8296
J. Phys. Chem. B 2006, 110, 8296-8301
Silica Nanocoils Yongquan Qu, Joshua D. Carter, and Ting Guo* Department of Chemistry, One Shields AVenue, UniVersity of California, DaVis, California 95616 ReceiVed: February 4, 2006; In Final Form: March 8, 2006
We report here the results of nanoparticle-catalyzed synthesis of a variety of silica nanocoils (SiNCs) under similar growth conditions to those of making regular straight silica/silicon nanowires (SiNWs). Synthesis of individual SiNCs with alternating coiled and straight segments was achieved with cobalt nanoparticle catalysts at alternating temperatures. In addition, a variety of SiNCs, including multiple SiNCs sharing the same nanoparticle catalyst, SiNC double helices, and individual SiNCs with varying pitches and diameters were made.
I. Introduction The intricate and rich interplays between nanoparticle catalysts and reactants in various growth environments have led to the discovery of many novel nanomaterials, including singlewalled carbon nanotubes (SWNTs) and metal and semiconductor nanowires. These discoveries have been the driving forces behind the rapid advancement of nanotechnology.1-6 Recently, nanostructures such as nanobelts and nanosprings have also been made using nanoparticles catalysts.7-9 It is certain that many more novel nanostructures will be produced catalytically. The catalysts and growth conditions have been known to greatly influence the outcome of the synthesis of these nanoand micromaterials. Carbon microcoils, for example, have been made when metal particle catalysts are used to make carbon fibers.10-14 Single- and double-walled carbon nanotubes can also be made from different nanoparticle catalysts.15 In another example, large diameter SWNTs are made with the addition of sulfur or other elements in the catalysts.16 Silica nanosprings with tight turns and triangular cross-sections have been made when a trace amount of carbon is present,7 which is different from another kind of nanosprings made from laser vaporization of Si powder at 1200 °C that have round cross sections.8 The growth of silica nanocoils (SiNCs) with round cross sections using a different synthetic method has also been recently reported.17 These examples have clearly demonstrated that controlling the very productive and yet complicated catalytic processes involving nanoparticle catalysts and nanostructures is the key to making new nanomaterials. Moreover, many of the growth parameters are interdependent. For example, parameters such as growth temperature can be used not only to control the rate of growth but also the geometry and the structures of the nanoparticle catalysts and the structure of nanomaterials thus produced. High temperatures and trace elements present in the growth environment may further complicate the phase diagram and the growth processes. In some sense, the task of fully describing a nanoparticle catalyst is similar to that of defining a macromolecule in biology: it can only be fully characterized by its whole structures for a specific catalytic function under certain conditions. In this report we will use experimental evidence to further illustrate the complex nature of catalytic processes, which in * Corresponding author. Phone: (530)-754-5283. E-mail: tguo@ ucdavis.edu.
turn can be used to produce novel nanostructures by controlling the catalytic growth conditions and the catalysts themselves. These results presented here suggest that even the same nanoparticles under different external physical conditions may possess totally different catalytic functions and hence can give rise to totally different end products. Such evidence shows that the size of premade nanoparticle catalysts is not the only factor to control the outcome of synthesis. Other properties such as surface composition and morphology, overall composition and structure, and physical state of all the nanomaterials involved are important. It is thus crucial to carefully design and control the catalytic growth environment in order to synthesize complex new nanostructures. II. Experimental Section Cobalt nanoparticles have been used to produce SWNTs and SiNWs.2,3,17-20 In this work we chose Co nanoparticles as the catalysts to produce SiNCs. Two kinds of Co nanoparticles were made and used. All the chemicals used here were purchased from Aldrich without further treatment. One type of Co nanoparticle (type A) was made via a high temperature thermal decomposition method from Co2(CO)8 in a mixture of oleic acid (OA), trioctylphosphine oxide (TOPO), and trioctylamine (TOA).21,22 Depending on the relative amounts of the surfactants in the mixture, Co nanoparticles of different sizes were made. For example, this method produced 3-12 nm Co nanoparticles in anhydrous dichlorobenzene (σ-DCB) solutions. Another type of Co nanoparticle (type B) was also made.23 Specifically, 0.5 g of Co(CH3COO)2‚4H2O and 0.75 mL of oleic acid were dissolved in 20 mL of diphenyl ether. The mixture was gradually heated to 200 °C in an Ar atmosphere, at which point 1 mL of trioctylamine was added to the mixture and the temperature was then raised to 250 °C. Then 2 g of reducing agent (1,2-dodecanediol) dissolved in 5 mL of diphenyl ether in a separate flask and heated to 80 °C in a water bath was quickly injected into the mixture. The system was held at 250 °C for 30 min. After the solution was allowed to cool to room temperature, ethanol was added to precipitate the nanoparticles. The precipitate was recovered via centrifugation and washed with ethanol three times. The nanoparticles were soluble in organic solvents such hexane, dichlorobenzene (σ-DCB), and chloroform (CHCl3). As shown later, the nanoparticles made
10.1021/jp0607519 CCC: $33.50 © 2006 American Chemical Society Published on Web 04/05/2006
Silica Nanocoils
J. Phys. Chem. B, Vol. 110, No. 16, 2006 8297
Figure 1. TEM image of type B Co nanoparticles. The average size is 12 ( 4 nm. The scale bar is 50 nm.
