Nickel Sulfide and Copper Sulfide Nanocrystal Synthesis and

Arrested precipitation of nickel sulfide nanocrystals gave cubic Ni3S4 (polydymite) .... The nanocrystals were drop cast onto a carbon-coated TEM grid...
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Langmuir 2005, 21, 9451-9456

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Nickel Sulfide and Copper Sulfide Nanocrystal Synthesis and Polymorphism Ali Ghezelbash 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

Langmuir 2005.21:9451-9456. Downloaded from pubs.acs.org by RUTGERS UNIV on 01/02/19. For personal use only.

Received May 4, 2005. In Final Form: July 29, 2005 Nickel sulfide and copper sulfide nanocrystals were synthesized by adding elemental sulfur to either dichlorobenzene-solvated (copper sulfide) or oleylamine-solvated metal(II) precursors (nickel sulfide) at relatively high temperature to produce the metal sulfide. Nickel sulfide nanocrystals are cubic Ni3S4 (polydymite) with irregular prismatic shapes, forming by a two-step reduction-sulfidation mechanism where Ni(II) reduces to Ni metal before sulfidation to Ni3S4. Despite extensive efforts to optimize the Ni3S4 nanocrystal size and shape distributions, polydisperse nanocrystals are produced. In contrast, copper sulfide nanocrystals can be obtained with narrow size and shape distributions. The copper sulfide stoichiometry depended on the Cu:S mole ratio used in the reaction: Cu:S mole ratios of 1:2 and 2:1 gave CuS (covellite) and Cu1.8S (digenite), respectively. CuS nanocrystals formed as hexagonal disks that assemble into stacked ribbons when cast from solution onto a substrate. CuS, Cu1.8S, and Ni3S4 differ from the Cu2S and NiS nanocrystals obtained by solventless decomposition of metal thiolate single source precursors, in terms of stoichiometry for copper sulfide, and both stoichiometry and morphology for nickel sulfide [Ghezelbash, A.; Sigman, M. B., Jr.; Korgel, B. A. Nano Lett. 2004, 4, 537-542. Sigman, M. B. Ghezelbash, A.; Hanrath, T.; Saunders, A. E.; Lee, F.; Korgel, B. A. J. Am. Chem. Soc. 2003, 125, 16050-16057].

Introduction Growing evidence suggests that both the shape and crystal phase of inorganic nanocrystals synthesized in solution by arrested precipitation can be modified to a significant extent by changing the precursor chemistry, concentrations, injection rates, reaction temperature, and solvent and capping ligand chemistry. For example, it was first observed by Peng et al. that subtle changes in ligand chemistry and processing conditions changed the shape of CdSe nanocrystals from spheres to rods.1 More complicated shape control was subsequently observed with the formation of CdSe tetrapods.2 Similar results have been observed for many other nanocrystals with anisotropic crystal structures, including CdS,3,4 PbS,5 CdTe,4,6 ZnSe,7 Fe2P,8 FeP,9,10 Co2P,10 Ni2P,10 MnP,10 GaP,11 Cu2S,12 Ni,13 Fe,14 Co,15 and TiO2.16 The crystal phase of nano* Corresponding author. E-mail: [email protected]. Tel: (512) 471-5633. Fax: (512) 471-7060. (1) Peng, X.; Manna, U.; Yang, W.; Wickham, J.; Scher, E.; Kadavanich, A.; Allvisatos, A. P. Nature 2000, 404, 59-61. (2) Manna, L.; Scher, E. C.; Alivisatos, A. P. J. Am. Chem. Soc. 2000, 122, 12700-12706. (3) Jun, Y.-W.; Lee, S.-M.; Kang, N.-J.; Cheon, J. J. Am. Chem. Soc. 2001, 123, 5150-5151. (4) Shieh, F.; Saunders, A. E.; Korgel, B. A. J. Phys. Chem. B 2005, 109, 8538-8542. (5) Lee, S.-M.; Jun, Y.-W.; Cho, W.-N.; Cheon, J. J. Am. Chem. Soc. 2002, 124, 11244-11245. (6) Peng, Z. A.; Peng, X. J. Am. Chem. Soc. 2001, 123, 183-184. (7) Cozzolli, D. D.; Manna, L.; Curri, M. L.; Kudera, S.; Giannini, C.; Striccoli, M.; Agostiano, A. Chem. Mater. 2005, 17, 1296-1306. (8) Park, J.; Koo, B.; Hwang, Y.; Bae, C.; An, K.; Park, J.-G.; Park, H. M.; Hyeon, T. Angew. Chem., Int. Ed. 2004, 43, 2282-2285. (9) Qian, C.; Kim, F.; Ma, L.; Tsui, F.; Yang, P. D.; Liu, J. J. Am. Chem. Soc. 2004, 126, 1195-1198. (10) Park, J.; Koo, B.; Yoon, K.-Y.; Hwang, Y.; Kang, M.; Park, J.-G.; Hyeon, T. J. Am. Chem. Soc. 2005, 127, 8433-8440, (11) Kim, Y.-H.; Jun, Y.-W.; Jun, B.-H.; Lee, S.-M.; Cheon, J. J. Am. Chem. Soc. 2002, 124, 13656-13657. (12) Liu, Z. P.; Xu, D.; Liang, J. B.; Shen, J. M.; Zhang, S. Y.; Qian, Y. T. J. Phys. Chem. B 2005, 109, 10699-10704. (13) Cordente, N.; Respaud, M.; Senocq, F.; Casanove, M. J.; Amiens, L.; Chaudret, B. Nano Lett. 2001, 1, 565-568.

