6593
2007, 111, 6593-6596 Published on Web 04/19/2007
Enhancement of the Ionization-Potential of K and Rb upon Chemisorption on a C60 Molecule Giannis Mpourmpakis† and George E. Froudakis Department of Chemistry, UniVersity of Crete, P.O. Box 1470, Heraklio, Crete, Greece 71409
Antonis N. Andriotis Institute of Electronic Structure and Laser, FORTH, P.O. Box 1527, 71110 Heraklio, Crete, Greece
Madhu Menon* Department of Physics and Astronomy, UniVersity of Kentucky, Lexington, Kentucky 40506-0055, Center for Computational Sciences, UniVersity of Kentucky, Lexington, Kentucky 40506-0045 ReceiVed: February 8, 2007; In Final Form: March 19, 2007
Ab initio computations are used to investigate the interaction of the alkali metal atoms Li, Na, K, and Rb as well as their ionic counterparts with the single wall carbon nanotubes and the C60 molecule. Our results lead to a novel feature, namely that of the enhancement of the ionization potential of some of the alkali metal atoms and, in particular, that of K and Rb, when adsorbed on a C60 molecule. This enhancement as well as its absence either in the case of Li and Na or in the case of the alkali metal atom adsorption on a nanotube is investigated, and it is demonstrated that this can be attributed to the synergy of the finite nature and the inherent properties of both the substrates (nanotube or C60) and the chemisorbed alkali atoms.
The chemisorption of alkali metal atoms (AMAs) onto a semiinfinite solid surface is a well studied topic and rich in theoretical descriptions based on both ab initio investigations and model descriptions.1-3 The studies of AMA chemisorption on a semiinfinite metal surface have provided a wealth of reports from which one can get a conclusive picture of the basic features of this physical process. Earlier results on the studies of AMA chemisorption onto a semi-infinite jellium metal surface have revealed that in such a process the AMA approaching the metal surface experiences a shift and a broadening of its own energy levels due to their mixing (interaction) with the continuum of the metal states. The strength of AMA-metal interaction depends on the ionization potential (IP) of the AMA and on the Fermi energy, EF, of the metal substrate. As a result of this interaction, significant electronic charge-transfer takes place from the AMA to the metal, leaving the AMA in a positively charged state. The importance of the interaction on the AMAs with carbon surfaces in catalysis has attracted much research interest in the past, from both theoretical and experimental point of view. Recently, there has been a renewed interest in this topic4,5 due to the discovery of novel forms of carbon such as fullerenes and single-wall carbon nanotubes (SWCNs). This interest is derived mainly from potential applications in catalysis as well as in energy storage and/or energy conversion.6,7 * To whom correspondence should be addressed. Electronic address:
[email protected]. † Present address: Center for Catalytic Science and Technology, Department of Chemical Engineering, University of Delaware, Newark, Delaware 19716.
