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N2O Adsorption on the Surface of MgO(001) Thin Films: An Infrared and TPD Study J. C. Lian,†,‡ E. Kieseritzky,† A. Gonchar,† M. Sterrer,† J. Rocker,† H.-J. Gao,‡ and T. Risse*,† Fritz-Haber-Insitut der MPG, Department of Chemical Physics, Faradayweg 4-6, 14195 Berlin, Germany, and Institute of Physics, Chinese Academy of Sciences, 100190 Beijing, China ReceiVed: NoVember 20, 2009
The adsorption of N2O on the surface of MgO(001) thin films has been studied at low temperature (60 K) using infrared reflection-absorption spectroscopy (IRAS) and temperature-programmed desorption (TPD). The observed infrared spectrum consists of several components indicating different adsorption sites for N2O. The different IR peaks can be related to particular thermal desorption features by combining the TPD spectra with temperature-dependent infrared measurement. By comparing spectra from films with different roughness and different supports, a band at 2236 cm-1 can be assigned to N2O adsorbed on the MgO terraces, while peaks at the higher frequency are assigned to molecules adsorbed on low-coordinated sites. 1. Introduction Understanding oxide surfaces at an atomistic level has been a very actively pursued field of research in the recent past1-3 due to the importance of these systems in a variety of technological applications, such as heterogeneous catalysis, corrosion protection, sensor development, or microelectronics. In particular, surface defects, such as steps or low-coordinated sites, are considered to play a pivotal role for the chemical and physical properties of these surfaces.4-6 Therefore, a characterization of surface defects is important to understanding the properties of oxide surfaces. Single-crystalline oxide films have shown to be valuable model systems to provide atomistic insight into such questions. In particular, MgO is an often used model system because it is considered the prototype of an ionic oxide with a simple rock salt structure, and extensive literature is available for well-defined powder material, bulk single crystals, and thin single crystalline films.7-12 Apart from often discussed defects, such as steps, corners, or kinks, it was recently shown that point defects, namely, oxygen vacancies, present on MgO(001) surfaces can influence the properties of metal deposits such as Au.13-15 Another class of point defects that is considered to be important for particular catalytic reactions, such as methane activation over Li-doped MgO, is O- radical ions.16-21 To this end, the interaction of metal oxide surfaces with N2O has attracted considerable attention because of its ability to serve as a source of atomic oxygen. In particular, the reaction of unpaired electrons trapped on MgO surfaces with N2O leads to the formation of O- radical ions and the release of molecular nitrogen. Despite the extensive literature on the reactivity of N2O on MgO surfaces, experimental and theoretical investigations characterizing the adsorption properties of N2O on welldefined MgO are rather scarce and focus, as far as the experimental investigations are concerned, on the analysis of a well-ordered N2O phase on the ideal terrace of MgO(100) surfaces.22,23 In the present work, we report on a combination of infrared spectroscopy (IRAS) and temperature-programmed desorption (TPD) of N2O adsorbed on 20 ML MgO(001) films grown on * To whom correspondence should be addressed. E-mail: risse@ fhi-berlin.mpg.de. † Fritz-Haber-Insitut der MPG. ‡ Chinese Academy of Sciences.
Mo(001). By comparing results obtained on annealed films with large terrace sizes with those on more defective pristine MgO films, it is possible to assign the IR bands to N2O adsorbed on different sites on the MgO(001) surface. 2. Experimental Section MgO(001) thin films were prepared on a Mo(001) substrate in a UHV system described in detail elsewhere.24 The MgO films were prepared according to recipes reported in the literature.15,25 Briefly, the Mo crystal was cleaned by oxidation in 1 × 10-6 mbar of O2 at 1500 K and subsequently flashed to 2200 K several times. MgO thin films were prepared by evaporating Mg in an oxygen ambient (1 × 10-6 mbar) at a substrate temperature of 600 K. Films of 20 ML (monolayer) thickness were grown for all experiments with a rate of 1 ML/ min. Films produced in this way are called “pristine” MgO film throughout the rest of this paper. To decrease the number of the low-coordinated sites, the freshly prepared film was annealed to 1100 K for 10 min. These systems are called annealed films, and the improvement of the lateral order was verified using lowenergy electron diffraction as well as IR spectroscopy using CO as a probe molecule. The sample was mounted to a He cryostat that allows cooling the sample to around 60 K. The temperature was measured by a thermocouple (type C) spotwelded to the upper edge of the Mo crystal using a commercial temperature controller (Schlichting Physikalische Instrumente). IR experiments were performed in grazing incidence reflectionabsorption geometry using a BioRad FTS 40 VM spectrometer. The spectral resolution was 4 cm-1, and 1000 scans were accumulated to obtain a reasonable signal-to-noise ratio. The heating rate in the TPD experiment was 0.5 K/s. 3. Results and Discussion Figure 1 shows infrared spectra after isothermal N2O adsorption on an annealed MgO film at 63 K as a function of dosage. The spectra reveal two main bands, which come from the asymmetric stretching vibration (ν3) and the symmetric stretching vibration (ν1) of N2O molecules. At very low coverage, the ν3 band has a sharp peak at 2249 cm-1 and a shoulder around 2270 cm-1, which are blue shifted by 22 and 43 cm-1 as compared with the gas phase (2223.9 cm-1), respectively. The
10.1021/jp911060x 2010 American Chemical Society Published on Web 01/29/2010
N2O Adsorption on the Surface of MgO(001) Thin Films
Figure 1. Infrared spectra taken after different amounts of N2O dosed at 63 K on the annealed MgO(001) films. N2O dosage in langmuirs (1 L ) 10-6 Torr s).
