Interaction of oxygen with ZrC ( 001 ) and VC ( 001 ) : Photoemission and first-principles studies

High-resolution photoemission and first-principles density-functional calculations were used to study the interaction of oxygen with ZrC 001 and VC 001 surfaces. Atomic oxygen is present on the carbide substrates after small doses of O2 at room temperature. At 500 K, the oxidation of the surfaces is fast and clear features for ZrOx or VOx are seen in the O 1s , Zr 3d , and V 2p3/2 core levels spectra, with an increase in the metal/carbon ratio of the samples. A big positive shift 1.3–1.6 eV was detected for the C 1s core level in O/ZrC 001 , indicating the existence of strong O↔C or C↔C interactions. A phenomenon corroborated by the results of first-principles calculations, which show a CZrZr hollow as the most stable site for the adsorption of O. Furthermore, the calculations also show that a C↔O exchange is exothermic on ZrC 001 , and the displaced C atoms bond to CZrZr sites. In the O/ZrC 001 interface, the surface C atoms play a major role in determining the behavior of the system. In contrast, the adsorption of oxygen induces very minor changes in the C 1s spectrum of VC 001 . The O↔V interactions are stronger than the O↔Zr interactions, and O↔C interactions do not play a dominant role in the O/VC 001 interface. In this system, C↔O exchange is endothermic. VC 001 has a larger density of metal d states near the Fermi level than ZrC 001 , but the rate of oxidation of VC 001 is slower. Therefore the O/ZrC 001 and O/VC 001 systems illustrate two different types of pathways for the oxidation of carbide surfaces.


I. INTRODUCTION
3][14][15][16][17][18][19][20][21][22][23][24][25] In broad terms, early transition metal carbides display a unique combination of the physical properties characteristic of noble metals and ceramics. 6,7,25Many early transition metal carbides are good electrical and thermal conductors while possessing ultrahardness and very high melting points. 6,7Furthermore, some of them are able to catalyze the transformation of hydrocarbons, 8,26 the conversion of methane to synthesis gas, 27,28 and desulfurization reactions. 8,23,29ypically, early transition metals are very reactive, are not stable under chemical environments containing light elements, and exhibit a tendency to form compounds ͑oxides, nitrides, sulfides, carbides, phosphides͒.The inclusion of C into the lattice of an early transition metal produces a substantial gain in stability and moderates the chemical reactivity of the system. 14Usually, the formation of metal-carbon bonds modifies the electronic properties of the metal, producing a decrease in its density of states near the Fermi level and a metal→ carbon charge transfer. 8,14,15,18,251][32][33] For example, the valence bands of VC show a strong hybridization of the V͑3d͒ and C͑2p͒ states as expected for a covalent compound, 25,31 but there is, nevertheless, a rather large degree of ionicity in the V -C bonds with a positive Mulliken charge of ϳ1e on the V atoms. 25Among the carbides, the magnitude of the metal→ carbon charge transfer increases following the VC Ͻ TiC Ͻ ZrC Ͻ TaC sequence, and there can be large variations in the relative density of the C͑2p͒ and metal nd states near the Fermi level. 25hese differences in the electronic properties can affect the chemical reactivity of these compounds.
The interaction of oxygen with surfaces of metal carbides is an important issue.7][28][29] In addition, oxygen also affects the performance of metal-carbide coatings used in the fabrication of mechanical and electronic devices. 6,7,21,22The generation of an oxide film on top of the carbide can lead to a degradation of the conductivity and hardness of the system, 6,7 and oxocarbides can have interesting physical properties on their own. 7,8,22Several experimental studies have investigated the interaction of oxygen with well-defined surfaces of metal carbides, 21,22,[34][35][36] but the microscopic or atomic details of the oxidation process are not fully understood.A key point is the relative importance of the oxygen↔ metal and oxygen↔ carbon interactions. 21,22,36Usually, it is assumed that the C sites of a metal carbide surface play a secondary or minor role in the chemical properties of the system. 5,8,18owever, recent photoemission studies for O / TiC͑001͒ point to the existence of strong oxygen↔ carbon interactions. 21,36A phenomenon corroborated by the results of first-principles density-functional ͑DF͒ calculations, which predict a CTiTi hollow as the most stable site for the adsorption of O. 36 In contrast, data of high-resolution electron energy loss spectroscopy ͑HREELS͒ for O / VC͑001͒ show the presence of V -O bonds and negligible oxygen↔ carbon interactions. 21The differences in the behavior of the O / TiC͑001͒ and O / VC͑001͒ systems could be a consequence of variations in the electronic properties of the metal carbides. 21,25In particular, one may wonder whether among the TM carbides, the behavior of O / TiC͑001͒ is atypical or, on the contrary, it is to be expected.In this work, we use high-resolution photoemission and first-principles DF calculations to study the adsorption and reaction of oxygen with ZrC͑001͒ and VC͑001͒ surfaces.ZrC and VC are among the most extensively studied transition metal carbides. 7,8,21,25,31They adopt a cubic NaCl lattice, 7,37 as TiC, and their ͑001͒ face contains the same number of metal and C atoms.
This paper is organized as follows.Section II gives a description of the technical details of the work, including the experimental and theoretical methods used.Section III presents synchrotron-based high-resolution photoemission spectra for a series of O / ZrC͑001͒ and O / VC͑001͒ systems, examining the effects of O adsorption on the carbon and metal surface atoms.In Sec.IV, density-functional calculations are used to examine the nature of the O -ZrC and O -VC bonds, and the energetics for the dissociation of O 2 and oxidation of the carbide surfaces is described in Sec.V. Finally, Sec.VI summarizes the main conclusions of the present work.

