VOLUME 4, NUMBER 4, APRIL 2004 © Copyright 2004 by the American Chemical Society
Solventless Synthesis of Nickel Sulfide Nanorods and Triangular Nanoprisms Ali Ghezelbash, Michael B. Sigman, Jr., and Brian A. Korgel* Department of Chemical Engineering, Texas Materials Institute, Center for Nano- and Molecular Science and Technology, The UniVersity of Texas at Austin, Austin, Texas 78712-1062 Received November 21, 2003; Revised Manuscript Received January 12, 2004
ABSTRACT Organic monolayer-coated rhombohedral NiS (millerite) nanorods and triangular nanoprisms were synthesized using a solventless thermolytic decomposition of nickel thiolate precursors in the presence of octanoate. The size and shape distributions are relatively narrow, with nanorod lengths that depend on the growth conditions, ranging from 15 to 50 nm and typically with aspect ratios of approximately 4. For example, a typical procedure yields nanorods 33.9 ± 8.6 nm long and 8.11 ± 1.6 nm wide. The approach also yields triangular nanoprisms under some reaction conditions with nearly a 1:1 ratio of nanorods to nanoprisms. FTIR spectra reveal that octanoate serves as a capping ligand that controls nanorod growth. X-ray diffraction (XRD) shows that the primary reaction byproduct in the synthesis is colloidal Ni3S4 in the form of misshapen needles and particulates. High-resolution transmission electron microscopy (HRTEM) confirm that the well-defined nanorods and triangular nanoprisms are composed solely of rhombohedral NiS (millerite) grown preferentially in the [110] direction.
Many properties of crystalline materials depend on the crystallographic direction, including, for example, the spontaneous polarization in ferroelectrics and the magnetic coercivity and remnance in ferromagnets.1-10 In nanorods, the preferred crystallographic orientation can be elongated relative to other orientations to optimize desired properties geometrically. On the nanoscale, the ability to control nanocrystal shape and produce anisotropic structures such as rods and disks provides an opportunity to test the coupling of shape anisotropy with quantum confinement effects. Colloidal solution synthetic routes have proven successful for producing nanorods of a range of materials, either by using reaction templates11-15 or seed particles16,17 to direct anisotropic growth or by careful control of the reaction * Corresponding author. E-mail:
[email protected]. Tel: (512) 4715633. Fax: (512) 471-7060. 10.1021/nl035067+ CCC: $27.50 Published on Web 02/24/2004
© 2004 American Chemical Society
conditions in the case of surfactant-directed arrested precipitation. Nanocrystal formation by arrested precipitation is a kinetically controlled process that can yield nanorods and other more complicated shapes with some degree of shape tunability through changes in the reaction parameters such as temperature, reaction time, concentration, and capping-ligand chemistry.8,18-23 We recently demonstrated a new solventless approach to Cu2S nanorod and nanodisk formation by the thermolysis of Cu-alkanethiolate precursors.24,25 In this letter, we extend this synthesis to another metal chalcogenide, NiS, using the solventless thermolysis of nickel thiolate precursors in the presence of octanoate. The synthesis produces NiS nanorods and triangular nanoprisms. The nanocrystals are relatively size- and shape-monodisperse because interparticle collisions are rare in the solventless environment and the particles grow
