Synthesis of NiS and MnS Nanocrystals from the Molecular Precursors

Synthesis of NiS and MnS Nanocrystals from the Molecular. Precursors (TMEDA)M(SC{O}C6H5)2 (M ) Ni, Mn). Lu Tian,† Lee Yong Yep,† Tien Teng Ong,† ...
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CRYSTAL GROWTH & DESIGN

Synthesis of NiS and MnS Nanocrystals from the Molecular Precursors (TMEDA)M(SC{O}C6H5)2 (M ) Ni, Mn)

2009 VOL. 9, NO. 1 352–357

Lu Tian,† Lee Yong Yep,† Tien Teng Ong,† Jiabao Yi,‡ Jun Ding,‡ and Jagadese J. Vittal*,† Department of Chemistry, National UniVersity of Singapore, Singapore 117543, and Department of Materials Science and Engineering, National UniVersity of Singapore, Singapore 117574 ReceiVed May 21, 2008; ReVised Manuscript ReceiVed October 3, 2008

ABSTRACT: The compounds (TMEDA)M(SC{O}C6H5)2 (M ) Ni, 1; Mn, 2) have been synthesized and characterized by X-ray crystallography. Both of the monomeric molecular precursors have been used to make the corresponding monodispersed NiS (two different phases) and cubic phase MnS nanocrystals (NCs) under specific experimental conditions. Changing the surfactants yielded low-temperature rhombohedral β-NiS NCs with millerite structure in dodecanethiol and high-temperature hexagonal R-NiS NCs with NiAs-type structure in oleic acid, while monodispersed hexagonal shaped R-MnS NCs with rock-salt structure were obtained in the presence of a combination of oleylamine and dodecanethiol surfactants. The NCs have been characterized by X-ray powder diffraction patterns, transmission electron microscopy, selected area electron diffraction patterns, and energy-dispersive X-ray analysis. Preliminary magnetic measurements show that the two phases of NiS NCs exhibit weak magnetism and hysteresis.

1. Introduction 1

Transition metal sulfides display appealing electronic, optical,2 thermoelectric3 and photoelectric properties.4 Among this large family, nickel sulfide is a potential cathode material for the rechargeable lithium battery,5 as a catalyst in the degradation of organic dyes,6 and in magnetic devices. The nickel sulfide system is very fascinating because of its numerous phases and stoichiometry, such as R-Ni3+xS2,7 β-Ni3S2,7 Ni3S4,8 NiS2,9 R-NiS (rhombohedral, millerite),10 β-NiS (hexagonal, NiAstype),10 Ni9S8,11 and Ni7S6.11 Manganese sulfide has attracted attention as diluted magnetic semiconductors because of its critical magnetic function.12 MnS exists in three forms: the most stable R-MnS with rock-salt type structure, and both β-MnS (sphalerite type) and γ-MnS (wurtzite type) as metastable tetrahedral structures.13,14 Recently the synthesis of nickel sulfide nanomaterials has been focused on micro/submicrometer scale with novel and complicated morphologies, such as flowerlike patterns of NiS nanostructures,15-17 NiS2 rings and parallelograms,18 and NiS hollow spheres.19,20 One-dimensional (1-D) NiS nanorods have been synthesized by the slow-precipitation method.21 Nickel sulfide nanotubes have also been reported by a directional infiltration self-assembly route employing anodic aluminum oxide (AAO) templates.22 Korgel et al. synthesized Ni3S4 nanoparticles with irregular prismatic shapes, but a wide size distribution.23 A mixture of NiS nanorods and triangular nanoprisms in a narrow size distribution was also obtained in Korgel’s group.24 Microsize manganese sulfide with various morphologies such as spheres, cubes, pyramids, octahedrons, flakes, flowers, and hexagonal rings have been synthesized by a solvothermal route.25-28 Interesting coral-shaped assemblies of MnS nanocrystals (NCs) have been obtained by a spray-based technique.29 The architectural control of MnS NCs with various shapes, including cubes, spheres, 1-D monowires, and branched wires, has been achieved by Cheon’s group.30 Hyeon et al. have * Corresponding author. E-mail: [email protected]. † Department of Chemistry. ‡ Department of Materials Science and Engineering.

