of the residual hexane
and methanol.
3. Results and discussion
As illustrated by the LRTEM images in Fig. 1A and S1,† mono-
disperse nanocuboids with a size of 5.9 nm were prepared
with high purity. The HRTEM image in Fig. 1B confirmed the
single-crystallinity of the nanocuboids. Based on indexing,
the distance measured between two adjacent fringes
were 0.70 nm and 0.50 nm, which should be assigned to the
(002) plane (0.61 nm) and (032) plane (0.45 nm) of Rickardite
Cu7Te5 (JCPDS #26-1117). The corresponding fast Fourier
transform (FFT) pattern (inset of Fig. 1B) contains diffraction
spots from the (032) and (030) planes of the orthorhombic
phase Cu7Te5, and belongs to [100].19 In other words, the
largest exposed faces of the crystal belong to highly-indexed
[100] planes.20 By adjusting the concentration of DDT ligand,
flexible size control was realized. As shown in Fig. 1C and D,
the LRTEM and HRTEM images exhibited single-crystalline
nanocuboids with a size of about 10–12 nm. The indexed lattice
spacings and the inserted electron diffraction pattern in
Fig. 1D are consistent with Fig. 1B. Fig. 1E shows nanocuboids
with a size of about 100 nm, and Fig. 1F shows the sub-
micrometer size produced without DDT addition. With increas-
ing size, the size range becomes bigger, and the rectangular
shape becomes more anisotropic, which finally produces
micrometer-sized nanoplate shapes. The rhombus shaped
nanoplates with large size range in Fig. 1F are reasonable
because of the orthorhombic crystal phase. The powder XRD
pattern of the nanocuboids in Fig. 2A also confirms the
Rickardite phase (JCPDS # 26-1117). The EDS pattern from large-
scale SEM analysis of the samples on a silicon substrate in
Fig. 2B exhibits a Cu : Te ratio of about 1.5, which is concordant
with the Cu7Te5 phase confirmed by the XRD and ED character-
izations. The strong Si signal comes from the silicon substrate.
The advantage of our synthesis method is not only the
flexible size control, but also the realization of self-assembly
of the as-prepared nanocuboids on a large scale. In this work,
we found that when the size of the as-prepared Cu7Te5 nano-
cuboids was ~5.9 nm, they were more similar to nanocubes
and the monodispersity was good. As illustrated by the size
analysis shown in Fig. 3A, more than 94% of the nano-
cuboids are in the range of 5.8–6.0 nm. Due to the good
monodispersity of nanocuboids, and DDT capping, which
can provide appropriate van der Waals interactions between
the nanocrystals,21–26 the large-scale self-assembly of nano-
cuboids is realized easily, as shown in Fig. 3A, B and S2.†
Here, DDT is an important factor to get large-scale self-
assembly. Fig. 3A exhibits the monolayer self-assembly. The
FFT pattern of the multi-layer self-assembly (inset in Fig. 3B)
confirmed that the face-to-face simple-cubic close packing
formed a superlattice. As studied by the Xia, Yang and Fang
groups,14–17 the dipole–dipole interactions promoted face–
face alignments. This kind of multi-layer superlattice can be
expanded on a bulk film scale, as shown in the digital photo
in the inset of Fig. 3B. This large-scale assembly of the nano-
cuboid film has potential for photothermal and photovoltaic
device applications.27,28 The superstructure formed by the
2.2 Top-Te organic precursor preparation
1 mmol (0.128 g) Te powder was dissolved in 3 mmol
(1.34 mL) TOP at 150 °C under nitrogen for more than 2 h,
until the Te powder was completely dissolved. Then we obtained
a clear yellow solution which was diluted in toluene solvent.
2.3 Synthesis of Cu7Te5 nanocuboids
Typically, 0.25 mmol CuSt2 was dissolved in 5 mL toluene to
obtain a clear solution, then 6 mL oleic acid (OA) was intro-
duced into the solution and 2 mL of the Top-Te precursor
was added. After this was stirred for 5 min, 2 mL dodecane-
thiol (DDT) was introduced to get a homogeneous solution.
Then, it was transferred into a Teflon-lined autoclave with
25 mL capacity. The autoclave was sealed and heated at
150 °C for 1 h. The product was washed with 20 mL of
methanol several times.
