ULTRAVIOLET PHOTOEMISSION STUDIES OF MOLECULAR … of Adsorbates on Oxides... · surface...

Pro9msr in Sutfacc Scimce. 1979. Vol. 9, pp. 143-W. Pergamon Press. Printed in Great Britain

ULTRAVIOLET PHOTOEMISSION STUDIES OF MOLECULAR ADSORPTION ON OXIDE SURFACES

VICTOR E. HENRICH Lkpatimmt of Engiwwing and Applied Science, Yale University,

New Haven, Connecticut 06520. U.S.A.

Abstract-This paper reviews the current status of ultraviolet photoemission (UPS) measurements of the interaction of molecules with the surfaces of metal oxides. A brief summary of the ekctronic, geometric and compositional properties of atomically ckan oxide surfaces is given, followed by a survey of the various adsorbatelsubstrate systems that have been investigated by UPS. Essentially all of that work to date has been on single-crystal oxide substrates. A brief description is also given of the various theoretical methods that are being used to study molecule/oxide interactions.

1. Introduction content!3

143

2. Structure of Clean Oxide Surfaces A. Composition B. Geometry C. Electronic structure

f: 145 I46

3. YoleeOar Adsorption on Single-crystal Oxides

B: Adsorption of 02 on Ti02 and SrTia C. Adsorption of Hz0 on TQ and SrTia D. Adsorption of Hz on TiO, E. Adsorption of 01 on TiO, (x - I) F. Adsorption of Hz0 on Ala G. Adsorption of C2H2 on NiO

147 147 IS2 I56 lS8 I59

E

4. Theoretical Calculations of Molecular ,Adsorption on Oxide .Surfaces 161

5. Future Directions

References

DV

LEED

&F

!z XPS

162

163

discrete variational electron-energy-loss spectrosc linear combination of atomic or “k

y tats

local density of states low-energy-electron diffraction self-comustent field scattered wave ultra-high vacuum ultraviolet photoemission spcctros~~p~ X-ray photoemission spectroscopy

1. Introduction

The surface properties of metal oxides, and particularly their interaction with adsorbed molecules, have been technologically important for

‘many years. The poisoning of a thermionic emit- ter: the activity and selectivity of an oxide catalyst: the sensitivity of an electron multiplier to ambient gases: the interaction of a small catalyst particle with its support: the long-term stability of MOS devices and negative-electron-affinity photo- cathodes; the efficiency with which hydrogen can be produced from sea water by photolysis-all of these involve the interaction of atoms and

molecules with oxide surfaces. Over the last few years, surface scientists have taken an increasing interest in oxides, and oxide surfaces have been studied using a wide range of techniques.

The present paper will review the current status of research on molecular adsorption on metal oxides by means of ultraviolet photoemission spectroscopy (UPS). The reason for restricting the scope of the paper to UPS studies is two-fold. First is simply a space requirement; a comprehen- sive treatise on studies of adsorption on oxides would occupy an entire book. Secondly, UPS has given us by far the most detailed information on

143

144 VICTOR E. HENRICH

the electronic nature of adsorbatelsubstrate inter- A. composition actions. The method is inherently surface sen- One of the main difficulties in working with sitive, has good energy resolution and is parti- oxides, as with any compound, is the possibility of cularly useful for studying electronic states in the a wide range of composition in the surface region. vicinity of the vacuum level (+-50eV). where the Many structures and compositions that cannot action is in chemisorption. While techniques, exist in the bulk are allowed in the region of such as infrared absorption, electron-spin reduced symmetry at a surface. Even surfaces that resonance, thermal- and electron-stimulated are ideal terminations of the bulk can have desorption, low-energy-electron diffraction different atomic arrangements and compobitions (LEED). X-ray photoemission (XPS), Auger and depending on which lattice plane is the topmost electron-energy-loss spectroscopy (ELS), work one. In the perovskite (ABC&) lattice, for example, function measurements. etc., are also extremely there are two possible (100) surfaces, one consis- useful, we will only mention a few results as they ting of a B02 plane and the other consisting of a relate to UPS measurements. plane with A0 composition.‘*’ Unlike the case of

The main technique that is used in ultraviolet the zincblende or wurtzite (A& lattices, where the photoemission studies of adsorption is UPS bond strengths are such that a crystal with (111) difference spectroscopy. This rather pretentious faces will always have an A-face on one side and a title simply refers to subtracting the UPS spectrum B-face on the other, there is an equal probability for a surface before adsorption (or some fraction of having either a B02 or an A0 face. An actual thereof, to compensate for adsorbate attenuation fractured surface will thus contain equal areas of of substrate emission) from the spectrum after each type. The two types of surfaces should have adsorption; the remainder is the “difference spec- quite different electronic and chemical proper- trum”.“’ In the limit of weak adsorbatelsubstrate ties,‘-’ and yet any experiment will sample both interaction, the difference spectrum reflects the surfaces simultaneously. It is then difficult, if pos- electronic structure of the adsorbed species; Since sible at all, to separate the features due to each the interactions are rarely very weak, care must be type of surface. taken in interpreting difference spectra, since fea- The stoichiomctry of the surface of a com- tures arising from changes in substrate electronic pound can be altered by the treatments that are structure will also be present. Also, when working usually used in preparing nearly perfect surfaces: with insulators or semiconductors, it is generally chemical etching. ion bombardment, annealing, necessary to shift the clean-surface spectrum in etc. In the case of SrTiG, for example, it has been energy relative to the adsorbed spectrum before found that simply rinsing a sample in distilled taking the difference to compensate for band water preferentially removes Sr ions from the bending: determining the amount of this shift is surface,“’ and it is very difficult to restore the not,always trivial, and small differences in align- surface composition to that of the bulk by means ment can lead to large changes in the difference of further processing.‘6”’ Hence, the only reliable spe+ra (see the discussion of adsorption on ZnO method of preparing surfaces having the stoi- in 53A). chiometry of the bulk is to cleave them in an

In 52, we briefly summarize the properties of ultra-high-vacuum (UHV) environment. Un- atomically clean oxide surfaces, since these must fortunately, many oxides do not cleave in the be understood in order to interpret adsorption sense of preferential fracture along a particular measurements. Section 3 reviews the particular crystal plane, and we will use the word “fracture” molecule/oxide systems that have been studied by to describe such samples; we will consider the UPS: almost all of that work to date has been geometry of fractured surfaces in 82 below. (Pho- performed on single-crystal substrates. Section 4 tographs of typical fractures for Ti02 and SrTiO, mentions the theoretical approaches that are being are reproduced in Ref. 8.) However, regardless of taken to study adsorption of oxides, while the geometric character of the fracture, one can $5 discusses some of the most fruitful directions assume that the composition of the bulk is for future research. retained. A surface prepared in any other manner

2. Structure of Clean Oxide Surfaces cannot be assumed u priori to have bulk stoichiometry, and its Auger or X-ray photoemission

Before discussing the adsorption of molecules (XPS) spectra should be compared with those for on oxide surfaces, we will briefly review what is a vacuum-fractured surface in order to determine known about the properties of atomically clean its actual composition. oxide surfaces. Due to the relatively small number Ion bombardment is especially troublesome of surfaces that have been studied carefully to with respect to altering surface composition. date, it is not possible to draw very many general The usual effect is a preferential removal of conclusions. but some trends are beginning to oxygen, resulting in a reduced, metal-rich sur- develop. The detailed properties of specific surface.‘6.7.e151 In the case of Ti02 and SrTiO3. faces will be treated in $3 when we discuss ad- this effect has been used to advantage to produce a sorption on those surfaces. controlled density of surface defects’6*‘0’ (see 03

Ultraviolet photoemission studies of mokcular adsorption on oxide surfaces 145

below), but it complicates the preparation of nearly perfect surfaces. For TiQ, it has been found that annealing to about lOOOK in vacuum after ion bombardment restores the surface to bulk stoichiometry by diffusion of oxygen ions from the bulk “a”) but a similar process does not work in the iase of SrTi0,.f6*” The only procedure that has been found to restore a SrTiG surface to bulk composition is inert gas ion bombardment at about 900K; this process has only been used on the SrTiG( I 11) surface.“’ For MgO, on the other hand, ion bombardment does not change the surface composition, and MgO(100) surfaces prepared by 5OO-eV Ar’-ion bombardment are indis- tinguishable from vacuum-cleaved surfaces.“~“~

Inert-gas ion bombardment has also been found to remove oxygen preferentially from ZnO sur- facesob”’ One procedure that has been adopted for the preparation of presumably stoichiometric ZnO surfaces is ion bombardment at 700 K followed by cooling to room temperature and subsequent annealing to 700 K.‘*’ An alternative procedure that has been used consists of room-temperature ion bombardment followed by annealing in vacuum at 1000 K and s&sequent annea&g in 8 x IO-’ Torr 02 at 700 K.““‘) The latter treatment was found to optimize the quality of LF$ED patterns from the (lOi0) face.“” However, direct comparisons between surfaces prepa& uaiug these procedures and vacuw surfaces have not yet been performed.

