Metal Ions Mediated Morphology and Phase Transformation of

Shi-Kui Han , Chao Gu , Songtao Zhao , Sen Xu , Ming Gong , Zhenyu Li , and Shu-Hong Yu. Journal of the American Chemical Society 2016 138 (39), 12913...
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Metal Ions Mediated Morphology and Phase Transformation of Chalcogenide Semiconductor: From CuClSe2 Microribbon to CuSe Nanosheet Yong-Qiang Liu,†,‡ Hao-Di Wu,†,‡ Yu Zhao,† and Ge-Bo Pan*,† †

Suzhou Institute of Nano-Tech and Nano-Bionics, Chinese Academy of Sciences, 215123 Suzhou, P. R. China University of Chinese Academy of Sciences, Beijing, P. R. China

‡

S Supporting Information *

ABSTRACT: Foreign ions are of significant importance in controlling and modulating the morphology of semiconductor nanocrystals during the colloidal synthesis process. Herein, we demonstrate the potential of foreign metal ions to simultaneously control the morphology and crystal phase of chalcogenide semiconductors. The results indicate that the introduction of Al3+ ions can induce the structural transformation from monoclinic CuClSe2 microribbons (MRs) to klockmannite CuSe nanosheets (NSs) and the growth of large-sized CuSe NSs. The as-prepared micrometer-sized CuSe NSs exhibit a high-conducting behavior, long-term durability, and environment stability. The novel properties enable CuSe NSs to open up a bright prospect for printable electrical interconnects and flexible electronic devices.

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INTRODUCTION The crystal phase- and shape-controlled synthesis of various micro/nanostructures has received extensive attention owing to their intriguing properties and various potential applications in photonics, electronics, catalysis, and biomedical research.1−6 To mediate well-defined nanostructures, organic molecules are often used that can selectively bind to specific crystal facet of the growing nanocrystal. In addition, some halide ions,7−11 metal ions,12−15 and foreign crystal seeds16−22 have been certified as efficient regulators of nanocrystal morphology, in particular for metal nanoparticles. Recent efforts have revealed that foreign can also induce the structural transformation of transition-metal chalcogenides nanostructures (TMCs).23−26 Besides the structural transformation, the morphology effect of foreign ions on TMCs nanostructures has been observed occasionally.18−21 The dual functions of foreign metal ions that act alone as a favorable regulator of morphology and phase transformation make them technologically effective and beneficial to rational design of hybrid nanocomposites with complex nanostructure. The discovery of graphene has stimulated popular interest in two-dimensional (2D) TMCs nanostructrues.27−30 CuSe represents one of the most important types of TMCs and is a typical narrow band gap semiconductor, which has both an indirect and a direct band gap in the range of 1.0−1.4 eV.31 To date, a few attempts have been made to synthesize 2D CuSe nanostructures. For instance, Xiao et al. prepared hexagonal CuSe nanoplates by reacting copper(II) acetate with selenium in oleylamine and 1-octadecene.32 Liu et al. synthesized CuSe nanoflakes with an edge length of 100−400 nm and a thickness © XXXX American Chemical Society

of 25−50 nm by using an ionic liquid precursor of 1-n-butyl-3ethylimidazolium methylselenite.33 We succeeded in the synthesis of hexagonal klockmannite CuSe nanosheets (NSs) by a microwave-assisted method.34 It is noted that CuSe nanostructures in previous studies only have small lateral dimensions ranging from dozens to hundreds of nanometers, which cannot afford as building blocks for construction of various complex nanostructures. Moreover, previous synthesis methods are often low-yield, time-consuming, and uneconomical. It is thus highly desirable and technologically important to develop simple and scalable strategies for the synthesis of largesized 2D CuSe nanostructures. Herein, we demonstrate the potential of foreign metal ions to simultaneously control the morphology and crystal phase of colloidal chalcogenide semiconductor during a facile microwave-assisted process. The monoclinic CuClSe2 microribbons (MRs) are the predominant phase in the absence of Al3+ ions, whereas the klockmannite CuSe NSs are obtained in the presence of Al3+ ions. The influence of both the amount of Al3+ ions and the reaction time on the morphology and crystal phase of the products is explored, and a probable mechanism accounting for the structural transformation is proposed. Besides, the optical and electrical properties of the as-prepared micrometer-sized CuSe NSs are investigated. Received: February 3, 2015 Revised: April 14, 2015