A. Photoemission and XPS experiments
The photoemission studies for the adsorption of oxygen on ZrC͑001͒ and VC͑001͒ were performed at the U7A beamline of the National Synchrotron Light Source ͑NSLS͒ at Brookhaven National Laboratory ͑BNL͒.This beamline is equipped with a conventional ultrahigh-vacuum ͑UHV͒ chamber ͑base pressure ϳ5 ϫ 10 −10 Torr͒ that contains a hemispherical electron energy analyzer with multichannel detection, instrumentation for low-energy electron diffraction ͑LEED͒, a quadrupole mass spectrometer, and a dual anode Mg/ Al K␣ x-ray source. 13,36The photoemission spectra reported in Sec.III were recorded using photon energies of 380 eV for the Zr͑3d͒ and C͑1s͒ core levels or 625 eV for the O͑1s͒ and V͑2p͒ ones. 13,36,38At these photon energies the excited electrons had kinetic energies in the range of 85-200 eV and, therefore, photoemission probed only the composition of the first 2 to 3 layers of the sample.The overall instrumental energy resolution in the photoemission experiments was ϳ0.3 eV.The binding energy scale in the photoemission spectra was calibrated by the position of the Fermi edge in the valence region.Additional experiments of XPS were carried out at the Tokyo Institute of Technology ͑TIT͒ using an UHV chamber ͑base pressure ϳ1 ϫ 10 −10 Torr͒ that has capabilities for this technique ͑Al K␣ x-ray source, hemispherical electron energy analyzer͒ plus LEED and Auger electron spectroscopy ͑AES͒. 13,36he surfaces of the carbides were prepared at the TIT.Bulk crystals of ZrC and VC were aligned by x-ray diffraction, then cut to within 1°of the desired crystallographic plane.The produced surfaces were polished with diamond paste down to a grit size of 0.15 m.In the UHV chambers at BNL and the TIT, the ZrC͑001͒ and VC͑001͒ surfaces were mounted and cleaned following the methodology described in previous works. 13,21,36Surface impurities were removed by ion sputtering and the metal/carbon ratio was kept equal to one by exposing the sputtered surfaces to small amounts of C 2 H 2 or C 2 H 4 at 800-900 K.At these high temperatures the C from the hydrocarbons is incorporated into the lattice of the metal carbide and the hydrogen evolves into gas phase. 8The cleaning procedure led to a clear 1 ϫ 1 diffraction pattern in LEED and no surface impurities in photoemission or XPS.The crystal growers estimated stoichiometries of ZrC 0.96-0.99 and VC 0.97-0.99 for the bulk samples, and, after cleaning, our quantitative XPS results showed surfaces with essentially a Zr/ C or V / C ratio of one. 21,39For surfaces prepared in this way, images of scanning tunneling microscopy ͑STM͒ give a square crystal lattice with terraces that are 480-710 Å wide, separated by single and double step heights. 39olecular oxygen ͑99.995% purity͒ was dosed to the ZrC͑001͒ and VC͑001͒ surfaces at 300 or 500 K using dosing tubes with apertures located ϳ5 mm away from the sample.These dosing systems provided a large enhancement ͑Ͼ10 times͒ in the mass adsorbed with respect to dosing by backfilling the UHV chambers with O 2 .The reported exposures of O 2 are based on the direct ion gauge readings without correction for the enhancement factors of the dosers.

