Surface Heterogeneity of C60 As Studied by Infrared Spectroscopy of

These included the void space between four, three, and two neighboring C60 molecules and the center of the hexagon and the pentagon on the C60 surface...
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Langmuir 1997, 13, 1118-1122

Surface Heterogeneity of C60 As Studied by Infrared Spectroscopy of Adsorbed CO and Adsorption Potential Calculations† M. Folman,* M. Fastow, and Y. Kozirovski Department of Chemistry, TechnionsIsrael Institute of Technology, Haifa 32000, Israel Received January 29, 1996. In Final Form: April 9, 1996X In our recent investigation of the IR spectrum of CO physically adsorbed on C60 films, two well-resolved absorption bands at 2135 and 2128 cm-1 were found, suggesting that the molecule is adsorbed on two different sites. To determine the nature of these adsorption sites, calculations of adsorption potentials and spectral shifts for the CO/C60 system were performed. The calculations were done for the fcc (100), fcc (111) hcp (001), and hcp (111) surface planes. In the calculations the 6-exponential and the LennardJones potentials were used. A number of adsorption sites were chosen. These included the void space between four, three, and two neighboring C60 molecules and the center of the hexagon and the pentagon on the C60 surface. The calculated potentials and spectral shifts clearly indicate that adsorption sites in the voids between the C60 molecules are energetically preferred over sites on top of single C60 molecules. Comparison is made between results obtained with the two potentials and with results obtained previously with the two other carbon allotropes: graphite and diamond.

1. Introduction Intense interest in fullerenes had begun in 1985 with the discovery that 60-atom clusters were present in the mass spectra of laser-vaporized graphite. The experiment was done by Richard Smalley’s group of Rice Institute, in collaboration with Kroto at the University of Sussex.1 In 1990, Kra¨tschmer and his colleagues2 succeeded in isolating macroscopic amounts of soluble fullerene mixtures by solvent extraction of a sooty deposit produced by arc vaporization of graphite in a helium atmosphere. These mixtures were composed mostly of C60 but also contained significant amounts of C70 and traces of other fullerenes. In contrast to infinite structures of diamond and graphite, the fullerenes represent a pure molecular form of the element. They are closed hollow cages comprising exactly 12 pentagons and any number of hexagons in which every carbon atom is sp2 hybridized. The 60 carbon atoms in C60 are chemically equivalent. The structure contains two distinct bond types: short double bonds, 1.39 Å, and single bonds, 1.44 Å. In very pure C60 the spherical molecules pack in a face-centered cubic (fcc) structure. This structure contains large interstitial cavities which account for nearly 27% of the unit cell volume and results in C60 being less than half as dense (1.65 g/cm3) as diamond (3.51 g/cm3). In the solid, intermolecular bonding is analogous to the weak interplanar bonding between graphite layers and is also due to van der Waals attraction. The distance between adjacent carbon cages in solid C60 is 2.9 Å as compared with the 3.35 Å gap between atomic planes in graphite. The electronic structure of C60 results in being a good electron acceptor and a weak oxidant, causing interesting physical and photophysical properties. Some of the group 1 and group 2 metal salts of C60 (in which the metal ions * To whom the correspondence should be addressed: e-mail, [email protected]; fax, (972-4) 8 233 735. † Presented at the Second International Symposium on Effects of Surface Heterogeneity in Adsorption and Catalysis on Solids held in Poland/Slovakia September 4-10, 1995. X Abstract published in Advance ACS Abstracts, September 15, 1996. (1) Kroto, H. W.; Heath, J. R.; O’Brien, S. C.; Curl, R. F.; Smalley, R. E. Nature (London) 1985, 318, 162. (2) Kra¨tschmer, W.; Lamb, L. D.; Fostiropoulos, K.; Huffman, D. R. Nature (London) 1990, 347, 354.