in this report (12 nm diameter) were slightly smaller than that originally reported (15 nm diameter). Piranha [1:4 H2O2 (30%):H2SO4 (concentrated)] cleaned Si(100) wafers (Virginia Semiconductor) were used as substrates for nanoparticle deposition and also as the source of silicon for making SiNCs. The wafers were immersed in σ-DCB solutions of Co nanoparticles for 15 min. They were then removed from the solution, dried with argon gas, and placed in an airtight quartz tube in a high-temperature furnace (Lindburg/Mini-Mite). Hydrogen (99.95%) and argon (99.997%) gas were fed through the quartz tube. The total pressure was maintained at ∼1 psi above the ambient pressure, and the gas flow rates were held at 280 sccm for Ar and 210 sccm for H2. Temperature and reaction time were adjusted using the furnace controller. The gases were continuously fed during temperature changes, and the rate of temperature change between 1000 and 1100 °C was approximately 25 °C/min. The time specified in the growth diagram did not include any residence time. The grown samples were inspected with a scanning electron microscope (SEM, FEI XL-30 SFEG) at 5 keV of electron acceleration voltage. Transmission electron microscopy (TEM) was used to identify the Co nanoparticles and the structure of nanowires and nanocoils. High resolution (HR) TEM (Philips CM-300) at the National Center for Electron Microscopy (NCEM) and another HR-TEM (JEOL, JEM-2500SE) at UC Davis as well as a lower resolution TEM (Philips CM-12) located at UC Davis were employed to inspect the samples. Small area electron diffraction (SAED) was used to inspect the crystallinity of the silica nanowires in SiNCs. III. Results The results of type A Co nanoparticles were shown earlier.18 The average size of these spherical Co nanoparticles was 3 ( 1 and 12 ( 2 nm, indicating that the sample contained nanoparticles of two sizes. A TEM (CM-12) image of type B Co nanoparticles is shown in Figure 1. The average size is 12 ( 4 nm. Some nanoparticles were darker than others under TEM, implying that they were crystalline. Most nanoparticles adopted nonspherical shapes. There was strong experimental evidence that suggested that the growth of SiNCs was caused by the nanoparticle catalysts
Figure 2. SEM images of SiNCs and SiNWs. (A) SEM image of SiNCs made at 1035 °C for 30 s, the lowest temperature at which SiNCs can be grown with relatively good yield. Curly short nanowires were seen with metal nanoparticles at their tips. SEM image of SiNCs made at 1035 °C for 45 min (B) and SiNWs made at 1100 °C for 45 min (C). Scale bars are 1 µm.
