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Electronic structure of Pt overlayers on (1 × 3) reconstructed TiO2(100) surfaces
surface science ELSEVIER
Surface Science 391 (1997) 196 203
Electronic structure of Pt overlayers on ( 1 x 3) reconstructed TiO2(100) surfaces Klaus-Dieter Schierbaum a . , Stefan Fischer a Paul Wincott b Peter Hardman Vin Dhanak b, Graham Jones b, Geoff Thornton b
b
" Institute qfPhysical and Theoretical Chemistry, Ebo'hard-Karls Unirersi O, q/Tiibingen, A~!/'der Morgenstelle 8, 72076 Tfibingen, Germany b Chemistt T Department, Manchester University, Manchester, M13 9PL, UK
Received 11 April 1997; accepted for publication 13 June 1997
Abstract The effect of Pt atoms on the Ti 3d emission of Ti 3+ states at ( 1 x 3) reconstructed TiO2(100) surfaces was studied by means of valence band photoemission spectroscopy using synchrotron radiation with excitation energies between 30 and 120 eV. A clean (1 x 3) reconstructed TIO2(100) surface was prepared by Ar + sputtering and annealing at T= 870 K as checked with low energy electron diffraction. Subsequently, Pt was evaporated in situ using a resistively heated Pt filament. Evaporation was performed stepwise up to a nominal coverage of 1.8 monolayers (ML) as checked with Auger electron spectroscopy (AES). The Pt 4f core levels are shifted to higher binding energies with respect to Pt metal, indicating the presence of small Pt clusters. Difference spectra show a dublet between the TiO2 valence band maximum and the Fermi energy which is attributed to Pt 5d states. We found that the position and the intensity of the Ti 3d band gap emission were not significantly altered upon evaporation of Pt. In addition, the valence band maximum of TiO, remains unchanged. These findings indicate the absence of an electronic charge transfer due to the high work function of TiO2(100)-(1 x 3) of 5.4 eV, in line with a simple metal-semiconductor contact theory..~2 1997 Elsevier Science B.V. Keywords." Clusters; Low index single crystal surfaces; Metal semiconductor interfaces; Photoelectron spectroscopy; Platinum:
Surface defects; Titanium oxide
1. Introduction S t a b l e m e t a l c o n t a c t s on s e m i c o n d u c t i n g o x i d e s w i t h c o n t r o l l e d electrical p r o p e r t i e s are r e q u i r e d in a v a r i e t y o f d i f f e r e n t a p p l i c a t i o n s such as c h e m i cal sensors a n d p h o t o c a t a l y s i s etc. O n e e x a m p l e is the S c h o t t k y c o n t a c t f o r m e d by p o r o u s Pt e l e c t r o d e s o n r u t i l - t y p e TiO2 [1]. C o r r e s p o n d i n g
* Corresponding author. Fax: (+49) 7071 296910: e-mail: [email protected] 0039-6028/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved. PH S0039-6028 (97)00483-4
devices m a y be used as s o l i d - s t a t e sensors to m o n i t o r 0 2 in the gas p h a s e [1,2]. In this c o n t e x t , TiO2 single crystals with ( s u b ) m i c r o m e t e r thick m e t a l l i c o v e r l a y e r s h a v e been used as a m o d e l system to s t u d y the e l e c t r o n i c s t r u c t u r e o f P t / T i O 2 i n t e r f a c e s a n d the influence o f e l e c t r o n i c surface states o n c h a r g e t r a n s f e r r e a c t i o n s [3]. Pt d o e s n o t u n d e r g o s o l i d - s t a t e r e a c t i o n s at the interface, i n d i c a t i n g a h i g h t h e r m o d y n a m i c stability o f this system [4,5]. C u r r e n t - v o l t a g e ( ~ V ) c u r v e s i n d i c a t e a S c h o t t k y b a r r i e r h e i g h t ~bsB o f 1.5 eV a n d an ideality f a c t o r n o f o n e if P t / T i O 2 ( l l 0 )
K.-D. Schierbaum et al. / SurJace Science 391 (1997) 196 203
diodes are operated in ultra-high vacuum ( U H V ) at 500 K [6]. Evidently, Pt overlayers act as acceptots and trap free TiO 2 conduction band electrons. This causes a depletion layer in the TiO 2 and a band bending - e A Vs of 0.9 eV. The barrier height is ca 0.7 eV higher than theoretically expected from ~sB = ~Pt - ZTio2
( 1)
where ~bpt=5.6 eV denotes the work function of the Pt overlayer and ZTiO2=4.8 eV, the electron affinity of TiO2(ll0) [3]. Recently, valence band photoemission studies have been performed on Pt/TiO2( 110)-( l x 1 ) [5,7]. They reveal only a very small band bending ( - eA Vs< 0.2 eV) at initial stages of the interface formation. The high Schottky barrier height of Pt/TiO2(ll0) diodes may formally be explained by an additional dipole contribution AZ to the electron affinity of TiO2 at the interface [3], the origin of which is not clear yet. A transition from this "ideal" Schottky diode behavior to a surface-state controlled one with values of n > 1 occurs in the presence of oxygen molecules, that is, after diffusion of 02 through the polycrystalline overlayer and formation of negatively charged atomic and molecular oxygen species O or 0 2 at the interface [3,6]. These adsorbates act as acceptor-type "extrinsic" surface states, ldeality values n > 1 correspond to a shift of the ~ V curves towards larger positive voltages if Pt/TiO2(110) diodes are forward biased (i.e. the positive pole of the voltage supply is connected to the Pt contact). The applied voltage leads to a depopulation of surface states while the space charge and hence the barrier height remain almost unaffected. Depopulation of donor-type "intrinsic" surface defects at Pt/TiO2(110) interfaces by an electronic charge transfer to the metallic Pt have been recently confirmed by photoemission and constant final-state photoemission spectroscopy [8]. In these experiments, Ti 3d band gap states arise from lowcoordinated surface Ti 3 + ions that were produced on TiO2(ll0) by weak Ar + sputtering at low kinetic energies of 500 eV and a low sputter dose. This is accompanied with a drastic decrease of the work function ~bto a value of 4 eV [8]. Subsequent evaporation of Pt leads to a strong damping of the Ti 3d band gap photoemission. The constant
197
position of the TiO2 valence band maximum with respect to the Fermi level E v indicates that free TiO2 conduction electrons are not involved in the electronic charge transfer reaction. In this paper, we report on the electronic structure and the absence of charge transfer effects between Pt overlayers and the (1 x 3) reconstructed TiO2(100) surface. We have employed normal-emission photoelectron spectroscopy at different photon energies [9]. TiO2(100)-(1 x 3 ) has a relatively large work function of 5.4 eV which is a little higher than the value of TiO2 (110)-(1 x l) (~b=5.2eV). The TiO2(100)-(1 x 3 ) surface contains rows of adjacent Ti 3+ ions at the topmost positions (Fig. 1 ). The microfacet surface model follows from recent scanning tunneling microscopy results [10] as well as grazing incidence X-ray diffraction studies [11] and is consistent with previous low energy electron diffraction (LEED) experiments [12]. Photoemission spectra show Ti 3d band gap states. Their substantial reduction during chemisorption of O2 [i.e. by exposing to a dose of 3000 L (1 L = 1 0 - 6 T o r r ) without changing the (1 x 3) reconstruction] with an accompanying work function change from 5.4 to 5.6 eV even at room temperature gives additional evidence that these states are localized at the surface.