B. First-principles density functional calculations
In this work, periodic DF calculations were performed using three different computational packages-CASTEP, VASP, and DMOL 3 -to take advantage of special features of these codes.The level of theory used here has been quite useful in previous studies that deal with the electronic properties and structure of carbide surfaces, 25,36 the dissociation of O 2 on TiC͑001͒, 36 and the bonding of small molecules to VC͑001͒ and other carbide surfaces. 13,23With CASTEP, 40,41 the valence electron densities were expanded in a plane wave basis set with k-vectors within a specified energy cutoff ͑E cut = 400 eV in our case͒. 13,23,36Tightly bound core electrons were represented by nonlocal ultrasoft pseudopotentials of the Vanderbilt type. 42In the calculations with VASP, 43 a plane-wave basis set was also used ͑E cut = 415 eV͒ 25 to expand the valence electronic states and the core electrons were represented by the projected augmented wave ͑PAW͒ method of Blöchl. 44Within the DMOL 3 code, the wave functions were expanded in a basis set of localized atomic orbitals. 45In the DMOL 3 calculations, all the electrons of C, O, Zr, and V were included and a numerical basis set of double-plus polarization quality was used to describe the valence orbitals of each element. 23,45The all-electron calculations with DMOL 3 ͑DF-AE in our notation͒ required more computer time than the pseudopotential calculations with CASTEP ͑DF-PSP in our notation͒ or the frozen-core calculations with VASP ͑DF-PAW in our notation͒.Thus, DMOL 3 calculations were used only to examine key systems, while a systematic theoretical study was carried out with CASTEP and VASP.By working with CASTEP, VASP, and DMOL, 3 we were able to obtain a robust theoretical description for the O / ZrC͑001͒ and O / VC͑001͒ interfaces that was independent of the type of basis set used, of the approximation employed to treat the core electrons or the exchange-correlation potential used ͑see below͒.
Following previous works, 13,23,25,36 we examined the bonding of O to the carbide surfaces using the generalizedgradient approximation ͑GGA͒ with the revised Perdew-Burke-Ernzerhof functional ͑RPBE͒, 46 CASTEP and DMOL 3 calculations, or the so-called Perdew-Wang functional ͑PW91͒, 47 VASP calculations.Our main interest here is in bonding-energy variations when oxygen moves from one adsorption site to another on the ZrC͑001͒ and VC͑001͒ surfaces.DF calculations using the RPBE functional predicted an adsorption energy for CO on VC͑001͒, 0.63 eV, 23 that is within 0.15 eV of the experimental value, 0.49 eV, 48 and gave a very good description of the bonding interactions of oxygen with TiC͑001͒. 36Nevertheless, even if one expects the RPBE functional to provide adsorption energies closer to experiment than those obtained by the PW91 method, it has also been pointed out that it tends to lead to worse results for bulk properties like lattice parameters and bulk moduli. 49lso, there are examples where the RPBE functional may even overcorrect adsorption energies, thus predicting incorrectly binding strengths. 50Therefore a comparison of results obtained using both PW91 and RPBE seems very convenient.
To model the ZrC͑001͒ and VC͑001͒ surfaces, we used the supercell approach 40,41 with a vacuum of 12 Å between the slabs. 13,23,25,36In test calculations with CASTEP, the adsorption of one monolayer of O on ZrC͑001͒ was investigated employing eight-, six-, and four-layer slabs, obtaining almost identical results independently of the slab thickness.The CASTEP and VASP results reported in Sec.IV are for sixand four-layer slabs, respectively.A four-layer slab was also used in the all-electron calculations with the DMOL 3 code.The adsorbed oxygen was set only on one side of the slabs.The geometry of the adsorbate and of the first two slab layers were completely relaxed in the DF calculations.The O coverage values are here reported with respect to the number of metal ͑M =Zr or V͒ atoms in the surface.Thus oxygen coverages of 1, 0.5, and 0.25 monolayer ͑ML͒ were modeled with the adlayer in p͑1 ϫ 1͒, p͑2 ϫ 1͒, and p͑2 ϫ 2͒ arrays with respect to the lattice of Zr or V surface atoms.The adsorption energy of oxygen was defined as , where E O/MC͑001͒ , E MC͑001͒ and E O represent the total energies of the adsorbed system, the clean relaxed ZrC͑001͒ or VC͑001͒ surface, and that of a free O atom, respectively.Spin polarization was used for calculating atomic O or molecular O 2 in gas phase.After several tests, we found no need for spin polarization in calculations involving O / ZrC͑001͒ and O / VC͑001͒.