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occupy the interstice) display superconductivity at low temperature (e.g., RbCs2C60, Tc ) 33 K). The C60 molecule is best described as a partly delocalized electron-deficient polyalkene rather than a superaromatic molecule. Much of the reported chemistry is consistent with this description. Despite the large amount of research devoted to C60 since its discovery and preparation in macroquantities, to our knowledge no adsorption studies of this material have been reported, except for a number of publications from our laboratory.3-5 Application of IR spectroscopy, one of the major tools in surface science research, posed some difficulties in the study of adsorbates on graphite and carbon films, due to low transparency and reflectivity of these materials. This difficulty is not encountered in the case of C60. Recently, we have reported on IR transmission spectra of CO and some other molecules adsorbed on high surface area films of C60. The films were prepared in a low-temperature adsorption cell, which served simultaneously for spectroscopic and adsorption studies.3 The high surface area C60 is deposited onto a cooled central window, usually CsBr, kept at 77 K (liquid nitrogen), by evaporating small C60 crystals (99.95% pure) from a small tantalum crucible placed opposite to the window during deposition. At a sublimation temperature of 670 K, the deposition of the film is usually completed within about 30 min. The films are annealed for several hours at 100-120 K to ensure their greater stability and reproducibility of results. The transparency of the films is about 50%. Four strong and narrow absorption bands appeared in the IR spectrum of the films at 1430, 1183, 580, and 530 cm-1, together with numerous weak absorptions. Due to very high symmetry of the C60 molecule, only four fundamental vibrational transitions are allowed and correspond to the strong absorption bands. The remaining peaks are due to other transitions which become partially allowed due to the presence of 13C isotopes, combination bands, and (3) Fastow, M.; Kozirovski, Y.; Folman, M.; Heidberg, J. J. Phys. Chem. 1992, 96, 6126. (4) Fastow, M.; Kozirovski, Y.; Folman, M. J. Electron Spectrosc. Relat. Phenom. 1993, 64/65, 843. (5) Fastow, M.; Kozirovski, Y.; Folman, M. Surf. Sci. 1995, 331-333, 121.

© 1997 American Chemical Society

Surface Heterogeneity of C60

Figure 1. IR spectrum of CO adsorbed on C60 at different equilibrium pressures.

overtones. The four main frequencies were found earlier by other researchers and have been cited in the literature.6 On adsorption of CO on these films the spectrum shown in Figure 1 was obtained at different surface coverages.3 The three absorptions were obtained for CO equilibrium pressures of 0.1, 0.7, and 1.7 Torr. All these pressures were much below the saturation pressure at the working temperature; thus condensation was excluded. Two partially superimposed and well-resolved absorption bands were recorded, positioned at 2135 ( 1 and 2128 ( 1 cm-1. The corresponding spectral shifts from the gas phase frequency are -8 and -15 cm-1. During the initial stage of adsorption the 2128 cm-1 band appeared first; the second peak, at 2135 cm-1, appeared as a weak shoulder. Upon further adsorption the latter one grew preferentially and at higher surface coverages became the dominant one. When the system was pumped at 77 K, the 2135 cm-1 absorption band disappeared first. Another feature of the spectrum is the stability of the peak frequencies, with increasing surface coverage. In our previous work on IR spectra of CO adsorbed on NaCl and on LiF, a single band appeared in the spectrum in the former case and a doublet for LiF. The bands shifted in frequency with increasing surface coverage. This finding was explained as due to static and dynamic dipole-dipole and higher multiple interactions. Such shifts were treated quantitatively, by Mahan and Lucas, by Heidberg, and by Ewing and his collaborators.7 The appearance of two absorption bands even at low coverages suggests that CO was adsorbed on two different sites. This assumption is supported by the observation of different growth rates for the two absorption bands. It appears that the site which gives rise to the 2128 cm-1 (6) Kra¨tschmer, W.; Fostiropoulos, K.; Huffman, D. R. Chem. Phys. Lett. 1990, 170, 167. (7) (a) Mahan, G. D.; Lucas, A. A. J. Chem. Phys. 1978, 68, 1344. (b) Disselkamp, R.; Chang, H. C.; Ewing, G. E. Surf. Sci. 1990, 240, 193. (c) Noda, Ch.; Ewing, G. E. Surf. Sci. 1990, 240, 181. (d) Heidberg, J.; Stahmer, K. N.; Stein, H.; Weiss, H. J. Electron Spectrosc. Relat. Phenom. 1987, 45, 87.