primarily by monomer addition and subsequent C-S bond cleavage. Like copper sulfide, nickel sulfide exhibits complicated compositional, structural, and magnetic phase behavior. Depending on the synthetic process, a variety of compositions can be obtained, including Ni3S2, Ni3+xS2, Ni4S3+x, Ni6S5, Ni7S6, Ni9S8, Ni3S4, and NiS.26-30 NiS exhibits two phases: the low-temperature rhombohedral (β-NiS, millerite) and high-temperature hexagonal (R-NiS) crystal structures.28 The high-temperature phase is antiferromagnetic and exhibits a metal-insulator transition.31-33 The ability to synthesize NiS nanocrystals with controlled shape, composition, and structure would enable a variety of size-dependent physical properties to be explored, including structural, electronic, and magnetic phase transitions as a function of size, composition, and shape. For example, other chalcogenides such as CdSe and CdS nanocrystals have exhibited phase behavior,34,35 such as melting temperatures and pressure-induced structural changes, that differs from that of the bulk. Because the available characterization tools generally probe ensembles of nanocrystals, these studies rely on nanocrystal samples with minimal impurities, thus reflecting the need for high-quality synthetic methods for a variety of different materials. The solventless synthesis of sterically stabilized nanocrystals represents a new effective route toward obtaining these materials. The solventless synthesis of NiS nanorods relies on the thermal degradation of nickel thiolate precursors in the presence of octanoate. The nickel precursors are prepared by the phase transfer of nickel cations from an aqueous salt solution to an organic phase using sodium octanoate as the phase-transfer catalyst. Once the nickel is transferred to the organic phase, alkanethiol is added to form the nickel thiolate compound that serves as the NiS precursor. In a typical experiment, 36 mL of an aqueous 20 mM Ni2+ solution (0.21 g of Ni(NO3)2‚xH2O in 36 mL of doubly distilled and deionized water (D-H2O)) is mixed with 24.5 mL of CHCl3, followed by the addition of 0.18 g of sodium octanoate (NaOOC(CH2)6CH3). After 30 min of vigorous stirring, the clear aqueous phase is discarded, and the green organic phase is retained. Dodecanethiol (C12H25SH, 240 µL) is added to the stirring organic phase. After vigorous stirring for 30 min, the organic solvent is evaporated on a rotary evaporator to leave a waxy residue consisting of the nickel thiolate and octanoate mixture. The precursor mixture is then heated in air at temperatures ranging from 150 to 190 °C for between 5 min and 5 h. After heating, the product appears as a darkbrown or black product, depending on the heating time and temperature. The product is dispersed in chloroform and precipitated in excess ethanol to remove reaction byproducts. After one precipitation in ethanol, the product is redispersed in excess chloroform and centrifuged to remove insoluble reaction byproducts and poorly capped particles. A typical preparation using dodecanethiol gives 4 mg of product (yield 5%). In some experiments, octadecanethiol (C18H37SH) was used in place of dodecanethiol. Transmission electron microscopy (TEM), X-ray diffraction (XRD), and elemental analysis were used to characterize the nanocrystal product. TEM images were obtained either 538
Figure 1. TEM images of NiS nanocrystals synthesized with octadecanethiol at Ni/thiol/octanoate concentrations of 58/80/86 mM at 190 °C for 1.5 h. The region on the TEM grid shown in (A) is enriched in prisms and the region on the TEM grid shown in (B) is enriched in rods.
Figure 2. X-ray diffraction pattern of the NiS nanorods. The ‡ symbol indicates NiS peaks, and the * symbol indicates Ni3S4 peaks. The sharp peaks at 2Θ ) ∼16 and 50° are the (001) and (003) peaks, respectively, of the synthetic quartz used as the substrate.
using a Philips 208 TEM with an 80-kV accelerating voltage or a JEOL 2010F TEM operating at 200 kV. Samples were prepared for TEM by dispersing the nanoparticles in chloroform and filtering the dispersed solution using a syringe filter with a GHP membrane having a pore size of 0.2 µm (Pall Gelman). The filtered solution was then dropcast on a 200-mesh carbon-film-coated copper TEM grid purchased from Ladd Research. XRD spectra were obtained using a Bruker-Nonius D8 Advance Theta-2Theta powder diffractometer with KR radiation and a scintillation detector. XRD of the product revealed that it consists primarily of Nano Lett., Vol. 4, No. 4, 2004
Figure 3. High-resolution TEM images of β-NiS nanorods.
