- Email: [email protected]
ZnO(0001) interface
iii!ili!iiiiii!iliiiii:i!i~iii~¸i!i!ili~li~i~i!~i~~ii~i!~li~?~i~il surface science Surface Science 323 (1995) 102-108
ELSEVIER
CO intermediates in the CO/ZNO(0001) interface P.J. M~ller *'~, S.A. Komolov 2, E.F. Lazneva 2, E.H. Pedersen Department of Chemistry, Uni~,ersityof Copenhagen, Unit,ersitetsparken 5, DK-2100 Copenl~agen O, Denmark Received 24 January 1994; accepted for publication 29 September 1994
Abstract Electron energy-loss and total current spectroscopy and photodesorption methods are employed in studies of CO adsorption on a ZnO(O001) surface. Predominantly CO, molecules are registered in the light-induced de sorption flow. In addition to the desorption stimulated by thermal action of light, a photodesorption component of CO 2 is observed. The spectroscopy data provides evidence for CO oxidation on the ZnO(0001) surface and for formation of a (CO~') chemisorption complex which is considered as an initial state for CO 2 photodesorp',it~n.
1. Introduction
The adsorption of carbon monoxide on the surface of the wide+gap semiconductor ZnO is related with various physical and chemical processes, including heterogeneous catalytic interactions on the surface and electronic interactions of the adsorbate with an electron-hole system of the adsorbent [1]. These properties of the CO/ZnO system become more pronounced under light illumination and one can deal with photo-induced catalytic reactions initiated by a photo-excited electron=hole system [2]. It was recently shown by a thermo- and laser-induced desorption experiment that CO adsorption on ZnO surfaces apparently is followed by catalytic CO oxidation [3,4]. In order to explain photoemission and
' Corresponding author. Fax: +81 53 474 0630, t Present address: Research Institute of Electronics, Shizuoka Un!versity, Hamamatsu 432, Japan. "On leave from Research Institute of Physics, St, Petersburg University, 19890,1+Petrodvorct:,, St. Petersburg, Russian Federation.
phutodesorption experiments a qualitative model for formation of a chemisorbed CO i complex was suggested [1,4,5], but there were no direct experimental evidence for a surface charge transfer and for the energy locations of chemisorption-induced electronic states on the surface. In this paper we present results from the CO/ZnO(0001) system obtained using a combination of iow-energy electron diffraction (LEED), electron energy-loss and total current spectroscopy (EELS and TCS, respectively) and photo-induced desorption. Observation of photo-induced CO,.. desorption provides evidence for CO oxidation on the ZnO(0001) surface. Characteristic changes in the EEL spectrum are discussed in terms of electronic transitions involving chemisorption-induced electronic states. In addition to electronic states located in the valence band region we observe a CO2-induced state located in the band gap of the ZnO (about 1.8 eV above the valence band maximum, VBM); these can trap electrons from the conduction band, resulting in negative charging of a CO~ chemisorption complex.
0039-6028/95/$09.50 © 1995 Elsevier Science B.V, All rights reserved SSDI 0039-6028(94)00632-6
P.J. M¢ller et aL / Surface Science 323 (1995) 102-108
2. Experimental methods For the adsorption-desorption experiment an ultrahigh-vacuum system (base pressure 10 -8 Pa) that combines different surface-analytical techniques was used (Fig. 1). A four-grid LEED system (1) may be switched between the diffraction mode and an operational mode in which the TCS spectra, the target-current derivative dJ(E1)/d(E l) vs. incident energy E I, are measured (recorded in the sample circuitry [6]). The four-grid energy analyzer was also used for EELS and Auger measurements at nermal incidence of the primary electron beam. In EELS, the second derivative d2N/dE 2 of the electron energy-distribution curve was recorded with a modulation of 0.5 eV, and the peak position determined within 0.3 eV. A quadrupole mass spectrometer, QMS, (2) is connected to the chamber for control of the residual gas composition and for measurements of the partialpressure changes during photo-induced desorption from the sample. The sample (3) was mounted on a manipulator (8) with auxiliary facilities for heating, temperature measurement, rotation and motion (the sample can be moved from the position 'in front of LEED' to the position 'in front of QMS'). The vacuum chamber was provided with a window for the purpose of external sample illumination through
I Tcs
i ILEEOEELSl I
I"""
I I °°"
I control ~-] control
~
I I |
Pump
Fig. 1. Schematic diagram of experimental setup. (l) LEED/EELS/TCS/AES; (2) quadrupole mass spectrometer; (3) sample; (4) window and focusing lens; (5) ion gun; (6) gas inlet manifold; (7) pressure gauge; (8) manipulator; (9) molecular /metal beam sources.
103
a lens (4), an ion gun for surface cleaning (5), a gas inlet manifold (6) and a gauge for pressure measurement (7). The setup was equipped with a system for thin film evaporation (9) and with a quartz crystal microbalance for film thickness measurement (not shown in Fig. 1). A transparent colorless zinc oxide crystal was used for these investigations. The crystals were cut to 0.5 mm thickness, 6 × 6 mm discshaped samples, oriented to within 0.3 ° to the (0001) surface, and diamond-polished to 1 /.tm followed by in situ thermal annealing in UHV. The CO was of 99.997% purity.
