Ultraviolet Photoemission Studies of Molecular Adsorption on Oxide Surfaces

Pro9msrin Sutfacc Scimce. 1979.Vol. 9, pp. 143-W. PergamonPress. Printed in Great Britain

ULTRAVIOLET PHOTOEMISSION STUDIES OF MOLECULAR ADSORPTION ON OXIDE SURFACES

VICTORE. 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) measurementsof the interaction of molecules with the surfaces of metal oxides. A brief summary of the ekctronic, geometric and compositionalproperties 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-crystaloxide substrates. A brief description is also given of the various theoretical methods that are being used to study molecule/oxide interactions. content!3 1. Introduction 143 2. Structure of Clean Oxide Surfaces A. Composition f: B. Geometry 145 C. Electronic structure I46

3. YoleeOar Adsorption on Single-crystal Oxides 147 147 B: Adsorption of 02 on Ti02 and SrTia IS2 C. Adsorption of Hz0 on TQ and SrTia I56 D. Adsorption of Hz on TiO, lS8 E. Adsorption of 01 on TiO, (x - I) I59 F. Adsorption of Hz0 on Ala G. Adsorption of C2H2on NiO E

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

5. Future Directions 162

References 163

DV discrete variational electron-energy-loss spectrosc y linear combination of atomic or“k tats local density of states LEED low-energy-electron diffraction self-comustent field &F scattered wave ultra-high vacuum !z ultraviolet photoemission spcctros~~p~ XPS X-ray photoemission spectroscopy

1. Introduction molecules with oxide surfaces. Over the last few The surface properties of metal oxides, and years, surface scientists have taken an increasing particularly their interaction with adsorbed mole- interest in oxides, and oxide surfaces have been cules, have been technologically important for studied using a wide range of techniques. ‘many years. The poisoning of a thermionic emit- The present paper will review the current status ter: the activity and selectivity of an oxide cata- of research on molecular adsorption on metal lyst: the sensitivity of an electron multiplier to oxides by means of ultraviolet photoemission ambient gases: the interaction of a small catalyst spectroscopy (UPS). The reason for restricting the particle with its support: the long-term stability of scope of the paper to UPS studies is two-fold. MOS devices and negative-electron-affinity photo- First is simply a space requirement; a comprehen- cathodes; the efficiency with which hydrogen sive treatise on studies of adsorption on oxides can be produced from sea water by photolysis-all would occupy an entire book. Secondly, UPS has of these involve the interaction of atoms and given us by far the most detailed information on

