J. Phys. Chem. 1987, 91, 2947-2950
2947
Spectroscopic Evidence for Surface Anion Radical Formation of Tetracyanoethylene Adsorbed on Cu(ll1) at 100 K: A High-Resolution Electron Energy Loss Study W. Erley* and H. Ibach Znstitut fur Grenzflachenforschung und Vakuumphysik, Kernforschungsanlage Julich, D-5170Julich, West Germany (Received: December 9, 1984; I n Final Form: February 2, 1987) Adsorption of tetracyanoethylene (TCNE) on a Cu(ll1) single-crystal surface at 100 K has been studied by high-resolution electron energy loss spectroscopy. Upon initial adsorption there is spectroscopic evidence for partial fragmentation of TCNE molecules. For increasing coverages up to one monolayer, a multiply charged TCNE ion is identified as the dominant species on the copper surface. Further exposure to TCNE produces vibrational modes Characteristic of TCNE anions, most likely adsorbed in a second layer. Finally, multilayer condensation for high exposures is indicated by the observation of virtually all vibrational modes of crystalline TCNE.
Introduction Tetracyanoethylene (TCNE) is one of the strongest a-acceptors known. It easily forms charge-transfer complexes with organic molecules' and metals,2 exhibiting a number of remarkable properties, the best known of which are the very low specific resistivities often observed in organic compounds of TCNE. Because of the strong interest in the fundamental properties of charge-transfer mechanisms, ground electron states, vibronicelectronic interactions, and r-bonding, TCNE and its anions have been the subjects of several vibrational studies.*" Other examples include the use of T C N E as a modifier to improve hydrogen storage in Mg2Nii2and of thin films of CuTCNE between metal electrodes to serve as fast optical ~ w i t c h e s . ' ~ Whereas conventional vibrational spectroscopies like infrared and Raman spectroscopy (the latter with the exception of resonance and surface enhanced effects) generally lack the sensitivity to measure the vibrational spectra of adsorbates on solid surfaces in the submonolayer range, inelastic electron tunneling spectroscopy (IETS) l4 yields both good resolution and high surface sensitivity over a wide spectral range. However, this method is limited to insulating surfaces, and the sample must be in the form of a tunnel junction. High-resolution electron energy loss spectroscopy (EELS)IS provides high surface sensitivity over a large spectral range, but with limited resolution ( 2 3 0 cm-I). The latter method has successfully been applied to measure the vibrational spectra of large organic molecules adsorbed on technical surfaces.I6 As is shown here, it is likewise well suited to study submonolayer as well as multilayer adsorption on surfaces. W e find that-after the occurrence of some initial fragmentation-TCNE adsorbs on Cu(1 l l ) at 100 K essentially as a multiply ionized molecular species up to monolayer coverage. A singly charged anion is identified as the dominant species in the second layer. A further increase of coverage leads to the condensation of neutral TCNE. (1) Looney, C. E.; Downing, J. R. J. Am. Chem. SOC.1958, 80, 2840. (2) Long, D. A,; George, W. 0. Spectrochim. Acta 1963, 19, 1717. (3) Miller, F.A.; Sala, 0.; Devlin, P.; Overend, J.; Lippert, E.; Liider, W.; Moser, H.; Varchmin, J. Spectrochim. Acta 1964, 20, 1233. (4) Takenaka, T.; Hayashi, S. Bull. Chem. SOC.Jpn. 1964, 37, 1216. (5) Rosenberg, A.; Devlin, J. P. Spectrochim. Acta 1965, 21, 1613. (6) Moore, J. C.;Smith, D.; Youhne, Y.; Devlin, J. P. J. Chem. Phys. 1971, 75, 325. (7) Hinkel, J. J.; Devlin, J. P. J . Chem. Phys. 1973, 58, 4750. (8) Khatkale, M. S.;Devlin, J. P. J . Chem. Phys. 1979, 83, 1636. (9) Yokojama, K.;Maeda, S. Bull. Chem. S o t . Jpn. 1980, 53, 1949. (IO) Michaelian, K. H.; Rieckhoff, K. E.; Voigt, E. M. Spectrosc. Lett. 1977. 10. 99. (1'1) Michaelian, K.H.; Rieckhoff, K. E.; Voigt, E. M. J . Mol. Spectrosc. ~
-.
