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2007, 111, 14612-14616 Published on Web 09/19/2007
Pressure-Induced Single-Walled Carbon Nanotube (n,m) Selectivity on Co-Mo Catalysts Bo Wang,† Li Wei,† Lu Yao,† Lain-Jong Li,‡ Yanhui Yang,† and Yuan Chen*,† School of Chemical and Biomedical Engineering, Nanyang Technological UniVersity, 637459, Singapore, and School of Materials Science and Engineering, Nanyang Technological UniVersity, 639798, Singapore ReceiVed: August 4, 2007; In Final Form: August 30, 2007
We report the selective growth of bulk single-walled carbon nanotube (SWCNT) samples enriched with three different dominant chiralities including (6,5), (7,5), and (7,6) through adjusting the pressure of carbon monoxide on Co-Mo catalysts from 2 to 18 bar. Semiconducting SWCNT abundance was evaluated via photoluminescence, optical absorption, and Raman spectroscopy based on a single-particle tight-binding theoretical model, indicating that the abundance of each chiral tube can be systematically altered by changing the carbon monoxide pressure.
One-dimensional SWCNT structures offer great potential in nanoelectronic applications. Their electronic and optical properties strongly depend on their diameter and chiral angle, which are identified as the chiral indices (n,m). Selective synthesis of single-walled carbon nanotubes (SWCNTs) with desired chiralities, which have specific electronic and optical properties, is one of the major challenges in nanotube science and applications.1-3 However, no growth method has been able to achieve precise control of SWCNT chirality. Identifying important factors in SWCNT chiral control is the first critical step to achieve the selective growth. Most synthesis methods result in a wide distribution of chiral species,4,5 even though narrower chiral distribution SWCNTs can be obtained on Co-Mo6 and CoMCM-417-9 catalysts. Various separation approaches have been developed for sorting SWCNTs by diameter, metallicity, and chirality,10-13 yet there is still a long way to the ultimate goal of total fractionation of a given nanotube mixture into its single chirality components.14 Selective synthesis of SWCNTs with narrow chiral distribution of specific chirality can provide effective starting materials for separation studies. Despite that, serious efforts have been undertaken to determine the nanotube growth mechanism; a detailed understanding of chiral-control mechanism during SWCNT growth is still not available. Several recent dynamic simulation and in situ observation studies15-18 have suggested that chiral selectivity may be related dynamically to catalyst particle-carbon network interactions. Many factors could influence these interactions, for example, growth temperature,19,20 catalytic supports,19 and carbon precursors.19,21 Although several experimental studies6-9,19-21 have demonstrated that narrow chiral distribution may be predominant in the same higher chiral angle region and that the chiral distribution can be changed, conditions required for selective growth of specific single chiral tubes remain unclear. We reason that because the chiral selectivity is a dynamic process, we may alter the chiral selectivity by changing the carbon feed rate to * To whom correspondence should be addressed. E-mail: chenyuan@ ntu.edu.sg. † School of Chemical and Biomedical Engineering. ‡ School of Materials Science and Engineering.
