11964
J. Phys. Chem. B 2004, 108, 11964-11970
Supercrystals of Uniform Nanorods and Nanowires, and the Nanorod-to-Nanowire Oriented Transition Narayan Pradhan and Shlomo Efrima* Department of Chemistry and the Ilse Katz Center for Meso and Nanoscale Science and Technology, Ben Gurion UniVersity of the NegeV, Beer-SheVa, Israel 84105 ReceiVed: January 22, 2004
Supercrystals of uniform nanorods and ordered assemblies of nanowires are presented. We describe a convenient method to produce highly uniform and extremely thin ZnS nanorods and nanowires. Because of their uniformity, they spontaneously assemble into crystalline (nanorods), as well as parallel and crossed (nanowire) structures. Most importantly, we present an intimate view of the stages of nanorod-to-nanowire transition via a unique, oriented head-to-head attachment mechanism.
1. Introduction Nanorods (of metals, seminconductors, and magnetic materials) recently have attracted interest for a variety of reasons. They offer tunability of material properties via control of shape, in addition to size-dependent quantum confinement effects. They are proposed as basic one-dimensional (1D) building blocks in nano-based structures and devices and their (self-)assembly properties and phases are of fundamental scientific interest. A current major challenge is the production of shape- and sizecontrolled uniform nanorods and nanowires and their assembly into well-defined and useful patterns and structures. Several efficient and general methods have been developed for the production of nanorods and nanowires.1-6 Most of them, however, usually require additional separation steps to obtain homogeneous populations of shape and size and often mandate strict exclusion of air and humidity (in the case of highly fluorescing semiconductor particles, for instance).7,8 Assemblies of nanoparticles involve capillary forces,9 template-directed patterning,10 surface derivatization processes,11 etc. Several systems of nanoparticles (nanodisks and rods) have been observed to assemble into nematic or smectic liquid-crystalline phases,5,6 which are of interest as intermediates in the formation of nanowires, and from the basic theoretical point of view.12 In the former context, it is interesting to note that an end-to-end oriented attachment mechanism of nanorods that leads to the production of elongated nanostructures has recently been proposed and demonstrated.4 In this communication, we describe highly uniform ZnS nanorods and nanowires of extremely small widths (∼1.2 nm), produced by a simple and convenient method that does not require any post-production separation stages. The synthesis itself is performed in the ambient at relative low temperatures and involves simple, commercially available, cheap precursors of relatively low toxicity. Furthermore, and most importantly, the nanorods and nanowires self-assemble into highly ordered newly observed crystalline phases. The interparticle spacing in these supercrystals can be tuned in subnanometer increments, using capping agents of varying alkyl chain lengths. Assembly occurs in a variety of circumstances: on transmission electron * Author to whom correspondence should be addressed. E-mail address:
[email protected].
microscopy (TEM) grids; on glass slides via Langmuir-Blodgett layer deposition; or by simply drying a solution, as crystals in precipitates, and even as aggregates existing in solution. The nanowires form large-scale parallel superstructures as well as crossed-wire, gridlike two-layer arrays. Most interestingly, we demonstrate the formation of nanowires from nanorods via a unique mechanism involving a rotated phase of the nanorods and a synchronous end-to-end attachment process. 2. Experimental Section 2.1. Chemicals and Instrumentation. High-purity ethylxanthic acid (potassium salt), zinc perchlorate, dodecylamine (DDA), tetradecylamine (TDA), and hexadecylamine (HDA) have been purchased from Aldrich and used without any further purification. Octadecylamine (ODA) (99%) was purchased from Fluka and used as received. Methanol, dichloromethane, and toluene of analytical-reagent grade are used for all experiments. Potassium salt of hexadecylxanthic acid was synthesized according to the method reported elsewhere and repeatedly recrystallized from methanol.13,14 Water with a resistivity of 18 MΩ cm was obtained from a Barnsted E-pure water purifier. Ultraviolet-visible (UV-vis) spectra were recorded with a Hewlett-Packard model HP 8452A diode array spectrophotometer in the wavelength range of 190-820 nm, with a resolution of 2 nm. Fluorescence was acquired using a Jobin Yvon Fluorolog-2. TEM analysis was performed using a JEOL model 2010 HR-TEM microscope system that was equipped with a Gatan multiscan charge-coupled device (CCD) camera. Energydispersive X-ray spectroscopy (EDS) was performed using an Oxford linked ISIS 6498 (version 1.4) spectrometer. A typical accelerating voltage of 200 kV was used, with magnifications of 100 000× (for obtaining size distributions) and 600 000× (for single-particle analysis). A drop of a suspension of the colloid in dichloromethane was placed on a Formvar lacey carbon-coated, 300-mesh copper grid (Ted Pella 01883-F) and allowed to evaporate. X-ray diffraction (XRD) was performed with a Philips model 1050/70 diffractometer using the Cu KR line. 2.2. Synthesis of Zinc Xanthate. A saturated solution of alkyl (C8 or C16) xanthic acid (potassium salt) was prepared in a 4:1 solution of methanol and water (for ethyl xanthic acid, only