3. Results
3.1. The clean ZnO(O001) surface The ZnO(0001) sample, as installed into the high-vacuum conditions, exhibited a strong electrical charging due to insulating properties of the crystals. After prolonged annealing at a temperature of 700750 K, the recovering of surface conductivity provided conditions for the electron spectroscopy measurements. Such an annealing procedure is in accordance with previous recommendations [7]. Through this annealing process we managed to trace an evolution of TCS and EELS spectra to shapes similar to those for the clean surface (Fig. 2, curve 1). A sharp LEED picture of the ZnO(0001) surface was obtained after additional brief annealing at 800 K, yielding pronounced pure singlets with six-fold symmetry, which is referred to as a 1 x 1 pattern [8,9]. With variation of the electron beam energy E I it is possible to trace the splitting of each of these LEED spots into a sub-sextet of six-fold symmetry. At a normal beam incidence, sharp pictures of these singlets were obtained at energies E 1 of 27, 45 and 87 eV, respectively. The splitting of the spots was more pronounced at 20 and 59 eV. In our previous investigations [10], LEED-picture variation with E 1 have been discussed in terms of diffracted reflection from a ZnO(0001) surface that contains many hexagonal pits, created by thermal etching during an extended anneal, the side-planes of the pits being formed by atomic steps. Thus we are dealing with a ZnO(0001) surface containing hexagonal pits created by thermal etching of the surface during the anneal-
104
P.I. Moiler et ai. / Surface Science 323 (1995) 102-108
3.2. The CO and ZnO(O001) interaction L. "500 ~_ r
=2500
....
-t~
.,= 'F,
2
._~
0
5
10
15
20
25
Energy loss (eV) Fig. 2. Electron energy-loss spectra at Ep--95 eV. (1) Clean ZnO(O001) surface; (2) after CO interaction with the ZnO(O001)
surface; (3) difference spectrum.
ing. Surface cleanness was checked also by AES and laser-induced desorption [4]. TCS curves measured at normal incidence from gnO(0001) is presented in a diagram (Fig. 4, curve 2) and compared with the calculated [11] (curve 1) density of empty electronic states above the vacuum level of ZnO. Energy-loss spectra were measured in the primary electron energy range 40 eV 130 eV we observe a loss of 93,5 eV which corresponds to a Zn 3p (M.,3) level ionization. Similar data for Zn 2p ionization in ZnS (E~ -- 94 eV) were earlier reported [16].
After extended annealing to 800 K a ZnO(0001) sample was exposed to CO. To make certain that the sample was saturated and to simulate a possible step in the initial heterogeneous catalytic synthesis e,f methanol the sample surface was saturated by CO at a pressure of 100 Pa for 4 h at room temperature (RT) followed by a raise of the sample temperature to 600 K during the last 10 min upon which the system was evacuated, and electron spectroscopy and photo-induced desorption experiments were performed. Thermal treatment used in our experiment followed previous investigations [3] in order to activate a CO activation with the ZnO(0001) surface and to form strongly bonded chemisorption states on the surface. It is emphasized that the CO adsorption experiment, which was performed without thermal activation (at RT), was characterized by a very low sticking probability (in accordance with Ref. [17]), and we did not observe a production of CO a during that process. Below (Section 3.2.2) we will discuss results obtained during thermal activation of CO adsorption. 3.2.1. LEED, TCS and EELS CO adsorption was followed by a weakening of the LEED spots, without any superstructure formation at the temperatures studied in the present investigation. At the same time it causes a strong attenuation of those TCS features that have a different origin, but the main peaks that reflected the density of empty states are still well resolved (diagram Fig. 4,-..curve 3). The observed behavior of the LEED picture and TCS curves may be related to a disordered CO adsorption. The primary peak in TCS exhibits a shift under CO adsorption which corresponds to the ~,,ork function increase of about 0.5 eV, and no surfao, band bending was observed (Fig. 4, curve 3). This wo:k function increase we explain by a dipole-layer fol:nation via negative charge t~ansfer to the surface. The loss spectrum after CO adsorption is shown in Fig. 2 (curve 2). The presented spectrum is normalized to the plasmon-peak intensity, since the plasmon peak was slightly lowered (by a factor of about 1.3) due to additional electron scattering in the adsorbed layer. Fig. 2 shows a difference spectrum
P.J. &lOlleret aL / Surface Science 323 (1995) 102-108
(curve 3) in which the spectrum before adsorption (curve 1) was subtracted from the spectrum after adsorption (curve 2). From the difft.~,~ce spectrum we clearly see formation of new adsorption-induced losses at 3.5 eV (F1), 11 eV (F2), 14 eV (F3) and 17 eV (F4), respectively.
3.2.2. Photo-induced desorption Desorption was induced by sample illumination from a high-pressure 300 W xenon lamp. The light beam was focused precisely on the sample surface so to prevent heating and desorption from the holder. A negligible amount of molecules desorbed by an action of multi-reflected light from the walls of the chamber was confirmed by a separate experiment. Adsorbed species were removed from the sample by heating of the sample holder up to 850 K with the help ef an electron beam. The subsequent light illumination did not increase partial pressures of CO and CO 2 in the chamber. The sample temperature increase induced by a thermal action of light was monitored by a thin chromel-alumel thermocouple squeezed in between the sample and the holder. Light illumination of the CO-containing surface was followed by the main desorption of CO 2. We conclude a possible CO oxidation on the ZnO surface which is in agreement with previous results [1,3,18]. A small amount of the desorbed oxygen may be related to oxygen ejection from the ZnO sample. In Fig. 3 the results of the light-induced desorption experiment are presented. Immediately after the illumination is switched on, within about 1 s, a jump in the CO 2 partial pressure takes place (Fig. 3, curve 2). This jump may obviously be caused by CO 2 photodesorption from the ZnO sample surface, since the sample surface temperature at this moment practically is equal to RT. The following raise in the CO 2 partial pressure is observed over the 100-200°C temperature range and may be directly connected with thermo-stimulated desorption of CO 2. Using the well-known procedure for analysis of thermo-desorption pressure vs. temperature curves [19,20], an activation energy of desorption for this phase is estimated to be 0.4 + 0.05 eV. The second illumination ef the same intensity (Fig. 3, curve 3) causes only desorption of the photo-induced component. One may conclude that after the action of the first illumi-
T/°C
i
.