10.1021/jp0762525 CCC: $37.00
nanometallic clusters, which serve as “seeds” for SWCNT growth. From two catalysts, Co-Mo and Co-MCM-41, which provide the narrowest SWCNT chirality distribution as reported so far,6,9 we selected the Co-Mo catalyst in this study for its simplicity in preparation. The preparation of Co-Mo catalyst and SWCNT synthesis method were modified from previous reported conditions.22,23 The silica- (Sigma-Aldrich SiO2 with 6 nm average pore size and Brunauer-Emmett-Teller surface area of 480 m2 g-1) supported Co-Mo catalysts (4.6 wt % Mo at a 1:3 Co/Mo molar ratio) were prepared by incipient wetness impregnation of an aqueous solution of cobalt nitrate and ammonium heptamolybdate as described in ref 21. In a typical SWCNT growth experiment, 200 mg of calcined Co-Mo catalysts were prereduced under 1 bar flowing H2 (80 sccm) using a temperature ramp of 10 °C/min to 500 °C. Once the temperature reached 500 °C, the reactor was purged using flowing Ar (250 sccm), while the temperature was further increased to 800 °C. A specially designed chemical vapor deposition (CVD) reactor was used in this study that allows the pressure to be increased from 1 bar to a desired pressure (up to 19 bar) in less than 3 s, avoiding SWCNT growth heterogeneity during the pressureincreasing period. The pressured CO (the carbonyls were eliminated by Nanochem Purifilter by Matheson Gas Products) flow of 50 sccm was kept for 30 min. Multiple runs under identical conditions were tested to validate the run-to-run fluctuation versus differences among runs under different pressures. The synthesized SWCNTs were refluxed in 1.5 mol/L NaOH solution to dissolve the silica supports and filtered by nylon membrane with 0.22 µm pore size to remove silica dissolved in NaOH. The purified SWCNTs were suspended in 1% sodium dodecyl benzene sulfonate (SDBS)/D2O (99.9 atom % D, Sigma-Aldrich) solution and sonicated using a cup-horn ultrasonicator (SONICS, VCX-130) at 20 W for 30 min. The solution was kept in an ice bath during sonication to avoid the sonication-induced changes in chiral distribution as pointed out by Strano et al.24 A series of controlled experiments21 using four different surfactants for various sonication durations were tested. (n,m) abundance distribution is not affected by different © 2007 American Chemical Society
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Figure 1. Pressure-induced single-walled carbon-nanotube (n,m) selectivity on Co-Mo catalyst monitored by fluorescence. Normalized photoluminescence intensity map as a function of excitation and emission wavelengths for SDBS-micellarized SWCNT samples in D2O. Locations of three dominate chiral tubes (6,5), (7,5), and (7,6) are shown. Pressures marked on the graph indicate the CO pressures under which SWCNTs were produced.
surfactants. After ultrasonication, the suspension was centrifuged for 1 h at ca. 53 000 g to remove the catalyst particles and other carbon impurities. Fluorescence spectroscopy measurements were conducted on a Jobin-Yvon Nanolog spectrofluorometer equipped with an IGA (InGaAs) near-infrared (NIR) detector. The intensities were corrected and normalized for instrumental variations in excitation intensity and detection sensitivity. The UV-vis-NIR absorption spectra were measured on a Varian Cary 5000 UV-vis-NIR spectrophotometer using the same SWCNT solutions as those used in Fluorescence spectroscopy. A 1% SDBS/D2O solution was used as a blank reference to be subtracted during the absorption measurement. A 2 nm bandpass and a 0.5 s integration time was adapted to obtain clean spectra. SWCNTs on filter membrane were also characterized by multiexcitation wavelength Raman spectroscopy. Spectra were collected with a Renishaw Ramanscope in the backscattering configuration using 514.5 nm (2.41 eV), 633 nm (1.96 eV), and 785 nm (1.58 eV) laser wavelengths over five random spots on each sample. Laser energies of 2.5-5 mW were used to prevent destroying SWCNT samples during measurement. Integration times of 20 s were adapted. As-grown SWCNTs on silica were also pressed into thin wafers and investigated by Raman spectroscopy; no significant difference was found in their spectra compared with SWCNTs on filter membrane. Figure 1 illustrates the photoluminescence (PL) intensity map for SWCNTs with excitation scanned from 500 to 800 nm and emission collected from 850 to 1350 nm. The resonance behavior of both excitation and emission events results in spikes corresponding to the transition pair from individual semiconducting (n,m) SWCNTs.25 Figure 1 suggests that SWCNTs of narrow chiral distribution are produced under CO pressure from 2 to 18 bar. Dominant PL peaks are originated from the (6,5),
