Large-Scale Synthesis of Single Crystalline CuSb(SxSe1–x)2

Article pubs.acs.org/JPCC

Large-Scale Synthesis of Single Crystalline CuSb(SxSe1−x)2 Nanosheets with Tunable Composition Yejun Zhang,† Lun Li,† Delu Li, and Qiangbin Wang* Key Laboratory of Nano-Bio Interface, Division of Nanobiomedicine and i-Lab, Suzhou Institute of Nano-Tech and Nano-Bionics, Chinese Academy of Sciences, Suzhou 215123, China S Supporting Information *

ABSTRACT: Alloying nanocrystals with multicomponent has been an effective way to tune the band gap of semiconductor nanocrystals, which promises their wide applications in optoelectronics, photovoltaics, etc. However, colloidal synthesis of homogeneous phase multicomponent nanocrystals in a large scale remains great challenge. Here, we declare the successful preparation of single-crystalline quaternary CuSb(SxSe1−x)2 nanosheets in a facile one-pot reaction with yield >3 g. The molar ratio of S/(S + Se) in CuSb(SxSe1−x)2 could be easily tuned from 1.5% to 13.7% by increasing the reaction temperature which enhances the reactivity of S source in the reaction, and accordingly, the band gap of the obtained CuSb(SxSe1−x)2 varies from 0.9 to 1.1 eV.

1. INTRODUCTION Alloying of pure semiconductor nanocrystals (NCs) with foreign elements has attracted extensive attention recently for the reason for its capability of engineering the NCs into desired band gaps by tunable compositions or phase structures.1−5 It is widely recognized that the homogeneous distribution of element constitute in the alloying NCs plays a key role to determine their unique properties, which could derive from or exceed their parent NCs.4−8 However, despite great progress,4 most of works still focused on the ternary NCs (such as CuSxSe1−x, CdSxSe1−x, Pb2‑xSnxS2, Bi2Te3‑ySey, and so on), and there are very rare reports of alloyed quaternary NCs.9−14 Comparing with ternary NCs, the successful synthesis of quaternary NCs was proved to be more challenging, in which better controllability over the reactivity of different elements was required than that of ternary NCs, since different growth rates of every potential composition would lead to the heterogeneous products, or even multiphase structures.15−18 Copper-based I−V−VI2 materials are less toxic and costeffective, and have attracted great attention owing to their wide range of applications such as photovoltaic devices, near-infrared (NIR) detectors, etc.19−22 Among them, the CuSbCh2 (Ch = S or Se) have a bandgap of 0.9−1.5 eV, which benefits the wide absorbance of light and transportation of electrons and makes it an excellent candidate for applications in photovoltaic devices and lithium ion batteries. Pure-phased CuSbS2 or CuSbSe2 with two-dimensional (2D) morphology (for example, sheet structure) have been prepared in the previous works.19 However, the simultaneous existence of the anions S and Se in the form of CuSb(SxSe1−x)2 with homogeneous element distribution has not been reported yet. Here, we report a facile © 2014 American Chemical Society

colloidal route to synthesize quaternary CuSb(SxSe1−x)2 nanosheets with yield >3 g in a single reaction, where atom S partially replace the position of atom Se in the ternary CuSbSe2. The obtained CuSb(SxSe1−x)2 nanosheets are single-crystalline without phase separation. Most importantly, it is found the molar ratio of S/(S+Se) in the CuSb(SxSe1−x)2 could be finely tuned from 1.5% to ∼13.7% by changing the reaction temperature and the resulted CuSb(SxSe1−x)2 nanosheets show composition-dependent band-gaps varying from 0.9 to 1.1 eV.

2. EXPERIMENTAL SECTION 2.1. Materials. All raw chemicals were used as received without further purification. Copper nitrate trihydrate (Cu(NO3)2·3H2O, 99.0%) was obtained from Sinopharm Chemical Reagent Co., Ltd. Antimony trichloride (SbCl3, 99.0%), selenium dioxide (SeO2, 99.9%) and oleylamine (OAM, 80− 90%) were purchased from Aladdin Chemistry Co., Ltd. 1Dodecanethiol (DT, 98.0%) was from Acros Organics. 2.2. Synthesis of CuSb(SxSe1−x)2 Nanosheets. In a typical reaction, stoichiometric amounts of Cu(NO3)2·3H2O (10 mmol), SbCl3 (10 mmol) and SeO2 (20 mmol) were added into the mixture of OAM (50 mL) and DT (50 mL) at room temperature. The mixture was sealed and continuously stirred for at least 30 min. Afterward, the solution was degassed at ∼120 °C by high purity nitrogen (N2) for 15 min. Then the reaction mixture was heated up to 200 °C at 10 °C/min, and Received: November 13, 2014 Revised: December 23, 2014 Published: December 23, 2014 1496

