H2O dissociation by SrTiO3 (100) catalytic step sites

Vacuum/volume38/numbers 4/5/pages 405 to 408/1988

0042-207X/88$3.00 + .00 Pergamon Press plc

Printed in Great Britain

H20 dissociation bySrTiO3(100) catalytic step sites N B B r o o k e s , Department of Chemistry, University of Manchester, Manchester M13 9PL, UK F M Q u i n n , Science and Engineering Research Council, Daresbury Laboratory, Warrington WA4 4AD, UK and G Thornton,*

Department of Chemistry, University of Manchester, Manchester M13 9PL, UK; Science and Engineering Research Council, Daresbury Laboratory, Warrington WA4 4AD, UK

The adsorption of H20 on fractured and planar surfaces of SrTiOa( l O0) has been studied using photoemission spectroscopy. Valence region spectra evidence molecular adsorption on the planar surface and dissociative adsorption on the fractured surface. Features arising from surface Sr (Ti) atoms on the SrO (Ti0 2) terraces of SrTi03( lO0) have been resolved in core-level photoemission spectra: the surface Sr 3d (Ti 3s) features fie to lower (higher) initial energy of the bulk-derived peaks by ca 1.0 eV (1.7 eV). These results are consistent with the expected enhancement of covalent bonding in the TiO 2-terrace surface. From the valence- and core-level spectra we deduce that step-site& which connect the two types of terrace, act as catalytic centres for H20 dissociation.

1. Introduction Achieving a fundamental understanding of oxide surface reactivity depends in large measure on understanding the role of minority sites, which are known to strongly influence the reactivity l'z. Theoretical studies have explored this phenomenon by modelling the interaction of molecules with step sites and oxygen vacancies 1-5. However, sufficiently specific experimental data has not been available to adequately test the results of this work. Experimental studies have mainly been focused on the effects of surface structural defects created by argon ion (Ar ÷) bombardment 1'2. Because a variety of defect sites are likely to be obtained by this method, the data are not easily compared with theoretical models. A new approach has been adopted in the present work, where we have used soft X-ray photoemission spectroscopy to directly compare the surface processes involved in the reaction of HzO with stepped and planar SrTiO3(100). Valence band photoemission spectra are used to identify the adsorption mode and photoemission core-level shifts 6-8 to identify Sr and Ti surface cations. This enables us to deduce the bonding sites of adsorbed species, and thus to determine that H 2 0 dissociates at step sites, in accord with theoretical expectations 5. The interaction of H 2 0 with surfaces of strontium titanate has attracted considerable attention, being of specific interest as a model photocatalysis system for the water-splitting reaction. Theoretical work has included surface electronic structure calculations of SrTiO 3(100) 9.1o, and electronic structure calculations of surface complexes which model H 2 0 and OH bound to Ti *To whom correspondence should be addressed.

atoms at terrace, step and oxygen vacancy sites 4'5. Previous experimental work has employed photoemission spectroscopies and low energy electron loss spectroscopy (LEELS) to study the interaction of H20 with the (100) 11-13 and (111) t4 SrTiO 3 surfaces. The results of this work indicate that the planar surfaces are inert 12-14, and that Ar + bombardment-induced defects dramatically enhance the reactivity, although the mode of H 2 0 adsorption, molecular or dissociative, has remained the subject of debate 11-14. This arises from the difficulty in distinguishing H20 from OH on oxide surfaces using photoemission or LEELS, even though they are the techniques of choice in such work.

