J. Phys. Chem. C 2007, 111, 6033-6040
6033
Oxygen Adsorption and Oxide Formation on V(100) Surfaces Min Shen, Qiang Ma, Ilkeun Lee, and Francisco Zaera* Department of Chemistry, UniVersity of California, RiVerside, California 92521 ReceiVed: January 18, 2007; In Final Form: February 27, 2007
The initial stages of the uptake of molecular oxygen on V(100) single-crystal surfaces were characterized by temperature programmed desorption (TPD), X-ray photoelectron spectroscopy (XPS), and ion scattering spectroscopy (ISS). As in previous reports on this system, a small amount of oxygen contamination was always detected on the surface, but that could be minimized by avoiding severe annealing after sputtering of the surface and appears to not affect the chemistry of the surface in any significant way, at least in terms of methanol conversion. Oxygen adsorption is dissociative even at liquid nitrogen temperatures. At low temperatures, saturation is reached at a coverage of approximately 1.0 ML, but above 170 K further uptake can occur, leading to the formation of a thin V2O3 + VO2 mixed oxide film. Thick oxide films, with thicknesses of the order of nanometers, could only be generated above 320 K and by using high oxygen doses. In general, the temperature at which the adsorption is carried out proved to play an important role in determining the surface stoichiometry and film thickness of the resulting surface oxides. Diffusion of oxygen into the bulk starts at approximately 500 K and displays an estimated activation energy of 132 kJ/mol.
1. Introduction Vanadium oxides are commonly used as catalysts for a number of industrial processes, in particular, for the partial oxidation of hydrocarbons,1,2 can also form on the surface of metallic vanadium, and can affect its performance as a hydrogen storage material.3,4 A better understanding of the nature of the oxygen species that form on vanadium surfaces could be useful in the interpretation of the information available on those applications; hence the recent interest shown by a handful of surface-science groups on the chemistry of oxygen-treated vanadium surfaces. Here we focus on the early stages of this oxygen uptake on V(100) single-crystal surfaces. A few reports have already appeared in the literature concerning the chemisorption of molecular oxygen on wellcharacterized vanadium surfaces, including those of V(100) single crystals.5-12 However, much of that work has focused on the characterization of the reconstructed surfaces that form upon the oxygen uptake, in particular a (5 × 1) superstructure commonly seen in this system, on the growth of oxide films, and on the influence of the structural changes induced by surface oxygen on hydrogen adsorption.7,9,13-15 Also, most of those studies have dealt with adsorption at room temperatures and usually with large oxygen exposures. A systematic characterization of the early stages of oxygen adsorption on vanadium, in particular to understand the influence of temperature and oxygen dosage on the chemistry of oxygen adsorption and oxide formation, is still not available. The present study was designed to help cover this deficiency. In our work, reasonably clean V(100) single-crystal surfaces could be obtained by extensive sputtering and annealing cycles, even though a small amount of oxygen was seen at all times, as in previous reports. That oxygen contaminant was determined by analysis using a combination of X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES), and ion scattering spectroscopy (ISS) to be located on the topmost * Corresponding author. E-mail:
[email protected].
layer of the surface and to exhibit a different reactivity toward methanol conversion from that seen after oxygen predosing directly from the gas phase. The interaction of oxygen with the clean V(100) surface was then explored systematically from 110 to 600 K. Four distinctive regions were identified according to the adsorption behavior observed: (1) from 110 to 170 K, where oxygen adsorbs dissociatively until reaching saturation at approximately 1 monolayer of coverage; (2) from 170 to 320 K, where a further thermally activated surface oxidation process yields a thin V2O3 layer; (3) from 320 to 500 K, a regime where the oxide layer can grow to a thickness of several nanometers; and (4) above 500 K, at which point oxygen starts to diffuse into the bulk, thus preventing the buildup of a thick surface oxide layer. The influence of both temperature and O2 exposures on this vanadium oxidation chemistry is discussed in detail below. 2. Experimental Section Most of the XPS, AES, and temperature-programmed desorption (TPD) experiments reported here were performed in an ultrahigh-vacuum (UHV) chamber with a base pressure of about 2 × 10-10 Torr. The equipment has been described in detail in previous publications.16,17 Both XPS and AES spectra were taken using a magnesium/aluminum dual anode X-ray source and a 50 mm hemispherical electron energy analyzer set up at a constant pass energy of 50 eV, resulting in an overall resolution of about 1.2 eV full-width-at-half-maximum. The energy scale was referenced to a binding energy value for Cu 2p3/2 of 932.7 eV.18 The resulting data were analyzed with the aid of the XPSPEAK fit software:19 a Shirley background was subtracted from the raw data, and the O 1s and V 2p peaks were then fitted with mixed Lorentizian-Gaussian curves. The O 1s and V 2p XPS data were processed together in order to minimize any mutual interferences,20 and the fitting parameters for the vanadium metallic and oxide forms were fixed for all of the spectra to ensure comparable results.