10.1021/jp0711040 CCC: $37.00
The chemisorption of an AMA onto a finite carbon surface is expected to add more features when compared to that of the chemisorption on a semi-infinite metal. This is because of the coupling between the discrete spectrum of the finite carbonsubstrate (in contrast to the semi-infinite metal continuum) and the significant substrate perturbation resulting from the AMA. These effects provide a unique opportunity to study interesting physical and chemical properties of this chemisorbed system and its potential for technological applications. Additionally, the carbon-AMA systems proved to be highly correlated systems whose study requires accurate computational techniques.4 The C60 molecule is known to have a large IP and high electron affinity (EA) (IP ) 7.61 eV and EA ) 2.65 eV, both experimental values; see, for example, ref 8), and AMA adsorption on this molecule may have many parallel features with AMA-metal chemisorption. However, the finite dimensions of the C60 as a substrate could become a pronounced feature leading to many interesting results. Most existing reports support the conclusion that the adsorption of Li, Na, and K on carbon surfaces is dominated by the charge transfer state leading to a heteropolar formation [AMA+ (C-surface)-] and that the most stable configuration for the AMA is to adsorb over the middle of an hexagon in graphite. Furthermore, it has been found that the Na atom makes weaker bonding to graphite than Li and K, whereas Li binds stronger than K.4 In the present work, we report results from a systematic study of the interaction of Li, Na, and K atoms and Li+, Na+, and K+ with a (5,5)-SWCN (consisting of 70 C-atoms and passivated © 2007 American Chemical Society
6594 J. Phys. Chem. C, Vol. 111, No. 18, 2007
Letters
TABLE 1: Mulliken Population Analysis of Alkali Metal Atoms Interacting with a Defected (5,5)-SWCN and a C60 Molecule Atop Different Carbon Ringsa SWCN alkali
hexagon
heptagon
pentagon
hexagon
(Li )
+0.61(+0.69)
+0.66(+0.70)
+0.63(+0.72)
+0.64(+0.71)
Na0 (Na+)
+0.81(+0.85)
+0.82(+0.84)
+0.81(+0.86)
+0.80(+0.85)
K0 (K+)
+0.87(+0.91)
+0.64(+0.70) [+0.64(+0.70)] +0.83(+0.86) [+0.83(+0.86)] +0.90(+0.92) [+0.90(+0.92)]
+0.89(+0.91)
+0.88(+0.92)
+0.88(+0.91)
Li0
a
C60
pentagon
+
Data in parentheses refer to alkali metal ions, and numbers in the square brackets refer to the same SWCN without defects.
TABLE 2: HOMO-LUMO Gap (eV) as in the Description of Caption of Table 1a SWCN alkali
pentagon
hexagon
C60 heptagon
pentagon
hexagon
Li0 (Li+) 1.44(1.54) 0.77(2.01) 1.26(1.72) 0.90(2.51) 0.92(2.55) [0.77(2.04)] Na0 (Na+) 1.39(1.66) [0.75(2.04)] 1.14(1.73) 0.88(2.57) 0.91(2.58) [0.75(2.09)] K0 (K+) 1.03(1.72) 0.74(2.05) 1.37(1.79) 0.87(2.61) 0.89(2.61) [0.74(2.11)] a HOMO-LUMO gap values for the pristine SWCN and C60 obtained using the same method are 2.17 and 2.73 eV, respectively.
with H atoms at both ends and ≈ 9.3 Å in length) as well as a C60 molecule. The SWCN is assumed to be either in a defectfree state (only hexagonal rings) or containing a Stone-Wales defect (pentagon-heptagon pair).9 The latter case allows us to compare the differences in the adsorption features of AMAs when adsorbed over a pentagon and an hexagon of a SWCN and a C60 molecule as well as over an heptagon in the case of the SWCN. All of the calculations are carried out using ab initio methods using the TURBOMOLE computational program package.10 The density functional approximation was implemented using the B3LYP hybrid functional11 together with the svp basis set.12 This combination of method and basis set can predict very accurate values for the alkali-π-face interactions, comparable to the MP2 level of theory as shown in our previous work.13 All of the structures were fully relaxed without any symmetry constraints. Our results are presented in Tables 1-4. From Table 1, it is observed that the charge transfer from the AMA to SWCN is practically the same as that toward the C60 molecule. It is also observed that the charged state of the AMAs onto either the SWCN or the C60 molecule is more or less the same as that of the alkali metal ions (AMIs) interacting with the SWCN or the C60 molecule. Thus, the charge state of the AMA and the AMI is found to be independent of the total charge Q of the system [i.e., AMA + C60 and AMA + SWCN: Q ) 0, AMI + C60 and AMI + SWCN: Q ) 1] or to the details of the C-surface and the adsorption position (on SWCN and C60, on carbon pentagons or hexagons). Although the charge state of the AMAs and AMIs remain the same, there is a preference for the AMAs and AMIs to adsorb stronger on
