Growth of Carbon Structures on Stepped (211)Co Surfaces - The

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Growth of Carbon Structures on Stepped (211)Co Surfaces Gustavo E. Ramı´rez-Caballero,† Juan C. Burgos, and Perla B. Balbuena* Department of Chemical Engineering, Materials Science and Engineering Program, Texas A&M UniVersity, College Station, Texas 77843 ReceiVed: March 30, 2009; ReVised Manuscript ReceiVed: May 3, 2009

Density functional theory is used to evaluate the adsorption of carbon on stepped (211) cobalt surfaces. It is found that the 4-fold step hollow sites on (100) planes are the most stable adsorption sites for carbon, followed by the 3-fold hcp sites located in (111) terraces where adsorption per carbon atom is 0.7-0.9 eV less stable than that on the step sites. When the carbon concentration over the surface increases, adsorption of carbon chains is also favorable, and at even higher carbon pressures, interaction of adsorbed chains may lead to the formation of graphene sheets parallel to the (100) plane or to the formation of horizontally aligned seminanotubes. Formation of these carbon structures is accompanied by oxidation of the cobalt atoms, especially those forming the 4-fold step hollow site, whereas cobalt terrace sites become negatively charged. We discuss the significance of our results in relation to the catalyzed growth of single-walled carbon nanotubes on cobalt nanoparticles. 1. Introduction One of the successful techniques for producing single-walled carbon nanotubes (SWCNTs) is the chemical vapor deposition method1 where a carbon-containing precursor gas (such as CO) flowing over a metal catalyst (Co, Fe, Ni, and others) at temperatures in the order of 1000K2,3 is decomposed, yielding carbon atoms that may diffuse inside the catalyst and eventually precipitate on the surface, forming specific carbon structures. Although the synthesis process is generally understood, details of the catalytic mechanism are still debated. One of the current challenges is trying to understand the origin of the selectivity of certain synthesis processes toward producing SWCNTs in a narrow range of chiralities.4 The chiral angle of a SWCNT is geometrically determined by rolling over a hypothetical graphite sheet to form the nanotube structure.5 Chirality, designated by the chiral indexes (n, m),6 is a very important feature because SWCNTs’ physical and chemical properties are strongly dependent on it, and therefore, its control would lead to a more specialized design of nanodevices. Recent advances in surface science techniques allow gains into the evolution of the microscopic structure of the catalyst and the nascent tubes during reaction.7,8 Also, classical9-11 and ab initio molecular dynamics12 and density functional theory13-15 studies play a significant role in elucidating important aspects of the growth mechanism. Both theoretical and experimental studies have suggested that the nature of the catalytic surface-mainly chemical composition and exposed crystallographic surface-may be closely related to the type of growing carbon nanostructure. Moreover, we have recently suggested that the catalytic metal surface, generally flexible at the high temperatures of the synthesis, is able to reconstruct following the pattern of the growing chiral carbon structure.14,15 However, the opposite is also possible: a chiral surface might lead to the formation of a chiral carbon structure compatible with its pattern. Moreover, both effects might operate simultaneously. * Corresponding author. E-mail: [email protected]. † Permanent address: Departamento de Ingenieria Quimica, Universidad Industrial de Santander, Bucaramanga, Colombia.

