Substrate-Dependent Reactivity of Water on Metal Carbide Surfaces

J. Phys. Chem. B 1999, 103, 11129-11140

11129

Substrate-Dependent Reactivity of Water on Metal Carbide Surfaces Stephen V. Didziulis*,† and Peter Frantz Materials Science Department, Mechanics and Materials Technology Center, The Aerospace Corporation, El Segundo, California 90245

Scott S. Perry,*,‡ Oussama El-bjeirami, Syed Imaduddin,§ and Philip B. Merrill| Department of Chemistry, UniVersity of Houston, Houston, Texas 77204-5641 ReceiVed: July 13, 1999; In Final Form: October 13, 1999

The interaction of water with two transition metal carbides, titanium carbide (TiC) and vanadium carbide (VC), has been investigated. The adsorption, reaction, and desorption of water on the (100) face of singlecrystal samples of these materials have been studied as a function of substrate temperature over the range 100-600 K. The adsorption state of water on these surfaces has been probed with high resolution electron energy loss spectroscopy (HREELS). The reactivity of water has been directly measured with HREELS and X-ray photoelectron spectroscopy (XPS). The desorption of molecular water and the products of surface reactions has been followed with temperature programmed desorption. Collectively, these measurements indicate that water adsorbs both molecularly and dissociatively on TiC and VC; however, a greater degree of reactivity at cryogenic temperatures is observed on TiC. Dissociation of water produces surface bound hydrogen and hydroxyl groups on both surfaces and a fully dissociated surface oxide on TiC. Furthermore, a greater participation of the surface carbon atoms is observed at the TiC surface through the evolution of COx species at elevated temperatures. The differences in surface bonding and desorption profiles are discussed in terms of differences in electronic structure of the two metal carbides. Some possible implications of these studies for the use of TiC and VC as tribological materials are also discussed.

Introduction Transition metal carbides are a unique class of materials with many desirable physical properties. In general, these materials are extremely hard and have high melting and boiling points.1 In addition, recent studies have demonstrated the ability to produce by vapor deposition techniques thin films of metal carbides that possess many of the same properties as the bulk materials.2-12 Together, these features qualify metal carbide materials as outstanding candidates for many coatings applications. Current examples include hard coatings in cutting tool applications,13 high-temperature, chemically stable coatings for the walls of nuclear reactors, and wear resistant coatings in advanced bearing technology.14 While significant effort has been applied to understanding the physical properties of metal carbides, comparatively little attention has been paid to understanding the surface chemical properties of these materials. Chemical reactions at metal carbide surfaces or interfaces are inherently related to a number of important phenomena: the oxidative stability of these materials used in high-temperature applications, the interfacial reactivity of carbide surfaces in tribological applications with lubricant species, and the chemical activity of metal carbides as catalytic materials.15-24 Recently we have initiated an investigation of the chemical reactivity of two carbide surfaces, titanium carbide * Corresponding authors. † E-mail: [email protected]. Fax: (310) 336-1636. ‡ E-mail: [email protected]. Fax: (713) 743-2709. § Present address: Institut fu ¨ r Physik der Technische Universita¨t-Ilmenau, Weimarer Strasse 32, PF 100565, D-98684, Ilmenau, Germany. | Present address: Evans Texas, 425 Round Rock West Drive, Suite 100, Round Rock, TX 78681.

(TiC) and vanadium carbide (VC), with respect to their use as hard coatings in tribological applications. We have reported on the reactivity of molecular oxygen with the (100) face of VC and TiC25 and the adsorption and reaction of water on the (100) face of TiC.26 These two adsorbates represent common atmospheric species that can potentially modify the tribological properties of carbide materials. In this paper, we present a comparison of the chemical reactivity of water with the nonpolar (100) faces of single crystal VC and TiC. Both of these carbide materials exist in the rock salt structure; a (100) termination of this structure produces a nonpolar surface with equal numbers of metal and carbon sites with 4-fold symmetry within the surface plane. While structurally similar, a significant difference exists in the electronic structure of these materials due to the one additional 3d valence electron per formula unit that is present in VC. The significance of this extra valence electron can be seen easily when considering a molecular orbital description of these materials possessing a polar covalent bond character.27 Such a treatment reveals that the highest lying occupied orbital will be associated predominantly with carbon sites for TiC. The additional valence electron of V, with respect to Ti, populates an orbital that is mostly 3d in character, and therefore the highest lying occupied orbital will be associated with the metal sites for VC. These differences in electronic structure have been further observed in band structure calculations as recently reviewed by Johansson.28 Our previous investigation of the reaction of molecular oxygen demonstrated that such differences in electronic structure can be used to understand observed differences in chemical reactiv-

10.1021/jp9923668 CCC: $18.00 © 1999 American Chemical Society Published on Web 11/19/1999

11130 J. Phys. Chem. B, Vol. 103, No. 50, 1999 ity.25 This model is extended in the present paper to describe the reactivity of water with these two carbide materials. Experimentally, the adsorption and reaction of water at cryogenic temperatures and the subsequent reaction at higher temperatures have been investigated with an array of surface analytical techniques. High resolution electron energy loss spectroscopy (HREELS) has been used to probe the vibrational structure of adsorbed species and to follow their evolution as a function of surface temperature. Temperature programmed desorption (TPD) has been used to document the desorption characteristics of water and other surface reaction products from these carbide surfaces. X-ray photoelectron spectroscopy (XPS) has been used to quantify the degree of dissociation of molecular water upon adsorption. Together, the results of these measurements demonstrate the reactivity of water on these carbide surfaces, even at cryogenic temperatures, and clearly reveal a difference in reactivity for the two carbide surfaces that can be described in terms of their relative electronic structure. Experimental Section This paper describes experiments performed at both The Aerospace Corporation and the University of Houston. Experiments were performed with (100) oriented single crystals of TiC and VC. Although, the experiments were not performed on identical samples, the crystals were cut and prepared from the same boule using identical procedures. Independent XPS measurements performed at the two locations have verified similar surface stoichiometries and cleanliness of the samples following in-situ cleaning procedures. The TiC(100) sample was obtained as a gift from UBE Industries of Japan. The growth techniques used to produce this sample have been described in the literature.29 The crystal growers estimated the bulk stoichiometry to be TiC0.9-0.92, although our XPS results indicate that the surface has a 1:1 stoichiometry (within the error of the technique). The VC(100) sample was obtained from the Linfield Research Institute, Linfield College, McMinnville, OR, and possessed a similar 1:1 surface stoichiometry, as measured by XPS. Both crystals were aligned by X-ray diffraction, then cut to within 1° of the desired crystallographic plane. They were polished with diamond paste down to a grit size of 0.25 µm and cleaned by rinsing with ethanol and acetone. Sample cleaning in ultrahigh vacuum entailed argon ion sputter/anneal cycles until the oxygen XPS signal was minimized. The accelerating potential of the Ar ion beam was 500 V, and the samples were bombarded while hot (870 K). This treatment was followed by electron beam heating to 1300 K or above for 60-300 s with a resistively heated tungsten filament mounted directly behind the sample stage on the manipulator. Cooling to ∼140 K in the HREELS system was provided through a copper braid attached to a liquid nitrogen cooled heat sink. In the TPD system, the samples were mounted directly on a liquid nitrogen filled annular dewar and thus allowed cooling to temperatures approximately 50 degrees colder than possible in the HREELS system. In both systems, the temperature was monitored with a type K thermocouple. Gas exposures (H2O, D2O) in the HREELS experiments were performed by placing the sample in the path of a gas-dosing system which consisted of a turbo-pumped gas supply, a leak valve, and a 24.8 cm long dosing tube terminated by a channel plate. This system provided an enhancement in the mass adsorbed of approximately 25 times the mass deposited through back-filling the chamber. Deionized, distilled H2O with a resistivity of 18 MΩ/cm was supplied by a water purification system. D2O was obtained from Aldrich, 99.96 atom % D.