III. OXYGEN ADSORPTION AND OXIDATION OF ZrC(001) AND VC(001): PHOTOEMISSION AND XPS STUDIES
The top panel in Fig. 1 shows the uptake of oxygen for ZrC͑001͒ and VC͑001͒ at 500 K.The data indicate that ZrC͑001͒ is somewhat more reactive than VC͑001͒.A similar trend was found when the dosing of O 2 was done at 300 K.The variation of the metal/carbon ratio as a function of O 2 exposure at 500 K is shown in the bottom panel of Fig. 1.The reported values were calculated using the total areas under the corresponding Zr͑3d͒, V͑2p͒, and C͑1s͒ photoemission features, and essentially reflect changes that occur in the first 2 to 3 layers of the samples ͑see Sec.II A͒.As the amount of adsorbed oxygen rises, there is an increase in the metal/carbon ratio that can be interpreted as a loss of carbon from the surface due to the formation of gaseous CO x species and the generation of oxide films. 21,36Indeed, the photoemission results discussed below indicate that Zr-C or V -C bonds are being replaced by Zr-O or V -O bonds.We found that the loss of carbon was much larger at 500 K than at room temperature.Images of STM taken at the Tokyo Institute of Technology show that at room temperature the oxidation process is mainly localized at the step edges between terraces of the surface. 39In contrast, at 500 K, there is also a substantial oxidation of regions within the wide terraces of the ZrC͑001͒ and VC͑001͒ samples. 39From the data in Fig. 1 we can conclude that the oxidation of ZrC͑001͒ is faster than the oxidation of VC͑001͒.
Figure 2 displays several O͑1s͒ core-level spectra ͑h = 625 eV͒ taken after dosing molecular oxygen to ZrC͑001͒ and VC͑001͒ surfaces at room temperature and 500 K.The position observed for the O͑1s͒ peak ͑531-528 eV͒ denotes the presence of atomic oxygen on/in the carbide surface/interface. 21,36In general, we found that at 300 or 500 K, O 2 adsorbs dissociatively on both ZrC͑001͒ and VC͑001͒ surfaces.In Fig. 2, there is a shift toward lower binding energy in the centroid of the O͑1s͒ features as the dosing temperature increases.This is clear for ZrC͑001͒ and less evident for VC͑001͒.Curve fitting, 51 after a simple background subtraction, indicates that there were at least two types of oxygen species in the carbide samples.In the O / ZrC͑001͒ systems, the O͑1s͒ peak located at ϳ528.8 eV is close to the position observed in our instrument for ZrO 2 . 38Thus one could have ZrO x species on the surface ͓O͑1s͒ features toward lower binding energy͔ together with O atoms chemisorbed on ZrC ͓O͑1s͒ features toward higher binding energy͔.This is because, according to simple charge transfer arguments, O͑1s͒ features in oxidelike species shift toward lower binding energy, although it is important to realize that other mechanisms do also affect core level shifts. 52n the case of O / VC͑001͒, the O͑1s͒ features toward lower binding energy probably correspond to VO x .This species has been detected in HREELS experiments for O / VC͑001͒. 21he relative intensity of the O͑1s͒ peaks in Fig. 2 clearly corroborates that the oxidation of the carbides proceeds faster at elevated temperatures and it is easier to oxidize ZrC͑001͒.Again, this is also less apparent for VC͑001͒.
The Zr͑3d͒ core-level spectra ͑h = 380 eV͒ acquired before and after dosing O 2 to ZrC͑001͒ at 500 K are shown in the top panel of Fig. 3.The reaction with oxygen produced a line-shape change in the photoemission peaks and features appeared towards higher binding energy, consistent with the formation of ZrO x species on the surface. 38The peaks for ZrO x become clearer in difference spectra.The Zr͑3d 5/2 ͒ peak position never reached the value of 182.5 eV characteristic of pure ZrO 2 , 38 even after dosing 100 L of O 2 at 500 K.The V͑2p 3/2 ͒ core-level data ͑h = 625 eV͒ displayed in the bottom panel of Fig. 3 indicate the presence of VO x in the O/VC͑001͒ system.The new O-induced V͑2p 3/2 ͒ features appear at 514.3 eV, well separated from the corresponding features of VC ͑513.1 eV͒, but not as deep in energy as the V͑2p 3/2 ͒ peaks reported for VO 2 ͑515.8eV͒ or V 2 O 5 ͑517.0 eV͒. 53,54revious XPS studies indicate that the C͑1s͒ core level of TiC͑001͒ is affected by the oxygen adsorption. 21,36This phe- nomenon is easier to detect with synchrotron-based highresolution photoemission than with standard XPS. 36In fact, at a photon energy of 380 eV, the emitted electrons have a kinetic energy of 90-95 eV and photoemission probes only the composition of the first 2 to 3 layers in the sample. 55ypical results of high-resolution photoemission are shown in Fig. 4. The C͑1s͒ spectrum for O / ZrC͑001͒ exhibits a distinctive line shape with two strong peaks at 283.4 and 281.9 eV.The peak at 281.9 eV is close in binding energy to the single peak found for clean ZrC͑001͒ at 281.7 eV.From the relative intensities of the two peaks it appears that, in O / ZrC͑001͒, a very large fraction ͑ϳ40% ͒ of the C atoms near the surface has been perturbed by the presence of oxygen.In general, we found C͑1s͒ shifts of 1.3-1.6 eV with respect to the main peak for the substrate.They point either to a strong interaction between O and C atoms ͑oxygen could be adsorbed directly on top of C sites͒ or to a O ↔ C exchange which could take place in the surface ͑Zr-C bonds being replaced by Zr-O and C -C bonds͒.These two possibilities will be examined in Sec.IV using DF calculations.In Fig. 4, the C͑1s͒ spectrum for O / VC͑001͒ is characterized by essentially a single peak at 282.1 eV and a very small feature near 283.5 eV.For clean VC͑001͒, the C͑1s͒ peak shows up at 282.0 eV.Thus it appears that the O ↔ C interactions are negligible in O / VC͑001͒.The results of photoemission are consistent with data of HREELS for O/VC͑001͒. 21The C͑1s͒ spectra in Fig. 4 suggest a big difference in the mechanisms for the oxidation of ZrC͑001͒ and VC͑001͒.The first-principles calculations presented in the next section also corroborate this finding.The calculated ⌬E's for the oxidation processes indicate that two reaction paths are possible for the C ↔ O exchange in O / ZrC͑001͒, while only one is allowed in O / VC͑001͒.