To investigate the role of the surface on the formation of carbon structures, we have chosen to analyze a stepped surface: Co(211). We, first, determine the most favorable sites for adsorption of single carbon atoms. Then we examine the adsorption of carbon chains under increasing carbon concentration. We discuss our results in relation to the formation of carbon structures over specific surface patterns. 2. Computational and System Details Calculations were performed within the framework of density functional theory (DFT) using the Vienna ab initio simulation package (VASP),16-19 which is a DFT code based on planewave basis sets. Electron-ion interactions are described using the projector-augmented wave (PAW) method,20 which was expanded within a plane-wave basis setting up to a cutoff energy of 400 eV. Electron exchange and correlation effects were described by the Perdew-Burke-Ernzerhof (PBE),21,22 a generalized gradient approximation (GGA)-type exchange-correlation function. Spin polarization was included in every simulation. The convergence criterion for the electronic self-consistent iteration was set to 10-4 eV and the forces were converged to 0.01 eV/Å, respectively. Brillouin zone integration was performed using a 9 × 9 × 1 Monkhorst-Pack grid23 and a MethfesselPaxton24 smearing of 0.2 eV. A Co(211) surface was modeled using 2 × 2 supercells with 24 total number of atoms. The systems consist of a four-layer slab model; the two layers at the bottom of the slab are fixed, whereas the other two layers, those at the top of the slab, are allowed to relax. The (211) surface has (111) terraces separated by monatomic (100) steps, as shown in Figure 1. The slab was taken as infinite in the x and y directions and finite in the z direction; periodic boundary conditions were used in the three directions. A vacuum space of 12 Å was included in the cell to separate the slab from those at the upper and lower cells, thus ensuring no interactions between the adsorbed species and the bottom surface of the next slab. We note that bulk Co is a hexagonal close-packed (hcp) structure;25 however, small clusters of diameters lower than 20 nm have been found to adopt a face-centered cubic (fcc) structure26 and similarly at temper-

10.1021/jp902878q CCC: $40.75  2009 American Chemical Society Published on Web 05/29/2009

Carbon Structures on Stepped (211)Co Surfaces

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Figure 1. Side view of Co(211) surface with (111) terraces separated by monatomic (100) steps.

atures above 450 °C, such as those in most SWCNT synthesis processes.27 Therefore, such an fcc structure is adopted here for the slabs, and the optimum lattice constant obtained from bulk DFT calculations of fcc Co was determined as 3.51 Å, in reasonable agreement with the experimental lattice constant (3.54 Å) for R-Co.28 The adsorption energies, Ead, of carbon were calculated using the following equation

Ead )

Eslab with adsorbed C - (Eclean slab + N · Ecarbon) N

(1)

where Eslab with adsorbed C stands for the total energy of the interacting Co(211) surface with N adsorbed C atoms, Eclean slab is the total energy of the bare Co(211) slab, and Ecarbon is the energy of gas-phase C in its ground state. Negative Ead values indicate favorable (exothermic) adsorption. To calculate the total electronic charge of an atom, we used the Bader analysis.29,30 This analysis defines an atom based on the electronic charge density using zero flux surfaces to divide atoms; the total electronic charge of an atom is approximately the charge enclosed within the Bader volume defined by zero flux surfaces. 3. Results and Discussion 3.1. Carbon Adsorption on Co(211). To find the most stable sites for adsorption of individual carbon atoms on the stepped Co(211) surface, several sites located over the terrace and over the step were chosen as initial locations. The total displacement of the C atom toward more energetically favorable sites and the changes in the total charge of each Co atom belonging to the top surface layer were registered after relaxation. Twelve sites were tested in total, as shown in Figure 2: three top positions were initially located approximately at 1.40 Å over

