J . Phys. Chem. 1985,89, 5025-5028
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An XPS Investigation of Ti02 Thin Films on Polycrystalline Pt C. M. Greenlief, J. M. White,* Department of Chemistry, University of Texas at Austin, Austin, Texas 78712
C.S. KO,and R. J. Gorte Department of Chemical Engineering, University of Pennsylvania, Philadelphia, Pennsylvania 19104 (Received: April 19, 1985)
X-ray photoelectron spectroscopy has been used to characterize Ti02 overlayers on polycrystalline Pt. Vacuum annealing of these thin films had two significant effects: (1) most of the Ti4+was reduced to Ti3+ and (2) an equilibrium coverage of the reduced species was reached for initial coverages of TiOz greater than about 0.9 monolayer. The O/Ti ratio after annealing was between 1.0 and 1.5. The reduced titania species present after annealing had a stoichiometry of T i 0 with titanium in a 3+ oxidation state. A shift in the Pt(4f7/2)peak to higher binding energy is interpreted as a Ti-Pt bonding interaction. Depth profiles of annealed and nonannealed surfaces indicate that the reduced titania species diffuses into the Pt.
Introduction Since the early work of Tauster and co-workers,' the study of the underlying reasons for strong metal-support interactions (SMSI) has been an active area of research. Several different explanations have been proposed. These include changes in particle morphology,2 charge transfer between the support and the metal,3 and the encapsulation of the metal by the support.@ Experiments carried out on powder catalysts have failed to unambiguously determine which of the possible explanations are responsible for SMSI behavior. So that the possible reason (or reasons) for SMSI could be elucidated, thin film model catalysts, prepared by vapor deposition of a metal onto a well-characterized oxide substrate in ultra-high vacuum (UHV), have been used. Belton et al.,'*1° using static secondary ion mass spectroscopy (SSIMS), demonstrated that encapsulation of Pt and Rh occurred when a TiO, ( x = 1) species was present. Sadeghi and Henrich* also saw evidence for the migration of a TiO, species in the Rh/TiOz system. The interaction of submonolayer amounts of TiOz on Pt has recently been investigated by Dwyer and co-workers." By combining low energy ion scattering spectroscopy and CO thermal desorption spectroscopy, they showed that simple site blocking of Pt sites by Ti02 played a dominant role. KO and G ~ r t e ' ~also , ' ~studied T i 0 2 deposited on bulk Pt and observed (by AES at the annealing temperature) the diffusion of a TiO, species into bulk Pt at temperatures greater than 700 K. Above 1100 K no titania species were detected within the AES sampling volume; however, the TiO, species reappeared on the (1) (a) S. J. Tauster, S. C. Fung, and R. L. Garten, J . Am. Chem. SOC., 100, 170 (1978). (b) S. J. Tauster and S. C. Fung, J . Catal., 55,29 (1978). (2) (a) R. T. K. Baker, E. B. Prestridge, and R. L. Garten, J. Catal., 56, 390 (1979). (b) R. T. K. Baker, E. B. Prestridge, and R. L. Garten, J . Catal., 59 293 (1979). (3) J. A. Horsley, J . Am. Chem. SOC.,101, 2870 (1979). (4) P. Meriaudeau, J. F. Dutel, M. Dufaux, and C. Naccache, in
'Metal-Support and Metal-Additive Effects in Catalysis", B. Imelik et al., Ed., Elsevier, Amsterdam, 1982, p 95. (5) J. Santos, J. Phillips, and J. A. Dumesic, J . Catal., 81, 147 (1983). (6) D. E. Resasco and G. L. Haller, J. Catal., 82, 279 (1983). (7) D. N. Belton, Y.-M. Sun, and J. M. White, J . Am. Chem. SOC.,106, 3059 (1984). (8) H. R. Sadeghi and V. E. Henrich, J . Card., 87, 279 (1984). (9) S. Takatani and Y. W. Chung, Appl. Surf. Sci., 19, 341 (1984). (10) D. N. Belton, Y.-M. Sun, and J. M. White, J. Phys. Chem., 88, 5172 ( 1984). (1 1) D. J. Dwyer, S. D. Cameron, and J. L. Gland, submitted J. Vac. Sci.