.
.
.
105 .
I
'
460
.o 300
....
~
i \
.
t ~Pcoz/Pa I 1.10-6 I-2.10"?
1.10.6 Lampoff 0 2'0 t Lamp on
4'0
do
do
time t/s
loo
1
,}o
Fig. 3. CO and C O 2 desorption induced by illumination of a CO/ZnO(0001) surface. Origin of the time scale corresponds to switching on the illumination. (1) Temperature curve; (2) and (3) CO 2 desorption due to the first and second illuminations, respectively; (4) and (5) CO desorption due to the first and second illuminations, respectively.
nation the adsorption sites supplying the thermodesorption flux become empty, and that the photodesorption component originates from the more strongly bonded chemisorption sites. We thus clearly have two desorption components; photo-induced (without time-delay) and thermo-induced. The shape of the CO desorption (curve 4) corresponds to the thermal origin of the observed desorption. The second illumination does not cause CO desorption during the first 40 s (curve 5). The observed increase in CO partial pressure for t > 40 s may be explained as a desorption from the sample holder whi~n is heated due to thermoconductivity. It is emphasized that the amount of desorbed CO molecules is approximately one order of magnitude less than that of CO 2. An estimated amount of desorbed CO 2 molecules, obtained from the analysis of desorption curves, corresponds to an initial sample coverage of about 0.3-0.5 monolayer of CO 2 on the ZnO(0001) surface under investigation.
106
P.J. M¢ller et ai. / Surface Science 323 (1995) 102-108
4. Discussion
The predominant CO 2 desorption observed in our experiment (Fig. 3) is a consequence of a heterogeneous catalytic CO oxidation at the ZnO surface, since the surface under investigation was exposed to CO adsorption only. In view of the obtained LEED results, the adsorption of CO molecules may be considered as an interaction with the ZnO(0001) surface which contains hexagonal-pit defects created by thermal etching of the surface during the annealing. The proposed formation of chemisorbed CO.;- is supported by our EELS results. Let us first consider a diagram of electronic transitions in relation to the density of states (DOS) in ZnO. We have measured an energy loss of 93.5 eV for the Zn 3p (M~) level ionization. Since the energy of this location is well known (86 eV below VBM) it is easy to determine the location of the final state for the corresponding electronic transition which is to be equal to 7.5 eV above the VI3M. This result is in accordance with a calculated position of the maximum a of the empty DOS [11], The calculated DOS is inserted in Fig. 4 (curve I). The energy positions of other DOS maxima placed above the vacuum level correspond well to maxima in the TCS curve (Fig. 4, curve 2), since there is a correlation between ~nergy locations of the band-structure critical points in the Briilouin zone and the maxima in the TCS curve [6,21]. The primary peak of the TCS curve gives a location of the vacuum level Evac (shown in Fig. 4 by a dotted line). The position of Evac shifts approximately 0.5 eV after CO interaction with the surface, showing an increase of the work function (Fig. 4, curve 3). The TCS structure, which is related to the band structure of the crystal, does not shift in energy under adsorption. Such a TCS behavior indicates that when the work function of the semiconductor changes as a result of electron-affinity variation then there is no shift in the semiconductor band structure relative to the Fermi level [6] and the band bending remains unchanged. Thus the observed work-function increase may be connected with a surface dipole-layer formation characterized by negative charge transfer to adsorbed species. The diagram of electronic transitions to the final state a is shown in Fig. 4. The arrows B, C and D correspond to transitions with energy losses 9.5, 12.8
IF4 T
E~ I F3
[ t
Ec
lEnac , J',
1 F2
-10
-5
0
5
10
15
Energy above Ev (eV) Fig. 4. Energy diagram combining TCS (dJ(Ez)/dE z vs. El), curves 2 and 3, with calculated (from Ref. [11]) DOS, curve 1, and schemes of electron transitions (indicated by arrows) for clean (lower part) and for CO-covered (upper part) ZnO(0001) surfaces in comparison with valence band (curve 4, from Ref. [20]) and difference (for CO, exposed surfaces, curve 5, from Ref. [25]) photoelectron spectra, a indicates the location of the final state. Ec and E,, are the edges of the conduction and valence bands, respectively, Ev~ the vacuum level, and A~ the work-function change due to CO interaction with the ZnO surface.