Co atoms in the upper layer (Top 1-3), two bridge sites (Bridge-1 and -2) were located over interatomic bonds along to the (111) terrace, and a third bridge site (Bridge-3 in Figure 2a) was positioned besides the bond that connects a terrace atom with an atom in the step edge (represented by purple spheres in Figure 2a,b). A fourth bridge site (Bridge-4) was tested on a bond connecting atoms over the step edge, as shown in Figure 2b. To determine hollow positions, the (111) terrace was divided in two zones: the top zone starts on the atoms that belong to the step edge and finishes on the atoms located at the middle of the terrace, and the bottom zone ends on the atoms located in the Top-3 sites in the lowest row of the monatomic (100) step. Each of these zones has two types of 3-fold hollow sites, hcp and fcc. A 4-fold hollow site is formed by two atoms from the step edge and two from the terrace (Figure 2b); we designate this hollow site as step. After relaxation, the twelve initial configurations previously mentioned were reduced to just five final atomic positions for carbon atoms that corresponded to the minimal energy configurations. None of the bridge sites were stable; in all of them, C atoms relaxed to more stable hollow sites. The C atom located at the Bridge-3 position, as well as that starting in an fcc-bottom hollow, was displaced toward their neighboring step hollow site, shown in Figure 3a (top view). The same position was kept by the C atom initially located in the step hollow site. C atoms initially located at a Bridge-1 or at a Bridge-4 site, as well as those at a Top-2 site, all converged to the same hcp-top hollow stable site where, also, the C atom that started at that position was found after relaxation, as shown in Figure 3b (top view). hcp hollow sites became strong favorable sites for adsorption of individual carbon atoms, considering that the other hcp hollow located in the bottom zone of the terrace (hcp-bottom) also remained at its initial position after relaxation and, additionally, when a C atom was located at a Top-3 site or at a Bridge-2 site, relaxed to the hcp-bottom hollow, as observed in Figure 3c (top view). Both hcp hollow sites located in the terrace and step hollow sites remained as the most preferred sites for C atoms. Results of adsorption energies using eq 1 are summarized in Figure 4 and show that C atoms on step hollow sites have an adsorption energy of -7.60 eV, whereas adsorption energies on hcp top and bottom sites are -6.88 and -6.64 eV, respectively. However, there were two sites where carbon atoms kept their initial position despite the existence of more favorable sites for adsorption in their vicinity. An atom located at fcc-top site did not change position, and its adsorption energy was calculated as -6.41 eV. The Top-1 site was the only top site where a

Figure 2. Locations of the different initial carbon positions tested over the (211)Co surface. (a) Side view: top sites and Bridge-1 to Bridge-3 sites. (b) Top view: Bridge-4 site and hollow sites. Purple spheres represent cobalt atoms located in the step edge.

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Figure 3. Side and top views of the most stable adsorption sites. (a) Step hollow: C atoms located on Bridge-3 (B3) and fcc-bottom (fcc-b) converge to this site. (b) hcp-top: C atoms located on Top-2 site (T2), Bridge-1 (B1), and Bridge-4 (B4) relax to this position. (c) hcp-bottom: C atoms located on Top-3 (T3) and Bridge-2 (B2) converge to this site after relaxation. Yellow spheres represent final C positions, whereas yellow circles enclosed by yellow dashed lines represent initial positions. Co atoms are gray, and those on the step edge are purple.

Figure 4. Adsorption energies according to the initial positions (shown on top of each bar). The same color bars indicate adsorption sites sharing the same final position.

carbon atom found a less stable local minimum of -4.96 eV, the weakest adsorption energy of all the tested sites. Therefore, in a stepped Co(211) surface, the adsorption of carbon atoms is more favorable on step hollow sites rather than in terrace sites, which is in agreement with previous works for C adsorption on (211)Ni surfaces.31 Small variations were found in the adsorption energies where the C atom converged to a given site starting from different initial locations. For example, when a C atom started and ended in a hcp-bottom site, the adsorption energy is -6.64 eV; however, when the C atom started from a Top-3 site, the value is -6.62 eV. It is also important to highlight that a carbon atom initially located on a Top-3 site was the only one causing a structural reconstruction in the (211) surface. In that case, a Co atom belonging to the step edge left its location, moving toward the terrace and building zigzag cobalt chains on the step edge, as shown in Figure 5b,c. This reconstruction phenomenon is similar to that where the addition of enantiomeric molecules led to flat surfaces to be reconstructed into chiral surfaces by formation of terraces, steps, and kinks;32-34 thus, the formation of zigzag cobalt chains due to adsorption of C atoms could be associated to the construction of kink sites along the step.

More detailed structural information about the most stable hollow sites is shown in Figure 6. Carbon atoms relaxed at hollow sites kept distances of around 1.80 Å to their nearest cobalt atoms, and even when the carbon atom was adsorbed on the step hollow, it had a close interaction with an atom in the second upper layer (atom 8). None of the C atoms located at hollow sites caused significant structural disorder in the surface; thus, Co atoms kept distances around 2.46 Å between nearest neighbors, and the surface also kept the initial orientation of the step and the terrace. However, as discussed above, C adsorption on the Top-3 site led to surface reconstruction, modifying the perfectly linear chain formed by the atoms 1, 2, and 1′ (Figure 6, with an 1-2-1′ angle of 180°) to a zigzag chain where the atoms 1, 2, and 1′ form an angle of 146.7° (Figure 5), with the distance 1-2 increasing from an initial of 2.46 Å to a final of 2.56 Å. According to the adsorption site, the carbon atom induces oxidation of its nearest cobalt atoms in the surface, as shown in Table 1. In several cases, the greatest oxidations are observed in Co atoms 1 and 2 (Figure 6) located at the step edge, especially when the C atom adsorbs on 4-fold step hollow or on 3-fold hollow sites belonging to the upper side of the terrace