Didziulis et al. For TPD measurements performed at the University of Houston, the adsorption of gases was limited to the crystal faces through use of a pinhole doser.30-32 This doser consisted of a 2 µm pinhole and a drift tube (5 mm ID) attached to a bellows. In a vacuum, the sample face was positioned within 0.5 mm of the end of the tube, such that all gas effusing from the tube encountered the sample face. A fixed diffusion rate through the pinhole was achieved by holding a constant pressure on the gas handling side, providing excellent reproducibility of coverage. With the sample situated approximately 2 cm from the ionizer of a quadrupole mass spectrometer (Fisons/V. G., Quartz 200), a linear increase in temperature (4 K/s) was applied. The massto-charge ratios (m/q) monitored in these experiments were 2(H2+,D+), 4(D2+), 18(H2O+, OD+), 20(D2O+), 28(CO+), 32(O2+), 44(CO2+). All data were collected using the same mass spectral sensitivities. An arbitrary y-offset has been utilized in the figures of this paper to display some of the multiple sets of data. XPS experiments, performed at the University of Houston in the same vacuum system that houses the TPD setup, were carried out with an Omicron EA 125 energy analyzer and a VG dual anode source that provided Mg KR radiation (1253.6 eV). Core level spectra were collected from a 1.5 mm spot on the sample surface with a 25 eV pass energy. Spectra were recorded with the energy analyzer situated 30° from surface normal. High resolution electron energy loss spectra (HREELS) were collected at the Aerospace Corporation with a double-pass spectrometer (LK 2000, LK Technologies, Inc.) with a current measured at the sample of 10-10 A, a resolution defined by the elastic peak full width at half-maximum of 11 meV, and a typical elastic peak intensity of 3 × 105 counts/s. The spectra were obtained with incident beam energies of 7 eV; they were collected digitally and typically took 1 h each. The pass energy of the analyzer was 2 eV. The angle of incidence of the electrons was fixed at 60° with respect to the surface normal, and the signal was collected along the specular reflection. Results and Analysis Water Reactivity on Vanadium Carbide. Figure 1 compares the HREELS data obtained from clean VC(100) to spectra of the same surface obtained after increasingly larger exposures to H2O vapor adsorbed at 148 K. The clean VC spectrum shows only one strong loss feature at 520 cm-1, which is attributed to the V-C stretch of the substrate atoms. Numerous features attributable to the adsorption of water on the VC surface are observed in Figure 1 after exposure of the surface to H2O vapor. At the lowest exposure of 0.1 L, small peaks at 3645, 1590, and 940 cm-1 are detected. In addition, shoulders appear on both the high and low energy sides of the V-C stretch. The peak at 3645 cm-1 is clearly an O-H stretch. This peak position is rather high energy when compared to literature data for adsorbed molecular water, and indicates the presence of isolated water molecules with minimal intermolecular or surface interactions (hydrogen bonding) or with the presence of surface hydroxyl groups created by the decomposition of water.33 The peak at 1590 cm-1 is assigned to the molecular water scissors deformation mode. After 0.5 L exposure, all of these features have increased in intensity, consistent with additional H2O adsorption on similar surface sites. The lower energy features have become quite prominent after this exposure, particularly the overlapping features between 500 and 1000 cm-1. Possible assignments for these peaks will be addressed below.

Reactivity of Water on Metal Carbide Surfaces

Figure 1. HREELS data obtained after increasing exposures of H2O on the VC(100) surface cooled to 148 K. From the bottom, the exposures are clean, 0.1, 0.5, 2.0, and 20 L. In these and all HREELS data in this paper, the spectra have been normalized to the intensity of their elastic peaks and the factors by which each spectrum has been multiplied are shown on the figure. The inset shows the O-H stretching region at higher sensitivity with each region further multiplied by a factor of either 4 or 8 relative to the underlying spectrum.

In the higher exposure data, more molecular water peaks become prominent. After 2 L exposure, an additional O-H stretch grows at 3475 cm-1 along with the broad feature at 2960 cm-1. Peaks lower than 3500 cm-1 are commonly observed for adsorbed molecular water as hydrogen bonding interactions between neighboring molecules perturb the O-H stretch, and we assign these two modes to distinct molecular water species. The scissors mode becomes stronger and shifts to 1565 cm-1, and very strong features are present at 900, 690, and 355 cm-1. Intense libration modes are often observed near 700 cm-1 when molecular water is adsorbed to a surface, and we assign the 690 cm-1 peak to such a mode.33 Peaks near 355 cm-1 have been ascribed to the hindered molecular translation of physisorbed water or the closely related surface-water stretch. The peaks at 900 and 355 cm-1 grow with the final exposure to 20 L, and the 2960 cm-1 peak grows relative to the other O-H stretches. In light of the relatively large exposures, comparison to the literature, and the TPD data presented below, we assign these prominent loss features after the 20 L exposure as resulting from physisorbed water. The intensity of the broad 2960 cm-1 tracks the 355 and 900 cm-1 features, and it likely arises from O-H stretches that are very strongly perturbed by hydrogen bonding interactions. The following TPD, temperature-dependent HREELS, and D2O HREELS data will help clarify more spectral assignments. Figure 2 presents the mass 20 TPD data obtained from various exposures of D2O to VC(100) cooled to 113 K. These data show

J. Phys. Chem. B, Vol. 103, No. 50, 1999 11131

Figure 2. (Top) The thermal desorption spectra of D2O (m/q ) 20 amu) adsorbed on VC(100) are plotted as a function of surface coverage. With increasing surface coverage, four desorption states associated with water adsorbed directly on the VC surface (175-283 K) are populated. At higher coverages, a second-layer state (160 K) and a multilayer state (148 K) are populated. (Bottom) Desorption traces of D2 (m/q ) 4 amu) were collected simultaneously with the D2O data and reveal a small but reproducible desorption pathway with an onset of ∼223 K, indicative of the decomposition of water on the VC surface.

six states resulting from the desorption of molecular water at temperatures ranging from 148 to 283 K. The lowest temperature peak centered at 148 K (β2) is populated last and does not saturate, identifying this feature as the desorption of multilayer water species. This multilayer desorption temperature is consistent with data in the literature.34 Other desorption maxima are observed at 160 K (β1), 175 K (R4), 202 K (R3), 243 K (R2), and 283 K (R1). The desorption temperature of β1 is consistent with this feature representing the loss of a physisorbed second layer of water, and we will tentatively assign it as such.35 Further assignments require a correlation to HREELS data obtained after warming the surface to specific temperatures to desorb specific surface species. In comparing HREELS and TPD data, it must be noted that the experiments were conducted in different vacuum systems, making an exact correlation of the absolute temperatures achieved problematic. For this reason, trends in the intensities of HREELS features will be compared to TPD features to assign surface species giving rise to desorption states. HREELS data obtained after warming a VC surface exposed to 25 L of H2O at 148 K to increasingly higher temperatures and recooling are presented in Figure 3. (Note that the spectral resolution in these data is not as good as those in Figure 1.) As the HREELS exposure temperature is clearly within the mul-