reported the synthesis of MnS nanorods and bullet- and hexagonshaped MnS NCs.31 In this paper, (TMEDA)M(SC{O}C6H5)2 (TMEDA ) tetramethylethylenediamine; M ) Ni, 1; Mn, 2) compounds were used as single-molecule precursors to synthesize the corresponding metal sulfide NCs. Low-temperature rhombohedral and high-temperature hexagonal phases of NiS NCs have been obtained in two different single capping agents. The NiS NCs had highly irregular shapes when oleic acid (OA) or dodecanethiol (DT) were used, but when a combination of oleylamine (OLA) and DT was employed, faceted cubic-shaped NiS NCs were formed. Hexagonal-shaped MnS NCs with rock-salt structure isolated in a mixture of OLA and octadecene can be achieved by fine-tuning the temperature. In addition, preliminary magnetic properties of the NiS NCs in both rhombohedral and hexagonal phases have also been reported.

2. Experimental Procedures Synthesis of (TMEDA)Ni(SC{O}C6H5)2 (1). Lithium thiobenzoate (PhC{O}S-Li+) was obtained by reacting LiOH (0.895 g, 0.037 mol) with C6H5COSH (4 mL, 0.034 mol) in 10 mL of MeOH. To this clear orange-red solution, TMEDA (2.7 mL, 0.017 mol) and then NiCl2 · 6H2O (3.847 g, 0.017 mol) in 10 mL of MeOH were added. A green precipitate was obtained immediately, and the contents were stirred for an hour. A 1:1 mixture of acetonitrile and dichloromethane was added slowly with warming in a water bath to dissolve the precipitate completely. The contents were left in a refrigerator for recrystallization overnight. Green crystals of 1 obtained on the following day were filtered, washed with Et2O, and dried under nitrogen. (Yield ) 5.6 g, 76%) Elemental analysis for C20H26NiN2O2S2 (Mol. Wt. 449.26), Calc. C, 53.47; H, 5.83; N, 6.24; S, 14.28. Found: C, 53.42, H, 5.62; N, 6.16, S, 14.27. Synthesis of (TMEDA)Mn(SC{O}C6H5)2 (2). Synthesis of 2 was performed following the procedure for 1. Yellow crystals of 2 were obtained. (Yield ) 4.9 g, 68%) Elemental analysis for C20H26MnN2O2S2 (Mol. Wt. 445.49), Calc.: C, 53.92; H, 5.88; N, 6.29; S, 14.40. Found: C, 53.98, H, 6.07; N, 6.29, S, 14.70. Synthesis of Metal Sulfide NCs. A typical synthesis of NiS NCs is described here. A similar procedure was used to prepare MnS NCs. The precursor 1 (50 mg, 0.11 mmol) was dissolved immediately in DT (1.35 mL, 5.62 mmol) and formed a greenish black solution. This solution was degassed for 15 min and then heated to 180 °C for 1 h under nitrogen atmosphere. At the end of the reaction, a black precipitate

10.1021/cg800536w CCC: $40.75  2009 American Chemical Society Published on Web 11/20/2008

NiS and MnS NCs from (TMEDA)M(SC{O}C6H5)2

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Table 1. Crystallographic Data and Structure Refinement Details for 1 and 2 complex formula M crystal system space group a, Å b, Å c, Å β, ° V, Å3 Z Dc, g cm-3 µ, (Mo KR), mm-1 Rint GOF on F2 final R [I > 2σ(I)]

1

2

C20H26N2NiO2S2 449.26 monoclinic P21/c 12.5865(4) 11.8616(4) 15.1205(5) 104.797(2) 2182.57(12) 4 1.367 1.097 0.0308 1.019 R1 ) 0.0493 wR2 ) 0.1186

C20H26MnN2O2S2 445.49 monoclinic P21/c 12.837(1) 11.8198(9) 15.0770(11) 104.772(2) 2212.0(3) 4 1.338 0.802 0.0306 1.057 R1 ) 0.0401 wR2 ) 0.096

