Phosphine-Free Synthesis and Characterization of Cubic-Phase

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Phosphine-Free Synthesis and Characterization of Cubic-Phase Cu2SnTe3 Nanocrystals with Optical and Optoelectronic Properties Wenliang Wang,†,‡,§,∥ Wenling Feng,† Tao Ding,†,‡,§,∥ and Qing Yang*,†,‡,§,∥ †

Hefei National Laboratory of Physical Sciences at the Microscale (HFNL), ‡Department of Chemistry, §Laboratory of Nanomaterials for Energy Conversion (LNEC), and ∥Synergetic Innovation Center of Quantum Information & Quantum Physics, University of Science and Technology of China (USTC), Hefei, Anhui 230026, The People’s Republic of China S Supporting Information *

Downloaded by RUTGERS UNIV on September 1, 2015 | http://pubs.acs.org Publication Date (Web): September 1, 2015 | doi: 10.1021/acs.chemmater.5b02743

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expected to open new avenues for the development of a versatile route for other metal telluride nanostructures. The phase, composition, and oxidation states of the synthesized Cu2SnTe3 NCs are determined by X-ray diffraction (XRD), energy dispersive X-ray spectroscopy (EDS), and X-ray photoelectron spectroscopy (XPS) measurements. As displayed in Figure 1a, the XRD peaks of the sample at 25.7°, 29.7°,

n recent years, copper-based multicomponent chalcogenide semiconducting NCs have received considerable attention owing to their size- and composition-tunable properties and applications in solar cell,1,2 thermoelectric devices.3−5 In particular, among these colloidal semiconductor NCs, the copper-based I2−IV−VI3 ternary compounds, such as Cu2SnS3, Cu2GeSe3, and Cu2SnSe3 due to their tunable band gap, high optical absorption coefficients, and hole mobilities6−8 have been intensively investigated and applied in lithium ion battery9 and photovoltaic devices.7 However, in comparison with Cu2MX3 (M = Ge, Sn, X = S, Se), synthesis of monodisperse and uniform Cu2SnTe3 NCs still remains a big challenge, and therefore their further characterization and application are also restricted. A possible reason may lie in the lack of simple and benign methods that employed reactivity-matching precursors for the growth of the homogeneous Cu2SnTe3 NCs. To the best of our knowledge, there is no report on the preparation of Cu2SnTe3 NCs performed by solution-based routes. Although Cu2SnTe3 crystals have been synthesized in high temperature solid-state reaction,10,11 this energy-intensive and time-consuming method usually resulted in products with irregular, large size and size distribution and uncontrolled aggregate formation, which would limit their potential applications. On the other hand, the solution synthesis of different kinds of metal telluride NCs mainly relies on air-sensitive alkylphosphines (such as TOP or TBP),1,12−14 which is harmful to the environment. Thus, to explore a new environmentally benign method for synthesizing high-quality Cu2SnTe3 NCs with monodisperse and uniform size is highly desirable. Fortunately, recent progress reveals that the hot-injection synthesis can act as a versatile methodology for the preparation of high-quality NCs with monodisperse and uniform size.1,13−16 This encourages us to synthesize Cu2SnTe3 NCs via such a hot-injection strategy. Herein, we have developed a facile, green, mild, and costeffective route to synthesize Cu2SnTe3 NCs for the first time. In our new strategy, the Te precursor is prepared by dissolving TeO2 in 1-dodecanethiol, avoiding the use of expensive and toxic alkylphosphines, which is rapidly injected into Cu−Sn complex solution under argon atmosphere at 210 °C to produce Cu2SnTe3 NCs (Supporting Information for details). It is well-known that one of the most distinctive features of the hot-injection method is the separation of the nanocrystal nucleation and growth stages.17−19 Taking advantage of these merits, as a result, the as-obtained Cu2SnTe3 NCs are nearly monodisperse and uniform size with an average diameter of around 25 nm (Figure S1). This new synthetic strategy is © XXXX American Chemical Society

Figure 1. (a) XRD pattern of the as-synthesized Cu2SnTe3 NCs along with standard JCPDS No. 89-2881, for reference. (b) Schematic representation of the Cu2SnTe3 crystal structure.

