J. Phys. Chem. C 2007, 111, 7963-7970
7963
Probing Surface Oxidation of Reduced Uranium Dioxide Thin Film Using Synchrotron Radiation S. D. Senanayake,†,§ G. I. N. Waterhouse,† A. S. Y. Chan,‡ T. E. Madey,‡ D. R. Mullins,§ and H. Idriss*,† The Department of Chemistry, The UniVersity of Auckland, PriVate bag 92019, Auckland, New Zealand, Laboratory for Surface Modification, The Department of Physics and Astronomy, Rutgers, The State UniVersity of New Jersey, Piscataway, New Jersey 08854, and Oak Ridge National Laboratory (ORNL), Oak Ridge, Tennessee 37830-6201 ReceiVed: December 21, 2006; In Final Form: April 11, 2007
The work presents a study of the formation of defects created by argon ion sputtering of UO2 thin film surfaces using high-resolution X-ray photoelectron spectroscopy with photon energy of 625 eV. Stable-defected surfaces of UO2-x were monitored by the uranium core level lines found at the low-binding energy side of the U+4 features. The lower end of these states gave binding energies at 377.1 and 387.7 eV attributed to U4f7/2 and U4f5/2 core levels peaks, respectively, for metallic uranium (U0). In between both states (U0 and U+4), the clear presence of other electronic states is observed. These lines are attributed to intermediate, metastable states of uranium ions between U0 and U+4. Temperature-programmed X-ray photoelectron spectroscopy is performed to observe the oxidation process of these reduced states with increasing temperature. The increase in the U+4 intensity is not correlated with the decrease of U ions signal with lower oxidation states indicating that the limitation is the photoelectron escape depth. In other words, sputtering has created reduced clusters or ridges on the surface of average dimensions larger than the escape depth of the 4f photoelectron with kinetic energy of 225 eV. The activation energy for surface oxidation to U+4 is ≈20 kJ mol-1. This value is close to other values reported for vacancy-type diffusion mechanism in the fluorite structure.
Introduction The study of the surface and near surface of the oxides of actinides is of considerable importance for several applied and fundamental reasons. In particular, the dynamics of oxygen and uranium ions affects the O to U ions distribution and hence the surface states affecting its reaction. Because U+4 atoms in UO2 have a 5f2 configuration, a significant hybridization occurs between the 5f and the O2p electrons causing a complex and yet not fully resolved electronic density of states.1 For example, there is up to now no satisfactory theoretical model totally describing the electronic properties of bulk UO2. As a result, there is no full treatment ab initio computational study of the surfaces of UO2 to date. Experimentally two main deviations of UO2 from stoichiometry have been observed: (i) the superstoichiometric phase, UO2+x, where extra oxygen atoms are accommodated in the bulk, and (ii) the substoichiometric phase, a less common feature (UO2-x) following a reduction reaction, thus forming a nominally substoichiometric compound.2 The reduction of UO2 is energy demanding, requiring temperatures over 1400 K and the presence of other metals,3 and thus the study of the changes of the U atoms in oxidation states lower than U+4 is difficult. Reduction, however, can be achieved with relative ease using ion sputtering as in this work. Sputtering phenomena on solid surfaces have been studied extensively and many aspects of it reported and reviewed.4 For * Author to whom correspondence should be addressed. E-mail: h.idriss@auckland. † The University of Auckland. ‡ Rutgers, The State University of New Jersey. § Oak Ridge National Laboratory (ORNL).