Figure 5. Surface reconstruction of a Co(211) surface by addition of a single carbon at a Top-3 site (see Figure 2): (a) side view of the initial state, (b) side view of the system after relaxation, (c) top view of the relaxed system. Note the formation of zigzag chains on the step edge. Color code as in Figure 3.

Figure 6. Structural information about C adsorption on the strongest adsorption sites. The figure shows the location of the four most stable sites (indicated by the position of the C atoms in yellow), whereas the Co surface atoms belonging to the upper layers are labeled 1-8. Note that numbers with an apostrophe represent periodic images of the given atom. The table shows distances in Å between C atoms located at the four most stable sites and their nearest-neighbor Co atoms in the upper layers. Color code as in Figure 3.

TABLE 1: Differences of Charge Calculated as ∆qi ) qfinal i - qclean i between Co Atoms Belonging to the Top Layers of the Clean Surface and the Charges of the Same Atoms after a Carbon Atom is Added to the System at the Indicated Location, i, Shown at the Top of Each Column Co no.

Top-1

Top-2

Top-3

Bridge-1

Bridge-2

Bridge-3

Bridge-4

hcp-bottom

fcc-bottom

hcp-top

fcc-top

step

1 2 3 4 5 6 7 8

0.25 0.15 0.04 0.02 -0.01 -0.03 -0.02 -0.01

0.18 0.24 0.19 0.10 -0.01 -0.04 -0.05 0.01

0.04 0.16 0.19 0.16 0.15 0.03 -0.03 -0.02

0.20 0.21 0.20 0.09 -0.01 -0.02 -0.01 -0.01

0.03 0.10 0.19 0.19 0.17 0.02 -0.01 -0.01

0.17 0.14 0.09 -0.02 0.12 0.12 -0.01 0.07

0.16 0.25 0.19 0.08 -0.02 -0.03 -0.02 -0.01

0.05 0.08 0.19 0.19 0.17 0.01 -0.01 -0.01

0.12 0.19 0.00 0.07 0.15 0.08 0.06 -0.01

0.25 0.17 0.20 0.10 -0.01 -0.01 -0.07 -0.04

0.12 0.18 0.17 0.19 0.03 -0.03 -0.02 0.01

0.19 0.12 0.07 0.01 0.09 0.14 0.00 0.06

(hcp-top and fcc-top). Furthermore, the Top-1 adsorption site previously reported as the weakest binding energy induces the greatest oxidation in the step Co atom 1. Note that the C atom adsorbed at a Top-1 site is located at 1.60 Å over Co1, and at this location, this C atom remains extremely far from the rest of the Co atoms in the surface; thus, it can be said that a C atom located on a Top-1 adsorption site interacts exclusively with the Co atom underneath. On the other hand, Co atoms located in the middle of the terrace (Co atoms 3 and 4) are mostly oxidized when the C atoms find their relaxed position at the bottom part of the terrace (hcp-bottom). In summary, the presence of a single C atom on the Co(211) stepped surface induces oxidation of its nearest neighbors, with the interior Co atoms remaining almost neutral (atoms 7 and 8 in Figure 6), with the exception of C adsorbed at the step hollow, where the close interaction between C and Co8 separated by only 2.10 Å generates a slight oxidation of an interior Co atom. 3.2. Formation of Carbon Structures. As the C concentration on the surface increases, a series of interesting carbon structures are formed over the (211)Co surface. Initially, we located four C atoms forming a zigzag chain near step sites,