-1982. - - - 7 -95. 1- -
(12) Imamura, H.; Takahashi, T.; Galleguillos, R.; Tsuchiya, S. J. LessCommun. Met. 1983, 89, 251. (13) Potember, R. S.;Poehler, T. 0.;Benson, R. C. Appl. Lett. 1982, .. Phys. . 41, 548. (14) Hansma, P.K.Phys. Rep. 1977, 30C, 145. (15) Ibach, H.;Mills, D. L. Electron Energy Loss Spectroscopy and Surface Vibrations; Academic: New York, 1982. (16) Wandass, J. H.; Gardella, Jr., J. A. Surf. Sci. 1985, 150, L107.
0022-3654/87/2091-2947$01.50/0
Experimental Section Experiments were performed in an ultrahigh vacuum (UHV) system which has been described in detail elsewhere.18 Lowenergy electron diffraction (LEED) and Auger electron spectroscopy (AES) were used to characterize the clean C u ( l l 1 ) surface and to monitor the relative coverage of TCNE. Highresolution electron energy loss spectra were measured in the specular direction (angle of reflection of 70') with an electron energy of 5.0 eV. The Cu( 111) single crystal (10 mm in diameter, 2 mm thick) was cleaned by repeated cycles of Ne+ ion bombardment and subsequent annealing to 1000 K. High-purity crystalline T C N E was carefully outgassed in a small glass bulb prior to exposing the copper sample ( T = 100 K) to the T C N E vapor via a heated doser tube. Results and Discussion Representative EELS spectra of T C N E adsorbed on a clean C u ( l l 1 ) single-crystal surface at 100 K are shown in Figures 1 and 2. The structure of the planar TCNE molecule is indicated on the upper right side of Figure 1. As a measure of the relative coverage of adsorbed TCNE the ratios of the peak-to-peak values of the C272and Cugz0Auger transitions are indicated on the right sides of the spectra. As will be shown below, monolayer coverage of T C N E is represented by an Auger value of about 0.1 1. Surface cleanliness of the copper sample prior to T C N E exposure is verified in the featureless spectrum shown in Figure la. For the sake of simplicity we start our discussion with the spectrum presented in Figure IC,as only a relatively small number of major bands is observed in this spectrum. In the region from 2000 to 2300 cm-I, which is characteristic for the symmetric C N stretching modes of the different ionization states of TCNE, an intense loss at 2035 cm-' is observed. According to the work of Kathkale and Devlin; this frequency lies between 2006 and 21 16 cm-' which are the characteristic C N stretching frequencies of tri- and dianions of TCNE, respectively. In our opinion this provides sufficient spectroscopic evidence for the presence of a multiply (approximately triply) charged TCNE ion as the dominant species adsorbed on the Cu surface in this coverage range. Consequently, we should expect a symmetric C=C stretching vibration to appear in the range from 1256 to 1346 cm-I, which are the frequencies reported for the doubly and triply ionized TCNE species, respectively. Indeed, strong mode is observed at 1275 cm-' (Figure IC) which falls in this range, however, indicating a charge closer to 2. This provides strong evidence for the presence of a multiply ionized TCNE anion radical as the primary species on the copper surface. In the following this species will be denoted as TCNEX-, where x is a value somewhere between 2 and 3. Before trying to assign further loss features to the characteristic modes of adsorbed TCNEX-, we must comment on the possible number of EELS active modes which may be observed. A free (17) Erley, W.;Ibach, H. Surf. Sci. 1986, 178, 565. (18) Erley, W.;McBreen, P. H.; Ibach, H. J . Catal. 1983, 84, 229.
