Identification of Defect Sites on MgO(100) Surfaces

Transcription

1 Langmuir 2002, 18, Identification of Defect Sites on MgO(100) Surfaces Y. D. Kim, J. Stultz, and D. W. Goodman* Department of Chemistry, Texas A& MUniversity, College Station, Texas Received December 10, In Final Form: February 20, 2002 Methodologies are reported that allow the identification of extended defects on MgO(100) surfaces. The techniques used include low-energy electron diffraction (LEED), metastable impact electron spectroscopy (MIES), D 2O and CO temperature-programmed desorption (TPD), and MIES of adsorbed Xe. LEED and MIES data for MgO(100), TPD spectra of D 2O and CO, and MIES spectra for adsorbed Xe are significantly altered as a function of the density of the extended defects. NO is shown to be an ineffective probe molecule for titration of defect sites on MgO(100). 1. Introduction Defects on oxide surfaces play a pivotal role as active sites for catalytic reactions 1,2 as well as nucleation centers for metal clusters. 3,4 Therefore, the identification and characterization of defect sites on oxide surfaces are crucial to obtaining a molecular level understanding of metal/ oxide catalyst systems. The existence of point defects, such as F-centers, on MgO can be identified relatively easily because additional peaks in the band gap region can be detected via optical absorption measurements, 5 electron energy loss spectroscopy (EELS), 6 ultraviolet photoelectron spectroscopy (UPS), and metastable impact electron spectroscopy (MIES). 7 The identification of extended defects such as steps and grain boundaries is difficult since no distinct additional features derived from extended defects can be observed using the above techniques. At most, broadening of the O(2P) band has been suggested by theoretical studies. 8 A possible route to the identification of extended defects is the use of probe molecules that uniquely interact with defect sites. Therefore, finding proper probe molecules to titrate extended defects is a key to understanding the properties of defects on oxide surfaces. In this regard, it has been suggested that NO 7,9,10 can be used for titration of defects on MgO(100) surfaces. In this paper, a methodology for identifying extended defects on MgO is described. Low-energy electron diffraction (LEED) data, the line shapes of MIES spectra, D 2 O and CO temperature-programmed desorption (TPD) spectra, and MIES spectra of adsorbed Xe significantly * To whom correspondence should be addressed. Goodman@mail.chem.tamu.edu. Fax: (979) Phone: (979) (1) Voskresenskaya, E. N.; Roguleva, V. G.; Anshits, A. G. Catal. Rev.sSci. Eng. 1995, 37, (2) Wu, M. C.; Truong, C. M.; Coulter, K.; Goodman, D. W. J. Am. Chem. Soc. 1992, 114, (3) Valden, M.; Lai, X.; Goodman, D. W. Science 1998, 281, (4) Haas, G.; Menck, A.; Brune, H.; Barth, J. V.; Venables, J. A.; Kern, K. Phys. Rev. B 2000, 61, (5) Chen, Y.; Williams, R. T.; Sibley, W. A. Phys. Rev. 1969, 182, 960. (6) Wu, M. C.; Truong, C. M.; Goodman, D. W. Phys. Rev. B 1992, 46, (7) Kolmakov, A.; Stultz, J.; Goodman, D. W. J. Chem. Phys. 2000, 113, (8) Kantorovich, L. N.; Shluger, A. L.; Sushko, P. V.; Stoneham, A. M. Surf. Sci. 2000, 444, (9) Rodriguez, J. A.; Jirsak, J.; Kim, J. Y.; Larese, J. Z.; Maiti, A. Chem. Phys. Lett. 2000, 330, (10) Rodriguez, J. A.; Jirsak, T.; Perez, M.; Gonzalez, L.; Maiti, A. J. Chem. Phys. 2001, 114, change in the presence of extended defects on MgO. Therefore, combinations of these experimental techniques are shown to be effective for identification of extended defects. In contrast to previous investigations, 7,9,10 NO is shown to be ineffective as a probe molecule to titrate defect sites on MgO. 2. Experimental Details The experiments were carried out in an ultrahigh vacuum (UHV) system with a base pressure of Torr. The UHV system consists of two interconnected chambers, one equipped with an ion-gun for sputtering, LEED, and TPD, and the second, for Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS), and MIES/UPS. MIES is a nondestructive and extremely surface sensitive technique that uses metastable helium atoms with thermal kinetic energy as a surface probe. For insulating materials, the intensity of the ejected electrons (resulting from the He atom de-excitation) versus their kinetic energies gives a direct image of the density of the occupied states of the topmost surface layer. For more details of MIES, the reader is referred to a recent review. 