Figure 4. HRTEM images of triangular β-NiS nanoprisms.
rhombohedral β-NiS (millerite) nanorods and in some cases triangular nanoprisms. Representative TEM images of NiS triangular nanoprisms and nanorods are shown in Figure 1. The NiS nanorods from the sample shown in the TEM image in Figure 1B had an average length and diameter of 36.7 ( 5.9 and 8.7 ( 1.4 nm, respectively. Figure 2 shows an XRD pattern of the NiS product, which reveals that it is composed of Ni3S4, which has a cubic structure, and NiS (millerite), which has a rhombohedral structure and is the low-temperature phase of NiS. Highresolution TEM images of the nanorods, such as those shown in Figures 3 and 4, revealed that the relatively monodisperse nanorods and triangular nanoprisms are composed solely of NiS. The nanorods exhibit lattice planes with a spacing of 4.8 Å orthogonal to the long axis of the rod, which corresponds to the d spacing of the (110) planes of NiS (millerite) (JCPDS file number 12-0041). HRTEM images reveal that the nanorods grow nearly exclusively in the [110] Nano Lett., Vol. 4, No. 4, 2004
direction. The orientation of the lattice planes found in the TEM images is also consistent with the XRD patterns, which exhibit an increased relative (110) peak intensity compared to the standard pattern for bulk millerite NiS. Like the nanorods, the triangular nanoprisms are NiS (millerite). Figure 5 shows a nanoprism imaged at different tilt angles. At - 5.9° tilt in the x direction, a lattice spacing of 2.79 Å appears, which could correspond to the d spacing between either the (113) Ni3S4 planes (2.85 Å according to JCPDS file number 43-1469) or the (300) NiS planes (2.77 Å according to JCPDS file number 12-0041). Tilting the nanoprism to +5.5° in the x direction reveals a different set of lattice planes with 4.8-Å spacing, characteristic of the (110) NiS planes. Tilting experiments on a number of nanoprisms gave the same results, indicating that both the triangular and rodlike particles are rhombohedral NiS. From the XRD patterns of the sample, there is a relatively large amount of Ni3S4 present in the sample. At first we believed 539
Figure 5. HRTEM images of a single triangular β-NiS nanoparticle tilted (A) -5.9° in the x direction and (B) +5.5° in the x direction. Table 1. XRD Peak Positions for β-NiS and Ni3S4 in the XRD Pattern in Figure 2 peak position (2Θ, deg) 16.0 18.6 26.8 30.5 31.6 32.3 35.9 38.1 40.6 49.1 52.8 54.8 56.4 57.7 59. 7
Ni3S4
NiS
5.53 Å, (111) 4.78 Å, (110) 3.32 Å, (022) 2.93 Å, (101) 2.83 Å, (113) 2.77 Å, (300) 2.50 Å, (021) 2.36 Å, (004) 2.22 Å, (211) 1.85 Å, (131) 1.73 Å, (401) 1.67 Å, (044) 1.63 Å, (321) 1.60 Å, (330) 1.55 Å, (012)
that the nanorods and nanoprisms were composed of different forms of nickel sulfide, but as discussed above, this is not the case. It turns out that Ni3S4 is present in the sample in the form of misshapen rods that we consider to be an unwanted reaction byproduct. These Ni3S4 particulates have proven to be extremely difficult to separate from the NiS nanorods because of their similar size relative to the size of nanorods and nanoprisms and because of the fact that they appear to be coated with organic capping ligands as well. Figure 6 shows a characteristic high-resolution TEM image of the Ni3S4 particulate material present in the sample. The polycrystalline nanoparticle in Figure 6A exhibits crystalline domains with lattice spacings of 3.38 and 2.39 Å, which correspond to the d spacing between the (022) and (004) planes of Ni3S4. The needle-shaped nanoparticle in Figure 6B exhibits a nonuniform diameter with lattice spacings that correspond to Ni3S4: 2.41 and 2.87 Å separate the (004) and (113) planes in Ni3S4. Presently, we do not have an effective way to isolate the NiS nanorods and nanoprisms completely from the similarly sized Ni3S4 misshapen rods; this represents an ongoing effort in our research. FTIR spectroscopy (Infinity Gold FTIR spectrometer, Thermo Mattson, model 960M0019) of the NiS product revealed that octanoate serves as the primary capping ligand 540
Figure 6. HRTEM images of (A) a polycrystalline nanoparticle and (B) a rodlike nanoparticle showing lattice planes with spacings that correspond to those of Ni3S4.