Damage produced by electron bombatdment (electron energy b 5 keV) is also a serious problem in the study of oxide surfaces.(~W’XHP-W As in the case of ion bombardment, electron bombardment tends to break cetion-anion bonds and remove oxygen preferentially from the surface. However, the response of a part&&u oxide to electrons can be entirely di&eut from its response to ions. A surface electronic state, which probably consists of a surface oxygen vacancy (or at least displacement of surface oxygen ions), can be produced by electron bombardment of MgWOO) surfaces, but no such state is produced by ion bombardment (at least for ion energks less than 1 keV).‘“’ Electron bombardment of SiO, produces drastic changes in composition, releasing oxygen and reducing much of the surface region to elemental silicon.-’ Ion bombardment, however, has essentially no dFect on the surface stoichiometry of bulk SiO, samplc~.~ The transition-metal oxides and perovskites exhibit changes in various electron emission spectra, due also to preferential removal of oxygen, after several minutes of exposure to the typical electron beams used in Auger and energy-loss spectroscopy: 1-5 keV, 10-50 ma/cmz~~~‘o*‘3*‘4’ However, it is possible to measure Auger and ELS spectra on TiO, and SrTi03 rapidly enough to obtain good signal-to-noise without producing measurable damage.'6.7.10,131 Electron-beam-induced changes in

surface composition have not been observed in Zn()."9*l"

Another effect that can change the stoichiometry of oxide surfaces is photodissociation accompanied by the release of oxygen. This effect seems to be strongest for photon energies close to (but larger than) the bandgap and is seen in ZIQ~’ TiG and a few other oxides. However, no effects of photodissociation of oxides in the photon energy range generally used for UPS (i.e. 15-U) eV) have been reported.

Annealing of oxides to temperatures where sublimation becomes appreciable can also alter surface composition. In the case of ZnO, heating to temperatures >750 K causes a preferential sublimation of oxygen, resulting in O-deficient surfaces.o”

In summary, anything that is done to an oxide surface can, and often will, change its composition. In almost all cases, the surface will become oxygen-deficient.

. R- Gcawry

The atomic geometry of a number of oxide surfaces has been examined qualitatively by low- energy&c&on diBa&on (LEED), but only a few surfaces have been studied quantitatively. LEED intensity proflIes have been measured for MgGWlO~~ and ZuO(OtKll)-Zn, woib0, (ioio) and (1 la),‘=‘- and these have been compared with dynamkal nruhiploscettering calculations.‘* y, For MgO(lOO), the LEED patterns exhibit the symmetry of the bulk,Ws’ and calculations in- corporating relax&m and rumpling (i.e. outward motion of anions relative to cations in the surface plane) indkate a relax&m of the top plane of less than3%andarumphngofnomorethanaafew percent.- The sensitivity of LEED spectra to rumpling has not yet been thoroughly investigated, howe~er.~’ For ZnO, comparison of LEED cal- culations_ with experimental I-V data indicates that the @001)-0 and (11%) surfaces are very nearly truncations of the bulk,- whik the (OOOl)-Zn face exhiits a relaxation of the top plane of Zn atoms by about 0.2 A inward,W and the (ioio) surface has the Zn ions relaxed 0.45 f 0.1 A inward and the 0 ions relaxed 0.05 & 0.1 A inward.“” Reconstruc- tion has been reported for the polar surfaces of ZnO, and steps have been observed on both polar and nonpolar faces*--”

Several Tiol surfaces have been studied qualitatively by LEED (i.e. only the symmetry of the LEED patterns has been examined).““*‘3”’ The (110) surface is stabk at all temperatures and exhibits only (1 X 1) LEED pattems.“‘“*“’ The (001) surface is unstable and reconstructs on heating to (110) and (100) facets.‘4’ The (100) surface exhibits three different reconstructions depending on annealing temperature: (1 x 3), (1 x 5) and (1 x 7) for temperatures of 900, 1100 and 1500 K, respectively.“” The SrTiO,( 111) surface exhibits a (1 x 1)

146 vlcroa E. HENNCH

pattern after annealing, but a complex, faceted pattern can be produced by ion bombardment at 900 K.” The SrTi~(100) surface also exhibits (1 x 1) LEED patterns (Fii l(b)).@’

The LEED patterns from vacuum-fmctured Tio2 and SrTiO, are generally poor (i.e. weak spot intensities relative to the background, spots somewhat difhlse). although they do exhiit the basic symmetry of the nominal fracture plane. A LEBD pattern for a fractured SfTiO3(100) surface is shown in Fig. l(a). After anneahng to about 12OOK. the LEED pattern improves to that expected for a nearly perfect surface [Fig. l(b)]. The poor quality of the LEED patterns on fractured TiG and SrTiQ surfaces is consistent with UPS determinations of the density of defect- associated electronic states on those surfaces (PZC).“‘”

c. Elecmmk- Compared to metals and semiconductors,

knowledge of the surface electronic properties of oxides is meager. UPS and ELS measurements on well characterized surfaces have only been pcr- formed for a few oxides. We will consider each of those surfaces in 03 when we discuss the UPS data on adsorption, but we will make a few general comments here. Nearly perfect surfaces of metal oxides that have empty cation-derived bulk conduction (bands (e.e MI@, TX&, SrTii, ZnO, St& AM) do not exhibit a gross transfer of charge between surface anions and cations relative to the bulk (i.e. if there is any transfer, it is less than about 0.1 electron per surface ion). Such a charge transfer would be seen in UPS spectra as occwied states in the bandnarr or conduction band

observed. Charge

FIG. I. LEED patterns (electron energy = 95 eV) for a vacuum-fractud SrTiOWO) surface (a) before and (b) after annealing to about I#)0 K.

Ultraviolet photoemission studies of molecular adsorntion on oxide surfaces 147

transfer has been found to accompany defect formation on those oxides,‘“*‘“.‘3’ and that will be discussed in 63 below. For oxides having partially filled conduction bands (e.g. NiO, T&a, TiO, (X = 1). MnO, Fe& etc.), anion-cation charge transfer between surface atoms would be much harder to determine from UPS spectra, since it would appear only as a change in the intensity or width of emission peaks due to bulk bands;“” no attempts have been made to date to determine the extent of any such transfer.

Very little is known about the spectra of empty surface states on nearly perfect oxide surfaces. Tia and SrTiO, do not exhibit any energy-loss features corresponding to empty final states in the bulk bandgap.‘6.‘o) In ZnO, one group has observed an ELS peak at 2.8eV,‘&’ less than the bulk bandgap of 3.2 eV, but that peak has not been seen by other groups. MgO exhibits a strong surface loss peak at 6.1 eV, less than the bandgap of 7.8eV, which clearly indicates an empty bandgap surface state.“” For SQ, three energy-loss peaks are observed at energies less than the bandgap on oxidized silicon,‘“’ vacuum-fractured fused silica, etc.,‘=’ but experiments have not yet been performed on vacuum-fractured single-crystal quartz. However, since the same three loss features are seen on all other types of SiO, surfaces, there is strong reason to believe that they do arise from empty intrinsic surface states.‘u’ Simii loss peaks have been observed for G& layers on Ge.“”

8.MoleeukrAdsorp&mou~Gxkks

Although one of the main goals of the surface scientist is to understand the mechanisms of heterogeneous catalysis involving complex molecules on real catalyst surfaces, basic studies begin with systems that can be interpreted as completely and unambiguously as possible-simple molecules on nearly perfect single-crystal surfaces. With the exception of the work on defect surfaces of TQ and SrTiO3 described below, all of the UPS studies to date of the changes in electronic structure during chemisorption have been performed on nearly perfect surfaces. We will discuss each oxide separately in the following sections.

A. ZOO

ZnO was the first oxide on which molecular adsorption was studied by UPS. The changes that are produced in its semiconducting properties by the adsorption of various gases have been studied by Tans of bulk. transport measurements for many .years.‘46’ The first UPS measurements on ZnO were made by Powell et al.“” in 1972, but they only studied vacuum-cleaved (IOiO) surfaces. Rubloff, Liith and Grobman at IBM began studying the adsorption of hydrocarbons on ZnO in 1975.‘“’ All of their work to date has been on as-grown, non-polar (ioio) “prism” surfaces

cleaned by direct resistive heating to about 1800 K in UHV. This procedure was shown to give the same ELS spectra as for (1010) surfaces cleaved in vacuum,(&) although a preferential sublimation of oxygen from (loio) surfaces at temperatures above 750 K has been reported by G&XL”* Table 1 lists the molecules whose adsorption has been studied by the IBM gro~p.‘~.‘~’

A sufficiently large number of molecuks has been examined to show clearl_y the trends in molecular adsorption on ZnO(1010).‘“s3’ Adsorp- tion was studied at both 120 K and 300 K. Some molecules (C&L, Ht, 02 and CO) produced no changes from the UPS spectra of clean ZnO(lOi0) at either temperature, except for 0.1-0.3 eV band bending, indicating essentially no adsorption. (Recent work by Gay et ul.“=‘is in disagreement with those results for CO; this will be discussed below.) Only C~HJN and HCOOH were found to adsorb at 300 K. The remainder of the molecules adsorbed only at 120 K.