Table 2. Selected Bond Distances (Å) and Angle (°) Ranges in 1 and 2

Bond Distances M-S M-O M-N S-C O-C Bond Angle Range O-M-N O-M-S N-M-S C-S-M C-O-M

1 (M ) Ni)

2 (M ) Mn)

2.429(1) and 2.466(1) 2.115(2) and 2.122(2) 2.124(3) and 2.133(3) 1.704(4) and 1.709(4) 1.250(4) and 1.251(4)

2.565(7) and 2.607(7) 2.215(7) and 2.231(7) 2.274(2) and 2.289(2) 1.707(3) and 1.712(2) 1.250(3) and 1.254(3)

88.30(2)-168.45(1) 67.39(7)-155.64(8) 96.54(1)-162.83(9) 74.96(3)-75.75(3) 97.7(2)-98.4(2)

86.10(7)-160.18(7) 64.07(5)-152.72(5) 97.02(6)-157.48(6) 75.33(8)-76.14(8) f 98.92(4)-100.13(5)

was formed. A small amount of toluene and a large excess of EtOH were added to the reaction solution, and NiS NCs were separated by centrifugation. The dark solid was washed with EtOH and dried under nitrogen. The reactions using OA or bisurfactants DT and OLA were performed following a procedure similar to that described above. As for the use of bisurfactants, 2 (42.5 mg, 0.095 mmol) was dissolved in a mixture of OLA (0.2 mL, 0.61 mmol) and DT (0.6 mL, 2.50 mmol) to get an orange solution. The solution was then degassed and heated to 300 °C for 2 h under nitrogen atmosphere. The solution turned orange brown almost instantaneously, and, within several minutes, dark green precipitate was observed in a gray solution. The contents were brought to room temperature, 2 mL of toluene and a large excess of EtOH were added to the reaction solution, and MnS NCs were separated by centrifugation and dried under nitrogen. The microanalytical laboratory at the Department of Chemistry, National University of Singapore, performed the microanalysis. Thermogravimetric (TG) analyses were performed (under a N2 atmosphere) using a SDT 2960 TGA apparatus with a sample size of 8-10 mg per run. The yields are reported with respect to the metal salt. X-ray powder diffraction (XRPD) patterns were obtained using a D5005 Bruker X-ray diffractometer equipped with Cu KR radiation. The accelerating voltage and current were 40 kV and 40 mA, respectively. A scan speed of 0.015°/s was used. Scanning electron microscopy (SEM) images were taken using a Jeol JSM-6700F field emission scanning electron microscope operated at 5 kV and 10 mA. Transmission electron microscopy (TEM) images were taken on a Philips CM 10 microscope operating at 100 kV. High resolution transmission electron microscopy (HRTEM) images, selected area electron diffraction (SAED), and energy-dispersive X-ray (EDX) analysis were obtained from a JEOL JSM-3010 instrument. X-ray Crystallography. Single crystals were obtained during the synthesis. The diffraction experiments were carried out at 22 °C on a Bruker SMART CCD diffractometer with a Mo KR sealed tube. The program SMART was used for collecting frames of data, indexing reflection, and determining lattice parameters, SAINT32 was used for integration of the intensity of reflections and scaling, SADABS33 was used for absorption correction, and SHELXTL34 was used for space

Figure 1. A perspective view of 2 along with an atom numbering scheme is shown. group and structure determination and least-squares refinements on F2. The relevant crystallographic data and refinement details are compiled in Table 1, and selected bond distance and angle ranges are given in Table 2.

