Chemisorption of organic molecules on ZnO(11̄00) surfaces: C5H5N, (CH3)2CO, and (CH3)2SO

Surface Science 74 (1978) 365-372 0 North-Holland Publishing Company

CHEMISORPTION CsHsN, (CHs),CO,

OF ORGANIC MOLECULES ON ZnO( 1 iO0) SURFACES: AND (CHa)aSO

H. LUTH *, G.W. RUBLOFF

and W.D. GROBMAN

IBM T.J. Watson Research Center, Yorktown Heights, New York, 10598, USA Received 11 December, 1977

Ultraviolet photoemission studies of pyridine, acetone, and dimethyl sulfoxide adsorbed on ZnO(1 iO0) show the involvement of molecular orbitals having n, n*, and nitrogen and oxygen lone-pan character in formation of the chemisorption bond. No decomposition products from adsorption of these molecules are observed on the surface.

1. Introduction In recent years a number of papers have demonstrated that ultraviolet photoemission spectroscopy (UPS) can be used to identify chemisorbed molecules on solid surfaces and the orbital bonding mechanisms responsible for chemisorption. The catalytic activity of metal oxide surfaces make them interesting systems for such investigations. In previous UPS work on ZnO(liOO) nonpolar surfaces, we have studied the decomposition of formic acid [l] and the chemisorption behavior of a variety of organic molecules containing n orbitals and/or oxygen lone-pair (n) orbitals [2]. Both kinds of orbitals have been found to be involved in forming chemisorption bonds to the surface. In the present paper we report further UPS results for the chemisorption of several organic molecules - pyridine, acetone, and dimethyl sulfoxide - on ZnO (liO0) surfaces. These measurements reveal a considerable variety of adsorbate molecular orbitals involved in the chemisorption, including nitrogen and oxygen lone-pair, ‘II, and n* orbitals, and furthermore suggest some trends which may be operative for the chemisorption of organic molecules in general.

2. Experimental

techniques

The ZnO single crystals (hexagonal wurtzite structure) were grown from the vapor phase without any intentional doping [3]. They were n-type with a bulk * Permanent

address: 2. Physikalisches Institut der Rheinisch-Westfalischen Hochschule Aachen, D-5 100 Aachen, Fed. Rep. Germany. 365

Technischen

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of organic molecules on ZnO (IIOO)

conductivity at room temperature between 10-r and low3 (!&cm)-’ and had the shape of hexagonal rods. These rods were cut to -10 mm length in the c-axis direction. The sides of these rods were the as-grown nonpolar (liO0) “prism” surfaces -2 X 10 mm2, one of which was used as the surface for study. Two gold contacts were pressure-bonded on the back side to permit direct resistance heating of the sample. One of these gold contacts was fused to the tip of a nickel wire at its junction with the ZnO to form a thermocouple in direct contact with the crystal in order to monitor the crystal temperature during annealing. The whole crystal holder could be cooled down to about 100 K by liquid nitrogen. The nonpolar (1 TOO) surfaces were cleaned by annealing to about 700°C in UHV (p < 1 X 10-r” Torr). This procedure produces surfaces which yield UPS spectra identical to those which have been measured on WV-cleaved surfaces [4]. The measurements were carried out in a stainless steel, ion-pumped ultrahigh vacuum (UHV) system with an operating pressure of 5 1 X lo-” Torr. Adsorbate gases were obtained from the equilibrium vapor pressure of reagent grade liquids. Partial pressures of input and residual gases were monitored with a quadrupole mass spectrometer. Exchange (displacement) reactions of adsorbate gases with residual gases on the walls of the W-IV system were prevented during exposure by predosing the system before producing a clean ZnO surface and by mass spectrometer checks of the input gases. Photoelectrons were produced by He I and He II radiation (photon energies hv = 21.2 and 40.8 eV respectively) from a differentially-pumped He resonance lamp, with the light beam at -45” angle of incidence to the surface. Kinetic energy distributions of the photoelectrons were measured using a double-pass cylindrical mirror analyzer at a fixed energy resolution -0.35-0.4 eV in conjunction with electron counting techniques. The axis of the analyzer was -90” from the axis of the incident photon beam. The normal to the sample surface was in the plane of these axes -45” from either one, so that the photoelectron intensity was averaged over a range of angles from 45” to grazing emission. Gas phase photoelectron spectra of the adsorbate molecules were also measured with a similar apparatus in order to provide a direct comparison with the chemisorption results at the same photon energies. In these gas phase measurements, the gas inlet was made by a jet which produced a high gas concentration near the image point of the analyzer, and the emission lines of Xe were used as a reference for the ionization potential (IP) scale [S] .