11132 J. Phys. Chem. B, Vol. 103, No. 50, 1999

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Figure 3. HREELS data obtained from a VC(100) surface after exposure to 25 L of H2O and heating to increasingly higher temperatures and then recooling to collect data. (a) From the bottom, data were obtained after heating to 148, 158, 168, and 178 K. (b) From the bottom, data were obtained after heating to 193, 233, 273, and 293 K. Once again, the insets show the O-H stretches multiplied by factors of 4 or 8.

tilayer TPD peak, we assert that our HREELS data are essentially free of multilayer water (β2), leaving only second layer and monolayer water species contributing to the data. Warming the sample in small increments from 148 to 178 K results in significant changes in the HREELS data of Figure 3a. Warming from 148 to 158 K results in a large intensity decrease of the 2960 and 900 cm-1 peaks, a decrease of the 1565 cm-1 peak, a shift of the low energy peak to 390 cm-1, and large relative growth of the peak at 690 cm-1. These HREELS spectral changes indicate the loss of phsyisorbed molecular water, so we assign the sharp β1 peak as second layer water desorption. The intensity loss of the 900 cm-1 peak is particularly revealing as Thiel and Madey state that this feature is uniquely related to a well-formed bilayer.33 Warming to 168 K results in a further drop in the 900 cm-1 peak, a further shift of the low energy peak to 420 cm-1, a large drop in the water scissors mode intensity and a peak shift to 1595 cm-1, and the complete absence of the 2960 cm-1 mode. At this temperature, all of the β1 TPD species must have desorbed. In addition, most, if not all, of the R4 species should have desorbed, although this peak is broad and some species associated with it may still remain. At 178 K, there is typically little scissors mode intensity, and the remaining peak appears to broaden to lower energy. The TPD indicates that all of the R4 species should now have desorbed. However, the 690 cm-1 mode is still strong, implying that either some molecular water remains or an energy loss from another species contributes to this feature. The remaining O-H feature has sharpened, and is centered above 3600 cm-1. In Figure 3b, the very weak or absent molecular scissors mode in the 193 K data and the numerous remaining spectral features indicate that water decomposition products are present on the VC surface at this temperature and above. Concurrently, the

R3 desorption state should be depopulated at this temperature. The O-H stretch has sharpened to a single feature at 3650 cm-1, large loss features are present at 460 and 690 cm-1, and a weak shoulder remains near 900 cm-1. The predominance of decomposition products requires that the remaining R2 and R1 desorption states in the TPD data of Figure 2 arise from recombinative water desorption. These data and others not shown indicate that the O-H stretch shows little change in intensity in the range of 173 to 233 K, but the emergence of a well-defined 1010 cm-1 peak indicates that a distinct chemical change has occurred in this temperature range. There has also been a dramatic drop in the intensity of the 690 cm-1 feature, and a very strong feature remains at 490 cm-1. Further temperature increase to 273 K results in a decrease in the O-H intensity, the decline of the 1010 cm-1 feature, and further loss of the 690 cm-1 feature. At this temperature, the surface species that contribute to the R2 desorption state should now be depopulated. Warming to room temperature eliminates all D2O desorption states, and the HREELS spectrum has again changed to reflect this. Specifically, only a very weak O-H stretch remains, the 1010 cm-1 peak is now gone and two remaining features are evident at 970 and 1370 cm-1. Understanding and assigning these complex spectra can be helped considerably through similar HREELS data obtained with D2O. Spectral features that shift with the isotopic substitution must arise from modes involving hydrogen. Data obtained after the saturation level adsorption of D2O on VC(100) and warming to selected temperatures are presented in Figure 4. The impact of the isotopic substitution is readily observable in the 143 K data, where the features assigned as monolayer and second layer molecular water O-D stretches have shifted to 2600-2700 and 2200 cm-1, respectively. The D2O scissors mode is evident at



Figure 5. The O 1s core level spectrum of the VC(100) surface following exposure to ∼1 mL of D2O at 133 K. Filled squares represent the raw data, the dashed lines represent the results of the deconvolution of the spectrum into two Lorentzian-broadened Gaussian peaks centered at 531.7 and 533.5 eV, and the solid line represents the resulting fit to the raw data. The feature at lower binding energy is assigned to surface hydroxyl species, while the higher binding energy feature, which does not saturate with higher coverages, is assigned to molecular water.

Figure 4. HREELS data obtained after adsorption of 25 L of D2O on the VC(100) and warming to increasingly higher temperatures. From the bottom, the data were obtained after heating to 143, 158, 178, 213, and 293 K. The O-H regions are shown at higher magnification, as is the energy loss region between 750 and 1500 cm-1 in the roomtemperature data.

1170 cm-1. The D2O features at lower energies show even greater overlap than in the H2O data, with peaks at 370, 530, and 675 cm-1. One would expect little isotopic effect in the hindered translational mode of physisorbed water, and this peak has apparently shifted to higher energy (370 cm-1), but this may be due to greater overlap with other features. The prominent feature assigned as the second layer libration mode exhibits an expected change from 900 to 675 cm-1. Further analysis of the complex overlapping features is difficult at this temperature. Warming the D2O/VC sample to 158 K practically eliminates the features at 2200 and 370 cm-1, and the 675 cm-1 peak has been greatly reduced as the second layer water is removed. Further temperature increases to 178 K eliminate more of the molecular water related features and sharpen the O-D stretch at 2690 cm-1. The data at 213 K and room temperature provide the greatest insight into the assignments of the spectral features of the decomposed water. At 213 K, a prominent, sharp feature is observed at 750 cm-1 that is not present in the H2O data at this temperature. In addition, the intense 1010 cm-1 feature observed in the H2O data is absent in the D2O data. The expected isotopic shift ratio is 1.4 for the H to D substitution, and the ratio of these peak energies is 1.35, leading us to conclude that these features are due to the same surface bound hydrogen species. The locations of this peak and its isotopic shift are quite similar to those observed when TiC (111) surfaces are exposed to molecular hydrogen.36 In fact, similar species have been observed before in H2O and D2O adsorption on TiC(100) and will be discussed more below.26 Similarly, the 690 cm-1 H2O mode has been red-shifted and is now overlapping with other

features, forming the large peak centered at 520 cm-1. The assignment of a D2O vibrational mode in addition to the libration that correlates to the 690 cm-1 H2O mode is clouded by the overlapping features in the D2O data. A possible assignment of this peak is the O-H(D) bending mode of the surface hydroxyl, which should display an isotopic shift to approximately 500 cm-1, overlapping with the V-C stretch. At room temperature, two relatively strong peaks remain on the VC surface exposed to D2O, in addition to the slightly broadened 520 cm-1 V-C stretch. These peaks are located at 970 and 1370 cm-1, virtually identical positions to those observed in the H2O results. The peaks were always present in the spectra of VC exposed to either form of water after warming to room temperature, even after a considerable period of time at room temperature. In fact, these peaks are observed in the 213 K (and possibly in the 178 K) D2O data, but are weaker, showing that these species form in the 213 K to room temperature regime. The lack of an isotopic shift indicates that these features result from bound oxygen atoms. The 970 cm-1 peak position is very similar to the strongest peak observed when VC(100) is exposed to molecular oxygen, and we assign it to the vanadyl (VdO) stretch as documented in our previous work.25 The 1370 cm-1 peak was not observed in our O2 experiments. The relatively high frequency suggests that this oxygen is bonded to a surface carbon atom. Quantifying HREELS data to determine the extent of surface reaction is quite difficult. To provide insight into the relative amounts of molecular and dissociative adsorption, XPS data were obtained from a VC(100) surface exposed to monolayer forming levels of D2O at 133 K. Although lower sample temperatures were obtainable in this experimental setup, the VC surface was purposefully held at this temperature during dosing to avoid the condensation of multilayers of water. Subsequent flash desorption of this coverage confirmed that the O 1s XPS spectrum shown in Figure 5 is representative of only monolayer species. The data show a relatively large peak centered at a binding energy of 533.5 eV with a lower binding energy shoulder. Fits to these data using Gaussian-broadened Lorentzian peaks reveal the presence of two species, one at a binding energy


Didziulis et al.