and 15.5 eV, respectively, and indicate the positions of initial states in the valence band of ZnO. They correspond well to two maxima in the valence-band DOS and to the Zn 3d band (Fig. 4, curve 4 presents a photoemission spectrum from the ZnO(0001) surface [22]). The energy loss of 15.5 eV was explained in previous investigations [13] as a transition from the Zn 3d band. The energy loss of 4.6 eV (in Fig. 4 indicated by arrow A) may be assigned to electron transitions from defect states in the gap (located at about 0.8 eV above VBM) to the final state at 5.4 eV above VBM. These final states correspond to highenergy DOS due to the flat band of empty states in the I"-A direction of the Brillouin zone [11] and to Zn dangling-bond states [1,24]. The location of the narrow band of electronic states, at about 2.5 eV below the bottom of the conduction band, in the gap of ZnO, has been observed by a photodischarge spectroscopy experiment [23] (the ZnO gap-width Eg is 3.3 eV). The same states may be responsible for a
P.J. M¢ller et al. / Surface Science 323 (1995) 102-108
visible photoluminescence of ZnO having its emission maximum at about 2.5 eV [24,25]. A proposed model for these states is a single electron trapped at an oxygen-ion vacancy in the ZnO crystal [26]. CO interaction with the ZnO(0001) surface creates new electronic states on the surface, and excitations of these states are characterized by the energy losses of 3.5 eV (F1), 11 eV (F2), 14 eV (F3) and 17 eV (F4) shown in Fig. 2 (curve 3). A corresponding scheme of electronic transitions is presented in Fig. 4 (arrows F l, F2, F3 and F4). The initial states for the transitions F2, Fa and F4 correspond well to the DOS of the chemisorbed CO 2 as is clearly seen from a comparison with the obtained [1,27] difference photoemission spectrum (Fig. 4, curve 5). The initial state for the transition F1 locates at about 1.9 eV above the VBM (i.e. in the vicinity of the middle of the gap) and may be directly associated with electronic states induced by CO 2 chemisorption. Electron trapping to these states provides the negative surface charging that was traced by the work-function increase. The obtained experimental r~,-~altsthus give support for the formation of the CO2 chemisorption complexes on the ZnO surface. Presence on the surface of CO2 chemisorbed species is a general condition for CO 2 photodesorption according to the photoelectron mechanism [5,28]. Our observation (Fig. 3, curve 3) of CO 2 photodesorption during the second illumination (when the thermodesorption component is absent) indicates that photodesorbed molecules originate from the strongly bonding chemisorption states which may be associated with the ZnCO~ complexes stabilized in the vicinity of positively charged oxygen vacancies [1]. Fundamental absorption of light quanta (h v > Eg) in the subsurface region of ZnO leads to creation of free electrons and holes. A capture of a photoexcited hole to the ZnCO~ complex is accompanied by both neutralization of the adsorbed particle and by its energy excitation due to import of the recombination energy which in our case is equal to 1.9 eV. Transition from a charged state to a neutral one proceeds according to the Franck-Condon principle. After neutralization, the particle is in an antibonding state and is being acted upon by forces of repulsion from the surface, and it starts to move away. On its way along the repulsing branch of the potential curve the particle will acquire excess kinetic energy [28]. This
107
energy determines the initial velocity of the desorbed particle which may significantly exceed the thermally equilibrated value of the velocity. Such a model explains the appearance of the fast CO e molecules which were observed in a recent laser-induced desorption investigation of the CO/ZnO(0001) system [4,29].
Acknowledgements The support of the Danish Science Research Councils through the Center for Surface Reactivity and by the Ministry of Science Research and High Schools of the Russian Federation is gratefully acknowledged.
References [1] W. G6pel, R.S. Bauer and G. Hansson, Surf. Sci. 99 (1980) 138. [2] T.J. Chuang, Surf. Sci. Rep. 3 (1983) 1. [3] W.H. Cheng and H.H. Kung, Surf. Sci. 122 (1982) 21. [4] P.J. Moiler, S.A. Komolov and E.F. Lazneva, Surf. Sci. 290 (1993) L677. [5] Y. Shapira, S.M. Cox and D. I.,ichtman, Surf. Sci. 54 (1976) 45. [6] S.A. Komolov, Total Current Spectroscopy of Surfaces (Gordon and Breach, Philadelphia, 1992). [7] H. Moorman, O. Kohl and G. lteiland, Surf. Sci. 182(1987) 458; P.J. Mollor and J. Nerlov, Surf. Sci. 307-309 (1994)591. [8] S.C. Chang and P. Mark, Surf. Sci. 46 (1974) 293. [9] C.T. Campbell, K.A. Daube and J.M. White, Surf. Sei. 182 (1987) 458. [10] P.J. Moiler, S.A. Komolov and E.F. Lazneva, Surf. Sci. 307-309 (1994) 1177. [11] S. Bloom and 1. Ortenburger, Phys. Stat. Sol. (b) 58 (1973) 561. [12] P.J. Moiler, S.A. Komolov and E.F. Lazneva, to be published. [13] A. Ebina and T. Takahashi, Surf. Sci. 74 (1978) 667. [14] Y. Margoniuski, Appl. Surf Sci. ~1/12 (1982) 784. [15] R. Dorn and H. Liith, J. Appl. Phys. 47 (1970) 5097. [16] Atlas of ionization losses, Eds. I.F. Koval, V.P. Lysenko, P.V. Meinik and N.G. Nakhodkin (Naukova Dumka, Kiev, 1989) [in Russian]. [17] S.S. Fu and G.A. Somorjai, Appl. Surf. Sci. 48/49 (1991) 93. [18] W. Hotan, W. G6pel and B. Haul, Surf. Sci. 83 (1979) 162. [19] V.N. Ageev and N.I. lonov, Progr. Surf. Sci. 5 (1), Ed. S.G. Davison (Pergamon, New York, 1974).
108
P.J. M¢ller et al. / Surface Science 323 (1995) 102-108
[20] A.M. de Jong and J.M. Niemantsvedriet, Surf. Sci. 233 (1990) 335. [21] S.A. Komolov and V.N. Strocov, Phys. Star. Sol. (b) 167 (1991) 6O5. [22] W. CRipei, J. Pollman, I. lvanov and B. Reihl, Phys. Rev. B 26 (1982) 3144. [23] R. Williams and A. Willis, J. Appl. Phys. 39 (1968) 373. [24] M. Anpo and Yu. Kubokawa, J. Phys. Chem. 88 (1984) 5556. [25] H. ldriss and M. Barteau, J. Phys. Chem. 96 (1992) 3382.
[26] W.F. Wei, Phys. Rev. B 15 (1977) 2250. [27] C.T. Au, W. Hirsch and W. Hirschwald, Surf. Sci. 197 (1988) 391. [28] E.F. Lazneva, Rad. Effects and Defects in Solids 115 (1991) 257. [29] P.J. M~ller, S.A. Komolov and E.F. Lazneva, in: Adsorption on Ordered Surfaces of Ionic Solids and Thin Films, Eds. H.-J. Freund and E. Umbach, Springer Ser. Surf. Sci., Vol. 33 (Springer, Berlin, 1993), p. 156.
