A Generic Method for Rational Scalable Synthesis of Monodisperse

A rational synthetic method is developed to produce monodisperse metal sulfide nanocrystals (NCs) in organic nonpolar solutions by using (NH4)2S as a ...
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A Generic Method for Rational Scalable Synthesis of Monodisperse Metal Sulfide Nanocrystals Haitao Zhang,† Byung-Ryool Hyun,‡ Frank W. Wise,‡ and Richard D. Robinson†,* †

Department of Materials Science and Engineering, Cornell University, Ithaca, New York 14853, United States School of Applied and Engineering Physics, Cornell University, Ithaca, New York 14853, United States

‡

S Supporting Information *

ABSTRACT: A rational synthetic method is developed to produce monodisperse metal sulfide nanocrystals (NCs) in organic nonpolar solutions by using (NH4)2S as a sulfide precursor. (NH4)2S is stabilized in an organic primary amine solution and exhibits high reactivity toward metal complexes. This novel technique exhibits wide applicability for organic phase metal sulfide NC synthesis: a large variety of monodisperse NCs have been synthesized, including Cu2S, CdS, SnS, ZnS, MnS, Ag2S, and Bi2S3. The stoichiometric reactions between (NH4)2S and metal salts afford high conversion yields, and large-scale production of monodisperse NCs (more than 30 g) can be synthesized in a single reaction. The high reactivity of (NH4)2S enables low temperature (3h) of anhydrous (NH4)2S oleylamine solution result in the slow appearance of light-yellow precipitates. These precipitates might come from the decomposition of (NH4)2S, which then reduces the concentration of the sulfide precursor. The (NH4)2S oleylamine solution is highly reactive toward a variety of metal salts, and metal sulfide NCs can be synthesized in the presence of appropriate surfactants. In the case of Cu2S NC synthesis, (NH4)2S reacts with CuCl (molar ratio of 1:2) in oleylamine at 50 °C (the lowest temperature to keep CuCl soluble in oleylamine) and produces ca. 4.5 ± 0.5 nm spherical NCs with weak crystallinity (Figure S1a, Supporting Information). ICP-MS studies on this NC sample reveal a Cu:S of 2.4:1, close to the stoichiometry of Cu2S. The reaction occurs spontaneously upon combining (NH4)2S and CuCl together at 50 °C, indicated by a sudden color change of reaction solution from light blue into black, due to the formation of Cu2S species. Heating up the reaction solution can increase the crystallinity (Figure S1c, Supporting Information) as well as the NCs size. Monodisperse Cu2S NCs with tunable size (4.6 ± 0.3 to 7.4 ± 0.4 nm; Figure 1a−c) are synthesized by this method and the chemical yield of NCs can reach 87% when the reaction solution is heated to 180 °C for 40 min. Such a heating-up synthetic process can be easily scaled up to produce more than 30 g of monodisperse Cu2S NCs in a single reaction without a size-sorting process (Figure 1d,e; Figure S1b, Supporting Information). For CdS NCs, the syntheses are performed by reacting (NH4)2S with Cd(OA)2 complexes in ODE solution. Figure S2a (Supporting Information) shows the UV−vis absorption spectra of CdS NCs synthesized between 100 and 200 °C. All spectra exhibit sharp band gap absorption features, indicating the formation of nearly monodisperse CdS NCs. TEM images reveal a near-spherical NC shape (Figure 1f) and XRD studies confirm the cubic CdS phase (Figure S2c). The size of CdS NCs can be tuned between 3 and 5 nm, with longer reaction times and higher reaction temperatures favoring