Technol. (12) C. S . KO and R. J. Gorte, J . Catal., 90 59 (1984). (13) C. S . KO and R. J. Gorte, Surf. Sci., 155, 296 (1985).
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Pt surface upon cooling to 300 K. They also demonstrated that torr of O2removed the TiO, heating the sample to 1100 K in species from the surface. Vannice and Sudhaker14 and Demmin et further showed that a Ti0,-covered Pt surface exhibited a methanation activity which was identical with actual titaniasupported Pt catalysts. To further characterize this reversible diffusion process, we have investigated thin films of TiOz on polycrystalline Pt using X-ray photoelectron spectroscopy (XPS) combined with vacuum annealing and depth profiling.
Experimental Section The experiments were performed in a VG ESCALAB spectrometer which was equipped with a hemispherical electron energy analyzer. The base pressure in both the preparation and spectrometer chambers was in the low 1O-Io torr range. The system was equipped with a dual X-ray anode for XPS, an ion gun for sputtering, and a quadrupole mass spectrometer (QMS). The Pt sample was spotwelded to a specially designed sample holder which was moved from the preparation chamber to the spectrometer chamber with the VG sample transfer system. In the preparation chamber, the sample was placed into a receptacle that made electrical and thermocouple connections to the Pt. Resistive heating to 1300 K was easily achieved and temperatures were monitored with a chromel-alumel thermocouple spotwelded to the back of the Pt foil. The sample was not heated during XPS analysis (X-ray beam absorption heated the sample by no more than 10 K). The Pt sample was cleaned by successive ion bombardment, oxidation, reduction, and annealing cycles until there were no detectable impurities as measured by XPS. Ti was vapor deposited onto the Pt foil (the latter at 300 K) by resistively heating a Ti-Ta alloy filament located =7 cm from and in line-of-sight of the sample surface. To assist the oxidation of Ti, the sample was exposed to torr of O2 during the Ti deposition. After deposition, the sample was held at 650 K in lo-' torr of O2 for 30 min. The thickness of the oxide overlayer was estimated from the attenuation of the Pt(4f7/,) XPS peak by assuming a mean free path of 13 XPS spectra were taken by using Mg Koc X-rays (1 253.6 eV) and the anode was operated at 280 W power. The hemispherical analyzer was operated in the constant analyzer energy mode with a fixed pass energy of 20 eV. The reported binding energies (BE) (14) M. A. Vannice and C. Sudhakar, J . Phys. Chem., 88, 2429 (1984). (15) R. A. Demmin, C. S.KO,and R. J. Gorte, J. Phys. Chem., 89, 1151 (1985). (16) M. P. Seah and W. A. Tench, Surf. Interface Anal., 1, 2 (1979).
0 1985 American Chemical Society
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Greenlief et al.