143 144 VICTORE. 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 cannot be assumed u priori to have bulk stoi- 2. Structure of Clean Oxide Surfaces chiometry, 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 surface composition have not been observed in nearly perfect surfaces. For TiQ, it has been Zn()."9*l" found that annealing to about lOOOK in vacuum Another effect that can change the stoi- after ion bombardment restores the surface to bulk chiometry of oxide surfaces is photodissociation stoichiometry by diffusion of oxygen ions from the accompanied by the release of oxygen. This effect bulk “a”) but a similar process does not work in seems to be strongest for photon energies close to the iase of SrTi0,.f6*” The only procedure that has (but larger than) the bandgap and is seen in been found to restore a SrTiG surface to bulk ZIQ~’ TiG and a few other oxides. However, no composition is inert gas ion bombardment at about effects of photodissociation of oxides in the pho- 900K; this process has only been used on the ton energy range generally used for UPS (i.e. SrTiG( I 11) surface.“’ For MgO, on the other 15-U) eV) have been reported. hand, ion bombardment does not change the sur- Annealing of oxides to temperatures where face composition, and MgO(100) surfaces prepared sublimation becomes appreciable can also alter by 5OO-eV Ar’-ion bombardment are indis- surface composition. In the case of ZnO, heating tinguishable from vacuum-cleaved surfaces.“~“~ to temperatures >750 K causes a preferential Inert-gas ion bombardment has also been found sublimation of oxygen, resulting in O-deficient to remove oxygen preferentially from ZnO sur- surfaces.o” facesob”’ One procedure that has been adopted In summary, anything that is done to an oxide for the preparation of presumably stoichiometric surface can, and often will, change its com- ZnO surfaces is ion bombardment at 700 K fol- position. In almost all cases, the surface will lowed by cooling to room temperature and sub- become oxygen-deficient. sequent annealing to 700 K.‘*’ An alternative pro- . cedure that has been used consists of room-tem- R- Gcawry perature ion bombardment followed by annealing The atomic geometry of a number of oxide in vacuum at 1000 K and s&sequent annea&g in surfaces has been examined qualitatively by low- 8 x IO-’ Torr 02 at 700 K.““‘) The latter treatment energy&c&on diBa&on (LEED), but only a few was found to optimize the quality of LF$ED pat- surfaces have been studied quantitatively. LEED terns from the (lOi0) face.“” However, direct intensity proflIes have been measured for comparisons between surfaces prepa& uaiug MgGWlO~~ and ZuO(OtKll)-Zn, woib0, (ioio) these procedures and vacuw surfaces and(1 la),‘=‘- and these have been compared have not yet been performed. with dynamkal nruhiploscettering calculations.‘* Damage produced by electron bombatdment y, For MgO(lOO), the LEED patterns exhibit the (electron energy b 5 keV) is also a serious problem symmetry of the bulk,Ws’ and calculations in- in the study of oxide surfaces.(~W’XHP-W As in corporating relax&m and rumpling (i.e. outward the case of ion bombardment, electron bombard- motion of anions relative to cations in the surface ment tends to break cetion-anion bonds and plane) indkate a relax&m of the top plane of less remove oxygen preferentially from the surface. than3%andarumphngofnomorethanaafew However, the response of a part&&u oxide to percent.- The sensitivity of LEED spectra to electrons can be entirely di&eut from its res- rumpling has not yet been thoroughly investigated, ponse to ions. A surface electronic state, which howe~er.~’ For ZnO, comparison of LEED cal- probably consists of a surface oxygen vacancy (or culations_with experimental I-V data indicates that at least displacement of surface oxygen ions), can the @001)-0 and (11%) surfaces are very nearly be produced by electron bombardment of truncations of the bulk,- whik the (OOOl)-Znface MgWOO) surfaces, but no such state is produced exhiits a relaxation of the top plane of Zn atoms by ion bombardment (at least for ion energks less by about 0.2 A inward,W and the (ioio) surface than 1 keV).‘“’ Electron bombardment of SiO, has the Zn ions relaxed 0.45 f 0.1 A inward and the produces drastic changes in composition, releasing 0 ions relaxed 0.05 & 0.1 A inward.“” Reconstruc- oxygen and reducing much of the surface region to tion has been reported for the polar surfaces of elemental silicon.-’ Ion bombardment, ZnO, and steps have been observed on both polar however, has essentially no dFect on the surface and nonpolar faces*--” stoichiometry of bulk SiO, samplc~.~ The tran- Several Tiol surfaces have been studied quali- sition-metal oxides and perovskites exhibit tatively by LEED (i.e. only the symmetry of the changes in various electron emission spectra, due LEED patterns has been examined).““*‘3”’ The also to preferential removal of oxygen, after (110) surface is stabk at all temperatures and several minutes of exposure to the typical electron exhibits only (1 X 1) LEED pattems.“‘“*“’The (001) beams used in Auger and energy-loss spectros- surface is unstable and reconstructs on heating to copy: 1-5 keV, 10-50 ma/cmz~~~‘o*‘3*‘4’However, it (110) and (100) facets.‘4’ The (100) surface exhibits is possible to measure Auger and ELS spectra on three different reconstructions depending on TiO, and SrTi03 rapidly enough to obtain good annealing temperature: (1 x 3), (1 x 5) and (1 x 7) signal-to-noise without producing measurable for temperatures of 900, 1100 and 1500K, respec- damage.'6.7.10,131 Electron-beam-induced changes in tively.“” The SrTiO,( 111) surface exhibits a (1 x 1) 146 vlcroa E. HENNCH pattern after annealing, but a complex, faceted c. Elecmmk- pattern can be produced by ion bombardment at Compared to metals and semiconductors, 900 K.” The SrTi~(100) surface also exhibits (1 x knowledge of the surface electronic properties of 1) LEED patterns (Fii l(b)).@’ oxides is meager. UPS and ELS measurements on The LEED patterns from vacuum-fmctured well characterized surfaces have only been pcr- Tio2 and SrTiO, are generally poor (i.e. weak spot formed for a few oxides. We will consider each of intensities relative to the background, spots those surfaces in 03 when we discuss the UPS somewhat difhlse). although they do exhiit the data on adsorption, but we will make a few general basic symmetry of the nominal fracture plane. A comments here. Nearly perfect surfaces of metal LEBD pattern for a fractured SfTiO3(100) surface oxides that have empty cation-derived bulk con- is shown in Fig. l(a). After anneahng to about duction (bands (e.e MI@, TX&, SrTii, ZnO, 12OOK. the LEED pattern improves to that St& AM) do not exhibit a gross transfer of expected for a nearly perfect surface [Fig. l(b)]. charge between surface anions and cations relative The poor quality of the LEED patterns on frac- to the bulk (i.e. if there is any transfer, it is less tured TiG and SrTiQ surfaces is consistent with than about 0.1 electron per surface ion). Such a UPS determinations of the density of defect- charge transfer would be seen in UPS spectra as associated electronic states on those surfaces occwied states in the bandnarror conduction band (PZC).“‘” 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 cleaned by direct resistive heating to about 1800 K formation on those oxides,‘“*‘“.‘3’and that will be in UHV. This procedure was shown to give the discussed in 63 below. For oxides having partially same ELS spectra as for (1010) surfaces cleaved in filled conduction bands (e.g. NiO, T&a, TiO, (X = vacuum,(&) although a preferential sublimation of 1). MnO, Fe& etc.), anion-cation charge transfer oxygen from (loio) surfaces at temperatures between surface atoms would be much harder to above 750 K has been reported by G&XL”* Table determine from UPS spectra, since it would ap- 1 lists the molecules whose adsorption has been pear only as a change in the intensity or width of studied by the IBM gro~p.‘~.‘~’ emission peaks due to bulk bands;“” no attempts A sufficiently large number of molecuks has have been made to date to determine the extent of been examined to show clearl_y the trends in any such transfer. molecular adsorption on ZnO(1010).‘“s3’ Adsorp- Very little is known about the spectra of empty tion was studied at both 120 K and 300 K. Some surface states on nearly perfect oxide surfaces. molecules (C&L, Ht, 02 and CO) produced no Tia and SrTiO, do not exhibit any energy-loss changes from the UPS spectra of clean ZnO(lOi0) features corresponding to empty final states in the at either temperature, except for 0.1-0.3 eV band bulk bandgap.‘6.‘o)In ZnO, one group has observed bending, indicating essentially no adsorption. an ELS peak at 2.8eV,‘&’ less than the bulk (Recent work by Gay et ul.“=‘is in disagreement with bandgap of 3.2 eV, but that peak has not been seen those results for CO; this will be discussed below.) by other groups. MgO exhibits a strong surface Only C~HJN and HCOOH were found to adsorb at loss peak at 6.1 eV, less than the bandgap of 300 K. The remainder of the molecules adsorbed 7.8eV, which clearly indicates an empty bandgap only at 120 K. surface state.“” For SQ, three energy-loss peaks As an example of the changes in the molecular are observed at energies less than the bandgap on orbital structure of molecules adsorbed on oxidized silicon,‘“’ vacuum-fractured fused silica, ZnOGOiO), the results for C& adsorption from etc.,‘=’ but experiments have not yet been per- Ref. 49 are shown in Fig. 2. Figure 2(a) shows the formed on vacuum-fractured single-crystal quartz. UPS spectra for clean (dashed curve) and C&Is- However, since the same three loss features are covered (solid curve) surfaces at 120 K. Figure 2(b) seen on all other types of SiO, surfaces, there is shows the difference spectrum (with the clean- strong reason to believe that they do arise from surface spectrum attenuated by 50%) for adsorbed empty intrinsic surface states.‘u’ Simii loss Cd&, and Fig. 2(c) shows the gas-phase CsHs peaks have been observed for G& layers on spectrum, measured with the same resolution as for Ge.“” the adsorbed spectra, for comparison. A uniform extra-molecular relaxation/polarization shift,‘lJ2’ 8.MoleeukrAdsorp&mou~Gxkks AE,,,, of 1.3 eV (toward smaller bindingenergy) is Although one of the main goals of the surface found for all but the three highest-lying molecular scientist is to understand the mechanisms of orbitals. The highest-lying (a) orbital is shifted heterogeneous catalysis involving complex mole- toward tighter binding by 0.4eV (A&), while the cules on real catalyst surfaces, basic studies begin next two orbitals, having mixed 7r and ‘a character, with systems that can be interpreted as completely have A& = 0.2 eV. The fact that the adsorbed and unambiguously as possible-simple molecules molecule clearly retains the basic molecular orbital on nearly perfect single-crystal surfaces. With the levels of the free molecule indicates that it does not exception of the work on defect surfaces of TQ dissociate upon adsorption. Since the three orbitals and SrTiO3 described below, all of the UPS that are shifted toward tighter binding relative to the studies to date of the changes in electronic struc- rest of the molecular spectrum are presumably the ture during chemisorption have been performed ones involved in bonding to the ZnO substrate, the on nearly perfect surfaces. We will discuss each molecule is most likely lying flat on the surface.‘49’ oxide separately in the following sections. The bonding shifts seen for the second and third orbitals in Cd& are somewhat unusual in that only A. ZOO the highest-lying orbital is shifted (in addition to ZnO was the first oxide on which molecular A&,) for most adsorbates. Figure 3 shows the adsorption was studied by UPS. The changes that more typical case of C&O and CH3OH adsorbed are produced in its semiconducting properties by the adsorption of various gases have been studied by Tans of bulk. transport measurements for TAFILE 1. Molecules adsorbed on many .years.‘46’ The first UPS measurements on Zn0(liOo) (Refs. 48-53) ZnO were made by Powell et al.“” in 1972, but C&N Co, they only studied vacuum-cleaved (IOiO) surfaces. % Rubloff, Liith and Grobman at IBM began study- H(%$O &3$0 ing the adsorption of hydrocarbons on ZnO in c&l (CH3)CCH 1975.‘“’ All of their work to date has been on CH,OH C% HzCO as-grown, non-polar (ioio) “prism” surfaces (CH3kCO HCOOH 148 VICTOR E. HENRIM 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), ‘24 I ,b ,’ ,‘6 ,I ,’ ,I 3 as well as on some metal surfaces.‘m Fii 4 23 I4 ELEckmeALER&ul shows data from Ref. 52 for chemisorbed H(CH,)CO compared to the UPS spectrum of the 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 gas-phase molecule. In addition to the bonding presence of a C& ambient solid curve, N.&E)); (b) shift of the highest-lying, O-lone-pair orbital diikrence spectrum N.&E)- I NdE): (c) UPS spectrum (which is expected), the peak at a binding energy for gas-phase CA at hv = 40.8 eV (from Ref. 49). 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