10.1021/jp0496708 CCC: $27.50 © 2004 American Chemical Society Published on Web 05/25/2004
Nanorods and Nanowires
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Figure 1. (A) UV-visible absorption spectrum and (B) corresponding PL spectrum of ZnS rods synthesized from zinc ethylxanthate (ZnEX) in hexadecylamine (HDA).
water was used). A separate solution of zinc acetate (or ZnClO4) was prepared in methanol. Dropwise addition of the latter into the former solution was continued until the zinc:xanthate molar ratio reached a value of 1:2. The white zinc xanthate was washed several times with alcohol, centrifuged, and dried at room temperature. 2.3. Synthesis of ZnS Rods and Wires. A solution of zinc ethylxanthate, ZnEX, (3.04 × 10-4 moles, 0.08 g) was prepared in molten ODA (5.6 × 10-3 mol, 1.53 g) at a temperature just below 60 °C. The addition of the white zinc xanthate initially resulted in a yellowish-white hazy solution that cleared after a few minutes. The temperature then was swiftly increased to 100 °C for nucleation and growth. After 5 min at 100 °C, a white turbidity appeared, indicating the formation of ZnS. After 20 min, allowing for the growth process to occur, the temperature was increased to 170 °C for 45 or 90 min of annealing, yielding rods and wires, respectively. The ZnS particles were harvested by flocculating the sample with methanol, centrifuging, and redispersing in hydrophobic solvents such as toluene, chloroform, dichloromethane, etc. We purged with nitrogen as a precaution, because the flashpoint of the amines is below the annealing temperature. ZnS rods appeared as a white power, whereas the wires were slightly yellowish. No size-selective precipitation was conducted. The same procedure and molar concentrations were used for all other amines and xanthates. The powders of ZnS rods and wires can be stored in dry containers for months without any apparent change. Their solubility (in toluene, for instance) remains unchanged, giving a clear and transparent solution after sonication for ∼1min. The transition of rods to wires depends on the concentration of the precursor, the alkyl length of the xanthate, the reaction temperature, the annealing temperature, and the annealing time. Long-chain xanthates serve better than ethylxanthate for achieving tight control of the particle shape and size. Mostly rods form when the nucleation and growth occurs in the range of 90-120 °C, with the concentration mentioned previously. The addition of zinc xanthates to the amine solvents at higher temperature (>160 °C), and the use of 10-fold-lower concentrations, result mostly in spherical particles.14 3. Results and Discussion Thermal decomposition of metal-xanthate leads to a metal sulfide.14 Alkylamine catalyses the reaction and reduces the decomposition temperature (to a degree that is dependent on the metal), resulting in high-quality metal sulfide particles. We have previously reported the formation of spherical ZnS particles from zinc hexadecylxanthate (ZnHDX) at temperatures of 10 µm long (figure not shown). Thus, we may conclude that both the rods and the wires selfassemble into highly ordered crystalline superstructures, even in solution. The question we address now is how the ∼5-nm-long ZnS rods transform to >100-nm-long wires. As reported previously, this transition occurs in solution by annealing the rods at 170 °C for >90 min. Our TEM studies seem to suggest an unusual mechanism that might occur, that is closely related to the oriented attachment mechanism proposed recently by Lee Penn and Banfield16 and demonstrated for ZnO.4 Panels D, E, and F in Figure 3 show a micrograph of ZnS nanoparticles harvested in the middle of the annealing of ZnS nanorods. One can see several different regions, three of which are worthy of note: region I (Figure 3 D) shows the supercrystal arrangements of the slightly rotated uniform nanorods, which we already discussed previously; in region II (Figure 3E), the rotated rods appear to be tilted and begin to merge at their tips; finally, in region III (Figure 3F), we observe fully formed nanowires that
Figure 12. Sketched diagram of the rods-to-wires transition: (A) superstructure arrangement of rods (corresponds to Figure 3D); (B) deformed superstructure (corresponds to Figure 3B); (C, C′) the clockwise (and counterclockwise) rotation of rods to a head-to-tail position (corresponds to Figure 3C-F); (D, D′) coalescence into wires (corresponds to Figure 3G-I); and (E) crossed wires.