The Journal of Physical Chemistry, Vol. 89, No. 23, 1985
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b
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Binding Energy ( e V )
Figure 1. Ti(2p) XPS region as a function of TiOl coverage. T h e coverages shown are (a) 0.5, (b) 1.1, (c) 1.5, (d) 2.0, (e) 3.1, (f) 4.2, and (g) 4.8 monolayers. ii
are referenced to the Fermi level. The Pt(4f7/2)BE for the clean Pt surface was 70.7 eV. Depth profiles were performed with a defocussed 2-kV, 800-nA, Ar+ ion beam. 450
Results Figure 1 shows a family of XPS curves for various TiOz coverages. At coverages greater than 3 monolayers (1 monolayer is assumed to be an evenly dispersed film 2.6 8, thick), the measured binding energy of 458.9 eV for the Ti(2p3,,) peak is in excellent agreement with the previously published res~lts.~’-’~ For coverages less than 2 monolayers, there is a significant shoulder (assigned to Ti3+)on the low binding energy side of the T i ( 2 ~ , , ~ ) XPS peak. Exposure of these surfaces to additional O2 at 650 K failed to fully oxidize the Ti3+. At higher coverages, however, the Ti3+ was easily oxidized as indicated by the narrowing of the full-width half-maxima and by the loss of Ti3+ intensity. With increasing coverage, the Ti(2p3,,) BE increases. This is attributed to reduced core hole screening by the Pt substrate. After the TiOZ overlayer had been characterized, either an annealing or a depth profiling experiment was performed. A typical annealing experiment involved heating to 1300 K under vacuum, cooling to 300 K, and XPS analysis. 1300 K was chosen as an annealing temperature because at this temperature no surface titania species rernain.l2J3 In a typical annealing experiment (Figure 2A), the estimated initial T i 0 2 coverage was 4.8 monolayers. Annealing to 1300 K for 15 min (and cooling to 300 K) reduced the total Ti(2p) intensity and increased the Ti3+intensity (456.2 eV). The description of oxygen from single crystal and powdered Ti02 is typical at these temperatureszhz3 and is probably the major pathway for the (17) J. A. Schreifels, D. N. Belton, and J. M. White, Chem. Phys. Lett., 90, 261 (1982). (18) C. D. Wagner, W. M. Riggs, L. E. Davis, J. F. Moulder, and G. E. Mullenberg, ‘Handbook of X-ray Photoelectron Spectroscopy”, Physical Electronics Industries, Inc., Eden Prairie, M N , 1979, p 68. (19) M. Murata, K. Wakino, and S. Ikeda, J. Electron Spectrosc., 10, 359 (1977).
460
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Binding Energy ( e V )
Figure 2. (A) The Ti(2p) XPS region for 4.8 monolayers of T i 0 2 on Pt, (i) before annealing and (ii) after annealing. The coverage after annealing is 0.6 monolayer. (B) The Ti(2p) XPS region for 1.2 monolayers of T i 0 2 on Pt, (i) before annealing and (ii) after annealing. The coverage after annealing is 0.8 monolayer.
reduction of Ti4+,particularly since 0, desorption from Pt occurs below 1000 K.24,25 For any initial TiO, coverage greater than 1 monolayer, the total amount of Ti present on the surface after annealing and cooling was consistently between 0.6 and 0.9 monolayer, indicating an equilibrium coverage of reduced Ti. This confirms the AES results of KOand G ~ r t e . ’ ~A. ’similar ~ annealing experiment (Figure 2B) with an initial Ti02 coverage of 1.2 monolayers shows the same general features. In this case the Ti3+ peak after annealing is particularly pronounced. For any TiO, coverage greater than 0.9 monolayer, the effect of annealing is generally the same. For coverages less than about 0.6 monolayer a reduced Ti species appeared upon annealing but there was no measurable decrease of the total Ti(2p) XPS intensity. The addition of Ti02 to the Pt surface (before annealing) shifted the Pt(4f) BE’S to higher values (Figure 3). The shift increased (20) B. A. Sexton, A. E. Hughes, and K. Foger, J . Catal., 77, 8 5 (1982).
(21) M. A. Vannice, P. Odier, M. Bujor, and J. J. Fripiat, presented at the 188th National Meeting of the American Chemical Society, Philadelphia, PA, August, 1984. (22) D. D. Beck, J. M. White, and C. T. Ratcliffe, submitted to J . P h i s . Chem. (23) D. D. Beck, Ph.D. Dissertation, University of Texas, Austin, TX, 1985. (24) S.-K. Shi, J. M. White, and R. L. Hance, J . Phys. Chern., 84, 2441 (1980). (25) J. L. Gland, Surf. Sci., 93 487 (1980).
