Ordered binary oxide films of V 2 O on Al 2 O 3

Transcription

1 Ordered binary oxide films of V 2 O on Al 2 O 3 Q. Guo, D. Y. Kim, S. C. Street, a) and D. W. Goodman b) Department of Chemistry, Texas A&M University, P.O. Box 30012, College Station, Texas Received 20 October 1998; accepted 14 December 1998 Ordered binary oxide films of vanadium oxide have been prepared on an aluminum oxide film supported on Mo 110 under ultrahigh vacuum conditions and characterized by various surface analytical techniques. Auger electron spectroscopy, low energy electron diffraction, high-resolution electron loss spectroscopy, x-ray photoelectron spectroscopy and ion scattering spectroscopy indicate that the vanadia films grow epitaxially on the Al 2 O 3 /Mo 110 surface as V 2 O The results of electronic structural measurements show an increase in the energy of the a 1g level in the 3d band at low temperatures, which is a possible explanation for the metal-to-insulator transition in V 2 O American Vacuum Society. S I. INTRODUCTION Various techniques have been used to characterize the chemical and physical properties of oxides due to their importance in many areas, such as in catalysis, ceramics, corrosion inhibition, and electronic materials. Surface analytical techniques are widely used to study the geometric and electronic structure of the surface region under controlled ultrahigh vacuum UHV conditions. However, electronic spectroscopies are often of very limited utility on bulk solid oxides because of surface-charging problems. To circumvent charging, ordered, thin ultrathin, 10 nm metal oxide films supported on refractory metals, chosen to limit the lattice mismatch with the overlayer oxide, have been successfully prepared, including layered binary oxide films. 1 8 Vanadium oxide catalyst is an industrial mainstay useful for a variety of chemical reactions; for example, the selective catalytic reduction of nitric oxide with ammonia. Vanadium pentoxide (V 2 O 5 ) is the commonly used vanadium oxide catalyst. However, the surface of this catalyst contains lower oxidation state vanadium oxide, which may play important roles in the catalytic activity. In actual binary catalysts monolayer or submonolayers of oxovanadium deposited on a substrate is thought to be the active center. 9 It has been proposed that some reactions might be controlled by the metal oxygen, basal-plane bond strength V O V rather than the strength of the terminal bond VvO and that the number of participating surface vanadium oxide sites is related to the reducibility of the V O-support bond. 10 In order to study the origin of this and other catalytically important properties, model vanadia films have been prepared in UHV. V 2 O 3 (0001) has been shown to grow epitaxially on Au 111, 11 whereas a disordered V 2 O 3 film and VO 111 on Cu 100 and Ni 110 have been reported Moreover, epitaxial VO 2 films grown on sapphire substrates also have been investigated. 15 In this article, we present the results of a study on a new, binary layered oxide film prepared in UHV, an ordered V 2 O 3 a Present address: Department of Chemistry, University of Alabama, Tuscaloosa, AL b Author to whom correspondence should be addressed; electronic mail: goodman@mail.chem.tamu.edu film grown on alumina supported by Mo 110. The films are characterized using Auger electron spectroscopy AES, low energy electron diffraction LEED, x-ray photoelectron spectroscopy XPS, ion scattering spectroscopy ISS, and high-resolution electron energy loss spectroscopy HREELS. II. EXPERIMENT The experiments were carried out in two ultrahigh vacuum chambers, one of which has a base pressure of Torr. This chamber is equipped with reverse view optics for LEED, Auger electron optics with a single-pass cylindrical mirror analyzer CMA for AES, and a LK-2000 model HREELS spectrometer for electron energy loss study. The second chamber has a base pressure of Torr and is equipped with dual-anode x-ray sources Mg and Al, a hemispherical analyzer for XPS, and an ion gun for ISS. LEED and AES are also available in this chamber. The Mo 110 sample, 10 mm in diameter, 1.5 mm thickness, was spot welded to Ta leads allowing resistive heating to 1500 K. The sample could be heated to 2500 K by electron bombardment and cooled to 90 K by contact with a liquid nitrogen (LN 2 ) reservoir. A W-5%Re/W-26%Re thermocouple was spot welded to the edge of the Mo 110 sample for temperature measurements. The surface