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Submonolayer-Pt on TiO2 (110) surfaces: electronic and geometric effects
B ELSEVIER
Sensors and Actuators B 31 (1996) 13-18
b~rfl"~--OR"
Submonolayer-Pt on TiO 2 (110) surfaces: electronic and geometric effects Stefan Fischer*, Klaus-Dieter Schierbaum, Wolfgang GOpel Institute of Physical and Theoretical Chemistry, Center of Interface Analysis and Sensors, University of Tiibingen, Auf der Morgenstelle 8, D-72076 Ttibingen, Germany
Abstract
For the fundamental understanding of processes that occur at metal/metal oxide interfaces it is helpful to reduce system parameters with the aid of model-system studies. Here we present an X-ray photoemission spectroscopy (XPS), ultraviolet photoemission spectroscopy (UPS), Auger electron spectroscopy (AES) and electron stimulated desorption (ESD) study of small Pt coverages on TiO2 (110) surfaces with respect to electronic and geometric properties of the metal/oxide interface.
Keywords: Oxide surfaces; Metal/oxide interfaces; TiO2 (110); Platinum
1. Introduction It is of particular interest in the field of chemical sensors and heterogeneous catalysis to improve the knowledge about interaction of metals with metal oxide surfaces as well as the interaction of these doped and undoped oxide surfaces with free molecules. In this study we focus on Pt dopants on well-ordered, thermodynamically stable, stoichiometric and electrostatically neutral TiO2 (110) surfaces which serve as a prerequisite of further improvement of oxide-based chemical sensors and oxidesupported noble metal catalysts. Three different electronic and geometric situations of the Pt/TiO 2 ( l l 0 ) interface may be adjusted. The 'Schottky state' is formed between geometrically ideal TiO2 ( l l 0 ) surfaces and Pt overlayers due to the high thermodynamic interface stability and due to a difference in the work functions of Pt ( ~ = 5.8 eV) and TiO 2 (110) ( ~ = 5.3 eV) [1]. The 'ohmic state' is characterized by Pt ions in the subsurface region thereby leading to electronic donor states in the band gap of TiO2. This diffusion layer links the electrically neutral Pt overlayer with the bulk oxide which leads to a decrease of the Debye length of electrons and hence causes narrow tunnelling barriers for electrons [2]. The 'SMSI state' of Pt/TiO2 (110) interfaces results from a localized charge transfer from Ti 3+ to * Corresponding author.
0925-4005/96/$15.00 © 1996 Elsevier Science S.A. All rights reserved SSDI 0 9 2 5 - 4 0 0 5 ( 9 5 ) 0 1 7 6 5 - 7
Pt atoms occupying oxygen lattice sites of the surface. It may be obtained by Pt doping of TiO 2 (110) surfaces with intrinsic point defects (i.e. oxygen vacancies). Strong metal/support interaction between Pt and TiO2 is an empirically well-known phenomenon and is found by hightemperature reduction of powder samples [3]. The three different situations and the pure TiO 2 (110) surface are schematically shown in Fig. 1. In this paper we focus on the initial stage of the formation of the Schottky state in order to study the electronic and geometric behaviour of Pt adatoms and small clusters on a well-defined oxide surface.
2. Experimental Using Mg Ka ( h v = 1253.6eV) and He I radiation (hv=21.21 eV), XPS and UPS experiments were performed in a combined high pressure/ultra-high vac/mm (UHV) system with details as reported previously [4]. The XPS spectra were obtained with a pass energy of 50 eV. In addition, X-ray induced Auger electron spectra (XAES) of Auger transitions were recorded in the XPS mode. Binding energies, E~, of photoelectrons are referred to the Fermi level. The AES and ESD experiments were carried out in an UHV system with a cylindrical mirror analyzer and a quadrupole mass spectrometer especially suited to detect and energetically analyze desorbed ions. Details have been published elsewhere [5].
14
S. Fischer et al. /Sensors and Actuators B 31 (1996) 13-18
6-fold coordinated Ti
5-fold CoordinatedTi
The TiO 2 (110) rutile samples were reduced in situ to prevent charging effects in electron spectroscopies and subsequently cleaned by A r + ion bombardment (500 eV, 0.5/~A). The surface stoichiometry was restored by heating the sample at 800 K in oxygen at a pressure of 10-5 mbar.
3. Results
"Ohmic state"
"Schottky diode state" [ I I(11
"SMSI state"
Fig. 1. Schematic presentation of the pure TiO2 (110) surface and three different kinds of interaction of Pt atoms with TiO2 (110) surfaces. The Schottky, ohmic and SMSI state show different electronic and catalytic properties. Further information is given in the text. Platinum was evaporated in situ from a resistively heated Pt filament (99.9%, Degussa) after several cleaning cycles in U H V at high temperatures T - 1900 K. Surface coverages, Opt, were determined from the attenuation o f the substrate-related XPS and A E S intensities and, for larger values Opt > 2 monolayers, by means of a quartz thickness monitor.