The Journal of Physical Chemistry, Vol. 89, No. 23, 1985 5027
XPS of Thin Films on Polycrystalline Pt
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(Current)(Time) 1 0 4 nA-sac
Figure 4. Depth profile of a nonannealed 1.O-monolayer TiO, overlayer
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Binding E n e r g y ( e V )
Figure 3. The Pt(4f) XPS region (a) for the clean surface, (b) with a TiOs overlayer of 4.8 monolayers, and (c) after annealing to 1300 K.
with Ti02coverage to a maximum of 0.25 eV at 1 monolayer and was constant for higher coverages (not shown). Figure 3 also shows the Pt(4f) region after annealing (lower spectra). Even after annealing, the Pt peaks remain shifted to higher binding energies. The increase in binding energy is most likely due to an initial state effect that involves Pt-TiO, bonding interactions (Le. some charge redistribution in the Pt in the presence of the Ti02 overlayer). Reduced final state screening of the photoelectrons could also cause a similar increase in the binding energy but this is considered unlikely because the presence of submonolayer amounts of a TiO, (1 .O I x I 2.0) species cannot significantly reduce the screening of Pt core holes several atomic layers beneath the interface. A similar problem must be confronted for the initial state interpretation we propose here. However, alteration of the electronic structure near the Fermi level can occur over longer distances than the effects of core hole screening. The O(1s) binding energy for the initial T i 0 2 overlayer was 530.3 eV. Upon annealing, it shifted down to 529.9 eV. The decrease is attributable to an increase in the surrounding electron density when Ti4+is replaced by reduced Ti and Pt atoms, resulting in increased final state screening of the O(1s) core holes. The O/Ti ratio (as determined by XPS peak areas) was stoichiometric (2.0) for initial TiO, overlayers thicker than 2 monolayers. Upon annealing the ratio dropped to between 1.O and 1.5 and a mixture of Ti3+and Ti4+ was observed. The area for each oxidation state of Ti was also determined. If we assume that two oxygen atoms are associated with each of the remaining Ti4+ ions, the O/Ti ratio for the reduced species is 1. The O/Ti ratio determined by Ti oxidation states agreed to within 5% of the ratio determined by the total Ti and 0 areas. Several annealing experiments were performed at 1050 K for the same length of time (1 5 min) as the 1300 K annealing experiments. Reduction of Ti4+was not as extensive as at 1300 K as expected for a strongly activated process. The depth profile for a nonannealed 1 monolayer TiO, overlayer is summarized in Figure 4. The initial flat region (Ti02overlayer) followed by a sharp change in slope and a steady decrease after 2 X lo4 nA s indicates that the overlayer has not diffused into the Pt. The second curve in Figure 4 shows the results of a depth profile on an annealed sample. The initial TiO, coverage was 1.5
(solid circles). The solid triangles indicate the Ti/Pt ratio during a depth profile of an annealed TiO, overlayer. The coverage before annealing was 1 . 5 monolayers.
monolayers and after annealing the coverage of the TiO, species was 0.9 monolayer. The concentration of titania is relatively constant until about 9 X lo4 nA s, after which the concentration drops more gradually as compared to the nonannealed case.