ELSEVIER
CO intermediates in the CO/ZNO(0001) interface P.J. M~ller *'~, S.A. Komolov 2, E.F. Lazneva 2, E.H. Pedersen Department of Chemistry, Uni~,ersityof Copenhagen, Unit,ersitetsparken 5, DK-2100 Copenl~agen O, Denmark Received 24 January 1994; accepted for publication 29 September 1994
Abstract Electron energy-loss and total current spectroscopy and photodesorption methods are employed in studies of CO adsorption on a ZnO(O001) surface. Predominantly CO, molecules are registered in the light-induced de sorption flow. In addition to the desorption stimulated by thermal action of light, a photodesorption component of CO 2 is observed. The spectroscopy data provides evidence for CO oxidation on the ZnO(0001) surface and for formation of a (CO~') chemisorption complex which is considered as an initial state for CO 2 photodesorp',it~n.
1. Introduction
The adsorption of carbon monoxide on the surface of the wide+gap semiconductor ZnO is related with various physical and chemical processes, including heterogeneous catalytic interactions on the surface and electronic interactions of the adsorbate with an electron-hole system of the adsorbent [1]. These properties of the CO/ZnO system become more pronounced under light illumination and one can deal with photo-induced catalytic reactions initiated by a photo-excited electron=hole system [2]. It was recently shown by a thermo- and laser-induced desorption experiment that CO adsorption on ZnO surfaces apparently is followed by catalytic CO oxidation [3,4]. In order to explain photoemission and
' Corresponding author. Fax: +81 53 474 0630, t Present address: Research Institute of Electronics, Shizuoka Un!versity, Hamamatsu 432, Japan. "On leave from Research Institute of Physics, St, Petersburg University, 19890,1+Petrodvorct:,, St. Petersburg, Russian Federation.
phutodesorption experiments a qualitative model for formation of a chemisorbed CO i complex was suggested [1,4,5], but there were no direct experimental evidence for a surface charge transfer and for the energy locations of chemisorption-induced electronic states on the surface. In this paper we present results from the CO/ZnO(0001) system obtained using a combination of iow-energy electron diffraction (LEED), electron energy-loss and total current spectroscopy (EELS and TCS, respectively) and photo-induced desorption. Observation of photo-induced CO,.. desorption provides evidence for CO oxidation on the ZnO(0001) surface. Characteristic changes in the EEL spectrum are discussed in terms of electronic transitions involving chemisorption-induced electronic states. In addition to electronic states located in the valence band region we observe a CO2-induced state located in the band gap of the ZnO (about 1.8 eV above the valence band maximum, VBM); these can trap electrons from the conduction band, resulting in negative charging of a CO~ chemisorption complex.
0039-6028/95/$09.50 © 1995 Elsevier Science B.V, All rights reserved SSDI 0039-6028(94)00632-6
P.J. M¢ller et aL / Surface Science 323 (1995) 102-108
2. Experimental methods For the adsorption-desorption experiment an ultrahigh-vacuum system (base pressure 10 -8 Pa) that combines different surface-analytical techniques was used (Fig. 1). A four-grid LEED system (1) may be switched between the diffraction mode and an operational mode in which the TCS spectra, the target-current derivative dJ(E1)/d(E l) vs. incident energy E I, are measured (recorded in the sample circuitry [6]). The four-grid energy analyzer was also used for EELS and Auger measurements at nermal incidence of the primary electron beam. In EELS, the second derivative d2N/dE 2 of the electron energy-distribution curve was recorded with a modulation of 0.5 eV, and the peak position determined within 0.3 eV. A quadrupole mass spectrometer, QMS, (2) is connected to the chamber for control of the residual gas composition and for measurements of the partialpressure changes during photo-induced desorption from the sample. The sample (3) was mounted on a manipulator (8) with auxiliary facilities for heating, temperature measurement, rotation and motion (the sample can be moved from the position 'in front of LEED' to the position 'in front of QMS'). The vacuum chamber was provided with a window for the purpose of external sample illumination through
I Tcs
i ILEEOEELSl I
I"""
I I °°"
I control ~-] control
~
I I |
Pump
Fig. 1. Schematic diagram of experimental setup. (l) LEED/EELS/TCS/AES; (2) quadrupole mass spectrometer; (3) sample; (4) window and focusing lens; (5) ion gun; (6) gas inlet manifold; (7) pressure gauge; (8) manipulator; (9) molecular /metal beam sources.
103
a lens (4), an ion gun for surface cleaning (5), a gas inlet manifold (6) and a gauge for pressure measurement (7). The setup was equipped with a system for thin film evaporation (9) and with a quartz crystal microbalance for film thickness measurement (not shown in Fig. 1). A transparent colorless zinc oxide crystal was used for these investigations. The crystals were cut to 0.5 mm thickness, 6 × 6 mm discshaped samples, oriented to within 0.3 ° to the (0001) surface, and diamond-polished to 1 /.tm followed by in situ thermal annealing in UHV. The CO was of 99.997% purity.