defected carbon rings of the SWCN. This has been attributed to the splitting of the degenerate orbitals which facilitates the hybridization of the adparticle’s orbitals.13 In Table 2 a differentiation is observed in the effect of the AMAs on the HOMO-LUMO gaps of the SWCN and the C60 (see also Figure 2). The differences appear as we compare the induced changes on the HOMO-LUMO gap of the adsorbed systems when the AMA or the AMI changes adsorption position on a defected SWCN. In the case of AMA, we find no significant dependence of the HOMO-LUMO gap on the kind of the AMA if the adsorption takes place on the defect-free SWCN or C60. However, the induced HOMO-LUMO gap values upon the adsorption of AMIs are found to be much larger than those of the AMAs (except for Li adsorbed on a StoneWales pentagon of the SWCN; see also Figure 2). The calculated binding energies (BEs) of the adsorbed AMAs and AMIs on (5,5)-SWCN and on a C60 molecule are presented in Table 3. The BE is defined as the energy of the interacting system (substrate plus ad-particle) minus the sum of the energies of the free substrate and the free ad-particle (AMA or AMI). The BE results of Table 3 indicate that, as in the case of graphite,4 the Na atoms bind weaker on the SWCN and the C60 than Li and K. It can, thus, be deduced that this exceptional behavior of Na does not depend on the details of the carbon surface (i.e., structure, curvature, adsorption site). The real cause of this anomaly is not well understood. On the one hand, it may be attributed to the details of the configuration interaction (CI), namely to the energetics of the dominant molecular orbitals (MOs) which participate in the CI description of the system.4,5 On the other hand, to the lack of combination between the sodium’s inner p orbitals and the C surface’s π molecular orbitals due to energetic differences.13 Furthermore our results for the BE in the AMIs case, follow the typical electrostatic trend of the interaction of the alkali cations with metallic π-MOs.14 That is, Li+ is found to bind stronger than Na+ and the latter stronger than K+. However, the most surprising feature implicit in the results of Table 3 is that of the IP of K as this becomes enhanced when K adsorbs on the C60 molecule either on top of an hexagon or on top of a pentagon.15 This is a novel phenomenon which, on a cursory look, may be attributed to the lowering of the IP of the AMAs in going from Li to K. To check this further, we
TABLE 3: Binding Energy (eV) as in the Description of Caption of Table 1 SWCN alkali
hexagon
heptagon
pentagon
hexagon
-0.78(-2.04) [-0.82(-2.04)] -0.25(-1.49) [-0.18(-1.48)] -0.50(-1.01) [-0.42(-1.01)] -0.39(-0.89)
-1.34(-2.14)
-1.29(-1.46)
-1.29(-1.46)
-0.78(-1.61)
-0.73(-0.98)
-0.79(-0.99)
-0.95(-1.10)
-0.98(-0.58)
-1.03(-0.60)
Li0
(Li )
-1.41(-2.16)
0
+
Na (Na )
-0.72(-1.54)
K0 (K+)
-0.94(-1.06)
+
Rb0 (Rb+)
C60
pentagon
-0.98(-0.50)
Letters
J. Phys. Chem. C, Vol. 111, No. 18, 2007 6595
Figure 1. Fully optimized structures of Li atom on SWCN containing a Stone-Wales defect. The figure on the left shows Li adsorbed over an hexagon which is far away from the defect (located at the bottom). The figure on the right shows Li adsorbed over a pentagon.
TABLE 4: Ionization Potentials (eV) as in the Description of Caption of Table 1 AMA on SWCN AMA on C60 free alkali pentagon hexagon heptagon pentagon hexagon atom Li Na K Rb
4.70 4.46 4.28
4.20 (4.24) 4.04 (3.98) 3.88 (3.81) 3.75
4.66 4.44 4.25
5.28 5.03 4.79
5.29 5.08 4.83 4.73
5.46 5.28 4.40 4.25
proceeded to evaluate the IP of Rb which follows K in the corresponding column of the periodic table. And indeed, Rb was found to behave like K, exhibiting an enhanced IP when adsorbed on a C60 molecule as shown in Table 4. However, the inherent relation and the contribution of the IP of the AMAs in establishing this novel phenomenon needs a deeper understanding and a more detailed examination of our results is necessary. Figure 3. Binding energies and ionization potentials (inset) in Tables 3 and 4, respectively.