0 1987 American Chemical Society
Erley and Ibach
2948 The Journal of Physical Chemistry, Vol. 91, No. 11, 1987
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assigned to neutral, singly, or multiply charged TCNE species are marked by (0), (-), or ( x - ) , respectively. TCNE molecule (& point group) has 24 internal modes, of which 5 are totally symmetric (A, modes). Generally, the symmetry of the surface complex formed after adsorption is lower than that of the gas-phase molecule. If one assumes that the molecule lies flat on the surface of the symmetry reduces at least to C2". The symmetry could reduce further when adsorption sites of lower symmetry are involved and the interaction with the surface atoms symmetry would be present if a T C N E is strong enough. A CzL. molecule (or one of its ions, all of which have virtually identical molecular geometry1*is adsorbed in a bridge or in an on-top position with its C=C bond parallel to the copper surface and with the four C N groups either parallel or symmetrically bent away from the surface (nontwisted). In this case, two other totally symmetric internal modes, the out-of-plane C C N bending mode and the out-of-plane wagging mode (the former B1, modes), should be observed in addition to the five totally symmetric A, modes of free TCNE molecules. A further mode would arise from the translational degree of freedom of the ad-species normal to the
m 2OOo ENERGY LOSS (cm" 1
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Figure 2. EELS spectra of TCNE adsorbed at 100 K on Cu(ll1) above
monolayer coverage. copper surface. After the assignment of the losses a t 2035 and 1275 cm-I to the symmetric C N and C=C stretching modes of the multiply ionized TCNE species, respectively, we identify the remaining symmetrical modes of TCNE"-. According to the studies of several author^,^^" the symmetric scissor mode and the out-of-plane wagging mode are expected to appear below 200 cm-I, bands which are generally difficult to observe with EELS. In the present case these bands would be certainly obscured by the strong mode at 245 cm-I, which does not result from an internal mode of the adsorbed molecule and will be discussed later. In our view there is only one reasonable choice for the symmetric C-C stretching mode of TCNE? the intense loss visible at 870 cm-l. This frequency is strongly blue-shifted by 191 cm-I with respect to 679 cm-l, a value that has been reported by various a ~ t h o r s ~for- ~the symmetric C-C stretching mode of neutral TCNE. So far, the assignment of the symmetric CN, C = C , and C-C stretching modes of the TCNE*- is at least qualitatively consistent with the bond order changes predicted from a simple Hueckel molecular orbital calculation by Penfold and Lipscomb.20 Their calculation predicts an increase in C-C and a decrease in C=C and C N bond orders. However, the blue shift of the C-C stretching mode in the present case is much larger than that (19) Philips, W. D.; Rowell, J. C.; Weissman, S . I. J . Chem. Phys. 1960, 33, 626. (20) Penfold, B R ; Lipscomb, W N. A d a Crysfaflogr.1961, 14, 589.
T C N E Adsorbed on Cu( 11 1) reported for a TCNE" species in the solid. This may be the consequence of an additional bonding between the C-C bond and the copper surface, including backdonation of electron charge into the C-C bonding orbital, leading to an increased C-C stretching frequency. Most authors assign a band a t 555 cm-' to the outof-plane C C N bending mode. Calculations* show that the corresponding modes for the different ionic species are not greatly shifted. Consequently, we assign the loss observed at 580 cm-l to the C C N bending mode of adsorbed TCNE"-. In search of a possible candidate of the in-plane C C N bending mode we can detect a weak shoulder at 470 cm-l on the low-frequency side of the loss at 510 cm-I. The intense loss of 510 cm-' may be attributed to the translational mode of the adsorbed TCNE species normal to the surface. All assignments made so far are consistent with the presence of a TCNE"- ad-species which forms a surface complex of C2, symmetry with the atoms of the C u ( l l 1 ) surface. However, there are some weaker losses observed in the spectrum of Figure I C which are obviously not due to the TCNE" species and which require further explanation. In the C N stretching region, we observe an additional weak loss at 2205 cm-'. This loss may be attributed either to neutral T C N E or to a TCNEanion (or both), as these species exhibit symmetric C N stretching frequencies at 2235 and 2000 cm-I, respectively. In the frequency region characteristic of C=C modes, we observe a shoulder at 1375 cm-I and a weak loss at 1565 cm-'. Whereas the loss at 1375 cm-I can definitely be assigned to the C=C stretching mode of the singly charged T C N E ion, the loss at 