10.1021/jp070444i CCC: $37.00 © 2007 American Chemical Society Published on Web 03/31/2007
6034 J. Phys. Chem. C, Vol. 111, No. 16, 2007 ISS (and additional corroborating XPS) experiments were carried out in a second, similar, UHV chamber, also described in detail before.21,22 The ISS measurements were performed by using a 30-50 nA, 500 eV He+ ion beam, a scattering angle of 90°, and a 100 mm hemispherical electron energy analyzer with a pass energy fixed at 125 V. The areas under the different ISS peaks were taken to be directly proportional to the concentrations of the corresponding elements on the topmost surface, and were used to quantify their surface coverages. Although this is not strictly accurate because ISS yields depend on screening factors reflective of shadowing and neutralization effects,23,24 it appears to be a reasonable approximation in this case: the results from ISS obtained here are consistent with the accompanying XPS and AES measurements, and linear relations between ISS signals and surface coverages have been reported for similar systems in the past.25,26 The V(100) single crystal (99.99% purity, Goodfellow Cambridge Limited, ∼12 mm diameter × ∼2 mm thickness) was cut and polished using standard procedures, and spot-welded to two tantalum rods attached to an on-axis manipulator capable of rotation and translation along all three x-y-z dimensions. The crystal could be cooled down to liquid-nitrogen temperatures and resistively heated to above 1300 K. The crystal temperature was measured by a chromel-alumel thermocouple spot-welded to the back of the crystal, and varied in a controlled manner by using homemade electronics. The oxygen (99.99% purity) and argon (99.999%) gases were purchased from Matheson and used as supplied. Methanol (99+%) was purchased from Aldrich and used after purification via several freeze-pump-thaw cycles. The purity of all of the chemicals was checked frequently in situ by mass spectrometry. Gas doses are reported in langmuirs (1 L ) 10-6 Torr‚s), uncorrected for differences in ion gauge sensitivities.
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Figure 1. AES, in raw N(E) (left panel) and differential dN(E)/dE (right panel) forms, from a V(100) single-crystal surface: (A) freshly sputtering, (B) after heating to 1250 K for 2 min, and (C) after sputtering and dosing with 2.0 L of O2 at 320 K. The peaks at 472 and 509 eV correspond to vanadium; the ones at 493 and 513 eV correspond to oxygen. Note that the surface right after sputtering contains only traces of oxygen but that it regains significant oxygen after annealing. All spectra were taken at 320 K.
3. Results 3.1. Characterization of the Sputtered and Annealed V(100) Surfaces. It has been previously noted that cleaning of the V(100) surface is difficult because it is nearly impossible to remove the final residual oxygen,9,27 and that proved to be the case in our studies as well. In this work, the sample was initially held at 1250 K for tens of hours to purge the hydrogen from the bulk, and then subjected to hundreds of cycles of sputtering at 500 K and annealing above 1250 K until no carbon, sulfur, or phosphorus could be detected within the probing ability of our instruments. At that point, a small amount of oxygen was still evident by a small XPS feature at 531.2 eV, as also reported before.5,9 A better estimate of the level of this contaminant was obtained by AES data such as those illustrated in Figure 1, which displays both raw N(E) (left) and differential dN(E)/dE (right) spectra obtained after the V(100) surface was subjected to three different treatments: (A) immediately after sputtering; (B) after annealing at 1250 K; and (C) after dosing 2.0 L of O2 at 320 K. It can be seen from those traces that the freshly sputtered vanadium surface contains only a negligible amount of oxygen, and that it is the subsequent heating of the sample to 1250 K that leads to oxygen segregation from the bulk to the surface, as clearly manifested by the appearance of peaks at 493 and 513 eV in the dN(E)/dE spectra (middle traces). This oxygen segregation continued even after many (several weeks) cleaning cycles, and proved reproducible within a 10% margin of error in both the AES and O 1s XPS data. For reference, spectra are also shown in Figure 1 for a surface exposed to 2.0 L of O2 at 320 K, which, according to reported calibrations,10,28 leads to a surface coverage of 1.67 ML of
Figure 2. ISS for the same three surfaces as in Figure 1. The peaks at 250 and 400 eV correspond to oxygen and vanadium atoms, respectively. Again, it is seen here that while the sputtered surface only shows traces of oxygen, significant oxygen concentrations are detected in the topmost surface after annealing.