2. Experimental The photoemission experiments were run on the IRC in Surface Science station 4.1 at the CCLRC, Daresbury Laboratory, using a toroidal grating monochromator with photon energies between 30 and 120eV and a SCIENTA analyzer [13]. The resolution of the analyzer is < 50 meV. The beamline contribution was ca 100 meV at 47 eV and ca 200 meV at 80 eV. The overall resolution was determined from the Fermi edge of a clean Pt foil. Binding energies E~ are referenced to the Fermi energy. Details of the sample preparation were described previously in Ref. [13]. Pt overlayers were evaporated in situ from a thermally heated filament under UHV conditions at a base pressure in the 10-i0 mbar range. The filament current was kept constant during evaporation. Coverages of Pt
198
IC-D. Schierbaum et aL / Smjhce Science 391 (1997) 196 203
[om]t
I l -----[OLO1
Fig. 1. Surface geometric structure of the (1 × 3) reconstructed TiO2(100) surface. Small black circles, Ti subsurface atoms: small hatched circles, Ti surface atoms; large gray circles, O subsurface atoms; large hatched circles, O in-plane surface atoms: and large white circles, O bridging surface atoms.
were estimated from Auger electron spectra o f the Ti L M M and Pt M N N transitions.
e-
EDC
by
f[100]
3. Results Fig. 2a displays the valence band spectrum of a clean (1 x l) nonreconstructed TiO2(100) surface in normal-emission, excited with 47 eV photons. By heating the sample in U H V at T = 870 K, the (100) surface easily reconstructs from a (1 x 1) to a (1 x 3) phase as the result of thermal activation [13]. The energy distribution curve o f TiO2(100)-(1 x 3 ) (Fig. 2b) indicates a change in the valence band structure, but an identical width. It exhibits a similar Ti 3d b a n d gap emission with larger intensity just below E v to Ti 3d states from defective TiO2(110) surfaces [8]. In a similar fashion, the Ti 3d photoemission intensity is greatly enhanced at p h o t o n energies ca 47 eV. This high sensitivity results from a resonance between the optical absorption Ti 3p 6 3d"--+[Ti 3p s 3d "+ 1], followed by a s u p e r - C o s t e r - K r o n i g transition [ T i 3 p S 3 d " + q * - - + T i 3 p 6 3 d " t + e and the direct photoemission Ti 3p 6 3d"--+Ti 3p 6 3d" l + e , which both have the same final state Ti [3p 6 3d °] [15]. Work functions o f 5.9 and 5.4 eV have been determined for (1 x 1) and (1 x 3 ) reconstructed (100) surfaces, respectively, from the zero cut-off o f the valence band spectra [ 16]. In subsequent evaporation experiments we have studied the effect o f Pt on the valence band spectra of TiO2( 100)-( 1 x 3) for different p h o t o n energies,
i
-!
TiO,(100) lx3
~.~
\
(b)
-
(lOO) l x l hv = 4 7 e V
-lO 4
-5 BINDING ENERGY ~
o [eV]
Fig. 2. Typical energy-distribution curves of normal-emission photoelectron spectra of (a) TiO2(100)-( 1 x 1), (b) TiO2(100)-(l x3) and (c) Ti203(1012) surfaces. The latter spectrum is taken from Ref. [14] and is used for the determination of the Ti3~ surface concentration in Section 4.2. Ev denotes the valence band maximum.
K.-D. Schierbaum el a/. /Sur/ace Science 391 (1997) 196 203
3 0 < h v < 9 0 e V . Fig. 3 shows a series of energy distribution curves in the valence band range with increasing Pt coverage. The maximum nominal coverage O, that was determined from the AES spectrum, is ca 1.8 monolayers (ML) for curve (k).
EDC Pt + TiO2(100) l x 3
ltSdh,~T [1°°] e-
1
hv
J(/\
= 4 7 eV
-(i)
° ~
e-
.6
A( \ -(h)
L
>-
-(g)
m
Z LU I-Z
increasing Pt c o v e r a g e
-(e)
J~-(d)
"_T-(c) J~(a) I -30
i
I -20
valence band i I i -10
3d
clean surface
EF I 0
B I N D I N G E N E R G Y E F [eV] Fig. 3. Typical energy-distribution curves of normal-emission photoelectron spectra of a clean (1 x 3)-reconstructed TiO_~(100) before (a) and after subsequent thermal evaporation of Pt in the submono- to monolayer range (b k) taken at by= 80eV. Total Pt exposing times are: (b) 30 s, (c) 1.5 rain, (d) 3 min, (e) 6 min, (f) 9 rnin, (g) 12 min, (h) 20 rain and (i) 30 rain and (k) 40 min.
199
This determination is based on relative AES p e a ~ peak strengths, taking into account the different escape depths but neglecting matrix effects. This then justifies our error estimate on the coverage numbers. Since the filament current and hence the evaporation rate was kept constant during evaporation, we estimate values of O to range from <0.1 (curve b), 0.1 (c), 0.2 (d), 0.3 (e), 0.4 (f), 0.5 (g), 0.9 (h) and 1.4 (i). The Ti 3p intensity shows intensity fluctuations which are not simply attributable to attenuation by the overlayer and hence Pt coverages cannot be estimated from the Ti 3p damping. The energy distribution curves of Fig. 3 were determined with 80 eV photon energy (i.e. above resonance). We found an increasing emission from Pt 5d states between the TiO2 valence band maximum, Ev, and the Fermi energy, a strong increase of the secondary electron background, a weak shift of the O 2s state (<0.2 eV to smaller kinetic energies) and slight changes of the TiO: valence band features at the high-binding energy side. Longer exposing times (t_> 15 min) of the sample to the Pt evaporation source (i.e. large Pt coverages) also produced a weak emission at 11 eV. The latter may be attributed to small amounts of Ti OH adsorbates (EV= 10.7eV [17]), resulting from water adsorption from the residual gas of the UHV chamber. Spectra obtained with 30 eV photon energy showed a strong secondary background and are therefore not evaluated here. We have also recorded core level spectra of Ti 3p (E~,= 38.7 eV) and of the spin-orbit split Pt 4f core levels (EbV= -- 71.8 eV for j = 7 / 2 ) at higher photon energies, hv, of 90 and 120 eV. The Pt 4f core level spectra indicated duplets with broad (full width half maximum = 1.4 eV) spin-orbit split peaks (j =5/2 and 7/2) separated by 3.2 eV. They showed a shift of 0.6 eV to higher binding energies when compared with the Pt 4f states of Pt bulk metal(EV= 71.2eV for j = 7 / 2 1 1 8 ] ) . The effect of Pt on the Ti 3d band gap state was determined at a photon energy of 47 eV (i.e. with the maximum value of the atomic subshell photoionization cross-section of Ti 3d [ 19]). Corresponding valence band spectra are indicated in Fig. 4. We found that the intensity of the Ti 3d state is not significantly altered in the initial stages of
200
K.-D. Sdfierbaum eta/. / Sur/ace Science 391 (1997) 196-203
EDC Pt + TiO2(100) l x 3
Pt5d hv~l x"
u~~ / ] ~ I ~ (k)
e-
[100]
t
I
hv = 80 e V
.~.