~H(CH&O hv=40.8eV 1 1 H I o=c’ -z

CH,CtiW PHASE I I\ h I 1

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). Pm. 4. UPS spectra at hv - 40.8 eV for H(CH&O; (a) on ZnO(lOiO) at 12OK.‘* only the most kkly chemisorbed on evaporated _W at 120 K. 68 L exposure: bound molecular orbital, which for both of these (b) chemisorbed on ZnO(1100) at 120 K. 5 x W’Torr molecules is an O-lone-pair orbital, is shifted, ambient: (c) gas phase at hv = 40.8 eV (from Ref. 52). with A&, = 0.6 eV and 0.4eV for GH.0 and 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 -C-AXIS- IO L (iL = 1 Langmuir = 10dTorr-set) of Fw. 6. Model for chemisorbed CO and 0 on ZnO(liOO) HCOOH at 120 K. The difference spectrum for (from Ref. 51). adsorbed HCOOH is the solid curve in Fig. 5(b). Unlike the other hydrocarbons studied, this difference spectrum does not resemble the gas- The type of adsorption pictured in Fig. 6 points phase HCOOH spectrum, suggesting that HCOOH up one of the main dil%ences between chem- has dissociatively chemisorbed. In an attempt to isorption on oxides and on metals or elemental determine the nature of the adsorbed species, CO semiconductors. On non-polar oxide faces, both and H2 were simultaneously adsorbed on a clean cations and anions are available for bonding, (1010) surface at 120 K (the mixture would not whereas elements offer only one type of substrate chemisorb at 3OOK), giving the difference spec- atom. The presence of cations and anions offers trum shown by the dashed curve in Fig. 5(b). the possibility of more complex bonding inter- (Recall that neither CO nor H2 alone would adsorb actions, as well as charge transfer between the at 120 K.) The difference spectra are compared to adsorbate and both positive and negative ions. The the gas-phase spectrum for CO in Fig. 5(c). L&h et full implications of this additional degree of al (” interpret the spectra as indicating that freedom are far from being understood at present. H&XH decomposes into CO and atomic 0, the Recently, Gay cf u!.~*“’ have been studying Wer illkmd from the peak in the difference the adsorption of CO, NH,, 02, HI, COZ and specmun at lOeV, which is absent for CO alone. Hi0 on the (OOOl)-Zn, (OOU&O, (1010) and Based on the observed bonding shifts, the authors (ll20) faces of ZnO by UPS. Their surfaces are propose the adsorption geometry shown in Fig. 6. prepared by cutting, polishing and etching of the desired face, followed by Ar+-ion bombardment at 7OOK. cooling to room temperature and sub- 10) sequently annealing to 7OOK in vacuum. This ZnO (IiOO)HCOOH .H 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 10m5Torr. 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

1 1 / 180 K in the absence of H2. CO does not physisorb 24 20 16'li 8 ' down to 80 K, and H2 does not’adsorb at all over ELECTRON BINDING ENERGY WI the temperature range investigated. The amount of FIG. 5. (a) UPS spectra of clean ZnO(l@IO) and after CO adsorbed was found to depend on ambient CO IOL exposure to HCOOH; (b) difference spectra for pressure rather than on exposure, necessitating 10 L HCOOH exposure at 300 K (solid curve) and co- exposure to CO+ Hz ambient (2 x lo-‘Ton) at 120 K measurements over a wide pressure range. (dashed curve); (c) UPS spectrum for gas-phase CO at Photoemission from the molecular orbitals of kv = 40.8 eV (from Ref. 51). adsorbed CO overlaps that from the strong Zn(3d) 150 VICTORE. HENRICH

band, complicating the determination of accurate band region (Fig. 7(c)), which results in three- UPS difference spectra. Slight shifts of the clean- peaked difference spectra similar to those of Liith surface spectrum relative to the spectrum with et al!” for adsorption on all four crystal faces. adsorbed CO can produce either a two- or three- Both groups thus agree that CO bonds to ZnO via peaked difference spectrum for adsorbed CO, as its So and la orbitals. shown in Fig. 7. It is not obvious which is the In spite of the complications discussed above, correct spectrum, since CO adsorption on metals the kinetics of CO adsorption can be studied by often results in a large bonding shift of the 50 monitoring the amplitude of the photoemission orbital, placing it on top of the 1~ orbital and peak due to the 4u orbital, which is well separated giving a two-peaked difference spectrum. from the ZnQd) band. Gay et d’= have studii However, weakly adsorbed CO should have a these kinetics for a wide range of temperatures three-peaked spectrum more like that of the free and pressures on all four faces, and Fig 8 shows a CO molecule [see Fig. 5(c)]. The experimental plot of the intensity of the 4a photoemission peak situation is further complicated by a narrowing of vs temperature for two different ambient CO the Zn(3d) band upon CO adsorption, which can pressures forthe(lOi0) face. Measurementsofthe result in spurious peaks in the difference spectra. change in work function with CO adsorption are (Such peaks are not spurious in the sense that they consistent with the intensity of the & peat’” The do represent a real narrowing of the Zn(34 band; adsorption kinetics were found to fit a Temkin they should not, however, be associated with the isotherm or isobar (i.e. Langmuir-type adsorption molecular orbital structure of adsorbed CO.) The plus a linear decrease of the heat of adsorption, narrowing is most likely due to depopulation of an AH&, with coverage). and the solid lines are Ats to intrinsic surface state on the clean ZnO surface, such an isobar. In comparing the data from various but the details of such a state have not been faces, it was necessary to assume that the investigated. A similar narrowing of the valence coverage dependence of AH,,,,,was the same on all band on adsorption, probably corresponding to faces. It is then possible to compare the amount of depopulation of a Wed intrinsic surface state at CO that would be chemisorbed at saturation the upper edge of the valence band, is also obser- coverage on diierent faces. The results are given ved on the (OtlOl)_Zn face. The subtraction pro- in Table 2, where coverage is expressed as number cedure finally adopted by Gay et al.‘” consists of of CO molecules per surface ZnO molecule, nor- maximixing the overlap of structum in the valence- 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 TOITand (0) IO-’ Torr. SoIid curves arc fits to Temkin isobars(from Ref. 20).