As an example of the changes in the molecular orbital structure of molecules adsorbed on ZnOGOiO), the results for C& adsorption from Ref. 49 are shown in Fig. 2. Figure 2(a) shows the UPS spectra for clean (dashed curve) and C&Is- covered (solid curve) surfaces at 120 K. Figure 2(b) shows the difference spectrum (with the clean- surface spectrum attenuated by 50%) for adsorbed Cd&, and Fig. 2(c) shows the gas-phase CsHs spectrum, measured with the same resolution as for the adsorbed spectra, for comparison. A uniform extra-molecular relaxation/polarization shift,‘lJ2’ AE,,,, of 1.3 eV (toward smaller binding energy) is found for all but the three highest-lying molecular orbitals. The highest-lying (a) orbital is shifted toward tighter binding by 0.4eV (A&), while the next two orbitals, having mixed 7r and ‘a character, have A& = 0.2 eV. The fact that the adsorbed molecule clearly retains the basic molecular orbital levels of the free molecule indicates that it does not dissociate upon adsorption. Since the three orbitals that are shifted toward tighter binding relative to the rest of the molecular spectrum are presumably the ones involved in bonding to the ZnO substrate, the molecule is most likely lying flat on the surface.‘49’

The bonding shifts seen for the second and third orbitals in Cd& are somewhat unusual in that only the highest-lying orbital is shifted (in addition to A&,) for most adsorbates. Figure 3 shows the more typical case of C&O and CH3OH adsorbed

TAFILE 1. Molecules adsorbed on Zn0(liOo) (Refs. 48-53)

% C&N Co,

c&l

H(%$O &3$0

(CH3)CCH

CH,OH C% HzCO (CH3kCO HCOOH

VICTOR E. HENRIM 148

‘24 I ,b ,’ ,‘6 ,I ,’ I4 ,I 3 23

ELEckmeALER&ul

Fffi. 2. (a) UPS spectra (kv = 40.8 ev) for the ZnOf liOO1 face at 120 K. clean (dashed curve, No(E)) and in the presence of a C& ambient solid curve, N.&E)); (b) diikrence spectrum N.&E)- I NdE): (c) UPS spectrum

for gas-phase CA at hv = 40.8 eV (from Ref. 49).

I CH,Cti W PHASE I\ h I

FOG. 3. (a) UPS difference sp@um for adsorbed C$&O on ZnO(liO0) at 120 K and kv =40.8eV, compared to the UPS spectrum of gas-phase CJGO at 40.8eV; 0~) UPS difference spectrum for adsorbed CHaOH on ZnO(l100) at 120 K and hv = 40.8 eV compared to the UPS spectNm of gas-phase CHsOH at hv = 40.8eV

(from Ref. 49).

on ZnO(lOiO) at 12OK.‘* only the most kkly bound molecular orbital, which for both of these molecules is an O-lone-pair orbital, is shifted, with A&, = 0.6 eV and 0.4eV for GH.0 and

CH,OH, respectively. The remainder of the orbitals have a uniform A& of 1.5 eV and 1.8 eV, respectively. Since only the O-lone-pair orbitals are involved in the bonding, the molecules are presumably standing on the surface with their 0 atoms down. There is clearly no dissociation of either molecule upon adsorption.

The above examples show the usual situation for molecules adsorbed on ZnO(10~0). The highest-lying, unsaturated ?r or lone-pair orbitals, which physically protrude from the molecule, are the dominant bonding orbitals.‘4BJ” The uniform relaxation/polarization shift of the remainder of the orbitals varies from 0.3 eV to 2.0 eV, values similar to those found for. adsorption on metals.‘s2’ With the exception of HCOOH, to be discussed below, the molecules do not dissociate.

Three molecules studied-H&O, H(CHX0 and (CH,#O-exhibit non-xero bonding shifts of deep-lying 0rbitaLs when adsorbed on ZnCXlOTO), as well as on some metal surfaces.‘m Fii 4 shows data from Ref. 52 for chemisorbed H(CH,)CO compared to the UPS spectrum of the gas-phase molecule. In addition to the bonding shift of the highest-lying, O-lone-pair orbital (which is expected), the peak at a binding energy of about 16eV is also shifted toward tighter binding. That peak turns out to arise from an in-plane 7~ orbital (see the diagram of the molecule in Fig. 4(a)), which has a large component of 0(2p,) character, as does the O-lone-pair orbital. It is thus not unreasonable that such an orbital should shift in a mater analogous to the lone-pair orbital. However, upon chemisorption, the photoemission peak at about 19 eV, shifts by 1 eV toward smuller bindin& relative to the other orbitals. This rather

I ~H(CH&O hv=40.8eV 1 1 H o=c’ -z

1

Pm. 4. UPS spectra at hv - 40.8 eV for H(CH&O; (a) chemisorbed on evaporated _W at 120 K. 68 L exposure: (b) chemisorbed on ZnO(1100) at 120 K. 5 x W’Torr

ambient: (c) gas phase at hv = 40.8 eV (from Ref. 52).

Ultraviolet photoemission studies of molecular adsorption on oxide surfaces 149

unexpected behavior can he understood from the fact that that orbital is a a& of purely s-character-the only orbital in the spectrum that is not of predominately p-character. It might therefore be expected to behave in a different manner from the others and more like the deeper-lying core ievels, which generally exhibit larger relaxationlpolariza- tion shifts than do valence levels.‘s2’

The dissociative chemisorption of HCOOH on ZnO(lOi0) has also been studied by UPS,‘_“’ and Fig. 5(a) shows the spectra for a clean surface (dashed curve) and that surface after exposure to IO L (iL = 1 Langmuir = 10dTorr-set) of HCOOH at 120 K. The difference spectrum for adsorbed HCOOH is the solid curve in Fig. 5(b). Unlike the other hydrocarbons studied, this difference spectrum does not resemble the gas- phase HCOOH spectrum, suggesting that HCOOH has dissociatively chemisorbed. In an attempt to determine the nature of the adsorbed species, CO and H2 were simultaneously adsorbed on a clean (1010) surface at 120 K (the mixture would not chemisorb at 3OOK), giving the difference spectrum shown by the dashed curve in Fig. 5(b). (Recall that neither CO nor H2 alone would adsorb at 120 K.) The difference spectra are compared to the gas-phase spectrum for CO in Fig. 5(c). L&h et al (” interpret the spectra as indicating that H&XH decomposes into CO and atomic 0, the Wer illkmd from the peak in the difference specmun at lOeV, which is absent for CO alone. Based on the observed bonding shifts, the authors propose the adsorption geometry shown in Fig. 6.

ZnO (IiOO) HCOOH .H 10)

1 1 24 20 16'li

/ 8 '

ELECTRON BINDING ENERGY WI

FIG. 5. (a) UPS spectra of clean ZnO(l@IO) and after IOL exposure to HCOOH; (b) difference spectra for 10 L HCOOH exposure at 300 K (solid curve) and co- exposure to CO+ Hz ambient (2 x lo-‘Ton) at 120 K (dashed curve); (c) UPS spectrum for gas-phase CO at

kv = 40.8 eV (from Ref. 51).

-C-AXIS-

Fw. 6. Model for chemisorbed CO and 0 on ZnO(liOO) (from Ref. 51).

The type of adsorption pictured in Fig. 6 points up one of the main dil%ences between chemisorption on oxides and on metals or elemental semiconductors. On non-polar oxide faces, both cations and anions are available for bonding, whereas elements offer only one type of substrate atom. The presence of cations and anions offers the possibility of more complex bonding interactions, as well as charge transfer between the adsorbate and both positive and negative ions. The full implications of this additional degree of freedom are far from being understood at present.

Recently, Gay cf u!.~*“’ have been studying the adsorption of CO, NH,, 02, HI, COZ and Hi0 on the (OOOl)-Zn, (OOU&O, (1010) and (ll20) faces of ZnO by UPS. Their surfaces are prepared by cutting, polishing and etching of the desired face, followed by Ar+-ion bombardment at 7OOK. cooling to room temperature and subsequently annealing to 7OOK in vacuum. This procedure produces surfaces exhibiting excellent LEED patterns. Since annealing is limited to 7OOK. the polar faces do not exhibit any of the reconstruction that has been seen for the (WOl)- Zn and (000&O faces annealed at temperatures greater than 900K.‘2910’ Gay ef al. have not yet compared their LEED intensities to those for either vacuum-cleaved surfaces or as-grown (1070) surfaces; the latter is important in interpreting the results for CO adsorption (or the lack thereof) on ZnO. To date they have studied the adsorption of CO and NH3 in detail on all four faces, for substrate temperatures from 80 K to 450 K and ambient gas pressures up to 10m5 Torr.

Gay et u/.~) find distinctly different results for CO adsorption than do L&h d aL”” They find that CO does chemisorb on all four faces up to about 180 K in the absence of H2. CO does not physisorb down to 80 K, and H2 does not’adsorb at all over the temperature range investigated. The amount of CO adsorbed was found to depend on ambient CO pressure rather than on exposure, necessitating measurements over a wide pressure range.

Photoemission from the molecular orbitals of adsorbed CO overlaps that from the strong Zn(3d)


band, complicating the determination of accurate UPS difference spectra. Slight shifts of the clean- surface spectrum relative to the spectrum with adsorbed CO can produce either a two- or three- peaked difference spectrum for adsorbed CO, as shown in Fig. 7. It is not obvious which is the correct spectrum, since CO adsorption on metals often results in a large bonding shift of the 50 orbital, placing it on top of the 1~ orbital and giving a two-peaked difference spectrum. However, weakly adsorbed CO should have a three-peaked spectrum more like that of the free CO molecule [see Fig. 5(c)]. The experimental situation is further complicated by a narrowing of the Zn(3d) band upon CO adsorption, which can result in spurious peaks in the difference spectra. (Such peaks are not spurious in the sense that they do represent a real narrowing of the Zn(34 band; they should not, however, be associated with the molecular orbital structure of adsorbed CO.) The narrowing is most likely due to depopulation of an intrinsic surface state on the clean ZnO surface, but the details of such a state have not been investigated. A similar narrowing of the valence band on adsorption, probably corresponding to depopulation of a Wed intrinsic surface state at the upper edge of the valence band, is also observed on the (OtlOl)_Zn face. The subtraction procedure finally adopted by Gay et al.‘” consists of maximixing the overlap of structum in the valence-

FIG. 7. -UPS difference spectra for CO chemisorbed on ZnOUOlO) for various shifts of the abscissa of the clean surface spectrum relative to the CO-covered Spectrum: (a) -0.025eV; (b) -0.075;‘4; (c) -0.125eV (from Ref.

band region (Fig. 7(c)), which results in three- peaked difference spectra similar to those of Liith et al!” for adsorption on all four crystal faces. Both groups thus agree that CO bonds to ZnO via its So and la orbitals.