(h) (g)
__.
(0
"~-:3e"
~
.6
Z
//I `o, increasing Ptcoverage
valence i -30
•
I -20
i
band I i
-10
F
clean
LEED images, that have been determined after each evaporation step, showed that the (100) surface does neither change its reconstruction nor displayed additional spots but did show an increase in background intensity.
surface
4. Discussion
4.1. Difference valence band spectra For the determination of the Pt-derived emission, difference valence spectra were calculated from clean and Pt coated TiO2(100) surfaces. We align all spectra to the binding energy of the O 2s core level (E~= - 2 2 . 5 eV) to account for the small changes in the kinetic energies of the O 2s photoelectrons. The latter may be attributed to a Pt-induced decrease in the electron affinity of TiO2, caused by a polarization of the Pt atoms. Typical results are shown in Fig. 5 which correspond to the spectrum (a) of the clean surface and (f) of the Pt covered ( 0 = 0 . 4 ML) surface taken from the Fig. 3. If we assume to a first approximation that the secondaries in the valence band region are produced only by the valence band photoelectrons (i.e. the background is sufficiently flat above and slightly below the valence band region) we may use the following background function for the inelastically scattered electrons (dotted lines)
[201: E~ g,
I
0
BINDING ENERGY E~ [eV]
Fig. 4. Typical energy-distribution curves of normal-emission photoelectron spectra of a clean (1 x3)-reconstructed TiO2(100) (a) before and (b k) after subsequent thermal evaporation of Pt taken at hv-47 eV.
interface formation (Fig. 4, curves b d). For higher values of the Pt coverage (curves i-k), an emission just below Ev is still observable, in contrast to the corresponding valence band spectra in Fig. 3. Evidently, there is a significant contribution of Ti 3d in this energy range even at higher Pt coverages.
Ib(EV)=A
i
v v. . . . [Itot(E~,)-Itot(Eb
)] d E v.
(2)
t) EbV.ma×
Here, /tot denotes the measured total photoemission intensity as a function of E~,, Ebv. . . . an energy above Ev where no signal contributes to the intensity and A is a constant, which is determined from the intensity slightly below the valence band (where the spectrum consists only of inelastically scattered electrons). Substraction of the background functions leads to energy distribution curves (a*) and (f*). We may then calculate the difference spectrum (f*-a*) and the difference spectrum (h* a*) from the corresponding spectrum (h) of Fig. 3 for larger Pt coverages (O =0.9 ML). We find negative inten-
P,s
K.ID. Schierbaum et aL / SurJace Science 391 (1997) 196 203
Pt clusters on Ag(110) after [20]
Pts
Pt~
(h* - a*)
(f* - a*)
(a)
30
(a*)-J I -30
,
i -20
i
I -10
L..... , 0 = EF
BINDING ENERGY E F [eV] Fig. 5. Background-substracted valence band spectra (hv= 80 eV) of Pt covered TiO2(100)-(l x 3) surfaces (a* and f*) and difference valence band spectra (f*-a*) and (h* a*). Further explanations are given in the text. sities in the TiO 2 valence band range between - 9 and - 4 eV (the latter value corresponds to the maximum valence band intensity). This finding may be attributed to a damping of the valence band emission rather than a change of the densityof-states in the TiO2 valence band since Pt atoms do not chemically interact with Ti and O surface
201
atoms [4,5]. Above - 4 e V and the onset of the Ti 3d band gap emission, the difference spectra show an emission that correspond to the widths and the separation of Pt 5d states in valence band difference spectra of small Pt, clusters on Ag(110) where n is the number of atoms per cluster [21]. These clusters have been deposited by a sizeselected assembly technique. In Ref. [21] valence band spectra are given for n = 1, 2, 3, 5, 10 and 15. Valence band spectra of Ptl, Pt5 and Pt15 are shown in the upper part of the Fig. 5. They show Pt 5d states at lower binding energy. If we assume that the escape depth of the emitting TiO2 valence band photoelectrons shows only negligible changes with kinetic energy and hence that the damping factor is constant over the entire valence band, we expect the Pt 5d state at - 3 eV slightly more intense than the observed peak. In conclusion, comparison of the difference spectra and the valence band spectra of Pt, clusters give evidence for the formation of a highly disperse Pt film with very small clusters of Pt on T i O z ( 1 0 0 ) - ( l x 3 ) under the experimental conditions (i.e a Volmer Weber growth of Pt). It also explains the absence of a Fermi edge in the valence band spectra which is expected for metallic Pt clusters. Further evidence of the formation of very small Pt clusters is given by core level spectra of the spin-orbit split Pt 4f states as well as the increasing L E E D background. They exhibit a shift of 0.6 eV in the absolute values to higher binding energies. This surface core level shift is consistent with the apparent absence of a Fermi edge at a binding energy of E ~ = 0 eV even at larger values of Opt. The latter is, however, aligned to the Fermi energy of the spectrometer. Heating the sample in U H V at T = 970 K and hence increasing the cluster sizes leads to metallic Pt 4f states as probed by the Pt 4f7/2 core level binding energy [22]. 4.2. Electronic structure and electronic charge transfer
The electronic structure of the d o metal oxide TiO 2 may be explained in terms of a simple oneelectron picture with a mainly O 2p derived valence band and a Ti 3d derived conduction band, with the band edges separated by a large band gap of
202
K.-D. Schierbaum et al. / Sur/ace Science 391 (1997) 196 203
3.1 eV [23]. A small concentration of electrically active donor-type bulk defects arise t¥om randomly distributed and noninteracting oxygen vacancies Vo [24]. They exhibit two ionization energies, E m = - 0 . 3 and ED2 = --0.5 eV, respectively, causing TiO2 to show n-type semiconducting properties with electrons as majority charge carriers. Temperature-dependent conductivity measurements on single crystals in UHV imply that the bulk states are singly occupied at room temperature (i.e. V o0 ~ V o+ + e - ) , thus leading to V(; as the predominant defect [24]. Typical bulk densities of donor states, ND, of slightly UHV reduced single crystals are in the order of 1016 cm 3 which adjusts the bulk position of the Fermi energy 0.25 eV below the conduction band minimum E c at T = 3 0 0 K [25]. Fig. 6a shows the electronic structure of TiO2, terminated with a (1 x 3) reconstructed (100) surface. Here, all binding energies are referred to the vacuum level, Eva c = 0. From the data presented in Fig. 2, we derive the surface position of the valence band maximum E v at -9.1 eV (i.e. 3.7 eV lower than Ev). Taking into account the bulk position E["~(eV)
®
® Ti 3 d
'Ti 3 d "s
_
EF ~ Ti 3 d ' I
_
ECEoi...::~___z,~Ev'-, _,.[_._ 1 ~ EL,~
I~Ti
Ev ~ ~ ~
-eAv,..l...:.: -,o
3d
~,.-eAV~
' 0 20
: o 2p .....