Langmuir, Vol. 13, No. 5, 1997 1119

band is the more energetic one. The shift of this band is larger, and its disappearance on desorption required much longer time. There are a number of possible adsorption sites on the C60 film. These include sites on the C60 cage itself and adsorption between the relatively large and very loosely packed C60 cages. Since CO is a small molecule, it can easily penetrate into the voids between the C60 cages. Before we treat this problem in a more quantitative way, we would like to stress that in a previous IR investigation of CO adsorbed on graphite and diamond films,8 only single absorptions were found. For graphite a band shifted by -4 cm-1 to lower frequency with respect to the gas phase absorption (2143 cm-1) was found; for diamond a single band shifted by only 1 cm-1 to higher frequencies was recorded. As already stressed for C60, as compared to graphite and diamond, much larger shifts in frequency were found. Since the spectral shift is a measure of the perturbation by the surface field of the adsorbent on the vibration of the adsorbate, it is only reasonable to assume that the interaction between CO and C60 is stronger than with the other two carbon allotropes. The observation of larger spectral shifts with C60 is consistent with the existence of adsorption sites between the C60 cages; in this case, the interaction energy being mainly of a dispersion type, depends directly on the number of neighboring atoms. 2. The Calculations In order to determine the nature of the adsorption sites, calculations of adsorption potentials and spectral shifts for the CO/C60 system were performed. The CO spectral shifts were calculated by means of the perturbation method, and were compared to the IR spectra of CO in the gas and in the adsorbed phase on C60, graphite, and diamond. Since the system under consideration represents a multiparticle problem, exact methods of adsorption potential calculations are difficult. Therefore, approximation methods have often been employed with quite good results. Low-temperature adsorption of CO on C60 and other carbon allotropes is physical in nature; therefore potentials suitable for this type of adsorption were chosen. Adsorption potential of CO on C60 was calculated by summing the dispersion and the repulsion potentials between the adsorbed molecule and each atom of the adsorbent. In the calculations, the 6-exponential and the Lennard-Jones potentials were used. The results obtained from the 6-exponential potential for the hcp (001) and hcp (111) planes were described very recently.5 In the present publication we present results obtained by means of the 6-exponential potential for the fcc (100) and fcc (111) planes, as well as results obtained from the Lennard-Jones potential for the different structures and surface planes. With the 6-exponential potential the total adsorption potential is given by

φA ) φD + φR

(1)

Here φD is the dispersion potential and φR the repulsion potential. The dispersion potential was calculated using the well-known Kirkwood-Mu¨ller expression, which is (8) (a) Tsidoni, E.; Kozirovski, Y.; Folman, M.; Heidberg, J. J. Electron Spectrosc. Relat. Phenom. 1987, 44, 89. (b) Gevirzman, R.; Kozirovski, Y.; Folman, M. Trans. Faraday. Soc. 1969, 65, 2206.

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given as a sum of pair interactions of the adsorbed molecule and each atom of the adsorbent:

φD ) 6mc2

RsRa Rs/χs + Ra/χa

∑i Ri-6

(2)

Here subscript a refers to the adsorbate (CO molecule) and the subscript s to the C atom of the substrate, m is the mass of the electron, c the velocity of light, Ra and Rs the polarizabilities, and χa and χs the diamagnetic susceptibilities of the adsorbate and the C atom in C60. For CO Ra ) 1.95 × 10-24 cm3 and χa ) -16.27 × 10-30 cm3. For C in C60, Rs was taken as 1.09 × 10-24 cm3, a value slightly higher than that of carbon in graphite or diamond. For χs, a value given in the literature was used, χs ) -10.54 × 10-3 cm3, close to the value in graphite. The lattice sums ΣR-6 were calculated by computer for different surface sites and crystallographic structures. The summation was performed over two planes; each plane contained 36 molecules. Due to the fast convergence of the sum, there was no need to extend the summation to additional planes. The repulsion potential was calculated from