2. Experimental Photoemission measurements were performed using the grazing incidence monochromator 15 (20 < hv < 200 eV) on station 6.1 at the Synchrotron Radiation Source, Daresbury Laboratory. A double-pass Cylindrical Mirror Analyser (Physical Electronics Inc) was used, the combined resolution (monochromator + analyser) being ca 1.0 and 0.4 eV [full width at half maximum (FWHM)] at photon energies of 180 and 110 eV, respectively. The CMA axis was at 90 ° to the photon beam, which was incident at 23 ° from the surface normal. Fracturing n-type SrTiO 3 par~tllel to the (100) planes in situ at a chamber pressure of < 8 x 10- 11 mbar yielded a stepped, unreconstructed (I00) face, as determined by low energy electron diffraction (LEED). The planar n-type SrTiO3(100 ) surface was prepared by Ar + bombardment and annealing in oxygen, yielding a clean, unreconstructed surface, as measured by Auger and 405

N B Brookes et al: H20 dissociation by SrTi03(100) catalytic step sites

LEED. Exposure to H20 was carried out using the vapour of doubly distilled water, dissolved atmospheric gases having been removed by several freeze/thaw cycles. For the fractured surface, the sample temperature remained at 300 K for H20 exposure and subsequent measurements. In contrast to adsorption on the stepped surface, where the initial sticking coefficient is close to unity, H , O does not react with Ar + bombarded/annealed SrTiO3(100) at 300 K. For this reason, H 2 0 exposure and photoemission measurements were performed with a sample temperature of ca 150 K. No noticeable change in the LEED patterns was observed following exposure to H,O, indicating the formation of disordered overlayers. 3. Results and discussion 3.1. Adsorption mode. A comparison of valence band photoemission spectra obtained from the stepped and planar (100) surfaces before and after exposure to H20 is shown in Figure 1 in the form of difference spectra. The data indicate that the mode of adsorption is different for the two surfaces, being molecular in the case of the planar surface, dissociative on the stepped surface. Whereas the adsorbate-induced structure in the planar surface spectrum 'fingerprints' molecular H20, a feature corresponding to lb 2 emission is absent in the stepped surface spectrum, indicating the presence of adsorbed hydroxyl species. The relative positions of the adsorbate-induced O 2s features are consistent with this interpretation, lying at lower initial energy in the planar surface spectrum, as expected for the more covalent, H20 species. The separation between the OH~ and OH~ peaks is 3.6 eV, 1.4 eV larger than for gas phase OH 16. The most straightforward explanation of this bonding shift involves the greater stabilization of the OH~ orbital via bonding to the substrate with the OH axis inclined towards the surface normal. The bonding shifts observed when comparing the spectrum of gas phase H20~ v with the planar surface difference spectrum are more difficult to interpret because H20 orbital energies are sensitive to the molecular bond angle. However, it is clear that H 2 0 is chemisorbed on planar SrTiO3(100 ) at 150 K and not simply condensed.

3.2. Surface core level shifts. Clean surface. Sr 3d and Ti 3s core level spectra of the stepped and planar surfaces are shown in Figures 2 and 3, along with spectra recorded after H 2 0 adsorption. The photon energies used yield Sr 3d and Ti 3s photoelectron kinetic energies of ca 40 eV, corresponding to an electron mean free path, 2, of ca 7 ,~. This value, which is typical of inorganic materials ~s, was estimated using the surface/bulk intensity ratios 7 and neglecting take-off angle effects by assuming electron emission normal to the surface. Spectra were also recorded at photoelectron kinetic energies of ca 10 eV, where ), is estimated to be ca 20 A, to isolate the bulk and surface contributions to the spectra shown in Figures 2 and 3. The Sr 3d (Ti 3s) spectra can all be adequately represented by four (two) overlapping symmetric Gaussian peaks. The results of peak fitting are summarized in Table 1. To improve the reliability of the fitting procedure, the widths of component peaks within each spectrum were constrained to be equal. This was the only constraint imposed. Except in the case of the Ti 3s planar-surface spectra the fitting procedure produced the same widths for the clean and HzO-dosed spectra. Fitting the Sr 3d spectra clearly leaves more room for error, although we note that the spin-orbit component separations and their branching ratios are close to those expected19. SrTi03 (1001stepped Ti3s h'u=110eV

...... bolk

:=.........oexpl./ FU ,/

g

/

f~,

Sr3d hlJ =180eV ~

/' \ \

1

' ' ",,\

/ -

J:A,

~\

r~

/: //

~ , t.-,

." ',\',, I /7',,/,',',

z ~,

,'

~.

"\

>[

,,

i?