llool
TiO2(100) lx3
=" [110]
TiOa(110)
with surface defects
Fig. 6. Electronic structure and electronic charge transfer reactions of Pt adsorbed on (a) TiO2( 100)-(1 x 3) and (b) nonstoichiometric TiO2(ll0) surfaces. Binding energies E~bac are referred to the vacuum energy Evac. Further explanations are given in the text.
of Ev, we obtain a band bending - e A Vs of 0.8 eV for this surface. This - eA Vs-effect may result from a partial ionization of the 3d states of Ti 3 + surface ions. Their concentration may be estimated from an evaluation of the corresponding valence band spectra. After accounting for the different photoemission cross-sections of Ti 3d and O 2p atomic states [19] and normalizing these cross-sections using the valence band spectrum of Ti203(10i-2) depicted in Fig. 2c (which has a known Ti 3d 1 concentration) [14], we obtain the fraction, F = 0.14, of missing O atoms (i.e. 0.28 of Ti 3+) on the ( 1 x 3) reconstructed TiO:(100) surface [16]. This estimation is based on the measured integrated photoemission intensity ratio of Ti 3 d / O 2 p = 0.019. Here, 2 = 10 and 11 A are taken as escape depths of Ti 3d and O 2p photoelectrons, respectively. The value F is in good accordance with the geometric model of the (1 x 3 ) reconstructed TiO2(100) depicted in Fig. 1 despite the neglect of effects like the mixing of Ti 3d states with the O 2p valence band. In contrast to the behavior of nonstoichiometric (110) surfaces [10], we do not find an electronic charge transfer from Ti 3d states of TiO2 ( 1 0 0 ) - ( I x 3 ) to Pt. This may result from the differences of the work functions of both, the TiO2(100) l x 3 surface (~b=5.4eV) and the Pt clusters. Their value is certainly smaller than the value we obtained previously for Pt overlayers ((~Pt ~-- 5.6 eV [3]). For comparison, Fig. 6b exhibits the corresponding electronic valence band structure of the nonstoichiometric (i.e. sputtered) TiO2(ll0) surface derived from previous photoemission data. This surface shows a lower Ti 3d band gap photoemission intensity and a significant lower work function. A typical value is 4 eV. According to simple work function arguments, an electron transfer from Ti 3+ to Pt clusters is expected to occur in this case. This explains the observed drastic decrease of the Ti 3d band gap photoemission of the nonstoichiometric TiO2(ll0) surface covered with Pt. In addition, the initial nucleation density and hence the growth mode of the evaporated Pt may differ for the different surfaces. This will be the subject of future scanning tunneling experiments.
K.-D. Schierbaum et al. / SutJ~tce Science 391 (1997) 196 203
5. Conclusions The photoemission results indicate drastic differences for electronic charge transfer reactions of Pt overlayers on nonstoichiometric (110) and ( l x 3 ) reconstructed (100) surfaces of rutile TiO2, both displaying Ti 3d states below the Fermi energy in the band gap. In the case of nonstoichiometric TiO2(ll0), Ti 3d behave like donor-type localized states, leading to a low work function, ~b=4 eV. The presence of Pt clusters leads to their partial depopulation. Ti 3d states of corresponding states of (1 x3) reconstructed TiO2(100) cause an accumulation layer as well and a corresponding band bending. However, the work function (~b=5.4 eV) of this surface is reduced only slightly when compared with TiO2 (100)-(1 x 1). In conclusion, TiO2(100)-(1 x3) behaves like the stoichiometric TiO2(ll0) surface and does not show an electronic charge transfer to Pt atoms evaporated onto the surface.
Acknowledgements We gratefully acknowledge the assistance of Dr Christopher A. Muryn during beam time at Daresbury. One of the authors (K-D.S.) would like to thank the Fonds der Chemischen Industrie for financial support.