crystals produced by arrested precipitation has also been found to be sensitive to the synthetic conditions for some materials, as in Co (cubic, hexagonal, or -Co),15,17,18 GaP (cubic or hexagonal),11 and ZnSe (cubic or hexagonal).7 From an industrial perspective, synthetic control over nanocrysal shape and crystal phase could be a powerful tool for tuning materials properties to suit applications. However, stoichiometric and structural polymorphism in nanocrystals is still relatively poorly understood. Many ceramics and minerals exhibit very complicated phase diagrams with large stoichiometric and structural diversity, and it is an open question as to how the crystal phase and shape of nanocrystals of these materials can be tuned in solution syntheses. Further obscuring the issue is the fact that nanocrystals can exhibit even greater phase complexity than the bulk materials due to size- and shapedependent effects, leading to effects such as spontaneous alloying as in Au/Ag core-shell nanocrystals at small diameters,19 size-dependent intraparticle phase separation,20 kinetically fast rod-to-sphere coalescence,4,15 and altogether new phases, such as the -Co phase.17 The relationship between the reaction chemistry and the nanocrystal shape, composition and phase is complicated. Iron-based nanocrystals have provided perhaps the best snapshot to date of the interplay between the reaction chemistry and the nanocrystal reaction product. For example, arrested precipitation by the thermal decom(14) Park, S.-J.; Kim, S.; Lee, S.; Khim, Z. G.; Char, K.; Hyeon, T. J. Am. Chem. Soc. 2000, 122, 8581-8582. (15) Puntes, V. F.; Zanchet, D.; Erdonmez, C. K.; Alivisatos, A. P. J. Am. Chem. Soc. 2002, 124, 12874-12880. (16) Cozzoli, P. D.; Kornowski, A.; Weller, H. J. Am. Chem. Soc. 2003, 125, 14539-14548. (17) Dinega, D. P.; Bawendi, M. G. Angew. Chem., Int. Ed. Engl. 1999, 38, 1788-1791. (18) Sun, S. H.; Murray, C. B. J. Appl. Phys. 1999, 85, 4325-4330. (19) Shibata, T.; Bunker, B. A.; Zhang, Z. Y.; Meisel, D.; Vardeman, C. F.; Gezelter, J. D. J. Am. Chem. Soc. 2002, 124, 11989-11996. (20) Shirinyan, A. S.; Wautalet, M. Nanotechnology 2004, 15, 17201731.

10.1021/la051196p CCC: $30.25 © 2005 American Chemical Society Published on Web 09/02/2005