3. Results
3.1. The clean ZnO(O001) surface The ZnO(0001) sample, as installed into the high-vacuum conditions, exhibited a strong electrical charging due to insulating properties of the crystals. After prolonged annealing at a temperature of 700750 K, the recovering of surface conductivity provided conditions for the electron spectroscopy measurements. Such an annealing procedure is in accordance with previous recommendations [7]. Through this annealing process we managed to trace an evolution of TCS and EELS spectra to shapes similar to those for the clean surface (Fig. 2, curve 1). A sharp LEED picture of the ZnO(0001) surface was obtained after additional brief annealing at 800 K, yielding pronounced pure singlets with six-fold symmetry, which is referred to as a 1 x 1 pattern [8,9]. With variation of the electron beam energy E I it is possible to trace the splitting of each of these LEED spots into a sub-sextet of six-fold symmetry. At a normal beam incidence, sharp pictures of these singlets were obtained at energies E 1 of 27, 45 and 87 eV, respectively. The splitting of the spots was more pronounced at 20 and 59 eV. In our previous investigations [10], LEED-picture variation with E 1 have been discussed in terms of diffracted reflection from a ZnO(0001) surface that contains many hexagonal pits, created by thermal etching during an extended anneal, the side-planes of the pits being formed by atomic steps. Thus we are dealing with a ZnO(0001) surface containing hexagonal pits created by thermal etching of the surface during the anneal-
104
P.I. Moiler et ai. / Surface Science 323 (1995) 102-108
3.2. The CO and ZnO(O001) interaction L. "500 ~_ r
=2500
....
-t~
.,= 'F,
2
._~
0
5
10
15
20
25
Energy loss (eV) Fig. 2. Electron energy-loss spectra at Ep--95 eV. (1) Clean ZnO(O001) surface; (2) after CO interaction with the ZnO(O001)
surface; (3) difference spectrum.
ing. Surface cleanness was checked also by AES and laser-induced desorption [4]. TCS curves measured at normal incidence from gnO(0001) is presented in a diagram (Fig. 4, curve 2) and compared with the calculated [11] (curve 1) density of empty electronic states above the vacuum level of ZnO. Energy-loss spectra were measured in the primary electron energy range 40 eV
After extended annealing to 800 K a ZnO(0001) sample was exposed to CO. To make certain that the sample was saturated and to simulate a possible step in the initial heterogeneous catalytic synthesis e,f methanol the sample surface was saturated by CO at a pressure of 100 Pa for 4 h at room temperature (RT) followed by a raise of the sample temperature to 600 K during the last 10 min upon which the system was evacuated, and electron spectroscopy and photo-induced desorption experiments were performed. Thermal treatment used in our experiment followed previous investigations [3] in order to activate a CO activation with the ZnO(0001) surface and to form strongly bonded chemisorption states on the surface. It is emphasized that the CO adsorption experiment, which was performed without thermal activation (at RT), was characterized by a very low sticking probability (in accordance with Ref. [17]), and we did not observe a production of CO a during that process. Below (Section 3.2.2) we will discuss results obtained during thermal activation of CO adsorption. 3.2.1. LEED, TCS and EELS CO adsorption was followed by a weakening of the LEED spots, without any superstructure formation at the temperatures studied in the present investigation. At the same time it causes a strong attenuation of those TCS features that have a different origin, but the main peaks that reflected the density of empty states are still well resolved (diagram Fig. 4,-..curve 3). The observed behavior of the LEED picture and TCS curves may be related to a disordered CO adsorption. The primary peak in TCS exhibits a shift under CO adsorption which corresponds to the ~,,ork function increase of about 0.5 eV, and no surfao, band bending was observed (Fig. 4, curve 3). This wo:k function increase we explain by a dipole-layer fol:nation via negative charge t~ansfer to the surface. The loss spectrum after CO adsorption is shown in Fig. 2 (curve 2). The presented spectrum is normalized to the plasmon-peak intensity, since the plasmon peak was slightly lowered (by a factor of about 1.3) due to additional electron scattering in the adsorbed layer. Fig. 2 shows a difference spectrum
P.J. &lOlleret aL / Surface Science 323 (1995) 102-108
(curve 3) in which the spectrum before adsorption (curve 1) was subtracted from the spectrum after adsorption (curve 2). From the difft.~,~ce spectrum we clearly see formation of new adsorption-induced losses at 3.5 eV (F1), 11 eV (F2), 14 eV (F3) and 17 eV (F4), respectively.
3.2.2. Photo-induced desorption Desorption was induced by sample illumination from a high-pressure 300 W xenon lamp. The light beam was focused precisely on the sample surface so to prevent heating and desorption from the holder. A negligible amount of molecules desorbed by an action of multi-reflected light from the walls of the chamber was confirmed by a separate experiment. Adsorbed species were removed from the sample by heating of the sample holder up to 850 K with the help ef an electron beam. The subsequent light illumination did not increase partial pressures of CO and CO 2 in the chamber. The sample temperature increase induced by a thermal action of light was monitored by a thin chromel-alumel thermocouple squeezed in between the sample and the holder. Light illumination of the CO-containing surface was followed by the main desorption of CO 2. We conclude a possible CO oxidation on the ZnO surface which is in agreement with previous results [1,3,18]. A small amount of the desorbed oxygen may be related to oxygen ejection from the ZnO sample. In Fig. 3 the results of the light-induced desorption experiment are presented. Immediately after the illumination is switched on, within about 1 s, a jump in the CO 2 partial pressure takes place (Fig. 3, curve 2). This jump may obviously be caused by CO 2 photodesorption from the ZnO sample surface, since the sample surface temperature at this moment practically is equal to RT. The following raise in the CO 2 partial pressure is observed over the 100-200°C temperature range and may be directly connected with thermo-stimulated desorption of CO 2. Using the well-known procedure for analysis of thermo-desorption pressure vs. temperature curves [19,20], an activation energy of desorption for this phase is estimated to be 0.4 + 0.05 eV. The second illumination ef the same intensity (Fig. 3, curve 3) causes only desorption of the photo-induced component. One may conclude that after the action of the first illumi-
T/°C
i
.
.
.
.
105 .
I
'
460
.o 300
....