(7,5), and (7,6) SWCNTs. The pressure-induced chiral selectivity can be identified among these three chiral tubes. (6,5) has the highest intensity under 18 bar CO, and its intensity decreases when the CO pressure decreases from 18 to 2 bar. In the contrast, (7,6) has the highest intensity under 2 bar CO, and its intensity goes down with the increase of CO pressure. The changing trend of (7,5) intensity lies in the middle, showing the highest intensity around 12 bar CO pressure. Initial studies have directly applied the PL intensity to determine the abundance of individual chiral SWCNTs.6,25 However, theoretical studies26,27 indicate that different chiral tubes possess different PL efficiencies. A recent study9 suggests that a theoretical model26 (based on the single-particle tightbinding theory) provides good analytical predictions about the relative PL quantum efficiency and absorption extinction coefficients of various chiral SWCNTs. It could be applied to evaluate the abundance profiles for semiconducting SWCNTs. We determined the strength of chiral tubes from Figure 1 by the amplitude of the partial derivative, as recommended by Arnold et al.,13 to minimize the effects of the slowly varying background. The relative PL intensities, calculated PL quantum yields, and (n,m) abundances for SWCNTs produced under different pressures are listed in Supporting Information Table S1. Several other (n,m) species at lower chiral angles, such as (8,3), (8,4), (9,4), and (8,6), also have pronounced PL intensity. However, their intensities are significantly lower than those from the three dominant species at higher chiral angles. Figure 2 shows that we can systematically increase the abundance of (6,5) tubes from less than 2% (among semiconducting tubes identified in PL analysis) to 48% by increasing the pressure from 2 to 18 bar. The abundance of (7,6) tubes decreases from 26% to 7% when CO pressures are altered in the same range. The (7,5)
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Figure 3. Optical absorbance spectra of SWCNTs produced under different CO pressures. The relative strengths of the E11 and E22 transition for semiconducting (6,5) (highlighted in blue), (7,5) (highlighted in green), and (7,6) (highlighted in red) tubes indicate that their abundances change with CO pressure variation.
Figure 2. Abundances of dominant (n,m) chiral tubes systematically altered with changing carbon monoxide pressure. Chirality map of aqueous suspensions of SWCNTs indicates three dominate chiral tubes identified in PL analysis ((6,5), (7,5), and (7,6)) and other tubes observed. The abundance evaluation is based on the single-particle tightbinding theory model26 considering the fluorescence quantum yield differences among semiconducting tubes.
tube abundance shows a volcano-shaped correlation with pressure, and the maximum abundance of (7,5) exists around 12 bar at 28%. The content of secondary (n,m) tubes identified in PL analysis (Figure 2) also shows the pressure dependence. Abundance of (8,3) tubes increases with pressure similar as that of (6,5) tubes. Abundance of (8,6) and (9,4) tubes decreases with pressure. And (8,4) tubes have the very similar pattern as that of (7,5) tubes. The relative PL intensity of zigzag, small chiral angle, and large diameter semiconducting SWCNTs is significantly lower than that from large chiral angle and small diameter SWCNTs.28-30 The absence of PL signal from these tubes in our samples may be due to their low PL quantum efficiency.26 To reconcile our findings, UV-vis-NIR absorption spectra of SDBS-micellarized SWCNT samples were taken. Figure 3 suggests that the three dominant chiral tubes identified in PL analysis are the main species produced. Their abundances vary with CO pressure in the same pattern as that in PL analysis. Deconvolution of the absorption spectra of SWCNTs into distinct chiral contribution is complicated because their transition energies are closely spaced. We used the abundance of each (n,m) tube determined in PL analysis to reconstruct the near-infrared Es11 absorption spectra. A good reconstruct was obtained when we included zigzag and smaller chiral tubes at the average abundance level of minor tubes identified in PL analysis, other than including them as the same abundance as dominant high chiral angle tubes.21 It reaffirms that the main (n,m) species produced are those high chiral angle tubes identified in PL analysis.