oxygen atoms (1 ML ) 1 adsorbed oxygen atom per vanadium surface atom). On the basis of that number, the oxygen coverages for the sputtered and annealed surfaces seen here are then estimated to correspond to approximately 5-10% of a monolayer and ∼0.5 ML, respectively. The nature and thermal behavior of the oxygen contamination was further characterized by ISS. Figure 2 shows the ISS spectra acquired for the freshly sputtered surface (top trace), for the same surface after heating to 1250 K for 2 min (middle trace), and for a sputtered surface predosed with 0.2 L of O2 at 320 K (bottom trace). Two broad peaks were seen in all cases around 250 and 400 eV, corresponding to surface oxygen and vanadium atoms, respectively. As in the AES studies, negligible (5-10%) amounts of oxygen are seen by ISS of the surface after extensive
O2 Adsorption & Oxide Formation on V(100) Surfaces
Figure 3. Methane TPD traces from 0.25 L of CH3OH adsorbed at 110 K on V(100) right after sputtering (middle trace), following annealing at 1250 K (bottom), and after sputtering and dosing 0.2 L of O2 at 320 K (top). The methane TPD from the sputtered and annealed surfaces are almost identical, whereas the one after the oxygen pretreatment shows an increase in the yield of methane production at high temperatures.
sputtering. On the other hand, a clear increase in surface oxygen is seen upon annealing to high temperatures, at which point an oxygen surface coverage of approximately 0.5 ML is reached. Notice that this value, while consistent with our AES measurements, is lower that those reported under similar conditions in previous studies.9,12 What can be said here is that ISS, with its particular surface sensitivity, provides specific information on the composition of the topmost layer of the surface; previous studies have relied on techniques that average the composition of a thin but several-layers thick film instead. The surface composition determined by ISS for the annealed surface is comparable to that obtained after dosing 0.2 L of O2 on the sputtered surface, suggesting similar structures, and indeed previous STM and LEED work have indicated the formation of a (5 × 1)-reconstructed surface structure in both cases,7,11,12 but the chemical studies reported next argue against this conclusion. Perhaps the possibility of removing the (5 × 1) surface reconstruction by gas adsorption may depend on the composition of the subsurface layers. The chemical behavior of these surfaces was briefly tested by using methanol as a probe, since the thermal activation of methanol adsorbed on V(100) produces methane with kinetics highly dependent on the surface conditions.29 Methane TPD spectra taken after 0.25 L methanol doses at cryogenic temperatures on the same three surfaces reported in Figure 2 are shown in Figure 3. The data show that the desorption of methane displays almost identical profiles, including similar peak areas and desorption temperatures, on the sputtered and annealed surfaces, even though the XPS, AES, and ISS spectra indicate a much higher oxygen concentration in the latter case. In contrast, on the oxygen-predosed surface, the high-temperature methane peak shifts upward and increases in yield at the expense of the methane production at lower temperatures. Analogous trends were observed for the production of hydrogen from adsorbed water in these systems (data not shown). It would appear that the subsurface of the annealed samples may be compositionally and/or structurally different from those obtained
J. Phys. Chem. C, Vol. 111, No. 16, 2007 6035
Figure 4. O 1s XPS from a V(100) single-crystal sputtered and annealed surface dosed with various amounts of O2 at 110 K. The O 1s XPS peak initially seen at approximately 531.2 eV shifts toward lower binding energies as the O2 exposure is increased. The inset shows a plot of both the oxygen coverage in monolayers and the O 1s XPS binding energy as a function of oxygen exposure. A Langmuir uptake is observed in this case, with a saturation coverage of approximately 1 ML after doses of approximately 2.0 L. No molecular adsorption is seen at any temperatures.