TABLET. -~~ No. of CO molecules adsorbed ZnO face Surface ZnO molecule

(lOi0) (1.00) FIG.7. -UPS difference spectra for CO chemisorbed on (JlW ZnOUOlO) for various shifts of the abscissa of the clean (0001 Zn :z!t surface spectrum relative to the CO-covered Spectrum: fooot- )-o 0:19 (a) -0.025eV; (b) -0.075;‘4; (c) -0.125eV (from Ref. (from Ref. 20.) Uhviokt photoemission studies of molecular adsorption on oxide surfaces 151 plitude of the CO peaks relative to the substrate has also been studied by Gay et d~mJ*J5’ In ad- emission for the non-polar faces also suggests dition to chemisorption, NH, physisorbs for tem- close to monolayer coverage at saturation.) The peratures below about 110 K. Figure 9(a) shows heats of adsorption at zero coverage were nearly the di&rence spectrum for chemisorption of NH3 the same for the four faces, ranging from 11.6 to on a (OOOl)_Znface at 246 K and an ambient NH, 12.4 kcallmole.~’ pressure of 5 X IOipTorr. The peaks corresponding The above results suggest an interesting pic%re to emission from the le and 3~ molecular orbitals of CO ajsorption on ZnO.m’ The non-polar (1010) of NH, are labelled (with 34: indicating the loca- and (I 120) faces, which adsorb a full monolayer of tion of the chemisorbed 3a, orbital). The two CO, have both coordinatively unsaturated Zn and peaks are shifted about 2eV closer together than 0 ions in the surface plane. The much lower in the free molecule,‘=) suggesting bonding via the saturation coverage on the polar (0001)_Zn and 3al N-lone-pair orbital. FQurc 9(b) shows the (OOOi)-Ofaces, where only Zn or only 0 ions are spectrum for a combination of chemisorbed and exposed, respectively, suggests that both a Zn and physisorbed NH3 for 2L exposure at 100 K. The 1e an 0 site are necessary for CO chemisorption, orbital lies at essentially the same energy for both analogous to the dissociative chemisorption of adsorption states, but the 3a1 orbital for physisor- HCOOH in Fig. 6. But if both Zn and 0 sites are bed NH2 is unshifted relative to the gas-phase required, then one would expect no adsorption on spectrum. (The other features in the difference the polar faces. However, it has been determined spectrum are “spurious” in that they arise from that these faces contain a large Aumber of steps, narrowing of the ZnQd) band and changes in the the height of which is predominately two double sfNcture of the valence band. As in the case of layers (or a full lattice constant along the c- CO adsorption, these changes have not yet been axis).‘W’7s’ Such steps expose non-polar (IOiO) thoroughly investigated.) facets, and CO could then adsorb at Zn-0 sites on The kinetics of NH, chemisorption have been the facets. The amount of CO adsorbed at satura- i%WlyzediIltheSZUtle- as those of CO.‘2o’ tion coverage would then be a measure of the They are found to fit a Temkin isotherm better density of steps on the polar faces. than a muir one, but there is significant scat- The reason that Gay d al. see chemisorption of ter in the data. The onset of physisorption below CO on ZnO, while Liith d al. do IX& has not been 110 K also restricts the range of data available. dew. Subtle differences in surface structure Values of AH.,,, for chemisorbed NH, are difficult or composition arc probably the crucial factor. to determine from the data, but they seem to lie in Although both groups report excellent qualitative the range of 25-U) k&/mole. It is difficult to LEED patterns, neither group has made I-V compare the ilitensities of the le orbital of NH3 qsurcments for comparison with theoretical adsorbed on the four faces of ZnO, due to the a&u&ions. difficulty of obtaining accurate difference spectra The adsorption of NH3 on all four faces of ZnO 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

Zn 0 KIOOl) -2n + NH, preliminary work function data do not corroborate this conclusion.m) 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 152 VICIDR E. HENRICH

zno mooT)-o

hv m2l.W

(a)

+O.SL NH,

(b)

