Langmuir 1987, 3, 159-163 before and after the destruction of the atomic scale roughness features by the deposition of a monolayer of Pb. It is not possible from the data reported here to unequivocally decipher the roles played by surface electronic properties or atomic scale roughness. However, it can be concluded that the SERS response of adsorbates in the presence of underpotentially deposited Pb appears to be sensitive to the chemical nature of the adsorbate and the
159
extent of adsorbate-metal interaction.
Acknowledgment. We acknowledge support of this research by the National &+~~ce l k ~ ~ M (CHEo n 8309454). The experimental assistance of Anita GUY and Mark A. Bryant is greatfully acknowledged. Registry No. Pb, 7439-92-1; Ag, 7440-22-4; C1-, 16887-00-6; Br-, 24959-67-9; SCN-, 302-04-5.
LEED Study of Benzene and Naphthalene Monolayers Adsorbed on the Basal Plane of Graphite U. Bardi, S. Magnanelli, and G. Rovida" Dipartimento di Chimica, Universitci d i Firenze, 50121 Firenze, Italy Received May 21, 1986. I n Final Form: November 6, 1986 The adsorption at low temperature of benzene and naphthalene on the basal plane of graphite has been studied by low-energy electron diffraction. One ordered monolayer phase was found for benzene and two distinct monolayer phases were found for naphthalene with different surface coverages. Benzene forms a hexagonal lattice where all molecules are adsorbed on unique sites of the substrate. In the case of naphthalene, in the low-coverage phase molecules are also adsorbed on unique sites. In the high-coverage phase, a long-range coincidence with the substrate periodicity is present. 1. Introduction The adsorption of aromatic molecules on the graphite basal plane is a subject of interest in the field of physisorption. This system represents also a simplified experimental model of the interactions of the carbon surface with aromatic surfactants used to obtain stable aqueous carbon slurries. In order to obtain informations on this subject, we carried out a study on the adsorption at low temperature of benzene and naphthalene on the basal plane of graphite, using low-energy electron diffraction (LEED) to obtain data on the structural parameters of the ordered phases formed. Benzene adsorption on the graphite (0001) surface has been already the object of several studies. Information about the structure of the adsorbed layer could be inferred from adsorption NMR ~ t u d i e s ,and ~ , ~PIES studies? Neutron diffraction7y8and X-ray diffraction9 gave direct structural information. However, no experimental data relative to naphthalene adsorption on graphite are available in the literature. Theoretical calculations relative to benzene and naphthalenelOJ1 indicate that aromatic (1)Pierce, C. J. Phys. Chem. 1969,73,813. (2)Isirikyan, A. A.;Kiselev, A. V. J. Phys. Chem. 1969,65,601. (3)Isirikyan, A. A.; Kiselev, A. V. J. Phys. Chem. 1962,66,205. (4)Boddenborg, B.;Moreno, J. A. Ber. Bunsenges. Phys. Chem. 1983, 87,83. (5)Tabony, J.; White, J. W.; Delachaume, J. C.; Coulon, M. Surf. Sci. 1980,95,L282. (6) Kubota, H.; Munakata, T.; Hirooka, T.; Kondow, T.; Kuchitsu, K.; Ohno, K.; Harada, Y. Chem. Phys 1984,87,399. (7)Monkenbusch, M.: Stockmever, R. Ber. Bunsenpes. Phvs. Chem. 1980,84,808. (8) Meehan. P.: Ravment. T.: Thomas. R. K. J . Chem. SOC..Faraday Trans. 1980,76,2011: (9)Gameson, I.; Rayment, T. Chem. Phys. Lett. 1986,123, 150. (10)Bondi, C.; Baglioni, P.; Taddei, G. Chem. Phys. 1986,96,277. (11)Bondi, C., tesi di Laurea in Chimica, Universitl di Firenze, 1984.