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EXPERIMENTAL SECTION

Chemicals. Copper(II) chloride dehydrate (CuCl2·2H2O, analytical reagent), selenium dioxide (SeO2, spectroscopic reagent), aluminum nitrate nonahydrate (Al(NO3)3·9H2O, analytical reagent), aluminum chloride hexahydrate (AlCl3·6H2O, analytical reagent), aluminum sulfate octadecahydrate (Al2(SO4)3·18H2O, analytical reagent), sodium nitrate (NaNO3, analytical reagent), and benzyl alcohol (analytical reagent) were purchased from Sinopharm Chemical Reagent Co., Ltd. Ammonium nitrate (NH4NO3, analytical reagent) was purchased from Aladdin Industrial Inc. All chemicals were used as received. Synthesis. All the synthesis was performed in a microwave system (2.45 GHz, 300 W, Discover S-Class, CEM). In a typical synthesis of CuSe NSs, 8.5 mg of CuCl2·2H2O, 5.5 mg of SeO2, 11.2 mg of Al (NO3)3·9H2O, and 5 mL of benzyl alcohol were mixed in a 10 mL microwave vial and capped. The mixed solution was magnetically stirred at room temperature overnight, and the vial was transferred into the microwave system and heated at 180 °C for 30 min. The resulting blackish-green products were centrifuged, washed with excess ethanol, and redispersed in ethanol for further characterization. For the synthesis of CuClSe2 MRs, the same procedures were performed only in the absence of Al(NO3)3·9H2O. Characterization. The products were characterized by X-ray diffraction pattern (XRD) on a Bruker D8 Advance powder X-ray diffractometer at a scanning rate of 0.04° s−1 in the 2θ range of 10− 70°, using Cu Kα radiation (λ = 1.5406 Å). The morphology and structure were examined on a Hitachi-S4800 scanning electron microscope (SEM) at 10 kV and a Tecnai G2 F20 S-Twin transmission electron microscope (TEM) at an acceleration of 200 kV. High-resolution TEM (HR-TEM) and selected area electron diffraction (SAED) pattern were taken simultaneously on Tecnai G2 F20 S-TEM. For SEM and TEM measurement, a drop of ethanol solution with the dispersed products was casted onto a piece of silicon and a carbon-coated Cu grid, respectively. The solvent was allowed to evaporate at room temperature in air. Energy-dispersive X-ray spectrometry (EDS) analysis was performed on the Quanta 400 FEG SEM at 20 kV. UV−vis−NIR absorption spectra and diffuse reflectance UV−vis absorption spectra (DRS) were performed on a PerkinElmer Lambda 750 equipped with a 60 mm integrating sphere in the range of 400−1600 nm. Raman spectra were obtained by LABRAM HR high-resolution Raman microscope with a focused laser (532 nm). The electrical measurement of CuSe NSs was measured by a two-probe method with the Keithley 4200 SCS and SUSS PM8 probe station at room temperature in air.

Figure 1. (a, b) SEM images and (c, d) XRD patterns of the products obtained by reacting 0.05 mmol of SeO2 and 0.05 mmol of CuCl2· 2H2O at 180 °C for 30 min in the absence/presence of 0.03 mmol of Al(NO3)3·9H2O.