\ 21 l7 0 9 9 IS IO 5 Ilwal alstgy, lv Ekeon binding OMrgy M FIG.10. UPS di&rence spectra for ZnO(OODi~at82 K FIG. 11. (a) UPS spectra of annealed ZnO(WOl~Zn with a CO ambient of 1 x IO-’ Torr: (a) before exposure before (dashed curve) and after (solid curve) lti L to NH,; (b) after 0.5 L NH, exposure; (cl after 1 L NH, exposure to Q; (b) dilTer~~cespectrum corresponding to exposure (from Ref. 20). (a) (from Ref. 57). been obtained in preliminary experiments by Gay spectra provide no information about the elec- ef al.@“’No interpretation of the observed spectra tronic IeveIs of the adsorbate. The highest-lying has been presented, but the 02 no doubt dis- core levels of Cs are the 5pla and Sp,,,, with sociates, since the UPS spectrum for molecular a binding energies (relative to I$) of 13.1 eV and exhibits four peaks over the energy range in- 11.4eV, respectively, which are too deep to be vestigated.‘W seen using this photon energy. The Cs(6s) electron One additional UPS measurement should be is presumably transferred to the substrate on ad- mentioned with regard to 2110. Cesiated ZnO has sorption, giving rise to the low value of work been found to have an extremely low work func- function. The two new peaks seen on the cesiated tion coupled with relative chemical stability!Ju” surface, s3 and S.. have been attributed to a max- properties of great interest for thermionic devices, imum in the ZnO conduction band density of and Powell and Spice? have studii the cesia- states and to inelastic scattering, respectively.‘60’ tion of ZnO by UPS. Figure 12 shows their spectra for cleaved (lO’lo>ZnO, ZnO powder and cesiated B. AdmrpthofChomTiOland!3rTiO, ZnO powder, all measured with a photon energy of The discovery in 1972 by Fujishima and 11.4eV. While showing clearly the reduction in HomW6’) that TiOr @utile) could be used as a work function brought about by Cs adsorption, the 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 1 FRACTURED SrTiO, (I@01 interest in the surface electronic properties of hv l 21.2eV 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. w The bulk band structures of Tio2 and SrTiO, are c very similar, because the Ti ions are in an octa- E, 2 b-0 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 x10 energy-level diagram for SrTiOa?*) the structure of TiG is essentially identical, except for the absence of Sr bands. Both filled and empty Sr (a) 0 t 1 I bands in SrTiG are too far from the Fermi level to 10 6 6 42, 2 E,=O play a significant role in chemisorption bond- INITIAL ENERGY, E (rV1 ing. (~45)The room-temperature bulk bandgaps for SrTiO, and TiOr are 3.17 and 3.05 eV, respectively. PK. 14. UPS spectra for vacuum-fractured SrTiO,(loO) (a) before and (b) after subtraction of 23.1 eV “ghost” The bulk Fermi level lies in the bandgap, and both spectrum (from Ref. 6). ’ materials are insulators when stoichiometric. Their bulk stoichiometry can be varied slightly by heating to several hundred K in vacuum (or a reducing origin, extends from 3 eV to 9eV. The small atmosphere); 0 then diffuses to the surface and is amount of emission in the bandgap region, which released, resulting in a small number of 0 vacan- is shown in Fig. 14(b) after correct&r for the cies in the bulk (up to about 1019/cm3at 900 K).‘W’ presence of a 23.1 eV line in the photon spectrum, These 0 vacancies act as donors, resulting in is believed to arise from surface &fee& present n-type material. (A bulk density of IO” elec- on fmchued surfaces (32B); as ‘shown. it virtually trons/cm3 is too small to be seen in a UPS pisappears upon 02 adsorption., The UPS spectra e&@ment.) for vacuum-fractured TiO, are similar to Fig. 14. R&e 14 shows the UPS spectrum of a Since the active states on TG and SrTii ph+ nmum-fractured surface of SrTi@ [roughly toelectrolysis elec&odes are believed, from elec- tM@)), measured with hv = 21.2 eV.“’ The zero of trochemical measurements,‘“‘~ to lie near the &s&l-state energy has been taken as the Fermi middle of the bulk bandgap. Henrich er (ll.“*‘O) kvel, EF, which is pinned at the bottom of the studied the electronic structure of surface defects conduction band by bulk 0 vacancies. The bulk on TioS and SrTiO,. They created defects in a valence band, which is predominately of O(2p) 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 Sr(56) vacuum-fractured SrTQ change with bombardment by 500 eV Ar’ ions for the times ~hown;‘~ 10 -_---_----_ VACUUM the t = 0 curve is essentially the same as Fig. 14. Fortimesgreaterthan6OOsec,therearenofurther Ti (3d) changes in the UPS spectra; a steady-state has E/O been reached and the sample surface is just being O(2p) 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- e Sr(4p) bardment time. When steady state has been -20 O(2s) 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). Ti(3p) The LEEDpattems disappear by t==3Osec, and -30 Auger spectra show a loss of both 0 and Sr from FM. 13. Schematic energy-level diagram of SrTiO, (from the surface as a result of ion bombardment. Ref. 6). Sii effects are seen for TiG (see Refs. 10 and VICTORE. 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). X10 ES (c) ly 13). with Auger spectra indicating a loss of 0 from i the surface. -4 E"=O 4 The defect surface states created on both INITIAL EN&Y W) SrTii and Tiol by ion bombardment correspond FIG. 16. UPS spectra for (a) annealed (nearly perfect) to a transfer of electrons from surface 0 ions to Ti02(110); (b) 500 eV Ar+-ion-bombarded TiO?( I 10) at surface ‘Ii ions, resulting in a Ti’+(3d’) electronic steady state (solid curve); (c) surface in (b) after con@uration.‘WW Henrich e? (Il.‘*ln in- exposure to lo’ L Oz (from Ref. IO). vestigated the SrTi~(100) and TiO4 10) surfaces, while Chung d u!.~*” studied the TiilOO) and pletely depopulate the bandgap surface state is (110) and SrTiO,(lll) surfaces. On the TiO#OO) smaller on SrTiO, (about 30 L) than on TiO, surface, defects were produced not only by ion (about lo’ L).‘=’ bombardment, but also by evaporation of sub- Detailed studies of the adsorption of 0, on Ti02 monolayer amounts of Ti.“” The defect surface and SrTia as a function of exposure at room states produced by both methods were consistent temperature have been carried out by Henrich et with the transfer of electrons to surface Ti ions; al.‘=’ Figure 17 shows a family of UPS spectra for we refer to such defects as Ti”/O-vacancy com- AC-ion-bombarded SrTiO,( 100) before exposure plexes. to a (0 L) and after exposures of 0.5 L to 108L; Since the defect surface states involve 0 the spectra have been aligned at the top edge of vacancies, they should interact strongly with OZ the valence band, E,, to eliminate the effects of molecules; this is just what is obser~ed.“‘~’ band bending during adsorption. The increase in figure 16 shows three UPS spectra for Tior(110) valence band emission and the depopulation of the after various treatments.“” Figure 16(a) is for a bandgap surface state can be clearly seen, indicat- nearly perfect surface, while Fig. 16(b) gives the ing electron transfer from the surface defect states steady-state spectrum after H)OeV AC-ion bom- to the adsorbed species. More detailed information bardment. Figure 46(c) shows the surface in (b) can be obtained from difference curves (always after exposure to lp L of 0~. The adsorption of subtracting the clean-surface spectrum), as shown 02 restores the valence-band emission to some- in Fig. 18. These spectra show that two distinct thing resembling that for the perfect surface, and adsorbed phases are formed at different the bandgap surface state has been completely exposures. The initial phase (I) is obtained for depopulated. (Geometric order is not restored to exposures up to about 100 L; its difference spectra the surface by 02 adsorption, of course, and the exhibit two peaks, separated by about 2.5 eV, surface in Fig. 16(c) exhibits no LEED patterns.) overlapping the bulk valence band [see also Fig. This behavior upon 02 adsorption is characteristic 19(a)]. The similarity of these difference spectra to of all SrTit& and Ti02 surfaces studied to date. the valence-band emission suggests that the ad- The amount of 02 exposure necessary to com- sorbed species may be ti-, since the bulk valence Ultraviolet photoemission studies of molecular adsorption on oxide surfaces 155 band is derived predominately from 02- ions. The depopulation of the Ti’+/O-vacancy surface state 0, ON BOMBARDED SrTi03(100) is consistent with the formation of a negatively I\ 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

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- 0 a 6 4 2 qeo -2 -4 defect-density surfaces and on nearly perfect sur- INITIAL ENERGY (&‘I faces can be seen from Fig. 20, which presents a Fffi. 17. UPS spectra (hv-5 21.2eV) for Ar’-ion-bom- family of difference spectra for 02 adsorption on Wded SrTi~(lO0) after successive exposures to Q vacuum-fractured SrTiOs( IOO).~) Two adsorbed (from Ref. 62). phases are seen here also, with phase I identical to that on a highdefectdensity surface. (The ab- solute intensity of photoemission from the ad- 4 ON SCMARDED SrTi4(1Wl sorbed phase on the fractured surface is less than h\ 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 INITIAL ENERGY WI one additional phase.‘=’ Charge transfer from the FIG. 18. UPS difference spectra for Ar’-ion-bombarded bandgap surface state is observed for these other SrTiQ(100) after successive exposures to 02 (from data phases, as well as for phase I. in Fig. 17) (from Ref. 62). Lo et al.“” have studied the adsorption of G on

JPSS Vol. 9. No. J/6-B 156 VKZTORE. HENRICH studies of Hz0 adsorption have been carried out a, ON FRACTURED by Lo et al!“’ on Ti02(100) and by Henrich et S r TI O,(lOO) al v”*7’1on TX&(110) and SrTQ( 100). Lo et al.“” measured UPS spectra for ICYL 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 spectra 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

I t I I I INITIAL ENERGY (cv) a+ - 0.8 rv FIG. 20. UPS ditference spectra for vacuum-fractured T.3OtY.K SrTii(100) after successive exposures to 4 (from Ref. qUctGl-G~3)+lo’Ln*O 62). 9 =4.9*v