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position of the iron precursor, Fe(CO)5, can give four different nanocrystal materials (Fe,14 FeP,9,10 Fe2P8 and γ-Fe2O321) depending on the solvent and the processing conditions. In pure TOPO (trioctylphosphine oxide), Fe(CO)5 decomposition at ∼300 °C gives Fe metal nanocrystals.14 The addition of TOP (trioctylphosphine) to the reaction yields FeP: the iron precursor first complexes with TOP to form a “single-source” precursor for FeP that forms upon thermolysis of the phosphorus-carbon bond.9,10 Interestingly, this same reaction, thermal decomposition of Fe(CO)5 in the presence of TOP, carried out without TOPO but rather in a mixture of octyl ether and oleylamine gives Fe2P nanorods.8 Fe(CO)5 thermal decomposition in octyl ether in the presence of oleic acid and trimethylamine oxide yields nanocrystals of the oxidized product γ-Fe2O3.21 In other materials, relationships have emerged between the nanocrystal shape and the composition obtained from small variations in the synthesis. For instance, Tang et al.22 showed that HfxZr1-xO2 nanocrystals produced with x < 0.5 were spherical with tetragonal crystal structure; whereas, nanocrystals synthesized with x > 0.5 were rodlike with the monoclinic phase. In the case of copper sulfide, subtle differences in precursor chemistry and solvent have been observed to give qualitatively different materials: Liu et al.12 decomposed CuS2CNEt2 in a mixture of dedecanethiol and oleic acid to obtaine wires of hexagonal Cu2S (high chalcocite), while Lou et al.23 decomposed Cu(S2CNEt2)2 in TOP/TOPO/TOPS to obtain spherical Cu1.8S (digenite) nanocrystals. Much more data is needed for more materials to start to develop a general understanding of what controls these observed differences in composition, phase and shape. Nickel sulfide and copper sulfide are two chemically simple binary compounds that exhibit rich phase diagrams with a number of thermodynamically stable crystal structures and stoichiometries,24 which makes them interesting and somewhat complicated model materials to study nanocrystal shape and phase polymorphism. We recently developed a solventless synthetic technique for size and shape-monodisperse copper and nickel sulfide nanocrystals from single-source metal thiolate precursors: NiS (millerite) nanorods and triangular nanoprisms25 and Cu2S (high chalcocite) nanorods and nanodisks.26,27 In the approach, metal alkylthiolate precursors are thermally decomposed in the presence of capping ligands (in the absence of solvent) to obtain NiS and Cu2S nanocrystals with nonspherical shapes that reflect their anisotropic internal crystal structure. Here we report the arrested precipitation of nickel sulfide and copper sulfide nanocrystals and compare the nanocrystal morphology and phases with those obtained in the solventless synthesis. Both nickel sulfide and copper sulfide nanocrystals obtained by arrested precipitation exhibited different stoichiometries than those obtained by the solventless method. Solution-phase arrested precipitation yielded cubic Ni3S4 (polydymite) nanocrystals and Ni metal as a (21) Hyeon, T.; Lee, S. S.; Park, J.; Chung, Y.; Bin Na, H. J. Am. Chem. Soc. 2001, 123, 12798-12801. (22) Tang, J.; Fabbri, J.; Robinson, R. D.; Zhu, Y. M.; Herman, I. P.; Steigerwald, M. L.; Brus, L. E. Chem. Mater. 2004, 16, 1336-1334. (23) Lou, Y. B.; Samia, A. C. S.; Cowen, J.; Banger, K.; Chen, X. B.; Lee, H.; Burda, C. Phys. Chem. Chem. Phys. 2003, 5, 1091-1095. (24) Binary Alloy Phase Diagrams, 2nd ed.; Scott, W. W., Jr.; ASM International: Materials Park, OH, 1990. (25) Ghezelbash, A.; Sigman, M. B., Jr.; Korgel, B. A. Nano Lett. 2004, 4, 537-542. (26) Larsen, T. H.; Sigman, M.; Ghezelbash, A.; Doty, R. C.; Korgel, B. A. J. Am. Chem. Soc. 2003, 125, 5638-5639. (27) Sigman, M. B.; Ghezelbash., A.; Hanrath, T.; Saunders, A. E.; Lee, F.; Korgel, B. A. J. Am. Chem. Soc. 2003, 125, 16050-16057.