3. Results and Discussion Both compounds have been prepared by a simple one-pot synthesis and isolated in moderate to high yields. The X-ray crystallographic data in Table 1 show that both 1 and 2 are isomorphous and isostructural. Hence a general structural description is given, and only differences are highlighted. Both are discrete monomers sustained by very weak interactions. Figure 1 shows a perspective view of 2. The metal ions have highly distorted octahedral geometry with S2O2N2 core, and the atoms of the same type have cis geometry. As observed in other thiobenzoate compounds, the COS atoms are out of plane with the benzene rings.35 The twists are more in 1, as seen from interplanar angles (10.6 and 22.5° in 1 and 6.7 and 19° in 2). Selected bond distances and angles are compiled in Table 2. The TG shows that 1 and 2 start decomposing at 174 and 171 °C, respectively, followed by rapid weight loss in the region 200-350 °C. The plateau is reached only after 450 °C due to tailing. However, the final residual weight of 21.7 and 23.0% observed respectively for 1 and 2 are slightly more than the calculated values for the formation of metal sulfides (20.4 and 19.4%), indicating that some organic residues are still associated with the final products. Nanosynthesis of Metal Sulfides. (a). Rhombohedral and Hexagonal Phases of NiS NCs. Rhombohedral and hexagonal phases of nickel sulfide NCs can be synthesized from the decomposition of the precursor 1 in DT and OA, respectively. Rhombohedral phase NiS NCs can also be synthesized using the same precursor in bisurfactants OLA and DT. The rhombodedral NiS NCs shown in Figure 2a were obtained when 1 was heated in DT (molar ratio of precursor/DT ) 1:50) at 180 °C. They have polyhedral morphology with a mean diameter of 41.5 ( 3.3 nm. The lattice fringes are clearly visible in the HRTEM image (Figure 2b), indicating that the individual NCs are single crystals. The interplanar distance of 2.98 Å from neighboring lattice fringes corresponds to the (101) plane. The strong ring patterns from SAED given in Figure 2c can be indexed to the rhombohedral phase of NiS (JCPDS 03-0653686). The XRPD pattern of this sample shown in Figure 3a is also consistent with the SAED results. EDX analysis of NCs in the electron microscope showed that the ratio of Ni and S is 1:1.06.36

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Figure 2. (a) Typical TEM images of rhombohedral NiS NCs, (b) its HRTEM image, and (c) SAED pattern of the NiS NCs. Figure 5. (a) Typical TEM image, (b) HRTEM images, and (c) SAED pattern of the rhombohedral NiS NCs obtained using bisurfactants.

Figure 3. Typical XRPD patterns of (a) rhombohedral NiS NCs prepared in DT and (b) hexagonal NiS NCs obtained in OA, along with standard reported patterns.

Figure 6. (a) TEM image and (b) HRTEM image of MnS nanocubes, and (c) SAED rings of MnS NCs obtained by heating 2 in a mixture of OLA and octadecene at 300 °C for 2 h.

Figure 4. (a) Typical TEM images of hexagonal NiS NCs, (b) HRTEM images, and (c) SAED pattern of the NiS NCs.

The XRPD patterns of both the rhombohedral and hexagonal NiS nanoparticles are shown in Figure 3. All the XRPD peaks can be indexed accordingly.

On the other hand, the hexagonal phase of NiS NCs was obtained by heating 1 in OA (molar ratio of precursor/OA ) 1:50) at 180 °C. As shown in Figure 4a, these NiS NCs have morphology similar to that of the rhombohedral phase, but different polyhedral shapes observed in the TEM image are more irregular, with a mean diameter of 26.4 ( 2.6 nm. The XRPD data shown in Figure 3b confirm that this sample belongs to the hexagonal phase of NiS (JCPDS 00-002-1280). The lattice fringes are visible in the HRTEM image shown in Figure 4b, indicating the single crystalline nature of these NCs. The interplanar distance, 2.96 Å, calculated from neighboring lattice fringes, corresponds to the (100) plane of the hexagonal phase. The hexagonal phase inferred from the XRPD data is also consistent with the indexed ring patterns of the SAED given in Figure 4c. In this method, the high-temperature hexagonal R-NiS with NiAs-type structure is stabilized at room temperature. The shape of the NCs obtained above using a single capping agent is irregular. However, Alivisatos et al. in 2000 used a mixture of hexylphosphonic acid and trioctylphosphine oxide

NiS and MnS NCs from (TMEDA)M(SC{O}C6H5)2

Figure 7. (a,b) TEM images, (c) HRTEM image, and (d) SAED rings of hexagonal-shaped MnS NCs prepared by heating 2 OLA and octadecene at 350 °C for 2 h.