Figure 2. Infrared spectra of 1 L of N2O at 63 K on the MgO(001) films after film deposition and after annealing to 1100 K.
ν1 band consists of a single peak at 1265 cm-1 that is red shifted with respect to the gas phase ν1 ) 1286 cm-1.26,27 With increasing coverage, the main peak as well as the shoulder in the ν3 region gains intensity and shifts slightly to lower frequencies. Concomitantly, a new peak appears at 2236 cm-1, which becomes the most prominent feature at saturation of the monolayer. At monolayer coverage, there is an indication for the presence of a shoulder around 2224 cm-1 that can be seen best in the lower trace of Figure 2. In contrast, virtually no change of the shape is found for the ν1 band, but its position shifts gradually up to 1276 cm-1 with increasing coverage. Beyond saturation of the monolayer, two new features at 2255 and 1298 cm-1 appear, whose intensity grows linearly with increasing dosage. These two peaks can be assigned to the respective N2O vibrations in the multilayer regime. The complexity in the ν3 band may indicate different adsorption sites of N2O molecules at low temperature. However, a direct assignment of the individual infrared peaks to particular adsorption sites is not straightforward. To shed some light on the nature of the adsorption sites, the spectra observed on the annealed films are compared to those measured on pristine films. The latter surface is characterized by a significantly larger fraction of structural defects, such as steps, edges, or kinks, which was previously inferred from LEED and infrared experiments using CO as a probe.12,28 The comparison of the infrared spectra obtained on both surfaces, dosing the same nominal
J. Phys. Chem. C, Vol. 114, No. 7, 2010 3149 amount of N2O, is shown in Figure 2. The line shapes of the two preparations differ considerably. Whereas the spectrum of the annealed film is dominated by the peak at 2236 cm-1, the pristine films exhibits a pronounced increase of the line around 2246 cm-1. On the basis of the structural knowledge acquired before, the line at 2236 cm-1 can be associated with N2O adsorbed on terrace sites, while the higher-frequency features should correspond to defect sites, such as steps or edges. This picture is corroborated by measurements on nominally 20 ML thick MgO(100) films grown on Ag(100). These films are known to exhibit a smaller number of defects due to the smaller lattice mismatch of the MgO film with the Ag substrate. The MgO films on Ag(100) show the same bands; however, the relative intensities of the peaks in the range of the ν3 vibration are different. The defect-related band at 2270 cm-1 becomes very weak, and the feature at 2246 cm-1 is decreased in intensity relative to the line at 2236 cm-1 assigned to terrace sites.29 In addition, the intensity of the shoulder at 2224 cm-1 is increased. The prominent reduction of the band at 2270 cm-1 indicates that the corresponding surface sites are almost absent on the films grown on Ag(100). On the basis of the structural investigations, this band is likely associated with N2O molecules adsorbed on dislocations that are much more prominent on the Mo substrate due to the larger lattice mismatch between MgO and the substrate.11,30 For N2O adsorbed on terrace sites of in situ cleaved MgO(001) single-crystal surfaces, Heidberg et al. have found ν3 bands shifted further to the red as compared with the ones observed here.22 The authors assign a pair of lines at 2217.0 and 2229.6 cm-1 to the antisymmetric and the symmetric combination vibration of the ν3 band split by a correlation field. It is interesting to note that similar values were also observed for N2O dimers in matrices.31 The existence of such a high coverage phase in which intermolecular interactions show up in the IR spectra seems to depend on the adsorption temperature as well as the defect density. Following the recipe of Heidberg and adsorbing the N2O at about 80 K does indeed produce a band around 2214 cm-1; however, the line is rather broad and much less intense than the one reported by Heidberg. This is in line with the significantly higher defect concentration on the MgO films as compared to UHV cleaved single crystals.12,30,32,33 On the other hand, the formation of such a densely packed, ordered N2O layer, which gives rise to a correlation field splitting in IR, is suppressed when adsorption of the molecules take place at low temperature, indicating a barrier for the formation of the ordered layer. Interesting additional information can be obtained