oxides of heavy elements, such as UO2, sputtering with argon ions largely removes oxygen atoms (because of mass difference) and consequently creates anion vacancies and the associated more reduced UO2-x surfaces and near surfaces. The reduced UO2-x surface has shown good activity to many reactions where the stoichiometric UO2 surface has shown none as in the case of CO coupling to C2 compounds5 and hydrogen production from H2O dissociation.6,7 The identification of the UO2-x phase using X-ray photoelectron spectroscopy (XPS) was initially reported by Chong et al.8 from argon ions sputtering of a UO2(111) single crystal at 300 K, showing a small broadening shoulder on the lower binding energy side of the U4f7/2 and U4f5/2 peaks at 377.4 and 387.9 eV, respectively. Although the result was intuitive as it is similar to many early metal oxides such as TiO2,9 the stability of these defects was very limited. In general, a few hours at 300 K at a background pressure of less than 5 × 10-10 Torr restored these defects (most likely because of water molecules in the ultrahigh vacuum (UHV) chamber). Subsequent work10 has considerably improved this result on the same surface at 300 K with more prominent peaks appearing in the same positions at 377.2 and 387.9 eV. A recently published study11 of a polycrystalline UO2 thin film reports a similar result and hence indicates that grain boundaries and the inherent defects in polycrystalline materials do not considerably change the defect distribution from those of a sputtered single crystal. Because of the limited resolution, all groups cited above have attributed the formation of the peak appearing at the low-binding energy side of U+4 to U metal formation (U0). This explanation originates from studies of U metal oxidation as in the work of
10.1021/jp068828g CCC: $37.00 © 2007 American Chemical Society Published on Web 05/16/2007
7964 J. Phys. Chem. C, Vol. 111, No. 22, 2007 Van den Berghe et al.12 and Miserque et al.13 where XPS shows U+4 core level (4f) peaks appearing with oxidation while U0 peaks disappear. If one compares UO2 to other transition metal oxides, one may seek into the possibilities of adding more electrons into the conduction band (having a 5f contribution) or, in other words, to make U ions in lower oxidation states than +4 (U5f2+n, where n is the added number of electrons). There is no reported oxide phase solely based on U3+ and U2+, unlike the case of titanium oxides where Ti2O3 and TiO are known compounds. Thus, any formation of these species will be of a discrete structure (cluster) rather than a phase formed on the top of the fluorite UO2 structure. Two main effects will thus influence these cationic states: the partial pressure of oxygen in the gas phase and O diffusion from the bulk. The activation energies for oxygen diffusion in UO2 (Ea, 50 kJ/mol), UO2+x (Ea, 62 kJ/mol), and UO2-x (Ea, 48.9 kJ/mol) have been previously reported14 and as such Ea has a weak dependence on the oxide overall stoichiometry. The aims of this work are (1) to study in detail the formation and stability of the different defect distributions created by argon ion bombardment as close as possible to the surface and in a relatively fast acquisition time (hence the importance of synchrotron radiation), (2) to determine the possible formation of other oxidation states in addition to +4 and 0 (using high-resolution photoelectron spectroscopy), and (3) to study the dynamics of these defects as a function of temperature. Experimental Section Experiments were performed using synchrotron radiation on beam line U12a at the National Synchrotron Light Source (NSLS). The UHV system used is a custom-made (Oak Ridge National Labs) stainless steel chamber that is maintained at 1 × 10-10 Torr with a CTI Cryotorr 8 cryopump and a Balzers TPU 170 turbo pump mounted on the chamber. A Physical Electronics PHI (20-115) sputter gun is used to clean and to cause surface defects in the samples under investigation. The beam of X-ray light enters the chamber by way of a 2 3/4 in. flange and a pneumatic gate valve. The VSW EA125 (EAC 2000 controller) concentric hemispherical electron energy analyzer with a multichannel detector system is mounted at 65° from the beam entrance. The sample mount used is composed of a copper heating/cooling block that is connected to a 360° free-rotatable xyz manipulator at the top of the chamber. The sample is spot welded onto tantalum wire (0.25 mm diameter) and is screwed to the heating/cooling block. Heating occurs resistively by current applied to the top of the copper unit at a power feedthrough using a Hewlett-Packard power supply interfaced to a RHK 310 temperature controller that allows for linear ramping of the applied current. Sample cooling is achieved by