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byproduct, which differs from the solventless process that gave NiS (millerite) as the primary product and Ni3S4 as a reaction byproduct. Copper sulfide arrested precipitation produced hexagonal CuS (covelite) and rhombohedral Cu1.8S (digenite), with stoichiometry that depended on the copper:sulfur mole ratio in the reaction. Experimental Details Chemicals. Oleylamine (OLA) and trioctylphosphine (TOP) were used as received from Fluka. Nickel(II) chloride (NiCl2) and copper(II) acetylacetonate (Cu(acac)2), elemental sulfur, o-dichlorobenzene (DCB), 1.0 M lithium triethylborohydride in tetrahydrofuran (THF) solution (Superhydride), and octenoic acid (OA) were used as received from Aldrich Chemical Co. Ni3S4 Nanocrystal Synthesis. In one three-neck flask, 0.13 g (1 mmol) of NiCl2 is dissolved in 7 mL of OLA and in a separate three-neck flask, 0.064 g (2 mmol) of elemental sulfur is dissolved in 2 mL of OLA at room temperature. Both solutions are purged of oxygen by bubbling nitrogen for 30 min. A total of 1.34 mL (3 mmol) of TOP and 0.15 mL (1 mmol) of OA are injected into the nickel-OLA mixture and serve as capping ligands. The nickelOLA and sulfur-OLA mixtures are heated to 110 °C and then combined. The reaction mixture is heated to 220 °C and maintained at that temperature for 1 h. The reaction is then quenched by withdrawing the colloidal solution using a glass syringe and metal needle and collecting it in a glass vial. A wide range of nickel sulfide synthesis conditions were explored, including reaction temperature, ligand chain length, and reactant mole ratios. Reactions at temperatures higher than 220 °C produced only insoluble black particulate material. Temperatures lower than 180 °C produced only a very low yield of nanocrystals. TOP and octenoic acid gave the highest quality nanocrystals, whereas shorter chain-length ligands such as tributylphosphine gave very poor quality nanocrystals with very rough surfaces and a very polydisperse size distribution. In some reactions, superhydride was added as a reducing agent prior to sulfur precursor injection into the Ni-OLA precursor solution. A total of 0.13 g (1 mmol) of NiCl2 was dissolved in 5 mL of OLA in a three-neck flask and heated to 200 °C. A total of 1.0 mmol of superhydride was dissolved in 1 mL of OLA and injected into the Ni-OLA mixture. Immediately following superhydride injection, the heating mantle was removed to allow the reaction mixture to cool. Once cooled to 110 °C, 2 mL of OLA with 0.08 g (2.3 mmol) of elemental sulfur was injected. The reaction mixture was heated at 220 °C for 1 h. CuS and Cu1.8S Nanocrystal Synthesis. In two different three-neck flasks, 0.13 g (1 mmol) of Cu(acac)2 is dissolved in 7 mL of DCB and 0.064 g (2 mmol) of elemental sulfur is dissolved in 3 mL of DCB at room temperature. Both flasks are purged of oxygen by bubbling nitrogen for 30 min. A total of 1.98 mL (6 mmol) of OLA and 0.075 mL (0.5 mmol) of OA are injected into the Cu-DCB mixture. The Cu-DCB and sulfur-DCB solutions are heated to 110 °C and combined. The reaction mixture is refluxed (182 °C) for 1 h. Cu1.8S nanocrystals are produced when the molar amount of Cu(acac)2 is increased to 2 mmol, and the molar amounts of elemental sulfur and OLA are decreased to 1 and 3 mmol, respectively. Nanocrystal Purification. Nanocrystals are purified by precipitation in excess ethanol followed by centrifugation at 7000 rpm for 7 min. The supernatant contains molecular reaction byproducts and is discarded. The nanocrystals are redispersed in 20 mL of hexane and centrifuged again. Poorly capped nanocrystals and large particulates settle during centrifugation, whereas the well-capped nanocrystals remain dispersed. The precipitate is discarded. The hexane-dispersed particles are reprecipitated in excess ethanol, with 0.05 mL of OLA and 0.02 mL of OA added to maintain good surface passivation during the precipitation step. The supernatant is discarded after centrifugation. The purified nanocrystals redisperse in various organic solvents, including hexane and chloroform. Solventless Synthesis of Cu2S and NiS Nanocrystals. NiS and Cu2S nanocrystals were synthesized using the solventless methods described in refs 1 and 2. In a typical preparation, between 32 and 36 mL of an aqueous metal(II) (M2+) nitrate solution (0.21 g of either Cu(NO3)2‚xH2O or Ni(NO3)2‚xH2O) are

Nickel Sulfide and Copper Sulfide Nanocrystals

Langmuir, Vol. 21, No. 21, 2005 9453 sions on 200 mesh carbon-coated copper TEM grids (Ladd Research). XRD patterns were obtained using a Bruker-Nonius D8 Advance θ-2θ Powder Diffractometer, with Cu KR (λ ) 1.54 Å) radiation, a Bruker Sol-X Si(Li) solid-state detector, and a rotating stage. XRD samples were prepared by dropcasting colloidal nanocrystal solutions onto a quartz (0001) substrate, and samples were scanned for 7 to 15 h with 0.02° angle increment and a scan rate of 12°/min.