molecules should be adsorbed on graphite with the ring plane parallel to the surface plane and that benzene and naphthalene should form an incommensurate cell with respect to the substrate. In a previous paper,12we reported the results of a LEED study of low-temperature benzene adsorption on graphite. In the present work we will review these results and report data relative to the adsorption of naphthalene on the same surface in the range 135-150 K. We found that one ordered adsorbed phase exists for benzene and two different ordered phases for naphthalene. 2. Experimental Section LEED measurements were carried out in an UHV chamber with base pressure in the torr range, equipped with three-grid LEED optics. The sample was a single-crystalgraphite platelet oriented along the (OOO1) plane, cleaved in air before introduction in the vacuum chamber. The sample was mounted on a tantalum plate which was in contact with a copper surface, cooled by liquid nitrogen circulation. The sample could also be annealed by heating the tantalum plate by resistive effect. The purity of the sample surface was controlled by AES spectroscopy, using a grazing incidence electron gun and the LEED optics as a retarding field analyzer of the Auger electrons. Temperatures were measured by means of a copper/constantan thermocouple spot welded on the tantalum plate. Due to the position of the thermocouple, temperature, measurements can be expected to be accurate within about f 5 K. Benzene and Naphthalene vapors were introduced directly in the vacuum chamber by means of a leak valve. The purity of the gases was monitored by a quadrupole mass spectrometer.
3. Results After annealing in vacuum at about 600 K, it was possible to find regions of the sample surface showing a sharp (12)Bardi, U.;Magnanelli, S.; Rovida, G. Surf. Sci. 1986,165, L7.
0743-7463/87/2403-0159$01.50/0 0 1987 American Chemical Society
160 Langmuir, Vol. 3, No.2, 1987
LEED pattern of hexagonal symmetry. The LEED pattern of the C(OOO1) plane should have trigonal symmetry, rather than he~agona1.l~However, the hexagonal symmetry is commonly observed on cleaved samples" as a result of the presence of more than one equivalent domain on the surface. Adsorption measurements were carried out, keeping the sample a t a temperature of 135 K in the case of benzene and in the region between 140 and 160 K for naphthalene. We observed that the LEED pattern of the graphite surface remained unaltered under the electron beam for extended periods of time. However, if benzene or naphthalene were condensed on the surface, the overlayer LEED pattern deteriorated and disappeared in periods on the order of a few minutes under the effect of the primary electron beam. Under these conditions, we noticed also a gradual irreversible deterioration of the pattern of the substrate, which indicates that the effect of the beam was not simply to cause the benzene or naphthalene desorption but also decomposition of the adsorbed molecules. The residuals of this decomposition could not be eliminated from the surface, even by thermal treatment over 600 K. T o minimize surface deterioration, all LEED observations reported here were performed with a low primary current. The beam could also be moved over different areas of the surface. Nevertheless, after a number of adsorption tests, it was necessary to restore the crystallinity of the substrate surface by a new cleavage in air. 3.1. Benzene. As described in ref 12, exposing the sample to a pressure of 3 X 10" torr of benzene for 30 s at 135 K led to the appearance of new beams in the LEED pattern due to the formation of an ordered benzene phase (Figure la). For this structure, the coincidences of the overlayer and substrate pattern permit a unequivocal determination of the overlayer unit cell parameters. Using the Wood notation, this pattern can be described as a (d7Xd7)R19.1° or in matrix notation as
Bardi et al.
P
Figure 1. LEED patterns observed after exposing the graphite surface to henzene and naphthalene: (a) benzene (39.5eV) (the indexes refer to the graphite substrate); (b) naphthalene lowcoverage structure (38 eV): (c) naphthalene high-caverageStNctUre (35 eV).