[

φR ) B exp(-cR) ) (BsBa)1/2 exp -

(cs + ca) R 2

]

(3)

The constants B and c were taken as geometrical and arithmetical means, respectively, of the corresponding constants for repulsion between the adsorbate molecules and the adsorbent atoms themselves. Reliable data exist for repulsion constants for CO in the gas phase and for crystalline CO. However, the constant Bs for repulsion between carbon atoms has to be calculated from the C60 intermolecular potential by equating the first derivative at minimum to zero. Thus Bs is given by

Bs ) -18mc2Rsχs/r607cs exp[- (csr60)]

(4)

here r60 is the closest distance between two carbon atoms, each of them located on adjacent C60 molecules (r60 ) 2.9 × 10-8 cm). cs was taken equal to that value given in the literature for carbon atom in graphite. Additional numerical values of the different constants are given in ref 5. The second calculation was done by means of the L-J equation

φLJ ) -AR-6 + BR-12

were (1) the void space between four neighboring C60 molecules, (2) the void space between two neighboring C60 molecules, (3) the center of the hexagon on the C60 surface, and (4) the center of the pentagon on the C60 surface. For the fcc (111) and hcp (001) surfaces the following sites were chosen for the calculations: (1) the void space between three neighboring C60 molecules; (2) the void space between two neighboring C60 molecules; (3) the center of the hexagon on the C60 surface; (4) the center of the pentagon on the C60 surface. Spectral Shift Calculations. The spectral shifts were calculated for CO adsorbed at the different adsorption sites and surface faces of the two structures. This was done by using the perturbation method in which the adsorption potential acts as the perturbation Hamiltonian. In the calculation it was assumed that the center of the molecule remains at the equilibrium distance from the surface, and during the vibration, the perturbation oscillates as a result of small changes in the polarizability of the molecule with the change in the vibration coordinate. Moreover, it was assumed that there was no interaction between the adsorbed molecules themselves (no lateral interaction) and also that there was no interaction between the internal mode and the vibration of the whole molecule with respect to the surface. Following Buckingham11 the total Hamiltonian of the adsorbed CO molecule is given by

H ) Ha + Hs where Ha is the Hamiltonian of the anharmonic oscillator and Hs the perturbation due to the adsorbent. The energy levels are given by the general expression

Wn ) Wna + Wns

where Wna represents the n energy level of the anharmonic oscillator and Wns the first- and second-order correction to the energy due to perturbation of the surface.

s

Wn )

∫ φn

a*

(9) Girifalco, L. A. J. Phys. Chem. 1991, 95, 5370. (10) Rice, W. E.; Hirschfelder, J. J. Chem. Phys. 1954, 22, 187.

a

Hsφn dr +

∑n

[

∫ φna*Hsφma dr]2 Wna - Wma

(7)

here r is the vibration coordinate. The shift in frequency resulting from this procedure is given by

(5)

The A and B constants were taken as geometrical means of the corresponding values for attraction and repulsion between the adsorbate molecules in the gas and the adsorbent atoms themselves. R is the distance between the adsorbed molecule and each carbon atom in C60. The constants A and B for C60 exist in the literature9 and those for CO were taken from previous calculations.5 The total adsorption potential φA is a sum of φLJ expressions. C60 crystallizes in a fcc structure, however, an hcp structure is obtained if some impurities, or gases, are present during condensation of C60 vapors, which was the case in our experiments. The adsorption potential calculations were performed for the two structures. Two surface planes, the (100) and the (111), were chosen as adsorption planes on which a number of sites were selected. For the fcc (100) and the hcp (111) structures the sites

(6)

∆ν )

( )

B0 (φ ′′ - 3aφA′) hν0 A

(8)

where B0 is the rotational constant of the molecule (B0 ) 1.93 cm-1), ν0 the harmonic frequency of the oscillator (ν0 ) 2170.2 cm-1), and a the anharmonicity constant.11 For pure vibrational transition, in absence of rotation, which applies to our system a ) -1, φ′ and φ′′ are the first and the second derivatives of the perturbing potential. Since the Lennard-Jones potential cannot be expanded in terms of the vibration coordinate of the adsorbate, spectral shifts for CO adsorbed at the different sites were calculated by means of the 6-exponential potential only. The spectral shift is given as a sum of contributions from dispersion and repulsion potentials. In the dispersion potential, the polarizability Ra is the property which changes with the vibration. For the differentiation of this potential, only Ra in the numerator was considered. The influence of the (11) Buckingham, A. D. Trans Faraday Soc. 1960, 56, 753.