Typical X-ray photoemission spectra o f the O Is, Ti 2pa~lt2 ) core levels o f clean and Pt covered stoichiometric TiO2 (110) surfaces are shown, respectively, in Fig. 2. As also shown in Fig. 2, the O l s intensities are attenuated by the Pt overlayer. Since an additional O l s photoemission at E l = 533 eV is not found, significant concentrations of surface OH groups [6] or O atoms which are bound to reduced Ti 3÷ ions adjacent to oxygen vacancies can be neglected [7]. Correspondingly, we did not observe low-energy shoulders o f the Ti 2p3/2 core level o f Ti 4+ at E F = 459.4 eV which might result from Ti 3÷ at EbF = 458.2 eV and its final state at E~ = 455.9 eV [8]. In Fig. 3 the increasing Pt 4 f photoemission intensities and in addition a shift o f Pt 4fTr~5/2) core levels can be observed with increasing Pt coverages. These findings may result from final state effects in small particles with diameters o f r < 1 nm. The positive hole created by the photoemission from the Pt 4 f core level is screened over the particle which leads to a particle size-dependent binding energy o f the Pt 4 f core level higher than 71.2 eV, i.e. the value for bulk Pt [9]. Initial state effects due to a charge transfer from Pt to TiO2 can be neglected since the latter is not indicated in the corresponding ultraviolet photoemission spectra. After heating the system up to 970 K an additional shift of the Pt core levels towards metallic values occurs due to clustering effects. Ultraviolet photoemission spectra o f the clean and Pt covered TiO2 (110) surface indicate neither a considerable shift of the zero-cut off to lower kinetic energies due to a change in the work function ~ , nor a change in the TiO2 valence band maximum, Ev, at E l = 3 eV. The position o f Ev remains constant at low Pt coverages in the sub-monolayer region. The UP (HeI) spectra of the clean and Pt-covered TiO2 (110) surface are shown in Fig. 4. Band bending effects upon Pt adsorption, which might result from an electron transfer, can therefore be excluded to occur during initial stages o f the Pt/TiO2 interface formation. This is in contrast to the electron-acceptor properties o f metallic Pt overlayers which cause a band bending and, hence, lead to a Schottky-diode behaviour of Pt
Fig. 2. X-Ray photoemission spectra (i.e. the intensity of emitted photoelectrons, of the O Is core level, and spin-orbit coupled Ti 2p core levels (j= 1/2 and 3/2)) in arbitrary units versus binding energy, E~ of the clean TiO2 (110) surface (Opt=0) and for increasing Pt coverages, 0.12 _
15
S. Fischer et al. / Sensors and Actuators B 31 (1996) 13-18
l
PtlTi02(110)
XPS h
>FCO Z ELI I-Z
Iit"l
Iii v
Z
'
'
540
'
'
I
. . . .
535
•~
I
. . . .
530
I
470
525
465
"-
EbF [eV]
460
4.55
BINDING ENERGYEb r [eV]
Fig. 2.
u-
,.,13
I.z.
LU
Iii
z
Pt/TiO2(110)
XPS
O P1=2.48 ML
hu = 1253.6eV
n
z
80 "~-~
75
7o
80
E~ [eV]
75
--
Fig. 3.
E bF [ e V ]
7O
16
S. Fischer et al. /Sensors and Actuators B 31 (1996) 13-18
UPS(He')
~.t.~vTe
ionic surfaces in ESD experiments can be described by the Feibelman-Knotek mechanism [10,11]. Incident electrons create a hole in the 3p core level of the Ti atom. Auger neutralization of the hole occurs by a corresponding transition of one O 2p electron into the Ti 3p hole. The excess of energy in this process leads to the ejection of two of the remaining O 2p electrons into the vacuum yielding two surface ionic species, Ti 4+ and O ÷. The highly repulsive Coulumbian interaction causes the desorption of O + ions from the surface. Deconvolution of the kinetic energy distribution of the desorbed O + ions shows two signals at 4 eV and 7 eV, respectively. They are attributed to O + ions desorbing from the in-plane and from bridging sites [12,13]. Curve (a) corresponds to the clean TiO2 (110) surface and curve (b) is obtained at a Pt coverage of Opt = 0.22. It can be clearly seen that after evaporation of Pt the total intensity of O + ions is significantly decreased. Additionally a drastic change of the intensity ratio of the 4 and 7 eV peak can be observed.
Pt/TiO2(110)
hv = 21.2 eV
t-
"/'--970K )Pt
03 Z W I'Z
cll,e a n ~ 20 15 10 5 BINDING ENERGY EbF [eV] .
.
.
.
.
I
.
.
.
.
I
.