manually filling a liquid N2 reservoir inside the mount through the top of the feedthrough and manipulator. A K-type thermocouple is drawn from the top of the chamber to the copper unit and spot welded onto the sample by way of a small drilled hole in the back of the mount. The surface investigated was a thin film of UO2 sputter deposited onto a molybdenum back plate of 1 cm × 1 cm dimension and 1 mm in total thickness. The sample was cleaned prior to experimentation by several cycles of argon ions sputter (1-2 kV, 15 mA, 10 µA drain current, 5 × 10-5 Torr of Ar) and annealing under O2 (1-5 × 10-5 Torr, 600 K). The stoichiometry and purity of the surface was confirmed by highresolution XPS (HRXPS) of U4f and by survey regions of the sample. All spectra are referenced to the Fermi edge recorded
Senanayake et al. from the copper mount at the respective photon energies. Scan parameters used for all HRXPS are as follows: U4f region, 10 eV pass energy (PE), 0.1 eV/step; O1s region, 5 eV PE, 0.1 eV/step; survey region, 10 eV PE, 0.5 eV/step. Instrument resolution is estimated close to 0.3 eV for the U4f and O1s lines, based on calibration done with a Ru single-crystal taken at 400 and 600 hν at pass energies of 10 eV. The defected surface was obtained by sputtering using argon ions (3-4 kV, 15 mA, 20 µA, 5 × 10-5 Torr Ar) for extended periods as the sample was maintained at 300 or 95 K as specified in the text. Temperature-programmed XPS (TPXPS) studies were undertaken by linearly heating (1K/sec) the sample to incremental temperatures. Peak fitting is performed using XPSpeak with a Shirley background, and a Gaussian to Lorentzian ratio of 80: 20 is used for all spectra.15 Peak area and full width halfmaximum values (fwhm) are obtained from analysis of spectra using the same program. Results A survey scan was performed of the UO2 stoichiometric surface with excitation energy of 625 eV. The predominant photoemission peaks were O1s, U4f5/2, U4f7/2, U5d, and U5f at 530, 392, 380, 96, and 1.3 eV, respectively. A number of Auger features was also present with apparent binding energies (kinetic energies) at 550 (75), 450 (175), 360 (275), 350 (285), 150 (475), 140 (485), and 111 eV (514 eV); the last two are for O while all the others are for U. These features were confirmed as Auger contributions by changing the excitation energy to 600 and 650 eV and identifying the features that did not shift (in kinetic energy). Figure 1 presents two sets of spectra for the U4f lines of the stoichiometric and the oxygen-defected U oxide surfaces. The top spectrum in Figure 1A presents the initial O2-annealed surface attributed to stoichiometric UO2 with 4f7/2, 4f5/2, and satellites (S2 and S1) at 380, 390.8, 387, and 397.5 eV, respectively. This surface is the closest possible to pure UO2 and will be considered as the “stoichiometric” surface. We have put stoichiometric in quotes because some deviation from UO2.0 may exist; any deviation will be toward superstoichiometry rather than substoichiometry because of the tendency of UO2 to accommodate extra oxygen.2,16,17 The middle spectrum shows the effect of sputtering (3 kV, 15 mA) the UO2 surface with argon ions at 300 K. The 4f7/2 and 4f5/2 lines shift to higher binding energy at 380.6 and 391.5 eV, respectively, and two shoulders appear on the lower binding energy side of the 4f peaks attributed to U+x 4f7/2 and U+x 4f5/2 at 377.2 and 387.7 eV, respectively. The shift of the U+4 lines to higher binding energies is due to the shift of the Fermi level closer to the conduction band as expected from an n-type semiconductor (UO2-x) (see refs 3, 8-9 for more details). Sputtering at 95 K results in the final spectrum shown with a further shift of the 4f peaks to 380.9 and 391.7 eV for U4f7/2 and U4f5/2, respectively. The broad lines at the lower binding energy sides of the U 4f7/2 and U 4f5/2 lines at ca. 380 and 391 eV are more prominent than at 300 K. The differences between the 300 and 95 K-sputtered surfaces are shown more clearly in Figure 1B, where the two spectra are aligned to the baselines. This graph shows an attenuation of the U+4 line for the 95 K-sputtered surface concomitant with a more pronounced U+n contribution (where n < 4). The corresponding O1s spectra of the stoichiometric UO2, 300 and 95 K argon-sputtered UO2-x surfaces were also studied (not shown). The stoichiometric UO2 surface has a sharp peak at 530.5 eV and with sputtering shifts to higher binding energies at 531.3 and 531.5 eV for the 300 and 95
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Figure 1. HRXPS U4f region of stoichiometric UO2 thin films, argon ion sputtered at 300 and 95 K. The presence of reduced states upon sputtering appears at the low binding energy side of the HRXPS U+4 f7/2 main peak.