In spite of the complications discussed above, the kinetics of CO adsorption can be studied by monitoring the amplitude of the photoemission peak due to the 4u orbital, which is well separated from the ZnQd) band. Gay et d’= have studii these kinetics for a wide range of temperatures and pressures on all four faces, and Fig 8 shows a plot of the intensity of the 4a photoemission peak vs temperature for two different ambient CO pressures forthe(lOi0) face. Measurementsofthe change in work function with CO adsorption are consistent with the intensity of the & peat’” The adsorption kinetics were found to fit a Temkin isotherm or isobar (i.e. Langmuir-type adsorption plus a linear decrease of the heat of adsorption, AH&, with coverage). and the solid lines are Ats to such an isobar. In comparing the data from various faces, it was necessary to assume that the coverage dependence of AH,,,,, was the same on all faces. It is then possible to compare the amount of CO that would be chemisorbed at saturation coverage on diierent faces. The results are given in Table 2, where coverage is expressed as number of CO molecules per surface ZnO molecule, nor- malized to unity for the cioio) face. (The am-

0 60 60

FIG. 8. Intensity of the CO(4a) UPS peak for CO adsorbed on Z&(1010) vs temperature for CO ambients of (x) lo-6 TOIT and (0) IO-’ Torr. SoIid curves arc fits to

Temkin isobars (from Ref. 20).

TABLET. -~~

ZnO face No. of CO molecules adsorbed Surface ZnO molecule

(lOi0) (JlW

(1.00)

(0001 Zn t-

:z!t fooo )-o 0:19

(from Ref. 20.)

Uhviokt photoemission studies of molecular adsorption on oxide surfaces 151

plitude of the CO peaks relative to the substrate emission for the non-polar faces also suggests close to monolayer coverage at saturation.) The heats of adsorption at zero coverage were nearly the same for the four faces, ranging from 11.6 to 12.4 kcallmole.~’

The above results suggest an interesting pic%re of CO ajsorption on ZnO.m’ The non-polar (1010) and (I 120) faces, which adsorb a full monolayer of CO, have both coordinatively unsaturated Zn and 0 ions in the surface plane. The much lower saturation coverage on the polar (0001)_Zn and (OOOi)-O faces, where only Zn or only 0 ions are exposed, respectively, suggests that both a Zn and an 0 site are necessary for CO chemisorption, analogous to the dissociative chemisorption of HCOOH in Fig. 6. But if both Zn and 0 sites are required, then one would expect no adsorption on the polar faces. However, it has been determined that these faces contain a large Aumber of steps, the height of which is predominately two double layers (or a full lattice constant along the c- axis).‘W’7s’ Such steps expose non-polar (IOiO) facets, and CO could then adsorb at Zn-0 sites on the facets. The amount of CO adsorbed at saturation coverage would then be a measure of the density of steps on the polar faces.

The reason that Gay d al. see chemisorption of CO on ZnO, while Liith d al. do IX& has not been dew. Subtle differences in surface structure or composition arc probably the crucial factor. Although both groups report excellent qualitative LEED patterns, neither group has made I-V qsurcments for comparison with theoretical a&u&ions.

The adsorption of NH3 on all four faces of ZnO

Zn 0 KIOOl) -2n + NH,

An interesting aspect of NH, adsorption on ZnO is that it “poisons” the surface for CO chemisorption.@Ow~” Fiire lo(a) shows a UPS difference spectrum for CO chemisorbed on the (OOO&O face at 82 K and lo-‘Torr. When the surface is exposed to 0.5 L of NH3 in the presence of the CO ambient, the difference spectrum in Fig. 10(b) results. Emission from the Co(&) orbital (the only CO orbital that does not overlap NH3 orbit&) drops as CO molecules are replaced by NH1 molecules; the other two peaks in Fig. 10(b) are combinations of CO and NHs orbitals. After I L NH3 exposure, Fig. lo(c), the C0(4a) has vanished, and the spectrum is purely that of NH,. The surface is then completely inactive for additional CO adsorption at any pressure-in other words, it is poisoned.

FIG. 9. UPS difference spectra for NH3 on Zno(O001)- Dmn et al.“’ have measured UPS spectra for 02 Zn; (a) 246 K. 5 x IO4 Torr NH3 ambient; (b) 100 K. 2 L adsorption on the (ooOl)-Zn face of ZnO; their

NH3 exposure (from Ref. 20). results are shown in Fig. 11. Similar results have

has also been studied by Gay et d~mJ*J5’ In addition to chemisorption, NH, physisorbs for temperatures below about 110 K. Figure 9(a) shows the di&rence spectrum for chemisorption of NH3 on a (OOOl)_Zn face at 246 K and an ambient NH, pressure of 5 X IOip Torr. The peaks corresponding to emission from the le and 3~ molecular orbitals of NH, are labelled (with 34: indicating the location of the chemisorbed 3a, orbital). The two peaks are shifted about 2eV closer together than in the free molecule,‘=) suggesting bonding via the 3al N-lone-pair orbital. FQurc 9(b) shows the spectrum for a combination of chemisorbed and physisorbed NH3 for 2L exposure at 100 K. The 1 e orbital lies at essentially the same energy for both adsorption states, but the 3a1 orbital for physisorbed NH2 is unshifted relative to the gas-phase spectrum. (The other features in the difference spectrum are “spurious” in that they arise from narrowing of the ZnQd) band and changes in the sfNcture of the valence band. As in the case of CO adsorption, these changes have not yet been thoroughly investigated.)

The kinetics of NH, chemisorption have been i%WlyzediIltheSZUtle- as those of CO.‘2o’ They are found to fit a Temkin isotherm better than a muir one, but there is significant scat- ter in the data. The onset of physisorption below 110 K also restricts the range of data available. Values of AH.,,, for chemisorbed NH, are difficult to determine from the data, but they seem to lie in the range of 25-U) k&/mole. It is difficult to compare the ilitensities of the le orbital of NH3 adsorbed on the four faces of ZnO, due to the difficulty of obtaining accurate difference spectra and the possible variation of the magnitude of A& between faces. UPS difftrence spectra in- dictate that NH3 adsorbs not only on the step but also on the terraces on the (ooOl)-Zn face, but preliminary work function data do not corroborate this conclusion.m)

VICIDR E. HENRICH 152

:

zno mooT)-o

+O.SL NH,

\ IS IO 5

Ilwal alstgy, l v

FIG. 10. UPS di&rence spectra for ZnO(OODi~ at 82 K with a CO ambient of 1 x IO-’ Torr: (a) before exposure to NH,; (b) after 0.5 L NH, exposure; (cl after 1 L NH,

exposure (from Ref. 20).

been obtained in preliminary experiments by Gay ef al.@“’ No interpretation of the observed spectra has been presented, but the 02 no doubt dis- sociates, since the UPS spectrum for molecular a exhibits four peaks over the energy range investigated.‘W

One additional UPS measurement should be mentioned with regard to 2110. Cesiated ZnO has been found to have an extremely low work function coupled with relative chemical stability!Ju” properties of great interest for thermionic devices, and Powell and Spice? have studii the cesia- tion of ZnO by UPS. Figure 12 shows their spectra for cleaved (lO’lo> ZnO, ZnO powder and cesiated ZnO powder, all measured with a photon energy of 11.4 eV. While showing clearly the reduction in work function brought about by Cs adsorption, the

hv m2l.W

(a)

(b)

21 l7 0 9 9

Ekeon binding OMrgy M

FIG. 11. (a) UPS spectra of annealed ZnO(WOl~Zn before (dashed curve) and after (solid curve) lti L exposure to Q; (b) dilTer~~ce spectrum corresponding to

(a) (from Ref. 57).

spectra provide no information about the electronic IeveIs of the adsorbate. The highest-lying core levels of Cs are the 5pla and Sp,,,, with binding energies (relative to I$) of 13.1 eV and 11.4eV, respectively, which are too deep to be seen using this photon energy. The Cs(6s) electron is presumably transferred to the substrate on adsorption, giving rise to the low value of work function. The two new peaks seen on the cesiated surface, s3 and S.. have been attributed to a max- imum in the ZnO conduction band density of states and to inelastic scattering, respectively.‘60’

B. AdmrpthofChomTiOland!3rTiO,

The discovery in 1972 by Fujishima and HomW6’) that TiOr @utile) could be used as a catalytic electrode in the photodecomposition of

Initial state energy relative ta valwm bgtd mamum

FIG. 12. UPS spectra for hv = 11.4eV for heat-clcancd ZnO powder. (Cs)ZnO powder and single- crystal ZnO(1010) cleaved in vacuum (from Ref. 60).

Ultraviolet photoemission studies of mokcukr adsorption on oxide surfaces 153

water @hotoelectmlysis) aroused a great deal of interest in the surface electronic properties of transition-metal oxides. The first UPS studies of the adsorption of G on Tiol and SrTiG were performed by Henrich ef (II.“Oa’ Subsequently, Lo et al.“” studied other surfaces of Tio2. Before the results of those experiments can be discussed, the electronic properties of atomically clean Ti02 and SrTiOs surfaces must be described in some detail.