into the CuSe crystal structure or surface facets. Apparently, the introduction of Al(NO3)3·9H2O leads to the transformation from CuClSe2 MRs to CuSe NSs. To determine the influence of either NO3− or Al3+ ions on the transformation from CuClSe2 MRs to CuSe NSs, a series of control experiments are carried out. Figure 2 shows the SEM images of the products obtained in the presence of four different salts: NH4NO3, NaNO3, Al(NO3)3·9H2O, and AlCl3. The products obtained in the presence of either NH4NO3 or NaNO3 are massive ribbon-shaped microcrystals (Figure 2a,b), which is comparable to the product obtained in the absence of foreign additives (Figure 1a) and different from a large quantity of NSs obtained in the presence of Al(NO3)3·9H2O (Figure 2c). The above observations demonstrate that Al3+ ions play an actual role in the transformation from CuClSe2 MRs to CuSe NSs. It is noted that the product obtained in the presence of AlCl3 is similar to those in the presence of either NH4NO3 or NaNO3. This is different from Cabot’s work that both Al(NO3)3·9H2O and AlCl3 are found to catalyze the formation of Cu3Se2 nanocubes.26 The low solubility of AlCl3 in the benzyl alcohol has been considered to induce the incomplete release of Al3+ ions. A similar phenomenon is observed in the presence of Al2(SO4)3·18H2O (Figure S2). Furthermore, the XRD patterns (Figure 2e) reveal that the NSs obtained in the presence of Al(NO3)3·9H2O are the pure klockmannite CuSe, while the other products are composed of both the monoclinic CuClSe2 and the klockmannite CuSe. The amount of Al(NO3)3·9H2O is found to affect the morphology and crystal phase of the products. Figure S3 shows a set of SEM images of the products obtained at different amount of Al(NO3)3·9H2O. When the amount of Al3+ ions is less than 0.03 mmol, a blend of bunchy and sheetlike structures are formed (Figure S3a). The content of bunchy structures in the blend decreases with the increase of the amount of Al3+ ions. When the amount of Al3+ ions is more than 0.03 mmol, only thin sheetlike structures are observed (Figure S3b). Further increasing the amount of Al3+ ions, the thinner NSs become less and the thicker ones become predominant (Figure S3c). When the amount of Al3+ ions is 0.07 mmol, only thicker sheets are observed (Figure S 3d). Therefore, to obtain the

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RESULTS AND DISCUSSION Figure 1 shows the typical SEM images and XRD patterns of the products obtained by reacting 0.05 mmol of SeO2 and 0.05 mmol of CuCl2·2H2O at 180 °C for 30 min in the absence/ presence of 0.03 mmol of Al(NO3)3·9H2O. The image (Figure 1a) indicates the formation of ribbon-shaped microcrystals in the absence of Al(NO3)3·9H2O. The as-obtained MRs have an average length of dozens of micrometers and are highly crystalline (Figure 1c). The primary diffraction peaks in the XRD pattern can be indexed to the monoclinic phase of CuClSe2 (JCPDS No. 23-0202) with the lattice constants of a = 7.735 Å, b = 4.665 Å, and c = 30.89 Å, and the two other peaks (marked with star symbol) are attributed to the klockmannite phase of CuSe. The EDS spectrum (Figure S1a) reveals the peaks of Cu, Se, and Cl elements. In contrast, the truncatedtriangular and hexagonal NSs are formed in the presence of Al(NO3)3·9H2O along with a few nanoparticles (Figure 1b). All the diffraction peaks in the XRD pattern (Figure 1d) can be indexed to the klockmannite phase of CuSe (JCPDS No. 340171, a = b = 3.939 Å, c = 17.25 Å). In addition, the EDS spectrum (Figure S1b) reveals only the peaks of Cu and Se elements, which confirms that the Al3+ ions do not incorporate B

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Figure 3. (a−e) SEM images and (f) XRD pattern of the products obtained in the presence of 0.03 mmol of Al (NO3)3·9H2O at different reaction times: (a) 15 s, (b) 1 min, (c) 5 min, (d) 15 min, (e) 30 min. As a reference, the peak positions of various phases are marked by (●) monoclinic CuClSe2, JCPDS No. 23-0202, and (○) klockmannite CuSe, JCPDS No. 34-0171.