TABLE 1 ID line no 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17
surface
transition
peak energy (eV)
peak area ratio O/U (CF)a
fwhm (eV)
UO2
U4+(4f7/2) S2 U4+(4f5/2) S1 O1s U4+(4f7/2) U4+(4f5/2) S1 Ux+(4f7/2) Ux+(4f5/2) O1s U4+(4f7/2) U4+(4f5/2) S1 Ux+(4f7/2) Ux+(4f5/2) O1s
380.0 386.9 390.8 397.5 530.5 380.6 391.5 398.2 377.2 387.7 531.3 380.9 391.7 398.3 377.9 388.7 531.5
2.00 (15.4)
2.0
1.40 (15.4)
1.5 2.5
1.45 (15.4)
1.9 3.8
UO2-x 300 K
UO2-x 95 K
2.0
a
CF denotes correction factor. Correction factors for U and O are semiempirical and may depend on the take off angle and nature of the analyzer. The Phi Handbook48 (calculated for X-rays at 54.7° relative to the analyzer) indicates a correction factor of 15.4 for O1s/U4f. In a previous work,49 the CF was found equal to 16.1 based on clean samples of U3O8 and UO2. In this work, the CF of 15.4 is taken assuming that the oxygen-annealed UO2 thin film is as close as possible to stoichiometry.
K-sputtered UO2-x surfaces, respectively. The O1s peak areas of the UO2-x are considerably reduced due to removal of oxygen atoms as compared to the UO2 surface. The decrease in total O/U peak area ratios (2.00 and ≈1.40 for UO2 and UO2-x (300 K), respectively) and the increase in fwhm (1.5 and 1.9 eV for UO2 and UO2-x (300 K), respectively) are clear evidence of this reduction. Table 1 shows the summary of peak positions in the U4f and O1s spectra for the stoichiometric UO2, 300 and 95 K argon-sputtered surfaces. Figure 2 presents the 4f HRXPS scan of the 95 K UO2-x surface closely. The question of the contribution of the multiple electronic states is addressed in this figure by the decomposition of the spectrum into minor peak contributions. The U47/2 peak is taken as the focus of attention as the U4f5/2 peak has combined contributions of the S2, U+4, and other U cation peaks. Initially, the U+4 peak can be positioned at 381.5 eV with a fwhm of
Figure 2. Curve fitting of the U4f region of the UO2 thin film that has been sputtered at 95 K. Dotted lines are for raw data, solid lines are for the fitted peaks and their sum. Shirley background was imposed on all peaks.
2.13 (it is worth indicating that the shift to higher binding energy of the U+4 with respect to those of the stoichiometric UO2 is due to the shift in the Fermi level). Following this, a second peak designated U0 can be set at 377.6 eV with a fwhm of 2.00. This peak is attributed to the lowest possible valence for uranium, which is the metallic state. The fwhm of U0 is smaller than that of U+4 and values range between 1.6 and 2.0 eV; for the purpose of this fit we have chosen a fwhm of 2.0 eV. Figure 2 demonstrates that there is a definite possibility to accommodate more states in between the U+4 and U0 peaks. The introduction of a third peak designated U+x at 378.5 eV at a fwhm of 2.00 eV and a fourth peak added in place at 380.1 eV (U+y) at a fwhm of 2.00 eV allows for the total peak contributions to fit the spectra well. In the Discussion, it is argued that these intermediate oxidation states for U cations can be assigned to
7966 J. Phys. Chem. C, Vol. 111, No. 22, 2007
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Figure 3. Effect of annealing temperature of the argon ion-sputtered UO2 thin film at 300 K on the oxidation of the surface and near surface, as monitored by U4f lines, by oxygen ion diffusion from the bulk.
formal charges of +3 and +2 for the +y and +x states, respectively. An additional peak could also have been introduced if the fwhm of the reduced states were taken between 1.6 and 1.8 eV. However, the purpose of this exercise is to demonstrate that between U+4 and U0 additional oxidation states are formed and are relatively stable. The oxidation with incremental annealing of the surfaces reduced at 300 and 95 K is shown in Figures 3-5. Figure 3 shows the U4f region of the surface that has been sputtered at 300 K with argon ions and subsequently annealed to 350, 400, 425, and 475 K. The gradual oxidation of the surface with the removal of the contribution of uranium atoms in oxidation states lower than +4 and the increase of the line attributed to U+4 is seen. The corresponding O1s peaks to those displayed in Figure 3 were collected and analyzed. There are three main changes in the O1s peak as the surface is oxidized: an increase in O1s peak intensity, a change from asymmetric to symmetric shape, and a shift to a lower binding energy position due to shift in the Fermi level. Peak shift and changes in peak intensity are quantitatively analyzed in Figure 4. Figure 4A presents the changes in peak positions with temperature of the U4f7/2 and O1s lines, and in Figure 4B the uncorrected O1s to U4f peak area ratios are presented. There is a 0.8 eV shift to lower binding energy (380.6-379.8 eV and 531.3-530.5 eV) for both spectra. It is interesting to note that the shift in binding energy position and the increase in the O/U peak intensity ratio track each other. In particular, there is a change in slope in both parameters at about 400 K. The change in slope is interpreted as an increase in the rate of oxygen diffusion from the bulk to the surface (as will be discussed latter). The TPXPS experiment of the 95 K reduced surface in Figure 5A,B presents the U4f and O1s scans with incremental heating of the 95 K-sputtered surface from 150 to 500 K, respectively. The oxidation of the surface with annealing is clear as tracked by both the U4f and O1s regions. The loss of the broad shoulders in the U4f region by 400 K and an increase of U+4 peak intensity with annealing are seen. The O1s also becomes more symmetric with increasing oxidation. There is a visible nonuniform/ nonlinear shift to lower binding energy with annealing for both the U4f and O1s lines.