Figure 2. Energy diagram of MO eigenvalues of AMAs and AMIs interacting with a C60 molecule. The increase in the HOMO-LUMO gap is apparent in the case of the C60-AMI systems as well as its variation in going from Li to K.
From such an examination, the following conclusions are worth elucidating: From Tables 1 and 2, it is observed that both the charged state of any of the adsorbed AMAs or the adsorbed AMIs as well as the value of the HOMO-LUMO gap that is related to their adsorption does not change significantly in going from a defect-free SWCN to C60. This means that the factors responsible for the observed enhancement of the IP of K (and Rb) should be mainly sought for, on the one hand, in those features of the C60 molecule that change as this interacts with the AMAs and AMIs and, on the other hand, in the relationship between these features and those properties of the AMAs that change along the AMA series in the periodic table (e.g., their IP). By plotting, as in Figure 3, the data of Table 3 (referring to the interaction of the defect-free SWCN and C60 with AMAs and AMIs atop the hexagonal carbon rings), it can be seen that the BE of the AMAs and AMIs follow a decrease in going from Li to Rb. Although this decrease is monotonic for the AMIs, this is not so for the AMAs as a significant dip appears in the case of Na as discussed in the above. However, in the absence of the Na-dip, the BE curve of the AMAs shows a monotonic decrease which is weaker (i.e., with smaller slope) than that of the corresponding curve for the AMIs. This behavior is also found in the case of the interactions of the AMAs and AMIs with the (5,5)-SWCN. A noticeable difference between the BE curves for the C60 and the SWCN is that the BE curves of AMAs
6596 J. Phys. Chem. C, Vol. 111, No. 18, 2007
Figure 4. Approximate binding energy curves (see text); blue doubledot-dash curve: BE ) Q*(EAC60 - IPAMA); red double-dot-dash curve: BE ) (1 - Q)*(IPC60 - IPAMA), where Q is the charge transfer given in Table 1.
and AMIs do not cross in the case of the SWCN. This is due to the large energy difference, δBE, between the BEs of AMAs and AMIs which cannot be overcompensated by the difference between the slopes of the BE curves for AMAs and AMIs. These observations make it amply clear that the observed enhancement of the IP of K and Rb (and maybe of Cs) can be attributed to (i) the weaker dependence of the AMA-C60 interaction on the kind of the AMA than that of the AMI-C60 interaction (this is reflected to the slope difference of the corresponding BE curves) and (ii) the relatively small values of δBE in the case of the C60 when compared to those of the SWCN. The quantitative picture described by our ab initio BE results can be roughly explained as the result of the energy balance associated with the charge transfer which dominates the chemisorption process. For example, in the case of the AMA chemisorption on the C60, the BE can be approximated as the energy difference between the energy spent by an AMA-charge Q in overcoming its ionization barrier and the energy gained upon its binding to the C60 (see Figure 4). The former is given by Q*IPAMA and the latter by Q*EAC60 where the subscripts specify the corresponding material. Similarly in the case of the AMIs.16 The BE curves drawn on this basis do not cross although their monotonic decrease along the alkali column is rediscovered (see Figure 4). This approximation can be improved by noting that, in contradistinction to the behavior of the semi-infinite metal substrate, the C60 or the SWCN substrate is finite and, therefore, experiences noticeable perturbations in the presence of the AMA or the AMI. As a result, the substrate energy levels are broadened and are shifted upward. Introducing such an appropriate shift in the approximate BE curves of Figure 4, one can achieve a picture similar to that of Figure 3. It should be noted that the ab initio picture incorporates the effects of additional factors not included in the approximate one. The factors with the most pronounced effects among them are as follows: (i) The charged state of the substrate (negative in the case of AMA-adsorption and positive in the AMI adsorption). (ii) The position of the HOMO level of the C60- which lies higher than the HOMO level of the AMI (see Figure 2) as well