Figure 2. (a−d) SEM images and (e) XRD patterns of the products obtained in the presence of different salts: (a) NH4NO3, (b) NaNO3, (c) Al(NO3)3·9H2O, and (d) AlCl3. As a reference, the peak positions of various phases are marked by (●) monoclinic CuClSe2, JCPDS No. 23-0202, and (○) klockmannite CuSe, JCPDS No. 34-0171.

monoclinic CuClSe2 gradually weakens and even disappears while the enhancement of (006) diffraction peak of klockmannite CuSe is followed. On the basis of the above discussion, it can be concluded that Al3+ ions play an essential role in the transformation from CuClSe2 MRs to CuSe NSs. Scheme 1 illustrates a possible mechanism for the structural transformation from CuClSe2 MRs to CuSe NSs. In a typical synthesis, both monoclinic CuClSe2 and klockmannite CuSe could nucleate and grow into the well-defined nanostructures synchronously. In the absence of Al3+ ions, the monoclinic

micrometer-sized CuSe NSs, the amount of Al(NO3)3·9H2O has to be limited in an appropriate range of 0.03−0.05 mmol. Accordingly, the XRD patterns (Figure S4) reveal the crystal phases of the products, which go through a transformation from a blend of monoclinic CuClSe2 and klockmannite CuSe, pure klockmannite CuSe, to a blend of berzelianite Cu2Se and klockmannite CuSe. As described previously,35,36 thermodynamics-induced phase transformation between CuSe and Cu2Se could be partly accounted for. To figure out the intrinsic mechanism of the transformation from the CuClSe2 MRs to CuSe NSs, the time-dependent experiments are performed. Figure 3 shows a set of SEM images of the products acquired at 15 s, 1 min, 5 min, 15 min, and 30 min. At the initial stage, many hexagonal NSs together with large irregular aggregates are formed (Figure 3a). The aggregates disappear quickly at the later stage, and a large number of NSs are obtained (Figure 3b). Further increasing the reaction time, both the population and size of NSs become larger through a ripening process (Figure 3c). The lateral size distribution of NSs keeps relatively stable from 5 to 30 min (Figure 3d,e), and this is possibly due to a drastic “sizedefocusing” process.37 Additionally, note that it takes only 5− 15 min for the preparation of large-sized CuSe NSs in high yield, which is far superior to previous methods.32,33 The XRD pattern (Figure 3f) demonstrates the evolution of the crystal phases with the reaction time. As the reaction goes on and up to 30 min, the characteristic (006) diffraction peak of

Scheme 1. Schematic Illustration of the Structural Transformation from CuClSe2 MRs to CuSe NSs

C

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Langmuir CuClSe2 acts as the predominant and kinetic dynamically controlled phase. While Al3+ ions are introduced into the reaction, the Al3+ ions could rapidly extract the Cl− ions from the precursor solution, decrease the concentration of Cl− ions, and further catalyze the high population of hexagonal klockmannite CuSe. That is, the transformation may be attributed to the formation of AlCl3 resulting from the coalescent between Al3+ ions and Cl− ions. Generally, AlCl3 is a kind of covalent compound and cannot easily dissolve in polar solvents such as benzyl alcohol, which has been verified in the case of AlCl3 (Figure 2d). As a result, such extraction arising from Al3+ ions can significantly retard the nucleation and growth of CuClSe2 phase, leading to the formation of irregular aggregates. Conversely, the extraction effect would spur the klockmannite CuSe phase to be the predominant product. The larger the concentration of Al3+ ions, the faster and the more profound the catalytic effect is. In addition, the excess Al3+ ions (or AlCl3) may even seize the Cl element inside the CuClSe2 molecule and accelerate the underway transformation. It should be emphasized that the formation of large-sized CuSe NSs is due to not only the intrinsic layered crystal structure of klockmannite CuSe but also the role of Al3+ ions (or AlCl3) that is likely to guide the crystal growth oriented along [001] direction and greatly enhance the Ostwald ripening rate.7,26 To confirm this assumption, contrast experiment, in which the precursor CuCl2·2H2O is replaced with Cu(CH3COO)2·H2O and the other reaction parameters kept the same, is carried out. Only small hexagonal nanoplates (klockmannite CuSe) are obtained (Figure S5), indicating that the extraction of Cl− ions arising from Al3+ ions do assist the above transformation and the formation of large-sized CuSe NSs. Figure 4a,b shows the typical TEM images of individual hexagonal CuSe NS. The hexagonal NSs have a large lateral dimension of several micrometers, and the lateral sizes of some NSs are even up to dozens of micrometers. The HR-TEM image and SAED pattern (Figure 4c) indicate that the NSs are single-crystalline with well-resolved hexagonal lattice structure.