Figure 4. Change in the binding energy, fwhm, and O to U intensity ratios as a function of annealing temperature.
Upon closer examination of the peak positions (see Figure 6A), the O1s shifts a total of 0.9 (531.5-530.6 eV) and the U4f by 0.8 (531.5-530.6 eV). (a) Between 90 and 200 K, both O1s and U4f peak positions shift exactly 0.2 eV. (b) This is followed by a plateau where negligible change in the peak position is seen. (c) Above 350 K, a gradual shift is seen again for both U4f and O1s lines. A similar trend is visible in the O/U peak area ratios as shown in Figure 6B. (a) Initially there is an increase in the O/U ratio indicating oxidation up to ∼200 K. (b) Beyond 200 K, this ratio mildly decreases to 375 K followed by a rapid increase up to ∼500 K. The initial oxidation is well matched to that of the shift and can be explained by the oxidation of the topmost surface. Above 200 K, the slight decrease is not necessarily related to the process of oxidation; it is most likely due to the desorption of some traces of water that were accumulated at 95 K during the study. The rapid oxidation above ∼400 K is attributed to bulk to surface oxygen diffusion. Deeper insights into the oxidation process in Figure 5A can be observed with the decomposition of the U4f peaks for all
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Figure 6. Change in the binding energy, fwhm, and O to U intensity ratios as a function of annealing temperature.
Figure 7 can be divided into three sections along the x-axis: [95-250] K, [250-370] K, and [370-500] K, denoted as region 1, 2 and 3, respectively. It is instructive to consider the following changes in the peak areas of the difference oxidation in the three regions Figure 5. Effect of annealing temperature of the argon ion-sputtered UO2 thin film at 95 K on the oxidation of the surface and near surface by oxygen ion diffusion from the bulk as monitored by their (a) HRXPS U4f lines and (b) HRXPS O1s lines.
spectra in the same manner as that of Figure 2. The fwhm of species U+4, U+y, U+x, and U0 are maintained at ∼2.00 eV and relative positions of each peak also fixed. The only variable that is changed with increasing temperature is the relative peak area. The result is shown in Figure 7. The U+4 peaks increase with temperature in a quasi exponential trend and this corresponds to the decrease in peak area of the peaks attributed to lower oxidation states. (b) The U0 is the first to disappear (390 K), followed by U+x (450 K) then by U+y (500 K). (c) The U+y is seen to increase in contribution up to 250 K from the oxidation of U+x and U0 but also gives rise to continued oxidation to U+4. This is similar to a reaction in series where A f B f C, in this case A is (U metal), B is U+y (might be attributed to U3+), and C is U+4, except that the variable is temperature and not time.