Results

Figure 1. TEM images of (A-C) Ni3S4 nanocrystals and (D) a polycrystalline Ni nanocrystal. combined with 25 mL of CHCl3 and 0.18 g of sodium octanoate. After 20 to 30 min of vigorous stirring, the clear aqueous phase is discarded and the colored organic phase containing the M2+ ions is retained. Dodecanethiol (C12H25SH, 240 µL) is then added to the stirring organic phase. After 30 min of vigorous stirring, the organic solvent is evaporated on a rotary evaporator, leaving a waxy residue consisting of the metal thiolate and octanoate mixture. The precursor mixture is heated in air at temperatures ranging from 140 to 200 °C for between 5 min and 5 h. After heating, the product appears dark brown or black depending on the heating time and temperature. The product is dispersed in chloroform and precipitated with excess ethanol and centrifuged at 8000 rpm for 5 min. The supernatant contains unreacted molecular precursors and reaction byproducts and is discarded. After one precipitation in excess ethanol, the product is redispersed in chloroform and centrifuged at 8000 rpm for 5 min to remove insoluble byproducts of the reaction and poorly passivated particles. The precipitate is discarded. Characterization. The nanocrystals were characterized using transmission electron microscopy (TEM), X-ray diffraction (XRD), and electron diffraction (ED). Low- to intermediateresolution TEM images were acquired using a Philips 208 TEM with 80 kV accelerating voltage with an AMT Advantage HR model CCD camera. High-resolution TEM images were acquired digitally (Gatan multipole scanning CCD camera) using a JEOL 2010F TEM operating at 200 kV. ED was performed on the JEOL 2010F TEM. TEM samples were prepared by drop-casting nanocrystals from either hexane, toluene, or chloroform disper-

Ni3S4 (Polydymite) Nanocrystals. Arrested precipitation of nickel sulfide nanocrystals gave cubic Ni3S4 (polydymite) particles with a range of irregular shapes, including cuboidal, trapezoidal, quasi-spherical, and quasitriangular. Polycrystalline Ni metal nanocrystals were observed as a reaction byproduct. Figure 1, panels A and B, shows two high-resolution TEM images of Ni3S4 nanocrystals with lattice spacings of 1.8 Å and 3.3 Å, corresponding to (115) and (022) d spacings of cubic Ni3S4 (JCPDS file 43-1469), and Figure 1D shows a TEM image of a polycrystalline Ni nanocrystal. The relative amount of metal Ni nanocrystals increases as the Ni:S ratio increases. Figure 2 shows XRD patterns of nanocrystals made with different Ni:S mole ratios. In the synthesis, it turns out that the Ni(II) precursor is first reduced to Ni metal nanocrystals by OA and then sulfidated to Ni3S4. At high Ni:S ratios, the reaction is starved for S and a larger amount of Ni byproduct forms (see Figure 2). The overall particle morphology also changed as the Ni:S mole ratio in the reaction was changed. To confirm the reduction-sulfidation reaction mechanism, we produced cubic Ni3S4 nanocrystals by adding superhydride as a strong reducing agent to first form Ni nanocrystals prior to adding the sulfur precursor. Nanocrystals prepared in this way were primarily polycrystalline cubic Ni3S4, in contrast to the single-crystal-domain nanocrystals obtained by reacting the nickel and sulfur precursors directly. However, the Ni nanocrystals were completely sulfidated upon addition of the sulfur source, as confirmed by XRD (Figure 3). CuS (Covellite) and Cu1.8S (Digenite) Nanocrystals. Arrested precipitation of copper sulfide nanocrystals gave either hexagonal CuS (covellite) or rhombohedral Cu1.8S (digenite) nanocrystals. The stoichiometry varied with the Cu:S mole ratio. 1:1 Cu:S gave a mixture of hexagonal CuS (covellite) and rhombohedral Cu1.8S (di-

Figure 2. TEM images and XRD patterns of Ni3S4 nanocrystals synthesized with TOP:OA ) 3 using Ni:S mole ratios of (A,D) 1:1, (B,E) 1:2 and (C,F) 1:4. The ‡ and * symbols mark peaks corresponding to cubic Ni3S4 and cubic Ni, respectively.

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Figure 3. (A-C) TEM images and (D) XRD pattern of Ni3S4 nanocrystals made when Superhydride was added. The nanocrystals are polycrystalline and visible lattice spacings corresponding to the (022) and (004) d spacing of cubic Ni3S4 have been labeled. In (D), the ‡ symbols mark diffraction peaks corresponding to Ni3S4 diffraction peaks (JCPDS file 43-1469).