The observed lattice spacing of 3.40 Å and an intersection angle of 60° are consistent with the {100} set of planes of klockmannite CuSe. Moreover, the SAED pattern reveals that CuSe NSs preferably grow normal to the [100] direction, which is verified by the strong (006) peak of XRD pattern in Figure 1d. The preferential orientation is determined intrinsically by the highly anisotropic structure nature of klockmannite CuSe similar to covellite CuS,38 in which klockmannite CuSe crystal consists of alternating CuSe3−Cu3Se−CuSe3 layers and Se−Se layers along the z-axis, and the interactions of CuSe3−Cu3Se− CuSe3 layers are covalent bonds while the interactions of Se−Se layers are van der Waals force (crystal structure model, Figure 4d). The TEM image (Figure S6a) shows the moiré patterns resulted from the interference between the crystalline lattices of stacked NSs, which implies the ultrathin character of some CuSe NSs. On the other hand, TEM images (Figure S6b,c) and SAED pattern (Figure S6d) display that the nanoparticles coexisting with larger hexagonal NSs are quasi-hexagonal (or hexagonal) and single-crystalline klockmannite CuSe. To further explore the optical properties of as-prepared CuSe NSs, UV−vis absorption and Raman spectrum are recorded. The absorption spectrum (Figure S7a) of colloidal CuSe NSs in ethanol shows a wide absorption band in the near-infrared spectral region. Also, the Raman spectrum (Figure S7b) exhibits a strong peak corresponding to Raman shifts of 262 cm−1, which probably originates from the longitudinal optical (LO) phonon mode of the Cu···Se bond.39 The optical band gap of as-prepared CuSe NSs, which is a crucial electronic parameter for semiconductor nanomaterials and their potential application, is investigated by diffuse reflectance spectroscopy. As shown in Figure S7c, the onset of absorption for CuSe NSs starts near 1400 nm. Applying the Kubelka−Munk transformation,40,41 a plot of [F(R)hv]1/2 versus energy yields an indirect band gap of 1.02 eV while a plot of [F(R)hv]2 versus energy yields a direct band gap of 1.43 eV (Figure S7d). By using the direct band gap method, the band edge of as-prepared CuSe NSs is blue-shifted by approximately 0.38 eV in relation to bulk CuSe (1.05 eV).42 Moreover, the electrical properties of micrometer-sized CuSe NSs are also explored. In order to perform the electrical measurements better, the as-prepared CuSe NSs are repeatedly washed with pyridine three times, ultrasonically dispersed in chloroform, drop-cast onto precleaned Au/Ti electrodes and dried naturally. Figure 5a shows a schematic illustration of CuSe NSs resting across the interdigital gold electrodes (finger dimensions: width 20 μm, length 200 μm, interfinger spacing 20 μm). Figure 5b shows the typical current−voltage (I−V) characteristics of micrometer-sized CuSe NS film. It is clear that all the measured I−V curves at different bias voltages show a linear dependence, and the slope is identical and independent of the bias voltage, indicating an Ohmic behavior of CuSe NSs. Also, the CuSe NS film displays superior electrical conductivity, namely that the current is 0.06 A at a bias of 0.5 V (σ = 9.1 × 104 S/m), which is comparable to that of vacuum-deposited gold electrode (Figure 5d). The high conductivity of CuSe NSs film is partly due to the pyridine pretreatment that is similar to short hydrocarbon ligands that contribute to enhance interparticle coupling and thereby improve the charge mobility in the film.43,44 Besides, while stored in air for 30 days, the conductivity of CuSe NSs film still keeps almost identical to the original one (Figure 5c), indicating long-term durability and environmental stability of CuSe NS film. The above results enlighten us to develop a novel and promising low-cost