Figure 7. FTIR spectra of (A) sodium octanoate and (B) NiS nanocrystals synthesized with sodium octanoate as the phasetransfer catalyst. Curve A is offset from curve B for clarity.
that controls the nanorod size and shape. Samples were prepared for FTIR spectroscopy by depositing thin films of nanoparticles dispersed in chloroform onto a (100) silicon substrate. Figure 7 shows the FTIR spectra of sodium octanoate compared to a sample of NiS nanoparticles Both spectra have characteristic C-H stretches at 2935, 2914, and 2850 cm-1. The NiS nanoparticle spectra has bands at 1715 and 1615 cm-1, which are characteristic of C-O stretching in the carboxylate ion, and bands at 1465 and 1380 cm-1 corresponding to C-H bending frequencies.37 There was no evidence of free dodecanethiol in the sample because the S-H stretching mode does not appear in the expected range of 2600-2550 cm-1. The alkanethiol serves as the sulfur source for particle growth, and octanoate serves as the capping ligand that controls growth, similar to what was Nano Lett., Vol. 4, No. 4, 2004
Figure 8. TEM images of NiS nanoparticles formed at 190 °C and 1.5 h for Ni/dodecanethiol/octanoate concentrations of (A) 28/ 40/43 mM and (B) 58/80/86 mM. All scale bars are 50 nm.
found by Sigman et al. in ref 25 for the solventless synthesis of Cu2S nanorods and nanodisks. To optimize the synthetic conditions and understand (at least experimentally) the rod-to-triangle shape transition, the effects of temperature, reactant concentration, and reaction time were explored. The total Ni, thiol, and octanoate concentrations and the Ni/thiol/octanoate ratio have been found to be the primary experimental parameters that affect the nanocrystal shape. As seen in Figure 8, samples produced at 190 °C and 1.5 h exhibit a shape distribution that includes significantly more triangular particles at a higher Ni/thiol/ octanoate concentration: 28/40/43 mM compared to 58/80/ 86 mM, respectively. (Note that the concentration values reported here are those in chloroform assuming complete phase transfer of the Ni2+ and octanoate species from the
aqueous phase.) Above a limiting Ni/thiol/octanoate concentration of 113/157/170 mM, a majority of the rodlike particles no longer exhibit uniform diameters but rather show protrusions along one or both of the long sides of the particle. As the dodecanethiol concentration is increased relative to the concentrations of Ni (28 mM) and octanoate (43 mM) from 40 to 80 mM, the rodlike particles also exhibit nonuniform diameters along the length of the rods (when reacted at 170 °C for 1.5 h). The reaction temperature is an important reaction parameter. It must be greater than the degradation temperature of the precursor, yet exceedingly high temperatures do not produce redispersible nanocrystalline material. The rate of particle growth was higher at higher temperature. For a reaction time of 1.5 h, increasing the reaction temperature from 150 to 190 °C increased the nanorod length from 19.1 nm (σ ) (22.4%) to 33.6 nm (σ ) (29.8%) (Figure 9AC). Higher reaction temperature also produced a greater number of rods with rounded or flat ends. The reaction time had only a minor effect on the particle shape and size. At relatively short reaction times, the particles were not crystalline. HRTEM revealed that many of the particles, both rodlike and triangular, were amorphous for samples produced at shorter reaction times of 5 and 15 min. At longer