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

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

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

I I 1 I v I I TiO@Ol

TiqLlOOl-W7l+lO’Ln,O the Ti~lOO)-(l x 3) surface, and Fig. 21 shows I -IO.8 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. J C. A&rptkmef&Oor~TiO,andSrl’KI, -12 -6 -4 Cktron 6indiq Lnu~y. *V IEpOb Clearly, the most important mokcule to study in Fro. 23. UPS difference spectra for Hfi adsorption on an attempt to understand the role Of the electrode three d&&rent TQ surfaces. Dashed curve is UPS surface in photoelectrolysis is HzO. Photoemission spectrum of gas-phase Hz0 (from Ref. 15). Ultraviolet photoemission studies of molecular adsorption on oxide surfaces 157 difference spectrum. The spectrum for the (1 X 3) r\ H-0 ON BOMBARDEDTiO, (110) 1 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 I,, 1 I -4 spectra exhibit two broad, weak peaks in the 10 8 6 4 2 E,=D -2 INITIAL ENERGY (eV) region of the bulk valence band. We believe that these peaks most likely correspond to adsorbed FIG. 25. UPS difference spectra for Ar+-ion-bombarded TiWllO) after successive exposuns to Hz0 (from data OH radicals, and that the Ha mole&es are dis- in Fig. 24) (from Ref. 70). so&%&y chemisorbed in that regime. Gas-phase UPS spectra of OH radicals exhibit two peaks having ionization potentials of 13.01 and temperature.m’ If the adsorbed species is assumed 152OcV,” the same spacing as the observed to be OH, the polarixatiqnlrelaxation shift peaks. Also, i&a-red absorption spectra show ob&ined is close to that found for molecularly thnt OH is a stable species on Ti% at room 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 ON BOMBARDED TI02 11101 hv * 21.2 lV 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 _E I 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-

INITIAL EWEWY (WI . axes of the molecule.) The four difference spectra

FIG. 24. UPS spectra (hu = 21.2eV) for ~k+-ion-bon+ are aligned with the b, orbital of the gas-phase barded Tio2(110) after successive exposures to H~O Hz0 spectrum in Fig. 26(a). We interpret all of (from Ref. 70). these spectra as arising from molecularly adsorbed VICTORE. HENRICH

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 BOMBARDED SrTii(lO0) 2.79 3.80 1.01 3.7 Bombarded and annealed SrTiQ( 100) 3.15 4.05 0.90 3.7

ANNEALED 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 Hz0 (GAS1 former case and OH radicals in the latter. We feel hv * 21.2 lV 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

20 10 16 14 12 likely results from more complicated adsorption

INITIAL ENERG’f WI products. The work function changes and FIG. 26. UPS difference spectra for various Ti0~ and SrTi4 surfaces exposed to 10’ L Ha (b-c) and (a) UPS polariultion/relaxation shifts measured for mole- spectrum of gas-phase Hz0 from Ref. 56 (from Ref. 70). cular 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 H10 rather than OH.“O*“’For both nearly perfect work function of Tit& which is consistent with TiOr(l10) (Fig. 26(b)) and Ar’-ion-bombarded the orientation of the adsorbed molecules as Tio2(110) (Fig. 26(c)), the spacing of the outermost determined above from binding energy shifts. On two peaks in the difference spectra is very nearly SrTiOJ, the work function is increased by HZ0 the same as that for the b, and b2 orbitals of adsorption; we have no interpretation for this gaseous Hto, but the central peak is shifted effect at present. The polarization/relaxation shifts toward tighter binding relative to the free-mokcuk in Table 4 exhibit an interesting dependence on the al orbital. The shift is l.OeV for the defect surface presence of d-electron surface states. Those sur- and 0.7 eV for the annealed surface. The b2 orbital faces that have filled Ti”(3d’) surface states have for adsorption on the nearly perfect TiwllO) similar shifts (3.7 2 0.2 eV), even though the work surface is also shifted about 0.2eV toward larger functions before HZ0 exposure differ by as much binding energy. We interpret these shifts as bond- as 2.3eV. The one surface having no d-electron ing of the H1O molecule predominately via its surface states has a shift of only 2.5 eV. It therein-plane O-lone-pair (a,) orbital, particularly on the fore appears that, even when there is no charge defect surface. The molecule would then be bound transfer between surface states and adsorbed to the surface with its 0 ion down and is H ions molecules, these states have a pronounced effect away from the surface. The b2 orbital, cor- on the polarization/relaxation of the molecules. responding to the O-lone-pair normal to the molecular plane, also enters in bonding on nearly per- D. Adsorption of HZ on TiG fect surfaces. For defect SrTiG(100) surfaces, Lo et aL”5’ have also studied HZ adsorption on both the ul and b2 orbitals are shifted by OJeV the TQ( 100X1 x 3) and Ar’-ion-bombarded relative to the b, orbital: there is also some (but TiC)2(100) surfaces, and their difference spectra for not total) depopulation of the bandgap surface 10 L HZ are shown in Fig. 27. Both are three- state. No shifts relative to gaseous Hz0 are seen peaked spectra, and both look very much like the for adsorption on Ar’-ion-bombarded and spectra for HZ0 adsorbed on the same surfaces annealed SrTiO,(loO) surfaces: this is a com- (Fig. 23). Lo et al.“” interpret the two lowest-lying plicated surface (see Ref. 6). however, and we peaks as due to OH formed by dissociative chemi- have not tried to interpret the result. sorption of HZ, with the H atoms bonding to Comparison of Fig. 23 with Fig. 26 shows that surface 0 ions. The highest-lying peak is attributed the difference spectra measured by Henrich el to a negative charge on the OH (i.e. OH-). Ultraviolet photoemission studies of molecular adsorption on oxide surfaces 159

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

Bixbyite Rutile Corundum Rocksalt Spine1 Other

TQ@natase) Ti02 - - - V20Y(orthorhombic) CrO,(orthorhombic)

3d’ - wh(Tr340 K) TiG - -

3d2 - v203 TiO,(x * 1) - 3d’ - ,9%&r Crz03 vo,tx= I) - 3d’ MnrOr - - 3d” - - a-FeG3 3d6 - - - 3d7 - - 3ds - - - NiO - - - I - - - CuO(monoclinic) - - - Cur0 (cubic) - 1 ZnO Wurtzite)