Discussion It is interesting to consider the possible reasons for the shift in the Pt binding energies. Since Pt-Ti alloys are well-known,26 it is not unreasonable to suggest a bonding interaction between the interfacial Pt and partially reduced Ti atoms which would lead to an initial state shift. The binding energy increased by 0.25 eV at 1 monolayer and was constant for higher coverages. This indicates that the Pt atoms at or very near the interface are involved because, as the coverage of the TiO, overlayer increases, the XPS sampling volume becomes more sensitive to the surface Pt as compared to the bulk Pt. For coverages less than 2 monolayers, it was not possible to convert all the titanium to Ti4+even when the oxide was exposed to lo-' torr of O2 at 650 K for up to 1 h. This suggests that the Ti3+is more stable than Ti4+at the oxide-metal interface under these conditions. When more oxide is added, the T i ( 2 ~ ~XPS ,~) BE increases and the fwhm decreases, indicating that the majority of the overlayer has changed to TiO,. However, the unchanged Pt(4f) BE suggests that the interface is still dominated by Pt-Ti3+ (Le., reduced Ti is still present at the interface). The equilibrium coverage of TiO, (1 .O < x < 1.5) after annealing and cooling was also observed by KO and Gorte12J3using AES. Recently Belton et al.' observed TiO, (1.0 5 x I 1.2) segregate to the surface when a Ti(0001) substrate covered by 40 A of TiO, and 40 A of Pt or Rh was vacuum annealed. The distribution of Ti species after annealing is also of interest. If we assume that it takes about 5 X lo4 nA s to sputter through 1 monolayer (based on the nonannealed surface), Figure 4 shows that the TiO, was uniformly distributed over the first two atomic layers. The distribution of TiO, then gradually fell off over the next 2-3 layers. Since one layer is 3-5 A thick, the TiO, is distributed as deep as 15-25 A. In the thin Pt overlayer studies by Belton et al.,' little Ti or 0 was detected in the Pt layer after annealing at 760 K. We propose that this is related to the annealing temperature and to the presence of reduced Ti oxides at the initial interface. For the ( 2 6 ) U. Bardi and P. N. Ross, accepted for publication in J . Vac. Sci. Technol.
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J . Phys. Chem. 1985, 89, 5028-5031
thin Pt overlayer case,' the oxide was also thin, probably highly defective and attached to bulk Ti. Heating to 760 K caused migration and segregation of the initially available reduced TiO, species which were then easily detected at the Pt surface. Further incorporation of TiO, in the Pt layer requires significantly higher temperatures and/or longer annealing t i r n e ~ . ~ 'On the basis of ~ Ti oxide the previous results of KO and G ~ r t e , ' ~a. ' reduced diffuses deeper than the sampling volume of AES at high temperatures. Upon cooling it returns to the surface region forming a uniform layer on the Pt. This would leave a concentration of TiO, beneath the surface as measured here. Taken together, the available data indicates that, once formed, the reduced TiO, (x = 1) species can easily diffuse into and out of the Pt. Since Ti-Pt alloys are well-known,26 one possibility for the formation of the reduced titanium oxide is as follows. If we assume that the high temperature annealing process removes two oxygen atoms from the TiO, unit cell, Ti30, is formed at the interface. Ti30, is assumed because it is consistent with the observed stoichiometry and involves breaking the fewest bonds at the interface. In equilibrium with this process, the formation of dilute Pt-Ti and Pt-0 compounds are proposed. The formation of these compounds provides enough energy for the overall process to be favorable thermodynamically. The diffusion of Ti and 0 would then occur through the Pt and away from the interface along the concentration gradient in a correlated but not simultaneous fashion. Bardi and Ross26have shown that a reduced TiO, species can diffuse through Pt3Ti. At high temperatures, entropy drives surface segregated TiO, into the bulk. Upon cooling the surface back to 300 K, enthalpy factors dominate and cause the surface segregation of the reduced species. For thin films of TiO, on Pt, this process terminates with roughly 1 monolayer of TiO, at the
surface with a concentration of TiO, distributed throughout the Pt . This model also predicts that the samll Pt particles present in Pt/Ti02 catalysts will incorporate TiO, when the substrate is reduced. In this way, all of the Pt atoms within the particles are altered and can account for the small Pt BE shifts sometimes observed in earlier XPS studies.20,28Extended thermal annealing of the thin film model catalysts supports this conclusion.