by direct dosing of O2 from the gas phase, as mentioned above. In fact, different bonding geometries have been reported for the oxygen atoms on the (5 × 1)-reconstructed V(100) surfaces prepared by high-temperature annealing versus by O2 adsorption, with some bonding on threefold coordinated sites on the former12 but only fourfold and bridge sites on the latter.10 Also, it has been reported that different sticking coefficients can be observed for hydrogen on surfaces with the same nominal oxygen coverage but prepared by different methods.9 These observations support our proposal that while V(100) single-crystal surfaces obtained by sputtering and annealing and by dosing O2 display similar oxygen coverages in the topmost layer, they may have different compositions in the subsurface region. 3.2. Temperature Dependence of the Oxygen Adsorption. Figure 4 displays the O 1s XPS data acquired after exposure of the annealed V(100) surface (0.0 L O2, top trace) to various small amounts of O2 at 110 K. It is seen here that in this case oxygen adsorption leads not only to an increase in O 1s XPS peak intensity but also to a shift in its binding energy toward lower values. This trend is more clearly illustrated in the inset, which summarizes the O 1s XPS peak area and position during the O2 uptake at 110 K. In fact, similar binding energy shifts are seen over the whole temperature range between 100 and 500 K (data not shown), and also on the sputtered surface (even though the initial amount of oxygen in that case is lower). In all cases, the small peak centered at 531.2 eV seen for the clean surface starts to shift with oxygen doses of as little as 0.1 L, to a final value of ∼530.4 eV after O2 saturation (which is reached at exposures around 2.0 L at 100 K and at lower values at high temperatures). The different O 1s binding energies measured for the surfaces before and after exposure to oxygen suggest the existence of different oxygen species in each case. Figure 5 displays the V 2p and O 1s XPS traces obtained from the V(100) single-crystal surface before and after treatments with 1000 L of O2 at a number of adsorption temperatures,
6036 J. Phys. Chem. C, Vol. 111, No. 16, 2007
Figure 5. V 2p and O 1s XPS taken after dosing the clean V(100) surface with 1000 L of O2 at different temperatures. The peaks at 512.0 and 515.2 eV are assigned to the V 2p3/2 photoelectrons from metallic vanadium and vanadium oxide, respectively, and those around 530 eV to the O 1s signal. An initial onset for surface oxidation is seen around 170 K, as evidenced by the appearance of a shoulder in the high binding energy end of the V 2p peaks and also by the shift in the O 1s XPS peak from 530.4 to 530.2 eV. A second, deeper, oxidation process is also identified above 320 K.
from 110 to 500 K. Adsorption at higher temperatures was found to result in extensive oxygen diffusion into the bulk (see below) and therefore are not reported in this figure. The data show that the main V 2p3/2 and 2p1/2 XPS peaks are always centered 512.0 and 519.2 eV, respectively, indicating the persistence of vanadium in its metallic form.20 O2 adsorption at low temperatures, below 170 K, results in the appearance of a small peak around 530.4 eV common for atomic surface oxygen, but the O 1s XPS signal grows and shifts gradually to lower binding energies, all the way to 530.1 eV, as the adsorption temperature is increased further (Figure 6, inset). This latter value has been previously recognized as typical for lattice oxygen in various metal oxides30,31 and also conforms well with reported O 1s XPS data for vanadium oxides.20,32,33 Additional and more direct evidence for the onset of such a thin oxide film formation at about 170 K is provided by the emergence of new small V 2p features at 515.2 and 522.5 eV at the expense of the signal due to the metallic phase. A second more extensive oxidation phase is then identified at higher temperatures, above 320 K, as reported previously9,13 and discussed in more detail below. It is also worth mentioning that the absence of any shoulder at higher binding energies in the O 1s XPS data at any temperatures points to the fact that no molecular oxygen states are ever present on the V(100) surface, not even at liquid nitrogen temperatures, a fact also corroborated by the lack of O2 desorption in our TPD results. The main frame of Figure 6 summarizes the oxygen uptake information derived from O 1s and V 2p XPS spectra such as those in Figure 5 for three different O2 exposures, namely, 0.5, 4.0, and 1000 L. Here, the oxygen coverages are reported in ML, calibrated against the case of 2.0 L oxygen adsorption at 320 K except for the data for 1000 L oxygen exposure at 500 K, for which the coverage was determined by calculating the oxide thickness as discussed later. The data in Figure 6 indicate that, for any fixed O2 exposure, the resulting oxygen coverage on the V(100) surface is influenced little by the
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Figure 6. Oxygen uptake curves on V(100) as a function of adsorption temperature after 0.5, 4.0, and 1000 L O2 exposures. The uptake coverage obtained after a given dose is almost constant at all temperatures below 170 K but then increases, in particular in the 1000 L case, until reaching a maximum value at 500 K. A decrease in signal is then seen above 550 K because of oxygen diffusion into the bulk. The inset shows the O 1s XPS binding energies obtained after the 1000 L O2 exposures at different adsorption temperatures. The shifts to lower binding energies seen at high temperatures are due to oxidation of the surface.