1565 cm-I can only be interpreted as the C=C stretch vibration of neutral TCNE.8 This provides good spectroscopic evidence that, in addition to the dominant TCNEx- species, there are small concentrations of both TCNE- anions and neutral TCNE species present on the Cu( 111) surface. All other modes correlated with adsorbed TCNEO and TCNE-, like e.g. the C-C stretching mode, may not be observable either because of low intensity or because of the limited instrumental resolution. Finally, there are only losses at 1055 and 1630 cm-' in spectrum I C which are left for interpretation. Both losses are more clearly observed in spectrum 1b for low T C N E coverage. Moreover, a careful inspection of a series of spectra taken at different TCNE exposures reveals that the intensities of the two losses observed at 1055 and 1630 cm-' are clearly correlated which implies that the two modes belong to a single surface species other than a T C N E ion. The mode at 1630 cm-l can only be explained as a C=C stretching mode. This value is higher than the corresponding one for neutral T C N E or of its ions.* One possibility would be a T C N E molecule from which one or even more C N groups have been ruptured upon adsorption on the Cu( 11 1) surface. This could result in a strengthening of the C=C bond, leading to a shift to a higher frequency. Simultaneously, the C-C stretching vibration of this T C N E fragment may be shifted to higher frequency, generating the loss observed at 1055 cm-I. A frequency of 1077 cm-I has been reported for the C-CN stretch vibration of ethyl cyanide.*l The mode observed at 2215 cm-' may be assigned to the symmetric C N stretching vibration of that intermediate. Upon partial fragmentation of T C N E in the beginning of the adsorption process, C N radicals should be formed on the copper surface. Indeed, the band observed at 2125 cm-' may be attributed to the C N stretching mode of an adsorbed C N radical. A C N stretching mode at 2125 cm-' has been reported for a C U ( C N ) complex ~ ion.22 The losses observed at 1275 and 2035 cm-l indicate that a small amount of TCNE"- is already present on the surface. Hence the spectrum shown in Figure l b suggests that upon initial exposure a part of the TCNE molecules are broken into C N and C=C(CN), (x = 1 to 3) fragments which are adsorbed on the copper surface together with a multiply charged T C N E ion. (21) Shimanouchi, T. Tables of Molecular Vibrational Frequencies, Consolidated Volume I; National Bureau of Standards: Washington, DC, 1972; NSRDS-NBS NO. 39. (22) Nakamoto, K. Infrared and Raman Spectra of Inorganic and Coordination Compounds; Wiley: New York, 1977.
The Journal of Physical Chemistry, Vol. 91, No. 11, 1987 2949 The loss observed at 1425 cm-', which falls in the range of C=C stretching frequencies, may be due to a further intermediate. The lack of further spectral information precludes a determination of its structure. For the later discussion it should be mentioned that no loss at 1375 cm-I is observed in the spectrum presented in Figure 1b for low TCNe coverage, e.g., TCNE- anions are not formed initially on the copper surface. Figure Id shows a spectrum obtained after further exposure to TCNE. As indicated on the left side of the spectrum, the intensity of the elastically reflected electron beam is very low, resulting in a very noisy spectrum. Most loss features are too weak to allow the unambiguous identification of the ad-species. We only want to point out that the former very intense loss observed at 245 cm-' is now visible only as a weak shoulder, whereas a new low-frequency mode has emerged at 175 cm-'. Upon further exposure (Figure 2b) the loss of 175 cm-' becomes the strongest feature in the spectrum and the loss at 245 cm-' disappears completely, demonstrating clearly that the spectrum shown in Figure Id represents a transition between two different adsorption states. This will be discussed below. In the spectrum shown in Figure 2a, the intensity of the elastically reflected electron beam has increased markedly and sharp losses are observed again in all relevant spectral regions. The loss at 1375 cm-l, which is characteristic for the symmetric C=C stretching mode of the TCNE- anion, has increased in intensity almost to the same level as the loss at 1275 cm-' which is due to the corresponding mode of the multiply ionized TCNE. The corresponding C N stretching mode (2215 cm-') may be hidden in the broad loss centered around 2255 cm-I. This suggests that at this coverage the process is dominated by the adsorption of singly charged TCNE ions. Most likely the TCNE- anions are adsorbed above the TCNE" ions in a second layer. This view will be confirmed later in connection with the interpretation of the intense low-frequency modes. In an IETS study23Mazur and Hipps reported