Surface Heterogeneity of C60

Langmuir, Vol. 13, No. 5, 1997 1121

Figure 2. 6-exponential potential curves of CO adsorbed on fcc (100) of C60: (a) site between four C60 molecules; (b) site between two C60 molecules; (c) site in the center of the hexagon.

expansions of Ra in the denominator is negligible and therefore was considered constant. The contribution of dispersion to ∆ν is given by:

∆νdisp )

[( ) ( ) ]

∂Ra B0M ∂2Ra 2 re + 3 r 2 hν0 ∂r ∂r e

(9)

where

M ≡ 6mc2

Rs Rs/χs + Ra/χa

∑i Ri-6

The first and the second derivatives of R with respect to r, the internuclear distance, are known both from theoretical expressions and from experiment (Raman band cross section). The contribution of the repulsion potential to the spectral shift was calculated by differentiating this potential with respect to the distance from the surface of the vibrating molecule:

∆νrep )

B0 Bc exp(-cRe) exp(cre)(c + 3) hν0

(10)

where B and c were defined in eq 3. 3. Results and Discussion As already mentioned the adsorption potentials and the spectral shifts were calculated for the CO molecule adsorbed on the fcc (100) and (111) and hcp (100) and (111) structures of C60. Detailed results obtained with the 6-exp potential for the (001) and (111) planes of the

Figure 3. Lennard-Jones potential curves of CO adsorbed on fcc (111) of C60: (a) site between three C60 molecules; (b) site between two C60 molecules; (c) site in the center of the hexagon.

hcp structure were described very recently.5 Here results for the fcc (100) and (111) planes using the 6-exp potential, together with results obtained by means of the L-J potential, will be given. Figure 2 shows the adsorption potential dependence on the CO distance from the fcc (100) surface plane for different adsorption sites. In Table 1 are summarized the numerical values: the equilibrium distance, the potential value at the minimum and the calculated spectral shifts for both fcc (100) and hcp (111). It is evident that the site of the lowest adsorption potential -5.8 kcal/mol is the void space between four neighboring C60 molecules on the fcc (100) surface plane. On this site the center of the CO molecule is located 3.8 Å below the surface plane (the surface plane is tangential to the outermost C60 molecules). For this configuration the calculated spectral shift amounts to ∆ν ) -15.3 cm-1. These values are quite close to the results obtained for CO on the hcp (111) surface plane (site between 4 molecules), for which the potential minimum is -5.9 kcal/ mol, the equilibrium distance is -3.4 Å, and the spectral shift is -15.9 cm-1. The remaining calculated results for the different adsorption sites on fcc (100) and (111) planes are very similar to those obtained for the hcp (111) and (001) planes. The next energetic site is for CO adsorbed in the void space between three C60 molecules on the fcc (111) and hcp (001) planes. The adsorption potential is -4.1 kcal/mol and the equilibrium distance is 0.1 Å for both structures. The calculated frequency shift is -13.5 cm-1 for the fcc structure and -13.8 cm-1 for the hcp structure.5 The L-J Potential. In Figures 3 and 4 are shown the adsorption potentials calculated by means of the Lennard-

Table 1. Equilibrium Distances, Adsorption 6-exp Potentials, and Spectral Shifts, Calculated for CO Adsorbed on the (100) and the (111) Planes of fcc C60 adsorption site

plane

equilibrium distance, Re (Å)

potential φA (kcal/mol)

spectral shift ∆ν (cm-1)

void space between four C60 molecules void space between two C60 molecules on top of C60 molecule above the center of the hexagon void space between three C60 molecules void space between two C60 molecules on top of C60 molecule above the center of the hexagon

(100) (100) (100) (111) (111) (111)