Figure 7. O 1s and V 2p XPS (left panel) and ISS (right panel) data from a V(100) after exposure to 4.0 L of O2 at 320 and 500 K. The solid lines in the XPS data correspond to fits obtained following the deconvolution analysis discussed in the text. The position of the V 2p XPS peaks obtained for the oxide phase, approximately 515.2 eV for the V 2p3/2 feature, is consistent with vanadium atoms in either V3+ or V4+ oxidation states. The ISS data indicates high oxygen coverages, over half a monolayer, for the topmost surface.
adsorption temperatures in the 100-170 K range but then increases systematically for exposures above those needed for monolayer saturation. Note again that the increase in oxygen coverage attained after high O2 exposures starts at about 170 K, the lowest temperature required for oxide formation according to the V 2p and O 1s XPS data in Figure 5. 3.3. Oxide Formation. The growth of vanadium oxide films upon exposure of the V(100) surface to O2 at higher temperatures was also characterized by XPS and ISS. Figure 7 shows the XPS (left panel) and ISS (right panel) spectra for the V(100)
O2 Adsorption & Oxide Formation on V(100) Surfaces TABLE 1: Fitting Parameters for the V 2p and O 1s XPS Peaks in the Spectra in Figure 7 XPS Peak
BE, eV
FWHM, eV
%L-Ga
V 2p3/2 (metal) V 2p1/2 (metal) V 2p3/2 (oxide phase) V 2p1/2 (oxide phase) O 1s
512.0 519.6 515.2 522.4 530.1
1.99 2.55 4.28 5.09 2.42
62 1 0 15 73
a Percent of Lorentzian component in fitted peak (relative to Gaussian component).
sample after 4.0 L doses of O2 at 320 and 500 K. The XPS data were deconvoluted as follows: (1) the XPS data for the clean vanadium surface were used to determine the fitting parameters for the metallic V 2p3/2 and V 2p1/2 XPS peaks (data not shown); and (2) the features for the oxide phase were extracted by subtraction of the metallic components from the traces after the O2 treatments. The entire remaining signal was considered to belong to the oxide film, since no O2 chemisorption takes place on the oxide surfaces. The parameters obtained by following this procedure, listed in Table 1, are in excellent agreement with previous studies;9,13 they were kept constant during the analysis of all of the other samples. Also, the deconvoluted data reported here conform well with those in Figure 5 in that both show that thicker oxide films with the same vanadium oxidation state are formed at a higher oxidation temperature. With respect to the ISS data in Figure 7 (right panel), a larger vanadium ISS peak is seen for the oxide formed at 500 K, suggesting a higher concentration of vanadium atoms in the topmost layer. Note, however, that in both cases the O peak dominates the spectra, and accounts for significantly more than half of the total He+ intensity. In terms of the nature of the oxide films that grow on the V(100) surface during these oxygen treatments, the V 2p XPS binding energies obtained for them in our studies are consistent with a V3+ oxidation state, although they could also be potentially assigned to V4+ species.20,33 Therefore, although exclusive formation of V2O3 has been assumed in previous publications,9,13 the growth of some VO2 cannot be ruled out. In fact, calculations based on the corresponding areas of the vanadium and oxygen XPS peaks indicate intermediate stoichiometries, suggesting the formation of mixtures of both oxides. The left panel of Figure 8 shows the oxygen-to-vanadium atomic ratio determined by using the V 2p3/2 signal of the oxide at 515.2 eV and known relative sensitivities34 as a function of O2 exposure for data sets taken at 320 and 500 K. Generally speaking, the oxides formed at both temperatures initially display relatively high amounts of oxygen, but increasing O2 exposures lead to decreases in the O/V atomic ratio. Importantly, although this decrease is faster in the case of 500 K, an asymptotic value of ∼1.8 is reached at both temperatures after exposures above ∼100 L. On the other hand, it is interesting to note that in the 500 K case the O/V ratio dips below that 1.8 value at intermediate O2 doses, perhaps because of the rapid diffusion of oxygen into the bulk seen at that temperature; according to the ISS data in the right panel of Figure 8, higher exposures are needed to reach surface saturation compared with those required at 320 K (see below). In any case, all of the V 2p XPS peak position and relative area data point to the conclusion that the oxide films grown by treating V(100) with O2 under vacuum conditions are most likely mixtures of V2O3 and VO2, or perhaps VO2 with some oxygen vacancies. There is, to the best of our knowledge, only one instance where a stoichiometry richer in oxygen than V2O3 has been reported for films prepared under vacuum.35 Most other oxide layers were produced at relatively