seem to run along the same direction of the rotated nanorods from which they apparently formed. Similar transitions are observed also in Figure 11. Thus, we seem to have captured the complete rod-to-wire transition; starting from the supercrystalline arrangement of uniform rods, which synchronously rotate and then coalesce. This is related to the oriented attachment head-to-head mechanism proposed recently by Lee Penn and Banfield.16 A scheme of the transition from rods to wires is shown in Figure 12. A clockwise rotation of the rods or a counterclockwise rotation would explain the appearance of crossed wires in the 2D TEM micrographs (see Figures 3 and 6-8), where an angle of ∼52° between the wires in the
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Pradhan and Efrima ordered aggregates (Figure 13). No intermixing of rods and wires is observed, even after sonication for hours. Thus, rods and wires phase-separate already in solution. Lastly, and perhaps more conclusively, we have never seen wires in TEM analyses of unannealled rods, whereas we do see the ordered supercrystalline arrangements of the rods. Thus, we can rule out the possibility that the rod-to-wire transition occurs on the grid. 4. Conclusion
Figure 13. TEM micrographs of (A) ZnS rods and (B) ZnS wires, taken from different regions of a TEM grid.
crossing layers is observed for ODA. This compares favorably with an angle of 48° calculated on the basis of the scheme in Figure 12 and the measured distances between the rods (3.9 ( 0.2 nm between neighbors and 5.2 ( 0.2 nm between layers in the nonrotated phases). In passing, it is important to note that the micrographs show that separate regions are observed, each involving exclusively one type of structure. This suggests three main possibilities: (i) a mixture of ZnS rods and wires deposits simultaneously with phase separation, which has has been observed in the past in the deposition of particles of different sizes or shapes;17 (ii) the ZnS nanoparticles are already segregated into separate clusters of rods and wires in solution; and (iii) the rod-to-wire coalescence occurs only on the grid. We can rule out the latter, based on our TEM studies of the rods and the wires and their mixtures and because we do not see any time/irradiation effects in our samples placed on the grids. We favor the second explanation, on the basis of our light-scattering measurements, which show that aggregates of ∼100-200 nm are present in solution of wires or rods, and the TEM results, which show regions of this size scale and suggest strong interaction between the wires or between the rods, as indicated by the long-range supercrystalline arrays. Two important questions now arise. First, how important is the length uniformity of the seed nanorods in this synchronous wire formation process? It stands to reason that a high degree of uniformity of the rod lengths is required to achieve the observed registry over large areas. The widths of the particles probably need not be highly uniform, as long as they are negligible, compared to the thickness of the capping layers. In a linear mechanism, where separate rods grow into separate wires, one rod at the time (the ZnO case, for instance4), it is conceivable that uniformity of length will be less critical. Second, does this oriented attachment mechanism also occur in solution, or is it a result of the deposition on the grid, forming the specific ordered superstructure and occurring perhaps under the action of the electron gun? We already know that the rodto-wire transition can occur in solution by annealing. Also, we do not see time/irradiation effects in the TEM measurements. Thus, the electron beam itself does not drive the transition. We also know that wires form in solution where aggregates of rods (and wires) are observed. In addition, nanowires form even when rods redispersed in toluene are heated at 70 °C for 45 min. The wires and the rods deposit on TEM grids in separate areas as