genite) nanocrystals (Figure 4, panels A and D). Decreasing the Cu:S mole ratio to 1:2 gave only CuS and increasing the Cu:S ratio to 2:1 gave Cu1.8S (OLA:OA ) 6:1). Both Cu1.8S and CuS nanocrystals exhibit disklike morphology, with either a “truncated triangle” or an “equilateral hexagon” shape. CuS nanocrystals exhibit primarily equilateral hexagon shape. The average diameter and thickness of the CuS nanodisks in a typical sample is 12.8 ( 1.9 nm (418 particles counted) and 4.6 ( 0.9 nm (358 particles counted) (aspect ratio ) 2.7). Decreased OLA: OA mole ratio gave broader size distributions, with a OLA: OA mole ratio of 12:1 giving the most size- and shapemonodisperse CuS nanocrystals. CuS nanodisks could be produced with relatively narrow size and shape distributions and were found to selfassemble upon drop casting from solution into supercrystals with columnar disk packing, as shown in Figure 5. When isolated on a substrate, the disks lie flat on their hexagonal faces and look spherical (or hexagonal), but when stacked together, they look like rods. Tilting the

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stacked disks in the TEM, as shown in Figure 6, confirms their disk-shape. Face-to-face platelet stacking is similar to what has been observed for Cu2S27 and Co15 nanodisks. Packing entropy (or free-volume entropy) favors orientational ordering, as the existence of the nematic phase was first suggested by Onsager many years ago28 and later confirmed by computer simulations of infinitely thin hard platelets by Frenkel and Eppenga,29 and experimentally for submicrometer hard disk colloids by Lekkerkerker and co-workers.30 Later computer simulations by Veerman and Frenkel31 and Zhang et al.32 revealed the columnar phase, which was experimentally observed by Lekkerkerker and co-workers33 in colloidal dispersions of submicrometer platelet particles. In the case of the CuS nanodisks, the van der Waals attraction is also greater for disks oriented face-to-face relative to disks oriented edge-to-edge due to the much greater interfacial contact area provided by this configuration.34 An additional energetic stabilization of face-to-face stacking might also be provided by electric dipole interactions, as the faces of the disks are the polar {001} crystal surfaces and the disk edges are nonpolar crystal faces, as proposed for Cu2S nanodisks in ref 27. One interesting observation was that neighboring supercrystals tended to form T- and L-shaped junctions such as those shown in Figure 5, which might be the result of polar interparticle interactions. The disk morphology reflects the internal hexagonal CuS crystal structure. High-resolution TEM images and electron diffraction patterns obtained from individual CuS nanodisks reveal that the [001] crystallographic direction (i.e, the c axis) is oriented in the direction of the short axis of the disk. Figure 7A shows a nanodisk lying on its hexagonal face. The electron diffraction pattern in Figure 7A shows a hexagonal array of spots 1.9 Å from the center spot corresponding to {110} diffraction spots from hexagonal CuS with the beam down the [001] zone axis. In the TEM image in Figure 7B, the 2.7 Å lattice spacing corresponds to the (006) d spacing of hexagonal CuS (covellite). When nanodisks are oriented on their sides, modulated fringes often appear due to double diffraction from stacking faults, as shown in Figure 7C. Similar modulated fringes have been observed in Au nanorods and Ag nanowires having twins extending down their lengths.35-37

Figure 4. TEM images and XRD patterns of copper sulfide nanocrystals synthesized with Cu:S mole ratios of 1:1 (A, D), 1:2 (B, E), 2:1 (C, F). The samples were produced with an OLA:OA mole ratio of 6:1 at 182 °C for 1 h. The ‡ and * symbols denote hexagonal CuS (covellite) and rhombohedral Cu1.8S (digenite) diffraction peaks. The average nanocrystal diameters were 12.4 ( 2.2 nm (Cu:S ) 1:1; 195 particles counted), 10.5 ( 1.8 nm (Cu:S)1:2; 135 particles counted), and 13.2 ( 1.8 nm (Cu:S ) 2:1; 349 particles counted). The peak at 2Θ ) 32.2° (panes D and E), corresponds to the (103) and (006) planes of hexagonal CuS; the peak at 2Θ ) 31.8° is broadened due to the small size of the nanocrystals and overlaps with the (006) peak (2Θ ) 32.8° from JCPDS file number 06-0464). The (006) peak (JCPDS file number 06-0464) is indicated by the arrow and ‡ symbol in panels D and E.