with two carbon atoms on top and two in bridge locations of the lowermost terrace layer (Figure 7a) separated by C-C distances of 1.38 Å and with a C-C-C angle of 122.80°. After relaxation, the carbon zigzag chain changed its geometry (Figure 7b,c); the C-C distance is the same for all four atoms, 1.42 Å, and the angle between three carbon atoms is 122.31°. The average carbon adsorption energy is -7.33 eV, in the order of that found for the most stable sites shown in Figure 4. Starting with the relaxed carbon zigzag chain described above (Figure 8a), we added a new chain of four carbon atoms parallel to the first one, as shown in Figure 8b. After relaxation, a second chain is formed on top of the uppermost terrace layer, parallel to the first carbon chain but at a very different orientation than that originally given (Figure 8c-e); the C-C distance of all four atoms belonging to the new chain is 1.30 Å, and the angle between three carbons is 150°. Note that the new chain is perpendicular to the surface. Because this new chain may play an important role in the formation of carbon structures, it was relaxed separately on a clean Co(211) surface, yielding an average carbon adsorption energy of -6.40 eV.

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Figure 7. (a) Initial geometry (side view) of a chain of four carbon atoms located near step top sites. (b) After relaxation, the resultant zigzag chain has a modified geometry shown in (c) from a top view. Color code as in Figure 3.

Figure 8. Four carbon atoms were added to the relaxed system depicted in view a, as shown in view b; after relaxation, the initial zigzag chain conserved its location and geometry, and a new zigzag chain on top of the uppermost terrace layer was formed, as is described in views c-e.

Figure 9. Evolution of higher C pressures on (211)Co surfaces. Each unit cell contains twelve carbon atoms. (a) and (b) are the side and top views, respectively, of the initial structure to which new C atoms are initially added at both sides of the chain, in the terraces next to and previous to the step. (c) and (d) are side and top views after relaxation. Two of the atoms bond to one of the terraces, but the other two stand out of the metal surface, with all of them forming a graphene surface that follows the orientation of the (100) lattice plane of the metal.

Starting from the relaxed system with two zigzag chains described in Figure 8, we added an additional four C atoms above the step, between both zigzag chains and approximately at the same distance from the surface, as depicted in Figure 9a. After optimization, a graphene surface was formed, starting from the second terrace layer and growing parallel to the (100) lattice planes of the metal. Interestingly, the graphene structure shown

in Figure 9 is extremely similar to those in experimental results recently published.35,36 Next, the procedure just described was repeated; from the relaxed system with two carbon chains, four carbon atoms were added, but this time in a different location: almost above the second terrace layer in the middle of both relaxed carbon chains (Figure 10a,b). After optimization, the system formed a curved



Figure 10. Evolution of carbon structures under increasing pressures of carbon over a (211)Co surface. (a) and (b) are side and top views of initial structures showing new addition of C atoms to the chain located in the terrace, and the relaxed side and top view structures are shown in (c-e).

Figure 11. Three carbon zigzag chains formed close to the surface step sites that may play an important role in the formation of carbon structures. Each one was separately relaxed for their study; carbon chains a and b were stable after relaxation but not c. Carbon chain c during relaxation evolved to carbon chain a, which is the most stable of all three zigzag chains.

carbon structure, shown in Figure 10c,d; this carbon structure has the form of a horizontally aligned semi-nanotube, which may be the beginning of other types of nanotube growth, for example, the formation of horizontally aligned SWCNTs on substrates, as recently reported.37,38 The carbon formations illustrated in Figures 9 and 10 suggest that the presence of higher C atmospheres on other stepped Co surfaces may lead to the formation of alternative structures resulting from specific relationships between the surface patterns and the growing structures that could influence the chirality of nascent nanotubes. 3.3. Geometry and Charge Analysis of Carbon Structures. During the formation of the carbon structures described in the previous section, we observed that three carbon zigzag chains formed close to the surface step site, suggesting a relationship between the surface geometry and that of the nascent carbon structures. To further investigate this point, each of the three carbon zigzag chains was separately relaxed on clean (211)Co surfaces. Interestingly, it was found that the carbon zigzag chain shown in Figure 11c is stable only when it is part of the carbon structure shown in Figure 10c,d but not alone; when this carbon zigzag chain was separately relaxed, its geometry changed to that shown in Figure 11a. The C zigzag chain in Figure 11a is parallel to the surface, with C-C distances of 1.42 Å and an C-C-C angle of 122.31°;