Figure 6. HREELS data obtained from the VC(100) surface on submonolayer species generated by exposure of a clean surface at 148 K to 0.5 L of H2O (solid line) and by warming a surface saturated with H2O at 148 K to 233 K (dotted line), and then recooling to obtain the data. The insets show the higher energy loss region multiplied by a factor of 8, showing similar O-H stretch intensities.

Figure 7. HREELS data obtained from the TiC(100) with increasing exposures of H2O at 148 K. From the bottom, the exposures are 0, 0.1, 0.5, 2.5, and 10 L. The insets show the high-energy region in more detail for selected data. Note the presence of the C-H stretch at 2750 cm-1, and the much stronger O-H stretch at 3650 cm-1 and hydride feature near 1000 cm-1 than was observed on VC in Figure 1.

of 533.5 eV and the second at 531.7 eV. These binding energies are consistent with literature values for adsorbed molecular water and adsorbed hydroxyl species, respectively.33 The integrated intensities of these features allow us to quantify the extent of surface decomposition. As the molecular water peak accounts for 75-80% of the total signal, the maximum amount of molecular water decomposition on VC(100) is approximately 20-25% at 138 K. These data are consistent with the HREELS data that suggest that molecular water predominates in this temperature and coverage regime, and the TPD data that show the bulk of the desorption of D2O from a saturated monolayer occurs from molecularly adsorbed species. The surface species present after submonolayer exposures of H2O on VC at low temperature are significantly different than those obtained by warming a saturated surface, indicating that a surface reaction accompanies molecular desorption as the surface is warmed. Figure 6 compares the HREELS spectra from the 0.5 L exposure at 148 K to the spectrum obtained after warming a saturated surface to 233 K. The main differences between the spectra are the obvious water scissors mode found only in the 148 K data and the very strong hydride peak in the 233 K data, which is either much weaker or absent in the 148 K data. The O-H stretch on the two surfaces is very similar, with a small blue-shift in the 233 K data. A weak peak is also evident near 1370 cm-1 at 233 K, as discussed above. These data show that surface chemical reactions are definitely occurring as the temperature is increased, with hydride species being produced. The data also indicate that the hydroxyl O-H stretch is not the best indicator of surface reaction, as little change is

evident for this peak despite the other significant spectral changes observed at the higher temperature. As demonstrated in the XPS data, there is limited decomposition of water on VC at low temperatures, but further reactions occur with increasing temperature, and the cryogenic reaction is not nearly as extensive as will be detailed for TiC below. Further evidence for this temperature-dependent surface reaction is observed in the mass 4 TPD data obtained simultaneously with the mass 20 data presented in Figure 2. The mass 4 data represent the desorption of D2 from the surface, an alternative reaction channel that is enabled by the presence of surface D atoms. These data at the bottom of Figure 2 show two broad desorption maxima centered at 238 and 363 K. The features are so broad that tying them to specific HREELS features is difficult, but the onset of large scale D2 desorption appears to be in the range of 193 to 213 K, which is the temperature regime in which the 1010 cm-1 (750 cm-1 for D2O) HREELS peak attributed to adsorbed hydrogen becomes plainly evident. The strength of this spectral feature at 273 K is consistent with the largest D2 desorption feature appearing at room temperature and above. D2 evolution provides additional evidence for irreversible surface reactions of water on VC, consistent with the presence of surface bound oxygen atoms at room temperature and above. Water Reactivity on Titanium Carbide. Figure 7 shows the HREELS data obtained from clean TiC(100) and the data obtained following sequential adsorption of H2O at 148 K. In the clean TiC spectrum, a large loss feature is evident at 520 cm-1 and is attributed to the Ti-C stretch in the carbide lattice.



TABLE 1: HREELS Loss Features (in cm-1) Obtained after Cryogenic H2O (D2O) Adsorption on VC(100) and Subsequent Annealing water species

H2O-surface

H2O librat.

H2O scissors

O-H stretch

V-OH

H-surface

VdO

(VC)-O

second layer first layer decomposition products

355 (370) 390-420

900 (675) 690 (520)

1565 (1165) 1590 (1180)

2960 (2200) 3475 (2580) 3650 (2690)

480-600

1010 (755)

970 (990)

1375 (1380)

TABLE 2: HREELS Loss Features (in cm-1) Obtained after Cryogenic H2O (D2O) Adsorption on TiC(100) and Subsequent Annealing water species

H2O-surface

H2O librat.

H2O scissors

O-H stretch

Ti-OH

H-surface

TidO

C-H

second layer first layer decomposition products

(340) 420 (420)

(750) 680 (500)

(1150) 1620 (1170)

(2190) 3510 (2550) 3630 (2680)

530-600

990 (710)

970 (975)

2750 (2110)

In addition, a small shoulder always evident in the TiC data is attributed to a persistent oxygen contaminant, as shown by XPS data.37 This oxygen typically accounted for approximately 5% of the surface composition. With water exposures, similar surface species as those discussed for VC are observed, however, with different relative intensities. At the lowest exposure (0.1 L), a weak O-H stretch located at 3630 cm-1 is attributed to the hydroxyl species from decomposed water. In addition, a stronger peak at 990 cm-1 is observed and is assigned as a surface hydride species. Further exposure to 0.5 L of H2O results in much stronger OH and H stretches, along with a weak molecular water scissors mode at 1620 cm-1 and a surface-water stretch at 420 cm-1. In addition, a weak feature near 2750 cm-1 is observed. As other features associated with a physisorbed second layer of water are absent from the spectrum, this feature cannot be assigned as the perturbed O-H stretch of water. Furthermore, this frequency is much lower than the 2960 cm-1 value found on VC. Instead, we attribute this feature to a surface C-H species from the decomposition of the water molecule. Further exposure to water vapor results in increased intensity of the water decomposition product peaks along with the growth of the molecular water peaks at 420, 680, and 1620 cm-1. Significantly, we see no indication of any second layer molecular water as the strong 355 and 900 cm-1 peaks observed on VC are absent in these data. Similar HREELS experiments have been performed with D2O on TiC(100) in addition to the analogous annealing experiments performed on VC(100). The results of these measurements have been previously presented26 and will only be summarized here. For adsorption of D2O on TiC(100), all of the spectral features of the water spectra were duplicated with the corresponding isotopic shifts. The frequencies of these features are listed in Table 2. Vibrational spectra collected as a function of annealing temperature revealed a decrease in the intensity of features associated with molecular water due to the desorption as well as the reaction of water within the monolayer. Reaction of water within the monolayer specifically was identified through the increase in the intensity of Ti-OD stretch, the metal hydride, and the O-D stretch. Upon annealing the water dosed surface to 298 K, the vibrational spectrum returned to that of the clean TiC(100) surface. In analogy to the desorption of water from the VC surface, the TPD data obtained for D2O desorption from TiC(100) included four states within the monolayer regime; however, at significantly different surface temperatures. These states, with peak desorption temperatures of 188, 233, 263, and 308 K, have been assigned to a number of different desorption pathways including both molecular and recombinative desorption and will