We also intended to quantify the (n,m) abundance using Raman spectroscopy. Raman spectra were taken for SWCNT samples under 514.5, 633, and 785 nm laser wavelengths shown in Supporting Information Figures S1-S3. Radical breathing mode peaks in Raman spectra suggest that smaller diameter tubes are produced under higher CO pressure. Yao et al.20 have used Raman spectroscopy to monitor the diameter and chirality variation of individual tubes with growth temperature changes. However, the abundance evaluation of (n,m) tubes for bulk SWCNT samples are more complicated requiring both continuous laser excitation Raman spectroscopy and intrinsic properties of each chiral species. Supporting Information Figure S4 indicates that the three laser energies we have used are not sufficient to determine the population of semiconducting tubes identified in PL analysis by Raman spectroscopy. To understand the mechanism of (n,m) selectivity under different CO pressure, we compared the SWCNT growth under the pure CO of 8 bar and a mixture (CO/Ar ) 1:1) of 16 bar. PL intensity maps are shown in Figure 4. The result shows that the similar chiral distribution is obtained under these two conditions. It suggests that the partial pressure of CO other than the total pressure in the CVD reactor is the determining factor in (n,m) selectivity. The (n,m) selectivity also appears to start at the early stage of the SWCNT structure formation. Because we introduced CO pressure gradually (in 1 min rather than 3 s) from 1 bar to the desired pressure, the distinct difference among various pressures was blurred. We also found that the chiral distribution of SWCNTs produced under 6 bar CO (pressureincreasing period is approximate 30 s)21 in a previous study is similar to that of SWCNTs produced under 4 bar CO in this study (pressure-increasing period is less than 3 s). Robertson et al.31 has proposed that the SWCNT growth on a size-fixed metallic cluster has four steps: (1) adsorption of the gas precursor molecule on the metal cluster surface, (2) dissociation of the precursor molecule, (3) diffusion of the adsorbed species on (or in the case of large particles, into) the catalyst particle, and (4) nucleation and incorporation of carbon into the growing structure. We reason that the direct impact of increasing CO pressure is to increase the striking coefficient of CO molecules on the Co clusters. It may lead to a faster dissociation rate of CO, releasing more active carbon atoms to interact with Co clusters. Theoretical calculations have shown
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Figure 4. Normalized photoluminescence (PL) intensity map as a function of excitation and emission wavelength for SWCNT samples produced (left) under pure CO of 8 bar and (right) under a gas mixture (CO/Ar ) 1:1) at 16 bar.
that the binding energies of CO to cobalt nanoparticles and preferred chemisorption sites depend on the cluster size and surface coverage.32 Maruyama et al.33 and Resasco et al.34 have proposed that carbon caps are formed when the catalytic clusters reaches saturation with carbon atoms. SWCNT growth may be initiated faster under higher CO pressure because of the higher concentration of active carbon atoms available, which leads to smaller diameter tubes like (6,5) and (8,3). When CO pressure is lower, it may take a longer period of time for enough active carbon atoms to form a stable carbon cap structure on a Co cluster surface. Because Co clusters are continuously aggregating during SWCNT growth owning to the high-temperature sintering, Co clusters might grow larger before the carbon caps could be stabilized on their surfaces. When the dynamic interaction between carbon caps and Co clusters is stabilized under lower CO pressure, larger diameter SWCNTs like (7,6) and (9,4) are produced. On the other aspect, the concentration of large chiral angle tubes determined in fluorescence (such as (6,5), (7,5), and (7,6)) is much higher than their small chiral angle tube counterparts (such as (8,3), (8,4), and (9,4), respectively) as shown in Figure 