0
Fig. 4. Ultraviolet photoemission spectra (Hel) of TiO 2 (110) surfaces as a function of the Pt coverages, Opt. The spectrum at the top was obtained after heating the Pt/TiO 2 system in UHV to 970 K.
contacts on TiO2 (110) single crystals [2]. Under the conditions chosen here, the only electronic changes that could be observed with UPS concerned an additional increased emission from electronic states in the band gap region of TiO 2 with increasing Pt-coverage. In a second set of experiments, we have investigated both the ion yield and the ion energy distribution of electron-stimulated desorption (ESD) of O ÷ ions from TiO2 (110) surfaces after evaporation of Pt in the submonolayer range. Typical results are presented in Fig. 5. The Pt coverages have been determined by Auger electron spectroscopy. The creation of the O ÷ ions which desorb from
4. Discussion In the O ls and Ti 2p3/2c1/2) X-ray photoemission spectra we found different attenuation of the signal intensities with increasing Pt coverage. No additional peaks due to new O or Ti species occurred with increasing Pt coverage and after heating. The ratio of O ls and Ti 2p photoemission intensity versus Pt coverage is shown in Fig. 6. It can be clearly seen that with increasing Pt coverage the Ti intensity is much more attenuated compared with O ls intensity. After heating the system this ratio recovers almost the value of the starting point. From this result a preferential occupation of the 5-fold coordinated surface Ti sites with Pt atoms may be concluded. As deduced from ultraviolet photoemission spectra of Pt/TiO2, no change occurred in the work function due to localized dipoles at the Pt/TiO 2 interface (changes in
(a) 77,02 (110) 60
f~
0.75
(b) R/-no2 (110) 8~ = (322
Pt/TiO=(110)
o.. {"4
_Z-7-
40
¢,]
(b) " ~
After heating T= 970 K, t = 30 min
0.70
o
E0
of Ti 02( 11O) 0
0
2
4
S
8
10
t~
0.65
KINETIC ENERGY E~,n [eV] Fig. 5. Ion kinetic energy distribution curves of the electron stimulated O + desorption of the clean TiO 2 (110) surface (a) and of the Pt covered TiO 2 (110) surface with Opt = 0.22 monolayers (b). The incident electron energy is 60 eV.
0 Pt [ m o n o l a y e r ] Fig, 6. Ratio IO Is / ITi 2p of the XPS intensities of the O Is and Ti 2p core levels for an increasing Pt coverage Opt.
S. Fischer et al. / Sensors and Actuators B 31 (1996) 13-18
electron affinity). Also, no delocalized charge transfer occurred upon Pt evaporation (changes in band bending). The lack of a charge transfer reaction Ptad - e- ~ Pt+ad, which would donate electrons e- from adsorbed Pt atoms to the TiO2 (110) conduction band and would lead to an observable band bending, can be explained in a simple ionic bond picture since the electron affinity Z = 4.8 eV of TiO2 (110) is small if compared with the first ionization energy I 1 = 8.89 eV of Pt atoms [ 14]. The lack of a charge transfer in the opposite direction, which would indicate the beginning of the creation of a Schottky contact between Pt and TiO2 may be explained with a non-metallic state of the adsorbed Pt species in the submonolayer and monolayer regions. A second explanation takes into consideration that hydroxyl groups at the surface of TiO 2 may play an important role at the initial stage of the interface formation of Pt/TiO 2. Co-adsorption experiments with submonolayer-Pt and water on TiO2 (110) indicate a significant decrease of the work function up to 1.2 eV due to localized dipoles at the surface. These results will be discussed in more detail in a further publication [15]. After heating the TiO2 sample at temperatures T > 970 K for t = 30 min, the formation of Pt clusters at the surface occurs. This reaction is driven by the gain of PtPt bond energies. Under these conditions, the cluster formation is indicated by a shift of the Pt 4f binding energy towards the metallic value observed with XPS (cf. Fig. 3) and a more pronounced Pt like valence band region with a clear Fermi-edge observed with UPS (cf. Fig. 4). In agreement with the X-ray photoemission spectroscopy measurements, the ion yield and ion kinetic energy distribution curves of the ejected O + ions indicate that Pt atoms occupy preferentially 5-fold coordinated Ti sites at low coverages (Opt < 0.3). This may also be concluded from the fact that the in-plane oxygen signal is reduced by two-thirds of the initial value after Pt evaporation whereas the bridging oxygen signal is reduced only by a quarter due to a blocking effect of these in-plane oxygen atoms which are located in direct neighbourhood of the 5fold coordinated Ti atoms at the surface. The Pt coverages have been calculated from the attenuation of the O 1s signal and the rising Pt 4f signal, by taking into account that the Pt 4f core level is significantly more affected by the particular growth mode of Pt than the attenuation of the substrate-derived XPS signals. During the initial stage of interface formation, a twodimensional growth mode can be concluded from relative intensity changes of the substrate-derived XPS and AES signals. Under these conditions, a high concentration of individual Pt atoms may be present at the surface. This has also been proved theoretically with a model which describes the XPS intensities /Ptaf of the Pt 4f core levels as a function Pt coverage and Pt cluster size [16]. A drastic increase of the cluster size results obviously at temperatures T = 970 K due to an enhanced surface mobility of the Pt atoms. This finding indicates weak interaction
17
energies of Pt with TiO2 surface atoms at the adsorption sites when compared with the thermal energy k T .