~
i \
.
t ~Pcoz/Pa I 1.10-6 I-2.10"?
1.10.6 Lampoff 0 2'0 t Lamp on
4'0
do
do
time t/s
loo
1
,}o
Fig. 3. CO and C O 2 desorption induced by illumination of a CO/ZnO(0001) surface. Origin of the time scale corresponds to switching on the illumination. (1) Temperature curve; (2) and (3) CO 2 desorption due to the first and second illuminations, respectively; (4) and (5) CO desorption due to the first and second illuminations, respectively.
nation the adsorption sites supplying the thermodesorption flux become empty, and that the photodesorption component originates from the more strongly bonded chemisorption sites. We thus clearly have two desorption components; photo-induced (without time-delay) and thermo-induced. The shape of the CO desorption (curve 4) corresponds to the thermal origin of the observed desorption. The second illumination does not cause CO desorption during the first 40 s (curve 5). The observed increase in CO partial pressure for t > 40 s may be explained as a desorption from the sample holder whi~n is heated due to thermoconductivity. It is emphasized that the amount of desorbed CO molecules is approximately one order of magnitude less than that of CO 2. An estimated amount of desorbed CO 2 molecules, obtained from the analysis of desorption curves, corresponds to an initial sample coverage of about 0.3-0.5 monolayer of CO 2 on the ZnO(0001) surface under investigation.
106
P.J. M¢ller et ai. / Surface Science 323 (1995) 102-108
4. Discussion
The predominant CO 2 desorption observed in our experiment (Fig. 3) is a consequence of a heterogeneous catalytic CO oxidation at the ZnO surface, since the surface under investigation was exposed to CO adsorption only. In view of the obtained LEED results, the adsorption of CO molecules may be considered as an interaction with the ZnO(0001) surface which contains hexagonal-pit defects created by thermal etching of the surface during the annealing. The proposed formation of chemisorbed CO.;- is supported by our EELS results. Let us first consider a diagram of electronic transitions in relation to the density of states (DOS) in ZnO. We have measured an energy loss of 93.5 eV for the Zn 3p (M~) level ionization. Since the energy of this location is well known (86 eV below VBM) it is easy to determine the location of the final state for the corresponding electronic transition which is to be equal to 7.5 eV above the VI3M. This result is in accordance with a calculated position of the maximum a of the empty DOS [11], The calculated DOS is inserted in Fig. 4 (curve I). The energy positions of other DOS maxima placed above the vacuum level correspond well to maxima in the TCS curve (Fig. 4, curve 2), since there is a correlation between ~nergy locations of the band-structure critical points in the Briilouin zone and the maxima in the TCS curve [6,21]. The primary peak of the TCS curve gives a location of the vacuum level Evac (shown in Fig. 4 by a dotted line). The position of Evac shifts approximately 0.5 eV after CO interaction with the surface, showing an increase of the work function (Fig. 4, curve 3). The TCS structure, which is related to the band structure of the crystal, does not shift in energy under adsorption. Such a TCS behavior indicates that when the work function of the semiconductor changes as a result of electron-affinity variation then there is no shift in the semiconductor band structure relative to the Fermi level [6] and the band bending remains unchanged. Thus the observed work-function increase may be connected with a surface dipole-layer formation characterized by negative charge transfer to adsorbed species. The diagram of electronic transitions to the final state a is shown in Fig. 4. The arrows B, C and D correspond to transitions with energy losses 9.5, 12.8
IF4 T
E~ I F3
[ t
Ec
lEnac , J',
1 F2
-10
-5
0
5
10
15
Energy above Ev (eV) Fig. 4. Energy diagram combining TCS (dJ(Ez)/dE z vs. El), curves 2 and 3, with calculated (from Ref. [11]) DOS, curve 1, and schemes of electron transitions (indicated by arrows) for clean (lower part) and for CO-covered (upper part) ZnO(0001) surfaces in comparison with valence band (curve 4, from Ref. [20]) and difference (for CO, exposed surfaces, curve 5, from Ref. [25]) photoelectron spectra, a indicates the location of the final state. Ec and E,, are the edges of the conduction and valence bands, respectively, Ev~ the vacuum level, and A~ the work-function change due to CO interaction with the ZnO surface.