Figure 1. (a−c) TEM images demonstrating size control for Cu2S NCs: a, 4.6 ± 0.3 nm; b, 6.4 ± 0.4 nm; c, 7.4 ± 0.4 nm. (d) Demonstration of ultralarge scale synthesis (39 g) of Cu2S NCs obtained from a single reaction. (e) Selected area electron diffraction (SAED) pattern of Cu2S NCs from part d, which are consistent with the (101), (102), (103), (110), and (112) planes of the Cu2S crystal structure. (f) TEM image of CdS NCs (4.9 ± 0.4 nm) synthesized by reacting Cd(OA)2 and (NH4)2S at 200 °C for 10 min. (g, h) TEM images demonstrating size control for SnS NCs: g, 6 ± 0.5 nm; h, 7.5 ± 0.5 nm. (i) TEM image of ZnS NCs (5.5 ± 0.4 nm). (j) TEM image of MnS NCs (20 × 12 nm). The scale bars represent 50 nm.

the formation of larger nanocrystals. The photoluminescence (PL) of CdS NCs (Figure S2b) shows both a sharp bandgap emission and a broad trap state emission centered around 500− 600 nm. The trap state emission is significantly depressed by increasing reaction temperature. Bandgap emission dominates the fluorescence of CdS NCs for synthetic temperatures over 150 °C. Similar to the synthesis of CdS NCs, monodisperse sub-10 nm SnS NCs are synthesized by adding (NH4)2S to Sn(OA)2 ODE solution. XRD studies show an orthorhombic SnS phase (PDF 65−3875; Figure S3b, Supporting Information), and the size of nanoparticles can be tuned between 6 ± 0.5 nm to 10.5 ± 1 nm by varying reaction temperatures (70− 5857

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105 °C; Figure 1g,h and Figure S3a, Supporting Information). We also prove that (NH4)2S can be used in high temperature hot-injection reactions: spherical Wurtzite ZnS nanocrystals (5.5 ± 0.4 nm) are obtained by adding (NH4)2S into a ZnCl2 oleylamine/TOP solution at 320 °C (Figure 1i, Figure S4a, Supporting Information), while injection of (NH4)2S into MnCl2 oleylamine/oleic acid solution at 250 °C results in rodshaped MnS nanocrystals with average size of ca. 20 × 12 nm (Figure 1j, Figure S4b). Based on metal precursors, the conversion yields can be over 90% when excess (NH4)2S (i.e., molar ratio of (NH4)2S:metal precursor = 2) are loaded in these high temperature hot-injection reactions. Compared to the conventional organic phase metal sulfide NC synthesis, our new technique exhibits much higher reactivity. For example, the room temperature reaction of (NH4)2S and Cd(OA)2 (molar ratio of 1:1) in octadecene (ODE) produces NCs (CdS-1) with a sharp band gap absorption peak at ca. 360 nm (full width at half-maximum (fwhm) = 15 nm; Figure 2a), which suggests a NC size of ca.