DOI: 10.1021/jp5113747 J. Phys. Chem. C 2015, 119, 1496−1499

Article

The Journal of Physical Chemistry C

layer-by-layer and present some bundle forms under electron microscope observation (Figures S1−S3). The high-resolution TEM image of one single CuSb(SxSe1−x)2 nanosheet from top view suggests its single-crystalline nature (Figure 1B). The interlayer spacing of two apparent planes with intersection angle of 89.6° are 0.32 and 0.21 nm, respectively, corresponding to the lattice fringe of (111) and (21̅3) plane. In addition, some Moiré patterns caused by the interference between the crystalline lattices of the neighboring sheets are also observed under high-resolution TEM (HR-TEM) observation (Figures S4). The selected area electron diffraction (SAED) pattern of this single sheet further confirms singlecrystalline feature and also suggests the orthorhombic phase, of which the as-prepared sheets orient normal to the (41̅3̅) plane direction (Figure 1C). The energy-dispersive spectroscopy (EDS) spectrum shows the composition of the CuSb(SxSe1−x)2 nanosheets as Cu1.02Sb1.05S0.032Se2 (Figure 1D), where despite trace amount, the element S is still detectable as marked by the red-dotted circle. To convince the successful synthesis of quaternary CuSb(SxSe1−x)2 nanosheets, we need to confirm that the element S detected in EDS spectrum is from CuSb(SxSe1−x)2, instead of that from the surface ligand DT. Powder XRD was first performed to characterize the crystalline structure of the product. Surprisingly, we found that the XRD pattern of the CuSb(SxSe1−x)2 nanosheets (black line in Figure 2A) lies in the

aged for another 60 min in N2 atmosphere. Once the reaction was finished and the solution was cooled down to room temperature naturally, the as-prepared products were collected with excess amount of ethanol by centrifugation, and washed by ethanol and cyclohexane for three times. Finally, the products were redispersed into cyclohexane (20 mL) for further use. 2.3. Characterization. The morphologies of the assynthesized CuSb(SxSe1−x)2 nanosheets were examined on a Tecnai G2 F20 S-Twin transmission electron microscope (TEM) at an acceleration of 200 kV, and a Quanta 400 FEG scanning electron microscopy (SEM) at 20 kV, respectively. Diffuse reflectance measurements were performed on a PerkinElmer Lambda 750 equipped with a 60 mm integrating sphere in the range of 400−1600 nm. X-ray photoelectron spectroscopy (XPS) was performed on a PerkinElmer PHI 5000C ESCA X-ray photoelectron spectrometer using Al Kα radiation (1486.6 eV) as the exciting source. The bonding energies of the CuSb(SxSe1−x)2 NSs obtained from the XPS analysis were corrected for specimen charging by referencing the C 1s to 284.80 eV. Powder X-ray diffraction (XRD) patterns of the as-obtained products were recorded on a Bruker D8 Advance powder X-ray diffractometer at a scanning rate of 2° min−1, using Cu Kα radiation (λ = 1.5406 Å).

3. RESULTS AND DISCUSSION A typical SEM image of the rectangle CuSb(Sx Se 1−x ) 2 nanosheets is shown in Figure 1A. The dimension in average

Figure 2. (A) XRD patterns of the CuSb(SxSe1−x)2 obtained at three different reaction temperatures. (B) Zoom-in image of 36.5°−41° marked by the green-color shadow in part A. (C) High-resolution XPS spectrum of the element S from the CuSb(SxSe1−x)2 nanosheets and the solvent DT.

Figure 1. (A) SEM image of the CuSb(SxSe1−x)2 nanosheets. (B) HRTEM image of one single CuSb(SxSe1−x)2 nanosheet, of which the SAED is shown in part C. (D) EDS pattern of the CuSb(SxSe1−x)2. The signal of element Si is from the substrate silicon.

middle position of the orthorhombic phase CuSbSe 2 (JCPDS:75−0992) and CuSbS2 (JCPDS:88−0822) and shifts a little bit toward larger angles. We attribute this shift to the decreased lattice constants of the crystallographic unit cell in comparison with CuSbSe2, in which the smaller-sized atom of S was incorporated into the CuSbSe2.15 Besides, all of the peak positions are found to shift toward larger angles with no peak splitting occurred, indicative of no phase separation existing in the quaternary CuSb(SxSe1−x)2 nanosheets.

is 2000 nm × 400 nm. A lower-magnification SEM image further illustrates the high monodispersity of the obtained products with no any other specific morphology observed (Figure S1). The average thickness of the CuSb(SxSe1−x)2 nanosheets is determined to be ∼40 nm under high-resolution SEM (Figure S2). Because of the highly uniform nature of the size, CuSb(SxSe1−x)2 nanosheets are often found to easily stack 1497