I It has a wide range of stoichiometry (0.6 IX 5 Ti 4 000) -II 13)t Id LH2 1.28) and has the NaCl structure for all com- A+:-0.4+0leV positions except those close to x = 1; it then has a AN(E) 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, I I resulting in a rather open structure.‘6J’ Ar s’~ttered TIQ 000) + IO5 LH2 Interest in the adsorption of 02 on TiO, arose A$= Olf0.1 eV indirectly because of a theoretical paper by Jen- ANyI\ , ~1 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 cal- -12 -8 -4 culations.“’ Their results agreed, however, with Electron Bindmg Energy, eV (EF = 0) XPS measurements of Ichikawa et ~1.~) on five FIG. 27. UPS difference spectra for Ti02(1OOHl X3) TiO, samples (0.84~ x Z=1.22). which showed no surface and A?-ion-bombarded TiOr(lOO) surface after electron emission from the region above the 16’ L H2 exposure at 300 K (from Ref. IS). valence band. This surprising result prompted Wertheim and Buchananm’ to perform XPS measurements on samples of arc-melted TiO,., Henrich et uL’~’ have tried to study HI ad- (whose surfaces had been abraded in the pre- sorption on TiQ and SrTiO, at room temperature, paration chamber of the XPS spectrometer) and but with little success. In all cases, the difference Henrich et 01.~’ to take UPS spectra for vacuum- spectra obtained were much weaker than, but fractured polycrystalline TioO., and TiOl.,s. Both essentially the same shape as, those for Hz0 ad- of these measurements showed strong emission sorption. Examination of the ambient in the from a Ti(3d) band lying 2.5 eV above the valence vacuum system with a quadropole mass spec- band. The question then arose of why Ichikawa et trometer during Hz exposure showed an increase al. ‘76’ had not seen this band. After having studied in the mass 18 (H20) peak whenever HZ was the effects of O2 on Tf+ defect states on TiOz, Hen- admitted to the vacuum system, even when the Hz rich et aLm) suspected that the exposure of the was passed through a liquid nitrogen trap prior to Ichikawa et nl. samples to air before measurement admission and the system had been thoroughly had resulted -in oxidation of Ti ions near the sur- baked. Presumably Hz was reacting with residual face to a Ti“(3dq co&uration. The open struc- O-containing molecules somewhere in the vacuum ture of TiO, might allow 0 atoms or ions to system, but the exact cause was never determined. migrate far enough into the sample (30 A or SO) to oxidize all of the Ti ions within the XPS sampling IL AdsorptionofOtonTiO.(x=l) depth. Thus, they exposed the vacuum-fractured TiO, (x = 1) is an interesting compound in surfaces to various amounts of 02. As seen in Fig. several respects. ‘=’ From its stoichiometry, the Ti 28 for TiOl.lJ, the Ti(3d) band disappears upon ions would be expected to have a Ti2+(3d2) exposure to G.m’ (It should be noted that configuration, which is rather unusual for Ti ions. 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 0 4 a I2 16 .20 24 sample was then heated for 14 hours in 5 x lo-’ Torr 02 at 850 K before adsorption. Fii 29(a) Kinotk omqy, eV shows the UPS spectrum for that surface, and Pii. Fm. 29. Experimental and theoretical UPS spectra of a 29(b) shows the difference spectrum after p&ally hydrated cr-Al& surface. The lower abscissa exposure to 2 x IO L of HsO. The results are gives the measured electron kinetic energy while the upper scale gives the electron binding energy with res- ditTtcult to interpret, since the A&O, sampk may pect to the experimental Fermi level of the substrate. not have been thoroughly dehydrated before ad- The four spectra represent (a) reference surface pn- sorption.‘m pamdbyhcating14hin5x10-~Torr4atJU)C;(b) UPS measurements of CHtOH adsorption on cm from reference after 7Smin exposure to 5X IO-’Tow Hfl vapor at room tew: (cl shifted AltO, have also been reported by Rogers and SCF-G-SW spectrum for A&(OHh (from Ref. 7!J). 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.

02 ON FRACTURED Ti O,,,, I

h*s 21.2rV ,!

I / i_ I6 14 12 IO 6 6 4 2 I4 ELECTRON 6lNOlNG ENERGY t&I !:’ 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 INITIAL ENERGY, E WI identified as arising from CH radicals adsorbed at Ni sites.“‘) C2H2 thus appears to chemisorb dis- FIO. 28. UPS spectra for vacuum-fractured TiOl.lJ after various Q exposures. Spectra have been aligned at EF sociatively on NiO(l1 l), unlike the associative (from Ref. 78). chemisorption observed for C2H2on ZnO(lOiO).‘m Ultraviolet photoemission studies of molecular adsorption on oxide surfaces 161

4. Theo&W Calculationsof Mohxdar Adsorption on Oxide Surfaces Several groups have considered the adsorption Gkii’ of molecules on oxide surfaces theoretically, and some of those calculations are sufficiently detailed 8I e - + to permit direct comparison with UPS data. We 1 +- will not attempt to describe the theoretical results 83------X2 in detail, but rather we will summarize the various approaches and conclusions. Morin and Wolframq’ considered molecular ad- (al (bl sorption on the (100) surfaces of d-band perovskites qualitatively, using a simple linear-com- pHn 7 bination-of-atomic-orbitals (LCAO) energy band 0nn calculation. Wolfram and Ellialtioght’4’ later e d+ extended the method to calculate the local density I + of states (LDOS) for atomic planes in the surface + 0 0 region, including Coulomb repulsion among d- + electrons, and these results agree favorably with ii 2 experimental UPS data for atomically clean sur- ILz faces.m’ They then calculated the LDOS for a (c 1 molecule consisting of one bonding and one anti- (d) bonding level with a Ti ion on a (100) perovskite hG. 32. Schematic of orbit& and geometries of typical molecular orbit& that interact witit x2 and z2 orbitals of surface;“’ Fig. 31 shows the results of this cal- a transition-metal ion. (a) and fb) illustrate overlap of the culation. The antibonding molecular orbital is xz state with antibonding molecular orbitals, and (c) and assumed to hybridize with the Ti(d,) orbital and (d) show the overlap between the z2 state and bonding the bondmg molecular orbital with the Tiid~) molecular orbitals. Arrows indicate tbe usual direction of orbital, as shown in Fig. 32. (They have not yet charge transfer when these orbitals hybridize to form a surface complex (from Ref. 65).

c&tWeti the LDOS for specific molecules ad- sr TQ sorbed on the perovskite surface.) Their cai- AEpO culations indicate that the adsorbed molecule par- ticipates in delocalized anface energy band states, which provides a mechanism for adsorbed mole- 0 cules to communicate electronically over large distances with each other or with substrate atoms. 3* Wolfram et al.‘“’ have also performed molecular 3 orbital cluster calculations for clusters represent- 10 ing the surface electronic levels of Tii, SrTiO3 and BaTicX, but they have not explicitly included c: 2 w the presence of an adsorbed molecule. Almy d af.m have performed self-consistent- Lo field (SCF)-Xa-scattered-wave (SW) cluster cal- 0 culations on the Al&OHh cluster (simulating Al& + HsO), shown in Fig. 33, in order to inter- 2 pret their experimental results for Hz0 adsorbed on A120&t001). The results of the calculation are shown in Fii. 29(c). The calculation, while show- 0 -4 -8 -12 ing moderate agreement with experiment, was not ddinitive, since some probable adsorption Ftc. 31. Local densities of states (LDOS) for a chemi- geometries were not considered.~’ Foyt and sorption process. (a) LDOS for the xz-component of a Whitea’ have also performed SCF-Xa-SW Cal- cation on a (100) oerovskite surface with A& = 0. where culations of CHJ(OH) adsorbed on Al& AE,, is a surface &rturbation parameter representing the Lee and Wang@) have calculated the LDOS for deviation of a surface from ideal truncation of the bulk (A& = 0); Same as (a), but for A& = - 2 eV (represent- an ordered monolayer of both H-like and Cl-like ing presence of surface defects): (c) LDOS for surface atoms adsorbed on MgO(lOO), using an LCAO cation (AI?,+= - 2 eV) after interaction with an antibond- interpolation scheme. Figure 34 shows the results ing molecular orbital; (d) LDOS for the interacting orbi- of their calculations for H-like atoms, along with tal of an adsorbed molecule. E,, and Ee are the antibonding and bonding states of the molecule-surface the bulk energy bands for MgO. There are, as yet, complex and EM is the energy of the molecular state no expciimentaf UPS data with which. to compare prior to interaction with the surface (from Ref. 65). their calculations. 162 VICTORE. HENRICH