The area of the unit cell for this phase is 36.7 A2. The unit cell of the adsorbate is the same as that derived from neutron diffraction',8 and X-ray diffractiong in the same range of temperature. However, LEED data furnish information which cannot be derived directly from diffraction data from powder samples, that is the angle formed by the overlayer and substrate crystal lattices. This phase was found to be stable at 135 K in UHV conditions for long periods of time if it was not exposed to the electron beam. No other ordered phases were observed. Higher exposures t o benzene vapor led to the gradual disappearance of all diffraction features, a result which can be interpreted as due to the formation of a disordered multilayer. The overlayer diffraction pattern also disappeared upon raising the temperature of the sample in a range between 140 and 150 K. Since the pattern reappeared upon cooling, this effect can be attributed to the 2D solid-2D liquid reversible phase transition reported in ref 7,8in the same range of temperatures. If the sample was brought to a temperature equal to or higher than 170 K, the overlayer pattem disappeared and did not reform upon cooling, indicating a complete desorption of the henzene layer. 3.2. Naphthalene. Exposing the sample to naphthalene vapor in the range of temperatures between 140 and 155 K, we observed the formation of two ordered phases. (13) Wu,N.J.; Ignatiev. A. Phys. Reo. B 1982,25.2983. (14) Lander. J. J.: Morrison, J. J. Appl. Phys. 1964.35, 3593.
The LEED pattern relative t o the first phase appeared after exposure to naphthalene at 152 K a t p = 6 X lo4 torr for 100 s (Figure lb). This pattern can be described as a (2\/3X2d3)R30° by using the Wood notation or in matrix notation as
(-$;) The symmetry of the pattern is apparently hexagonal, however, since naphthalene molecules have a 2-fold symmetry, assuming one molecule per cell, the observed symmetry must derive from the simultaneous observation of diffraction from several domains with different orientations. Therefore the unit cell of this phase should be described as rhombic with an angle of 120°. The base vectors form an angle of 30' with the base vectors of the substrate. The periodicity of the overlayer indicates that naphthalene molecules in this structure are adsorbed in unique sites (assuming one molecule per unit cell). Further exposure to naphthalene in the same conditions of pressure and temperature led to the appearance of new beams (Figure IC) in the pattern. These beams initially coexisted with those of the first pattem, subsequently they increased in relative intensity, eventually leading to the complete disappearance of the first pattem. This second naphthalene phase should correspond to a higher surface coverage, since a higher exvosure to naDhthalene is necessaryto form it. For this Dhase. onlv fmtorder diffraction features could be detected. Therefore it was not possible to determine
LEED Study of Benzene and Naphthalene Monolayers 7
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A
A
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n
n
n
Langmuir, Vol. 3, No. 2, 1987 161 r-
Figure 2. Comparison of the unit cells of the low-coverage (structure 1)and high-coverage (structure 2) phases of naphthalene on graphite. For both phases, four unit cells of the adsorbate are drawn, in order to show the coincidence mesh for structure 2. Two equivalent domains of structure 2 are shown, for one of them (broken lines) the possible interpretation with a rectangular cell (heavy broken lines) is also shown. i
the unit cell parameters as univocally as for the low-coverage phase, where well-defined coincidences with the substrate could be detected. The LEED pattern of the high-coverage phase can be interpreted with a rhombic unit cell with 120’ angle. This cell can be derived from the low-coverage cell by means of a contraction of the length of the base vectors and a rotation. The contraction can be estimated to range from 4 % to 6% with respect to the base vectors of the low-coverage cell and the rotation as 22.5’ f 0.5’, again with respect to the first observed phase. However, as it will be discussed in detail in the next section, it is also possible to interpret this LEED pattern with a rectangular unit cell, with two molecules per cell. Both these interpretations are shown in Figure 2, together with the unit cell of the low-coverage phase. As it is evident from the figure, for both rectangular or rhombic interpretations of the unit cell, a well-defined long-range coincidence of the substrate and overlayer lattices appears to exist. Because of this coincidence, one naphthalene molecule every four can be adsorbed on the same type of site. The rhombic coincidence cell for the high coverage phase can be written in matrix notation as
(-! i) Therefore the primitive rhombic unit cell for the adsorbed layer is
The matrix describing the rectangular cell for the highcoverage phase can be written as
Structural models relative to these phases will be examined in the next section. 4. Discussion Most of the data reported in literature about benzene adsorption on the basal plane of graphite lead to a model of the surface structure, under or a t the monolayer coverage, where the aromatic ring lies parallel to the substrate surface. This conclusion derives from the determination of the area per molecule from adsorption isothermslg and from the parameters of the unit cell observed by neutron diffraction'^^ and X-ray diffraction? The calculations by
i
Figure 3. Model for the structure of the (d7Xd7)R19.lophase of benzene adsorbed on graphtie. Minimum H-H intermolecular distance is 2.5 i\.