J. Phys. Chem. C, Vol. 111, No. 16, 2007 6037 high temperatures, so perhaps this effect may have been partially masked by a competing oxygen diffusion into the bulk (see below). The right panel of Figure 8 displays the evolution of the oxygen-to-vanadium ISS signal intensities ratios as a function of oxygen exposure for the same two adsorption temperatures. As expected, the oxygen coverage on the topmost layer increases in both cases with O2 dose, but reach a saturation value of threequarters of a monolayer. Also, in agreement with the XPS data, it is seen here that the saturation of the topmost layer is reached earlier at 320 K than at 500 K. It is clear that at 500 K diffusion into the bulk is already appreciable and that some of the adsorbed oxygen diffuses into the bulk to form a thin VOx film. This is why we think that the O/V ratio estimated by XPS and reported in the right panel of Figure 8 displays a dip at intermediate O2 doses (see above). The thickness of the oxide films grown by O2 treatments, estimated from the ratio of the areas of the V 2p3/2 XPS peaks at 512.0 (metallic V) versus 515.2 eV (oxidized V) using a standard analysis,36,37 are reported in Figure 9. Clearly, the oxide growth is quite fast at the beginning of the O2 dosing procedure but then levels off after higher exposures. One thing that is clear from this figure is the fact that thicker films can be grown at 500 K (∼2.2 nm) than at 320 K (∼1.0 nm), even if that also requires higher exposures (∼200 vs 50 L). This is consistent with the previous observations that indicate two oxide film growth regimes, one self-limiting after the growth of a few layers at lower temperatures (between 170 and 320 K) and a second above 320 K leading to the formation of thicker oxide films. On the other hand, the initial stages of the oxygen uptake appear to be more efficient at low temperatures (Figure 9, inset and Figure 8, right panel), again perhaps because at 500 K some of the oxygen diffuses into the bulk. Alternative estimates of the film thickness using the O 1s XPS peak areas38 yielded values consistently 5-10% higher than those calculated with the V XPS data but corroborated all of the trends reported in Figure 9. It should be pointed out that these film thicknesses were estimated by assuming a homogeneous oxide film on top of the vanadium metal. Although this is most certainly not the case here, since the data in Figure 8 show surface saturation before the bulk reaches its final composition, the approximation is quite acceptable for high oxygen exposures (>100 L) and qualitatively useful in showing trends at lower doses (where oxygen gradients are likely to form as a function of distance from the surface). 3.4. Oxygen Diffusion. As already briefly reported in Figure 6, the amount of oxygen in the upper layers of these oxygentreated vanadium surfaces decreases considerably after heating above 500 K. This effect has been seen before already and explained by oxygen diffusion into the bulk.9 Our data are consistent with this interpretation and not with any desorption into the gas phase, as suggested previously,13 since no atomic or molecular oxygen was ever detected in our TPD experiments. A detailed isothermal kinetic study of this process was carried out by initially exposing the V(100) surface to 8.0 L of O2 at 500 K and then heating it to a predetermined and fixed temperature while monitoring the XPS signal at 530.1 eV binding energy, the maximum of the O 1s XPS peak, versus time. Figure 10 reports the data obtained this way for a number of temperatures between 530 and 630 K in the form of the temporal evolution of the oxygen coverage relative to its initial value. These data clearly show that oxygen diffusion into the vanadium bulk starts at temperatures between 530 and 550 K, and that it follows approximately first-order kinetics. Rate
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Figure 8. Average stoichiometries of the vanadium oxide films generated by exposure of the V(100) surface to various O2 doses at 320 (triangles) and 500 K (circles) as estimated by XPS (left panel) and by ISS (right panel). The oxides produced by low oxygen exposures are quite rich in oxygen, especially in the case of 320 K, but the average O/V ratio estimated by XPS decreases gradually with increasing exposures and asymptotically reaches a value of approximately 1.8 at both temperatures. This value suggests that the oxide films may consist of mixtures of V2O3 and VO2 on the surface. Also, according to the ISS data, the topmost surface saturates at a coverage of approximately 3/4 of a monolayer.