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Films, Springer Series in Surface Science, Vol. 33. Springer, Berlin (1993), p. 268. [4] H.-P. Steinrt~ck, F. Pesty, L. Zhang, Th.E. Madey, Phys. Rev. B. 51 (1995) 2427. [5] K.D. Schierbaum, S. Fischer, M.C. Torquemada, J.L. de Segovia, E. Roman, J.A. Martin-Gago, Surf. Sci. 345 (1996) 261. [6] K.D. Schierbaum, X. Wei-Xing, W. G6pel, Ber. Bunsenges. Phys. Chem. 97 (1993) 363. [7] S. Fischer, J.A. Martin-Gago, E. Romfin, K.D. Schierbaum, J.L. de Segovia, J. Electron Spectrosc. Relat. Phenom. 83 (1997) 217. [8] S. Fischer, F. Schneider, K.D. Schierbaum, Vacuum 47 (1996) 1149. [9] P.W. Murray, F.M. Leibsle, H.J. Fisher, C.F.J. Flipse, Ch.A. Muryn, G. Thornton, Phys. Rev. B 46 (1992) 12877. [10] P.W. Murray, F.M. Leibsle, H.J. Fisher, C.F.J. Flipse, Ch.A. Muryn, G. Thornton, Phys. Rev. Lett. 72 (1994) 689. [11] P. Zschak, J.B. Cohen, Y.W. Chung, Surf. Sci. 262 (1991) 395. [12] Y.W. Chung, W.J. Lo, G.A. Somorjai, Surf. Sci. 64 (1977) 588. [13] C.A. Muryn, P.J. Hardman, J.J. Crouch, G.N. Raiker, G. Thornton, Surf. Sci. 251/252 (1991) 747. [14] J.D. McKay, M.H. Mohamed, V.E. Henrich, Phys. Rev. B 35 (1987) 4304. [15] Z. Zhang, S.-P. Jeng, V.E. Henrich, Phys. Rev. B 43 (1991) 12004. [16] Ch.A. Muryn, Thesis, University of Manchester, Manchester, UK, 1991. [17] Ch.A. Muryn, P.J. Hardman, J.J. Crouch, G.N. Raiker, G. Thornton, Surf. Sci. 251/252 (1991) 747. [18] D. Briggs, in: D. Briggs, M.P. Seah (Eds.), Practical Surface Analysis Vol. I, Auger and X-ray Photoelectron Spectroscopy. Wiley, Chichester, 1990. [19] J.J. Yeh, I. Lindau, Atomic and Nuclear Data Tables 32 (1985) 1 155. [20] X. Li, Z. Zhang, V.E. Henrich, J. Electron Spectrosc. Relat. Phenom. 63 (1993) 253. [21] W.D. Schneider, Appl. Phys. A 59 (1994) 463. [22] S.B. DiCenzo, G.K. Wertheim, in: H. Haberland (Ed.), Clusters of Atoms and Molecules II, Vol. 56. SpringerVerlag, Berlin, 1994. [23] C.J. Kevan, Phys. Rev. A 133 (1964) 1431. [24] W. G6pel, G. Rocker, R. Feierabend, Phys. Rev. B 28 (1983) 3427. [25] U. Berner, Diploma Thesis, University of T0_bingen, T/ibingen, Germany, 1996.
Surface Science 391 (1997) 196 203
Electronic structure of Pt overlayers on ( 1 x 3) reconstructed TiO2(100) surfaces Klaus-Dieter Schierbaum a . , Stefan Fischer a Paul Wincott b Peter Hardman Vin Dhanak b, Graham Jones b, Geoff Thornton b
b
" Institute qfPhysical and Theoretical Chemistry, Ebo'hard-Karls Unirersi O, q/Tiibingen, A~!/'der Morgenstelle 8, 72076 Tfibingen, Germany b Chemistt T Department, Manchester University, Manchester, M13 9PL, UK
Received 11 April 1997; accepted for publication 13 June 1997
Abstract The effect of Pt atoms on the Ti 3d emission of Ti 3+ states at ( 1 x 3) reconstructed TiO2(100) surfaces was studied by means of valence band photoemission spectroscopy using synchrotron radiation with excitation energies between 30 and 120 eV. A clean (1 x 3) reconstructed TIO2(100) surface was prepared by Ar + sputtering and annealing at T= 870 K as checked with low energy electron diffraction. Subsequently, Pt was evaporated in situ using a resistively heated Pt filament. Evaporation was performed stepwise up to a nominal coverage of 1.8 monolayers (ML) as checked with Auger electron spectroscopy (AES). The Pt 4f core levels are shifted to higher binding energies with respect to Pt metal, indicating the presence of small Pt clusters. Difference spectra show a dublet between the TiO2 valence band maximum and the Fermi energy which is attributed to Pt 5d states. We found that the position and the intensity of the Ti 3d band gap emission were not significantly altered upon evaporation of Pt. In addition, the valence band maximum of TiO, remains unchanged. These findings indicate the absence of an electronic charge transfer due to the high work function of TiO2(100)-(1 x 3) of 5.4 eV, in line with a simple metal-semiconductor contact theory..~2 1997 Elsevier Science B.V. Keywords." Clusters; Low index single crystal surfaces; Metal semiconductor interfaces; Photoelectron spectroscopy; Platinum:
Surface defects; Titanium oxide
1. Introduction S t a b l e m e t a l c o n t a c t s on s e m i c o n d u c t i n g o x i d e s w i t h c o n t r o l l e d electrical p r o p e r t i e s are r e q u i r e d in a v a r i e t y o f d i f f e r e n t a p p l i c a t i o n s such as c h e m i cal sensors a n d p h o t o c a t a l y s i s etc. O n e e x a m p l e is the S c h o t t k y c o n t a c t f o r m e d by p o r o u s Pt e l e c t r o d e s o n r u t i l - t y p e TiO2 [1]. C o r r e s p o n d i n g
* Corresponding author. Fax: (+49) 7071 296910: e-mail: [email protected] 0039-6028/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved. PH S0039-6028 (97)00483-4
devices m a y be used as s o l i d - s t a t e sensors to m o n i t o r 0 2 in the gas p h a s e [1,2]. In this c o n t e x t , TiO2 single crystals with ( s u b ) m i c r o m e t e r thick m e t a l l i c o v e r l a y e r s h a v e been used as a m o d e l system to s t u d y the e l e c t r o n i c s t r u c t u r e o f P t / T i O 2 i n t e r f a c e s a n d the influence o f e l e c t r o n i c surface states o n c h a r g e t r a n s f e r r e a c t i o n s [3]. Pt d o e s n o t u n d e r g o s o l i d - s t a t e r e a c t i o n s at the interface, i n d i c a t i n g a h i g h t h e r m o d y n a m i c stability o f this system [4,5]. C u r r e n t - v o l t a g e ( ~ V ) c u r v e s i n d i c a t e a S c h o t t k y b a r r i e r h e i g h t ~bsB o f 1.5 eV a n d an ideality f a c t o r n o f o n e if P t / T i O 2 ( l l 0 )
K.-D. Schierbaum et al. / SurJace Science 391 (1997) 196 203
diodes are operated in ultra-high vacuum ( U H V ) at 500 K [6]. Evidently, Pt overlayers act as acceptots and trap free TiO 2 conduction band electrons. This causes a depletion layer in the TiO 2 and a band bending - e A Vs of 0.9 eV. The barrier height is ca 0.7 eV higher than theoretically expected from ~sB = ~Pt - ZTio2
( 1)