5. Summary and outlook We found that Pt overlayers are thermodynamically stable at room temperature and do not induce the formation of Ti suboxides or the formation of surface oxygen vacancies. At submonolayer coverages, Pt atoms adsorb preferentially on top of five-fold coordinated Ti surface atoms. At these adsorption sites no localized charge transfer occurs, which would lead to the formation of surface dipole moments or charge transfer reactions (the latter would involve free conduction band electrons of TiO z Localized electronic states of Pt atoms occur in the band gap of TiO2. Chemical bonds between Pt and TiO2 surface atoms are not formed under these conditions. At temperatures T < 950 K, metallic Pt clusters are formed at the surface. In a next step STM/STS experiments will now be performed in arder to image the pure and Pt-doped TiO2 (110) surface. With this technique it will be possible to identify the geometric effects with atomic resolution and also identify the underlying electronic structure locally from the corresponding spectroscopic STS results.
Acknowledgements The authors would like to thank J.L. de Segovia, M.C. Torquemada, E. Roman and J.A. Martin-Gago for very helpful discussions in the field of ESD spectroscopy and support in performing ESD measurements.
References [1] H. Kobayashi, K. Kishimoto, Y. Nakato and H. Tsubomura, Mechanism of hydrogen sensing by Pd/TiO2 Schottky diodes, Sensors and Actuators B, 13-14 (1993) 125-127. [2l K.D. Schierbaum, Xu-Wei Xing, S. Fischer and W. GOpel, in: E. Umbach and H.J. Freund (eds.), Adsorption on Ordered Surfaces of Ionic Solids and Thin Films, Springer Series in Surface Science, Vol. 33, Springer-Vedag, Berlin, 1993, p. 268.
[3] S.J. Tauster, S.C. Fung, R.T.K. Baker and J.A. Horsley, Strong interactions in supported metal catalysts, Science, 211 (1981) 1121. [4] W. G6pel, Charge transfer reactions at semiconductor surfaces: implications to design gas sensors, Prog. Surf. Sci., 20 (1985) 9. [5] MC. Torquemada and J.L. de Segovia, Ion kinetic energy distribution of electron stimulated desorption of O+ from TiO2 (110)SO2, J. Vac. Sci. Technol. A, 12 (1994) 2318. [6] A. Pfau, K.D. Schierbaum and W. GOpel, The electronic structure of CeO2 thin films: the influence of Rh surface dopants, Surf.. Sci., 331-333 (1995) 1479. [7] G. Rocker and W. G6pel, Titanium overlayers on TiO2 (110), Surf. Sci., 181 (1987) 530-558. [8] V.E. Henrich, The shape of Ti2p core levels agrees with those obtained from Ti203 surfaces for which the presence of Ti2÷ states can be ruled out. Private communication. 19] M. Peuckert and H.P. Bonzel, Characterization of oxidized platinum surfaces by x-ray photoelectron spectroscopy, Surf.. Sci., 145 (1984) 239.
18
S. Fischer et al. / Sensors and Actuators B 31 (1996) 13-18
[10] R.J. Feibelman and M.L. Knotek, Reinterpretation of electronstimulated desorption data from chemisorption, Phys. Rev. B, 18 (1978) 6531. [11] M.L. Knotek and R.J. Feibelman, Ion desorption by core-hole auger decay, Phys. Rev. Lett., 40 (1978)964. [12] M.C. Torquemada, J.L. de Segovia, E. Rom~, G. Thornton, E.M. Williams and S.L. Bennet, in A.R. Burns, E.B. Stechel and D.R. Jennison (eds.), Desorption Induced by Electronic Transitions, DIET V, Springer Series in Surface Science, Vol. 31, SpringerVerlag, Berlin, 1993, p. 289. [13] D'Anx-Lax, Since I 1 of adsorbed Pt atoms is not known the value
of the free Pt atom is taken here as a first approximation, Taschenbuch flit Chemiker und Physiker, Bd. I11, Eigenschaften yon Atomen und Molekeln, Springer-Vedag, Berlin, 1970, p. 348. [14] S. Fischer, K.D. Schierbaum and W. G6pel, The influence of OHgroups on adsorption phenomena of Pt on TiO 2 (110) surfaces, unpublished material. [15] K.D. Schierbanm, S. Fischer, M.C. Torquemada, E. Roman, J.A. Martin-Gago and J.L. de Segovia, The interaction of Pt with TiO 2 (110) surfaces: a comparative SPS, UPS, ISS and ESD study, Su~ Sci. (1995) in press.