and 15.5 eV, respectively, and indicate the positions of initial states in the valence band of ZnO. They correspond well to two maxima in the valence-band DOS and to the Zn 3d band (Fig. 4, curve 4 presents a photoemission spectrum from the ZnO(0001) surface [22]). The energy loss of 15.5 eV was explained in previous investigations [13] as a transition from the Zn 3d band. The energy loss of 4.6 eV (in Fig. 4 indicated by arrow A) may be assigned to electron transitions from defect states in the gap (located at about 0.8 eV above VBM) to the final state at 5.4 eV above VBM. These final states correspond to highenergy DOS due to the flat band of empty states in the I"-A direction of the Brillouin zone [11] and to Zn dangling-bond states [1,24]. The location of the narrow band of electronic states, at about 2.5 eV below the bottom of the conduction band, in the gap of ZnO, has been observed by a photodischarge spectroscopy experiment [23] (the ZnO gap-width Eg is 3.3 eV). The same states may be responsible for a
P.J. M¢ller et al. / Surface Science 323 (1995) 102-108
visible photoluminescence of ZnO having its emission maximum at about 2.5 eV [24,25]. A proposed model for these states is a single electron trapped at an oxygen-ion vacancy in the ZnO crystal [26]. CO interaction with the ZnO(0001) surface creates new electronic states on the surface, and excitations of these states are characterized by the energy losses of 3.5 eV (F1), 11 eV (F2), 14 eV (F3) and 17 eV (F4) shown in Fig. 2 (curve 3). A corresponding scheme of electronic transitions is presented in Fig. 4 (arrows F l, F2, F3 and F4). The initial states for the transitions F2, Fa and F4 correspond well to the DOS of the chemisorbed CO 2 as is clearly seen from a comparison with the obtained [1,27] difference photoemission spectrum (Fig. 4, curve 5). The initial state for the transition F1 locates at about 1.9 eV above the VBM (i.e. in the vicinity of the middle of the gap) and may be directly associated with electronic states induced by CO 2 chemisorption. Electron trapping to these states provides the negative surface charging that was traced by the work-function increase. The obtained experimental r~,-~altsthus give support for the formation of the CO2 chemisorption complexes on the ZnO surface. Presence on the surface of CO2 chemisorbed species is a general condition for CO 2 photodesorption according to the photoelectron mechanism [5,28]. Our observation (Fig. 3, curve 3) of CO 2 photodesorption during the second illumination (when the thermodesorption component is absent) indicates that photodesorbed molecules originate from the strongly bonding chemisorption states which may be associated with the ZnCO~ complexes stabilized in the vicinity of positively charged oxygen vacancies [1]. Fundamental absorption of light quanta (h v > Eg) in the subsurface region of ZnO leads to creation of free electrons and holes. A capture of a photoexcited hole to the ZnCO~ complex is accompanied by both neutralization of the adsorbed particle and by its energy excitation due to import of the recombination energy which in our case is equal to 1.9 eV. Transition from a charged state to a neutral one proceeds according to the Franck-Condon principle. After neutralization, the particle is in an antibonding state and is being acted upon by forces of repulsion from the surface, and it starts to move away. On its way along the repulsing branch of the potential curve the particle will acquire excess kinetic energy [28]. This
107
energy determines the initial velocity of the desorbed particle which may significantly exceed the thermally equilibrated value of the velocity. Such a model explains the appearance of the fast CO e molecules which were observed in a recent laser-induced desorption investigation of the CO/ZnO(0001) system [4,29].
Acknowledgements The support of the Danish Science Research Councils through the Center for Surface Reactivity and by the Ministry of Science Research and High Schools of the Russian Federation is gratefully acknowledged.
References [1] W. G6pel, R.S. Bauer and G. Hansson, Surf. Sci. 99 (1980) 138. [2] T.J. Chuang, Surf. Sci. Rep. 3 (1983) 1. [3] W.H. Cheng and H.H. Kung, Surf. Sci. 122 (1982) 21. [4] P.J. Moiler, S.A. Komolov and E.F. Lazneva, Surf. Sci. 290 (1993) L677. [5] Y. Shapira, S.M. Cox and D. I.,ichtman, Surf. Sci. 54 (1976) 45. [6] S.A. Komolov, Total Current Spectroscopy of Surfaces (Gordon and Breach, Philadelphia, 1992). [7] H. Moorman, O. Kohl and G. lteiland, Surf. Sci. 182(1987) 458; P.J. Mollor and J. Nerlov, Surf. Sci. 307-309 (1994)591. [8] S.C. Chang and P. Mark, Surf. Sci. 46 (1974) 293. [9] C.T. Campbell, K.A. Daube and J.M. White, Surf. Sei. 182 (1987) 458. [10] P.J. Moiler, S.A. Komolov and E.F. Lazneva, Surf. Sci. 307-309 (1994) 1177. [11] S. Bloom and 1. Ortenburger, Phys. Stat. Sol. (b) 58 (1973) 561. [12] P.J. Moiler, S.A. Komolov and E.F. Lazneva, to be published. [13] A. Ebina and T. Takahashi, Surf. Sci. 74 (1978) 667. [14] Y. Margoniuski, Appl. Surf Sci. ~1/12 (1982) 784. [15] R. Dorn and H. Liith, J. Appl. Phys. 47 (1970) 5097. [16] Atlas of ionization losses, Eds. I.F. Koval, V.P. Lysenko, P.V. Meinik and N.G. Nakhodkin (Naukova Dumka, Kiev, 1989) [in Russian]. [17] S.S. Fu and G.A. Somorjai, Appl. Surf. Sci. 48/49 (1991) 93. [18] W. Hotan, W. G6pel and B. Haul, Surf. Sci. 83 (1979) 162. [19] V.N. Ageev and N.I. lonov, Progr. Surf. Sci. 5 (1), Ed. S.G. Davison (Pergamon, New York, 1974).
108
P.J. M¢ller et al. / Surface Science 323 (1995) 102-108
[20] A.M. de Jong and J.M. Niemantsvedriet, Surf. Sci. 233 (1990) 335. [21] S.A. Komolov and V.N. Strocov, Phys. Star. Sol. (b) 167 (1991) 6O5. [22] W. CRipei, J. Pollman, I. lvanov and B. Reihl, Phys. Rev. B 26 (1982) 3144. [23] R. Williams and A. Willis, J. Appl. Phys. 39 (1968) 373. [24] M. Anpo and Yu. Kubokawa, J. Phys. Chem. 88 (1984) 5556. [25] H. ldriss and M. Barteau, J. Phys. Chem. 96 (1992) 3382.
[26] W.F. Wei, Phys. Rev. B 15 (1977) 2250. [27] C.T. Au, W. Hirsch and W. Hirschwald, Surf. Sci. 197 (1988) 391. [28] E.F. Lazneva, Rad. Effects and Defects in Solids 115 (1991) 257. [29] P.J. M~ller, S.A. Komolov and E.F. Lazneva, in: Adsorption on Ordered Surfaces of Ionic Solids and Thin Films, Eds. H.-J. Freund and E. Umbach, Springer Ser. Surf. Sci., Vol. 33 (Springer, Berlin, 1993), p. 156.