where ~bpt=5.6 eV denotes the work function of the Pt overlayer and ZTiO2=4.8 eV, the electron affinity of TiO2(ll0) [3]. Recently, valence band photoemission studies have been performed on Pt/TiO2( 110)-( l x 1 ) [5,7]. They reveal only a very small band bending ( - eA Vs< 0.2 eV) at initial stages of the interface formation. The high Schottky barrier height of Pt/TiO2(ll0) diodes may formally be explained by an additional dipole contribution AZ to the electron affinity of TiO2 at the interface [3], the origin of which is not clear yet. A transition from this "ideal" Schottky diode behavior to a surface-state controlled one with values of n > 1 occurs in the presence of oxygen molecules, that is, after diffusion of 02 through the polycrystalline overlayer and formation of negatively charged atomic and molecular oxygen species O or 0 2 at the interface [3,6]. These adsorbates act as acceptor-type "extrinsic" surface states, ldeality values n > 1 correspond to a shift of the ~ V curves towards larger positive voltages if Pt/TiO2(110) diodes are forward biased (i.e. the positive pole of the voltage supply is connected to the Pt contact). The applied voltage leads to a depopulation of surface states while the space charge and hence the barrier height remain almost unaffected. Depopulation of donor-type "intrinsic" surface defects at Pt/TiO2(110) interfaces by an electronic charge transfer to the metallic Pt have been recently confirmed by photoemission and constant final-state photoemission spectroscopy [8]. In these experiments, Ti 3d band gap states arise from lowcoordinated surface Ti 3 + ions that were produced on TiO2(ll0) by weak Ar + sputtering at low kinetic energies of 500 eV and a low sputter dose. This is accompanied with a drastic decrease of the work function ~bto a value of 4 eV [8]. Subsequent evaporation of Pt leads to a strong damping of the Ti 3d band gap photoemission. The constant
197
position of the TiO2 valence band maximum with respect to the Fermi level E v indicates that free TiO2 conduction electrons are not involved in the electronic charge transfer reaction. In this paper, we report on the electronic structure and the absence of charge transfer effects between Pt overlayers and the (1 x 3) reconstructed TiO2(100) surface. We have employed normal-emission photoelectron spectroscopy at different photon energies [9]. TiO2(100)-(1 x 3 ) has a relatively large work function of 5.4 eV which is a little higher than the value of TiO2 (110)-(1 x l) (~b=5.2eV). The TiO2(100)-(1 x 3 ) surface contains rows of adjacent Ti 3+ ions at the topmost positions (Fig. 1 ). The microfacet surface model follows from recent scanning tunneling microscopy results [10] as well as grazing incidence X-ray diffraction studies [11] and is consistent with previous low energy electron diffraction (LEED) experiments [12]. Photoemission spectra show Ti 3d band gap states. Their substantial reduction during chemisorption of O2 [i.e. by exposing to a dose of 3000 L (1 L = 1 0 - 6 T o r r ) without changing the (1 x 3) reconstruction] with an accompanying work function change from 5.4 to 5.6 eV even at room temperature gives additional evidence that these states are localized at the surface.
2. Experimental The photoemission experiments were run on the IRC in Surface Science station 4.1 at the CCLRC, Daresbury Laboratory, using a toroidal grating monochromator with photon energies between 30 and 120eV and a SCIENTA analyzer [13]. The resolution of the analyzer is < 50 meV. The beamline contribution was ca 100 meV at 47 eV and ca 200 meV at 80 eV. The overall resolution was determined from the Fermi edge of a clean Pt foil. Binding energies E~ are referenced to the Fermi energy. Details of the sample preparation were described previously in Ref. [13]. Pt overlayers were evaporated in situ from a thermally heated filament under UHV conditions at a base pressure in the 10-i0 mbar range. The filament current was kept constant during evaporation. Coverages of Pt
198
IC-D. Schierbaum et aL / Smjhce Science 391 (1997) 196 203
[om]t
I l -----[OLO1
Fig. 1. Surface geometric structure of the (1 × 3) reconstructed TiO2(100) surface. Small black circles, Ti subsurface atoms: small hatched circles, Ti surface atoms; large gray circles, O subsurface atoms; large hatched circles, O in-plane surface atoms: and large white circles, O bridging surface atoms.
were estimated from Auger electron spectra o f the Ti L M M and Pt M N N transitions.
e-
EDC
by
f[100]
3. Results Fig. 2a displays the valence band spectrum of a clean (1 x l) nonreconstructed TiO2(100) surface in normal-emission, excited with 47 eV photons. By heating the sample in U H V at T = 870 K, the (100) surface easily reconstructs from a (1 x 1) to a (1 x 3) phase as the result of thermal activation [13]. The energy distribution curve o f TiO2(100)-(1 x 3 ) (Fig. 2b) indicates a change in the valence band structure, but an identical width. It exhibits a similar Ti 3d b a n d gap emission with larger intensity just below E v to Ti 3d states from defective TiO2(110) surfaces [8]. In a similar fashion, the Ti 3d photoemission intensity is greatly enhanced at p h o t o n energies ca 47 eV. This high sensitivity results from a resonance between the optical absorption Ti 3p 6 3d"--+[Ti 3p s 3d "+ 1], followed by a s u p e r - C o s t e r - K r o n i g transition [ T i 3 p S 3 d " + q * - - + T i 3 p 6 3 d " t + e and the direct photoemission Ti 3p 6 3d"--+Ti 3p 6 3d" l + e , which both have the same final state Ti [3p 6 3d °] [15]. Work functions o f 5.9 and 5.4 eV have been determined for (1 x 1) and (1 x 3 ) reconstructed (100) surfaces, respectively, from the zero cut-off o f the valence band spectra [ 16]. In subsequent evaporation experiments we have studied the effect o f Pt on the valence band spectra of TiO2( 100)-( 1 x 3) for different p h o t o n energies,
i
-!
TiO,(100) lx3
~.~
\
(b)
-
(lOO) l x l hv = 4 7 e V
-lO 4
-5 BINDING ENERGY ~
o [eV]
Fig. 2. Typical energy-distribution curves of normal-emission photoelectron spectra of (a) TiO2(100)-( 1 x 1), (b) TiO2(100)-(l x3) and (c) Ti203(1012) surfaces. The latter spectrum is taken from Ref. [14] and is used for the determination of the Ti3~ surface concentration in Section 4.2. Ev denotes the valence band maximum.
K.-D. Schierbaum el a/. /Sur/ace Science 391 (1997) 196 203
3 0 < h v < 9 0 e V . Fig. 3 shows a series of energy distribution curves in the valence band range with increasing Pt coverage. The maximum nominal coverage O, that was determined from the AES spectrum, is ca 1.8 monolayers (ML) for curve (k).
EDC Pt + TiO2(100) l x 3
ltSdh,~T [1°°] e-
1
hv
J(/\
= 4 7 eV
-(i)
° ~
e-
.6
A( \ -(h)
L
>-
-(g)
m
Z LU I-Z
increasing Pt c o v e r a g e
-(e)
J~-(d)
"_T-(c) J~(a) I -30
i
I -20
valence band i I i -10
3d
clean surface
EF I 0
B I N D I N G E N E R G Y E F [eV] Fig. 3. Typical energy-distribution curves of normal-emission photoelectron spectra of a clean (1 x 3)-reconstructed TiO_~(100) before (a) and after subsequent thermal evaporation of Pt in the submono- to monolayer range (b k) taken at by= 80eV. Total Pt exposing times are: (b) 30 s, (c) 1.5 rain, (d) 3 min, (e) 6 min, (f) 9 rnin, (g) 12 min, (h) 20 rain and (i) 30 rain and (k) 40 min.