ShrvY__lO(I00)-H2 s 0

/--, L ~ ~\

-63

\

~

-61 -59 -57 INITIAL ENERGY(eV)

/'

,

;,,

\j

\

-136 -131, -132 -130 INITIAL ENERGY(eV)

Figure 2. Ti 3s and Sr 3d photoemission spectra of clean and H20-dosed stepped SrTiO3 (100), recorded at hv= ll0eV (Ti 3s) and hv= 180eV (Sr 3d). The spectra have been background subtracted and fitted to symmetricGaussian peaks. Initial energy is referencedto the valenceband maximum. Both the bulk and surface Sr 3d features are spin-orbit split by cal.8eV. 1 L=l.32xlO-6mbars.

",.j

'

-I~

'

v

-Sd

\

'

H20 hv =28-&eV

-L

INITIAL ENERGY(eVI

'

'Ev~ ~

Figure 1. UPS differencespectra (H20 dosed-clean) for (a) I0 L H20 on stepped SrTiO3(100) adsorbed at 300 K and (b) 0.5 L H20 on planar SrTiO3(100) adsorbed at 150 K, compared with a gas-phase photoelectron spectrum of H20 from ref 17, aligned at the lb 2 energy; 1 L= 1.32 x J.0 - 6 mbar s. 406

Let us first consider the origin of surface features in Figures 2 and 3. We can immediately deduce that surface features in the stepped surface spectra arise from Sr and Ti atoms occupying terrace sites since they have identical chemical shifts to those observed from the planar surface, which contains a very low density of step sites. The assignment of the Ti 3s surface chemical shift to terrace sites is in line with the conclusions reached in a previous study z° of planar SrTiOa(100). Having the simple-cubic perovskite structure, SrTiO s contains two types of crystal plane parallel to (100), one of which contains Sr and O atoms (SrOtype), the other containing Ti and O atoms (TiO2-type) ~. Hence, the surface features in Figures 2 and 3 arise from cations on their respective terraces, and their intensities relative to the bulk

N B Brookes et al: H20 dissociation by SrTi03(1 00) catalytic step sites

SrTiO3 (100)planar Ti3s ~-h~)=110eV / ' ~ ..... bulk / '# .... surface/ ,\ fofa[ , ~\

........ xpt./

I CLEAN

OOK

',\

\ , . ,/~I /.\ / t

,// ./

/,,-/ }, \

+H~O

/ /

Sr3d

Ih'O=180eV /

160K

/

/%J,' \ ~, /

-63

-61 -59 -57 -136 -13t~ -132 -130 INITIAL ENERGY (eV) INITIAL ENERGY(eV} F'igure 3. Same as Figure 2 for planar SrTi03(lO0 ). Calibration problems prevent us from quoting a meaningful value of H20 exposure.

"]?able 1. Fitting parameters for the core-level spectra of SrTiO3(100) shown in Figures 2 and 3. Symmetric Gaussian peaks were used, constrained to be of equal width within each spectrum. Only for the Ti 3s spectra of the planar surface were different peak widths obtained from the clean (C) and HzO-dosed (D) spectra. Surface core-level shifts refer to surface-bulk initial energies. All energies are in eV Stepped

Planar

Sr 3d5/2

Bulk initial energy: - 132.39 Spin-orbit splitting: bulk surface Branching ratio: bulk surface Peak widths (FWHM) Surface core-level shift Surface/bulk intensity: clean +H20

1.78 1.78 0.67 0.64 1.39 --1.00 0.45 0.17

1.74 1.69 0.63 0.62 1.35 -0.96 0.74 0.06

75 3s

Bulk initial energy: - 60.30 Peak widths (FWHM)