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Figure 5. (A-C) TEM images of CuS nanodisks self-assembled into supercrystals with columnar packing with nanodisks stacked face-to-face and lying on their sides. Panels A-C show three different self-assembled structures. The nanodisk supercrystals are often found to align perpendicular to other supercrystals, forming (A, C) T structures and (B) L structures. The nanocrystals were drop cast onto a carbon-coated TEM grid from toluene at concentrations of 3.75 mg/mL (A, B) and 5 mg/mL (C). The nanodisks have a diameter of 15.5 ( 1.4 nm and a thickness of 6.0 ( 1.0 nm. Figure 7. TEM images of CuS nanodisks. (A) A CuS hexagonal nanodisk oriented on its face and a corresponding electron diffraction pattern; (B) a row of nanodisks stacked on their sides. (C) Two CuS hexagonal nanodisks stacked on their sides with several stacking faults, which give rise to double diffraction and the modulated fringes in the particle on the left.

Figure 6. TEM images of a row of stacked CuS nanodisks imaged at different tilt angles: (A) +18.5° and (B) -18.5° in the x direction. (C, D) Schematic illustration of the nanodisk morphology and how their overlap affects the image.

The stacking faults can help promote disk growth by providing a high energy location for nucleation and growth. Stacking fault planes have also been observed in Ag nanodisks38 and have been suggested as a contributing factor to their shape and radial growth. However, stacking

faults are probably not a necessity for CuS disk growth. Due to the hexagonal crystal structure, {001} and {100} faces have different surface energies and reactivities as discussed by Sigman et al.27 for Cu2S, which has a similar crystal structure. In the solventless synthesis, Cu2S nanodisks appear to result from faster C-S bond cleavage on the more energetic {100} facets compared to the {001} facets.27 Matysina found that hexagonal metals with a c/a ratio greater than 1.63 have {101} and {100} surfaces with 1.5 times greater surface energy than the {001} facets.39 The c/a ratio for CuS is 4.31, and 6-fold symmetric growth of CuS nanodisks that increases the {001}/{100} surface area ratio is expected based on these guidelines. Additionally, increased OLA:OA mole ratio from 6:1 (shown in Figure 4B) to 12:1 gave a much higher yield of nanodisks (shown in Figure 5) but did not lead to a change in CuS crystal structure. Therefore, it is possible that the amine preferentially binds to the {001} facets to inhibit [001] directed growth as Puntes et al. observed for Co nanodisks grown in the presence of amine capping ligands.15 However, there are other arrested precipitation examples of preferential ligand binding to the nonpolar crystal facets, as in the case of CdSe where phosphonic acid promotes crystallization in the [001] direction leading to rod growth.1 (28) Onsager, L. Phys. Rev. 1942, 62, 558.

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Figure 8. TEM images of nickel sulfide (top row) and copper sulfide (bottom row) produced by (A,C) solution-phase (Ni3S4 and CuS) and (B, D) solventless routes (NiS and Cu2S).

Discussion Figure 8 compares TEM images of Ni3S4 and CuS nanocrystals produced by arrested precipitation and NiS and Cu2S nanocrystals produced by the solventless thermolytic degradation of metal alkylthiolate singlesource precursors. Nickel Sulfide Nanocrystals. In solution, OA reduces the Ni(II) precursor to Ni metal nanocrystals, which are subsequently sulfidated to Ni3S4. This reaction pathway contrasts the solventless synthesis, in which NiS nanorods precipitate directly as a single source metal thiolate precursor undergoes C-S thermolysis. When the singlesource nickel thiolate precursor was tried as the nickel sulfide source in solution, both Ni3S4 nanocrystals and Ni byproduct were produced, apparently decomposing by the same reduction-sulfidation mechanism. The solventless synthesis yields rhombohedral NiS (millerite) nanorods with measurable amounts of cubic Ni3S4 (polydymite) particles, which are in fact the expected equilibrium phases for the Ni:S ratio in the synthesis (Ni:S ) 0.7:1).25 When Ni:S ) 1:1 is used in the solution-phase reaction, there is not enough sulfur available to sulfidate all of the Ni initially formed, and therefore it is not surprising that residual unreacted Ni metal appears as a reaction byproduct. It is somewhat surprising, however, that the nickel sulfide phase formed in the reaction is cubic Ni3S4. Ni3S2 is actually the sulfide phase in equilibrium with pure Ni metal. Considering a Ni sulfidation reaction and simply considering thermodynamics based on a bulk phase diagram, one would expect Ni3S2 as a product. In fact, this is what Tilley et al.40 found when they sulfidated larger (40∼60 nm diameter) Ni particles. The Ni3S4 (29) Frenkel, D.; Eppenga, R. Phys. Rev. Lett. 1982, 49, 1089-1092. (30) Van der Kooij, F. M.; Lekkerkerker, H. N. W. J. Phys. Chem. B 1998, 102, 7829-7832. (31) Veerman, J. A. C.; Frenkel, D. Phys. Rev. A 1992, 45, 56325648. (32) Zhang, S.-D.; Reynolds, P. A.; van Duijneveldt, J. S. J. Chem. Phys. 2002, 117, 9947-9958. (33) Van der Kooij, F. M.; Kassapidou, K.; Lekkerkerker, H. N. W. Nature 2000, 406, 868-871. (34) Israelachvili, J. Intermolecular & Surface Forces; Academic Press: London, 1992. (35) Chen, H.; Gao, Y.; Zhang, H.; Liu, L.; Yu, H.; Tian, H.; Xie, S.; Li, J. J. Phys. Chem. B 2004, 108, 12038-12043. (36) Johnson, C. J.; Dujardin, E.; Davis, S. A.; Murphy, C. J.; Mann, S. J. Mater. Chem. 2002, 12, 1765-1770. (37) Sun, Y.; Mayers, B.; Herricks, T.; Xia, Y. Nano Lett. 2003, 3, 955-960. (38) Germain, V.; Li, J.; Ingert, D.; Wang, Z. L.; Pileni, M. P. J. Phys. Chem. B 2003, 107, 8717-8720. (39) Matysina, Z. A. Mater. Chem. Phys. 1999, 60, 70-78.