was prepared by annealing in Torr O 2 at 1200 K, with a subsequent flash to 2000 K. After several treatments, no impurities were detected by AES. The Al doser was made from a ceramic tube packed with Al wire and tightly wound on the outside with a tungsten wire. The V doser consisted of a metal wire wrapped tightly around a tungsten filament. Before evaporation, the Al and V sources were thoroughly degased. Alumina films were grown in UHV by evaporating Al onto the Mo 110 at K in Torr O 2 background followed by an anneal to 1100 K with 10 6 Torr O 2. This high temperature anneal improves the crystalline quality of the film. Subsequent LEED showed hexagonal symmetry, indicating that the Al 2 O 3 grows epitaxially on Mo 110. Prior to growth of a V 2 O 3 film, an Al 2 O 3 film with a monolayer equivalent MLE thickness of alumina on the Mo 110 substrate was 1887 J. Vac. Sci. Technol. A 17 4, Jul/Aug /99/17 4 /1887/6/$ American Vacuum Society 1887

2 1888 Guo et al.: Ordered binary oxide films of V 2 O on Al 2 O prepared. The V 2 O 3 film was grown on the Al 2 O 3 /Mo 110 by evaporation of V in 10 6 Torr O 2 ambient at K followed by K annealing in 10 6 Torr O 2. The film was further annealed to K in vacuum to enhance surface ordering. The amount of Al and V deposition was controlled by adjusting the exposure time using a fixed filament current through the doser. A primary electron energy of 3 kev was used in all AES measurements; a Mg K source was used for XPS. Binding energies BE were calibrated using the Au 4 f BE 84.0 ev and Cu 2p BE ev features of metallic Au and Cu, respectively. A pass energy of ev was used while collecting all data. He with a primary energy of 550 ev was used for ISS. During HREELS measurements, incident energies, E p, between 4.5 and 6.0 ev were used, with a typical resolution of 8 12 mev (65 97 cm 1 ) at full-width at halfmaximum FWHM of the elastic peak. To study the electronic structure, an E p of 25 ev was used, and the resolution in this case was 25 mev ( cm 1 ) at FWHM of the elastic peak. The HREELS data were recorded at 60 to the surface normal with specular reflection. FIG. 1. LEED data from: a a clean Mo 110 substrate, E p 85.5 ev; b a 20 MLE Al 2 O 3 film on the Mo 110. E p 84.3 ev; and c a20mle vanadia film on Al 2 O 3 /Mo 110. E p 92.1 ev. III. RESULTS AND DISCUSSION An alumina film 20 MLE was grown on the Mo 110 prior to the vanadia film growth. This film has been fully characterized previously by HREELS, AES, and LEED 16 which show that stoichiometric, ordered Al 2 O 3 films are formed. Figure 1 shows a 1 1 LEED pattern of clean Mo 110, a 1 1 hexagonal LEED pattern of an alumina film of 20 MLE grown on the Mo 110 surface, and a hexagonal 3 3 R30 LEED structure of a vanadia oxide layer on alumina. Bulk molybdenum has a cubic closepacked structure with a lattice constant of a nm. The 110 face of Mo has 1 1 structure with lattice constants of a nm and b nm. The structure of Al 2 O 3 differs depending on its phase. For instance, -Al 2 O 3 has the corundum structure that ideally is formed by closepacked oxygen anions with a hexagonal symmetry; and -Al 2 O 3 is cubic with essentially the spinel structure. 17 LEED reveals that the lattice constant of the as-grown alumina film, with respect to the O O distance, is approximately 0.28 nm, the same as bulk alumina. Note that these LEED data are not the same as seen for single crystal -Al 2 O 3 (0001), where the lattice constant is significantly larger the reciprocal space in LEED is smaller than that shown in Fig. 1 b. This means that our alumina films are bulk-like and probably oxygen terminated. However, the exact surface structure is uncertain, given only LEED data. During vanadium oxide growth on the alumina film a hexagonal 3 3 R30 LEED pattern was observed see Fig. 1 c. With increasing deposition of V in ambient O 2, the Al signal gradually decreased in AES. For 20 to 80 MLE coverages of vanadia, AES showed only V and O AES transitions, as depicted in Fig. 2. The various vanadium oxides are of either Wadsley phase (V 2n O 5n 2 ) or Magneli phase (V n O 2n 1 ). Due to the variable valence of vanadium, there are many possible stochiometries of vanadium oxide, e.g., VO, VO 2,V 2 O 3, and V 2 O 5, with different structures. Among them, VO has the rocksalt structure; VO 2, the rutile structure like titanium oxide; V 2 O 3, the corundum crystal structure with hexagonal symmetry; and V 2 O 5, the layered orthorhombic