3. Results and discussion In the following we concentrate on the UPS “difference spectra” (basically adsorbate-covered minus clean surface spectra) for the chemisorbed molecule and their comparison to the gas phase UPS spectra. The procedure for deducing these difference spectra involves (i) shifting the clean surface spectrum in energy to

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account for chemisorption-induced changes in the band-bending at the ZnO( 1iO0) surface, (ii) scaling the adsorbate-covered surface spectrum by a constant factor (-1.1-1.4) to roughly compensate for adsorbate-induced attenuation of the primary ZnO emission, and (iii) then subtracting from this the clean surface spectrum. These procedures have been described in more detail previously [1,2]. To compare the difference spectra to gas phase spectra one must also take into account an approximately uniform extramolecular relaxation/polarization shift toward smaller binding energy for the orbitals of the chemisorbed molecule [6]. In all spectra shown for adsorbed species, the electron binding energy (BE) scale is referred to a zero at the vacuum level of the adsorbate-covered surface and the adsorbate coverage is saturation for the sample temperature stated. Because the UPS spectrum for hv = 40.8 eV (He II) reveals more deeply bound molecular orbitals than that for hv = 21.2 eV (He I), we present only the He II data in this paper. 3.1. Pyridine Fig. la shows the UF’S difference spectrum for adsorbed pyridine (CsHsN) on ZnO(liO0) measured at 300 K in an ambient of 2 X lo-’ Torr CsHsN. This spec-

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ELECTRON BINDINGENERGY(eV)

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Fig. 1. UPS spectra at hv = 40.8 eV for adsorbed and gas phase pyridine: (a) difference spectrum for chemisorbed C5HgN on ZnO(li00) at 300 K; (b) difference spectrum for C5HsN condensed at 120 K over the chemisorbed species; (c) gas phase spectrum of CsHgN with ionization potentials given by the IP scale. EV designates the position of the ZnO valence band maximum for the adsorbate-covered surface, and the vacuum level for the covered surface is taken as the zero of binding energy.

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trum represents the saturation coverage at low pressure since it remains unchanged after pumping down into the 10-r’ Torr range. The work function change caused by adsorption at 300 K is A$ = 0.4 eV. Cooling the sample to 120 K in the same CsHsN ambient causes a strong increase in the adsorbate emission intensity as a thicker overlayer is formed, but further work-function changes A$ are small (
H. Liith et al. / Chemisorption of organic molecules on ZnO (lfO0)

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19

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II

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7

ELECTRONBiNDINGiNERGY(eV)

Fig. 2. UPS difference spectra at hv = 40.8 eV for chemisorbed (a) acetone and (b) dimethyl sulfoxide on ZnO(liO0) at 120 K, compared to the corresponding gas phase spectra.

120 K. This adsorption is accompanied by a downwards band-bending change at the surface of -0.2 eV and a work function increase A$ -1.4 eV. The highest-lying orbital in the gas phase, near 9.8 eV on the IP scale, is the oxygen lone-pair, which lies in the central O-C-C-C plane of the molecule, perpendicular to the O=C bond. It undergoes a chemical bonding shift AEn S 0.7 eV downwards relative to the other orbitals. This suggests that the molecule is chemisorbed via the lone-pair orbital and that the oxygen end of the molecule probably lies toward the surface. The orbital immediately below it near 12.5 eV on the IP scale is the C=O rr orbital, which lies perpendicular to the central plane of the molecule. It seems to be much less perturbed by the chemisorption. In the (CHs)aCO molecule, which has both lone-pair and n orbitals, it appears that the former are more chemically active in chemisorption bonding to the ZnO(li00) surface, a situation similar to the behavior of the corresponding orbitals in aldehydes and ketones on Pd surfaces [8]. 3.3. Dimethyl sulfoxide The UPS difference spectrum at hv = 40.8 eV for a lo-’ Torr ambient exposure of dimethyl sulfoxide ((CHs)aSO) at 120 K is shown in fig. 2b together with the gas