Figure 8. XRPD patterns of hexagonal-shaped MnS NCs.

to grow different shapes of CdSe NCs.37 Recently, we have reported the synthesis of a new orthorhombic phase of AgInSe2 nanorods using a combination of OLA and DT,38 and monodispersed toluene-soluble AgInS2 NCs were grown from a mixture of OA and DT.39 Since these reports demonstrated that perfect matching of two (or more) surfactants can yield highquality nanoparticles, the synthesis of NiS NCs have been investigated using a bisurfactant system in order to get welldefined shape. We have succeeded in getting a more regularshaped NiS NCs in this way, and the results are discussed below. According to the TEM image shown in Figure 5a, the rhombohedral NiS NCs obtained from thermal decomposition of 1 in a mixture of OLA and DT at 300 °C are well-defined

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monodispersed particles with faceted cubic shapes. The rhombohedral phase was confirmed by XRPD patterns.36 The average size of the rhombohedral NiS NCs is 16.4 ( 1.6 nm. The interplanar distance, 1.86 Å, calculated from the lattice fringes shown in Figure 5b corresponds to the (131) plane in rhombohedral NiS. The indexed SAED pattern shown in Figure 5c is consistent with the rhombohedral NiS (JCPDS 03-065-3686). The use of OLA only, however, resulted in uncontrollable nucleation and growth of polydispersed NCs, which may be attributed to the rapid decomposition of 1. Further, the use of single surfactant failed to furnish well-defined shapes. When a mixture of OLA and DT was used, the rhombohedral phase was maintained while the shape became relatively regular. The obvious reaction between the basic OLA and acidic DT is the formation of OLAH+ and dodecane thiolate anion. This anion can bind the surfaces of the NiS crystals better than the neutral DT, thereby providing isotropic growth of NCs. When the heating duration was increased from 0.5 to 3 h, there was a distinct shape evolution from microrods into faceted nanocubes.36 However, prolonged heating beyond 3 h and up to 6 h did not change the shape of the NiS nanoparticles significantly but only resulted in a wider size distribution of the faceted nanocubes. Thus, the optimum time duration for heating at 300 °C was eventually found to be 3 h to obtain the NiS faceted nanocubes with the narrow size distribution. The XRPD patterns of the products obtained at 200-270 °C could not be matched to any of the known phases of nickel sulfides in the JCPDS database.36 It is proposed that this could be attributed to some intermediate species before complete decomposition to metal sulfide. Cubic Phase MnS NCs. The monodispersed MnS faceted nanocubes were obtained from 2 in a mixture of OLA and octadecene at 300 °C. The XRPD patterns of the sample confirm that the MnS NCs have rock-salt structure, and the EDX analysis gave an atomic ratio of 1.03 Mn:1 S and thus confirmed the purity of MnS.36 As shown in Figure 6a, the majority of MnS NCs have faceted cubic morphology followed by a small amount of hexagons and regular cubes. These NCs are fairly monodispersed with an average diameter of 24.0 ( 1.5 nm. In the HRTEM image in Figure 6b, the lattice fringes are visible, and lattice spacing was measured to be 2.60 Å, which corresponds to the (200) plane of cubic MnS (JCPDS 00-006-0518). The SAED pattern in Figure 6c conspicuously exhibits polycrystalline diffraction rings indexed to (200), (220), (222), (400), (420), and (422) planes of cubic MnS. When the reaction temperature was increased to 350 °C, highly monodispersed hexagonal-shaped MnS NCs were formed as shown in Figure 7a,b. The interplanar distance 2.61 Å calculated from the lattice fringes corresponds to the (200) plane of cubic phase MnS (JCPDS 00-006-0518). The SAED patterns shown in Figure 7d exhibit polycrystalline diffraction rings.

Figure 9. Hysteresis loop of (a) rhombohedral and (b) hexagonal NiS at room temperature. Each inset is the enlarged magnetization curve.

