Growth and Characterization of High Surface Area Titanium Carbide

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Growth and Characterization of High Surface Area Titanium Carbide David W. Flaherty,† Nathan T. Hahn,† Domingo Ferrer,‡ Todd R. Engstrom,† Paul L. Tanaka,† and C. Buddie Mullins*,† Department of Chemical Engineering, 1 UniVersity Station, CO400, UniVersity of Texas at Austin, Austin, Texas 78712, and Department of Electrical Engineering, Microelectronics Research Center, UniVersity of Texas at Austin, Austin, Texas 78758-4445 ReceiVed: May 6, 2009; ReVised Manuscript ReceiVed: May 26, 2009

High surface area, porous titanium carbide (TiC) films have been synthesized employing physical vapor deposition of titanium at glancing angles under high vacuum within an ethylene ambient. The composition, surface area, and morphology of the TiC films were studied as a function of deposition conditions including ethylene pressure, titanium deposition angle, substrate temperature during growth, and postdeposition annealing temperature. At high or glancing deposition angles (∼80-85°) synthesis produces films composed of arrays of porous nanocolumns of TiC, while deposition at more moderate angles, less than 70°, results in continuous, reticulated films. The maximum specific surface area (840 m2/g) is obtained by growth with an incident titanium deposition angle of 65°, an ethylene pressure of 1.5 × 10-7 Torr, and a substrate growth temperature of ∼350 K. This result is in contrast to previous investigations using related physical vapor deposition techniques which have generally shown that films with the greatest porosity and surface area are grown by deposition at cryogenic temperatures (T e 77 K). The fact that the surface area is maximized at this uncharacteristically high growth temperature implies that thermally induced decomposition of ethylene and the subsequent desorption of reaction byproducts are important steps for the synthesis of these materials. Not only does deposition of TiC at 350 K result in high specific surface areas, but electron diffraction measurements indicate that these films are polycrystalline. Titanium carbide films created in this study are thermally robust and resistant to sintering, retaining greater than 70% of their initial surface area after annealing to 1000 K. The ability to deposit TiC near room temperature should allow these films to be deposited onto a wide variety of substrates. Transition-metal carbides have attracted significant interest following seminal research by Boudart and co-workers, who reported platinum-like reactivity of tungsten carbide catalysts.1 Since this discovery, numerous research groups have investigated the reactivity of transition-metal carbides (TMCs) with a focus on group VI TMCs, namely, tungsten and molybdenum carbides (WC, MoC).2-4 Early transition metals have very high binding energies for many reactant molecules, preventing facile desorption and repeated reaction. However, the binding energy can be tempered by the addition of carbon and subsequent carbide formation, leading to improved catalytic behavior. Many investigations have shown that transition-metal carbides can catalyze a number of reactions (many at rates matching or exceeding the best known group VIII transition metals) involving hydrogen transfer (hydrogenation, isomerization, hydrodesulfurization), reactions of alcohols, and other chemical transformations.5 Further, TMCs have the added benefits of comparatively low cost (with respect to Pt group metals), high thermal stability, mechanical durability, and greater tolerance to common catalyst poisons.2,6 Beside the immediate economic benefits of using TMCs as replacements for precious-metal catalysts, it is necessary to consider the sustainability of relying on such rare materials. The U.S. Geological Survey estimates the relative abundance of Pd, Pt, Rh, and Ir as 7 × 10-4, 7 × 10-4, 2 × 10-4, and 1 × 10-5 (the relative abundance of each * To whom correspondence should be addressed. Phone: (512) 471-5817. Fax: (512) 471-7060. E-mail: [email protected]. † Department of Chemical Engineering. ‡ Microelectronics Research Center.

element is expressed as the number of atoms of the element per 106 atoms of Si in the earth’s upper crust).7 In sharp contrast Ti, Mo, and W (a few of the metals used for TMC catalysts) have a natural abundance of 7 × 103, 2, and 1, making them ∼103-109 times more abundant than the Pt group elements. Titanium carbide (TiC) has received considerably less attention than tungsten and molybdenum carbides. Consequently, its surface chemistry and catalytic properties are relatively unknown. However, recent reports indicate that TiC is active for select reactions. Early investigations showed that the TiC(111) surface is highly active for dissociative adsorption of hydrogen (initial sticking probability of ∼0.6) at room temperature.8 Further work has illustrated that bulk TiC (as well as other TMCs) is catalytically active for the hydrogenation of CO9 and C2H4.5,10 Perry and co-workers have studied the adsorption of several molecules on the nonpolar TiC(100) surface including water,11,12 methanol,13 ethanol and 2-propanol,14 NH3,15 CO,16 and methyl formate.17 Vin˜es et al.18,19 and Rodriguez et al.20,21 have studied the adsorption and dissociation of O2 and SO2 on TiC(001) and other TMC surfaces. Chen and co-workers have investigated the adsorption and subsequent reaction of ethylene, cyclohexene, and methanol on the carbon-modified Ti(001) single-crystal surface.22,23 Their results indicate the titanium carbide model catalyst has a reactivity similar to that of Pt surfaces and for some reactions (cyclohexene dehydrogenation) displays greater selectivity for the desired products.22,23 Additionally, titanium carbide metallocarbohedrenes (metcars) such as Ti8C12 and Ti14C13 have attracted attention since their discovery by Castleman and co-workers.24,25 Zhao and co-

10.1021/jp904236v CCC: $40.75  2009 American Chemical Society Published on Web 06/11/2009