DOI: 10.1021/jp5113747 J. Phys. Chem. C 2015, 119, 1496−1499

Article

The Journal of Physical Chemistry C

single-crystalline nature of CuSb(SxSe1−x)2 with homogeneous element distribution. Because the reaction temperature could finely tune the reactivity of S and Se in the reaction and thus the molar ratio of S/(S+Se) in the CuSb(SxSe1−x)2, the relationship between reaction temperature and composition is then carefully studied by using EDS. As shown in Figure 4, the S content in the

Interestingly, if the reaction temperature was increased, for example, to 240 and 280 °C, while keeping other parameters unchanged, it is found that all of the XRD peaks continue shifting to the larger degrees toward CuSbS2 (JCPDS, 88− 0822) as shown in Figure 2A. A zoom-in image from 36.5° to 41° including (015) and (212) peaks further reveals this shift process (Figure 2B), which indicates the increasing S content in the CuSb(SxSe1−x)2 nanosheets at higher reaction temperature. It is understandable that the temperature plays a key role to tune the molar ratio of S/(S+Se) in the CuSb(SxSe1−x)2 sheets. Because of the enhanced reactivity of the chemical speices at higher temperature, more S element from DT will be incorporated into the as-formed sheets. X-ray photoelectron spectroscopy (XPS) of the CuSb(SxSe1−x)2 nanosheets is further analyzed to accurately determine the valent state of the elements in the product. The charge state of Cu+, Sb3+ and Se2− are first approved by their high-resolution XPS (Figure S5). In particular, the highresolution XPS spectrum of element S in the CuSb(SxSe1−x)2 nanosheets depicts the binding energies of S 2p1/2 (166.0 eV) and S 2p3/2 (160.2 eV), respectively (Figure 2C). In comparison with that from the pure solvent DT, both of the S 2p peaks are found to be red-shifted about 2.27 eV. This suggests the sulfur element from DT have been partially reduced to be anion S2−, which are then incorporated into the as-formed CuSb(SxSe1−x)2 nanosheets. Despite the fact that the S2− has been verified to be incorporated into the quaternary CuSb(SxSe1−x)2 nanosheets, the homogeneous distribution of elements of Cu, Sb, S, and Se in CuSb(SxSe1−x)2 has also been testified to by using scanning transmission electron microscopy-energy disperse spectrum (STEM-EDS). As shown in Figure 3B−E, the elemental mapping of the elements Cu, Sb, S, and Se from a selected area of a single CuSb(S x Se 1−x ) 2 nanosheet in Figure 3A unambiguously proves that the four elements are homogeneously distributed in the whole area with no apparent element separations or aggregations observed. This result suggests the

Figure 4. Temperature-dependent profiles of the S/(S+Se) for the CuSb(SxSe1−x)2 nanosheets.

obtained nanosheets is about 1.5% at the reaction temperature of 200 °C. Whereas, as the reaction temperature increases to 280 °C, the S content in the product dramatically increases to ∼13.7%. This indicates the enhanced DT reactivity at higher temperature promotes the formation of S-riched nanosheets. Theoretically, the tunable compositions could tune the band gaps of the obtained CuSb(SxSe1−x)2 nanosheets. Therefore, diffuse reflectance spectroscopy (DRS) was performed to investigate their component-dependent band gap profiles. The original DRS spectrum of the CuSb(SxSe1−x)2 obtained at 200, 240, and 280 °C are shown in Figure S6, in which the onset absorption begins near 1150 nm. By performing Kubelka− Munk transformation,23 the direct band gaps of the three samples are calculated to be 0.95, 1.03, and 1.08 eV (Figure 5A−C), respectively, suggesting a strong relationship between the band gaps and the reaction temperature as shown in Figure 5D.

Figure 3. (A) STEM image of one single CuSb(SxSe1−x)2 nanosheet. (B−E) Elemental mapping of the Cu, Sb, Se, and S from the selected area of the single nanosheet in part A.

Figure 5. Diffuse reflectance spectroscopy of three samples obtained at (A) 200, (B) 240, and (C) 280 °C and (D) band-gaps of the three CuSb(SxSe1−x)2 samples. 1498

DOI: 10.1021/jp5113747 J. Phys. Chem. C 2015, 119, 1496−1499

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The Journal of Physical Chemistry C