the existence of mono- and dianions of T C N E adsorbed on an alumina surface. In a more recent Raman study of the T C N E / N i ( l l l ) system, Fu-Ming Pan et al.24 found spectroscopic evidence for a TCNE- anion radical layer underlying the physisorbed T C N E multilayers. However, the presence of any multiply ionized T C N E species has not been reported. The broad loss centered at 2255 cm-', in which the symmetric C N stretching mode of the TCNE- may be hidden, must be attributed to C N stretching modes of neutral TCNE, which starts now to condense as multilayers. This becomes quite evident by inspection of the spectrum of increased T C N E coverage shown in Figure 2b. Whereas a shoulder at 1375 cm-' due to the TCNEremains still visible, new intense losses which are obviously due to neutral TCNE appear at 965, 1165, and 1305 cm-I. Finally, the spectrum shown in Figure 2c, obtained after extended TCNE exposures, shows all relevant modes due to neutral TCNE. The assignment of modes is identical with that given by Miller et al.3 It is remarkable that all four different C-C stretching modes of neutral T C N E are clearly observed (vj, q0,v16,and vzO). Consequently, four different C N stretching should principally be observable. However, due to the limited resolution of our instrument, only a single broad loss is observed in the spectrum. Losses which cannot be assigned to vibrational modes of TCNEO may either be attributed to overtones (350 cm-I) or to combination bands (745, 870, 1750, and 2445 cm-I). The observation of an intense loss at 175 cm-' has been reported by Hinkel and Devlin' for the sodium and potassium salts of TCNE. These authors attribute the loss to a librational lattice mode. In our recent paper on the TCNQ/Cu(l 11) system," an intense loss observed at 170 cm-' has been interpreted as the hindered translation between the neutral TCNQ layers. We follow this interpretation in the present case and attribute the loss at 175 cm-' to the hindered translation between the multilayers of neutral TCNE. The loss observed at 245 cm-' at lower TCNE coverage is consequently attributed to the hindered translation between the first and second layer of (23) Mazur, U.; Hipps, K. W. J . Phys. Chem. 1984, 88, 1555. (24) Pan, Fu-Ming; Hemminger, J. C.; Ushioda, S.J . Phys. Chem. 1985, 89, 862.
J. Phys. Chem. 1987, 91, 2950-2954
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cules among the different layers, assuming complete sticking. TCNE. The spectrum shown in Figure 2a, where both low-frequency losses are observed simultaneously as weak shoulders, represents the transition between the first and the second layer. The reason why the loss at 245 cm-I is already observed at relatively low coverage, long before the losses due to the TCNE”reach their maximum intensity (Figure IC), is that the TCNEanions start to adsorb on top of the TCNEX-species in the first layer long before the first layer is completed. This is demonstrated in Figure 3 by applying simple statistics. The calculation assumes
that all impinging molecules stick on the surface and that a flat molecule like T C N E is adsorbed immobile at low temperature. The fact that a loss at 1375 cm-I due to the C=C stretching mode of TCNE- species is observed before the completion of the first layer is now easily understood. If we take the spectrum shown in Figure IC to be representative for the maximum TCNE“ coverage the corresponding C272/Cu920 Auger value is 0.11. On the other hand, taking the spectrum shown in Figure 2a to be representative for the maximum TCNEcoverage in the second layer, the corresponding Auger value is 0.24, which is roughly twice the value of 0.1 1. In order to make an estimation of the number of adsorbed TCNE species per surface copper atoms in the first layer, we follow here the same procedure used in our recent work on the adsorption of TCNQ on Cu(l1 l).17 In this work the carbon Auger signal of adsorbed carbon monoxide was calibrated against the coverage via two well-ordered lowenergy electron diffraction pattern. The result was that a complete monolayer of C O on the Cu( 11 1) surface (1 :1 correspondence) would be represented by an C272/Cu920Auger signal of 0.25. usingthis value and by taking into account that a TCNE contains six carbon atoms, we estimate that a single TCNE”- sDecies adsorbed in the first layer occupies an averageof 13 copper surface atoms. This number is considerably smaller than that reported for the adsorbed TCNQ molecules (20 Cu atoms, a fact which simply reflects the different size of the two molecules. This result is fully compatible with the assumption that the formation of a TCNE monolayer is governed by its shortest intermolecular C-N distance of 3.41 A, as reported for crystalline TCNE.25 ( 2 5 ) Bekoe, D. A,; Trueblocd, K.