11 MIES/UPS spectra were measured simultaneously using a cold-cathode discharge source 12,13 that provides both ultraviolet photoelectron and metastable He 2 3 S (E* ) 19.8) atoms with thermal kinetic energy. Metastable and photon excited contributions to the signal are separated by a time-of-flight method using a mechanical chopper. MIES and UPS spectra were acquired with the photon/metastable beams incident at 45 with respect to the surface normal and, typically, in a retarding-field mode (constant pass energy) using a doublepass cylindrical mirror analyzer (CMA). For acquiring the data of Figure 6, however, the non-retarding-field mode was used. By measurement of the width of the Fermi edge of the clean Mo- (100) surface, the energy resolution of the analyzer was determined to be ca. 0.9 and 0.4 ev for the retarding- and non-retardingfield modes, respectively. The energy denoted by E F in the spectra corresponds to electrons emitted from the Fermi level of the Mo- (100) substrate and is the reference point for all the MIES and UPS data. The Mo(100) sample was cleaned by repeated heating to 2200 K. Sample cleanliness was determined using AES. In addition, the LEED pattern of the clean sample showed a (1 1) periodicity with high spot and low background intensities. D 2O (Aldrich 99.9%) was purified by several freeze-pump-thaw cycles and was dosed via backfilling the chamber. 3. Results and Discussions 3.1. Characterizations of Various MgO(100) Surfaces Using LEED and MIES. The MgO films were (11) Harada, Y.; Masuda, S.; Ozaki, H. Chem. Rev. 1997, 97, (12) Mausfriedrichs, W.; Dieckhoff, S.; Kempter, V. Surf. Sci. 1991, 249, (13) Mausfriedrichs, W.; Wehrhahn, M.; Diecjhoff, S.; Kempter, V. Surf. Sci. 1990, 237, /la CCC: $ American Chemical Society Published on Web 04/18/2002

2 4000 Langmuir, Vol. 18, No. 10, 2002 Kim et al. Figure 2. (a) MIES spectra for the as-grown, vacuum-annealed, and sputter-damaged MgO(100) films on Mo(100); (b) magnified view of the low binding energy portion of (a). Figure 1. (a) LEED data of an as-grown MgO(100) film on Mo(100); (b) LEED data of a vacuum-annealed MgO(100) film. Primary electron energy ) 111 ev. synthesized by depositing Mg on Mo(100) in Torr of ambient O 2 at 600 K. The thickness of the film was estimated to be ca. 15 monolayers. AES showed that under these conditions Mg is completely oxidized, that is, no feature for metallic Mg was evident at 44 ev. 14 Subsequently, MgO films prepared in this way will be referred to as as-grown MgO films. These as grown films exhibited a(1 1) LEED pattern, consistent with the epitaxial growth of MgO with the 100 plane parallel to the Mo- (100) surface (see Figure 1a). However, the low intensities of the primary LEED spots and the accompanying high background LEED intensities indicate that the MgO(100) surface is very defective. (14) Wu, M. C.; Corneille, J. S.; Estrada, C. A.; He, J. W.; Goodman, D. W. Chem. Phys. Lett. 1991, 182, After annealing the as-grown MgO film to 1150 K (vacuum-annealed MgO film), a distinct, high-intensity (1 1) LEED pattern with a low background intensity was evident, showing that the high-temperature anneal orders the MgO(100) surface (Figure 1b). After sputtering the vacuum-annealed MgO film at room temperature, followed by a brief anneal to 600 K, the LEED data still showed a (1 1) periodicity; however, the spot intensities decreased and the background intensity increased compared to the vacuum-annealed film. The MIES spectra for the various MgO(100) surfaces are compared in Figure 2. The MIES spectra of the asgrown film exhibit a relatively broad O(2P) peak at ca. 5.5 ev. After multiple anneals at 1150 K, the O(2P) peak became narrower, indicating that the surface becomes more ordered with annealing. With sputtering, the O(2P) feature broadens significantly. The changes of the MIES spectra in Figure 2 are consistent with the previously described LEED data. MIES studies have shown that electron bombardment of a MgO(100) surface at 95 K and thermal quenching can create F-centers. 7 The F-centers were detected as an additional feature in the MIES spectrum between 1 and 2.5 ev below the Fermi level. 7 In the present work, no evidence for F-centers was found for any of the sample preparations (Figure 2b); therefore, we conclude that the F-center density for the MgO(100) surfaces studied is negligibly small. The defect sites on the MgO(100) surfaces then are presumed to be extended defects such as steps, corners, and grain boundaries D 2 O Adsorption on the Vacuum-Annealed and the Sputter-Damaged MgO(100) Surfaces. To investigate the chemical properties of the MgO(100) thin films with various defect densities, D 2 O TPD experiments on