Figure 9. Thickness of the oxide films grown by oxidation of V(100) at 320 and 500 K as a function of O2 exposure. The oxidation process is more extensive at 500 K, as indicated by the higher thickness of the film ultimately reached. On the other hand, the initial uptake is more efficient at 320 K, as shown by the plot of the oxygen coverage as a function of exposure below 5.0 L.
constants k were estimated from the slopes of the semilogarithmic plots in the main frame of Figure 10 and plotted in Arrhenius form in the inset, and from that figure an activation energy of about 132 kJ/mol was estimated for this oxygen diffusion, close to values estimated using temperature-programmed methods.9,39 On the other hand, the pre-exponential factor extracted from our data is fairly low, on the order of 1 × 10-10 s-1, perhaps a couple of orders of magnitude lower than previously reported.9 Our measurements, having been taken using isothermal conditions, are expected to yield more reliable kinetics than those based on temperature ramping, the method used before.
Figure 10. Isothermal changes in the XPS signal at 530.1 eV, a measure of the coverage of oxygen on the surface, as a function of time after dosing 8.0 L of O2 on the V(100) surface at 500 K. Kinetic runs are reported for several temperatures between 530 and 630 K. The inset shows the resulting Arrhenius plot for the oxygen diffusion rate constants obtained from these experiments versus temperature. An estimated activation energy of around 132 kJ/mol was calculated for this diffusion process.
4. Discussion In this work, the adsorption of oxygen at different temperatures was systematically characterized by XPS and ISS. Although there have been a few previous studies of this system, ours adds new facts to the understanding of the early stages of the oxidation of vanadium surfaces. Three main conclusions were derived here: (1) the incorporation of oxygen on V(100) surfaces displays four distinguishable temperature ranges, including two distinct oxide film formation regimes; (2) the oxide film formed upon treatments with O2 at high temperatures appears to consist of a mixture of V2O3 and VO2, the exact composition being dependent on the conditions (temperature
O2 Adsorption & Oxide Formation on V(100) Surfaces and oxygen exposure) of the deposition; and (3) the V(100) surfaces resulting from oxygen segregation upon annealing of sputtered surfaces are different, in particular in the immediate subsurface region, than those obtained from O2 dosed directly from the gas phase; they display different chemistry. In terms of the different temperature regimes identified during the oxygen uptake on V(100), it was determined that adsorption below 170 K only leads to the generation of a monolayer of atomic oxygen on the surface (Figure 4). On the other hand, molecular adsorption was never detected. Oxygen dissociation upon adsorption at low temperatures is common on early transition-metal surfaces,40 but molecular adsorption has been reported on several body centered cubic (110) surfaces, including V(110).41 The absence of molecular adsorption on the V(100) surface, as reported here, suggests that the stability of molecular O2 possibly depends not only on the nature of the metal but also on its surface structure. In any case, it was also found in this work that the formation of oxide films on V(100) only takes place at temperatures above 170 K (Figures 5 and 6). On the other hand, this initial oxidation leads only to the formation of a thin surface oxide film; for the growth of thicker films it was determined that temperatures above 320 K are needed (Figures 5 and 9). The latter is the oxide film growing regime explored in previous publications.9,13 It could be argued that, while thin oxide film formation only requires molecular oxygen activation (and perhaps a local atomic rearrangement on the surface), the build up of a thicker film may be rate limited by the typical metal oxidation steps, which include the traveling of electrons from the bulk into the surface to reduce the adsorbed oxygen atoms into negative ions and the transport of vanadium ions to maintain electronic balance.42,43 Certainly, the initial stage of the oxygen uptake is more efficient at 320 than at 500 K (Figure 9), suggesting that, before a thick oxide film is formed, the sticking coefficient of the oxygen molecules may be what defines the oxidation rates. In contrast, oxygen uptake continues at 500 K long after the film grown at 320 K reaches saturation (Figure 9), indicating atomic (or ionic) rearrangements on a larger dimensional scale. The suggestion (also based on the ISS data in Figure 8) is that at 500 K some atomic diffusion may play a role in the overall kinetics of the oxygen uptake and oxide thin film formation, even though appreciable