42.6°, 50.2°, 61.6°, 67.8°, and 77.6° are in good agreement with (111), (200), (220), (311), (400), (331), and (422) planes of the cubic-structured Cu2SnTe3 (JCPDS No. 89-2881, cubic, a = b = c = 6.04 Å),20 respectively. The corresponding schematic crystal structure of cubic Cu2SnTe3 NCs is displayed in Figure 1b. It is worth noting that Cu+ and Sn4+ ions occupy the same position. The EDS spectrum shows the presence of only Cu, Sn, Te, Mo, and C and the existence of molybdenum, and carbon signals arise from the TEM grid (Figure S2). The atom ratio of Cu:Sn:Te is around 2:1:3, which further verifies that the resulting products are Cu2SnTe3. In addition, the XPS measurement is performed to study the oxidation states and purity of the sample. As shown in Figure S3, no peaks of other elements except Cu, Sn, Te, C, and O are observed in a typical XPS survey spectrum of the product, demonstrating the high purity of the as-synthesized Cu2SnTe3 NCs. The two distinct Cu 2p peaks appeared at 932.1 (2p3/2) and 951.9 eV (2p1/2) with a binding energy splitting of 19.8 eV identifying the presence of Cu(I).1,21 The peaks of Sn 3d located at 486.5 (3d5/2) and 494.9 eV (3d3/2) with its characteristic peak separation of 8.4 eV confirm the Sn(IV) state.22,23 The peaks of Te 3d5/2 and Te 3d3/2 located at 571.8 and 582.1 eV, Received: July 17, 2015 Revised: August 26, 2015

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DOI: 10.1021/acs.chemmater.5b02743 Chem. Mater. XXXX, XXX, XXX−XXX

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Chemistry of Materials


respectively, are in good agreement with values reported for the (−2) oxidation state of Te. It is worth mentioning that the small peaks at 575.6 and 586.1 eV are characteristics of oxidized layers due to exposure to atmosphere.24,25 These results demonstrate that Cu2SnTe3 NCs have been successfully synthesized. Transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), and selected area electron diffraction (SAED) are performed to further study the morphology and structure of Cu2SnTe3 NCs. A typical TEM image of products in Figure 2a displays that the as-synthesized

Figure 3. (a) UV−vis−NIR absorption spectrum of the as-prepared Cu2SnTe3 NCs dispersed in toluene; the inset spectrum shows the determination of band gap by direct band gap method. (b) ATR-FTIR spectra of the as-synthesized Cu2SnTe3 NCs; blue and red curves refer to the results of the Cu2SnTe3 product before and after ligand exchange with MPA.

semiconductor nanoparticles, it still remains at the nanoparticle interface and forms an insulating layer that limits the charge carrier transfer of the as-deposited OAm-capped Cu2SnTe3 nanocrystal thin film. In order to improve the charge transport in the nanocrystal thin film, a ligand-exchange strategy that long ligand is replaced by short ligand to reduce interparticle distance and enhance interparticle coupling has been delicately designed. We select a small thiol-based molecule, 3mercaptopropionic acid (MPA), as the short ligand. MPA readily replaces the OAm ligand of the semiconductor nanocrystal.15 The effectiveness of the ligand-exchange method has been examined by attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR). As displayed in Figure 3b, the peaks at 3358 and 1646 cm−1 can be assigned to the N−H stretching and bending modes revealing the OAm on the surfaces of Cu2SnTe3 NCs (blue curve).15,27 After the ligand-exchange process, the peaks for N−H stretching and bending disappear (red curve), whereas two new peaks at 1556 and 1403 cm−1 are observed, corresponding to the stretching bands of the RCOO¯ group.19 Moreover, the peak at 1697 cm−1 is in good agreement with the free carbonyl stretching mode, demonstrating the existence of free carboxylic acid.15,19 The peaks at 2800−3000 cm−1 mainly derive from the C−H symmetric and asymmetric stretching modes.19 Therefore, such results confirm that the short-chain MPA has become the dominant ligand anchoring onto the Cu2SnTe3 NCs surfaces after ligand exchange. In addition, the photocurrent and photoresponse properties of the Cu2SnTe3 NCs have been investigated for the first time. A photodetector is fabricated to preliminarily explore the optoelectronic properties by drop-casting a concentrated toluene solution of Cu2SnTe3 NCs on glass substrate and employed two adjacent ITO film as conductive electrodes.28 A xenon lamp equipped with a cutoff filter of 400 nm is used as a white light source, which provides visible light ranging from 400 to 780 nm. Figure 4a,c shows the current versus voltage curves of the Cu2SnTe3 NCs before and after ligand exchange in the dark and under irradiation at a bias of 1.0 V, respectively. It is clearly observed that the dark current of the Cu2SnTe3 NCs is improved from about 1.5 to 62.5 nA before and after ligand exchange, respectively. Meanwhile, the photocurrent is also enhanced from 4.6 to 145.5 nA. Such results demonstrate that the ligand-exchanged Cu2SnTe3 NCs indeed improve the charge carrier transfer compared with the pristine. Figure 4b,d displays the current versus time curves obtained from the Cu2SnTe3 NCs before and after ligand exchange, respectively. The photoresponse of the Cu2SnTe3 NCs before and after ligand exchange has been measured under irradiation with

Figure 2. (a) TEM image of the Cu2SnTe3 NCs. (b) HRTEM image of a Cu2SnTe3 nanocrystal with the corresponding SAED pattern (inset). (c) STEM image and STEM-EDS elemental mappings of the Cu2SnTe3 NCs. (d) STEM-EDS line scan of one single Cu2SnTe3 nanocrystal, and the inset is the corresponding STEM image.