structure. Comparing our vanadia films grown on the ordered Al 2 O 3 /Mo 110 with these structures, only VO 111 and V 2 O 3 (0001) exhibits a hexagonal LEED pattern, therefore the VO 2 and V 2 O 5 forms can be excluded. The lattice parameter for the bulk VO 111 surface is 0.29 nm, which is much shorter than the lattice parameter 0.51 nm of the 0001 face of V 2 O 3. From our LEED data the lattice constant of the vanadia film is estimated to be 0.5 nm, a value very close to 0.51 nm found for V 2 O 3 (0001). The relative intensities of the oxygen to vanadium AES transitions has been used to ascertain the surface composi- J. Vac. Sci. Technol. A, Vol. 17, No. 4, Jul/Aug 1999

3 1889 Guo et al.: Ordered binary oxide films of V 2 O on Al 2 O FIG. 2. Auger electron spectrum of vanadium oxide film on Al 2 O 3 /Mo 110 at RT. A Clean Al 2 O 3 /Mo(110) surface; B 30 MLE vanadia film on Al 2 O 3 /Mo 110. The primary energy is 3 kev. FIG. 4. HREELS spectrum for a thick film, 80 MLE coverage, of V 2 O 3 on Al 2 O 3 /Mo 110. The spectrum was collected at 300 K. E p 6.0 ev. tion of various oxides. 18 Although quantitative analysis is difficult due to the overlapping AES features from oxygen and vanadium, the ratio of the vanadium 473 ev feature to the 437 ev feature can be used to distinguish the various vanadium oxides. In the AES spectrum of V 2 O 3 film grown on Au 111, the V 473 :V 437 peak-to-peak ratio is The V 473 :V 437 marked a and b in Fig. 2 B peak-topeak ratio is 1.0 for films of 10 to 80 MLE, consistent with V 2 O 3. The alumina supported vanadia films were further characterized with HREELS Fig. 3. The spectra were obtained at room temperature at an incidence angle of 60 from the surface normal. For the ordered Al 2 O 3 film on Mo 110 prior to vanadia film growth the loss energy of the dominant peak is at 896 cm 1 Fig. 3 a, with a second intense loss at 658 cm 1. Those values are in good agreement with results reported previously. 16,19 22 The commonly observed threepeak phonon spectra from alumina ultrathin films have been FIG. 3. HREELS spectra of vanadia film grown on Al 2 O 3 /Mo 110 at RT. a Clean Al 2 O 3 film with 30 MLE thickness on Mo 110 ; b 5 MLE vanadia film on Al 2 O 3 /Mo 110 ; c 10 MLE vanadia film on Al 2 O 3 /Mo 110 ; d 30 MLE vanadia film on Al 2 O 3 /Mo 110. shown to be due to oxide lattice vibrations. 22 Figure 3 a shows a third fundamental loss with reduced intensity at 390 cm 1. This arises from symmetry mixing with dispersion of the phonon modes away from the point, and surface defects. 22 Single crystalline -Al 2 O shows only two substantial surface phonon features at 806 and 496 cm 1. 23,24 With increasing deposition of V in O 2 ambient on the ordered Al 2 O 3 film the main surface phonon features are gradually attenuated. A new dominant loss at 640 cm 1 appears at vanadia film coverages 10 MLE see Figs. 3 b 3 d. A second fundamental loss is located at 386 cm 1. Figure 3 d shows the surface phonons for a vanadia film coverage of 30 MLE. Overtones and combinations, at 1036 and 1290 cm 1 also can be seen in the spectra. To ensure that there is no influence of the alumina support on the vibrational features, thicker 80 MLE vanadia films were prepared. These films were annealed to 900 K for one hour to enhance surface ordering and the HREELS measured at room temperature Fig. 4. On these surfaces, two main losses at 382 and 631 cm 1 are observed; the features at 1034 and 1264 cm 1 are combinations and/or overtones. The electron energy gain is also presented in Fig. 4. It should be noted that the phonons from V 2 O with a layered structure are quite different from those of our surface. On the V 2 O surface, nine fundamental phonon frequencies and six gain peaks with very small intensities were observed, 25 the most pronounced at 585 cm 1. These HREELS data were compared with infrared reflection data, and the single loss frequencies assigned. Comparing our data with those found previously for V 2 O 5 confirms that our vanadia films indeed are not V 2 O 5. The phonon spectrum of the vanadia film shown in Fig. 4 is significantly different from that of an alumina film in which three fundamental modes are apparent discussed above. On the other hand, the vibrational modes are quite similar to those of single crystal -Al 2 O and -Fe 2 O ,24,26 These materials exhibit the corundum structure as well, with two main fundamental surface phonon modes. A similar result for a Cr 2 O film grown on JVST A - Vacuum, Surfaces, and Films