High Surface Area Titanium Carbide workers have predicted, using density functional theory (DFT), that TiC metcars of selected sizes can adsorb both atomic hydrogen and molecular hydrogen with adsorption amounts reaching 7.7 wt %, an indication that nanostructured TiC may be a promising hydrogen storage material.26 Presumably, the Ti atoms within each cluster dissociatively adsorb hydrogen, which reacts further to form carbon hydrides. Liu, Rodriguez, and Muckerman have utilized DFT calculations to study the reactivity of TMC (and especially TiC) surfaces and metcars for use in hydrodesulfurization (HDS) reactions.27,28 The results of Liu et al. suggest that specific metcars (Ti8C12) may have greater catalytic activity and stability for HDS of thiophene than current commercial catalysts (e.g., NiMoS and CoMoS).28 Expanding on this work, White and co-workers have experimentally investigated the gas-phase reactivity of TiC metcars with small sulfur-containing molecules.29 Liu et al. also compared the reactivities of CO, NH3, and H2O on the extended TiC(001) surface and metcars.30 Their calculations indicate that metcars are much more reactive toward these probe molecules due to the change in coordination number of Ti as well as strain effects. Very recently, Illas and co-workers utilized DFT to investigate the water-gas shift (WGS) reaction over TiC nanoparticles and extended surfaces.31 Their work showed that the WGS reaction could proceed readily via the associative mechanism (carboxyl intermediate) on TiC(001); however, TiC nanoparticles initiate a redox mechanism with unfavorable energetics and low reaction rates. All together these studies indicate that TiC is catalytically active for many reactions involving hydrogen transfer, with the specific form and structure (single crystal surfaces, thin films, metcars, or powders) greatly influencing their reactivity. Certainly there is much work remaining to fully characterize this promising material, and techniques for the synthesis of TiC with direct control of morphology and structure will play an important role in uncovering links between material structure and catalytic activity. In this paper, we describe the synthesis and physical characterization of high surface area (up to ∼840 m2/g), porous TiC films grown using physical vapor deposition in a gaseous ethylene ambient. Through control of three growth parameters (ethylene pressure, deposition angle, and deposition temperature) it is possible to manipulate the film composition, structure, and morphology, which can impact catalyst activity and selectivity. A key factor for synthesizing high surface area films grown by physical vapor deposition is the limitation of adatom surface diffusion during film growth. When the surface is cooled to cryogenic temperatures, surface diffusion is restrained and nonequilibrium surfaces can be generated. This growth scheme is evocatively described as “hit-and-stick” or ballistic deposition (BD).32 In this scenario, vapor-phase atoms or molecules travel from their source along straight-line trajectories to the deposition surface where each is incorporated in close proximity to its original landing site.32 Initial surface “roughening” occurs randomly, and subsequent development of the film is highly dependent on the deposition angle. At oblique deposition angles topographically elevated points, created stochastically, preferentially intercept the incoming atoms while shadowing lower regions.33 This self-shadowing growth process results in porous films. Brett et al. have grown highly sculptured films using ballistic deposition at glancing deposition angles to create arrays of micrometer-scale columns, zig-zags, and helices.34 This growth technique, sometimes referred to as glancing angle deposition (GLAD), has been the subject of two recent reviews by Abelmann and Lodder33 as well as Hawkeye and Brett.35

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Figure 1. Schematic of the experimental apparatus.

The capabilities of ballistic deposition can be further extended by directionally depositing metal in a reactive ambient (O2, C2H4, etc.) as described by Dohna´lek and Kay.36 In this way, metal-compound thin films can be grown through the surface reaction of the metal adatoms and the ambient species. Aptly, this technique is referred to as reactive ballistic deposition (RBD).36 Using ballistic deposition or reactive ballistic deposition, highly structured films have been grown from a wide variety of materials including Pd,37 Cr,38 Cu,38 Fe,39 Ti,40 TiO2,41,42 Mn,43 MgO,36 Ta2O5,44 WO3,44 SiOX,45,46 MgF2,43 CaF2,43 and amorphous solid water (ASW).47-49 The resulting films often have optical, electronic, and magnetic properties which differ from those of dense, nonstructured films of similar composition. In addition, the films demonstrate unique chemiphysical surface properties; however, surface characterization studies have only been conducted for porous films of TiO2,41 Pd,37 MgO,36 and ASW.47-49 Recently, Dohna´lek et al. have shown that Pd films deposited in this manner have unprecedented activity for the hydrogenation of C2H4 due in part to a high fraction of undercoordinated surface sites.50 A key point of these studies has been that films with the greatest degree of porosity and the highest surface area are synthesized at more glancing angles of deposition (75°-85°) using the lowest substrate temperature achievable, because the combination of these two factors increases the effectiveness of the selfshadowing growth process. Experimental Methods We have investigated the growth of titanium carbide films grown using the RBD scheme described above employing an ultra-high-vacuum (UHV) molecular beam surface scattering apparatus, depicted in Figure 1. The apparatus is comprised of five individually pumped vacuum chambers which are divided into three sections: a separable section for generating molecular beams (chambers 1 and 2), a bakeable UHV section for molecular beam scattering and reflection absorption infrared spectroscopy (RAIRS) (chambers 3 and 4), and a UHV sample synthesis chamber (chamber 5) accessed by translating the sample vertically and which can be isolated from chamber 4 using a gate valve. The molecular beam source section is sealed to the UHV section using a Viton O-ring at the mating surfaces of chambers 2 and 3. By closing two custom-designed gate valves installed on the outer wall of chamber 3, it is possible to separate the two sections from one another while preserving the vacuum in chambers 3-5. This is advantageous by allowing operators to break vacuum in the source chamber to perform maintenance without interrupting the operation of the UHV section.

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The molecular beams are triply differentially pumped to minimize the effusive component of the beam that reaches the sample. Two molecular beams (A and B) can be generated in chamber 1 by expanding gas through separate nozzles (the current nozzles, A and B, have aperture diameters of 100 and 1000 µm, respectively). Within the molecular beam source section, chamber 1 is pumped by P1, a 2400 L/s diffusion pump (Varian VHS-6), and chamber 2, the second differential pumping stage, is pumped by P2, a 1500 L/s diffusion pump (Varian M6). The final differential pumping section, chamber 3, comprises part of the UHV section and is pumped by P3, a 350 L/s Leybold-Heraeus turbo molecular pump. The beam scattering chamber (chamber 4) is pumped by P4, a 210 L/s turbo molecular pump (Pfeiffer TMU 261), and typically has a base pressure less than 1 × 10-10 Torr as measured by a nude ionization gauge and controller (GranvillePhillips model 307). Chamber 4 contains a quadrupole mass spectrometer (QMS; Extrel EXM 3) and capabilities for RAIRS. Infrared light is admitted into and out of the UHV chamber from a Fourier transform infrared spectrometer (FTIR; Bruker model Tensor 27) via differentially pumped KRS-5 windows, and spectra are collected from a liquid nitrogen cooled, mercury-cadmium-telluride detector (MCT). In addition, an inert stainless steel flag is mounted in chamber 4 and can be pneumatically actuated to intercept the molecular beam before it reaches the sample surface, allowing for King and Wells type measurements.51 Chamber 5, used for film synthesis and analysis, is located above chamber 4. This chamber is pumped by P5, a 550 L/s turbo molecular pump (Varian Navigator 551). Chamber 5 contains tools for thin film deposition and characterization including a “mini” electron beam evaporator (E-beam; Tectra e-flux model), a quartz crystal microbalance (QCM) and controller (Maxtek Inc.), an Auger electron spectrometer (AES; Perkin-Elmer model 10-155), and an ion gun (Ar+; RBD Enterprises model 04-165). Titanium carbide films are deposited onto a polished polycrystalline tantalum plate (ESPI) with dimensions of approximately 10 mm × 10 mm × 0.75 mm. The tantalum plate is spot-welded to adjacent 1 mm tantalum posts and mounted on a liquid nitrogen cooled probe. A home-built manipulator provides sample motion in the x-, y-, and z-directions with positioning within ∼0.01 in. The sample manipulator is mounted atop a home-built differentially pumped rotary seal that allows 360° of rotation with a precision of ∼0.5°. The sample temperature is measured with a type-C thermocouple spotwelded to the backside of the Ta plate and controlled by resistive heating over the range 77-1850 K. The absolute temperature ((2 K) is verified using the known multilayer desorption temperatures of small molecules. Titanium carbide films were deposited using the reactive ballistic deposition scheme.36 Briefly, the electron beam evaporator is used to deposit metallic titanium at a constant rate of ∼1.8 monolayers (ML)/min (1 ML ) 1.48 × 1015 Ti atoms/ cm2), as calibrated by the QCM. A continuous measure of the deposition rate is obtained by monitoring the flux of titanium ions generated by electron beam bombardment with an electrode integrated within the evaporator. During Ti deposition, the sample is held at constant temperature while C2H4 is introduced by a leak valve to a pressure of ∼1.5 × 10-7 Torr. Under these conditions, the flux of ethylene to the surface is much greater than needed to carburize the titanium, ensuring a sufficient supply of carbon; however, ethylene multilayers do not accumulate at any of the deposition temperatures used (77-700