Figure 8. The thermal desorption spectra (m/q ) 20 amu) of ∼1.3 mL of water (D2O) are compared for the TiC(100) and VC(100) surfaces. The vertical dashed line represents an approximate lower temperature limit to recombinative desorption and highlights the difference in molecular versus dissociative adsorption of water on TiC and VC.

be discussed in the following section in the context of reactivity differences between VC and TiC. Furthermore, D2, CO, and CO2 were observed as desorption products arising from the surface reactions of water on TiC(100). A small D2 desorption feature was observed at elevated temperatures (500 K) while broad CO and CO2 desorption features were observed between 175 and 700 K. A complete discussion of these results can be found elsewhere.26 For comparative purposes, only the mass 20 TPD data for the desorption of ∼1.35 mL of water from TiC(100) are shown in Figure 8. This spectrum is displayed together with results for the desorption of a similar coverage of water from VC(100) (from Figure 2). As with VC, the adsorption temperature for the TPD measurements on TiC was 98 K. From these spectra, we note the similarity in the multilayer desorption peak (β1), the relative differences in monolayer desorption temperatures, and the absence of the second layer desorption feature observed on VC(100). The degree of reactivity of water on TiC(100) was further evaluated through XPS measurements. Figure 9 displays the O 1s spectrum following the adsorption of approximately one monolayer of water on TiC(100) at 133 K. The spectrum of the “clean” surface obtained under identical conditions prior to water exposure has been subtracted from this spectrum to account for the presence of residual oxygen at the surface. As in the experiments performed on VC, the surface was held at this slightly elevated temperature during dosing to avoid the condensation of multilayers of water. The O 1s spectrum contains a number of features that can be deconvoluted with


Figure 9. The O 1s core level spectrum of the TiC(100) surface following exposure to ∼1 mL of D2O at 133 K. Filled squares represent the raw data, the dashed lines represent the results of the deconvolution of the spectrum, and the solid line represents the resulting fit to the raw data. This procedure indicates the presence of three species on the surface, giving rise to peaks centered at 530.6, 531.7 and 533.5 eV. The lowest binding energy feature is assigned as surface-bound oxygen while the other two are assigned as hydroxyl and molecular water features as in the case of adsorption on VC.

the guidance of the results obtained for water adsorption on VC. The data have been fit again using Gaussian-broadened Lorentzian peaks. The first two peaks were fixed in peak position (533.5 and 531.7) and full-width half-maximum, and they were allowed to vary only in intensity. These peaks are assigned again to oxygen of molecular water and of adsorbed hydroxyl groups, respectively. As the data could not be adequately fit using only these features, a third peak was assigned at 530.6 and attributed to chemisorbed oxygen on or within the TiC surface. Integration of these spectral features yielded the following results at 133K: 60% molecular water, 30% hydroxyl, 10% chemisorbed oxygen. Both the presence of an additional oxygen species and the greater concentration of surface hydroxyls are indicative of a greater reactivity of water with TiC as compared to VC at this temperature. Discussion As described in the previous section, an array of experimental techniques has been used to characterize the interaction of water vapor with the nonpolar (100) faces of VC and TiC. The adsorption, reaction, and desorption of water and reaction products have been investigated as a function of surface temperature over the range of 98-600 K. Together, the results from the different experimental approaches allow us to establish both the degree of surface reactivity as well as the reaction path for water adsorbed on the two carbide surfaces. The following sections discuss the nature of surface reactivity at cryogenic temperatures, temperature driven surface reactivity, and the driving forces responsible for differences in the reactivity of the two carbide surfaces. Surface Reactions at Cryogenic Temperatures. One of the distinct differences in the reaction of water on VC(100) and TiC(100) at cryogenic temperatures is the extent of reaction. The surface reaction observed on both materials can be described in general terms as dissociative adsorption to produce surface hydrogen and hydroxyl species. In addition to this reaction, molecular or nondissociative adsorption is observed

Didziulis et al. on both surfaces. In some instances, the binding site of the adsorbed species is revealed through vibrational or photoelectron studies; however, the complex array of adsorption sites on these compound surfaces precludes the complete assignment of adsorption sites. The identity of reaction products will be discussed below in the context of the different reactivities. The first measure of surface reactivity is provided through a comparison of the vibrational spectra of low coverages of water adsorbed at cryogenic temperatures. Figures 1 and 7 present, respectively, the vibrational spectra of the VC(100) and TiC(100) surfaces at 148 K as a function of water coverage. The difference in surface reactivity is most clearly seen in submonolayer coverages resulting from 0.5 L exposures. On VC(100), the spectrum (Figure 1) is dominated by overlapping vibrational losses in the region of 500-1000 cm-1 and the molecular water scissors mode at 1590 cm-1. The weak 900 cm-1 feature may be assigned as the O-H bending mode from dissociated water, or it may indicate the onset of second layer adsorption as this feature dramatically increases in intensity with increasing exposures. At higher frequencies, an O-H stretch is observed at 3650 cm-1, a frequency that is characteristic of surface hydroxyls, indicating some dissociative adsorption. Features associated with other decomposition products (VdO at 970 cm-1 or a surface hydride at 1010 cm-1) are absent or are weak and hidden by other loss peaks. In contrast to the VC data, the vibrational features of the dissociation products of water dominate the HREELS spectrum of the TiC(100) surface following exposure to 0.5 L of H2O (Figure 7). These features include the surface hydride stretch at 990 cm-1, the C-H stretch near 2750 cm-1, and the hydroxyl O-H stretch at 3630 cm-1. The molecular water modes present in the 0.5 L TiC spectrum are much weaker relative to the reaction product features than were observed on VC. As the exposure increases to saturation at 140 K, the intensity of reaction product features continues to increase, but the molecular water features grow at a faster rate, indicating more molecular adsorption as coverage increases. A quantitative assessment of the extent of this reaction is difficult with HREELS due to potential differences in scattering cross sections and uncharacterized angular dependencies of the vibrational intensities of some surface features. Instead, XPS and TPD have been used to corroborate these results and provide a semiquantitative measure of the reaction yields for the dissociation of water on these surfaces. The XPS results for the O 1s region presented in Figures 5 and 9 again illustrate the difference in the extent of dissociation upon exposure of the VC and TiC surfaces to water at 130140 K. As previously described, the O 1s spectrum obtained from VC contains two features assigned as hydroxide and molecular water moieties. Integration of these respective features results in a surface monolayer composed of ∼20% surface hydroxyls and ∼80% molecular water, in qualitative agreement with the HREELS results described above. The O 1s spectrum obtained from the TiC surface following exposure to ∼1 mL contains the two features present on the VC surface and, in addition, a feature at lower binding energy (530 eV) which has been assigned to surface bound oxygen, resulting from the complete dissociation of water. Integration of the hydroxyl and surface oxygen peaks present on the TiC surface indicates that ∼40% of the monolayer is composed of dissociation products. While the HREELS hydride peak intensity would suggest an even greater extent of reaction on TiC, the XPS results represent only a lower limit to the degree of dissociation at these temperatures as some oxygen may be lost through the evolution