Region 1:
∆(U0 + U+x) ∆(U+y + U+4) ≈∆T ∆T
Region 2:
∆(U+4) ∆(U0 + U+x + U+y) ≈∆T ∆T
Region 3:
∆(U+x + U+y) ∆(U+4) ≈∆T ∆T
(1)
(2) (3)
The dashed lines in Figure 7 present the changes in these regions. It is assumed that variations in photoemission (photoionization) cross sections due to changes in electronic distributions associated with the different U oxidation states are negligible compared to the observed variations in the signal. For example, it has been reported that photoemission cross section for the different oxidation states of Si+x (where 0 e x e 4) are quasi constant in the 130-175 eV range50 as well as with Al KR radaition.51 It is clear that the results deviate from those projected using the above equations (i.e., the increase in U+4 is greater than the decrease in the lower oxidation states). The appearance of U+4 relies on the conversion of other reduced
7968 J. Phys. Chem. C, Vol. 111, No. 22, 2007
Senanayake et al. TABLE 2 x UO2-x UO2 UO2+x PuO2-x CeO2-x UO2-x (thin film)
0.01 (0.05) 0.006 0.01 (0.05) 0.08 (0.2) x ∈ [0,0.25]b
Ev Ei (kJ mol-1) (kJ mol-1)a 50 (50) 24 46 (45) 50 (15) 20
67 96
AV × 105 cm2 s-1
refs
0.22 (0.65) 42 43, 44 100 41 0.5 (1.6) 46 1.5 (0.62) 45 this work
a Ei is compositional dependent and values up to 126 kJ mol-1 have been reported.47 bx is computed for the first three/four atomic layers and is found close to 0.25 at 370 K and assumed equal to 0 at 550 K. Ei and Ev: activation energy for interstitial and anion vacancy mechanism respectively. Av: pre-exponential factor for the diffusion equation (Dv ) Avθv(1 - θv) exp(-Ev/RT) where θ is the anion vacancy concentration (site fraction)) reported in ref 42. Numbers in parentheses track each other.
Figure 7. Change in the distribution of U ion oxidation states as a function of annealing temperatures for the 95 K-sputtered UO2 thin i)250K film. The dashed lines represent the computed peak areas of: ∑i)95K i)500K +4 +y +x 0 +4 [(Ui + Ui ) - (Ui + Ui )] for region 1 and ∑i)300K [Ui - (Ui+y + Ui+x + Ui0)] for regions 2 and 3.
Figure 8. The ln(∆U+4) as a function of 1/T in the [370-500] K temperature domain.
sites not monitored during the experiment. The reason is plausibly related to the escape depth of the photoelectron with kinetic energy of about 225 eV. Some of the reduced species would not be detected if they were formed at layers deeper than ≈20 Å in an island-type distribution. It is thus highly likely that U atoms in reduced states are in the form of clusters of size larger than this depth. The considerable deviation of the dashed lines from zero and their steady increase with increasing temperature may also be explained by the increasing rate of oxygen diffusion from the deeper layers in the bulk to the surface. At high temperatures, the U+4 fraction becomes dominant. Their change with respect to temperature is thus most likely tracking O diffusion from the near surface region where UO2-x (x f 0) converted to UO2. A plot of ln (U+4 U+4370 K), where U+4 is represented by the integrated peak area of the totality of the U4f region as a function of 1/T is shown in Figure 8. The linear fitting is clear; from the curve the activation energy, Ea, for U+4 formation is computed equal to ≈20 kJ/mol. The process of oxidation of U atoms in lower oxidation state (Uy+ for example) to U+4 depends on (i) the rate of diffusion of O atoms within the solid and (ii) the activation energy for oxidation with O atoms (Uy+ + O f U4+). Oxygen diffusion in heavy metals, such as uranium, has been studied in detail because of important technological applications in which the O/M ratio changes considerably the fundamental
properties of the solids. However, although oxygen diffusion in superstoichiometric uranium oxides (UO2+x) has been studied in detail18,19 that of UO2-x has received very little work.20 This is because UO2-x, unlike its isostructure CeO2-x, is stable only at very high temperatures; at these temperatures, O diffusion is very large and renders conventional methods unreliable. There are two types of diffusion in UO2(x: anion vacancy migration (that are termed here oxygen diffusion) and interstitialcy.18-20 Many calculations have shown that interstitialcy dominates in UO2+x while vacancy migration dominates in UO2-x. The enthalpy for these two types of diffusion has been studied experimentally and computed by theoretical methods and modeling on many oxides including UO2. Table 2 presents some of the computed and experimentally extracted numbers. It is also worth mentioning that while the activation energy for interstitialcy shows dependency on the O/U ratio, that of oxygen vacancy migration is relatively independent. The effect of electron hopping has also been considered but its effect only becomes noticeable at very high temperatures (above 1800 °C, the temperature at which the specific heat displays an unusually rapid increase).21 Discussion While the stability of oxide surfaces is crucially important for the understanding of surface reactions, the dynamics between the reduced and oxidized states of uranium and its oxides has not been the subject of dedicated studies by high-resolution spectroscopy. Examples for surface reactions directly related to the dynamics between the reduced and oxidized states are not scarce. One chief example of these reactions is that of CO oxidation to CO2 using either n-type (O-defected) or p-type (Oexcess) semiconductors.22-25 Other examples include reductive coupling of carbonyl compounds on O-defected TiO2 and UO2 surfaces,16-17,26-30 and the coupling of CO2 to ethylene has also been observed.31 An example that so far was only observed on O-defected U oxide is that of the coupling of CO to acetylene and acetaldehyde to butenes.5,32,33 All these processes rely on the dynamic between the oxidation and reduction of the solid surface. As seen in this work the reduced surface that is formed upon Ar ion bombardment is oxidized by thermal annealing and becomes stoichiometric when the annealing temperature approaches 500 K. Sputtering at 90 K gave consistently more reduced states than sputtering at 300 K. At 90 K, most background water has been frozen on the sample mount while at 300 K some water is still left in the background even at 10-10 Torr. As recently pointed out on a rutile TiO2(110) single-crystal
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Figure 10. XPS U4f of a polycrystalline U metal that has been exposed to air and then annealed at 800 K for ≈30 min each time. Initially, the surface consists of mainly UO2 (a) and upon successive annealing O diffusion from the bulk to the vacuum occurs resulting in partial restoration of the U metal (j).