Figure 12. Geometries of carbon structures: (a) corresponds to graphene layer in Figure 9, (b) corresponds to carbon structure in Figure 10.

its geometry results from alternating bonds between one Co from the step and one Co atom from the terrace. The Co1-C12, Co2-C11, Co4-C9, and Co3-C10 distances are equal to 1.93 Å; this carbon chain is the most stable with an average carbon adsorption energy of -7.33 eV. Figure 10b is a carbon zigzag chain perpendicular to the surface, formed on top of the uppermost terrace layer with C-C distances of 1.30 Å and a C-C-C angle of 150°. C12 and C11 are on top of Co1 and Co2, separated by 2.04 Å, whereas C9 and C10 are on bridge locations separated 2.12 Å from their

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TABLE 2: C-C Distances (in Å) for Structures of Figure 12a,b carbon-carbon location C1-C2 C2-C3 C3-C4 C4-C5 C2-C7 C8-C7 C7-C6 C6-C5 C8-C9 C6-C11 C9-C10 C10-C11 C11-C12 Figure 12a Figure 12b

1.44 1.43

1.44 1.43

1.44 1.43

1.46 1.44

1.46 1.44

1.43 1.43

nearest Co atoms. The average adsorption energy for this carbon chain is -6.40 eV. The last C zigzag chain is depicted in Figure 11c with a C-C distance of 1.46 Å. This carbon chain has two C atoms located on a hollow site of the (100) plane, which, as shown in Section 3.1, corresponds to the strongest binding energy, and the other two C atoms are located in bridge locations. However, despite the fact that this carbon chain involves a favorable location for adsorption of one of the carbon atoms, the isolated chain is not stable, probably due to the location of the other C atoms on unstable bridge locations. During relaxation, it evolved to become identical to that depicted in Figure 11a. It is only when it is part of a carbon structure, as described in Figure 10, that the chain of Figure 11c can be stabilized due to the interactions with C atoms from neighboring chains. It is observed that the C-C bond length differs depending on carbon chain geometry; these differences of carbon bond length may be due to the formation of covalent bonds, being shorter when more electrons are involved. Experimentally, C-C bond lengths are in the range of 1.20-1.54 Å, depending on the nature of the covalent bond. The geometries of the carbon structures of Figures 9 and 10 are depicted in Figure 12, and the corresponding C-C distances of each structure are listed in Table 2. As expected, the C-C distances vary as a consequence of C-Co interactions and surface geometry; for example, the bottom graphene carbons C1, C2, C3, and C4 bonded to cobalt atoms have longer C-C distances (1.44 Å) than those of the uppermost atoms C9, C10, C11, and C12 not bonded to Co atoms (1.39 Å). In addition, in the horizontally aligned semi-nanotube, the C12, C11, C10, and

1.43 1.43

1.43 1.43

1.48 1.43

1.48 1.43

1.39 1.46

1.39 1.46

1.39 1.46

C9 atoms bonded to the step surface have longer C-C distances (1.46 Å) than those of the C4, C3, C2, and C1 atoms bonded to the uppermost terrace layer (1.43 Å). To study the surface electronic distribution, a Bader charge analysis was performed to the systems depicted in Figure 13. In each illustrated system, the Co and C atoms are labeled and their charges are listed in Table 3. Because the Co atomic charges of the (211) clean surface were calculated, it is possible to know changes in the oxidation state of the metal atoms in the surface due to the carbon presence; these changes of oxidation are listed in Table 3 as ∆q. Changes in the oxidation state of the metal atoms in contact with the carbon structures shown in Table 3 are in qualitative agreement with those reported previously.14,39 However, important differences are observed in the charge distribution among the various cases shown in Figure 13. In all cases, the Co atoms belonging to the most stable 4-fold step hollow (1,2,5,6) become oxidized, with different degrees of positive charges on each atom, whereas those of terrace sites become negatively charged. For this reason, the C-chain/surface system in cases b and c is seen as polarized, with the C chain displaying alternative negative and positive charges. The difference between cases b and c is that case c has a much less symmetric distribution of charges than that in case b as a consequence of the different chain-surface interactions in each case. Cases d and e illustrate the effect of chain-chain interactions on the systems. In the case of the graphene plane (case d), we could divide the C atoms into three chains; the external does not interact with Co atoms, where the C atoms are almost neutral, whereas the two other chains (central and internal) have charges of alternating signs