at the ends of the nanowires suspended in space. Such observation implied that the growth followed a tip-growth mechanism. Figure 2A shows the result of SEM investigation after a short growth time (30 s at 1035 °C) using type A Co nanoparticles. Metal-containing nanoparticles, shown here as bright spots, were seen at the tips of the leading sections of short nanowires. Short SiNCs began to develop, as shown by the curved nanowires in Figure 2A. In many cases, crystalline cobalt silicide nanoparticles wrapped in a sheath of amorphous SiO2 were observed with the high-resolution TEM. Similar results were observed in other growth conditions.24 In general, the overall size of the catalyst heads at the tip of SiNCs was 10-100% larger than the average diameter of nanowires. The size of the metal-containing core in these heads was smaller, as revealed by high-resolution TEM. It is worth noting that because the samples were inspected after they were cooled to room temperature, these structural identifications may not reflect the true structure of the metal nanoparticles and their immediate environment during the growth. Future in situ studies will be needed in order to fully understand the structure of nanoparticle catalysts during the growth of SiNCs. SiNWs of different morphologies can be made from type A nanoparticles. For example, many SiNCs were visible when the samples were grown at 1035 °C. An example is shown in Figure 2B. These SiNCs were mixed with straight SiNWs. For the results shown in Figure 2B, most SiNCs were on the top of a thick layer of nanowires, which also contained a large quantity of straight SiNWs. It was difficult to directly estimate the yield of SiNCs in this case. On the other hand, predominantly (over 99%) straight SiNWs were made at 1100 °C (Figure 2C), and almost no SiNCs were detected. Although overwhelmingly straight SiNWs were grown at 1100 °C, more SiNCs were made between 1035 and 1050 °C using type A nanoparticle catalysts at a high deposition density of nanoparticles, as shown in Figure 2B. To further investigate the growth of SiNCs as a function of temperature, synthesis
8298 J. Phys. Chem. B, Vol. 110, No. 16, 2006
Qu et al.
Figure 4. High-resolution TEM images of SiNCs. Round cross section (A) and regular noncircular cross section (B) SiNCs are shown.
Figure 3. SEM image of a SiNW made from low-density type A Co nanoparticle deposition (A). The growth temperatures and times are shown in B (1050 °C for 10 min, 1100 °C for 10 min, 900 °C for a few min; repeated three times).The last growth was done at 1100 °C, and the last segment is a straight nanowire (#6). This was preceded by a coiled segment (#5) and then a straight segment (#4) and another highly coiled segment (#3). The first two segments of the nanowire were less visible (#2 and #1). The scale bar is 1 µm. Another coilstraight-coil SiNW is shown in C, as the result of a similar temperature varying growth experiment.
was performed at a much lower nanoparticle deposition density and varying growth temperatures. Figure 3A shows the result of the growth of a single SiNC produced at alternating temperatures between 1050 and 1100 °C. The temperature history is plotted in Figure 3B. The 700 °C cooling periods of 2-3 min were inserted into the growth process to make sure that growth was totally quenched after each heating cycle at 1100 °C. Because nanoparticle catalysts were observed at the tips of SiNCs, isolated from the wafer surface and other Co nanoparticles, they should remain unchanged during the whole growth period. Consequently, the same nanoparticle catalyst should be responsible for the growth of the whole nanowire shown in Figure 3A. Because it was the same nanoparticle, one may speculate that only the growth rate of the nanowire would change as the reaction temperature was lowered or raised. However, this was clearly not the case. As shown in Figure 3A, there are two distinct morphologies in this nanowire: two straight (#4 and #6) and two coiled (#3 and #5) segments, following closely the profile of the temperature history. Segments 1 and 2, grown during the first temperature cycle, were less visible. These results suggested that the same Co or cobalt silicide nanoparticle catalyst (shown at the end of #6 in Figure 3A) catalyzed the growth of coiled segments (#3 and #5) at 1050 °C and straight ones at 1100 °C (#4 and #6). Furthermore, even the coiled segments of this nanowire had two different