2.4 Characterization
Samples for characterization by TEM and electron diffraction
were prepared by adding one drop of toluene colloid
containing the product onto a 300 mesh nickel grid with car-
bon support film. Low resolution transmission electron micros-
copy (LRTEM), carried out by a JEOL JEM-1200EX instrument
working at 100 kV, and high-resolution transmission electron
microscopy (HRTEM), carried out by a FEI Tecnai G2 F20
S-Twin instrument working at 200 kV, were utilized to charac-
terize the morphology, monodispersity, self-assembly and
crystallization details. SEM images and energy-dispersive
spectrometry (EDS) analysis were obtained from the sample
on a silicon substrate, using a Hitachi FE-SEM 4800 instru-
ment. Samples for XRD measurements were prepared by
adding several drops of concentrated product onto silicon
(100) wafers and drying at room temperature. The phases of
the products were determined by XRD on a Bruker D8
Advance X-ray power diffractometer with CuKα radiation
(λ = 1.5418 Å). The ultraviolet-visible-near-infrared (UV-vis-
NIR) absorption spectra of the Cu7Te5 nanocuboid toluene
colloids were recorded on a Shimadzu UV3600 UV-vis-NIR
spectrophotometer at room temperature. Confirmation of the
p-type conductivity of the Cu7Te5 nanocuboids was carried
out by measurement of the Mott–Schottky impedance poten-
tial curves. These were measured on an IM6e electrochemical
workstation (Zahner, Germany) with
a standard three-
electrode system, namely, the working electrode, the Pt
counter electrode, and the Ag/AgCl reference electrode.
As-prepared Cu7Te5 nanocuboid film (which underwent vac-
uum drying treatment at 100 °C for 3 h) on indium tin oxide
(ITO) glass was used as the working electrode. The electrolyte
was a 200 ml solution of 0.1 M NaOH and 0.1 M Na2S.
9442 | CrystEngComm, 2014, 16, 9441–9445
This journal is © The Royal Society of Chemistry 2014

View Article Online
CrystEngComm
Communication
Fig. 1 LRTEM and HRTEM morphologies (A–E) and SEM morphology (F) of Cu7Te5 nanocuboids, from nanometer size to sub-micrometer size.
The FFT pattern and ED pattern are shown in the insets of Fig. B and D.
hierarchical assembly of the nanocuboids was further
characterized by scanning TEM (STEM), as shown in Fig. 3C.
Through DDT capping, which enabled three-dimensional
ordered alignment of the nanocuboids, the micrometer-sized
superstructures could form (see Fig. S3†). As with the shape
evolution shown in Fig. 1, when the size becomes bigger, the
anisotropic shape of the nanocuboid becomes more and
more distinct. Without good monodispersity, the nano-
cuboids in Fig. 1C and E have no long-range-ordered self-
assembly. However, the micrometer-sized nanoplates in
Fig. 1D assemble face-to-face into superstructures on a large
scale, as shown in Fig. 3D.
The LSPR properties of the Cu2−xTe nanocrystals in the
NIR region could be utilised in a broad range of applications,
from optoelectronics and photothermal therapy to
biomedicine.1–4,27,28 Fig. 4A shows the UV-vis-NIR spectra of
colloidal Cu7Te5 nanocuboids with two sizes. A strong absorp-
tion band centered at 900–1000 nm is clearly observed. This
is attributed to the LSPR of the nanocuboids. It was already
confirmed that the nanocube shape had a stronger LSPR than
the nanoplate and nanorod shaped nanocrystals in the NIR
region.4 Therefore, the as-prepared monodisperse Cu7Te5
nanocuboids, which are similar to nanocubes, and their self-
assembly, have the potential for use in surface enhanced
Raman scattering (SERS) detection and photothermal therapy.