Flaherty et al. K) as confirmed by temperature-programmed desorption (TPD).52 Rotation of the sample manipulator around the vertical axis allows the deposition angle to vary from 0° to 90° from the sample normal. We have investigated the effect of the ethylene pressure and surface temperature on the composition of the TiC films using Auger electron spectroscopy (AES; see the Results and Discussion). Before each film is deposited, the Ta substrate is annealed to 1800 K, which provides a smooth, clean surface (as confirmed by AES and cyclohexane TPD) by removing previously deposited material through a combination of diffusion into the bulk and evaporation from the surface.23 Following each film growth, the surface of each film is covered with molecular ethylene and hydrocarbon fragments (C1 and C2 species and hydrogen) resulting from the surface reaction with metallic Ti. The majority of the remaining hydrocarbon species can be removed or decomposed by annealing the film to 400 K. Measurements of the surface area and binding site energy distribution of the TiC films are conducted by adsorption of cyclohexane and subsequent temperature-programmed desorption as described later. Cyclohexane (C6H12) is dosed using a mildly supersonic, 300 K molecular beam. For all C6H12 TPD experiments, the sample is linearly heated at 1 K/s while the C6H12 desorption rate is measured by monitoring 56 m/z and 84 m/z with the QMS. Additionally, Auger electron spectroscopy is utilized to determine the chemical environment of the carbon within the deposited films since AES is a sensitive probe of the valence electrons in carbon and can ascertain its graphitic or carbidic nature. Complementing the in situ measurements, additional TiC films are grown under identical conditions within a separate high-vacuum chamber designed to deposit films for ex situ analysis. This chamber is equipped with electron beam evaporators, an x-y-z sample manipulator mounted upon a rotary seal, and a quartz crystal microbalance. Additionally, the substrate temperature can be controlled from 77 to 1200 K (measured by a type-K thermocouple) through a combination of liquid nitrogen cooling and resistive heating. The chamber has a typical base pressure of ∼5 × 10-9 Torr. TiC films are deposited onto Si(100) substrates which are removed from the chamber and examined with scanning electron microscopy (SEM) and transmission electron microscopy (TEM) in combination with selected area electron diffraction (SAED). Results and Discussion Electron Microscopy and Diffraction. Figure 2 displays scanning electron micrographs of representative films grown by depositing 300 ML of titanium carbide at 77 K in an ethylene background pressure of 1.5 × 10-7 Torr with deposition angles of 70° (Figure 2A,C) and 85° (Figure 2B,D). It is clear from this work and that of previous investigations that the morphology of the deposited films is very sensitive to the deposition angle during film growth.34,36-49 As the deposition angle is gradually increased from 0° (perpendicular to the surface normal) to 90° (parallel to the surface), the film morphology progresses from a completely dense material into a continuous, reticular structure with pores with estimated diameters of several nanometers and then finally into an array of discrete, porous columns with characteristic diameters in the range of 50-100 nm. Additionally, TEM images were taken in bright-field mode and complemented by SAED utilizing a Schottky field emitter based FEI TF20 (200 kV) transmission electron microscope, equipped with a scanning transmission electron microscopy (STEM) unit, high-angle annular dark-field (HAADF) detector,

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Figure 2. Scanning electron microscopy images of films grown by reactive ballistic deposition of 300 ML of TiC at 77 K in an ethylene background pressure of 1.5 × 10-7 Torr: (A, C) top and profile views, respectively, of a film grown at 70° with an overall film thickness of 275 nm, (B, D) top and profile views, respectively, of a film grown at 85° with an overall film thickness of 700 nm.

and X-Twin lenses. The FEI transmission electron microscope includes a temperature-controlled sample stage capable of heating to 1200 K which was employed to perform in situ hightemperature electron microscopy and analyze the evolution of the films’ structure with annealing. Samples were prepared for TEM in one of two ways. In the first approach, portions of the film were removed from the Si(100) substrate, sonicated in ethanol, and drop cast onto lacey carbon-coated copper grids. Figure 3A displays a TEM image acquired of nanocolumns prepared in this manner from a film synthesized by deposition of TiC at 77 K and 85° and preannealed to 400 K. The second preparation technique provides cross-sectional slices of the Si(100) wafer with the TiC film still intact using a three-step process. First, the TiC sample is covered with several hundred nanometers of amorphous silica by chemical vapor deposition to protect the structure of the nanocolumns during subsequent processing. Second, a dicing saw is used to prepare a 300 µm thick slice of the Si(100)-supported sample, followed by focused ion-beam milling (employing 30 kV gallium ions) to reduce the sample thickness to ∼100 nm to allow for electron transmission. Third, the prepared sample is briefly cleaned with a low-pressure Ar+ plasma to remove residual Ga. Figure 3B displays a TEM image of a sample prepared in this way containing a number of TiC nanocolumns grown by deposition of TiC at 350 K and 85°. Inspection of this image reveals that the columns are fairly uniform and well aligned. Selected area electron diffraction illustrates that films deposited at 77 K are amorphous after heating to 400 K; however, their crystallographic order improves as they are annealed to 900 and 1200 K as shown in parts C, D, and E, respectively, of Figure 3. However, deposition at increased temperatures can result in crystalline films as illustrated by SAED patterns acquired from a film grown by deposition of TiC at 350 K and