by combining the IR results with data from thermal desorption experiments. Figure 3 gives the thermal desorption behavior of N2O from annealed films taken directly after the adsorption experiment shown in Figure 1. To ensure that the signal of mass 44 amu originated from N2O, the cracking fragment NO+ (30 amu) was monitored simultaneously. The constant intensity ratio between the mass 44 and the mass 30 signal shown in Figure 3 indicates the observed signal to be due to N2O desorption and not obscured by CO2 adsorption from the background. The huge desorption peak at 77 K, which does not saturate with increasing coverage, can be assigned to N2O multilayer desorption. The multilayer peak exhibits a small shoulder on the hightemperature site, around 84 K, indicating the presence of a desorption species that is largely buried by the multilayer peak. Another desorption peak is found at 100 K, followed by a broad feature ranging up to 140 K that may consist of several components. In an attempt to correlate the TPD peaks with the observed IR signals, IR spectra were taken after annealing to
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Figure 3. TPD spectra taken after dosing 3 L of N2O at 63 K on the annealed MgO(001) films. The heating rate was 0.5 K/s. Black, 44 amu; gray, 30 amu.
Figure 4. Infrared spectra taken after dosing 2.5 L of N2O at 65 K on the annealed MgO(001) films and then annealing to different temperatures. The spectra are all measured at 65 K.
different temperatures. Figure 4 shows the infrared spectra taken after dosing 2.5 L of N2O on annealed films at 65 K. Prior to the measurements performed at 65 K, the sample was annealed to the temperature indicated in the plot. The spectrum taken directly after adsorption of N2O has all the bands mentioned above. Annealing to 80 K depletes the multilayer peaks at 2255 and 1298 cm-1 selectively, which is in line with the desorption peak at 77 K. The peak at 2236 cm-1 that was assigned to molecules adsorbed on terrace sites is reduced after annealing to 80 K and vanishes completely after heating to 90 K. Thus, we correlate this peak to the shoulder in the TPD spectrum around 84 K. After heating up to 105 K, the broad infrared band around 2270 cm-1 vanished and the spectrum consists of a peak at 2249 cm-1 that disappears after annealing to 150 K. During the annealing process, the ν1 band loses intensity and shifts back to the red with increasing temperature, which is just the reverse behavior compared with the adsorption experiment shown in Figure 1. Within experimental accuracy, the intensity ratio of the ν3 and ν1 bands remains constant at around 6:1. This renders an observation of the ν1 band after annealing above 105 K difficult due the signal-to-noise limitations. As far as IR intensities are concerned, it is interesting to note that the total IR intensity after annealing to 105 K is only about 15% of the intensity found after annealing to 90 K. However, the N2O surface coverage, as judged by the TPD spectra, is reduced only by about 50% in this temperature range. This indicates
Lian et al. significantly different relative intensities of N2O molecules for these two adsorption sites, which may be due to differences in oscillator strength or adsorption geometries. The desorption energy of N2O from different sites can be estimated from the TPD data using Readhead’s equation.34 Assuming a first-order desorption and frequency factor of 1013, molecules adsorbed on the terraces associated with an infrared peak at 2236 cm-1 have the lowest desorption energy of ∼0.23 eV. The TPD peak at 100 K, which is correlated with the infrared peak at 2270 cm-1, corresponds to a binding energy of ∼0.27 eV. The infrared peak at 2249 cm-1 originates from molecules bound strongest with a desorption energy of more than 0.3 eV. This result is consistent with the idea that gas molecules can bind stronger on the low-coordinated sites than the terrace site of the MgO(001) surface, which is wellestablished for CO adsorbed to MgO(001).35 This is in agreement with the results calculated by Xu.23 The calculations suggest that N2O molecules prefer to adsorb with the N end down on Mg2+ sites of the MgO(001) surface, irrespective of the adsorption site. However, the energetic differences between the N down and the O down configuration are typically only 50 meV, which is at least close to the error margin of the calculation. For terrace, edge, and corner sites, the calculated adsorption energies are 0.22, 0.25, and 0.61 eV, respectively. Although the