reaction times, the particle shape appeared to evolve. After 5 h, the product obtained at 190 °C consisted of only rodlike and quasi-spherical particles versus the mixture of rodlike and triangular particles obtained after 1.5 h. Although we do not fully understand all of the factors that control the shape in this synthetic system, the particle shape reflects the anisotropic crystal structure. This is similar to our findings with respect to the solventless synthesis of Cu2S nanodisks using copper thiolate precursors.25 The disk morphology reflects the anisotropy of the hexagonal lattice. In the Cu2S system, it appears that the shape anisotropy also relates to significantly different thiol thermolysis rates on different crystallographic faces, which have been observed
Figure 9. TEM images of NiS nanoparticles synthesized by reacting for 1.5 h, with (A-C) 28/40/43 mM and (D-F) 58/80/86 mM Ni/dodecanethiol/octanoate concentrations at (A, D) 150 °C, (B, E) 170 °C, and (C, F) 190 °C. Scale bars are 50 nm. Nano Lett., Vol. 4, No. 4, 2004
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in thiol adsorption studies on monolithic copper substrates.25 Much less is known about alkanethiol monolayer adsorption and reactivity on nickel surfaces. Although we can speculate that the shape differences between rods and triangular prisms may be the result of differences in face-sensitive reactivity, at this point we do not have a predictive understanding of the factors that control NiS nanorod and nanoprism formation. In conclusion, rhombohedral NiS (millerite) nanorods and triangular nanoprisms were synthesized by the solventless thermal decomposition of nickel thiolate precursors in the presence of octanoate. In addition to NiS nanorods and triangular nanoprisms, a significant amount of Ni3S4 with irregular particle shape is produced. Because the irregular Ni3S4 particulate material is also capped with organic ligands and is close to the same size as the NiS material, it is challenging to separate from the desired product. The synthetic method, however, can produce relatively large amounts of size- and shape-monodisperse nanorods and nanoprisms of NiS. These materials should prove to be interesting model systems to using in studying size- and shape-dependent magnetic and structural phase behavior. Acknowledgment. We thank T. Hanrath, F. Mikulec, and D. Jurbergs for valuable discussions and J. P. Zhou for assistance with the HRTEM. We also acknowledge the NSF, the Welch Foundation, and the Texas Higher Education Coordinating Board through their ATP program for financial support of this work. References (1) Sasaki, T.; Katsuragi, A.; Mochizuki, O.; Nakazawa, Y. J. Phys. Chem. B 2003, 107, 7659. (2) Kim, H.-W.; Lee, H.-S.; Kim, J.-D. Liq. Cryst. 2002, 29, 413. (3) Leslie-Pelecky, D. L.; Rieke, R. D. Chem. Mater. 1996, 8, 1770. (4) Cullity, B. D. Introduction to Magnetic Materials; Addison-Wesley: Reading, MA, 1972. (5) Park, S.-J.; Kim, S.; Lee, S.; Khim, Z. G.; Char, K.; Hyeon, T. J. Am. Chem. Soc. 2000, 122, 8581. (6) Jun, Y.-w.; Jung, Y.-y.; Cheon, J. J. Am. Chem. Soc. 2002, 124, 615. (7) Park, J.-I.; Kang, N.-J.; Jun, Y.-W.; Oh, S. J.; Ri, H.-C.; Cheon, J. ChemPhysChem 2002, 3, 543. (8) Cordente, N.; Respaud, M.; Senocq, F.; Casanove, M.-J.; Amiens, C.; Chaudret, B. Nano Lett. 2001, 1, 565. (9) Bieniulis, M. Z. C., C. E.; Hoskins, E. R. Geophys. Res. Lett. 1987, 14, 135. (10) Corry, C. E. J. Appl. Geophys. 1994, 32, 55.