ca. 360 nm bandgap absorption in both nonpolar (e.g., ODE) and polar (e.g., octyl ether, TOPO) solvents, using various surfactant ligands (e.g., oleic acid, trioctylphosphine (TOP)), different cadmium complexes, and even at slightly higher temperatures (60 °C). Such small and extremely size-stable NCs have been reported for II−VI semiconductor NCs, called magic-size NCs (MSNs).28−32 The room temperature reactions of (NH4)2S with Cd(OA)2 can produce different size CdS NCs by varying the Cd(OA)2 to (NH4)2S ratios. Monodisperse CdS NCs with bandgap absorption at 326 nm (CdS-2) and 305 nm (CdS-3) (Figure 2c; suggesting NC sizes of 1.3 and 0.9 nm, respectively)27 are synthesized in pure form by the reaction of Cd(OA)2 and (NH4)2S at molar ratios of 2.8:1 and 4.2:1, respectively. A higher Cd(OA)2 to (NH4)2S ratio forms smaller CdS NCs. However, the size of CdS NCs cannot be continuously changed for a room temperature reaction: an increase of the Cd(OA)2 to (NH4)2S ratio to 8.3:1 still produces NCs with the same size of CdS-3, while an intermediate Cd(OA)2 to (NH4)2S ratio (2:1) results in a mixture of products of CdS-1, CdS-2 and CdS-3 (Figure S7, Supporting Information). Such a discontinuous stepwise growth has been observed in the other MSNs synthesis,29 in which the NCs have a number of thermodynamically stable configurations and, instead of continuous growth, their sizes jump from one configuration to the next one. Our highly thermodynamically favored, low-temperature synthetic technique provides a novel method to produce small size metal sulfide quantum dots, some of which are difficult to be obtained by the conventional high temperature reactions. For example, the synthesis of Ag2S NCs within the quantum confinement regime remains difficult and only a very few examples are known.33,34 We have found that small size Ag2S quantum dots can be obtained by reacting (NH4)2S with AgCl in a solvent mixture of oleylamine and TOP at room temperature. XRD results of as-prepared Ag2S NCs show broad peaks which are consistent with the pattern of monoclinic αAg2S (PDF 14−0072; Figure S8a,b, Supporting Information). HRTEM studies reveal the lattice with d-spacing of 2.4 Å, which can be indexed as the (−103) facet of monoclinic α-Ag2S (inset of Figure 3a). Inductively coupled plasma mass spectrometry (ICP-MS) studies also confirm that NC products consistent of Ag and S elements with the atomic ratio of Ag:S equaling to 2.8:1. Similar to our CdS MSNs synthesis, the size of Ag2S NCs can be tuned discontinuously by varying AgCl: (NH4)2S ratios: Ag2S NCs with average sizes of 2.1 ± 0.3 nm (Ag2S-1; Figure S8a, Supporting Information) and 2.8 ± 0.4 nm (Ag2S-2; Figure 3a) are obtained by using AgCl:(NH4)2S ratios of 16:1, and 8:1 respectively. Ag2S-1 NCs exhibit clear excitonic absorption peaks at 580 nm, while this peak red-shifts to 635 nm for the bigger size Ag2S-2 NCs (Figure 3b). Compared to the band gap energy of bulk α-Ag2S (0.9−1.1 eV),33 these excitonic absorption peaks of Ag2S-1 and Ag2S-2 show significant blue shifts, indicating that our Ag2S NCs products behave within the quantum-confinement regime.35 The photoluminescence (PL) of Ag2S-1 and Ag2S-2 shows NIR emission centered at 940 nm, with symmetric emission peaks (Figure S8c, Supporting Information). These NIR emissions cannot be tuned by changing the size of NCs, indicating that they are likely caused by surface trap states. Our techniques can also be used to produce Bi2S3 quantum dots. Although Bi2S3 NCs with different morphologies have been well studied,36−39 quantum confinement in Bi2S3 NCs has previously only been observed in very limited examples.37,39

Figure 2. (a) Optical absorption spectrum of CdS-1. Inset of part a shows the XRD pattern of CdS-1. (b) HR-TEM image of CdS-1. The crystalline lattice in the inset of part b corresponds to (111) facet of cubic CdS phase. (c) Optical absorption spectra of CdS-2 and CdS-3. (d) Demonstration of ultralarge scale synthesis of CdS-1 obtained from a single reaction: 35 g of product.

2.5 nm.27 The synthesis can be carried out in air by simply combining (NH4)2S and Cd(OA)2 solutions together in a capped vial. The reaction can be easily scaled up (Figure 2d), and has a conversion yield of over 90%. HRTEM studies (Figure 2b) show that the NCs have an elongated quasispherical shape (ca. 3 × 2.5 nm) with high crystallinity. The lattice value of 3.2 Å agrees with the characteristic d-spacing for the (111) plane of bulk cubic CdS. The aliquots studies show that the room temperature reaction of (NH4)2S and Cd(OA)2 (at molar ratio 1:1) initially produces CdS NCs with band gap absorption of 360 nm and a broad fwhm, and then the CdS NCs exhibit a size self-focusing process with the fwhm decreasing from ca. 50 nm to ca. 15 nm in 30 min (Figure S5, Supporting Information). The initially formed (in ca. 1 min) NC product does not show further growth and its band gap absorption remains at 360 nm during a 5 h reaction. This reaction is highly stable for a variety of synthetic parameters (Figure S6, Supporting Information), producing CdS NCs with 5858

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metal sulfide NCs which are difficult to be obtained by the conventional high temperature methods.