IV. OXYGEN ON ZrC(001) AND VC(001): DENSITY FUNCTIONAL STUDIES
A detailed theoretical study of the structural parameters for the clean ZrC͑001͒ and VC͑001͒ surfaces is presented elsewhere. 25Here, we only summarize the main features which are relevant to the present study.The DF results show some surface relaxation in the transition metal carbide surfaces: the topmost C atoms displace outward and the Zr or V atoms inward with respect to the truncated bulk atomic positions ͑see Fig. 5͒.Table I lists the calculated amplitudes for this rippling ͑R in Fig. 5͒.The three different methods used in this study, all electron ͑DF-AE͒ and pseudopotential ͑DF-PSP͒ or frozen-core ͑DF-PAW͒ calculations, indicate that the rippling is of ϳ0.07 Å in ZrC͑001͒ and ϳ0.18 Å in VC͑001͒.In general, these values are consistent with the results of other theoretical works. 25,56,57][58] For the bulk carbides, recent DF calculations show a metal→ carbon charge transfer ͑ϳ1.3 e in ZrC and 1.0 e in VC͒. 25 At the ZrC͑001͒ and VC͑001͒ surfaces, the metal atoms have a positive charge but the metal-carbon bonds still exhibit a strong covalent character with Zr͑4d͒-C͑2p͒ and V͑3d͒-C͑2p͒ hybridizations. 25The moderate positive charge on the metal centers ͑0.8-1.1 e͒ is not large enough to prevent a metal→ oxygen electron transfer.In other words, the metal can be further oxidized.On the other hand, the outward position of the C atoms in the ZrC͑001͒ and VC͑001͒ surfaces suggests that they may play an important role in the oxidation process.The highest occupied electronic states have a strong carbon character in the case of ZrC͑001͒, see top of Fig. 6, but in the case of VC͑001͒ the extra electron in the valence band resides predominantly in a V͑3d͒ level, 25 bottom of the same figure.Thus on the basis of these struc-   We examined the bonding of O atoms to the following adsorption sites of the ZrC͑001͒ and VC͑001͒ surfaces: on top of Zr or V, on top of C, bridging two metal or two C atoms, bridging metal and carbon atoms and threefold hollow sites including either two metals and one carbon or two carbons and one metal atoms.The oxygen overlayer was in a p͑2 ϫ 2͒, p͑2 ϫ 1͒, or p͑1 ϫ 1͒ array with respect to the lattice of Zr or V surface atoms.The geometry of the adlayer and carbide surface two outermost atomic layers was always fully relaxed.Tables II and III display a summary of the DF results.The energy released by the bonding of O atoms to the carbide surfaces is more than enough to dissociate the O 2 molecule 36 and the O 2,gas → 2O ads reaction is very exothermic ͑Ͼ3 eV͒.The DF-PSP, DF-PAW, and DF-AE calculations for O / ZrC͑001͒ revealed that O adsorption directly on top of C and on bridge sites ͑Zr-Zr or C-C͒ was unstable and the adatoms spontaneously moved onto the kind of CZrZr hollow sites shown in Fig. 7.The off center site found here is logical because of the different atomic radii for Zr and C. 59 Our first-principles calculations indicate that this is by far the most stable adsorption site for O on ZrC͑001͒.These theoretical results are consistent with photoemission experiments ͑Fig.4͒ that point to a strong O ↔ C interaction in the oxygen/ZrC͑001͒ interface, but the DF calculations complement the experiment by revealing a more complex situation since the adsorbate is bonded simultaneously to both C and Zr ͑Table III͒.In contrast to the description above for O / ZrC͑0001͒, for the interaction of O with the VC͑001͒ surface the DF calculations show no big differences for the O adsorption energies on top of V and on a CVV hollow.Hence the O ↔ V interactions are stronger than the O ↔ Zr interactions, and from this one may deduce that the O ↔ C interactions are not really essential for the binding of the adsorbate in O/VC͑001͒.Moreover, the VC͑001͒ valence band has one more electron than that of ZrC͑001͒, 25 and it occupies states that favor V -O bonding.In Fig. 4, the C 1s spectrum of O/VC͑001͒ exhibits a small shoulder around 283.5 eV.This feature points to a C ↔ O interaction, as seen in the case of O / ZrC͑001͒, but on VC͑001͒ there is multisite adsorption since the associated energetics does not force the O onto CVV sites.
The results in Table II indicate that the oxygen coverage has a substantial effect on the oxygen adsorption energy.Such a phenomenon is more clearly seen in Fig. 8, which also includes the calculated Mulliken charges 60 for O adsorbed on the carbide surfaces as a function of coverage ͑DF-PSP calculations͒.Due to its large electronegativity, 61 oxygen withdraws electrons from the ZrC͑001͒ and VC͑001͒ FIG. 6. DF-PSP calculated density-of-states for ZrC͑001͒ and VC͑001͒ surfaces.surfaces.This is consistent with experimental measurements that show an increase in the work function of the carbide surfaces upon the bonding of oxygen. 39There is a limit in the number of electrons that the carbide surfaces can provide to an adsorbate, and as the oxygen coverage increases there is a reduction in the negative charge on the individual O adatoms.Another effect that probably reduces the amount of charge transfer for higher coverages is the Coulomb repulsion among the adsorbates.The reduction of the carbide → oxygen charge transfer is accompanied by a weakening of the bonding interactions between the adsorbate and the surface, but there is not a linear relationship between the magnitude of the charge transfer and the strength of the adsorption bond.This reflects the strong covalent character in the O -ZrC͑001͒ and O -VC͑001͒ bonds.