Reactivity of Water on Metal Carbide Surfaces of volatile reaction products. The greater intensity of the O 1s peaks arising from surface reaction products, together with the presence of the additional surface oxygen feature, indicate a greater degree of cryogenic reactivity of water with the TiC as well as differences in the specific reaction pathways on the two surfaces. The temperature-dependent evolution of these photoelectron spectra will be more fully discussed in a subsequent paper.38 The difference in surface reactivity at cryogenic temperatures can also be seen in the TPD data. Although the TPD experiments may also sample some degree of a temperature driven surface reaction, a comparison of the desorption results for the molecular (D2O) channel does suggest significant differences in the composition of the water monolayer at cryogenic temperatures. These differences are illustrated in the TPD spectra for the desorption of ∼1.3 mL of water from the VC and TiC surfaces shown in Figure 8. Four monolayer desorption states are observed on each surface. On TiC, we have previously assigned R4 as arising from molecular water in the monolayer and R1R3 as arising from a recombinative desorption process involving surface hydride and hydroxide species. We note that this assignment of R3 is at the lower limit of the temperature range for a desorption process that involves bond breaking on metal surfaces. However the evolution of other gaseous reaction products at even lower temperatures on these materials (H2 on VC, CO on TiC) indicates surface diffusion and reaction are occurring. On the VC surface, R4 and R3 are both assigned as desorption of molecular water. As the two states differ in desorption temperature, they likely originate from different adsorption sites (possibly including step edges),39 although the exact nature of the adsorption site cannot be exclusively determined from this type of experiment. Nonetheless, the low desorption temperatures coupled to the significant loss of molecular water features in the temperature-dependent HREELS spectra from 140 to 213 K (Figure 3a and 3b) strongly suggest that R3 and R4 arise from molecular water desorption. A comparison of the temperature ranges of these desorption processes on VC and TiC (Figure 8) highlights the differences in surface reactivity and resulting monolayer species. The vertical dashed line shown in Figure 8 at 223 K represents the approximate lower limit to desorption processes arising from a recombinative process and illustrates the separation in the temperature ranges of the predominant water desorption from the two surfaces. Although a number of assumptions regarding peak shape must be made, the integration of 1 mL TPD data reveals that the molecular R4 and R3 desorption features from the VC surface account for ∼80% of the total intensity in this channel, while the R4 desorption feature from TiC accounts for only ∼25% of the desorption intensity.26 In contrast to VC, the recombinative R3 desorption state on TiC accounts for approximately 55% of the evolved water. The difference between the XPS results (40% decomposed water in the monolayer) and the TPD results (75% decomposed water, R1-R3) for the TiC surface is ascribed to (i) the subsequent decomposition of water on TiC as the temperature is ramped during the TPD experiment, (ii) the possible overlap of oxygen XPS peaks of reaction products containing C-O bonds with the molecular water peak, and (iii) the possibility that a small amount of physisorbed water may be present in the XPS data. In summary, all of the techniques used in this work indicate that water dissociation proceeds to a greater extent on TiC at cryogenic temperatures. The driving force for such reactivity is discussed in a following section. Temperature Driven Surface Reactivity. The HREELS

J. Phys. Chem. B, Vol. 103, No. 50, 1999 11137 spectra measured as a function of temperature and the TPD results indicate that additional surface reactions occur with increasing surface temperature. These reactions are observed to proceed along different pathways for the two materials. The HREELS spectra measured as a function of increasing surface temperature of VC (Figure 3) reveal a number of changes in the surface monolayer over the range of 148 to 293 K. As previously discussed, the predominant change observed over the range of 148 to 193 K is the loss of molecular water features due primarily to molecular desorption. At a surface temperature of 203 K, the surface hydride is now clearly resolved and the O-H stretch at 3650 cm-1 has increased in intensity. Ultraviolet and X-ray photoelectron spectra obtained over a similar temperature range indicate that reaction of water in the monolayer, beyond that which occurs at the adsorption temperature, occurs with increasing temperature.38 The issue of greater reactivity at higher surface temperatures is illustrated in the HREELS data of Figure 6. These spectra compare the submonolayer surface species formed by heating a saturated surface to 233 K with the submonolayer surface species formed by 0.5 L exposure at 135 K. The greater intensity of the hydride feature (1010 cm-1) on the surface warmed to 233 K clearly indicates that subsequent reaction of water within the monolayer is a competing process to molecular desorption. We believe that these differences show that reactivity on VC is not limited by surface coverage at cryogenic temperatures, but requires activation. The hydride and hydroxyl spectral features begin to decrease in intensity around 273 K and are absent by 293 K, consistent with temperature range of the final m/q ) 20 amu TPD feature (R1) arising from recombinant desorption. The presence of oxygen-related HREELS features at 293 K (Figure 3b) and the onset of significant D2 desorption (Figure 2B) near room temperature reveal a second surface reaction on VC, beyond water dissociation. After annealing to 293 K, the feature at 970 cm-1 is produced by the reaction of either D2O or H2O on VC and is assigned to a surface vanadyl (VdO). This feature has been previously observed in related studies of the dissociative adsorption of molecular oxygen on VC(100).25 The formation the vanadyl species could follow from either the disproportionation reaction of surface hydroxyls to produce gasphase water and surface-bound oxygen, or from the simple decomposition of surface hydroxyls leading to the evolution of hydrogen. An additional oxygen species was observed with a vibrational frequency of 1380 cm-1, which we tentatively assigned as an oxygen atom bound to both V and C. Similar C-O stretching frequencies have been observed in organometallic complexes when face-bonded C-O species bridge to adjacent metal atoms.40 The presence of residual oxygen species at room temperature and above, as well as the desorption of molecular hydrogen near room temperature, is distinct from the additional reactivity observed on TiC. We note that the presence of residual species necessitated sputtering and annealing treatments between successive TPD runs on VC in order to obtain reproducible data, unlike the case of TiC. Temperature-mediated surface reactivity of water on TiC(100) was observed to proceed along a number of reaction pathways. Similar to the temperature-driven dissociation of water observed on VC, molecular water found in the monolayer at cryogenic temperatures on TiC also undergoes dissociation to produce hydride and hydroxyl species. This reaction, discussed in detail elsewhere,26 is identified by the increase in the hydride and hydroxyl spectral intensities in the temperature range of 153 to 193 K. While this reaction channel is similar on the two carbide surfaces, other reactions on TiC substantially differ from

11138 J. Phys. Chem. B, Vol. 103, No. 50, 1999 those observed on VC. First, the cryogenic decomposition of water on TiC proceeds further to produce the 530 eV surface oxygen species observed in the O 1s core level spectrum (Figure 9), in addition to hydride and hydroxyl species observed through the HREELS studies. Second, increasing surface temperatures lead to a reaction of oxygen with surface carbon. This reaction channel is most clearly revealed through the TPD data obtained from TiC which show the evolution of CO and CO2 over the temperature range of 200-700 K. However, since this reaction and the changes in surface composition that must accompany it seem to have little impact on subsequent TPD experiments, the amount of carbon oxide evolution must be quite small. In addition, the HREELS data show that the O-H stretching frequency of the molecularly adsorbed water is strongly perturbed by hydrogen bonding interactions, presumably with neighboring adsorbed molecules. It follows that the water molecules tend to cluster as coverage increases, perhaps limiting reaction with neighboring surface sites. Decomposition of adsorbed water species could then be promoted by the onset of molecular desorption with increased temperature, enabling further breakdown that is evident in our spectroscopic results. We surmise that the initial water decomposition reaction on TiC is limited by surface coverage in contrast to VC. Thermodynamics of Surface Reactions. In considering the surface reactions of water with both VC and TiC(100), the role of thermodynamics must be considered. In general, the formation of metal oxides is thermodynamically favorable for most metal compounds. In the case of TiC reacting with water, this is clearly the case as the ∆G298 for each of the following reactions is negative:

TiC(s) + 3H2O(g) f TiO2(s) + 3H2(g) + CO(g) TiC(s) + 4H2O(g) f TiO2(s) + 4H2(g) + CO2(g) The above reaction products were chosen as they have been observed in this system, but other reactions are also possible. In the case of VC, the numerous oxides of vanadium allow for several potential reaction pathways, many of which are thermodynamically favored at room temperature. Therefore, at equilibrium these materials would be expected to react with water to form surface oxides. We have observed that TiC is more effective than VC in breaking down water at low temperature and is more prone to losing carbon through the formation of carbon oxides, but it must be emphasized that our reactions are occurring far from equilibrium. We must conclude, therefore, that the activation barrier for this reaction is lower on TiC than on VC. When compared to our earlier work studying the surface chemistry of molecular oxygen, the oxygen surface products eventually formed from the water reaction on VC are similar. In particular, the surface vanadyl species is formed as a stable intermediate that appears to retard further surface oxidation. TiC undergoes significant surface oxidation upon oxygen exposures (on the order of tens of Langmuirs) that is not readily evident after monolayer levels of water adsorption and reaction. As noted in the reactions above, three or four water molecules are required to form one formula unit of CO or CO2 and TiO2. As a result, the relatively low level of water exposure and adsorption allows the competing reaction pathways involving molecular desorption and the recombination of surface species leading to water desorption to dominate the TiC chemistry. On VC, molecular water desorption dominates at low temperatures, but some decomposition does occur with increased temperature. This decomposition leads to some recombinative desorption of water,

Didziulis et al. but also to a significant amount of irreversible chemistry leading to strongly adsorbed surface oxygen species and the evolution of much more hydrogen than was observed on TiC.

VC(s) + H2O(g) f (VC)O(a) + H2(g) Surface Bonding and Activation. The electronic structure framework that was set forward in the Introduction as a means of understanding the surface adsorption and reaction properties of these materials can be applied to the results of water reactivity in a limited fashion. In doing so, we must focus on an atomic and molecular level to describe localized bonding effects while understanding that these extended lattice systems have electronic states that are delocalized. It is widely accepted that molecular water coordinates with a surface by donating essentially oxygen lone pair electrons in the 1b1 or 3a1 molecular orbitals.33 This σ-donation interaction must occur with an empty surface “molecular” orbital, which must be located predominantly on the metal atom for either of the metal carbides in this work.41 Hence, the initial surface interaction of molecular water with either VC or TiC is bonding to a metal atom via an σ-interaction with the empty orbitals having predominantly metal 4s, 4pz, or 3dz2 character. This is borne out in the metal-water stretching frequencies observed in our HREELS data. The TPD data show that molecular water is desorbed at higher temperatures on TiC, indicating that the metal-water bond is stronger on TiC, perhaps because of the lesser electron density at the Ti atom due to both greater ionicity of TiC and the extra valence electron present in VC. These factors may enable the formation of a stronger σ-donor bond and could potentially activate the water molecule by withdrawing electron density to facilitate proton transfer on TiC. As observed in our previous oxygen work, the carbon atom on TiC participates in surface chemistry with water to a much greater extent than the carbon atoms of the VC surface. We have presented evidence for the formation of C-H species on TiC; however, the evolution of carbon oxides indicates that surface C-O interactions must also be occurring. The more accessible electron density on the carbon atoms of TiC could work in concert with the greater electron donation from molecular water to enable more facile proton transfer to a basic carbon site. The lack of observed C-H species on VC is consistent with this picture. However, the most stable surface hydrogen species on either surface is the multicoordinate hydride characterized by the 1000 cm-1 vibrational frequency. The lack of knowledge regarding the actual surface site of this hydride limits our ability to speculate further on this chemistry. The hydroxyl group formed after loss of a hydrogen atom or proton is also electrophillic, and hence would seek out electron rich surface sites. This availability of the d-electron on VC could cause the OH group to remain in place, eventually leading to vanadyl formation. On TiC, the hydroxyl may well migrate to the electron rich carbon, initiating the reaction that leads to carbon oxide evolution. The chemistry of water on TiC(100) under the conditions studied in this work does not leave behind significant amounts of surface species at a level detectable with HREELS. The evolution of carbon oxides agrees with the previous studies of oxygen chemistry and the understanding of the electrophillic nature of a hydroxyl species. In contrast, the VC(100) surface does retain surface oxygen, particularly in the form of VdO and possibly V-O-C species. For the low water exposures studied here, these surface species on VC are stable enough to prevent significant surface oxidation (removal of surface C).

Reactivity of Water on Metal Carbide Surfaces The kinetic course of the reaction, therefore, is slowed by the formation of these metastable reaction intermediates on VC. In our discussions, we have ignored the role of surface defects in understanding surface chemistry. Previous work has clearly shown that a more highly defective TiC surface will produce much more hydrogen than a well-prepared surface.26 The most common defect in these materials is likely to be a carbon vacancy. When such a vacancy is present, the neighboring titanium atoms will gain electron density and will be more likely to react in analogy to sites of a metal (Ti or V) surface.41 An interesting possibility to consider is the potential similarity of such sites to the vanadium atoms of VC. In fact, in the oxygen work discussed throughout this paper, a significant number of TidO species were evident in our HREELS studies, which we attributed to defect sites. The formation of any stable Ti-O surface species without the loss of surface carbon would be expected to produce hydrogen, similar to our water results on VC. Eventually, however, this reaction pathway will be poisoned by the oxidation of the surface. The low level of hydrogen evolution on a well-prepared TiC surface suggests that an additional reaction channel may be present producing hydrogencontaining molecules (for example, formaldehyde) that were not monitored in our TPD work. This work has furthered our understanding of the chemical reaction of these carbides with atmospheric gases and provided a framework for understanding the adsorption and reactivity of lubricant species containing oxygen functionalities. Water is observed to release carbon oxides from TiC(100), while stable surface species are formed on VC(100). In earlier work, O2 uptake was observed to continue to increase for the TiC(100) surface, but the TiC (111) surface, which is terminated with Ti atoms, developed a stable surface layer that retarded further oxidation.42 Whether water will follow the same trend is unclear, as larger exposures at room temperature were not conducted. However, should water continue to oxidize the surface, then the further breakdown of water may eventually become less likely, as evidenced by the limited reactivity of water with TiO2 surfaces relative to TiC.43 Regarding the interaction of oxygencontaining lubricants and additives, our data show that TiC is capable of forming strong chemical bonds with oxygen species. The question that remains to be addressed is the potential for chemical reaction of the remainder of the lubricant species, which will depend on the nature of the molecule in question, to determine whether beneficial or harmful reactions of either the lubricant or the surface occur. For example, the tendency for the substrate to break C-H bonds of hydrocarbon chains or C-F bonds in fluorinated materials will impact the performance of the lubricant. This work has indicated that the reaction of VC with atmospheric gases is retarded by the formation of stable reaction intermediates, at least under the conditions we have studied. The inclusion of VC as a component of a tribological coating, therefore, would possibly help the coating resist oxidation. Alternatively, the aggressive nature of VC in forming the stable VdO species might also make it prone to degrading oxygen containing lubricant species. The formation of a passivating surface layer, however, could possibly prevent further degradation of lubricant species. Additional studies aimed at clarifying these potential reactions are currently underway. Conclusions The adsorption and reaction of water vapor on the (100) surfaces of VC and TiC have been studied with a variety of techniques. The results of these studies show that water