Bondi et a1.loJ1 also lead to the same conclusion. A different model is presented in a NMR study4 where it is proposed that benzene molecules can be adsorbed on graphite with the aromatic plane normal to the substrate. This discrepancy in the interpretation of the data can be tentatively explained as due to a high concentration of defects on the surface of the sample used in ref 4. High density of defects and/or lack of homogeneity of the surface may trigger the formation of areas covered by a multilayer even before the coverage of the whole substrate surface is completed. In such case the surface structure may be quite different, since neutron diffra~tion,~ NMR,5 and PIES6 concur in suggesting that, for coverages higher than the monolayer, the benzene layer forms a structure where the ring plane is approximately normal to the substrate plane. Most of the data existing in literature do not permit to take definitive conclusions about the relation of the overlayer and substrate periodicity and in particular if aromatic molecules are adsorbed on specific sites. Theoretical calculations by Bondi et al.IOhave suggested that benzene and naphthalene form incommensurate structures (e.g., where molecules do not occupy specific sites). On the contrary, diffraction data for b e n ~ e n e l -have ~ been interpreted with commensurate structure models (molecules adsorbed “on site”). However, only LEED data can provide a definitive proof of this point. Considering first the case of benzene, the unit cell of the observed phase is compatible with a model where molecules are adsorbed flat on the surface. Assuming that the molecule is not distorted upon adsorption, the lowest intermolecular repulsion occurs for the model shown in Figure 3, where the minimum intermolecular H-H distance assumes a value of 2.5 A. This model is the same as the one proposed on the basis of neutron diffraction data.Ip8 Benzene molecules can assume this orientation without a strong repulsive intermolecular interaction considering that the minimum H-H intermolecular distance in solid aromatic hydrocarbons is reported to vary between 2.02 and 2.10 A.15 Models considering a radically different orientation of the benzene molecules can be discarded as they would need considerably smaller H-H intermolecular distances. (15)Bondi, A. J.Phys. Chem. 1964, 68, 441.
162 Langmuir, VoE. 3, No. 2, 1987
Bardi et a1
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Figure 4. Models for the low-coverage phase of naphthalene adsorbed on graphite. In (a) the long axis of the naphthalene molecules forms an angle of 8 O with the unit cell side. Minimum intermolecularH-H distance is 2.15 A. In (b) the same axis forms an angle of 30° with the cell side. Minimum H-H intermolecular distance is 1.9 A.
The periodicity of the d7Xd7R19.1° benzene phase requires that all molecules are adsorbed on the same type of site. From our LEED data the actual site of adsorption cannot be derived. However, the calculations reported in ref 7 and ref 10 and 11 indicate that an isolated benzene molecule should lie with the center of the ring on top of a carbon atom of the substrate. Accordingly, this type of site has been chosen for the model shown in Figure 3. Considering now the case of naphthalene, comparing the size of a molecule lying flat on the surface and the periodicity and area of the unit cell of the first observed phase ("low-coverage'' phase: (2d3X2d3)R30°), it appears possible, as in the case of benzene, to fit one naphthalene molecule per cell, provided that the molecular axis is oriented in such a way to minimize intermolecular repulsion. Using structural data for the naphthalene molecule reported in ref 16 and assuming that the interatomic distances in the molecule do not change upon adsorption, it is possible to find the molecular orientation which maximizes the intermolecular H-H distances and therefore minimizes steric repulsion. The best model in this sense for the low-coverage phase is shown in Figure 4a. Here the long axis of the naphthalene molecule forms an angle of approximately 8 O with the side of the overlayer unit cell. In such an arrangement the minimum H-H intermolecular distance is 2.15 A. For comparison, in solid naphthalene the shortest H-H intermolecular distance is 2.02 A15 Another orientation of the molecules which produces a low intermolecular repulsion can be found for a model where the long axis of the naphthalene molecule forms an angle (16) Wyckoff, R. W. G. Crystal Structures; Wiley: New York, 1951.
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