The bulk band structures of Tio2 and SrTiO, are very similar, because the Ti ions are in an octa- hedral oxygen environment in both the rutile (Ti0.J and perovskite (SrTiG) lattices, and the ions in both materials have a Ti4’(3dq electronic configuration.“~’ Figure 13 shows a schematic energy-level diagram for SrTiOa?*) the structure of TiG is essentially identical, except for the absence of Sr bands. Both filled and empty Sr bands in SrTiG are too far from the Fermi level to play a significant role in chemisorption bonding. (~45) The room-temperature bulk bandgaps for SrTiO, and TiOr are 3.17 and 3.05 eV, respectively. The bulk Fermi level lies in the bandgap, and both materials are insulators when stoichiometric. Their bulk stoichiometry can be varied slightly by heating to several hundred K in vacuum (or a reducing atmosphere); 0 then diffuses to the surface and is released, resulting in a small number of 0 vacancies in the bulk (up to about 1019/cm3 at 900 K).‘W’ These 0 vacancies act as donors, resulting in n-type material. (A bulk density of IO” electrons/cm3 is too small to be seen in a UPS e&@ment.)

R&e 14 shows the UPS spectrum of a nmum-fractured surface of SrTi@ [roughly tM@)), measured with hv = 21.2 eV.“’ The zero of &s&l-state energy has been taken as the Fermi kvel, EF, which is pinned at the bottom of the conduction band by bulk 0 vacancies. The bulk valence band, which is predominately of O(2p)

Sr(56)

10 VACUUM -_---_----_

Ti (3d)

E/O

O(2p)

e Sr(4p)

-20 O(2s)

Ti(3p) -30

FM. 13. Schematic energy-level diagram of SrTiO, (from Ref. 6).

1 FRACTURED SrTiO, (I@01

hv l 21.2eV

w

c

E, 2 b-0

x10

(a) 0 t 1 I

10 6 6 42, 2 E,=O

INITIAL ENERGY, E (rV1

PK. 14. UPS spectra for vacuum-fractured SrTiO,(loO) (a) before and (b) after subtraction of 23.1 eV “ghost”

spectrum (from Ref. 6). ’

origin, extends from 3 eV to 9eV. The small amount of emission in the bandgap region, which is shown in Fig. 14(b) after correct&r for the presence of a 23.1 eV line in the photon spectrum, is believed to arise from surface &fee& present on fmchued surfaces (32B); as ‘shown. it virtually pisappears upon 02 adsorption., The UPS spectra for vacuum-fractured TiO, are similar to Fig. 14.

Since the active states on TG and SrTii ph+ toelectrolysis elec&odes are believed, from elec- trochemical measurements,‘“‘~ to lie near the middle of the bulk bandgap. Henrich er (ll.“*‘O) studied the electronic structure of surface defects on TioS and SrTiO,. They created defects in a controlled manner by Ar’-ion bombardment and found that several different surface defect phases occurred as the defect density was varied. Figure 15 shows the mannerinwhichtheUPSspectrafor vacuum-fractured SrTQ change with bombardment by 500 eV Ar’ ions for the times ~hown;‘~ the t = 0 curve is essentially the same as Fig. 14. Fortimesgreaterthan6OOsec,therearenofurther changes in the UPS spectra; a steady-state has been reached and the sample surface is just being etched away by the ions. Emission from the valence band decreases in intensity with ion born bardment, and an emission peak appears in the regionofthebulkbandgapandgrowswithbom- bardment time. When steady state has been reached, the amplitude of the bandgap emission peak corresponds to l-l.5 electrons per surface unit cell (see Ref. 6 for a complete discussion). The LEEDpattems disappear by t==3Osec, and Auger spectra show a loss of both 0 and Sr from the surface as a result of ion bombardment. Sii effects are seen for TiG (see Refs. 10 and

VICTOR E. HENRICH

FIG. IS. UPS spectra for vacuum-fractured SrTiQ(lO0) for various SOOeV Ar+-ion-bombardment times. Spectra

are aligned at EF (from Ref. 6).

13). with Auger spectra indicating a loss of 0 from the surface.

The defect surface states created on both SrTii and Tiol by ion bombardment correspond to a transfer of electrons from surface 0 ions to surface ‘Ii ions, resulting in a Ti’+(3d’) electronic con@uration.‘WW Henrich e? (Il.‘*ln investigated the SrTi~(100) and TiO4 10) surfaces, while Chung d u!.~*” studied the TiilOO) and (110) and SrTiO,(lll) surfaces. On the TiO#OO) surface, defects were produced not only by ion bombardment, but also by evaporation of sub- monolayer amounts of Ti.“” The defect surface states produced by both methods were consistent with the transfer of electrons to surface Ti ions; we refer to such defects as Ti”/O-vacancy com- plexes.

Since the defect surface states involve 0 vacancies, they should interact strongly with OZ molecules; this is just what is obser~ed.“‘~’ figure 16 shows three UPS spectra for Tior(110) after various treatments.“” Figure 16(a) is for a nearly perfect surface, while Fig. 16(b) gives the steady-state spectrum after H)OeV AC-ion bombardment. Figure 46(c) shows the surface in (b) after exposure to lp L of 0~. The adsorption of 02 restores the valence-band emission to some- thing resembling that for the perfect surface, and the bandgap surface state has been completely depopulated. (Geometric order is not restored to the surface by 02 adsorption, of course, and the surface in Fig. 16(c) exhibits no LEED patterns.) This behavior upon 02 adsorption is characteristic of all SrTit& and Ti02 surfaces studied to date. The amount of 02 exposure necessary to com-

X10

ES (c) ly

i -4 E"=O 4

INITIAL EN&Y W)

FIG. 16. UPS spectra for (a) annealed (nearly perfect) Ti02(1 10); (b) 500 eV Ar+-ion-bombarded TiO?( I 10) at steady state (solid curve); (c) surface in (b) after

exposure to lo’ L Oz (from Ref. IO).

pletely depopulate the bandgap surface state is smaller on SrTiO, (about 30 L) than on TiO, (about lo’ L).‘=’

Detailed studies of the adsorption of 0, on Ti02 and SrTia as a function of exposure at room temperature have been carried out by Henrich et al.‘=’ Figure 17 shows a family of UPS spectra for AC-ion-bombarded SrTiO,( 100) before exposure to a (0 L) and after exposures of 0.5 L to 108 L; the spectra have been aligned at the top edge of the valence band, E,, to eliminate the effects of band bending during adsorption. The increase in valence band emission and the depopulation of the bandgap surface state can be clearly seen, indicating electron transfer from the surface defect states to the adsorbed species. More detailed information can be obtained from difference curves (always subtracting the clean-surface spectrum), as shown in Fig. 18. These spectra show that two distinct adsorbed phases are formed at different exposures. The initial phase (I) is obtained for exposures up to about 100 L; its difference spectra exhibit two peaks, separated by about 2.5 eV, overlapping the bulk valence band [see also Fig. 19(a)]. The similarity of these difference spectra to the valence-band emission suggests that the adsorbed species may be ti-, since the bulk valence


band is derived predominately from 02- ions. The depopulation of the Ti’+/O-vacancy surface state is consistent with the formation of a negatively charged adsorbed species. However, the data do not rule out other negatively charged species, such as a-.

For exposures greater than IOOL. a second phase (II) appears, whose difference spectra are characterized by a third peak below the bulk valence band (7-g eV) and by changes in the relative heights of the peaks near 2 eV and 4eV. If one assumes that phase II adsorbs in addition to phase I, its difference spectrum can be obtained by

0 a 6 4 2 qeo -2 -4

INITIAL ENERGY (&‘I

Fffi. 17. UPS spectra (hv-5 21.2eV) for Ar’-ion-bom- Wded SrTi~(lO0) after successive exposures to Q

(from Ref. 62).

h\ 4 ON SCMARDED

SrTi4(1Wl

INITIAL ENERGY WI

FIG. 18. UPS difference spectra for Ar’-ion-bombarded SrTiQ(100) after successive exposures to 02 (from data

in Fig. 17) (from Ref. 62).

JPSS Vol. 9. No. J/6-B

I\ 0, ON BOMBARDED SrTi03(100)

II / 1 I>, 8 s 4, I iI e 6 4 2 E,.O -2 -4

INITIAL ENERGY IW

FIG. 19. UPS difference spectra for Ar+ -ion-bombarded SrTiO,(loO): (a) phase I (30 L-O L); (b) phase II (10’

L - 30 L) (from Ref. 62).

subtracting the UPS spectrum for 30L 02 exposure from that for IO’ L; this has been done in Fig. 19(b). We have not been able to identify the adsorbed species in phase II. It is probably not neutral 02, since the free molecule has a four- peaked UPS spectrum,c16) although we cannot rule out a severely perturbed (or perhaps dissociated) 02 molecule.

The difference between 02 adsorptiok on high- defect-density surfaces and on nearly perfect surfaces can be seen from Fig. 20, which presents a family of difference spectra for 02 adsorption on vacuum-fractured SrTiOs( IOO).~) Two adsorbed phases are seen here also, with phase I identical to that on a highdefectdensity surface. (The ab- solute intensity of photoemission from the adsorbed phase on the fractured surface is less than that on the defect surface, however.) The features in the difference spectra for 100 L and greater are quite diierent in Fig. 20 than in Fig. 18, however. On the fractured surface, the peak near 1 eV has completely vanished by lb L, and the difference spectrum for lo’ L in Fii. 20 is very similar to the spectrum for phase II alone in Fig. 19(b). It therefore appears that the same two adsorbed phases are present on both surfaces, but that phase II displaces phase I on nearly perfect surfaces. A third case has been seen for ol adsorbed on ion- bombarded and annealed SrTi6 (100) surfaces. There, a similar initial adsorbed phase is seen, but no second phase adsorbs.