Acknowledgment. J.M.W. acknowledges support of this research by the Robert A. Welch Foundation. The XPS instrumentation used in this research was supported in part by the National Science Foundation, Grant C H E 8201 179. C.M.G. also thanks D. Belton for several insightful conversations. C. S. KO and R. J. Gorte acknowledge the support of the DOE, Grant DE-AC21-82MC19204. Registry No. Pt, 7440-06-4; Ti, 7440-32-6;TiOz, 13463-67-7.
(27) Y.-M. Sun, D. N. Belton, and J. M. White, to be submitted for publication.
1 (1980).
Summary
X-ray photoelectron spectroscopy has been used to characterize Ti02 overlayers on polycrystalline Pt. The addition of TiO, increases (I 0.25 eV) the Pt(4f) BE. This is attributed to bonding between Pt and Ti3+at the interface. Annealing to 1300 K under vacuum causes a significant increase in the Ti3+surface concentration and a decrease in the total amount of Ti in the XPS sampling volume. For initial Ti02 coverages greater than 1 monolayer an equilibrium amount of reduced Ti species is present on the surface after annealing (between 0.6 and 0.9 monolayer). Depth profiles indicate that diffusion of a reduced TiO, (x = 1) species (with titanium in a 3+ oxidation state) occurs during annealing.
(28) C. C. Kao, S. C. Tsai, M. K. Bahl, and Y. W. Chung, Surf Sci., 95,
Surface Analysis of Polymer Systems. 3. ZnS.CdS/Naflon N. Kakuta, K.-H. Park, M. F. Finlayson, A. J. Bard, A. Campion, M. A. Fox, S. E. Webber, and J. M. White* Department of Chemistry, University of Texas, Austin, Texas 78712 (Received: April 23, 1985; I n Final Form: July 17, 1985)
X-ray photoelectron spectroscopy has been used to characterize the surfaces of Nafion films into which mixtures of CdS and ZnS have been precipitated by exposing ion-exchanged Nafion films to H2S. For all the cases studied, precipitation led to segregation of the metals from interior ion-exchange sites to the surface where small sulfide particles were formed. Irradiation of the films with visible light in the presence of aqueous 0.1 M Na2S solution produced H2 and some Cdo. In the absence of ZnS, there was neither H2 nor Cdo production. When ZnS and CdS were coprecipitated or when ZnS was precipitated after CdS, very active films were formed and irradiation changed the surface Zn/Cd ratio to a value near 5. When CdS followed ZnS precipitation, irradiation did not change the Zn/Cd ratio from its initial value of unity and the films were inactive for photoassisted hydrogen generation. The results are discussed in light of other measurements on these materials which point to the importance of CdS surface state alteration by Zn.
Introduction In previous papers, we have reported on mixed semiconductor catalysts supported on a variety of materials (particularly coprecipitated ZnSCdS/Nafion and ZnSCdS/Si02) that are active hydrogen evolution catalysts when irradiated with visible light.'s2 ( 1 ) N. Kakuta, K.-H. Park, M. F. Finlayson, A. Ueno, A. J. Bard, A. Campion, M. A. Fox, S. E. Webber, and J. M. White, J . Phys. Cfiern.,89, 732 (1985). (2) A. Ueno, N. Kakuta, K.-H. Park, M. F. Finlayson, A. J. Bard, A. Campion, M. A. Fox, S. E. Webber, and J. M. White, submitted for publication.
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These catalysts have activities that are comparable with Pt. CdS/Nafion films and do not depend strongly on the support. These interesting materials have been studied in a variety of ways in an effort to understand the key features. In the absorption spectrum, two onsets were found corresponding to bulk CdS and ZnS interband transitions. Luminescence measurements indicated lattice vacancies and interstitials that participated in radiative relaxation. No evidence for solid solutions has been f o ~ n d . ~ , ~ (3) M. F. Finlayson, K.-H. Park, N. Kakuta, A. J. Bard, A. Campion, M. A. Fox, S. E. Webber, and J.
M. White, to be submitted for publication.
0 1985 American Chemical Society