Figure 9. Change in the diffusion coefficient, D in nm2 s-1, with temperature at different preset activation energies (20-70 kJ mol-1). The plots with activation energies greater than 20 kJ mol-1 were multiplied by the indicated factor to fit the y-axis. D0 was fixed at the experimental value of 1.5 × 107 nm2 s-1 from ref 39.
surface, after careful scanning tunneling microscopy studies, surface hydroxyls are formed on the bridging O atoms after dissociative adsorption of water present in the background of the UHV chamber even at pressures in the high 10-11 Torr range.34,35 We have previously investigated the effect of water on the oxidation/reduction mechanism and have found oxidation of U ions in oxidation states lower than +4 to occur at a temperature as low as 100 K.36 Thus, water presence in the background might contribute in the observed lower yield of reduced states upon sputtering with Ar ions at 300 K, compared to those observed upon sputtering at 90 K. Early work on U metal by XPS has also shown that even during XPS collection, fresh U surfaces are oxidized if the pressure is not in the 10-11 Torr range.37,38 Another factor that might be behind the observation that there is less reduction at 300 K compared to 85 K is related to oxygen diffusion. Within a simplified equation, oxygen diffusion is determined by the pre-exponential factor (diffusion coefficient, Do), and the activation energy for O diffusion, Ed
D ) Do exp(-Ed/RT)
(4)
Figure 9 presents a plot of the diffusion coefficient, D, as a function of temperature, T, between 100 and 300 K with changing activation energy from 20 to 70 kJ mol-1. In this figure Do is taken from ref 39 as 1.5 × 107 nm2 s-1 and is assumed independent of temperature. Although Do should show small changes due to deviation from stoichiometry, it has been assumed independent in many works.39 The activation energy, as expected, has a dramatic effect on the diffusion of oxygen. Successive HRXPS of the U4f region of the same surface at 300 K indicated a similar distribution of U ions; in other words, oxygen diffusion from the bulk to the surface at 300 K is not fast enough to alter the U and O HRXPS signal of the surface and near surface. To rationalize this observation and the numbers presented in Figure 9, the activation energy for oxygen diffusion
for this defected surface would be 40 kJ mol-1 or higher with a diffusion coefficient of 1.5 × 107 nm2 s-1, or the near the surface region is depleted of large amounts of oxygen atoms making their diffusion kinetically slow. Another point that this work has addressed is the nature and distribution of the uranium states upon sputtering and annealing. Figure 10 presents conventional XP spectra of a polycrystalline uranium metal that has been exposed to air for a long period of time, then annealed in UHV at 800 K at successive time intervals. Oxygen removal results in the formation of U atoms in lower oxidation states than U+4 and lines attributed to U metal at 377.0 eV can be clearly identified. It is, however, difficult to determine whether there are partially oxidized species between U+4 and U0 using a conventional XPS source. This task was made possible using synchrotron light. Curve fitting indicates that at least two states are present between U metal and U+4. Although it is tempting to attribute them to U3+ and U2+, there is no reference in the solid state for these two oxidation states. However, U3+ containing compounds have been made by organometallic chemistry and are interestingly very active for reductive trimerisation of CO.40 It is worth indicating that the reduced surface of this work has been shown very active for the reductive coupling of CO to a