2. A theoretical calculation16 has compared the total energy of nanotube caps on a Ni(111) surface and found that large chiral angle tubes such as (7,5) and (7,6) have lower formation energies compared with small chiral angle tubes. On the basis of this calculation,16 Reich et al. proposed that chiral selectivity must start during nucleation, and once a cap is formed it is quenched in and grows into a unique nanotube. The selectivity of larger chiral angle tubes shown in our study may be owing to the higher stability between large chiral angle carbon caps with Co clusters surface. However, further work is needed to clarify how the carbon monoxide pressure plays a role in the dynamic process of (n,m) selection. A strategy to obtain SWCNT samples enriched with smaller large chiral tubes (such as (5,4) or (6,4)) or larger diameter large chiral tubes (such as (8,6) or (8,7)) is to further increase or decrease the active carbon atoms’ concentration on the cobalt cluster surface while at the same time sustaining the activity of Co clusters by novel catalyst design. Such work is in process in our lab. In summary, we have discovered that CO pressure is a critical factor in SWCNT (n,m) control for Co-Mo catalysis. We are able to obtain bulk SWCNT samples enriched with three dominant tubes at (6,5), (7,5), and (7,6). Semiconducting tube abundance was evaluated by photoluminescence and absorption
spectroscopy. Selective growth of SWCNTs enriched with different chiral tubes can be achieved. By increasing the pressure from 2 to 18 bar, we can increase the amount of the (6,5) with respect to the amount of the (7,6). Using the strategy reported here, it is feasible to map out particular growth conditions required for individual (n,m) tubes. Combining with various separation technologies, it is expected that SWCNTs with monodisperse electronic and optical properties can be obtained for various technological applications. Acknowledgment. This project is supported by the Nanyang Technological University CoE-SUG and AcRF Grant RG38/ 06 and RG106/06. Supporting Information Available: Tabulated values of the relative photoluminescence intensities, calculated PL intensities from the electrophonon interaction model,26 and (n,m) abundance for SWCNT samples produced under different pressures; Raman spectra and analysis of SWCNT samples under 514.5, 633, and 785 nm laser excitation. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Dresselhaus, M. S.; Dresselhaus, G.; Avouris, P., Eds.; Carbon Nanotubes: Synthesis, Structure, Properties, and Applications. Topics in Applied Physics 80; Springer: Berlin, 2001; p 447. (2) Tans, S. J.; Verschueren, A. R. M.; Dekker, C. Room-temperature transistor based on a single carbon nanotube. Nature (London) 1998, 393 (6680), 49-52. (3) Gruner, G. Carbon nanonets spark new electronics. Sci. Am. 2007, 296 (5), 76-83. (4) Henrard, L.; Loiseau, A.; Journet, C.; Bernier, P. What is the chirality of singlewall nanotubes produced by arc discharge? An electron diffraction study. Synth. Met. 1999, 103 (1-3), 2533-2536. (5) Nikolaev, P.; Bronikowski, M. J.; Bradley, R. K.; Rohmund, F.; Colbert, D. T.; Smith, K. A.; Smalley, R. E. Gas-phase catalytic growth of single-walled carbon nanotubes from carbon monoxide. Chem. Phys. Lett. 1999, 313 (1,2), 91-97. (6) Bachilo, S. M.; Balzano, L.; Herrera, J. E.; Pompeo, F.; Resasco, D. E.; Weisman, R. B. Narrow (n,m)-distribution of single-walled carbon nanotubes grown using a solid supported catalyst. J. Am. Chem. Soc. 2003, 125 (37), 11186-11187. (7) Chen, Y.; Ciuparu, D.; Lim, S.; Yang, Y.; Haller, G. L.; Pfefferle, L. Synthesis of uniform diameter single-wall carbon nanotubes in Co-MCM41: effects of the catalyst prereduction and nanotube growth temperatures. J. Catal. 2004, 225 (2), 453-465. (8) Chen, Y.; Ciuparu, D.; Lim, S.; Yang, Y. H.; Haller, G. L.; Pfefferle, L. Synthesis of uniform diameter single wall carbon nanotubes in Co-MCM41: effects of CO pressure and reaction time. J. Catal. 2004, 226 (2), 351362.
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