3 Defect Sites on MgO(100) Surfaces Langmuir, Vol. 18, No. 10, Figure 3. (a) D 2O TPD spectra from a vacuum-annealed MgO- (100) film on Mo(100); (b) D 2O TPD spectra from a sputterdamaged MgO(100) film on Mo(100). the vacuum-annealed and the sputter-damaged MgO(100) surfaces were carried out. The D 2 O TPD spectra taken from the vacuum-annealed MgO(100) film are shown in Figure 3a. At lower coverages, a sharp peak at 240 K appears and is assigned to the first D 2 O monolayer. After the feature at 240 K essentially saturates, two additional peaks grow at 180 and 150 K. In previous LEED studies, the 180 K peak was assigned to the partial desorption of water during the transition from the c(4 2) overlayer to the p(3 2) phase. 15 The appearance of the peak at 150 K results from the formation of multilayers. The monolayer peak saturates before the multilayer peak grows in, suggesting a layer-by-layer growth mode of D 2 O on MgO- (100)/Mo(100). TPD spectra taken from the sputter-damaged MgO- (100) film are substantially different from those of the vacuum-annealed MgO(100) surface (Figure 3b). At lower D 2 O coverages, a single peak at 220 K appears. With increasing D 2 O exposures, two peaks at 160 and 220 K grow simultaneously. The feature between the monolayer and multilayer features, which is seen clearly in Figure 3a, is not observed on the sputter-damaged surface. For the sputter-damaged MgO(100) surface, the feature at 220 K is assigned to correspond to monolayer desorption, and the feature at 160 K, to multilayer desorption. The D 2 O monolayer peak on the sputter-damaged surface shifts to a lower temperature by 20 K with respect to that of the vacuum-annealed MgO(100) surface. In addition, a broadening of the D 2 O monolayer peak is apparent after sputtering. This broadening implies that the D 2 O adsorption sites on the sputter-damaged surface are heterogeneous, in agreement with the MIES and LEED data indicating that the MgO(100) surface is more defective (15) Ferry, D.; Picaud, S.; Hoang, P. N. M.; Girardet, C.; Giordano, L.; Demirdjian, B.; Suzanne, J. Surf. Sci. 1998, 409, Figure 4. D 2O TPD spectra from a sputter-damaged MgO- (100) film on Mo(100): (a) 4 langmuir, (b) 10 langmuir, and (c) after several cycles of D 2O deposition and brief heating to 400 K. The water was dosed at a MgO(100) surface temperature of 100 K. after sputtering. In contrast to the vacuum-annealed MgO- (100) surface, the monolayer and multilayer features grow simultaneously on the sputter-damaged MgO(100) surface, indicating that D 2 O grows three-dimensionally rather than layer-by-layer on the defective surface. More detailed information from MIES/UPS studies regarding the properties of water adsorbed on both surfaces is described elsewhere. 16 At very high D 2 O coverages, additional desorption peaks are evident between 400 and 600 K (Figure 4). These TPD features are more clearly visible following several cycles of D 2 O exposure at 95 K and subsequent heating to 400 K (Figure 4). Because these peaks appear only at very high water coverages, this desorption feature is assumed to be related to multilayer D 2 O, most likely to the recombination of hydroxyl groups formed by multilayerinduced dissociation. 17,18 These features are negligible on well-ordered MgO(100) surfaces. 17 Therefore, the watermultilayer-induced dissociation of water can take place more easily on defective MgO(100) surfaces. These results show that the adsorptive behavior of D 2 O on MgO significantly changes as a function of defect concentration. Extended defects, then, can be probed on MgO(100)/Mo- (100) by the adsorption of D 2 O and its subsequent TPD. Our results of D 2 O TPD experiments on both the vacuum-annealed and the sputter-damaged surfaces are consistent with comparable data acquired from atomically flat and well-ordered MgO(100) films as well as defective MgO(100) single-crystal surfaces. 17 Essentially, the structural and chemical properties of the MgO(100) thin films are identical to those of comparably prepared MgO(100) single crystals. These results suggest that the adsorptive properties of MgO thin films then are equivalent to those of MgO single crystals CO TPD from Various MgO(100) Surfaces. It has been shown that CO adsorbs selectively on defect sites of MgO(100) surfaces at 90 K, suggesting that CO can be used as a probe molecule to titrate certain defect sites. (16) Kim, Y. D.; Stultz, J.; Goodman, D. W. J. Phys. Chem. B 2002, 106, (17) Ahmed, S. I. U.; Perry, S. S.; El Bjeirami, O. J. Phys. Chem. B 2000, 104, (18) Johnson, M. A.; Stefanovich, E. V.; Truong, T. N.; Gunster, J.; Goodman, D. W. J. Phys. Chem. B 1999, 103,