-3.8 +1.1 +3.2 +0.1 +1.1 +3.2

-5.76 -2.79 -1.82 -4.05 -2.91 -1.85

-15.3 -9.2 -6.3 -13.5 -9.1 -6.4

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Table 2. Equilibrium Distances, Adsorption L-J Potentials Calculated for CO Adsorbed on the (100) and the (111) Planes of fcc C60 adsorption site

plane

equilibrium distance, Re (Å)

potential, φA (kcal/mol)

void space between four C60 molecules void space between two C60 molecules on top of C60 molecule above the center of the hexagon void space between three C60 molecules void space between two C60 molecules on top of C60 molecule above the center of the hexagon

(100) (100) (100) (111) (111) (111)

-3.4 +1.7 +3.6 +0.8 +1.7 +3.6

-5.05 -2.12 -1.31 -3.00 -2.24 -1.34

Table 3. Equilibrium Distances, Adsorption L-J Potentials Calculated for CO Adsorbed on the (111) and the (001) Planes of hcp C60 adsorption site

plane

equilibrium distance, Re (Å)

potential, φA (kcal/mol)

void space between four C60 molecules void space between two C60 molecules on top of C60 molecule above the center of the hexagon void space between three C60 molecules void space between two C60 molecules on top of C60 molecule above the center of the hexagon

(111) (111) (111) (001) (001) (001)

-3.4 +1.7 +3.6 +0.8 +1.7 +3.6

-5.79 -2.13 -1.31 -3.01 -2.25 -1.34

Figure 4. Lennard-Jones potential curves of CO adsorbed on hcp (111) of C60: (a) site between four C60 molecules; (b) site between two C60 molecules; (c) site in the center of the hexagon. The dashed curves were obtained for the same sites by means of the 6-exp potential.

Jones expression. The numerical results are summarized in Tables 2 and 3. It is quite evident that for adsorption sites in voids between four C60 molecules (fcc (100) and hcp (111)) the calculated values are quite similar to those obtained from the 6-exp potential, whereas for the other sites the values obtained from the two potentials differ. In general, the values obtained by means of the 6-exp potential are lower (higher absolute values) and the equilibrium distances are smaller as compared to the results from the L-J potential. For example, for the site between three C60 molecules on the fcc (111) plane, the potential is -4.16 kcal/mol at 0.1 Å equilibrium distance, from the 6-exp potential, and -3.0 kcal/mol at 0.8 Å from the L-J potential.

Comparison with Experimental Results. From comparison of the calculated spectral shifts with the experimental ones, it is evident that the absorption band shifted by -15 cm-1 may be attributed to CO molecules adsorbed in the void spaces between four C60 molecules either on the hcp (111) plane or on the fcc (100) plane. The second band seen in the spectrum is shifted by -8 cm-1 from the gas phase value. It may be attributed to CO adsorbed between two C60 molecules on the hcp or fcc plane. As already mentioned, the C60 films were composed of small crystallites; therefore all the adsorption sites pertinent to these structures might be present. As already mentioned, with two other carbon allotropes, the IR spectrum of adsorbed CO on graphite showed only one absorption band positioned at 2139 cm-1 and one band at 2144 cm-1 for diamond. The corresponding shifts were ∆ν ) -4 cm-1 and ∆ν ) +1 cm-1. The much smaller shifts in these cases may be explained by the weaker interaction of the CO molecule with the two adsorbents. The adsorption sites on graphite are similar to those on top of a single C60 molecule. Summary The calculated adsorption potentials by means of the 6-exponential and Lennard-Jones expressions for CO adsorbed on C60, hcp and fcc structures, indicated that the most energetic adsorption sites are in void spaces between four C60 molecules (hcp (111) and fcc (100)) or in void spaces between three C60 molecules (hcp (001) and fcc (111)). The next energetic sites are in voids between two C60 molecules on all the surface planes. The two potentials gave quite close values for sites between four C60 molecules. For other sites the 6-expotential gave lower values (higher energies). The calculated spectral shifts (for adsorbed CO) are in good agreement with the previously obtained experimental values. LA960092Z