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phase (CHs)aSO spectrum (shifted up by A,Yn% 1.6 ev). As with (CH&CO, only minor spectral changes were observed for exposure at 300 K, indicating very little adsorption. Saturation adsorption at 120 K produced a work function change A$ = 1.4 eV and a small downward band-bending change -0.1 eV. The similarity between the spectra for the adsorbed and gas phase (CHs),SO suggests that the molecule is not decomposed upon adsorption. However, the highest-lying gas orbital at 9.1 eV on the IP scale is not replicated in the spectrum for the adsorbed phase. Furthermore, the orbitals near 19 and 23 eV IP in the gas phase appear to undergo larger extramolecular relaxation/polarization shifts than those at smaller IP, as noted elsewhere for this and similar molecules [6]. The results for (CHs)aSO are more complicated to interpret than those for (CH&CO because the two extra valence electrons from the sulfur atom (compared to carbon) must produce an additional occupied orbital, expected at small BE. This accounts for the two peaks near 9.1 eV and 10.3 eV IP in the gas phase spectrum of fig. 2b as compared to the single oxygen lone-pair orbital near 9.8 eV IP in gas phase (CH&CO. However, the difference curve for adsorbed (CHs)aSO shows a single peak at -8.5 eV BE in this energy re8ion, essentially identical to the structure observed in the same region for (CH&CO. The most reasonable interpretation seems to be that the crest-lying peak in the gas phase (CHs)aSO spectrum is shifted downward (to larger BE) in the adsorbed phase due to a bonding shift A.!& E 1.2 eV, indicative of chemisorption. This would make it degenerate with the orbital just below. Alternatively one might suggest that the highest-lying orbital in the gas phase spectrum might be emptied of its charge during che~sorption. However, such a large charge tranfer would produce work function and band-bending changes much larger than those typically observed (e.g. for (CHs)aCO), contrary to observation. Previous photoelectron spectroscopy work on gas phase (CH&SO had identified the orbitals at 9.11, 10.26, and 12.58 eV on the gas phase IP scale respectively with a sulfur lone-pair, an oxygen lone-pair, and a II orbital associated with the S=O double bond [9]. This ordering of oxygen and sulfur lone-pair orbitals is supported by phOtOeleCtrOnstudies of the positions of such orbitah in chalcogen-substituted benzene in which the S and 0 atoms are isoelectronic. In (CHs)$O, however, the S and 0 atoms cannot behave as if they were isoelectronic because their molecular en~ronments are very different. Furthermore, it is difficult to ~derst~d how a three-fold coordinated sulfur atom (with one bond a double bond to the oxygen) can have a lone-pair orbital. To clarify this question and to understand the nature of the highest-lying orbital which appears to be the one most strongly affected by Che~sorptiOn, we have calculated the ground state energy level structure and the character of the valence orbitals in (CHs)aCO and (CHs)aSO using ab-initio SCF-MO Gaussian-70 calculations [lo]. Such calculations generally provide at least qualitative agreement with the relative spacing and ordering of Orbitah in the gas phase phOtOelectrOn spectrum. The results show clearly that no high-lying sulfur lone-pair orbital is formed.