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ASSOCIATED CONTENT

S Supporting Information *

Materials, nanocrystal synthesis, and characterization techniques described in the text, additional UV−vis, PL, and XRD spectra, and additional TEM images. 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]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported in part by the National Science Foundation under award number CHE-1152922, and the Cornell Center for Materials Research (CCMR) with funding from the Materials Research Science and Engineering Center program of the National Science Foundation (cooperative agreement DMR 1120296). We also acknowledge support of Energy Materials Center at Cornell (EMC2), an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Science under Award Number DE-SC0001086. Thanks to Christian Ocier for his help.

Figure 3. (a) TEM image of Ag2S-2 NCs (2.8 ± 0.4 nm). The crystalline lattice of Ag2S-2 in inset of part a corresponds to the (−103) facet. (b) Optical absorption spectra of Ag2S-1 (I) and Ag2S-2 (II). (c) TEM image of Bi2S3-1 NCs (4.5 ± 0.5 nm). The crystalline lattice of Bi2S3-1 in inset c corresponds to (002) facet. (d) Optical absorption spectra of Bi2S3-1 (I) and Bi2S3-2 (II).

The room temperature reaction of (NH4)2S and bismuth dodecanethiolate complex produces uniform Bi2S3 NCs, with an average size of ca. 4.5 ± 0.5 nm (Bi2S3-1; Figure 3c). The XRD results of Bi2S3-1 closely resemble the pattern reported for ultrathin Bi2S3 necklace nanowires,37 and the broadened peaks can be assigned to orthorhombic Bi2S3 (PDF 17−0320; Figure S9a, Supporting Information). The HRTEM image in Figure 3c shows a lattice with d-spacing of 1.98 Å, which can be indexed to (002) facet of orthorhombic Bi2S3. The UV−vis absorption spectrum of Bi2S3-1 NCs displays a clear shoulder near the onset of absorption at ca. 620 nm (I of Figure 3d), which is similar to the absorption spectrum of reported quantum confined Bi2S3 necklace NCs37 and can be ascribed to the lowest-energy excitonic absorption. Relative to the bandgap energy of orthorhombic bulk Bi2S3 (1.3 eV),37 the excitonic absorption of this Bi2S3 NCs has a 0.7 eV blue shift as a consequence of quantum confinement. The size of Bi2S3 NCs can be tuned by using different bismuth complex precursors in synthesis. The reaction of Bi(OA)3 and (NH4)2S at 60 °C affords Bi2S3 NCs of ca. 3.3 ± 0.4 nm (Bi2S3-2; Figure S9b, Supporting Information). XRD studies of as-prepared Bi2S3-2 reveal a similar pattern as that of Bi2S3-1 (inset of Figure S9b, Supporting Information). The exitonic absorption of the smaller Bi2S3-2 particle is at ca. 590 nm (II of Figure 3d), displaying a clear blue shift compared to the bigger Bi2S3-1 NCs. In conclusion, we have reported the first use of (NH4)2S as a generic sulfide precursor in organic nonpolar-phase nanocrystals synthesis. This novel method offers rational syntheses to a variety of monodisperse metal sulfide NCs. Compared to conventional methods, the simple and clear reaction mechanism of our synthesis has brought important advantages, such as good reproducibility, high conversion yields, and ultralarge scale production of monodisperse NCs. This will benefit the applications of semiconductor colloidal nanocrystals in hightech devices that need copious amount of materials. Moreover, (NH4)2S exhibits very high reactivity toward metal complexes and the synthesis in general can be performed at low temperatures, which opens new opportunities for producing

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