adsorption energies for terrace and edge sites compare well with the experimental results, the calculated adsorption energy for N2O molecules on corner sites deviates beyond the experimental uncertainty. It is worth noting that Scagnelli et al. failed to find bound N2O molecules on terrace sites using a very similar theoretical approach, which renders it difficult to judge the reliability of the theoretical results.36 At full coverage, the ν3 band has several components ranging from 2236 cm-1 up to the shoulder around 2270 cm-1, which are blue shifted as compared with the gas phase, 2223 cm-1. A part of this shift can be understood by dipolar interactions between molecules, as judged by comparison with the multilayers at 2255 cm-1. The increasing blue shift of the ν3 band for lower-coordinated adsorption sites indicates that additional electrostatic and polarization effects play a considerable role for the adsorption of N2O, which is also reflected in the different adsorption energies, as discussed above. The ν1 band shows a single line at 1276 cm-1, indicating that this vibration is much less sensitive to the details of the adsorption site as compared with the ν3 vibration. On the other hand, this band is red shifted with respect to the gas phase (1286 cm-1) and the multilayer (1298 cm-1) alike. On metal surfaces, such as Pt(111), Ir(111), or Ru(0001), the ν1 vibration was observed very close to the value found for multilayers, which indicates little perturbation of this vibration as compared to free molecules.37-39 The red shift of the ν1 band, however, is not readily explainable by intermolecular interactions. These interactions are present and result in the expected blue shift of the line with increasing coverage. The observed red shift as compared to the gas phase indicates a modification of the electronic structure induced by the bonding. Similar red shifts of the ν1 band have also been observed on NaCl(001), R-Cr2O3 powder, as well as in molecular complexes, where the N2O is bound to cationic transition metals, such as Ru.40-43 In the case of R-Cr2O3 powder, the red shifted bands in the ν1 stretch region were assigned to N2O molecules bound with the oxygen end down.42,43 4. Conclusions In summary, we have presented a combined infrared and thermal desorption study of N2O molecules adsorbed on the
N2O Adsorption on the Surface of MgO(001) Thin Films surface of MgO(001) thin films at low temperature. The infrared spectra show several bands related to molecules adsorbed on different surface sites. From TPD spectra and temperaturedependent infrared measurement, the desorption energies corresponding to each infrared band can be deduced and assigned to molecules bound on regular surfaces sites as well as structural defects, which is in reasonable agreement with theoretical results. Acknowledgment. J.C.L. and A.G. thank the IMPRS “Complex Surfaces in Materials Science” for financial support. E.K. thanks the Studienstiftung des deutschen Volkes for a scholarship. This work was partially supported by the European Union through the center of excellence IDECAT and the COST program D41. The authors also acknowledge support from the Cluster of Excellence “Unifying Concepts in Catalysis” coordinated by the Technische Universita¨t Berlin and funded by the Deutsche Forschungsgemeinschaft. References and Notes (1) Woodruff, D. P., Ed. The Chemical Physics of Surfacs: Oxide Surfaces; Elsevier: Amsterdam, 2001; Vol. 9. (2) Goodman, D. W. J. Catal. 2003, 216, 213. (3) Nilius, N. Surf. Sci. Rep. 2009, 64, 595. (4) Pacchioni, G. ChemPhysChem 2003, 4, 1041. (5) Freund, H. J.; Kuhlenbeck, H.; Staemmler, V. Rep. Prog. Phys. 1996, 59, 283. (6) Pang, C. L.; Thornton, G. Surf. Sci. 2009, 603, 3255. (7) Coluccia, S.; Barton, A.; Tench, A. J. J. Chem. Soc., Faraday Trans. 1 1981, 77, 2203. (8) Chiesa, M.; Giamello, E.; Di Valentin, C.; Pacchioni, G.; Sojka, Z.; Van Doorslaer, S. J. Am. Chem. Soc. 2005, 127, 16935. (9) Heidberg, J.; Kandel, M.; Meine, D.; Wildt, U. Surf. Sci. 1995, 331-333, 1467. (10) Wu, M. C.; Corneille, J. S.; He, J. W.; Estrada, C. A.; Goodman, D. W. J. Vac. Sci. Technol., A 1992, 10, 1467. (11) Wollschla¨ger, J.; Viernow, J.; Tegenkamp, C.; Erdo¨s, D.; Schro¨der, K. M.; Pfnu¨r, H. Appl. Surf. Sci. 1999, 142, 129. (12) Sterrer, M.; Risse, T.; Freund, H.-J. Surf. Sci. 2005, 596, 222. (13) Sterrer, M.; Yulikov, M.; Fischbach, E.; Heyde, M.; Rust, H. P.; Pacchioni, G.; Risse, T.; Freund, H.-J. Angew. Chem., Int. Ed. 2006, 45, 2630.
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