Figure 4. (a, b) TEM images and (c) HRTEM image of CuSe NSs obtained by reacting 0.05 mmol of SeO2 and 0.05 mmol of CuCl2· 2H2O in the presence of 0.03 mmol of Al(NO3)3·9H2O at 180 °C for 30 min. (d) Crystal structure model of klockmannite CuSe. D

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(2) Gao, M. R.; Jiang, J.; Yu, S. H. Solution-Based Synthesis and Design of Late Transition Metal Chalcogenide Materials for Oxygen Reduction Reaction (ORR). Small 2012, 8, 13−27. (3) Gao, M. R.; Xu, Y. F.; Jiang, J.; Yu, S. H. Nanostructured Metal Chalcogenides: Synthesis, Modification, and Applications in Energy Conversion and Storage Devices. Chem. Soc. Rev. 2013, 42, 2986− 3017. (4) Zhuang, Z. B.; Peng, Q.; Li, Y. D. Controlled Synthesis of Semiconductor Nanostructures in the Liquid Phase. Chem. Soc. Rev. 2011, 40, 5492−5513. (5) Tian, Q. W.; Hu, J. Q.; Zhu, Y. H.; Zou, R. J.; Chen, Z. G.; Yang, S. P.; Li, R. W.; Su, Q. Q.; Han, Y.; Liu, X. G. Sub-10 nm Fe3O4@ Cu2‑xS Core-Shell Nanoparticles for Dual-Modal Imaging and Photothermal Therapy. J. Am. Chem. Soc. 2013, 135, 8571−8577. (6) Tian, Q. W.; Tang, M. H.; Sun, Y. G.; Zou, R. J.; Chen, Z. G.; Zhu, M. F.; Yang, S. P.; Wang, J. L.; Wang, J. H.; Hu, J. Q. Hydrophilic Flower-Like CuS Superstructures as an Efficient 980 nm Laser-Driven Photothermal Agent for Ablation of Cancer Cells. Adv. Mater. 2011, 23, 3542−3547. (7) Kim, M. H.; Lim, B.; Lee, E. P.; Xia, Y. N. Polyol Synthesis of Cu2O Nanoparticles: Use of Chloride to Promote the Formation of a Cubic Morphology. J. Mater. Chem. 2008, 18, 4069−4073. (8) Loukrakpam, R.; Chang, P.; Luo, J.; Fang, B.; Mott, D.; Bae, I. T.; Naslund, H. R.; Engelhard, M. H.; Zhong, C. J. Chromium-Assisted Synthesis of Platinum Nanocube Electrocatalysts. Chem. Commun. 2010, 46, 7184−7186. (9) Zhang, H.; Jin, M. S.; Wang, J. G.; Li, W. Y.; Camargo, P. H. C.; Kim, M. J.; Yang, D. R.; Xie, Z. X.; Xia, Y. N. Synthesis of Pd-Pt Bimetallic Nanocrystals with a Concave Structure through a BromideInduced Galvanic Replacement Reaction. J. Am. Chem. Soc. 2011, 133, 6078−6089. (10) Xie, S. F.; Lu, N.; Xie, Z. X.; Wang, J. G.; Kim, M. J.; Xia, Y. N. Synthesis of Pd-Rh Core-Frame Concave Nanocubes and Their Conversion to Rh Cubic Nanoframes by Selective Etching of the Pd Cores. Angew. Chem., Int. Ed. 2012, 51, 10266−10270. (11) Yin, J.; Wang, J. H.; Li, M. R.; Jin, C. Z.; Zhang, T. Iodine Ions Mediated Formation of Monomorphic Single-Crystalline Platinum Nanoflowers. Chem. Mater. 2012, 24, 2645−2654. (12) Guo, H. Z.; Chen, Y. Z.; Ping, H. M.; Jin, J. R.; Peng, D. L. Facile Synthesis of Cu and Cu@Cu-Ni Nanocubes and Nanowires in Hydrophobic Solution in the Presence of Nickel and Chloride Ions. Nanoscale 2013, 5, 2394−2402. (13) Grass, M. E.; Yue, Y.; Habas, S. E.; Rioux, R. M.; Teall, C. I.; Yang, P. D.; Somorjai, G. A. Silver Ion Mediated Shape Control of Platinum Nanoparticles: Removal of Silver by Selective Etching Leads to Increased Catalytic Activity. J. Phys. Chem. C 2008, 112, 4797− 4804. (14) Chen, J. Y.; Herricks, T.; Geissler, M.; Xia, Y. N. Single-Crystal Nanowires of Platinum Can Be Synthesized by Controlling the Reaction Rate of a Polyol Process. J. Am. Chem. Soc. 2004, 126, 10854−10855. (15) Beberwyck, B. J.; Surendranath, Y.; Alivisatos, A. P. Cation Exchange: A Versatile Tool for Nanomaterials Synthesis. J. Phys. Chem. C 2013, 117, 19759−19770. (16) Wang, C.; Daimon, H.; Lee, Y. M.; Kim, J. M.; Sun, S. H. Synthesis of Monodisperse Pt Nanocubes and Their Enhanced Catalysis for Oxygen Reduction. J. Am. Chem. Soc. 2007, 129, 6974−6975. (17) Teng, X. W.; Yang, H. Synthesis of Platinum Multipods: An Induced Anisotropic Growth. Nano Lett. 2005, 5, 885−891. (18) Lim, S. I.; Jiménez, I. O.; Varon, M.; Casals, E.; Arbiol, J.; Puntes, V. Synthesis of Platinum Cubes, Polypods, Cuboctahedrons, and Raspberries Assisted by Cobalt Nanocrystals. Nano Lett. 2010, 10, 964−973. (19) Han, S. K.; Gu, C.; Gong, M.; Wang, Z. M.; Yu, S. H. Colloidal Synthesis of Ternary AgFeS2 Nanocrystals and Their Transformation to Ag2S-Fe7S8 Heterodimers. Small 2013, 9, 3765−3769. (20) Han, S. K.; Gong, M.; Wang, Z. M.; Gu, C.; Yu, S. H. Colloidal Synthesis of Cu2SxSe1‑x Hexagonal Nanoplates and Their Trans-