H o/0

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

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- FIG. 34. Energy-band structure for a monolayer of H- clude the effects of Ti-Ti interactions in both bulk like atoms on MgO(IaO). Shaded area represents sub- and surface geometries. strate continua (from Ref. 84). J.FutureDirWtions Kunz and co-workers have investigated the ad- It is clear from the work reviewed above that sorption of H on Mg@=’ and Ni@=’ by means of the study of molecular adsorption on oxide sur- SCF-unrestricted-Hartree-Fock cluster caicula- faces by means of photoemission spectroscopy is tions. These systems have not yet been studied a rather young field. Relatively few oxide surfaces experimentally by UPS, so no comparison of have been studied, and little is known about the theory and experiment is possible. details of adsorption sites. Only on the Zno(lOi0) Clwter calculations of OH- adsorbed onto a surface have a wide enough range of adsorbed defect site on TiG have beep performed by Kawai molecules been studied to see trends in adsor- et al.,“) using the discrete-variational (DV)_Xa bate/substrate interactions,‘-” and even then the method. They have performed calculations for details of bonding are not well understood. The both an atomically clean surface (TiO, cluster) and field is ripe for expansion in a number of different for a surface with adsorbed OH- (TiOrOH- clus- directions. ter), and taken the difference between them to Space limitations in the present paper have simulate UPS difference spectra. The results are prevented us from discussing surface analysis shown in Fig. 35 along with the UPS data of techniques other than UPS in any detail. Most of Henrich et al.“O’ Since their calculations yielded those techniques, such as the ones mentioned in essentially a three-peaked spectrum, they con- $1, have been applied in at least some degree to cluded that only OH is adsorbed on TQ and oxides, and a coupling of the various methods is SrTia surfaces, as opposed to the interpretation obviously necessary in order to gain as complete given by Henrich et d.(m) They did not perform an understanding of adsorption on oxides as pos- calculations for a cluster simulating adsorbed sible. Most groups already employ a number of molecular H20. dierent techniques, and that trend is continuing. Kowalski and Johnson’p’ have performed scat- Just within the field of ultra-violet photoemis- tered-wave (SW)-Xa calculations for both Hz0 and sion spectroscopy, however, there are several OH adsorption on TiO. clusters that simulate both directions of research that should greatly increase perfect and defect surfaces of Ti(x and SrTia. our understanding of molecule/oxide interactions. They find ‘that molecular Hz0 gives the expected One is to perform more detailed UPS measure- three-peaked spectrum, but that the peaks are at ments on a few selected systems. At@e-resolved larger binding energies than those seen by UPS. photoemission, using tunable synchrotron radia- Adsorbed OH gives a two-peaked spectrum, which tion, should be applied to relatively simple sys- Ultraviolet photoemission studies of molecular adsorption on oxide surfaces 163

terns, such as CO on the four major faces of ZnO. T. WOLFRAM,E. A. Krutrt and F. J. MORIN, Phys. These techniques are now sufficiently well Rev:B7, 1677 (1973). developed, as a result of work on metal and semi-. F. J. MOJUNand T. WOLFRAM,.P~~~.Rev. L&r. 30, 1214 (1973). conductors, that they can easily be applied to Ti W&MM and S. ELLLUI~CLU,A&. Ms. 13, oxides. Such measurements would give information about both filled and empty surface and D. M. TENCHand D. 0. RALEIGH,Elrctmotalysis on molecular levels as well as bonding geometry. Non-h&aNic Surfaces, Natl. Bur. Stand. Spec. Publ. No. 455, p. 229 (U.S. GPO, Washin%on, D.C., Quantitative LEED measurements and com- 1976). parison with theoretical calculations would also be 6. V. E. HENRICH,G. I)RESSELHAUSand H. J. ZEIGER, necessary on those systems in order to better Phys. Rev. B17,4988 (1978). define the surface geometry. 7. W. J. LQ and G. A. SOMORIAI.Phys. Rev. B17,4942 Another general approach that should prove (1978). 8. G. F. DEREIENWICK.Ph.D. thesis (Stanford Uni- very useful is the study of a number of different versity, 1970) (unpublished). crystal faces of the same oxide. Since adsorption 9. S. THOMAS,Surf. Sci. 55,754 (1976). sites on oxides, or on any compound, can involve 10. V. E. HENRICH,G. DRESSELHAUSand H. J. ZEIGER, two or more different types of atoms, it is im- Phys. Rev. Lerr. 36, 1335 (1976). 11. M. L. KNOTEK,in Proceedings of the Symposium on portant to examine the effect of changes in the Eiecrtvde Marerkk and Ptvcesses for Eneqy Con- relative position of the substrate atoms, the direc- version and storage. eds. J. D. E. MCINTYRE,S. tion of their bonding and antibonding orbitals, etc., SRINIVASANand F. G. WILL, p. 234, (Tbc Elec- on adsorption. To date, only the adsorption of CO trochemical Society, Princeton, NJ, 1977). and NH3 on ZnO have been studied in such a 12. M. L. KNUIXKand J. E. HOUSTON,Phys. Rev. B15, 4588 (1977). systematic manner.(203rJs) Work of this type 13. Y. W. CHUNG,W. J. Lo and G. A. !SOMOIUAI, Surf should be extended to a number of other mole- sci. 64,588 (1977). cule/oxide systems. 14. H. J. MATHIELJ,J. B.. MAIXIEU,D. E. MCCLUREand In addition to varying the surface geometry (i.e. D. LANDOLT,J. Vat. Sci. Technof. 14. 1023 (1977). 15. W. J. Lo, Y. W. CHIJNGand G. A. !SOMORJAI, Srtrf crystal face) on a single oxide, it is possible in Sci. 71. 199 (1978). some oxide systems to vary the electronic 16. V. E. HENRICH,G. DRESELHAUS and H. J. ZEIGER, configuration of the surface without changing its Phys. Rcu. L&r. 36. 158 (1976). crystal structure. The fourth-row transition-metal 17. P. W. PALMBERG,personal communication. oxides are an excellent case in point. The cation 3d 18. w. OPEL. surf. sci. 62.165 f1977). 19. R. R. GAY; personal c&nun&ati~n. levels fill progressively as one moves across the 20. R. R. GAY, E. 1. SOLOMON,V. E. HENRICHand H. J. periodic table from SC203to ZnO. The fourth-row ZEK~ER,(unpublished). @&ion-metal oxides are organized according to W. G~PEL, J. Vat. Sci. TechnoL 15. 1298 (1978). their d-electron configuration and crystal structure g: C. C. CHANG,Surf Sci. 25,53 (1971). 23. S. THOMAS.J. ADD!.Phvs. 48. 161 (1974). in Table 4. Large isostructural families exist in this 24. V. M. B&&‘and V. H. Rtti! (&pubJisbcd). system, and it should be posstble to vary the 25. V. E. HENRICH,G. DRI%SEL.HAUSand H. J. ZEIGER cation d-electron population for a particular sur- (unpublished). face geometry. Such studies have not been per- 26. G. HENAND, P~vceedings of the Second Inremarional formed, and the structure of a particular crystal Confemnceon Ekcrrophorog~hy, p. 117. 27. K. 0. Leon, M. PR~JT~~Nand C. KINNIBURGH,1. face may well turn out to be sensitive to dclec- Phys. C7,4236 (1974). tron configuration, but it is certainly a promising 28. S. C. CHANGand P. MARK,Surf. Sci. 45,721 (1974). direction in which to proceed in order to determine 29. S. 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