V. OXIDATION OF ZrC(001) AND VC(001): DENSITY FUNCTIONAL STUDIES
The photoemission spectra described in Sec.III indicate that the adsorption of oxygen can lead to oxidation of the ZrC͑001͒ and VC͑001͒ systems.Using DF-PSP calculations we investigated the energetics of the reactions associated  The reaction shown in Fig. 9 leads to oxidation of the carbide without changing the relative concentration of oxygen and carbon in the O / ZrC͑001͒ interface.In principle, the C atoms displaced from the surface could react with O adatoms to form gaseous CO x species. 21This is a convenient hypothesis to explain the increase in the Zr͑3d͒ /C͑1s͒ intensity ratio seen in photoemission ͑bottom panel in Fig. 1͒.Thus, in the system of Fig. 9͑a͒, the O and C could combine to generate adsorbed CO, see Fig. 10.We found that this reaction pathway was highly endothermic on ZrC͑001͒ and VC͑001͒.For these surfaces, the removal of carbon by a single oxygen atom is essentially impossible.The system in Fig. 10 is unstable due to the presence of C vacancies in the carbide surface.These vacancies can be avoided if the oxygen coverage is large.In the reaction scheme of Fig. 11, two O atoms work in a cooperative way to remove one C atom from the surface.In the p͑2 ϫ 2͒ cell of Fig. 11͑a͒, there are two O adatoms.One of the oxygen atoms can become embedded into the carbide surface and the displaced carbon reacts with the second oxygen to form adsorbed CO ͓Fig.11͑b͒, ⌬E = −1.26eV for ZrC͑001͒ and −0.98 eV for VC͑001͒, DF-PSP͔.In the final step ͓Fig.11͑c͔͒, the CO molecule desorbs ͓⌬E = 0.4 eV for ZrC͑001͒ and 0.6 eV for VC͑001͒, DF-PSP͔ leaving behind an oxidized system with a larger metal/carbon ratio than in stoichiometric ZrC͑001͒ or VC͑001͒.The adsorption energies of CO on the oxidized carbides are close to those calculated and measured experimentally for CO on pure VC͑001͒, −0.6 and −0.5 eV, 23,48 respectively.The CO molecule is not stable on the VC͑001͒ surface at temperatures above 250 K 48 and, thus, it could not be detected in the photoemission experiments of Figs. 2 and  4.
The results of the DF calculations indicate that the two pathways for the oxidation of the carbide surfaces are more exothermic for ZrC than for VC, a fact consistent with the trends observed in the photoemission data.The C͑1s͒ spectra and the calculated ⌬E's indicate that pathway ͑1͒ plays a minor role during the oxidation of VC͑001͒.For this oxidation process the formation of CO is essential.In VC͑001͒ the C atoms protrude from the surface more than the C atoms in ZrC͑001͒, see Table I.This structural difference does not have a major effect on the chemical behavior of the carbide surfaces, which seems to be determined by their electronic properties, see Fig. 6.By extrapolating from the behavior found for metal oxides, 1,3,10 it is frequently assumed that the metal centers in carbide surfaces carry out "the chemistry" and the C centers are simple spectators. 7,8,26The type of interactions seen in the O / VC͑001͒ interface are consistent with this assumption, since the C sites in the VC͑001͒ substrate do not play a predominant role in the bonding of O and are not efficient for the bonding of the C adatoms produced by a C ↔ O exchange.Interestingly, a comparison of the electronic proper- ties and chemical behaviors of ZrC͑001͒ and VC͑001͒ indicates that the carbide surface with the larger density-of-metal d states near the Fermi level or the smaller positive charge on the metal centers ͑VC͒ is not necessary the more reactive.In ZrC͑001͒, the C atoms are not simple spectators and help to overcome the chemical deficiencies of the metal centers.In the structures of Figs. 7 and 9, the O or C adatoms are si-multaneously bonded to C and metal surface atoms.A similar phenomenon was seen previously for the O / TiC͑001͒ interface. 36In contrast, preliminary DF calculations for the O / TaC͑001͒ system show a behavior similar in many aspects to that described above for O / VC͑001͒. 62Thus the O / ZrC͑001͒ and O / VC͑001͒ interfaces illustrate different types of phenomena that can occur during the oxidation of carbide surfaces.