85° and incrementally annealed to 400, 900, and 1200 K, shown respectively in parts F, G, and H of Figure 3. As the annealing temperature increases from 400 to 900 and 1200 K, the average size of the crystallites also increases, as indicated by the appearance of distinct spots in the diffraction pattern. The interlattice spacings or “d-spacings” extracted from the SAED measurements confirm that the structure of the deposited films matches the TiC “rock-salt” structure, which consists of two interlaced face-centered cubic lattices of carbon and titanium. Films deposited at 70° at 350 K display similar polycrystallinity, suggesting that the film crystal structure is not related to the overall film morphology. In addition, energy-dispersive spectroscopy (EDS) within the transmision electron microscope revealed that the films were comprised almost entirely of titanium and carbon, with trace amounts of oxygen. Comparison of the diffraction patterns acquired from samples prepared by either technique confirms that the manner of sample preparation for TEM (drop casting onto Cu grids or the combination of dicing and Ga+ sputtering) has no discernible effect on the electron diffraction patterns. The combined results of the film composition and crystallographic structure confirm that the deposited films consist of TiC. Auger Electron Spectroscopy. We have utilized AES to determine the composition of films deposited within the molecular beam surface scattering apparatus. Auger electron spectroscopy can directly probe the nature of the local chemical environment for many carbon-containing compounds. This is atypical of most experimental situations in which AES is primarily useful for determining the composition of a material but not the chemical state. However, in the case of carbon, valence electrons are involved in the KLL Auger process (Lshell, 2s, and 2p) and reflect changes in the hybridization of the carbon L-shell electrons induced by bonding with either other

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Figure 4. Auger electron spectra acquired from TiC synthesized by deposition of titanium in C2H4 at various background pressures: (top) films annealed to 400 K to desorb or decompose residual hydrocarbons on the surface, (bottom) films annealed to 900 K to illustrate evolution of carbide.

Figure 3. Transmission electron microscopy images and electron diffraction patterns of TiC nanocolumns. (A) Transmission electron micrograph of a cluster of TiC nanocolumns removed from a film deposited at 85° at 77 K and drop cast onto a Cu grid. (B) Crosssectional view of TiC nanocolumns deposited on Si(100) and prepared as described in the text. At the top of the image a protective silica coating is visible which was deposited during sample preparation for TEM. SAED patterns acquired from the columns as a function of the deposition temperature and annealing temperature within the transmission electron microscope. The patterns are from films: TiC deposited at 85° at 77 K and annealed to (C) 400 K, (D) 900 K, and (E) 1200 K, as well as TiC deposited at 85° at 350 K and annealed to (F) 400 K, (G) 900 K, and (H) 1200 K.

carbon atoms or transition metals, resulting in sp2, sp3, or d-orbital hybridization, respectively.53,54 Consequently, the carbon (KLL) feature line shape is indicative of these changes, and in the case of transition-metal carbide formation displays a characteristic triple-peak feature near 273 eV.53 By monitoring the carbon feature, we can confirm the presence of a carbidic phase and qualitatively correlate it to the deposition conditions and postgrowth treatment. To create a standard for comparison, ∼40 monolayers of titanium were deposited in vacuum onto the tantalum substrate at 600 K. Following the procedure of Hwu and Chen, the titanium film was dosed with 60 langmuirs of ethylene at 600 K. The sample was then incrementally annealed to temperatures as high as 1200 K, and the acquired

AES agreed closely with published results from surface carburization of Ti(0001).23 Employing AES, we investigated the impact of the C2H4 background pressure, deposition temperature, and annealing temperature on the film stoichiometry and composition (graphitic or carbidic). Titanium was deposited at normal incidence to the substrate at a constant rate of 1.5 ML/min to form 15 ML thick films at 77 K at a set C2H4 pressure for each deposition. Each film was preannealed to 400 K before the spectrum was acquired to desorb or decompose residual hydrocarbon surface species. Figure 4 (top) shows spectra for films grown over a range of ethylene pressures. Clearly, the film composition is strongly dependent on the ethylene pressure up to a pressure of ∼3 × 10-8 Torr, at which point the carbon content saturates. The chemical nature of the carbon (i.e., graphitic or carbidic) in the TiC film is sensitive to the deposition and annealing conditions. Figure 4 (bottom) displays AE spectra acquired after annealing of the films in Figure 4 (top) to 900 K. The annealing treatment induces a clear change in the intensities of the three Auger peaks at 253, 262, and 273 eV. The Auger spectra indicate that films deposited at 77 K and preannealed to 400 K contain a significant amount of graphitic or amorphous carbon, but annealing to 900 K dramatically increases the carbidic nature of the film. Additional experiments were conducted in which we varied the deposition temperature over a range from 77 to 600 K (data not shown). Even with moderate increases in the deposition temperature (77 f 150 K), the carbide quality notably improves. In general, we find that high-temperature deposition and annealing favor the formation of titanium carbide; however, sufficiently high temperature treatments can be detrimental to the surface area and porosity of the films. It should be noted that the AES measurements of porous films grown by deposition in C2H4 ambient reflect carbon to titanium ratios greater than expected for stoichiometric TiC (carbon uptake saturates at a C:Ti of ∼2.5, or ∼70% carbon content). In comparison, dense TiC films prepared in a way similar to that used by Hwu and Chen (deposition of titanium in vacuum,

High Surface Area Titanium Carbide

Figure 5. Series of temperature-programmed desorption spectra of cyclohexane from films synthesized by reactive ballistic deposition of identical quantities of TiC, 50 ML: (top) desorption from a porous film deposited at 65° at 350 K, (bottom) desorption from a dense film grown at 0° at 600 K. Cyclohexane was adsorbed onto the surface at 77 K by time-resolved doses employing a molecular beam. Samples were linearly heated at 1 K/s.