Letters as the position of the outer-s level of the AMAs. (iii) The larger gap openings (due to symmetry reasons) combined with degenerate level-splittings when the AMAs/AMIs interact with the C60 molecule than with the SWCN. As a result, a net gain appears in favor of the AMIs interacting with the C60 than with the SWCNs. This gain reduces the value of δBE. (iv) The smaller IP ()5.66 eV) and EA ()1.58 eV) values of SWCNs when compared to those of the C60. In conclusion, the enhancement of the IP of some AMAs when adsorbed on a C60 molecule can be attributed to contributions which originate from both the ordinary intrinsic changes taking place in the AMAs due to their interaction with the substrate (energy level shift and broadening) and the finite character of the substrate (C60) and, specifically, due to its perturbation from the presence of the adparticle. The apparent enhancement of the IP of some AMAs interacting with the C60 is the result of the energy balance between these contributions. In comparison with the chemisorption of the AMAs and AMIs on infinite metal substrates it is mainly the finite nature of the C60 that makes it sensitive to outer perturbations which, in turn, are reflected as noticeable contributions to the AMA and AMI chemisorption processes. The present work is supported through grants by NSF (ITR-0221916), DOE (DE-FG02-00ER45817), and US-ARO (W911NF-05-1-0372). Supporting Information Available: (i) A schematic representation of ionization potential for K when bonded to the C60 molecule and SWCN and (ii) the optimized geometries of all the structures used in the present work. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Lang, N. D.; Williams, A. R. Phys. ReV. Lett. 1976, 37, 212. (2) Bennet, A. J.; Falikov, L. M. Phys. ReV. 1966, 151, 512. (3) Newns, D. M. Phys. ReV. 1969, 178, 1123. (4) Zhu, Z. H; Lu, G. Q. Langmuir 2004, 20, 10751. (5) Pitarch-Ruiz, J.; Evangelisti, S.; Maynau, D. J. Chem. Theory Comput. 2005, 1, 1079. (6) Zhao, J.; Buldum, A.; Han, J.; Lu, J. P. Phys. ReV. Lett. 2000, 85, 1706. (7) Froudakis, G. E. Nano Lett. 2001, 1, 531. (8) Zhao, J.; Han, J.; Lu, J. P. Phys. ReV. B 2002, 65, 193401. (9) Stone, A. J.; Wales, D. J. Chem. Phys. Lett. 1986, 128, 501. (10) Ahlrichs, R.; Bar, M.; Haser, M.; Horn, H.; Kolmel, C. Chem. Phys. Lett. 1989, 162, 165. (11) Becke, A. D. Phys. ReV. A 1988, 38, 3098. (12) Schafer, A.; Horn, H.; Ahlrichs, R. J. Chem. Phys. 1992, 97, 2571. (13) Mpourmpakis, G.; Froudakis, G. E. J. Chem. Phys. 2006, 125, 204707. (14) Kumpf, R. A.; Dougherty, D. A. Science 1993, 261, 1708. (15) The IP of the adsorbed AMA, IP(ads) is defined as follows: IP(ads) ) IP(free) + BE(AMI) - BE(AMA) where IP(free) is the IP of the free AMA. (16) In the AMI case, however, one may wonder how is it possible for some charge to leave the C60 and move onto the AMI, while the latter exhibiting EA smaller than the IP of the C60. One could qualitatively justify this by attributing it to the electron correlation effects in the form of Coulombic image charge interaction (see, for example, ref 17 and references therein) which could reduce the IP of the C60 to values smaller than the IPs of the AMAs. Alternatively, one can employ the quantum mechanical approach of the Coulombic cation-π interaction and justify this charge transfer.18 (17) Andriotis, A. N.; Nicolaides, C. A. Phys. ReV. B 1987, 35, 2583. (18) Gokel, G. W; De Wall, S. L; Meadows, E. S. Eur. J. Org. Chem. 2000, 17, 2967.