Ghezelbash and Korgel

nanocrystals produced here are an order of magnitude smaller than Tilley’s particles, and it is possible that the smaller size facilitates faster sulfidation kinetics that leads to more extensive sulfidation. Anyway, the solventless synthesis appears to yield nanocrystals with the expected equilibrium phases; whereas, arrested precipitation gives products that deviate from thermodynamic expectations and are most likely determined by kinetic factors derived from the details of the reaction chemistry related to the precursors, solvent, capping ligands, and termperature. Copper Sulfide Nanocrystals. Solution-phase arrested precipitation of copper sulfide also gave different stoichiometry than the solventless synthesis, forming covellite (CuS) and digenite (Cu1.8S) nanocrystals, as opposed to hexagonal Cu2S (high chalcocite). Covellite and digenite are the expected equilibrium phases at the synthesis temperature (182 °C) and room temperature when an excess of sulfur is present in the reaction, i.e., Cu:S ) 1:2.24 However, at higher Cu:S ratios, the observed digenite phase differs from the expected equilibrium phases at room temperature: Cu1.96S and Cu1.75S are the room-temperature equilibrium phases with Cu:S ) 2:1, and Cu1.75S is the equilibrium phase with Cu:S ) 1:1. The digenite nanocrystals appears to be metastable and do not undergo a phase change upon cooling to room temperature. This is similar to Cu2S nanodisks formed by the solventless synthesis, which maintained the hightemperature hexagonal crystal structure when cooled to room temperature,27 as opposed to the thermodynamically stable monoclinic Cu2S structure. The similarities in morphology between the Cu2S and CuS nanocrystals, despite the different reaction pathways, can be attributed to both the internal hexagonal crystal structure and the greater surface energy of the {100} and {101} facets compared to the {001} facets for both materials. Conclusions Solution-phase arrested precipitation of copper sulfide and nickel sulfide nanocrystals gave hexagonal CuS (covellite), rhombohedral Cu1.8S (digenite) and cubic Ni3S4 (polydymite) nanocrystals. The Ni3S4 nanocrystals formed by a two-step reduction-sulfidation process and exhibited irregular prismatic shapes. Hexagonal CuS (covellite) and rhombohedral Cu1.8S (digenite) nanocrystals were produced with stoichiometries that varied with the Cu:S mole ratio. CuS nanodisks could be produced with relatively narrow size and shape distributions. The nickel sulfide nanocrystals produced by solventless nickel alkylthiolate decomposition are those expected based on bulk phase compositions; whereas, arrested precipitation gave unexpected nickel sulfide phases. Both solution-phase and solventless synthetic routes can produce relatively size and shape monodisperse copper sulfide nanodisks under certain reaction conditions. The nanodisk morphology relates in both cases to the internal hexagonal crystal structure of CuS and Cu2S, with higher {101} and {100} facet surface energies relative to the {001} facets that promotes 6-fold symmetric growth perpendicular to the c axis. The results presented here further demonstrate that for many materials a wide range of nanocrystal shapes and phases can be obtained by manipulating the reaction chemistry and the processing conditions. Although much still remains to be learned about the detailed mechanisms of nanocrystal formation, these data add to the rapidly growing body of knowledge exploring this important topic. LA051196P (40) Tilley, R. D.; Jefferson, D. A. J. Phys. Chem. B 2002, 106, 1089510901.