followed by annealing in C2H4 at 600 K) result in carbon to titanium ratios of 2.5 and 1 after annealing in a vacuum at 600 and 850 K, respectively, due to the inward diffusion of carbon into the bulk.23 However, SAED results obtained within the transmission electron microscope, Figure 3, do confirm that the porous films are polycrystalline TiC. Further, diffraction representative of graphitic carbon is not apparent, even after annealing to 1200 K. Altogether, these results suggest that the surface composition of the film, as probed by AES, does not match the bulk composition which is responsible for electron diffraction. Interpretation of the AES results using a homogeneous attenuation model for the Auger electrons55 suggests that the films consist of near stoichiometric carbide (C:Ti of 1:1) with a single atomic carbon overlayer. Presumably, this is due to decomposition of C2H4 on the film even after the film has a fully carburized surface.23 While surface carbon will affect adsorption geometries and reaction processes, the quantification and study of surface area as a function of deposition conditions as presented here is not impacted (the technique used for surface area calculations is described in the paper). We are currently investigating schemes for removing excess surface carbon and fully activating the TiC catalyst. Temperature-Programmed Desorption of Cyclohexane. TPD of cyclohexane (C6H12) was performed by adsorbing C6H12 onto the titanium carbide films using a neat supersonic molecular beam at low surface temperatures (77-140 K), after which the sample was heated at 1 K/s while the desorption products were monitored with the QMS. C6H12 desorbs molecularly from titanium carbide films when dosed at temperatures of less than 300 K, as determined by monitoring for multiple dissociation fragments (cyclohexene, benzene, hydrogen, and C2 and C3 fragments). Because C6H12 is inert on these TiC films, we employ it as a probe molecule to investigate the surface area, porosity, and surface binding site energy distribution as a function of the deposition and annealing conditions. Figure 5 (top) shows TPD spectra from a porous TiC film grown by depositing 50 ML of TiC at a sample temperature of 350 K with a deposition angle of 65°, which will be described

J. Phys. Chem. C, Vol. 113, No. 29, 2009 12747 with the nomenclature 50 ML, 350 K, 65° for the remainder of the paper. Figure 5 (bottom) displays TPD spectra from dense TiC (50 ML, 600 K, 0°) also grown using RBD. Each spectrum was acquired after a beam of C6H12 was impinged onto the sample at 77 K. At 77 K, the sticking probability of C6H12 on the sample surface is independent of the surface coverage (S ≈ 0.95); therefore, the resulting C6H12 coverage is directly proportional to the dosing time. The data shown in Figure 4 reveal several distinct properties of TiC films. First, for both surfaces, C6H12 desorbs over a wide temperature range, ∼135-300 K, indicating a broad distribution of surface site binding energies. In addition, C6H12 preferentially fills the highest energy binding sites first, suggesting that transport of C6H12, a relatively large molecule, is facile on both films even at 77 K. Second, the TPD spectra from both films contain a zero-order desorption feature at ∼138 K due to condensation of C6H12 multilayers on the surface. However, the porous film contains an additional desorption peak located at 155 K which is absent from the dense TiC films. Previous investigations of porous films grown with RBD (MgO and TiO2) or BD (amorphous solid water and Pd) have shown related features in N2 TPD spectra.37,41,48,56 For these particular systems, desorption peaks due to pore condensation occurred at temperatures ∼5 K greater than that of the multilayer desorption feature. The shift in desorption temperature from pore condensation should increase as the characteristic diameter of the pore decreases (as predicted by the Kelvin equation). Considering the morphological similarities of these systems, the C6H12 TPD features are identified: a multilayer desorption peak at 138 K, a pore condensate desorption peak at 155 K, and surface desorption primarily above 170 K. Temperature-programmed desorption of C6H12 can also be utilized to investigate other aspects of the films’ structure and their dependence on parameters concerning growth (deposition angle and temperature) and postgrowth (annealing temperature) treatment. Since C6H12 multilayers desorb at 138 K, their formation can be prevented by maintaining the sample at 140 K during C6H12 exposure while still allowing for surface adsorption and condensation within the pores. A comparison of the integrated TPD spectrum acquired in this manner on a porous film to that acquired from a dense, flat surface provides a relative measure of the porous film’s surface area. The specific surface area (m2/g) can be determined using relevant physical properties of TiC; however, the details of this calculation will be discussed in the following section and will be presented with the derived values. To summarize, temperature-programmed desorption spectra of C6H12 from TiC surfaces provide quantitative measures of the surface binding site energy distribution from the TPD line shape, the specific surface area from the integral areas of each TPD, and a qualitative description of each film’s morphology. Titanium carbide films bind C6H12 up to ∼300 K. The TPD spectra indicate that films deposited at glancing angles are porous, and as illustrated in Figure 5, the quantity of cyclohexane adsorbed directly on the film surface is much greater on the film deposited at glancing incidence (deposition angle of 65°) as opposed to the dense film (deposition angle of 0°). Note that the y-axis, indicating the desorption rate, from the dense film has been multiplied by a factor of 10. Angular Dependence on the Film Morphology. As illustrated by the SEM images in Figure 2, the structure of films grown by RBD is extremely sensitive to the angle of deposition, because the deposition angle affects the extent of the selfshadowing mechanism. Interestingly, these morphological al-

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Figure 6. TPD spectra from RBD films grown at varying angles by depositing 50 ML of TiC at 77 K. From top to bottom the films were deposited at 75°, 65°, 60°, 55°, 50°, and 0°. Pore condensation features are indicated at ∼148 and ∼165 K. Each spectrum was acquired after saturation at 140 K with C6H12.

Figure 7. Total amount of C6H12 desorption from TiC films as a function of the titanium deposition angle. Each film was grown from a total of 50 ML of TiC. Cyclohexane desorption is expressed as monolayer equivalents, where 1 ML is equal to saturation coverage at 140 K on dense TiC.

terations manifest as subtle changes in the C6H12 TPD spectra as well. To explore this effect, titanium carbide films were synthesized by depositing equivalent amounts of TiC (50 ML) by RBD over a wide range of deposition angles (0-85°) at 77 K. Figure 6 shows C6H12 TPD spectra from TiC grown at selected deposition angles and preannealed to 400 K. Each spectrum was acquired after saturation of the film with C6H12 at 140 K. As shown in Figure 6, the C6H12 TPD spectra from the RBD TiC films vary greatly with the deposition angle. Both the magnitude and line shape of the spectra depend on the deposition angle; however, all films deposited at angles greater than 50° have common aspects including one or two desorption peaks at low temperatures (∼145-170 K) from pore condensation and a high-temperature shoulder extending to ∼300 K. The sensitivity to the deposition angle is evident when the pore condensation features in the spectra located at 148 and 165 K are examined. As the deposition angle increases from 0° to 55°, a pore condensation feature appears at 165 K. Further increases in the deposition angle result in a slight rise in the intensity of the 165 K feature. When the deposition is greater than 60°, the 148 K feature emerges clearly and is accompanied by a simultaneous decrease in the 165 K peak (films were deposited over a wide range of angles extending from 0° to 85° and are included as Figure 1 in the Supporting Information). We interpret this effect as indicative of an increase in the average pore diameter in the TiC film. Consistent observations were made by Kimmel et al. in simulations and experiments with ballistic deposition of ASW films.48,57 Generally, simulations illustrate that the average pore diameters in films grown by BD57,58 increase with increasing deposition angle, which would lead to a decrease in desorption energy, as qualitatively described by the Kelvin equation.59 The distinction between pore sizes is supported by similar experiments conducted on identical films using the smaller adsorbate cyclopropane, C3H6. Temperature-programmed desorption of both C6H12 and C3H6 was examined from an RBD TiC film deposited at 77 K with a deposition angle of 62.5°. During these experiments the sample temperature was controlled to prevent multilayer condensation during dosing (140 K for C6H12 and 88 K for C3H6). Resulting TPD spectra display two pore condensation features for C6H12 at 148 and 165 K, while C3H6 TPD spectra display two peaks at 98 and 109 K (Figure 2 in the Supporting Information). On the basis of these results, we believe that the size of the probe molecule in relation to the pore diameter impacts the observed spectra. Of course, if a molecule is too large, it will be excluded from the pores, and if