199
This determination is based on relative AES p e a ~ peak strengths, taking into account the different escape depths but neglecting matrix effects. This then justifies our error estimate on the coverage numbers. Since the filament current and hence the evaporation rate was kept constant during evaporation, we estimate values of O to range from <0.1 (curve b), 0.1 (c), 0.2 (d), 0.3 (e), 0.4 (f), 0.5 (g), 0.9 (h) and 1.4 (i). The Ti 3p intensity shows intensity fluctuations which are not simply attributable to attenuation by the overlayer and hence Pt coverages cannot be estimated from the Ti 3p damping. The energy distribution curves of Fig. 3 were determined with 80 eV photon energy (i.e. above resonance). We found an increasing emission from Pt 5d states between the TiO2 valence band maximum, Ev, and the Fermi energy, a strong increase of the secondary electron background, a weak shift of the O 2s state (<0.2 eV to smaller kinetic energies) and slight changes of the TiO: valence band features at the high-binding energy side. Longer exposing times (t_> 15 min) of the sample to the Pt evaporation source (i.e. large Pt coverages) also produced a weak emission at 11 eV. The latter may be attributed to small amounts of Ti OH adsorbates (EV= 10.7eV [17]), resulting from water adsorption from the residual gas of the UHV chamber. Spectra obtained with 30 eV photon energy showed a strong secondary background and are therefore not evaluated here. We have also recorded core level spectra of Ti 3p (E~,= 38.7 eV) and of the spin-orbit split Pt 4f core levels (EbV= -- 71.8 eV for j = 7 / 2 ) at higher photon energies, hv, of 90 and 120 eV. The Pt 4f core level spectra indicated duplets with broad (full width half maximum = 1.4 eV) spin-orbit split peaks (j =5/2 and 7/2) separated by 3.2 eV. They showed a shift of 0.6 eV to higher binding energies when compared with the Pt 4f states of Pt bulk metal(EV= 71.2eV for j = 7 / 2 1 1 8 ] ) . The effect of Pt on the Ti 3d band gap state was determined at a photon energy of 47 eV (i.e. with the maximum value of the atomic subshell photoionization cross-section of Ti 3d [ 19]). Corresponding valence band spectra are indicated in Fig. 4. We found that the intensity of the Ti 3d state is not significantly altered in the initial stages of
200
K.-D. Sdfierbaum eta/. / Sur/ace Science 391 (1997) 196-203
EDC Pt + TiO2(100) l x 3
Pt5d hv~l x"
u~~ / ] ~ I ~ (k)
e-
[100]
t
I
hv = 80 e V
.~.
(h) (g)
__.
(0
"~-:3e"
~
.6
Z
//I `o, increasing Ptcoverage
valence i -30
•
I -20
i
band I i
-10
F
clean
LEED images, that have been determined after each evaporation step, showed that the (100) surface does neither change its reconstruction nor displayed additional spots but did show an increase in background intensity.
surface
4. Discussion
4.1. Difference valence band spectra For the determination of the Pt-derived emission, difference valence spectra were calculated from clean and Pt coated TiO2(100) surfaces. We align all spectra to the binding energy of the O 2s core level (E~= - 2 2 . 5 eV) to account for the small changes in the kinetic energies of the O 2s photoelectrons. The latter may be attributed to a Pt-induced decrease in the electron affinity of TiO2, caused by a polarization of the Pt atoms. Typical results are shown in Fig. 5 which correspond to the spectrum (a) of the clean surface and (f) of the Pt covered ( 0 = 0 . 4 ML) surface taken from the Fig. 3. If we assume to a first approximation that the secondaries in the valence band region are produced only by the valence band photoelectrons (i.e. the background is sufficiently flat above and slightly below the valence band region) we may use the following background function for the inelastically scattered electrons (dotted lines)
[201: E~ g,
I
0
BINDING ENERGY E~ [eV]
Fig. 4. Typical energy-distribution curves of normal-emission photoelectron spectra of a clean (1 x3)-reconstructed TiO2(100) (a) before and (b k) after subsequent thermal evaporation of Pt taken at hv-47 eV.
interface formation (Fig. 4, curves b d). For higher values of the Pt coverage (curves i-k), an emission just below Ev is still observable, in contrast to the corresponding valence band spectra in Fig. 3. Evidently, there is a significant contribution of Ti 3d in this energy range even at higher Pt coverages.
Ib(EV)=A
i
v v. . . . [Itot(E~,)-Itot(Eb
)] d E v.
(2)
t) EbV.ma×
Here, /tot denotes the measured total photoemission intensity as a function of E~,, Ebv. . . . an energy above Ev where no signal contributes to the intensity and A is a constant, which is determined from the intensity slightly below the valence band (where the spectrum consists only of inelastically scattered electrons). Substraction of the background functions leads to energy distribution curves (a*) and (f*). We may then calculate the difference spectrum (f*-a*) and the difference spectrum (h* a*) from the corresponding spectrum (h) of Fig. 3 for larger Pt coverages (O =0.9 ML). We find negative inten-
P,s
K.ID. Schierbaum et aL / SurJace Science 391 (1997) 196 203
Pt clusters on Ag(110) after [20]
Pts
Pt~
(h* - a*)
(f* - a*)
(a)
30
(a*)-J I -30
,
i -20
i
I -10
L..... , 0 = EF
BINDING ENERGY E F [eV] Fig. 5. Background-substracted valence band spectra (hv= 80 eV) of Pt covered TiO2(100)-(l x 3) surfaces (a* and f*) and difference valence band spectra (f*-a*) and (h* a*). Further explanations are given in the text. sities in the TiO 2 valence band range between - 9 and - 4 eV (the latter value corresponds to the maximum valence band intensity). This finding may be attributed to a damping of the valence band emission rather than a change of the densityof-states in the TiO2 valence band since Pt atoms do not chemically interact with Ti and O surface
201