We have reported on a convenient method to produce highly uniform ZnS nanorods and nanowires that self-assemble already in solution into crystalline-like superstructures. The structure is observed in precipitates, as well as deposits, on TEM grids and glass slides (by evaporation or the Langmuir-Blodgett technique). The distances between the rods and the wires in these supercrystals can be precisely controlled in increments of 0.4 nm. Wires form rather flexible single-layer parallel arrays, as well as two-layer crossed two-dimensional nets. The rod-towire transition was caught in action, demonstrating a unique synchronous oriented end-to-end attachment mechanism that occurs in ordered phases of rods and involves their synergistic rotation. Acknowledgment. We thank B. Katz and N. R. Jana for their help, and S. Acharya for the preliminary LB work. References and Notes (1) Puntes, V. F.; Krishnan, K. M.; Alivisatos, A. P. Science 2001, 291, 2115. (2) (a) Li, M.; Schnablegger, H.; Mann, S. Nature 1999, 402, 393. (b) Nikoobakht, B.; Wang, Z. L.; El-Sayed, M. A. J. Phys. Chem. B 2000, 104, 8635. (c) Wang, Z. L. J. Phys. Chem. B 2000, 104, 1153. (d) Motte, L.; Lacaze, E.; Maillard, M.; Pileni, M. P. Langmuir 2000, 16, 3803. (e) Andres, R. P.; Bielefeld, J. D.; Henderson, J. I.; Janes, D. B.; Kolagunta, V. R.; Kubiak, C. P.; Mahoney, W. J.; Osifchin, R. G. Science 1996, 273, 1690. (f) Yu, S. H.; Antonietti, M.; Colfen, H.; Hartmann, J. Nano Lett. 2003, 3, 379. (3) (a) Murphy, C. J.; Jana, N. R. AdV. Mater. 2002, 14, 80. (b) Jana, N. R.; Gearheart, L. A.; Obare, S. O.; Johnson, C. J.; Edler, K. J.; Mann, S.; Murphy, C. J. J. Mater. Chem. 2002, 12, 2909. (c) Korgel, B. A.; Fitzmaurice, D. AdV. Mater. 1998, 10, 661. (d) Springholz, F.; Holy, V.; Pinczolits, M.; Bauer, G. Science 1998, 282, 734. (4) Pacholski, C.; Kornowski, A.; Weller, H. Angew. Chem., Int. Ed. 2002, 41, 1188. (5) Nikoobakht, B.; Wang, Z. L.; El-Sayed, M. A. J. Phys. Chem. B 2000, 104, 8635. (6) Puntes, V. F.; Zanchet, D.; Erdonmez, C. K.; Alivisatos, A. P. J. Am. Chem. Soc. 2002, 124, 12874. (7) Adams, M.; Dogic, Z.; Keller, S.; Fraden, S. Nature 1998, 393, 349. (8) Bates, M. A.; Frenkel, D. J. Chem. Phys. 2000, 112, 10034. (9) Bowden, N.; Arias, F.; Deng, T.; Whitesides, G. M. Langmuir 2001, 17, 1757. (10) Alivisatos, A. P.; Johnsson, K. P.; Peng, X.; Wilson, T. E.; Loweth, C. J.; Bruchez, M. P.; Schultz, P. G. Nature 1996, 382, 609. (11) (a) Collier, C. P.; Vossmeyer, T.; Heath, J. R. Annu. ReV. Phys. Chem. 1998, 49, 371. (b) Pileni, M. P. J. Phys. Chem. B 2001, 105, 3358. (12) Rabani, E.; Reichman, D. R.; Geissler, P. L.; Brus, L. E. Nature 2003, 426, 271. (13) Pradhan, N.; Efrima, S. J. Am. Chem. Soc. 2003, 125, 2050. (14) Pradhan, N.; Efrima, S. J. Phys. Chem. B 2003, 107, 13843. (15) Israelachvili, J. Intermolecular and Surface Forces, 2nd ed.; Academic Press: London, 1991; p 370. (16) Lee Penn, R.; Banfield, J. F. Science 1998, 281, 969. (17) Korgel, B. A.; Fitzmaurice, D. AdV. Mater. 1998, 10, 661.