4 4002 Langmuir, Vol. 18, No. 10, 2002 Kim et al. Figure 5. CO TPD from MgO surfaces with varying defect densities. In Figure 5, CO TPD results from various MgO(100) films are shown. A CO TPD spectrum acquired from a sputter-damaged surface exhibits a CO desorption feature at 120 K. After annealing the sputter-damaged surface to 1050 K, the intensity of the CO desorption peak decreases, indicating that annealing reduces the number of defect sites. An anneal to 1150 K reduces the intensity of the CO desorption peak even further. Annealing then to K significantly reduces the number of defect sites on MgO(100), the site density of which can be assessed by using CO as a probe MIES of Adsorbed Xe. In Figure 6a, the MIES spectrum for the vacuum-annealed MgO(100) surface is compared with that from a comparably prepared surface with adsorbed Xe. The MIES spectrum for the adsorbed Xe monolayer was acquired in a Xe background pressure of Torr at a sample temperature of 80 K. Xe on the surface typically gives rise to sharp doublet features originating from the 5P 1/2 and 5P 3/2 states of adsorbed Xe atoms. 22 In general, the 5P 3/2 signal splits into two sublevels causing broadening of the 5P 3/2 peak. 23 As shown in Figure 6a, the MIES spectrum collected for Xe/MgO yields a sharp doublet feature from the Xe 5P states. The O(2P) peak from the substrate completely disappears indicating that the surface is almost completely covered by Xe. The relatively high intensity of the 5P 3/2 peak is probably due to the relatively high cross section of this state for the Auger de-excitation (AD) process. A similar ratio of intensities of the 5P 1/2 and 5P 3/2 levels was found in a previous MIES study of Xe multilayers. 24 Figure 6b shows MIES spectra for bare and Xe-adsorbed MgO surfaces grown at 300 K without further annealing. The broader O(2P) feature in Figure 6b compared to that of Figure 6a indicates that the surface for Figure 6b is more defective. The MIES spectrum of adsorbed Xe (Figure 6b) exhibits a doublet feature originating from the 5P 1/2 and 5P 3/2 levels of Xe. However, the Xe features on this surface are broader compared with those of the vacuumannealed MgO surface. The broadening of the Xe peaks in MIES indicates that the adsorption sites for the Xe atoms are more heterogeneous. These results show that (19) Dohnalek, Z.; Kimmel, G. A.; Joyce, S. A.; Ayotte, P.; Smith, R. S.; Kay, B. D. J. Phys. Chem. B 2001, 105, (20) Wichtendahl, R.; Rodriguez-Rodrigo, M.; Hartel, U.; Kuhlenbeck, H.; Freund, H. J. Phys. Status Solidi A 1999, 173, (21) Kim, Y. D.; Stultz, J.; Goodman, D. W. Surf. Sci., in press. (22) Hulse, J.; Kuppers, J.; Wandelt, K.; Ertl, G. Appl. Surf. Sci. 1980, 6, (23) Scheffler, M.; Horn, K.; Bradshaw, A. M.; Kambe, K. Surf. Sci. 1979, 80, (24) Oro, D. M.; Soletsky, P. A.; Zhang, X.; Dunning, F. B.; Walters, G. K. Phys. Rev. A 1994, 49, the valence band structure for an adsorbed Xe monolayer can be used to estimate the uniformity of the surface NO Adsorption on the Vacuum-Annealed and Sputter-Damaged MgO(100) Surfaces. Recently, MIES/ UPS and TPD studies suggested that NO can be used as a probe molecule to titrate defects on MgO(100). 7 NO was purported to adsorb selectively on extended defects such as steps and grain boundaries of MgO(100)/Mo(100) to form N 2 Oat90K. 7 In contrast to ref 7, XPS experiments of Rodriguez et al. reported that extended defects on MgO- (100) facilitate molecular adsorption of NO without any N 2 O formation at K. 9 The TPD study from Rodriguez et al. showed only NO adsorption without formation of N 2 O. 10 TPD studies of Wichtendahle et al. using MgO(100) single crystals suggested that defect sites are responsible for a broad NO desorption feature above 80 K, while NO from an ideal MgO(100) surface desorbs below 60 K. 20 These results imply that NO exclusively interacts with defect sites above 80 K. 20 From these previous studies, it is not clear whether NO undergoes conversion to N 2 O on MgO(100) above 80 K. However, these previous studies do suggest that NO selectively interacts with defect sites above 80 K. As mentioned above, the D 2 O TPD spectra for a vacuumannealed MgO(100) film are identical to those from atomically flat and well-ordered MgO(100) single-crystal surfaces. No CO adsorption can be detected on the vacuumannealed MgO(100) surface using MIES/UPS suggesting very low defect density. 