2.45

Surface core-level shift Surface/bulk intensity: clean +HzO

1.70 0.38 0.08

2.25C 2.15D 1.70 0.12 0.14

features can therefore be used to estimate the relative area of each type of terrace. The surface to bulk ratios obtained from the stepped spectra (see Table 1) indicate the presence of roughly equal areas of SrO- and TiO2-type terrace, as expected. In contrast, the planar surface data indicate an 85% SrO, 15% TiO 2 composition. The detailed origin of polar-surface coreqevel shifts is, in general, likely to be very different from those of semiconductor and metal surfaces, the principal contribution arising from the different Madelung potential experienced by a surface atom. Here we compare our results with calculations9 of the potentials at Sr and Ti sites in the bulk and at unrelaxed terrace sites of SrTiO3(100 ). In this comparison we neglect the effect of possible surface relaxation and differences between bulk and surface final-

state screening. Only the latter assumption can be justified, and only to a degree, by the conclusions reached in a study of GaAs(110) 8. The calculations9 for SrTiO3(100) indicate that the potential at sub-surface sites is almost identical to that of the bulk sites. Hence in the absence of a measurable contribution from step or defect sites, only one Ti and Sr chemically shifted feature is expected, as observed, arising from atoms on TiO 2 and SrO terraces, respectively. The predicted chemical shift of surface Sr atoms is 1.23 eV to lower initial energy, in reasonable agreement with our observed shift of 1.0+_0.05 eV (see Table 1). The calculations also predict a chemical shift for surface Ti atoms of 2.0 eV to lower initial energy, to be compared with the observed shift of 1.7±0.1 eV to higher initial energy. However, later calculations by Wolfram et al 1°, who employed an empirically tested :t bulk band structure as a basis 22, indicate that the surface Ti shift will be significantly affected by surface-enhanced covalency. Wolfram et al 1° express the subsequent difference between the bulk and surface potential ofa Ti 3d electron in terms of a surface perturbation parameter, AE~, which is numerically equivalent to the surface chemical shift (surface-bulk initial energy)*. While this simple model explains the trend in Sr and Ti surface shifts, it does not adequately account for the magnitude of the Ti shift. H20 adsorption. The effect of HzO adsorption on the terrace site core level features is interpreted in light of the valence band photoemission results, which indicate molecular adsorption on the planar surface and dissociative adsorption on the stepped surface. The planar-surface core-level spectra (Figure 3) show that the Sr 3d surface feature is strongly attenuated relative to the bulk feature on H 2 0 adsorption, whereas the relative intensity of the Ti 3s surface feature increases slightly (see Table 1). This is consistent with HaO bonding to SrO terrace-cation sites, which will modify the cation-site potential and slightly attenuate Ti 3s photoemission from underlying bulk sites. If adsorbate effects on the sampling depth are taken to be small, the total intensity of core-level emission should be conserved on H 2 0 adsorption. With this assumption, the data indicate that the intensity lost from the Sr 3d surface feature on adsorption is transferred to the bulk peak, indicating an Sr-OH 2 cation potential similar to the bulk value, and an 80% coverage of Sr terrace sites. The steppedsurface spectra (Figure 2) present a very different picture, with both Sr 3d and Ti 3s terrace-site features being attenuated on HzO adsorption. Making the same assumption regarding adsorbate effects on the sampling depth, both the Ti 3s and Sr 3d data indicate that intensity is transferred from the surface-peak to the bulk-peak position on adsorption. This suggests the formation of T i - O H and Sr-OH species, where the cation potentials would be similar to the bulk values, with H atoms bound to oxygen terrace sites. The estimated coverage is approximately I/2 (2/3) of Sr (Ti) terrace sites. 3.3 Mechanism for dissociation. By reference to only the valence band spectra of the planar and stepped surfaces it might reasonably be proposed that steps activate H 2 0 dissociation. However, the surface core-level shift data presented here allow us to exclude the additional possibility that active sites are located on a (100) terrace-type which is not present on the planar surface. A further possible source of active sites are oxygen vacancies created *For the purposes of this work, the Coulomb terms applicable to Ti 3s and 3d electrons can be considered identical. This was determined using a Dirac-Fock computer code23. 407