Figure 5. (a) Schematic illustration of the device based on CuSe NSs film for electrical measurement. (b) I−V characteristics of CuSe NSs film at different bias voltages. (c) I−V characteristics of CuSe NSs film versus storage time. (d) I−V characteristics of vacuum-deposited Au electrode.

semiconductor material in place of traditional noble metals (Au, Ag, Cu, etc.) for high-performance electronic conductors.

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CONCLUSION The potential of foreign metal ions to simultaneously control the morphology and crystal phase of colloidal chalcogenide semiconductor was explored for the first time. The extraction effect arising from Al3+ ions can contribute to the transition from monoclinic CuClSe2 MRs to klockmannite CuSe NSs. The appropriate amount (0.03−0.05 mmol) of Al3+ ions can lead to the micrometer-sized CuSe NSs in high yield. The micrometer-sized CuSe NSs display a high-conducting behavior, long-term durability, and environment stability, which could compete with vacuum-deposited gold electrode. The novel properties enabled the CuSe NSs to open up a bright prospect for printable electrical interconnects and flexible electronic devices.

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ASSOCIATED CONTENT

S Supporting Information *

Additional SEM images, TEM images, EDX spectra, XRD pattern, and UV−vis absorption spectra and Raman spectrum of products. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (G.-B.P.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (No. 21273272) and the Chinese Academy of Sciences.

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REFERENCES

(1) Kwon, S. G.; Hyeon, T. Colloidal Chemical Synthesis and Formation Kinetics of Uniformly Sized Nanocrystals of Metals, Oxides, and Chalcogenides. Acc. Chem. Res. 2008, 41, 1696−1709. E

DOI: 10.1021/acs.langmuir.5b00373 Langmuir XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.langmuir.5b00373 Langmuir XXXX, XXX, XXX−XXX