oxygen diffusion into the bulk is only seen above 550 K (Figure 10). That diffusion may be only apparent, though, since thick oxide film growth is more likely to occur via the diffusion of vanadium ions toward the surface.2,43 Another interesting observation from the research presented here concerns the nature of the vanadium oxide films that form on V(100) upon oxygen treatments under different conditions. In particular, it was seen that the temperature of the O2 dose clearly affects the stoichiometry of the resulting films: higher adsorption temperatures (T ∼ 500 K) yield thicker films but with higher vanadium concentrations (Figure 8). Again, this may be because oxygen dissolution into the bulk plays a role at those temperatures, even if measurable diffusion is only seen above 550 K (Figure 10). Regardless, the O/V ratio is also seen to vary with oxygen exposure at each individual temperature, but to asymptotically reach a constant value of approximately 1.8 corresponding to a composition intermediate between V2O3 and VO2 (Figure 8). The position of the V 2p XPS peaks for the oxide phase of these films suggests an oxidation state for the vanadium ions in those films of V3+ (Figure 7), consistent with the formation of V2O3, but the possibility of some VO2 cannot be ruled out. In fact, it may as well be that the final films
J. Phys. Chem. C, Vol. 111, No. 16, 2007 6039 prepared at the lower (∼ 300 K) temperatures, which always show high O/V ratios (Figure 8), consist of a VO2 layer with some oxygen vacancies. On the other hand, the films obtained at 500 K, which display low O/V ratios even after less than 2 L O2, are more likely to be oxygen-rich V2O3. Finally, there is the issue of oxygen segregation toward the surface upon annealing of the V(100) single crystals. This has been seen repeatedly in the past9,10,12 and makes cleaning of those surfaces almost impossible. On the other hand, the data reported here suggest that the concentration of that oxygen in the subsurface region may not be particularly high. For one, it was seen that in the early stages of O2 dosing a gradual shift is observed in the O 1s XPS peak, from an initial binding energy of 531.2 eV to a value of 530.3 eV (Figures 4 and 6). While the latter is clearly associated with surface oxygen, the former is suggestive of oxygen in a different environment. That, in addition to the similar reactivity of the sputtered and annealed V(100) surfaces toward methanol decomposition, different than on oxygen-pretreated surfaces (Figure 3), leads us to suggest that perhaps the oxygen atoms that segregate upon heating mostly migrate all the way to the topmost surface and do not significantly modify the region right underneath. At this point, however, this can only be taken as a tentative conclusion in need of further corroboration. 5. Conclusions In summary, it was determined that V(100) surfaces can be cleaned almost completely by sputtering and annealing cycles. Small and reproducible amounts of oxygen impurities remained at all times, but amount to a coverage of less than 10% on the topmost surface right after sputtering. Significantly more oxygen resurface upon annealing of the crystal, but those atoms display unusually high O 1s XPS binding energies, and perhaps are only on the topmost surface. Certainly, the annealed surfaces are different than those obtained by oxygen dosing from the gas phase and do not significantly affect the chemistry of the surface, at least in terms of methanol conversion. In terms of O2 adsorption on V(100), its uptake on the clean surface can be categorized into four temperature regimes: (1) from 110 to 170 K, where dissociative adsorption leads to the saturation of a monolayer of atomic oxygen after exposures of around 2.0 L of oxygen; (2) from 170 to 320 K, when the formation of a thin oxide layer is feasible; (3) from 320 to 500 K, a second oxide formation regime characterized by the growth of thicker but less oxygen-rich layers; and (4) above 500 K, at which point metal oxidation competes with oxygen diffusion into the bulk. Finally, the oxide that results from O2 treatment in the intermediate temperature regime and under vacuum was determined by XPS to have a stoichiometry of VO2-x, perhaps reflecting the growth of either oxygen-rich V2O3 or VO2 with some oxygen vacancies depending upon the actual conditions used for its growth. Acknowledgment. Financial support for this project was provided by the U.S. National Science Foundation. References and Notes (1) Hucknall, D. J. SelectiVe Oxidation of Hydrocarbons; Academic Press: New York, 1974. (2) Bielanski, A.; Haber, J. Oxygen in Catalysis; Marcel Dekker: New York, 1991. (3) Tsukahara, M.; Takahashi, K.; Isomura, A.; Sakai, T. J. Alloys Compd. 1998, 265, 257.
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