VI. SUMMARY AND CONCLUSIONS
High-resolution photoemission and first-principles density-functional slab calculations were used to study the interaction of oxygen with ZrC͑001͒ and VC͑001͒ surfaces.Atomic oxygen is present on the carbide substrates after small doses of O 2 at room temperature.At 500 K, the oxidation of the surfaces is fast and clear features for ZrO x or VO x are seen in the O͑1s͒, Zr͑3d͒, and V͑2p 3/2 ͒ spectra, with an increase in the metal/carbon ratio of the samples.In spite of the substantial oxidation of ZrC͑001͒ and VC͑001͒, no signal was seen for the formation ZrO 2 , VO 2 , and V 2 O 5 .
A big positive shift ͑1.3-1.6 eV͒ was detected for the C 1s core level in O / ZrC͑001͒, indicating the existence of strong O ↔ C or C↔ C interactions, a phenomenon corroborated by the results of first-principles calculations, which show a CZrZr hollow as the most stable site for the adsorption of O. Furthermore, the calculations also show that a C ↔ O exchange is exothermic on ZrC͑001͒, and the displaced C atoms bond to CZrZr sites.In the O / ZrC͑001͒ interface, the surface C atoms play a major role in determining the behavior of the system.
The adsorption of oxygen induces very minor changes in the C͑1s͒ spectrum of VC͑001͒.The O ↔ V interactions are stronger than the O ↔ Zr interactions, and O ↔ C interactions do not play a dominant role in the O / VC͑001͒ interface.In this system a C ↔ O exchange is endothermic.VC͑001͒ has a larger density-of-metal d states near the Fermi level than ZrC͑001͒, but the rate of oxidation of VC͑001͒ is slower.The O / ZrC͑001͒ and O / VC͑001͒ systems show two different kinds of mechanisms for the oxidation of carbide surfaces.
The mechanism for the removal of a C atom from ZrC͑001͒ or VC͑001͒ as CO gas involves a minimum of two O adatoms.One to take the place of the carbon in the surface, and the other for the generation of CO.Due to the high stability of ZrC͑001͒ and VC͑001͒, an O adatom alone cannot induce the creation of a surface C vacancy through the formation of CO.