Sensors and Actuators B 31 (1996) 13-18
b~rfl"~--OR"
Submonolayer-Pt on TiO 2 (110) surfaces: electronic and geometric effects Stefan Fischer*, Klaus-Dieter Schierbaum, Wolfgang GOpel Institute of Physical and Theoretical Chemistry, Center of Interface Analysis and Sensors, University of Tiibingen, Auf der Morgenstelle 8, D-72076 Ttibingen, Germany
Abstract
For the fundamental understanding of processes that occur at metal/metal oxide interfaces it is helpful to reduce system parameters with the aid of model-system studies. Here we present an X-ray photoemission spectroscopy (XPS), ultraviolet photoemission spectroscopy (UPS), Auger electron spectroscopy (AES) and electron stimulated desorption (ESD) study of small Pt coverages on TiO2 (110) surfaces with respect to electronic and geometric properties of the metal/oxide interface.
Keywords: Oxide surfaces; Metal/oxide interfaces; TiO2 (110); Platinum
1. Introduction It is of particular interest in the field of chemical sensors and heterogeneous catalysis to improve the knowledge about interaction of metals with metal oxide surfaces as well as the interaction of these doped and undoped oxide surfaces with free molecules. In this study we focus on Pt dopants on well-ordered, thermodynamically stable, stoichiometric and electrostatically neutral TiO2 (110) surfaces which serve as a prerequisite of further improvement of oxide-based chemical sensors and oxidesupported noble metal catalysts. Three different electronic and geometric situations of the Pt/TiO 2 ( l l 0 ) interface may be adjusted. The 'Schottky state' is formed between geometrically ideal TiO2 ( l l 0 ) surfaces and Pt overlayers due to the high thermodynamic interface stability and due to a difference in the work functions of Pt ( ~ = 5.8 eV) and TiO 2 (110) ( ~ = 5.3 eV) [1]. The 'ohmic state' is characterized by Pt ions in the subsurface region thereby leading to electronic donor states in the band gap of TiO2. This diffusion layer links the electrically neutral Pt overlayer with the bulk oxide which leads to a decrease of the Debye length of electrons and hence causes narrow tunnelling barriers for electrons [2]. The 'SMSI state' of Pt/TiO2 (110) interfaces results from a localized charge transfer from Ti 3+ to * Corresponding author.
0925-4005/96/$15.00 © 1996 Elsevier Science S.A. All rights reserved SSDI 0 9 2 5 - 4 0 0 5 ( 9 5 ) 0 1 7 6 5 - 7
Pt atoms occupying oxygen lattice sites of the surface. It may be obtained by Pt doping of TiO 2 (110) surfaces with intrinsic point defects (i.e. oxygen vacancies). Strong metal/support interaction between Pt and TiO2 is an empirically well-known phenomenon and is found by hightemperature reduction of powder samples [3]. The three different situations and the pure TiO 2 (110) surface are schematically shown in Fig. 1. In this paper we focus on the initial stage of the formation of the Schottky state in order to study the electronic and geometric behaviour of Pt adatoms and small clusters on a well-defined oxide surface.
2. Experimental Using Mg Ka ( h v = 1253.6eV) and He I radiation (hv=21.21 eV), XPS and UPS experiments were performed in a combined high pressure/ultra-high vac/mm (UHV) system with details as reported previously [4]. The XPS spectra were obtained with a pass energy of 50 eV. In addition, X-ray induced Auger electron spectra (XAES) of Auger transitions were recorded in the XPS mode. Binding energies, E~, of photoelectrons are referred to the Fermi level. The AES and ESD experiments were carried out in an UHV system with a cylindrical mirror analyzer and a quadrupole mass spectrometer especially suited to detect and energetically analyze desorbed ions. Details have been published elsewhere [5].
14
S. Fischer et al. /Sensors and Actuators B 31 (1996) 13-18
6-fold coordinated Ti
5-fold CoordinatedTi
The TiO 2 (110) rutile samples were reduced in situ to prevent charging effects in electron spectroscopies and subsequently cleaned by A r + ion bombardment (500 eV, 0.5/~A). The surface stoichiometry was restored by heating the sample at 800 K in oxygen at a pressure of 10-5 mbar.
3. Results
"Ohmic state"
"Schottky diode state" [ I I(11
"SMSI state"
Fig. 1. Schematic presentation of the pure TiO2 (110) surface and three different kinds of interaction of Pt atoms with TiO2 (110) surfaces. The Schottky, ohmic and SMSI state show different electronic and catalytic properties. Further information is given in the text. Platinum was evaporated in situ from a resistively heated Pt filament (99.9%, Degussa) after several cleaning cycles in U H V at high temperatures T - 1900 K. Surface coverages, Opt, were determined from the attenuation o f the substrate-related XPS and A E S intensities and, for larger values Opt > 2 monolayers, by means of a quartz thickness monitor.