The adsorption of 02 on TiG is more complex than on SrTio3, and has not been as thoroughly studied.‘~ The same initial phase (I) is observed, but exposures greater than about 100 L give complicated spectra, perhaps consisting of more than one additional phase.‘=’ Charge transfer from the bandgap surface state is observed for these other phases, as well as for phase I.

Lo et al.“” have studied the adsorption of G on

156 VKZTOR E. HENRICH

a, ON FRACTURED

S r TI O,(lOO)

studies of Hz0 adsorption have been carried out by Lo et al!“’ on Ti02(100) and by Henrich et al v”*7’1 on TX&(1 10) and SrTQ( 100).

Lo et al.“” measured UPS spectra for ICY L of Hz0 on the TiGWtV-(l x 3) surface (which has no T?+ surface ions), the Ti02(100)41 x 7) surface (which exhibits a small concentration of Ti’+ surface ions) and on Tideposited and Ar’-ion-bombarded Tio2(100) surfaces (both of which have a large number of surface Ti’+ ions and exhibit similar UPS difference spectra). Figure 22 shows the UPS spectra for the TiO#OOHl x 3) surface before and after lo’ L Hz0 exposure. Three peaks are seen, two overlapping the substrate valence band and one lying below it. The difference spec- , tra for l@ L Hz0 on the three different surfaces are shown in Fig. 23. While the Ti01(100Hl x 3) and Ar’-ion-bombarded (or Tideposited) surfaces both give three-peaked difference spectra, the Ti02(100)-(1 x 7) surface exhibits four peaks in the

INITIAL ENERGY (cv)

FIG. 20. UPS ditference spectra for vacuum-fractured SrTii(100) after successive exposures to 4 (from Ref.

62).

I t I I I

a+ - 0.8 rv

k

T.3OtY.K

qUctGl-G~3)+lo’Ln*O 9 =4.9*v

A+* * 0.2 l v T.300.K

-12 -6 -4 2tectron Binding Enwgy. eV

I

WO

FIG. 22. UPS spectra of Tio2(100)-(1 x 3) surface before and after l@ L Hz0 exposure at 300 K (from Ref. 15).

v I I I

-16 -12 -6 -4 -0

Ektron 6imsq Enugy. rv

FIG. 21. UPS spectra for TiO&OOHl x3) surface before and after 10’ L 4 exposure at 3C0K (from Ref.

15).

the Ti~lOO)-(l x 3) surface, and Fig. 21 shows their UPS spectra for the clean surface and for lol L of 02 at room temperature. The three peaks due to 0, adsorption in Fig. 21 are similar to the spectra for comparable exposure in Fig. 19(b) and Fig. 20. Similar structure was seen for G adsorption on Ar+-ion-bombarded TiOz(lOO).“n Lo et ol. only used a single 02 exposure of lo’, and no model for the adsorbed species was postulated.

I TiqLlOOl-W7l+lO’Ln,O

-IO.8

J

C. A&rptkmef&Oor~TiO,andSrl’KI, -12 -6 -4

Clearly, the most important mokcule to study in Cktron 6indiq Lnu~y. *V IEpOb

an attempt to understand the role Of the electrode Fro. 23. UPS difference spectra for Hfi adsorption on three d&&rent TQ surfaces. Dashed curve is UPS

surface in photoelectrolysis is HzO. Photoemission spectrum of gas-phase Hz0 (from Ref. 15).

I I 1

-11.2 TiO@Ol<l~3) +IO’L%O


difference spectrum. The spectrum for the (1 X 3) surface is compared with the UPS spectrum for gas-phase Hz0 (dashed curve) in Fig. 23(a). Lo er al.“” concluded that Hz0 is adsorbed molecularly on the (1 x 3) surface, with the b, orbital dominant in binding to the surface. They interpret the spectra for the other two surfaces as indicating dissociative chemisorption, resulting in adsorbed hydroxyl groups (OH or OH-). They base the latter conclusion on thermal- and electron-stjmu- lated desorption measurements and i&a-red studies, as well as UPS data.“”

Henrich et ul:m*7” have studied Hz0 adsorption on Ti%(!!O) and SrTiO&OO) surfaces for a wide range of exposures. Figure 24 shows a family of UPS spectra for Hz0 on an Ar’-ion-bombarded TiMllO) surface. The changes that occur upon HD adsorption are similar to those for 4 on Ar+-ion-bombarded SrTia(100) shown in Fig. 17, but there is no depopulation of the bandgap surface state for Hz0 adsorption. The detailed changes c+n be seen more ckarly in Fig 25, which presents the difference spectra corresponding to FQ. 24. For exposures up to 10 L, the difference spectra exhibit two broad, weak peaks in the region of the bulk valence band. We believe that these peaks most likely correspond to adsorbed OH radicals, and that the Ha mole&es are dis- so&%&y chemisorbed in that regime. Gas-phase UPS spectra of OH radicals exhibit two peaks having ionization potentials of 13.01 and 152OcV,” the same spacing as the observed peaks. Also, i&a-red absorption spectra show thnt OH is a stable species on Ti% at room

E _ I

Hz0 ON BOMBARDED TI02 11101

hv * 21.2 l V

INITIAL EWEWY (WI .

FIG. 24. UPS spectra (hu = 21.2eV) for ~k+-ion-bon+ barded Tio2(110) after successive exposures to H~O

(from Ref. 70).

r\ H-0 ON BOMBARDEDTiO, (110) 1

I,, 1 I

10 8 6 4 2 E,=D -2 -4

INITIAL ENERGY (eV)

FIG. 25. UPS difference spectra for Ar+-ion-bombarded TiWllO) after successive exposuns to Hz0 (from data

in Fig. 24) (from Ref. 70).

temperature.m’ If the adsorbed species is assumed to be OH, the polarixatiqnlrelaxation shift ob&ined is close to that found for molecularly adsorbed HD at higher exposures (see below). The same two-peaked spectrum is seen on nearly perfect TiwllO) surfaces, but it is not present on any of the SrTiO, surfaces studied, suggesting that Hz0 is not dissociatively chemisorbed on SrTioS. This is a surprising result, and it must be examined in more detail before reliable conclusions can be drawn.

For HzO exposures greater than about 3OL, a thirdpe.akappearsabout7eVbelowE,andgrows until at lb L a three-peaked spectrum, similar to that for gas-phase H20, is obtained. This three- peaked spectrum has been observed on all TioZ and SrTia surfaces studied,~’ as shown in Fig. 26. (A background emission has been subtracted in Fig. 26 to facilitate peak location. The three- peaked spectra at 1o”L are sufhciently more in- tense than the two-peaked spectra for low exposures that essentially the same lb L difference spectra are obtained whether the clean surface spectrum or that for 10 L is subtracted. Also, note the change in the labelling of the b, and b2 orbitals in Fig 26 compared to Fig. 23; this corresponds to different conventions for labelling the x- and y- axes of the molecule.) The four difference spectra are aligned with the b, orbital of the gas-phase Hz0 spectrum in Fig. 26(a). We interpret all of these spectra as arising from molecularly adsorbed

VICTOR E. HENRICH

BOMBARDED

ANNEALED

Hz0 (GAS1

hv * 21.2 l V

20 10 16 14 12

INITIAL ENERG’f WI

FIG. 26. UPS difference spectra for various Ti0~ and SrTi4 surfaces exposed to 10’ L Ha (b-c) and (a) UPS spectrum of gas-phase Hz0 from Ref. 56 (from Ref. 70).

H10 rather than OH.“O*“’ For both nearly perfect TiOr(l10) (Fig. 26(b)) and Ar’-ion-bombarded Tio2(110) (Fig. 26(c)), the spacing of the outermost two peaks in the difference spectra is very nearly the same as that for the b, and b2 orbitals of gaseous Hto, but the central peak is shifted toward tighter binding relative to the free-mokcuk al orbital. The shift is l.OeV for the defect surface and 0.7 eV for the annealed surface. The b2 orbital for adsorption on the nearly perfect TiwllO) surface is also shifted about 0.2eV toward larger binding energy. We interpret these shifts as bonding of the H1O molecule predominately via its in-plane O-lone-pair (a,) orbital, particularly on the defect surface. The molecule would then be bound to the surface with its 0 ion down and is H ions away from the surface. The b2 orbital, corresponding to the O-lone-pair normal to the molecular plane, also enters in bonding on nearly perfect surfaces. For defect SrTiG(100) surfaces, both the ul and b2 orbitals are shifted by OJeV relative to the b, orbital: there is also some (but not total) depopulation of the bandgap surface state. No shifts relative to gaseous Hz0 are seen for adsorption on Ar’-ion-bombarded and annealed SrTiO,(loO) surfaces: this is a complicated surface (see Ref. 6). however, and we have not tried to interpret the result.

Comparison of Fig. 23 with Fig. 26 shows that the difference spectra measured by Henrich el

TABLE 3. Work functions and relaxation- polarization shifts for Hz0 on TiQ and SrTi4

(energies in eV) (from Ref. 70.) @ ckm @HP A4w10 A&

Bombarded Tia( 110) 5.06 4.56 -0.50 3.6

Annealed TI02(110) 5.32 4.80 -0.44 2.5

Bombarded SrTii(lO0) 2.79 3.80 1.01 3.7

Bombarded and annealed SrTiQ( 100) 3.15 4.05 0.90 3.7

al.“” on ion-bombarded TiO.#lO) are virtually identical to those measured by Lo ef al.“” on ion-bombarded TiMlOO), and yet the data have been interpreted as due to molecular HiO in the former case and OH radicals in the latter. We feel that data of the type shown in Fig. 25, which show two adsorbed phases, support the interpretation of three-peaked difference spectra as due to molecular H,O. However, the difference spectrum in Fig. 23(b) for the TiO#OO)-(l x 7) surface”” most likely results from more complicated adsorption products.