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The calculations indicate that, if the central frame of the (CH&SO molecule were planar as in (CHs)&O, the two extra electrons present in the molecule would simply fil an antibonding S=O n* orbital, i.e. the lowest unoccupied orbital in (CH& CO. However, populating the n* orbital destabilizes the planar structure, causing the molecule to assume a pyrimidal geometry (the 0 atom is out of the C-S-C plane so that 4 OSC = 107” and 4 CSC = 100”). As a result, much of the charge density of the n* orbital is shifted onto the oxygen atom, making it more like an additional oxygen lone-pair perpendicular to the S=O bond and having some a* character. (The charge density of the n (fourth) orbital is accordingly shifted toward the sulfur atom.) In the actual (pyrimidal) geometry, then, the two extra electrons mostly fill the empty 2p states of the oxygen atom. It is most likely that the highest-lying peak in the gas phase UPS spectrum of (CHs),SO (at 9.11 eV IP) is composed of both the oxygen lone-pair/n* orbital and the second molecular orbital, an oxygen lone-pair also perpendicular to the S=O bond. These appear rather close in energy in the calculations. Thus the chemisorption bonding of (CHs)$O on ZnO(liO0) appears to involve oxygen lone-pair orbitals, the higher of which has some rr* character. The calculations also suggest that the orbitals at -19 and 23 eV IP in the gas phase spectrum are essentially pure s-like orbitals, the former 0(2s)-S(3s) antibonding and the latter C(2s)-C(2s) antibonding. As noted elsewhere [6], such pure s-like molecular orbitals often exhibit larger extramolecular relaxation/polarization shifts than do valence orbitals containing both s- and p-like atomic character. This seems to be the case here for (CHs),SO as observed here.

4. Conclusions

These UPS studies of pyridine, acetone, and dimethyl sulfoxide on ZnO(li00) reveal the participation of a variety of adsorbate molecular orbitals in chemisorption bonding, including rr, 1~*, and nitrogen and oxygen lone-pair orbitals. These results, and others previously published [8], also suggest that in organic molecules like aldehydes and ketones the lone-pair orbitals are more strongly affected in chemisorption than are the II orbitals. These studies present further evidence, in accordance with the observations of a considerable number of previous investigations [ 111, that the highest-lying adsorbate molecular orbitals in the UPS spectrum - generally those least contributing to the intramolecular bonding - are chemically the most active orbitals in forming the chemisorption bond. Finally, it is interesting to note that one does not generally observe decomposition products from adsorption of organic molecules on the ZnO(liO0) surface [ 1,2]. This behavior contrasts that seen for organic molecules on transition metal surfaces [8,11].

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Acknowledgement We are grateful for stimulating discussions with D.E. Eastman and J.E. Demuth, and we thank G. Heiland for supplying the ZnO crystals.

References [l] H. Liith, G.W. Rubloff and W.D. Grobman, Solid State Commun. 18 (1976) 1427. [2] G.W. Rubloff, H. Liith and W.D. Grobman, Chem. Phys. Letters 39 (1976) 493. [ 31 The crystal were grown in the II. Physikalisches Institut der RWTH Aachen. [4] J. Freeouf, M. Erbudak and D.E. Eastman, to be published. [5] D.W. Turner et al., Molecular Photoelectron Spectroscopy (Wiley, New York, 1970). [6] G.W. Rubloff, W.D. Grobman and H. Liith, Phys. Rev. B15 14 (1976) 1450. [ 71 J. AhnlBf, B. Ross, U. Wahlgren and H. Johansen, J. Electron Spectros. 2 (1973) 51; A.D. Baker and D.W. Turner, Phil. Trans. Roy. Sot. London A268 (1970) 131. [8] H. Ltith, G.W. Rubloff and W.D. Grobman, Surface Sci. 63 (1977) 325. [9] G.W. Mines, R.K. Thomas and Sir Harold Thompson, Proc. Roy. Sot. (London) A329 (1972) 275. [lo] W.J. Hehre, W.A. Lathan, R. Ditchtield, M.D. Newton and J.A. Pople, Gaussian 70: ab-initio SCF-MO Calculations on Organic Molecules, Program 236, Quantum Chemistry Program Exchange, Indiana University (1974). [ 1 l] See references listed in ref. [6].