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(11) Ye, X.-R.; Lin, Y.; Wang, C.; Wai, C. M. AdV. Mater. 2003, 15, 316. (12) Satishkumar, B. C.; Govindaraj, A.; Nath, M.; Rao, C. N. R. J. Mater. Chem. 2000, 10, 2115. (13) Zhang, Y.; Zhu, J.; Zhang, Q.; Yan, Y.; Wang, N.; Zhang, X. Chem. Phys. Lett. 2000, 317, 504. (14) Han, W.; Fan, S.; Li, Q.; Gu, B.; Zhang, X.; Yu, D. Appl. Phys. Lett. 1997, 71, 2271. (15) Han, W.; Fan, S.; Li, Q.; Hu, Y. Science 1997, 277, 1287. (16) Nikoobakht, B.; El-Sayed, M. A. Chem. Mater. 2003, 15, 1957. (17) Jana, N. R.; Gearheart, L.; Murphy, C. J. J. Phys. Chem. B 2001, 105, 4065. (18) Jun, Y.-w.; Lee, S.-M.; Kang, N.-J.; Cheon, J. J. Am. Chem. Soc. 2001, 123, 5150. (19) Kim, Y.-H.; Jun, Y.-w.; Jun, B.-H.; Lee, S.-M.; Cheon, J. J. Am. Chem. Soc. 2002, 124, 13656. (20) Peng, X.; Manna, U.; Yang, W.; Wickham, J.; Scher, E.; Kadavanich, A.; Allvisatos, A. P. Nature 2000, 404, 59. (21) Puntes, V. F.; Zanchet, D.; Erdonmez, C. K.; Alivisatos, A. P. J. Am. Chem. Soc. 2002, 124, 12874. (22) Puntes, V. F.; Krishnan, K. M.; Alivisatos, A. P. Science 2001, 291, 2115. (23) Holmes, J. D.; Ziegler, K. J.; Doty, R. C.; Pell, L. E.; Johnston, K. P.; Korgel, B. A. J. Am. Chem. Soc. 2001, 123, 3743. (24) Larsen, T. H.; Sigman, M.; Ghezelbash, A.; Doty, R. C.; Korgel, B. A. J. Am. Chem. Soc. 2003, 125, 5638. (25) Sigman, M. B., Jr.; Ghezelbash, A.; Hanrath, T. H.; Saunders, A. E.; Lee, F.; Korgel, B. A. J. Am. Chem. Soc. 2003, 125, 1605016057. (26) Tilley, R. D.; Jefferson, D. A. J. Phys. Chem. B 2002, 106, 10895. (27) Olivas, A.; Cruz-Reyes, J.; Petranovskii, V.; Avalos, M.; Fuentes, S. J. Vac. Sci. Technol., A 1998, 16, 3515. (28) Kullerud, G.; Yund, R. A. J. Petrol. 1962, 3, 126. (29) Seim, H.; Fjellvag, H.; Groenvold, F.; Stoelen, S. J. Solid State Chem. 1996, 121, 400. (30) Stolen, S.; Fjellvag, H.; Gronvold, F.; Seim, H. J. Chem. Thermodyn. 1994, 26, 987. (31) Sparks, J. T.; Komoto, T. Phys. Lett. A 1967, 25, 398. (32) Trahan, J.; Goodrich, R. G.; Watkins, S. F. Phys. ReV. B 1970, 2, 2859. (33) McWhan, D. B.; Marezio, M.; Remeika, J. P.; Dernier, P. D. Phys. ReV. B 1972, 5, 2552. (34) Tolbert, S. H.; Alivisatos, A. P. Science 1994, 265, 373. (35) Ricolleau, C.; Audinet, L.; Gandais, M.; Gacoin, T.; Boilot, J. P. J. Cryst. Growth 1999, 203, 486. (36) TEM images were obtained either using a Philips 208 TEM with an 80-kV accelerating voltage or a JEOL 2010F TEM operating at 200 kV. TEM samples were prepared by dispersing the nanoparticles in chloroform and filtering the dispersed solution using a syringe filter with a GHP membrane with a pore size of 0.2 µm (Pall Gelman). The filtered solution was then dropcast on a 200-mesh carbon-filmcoated copper TEM grid purchased from Ladd Research. XRD spectra were obtained using a Bruker-Nonius D8 Advance Theta-2Theta powder diffractometer with KR radiation and a scintillation detector. (37) Silverstein, R. M.; Webster, F. X. Spectrometric Identification of Organic Compounds, 6th ed.; Wiley & Sons: New York, 1998.
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