Typical X-ray photoemission spectra o f the O Is, Ti 2pa~lt2 ) core levels o f clean and Pt covered stoichiometric TiO2 (110) surfaces are shown, respectively, in Fig. 2. As also shown in Fig. 2, the O l s intensities are attenuated by the Pt overlayer. Since an additional O l s photoemission at E l = 533 eV is not found, significant concentrations of surface OH groups [6] or O atoms which are bound to reduced Ti 3÷ ions adjacent to oxygen vacancies can be neglected [7]. Correspondingly, we did not observe low-energy shoulders o f the Ti 2p3/2 core level o f Ti 4+ at E F = 459.4 eV which might result from Ti 3÷ at EbF = 458.2 eV and its final state at E~ = 455.9 eV [8]. In Fig. 3 the increasing Pt 4 f photoemission intensities and in addition a shift o f Pt 4fTr~5/2) core levels can be observed with increasing Pt coverages. These findings may result from final state effects in small particles with diameters o f r < 1 nm. The positive hole created by the photoemission from the Pt 4 f core level is screened over the particle which leads to a particle size-dependent binding energy o f the Pt 4 f core level higher than 71.2 eV, i.e. the value for bulk Pt [9]. Initial state effects due to a charge transfer from Pt to TiO2 can be neglected since the latter is not indicated in the corresponding ultraviolet photoemission spectra. After heating the system up to 970 K an additional shift of the Pt core levels towards metallic values occurs due to clustering effects. Ultraviolet photoemission spectra o f the clean and Pt covered TiO2 (110) surface indicate neither a considerable shift of the zero-cut off to lower kinetic energies due to a change in the work function ~ , nor a change in the TiO2 valence band maximum, Ev, at E l = 3 eV. The position o f Ev remains constant at low Pt coverages in the sub-monolayer region. The UP (HeI) spectra of the clean and Pt-covered TiO2 (110) surface are shown in Fig. 4. Band bending effects upon Pt adsorption, which might result from an electron transfer, can therefore be excluded to occur during initial stages o f the Pt/TiO2 interface formation. This is in contrast to the electron-acceptor properties o f metallic Pt overlayers which cause a band bending and, hence, lead to a Schottky-diode behaviour of Pt
Fig. 2. X-Ray photoemission spectra (i.e. the intensity of emitted photoelectrons, of the O Is core level, and spin-orbit coupled Ti 2p core levels (j= 1/2 and 3/2)) in arbitrary units versus binding energy, E~ of the clean TiO2 (110) surface (Opt=0) and for increasing Pt coverages, 0.12 _
15
S. Fischer et al. / Sensors and Actuators B 31 (1996) 13-18
l
PtlTi02(110)
XPS h
>FCO Z ELI I-Z
Iit"l
Iii v
Z
'
'
540
'
'
I
. . . .
535
•~
I
. . . .
530
I
470
525
465
"-
EbF [eV]
460
4.55
BINDING ENERGYEb r [eV]
Fig. 2.
u-
,.,13
I.z.
LU
Iii
z
Pt/TiO2(110)
XPS
O P1=2.48 ML
hu = 1253.6eV
n
z
80 "~-~
75
7o
80
E~ [eV]
75
--
Fig. 3.
E bF [ e V ]
7O
16
S. Fischer et al. /Sensors and Actuators B 31 (1996) 13-18
UPS(He')
~.t.~vTe
ionic surfaces in ESD experiments can be described by the Feibelman-Knotek mechanism [10,11]. Incident electrons create a hole in the 3p core level of the Ti atom. Auger neutralization of the hole occurs by a corresponding transition of one O 2p electron into the Ti 3p hole. The excess of energy in this process leads to the ejection of two of the remaining O 2p electrons into the vacuum yielding two surface ionic species, Ti 4+ and O ÷. The highly repulsive Coulumbian interaction causes the desorption of O + ions from the surface. Deconvolution of the kinetic energy distribution of the desorbed O + ions shows two signals at 4 eV and 7 eV, respectively. They are attributed to O + ions desorbing from the in-plane and from bridging sites [12,13]. Curve (a) corresponds to the clean TiO2 (110) surface and curve (b) is obtained at a Pt coverage of Opt = 0.22. It can be clearly seen that after evaporation of Pt the total intensity of O + ions is significantly decreased. Additionally a drastic change of the intensity ratio of the 4 and 7 eV peak can be observed.
Pt/TiO2(110)
hv = 21.2 eV
t-
"/'--970K )Pt
03 Z W I'Z
cll,e a n ~ 20 15 10 5 BINDING ENERGY EbF [eV] .
.
.
.
.
I
.
.
.
.
I
.