pitches: Segment 3 was slightly larger than segment 5. This difference was possibly caused by subtle changes in the nanoparticle catalyst during temperature cycling. An SEM image of another hybrid nanowire grown under a similar condition is shown in Figure 3C, and again coiled and straight sections are clearly observed. We found that while many of the cobalt nanoparticle catalysts at the tips of SiNCs after growth were crystalline (HR-TEM), the SiNCs were amorphous (HR-TEM and SAED). Energydispersive X-ray analyses (EDX) using both SEM and TEM were used to identify the composition of the samples, and the results showed that they were nearly SiO2. Furthermore, the cross-section of these SiNCs may adopt the conventional cylindrical or other patterned shapes, as shown in Figure 4A,B. This could imply that the as-grown SiNCs were crystalline, and they were subsequently oxidized to SiO2 when they were exposed in air. However, as shown by other work, even crystalline Si nanowires could be perfectly straight, demonstrating that it is more than the crystallinity of the nanowires that controls their morphology.25,26 The average diameter of SiNCs (53 ( 10 nm) was generally larger than that of straight nanowires (40 ( 5 nm) when type A Co nanoparticles were used. The yield of SiNCs made with type A nanoparticles at low deposition density was still low, generally on the order of 10%. When type B Co nanoparticle stock material was used under low nanoparticle deposition conditions and at ∼1050 °C, the yield of SiNCs was over 50%. An SEM image of one such sample is displayed in Figure 5. Even more interesting was that SiNCs of various helicities were made. In Figure 5, all eight coiled SiNCs had different helicities and coil diameters. The pitch of the tightest coil (#1) was 20 nm, with a 20-nm coil diameter. The diameter of the nanowire in this SiNC was less than 10 nm. This was one of the most tightly coiled SiNCs we observed so far. We did not observe a strong correlation between the coil diameter and the helicity, although it seemed that thicker nanowires are more tightly coiled, as sample #1 in Figure 5 may have suggested. SiNCs may also adopt a varying helicity within the same nanowire, as indicated by sample #8. Nanoparticles were visible on some SiNCs, as shown by the nanoparticle at the end of coil #4. When type B nanoparticles were used, the difference between the diameters of nanowires in coiled and straight nanowires was less than 5%. It is also possible to produce various shaped SiNCs. As shown in Figure 6, there are several shapes that are worth mentioning. We observed nanowires sharing a nanoparticle catalyst, resembling a thermal couple tip (Figure 6A). There were also SiNC double helices, one of which is shown in Figure 6B. Figure 6C shows a clean single SiNC that was several microns long. Samples of this format can facilitate mechanical property measurements using atomic force microscopy (AFM). Figure
Silica Nanocoils
J. Phys. Chem. B, Vol. 110, No. 16, 2006 8299
Figure 7. Four isolated individual SiNCs on a Si wafer grown from type B Co nanoparticles.
Figure 5. SEM image of eight SiNCs on a Si wafer. Type B Co nanoparticles were used. The scale bar is 200 nm.
Figure 8. Weight (fg) and length (mm) of SiNCs as a function of growth time (min). Both exhibit a near-linear relationship.
temperature. A small kink is visible near the transition point in Figure 6D, indicating a possible structural change at that place. We also used very low density deposition of type B nanoparticles to limit the coalescence of Co nanoparticles on the surface. In this case, only isolated SiNCs were visible, as shown in Figure 7. Each SiNC again possessed a different coil diameter and helicity or pitch. This growth configuration was suited for making prototype nanoscale devices from these SiNCs. The length of the SiNCs may not be a good parameter to define the growth rate because of the differences in pitch, coil, and nanowire diameters. Therefore, we used other parameters such as the length and volume of the nanowires in those SiNCs or the overall weight (assuming the density is the same as regular SiO2) of the SiNCs to calibrate the growth rate. Figure 8 shows the relationship between the growth time and the nanowire length and weight of the SiNCs. Both length and weight seemed to follow a near-linear relationship with the growth time. IV. Discussion
Figure 6. SEM images of several different SiNCs, including multiple SiNCs grown from the same nanoparticles (A), SiNC double helix (B), an isolated long SiNC (C), and a SiNC with two pitches (D). The scale bars are 500 nm in A and B and 1 µm in C and D.
6D shows another hybrid SiNC. In this case, the pitch changes from tight to loose in the middle of the growth even at the same