P-type Cu2−xTe nanocrystals, due to copper vacancy deter-
mined efficient charge transport, can potentially be used for
p/n heterojunction solar cell applications. According to the
Mott–Schottky equation, for a p-type semiconductor,29,30
1
2
kT
e


 
E  EFB 
(1)




2
Csc
0r NA A2
Fig. 2 A) XRD pattern of the as-prepared Cu7Te5 nanocuboids. The
pattern for bulk Rickardite Cu7Te5 (JCPDS # 26-1117) is provided for
reference and comparison. B) EDS pattern and Cu : Te ratio by SEM analysis.
where Csc is the space charge region capacitance, ε0 and εr
are the vacuum and relative dielectric permittivities, A is the
This journal is © The Royal Society of Chemistry 2014
CrystEngComm, 2014, 16, 9441–9445 | 9443

View Article Online
Communication
CrystEngComm
Fig. 3 LRTEM and STEM images of the monolayer self-assembly (A), large-scale multi-layer superlattice (B, C), and large-scale hierarchical
assembly of Cu7Te5 nanocuboids into superstructures (D). The size analysis for the ~5.9 nm nanocuboids is shown in the inset of Fig. A. The
drawings in the insets of Fig. A, C and D are provided to highlight the face-to-face assembly. The FFT pattern of the assembly and a photograph
of the bulk-sized film on the flexible substrate are shown in the insets of Fig. B.
surface area of the sample in the electrolyte, NA is the con-
centrations of acceptors, k is the Boltzmann constant, T is the
4. Conclusion
temperature, E is the potential, and EFB is flat band potential.
A typical Mott–Schottky plot for the Cu7Te5 nanocuboid film
(1/Csc versus the applied potential, where Csc is the space
charge region capacitance) is shown in Fig. 4B, from which
the n or p-type conductivity can be identified.30 The negative
slope of all of the linear parts confirmed the p-type conduc-
tivity of the as-prepared Cu7Te5 nanocuboids. Based on the
confirmation of the p-type conductivity, potential use in a p/n
heterojunction solar cell and optoelectronic device applica-
tions could be carried out next.
In summary, we developed a low-temperature solvothermal
method to synthesize single-crystalline Cu7Te5 nanocuboids
with flexible size control. Thanks to DDT capping and shape
uniformity, dipole–dipole interactions promote large-scale
self-assembly of the nanocuboids. The strong LSPR properties
exhibited in the NIR region indicate the potential of
as-prepared Cu7Te5 nanocuboids in sensitive detection and
photothermal therapy applications. Confirmation of the
p-type conductivity suggests potential application in photo-
voltaic devices.
2
Acknowledgements
This work was supported by the Natural Science Foundation
of China (91123001, 21322105, and 51371015), funding
support from the Ministry of Education, China (NCET-11-
0793, 2011101120016), and the Program for Beijing Excellent
Talents (no. 2012D009011000007).
References
Fig. 4 A) UV-vis-NIR spectra of the as-prepared Cu7Te5 nanocuboid
colloid. B) Typical Mott–Schottky plot for the Cu7Te5 nanocuboid film
in 0.1 M Na2S and 0.1 M NaOH at 1 kHz. The potentials are measured
with respect to the standard calomel electrode (SCE).
1 J. M. Luther, P. K. Jain, T. Ewers and A. P. Alivisatos,
Nat. Mater., 2011, 10, 361–366.
9444 | CrystEngComm, 2014, 16, 9441–9445
This journal is © The Royal Society of Chemistry 2014

View Article Online
CrystEngComm
Communication
2 Y. Zhao and C. Burda, Energy Environ. Sci., 2012, 5,
5564–5576.
16 M. Rycenga1, J. M. McLellan and Y. N. Xia, Adv. Mater.,
2008, 20, 2416–2420.
3 I. Kriegel, C. Jiang, J. Rodríguez-Fernández, R. D. Schaller,
D. V. Talapin, E. da Como and J. Feldmann, J. Am. Chem.
Soc., 2012, 134, 1583–1590.
4 W. H. Li, R. Zamani, P. R. Gil, B. Pelaz, M. Ibáüez,
D. Cadavid, A. Shavel, R. A. Alvaerz-Puebla, W. J. Parak,
J. Arbiol and A. Cabot, J. Am. Chem. Soc., 2013, 135,
7098–7101.
5 H. Yang, C. Chen, F. Yuan and H. Tuan, J. Phys. Chem. C,
2013, 117, 21955–21964.