21 The sharp doublet features for MIES of adsorbed Xe also imply that the vacuum-annealed MgO(100) surface is uniform. Assuming that NO exclusively interacts with defect sites of MgO(100) at 90 K, very little change in the MIES spectrum for MgO(100) should occur then upon exposing the vacuum-annealed MgO(100) to NO at 90 K. However, the MIES spectra of the vacuum-annealed MgO(100) film change significantly upon NO exposure (Figure 7). After dosing 130 langmuir of NO, the O(2P) peak of the vacuum-annealed MgO(100) surface virtually disappears. Concomitantly, three new peaks appear, corresponding to the electronic states of N 2 O. The attenuation of the O(2P) feature upon exposure to 130 langmuir of NO is approximately 90%. To acquire a better understanding regarding the interaction between NO and MgO(100), NO adsorption experiments on a sputter-damaged MgO(100) surface were carried out. As illustrated in Figure 8, the change of the MIES spectra induced by the N 2 O formation is more pronounced on the vacuum-annealed MgO(100) surface, indicating that the amount of N 2 O(ad) formed on this surface decreases with increasing defect density. The MIES data imply that N 2 O formation takes place on terrace sites rather than defect sites. However, these data are in disagreement with previous studies, 7,9,10,20 where NO was found to interact exclusively with defect sites above 80 K. It is plausible that MgO surfaces with various types of defects can result from different surface preparations. Some defects may interact strongly with NO, whereas some defects may adsorb NO only weakly or not at all. Moreover, the mechanism of the N 2 O formation can consist of several steps involving the interaction of NO with terraces, point defects, extended defects, or other types of defects in a complicated manner. Further studies are required to illuminate these issues. 4. Conclusion Various techniques have been used to characterize low and highly defective MgO(100) surfaces. Broadening of

5 Defect Sites on MgO(100) Surfaces Langmuir, Vol. 18, No. 10, Figure 6. (a) A MIES spectrum for the vacuum-annealed MgO(100) surface compared with that of adsorbed Xe on the same surface; (b) a MIES spectrum for the defective MgO(100) surface compared with that of adsorbed Xe on the same surface; (c) MIES spectra for adsorbed Xe on low and highly defective MgO(100) surfaces. Figure 7. MIES spectra from a vacuum-annealed MgO(100) surface collected as a function of NO exposure. the O(2P) band upon sputtering indicates that defects are formed on the surface; however, no evidence of F-centers following sputtering was apparent with MIES/UPS. Because of the strong influence of defects on the adsorption characteristics, D 2 O TPD is demonstrated to effectively reveal defect sites on MgO(100). In addition, D 2 O TPD Figure 8. MIES spectra acquired after exposing the vacuumannealed and sputter-damaged MgO(100) surfaces to 130 langmuir of NO at 90 K. spectra from vacuum-annealed and sputter-damaged MgO(100) surfaces are identical to those observed for ideal and defective MgO(100) single-crystal surfaces, 17 suggesting that results from MgO(100) thin films should correspond to analogous results for MgO(100) single crystals.

6 4004 Langmuir, Vol. 18, No. 10, 2002 Kim et al. Extended defects on MgO(100) can also be detected using CO as a probe molecule. For example, the intensity of the CO TPD peak decreases as the number of defect sites decreases. Furthermore, MIES of adsorbed Xe as a function of the surface defect density shows that Xe atoms yield features at unique binding energies, confirming that MIES of adsorbed Xe can be used as well to identify extended defects. In contrast to previous studies, 7,9 NO adsorption on vacuum-annealed and sputter-damaged surfaces indicates that the amount of N 2 O formed from NO exposures is not an accurate measure of surface defect densities. That is, NO is not a reliable probe molecule for the titration of defect sites on MgO(100). In particular, the reaction mechanism of the conversion of NO to N 2 O on MgO(100) is not obvious. Further studies are required to explore the mechanism of N 2 O formation from NO on MgO(100). Acknowledgment. Funding for this work was provided by the Department of Energy, Office of Basic Energy Sciences, Division of Chemical Sciences, and the Robert A. Welch Foundation. LA