The work function changes and polariultion/relaxation shifts measured for molecular HZ0 adsorbed on Ti02 and SrTiO, by Hen- rich el al!‘“” are given in Table 3. (All values are for 1O’L exposure.) HZ0 is seen to decrease the work function of Tit& which is consistent with the orientation of the adsorbed molecules as determined above from binding energy shifts. On SrTiOJ, the work function is increased by HZ0 adsorption; we have no interpretation for this effect at present. The polarization/relaxation shifts in Table 4 exhibit an interesting dependence on the presence of d-electron surface states. Those surfaces that have filled Ti”(3d’) surface states have similar shifts (3.7 2 0.2 eV), even though the work functions before HZ0 exposure differ by as much as 2.3eV. The one surface having no d-electron surface states has a shift of only 2.5 eV. It therefore appears that, even when there is no charge transfer between surface states and adsorbed molecules, these states have a pronounced effect on the polarization/relaxation of the molecules.

D. Adsorption of HZ on TiG

Lo et aL”5’ have also studied HZ adsorption on the TQ( 100X1 x 3) and Ar’-ion-bombarded TiC)2( 100) surfaces, and their difference spectra for 10 L HZ are shown in Fig. 27. Both are three- peaked spectra, and both look very much like the spectra for HZ0 adsorbed on the same surfaces (Fig. 23). Lo et al.“” interpret the two lowest-lying peaks as due to OH formed by dissociative chemisorption of HZ, with the H atoms bonding to surface 0 ions. The highest-lying peak is attributed to a negative charge on the OH (i.e. OH-).

Ultraviolet photoemission studies of molecular adsorption on oxide surfaces

TABLE 4. d-Electron confi.guration vs crystal structure for fourth-period transition-metal oxides

159

Bixbyite Rutile Corundum Rocksalt Spine1 Other

3d’ - 3d2 - 3d’ - 3d’ MnrOr 3d” - 3d6 - 3d7 - 3ds -

Ti02 TQ@natase)

- - - V20Y(orthorhombic) CrO,(orthorhombic)

wh(Tr340 K) TiG - -

,9%&r v203 TiO,(x * 1) - Crz03 vo,tx= I) -

- - - a-FeG3 - -

- - - NiO - - - I - - - CuO(monoclinic) - - -

- 1 Cur0 (cubic)

ZnO Wurtzite)

Ti 4 000) -II 13)t Id LH2

A+:-0.4+0leV

I

AN(E)

I

Ar s’~ttered TIQ 000) + IO5 LH2 I

A$= Olf0.1 eV

ANyI\ , ~1

-12 -8 -4 Electron Bindmg Energy, eV (EF = 0)

FIG. 27. UPS difference spectra for Ti02(1OOHl X3) surface and A?-ion-bombarded TiOr(lOO) surface after

16’ L H2 exposure at 300 K (from Ref. IS).

Henrich et uL’~’ have tried to study HI adsorption on TiQ and SrTiO, at room temperature, but with little success. In all cases, the difference spectra obtained were much weaker than, but essentially the same shape as, those for Hz0 adsorption. Examination of the ambient in the vacuum system with a quadropole mass spectrometer during Hz exposure showed an increase in the mass 18 (H20) peak whenever HZ was admitted to the vacuum system, even when the Hz was passed through a liquid nitrogen trap prior to admission and the system had been thoroughly baked. Presumably Hz was reacting with residual O-containing molecules somewhere in the vacuum system, but the exact cause was never determined.

IL AdsorptionofOtonTiO.(x=l)

TiO, (x = 1) is an interesting compound in several respects. ‘=’ From its stoichiometry, the Ti ions would be expected to have a Ti2+(3d2) configuration, which is rather unusual for Ti ions.

It has a wide range of stoichiometry (0.6 IX 5 1.28) and has the NaCl structure for all compositions except those close to x = 1; it then has a monoclinic structure. Over the entire composition range, the crystal structure contains an extremely high density of lattice vacancies. For x = I, about 15% of both cation and anion sites are vacant, resulting in a rather open structure.‘6J’

Interest in the adsorption of 02 on TiO, arose indirectly because of a theoretical paper by Jen- nison and Kunzo‘) on the bulk band structure of TiO. Their results indicated that there should not be a partially filled TiQd) band above the O(2p) valence band, in disagreement with other calculations.“’ Their results agreed, however, with XPS measurements of Ichikawa et ~1.~) on five TiO, samples (0.84~ x Z= 1.22). which showed no electron emission from the region above the valence band. This surprising result prompted Wertheim and Buchananm’ to perform XPS measurements on samples of arc-melted TiO,., (whose surfaces had been abraded in the preparation chamber of the XPS spectrometer) and Henrich et 01.~’ to take UPS spectra for vacuum- fractured polycrystalline TioO., and TiOl.,s. Both of these measurements showed strong emission from a Ti(3d) band lying 2.5 eV above the valence band. The question then arose of why Ichikawa et al. ‘76’ had not seen this band. After having studied the effects of O2 on Tf+ defect states on TiOz, Hen- rich et aLm) suspected that the exposure of the Ichikawa et nl. samples to air before measurement had resulted -in oxidation of Ti ions near the surface to a Ti“(3dq co&uration. The open structure of TiO, might allow 0 atoms or ions to migrate far enough into the sample (30 A or SO) to

oxidize all of the Ti ions within the XPS sampling depth. Thus, they exposed the vacuum-fractured surfaces to various amounts of 02. As seen in Fig. 28 for TiOl.lJ, the Ti(3d) band disappears upon

exposure to G.m’ (It should be noted that exposure of T&O,, which is a “tight” lattice, to O2

160 VKTOR E. HENRICH

decreases but does not eliminate the Ti’+ emission peak.‘=) The UPS spectra in Fig. 28 are remark- ably like those for % adsorption on Ar’-ion-bombarded Tia (Fig. 6 of Ref. 62) for all exposures. Wertheim and Buchanan also reported the absence of the Ti(3d) band for TiO,., samples prepared in air.”

F. Adsorption of I&O on AId&

One study of the room temperature adsorption of Hz0 on Al$&(OOOl) has been reported by Ahny et al.‘ml AIla is of particular interest because of its widespread use as a catalyst as well as a catalyst support. The surfaces studied were orien- ted to within 25” of the (0001) face, cut and polished (no mention was made of subsequent etching, either chemical or ion-bombardment). The sample was then heated for 14 hours in 5 x lo-’ Torr 02 at 850 K before adsorption. Fii 29(a) shows the UPS spectrum for that surface, and Pii. 29(b) shows the difference spectrum after exposure to 2 x IO L of HsO. The results are ditTtcult to interpret, since the A&O, sampk may not have been thoroughly dehydrated before adsorption.‘m

UPS measurements of CHtOH adsorption on AltO, have also been reported by Rogers and White.‘”

G. Adso@enefC~~onNlO

In the course of studying the chemisorption of C2Hz on single-crystal Ni surfaces, Demuthg” oxidized a NitIll) surface at room temperature to obtain an epitaxial NiO layer consisting of NiO islands having the (111) surface exposed and the same crystallographic orientation as the substrate. He then adsorbed CzHl on the NiO(ll1) surface and obtained the UPS spectra shown in Fig 30.

I 02 ON FRACTURED Ti O,,,,

h*s 21.2rV

INITIAL ENERGY, E WI

FIO. 28. UPS spectra for vacuum-fractured TiOl.lJ after various Q exposures. Spectra have been aligned at EF

(from Ref. 78).

0 4 a I2 16 .20 24

Kinotk omqy, eV

Fm. 29. Experimental and theoretical UPS spectra of a p&ally hydrated cr-Al& surface. The lower abscissa gives the measured electron kinetic energy while the upper scale gives the electron binding energy with respect to the experimental Fermi level of the substrate. The four spectra represent (a) reference surface pn- pamdbyhcating14hin5x10-~Torr4atJU)C;(b) cm from reference after 7Smin exposure to 5X IO-’ Tow Hfl vapor at room tew: (cl shifted SCF-G-SW spectrum for A&(OHh (from Ref. 7!J).

I / I6 14 12 IO 6 6 4 2 I

ELECTRON 6lNOlNG ENERGY t&I

!:’ I.

,!

i_ 4

FIG. 30. (a) UPS spectra (hv-40.8.eV) for NiO (on thermally oxidized Ni( I I I)) before (solid curve) and after 3 L exposure of &Hz; (b) UPS difference spectrum from (a) for hv = 40.8 cl!; (c) UPS difference spectrum for

hv = 21.2 eV (from Ref. 81).

The main features of the difference spectrum in Fig 30(b) are two peaks at 10.2eV and 16.8eV .below EF. By comparison with experimental UPS results of CzHz adsorption on Ni, and by using molecular orbital arguments, the peaks were identified as arising from CH radicals adsorbed at Ni sites.“‘) C2H2 thus appears to chemisorb dissociatively on NiO(l1 l), unlike the associative chemisorption observed for C2H2 on ZnO(lOiO).‘m


4. Theo&W Calculations of Mohxdar Adsorption on Oxide Surfaces

Several groups have considered the adsorption of molecules on oxide surfaces theoretically, and some of those calculations are sufficiently detailed to permit direct comparison with UPS data. We will not attempt to describe the theoretical results in detail, but rather we will summarize the various approaches and conclusions.