Cu2SnTe3 NCs are nearly monodisperse and uniform size nanoparticles. As can be seen in Figure 2b, an HRTEM image of an individual Cu2SnTe3 nanocrystal verifies the crystalline with observed interplanar distances 0.35 and 0.30 nm corresponding to (111) and (200) planes of cubic Cu2SnTe3, respectively. Meanwhile, the corresponding SAED pattern in the [011]̅ zone axis as shown in the inset of Figure 2b further reveals that the products are of single crystalline nature. Moreover, STEM-EDS elemental mapping and line scan are applied to clarify the element distribution of the as-prepared Cu2SnTe3 NCs, as displayed in Figure 2c,d, revealing the homogeneous distribution of the three elements with the Cu:Sn:Te ratio close to the expected 2:1:3. The above results confirm that the chemical composition of sample is consist with the stoichiometric composition. In order to investigate the optical properties of the asobtained Cu2SnTe3 NCs, the room temperature UV−vis−NIR spectrum is measured. As can be seen in Figure 3a, the assynthesized Cu2SnTe3 NCs have obvious optical absorption from the entire visible to near-infrared regions. The optical band gap of Eg = 1.18 eV (the inset in Figure 3a) is determined by using a method based on the relation of (αhν)2 versus hν (where α is absorption coefficient, h is Planck’s constant, and ν is frequency), 21,26 which has potential applications in optoelectronic and photovoltaic devices. Nevertheless, it is well-known that the organic ligands anchored on the surface of NCs usually militate against efficient carrier transport in nanocrystal thin films, thus decreasing the photocurrent. In the present work, although the employment of OAm as capping ligand may be beneficial for controlling and stabling Cu2SnTe3 B


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Chemistry of Materials

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TEM image, and EDX and XPS spectra of the assynthesized Cu2SnTe3 NCs (PDF)

AUTHOR INFORMATION

Corresponding Author

*(Q.Y.) E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by National Basic Research Program of China (2012CB922001) and National Nature Science Foundation of China (51271173, 21571166, 21071136). We thank Prof. Shuji Ye in USTC for technical assistance on ATRFTIR measurements.

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Figure 4. (a, c) Current versus voltage curves of the Cu2SnTe3 NCs before and after ligand exchange with MPA (blue and red lines), respectively. In both cases, the measurements are carried out in the dark and under white light illumination, and the light power intensity is kept at 5.6 mW cm−2 when the xenon lamp is turned on. Inset in part a shows schematic of the device configuration. (b, d) Current versus time curves of the Cu2SnTe3 NCs before and after ligand exchange with MPA (blue and red lines) during ON and OFF cycles, respectively.

continuous light switching ON and OFF measurement, respectively. In both cases, the current increases in response to the ON/OFF operation for a duration of 50 s. Moreover, the photocurrent can still be changed by irradiation switching, even after a number of cycles. These results reveal that the photodetector based on the Cu2SnTe3 NCs demonstrates excellent sensitivity, stability, and repeatability. In summary, we report on the successful synthesis of highquality Cu2SnTe3 NCs via a facile solution phase method for the first time. An innovative “green” strategy has been proposed by dissolving TeO2 into 1-dodecanethiol as Te precursor without the use of expensive and air-sensitive alkylphosphines. Using this method, the as-prepared Cu2SnTe3 NCs are of monodisperse and uniform size. XRD, XPS, TEM, and EDS measurements confirm that the products are pure cubic Cu2SnTe3 NCs with a well-defined stoichiometric composition. The as-prepared Cu2SnTe3 NCs exhibit strong absorption in vis−NIR regions and possess an optical band gap of 1.18 eV. We investigate the surface property of Cu2SnTe3 NCs by ATRFTIR and demonstrate that various ligands bind to the Cu2SnTe3 NCs’ surface heavily influence the charge carrier transport. In addition, optoelectronic measurements verify that the photodetector based on the Cu2SnTe3 NCs demonstrates excellent stability and repeatability. Hopefully, such new synthetic strategy can fill the void in the gallery of solutiongrown Cu2SnTe3 NCs and open new avenues for the development of a versatile route for other metal telluride nanostructures.

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ASSOCIATED CONTENT

S Supporting Information *

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Phosphine-Free Synthesis and Characterization of Cubic-Phase

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