J. Phys. Chem. B, Vol. 103, No. 50, 1999 11139 decomposition occurs on both materials, but the decomposition is more extensive and occurs at lower temperatures on TiC. We conclude, therefore, that the reaction barrier for water decomposition is lower on TiC than on VC. The formation of the stable VdO reaction product on VC at higher temperatures produces significant amounts of gaseous hydrogen and retards surface oxidation under the conditions studied, while the greater participation of the surface carbon on TiC leads to some CO and CO2 evolution. Both of these findings can be described in terms of the surface electronic structure of these materials, with the most accessible electron density residing on the V atoms in VC and on the C atoms on TiC. The implications of these results for lubricant chemistry require further study, but one could expect significantly different interactions with oxygen containing lubricant and additive species based on the differing tendencies of these material surfaces to bond and react with electrophilic species. Acknowledgment. This work is supported by the AFOSR under contract F49620-97-0029. Partial support has also been provided through The Aerospace Corporation IR&D program funded by the Space and Missile Systems Center (SMC) of the USAF under contract number F04701-93-C-0094. The authors thank Paul Adams of The Aerospace Corporation for assistance with sample preparation. References and Notes (1) Toth, L. E. Transition Metal Carbides and Nitrides; Academic Press: New York, 1991. (2) Sessler, W. J.; Donley, M. S.; Zabinski, J. S.; Walck, S. D.; Dyhouse, V. J. Surf. Coat. Technol. 1993, 56, 125. (3) Rebenne H. E.; Bhat, D. G. Surf. Coat. Technol. 1994, 63, 1. (4) Bernt, H.; Zeng, A.-Q.; Stock, H.-R.; Mayr, P. J. Phys. IV, Collque C3, Suppl. J. Phys. II 1993, 3, 313. (5) Bae, Y. W.; Lee., W. Y.; Besmann, T. M.; Blau, P. J. Applied Phys. Lett. 1995, 66, 1895. (6) Efeoglu, I.; Arnell, R. D.; Tinston, S. F.; Teer, D. G. Surf. Coat. Technol. 1993, 57, 61. (7) Radhakrishnan, G.; Adams, P. M. SPIE Proceedings - High Power Lasers and Applications, 1999 in press. (8) Voevodin, A. A.; Capano, M. A.; Laube, S. J. P.; Donley, M. S.; Zabinski, J. S. Thin Solid Films 1997, 298, 107. (9) Vetter, J.; Rochotzki, R. Thin Solid Films 1990, 192, 253. (10) Voevodin, A. A.; Zabinski, J. S. J. Mater. Sci. 1998, 33, 319. (11) Zabinski, J. S.; Voevodin, A. A. J. Vac. Sci. Technol. A 1998, 16, 1890. (12) Voevodin, A. A.; O’Neill, J. P.; Zabinski, J. S. Thin Solid Films 1999, 342, 195. (13) For example, see Kawata, K. Surf. Coat. Technol. 1992, 54/55, 604. (14) Boving, H. J.; Hintermann, H. E. Tribol. Int. 1990, 23, 129, and references therein. (15) Ribeiro, F. H.; Dalla Betta, R. A.; Boudart, M.; Baumgartner, J.; Iglesia, E. J. Catal. 1991, 130, 86. (16) Lee, J. S.; Locatelli, S.; Oyama, S. T.; Boudart, M. J. Catal. 1990, 125, 157. (17) Iglesia, E.; Baumgartner, J. E.; Ribeiro, F. H.; Boudart, M. J. Catal. 1991, 131, 523. (18) Boudart, M.; Lee, J. S.; Imura, K.; Yoshida, S. J. Catal. 1987, 103, 30. (19) Wang, J.; Castonguay, M.; Deng, J.; McBreen, P. H. Surf. Sci. 1997, 374, 197. (20) Wang, J.; Castonguay, M.; Deng, J.; McBreen, P. H.; Ramanathan, S.; Oyama, S. T. Monograph on the Chemistry of Metal Nitrides and Carbides; Chapman & Hall: London, 1996; p 426. (21) Fruhberger, B.; Chen, J. G. Chem. ReV. 1992, 96, 362. (22) Chen, J. G. J. Catal. 1995, 154, 80. (23) Chen, J. G.; Kim, C. M.; Fruhberger, B.; DeVries, B. D.; Touville, M. S. Surf. Sci. 1994, 321, 145. (24) Chen, J. G.; Fruhberger, B.; Weisel, M. D.; Baumgartner, J. E.; DeVries, B. D. Monograph on the Chemistry of Metal Nitrides and Carbides; Chapman & Hall: London, 1996; p 439. (25) Frantz, P. P.; Didziulis, S. V. Surf. Sci. 1998, 412-413, 384. (26) Merrill, P. B.; Perry, S. S.; Frantz, P.; Didziulis, S. V. J. Phys. Chem. B 1998, 102, 7606.

11140 J. Phys. Chem. B, Vol. 103, No. 50, 1999 (27) Gubanov, V. A.; Connolly, J. W. D. Chem. Phys. Lett. 1976, 44, 139. (28) Johansson, L. I. Surf. Sci. Rep. 1995, 21, 177. (29) Otani, S.; Honma, S.; Tanaka, T.; Ishizawa, Y. J. Cryst. Growth 1983, 61, 1. (30) Olle, L.; Salmeron, M.; Baro, A. M. J. Vac. Sci. Technol. A 1985, 3, 1866. (31) Wu, M.-C.; Estrada, C. A.; Goodman, D. W. Phys. ReV. Lett. 1991, 67, 2910. (32) Yates, J. T., Jr. J. Vac. Sci. Technol. A 1987, 5, 1. (33) Thiel, P. A.; Madey, T. E. Surf. Sci. Reports 1987, 7, 211. (34) Stulen, R. H.; Thiel, P. A. Surf. Sci. 1985, 157, 99. (35) Thiel, P. A.; DePaola, R. A.; Hoffmann, F. M. J. Chem. Phys. 1984, 80, 5326.

Didziulis et al. (36) Oshima, C.; Aono, M.; Otani, S.; Ishizawa, Y. Solid State Commun. 1983, 48, 911. (37) Frantz, P.; Didziulis, S. V.; Merrill, P. B.; Perry, S. S. Tribol. Lett. 1998, 4, 141. (38) Perry, S. S.; El-bjeirami, O., to be published. (39) STM images of the VC(100) surface reveal terrace widths of ∼100 Å and indicate a significant presence of step sites across this surface. (40) Horwitz, C. P.; Shriver, D. F. AdV. in Organomet. Chem. 1984, 23, 219. (41) Janson, S. A.; Hoffmann, R. Surf. Sci. 1988, 197, 474. (42) Souda, R.; Aizawa, T.; Otani, S.; Ishizawa, Y.; Oshima, C. Surf. Sci. 1991, 256, 19. (43) Henderson, M. A. Surf. Sci. 1994, 319, 315.

Substrate-Dependent Reactivity of Water on Metal Carbide Surfaces

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