We performed yield measurements and found that almost no nanowires could be produced at temperatures below 1030 °C. Because more SiNCs were grown at a temperature close to the lowest possible temperature to produce any nanowires, we speculate that the growth of SiNCs was caused by anisotropy of the nanoparticles (metal or metal silicides) at near melting point (1035 °C). Since Si-Co bulk binary alloys have a melting point of 1239 °C, a ∼200 °C decrease in melting temperature indicates a moderate quantum effect for the nanoparticle catalysts used in this work.27 Because catalytic growth of SiNWs requires the migration of Si through catalysts,28-30 the result of no nanowires below 1030 °C implies that the rate of migration might be too low, thus suggesting that the melting point was below but very close to 1035 °C for most catalytic nanoparticles
8300 J. Phys. Chem. B, Vol. 110, No. 16, 2006 in the size range of 10-80 nm. As the temperature approaches the melting point, nanoparticles may not take the form of molten spheres to produce straight nanowires (symmetric with respect to the growth axis). Instead, the inhomogeneity in these nanoparticles at near melting point results in anisotropic growth, which makes the nanowires coil. As a result, nanowires produced at near melting point are more coiled, and those produced at a higher temperature at which the nanoparticle catalysts are totally molten are straight. For smaller nanoparticles, the melting temperatures are more likely lower than 1035 °C, due to a larger quantum effect, and hence they only catalyze the growth of straight nanowires. However, one cannot rule out the possibility of impurities in those small nanoparticles, which may result in an increased melting temperature. This may explain why some SiNCs were made of small diameter nanowires. Other possibilities include the contact angle anisotropy (CAA), which may control the growth even when the nanoparticles are molten.31 The improvement of type B Co nanoparticle over type A in catalyzing the growth of SiNCs may be understood as follows. When type A nanoparticles were used, many of them had to undergo coalescence to form larger nanoparticles in order to make SiNCs at ∼1050 °C because of the requirement for anisotropy at near melting temperature. Coalescence could happen when the substrate temperature was raised above 500 °C, as shown by an in situ TEM study.32 As a result, uncoalesced small nanoparticles in the sample (having a lower melting temperature) catalyzed the growth of small diameter straight SiNWs at 1035 °C. Coalesced larger nanoparticles, on the other hand, catalyzed the growth of SiNCs at 1035 °C, as shown in Figure 2. At 1100 °C, all the nanoparticles catalyzed the growth of straight SiNWs. For type B Co nanoparticles used in this work, few nanoparticles were smaller than 10 nm. Therefore, the yield of SiNCs using type B Co nanoparticles was relatively higher. The results presented here also suggest that the same nanoparticle can be responsible for the growth of straight SiNWs and SiNCs at two different temperatures. More precisely, nanoparticles in the size range of 20-50 nm may favor the synthesis of SiNCs at low growth temperatures near 1050 °C. On the other hand, smaller size nanoparticles (below 20 nm) most likely only catalyze the growth of straight SiNWs. It is evident that most nanoparticles would catalyze the growth of straight nanowires at 1100 °C. As shown in Figure 6D, the growth may be complex and influenced by the subtle changes to nanoparticle catalysts even at the same temperature. Because these samples were produced from regular Si wafers and were only inspected with SEM, new samples grown on ion-milled Si wafers may be needed for high-resolution TEM inspection to identify the subtle structural changes that occurred to these SiNCs. High-yield SiNCs are generally more difficult to make than straight SiNWs because the growth conditions have to be tightly controlled. If the size of the nanoparticles and growth temperature and the compositions are correctly arranged, then SiNCs can be made. As described earlier, some of these conditions were met in the laser vaporization experiments or via the addition of a trace amount of methane, respectively. To grow high-yield SiNCs with controlled nanowire diameter, pitch, and even helicity, however, control over the size of nanoparticles and growth temperature have to be carefully exercised. For this purpose, employing premade nanoparticles under chemical vapor deposition or similar conditions as presented here is more favorable.