the probe molecule is very small, it will penetrate most pores. However, if the average pore diameter is only a few multiples of the characteristic diameter of the probe molecule, then the narrower pores will display very strong capillary action and wider pores may show a weaker interaction. For the two probe molecules, the ratio of the intensity of the high-temperature peak to the intensity of the low-temperature peak is 7:5 and 1:4 for C6H12 and C3H6, respectively. Our interpretation is that C3H6, with an estimated diameter of ∼0.42 nm, is weakly condensed in the majority of the pores due to its small size (98 K peak), while a few of the smaller pores may hold it more tightly (109 K peak). Accordingly, C6H12, which has a diameter of ∼0.55 nm, is more strongly condensed in a fraction of the pores (165 K peak), is weakly bound in a smaller fraction (148 K peak), and may be completely excluded from the smallest pores. We are currently applying other ex situ experimental methods to quantify the pore size dependence on the deposition angle. Figure 7 displays the total C6H12 uptake for 50 ML TiC films (synthesized at 77 K and preannealed to 400 K) expressed as monolayer equivalents (i.e., the amount of C6H12 adsorbed on a flat surface). As described earlier, multilayer formation is prevented by saturating the films with C6H12 at 140 K; therefore, the quantities presented in Figure 7 only account for direct surface adsorption and pore condensation. For small deposition angles (θdep < 50°) there is little increase in the surface area of the film. However, upon passing 50°, the surface area increases dramatically, reaching a maximum at ∼65-75°, after which there is a small decrease of ∼15%. Previous studies on Pd films displayed a surface area which increased monotonically from 45° to 85°,37 while films of TiO241 and ASW47,57 exhibited maxima at ∼70°. It is important to note that films grown at 77 K with a deposition angle of 0° adsorb 2.4 times the amount of C6H12 adsorbed onto the bare, smooth tantalum substrate which was used as a standard. While the self-shadowing growth mode is not in effect in this case, a degree of surface roughening occurs due to limited surface diffusion. As mentioned previously, the relative amounts of C6H12 adsorption/desorption can be converted into a measure of the specific surface area. This is accomplished by first calculating the amount of titanium deposited onto the sample using the QCM (g/cm2). Then assuming a titanium to carbon ratio of 1:1 for the bulk stoichiometry, the total mass per square centimeter of deposited TiC is calculated using the difference in the molar masses of Ti (47.87 g/mol) and TiC (59.88 g/mol). The quantity of C6H12 desorption from each porous film saturated at 140 K is calculated by integration of each TPD spectrum and includes

High Surface Area Titanium Carbide contributions from surface adsorption as well as pore condensation. Then, this integral is divided by the quantity of C6H12 which desorbs from the flat, polished, polycrystalline tantalum surface saturated at 140 K, which results in the adsorption of one molecular layer of C6H12. The ratio of these two quantities (C6H12 from porous TiC:C6H12 from Ta) is the amount of enhanced surface are per geometric area (this quantity is unitless) and can be expressed as the number of monolayer equivalents with respect to the Ta surface. The specific surface area is found by dividing the number of monolayer equivalents (unitless) by the mass of TiC deposited (g/cm2). This technique has the advantage of not requiring any knowledge of the specific adsorption geometry of C6H12 on either surface. Rather it only relies on the fact that C6H12 physically adsorbs as an intact molecule at 140 K on the surface and on average occupies a constant molecular cross-sectional area on each surface. The total C6H12 desorption from a film grown by depositing 50 ML of TiC at 77 K and 75° (50 ML, 77 K, 75°) corresponds to a specific surface area of ∼440 m2/g. As mentioned previously, this value includes contributions from pore condensation in addition to desorption directly from the surface. Following inspection of the TPD line shapes and integrals for C6H12 from TiC films, we estimate that pore condensation accounts for 15-30% of desorption from these TiC films. For the sake of comparison, we note that several investigators have reported approaches for synthesis of high surface area transition-metal carbides including tungsten carbide powders with surface areas of 30-100 m2/g,60 silicon carbide with a surface area of 140 m2/g,61 vanadium carbide (with residual carbon) with a surface area >200 m2/g,62 and molybdenum and tungsten carbides with surface areas of 100-200 m2/g.63 One important point is that all of the high surface area TiC films presented here must be finely divided and highly porous to attain such high surface areas. In particular for films deposited at very glancing angles (80-85°), resulting in the formation of individual columns (Figures 2 and 3), these nanocolumns themselves must contain a large internal surface area. A simple calculation assuming cylindrical columns with diameters ranging from 20 to 50 nm, and using the bulk density of TiC (4.96 g/cm3), shows that a smooth nonporous column would have a specific surface area of only ∼16 - 40 m2/g, far short of the values measured. Therefore, each of these nanocolumns must have a high degree of internal porosity as well. Effects of Annealing. Transition-metal carbides, such as TiC, have potential as catalysts for a number of reactions, many of which operate at high temperature (T > 800 K). To be useful, potential catalytic materials must withstand annealing at temperatures in this range. The thermal stability of the porous TiC films was investigated by incrementally annealing from 400 to 1750 K. Figure 8 shows a set of C6H12 TPD spectra from a single TiC film deposited at 77 K and a deposition angle of 75°. The line shape of the TPD spectra changes very little as the film is progressively annealed up to 1100 K, but the film does lose ∼18% of its surface area. The pore condensation feature at ∼148 K is still present after annealing to 1200 K, suggesting that even at these high temperatures the TiC films remain porous. By 1400 K, the pore condensation feature has disappeared, showing that the film is dense. After annealing to 1750 K, all traces of TiC have been removed (as determined by AES) and the C6H12 TPD again resembles that of the clean tantalum support surface. The quantity of C6H12 desorption is plotted in Figure 9 with respect to the annealing temperature for a number of films deposited at 77 K and at deposition angles ranging from 0° to

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Figure 8. Series of C6H12 TPD spectra from a single TiC film grown by reactive ballistic deposition of 50 ML of TiC at 75° and 77 K. The film was incrementally annealed from 400 to 1750 K. Each spectrum is from a saturation dose of C6H12 at 140 K. From top to bottom the spectra correspond to annealing temperatures of 400, 600, 1100, 1200, 1400, and 1750 K.