(11) Pan, D. C.; Weng, D.; Wang, X. L.; Xiao, Q. F.; Chen, W.; Xu, C. L.; Yang, Z. Z.; Lu, Y. F. Alloyed Semiconductor Nanocrystals with Broad Tunable Band Gaps. Chem. Commun. 2009, 28, 4221−4223. (12) Fan, F. J.; Wang, Y. X.; Liu, X. J.; Wu, L.; Yu, S. H. Large-Scale Colloidal Synthesis of Non-Stoichiometric Cu2ZnSnSe4 Nanocrystals for Thermoelectric Applications. Adv. Mater. 2012, 24, 6158−6163. (13) Wu, X. J.; Huang, X.; Qi, X. Y.; Li, H.; Li, B.; Zhang, H. CopperBased Ternary and Quaternary Semiconductor Nanoplates: Templated Synthesis, Characterization, and Photoelectrochemical Properties. Angew. Chem., Int. Ed. 2014, 53, 8929−8933. (14) Ratchford, D.; Dziatkowski, K.; Hartsfield, T.; Li, X. Q.; Gao, Y.; Tang, Z. Y. Photoluminescence Dynamics of Ensemble and Individual CdSe/ZnS Quantum Dots with an Alloyed Core/Shell Interface. J. Appl. Phys. 2011, 109, 103509. (15) Wang, J. J.; Xue, D. J.; Guo, Y. G.; Hu, J. S.; Wan, L. J. Bandgap Engineering of Monodispersed Cu2‑xSySe1‑y Nanocrystals Through Chalcogen Ratio and Crystal Structure. J. Am. Chem. Soc. 2011, 133, 18558−18561. (16) Shen, S. L.; Zhang, Y. J.; Peng, L.; Du, Y. P.; Wang, Q. Matchstick-Shaped Ag2S-ZnS Heteronanostructures Preserving Both UV/Blue and Near-Infrared Photoluminescence. Angew. Chem., Int. Ed. 2011, 50, 7115−7118. (17) Zhang, Y. J.; Shen, S. L.; Wang, Q. Controllable Growth of Ag2S-CdS Heteronanostructures. CrystEngComm 2014, 16, 9501− 9505. (18) Han, W.; Yi, L. X.; Zhao, N.; Tang, A. W.; Gao, M. Y.; Tang, Z. Y. Synthesis and Shape-Tailoring of Copper Sulfide/Indium SulfideBased Nanocrystals. J. Am. Chem. Soc. 2008, 130, 13152−13161. (19) Xu, D. Y.; Shen, S. L.; Zhang, Y. J.; Gu, H. W.; Wang, Q. Selective Synthesis of Ternary Copper-Antimony Sulfide Nanocrystals. Inorg. Chem. 2013, 52, 12958−12962. (20) Deng, M. J.; Shen, S. L.; Zhang, Y. J.; Xu, H. R.; Wang, Q. A Generalized Strategy for Controlled Synthesis of Ternary Metal Sulfide Nanocrystals. New J. Chem. 2014, 38, 77−83. (21) Zhang, Y. S.; Ozolins, V.; Morelli, D.; Wolverton, C. Prediction of New Stable Compounds and Promising Thermoelectrics in the CuSb-Se System. Chem. Mater. 2014, 26, 3427−3435. (22) Temple, D. J.; Kehoe, A. B.; Allen, J. P.; Watson, G. W.; Scanlon, D. O. Geometry, Electronic Structure, and Bonding in CuMCh(2) (M = Sb, Bi; Ch = S, Se): Alternative Solar Cell Absorber Materials? J. Phys. Chem. C 2012, 116, 7334−7340. (23) Vaughn, D. D., II; Patel, R. J.; Hickner, M. A.; Schaak, R. E. Single Crystal Colloidal Nanosheets of GeS and GeSe. J. Am. Chem. Soc. 2010, 132, 15170−15172.

4. CONCLUSIONS In summary, we report a colloidal route to prepare rectangle CuSb(SxSe1−x)2 nanosheets in a large scale with tunable molar ratio of S/(S+Se) from 1.5% to 13.7%. It is found that, with increasing the reaction temperature, more S element from the solvent DT is incorporated to form sulfur-enriched CuSb(SxSe1−x)2 nanosheets. The as-prepared CuSb(SxSe1−x)2 nanosheets show composition-dependent band gaps varying from 0.9 to 1.1 eV. We expect such a facile methodology can be extended to the preparation of other multicomponent nanocrystals and exploration of their new properties.

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ASSOCIATED CONTENT

S Supporting Information *

Figures showing SEM/TEM/XPS/diffuse reflectance spectroscopy data. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*(Q.W.) E-mail: [email protected]. Author Contributions †

These authors contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge funding by the CAS “Strategic Priority Research Program” (Grant No. XDA 01030200), MOST (Grant No. 2011CB965004), NSFC (Grant No. 21425103, 21303249, 21301187), and NSF of Jiangsu Province (Grant No. BK2012007).

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REFERENCES

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DOI: 10.1021/jp5113747 J. Phys. Chem. C 2015, 119, 1496−1499