4 1890 Guo et al.: Ordered binary oxide films of V 2 O on Al 2 O FIG. 5. XPS on V 2 O 3 film with 20 MLE thickness on Al 2 O 3 /Mo 110. Mg K source, h ev. Cr 110 has also been reported, in which two primary Fuchs Kliewer surface phonon modes were found. 27 -Al 2 O 3, -Fe 2 O 3,Cr 2 O 3, and V 2 O 3 are of the same crystal structure, and thus should exhibit typical Fuchs Kliewer phonon modes whose properties are determined primarily by the bulk properties, e.g., the transverse optical phonon frequency ( ro ). These results suggest that the films in our study are very close to the bulk vanadia structure. The LEED data from supported V 2 O 3 ultrathin films are essentially the same as from bulk V 2 O 3. In contrast, for the alumina ultrathin film, LEED data are as expected from an oxygenterminated surface oxygen-to-oxygen distances, unlike that of single crystal alumina. The dominant loss feature 631 cm 1 from the vanadia film of Fig. 4 is anticipated to be lower than that from alumina 806 cm 1 23,24 but higher than that for -Fe 2 O cm 1 26 based on mass differences, i.e., a V atom is more massive than an Al atom, but less massive than an Fe atom. It should be noted that the primary phonon loss feature from V 2 O 3 is quite close to the 629 cm 1 feature of -Fe 2 O 3 and also this feature in Fig. 4 is not symmetric. These arise because the V 2 O 3 surface phonon loss changes as a function of temperature. That is, the phonon frequency varies as the lattice constant of V 2 O 3 changes with temperature. This correlation will be discussed further below. XPS data for a 20 MLE vanadia film on Al 2 O 3 /Mo 110 are shown in Fig. 5. Vanadias, such as VO 2,V 2 O 3,V 2 O 5, and V 6 O 13, have been investigated by photoemission studies. XPS yields binding energies of V 2p 3/2 of 516.7, 515.5, and ev for V 2 O 5, V 2 O 3, and VO 2, respectively In Fig. 4 the binding energy of the V 2p 3/2 feature is ev and the O 1s, ev. These values agree with previous XPS studies on the V 2 O 3 surface. The peak shapes are very similar to those obtained from single crystals of V 2 O 3, 33 consistent with our films being V 2 O 3. Various vanadium oxides have different thermal stabilities. For example, some vanadium oxides are easily transformed to a different phase after heating in UHV. V 2 O 5 can be transformed to V 4 O 9 by heating at 660 K in UHV. On the FIG. 6. ISS of vanadia film on Al 2 O 3 /Mo 110. Primary energy of He ion is 550 ev. other hand, VO 2 is stable to 500 K in UHV, while V 2 O 3 polycrystalline is stable at 850 K for 14 h in UHV. 28 The vanadium oxide films in this study are stable to 950 K, consistent with them being V 2 O 3. ISS was also used to characterize the vanadia films. Figure 6 shows ISS data as a function of the vanadia coverage. At 1.0 MLE of vanadia on the alumina/mo 110 surface, the Al signal is considerably attenuated, and at 2.0 MLE coverage, the Al signal has essentially disappeared. These results are consistent with layer-by-layer growth of vanadia on the alumina film. V 2 O 3 undergoes a metal insulator transition as a function of temperature. 34 It is metallic at room temperature, and converts to an insulator at temperatures between 140 and 168 K. 29,34 41 It has been found that the metal insulator transition of V 2 O 3 is similar to that of VO 2 and Ti 2 O 3. In the case of VO 2, for example, the transition can be explained by the changes in the 3d-band structure. 42 The 3d valence band consists of two features in metallic VO 2. The high density of states at the Fermi level is associated with the * band. Upon converting to the insulator phase, the * band of VO 2 separates from the Fermi level and empties. Ultraviolet photoelectron spectroscopy UPS, in turn, shows a single, occupied 3d band assigned to the lower d band. 33,43 Since the films in this study have been shown to be V 2 O 3, it was of interest to probe the electronic structure using HREELS. It is believed that the metal insulator transition is due to changes in the 3d band. In V 2 O 3, the width of the 3d state is 3 ev accompanied by overlap of the a 1g and e g states; the density of 3d states at the Fermi level is very low. J. Vac. Sci. Technol. A, Vol. 17, No. 4, Jul/Aug 1999