N B Brookes et a/. H20 dissociation by SrTi03(100) catalytic step sites

in sample p r e p a r a t i o n , a l t h o u g h this can be excluded on the g r o u n d s t h a t the stepped a n d p l a n a r surfaces have a similar c o n c e n t r a t i o n of such defects. This was estimated from their c o n t r i b u t i o n to the b a n d - g a p region of the p h o t o e m i s s i o n spectra 1, their c o n c e n t r a t i o n being too low to observe the c o r r e s p o n d i n g features 2° in the core-level spectra. We therefore conclude t h a t steps are the active sites for H 2 0 dissociation. F r o m the initial sticking coefficient a n d core-level shift data we deduce a m e c h a n i s m of H z O a d s o r p t i o n on the stepped surface which involves diffusion of H 2 0 across terraces to steps, the dissociation p r o d u c t s subsequently m i g r a t i n g back to terrace sites. Hence the step sites are renewable a n d are acting as true catalytic centres. In seeking a n e x p l a n a t i o n for the e n h a n c e d reactivity of the step sites, it seems m o r e likely t h a t the l o w - c o o r d i n a t i o n Ti sites are reactive, since here the 3d-orbital o c c u p a t i o n will be maximized. Following the ideas of Kowalski et al s, the O - H 2 b o n d s would be weakened by interaction of occupied 3d orbitals with H 2 0 a n t i b o n d i n g orbitals.

Acknowledgements This work was s u p p o r t e d by the Science a n d Engineering Research Council (UK). Additional s u p p o r t was received from the U n i t e d K i n g d o m Atomic Energy A u t h o r i t y and V a c u u m Science W o r k s h o p Ltd.

References 1 V E Henrich, Proy SurfSci, 9, 143 (1979); Proff SurfSci, 14, 175 (1983); Rep Proff Phys, 48, 1481 (1985).

408

2 G Heiland and H L/ith, in The Chemical Physics of Solid Surfaces and Heterogeneous Catalysis (Edited by D A King and D P Woodruff), Vol IIIb. Elsevier, Amsterdam (1984). 3 V M Bermudez, Pro9 SurfSci, 11, 1 (1981). 4 M Tsukada, H Adachi and C Satoko, Pro9 SurfSci, 14, 113 (1983). 5 j M Kowalski, K H Johnson and H L Tuller, J Electrochem Soc, 127, 1969 (1980). 6 D Spanjaard, C Guillot, M-C D6sjonqu6res, G Treglia and J Lecante, SurfSci Rep, 5, 1 (1985). 7 j F Van der Veen, F J Himpsel and D E Eastman, Phys Rev Lett, 44, 189 (1980). s D E Eastman, T-C Chiang, P Heimann and F J Himpsel, Phys Rev Lett, 45, 656 (1980). 9 E A Kraut, T Wolfram and W Hall, Phys Rev B, 6, 1499 (1972). ~o T Wolfram and S Ellialtioglu, Appl Phys, 13, 21 (1977). 1~ V E Henrich, G Dresselhaus and H J Zeiger, Solid St Communs,24, 623 (1977). 12 C Webb and M Lichtensteiger, SurfSci, 107, L345 (1981). a3 R G Egdell and P D Naylor, Chem Phys Lett, 91, 200 (1982). i4 S Ferrer and G A Somorjai, SurfSci, 94, 41 (1980). J5 M R Howells, D Norman, G P Williams and J B West, J Phys E, 11, 199 (1978). 16 S Katsumata and D R Lloyd, Chem Phys Lett, 45, 519 (1977). 17C M Truesdale, S H Southworth, P H Kobrin, D W Lindle, G Thornton and D A Shirley, J Chem Phys, 76, 860 (1982). is M P Seah and W A Dench, SurfInterfac Analysis, 1, 1 (1979). 19 j C Fuggle and N M~rtensson, J Electron Spectrosc Relat Phenom, 21, 275 (1980). 2o B Cord and R Courths, SurfSei, 162, 34 (1985). zl N B Brookes, D S-L Law, T S Padmore, D R Warburton and G Thornton, Solid St Communs, 57, 473 (1986). 2z T Wolfram, E A Kraut and F J Morin, Phys Rev B, 7, 1677 (1973). 23 D A Liberman, D T Cromer and J T Waber, Comput Phys Communs,2, 107 (1971),