6 I. Kriegel, J. Rodríguez-Fernández, A. Wisnet, H. Zhang,
C. Waurisch, A. Eychmüller, A. Dubavik, A. O. Govorov and
J. Feldmann, ACS Nano, 2013, 7, 4367–4377.
17 Y. Nakagawa, H. Kageyama, Y. Oaki and H. Imai, J. Am.
Chem. Soc., 2014, 136, 3716–3719.
18 S. Li, H. Z. Wang, W. W. Xu, H. L. Si, X. J. Tao, S. Y. Lou,
Z. L. Du and L. S. Li, J. Colloid Interface Sci., 2009, 330,
483–487.
19 G. X. Hu, X. Cai and Y. H. Rong, Fundamentals of Materials
Science, Shanghai Jiao Tong University Press Co., Ltd, China,
2th edn, 2006, pp. 19–35.
20 F. R. Zhou, P. L. Zhu, X. Z. Fu, R. Q. Chen, R. Sun and
C. P. Wong, CrystEngComm, 2014, 16, 766–774.
21 Z. Zhuang, P. Qing, B. Zhang and Y. D. Li, J. Am. Chem. Soc.,
2008, 130, 10482–10484.
7 J. Liu, J. Xu, H. Liang, K. Wang and S. Yu, Angew. Chem., Int.
Ed., 2012, 51, 7420–7425.
22 Q. Zhao, J. T. Zhang and H. S. Zhu, Prog. Nat. Sci.: Mater.
Int., 2013, 23, 588–592.
8 Q. Wang, G. Chen, D. Chen and R. Jin, CrystEngComm,
2012, 14, 6962–6973.
9 Y. Zhang, Y. Ni, X. Wang, J. Xia and J. Hong, Cryst. Growth
Des., 2011, 11, 4368–4377.
10 L. Zhang, Z. Ai, F. Jia, L. Liu, X. Hu and J. C. Yu,
Chem. – Eur. J., 2006, 12, 4185–4190.
11 H. Shen, X. Jiang, S. Wang, Y. Fu, C. Zhou and L. S. Li,
J. Mater. Chem., 2012, 22, 25050–25056.
23 X. X. Xu and X. Wang, J. Mater. Chem., 2009, 19, 3572–3575.
24 Z. Zhuang, Q. Peng, X. Wang and Y. D. Li, Angew. Chem., Int.
Ed., 2007, 46, 8174–8177.
25 E. V. Shevchenko, D. V. Talapin, N. A. Kotov, S. O'Brien and
C. B. Murray, Nature, 2006, 40, 685–689.
26 Y. S. Xia, T. D. Nguyen, M. Yang, B. Lee, A. Santos,
P. Podsiadlo, Z. Tang, S. C. Glotzer and N. A. Kotov,
Nat. Nanotechnol., 2011, 6, 580–585.
12 H. B. Li, R. Brescia, M. Povia, M. Prato, G. Bertoni, L. Manna
and I. Moreels, J. Am. Chem. Soc., 2013, 135, 12270–12278.
13 M. P. Pileni, J. Mater. Chem., 2011, 21, 16748–16758.
14 J. Henzie, M. Grünwald, A. Widmer-Cooper, P. L. Geissler
and P. D. Yang, Nat. Mater., 2012, 11, 131–137.
15 Z. Quan, H. W. Xu, C. Y. Wang, X. D. Wen, Y. X. Wang,
J. L. Zhu, R. P. Li, C. J. Sheehan, Z. W. Wang, D. Smilgies,
Z. P. Luo and J. Y. Fang, J. Am. Chem. Soc., 2014, 136,
1352–1359.
27 W. Lu, Q. Huang, C. Li and W. Chen, Nanomedicine, 2010, 5,
1161–1171.
28 C. Lin, W. Lee, M. Lu, S. Chen, M. Hung, T. Chan, H. Tsai,
Y. Chueh and L. Chen, J. Mater. Chem., 2012, 22,
7098–7100.
29 D. F. Underwood, T. Kippeny and S. J. Rosenthal, J. Phys.
Chem. B, 2001, 105, 436–440.
30 A. Izgorodin, O. Winther, B. Winther and D. R. MacFarlane,
Phys. Chem. Chem. Phys., 2009, 11, 8532–8536.
This journal is © The Royal Society of Chemistry 2014
CrystEngComm, 2014, 16, 9441–9445 | 9445