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These diffraction rings can be indexed to (200), (220), (222), (400), (420), and (422) planes of the MnS with rock-salt structure (JCPDS 00-006-0518), and the atomic weight ratio of Mn/S of 1:1.02 obtained by EDX analysis confirms the stoichiometry.36 The XRPD patterns shown in Figure 8 can be unequivocally indexed to rock-salt structure. The R-MnS (alabandite) is the stable form with rock-salt structure. Cheon and co-workers30 have obtained different shapes of MnS NCs by decomposing the molecular precursor in Mn(S2CNEt2)2 in hexadecylamine (HDA) by varying the growth temperature and the reaction time. At temperatures greater than 250 °C for 2 h of heating, MnS nanocubes with rock-salt structure were formed. On the other hand, Hyeon’s laboratory31 reported hexagonal shaped MnS having wurtzite structure by heating a mixture of MnCl2 and sulfur in OLA at 280 °C for 6 h. Here we have optimized the condition to get highly monodispersed hexagonal shaped cubic phase MnS NCs using a combination of OLA and octadecene. In this method OLA played the role of a capping as well as shape-controlling agent, while octadecene acts as a high boiling solvent. By varying the ratio of OLA and octadecene, it has been found that certain octadecenes support the growth of monodispersed MnS nanocubes.36 When the heating time was increased from 1 to 16 h, reduction in size was observed.36 The optimum time was found to be 3 h, which produced hexagonal shaped MnS NCs with a narrow size distribution. The intraparticle ripening may be responsible for the decrease in size while increasing heating time. Moreover, the reaction temperatures, 300 and 350 °C, are proved to be the optimum temperature for the successful morphology control of faceted cube and hexagonal shaped MnS NCs. Magnetic Properties of NiS NCs. Figure 9a,b shows the room temperature hysteresis loops of rhombohedral and hexagonal NiS NCs at room temperature. Rhombohedral NiS NCs (Figure 9a) has a magnetization of 1 emu/g at a field of 5 kOe and a coercivity of 10 Oe. Hexagonal NiS NCs (Figure 9b) has a magnetization of 1.7 emu/g at a field of 5 kOe and a coercivity of 50 Oe. The loops indicate that both samples have weak ferromagnetism at room temperature. It is known that NiS is one of the antiferromagnetic materials with a Ne´el temperature of 265 K.40 Weak ferromagnetism has been found in nanostructured antiferromagnetic materials as a result of the surface spins.41 However, the weak magnetization generally can only be observed at low temperature. In addition, its Ne´el temperature (265 K) is lower than room temperature. Hence, the room temperature weak ferromagnetism in this work may be related to the interaction between the surfactant and NiS nanoparticles since weak room temperature ferromagnetism has been reported in nonmagnetic nanoparticles as a result of the surfactant bonding.42

4. Conclusions The synthesis and the crystal structures of the molecular precursors (TMEDA)M(SC{O}C6H5)2 (where M ) Mn and Ni) are described. These molecular precursors have been thermally decomposed in suitable surfactants to obtain MnS and NiS NCs. The type of surfactants does affect the crystalline phase of NiS formed: rhombohedral phase formed in the presence of DT as well as in the DT and OLA bisurfactant system, but hightemperature hexagonal phase was formed in OA exclusively. Using an OLA and octadecene bisurfactant system, highly monodispersed faceted cube-shaped and hexagonal shaped MnS NCs were produced by merely heating at 300 and 350 °C respectively. In the magnetic properties studies, noticeable

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magnetic signals have been detected for both rhombohedral and hexagonal phase of NiS, but the rhombohedral phase exhibits stronger magnetization than the hexagonal phase. Acknowledgment. We would like to thank the Ministry of Education, Singapore, for their generous financial support through the NUS-ARF fund (Grant No. R-143-000-283-112). Supporting Information Available: Two X-ray crystallographic files (CIFs) along with XRPD, TEM, and EDX Figures (4 pages) of metal sulfide nanocrystals are available free of charge via the Internet at http://pubs.acs.org.

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