Figure 9. Total amount of C6H12 desorption from TiC films as a function of annealing temperature and deposition angle. Each film was grown from a total of 50 ML of TiC deposited at 77 K. From top to bottom the films were deposited at 75°, 70°, 65°, 60°, 55°, 50°, and 0°.

75°. Films were also deposited at 80° and 85°, but the data are omitted for clarity. Films deposited at angles from 65° to 75° exhibit the greatest C6H12 uptake at all annealing temperatures. As the films are annealed, the amount of C6H12 uptake decreases slightly, typically leading to a loss of 20% or less by 1000 K. Films deposited at more oblique angles appear to display slightly greater thermal stability and suffer from a relatively smaller loss in surface area. By 1400 K, the films are completely dense but still display surface roughening, which contributes an additional ∼30% to the C6H12 uptake. Final annealing to 1750 K is necessary to completely remove TiC from the tantalum support, exposing the smooth tantalum surface. Effect of the Deposition Temperature and Growth Mechanism. Previously, we outlined the self-shadowing growth mechanism and how it can be affected by the deposition angle as well as the substrate temperature. Prior investigations of BD and RBD demonstrated without exception that the greatest porosity and highest surface area were achieved at the lowest deposition temperatures. Notably, this is not the case for RBD of TiC films, as illustrated in Figure 10. Each film was grown by RBD of 50 ML of TiC at a deposition angle of 65° and a surface temperature ranging from 77 to 700 K. All films were annealed to 800 K prior to the surface area measurements. Clearly, there is a strong dependence on the deposition temperature; however, the maximum surface area occurs at a deposition temperature of 350 K. In fact, increasing the deposition temperature from 77 to 350 K, while keeping all

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Figure 10. Total C6H12 desorption as a function of the deposition temperature. Each film was synthesized by deposition of 50 ML of TiC at 65°. The quantity of C6H12 desorption is expressed as the calculated specific surface area, as described in the text, on the vertical axis on the right. Films were annealed to 800 K.

other conditions constant, increases the surface area of the films by ∼100%. Incredibly, the specific surface area of a film grown by depositing 50 ML of TiC at 350 K at a deposition angle of 65° is approximately ∼840 m2/g, a very large value considering the previously mentioned maximum surface areas for TMCs. The fact that the surface area does not depend monotonically on the deposition temperature indicates that the film growth involves a series of surface processes with opposing temperature dependence. One of these processes is surface diffusion of Ti adatoms before they are incorporated into the film. The rate of Ti adatom diffusion will increase with increasing surface temperature, leading to smoother films and correspondingly lower surface areas. This is the likely cause of the decrease in surface area from 350 to 700 K. However, there must be at least one additional process not encountered in prior investigations which is responsible for the dramatic increase in surface area from 77 to 350 K. Deposition of TiC differs in one distinct aspect from earlier work with metal oxides, Pd, and ASW. Metal oxides were synthesized by deposition of metal in an oxygen background, and Pd and ASW were deposited in ultrahigh vacuum. Neither situation involves chemisorbed surface species which are not ultimately incorporated into the film. On the other hand, deposition of titanium in a C2H4 background not only carburizes the titanium, leading to TiC, but also generates a number of by products which are observed desorbing after film growth, such as CXHY and H2. As shown in Figure 11 (top), synthesis of TiC by reactive ballistic deposition at a surface temperature of 77 K produces a significant amount of ethane which desorbs with a peak at ∼250 K. Additionally, a much smaller amount of propane and propylene desorb at ∼240 K. Ethylene evolves over a wide temperature range (100-1200 K) with a peak at ∼400 K. While the hydrocarbon species exhibit a single desorption peak, hydrogen exhibits a very broad peak at ∼360 K with a second less intense feature at ∼900 K, suggesting that hydrogen is produced from the decomposition of two distinct surface moieties. Figure 11 (bottom) shows postgrowth desorption of products from a film deposited under similar conditions except the surface was maintained at 350 K during deposition. Not surprisingly, significantly less desorption is seen overall. Ethylene, ethane, and hydrogen are still seen; however, neither propylene nor propane is observed. Evidently, the increased deposition temperature may influence the growth process by allowing continuous desorption of C2H4 and byproducts formed during the carburization reaction.

Figure 11. TPD of species formed as a product of reactive ballistic deposition of titanium in a C2H4 background. Top: titanium deposited at 77 K. Ethane, ethylene, hydrogen, propylene, and propane are evident in the TPD spectrum. Bottom: titanium deposited at 350 K. Only ethane, ethylene, and hydrogen desorptions are observed. The desorption spectra were acquired by heating the sample at 3 K/s from 77 to 1500 K after film growth. The majority of hydrocarbon desorption above 600 K is attributed to desorption from the sample mount.

The surface temperature will also impact the decomposition of surface-bound intermediates. Low deposition temperatures may limit surface diffusion, which is advantageous for generating high surface area films. On the other hand, thermal energy is necessary to drive the kinetics of titanium carburization. In a related investigation, Chen et al. studied the decomposition of C2H4 over Ti(0001) and carburized Ti(0001) utilizing a combination of high-resolution electron energy loss spectroscopy (HREELS) and TPD.23 They found that C2H4 adsorbs onto Ti(0001) in both π- and di-σ-bond configurations. As the surface temperature increases to 200 K, vibrational features associated with Ti-C increase, signaling a degree of C2H4 decomposition. Interestingly, upon heating to 300 K, the CdC bond vibrational mode disappears, most likely indicating desorption of π-bonded C2H4.23 After heating to 450 K, the HREELS spectra indicate that the remaining di-σ-bonded C2H4 undergoes complete carbon-carbon bond cleavage; however, C-H2 vibrations are still present. Related spectra acquired following the adsorption of C2H4 on the carburized Ti(0001) surface suggest a different reaction pathway resulting in the formation of an ethylidyne (CCH3) intermediate which is stable to 300 K. Heating to higher temperatures decomposes ethylidyne and generates CXHY fragments and surface-bound atomic hydrogen (Ti-H) which persist to at least 450 K. The authors attributed the lack of desorption products to diffusion of atomic hydrogen into the titanium bulk.23 Similarly, C2H4 adsorption on a carburized Mo(110) surface leads to the formation of the ethylidyne intermediate at ∼260-300 K; however, this species decomposed after heating to 350 K as seen by changes in the HREEL spectra and hydrogen desorption during TPD.64 A simple experiment was performed to relate the observations of Chen and co-workers on adsorption of C2H4 onto Ti(0001) surfaces to the synthesis of TiC by reactive ballistic deposition of titanium in a C2H4 ambient. By depositing titanium onto the sample at 77 K in a molecular hydrogen background, we generated a number of hydrogenated films. Hydrogen desorption was easily observable from these thin films, in contrast to the