0
Fig. 4. Ultraviolet photoemission spectra (Hel) of TiO 2 (110) surfaces as a function of the Pt coverages, Opt. The spectrum at the top was obtained after heating the Pt/TiO 2 system in UHV to 970 K.
contacts on TiO2 (110) single crystals [2]. Under the conditions chosen here, the only electronic changes that could be observed with UPS concerned an additional increased emission from electronic states in the band gap region of TiO 2 with increasing Pt-coverage. In a second set of experiments, we have investigated both the ion yield and the ion energy distribution of electron-stimulated desorption (ESD) of O ÷ ions from TiO2 (110) surfaces after evaporation of Pt in the submonolayer range. Typical results are presented in Fig. 5. The Pt coverages have been determined by Auger electron spectroscopy. The creation of the O ÷ ions which desorb from
4. Discussion In the O ls and Ti 2p3/2c1/2) X-ray photoemission spectra we found different attenuation of the signal intensities with increasing Pt coverage. No additional peaks due to new O or Ti species occurred with increasing Pt coverage and after heating. The ratio of O ls and Ti 2p photoemission intensity versus Pt coverage is shown in Fig. 6. It can be clearly seen that with increasing Pt coverage the Ti intensity is much more attenuated compared with O ls intensity. After heating the system this ratio recovers almost the value of the starting point. From this result a preferential occupation of the 5-fold coordinated surface Ti sites with Pt atoms may be concluded. As deduced from ultraviolet photoemission spectra of Pt/TiO2, no change occurred in the work function due to localized dipoles at the Pt/TiO 2 interface (changes in
(a) 77,02 (110) 60
f~
0.75
(b) R/-no2 (110) 8~ = (322
Pt/TiO=(110)
o.. {"4
_Z-7-
40
¢,]
(b) " ~
After heating T= 970 K, t = 30 min
0.70
o
E0
of Ti 02( 11O) 0
0
2
4
S
8
10
t~
0.65
KINETIC ENERGY E~,n [eV] Fig. 5. Ion kinetic energy distribution curves of the electron stimulated O + desorption of the clean TiO 2 (110) surface (a) and of the Pt covered TiO 2 (110) surface with Opt = 0.22 monolayers (b). The incident electron energy is 60 eV.
0 Pt [ m o n o l a y e r ] Fig, 6. Ratio IO Is / ITi 2p of the XPS intensities of the O Is and Ti 2p core levels for an increasing Pt coverage Opt.
S. Fischer et al. / Sensors and Actuators B 31 (1996) 13-18
electron affinity). Also, no delocalized charge transfer occurred upon Pt evaporation (changes in band bending). The lack of a charge transfer reaction Ptad - e- ~ Pt+ad, which would donate electrons e- from adsorbed Pt atoms to the TiO2 (110) conduction band and would lead to an observable band bending, can be explained in a simple ionic bond picture since the electron affinity Z = 4.8 eV of TiO2 (110) is small if compared with the first ionization energy I 1 = 8.89 eV of Pt atoms [ 14]. The lack of a charge transfer in the opposite direction, which would indicate the beginning of the creation of a Schottky contact between Pt and TiO2 may be explained with a non-metallic state of the adsorbed Pt species in the submonolayer and monolayer regions. A second explanation takes into consideration that hydroxyl groups at the surface of TiO 2 may play an important role at the initial stage of the interface formation of Pt/TiO 2. Co-adsorption experiments with submonolayer-Pt and water on TiO2 (110) indicate a significant decrease of the work function up to 1.2 eV due to localized dipoles at the surface. These results will be discussed in more detail in a further publication [15]. After heating the TiO2 sample at temperatures T > 970 K for t = 30 min, the formation of Pt clusters at the surface occurs. This reaction is driven by the gain of PtPt bond energies. Under these conditions, the cluster formation is indicated by a shift of the Pt 4f binding energy towards the metallic value observed with XPS (cf. Fig. 3) and a more pronounced Pt like valence band region with a clear Fermi-edge observed with UPS (cf. Fig. 4). In agreement with the X-ray photoemission spectroscopy measurements, the ion yield and ion kinetic energy distribution curves of the ejected O + ions indicate that Pt atoms occupy preferentially 5-fold coordinated Ti sites at low coverages (Opt < 0.3). This may also be concluded from the fact that the in-plane oxygen signal is reduced by two-thirds of the initial value after Pt evaporation whereas the bridging oxygen signal is reduced only by a quarter due to a blocking effect of these in-plane oxygen atoms which are located in direct neighbourhood of the 5fold coordinated Ti atoms at the surface. The Pt coverages have been calculated from the attenuation of the O 1s signal and the rising Pt 4f signal, by taking into account that the Pt 4f core level is significantly more affected by the particular growth mode of Pt than the attenuation of the substrate-derived XPS signals. During the initial stage of interface formation, a twodimensional growth mode can be concluded from relative intensity changes of the substrate-derived XPS and AES signals. Under these conditions, a high concentration of individual Pt atoms may be present at the surface. This has also been proved theoretically with a model which describes the XPS intensities /Ptaf of the Pt 4f core levels as a function Pt coverage and Pt cluster size [16]. A drastic increase of the cluster size results obviously at temperatures T = 970 K due to an enhanced surface mobility of the Pt atoms. This finding indicates weak interaction
17
energies of Pt with TiO2 surface atoms at the adsorption sites when compared with the thermal energy k T .