N. Z . Kristallogr. 1960,ZZ3, 1.
Proton NMR Study on the Structure of Water in the Stern Layer of Negatively Charged Micelles Omar A. El Seoud,* JoHo P. S. Farah,lr Paulo C. Vieira,lb and Monica I. El Seoud Instituto de Quimica, Universidade de Siio Paulo, C.P. 20.780, 01498 Siio Paulo, S.P., Brazil (Received: December 26, 1986; In Final Form: January 5, 1987)
The proton chemical shift of water was studied as a function of the concentration of three negatively charged surfactants (sodium dodecyl sulfate, sodium dodecylbenzenesulfonate,and sodium perfluorooctanoate) and the deuterium content of the solvent. The effect of the simple presence of ionic head groups of the surfactants (Le., in the absence of the micellar interface) on the structure of water was determined by studying the behavior of the model short-chain compounds: sodium butyl sulfate, sodium p-toluenesulfonate, and sodium perfluorobutyrate. Graphs of the chemical shifts vs. the mole fractions of the solubilizates were linear in all cases and the slopes were used to calculate the so-called “fractionation factor, cp” ( c p = 1 for the bulk solvent) of the water of hydration. Little structure perturbation was produced by the solubilization of the short-chain compounds, the sulfate and the sulfonate anions being slight structure breakers (cp = 0.97 f 0.04) whereas the perfluorobutyrate was found to marginally enhance the structuring (cp = 1.02 f 0.03). The water in the Stern layer of the micelles was found, however, to be more organized than bulk water (cp = 1.07 f 0.04), in agreement with other measurements of NMR chemical shifts, relaxation times, and self-diffusion coefficients. The utility as well as some possible limitations of this new approach is discussed.
Introduction Knowledge of the properties of water in the Stern layer of aqueous micelles is important for a better of both the physical of the micelle itself for instance, the problem of water penetration)Z and of the interactions oCcurring therein (e.g., catalysis, indicator equilibria),3 (1) (a) Present address: Univ. Estadual de Maringl, Parani, Brazil. (b) Present address: Univ. Federal SBo Carlos, SBo Carlos, S.P., Brazil. (2) Menger, F. M. In Surfactants in Solution, Mittal, K. L., Lindman, B., Eds.; Plenum: New York, 1984; pp 3, 347.
0022-3654/87/2091-2950$01.50/0
Extrinsic lipophilic or water-insoluble reporter molecules have been used to Probe the microscopic Properties (e4.3 viscosity, polarity) of water associated with the micellar pseudo-phase and to evaluate the extent of hydrocarbon chain-water ~ o n t a c t . ~The . ~ interpretation of the results thus obtained is, however, not always unequivocal mainly because of the difficulty in identifying the loci of their solubilization in the m i ~ e l l e . A ~ more straightforward (3) (a) Fendler, J. H.; Fendler, E. J. Catalysis in Micellar and Macromolecular Systems; Academic: New York, 1975. (b) Fendler, J. H. Membrane Mimic Chemistry; Wiley: New York, 1982.
0 1987 American Chemical Society