FIG. 1 .
FIG. 1. Top panel: Variation of the intensity for the O͑1s͒ signal for O / ZrC͑001͒ and O / VC͑001͒ as a function of the O 2 dose at 500 K. Bottom panel: Effects of oxygen adsorption on the Zr͑3d͒ /C͑1s͒ and V͑2p 3/2 ͒ /C͑1s͒ intensity ratios.The dosing of molecular oxygen was done at 500 K and all the data were acquired with the same electron energy analyzer.Error bars are also indicated.

FIG. 2 .
FIG. 2. O͑1s͒ photoemission spectra for the dissociative adsorption of O 2 on ZrC͑001͒ and VC͑001͒ at 300 or 500 K.The spectra were curve fitted following the procedure described in Ref. 36.A photon energy of 625 eV was used to excite the electrons.FIG. 3. Zr͑3d͒ and V͑2p 3/2 ͒ photoemission spectra ͑b͒ for the adsorption of O 2 on ZrC͑001͒ and VC͑001͒ at 500 K.At the top of each panel are shown difference spectra, denoted as ͑b-a͒ and obtained after subtracting from ͑b͒ the normalized features ͑a͒ for clean ZrC͑001͒ or VC͑001͒ ͑Ref.36͒.The electrons were excited using photon energies of 380 or 625 eV for the Zr͑3d͒ and V͑2p 3/2 ͒ levels, respectively.

FIG. 4 .
FIG. 4. C͑1s͒ photoemission data recorded after exposingZrC͑001͒ and VC͑001͒ to O 2 at 500 K.A photon energy of 380 eV was used to excite the electrons.
tural and electronic properties, it is difficult to make an a priori prediction of the relative strength of the O ↔ C, O ↔ Zr, and O ↔ V interactions.

FIG. 7 .
FIG. 7. ͑Color online͒ Most stable configuration for an O atom on the ZrC͑001͒ surface.Black ͑red in the web͒ spheres represent the adsorbate, which is bonded to one C atom ͑represented as dark gray spheres͒ and two Zr atoms ͑represented as light gray spheres͒.

FIG. 9 .
FIG. 9. ͑Color online͒ Initial and final structures for a C ↔ O exchange in a ZrC͑001͒ surface with an oxygen coverage of 0.5 ML.The O atoms in the unit cell are shown as black ͑red on the web͒ spheres.C atoms are represented as dark gray spheres, while light gray spheres denote Zr atoms.

FIG. 11 .
FIG. 11. ͑Color online͒ Formation of CO in a O / ZrC͑001͒ or O/VC͑001͒ interface.In the initial state ͑a͒, 0.5 ML of O atoms are located on CMM hollow sites of the surface ͑M =Zr or V͒.Each unit cell contains two oxygen atoms.Then, one of the oxygens inserts into the surface and the other forms CO with the displaced carbon ͑b͒.In the final step ͑c͒, the CO has desorbed from the surface.The O atoms are shown as black ͑red on the web͒ spheres.C atoms are represented as dark gray spheres, while light gray spheres denote Zr or V atoms.

TABLE I .
Surface rippling ͑Å͒ in ZrC͑001͒ and VC͑001͒: Firstprinciples results.a a For a definition of the rippling ͑R͒ see Fig.5.b Calculations using the RPBE functional, core pseudopotentials, and a plane wave basis set ͑DF-PSP͒.c Calculations using the PW91 functional, core electrons represented by the projected augmented wave method, and a plane wave basis set ͑DF-PAW͒.d Calculations using the RPBE functional, including all the electrons with a numerical basis set ͑DF-AE͒.

TABLE II .
Adsorption energy ͑eV͒ of O on ZrC͑001͒ and VC͑001͒: First-principles results.
a M is Zr or V.

TABLE III .
Bond distances of O ͑in Å͒ on ZrC͑001͒ and VC͑001͒: First-principles results.a Calculations using the RPBE functional, pseudopotentials, and a plane wave basis set ͑DF-PSP͒.Results obtained from the PW91 funcional, PAW cores and a plane wave basis set ͑DF-PAW͒ are given in parenthesis.
a b M is Zr or V.