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J. Phys. Chem. C, Vol. 113, No. 29, 2009 12751 may present a more corrugated surface potential, lead to greater barriers for diffusion, and consequently result in shorter diffusion lengths. However, while it is clear that the surface temperature does affect the properties of the TiC films synthesized here, the experimental techniques utilized in this investigation are not capable of illustrating how this manifests in the film growth mechanism. Conclusions

Figure 12. TPD spectra of molecular hydrogen as a result of depositing titanium onto the sample at 77 K in ethylene or hydrogen ambients. Desorption spectra were acquired by heating the sample at 3 K/s to 1500 K after deposition of the film.

Ti(0001) sample utilized by Chen et al., perhaps due to the limited volume of the thin films deposited in this study. Temperature-programmed desorption of hydrogen from these surfaces resulted in a single desorption feature at ∼850 K. A single spectrum acquired in this manner is shown in Figure 12 in comparison to a TPD spectrum of hydrogen taken after deposition of a TiC film at 77 K. The hydrogen desorption peak seen at ∼360 K for films deposited in C2H4 is absent from the hydrogenated film and must be affiliated with decomposition of hydrocarbon surface intermediates. As ethylidyne is observed to decompose between 300 and 450 K on carburized Ti(0001),23 we attribute hydrogen desorption at low temperatures in part to CCH3 dissociation. Further, the correlation between hydrogen desorption from the two films in Figure 12 suggests that the high-temperature feature is associated with reaction-limited recombinative desorption of hydrogen from Ti-H bond cleavage. The critical temperature range (T > 300 K) observed by Chen et al. for C2H4 decomposition or desorption correlates with the optimum deposition temperature for RBD TiC, 350 K.23 This may indicate that at lower temperatures (T < 350 K) the advantageous effects on the reaction kinetics of film deposition outweigh the detrimental aspects of enhanced surface diffusion. Indeed, temperatures in excess of 300 K are necessary for complete carbon-carbon bond cleavage on both Ti(0001) and carburized Ti(0001). Therefore, it appears that total dissociation of C2H4 (C2 f C1) on the surface during film synthesis plays an important role. We can only speculate how this impacts the surface area of the film. Two general mechanisms are possible. The first scheme considers that the synthesis of TiC may be limited by the decomposition of the carbon precursor, C2H4. If this is the case, the highest surface areas may be achieved when the surface readily dissociates the hydrocarbon fragments containing C-C bonds. Following the observations of Chen et al., thermally induced C-C bond cleavage occurs at temperatures greater than 300 K, so the sample may not form a stable TiC structure until it is heated sufficiently to decompose C2H4. This scheme may be thought of as reaction-limited growth. The second scheme considers that the surface temperature affects the surface composition (through desorption and reaction of surface species) as well as adatom diffusion. For a given potential energy landscape, an increase in temperature will directly increase the rate of adatom diffusion, which would decrease the film’s surface area. However, in the case of C2H4 decomposition over Ti and TiC, an increase in surface temperature also necessarily changes the chemical nature and composition of the surface as C2H4 fragments react and desorb. For TiC, the “clean” surface

We have studied the synthesis of high surface area, porous, and thermally robust titanium carbide films created by the reactive ballistic deposition technique. The chemical nature of carbon within the film is dependent upon the deposition temperatures and annealing with higher temperatures, resulting in a more ordered carbide structure. Important parameters that impact the film structure include the deposition angle and temperature, as well as postgrowth annealing treatment. A maximum surface area of ∼840 m2/g was observed for films grown with a deposition angle of ∼65° and a deposition temperature of ∼350 K, in contrast to previous investigations where the surface area was maximized at cryogenic temperatures. The influence of the deposition temperature is attributed to incomplete decomposition of C2 surface intermediates at temperatures of 300 K or lower, as well as changes in Ti adatom surface diffusion influenced by desorption of byproducts formed via the carburization reaction. The exact manner by which the deposition temperature affects the film growth remains unclear. These TiC films are thermally robust and retain their structure and substantial surface area after annealing to temperatures in excess of 1000 K. Titanium carbide grown in this manner may be suited as a material for heterogeneous catalysis due to its high surface area and thermal stability. Further, this low-temperature growth process may have advantages for producing coatings on substrate materials not compatible with the traditional, high-temperature techniques used for synthesizing transition-metal carbides. We are currently investigating the catalytic activity of these films for relevant model reactions including hydrogenation, dehydrogenation, and hydrogenolysis. Acknowledgment. We thank Dr. Jinlong Gong for helpful discussions and Chris Earle for experimental assistance. We also acknowledge the Defense Threat Reduction Agency (Grant CBT070005974), the National Science Foundation (Grant CTS0553243), and the Welch Foundation (Grant F-1436) for their generous support. Finally, D.W.F. acknowledges the Bruce B. Jackson Endowed Graduate Fellowship in Engineering. Supporting Information Available: Temperature-programmed desorption spectra of cyclohexane and cyclopropane, illustrating the effect of the deposition angle on the characteristic pore diameter. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Levy, R. B.; Boudart, M. Science 1973, 181, 547. (2) The Chemistry of Transition Metal Carbides and Nitrides; Oyama, S. T., Ed.; Blackie Academic and Professional: London, 1996. (3) Chen, J. G.; Fruhberger, B.; Eng, J. J.; Bent, B. E. J. Mol. Catal. A 1998, 131, 285. (4) Hwu, H. H.; Chen, J. G. Chem. ReV. 2005, 105, 185. (5) Oyama, S. T. Catal. Today 1992, 15, 179. (6) Chen, J. G. Chem. ReV. 1996, 96, 1477. (7) Haxel, G. B.; Hedrick, J. B.; Orris, G. J. Rare Earth ElementssCritical Resources for High Technology. USGS Fact Sheets; U.S. Geological Survey: Reston, VA, 2002; Vol. 087-02.

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