Figure 13. Surface and carbon structure geometries used to illustrate the electronic charge analysis shown in Table 2: (a) outermost Co atoms of the (211) surface, (b) zigzag carbon chain where each of the atoms in the zigzag is alternatively bonded to a step and to a terrace site, (c) zigzag carbon chain bonded on top of the outermost terrace layer, (d) graphene bonded to the Co surface, and (e) horizontally aligned semi-nanotube bonded to the Co surface.



TABLE 3: Electronic Charges of Each Surface Atom Depicted in Figure 13a species

charge (a)

charge (b)

charge (c)

charge (d)

charge (e)

Co1 Co2 Co3 Co4 Co5 Co6 Co7 Co8 C9 C10 C11 C12 C13 C14 C15 C16 C17 C18 C19 C20

0.01 0.01 -0.01 -0.01 0.01 0.01 0.02 0.02

0.38 (∆q ) 0.37) 0.39 (∆q ) 0.38) -0.91 (∆q ) -0.90) -0.85 (∆q ) -0.84) 0.71 (∆q ) 0.70) 0.65 (∆q ) 0.64) -0.16 (∆q ) -0.18) -0.16 (∆q ) -0.18) -0.62 -0.66 0.22 0.16

0.28 (∆q ) 0.27) 0.88 (∆q ) 0.87) -0.50 (∆q ) -0.49) -0.45 (∆q ) -0.44) 1.63 (∆q ) 1.62) -0.12 (∆q ) -0.13) -0.95 (∆q ) -0.97) -1.01 (∆q ) -1.03) -0.65 -0.70 0.32 0.31

0.42 (∆q ) 0.41) 0.44 (∆q ) 0.43) -0.14 (∆q ) -0.13) -0.14 (∆q ) -0.13) 0.56 (∆q ) 0.55) 0.54 (∆q ) 0.53) -0.48 (∆q ) -0.50) -0.49 (∆q ) -0.51) -0.49 -0.49 0.37 0.36 -0.13 -0.13 0.075 0.075 -0.97 -0.96 0.49 0.49

0.77 (∆q ) 0.76) 0.84 (∆q ) 0.83) -0.98 (∆q ) -0.97) -0.88 (∆q ) -0.87) 0.92 (∆q ) 0.91) 1.02 (∆q ) 1.01) -0.62 (∆q ) -0.64) -0.61 (∆q ) -0.63) 0.12 0.13 0.25 0.25 -0.67 -0.70 0.27 0.23 -1.11 -1.06 0.35 0.36

a

∆q is the change of oxidation of Co surface atoms. Bold numbers correspond to Co atoms bonded to carbons. Charges a-e refer to Figure

13.