variety of organics.5,32,33 UO exists in the gas phase but is not known to exist in the solid state. It is possible that small clusters of uranium oxides, containing a relatively small number of atoms, would have uranium atoms that collectively manifest at lower oxidation states than +4. Some of these may thus have nominal stoichiometry of UO and U2O3. Conclusions Deviation from stoichiometry is crucial in understanding surface reactions of most metal oxide materials. In this work, we have shown that a substoichiometric uranium oxide thin film is transformed to stoichiometric oxide by heating in UHV conditions in the absence of gas-phase oxygen. Oxygen diffusion from the bulk to the surface is the main reason. The activation energy for oxygen diffusion at near stoichiometry is computed from the increase of the peak areas of the U4f lines upon annealing in the 370-500 K range and is found close to 20 kJ mol-1 indicating that O diffusion follows a vacancy-type mechanism. The formation of uranium ions in oxidation states lower than +4 is clearly seen using HRXPS U4f lines. These
7970 J. Phys. Chem. C, Vol. 111, No. 22, 2007 lines are tentatively assigned to metallic uranium, U2+, and U3+. These lines disappear upon annealing to 370, 450, and 500 K, respectively. Acknowledgment. S.D.S. and H.I. would like to thank the University of Auckland Vice Chancellors Strategic Development Fund and the New Zealand Institute of Chemistry (NZIC) for their financial contribution. G.I.N.W. thanks the Foundation for Research Science and Technology (FORST) for the provision of a postdoctoral fellowship (Contract No. UOAX0412) and funding to undertake this work. Research was supported (in whole or in part) by the U.S. Department of Energy, Division of Materials Sciences and Division of Chemical Sciences. T.E.M. acknowledges support from DE-FG02-93ER14331. D.R.M. and the U12a beamline are supported by Contract DEAC05-00OR22725 with Oak Ridge National Laboratory, managed and operated by UT-Battelle, LLC. The National Synchrotron Light Source at Brookhaven National Laboratory is supported by DE-AC02-98CH10886. The authors would also like to extend thanks to Qing-yi Dong and Gary Nintzel for technical assistance at BNL/NSLS. References and Notes (1) Kudin, K. N.; Scuseria, G. E.; Martin, R. L. Phys. ReV. Lett. 2002, 89 (26), 266402-1. (2) Colmenares, C. A. Prog. Solid State Chem. 1975, 9, 139; Prog. Solid State Chem. 1984, 15, 257. (3) Restivo, T. A. G.; Capocchi, J. D. T. J. Nucl. Mater. 2004, 334, 189. (4) Smentkowski, V. S. Prog. Surf. Sci. 2000, 64, 1. (5) Senanayake, S. D.; Soon, A.; Kohlemeyer, A.; Sohnel, T.; Idriss, H. J. Vac. Sci. Technol., A 2005, 23, 1. (6) Senanayake, S. D.; Idriss, H. Surf. Sci. 2004, 563, 135. (7) Stultz, J.; Paffett, M. T.; Joyce, S. A. J. Phys. Chem. B 2004, 108 (7), 2362. (8) Chong, S. V.; Idriss, H.; Barteau, M. A. Surf. Sci. Spectra 2001, 8, 297. (9) Idriss, H.; Barteau, M. A. AdV. Catal. 2000, 45, 261. (10) Senanayake, S. D.; Idriss, H. Surf. Sci. Spectra, in press. (11) Hedhili, M. N.; Yakshinskiy, B. V.; Schlereth, T. W.; Gouder, T.; Madey, T. E. Surf. Sci. 2005, 574, 17. (12) Van den Berghe, S.; Miserque, F.; Gouder, T.; Gaudreau, B.; Verwerft, M. J. Nucl. Mater. 2001, 294, 168. (13) Miserque, F.; Gouder, T.; Wegen, D. H.; Bottomley, P. D. W. J. Nucl. Mater. 2001, 298, 280. (14) Murch, G. E.; Catlow, C. R. A. J. Chem. Soc., Faraday Trans. 2 1987, 83, 1157. (15) Department of Physics the Chinese University of Hong Kong. http://sun.phy.cuhk.edu.hk/∼surface/XPSPEAK/XPSPEAKusersguide.doc (accessed Jan 2001). (16) Chong, S. V.; Idriss, H. J. Vac. Sci. Technol., A 2001, 19, 1933.
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