Morin and Wolframq’ considered molecular adsorption on the (100) surfaces of d-band perovskites qualitatively, using a simple linear-combination-of-atomic-orbitals (LCAO) energy band calculation. Wolfram and Ellialtioght’4’ later extended the method to calculate the local density of states (LDOS) for atomic planes in the surface region, including Coulomb repulsion among d- electrons, and these results agree favorably with experimental UPS data for atomically clean surfaces.m’ They then calculated the LDOS for a molecule consisting of one bonding and one antibonding level with a Ti ion on a (100) perovskite surface;“’ Fig. 31 shows the results of this calculation. The antibonding molecular orbital is assumed to hybridize with the Ti(d,) orbital and the bondmg molecular orbital with the Tiid~) orbital, as shown in Fig. 32. (They have not yet

sr TQ

AEpO

0

3* 3 1 0

c: 2 w

Lo

0

2

0 -4 -8 -12

Ftc. 31. Local densities of states (LDOS) for a chemisorption process. (a) LDOS for the xz-component of a cation on a (100) oerovskite surface with A& = 0. where AE,, is a surface &rturbation parameter representing the deviation of a surface from ideal truncation of the bulk (A& = 0); Same as (a), but for A& = - 2 eV (representing presence of surface defects): (c) LDOS for surface cation (AI?,+ = - 2 eV) after interaction with an antibonding molecular orbital; (d) LDOS for the interacting orbital of an adsorbed molecule. E,, and Ee are the antibonding and bonding states of the molecule-surface complex and EM is the energy of the molecular state

prior to interaction with the surface (from Ref. 65).

8 I

e I

(al

pHn d + + IL 0

e 1 Gkii’ - + 83 + -

-------X2

(bl

7 0 nn

+

ii

0

+ 2

z

(c 1 (d) hG. 32. Schematic of orbit& and geometries of typical molecular orbit& that interact witit x2 and z2 orbitals of a transition-metal ion. (a) and fb) illustrate overlap of the xz state with antibonding molecular orbitals, and (c) and (d) show the overlap between the z2 state and bonding molecular orbitals. Arrows indicate tbe usual direction of charge transfer when these orbitals hybridize to form a

surface complex (from Ref. 65).

c&tWeti the LDOS for specific molecules adsorbed on the perovskite surface.) Their cai- culations indicate that the adsorbed molecule par- ticipates in delocalized anface energy band states, which provides a mechanism for adsorbed molecules to communicate electronically over large distances with each other or with substrate atoms. Wolfram et al.‘“’ have also performed molecular orbital cluster calculations for clusters representing the surface electronic levels of Tii, SrTiO3 and BaTicX, but they have not explicitly included the presence of an adsorbed molecule.

Almy d af.m have performed self-consistent- field (SCF)-Xa-scattered-wave (SW) cluster calculations on the Al&OHh cluster (simulating Al& + HsO), shown in Fig. 33, in order to interpret their experimental results for Hz0 adsorbed on A120&t001). The results of the calculation are shown in Fii. 29(c). The calculation, while showing moderate agreement with experiment, was not ddinitive, since some probable adsorption geometries were not considered.~’ Foyt and Whitea’ have also performed SCF-Xa-SW Cal- culations of CHJ(OH) adsorbed on Al&

Lee and Wang@) have calculated the LDOS for an ordered monolayer of both H-like and Cl-like atoms adsorbed on MgO(lOO), using an LCAO interpolation scheme. Figure 34 shows the results of their calculations for H-like atoms, along with the bulk energy bands for MgO. There are, as yet, no expciimentaf UPS data with which. to compare their calculations.


o/ H

0

FIG. 33. Geometry of the Al&(OHk cluster used to calculate interactions of Ha with AI& (from Ref. 79).

FIG. 34. Energy-band structure for a monolayer of H- like atoms on MgO(IaO). Shaded area represents sub-

strate continua (from Ref. 84).

Kunz and co-workers have investigated the adsorption of H on Mg@=’ and Ni@=’ by means of SCF-unrestricted-Hartree-Fock cluster caicula- tions. These systems have not yet been studied experimentally by UPS, so no comparison of theory and experiment is possible.

Clwter calculations of OH- adsorbed onto a defect site on TiG have beep performed by Kawai et al.,“) using the discrete-variational (DV)_Xa method. They have performed calculations for both an atomically clean surface (TiO, cluster) and for a surface with adsorbed OH- (TiOrOH- cluster), and taken the difference between them to simulate UPS difference spectra. The results are shown in Fig. 35 along with the UPS data of Henrich et al.“O’ Since their calculations yielded essentially a three-peaked spectrum, they concluded that only OH is adsorbed on TQ and SrTia surfaces, as opposed to the interpretation given by Henrich et d.(m) They did not perform calculations for a cluster simulating adsorbed molecular H20.

Kowalski and Johnson’p’ have performed scattered-wave (SW)-Xa calculations for both Hz0 and OH adsorption on TiO. clusters that simulate both perfect and defect surfaces of Ti(x and SrTia. They find ‘that molecular Hz0 gives the expected three-peaked spectrum, but that the peaks are at larger binding energies than those seen by UPS. Adsorbed OH gives a two-peaked spectrum, which

v8lence band

FIG. 35. (a) Calculated state density difference (4) for OH- adsorbed on a TiO, cluster; (b) UPS difference spectrum for 10’ L Ha on Ar+-ion-bombarded TiQ

(adapted from Henrich ef a/., Ref. 70) (from Ref. 87).

agrees quite well with the initial stages of adsorption seen by UPS. They are currently extend- ing their method to larger clusters (T&OS) to in- clude the effects of Ti-Ti interactions in both bulk and surface geometries.

J.FutureDirWtions

It is clear from the work reviewed above that the study of molecular adsorption on oxide surfaces by means of photoemission spectroscopy is a rather young field. Relatively few oxide surfaces have been studied, and little is known about the details of adsorption sites. Only on the Zno(lOi0) surface have a wide enough range of adsorbed molecules been studied to see trends in adsorbate/substrate interactions,‘-” and even then the details of bonding are not well understood. The field is ripe for expansion in a number of different directions.

Space limitations in the present paper have prevented us from discussing surface analysis techniques other than UPS in any detail. Most of those techniques, such as the ones mentioned in $1, have been applied in at least some degree to oxides, and a coupling of the various methods is obviously necessary in order to gain as complete an understanding of adsorption on oxides as possible. Most groups already employ a number of dierent techniques, and that trend is continuing.

Just within the field of ultra-violet photoemission spectroscopy, however, there are several directions of research that should greatly increase our understanding of molecule/oxide interactions. One is to perform more detailed UPS measurements on a few selected systems. At@e-resolved photoemission, using tunable synchrotron radia- tion, should be applied to relatively simple sys-


terns, such as CO on the four major faces of ZnO. These techniques are now sufficiently well developed, as a result of work on metal and semi-. conductors, that they can easily be applied to oxides. Such measurements would give information about both filled and empty surface and molecular levels as well as bonding geometry. Quantitative LEED measurements and comparison with theoretical calculations would also be necessary on those systems in order to better define the surface geometry.

Another general approach that should prove very useful is the study of a number of different crystal faces of the same oxide. Since adsorption sites on oxides, or on any compound, can involve two or more different types of atoms, it is important to examine the effect of changes in the relative position of the substrate atoms, the direction of their bonding and antibonding orbitals, etc., on adsorption. To date, only the adsorption of CO and NH3 on ZnO have been studied in such a systematic manner.(203rJs) Work of this type should be extended to a number of other molecule/oxide systems.

6.

7.

8.

9. 10.

11.

12.

13.

14.

In addition to varying the surface geometry (i.e. crystal face) on a single oxide, it is possible in some oxide systems to vary the electronic configuration of the surface without changing its crystal structure. The fourth-row transition-metal oxides are an excellent case in point. The cation 3d levels fill progressively as one moves across the periodic table from SC203 to ZnO. The fourth-row @&ion-metal oxides are organized according to their d-electron configuration and crystal structure in Table 4. Large isostructural families exist in this system, and it should be posstble to vary the cation d-electron population for a particular surface geometry. Such studies have not been performed, and the structure of a particular crystal face may well turn out to be sensitive to dclec- tron configuration, but it is certainly a promising direction in which to proceed in order to determine the role of d-electrons in chemisorption and catalysis.

15.

16.

17. 18. 19. 20.

g: 23. 24. 25.

26.

27.

28. 29. 30.

The study of the interaction of molecules with defects on oxide surfaces is also an important direction for future research. Some work of this type. has already been done on Ti02 and srTiO~,‘6.‘O.‘Sb2.~II) but much more is needed, particularly on well-defined defect sites. Unless the geometry of defect sites is fairly well understood, it will be difficult to make meaningful contact between experimental results and theoretical cai- culations. This is a difficult area of research, but it is one that must be tackled in order to gain an understanding of real oxide surfaces.

Rdtrtntts

31.

32.

33.

34.

z:

37. 38.

39.

40.

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ULTRAVIOLET PHOTOEMISSION STUDIES OF MOLECULAR … of Adsorbates on Oxides... · surface...

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Transcript of ULTRAVIOLET PHOTOEMISSION STUDIES OF MOLECULAR … of Adsorbates on Oxides... · surface...