XPS study of reactive and non-reactive SrTiO3 (100) surfaces: Adsorption of H2O

Surface Science 107 (1981) L345-L349 North-Holland Publishing Company

SURFACE SCIENCE LETTERS UPS/XPS STUDY OF REACTIVE AND NON-REACTIVE SrTiO3 (100) SURFACES: ADSORPTION OF H20 C. WEBB and M. LICHTENSTEIGER Coulter Systems Corporation, Bedford, Massachusetts 01730, USA

Received 4 December 1980; accepted for publication 6 February 1981

SrTiO3 surfaces, prepared by ion bombardment alone and in conjunction with annealing, exhibit very different properties upon exposure to H20. In the case of ion bombardment only, H20 adsorption occurs, signaled by the appearance of a peak in the Hell spectrum at 10.8 eV binding energy which saturates for dosages lower than 5 L. Our data indicate that this adsorption is non-dissociative. No adsorption was detected for bombarded and annealed samples. The difference in behavior is believed to be related to the oxidation state of Ti. The significance of these results for previously published data is discussed.

Strontium titanate has become of interest recently because of its action as a photocatalyst for the decomposition of water [1 ]. Several studies have been made including those of Henrich, Dresselhaus and Zeiger (HDZ-1) [2] who investigated chemisorption of water on both TiO~ and SrTiO3 surfaces. (The Ti environment in both compounds is very similar.) The main feature introduced into their HeI spectra by H20 adsorption is a shoulder on the secondary background near 11 eV binding energy [3]. They interpreted this peak as indicating the presence of non-dissociated H20 as opposed to OH-. Ferret and Somorjai (FS) [4] studied chemisorption of oxygen, hydrogen and water on SrTiO3 surfaces and, by means of a comparison with NaOH, identified OH- as being present on reduced surfaces in all cases though they offered no mechanism for the production of the hydroxyl ions found in the first two cases. Again, an essential feature of their data is a structure similar to the one noted by HDZ-1 which they observed even for "clean" reduced surfaces. Henrich et al. (HDZ-2) [5] have, in addition, reported results for the chemisorption of oxygen on SrTiO3 identifying two separate chemisorbed states which they termed "Phase I" and "Phase II". The latter was observed for large exposures to oxyge n, 104 L, and was again signaled by a 11 eV shoulder. This feature at 11 eV binding energy thus occurs frequently in the literature but with almost as many interpretations. In this letter we report a UPS/XPS study of the chemisorption o f H20 on SrTiO3 surfaces prepared by ion bombardment alone and in combination with prolonged annealing (~15 h) to 500°C. Following FS we refer to bombarded and

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C. Webb, M. Lichtensteiger / UPS/XPS and SrTiO a (100) surfaces

annealed surfaces as "reduced" and "stoichiometric" respectively [6]. We find that the reduced surfaces are extremely sensitive to H20 adsorption with a sticking coefficient near unity, while the stoichiometric surfaces exhibit no measurable changes in the UP spectra after exposures up to 500 L of H20. This difference in surface reactivity is in contrast with the results reported by HDZ-1, but agrees with the findings of FS. The experiments were carried out on a SrTiO3 (100) single crystal which for studies of reduced surfaces was freshly ion etched for each adsorption dosage. It was then immediately exposed to water vapor in a preparation chamber followed by rapid transfer to an analytical chamber at operating pressures of 2 × 10 -1° Torr for XPS and 5 X 10 -l° (uncorrected ion gauge reading) for UPS with helium making up the difference. Data were routinely acquired by multiple scans in not more than 10 min and stored at an intermediate stage of acquisition so that checks could be made against possible changes. As noted below, certain changes did occur in the UP spectra as a function of time, apparently as a consequence of the UV irradiation. Fig. 1 shows the effect of H20 adsorption on reduced SrTiO3, both HeI and HeII data are shown. H20 induces a peak which is not resolved from the secondary background in the HeI spectra, and is more readily seen in the HeII data at 10.8

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eV. This peak saturates within about 5 L of H20 exposure and was observed to decrease in intensity by some 30% after 30 minutes irradiation by the UV source: No such peak was found for the stoichiometric surface even after 500 L exposure• The reduced surfaces, unlike the stoichiometric ones, are characterized by the presence of Tia+(3dt) as evidenced by a chemically shifted Ti(2p) component (fig. 2) and a small threshold peak in the HeI spectra [7] (around 1 eV BE in fig. la). The major peak in the UP spectrum, occurring at 5 - 7 eV binding energy, is assigned to O(2p) bands. It may be observed from fig. 1 that H20 adsorption is accompanied by a reduction of the theshold peak attributed to Ti 3+. This suggests interaction of H20 with Ti 3+ sites which is supported by the absence of adsorption in the stoichiometric case. The origin of the peak near 11 eV is clearly associated with the adsorption of H20; however, it has been attributed to both dissociative [4] and non-dissociative [2] adsorption• In fig. 3, we show an extended energy range Hell scan at the 5 L dosage (the acquisition time for this spectrum was much longer than for the others, ~90 min), together with energy level positions for H20 adsorbed on TiO2 and OHon TiO2, obtained from the small cluster calculations of Kowalski, Johnson and Tuller [8]. On the basis of these data, we conclude that non-dissociative adsorption occurs in agreement with HDZ-1, but our assignment of levels differs: We attribute the 11 eV peak to the al level. This association virtually removes the discrepancy noted by Kowalski et al. in the location of the energy levels deduced by HDZ-1 (they placed b2 at 11 eV) and also agrees with the relative weighting of at/b2 levels