Qu et al. As mentioned before, the nanoparticles observed at the tips of these short nanowires were most likely not the same as those originally deposited on the Si wafer, because those nanoparticles might migrate, sinter, and break up on the surface en route to high-temperature growth conditions. Future work is needed to study the synthesis and anchoring of uniformly sized nanoparticle catalysts to guarantee controlled growth of only SiNCs. The growth mechanisms will have to be studied with in situ methods such as in situ TEM or X-ray absorption spectroscopy because the structures of nanoparticles and SiNCs at the growth temperature are needed.33,34 The coiled and straight hybrid silicon-based nanowires with varying pitches can be used as composite nanomaterials or templates for making other nanomaterials. SiNCs of different tensile strengths can be used in nanoscale mechanical, photonic, and electronic devices. Furthermore, functionalized small coil diameter SiNCs may be developed to mimic the function of biological macromolecules. Acknowledgment. This work is partially supported by NSF CAREER award (CHE0135132) and University of California Campus-Laboratory Exchange grants. We thank the National Center for Electron Microscopy (NCEM) for their support of this work. We thank Drs. C. Song and V. Radmilovic at NCEM for their assistance. The NCEM is a DOE facility. We thank D. Masiel, Dr. C. Mitterbauer, and Prof. N. Browning at UC Davis for their assistance in TEM measurements. J.D.C. thanks Tyco for the Tyco Electronic Fellowship for Research in Fundamental Materials. References and Notes (1) Iijima, S.; Ichihashi, T. Nature 1993, 364, 737. (2) Bethune, D. S.; Kiang, C. H.; Devries, M. S.; Gorman, G.; Savoy, R.; Vazquez, J.; Beyers, R. Nature 1993, 363, 605. (3) Guo, T.; Nikolaev, P.; Thess, A.; Colbert, D. T.; Smalley, R. E. Chem. Phys. Lett. 1995, 243, 49. (4) Morales, A. M.; Lieber, C. M. Science 1998, 279, 208. (5) Wu, Y. Y.; Yan, H. Q.; Huang, M.; Messer, B.; Song, J. H.; Yang, P. D. Chem.-A Eur. J. 2002, 8, 1261. (6) Alivisatos, A. P. Science 1996, 271, 933. (7) Zhang, H. F.; Wang, C. M.; Buck, E. C.; Wang, L. S. Nano Lett. 2003, 3, 577. (8) Tang, Y.; Zhang, Y.; Wang, N.; Lee, C.; Han, X.; Bello, I.; Lee, S. J. Appl. Phys. 1999, 85, 7981. (9) Gao, P.; Ding, Y.; Mai, W.; Hughes, W.; Lao, C.; Wang, Z. Science 2005, 309, 1700. (10) Kuzuya, C.; In-Hwang, W.; Hirako, S.; Hishikawa, Y.; Motojima, S. Chem. Vapor Depos. 2002, 8, 57. (11) Qin, Y.; Jiang, X.; Cui, Z. J. Phys Chem B 2005, 109, 21749. (12) Varadan, V.; Xie, J. Smart Mater. Struct. 2002, 11, 728. (13) Yang, S.; Chen, X.; Motojima, S.; Iwanaga, H. J. Nanosci. Nanotechnol. 2004, 4, 167. (14) Chen, X.; Zhang, S.; Dikin, D.; Ding, W.; Ruoff, R.; Pan, L.; Nakayama, Y. Nano Lett. 2003, 3, 1299. (15) Ago, H.; Nakamura, K.; Imamura, S.; Tsuji, M. Chem. Phys. Lett. 2004, 391, 308. (16) Kiang, C. H. J. Phys. Chem. A 2000, 104, 2454. (17) Carter, J. D.; Qu, Y.; R., P.; Hoang, L.; Masiel, D. J.; Guo, T. Chem. Commun. 2005, 2274. (18) Carter, J. D.; Cheng, G.; Guo, T. J. Phys. Chem. B 2004, 108, 6901. (19) Carter, J. D.; Shan, F.; Guo, T. J. Phys. Chem. B 2004, 109, 4118. (20) Cheng, G.; Guo, T. J. Phys. Chem. B 2002, 106, 5833. (21) Puntes, V. F.; Krishnan, K. M.; Alivisatos, A. P. Science 2001, 291, 2115. (22) Cheng, G.; Carter, J. D.; Guo, T. Chem. Phys. Lett. 2004, 400, 122. (23) Wu, N.; Fu, L.; Su, M.; Aslam, M.; Wong, K.; Dravid, V. Nano Lett. 2004, 4, 383. (24) Li, C.; Wang, N.; Wong, S.; Lee, C.; Lee, S. AdV. Mat. 2002, 14, 218. (25) Sharma, S.; Sunkara, M. Nanotechnology 2004, 15, 130.
Silica Nanocoils (26) Holmes, J. D.; Johnston, K. P.; Doty, R. C.; Korgel, B. A. Science 2000, 287, 1471. (27) Massalski, T. B.; Okamoto, H.; Subramanian, P. R.; Kacprzak, L. Binary Alloy Phase Diagrams; ASM International: Materials Park, OH, 1990; Vol. 2. (28) Wagner, R. S.; Ellis, W. C. Appl. Phys. Lett. 1964, 4, 89. (29) Givargizov, E. J. Alloys Compds. 1975, 31, 20. (30) Johansson, J.; Svensson, C.; Martensson, T.; Samuelson, L.; Seifert, W. J. Phys. Chem. B 2005, 109, 13567.
J. Phys. Chem. B, Vol. 110, No. 16, 2006 8301 (31) McIlroy, D.; Alkhateeb, A.; Zhang, D.; Aston, D.; Marcy, A.; Norton, M. J. Phys.-Condensed Matter 2004, 16, R415. (32) Guo, T. Dual Catalytic Role of Co Nanoparticles in Bulk Synthesis of Si-Based Nanowires; Kluwer Academic: Norwell, MA, 2005; Vol. 3. (33) Persson, A.; Larsson, M.; Stenstrom, S.; Ohlsson, B.; Samuelson, L.; Wallenberg, L. Nat. Mater. 2004, 3, 677. (34) Helveg, S.; Lopez-Cartes, C.; Sehested, J.; Hansen, P.; Clausen, B.; Rostrup-Nielsen, J.; Abild-Pedersen, F.; Norskov, J. Nature 2004, 427, 426.