5 1891 Guo et al.: Ordered binary oxide films of V 2 O on Al 2 O FIG. 7. Comparison of the electron transitions for a 80 MLE V 2 O 3 film on the Al 2 O 3 /Mo 110 as a function of temperature. Primary beam energy, E p 25 ev. The 3d band consists of in increasing energy order e g, a 1g, e g * and a 1g *. 33,36,37,39,40 Figure 7 shows the electronic structural change of the V 2 O 3 film as a function of temperature. The spectra were collected with a primary beam energy, E p, of 25 ev from the identical surface at 97 and 300 K, respectively. At room temperature, two losses are distinguishable at 0.9 and 1.6 ev; at 97 K a new loss feature at 1.2 ev is observed. The loss corresponding to the 1.6 ev feature remains unchanged at the lower temperature. These losses are tentatively assigned to d d transition from the 3d band of vanadium. The loss at 0.9 ev is attributed to the e g to a 1g transition, and that at 1.6 ev to the e g to a g * transition. Interestingly, the loss at 0.9 ev at 300 K shifts to 1.2 ev at 97 K. Apparently the d d transition depends strongly on temperature, as shown in Fig. 7. At 300 K, the d d transitions present a continuous manifold, but at low temperatures, e.g., 97 K, there is a gap of 0.7 to 0.8 ev. At 97 K, the width of the d d transition band is 2.6 ev, much narrower than that at 300 K. There are two possibilities which can account for the loss at 0.9 ev shifting to 1.2 ev at lower temperatures. One is separation of the e g * feature from the Fermi level, as for VO 2. The e g * energy level destabilizes leading to a higher apparent transition energy. Another possibility is destablization of the a 1g state. Since the loss at 1.6 ev was unchanged for the two phases, it is most likely that the a 1g is destablized i.e., the entire manifold is not shifted, therefore it is more likely that the relative energy levels are changing. The a 1g shifts to higher energy and empties resulting in an increase in the band gap. This is likely the result of a lattice expansion along the c axis. 42 In fact, as the metalinsulator occurs, changes in the surface phonon region of the vibrational spectra are apparent. Figure 8 shows HREELS spectra from a vanadia/alumina surface as a function of temperature. The primary loss at 631 cm 1 at 300 K shifts to 662 cm 1 at 97 K. Furthermore the loss feature is symmetric and enhanced in intensity. The overtone and the combination of the surface phonons change as well, due to the shift of the primary loss. These are attributable to a change in the lattice constant, which is a consequence of increased hybridization. FIG. 8. HREELS spectra changes as a function of the temperature. The thickness of the V 2 O 3 film on Al 2 O 3 /Mo 110 is 80 MLE. E p 6 ev. This film was annealed to 910 K for 1 h prior to taking the spectra; a received at room temperature; b received at 97 K. IV. CONCLUSION Ordered binary oxide film of vanadia films on alumina have been prepared. Surface analysis techniques show that the vanadia films grow epitaxially on Al 2 O 3 /Mo 110 as V 2 O 3. LEED data show the V 2 O 3 films are oriented At low temperatures, destabilization of the a 1g state in the 3d band is suggested by changes in the electronic structure as measured by HREELS. These films provide an excellent model for further studies in materials science and catalysis. ACKNOWLEDGMENTS The authors gratefully acknowledge the support of this work by the Department of Energy. Q.G. acknowledges support from an AWU Fellowship at Pacific Northwest National Laboratories. D.Y.K. acknowledges the support from Hallym Academy of Science, Hallym University. 1 D. W. Goodman, Chem. Rev. 95, H. J. Freund, Angew. Chem. Int. Ed. Engl. 36, H. J. Freund, H. Kuhlenbeck, and V. Staemmler, Rep. Prog. Phys. 59, M. L. Burke and D. W. Goodman, Surf. Sci. 311, Ch. Grundling, J. A. Lercher, and D. W. Goodman, Surf. Sci. 318, Q. Guo, C. Xu, and D. W. Goodman, Langmuir 14, S. C. Street, C. Xu, and D. W. Goodman, Annu. Rev. Phys. Chem. 48, P. L. J. Gunter, J. W. Niemantsverdriet, F. H. Ribeiro, and G. A. Somorjai, Catal. Rev. Sci. Eng. 39, G. Deo and I. E. Wachs, J. Catal. 146, K. Tran, M. A. Hanning-Lee, A. Biswas, A. E. Stiegman, and G. W. Scott, J. Am. Chem. Soc. 117, K. B. Lewis, S. T. Oyama, and G. A. Somorjai, Surf. Sci. 233, K. Kishi and K. Fujiwara, J. Electron Spectrosc. Relat. Phenom. 71, K. Kishi, Y. Hayakawa, and K. Fujiwara, Surf. Sci. 356, K. Kishi and K. Fujiwara, J. Electron Spectrosc. Relat. Phenom. 85, H. L. M. Chang, H. You, J. Guo, and D. J. Lam, Appl. Surf. Sci. 48/49, M-C. Wu and D. W. Goodman, J. Phys. Chem. 98, JVST A - Vacuum, Surfaces, and Films

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