L348

C. Webb, M. Lichtensteiger / UPS/XPS and SrTi03 (100) surfaces

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found in gas phase spectra [2]. In fact, FS and HDZ all made use of difference spectra in order to isolate the emissions due to the adsorbate from the secondary background and O(2p) band. We did not attempt this as small changes in the region of O(2p) must be considered of lower significance than peaks seen on an otherwise featureless background. The use of Hell has clear advantages over HeI in that the secondary background is much less serious. Knotek [9] has presented evidence from electron stimulated desorption studies that H20 dissociates on TiO2. If, as we believe, TiO2 and SrTiO3 behave similarly, then this represents a possible conflict with our findings, but the observed dissociation may be a result of electron bombardment. We noted above the pre-eminence of an 11 eV peak in published data on SrTiO3 and TiO2. The high sensitivity of bombarded SrTiOa surfaces" to H20 which we have found may account for this. "Phase II" adsorption observed by HDZ-2 for large oxygen doses could be explained by the typical 5 ppm H20 found in research grade oxygen; thus l0 s L of O2 would also provide 5 L of H20. The observation of a similar feature for "clean" surfaces by FS could be accounted for if an appreciable H20 residual background existed. Knotek's [9] studies of hydrogen on TiO2

C lCebb, M. Lichtensteiger/UPS/XPS and SrTi03 (100) surfaces

L349

surfaces led him to suggest that hydrogen, to which most surface techniques are insensitive, may be present to a significant extent in the surface region o f a wide range o f compounds. Clearly, techniques insensitive to hydrogen will be similarly deficient in distinguishing between oxygen and water. We find it interesting to note that in a recent paper b y Su et al. [10] considering adsorption o f O 2 on GaAs two phases o f adsorption were observed; one, associated with exposures up to 106 L 02, was attributed to interaction with defect sites. The main signature of this phase was a peak at 10.4 eV binding energy. In addition, we have found that surfaces o f CdS, a material which has been considered for use in the photocatalytic decomposition o f water, can be made to exhibit a high reactivity towards H20 [11]. Generally no detectable adsorption occurs but, in the presence of simultaneous electron bombardment, the surface becomes responsive to even extremely low H20 partial pressures. In summary, we find reduced SrTiOa surfaces to be extremely sensitive to H20 adsorption while stoichiometric surfaces appear inert. This difference has been related to the presence or absence o f Ti 3÷, respectively. We present evidence that the peak at 11 eV binding energy is indicative o f non-dissociative adsorption o f H20. The data suggest further that caution should be exercised in semiconductor studies where large doses of oxygen (or other gases) are involved or where there may be a significant residual H~O pressure. We are indebted to Dr. A. Linz, Massachusetts Institute o f Technology, for supplying the SrTiO3 crystal used in this work.

References [1] J.M. Kowalski and H.L. Tuller, in: Energy and Ceramics, Proc. 4th Intern. Conf. on Modern Ceramics, Ed. P. Vincencini (Elsevier, Amsterdam, 1980) p. 1027. [2] V.E. Hertrich, G. Dresselhaus and H.J. Zeiger, Solid State Commun. 24 (1977) 623. [3] HDZ reference their energy scale to the valence band edge. We added about 3.5 eV to their energies to take account of this. [4] S. Fetter and G.A. Somorjai, Surface Sci. 94 (1980) 41. [5] V.E. Heraich, G. Dresselhaus and H.J. Zeiger, J. Vacuum Sci. Technol. 17 (1980) 936. [6] It may be noted that the sample preparation methods described in ref. [4] differ from those used here but probably not significantly. [7] This peak is masked in the Hell spectra by emission due to the 48.4 eV line. [8} J.M. Kowalski, K.H. Johnson and H.L. TuUer, J. Electrochem. Soc. 127 (1980) 1969. [9] M.L. Knotek, Surface Sci. 91 (1980) L17. [10] C.Y. Su, I. Lindau, P.R. Skeath, P.W. Chye and W.E. Spicer, J. Vacuum Sci. Technol. 17 (1980) 936. [11] M. Lichtensteiger, C. Webb and J. Lagowski, Surface Sci. 97 (1980) L375.