atoms [4,5]. Above - 4 e V and the onset of the Ti 3d band gap emission, the difference spectra show an emission that correspond to the widths and the separation of Pt 5d states in valence band difference spectra of small Pt, clusters on Ag(110) where n is the number of atoms per cluster [21]. These clusters have been deposited by a sizeselected assembly technique. In Ref. [21] valence band spectra are given for n = 1, 2, 3, 5, 10 and 15. Valence band spectra of Ptl, Pt5 and Pt15 are shown in the upper part of the Fig. 5. They show Pt 5d states at lower binding energy. If we assume that the escape depth of the emitting TiO2 valence band photoelectrons shows only negligible changes with kinetic energy and hence that the damping factor is constant over the entire valence band, we expect the Pt 5d state at - 3 eV slightly more intense than the observed peak. In conclusion, comparison of the difference spectra and the valence band spectra of Pt, clusters give evidence for the formation of a highly disperse Pt film with very small clusters of Pt on T i O z ( 1 0 0 ) - ( l x 3 ) under the experimental conditions (i.e a Volmer Weber growth of Pt). It also explains the absence of a Fermi edge in the valence band spectra which is expected for metallic Pt clusters. Further evidence of the formation of very small Pt clusters is given by core level spectra of the spin-orbit split Pt 4f states as well as the increasing L E E D background. They exhibit a shift of 0.6 eV in the absolute values to higher binding energies. This surface core level shift is consistent with the apparent absence of a Fermi edge at a binding energy of E ~ = 0 eV even at larger values of Opt. The latter is, however, aligned to the Fermi energy of the spectrometer. Heating the sample in U H V at T = 970 K and hence increasing the cluster sizes leads to metallic Pt 4f states as probed by the Pt 4f7/2 core level binding energy [22]. 4.2. Electronic structure and electronic charge transfer
The electronic structure of the d o metal oxide TiO 2 may be explained in terms of a simple oneelectron picture with a mainly O 2p derived valence band and a Ti 3d derived conduction band, with the band edges separated by a large band gap of
202
K.-D. Schierbaum et al. / Sur/ace Science 391 (1997) 196 203
3.1 eV [23]. A small concentration of electrically active donor-type bulk defects arise t¥om randomly distributed and noninteracting oxygen vacancies Vo [24]. They exhibit two ionization energies, E m = - 0 . 3 and ED2 = --0.5 eV, respectively, causing TiO2 to show n-type semiconducting properties with electrons as majority charge carriers. Temperature-dependent conductivity measurements on single crystals in UHV imply that the bulk states are singly occupied at room temperature (i.e. V o0 ~ V o+ + e - ) , thus leading to V(; as the predominant defect [24]. Typical bulk densities of donor states, ND, of slightly UHV reduced single crystals are in the order of 1016 cm 3 which adjusts the bulk position of the Fermi energy 0.25 eV below the conduction band minimum E c at T = 3 0 0 K [25]. Fig. 6a shows the electronic structure of TiO2, terminated with a (1 x 3) reconstructed (100) surface. Here, all binding energies are referred to the vacuum level, Eva c = 0. From the data presented in Fig. 2, we derive the surface position of the valence band maximum E v at -9.1 eV (i.e. 3.7 eV lower than Ev). Taking into account the bulk position E["~(eV)
®
® Ti 3 d
'Ti 3 d "s
_
EF ~ Ti 3 d ' I
_
ECEoi...::~___z,~Ev'-, _,.[_._ 1 ~ EL,~
I~Ti
Ev ~ ~ ~
-eAv,..l...:.: -,o
3d
~,.-eAV~
' 0 20
: o 2p .....
llool
TiO2(100) lx3
=" [110]
TiOa(110)
with surface defects
Fig. 6. Electronic structure and electronic charge transfer reactions of Pt adsorbed on (a) TiO2( 100)-(1 x 3) and (b) nonstoichiometric TiO2(ll0) surfaces. Binding energies E~bac are referred to the vacuum energy Evac. Further explanations are given in the text.
of Ev, we obtain a band bending - e A Vs of 0.8 eV for this surface. This - eA Vs-effect may result from a partial ionization of the 3d states of Ti 3 + surface ions. Their concentration may be estimated from an evaluation of the corresponding valence band spectra. After accounting for the different photoemission cross-sections of Ti 3d and O 2p atomic states [19] and normalizing these cross-sections using the valence band spectrum of Ti203(10i-2) depicted in Fig. 2c (which has a known Ti 3d 1 concentration) [14], we obtain the fraction, F = 0.14, of missing O atoms (i.e. 0.28 of Ti 3+) on the ( 1 x 3) reconstructed TiO:(100) surface [16]. This estimation is based on the measured integrated photoemission intensity ratio of Ti 3 d / O 2 p = 0.019. Here, 2 = 10 and 11 A are taken as escape depths of Ti 3d and O 2p photoelectrons, respectively. The value F is in good accordance with the geometric model of the (1 x 3 ) reconstructed TiO2(100) depicted in Fig. 1 despite the neglect of effects like the mixing of Ti 3d states with the O 2p valence band. In contrast to the behavior of nonstoichiometric (110) surfaces [10], we do not find an electronic charge transfer from Ti 3d states of TiO2 ( 1 0 0 ) - ( I x 3 ) to Pt. This may result from the differences of the work functions of both, the TiO2(100) l x 3 surface (~b=5.4eV) and the Pt clusters. Their value is certainly smaller than the value we obtained previously for Pt overlayers ((~Pt ~-- 5.6 eV [3]). For comparison, Fig. 6b exhibits the corresponding electronic valence band structure of the nonstoichiometric (i.e. sputtered) TiO2(ll0) surface derived from previous photoemission data. This surface shows a lower Ti 3d band gap photoemission intensity and a significant lower work function. A typical value is 4 eV. According to simple work function arguments, an electron transfer from Ti 3+ to Pt clusters is expected to occur in this case. This explains the observed drastic decrease of the Ti 3d band gap photoemission of the nonstoichiometric TiO2(ll0) surface covered with Pt. In addition, the initial nucleation density and hence the growth mode of the evaporated Pt may differ for the different surfaces. This will be the subject of future scanning tunneling experiments.
K.-D. Schierbaum et al. / SutJ~tce Science 391 (1997) 196 203
5. Conclusions The photoemission results indicate drastic differences for electronic charge transfer reactions of Pt overlayers on nonstoichiometric (110) and ( l x 3 ) reconstructed (100) surfaces of rutile TiO2, both displaying Ti 3d states below the Fermi energy in the band gap. In the case of nonstoichiometric TiO2(ll0), Ti 3d behave like donor-type localized states, leading to a low work function, ~b=4 eV. The presence of Pt clusters leads to their partial depopulation. Ti 3d states of corresponding states of (1 x3) reconstructed TiO2(100) cause an accumulation layer as well and a corresponding band bending. However, the work function (~b=5.4 eV) of this surface is reduced only slightly when compared with TiO2 (100)-(1 x 1). In conclusion, TiO2(100)-(1 x3) behaves like the stoichiometric TiO2(ll0) surface and does not show an electronic charge transfer to Pt atoms evaporated onto the surface.
Acknowledgements We gratefully acknowledge the assistance of Dr Christopher A. Muryn during beam time at Daresbury. One of the authors (K-D.S.) would like to thank the Fonds der Chemischen Industrie for financial support.
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