5. Summary and outlook We found that Pt overlayers are thermodynamically stable at room temperature and do not induce the formation of Ti suboxides or the formation of surface oxygen vacancies. At submonolayer coverages, Pt atoms adsorb preferentially on top of five-fold coordinated Ti surface atoms. At these adsorption sites no localized charge transfer occurs, which would lead to the formation of surface dipole moments or charge transfer reactions (the latter would involve free conduction band electrons of TiO z Localized electronic states of Pt atoms occur in the band gap of TiO2. Chemical bonds between Pt and TiO2 surface atoms are not formed under these conditions. At temperatures T < 950 K, metallic Pt clusters are formed at the surface. In a next step STM/STS experiments will now be performed in arder to image the pure and Pt-doped TiO2 (110) surface. With this technique it will be possible to identify the geometric effects with atomic resolution and also identify the underlying electronic structure locally from the corresponding spectroscopic STS results.
Acknowledgements The authors would like to thank J.L. de Segovia, M.C. Torquemada, E. Roman and J.A. Martin-Gago for very helpful discussions in the field of ESD spectroscopy and support in performing ESD measurements.
References [1] H. Kobayashi, K. Kishimoto, Y. Nakato and H. Tsubomura, Mechanism of hydrogen sensing by Pd/TiO2 Schottky diodes, Sensors and Actuators B, 13-14 (1993) 125-127. [2l K.D. Schierbaum, Xu-Wei Xing, S. Fischer and W. GOpel, in: E. Umbach and H.J. Freund (eds.), Adsorption on Ordered Surfaces of Ionic Solids and Thin Films, Springer Series in Surface Science, Vol. 33, Springer-Vedag, Berlin, 1993, p. 268.
[3] S.J. Tauster, S.C. Fung, R.T.K. Baker and J.A. Horsley, Strong interactions in supported metal catalysts, Science, 211 (1981) 1121. [4] W. G6pel, Charge transfer reactions at semiconductor surfaces: implications to design gas sensors, Prog. Surf. Sci., 20 (1985) 9. [5] MC. Torquemada and J.L. de Segovia, Ion kinetic energy distribution of electron stimulated desorption of O+ from TiO2 (110)SO2, J. Vac. Sci. Technol. A, 12 (1994) 2318. [6] A. Pfau, K.D. Schierbaum and W. GOpel, The electronic structure of CeO2 thin films: the influence of Rh surface dopants, Surf.. Sci., 331-333 (1995) 1479. [7] G. Rocker and W. G6pel, Titanium overlayers on TiO2 (110), Surf. Sci., 181 (1987) 530-558. [8] V.E. Henrich, The shape of Ti2p core levels agrees with those obtained from Ti203 surfaces for which the presence of Ti2÷ states can be ruled out. Private communication. 19] M. Peuckert and H.P. Bonzel, Characterization of oxidized platinum surfaces by x-ray photoelectron spectroscopy, Surf.. Sci., 145 (1984) 239.
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[10] R.J. Feibelman and M.L. Knotek, Reinterpretation of electronstimulated desorption data from chemisorption, Phys. Rev. B, 18 (1978) 6531. [11] M.L. Knotek and R.J. Feibelman, Ion desorption by core-hole auger decay, Phys. Rev. Lett., 40 (1978)964. [12] M.C. Torquemada, J.L. de Segovia, E. Rom~, G. Thornton, E.M. Williams and S.L. Bennet, in A.R. Burns, E.B. Stechel and D.R. Jennison (eds.), Desorption Induced by Electronic Transitions, DIET V, Springer Series in Surface Science, Vol. 31, SpringerVerlag, Berlin, 1993, p. 289. [13] D'Anx-Lax, Since I 1 of adsorbed Pt atoms is not known the value
of the free Pt atom is taken here as a first approximation, Taschenbuch flit Chemiker und Physiker, Bd. I11, Eigenschaften yon Atomen und Molekeln, Springer-Vedag, Berlin, 1970, p. 348. [14] S. Fischer, K.D. Schierbaum and W. G6pel, The influence of OHgroups on adsorption phenomena of Pt on TiO 2 (110) surfaces, unpublished material. [15] K.D. Schierbanm, S. Fischer, M.C. Torquemada, E. Roman, J.A. Martin-Gago and J.L. de Segovia, The interaction of Pt with TiO 2 (110) surfaces: a comparative SPS, UPS, ISS and ESD study, Su~ Sci. (1995) in press.