on their C atoms. Case e shows the highest degree of oxidation for Co atoms forming a step-hollow site, and those Co atoms (such as 1 and 2) have a large number of short interactions with C atoms (Figure 13e). Charges of the Co atoms located in terrace sites are highly negative. In this case, probably due to the curvature of the semi-nanotube, all the C atoms on top (9 to 12) bear positive charges, whereas those in contact with surface atoms have alternating signs as in all the other cases. In relation to our discussion in the Introduction (Section 1), we would like to point out a series of interesting phenomena that, although, at this point, are mostly speculative, provide further insights with respect to the catalyzed growth of SWCNTs. First, we have shown that, under certain conditions, C adsorption may lead to surface reconstruction. Second, adsorption of carbon chains is very favorable, and the chain geometry and chemical structure depend on the nature of the interacting surface. Chain adsorption leads to formation of stable structures, such as those shown in Figures 9 and 10. Previous reports35,36 have attributed the formation of graphene planes that we showed in Figure 9 as the initial stage for the formation of SWCNTs, whereas others have synthesized horizontally aligned SWCNTs,37,38 of which the structures reported in Figure 10 could be the predecessors. Thus, it is clear that this is a strongly interactive system where a specific surface pattern may very well induce the growth of a chiral nanotube. Our current work is oriented to investigate growth on chiral surfaces. 4. Conclusions Carbon atoms adsorb preferentially on 4-fold step hollow sites on (100) planes. The next most favorable sites are 3-fold hcp hollow sites on terraces, but the difference in adsorption energies is 0.7-0.9 eV with respect to the most stable step hollow sites. Adsorptions on top sites are local minima that induce strong surface reconstruction on the step edge. Chains of carbon atoms are stably adsorbed either parallel to the surface or perpendicular to it; the first is the most stable, and the C-C bond length is in the order of those in benzene. The second chain has shorter bond lengths closer to that of a double bond in an alkene. In both cases, the C chains are also linked to the surface Co atoms. When the density of chains increases on the surface, higher-

order carbon structures are formed. Depending on the location of the interacting chains, we found that a graphene sheet may grow parallel to the (100) plane of the step, and other structures resembling horizontally aligned semi-nanotubes are also possible to be formed on the (211) surface. The carbon-surface interaction causes oxidation of the cobalt atoms; generally, the 4-fold step hollow site atoms bear the highest positive charges. Both high-order carbon structures found, a graphene sheet and a semi-nanotube, may be predecessors of the formation of SWCNTs. Acknowledgment. This work was supported by the U.S. Department of Energy, Basic Energy Sciences, under Grant No. DE- FG02-06ER15836. Computational resources from TAMU Supercomputer Facility and from the National Energy Research Scientific Computing Center, which is supported by the Office of Science of the U.S. Department of Energy under Contract No. DE-AC03-76SF00098, are gratefully acknowledged. References and Notes (1) Dresselhaus, M. S.; Avouris, P. Carbon Nanotubes: Synthesis, Structure, Properties and Applications; Springer: Berlin, 2001. (2) Hafner, J. H.; Bronikowski, M. J.; Azamian, B. R.; Nikolaev, P.; Rinzler, A. G.; Colbert, D. T.; Smith, K. A.; Smalley, R. E. Catalytic growth of single-wall carbon nanotubes from metal particles. Chem. Phys. Lett. 1998, 296, 195. (3) Resasco, D. E.; Alvarez, W. E.; Pompeo, F.; Balzano, L.; Herrera, J. E.; Kitiyanan, B.; Borgna, A. A scalable process for production of singlewalled carbon nanotubes (SWNTs) by catalytic disproportionation of CO on a solid catalyst. J. Nanopart. Res. 2002, 4, 131. (4) Lolli, G.; Zhang, L. A.; Balzano, L.; Sakulchaicharoen, N.; Tan, Y. Q.; Resasco, D. E. Tailoring (n,m) structure of single-walled carbon nanotubes by modifying reaction conditions and the nature of the support of CoMo catalysts. J. Phys. Chem. B 2006, 110, 2108. (5) Saito, R.; Dresselhaus, G.; Dresselhaus, M. S. Physical Properties of Carbon Nanotubes; Imperial College Press: London, 1998. (6) Dukovic, G.; Balaz, M.; Doak, P.; Berova, N. D.; Zheng, M.; Mclean, R. S.; Brus, L. E. Racemic single-wall carbon nanotubes exhibit circular dichroism when wrapped with DNA. J. Am. Chem. Soc. 2006, 128, 9004. (7) Hofmann, S.; Sharma, R.; Ducati, C.; Du, G.; Mattevi, C.; Cepek, C.; Cantoro, M.; Pisana, S.; Parvez, A.; Cervantes-Sodi, F.; Ferrari, A. C.; Dunin-Borkowski, R.; Lizzit, S.; Petaccia, L.; Goldoni, A.; Robertson, J. In situ observations of catalyst dynamics during surface-bound carbon nanotube nucleation. Nano Lett. 2007, 7, 602.

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